Polycaprolactone Nanocomposites Reinforced with Cellulose

Subscriber access provided by Gothenburg University Library

Article

Polycaprolactone nanocomposites reinforced with cellulose nanocrystals surface-modified via covalent grafting or physisorption - A comparative study Assya Boujemaoui, Carmen Cobo Sanchez, Joakim Engström, Carl Bruce, Linda Fogelström, Anna Carlmark, and Eva Malmström ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09009 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Polycaprolactone nanocomposites reinforced with cellulose nanocrystals surface-modified via covalent grafting or physisorption - A comparative study Assya Boujemaoui,†§ Carmen Cobo Sanchez,†§ Joakim Engström,‡ Carl Bruce, † Linda Fogelström,†‡ Anna Carlmark†* and Eva Malmström†* KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, † Division of Coating Technology ‡ Wallenberg Wood Science Center, SE-100 44 Stockholm, Sweden

ABSTRACT

In the present work, cellulose nanocrystals (CNCs) have been surface-modified either via covalent grafting or through physisorption of poly(n-butyl methacrylate) (PBMA) and employed as reinforcement in PCL. Covalent grafting was achieved by surface-initiated atom transfer radical polymerization (SIATRP). Two approaches were utilized for the physisorption; either using micelles of poly(dimethyl aminoethylmethacrylate)-block-poly(n-butyl methacrylate) (PDMAEMA-b-PBMA) or latex nanoparticles of

poly(dimethyl

aminoethylmethacrylate-co-methacrylic

acid)-block-poly(n-butyl

methacrylate)

(P(DMAEMA-co-MAA)-b-PBMA). Block copolymers (PDMAEMA-b-PBMA)s were obtained by ATRP and subsequently micellized. Latex nanoparticles were produced via reversible addition-fragmentation 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

chain-transfer (RAFT) mediated surfactant-free emulsion polymerization, employing polymer-induced self-assembly (PISA) for the particle formation. For a reliable comparison, the amounts of micelles/latex particles adsorbed and the amount of polymer grafted onto the CNCs were kept similar. Two different chain lengths of PBMA were used, below and above the critical molecular weight for chain entanglement of PBMA (Mn,c ~ 56,000 g mol-1), were targeted. Poly(ε-caprolactone) (PCL) nanocomposites reinforced with unmodified and modified CNCs in different weight percentages (0.5, 1, and 3 wt%) were prepared via melt extrusion. The resulting composites were evaluated by UV-Vis, SEM, TGA, and tensile testing. All materials resulted in higher transparency, greater thermal stability and stronger mechanical properties than unfilled PCL and nanocomposites containing unmodified CNCs. The degradation temperature of PCL reinforced with grafted CNCs was higher than that of micelle-modified CNCs, and the latter was higher than that of latex-adsorbed CNCs with a long PBMA chain length. The results clearly indicate that covalent grafting is superior to physisorption with regard to thermal and mechanical properties of the final nanocomposite. This unique study is of great value for the future design of CNC-based nanocomposites with tailored properties.

KEYWORDS: cellulose nanocrystals (CNCs), covalent grafting, physisorption, reversible-deactivation radical polymerization (RDRP), poly(ε-caprolactone) (PCL), nanocomposites. INTRODUCTION Cellulose nanocrystals (CNCs) are rod-like nanoparticles of high interest owing to their abundance, renewability, degradability, light weight, and low production cost.1-2 The combination of these features, together with their high Young’s modulus, large specific surface area, and high aspect ratio, renders CNCs an ideal reinforcing agent for nanocomposite applications.3-5 CNCs are generally prepared by acid hydrolysis of cellulose fibers using sulfuric acid, generating negatively charged sulfate groups on the surface and thus offering good stability to aqueous suspensions of CNCs.6-7

2


Page 2 of 49

Page 3 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In several studies, CNCs have been incorporated into polymer matrices as a reinforcing agent.8-10 However, due to the hydrophilic character of CNCs, chemical surface modification, either via covalent grafting or physisorption of low or higher molecular weight molecules, has been attempted to achieve good dispersion and adhesion in the non-polar matrices.11-14 It is worth mentioning that simultaneous preparation and functionalization of CNCs using low molecular weight compounds have been reported,1516

. In recent years, much attention has been directed to cellulose surface modification with controlled and

well-defined homo-or block-copolymers prepared by reversible-deactivation radical polymerization (RDRP) techniques, as first reported by Carlmark and Malmström.17-18 For example, polystyrene (PS)19 and poly(methyl methacrylate) (PMMA)15 have been covalently grafted onto CNCs via atom transfer radical polymerization (ATRP), and poly(N-isopropylacrylamide) (PNIPAM), poly(acrylic acid) (PAA), and PNIPAM-co-PAA via reversible addition-fragmentation chain-transfer polymerization (RAFT).20 Cellulose surfaces can also be modified by the physisorption of polyelectrolytes or amphiphilic molecules, and several block-copolymers based on a hydrophobic polymer and a cationic block have been synthesized via RDRP for this purpose.21-22 To the best knowledge of the authors, CNC-surface modification via physisorption of block copolymers synthesized via RDRP has not previously been attempted, although this approach has been widely used for the surface modification of other cellulose substrates, such as cellulose nanofibrils (CNF) and cellulose-model surfaces.21-22 For example, micelles of block-copolymers with quaternized poly(dimethyl aminoethylmethacrylate) (PDMAEMA) as hydrophilic cationic block and various hydrophobic blocks, such as polybutadiene23 and polystyrene (PS),24 have been synthesized via ATRP and adsorbed onto cellulose surfaces. Latex particles of block copolymers have also been studied, for example poly(dimethyl aminoethylmethacrylate-co-methacrylic acid)-block-poly(methyl methacrylate) (P(DMAEMA-co-MAA)-b-PMMA) latex particles have been synthesized via RAFTmediated surfactant-free emulsion polymerization employing polymerization-induced self-assembly (PISA) and adsorbed onto a cellulose model surface.25 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Good adhesion between CNCs and the host matrix, i.e. a strong interface, is a key parameter to improve the stress transfer between the constituents of the nanocomposite, and thus to achieve good material performance.2, 5 In earlier work, it has been shown that the molecular weight of the polymer grafted on the cellulose surface has a significant impact on the interfacial toughness at the polymer-polymer interface,26 and that the mechanical properties of the nanocomposite were significantly improved for the longest polymer graft length.27 Melt-compounding, such as extrusion, is attractive for the industrial production of CNC-reinforced nanocomposites.14,

28

The major drawback of the extrusion process is the low thermal stability of the

CNCs; the sulfate groups on the CNC surface generate sulfuric acid upon heating and thus, induce cellulose-chain degradation.29-30 To overcome this, shielding of the sulfate groups using high molecular weight molecules attached to the CNCs, either via covalent grafting or physisorption, has been proposed. In a recent study by Dufresne and Lin, CNCs was modified either by the covalent attachment of methoxypoly(ethylene glycol) amine (MPEG) (Mw = 5 kg mol-1) via a carboxylation-amidation reaction or by physisorption of polyoxyethylene (PEO) (Mw = 5 000 kg mol-1) taking advantage of the hydrophilic affinity between PEO and cellulose. The modified CNCs were incorporated in a PS matrix and the resulting nanocomposites showed improved thermomechanical properties.31 It is hypothesized that the methodology used for the modification will influence the interface between CNCs and the host matrix, and thereby the overall performance of the nanocomposite. The grafting-to and grafting-from methodologies are compared for cellulose modification32-33 but to the best knowledge of the authors, no studies comparing covalent grafting and modification by physisorption with respect to nanocomposite performance, have previously been reported. The aim of the present work is to investigate how the method used to modify CNCs affects the performance of the final nanocomposite. CNCs were modified via physisorption of micelles based on poly(dimethyl aminoethylmethacrylate)-block-poly(nbutyl methacrylate) (PDMAEMA-b-PBMA), and latex particles P(DMAEMA-co-MAA)-b-PBMA, and by covalent grafting of PBMA. In the block-copolymers, i.e. micelles and latex particles, the PDMAEMA 4


Page 4 of 49

Page 5 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


block length was kept constant while the molecular weight (Mn) of the hydrophobic PBMA block was varied. Two different PBMA lengths were studied, aiming for Mn above and below the critical molecular weight for chain entanglement of PBMA, ~ 56 000 g mol-1.34 To render it possible to perform reliable comparisons, it is crucial that samples prepared with different methods contain the same fraction of CNCs. Nanocomposites were prepared by melt extrusion of modified CNCs and PCL and the properties (transparency, thermal stability, mechanical performance) of the of the resulting nanocomposites were investigated.

EXPERIMENTAL SECTION Materials Butyl methacrylate (n-BMA, 99%) and 2-(dimethylamino)ethyl methacrylate (DMAEMA, 98%) were purchased from Sigma-Aldrich, passed through basic alumina and stored at -20 °C prior to use. Poly(εcaprolactone) (PCL, Mn 80 000 g mol-1, ÐM 99%), α-bromoisobutyryl bromide (BIB, 98%), ethyl αbromoisobutyrate

(EBIB,

98%),

triethylamine

(TEA,

≥99%),

1,1,4,7,10,10-

hexamethyltriethylenetetramine (HMTETA, 97%), copper(I) bromide (CuBr, ≥99%), copper(II)bromide (CuBr2, 99%), 2,2’-azobis(2-methylpropionamidine) dihydrochloride (AIBA, 97%), sulfuric acid (H2SO4, pure), hydrochloric acid (HCl, 37%), N,N-dimethylformamide (DMF, HPLC grade), tetrahydrofuran (THF, HPLC grade), toluene (HPLC grade), methanol (HPLC grade), and filter paper (Whatman No. 1) were used as received from Aldrich. Methods Proton Nuclear Magnetic Resonance (1H-NMR) was conducted on a Bruker AM 400 at 400 MHz using deuterated chloroform (CDCl3) as solvent to determine the conversion of the non-grafted PBMA, PDMAEMA and PDMAEMA-b-PBMA. The solvent signal was used as an internal standard. The theoretical molecular weights (Mn,th) of macroinitiator and block-copolymers were estimated from the monomer conversions as assessed by 1H-NMR. 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 49

Size Exclusion Chromatography (SEC) with dimethyl formamide (DMF) as mobile phase was utilized to determine molecular weights and molar-mass dispersities (ÐM). The analyses were performed on a TOSOH EcoSEC HLC-8320GPC system equipped with an EcoSEC RI detector and three columns (PSS PFG 5µm; Microguard, 100 Å, and 300 Å) (Mw resolving range: 100-300 000 g mol-1) from PSS GmbH, using DMF (0.2 mL min-1) with 0.01 M LiBr as the mobile phase at 50 °C. A conventional calibration method was created using narrow, linear poly(methyl methacrylate) standards ranging from 700 to 2 000 000 g mol-1. Corrections for flow-rate fluctuations were made using toluene as an internal standard. PSS WinGPC Unity software version 7.2 was used to process data. Polyelectrolyte titration (PET) of PDMAEMA-b-PBMA and P(DMAEMA-co-MAA)-b-PBMA samples was conducted with a 716 DMS Titrino from Metrohm (Switzerland). Ortho-toluidine blue (OTB) and potassium polyvinyl sulfate (KPVS) were used as indicator and titrant, respectively. Fotoelektrischer Messkopf 2000 (BASF) was used to record the color change from which the amount of KPVS needed to reach the equilibrium point was determined, according to the method developed by Horn et al.35 Quartz crystal microbalance with dissipation (QCM-D) was utilized to monitor the adsorption in a QCM-E4 from Q-sense AB with a continuous flow of 0.15 mL min-1. This instrument measures the change in resonance frequency of the crystal, corresponding to a change in mass attached to the surface. To convert a change in frequency to its corresponding change in adsorbed mass per area unit, the Sauerbrey model36 was used: ∆

=

(1)

where C is a sensitivity constant, -0.177 mg (m2 Hz)−1, ∆f the change in resonance frequency (Hz), and n the overtone number. The dissipation is related to the viscoelastic properties of the adsorbed layer. A thin, rigid attached film is expected to yield a low change in dissipation. A more water-rich and mobile film is expected to yield a larger change in dissipation. The dissipation factor, D, is defined as:

6


Page 7 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


=−

(2)

where Edissipated is the energy dissipated during one oscillation period, and Estored is the energy stored in the oscillating system. The Sauerbrey model assumes rigidly attached layers, and the attached amount determined contains not only polymer but also other compounds coupled to the surface. Earlier work has shown that this model is also valid for layers with higher dissipations and is comparable to more advanced models.37 The silica-coated QCM crystals were rinsed with MilliQ water, ethanol and finally MilliQ water again, and thereafter dried under N2. The crystals were then placed in an air plasma cleaner (Model PDC 002, Harrick Scientific Corporation, NY, USA) under reduced air pressure for 120 s at 30 W. Dynamic light scattering (DLS) analysis of PDMAEMA-b-PBMA micelles and P(DMAEMA-coMAA)-b-PBMA latex was performed on a Malvern Zetasizer NanoZS in order to determine their hydrodynamic radii and size distribution based on Stokes-Einstein equation, assuming spherical particles. Fourier transform-infrared spectroscopy (FT-IR) was performed on a Perkin-Elmer Spectrum 2000 FTIR equipped with a MKII Golden Gate, Single Reflection ATR system from Specac Ltd, London, U.K. All spectra were the average of 32 scans and were normalized with respect to the ATR crystal absorption region (2300-1900 cm-1). Thermal gravimetric analysis (TGA) was performed on a TGA/DSC 1 Mettler Toledo AG, Analytical Switzerland, to determine the thermal decomposition of the modified CNCs. All samples were heated from 40 °C to 700 °C at a heating rate of 10 °C min−1 in oxygen with a flow rate of 80 ml min−1. Differential scanning calorimetry (DSC) was performed on a Mettler Toledo DSC 820 equipped with a Sample Robot TSO801RO, calibrated using standard procedures, with cooling and heating rate of 10 °C min-1. The sample was heated from -60 °C to 120 °C and equilibrated for 3 min, and thereafter cooled to 60 °C. After being equilibrated for 3 min the sample was then reheated to 120 °C. The degree of crystallization (XC) was estimated from the crystallization transition according to the equation: = ΔH/(w ⨯ ΔH !!! )

(3)

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

where ∆H is the heat of fusion of the sample, w is the weight fraction of PCL, and ∆H0100 is the heat of fusion for a 100 % crystalline PCL, 136 J g-1 according to the literature.38 Ultraviolet-visible spectroscopy (UV-Vis) was conducted on a Shimadzu UV-2550 UV-VIS Spectrophotometer (Kyoto, Japan), the software UVProbe 2.0 being used to assess the data. Tensile tests were performed with an Instron 5944 tensile tester. Pure PCL and nanocomposite films (130 µm thickness and 5 mm width) were tested at least in quadruplicate, with a 500 N load cell, a gauge length of 25 mm and a strain rate of 80 % min-1. The samples were conditioned at 23 ± 1 ºC and 50 % relative humidity (RH) for three days or at 98 % RH for one week prior to testing. The surface morphology and cross section of polymer films were observed by field-emission scanning electron microscopy (FE-SEM) using a Hitachi S-4800 equipped with a cold field emission electron source. Images were captured for samples coated with a ca. 5 nm layer of platinum-palladium using an Agar HR sputter coater. Atomic force microscopy (AFM) with tapping-mode AFM (Multimode IIIa, Veeco Instruments, Santa Barbara, CA) was utilized to determine the CNC dimensions. Dilute CNC-water dispersions (0.0001 wt%) was applied on mica surfaces, modified with poly(l-lysine) (1 %) and the excess was washed off with Milli-Q water. The frieze dried grafted CNCs (CNC-Graft) and physisorbed micelle on CNCs (CNCMicelle) were separately dispersed in THF and then deposited on mica surface. The sample surface was dried under air flow and scanned with a cantilever having a tip radius 8 nm and typical spring constant 40 N m−1. NanoScope Analysis 1.5 software was used for particle size determination. XRD diffractograms were collected utilizing a ARL™ X'TRA Powder Diffractometer (Thermo Fisher Scientific Inc., USA) with operated at 45 kV and 40 mA. The samples were scanned at 2θ angular range 10–60º and the increment was 0.02º. The plotted diffractograms are represented from 2θ angular range 10–40º as there was no useful information in the range of 40–60º.

Preparation of negatively charged CNCs 8


Page 8 of 49

Page 9 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cellulose nanocrystals were prepared according to the method described in the literature with slight modification.7 Briefly, ground filter paper (20 g) was added to a round-bottomed flask equipped with a magnetic stirrer. Thereafter, a solution of sulfuric acid (64 wt%, 175 mL) was added, and the reaction flask was immersed in an oil bath, preheated to 45 °C, for 45 min. Thereafter, the cellulose suspension was diluted 10-fold with deionized water and washed by repeated centrifugation and dispersion. The CNCs were then dialyzed against deionized water for 10 days, to remove traces of sulfuric acid and hydrolysis degradation byproducts. The pH of the CNC suspension after dialysis was similar to deionized water pH (around 6). The membrane used for dialysis was 6.4 mL cm-1 regenerated cellulose with a 12-14 KD cut-off. The CNC suspension (1 wt%) was sonicated for 30 min at an amplitude of 28 % and 5 s pulse, and finally filtered over a pore 1 glass filter. The charge of the CNCs was determined to be 263 ± 6 µeq g-1 and the estimated size by DLS was 229 ± 5 nm and by AFM 112 ± 57 nm in length, 12.5 ± 7.5 nm in width (Figure S1). The crystallinity index (CrI) of the CNCs was 82 % according to XRD results (Figure S4 and Table S2). Preparation of CNC-Br Freeze-dried CNCs (2.00 g), TEA (48.0 g, 0.474 mol), and DMF (200 mL) were added to a roundbottomed flask equipped with a magnetic stirrer, after which BIB (37.2 g, 0.162 mol) was added dropwise under stirring. Thereafter, the reaction mixture was stirred at 70 °C for 24 h, filtered through an extraction thimble and purified by successive Soxhlet extractions with dichloromethane for two days followed by another two days with ethanol. The modified CNCs (CNC-Br) were solvent-exchanged to acetone and then to toluene by repeated dispersion/filtration cycles 10 times for each solvent, and finally stored at 4 °C prior to use. The resulting CNCs bearing the ATRP initiator functionality were denoted CNC-Br. Covalent grafting of PBMA on CNC-Br by ATRP In a typical procedure, CNC-Br (0.50 g) was dispersed in toluene (25.0 g) in a round-bottomed flask (100 mL) equipped with a magnetic stirrer and the dispersion was sonicated in a sonication bath for 5 min prior to the reaction. Thereafter, BMA (25.0 g, 176 mmol) was added, and the flask was immersed in an 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 49

ice/water bath, whereafter EBIB (137 mg, 0.70 mmol) (sacrificial initiator) and HMTETA (203 mg, 0.88 mmol) were added to the mixture. The reaction flask was degassed by one vacuum/argon cycle after which Cu(I)Br (100 mg, 0.70 mmol) and Cu(II)Br (39.3 mg, 0.18 mmol) were added under argon flow. Finally, the round-bottomed flask was sealed with a rubber septum, degassed with 2 vacuum/argon cycles and then immersed in an oil bath pre-heated to 80 °C. The monomer conversion was monitored by withdrawing 1H-NMR samples and stopped at a conversion at about 70 % by exposing the reaction mixture to air and diluting the reaction mixture with DCM. The polymer-grafted CNCs (CNC-Graft(S or L)) were separated and purified from the free polymer by dispersing the reaction mixture in DCM and then filtering. The polymer-grafted CNCs were purified by Soxhlet extraction with THF for 24 h to remove any unbound polymer, and then with methanol for 24 h to remove traces of copper ions. The free polymer was passed through a column of neutral aluminum oxide to remove copper and was then precipitated in cold methanol, decanted and finally dried under vacuum at 50 °C overnight. Two graft lengths were targeted and the grafted CNC samples are denoted CNC-Graft(S) and CNC-Graft(L) for short and long grafts, respectively.

Synthesis of PBMA latex by RAFT-mediated surfactant-free emulsion polymerization employing polymerization-induced self-assembly (PISA) PBMA latex nanoparticles were synthesized according to a previously reported method.39 In a typical experiment, P(DMAEMA-co-MAA) macroRAFT agent25, 39 (0.30 g, 72.5 µmol) was added to a roundbottomed flask (50 mL) equipped with a magnetic stirrer bar, followed by deionized water (37.2 mL). Thereafter, the BMA monomer (7.27 g, 51.1 mmol) and an aqueous solution of AIBA initiator (3.4 g L-1) were added to the reaction mixture (2.4 mg, 8.8 µmol in 1:8.25 molar ratio to the macroRAFT) to reach a total final solids content of 16.7 wt%. The reaction mixture was degassed with argon on an ice bath for 30 min, whereafter the flask was immersed in an oil bath pre-heated to 70 °C. All the reactions were run for 120 min. The monomer conversion was monitored by gravimetric analysis of the dry weight by 10


Page 11 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


withdrawing samples during the polymerization. The latex block copolymers formed were analyzed by 1

H-NMR and DMF-SEC.

Synthesis of PDMAEMA-b-PBMA by ATRP PDMAEMA macroinitiator (4220 g mol-1, ÐM = 1.14) was synthesized according to a procedure described elsewhere.40 The general procedure used for the polymerization of BMA from the PDMAEMA macroinitiator was as follows: To a round-bottomed flask (50 mL), PDMAEMA (422 mg, 0.10 mmol) dissolved in acetone (50 wt% in relation to BMA) was added, and the flask was immersed in an ice/water bath. HMTETA (28.7 µL, 0.11 mmol) and BMA (5.00 g, 35.2 mmol) were added and the reaction flask was sealed and degassed with one cycle of vacuum/argon. Thereafter, Cu(I)Cl (10.4 mg, 0.11 mmol) was added and the reaction mixture was degassed with two additional cycles of vacuum/argon, after which the flask was placed in an oil bath pre-heated to 50 °C and the mixture was left to react until the desired degree of monomer conversion (30 %) had been reached. The reaction was monitored by withdrawal of aliquots for 1H-NMR analysis. The reaction was terminated by exposure to air, acetone was removed by rota-evaporation, and the reaction mixture was dissolved in THF before being passed through a neutral aluminum oxide column to remove copper complexes. The product was concentrated by rota-evaporation, dissolved in DCM and then precipitated twice in cold diethyl ether. The final polymer was dried under reduced pressure overnight at room temperature. The PDMAEMA-b-PBMA was analyzed by 1H-NMR and DMF-SEC and quaternized according to a procedure described elsewhere.40 Micelle aqueous solution (0.1 mg mL-1, which is above the critical micelle concentration (Figure S2)) were prepared by drop-wise addition to water of quaternized PDMAEMA-b-PBMA, with short or long PBMA block, dissolved in THF. Physisorption of P(DMAEMA-co-MAA)-b-PBMA latex particles and PDMAEMA-b-PBMA micelles on negatively charged CNCs The physisorption of P(DMAEMA-co-MAA)-b-PBMA latex particles or PDMAEMA-b-PBMA micelles was performed according to the following procedure: typically, to a CNC aqueous dispersion 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 49

(0.01 wt%) a dispersion of P(DMAEMA-co-MAA)-b-PBMA latex particles or PDMAEMA-b-PBMA micelles was slowly added under rigorous stirring. Thereafter, the mixture was stirred for one hour followed by freeze-drying overnight. Samples, denoted according to the degree of polymerization of the PBMA segment as short(S) or long (L): CNC-Latex(S), CNC-Latex(L), CNC-Micelle(S), CNCMicelle(L), were characterized by FT-IR and TGA. Formulation and preparation of PCL composites reinforced with CNCs PCL pellets (5 g) were mixed together with the modified and unmodified dry CNCs in a twin miniextruder operated at 100 rpm, at 110 ºC for 6 min. Nanocomposites with a CNC-content of 0.5, 1, or 3 wt% were prepared. A reference sample of neat PCL was produced using the same procedure. The extruded materials were then hot-pressed into 130 µm thick films (200 kN, 80 ºC for 10 min). The nanocomposite samples were denoted PCL-xCNC-Graft/Micelle/Latex)(S/ L) where x is the CNC content (wt%) in the nanocomposite, Graft/Micelle/Latex corresponds to the method used for the surface modification, and S and L refer to the length of the PBMA-chain, S(short) and L(long).

RESULTS AND DISCUSSION Adequate compatibility and adhesion between the components in the nanocomposites are key factors to achieve good dispersion and uniform distribution of the nanosized reinforcing agent within the polymer matrix. The main goal of this work was to investigate how the properties of CNC-reinforced nanocomposites, based on PCL, are influenced by polymer modification of CNCs. Three different approaches have been explored: covalent grafting of poly(butyl methacrylate) (PBMA) or by physiosorption of PDMAEMA-b-PBMA micelles or P(DMAEMA-co-MAA)-b-PBMA latex particles, Scheme 1. The grafting method provides a stable modification but suffers from a more tedious and less green protocol involving organic solvents, for manufacturing. The micellized block copolymers and the nanolatex can be obtained as aqueous dispersions that can be readily added to the CNC dispersion but is

12


Page 13 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hypothesized to provide a less stable final product. PBMA was chosen as the hydrophobic polymer modifying the CNCs and PCL as the matrix polymer. A series of modified CNCs was successfully prepared, two different molecular weights of the grafted and adsorbed polymers (micelles and latex particles) were targeted; based on the PBMA critical molecular weight for chain entanglement, Mc,n of about 56 000 g mol-1 corresponding to a DP of 40034, the targeted molecular weights of the PBMA-based polymers were selected to be above and below the Mc,n, and the synthesized polymers were denoted long (L) and short (S), respectively.

Scheme 1. The different surface-modification routes utilized in this work. The size of unmodified CNCs is 112 ± 57 nm in length and 12.5 ± 7.5 nm in width as determined by AFM. Surface modification of CNCs The successful synthesis of PBMA-grafted CNCs via surface-initiated ATRP (SI-ATRP) with two different DP’s is reported here. In an initial step, the ATRP initiator was anchored to the CNC surface 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 49

according to a previously described method19, and the product was denoted CNC-Br. Thereafter, SI-ATRP of BMA was carried out in the presence of a sacrificial initiator in order to control the molecular weight of the polymer grafts, targeting a DP either above or below the critical DP for chain entanglement, i.e. 400. The characteristics of the free PBMA polymers are presented in Table 1. The amount of PBMA graftedfrom CNCs was determined gravimetrically and was estimated to be 4 wt% and 28 wt% for low (DP 110) and high (DP 487) molecular weight, respectively. The physisoprtion approach is an alternative method for surface modification of CNCs, in which selfassemblies of amphiphilic block copolymers, containing both a hydrophobic and a cationically-charged block can be employed. The main advantage of this approach is that the surface modification is conducted under aqueous conditions. In this work, PDMAEMA-b-PBMA micelles and P(DMAEMA-co-MAA)-bPBMA latex particles were successfully synthesized via ATRP and RAFT-mediated surfactant-free emulsion polymerization employing polymerization-induced self-assembly (PISA), respectively. The PDMAEMA macroinitiator and the P(DMAEMA-co-MAA) macroRAFT agent were first synthesized individually. Thereafter, chain extension with the hydrophobic polymer was performed, leading to an amphiphilic block copolymer. For PDMAEMA-b-PBMA, the hydrophilic block was subsequently quaternized inducing cationic charges on its backbone. P(DMAEMA-co-MAA) were cationically charged through protonation of the PDMAEMA group, i.e. by pH regulation. The characteristics of the PDMAEMA

macroinitiator,

P(DMAEMA-co-MAA)

macroRAFT,

PDMAEMA-b-PBMA,

and

P(DMAEMA-co-MAA)-b-PBMA are summarized in Table 1. It was found that the molar masses of PDMAEMA-b-PBMA were in the same range as the targeted values, but that the P(DMAEMA-co-MAA)b-PBMA molecular weights were considerably higher than the theoretical values (Mn,th) calculated from conversion measurements. The calibration of the SEC instrument was conducted with linear PMMA standards which are anticipated to have a different hydrodynamic volume than the synthesized blockcopolymers. Moreover, P(DMAEMA-co-MAA) was found to be poorly soluble in DMF. Both these

14


Page 15 of 49

observations may be reasons for the differences observed between the Mn,th and Mn of the P(DMAEMAco-MAA)-b-PBMA block copolymers.

Table 1. Theoretical molecular weight (Mn,th), molecular weight and molar-mass dispersity (ÐM), charge, and micelle/particle size of free PBMA, PDMAEMA-b-PBMA, and P(DMAEMA-co-MAA)-b-PBMA.

Micelles Graft

Polymera PBMA110 (short) PBMA487 (long)

Mn,thb (g mol-1) 15 640 69 170

Mnc (g mol-1) 17 480 80 260

PDMAEMA27 (Macroinitiator) PDMAEMA27-b-PBMA93 PDMAEMA27-b-PBMA523

3 930 17 100 78 610

4 220 28 500 89 000

ÐMc 1.11 1.09

Charged (meq g-1) _ _

Sizee (nm) _ _

1.14 1.06 1.20

_ 1.23 0.22

_ 58 97

P(DMAEMA-co-MAA)25 (MacroRAFT) 4 200f _* _* _ _ P(DMAEMA-co-MAA)25-b-PBMA160 25 910 48 400 1.8 0.43 42 P(DMAEMA-co-MAA)25-b-PBMA684 101 380 223 100 1.39 0.14 79 a The suffix denotes the number average degree of polymerization for PBMA, PDMAEMA and PBMA blocks determined by SEC. bEstimated from the conversion with 1H-NMR. cDetermined by DMF-SEC. d Determined by PET. eDetermined by DLS. fEstimated from gravimetric measurements. *Polymer was insoluble in DMF. Latex

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Micelle formation was achieved by the drop-wise addition to water of quaternized PDMAEMA-bPBMA dissolved in THF. The surfactant-free emulsion polymerization of BMA employing RAFT-PISA starting from a positively charged macroRAFT agent led to the direct formation of P(DMAEMA-co15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 49

MAA)-b-PBMA latex particles. Particle sizes were determined by DLS and the results are presented in Table 1. As expected, larger particles were obtained for block-copolymers of higher molecular weights. A dense structure of the latex nanoparticles may explain their relatively small size compared with that of the micelles with similar PBMA molecular weights. As expected, the charge densities of micelles and latex particles, estimated by polyelectrolyte titration (Table 1) decrease with increasing molecular weight of the hydrophobic block. It is worth noting that the charge density was lower for the latex particles than for the micelles with PBMA blocks of similar molecular weight. This could be due to the quaternization of the macroinitiator, whereas the macroRAFT agent is cationically charged from protonation at a pH below the pKa-value of PDMAEMA. Unlike the PDMAEMA block in the micelles, the P(DMAEMA-co-MAA) contained MAA units which may lower the overall cationic charge density of the macroRAFT agent, and subsequently that of the latex particles. To further investigate the adsorption behavior, PDMAEMA-b-PBMA micelles and P(DMAEMA-coMAA)-b-PBMA latexes were adsorbed onto negatively charged silicon-oxide quartz crystals in the QCMD (Figure S3). The difference in adsorption behavior between latexes and micelles can be compared on smooth silicon dioxide surfaces, and such surfaces have previously been used to investigate trends in adsorption behavior.41 The adsorbed masses according to the Sauerbrey model using the third overtone were found to be 1.4 and 9.1 mg m-2 for PDMAEMA-b-PBMA micelles, and 11.6 and 31.7 mg m-2 for P(DMAEMA-co-MAA)-b-PBMA latexes, for short and long PBMA blocks, respectively. From this experiment it can be concluded that the adsorption of micelles and latexes to surfaces is significantly different, and therefore there will most probably be a difference in the mass adsorbed onto the CNCs, when comparing the different grafting approaches. Both micelles and latex particles showed a larger adsorbed mass with a longer hydrophobic block, Figure S3A, which has also been shown in previous work.39 As a larger mass of latex adsorbs, compared with respective micelles, this gives rise to different surface properties in terms of viscoelasticity, seen from the change in dissipation during adsorption, Figure S3B. 16


Page 17 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Characterization of modified CNCs After the SI-ATRP, the PBMA-grafted CNCs were washed with THF to remove any unbound PBMA and MeOH to remove residual copper. The PBMA-grafted CNCs were solvent-exchanged to water and freeze-dried to give a powder-like material with a faint green color from copper traces. The latex particles and micelles were adsorbed onto the CNCs under aqueous conditions, where after the mixtures were subsequently freeze-dried, resulting in white fluffy powder and lumpy cotton-like products, for the micelles and latex particles, respectively. Figure 1 shows freeze-dried CNCs redispersed in deionized water, and unmodified and modified CNCs in THF (2 mg mL-1) immediately after stirring, and after 30 min. As expected, unmodified CNCs aggregate in THF but not in water, due to their hydrophilic character. On the other hand, dispersion of modified CNCs showed a better stability in THF than pristine CNCs, and dispersions of modified CNCs with long PBMA chains were more stable than those with short PBMA chains. Interestingly, the sample with physisorbed latex with short PBMA showed the lowest stability of all modified samples, which could be due to insufficient coverage of the cellulose surface, indicating that the hydrophilic character of the CNCs was not altered to the same extent in this sample. The hydrophilicity of the modified CNCs was further investigated by contact angle measurements (CA), as shown in Figure 2. The contact angles obtained on compressed discs of pristine and modified CNCs, prepared using a hydraulic press for KBr pellet preparation, against water varied between 43 and 91°, which are all significantly higher than corresponding value for neat CNCs, 33°. The CA increased with increasing PBMA chain length for micelles and latex particles. The significantly highest contact angle was observed for PBMA-grafted CNCs, irrespectively of chain length of PBMA. One plausible explanation can be that the surface coverage on PBMA-grafted CNCs is more efficient than in the physisorption techniques. Furthermore, the surfaces with micelle-adsorbed CNCs showed a larger contact angle than latex-adsorbed CNCs, which is in agreement with the results observed for the dispersion stability in THF. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Neat and modified CNCs (2 mg mL-1), directly after stirring (top) and after settling for 30 min (bottom): (a) neat CNCs in water, (b) neat CNCs in THF (c) CNC-Graft(S), (d) CNC-Micelle(S), (e) CNC-Latex(S), (f) CNC-Graft(L), (g) CNC-Micelle(L) and (h) CNC-Latex(L) in THF.

18


Page 18 of 49

Page 19 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Contact angle measurements and inserted photographs taken 30 s after the water drops were placed on unmodified and modified CNC discs. The CNC samples were also analyzed by AFM. Water suspensions of modified CNCs, with either micelles or latex particles, were placed on freshly cleaved mica surfaces and then dried under air flow. The grafted CNC samples were dispersed in THF and a drop was applied on freshly cleaved mica surface and then dried under air flow. The morphology of unmodified and modified CNCs was observed by AFM in tapping mode (Figure 3). The size of the CNCs increased after modification by physisorption of micelles or covalent grafting with formation of aggregates of modified CNCs. The AFM images of CNCLatex clearly show adsorbed latex particles on the CNC surface. Moreover, some CNCs are unbound to the latex particles which may explain the lower hydrophobicity of these samples compared with micelle modified CNCs. The CNC-Micelle nanoparticles exhibit different shapes, from rod like to more round shapes. In both cases the average width of the particles is larger than the unmodified CNCs which indicates that the micelles are successfully adsorbed on the CNCs surface. However, the presence of nonphysisorbed micelles could be noticed. The observed aggregation for all the samples could be related to 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 49

the sample preparation and the drying method, especially for the grafted CNCs due to bad adhesion of the sample to the mica surface and fast evaporation of THF. It is worth nothing that difficulties imaging modified CNCs, especially for the grafted CNCs, were encountered during AFM analysis. This is related to the soft nature of the PBMA attached to CNCs and the high surface coverage of CNCs for CNC-Graft sample compared to the latex and micelles adsorbed CNCs.

Figure 3. Height (left) and phase (right) AFM images 5 ×5 µm of unmodified CNCs and modified CNCs with micelles (CNC-Micelle), latex particles (CNC-Latex) and grafted CNCs (CNC-Graft). The modified CNCs were further characterized by XRD, FT-IR and TGA. The crystallinity, as assessed by XRD, of the modified CNCs decreased (Figure S4 and Table S2) as was expected since the attached polymer, PBMA, is an amorphous polymer.42 The decrease in crystallinity was more pronounced for the grafted CNCs compared with the CNCs modified via physisorption. This may be due to cleavage of cellulose chains after modification as previously reported in the literature.6 The FT-IR spectra, Figure 4, of the unmodified CNCs shows a broad peak between 3330–3306 cm−1 and a peak from 2923 to 2897 cm−1 ascribed to the O-H stretch and C-H stretch, respectively. The FT-IR spectra of the modified CNCs show a 20


Page 21 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


slight decrease of the intensity of O-H band and an increase in the intensity of C-H stretch due to the contribution from the attached polymers. In addition, modified CNCs, show an increase in the intensity of the peak around 1725 cm-1, ascribed to the carbonyl group in PBMA. As expected, the intensity of this peak increased with increasing amount of PBMA attached to the CNCs. The intensity of the peak at 1725 cm-1 for micelle-adsorbed CNCs with long PBMA chains was found to be lower than that for the graftedCNCs and latex adsorbed-CNCs, possibly because the sample texture is different, and this may influence the area of the sample in contact with the ATR crystal, which might affect the spectrum acquisition. In this case, the FT-IR could be considered only as a qualitative, rather than quantitative, method.

Figure 4. FT-IR spectra of unmodified CNCs, ATRP-initiated CNCs (CNC-Br), grafted CNCs (CNCGraft), latex-adsorbed CNCs (CNC-Latex) and micelle-adsorbed CNCs (CNC-Micelle). The letters L and S after the sample names indicate long and short PBMA chains, respectively. The inset spectra show an enlargement of the 1760-1680 cm-1 region. Unmodified CNCs undergo thermal degradation starting at 150 °C due to the presence of sulfate groups on its surface.29 When heated, these charged groups generate corrosive species which catalyze the thermal degradation of the CNCs. The thermal degradation, assessed by TGA, of CNCs before and after chemical 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 49

surface modification is shown in Figure 5. The grafted-CNC samples showed a better thermal stability than most of the CNCs treated by physisorption; 5 wt% was lost at 235 °C compared with about 40 wt% for the unmodified CNCs and for the micelle/latex-adsorbed CNCs with short PBMA blocks, presumably due to a better coverage of the sulfate groups by covalent grafting even with short PBMA chains. It is also probable that the sulfate groups were uncharged during the covalent grafting of the CNCs through, for example,

complexation with copper ions.43 Interestingly, the thermal stability of the micelle/latex-

adsorbed CNCs with long PBMA blocks, where the weight losses were 5 wt% and 35 wt% at 235 °C respectively, was significantly better than that of the CNCs adsorbed with short blocks. This was expected as the amount of micelles/latexes with long PBMA blocks was higher than the amount with short PBMA blocks. Moreover, the thermal behavior of micelle-adsorbed CNCs with long PBMA blocks was similar to that of the corresponding grafted CNCs and showed significantly better stability than the latex-adsorbed CNCs with similar PBMA chain length, arguably due to the adsorption mechanism in combination with the glass transition temperature (Tg) of the attached PBMA affecting the coverage of the CNCs. The Tg of the micelles and latex particles were assessed by DSC, and the results are summarized in Table SI1. The Tg of the micelles was 28 °C while that of the latexes was found to be 6 degrees higher, probably due to the more restricted chain mobility in the dense core of the latex particles. The higher Tg of the latex particles limits the spreading of PBMA on the CNCs at room temperature, and thus gives poorer coverage of the sulfate groups compared with the micelles. To test this hypothesis, latex- and micelle- adsorbed CNCs were annealed at 50 °C for 2 hours prior to TGA analysis. Figure 5 presents thermograms of latexand micelle-adsorbed CNCs and of the corresponding annealed samples. As expected, no difference was observed in the case of the micelle-adsorbed CNCs, indicating that micelles were spread over the surface upon adsorption. A similar observation was made in the case of the short PBMA latex-adsorbed CNCs, where the thermal stability was unaltered by the annealing, probably due to the small amount of attached latex particles, i.e. 4 wt%. Interestingly, annealed long PBMA latex-adsorbed CNCs exhibited a higher thermal stability than the non-annealed sample, confirming that annealing of PBMA latex at a temperature 22


Page 23 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


higher than the Tg induced polymer spreading on the CNC surface, and thus a better shielding of the sulfate groups.

Figure 5. TGA thermograms of unmodified CNCs, grafted CNCs (CNC-Graft), latex-adsorbed CNCs (CNC-Latex) and micelle-adsorbed CNCs (CNC-Micelle). The letters L and S after the sample name indicate long and short PBMA chains, respectively. (*) samples annealed at 50 °C for 2 hours. Characterization of PCL nanocomposites reinforced with unmodified and modified CNCs

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 49

PCL nanocomposites with modified CNCs (i.e. CNC-Graft, CNC-Micelle and CNC-Latex) at 0.5, 1 and 3 wt%, were prepared via melt extrusion and subsequent hot-pressing. Two reference samples, unfilled PCL and PCL filled with pristine CNCs, were also produced. The resulting nanocomposites were compared in terms of their transparency, morphology, thermal and mechanical performance. The appearances of the nanocomposite films are shown in Figure S5. Regardless of the content of CNCs, it can be observed that all the films contain “spots”, indicating uneven dispersion of the CNCs within the matrix. These aggregates were dark in nanocomposites containing unmodified CNCs, CNC-Latex and CNCMicelle(S) which indicates that thermal degradation of the CNCs occurred during extrusion at 110 °C, probably due to the lower degradation temperature of these CNCs as discussed earlier. In a previous study, a similar observation was reported for PS reinforced with CNCs modified with PEO and/or PEG where the dark color of the dots was intensified with increasing CNCs content.31 Figure 6 shows that reinforced PCL films exhibit a greater UV-transmittance than unfilled PCL, even at 3 wt% loading. The transparency was higher for all long PBMA samples. This behavior has previously been discussed in the literature and is attributed to be an effect of the better compatibility between the nano-reinforcing agent and the matrix.44 For PCL-CNC and PCL-Graft the transparency increased with a CNC content up to 1 wt% and thereafter decreased whereas micelles and latexes show a steady increase with increasing wt% of CNCs. The efficient surface coverage obtained for CNC-Graft might promote adhesion of modified CNC nanoparticles to each other inducing agglomeration and thus decreasing the transparency at higher loadings. Furthermore, changes in crystallinity can also influence the transparency of the nanocomposite films (Table S3).

24


Page 25 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. UV spectra for PCL, PCL-CNC, PCL-CNC-Latex, PCL-CNC-Micelle and PCL-CNC-Graft with 0.5, 1 and 3 wt% of CNCs. The letters L and S after the sample name denotes long and short PBMA chains, respectively.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 49

Figure 7. SEM micrographs (10000× magnification) of annealed (a) PCL, (b) PCL-3CNC-Graft(S), (c) PCL-3CNC-Micelle(S), (d) PCL-3CNC-Latex(S), (e) PCL-3CNC, (f) PCL-3CNC-Graft(L), (g) PCL3CNC-Micelle(L) and (h) PCL-3CNC-Latex(L). SEM micrographs of annealed PCL films reinforced with 3 wt% of pristine or modified CNCs are shown in Figure 7. For the sample prepared with pristine CNCs, non-homogeneously dispersed white “dots” are observed in Figure 7e, which is accordance with prior observations.45 PCL nanocomposites reinforced with modified CNCs are significantly more homogeneous. In Figures 7f-h, (samples with long PBMA chains) the CNCs appear to be embedded in the PCL matrix rather than exposed at the surface, in contrast to PCL reinforced with CNCs, physisorbed with short PBMA chains (Figures 7c and d). This suggests that the compatibility between the treated CNCs treated by physisorption and the host matrix increases with increasing PBMA length. Additionally, individual “spots” can be observed in PCL reinforced with physisorbed CNC samples (Figure S7 c-d and g-h), possibly due to the formation of a separate phase of unabsorbed/detached micelles or latex particles and the PCL matrix. Figure 8 shows representative SEM micrographs of the cross sections of cryo-fractured PCL nanocomposite films with 3 wt% of pristine or CNCs modified with long PBMA blocks. Smaller particles with a rod-like structure could be observed in the samples containing modified CNCs. Images captured at 400× magnification are shown in Figure S8. At low magnification, all films reinforced with modified CNCs showed a morphology similar to that of the neat PCL film. However, larger aggregates were observed in the case of unmodified CNCs indicating, as expected, poor compatibility between the composite components.

26


Page 27 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. SEM cross-section images (20000× magnification) for (a) PCL-3CNC, (b) PCL-3CNCGraft(L), (c) PCL-3CNC-Micelle(L) and (d) PCL-3CNC-Latex(L). The thermal characterization of PCL nanocomposites was carried out by DSC analysis in duplicates. The average values of the crystallinity temperature (Tc), the melting temperature from both the first and second heatings (Tm1 and Tm2) and the degree of crystallinity from both the first and the second heatings (Xc1 and Xc2) are listed in Table S3. A slight increase in Tc and Tm1 compared with neat PCL film was observed in all the PCL nanocomposites, and the Xc1 increased from 54 % to 60 % with increasing content of unmodified CNCs. The increase in Tc and Xc1 can be attributed to the action of the CNCs as nucleating agents. A decrease in Xc1 was observed for all the PCL reinforced with modified CNCs compared with unmodified CNCs which is in agreement with literature.46 A similar trend was observed for Xc2, but the values of Xc2 were lower than Xc1 most likely as an effect of the lower cooling rate after hot-pressing, around 5 °C min-1, than the cooling rate (10 °C min-1) in the DSC analysis. For the same reason, the Tm2 values were about four degrees lower than Tm1. No significant difference was observed between the grafted CNCs and the physisorbed CNCs with regard to the Xc of the PCL nanocomposite samples.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 49

TGA thermograms of the PCL-CNC nanocomposites are shown in Figure S6. CNC-Micelle(L) and the two grafted CNCs resulted in thermally more stable nanocomposite films than films containing pristine CNCs and physisorbed micelle(S) and the two latexes. Tensile testing was conducted on PCL-based nanocomposite films at room temperature and 50 % relative humidity (RH). Tensile Young’s modulus, strength and strain-at-break are shown in Figure 9. Above Tg (-60 °C), unfilled PCL was able to undergo large elongation (ca. 1200 %) prior to break, and showed a relatively modest Young’s modulus and strength of ca. 280 MPa and 25 MPa, respectively, which is characteristic of a semicrystalline polymer. The reinforcement of the PCL matrix with unmodified CNCs increased the Young’s modulus by 28 % with 3 wt% loading, but the addition of CNCs considerably reduced the strength and strain-at-break. With 3 wt% CNCs, the strain-at-break was reduced to 79 % of the value for the unfilled material. It is reported in the literature that poor adhesion between the reinforcing agent and the matrix generates microvoids in the composite structure, and that this reduces the stress transfer from matrix to filler. In addition, unmodified CNCs are more prone to aggregate in hydrophobic matrices, leading to the formation of defects and weak points, and increasing the brittleness. The addition of modified CNCs generally increased the Young’s modulus and strength without any detrimental effect on the ductility. The addition of 3 wt% modified CNCs increased the modulus by 27 %, the strength by 600 % and strain-at-break by 1200 % compared with unmodified CNCs, which is indicative of a better compatibility between the nanoparticles and the PCL matrix, as a result of the chemical modification of the CNCs. The results are in agreement with data in the literature; in a previous study, the surface hydrophilicity of CNCs was altered by attaching n-octadecyl isocyanate. The addition of 3 wt% and 12 wt% of n-octadecyl-modified CNCs improved the Young’s modulus by 15 % and 45 %, respectively.47 Malmström et al. reported surface modification of cellulose nanofibers (CNFs) via grafting from PCL by surface-initiated ring opening polymerization (SI-ROP), where the incorporation of 3 wt% of the modified CNFs in a PCL matrix led to an increase in the modulus from 20 % to 35 % when the chain length of the grafted PCL was increased.27 Carlmark, Destarac and co-workers showed that the 28


Page 29 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


grafting of PVAc onto CNCs improved the dispersibility and the mechanical performance of CNCreinforced PVAc, compared with the neat CNCs.48 Gao and co-workers investigated CNCs grafted with polystyrene in PMMA matrix. It was found that with the addition of more than 2 wt% of CNCs, the mechanical properties decreased compared with unfilled PMMA.49 Interestingly, in the present study no clear trend was observed when the moduli of nanocomposites including CNCs with long and short PBMA chains were compared, although the strength and strain-at-break were generally higher with short rather than long PBMA chains, which is again in agreement with reported observations.27 On the other hand, better mechanical properties are expected to be obtained with the higher molecular weight material due to the possibility of chain entanglements with the PCL chains from the matrix. Dufresne and Habibi also showed that the strength and strain-at-break decreased when the length of the PCL grafted onto the CNCs was increased.50 Another possible reason is chain entanglement between adjacent PBMA chains, attached either to the same or to another nanoparticle, favored by the dissimilarity in chemical structures of PBMA and PCL.51 PCL reinforced with covalently grafted PBMA on CNCs showed a better mechanical performance than its physisorbed counterparts, and the latex-adsorbed CNCs gave the lowest performance. All the physisorbed CNCs exhibited a higher strength and strain-at-break than the unmodified CNCs. The difference observed between covalent grafting and physisorption could be related to a better coverage of the CNC surface in the former case, as indicated by the larger contact angle of water. As discussed earlier, the latex particles attach to the cellulose surface without significantly altering its morphology and retains its structural integrity, whereas the micelles (lower Tg) tend to spread out on the surface upon adsorption. In order to achieve a better spreading of the hydrophobic PBMA block, the samples should be annealed prior to their incorporation in the host matrix. The latex-adsobed CNCs were not annealed in this study in order to obtain a reasonable and simple comparison with the micelles. In order to investigate the mechanical properties of the films under high humidity conditions (98 % RH), tensile tests were conducted on PCL reinforced with 1wt% unmodified and modified CNCs with 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 49

short PBMA chain length. The test was performed on these samples, since they showed the best mechanical performance at 50 % RH. At 98 % RH, the PCL strength was considerably higher while its Young’s modulus and strain-at-break remained in the same range as at 50 % RH (Figure 10). This could be due to strong hydrogen bonding between the adsorbed water and the carbonyl groups on the PCL chains. In all cases, Young’s modulus was slightly lower after conditioning at high RH and similar to the modulus of unfilled PCL. Samples with physisorbed latex particles and micelles on CNCs showed a decrease in both strength and strain-at-break at high humidity. Since both micelles and latex particles contain hydrophilic PDMAEMA blocks, the lower mechanical strength and elongation at high RH could be attributed to a favored adsorption of water by these blocks, reducing the compatibility between CNCs and the matrix. This conclusion was supported by the fact that the mechanical properties of the nanocomposite film containing 1 wt% of covalently grafted PBMA-CNC were maintained at high RH, i.e. these nanocomposites were unaffected by the adsorption of moisture.

30


Page 31 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Mechanical properties at 50% RH of PCL, PCL-CNC, PCL-CNC-Latex (PCL-CNC_L), PCLCNC-Micelle (PCL-CNC_M) and PCL-CNC-Graft (PCL-CNC_G) with 0.5, 1 and 3 wt% of CNCs.

32


Page 32 of 49

Page 33 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Mechanical properties of PCL and PCL reinforced with 1 wt% unmodified CNCs, grafted CNCs (CNC_G), latex-adsorbed CNCs (CNC_L) and micelles-adsorbed CNCs (CNC_M), with short PBMA length, at 50 % RH (light blue) and 98 % RH (dark blue). CONCLUSIONS In this work, cellulose nanocrystals were modified with well-defined polymers using two methods, covalent grafting and physisorption. PBMA was grafted onto CNCs via SI-ATRP, targeting PBMA graft lengths above and below the critical molecular weight for chain entanglement. 4 wt% and 28 wt% of PBMA with short and long graft lengths, respectively, were successfully grafted on CNCs. (PDMAEMAb-PBMA) micelles and (P(DMAEMA-co-MAA)-b-PBMA) latex particles were synthesized via ATRP and RAFT-mediated surfactant-free emulsion polymerization employing PISA, respectively, and were subsequently adsorbed onto the CNC surface through electrostatic interaction. Successful modification of CNCs via covalent grafting and physisorption was confirmed by FT-IR, by their redispersibility in THF and by the larger contact angle of water compared with unmodified CNCs. All modified CNCs exhibited better thermal stability than the pristine CNCs, which promotes their utilization in nanocomposite processing via melt-compounding techniques. Moreover, micelle adsorption of long PBMA chains and covalent grafting led to the best thermal stability, due presumably to better surface coverage and the shielding of sulfate groups. Noteworthy is that covalent grafting led to CNC films exhibiting contact angles as high as 90°, which was higher than that observed on corresponding films with physisorbed PBMA. In order to investigate the effect of the chemical modification methodology on the final properties of a nanocomposite material, pristine and modified CNCs were incorporated in a PCL matrix via extrusion. In order to produce films, the extruded materials were further hot pressed. All the nanocomposite films containing modified CNCs showed a greater transparency, thermal stability and mechanical performance than the films containing unmodified CNCs. Interestingly, nanocomposites with PBMA-grafted CNCs showed a better mechanical performance at 50 % RH than those with physisorbed PBMA, especially with short PBMA lengths. Furthermore, the mechanical performance of 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 49

nanocomposites reinforced with 1 wt% grafted CNCs was maintained at high RH (98 %), while the strength and strain-at-break decreased considerably with physisorbed PBMA. The unique comparative study investigated in this work could be of great interest for the design of CNC-based nanocomposites with tailored properties. ASSOCIATED CONTENT Supporting Information AFM images of pristine CNCs, plots of CMC measurements of PDMAEMA-b-PBMA, plots of the QCM measurements, glass transition temperature of the attached polymer, photographs of the nanocomposite films, TGA thermograms, DSC results and SEM micrographs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.C.) and [email protected] (E.M). Tel: +46 8 7908027 (A.C.) and +46 8 7907225 (E.M). Fax: +46 8 7908283. Author Contributions The manuscript was written through contributions of all authors. All the authors have given their approval of the final version of the manuscript. § These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Wilhelm Beckers Jubileumsfond, The Swedish Foundation for Strategic Research (SSF, EM11-0022), and the Wallenberg Wood Science Centre (WWSC) are gratefully acknowledged for financial support.

34


Page 35 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1. Fleming, K.; Gray, D. G.; Matthews, S., Cellulose Crystallites. Chemistry - A European Journal 2001, 7 , 1831-1835. 2. Siqueira, G.; Bras, J.; Dufresne, A., Cellulosic Bionanocomposites: A Review of Preparation, Properties and Applications. Polymers (Basel, Switzerland) 2010, 2, 728-765. 3. Mariano, M.; El Kissi, N.; Dufresne, A., Cellulose Nanocrystals and Related Nanocomposites: Review of Some Properties and Challenges. Journal of Polymer Science Part B: Polymer Physics 2014, 52, 791-806. 4. Eichhorn, S. J., Cellulose Nanowhiskers: Promising Materials for Advanced Applications. Soft Matter 2011, 7, 303-315. 5. Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Toman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T., Review: Current International Research into Cellulose Nanofibres and Nanocomposites. Journal of Materials Science 2010, 45, 1-33. 6. Dufresne, A., Nanocellulose: From Nature to High Performance Tailored Materials. De Gruyter: 2012. 7. Beck-Candanedo, S.; Roman, M.; Gray, D. G., Effect of Reaction Conditions on the Properties and Behavior of Wood Cellulose Nanocrystal Suspensions. Biomacromolecules 2005, 6, 1048-1054. 8. Lin, N.; Huang, J.; Dufresne, A., Preparation, Properties and Applications of Polysaccharide Nanocrystals in Advanced Functional Nanomaterials: a Review. Nanoscale 2012, 4, 3274-3294. 9. Xu, X.; Liu, F.; Jiang, L.; Zhu, J. Y.; Haagenson, D.; Wiesenborn, D. P., Cellulose Nanocrystals vs. Cellulose Nanofibrils: A Comparative Study on Their Microstructures and Effects as Polymer Reinforcing Agents. ACS Applied Materials & Interfaces 2013, 5, 2999-3009. 10. Oksman, K.; Mathew, A. P.; Sain, M., Novel Bionanocomposites: Processing, Properties and Potential Applications. Plastics, Rubber and Composites 2009, 38, 396-405. 11. Habibi, Y., Key Advances in The Chemical Modification of Nanocelluloses. Chem. Soc. Rev. 2014, 43, 1519-42. 12. Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J., Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chem. Soc. Rev. 2011, 40, 3941-3994. 13. Goffin, A. L.; Raquez, J. M.; Duquesne, E.; Siqueira, G.; Habibi, Y.; Dufresne, A.; Dubois, P., Poly(ϵcaprolactone) Based Nanocomposites Reinforced by Surface-Grafted Cellulose Nanowhiskers via Extrusion Processing: Morphology, Rheology, and Thermo-mechanical Properties. Polymer 2011, 52, 1532-1538. 14. Nagalakshmaiah, M.; Pignon, F.; El Kissi, N.; Dufresne, A., Surface Adsorption of Triblock Copolymer (PEO-PPO-PEO) on Cellulose Nanocrystals and Their Melt Extrusion with Polyethylene. RSC Advances 2016, 6, 66224-66232. 15. Boujemaoui, A.; Mongkhontreerat, S.; Malmström, E.; Carlmark, A., Preparation and Characterization of Functionalized Cellulose Nanocrystals. Carbohydr Polym 2015, 115, 457-64. 16. Braun, B.; Dorgan, J. R., Single-Step Method for the Isolation and Surface Functionalization of Cellulosic Nanowhiskers. Biomacromolecules 2009, 10, 334-341. 17. Carlmark, A.; Malmström, E., Atom Transfer Radical Polymerization from Cellulose Fibers at Ambient Temperature. Journal of the American Chemical Society 2002, 124, 900-901. 18. Malmström, E.; Carlmark, A., Controlled Grafting of Cellulose Fibres - An Outlook Beyond Paper and Cardboard. Polymer Chemistry 2012, 3, 1702-1713. 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 49

19. Morandi, G.; Heath, L.; Thielemans, W., Cellulose Nanocrystals Grafted with Polystyrene Chains Through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP). Langmuir 2009, 25, 8280-6. 20. Zeinali, E.; Haddadi-Asl, V.; Roghani-Mamaqani, H., Nanocrystalline Cellulose Grafted Random Copolymers of N-isopropylacrylamide and Acrylic Acid Synthesized by RAFT Polymerization: Effect of Different Acrylic Acid Contents on LCST Behavior. RSC Advances 2014, 4, 31428. 21. Carlmark, A., Tailoring Cellulose Surfaces by Controlled Polymerization Methods. Macromolecular Chemistry and Physics 2013, 214, 1539-1544. 22. Hatton, F. L.; Malmström, E.; Carlmark, A., Tailor-Made Copolymers for The Adsorption to Cellulosic Surfaces. Europ. Pol. J. 2015, 65, 325-339. 23. Wang, M.; Olszewska, A.; Walther, A.; Malho, J. M.; Schacher, F. H.; Ruokolainen, J.; Ankerfors, M.; Laine, J.; Berglund, L. A.; Osterberg, M.; Ikkala, O., Colloidal Ionic Assembly Between Anionic Native Cellulose Nanofibrils and Cationic Block Copolymer Micelles into Biomimetic Nanocomposites. Biomacromolecules 2011, 12, 2074-81. 24. Utsel, S.; Carlmark, A.; Pettersson, T.; Bergström, M.; Malmström, E. E.; Wågberg, L., Synthesis, Adsorption and Adhesive Properties of a Cationic Amphiphilic Block Copolymer for Use as Compatibilizer in Composites. European Polymer Journal 2012, 48, 1195-1204. 25. Carlsson, L.; Fall, A.; Chaduc, I.; Wågberg, L.; Charleux, B.; Malmström, E.; D'Agosto, F.; Lansalot, M.; Carlmark, A., Modification of Cellulose Model Surfaces by Cationic Polymer Latexes Prepared by RAFT-mediated Surfactant-free Emulsion Polymerization. Polym. Chem. 2014, 5, 6076-6086. 26. Nordgren, N.; Carlsson, L.; Blomberg, H.; Carlmark, A.; Malmström, E.; Rutland, M. W., Nanobiocomposite Adhesion: Role of Graft Length and Temperature in a Hybrid Biomimetic Approach. Biomacromolecules 2013, 14, 1003-1009. 27. Lönnberg, H.; Larsson, K.; Lindström, T.; Hult, A.; Malmström, E., Synthesis of PolycaprolactoneGrafted Microfibrillated Cellulose for Use in Novel Bionanocomposites–Influence of the Graft Length on the Mechanical Properties. ACS Applied Materials & Interfaces 2011, 3, 1426-1433. 28. Pereda, M.; Kissi, N. E.; Dufresne, A., Extrusion of Polysaccharide Nanocrystal Reinforced Polymer Nanocomposites through Compatibilization with Poly(ethylene oxide). ACS Applied Materials & Interfaces 2014, 6, 9365-9375. 29. Roman, M.; Winter, W. T., Effect of Sulfate Groups from Sulfuric Acid Hydrolysis on the Thermal Degradation Behavior of Bacterial Cellulose. Biomacromolecules 2004, 5, 1671-1677. 30. Wang, N.; Ding, E.; Cheng, R., Thermal degradation behaviors of spherical cellulose nanocrystals with sulfate groups. Polymer 2007, 48, 3486-3493. 31. Lin, N.; Dufresne, A., Physical and/or Chemical Compatibilization of Extruded Cellulose Nanocrystal Reinforced Polystyrene Nanocomposites. Macromolecules 2013, 46, 5570-5583. 32. Benkaddour, A.; Jradi, K.; Robert, S.; Daneault, C., Study of the Effect of Grafting Method on Surface Polarity of Tempo-Oxidized Nanocellulose Using Polycaprolactone as The Modifying Compound: Esterification versus Click-Chemistry. Nanomaterials 2013, 3, 638-654, 17 pp. 33. Hansson, S.; Trouillet, V.; Tischer, T.; Goldmann, A. S.; Carlmark, A.; Barner-Kowollik, C.; Malmström, E., Grafting Efficiency of Synthetic Polymers onto Biomaterials: A Comparative Study of Grafting-from versus Grafting-to. Biomacromolecules 2013, 14, 64-74. 34. Vinogradov, G. V.; Dreval, V. E.; Borisenkova, E. K.; Kurbanaliev, M. K.; Zabugina, M. P., Tensile Flow and Stress–Strain Behavior Over a Wide Temperature Range for Poly(butyl methacrylate) with a Broad Molecular Weight Distribution. Journal of Polymer Science Part B: Polymer Physics 1986, 24, 19711981. 35. Horn, D., Optisches Zweistrahlverfahren zur Bestimmung von Polyelektrolyten in Wasser und zur Messung der Polymeradsorption an Grenzflächen. Progr Colloid Polym Sci 1978, 65, 251-264. 36


Page 37 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


36. Sauerbrey, G., Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. Z. Physik 1959, 155, 206-222. 37. Aulin, C.; Varga, I.; Claesson, P. M.; Wågberg, L.; Lindström, T., Buildup of Polyelectrolyte Multilayers of Polyethyleneimine and Microfibrillated Cellulose Studied by in Situ Dual-Polarization Interferometry and Quartz Crystal Microbalance with Dissipation. Langmuir 2008, 24, 2509-2518. 38. Wunderlich, B., Macromolecular Physics; Academic press: New York 1980, 3: Crystal Melting, p 363. 39. Engström, J.; Hatton, F. L.; Wagberg, L.; D'Agosto, F.; Lansalot, M.; Malmström, E.; Carlmark, A., Soft and Rigid Core Latex Nanoparticles Prepared by RAFT-Mediated Surfactant-Free Emulsion Polymerization for Cellulose Modification - A Comparative Study. Polymer Chemistry 2017, 8, 1061-1073. 40. Larsson, E.; Sanchez, C. C.; Porsch, C.; Karabulut, E.; Wågberg, L.; Carlmark, A., ThermoResponsive Nanofibrillated Cellulose by Polyelectrolyte Adsorption. European Polymer Journal 2013, 49, 2689-2696. 41. Galván, M. V.; Peresin, M. S.; Mocchiutti, P.; Granqvist, N.; Zanuttini, M. Á.; Tammelin, T., Effects of Charge Ratios of Xylan-Poly(allylamine hydrochloride) Complexes on Their Adsorption Onto Different Surfaces. Cellulose 2015, 22, 2955-2970. 42. Hatton, F. L.; Kedzior, S. A.; Cranston, E. D.; Carlmark, A., Grafting-from Cellulose Nanocrystals via Photoinduced Cu-Mediated Reversible-Deactivation Radical Polymerization. Carbohydrate Polymers 2017, 157, 1033-1040. 43. Sheikhi, A.; Safari, S.; Yang, H.; van de Ven, T. G. M., Copper Removal Using Electrosterically Stabilized Nanocrystalline Cellulose. ACS Applied Materials & Interfaces 2015, 7, 11301-11308. 44. Cobo Sánchez, C.; Wåhlander, M.; Taylor, N.; Fogelström, L.; Malmström, E., Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE. ACS Applied Materials & Interfaces 2015, 7, 25669-25678. 45. Ansari, F.; Salajková, M.; Zhou, Q.; Berglund, L. A., Strong Surface Treatment Effects on Reinforcement Efficiency in Biocomposites Based on Cellulose Nanocrystals in Poly(vinyl acetate) Matrix. Biomacromolecules 2015, 16, 3916-3924. 46. Zoppe, J. O.; Peresin, M. S.; Habibi, Y.; Venditti, R. A.; Rojas, O. J., Reinforcing Poly(εcaprolactone) Nanofibers with Cellulose Nanocrystals. ACS Applied Materials & Interfaces 2009, 1, 19962004. 47. Siqueira, G.; Bras, J.; Dufresne, A., Cellulose Whiskers versus Microfibrils: Influence of the Nature of the Nanoparticle and its Surface Functionalization on the Thermal and Mechanical Properties of Nanocomposites. Biomacromolecules 2009, 10, 425-432. 48. Boujemaoui, A.; Mazières, S.; Malmström, E.; Destarac, M.; Carlmark, A., SI-RAFT/MADIX Polymerization of Vinyl Acetate on Cellulose Nanocrystals for Nanocomposite Applications. Polymer 2016, 99, 240-249. 49. Yin, Y.; Tian, X.; Jiang, X.; Wang, H.; Gao, W., Modification of Cellulose Nanocrystal via SI-ATRP of Styrene and The Mechanism of its Reinforcement of Polymethylmethacrylate. Carbohydrate Polymers 2016, 142, 206-212. 50. Habibi, Y.; Dufresne, A., Highly Filled Bionanocomposites from Functionalized Polysaccharide Nanocrystals. Biomacromolecules 2008, 9, 1974-1980. 51. Mamun, A.; Bazuin, C. G.; Prud’homme, R. E., Morphologies of Various Polycaprolactone/Polymer Blends in Ultrathin Films. Macromolecules 2015, 48, 1412-1417.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents 129x165mm (300 x 300 DPI)


Page 38 of 49

Page 39 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The different surface-modification routes utilized in this work. The size of unmodified CNCs is 112 ± 57 nm in length and 12.5 ± 7.5 nm in width as determined by AFM. 155x112mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Neat and modified CNCs (2 mg mL-1), directly after stirring (top) and after settling for 30 min (bottom): (a) neat CNCs in water, (b) neat CNCs in THF (c) CNC-Graft(S), (d) CNC-Micelle(S), (e) CNC-Latex(S), (f) CNCGraft(L), (g) CNC-Micelle(L) and (h) CNC-Latex(L) in THF. 254x190mm (96 x 96 DPI)


Page 40 of 49

Page 41 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Contact angle measurements and inserted photographs taken 30 s after the water drops were placed on unmodified and modified CNC discs. 254x190mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Height (left) and phase (right) AFM images 5 ×5 µm of unmodified CNCs and modified CNCs with micelles (CNC-Micelle), latex particles (CNC-Latex) and grafted CNCs (CNC-Graft). 338x190mm (96 x 96 DPI)


Page 42 of 49

Page 43 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FT-IR spectra of unmodified CNCs, ATRP-initiated CNCs (CNC-Br), grafted CNCs (CNC-Graft), latex-adsorbed CNCs (CNC-Latex) and micelle-adsorbed CNCs (CNC-Micelle). The letters L and S after the sample names indicate long and short PBMA chains, respectively. The inset spectra show an enlargement of the 1760-1680 cm-1 region. 254x190mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TGA thermograms of unmodified CNCs, grafted CNCs (CNC-Graft), latex-adsorbed CNCs (CNC-Latex) and micelle-adsorbed CNCs (CNC-Micelle). The letters L and S after the sample name indicate long and short PBMA chains, respectively. (*) samples annealed at 50 °C for 2 hours. 254x190mm (96 x 96 DPI)


Page 44 of 49

Page 45 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


UV spectra for PCL, PCL-CNC, PCL-CNC-Latex, PCL-CNC-Micelle and PCL-CNC-Graft with 0.5, 1 and 3 wt% of CNCs. The letters L and S after the sample name denotes long and short PBMA chains, respectively. 289x223mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SEM micrographs (10000× magnification) of annealed (a) PCL, (b) PCL-3CNC-Graft(S), (c) PCL-3CNCMicelle(S), (d) PCL-3CNC-Latex(S), (e) PCL-3CNC, (f) PCL-3CNC-Graft(L), (g) PCL-3CNC-Micelle(L) and (h) PCL-3CNC-Latex(L). 254x190mm (96 x 96 DPI)


Page 46 of 49

Page 47 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SEM cross-section images (20000× magnification) for (a) PCL-3CNC, (b) PCL-3CNC-Graft(L), (c) PCL-3CNCMicelle(L) and PCL-3CNC-Latex(L). 254x190mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Mechanical properties at 50% RH of PCL, PCL-CNC, PCL-CNC-Latex (PCL-CNC_L), PCL-CNC-Micelle (PCLCNC_M) and PCL-CNC-Graft (PCL-CNC_G) with 0.5, 1 and 3 wt% of CNCs. 266x362mm (96 x 96 DPI)


Page 48 of 49

Page 49 of 49

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mechanical properties of PCL and PCL reinforced with 1 wt% unmodified CNCs, grafted CNCs (CNC_G), latex-adsorbed CNCs (CNC_L) and micelles-adsorbed CNCs (CNC_M), with short PBMA length, at 50 % RH (light blue) and 98 % RH (dark blue). 274x271mm (96 x 96 DPI)


Polycaprolactone Nanocomposites Reinforced with Cellulose

Recommend Documents