Synthesis of Poly(Methylacrylate-b-"-Caprolactone) and Application to Compatibilizer for Poly(L-Lactide)/Poly("-Caprolactone) Blend System

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1 Materials Transactions, Vol. 46, No. 12 (2005) pp to 2672 Special Issue on Growth of Ecomaterials as a Key to Eco-Society II #2005 The Japan Institute of Metals Synthesis of Poly(Methylacrylate-b-"-Caprolactone) and Application to Compatibilizer for Poly(L-Lactide)/Poly("-Caprolactone) Blend System Naohisa Tamura, Kazuhiro Ban, Shinya Takahashi and Tomoyuki Kasemura Department of Applied Chemistry, Faculty of Engineering, Gifu University, Gifu , Japan Poly(L-lactic acid) (PLLA) was blended with poly("-caprolactone) (PCL) using a single-screw extruder in order to modify poor characteristic of these polymers. When the polymer was blended, the block copolymer that is synthesized by methyl acrylate (MA) and "-caprolactone ("-CL) via an atom transfer radical polymerization was used as a novel compatibilizer. The structure of the synthesized compatibilizer is determined by 1 Hor 13 C NMR. From this result, it was found that the ring-opening polymerization of the "-CL was taken place in the hydroxyl end group of MA. Moreover, the morphologies of the PLLA/PCL solvent-cast blend films were observed by the optical microscope and SEM. From the optical microscopic observation, the morphologies of the solvent-cast blend films with the synthesized compatibilizer were more homogeneous than that of the solvent-cast blend films without the compatibilizer. It was confirmed that the phase structure of the solvent-cast blend films with the compatibilizer was more stable than that of solvent-cast blend films without the compatibilizer. (Received June 20, 2005; Accepted August 22, 2005; Published December 15, 2005) Keywords: poly(l-lactide), poly("-caprolactone), poly(methylacrylate-b-"-caprolactone), compatibilizer 1. Introduction Recently, a lot of plastics have been produced all over the world. The amount extends to 100 million ton in a year. In order to respond to the diversified required performance, about 30% of produced plastics are a multi-component system containing two or more kinds of polymer. The percentage will increase in the future. From this background, many researchers are interested in the study of polymer blends due to the importance in scientific and technical aspect. It is possible to modify poor characteristic of polymers by polymer blends. Generally, when the different polymers are blended, the blends system usually leads to phase separation probably due to the small entropy of mixing. 1) However, the compatibilizer, which makes polymer blend miscible, has also been synthesized, and effectively used in a practical use. 2) Aliphatic polyesters such as poly(l-lactic acid) (PLLA), poly("-caprolactone) (PCL), poly(butylenesuccinate) (PBS) and so on have been noticed because of their bio-degradability. However, these polymers do not have enough properties for practical application. For example, PLLA is too rigid and brittle, PCL has too low melting temperature and PBS is too flexible. Therefore, many blend pair such as PLLA and PDLLA, 3,4) PLLA and poly(l-lactide-co-glycolide) and PCL and poly(l-lactide-co-glycolide) 5) have been investigated to improve these disadvantages by blending each other. In our previous reports, PLLA was blended with PCL or PBS using a single-screw extruder in order to modify poor characteristic of these polymers. Furthermore, when the polymer was blended, the copolymer (LA-CL block copolymer) consisted of L-lactide (L-LA) and "-caprolactone ("-CL) was used as a compatibilizer. Then, we could spin the blend fibers and prepare the blend films of PLLA/PCL and PLLA/PBS having uniform thickness. 6) In this paper, we focused on poly(methyl acrylate) (PMA) that was miscible with PLLA. We tried to synthesize the block copolymers composed of methyl acrylate (MA) and "- CL which is used as a compatibilizer for blending of PLLA with PCL. The copolymer was synthesized via an atom transfer radical polymerization (ATRP) of methyl acrylate and "-CL. 7 9) Then, it was characterized via NMR and GPC. Moreover, we tried to modify the PLLA/PCL blends prepared by solvent cast method. We discuss that the block copolymer acts as a compatibilizer. 2. Experimental 2.1 Materials "-CL was purchased from Aldrich, distilled under the reduced pressure less than 2.0 mmhg and stored over molecular sieves 4A 1/16 (Wako Pure Chemical Industries, Ltd.) prior to synthesis. Methyl acrylate (MA) was purchased from Aldrich, dried over CaH 2 and distilled under pressure before use. 2,2,2-tribromoethanol (CBr 3 CH 2 OH) and dibromobis(triphenylphosphine)nickel(ii) (NiBr 2 (PPh 3 ) 2 ) were purchased from Aldrich and used as received without further purification. Triethyl aluminum (Al(Et) 3 ) was purchased from TOKYO KASEI KOGYO CO., LTD. and used as received without further purification. Tin(II) octoate [stannous 2-ehtylhexanoate; Sn(Oct) 2 ] was purchased from Wako Pure Chemical Industries, Ltd., used as a catalyst and used as received without further purification. All the other chemicals were used as received. PLLA (LACEA H-100) and PCL (Celgreen P-H7) were provided from Mitsui Chemicals, Inc., and DAICEL CHEM- ICAL INDUSTRIES, LTD., respectively and were used as received. Their molecular weights were determined by GPC as shown in Table 1. PMA, which have a weight average molecular weight of approximately 40,000, was purchased Table 1 Molecular weight of PLLA and PCL. Polymer M n (10 4 ) M w (10 4 ) M w =M n PLLA PCL Determined by GPC

2 Synthesis of Poly(Methylacrylate-b-"-Caprolactone) and Application to Compatibilizer 2669 from Aldrich. This sample was used for blending with PLLA. The sample was purified by precipitation into ethanol before use. 2.2 Preparation of PLLA/PMA blends If PMA is miscible with PLLA, it is expected that PMA can be used as one component of the compatibilizer for blending of PLLA with PCL. Therefore, the PLLA/PMA blend pellet were prepared using a single-screw extruder at C. PLLA/PMA blending ratios were 100/0, 75/25, 50/50, 25/75, and 0/100 by weight. The composition of blends was calculated by 1 H NMR. The composition of the PLLA/PMA = 75/25, 50/50 and 25/75 was 84/16, 60/40 and 38/62, respectively. 2.3 Synthesis of compatibilizer Polymerization was carried out via two-step process. First, PMA homopolymer was synthesized by radical polymerization (RP) and subsequently "-CL was block-copolymerized with terminal OH group of PMA through ring-opening polymerization (ROP). For this process, CBr 3 CH 2 OH was used as an initiator. This chemical acts as a dual-purpose initiator that is able to induce both RP and ROP. 7 9) In the first step, a given amount of MA, CBr 3 CH 2 OH, NiBr 2 (PPh 3 ) 2 were added into the special bottle and were dissolved into tetrahydrofuran (THF). And then, the bottle was sealed and the inner gas was replaced by argon. This polymerization was performed in the oil bath preset at 75 C for 24 h with stirring. In the second step, PMA-block-PCL was synthesized by in situ copolymerization as follows. After an additional THF and Al(Et) 3 as a catalyst were added into the first step product, the system was maintained for 2.5 h with stirring at R.T. Next, a given amount of "-CL was added and the bottle containing the reactant was sealed according to the above. The polymerization was performed for 72 h with stirring at R.T. and was terminated by adding an excess of HCl (0.1 M/ aq.). The whole product was poured into a large amount of hexane, and PMA-b-PCL was isolated by precipitating from it. The isolated PMA-b-PCL was sufficiently dried in vacuum at R.T. (yield: 72%) 2.4 Preparation of PLLA/PCL blends Solvent-cast blend film of PLLA/PCL was also prepared by dissolving in chloroform and casting on glass petri dish at room temperature for 3 d, and then dried at room temperature. Simultaneously, the synthesized compatibilizer (1, 3 and 5 mass% of total polymer) was added to the blends. PLLA/PCL blending ratio were 100/0, 80/20, 60/40, 50/50, 40/60, 20/80 and 0/100 by mass. 2.5 Measurements 1 H NMR spectra were recorded on a Varian 400 and 500 MHz spectrometers. Tetramethylsilane was used as an internal standard. The number and weight average molecular weights (M n and M w ) and the polydispersities (M w =M n ) were determined by gel permeation chromatography (GPC) using a Tri SEC Model 300TDA (Asahi Techneion Co., Ltd.) and chloroform as an eluent. These molecular weights are relative values calibrated with polystyrene standard. The morphologies of solvent-cast blend film were observed with a polarized microscope (SHIMADZU KALNEW MODEL EP) equipped with crossed polarizers and a CCD camera (ELMO MODEL TSN 401A). The surface morphologies of solvent-cast blend film to which the PCL component was extracted by methylethylketone were evaluated by SEM (HITACHI S-4300) observation. The electron gun voltage was 1.0 kv. The samples were coated with gold before observation. The tensile tests were performed using a TMI UTM-3 tensile testing machine according to JIS K7161. The mechanical properties of blend films were measured using cross-head speed of 20 mmmin 1 at approximately 20 C. Thermal properties of the blends were measured by DSC analysis using SII NanoTechnology Inc. EXSTAR 6000 DSC instrument. Each samples (approximately 5 mg) was presealed in an aluminum pan, and then was heated from 50 to 200 C at a rate of 10 Cmin 1 (1st scan). After heating, the samples were rapidly cooled at from 200 to 50 C and then were heated from 50 to 200 C at a rate of 10 Cmin 1 again (2nd scan). 3. Results and Discussion 3.1 Thermal properties of PLLA/PMA blends Figure 1 illustrates the DSC thermograms as a function of temperature. The DSC thermogram of pure PLLA represented the glass transition temperature (T g ) at approximately 65 C when heated from 50 to 200 C. It also showed an endothermic peak of melting temperature (T m ) at approximately 170 C. The DSC thermogram of pure PMA showed the glass transition only at approximately 10 C. The DSC thermograms of blends represented the single T g of miscible PLLA/PMA at approximately 40 C. It also showed an endothermic peak of T m at approximately 165 C. The exothermic peak represents the crystallization of the PLLA phase. These results were shown in Table 2. The deference was observed in the magnitude of T g of the PLLA/ PMA = 60/40 blends between the first and second scans. This was due to relaxation of each polymer toward equilibrium after heating the blend. The relaxation process results in a decrease in thermodynamics quantities such as enthalpy and volume. 10) The T g of PLLA phase decreased with an increase in the PMA content. And the single T g was observed. The crystallization temperature of the PLLA phase also decreased. From these results, it was suggested that PLLA could be miscible with PMA. Therefore, it was expected that the synthesized block copolymer (PMA-b- PCL) in this study could act as compatibilizer for PLLA/PCL blend system H NMR of synthesized PMA and block copolymer 1 H NMR measurement was carried out for both PMA that was obtained from first step polymerization and PMA-b-PCL that was obtained from second step polymerization. The peak, corresponding to methoxy group ( OCH 3 ) appeared at about 3.7 ppm in the PMA sample. Moreover, the peaks of methylene group ( CH 2 ) of PCL backbone appeared to about 2.2 and 4.1 ppm as well as the peak of the PMA sample

3 2670 N. Tamura, K. Ban, S. Takahashi and T. Kasemura Table 3 Molecular weight of PMA and PMA-b-PCL. PLLA 100% Polymer Molecular weight M n 10 4 M w 10 4 M w =M n PMA PMA-b-PCL Determined by GPC Endothermic PLLA/PMA = 84/16 PLLA/PMA = 60/40 (1st) PLLA/PMA = 60/40 (2nd) Tensile strength, σ/mpa without PMA-b-PCL PMA-b-PCL 1wt% PMA-b-PCL 3wt% PMA-b-PCL 5wt% PLLA/PMA = 38/62 PMA 100% PLLA content (mass%) PLLA/PMA Table Temperature, T/ C Fig. 1 T g, PMA 1 DSC thermograms of PLLA/PMA blend. Thermal properties of PLLA/PMA blend. T g, blend 2 T g, PLLA 3 T c, PLLA 4 T m, PLLA 5 100/ , / , /40 (1st) , /40 (2nd) , / , / Glass transition temperature of PMA, 2 Glass transition temperature of PLLA 3 Crystallization temperature of PLLA, 4 Melting temperature of PLLA 1, 2, 3, 4 and 5 obtained by DSC Fig. 2 Elongation percentage (%) Tensile strength of PLLA/PCL blend films with PMA-b-PCL without PMA-b-PCL PMA-b-PCL 1wt% PMA-b-PCL 3wt% PMA-b-PCL 5wt% PLLA content (mass%) in the PMA-b-PCL sample. Furthermore, the compositions of MA and "-CL repeating unit involved in the PMA-b-PCL were approximately estimated from methoxy proton resonance (3.7 ppm) and methylene proton resonance (4.1 ppm), respectively. Consequently, the MA/CL unit molar ratio was about GPC of synthesized PMA and block copolymer GPC measurement was carried out for both PMA that was obtained from first step polymerization and PMA-b-PCL that was obtained from second step polymerization. Their molecular weights (number average molecular weight M n and Fig. 3 Elongation percentage at break of PLLA/PCL blend films with PMA-b-PCL. weight average molecular weight M w ) and polydispersity index M w =M n were determined by GPC as shown in Table 3. The molecular weight of PMA-b-PCL was larger than that of PMA before it was reacted with "-CL. 3.4 Mechanical properties of PLLA/PCL films with synthesized block copolymer The mechanical properties such as Young s modulus, maximum strength, elongation at maximum strength and

4 Synthesis of Poly(Methylacrylate-b-"-Caprolactone) and Application to Compatibilizer 2671 (a) (b) 500µm Fig. 4 Optical micrographs of PLLA/PCL = 60/40 solvent-cast film (a) without and (b) with PMA-b-PCL. elongation at break were evaluated from stress strain curves. The experimental results of maximum strength and elongation at break were shown in Figs. 2 and 3, respectively. According to the data, for PLLA and PCL homopolymer films, the maximum strength of PLLA was greater than that of PCL, while the elongation at break of PCL was greater than that of PLLA. As the our previous study of DSC also showed, 11) it is because T g of PLA is higher than the room temperature and is in a glass state at room temperature. 12) Moreover, it is because the molecular chain of PLLA is relatively rigid and the entanglement density is small. On the contrary, since T g of PCL is below the room temperature and it is flexible at the room temperature, the maximum strength showed the low value. The maximum strength of the blend films with PMA-b- PCL was improved as compared with that of the blend films without PMA-b-PCL when the PLLA composition was rich in the blend systems (for example, PLLA/PCL = 60/40 and 80/20). Moreover, in those compositions, the maximum strength was improved with an increase in PMA-b-PCL. The elongation at break of the blend films with PMA-b- PCL was improved as compared with that of the blend films without PMA-b-PCL when the PCL composition was rich in the blend systems (for example, PLLA/PCL = 20/80 and 40/60). From these results, the strength of the PLLA/PCL = 60/ 40 blend film could be improved by the addition of PMA-b- PCL compared with other compositions. As will be described in the next section, this was due to the formation of microseparation structure and stabilization of the phase structure of the film. It was considered that the generation of the crack was relieved by the stabilization of phase structure when the blend film was broken. Thus, it was thought that the strength of the blend film was improved. The elongation of the PLLA/ PCL = 20/80 and 40/60 blend films could be notably improved by the addition of PMA-b-PCL. On the other hand, that of the PLLA/PCL = 60/40 and 80/20 blend films showed no change. It was considered that a smaller amount of component was dispersed by addition of PMA-b-PCL in the blend. Therefore, it was considered that the property of a larger component was notably expressed in the blend films. 3.5 Optical micrographs of a solvent-cast blend film The structures of a solvent-cast blend films with and without PMA-b-PCL were observed by optical microscope. The optical micrographs of PLLA/PCL = 60/40 solventcast blend film without and with PMA-b-PCL were shown in Figs. 4(a) and (b), respectively. The phase separation structure was observed clearly in Fig. 4(a). Moreover, it was found that the size was remarkably large. And the spherulite was heterogeneous. However, although the phase separation was also observed in Fig. 4(b), it was confirmed that the size was small as compared with Fig. 4(a). It was considered that this is the reason that PMA-b-PCL exists in the interface between the PLLA and the PCL component, and interfacial free energy between the PLLA and the PCL component decreased. Therefore, it was thought that the film formed micro-phase separation structure. From these results, since the synthesized block copolymer in this study stabilized the phase structure of the film effectively, it was suggested that it acted as a compatibilizer. 3.6 SEM photographs of a solvent-cast blend film The surface morphology of a solvent-cast blend films without and with compatibilizer (1 and 3 mass%) were observed by SEM. In order to observe the surface morphology more clearly, the blend films were extracted by methylethylketone (MEK), which was non-solvent to PLLA, to remove only the PCL component. SEM photographs of these films were shown in Figs. 5(a), (b) and (c), respectively. In Fig. 5(a), only smooth surface was observed. As the surface tension of PLLA was lower than PCL for the PLLA/PCL blend system, PLLA was predominant at blend surface without compatibilizer. Therefore, the surface could not be extracted by MEK that could not solve PLLA. Contrary, for the blend systems with compatibilizer, PCL component could be existed in the surface region, the etched surface could be observed as shown in Figs. 5(b) and (c). For the system containing 1 mass% compatibilizer, the particles smaller than 200 mm were observed. This phase separation was not larger than the system without compatibilizer that showed macroscopic phase separation as shown in Fig. 4. While the system containing 3 mass% compatibilizer the phase separation could be observed as shown in Fig. 5(c). From these results, it was shown that the block copolymer effectively acts as a compatibilizer.

5 2672 N. Tamura, K. Ban, S. Takahashi and T. Kasemura (a) (b) strength of the PLLA/PCL = 60/40 blend film could be improved by the addition of PMA-b-PCL compared with other compositions. This was due to the formation of microseparation structure and stabilization of the phase structure of the film. The elongation of the PLLA/PCL = 20/80 and 40/ 60 blend films could be notably improved by the addition of PMA-b-PCL. On the other hand, that of the PLLA/ PCL = 60/40 and 80/20 blend films showed no change. It was considered that a smaller amount of component was dispersed by addition of PMA-b-PCL in the blend. Therefore, it was considered that the property of a larger component was notably expressed in the blend films. From the observation of optical microscope and SEM, the synthesized block copolymer in this study stabilized the phase structure of the film effectively, it was suggested that it acted as a compatibilizer. Acknowledgement (c) 4. Conclusions 500µm Fig. 5 SEM photographs of PLLA/PCL = 80/20 solvent-cast film (a) without, (b) with 1 mass%, and (c) with 3 mass% PMA-b-PCL, respectively. It was found that PLLA was miscible with PMA by DSC measurements of PLLA/PMA blends. From NMR and GPC results, PMA-b-PCL block copolymer could be synthesized by ATRP. From the tensile test results, it was found that the I want to thank all people for all the things everyone did for me. REFERENCES 1) M. Ballauff and J. R. Dorgan: Polymer Blends Volume 1: Formulation (John Wiley & Sons, Inc., New York, 2000) pp ) T. Nishi: High-performance Polymer Alloy (MARUZEN CO., LTD, Japan, 1991), pp ) H. Tsuji and Y. Ikada: J. Appl. Polym. Sci. 60 (1996) ) H. Tsuji and Y. Ikada: Macromolecules 25 (1992) ) Y. Cha and CG. Pitt: Biomaterials 11 (1990) ) N. Kuriyama, S. Takahashi, T. Kasemura, Y. Nishimura and K. Kato: Proceedings of The Fourth International Conference on ECOMATE- RIALS (Gifu, Japan, 1999) pp ) C. J. Hawker, J. L. Hedrick, E. E. Malmstöm, M. Trollsas, D. Mecerreyes, G. Mineau, Ph. Dubois and R. Jérôme: Macromlecules 31 (1998) ) D. Mecerreyes, G. Mineau, Ph. Dubois, R. Jérôme, J. L. Hedrick, C. J. Hawker, E. E. Malmstöm and M. Trollsas: Angew. Chem. Int. Ed. 37 (1998) ) W. Wang, Z. Yin, C. Detrembleur, P. Lecomte, X. Lou and R. Jérôme: Macromol. Chem. Phys. 203 (2002) ) J. L. Eguiburu, J. J. Iruin, M. J. Fernandez-Berridi and J. S. Román: Polymer 39 (1998) ) N. Tamura, T. Chitose, K. Komai, S. Takahasi, T. Kasemura and S. Obuchi: Trans. Mater. Res. Soc. Jpn. 29 (2004) ) J. Takagi: Biodegradable Plastic Handbook (NTS Co., Ltd., Japan, 1995) pp