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Melt stability of 8-arms star-shaped stereocomplex polylactide with three-dimensional core structures
Polymer Degradation and Stability 98 (2013) 1097e1101
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Short communication
Melt stability of 8-arms star-shaped stereocomplex polylactide with three-dimensional core structures Purba Purnama a, Youngmee Jung a, Soo Hyun Kim a, b, * a b
Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2012 Received in revised form 22 January 2013 Accepted 18 February 2013 Available online 27 February 2013
The melt stability of star-shaped stereocomplex polylactide with different core structure by tripentaerythritol (TPE) and polyhedral oligomeric silsesquioxane-octahydroxypropyldimethylsilyl (POSS(OH)8) compounds was investigated. The three-dimensional (3D) core structures affect the ability of star-shaped stereocomplex polylactide to preserve stereocomplex crystallite after melted. The exact position and proper space distribution of arm result in low chain freedom of itself which affects the ability of each arm to reassemble stereocomplex crystallites after melted. The result confirmed that the inorganic POSS(OH)8 core molecule has better support to maintain melt stability compare to TPE core. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Melt stability Polylactide Stereocomplex Star polymers Structure
1. Introduction Today, the bio-based polymers have received much interest because it made from renewable resources [1]. Polylactide (PLA) is one of the interesting synthetic biodegradable polymers. It has advantages in sustainable resources, non-toxic, biocompatibility, and biodegradability. The improvement on PLA materials has been studied for a long time through nanocomposite [2,3] and stereocomplexation approaches [4e7]. Stereocomplexation is one of the approaches to improve PLA properties by mixing enantiomeric polymer blend of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) [4e7]. Stereocomplexation can be formed by common methods, such as melt blending [8e10] and solution casting [4,11,12]. In our laboratory, supercritical carbon dioxide e dichloromethane (sc-CO2eDCM) was utilized as a new method to produce stereocomplex PLA (s-PLA) from highmolecular-weight PLA homopolymers resulted in perfect degree of s-PLA products in relatively short-time [13,14].
* Corresponding author. Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea. Tel.: þ82 2 958 5343; fax: þ82 2 958 5308. E-mail addresses: [email protected] (P. Purnama), [email protected] (S.H. Kim).
S-PLA has melting temperature (Tm) approximately 50 C higher than the Tm of either PLLA or PDLA [4,7]. Those improvement can be achieved for many type of s-PLA, such as s-PLA using block copolymer [15e21], star-shaped [22,23], and cyclic based-PLA [24]. In other hand, s-PLA has weakness in the melt stability due to limitation of memory to re-form s-PLA after melted. This weakness should be solved because melt process is the regular processing in industry. Some researchers reported some approaches to solve melt stability limitation. Biela et al. proposed the 13-arms or more of star-shaped s-PLA to maintain melt stability by hardlock-type interactions through parallel or anti-parallel orientations [22]. The number of arm is important factor to control the interaction between PLLA and PDLA in the s-PLA [22]. Up to date, the melt stable star-shaped s-PLA requires star-shaped homopolymers with 13-arms or more. Recently, there is no report of the melt stable star-shaped s-PLA with number of arm is lower than 13-arm. With regard to star-shaped s-PLA, the core structures are expected has big differences in the melt stability properties of star-shaped s-PLA. Herein, we studied the melt stability of 8-arm star-shaped s-PLA with different 3D core structure: organic and inorganic core of star-shaped s-PLA. We addressed whether 3D core structure affects the melt stability of the star-shaped s-PLA.
0141-3910/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.02.010
1098
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101
2. Experimental sections
2.4. Characterizations
2.1. Materials High purity L-Lactide (Purac Biochem BV, Gorinchem, The Netherlands) and D-lactide (Musashino Chemical) were used as received. Tripentaerythritol (TPE) (Sigma Aldrich, technical grade) and polyhedral oligomeric silsesquioxane-octahydroxypropyldimethylsilyl (POSS(OH)8) (Sigma Aldrich) were used as received. Tin(II)bis(2-ethylhexanoate) (Sn(Oct)2) (Sigma Chemical Co., St. Louis, MO, USA, purity 99%) was purified by distillation under reduced pressure and dissolved in dry toluene. Chloroform, methanol (Daejung Chemicals & Metal Co. Ltd. with a purity >99.5%), dichloromethane (DCM) (JT Baker, HPLC grade), and carbon dioxide gas (CO2) (Shin Yang Oxygen Industry Co. Ltd., minimum purity 99.9%) were used as received.
The molecular weight of star-shaped PLAs was analyzed by gel permeation chromatography (GPCmax, Viscotek VE 2001) equipped with micro-styragel columns calibrated with polystyrene. Chloroform was used as a mobile phase at a flow rate of 1.0 mL/min at 40 C. The 1H NMR of star-shaped structure from the polymerized materials was evaluated by 600 MHz NMR Spectrometer with CDCl3 as solvent. The stereocomplex formation of star-shaped sPLA was investigated by Wide-angle X-ray spectroscopy (WAXS) with an X-ray diffractometer Rigaku D/Max-2500 composed of Cu A, 30 kV, 100 mA) source, a quartz monochromator, Ka (l ¼ 1.54056 and a goniometric plate. The melt stability evaluation was performed by differential scanning calorimeter (DSC) (Modulated DSC 2910, TA Instrument) under nitrogen gas. The heating and cooling rate of DSC analysis were fixed at 10 C/min.
2.2. Ring-opening polymerization of star-shaped polylactides
3. Results and discussion
D-lactide or L-lactide was added into a 100 mL glass ampoule reactor equipped with magnetic stirring. Sn(Oct)2 and co-initiator were added into the ampoule at desired ratio. TPE was used as a co-initiator to produce organic core PLAs and POSS(OH)8 was used as a co-initiator to produce inorganic core PLAs. The reactor was purged with nitrogen for 3 times and vacuumed for 6 h. The reactor was sealed under vacuum. The ring-opening polymerization was conducted at 130 C for 24 h. After the reaction, mixture was allowed to cool to room temperature. The product was dissolved in chloroform, and poured into methanol for purification. The polymer was collected and dried under vacuum to constant weight.
The 8-arms organic and inorganic core of PLLA and PDLA were synthesized by stannous octoate initiated ring-opening polymerization at 130 C (Scheme 1). TPE was selected as 8-arms organic core and POSS(OH)8 as 8-arms inorganic core. TPE is the molecules consist of three molecules of pentaerythritol connected by ether bond. The backbone structure is single bond structure of CeC and CeO bonds. The POSS(OH)8 is an octa-functionalized hydroxyl group of POSS molecules. The POSS molecule families are real three-dimensional (3D) cage compound with silicon-oxide backbones [25e27]. The TPE and POSS(OH)8 with 8 eOH functional groups act as a co-initiator which lead polymerization growth in star-shaped form. The polymer chains were synthesized by in situ polymerization initiated by functional group from the core compounds [28,29]. Table 1 shows the melting point (Tm) and molecular weight of the synthesized materials. The structure of star-shaped PLA was confirmed by 1H NMR data as shown in Fig. 1. The peaks at 1.6 ppm and 5.2 ppm represented the methyl (a) and methine proton (b) resonances of the lactate units from linear PLA, respectively, in agreements with previous literature [28,30,31]. From 1H NMR spectrum of PLAePOSS
2.3. Stereocomplex formation Stereocomplexation of 8-arm star-shaped PLLA and 8-arm starshaped PDLA was conducted in sc-CO2eDCM at 65 C and 350 bar for 5 h in the 40 mL stainless steel high-pressure reactor equipped with magnetic stirring and electrical heating mantle as describe in previous work [13]. The reactor was opened immediately after the reaction had finished. The s-PLA was collected and vacuumed at 40 C for 1 night.
Scheme 1. Synthesis of 8-arms star-shaped PLA through bulk polymerization at 130 C. a. Using inorganic core (POSS(OH)8); b. Using organic core (TPE).
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101 Table 1 Summary of synthesized star-shaped PLA’s properties. Materials
[M:core] [mol]
Tma [ C]
Mnb [103]
PDIb
PLLAeTPE PDLAeTPE PLLAePOSS_A PDLAePOSS_A PLLAePOSS_B PDLAePOSS_B PLLAePOSS_C PDLAePOSS_C
420 420 420 420 560 560 840 840
163.33 162.64 164.44 164.21 165.08 167.22 172.28 170.71
63 77 49 65 72 83 156 132
1.47 1.74 2.38 1.63 2.33 1.26 1.26 1.70
a b
Measured by DSC at first scan with heating rate at 10 C/min. Measured by GPC.
(represented by PLLAePOSS_C), the peaks at 1.6 ppm and 5.2 ppm showed the characteristic of PLA absorption. The spectrum also has new peaks at 0.1 ppm and 0.6 ppm corresponds to eSiCH2 and e SiCH3 absorption peak, respectively. There is no other peak which refers to unreacted OH group of POSS(OH)8. By comparing the area of eCHe (lactate) and eSiCH2 absorption peak, the obtained molecular weight of PLLAePOSS_C is 156,802 g/mol. For organic starshaped PLA (PLLAeTPE), the presence of peaks at 4.1 ppm and
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3.4 ppm refers to the absorption of eCH2e at arms and backbone, respectively. By comparing the area of eCHe (lactate segment) and eCH2e (TPE backbone) absorption peaks, the obtained molecular weight of PLLAeTPE is 78,132 g/mol. These data showed that PLA fragment attached to all arms of the core materials. We successfully formed star-shaped s-PLA from star-shaped PLLA and star-shaped PDLA via sc-CO2eDCM (5 wt.%) at 65 C and 350 bar for 5 h. The s-PLAeTPE denotes the star-shaped s-PLA from PLLAeTPE and PDLAeTPE. The star-shaped s-PLA from PLLAePOSS A and PDLAePOSS A is denoted as s-PLAePOSS A, and soon. The dry and powder-like shape of star-shaped s-PLA was successfully obtained. The stereocomplexation of star-shaped s-PLA was confirmed by WAXS analysis (Fig. 2a). The observed strong peaks at 11.86 , 20.56 , and 23.86 of 2q without any characteristic peaks of homopolymers at 16.52 and 19.08 of 2q proved the formation of s-PLA, in agreement with previous reports [13]. The absence of homocrystal peaks demonstrated that star-shaped s-PLA was perfectly obtained. The melt stability is important factor to maintain physical properties and process ability of materials in industrial applications. The evaluation on melt stability of 8-arms star-shaped s-PLA with organic (s-PLAeTPE) and inorganic core (s-PLAePOSS A) was performed by a DSC as shown in Fig. 2b. The DSC thermograms
Fig. 1. The 1H NMR spectra of PLLAePOSS_B, PLLA, and PLLAeTPE.
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P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101
Fig. 2. (a) WAXS patterns of star-shaped s-PLA by organic core (s-PLAeTPE) and inorganic core (s-PLAePOSS A) prepared in sc-CO2eDCM at 350 bar and 65 C for 5 h. The WAXS pattern of PLA homopolymer was added as a control. (b) DSC scans of sPLAeTPE and s-PLAePOSS A before and after melted at 260 C. The heating and cooling rate were fixed 10 C/min.
Table 2 Summary of degree of s-PLA inorganic star-shaped s-PLA before and after melted. Materials
S-PLAePOSS A S-PLAePOSS B S-PLAePOSS C
Before
After
Deg of s-PLAa [%]
DHma
Deg of s-PLAa [%]
DHma
[J/g]
100 100 91.34
71.68 57.05 53.82
100 100 92.11
31.12 29.39 54.56
[J/g]
a Measured by DSC under nitrogen. Heating and cooling rate are fixed at 10 C/min.
before melted (Fig. 2b, left) confirmed the formation of star-shaped s-PLA in a perfect s-PLA degree. This result was in agreement with above WAXS result. After melted and recrystallized, there is a big difference in the degree of s-PLA crystallites from inorganic core and organic core of star-shaped s-PLA (Fig. 2b, right). The s-PLAeTPE remains degree of s-PLA about 84.95% but s-PLAePOSS A remains degree of s-PLA about 100% which means inorganic core of star-shaped s-PLA has better melt stability compare to the organic core of star-shaped sPLA. We assumed the 3D core structure of s-PLA will lead to different ability in reforming s-PLA after melted. We also continued to examine the inorganic core star-shaped sPLA in different molecular weight as shown in Table 2. We obtained high s-PLA degree of inorganic core star-shaped s-PLAs by sc-CO2e DCM with 5 h processing time. Decreasing s-PLA crystallites degree of s-PLAePOSS C due to higher molecular weight requires longer processing time in sc-CO2eDCM system [14]. Regarding DSC data, s-PLAePOSS A and s-PLAePOSS B are able to re-form the s-PLA crystallite completely after melted. The s-PLAePOSS C, in the first heating the s-PLA portion is about 91.34% and in the second heating is about 92.11%. These phenomena informed that the octafunctionalized POSS core structure has strong ability of s-PLLAe POSSs from low- and high-molecular-weight star-shaped PLAs to preserve s-PLA crystallites after melted. In the previous report related to star-shaped s-PLA, the proposed mechanisms in maintaining melt stability are hardlock-type interactions through parallel and anti-parallel orientation which are suitable for star-shaped having 13-arms or more [22]. The parallel orientation is only suitable for star-shaped s-PLA in twodimensional (2D) views. The 3D structure of organic and inorganic core of star-shaped s-PLA does not allow to re-form s-PLA through parallel orientation due to steric hindrance. By this work, the data exactly proved the possible orientation to re-form s-PLA after melted is anti-parallel orientation. As above explanation, the s-PLAeTPE and s-PLAePOSSs showed the different result in melt stability. It may be caused by the different core structure of TPE and POSS(OH)8 compounds in 3D view as illustrated in Fig. 3a. The organic core, TPE, has 3 arms in each side and 2 arms in the middle part. The 2 arms at the middle part of organic core have wider space compare to the other 6-arms at side part. The backbone structure with single bond CeC and CeO of TPE may also support the arms at the middle part to rotate. These conditions affected the hardlock interaction of the arms at the middle part which mean those two arms are having “chain
Fig. 3. (a) Illustrated 3D core structure of TPE and POSS(OH)8 compounds. (b) DSC thermograms of cooling process of inorganic core star-shaped s-PLA after melted at 260 C. The cooling rate was fixed 10 C/min.
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101
freedom” close to linear s-PLA. Consequently, the degree of starshaped s-PLA with organic core was slightly decreased (w15%). For inorganic core, POSS(OH)8 has real 3D cage structure with silicon-oxide backbones and 8 eOH functional groups in each corner. It leads each arm distributed at certain position and has similar free space distribution. Each arm has lower “chain freedom” compare to linear s-PLA. Hence, it supports each arm to reassemble of s-PLA easily after melted. The observation of cooling process in DSC analysis (Fig. 3b) suggested recrystallization mechanism for s-PLAePOSSs with different molecular weight. S-PLAePOSS A and s-PLAePOSS B have single recrystallization peak, but s-PLAePOSS C has two recrystallization peaks. In the melt state, the star-shaped PLAs were unzipped from star-shaped s-PLA, and then recrystallized during cooling process. For high-molecular weight star-shaped PLAs, each arm has longer chain length compared to the low-molecular weight star-shaped PLAs. The longer chain also decreases the chain freedom of unzipped chain. The different “chain freedom” causes the high-molecular-weight easier to recrystallize represent by different recrystallization peak. The recrystallization peak at 110 C of s-PLAePOSS C is belongs to the unstereocomplexed star-shaped PLAs and the peak at 146 C is belongs to recrystallization peak of unzipped star-shaped PLAs from the s-PLAePOSS C. Recrystallization of unstereocomplexed star-shaped PLAs during cooling process leads to small increasing s-PLAePOSS C portion after melted. In the s-PLAePOSSs materials, the space distribution and the chain length have simultaneous effect to reduce the “chain freedom”. 4. Conclusions In conclusion, the 8-arm star-shaped s-PLA was successfully synthesized using different core compounds. However, the inorganic POSS(OH)8 core has better support to maintain melt stability of 8-arm star-shaped s-PLA compare to organic TPE core. The 3D core structure affects the melt stability of star-shaped s-PLA. In the 8-arm star-shaped s-PLA, the arm distribution and chain freedom influence the ability of each arm to re-form s-PLA after melted. The real 3D core structure also proved the anti-parallel orientation as the suitable mechanism to enhance melt stability of star-shaped s-PLA. The use of inorganic materials as a core material opens a new way to develop hybrid polylactide materials with good melt stability. Acknowledgment This study was supported by the National Research Foundation of Korea Grant funded by the Korea Government (MEST), NRF2010-C1AAA001-0028939. References [1] Tsuji H, Ikada Y. Properties and morphologies of poly(L-lactide):1. Annealing condition effects on properties and morphologies of poly(L-lactide). Polymer 1995;36:2709e16. [2] Messermith PB, Giannelis EP. Synthesis and barrier properties of poly(ε-caprolactone)-layered silicate nanocomposites. J Polym Sci Part A Polym Chem 1995;33:1047. [3] Brown JM, Curliss D, Vaia RA. Thermoset-layered silicate nanocomposites. Quaternary ammonium montmorillonite with primary diamine cured epoxies. Chem Mater 2000;12:3376.
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[4] Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Stereocomplex formation between enantiomeric poly(lactides). Macromolecules 1987;20:904e6. [5] Murdoch JM, Loomis GL. Du Pont de Nemours & Co. US Patent 4,766,182; 1988. [6] Murdoch JM, Loomis GL. Du Pont de Nemours & Co. US Patent 4,719,246; 1988. [7] Tsuji H. Poly(lactide) stereocomplexes: formation, structure, properties, degradation, and applications. Macromol Biosci 2005;5:569e97. [8] Tsuji H, Ikada Y. Stereocomplex formation between enantiomeric poly(lactic acid) s. 6. Binary blends from copolymers. Macromolecules 1992;25:5719e9723. [9] Anderson KS, Hillmyer MA. Melt preparation and nucleation efficiency of polylactide stereocomplex crystallites. Polymer 2006;47:2030e5. [10] Fukushima K, Kimura Y. An efficient solid-state polycondensation method for synthesizing stereocomplexed poly(lactic acids) with high molecular weight. J Polym Sci Part A Polym Chem 2008;46:3714e22. [11] Tsuji H, Ikada Y. Stereocomplex formation between enantiomeric poly(lactic acid)s. 9. Stereocomplexation from the melt. Macromolecules 1993;26: 6918e26. [12] Yamane H, Sasai K. Effect of the addition of poly(D-lactic acid) on thermal property of poly(L-lactic acid). Polymer 2003;44:2569e75. [13] Purnama P, Kim SH. Stereocomplex formation of high-molecular-weight polylactide using supercritical fluid. Macromolecules 2010;43:1137e42. [14] Purnama P, Kim SH. Rapid stereocomplex formation of polylactide using supercritical fluid technology. Polym Int 2012;61:939e42. [15] Pensec S, Leroy M, Akkouche H, Spassky N. Stereocomplex formation in enantiomeric diblock and triblock copolymer of poly(ε-caprolactone) and polylactide. Polym Bull 2000;45:373e80. [16] Fukushima K, Chang YH, Kimura Y. Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid). Macromol Biosci 2007;7:829e35. [17] Tsuji H, Yamamoto S, Okumura A, Sugiura Y. Heterostereocomplexation between biodegradable and optically active polyesters as a versatile preparation method for biodegradable materials. Biomacromolecules 2010;11:252e8. [18] Tsuji H, Shimizu K, Sakamoto Y, Okumura A. Hetero-stereocomplex formation of stereoblock copolymer of substituted and non-substituted poly(lactide)s. Polymer 2011;52:1318e25. [19] Portinha D, Belleney J, Bouteiller L, Pensec S, Spassky N. Formation of nanoparticles of polylactide-containing diblock copolymers: is stereocomplexation the driving force? Macromolecules 2002;35:1484e6. [20] Kim SH, Nederberg F, Zhang L, Wade CG, Waymouth RM, Hedrick JL. Hierarchical assembly of nanostructured organosilicate networks via stereocomplexation of block copolymers. Nano Lett 2008;8:294e301. [21] Kang N, Perron M-E, Prud’homme RE, Zhang Y, Gaucher G, Leroux J-C. Stereocomplex block copolymer micelles: core-shell nanostructures with enhanced stability. Nano Lett 2005;5:315e9. [22] Biela T, Duda A, Penczek S. Enhanced melt stability of star-shaped stereocomplexes as compared with linear stereocomplexes. Macromolecules 2006;39:3710e3. [23] Tan BH, Hussain H, Lin TT, Chua YC, Leong YW, Tjiu WW, et al. Stable dispersions of hybrid nanoparticles induced by stereocomplexation between enantiomeric poly(lactide) star polymers. Langmuir 2011;27:10538e47. [24] Shin EJ, Jones AE, Waymouth RM. Stereocomplexation in cyclic and linear polylactide blends. Macromolecules 2012;45:595e8. [25] Cordes D, Lickiss P, Rataboul F. Recent developments in the chemistry of cubical polyhedral oligosilsesquioxanes. Chem Rev 2010;110:2081e254. [26] Laine RM. Nanobuilding blocks based on the [OSiO1.5]x (x ¼ 6, 8, 10) octasilsesquioxanes. J Mater Chem 2005;15:3725e44. [27] Baney RH, Itoh M, Sakakibara A, Suzuki T. Silsesquioxanes. Chem Rev 1995;95: 1409e30. [28] Goffin A-L, Duquesne E, Moins S, Alexandre M, Dubois Ph. New organicinorganic nanohybrids via ring opening polymerization of (di)lactones initiated by functionalized polyhedral oligomeric silsesquioxane. Eur Polym J 2007;43:4103e13. [29] Goffin A-L, Duquesne E, Raquez J-M, Miltner HE, Ke X, Alexandre M, et al. From polyester grafting onto POSS nanocage by ring-opening polymerization to high performance polyester/POSS nanocomposites. Mater Chem 2010;20: 9415e22. [30] Biela T, Duda A, Pasch H, Rode K. Star-shaped poly(L-lactide)s with variable numbers of hydroxyl groups at polyester arms chain-end and directly attached to the star-shaped core e controlled synthesis and characterization. J Polym Sci Part A Polym Chem 2005;43:6116e33. [31] Kim SH, Han Y-K, Kim YH, Hong SI. Multifunctional initiation of lactide polymerization by stannous octoate/pentaerythritol. Makromol Chem 1992;193:1623e31.
Contents lists available at SciVerse ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Short communication
Melt stability of 8-arms star-shaped stereocomplex polylactide with three-dimensional core structures Purba Purnama a, Youngmee Jung a, Soo Hyun Kim a, b, * a b
Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2012 Received in revised form 22 January 2013 Accepted 18 February 2013 Available online 27 February 2013
The melt stability of star-shaped stereocomplex polylactide with different core structure by tripentaerythritol (TPE) and polyhedral oligomeric silsesquioxane-octahydroxypropyldimethylsilyl (POSS(OH)8) compounds was investigated. The three-dimensional (3D) core structures affect the ability of star-shaped stereocomplex polylactide to preserve stereocomplex crystallite after melted. The exact position and proper space distribution of arm result in low chain freedom of itself which affects the ability of each arm to reassemble stereocomplex crystallites after melted. The result confirmed that the inorganic POSS(OH)8 core molecule has better support to maintain melt stability compare to TPE core. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Melt stability Polylactide Stereocomplex Star polymers Structure
1. Introduction Today, the bio-based polymers have received much interest because it made from renewable resources [1]. Polylactide (PLA) is one of the interesting synthetic biodegradable polymers. It has advantages in sustainable resources, non-toxic, biocompatibility, and biodegradability. The improvement on PLA materials has been studied for a long time through nanocomposite [2,3] and stereocomplexation approaches [4e7]. Stereocomplexation is one of the approaches to improve PLA properties by mixing enantiomeric polymer blend of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) [4e7]. Stereocomplexation can be formed by common methods, such as melt blending [8e10] and solution casting [4,11,12]. In our laboratory, supercritical carbon dioxide e dichloromethane (sc-CO2eDCM) was utilized as a new method to produce stereocomplex PLA (s-PLA) from highmolecular-weight PLA homopolymers resulted in perfect degree of s-PLA products in relatively short-time [13,14].
* Corresponding author. Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea. Tel.: þ82 2 958 5343; fax: þ82 2 958 5308. E-mail addresses: [email protected] (P. Purnama), [email protected] (S.H. Kim).
S-PLA has melting temperature (Tm) approximately 50 C higher than the Tm of either PLLA or PDLA [4,7]. Those improvement can be achieved for many type of s-PLA, such as s-PLA using block copolymer [15e21], star-shaped [22,23], and cyclic based-PLA [24]. In other hand, s-PLA has weakness in the melt stability due to limitation of memory to re-form s-PLA after melted. This weakness should be solved because melt process is the regular processing in industry. Some researchers reported some approaches to solve melt stability limitation. Biela et al. proposed the 13-arms or more of star-shaped s-PLA to maintain melt stability by hardlock-type interactions through parallel or anti-parallel orientations [22]. The number of arm is important factor to control the interaction between PLLA and PDLA in the s-PLA [22]. Up to date, the melt stable star-shaped s-PLA requires star-shaped homopolymers with 13-arms or more. Recently, there is no report of the melt stable star-shaped s-PLA with number of arm is lower than 13-arm. With regard to star-shaped s-PLA, the core structures are expected has big differences in the melt stability properties of star-shaped s-PLA. Herein, we studied the melt stability of 8-arm star-shaped s-PLA with different 3D core structure: organic and inorganic core of star-shaped s-PLA. We addressed whether 3D core structure affects the melt stability of the star-shaped s-PLA.
0141-3910/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.02.010
1098
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101
2. Experimental sections
2.4. Characterizations
2.1. Materials High purity L-Lactide (Purac Biochem BV, Gorinchem, The Netherlands) and D-lactide (Musashino Chemical) were used as received. Tripentaerythritol (TPE) (Sigma Aldrich, technical grade) and polyhedral oligomeric silsesquioxane-octahydroxypropyldimethylsilyl (POSS(OH)8) (Sigma Aldrich) were used as received. Tin(II)bis(2-ethylhexanoate) (Sn(Oct)2) (Sigma Chemical Co., St. Louis, MO, USA, purity 99%) was purified by distillation under reduced pressure and dissolved in dry toluene. Chloroform, methanol (Daejung Chemicals & Metal Co. Ltd. with a purity >99.5%), dichloromethane (DCM) (JT Baker, HPLC grade), and carbon dioxide gas (CO2) (Shin Yang Oxygen Industry Co. Ltd., minimum purity 99.9%) were used as received.
The molecular weight of star-shaped PLAs was analyzed by gel permeation chromatography (GPCmax, Viscotek VE 2001) equipped with micro-styragel columns calibrated with polystyrene. Chloroform was used as a mobile phase at a flow rate of 1.0 mL/min at 40 C. The 1H NMR of star-shaped structure from the polymerized materials was evaluated by 600 MHz NMR Spectrometer with CDCl3 as solvent. The stereocomplex formation of star-shaped sPLA was investigated by Wide-angle X-ray spectroscopy (WAXS) with an X-ray diffractometer Rigaku D/Max-2500 composed of Cu A, 30 kV, 100 mA) source, a quartz monochromator, Ka (l ¼ 1.54056 and a goniometric plate. The melt stability evaluation was performed by differential scanning calorimeter (DSC) (Modulated DSC 2910, TA Instrument) under nitrogen gas. The heating and cooling rate of DSC analysis were fixed at 10 C/min.
2.2. Ring-opening polymerization of star-shaped polylactides
3. Results and discussion
D-lactide or L-lactide was added into a 100 mL glass ampoule reactor equipped with magnetic stirring. Sn(Oct)2 and co-initiator were added into the ampoule at desired ratio. TPE was used as a co-initiator to produce organic core PLAs and POSS(OH)8 was used as a co-initiator to produce inorganic core PLAs. The reactor was purged with nitrogen for 3 times and vacuumed for 6 h. The reactor was sealed under vacuum. The ring-opening polymerization was conducted at 130 C for 24 h. After the reaction, mixture was allowed to cool to room temperature. The product was dissolved in chloroform, and poured into methanol for purification. The polymer was collected and dried under vacuum to constant weight.
The 8-arms organic and inorganic core of PLLA and PDLA were synthesized by stannous octoate initiated ring-opening polymerization at 130 C (Scheme 1). TPE was selected as 8-arms organic core and POSS(OH)8 as 8-arms inorganic core. TPE is the molecules consist of three molecules of pentaerythritol connected by ether bond. The backbone structure is single bond structure of CeC and CeO bonds. The POSS(OH)8 is an octa-functionalized hydroxyl group of POSS molecules. The POSS molecule families are real three-dimensional (3D) cage compound with silicon-oxide backbones [25e27]. The TPE and POSS(OH)8 with 8 eOH functional groups act as a co-initiator which lead polymerization growth in star-shaped form. The polymer chains were synthesized by in situ polymerization initiated by functional group from the core compounds [28,29]. Table 1 shows the melting point (Tm) and molecular weight of the synthesized materials. The structure of star-shaped PLA was confirmed by 1H NMR data as shown in Fig. 1. The peaks at 1.6 ppm and 5.2 ppm represented the methyl (a) and methine proton (b) resonances of the lactate units from linear PLA, respectively, in agreements with previous literature [28,30,31]. From 1H NMR spectrum of PLAePOSS
2.3. Stereocomplex formation Stereocomplexation of 8-arm star-shaped PLLA and 8-arm starshaped PDLA was conducted in sc-CO2eDCM at 65 C and 350 bar for 5 h in the 40 mL stainless steel high-pressure reactor equipped with magnetic stirring and electrical heating mantle as describe in previous work [13]. The reactor was opened immediately after the reaction had finished. The s-PLA was collected and vacuumed at 40 C for 1 night.
Scheme 1. Synthesis of 8-arms star-shaped PLA through bulk polymerization at 130 C. a. Using inorganic core (POSS(OH)8); b. Using organic core (TPE).
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101 Table 1 Summary of synthesized star-shaped PLA’s properties. Materials
[M:core] [mol]
Tma [ C]
Mnb [103]
PDIb
PLLAeTPE PDLAeTPE PLLAePOSS_A PDLAePOSS_A PLLAePOSS_B PDLAePOSS_B PLLAePOSS_C PDLAePOSS_C
420 420 420 420 560 560 840 840
163.33 162.64 164.44 164.21 165.08 167.22 172.28 170.71
63 77 49 65 72 83 156 132
1.47 1.74 2.38 1.63 2.33 1.26 1.26 1.70
a b
Measured by DSC at first scan with heating rate at 10 C/min. Measured by GPC.
(represented by PLLAePOSS_C), the peaks at 1.6 ppm and 5.2 ppm showed the characteristic of PLA absorption. The spectrum also has new peaks at 0.1 ppm and 0.6 ppm corresponds to eSiCH2 and e SiCH3 absorption peak, respectively. There is no other peak which refers to unreacted OH group of POSS(OH)8. By comparing the area of eCHe (lactate) and eSiCH2 absorption peak, the obtained molecular weight of PLLAePOSS_C is 156,802 g/mol. For organic starshaped PLA (PLLAeTPE), the presence of peaks at 4.1 ppm and
1099
3.4 ppm refers to the absorption of eCH2e at arms and backbone, respectively. By comparing the area of eCHe (lactate segment) and eCH2e (TPE backbone) absorption peaks, the obtained molecular weight of PLLAeTPE is 78,132 g/mol. These data showed that PLA fragment attached to all arms of the core materials. We successfully formed star-shaped s-PLA from star-shaped PLLA and star-shaped PDLA via sc-CO2eDCM (5 wt.%) at 65 C and 350 bar for 5 h. The s-PLAeTPE denotes the star-shaped s-PLA from PLLAeTPE and PDLAeTPE. The star-shaped s-PLA from PLLAePOSS A and PDLAePOSS A is denoted as s-PLAePOSS A, and soon. The dry and powder-like shape of star-shaped s-PLA was successfully obtained. The stereocomplexation of star-shaped s-PLA was confirmed by WAXS analysis (Fig. 2a). The observed strong peaks at 11.86 , 20.56 , and 23.86 of 2q without any characteristic peaks of homopolymers at 16.52 and 19.08 of 2q proved the formation of s-PLA, in agreement with previous reports [13]. The absence of homocrystal peaks demonstrated that star-shaped s-PLA was perfectly obtained. The melt stability is important factor to maintain physical properties and process ability of materials in industrial applications. The evaluation on melt stability of 8-arms star-shaped s-PLA with organic (s-PLAeTPE) and inorganic core (s-PLAePOSS A) was performed by a DSC as shown in Fig. 2b. The DSC thermograms
Fig. 1. The 1H NMR spectra of PLLAePOSS_B, PLLA, and PLLAeTPE.
1100
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101
Fig. 2. (a) WAXS patterns of star-shaped s-PLA by organic core (s-PLAeTPE) and inorganic core (s-PLAePOSS A) prepared in sc-CO2eDCM at 350 bar and 65 C for 5 h. The WAXS pattern of PLA homopolymer was added as a control. (b) DSC scans of sPLAeTPE and s-PLAePOSS A before and after melted at 260 C. The heating and cooling rate were fixed 10 C/min.
Table 2 Summary of degree of s-PLA inorganic star-shaped s-PLA before and after melted. Materials
S-PLAePOSS A S-PLAePOSS B S-PLAePOSS C
Before
After
Deg of s-PLAa [%]
DHma
Deg of s-PLAa [%]
DHma
[J/g]
100 100 91.34
71.68 57.05 53.82
100 100 92.11
31.12 29.39 54.56
[J/g]
a Measured by DSC under nitrogen. Heating and cooling rate are fixed at 10 C/min.
before melted (Fig. 2b, left) confirmed the formation of star-shaped s-PLA in a perfect s-PLA degree. This result was in agreement with above WAXS result. After melted and recrystallized, there is a big difference in the degree of s-PLA crystallites from inorganic core and organic core of star-shaped s-PLA (Fig. 2b, right). The s-PLAeTPE remains degree of s-PLA about 84.95% but s-PLAePOSS A remains degree of s-PLA about 100% which means inorganic core of star-shaped s-PLA has better melt stability compare to the organic core of star-shaped sPLA. We assumed the 3D core structure of s-PLA will lead to different ability in reforming s-PLA after melted. We also continued to examine the inorganic core star-shaped sPLA in different molecular weight as shown in Table 2. We obtained high s-PLA degree of inorganic core star-shaped s-PLAs by sc-CO2e DCM with 5 h processing time. Decreasing s-PLA crystallites degree of s-PLAePOSS C due to higher molecular weight requires longer processing time in sc-CO2eDCM system [14]. Regarding DSC data, s-PLAePOSS A and s-PLAePOSS B are able to re-form the s-PLA crystallite completely after melted. The s-PLAePOSS C, in the first heating the s-PLA portion is about 91.34% and in the second heating is about 92.11%. These phenomena informed that the octafunctionalized POSS core structure has strong ability of s-PLLAe POSSs from low- and high-molecular-weight star-shaped PLAs to preserve s-PLA crystallites after melted. In the previous report related to star-shaped s-PLA, the proposed mechanisms in maintaining melt stability are hardlock-type interactions through parallel and anti-parallel orientation which are suitable for star-shaped having 13-arms or more [22]. The parallel orientation is only suitable for star-shaped s-PLA in twodimensional (2D) views. The 3D structure of organic and inorganic core of star-shaped s-PLA does not allow to re-form s-PLA through parallel orientation due to steric hindrance. By this work, the data exactly proved the possible orientation to re-form s-PLA after melted is anti-parallel orientation. As above explanation, the s-PLAeTPE and s-PLAePOSSs showed the different result in melt stability. It may be caused by the different core structure of TPE and POSS(OH)8 compounds in 3D view as illustrated in Fig. 3a. The organic core, TPE, has 3 arms in each side and 2 arms in the middle part. The 2 arms at the middle part of organic core have wider space compare to the other 6-arms at side part. The backbone structure with single bond CeC and CeO of TPE may also support the arms at the middle part to rotate. These conditions affected the hardlock interaction of the arms at the middle part which mean those two arms are having “chain
Fig. 3. (a) Illustrated 3D core structure of TPE and POSS(OH)8 compounds. (b) DSC thermograms of cooling process of inorganic core star-shaped s-PLA after melted at 260 C. The cooling rate was fixed 10 C/min.
P. Purnama et al. / Polymer Degradation and Stability 98 (2013) 1097e1101
freedom” close to linear s-PLA. Consequently, the degree of starshaped s-PLA with organic core was slightly decreased (w15%). For inorganic core, POSS(OH)8 has real 3D cage structure with silicon-oxide backbones and 8 eOH functional groups in each corner. It leads each arm distributed at certain position and has similar free space distribution. Each arm has lower “chain freedom” compare to linear s-PLA. Hence, it supports each arm to reassemble of s-PLA easily after melted. The observation of cooling process in DSC analysis (Fig. 3b) suggested recrystallization mechanism for s-PLAePOSSs with different molecular weight. S-PLAePOSS A and s-PLAePOSS B have single recrystallization peak, but s-PLAePOSS C has two recrystallization peaks. In the melt state, the star-shaped PLAs were unzipped from star-shaped s-PLA, and then recrystallized during cooling process. For high-molecular weight star-shaped PLAs, each arm has longer chain length compared to the low-molecular weight star-shaped PLAs. The longer chain also decreases the chain freedom of unzipped chain. The different “chain freedom” causes the high-molecular-weight easier to recrystallize represent by different recrystallization peak. The recrystallization peak at 110 C of s-PLAePOSS C is belongs to the unstereocomplexed star-shaped PLAs and the peak at 146 C is belongs to recrystallization peak of unzipped star-shaped PLAs from the s-PLAePOSS C. Recrystallization of unstereocomplexed star-shaped PLAs during cooling process leads to small increasing s-PLAePOSS C portion after melted. In the s-PLAePOSSs materials, the space distribution and the chain length have simultaneous effect to reduce the “chain freedom”. 4. Conclusions In conclusion, the 8-arm star-shaped s-PLA was successfully synthesized using different core compounds. However, the inorganic POSS(OH)8 core has better support to maintain melt stability of 8-arm star-shaped s-PLA compare to organic TPE core. The 3D core structure affects the melt stability of star-shaped s-PLA. In the 8-arm star-shaped s-PLA, the arm distribution and chain freedom influence the ability of each arm to re-form s-PLA after melted. The real 3D core structure also proved the anti-parallel orientation as the suitable mechanism to enhance melt stability of star-shaped s-PLA. The use of inorganic materials as a core material opens a new way to develop hybrid polylactide materials with good melt stability. Acknowledgment This study was supported by the National Research Foundation of Korea Grant funded by the Korea Government (MEST), NRF2010-C1AAA001-0028939. References [1] Tsuji H, Ikada Y. Properties and morphologies of poly(L-lactide):1. Annealing condition effects on properties and morphologies of poly(L-lactide). Polymer 1995;36:2709e16. [2] Messermith PB, Giannelis EP. Synthesis and barrier properties of poly(ε-caprolactone)-layered silicate nanocomposites. J Polym Sci Part A Polym Chem 1995;33:1047. [3] Brown JM, Curliss D, Vaia RA. Thermoset-layered silicate nanocomposites. Quaternary ammonium montmorillonite with primary diamine cured epoxies. Chem Mater 2000;12:3376.
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