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Stereocomplex crystallization and homo-crystallization of enantiomeric poly(2-hydroxybutyrate)s: Effects of molecular weight and crystallization conditions
Polymer 53 (2012) 5385e5392
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Stereocomplex crystallization and homo-crystallization of enantiomeric poly(2-hydroxybutyrate)s: Effects of molecular weight and crystallization conditions Hideto Tsuji*, Satoru Shimizu Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2012 Received in revised form 5 September 2012 Accepted 17 September 2012 Available online 23 September 2012
Enantiomeric substituted polylactides, poly(L-2-hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) [P(D-2HB)] with different weight-average molecular weights (Mw ¼ 1.8 103e4.1 104 g mol1) were synthesized by acid-catalyzed polycondensation. The effects of molecular weights of P(L-2HB) and P(D-2HB) and crystallization conditions on stereocomplex (SC) crystallization and homo-crystallization of neat P(L2HB), neat P(D-2HB), and their equimolar blends were investigated by means of differential scanning calorimetry and wide-angle X-ray scattering. Only SC crystallites were formed during solution-crystallization of the blends without the formation of homo-crystallites, irrespective of average Mw. SC crystallites were only crystalline species formed during melt-crystallization of the blends, in the crystallization temperature (Tc) range of 55e160 C (except at 70 C). At Tc ¼ 70 C, in addition to SC crystallites, a significant amount of homo-crystallites were formed for average Mw exceeding 2.7 104 g mol1, and equal amounts of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 104 g mol1. However, a critical average Mw above which predominant homo-crystallization and no SC crystallization occur was not observed. The equilibrium melting temperature of SC crystallites obtained by the HoffmaneWeeks procedure was 239.1 C, which was lower than the highest value (279 C) reported for polylactide SC crystallites. In solution-crystallization of the neat P(L-2HB) and P(D-2HB), a new type of homo-crystallites were formed for Mw below 1 104 g mol1 and the normal type of homo-crystallites were formed for Mw above 1 104 g mol1. In contrast, melt-crystallization of the neat P(L-2HB) and P(D-2HB) generated the new and the normal type homo-crystallites for Mw ¼ 9 103e1 104 g mol1 and Mw above 9 103 g mol1, respectively. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Poly(hydroxybutyric acid) Poly(hydroxybutanoic acid) Stereocomplexation
1. Introduction Poly(L-lactide), i.e, poly(L-lactic acid) (PLLA) is a biodegradable polyester produced from renewable carbohydrate resources of such as corn starch [1e4]. Stereocomplex (SC) crystallization in blends of PLLA and poly(D-lactide), i.e., poly(D-lactic acid) (PDLA), or in the stereoblock copolymer PLLA-b-PDLA effectively improves the mechanical properties, thermal/hydrolytic degradation resistance, and gas barrier properties of poly(lactide), i.e., poly(lactic acid) (PLA)-based materials [5e8]. Poly(2-hydroxybutyrate), i.e., poly(2hydroxybutanoic acid) [P(2HB)] is a biodegradable polyester having a structure similar to that of PLA except that the methyl groups are substituted by ethyl groups. As in the case of PLA [5e8], SC crystallites
* Corresponding author. E-mail address: [email protected] (H. Tsuji). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.09.039
can be formed by blending enantiomeric P(2HB)s, i.e., poly(L-2hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) [P(D2HB)] [9,10]. Moreover, hetero-SC crystallization was found to occur in P(L-2HB)/PDLA or P(D-2HB)/PLLA blends [11] and a P(D-2HB)-b-PLLA block copolymer [12]. Very recently, ternary SC crystallization was reported to take place in ternary P(L-2HB)/P(D-2HB)/PDLA blends [13]. The melting temperature (Tm) of the P(L-2HB)/P(D-2HB) SC crystallites (about 200 C) is higher than those of neat P(L-2HB) and P(D-2HB) (about 100 C) [9,10]. P(L-2HB)/P(D-2HB) blend exhibits enhanced radial growth rate of spherulites (G) and hydrolytic/ thermal degradation resistance relative to neat P(L-2HB) and P(D-2HB) [10]. This indicates that the intermolecular interaction between the P(L-2HB) and P(D-2HB) chains, having opposite configurations, in the amorphous regions or in the molten state is higher than that between the iso-structural chains of P(L-2HB) or P(D-2HB). In the case of PLA, the relative amount of SC crystallites versus homo-crystallites of either PLLA or PDLA produced is determined by the molecular weights of the enantiomeric PLAs
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Table 1 Molecular characteristics of P(L-2HB)s and P(D-2HB)s. Polymer
Code
Reactiona time (h)
Mwb (g mol1)
P(L-2HB)
L-41
48 24 20 12 2 2
4.08 3.12 3.02 1.97 1.00 1.82
48 24 20 12 2 2
3.36 3.26 2.30 1.86 8.61 3.33
L-31 L-30 L-20 L-10 d L-2
P(D-2HB)
c
D-34 D-33 D-23 D-19 c D-9 d D-3
a b c d
Mw/Mnb
Yield (%)
104 104 104 104 104 103
1.60 1.42 1.70 1.38 2.34 2.41
50.9 42.5 46.4 27.7 15.7 e
104 104 104 104 103 103
1.55 1.34 1.57 1.32 2.17 3.38
46.9 27.6 38.1 28.0 12.6 e
Reaction time for second step of polycondensation. Mw and Mn are weight- and number-average molecular weights, respectively. Sample undissolved when extracted with methanol. Sample dissolved when extracted with methanol.
and the crystallization conditions including the protocol employed (e.g., solution-crystallization or melt-crystallization), as well as the solvent type and polymer concentrations, and crystallization temperature (Tc). The use of high molecular weight PLAs is beneficial for enhanced performance of polymeric materials. However, homo-crystallization prevails over SC crystallization during the melt- and solution-crystallization processes when the molecular weights of both PLLA and PDLA exceed 1 104 g mol1 and 1 105 g mol1 for the respective processes [14e16], in contrast to the facile SC crystallization in stereoblock copolymers [17e19]. Numerous approaches have been attempted as potential
resolutions of this issue, including thermal drawing [20,21] and annealing [22e24], electrospinning [25e27], solution casting with solvent/non-solvent mixtures [28] or supercritical carbon dioxide [29], and repeated solution casting [30]. To the best of our knowledge, there is no existing report addressing the effects of molecular weight and crystallization conditions on SC crystallization and homo-crystallization in P(L-2HB)/P(D-2HB) blends or on homocrystallization in neat P(L-2HB) and P(D-2HB). In the present study, to investigate such effects, P(L-2HB) and P(D2HB) with different weight-average molecular weight (Mw) values that ranged from 1.8 103 to 4.1 104 g mol1 were synthesized. The SC crystallization and homo-crystallization of equimolar blends of P(L-2HB) and P(D-2HB) with the similar molecular weights, together with the homo-crystallization of neat P(L-2HB) and P(D-2HB) were investigated under different crystallization conditions. Differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) techniques were used to monitor the SC crystallization and homocrystallization during solvent evaporation (solution-crystallization) and at different Tc values from the melt (melt-crystallization). The information obtained in the present study will facilitate the design and processing of biodegradable materials from neat P(L2HB), neat P(D-2HB), and their blends with a wide variety of physical and hydrolytic degradation properties for use in biomedical, pharmaceutical, and environmental applications. 2. Materials and methods 2.1. Materials P(L-2HB) and P(D-2HB) were synthesized by the polycondensation of (S)- and (R)-2-hydroxybutanoic acids (2-hydroxybutyric acids)
Fig. 1. WAXS profiles of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f).
H. Tsuji, S. Shimizu / Polymer 53 (2012) 5385e5392
(97.0%, enantiomeric ratio 99:1, SigmaeAldrich Co.), respectively, at 130 C in the presence of catalytic p-toluenesulfonic acid monohydrate (5wt% of monomer, guaranteed grade, Nacalai Tesque Inc., Kyoto, Japan) under a nitrogen gas flow, according to a previously reported method [31,32]. P(L-2HB) and P(D-2HB) polymers with various molecular weights were obtained by varying the reaction time of the second step of the polycondensation under reduced pressure from 2 to 48 h, subsequent to the first step of the polycondensation under atmospheric pressure for 5 h. The synthesized polymers were purified by reprecipitation using chloroform and methanol as solvent and precipitant, respectively [9,10]. In the case of polymers synthesized using a reaction time of 2 h in the second step of the polycondensation, the low-molecular weight oligomers were removed by extraction with methanol. The extracted low-molecular weight oligomers were recovered by solvent evaporation. The purified polymers and recovered oligomers were dried under reduced pressure for at least 7 days. Thus prepared P(L-2HB) and P(D-2HB) samples were abbreviated as L- and D-Mw/103, respectively (Table 1). Samples of the neat polymers and polymer blends (thickness of ca. 50 mm) were prepared by solution casting according to a previously reported method [9e11]. Separate equimolar solutions of the purified and recovered P(L-2HB) and P(D-2HB) samples were prepared in dichloromethane to give a polymer concentration of 1 g dL1. The P(L-2HB)/P(D-2HB) blend samples were prepared by mixing the P(L-2HB) and P(D-2HB) solutions in an equimolar ratio under vigorous stirring. The solutions were cast onto petri dishes, followed by solvent evaporation at 25 C for approximately one day. Neat P(L-2HB) and P(D-2HB) samples were also prepared using the same procedure without mixing the solutions. Residual solvent in the as-cast samples was removed by evacuation under reduced
100
a Solution-crystallized P(L-2HB)
pressure for at least 7 days and the samples were stored in a desiccator prior to use. These samples were designated as “solutioncrystallized samples”. Melt-crystallization of the solutioncrystallized samples, which were sealed under reduced pressure, was carried out at Tc ¼ 55e160 C for 10 h after melting at 240 C for 3 min. After crystallization, samples were quenched at 0 C in iced water for 10 min to stop further crystallization. These samples were designated as “melt-crystallized samples”. 2.2. Physical measurements The Mw and number-average molecular weight (Mn) of the polymers and oligomers were evaluated in chloroform at 40 C using a Tosoh (Tokyo, Japan) gel permeation chromatography (GPC) system (refractive index monitor: RI-8020) having two TSK Gel columns (GMHXL) using polystyrene standards. The molecular characteristics of the polymers used in the present study are summarized in Table 1. The glass transition and melting temperatures (Tg and Tm, respectively) and the melting enthalpies of the homo-crystallites and SC crystallites [DHm(H) and DHm(S), respectively] of the samples were determined using a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter. The samples were heated at a rate of 10 C min1 under a nitrogen gas flow of 50 mL min1 for DSC measurements. The Tm values of the samples were calibrated using tin, indium, and benzophenone as standards. The crystalline species and crystallinity (Xc) values of the samples were estimated using WAXS. The WAXS measurements were performed at 25 C using a Rigaku (Tokyo, Japan) RINT-2500 instrument equipped with a Cu-Ka source (l ¼ 0.1542 nm), which was operated at 40 kV and 200 mA. In the 2q range of 7.5e27, the
100
b Solution-crystallized P(D-2HB)
80
80
60
60
60
Xc (%)
80
Xc (%)
Xc (%)
100
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40
40
40
20
20
20
c Solution-crystallized blends
Xc(S) Xc(H) Xc(total) 0 1x103
0 1x103
5x104
100
d Melt-crystallized P(L-2HB)
1x104 Mw (g mol-1)
0 1x10 3
5x104
100
e Melt-crystallized P(D-2HB)
80
80
60
60
60
Xc (%)
80
Xc (%)
Xc (%)
100
1x104 Mw (g mol-1)
40
40
40
20
20
20
0 1x10 3
1x10 4 Mw (g mol-1)
5x10 4
0 1x103
1x10 4 Mw (g mol-1)
5x104
1x10 4 Mw (g mol-1)
f
0 1x10 3
5x10 4
Melt-crystallized blends
1x10 4 Mw (g mol-1)
5x10 4
Fig. 2. Crystallinity of SC and homo-crystallites [Xc(S) and Xc(H), respectively] and total crystallinity [Xc(total)] of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f) as a function of Mw.
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crystalline diffraction peak areas for the SC crystallites at 2q values of ca. 10.7, 18.5 [19.3 (subpeak)], and 21.5 , and those of the homo-crystallites of P(L-2HB) or P(D-2HB) at 2q values of ca. 14.7, 17.2 , 23.2 , and 25.6 (and/or around 11.2 , 12.7, 16.5 , 18.2 , 18.9 , 20.4 , 21.5 , and 23.3 for a new type of homo-crystallites) were used to estimate the Xc values of the SC crystallites [Xc(S)] and P(L2HB) or P(D-2HB) homo-crystallites [Xc(H)], respectively, based on the intensity relative to the total area between the diffraction profile and the baseline [9,10]. 3. Results and discussion 3.1. Effects of molecular weight The effects of molecular weight on the crystalline species and Xc of the solution- and melt-crystallized samples were evaluated using WAXS measurements. Fig. 1 shows the WAXS profiles of the solution- and melt-crystallized neat P(L-2HB), P(D-2HB), and their equimolar blend samples. Based on the observation that homocrystallites formation was observed only at Tc ¼ 70 C for the L41/D-34 blend samples having the highest molecular weights (vide infra), the Tc of the melt-crystallized samples presented in Fig. 1 was fixed to 70 C. In the case of the neat P(L-2HB) and P(D-2HB) samples, except the solution- and melt-crystallized neat L-2 and D-3 samples, crystalline diffraction peaks of P(L-2HB) or P(D-2HB)
normal homo-crystallites were observed at 2q values around 14.7, 17.2 , 23.2 , and 25.6 , irrespective of the crystallization method [Fig. 1(a), (b), (d) and (e)] [9e12]. The solution-crystallized neat L-2 and D-3 samples exhibit crystalline diffraction peaks at 2q values around 11.2 , 12.7, 16.5 , 18.2 , 18.9 , 20.4 , 21.5 , and 23.3 [Fig. 1(a) and (b)], most of which were incongruent with those reported for P(L-2HB) or P(D-2HB) normal homo-crystallites [9e12]. These new crystalline peaks are ascribed to a new type of P(L-2HB) or P(D-2HB) homo-crystallites (vide infra). In addition to the crystalline diffraction peaks of P(L-2HB) or P(D-2HB) normal homocrystallites, the solution-crystallized and melt-crystallized neat L10 and D-9 samples also exhibited crystalline diffraction peaks corresponding to the new type of homo-crystallites. In contrast, no crystalline diffraction peaks were observed for the meltcrystallized neat L-2 and D-3 samples, reflecting that their amorphous nature [Fig. 1(d) and (e)]. Without exception, in the average Mw range of 2.6 103 to 3.7 104 g mol1, the solution-crystallized blends exhibited crystalline peaks at 2q values around 10.7, 18.5 [19.3 (subpeak)], and 21.5 only [Fig. 1(c)], which were consistent with the values reported for P(L-2HB)/P(D-2HB) SC crystallites [9,10]. These results are indicative of the sole formation of SC crystallites, in the stated Mw range during solution-crystallization, without the formation of P(L-2HB) or P(D-2HB) homo-crystallites. Similar to the solutioncrystallized blends, SC crystalline peaks were observed for all of
Fig. 3. DSC thermograms of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f).
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the melt-crystallized blends [Fig. 1(f)]. However, in addition to the SC crystalline peaks, peaks corresponding to normal homocrystallites were also observed in the case of the melt-crystallized L-41/D-34, L-31/D-33, and L-30/D-23 blends, reflecting the formation of both types of crystalline species for the average Mw range of 2.7 104 to 3.7 104 g mol1 during melt-crystallization. The Xc(S) and Xc(H) values evaluated from the WAXS profiles in Fig. 1 are plotted in Fig. 2 as a function of Mw. In the case of the blend samples, the arithmetic means of the Mw values of P(L-2HB) and P(D-2HB) were used in the plot. For the solution-crystallized neat P(L-2HB) and P(D-2HB) samples, the Xc(H) values of 16e65% were obtained for the solution-crystallized neat P(L-2HB) and P(D2HB) samples, and there was a monotonous decrease with increasing Mw [Fig. 2(a) and (b)], indicating disruption or delay of the homo-crystallization of the neat P(L-2HB) and P(D-2HB) samples with high molecular weight and low chain mobility during solvent evaporation. However, for the melt-crystallized neat P(L2HB) and P(D-2HB) samples, the Xc(H) values that were nil for Mw ¼ 1.8 103 and 3.3 103 g mol1, respectively, increased remarkably with increasing Mw, to reach 60% at respective Mw values of 1.0 104 and 8.6 103 g mol1, saturation values of ca. 65% was observed at Mw values exceeding 2 104 g mol1. For the solution-crystallized P(L-2HB)/P(D-2HB) blend samples, the Xc(S) and Xc(H) values were 65e85% and 0%, respectively [Fig. 2(c)], reflecting facile and predominant SC crystallization during solvent evaporation, irrespective of average Mw. An Xc(S) value of 89% was obtained for the melt-crystallized blend samples at an average Mw of 2.6 103 g mol1; this value gradually decreased with increasing average Mw, to reach 28% at an average Mw of 3.7 104 g mol1. The Xc(S) and Xc(total) [¼Xc(S) þ Xc(H)]
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values of the blends were higher than the Xc(H) values of the homocrystallites in the neat P(L-2HB) and P(D-2HB) samples, indicating more facile crystallization of SC crystallites in the blends relative to the homo-crystallites in the neat P(L-2HB) and P(D-2HB) samples. It is noteworthy that both SC crystallites and homo-crystallites were simultaneously formed during melt-crystallization for average Mw above 2.7 104 g mol1, and an equal amount of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 104 g mol1. Although there have been prior reports of similar critical molecular weights above which both SC crystallites and homo-crystallites (not just SC crystallites) form for solutioncrystallized and melt-crystallized PLLA/PDLA blends [14e16], this is the first report for P(L-2HB)/P(D-2HB) blends. In the case of PLLA/ PDLA blends, the critical molecular weight is higher for solutioncrystallized samples than that for melt-crystallized samples [14e16]. The radius of polymer in the presence of solvent should be larger than that without solvent in the melt and the P(L-2HB) and P(D-2HB) chains in the solution can readily interpenetrate, resulting in the ready formation of stereocomplex crystallites. Therefore, in the average Mw range studied here, even the highest average Mw values (3.7 104 g mol1) for P(L-2HB) and P(D-2HB) should be too low for the solution-crystallized blends to form homo-crystallites. Furthermore, predominant formation of homo-crystallites without the formation of SC crystallites, as reported for PLLA/ PDLA blends at high average Mw such as 105 g mol1 [14e16], was not observed in the present study. This may be due to the fact that even the highest average Mw values of P(L-2HB) and P(D-2HB) will be too low to cause predominant formation of homo-crystallites. The thermal properties of the samples were evaluated using DSC. Fig. 3 shows the DSC thermograms of neat P(L-2HB), neat
Fig. 4. Melting temperatures of SC and homo-crystallites [Tm(S) and Tm(H), respectively] of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f) as a function of Mw.
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P(D-2HB), and their blend samples. Melting peaks of homocrystallites were observed in the range of 77e112 C for both the solution- and melt-crystallized neat P(L-2HB) and P(D-2HB) samples (excluding the melt-crystallized neat L-2 and D-3, which exhibited no melting peak), and for the solution-crystallized L-2/D-3 blend and the melt-crystallized L-41/D-34, L-31/D-33, and L-30/D-23 blends. Melting peaks of SC crystallites were observed in the range of 172e219 C for all the solution- and melt-crystallized blends. The presence of melting peak for the homo-crystallites despite the absence of WAXS peaks for the homo-crystallites in the solution-crystallized L-2/D-3 blend indicates the formation of homo-crystallites during DSC heating, although an explicit cold crystallization peak was not observed. The melting temperatures of homo-crystallites and SC crystallites [Tm(H) and Tm(S), respectively] estimated from the DSC thermograms in Fig. 3 are plotted in Fig. 4 as a function of Mw. As elaborated below, the Tm(H) values for the neat L-2 and D-3 and the higher Tm(H) values for the neat L-9 and D10 are attributed to the melting of the new type of homocrystallites, which have a crystalline structure that is different from the crystalline structure of the normal homo-crystallites formed in neat P(L-2HB) and P(D-2HB) for Mw above 1.9 104 g mol1 Fig. 4 demonstrates that the Tm(H) and Tm(S) values increased with an increase in Mw and the dependence of the Tm(H) and Tm(S) on Mw was stronger at low Mw. 3.2. Effects of crystallization temperature
Fig. 6. Crystallinity (Xc) of L-41/D-34 equimolar blend samples crystallized from the melt as a function of Tc.
The effects of Tc on crystalline species and Xc were evaluated using WAXS measurements. Fig. 6 shows the WAXS profiles of the L-41/D-34 blend samples crystallized at different Tc values. The Xc values of each crystalline species estimated from the WAXS profiles in Fig. 5 are plotted in Fig. 6 as a function of Tc. A non-zero Xc(H), which was higher than that of Xc(S), was observed only at Tc ¼ 70 C; this indicates that the induction period for crystallization was shorter and the crystallization rate was higher for homocrystallites than for SC crystallites only at this Tc, whereas for Tc
values of 55e160 C, with the exception of 70 C, only Xc(S) had positive values, and it increased with an increase in Tc. The equilibrium melting temperature of SC crystallites [T0m(S)] was assessed based on DSC analysis of the L-31/D-33 blend samples crystallized at different Tc values (Fig. 7). Broad crystallization peaks were observed in the temperature range 150e200 C for the blends crystallized at Tc ¼ 100e120 C. This strongly suggests that the melting peaks observed near 220 C can be ascribed to the melting of SC crystallites recrystallized during DSC heating. The Tm(S) values
Fig. 5. WAXS profiles of L-41/D-34 equimolar blend samples crystallized at different Tc values from the melt.
Fig. 7. DSC thermograms of L-31/D-33 equimolar blend samples crystallized at different Tc values from the melt.
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3.3. A new type of homo-crystallites
Fig. 8. Tm(S) of L-31/D-33 equimolar blend samples as a function of Tc.
estimated from the DSC thermograms in Fig. 7 are plotted in Fig. 8 as a function of Tc. The Tm(S) shifted gradually from 209 C to 214 C with increasing Tc, in the Tc range of 130e170 C, then increased rapidly, from 214 C to 225 C in the Tc range of 170e200 C and finally exhibited a rapid decline to the lower temperature of about 210 C at Tc ¼ 210 C. The HoffmaneWeeks plot of the Tm(S) values of the L-31/D-33 blend samples is shown in Fig. 8. Extrapolation of the four linear points in the Tc range of 170e200 C (where the dependence of Tm(S) on Tc was stronger than that in the Tc range of 130e170 C) to Tm(S) ¼ Tc gives the T0m(S) value of 239.1 C. This value is comparable to those reported for PLLA/PDLA SC crystalline residues (or extended chain crystallites) after hydrolytic degradation by the HoffmaneWeeks procedure (233e237 C) [33], but it is much lower than that obtained by extrapolation of T0m(S) values of L-lactide-rich PLA/D-lactide-rich PLA blends to 100% optical purity (i.e., PLLA/PDLA blend) (279 C) [22].
New crystalline diffraction peaks with 2q angles different from those of normal homo-crystallites were observed at 2q values of ca. 11.2 , 12.7, 16.5 , 18.2 , 18.9 , 20.4 , 21.5 , and 23.3 in the WAXS profiles of the solution-crystallized neat L-10, L-2, D-9, and D-3 samples and the melt-crystallized neat L-10 and D-9 samples [Fig. 1(a), (b), (d) and (e)], suggesting the formation of a new type of homo-crystallites. Multiple melting peaks were observed for the solution- and melt-crystallized neat L-10 and D-9 samples [Fig. 3(a), (b), (d) and (e)]. To assign the new crystalline diffraction peaks in the WAXS profiles and multiple melting peaks in the DSC thermograms, solution-crystallized neat L-10, as a typical sample, was annealed at 100 C (just above the endset temperature of the first melting peak) for 3 min and then quenched at 0 C for 10 min. The WAXS profiles and DSC thermograms of L-10 before and after annealing at 100 C are shown in Fig. 9, conjunction with the reference WAXS profile and DSC thermogram of solutioncrystallized neat L-41, which contains the normal type of P(L-2HB) homo-crystallites. The changes in the WAXS profiles and DSC thermograms of neat solution-crystallized D-9 (with Mw similar to that of L-10) before and after the annealing process were similar to those of L-10 (data not shown here). For reference, Fig. 9 also shows the WAXS profiles of (S)-2-hydroxybutanoic acid and p-toluenesulfonic acid, which are abbreviated as (S)-2HB and p-TSA, respectively. The crystalline diffraction angles of L-10 before and after annealing were different from those of (S)-2HB and p-TSA. This indicates that the new WAXS crystalline peaks of L-10 before and after annealing were not due to the presence of the monomer or catalyst and strongly suggests that they are ascribed to the new type of homo-crystallites. As seen in Fig. 9, subsequent to annealing, no crystalline diffraction peaks corresponding to the normal type of homo-crystallites at 2q values around 14.7, 17.2 , 23.2 , and 25.6 are observed for the L-10 samples, whereas these peaks were observed for L-41 (shown with broken lines), and the lower temperature melting peak in the DSC thermogram of L-10 almost disappeared. These findings indicate that the higher-temperature melting peak in the DSC thermogram of the solution-crystallized neat L-10 is attributable to the new type of homo-crystallites, whereas the lower temperature melting peak can be ascribed to the normal type of home-crystallites.
Fig. 9. WAXS profiles (a) and DSC thermograms (b) of solution-crystallized neat L-41 and L-10, and L-10 annealed at 100 C and quenched at 0 C. WAXS profiles of (S)-2Hydroxybutanoic acid [(S)-2HB] and p-toluenesulfonic acid (p-TSA) are shown as references in part (a) and crystalline diffraction angles of P(L-2HB) or P(D-2HB) normal homocrystallites are shown in broken lines.
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The lowest-molecular weight solution-crystallized neat L-2 and showed the crystalline diffraction peaks of the new type homocrystallites only [Fig. 1(a) and (b)], whereas these samples became amorphous when crystallized from the melt at Tc ¼ 70 C [Fig. 1(d) and (e)]. In contrast, the crystalline diffraction peaks of the new type of homo-crystallites were observed for the relatively higher molecular weight solution-crystallized neat L-10 and D-9 samples [Fig. 1(a) and (b)], whereas these peaks of very low intensity in the corresponding melt-crystallized samples [Fig. 1(d) and (e)], even though relatively intense crystalline diffraction peaks corresponding to the normal type of homo-crystallites were observed for both the solution- and melt-crystallized samples [Fig. 1(a), (b), (d) and (e)]. The WAXS profiles in Fig. 1 and the assignment of the crystalline diffraction peaks indicate that in the solution-crystallization of neat P(L-2HB) and P(D-2HB), a new type of homo-crystallites were formed for Mw below 1 104 g mol1 and the normal type of homocrystallites were formed for Mw above 1 104 g mol1. On the other hand, in the melt-crystallization of the neat P(L-2HB) and P(D-2HB) at Tc ¼ 70 C, the new type of homo-crystallites were formed for Mw ¼ 9 103e1 104 g mol1 and the normal type homocrystallites were formed for Mw above 9 103 g mol1. Here, we assumed that the effects of Mw on the crystalline species are same for neat P(L-2HB) and P(D-2HB). D-3
4. Conclusions The following conclusions can be derived from the present study for the effects of molecular weight and crystallization conditions on the crystallization of neat P(L-2HB), P(D-2HB), and their blends with Mw ¼ 1.8 103e4.1 104 g mol1. Only SC crystallites were formed during the solution-crystallization of the blends, without the formation of homo-crystallites, irrespective of Mw. SC crystallites were the only crystalline species formed during the melt-crystallization of the blends in the Tc range of 55e160 C (except at 70 C), whereas homo-crystallites were not formed. On the other hand, at Tc ¼ 70 C, in addition to the SC crystallites, a significant amount of homo-crystallites were formed for average Mw exceeding 2.7 104 g mol1, and an equal amount of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 104 g mol1. However, the critical average Mw, above which predominant homo-crystallization and no SC crystallization occurs was not observed. The T0m(S) of SC crystallites obtained by the HoffmaneWeeks procedure was 239.1 C, which was lower than the highest T0m(S) value (279 C) reported for PLLA/ PDLA SC crystallites. In solution-crystallization of neat P(L-2HB) and P(D-2HB), a new type of homo-crystallites were formed for Mw below 1 104 g mol1 and the normal type of homo-crystallites were formed for Mw above 1 104 g mol1. In contrast, during melt-crystallization of neat P(L-2HB) and P(D-2HB), the new and
normal types of homo-crystallites were formed for Mw ¼ 9 103e 1 104 g mol1 and Mw above 9 103 g mol1, respectively. Acknowledgments This research was supported by a Grant-in-aid for Scientific Research, Category “C”, No. 24550251, from the Japan Society for the Promotion of Science (JSPS).
References [1] Kharas GB, Sanchez-Riera F, Severson DK. In: Mobley DP, editor. Plastics from microbes. New York: Hanser Publishers; 1994. p. 93e137. [2] Hartmann MH. In: Kaplan DL, editor. Biopolymers from renewable resources. Berlin: Springer; 1998. p. 367e411. [3] Tsuji H. In: Doi Y, Steinbüchel A, editors. Polyesters 3. Biopolymers, vol. 4. Weinheim (Germany): Wiley-VCH; 2002. p. 129e77. [4] Auras R, Lim L-T, Selke SEM, Tsuji H, editors. Poly(lactic acid): synthesis, structures, properties, processing, and applications, Wiley series on polymer engineering and technology. New Jersey: John Wiley & Sons, Inc.; 2010. [5] Slager J, Domb AJ. Advanced Drug Delivery Reviews 2003;55:549e83. [6] Tsuji H. Macromolecular Bioscience 2005;5:569e97. [7] Fukushima K, Kimura Y. Polymer International 2006;55:626e42. [8] Tsuji H, Ikada Y. In: Yu L, editor. Biodegradable polymer blends from renewable resources. New Jersey: John Wiley & Sons, Inc.; 2009. p. 165e90. [9] Tsuji H, Okumura A. Macromolecules 2009;42:7263e6. [10] Tsuji H, Okumura A. Polymer Journal 2011;43:317e24. [11] Tsuji H, Yamamoto S, Okumura A, Sugiura Y. Biomacromolecules 2010;11: 252e8. [12] Tsuji H, Shimizu K, Sakamoto Y, Okumura A. Polymer 2011;52:1318e25. [13] Tsuji H, Hosokawa M, Sakamoto Y. ACS Macro Letters 2012;1:687e91. [14] Tsuji H, Hyon S-H, Ikada Y. Macromolecules 1991;24:5651e6. [15] Tsuji H, Ikada Y. Macromolecules 1993;25:6918e26. [16] Tsuji H, Ikada Y. Polymer 1999;40:6699e708. [17] Yui N, Dijkstra PJ, Feijen J. Makromolekulare Chemie 1990;191:481e8. [18] Fukushima K, Hirata M, Kimura Y. Macromolecules 2007;40:3049e55. [19] Tsuji H, Wada T, Sakamoto Y, Sugiura Y. Polymer 2010;51:4937e47. [20] Tsuji H, Ikada Y, Hyon SH, Kimura Y, Kitao T. Journal of Applied Polymer Science 1994;51:337e44. [21] Takasaki M, Ito H, Kikutani T. Journal of Macromolecular Science: Physics 2003;42B(3, 4 SPEC):403e20. [22] Tsuji H, Ikada Y. Macromolecular Chemistry and Physics 1996;197:3483e99. [23] Zhang J, Tashiro K, Tsuji H, Domb AJ. Macromolecules 2007;40:1049e54. [24] Fujita M, Sawayanagi T, Abe H, Tanaka T, Iwata T, Ito K, et al. Macromolecules 2008;41:2852e8. [25] Tsuji H, Nakano M, Hashimoto M, Takashima K, Katsura S, Mizuno A. Biomacromolecules 2006;7:3316e20. [26] Ishii D, Ying TH, Mahara A, Murakami S, Yamaoka T, Lee W, et al. Biomacromolecules 2009;10:237e42. [27] Spasova M, Manolova N, Paneva D, Mincheva R, Dubois P, Rashkov I, et al. Biomacromolecules 2010;11:151e9. [28] Furuhashi Y, Yoshie N. Polymer International 2012;6:301e6. [29] Purnama P, Kim SH. Macromolecules 2010;43:1137e42. [30] Tsuji H, Yamamoto S. Macromolecular Materials and Enginering 2011;296: 583e9. [31] Tsuji H, Matsuoka H, Itsuno S. Journal of Applied Polymer Science 2008;110: 3954e62. [32] Tsuji H, Eto T, Sakamoto Y. Materials 2011;4:1384e98. [33] Tsuji H, Tsuruno T. Polymer Degradation and Stability 2010;95:477e84.
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Stereocomplex crystallization and homo-crystallization of enantiomeric poly(2-hydroxybutyrate)s: Effects of molecular weight and crystallization conditions Hideto Tsuji*, Satoru Shimizu Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2012 Received in revised form 5 September 2012 Accepted 17 September 2012 Available online 23 September 2012
Enantiomeric substituted polylactides, poly(L-2-hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) [P(D-2HB)] with different weight-average molecular weights (Mw ¼ 1.8 103e4.1 104 g mol1) were synthesized by acid-catalyzed polycondensation. The effects of molecular weights of P(L-2HB) and P(D-2HB) and crystallization conditions on stereocomplex (SC) crystallization and homo-crystallization of neat P(L2HB), neat P(D-2HB), and their equimolar blends were investigated by means of differential scanning calorimetry and wide-angle X-ray scattering. Only SC crystallites were formed during solution-crystallization of the blends without the formation of homo-crystallites, irrespective of average Mw. SC crystallites were only crystalline species formed during melt-crystallization of the blends, in the crystallization temperature (Tc) range of 55e160 C (except at 70 C). At Tc ¼ 70 C, in addition to SC crystallites, a significant amount of homo-crystallites were formed for average Mw exceeding 2.7 104 g mol1, and equal amounts of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 104 g mol1. However, a critical average Mw above which predominant homo-crystallization and no SC crystallization occur was not observed. The equilibrium melting temperature of SC crystallites obtained by the HoffmaneWeeks procedure was 239.1 C, which was lower than the highest value (279 C) reported for polylactide SC crystallites. In solution-crystallization of the neat P(L-2HB) and P(D-2HB), a new type of homo-crystallites were formed for Mw below 1 104 g mol1 and the normal type of homo-crystallites were formed for Mw above 1 104 g mol1. In contrast, melt-crystallization of the neat P(L-2HB) and P(D-2HB) generated the new and the normal type homo-crystallites for Mw ¼ 9 103e1 104 g mol1 and Mw above 9 103 g mol1, respectively. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Poly(hydroxybutyric acid) Poly(hydroxybutanoic acid) Stereocomplexation
1. Introduction Poly(L-lactide), i.e, poly(L-lactic acid) (PLLA) is a biodegradable polyester produced from renewable carbohydrate resources of such as corn starch [1e4]. Stereocomplex (SC) crystallization in blends of PLLA and poly(D-lactide), i.e., poly(D-lactic acid) (PDLA), or in the stereoblock copolymer PLLA-b-PDLA effectively improves the mechanical properties, thermal/hydrolytic degradation resistance, and gas barrier properties of poly(lactide), i.e., poly(lactic acid) (PLA)-based materials [5e8]. Poly(2-hydroxybutyrate), i.e., poly(2hydroxybutanoic acid) [P(2HB)] is a biodegradable polyester having a structure similar to that of PLA except that the methyl groups are substituted by ethyl groups. As in the case of PLA [5e8], SC crystallites
* Corresponding author. E-mail address: [email protected] (H. Tsuji). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.09.039
can be formed by blending enantiomeric P(2HB)s, i.e., poly(L-2hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) [P(D2HB)] [9,10]. Moreover, hetero-SC crystallization was found to occur in P(L-2HB)/PDLA or P(D-2HB)/PLLA blends [11] and a P(D-2HB)-b-PLLA block copolymer [12]. Very recently, ternary SC crystallization was reported to take place in ternary P(L-2HB)/P(D-2HB)/PDLA blends [13]. The melting temperature (Tm) of the P(L-2HB)/P(D-2HB) SC crystallites (about 200 C) is higher than those of neat P(L-2HB) and P(D-2HB) (about 100 C) [9,10]. P(L-2HB)/P(D-2HB) blend exhibits enhanced radial growth rate of spherulites (G) and hydrolytic/ thermal degradation resistance relative to neat P(L-2HB) and P(D-2HB) [10]. This indicates that the intermolecular interaction between the P(L-2HB) and P(D-2HB) chains, having opposite configurations, in the amorphous regions or in the molten state is higher than that between the iso-structural chains of P(L-2HB) or P(D-2HB). In the case of PLA, the relative amount of SC crystallites versus homo-crystallites of either PLLA or PDLA produced is determined by the molecular weights of the enantiomeric PLAs
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Table 1 Molecular characteristics of P(L-2HB)s and P(D-2HB)s. Polymer
Code
Reactiona time (h)
Mwb (g mol1)
P(L-2HB)
L-41
48 24 20 12 2 2
4.08 3.12 3.02 1.97 1.00 1.82
48 24 20 12 2 2
3.36 3.26 2.30 1.86 8.61 3.33
L-31 L-30 L-20 L-10 d L-2
P(D-2HB)
c
D-34 D-33 D-23 D-19 c D-9 d D-3
a b c d
Mw/Mnb
Yield (%)
104 104 104 104 104 103
1.60 1.42 1.70 1.38 2.34 2.41
50.9 42.5 46.4 27.7 15.7 e
104 104 104 104 103 103
1.55 1.34 1.57 1.32 2.17 3.38
46.9 27.6 38.1 28.0 12.6 e
Reaction time for second step of polycondensation. Mw and Mn are weight- and number-average molecular weights, respectively. Sample undissolved when extracted with methanol. Sample dissolved when extracted with methanol.
and the crystallization conditions including the protocol employed (e.g., solution-crystallization or melt-crystallization), as well as the solvent type and polymer concentrations, and crystallization temperature (Tc). The use of high molecular weight PLAs is beneficial for enhanced performance of polymeric materials. However, homo-crystallization prevails over SC crystallization during the melt- and solution-crystallization processes when the molecular weights of both PLLA and PDLA exceed 1 104 g mol1 and 1 105 g mol1 for the respective processes [14e16], in contrast to the facile SC crystallization in stereoblock copolymers [17e19]. Numerous approaches have been attempted as potential
resolutions of this issue, including thermal drawing [20,21] and annealing [22e24], electrospinning [25e27], solution casting with solvent/non-solvent mixtures [28] or supercritical carbon dioxide [29], and repeated solution casting [30]. To the best of our knowledge, there is no existing report addressing the effects of molecular weight and crystallization conditions on SC crystallization and homo-crystallization in P(L-2HB)/P(D-2HB) blends or on homocrystallization in neat P(L-2HB) and P(D-2HB). In the present study, to investigate such effects, P(L-2HB) and P(D2HB) with different weight-average molecular weight (Mw) values that ranged from 1.8 103 to 4.1 104 g mol1 were synthesized. The SC crystallization and homo-crystallization of equimolar blends of P(L-2HB) and P(D-2HB) with the similar molecular weights, together with the homo-crystallization of neat P(L-2HB) and P(D-2HB) were investigated under different crystallization conditions. Differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) techniques were used to monitor the SC crystallization and homocrystallization during solvent evaporation (solution-crystallization) and at different Tc values from the melt (melt-crystallization). The information obtained in the present study will facilitate the design and processing of biodegradable materials from neat P(L2HB), neat P(D-2HB), and their blends with a wide variety of physical and hydrolytic degradation properties for use in biomedical, pharmaceutical, and environmental applications. 2. Materials and methods 2.1. Materials P(L-2HB) and P(D-2HB) were synthesized by the polycondensation of (S)- and (R)-2-hydroxybutanoic acids (2-hydroxybutyric acids)
Fig. 1. WAXS profiles of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f).
H. Tsuji, S. Shimizu / Polymer 53 (2012) 5385e5392
(97.0%, enantiomeric ratio 99:1, SigmaeAldrich Co.), respectively, at 130 C in the presence of catalytic p-toluenesulfonic acid monohydrate (5wt% of monomer, guaranteed grade, Nacalai Tesque Inc., Kyoto, Japan) under a nitrogen gas flow, according to a previously reported method [31,32]. P(L-2HB) and P(D-2HB) polymers with various molecular weights were obtained by varying the reaction time of the second step of the polycondensation under reduced pressure from 2 to 48 h, subsequent to the first step of the polycondensation under atmospheric pressure for 5 h. The synthesized polymers were purified by reprecipitation using chloroform and methanol as solvent and precipitant, respectively [9,10]. In the case of polymers synthesized using a reaction time of 2 h in the second step of the polycondensation, the low-molecular weight oligomers were removed by extraction with methanol. The extracted low-molecular weight oligomers were recovered by solvent evaporation. The purified polymers and recovered oligomers were dried under reduced pressure for at least 7 days. Thus prepared P(L-2HB) and P(D-2HB) samples were abbreviated as L- and D-Mw/103, respectively (Table 1). Samples of the neat polymers and polymer blends (thickness of ca. 50 mm) were prepared by solution casting according to a previously reported method [9e11]. Separate equimolar solutions of the purified and recovered P(L-2HB) and P(D-2HB) samples were prepared in dichloromethane to give a polymer concentration of 1 g dL1. The P(L-2HB)/P(D-2HB) blend samples were prepared by mixing the P(L-2HB) and P(D-2HB) solutions in an equimolar ratio under vigorous stirring. The solutions were cast onto petri dishes, followed by solvent evaporation at 25 C for approximately one day. Neat P(L-2HB) and P(D-2HB) samples were also prepared using the same procedure without mixing the solutions. Residual solvent in the as-cast samples was removed by evacuation under reduced
100
a Solution-crystallized P(L-2HB)
pressure for at least 7 days and the samples were stored in a desiccator prior to use. These samples were designated as “solutioncrystallized samples”. Melt-crystallization of the solutioncrystallized samples, which were sealed under reduced pressure, was carried out at Tc ¼ 55e160 C for 10 h after melting at 240 C for 3 min. After crystallization, samples were quenched at 0 C in iced water for 10 min to stop further crystallization. These samples were designated as “melt-crystallized samples”. 2.2. Physical measurements The Mw and number-average molecular weight (Mn) of the polymers and oligomers were evaluated in chloroform at 40 C using a Tosoh (Tokyo, Japan) gel permeation chromatography (GPC) system (refractive index monitor: RI-8020) having two TSK Gel columns (GMHXL) using polystyrene standards. The molecular characteristics of the polymers used in the present study are summarized in Table 1. The glass transition and melting temperatures (Tg and Tm, respectively) and the melting enthalpies of the homo-crystallites and SC crystallites [DHm(H) and DHm(S), respectively] of the samples were determined using a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter. The samples were heated at a rate of 10 C min1 under a nitrogen gas flow of 50 mL min1 for DSC measurements. The Tm values of the samples were calibrated using tin, indium, and benzophenone as standards. The crystalline species and crystallinity (Xc) values of the samples were estimated using WAXS. The WAXS measurements were performed at 25 C using a Rigaku (Tokyo, Japan) RINT-2500 instrument equipped with a Cu-Ka source (l ¼ 0.1542 nm), which was operated at 40 kV and 200 mA. In the 2q range of 7.5e27, the
100
b Solution-crystallized P(D-2HB)
80
80
60
60
60
Xc (%)
80
Xc (%)
Xc (%)
100
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40
40
40
20
20
20
c Solution-crystallized blends
Xc(S) Xc(H) Xc(total) 0 1x103
0 1x103
5x104
100
d Melt-crystallized P(L-2HB)
1x104 Mw (g mol-1)
0 1x10 3
5x104
100
e Melt-crystallized P(D-2HB)
80
80
60
60
60
Xc (%)
80
Xc (%)
Xc (%)
100
1x104 Mw (g mol-1)
40
40
40
20
20
20
0 1x10 3
1x10 4 Mw (g mol-1)
5x10 4
0 1x103
1x10 4 Mw (g mol-1)
5x104
1x10 4 Mw (g mol-1)
f
0 1x10 3
5x10 4
Melt-crystallized blends
1x10 4 Mw (g mol-1)
5x10 4
Fig. 2. Crystallinity of SC and homo-crystallites [Xc(S) and Xc(H), respectively] and total crystallinity [Xc(total)] of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f) as a function of Mw.
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crystalline diffraction peak areas for the SC crystallites at 2q values of ca. 10.7, 18.5 [19.3 (subpeak)], and 21.5 , and those of the homo-crystallites of P(L-2HB) or P(D-2HB) at 2q values of ca. 14.7, 17.2 , 23.2 , and 25.6 (and/or around 11.2 , 12.7, 16.5 , 18.2 , 18.9 , 20.4 , 21.5 , and 23.3 for a new type of homo-crystallites) were used to estimate the Xc values of the SC crystallites [Xc(S)] and P(L2HB) or P(D-2HB) homo-crystallites [Xc(H)], respectively, based on the intensity relative to the total area between the diffraction profile and the baseline [9,10]. 3. Results and discussion 3.1. Effects of molecular weight The effects of molecular weight on the crystalline species and Xc of the solution- and melt-crystallized samples were evaluated using WAXS measurements. Fig. 1 shows the WAXS profiles of the solution- and melt-crystallized neat P(L-2HB), P(D-2HB), and their equimolar blend samples. Based on the observation that homocrystallites formation was observed only at Tc ¼ 70 C for the L41/D-34 blend samples having the highest molecular weights (vide infra), the Tc of the melt-crystallized samples presented in Fig. 1 was fixed to 70 C. In the case of the neat P(L-2HB) and P(D-2HB) samples, except the solution- and melt-crystallized neat L-2 and D-3 samples, crystalline diffraction peaks of P(L-2HB) or P(D-2HB)
normal homo-crystallites were observed at 2q values around 14.7, 17.2 , 23.2 , and 25.6 , irrespective of the crystallization method [Fig. 1(a), (b), (d) and (e)] [9e12]. The solution-crystallized neat L-2 and D-3 samples exhibit crystalline diffraction peaks at 2q values around 11.2 , 12.7, 16.5 , 18.2 , 18.9 , 20.4 , 21.5 , and 23.3 [Fig. 1(a) and (b)], most of which were incongruent with those reported for P(L-2HB) or P(D-2HB) normal homo-crystallites [9e12]. These new crystalline peaks are ascribed to a new type of P(L-2HB) or P(D-2HB) homo-crystallites (vide infra). In addition to the crystalline diffraction peaks of P(L-2HB) or P(D-2HB) normal homocrystallites, the solution-crystallized and melt-crystallized neat L10 and D-9 samples also exhibited crystalline diffraction peaks corresponding to the new type of homo-crystallites. In contrast, no crystalline diffraction peaks were observed for the meltcrystallized neat L-2 and D-3 samples, reflecting that their amorphous nature [Fig. 1(d) and (e)]. Without exception, in the average Mw range of 2.6 103 to 3.7 104 g mol1, the solution-crystallized blends exhibited crystalline peaks at 2q values around 10.7, 18.5 [19.3 (subpeak)], and 21.5 only [Fig. 1(c)], which were consistent with the values reported for P(L-2HB)/P(D-2HB) SC crystallites [9,10]. These results are indicative of the sole formation of SC crystallites, in the stated Mw range during solution-crystallization, without the formation of P(L-2HB) or P(D-2HB) homo-crystallites. Similar to the solutioncrystallized blends, SC crystalline peaks were observed for all of
Fig. 3. DSC thermograms of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f).
H. Tsuji, S. Shimizu / Polymer 53 (2012) 5385e5392
the melt-crystallized blends [Fig. 1(f)]. However, in addition to the SC crystalline peaks, peaks corresponding to normal homocrystallites were also observed in the case of the melt-crystallized L-41/D-34, L-31/D-33, and L-30/D-23 blends, reflecting the formation of both types of crystalline species for the average Mw range of 2.7 104 to 3.7 104 g mol1 during melt-crystallization. The Xc(S) and Xc(H) values evaluated from the WAXS profiles in Fig. 1 are plotted in Fig. 2 as a function of Mw. In the case of the blend samples, the arithmetic means of the Mw values of P(L-2HB) and P(D-2HB) were used in the plot. For the solution-crystallized neat P(L-2HB) and P(D-2HB) samples, the Xc(H) values of 16e65% were obtained for the solution-crystallized neat P(L-2HB) and P(D2HB) samples, and there was a monotonous decrease with increasing Mw [Fig. 2(a) and (b)], indicating disruption or delay of the homo-crystallization of the neat P(L-2HB) and P(D-2HB) samples with high molecular weight and low chain mobility during solvent evaporation. However, for the melt-crystallized neat P(L2HB) and P(D-2HB) samples, the Xc(H) values that were nil for Mw ¼ 1.8 103 and 3.3 103 g mol1, respectively, increased remarkably with increasing Mw, to reach 60% at respective Mw values of 1.0 104 and 8.6 103 g mol1, saturation values of ca. 65% was observed at Mw values exceeding 2 104 g mol1. For the solution-crystallized P(L-2HB)/P(D-2HB) blend samples, the Xc(S) and Xc(H) values were 65e85% and 0%, respectively [Fig. 2(c)], reflecting facile and predominant SC crystallization during solvent evaporation, irrespective of average Mw. An Xc(S) value of 89% was obtained for the melt-crystallized blend samples at an average Mw of 2.6 103 g mol1; this value gradually decreased with increasing average Mw, to reach 28% at an average Mw of 3.7 104 g mol1. The Xc(S) and Xc(total) [¼Xc(S) þ Xc(H)]
5389
values of the blends were higher than the Xc(H) values of the homocrystallites in the neat P(L-2HB) and P(D-2HB) samples, indicating more facile crystallization of SC crystallites in the blends relative to the homo-crystallites in the neat P(L-2HB) and P(D-2HB) samples. It is noteworthy that both SC crystallites and homo-crystallites were simultaneously formed during melt-crystallization for average Mw above 2.7 104 g mol1, and an equal amount of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 104 g mol1. Although there have been prior reports of similar critical molecular weights above which both SC crystallites and homo-crystallites (not just SC crystallites) form for solutioncrystallized and melt-crystallized PLLA/PDLA blends [14e16], this is the first report for P(L-2HB)/P(D-2HB) blends. In the case of PLLA/ PDLA blends, the critical molecular weight is higher for solutioncrystallized samples than that for melt-crystallized samples [14e16]. The radius of polymer in the presence of solvent should be larger than that without solvent in the melt and the P(L-2HB) and P(D-2HB) chains in the solution can readily interpenetrate, resulting in the ready formation of stereocomplex crystallites. Therefore, in the average Mw range studied here, even the highest average Mw values (3.7 104 g mol1) for P(L-2HB) and P(D-2HB) should be too low for the solution-crystallized blends to form homo-crystallites. Furthermore, predominant formation of homo-crystallites without the formation of SC crystallites, as reported for PLLA/ PDLA blends at high average Mw such as 105 g mol1 [14e16], was not observed in the present study. This may be due to the fact that even the highest average Mw values of P(L-2HB) and P(D-2HB) will be too low to cause predominant formation of homo-crystallites. The thermal properties of the samples were evaluated using DSC. Fig. 3 shows the DSC thermograms of neat P(L-2HB), neat
Fig. 4. Melting temperatures of SC and homo-crystallites [Tm(S) and Tm(H), respectively] of solution-crystallized (aec) and melt-crystallized (Tc ¼ 70 C) (def) neat P(L-2HB) (a, d), P(D-2HB) (b, e), and their equimolar blend samples (c, f) as a function of Mw.
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P(D-2HB), and their blend samples. Melting peaks of homocrystallites were observed in the range of 77e112 C for both the solution- and melt-crystallized neat P(L-2HB) and P(D-2HB) samples (excluding the melt-crystallized neat L-2 and D-3, which exhibited no melting peak), and for the solution-crystallized L-2/D-3 blend and the melt-crystallized L-41/D-34, L-31/D-33, and L-30/D-23 blends. Melting peaks of SC crystallites were observed in the range of 172e219 C for all the solution- and melt-crystallized blends. The presence of melting peak for the homo-crystallites despite the absence of WAXS peaks for the homo-crystallites in the solution-crystallized L-2/D-3 blend indicates the formation of homo-crystallites during DSC heating, although an explicit cold crystallization peak was not observed. The melting temperatures of homo-crystallites and SC crystallites [Tm(H) and Tm(S), respectively] estimated from the DSC thermograms in Fig. 3 are plotted in Fig. 4 as a function of Mw. As elaborated below, the Tm(H) values for the neat L-2 and D-3 and the higher Tm(H) values for the neat L-9 and D10 are attributed to the melting of the new type of homocrystallites, which have a crystalline structure that is different from the crystalline structure of the normal homo-crystallites formed in neat P(L-2HB) and P(D-2HB) for Mw above 1.9 104 g mol1 Fig. 4 demonstrates that the Tm(H) and Tm(S) values increased with an increase in Mw and the dependence of the Tm(H) and Tm(S) on Mw was stronger at low Mw. 3.2. Effects of crystallization temperature
Fig. 6. Crystallinity (Xc) of L-41/D-34 equimolar blend samples crystallized from the melt as a function of Tc.
The effects of Tc on crystalline species and Xc were evaluated using WAXS measurements. Fig. 6 shows the WAXS profiles of the L-41/D-34 blend samples crystallized at different Tc values. The Xc values of each crystalline species estimated from the WAXS profiles in Fig. 5 are plotted in Fig. 6 as a function of Tc. A non-zero Xc(H), which was higher than that of Xc(S), was observed only at Tc ¼ 70 C; this indicates that the induction period for crystallization was shorter and the crystallization rate was higher for homocrystallites than for SC crystallites only at this Tc, whereas for Tc
values of 55e160 C, with the exception of 70 C, only Xc(S) had positive values, and it increased with an increase in Tc. The equilibrium melting temperature of SC crystallites [T0m(S)] was assessed based on DSC analysis of the L-31/D-33 blend samples crystallized at different Tc values (Fig. 7). Broad crystallization peaks were observed in the temperature range 150e200 C for the blends crystallized at Tc ¼ 100e120 C. This strongly suggests that the melting peaks observed near 220 C can be ascribed to the melting of SC crystallites recrystallized during DSC heating. The Tm(S) values
Fig. 5. WAXS profiles of L-41/D-34 equimolar blend samples crystallized at different Tc values from the melt.
Fig. 7. DSC thermograms of L-31/D-33 equimolar blend samples crystallized at different Tc values from the melt.
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Fig. 8. Tm(S) of L-31/D-33 equimolar blend samples as a function of Tc.
estimated from the DSC thermograms in Fig. 7 are plotted in Fig. 8 as a function of Tc. The Tm(S) shifted gradually from 209 C to 214 C with increasing Tc, in the Tc range of 130e170 C, then increased rapidly, from 214 C to 225 C in the Tc range of 170e200 C and finally exhibited a rapid decline to the lower temperature of about 210 C at Tc ¼ 210 C. The HoffmaneWeeks plot of the Tm(S) values of the L-31/D-33 blend samples is shown in Fig. 8. Extrapolation of the four linear points in the Tc range of 170e200 C (where the dependence of Tm(S) on Tc was stronger than that in the Tc range of 130e170 C) to Tm(S) ¼ Tc gives the T0m(S) value of 239.1 C. This value is comparable to those reported for PLLA/PDLA SC crystalline residues (or extended chain crystallites) after hydrolytic degradation by the HoffmaneWeeks procedure (233e237 C) [33], but it is much lower than that obtained by extrapolation of T0m(S) values of L-lactide-rich PLA/D-lactide-rich PLA blends to 100% optical purity (i.e., PLLA/PDLA blend) (279 C) [22].
New crystalline diffraction peaks with 2q angles different from those of normal homo-crystallites were observed at 2q values of ca. 11.2 , 12.7, 16.5 , 18.2 , 18.9 , 20.4 , 21.5 , and 23.3 in the WAXS profiles of the solution-crystallized neat L-10, L-2, D-9, and D-3 samples and the melt-crystallized neat L-10 and D-9 samples [Fig. 1(a), (b), (d) and (e)], suggesting the formation of a new type of homo-crystallites. Multiple melting peaks were observed for the solution- and melt-crystallized neat L-10 and D-9 samples [Fig. 3(a), (b), (d) and (e)]. To assign the new crystalline diffraction peaks in the WAXS profiles and multiple melting peaks in the DSC thermograms, solution-crystallized neat L-10, as a typical sample, was annealed at 100 C (just above the endset temperature of the first melting peak) for 3 min and then quenched at 0 C for 10 min. The WAXS profiles and DSC thermograms of L-10 before and after annealing at 100 C are shown in Fig. 9, conjunction with the reference WAXS profile and DSC thermogram of solutioncrystallized neat L-41, which contains the normal type of P(L-2HB) homo-crystallites. The changes in the WAXS profiles and DSC thermograms of neat solution-crystallized D-9 (with Mw similar to that of L-10) before and after the annealing process were similar to those of L-10 (data not shown here). For reference, Fig. 9 also shows the WAXS profiles of (S)-2-hydroxybutanoic acid and p-toluenesulfonic acid, which are abbreviated as (S)-2HB and p-TSA, respectively. The crystalline diffraction angles of L-10 before and after annealing were different from those of (S)-2HB and p-TSA. This indicates that the new WAXS crystalline peaks of L-10 before and after annealing were not due to the presence of the monomer or catalyst and strongly suggests that they are ascribed to the new type of homo-crystallites. As seen in Fig. 9, subsequent to annealing, no crystalline diffraction peaks corresponding to the normal type of homo-crystallites at 2q values around 14.7, 17.2 , 23.2 , and 25.6 are observed for the L-10 samples, whereas these peaks were observed for L-41 (shown with broken lines), and the lower temperature melting peak in the DSC thermogram of L-10 almost disappeared. These findings indicate that the higher-temperature melting peak in the DSC thermogram of the solution-crystallized neat L-10 is attributable to the new type of homo-crystallites, whereas the lower temperature melting peak can be ascribed to the normal type of home-crystallites.
Fig. 9. WAXS profiles (a) and DSC thermograms (b) of solution-crystallized neat L-41 and L-10, and L-10 annealed at 100 C and quenched at 0 C. WAXS profiles of (S)-2Hydroxybutanoic acid [(S)-2HB] and p-toluenesulfonic acid (p-TSA) are shown as references in part (a) and crystalline diffraction angles of P(L-2HB) or P(D-2HB) normal homocrystallites are shown in broken lines.
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The lowest-molecular weight solution-crystallized neat L-2 and showed the crystalline diffraction peaks of the new type homocrystallites only [Fig. 1(a) and (b)], whereas these samples became amorphous when crystallized from the melt at Tc ¼ 70 C [Fig. 1(d) and (e)]. In contrast, the crystalline diffraction peaks of the new type of homo-crystallites were observed for the relatively higher molecular weight solution-crystallized neat L-10 and D-9 samples [Fig. 1(a) and (b)], whereas these peaks of very low intensity in the corresponding melt-crystallized samples [Fig. 1(d) and (e)], even though relatively intense crystalline diffraction peaks corresponding to the normal type of homo-crystallites were observed for both the solution- and melt-crystallized samples [Fig. 1(a), (b), (d) and (e)]. The WAXS profiles in Fig. 1 and the assignment of the crystalline diffraction peaks indicate that in the solution-crystallization of neat P(L-2HB) and P(D-2HB), a new type of homo-crystallites were formed for Mw below 1 104 g mol1 and the normal type of homocrystallites were formed for Mw above 1 104 g mol1. On the other hand, in the melt-crystallization of the neat P(L-2HB) and P(D-2HB) at Tc ¼ 70 C, the new type of homo-crystallites were formed for Mw ¼ 9 103e1 104 g mol1 and the normal type homocrystallites were formed for Mw above 9 103 g mol1. Here, we assumed that the effects of Mw on the crystalline species are same for neat P(L-2HB) and P(D-2HB). D-3
4. Conclusions The following conclusions can be derived from the present study for the effects of molecular weight and crystallization conditions on the crystallization of neat P(L-2HB), P(D-2HB), and their blends with Mw ¼ 1.8 103e4.1 104 g mol1. Only SC crystallites were formed during the solution-crystallization of the blends, without the formation of homo-crystallites, irrespective of Mw. SC crystallites were the only crystalline species formed during the melt-crystallization of the blends in the Tc range of 55e160 C (except at 70 C), whereas homo-crystallites were not formed. On the other hand, at Tc ¼ 70 C, in addition to the SC crystallites, a significant amount of homo-crystallites were formed for average Mw exceeding 2.7 104 g mol1, and an equal amount of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 104 g mol1. However, the critical average Mw, above which predominant homo-crystallization and no SC crystallization occurs was not observed. The T0m(S) of SC crystallites obtained by the HoffmaneWeeks procedure was 239.1 C, which was lower than the highest T0m(S) value (279 C) reported for PLLA/ PDLA SC crystallites. In solution-crystallization of neat P(L-2HB) and P(D-2HB), a new type of homo-crystallites were formed for Mw below 1 104 g mol1 and the normal type of homo-crystallites were formed for Mw above 1 104 g mol1. In contrast, during melt-crystallization of neat P(L-2HB) and P(D-2HB), the new and
normal types of homo-crystallites were formed for Mw ¼ 9 103e 1 104 g mol1 and Mw above 9 103 g mol1, respectively. Acknowledgments This research was supported by a Grant-in-aid for Scientific Research, Category “C”, No. 24550251, from the Japan Society for the Promotion of Science (JSPS).
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