Research Note pubs.acs.org/IECR
Styrene−Butadiene−Styrene Triblock Copolymer Latex via Reversible Addition−Fragmentation Chain Transfer Miniemulsion Polymerization Renzhong Wei, Yingwu Luo,* Wang Zeng, Feizhou Wang, and Shaohong Xu The State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, China ABSTRACT: It was demonstrated that gel-free styrene−butadiene−styrene triblock copolymer (SBS) latex with a molecular weight about 90 kg/mol and styrene composition about 34% could be synthesized by reversible addition−fragmentation chain transfer miniemulsion polymerization. The resulted SBS might be an excellent thermoplastic elastomer as indicated by its mechanical properties with ultimate tensile strength of 16 MPa and about 800% elongation at break.
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INTRODUCTION Controlled/living radical polymerization (CLRP) represented by nitroxide-mediated polymerization (NMP),1,2 atom transfer radical polymerization (ATRP),3,4 and reversible addition− fragmentation chain transfer (RAFT) polymerization5 was invented in 1990s. It has been widely expected that CLRP would play a significant influence on the polymer industry due to the versatility of the available monomers and mild polymerization process conditions. After almost two-decade extensive investigations, CLRP has now become a powerful tool to control (co)polymer chain microstructures like preset molecular weight, narrow molecular weight distribution,6,7 block copolymer,8 composition profiles along copolymer chains in the case of gradient copolymer,9 and telelic polymer. However, very few commercial products based on CLRP have appeared.10 The block copolymer, which can be used as the compatibilizer for polymer blends and thermoplastic elastomers (TPE), is thought to be the main promising products derived from CLRP.6 Generally speaking, the molecular weight of the block copolymer used as TPE and the compatiblizer should be over 100 kg/mol. Unfortunately, with CLRP it is difficult to achieve such a high molecular weight due to the existence of the irreversible termination of radicals. Actually, only in a few reports among a huge number of the CLRP papers11,12 was the molecular weight targeted at over 50 kg/mol. Thanks to the segregation effect of radicals,13−18 RAFT polymerization in a (mini)emulsion can be an effective technique to dramatically increase the molecular weight at little expense of the low polymerization rate while retaining a low fraction of dead chains once the structure of RAFT agent was wisely selected. The exploitation of RAFT (mini)emulsion polymerization was frustrated by the colloidal instability and loss of control over the molecular weight.19,20 More recently, the colloidal instability often observed during RAFT in an ab inito emulsion and miniemulsion was explained by superswelling of the particles in the early stage of the polymerization, which was revealed by the thermodynamic simulations.21,22 The RAFT ab initio emulsion and miniemulsion polymerization of styrene, butyl acrylate, and butadiene has been successfully achieved in terms of predicted molecular weight, low PDI, stable latex, and fast polymerization rate by using the rationally designed structures of amphiphilic © 2012 American Chemical Society
RAFT agent in the case of ab initio emulsion polymerization or the modified recipe in the miniemulsion polymerization after the continuing efforts of several groups.23−30 The wellcontrolled polystyrene-b-poly(n-butyl acrylate)-b-polystyrene triblock copolymers (SBAS) with molecular weight up to 300 kg/mol and only a few percent of dead chains were prepared by ab initio emulsion polymerization within four hours.31 SBAS was demonstrated to be a promising novel TPE material as evidenced by the ultimate tensile strength reached over 10 MPa with 500% elongation at break . Among TPEs, styrene−butadiene−styrene triblock copolymer (SBS) is the most widely used. SBS is commercially produced in an anionic solution polymerization process.32 Anionic polymerization is notorious for the intolerance to monomer functionality and impurities, requiring rigorous purification of reagents and exhausting removal of moisture, which cost a large amount of energy. On the other hand, SBS molecules are nonpolar, which limits its use in many applications like the modification of polar polymers and adhesives for polar substrate. Because anionic polymerization, where the catalyst is highly sensitive to water, cannot be carried out in (mini)emulsion polymerization, SBS latex has not been synthesized yet. In this paper, we exploit the RAFT miniemulsion polymerization to synthesize the latex of SBS. Unlike in the anionic polymerization, two reactive double bonds on a butadiene molecule could lead to the cross-linked product in the RAFT polymerization of butadiene.33 Only a few reports on RAFT polymerization in the (mini)emulsion of butadiene have been documented. Gilbert et al.34 tried to prepare poly(styrene150-bbutadiene250) by the semibatch emulsion RAFT copolymerization. Unfortunately, the cross-linked product was obtained. More recently, Luo et al. reported that the gelation point was able to be much retarded with the highest accessible molecular weight increased up to 23 kg/mol in the RAFT emulsion homopolymerization of butadiene.28 In another paper from Received: Revised: Accepted: Published: 15530
August 5, 2012 October 16, 2012 November 11, 2012 November 12, 2012 dx.doi.org/10.1021/ie302067n | Ind. Eng. Chem. Res. 2012, 51, 15530−15535
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The Films for Tensile Test. The film of styrene− butadiene−styrene triblock copolymer having a thickness of 1.0 mm was cast from 10 wt % tetrahydrofuran (THF) solution in a glass dish. The solvent was slowly evaporated off at room temperature for three days. Then the film was annealed in a vacuum oven at 100 °C for 12 h. Characterization. Gel Permeation Chromatography (GPC) Analysis. The molecular weight and molecular weight distribution were determined by GPC (Waters 2487/630C) with three PL columns (104, 103, and 500 Å) and a refractive index (RI) detector. The eluent was THF with a flow rate of 1 mL/min at 30 °C. Molecular weights were calibrated using narrow polystyrene standards (Polymer Laboratory) with molecular weight ranging from 580 to 710 000 g/mol. The sample solution was subjected to the filtration through a 0.45 μm pore-size membrane. The concentration of the samples is 3 mg·mL−1. The data were referred to polystyrene. Nuclear Magnetic Resonance Spectroscopy (NMR) Analysis. Molar compositions of the copolymers were determined by 1H NMR spectroscopy (NMK/300 MHz) in CDCl3 solution (internal reference: tetramethylsilane (TMS), 1 wt % solution in CDCl3) at room temperature. 1H NMR signals were assigned as follows (in ppm): 7.15 (2 ortho-H and 1 para-H, −C 6 H 5 of polystyrene), 6.65 (2 meta-H, −C 6 H 5 of polystyrene), 5.61 (1H, −CHCH2 of vinyl-1,2 polybutadiene), 5.37(2H,−CHCH−of trans-1,4 polybutadiene), 5.32(2H,−CHCH− of cis-1,4 polybutadiene), 4.92 (2H, −CHCH2 of vinyl-1,2 polybutadiene). The molar ratio of butadiene and styrene units was calculated by the ratio of the summation of H signals of −CHCH− from the 1,4 polybutadiene (5.37 ppm, 5.32 ppm) and H of −CHCH2 from the vinyl-1,2 polybutadiene (4.92 ppm) and meta-H signals of −C6H5 from polystyrene (6.65 ppm). Differential Scanning Calorimetry (DSC). Glass transition temperature (Tg) of the block copolymers were determined by differential scanning calorimetry (DSC) on a TA Instrument equipped with DSC Q100 module. The sample was scanned from −100 to 120 °C with the heating rate of 20 °C min−1. Before the measurement, the thermal history of the sample was removed by heating the sample to 120 °C and holding the temperature for 2 min. Transmission Electron Microscopy (TEM) Observation. The phase morphology of triblock copolymer samples was examined by TEM (80 kV, J EOL, JEM-1230). The ultrathin samples were microtomed and stained by osmium tetroxide (OsO4) vapor for a few hours. Particle Size Analysis. The particle size was measured by Malvern Zetasizer 3000 HAS at 25 °C. Tensile Test. The tensile test was carried out by a universal tensile machine (Zwick/Roell Z020). The testing samples were strained at 50 mm/min at 25 °C according to the testing method GB16421-1996.
Luo’s group, nanostructured particles of poly(styrene-bbutadiene) was synthesized via RAFT miniemulsion polymerization.35 So far, it is not sure whether one can achieve the reasonably high molecular weight and the suitable composition for SBS TPE via RAFT polymerization. On the other hand, the triblock copolymer via RAFT emulsion polymerization was relatively broad in molecular weight distribution.31 It is not clear how this broad molecular weight would change the morphology and mechanical properties of TPE. In this communication, it is demonstrated, for the first time, that the noncross-linked SBS latex with the reasonable composition and acceptable mechanical properties could be synthesized by RAFT miniemulsion polymerization.
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EXPERIMENTAL SECTION Materials. Styrene was purified by distillation under reduced pressure. Butadiene was distilled directly from a fiveliter storage vessel into a cooled steel container. Potassium persulfate (KPS, initiator, > 99%), sodium bicarbonate (NaHCO3, pH value buffer, > 99%), and sodium dodecyl sulfate (SDS, surfactant), hexadecane (HD, costabilizer, from Aldrich) were used without further purification. 1-Phenylethyl phenyl dithioacetate (PEPDTA, RAFT agent) was synthesized and purified as previously reported.30 Synthesis of Styrene−Butadiene−Styrene Triblock Copolymer. Styrene (24g, 0.23 mol) was first mixed with HD (1.2 g, 5.31 × 10−3 mol) and PEPDTA (0.42 g, 1.54 × 10−3 mol). This oil solution was then added to the SDS aqueous solution (2.16 g, 7.5 × 10−3 mol SDS in 126 g water) under gentle stirring for 20 min. The resulted coarse emulsion was then subjected to ultrasonication (KS600 sonifier, amplitude 70%, 600 W) for 15 min in an ice−water bath to form miniemulsion. The obtained miniemulsion was immediately transferred to a 500 mL five-neck flask equipped with a condenser, a thermometer, a nitrogen inlet, and a mechanical stirrer. The miniemulsion was stirred with the purge of high pure nitrogen at room temperature for 10 min and then immersed in a thermostatic water bath at 70 °C. The addition of KPS (0.08 g, 2.96 × 10−4 mol) and NaHCO3 (0.08 g, 9.5 × 10−4 mol, pH buffer) dissolved in 3 g of water initiated the polymerization. After 150 min, when styrene conversion was about 80%, the resulted latex was cooled down to room temperature to be PSt-RAFT seed. The diluted PSt-RAFT seed latex (600 g, 4 wt % solid content), KPS (0.08 g, 2.96 × 10−4 mol) and NaHCO3 (0.08 g, 9.5 × 10−4 mol, pH buffer) were then charged into a 1 L cylindrical autoclave (diameter = 100 mm, height = 180 mm) with a 2-blade skewed propeller (diameter = 50 mm). The sealed autoclave was purged with high pure nitrogen for 30 min and then was vacuumized. The distilled butadiene (300 g) was then pumped into the autoclave. The PSt-RAFT seeded latex particles were swollen by butadiene under gentle stirring for 2 h at room temperature before the polymerization. The polymerization was carried out for 5 h at 70 °C and the conversion of butadiene was about 30%. Then the reaction was stopped by cooling the system down to 20 °C before releasing the pressure. The residue of butadiene was swept off by purging the autoclave with high pure nitrogen for 2 h under stirring. An 80 g portion of styrene was then injected to the autoclave and swelled the latex for 2 h before the reaction temperature was raised again to 70 °C. The polymerization was carried on for 40 and 90 min, respectively, to obtain two samples of the triblock copolymer latexes at styrene conversions 30% and 78%.
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RESULTS AND DISCUSSION The polystyrene composition in an styrenic thermoplastics elastomer exerts significant influence on the physical properties.36 For most commercial SBS products, the total molecular weight of SBS chains is about 100 K and the polystyrene block weight percentage is from 30% to 40% to balance the mechanical and processing properties. Our previous work showed that the gel point was reached when the molecular weight of polybutadiene block was about 60 kg/mol during the formation of polybutadiene block in the miniemulsion RAFT 15531
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polymerization.35 Accordingly, the molecular weight of the polystyrene blocks in SBS was designed to be 12 kg/mol and the molecular weight of PB was designed to be 60 kg/mol. SBS was synthesized by adding styrene, butadiene, and styrene in sequence. During the whole course, the colloidal systems remained stable. The detailed information of the resulted SBS is summarized in Table1. The particle size of the polystyrene seed
polymerization, where the monomer was switched from butadiene to styrene, no gel was formed. Actually, when the third block of polystyrene further grew up to 39 kg/mol in molecular weight, the gel had not appearred yet though the PDI was further increased (refer to Table 1). Most likely, the microphase separation had occurred soon after the third stage was started so that the radicals were confined within the PS microphases in the third stage. Thus, the double bonds on polybutadiene blocks were difficult to be accessed to by the radicals due to the microphase separation. As shown in Figure 1, the GPC curves moved to the higher molecular weight from PS, through PS-b-PB to PS-b-PB-b-PS
Table 1. The Characteristics of Styrene−Butadiene−Styrene Triblock Copolymers calculated structurea PS compositionb (wt %) PSc Mn PDI DV/Dn PS-b-PBc Mn PDI DV/Dn PS-b-PB-b-PSc Mn PDI DV/Dn structure by CPC
SBS1
SBS2
12K-62K-15K 34%
12K-62K-39K 48%
11 1.15 87.4/74.9
11 1.15 87.4/74.9
78 2.60 132.1/124.5
78 2.60 132.1/124.5
90 2.73 / 11K-67K-12K
100 2.80 156.4/145.7 11K-67K-22K
a
The number representing the designed molecular weight of each block, and the molecular weights of each block were calculated by Mn = [M]0x/[RAFT], where [M]0, x, [RAFT] are the initial monomer concentration, monomer conversion, and RAFT agent concentration, respectively. bPS compositions were calculated by 1H NMR analysis. c The number-average molecular weights and PDIs were determined by GPC analysis. dPD is the molar percentage of PS-b-PB dead chains. e Dv is the volume average diameter of latex and Dn is the number average diameter, both by DLS.
Figure 1. Molecular weight distributions during synthesis of styrene− butadiene−styrene triblock copolymers via sequential monomer addition.
(SBS) as a whole, indicating the formation of the wellcontrolled triblock copolymer. In triblock copolymer, the contents of the homopolymer and diblock copolymer, which are the dead chains produced from the side-reactions-like irreversible termination is an important factor affecting the properties. Unfortunately, it is difficult to quantify the fraction of those dead chains, though for the specific system of styrene/butyl acrylate, the liquid adsorption chromatogram was developed to separate homopolystyrene and
was about 87 nm (Dv) and the particle polydispersity was 1.17 (Dv/Dn). The particle diameter of the resulted SBS latex (SBS2) was about 156 nm and polydispersity was 1.07. Both samples of SBS were gel-free. It must be noted that styrene conversion in the first step was about 80% so a small transition copolymer segment with gradient composition formed between the first block (polystyrene) and the second block (polybutadiene).35 In the second step, the residue of butadient was swept off so that there was no transition segment between polybudiene and polystyrene. The microstructures of SBS chains were measured by GPC and 1H NMR, respectively. By GPC, the molecular weights of the first block of PS and PS-bPB were estimated to be about 11 kg/mol and 78 kg/mol (refer to Table 1), respectively, both in agreement with the designed values. It must be pointed out that the molecular weights of PSb-PB and PS-b-PB-b-PS listed in Table 1 are relative to PS standards. Because of the different Mark-Howink constants of PB and PS37 and the branching structures of PB block,33 some errors might occur in the PS-b-PB and PS-b-PB-b-PS molecular weights. The PDIs of the first PS block and PS-b-PB were 1.1 and 2.6 (refer to Table 1), respectively. The molecular weight distribution was much broadened during the formation of PS-bPB due to the branching reactions on PB. To increase the PB composition, the second stage of the polymerization, that is, the stage of the formation of PB block, had to extend close to the gel point at a molecular weight about 60 kg/mol. Even so, it was very interesting to note that during the third stage of the
Table 2. The Molar Fraction of the Dead Chains Formed at Each Polymerization Stage SBS1 SBS2
PS
PS-b-PB
PS-b-PB-b-PS
1.8% 1.8%
5.6% 5.6%
0.7% 1.5%
homobutyl acrylate from their diblock copolymer.38 In Table 2, the molar fraction of the dead chains was estimated by PD% =
fini [I](1 − e−kdt ) [RAFT]0 + fini [I](1 − e−kdt )
100
where f ini is initiation efficiency of initiator and f ini = 0.5;39 [RAFT]0 is the initial concentrations of RAFT agent; [I] is the concentrations of the initiator; t is the polymerization time; kd is decomposition rate constant of initiator and kd = 2.33 × 10−5 s−1.40 It is assumed that radicals be terminated by combination 15532
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Figure 4 shows the tensile curves of SBS1 and SBS2, which present the typical mechanical behavior of the styrenic
and the transfer reactions are ignored. From Table 2, it seems that the dead chains from the termination should be minor, supporting that the RAFT polymerization in emulsion could much suppress the termination reactions. Since the transfer reactions might be quite significant, the dead chain fractions might be higher. Figure 2 shows the 1H NMR spectra of the block copolymers. According to the spectra, polystyrene composi-
Figure 4. Stress−strain curves of SBS.
thermoplastics elastomer: large strain, yielding behavior, and strain-induced plastic-to-rubber transition. Taking SBS1 as an example, the ultimate tensile strength reached 16 MPa with a large strain of about 800%, and the stress at 300% strain remained about 4 MPa. These data are the best reported data for TPE from CLRP. The ultimate tensile strength are inferior to those of SBS by anionic polymerization (over 20 MPa)42 but still good enough for most application fields. It is also worthy to note that the resulted SBS latex cannot be synthesized previously. Such latex could be very useful in the adhesive and coating areas. As demonstrated above, the gel-free SBS with the total molecular weight about 90 K and the polystyrene block weight percentage from 30% to 50% could be synthesized in a RAFT miniemulsion polymerization. In terms of chain architechture, the SBS that results is a mixture of linear chains and multiarm star macromolecules. More interestingly, the mechanical properties are comparable with the product from aionic solution polymerization. Considering that SBS has unique properties and has been widely used in many applications, SBS latex might be an interesting product derived from CLRP with high industrial impacts. Not only could SBS from RAFT (mini)emulsion polymerization replace those from anionic
Figure 2. 1H NMR spectra between 4.5−8.0 ppm in CDCl3 of styrenebutadiene diblock copolymer and styrene−butadiene−styrene triblock copolymers.
tions in SBS1 and SBS2 were 34% and 48%, respectively. From 1 H NMR data, three types of isomeric structures in the polybutadiene block were estimated to be 12% vinyl-1,2 structure, 50% trans-1,4 structure, and 38% cis-1,4 structure, respectively. As a comparison, the anonic polymerization of butadiene yields 35% cis-1,4, 52% trans-1,4, and 13% vinyl-1,2 structures (Li/n-pentane, 0 °C).41 The microphase separation of two kinds of SBS was observed by TEM, as shown in Figure 3. Polystyrene morphology (light regions) in SBS1 (PS wt % = 34%) was worm-like, which might be related with the rather broad molecular weight distribution. With the increasing content of polystyrene segment (PS wt % = 48%), partial lamella morphology occurred in the SBS2.
Figure 3. TEM images of SBS1 and SBS2, stained by OsO4. 15533
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polymerization, but also the latex form of SBS has the unique advantage to open new application opportunities such as waterbased coatings and adhesives.
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CONCLUSION For the first time, we demonstrated that the gel-free styrene− butadiene−styrene triblock copolymer (SBS) latex with the total molecular weight about 90 K and the polystyrene block weight percentage from 30% to 50% could be synthesized in a RAFT miniemulsion polymerization. The triblock copolymer exhibited to be a promising TPE material with comparable mechanical properties to the product from the anonic solution polymerization. It is worth to noting that the resulted SBS latex could not be synthesized previously. Such latex could be very useful in the adhesive and coating areas. It is expected that SBS latex might be a product from CLRP with high industrial impact.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: 86-571-87951612. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was supported by National Program on Key Basic Research Project (973 program, 2011CB606002), the National Science Foundation of China (NSFC) for Award Nos. 21076181, 20836007, 21125626, and Program for Changjiang Scholars and Innovative Research Team in University.
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