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Well-defined diblock copolymers of poly(tert-butyldimethylsilyl methacrylate) and poly(dimethylsiloxane) synthesized by RAFT polymerization
Polymer 55 (2014) 39e47
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Well-defined diblock copolymers of poly(tert-butyldimethylsilyl methacrylate) and poly(dimethylsiloxane) synthesized by RAFT polymerization The Hy Duong, Christine Bressy*, André Margaillan Université de Toulon, Laboratoire MAPIEM, EA 4323, 83957 La Garde, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2013 Received in revised form 20 November 2013 Accepted 22 November 2013 Available online 3 December 2013
We report the application of reversible addition-fragmentation chain transfer (RAFT) polymerization using poly(dimethylsiloxane) (PDMS) chain transfer agents toward the synthesis of a variety of diblock copolymers containing tert-butyldimethylsilyl methacrylic (MASi) monomer units. The methodology relies on the synthesis of PDMS monofunctional chain transfer agents easily available in one synthetic step from commercially available hydroxylated PDMSs. The RAFT process enables access to polymer chains with narrow molar mass distributions and high conversions. Data from differential scanning calorimetric measurements revealed that the diblock copolymers exhibited two glass transition temperatures, corresponding to the PDMS- and PMASi-enriched phases, respectively. Copolymerizations of MASi and butyl methacrylate (BMA) within the second block led to immiscible phases with lower glass transition temperatures than PDMS-block-PMASi copolymers. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: RAFT PDMS Tert-butyldimethylsilyl methacrylate
1. Introduction Hydrolyzable tri-alkylsilylester (meth)acrylic monomers have been recently used in the synthesis of (meth)acrylic acid-based polymers with various tacticities [1], and are currently used in marine antifouling coatings as seawater erodible matrixes [2,3]. Depending on the steric hindrance of the alkyl groups, the trialkylsilyl (meth)acrylate-based polymers range from readily hydrolyzable to quite stable polymers in seawater [4]. In our research group, Bressy and coworkers have synthesized tri-alkylsilylesterbased statistical copolymers by conventional radical polymerization and several diblock copolymers using the reversible additionfragmentation chain transfer (RAFT) polymerization for developing erodible coatings [4,5]. Methyl methacrylate (MMA) [4,6,7] and poly(dimethylsiloxane) methacrylate (PDMSMA) [5] were previously used as co-monomers in diblock and graft copolymers, respectively. Poly(dimethylsiloxane) blocks inserted in silylated-based polymers provide access to a wide variety of materials with tunable properties. The fundamental properties of PDMS chains are related to the specific characteristics of the siloxane bonds, i.e. the combination of a flexible backbone and low surface energy side groups.
* Corresponding author. Tel.: þ33 4 94 14 25 80; fax: þ33 4 94 14 24 48. E-mail address: [email protected] (C. Bressy). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.11.034
PDMS with a glass transition temperature Tg ranging from 127 C to 100 C exhibits the most flexible backbone among the common polymers. As a result, it can readily adapt the lowest surface energy configuration through a close packing of the pendant methyl groups at the PDMS/air interface. Then, PDMSs increase hydrophobicity and improve water resistance and thermal stability of many materials. PDMS has been extensively associated with other monomer units in graft and block copolymers [8,9]. PDMS-based block copolymers have been mainly synthesized via anionic polymerization with styrene [10] and 4-vinylpyridine [11]. Atom transfer radical polymerization (ATRP) has been used to prepare block copolymers with styrene [12], methyl methacrylate and N, Ndimethylaminoethyl methacrylate [12e14]. Reversible additionfragmentation chain transfer (RAFT) polymerization has been studied especially to synthesize block copolymers containing PDMS and N,N-dimethylacrylamide (DMA) [15,16], 2-(dimethylamino) ethyl acrylate (DMAEA) [17] and hydroxyethylacrylate (HEA) [18]. This article describes the synthesis of well-defined diblock copolymers from copolymerizations of tert-butyldimethylsilyl methacrylate (MASi) and butyl methacrylate on PDMS macro-RAFT agents. MASi was known to easily polymerize by RAFT polymerization [19] and copolymerized with methacrylic co-monomers [5,7]. Its ability to copolymerize with BMA from PDMS macroRAFT agents is investigated. As these copolymers might be used in marine antifouling coatings, their ability to form films without cracking is required.
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Therefore, the thermal properties of the diblock copolymers are characterized in terms of glass transition temperatures and crystallization temperatures. The effects of the copolymer composition and the PDMS length on the thermal behavior are studied.
633 nm, was used for the scattering experiments which were performed at room temperature, whereas the scattering intensity at 90 was recorded. Prior to the light scattering measurements, the polymer solutions were filtered through a 0.2 mm PTFE syringe filter.
2. Experimental 2.1. Materials Tert-butyldimethylsilyl methacrylate (MASi) was synthesized as described elsewhere [19], distilled under reduced pressure and stored under argon before use. a-hydroxyethylpropoxyl-u-propyl poly(dimethylsiloxane)s (PDMS-OH) of 1000 g mol1 (1k PDMSOH), 5000 g mol1 (5k PDMS-OH), 10,000 g mol1 (10k PDMSOH) and a,u-dihydroxyethylpropoxyl-PDMS (1k HO-PDMS-OH, 5k HO-PDMS-OH and 10k HO-PDMS-OH) (Gelest) were used as received. The molar masses of the PDMSs were furnished by the supplier. 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid (CTA) (Strem Chemicals) was used as received. Dicyclohexylcarbodiimide (DCC), tert-butyl(chloro)dimethylsilane, 4dimethylamino pyridine (DMAP), hexane, methanol, acetonitrile and ethyl acetate were purchased from SigmaeAldrich and used as received. 2,20 -azobis(isobutyronitrile) (AIBN) was purchased from SigmaeAldrich and purified by recrystallization from methanol. Butyl methacrylate (BMA), toluene and dichloromethane (DCM) were purchased from SigmaeAldrich and distilled under reduced pressure to remove inhibitors before use. 2.2. Characterization methods 1
H NMR and 13C NMR measurements were carried out on a Brüker Advance 400 (400 MHz) spectrometer with deuterated chloroform (CDCl3) as solvent at room temperature. The number-average molar mass (Mn) and dispersity (Ð) of polymers were determined by triple detection size exclusion chromatography (TD-SEC). Analyses were performed on a Viscotek apparatus, composed of a GPC Max (comprising a degasser, a pump and an autosampler) with a TDA-302 (RI refractive index detector, right and low angle light scattering detector at 670 nm and viscometer) and an UV detector (l ¼ 298 nm). The following columns were used: a Viscotek HHR-H precolumn and two Viscotek ViscoGel GMHHR-H columns. THF was used as the eluent with a flow rate of 1.0 mL min1 at 30 C. For each precipitated polymer, the refractive index increment (dn/dc) was determined using the OmniSec software, from a solution of known concentration (ca. 10 mg mL1) filtered through a 0.2 mm PTFE filter. Differential scanning calorimetric (DSC) measurements were performed on a DSC Q10 apparatus from TA Instruments calibrated with indium. Polymer samples weighing 15e20 mg were run at equal heating and cooling rates, 10 C min1, under a constant stream of nitrogen. The polymer samples were first scanned from room temperature to 100 C (PDMS-block-P(MASi-stat-BMA)) and from room temperature to 140 C (PDMS-block-PMASi). The samples were then cooled to 165 C. This temperature was held for 5 min to allow the system to attain thermal equilibrium before the second heating scan. The first heating ramp of each sample was discarded for this work. The glass transition temperature (Tg) values were determined as the midpoint between the onset and the end of a step transition using the TA Instruments Universal Analysis 2000 software. The melting and crystallization temperatures were recorded at the maximum value of the endothermic and exothermic peaks, respectively. DCM solutions of the PDMS precursors (28 g dL1) were characterized using dynamic light scattering (DLS). A Zetasizer Nano S (Malvern Instruments), equipped with a diode laser operating at
2.3. Synthesis of the macro-chain transfer agents (macro-RAFT agents) Macro-RAFT agents were synthesized by the esterification of monohydroxyl-terminated poly(dimethylsiloxane)s with a carboxylic acid end-functionalized trithiocarbonate RAFT agent catalyzed by DCC/DMAP. The synthesis of macro-RAFT agents was adapted from the scientific paper of Magenau et al. [20] with some modifications. Molar ratio of reactive agents [OH]:[COOH]: [DCC]:[DMAP] were selected as 1:1.8:1.8:0.1 and 1:1.5:2:0.2 successively. Two reactive agent feed procedures were used. In procedure (a) all of the raw materials were introduced together into a two-neck round bottom flask. The flask was evacuated under vacuum and stirred overnight. In procedure (b) the feed of the raw materials was done as follows: 5.568 g of 5k PDMS-OH (1.1 mmol), 0.029 g of DMAP (0.24 mmol), and 20 mL of distilled methylene chloride were added to a 100 mL two-neck round bottom flask equipped with a magnetic stir bar under a dried argon atmosphere. 0.682 g of CTA (1.69 mmol) in 10 mL distilled methylene chloride and 0.463 g of DCC (2.24 mmol) in 10 mL distilled methylene chloride were introduced in two different 50 mL vessels ([OH]:[COOH]:[DCC]:[DMAP] ¼ 1:1.5:2:0.2). The solutions of CTA and DCC were then added dropwise into the reactor while maintaining the argon flow rate. Then, the reaction mixture was placed in an oil bath preheated at 30 C, 40 C or 60 C for 15 h. The corresponding temperatures of the reaction mixture were 29 C, 38 C and 40 C (at reflux), respectively. The flask was then cooled in the freezer and the reaction mixture was filtered to remove any solid impurities. The solvent was removed under reduced pressure. 50 mL of hexane was added and the mixture was filtered one more time. The resulting solution was diluted with 200 mL of hexane and 100 mL of methanol was added to extract some unreacted CTA and DMAP. These two solvents are rather miscible. Then, this step was repeated three times. The hexane-rich phase containing the macroRAFT agent was added dropwise into methanol for further purification. Because of the dissolution of the 1k PDMS-RAFT agent in the methanol/hexane mixture, acetonitrile was used in place of methanol for this sample. The macro-RAFT agent was dissolved in hexane and washed first with a saturated NaCl solution and then with deionized water. The solution was dried over magnesium sulfate, filtered, and the hexane was eliminated under reduced pressure until a constant weight was reached. Finally, the product was passed through a silica gel column with a mixture of hexane/ ethyl acetate as eluent. A final pale yellow liquid product was obtained by drying under vacuum. The short names of the macroRAFT agents are summarized in Table 1. 2.4. Synthesis of PDMS-based diblock copolymers PDMS-block-PMASi (MCxMy) and PDMS-block-P(MASi-statBMA) (MCxMBy) copolymers with target Mn values were Table 1 Short names of macro-RAFT agents. molar mass of PDMS precursor (1H NMR) (g mol1)
Name of macro-RAFT agent
1,400 5,100 10,800
MC1 MC2 MC3
T.H. Duong et al. / Polymer 55 (2014) 39e47
synthesized using MC1, MC2 and MC3 as macro-RAFT agents. The macro-RAFT agent concentration was estimated from Eqn. (1) for a given target Mn value of the diblock copolymer. Mn,target is defined at 100% of monomer conversion.
Mn; target ¼
½M0 Mmonomer r þ MCTA ½CTA0
(1)
Where [M]0 is the initial monomer concentration, Mmonomer is the molar mass of the monomer, r is the fractional conversion, [CTA]0 is the initial concentration of macro-RAFT agent, and MCTA is the molar mass of the macro-RAFT agent. Monomer concentration and molar ratio of macro-RAFT agent/ AIBN and BMA/MASi (if BMA is used) were fixed to 1.5 M, 5 and 6, respectively. The typical procedure was used as follows: 10 mL of a solution composed of AIBN (0.053 g, 0.032 mmol), MC2 (0.955 g, 0.16 mmol), MASi (2.997 g, 15.0 mmol) in toluene were charged into a dried twoneck flask along with a magnetic stirrer bar. The solution was then deoxygenated by bubbling argon for 40 min at room temperature. The reaction flask was placed in an oil bath preheated to 70 C. Samples were taken periodically with a degassed syringe. At the end of the reaction, the reaction mixture was cooled to room temperature and precipitated in methanol. The obtained polymer was rinsed with methanol three times and dried under vacuum at 40 C to a constant weight. The molar masses and dispersity values of purified diblock copolymers were determined by TD-SEC. For kinetics, the molar masses and dispersity values of polymer chains were determined by TD-SEC using dn/dc values reported in Supporting information. The monomer conversion was evaluated by 1H NMR comparing the signals of the reactive double bond (5.65 ppm for MASi and 5.6 ppm for BMA) from monomers with the methyl protons eSi(CH3)2e from MASi and PMASi (from 0.34 to 0.15 ppm) and the methylene protons eOeCH2e from BMA and PBMA (from 4.2 to 4.0 ppm). The reactivity ratios of MASi (r1) and BMA (r2) were assessed by stopping the reaction at low conversion values. Five copolymers of MASi and BMA having similar molar ratio values of monomer/ macro-RAFT agent/AIBN of 950/5/1 but different molar fractions of monomers in the feed ranging from 0.2 to 0.8 were prepared. 3. Results and discussion 3.1. Synthesis of the macro-RAFT agents RAFT agent with a carboxylic acid function and PDMSs carrying a hydroxyl group at the chain end were used for the synthesis of the MC1, MC2 and MC3 macro-RAFT agents. There are two conventional esterification pathways of hydroxylated polymers with carboxylic acid compounds with high conversion. The first pathway is the transformation of the carboxylic acid group into an acyl chloride one using oxalyl chloride at ambient temperature and then esterification with hydroxylated polymers [16,18,21]. Pavlovi c et al. [16] reported that unreacted hydroxylated PDMS chains should be removed when the conversion is lower than 100%. The second pathway is the use of DMAP as a catalyst and DCC as a dehydrating agent [20,22e27]. The reaction could be done at room temperature. Most of the published works demonstrated that near 100% of the hydroxyl groups were converted into ester functions [20,22,23,25]. This latter pathway was thus selected to synthesize macro-RAFT agents (Scheme 1). The conversion of the esterification reaction was calculated using 1 H NMR spectrometry (Fig. 1). When the hydroxyl function of PDMS is converted to an ester function, the methylene protons adjacent to the hydroxyl group of the PDMS chain are shifted from approximately 3.73 ppm (H-100 ) to 4.25 ppm (H-10). The conversion of the
41
reaction was then calculated from the integration of these two 1H NMR peaks. If the conversion is 100%, the peak located at 3.73 ppm disappears in the NMR spectrum of the product. The conversion of carboxylic acid function into ester function was also observed using 13 C NMR spectrometry (see Supporting information). 5k and 10k PDMS-OHs were chosen to optimize the conditions of the esterification reaction. At first, molar ratio of reactive agents [OH]:[COOH]:[DCC]:[DMAP] and reaction temperature were selected as 1:1.8:1.8:0.1 and 29 C, respectively. All of the raw materials were introduced together into a two-neck round bottom flask. The flask was evacuated under vacuum overnight with a continuous stirring to remove any trace amounts of water (procedure a) [22]. However, conversions were ranging from 50% to 70%. So, a molar ratio of reactive agents [OH]:[COOH]:[DCC]:[DMAP] of 1:1.5:2:0.2 was used and the results were still not as good as expected (Table 2, run 1). A dropwise addition of both CTA and DCC solutions (procedure b) was carried out to obtain 100% of conversion, except for 10k PDMS-OH (Table 2, run 2). The result of light scattering experiment using a solution of 10k PDMS-OH in DCM at the same concentration and temperature used above shows the existence of aggregates with mean dimensions of 900 nm. If the solution is diluted twice, smaller aggregates with mean dimensions of 350 nm were obtained at 30 C together with a conversion of the reaction of 92%. The formation of aggregates might be considered as the main cause of the low conversion of the esterification reaction of 10k PDMS-OH. In addition, the conversion of this PDMS-OH was 100% when the reaction temperature increased to 40 C (reflux temperature of DCM) (Table 2, run 4). The final yields of MC1, MC2, MC3 were 21%, 63%, and 73%, respectively. The lower yield obtained for MC1 sample was attributed to its solubility in the methanol/ hexane mixture during its purification step. Acetonitrile was used instead of methanol and led to a final yield of 60% for MC1. As raw hydroxylated PDMSs could exhibit a broad molar mass distribution, 1H NMR investigations were also done to estimate the absolute number-average molar mass of the purified macro-RAFT agents. The peak intensity of the dimethylsiloxane repeat units located at w0.08 ppm (H-5) and the peak intensity of the methylene group at 4.25 ppm (H-10) were used to calculate the number of dimethylsiloxane repeat units per chain and therefore the absolute number-average molar mass of the macro-RAFT agents. It was found that the average number of the dimethylsiloxane repeat units (Xn) in MC1 and MC2 was higher than the one estimated in the pristine hydroxylated PDMS. Xn values of 20, 74, and 142 were obtained for MC1, MC2 and MC3 whereas Xn values of 16, 67, and 143 were obtained for the corresponding PDMS precursors, respectively. This means that some lower molar mass PDMS chains were removed during the purification step. The integration of the peak of methylene protons of the RAFT agents at 3.25 ppm (H-14, e CH2eSe) and the integration of the peak at 4.25 ppm (H-10) have similar values, indicating that there was one RAFT agent molecule per PDMS chain and no residual unreacted CTA (Fig. 1). In addition, a white precipitate was appearing in the final product over storage time if the product was not passed through a silica gel column. The formation of a white precipitate was also reported by Wadley and Cavicchi [22] during the synthesis of macro-RAFT agents using 5k and 10k PDMS-OH and 2-(dodecylthiocarbonothioylthio)2-methylpropionic acid in the presence of DCC and DMAP at 60 C. Moreover, the products dried at temperatures above 50 C under vacuum show a color modification from pale to dark. 3.2. Synthesis of PDMS-based diblock copolymers Once the macro RAFT agents are obtained, the MASi polymerization is started. Fig. 2 shows the control of the growth of PMASi
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T.H. Duong et al. / Polymer 55 (2014) 39e47
Scheme 1. Synthetic pathway of macro-RAFT agents and diblock copolymers.
Fig. 1. 1H NMR spectrum of MC2.
T.H. Duong et al. / Polymer 55 (2014) 39e47 Table 2 Synthesis of macro-RAFT agents. Run
1 2 3 4
Temperature ( C)
29 29 38 At reflux
Procedure
a b b b
43
Table 3 Characteristics of PDMS-block-PMASi. Mn (1H-NMR) of MC2 ¼ 6,000 g mol1. Time (h)
15 15 15 15
Conversion (%) MC1
MC2
MC3
e 100 e 100
76 100 e 100
60 77 94 100
chains during the polymerization. The monomer consumption follows the first order kinetic together with a linearly increase of the Mn value with monomer conversion (Fig. 2a). The Ð value decreases rapidly in the early stage of the reaction and stays unchanged in the later stage (Fig. 2b). In addition, it can be seen from Fig. 2a that the chain length of PDMS precursors within the range being used does not seem to influence the polymerization rate. The conversions after 9 h are 79%, 81%, and 79% when using MC1, MC2, MC3, respectively. Table 3 shows the control of the polymerization over a wide range of molar masses using MC2 with Ð values lower than 1.2. On the RI peaks of the TD-SEC it is clear that the molar mass increases monotonically with polymerization time as peaks remain narrow and monomodal (Fig. 3). Due to the high Tg of PMASi (142 C) [4], films based on PDMSblock-PMASi are brittle and cracks appear. So, one way to reduce the Tg value of the second block is to introduce a co-monomer. BMA was chosen for this purpose because of the low Tg value of PBMA (20 C) [28]. In addition, this monomer belongs to the methacrylic family. The BMA/MASi molar ratio was fixed at 6 as mentioned above. The control of the growth of the second block from the macro-RAFT
Diblock copolymer
Conversion (%) (1H NMR)
Mn,target (g mol1)
Mn (NMR) (g mol1)
Mn(TD-Sec) (g mol1)
Ð
MC2M1 MC2M2 MC2M3 MC2M4 MC2M5
98.5 97.2 85.5 91.5 90.2
14,900 24,400 30,500 42,300 50,400
14,500 22,900 28,800 39,350 45,550
18,800 29,000 38,750 46,300 55,400
1.07 1.13 1.12 1.13 1.16
agents was also observed when using BMA as a co-monomer; The Mn value increases linearly with monomer conversion; A low Ð value is obtained all polymerization long (Fig. 4); the RI peak of TDSEC moves to shorter retention times and remains narrow and monomodal (Fig. 5). In addition, Fig. 6 clearly shows that most of the macro-RAFT agent chains were converted into block copolymer chains when using the shorter PDMS precursor and increasing the chain length of the second block. The molar proportion of the residual macro-RAFT agent was estimated lower than 10 mol.% by integrating the respective area of the two peaks for all copolymers. Fig. 7 shows the conversion of the two monomers over RAFT polymerization time. The conversion of MASi is always slightly higher than the one of BMA. Thus, there is a drift in the comonomer composition toward the less reactive monomer as the degree of conversion increases. Reactivity ratios of MASi (r1 ¼ 1.13 0.04) and BMA (r2 ¼ 0.90 0.01) in RAFT polymerization were obtained from the NLLS (non-linear least square) method based on the Mayo-Lewis copolymerization equation using the terminal model of copolymerization [7,29] (See Supporting information). The copolymer produced is slightly richer in MASi than the feed because MASi has a monomer reactivity ratio larger than 1 and BMA lower than 1. The feed becomes richer in BMA with conversion, leading to an increase in the BMA content of the copolymer with conversion. On the other hand, r2 was shown to slightly increase when using AIBN-initiated conventional radical polymerization. This leads to a slight decrease of the drift composition (see drift composition in Supporting information). Fig. 8 shows the 1H NMR spectrum of one PDMS-block-P(MASistat-BMA). The number-average molar mass and the molar compositions of all block copolymers were calculated using Eqn. (2):
Mn ðNMRÞ ¼ NðId MMASi þ 3 Ih MBMA Þ=Ia þ Mn;CTA
Fig. 2. (a) ln([M]0/[M]) vs time of the polymerization of MASi using MC1(:), MC2 (C), MC3(D). (b) Mn (TD-SEC) (D) and Ð (C) vs MASi conversion using MC2. Monomer/ macro-RAFT agent/initiator molar ratio of 475: 5:1. Target number-average molar mass value of PMASi segment of 19,000 g mol1 at 100% conversion. Solid lines are linear fits to the data. The dashed line is corresponding to the theoretical line.
(2)
Fig. 3. SEC traces (RI signal) of PMASi chain growth using MC2. Monomer/macro-RAFT agent/initiator molar ratio of 475: 5:1. Target number-average molar mass value of PMASi segment of 19,000 g mol1 at 100% conversion. The peaks from left to right correspond to a polymerization time of 9 h, 3 h, 1.5 h, and 0 h, respectively.
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Fig. 4. Mn (TD-SEC) (D) and Ð (C) vs monomer (MASi and BMA) conversion using MC2 as macro-RAFT agent. Monomer/macro-RAFT agent/initiator molar ratio of 300: 5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 9,000 g mol1 at 100% conversion. The dashed line is corresponding to the theoretical line. The solid line is a linear fit to the data.
Where N is the dimethylsiloxane unit number within the macroRAFT agent, Ia, Id and Ih represent the intensity of the methyl protons “a” of PDMS, the methyl protons “d” of PMASi, and the methylene protons “h” of PBMA; Mn,CTA is the average-number molar mass of the macro-RAFT agent. N Id/Ia and N 3 Ih/Ia are the numbers of MASi and BMA units in copolymers, respectively. Table 4 shows that the molar mass of polymers is close to those expected and that Ð values are lower than 1.15 indicating that the control of the chain growth was obtained for all of the second blocks over a wide range of molar masses. The synthesized copolymers exhibit trithiocarbonate chain ends. Investigations done on the UV signal of TD-SEC revealed that these chain ends were cleaved after five days of storage in tetrahydrofuran (THF) at the ambient temperature. Similar attempts were previously reported by Gruendling et al. [30] on cumyldithiobenzoate-terminated poly(methyl methacrylate) and polystyrene. The degradation rate was shown to strongly depend on the hydroperoxide-content of the solvent. 3.3. Thermal behavior of PDMS-based diblock copolymers Differential scanning calorimetry (DSC) was used to study the thermal behavior of the diblock copolymers. First, thermograms of PDMS precursors were recorded. Tg values ranging from 120 to 125 C are in good agreement with previously reported values
Fig. 5. SEC traces (RI signal) of P(MASi-stat-BMA) chain growth using MC2. Monomer/ macro-RAFT agent/initiator molar ratio of 300:5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 9,000 g mol1at 100% conversion. The peaks from left to right correspond to a polymerization time of 9 h, 3.5 h, 1.5 h, and 0 h, respectively.
Fig. 6. SEC traces (UV signal) of P(MASi-stat-BMA) using MC1. Monomer/macro- RAFT agent/initiator molar ratio of 630:5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 19,000 g mol1 at 100% conversion. The peaks from left to right correspond to a polymerization time of 68 h and 0 h, respectively.
(Table 5) [31]. While 1k PDMS-OH exhibits no crystallization during the cooling or the heating step, 5k PDMS-OH crystallizes at 71 C and melts at 41 C during the heating scan. 10k PDMS-OH exhibits one peak of crystallization at 86 C during the cooling step, one peak of the so-called cold crystallization at 83 C, and two melting peaks at 47 C and 36 C recorded during the heating step. The cold crystallization corresponds to a crystallization of metastable crystals formed during the cooling step, which finally leads to two melting peaks (Tmelt1 and Tmelt2) [32]. Clarson et al. [33] reported Tgs of 22 methyl end-chain linear poly(dimethylsiloxane)s with molar masses ranging from 160 g mol1 to 25,460 g mol1. According to Clarson’s work, Tg of the 1k PDMS is about 130 C. A variation of 7 C was obtained between this value and our value. This difference may result either from a difference in the cooling rate or in the type of chemical groups at the end of the PDMS chains. Aranguren [34] investigated the effect of the cooling rate (3 C min1, 8 C min1, and ca. 50 C min1) on the Tg values of PDMS with molar masses of 17k, 88k, and 118k. Results showed that Tgs did not change with the cooling rate. To define the effect of the end-groups on the Tg of PDMS, 1k, 5k, and 10k dihydroxyl-terminated poly(dimethylsiloxane)s (HO-PDMS-OH) were used. DSC results summarized in Table 5 show that the Tg values of HO-PDMS-OH are always higher than the corresponding monohydroxylated PDMS. The difference in Tgs between these two types of PDMS increases with decreasing the molar mass of PDMS. It is known that the free
Fig. 7. Ln([M]0/[M]) of MASi (C) and BMA (A) vs time of the copolymerization of MASi and BMA using MC2. Monomer/macro-RAFT agent/initiator molar ratio of 300:5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 9,000 g mol1 at 100% conversion. The solid lines are linear fits to the data.
T.H. Duong et al. / Polymer 55 (2014) 39e47
45
Fig. 8. 1H NMR spectrum of MC2MB1.
volume linked to Tg of polymers is mainly affected by chain end groups when the molar mass of the polymer is relatively low. This free volume depends also on the attractive forces between the macromolecules. The more strongly the macromolecules are bound together, the more thermal energy must be applied to produce motion. The PDMSs used by Clarson et al. [33] exhibited a methyl group at the end of the PDMS chains. When there is a hydroxyl group at the end of PDMS chains, hydrogen bonds could limit the motion of the macromolecules. Therefore, the free volume of hydroxylated PDMS is lower than the one of methylated PDMS leading to higher Tg values. This effect is more noticeable at lower molar masses because of the higher concentration of chain end-groups. The amount of hydroxyl groups seems also to influence the crystallization of PDMS. While 5k PDMS-OH exhibits a cold crystallization, 5k HO-PDMS-OH does not exhibit one. Similarly, 10k PDMS-OH has a crystallization peak during the cooling step, a cold crystallization peak, and two separated melting peaks during the heating step, but the crystallization peak during the cooling step disappears and two melting peaks during heating overlap in the case of 10k HO-PDMS-OH. Furthermore, the cold crystallization of 10k HO-PDMS-OH occurs at a higher temperature. This means that the increase of hydrogen bonds in PDMS system prevents the PDMS chains from arrangement to form crystals. Table 6 shows that diblock copolymers exhibit two glass transition temperatures: one at low temperature (from 127 C to 124 C) and the other one at higher temperature (from 25 to 127 C), corresponding to the PDMS- and P(MASi-stat-BMA)- or PMASi-enriched phases, respectively. For PDMS-block-PMASi copolymers, the two Tgs were close to the Tg of PDMS precursors (from 125 C to 123 C) and of MASi homopolymers. PMASi of molar masses ranging from 12,800 to 52,000 g mol1 exhibit Tg values ranging from 114 C to 138 C (internal data soon published assessed from DSC curves of MASi homopolymers). Tg2 calc. values were estimated using the FoxeFlory equation (Eqn. (3)) [35] with
TgN and K values equal to 143 C and 376,061 respectively. Experimental Tg2 values of PMASi blocks were shown to be close to Tg2 calc. values for all PDMS-block-PMASi copolymers.
Tg ¼ TgN
K Mn
(3)
The existence of two Tgs demonstrated immiscibility between the PDMS and PMASi blocks, and therefore a phase separation in such copolymers. Similar results were recently reported for PMASigraft-PDMS copolymers [5]. In the DSC profiles of PDMS-block-P(MASi-stat-BMA) copolymers, two distinct Tgs were observed (Fig. 9); The lower Tg in the range from 127 C to 123 C is quite similar to the one of the PDMS precursor, and the higher Tg is assigned to the P(MASi-statBMA) block. As expected, the Tg values of P(MASi-stat-BMA) block are lower than the one of the PMASi block of PDMS-block-PMASi copolymers. The Tg value of the P(MASi-stat-BMA) block increases with increasing its molar mass. These values are close to the Tg of P(MASi-stat-BMA) copolymers of molar masses ranging from 11,100 to 49,500 g mol1 (from 34 C to 48 C). Tg2calc. values of P(MASistat-BMA) block were estimated using the Fox equation (Eqn. (4)) [36], in which Tg1 and Tg2 are the Tgs of homopolymers corresponding to MASi and BMA components exhibiting Mn1 and Mn2 values, respectively (Eqn. (3)). TgN and K values for BMA homopolymers were estimated equal to 33 C and 64,804, respectively using a series of PBMA with molar masses ranging from 6600 g mol1 to 42,900 g mol1 (internal data soon published). Mn1 and Mn2 values of MASi and BMA components in the methacrylicbased block were evaluated from 1H NMR (see Eqn. (2)).
1 W W ¼ 1þ 2 Tg Tg1 Tg2
(4)
W1, W2 are the weight fractions of each component.
Table 4 Characteristics of PDMS-block-P(MASi-stat-BMA). Copolymer
Mn of PDMS-CTA (g mol1) NMR
Conversion r (%) MASi
BMA
DMS
BMA
MASi
MC1MB1 MC1MB6 MC2MB1 MC2MB6 MC3MB1 MC3MB6
2,000
97.8 93.8 97 90.4 99.4 97.1
96 84.2 94.2 83 96.4 88.3
22 6 56 21 72 31
67 78 38 67 24 59
11 16 6 12 4 10
6,000 11,100
Molar compositions (%)
Ð
Mn (g mol1) NMR
TD-Sec
Target at r
1.09 1.11 1.12 1.14 1.04 1.1
12,900 48,300 15,000 48,900 19,200 57,600
10,400 42,700 16,800 52,300 23,600 59,700
10,600 45,500 14,500 49,000 19,700 58,700
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T.H. Duong et al. / Polymer 55 (2014) 39e47
Table 5 Thermal properties dimethylsiloxane)s.
of
monohydroxyl-
and
dihydroxyl-terminated
PDMS
Mn (g mol1)
Tg ( C)
Tcryst. ( C)
Tcold ( C)
PDMS-OH
1k 5k 10k 1k 5k 10k
123 124 125 114 122 124
No No 86 No No No
No 71 83 No No 75
HO-PDMS-OH
cryst
poly(-
Tmelt1 ( C)
Tmelt2 ( C)
No No 47 No No 45
No 41 36 No No 38
Unfortunately, differences in Tgs for the P(MASi-stat-BMA) block were noticed between experimental values and calculated values. These differences become smaller when the molar mass of the second block and therefore its weight composition is higher. Strong differences were attributable to the low amount of MASi monomer units within the second block. This result reveals that Eqn. (3) is not usable for PMASi with Mn 2,000 g mol1. In addition, the Tg of the PDMS block was not visible when the wt% of PDMS is lower than 39%. Muppalla et al. [37] reported that the Tg of the PDMS block was not visible in the storage modulus-temperature, tan deltaetemperature curves and also in DSC profiles when the wt% of PDMS in triblock and pentablock copolymers was at ca. 17%. Krause et al. [31] could not see the Tg of PDMS parts in the DSC curves of PDMS-blockPS copolymers containing less than 22 wt % of DMS monomer unit. In the case of PMMA-block-PDMS-block-PMMA, the Tg of the PDMS block was not visible even when the content of PDMS in the triblock copolymers is up to 30 wt% [14]. This last result was due to the
crystallization of PDMS microphases under experimental conditions used. Fig. 9 shows that the crystallization of the PDMS segment in block copolymers differs from the one of PDMS precursors. For example, 10k PDMS-OH crystallizes during the cooling step, but the copolymers based on this PDMS precursor do not crystallize. In addition, the cold crystallization of the diblock copolymer occurs at higher temperatures and there is only one melting peak during the heating step. So, one can conclude that the second block strongly influences the thermal behavior of PDMS as well as end-groups. The presence of the two distinct Tgs on DSC curves demonstrates the immiscibility between the PDMS and P(MASi-stat-BMA) blocks. 4. Conclusions In conclusion, the synthesis of MASi and PDMS-based diblock copolymers was effectively controlled by RAFT polymerization, using PDMS macro-RAFT agents with different molar masses. Diblock copolymers with controlled molar masses and low dispersity were obtained. DSC analyses demonstrated the block-like architecture of the samples and the influence of the methacrylic block on the thermal behavior of PDMS-based diblock copolymers. Since these diblock copolymers exhibit two distinct Tgs on DSC curves, microphase separations can occur between blocks. Their morphologies will be investigated in a future paper. Acknowledgments The authors acknowledge the Vietnamese government for the financial support of this research work. We would like to thank Dr. Gérald Culioli for his help to assign NMR spectra.
Table 6 Thermal properties of diblock copolymers. Diblock copolymer
MC2M1 MC2M2 MC2M3 MC2M4 MC2M5 MC1MB1 MC1MB6 MC2MB1 MC2MB6 MC3MB1 MC3MB6 a b
Mass compositions (%)
Tg (DSC) ( C)
DMS
MASi
BMA
Tg1
Tg2
39 25 19 14 12 12 3 39 11 57 18
61 75 81 86 88 16 21 11 18 8 16
0 0 0 0 0 72 76 50 71 35 66
127 e e e e e e 126 e 124 e
91 111 115 122 127 25 41 29 44 32 46
Tg2 calc. ( C) 98a 120a 126a 132a 133a 10b 45b 8b 42b 21b 43b
Calculated from FoxeFlory equation (Eqn. (3)). Calculated from Fox equation (Eqn. (4)).
Fig. 9. DSC thermograms of 10k PDMS-OH and MC3MB1 diblock copolymer.
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2013.11.034. References [1] Ishitake K, Satoh K, Kamigaito M, Okamoto Y. Stereospecific free radical and RAFT polymerization of bulky silyl methacrylates for tacticity and molecular weight controlled poly(methacrylic acid). Macromolecules 2011;44(23): 9108e17. [2] Bressy C, Margaillan A. Erosion study of poly(trialkylsilyl methacrylate)-based antifouling coatings. Prog Org Coat 2009;66(4):400e5. [3] Bressy C, Margaillan A, Faÿ F, Linossier I, Vallée-Réhel K. Tin-free self-polishing marine antifouling coatings. In: Hellio C, Yebra DM, editors. Advances in marine antifouling coatings and technologies. Woodhead Publishing Limited, CRC Press; 2009. pp. 445e91. [4] Bressy C, NGuyen MN, Tanguy B, Ngo VG, Margaillan A. Poly(trialkylsilyl methacrylate)s: a family of hydrolysable polymers with tuneable erosion profiles. Polym Degrad Stab 2010;95(7):1260e8. [5] Lejars M, Margaillan A, Bressy C. Well-defined graft copolymers of tertbutyldimethylsilyl methacrylate and poly(dimethylsiloxane) macromonomers synthesized by RAFT polymerization. Polym Chem 2013;4(11): 3282e92. [6] Durand P, Margaillan A, Camail M, Vernet JL. Triorganosilyl methacrylates: kinetic studies of radical homopolymerization and radical copolymerization with methyl methacrylate. Polymer 1994;35(20):4392e6. [7] Nguyen MN, Bressy C, Margaillan A. Synthesis of novel random and block copolymers of tert-butyldimethylsilyl methacrylate and methyl methacrylate by RAFT polymerization. Polymer 2009;50(14):3086e94. _ McGrath JE. Polysiloxane containing copolymers: a survey of recent [8] Yilgör I, developments. In: Polysiloxane copolymers/anionic polymerization. Berlin Heidelberg: Springer; 1988. pp. 1e86. [9] Pouget E, Tonnar J, Lucas P, Lacroix-Desmazes P, Ganachaud F, Boutevin B. Well-architectured poly(dimethylsiloxane)-containing copolymers obtained by radical chemistry. Chem Rev 2010;110(3):1233e77. [10] Ninago MD, Satti AJ, Ciolino AE, Vallés EM, Villar MA, Vega DA, et al. Synthesis and morphology of model PS-b-PDMS copolymers. J Polym Sci Part A Polym Chem 2010;48(14):3119e27.
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Well-defined diblock copolymers of poly(tert-butyldimethylsilyl methacrylate) and poly(dimethylsiloxane) synthesized by RAFT polymerization The Hy Duong, Christine Bressy*, André Margaillan Université de Toulon, Laboratoire MAPIEM, EA 4323, 83957 La Garde, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2013 Received in revised form 20 November 2013 Accepted 22 November 2013 Available online 3 December 2013
We report the application of reversible addition-fragmentation chain transfer (RAFT) polymerization using poly(dimethylsiloxane) (PDMS) chain transfer agents toward the synthesis of a variety of diblock copolymers containing tert-butyldimethylsilyl methacrylic (MASi) monomer units. The methodology relies on the synthesis of PDMS monofunctional chain transfer agents easily available in one synthetic step from commercially available hydroxylated PDMSs. The RAFT process enables access to polymer chains with narrow molar mass distributions and high conversions. Data from differential scanning calorimetric measurements revealed that the diblock copolymers exhibited two glass transition temperatures, corresponding to the PDMS- and PMASi-enriched phases, respectively. Copolymerizations of MASi and butyl methacrylate (BMA) within the second block led to immiscible phases with lower glass transition temperatures than PDMS-block-PMASi copolymers. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: RAFT PDMS Tert-butyldimethylsilyl methacrylate
1. Introduction Hydrolyzable tri-alkylsilylester (meth)acrylic monomers have been recently used in the synthesis of (meth)acrylic acid-based polymers with various tacticities [1], and are currently used in marine antifouling coatings as seawater erodible matrixes [2,3]. Depending on the steric hindrance of the alkyl groups, the trialkylsilyl (meth)acrylate-based polymers range from readily hydrolyzable to quite stable polymers in seawater [4]. In our research group, Bressy and coworkers have synthesized tri-alkylsilylesterbased statistical copolymers by conventional radical polymerization and several diblock copolymers using the reversible additionfragmentation chain transfer (RAFT) polymerization for developing erodible coatings [4,5]. Methyl methacrylate (MMA) [4,6,7] and poly(dimethylsiloxane) methacrylate (PDMSMA) [5] were previously used as co-monomers in diblock and graft copolymers, respectively. Poly(dimethylsiloxane) blocks inserted in silylated-based polymers provide access to a wide variety of materials with tunable properties. The fundamental properties of PDMS chains are related to the specific characteristics of the siloxane bonds, i.e. the combination of a flexible backbone and low surface energy side groups.
* Corresponding author. Tel.: þ33 4 94 14 25 80; fax: þ33 4 94 14 24 48. E-mail address: [email protected] (C. Bressy). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.11.034
PDMS with a glass transition temperature Tg ranging from 127 C to 100 C exhibits the most flexible backbone among the common polymers. As a result, it can readily adapt the lowest surface energy configuration through a close packing of the pendant methyl groups at the PDMS/air interface. Then, PDMSs increase hydrophobicity and improve water resistance and thermal stability of many materials. PDMS has been extensively associated with other monomer units in graft and block copolymers [8,9]. PDMS-based block copolymers have been mainly synthesized via anionic polymerization with styrene [10] and 4-vinylpyridine [11]. Atom transfer radical polymerization (ATRP) has been used to prepare block copolymers with styrene [12], methyl methacrylate and N, Ndimethylaminoethyl methacrylate [12e14]. Reversible additionfragmentation chain transfer (RAFT) polymerization has been studied especially to synthesize block copolymers containing PDMS and N,N-dimethylacrylamide (DMA) [15,16], 2-(dimethylamino) ethyl acrylate (DMAEA) [17] and hydroxyethylacrylate (HEA) [18]. This article describes the synthesis of well-defined diblock copolymers from copolymerizations of tert-butyldimethylsilyl methacrylate (MASi) and butyl methacrylate on PDMS macro-RAFT agents. MASi was known to easily polymerize by RAFT polymerization [19] and copolymerized with methacrylic co-monomers [5,7]. Its ability to copolymerize with BMA from PDMS macroRAFT agents is investigated. As these copolymers might be used in marine antifouling coatings, their ability to form films without cracking is required.
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T.H. Duong et al. / Polymer 55 (2014) 39e47
Therefore, the thermal properties of the diblock copolymers are characterized in terms of glass transition temperatures and crystallization temperatures. The effects of the copolymer composition and the PDMS length on the thermal behavior are studied.
633 nm, was used for the scattering experiments which were performed at room temperature, whereas the scattering intensity at 90 was recorded. Prior to the light scattering measurements, the polymer solutions were filtered through a 0.2 mm PTFE syringe filter.
2. Experimental 2.1. Materials Tert-butyldimethylsilyl methacrylate (MASi) was synthesized as described elsewhere [19], distilled under reduced pressure and stored under argon before use. a-hydroxyethylpropoxyl-u-propyl poly(dimethylsiloxane)s (PDMS-OH) of 1000 g mol1 (1k PDMSOH), 5000 g mol1 (5k PDMS-OH), 10,000 g mol1 (10k PDMSOH) and a,u-dihydroxyethylpropoxyl-PDMS (1k HO-PDMS-OH, 5k HO-PDMS-OH and 10k HO-PDMS-OH) (Gelest) were used as received. The molar masses of the PDMSs were furnished by the supplier. 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid (CTA) (Strem Chemicals) was used as received. Dicyclohexylcarbodiimide (DCC), tert-butyl(chloro)dimethylsilane, 4dimethylamino pyridine (DMAP), hexane, methanol, acetonitrile and ethyl acetate were purchased from SigmaeAldrich and used as received. 2,20 -azobis(isobutyronitrile) (AIBN) was purchased from SigmaeAldrich and purified by recrystallization from methanol. Butyl methacrylate (BMA), toluene and dichloromethane (DCM) were purchased from SigmaeAldrich and distilled under reduced pressure to remove inhibitors before use. 2.2. Characterization methods 1
H NMR and 13C NMR measurements were carried out on a Brüker Advance 400 (400 MHz) spectrometer with deuterated chloroform (CDCl3) as solvent at room temperature. The number-average molar mass (Mn) and dispersity (Ð) of polymers were determined by triple detection size exclusion chromatography (TD-SEC). Analyses were performed on a Viscotek apparatus, composed of a GPC Max (comprising a degasser, a pump and an autosampler) with a TDA-302 (RI refractive index detector, right and low angle light scattering detector at 670 nm and viscometer) and an UV detector (l ¼ 298 nm). The following columns were used: a Viscotek HHR-H precolumn and two Viscotek ViscoGel GMHHR-H columns. THF was used as the eluent with a flow rate of 1.0 mL min1 at 30 C. For each precipitated polymer, the refractive index increment (dn/dc) was determined using the OmniSec software, from a solution of known concentration (ca. 10 mg mL1) filtered through a 0.2 mm PTFE filter. Differential scanning calorimetric (DSC) measurements were performed on a DSC Q10 apparatus from TA Instruments calibrated with indium. Polymer samples weighing 15e20 mg were run at equal heating and cooling rates, 10 C min1, under a constant stream of nitrogen. The polymer samples were first scanned from room temperature to 100 C (PDMS-block-P(MASi-stat-BMA)) and from room temperature to 140 C (PDMS-block-PMASi). The samples were then cooled to 165 C. This temperature was held for 5 min to allow the system to attain thermal equilibrium before the second heating scan. The first heating ramp of each sample was discarded for this work. The glass transition temperature (Tg) values were determined as the midpoint between the onset and the end of a step transition using the TA Instruments Universal Analysis 2000 software. The melting and crystallization temperatures were recorded at the maximum value of the endothermic and exothermic peaks, respectively. DCM solutions of the PDMS precursors (28 g dL1) were characterized using dynamic light scattering (DLS). A Zetasizer Nano S (Malvern Instruments), equipped with a diode laser operating at
2.3. Synthesis of the macro-chain transfer agents (macro-RAFT agents) Macro-RAFT agents were synthesized by the esterification of monohydroxyl-terminated poly(dimethylsiloxane)s with a carboxylic acid end-functionalized trithiocarbonate RAFT agent catalyzed by DCC/DMAP. The synthesis of macro-RAFT agents was adapted from the scientific paper of Magenau et al. [20] with some modifications. Molar ratio of reactive agents [OH]:[COOH]: [DCC]:[DMAP] were selected as 1:1.8:1.8:0.1 and 1:1.5:2:0.2 successively. Two reactive agent feed procedures were used. In procedure (a) all of the raw materials were introduced together into a two-neck round bottom flask. The flask was evacuated under vacuum and stirred overnight. In procedure (b) the feed of the raw materials was done as follows: 5.568 g of 5k PDMS-OH (1.1 mmol), 0.029 g of DMAP (0.24 mmol), and 20 mL of distilled methylene chloride were added to a 100 mL two-neck round bottom flask equipped with a magnetic stir bar under a dried argon atmosphere. 0.682 g of CTA (1.69 mmol) in 10 mL distilled methylene chloride and 0.463 g of DCC (2.24 mmol) in 10 mL distilled methylene chloride were introduced in two different 50 mL vessels ([OH]:[COOH]:[DCC]:[DMAP] ¼ 1:1.5:2:0.2). The solutions of CTA and DCC were then added dropwise into the reactor while maintaining the argon flow rate. Then, the reaction mixture was placed in an oil bath preheated at 30 C, 40 C or 60 C for 15 h. The corresponding temperatures of the reaction mixture were 29 C, 38 C and 40 C (at reflux), respectively. The flask was then cooled in the freezer and the reaction mixture was filtered to remove any solid impurities. The solvent was removed under reduced pressure. 50 mL of hexane was added and the mixture was filtered one more time. The resulting solution was diluted with 200 mL of hexane and 100 mL of methanol was added to extract some unreacted CTA and DMAP. These two solvents are rather miscible. Then, this step was repeated three times. The hexane-rich phase containing the macroRAFT agent was added dropwise into methanol for further purification. Because of the dissolution of the 1k PDMS-RAFT agent in the methanol/hexane mixture, acetonitrile was used in place of methanol for this sample. The macro-RAFT agent was dissolved in hexane and washed first with a saturated NaCl solution and then with deionized water. The solution was dried over magnesium sulfate, filtered, and the hexane was eliminated under reduced pressure until a constant weight was reached. Finally, the product was passed through a silica gel column with a mixture of hexane/ ethyl acetate as eluent. A final pale yellow liquid product was obtained by drying under vacuum. The short names of the macroRAFT agents are summarized in Table 1. 2.4. Synthesis of PDMS-based diblock copolymers PDMS-block-PMASi (MCxMy) and PDMS-block-P(MASi-statBMA) (MCxMBy) copolymers with target Mn values were Table 1 Short names of macro-RAFT agents. molar mass of PDMS precursor (1H NMR) (g mol1)
Name of macro-RAFT agent
1,400 5,100 10,800
MC1 MC2 MC3
T.H. Duong et al. / Polymer 55 (2014) 39e47
synthesized using MC1, MC2 and MC3 as macro-RAFT agents. The macro-RAFT agent concentration was estimated from Eqn. (1) for a given target Mn value of the diblock copolymer. Mn,target is defined at 100% of monomer conversion.
Mn; target ¼
½M0 Mmonomer r þ MCTA ½CTA0
(1)
Where [M]0 is the initial monomer concentration, Mmonomer is the molar mass of the monomer, r is the fractional conversion, [CTA]0 is the initial concentration of macro-RAFT agent, and MCTA is the molar mass of the macro-RAFT agent. Monomer concentration and molar ratio of macro-RAFT agent/ AIBN and BMA/MASi (if BMA is used) were fixed to 1.5 M, 5 and 6, respectively. The typical procedure was used as follows: 10 mL of a solution composed of AIBN (0.053 g, 0.032 mmol), MC2 (0.955 g, 0.16 mmol), MASi (2.997 g, 15.0 mmol) in toluene were charged into a dried twoneck flask along with a magnetic stirrer bar. The solution was then deoxygenated by bubbling argon for 40 min at room temperature. The reaction flask was placed in an oil bath preheated to 70 C. Samples were taken periodically with a degassed syringe. At the end of the reaction, the reaction mixture was cooled to room temperature and precipitated in methanol. The obtained polymer was rinsed with methanol three times and dried under vacuum at 40 C to a constant weight. The molar masses and dispersity values of purified diblock copolymers were determined by TD-SEC. For kinetics, the molar masses and dispersity values of polymer chains were determined by TD-SEC using dn/dc values reported in Supporting information. The monomer conversion was evaluated by 1H NMR comparing the signals of the reactive double bond (5.65 ppm for MASi and 5.6 ppm for BMA) from monomers with the methyl protons eSi(CH3)2e from MASi and PMASi (from 0.34 to 0.15 ppm) and the methylene protons eOeCH2e from BMA and PBMA (from 4.2 to 4.0 ppm). The reactivity ratios of MASi (r1) and BMA (r2) were assessed by stopping the reaction at low conversion values. Five copolymers of MASi and BMA having similar molar ratio values of monomer/ macro-RAFT agent/AIBN of 950/5/1 but different molar fractions of monomers in the feed ranging from 0.2 to 0.8 were prepared. 3. Results and discussion 3.1. Synthesis of the macro-RAFT agents RAFT agent with a carboxylic acid function and PDMSs carrying a hydroxyl group at the chain end were used for the synthesis of the MC1, MC2 and MC3 macro-RAFT agents. There are two conventional esterification pathways of hydroxylated polymers with carboxylic acid compounds with high conversion. The first pathway is the transformation of the carboxylic acid group into an acyl chloride one using oxalyl chloride at ambient temperature and then esterification with hydroxylated polymers [16,18,21]. Pavlovi c et al. [16] reported that unreacted hydroxylated PDMS chains should be removed when the conversion is lower than 100%. The second pathway is the use of DMAP as a catalyst and DCC as a dehydrating agent [20,22e27]. The reaction could be done at room temperature. Most of the published works demonstrated that near 100% of the hydroxyl groups were converted into ester functions [20,22,23,25]. This latter pathway was thus selected to synthesize macro-RAFT agents (Scheme 1). The conversion of the esterification reaction was calculated using 1 H NMR spectrometry (Fig. 1). When the hydroxyl function of PDMS is converted to an ester function, the methylene protons adjacent to the hydroxyl group of the PDMS chain are shifted from approximately 3.73 ppm (H-100 ) to 4.25 ppm (H-10). The conversion of the
41
reaction was then calculated from the integration of these two 1H NMR peaks. If the conversion is 100%, the peak located at 3.73 ppm disappears in the NMR spectrum of the product. The conversion of carboxylic acid function into ester function was also observed using 13 C NMR spectrometry (see Supporting information). 5k and 10k PDMS-OHs were chosen to optimize the conditions of the esterification reaction. At first, molar ratio of reactive agents [OH]:[COOH]:[DCC]:[DMAP] and reaction temperature were selected as 1:1.8:1.8:0.1 and 29 C, respectively. All of the raw materials were introduced together into a two-neck round bottom flask. The flask was evacuated under vacuum overnight with a continuous stirring to remove any trace amounts of water (procedure a) [22]. However, conversions were ranging from 50% to 70%. So, a molar ratio of reactive agents [OH]:[COOH]:[DCC]:[DMAP] of 1:1.5:2:0.2 was used and the results were still not as good as expected (Table 2, run 1). A dropwise addition of both CTA and DCC solutions (procedure b) was carried out to obtain 100% of conversion, except for 10k PDMS-OH (Table 2, run 2). The result of light scattering experiment using a solution of 10k PDMS-OH in DCM at the same concentration and temperature used above shows the existence of aggregates with mean dimensions of 900 nm. If the solution is diluted twice, smaller aggregates with mean dimensions of 350 nm were obtained at 30 C together with a conversion of the reaction of 92%. The formation of aggregates might be considered as the main cause of the low conversion of the esterification reaction of 10k PDMS-OH. In addition, the conversion of this PDMS-OH was 100% when the reaction temperature increased to 40 C (reflux temperature of DCM) (Table 2, run 4). The final yields of MC1, MC2, MC3 were 21%, 63%, and 73%, respectively. The lower yield obtained for MC1 sample was attributed to its solubility in the methanol/ hexane mixture during its purification step. Acetonitrile was used instead of methanol and led to a final yield of 60% for MC1. As raw hydroxylated PDMSs could exhibit a broad molar mass distribution, 1H NMR investigations were also done to estimate the absolute number-average molar mass of the purified macro-RAFT agents. The peak intensity of the dimethylsiloxane repeat units located at w0.08 ppm (H-5) and the peak intensity of the methylene group at 4.25 ppm (H-10) were used to calculate the number of dimethylsiloxane repeat units per chain and therefore the absolute number-average molar mass of the macro-RAFT agents. It was found that the average number of the dimethylsiloxane repeat units (Xn) in MC1 and MC2 was higher than the one estimated in the pristine hydroxylated PDMS. Xn values of 20, 74, and 142 were obtained for MC1, MC2 and MC3 whereas Xn values of 16, 67, and 143 were obtained for the corresponding PDMS precursors, respectively. This means that some lower molar mass PDMS chains were removed during the purification step. The integration of the peak of methylene protons of the RAFT agents at 3.25 ppm (H-14, e CH2eSe) and the integration of the peak at 4.25 ppm (H-10) have similar values, indicating that there was one RAFT agent molecule per PDMS chain and no residual unreacted CTA (Fig. 1). In addition, a white precipitate was appearing in the final product over storage time if the product was not passed through a silica gel column. The formation of a white precipitate was also reported by Wadley and Cavicchi [22] during the synthesis of macro-RAFT agents using 5k and 10k PDMS-OH and 2-(dodecylthiocarbonothioylthio)2-methylpropionic acid in the presence of DCC and DMAP at 60 C. Moreover, the products dried at temperatures above 50 C under vacuum show a color modification from pale to dark. 3.2. Synthesis of PDMS-based diblock copolymers Once the macro RAFT agents are obtained, the MASi polymerization is started. Fig. 2 shows the control of the growth of PMASi
42
T.H. Duong et al. / Polymer 55 (2014) 39e47
Scheme 1. Synthetic pathway of macro-RAFT agents and diblock copolymers.
Fig. 1. 1H NMR spectrum of MC2.
T.H. Duong et al. / Polymer 55 (2014) 39e47 Table 2 Synthesis of macro-RAFT agents. Run
1 2 3 4
Temperature ( C)
29 29 38 At reflux
Procedure
a b b b
43
Table 3 Characteristics of PDMS-block-PMASi. Mn (1H-NMR) of MC2 ¼ 6,000 g mol1. Time (h)
15 15 15 15
Conversion (%) MC1
MC2
MC3
e 100 e 100
76 100 e 100
60 77 94 100
chains during the polymerization. The monomer consumption follows the first order kinetic together with a linearly increase of the Mn value with monomer conversion (Fig. 2a). The Ð value decreases rapidly in the early stage of the reaction and stays unchanged in the later stage (Fig. 2b). In addition, it can be seen from Fig. 2a that the chain length of PDMS precursors within the range being used does not seem to influence the polymerization rate. The conversions after 9 h are 79%, 81%, and 79% when using MC1, MC2, MC3, respectively. Table 3 shows the control of the polymerization over a wide range of molar masses using MC2 with Ð values lower than 1.2. On the RI peaks of the TD-SEC it is clear that the molar mass increases monotonically with polymerization time as peaks remain narrow and monomodal (Fig. 3). Due to the high Tg of PMASi (142 C) [4], films based on PDMSblock-PMASi are brittle and cracks appear. So, one way to reduce the Tg value of the second block is to introduce a co-monomer. BMA was chosen for this purpose because of the low Tg value of PBMA (20 C) [28]. In addition, this monomer belongs to the methacrylic family. The BMA/MASi molar ratio was fixed at 6 as mentioned above. The control of the growth of the second block from the macro-RAFT
Diblock copolymer
Conversion (%) (1H NMR)
Mn,target (g mol1)
Mn (NMR) (g mol1)
Mn(TD-Sec) (g mol1)
Ð
MC2M1 MC2M2 MC2M3 MC2M4 MC2M5
98.5 97.2 85.5 91.5 90.2
14,900 24,400 30,500 42,300 50,400
14,500 22,900 28,800 39,350 45,550
18,800 29,000 38,750 46,300 55,400
1.07 1.13 1.12 1.13 1.16
agents was also observed when using BMA as a co-monomer; The Mn value increases linearly with monomer conversion; A low Ð value is obtained all polymerization long (Fig. 4); the RI peak of TDSEC moves to shorter retention times and remains narrow and monomodal (Fig. 5). In addition, Fig. 6 clearly shows that most of the macro-RAFT agent chains were converted into block copolymer chains when using the shorter PDMS precursor and increasing the chain length of the second block. The molar proportion of the residual macro-RAFT agent was estimated lower than 10 mol.% by integrating the respective area of the two peaks for all copolymers. Fig. 7 shows the conversion of the two monomers over RAFT polymerization time. The conversion of MASi is always slightly higher than the one of BMA. Thus, there is a drift in the comonomer composition toward the less reactive monomer as the degree of conversion increases. Reactivity ratios of MASi (r1 ¼ 1.13 0.04) and BMA (r2 ¼ 0.90 0.01) in RAFT polymerization were obtained from the NLLS (non-linear least square) method based on the Mayo-Lewis copolymerization equation using the terminal model of copolymerization [7,29] (See Supporting information). The copolymer produced is slightly richer in MASi than the feed because MASi has a monomer reactivity ratio larger than 1 and BMA lower than 1. The feed becomes richer in BMA with conversion, leading to an increase in the BMA content of the copolymer with conversion. On the other hand, r2 was shown to slightly increase when using AIBN-initiated conventional radical polymerization. This leads to a slight decrease of the drift composition (see drift composition in Supporting information). Fig. 8 shows the 1H NMR spectrum of one PDMS-block-P(MASistat-BMA). The number-average molar mass and the molar compositions of all block copolymers were calculated using Eqn. (2):
Mn ðNMRÞ ¼ NðId MMASi þ 3 Ih MBMA Þ=Ia þ Mn;CTA
Fig. 2. (a) ln([M]0/[M]) vs time of the polymerization of MASi using MC1(:), MC2 (C), MC3(D). (b) Mn (TD-SEC) (D) and Ð (C) vs MASi conversion using MC2. Monomer/ macro-RAFT agent/initiator molar ratio of 475: 5:1. Target number-average molar mass value of PMASi segment of 19,000 g mol1 at 100% conversion. Solid lines are linear fits to the data. The dashed line is corresponding to the theoretical line.
(2)
Fig. 3. SEC traces (RI signal) of PMASi chain growth using MC2. Monomer/macro-RAFT agent/initiator molar ratio of 475: 5:1. Target number-average molar mass value of PMASi segment of 19,000 g mol1 at 100% conversion. The peaks from left to right correspond to a polymerization time of 9 h, 3 h, 1.5 h, and 0 h, respectively.
44
T.H. Duong et al. / Polymer 55 (2014) 39e47
Fig. 4. Mn (TD-SEC) (D) and Ð (C) vs monomer (MASi and BMA) conversion using MC2 as macro-RAFT agent. Monomer/macro-RAFT agent/initiator molar ratio of 300: 5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 9,000 g mol1 at 100% conversion. The dashed line is corresponding to the theoretical line. The solid line is a linear fit to the data.
Where N is the dimethylsiloxane unit number within the macroRAFT agent, Ia, Id and Ih represent the intensity of the methyl protons “a” of PDMS, the methyl protons “d” of PMASi, and the methylene protons “h” of PBMA; Mn,CTA is the average-number molar mass of the macro-RAFT agent. N Id/Ia and N 3 Ih/Ia are the numbers of MASi and BMA units in copolymers, respectively. Table 4 shows that the molar mass of polymers is close to those expected and that Ð values are lower than 1.15 indicating that the control of the chain growth was obtained for all of the second blocks over a wide range of molar masses. The synthesized copolymers exhibit trithiocarbonate chain ends. Investigations done on the UV signal of TD-SEC revealed that these chain ends were cleaved after five days of storage in tetrahydrofuran (THF) at the ambient temperature. Similar attempts were previously reported by Gruendling et al. [30] on cumyldithiobenzoate-terminated poly(methyl methacrylate) and polystyrene. The degradation rate was shown to strongly depend on the hydroperoxide-content of the solvent. 3.3. Thermal behavior of PDMS-based diblock copolymers Differential scanning calorimetry (DSC) was used to study the thermal behavior of the diblock copolymers. First, thermograms of PDMS precursors were recorded. Tg values ranging from 120 to 125 C are in good agreement with previously reported values
Fig. 5. SEC traces (RI signal) of P(MASi-stat-BMA) chain growth using MC2. Monomer/ macro-RAFT agent/initiator molar ratio of 300:5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 9,000 g mol1at 100% conversion. The peaks from left to right correspond to a polymerization time of 9 h, 3.5 h, 1.5 h, and 0 h, respectively.
Fig. 6. SEC traces (UV signal) of P(MASi-stat-BMA) using MC1. Monomer/macro- RAFT agent/initiator molar ratio of 630:5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 19,000 g mol1 at 100% conversion. The peaks from left to right correspond to a polymerization time of 68 h and 0 h, respectively.
(Table 5) [31]. While 1k PDMS-OH exhibits no crystallization during the cooling or the heating step, 5k PDMS-OH crystallizes at 71 C and melts at 41 C during the heating scan. 10k PDMS-OH exhibits one peak of crystallization at 86 C during the cooling step, one peak of the so-called cold crystallization at 83 C, and two melting peaks at 47 C and 36 C recorded during the heating step. The cold crystallization corresponds to a crystallization of metastable crystals formed during the cooling step, which finally leads to two melting peaks (Tmelt1 and Tmelt2) [32]. Clarson et al. [33] reported Tgs of 22 methyl end-chain linear poly(dimethylsiloxane)s with molar masses ranging from 160 g mol1 to 25,460 g mol1. According to Clarson’s work, Tg of the 1k PDMS is about 130 C. A variation of 7 C was obtained between this value and our value. This difference may result either from a difference in the cooling rate or in the type of chemical groups at the end of the PDMS chains. Aranguren [34] investigated the effect of the cooling rate (3 C min1, 8 C min1, and ca. 50 C min1) on the Tg values of PDMS with molar masses of 17k, 88k, and 118k. Results showed that Tgs did not change with the cooling rate. To define the effect of the end-groups on the Tg of PDMS, 1k, 5k, and 10k dihydroxyl-terminated poly(dimethylsiloxane)s (HO-PDMS-OH) were used. DSC results summarized in Table 5 show that the Tg values of HO-PDMS-OH are always higher than the corresponding monohydroxylated PDMS. The difference in Tgs between these two types of PDMS increases with decreasing the molar mass of PDMS. It is known that the free
Fig. 7. Ln([M]0/[M]) of MASi (C) and BMA (A) vs time of the copolymerization of MASi and BMA using MC2. Monomer/macro-RAFT agent/initiator molar ratio of 300:5:1. Target number-average molar mass value of P(MASi-stat-BMA) segment of 9,000 g mol1 at 100% conversion. The solid lines are linear fits to the data.
T.H. Duong et al. / Polymer 55 (2014) 39e47
45
Fig. 8. 1H NMR spectrum of MC2MB1.
volume linked to Tg of polymers is mainly affected by chain end groups when the molar mass of the polymer is relatively low. This free volume depends also on the attractive forces between the macromolecules. The more strongly the macromolecules are bound together, the more thermal energy must be applied to produce motion. The PDMSs used by Clarson et al. [33] exhibited a methyl group at the end of the PDMS chains. When there is a hydroxyl group at the end of PDMS chains, hydrogen bonds could limit the motion of the macromolecules. Therefore, the free volume of hydroxylated PDMS is lower than the one of methylated PDMS leading to higher Tg values. This effect is more noticeable at lower molar masses because of the higher concentration of chain end-groups. The amount of hydroxyl groups seems also to influence the crystallization of PDMS. While 5k PDMS-OH exhibits a cold crystallization, 5k HO-PDMS-OH does not exhibit one. Similarly, 10k PDMS-OH has a crystallization peak during the cooling step, a cold crystallization peak, and two separated melting peaks during the heating step, but the crystallization peak during the cooling step disappears and two melting peaks during heating overlap in the case of 10k HO-PDMS-OH. Furthermore, the cold crystallization of 10k HO-PDMS-OH occurs at a higher temperature. This means that the increase of hydrogen bonds in PDMS system prevents the PDMS chains from arrangement to form crystals. Table 6 shows that diblock copolymers exhibit two glass transition temperatures: one at low temperature (from 127 C to 124 C) and the other one at higher temperature (from 25 to 127 C), corresponding to the PDMS- and P(MASi-stat-BMA)- or PMASi-enriched phases, respectively. For PDMS-block-PMASi copolymers, the two Tgs were close to the Tg of PDMS precursors (from 125 C to 123 C) and of MASi homopolymers. PMASi of molar masses ranging from 12,800 to 52,000 g mol1 exhibit Tg values ranging from 114 C to 138 C (internal data soon published assessed from DSC curves of MASi homopolymers). Tg2 calc. values were estimated using the FoxeFlory equation (Eqn. (3)) [35] with
TgN and K values equal to 143 C and 376,061 respectively. Experimental Tg2 values of PMASi blocks were shown to be close to Tg2 calc. values for all PDMS-block-PMASi copolymers.
Tg ¼ TgN
K Mn
(3)
The existence of two Tgs demonstrated immiscibility between the PDMS and PMASi blocks, and therefore a phase separation in such copolymers. Similar results were recently reported for PMASigraft-PDMS copolymers [5]. In the DSC profiles of PDMS-block-P(MASi-stat-BMA) copolymers, two distinct Tgs were observed (Fig. 9); The lower Tg in the range from 127 C to 123 C is quite similar to the one of the PDMS precursor, and the higher Tg is assigned to the P(MASi-statBMA) block. As expected, the Tg values of P(MASi-stat-BMA) block are lower than the one of the PMASi block of PDMS-block-PMASi copolymers. The Tg value of the P(MASi-stat-BMA) block increases with increasing its molar mass. These values are close to the Tg of P(MASi-stat-BMA) copolymers of molar masses ranging from 11,100 to 49,500 g mol1 (from 34 C to 48 C). Tg2calc. values of P(MASistat-BMA) block were estimated using the Fox equation (Eqn. (4)) [36], in which Tg1 and Tg2 are the Tgs of homopolymers corresponding to MASi and BMA components exhibiting Mn1 and Mn2 values, respectively (Eqn. (3)). TgN and K values for BMA homopolymers were estimated equal to 33 C and 64,804, respectively using a series of PBMA with molar masses ranging from 6600 g mol1 to 42,900 g mol1 (internal data soon published). Mn1 and Mn2 values of MASi and BMA components in the methacrylicbased block were evaluated from 1H NMR (see Eqn. (2)).
1 W W ¼ 1þ 2 Tg Tg1 Tg2
(4)
W1, W2 are the weight fractions of each component.
Table 4 Characteristics of PDMS-block-P(MASi-stat-BMA). Copolymer
Mn of PDMS-CTA (g mol1) NMR
Conversion r (%) MASi
BMA
DMS
BMA
MASi
MC1MB1 MC1MB6 MC2MB1 MC2MB6 MC3MB1 MC3MB6
2,000
97.8 93.8 97 90.4 99.4 97.1
96 84.2 94.2 83 96.4 88.3
22 6 56 21 72 31
67 78 38 67 24 59
11 16 6 12 4 10
6,000 11,100
Molar compositions (%)
Ð
Mn (g mol1) NMR
TD-Sec
Target at r
1.09 1.11 1.12 1.14 1.04 1.1
12,900 48,300 15,000 48,900 19,200 57,600
10,400 42,700 16,800 52,300 23,600 59,700
10,600 45,500 14,500 49,000 19,700 58,700
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T.H. Duong et al. / Polymer 55 (2014) 39e47
Table 5 Thermal properties dimethylsiloxane)s.
of
monohydroxyl-
and
dihydroxyl-terminated
PDMS
Mn (g mol1)
Tg ( C)
Tcryst. ( C)
Tcold ( C)
PDMS-OH
1k 5k 10k 1k 5k 10k
123 124 125 114 122 124
No No 86 No No No
No 71 83 No No 75
HO-PDMS-OH
cryst
poly(-
Tmelt1 ( C)
Tmelt2 ( C)
No No 47 No No 45
No 41 36 No No 38
Unfortunately, differences in Tgs for the P(MASi-stat-BMA) block were noticed between experimental values and calculated values. These differences become smaller when the molar mass of the second block and therefore its weight composition is higher. Strong differences were attributable to the low amount of MASi monomer units within the second block. This result reveals that Eqn. (3) is not usable for PMASi with Mn 2,000 g mol1. In addition, the Tg of the PDMS block was not visible when the wt% of PDMS is lower than 39%. Muppalla et al. [37] reported that the Tg of the PDMS block was not visible in the storage modulus-temperature, tan deltaetemperature curves and also in DSC profiles when the wt% of PDMS in triblock and pentablock copolymers was at ca. 17%. Krause et al. [31] could not see the Tg of PDMS parts in the DSC curves of PDMS-blockPS copolymers containing less than 22 wt % of DMS monomer unit. In the case of PMMA-block-PDMS-block-PMMA, the Tg of the PDMS block was not visible even when the content of PDMS in the triblock copolymers is up to 30 wt% [14]. This last result was due to the
crystallization of PDMS microphases under experimental conditions used. Fig. 9 shows that the crystallization of the PDMS segment in block copolymers differs from the one of PDMS precursors. For example, 10k PDMS-OH crystallizes during the cooling step, but the copolymers based on this PDMS precursor do not crystallize. In addition, the cold crystallization of the diblock copolymer occurs at higher temperatures and there is only one melting peak during the heating step. So, one can conclude that the second block strongly influences the thermal behavior of PDMS as well as end-groups. The presence of the two distinct Tgs on DSC curves demonstrates the immiscibility between the PDMS and P(MASi-stat-BMA) blocks. 4. Conclusions In conclusion, the synthesis of MASi and PDMS-based diblock copolymers was effectively controlled by RAFT polymerization, using PDMS macro-RAFT agents with different molar masses. Diblock copolymers with controlled molar masses and low dispersity were obtained. DSC analyses demonstrated the block-like architecture of the samples and the influence of the methacrylic block on the thermal behavior of PDMS-based diblock copolymers. Since these diblock copolymers exhibit two distinct Tgs on DSC curves, microphase separations can occur between blocks. Their morphologies will be investigated in a future paper. Acknowledgments The authors acknowledge the Vietnamese government for the financial support of this research work. We would like to thank Dr. Gérald Culioli for his help to assign NMR spectra.
Table 6 Thermal properties of diblock copolymers. Diblock copolymer
MC2M1 MC2M2 MC2M3 MC2M4 MC2M5 MC1MB1 MC1MB6 MC2MB1 MC2MB6 MC3MB1 MC3MB6 a b
Mass compositions (%)
Tg (DSC) ( C)
DMS
MASi
BMA
Tg1
Tg2
39 25 19 14 12 12 3 39 11 57 18
61 75 81 86 88 16 21 11 18 8 16
0 0 0 0 0 72 76 50 71 35 66
127 e e e e e e 126 e 124 e
91 111 115 122 127 25 41 29 44 32 46
Tg2 calc. ( C) 98a 120a 126a 132a 133a 10b 45b 8b 42b 21b 43b
Calculated from FoxeFlory equation (Eqn. (3)). Calculated from Fox equation (Eqn. (4)).
Fig. 9. DSC thermograms of 10k PDMS-OH and MC3MB1 diblock copolymer.
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