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Time-Dependent Polymerization Kinetic Study and the Properties of Hybrid Polymers with Functional Silsesquioxanes Khine Y. Mya,* Esther M. J. Lin, Chakravarthy S. Gudipati, Lu Shen, and Chaobin He* Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 3-Research Link, Singapore 117602, Singapore ReceiVed: March 26, 2010; ReVised Manuscript ReceiVed: May 26, 2010
Methacrylate-functionalized cubic silsesquioxane homopolymers [p(MA-CSSQ)] were synthesized by reversible addition-fragmentation chain transfer (RAFT)-mediated living radical polymerization in the presence of dodecyl(dimethylacetic acid)trithiocarbonate (DDTA) chain transfer agent, and their polymerization kinetics were studied. The DDTA-terminated p(MA-CSSQ) was then employed as a macro-RAFT agent in the polymerization of methylmethacrylate (MMA) for the synthesis of a brushlike p(MA-CSSQ)-b-PMMA block copolymer. The kinetics study of p(MA-CSSQ) showed that the monomer to polymer conversion, evaluated by 1H NMR, was found to be ∼80% with the maximum number average molecular weight (Mn) of 24000 and 32300 Da, for the [MA-CSSQ]/[DDTA] ratios of 100 and 200, respectively, as determined by gel permeation chromatography (GPC). The broadening of molecular weight distributions in p(MA-CSSQ) homopolymer GPC traces was observed, presumably due to the presence of the radical-radical termination products. The resultant homopolymer and block copolymer exhibited excellent thermal stability as evidenced by thermogravimetric and differential scanning calorimetric analyses. The surface properties of p(MA-CSSQ) homopolymer and p(MA-CSSQ)-b-PMMA block copolymer, determined by water contact angle and atomic force microscopy (AFM) measurements, strongly indicated the surface enrichment of the hydrophobic silsesquioxane groups. The AFM images showed the microsized granular domains of p(MA-CSSQ) homopolymer, whereas the islandlike phase-separated domains were observed in p(MA-CSSQ)-b-PMMA block copolymer. Introduction In the past two decades, living radical polymerization (LRP) has evolved as an effective technique to control the molecular weights and the molecular weight distributions of polymers as well as to tailor the polymer architectures. The LRP method is based on a free radical process; therefore, it derives an advantage from its tolerance toward a wide range of functionalities and reaction conditions.1 Some of the well-studied LRP methods include atom transfer radical polymerization (ATRP),2 nitroxidemediatedpolymerization(NMP),3 andreversibleaddition-fragmentation chain transfer (RAFT) polymerization.4–7 Among the LRP techniques, the RAFT process is the more recent LRP methodology that has been utilized for the preparation of linear block copolymers,8–10 star-branched,11,12 shell-cross-linked micelles,13,14 core-cross-linked materials,15 and many other systems.16–18 The stability and chemical versatility inherent in RAFT agents offer RAFT-based synthesis potentially attractive for the generation of advanced materials with specific polymer architectures. Inorganic-organic hybrid materials, cubic silsesquioxanes (CSSQ), have gained tremendous interest in both academic and technical fields because of their well-defined, compact hybrid chemical structures with unique inorganic silica cores and surrounding organic functional groups.19–22 A variety of CSSQ have been widely employed as building blocks for precisely defined nanostructured functional materials22–30 as well as for the modification of properties in most of the organic polymeric materials.31–37 For example, Xu et al.32 reported the soluble * To whom correspondence should be addressed. (K.Y.M.) Fax: 6568727744. E-mail:
[email protected]. (C.H.) Fax: 65-68727528. E-mail:
[email protected].
p(MMA-co-octavinyl-CSSQ) hybrid copolymers, synthesized by a conventional free radical polymerization technique. The improvement in thermal stability was observed due to the incorporation of nanocage CSSQ uniformly dispersed at the molecular level in the polymer matrix. Recently, perfluoroalkylthioether-substituted CSSQ compounds were reported with high yields via coupling of the octa(vinyl)-CSSQ/octa(vinyldimethylsilyloxy)-CSSQ with perfluoroalkylthiol in the presence of 2,2′-azobisisobutyronitrile (AIBN).23 The studies showed a promising new class of highly hydrophobic hybrid materials. Of particular relevance to the present paper, Pyun and Matyjaszewski38 reported the synthesis of p(MA-b-(MA-CSSQ)) and p((MA-CSSQ)-b-BA-b-(MA-CSSQ)) by ATRP technique, but no details regarding the polymerization kinetics were reported. The Hirai group also described the synthesis and hierarchical self-assembled structures of PMMA-b-P(MACSSQ) block copolymers by living anionic polymerization (LAP).31 Their morphological study showed that the spherical, cylindrical, and lamellae structures were obtained by changing the length of MA-CSSQ. To the best of our knowledge, most of the previous works have dealt with the copolymerization of CSSQ with PMMA by ATRP and LAP techniques. In this study, we made an attempt to synthesize p(MA-CSSQ) homopolymers by RAFT-mediated LRP in the presence of dodecyl(R,R′dimethylacetic acid)trithiocarbonate (DDTA) chain transfer agent (CTA). Afterward, the dodecylthiocarbonate-terminated p(MA-CSSQ) was employed as a macro-RAFT agent in the polymerization of methylmethacrylate (MMA) for the synthesis of p(MA-CSSQ)-b-PMMA block copolymer. Our interest of employing the RAFT process is 2-fold: (i) to gain deeper insight
10.1021/jp102731e 2010 American Chemical Society Published on Web 06/24/2010
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into the feasibility and kinetics of RAFT polymerization of MACSSQ and (ii) to develop a p(MA-CSSQ)-b-PMMA block copolymer using dodecylthiocarbonate-terminated p(MA-CSSQ) as a macro-RAFT agent. The characterization of these homopolymers and block copolymer was performed by spectroscopy, chromatography, and thermal analysis techniques. Additionally, the surface properties of p(MA-CSSQ) and p(MACSSQ)-b-PMMA were investigated by using water contact angle and atomic force microscopy (AFM). Experimental Section Materials. Trisilanolisobutyl silsesquioxane was purchased from Hybrid Plastics Inc. (product #SO1450). Triethylamine (g99.5%), trichlorosilylpropyl methacrylate (g90%), dodecanethiol (g98%), tricaprylylmethylammonium chloride, and MMA (99.5%) were supplied by Sigma-Aldrich. MMA was passed through a basic alumina column to remove the inhibitor. Trisilanolisobutyl silsesquioxane was dried in a vacuum oven at 40 °C for 1 day before use. Triethylamine was distilled from calcium hydride (CaH2) under nitrogen prior to use. Toluene and tetrahydrofuran (THF) were freshly dried with sodium/ benzophenone under nitrogen atmosphere. AIBN was used as an initiator for polymerization, which was recrystallized from ethanol before use. DDTA was synthesized according to the previously reported procedure.39 MA-CSSQ was prepared by the method reported by Lichtenhan,40 and the detailed procedure is shown in the Supporting Information. Polymerization of Poly(isobutylpropylmethacryl cubic silsesquioxane) [p(MA-CSSQ)]. The MA-CSSQ polymerizations were carried out by using [MA-CSSQ]/[DDTA] molar ratios of 100 and 200, respectively. The required amount of MA-CSSQ, DDTA, and AIBN was weighed in a Schlenk tube containing a magnetic stir bar and dissolved in a small amount of anhydrous toluene. The solutions were degassed through five freeze-pump-thaw cycles and sealed under vacuum. The reactions were performed by heating the solutions in an oil bath at 70 °C for 5 h in an argon atmosphere. The polymerization reactions were quenched by freezing the solutions in liquid nitrogen and exposing them to air. A small amount of THF was then added to each solution, and the excess solvent was removed by rotary evaporator. This process was repeated several times until all of the toluene had been removed. The resultant products were purified by precipitation in a large excess of cold methanol for 60 min. The final products, p(MA-CSSQ), were obtained as white powders after they were filtered and dried under vacuum overnight at 40 °C. Polymerization of Poly(isobutylpropylmethacryl cubic silsesquioxane)-block-poly(methyl methacrylate) [p(MACSSQ)-b-PMMA]. MMA (1.01 g, 10.35 mmol), p(MA-CSSQ) (0.93 g, 1.38 × 10-2 mmol) macro-RAFT agent, and AIBN (0.755 mg, 4.60 × 10-3 mmol) were added to a Schlenk flask and dissolved in 5 mL of anhydrous toluene. The solution was then degassed through five freeze-pump-thaw cycles and sealed under vacuum. The flask was placed in an oil bath preheated at 70 °C for 5 h in argon atmosphere, and the reaction was quenched by freezing the solution in liquid nitrogen and exposing it to air. The solvent was removed by a rotary evaporator, and the resulting viscous solution was precipitated in cold methanol. A white powder product was obtained after drying it under vacuum at 40 °C for 24 h. The synthetic route for the preparation of MA-CSSQ, p(MA-CSSQ), and p(MACSSQ)-b-PMMA is illustrated in Figure 1. Instrumentation. 1H-, 13C-, and 29Si NMR were performed in deuterated chloroform (CDCl3) and recorded by Bruker DRX
Mya et al. 400 MHz NMR spectrometer with tetramethylsilane (TMS) as the internal standard. Molecular weights (Mw and Mn) and molecular weight distributions were determined by a Waters gel permeation chromatography (GPC) system (model 2690) with a 410 refractive index (RI) detector. The system was calibrated with polymethylmethacrylate (PMMA) standards, and THF was used as the eluant. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer Instruments (model TGA 7), and the data were recorded over a temperature range of 50-800 °C under a nitrogen atmosphere at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) measurements were conducted on a DSC 2920 modulated differential scanning calorimeter from TA Instruments under nitrogen atmosphere. The samples of approximately 5-10 mg were sealed in aluminum pans and run in the temperature range of ∼30 to 200 °C with a heating rate of 10 °C/min. For water contact angle and AFM studies, p(MA-CSSQ) homopolymer and p(MA-CSSQ)-b-PMMA block copolymer were dissolved in THF (2 wt %) and spin-coated on a cleaned glass substrate. The glass substrates were freshly cleaned by ultrasonification in isopropanol for 15 min and rinsed with deionized water, followed by blow-drying with nitrogen. The samples were spun at 3000 rpm for 30 s using a spin coater purchased from Specialty Coating System. Afterward, the samples were dried in a vacuum oven for 24 h and kept in a humidity box before AFM and water contact angle studies. AFM was performed using a Multimode scanning probe microscope with Nanoscope IV Controller (Digital Instruments). The samples were imaged in tapping mode with high aspect ratio sharp tips (Nanosensors, type PPP-NCH-50 pointprobe-plus silicon-SPM-sensor) to enable enhanced image resolution. The spring constant of the tip used was 42 N m-1 with a tip height of 10-15 µm, and the AFM scanning rate was 0.5-1.0 Hz. The static water contact angle measurements were performed by a rame-hart digital goniometer using DROPimage software (rame-hart Instrument Co.) at ambient humidity and temperature. The results were evaluated from the direct analysis of the shape of the water drop by placing a drop size of 3 µL of deionized water (18 MΩ cm) using an autopipetting system, and the contact angle of the water droplet was measured within 30 s. The measurements were repeated at least five times on each sample. Results and Discussion Polymerization of p(MA-CSSQ). The MA-CSSQ polymerizations were performed in the presence of the DDTA CTA at 70 °C for 5 h in toluene by using [MA-CSSQ]/[DDTA] molar ratios of 100 and 200, while the initial [MA-CSSQ] was 1.06 M. Figure 2 shows an example of the representative GPC traces for the p(MA-CSSQ) homopolymerization with a [MA-CSSQ]/ [DDTA] ratio of 200. The molecular weight distribution shifted to a larger molecular weight with an increase in reaction time, particularly at a shorter reaction time (within 3 h) (insert in Figure 2). Long tails were observed at high retention times due to the formation of low molecular weight species, which could be attributed to the radical-radical termination products, which do not contribute toward the growth of the polymer (dead chains). The slowdown of the RAFT equilibrium was observed after 3 h of reaction time. It was speculated that the dodecylthiocarbonate-terminated groups may be inhibited from growing p(MA-CSSQ) chains by a bulky group of CSSQ during polymerization, particularly at high conversion. Pyun et al.41 also described similar results of a limiting degree of polymerization (DP) due to the steric hindrance of the silsesquioxane cage in the ATRP of MA-POSS macromonomers.
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Figure 1. Synthetic route of the RAFT-based polymerization.
The relationship between number average molecular weight (Mn) (measured by GPC analysis) and monomer conversion is illustrated in Figure 3. The monomer to polymer conversion was determined by 1H NMR analysis. The values of Mn increased rapidly to high molecular weights of ∼12000 and ∼20000 Da for the [MA-CSSQ]/[DDTA] ratios of 100 and 200 in the first 30 min of polymerization. Such an initial increase
to a high molecular weight at low monomer conversions could be attributed to the hybrid polymerization effect.9,42 The rapid jump in Mn at the initial polymerization process is also believed to be dependent on the concentration of the RAFT agent. The effect of the initial increase was lower at a higher concentration of DDTA as shown in Figure 3. Similar behavior was also presented in the RAFT homopolymerization of DEGMA.43 The
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Figure 2. GPC traces of the p(MA-CSSQ) homopolymer with a [MA-CSSQ]/[DDTA] ratio of 200.
Figure 3. Dependence of molecular weight and polydispersity of p[MA-CSSQ] on monomer conversions. [MA-CSSQ]/[DDTA] ) 100 (9) and [MA-CSSQ]/[DDTA] ) 200 ((). The solid and dashed lines represent the theoretically calculated Mn for [MA-CSSQ]/[DDTA] ratios of 100 and 200, respectively.
positive deviations from the straight lines could be due to the incomplete usage of CTA (hybrid behavior).44 As expected, the molecular weights for a [MA-CSSQ]/[DDTA] ratio of 200 were found to be higher than those for the ratio of 100. However, the deviations from the theoretically calculated Mn values and the experimental Mn data were detected in both ratios of [MACSSQ]/[DDTA] as indicated in Figure 3. At the initial stage of polymerization (
