Blending of PLLA and starch dried in a vacuum oven for 48 h was performed on a counter-rotating mixer, and the similar details of the preparation process of PLLA/starch composites were described in our earlier work [9Cai YH. Synthesis of N, N′-bis(benzoyl) azelaic acid dihydrazide and Its Effect on Performance of Poly(L-lactic acid). J Chem Soc Pak 2013; 35(4): 1174-8., 10Cai YH, Yan SF, Fan YQ, Yu ZY, Chen XS, Yin JB. The Nucleation Effect of N, N′-Bis (benzoyl) Alkyl Diacid Dihydrazide on Crystallization of Biodegradable Poly (L-lactic acid). Iran Polym J 2012; 21(7): 435-44.
[http://dx.doi.org/10.1007/s13726-012-0046-x] ].

Wide angle X-ray diffraction (WAXD): WAXD measurement of PLA/starch composites after different heating treatments was performed on a diffractometer (D/MAX2550, Rigaku, Japan) using Cu K_α radiation (wavelength, 1.54 Å) in the range of 2θ=10-80 º with the scanning rate of 2 º/min.

Crystallization Measurement: Crystallization of PLLA/starch composites was investigated by GJY-IIIoptical depolarizer (Donghuang University, China) in the region from 100 °C to 120 °C.

Melt Index: The melt index instrument (Beijing Guance Testing Instrument Co., LTD, China) was used to measure the fluidity of melting PLLA/starch composites. The measurement parameters are as follows: the measurement temperature was 150 °C, and the load was 10 Kg.

The fluidity of PLLA/starch composites was measured using melt index instrument, and the results are shown in (Fig. 1). As shown in (Fig. 1), the melt mass flow rate (MFR) of the neat PLLA is slow, which indicates that the fluidity of the neat PLLA is poor at 150 °C; the possible reason is that the low measurement temperature makes the fluidity of the neat PLLA poor. However, the data of (Fig. 1) shows that addition of starch makes the fluidity of PLLA significantly increased, especially upon the 5 wt% starch, the MFR of PLLA/starch composites has the maximum value of 13.36 g/10min. Then, the value begins becoming small. This result indicates that 5 wt% starch makes the PLLA have the best fluidity, and the reason is that the starch would contribute to the fluidity of PLLA improvement, but too much starch also would block the movement of PLLA which affects the fluidity of PLLA. In addition, it is clear from (Fig. 1) that the fluidity of PLLA/starch is better than that of PLLA at low temperature, which indicates that addition of starch would be effective to avoid the degradation of PLLA and additives with low decomposition temperature because of high processing temperature, and widen the application field of PLLA materials.

Influence of starch on the crystallization of PLA had been reported in literature [11Park JW, Lee DJ, Yoo ES, Im SS, Kim SH, Kim YH. Biodegradable polymer blends of poly(lactic acid) and starch. Korea Polymer Journal 1999; 7(2): 93-101., 12Kang KS, Lee SI, Lee TJ, Narayan R, Shin BY. Effect of biobased and biodegradable nucleating agent on the isothermal crystallization of poly(lactic acid). Korean J Chem Eng 2008; 25(3): 599-608.
[http://dx.doi.org/10.1007/s11814-008-0101-7] ], and the research results showed that starch could serve as nucleating agent of PLA, improving the degree of crystallization and crystallization rate of PLLA. Similarly, the crystallization performance of PLLA with starch was investigated using optical depolarizer, and the rate of measurement is shown in (Fig. 2). It is observed that the half-time of overall crystallization t_1/2 of PLLA is long, and the crystallization rate of neat PLLA appears to be maximum at value 115 °C. However, the addition of starch can increase the crystallization rate; this result is consistent with other literatures results [11Park JW, Lee DJ, Yoo ES, Im SS, Kim SH, Kim YH. Biodegradable polymer blends of poly(lactic acid) and starch. Korea Polymer Journal 1999; 7(2): 93-101., 12Kang KS, Lee SI, Lee TJ, Narayan R, Shin BY. Effect of biobased and biodegradable nucleating agent on the isothermal crystallization of poly(lactic acid). Korean J Chem Eng 2008; 25(3): 599-608.
[http://dx.doi.org/10.1007/s11814-008-0101-7] ]. It is worth noting that the maximum value of crystallization rate of PLLA/starch composites appears in low crystallization temperature zone. This phenomenon suggests that the crystallization of PLLA/starch composites is significantly affected by starch and crystallization temperature, and the addition of starch makes the optimization crystallization temperature shift to low temperature zone. Compared to the neat PLLA, 10% starch can make the t_1/2 of PLLA decrease from 3999.4 s to 421.4 s.

Fig. (1) The fluidity of PLLA/starch composites at 150 °C.

Fig. (2) The effect of temperature and starch on crystallization of PLLA.

Meantime, Avrami equation is used to describe the kinetics of crystallization behavior of PLLA/starch composites, and the Avrami plots of PLLA/starch composites are shown in Fig. (3); it is clear from Fig. (3) that the relationship of log[−ln(1−X_t)] versus logt is linear, which indicates that Avrami equation may be feasible to describe the crystallization behavior of PLLA/starch composites.

In addition, the crystallization of PLLA/starch composites was also investigated by WAXD. Fig. (4) shows the XRD curves of PLLA and PLLA/starch samples without heating treatment, in which it can be observed that the trend of XRD curves of PLLA and PLLA with different starch contents is similar, and the peaks are very wide, which indicates that the degree of crystallization of all samples is low, and this low degree of crystallization results from rapid cooling molding.

Fig. (3) Avrami plots for PLLA/starch composites at different temperature.

Fig. (4) XRD curves of PLLA and PLLA/starch without heating treatment.

However, the XRD curves of PLLA and PLLA/starch after isothermal crystallization at 100 °C for 5 min are very different. As shown in Fig. (5), the neat PLLA only exhibits a peak at 2θ= 16.6 ° due to diffraction from (110) plane, and the intensity of (110) plane diffraction peak is very weak. These results indicate that the crystal of the neat PLLA after isothermal crystallization at 100 °C for 5min is not perfect. However, all PLLA/starch samples exhibit peaks not only at 2θ= 16.6 °, but also at 2θ=14.7 °, 18.9 ° and 22.2 ° occurring from the (010) plane, (203) plane and (205) plane, respectively [13Hergeth WD, Lebek W, Stettin E, Witkowski K, Schmutzler K. Particle formation in emulsion polymerization, 2. Aggregation of primary particles. Makromol Chem 1992; 193(7): 1607-21.
[http://dx.doi.org/10.1002/macp.1992.021930705] ].Moreover, the intensity of diffraction peak is very strong, resulting from the nucleating effect of starch. With increasing of crystallization time, it is clear from Fig. (6) that the diffraction peak of the neat PLLA further strengthens because of increasing crystal perfection of the neat PLLA. However, compared to the diffraction peaks of PLLA/starch after isothermal crystallization at 100 °C for 5 min, the diffraction peaks of PLLA/starch composites after isothermal crystallization at 100 °C for 15 min strengthen slightly, which indicates that the crystal of PLLA/starch after isothermal crystallization at 100 °C for 5 min has become relatively perfect.

Fig. (5) XRD curves of PLLA and PLLA/starch after isothermal crystallization at 100 °C for 5 min.

Fig. (6) XRD curves of PLLA and PLLA/starch after isothermal crystallization at 100 °C for 15 min.