Solid

Article pubs.acs.org/IECR

Synthesis of High Molecular Weight Poly(L-lactic acid) via Melt/Solid State Polycondensation. II. Effect of Precrystallization on Solid State Polycondensation Bo Peng, Hongbing Hou, Fangchao Song, and Linbo Wu* State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: To enhance the reaction rate of solid state polycondensation (SSP) of poly(L-lactic acid) (PLLA) and thus to prepare high molecular weight PLLA by SSP in a short time, the effects of precrystallization as well as SSP temperature (Ts) on the SSP of PLLA were investigated. It was found that precrystallization at low Tc (70 °C) and then SSP at step-raised Ts could greatly favor the growth of the molecular weight of PLLA. After precrystallization at 70 °C for 60 min and then SSP at 150−160 °C for 20 h, the product’s weight average molecular weight (Mw) exceeds 200 000 g·mol−1 while its optical purity remained as high as 90%. The prepolymer precrystallized at 70 °C for 60 min had smaller crystal size and looser packing of lamellae structure than that at Tcmax (110 °C) and moderate crystallinity of 45%. Such crystal properties may result in enhanced concentration and mobility of chain-end groups and improved diffusion of byproduct water in an amorphous area, so the SSP reaction is accelerated. PLLA continues to crystallize during SSP. The crystallinity and melting temperature increase rapidly at the early stage and slowly at the late stage, implying that the effect of precrystallization on the SSP rate is significant at the early stage, and weakens gradually at the late stage. late),21,22,24,28,29,32 polyamide,25,27 polycarbonate,19,23,26,33 poly(trimethylene terephthalate),30 and poly(butylene terephthalate)31 have been extensively studied, but more work needs to be done for the SSP of PLLA, especially on the effects of its crystal properties. The crystallinity (γc) of PLLA may have a crucial effect on its SSP rate. There should be an optimal initial crystallinity (γc0) and an appropriate evolution of γc which are favorable to enhance the SSP rate. However, in previous studies,7−15 the precrystallization of PLLA was all conducted at around 110 °C at which the precrystallization was too fast to control the γc0 and thus making it hard to investigate its effect on the SSP.8,14 On the other hand, the crystal size may also play an important role in the SSP rate. Small crystal size may be helpful in enhancing the SSP rate because it may leave more end groups in the amorphous phase and facilitate the diffusion of water. However, the effect of the crystal size of PLLA on its SSP is also unknown. In our previous studies, we reported melt polycondensation and crystallization behavior of PLLA 34−36 and its SiO2 nanocomposites.37,38 In this article, we conducted precrystallization at low temperature (70 °C) so as to control the γc0 readily and to get small crystal size, and then investigated the effects of precrystallization conditions and crystal properties as well as reaction temperature on SSP of PLLA, aiming to enhance the growth rate of molecular weight.

1. INTRODUCTION Poly(L-lactic acid) (PLLA) is an important biobased and biodegradable polymer having many end-uses in disposable commodities and package materials, fibers, and biomaterials because of its reasonably good properties and excellent biodegradability and biocompatibility.1−5 Although PLLA has been industrially produced by the ring-opening polymerization of L,L-lactide,1,2 many efforts have been made in the past decade to develop more cost-effective synthetic methods in order to promote its applications in large scale. Among these attempts, melt/solid state polycondensation (MP/SSP) was regarded as a promising route due to its reasonably low cost and high product quality.6−16 Moon SI et al.7 reported SSP of PLLA for the first time and successfully synthesized high molecular weight (MW) PLLA. They optimized the reaction conditions8 and developed a 10 L bench-scale MP/SSP process,9 and utilized the SSP technology in the synthesis of stereocomplexes10−12 and copolymers13 of PLLA. The progress has been reviewed recently.16 However, the SSP rate is still not fast enough, and SSP technology of PLLA is far from industrialized up to now. A long SSP time, for example, 20−40 h,7−15 is often required to reach enough molecular weight (70 000 g/mol or higher) to endow PLLA with satisfactory properties.6 From the viewpoint of engineering, it is essential to enhance the SSP rate in order to industrialize it. In general, the SSP rate depends on the (1) diffusion and chemical reaction of end groups in the amorphous phase and the (2) internal and external diffusion of the byproduct.17,18 The two rate-determining steps are determined by the initial molecular weight19 and reaction conditions including reaction temperature,8,20 crystallinity,21−24 catalyst concentration,25−27 particle size,28,29 and flow rate of sweeping inert gas.27,28,31,32 Their effects on the SSP of poly(ethylene terephtha© 2012 American Chemical Society

Received: Revised: Accepted: Published: 5190

September 26, 2011 March 9, 2012 March 19, 2012 March 19, 2012 dx.doi.org/10.1021/ie202192q | Ind. Eng. Chem. Res. 2012, 51, 5190−5196

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION 2.1. Materials. LLA aqueous solution (90 wt %) with an optical purity of 98% was purchased from Jiangxi Musashimo Bio-Chem Co. Ltd., China. Tin(II) chloride dihydrate (SnCl2·2H2O), toluene-4-sulfonic acid (TSA), and chloroform were all reagent-grade chemicals and used as received. 2.2. Melt and Solid-State Polycondensation. The prepolymer synthesis was carried out in a 500 mL flask. First, 200 g of LLA solution were added into the flask, and the dehydration was run at 130 °C/∼103 Pa for 2 h, followed by oligomerization at 150 °C/∼300 Pa for 2 h. After that, a clarified oligomer (OLLA) was obtained. Subsequently, SnCl2·2H2O (0.33 wt % of LLA) and TSA (TSA/SnCl2·2H2O = 1:1 in mol) were added into the flask, and the reaction temperature was gradually raised to 180 °C. At the same time, the pressure was reduced to about 200 Pa to remove the water formed during the polycondensation. To avoid the escape of volatile lactide formed, a condenser was connected to the byproduct outlet of the flask and heated to 83 °C to help the reflux of lactide. After a 4 h melt polycondensation, the molten prepolymer was poured into liquid nitrogen to obtain an amorphous sample. The sample was dried at 45 °C under vacuum for 24 h. Its weight average molecular weight (Mw) was 21 300 g·mol−1, and it had an optical purity of 95% ee. The amorphous prepolymer was pulverized in a high speed pulverizer and then sieved. About 5 g of granules with particle size of 0.4−0.5 mm (35−45 mesh) was spread evenly in a Petri dish preheated in an oven to precrystallize at 110 or 70 °C for a predetermined time. The molecular weight of prepolymer did not exhibit a clear change after precrystallization. For convenience, the prepolymer precrystallized at temperature (Tc) for time of tc is denoted as pcPLLATc/tc. After the precrystallization, the prepolymer was immediately transferred into a stainless steel tubular fixed-bed reactor which had already been preheated to the SSP reaction temperature. A schematic of the SSP apparatus was shown in Figure 1. The

reactor to remove the water and heat up the reactant system. The N2 flow rate was 0.04 L·g−1·min−1 at which the outer diffusion in the gas phase can be ignored (data not shown). After the N2 flow rate and temperature reached predetermined values, the inner tube charged with the precrystallized PLLA granules was inserted into the outer tube and the SSP was started. Upon the completion of SSP, the inner tube was removed rapidly and quenched with liquid nitrogen in order to retain its heat history for the DSC measurement. After drying at 45 °C for 24 h under vacuum, the final product was subject to other analyses. 2.3. Characterization. Intrinsic viscosity of PLLA was determined in chloroform at 30 °C using an Ubbelohde viscometer. Its weight average molecular weight (Mw) was calculated from the intrinsic viscosity (eq 1).36 [η] = 1.13 × 10−4Mw 0.778

(1)

([α]D25)

Specific optical rotation of PLLA was measured in a 1.0 g·dL−1 chloroform solution at 25 °C using an automatic polarimeter (PE 341LC, Perkin-Elmer Co.) at a wavelength of 589.3 nm. The optical purity (OP, %ee) was calculated from eq 2 based on the fact that the specific optical rotation of pure PLLA is −156°.39 OP (%ee) =

[α]25 D × 100 −156

(2)

The thermal transition of PLLA was recorded with a differential scanning calorimeter (DSC7, Perkin-Elmer Co.) which was calibrated using the melting temperature and enthalpy of indium before use. The heating scan was performed at a rate of 10 °C/min from room temperature to 200 °C. The crystallinity (γc) was calculated from the melt enthalpy (deducting crystallization enthalpy if a crystallization peak appeared) by rating it to the reference (100% crystalline PLLA (93.6 J·g−1)40). WAXD analysis was carried out on a XPert-PRO, working at 40 kV and 200 mA with Cu Kα radiation. Scans were made between Bragg angles of 3° and 30° at a rate of 1°/min. The crystallite size of prepolymer was estimated on a relative scale with the Scherrer equation (eq 3), relating crystallite dimensions to the width of the reflections,41 L=

0.9λ (B − B0)cos θ

(3)

Where L is the crystallite size perpendicular to the reflecting plane, λ is the wavelength of Cu Kα radiation (λ = 0.154 nm), θ is the half diffraction angle of corresponding diffraction peaks, B is the observed peak width at half-maximum intensity, and B0 is the instrumental line width (0.09°). Figure 1. Schematic diagram of the apparatus for SSP of PLLA: (1) N2 cylinder; (2) pressure reducing valve; (3) dehydrator; (4) deoxidizer; (5) equalizer valve; (6) rotary flowmeter; (7) heating coil pipe; (8) fixed bed reactor; (9−10) thermocouple; (11) stirrer; (12) heating rod; (13) oil bath body; (14) temperature control unit.

3. RESULTS AND DISCUSSION 3.1. Precrystallization at Different Temperatures. To avoid fusion and the sticking of prepolymer particles during SSP, the prepolymer should be precrystallized before SSP to reach a sufficient initial crystallinity (γc0). In general, the precrystallization is conducted around Tc,max, a temperature at which the prepolymer has a maximum crystallization rate. For the PLLA, its Tc,max is around 110 °C,36 so it is often precrystallized around 110 °C in solid state polycondensation studies.7−15 Precrystallization at 110 °C enables PLLA to finish the crystallization process very rapidly. Figure 2A illustrates the

reactor was composed of three parts: an inner tube, an outer tube, and a heating coil pipe. The outer tube was fixed in an oil bath and its conic bottom end was connected with the heating coil pipe. The inner tube can be facilely inserted into and removed from the outer one and acted as the actual reactor. N2 from a cylinder was dehydrated, deoxidized, metered, and then preheated in the coil pipe, and finally allowed to enter the 5191

dx.doi.org/10.1021/ie202192q | Ind. Eng. Chem. Res. 2012, 51, 5190−5196


Article

Figure 2. DSC heating curves of pcPLLA110/tc (A) and pcPLLA70/tc (B).

prepolymer started to melt at 140 °C according to the DSC diagram (see Figure 2), no particle fusion and sticking phenomena were observed when the SSP was conducted from 150 °C, probably due to the minor fusion enthalpy up to 150 °C. But higher SSP temperatures (Ts) led to obvious particle fusion and sticking, therefore, 150 °C was selected as the (starting) Ts. The growth of weight average molecular weight (Mw) of pcPLLA70/tc during SSP is shown in Figure 3. For the SSP at

DSC heating curves of pcPLLA110 precrystallized for 5−120 min. No exothermal crystallization peak is observed in the heating scans, ascribing to the fast and nearly complete crystallization at 110 °C. The crystallinity reaches 56% at a precrystallization time (tc) of only 5 min, and it only increases slightly from 56% to 62% when tc further prolongs to 120 min (see Table 1). Table 1. Pre-crystallization Data of pcPLLA110/tc and pcPLLA70/tca precrystallization at 110 °C

precrystallization at 70 °C

tc (min)

γc0 (%)

Tm,L (°C)

Tm,H (°C)

tc (min)

γc (%)

Tm,L (°C)

Tm,H (°C)

5 10 30 60 120

56 56 57 62 62

152 152 154 156 157

160 160 161 161 161

15 30 60 90

14 25 45 48

149 145 145 145

160 160 160 160

a TmL and TmH are the low and high melting temperature of the double melting peaks, respectively.

The too fast precrystallization at Tc,max results in poor flexibility in tuning crystallinity with tc and consequent difficulty in investigating the effect of crystallinity on SSP. The crystallization of PLLA slows down at lower (120 °C) Tc.36 Considering lower Tc can lead to not only better controllability of crystallinity, but also smaller crystallite size and looser packing of lamellae which may be favorable to chain-end mobility and reactivity, a Tc of 70 °C is adopted in this study. As seen in Figure 2B, the precrystallization at 70 °C is much slower. A clear crystallization peak appears in the heating scan at tc from 15 to 30 min. The crystallization peak diminishes gradually with prolonging the tc, is barely visible at 60 min, and completely disappears at 90 min. These results indicate the PLLA crystals are developing and growing in the precrystallization process, and as well there is a continuous increase of crystallinity with increasing tc at 70 °C. The crystallinity reaches 14%, 25%, and 45% at 15, 30, and 60 min, and finally amounts to 48% when tc comes to 90 min. Such dependence of crystallinity on tc gives us easy access to control the initial crystallinity (denoted as γc0) of a PLLA prepolymer and then to investigate the effect of γc0 on SSP. 3.2. Effect of SSP Temperature and Prepoloymer Crystallinity. The SSPs of pcPLLA70 were conducted at constant temperature (150 °C/20 h) and step-raised temperature (150 °C/5 h + 155 °C/5 h + 160 °C/10 h). Although the

Figure 3. Mw growth of pcPLLA70/tc during SSP at constant (150 °C/20 h) and step-raised temperature (150 °C/5 h + 155 °C/5 h + 160 °C/10 h).

the constant temperature of 150 °C, the Mw grows in an approximately linear manner at a rate of about 7000 g/mol per hour, reaching 87 000 g/mol and 166 000 g/mol at 10 and 20 h, respectively. In comparison, the Mw grows more rapidly at step-raised Ts than that at the constant Ts. After precrystallization at 70 °C for 60 min and then SSP at 150−160 °C for 20 h, the Mw reaches 211 000 g·mol−1, clearly higher than the one (166 000 g·mol−1) obtained at constant Ts of 150 °C. Figure 3 also illustrates the effect of precrystallization time or initial crystallinity (γc0) on the SSP rate of pcPLLA70/tc. The Mw growth of PLLA gets more rapid with prolonged tc, reaching a maximum growth rate at a tc of 60 min (or γc0 = 45%) but then decreases at longer tc (90 min, or γc0 = 48%). The initial crystallinity plays a key role on the SSP rate, but it has a two-sided effect on SSP.23 Increasing γc0 might increase SSP rate by increasing the concentrations of both reactive end groups and catalyst in the amorphous phase that are rejected from the crystalline phase, resulting in an increase in SSP rate. But on the other hand, an overhigh γc0 can decrease the SSP rate by limiting the byproduct escape and chain-ends mobility. So an appropriate γc0 exists for SSP. For PLLA, a γc0 around 45% may be favorable for chain ends of PLLA and catalyst to 5192



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increases rapidly at an early SSP stage and slowly at a later stage, reaching as high as 85%. The optical purity decreases slowly with ts, from an initial 94.9% to a final 89.6%. For the SSP at the step-raised temperature, the crystallinity and melting temperature also increases as ts goes by, reaching 83% and 182 °C, respectively, after SSP for 20 h, as seen in Table 3. 3.3. Effect of Precrystallization Temperature on SSP. To study the effect of precrystallization temperature on SSP, the SSP at constant and step-raised temperatures were also carried out for pcPLLA110, and the results are compared with those of pcPLLA70. The results of SSP of pcPLLA110 at 150 °C for 20 h are shown in Table 4. Similar to the SSP of pcPLLA70, the Mw of

concentrate in as well as for water to escape from the amorphous phase, leading to highest SSP rate. It has also been reported that the appropriate γc0 locates in the range of 35% to 45% for PET. 24 Besides, it should be noted that a precrystalllization time of 60 min is very suitable for industrial practice. The DSC first heating scans of the as-formed PLLA products after SSP of pcPLLA70/60 at 150 °C for various times are illustrated in Figure 4. Double melting peaks appear at early

Table 4. Properties of SSP Products of pcPLLA110/tc after SSP at 150 °C for 20 h

Figure 4. DSC heating scans of pcPLLA70/60 and then SSP at 150 °C for various times.

tc (min)

Mw (103g/mol)

Tm (°C)

γc (%)

5 10 30 60 120

86.0 92.0 105 103 85.0

179 179 178 179 176

85 83 84 82 82

PLLA increases with increasing tc, reaches a maximum of 105 000 g·mol−1 when tc arrives at 30 min at which the γc0 is 57%, and decreases thereafter. The difference is that the γc0 is higher and its tc-dependence is much weaker at 110 °C than at 70 °C, as shown in Table 1. So the SSP of pcPLLA110/30 was further carried out at constant and step-raised temperatures and compared with the results of pcPLLA70/60 (see Figure 5). Obviously, the SSP rate

stage, but the low temperature melting peak gradually diminishes and vanishes up to an SSP time (ts) of 7.5 h and only single melting peak exists thereafter. Both Tm,L and Tm,H increase with prolonging ts. The Tm of the final product exceeds 180 °C. The continuous increase of Tm is the reason why PLLA can endure increasing Ts in the SSP at step-raised temperature program. As listed in Table 2, the crystallinity (γc) of PLLA Table 2. Properties of pcPLLA70/60 and then SSP at 150 °C for Various Timesa

a

ts (h)

Mw (103g/mol)

Tm,L (°C)

Tm,H (°C)

γc (%)

OP (%ee)

0 0.5 1.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0

21.3 nd 25.0 39.8 45.5 57.1 87.0 111 147 161 166

145 138 139 140 143 145 nd nd nd nd nd

159 164 167 170 173 174 177 179 180 180 180

46 61 67 68 75 76 77 80 83 85 85

94.9 93.8 92.3 92.9 92.1 92.1 90.6 91.0 90.3 90.3 89.6

Figure 5. Mw growth during SSP at constant and step-raised temperatures for pcPLLA70/60 and pcPLLA110/30, respectively: (○) 70 °C/60 min + 150 °C/20 h; (□) 110 °C/30 min + 150 °C/40 h; (●) 70 °C/60 min + 150 °C/5 h + 155 °C/5 h + 160 °C/10 h; (■) 110 °C/30 min + 150 °C/5 h + 155 °C/5 h + 160 °C/5 h + 165 °C/5 h.

Abbreviations: nd, not detected.

Table 3. Properties of pcPLLA70/60 and then SSP at a Step-Raised Temperature Program: 150 °C/5h + 155 °C/5h + 160 °C/ 10h ts = 5 h

a

ts = 10 h

ts = 20 h

tc (min)

Mw × 10−3 (g/mol)

Tm (°C)a

γc (%)

Mw × 10−3 (g/mol)

Tm (°C)

γc (%)

Mw × 10−3 (g/mol)

Tm (°C)

γc (%)

15 30 60 90

48.0 50.0 56.0 50.0

145/173 146/173 147/174 147/178

65 67 68 70

112 126 141 127

178 178 179 179

78 78 77 76

185 199 211 197

182 182 182 182

84 83 83 82

TmL and TmH. 5193



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of pcPLLA70/60 is significantly higher than the rate for pcPLLA110/30 in both SSP methods. For thermostatic SSP at 150 °C, the Mw reaches 166 000 g·mol−1 after 20 h for the pcPLLA70/60 system, but only 115 000 g·mol−1 for the pcPLLA110/30 system. Only when the SSP time of the latter is prolonged to about 40 h, does the the Mw (176 000 g·mol−1) overtake the one (166 000 g·mol−1) of the former at 20 h. This means the SSP time can be greatly shortened when the precrystallization is moved from Tc,max (∼110 °C) to a lower temperature (70 °C). For step-raised SSP, a similar conclusion can be drawn. The Mw reaches 141 000 and 211 000 g·mol−1 at a ts of 10 and 20 h, respectively, for pcPLLA70/60, but only 97 300 and 152 000 g·mol−1, respectively, for pcPLLA110/30. In addition, it should be noted that too high Ts results in discoloration and thermal degradation of PLLA, so the Mw stops to grow or even drops down when the Ts is raised to 165 °C. The possible reasons for higher SSP rate at low Tc may be as follows. First, the initial crystallinity (γc0) is critical for SSP, especially at the very early stage of SSP. A moderate γc0 (∼45%) is obtained at low Tc but a high γc0 exceeding 55% is obtained at Tc,max. As discussed above, the too high γc0 may restrict the diffusion of byproduct water and limit the movement of chain ends, thus the polycondensation between −COOH and −OH end groups is inevitably impeded. Second, there exists differences in crystal size and lamellae structure that may affect the SSP reaction. For melt crystallization of PLLA, the spherulite size can be observed by polarized optical micrographs (POM), and it is well-known that it decreases with decreasing crystallization temperature.42,43 For cold crystallization at low temperature, it is difficult to observe spherulite size by POM because of very high crystallite density. But the crystallite size can be measured by TEM44 or WAXD.45 Through TEM observation, J. M. Zhang et al.44 reported that PLLA crystallites formed by coldcrystallization at 80 °C are clearly smaller than those formed at 140 °C. In this study, we estimated the crystallite size perpendicular to the reflecting plane using the Scherrer equation from the XRD patterns of pcPLLA70/60 and pcPLLA110/30 (see Figure 6). As can be seen from Table 5,

Table 5. Crystallize Size of the Reflecting Planes Originated from WAXD Data of pcPLLA70/60 and pcPLLA110/30

pcPLLA70/60 pcPLLA110/30 pcPLLA70/60 pcPLLA110/30 pcPLLA70/60 pcPLLA110/30

reflecting plane

da (nm)

2θ (deg)

Bdeg (deg)b

L (nm)

010

5.98 6.03 5.36 5.35 4.72 4.69

14.8 14.7 16.6 16.6 18.8 18.9

0.32 0.28 0.27 0.25 0.45 0.42

34.8 42.2 44.6 50.2 22.4 24.4

110/200 203

a The interplanar spacing. bPeak width at half-maximum intensity at the scattering angle of corresponding diffraction peaks.

phase, while a few inactive end groups and chain sections are trapped in the crystalline phase.28 H. B. Zhang and S. A. Jabarin18 have studied how the SSP rate is affected by the reactivity and mobility of chain ends from the view of molecule dynamics. They proposed that the mobile radius (R0) of end groups in the amorphous phase is associated with the chain end length. Longer chain end length leads to bigger R0 and therefore, higher reactivity of the end groups. Since pcPLLA70 has smaller crystallite size, higher specific interface area and less ordered and looser chain packing, it is reasonable to speculate that, compared with the fully developed crystals, pcPLLA70 has more active chain-end groups with bigger mobile radius. That means the concentration and mobility of chain-end groups is enhanced. Therefore, the SSP rate of pcPLLA70 is improved. On the other hand, the byproduct water is also diffused out more easily as a larger crystallite interface may reduce the internal diffusion resistance. From both sides, the SSP reaction rate is notably promoted. 3.4. Evolution of Crystallinity. Figure 7 summarizes the evolution of γc and Tm of PLLA during SSP under different conditions. In the DSC heating scan of the as-formed PLLA product, double melting peaks appear at an early SSP stage up to ∼7.5 h, and then only single melting peak appears. Both γc and Tm,H increase with SSP time, and the increases are significant in the early stages of SSP, then slow down and level off when the SSP time exceeds 15 h or more. Though the initial values are distinct, they come close with each other after 5 h or more. In other words, the final γc and Tm are independent of their initial values. They exceed 80% and 180 °C, respectively. Similar evolutions have been reported in SSP of PET22 and polycarbonate.23 The results suggest that the PLLA crystals continue to grow during SSP, and the crystal structure develops into a more perfect one because of recrystallization at SSP temperature. The results also imply that the effect of γc0 of the prepolymer on the SSP rate is significant at an early stage, and weakens gradually at a late stage. This is supported by the data shown in Figure 5, in which the growth rate of Mw for pcPLLA70 is clearly higher at the early stage, but the difference weakens gradually at the late stage. However, this phenomenon does not weaken the significance of the effects of crystal properties on the SSP rate. 3.5. Evolutions of MW during the Entire MP/SSP Process. The full evolutions of molecular weight and optical purity of PLLA during the entire process of dehydration/ oligomerization (D/O), melt polycondensation (MP), and SSP under optimized conditions, D/O = 140−160 °C, MP = 180 °C, precrystallization = 70 °C/60 min, SSP = 150−160 °C, are illustrated in Figure 8. It can be seen that the growth rate of molecular weight in the SSP at 150−160 °C is nearly close to

Figure 6. WAXD patterns of pcPLLA70/60 and pcPLLA110/30.

PLLA precrystallized at 70 °C has smaller crystal sizes. On the other hand, the reflections of the 010 and 015 planes of pcPLLLA70 are weaker than those of pcPLLA110. This indicates that more α′ crystal is formed at 70 °C and the packing of the chains of the α′ crystal is less ordered and looser.44 In the SSP, catalysts, small molecules, and most of the polymer chain end groups are excluded in the amorphous 5194



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Figure 7. Evolution of crystallinity (A) and melting point (B) of pcPLLA70 and pcPLLA110 during SSP under indicated conditions.

PLLA can be raised from 20 000 g/mol to 140 000 to 200 000 g·mol−1.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-571-8795 2631. Fax: +86-571-8795 1612. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Basic Research Program of China (973 Program, 2011CB606004), the National Nature Science Foundation of China (20406018, 20304012, 20674067) and PCSIRT for financial support.

Figure 8. Evolution of molecular weight and optical purity of pcPLLA70 during the whole process of dehydration/oligomerization (D/O), melt polycondensation (MP), and solid state polycondensation (SSP) under optimized condition: D/O, 140−160 °C; MP, 180 °C; precrystallization, 70 °C/60 min; SSP, 150 °C/5 h + 155 °C/5 h + 160 °C/10 h.

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REFERENCES

(1) Lunt, J. Large-scale production, properties, and commercial applications of polylactic acid polymers. Polym. Degrad. Stab. 1998, 59, 145−152. (2) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Polylactic acid technology. Adv. Mater. 2000, 12, 1841−1846. (3) Södergård, A.; Stolt, M. Properties of lactic acid based polymers and their correlation with composition. Prog. Polym. Sci. 2002, 27, 1123−1163. (4) Auras, R.; Harte, B.; Selke, S. An overview of polylactides as packaging materials. Macromol. Biosci. 2004, 4, 835−864. (5) Mehta, R.; Kumar, V.; Bhunia, H.; Upadhyay, S. N. Synthesis of poly(lactic acid): A review. J. Macromol. Sci., Part C: Polym. Rev. 2005, 45, 325−349. (6) Moon, S. I.; Lee, C. W.; Miyamoto, M.; Kimura, Y. Melt polycondensation of L-lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight poly(L-lactic acid). J. Polym. Sci. A: Polym. Chem. 2000, 38, 1673−1679. (7) Moon, S. I.; Lee, C. W.; Taniguchi, I.; Miyamoto, M.; Kimura, Y. Melt/solid polycondensation of L-lactic acid: An alternative route to poly(L-lactic acid) with high molecular weight. Polymer 2001, 42, 5059−5062. (8) Moon, S. I.; Taniguchi, I.; Miyamoto, M.; Kimura, Y.; Lee, C. W. Synthesis and properties of high-molecular-weight poly(L-lactic acid) by melt/solid polycondensation under different reaction conditions. High Perform. Polym. 2001, 13, 189−196. (9) Fukushima, T.; Sumihiro, Y.; Koyanagi, K.; Hashimoto, N.; Kimura, Y.; Sakai, T. Development of a direct polycondensation process of poly(L-lactic acid). Intern. Polym. Proc. 2000, 15, 380−385. (10) Fukushima, K.; Furuhashi, Y.; Sogo, K.; Miura, S.; Kimura, Y. Stereoblock poly(lactic acid): Synthesis via solid-state polycondensation of a stereocomplexed mixture of poly(L-lactic acid) and poly(Dlactic acid). Macromol. Biosci. 2005, 5, 21−29. (11) Fukushima, K.; Kimura, Y. A novel synthetic approach to stereoblock poly(lactic acid). Macromol. Symp. 2005, 224, 133−143.

that in the MP at 180 °C. It tooks 4, 4, and 20 h in D/O, MP, and SSP phases, respectively, to finally get a product with Mw over 200 000 g·mol−1. The optical purity decreases slowly with reaction time, ultimately approaching a value close to 90% ee after a reaction for 28 h.

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CONCLUSIONS The effects of precrystallization temperature and time as well as SSP temperature on the solid state polycondensation of PLLA were investigated, and the following conclusions are drawn. (1) With comparison to precrystallization at Tc,max, precrystallization at lower Tc (70 °C) results in slower crystallization rate, smaller crystal size, and looser packing of lamellae, but controllable and appropriate crystallinity (45%). These may improve the concentration and mobility of the chain ends in the amorphous phase and facilitate the diffusion of water, and consequently, are favorable to enhance the growth rate of molecular weight of PLLA in the SSP. The effect of the initial crystal properties of prepolymer on the SSP rate is significant at an early stage of SSP, and weakens gradually at a later stage. (2) The PLLA crystals continue to grow during SSP, and the crystal structure develops into a more perfect one, leading to a continuous increase in crystallinity and melting temperature. The final crystallinity and melting temperature exceed 80% and 180 °C, respectively. (3) The SSP temperature Ts also plays a very significant role in enhancing the SSP rate. Because of the continuous increase in melting temperature during SSP, the SSP rate can be further increased by raising Ts. (4) After precrystallization at 70 °C for 60 min and then SSP at a stepraised temperature of 150−160 °C for 10−20 h, the Mw of 5195



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