Straightforward synthesis of poly(lauryl acrylate)-b-poly(stearyl acrylate) diblock copolymers by ATRP

European Polymer Journal 47 (2011) 343–351

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Straightforward synthesis of poly(lauryl acrylate)-b-poly(stearyl acrylate) diblock copolymers by ATRP Fabien Dutertre a, Pierre-Yves Pennarun b, Olivier Colombani a,⇑, Erwan Nicol a,⇑ a b

Université du Maine – Polymères, Colloïdes, Interfaces – UMR CNRS 6120, 3ème étage, Bâtiment Chimie, Avenue Olivier Messiaen, 72085 Le Mans Cédex 09, France 915 Route de Moundas, 31600 Lamasquère, France

a r t i c l e

i n f o

Article history: Received 22 September 2010 Received in revised form 23 November 2010 Accepted 9 December 2010 Available online 15 December 2010 Keywords: Lauryl acrylate/dodecyl acrylate Stearyl acrylate/octadecyl acrylate Atom transfer radical polymerization (ATRP) Block copolymer Semi-crystalline polymer Thermoplastic elastomer

a b s t r a c t Lauryl (LA) and stearyl (SA) acrylates were successfully polymerized by atom transfer radical polymerization (ATRP), leading to well defined homopolymers and diblock copolymers (PDI < 1.2). Interestingly, the polymerization was very well controlled using N,N,N0 ,N00 ,N00 pentamethyldiethylenetriamine (PMDETA), a ligand which had initially been reported to be unadvisable for the polymerization of such monomers. Both kinetic studies and chain extension reactions supported our conclusions. A PLA65-b-PSA47 diblock copolymer was characterized by differential scanning calorimetry and dynamic thermo-mechanical analysis, revealing that both blocks exhibit side-chain crystallinity and phase segregate in the crystalline state. The diblock behaves as a brittle rigid polymer when both blocks are crystalline, as a ductile material after the melting of the PLA phase and becomes a viscous liquid when both blocks are molten. This work could be extended to the preparation of PSA-b-PLA-b-PSA bio-issued thermoplastic elastomers. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The synthesis and application of polymers based on lauryl or stearyl (meth)acrylates have been the object of growing interest since their preparation by controlled radical polymerizations, such as atom transfer radical polymerization (ATRP) [1,2], nitroxide mediated polymerization (NMP) [3] and radical addition fragmentation transfer polymerization (RAFT) [4], was reported. Their monomers can indeed be derived from fatty alcohols which are renewable resources [2,5]. Moreover, due to the long alkyl side chains borne by the monomer units, these polymers are highly hydrophobic and exhibit a low glass transition temperature (Tg). Finally, poly(lauryl acrylates) and poly(stearyl (meth)acrylates) are semi-crystalline polymers because their side chains may crystallize in spite of an amorphous backbone [6]. Owing to these properties, block copolymers consisting of lauryl or stearyl ⇑ Corresponding authors. E-mail addresses: [email protected] (O. Colombani), [email protected] (E. Nicol). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.12.003

(meth)acrylate blocks may be used as flow depressants in organic oils [7], self-assembling amphiphilic polymers [8] or particle stabilizers [9,10] for example. A few articles showed that poly(lauryl methacrylate) (PLMA) based triblock copolymers exhibit interesting thermoplastic elastomer behavior [11,12]. In these copolymers; the PLMA block acted as a soft amorphous block (Tg  60 °C), while the poly(methyl methacrylate) or poly(tert-butyl methacrylate) ones acted as the rigid glassy blocks at room temperature. Investigating the thermo-mechanical behavior of block copolymers based on long alkyl side chain poly(alkyl acrylates) that are known to have a higher capacity to crystallize than their poly(alkyl methacrylate) homologues would be even more interesting. In this case, the mechanical properties are indeed expected to come from crystalline domains rather than from glassy ones. Beers and Matyjaszewski [1] were the first to report the successful polymerization of lauryl acrylate (LA) by ATRP, using CuBr/4,40 -di(5-nonyl)-2,20 -bipyridyne (CuBr/dNbpy) as catalytic complex in almost bulk conditions (5 vol% toluene). They also tried to use N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA) as ligand instead of dNbpy,

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but the control of the polymerization in acetone was poor because the polymer eventually precipitated in this solvent [1]. They concluded that PMDETA was not a suitable ligand for the polymerization of long chain alkyl (meth)acrylates because their polymers were not soluble in polar media, whereas PMDETA was not able to dissolve completely the catalytic complex in a non polar medium. Similarly, Biedron and Kubisa [13] reported that the polymerization of LA in ionic liquids using CuBr/PMDETA led to a heterogeneous reaction medium and poor control of the polymerization. Since this first article by Beers and Matyjaszewski [1], very few authors attempted the polymerization of lauryl or stearyl (meth)acrylates by ATRP using the CuBr/PMDETA catalytic complex. On one hand, Chatterjee and Mandal [12,14] developed an interesting system combining the PMDETA ligand with a highly hydrophobic quaternary ammonium (tricaprylylmethylammonium chloride, AliquatÒ336), both commercially available. The quaternary ammonium afforded a perfect solubility of the catalytic complex even in pure LMA or SMA at 35 °C and enabled the bulk polymerization of these monomers with a very good control. On the other hand, Raghunadh et al. [15] compared several ligands for the polymerization of LMA at 95 °C in toluene with CuBr catalyst, including PMDETA. They showed that PMDETA was more suitable than 2,20 bipyridyl (bpy) or 4,40 -dimethyl-2,20 -bipyridyl (dMebpy), since bpy and dMebpy led to heterogeneous catalytic complexes and polymers with broad molecular weight distributions. However, the authors concluded that N-(npropyl)-2-pyridylmethanimine (PPMI) was more suitable than PMDETA, because the polymerization with PMDETA occurred with significant termination reactions. Following Beers and Matyjaszewski first report [1], dNbpy was preferred to PMDETA as ligand and used for the polymerization of lauryl methacrylate [16], stearyl acrylate [17] or stearyl methacrylate [10,17] either in bulk or in non polar solvents. However, dNbpy is not commercially available and difficult to prepare [18]. Other ligands, not commercially available but easier to synthesize and still able to dissolve efficiently CuBr in non polar medium, were proposed. N-(n-Octyl)-2-pyridylmethanimine (OPMI) allowed a good control of the polymerization of LA in xylene [19] and of SMA either in bulk [20] or in toluene [9]. Similarly, the ATRP polymerization of LMA was well controlled in toluene at 95 °C with CuBr/N-(n-propyl)-2pyridylmethanimine (CuBr/PPMI) [15]. PPMI was however not sufficiently hydrophobic to allow a good control of the ATRP of SMA either in bulk or in the presence of xylene or dimethylformamide (DMF) [20]. Herein, we report that CuBr/PMDETA actually allows the straightforward preparation of well defined homopolymers of poly(lauryl acrylate) and poly(stearyl acrylate) without any other additive than a moderately polar solvent (anisole) and although the deactivator complex CuBr2/ PMDETA is not fully soluble in this reaction medium. PMDETA should thus not be systematically discarded for the ATRP of LA or SA contrarily to what seems to be taken for granted in the literature. Moreover, PLA-b-PSA diblock copolymers were prepared with an initiator efficiency close to 100%. To the best of our knowledge, it is the first

time that PLA-b-PSA diblock copolymers are prepared by controlled radical polymerization, although PLA-b-PSMA diblock copolymers were already prepared by NMP and used as low temperature flow modifiers in lubricating oils [7]. The thermal behavior and thermo-mechanical properties of a symmetric PLA-b-PSA diblock copolymer are presented, showing that both blocks crystallize independently. This latter phenomenon could be used advantageously for the elaboration of PSA-b-PLA-b-PSA triblock copolymers which are potential bio-issued thermoplastic elastomers. 2. Material and methods 2.1. Reagents Lauryl acrylate (technical grade, Aldrich P90%) was eluted on a SiO2 column (63–200 lm, Fluka, chromatography grade) to remove the 4-methoxyphenol inhibitor it contains. Stearyl acrylate (Aldrich, 97%, free of inhibitor), copper I bromide (CuBr, Aldrich, 99.999%), N,N,N0 ,N0 ,N00 -pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), methyl-2bromopropionate (MBP, Aldrich, 98%), copper II bromide (CuBr2, Acros Organics, 99%), anisole (Sigma–Aldrich, 99%), methanol (Fisher Scientific, 99.99%), dichloromethane (Fisher Scientific, 99.99%), cyclohexane (Sigma–Aldrich, 99%), chloroform (Fischer Scientific, 99.99%) and tetrahydrofurane (THF, Fisher Scientific, 99.99%) were used as received. 2.2. Syntheses of homopolymers 2.2.1. PLA39 homopolymer Typically, for the synthesis of PLA39, a single-neck 50 mL flask was filled with CuBr (0.123 g, 8.57  104 mol). After introduction of a magnetic stirrer, the flask was closed with a screw-cap equipped with a septum and argon was flushed through the septum for 10 min in order to remove dioxygen. In a second flask, CuBr2 (0.0102 g, 4.57  105 mol), MBP (0.163 g, 9.76  104 mol), PMDETA (0.175 g, 1.01  103 mol), anisole (15.62 g) were stirred together, yielding a green homogeneous solution. LA (12.292 g, 5.11  102 mol) was finally added, which triggered the precipitation of part of the CuBr2 complex: dark green particles were precipitated, but the solution remained green. The flask was closed with a screw-cap equipped with a septum and degassed by argon bubbling for 10 min. A small part of the solution (0.1 mL) was taken as sample t0 and the rest was transferred with a canula under argon (while stirring) into the first flask containing degassed Cu(I)Br. After complete dissolution of the Cu(I)Br/PMDETA complex, the flask was dipped in an oil bath at 60 °C and samples were withdrawn with argon-degassed syringes throughout the reaction in order to follow the kinetic. The samples were cooled immediately in ice/ water for several seconds and opened to air to stop the reaction. Note that although the CuBr/PMDETA complex was fully soluble in these conditions, part of the CuBr2/ PMDETA complex remained insoluble even at 60 °C. At about 70% conversion, the reaction mixture was cooled

F. Dutertre et al. / European Polymer Journal 47 (2011) 343–351

down in ice/water, opened to air, diluted with dichloromethane and passed through a SiO2 column rinsed with dichloromethane (150 mL) to remove copper salts. After elimination of most of the solvents by evaporation under reduced pressure, the polymer was precipitated in methanol and rinsed several times with this non solvent. As 1H NMR revealed the presence of residual LA monomer, the polymer was dissolved again in dichloromethane and precipitated in a methanol/cyclohexane mixture (90/10 vol/ vol). Then, it was rinsed with the same mixture several times and finally dried at 40 °C under vacuum to constant weight, ultimately yielding 4.0 g of a viscous colorless liquid (yield = 47% taking into account the conversion). The low yield may be partly explained by the fact that some of the polymer was lost in the SiO2 column. It was indeed checked that little polymer was lost during the precipitation in the methanol/cyclohexane mixture (yield of the second precipitation = 92%). Although the yields were not fully optimized, they were improved in the case of PLA65 and PLA65-b-PSA47 by rinsing the SiO2 column respectively with more dichloromethane or with chloroform. 2.2.2. PLA65 homopolymer The reaction was conducted in the same way as for PLA39. However, this reaction had to be stopped exactly at 50% conversion to reach the desired degree of polymerization. As this required 11 h of reaction, the reaction mixture was stored overnight at 18 °C (without opening it to air) after having reached 35% conversion and allowed to proceed at 60 °C the day after. According to gas chromatography, the conversion increased by about 3% during the night, but the reaction was not disturbed further by being stored overnight at low temperature. Purification was conducted as for PLA39 except that the SiO2 column was rinsed with more dichloromethane (300 mL), increasing the yield to 59%. 2.2.3. PSA32 homopolymer The reaction was conducted in the same way as for PLA and stopped at 65% conversion. Purification was conducted as for PLA39, rinsing the SiO2 column with 150 mL of dichloromethane. Two precipitations were also necessary: the first in methanol was absolutely not efficient to remove the SA residual monomer and should be suppressed, whereas the second in a methanol/cyclohexane mixture (90/10 vol/vol) proved efficient. The yield was satisfactory in this case (71%), indicating that PSA-based polymers may adsorb less on the SiO2 column and precipitate more easily in methanol because of their lower polarity compared to PLA ones. 2.3. Synthesis of the PLA-b-PSA diblocks Diblock copolymers were prepared by polymerizing SA starting from a PLA macroinitiator in the same conditions as those described for the homopolymer PSA32. PLA39-bPSA181 was not purified. PLA65-b-PSA47 was purified similarly to PLA65, except that the SiO2 column was rinsed with 300 mL chloroform instead of dichloromethane. One precipitation in methanol/cyclohexane (90/10 vol/vol) proved efficient. The final yield was 78%.

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2.4. Estimation of the conversion using gas chromatography One drop of each kinetic sample was diluted in 2 mL of chloroform and injected in gas chromatography in order to determine the conversion using anisole as internal standard. A GC-2014 apparatus from Shimadzu equipped with an AOC-20i auto-injector and an Equity-1 capillary column (length: 30 m, 0.25 mm int. diam., film thickness 0.25 lm) was used. Injector and detector temperatures were set at 250 °C. For LA, the heating ramp consisted of three plateaus: 60 °C for 1 min, 150 °C for 1 min (heating from 60 to 150 °C at 60 °C/min) and 240 °C for 2.75 min (heating from 150 to 240 °C at 40 °C/min). For SA, the three plateaus were: 60 °C for 1 min, 150 °C for 0.5 min and 240 °C for 9.75 min with the same heating rates between each plateau. See Ref. [21] for further details about the determination of the conversion and precautions which should be taken. 2.5. Determination of the molecular weight The kinetic samples were finally characterized by SEC in THF (flow rate = 1 mL/min) after removal of the copper complex by flash chromatography on a small SiO2 column rinsed with chloroform. Importantly, for the kinetic samples, no other purification than flash chromatography, and in particular no precipitation, was conducted before SEC analysis in order to be sure that the results faithfully represent the characteristics of the polymer in each sample. The home-made SEC apparatus was equipped with UV and RI detectors. Homopolymers were eluted on JORDI-gel columns (500 Å pore size, 5 lm particle size, 50 cm length), whereas diblock copolymers were eluted on PL-gel Mixed C (mixed pore dimensions, 5 mm, 60 cm) ones. For comparison of the diblock copolymers with their macroinitiator and correct determination of the molecular weights of PLA39 and PLA65 after precipitation, the latter were also analyzed on the PL-gel Mixed C columns. Polystyrene-equivalent molecular weights were first determined from the RI signal using polystyrene (PS) standards for calibration. The JORDI-gel columns were calibrated with 5 PS standards ranging from 700 to 43 000 g/ mol, leading to the following equation of the calibration curve: log M ¼ 14:52  1:158  V e þ 3:976  102  V 2e  5:017  104  V 3e . The PL-gel Mixed C columns were calibrated with 9 PS standards ranging from 700 to 870,000 g/mol, leading to the following equation of the calibration curve: log M = 1.070  4.359  Ve. Moreover, the principle of universal calibration [22] was used to calculate true molecular weights rather than PS-equivalent ones from the SEC data according to Eq. (1) [23] and knowing the Mark–Houwink–Kuhn–Sakurada (MHKS) parameters of PS [24] and PLA [24]. Using the MHKS parameters of PSA would have been better to determine true molecular weights for the PSA samples, but they were not found in the literature. However, SEC-experimental molecular weights were in fair agreement with theoretical ones for PSA after correction using the MHKS parameters of PLA, indicating that the PSA parameters are probably not too different from those of PLA. The MHKS

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Table 1 Characteristics of the polymers prepared by ATRP in anisole at 60 °C using CuBr/PMDETA as catalytic complex. Polymera

Theoretical Mn (g/mol)b

PSA32 PLA39-b-PSA181 PLA65-b-PSA47

Mn RMN 1H (g/mol)c

Mn SEC (g/mol)d

Dispersity SECd

First blocke

Diblock

First blocke

Diblock

First blocke

Diblock

First blocke

Diblock

11,000 8900 15,600

– 72,000 32,000

12,500 11,000 14,600

– n.a.f 28,000

10,500 9600g 15,900g

– 68,000 31,000

1.17 1.14 1.16

– 1.16 1.15

a

The indexes correspond to the DP of each block determined by universal SEC (using the MHKS parameters of PS and PLA for all corrections). Calculated Mn based on the conversion of each monomer determined by GC and using the molecular weights of the MBP initiator (167 g/mol) or of the 1st block (determined by universal SEC), and of the LA (240.4 g/mol) and SA (324.6 g/mol) units. c Calculated from the DP of each block determined, after precipitation of the polymers, by 1H NMR (see text). d Determined by SEC in THF using the principle of universal calibration and using the MHKS parameters of PLA. e For the diblocks, PLA was polymerized first and used as macroinitiator for the polymerization of the PSA block. f This diblock was not purified and consequently not analyzed by 1H NMR. g The molecular weights given here were obtained after precipitation of the polymers. They slightly differ from values obtained before precipitation (Mn = 8400 g/mol for PLA39 and Mn = 14,900 g/mol for PLA65), indicating that the polymers could have been slightly fractionated during their purification. b

parameters of PLA were thus used in order to correct the PS-equivalent molecular weights of PLA, PSA and PLA-bPSA using the principle of universal calibration.

logðM PLA Þ ¼

  1 K PS aPS þ 1 þ log logðM PS Þ aPLA þ 1 K PLA aPLA þ 1

of the sample volume. A deformation sweep experiment was lead at the frequency of 1 rad s1, prior to temperature sweep measurements, to determine the linear domain of the material.

ð1Þ

Eq. (1). Determination of the ‘‘true’’ molecular weight MPLA from the PS-equivalent molecular weight MPS, using the Mark–Houwink–Kuhn–Sakurada (MHKS) parameters of the PS standards (KPS = 11.4  105 dL g1 and aPS = 0.716) and of PLA (KPLA = 29.2  105 dL g1 and aPLA = 0.585) given in Ref. [24]. 2.6. 1H NMR spectroscopy 1 H NMR spectra were recorded on a Brücker 400 MHz spectrometer in CDCl3. Tetramethylsilane was used as standard for the calibration of the chemical shifts (d = 0 ppm).

2.7. Thermal and thermo-mechanical characterization 2.7.1. Differential scanning calorimetry Heats of crystallization/melting and transition temperatures were measured on a TA Instrument modulated 2920 DSC in normal mode. Temperature ramps from 1 to 10 °C/ min were tested. Transition temperatures were determined at the extremum of the thermal transitions. Crystallinities v were determined as the ratio between the heat of transition of the sample determined by DSC and the heat of transition for a 100% crystalline polymer. This latter reference was reported by Jordan et al. [6] to be DHref(PLA) = 215 J/g for the PLA and DHref(PSA) = 218 J/g for the PSA. For the PLA65-b-PSA47 diblock copolymer, crystallinities of each block were determined taking into account the weight fraction of the block. 2.7.2. Dynamic thermo-mechanical analysis Dynamic shear measurements were made on a Rheometrics RDA II dynamic rheometer using parallel-plate geometry (diameter 8 mm) at temperatures between 25 °C and +50 °C. The so-called hold mode was used where the gap was corrected for temperature variations

3. Results and discussion 3.1. Syntheses LA and SA were first homopolymerized independently in order to evaluate the degree of control by ATRP for each monomer. Both monomers were polymerized at 60 °C in anisole (monomer/anisole  50/50 wt%) using CuBr/PMDETA as catalytic complex and methyl-2-bromopropionate (MBP) as initiator. About 5 mol% (compared to CuBr) of deactivator CuBr2 was added from the beginning of the reaction in order to improve the control of the polymerization. The following [Monomer]:[MBP]:[CuBr]:[CuBr2]: [PMDETA] ratios were used for both cases: 52:1:0.9: 0.05:1.0. The reactions were stopped at around 65–70% conversion, leading respectively to PLA39 and PSA32 homopolymers (Table 1), where the subscripts stand for the degrees of polymerization. For both polymerizations, the increase of ln([M]0/[M]) versus time was linear, indicating a constant concentration of propagating species (Fig. 1a and b).1 Moreover, the dispersity decreased throughout the reaction and reached values lower than 1.2 (Fig. 1b/Fig. 2b and Table 1). The PSequivalent number average molecular weight (Mn-eq PS, determined by size exclusion chromatography (SEC)) increased with the conversion and deviated from linearity at high conversion (Figs. 1b and 2b). It would be tempting to attribute this to the existence of significant transfer reactions. However, several observations contradict this hypothesis. First, significant transfer reactions (except transfer to polymer) would have resulted in the formation of ‘‘deadchains’’, which was not the case at least for the PLA 1 It should be highlighted that the data in Fig. 1a do not fit perfectly with a straight-line going through the origin: the conversion only starts to grow after about 15 min. This can probably be explained by thermal inertia (time t0 corresponds to the moment when the reaction medium, initially at room temperature, is dipped in an oil bath at 60 °C) or to a very small error on the determination of the conversion by GC (see Colombani et al. [21]).

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Fig. 1. Kinetic study for the ATRP of PLA39: LA/anisole = 44/56 wt%, 60 °C, [LA]:[MBP]:[CuBr]:[CuBr2]:[PMDETA] = 52:1:0.9:0.05:1.0. (a) Evolution of ln ([M]0/[M]) versus time (j), fitted linearly (–). [M]0 and [M] stand for the monomer concentrations at time t0 and t, respectively. (b) Evolution of the number average molecular weight Mn (filled symbols) and dispersity (empty symbols), determined by SEC, versus conversion: (j, h) PSequivalent values and (N, 4) ‘‘true’’ values corrected using the universal calibration principle and the MHKS parameters of PS and PLA respectively. The straight-line represents the theoretical molecular weight calculated from the conversion and the initial monomer to initiator ratio.

homopolymer as revealed by a chain extension reaction (Fig. 3, see text below). Moreover, the dispersity usually increases with the occurrence of more transfer reactions, which was not the case for either polymers (Figs. 1b and 2b). Furthermore, the number average molecular weights of the purified PLA and PSA homopolymers determined by 1 H NMR were in fair agreement with the theoretical values (Table 1). The molecular weights determined by 1H NMR were actually even higher than the theoretical ones, which is contradictory with the occurrence of transfer reactions and might rather be explained by a fractionation of the polymers during the purification by precipitation. Finally, it is well known that SEC does not separate polymer chains according to their molecular weight but rather according to their hydrodynamic volume [22]. As a consequence, molecular weights determined by SEC using PS standards correspond to PS-equivalent molecular weights and not to true ones. Guillaneuf and Castignolles [23] highlighted that PS-equivalent molecular weights should be used with care to conclude about the ‘‘living’’ nature of a controlled radical polymerization such as ATRP. Indeed, they showed that a well-controlled polymerization

347

Fig. 2. Kinetic study for the ATRP of PSA32: SA/anisole = 49/51 wt%, 60 °C, [SA]:[MBP]:[CuBr]:[CuBr2]:[PMDETA] = 52:1:0.9:0.05:1.0. (a) Evolution of ln ([M]0/[M]) versus time (j), fitted linearly (–). [M]0 and [M] stand for the monomer concentrations at time t0 and t respectively. (b) Evolution of the number average molecular weight Mn (filled symbols) and dispersity (empty symbols), determined by SEC, versus conversion: (j, h) PSequivalent values and (N, 4) ‘‘true’’ values corrected using the universal calibration principle and the MHKS parameters of PS and PLA respectively. The straight-line represents the theoretical molecular weight calculated from the conversion and the initial monomer to initiator ratio.

Fig. 3. Comparison of the SEC chromatograms of the PLA39 macroinitiator with those of kinetic samples withdrawn from the reaction medium during the synthesis of the PLA39-b-PSA181 diblock. The abscissa corresponds to PLA-equivalent molecular weights, calculated from the MHKS of PS and PLA.

may look as if transfer reactions were significant when using PS-equivalent molecular weights rather than true ones because a linear evolution of Mn(true) versus

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conversion may be transformed into a non linear increase of Mn(eq-PS) versus conversion. Such deviations are usually observed if the architecture of the standards differs from that of the analyte or if the quality of the solvent is not the same for both polymers, which is the case for our system according to the Mark–Houwink–Kuhn–Sakurada (MHKS) parameters of PS (KPS = 11.4  105 dL g1 and aPS = 0.716) [24] and PLA (KPLA = 29.2  105 dL g1 and aPLA = 0.585) [24]: THF is a far better solvent for PS than for PLA. Following Guillaneuf and Castignolles advises [23], the PS-equivalent molecular weights determined by SEC were corrected to true molecular weights for PLA relying on the principle of universal calibration [22] and using the Mark–Houwink–Kuhn–Sakurada (MHKS) parameters of PS (KPS = 11.4  105 dL g1 and aPS = 0.716) [24] and PLA (KPLA = 29.2  105 dL g1 and aPLA = 0.585) [24] (Table 1, Fig. 1b, details are given in ‘‘Materials and methods’’). As highlighted in Fig. 1b, the corrected molecular weights were in good agreement with the theoretical curve and followed an almost perfectly linear increase versus conversion, at least up to 50% conversion. Transfer reactions were thus negligible up to this conversion at least. The MHKS parameters of PSA were not found in the literature. However, as the structures of PLA and PSA do not differ too much, it was attempted to correct the PS-equivalent molecular weights of the PSA samples to PLA-equivalent ones. Fig. 2b shows that ‘‘PLA-corrected’’ molecular weights were in good agreement with the theoretical ones and followed a linear increase with conversion. Using the MHKS parameters of PLA to correct the PS-equivalent molecular weights of PSA to true values is thus a reasonable approximation. As a conclusion, after having taken the precaution to convert PS-equivalent molecular weights into PLA-equivalent ones, it can be concluded safely that all results of the kinetic study point at a controlled ATRP polymerization both for LA and SA. PMDETA thus revealed an efficient ligand for the polymerization of LA and SA in the presence of anisole. In order to get a better insight on the degree of control of the polymerization and confirm that neither transfer nor termination reactions were significant, the PLA39 homopolymer was purified and then used for the initiation of the polymerization of a PSA181 second block. This chain extension reaction was also conducted at 60 °C in 50/ 50 wt% anisole and with [SA]:[PLA39]:[CuBr]:[CuBr2]: [PMDETA] = 338:1:1.2:0.08:1.9. The monomer to initiator ratio was strongly increased compared to the conditions used for the PLA39 block in order to shift the SEC trace of the diblock sufficiently from that of the macroinitiator and thus detect any traces of residual PLA39. Fig. 3 compares the SEC chromatogram of the PLA39 macroinitiator with those of kinetic samples withdrawn from the reaction medium during the synthesis of the PSA181 second block. It confirms that PLA39 was prepared with a high degree of control and with a negligible proportion of termination or transfer reactions: the chromatograms of the kinetic samples shifted toward higher molecular weights with conversion and the final PLA39-b-PSA181 diblock copolymer did not contain any significant trace of PLA39 macroinitiator. This latter result definitely confirms that transfer or

termination reactions were negligible during the preparation of PLA39. It must be highlighted that the kinetic of the polymerization of the second block PSA181 was also followed (Fig. 4). Again, PS-equivalent molecular weights were corrected to PLA-equivalent molecular weights using the MHKS parameters of PS and PLA. Considering that the MHKS parameters of PLA and PSA are very similar, according to the results presented above, the use of the MHKS parameters of PLA for the determination of the true molecular weight of the diblock copolymers seems an acceptable approximation. Contrarily to what was observed for the shorter PSA32 homopolymer (Fig. 2), the polymerization was very slow and the increase of ln ([M]0/[M]) versus time was not linear for the PSA181 second block (Fig. 4a). The number of termination reactions occurring during the polymerization of the second block was indeed not completely negligible as confirmed by the dragging of the SEC chromatograms of the kinetic samples toward low molecular weights (Fig. 3). However, the PLA-equivalent number average molecular weight grew linearly with conversion and fairly followed the theoretical curve, whereas the

Fig. 4. Kinetic study for the synthesis of the second block PSA181 of the PLA39-b-PSA181 diblock: SA/anisole = 48/52 wt%, 60 °C, [SA]:[PLA39]: [CuBr]:[CuBr2]:[PMDETA] = 338:1:1.2:0.08:1.9. (a) Evolution of ln ([M]0/ [M]) versus time (j), fitted linearly (–). [M]0 and [M] stand for the monomer concentrations at time t0 and t respectively. (b) Evolution of the number average molecular weight Mn (filled symbols) and dispersity (empty symbols), determined by SEC, versus conversion: (j, h) PSequivalent values and (N, 4) ‘‘true’’ values corrected using the universal calibration principle and the MHKS parameters of PS and PLA respectively. The straight-line represents the theoretical molecular weight calculated from the conversion and the initial monomer to initiator ratio.

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dispersity remained lower than 1.16 throughout the polymerization (Fig. 4b). Again, the PS-equivalent molecular weights were not in agreement with the theoretical values because of the discrepancy between the MHKS parameters of PS and PLA/PSA (Fig. 4b). As a conclusion, termination reactions were clearly more numerous when a longer PSA block was targeted, but the control of the polymerization was still satisfying. Finally, a PLA65-b-PSA47 diblock copolymer was prepared in similar conditions, targeting a symmetric polymer where both blocks had a molecular weight of about 15,000 g/mol. Particular care was taken to stop the polymerization of each block at 50% conversion in order to reach the desired degrees of polymerization and to minimize the risks of side reactions occurring at higher conversions in controlled radical polymerization. The kinetic study for the polymerization of both blocks is given in Supplementary data and revealed again a proper control. The SEC of the PLA65 macroinitiator and of the PLA65-b-PSA47 diblock copolymer corresponded to monodisperse polymers (Fig. 5). The molecular weights of the PLA65 macroinitiator and of the PLA65-b-PSA47 diblock were finally determined both by universal SEC and by 1H NMR spectroscopy (Table 1) and were in good agreement with theoretical values. For 1H NMR, the degree of polymerization of each block was determined by comparing, at each step of the diblock synthesis, the intensity of the methoxy group of the initiator at 3.65 ppm to that of the first methylene group of the alkyl chains at 4.0 ppm (Fig. 6). It is also interesting to note that, according to the assignment of the main chain signals reported by Tabuchi et al. [25] and Coelho et al. [26], the ratio racemo/meso was equal to 55:45. There was thus no stereoselectivity of the polymerization, leading to atactic polymers. 3.2. Thermo-mechanical properties The purpose of this article is not the detailed analysis of the thermal and thermo-mechanical properties of PLA-bPSA copolymers and a brief overview of these properties is simply given hereafter. PLA39 and PSA32 homopolymers were characterized by DSC, whereas the PLA65-b-PSA47 di-

f, f'

g, g' d, d' (b, b') racemo + e, e'

a 4.1

4.0

3.9

3.8

3.7

3.6

δ (ppm)

c, c'

5

4

(b, b') meso

3

2

1

0

δ (ppm) Fig. 6. 1H NMR spectrum of PLA65-b-PSA47 block copolymer. Insert shows the two peaks that allow the determination of DPn.

block copolymer was studied by means of DSC and dynamic thermo-mechanical analysis (DMTA). As expected, the homopolymers exhibited a semi-crystalline behavior (Table 2). As the polymers are atactic (see above), their crystallinity necessarily only results from the crystallization of the C12 and C18 side chains rather than from the backbone, as already reported [6]. The phase transitions occured at lower temperature than those measured by Jordan who found 12 °C and 56 °C for the melting of PLA and PSA respectively [6], whereas we found 6 °C and 54 °C. This might be due to the shorter degrees of polymerization of the polymers studied here. The melting temperature of PLA seems to be more influenced by the length of the main chain. Surprisingly, the degrees of crystallinity v were not affected by the degrees of polymerization and were in very good agreement with the literature data (17% and 41% of crystallinity for the PLA and PSA, respectively) [6]. The PLA65-b-PSA47 copolymer exhibits two transitions when cooling or heating ramps were applied (Fig. 7). The endotherm occurring at the lowest temperature (around 10 °C) was attributed to the melting of the PLA block; the other peak was attributed to the melting of the PSA block (around 50 °C). This observation revealed the incompatibility and, thus, the micro-phase separation of the two blocks; at least as soon as crystallization began. The results of the influence of the heating and cooling rates on the thermal transitions are gathered in Table 3. Both the crystallization and melting temperatures of the PSA block were slightly modified in the diblock compared to the homopolymer. This may indicate that the PLA block Table 2 Thermal characteristics of the homopolymers measured by DSC at 10 °C/ min. Polymer

Fig. 5. Comparison of the SEC chromatograms of the PLA65 macroinitiator with that of PLA65-b-PSA47. The abscissa corresponds to PLA-equivalent molecular weights, calculated from the MHKS of PS and PLA.

PLA39 PSA32

Crystallization

Melting

Tc (°C)

D Hc (J/g)

vc

Tm (°C)

D Hm (J/g)

vm

(%)

9 38

34 95

16 43

6 54

40 89

18 41

(%)

350

F. Dutertre et al. / European Polymer Journal 47 (2011) 343–351

Fig. 7. DSC thermograms of PLA65-b-PSA47 copolymer heated at 2 °C/min (straight line) and cooled at 2 °C/min (dashed line).

Table 3 Thermal characteristics of the PLA65-b-PSA47 block copolymer. Measurement rate (°C/min)

1 2 5 10 5a

PLA block

PSA block

Tc (°C)

vc

Tm (°C)

vm

Tc (°C)

vc

Tm (°C)

vm

5 5 4 4

20% 23% 22% 19%

n.d. 11 11 14 9

n.d. 23% 21% 19% 18%

44 44 42 41

30% 29% 29% 26%

n.d. 48 49 50 48

n.d. 31% 30% 28% 40%

n.d.: Not determined. a After annealing at 20 °C for 24 h.

influences the crystalline properties of the PSA one. A similar observation was made by Sun et al. on poly(L-lactide)-b-poly(ethylene glycol) diblock copolymers [27]. Moreover, the degree of crystallinity of the PSA47 block drops to 30% independently of the cooling rate instead of 40% in the homo-PSA32. However, this is a kinetic phenomenon: when the block copolymer was annealed at 20 °C for 24 h, the crystallinity of the PSA block raised to 40% as was observed in the homopolymer (Table 3). This kinetic phenomenon is further discussed below. Both crystallization and melting temperatures of the PLA block were significantly increased by comparison to the short homopolymer and were weakly affected by the measurement rate. First, the increase of the melting temperature of the PLA in the diblock compared to what happens in the homopolymer might be partly explained by a chain-end effect. In the case of PLA65-b-PSA47, the PLA block is indeed longer than in the homopolymer and attached to the PSA block, both effects reducing the mobility excess brought by the chain-end. The fact that the crystallinity of the PSA in the diblock dropped to 30% (instead of 40% in the homopolymer) unless annealed for 24 h at room temperature may hint at a second phenomenon. Indeed, this observation was concomitant with an apparent increase of the degree of crystallinity of the PLA block, and this the more when the rate of heating/cooling was decreased. But, after 24 h of annealing at room temperature, both blocks in the PLA65-b-PSA47 retrieved the same crystallinity as in their respective homopolymers. It can be

concluded that a small part of the PSA material was crystallized and melted simultaneously with the PLA. Whether this involves partial co-crystallization of both blocks or simply implies that the PSA block needs time to crystallize because of the presence of the PLA block is difficult to conclude without further investigation (by wide angle X-ray scattering, WAXS, for example). However, it is clear that this is only a kinetic effect. Indeed, the fact that both PLA and PSA blocks retrieved the same crystallinity as their respective homopolymers within 24 h at room temperature indicates that both blocks fully phase segregate at equilibrium. The chain end-effect, alone, could explain the shift of the melting temperature of the PLA block, but cannot account for such a high upper shift of its Tc (14 °C, Tables 2 and 3). The presence of the PSA thus clearly modifies the crystallization of the PLA phase, either because the PSA crystallites already formed act as nucleating centers, or because the PSA part which has not yet crystallized cocrystallizes with the PLA. Again, it is difficult to conclude between these two hypotheses without further investigations. The thermo-mechanical properties of PLA65-b-PSA47 were studied by dynamic mechanical thermal analysis (DMTA). The linear domain was determined at 30 °C for a frequency of 1 rad s1 and the deformation rate was set to 0.02% accordingly. The evolution of the storage (G0 ) and loss (G00 ) moduli as a function of temperature was measured at the rate of 1 °C/min during a cooling ramp, followed by a heating one (Fig. 8). Two transitions were observed for each ramp and were attributed to the crystallization (cooling ramp) and melting (heating ramp) of both blocks. The transition temperatures associated to these processes were indeed very similar to those measured by DSC for the crystallization/melting of the PLA and PSA blocks. Below 0 °C, the polymer was rigid and brittle. At room temperature, above the melting of the PLA block, the elastic modulus was around 107 Pa and ductile properties were expected for this material. Above 50 °C (Tm of the PSA block), a liquid behavior was observed. The cooling

Fig. 8. Evolution of the storage and loss moduli as a function of temperature during a cooling (blue) or heating (red) ramp at 1 °C/min (x = 1 rad s1, c = 0,02%). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

F. Dutertre et al. / European Polymer Journal 47 (2011) 343–351

experiment led at 2 °C/min shows a small shift of the transitions to lower temperatures (3.5 °C lower for the crystallization of the PSA block and 6 °C lower for those of PLA) indicating that the mechanical response is more sensitive to the experiment rate than the heat flow measurements (see Supplementary data).

351

Appendix A. Supplementary data Kinetics and SEC of PLA65-b-PSA47. DMTA of PLA65-bPSA47 at 2 °C/min. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.eurpolymj.2010.12.003. Reference

4. Conclusion Since the first article dealing with this topic by Beers and Matyjaszewski [1], PMDETA was considered as an improper ligand for the ATRP of lauryl (meth)acrylate or stearyl (meth)acrylate. In this paper, it was shown by conducting thorough kinetic studies and chain extension reactions that CuBr/PMDETA is actually a suitable catalyst for the preparation of both homopolymers and diblock copolymers of PLA and PSA by ATRP. As suggested by Guillaneuf and Castignolles [23], PS-equivalent molecular weights obtained by SEC had however to be converted into true values using the principle of universal calibration [22] in order to conclude properly concerning the control of the polymerization. Importantly, it is the first time that a PLAb-PSA diblock copolymer could be prepared by controlled radical polymerization and its thermo-mechanical properties were studied. DSC and thermo-mechanical analysis revealed that PLA and PSA blocks phase segregate in the crystalline state. Whether both blocks initially co-crystallize is not clear at the moment and would require thorough investigations by WAXS. However, it is clear that if co-crystallization occurs, it is only a metastable state. At equilibrium, both blocks form distinct crystalline phases. Still, the crystallization and melting of both blocks are definitely influenced by the presence of the other block. Phase segregation of both blocks results in interesting thermomechanical properties: the polymer behaves as a brittle solid as long as both blocks are crystallized, as a ductile material after the melting of the PLA phase, and finally as a viscous liquid when both blocks are amorphous. This work could be extended to the preparation of PSA-b-PLAb-PSA triblock copolymers which may reveal interesting bio-issued thermoplastic elastomers.

Acknowledgements Magali Martin is thanked for carrying the SEC experiments. Patrice Castignolles is acknowledged for helpful discussions concerning PS-equivalent molecular weights versus true ones.

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