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Controlled Architecture of Glass Fiber/Poly(Glycidyl Methacrylate) Composites via Surface-initiated ICAR ATRP Mediated by Mussel-Inspired Polydopamine Chemistry Wenqing Wang, Paziliya Julaiti, Gang Ye, Xiaomei Huo, Yuexiang Lu, and Jing Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03065 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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Industrial & Engineering Chemistry Research

Controlled Architecture of Glass Fiber/Poly(Glycidyl Methacrylate) Composites via Surface-initiated ICAR ATRP Mediated by Mussel-Inspired Polydopamine Chemistry Wenqing Wanga, b, Paziliya Julaitia, c, Gang Yea,

b*

, Xiaomei Huoa, Yuexiang Lua, b,

Jing Chena, b* a

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute

of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China b

Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing,

100084, China c

Faculty of Chemical Science and Engineering, China University of Petroleum,

Beijing, 102249, China

KEYWORDS. Composites; Surface-initiated ATRP; Polydopamine; Poly(Glycidyl Methacrylate)

ABSTRACT. This study presents a new strategy by integrating surface-initiated atom transfer radical polymerization (SI-ATRP) with bio-inspired polydopamine chemistry to prepare well-defined glass fiber/polymer composites. Firstly, a homogenous PDA layer, which served as a favorable platform facilitating ATRP initiators anchoring, was deposited onto the glass fibers. Controlled growth of poly(glycidyl methacrylate) 1

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(PGMA) brushes from the glass fibers were then performed using the initiators for continuous activator regeneration (ICAR) ATRP method. PGMA brushes with defined structure, grafting density (5 wt.% ~ 25 wt.%) and well preservation of chain end-functionality could be obtained by reducing radical initiator ratio and using a high dilution strategy (solvent: monomer=10:1 (V:V)). And, a narrow distribution of molecular weight (PDI≤1.2) could be attained within reduced polymerization time. Laccase, as an enzyme model, was then covalently immobilized to the glass fiber/PGMA composites. The developed bio-composites showed improved enzymatic stability and enhanced catalytic activity toward the degradation of 2,6-dimethoxy phenol.

1. INTRODUCTION

Glass fiber (GF) is known as a widely used reinforcing agent in polymer synthesis for its excellent mechanical and thermal properties.1-3 Meanwhile, due to the high surface-area-to-volume ratio, easy accessibility, and low cost, it is also an attractive substrate for fabricating versatile composite materials applied in domains such as gas or water filtration,4-6 antifouling membranes,7,8 and oil-water separation.9 In recent years, the rapid development of controlled/living radical polymerization (LRP) opens a new avenue for surface modification/functionalization of GFs, which is envisioned to further broaden their applications. Taking advantage of the surface-initiated LRP (“grafting-from”) strategy, a wide variety of surface-induced polymers with controllable composition and topological architectures, such as block copolymer, 2


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hierarchical comb brushes and diverse polymer patterns, have been fabricated.10-13

Among the well-established LRP methods, atom transfer radical polymerization (ATRP) is widely used for surface-initiated synthesis of polymer brushes with predetermined degrees of polymerization, low polydispersities and high chain end-functionality (CEF), which benefits further modification for preparing specific functional materials.14-16 Until recently, surface-initiated ATRP has been reported for preparation of polymer composite and hybrid materials from both macro- and nano-scopic substrates, including metal oxides,17 mesoporous carbons,18 silica nanoparticles,19 and fibers.20 Basically, performing surface-initiated polymerization from nano-scopic substrates with complex surface curvatures and confined spaces is quite challenging because of the steric hindrance and mass transfer resistance.21,22 In comparison, controlled growth of polymer brushes from macro-scopic substrates, especially those with flat surfaces, can be more easily operated using “benchtop” methods, showing greater potential for well controllable preparation of functional polymer composites for specific applications.

In aspect of polymer chain end-functionality preservation for targeted functionality, initiators for continuous activator regeneration (ICAR) ATRP enables well-controlled growth of polymer chains with preserved chain end-functionality.22 Compared to normal ATRP methods, ICAR ATRP allows the use of low concentration of Cu catalyst, greatly reducing the residual Cu catalyst in the products while preventing the tedious purification procedures. The polymerization mechanism of ICAR ATRP 3



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belongs to degenerative chain transfer polymerization.23-26 Due to the introduction of a radical initiator (2,2’-azobis(isobutyronitrile), AIBN), it requires less rigorous oxygen-free conditions in a ICAR ATRP system.

Effective attachment of halogen-containing initiators onto the substrates is crucial for the following ICAR ATRP process. For the surface-initiated polymer growth from glass fibers with abundant Si-OH groups on the surfaces, traditional methods rely on the use of silane coupling agents for initiator anchoring. For instance, aminoalkane -substituted silanes, such as 3-aminopropyltriethoxysilane (APTS), are broadly used as coupling agents in glass fiber treatment,27 followed by amidization with 2-bromoisobutyryl bromide.28 Lately, polydopamine (PDA) chemistry inspired by mussel foot proteins has been found to be an effective approach to surface modification, in which adherent PDA coatings are deposited to diverse substrates and serve as a versatile platform for secondary surface-mediated reactions.29-31 The PDA layers generated in mild experimental conditions possess abundant catechol and amine groups, which provide sufficient “active sites” for the attachment of halogen-containing initiators. This is expected to increase the number of the initiators anchored onto the surfaces of glass fibers, while efficiently improving the grafting density of the polymer chains compared to aforementioned silane-mediated strategy,32,33 In addition, the covalent cross-linking and non-covalent interactions between PDA and the substrate surfaces enable the composite materials with reinforced mechanical strength.34

4


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In this study, controlled growth of poly(glycidyl methacrylate) (PGMA) brushes from glass fibers by using surface-initiated ICAR ATRP, mediated by the mussel-inspired PDA chemistry, was reported for the first time. Firstly, a uniform PDA coating layer was generated on the surface of the GFs (GF@PDA), followed by anchoring of ATRP initiator, 2-bromoisobutyryl bromide (BiBB), labeled as GF@PDA-Br. Glycidyl methacrylate, a versatile ATRP monomer for post-synthetic functionalization through epoxide ring-opening,35,36 was chosen to study the growth behavior of polymer brushes from the surfaces of GFs, for fabricating well-defined functional glass fiber/polymer composites. Controlled growth of PGMA brushes from the BiBB-anchored GF@PDA was achieved using ICAR ATPR, and further modification was performed.

Bio-composites with “active surfaces” are appealing materials for many applications, such as catalysis, medicine, bioremediation, etc.37-39 Immobilization of functional biomolecules onto solid supports is considered as a promising approach to design “active surfaces”, of which covalent bonding to modified surfaces is more favorable with stronger biomolecules attachment.40 Here, laccase from Trametes versicolor, as an enzyme model, was immobilized to the glass fiber/polymer composites via the reaction between the epoxy groups in PGMA brushes and the amino groups of enzyme. The obtained bio-composites showed favorable catalytic activity toward the degradation of 2,6-dimethoxy phenol.41-43 And, with high surface-area-to-volume ratio and nonporous properties, the laccase functionalized glass fiber/PGMA composites efficiently overcome the diffusional limitations of 5



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porous materials.44 More importantly, except for preparation of the functional glass fiber/polymer composites, the new methodology developed in this study would offer the potential for fabricating more well-defined materials with specific functionality and easily accessible surfaces for diverse applications.

2. EXPERIMENTAL SECTION

2.1 Materials Nonwoven glass fiber mat with 7.0 wt.% PVA binder (Craneglass 230) was provided by Crane & Co. Inc (Dalton, MA). Dopamine hydrochloride (DA, ～98%), tris (hydroxymethyl) aminomethane (Tris, 99.5%), 2-bromoisobutyryl bromide (BiBB, 98%), trimethylamine (TEA, 99%), glycidyl methacrylate (GMA, 96%), ethyl 2-bromo-2-methylpropionate (EBiB, 98%), 2,2’-azobis(isobutyronitrile) (AIBN, 98%), tris(2-pyridylmethyl) amine (TPMA, 98%), copper bromide(CuBr2, 99%), N,N-Dimethylformamide (DMF, 99.8%), anisole (99%), butylene oxide (THF, 99.9%), ethanol (99.9%), laccase from trametes versicolor (80 U·mg-1), 2,6-dimethoxy phenol (DMP, 98%) were all purchased from J&K Scientific Ltd. (Beijing, China) and used as received except GMA, which was pretreated by basic aluminium oxide. Other reagents, including sodium periodate (NaIO4, 99%), cupric sulfate (CuSO4, 98%), hydrogen peroxide (H2O2, 30 wt.% in water) were analytical purity.

2.2 Synthesis of GF@PDA Polydopamine, with GF@PDA was prepared via mussel-inspired PDA chemistry. Prior to use, glass fiber mats were cut into pieces of about 2.0 cm2 area. A determined 6


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amount of dopamine hydrochloride (Conc.=2.0 g·L-1) was dissolved in 1.0 mL Tris buffer solution (pH=8.5, 10 mM) and added into 24.0 mL Tris buffer solution dropwise under magnetic stirring. Glass fiber mats were transferred into the aforementioned solution, and PDA coating layer on glass fiber mat was obtained through the self-polymerization of dopamine in a thermostatic oscillator at 25 ºC for 24 h. The resulting GF@PDA was separated and rinsed with deionized water triple times, followed by being dried in vacuum (-100 Kpa) at 50 ºC for 12 h. In order to optimize PDA coating, oxidant, such as NaIO4 or CuSO4/H2O2, was introduced into the dopamine solution under certain pH value of 5.0 before glass fiber mats were impregnated.

2.3 Anchoring of ATRP initiator Halogen-containing initiator BiBB was anchored to the surface of GF@PDA through amidation with the amino group in the PDA. DMF rinsed GF@PDA was added to 60.0 mL DMF in a 100 mL Schlenk flask, followed by addition of 1.5 mL TEA. After lowering the temperature to 0 ºC, 1.5 mL BiBB dissolved in 5 mL DMF was fed dropwise under argon protection and magnetic stirring. GF@PDA-Br was separated after reaction for 24 h at room temperature and rinsed with DMF, ethanol several times to remove the residual reactants. The final product was dried in vacuum (-100 Kpa) at 50 ºC for 12 h.

2.4 Surface-initiated ICAR ATRP growth of PGMA Glass fiber/PGMA composites were prepared via ICAR ATRP using CuBr2/TPMA

7



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catalytic/ligand system and AIBN. GF@PDA-Br was added in a 50 mL Schlenk flask, followed by adding 3.0 mL anisole, 3.0 mL GMA, 33.0 µL EBiB, 0.74 mL of 1.0 g·L-1AIBN/DMF sequentially. The reaction system was sealed and vacuum-argon inflation cycle was operated for several times. In argon protection, 1.0 mL of 1.0 g·L-1 CuBr2 /TPMA (wt:wt=1:3) dissolved DMF mixture was added to flask through a syringe.

The

molecular

ratio

of

the

reagents

was

as

following:

[GMA]:[EBiB]:[macroinitiator]:[AIBN]:[CuBr2]:[TPMA]=100:1:0.01:0.02:0.02:0.05. The reaction was kept in oil bath at 60 ºC for pre-set time. The obtained GF@PDA@PGMA composites were separated and rinsed with THF, DMF, and ethanol in turn for several times, followed by being dried under vacuum (-100 KPa) at 50 ºC for 12 h. Free PGMA product in solution was precipitated in ethanol and collected for estimation of molecular weight and polydispersity. To study the growth of PGMA brushes from the glass fibers, the ratio of AIBN to Cu and the volume amount of solvent anisole were altered.

2.5 Immobilization of laccase onto GF@PDA@PGMA composites 50.0 mg laccase from Trametes versicolor was dissolved in 25.0 mL of 0.1 M acetic buffer solution and transferred in 50 mL Schlenk flask. GF@PDA@PGMA composites were added in the flask under argon protection and reacted for 24 h at 18ºC. Laccase immobilized GF@PDA@PGMA was separated and rinsed with deionized water, being dried at room temperature for 24 h.

2.6 Characterizations

8


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Surface morphology of the intermediate and final products was observed using ZEISS Merlin Scanning Electron Microscopy (SEM) with an accelerating voltage of 30 kV. Prior to each characterization, the samples were dried overnight at 50 ºC in vacuum followed by conductive coating using a gold palladium plasma spray for 20 s. The BET specific surface area of glass fiber mats was measured using N2 adsorption at 77 K in an Autosorb-1 volumetric sorption analyzer controlled by Autosorb-1 software (Quantachrome Corp.) The sample was degassed at 150 ºC until the outgas pressure was below 5 µm·Hg min-1 prior to analysis. The surface elemental information was obtained through PHI-5300 ESCA X-ray Photoelectron Spectroscopy (XPS) with a monochromatic Al X-ray source and the binding energy scale was referenced by setting the C1s peak at 284.8 eV. The element content was determined using a Vario EL Elemental Analyser (EA) in CHNS mode and the weight loss ratio in N2 or Air atmosphere was measured in TA Instrument SDT Q600 Thermogravimetric Analysis (TGA) with a heating rate of 10 ºC·min-1 from 25 ºC to 800 ºC. The molecular weight and polydispersity index (PDI) were determined using a gel permeation chromatography (GPC) system equipped with two KF-804 and KF-805 column in series and a Waters 2424 refractive index detector. THF with 0.05% LiBr was used as an eluent at a flow rate of 1.0 mL·min-1 and the calibration curve was obtained using polystyrene standard samples and fitted by triple-order polynomial equation. Contact angle measurements were performed using a CAM 100 Goniometer based on a CDD camera with 50 mm optics using deionized water as media at 1.0 µL liquid volume through pendant drop method. 9



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2.7 Activity of Free and Immobilized Enzyme Activities of laccase immobilized to glass fiber/polymer composites and the free counterpart were investigated with DMP as substrate. 11.7 mg DMP (0.6 mM) was dissolved in 100 mL 0.1 M acetic buffer solution. Free laccase and GF@PDA@PGMA-laccase were added into aforementioned solution and kept for 24h at room temperature. After filtration, the UV-visible spectra of DMP degradation products were then recorded in the wavelength range from 200 nm to 800 nm using a Cary 6000i Agilent spectrophotometer. The stability for laccase immobilized GF@PGA@PGMA was carried out under shear flow condition by mechanically stirring for different period of time and 5.0 mL of solution was taken out after a pre-set time interval for DMP degradation. DMP concentration was 0.6 mM in addition of 1 mL 0.36 mM DMP acetic buffer solution to the aforementioned solution. The absorbance at 470 nm for these samples was record.

3 RESULTS AND DISCUSSION

3.1 Preparation of PDA coated glass fibers

Mussel-inspired polydopamine coating offers a promising strategy to modify various surfaces with multi-functionality. By controlling the auto-oxidative self-polymerization of dopamine, robust PDA coatings with tunable thickness can be deposited on substrates, thereby forming strong covalent and noncovalent interactions.29 However, polydopamine coating suffers from poor homogeneity and long time of reaction in basic buffer solutions.45 Herein, PDA deposition onto glass 10


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fiber via chemical oxidation was investigated in slightly acidic conditions using sodium periodate and copper sulfate/hydrogen peroxide as oxidants. For convenience, GF@PDA-O2 is used to denote the PDA coated glass fiber prepared in traditional Tris buffer solution (pH=8.5). GF@PDA-SP and GF@PDA-CS/H2O2 refer to the PDA coated glass fiber through chemical oxidation in acidic solutions (pH=5.0) in the presence of sodium periodate and copper sulfate/ hydrogen peroxide, respectively.

Surface morphology of the PDA coated glass fiber under different synthetic conditions is shown in Figure 1. Apparently, the PDA coating of GF@PDA-O2 obtained in Tris buffer solution (pH=8.5) was not uniform. Large-sized PDA agglomerates were observed scattering on the surface of the glass fiber (Figure 1a). Compared to GF@PDA-O2, GF@PDA-CS/H2O2 (Figure 1b) and GF@PDA-SP (Figure 1c) possessed relatively homogeneous PDA deposition composed of PDA nanoparticles of ~ 20 nm diameter as shown in Figure 1d. This may be attributed to the more complete generation of dopamine quinine with strong oxidant via accelerated reaction kinetics, which allows efficient cyclization to dopaminochrome and subsequent conversion to indole-type units, resulting in homogenous nucleation followed by the quick deposition of PDA nanoparticles.46 Besides, PDA layer of GF@PDA-SP obtained in sodium periodate oxidation (two-electron oxidation) condition was thicker than that of GF@PDA-CS/H2O2 in copper sulfate/hydrogen peroxide (one-electron oxidation) system within 2 h. From the UV-vis spectra of dopamine solutions at predetermined time using sodium periodate at acidic pH (Figure S1, Supporting Information), the absorbance at 305 nm and 480 nm 11



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corresponding to dopaminochrome kept increasing within 1 h. After that, the 480 nm peak disappeared, indicating a conversion to indole-type units. Thus, PDA deposition onto glass fibers in sodium periodate containing acid solution was a more favorable approach for preparing GF@PDA with uniform homogenous PDA layer within a relative short period. Besides, the mechanical strength of the glass fiber matrix was efficiently improved with the PDA layer. In the following modification/functionality, GF@PDA prepared in 2.0 g·L-1 dopamine solution with pH value of 5.0 by using NaIO4 as oxide for 2 h was applied to prepare glass fiber/Poly(Glycidyl Methacrylate) composites.

Figure 1 SEM images of PDA coated GFs prepared under different conditions: (a) GF@PDA-O2-24 h; (b) GF@PDA-CS/H2O2-2 h; (c, d) GF@PDA-SP-2 h (DA 2 g·L-1; CS: 5 mM; H2O2: 19.6 mM; SP: 20 mM) 3.2 ICAR ATRP synthesis of GF@PDA@PGMA

To get an increased functionality degree, polymer chains with abundant grafting 12


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sites in its constitutional unit were considered as an efficient medium to realize the controlled laccase grafting via manipulated growth of PGMA brushes on GF@PDA surfaces. The PDA layer deposited on the glass fiber can act as a versatile platform, facilitating subsequent surface modification and/or functionalization. On this basis, controlled growth of poly(glycidyl methacrylate) (PGMA) brushes from glass fibers was performed by using surface-initiated ICAR ATRP. The synthetic scheme was illustrated in Figure 2. After the PDA layer was generated on the surface of GFs, ATRP initiator was introduced via amidation and/or esterification reactions with the amino and catechol groups in PDA structure. Subsequently, GMA polymerization was conducted by ICAR ATRP from the initiator anchored PDA layer using CuBr2/TPMA catalyst/ligand

system

and

AIBN

at

60

ºC.

The

obtained

composite

GF@PDA@PGMA could be further modified with rich epoxy groups ring-opening reaction to prepare specific functionalized glass fiber/polymer composites. O Br

NaIO4; pH 5.0; RM

DMF; Et3N; RM

Rinsed by Ethanol

GF

GF@PDA

AIBN; Cu (II)/TPMA;

GF@PDA-Br

DMF; 60 ºC;

Rinsed by DMF

GF@PDA-Br

Acatic buffer; RM

GF@PDA@PGMA

Rinsed by DMF/THF

Rinsed by DIW; Dry at RM

O Br

13


GF@PDA@PGMA -laccase


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Figure 2 Illustration of ICAR ATRP synthesis of GF@PDA@PGMA and post laccase immobilization The digital photos in Figure 3 recorded the change of glass fiber mat during surface modification. Firstly, a black PDA coating was observed for GF@PDA (Figure 3b). Because the self-polymerization of dopamine was carried out under mild conditions, GF@PDA well reserved the physical form of the glass fiber mat. The robust and reactive PDA platform enabled the anchoring of ATRP initiator in organic solvent, without compromising the texture of the substrate (Figure 3c). After ICAR ATRP growth of PGMA, white polymers were observed on the surface of the glass fiber mat (Figure 3d). SEM morphology of the glass fiber and the counterparts during surface modification were shown in Figure 4. It can be seen that glass fiber was an extremely smooth cylinder with diameter of 6.0 µm (Figure 4a). A uniform PDA layer with thickness of about 0.5 µm was observed in Figure 4b and the morphology of GF@PDA-Br had little change after BiBB grafting (Figure 4c) except that some PDA loss. PDA is accepted as supramolecular structure of oligomers arranged via multiple mechanisms including hydrogen bonding, π-π stacking, etc.,47 rather than real polymers with high molecular weight. Moreover, when PDA was used as coating material, the interaction between PDA and the substrates includes both covalent and non-covalent bonding. Thus, under harsh environment such as mechanical stirring and long-time organic solvent impregnation, the PDA coating layer may be partially damaged. Through ICAR ATRP reaction, a dense PGMA polymer layer of 2.1 µm was

observed

(Figure

4d),

indicating

successful

14


preparation

of

the

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GF@PDA@PGMA composite. The loss of homogeneity for GF@PDA@PGMA could be explained by the partial damage of PDA mentioned above and non-uniform distribution of the initiators. ATRP initiator attachment sites in layered-stacking polydopamine could not be simply regarded as monolayer module. Under this circumstance, PGMA chains, which grew from these ATRP initiators, could not be observed as smooth polymer layer even though all the PGMA chains were bomb-like and owned a narrow distribution of molecular weight.

Figure 3 Photos for (a) GF mat; (b) GF@PDA; (c) GF@PDA-Br and (d) GF@PDA@PGMA (VGMA=VAnisloe=3.0 mL; nAIBN:nCu(II)=1:1)

15



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Figure 4 SEM images for (a) GF mat; (b) GF@PDA; (c) GF@PDA-Br and (d) GF@PDA@PGMA (VGMA=VAnisloe=3.0 mL; nAIBN:nCu(II)=1:1) 3.3 Controlled growth of PGMA brushes from glass fiber through ICAR ATRP

Good control of polymerization on macro-initiator anchored surfaces is of great significance for building defined spatial topological structure and tuning polymer chain density. To facilitate the effective post-modification/functionalization of the glass fiber/polymer composites, a uniform comb brush like structure and retention of chain end-functionality for PGMA chains are preferred, while the knotted chains would influence the accessibility of the epoxy groups and hinder their nucleophilic addition reactions for the grafting of target functionalities. The polymerization of GMA monomers from GF@PDA-Br was conducted through ICAR ATRP in the presence of a sacrificial ATRP initiator. Free PGMA generated in the reaction solution was separated for measurements to monitor the molecular weight (Mn) and polydispersity (Mw/Mn). In our previous study, a high dilution strategy was employed 16


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for an improved control of PGMA growth in ICAR ATRP system, where the amount of AIBN was a vital factor affecting the rate of activator regeneration.22 Thus, in this study, the influences of GMA monomers and AIBN, were systematically investigated for the controllable growth of PGMA brushes from glass fibers. The experimental conditions and results were summarized in Table 1.

Table 1 Experimental conditions and results of the polymerization of GMA on glass fiber by ICAR ATRP

Samples

[AIBN]/

anisole/

[CuBr2]

GMA

Grafting

Time/h

Mn

Mw/Mn

(V/V)

contentb/

(wt.%)

GF@[email protected]

0.5

1

24

15652

1.51

18.3

GF@PDA@PGMA-R-1

1

1

24

15168

1.41

19.1


2.5

1

24

19532

1.59

24.7

GF@PDA@PGMA-R-5

5

1

24

18752

1.53

18.6

GF@PDA@PGMA-D-1

1

1

24

13514

1.47

13.5


1

2.5

24

15716

1.62

8.84

GF@PDA@PGMA-D-5

1

5

24

15852

1.60

10.6

GF@PDA@PGMA-D-10

1

10

24

15729

1.35

5.08

GF@PDA@PGMA-2

1

1

2

6492

1.19

7.93

GF@PDA@PGMA-4

1

1

4

9141

1.23

9.52

GF@PDA@PGMA-6

1

1

6

11063

1.23

10.4

Note: EBiB was introduced as the sacrificial initiator; the amount of macro initiator 17



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was obtained through Br content in XPS analysis; The molar ratio of monomer, sacrificial

initiator,

macro

initiator,

Cu

and

ligand

was:

[GMA]:[EBiB]:[macroinitiator]:[CuBr2]:[TPMA]=100:1:0.01:0.02:0.05; [GMA]=20 mM; aMn and Mw/Mn were measured through GPC; bThe grafting contents of PGMA on GF@PDA-Br were calculated from TGA curves.

Compared to normal ATRP, ICAR ATRP introduces a radical initiator, such as 2,2’-azobis(isobutyronitrile) (AIBN), as reducing agent to convert the accumulated deactivator CuIILn back to the activator CuILn, which allows the use of low concentration of copper catalyst.24,48 On the other side, the radical initiators continuously generate new chains I-X and I-P-X along with the extendable R-X chains generated from original (macro) initiator.49 The concentration of AIBN plays an important role in ICAR ATRP system, which significantly influences the polymerization kinetics as well as chain end-functionality. The effect of AIBN ratio to Cu catalyst on the GMA polymerization was shown in Table 1, corresponding to the data series of GF@PDA@PGMA-R-x (x=0.5, 1, 2.5, 5). When the ratio of AIBN to Cu was lower than 1.0, the increase of AIBN was beneficial for the homogenous polymerization. From the GPC results of free polymer chains, relatively lower value of Mn and polydispersity were obtained when [AIBN]/[CuBr2]=1.0. Further increase of AIBN could efficiently regenerate copper catalyst Cu(I) and accelerate chain growth from GF@PDA-Br. But, since AIBN enables polymer chain growth as a radical initiator, excess amount of AIBN in ICAR ATRP system brought about broader molecular weight distribution and more bimolecular termination, resulting in 18


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the loss of the

-R end group and increase of dead chain fraction (DCF). Basically, to

facilitate the effective post-modification/functionalization, uniform layer of PGMA brushes from GF@PDA-Br with controllable molecular weight and narrow polydispersity is expected. From this aspect, controlled growth of PGMA on the glass fibers via ICAR ATRP should be carried out in a system with optimized amount of AIBN.

According to the discussion aforementioned, polymerization rate is also closely associated with the degree of polymerization and chain end-functionality reservation of polymer products obtained in LRP systems. Here, a high dilution strategy was put forward for a better control over the ATRP growth of PGMA from the surfaces of glass fibers. The products in this series of experiments were donated as GF@PDA@PGMA-D-y, where y represents the volume ratio of anisole to GMA monomer (Table 1). A smaller Mw/Mn of 1.35 was observed when the volume ratio of anisole to GMA was 10-fold, implying a better control in diluted polymerization systems. However, because of the lower concentration of GMA monomer in the reaction mixture, reduced grafting content of PGMA brushes on the surfaces of GF@PDA-Br was obtained.

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

GF@PDA@PGMA-2 Mn=6497 Mw/Mn=1.19

0.00

GF@PDA@PGMA-6 Mn=11063 Mw/Mn=1.23

2.50x104

5.00x104

Molecular weight

Figure 5 Molecular weight distribution of free PGMA formed in the solution

To monitor the PGMA growth process in ICAR ATRP system, the Mn, polydispersity, and grafting content of PGMA obtained at pre-determined polymerization time interval were analyzed using GPC and TGA. Within 6 h, 80% conversion was obtained and the control of Mw/Mn was kept in the range of 1.19~1.23 (Figure 5). This suggested a typical LRP kinetics and steady polymer chain growth with relatively lower ratio of dead chain generation in ICAR ATRP system. With the increase of polymerization time up to 24 h, broadened molecular weight distribution (GF@PDA@PGMA-D-1 data in Table 1) was obtained due to the increased chain termination of the propagating radicals (I and R ) and reduced free GMA monomers.

In summary, the growth of PGMA brushes from glass fibers through surface-initiated ICAR ATRP is correlated to many factors in this heterogeneous polymerization system, including the ratio of GMA monomer, macro initiator, copper catalyst and ligands, the amount of solvent anisole and polymerization time. 20


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Performing the ICAR ATRP using a high dilution strategy with an appropriate GMA monomer to AIBN ratio and reduced reaction time is beneficial to prepare well-defined PGMA brushes with controlled molecular weights and narrow molecular weight distribution.

3.4 Structural and compositional characterizations

Elemental analysis results of the glass fiber/PGMA composite and the counterparts during surface modification were listed in Table 2. It can be seen that, the carbon content increases with the introduction of organic components on the glass fibers. Especially after the ATRP growth of PGMA brushes, the C wt.% increases from 5.42 wt.% to 40.21 wt.%. Similar tendency was also found for change of the hydrogen content. For pristine glass fiber, the existence of SiO2, which was the main composition of glass fiber, led to 11.6% Si content. When PDA coating was deposited onto glass fiber, N content obviously increased. This could be attributed to the amino groups in PDA. Following, Br content showed an increase from 0.06% up to 1.85% from XPS analysis (Figure S2, Supporting Information), indicating the successful introduction of initiator BiBB. Owing to the epoxy and ester group of GMA monomers, abundant surface O and C were detected, suggesting the growth of PGMA brushes from the surfaces of GF@PDA-Br. Figure 6 shows the FTIR spectra of the glass fiber/PGMA composite. Evidently increased C=O signal at 1728 cm-1 and epoxy signal at 874.9 cm-1 were observed for GF@PDA@PGMA (c), further confirming the successful generation of PGMA.

21



Table 2 Compositional analysis of the glass fiber/PGMA composite and the counterparts during surface modification by XPS and EA

Compositional analysis

Samples

XPS Surface Atomic%

EA wt.%

Si

Br

C

N

O

C

H

N

GF

11.6

Null

52.12

Null

34.70

3.63

0.24

0.09

GF@PDA

2.88

0.06

68.96

7.21

20.89

5.29

0.31

0.26

GF@PDA-BiBB

5.22

1.85

61.21

6.17

25.55

5.42

0.39

0.30

GF@PDA@PGMA

4.31

0.14

64.62

1.91

29.02

40.21

4.30

0.17

a GF b GF@PDA c GF@PDA@PGMA Transmittance (%)

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 38

(a) (b) (c) -1

1728 cm

874.9 cm-1

3500

3000

2500

2000

1500

1000

-1

Wavenumber (nm )

Figure 6 FTIR spectra for GF (a), GF@PDA (b) and GF@PDA@PGMA (c) The verification of each modification of glass fiber could also be conducted from XPS analysis through C and O peak-differentiation-imitating and the surface chemistry for all semi-products was shown in Figure 7 and Figure S3 (Supporting Information). For pristine glass fiber mat, C-O and C-C groups were introduced by 22


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the binder of PVA, and the O-C=O group may come from the other stabilizer (Figure 7a). When PDA was deposited onto glass fiber mat, C-O group on the surface ( C 1s peak at 286.23 eV) was enhanced with the emergence of C 1s peak at 284.87 and 287.54 eV corresponding to the C=C species and the

interactions, respectively

(Figure 7b). This was also proved by the appearance of O 1s peak at 532.9 eV and 531.7 eV, which was assigned to C=O and C-O group, respectively (Figure S3b). Meanwhile, N 1s peak was observed at 400.42eV in the spectrum of GF@PDA (Figure S2, Supporting Information). This was attributed to the benzene rings and quinoid structure of PDA. When following BiBB was successfully grafted, new peaks at 288.7 eV and 534.3 eV indicative of C 1s and O 1s signals of O-C=O group showed up (Figure 7c and Figure S3c), providing the evidence of anchored initiator. From the deconvolution of C 1s high resolution spectra of GF@PDA@PGMA (Figure 7d), the evident enhancement of C-O signal due to the introduction of epoxy groups indicated the generation of PGMA layers on the surfaces of the GF@PDA-Br.

23



2.0E4

2.0E4

(a)

(b)

C-C 1.6E4

Intensity (Arb. units)


1.5E4

1.0E4

5.0E3

C-O

0.0 290

288

286

284

282

280

C=C

1.2E4

C-O

8.0E3

4.0E3

O-C=O

0.0

C-N C-C

C=O π−π∗

290

288

286

284

282

280

Binding Energy (eV)

Binding Energy (eV) 2.0E4

6E3

(c)

5E3

1.5E4

C=C 1.0E4



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 38

C-N C-C

C-O C=O

5.0E3

O-C=O

(d)

C-O C=O

4E3 3E3

O-C=O

2E3 1E3

π−π∗ 0.0 290

288

286

284

282

280

Binding Energy (eV)

0

290

288

286

284

282

280

Binding Energy (eV)

Figure 7 XPS analysis using C 1s peak for (a) GF; (b) GF@PDA; (c) GF@PDA-Br and (d) GF@PDA@PGMA

TG/DTG measurements were performed to investigate the composition of each modification product and the results were given in Figure 8. For pristine glass fiber mat, 6.18 wt.% weight loss ratio was observed from 0~800 ºC (Figure 8a), resulting from the consuming of PVA binder and other stabilizer in the initial mat. After PDA coating, 10.37 wt.% weight loss ratio was found till 800 ºC (Figure 8b), indicating a PDA layer of 4.19 wt.% was deposited on the glass fiber. From the TGA curve of GF@PDA, the thermal decomposition of PDA mainly happened in the temperature range of 300~450 ºC compared with PVA loss around 250 ºC. This was in accordance with previously reported thermal properties of PDA.30 There was a slight increase (0.8 wt.%) in weight loss after grafting of BiBB onto GF@PDA (Figure 8c). And, the 24


Page 25 of 38

weight loss increased up to 19.1 wt.% after controlled growth of PGMA brushes (Figure 8d), which was attributed mainly to PGMA thermal decomposition in a temperature range between 200 and 450 ºC (Figure S4, Supporting Information) and this indicated a successful generation of PGMA brushes via ICAR ATRP onto the glass fibers. Besides, from TG analysis, the grafting content of PGMA in the glass fiber/polymer composites could be estimated which was shown in Table 1. Besides, from Figure S5 (Supporting Information), BET surface area for glass fiber mats was 5.19 m2·g-1, and this small BET value was attributed to the non-porous property of glass fiber substrate. Based on TG data from Figure 8, the grafted amount of PGMA was determined at 19.1 wt% on the condition of 3.0 mL GMA monomer and 3.0 mL anisole. Thus, PGMA grafting density of 3.68 µg·cm-2 was calculated. 104

(a)

100 °C

0.15

(b) 355 °C

88

(c)

0.10 80 248 °C

0.05

72

(c)

Deriv. Weight (% /°C)

0.20

96

Weight loss (%)

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


(d)

(b) 188 °C (d)

64 0

100

200

0.00 300

400

500

600

700

800

T ( °C)

Figure 8 TG and DTG curves for (a) GF; (b) GF@PDA; (c) GF@PDA-Br; (d) GF@PDA@PGMA (GMA: anisole=1:1 (V/V)) (All the data has been corrected in consideration of PDA loss in the synthesis )

25



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Page 26 of 38

3.5 Enzyme immobilization to GF@PDA@PGMA

Laccase, used as an enzyme model, was introduced to establish “active surfaces” through covalent binding between the substrate GF@PDA@PGMA and the biomolecules. Laccase was successfully immobilized onto GF@PDA@PGMA-R (19.1 wt.% PGMA grafting content) by the nucleophilic attack of amine groups of laccase

to

epoxy

groups

of

matrix.

Through

TG

characterization

of

GF@PGA@PGMA before and after laccase grafting (Figure S6, Supporting Information), 2.24 wt.% grafting ratio of laccase (equal to 22.4 mg·g-1 enzyme stabilization capacity) was obtained in acetic buffer solution with 2.0 mg·mL-1 concentration of laccase at a grating density of 0.43 µg·cm-2. From XPS characterization (Figure 9), an obvious new peak of N 1s was observed for the laccase immobilized GF@PDA@PGMA composite, compared to the unmodified counterpart. It is noteworthy that, the GF@PDA@PGMA composite showed relatively hydrophobic surface, and, the contact angle was slightly increased and protein repellent after the immobilization of laccase (Figure S7, Supporting Information).

26


Page 27 of 38

O1s GF@PDA@PGMA GF@PDA@PGMA-Laccase

C1s

C/S

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


N1s

1400

1200

1000

800

600

400

200

0

Binding Energy (eV)

Figure 9 XPS spectra for GF@PDA@PGMA and GF@PDA@PGMA-laccase

Laccase activity was investigated through the oxidation of 0.6 mM DMP substrate in 20.0 mL solution. The spectroscopic curve of the oxidative product under UV-vis was measured and given in Figure 10. The laccase immobilized GF@PDA@PGMA composite and free laccase in form of powder and liquid were used for DMP degradation (equivalent amount of laccase ~ 0.5 mg). It was found that enzymatic activity of the laccase immobilized on the glass fiber composite was evidently enhanced compared to the powder and liquid laccase, showing the highest absorbance value at 470 nm. The immobilization of laccase onto GF@PDA@PGMA provides a larger surface area, which is presumably to be beneficial for the substrate accessibility. As discussed before, an obvious adsorption peak would appear at 470 nm when laccase existed and proceeded DMP degradation. However, the absorbance values for different samples acquired in shear flow experiment were close to zero (Table S1, Supporting Information), suggesting no laccase dropped out even in long time shear 27



flow condition. On the other side, this could be the evidence illustrating the stability of laccase after covalent bonding to GF@PDA@PGMA.

0.12 DMP Laccase (Powder) Laccase (Liquid)

0.09

Absorbance

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 38

GF@PDA@PGMA-Laccase

0.06

0.03

0.00 300

350

400

450

500

550

600

650

Wavelength (nm)

Figure 10 UV-vis spectra for DMP oxidation product by free laccase (powder form-red; liquid form-blue) and immobilized laccase onto GF@PDA@PGMA (pink)

4 CONCLUSIONS

High surface-area-to-volume glass fiber/polymer composites with active surfaces were prepared, for the first time, through surface-initiated ICAR ATRP integrated with bio-inspired polydopamine chemistry. By manipulating the self-polymerization of dopamine under acidic conditions in the presence of different oxidants, uniform and robust PDA layers were deposited on the surfaces of glass fibers. Compared to traditional silane coupling agents, PDA chemistry provided a facile and benign way for surface modification of glass fibers. The abundant amine groups in the PDA structure facilitated the efficient anchoring of ATRP initiators. Controlled growth of 28


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PGMA brushes were then performed from the initiator-anchored surfaces of glass fibers by using ICAR ATRP method. To obtain polymer brushes with defined grafting density, low polydispersity, and high chain end-functionality, influences of free radical initiator AIBN, monomer concentration, and polymerization time were systematically investigated. Determined grafting content of PGMA on the glass fibers could be tuned, varying from 5 wt.% ~ 25 wt.%. Better control over the PGMA growth and preservation of CEF could be achieved by using a high dilution strategy (anisole: GMA=10:1 (V:V)) at lower conversion, with the polymerization time less than 6 h. The glass fiber/PGMA composites with rich epoxy groups provided an ideal platform for accommodating biomolecules for biochemical and biological applications. Laccase, as an enzyme model, was covalently immobilized through the mild ring-opening reaction of the epoxy groups. The obtained bio-composites showed evidently enhanced catalytic activity toward DMP degradation, compared to the free laccase in powder form or in solution.

Overall, this study established a new methodology using surface-initiated ICAR ATRP and bio-inspired PDA chemistry for surface functionalization of glass fibers to build active (bio-)composites. The synthetic strategy can integrate versatile organic functionalities with the high surface-area-to-volume glass fibers for more broadened applications.

ASSOCIATED CONTENT

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

Supporting Information. UV-vis spectra for self-polymerization of dopamine, XPS Spectra, TG curves, and contact angle measurements for the glass fiber/PGMA composites. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

E-mail: [email protected]; [email protected].

ACKNOWLEDGMENT

This study was financially supported by the Changjiang Scholars and Innovation Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), National Natural Science Foundation of China under Projects U1430234, 51673109, and 51473087.

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Controlled architecture of glass fiber/PGMA composites via SI- ATRP

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Poly(glycidyl ... - ACS Publications

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