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Crystallization behavior and physical properties of linear 2-arm and branched 4-arm poly(l -lactide)s: Effects of branching
Polymer 54 (2013) 2422e2434
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Crystallization behavior and physical properties of linear 2-arm and branched 4-arm poly(L-lactide)s: Effects of branching Yuzuru Sakamoto, Hideto Tsuji* Department of Environmental and Life Sciences, Graduate School of Technology, 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 14 November 2012 Received in revised form 4 February 2013 Accepted 26 February 2013 Available online 13 March 2013
The pure effects of branching architecture on crystallization and thermal properties were investigated using linear 2-arm and branched 4-arm poly(L-lactide) (2L and 4L, respectively) polymers having a wide range of number-average molecular weight (Mn) values of 5.0 103e6.0 104 g mol1. 2L and 4L were synthesized by bulk ring-opening polymerization of L-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) in the presence of bifunctional coinitiator of 1,3-propanediol and tetrafunctional coinitiator of pentaerythritol with the identical carbon numbers between the hydroxyl groups. Branching architecture of 4L delayed or disturbed non-isothermal crystallization during heating and isothermal crystallization, compared to those of linear architecture of 2L. In contrast, the glass transition temperature or segmental mobility, melting temperature or crystalline thickness, transition crystallization temperatures of crystalline form, and crystal growth mechanism were not affected by the presence of branching, but depended on Mn or Mn per one arm. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Branching Crystallization Poly(lactic acid)
1. Introduction Poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] is attracting much attention because it is biomass-derived, biodegradable, compostable, and nontoxic to the human body and the environment [1e10]. Linear 1-arm high-molecular weight PLLA has high mechanical performance comparable with that of commercial polymers such as polystyrene and poly(ethylene terephthalate) and, therefore, is utilized as commodity and industrial materials as well as biomedical materials for tissue regeneration and matrixes for drug delivery systems (DDS). Crystallization behavior of linear 1-arm and 2-arm and branched multi-arm PLLA is a matter of concern because its crystallinity affects in vivo degradation behavior and drug release profile. For linear 1-arm PLLA, various parameters’ effects on the crystallization and spherulite growth behavior have been studied intensively and a great amount of information has been accumulated [11e33]. On the other hand, linear 2-arm and branched multi-arm PLLA and L-lactide copolymers have been prepared by polymerization of L-lactide and comonomers by various procedures using diols and polyols as coinitiators, or spirocyclic initiators [1,5,9,25,34e40]. In a previous study, for Mn estimated by GPC [Mn(GPC)] below 1.5 104 g mol1, the non-isothermal crystallization during heating * Corresponding author. E-mail address: [email protected] (H. Tsuji). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.02.044
and isothermal spherulite growth were disturbed in linear 2-arm PLLA compared to those in linear 1-arm PLLA [41]. The result suggested that the reported low crystallizability of multi-arm PLLA compared to that of 1-arm PLLA is caused not only by presence of branching but also by the chain directional change, the incorporation of coinitiator moiety in the middle of the molecules, and their mixed effect (abbreviated as “chain directional effects”). Hao et al. found that Avrami exponent n values observed to strongly depend on the star-shaped structures with different arm numbers, implying their distinct nucleation mechanisms, and the more arm numbers of a star-shaped PLLA led to a lower isothermal crystallization rate [39]. Moreover, Wang et al. found that for 1, 2, 4, and 6-arm PLLAs, the melting temperature (Tm), the cold crystallization temperature (Tcc), and the degree of crystallinity (Xc) of the PLLAs decreased with increasing the number of arms at a fixed numberaverage molecular weight (Mn), and the sperulite growth rate (G) of 6-arm PLLA slightly increased with increasing Mn [40]. However, in these studies, G was monitored at a fixed crystallization temperature (Tc) of 130 and 120 C, respectively for all star-shaped PLLA with different Mn and Tm values and, therefore, the effects of Mn were compared at different supercooling (DT ¼ TmTc) values, and the effects of branching on crystal growth mechanisms were not estimated by regime analysis. In addition, the comparison was performed for the linear and branched PLLAs in terms of total molecular weight, although Tm values are expected to depend on the molecular weight per one arm [Mn(arm)].
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PLLA shows the various types of crystalline forms [a, b, or g-forms] depending on the processing conditions [42e53]. The mechanical, thermal, and optical properties of polymorphic polymers strongly depend on the crystalline form and morphology [54,55]. Besides, biodegradability is also influenced by the crystalline form in the case of biodegradable polymers including PLLA [56]. Pan et al. reported that the disordered a0 -form was formed in the PLLA samples crystallized at low temperature, and the crystallization kinetics of a0 and a crystals are different [57,58]. Recently, Wasanasuk et al. proposed based on the wide angle X-ray diffraction (WAXD) measurements that the disordered a0 -form is a crystal modification independent of the ordered a-form and, therefore, should be named as a d-form, and the d-form transforms discontinuously to more regular the a-form at higher temperature [59]. The effects of molecular weight [58] and polymer blending with PDLLA [60,61], and with PDLA [62] on the transition temperature from the d-form to the a-form were reported. To the best of our knowledge, detailed study on the pure effect of branching on that the crystallization behavior and physical properties of PLLAs with a wide range of molecular weight has not been reported so far. The purpose of the present study is to investigate in detail the pure effects of the branching on the crystallization behavior (spherulite growth rate and crystalline growth mechanism), crystalline form, and physical properties of PLLAs with a wide range of molecular weight [Mn and Mn(arm)]. For this purpose, the chain directional effects, as in the case of linear 1- and 2-arm PLLA [41], and the effects of the difference in carbon number in the main chain of coinitiator moiety should be excluded. In the present study, we synthesized linear 2-arm (not 1-arm) and branched 4-arm PLLA (abbreviated as 2L and 4L, respectively) having a wide range of molecular weight by bulk ringopening polymerization of L-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) in the presence of bifunctional coinitiator of 1,3-propanediol and tetrafunctional coinitiator of pentaerythritol, respectively. 1,3-propanediol and pentaerythritol have the identical carbon numbers between the hydroxyl groups. The non-isothermal crystallization and the isothermal spherulite growth behavior, crystallinity and crystal modification at different crystallization temperatures were investigated by the use of differential scanning calorimetry (DSC) and polarized optical microscopy (POM), and WAXD.
Fig. 1. DSC thermograms of melt-quenched 2L (a) and 4L (b) during heating.
2. Experimental section
Gorinchem, The Netherlands) in bulk at 140 C initiated with 0.03 wt% of tin(II) 2-ethylhexanoate (Nacalai Tesque, Inc., Kyoto, Japan) in the presence of different amounts of 1,3-propanediol (SigmaeAldrich Co., St. Louis, MO) and pentaerythritol (Sigmae Aldrich Co., St. Louis, MO), respectively, as coinitiators. Before polymerization, L-lactide was purified by recrystallization using ethylacetate as the solvent, whereas tin(II) 2-ethylhexanoate was purified by distillation under reduced pressure. These alcohols as coinitiators were used as received. Synthesized polymers were purified by reprecipitation using chloroform and methanol as the solvent and non-solvent, respectively. The purified polymers were dried in vacuo for at least 1 week. In the present study, the numbers immediately following the codes 2L and 4L are the number-average molecular weight [Mn(NMR)]/103 g mol1. Here, the Mn(NMR) values are those estimated by 1H NMR spectroscopy. The molecular characteristics and thermal properties of PLLAs used in this study are listed in Table 1. For preparation of melt-quenched and crystallized samples, blend films were sealed in test tubes under reduced pressure, melted at 200 C for 5 min, and quenched at 0 C for 5 min (melt-quenched samples for DSC) or crystallized at different Tc of 80e140 C for 10 h, and quenched at 0 C for 5 min (crystallized samples for WAXD).
2.1. Materials
2.2. Measurements and observation
Linear 2L and branched 4L were synthesized by ring-opening polymerization of L-lactide (PURASORB LÒ, Purac Biomaterials,
The Mn(NMR) values of the synthesized polymers were determined from the 400 MHz 1H NMR spectra obtained in deuterated
Table 1 Characteristics and properties of 2-arm PLLAs (2L) and 4-arm PLLAs (4L). Architecture
Code
LLA/alcohol in the feeda [mol/mol]
Mn(th)b [g mol1]
Linear
2L5 2L11 2L20 2L38 4L6 4L9 4L18 4L30 4L57
42/1 69/1 139/1 278/1 35/1 69/1 139/1 216/1 461/1
6.0 1.0 2.0 4.0 5.0 1.0 2.0 3.1 6.6
Branched
a b c d e f
103 104 104 104 103 104 104 104 104
Mn(NMR)c [g mol1] 5.2 1.1 2.0 3.8 6.1 9.5 1.8 3.0 5.7
103 104 104 104 103 103 104 104 104
Mn(GPC)d [g mol1] 7.2 1.9 3.4 6.2 6.9 1.2 2.4 4.7 9.8
103 104 104 104 103 104 104 104 104
Mw(GPC)e/Mn(GPC)
Tgf [ C]
Tccf [ C]
Tmf [ C]
1.2 1.2 1.2 1.4 1.1 1.2 1.1 1.1 1.1
38.1 52.9 55.8 56.8 43.5 50.4 54.0 57.0 58.4
99.6 95.8 101.6 107.3 e 118.0 115.0 110.8 112.4
142.2 157.8 167.2 173.1 e 138.5 152.4 163.0 169.7
LLA represents L-lactide units (molecular weight ¼ 144.1 g mol1). The number-average molecular weight calculated from LLA/alcohol in the feed. The number-average molecular weight determined by NMR spectroscopy. The number-average molecular weight determined by GPC. The weight-average molecular weight determined by GPC. The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) were obtained using DSC from the first run of amorphous-made samples.
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chloroform (50 mg mL1) by a Bruker BioSpin (Kanagawa, Japan) AVANCE III 400 using tetramethylsilane as the internal standard. The Mn(NMR) values of 2L and 4L were estimated according to the following equation using the peak intensity for methylene protons of coinitiators eCH2Oe connected to L-lactide units (I1) at around 4.2 and methine protons of L-lactide units inside the chains and at chain terminals (I2), observed at around 5.2 and 4.4 ppm [63e65]:
respectively). The M(coinitiator) values are 76.1 and 136.1 g mol1 for 1,3-propanediol and pentaerythritol, respectively. The theoretical Mn [Mn(th)] values (Table 1) were calculated using the following equation, assuming that all the alcohol molecules acted as coinitiators, that all the hydroxyl groups in the alcohols acted as initiating sites of L-lactide polymerization, and that the alcohol molecules were incorporated in the synthesized polymers:
Mn ðNMRÞ ¼ MðcoinitiatorÞ þ ð144:1=2Þ f ð2I2 =I1 Þ;
Mn ðthÞ ¼ MðcoinitiatorÞ þ 144:1 ½l-lactide=coinitiator ðmol=molÞ in the feed:
(1)
where M(coinitiator) is the molecular weight of the coinitiator, 144.1 g mol1 is the molecular weight of L-lactide, and f is the arm number (2 and 4 for 1,3-propanediol and pentaerythritol,
(2)
For reference, the weight-average molecular weight [Mw(GPC)] and Mn(GPC) values of the synthesized polymers were evaluated in
Fig. 2. Tg (a) and Tcc (b) as a function of Mn and Tm as functions of Mn (c) and Mn(arm) (d) and DHm(e) as a function of Mn.
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chloroform at 40 C by a Tosoh (Tokyo, Japan) GPC system with two TSK gel columns (GMHXL) using polystyrene standards. Therefore, Mw(GPC) and Mn(GPC) values are those relative to polystyrene. In the following section, “Mn” means “Mn(NMR)” not “Mn(GPC)”. The glass transition, cold crystallization, melting temperatures (Tg, Tcc, and Tm, respectively), and enthalpies of cold crystallization and melting (DHcc and DHm, respectively) of PLLA samples were determined with a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter under a nitrogen gas flow at a rate of 50 mL min1. For investigating non-isothermal crystallization during heating, about 3 mg of melt-quenched samples were heated at a rate of 10 C min1 from room temperature to 200 C (glass transition, cold crystallization, and melting were monitored here). The Tg, Tcc, and Tm values were calibrated using tin, indium, and benzophenone as standards. The crystalline form and crystallinity (Xc) of isothermally crystallized samples were estimated by the use of WAXD. The WAXD measurements were performed for a 2q range of 10e30 at 25 C using a RINT-2500 equipped with a Cu-Ka source (l ¼ 0.15418 nm), which was operated at 40 kV and 200 mA (Rigaku Co., Tokyo, Japan). The Xc values of crystallized films were estimated using the following equation:
1 (b) of melt-quenched 2L and 4L during heating as a function of M1 . Fig. 3. Tg (a) and Tm n
Xc ¼ 100 Sc =ðSc þ Sa Þ;
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(3)
where Sc and Sa are crystalline and amorphous diffraction peak areas, respectively. The isothermal spherulite growth of the PLLA samples was observed using an Olympus (Tokyo, Japan) polarized optical microscope (BX50) equipped with a heating-cooling stage and a temperature controller (LKe600PM, Linkam Scientific Instruments, Surrey, UK) under a constant nitrogen gas flow. The powdery PLLA polymers were heated to 200 C at 100 C min1, held at this temperature for 3 min, cooled at 100 C min1 to a desired crystallization temperature (Tc) in the range of 80e140 C, and then held at the Tc (spherulite growth was observed here). 3. Results and discussion 3.1. Differential scanning calorimetry Fig. 1 shows the typical DSC thermograms of melt-quenched linear 2L and branched 4L. All PLLA samples had glass transition, cold crystallization, and melting peaks in the temperature ranges of 30e60, 90e120, and 130e180 C, respectively, except for the thermogram of 4L6 with the lowest Mn, in which neither cold crystallization nor melting peaks was observed. Similarly, neither cold crystallization or melting peak was observed for 2-arm, 3-arm and 5-arm PLLA, when their Mn(GPC) values were lowered to 2.7 103, 5.4 103 and 2.6 103 g mol1, respectively [36,37].
Fig. 4. WAXD profiles for 2q ¼ 14e22 of 2L5 (a), 2L11 (b), 2L20 (c), and 2L38 (d) crystallized at different Tc.
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The Tg, Tcc, and Tm values of PLLA were estimated from the DSC thermograms in Fig. 1 and are plotted in Fig. 2 as a function of Mn or Mn(arm). When two or more melting peaks were observed in the DSC thermograms, we assumed the highest peak temperature of the melting peak as the melting temperature because some PLLA samples have only one melting peak even when the data were expected to show multiple peaks, and we intended to evaluate the highest melting temperature of the crystallites with the largest crystalline size, which were formed during heating of amorphousmade samples. We found in the previous study that the selection of the highest or lowest peak will not give a difference with respect to the estimation of the melting temperature at an infinite molecular weight [25]. As can be expected from the results reported for linear 1-arm PLLA [28,41], the Tg (indicator of chain mobility), Tcc (indicator of crystallizability during heating), Tm (indicator of crystalline thickness), and DHm (indicator of final crystallinity during heating) of the melt-quenched 2L and 4L (except for Tcc of 2L below 1.0 104 g mol1 and of 4L below 3.0 104 g mol1) increased with increasing Mn or Mn(arm) (Fig. 2). With decreasing the molecular weight, the crystallization of PLLA will be accelerated by the enhanced chain mobility but decelerated by the increased chain directional effects (2L and 4L) due to elevated number of chain directional changing points per unit mass and by increased branching effects (4L) due to elevated number of branching points per unit mass. The increment of Tcc values or decelerating crystallization with decreasing Mn at relatively low molecular weights indicates that the disturbance of crystallization of 2L by the chain
directional effects and that of 4L by the chain directional effects and branching effects overcome the accelerating effects by the increased chain mobility. The Tg values of the 2L and 4L were very similar to each other [Fig. 2(a)]. In contrast, the Tm and DHm values of 2L were higher than those of 4L, whereas the Tcc values of 2L were lower than those of 4L [Fig. 2(b), (c), and (e)]. Furthermore, Mn below which Tcc increases with decreasing Mn was higher for 4L (3.0 104 g mol1) than for 2L (1.0 104 g mol1). These results mean that branching have no significant effect on the segmental mobility but disturbed the crystallization during heating, when the branching effects were estimated in terms of Mn. However, the Tm values of 2L were similar to those of 4L when compared at the similar Mn(arm) [Fig. 2(d)]. This indicates that the crystalline thickness is determined by Mn(arm) but not by Mn and that branching had no significant effect on crystalline thickness when compared in terms of Mn(arm). To obtain the Tg and Tm values of the 2L and the 4L at infinite N , respectively), the T and T 1 values molecular weight (TgN and Tm g m are plotted in Fig. 3 as a function of Mn1 according to the Flory-Fox [66] and Flory [67] equations, respectively:
Fig. 5. WAXD profiles for 2q ¼ 14e22 of 4L9 (a), 4L18 (b), 4L30 (c), and 4L57 (d) crystallized at different Tc.
Fig. 6. Enlarged WAXD profiles for 2q ¼ 10e14 and 20e26 of 2L5 (a), 2L11 (b), 2L20 (c), and 2L38 (d) crystallized at different Tc.
Tg ¼ TgN K=Mn
(4)
N 1 1 ¼ Tm 2RM 0 =ðDHm Mn Þ; Tm
(5)
Y. Sakamoto, H. Tsuji / Polymer 54 (2013) 2422e2434
where K is a constant representing the excess free volume of the end groups of the polymer chains, M0 is the molecular weight of a half lactide unit (72.1 g mol1), and R is the gas constant. For 105Mn1 below 10 g1 mol, the TgN values estimated using Equation (4) from Fig. 3(a) were 58.5 and 60.6 C for the 2L and 4L, respectively, suggesting that branching does not affect the chain mobility of PLA at infinite Mn. The estimated TgN values are in agreement with the 58 C reported by Jamshidi et al. for linear 1-arm PLLA prepared by polycondensation [68]. The K values evaluated using Equation (4) from Fig. 3(a) were 5.9 104 and 9.1 104 K g mol1 for the 2L and 4L, respectively, reflecting the larger excess free volume for the branching architecture. The K values estimated for 2L in the present study is very close to the 5.8 104 K g mo11 reported for linear 2-arm PLLA prepared with ethylene glycol [41]. N values obtained using Equation (5) from On the other hand, the Tm Fig. 3(b) was 179.4 and 178.0 C for the 2L and 4L, respectively. The values were similar to 178 and 176 C that reported for linear 1- and 2-arm PLLA prepared with 1-dodecanol and ethylene glycol, N values for the 2L and 4L respectively. However, the evaluated Tm are slightly lower than the 184 C reported for linear 1-arm [68]. 3.2. Wide-angle X-ray diffraction
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WAXD profiles were obtained (Figs. 4 and 5). The sharp crystalline peaks were observed at 2q values of around 17 and 19 for both 2L and 4L (except 2L5 and 4L9 crystallized at Tc ¼ 140 C). These two peaks are attributed to the diffraction from (110)/(200) and (203) lattice planes, respectively, of the d- or a-form of PLLA, in good agreement with the reported results [50,53,57]. On the other hand, no crystalline diffraction peaks were observed for 2L5 and 4L9 crystallized at Tc ¼ 140 C, indicating that they were amorphous when crystallized at Tc very close to Tm (142.2 and 138.5 C, respectively). Assignments of the observed diffraction are based on the crystalline structure reported for a-form by Miyata and Masuko [52]. With increasing Tc, several major changes were observed for relatively weak crystalline diffraction peaks in 2q ranges of 10e14 and 20e26 , as shown in Figs. 6 and 7. Firstly, the crystalline peaks attributed to the diffraction from (004)/(103), (204), (115), (016), and (206) lattice planes of a-form (not d-form) were observed at around 12.5 , 20.8 , 23.0 , 24.1 and 25.1. Secondly, as for crystalline peaks ascribed to the diffraction from (010) and (015) lattice planes of the more ordered and compact structure of a-form at around 14.8 and 22.4 , respectively, becomes more distinct. Thirdly, weak crystalline peak attributed to diffraction from (206)
To investigate the effect of Tc on the crystalline form (the d- or a-forms) and Xc values, 2L and 4L were crystallized from the melt at different Tc values in the range of 80e140 C for 10 h and their
Fig. 7. Enlarged WAXD profiles for 2q ¼ 10e14 and 20e26 of 4L9 (a), 4L18 (b), 4L30 (c), and 4L57 (d) crystallized at different Tc.
Fig. 8. Tc values at which the transition from d-form to a-form took place [Tc(a)] of 2L and 4L as functions of Mn (a) and Mn(arm) (b).
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lattice plane of the d-form [58] at nearby 24.6 (broken lines) moves to higher 2q of the a-form (dotted lines). The two distinct crystalline diffraction peaks of the a-form at around 14.8 and 22.5 and some weak crystalline diffraction peaks of the a-form at around 12.5 , 20.8 , 23.0 , 24.1, and 25.1 were observed for 2L5 and 4L9 crystallized at Tc ¼ 90 C [Figs. 6(a) and 7(a)], and 2L11 and 4L18 at Tc ¼ 110 C [Figs. 6(b) and 7(b)], and 2L20, 2L38, 4L30, 4L57 crystallized at Tc ¼ 120 C [Figs. 6(c,d) and 7(c,d)], respectively, indicating these samples were crystallized in the a-form. On the other hand, the crystalline peak at 24.6 of the d-form was observed for 2L5 and 4L9 crystallized at Tc 80 C [Figs. 6(a) and 7(a)], and 2L11 and 4L18, 4L30, 4L57 at Tc 100 C [Figs. 6(b) and 7(bed)], and 2L20, 2L38 crystallized at Tc 110 C [Figs. 6(c,d)], respectively, indicating these samples were crystallized in the d-form. However, the two weak crystalline diffraction peaks of the a-form at around 20.8 and 23.0 and a crystalline peak of the d-form at 24.6 the are not observed for 4L30 and 4L57 at Tc ¼ 110 C. The Tc values at which the transition from the d-form to the a-form took place [Tc(a)] were estimated from Figs. 6 and 7 and are plotted in Fig. 8(a) and (b) as functions of Mn and Mn(arm), respectively. Tc(a) is defined as the Tc above which samples were crystallized in only a-form. For Mn below 2.0 104 g mo11, the Tc(a) values of 2L were higher than that of 4L when plotted as a function of Mn. However, when plotted as a function of Mn(arm), the Tc(a) values of both 2L and 4L were very similar to each other and increased with increasing Mn(arm) for Mn(arm) below 1.0 104 g mol1. This means that the branching parts in 4L have no significant effects on Tc(a) and Tc(a) is determined only by the crystallizable PLLA segmental length as in the case of Tm. As
reported, the d- and a-forms are formed at relatively low and high Tc values, respectively [50,53,57]. Zhang et al. [50], Kawai et al. [53] and Pan et al. [57] showed that the transition of linear 1-arm PLLA from d- to a-form takes place at Tc of 110 and 120 C. Pan et al. reported that for low molecular weight PLLA with higher segmental mobility the transition from the d-form to a-form is more rapid and proceeds prominently even when annealed at relatively low Tc [58], in consistent with the results for 2L and 4L in the present study. The Xc values of 2L and 4L are plotted in Fig. 9 as functions of Tc and DT. In the crystallizable Tc range, the Xc values of 2L were higher than that of 4L, whereas the values were practically independent of DT and Mn. The result indicates that branching reduced the ratio of polymer chains which formed crystalline regions. 3.3. Spherulite growth Figs. 10 and 11 show the typical polarized photomicrographs of the spherulites of 2L and 4L crystallized isothermally in the a-form and regime II at DT in the range of 32e35 C. As seen in Figs. 10 and 11, normal spherulites were formed in all 2L and 4L having Mn exceeding 1.0 104 g mol1, whereas rather disordered spherulites were seen in 4L9. The latter disorder strongly suggests that the branching in 4L9 caused the macroscopic structural defects in the spherulites. The radius growth rate of spherulites (G) and the induction period of spherulite formation (ti) of 2L and 4L are plotted in Figs. 12 and 13, respectively, as functions of Tc and DT. Here, the G values were estimated from the slopes of spherulite radius plotted as a function of crystallization time, whereas the ti values were
Fig. 9. Crystallinity (Xc) of 2L [(a), (c)] and 4L [(b), (d)] as functions of Tc [(a), (b)] and DT [(c), (d)]. Open and closed symbols are values for d- and a-form, respectively.
Fig. 10. Polarized photomicrographs of spherulites of 2L crystallized at the shown temperatures and times (crystallized in a-form and regime II).
Fig. 11. Polarized photomicrographs of spherulites of 4L crystallized at the shown temperatures and times (crystallized in a-form and regime II).
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Fig. 12. Radius growth rates of spherulites (G) of 2L [(a), (c)] and 4L [(b), (d)] as functions of Tc [(a), (b)] and DT [(c), (d)]. Open and closed symbols are values for d- and a-forms, respectively.
evaluated from extrapolation of the spherulite radius lines plotted against crystallization time to a radius of 0 mm [22]. As seen Fig. 12(c), the G values crystallized in the d-form [G(d)] (open symbols) of 2L decreased in the following order: 2L11 z 2L20 z 2L38 > 2L5. However, the G values crystallized in the a-form [G(a)] (closed symbols) of 2L for DT exceeding 30 C became lower in the following order: 2L11 > 2L5 z 2L20 > 2L38, whereas that of 2L for DT below 30 C decreased in the following order: 2L5 z 2L11 > 2L20 > 2L38. As stated earlier, with decreasing molecular weight, two or three factors compete. That is, with lowering molecular weight, the coinitiator moieties as chain directional changing or branching points per unit mass increase, which decelerates the spherulite growth, whereas the increased segmental mobility accelerates the spherulite growth. The results indicate that, for Mn exceeding 1.0 104 g mol1, the accelerating effect of spherulite growth of 2L crystallized in the pure d-form by increased segmental mobility through decreased molecular weight was balanced with the decelerating effect by increased chain directional changing points per unit mass, whereas the accelerating effect of spherulite growth of 2L crystallized in the a-form by increased segmental mobility through decreased molecular weight was higher than the decelerating effect by increased chain directional changing points. In other words, the accelerating effect of enhanced segmental mobility relative to the decelerating effect of increased chain directional changing points per unit mass increased with decreasing DT. As seen in Fig. 12(d), G(d) values of 4L became lower in the following order: 4L57 > 4L30 > 4L18. The G(a) values of 4L for DT
exceeding 30 C became lower in the following order: 4L57 z 4L30 z 4L18 > 4L9, whereas that of 4L for DT below 30 C decreased in the following order 4L18 > 4L30 > 4L9 > 4L57. These results indicate that for Mn exceeding 1.0 104 g mol1, the accelerating effect of spherulite growth of 4L crystallized in the d-form by increased segmental mobility through decreased molecular weight was lower than the decelerating effect by increased branching points per unit mass, and the accelerating effect of 4L crystallized in the a-form for DT exceeding 30 C by increased segmental mobility through decreased molecular weight was balanced with the decelerating effect by increased branching density, whereas the accelerating effect of 4L crystallized in the aform for DT below 30 C by increased segmental mobility through decreased molecular weight was higher than the decelerating effect by increased branching density. In other words, the accelerating effect of enhanced segmental mobility relative to the decelerating effect of increased branching density increases with decreasing DT. The G values were compared using 2L and 4L having similar Mn values, i.e., 2L20 and 4L18, 2L38 and 4L30. The G(d) values of 2L and 4L as a function of DT decreased in the following order: 2L20 z 2L38 > 4L30 > 4L18. The G(a) values for DT exceeding 30 C decreased in the following order: 2L20 > 2L38 z 4L18 z 4L30, whereas that for DT below 30 C decreased in the following order: 2L20 > 4L18 > 2L38 z 4L30. These results suggest that disturbance effect of spherulite growth of 4L by branching decreased with increasing Mn and decreasing DT. As seen in Fig. 13, the ti values exhibited the positive values when Tc approached Tm. The ti
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Fig. 13. Induction period for spherulite formation (ti) of 2L [(a), (c)] and 4L [(b), (d)] as functions of Tc [(a), (b)] and DT [(c), (d)]. Open and closed symbols are values for d- and aforms, respectively.
increased at lower DT with increasing Mn, as can be expected, and this trend was more remarkable for 2L than for 4L. The latter finding may be ascribed to the fact that the main factor determining ti is not Mn but Mn(arm). 3.4. Nucleation and front constants We estimated the nucleation constant (Kg) and the front constant (G0) for 2-arm and 4-arm PLLA using the nucleation theory established by Hoffman et al. [69,70], in which G can be expressed by the following equation:
h i G ¼ G0 exp U * =RðTc TN Þ exp Kg =Tc DTf ;
(6)
0 T when T 0 is equilibrium T , f is the where DT is supercooling Tm c m m 0 factor expressed by 2Tc/(Tm þ Tc) which accounts for the changes in 0 , U* is the the heat of fusion as the temperature is decreased below Tm activation energy for the transportation of segments to the crystallization site, R is the gas constant, and TN is the hypothetical temperature where all motion associated with viscous flow ceases. Fig. 14 illustrates the ln G þ 1500/R(TcTN) of 2L and 4L as a function 0 is 212 C [13]. Here, we used the of 1/(TcDTf), assuming that Tm universal value of U* ¼ 1500 cal mol1 and TN ¼ Tg30 K for comparison with the reported values [11,18,22,25,26,28], although Urbanovici et al. suggested that U* has to be temperature-dependent (not a constant) and that instead of TN ¼ Tg30 K, Tg should be used for TN [16]. The plots in this figure give Kg as a slope and the intercept ln G0. The estimated Kg and G0 values are tabulated in Table 2.
The experimental data of 2L and 4L were composed of two or three lines having different slopes. The slope difference is caused by the kinetic difference in the crystal growth of regimes III and II or regimes II and I. The reported transition temperatures of linear 1-arm PLLA for regimes IIIeII and regimes IIeI are respectively around 120 C and around 150 C or higher [11,15,18,20,25,28]. In the present study, we have estimated the Kg and G0 values from the two or three lines, assuming that the lines having high and low slopes are for regime III or I and regime II kinetics, respectively [11,18,25,28]. Both 2L and 4L having Mn(arm) of 2.5 103e 9.0 103 g mol1 (i.e., Mn of 5.0 103e1.8 104 g mol1 for 2L and Mn of 1.0 104e3.6 104 g mol1 for 4L) had three different Kg values or regimes, whereas both 2L and 4L having Mn(arm) exceeding 9.0 103 g mol1 (i.e., Mn exceeding 1.8 104 g mol1 for 2L and Mn exceeding 3.6 104 g mol1 for 4L) and 4L having Mn(arm) below 2.5 103 g mol1 (i.e., Mn below 1.0 104 g mol1) had two different Kg values or regimes. The Kg values for regime II [Kg(II)] (1.87e2.57 105 K2) and the Kg values for regime III [Kg(III)] (4.97e6.70 105 K2) of 2L and 4L were approximately in the ranges of the Kg(II) values (2.27e2.55 105 K2) and Kg(III) values (4.20e5.51 105 K2) reported for 1-arm PLLA or PDLA prepared with and without 1-dodecanol and having Mn(GPC) of 7.7 103e5.6 105 g mol1 [22,25,28], and Kg(II) values (2.20e2.59 105 K2) and Kg(III) values (4.48e6.57 105 K2) reported for 1-arm PLLA prepared with 1-dodecanol and L-lactic acid and 2-arm PLLA prepared with ethylene glycol and having Mn(GPC) of 1.5 104 g mol1 [41]. The Kg values for regime I [Kg(I)] (3.72e4.03 105 K2) of 2L and 4L having Mn(arm) of 4.5 103e 1.0 104 g mol1 were similar with the Kg(I) values (4.87 105 K2)
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Y. Sakamoto, H. Tsuji / Polymer 54 (2013) 2422e2434
Fig. 14. ln G þ 1500/R(TcTN) of 2L [(a), (c)] and 4L [(b), (d)] having Mn below 1.8 104 g mol1 [(a), (b)] and exceeding 1.8 104 g mol1 [(c), (d)] as a function of 1/(Tc DTf). Open and closed symbols are values for d- and a-forms, respectively.
reported for 1-arm PLLA prepared without coinitiator having viscosity average molecular weight of 1.5 105 g mol1 [11]. The Tc values at which the transition from regime III kinetics to regime II kinetics took place [Tc(IIIeII)] and from regime II kinetics to regime I kinetics took place [Tc(IIeI)] obtained from Fig. 14 are summarized in Table 3 and Tc(IIIeII) values are plotted in Fig. 15 as functions of Mn and Mn(arm), together with Tc(a). The dependence of Tc(IIIeII) values of 2L and 4L on Mn(arm) was very close to Tc(a). This strongly suggests that the transition of the a-form to the d-form should has changed the crystalline growth mechanism from regime III to II. In contrast, transition from regime II to I of 2L and 4L occurred without transition of crystalline form. The Tc(IIIeII) and Tc(III) values of 4L30 (115 and 140 C, respectively) were almost similar with the those reported for 1-arm PLLA having Mn(GPC) of
Table 3 Tc which gives maximum G (Gmax) [Tc(max)], DTc(max) ¼ TmTc(max), and the transition Tc from Regime III to II and from Regime II to I [Tc(IIIII) and Tc(III), respectively]. Code
Mn(arm)
2L5 2L11 2L20 2L38 4L9 4L18 4L30 4L57
2.6 5.3 9.8 1.9 2.3 4.4 7.5 1.4
a
103 103 103 104 103 103 103 104
Tc(max) [ C]
DTc(max) [ C]
Gmax [mm min1]
Tc(IIIII)a [ C]
Tc(III)a [ C]
115 110 110 115 110 120 135 110
27 48 57 58 29 32 28 60
13.9 18.2 13.0 8.5 3.3 6.7 5.3 5.0
100 115 120 120
115 130
105 115 115
120 120 140
Tc(IIIII) and Tc(III) were estimated from Fig. 14.
Table 2 Front constant (G0) and nucleation constant (Kg) of 2L and 4L. Code
Mn(arm)
2L5 2L11 2L20 2L38 4L9 4L18 4L30 4L57
2.6 5.3 9.8 1.9 2.3 4.4 7.5 1.4
103 103 103 104 103 103 103 104
G0(III) [mm min1] 4.57 5.12 3.96 9.97
1010 1011 1011 1010
9.27 1010 6.27 1010 1.14 1013
G0(II) [mm min1] 7.28 1.29 6.01 2.57 4.73 5.65 1.37 4.22
106 107 107 107 108 106 107 107
G0(I) [mm min1]
Kg(III) [K2]
9.76 1011 7.28 109
5.19 5.42 5.34 4.97
1.08 1013 4.92 109 3.15 109
105 105 105 105
5.28 105 5.07 105 6.70 105
Kg(II) [K2] 2.02 1.94 2.51 2.37 3.60 1.87 2.25 2.57
105 105 105 105 105 105 105 105
Kg(I) [K2] 5.96 105 3.85 105
6.87 105 4.03 105 3.72 105
Y. Sakamoto, H. Tsuji / Polymer 54 (2013) 2422e2434
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(3) Regime analysis exhibited that crystal growth mechanism depends on Tc, crystalline form, the molecular weight and presence or absence of branching. References
Fig. 15. Tc values at which the transition from regime III to II took place [Tc(IIIII)] (closed symbols) and Tc(a) (open symbols) of 2L and 4L as functions of Mn (a) and Mn(arm) (b).
1.7 104e3.34 105 g mol1 (120 and 147 C, respectively) [18]. However, Tc(IIIeII) and Tc(III) (100120 and 115140 C, respectively) values of 2L and 4L decreased with decreasing Mn, whereas those of 1-arm PLLA reported are independent of Mn [18]. These results indicate that crystal growth mechanism depends on Tc, crystalline form, the molecular weight and presence or absence of branching.
4. Conclusions The following conclusions can be derived for the crystallization and physical properties of linear 2-arm PLLA (2L) and branched 4-arm PLLA (4L) from the aforementioned experimental results: (1) Branching architecture of 4L delayed or disturbed the nonisothermal and isothermal crystallization, compared to linear architecture of 2L. This resulted in the higher Tcc values, lower Xc, and G values of 4L compared to those of 2L. (2) The Tg or segmental mobility, Tm or crystalline thickness, and Tc(a) were not affected by the presence of branching. Also, the Tg depended Mn, whereas the Tm or crystalline thickness, and Tc(a) of 4L depended on Mn(arm) not Mn, and were similar to those of 2L, when plotted as a function of Mn(arm).
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Crystallization behavior and physical properties of linear 2-arm and branched 4-arm poly(L-lactide)s: Effects of branching Yuzuru Sakamoto, Hideto Tsuji* Department of Environmental and Life Sciences, Graduate School of Technology, 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 14 November 2012 Received in revised form 4 February 2013 Accepted 26 February 2013 Available online 13 March 2013
The pure effects of branching architecture on crystallization and thermal properties were investigated using linear 2-arm and branched 4-arm poly(L-lactide) (2L and 4L, respectively) polymers having a wide range of number-average molecular weight (Mn) values of 5.0 103e6.0 104 g mol1. 2L and 4L were synthesized by bulk ring-opening polymerization of L-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) in the presence of bifunctional coinitiator of 1,3-propanediol and tetrafunctional coinitiator of pentaerythritol with the identical carbon numbers between the hydroxyl groups. Branching architecture of 4L delayed or disturbed non-isothermal crystallization during heating and isothermal crystallization, compared to those of linear architecture of 2L. In contrast, the glass transition temperature or segmental mobility, melting temperature or crystalline thickness, transition crystallization temperatures of crystalline form, and crystal growth mechanism were not affected by the presence of branching, but depended on Mn or Mn per one arm. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Branching Crystallization Poly(lactic acid)
1. Introduction Poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] is attracting much attention because it is biomass-derived, biodegradable, compostable, and nontoxic to the human body and the environment [1e10]. Linear 1-arm high-molecular weight PLLA has high mechanical performance comparable with that of commercial polymers such as polystyrene and poly(ethylene terephthalate) and, therefore, is utilized as commodity and industrial materials as well as biomedical materials for tissue regeneration and matrixes for drug delivery systems (DDS). Crystallization behavior of linear 1-arm and 2-arm and branched multi-arm PLLA is a matter of concern because its crystallinity affects in vivo degradation behavior and drug release profile. For linear 1-arm PLLA, various parameters’ effects on the crystallization and spherulite growth behavior have been studied intensively and a great amount of information has been accumulated [11e33]. On the other hand, linear 2-arm and branched multi-arm PLLA and L-lactide copolymers have been prepared by polymerization of L-lactide and comonomers by various procedures using diols and polyols as coinitiators, or spirocyclic initiators [1,5,9,25,34e40]. In a previous study, for Mn estimated by GPC [Mn(GPC)] below 1.5 104 g mol1, the non-isothermal crystallization during heating * Corresponding author. E-mail address: [email protected] (H. Tsuji). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.02.044
and isothermal spherulite growth were disturbed in linear 2-arm PLLA compared to those in linear 1-arm PLLA [41]. The result suggested that the reported low crystallizability of multi-arm PLLA compared to that of 1-arm PLLA is caused not only by presence of branching but also by the chain directional change, the incorporation of coinitiator moiety in the middle of the molecules, and their mixed effect (abbreviated as “chain directional effects”). Hao et al. found that Avrami exponent n values observed to strongly depend on the star-shaped structures with different arm numbers, implying their distinct nucleation mechanisms, and the more arm numbers of a star-shaped PLLA led to a lower isothermal crystallization rate [39]. Moreover, Wang et al. found that for 1, 2, 4, and 6-arm PLLAs, the melting temperature (Tm), the cold crystallization temperature (Tcc), and the degree of crystallinity (Xc) of the PLLAs decreased with increasing the number of arms at a fixed numberaverage molecular weight (Mn), and the sperulite growth rate (G) of 6-arm PLLA slightly increased with increasing Mn [40]. However, in these studies, G was monitored at a fixed crystallization temperature (Tc) of 130 and 120 C, respectively for all star-shaped PLLA with different Mn and Tm values and, therefore, the effects of Mn were compared at different supercooling (DT ¼ TmTc) values, and the effects of branching on crystal growth mechanisms were not estimated by regime analysis. In addition, the comparison was performed for the linear and branched PLLAs in terms of total molecular weight, although Tm values are expected to depend on the molecular weight per one arm [Mn(arm)].
Y. Sakamoto, H. Tsuji / Polymer 54 (2013) 2422e2434
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PLLA shows the various types of crystalline forms [a, b, or g-forms] depending on the processing conditions [42e53]. The mechanical, thermal, and optical properties of polymorphic polymers strongly depend on the crystalline form and morphology [54,55]. Besides, biodegradability is also influenced by the crystalline form in the case of biodegradable polymers including PLLA [56]. Pan et al. reported that the disordered a0 -form was formed in the PLLA samples crystallized at low temperature, and the crystallization kinetics of a0 and a crystals are different [57,58]. Recently, Wasanasuk et al. proposed based on the wide angle X-ray diffraction (WAXD) measurements that the disordered a0 -form is a crystal modification independent of the ordered a-form and, therefore, should be named as a d-form, and the d-form transforms discontinuously to more regular the a-form at higher temperature [59]. The effects of molecular weight [58] and polymer blending with PDLLA [60,61], and with PDLA [62] on the transition temperature from the d-form to the a-form were reported. To the best of our knowledge, detailed study on the pure effect of branching on that the crystallization behavior and physical properties of PLLAs with a wide range of molecular weight has not been reported so far. The purpose of the present study is to investigate in detail the pure effects of the branching on the crystallization behavior (spherulite growth rate and crystalline growth mechanism), crystalline form, and physical properties of PLLAs with a wide range of molecular weight [Mn and Mn(arm)]. For this purpose, the chain directional effects, as in the case of linear 1- and 2-arm PLLA [41], and the effects of the difference in carbon number in the main chain of coinitiator moiety should be excluded. In the present study, we synthesized linear 2-arm (not 1-arm) and branched 4-arm PLLA (abbreviated as 2L and 4L, respectively) having a wide range of molecular weight by bulk ringopening polymerization of L-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) in the presence of bifunctional coinitiator of 1,3-propanediol and tetrafunctional coinitiator of pentaerythritol, respectively. 1,3-propanediol and pentaerythritol have the identical carbon numbers between the hydroxyl groups. The non-isothermal crystallization and the isothermal spherulite growth behavior, crystallinity and crystal modification at different crystallization temperatures were investigated by the use of differential scanning calorimetry (DSC) and polarized optical microscopy (POM), and WAXD.
Fig. 1. DSC thermograms of melt-quenched 2L (a) and 4L (b) during heating.
2. Experimental section
Gorinchem, The Netherlands) in bulk at 140 C initiated with 0.03 wt% of tin(II) 2-ethylhexanoate (Nacalai Tesque, Inc., Kyoto, Japan) in the presence of different amounts of 1,3-propanediol (SigmaeAldrich Co., St. Louis, MO) and pentaerythritol (Sigmae Aldrich Co., St. Louis, MO), respectively, as coinitiators. Before polymerization, L-lactide was purified by recrystallization using ethylacetate as the solvent, whereas tin(II) 2-ethylhexanoate was purified by distillation under reduced pressure. These alcohols as coinitiators were used as received. Synthesized polymers were purified by reprecipitation using chloroform and methanol as the solvent and non-solvent, respectively. The purified polymers were dried in vacuo for at least 1 week. In the present study, the numbers immediately following the codes 2L and 4L are the number-average molecular weight [Mn(NMR)]/103 g mol1. Here, the Mn(NMR) values are those estimated by 1H NMR spectroscopy. The molecular characteristics and thermal properties of PLLAs used in this study are listed in Table 1. For preparation of melt-quenched and crystallized samples, blend films were sealed in test tubes under reduced pressure, melted at 200 C for 5 min, and quenched at 0 C for 5 min (melt-quenched samples for DSC) or crystallized at different Tc of 80e140 C for 10 h, and quenched at 0 C for 5 min (crystallized samples for WAXD).
2.1. Materials
2.2. Measurements and observation
Linear 2L and branched 4L were synthesized by ring-opening polymerization of L-lactide (PURASORB LÒ, Purac Biomaterials,
The Mn(NMR) values of the synthesized polymers were determined from the 400 MHz 1H NMR spectra obtained in deuterated
Table 1 Characteristics and properties of 2-arm PLLAs (2L) and 4-arm PLLAs (4L). Architecture
Code
LLA/alcohol in the feeda [mol/mol]
Mn(th)b [g mol1]
Linear
2L5 2L11 2L20 2L38 4L6 4L9 4L18 4L30 4L57
42/1 69/1 139/1 278/1 35/1 69/1 139/1 216/1 461/1
6.0 1.0 2.0 4.0 5.0 1.0 2.0 3.1 6.6
Branched
a b c d e f
103 104 104 104 103 104 104 104 104
Mn(NMR)c [g mol1] 5.2 1.1 2.0 3.8 6.1 9.5 1.8 3.0 5.7
103 104 104 104 103 103 104 104 104
Mn(GPC)d [g mol1] 7.2 1.9 3.4 6.2 6.9 1.2 2.4 4.7 9.8
103 104 104 104 103 104 104 104 104
Mw(GPC)e/Mn(GPC)
Tgf [ C]
Tccf [ C]
Tmf [ C]
1.2 1.2 1.2 1.4 1.1 1.2 1.1 1.1 1.1
38.1 52.9 55.8 56.8 43.5 50.4 54.0 57.0 58.4
99.6 95.8 101.6 107.3 e 118.0 115.0 110.8 112.4
142.2 157.8 167.2 173.1 e 138.5 152.4 163.0 169.7
LLA represents L-lactide units (molecular weight ¼ 144.1 g mol1). The number-average molecular weight calculated from LLA/alcohol in the feed. The number-average molecular weight determined by NMR spectroscopy. The number-average molecular weight determined by GPC. The weight-average molecular weight determined by GPC. The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) were obtained using DSC from the first run of amorphous-made samples.
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chloroform (50 mg mL1) by a Bruker BioSpin (Kanagawa, Japan) AVANCE III 400 using tetramethylsilane as the internal standard. The Mn(NMR) values of 2L and 4L were estimated according to the following equation using the peak intensity for methylene protons of coinitiators eCH2Oe connected to L-lactide units (I1) at around 4.2 and methine protons of L-lactide units inside the chains and at chain terminals (I2), observed at around 5.2 and 4.4 ppm [63e65]:
respectively). The M(coinitiator) values are 76.1 and 136.1 g mol1 for 1,3-propanediol and pentaerythritol, respectively. The theoretical Mn [Mn(th)] values (Table 1) were calculated using the following equation, assuming that all the alcohol molecules acted as coinitiators, that all the hydroxyl groups in the alcohols acted as initiating sites of L-lactide polymerization, and that the alcohol molecules were incorporated in the synthesized polymers:
Mn ðNMRÞ ¼ MðcoinitiatorÞ þ ð144:1=2Þ f ð2I2 =I1 Þ;
Mn ðthÞ ¼ MðcoinitiatorÞ þ 144:1 ½l-lactide=coinitiator ðmol=molÞ in the feed:
(1)
where M(coinitiator) is the molecular weight of the coinitiator, 144.1 g mol1 is the molecular weight of L-lactide, and f is the arm number (2 and 4 for 1,3-propanediol and pentaerythritol,
(2)
For reference, the weight-average molecular weight [Mw(GPC)] and Mn(GPC) values of the synthesized polymers were evaluated in
Fig. 2. Tg (a) and Tcc (b) as a function of Mn and Tm as functions of Mn (c) and Mn(arm) (d) and DHm(e) as a function of Mn.
Y. Sakamoto, H. Tsuji / Polymer 54 (2013) 2422e2434
chloroform at 40 C by a Tosoh (Tokyo, Japan) GPC system with two TSK gel columns (GMHXL) using polystyrene standards. Therefore, Mw(GPC) and Mn(GPC) values are those relative to polystyrene. In the following section, “Mn” means “Mn(NMR)” not “Mn(GPC)”. The glass transition, cold crystallization, melting temperatures (Tg, Tcc, and Tm, respectively), and enthalpies of cold crystallization and melting (DHcc and DHm, respectively) of PLLA samples were determined with a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter under a nitrogen gas flow at a rate of 50 mL min1. For investigating non-isothermal crystallization during heating, about 3 mg of melt-quenched samples were heated at a rate of 10 C min1 from room temperature to 200 C (glass transition, cold crystallization, and melting were monitored here). The Tg, Tcc, and Tm values were calibrated using tin, indium, and benzophenone as standards. The crystalline form and crystallinity (Xc) of isothermally crystallized samples were estimated by the use of WAXD. The WAXD measurements were performed for a 2q range of 10e30 at 25 C using a RINT-2500 equipped with a Cu-Ka source (l ¼ 0.15418 nm), which was operated at 40 kV and 200 mA (Rigaku Co., Tokyo, Japan). The Xc values of crystallized films were estimated using the following equation:
1 (b) of melt-quenched 2L and 4L during heating as a function of M1 . Fig. 3. Tg (a) and Tm n
Xc ¼ 100 Sc =ðSc þ Sa Þ;
2425
(3)
where Sc and Sa are crystalline and amorphous diffraction peak areas, respectively. The isothermal spherulite growth of the PLLA samples was observed using an Olympus (Tokyo, Japan) polarized optical microscope (BX50) equipped with a heating-cooling stage and a temperature controller (LKe600PM, Linkam Scientific Instruments, Surrey, UK) under a constant nitrogen gas flow. The powdery PLLA polymers were heated to 200 C at 100 C min1, held at this temperature for 3 min, cooled at 100 C min1 to a desired crystallization temperature (Tc) in the range of 80e140 C, and then held at the Tc (spherulite growth was observed here). 3. Results and discussion 3.1. Differential scanning calorimetry Fig. 1 shows the typical DSC thermograms of melt-quenched linear 2L and branched 4L. All PLLA samples had glass transition, cold crystallization, and melting peaks in the temperature ranges of 30e60, 90e120, and 130e180 C, respectively, except for the thermogram of 4L6 with the lowest Mn, in which neither cold crystallization nor melting peaks was observed. Similarly, neither cold crystallization or melting peak was observed for 2-arm, 3-arm and 5-arm PLLA, when their Mn(GPC) values were lowered to 2.7 103, 5.4 103 and 2.6 103 g mol1, respectively [36,37].
Fig. 4. WAXD profiles for 2q ¼ 14e22 of 2L5 (a), 2L11 (b), 2L20 (c), and 2L38 (d) crystallized at different Tc.
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The Tg, Tcc, and Tm values of PLLA were estimated from the DSC thermograms in Fig. 1 and are plotted in Fig. 2 as a function of Mn or Mn(arm). When two or more melting peaks were observed in the DSC thermograms, we assumed the highest peak temperature of the melting peak as the melting temperature because some PLLA samples have only one melting peak even when the data were expected to show multiple peaks, and we intended to evaluate the highest melting temperature of the crystallites with the largest crystalline size, which were formed during heating of amorphousmade samples. We found in the previous study that the selection of the highest or lowest peak will not give a difference with respect to the estimation of the melting temperature at an infinite molecular weight [25]. As can be expected from the results reported for linear 1-arm PLLA [28,41], the Tg (indicator of chain mobility), Tcc (indicator of crystallizability during heating), Tm (indicator of crystalline thickness), and DHm (indicator of final crystallinity during heating) of the melt-quenched 2L and 4L (except for Tcc of 2L below 1.0 104 g mol1 and of 4L below 3.0 104 g mol1) increased with increasing Mn or Mn(arm) (Fig. 2). With decreasing the molecular weight, the crystallization of PLLA will be accelerated by the enhanced chain mobility but decelerated by the increased chain directional effects (2L and 4L) due to elevated number of chain directional changing points per unit mass and by increased branching effects (4L) due to elevated number of branching points per unit mass. The increment of Tcc values or decelerating crystallization with decreasing Mn at relatively low molecular weights indicates that the disturbance of crystallization of 2L by the chain
directional effects and that of 4L by the chain directional effects and branching effects overcome the accelerating effects by the increased chain mobility. The Tg values of the 2L and 4L were very similar to each other [Fig. 2(a)]. In contrast, the Tm and DHm values of 2L were higher than those of 4L, whereas the Tcc values of 2L were lower than those of 4L [Fig. 2(b), (c), and (e)]. Furthermore, Mn below which Tcc increases with decreasing Mn was higher for 4L (3.0 104 g mol1) than for 2L (1.0 104 g mol1). These results mean that branching have no significant effect on the segmental mobility but disturbed the crystallization during heating, when the branching effects were estimated in terms of Mn. However, the Tm values of 2L were similar to those of 4L when compared at the similar Mn(arm) [Fig. 2(d)]. This indicates that the crystalline thickness is determined by Mn(arm) but not by Mn and that branching had no significant effect on crystalline thickness when compared in terms of Mn(arm). To obtain the Tg and Tm values of the 2L and the 4L at infinite N , respectively), the T and T 1 values molecular weight (TgN and Tm g m are plotted in Fig. 3 as a function of Mn1 according to the Flory-Fox [66] and Flory [67] equations, respectively:
Fig. 5. WAXD profiles for 2q ¼ 14e22 of 4L9 (a), 4L18 (b), 4L30 (c), and 4L57 (d) crystallized at different Tc.
Fig. 6. Enlarged WAXD profiles for 2q ¼ 10e14 and 20e26 of 2L5 (a), 2L11 (b), 2L20 (c), and 2L38 (d) crystallized at different Tc.
Tg ¼ TgN K=Mn
(4)
N 1 1 ¼ Tm 2RM 0 =ðDHm Mn Þ; Tm
(5)
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where K is a constant representing the excess free volume of the end groups of the polymer chains, M0 is the molecular weight of a half lactide unit (72.1 g mol1), and R is the gas constant. For 105Mn1 below 10 g1 mol, the TgN values estimated using Equation (4) from Fig. 3(a) were 58.5 and 60.6 C for the 2L and 4L, respectively, suggesting that branching does not affect the chain mobility of PLA at infinite Mn. The estimated TgN values are in agreement with the 58 C reported by Jamshidi et al. for linear 1-arm PLLA prepared by polycondensation [68]. The K values evaluated using Equation (4) from Fig. 3(a) were 5.9 104 and 9.1 104 K g mol1 for the 2L and 4L, respectively, reflecting the larger excess free volume for the branching architecture. The K values estimated for 2L in the present study is very close to the 5.8 104 K g mo11 reported for linear 2-arm PLLA prepared with ethylene glycol [41]. N values obtained using Equation (5) from On the other hand, the Tm Fig. 3(b) was 179.4 and 178.0 C for the 2L and 4L, respectively. The values were similar to 178 and 176 C that reported for linear 1- and 2-arm PLLA prepared with 1-dodecanol and ethylene glycol, N values for the 2L and 4L respectively. However, the evaluated Tm are slightly lower than the 184 C reported for linear 1-arm [68]. 3.2. Wide-angle X-ray diffraction
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WAXD profiles were obtained (Figs. 4 and 5). The sharp crystalline peaks were observed at 2q values of around 17 and 19 for both 2L and 4L (except 2L5 and 4L9 crystallized at Tc ¼ 140 C). These two peaks are attributed to the diffraction from (110)/(200) and (203) lattice planes, respectively, of the d- or a-form of PLLA, in good agreement with the reported results [50,53,57]. On the other hand, no crystalline diffraction peaks were observed for 2L5 and 4L9 crystallized at Tc ¼ 140 C, indicating that they were amorphous when crystallized at Tc very close to Tm (142.2 and 138.5 C, respectively). Assignments of the observed diffraction are based on the crystalline structure reported for a-form by Miyata and Masuko [52]. With increasing Tc, several major changes were observed for relatively weak crystalline diffraction peaks in 2q ranges of 10e14 and 20e26 , as shown in Figs. 6 and 7. Firstly, the crystalline peaks attributed to the diffraction from (004)/(103), (204), (115), (016), and (206) lattice planes of a-form (not d-form) were observed at around 12.5 , 20.8 , 23.0 , 24.1 and 25.1. Secondly, as for crystalline peaks ascribed to the diffraction from (010) and (015) lattice planes of the more ordered and compact structure of a-form at around 14.8 and 22.4 , respectively, becomes more distinct. Thirdly, weak crystalline peak attributed to diffraction from (206)
To investigate the effect of Tc on the crystalline form (the d- or a-forms) and Xc values, 2L and 4L were crystallized from the melt at different Tc values in the range of 80e140 C for 10 h and their
Fig. 7. Enlarged WAXD profiles for 2q ¼ 10e14 and 20e26 of 4L9 (a), 4L18 (b), 4L30 (c), and 4L57 (d) crystallized at different Tc.
Fig. 8. Tc values at which the transition from d-form to a-form took place [Tc(a)] of 2L and 4L as functions of Mn (a) and Mn(arm) (b).
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lattice plane of the d-form [58] at nearby 24.6 (broken lines) moves to higher 2q of the a-form (dotted lines). The two distinct crystalline diffraction peaks of the a-form at around 14.8 and 22.5 and some weak crystalline diffraction peaks of the a-form at around 12.5 , 20.8 , 23.0 , 24.1, and 25.1 were observed for 2L5 and 4L9 crystallized at Tc ¼ 90 C [Figs. 6(a) and 7(a)], and 2L11 and 4L18 at Tc ¼ 110 C [Figs. 6(b) and 7(b)], and 2L20, 2L38, 4L30, 4L57 crystallized at Tc ¼ 120 C [Figs. 6(c,d) and 7(c,d)], respectively, indicating these samples were crystallized in the a-form. On the other hand, the crystalline peak at 24.6 of the d-form was observed for 2L5 and 4L9 crystallized at Tc 80 C [Figs. 6(a) and 7(a)], and 2L11 and 4L18, 4L30, 4L57 at Tc 100 C [Figs. 6(b) and 7(bed)], and 2L20, 2L38 crystallized at Tc 110 C [Figs. 6(c,d)], respectively, indicating these samples were crystallized in the d-form. However, the two weak crystalline diffraction peaks of the a-form at around 20.8 and 23.0 and a crystalline peak of the d-form at 24.6 the are not observed for 4L30 and 4L57 at Tc ¼ 110 C. The Tc values at which the transition from the d-form to the a-form took place [Tc(a)] were estimated from Figs. 6 and 7 and are plotted in Fig. 8(a) and (b) as functions of Mn and Mn(arm), respectively. Tc(a) is defined as the Tc above which samples were crystallized in only a-form. For Mn below 2.0 104 g mo11, the Tc(a) values of 2L were higher than that of 4L when plotted as a function of Mn. However, when plotted as a function of Mn(arm), the Tc(a) values of both 2L and 4L were very similar to each other and increased with increasing Mn(arm) for Mn(arm) below 1.0 104 g mol1. This means that the branching parts in 4L have no significant effects on Tc(a) and Tc(a) is determined only by the crystallizable PLLA segmental length as in the case of Tm. As
reported, the d- and a-forms are formed at relatively low and high Tc values, respectively [50,53,57]. Zhang et al. [50], Kawai et al. [53] and Pan et al. [57] showed that the transition of linear 1-arm PLLA from d- to a-form takes place at Tc of 110 and 120 C. Pan et al. reported that for low molecular weight PLLA with higher segmental mobility the transition from the d-form to a-form is more rapid and proceeds prominently even when annealed at relatively low Tc [58], in consistent with the results for 2L and 4L in the present study. The Xc values of 2L and 4L are plotted in Fig. 9 as functions of Tc and DT. In the crystallizable Tc range, the Xc values of 2L were higher than that of 4L, whereas the values were practically independent of DT and Mn. The result indicates that branching reduced the ratio of polymer chains which formed crystalline regions. 3.3. Spherulite growth Figs. 10 and 11 show the typical polarized photomicrographs of the spherulites of 2L and 4L crystallized isothermally in the a-form and regime II at DT in the range of 32e35 C. As seen in Figs. 10 and 11, normal spherulites were formed in all 2L and 4L having Mn exceeding 1.0 104 g mol1, whereas rather disordered spherulites were seen in 4L9. The latter disorder strongly suggests that the branching in 4L9 caused the macroscopic structural defects in the spherulites. The radius growth rate of spherulites (G) and the induction period of spherulite formation (ti) of 2L and 4L are plotted in Figs. 12 and 13, respectively, as functions of Tc and DT. Here, the G values were estimated from the slopes of spherulite radius plotted as a function of crystallization time, whereas the ti values were
Fig. 9. Crystallinity (Xc) of 2L [(a), (c)] and 4L [(b), (d)] as functions of Tc [(a), (b)] and DT [(c), (d)]. Open and closed symbols are values for d- and a-form, respectively.
Fig. 10. Polarized photomicrographs of spherulites of 2L crystallized at the shown temperatures and times (crystallized in a-form and regime II).
Fig. 11. Polarized photomicrographs of spherulites of 4L crystallized at the shown temperatures and times (crystallized in a-form and regime II).
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Fig. 12. Radius growth rates of spherulites (G) of 2L [(a), (c)] and 4L [(b), (d)] as functions of Tc [(a), (b)] and DT [(c), (d)]. Open and closed symbols are values for d- and a-forms, respectively.
evaluated from extrapolation of the spherulite radius lines plotted against crystallization time to a radius of 0 mm [22]. As seen Fig. 12(c), the G values crystallized in the d-form [G(d)] (open symbols) of 2L decreased in the following order: 2L11 z 2L20 z 2L38 > 2L5. However, the G values crystallized in the a-form [G(a)] (closed symbols) of 2L for DT exceeding 30 C became lower in the following order: 2L11 > 2L5 z 2L20 > 2L38, whereas that of 2L for DT below 30 C decreased in the following order: 2L5 z 2L11 > 2L20 > 2L38. As stated earlier, with decreasing molecular weight, two or three factors compete. That is, with lowering molecular weight, the coinitiator moieties as chain directional changing or branching points per unit mass increase, which decelerates the spherulite growth, whereas the increased segmental mobility accelerates the spherulite growth. The results indicate that, for Mn exceeding 1.0 104 g mol1, the accelerating effect of spherulite growth of 2L crystallized in the pure d-form by increased segmental mobility through decreased molecular weight was balanced with the decelerating effect by increased chain directional changing points per unit mass, whereas the accelerating effect of spherulite growth of 2L crystallized in the a-form by increased segmental mobility through decreased molecular weight was higher than the decelerating effect by increased chain directional changing points. In other words, the accelerating effect of enhanced segmental mobility relative to the decelerating effect of increased chain directional changing points per unit mass increased with decreasing DT. As seen in Fig. 12(d), G(d) values of 4L became lower in the following order: 4L57 > 4L30 > 4L18. The G(a) values of 4L for DT
exceeding 30 C became lower in the following order: 4L57 z 4L30 z 4L18 > 4L9, whereas that of 4L for DT below 30 C decreased in the following order 4L18 > 4L30 > 4L9 > 4L57. These results indicate that for Mn exceeding 1.0 104 g mol1, the accelerating effect of spherulite growth of 4L crystallized in the d-form by increased segmental mobility through decreased molecular weight was lower than the decelerating effect by increased branching points per unit mass, and the accelerating effect of 4L crystallized in the a-form for DT exceeding 30 C by increased segmental mobility through decreased molecular weight was balanced with the decelerating effect by increased branching density, whereas the accelerating effect of 4L crystallized in the aform for DT below 30 C by increased segmental mobility through decreased molecular weight was higher than the decelerating effect by increased branching density. In other words, the accelerating effect of enhanced segmental mobility relative to the decelerating effect of increased branching density increases with decreasing DT. The G values were compared using 2L and 4L having similar Mn values, i.e., 2L20 and 4L18, 2L38 and 4L30. The G(d) values of 2L and 4L as a function of DT decreased in the following order: 2L20 z 2L38 > 4L30 > 4L18. The G(a) values for DT exceeding 30 C decreased in the following order: 2L20 > 2L38 z 4L18 z 4L30, whereas that for DT below 30 C decreased in the following order: 2L20 > 4L18 > 2L38 z 4L30. These results suggest that disturbance effect of spherulite growth of 4L by branching decreased with increasing Mn and decreasing DT. As seen in Fig. 13, the ti values exhibited the positive values when Tc approached Tm. The ti
Y. Sakamoto, H. Tsuji / Polymer 54 (2013) 2422e2434
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Fig. 13. Induction period for spherulite formation (ti) of 2L [(a), (c)] and 4L [(b), (d)] as functions of Tc [(a), (b)] and DT [(c), (d)]. Open and closed symbols are values for d- and aforms, respectively.
increased at lower DT with increasing Mn, as can be expected, and this trend was more remarkable for 2L than for 4L. The latter finding may be ascribed to the fact that the main factor determining ti is not Mn but Mn(arm). 3.4. Nucleation and front constants We estimated the nucleation constant (Kg) and the front constant (G0) for 2-arm and 4-arm PLLA using the nucleation theory established by Hoffman et al. [69,70], in which G can be expressed by the following equation:
h i G ¼ G0 exp U * =RðTc TN Þ exp Kg =Tc DTf ;
(6)
0 T when T 0 is equilibrium T , f is the where DT is supercooling Tm c m m 0 factor expressed by 2Tc/(Tm þ Tc) which accounts for the changes in 0 , U* is the the heat of fusion as the temperature is decreased below Tm activation energy for the transportation of segments to the crystallization site, R is the gas constant, and TN is the hypothetical temperature where all motion associated with viscous flow ceases. Fig. 14 illustrates the ln G þ 1500/R(TcTN) of 2L and 4L as a function 0 is 212 C [13]. Here, we used the of 1/(TcDTf), assuming that Tm universal value of U* ¼ 1500 cal mol1 and TN ¼ Tg30 K for comparison with the reported values [11,18,22,25,26,28], although Urbanovici et al. suggested that U* has to be temperature-dependent (not a constant) and that instead of TN ¼ Tg30 K, Tg should be used for TN [16]. The plots in this figure give Kg as a slope and the intercept ln G0. The estimated Kg and G0 values are tabulated in Table 2.
The experimental data of 2L and 4L were composed of two or three lines having different slopes. The slope difference is caused by the kinetic difference in the crystal growth of regimes III and II or regimes II and I. The reported transition temperatures of linear 1-arm PLLA for regimes IIIeII and regimes IIeI are respectively around 120 C and around 150 C or higher [11,15,18,20,25,28]. In the present study, we have estimated the Kg and G0 values from the two or three lines, assuming that the lines having high and low slopes are for regime III or I and regime II kinetics, respectively [11,18,25,28]. Both 2L and 4L having Mn(arm) of 2.5 103e 9.0 103 g mol1 (i.e., Mn of 5.0 103e1.8 104 g mol1 for 2L and Mn of 1.0 104e3.6 104 g mol1 for 4L) had three different Kg values or regimes, whereas both 2L and 4L having Mn(arm) exceeding 9.0 103 g mol1 (i.e., Mn exceeding 1.8 104 g mol1 for 2L and Mn exceeding 3.6 104 g mol1 for 4L) and 4L having Mn(arm) below 2.5 103 g mol1 (i.e., Mn below 1.0 104 g mol1) had two different Kg values or regimes. The Kg values for regime II [Kg(II)] (1.87e2.57 105 K2) and the Kg values for regime III [Kg(III)] (4.97e6.70 105 K2) of 2L and 4L were approximately in the ranges of the Kg(II) values (2.27e2.55 105 K2) and Kg(III) values (4.20e5.51 105 K2) reported for 1-arm PLLA or PDLA prepared with and without 1-dodecanol and having Mn(GPC) of 7.7 103e5.6 105 g mol1 [22,25,28], and Kg(II) values (2.20e2.59 105 K2) and Kg(III) values (4.48e6.57 105 K2) reported for 1-arm PLLA prepared with 1-dodecanol and L-lactic acid and 2-arm PLLA prepared with ethylene glycol and having Mn(GPC) of 1.5 104 g mol1 [41]. The Kg values for regime I [Kg(I)] (3.72e4.03 105 K2) of 2L and 4L having Mn(arm) of 4.5 103e 1.0 104 g mol1 were similar with the Kg(I) values (4.87 105 K2)
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Fig. 14. ln G þ 1500/R(TcTN) of 2L [(a), (c)] and 4L [(b), (d)] having Mn below 1.8 104 g mol1 [(a), (b)] and exceeding 1.8 104 g mol1 [(c), (d)] as a function of 1/(Tc DTf). Open and closed symbols are values for d- and a-forms, respectively.
reported for 1-arm PLLA prepared without coinitiator having viscosity average molecular weight of 1.5 105 g mol1 [11]. The Tc values at which the transition from regime III kinetics to regime II kinetics took place [Tc(IIIeII)] and from regime II kinetics to regime I kinetics took place [Tc(IIeI)] obtained from Fig. 14 are summarized in Table 3 and Tc(IIIeII) values are plotted in Fig. 15 as functions of Mn and Mn(arm), together with Tc(a). The dependence of Tc(IIIeII) values of 2L and 4L on Mn(arm) was very close to Tc(a). This strongly suggests that the transition of the a-form to the d-form should has changed the crystalline growth mechanism from regime III to II. In contrast, transition from regime II to I of 2L and 4L occurred without transition of crystalline form. The Tc(IIIeII) and Tc(III) values of 4L30 (115 and 140 C, respectively) were almost similar with the those reported for 1-arm PLLA having Mn(GPC) of
Table 3 Tc which gives maximum G (Gmax) [Tc(max)], DTc(max) ¼ TmTc(max), and the transition Tc from Regime III to II and from Regime II to I [Tc(IIIII) and Tc(III), respectively]. Code
Mn(arm)
2L5 2L11 2L20 2L38 4L9 4L18 4L30 4L57
2.6 5.3 9.8 1.9 2.3 4.4 7.5 1.4
a
103 103 103 104 103 103 103 104
Tc(max) [ C]
DTc(max) [ C]
Gmax [mm min1]
Tc(IIIII)a [ C]
Tc(III)a [ C]
115 110 110 115 110 120 135 110
27 48 57 58 29 32 28 60
13.9 18.2 13.0 8.5 3.3 6.7 5.3 5.0
100 115 120 120
115 130
105 115 115
120 120 140
Tc(IIIII) and Tc(III) were estimated from Fig. 14.
Table 2 Front constant (G0) and nucleation constant (Kg) of 2L and 4L. Code
Mn(arm)
2L5 2L11 2L20 2L38 4L9 4L18 4L30 4L57
2.6 5.3 9.8 1.9 2.3 4.4 7.5 1.4
103 103 103 104 103 103 103 104
G0(III) [mm min1] 4.57 5.12 3.96 9.97
1010 1011 1011 1010
9.27 1010 6.27 1010 1.14 1013
G0(II) [mm min1] 7.28 1.29 6.01 2.57 4.73 5.65 1.37 4.22
106 107 107 107 108 106 107 107
G0(I) [mm min1]
Kg(III) [K2]
9.76 1011 7.28 109
5.19 5.42 5.34 4.97
1.08 1013 4.92 109 3.15 109
105 105 105 105
5.28 105 5.07 105 6.70 105
Kg(II) [K2] 2.02 1.94 2.51 2.37 3.60 1.87 2.25 2.57
105 105 105 105 105 105 105 105
Kg(I) [K2] 5.96 105 3.85 105
6.87 105 4.03 105 3.72 105
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(3) Regime analysis exhibited that crystal growth mechanism depends on Tc, crystalline form, the molecular weight and presence or absence of branching. References
Fig. 15. Tc values at which the transition from regime III to II took place [Tc(IIIII)] (closed symbols) and Tc(a) (open symbols) of 2L and 4L as functions of Mn (a) and Mn(arm) (b).
1.7 104e3.34 105 g mol1 (120 and 147 C, respectively) [18]. However, Tc(IIIeII) and Tc(III) (100120 and 115140 C, respectively) values of 2L and 4L decreased with decreasing Mn, whereas those of 1-arm PLLA reported are independent of Mn [18]. These results indicate that crystal growth mechanism depends on Tc, crystalline form, the molecular weight and presence or absence of branching.
4. Conclusions The following conclusions can be derived for the crystallization and physical properties of linear 2-arm PLLA (2L) and branched 4-arm PLLA (4L) from the aforementioned experimental results: (1) Branching architecture of 4L delayed or disturbed the nonisothermal and isothermal crystallization, compared to linear architecture of 2L. This resulted in the higher Tcc values, lower Xc, and G values of 4L compared to those of 2L. (2) The Tg or segmental mobility, Tm or crystalline thickness, and Tc(a) were not affected by the presence of branching. Also, the Tg depended Mn, whereas the Tm or crystalline thickness, and Tc(a) of 4L depended on Mn(arm) not Mn, and were similar to those of 2L, when plotted as a function of Mn(arm).
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