Evaluation of Relationship Between Crystallization Structure and Thermal-Mechanical Performance of PLA with MCC Addition

Poly(lactic acid) (PLA) biocomposite with 1 wt % microcrystalline cellulose (MCC) addition were prepared by melt-mixed method using torque rheometer and annealed under different isothermal conditions. The morphology, crystallization behaviors, thermal and dynamic-mechanical properties were evaluated by techniques of polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). Nucleus density, crystallization rate and crystallinity (X _c) of PLA matrix were significantly increased by addition of MCC. It was found that MCC could improve the activity of PLA segments at low crystallization temperature (T _c), and the stability of segments in crystal lattice was also enhanced at high T _c. DSC and XRD analysis supported that MCC contributed to the transformation of α’ form into α form in PLA during annealing. The temperature range in which α crystal form appeared for pure PLA was 125 to 135 °C and it was enlarged from 105 to 145 °C for PLA with MCC addition. The Avrami exponent (n) and crystallization rate constant (k) for MCC/PLA decreased and increased respectively, supporting that the time dimension in nucleation mode was inhibited and crystallization kinetics was improved. MCC/PLA showed an increased stiffness and the highest value of storage modulus (E’) was 52 GPa at T _c of 125 °C, which is basically consistent with the condition of maximum X _c. DMA analysis supported that at high T _c, the addition of MCC greatly increased the crystallization ability of PLA and the elasticity and toughness were also improved.