Physical Properties, Crystallization, and Spherulite Growth of Linear

Biomacromolecules 2005, 6, 244-254

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Physical Properties, Crystallization, and Spherulite Growth of Linear and 3-Arm Poly(L-lactide)s Hideto Tsuji,* Tatsuhiro Miyase, Yasufumi Tezuka, and Swapan Kumar Saha Department of Ecological Engineering, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Received August 4, 2004; Revised Manuscript Received October 26, 2004

The physical properties, crystallization, and spherulite growth behavior and mechanism of linear and 3-arm poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] have been investigated using absolute molecular weight as a molecular index. The branching reduces the chain mobility of PLLA and must be excluded from the crystalline regions. The former factor gives the higher glass transition temperature (Tg) and starting temperature for thermal degradation (Td,S) of 3-arm PLLA compared with those of linear PLLA. On the other hand, both the former and the latter factors lead to the higher cold crystallization temperature (Tcc), the longer induction period for spherulite growth (ti), the lower melting temperature (Tm), crystallinitiy (Xc), and radius growth rate of the spherulties (G) for the 3-arm PLLA compared with those for the linear PLLA. The G of 3-arm PLLA showed the vague dependence on number-average molecular weight (Mn), probably because the branching effect was balanced with the molecular weight effect. At the Mn exceeding critical values, the linear and 3-arm PLLA crystallize in regime II or regime III kinetics, depending on crystallization temperature (Tc). In contrast, at the Mn below critical values, the linear and 3-arm PLLA crystallize according solely to regime III and regime II kinetics, respectively, for all the Tc. Introduction Poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] is attracting much attention because it is biodegradable, compostable, producible from renewable resources, and nontoxic to the human body and the environment.1-6 Moreover, linear PLLA has high mechanical performance comparable with that of commercial polymers such as polystyrene and poly(ethylene terephthalate) and therefore is utilized as biomedical materials for tissue regeneration and matrixes for drug delivery systems (DDS) as well as alternatives for commercial polymers. Physical properties including thermal properties, crystallization, spherulite growth, and hydrolysis of linear PLLA have been studied intensively and a great amount of information has been accumulated for various applications.1-6 On the other hand, multiarm or branched PLLA and L-lactide copolymers have been prepared by homo- and copolymerization of L-lactide by various procedures.1-9 Differential scanning calorimetry (DSC),10-25 thermogravimetry (TG),13,22 wide-angle X-ray diffractometry (WAXD),26 viscometry,10,15,26,27 rheological measurements,26 tensile and impact testing,10,11,16,18,24,28 and swelling measurements15 were applied for multiarm PLLA and L-lactide copolymers to investigate the effects of branching on the physical properties and highly ordered structures. Kim et al. found that in dilute solution, 4-arm PLLA exhibited the lower second virial coefficient and intrinsic viscosity than those of linear PLLA, whereas in concentrated solution, 4-arm PLLA gave higher values of dynamic viscosity, storage, and loss moduli than those of * To whom correspondence [email protected].

should

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linear PLLA.10,26 For 4-arm and 16-21-arm PLLA, Kim et al.26 and Zhao et al.22 found that multiarm PLLA has lower glass transition temperature (Tg), melting temperature (Tm), crystallinity (Xc), and thermal degradation temperature compared with those of linear PLLA. Grijpma and Penninngs observed increased impact strength for L- and D-lactide copolymers synthesized with spiro-bis-dimethylene-carbonate compared with that of linear copolymers,24 whereas Joziasse et al. showed that branching of poly(D,L-lactide-co-glycolide) hardly affected the tensile and impact properties of its blends with poly(trimethylnene-co--caprolactone).16 Despite the above-mentioned intensive studies, as far as we are aware, there have been few reports on the effects of branching on physical properties, crystallization, and spherulite growth of PLLA studied for a wide range of molecular weights, where absolute molecular weights not polystyrene (PS) (or other polymers)-standardized molecular weights were used. In some of the reported articles, the molecular weight effects of multiarm PLLA rather than the multiarm effects of PLLA on the physical properties and crystallization have been investigated, or the effects of branching were studied at a fixed molecular weight. The objective of this study was to investigate the pure effects of multiarm on the physical properties, crystallization, and spherulite growth behavior and mechanism of PLLA. For this purpose, we have selected 3-arm PLLA for the simplest model of multiarm PLLA and prepared linear and 3-arm PLLA having wide ranges of molecular weights by ringopening polymerization of L-lactide initiated with stannous octoate in the presence of lauryl alcohol and glycerol as coinitiators,13,29-31 respectively. Then, their physical properties

10.1021/bm049552q CCC: $30.25 © 2005 American Chemical Society Published on Web 12/14/2004

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Linear and 3-Arm Poly(L-lactide)s Table 1. Characteristics of Linear and 3-arm PLLA code L420c L57c L20c L5c 3L36d 3L32d 3L12d 3L9d c

LLA/alcohol in the feeda (mol/mol)

[R] 58925 (deg dm-1 g-1 cm3)

259/0 259/1 129/1 43/1 375/1 250/1 100/1 50/1

-155 -154 -153 -139 -151 -149 -146 -147

Mn, calb (g mol-1)

Mn(abs) (g mol-1)

3.75 × 104 1.88 × 104 6.38 × 103 5.41 × 104 3.61 × 104 1.45 × 104 7.30 × 103

4.2 × 5.7 × 104 2.0 × 104 5.1 × 103 3.6 × 104 3.2 × 104 1.2 × 104 9.4 × 103 105

Mw(abs)/Mn(abs)

Mn(PS) (g mol-1)

Mw(PS)/Mn(PS)

2.9 1.4 1.3 1.2 1.0 1.1 1.1 1.3

5.6 × 1.0 × 105 3.1 × 104 9.2 × 103 6.3 × 104 5.9 × 104 2.1 × 104 1.3 × 104

1.9 1.6 1.8 1.3 1.1 1.2 1.1 1.2

105

a LLA represents L-lactide unit (molecular weight ) 144.1 g mol-1). b The number-average molecular weight calculated from LLA/alcohol in the feed. Linear poly(L-lactide). d 3-arm poly(L-lactide).

have been studied by the use of DSC, TG, and polarimetry. Moreover, their spherulite growth behavior and mechanism have been traced using a polarization optical microscope equipped with a heating-cooling stage and a temperature controller. As far as we know, this is the first report for the spherulite growth of multiarm PLLA, although there have been numerous reports on the spherulite growth of linear PLLA and L-lactide copolymers.32-48 Experimental Section Materials. Linear and 3-arm PLLA having different molecular weights were synthesized by ring-opening polymerization of L-lactide in bulk at 140 °C initiated with stannous octoate (0.03 wt %) in the presence of glycerol (Guaranteed grade, 99%, Nacalai Tesque Inc., Kyoto, Japan) and lauryl alcohol (Guaranteed grade, 98%) Nacalai Tesque Inc. as co-initiators,13,29-31 respectively. The 3-arm PLLA polymers used in this study were synthesized using the same procedure reported by Arvanitoyanis et al.13 Synthesized polymers were purified by reprecipitation using methylene chloride and methanol as solvent and nonsolvent, respectively. The films (10 and 50 µm thick) for physical measurements and crystallization experiments were prepared with the method described in previous papers using methylene chloride as a solvent.35,38,48 The obtained films were dried in vacuo for at least 1 week. The molecular characteristics of the polymers utilized in this study are listed in Table 1. In the present study, codes L and 3L stand for the linear and the 3-arm PLLA, respectively, and numbers right behind the codes mean the indexes of absolute number-average molecular weight [Mn(abs)]. The films for DSC measurements were made amorphous by melting at 200 °C for 3 min and subsequent quenching at 0 °C or were prepared by melting at 200 °C for 3 min and subsequent crystallization at a desired crystallization temperature (Tc) for 600 min, followed by quenching at 0 °C. Measurements and Observation. The weight- and numberaverage molecular weights [Mw(PS) and Mn(PS), respectively] of polymers were evaluated in chloroform at 40 °C by a Tosoh (Tokyo, Japan) GPC system with two TSK gel columns (GMHXL) using polystyrene standards. The calculated Mn (Mn, cal) using the following equations assuming that all of the alcohol molecules acted as co-initiators and were incorporated in the polymers are listed in Table 1

Mn,cal ) 186.3 + [L-lactide/alcohol (mol/mol) in the feed] × 144.1 (for linear PLLA) (1) Mn, cal ) 92.1 + [L-lactide/alcohol (mol/mol) in the feed] × 144.1 (for 3-arm PLLA) (2) where 144.1, 186.3, and 92.1 g mol-1 are molecular weights of L-lactide, lauryl alcohol, and glycerol, respectively. The specific optical rotation of polymers ([R]58925) was measured in chloroform at a concentration of 1 g dL-1 and 25 °C using a JASCO DIP-140 polarimeter at a wavelength of 589 nm. The glass transition, cold crystallization, melting temperatures (Tg, Tcc, and Tm, respectively), and enthalpies of cold crystallization and melting (∆Hcc and ∆Hm) of meltquenched and crystallized PLLA were determined with a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter. The specimens (sample weight ca. 3 mg) were heated from room temperature to 200 °C at a rate of 10 °C min-1 under a nitrogen gas flow of 50 mL min-1. The Tg, Tcc, Tm, ∆Hcc, and ∆Hm values were calibrated using tin, indium, and benzophenone as standards. The crystallinity (Xc) of films was evaluated according to the following equation:49 Xc(%) ) 100(∆Hcc + ∆Hm)/∆Hm(100%)

(3)

where ∆Hm(100%) is the enthalpy of melting of PLLA crystal having infinite crystal thickness. We used a value of 135 J g-1 reported by Miyata and Masuko as ∆Hm(100%).41 By definition, ∆Hcc and ∆Hm are negative and positive, respectively. The thermal degradation behavior of as-cast films was traced by TG using a Shimadzu DTG-50. The specimens (sample weight ca. 3 mg) were heated from room temperature to 400 °C at a rate of 10 °C min-1 under a nitrogen gas flow of 50 mL min-1. In our previous study, it was confirmed that the deviation values from the averaged ones for the Tg, Tcc, Tm, and thermal degradation temperatures and Xc of PLLA specimens were below 0.3 °C, 0.3 °C, 0.3 °C, and 0.5 °C and 1%, respectively. The thermal properties of liner and 3-arm PLLA are summarized in Table 2. The spherulite growth in the films was observed using an Olympus polarization optical microscope (BX50) equipped with a heating-cooling stage and a temperature controller (LK-600PM, Linkam Scientific Instruments, Surrey, U.K.) under a constant nitrogen gas flow. The crystallization of the films was performed as follows. The specimens were first heated to 200 °C at 100 °C min-1, held at this

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Figure 1. DSC thermograms of amorphous-made linear PLLA (a) and 3-arm PLLA (b). Table 2. Thermal Properties of Linear and 3-arm PLLA code

Tga (°C)

Tcca (°C)

Tma (°C)

Tm0b (°C)

Td,Sc (°C)

Td,Ed (°C)

L420e L57e L20e L5e 3L36f 3L32f 3L12f 3L9f

57.3 56.1 49.5 46.7 57.5 57.6 54.0 50.0

111.7 114.0 88.1 82.9 125.9 113.3 115.4 117.4

177.7 176.4 170.4 155.5 167.5 161.0 145.6 135.1

210.4 191.7 180.9 169.3 178.2 176.0 162.1 149.2

330.0 292.4 297.2 282.5 314.3 341.2 292.1 300.3

373.5 347.7 353.7 330.8 360.3 375.5 329.0 333.0

a The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) were obtained using DSC from the first run of amorphous-made films. b The equilibrium melting temperature (Tm0) was estimated using the Hoffman-Weeks procedure. c The starting temperature for thermal degradation traced using TG for as-cast films. d The ending temperature for thermal degradation traced using TG for as-cast films. e Linear poly(L-lactide). f 3-arm poly(L-lactide).

temperature for 3 min to destroy thermal history, cooled at 100 °C min-1 to a desired Tc in the range of 100-160 °C, and then held at the Tc. Results and Discussion Differential Scanning Calorimetry. Figure 1 shows the DSC thermograms of amorphous-made linear and 3-arm PLLA. All of the linear and 3-arm PLLA specimens have glass transition, cold crystallization, and melting peaks in the temperature ranges of 45-60 °C, 80-130 °C, and 130180 °C, respectively. The Tg, Tcc, and Tm values of linear and 3-arm PLLA were estimated from Figure 1 and are plotted in Figure 2 as a function of Mn(abs), together with the [R]58925 values. When two or more melting peaks were observed in a DSC thermogram, we assumed the lowest melting peak as a real melting peak and other peaks were

ascribed to the melting of the crystallites formed or thickened during DSC scanning. When compared at the same Mn(abs), the Tg and Tcc of 3-arm PLLA were higher than those of linear PLLA, whereas the Tm and [R]58925 of 3-arm PLLA were lower than those of linear PLLA excluding the [R]58925 value for 3-arm PLLA with the lowest Mn(abs). With respect to Tm, the obtained result agrees with the reported ones, whereas as for Tg the obtained result contradicts the reported ones.22,26 The latter contradiction may be caused by the difference in branching number, specimen preparation procedure, or estimation method of molecular weight. The Tg, Tcc, Tm, and [R]58925 of both the linear and 3-arm PLLA excluding the Tcc of 3-arm PLLA became higher with increasing Mn(abs). For linear PLLA in viscosity-average molecular weight (Mv) range of 1.8 × 104-4.3 × 105 g mol-1, Migliaresi et al. reported the similar dependence of Tcc and Tm on Mv.50 For 3-arm PLLA in Mn(PS) range from 7.6 × 103 to 1.4 × 104 g mol-1, Avranitoyannis et al. reported significant dependence of Tm and Tg on Mn(PS) but vague dependence of Tcc on Mn(PS).13 Also, they observed that the 3-arm PLLA loses crystallizability during DSC heating for the Mn(PS) lower than 5.4 × 103 g mol-1. On the other hand, although Fukuzaki et al. found the similar dependence of [R]58920 on Mn(PS) for linear PLLA with low Mn(PS) below 1.0 × 103 g mol-1, the [R]58920 saturated around 125 deg dm-1 g-1 cm3 for Mn(PS) exceeding 1.5 × 103 g mol-1,51 in marked contrast with our result, where the monotonic increase of [R]58925 with Mn(abs) was observed for Mn(abs) up to 4.2 × 105 g mol-1. For 3-arm PLLA, with increasing molecular weight, two factors compete with each other; that is, the prolonged chain length reduces the chain mobility, whereas the decreased branching number per unit mass (branching density) elevates


Linear and 3-Arm Poly(L-lactide)s

Figure 2. Tg (a), Tcc (b), Tm (c), and [R]58925 (d) of amorphous-made linear and 3-arm PLLA as a function of Mn(abs).

the chain mobility and lowers the amount of defect points which must be excluded from the crystallite nuclei. The former one factor and the latter two factors respectively increases and decrease the Tcc. Therefore, the decrease in Tcc with increasing Mn(abs) from 9.4 × 103 to 3.2 × 104 g mol-1 means that the branching density is a main factor to determine Tcc, whereas the rapid increase in Tcc with increasing Mn(abs) from 3.2 × 104 to 3.6 × 104 g mol-1 reflects that the length of the molecules is a major factor to determine Tcc. As mentioned earlier, the Tg and Tcc were higher for the 3-arm PLLA than for the linear PLLA, when compared at the same Mn(abs). The higher Tg of 3-arm PLLA compared with that of linear PLLA reveals that the branching disturbs the segmental motion, whereas the higher Tcc and lower Tm of 3-arm PLLA compared with those of linear PLLA indicate that the branching disturbs thickening (or growth) of the crystallites during DSC scanning or increases the lattice disorder of the formed crystallites. To obtain the Tg, [R]58925, and Tm values of linear and 3-arm PLLA at infinite molecular weight (Tg∞, [R]58925∞, and Tm∞, respectively), the Tg and [R]58925 values and the Tm-1 values are plotted in Figure 3, parts (a) and (d), and part (c) as a function of Mn(abs)-1 according to the Flory-Fox52 and Flory53 equations, respectively: Pp ) Pp∞ - K/Mn(abs)

(4)

Tm-1 ) (Tm∞)-1 - 2R M0/[∆Hm Mn(abs)]

(5)

where Pp and Pp∞ are physical property and that at infinite molecular weight, respectively, K is a constant representing the excess free volume of the end groups of polymer chains, M0 is molecular weight of a half lactide unit (72.1 g mol-1), and R is the gas constant. The Flory-Fox plot for Tg∞ holds approximately well for polymers having high Mn(abs). The Tg∞ values estimated using eq 4 from Figure 3a were 58 and 60 °C for the linear and 3-arm PLLA, respectively, confirming that branching reduces the chain mobility of PLLA even at infinite Mn(abs). The K values evaluated using eq 4 from Figure 3a were 1.7 × 105 and 9.0 × 104 K g mol-1 for the linear and 3-arm PLLA, respectively, reflecting the larger excess free volume of terminal groups for the 3-arm PLLA. The estimated Tg∞ value for the linear PLLA is in complete agreement with 58 °C reported by Jamshidi et al., whereas both the evaluated K values for the linear and 3-arm PLLA are lower than 5.5 × 105 K g mol-1 reported for linear PLLA.54 The [R]58925 of both the linear and 3-arm PLLA monotonically decreased with Mn(abs)-1. The Flory-Fox plot for [R]58925 holds well solely for the linear PLLA, and the [R]58925∞ value evaluated using eq 4 from Figure 3d was 156 deg dm-1 g-1 cm3, respectively. On the other hand, the Tm∞ values obtained using eq 5 from Figure 3b, 178 °C both for the linear and 3-arm PLLA. The estimated Tm∞ value for linear PLLA is slightly lower than 184 °C reported by Jamshidi et al.54 In Figure 3b, the Tcc of both linear and 3-arm PLLA are plotted as a function of Mn(abs)-1 assuming the Flory-Fox relationship [eq 4]. However, as expected from

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Figure 3. Tg (a), Tcc (b), and Tm-1 (c), and [R]58925 (d) of amorphous-made linear and 3-arm PLLA as a function of Mn(abs)-1.

Figure 2b, the dependence of Tcc on Mn(abs)-1 is also vague in Figure 3b, confirming the molecular weight is not a sole factor to determine Tcc. Figure 4 illustrates the Tm and Xc of L20, L5, and 3L9 as a function of Tc. Here, we have selected the linear PLLA specimens having the similar Mn(abs) with the 3-arm PLLA 3L9 and the specimens were crystallized at a desired Tc for 600 min from the melt at 200 °C. As evident from this figure, the Tm and Xc values of 3-arm PLLA were much lower than those of linear PLLA, when compared at the same Tc. Such result of Tm for the crystallized specimens is in agreement with that for the amorphous-made specimens mentioned earlier. These findings support that branching disturbs thickening (or growth) of the crystallites at a constant Tc from the melt or increases the lattice disorder of the formed crystallites. Moreover, the highest Tc where crystallization took place was lower for the 3-arm PLLA than for the linear PLLA, meaning that the branching decreases the stability of crystallites at high Tc. This is consistent with result for L-lactide copolymers, where comonomer units lower the stability of crystallites.39 The equilibrium melting temperature (Tm0) of linear and 3-arm PLLA was estimated from the Hoffman-Weeks procedure and is given in Table 2. This procedure may give Tm0 values slightly different from real Tm0 values as suggested by Alamo et al.55 and Marand et al.56 However, we have used this simple procedure to obtain approximate Tm0 values, since the difference between the values using this procedure (478-485 K) and the Gibbs-

Thomson equation (478 K) was very small for linear PLLA.49 Similarly to the result for Tm values, the Tm0 values were higher for the linear PLLA than for the 3-arm PLLA when compared at the similar Mn(abs). Thermogravimetry. Figure 5 gives the TG curves of ascast linear and 3-arm PLLA. The starting and ending temperatures (Td,S and T d,E, respectively) for thermal degradation of the specimens were evaluated from this figure by extrapolation of the straight line parts of the degradation curves to remaining weights of 100 and 0%, respectively, as schematically depicted by Cam and Marucci in the literature.57 The evaluated Td,S and Td,E are listed in Table 2 and plotted in Figure 6 as a function of Mn(abs). The T d,S and Td,E values obtained for linear PLLA in this study were very similar to previously reported values.58-61 The fluctuation of data in Figure 6 may be due to the slight difference in the amount of tin catalyst and/or L-lactide remaining after the purification by precipitation.54,58,62-66 Although such factors may slightly affect the comparison of thermal stability between the linear and 3-arm PLLA, such factors will not vary the dependence of thermal stability on the Mn(abs) as a whole, since the specimens were synthesized and purified under the same conditions excluding the co-initiator (alcohol) content. When compared at the same Mn(abs), the Td,S of 3-arm PLLA was higher than that of linear PLLA, whereas the Td, E of the 3-arm PLLA was higher and lower than that of the linear PLLA for the Mn(abs) higher and lower than 2 × 104


Figure 4. Tm (a) and Xc (b) of L20, L5, and 3L9 crystallized at Tc from the melt as a function of Tc.

g mol-1, respectively. The Td,S and Td,E decreased with decreasing Mn(abs) and their slopes were steeper for the 3-arm PLLA. In other words, the Td,S and Td,E of 3-arm PLLA were more sensitive to Mn(abs) than those of linear PLLA. The decreased Td,S and Td,E of both linear and 3-arm PLLA with decreasing Mn(abs) can be ascribed to the enhanced chain mobility and the increased number of terminal groups per unit mass (density of the terminal groups). These two factors increase the probability of back-biting or formation of lactides as volatile components.54 The steeper decrease in Td,S and Td,E of 3-arm PLLA with decreasing Mn(abs) compared with that of linear PLLA may be due to the rapid increase in density of the terminal groups of 3-arm PLLA compared with the slow increase in density of the terminal groups of linear PLLA. However, the higher Td,S and Td,E [Mn(abs) > 2 × 104 g mol-1] of 3-arm PLLA contradict the effect of its higher density of the terminal groups. It is probable that the reduced chain mobility by the branching as evident from the Tg values of 3-arm PLLA [Figure 2a] raises the Td,S and Td,E [Mn(abs) > 2 × 104 g mol-1], overcoming the effect of its higher density of the terminal groups. In contrast, the expected result was obtained for 1621-arm PLLA,22 in which the multiarm PLLA had a lower thermal stability than that of linear PLLA. In such hyperbranched PLLA, the effect of increased density of the terminal groups must be more remarkable compared with that in our 3-arm PLLA and the branching will not reduce the chain mobility.


Figure 5. TG curves of as-cast linear PLLA (a) and 3-arm PLLA (b).

Spherulite Growth. The typical polarization photomicrographs of linear and 3-arm PLLA are shown in Figure 7. The normal spherulites were formed in L57, L20, and 3L32 films. However, the contrast between the bright and dark regions of Maltese cross was lower for the spherulites of 3-arm PLLA compared with those of linear PLLA and the structure of spherulite center of 3-arm PLLA was more disordered than that of linear PLLA, meaning that the branching causes the macroscopic defects in the PLLA spherulites. Such contrast reduction in the spherulites is attributable to the decreased orientation of lamellae and has been observed for L-lactide copolymers with D-lactide or other lactones.39,67 Figures 8 and 9 depict the radius growth rate of spherulites (G) and the induction period of spherulite formation (ti) for the linear and 3-arm PLLA. Here, we have evaluated the ti from extrapolation of the spherulite radius lines plotted as a function of crystallization time (tc) to a radius of 0 µm. Also, in Table 3, the Tc which gave maximum G (Gmax) [Tc(max)] is given. The G of linear PLLA was in the range of 1-70 µm min-1 and increased monotonically with decreasing Mn(abs),whereas the G of 3-arm PLLA was in the range of 1-8 µm min-1 with unclear dependence on Mn(abs). The G of 3-arm PLLA was much lower than that of linear PLLA, when compared at the similar Mn(abs). It is probable that exclusion of the branching points from the crystalline regions reduces the G value. Moreover, for the 3-arm PLLA the enhancement of spherulite growth by the increased chain mobility through the lowered molecular weight was balanced with the

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Figure 6. Td,S (a) and Td,E (b) of as-cast linear and 3-arm PLLA as a function of Mn(abs).

disturbance of spherulite growth through increased branching density, resulting in unclear dependence of G on Mn(abs). The Gmax value 7.5 µm min-1 for 3L12 was much lower than 19 µm min-1 for L20 having a higher molecular weight than that of 3L12. This fact also indicates that branching disturbs the spherulite growth. The G curves with two maxima were observed for 3L36, 3L32, L57, and L20, which are probably due to regime transition. Such multi peaks were seen for the linear and 3-arm PLLA with the Mn(abs) range of (2-6) × 104 g mol-1. On the other hand, the ti [Figure 9b] was longer for the 3-armPLLA (excluding 3L32) than for the linear PLLA, when compared at the similar Mn(abs), reflecting that the formation of spherulite nuclei was disturbed by the presence of branching. As mentioned in the section of Tcc result, the prolonged ti of 3-arm PLLA must be ascribed to the reduction of chain mobility and the exclusion of branching points for the formation of spherulite nuclei. The large minus value of the ti for 3L9 may be due to the growth of spherulties during cooling from 110 to 100 °C. Nucleation and Front Constants. We have estimated the nucleation constant (Kg) and the front constant (G0) for linear and 3-arm PLLA by the use of nucleation theory established by Hoffman et al.,68,69 in which G can be expressed by the following equation: G ) G0 exp[-U*/R(Tc - T∞)] exp[-Kg/(Tc∆Tf)] (6)

Figure 7. Polarization photomicrographs of spheruiltes of linear and 3-arm PLLA crystallized at Tc ) 130 °C from the melt; (A) L57, tc ) 11 min; (B) L20, tc ) 5 min; (C) 3L32, tc ) 11 min.

where ∆T is undercooling Tm0 - Tc where Tm0 is equilibrium Tm, f is the factor expressed by 2Tc/(Tm0 + Tc) that accounts for the change in heat of function as the temperature is decreased below Tm0, U* is the activation energy for transportation of segments to the crystallization site, R is the gas constant, and T∞ is the hypothetical temperature where all motion associated with viscous flow ceases. For Fixed Tm0. Figure 10 illustrates the ln G + 1500/ R(Tc - T∞) of linear and 3-arm PLLA as a function of 1/(Tc∆Tf) assuming that Tm0 is 212 °C.36,48 Here, we used the universal values of U* ) 1500 cal mol-1 and T∞ ) Tg 30 K for comparison with the reported values,34,45 although Urbanovici et al. suggested that U* has to be temperaturedependent not a constant and that instead of T∞ ) Tg - 30



Figure 8. Radius growth rate of the spherulites (G) for linear PLLA (a) and 3-arm PLLA (b).

K, Tg should be used for T∞.70 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 4. The ln G + 1500/R(Tc - T∞) of linear and 3-arm PLLA respectively shifted to lower and upper positions with increasing Mn(abs). The data of L420, L57, and L20 were composed of two lines having different slopes. In these cases, we have estimated Kg and G0 values for the two lines and assumed the lines having high and low slopes were for regime III and regime II kinetics, respectively, and the transition Tc from regime II kinetics to regime III kinetics [Tc(II-III)] obtained in Figure 10 is summarized in Table 3. This assumption is validated in the case of linear PLLA because the Kg values evaluated for regime III [Kg(III)] were twice those for regime II [Kg(II)] (Table 4). In addition, the obtained Tc(II-III) was 120 °C, in good agreement with those reported for relatively high-molecular-weight linear PLLA in previous articles.34,45,71,72 Namely, Abe et al. reported that the crystallization of PLLA proceeds by regime II kinetics at the temperature range of 120-147 °C,45 whereas Vasanthkumari and Pennings34 found that the transition from regime II to regime I takes place at 163 °C. Moreover, Di Lorenzo71 and Iannace and Nicolais72 reported that the transition from regime III to regime II occurs at 115 and 120 °C, respectively. These reported results confirmed that, at the Tc ranges below and above 120 °C, the linear PLLA crystallizes according to regime III and regime II kinetics, respectively.

Figure 9. Induction period of the spherulite formation (ti) for linear PLLA (a) and 3-arm PLLA (b). Table 3. Tc Which Gives Maximum G (Gmax) [Tc(max)] and the Transition Tc from Regime II to Regime III [Tc(II-III)] code

Tc(max) (°C)

Gmax (µm min-1)

Tc(II-III)a (°C)

L420b L57b L20b L5b 3L36d 3L32d 3L12d 3L9d

130 110 115

2.7 7.5 19 67c 4.9 6.2 7.5 5.4

120 120 120

110 110 120 120

120 120

aT b c c(II-III)was estimated from Figure 10. Linear poly(L-lactide). G value at Tc ) 110 °C. G value at Tc below 110 °C could not be estimated because of too high density of the spherulites. d 3-arm poly(L-lactide).

L5 having a lowest Mn(abs) of 5.1 × 103 g mol-1 has no apparent Tc(II-III) (Figure 10) and shows the Kg value 4.60 ×105 K2 similar to Kg(III) values 4.73-5.51 × 105 K2 for other PLLA having the higher molecular weights, suggesting that L5 crystallizes according to regime III kinetics for all the Tc (110-140 °C) studied here. This result implies that the Kg values 4.95 × 105 and 4.20 × 105 K2 reported for PLLA and poly(D-lactide) having Mn(PS) of 7.7 × 103 and 1.5 × 104 g mol-1, respectively, in the Tc range of 110140 °C may be for regime III kinetics not for regime II kinetics.45 This can be expected from their lower Mn values than that of L20. The molecular weight effect on regime kinetics mentioned above reveals that PLLA crystallizes according to solely regime III kinetics for all the Tc when

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Table 4. Front Constant (G0) and Nucleation Constant (Kg) Values Estimated for a Fixed and Each Tm0 Values for a fixed Tm0 value (212 °C)45 code L420c L57c L20c L5c 3L36d 3L32d 3L12d 3L9d c

G0(II)b (µm min-1)

G0(III)b (µm min-1)

Kg(II)b (K2)

1.79 × 107 3.98 × 107 2.42 × 107

1.78 × 1010 4.81 × 1011 5.32 × 1011 1.19 × 1011 7.20 × 1010 5.33 × 1010

2.52 × 105 2.55 × 105 2.27 × 105

1.79 × 107 3.26 × 107 1.32 × 108 4.39 × 108

2.29 × 105 2.44 × 105 2.87 × 105 3.53 × 105

for each Tm0 valuea (III)b

Kg (K2)

4.73 × 105 5.51 × 105 5.45 × 105 4.60 × 105 4.93 × 105 4.81 × 105

G0(II)b (µm min-1)

G0(III)b (µm min-1)

Kg(II)b (K2)

Kg (III)b (K2)

1.33 × 107 1.79 × 106 5.40 × 105

1.19 × 1010 3.96 × 109 7.24 × 108 1.09 × 107 8.86 × 107 5.94 × 107

2.35 × 105 1.25 × 105 7.25 × 104

4.49 × 105 3.18 × 105 2.28 × 105 8.88 × 104 1.81 × 105 1.66 × 105

1.99 × 105 1.99 × 105 4.42 × 105 1.21 × 105

5.63 × 104 5.37 × 104 4.90 × 104 2.78 × 104

a T 0 values estimated by the Hoffmann-Weeks procedure (Table 1) were used for the calculation. b (II) and (III) represent regimes II and III, respectively. m Linear poly(L-lactide). d 3-arm poly(L-lactide).

Figure 10. ln G + 1500/R(Tc - T∞) of linear PLLA (a) and 3-arm PLLA (b) as a function of 1/(Tc∆Tf).

the Mn(abs) is lowered below 5 × 103 g mol-1, whereas PLLA crystallizes in regime II or regime III kinetics, depending on Tc, when the Mn(abs) is elevated over 2.0 × 104 g mol-1. The Kg(II) values of L420, L57, and L20, 2.52 × 105, 2.55 × 105, and 2.27 × 105 K2, respectively, are in good agreement with reported Kg(II) values, 2.29-2.44 × 105 K2 [Mv ) 1.5 × 105-6.9 × 105 g mol-1, Vasanthakumari and Pennings],34 1.92 × 105 K2 [Mn(PS) ) 6.6 × 104 g mol-1, Baratian et al.],46 2.4 × 105 K2 [Mn(PS) ) 6.6 × 104 g mol-1, Huang et al.],42 1.85 × 105 K2 [Mw(unspecified method) ) 1.0 × 105 g mol-1, Di Lorenzo],71 whereas a half of Kg(II) values 4.64-4.97 × 105 K2 reported for PLLA having Mn-

(PS) of 0.17-3.3 × 105 g mol-1 (Abe et al.).45 The Kg(II) and Kg(III) values are constant irrespective of molecular weight of PLLA, in agreement with the results reported by Vasanthakumari and Pennings34 and by Abe et al.,45 whereas the G0 for regime II and regime III [G0(II) and G0(III), respectively] did not have explicit dependence on the molecular weight, in contrast with the reported monotonic G0(II) increase with decreasing Mv or M(PS).34,45 On the other hand, the ln G+1500/R(Tc - T∞) of relatively high-molecular-weight 3L36 and 3L32 were composed of two lines, whereas relatively low-molecular-weight 3L12 and 3L9 consist of one line. From the comparison between the results of linear and 3-arm PLLA, it seems that 3L36 and 3L32 crystallize according to regime II or III kinetics depending on Tc and that 3L12 and 3L9 crystallize solely in regime II or regime III kinetics for all the Tc. Further comparison of the estimated Kg and G0 values between the 3-arm PLLA not only for the fixed Tm0 but also for each Tm0 (Table 4) and the consistency of their dependence on Mn(abs) strongly suggest that 3L12 and 3L9 crystallize according to the regime II kinetics (not the regime III kinetics) for all the Tc. As shown in Table 4, the obtained Kg(II) values of 3-arm PLLA were in the range of 2.29-3.53 × 105 K2 and the Kg(II) increased with decreasing Mn, in agreement with the results for linear PLLA. The obtained Kg(III) values 4.93 × 105 and 4.81 × 105 K2 for 3L36 and 3L32, respectively, were twice those of the obtained Kg(II) values 2.29 × 105 and 2.44 × 105 K2, respectively. This finding supports the assumption that 3L36 and 3L32 crystallize in regime II or III kinetics, depending on Tc. The G0(II) values (1.79 × 1074.39 × 108 µm min-1) of the 3-arm PLLA increased with decreasing Mn. This inclination is consistent with the results reported by Vasanthakumari and Pennings34 and by Abe et al.45 The G0(III) values 7.20 × 1010 and 5.33 × 1010 µm min-1 for the 3-arm PLLA, 3L36 and 3L32 were 1 order of magnitude lower than the G0(III) values 4.81 × 1011, 5.32 × 1011, and 1.19 × 1011 µm min-1 for the linear L57, L20, and L5, respectively, but are on the same order with the G0(III) value 1.78 × 1010 µm min-1 for the linear PLLA L420 having the highest Mn. For Each Tm0. Some reported articles used each Tm0 value, which varies depending on the molecular structures and characteristics. For further comparison, we have carried out the kinetic analysis using the eq 6 with Tm0 obtained by the


Hoffman-Weeks procedure (Table 4). With the deviation of Tm0 from the linear PLLA having the highest molecular weight (i.e., with decreasing molecular weight and in the presence of branching), the large deviation in Kg(II), Kg(III), G0(II), and G0(III) from the linear PLLA having the highest molecular weight occurred. The Tc(II-III) did not vary, whereas the Kg(II), Kg(III), G0(II), and G0(III), excluding G0(II) of 3L12, decreased with decreasing Mn(abs), irrespective of the presence of branching. Huang et al.42 and Baratian et al.46 found that the Kg(II) of poly(L-lactide-co-D-lactide) and poly(L-lactide-co-meso-lactide) decreases with increasing amount of the comonomers. The results obtained here and the reported ones strongly suggest that the Kg estimated with each Tm0 value decreases with increasing the amount of defects (branching, comonomers, and terminal groups) for crystallization. Conclusions The following conclusions can be derived for the physical properties, crystallization, and spherulite growth of linear and 3-arm PLLA from the above-mentioned experimental results: (1) The branching reduces the chain mobility of PLLA and must be excluded from the crystalline regions. The former factor gives the higher Tg and Td,S of 3-arm PLLA compared with that of linear PLLA. On the other hand, both the former and the latter factors disturb the nuclei formation of crystallites and spherulites, the thickening of crystallites, and the growth of spherulites, and increase the lattice disorder. These can be traced as the higher Tcc, the longer ti, the lower Tm, Xc, and G for 3-arm PLLA compared with those for linear PLLA. (2) The G of 3-arm PLLA showed the vague dependence on molecular weight, probably because the branching effect was balanced with the molecular weight effect, in marked contrast with the monotonic increase of the G of linear PLLA with decreasing molecular weight. (3) At the Mn(abs) exceeding critical values [2.0 × 104 g mol-1 (linear PLLA) and 3.2 × 104 g mol-1 (3-arm PLLA)], the linear and 3-arm PLLA crystallizes in regime II or regime III kinetics, depending on Tc, whereas at the Mn(abs) below critical values [5 × 103 g mol-1 (linear PLLA) and 1.2 × 104 g mol-1 (3-arm PLLA)], the linear and 3-arm PLLA crystallized according solely to regime III and regime II kinetics, respectively, for all the Tc. Acknowledgment. The authors thank Dr. Eiichi Takatori and Mr. Nobuyuki Kagawa, from TOSOH Analysis and Research Center Co. (Yokkaichi, Japan), for their GPC and light scattering measurements, and Professor Dr. Shinichi Itsuno, from Department of Materials Science, Faculty of Engineering at Toyohashi University of Technology, for the use of the polarimeter facility. This research was supported by a Grant-in-Aid for Scientific Research on Priority Area, “Sustainable Biodegradable Plastics” No. 11217209, and The 21st Century COE Program, “Ecological Engineering for Homeostatic Human Activities”, from the Ministry of Education, Culture, Sports, Science and Technology (Japan).


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Physical Properties, Crystallization, and Spherulite Growth of Linear

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