Stereocomplex Formation between Enantiomeric Poly(lactic acid)s. 12

Jun 12, 2004 - The front factor (G0) and nucleation constant (Kg) for the stereocomplex crystallites in the temperature range of 140−190 °C were es...
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Biomacromolecules 2004, 5, 1181-1186

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Stereocomplex Formation between Enantiomeric Poly(lactic acid)s. 12. Spherulite Growth of Low-Molecular-Weight Poly(lactic acid)s from the Melt Hideto Tsuji* and Yasufumi Tezuka Department of Ecological Engineering, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Received March 19, 2004; Revised Manuscript Received May 10, 2004

The spherulite growth of stereocomplex crystallites in the blend from low-molecular-weight poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e., poly(D-lactic acid) (PDLA)] from the melt, together with that of the homocrystallites in pure PLLA and PDLA films, was investigated using polarization optical miscroscopy. The spherulite growth of stereocomplex crystallites occurred at a wider temperature range (e190 °C) compared with that of homocrystallites (e140 °C). At 140 °C, the spherulite radius growth rate (G) for the stereocomplex crystallites (136.4 µm min-1) was an order of magnitude higher than those for the homocrystallites of PLLA (11.8 µm min-1) and PDLA (15.7 µm min-1), whereas the induction period was shorter for the spherulties of stereocomplex crystallites (0.0 min) than for the spherulties of homocrystallites of PLLA (2.6 min) and PDLA (0.7 min). In addition to these two factors, the higher spherulite density of stereocomplex crystallites compared with those of the homocrystallites of PLLA and PDLA resulted in rapid completion of overall crystallization of stereocomplex. The front factor (G0) and nucleation constant (Kg) for the stereocomplex crystallites in the temperature range of 140-190 °C were estimated to be 3.56 × 1012 µm min-1 and 8.42 × 105 K2, respectively. The G0 value for stereocomplex crystallites was 1 and 2 orders of magnitude higher than those for the homocrystallites of PLLA (9.69 × 1011 µm min-1) and PDLA (8.79 × 1010 µm min-1), whereas the Kg value for stereocomplex crystallites was twice those for the homocrystallites of PLLA (4.95 × 105 K2) and PDLA (4.20 × 105 K2). Introduction The stereocomplex crystallization or racemic crystallization can occur between enantiomeric polymers when the interaction between the two polymers having different configuration is stronger than that between those having the same configuration.1-3 Since Ikada et al.4 found stereocomplex crystallization between enantiomeric poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e., poly(D-lactic acid) (PDLA)], intensive and numerous studies on the stereocomplex crystallization have been carried out.1-3,5 Such stereocomplex crystallization was found to enhance the mechanical performance,6 thermal stability,7 and hydrolysisresistance8,9 of the poly(lactide)s [i.e., poly(lactic acid)s (PLAs)] materials, which are producible from renewable resources and degradable in the environment as well as in the human body.10-19 From the studies of stereocomplex crystallization from the melt in the past, the following results were disclosed: (1) The overall stereocomplex crystallization completes in a shorter period compared with that required for the completion of overall homocrystallization of pure PLLA and PDLA;20 (2) stereocomplex crystallites can be formed epitaxially on the homocrystallites of PLLA;21 (3) talc can enhance the spherulite ncleation of stereocomplex crystallites.22 Here, “homocrystallites” are defined to be composed * To whom correspondence should be addressed.

either of PLLA or PDLA chains and “homocrystallization” means formation of homocrystallites. The first result may be ascribed to the higher spherulite density of stereocomplex crystallites, which can be seen from the fact that the final spherulite sizes of stereocomplex crystallites were smaller than those of homocrystallites either of pure PLLA and PDLA.20,23 Moreover, it was found that stereocomplex crystallites formed in PLLA materials by addition of PDLA act as the nucleator of PLLA spherulites.24,25 For the spherulites growth of PLLA and L-lactide copolymers and that of PLLA in the blends, numerous studies including regime analysis have been performed,20,23,26-36 and thereby, a sufficiently great amount of information has been obtained and accumulated. However, as far as we are aware, a detailed study on spherulite growth of the PLA stereocomplex crystallites has not been reported so far. The information on the growth rate, induction period, crystallizable temperature range, and morphology, which can be obtained from the detailed study, must be useful in preparation of stereocomplexed PLA materials. The objectives of this study were to investigate the spherulite growth of PLA stereocomplex crystallites in comparison with that of the homocrystallites of pure PLLA and PDLA and to elucidate the mechanisms for rapid completion of overall stereocomplex crystallization compared with prolonged completion of overall homocrystallization of pure PLLA and PDLA. For these purposes, a PLLA/PDLA

10.1021/bm049835i CCC: $27.50 © 2004 American Chemical Society Published on Web 06/12/2004

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Table 1. Characteristics and Properties of As-cast L, D, and L/D Films Init. conca code (wt %) Ld De L/Df

3.0 2.5

Mw (g/mol) 1.0 × 104 2.2 × 104

[R]58925 Tgb Tmc Mw/Mn (deg dm-1 g-1 cm3) (°C) (°C) 1.3 1.5

-149 +155

63 64 67

158 165 224

a Initiator (lauryl alcohol) concentration for polymerization. b Glass transition temperature. c Melting temperature. d Poly(L-lactide) (PLLA) film. e Poly(D-lactide) (PDLA) film. f Equimolar blend film of PLLA and PDLA.

(1/1) blend film and pure PLLA and PDLA films having low molecular weights were prepared and their spherulite growth was monitored by the use of polarization optical microscope equipped with a heating-cooling stage. Relatively low-molecular-weight PLLA and PDLA (in the order of 104 g mol-1) were used in this study for film preparation because the ratio of stereocomplex crystallites to homocrystallites depends on the molecular weight of PLLA and PDLA, and solely stereocomplex crystallization takes place when the molecular weights of PLLA and PDLA are both in the order of 103-104. 20 Experimental Section Materials. Synthesis and purification of PLLA and PDLA used in this work were described in previous papers.4,23 Namely, ring-opening polymerization of D- and L-lactides was performed in bulk at 140 °C initiated by stannous octoate (0.03 wt %) in the presence of lauryl alcohol as coinitiator.37-39 Polymerization conditions and molecular characteristics of PLLA and PDLA utilized in this study are listed in Table 1. Films (10 µm thick) used for crystallization experiments were prepared with the method described in previous papers.6,8,9,20 Briefly, each solution of PLLA and PDLA was separately prepared to have a polymer concentration of 1.0 g/dL and then admixed with each other under vigorous stirring in the case of blend film preparation. Methylene chloride was used as a solvent, and the mixing ratio of PLLA and PDLA was fixed to 1:1 for the blend film. The solutions were cast onto Petri dishes, followed by solvent evaporation at 25 °C for approximately 1 day. The obtained films were dried in vacuo for at least 1 week. In this study, L, D, and L/D stand for pure PLLA and PDLA films and their 1:1 blend film, respectively. Physical Measurements and Optical Observation The weight- and number-average molecular weights (Mw and Mn, respectively) of polymers were evaluated in chloroform at 40 °C using a Tosoh (Tokyo, Japan) GPC system with two TSK gel columns (GMHXL) and polystyrene standards. Therefore, the Mw and Mn values are given relative to polystyrene. Specific optical rotation [R] of polymers was measured in chloroform at a concentration of 1 g dL-1 and 25 °C using a JASCO DIP-140 polarimeter at a wavelength of 589 nm. Melting and glass transition temperatures (Tm and Tg, respectively) and enthalpies of cold crystallization and melting (∆Hcc and ∆Hm, respectively) of the as-cast films and melt-crystallized films were determined with a Shimadzu

(Kyoto, Japan) DSC-50 differential scanning calorimeter. Films were heated at a rate of 10 °C min-1 under a nitrogen gas flow for DSC measurements. The Tm, Tg, and ∆Hm values were calibrated using tin, indium, and benzophenone as standards. The DSC measurements revealed that the as-cast and melt-crystallized L/D films showed a glass transition peak (50-70 °C) and a melting peak at around 220 °C with ∆Hm of 68-86 J g-1 but no melting peak at around 170 °C, indicating that the L/D films contained solely stereocomplex crystallites.4-6 In addition to these peaks, for the meltcrystallized L/D films a very small cold crystallization peak with ∆Hcc of 0.5-1.5 J g-1 was observed above the temperature applied for crystallization from the melt, reflecting that slight thickening or reduction of lattice disorder of the stereocomplex crystallites occurred during DSC heating. The characteristics and properties of the as-cast films are summarized in Table 1. The spherulite growth in the films (thickness ca. 10 µm) was observed using an Olympus (Tokyo, Japan) polarization microscope (BX50) equipped with a heating-cooling stage and a temperature controller (Linkam LK-600PM) under a constant nitrogen gas flow. The crystallization of the films was performed as follows. The films were first heated to 200 and 250 °C for L and D films and L/D film, respectively, at 100 °C min-1, held at these temperatures for 3 min to destroy thermal history, cooled at 100 °C min-1 to an arbitrary crystallization temperature (Tc) in the range of 100200 °C, and then held the Tc. Results and Discussion Spherulite Morphology and Growth. Figure 1 shows the polarization photomicrographs of L, D, and L/D films. At 140 °C, which temperature was critical temperature below this temperature spherulite growth occurred in L and D films, isolated and slightly deformed spherulites (hexagonal crystallite assemblies) were observed in the D film at 6 min and the L film at 11 min, whereas the spherulites of stereocomplex crystallites covered L/D film in a very short period of 0.5 min. At 190 °C and 12 min, the well-defined spherulites of sterecomplex crystallites were observed in the L/D film, whereas at Tc exceeding 190 °C no spherulite was formed in L/D film. The spherulite density of stereocomplex crystallites was higher at 140 °C than at 190 °C. Such temperature dependence of spherulite density has been reported for pure L and D films.20,29 Figure 2 gives the spherulite radius of of L, D, and L/D films as a function of crystallization time (tc). Evidently, the spherulites of L, D, and L/D films grew linearly with tc, irrespective of Tc. The spherulites of stereocomplex crystallites can grow at a temperature below 130 °C, but too high of a density and rapid growth completion of the spherulites disturbed the evaluation of time change of the radii. The radius growth rates of the spherulites (G) were estimated from the slope of spherulite radii in Figure 2, whereas the induction periods for the spherulite growth (ti) were evaluated from extrapolation of the spherulite radius lines in Figure 2 to a radius of 0 µm. The obtained G and ti values are plotted in Figures 3 and 4, respectively, as a function of Tc.

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Figure 1. Polarization photomicrographs of the L, D, and L/D films: (A) L film, Tc ) 140 °C, tc ) 11 min; (B) D film, Tc ) 140 °C, tc ) 6 min; (C) L/D film, Tc ) 140 °C, tc ) 0.5 min; (D) L/D film, Tc ) 190 °C, tc ) 12 min. Table 2. Maximum G (Gmax), Tc Which Gives Gmax [Tc(max)], Front Factor (G0), and Nucleation Constant (Kg) specimens

researchers

Mw or Mv (g/mol)

PLLA

Vasanthakumari and Pennings28 Miyata and Masuko32 Huang et al.33 Abe et al.34 Baratian et al.35 Di Lorenzo36 this study

1.5 × 105-6.9 × 105 (Mv) 5.0 × 104-2.0 × 105 (Mw) 1.3 × 105 (Mw) 2.2 × 104-7.1 × 105 (Mw) 1.3 × 105 (Mw) 1.0 × 105 (Mw) 1.0 × 104 (Mw) 2.2 × 104 (Mw)

Lc Dd L/De

Tc(max) (°C)

Gmax (µm min-1)

G0a (µm min-1)

K ga (K2)

130 120-125 130 130b 130 130 120 120 140f

2.5-5.1 3.1-7.5 4.5 4.1-19.0 4.6 6.8 37.8 36.1 136

1.56-3.38 × 107

2.29-2.44 × 105

1.53-2.70 ×

9.69 × 1011 8.79 × 1010 3.56 × 1012

1010

2.4 × 105 4.64-5.01 × 105 1.92 × 105 1.85 × 105 4.95 × 105 4.20 × 105 8.42 × 105

a G and K values are for regime II. b T 0 g c(max) values could not be estimated for PLLAs with lower molecular weights because the transition from regime II to regime III occurred. c Poly(L-lactide) (PLLA) film. d Poly(D-lactide) (PDLA) film. e Equimolar blend film of PLLA and PDLA. f G values at the Tc lower than 140 °C could not be obtained because of rapid completion of overall stereocomplex crystallization.

Spherulite Radius Growth Rate. As seen in Figure 3, the G of L and D films gave maximum values of 37.8 and 36.1 µm/min, respectively, at around 120 °C, whereas no spherulite growth occured at the Tc exceeding 140 °C. Table 2 listed the maximum G (Gmax) and Tc which gives Gmax [Tc(max)] in the present study and previous articles.28,32-36 These Gmax values in the present study are higher than 19.0 µm/min which is the highest value reported by Abe et al. for PLLA with Mw of 2.2 × 104 g mol-1.34 The high Gmax value of the L film can be simply ascribed to its low molecular weight (Mw ) 1.0 × 104 g mol-1), which enhances the molecular motion. In contrast, probable reasons for the high Gmax value of the D film (Mw ) 2.5 × 104 g mol-1)

compared with that of PLLA having a similar molecular weight may be differences in the molecular weight distribution and optical purity of the polymers, the kind and amount of impurities in the polymers, the thickness of the films, and/ or crystallization circumstances. On the other hand, the G of the stereocomplex crystallites in the L/D film decreased monotonically and dramatically with Tc from 136 µm/min at 140 °C to 7.3 µm/min at 190 °C. Surprisingly, at 140 °C, the G value (136 µm/min) of stereocomplex crystallites is 11.5 and 8.7 times those of homocrystallites in L and D films (11.8 and 15.7 µm/min, respectively). Moreover, it is interesting to note that the spherulites of stereocomplex crystallites can grow even at

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Figure 3. Radius growth rate of spherulites (G) of L, D, and L/D films as a function of crystallization temperature (Tc).

Figure 4. Induction period of spherulites (ti) of L, D, and L/D films as a function of crystallization temperature (Tc).

Figure 2. Radius of spherulites of L (a), D (b), and L/D (c) films as a function of crystallization time (tc).

the Tc exceeding 140 °C, at which no spherulite of homocrystallites was formed in L and D films and even at a Tc exceeding 170 °C, which is higher than the Tm values of homocrystallites in L and D films (158 and 165 °C, respectively). In other words, the stereocomplex crystallites and their nuclei must be stable at a wide tempera-ture

range below 190 °C. It is probable that the G of stereocomplex crystallties in the L/D film becomes higher than 136 µm/min at 140 °C, when the Tc is decreased below 140 °C. This is expectedfrom the temperature dependence of the G of L and D films, in which the Gmax values were given at Tc(max) ) 120 °C. However, extremely rapid completion of overall stereocomplex crystallization disturbed the estimation of G values at Tc below 140 °C. Yamane and Sasai reported the G of PLLA/PDLA (99/1-95/5) blend films, in which the spherulites of PLLA homocrystallites not those of stereocomplex crystallites were formed and the sterecocomplex crystallites acted as the nucleator of PLLA spherulites.25 Induction Period. The ti values of homocrystallites in L and D films were below 1 min excluding that of the L film

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Figure 5. ln G + 1500/R(Tc - T∞) of L, D, and L/D films as a function of 1/(Tc∆Tf).

at 140 °C (2.7 min) and gave a minimum at about 0 min at 130 °C (Figure 4). In contrast, the ti values of stereocomplex crystallites in the L/D film were approximately 0 min in the Tc range of 140-180 °C and increased dramatically with Tc to 4.7 min at 190 °C. Practically nil ti values of stereocomplex crystallites in the L/D film in the Tc range of 140-180 °C reflects a higher stability of the formed nuclei compared with that of the homocrystallites in L and D films. This can be expected from higher Tm of the sterocomplex crystallites than those of the homocrystallites. Here, we can conclude that the reasons for rapid completion of overall stereocomplex crystallization compared with prolonged completion of overall homocrystallization at arbitrary Tc above 140 °C is ascribed to high spherulite density, short ti, and high G value. Such rapid overall crystallization of the stereocomplex at the high Tc range disturbed the preparation of amorphous films especially for PLLA and PDLA having low molecular weights even by a quenching process from the melt.20 Nucleation and Front Constants. We have estimated the nucleation constant (Kg) and the front constant (G0) for L, D, and L/D films by the use of nucleation theory established by Hoffman et al.,40,41 in which G can be expressed by the following equation: G ) G0 exp[-U*/R(Tc - T∞)] exp[-Kg/(Tc∆Tf)]

(1)

where ∆T is undercooling T 0m - Tc where Tm0 is equilibrium Tm, f is the factor expressed by 2Tc/(T 0m + Tc) that accounts for the change in heat of function as the temperature is decreased below T 0m, U* is the activation energy for transportation of segments to the crystallization site, R is the gass constant, and T∞ is the hypothetical temperature where all motion associated with viscous flow ceases. Figure 5 illustrates ln G + 1500/R(Tc - T∞) of L, D, and L/D films as a function of 1/(Tc∆Tf). This plot will give Kg as a slope and the intercept ln G0. Here, we used the universal values of U* ) 1500 cal mol-1 and T∞ ) Tg - 30 K for comparison

with the reported values, though Urbanovici et al. suggested that U* has to be temperature-dependent not a constant and that instead of T∞ ) Tg - 30 K, Tg should be used for T∞.42 The Kg values 4.95 × 105 and 4.20 × 105 K2 obtained for L and D films are in complete agreement with 4.644.97 × 105 K2 reported for PLLA (Mw ) 0.22-7.1 × 105 g mol-1) crystallized according to regime II kinetics (Abe et al.)34 but are twice that of 1.85-2.44 × 105 K2 reported for PLLA (Mv ) 1.5 × 105-6.9 × 105 g mol-1, Mw ) 1.0-1.3 × 105 g mol-1) crystallized according to regime II kinetics (Vasanthakumari and Pennings,28 Huang et al.,33 Baratian et al.,35 and Di Lorenzo36). The molecular weight range of PLLA and PDLA used in this study was much lower than those used in the latter studies,28,33,35,36 which may have caused the Kg value difference. Abe et al. reported that the crystallization of PLLA proceeds by regime II kinetics at the temperature range of 120-147 °C,34 whereas Vasanthkumari and Pennings28 found that the transition from regime II to regime I takes place at 163 °C. Moreover, Di Lorenzo36 and Iannace and Nicolais43 reported that the transition from regime III to regime II occurs at 115 and 120 °C, respectively. In addition, the spherulite morphology of homocrystallites of L and D films at Tc ) 140 °C (Figure 1) was different from the axialites and single crystals observed for regime I kinetics.28 These findings together with the low molecular weights of L and D films in the present study strongly suggest that in the Tc range of 110-140 °C pure PLLA and PDLA crystallize according to regime II kinetics.28,34 On the other hand, the Kg value for the L/D film 8.42 × 105 K2 was twice that of L and D films. From the spherulitic morphology of crystallite assemblies in Figure 1, parts c and d, stereocomplex crystallization proceeds according to the regime II or III kinetics. If we assume regime II kinetics for L, D, and L/D films in the temperature range of 110-190 °C, Kg can be expressed as follows:40,41 Kg ) 2bσσeT 0m/∆hfk

(2)

where b is the layer thickness, σ is the lateral surface free energy, σe is the fold surface free energy, ∆hf is the heat of melting per unit volume, and k is the Boltzmann constant. The higher Kg value for stereocomplex crystallites can be partly due to its higher T 0m (279 °C)23 compared with that of homocrystallites (205-227 °C)27,29-31,34 and the higher b value of stereocomplex crystallites (0.74 nm assuming that the crystallites grow along the 〈110〉 direction)44 compared with that of homocrystallites (0.53 nm assuming that the crystallites grow along the 〈110〉 direction).45 Large σ and σe values and/or a small ∆hf value of stereocomplex crystallties compared with those of homo crysatllites may be the causes for high Kg value of stereocomplex crystallites. However, the ∆hf value reported for stereocomplex crystallites (142 J/g)46 is in the ∆hf value range reported for homocrystallites (93-203 J/g),26,32,33,45-47 and there has been no report for the σ and σe values of steterocomplex crystallties. The G0 values estimated for L and D films were 9.69 × 1011 and 8.79 × 1010 µm min-1. The former and latter G0

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value were respectively 1 order of magnitude higher than and in the same order of 1.53-2.70 × 1010 µm min-1 reported by Abe et al.34 The fact that the L film having a lower molecular weight than the D film had the higher G0 value than that of D film is in consistent with the fact that G0 increases with decreasing the molecular weight.28,34 However, the reason for the fact that our G0 values of L and D films were much higher than the G0 values 1.56-3.38 × 107 µm min-1 reported for regime II kinetics by Vasanthakumari and Pennings28 is not clear to us. On the other hand, the G0 value for stereocomplex crystallites in the L/D film (3.56 × 1012 µm min-1) was much higher than 9.69 × 1011 and 8.79 × 1010 µm min-1 estimated for homocrystallites in L and D films, respectively. Therefore, it is obvious that the higher Kg and G0 values of stereocomplex crystallites compared with those of homocrystallites contribute the high G value as can be seen from eq 1. However, further investigations are required to determine which basic factors such as included in eq 2 are crucial to increase G value of stereocomplex crystallites. In conclusion, this study revealed that the stereocomplex crystallites have much higher spherulite growth rate and shorter induction period than those of the homocrystallites and that the rapid completion of the stereocomplex crystallization compared with that of homocrystallization can be ascribed to the very short induction period, extremely high growth rate and density of the stereocomplex spherulites. Detailed investigations with respect to the effects of molecular weight on the spherulite growth of stereocomplex will be published soon. Acknowledgment. The encouragement and support of Prof. Dr. Yoshito Ikada, Suzuka University of Medical Science, for this study are greatly appreciated. The authors thank Daicel Chemical Industries, Ltd. (Japan) for supplying methyl D-lactate, Professor Dr. Shinichi Itsuno, from Department of Materials Science, Faculty of Engineering at Toyohashi University of Technology, for the use of the polarimeter facility, and Dr. Takumi Okihara, from Department of Applied Chemistry, Faculty of Engineering, Okayama University, for his significant and precious suggestions for a crystallization mechanism of stereocomplex. This research was supported by a Grant-in-Aid for Scientific Research on Priority Area, “Sustainable Biodegradable Plastics” No. 11217209, a Grant-in-Aid for Scientific Research (A)(2) No. 15201017, and The 21st Century COE Program, “Ecological Engineering for Homeostatic Human Activities”, from the Ministry of Education, Culture, Sports, Science and Technology (Japan). References and Notes (1) Slager, J.; Domb, A. J. AdV. Drug DeliVery ReV. 2003, 55, 549583. (2) Slager, J.; Gladnikoff, M.; Domb, A. J. Macromol. Symp. 2001, 175, 105-116. (3) Tsuji, H. In Research AdVances in Macromolecules; Mohan, R. M., Ed.; Grobal Research Network: Thiruvanathapuram, India, 2000; Vol. 1, pp 25-48. (4) Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S.-H. Macromolecules 1987, 20, 904-906. (5) Tsuji, H. In Polyesters 3 (Biopolymers, Vol. 4); Doi, Y., Steinbu¨chel, A., Eds.; Wiley-VCH: Weinheim, Germany, 2002; pp 129-177.

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