Study On The Influence Of Memory Effect On The Crystallization Behavior Of Poly(Lactic Acid)

Poly(lactic acid)(PLA) is a biodegradable semicrystalline polyester derived from renewable resources, with growing importance in biomedical and commodity applications. However, its intrinsic slow crystallization rate limits its wider application. The addition of a nucleating agent is a common approach to improve the crystallization rate of PLA. However, most of them are toxic to the human body and are not environmentally friendly. Actually, utilizing the memory effect to the original crystal structure remained after the partial melting of polymer crystals, can effectively improve the crystallization rate of the material also. In this thesis, the partial melting and subsequent crystallization behavior of PLA at various pre-crystallized states was investigated by differential scanning calorimetry(DSC) and polarized optical microscopy(POM). The main results are as follows:Firstly, the partial melting and subsequent crystallization behavior of PLA crystals, obtained by nonisothermal crystallzation from the melt, was investigated. The results show that the partial melting temperature with self-nucleating effect ranges from 176 oC to 180 oC for PLA pre-cystallized nonisothermally. After partial melting, the maximum crystallization peak shifts to higher temperature by 29.8 oC during subsequent cooling, compared with the PLA completely melted, indicating that self-nucleation significantly improved crystallization rate of PLA. Partial melting at 178 oC, 176 oC or 175 oC leads to predetermined athermal nucleation at higher temperature, while complete melting at 200 oC results in a sporadic nucleation at lower temperature. Irregular spherulites are formed for PLA partially melted at 176 oC during subsequent isothermal crystallization, and the nucleation density decreases with increasing crystallization temperature.Secondly, the partial melting and subsequent crystallization behavior of PLA, obtained by isothermal crystallzation from the melt, was investigated. The results show that the partial melting temperature with self-nucleating effect ranges from 175 oC to 179 oC and from 176 oC to 179 oC for PLA isothermally crystallizated at 135 oC and 110 oC, respectively; The lower of the partial melting temperature, the higher of the subsequent crystallization peak temperature; After partial melting, the maximum crystallization peak shifts to higher temperature by 34.4 oC and 31.1 oC, respectively, during subsequent cooling, compared with the PLA completely melted, indicating that self-nucleation significantly improved crystallization rate of PLA. For both PLA isothermally crystallzed at 135 oC and 110 oC, respectively, the overall isothermal crystallization rate at 158 oC, after partial melting, decreases with the increasing of the partial melting temperature. Moreover, at the same partial melting temperature, the overall isothermal crystallization rate of the former was lower than the latter. The subsequent isothermal crystallization data were further analyzed by the Avrami model. It is found that the values of the Avrami exponent keep almost no change, while the partial melting temperature changes.Finally, the partial melting and the subsequent isothermally crystallization behavior of single PLA spherulite, isothermally crystallized at 135 oC for 15 min, was investigated by POM. The results show that the partial melting temperature and time could have significant effect on the crystalline morphologies formed after recrystallization. Some representative crystalline morphologies include:(1) irregular morphology with a twisted spherulitic-like inner part and a transcrystalline outer part;(2) sandwiched morphology with a spherulitic inner part, a broad circle composed of thousands of granular structures in the middle part and a round transcrystalline outer part;(3) ellipsoidal morphology with an inner part composed of thousands of granular structures and a transcrystalline outer part;(4) a large number of small spherulites(granular structure) appeared within the outline of the original spherulite but the intertior part is almost empty. From the above results, it can be deduced prilimilarily that the distribution of the structure in the radial direction is non-uniform, even for a single PLA spherulite obtained by isothermal crystallization from the melt.