Study On The Preparation And Properties Of Poly (Lactic Acid)/Tetrapod-like Zinc Oxide Whisker Composites

As a green biocompatible material, poly (lactic acid) (PLA) can be biodegradable absorbed, enventrually decomposing into carborn dioxide and water. The PLA materials have been used in many realms, for example of clothing, packaging, household and health care products. Nevertheless, because of its low mechanical characteristics and its poor thermal stability, PLA was limited in some practical fields. Tetrapod-like zinc oxide whisker (T-ZnOw) was used as enhance filler in this paper, for its unique three-dimensional tetrapod-shaped structure and its high strength and modulus, aiming at improving the mechanical properties and other properties of the matrix material PLA.In this dissertation,the poly(lactic acid)/tetrapod-like zinc oxide whisker composites (PLA/T-ZnOw) were prepared by melt blending with the double-screw extruder, and their mechanical properties,dynamic mechanical behaviors,rheological behaviors and crystallization behaviors were investigated by dynamic mechanical analysis (DMA),rotating rheometry,differential scanning calorimetry(DSC),polarizing optical microscopy(POM) and scan electron microscope(SEM).It was found that the modified T-ZnOw took some advantages over the mechanical properties of the PLA/T-ZnOw composites. When the additive amount of T-ZnOw was 3%, the tensile strength of the materials reached the maximum 64.65 MPa, 20.7% higher than that of pure PLA. To some degree, the impact strength under normal temperature of the PLA/T-ZnOw composites improved slightly. It suggests that the T-ZnOw has certain advantageous effects on PLA.Heat distortion temperature (HDT)/VICAT results showed that the heat distortion temperature of PLA/T-ZnOw composites grew up to the maximum 55.7℃when the additive amount of T-ZnOw was 3%, 9.87% higher than that of pure PLA. Dynamic mechanical analysis (DMA) results showed that the the glass transition temperature of the materials ascended to the maximum 68.7℃when the additive amount of T-ZnOw was 3%, more 6℃than that of pure PLA. By comparison with the pure PLA, the storage modulus of the PLA/T-ZnOw composites increased while the loss modulus decreased.The experimental research results of rheological behaviors revealed that both pure PLA and its PLA/T-ZnOw composites are pseudo-plasticity fluid, showing the shear-thinning performance. With the increased amount of T-ZnOw, the non-Newtonian index numbers of PLA/T-ZnOw composites were lower than that of PLA at first, and then went up gradually getting closer to the integer 1. It indicated that the composites are more sensitive to the shear rate with lower T-ZnOw lever, displaying non-Newtonian fluid feature apparently, while the composites was getting nearly to the Newton fluid when the additive amount of T-ZnOw grew slightly higher, weakening its sensitivity to shear rate. At the same shear rate, the composit viscosity of the composites increased initiately and then decreased with the increased T-ZnOw. In the scaning area of angular frequency 0.1s-1~100s-1, both the storage modulus and loss modulus of the composites are increased originally and then decreased as the additive of T-ZnOw increased.The observation results obtained from the polarized optical microscopy (POM) showed that the PLA spherulites in the composites increased while the size diminished. It revealed that T-ZnOw palyed a role of heterogeneous nucleation in the composites and promoted the nucleation process of the composites crystalloid, while it didn't contribute to its growup.The analysis of isothermal cold crystallization showed that the avrami equation can describe the isothermal cold crystallization behavior of pure PLA and the PLA/T-ZnOw composites perfectly.The calculated avrami index value was 1.78～3.04, suggesting that the growth mode of the pure PLA crystals and the composites'may be two-dimensional discotic and three-dimensional spherulitic growth mode. The equilibrium melting point of PLA/T-ZnOw composites obtained by the Hoffman-Weeks theory has little improvation compared with that of pure PLA. All the melting curves after the isothermal crystallization exhibited double melting peaks under higher crystallization temperature and triple melting peaks under lower crystallization temperature. The Hoffman-Lauritzen theory revealed that the addition of T-ZnOw increased the folding surface free energy and restricts the movement of chain segments.The investigation of non-isothermal crystallization showed that Ozawa equation is not suitable for describing the non-isothermal cold crystallization process, while the modified Avrami and Mo equations can take great advantages over the non-isothermal cold crystallization kinetics. All the melting curves after the non-isothermally cold crystallization exhibited double or triple melting peaks and the incorporation of T-ZnOw promoted the vanishment of the double melting peak. The Hoffman-Lauritzen theory and its deduction reveal that the absolute value of crystallization activation energy of the composites was larger than that of pure PLA, indicating that the addition T-ZnOw restricted the movements of the polymer chain segments and increased the needed activation energy.