Journal of Colloid and Interface Science 453 (2015) 226–236

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Synthesis and characterization of brush-like multigraft copolymers PnBA-g-PMMA by a combination of emulsion AGET ATRP and emulsion polymerization Hui Li a, Wenwen Wang b, Chunmei Li a, Jiaojun Tan a, Dezhong Yin a, Hepeng Zhang a, Baoliang Zhang a, Changjie Yin a, Qiuyu Zhang a,⇑ a b

Key Laboratory of Applied Physics and Chemistry in Space of Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an 710072, China School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 10 April 2015 Accepted 23 April 2015 Available online 7 May 2015 Keywords: Multigraft copolymer Brushlike Emulsion polymerization AGET ATRP Thermoplastic elastomer

a b s t r a c t In this paper, poly(n-butyl acrylate)-g-poly(methyl methacrylate) multigraft copolymers were synthesized by macromonomer technique and miniemulsion copolymerization. The PMMA macromonomers were obtained by an activator generated by electron transfer atom transfer radical polymerization (AGET ATRP) in emulsion system and subsequent allylation. Then the copolymerization of different macromonomers with nBA was carried out in miniemulsion system, obtaining multigraft copolymers with high molecular weight. The latex particles and distribution of emulsion AGET ATRP and miniemulsion copolymerization were characterized using laser light scattering. The molecular weight and polydispersity indices of macromonomers and multigraft copolymers were analyzed by gel permeation chromatography, and the number-average molecular weight range is 187,600–554,800 g/mol for PnBA-g-PMMA copolymers. In addition, the structural characteristics of macromonomer and brush-like copolymers were determined by infrared spectra and 1H nuclear magnetic resonance spectroscopy. The thermal performance of brush-like copolymers were characterized by differential scanning calorimetry and thermogravimetric analysis. Atomic force microscopy results showed that the degree of microphase separation was varying with increasing PMMA content in PnBA-g-PMMA. The dynamic rheometer analysis revealed that multigraft copolymer with PMMA content of 31.4% exhibited good

⇑ Corresponding author. E-mail address: [email protected] (Q. Zhang). http://dx.doi.org/10.1016/j.jcis.2015.04.051 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

227

elastomeric properties to function as a TPE. These multigraft copolymers show a promising low cost and environmental friendly thermoplastic elastomer. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction In recent years, much attention has been focused on the synthesis of different topological polymers such as star, gradient, hyper-branched, dendritic, cyclic and graft copolymers due to facilitating to study the materials with new and different properties from those of linear polymers [1–6]. Especially, since the grafting density, the length of side chain and backbone, the number of junction points can be tuned by molecular design, graft copolymers have potential applications as compatibilizers, thermoplastic elastomers, photonic materials, renewable materials and impactresistant plastics [7–9]. Thermoplastic elastomers (TPEs) that have the unique property of being spontaneously and thermoreversibly crossed-linked materials are widely used for a pyramid of important applications. It was demonstrated that thermoplastic elastomers based on graft copolymers may exhibit improved properties in comparison with the conventional linear ABA triblock copolymers, such as larger elongation at break, lower residual strain, and high modulus [10– 12]. Kramer et al. [7] prepared well-defined elastomeric polyethylene (PE) macroinitiators and subsequently generated a series of poly (ethylene-graft-(n-butyl acrylate)) (PE-g-PnBA) materials by grafting oligomers of n-butyl acrylate (nBA) via ATRP and the resulting highest performing acrylate containing PE-g-PnBA elastomer exhibited an elongation at break of 780% strain and a recovery of 83% at 400% strain. The third-generation thermoplastic elastomers based on multigraft copolymers with spaced tri-, tetra-, and hexafunctional functionality were recently developed quickly, 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 [11]. These multigraft copolymers, having trifunctional, tetrafunctional, and hexafunctional branch points are called as ‘‘comb’’, ‘‘centipede’’, and ‘‘barbwire’’ architectures, respectively. Hadjichristidis and Mays et al. synthesized PI-g-PS multigraft copolymers with regularly spaced branch points by employing high vacuum living anionic polymerization and chlorosilane linking chemistry. Mechanical properties studies revealed that these multigraft copolymers are a new class of thermoplastic elastomers (TPEs) endowed with superior elongation at break and low residual strains when compared to conventional TPEs [13–18]. However, the synthesis of these multigraft copolymers require both stringent high vacuum anionic

polymerization and extensive fractionating of graft copolymers and homopolymers. According to the features of synthetic approaches, grafting-from, grafting-onto and grafting-through techniques could be employed to synthesize well-defined graft copolymers. Grafting-through strategy is a useful method for synthesizing graft copolymers by utilizing macromonomer. Macromonomer is an oligomer bearing polymerizable terminal functional groups that can copolymerize with monomers using free radical polymerization methods to form graft copolymers. Since Schulz and Milkovich [19] proposed the method of preparing well-defined graft copolymers with macromonomer technique, many different topological polymers has been obtained extensively by this useful method. Xie and Zhou [20] synthesized PMMA macromonomer by radical polymerization with thioglycolic acid as a chain transfer agent, and followed by reaction with glycidyl methacrylate. And the graft copolymers as thermoplastic elastomer with good elastic properties were obtained by the copolymerization of PMMA macromonomers with n-butyl acrylate in benzene. Schulze et al. [21] reported that PS macromonomers were prepared by ATRP in toluene. Moreover, a variety of macromonomers with topologies obtained by high vacuum anionic polymerization techniques have been studied by many researchers [8,22–25]. But the preparation of macromonomers is carried out in solvent and the conditions for anionic polymerization are too demanding. And it is desirable to replace these hazardous organic solvents with water [26,27]. Controlled/living radical polymerization in emulsion system has been greatly developed, in particular emulsion ATRP [26,28–31] and RAFT emulsion polymerization [32–37]. AGET ATRP was successfully implemented in an ab initio emulsion system by Krzysztof Matyjaszewski [38], which has overcome the inefficient transport of Cu-based catalysts across the aqueous phase to micelles or polymerizing particles. In addition, AGET ATRP has been widely adopted for preparing well-defined polymers regardless of the presence of a limited amount of air [38–40], and this approach is more compatible in monomers types and is more insensitive to impurities [41]. Therefore, the macromonomers for graft copolymers in this paper were obtained by ab initio emulsion AGET ATRP. Besides, our group has synthesized successfully combs and centipede multigraft copolymers by emulsion copolymerization of macromonomers, which can be functioned as thermoplastic elastomer [8,23]. But the macromonomers were prepared by using

Scheme 1. Synthesis of Allyl-terminated polymethyl methacrylate macromonomers and their copolymerization with n-butyl acrylate.

228

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

Scheme 2. Schematic illustration of emulsion AGET ATRP of MMA.

Scheme 3. Scheme for synthesis of brush-like graft copolymers Poly((n-butyl acrylate)-g-polymethyl methacrylate) in miniemulsion polymerization using PMMA macromonomer.

high-vacuum anionic polymerization. Inspired by the previous work, our group describe the synthesization of macromonomers and preparation of multigraft copolymers in emulsion system, which is very environmental friendly. In this paper, we report the synthesis of brush-like poly(n-butyl acrylate-g-methyl methacrylate) multigraft copolymers with PnBA as the backbone and PMMA as side chains by macromonomer technique combining with emulsion ATRP and conventional emulsion copolymerization. The well-defined PMMA macromonomers were obtained via ab initio emulsion AGET ATRP (Schemes 1 and 2) and subsequently chain end modification. Then PnBA-g-PMMA brush-like multigraft copolymers having different molecular weight were prepared by eco-friendly miniemulsion copolymerization, initiated by azobis-(isobutyronitrile) (AIBN) (Scheme 3). The structural characteristics of PMMA macromonomers and brush-like multigraft copolymers were characterized. The microphase separation morphology for brush-like multigraft copolymers with different PMMA content was analysized. Dynamic rheometer results showed that brush-like multigraft copolymers were promising for application as thermoplastic elastomers.

2. Experimental section 2.1. Chemicals Methyl methacrylate (MMA) and n-butyl acrylate (nBA) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. and were purified by passing through a neutral alumina column to remove stabilizers. 2,20 -Azobisisobutyronitrile (AIBN, Aldrich, 90%) was recrystallized in methanol before use. Copper (II) bromide, L-ascorbic acid, SDS, and titanium tetrachloride (all from Sinopharm Chemical Reagent Co., Ltd.), Brij98 (Acros), 4,40 -Dinonyl-2, 20 -bipyridyl (dNbpy, Alfa, P97%), Ethyl 2-bromopropionate (EBiB, Aldrich, P99%), allyltrimethylsilane

(ATMS, Aldrich, P98%) were used as received without further purification. Dichloromethane was dried by refluxing over calcium hydride and was distilled under nitrogen atmosphere. Toluene, methanol, and tetrahydrofuran were used as received.

2.2. Synthesis of macromonomers Polymethyl methacrylate with bromine chain end was synthesized by ab initio emulsion atom transfer radical polymerization (Schemes 1 and 2). The initiator was ethyl 2-bromopropionate (EBiB) and the catalyst was copper (II) bromide complexed with dNbpy. In a typical reaction, the Cu (II) complex was prepared by dissolving CuBr2 (13.9 mg, 0.061 mmol) and dNbpy (51.6 mg, 0.123 mmol) in MMA (2 ml, 18.9 mmol) at room temperature. Then, the initiator, ethyl 2-bromoisobutyrate (EBiB, 24.76 lL, 0.188 mmol) was dissolved in above complex. The resulting solution was slowly added to an aqueous solution of polyoxyethylene (20) oleyl ether (Brij98, 15.19 ml, 0.087 mol/L) and sodium chloride (NaCl, 2 g/L) under stirring to form steady emulsion. After bubbling the emulsion with ultrahigh-purity nitrogen for 30 min, the flask was immersed in oil bath thermostated at 60 °C. A predeoxygenated aqueous solution (1 ml) of L-ascorbic acid (4.5 mg) was injected into the emulsion to start the polymerization. And the variation of appearance of emulsion can be clearly observed in Scheme 2 before and after adding L-ascorbic acid, which meant that the copper (II) was successfully reduced to copper (I). About twenty minutes later, the predeoxygenated second part of monomer MMA (3 ml, 28.3 mmol) and L-ascorbic acid (5.0 mg) was added to the emulsion with a syringe to form ab initio emulsion system. The samples were withdrawn periodically to measure the monomer conversion gravimetrically. The resulting polymers were dissolved in THF, and the solution was purified by passing through a neutral alumina column. The polymers were precipitated into methanol. The precipitate was dissolved in

229

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

tetrahydrofuran (THF) and then reprecipitated three times in methanol, and the final precipitate was dried in a vacuum oven at 40 °C for 24 h. The halogen-terminated polymers (PMMA–Br) were dissolved in dichloromethane and it was cooled to 0 °C under ultrahigh-purity dry nitrogen atmosphere. Then TiCl4 was added to the stirred solutions and the reaction mixture was aged for 30 min before charging prechilled ATMS. About 1 h later the flask was opened and some methanol was added to deactivate TiCl4. The resulting solutions was filtered off and subsequently dichloromethane and methanol were evaporated off. The resulting polymers were dissolved in dichloromethane and the solution was purified by passing through a neutral alumina column. The polymers were precipitated into methanol. The precipitate was dissolved in dichloromethane and then reprecipitated twice in methanol, and the final precipitate was dried in a vacuum oven at 50 °C for 24 h. The concentrations of reagents and other reaction conditions for synthesizing of PMMA macromonomers with different molecular weights are summarized in Table 1. 2.3. Synthesis of brush-like multigraft copolymers PnBA-g-PMMA graft copolymers were prepared by miniemulsion copolymerization (Scheme 3). In a typical copolymerization, the stable emulsions for copolymerization were obtained by adding organic solutions of toluene containing PMMA macromonomer, HD, AIBN and nBA dropwisely to aqueous solutions containing SDS and mixing simultaneously with magnetic stirring. The mixture was mixed under sonication in an ice bath for 10 min and then quickly transferred to a water bath at 80 °C for 12 h. The resulting products was obtained as a precipitate by adding the reaction mixture dropwisely to excess methanol to remove the surfactant. The final graft copolymers were purified by fractional precipitation to remove the unreacted macromonomer. 2.4. Characterization The molecular weights and polydispersity indices were measured by Gel Permeation Chromatography (GPC) equipped with a differential refractometer (Waters, 2414). A calibration curve was obtained with linear polySt standards and Chromatographic grade tetrahydrofuran was used as the mobile phase at a rate of 1 mL/min. The particle size distributions of latex particle were measured at room temperature by Laser Light Scattering (LLS) with a Delsa™ Nano C from Beckman Coulter. The 1H NMR spectra was obtained on a BRUKER AVANCE-600 spectrometer in CDCl3 at room temperature, using TMS as a standard. Infrared spectra of macromonomer and graft copolymer were obtained at room temperature on a BRUKER TENSOR 27 FT-IR spectrometer. The glass transition temperature and the heat of fusion were determined by differential scanning calorimetry (DSC) using a Mettler Toledo DSC 823 instrument with a heating and cooling rate of 10 °C/min from 80 °C to

150 °C under nitrogen. Atomic Force Microscopy (AFM) characterization for all thin films of graft copolymers for were recorded using scanning force microscope (SPI3800-SPA-400, Japan, NSK Ltd.) in tapping mode equipped with Olympus cantilevers (spring constant is 1.6 N/m). The thermal properties of polymers were characterized using TGA (TGA/DSC STARe apparatus, Mettler Toledo) with a heating rate of 10 °C/min from 25 °C to 700 °C under N2. The rheological properties of graft copolymers were analyzed by plate to plate mode rotational rheometer (MCR 302, Anton Paar) and sweep frequency from 0.1 to 100 Hz was chosen. These samples were measured by strain-fixed with sample thickness of 0.5–2 mm.

3. Results and discussion 3.1. Synthesis and characterization of macromonomer The reaction route for the synthesis of poly((n-butyl acrylate)-g-methyl methacrylate) (PnBA-g-PMMA) is shown in Scheme 1. For synthesizing PMMA macromonomer, polymethyl methacrylate macroinitiators (PMMA–Br) were prepared firstly by ab initio emulsion AGET ATRP38 (Scheme 2) using the EBiB/CuBr2/dNbpy initiating system. Then these polymethyl methacrylate macroinitiators were transformed to allyl–ended polymethyl methacrylate macromonomers (PMMA–allyl) by quantitative carbocationic allylation with ATMS in the presence of TiCl4 Lewis acid [44]. The preparation conditions and some characteristics of ab initio emulsion AGET ATRP are listed in Table 1. Table 1 indicates that there was no coagulum with different reaction time in the process of emulsion AGET ATRP, and the final conversion was expected. Moreover, the size and distribution of latex particles obtained from emulsion AGET ATRP are shown in Fig. 1 and Table 1 shows the mean diameter and C.V. value of each sample. It can be seen that the particles are homogeneous and the mean diameters are from 78 to 112 nm with low C.V. value (23.4–25.8%). Hence, it is successful to proceed ab initio emulsion AGET ATRP and the emulsion latex was stable enough. However, the C.V. value of these four samples with different molecular weight is varying, and the latex particles of macroinitiator 1 were better monodisperse with the lowest C.V. value of 23.4%. This can be due to the molecular weight of macroinitiator 1 being lower than that of others, these latex particles with lower molecular weight kept the emulsion stable to stop them from aggregating with each other. The GPC curves of different macromonomers are shown in Fig. 2, and the molecular weights and polydispersity indices are given in Table 2. From Fig. 2 and Table 2, we can observe that the GPC curves of PMMA macromonomers with different molecular weights are unimodal and the polydispersity indices are small (Mw/Mn = 1.11–1.23), which were as expected when using emulsion AGET ATRP. Besides, it can be seen clearly from Table 2 that the molecular weight of macromonomers after allylation was slightly higher than those before allylation, which proves the

Table 1 Synthesis conditions of ATRPa in the system of emulsion. Sample Macroinitiator Macroinitiator Macroinitiator Macroinitiator a b c d

1 2 3 4

mMMA (g)

EBiB (lL)

CuBr2 (g)

t (min)

Conversionb (%)

Coagulum (%)

Latex particles sizec (nm)

C.V.d (%)

3.78 3.78 4.72 5.66

99.05 49.52 41.23 49.44

0.0111 0.0111 0.0139 0.0167

210 360 450 600

41.2 52.3 51.6 63.4

0 0 0 0

78 83 108 112

23.4 24.5 25.8 25.2

All polymerizations were conducted with CuBr2:dNbpy:ascorbic acid = 1/2/0.5, 60 °C. Brij 98 = 3 wt% vs monomer. Solid content = 12% (based on 100% conversion). Conversion was determined by gravimetric analysis. Latex particle size and distribution were measured by Laser Light Scattering. Latex particle size and distribution were measured by Laser Light Scattering.

230

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

successful preparation of macromonomers containing allyl groups. It was also revealed that the PDI value became smaller with increasing reaction time. The 1H NMR spectrum of macromonomer 1 is shown in Fig. 3. The characteristic peaks for vinyl protons from allyl chain end (CH2@, d = 4.6–4.8 ppm; @CHA, d = 5.9–6.0 ppm) [45] and methoxy protons from polymethyl methacrylate (CH3OA, d = 3.6 ppm) [46] can be seen clearly in Fig. 3. In addition, the characteristic peaks of backbone methylene protons (ACH2A, d = 1.7–1.85 ppm) [46] can be observed. The 1H NMR spectrum exhibits signals at 0.8–1.06 ppm, which are assigned to methyl protons from ethyl2-bromopropionate segment and polymethyl methacrylate.

mean diameters range from 113 to 237 nm. However, the size distribution for MG-3-1 and MG-3-10 is more narrow than other samples. This can be due to a reason that the molecular weight of macromonomer using in the copolymerization of MG-3-1 and

3.2. Synthesis and structure of multigraft copolymers PnBA-g-PMMA 3.2.1. Characterization of latex particles obtained in the miniemulsion copolymerization Miniemulsion polymerization of PMMA macromonomers and nBA was carried out in order to get multigraft copolymers with higher molecular weight. Fig. 4 shows latex particles distribution and the corresponding emulsion appearance, and the average particle diameter of each sample are shown in Table 3. It can be seen obviously from Table 3 and Fig. 4 that the latex particles for these samples are heterogenous with wide size distribution, and the

Fig. 2. GPC curves of PMMA macromonomers.

Macroinitiator 1

Volume/%

Volume/%

Macroinitiator 2

Particle Diameter/nm

Particle Diameter/nm

Macroinitiator 4

Volume/%

Volume/%

Macroinitiator 3

Particle Diameter/nm Fig. 1. Latex particles size distribution of different samples.

Particle Diameter/nm

231

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236 Table 2 Synthesis conditions of allyl-terminated polymethyl methacrylate by carbocationic allylation. Sample Macromonomer Macromonomer Macromonomer Macromonomer a b

1 2 3 4

nATMS (equiv)

nTiCl4 (equiv)

Mna (g/mol)

Mw /Mna

Mnb (g/mol)

Mw /Mnb

5 5 10 15

5 5 10 15

8635 9712 15,795 24,382

1.14 1.25 1.09 1.07

8718 9817 16,212 24,806

1.16 1.23 1.12 1.11

Molecular weight and polydispersity indices were calculated before allylation. Molecular weight and polydispersity indices were calculated after allylation.

Fig. 3. 600 MHz 1H NMR spectrum of macromonomer.

MG-3-10 is less than the other three samples, resulting in non-significant steric effect compared with other samples. Furthermore, with the molecular weight of macromonomers increasing and molecular chain longer, the stability of emulsion copolymerization decreased and phase separation happened, resulting in a small fraction of aggregates, which leads to latex particles heterogenous and wider distribution [42,43]. 3.2.2. Characterization of molecular weight and composition of PnBAg-PMMA The molecular weights and polydispersity indices of brush-like multigraft copolymers were characterized by GPC. Fig. 5 shows the GPC curves after fractionation for all samples obtained in our work, and the GPC curves of PMMA macromonomers are also shown in the same figure for comparison. Moreover, the molecular weights of different multigraft copolymers as measured by GPC are presented in Table 4. As shown in Fig. 5, the GPC curves of multigraft copolymers almost show unimodal after several fractionation except that MG-3-4 exhibits a little peak for PMMA macromonomer. Residual PMMA macromonomers reflects the longer polymer chains of such species have some influence on reactivity. As previous studies reported, Mays et al. had synthesized PI-g-PS multigraft copolymers having regularly spaced trifunctional, tetrafunctional and hexafunctional branch points [11,47,48]. However, the branch points of multigraft copolymers prepared by miniemulsion polymerization are considered to be random in our work. Because of PMMA macromonomer with high molecular weight, there is a big steric hindrance effect for copolymerization [49], which makes its reactivity lower than methyl methacrylate. Furthermore, the activity of allyl copolymerizing with acrylate monomers is low, and especially when the macromonomers were obtained by termination with ATMS, which means that the activity of macromonomers is lower than acrylate monomers in the process of radical copolymerization. These effects have been verified by our

experimental data. Furthermore, from Table 2, the molecular weights for macromonomer 3 and macromonomer 4 are 16,212 g/mol and 24,806 g/mol, but only the multigraft copolymers with low weight content of PMMA, such as 2.04% in MG-3-3 and 1.03% in MG-3-4, were obtained due to steric effect, which made a large number of macromonomers residual. As implied in Table 4, multigraft copolymers with different characteristics were prepared successfully by adjusting the weight ratio of macromonomer to n-butyl acrylate and kinds of macromonomers. The FT-IR spectra of PnBA-g-PMMA after purification are shown in Fig. 6 [20]. In Fig. 6, the peak at 1733 cm1 is assigned to the characteristic peak of carbonyl group (C@O) in methyl methacrylate and n-butyl acrylate. Additionally, the absorption peaks of symmetric and anti-symmetric vibration of CAOAC are at 1160 cm1 and 1252 cm1, respectively. Peaks at 2876 cm1 and 2960 cm1 can be assigned to anti-symmetric stretching vibration absorption peak of methylene group (ACH2A) and methyl group (ACH3). Characteristic vibration absorption peaks at 750 cm1 (ACH2A) and at 1388 cm1 (CACH3) appear. As can be seen, the structure of PnBA-g-PMMA graft copolymers cannot be clearly distinguished by FTIR spectra. Thus, 1H NMR spectra was used to confirm the structure and composition of PnBA-g-PMMA graft copolymers. Moreover, 1H NMR was used to characterize the structure of PnBA-g-PMMA and calculate the content of PMMA in the multigraft copolymers. Fig. 7 shows 1H NMR spectra of MG-3-1 in CDCl3. Signals at 0.9–0.95 ppm are attributed to alfa-methyl protons (dH) [50]. The peak at 1.36–1.41 ppm is ascribed to protons (cH) of methylene PnBA segments [50]. The absorption at 1.54– 1.66 ppm is assigned methylene protons (fH, hH) in PMMA and PnBA segments. The peaks at 1.75–1.89 ppm and 2.08–2.31 ppm are due to chemical shift of the methylene protons (gH) and methane proton (eH) [23,51]. In addition, the characteristic peaks for methylene ester protons (aH) from PnBA unit and methoxy protons (bH) from PMMA unit can be seen clearly at 4.0 ppm [23] and 3.6 ppm [46,50], respectively, which means, combined with GPC data, that copolymerization of PMMA macromonomer with nBA was successful. The molar ratio of PMMA to PnBA for final multigraft copolymer can be calculated from 1H NMR according to Eq. (1) (Sb is the integration area of three protons of methoxy protons of PMMA side chains at 3.6 ppm, and Sa is the integration area of two protons of methylene ester protons of PnBA at 4.0 ppm). Then the weight composition of graft copolymers can be obtained as listed in Table 4.

gPMMA =gPnBA ¼ 2Sb =3Sa

ð1Þ

3.2.3. Thermal properties of PnBA-g-PMMA The thermal property is one of the most important factors to be considered for the application of multigraft copolymers. Fig. 8 shows the TGA thermograms and the decomposition temperature of 5% mass loss (T5d) for different samples are shown in Table 4. From Table 4, it can be observed that the T5d for all multigraft copolymers is 345–352 °C, which are much higher than that for PnBA (329 °C). Another significant finding is that the synthesized

232

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

Fig. 4. Size distributions of latex particles for different samples by LLS.

MG-3-10 and MG-3-3 samples shows slightly higher T5d (352 and 350 °C, respectively) than that for PMMA (348 °C). The above results imply that the thermal stability for multigraft copolymers was improved after copolymerization. Glass transition temperature (Tg) is a significant physical parameter that has a great influence on the application performance of material, especially for thermoplastic elastomer. Fig. 9 displays DSC heat flow curves for all samples, and DSC curves for

PnBA homopolymer and PMMA macromonomer are also presented in Fig. 9 for comparison. The glass transition temperature for PnBA and PMMA are 51.2 °C and 119.3 °C, respectively. It can be obviously seen that all Tg values of multigraft copolymers shift toward larger values with increasing weight content of PMMA, which is ascribed to that the rigid PMMA chains as physical crosslinking points hindered the mobility of the entire graft copolymers chains, leading to increasing Tgs. On the other hand, the DSC curves for

233

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236 Table 3 Miniemulsion Radical Copolymerization of nBA in the presence of PMMA Macromonomers for Multigraft Copolymers.

a b

Samples

SDS (g)

HD (g)

DI water (g)

nBA (g)

MacroPMMA (g)

AIBN (g)

CnBAa (%)

Latex particles sizeb (nm)

MG-3-1 MG-3-10 MG-3-2 MG-3-3 MG-3-4

0.0494 0.0821 0.0459 0.0712 0.0714

0.0343 0.0570 0.0318 0.0494 0.0496

20.07 20.01 20.10 20.08 20.07

1.2317 2.4213 1.0250 1.8689 1.8745

0.4135 0.3160 0.5034 0.5042 0.5064

0.0247 0.0411 0.0229 0.0356 0.0357

84.6 87.4 82.8 86.2 83.6

113 148 237 124 136

Conversion of nBA was determined by gravimetric analysis. Latex particle size and distribution were measured by Laser Light Scattering.

Fig. 5. GPC curves for macromonomers and the corresponding multigraft copolymers.

Table 4 Molecular characteristics of multigraft copolymers.

a b c d

Sample

Mn (g/mol)

Mw (g/mol)

Mw Mn

PMMAa (wt%)

T5db (°C)

Tgc (°C)

NPnBA-g-PMMAd

MG-3-1 MG-3-10 MG-3-2 MG-3-3 MG-3-4

266,400 314,000 187,600 416,200 554,800

648,600 949,400 423,400 1,242,600 1,324,200

2.43 3.02 2.25 2.98 2.38

31.4 12.4 40.7 2.0 1.0

347 352 345 350 345

13.6 20.5 8.1 39.4 47.8

9.6 3.4 7.8 0.5 0.2

The mass ratio of PMMA in the graft copolymer was determined by 1H NMR. The decomposition temperature of 5% weight loss (T5d) was calculated by TGA. Determined by DSC analysis. Number of junction points per molecule (NPnBA-g-PMMA) were measured according to the total Mn of multigraft copolymers and the mass ratio of PMMA and PnBA.

graft copolymers exhibited only one Tg which was between Tg of PnBA and PMMA macromonomer. Other literatures [8,39] has reported similar results and the corresponding explanation is as follows. Taking samples of MG-3-10 , MG-3-1 and MG-3-2 into consideration, it may be due to the relatively large PDIs in Table 4, which results in a large number of PMMA short side chains dissolving in neighboring PnBA microdomains, thus showing one Tg. However, the weight content of PMMA was 1.0% and 2.0% for MG-3-4 and MG-3-3, hence a very small amount of PMMA was

embedded in substantial PnBA segments, making it difficult for DSC to detect another Tg. 3.2.4. Microphase separation of PnBA-g-PMMA Microphase separation morphologies of PnBA-g-PMMA multigraft copolymers were investigated by AFM, as shown in Fig. 10. From Fig. 10, microphase separation morphologies of multigraft copolymers exhibit apparent difference with increasing PMMA component. It can be seen clearly that microphase separation

234

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

Fig. 6. FTIR spectra of PnBA-g-PMMA. Fig. 9. DSC heat flow curves for PnBA homopolymer, PMMA macromonomer, and multigraft copolymers.

that rigid PMMA component facilitated the microphase separation for PnBA-g-PMMA due to its immiscibility with PnBA component. Thus, the degree of microphase separation increased with increasing PMMA content.

Fig. 7. 600 MHz 1H NMR spectrum of PnBA-g-PMMA.

Fig. 8. TGA curves for PnBA-g-PMMA multigraft copolymers.

occurred with wormlike cylindrical domains for MG-3-1 and MG-3-2 but rather disordered domains for MG-3-3 and MG-3-10 . In addition, from Table 4, PMMA content of MG-3-1 and MG-3-2 is 31.4% and 40.7%, but for MG-3-3 and MG-3-10 , the content of PMMA is only 2.0% and 12.4%, respectively. These results suggest

3.2.5. Rheological properties of PnBA-g-PMMA In Fig. 11, we show the dynamic viscoelastic property of PnBA-g-PMMA copolymers. The frequency dependence of the storage modulus (G0 ) and loss modulus (G00 ) for multigraft copolymers was obtained by dynamic rheometer at room temperature. From Fig. 11, the fact that G0 is larger than G00 over the frequency range for all the samples except for MG-3-2 indicates that the PnBA-g-PMMA copolymers exhibit good elastic properties at room temperature [52]. However, when the sweep frequency is higher than 30 Hz, G00 is larger than G0 for MG-3-2 and therefore elastic properties are weakened. In addition, it is evident that G0 increases with increasing frequency for different samples. This is ascribed to that the mutual entangled chains have enough time to extend within the sweep frequency range, which signifies G0 is smaller at low frequency. On the contrary, the entangled chains has less time to reorientate at high frequency, leading to higher G0 values [53]. Fig. 12 shows the dynamic viscoelastic curves of the damping factor (tan d) as a function of frequency for PnBA-g-PMMA copolymers. As can be seen from Fig. 12a and b, the damping factor (tan d) of PnBA-g-PMMA copolymers (MG-3-1 and MG-3-2) increases with increasing sweep frequency, which implies that the elastic properties decline with increasing frequency. Furthermore, the tan d for MG-3-1 is almost constant over the frequency range in Fig. 12a, indicating that MG-3-1 copolymer has good elastic performance with increasing frequency. And we can see from Fig. 12b that the tan d for MG-3-3 and MG-3-4 decreases slightly as the frequency increases, which shows that the elastic performance for MG-3-3 and MG-3-4 is getting better with increasing frequency [54]. Besides, it can be seen obviously that the elastic performance of MG-3-3, MG-3-4 and MG-3-10 is much better than that of MG-3-2 and MG-3-1 according to the fact that the damping factor for MG-3-2 and MG-3-1 is higher than MG-3-3, MG-3-4 and MG-3-10 , respectively, in Fig. 12a and b. The main reason for these results are due to different PnBA content in PnBA-g-PMMA copolymers. As is known to all, PnBA component in graft copolymers is used as the elastomeric segments at room temperature while PMMA segments show glassy state. Increasing PnBA component in graft copolymers will help to improve the elastic properties. Moreover, the length of side chains and the number of junction points per molecule will also affect the elastic performance of different graft

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

235

Fig. 10. AFM height images of thin film of PnBA-g-PMMA from different samples.

Fig. 11. Rheological property of on PnBA-g-PMMA multigraft copolymers as a function of frequency. Storage modulus G0 is shown with solid symbols, and the loss modulus G00 is shown with hollow symbols.

Fig. 12. Plots of damping factor (tan d) versus frequency for multigraft copolymers.

copolymers. When PMMA content in graft copolymers is very low, such as 1.0% in MG-3-4, 2.0% in MG-3-3 and 12.4% in MG-3-10 , the samples are very sticky, which means that these graft copolymers may be used as adhesives. Multigraft copolymer for MG-3-1 with PMMA component of 31.4% reveals better elastic performance than MG-3-2 with PMMA component of 40.7%, indicating that it was promising for the application as TPE. 4. Conclusion In summary, PnBA-g-PMMA multigraft copolymers were synthesized by adopting macromonomer technique and miniemulsion copolymerization. The macromonomers were prepared by

emulsion AGET ATRP and subsequent allylation, and then the copolymerization of different macromonomers with nBA was proceeded in miniemulsion system, obtaining PnBA-g-PMMA multigraft copolymers with high molecular weight and different PMMA component successfully. The number-average weight range is 187,600–554,800 g/mol for PnBA-g-PMMA copolymers. The structure and the weight content of PMMA in PnBA-g-PMMA copolymers were determined by FT-IR and 1H NMR. The degree of microphase separation was different with the increasing content of PMMA in PnBA-g-PMMA copolymers. The dynamic rheometer analysis showed that multigraft copolymers had elastic performance, wherein MG-3-1 with PMMA content of 31.4% exhibited better elasticity to function as a TPE. For the synthesis of

236

H. Li et al. / Journal of Colloid and Interface Science 453 (2015) 226–236

macromonomers and preparation of multigraft copolymers, the whole process was carried out in emulsion system. Thus it is very environmental friendly to carry out reaction in an aqueous medium. On the other hand, multigraft copolymers with high molecular weight and large numbers of junction points can be obtained. Therefore, it appears that emulsion system is a promising low cost and environmental alternative to prepare thermoplastic elastomer. In future work, we will synthesize macromonomers having different structures and high activity for free radical polymerization so as to prepare graft copolymers with more complex structures, such as ‘‘centipede’’ and ‘‘barbwire’’ architectures. Acknowledgment Financial support from the National Natural Science Foundation of China (No. 51433008) is highly appreciated. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.04.051. References [1] T. Higashihara, K. Sugiyama, H.-S. Yoo, M. Hayashi, A. Hirao, Macromol. Rapid Commun. 31 (2010) 1031–1059. [2] C. Feng, Y. Li, D. Yang, J. Hu, X. Zhang, X. Huang, Chem. Soc. Rev. 40 (2011) 1282–1295. [3] G. Morandi, S. Pascual, V. Montembault, S. Legoupy, N. Delorme, L. Fontaine, Macromolecules 42 (2009) 6927–6931. [4] R. Wei, Y. Luo, W. Zeng, F. Wang, S. Xu, Ind. Eng. Chem. Res. 51 (2012) 15530– 15535. [5] M.B. Runge, S. Dutta, N.B. Bowden, Macromolecules 39 (2006) 498–508. [6] G. Morandi, V. Montembault, S. Pascual, S. Legoupy, L. Fontaine, Macromolecules 39 (2006) 2732–2735. [7] Y. Schneider, N.A. Lynd, E.J. Kramer, G.C. Bazan, Macromolecules 42 (2009) 8763–8768. [8] W. Wang, W.Y. Wang, H. Li, X. Lu, J. Chen, N.G. Kang, Q. Zhang, J. Mays, Ind. Eng. Chem. Res. 54 (2015) 1292–1300. [9] H. Shinoda, K. Matyjaszewski, Macromolecules 34 (2001) 6243–6248. [10] D. Uhrig, J. Mays, Polym. Chem. 2 (2010) 69–76. [11] D. Uhrig, R. Schlegel, R. Weidisch, J. Mays, Eur. Polym. J. 47 (2011) 560–568. [12] R. Schlegel, D. Wilkin, Y. Duan, R. Weidisch, G. Heinrich, D. Uhrig, J.W. Mays, H. Iatrou, N. Hadjichristidis, Polymer 50 (2009) 6297–6304. [13] R. Schlegel, Y.X. Duan, R. Weidisch, S. Hölzer, K. Schneider, M. Stamm, D. Uhrig, J.W. Mays, G. Heinrich, N. Hadjichristidis, Macromolecules 44 (2011) 9374–9383. [14] Y.X. Duan, M. Thunga, R. Schlegel, K. Schneider, E. Rettler, R. Weidisch, H.W. Siesler, M. Stamm, J.W. Mays, N. Hadjichristidis, Macromolecules 42 (2009) 4155–4164. [15] R. Schlegel, U. Staudinger, M. Thunga, R. Weidisch, G. Heinrich, D. Uhrig, J.W. Mays, H. Iatrou, N. Hadjichristidis, Eur. Polym. J. 45 (2009) 2902–2912. [16] Y.X. Duan, E. Rettler, K. Schneider, R. Schlegel, M. Thunga, R. Weidisch, H.W. Siesler, M. Stamm, J.W. Mays, N. Hadjichristidis, Macromolecules 41 (2008) 4565–4568.

[17] Y.Q. Zhu, E. Burgaz, S.P. Gido, Macromolecules 39 (2006) 4428–4436. [18] J.W. Mays, D. Uhrig, S. Gido, Y.Q. Zhu, R. Weidisch, H. Iatrou, N. Hadjichristidis, K.L. Hong, F. Beyer, R. Lach, M. Buschnakowski, Macromol. Symp. 215 (2004) 111–126. [19] G.O. Schulz, R. Milkovich, J. Appl. Polym. Sci. 27 (1982) 4773–4786. [20] H.Q. Xie, S.B. Zhou, J. Macromol. Sci. Part A – Chem. 27 (1990) 491–507. [21] T. Fónagy, U. Schulze, H. Komber, D. Voigt, J. Pionteck, B. Iván, Macromolecules 40 (2007) 1401–1407. [22] K. Ito, Prog. Polym. Sci. 23 (1998) 581–620. [23] W. Wang, W.Y. Wang, X.Y. Lu, S. Bobade, J.H. Chen, N.G. Kang, Q.Y. Zhang, J. Mays, Macromolecules 47 (2014) 7284–7295. [24] D. Pantazis, I. Chalari, N. Hadjichristidis, Macromolecules 36 (2003) 3783– 3785. [25] N. Hadjichristidis, M. Pitsikalis, S. Pispas, H. Iatrou, Chem. Rev. 101 (2001) 3747–3792. [26] K. Min, K. Matyjaszewski, Cent. Eur. J. Chem. 7 (2009) 657–674. [27] X.S. Wang, S.P. Armes, Macromolecules 33 (2000) 6640–6647. [28] J. Qiu, S.G. Gaynor, K. Matyjaszewski, Macromolecules 32 (1999) 2872–2875. [29] Y. Kwak, K. Matyjaszewski, Polym. Int. 58 (2009) 242–247. [30] H. Eslami, S. Zhu, J. Polym. Sci., Part A: Polym. Chem. 44 (2006) 1914–1925. [31] W. Li, K. Matyjaszewski, Macromolecules 44 (2011) 5578–5585. [32] Y. Luo, J. Tsavalas, F.J. Schork, Macromolecules 34 (2001) 5501–5507. [33] Y. Luo, R. Wang, L. Yang, B. Yu, B. Li, S. Zhu, Macromolecules 39 (2006) 1328– 1337. [34] L. Yang, Y. Luo, X. Liu, B. Li, Polymer 50 (2009) 4334–4342. [35] J. Huang, S. Zhao, X. Gao, Y. Luo, B. Li, Ind. Eng. Chem. Res. 53 (2014) 7688– 7695. [36] C.J. Ferguson, R.J. Hughes, B.T.T. Pham, B.S. Hawkett, R.G. Gilbert, A.K. Serelis, C.H. Such, Macromolecules 35 (2002) 9243–9245. [37] C.J. Ferguson, R.J. Hughes, D. Nguyen, B.T.T. Pham, R.G. Gilbert, A.K. Serelis, C.H. Such, B.S. Hawkett, Macromolecules 38 (2005) 2191–2204. [38] K. Min, H. Gao, K. Matyjaszewski, J. Am. Chem. Soc. 128 (2006) 10521–10526. [39] F. Jiang, Z. Wang, Y. Qiao, Z. Wang, C. Tang, Macromolecules 46 (2013) 4772– 4780. [40] W. Li, K. Matyjaszewski, J. Am. Chem. Soc. 131 (2009) 10378–10379. [41] H. Eslami, S. Zhu, Polymer 46 (2005) 5484–5493. [42] K. Ishimoto, M. Arimoto, T. Okuda, S. Yamaguchi, Y. Aso, H. Ohara, S. Kobayashi, M. Ishii, K. Morita, H. Yamashita, N. Yabuuchi, Biomacromolecules 13 (2012) 3757–3768. [43] Q. Zhang, J.M. Galvan-Miyoshi, F. Pezzotti, W. Ming, Eur. Polymer J. 49 (2013) 2327–2333. [44] U. Schulze, T. Fónagy, H. Komber, G. Pompe, J. Pionteck, B. Iván, Macromolecules 36 (2003) 4719–4726. [45] L. Vojtova, N.J. Turro, J.T. Koberstein, Mater. Res. Soc. Symp. Proc. 774 (O1) (2003) 8. [46] C. Moineau, M. Minet, P. Teyssie’, R. Jeérôme, Macromolecules 32 (1999) 8277– 8282. [47] H. Iatrou, J.W. Mays, N. Hadjichristidis, Macromolecules 31 (1998) 6697–6701. [48] D. Uhrig, J.W. Mays, Macromolecules 35 (2002) 7182–7190. [49] J.L. Fuente, M. Fernández-García, M. Fernández-Sanz, E.L. Madruga, Macromolecules 34 (2001) 5833–5837. [50] W.W. Simons, M. Zhang, The Sadtler Guide to the NMR Spectra of Polymers, Sadtler Research Laboratories, Philadelphia, 1973. [51] N.D. Koromilas, G.C. Lainioti, E.K. Oikonomou, G. Bokias, J.K. Kallitsis, Eur. Polym. J. 54 (2014) 39–51. [52] Y. Dong, J. Yin, X. Zhao, J. Mater. Chem. A 2 (2014) 9812–9819. [53] W. Shi, N.A. Lynd, D. Montarnal, Y. Luo, G.H. Fredrickson, E.J. Kramer, Macromolecules 47 (2014) 2037–2043. [54] C.W. Macosko, Rheology principles, Measurements, and Applications, 1994.