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UV-absorbing cellulose nanocrystals as functional reinforcing fillers in poly(vinyl chloride) films Zhen Zhang, Gilles Sèbe, Xiaosong Wang, and Kam (Michael) Chiu Tam ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00126 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Applied Nano Materials

UV-absorbing cellulose nanocrystals as functional reinforcing fillers in poly(vinyl chloride) films Zhen Zhang, †, ‡ Gilles Sèbe *, ‡, § Xiaosong Wang, *, † Kam C. Tam, *, ⊥ † Department of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1. ‡ Laboratoire de Chimie des Polymères Organiques, Université de Bordeaux, 16 avenue PeyBerland, F-33600 Pessac, France. §CNRS, LCPO, UMR 5629, F-33600 Pessac, France

⊥ Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada, N2L3G1 KEYWORDS: poly(vinyl chloride), thermal stability, UV resistance, UV-absorbing, Cellulose Nanocrystals, SI-ATRP

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ABSTRACT: Ultraviolet (UV)-responsive poly(cinnamoyloxy ethyl methacrylate) (PCEM) was grafted on cellulose nanocrystals (CNCs) via the Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) technique. The resultant PCEM-grafted CNCs (PCEM-g-CNCs) exhibit high UV absorption properties and undergoes crosslinking when subjected to UV radiation. When the PCEM-g-CNC nanohybrids were incorporated into poly(vinyl chloride) matrix, transparent nanocomposite films with UV-blocking characteristics were obtained. Comparison of the optical and mechanical properties of the films before and after UV-irradiation confirmed that the PCEM-g-CNCs were excellent thermal and UV-stabilizers for PVC. In addition, the tensile properties of the PVC film were increased significantly, and further enhanced after UV-irradiation.

INTRODUCTION Poly (vinyl chloride) (PVC) is the third-most widely used synthetic plastic around the world, after polyethylene and polypropylene.1-3 Due to its outstanding mechanical properties, excellent chemical resistance, durability, non-flammability, formulating versatility and low price, PVC is widely used for pipes, window frames, packings, wires, coatings and plastic cards.2-7 However, PVC exhibits very low thermal stability and begins to degrade at the glass transition temperature (Tg) of the polymer, which poses many challenges during manufacturing.8-10 Moreover, its gradual degradation by weathering, when exposed to heat and ultraviolet (UV) light, reduces the lifetime of PVC-based products. The degradation of PVC results in discoloration, embrittlement and micro-cracking, leading to changes in appearance and reduction in mechanical performance.2,

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Therefore, thermal and UV stabilizers are often required in PVC matrices.

However, many stabilizers contain toxic heavy metals, such as lead, which may leach out in drinking water that lead to safety and environmental issues.8 Titanium dioxide (TiO2) is an


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essential additive in PVC matrices due to its outstanding UV absorption capability.4, 11 However, TiO2 also acts as a photo-catalyst and accelerates the weathering degradation of PVC surfaces.4, 12-13

Therefore, there is a critical need to develop environment-friendly and efficient thermal and

UV stabilizers for PVC. Due to the increasing concern on climate change and environmental pollution, it is highly desirable to develop sustainable materials with good performance. Cellulose nanocrystals (CNCs) are bio-based nanoparticles produced by sulfuric acid hydrolysis of cellulosic substrates, such as microcrystalline cellulose, paper or pulp.14-16 These rod-shaped nanoparticles are renewable, biocompatible, possess high tensile strength and elastic modulus, have a low density and low coefficient of thermal expansion, and may self-assemble to yield nematic phases.17-18 As a result, CNCs offer many attractive features that can be exploited in the field of cosmetics, emulsions, electronics, liquid crystals, composites, foods, and catalysts.14-17, 19 In particular, the excellent mechanical properties of CNCs make them ideal candidates for use as reinforcing agents in polymer matrices.16,

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However, the hydrophilic surface of CNCs limits their

compatibility with hydrophobic matrices, and the hydroxylated nanoparticles are prone to selfaggregation via hydrogen bonding. Thus, surface modification of CNCs is generally required to improve the filler/matrix interfacial adhesion, and to introduce novel functionalities necessary for many advanced engineering applications.21-23 To date, only limited studies have reported on the utilization of CNCs as reinforcing agents in PVC matrices.24 Photo-responsive molecules containing cinnamate functional groups have demonstrated interesting properties in the field of light-induced shape memory materials,25-28 polymers with crack healing properties,29 or hydrogels.30-33 Moreover, the cinnamate moiety exhibits a strong absorption in the UV region and can undergo [2π+2π] photodimerization and trans-cis


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photoisomerization upon UV irradiation (wavelength λ> 280 nm).34 Compared with the trans-cis photoisomerization, the photodimerization is the predominant process during UV irradiation.30, 35 When pendent cinnamate groups are attached to polymers, the mechanical properties of the material can be greatly enhanced by crosslinking, via the photodimerization process.36-37 Cinnamate esters are considered non-toxic and biocompatible and are widely used in the flavor, perfume and pharmaceutical industries.25 The grafting of photo-responsive cinnamate moieties at the CNCs surface could then impart additional UV-absorbing and photo-crosslinking properties to the nanoparticle, which could have a positive impact on the mechanical and photo-stability of PVC matrices reinforced with this filler. Different methods can be used to functionalize the CNCs reinforcing particles, but the surface grafting of polymers chains is particularly attractive, as it provides a method of strengthening the interfacial adhesion by (i) adjusting properly the surface wettability of the CNCs and (ii) promoting chain entanglements at the filler/matrix interface.38-39 In particular, Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) is a “grafting from” method that allows for the preparation of polymer-grafted CNCs with good control.21-22, 40-42 It could therefore serve as an efficient tool to graft polymers bearing cinnamate moieties at the CNCs surface. Herein, poly(cinnamoyloxy ethyl methacrylate)-grafted CNC nanohybrids (PCEM-g-CNCs) were prepared by SI-ATRP and employed as a UV/thermal stabilizer and reinforcing agent in PVC films. The grafted nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and water contact angle (WCA) measurements. The photoactivity of the grafted cinnamate moieties was demonstrated by UV spectroscopy, FTIR, TGA, and DSC, through the analysis of the grafted material before and after UV-irradiation. The PCEM-g-CNC nanohybrids


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were then incorporated in a PVC matrix to produce transparent PVC films. The optical and mechanical properties of the films before and after UV-irradiation were subsequently investigated and compared with the properties of PVC and PVC/CNC reference materials. EXPERIMENTAL SECTION Materials. 2-Hydroxyethyl methacrylate (HEMA), cinnamoyl chloride, α-bromoisobutyryl bromide (BIBB), 4-(dimethylamino) pyridine (DMAP), ethyl α-bromoisobutyrate (EBiB), triethylamine (TEA), N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (PMDETA), CuBr, CuBr2, poly(vinyl chloride) carboxylated (PVC) (Mw =220,000 g/mol) and all solvents were purchased from Sigma. CuBr was purified with acetic acid at 80 oC under N2 for 24 h, and washed with acetic acid and dried under vacuum at 50 oC. HEMA was purified by distillation in calcium hydride under reduced pressure, and all the other chemicals were used as received. Synthesis of monomer: cinnamoyloxy ethyl methacrylate (CEM). TEA (18.4 mL, 0.12 mol) was added to 100 ml dichloromethane (DCM) solution containing HEMA (13 g, 0.1 mol) under argon. The mixture was cooled in an ice bath. DCM solution (80 mL) of cinnamoyl chloride (16.7 g, 0.1 mol) was added dropwise within 1 hour in the ice bath. After stirring overnight, the triethylamine hydrochloride salt was removed by filtration. The filtrate was concentrated in a rotary evaporator under reduced pressure. The CEM was purified in a silica gel chromatography using a 10:1 (v/v) petroleum ether: ethyl acetate mixture as eluent. After drying at 50oC under reduced pressure overnight, 19.5 g viscous liquid was obtained as the final CEM with a yield of 68.0%. 1H NMR (400 MHz, CDCl3): δ 1.89 (3H, t, CH3); 4.37 (4H, m, -O-CH2CH2-); 5.53 (1H, quin, =CH), 6.08(1H, quin, =CH), 6.39 (1H, d, =CH-Ar), 7.64 (1H, d, =CHCO-), 7.3~7.5 (5H, m, ArH).

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C NMR (400 MHz, CDCl3: δ 18.30 (-CH3); 62.21, 62.52 (-O-


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CH2-CH2-O-); 117.54, 145.42, (-HC=CH-Ar); 126.10 (=CH2); 128.16 (2CAr); 128.92 (2CAr); 130.45, 134.20 (2CAr); 135.96 (=C-); 166.69, 167.18 (2C=O). Preparation of the brominated CNC nano-initiator: CNC-Br. CNC-Br was prepared by modifying the surface of CNCs with BIBB. CNCs (1.2 g) were dispersed in DMF (100 mL) by sonication. Then TEA (3.1 mL, 22.2 mmol) and DMAP (1.36 g, 11.1 mmol) were added. The solution of BIBB (1.36 mL, 11.1 mmol) in DMF (20 mL) was added dropwise to the dispersion of CNCs kept in an ice bath. After 24 h, H2O was added and the resultant CNC-Br was recovered by centrifugation and subsequently washed with THF for 24 h using a Soxhlet extractor. The product was further dialyzed against deionized H2O for 5 days. Finally, the purified CNC-Br was obtained via freeze-drying. Preparation of PCEM-g-CNCs and PCEM. The SI-ATRP of PCEM from CNC-Br was conducted in the absence of sacrificial initiator to enhance the utilization efficiency of the monomer. Therefore, polymerization was only initiated from the surface of CNC-Br and there was no free PCEM produced in the SI-ATRP procedure. In detail, CNC-Br (700 mg) was dispersed in DMF (70 mL) by sonication. CuBr2 (22 mg, 0.1 mmol) and CEM (1.3 g, 5 mmol) were added. The mixture was purged with argon for 20 min to remove oxygen. Then CuBr (58 mg, 0.4 mmol) and PMDETA (0.21 mL, 1 mmol) were added under the protection of argon. Three freeze-pump-thaw cycles were conducted to remove the oxygen in the mixture. The polymerization was conducted at 40oC. After 12 hours, the reaction was terminated by exposing the mixture to air. PCEM-g-CNCs were then collected by centrifugation, washed with ethanol (sonication and centrifugation cycle) three times, and dialyzed against water for 6 days (molecular weight cut off is about 14,000 g/mol). Finally, the PCEM-g-CNCs were freeze-dried.


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According to the 1H NMR spectrum, the monomer conversion of the SI-ATRP reaction is ca. 35%. Therefore, the weight ratio of PCEM in PCEM-g-CNCs is ca. 39%. PCEM was synthesized separately with EBiB as an initiator, using a similar ATRP procedure. The molar ratio of [CEM]: [CuBr]: [CuBr2]: [PMDETA]: [EBiB] was 88: 1: 1: 4: 2. After polymerization at 40 oC for 2 hours, the monomer conversion is 42.8% (determined by 1H NMR). The average number molecular weight (Mn) of PCEM determined by GPC is 6404 Da, with a dispersity of ca. 1.3. Preparation of the PVC films. The PVC films reinforced with CNCs (PVC/CNC) or PCEMg-CNCs (PVC/PCEM-g-CNC) were prepared by solution casting. CNCs or PCEM-g-CNCs (25 mg) were dispersed in DMF (5 mL) by sonication. After the addition of PVC (500 mg; weight percentage of CNCs or PCEM-g-CNCs relative to PVC = 5%), the mixture was stirred for 24 hours, to dissolve the PVC. The mixture was then sonicated in an ice bath for 2 min set at 16 W to complete the dissolution. The mixture was degassed for 10 min under vacuum. The degassed mixture was then poured into a round silica mold of diameter 65 mm. The homogeneous films were obtained after removing the solvent in the oven at 60 oC for 12 hours, followed by annealing at 80 oC for 10 hours. A neat PVC film was also prepared by the same procedure as a reference. The thickness of the films was ca. 90 µm, evaluated from the cross section analyzed by optical microscopy. The PVC films were cut into samples with rectangular dimensions of 1 cm × 4 cm for the tensile test measurements. UV irradiation experiments. The impact of UV irradiation on the FTIR spectra and DSC curves of PCEM and PCEM-g-CNC samples was investigated from their THF solution (10 mg/mL) or dispersion (10 mg/mL), respectively. The solution or dispersion was irradiated at 365


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nm for 12 hours, using a UV-F400 system (with a power of 450 W), leading to a concurrent evaporation of the solvent. The recovered dry material was then analyzed by FTIR and DSC. The impact of UV irradiation on the cross-linking of PCEM and PCEM-g-CNC samples was investigated from films prepared by spin-coating THF solution/dispersion of PCEM/PCEM-gCNCs on quartz. The films deposited on the quartz support were then irradiated at 365 nm, using a handheld UV lamp (UVP) with a power of 6 W. The absorbance of the films was then measured at different time intervals with a UV-vis spectrometer in order to follow the evolution upon irradiation of the conjugated C=C absorption at 276 nm. The PVC films were irradiated at 365 nm wavelength for 24 hours, using a UV lamp with a power of 100 W. To protect the films from the heat produced by the UV lamp, the distance between the UV lamp and the PVC films was set to 20 cm. Characterization. 1H NMR and

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C NMR spectra were recorded using a Bruker AC- 400

NMR at room temperature by dissolving the samples in CDCl3. TGA was performed on a TGA-Q50 system (TA Instruments) under nitrogen atmosphere with a balance gas flow rate of 60.0 mL/min. A 5 ~ 10 mg sample was placed on a platinum pan and heated from room temperature to 700

o

C at a heating rate of 10oC/min. Derivative

Thermogravimetric Analysis (DTG) curves were obtained from the derivation of the TGA curve. Differential scanning calorimetry (DSC) was measured using a DSC Q100 apparatus (TA Instruments). The DSC was calibrated with indium sample. All samples were first heated to 150 o

C to remove the moisture and then cooled down to 0 oC and heated to 200 oC with nitrogen flow

rate of 40.0 mL/min. The glass transition temperature (Tg) values were determined from the second heating run of DSC.


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Gel Permeation Chromatography (GPC) analysis was performed in THF with LiBr at room temperature on a PL-GPC 50 plus Integrated GPC (Polymer Laboratories-Varian). The elution of the filtered samples was monitored using a simultaneous refractive index and UV detection systems. The elution times were converted to molar mass using a calibration curve based on low dispersity polystyrene standards. Fourier transform infrared spectra (FTIR) of all the samples were acquired after they were pressed into KBr pellets using the Thermo Nicolet Avatar 970 FTIR spectrometer with a resolution of 8 cm-1 (64 scans). The samples for water contact angle (WCA) measurements were prepared by spin coating the CNCs aqueous dispersion or PCEM-g-CNCs THF dispersion on glass. The static WCA was measured by analyzing the images of 3 water droplets using MATLAB. Ultraviolet−visible (UVvis) spectra were obtained on an Agilent Model 8453 UV-vis spectrometer. Dynamic light scattering (DLS) and zeta-potential experiments were performed using a Malvern Instrument Zetasizer Nanoseries. The tensile testing of the PVC films was performed using a computer-controlled TA materials testing instrument at room temperature. Three rectangular samples with dimensions of 1 cm × 4 cm are used for the tensile testing. The effective length for the tensile testing was 2 cm. A constant crosshead speed of 20 mm/min was maintained for all samples. Scanning electron microscopy (SEM) measurements were performed on a FEI Inspect F50 scanning electron microscope, operating at 2.0 kV. RESULTS AND DISCUSSION Preparation and characterizations of the PCEM-g-CNC nanohybrids. The CNCs used in this study were isolated by the sulfuric acid hydrolysis of wood pulp, according to a general


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procedure described in the literature.43 They consist of rod-like particles with estimated dimensions of 110.3 ± 47.7 nm in length and 4.8 ± 1.1 nm in diameter, based on a study performed with particles taken from the same batch (estimated from AFM pictures).44 The amount of accessible hydroxyl groups at their surface was estimated to be 3.10 ± 0.11 mmol.g-1, using a method based on phosphorylation coupled with 31P NMR and FTIR analysis.44 The CNCs are also negatively charged (zeta-potential in water = - 48 mV) due to the presence of sulfate ester groups at the surface, imparted by the sulphuric acid treatment. Scheme 1. Chemical route used to prepare the PCEM-g-CNC hybrid nanoparticles.

The chemical route used to prepare the PCEM-g-CNC nanohybrids is summarized in Scheme 1. The CEM monomer was synthesized by acylating HEMA with cinnamoyl chloride. The successful synthesis of CEM was confirmed by the 1H NMR and

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C NMR (Figure S1 and

Figure S2 in the supporting information). To graft the polymer by SI-ATRP, Br initiating sites were first introduced on the surface of CNCs by esterification with BIBB (CNC-Br). The success of the reaction was confirmed by FTIR, through the observation of the carbonyl stretching vibration of the grafted bromoisobutyrate moiety at 1739 cm-1 (Figure 1A). The SI-ATRP


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grafting of PCEM from CNC-Br was performed in the absence of sacrificial initiator. Therefore, there is no free PCEM produced in the SI-ATRP process, and only PCEM brushes are initiated from the surface of CNC-Br. A reference PCEM polymer was nevertheless synthesized separately, using the same ATRP process, with ethyl α-bromoisobutyrate (EBiB) as an initiator. The FTIR spectra of PCEM-g-CNCs and PCEM are shown in Figure 1. Compared with CNC-Br, the FTIR spectrum of PCEM-g-CNCs displayed additional vibration bands characteristic of the PCEM polymer at 1716 cm-1 (C=O stretching of the two carbonyl groups), 1635 cm-1 (C=C stretching) and 768 cm-1 (out-of-plane C-H bending of the aromatic ring), suggesting that the grafting was successful.32

Figure 1. FTIR spectra (A) and DSC curves (B) of CNCs, CNC-Br, PCEM-g-CNCs and PCEM. The glass transition temperature (Tg) of PCEM and PCEM-g-CNCs were measured by DSC, from the second heating scan (Figure 1B). As expected, the crystalline CNC and CNC-Br materials did not display any glass transition. PCEM-g-CNCs possessed an enhanced apparent Tg of 52.9 oC compared with free PCEM (Tg = 32.8 oC), which is attributed to the restriction of the chains movement after the grafting at the CNC surface.


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The surface wettability of CNCs and PCEM-g-CNCs was evaluated by WCA measurements performed at the surface of films prepared by spin-coating a water or THF dispersion (respectively) of the nanoparticles on glass. The water droplet images on the different surfaces are shown in Figure 2. The WCA on the pristine glass support is ca. 28.6o. The average WCA on CNC-coated glass is only ca. 14.0o, confirming the hydrophilic nature of the unmodified CNC. On the other hand, the WCA of PCEM-g-CNCs coated glass is about 66.6o, indicating that the polymers grafted at the CNC surface imparted some hydrophobic properties to the material.

Figure 2. Images of the water droplets deposited on the glass support, before and after spincoating with CNCs and PCEM-g-CNCs. The dispersibility of CNCs and PCEM-g-CNCs in water and DMF was studied by DLS, respectively. The optical images of CNCs and PCEM-g-CNCs dispersion in water and DMF (5mg/mL) are shown in Figure S1 (supporting information). The CNCs dispersion in water and DMF are homogeneous and transparent, and the PCEM-g-CNCs were well-dispersed in DMF. However, the PCEM-g-CNCs cannot be dispersed in water even after extensive sonication, and the PCEM-g-CNCs immediately precipitated from the turbid aqueous dispersion (Figure S1). The size distribution (based on intensity) of the CNCs and PCEM-g-CNCs aqueous or DMF dispersion (0.2 mg/mL) characterized by DLS is shown in Figure S2. The average apparent hydrodynamic diameter (Dh) of CNCs in aqueous suspension was about 90.4 nm, which is consistent with the estimated length of the CNCs. The size distribution in intensity of CNCs in


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DMF exhibited two peaks, and the Dh of CNCs in DMF was about 258.4 nm, indicating a slight aggregation of CNCs in DMF suspension. The Dh of PCEM-g-CNCs in DMF was about 182.9 nm, indicating a better dispersibility than CNCs in DMF. However, the PCEM-g-CNCs displayed a broad size distribution with an average size greater than 1000 nm, suggesting the poor dispersibility of PCEM-g-CNCs in water. The thermal stability of PCEM-g-CNCs was investigated by TGA and compared with that of CNCs, CNC-Br, and PCEM. The thermograms and corresponding DTG derivatives are shown in Figure 3. The thermogram of pristine CNCs is consistent with the thermal behavior generally observed for CNCs bearing sulfate ester groups.45 The CNC-Br material displays a decreased thermal stability associated with the labile bromine, which may form HBr upon heating and can catalyze cellulose degradation.42, 46 The unmodified CNCs exhibit a 6% weight loss below 100 o

C, associated to the absorbed moisture. This weight loss is absent or small in the thermograms

of PCEM, CNC-Br, and PCEM-g-CNC, due to the more hydrophobic character of these samples. The PCEM degradation occurs in two step, consistent with literature data.47 The first weight loss of low decomposition rate (from 190 to 300°C) is associated with the cleavage of the cinnamate moieties, while the second weight loss of higher decomposition rate (from 300 to 450 °C) corresponds to the decomposition of the main polymer chains. Compared with the CNC-Br nanoinitiators, the PCEM-g-CNC nanoparticles exhibit a much better thermal stability, with a decomposition process approaching that of the free polymer.


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Figure 3. TGA thermograms (A) and DTG curves (B) of pristine CNC, CNC-Br, PCEM and PCEM-g-CNC. Impact

of

UV

irradiation

on

PCEM-g-CNC.

PCEM

is

known

to

undergo

photoisomerization and photodimerization when irradiated by UV light (wavelength λ >280 nm). Upon irradiation at 365 nm, the cinnamate moiety can isomerize from trans to cis form and/or dimerize via [2π+2π] photocyclization (Scheme 2). The photodimerization process is believed to be the predominant process during UV irradiation. Scheme 2. Photoisomerization and photodimerization of PCEM-g-CNCs under UV irradiation at 365 nm.


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Since the conjugated C=C bond in the cinnamate moiety displays a UV absorption band at 276 nm due to the π-π* electronic transition, the photodimerization process can be monitored by UV spectroscopy. Hence, PCEM and PCEM-g-CNC films – prepared by spin-coating THF solutions or dispersions of the polymer or nanohybrids – were irradiated at 365 nm for several hours. The evolution of their absorbance was then recorded with time, with a UV-Vis spectrometer (Figure 4A and C). As expected, the UV absorbance of the conjugated C=C bond of both the free and grafted polymer is observed between 240 and 320 nm, but its intensity tends to decrease with increasing irradiation time at 365 nm. This decrease was assigned to the [2π+2π] photocyclization of neighboring cinnamate moieties within the free or grafted polymers. The evolution of the maximum UV absorbance (At) and reciprocal (1/At) at 276 nm, were then plotted as a function of irradiation time for both PCEM and PCEM-g-CNC films (Figure 4B and D). 1/At increases proportionally with time, indicating that the photodimerization occurred in bulk, and followed second-order kinetics. The slope of the fitting line of 1/At vs. time is much smaller for the PCEM-g-CNCs than for PCEM, suggesting that the photodimerization of PCEMg-CNCs occurred at a much slower rate.


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Figure 4. Evolution of the UV absorption of the conjugated C=C bond in PCEM (A) and PCEMg-CNCs (C) films irradiated at 365 nm for several hours; and evolution upon irradiation of the maximum absorbance () and reciprocal (▲) at 276 nm, for both PCEM (B) and PCEM-g-CNCs (D) films. To ensure PCEM and PCEM-g-CNCs were sufficiently crosslinked, the impact of UV irradiation on the FTIR spectra and DSC curves of PCEM and PCEM-g-CNC samples was investigated from their THF solution or dispersion, respectively. The solution or dispersion was irradiated at 365 nm for 12 hours, leading to a concurrent evaporation of the solvent. The recovered dry material was then analyzed by FTIR and DSC.


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The FTIR spectra of the PCEM and PCEM-g-CNC samples, before and after irradiation at 365 nm are presented in Figure 5A. In both cases, the intensity of the stretching vibration of the conjugated C=C at 1635 cm-1 decreases after UV irradiation, consistent with the expected photodimerization. Meanwhile, the conjugated C=O at 1716 cm-1 is up-shifted to 1731 cm-1, which is consistent with the loss of conjugation expected after the cycloaddition.48 The decrease in the intensity of the trans-vinylene C-H deformation at 980 cm-1 after irradiation, could be related to both the dimerization and trans-cis isomerization processes.35 This band completely disappears from the spectrum of PCEM-g-CNCs after irradiation, while a weak C=C signal is still observed, suggesting that partial photoisomerization may also have occurred in that case. However, since no distinguishable cis-vinylene C-H band could be detected in the spectra of both irradiated samples, photodimerization should be the main process. With PCEM-g-CNC nanohybrids, the photo-crosslinking should be highly favored between neighboring chains grafted at the surface of the single nanoparticles. With the PCEM solutions, the inter-chain contact should be more difficult, which may explain why a significant amount of cinnamate moieties remains uncross-linked after UV-radiation (the intensity of the C=C band remains quite high). The DSC curves of the PCEM and PCEM-g-CNC samples before and after irradiation at 365 nm are shown in Figure 5B. The Tg of both samples disappeared after UV irradiation in the temperature range investigated. The movement of the polymer chains being restrained by the crosslinking, the Tg is expected to shift to a higher temperature or completely disappear, depending on the degree of crosslinking.


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Figure 5. FTIR spectra (A) and DSC curves (B) of PCEM and PCEM-g-CNC samples, before and after irradiation at 365 nm of their solution/dispersion (overnight irradiation accompanied by an evaporation of the solvent). Preparation and properties of composite PVC films filled with PCEM-g-CNC nanohybrids. Composite PVC films were prepared via the solution casting method. DMF was chosen as the co-solvent to favor the miscibility between the cellulosic nanoparticles and PVC. The PVC solution in DMF is transparent and colorless, while the PVC/CNC dispersion is opaque, and the PVC/PCEM-g-CNCs dispersion is slightly translucent. As shown in Figure 6A, homogeneous and highly transparent PVC films were obtained after evaporation of the DMF and subsequent heat annealing at 80 oC. The thickness of the films was ca. 90 µm, evaluated from the cross section analyzed by optical microscopy. The transparency of the PVC films suggests that there is negligible aggregation of CNC or PCEM-g-CNCs in the PVC matrices, indicating the good miscibility between CNCs or PCEM-g-CNCs and PVC. Moreover, the SEM of PVC films was also conducted to study the miscibility of CNC or PCEM-g-CNCs in the PVC matrices. As shown in Figure 6B, no CNC or PCEM-g-CNC aggregates were observed in the PVC films,


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suggesting the good miscibility between CNCs or PCEM-g-CNCs and PVC matrices by the solution casting method.

Figure 6. (A) PVC, PVC/CNC (5 wt. %) and PVC/PCEM-g-CNC (5 wt. %) composite films obtained by solution casting of the corresponding DMF solutions/suspensions followed by heat annealing; (B) the SEM images of PVC/CNC film and PVC/PCEM-g-CNC film (the scale bar is 5 µm). The effect of weathering on PVC has been widely studied and is influenced by many parameters, such as temperature, sunlight intensity, moisture, oxygen, additives etc.6,

49

Generally, two main degradation reactions occur during the weathering of PVC: (1) through the action of heat, hydrogen chloride is eliminated while conjugated polyenes are formed, leading to the discoloration of the material;50 (2) through the action of light, the photo-induced free radicals in the polyenes are rapidly quenched by oxygen or crosslinked, leading to a bleaching of the


19


Page 20 of 34

material.49 A small exposure to heat or light can result in a significant color change of the PVC material. Therefore, the color change of the PVC induced by heat or light could be employed to represent the degradation of PVC.

Figure 7. Transmittance of the PVC, PVC/CNC and PVC/PCEM-g-CNC films before and after heat annealing at 80 oC (A) and before and after UV irradiation (B) The optical properties of the PVC films before annealing were investigated by UV-vis spectroscopy. As shown in Figure 7A, all unheated films exhibited a high transmittance in the visible light range. The addition of CNCs and PCEM-g-CNCs in the PVC matrix further enhanced the transmittance in this range, the best transparency being obtained with the PCEM-gCNC fillers. Unlike other materials, the transmittance of the PVC/PCEM-g-CNC film was significantly reduced in the UVA range (320 - 400 nm) and all UVB and UVC (below 320 nm) were blocked. Therefore, the PCEM-g-CNC fillers acted as a UV-blocking agent within the PVC film before annealing, as shown schematized in Figure 8. It could find applications where both high visible light transparency and UV-blocking properties are required, such as film for the glass of car. After annealing, the transmittance of all PVC films decreased, but the PVC/PCEMg-CNC film remained highly transparent in the visible region and displayed UV-blocking


20

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properties. The reduction in transmittance is assigned to the formation of conjugated polyenes after dehydrochlorination, when the temperature approached 70-80 oC.8-9 Since 555 nm is the most sensitive wavelength for humans, it was used as a reference to track the evolution of optical properties of PVC films in the visible range, before and after annealing. The PVC/PCEM-g-CNC film exhibited the lowest transmittance change after annealing, indicating that the PCEM-g-CNC fillers also acted as a thermal stabilizer of PVC. The change in the transmittance decreased after heat annealing for both PVC/CNC and PVC/PCEM-g-CNC film due to dehydrochlorination. The PVC/PCEM-g-CNC exhibited a minimum reduction in the transmittance, indicating the dehydrochlorination was suppressed in PVC/PCEM-g-CNC film due to the enhanced UVresistance.

Figure 8. Schematic representation depicting the UV-filtering and reinforcing effect of the PCEM-g-CNC fillers in PVC, before and after heat annealing. The transmittances given were measured at 320 nm (UV) and 555 nm (Visible). The optical properties of the annealed PVC films before and after 24h UV irradiation at 365 nm were compared. As shown in Figure 7B, all the PVC films displayed a higher transmittance in the UV-visible region after UV irradiation. This increase was assigned to the degradation of


21


Page 22 of 34

PVC due to the bleaching effect associated with the photo-induced oxidation and crosslinking of the conjugated polyenes.49 The evolution of the transmittance at 555 nm of the PVC films before and after UV-irradiation, are summarized in Table 1. The PVC/CNC and PVC/PCEM-g-CNC samples exhibited a lower transmittance change after UV-irradiation compared with pure PVC, indicating that both cellulosic fillers photo-stabilized the PVC to some extent. The best results were obtained with PCEM-g-CNC, most probably because the grafted cinnamate moieties acted as UV-absorbers within the PVC matrix. Moreover, PVC/PCEM-g-CNC films exhibited increased transmittance in the 320-420 nm region after the UV irradiation, which is associated with consumption of the conjugated C=C in PCEM-g-CNC. This was attributed to the crosslinking among the cinnamate groups or between the cinnamate groups and the PVC chains. The photo-induced radicals in the PVC matrices could be trapped and crosslinked with the cinnamate moieties of PCEM-g-CNCs under UV irradiation. Table 1. Evolution of the transmittance at 555 nm of the PVC films, before and after heat annealing or UV-irradiation. Before annealing (%)

After annealing (%)

Transmittance variation during annealing (%)

After UV irradiation (%)

Transmittance variation during UV irradiation (%)

Neat PVC film

72.2

67.2

-7.0

75.8

12.9

PVC/CNC film

73.8

68.8

-6.8

71.5

4.2

PVC/PCEM-gCNC film

79.4

75.7

-4.7

77.2

2.0

Samples

The mechanical performance of the PVC films were evaluated by tensile tests measurements. The tensile stress-strain curves obtained before and after 24h UV irradiation of the films at 365 nm are shown in Figure 9, while the mechanical characteristics – including yield strength (σy),


22

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Young’s modulus (EY), and elongation at break (εb) – are summarized in Table 2. The neat PVC film displays a typical ductile tensile curve with a prominent plastic deformation.4, 11 After the addition of CNCs or PCEM-g-CNCs, both PVC/CNC and PVC/PCEM-g-CNC composite films exhibited higher σy, EY and εb values, indicating that both cellulosic fillers acted as reinforcing agents in the PVC matrix. PVC/PCEM-g-CNC film shows much better εb than PVC/CNC, which is due to the better miscibility between PVC and PCEM-g-CNCs than that between PVC and CNCs.24, 51

Figure 9. Tensile stress-strain curves obtained after analysis of the PVC, PVC/CNC and PVC/PCEM-g-CNC films before and after the UV irradiation. After the UV irradiation, both neat PVC and PVC/CNC films displayed a slight increase in σy (ca. 5-6%) and EY (ca. 2-3%), which is associated with the UV-induced crosslinking among the PVC chains.52 The crosslinking of PCEM-g-CNC may be verified by the FTIR. However, the


23


Page 24 of 34

vibrations of un-crosslinked PCEM and crosslinked PCEM overlapped, therefore, it is difficult to precisely quantify the crosslinking degree by FTIR and DMA. Moreover, compared with neat PVC, the εb of the PVC/CNC was significantly decreased after the UV irradiation, which may be attributed to the formation of micro-crack between CNCs and PVC during the UV irradiation.6 On the other hand, the PVC/PCEM-g-CNC exhibited remarkably increase in σy (ca. 18.6%) and EY (ca. 15.3%), which is mostly due to the UV-induced crosslinking among the cinnamate moieties of PCEM-g-CNCs and the PVC chains. The radicals induced by the UV irradiation in the PVC matrices were trapped by the cinnamate moieties of PCEM-g-CNC, leading to crosslinking between the PCEM-g-CNCs and PVC under UV irradiation. Moreover, the PVC/PCEM-g-CNCs exhibited a relatively high εb after the UV irradiation, suggesting a good miscibility between PCEM-g-CNCs and PVC even after the UV irradiation. Therefore, the mechanical performance of PVC/PCEM-g-CNCs were further improved due to the UV-induced crosslinking. Table 2. Summary of the tensile mechanical characteristics of the PVC films before and after UV-irradiation. Neat PVC Samples

PVC/CNC

PVC/PCEM-g-CNC

Before UV After UV Before UV After UV Before UV After UV irradiation irradiation irradiation irradiation irradiation irradiation

σy (MPa)

5.2±0.1

5.5±0.2

6.0±0.2

6.3±0.1

5.9±0.1

7.0±0.2

EY (MPa)

114.9±1.1 118.0±1.6 139.3± 0.9 147.6± 2.1 135.6± 2.4 156.3±0.1

εb (%)

68.8±1.8

66.5±1.3

90.0±4.2

33.9±3.3

156.8± 3.8 105.9± 2.7

CONCLUSION


24

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Poly(cinnamoyloxy ethyl methacrylate)-grafted CNC nanohybrids (PCEM-g-CNCs) were prepared by SI-ATRP and employed as a UV/thermal stabilizers and reinforcing agents in PVC films. WCA measurements indicated that the grafting of PCEM on CNCs – confirmed by FTIR, TGA and DSC – led to a more hydrophobic surface. The photoactivity of the grafted cinnamate moieties was demonstrated by analyzing PCEM-g-CNCs film before and after irradiation at 365 nm. The [2π+2π] photocyclization of neighboring cinnamate moieties was revealed by the decrease in the intensities of the C=C absorbance (UV spectroscopy) and C=C stretching vibration (FTIR) upon irradiation; the Tg of the grafted polymer also disappeared; confirming the cross-linking of the material. When the PCEM-g-CNC nanohybrids were incorporated in PVC, transparent composite films were obtained. The comparison of optical and mechanical properties of the films before and after UV-irradiation at 365 nm showed that PCEM-g-CNC nanoparticles acted as thermal and UV-stabilizers for PVC. The material also displayed UV-blocking properties, while the high transparency in the visible range was preserved. Also, the mechanical properties of the PVC film were significantly improved after incorporation of the PCEM-g-CNC fillers. A further increase in the mechanical performance of the composite film was observed after UV-irradiation, resulting from the radical coupling between the grafted PCEM chains and the PVC matrix. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Proton and carbon NMR spectra, optical picture of dispersion of CNC and PCEM-CNC in water and DMF, Particle size analysis of dispersions. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected] Funding Sources NSERC Canada, and IDS FunMat program and the European commission. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the University of Waterloo and the IDS FunMat program and the European commission for providing financial support for this project. The funding from NSERC is also acknowledged. REFERENCES 1.

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33

UV 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

Visible

σy 5.2 Mpa, EY 114.9 Mpa, and


UV-absorbing cellulose nanocrystals

εb 68.8% Neat PVC film

47.0%

67.2%

UV irradiation


Visible

UV

Page 34 of 34

σy 7.0 Mpa, EY 156.3 Mpa, and εb 105.9% Reinforced PVC film with UVabsorbing cellulose nanocrystals

0%

77.2%

UV-Absorbing Cellulose Nanocrystals as ... - ACS Publications

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