A Novel Architecture toward Third-Generation Thermoplastic

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A Novel Architecture toward Third-Generation Thermoplastic Elastomers by a Grafting Strategy Feng Jiang,†,§ Zhongkai Wang,†,§ Yali Qiao,‡ Zhigang Wang,*,† and Chuanbing Tang*,‡ †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 China ‡ Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: Thermoplastic elastomers (TPEs) are ever sought using a simple robust synthetic approach. Widely successful first-generation TPEs rely on microphase-separated ABA triblock copolymers (Architecture I). Recent multigraft copolymers represent the second-generation TPEs in which multiple branched rigid segments are dispersed in a rubbery backbone matrix (Architecture II). This paper reports our discovery of the third-generation TPEs that are based on rigid backbone dispersed in a soft grafted matrix. This Architecture III allows the use of random copolymers as side chains to access a wide spectrum of TPEs that cannot be achieved by architecture designs of the first two generations. In this report, random copolymer-grafted cellulose, cellulose-graft-poly(n-butyl acrylate-co-methyl methacrylate) copolymers with only 0.9−3.4 wt % cellulose prepared by activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP), as novel thermoplastic elastomers are investigated.

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copolymer-based TPEs.9−12 However, synthesis of these triblock copolymers requires high chain-extension efficiency among blocks and/or stringent polymerization conditions, thus limiting to just a few block copolymer systems. In Architecture II, a new TPE system was recently developed based on multigraft copolymers with spaced tri-, tetra-, and hexafunctional junction points, in which a rubbery backbone (e.g., polyisoprene) as matrix anchors with multiple glassy domains from branched segments (e.g., polystyrene) at each junction point. These discrete hard domains are indeed physical cross-linking, similar to conventional microphase separated triblock copolymers.13−17 However, the synthesis of these multigraft copolymers is challenging, requiring both stringent anionic polymerization conditions and extensive fractionation of graft homo- and copolymers. Inspired by the second TPE system, we hypothesize that properties of TPEs can be achieved if we can reverse the above Architecture II design, that is, to use a rigid backbone as

INTRODUCTION In this paper, we conceptualize an unprecedented and robust strategy to prepare the first third-generation thermoplastic elastomers (TPEs). We design a novel architecture based on random copolymer matrix grafting from rigid minority substrates. Such architecture design circumvents the conventionally challenging synthesis of TPEs and has the greater potential in putting into practice for large-scale production. Thermoplastic elastomers (TPEs) that combine elasticity with thermoplastic properties are widely used for a pyramid of important applications.1−4 There are two major architecture designs as illustrated in Scheme 1a,b. In Architecture I, it is known that microphase-separated ABA triblock copolymers, which consist of hard domains dispersed in a soft matrix, can be used to prepare TPEs. Polystyrene-b-polybutadiene-b-polystyrene (SBS) and polystyrene-b-polyisoprene-b-polystyrene (SIS) triblock copolymers prepared by living anionic polymerization are widely used engineering TPEs.5−8 Poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly(methyl methacrylate) (PMMA-b-PBA-b-PMMA) copolymers with linear or star chain architectures synthesized by atom transfer radical polymerization (ATRP) are another class of ABA triblock © XXXX American Chemical Society

Received: April 10, 2013 Revised: May 25, 2013

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Scheme 1. Illustration of (a) Architecture I, (b) Architecture II, and (c) Architecture III for Making Thermoplastic Elastomers

Scheme 2. Synthesis of Graft Copolymers Cellulose-g-P(BA-co-MMA) as Novel Thermoplastic Elastomers

chains. The cellulose rigid chain backbone is able to reinforce the soft P(BA-co-MMA) copolymer matrix. The versatility of random copolymers including chemistry and compositions as well as synthetic simplicity is manifested as a few advantages. This approach can be generalized to many other rigid substrates coupled with soft side chains, thus having a significant impact in developing novel thermoplastic elastomers.

minority physical cross-linker and grafted chains from this backbone as a soft matrix, for which we call Architecture III (see Scheme 1c). In principle, this rigid backbone can be single rigid polymer chains or stiff nanoobjects, while the grafted chains can be homopolymers or copolymers with relatively low glass transition temperature. If this architecture design indeed leads to the formation of TPEs, it would provide a powerful platform for installation of a variety of polymeric compositions using simple grafting chemistry. Note that Bazan and coworkers have recently prepared elastomers by grafting n-butyl acrylate from polyethylene (PE) macroinitiator copolymers, and these semicrystalline PE containing elastomers possess good mechanical properties.18 Herein we report the first efforts to prepare random copolymer-based TPEs, in which soft random copolymers are grafted from rigid cellulose chains. By manipulating the compositions of random copolymers, we can access a broad spectrum of TPEs that are almost impossible to be achieved by the first two generations of architecture designs. Specifically, we use activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP) “grafting-from” approach to prepare graft copolymers, cellulose-graf t-poly(nbutyl acrylate-co-methyl methacrylate) (cellulose-g-P(BA-coMMA)), as shown in Scheme 2. The key design of our unique approach is the use of a rigid cellulose as the backbone and rubbery P(BA-co-MMA) random copolymers as grafted side

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RESULTS

Synthesis and Thermal Properties of Graft Copolymers Cellulose-graf t-poly(n-butyl acrylate-co-methyl methacrylate). We carried out the grafting-from polymerization of different monomers from cellulose backbone using activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP).19 We first used ARGET ATRP to graft PMMA and PBA homopolymers from cellulose. The polymerization of MMA or BA on cellulose backbone using CuIIBr2 as catalysts, with Cu0 as a reducing agent, was performed with a molar ratio of Cell-BiB/M/Cu0/ CuIIBr2/bpy = 1/1000/−/0.05/0.3 (M: MMA or BA). The cellulose macroinitiator (Cell-BiB) was prepared according to the literature,20 and CuIIBr2 with a concentration of 50 ppm was chosen to conduct the polymerization.21,22 The structure of Cell-BiB was characterized by FT-IR (Figure S1) and NMR (Figure S2). Figure 1 shows linear kinetic plots for both B

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in Table 1. The cellulose contents in the samples were controlled by tuning the total conversions of BA and MMA monomers. For comparison, the synthesized cellulose-g-PMMA (BA0-Cell1.3) and cellulose-g-PBA (BA1000-Cell1.0) are also listed in Table 1. The details of their linear copolymer counterparts are summarized in Table S2. Thermal stability is one of the most important limiting factors for the application of polymeric materials. Thermogravimetric analysis (TGA) was applied to study the thermal stability and decomposition behaviors of cellulose, macroinitiator Cell-BiB, cellulose-g-PBA, cellulose-g-PMMA, and cellulose-g-P(BA-co-MMA) copolymers. Figure 2 shows the Figure 1. Semilogarithmic dependences of monomer consumption versus time for polymerization of MMA or BA and the copolymerization of BA and MMA initiated by Cell-BiB in DMF at 70 °C. Molar ratio of Cell-BiB/M/Cu0/CuIIBr2/bpy = 1/1000/ −/0.05/0.3. For the copolymerization, this [M] is a sum of BA and MMA, and the molar ratio of BA/M = 600/400.

monomers, indicating the controlled and living character. Both graft polymerizations on macroinitiator Cell-BiB exhibit induction periods. The reason for the presence of an induction period is that the initiating groups are less accessible after anchored onto cellulose chains. A similar phenomenon was also observed by another group.23 Meanwhile, the reactivity ratios of MMA and BA were investigated (Figure S3 and Table S1). The success of graft copolymerization of BA and MMA on macroinitiator Cell-BiB is also confirmed by FT-IR (Figure S5) and NMR (Figure S6), as detailed in the Supporting Information. Figure 1 also shows a typical semilogarithmic dependence of monomer consumption versus time for copolymerization of BA and MMA initiated by Cell-BiB with a molar ratio of Cell-BiB/ BA/MMA/Cu0/CuIIBr2/bpy = 1/600/400/−/0.05/0.3. It can be seen that during copolymerization ln([M]0/[M]) increases linearly with time, indicating that the BA and MMA copolymerization also obeys the first-order dependence on the total monomer concentration. An induction period was also observed. Figure S4 further shows individual monomer conversions with time for copolymerization of BA and MMA. Given the apparent success of controlled copolymerization of BA and MMA, a series of cellulose-g-P(BA-co-MMA) copolymers were synthesized according to the formulas listed

Figure 2. TGA curves for cellulose, Cell-BiB, cellulose-g-PBA (sample code BA1000-Cell1.0 in Table 1), cellulose-g-PMMA (sample code BA0-Cell1.3 in Table 1), and cellulose-g-P(BA-co-MMA) (sample code BA600-Cell1.0 in Table 1) at a heating rate of 10 °C/min under a nitrogen atmosphere.

TGA curves for cellulose, Cell-BiB, cellulose-g-PBA, cellulose-gPMMA, and cellulose-g-P(BA-co-MMA). The maximum degradation of cellulose occurs at 367 °C. Thermal stability of macroinitiator Cell-BiB decreases significantly as confirmed from its maximum degradation temperature at 267 °C if compared with cellulose, which results from the introduction of relatively unstable BiB groups. The maximum decomposition of cellulose-g-PMMA copolymer occurs at the temperature of 367 °C, which is similar to that of cellulose. A more significant finding is that the synthesized cellulose-g-PBA and cellulose-gP(BA-co-MMA) samples show much higher decomposition temperatures (402 and 396 °C, respectively) than those of

Table 1. Synthetic Formulas and Characteristics of the Prepared Cellulose-g-PBA, Cellulose-g-PMMA, and Cellulose-g-P(BA-coMMA) Copolymers sample codea

molar ratiob

cellulose content (wt %)

PMMA contentc (wt %)

total conv (%)

Mntheor (grafts)d (g/mol)

Tge (°C)

BA1000-Cell1.0 BA700-Cell1.1 BA650-Cell0.9 BA650-Cell1.2 BA650-Cell2.5 BA650-Cell3.4 BA600-Cell1.0 BA550-Cell1.1 BA500-Cell1.0 BA0-Cell1.3

1/1000/0/−/0.05/0.3 1/700/300/−/0.05/0.3 1/650/350/−/0.05/0.3 1/650/350/−/0.05/0.3 1/650/350/−/0.05/0.3 1/650/350/−/0.05/0.3 1/600/400/−/0.05/0.3 1/550/450/−/0.05/0.3 1/500/500/−/0.05/0.3 1/0/1000/−/0.05/0.3

1.0 1.1 0.9 1.2 2.5 3.4 1.0 1.1 1.0 1.3

0 39.6 45.8 46.1 47.3 47.5 48.3 54.8 59.7 98.7

24.8 22.5 27.9 20.9 9.9 7.2 25.4 23.4 26.0 19.2

31 800 26 000 32 200 24 300 11 700 8600 30 400 27 500 30 300 19 200

−46 2 6 11 14 16 21 32 40 128

Sample codes are defined as follows: the numbers behind “BA” stand for BA molar number, and the numbers behind “Cell” stand for cellulose content (percentage). bMolar ratio: Cell-BiB/BA/MMA/Cu0/CuBr2/bpy. cMeasured by 1H NMR spectroscopy. dTheoretical Mn of grafts estimated from monomer conversion and DS of macroinitiator Cell-BiB. eMeasured by DSC. a

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Mechanical properties of linear P(BA-co-MMA) (see Figures S9 and S10) and cellulose-g-P(BA-co-MMA) copolymers (Figure 4) were measured by two types of tests: monotonic and step cyclic tensile tests. The monotonic stress−strain curves for cellulose-g-P(BA-co-MMA) copolymers in Figure 4a clearly indicate the elastomeric behavior of these samples with elongations at break all above 500%. Table S3 and Table 2 summarize the mechanical properties of the linear P(BA-coMMA) and cellulose graft copolymers, respectively. Note that for BA500-Cell1.0 sample with the initial feed molar ratio of Cell-BiB/BA/MMA of 1/500/500, the ultimate tensile strength and elongation at break are 20.6 MPa and 10%, respectively (stress−strain curve not shown), which indicates the absence of elasticity of the sample, probably due to too high MMA content in the side chains. The MMA content has an important effect on the tensile properties of cellulose-g-P(BA-co-MMA) copolymers. Figure 4a and data in Table 2 clearly indicate that the elongation at break decreases with the decrease of BA content, while the ultimate tensile strength accordingly increases with the increase of MMA content. This result is in accordance with the change of Tg of these cellulose graft copolymers as shown in Figure 3b. Thus, the mechanical properties of these materials can be easily tailored by adjusting compositions of random copolymers. The mechanical properties of these cellulose graft copolymers are also related to the cellulose content as shown in Figure 4b. The ultimate tensile strength increases with the increase of cellulose content, when the molecular mass and compositions of random copolymers are maintained constant. However, the elongation at break decreases with increasing cellulose content. Similarly to the discussion on the change of Tg with cellulose content for the cellulose graft copolymers shown in Figure S7, the above stress−strain curve upturn shown in Figure 4b is caused mainly by the increase of cellulose content in the samples. It should be noted that the sample BA1000-Cell1.0 (without MMA unit) was viscous, and the mechanical property test could not be performed on this sample, which indicates that the effect of MMA component is crucial for the desired mechanical properties. Another crosshead speed of 10 mm/min was applied to examine its effect on the monotonic tensile mechanical properties (Figure S12) for cellulose-g-P(BA-coMMA) copolymers. The results indicate that the strain hardening occurs more obviously at higher crosshead speed because the stress develops more rapidly with the higher crosshead speed. Figure 4c shows typical nominal stress−strain curve for sample BA550-Cell1.1 during cyclic tensile deformation with maximum strains of 50, 100, 150, 200, 250, 300, 350, and 400%. Obviously, in a given cycle the first loading curve and subsequent loading curves are quite different, and the residual strain at zero stress is progressively larger caused by plastic deformation. This phenomenon is called the Mullins effect.24 Interestingly, these cellulose graft copolymers exhibit improved elastic recovery (ER) reaching plateau regions at higher strains, as can be seen from Figure 4d. The strain for approaching the plateau regions increases with the increase of MMA content. Nevertheless, the ER values of the cellulose graft copolymers are all above 90% at a strain of 250%. As shown in Figure 4e, the cellulose-g-P(BA-co-MMA) copolymer sample is transparent and clear; it can be easily bended and the deformation can be recovered to original shape and the transparency maintains, indicating a sufficient elastic recovery property. The changes of elastic recovery with maximum strain in each cycle

cellulose or Cell-BiB, which implies that the cellulose backbones have been successfully wrapped by PMMA, PBA, or P(BA-co-MMA) grafting chains. Glass transition temperature (Tg) is an important physical parameter that determines the application properties of polymeric materials, especially for the elastomers. Figure 3

Figure 3. DSC heat flow curves for (a) linear copolymers and (b) cellulose-g-P(BA-co-MMA) copolymers prepared with varied monomer feed compositions and with similar cellulose contents. The heating rate was 10 °C/min.

shows DSC heat flow curves for these linear copolymers (Table S2) and cellulose graft copolymers (Table 1). The dashed lines marked on the heat flow curves indicate Tg. It can be obviously seen that all Tg values of linear copolymers and cellulose graft copolymers shift toward higher values with decreasing feed molar ratio of BA to MMA (Figure S8). On the other hand, at the same feed molar ratio, the Tg of cellulose-g-P(BA-co-MMA) copolymer is slightly higher than that of linear P(BA-co-MMA) copolymer. It is well-known that Tg is related to polymer chain segmental mobility. For cellulose-g-P(BA-co-MMA) copolymer cellulose as the rigid backbones reduces the flexibility of the graft side chains, leading to slightly higher Tg than the linear counterpart. Figure S7 shows that Tg increases from 6 to 16 °C as the cellulose content increases. As a matter of fact, for these four samples shown in Figure S7, the MMA content (more than 45 wt %) also increases with increasing cellulose content. The Tg variations with MMA content shown in Figure 3 indicate that the Tg increase is actually mainly caused by increasing MMA content (Table 1). The increasing cellulose content might affect the increase of Tg somewhat; however, the effect is minor because cellulose contents (0.9−3.4 wt %) are much lower than those of MMA contents (more than 45 wt %) in cellulose-g-P(BA-co-MMA) copolymers. Mechanical Properties of Graft Copolymers Cellulosegraf t-poly(n-butyl acrylate-co-methyl methacrylate). D

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Figure 4. Monotonic stress−strain curves for cellulose-g-P(BA-co-MMA) copolymers prepared with (a) varied monomer feed compositions and similar cellulose contents and (b) the same monomer feed composition and varied cellulose contents (note that the samples for the monotonic tests break at the end of the tests). (c) Nominal stress−strain curve for sample BA550-Cell1.1 during step cyclic tensile deformation with maximum strains of 50, 100, 150, 200, 250, 300, 350, and 400%. (d) Changes of elastic recovery with maximum strain in each cyclic step for these cellulose-gP(BA-co-MMA) copolymers. The short dashed line indicates the elastic recovery value of 90%. (e) Photos of transparent BA550-Cell1.1 tensile test sample in (A), the bended shape by fingers, and in (B) the recovered shape after bending in (A).

ultimate tensile strength and maximum elongation at break are 6.4 MPa and 430%, respectively.11 Compared to these elastomers, the cellulose-g-P(BA-co-MMA) copolymers possess better mechanical properties, with higher ultimate tensile strength and higher elongation at break. Morphology of Graft Copolymers Cellulose-graf tpoly(n-butyl acrylate-co-methyl methacrylate). Morphologies of cellulose-g-P(BA-co-MMA) copolymers were investigated by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM), as shown in Figure 5a, Figure 5b, and Figure S14. Most graft copolymers show the presence of a strong correlation peak

step for linear P(BA-co-MMA) copolymers are shown in Figure S11. Mechanical properties including the ultimate tensile strengths and maximum elongations at break for P(BA-bMMA) star block copolymers with similar compositions but varying numbers of arms have been investigated by Matyjaszewski and co-workers.11 In their work the ultimate tensile strength and elastic modulus increase with the increase of number of arms. For linear PMMA-b-PBA-b-PMMA triblock copolymers the ultimate tensile strength and maximum elongation at break are only 4.2 MPa and 520%, respectively,12 and for a 10-arm P(BA-b-MMA) star block copolymer the E

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DISCUSSION Typically, a well-ordered microphase-separated morphology is required to obtain acceptable TPEs for the triblock copolymer system. However, in the case of densely grafted copolymers such as the studied cellulose-g-P(BA-co-MMA) copolymers, a large number of branched points are necessary rather than a well-ordered morphology.25 For cellulose-g-P(BA-co-MMA) copolymers, the number of branch points is large enough to form sufficient physical multi-cross-links in the bulk materials. The multi-cross-links are formed through hydrogen bonding between OH groups of cellulose and carbonyl group of acrylic side chains among cellulose-g-P(BA-co-MMA) copolymers. Both MMA and cellulose contents in cellulose-g-P(BA-coMMA) copolymers possesses important effects on the observed mechanical properties. The increase of tensile strength and decrease of elongation at break clearly depend on Tg. Moreover, the viscoelastic properties and the Young’s modulus (influenced by Tg) are mainly controlled by the proportion of MMA in the graft chains while the large strain properties and the strain hardening are controlled by the cellulose rigid segments. Both MMA and cellulose act as physical cross-links, which are the glassy domains of MMA and aggregated domains of cellulose, respectively. During the tensile deformation, the initial tensile strength is related to the proportion of MMA, as the Young’s modulus increases with the increase of MMA content. If MMA content is high, the ability of the soft side chains to dissipate energy is low, which explains the low elongation at break. Moreover, the lower elastic recovery of the copolymers with a higher MMA content is explained by the breakage of the structure made by the glassy MMA domains at lower strain when the load is mainly sustained by the MMA segments. At higher strain, the load is transmitted to the cellulose backbones and the strain hardening is the result of the final extensibility of the soft segments between two cellulose strands. Because the mechanical properties of these cellulose-based copolymers are dominated by the proportions of MMA and cellulose rigid chains, it is necessary to clarify what are the roles that MMA and cellulose play for these cellulose-g-P(BA-coMMA) copolymer elastomers at different stages during tensile deformation. The detailed explanation on the mechanism involved during stretching, starting with the small strain behavior until the strain hardening and final failure, could be explained as follows. Before the stretching, cellulose rigid chains and P(BA-co-MMA) copolymer side chains are dispersed

Table 2. Mechanical Properties of Cellulose-g-P(BA-coPMMA) Copolymers sample code BA700-Cell1.1 BA650-Cell0.9 BA650-Cell1.2 BA650-Cell2.5 BA650-Cell3.4 BA600-Cell1.0 BA550-Cell1.1 BA500-Cell1.0

ERa at 200% strain

ER at 350% strain

av % ERb

96 95 97 97 96 92 87

96 96 99 99 99 97 96

93 95 95 94 92 86 81

stress at break (MPa)c

strain at break (%)c

2.3 2.5 3.3 4.6 6.6 7.9 11.1 20.6

1000 890 811 710 660 630 550 10

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Calculated from ER = 100%(εmax − ε(0, εmax))/εmax, where εmax is the maximum strain and ε(0,εmax) is the strain in the cycle at zero stress after the maximum strain εmax. bDetermined from step cyclic stress− strain curves. cDetermined from monotonic stress−strain curves. a

with d-spacing dependent on copolymer compositions (Figure S13). Because there are no additional higher order peaks, it can be concluded that these copolymers do not form well-ordered structures, but rather disordered domains, except the sample BA650-Cell3.4 which shows a second-order peak. The TEM image of sample BA650-Cell3.4 shows a weakly segregated morphology. Given that most graft copolymers contain only 0.9−3.4 wt % rigid cellulose, such disordered (but not homogeneous) morphology is not surprising. AFM phase images show homogeneous surface morphology for all studied grafted copolymers. Given that rigid cellulose domains, if phase-separated, are embedded in the rubbery matrix, only a homogeneous matrix can be observed on the surface. These results suggest that rigid cellulose component assists the microphase separation for cellulose-g-P(BA-co-MMA) copolymers as illustrated in Figure 5c. Note that in Figure 5a the SAXS profiles indicate that the d-spacing increases with increasing cellulose content. Cellulose component (rigid chain segments) as a polar component, immiscible with MMA and BA components, is considered to assist the microphase separation for cellulose-g-P(BA-co-MMA) copolymers. Therefore, the degree of microphase separation increases with the increase of cellulose content, resulting in the increase of d-spacing.

Figure 5. (a) SAXS profiles (I(q)q2 vs q curves) for cellulose-g-P(BA-co-MMA) copolymers prepared with the same monomer feed composition and varied cellulose contents. (b) TEM image of sample BA650-Cell3.4. (c) Illustration of the microphase-separated morphology for cellulose-g-P(BAco-MMA) copolymers. F

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and cellulose-g-P(BA-co-MMA) were recorded using a Bruker Tensor 27 FTIR spectrophotometer by the KBr sample holder method. Nuclear Magnetic Resonance Spectroscopy (NMR). 1H and 13 C NMR spectra of Cell-BiB and 1H NMR spectra of cellulose-gPMMA, cellulose-g-PBA, and cellulose-g-P(BA-co-MMA) in solutions of DMSO-d6 or CDCl3 were recorded on a Bruker AVANCE 400 spectrometer. The spectra were internally referenced to tetramethylsilane (TMS) standard. Gel Permeation Chromatography (GPC). Molecular masses and molecular mass distributions of linear P(BA-co-MMA) copolymers and the cleaved P(BA-co-MMA) grafts from cellulose-g-P(BA-co-MMA) copolymers were determined on a Waters 150C GPC apparatus equipped with three columns (500, 103, and 104 Å) and RI 2414 detector, using monodispersed polystyrenes as the calibration standards. THF was used as the eluent at a flow rate of 1 mL/min. Thermogravimetric Analysis (TGA). TGA measurements were carried out on a TA Q5000IR thermogravimetric analyzer (TA Instruments). Samples of about 5 mg were heated from room temperature to 100 °C at a heating rate of 10 °C/min under a nitrogen atmosphere, held at this temperature for 30 min, and then cooled to 40 °C at a rate of 10 °C/min. Then the samples were heated from 40 to 600 °C at a heating rate of 10 °C/min. Differential Scanning Calorimetry (DSC). DSC measurements were conducted using a TA Q2000 DSC (TA Instruments) under a nitrogen atmosphere. Approximately 5 mg of each sample was used in the measurement. First, the sample was heated from room temperature to 200 °C, held at this temperature for 5 min to remove thermal history, and then cooled to −80 °C at a rate of 10 °C/min. Then the sample was heated from −80 to 200 °C at a heating rate of 10 °C/min. To determine the glass transition temperatures (Tg), the DSC data were analyzed using the Universal Analysis 2000 Software (TA Instruments). Small-Angle X-ray Scattering (SAXS). The SAXS measurements on the samples were carried out on a Bruker Nanostar SAXS instrument at room temperature. The SAXS intensity profiles were output as the plots of the scattering intensity (I) versus the scattering vector (q). The SAXS profiles were corrected for the background scattering. Transmission Electron Microscopy (TEM) Imaging. The linear or cellulose-based copolymer was added into the THF to a concentration of 10 mg/mL, and the solution was left to stand for 24 h prior to use. Thin films were generated on silicon wafer by spincoating at 4000 rpm from the solution. To get the film from the silicon wafer, the silicon wafer was placed on the water surface including 5% hydrofluoric acid. After a while, the silicon layer on the silicon wafer was corroded, and the films floated on the water surface were directly collected with carbon-coated copper grids. The films were stained with RuO4 at room temperature for 4 h. TEM measurements were performed on a JEOL JEM-1011 transmission electron microscope operated at an acceleration voltage of 100 kV. Atomic Force Microscopy (AFM) Imaging. Tapping mode AFM experiments were carried out using a Multimode Nanoscope V system (Veeco (now Bruker), Santa Barbara, CA). The measurements were performed using commercial Si cantilevers with a nominal spring constant and resonance frequency at 20−80 N/m and 230−410 kHz, respectively (TESP, Bruker AFM Probes, Santa Barbara, CA). The height and phase images were acquired simultaneously at the set-point ratio A/A0 = 0.9−0.95, where A and A0 refer to the “tapping” and “free” cantilever amplitudes, respectively. Thin films were prepared by spin-coating in DMF (15 mg/mL) and thermally annealed at 150 °C under vacuum for about 24 h. Mechanical Tensile Property Tests. Monotonic tensile deformation of the dumbbell film samples was performed on an Instron Model 1185 universal testing machine. The film samples with thicknesses of 0.2−0.6 mm were obtained by casting DMF solutions of 10 wt % cellulose-g-P(BA-co-MMA) copolymers on polytetrafluoroethylene membrane followed by removal of the solvent at 40 °C for 48 h. The films were further dried at 60 °C under vacuum until constant masses. Dumbbell specimens with width of 4 mm and length of 15 mm were cut from the cast films, which were subjected to the

relatively homogeneously in the samples. When the stretching starts, at the initial stage (small strain), the deformation is attributed to the random copolymer chain segments; thus, the stress needed is low. When the strain becomes higher, the physical cross-links by MMA domains (high Tg) and cellulose rigid chains (through hydrogen bonding between OH groups of cellulose and carbonyl group of acrylic side chains) begin to play roles, and the stress becomes higher. With further tensile deformation, the loading is transmitted to the cellulose rigid chains, and the cellulose backbones start to be stretched, leading to the highly needed stresses (strain hardening). In addition, during this stage, the strain hardening also result from the final extensibility of the stretched soft segments between cellulose strands because the MMA domains (from different graft copolymers) that make the connection between the rigid cellulose strands can be stretched as well. Further tensile deformation leads to the failure of the elastomers. On the other hand, the high aspect ratio cellulose rigid backbone chains can indeed act as reinforcing fibers for cellulose-g-P(BA-co-MMA) copolymers. Such a kind of reinforcement depends on the dispersion of cellulose in the polymer matrix. Cellulose as a polar rigid chain biopolymer is predicted to be immiscible with linear P(BA-co-MMA) copolymers. Therefore, the obviously enhanced mechanical properties for cellulose-g-P(BA-co-MMA) copolymer elastomers are not possible for a simple mixture of cellulose and linear P(BA-co-MMA) copolymers, especially at much low cellulose contents of 1−3 wt %. Cellulose-g-PMMA copolymers were used to reinforce PMMA with much less effects than that in our case.26

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CONCLUSIONS In summary, we report the discovery of the third-generation thermoplastic elastomers that are based on novel architecture design, in which cellulose acts as minority rigid domains (only 0.9−3.4 wt %) and grafted P(BA-co-MMA) random copolymer side chains serve as the rubbery matrix. These materials exhibit desired mechanical properties of TPEs. The simple synthetic approach coupled with easily accessible architectures could be further applied for copolymerization of different types of monomers grafted from various rigid substrates to obtain thermoplastic elastomers with tailored performances and potential scale-up.

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EXPERIMENTAL SECTION

Materials. The cellulose (wood pulp) was supplied by Hubei Chemical Fiber Co., Ltd. (Hubei, China). The viscosity-averaged degree of polymerization (DP) of the cellulose in cupriethylenediamine hydroxide solution was about 650, measured by using an Ubbelohde viscometer. The ionic liquid 1-allyl-3-methylimidazolium chloride (AMIMCl) was supplied by Lanzhou Greenchem ILS, LICP, CAS (Lanzhou, China). Both cellulose and AMIMCl were dried at 70 °C in vacuum for 2 days prior to use. Methyl methacrylate (MMA), nbutyl acrylate (BA), N,N-dimethylformamide (DMF), CuIIBr2, copper wire (diameter 0.5 mm), and 2,2′-bipyridyl (bpy) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). MMA and BA were passed through the column of alumina and distilled under vacuum, respectively. DMF was dried and distilled under reduced pressure. 2-Bromoisobutyryl bromide (98%, Aldrich), ethyl 2-bromoisobutyrate (EBiB, 98%, TCI), and other reagents were all analytical grade and used as received. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra of cellulose, Cell-BiB, cellulose-g-PMMA, cellulose-g-PBA, G

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monotonic tensile tests at room temperature with the crosshead speed of 25 mm/min. Stress versus strain curves were recorded. Data analyses were based on five measurements on each sample performed at the same conditions. Cyclic tensile processing of the samples was performed stepwise to progressively higher tensile strains at room temperature using a Linkam TST350 tensile stage with a 200 N load cell. Rectangular specimens with width of 4 mm and length of 15 mm were cut from the cast films. In each step the specimen was extended up to strains of 50, 100, 150, 200, 250, 300, 350, and 400% at a constant strain rate of 0.02 s−1. Once the specimen reached the appropriate strain, the crosshead direction was reversed and the strain was decreased at the same strain rate until zero stress was achieved. After that, the specimen was extended again at the same constant strain rate until it reached the next targeted strain. The cyclic tensile processing was continued until the final strain of 400% was reached. The elastic recovery (ER) values of these thermoplastic elastomers were obtained from these step cyclic tests. Synthesis of Macroinitiator Cellulose 2-Bromoisobutyrylate (Cell-BiB). Cell-BiB was synthesized by homogeneous acylation of cellulose with 2-bromoisobutyryl bromide (Scheme 1) according to the procedure reported.20 Cellulose (1.0 g, 6.2 mmol) was dissolved in AMIMCl (20.0 g) with stirring at 80 °C for 1 h in a 100 mL roundbottom flask. DMF (10.0 mL) was added as cosolvent to obtain a clear yellow solution, which was then cooled to room temperature. The flask was put in an ice/water bath, and 2-bromisobutyryl bromide (7.1 g, 31.0 mmol) was added dropwise into the solution. After 36 h, the solution was poured into excessive deionized water and the precipitated white floccules were obtained. After washed thoroughly with deionized water, the white floccules (the macroinitiator Cell-BiB) was filtered and dried under vacuum at 50 °C for 12 h. The degree of substitution (DS) of Cell-BiB was calculated from the 1H NMR spectrum of Cell-BiB as the ratio of the integral of the methyl group to the integral of protons of glucose. Synthesis of Cellulose-g-P(BA-co-MMA). For a typical polymerization, Cell-BiB (30 mg, 0.1 mmol of Br), MMA (4.00 g, 40.0 mmol), BA (7.68 g, 60.0 mmol), CuIIBr2 (1.1 mg, 0.005 mmol), bpy (4.7 mg, 0.03 mmol), and DMF (12 mL) were introduced to a flask equipped with a magnetic stirring bar. A copper wire of 20 cm was added after the Cell-BiB was completely dissolved. Subsequently, the solution was degassed by three freeze−pump−thaw cycles and sealed. Thereafter, the flask was immersed into an oil bath set at 70 °C. After 7 h, the polymerization was stopped by opening the flask and exposing the reactive mixture to air. THF was then added to the mixture, and the product was precipitated into a large amount of cold methanol. After washed for several times, the resulting product was collected and dried under vacuum at 40 °C. Polymerizations of BA and MMA with different molar ratios on Cell-BiB were performed in a similar way. Linear copolymers P(BA-co-MMA) were prepared via ARGET ATRP using ethyl 2-bromoisobutyrate (EBiB) as initiator. In a typical polymerization, EBiB (19.5 mg, 0.1 mmol), MMA (4.00 g, 40.0 mmol), BA (7.68 g, 60.0 mmol), CuIIBr2 (1.1 mg, 0.005 mmol), bpy (4.7 mg, 0.03 mmol), DMF (12 mL), and a copper wire of 20 cm were introduced to a flask equipped with a magnetic stirring bar. Then the solution was degassed by three freeze−pump−thaw cycles and sealed. Thereafter, the flask was immersed into an oil bath set at 70 °C. During preparation of linear copolymers THF was added after the polymerization was stopped, and the solution was purified by passing through neutral alumina column. The final products were precipitated into cold methanol and dried under vacuum at 40 °C until constant masses.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.W.); [email protected] (C.T.). Author Contributions §

F.J. and Z.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z. G. Wang acknowledges the financial support from the National Basic Research Program of China with Grant 2012CB025901 and NSFC with Grant 51073145. The project was also funded by State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Dong Hua University, China. C. B. Tang acknowledges the financial support from U.S. National Science Foundation with Career Award DMR1252611 and U.S. Department of Agriculture with NIFA Award 2011-51160-31205.

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dx.doi.org/10.1021/ma4007472 | Macromolecules XXXX, XXX, XXX−XXX