| Summary: | 碩士 === 崑山科技大學 === 材料工程研究所 === 99 === The PLA/α-cellulose composites have been fabricated using twin-screw extruder. The α-cellulose in this study will play the role of toughening the PLA matrix, and the contents of α-cellulose in PLA matrix will range from 0.2 to 10 wt%. To achieve this purpose α-cellulose was first subjected to surface modification with stearic acid to impart a good interfacial compatibility between the PLA matrix and α-cellulose. The resulting PLA/α-cellulose composites exhibit softer characteristics than the neat PLA. The mechanical properties, including ultimate tensile strength (UTS) and Young’s modulus (E), of the PLA/α-cellulose composites are much lowered at lower α-cellulose contents, and then these properties would climb up gradually at higher α-cellulose contents. However, the values of UTS and E of the resulting composites are significantly lower than those of the neat PLA polymer. Meanwhile, the ε (elongation at break) values of PLA/α-cellulose composites for all of the α-cellulose contents are higher than that of the neat PLA. It is intentional that the inclusion of α-cellulose into PLA matrix is aimed to increase the microcrack length and to improve the toughness of the PLA polymer. The SEM micrographs show that the fractural surfaces for the PLA/α-cellulose composites present the dimple-like morphology as compared to the somewhat flatten-like for the neat PLA polymer. The impact test shows that the impact strength for PLA/α-cellulose composite filled with 2 wt% modified α-cellulose would obtain an enhancement about 310% as compared to that of the neat PLA. Moreover, the damping factor from the DMA test for PLA/α-cellulose composite filled with 4 wt% modified α-cellulose is higher than that of the neat PLA by 28.6%. From the results of impact and DMA tests, the inclusion of α-cellulose could substantially and significantly increase the microcrack length and, as a result, improve the toughness and damping characteristics of the PLA polymer. Furthermore, α-cellulose modified with stearic acid would exhibit better performance in toughering property than that of the unmodified counterparts. As for the isothermal crystallization behavior of the PLA/α-cellulose composites, the inclusion of α-cellulose would decrease the spherulite dimensions and increase the growth rates of spherulite of the resulting composites. Moreover, there is a max. radial growth rate of spherulite for neat PLA and PLA/α-cellulose composites. The inclusion of α-cellulose could significantly increase the crystallinities Xc and the crystallization growth rates K of the resulting composites. Furthermore, irrespective of neat PLA and PLA/α-cellulose composites, there are double melting (Tm1，Tm2) occurrences for the crystallization temperatures between 95~115oC. As the temperatures raising higher than this range, the PLA molecules would possess enough kinetic energy to undergo the melt-recrystallization process, and under this condition the unsteable Tm1 would vanish and the Tm1，Tm2 come into single melting peak. The investigations on the non-isothermal crystallization kinetic behavior have been conducted by means of differential scanning calorimeter. The Avrami, Ozawa, and combined Avrami and Ozawa equations were applied to describe the crystallization kinetics and to determine the crystallization parameters of the α-cellulose filled PLA composites. It is found that the inclusion of α-cellulose can decrease the growth rate Zc due to hindrance of the polymer chain mobility. On the other hand, the composites show a higher Avrami value than that of the neat PLA, implying a more complex crystallization configuration. Moreover, the combined Avrami and Ozawa equation can successfully describe the crystallization model under the non-isothermal crystallization.
|
|---|
