Thermoresponsive Copolymer Poly(N-Vinylcaprolactam) Grafted

Research Article pubs.acs.org/journal/ascecg

Thermoresponsive Copolymer Poly(N‑Vinylcaprolactam) Grafted Cellulose Nanocrystals: Synthesis, Structure, and Properties Jinlong Zhang,† Qinglin Wu,*,† Mei-Chun Li,† Kunlin Song,† Xiuxuan Sun,† Sun-Young Lee,‡ and Tingzhou Lei*,§ †

School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803, United States ‡ Department of Forest Products, Korea National Institute of Forest Science, Seoul, 130-712, Republic of Korea § Key Biomass Energy Laboratory of Henan Province, Zhengzhou, 450008, Henan, China S Supporting Information *

ABSTRACT: A novel thermoresponsive copolymer poly(Nvinylcaprolactam) grafted cellulose nanocrystals (PVCL-gCNCs) was synthesized using surface-initiated atom transfer radical polymerization, and its temperature responsive behavior was studied in this work. The chemical structure characterization by Fourier transform infrared spectroscopy, solid-state CP/MAS 13C NMR spectroscopy, and X-ray photoelectron spectroscopy confirmed the presence of covalently grafting PVCL chains on the CNC surface. The crystalline structure and nanorod-shaped morphology of CNCs were well preserved after polymerization. Transmission electron microscope results indicated that the surface of CNCs was covered with grafted PVCL brushes. The viscoelastic properties of PVCL-g-CNC aqueous suspensions (1.0 wt %) by dynamic rheology measurements confirmed the thermally induced phase transition behavior. The work presented herein paves the way to design CNC-based advanced functional materials benefiting both from the intrinsic characteristics of CNCs and the new properties imparted by the temperature sensitive grafted polymer chains. KEYWORDS: Poly(N-vinylcaprolactam), Thermally induced phase transition, Cellulose nanocrystals, Atom transfer radical polymerization, Temperature responsive polymer

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tion10), ring open polymerization,11,12 olefin metathesis polymerization (ring open metathesis polymerization13 and acyclic diene metathesis polymerization14,15), and postmodification by click chemistry (copper catalyzed azide−alkyne cycloaddition, diels-alder reaction, and thiol−ene reaction).16,17 Among these methods, the ATRP technique has emerged as a robust synthesis tool, which is compatible with a wide range of initiators, monomers, and solvents.18,19 ATRP is also a desirable surface functionalization technique as it does not compromise the surface morphology and structure of the substrate.20 The robustness and versatility of ATRP can lead to precise control over the molecular architecture in terms of chain topology, composition, and functionality.7,21 Covalently functionalized CNCs with polymer brushes to tune the hydrophilic surface of CNCs were widely reported in recent years. The functionalized CNCs with polystyrene (PS) by ATRP strategy were first published by Yi et al.22 Subsequent investigations were reported by Morandi et al. concerning the optimizing surface initiated

INTRODUCTION Cellulose nanocrystals (CNCs), as an inexpensive, renewable, and biodegradable nanomaterial, have received increasing attention in recent years. CNCs possess many attractive properties, such as a large specific surface area, high modulus, unique optical properties, and the ability to self-assemble into liquid crystalline phases, which makes them useful in nanocomposites.1−3 However, as a consequence of poor dispersibility of CNCs in hydrophobic matrices, and weak interfacial adhesion among different phases, their composites generally exhibit poor mechanical properties. A feasible approach is to modify the CNC surface with hydrophobic monomers, leading to improved stability and dispersibility of CNCs in hydrophobic matrices.4 A diverse set of powerful polymerization tools have been used to tailor the surface of CNCs and other nanocellulose materials for creating uniquely surface-functionalized nanomaterials. These techniques include reversible deactivation radical polymerization (atom transfer radical polymerization (ATRP),5−8 reversible addition−fragmentation chain transfer polymerization,9 and nitroxide-mediated radical polymeriza© 2017 American Chemical Society

Received: June 21, 2017 Published: June 26, 2017 7439

DOI: 10.1021/acssuschemeng.7b02033 ACS Sustainable Chem. Eng. 2017, 5, 7439−7447

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis Routes of PVCL-g-CNC Copolymer

PVCL in the environmental field is to remove toxic wastes from water. PVCL was used to remove phenols from wastewater due to its complexing ability with phenols.46 Taking advantages of its biocompatibility as well as thermoresponsive properties, PVCL has also been used for the environmentally friendly flocculating polymer in combination with chitosan, a polyelectrolyte.47 Although an increasing number of studies concerning PVCL appeared recently, cellulose or cellulose nanocrystal (CNC) based thermoresponsive PVCL hybrid materials were only reported a few times up to now. Studies on grafting of PVCL on cellulose (polysaccharide) or its derivatives were reported from the conventional free radical polymerization, such as PVCL grafted cotton fabrics, PVCL grafted dextran, and PVCL grafted sodium alginate copolymers.48−51 Although the synthesis and characterization of PVCL/CNCs nanocomposite hydrogels by the frontal polymerization were reported.52 That particular paper was mainly focused on the reinforcement and dispersion of CNCs in the PVCL/CNCs nanocomposite hydrogels through dynamic viscoelastic measurements. However, there was a lack of detailed discussion on the chemical structure of the hydrogels with the frontal polymerization and their thermoresponsive behavior. Compared with the frontal polymerization/conventional free radical polymerization, the controlled radical polymerization could precisely control the molecular structure of grafted PVCL brushes on the surface of CNCs. To our knowledge, there are very few reports dealing with the surface initiated ATRP of NVCL-based polymers. The graphene oxide functionalized with PVCL by the ATRP strategy was reported by Kavitha and co-workers.53 Karesoja and co-workers reported the surface modification of mesoporous silica particles with PEO-b-PVCL copolymers to prepare thermoresponsive hybrid nanoparticles through the ATRP and click reaction.54 To the best of our knowledge, this is the first paper for the exploration of synthesis via the ATRP strategy and thermal responsive behavior of copolymer PVCLg-CNCs. The biocompatible, biodegradable, and thermosresponsive properties of PVCL-g-CNCs are useful in controlled drug delivery.

ATRP (SI-ATRP) of polymer brushes from the CNC surface.5,23 Different types of polymers have been grafted from CNCs through the ATRP method after these developments such as PS,5,22,24,25 poly(methyl methacrylate),26 poly(methyl acrylate),27 poly(2-aminoethylmethacrylamide),28,29 tert-butyl acrylate,7,30 poly(butyl acrylate),31 poly(2-aminoethyl methacrylate) and poly(2-aminoethylmethacrylamide),32 poly(6-(4-(4-methoxyphenylazo)phenoxy)hexyl methacrylate,33 soybean amide methacrylate,8 and poly(methyl methacrylate)co-butyl acrylate.34 The grafting of hydrophobic polymers onto the surface of CNCs promoted the compatibility between CNCs and hydrophobic matrices, which facilitated the dispersibility of CNCs in the hydrophobic matrices, enhanced the interfacial adhesion, and subsequently improved the toughness of CNCs based nanocomposites. Recently, the surface functionalized CNCs with responsive polymers, which respond to external stimuli such as pH,35 light,36,37 and temperature,38 have been of significant interest. Temperature responsive CNCs have been synthesized with polymers such as poly(N,N-diethylacrylamide)9 and poly(Nisopropylacrylamide) (PNIPAM),20,39 and poly(oligoethylene glycol) methyl ether acrylate (POEGMA),40 which possess a well-defined thermal induced phase transition. Compared to the extensive studies of PNIPAM based thermoresponsive hybrid materials, the research on the thermoresponsive properties of poly(N-vinylcaprolactam) (PVCL) based hybrid materials is still in its infancy. PVCL has many unique characteristics, such as solubility in water and organic solvents, nontoxicity, biocompatibilility, nonionic nature, temperature sensitivity, and the presence of both hydrophilic and hydrophobic groups in its structure.41−43 These properties make it an interesting candidate for the applications in the biomedical and environmental areas. PVCL grafted carbohydrate polymer (chitosan, alginate, or dextrans) hydrogel, nanoparticles, microspheres, and beads have been studied for the application of drug delivery.44,45 The chitosan-g-PVCL beads in the controlled release experiment showed a slow and steady release of the drug from the beads as its strong hydrophobic interactions with the drug molecules.45 Another application of 7440


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ACS Sustainable Chemistry & Engineering

three freeze−pump−thaw cycles. Thereafter, the flask was immersed into an oil bath preheated at 75 °C. The polymerization was stopped after 12 h by exposing the mixture to air, and DMF was subsequently added to dilute the solution. The product was passed through an activated alumina column to remove copper complex, condensed by rotary evaporation and then precipitated in excess n-hexane. The resulting precipitation was dissolved in dichloromethane, strongly sonicated for 5 min and then centrifuged three times to remove unreacted NVCL monomer and PVCL homopolymer. Finally, the resultant PVCL-g-CNC copolymer was collected and dried under vacuum at room temperature to a constant weight. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. ATR-FTIR analysis was conducted in the transmittance mode. The scanning range of wavenumbers was from 4000 to 800 cm−1 with a resolution of 2 cm−1 and accumulation of 132 scans. The spectra were collected using the FTIR spectrophotometer (Nicolet Nexus 670-FTIR, Thermo Electron Corporation, Gormley, Canada). Solid-State CP/MAS 13C NMR Spectroscopy. Solid-state 13C NMR spectroscopy measurements were performed with a Bruker Avance 400 WB instrument operated at 100.6 MHz at room temperature. The CP/MAS experiments were performed with a relaxation delay of 4.0 s and a contact time of 2.0 ms. Magic angle spinning (MAS) was achieved at a rate of 4 kHz. The chemical shift was in parts per million related to an external sample of tetramethylsilane (TMS). The change of CNCs in crystallinity before and after the esterfication reaction was further calculated. The crystallinity index (CrI)27,55 was determined according to

The objective of this study was to functionalize CNCs with PVCL and to elucidate its temperature responsive behavior. The initiator modified CNCs (Br-CNCs) were first prepared through esterification reaction. PVCL was then grafted from macroinitiator Br-CNCs through SI-ATRP. The chemical structure of Br-CNCs and PVCL-g-CNCs was confirmed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), solid-state CP/MAS 13C NMR spectroscopy, and X-ray photoelectron spectroscopy (XPS). The crystalline structure and morphology as well as thermal properties were subsequently characterized by wide-angle Xray diffraction (WAXD), solid-state CP/MAS 13C NMR spectroscopy, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The temperature-induced phase transition behavior was further investigated through rheology measurement.

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EXPERIMENTAL SECTION

Materials. Microfibrillated cellulose (Celish KY 100-S grade, 25% solid content) was purchased from Daicel Chemical Industries, Ltd. (Tokyo, Japan). Copper bromide (CuBr, 98%, Acros Organics, Morris Plains, NJ, USA) was purified by stirring in glacial acetic acid, filtering and washing with methanol, followed by drying in vacuum at 30 °C overnight. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA, 99%, Sigma-Aldrich, St. Louis, MO, USA), N-vinylcaprolactam (NVCL, 98%, Sigma-Aldrich, St. Louis, MO, USA), triethylamine (TEA, ≥99%, Sigma-Aldrich, St. Louis, MO, USA), 2-bromoisobutyryl bromide (BIBB, 98%, Acros Organics, Morris Plains, NJ, USA), and 4dimethylaminopyridine (DMAP, 99%, Acros Organics, Morris Plains, NJ, USA) were used without further purification. N,N-Dimethylformamide (DMF, anhydrous, ≥ 99.8%, Sigma-Aldrich, St. Louis, MO, USA) was purified with the solvent purification system. Other reagents were used as received. Preparation of CNCs. Microfibrillated cellulose was mixed with sulfuric acid (64 wt %), followed by mechanical stirring at 50 °C for 3 h to allow its hydrolysis. The obtained suspensions were subsequently diluted with excessive deionized water and then dialyzed with deionized water using semipermeable membrane (cutoff molar mass 12000−14000) to remove residual sulfuric acid for 7 days until neutrality was reached. The resultant CNC suspensions were further treated using a high pressure homogenizer (Microfluidizer M-110P, Microfluidics Corp., Newton, MA, USA). CNCs were finally recovered from the resultant suspensions by freeze-drying the suspensions for 3 days. Synthesis of Br-CNCs and PVCL-g-CNC Copolymer. The synthesis of the PVCL-g-CNC copolymer was performed in two steps as illustrated in Scheme 1. Dry CNCs (3.10 mmol) were dispersed in DMF (15.0 mL) under stirring, and TEA (10.04 mmol) was subsequently added after CNCs were completely dispersed, followed by the addition of DMAP (2.21 mmol). 2-Bromoisobutyryl bromide (9.71 mmol) was then added dropwise to the above CNC suspensions in an ice/water bath. The mixture was stirred at room temperature for 24 h. The resultant mixture was poured into an excess amount of deionized water to remove any residual small molecules and then precipitated as white floccules. Purification of the modified CNCs was carried out by redispersing them in DMF and reprecipitating them in deionized water for three times. Yellow powder of Br-CNCs was finally obtained after drying the precipitated materials under vacuum at 30 °C for 24 h to a constant weight. The formed macroinitiator Br-CNCs then initiated the grafted polymerization of NVCL through a SI-ATRP procedure. The BrCNCs (0.312 mmol), N-vinylcaprolactam (31.2 mmol), and PMDETA (0.312 mmol) were mixed in DMF in a 25 mL roundbottom flask under magnetic stirring. The mixture was then degassed with three freeze−pump−thaw cycles after Br-CNCs were completely dispersed. CuBr (0.312 mmol) was then quickly added into the flask under nitrogen atmosphere, and the flask was degassed again with

CrI =

area of crystalline region area of crystalline region + area of amorphous region

(1) where the areas of the crystalline and amorphous regions corresponded to the integrals of C4 in the range of 86−92 and 80− 86 ppm, respectively. X-ray Photoelectron Spectroscopy. XPS spectra were recorded using a Specs PHOIBOS-100 spectrometer (SPECS, Berlinm, Germany) with Al Ka irradiation (1486.61 eV) at 10 kV and 10 mA. Survey scans were conducted from 1200 to 0 eV with pass energy of 40 eV and scan step of 1.0 eV. The high resolution C 1s spectra were recorded at pass energy of 40 eV and scan step of 0.1 eV. Thermogravimetric Analysis. TGA was carried out using a TA Q50 thermogravimetric analyzer (TA Instruments Inc., New Castle, DE, USA) from 25 to 600 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Wide-Angle X-ray Diffraction. WAXD patterns were measured using a Bruker Siemens D5000 X-ray diffractometer (Bruker AXS Inc., Madison, WI, USA) operated at the Cu Ka radiation (λ = 0.154 nm) with 40 kV and 30 mA in a 2θ range from 4 to 40° at a step size of 0.02°. Transmission Electron Microscopy. The morphology was observed with a transmission electron microscope (JEM 1400, JEOL, Peabody, MA, USA) at an accelerating voltage of 120 kV. Drops of about 0.001 wt % CNC or PVCL-g-CNC suspension were deposited onto glow-discharged carbon-coated TEM grids. The liquid in excess was absorbed with filter paper, and a drop of 2% uranyl acetate was deposited onto the specimen prior to drying. The stain in excess was blotted, and the remaining thin liquid film was allowed to dry. The TEM grids were allowed to dry at room temperature for 3 h before imaging. Rheology. Rheology measurements were carried out using a TA Instruments Rheometer (AR 2000, TA Instruments Inc., New Castle, DE, USA). A cone-and plate geometry with a cone diameter of 40 mm and a 2° truncation angle was employed at a constant shear rate of 150 s−1, and the temperature was controlled by a Peltier plate. The dynamic viscoelastic properties (dynamic storage modulus G′ and loss modulus G″) of the sample were measured by oscillatory shear experiments performed at a fixed frequency of 1 Hz in a temperature ramp of 25 to 90 °C with a heating or cooling rate of 3 °C/min. The 1.0 wt % aqueous suspensions were prepared by dispersing the powder 7441


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onto the surface of CNCs.21 In addition, the broad stretching band of −OH groups at 3600−3100 cm−1 in CNCs was significantly decreased, demonstrating that the hydroxyl groups were substituted and converted to ester bonds. The peak of double bond (CC) in the spectrum of NVCL was observed at 1651 cm−1, and the vinyl peaks (CH and  CH2) were located at 3104 and 991 cm−1.57 However, the peaks of the double bond disappeared after polymerization. It can be concluded that the polymerization of PVCL was successfully achieved. The solid state 13C NMR spectra of CNCs, Br-CNCs, NVCL, and PVCL-g-CNC copolymer are displayed in Figure 2. By comparison of CNCs and Br-CNCs, the new peaks in BrCNCs at 56 ppm (C7), 31 ppm (C8) and around 170 ppm (C9) were assigned to the quaternary carbon, methyl groups, and carbonyl carbon of the attached 2-bromoisobutyryl ester groups, which indicated that the 2-bromoisobutyryl ester moiety reacted with the hydroxyl groups of CNCs. In addition, no substantial modifications were observed for the CNC backbone in the 60−110 ppm region, indicating that the esterification reaction did not remarkably affect the backbone structure of CNCs. For PVCL-g-CNC copolymer, the peaks for CNC backbone were clearly observed. The characteristic peaks belonging to vinyl groups in NVCL shifted to the range of 38− 45 ppm (C10 and C11) in the PVCL-g-CNC copolymer. The above analysis confirmed the successfully grafted polymerization of PVCL onto the surface of CNCs. The solid-state 13C NMR spectroscopy is used not only for structural identification but also for the determination of degree of substitution (DS) and CrI.58,59 The DS value of Br-CNCs was determined by the amount of 2-bromoisobutyryl bromide grafted and calculated according to

sample into deionized water with a strong stirring for more than 10 min in order to get a homogeneous suspension as much as possible.

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RESULTS AND DISCUSSION Chemical Structure. The FTIR spectra of CNCs, BrCNCs, NVCL, and PVCL-g-CNCs as well as digital photographs of CNCs, Br-CNCs, and PVCL-g-CNCs are shown in Figure 1a and b. Compared to the spectrum of CNCs, the

Figure 1. FTIR spectra of CNCs, Br-CNCs, NVCL, and PVCL-gCNC copolymer (a) and digital pictures of CNCs, Br-CNCs, and PVCL-g-CNC copolymer (b).

appearance of the CO group at 1736 cm−1 in Br-CNCs indicated that the bromoisobutyryl ester moiety was bonded

DS =

ICH3 2IO − CH − O

(2)

Figure 2. Solid-state CP/MAS 13C NMR spectra of CNCs (a), Br-CNCs (b), NVCL (c), and PVCL-g-CNC copolymer (d). 7442


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Figure 3. XPS full survey spectra of CNCs, Br-CNCs, and PVCL-g-CNC copolymer (a) and deconvolved curves of C 1s in the high resolution spectrum of CNCs (b), Br-CNCs (c), and PVCL-g-CNC copolymer (d).

Crystallinity Structure and Morphology. WAXD patterns for CNCs, Br-CNCs, and PVCL-g-CNCs are shown in Figure S1. Both CNCs and Br-CNCs had typical patterns of the native crystalline form of cellulose I.20 There were 2θ diffraction peaks at 14.8°, 16.5°, 22.6°, and 34.4°, which represent the lattice planes (110), (11̅0), (200), and (400), respectively.55,64 These results verified that esterification reaction just occurred on the surface of CNCs and did not significantly alter the crystallite region of CNCs. In addition, the X-ray patterns of CNCs did not show a considerable change after being grafted with PVCL, which indicated that the polymerization of PVCL only took place on the surface of CNCs. The TEM images of CNCs and PVCL-g-CNCs are shown in Figure 4. The particle lengths of CNCs were between 100 and 300 nm. The particle widths were fairly uniform in the range of 15−20 nm. These measurements are in close agreement with the dimensions of CNCs derived from microcrystalline cellulose, as observed in the literature.1 For PVCL-g-CNCs, the particle lengths and widths were about 172 and 32 nm, respectively. Compared to CNCs, PVCL-g-CNCs had similar shape and size with well-preserved nanorod-shaped morphology (except width increase), which was also consistent with the above results of WAXD and solid-state 13C NMR measurements. These results also indicated that the morphology of CNCs after grafting polymerization of PVCL was not apparently destroyed. On the other hand, CNCs had a relatively smooth surface, while a rough surface for the grafting PVCL layers was observed. These results indicated that the grafted PVCL brushes were covered on the surface of CNCs. Thermal Stability Properties. Thermogravimograms of CNCs, Br-CNCs, and PVCL-g-CNC copolymer are shown in Figure 5. The CNCs showed two separate pyrolyses within a wider temperature range rather than one pyrolysis because trace residue of sulfuric acid in the CNCs reduced the degradation temperature. The decomposition ranges were from 150 to 230 °C and from 250 to 375 °C with a maximum

where ICH3 represents the integral area of the carbon peak of methyl group from the grafted 2-bromoisobutyryl ester moiety and IO−CH−O denotes the integral area of the carbon peak of C1 in the backbone of CNCs. The DS value was calculated to be 0.78.60 The relatively low DS may be explained by the fact that most hydroxyl groups in CNCs were not available for the reaction due to the crystalline character and hydrogen bonding. The CrI values for CNCs and Br-CNCs were about 54% and 59%, respectively, which were in the typical crystallinity value range of 55−80%.61 The CrI values before and after esterification reaction confirmed that no damage occurred on the backbone structure of CNCs after the immobilization of 2bromoisobutyryl ester moiety onto the surface of CNCs. The XPS spectra of CNCs, Br-CNCs, and PVCL-g-CNC copolymer are shown in Figure 3. For Br-CNCs, the bromine element as new peaks at around 182 eV (3p) and 70 eV (3d) was observed except for the same peaks in CNCs. In addition, a new peak for the CO (288 eV) bond appeared in the high resolution spectrum of Br-CNCs. As shown in Table S1, the value for the O/C ratio in Br-CNCs was slightly reduced compared with that of CNCs. These results indicated the immobilization of 2-bromoisobutyryl ester moiety onto the surface of CNCs. For the PVCL-g-CNC copolymer, the presence of a distinctive nitrogen signal was a further indication that PVCL chains were presented on the surface of CNCs. It should be pointed out that the molecular weight of grafted PVCL chains is not available here as the process for sufficiently cleaved PVCL chains did not succeed. PVCL-g-CNCs were attempted for hydrolysis to cleave PVCL grafting chains from the CNC backbones for gel permeation chromatography test as the ester bond between CNCs and PVCL chains was the obvious site to hydrolyze.20,39 It is possible that CNC backbones were tightly wrapped around the grafted PVCL chains, resulting in the ester linkage being less accessible. Similar issues were also reported for the hydrolysis of cellulose or CNC based grafted polymer brushes.56,62,63 7443


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Figure 6. Viscoelastic properties (G′ and G″) for 1.0 wt % aqueous suspensions of CNCs and PVCL-g-CNCs from oscillatory shear experiments at a frequency of 1 Hz, a strain amplitude of 1.0 and a heating rate of 3 °C/min.

although both CNC and PVCL are hydrophilic polymers. The heterogeneous suspensions should have some influence on the performance of the rheology test. A similar situation was found for the PNIPAM-g-CNCs reported in ref 60. Above the LCST, PVCL chains collapsed into a thin hydrophobic layer on the CNC surface, and hence, G′ and G″ significantly increased. The mechanism for temperature induced phase transition behavior of PVCL-g-CNCs is proposed as shown in Figure 7a.

Figure 4. TEM images of CNCs (a and b) and PVCL-g-CNCs (c and d).

Figure 5. Thermal stability (TG (a) and DTG (b)) of CNCs, BrCNCs, and PVCL-g-CNC copolymer.

decomposition rate at 301 °C and a residual weight of 6.4 wt %. The Br-CNCs showed an initial pyrolysis at 169 °C with a maximum decomposition rate at 264 °C and a higher residual weight of 22.0 wt %. The decomposition shifted to lower temperature due to the lower thermal stability of grafted initiator (2-bromoisobutyryl ester moiety) on the surface of CNCs. The decomposition of PVCL-g-CNCs displayed two stages with the first stage from 200 to 400 °C for CNCs and the second stage from 450 to 577 °C for grafted PVCL.65 The maximum decomposition rate of PVCL-g-CNC copolymer was about 281 °C, and its residual weight was 1.64 wt %. Thermal Responsive Behavior. The viscoelastic properties (storage modulus G′ and loss modulus G″) as a function of temperature for 1.0 wt % aqueous suspensions of CNCs and PVCL-g-CNCs are shown in Figure 6. For CNCs, the values of G′ and G″ were nearly constant as the temperature raised from 25 to 60 °C. However, in the case of PVCL-g-CNC copolymer, below its low critical solution temperature (LCST, 36 °C determined at the point, G′ > G″), the PVCL chains were extended due to their hydrophilic nature, and it had low G′ and G″. The LCST of 36 °C was relatively higher than that of the PVCL homopolymer reported in the literature (32 °C).42,44 The slightly higher LCST observed in PVCL-g-CNCs may be due to solubility issues. The PVCL-g-CNC was not soluble in water and appeared in the suspensions at room temperature

Figure 7. Elucidation of temperature responsive phase transition of PVCL-g-CNC copolymer influenced by temperature change (a) and digital photographs of aqueous suspensions of PVCL-g-CNCs (1.0 wt %) at 25 (b) and 70 °C (c).

At a temperature below the LCST, the hydrophilic PVCL chains were extended. Entropic repulsive interactions between particles made the aggregation or connection between particles unfavorable.66 Therefore, they had low G′ and G″ as confirmed by the dynamic rheology measurement. Upon heating above the LCST, the hydrophobic nature of the PVCL globules 7444



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dominated the interparticle attractions among CNC particles, and the absence of any electrostatic repulsion formed coagulation of PVCL-g-CNCs with evidence of the increased G′ and G″ in the dynamic rheology measurement.39 In addition, the digital photographs of aqueous suspensions of PVCL-g-CNCs (1.0 wt %) in Figure 7b and c displayed the opaque-to-transparent transition, which also confirmed the temperature responsive phase transition behavior.

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CONCLUSIONS

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ASSOCIATED CONTENT

REFERENCES

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Temperature responsive PVCL-g-CNC copolymer was synthesized through SI-ATRP. The peaks of double bond as well as the vinyl group disappeared in the spectrum of CNCs-g-PVCL copolymer by the ATR-FTIR spectroscopy and solid-state 13C NMR spectroscopy measurement confirmed grafting polymerization of PVCL on the surface of CNCs. A new signal of nitrogen element observed in the spectrum of PVCL-g-CNC copolymer by the XPS measurement also supported the grafting polymerization. The crystalline structure and nanorod-shaped morphology of CNCs before and after polymerization were well-preserved, confirmed by the WAXD, solidstate 13C NMR spectroscopy, and TEM measurements. The dynamic rheology measurement confirmed the temperature responsive phase transition behavior for aqueous suspensions of PVCL-g-CNCs (1.0 wt %). The mechanism for temperature responsive phase transition was that the hydrophilic PVLC chains transformed into hydrophobic ones upon its LCST, and then collapsed into a thin hydrophobic layer on the surface of CNCs. The temperature sensitivity of the aggregation properties provides the polymer-decorated CNCs as an opportunity for preparation of advanced CNC-based functional materials.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02033.

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Research Article

Atomic percentages and WAXD patterns of CNCs, BrCNCs, and PVCL-g-CNC copolymer (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.W.). *E-mail: [email protected] (T.L.). ORCID

Qinglin Wu: 0000-0001-5256-4199 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from Louisiana Board of Regents [LEQSF(2015-17)-RD-B-01], LEQSF(2016-2017)-ENH-TR-01, LEQSF(2017-2018)-RD-A01), Louisiana State University EDA program, and Korea National Institute of Forest Science through a cooperative project to the LSU AgCenter. Appreciation is also given to Dr. Donghui Zhang and her graduate student Zhaoyuan Liu in the Department of Chemistry at LSU for the technical help and for access to the solvent purification system in her group. 7445


Research Article

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Thermoresponsive Copolymer Poly(N-Vinylcaprolactam) Grafted

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