Enhancements Of Crystallization Kinetics Of Polylactide Through Multilayer Film Method And Formation Mechanism Of The Ring-Banded Spherulites

Polylactide (PLA) is a new kind of biodegradable material, which is considered as a new promising material to be a substitute of the petroleum-based polymers. Some weakness of PLA has prevented its applications such as slow crystallization rate and poor toughness. In this thesis, the crystallization rate of PLA can be accelerated through applying of a layer-by-layer method. The nucleation and growth of spherulites of PLA in the double-layer films during isothermal crystallization at various temperatures have been investigated by using polarized optical microscopy, and the miscibility for these polymer pairs in double-layer films has been investigated by using differential scanning calorimetry (DSC) and phase contrast optical microscope (PCOM). At last, the layer structures of fractured surface of the double-layer films have been observed from scanning electron microscopy. The main contents are described as follows:1. The nucleation and growth of spherulites for the beneath polylactide (PLA) layer in poly(ethylene oxide)/polylactide (PEO/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PEO have been investigated by using polarized optical microscopy, with the particular results compared with that for neat PLA and PLA/PEO blend films. It is interesting to find that the top covering molten PEO layer can greatly accelerate the spherulitic growth rate (G) of the beneath PLA layer. Another significant result is that the temperature for the measurable nucleation and spherulitic growth of PLA in the double layer films can be eventually pushed down close to the glass transition temperature of neat PLA. The changes of glass transition temperature, Tg for PEO/PLA multilayer films have been measured by using modulated differential scanning calorimetry and dynamic mechanical analysis, which reveal slight decreases of Tg for PLA layer due to the influence of PEO layer. The layer structures of fractured surface of the double-layer films are analyzed on the basis of the observation from scanning electron microscopy and the existence of interdiffusion areas with irregular boundary between PEO and PLA layers is the key clue to understanding the significant acceleration of G for PLA. The layer-by-layer film method infers promising applications, which might be considered to well replace the blending method. 2. The spherulitic growth rates for the beneath polylactide (PLA) layer in poly(ε-caprolactone)/polylactide (PCL/PLA), poly(ethylene oxide)/polylactide (PEO/PLA), and poly(ethylene glycol)/polylactide (PEG/PLA) double-layer films during isothermal crystallization at various temperatures above the melting points of PCL, PEO and PEG layers have been measured by using polarized optical microscope (POM), with the particular results compared with that for neat PLA films. The PCL/PLA, PEO/PLA and PEG/PLA double-layer films were in-situ prepared by covering PCL, PEO and PEG films, respectively on PLA films at180℃and holding for5min before quenched to isothermal crystallization temperatures for POM observations. It is interesting to find that the covering molten PEG layer can greatly accelerate the spherulitic growth rates for beneath PLA layer and PEO layer shows a similar effect for PEO/PLA double-layer films. However, the effect of PEO layer is weaker than that of PEG layer for acceleration of PLA spherulitic growth rates in the double-layer films. Covering a molten PCL layer on PLA layer can only slightly increase the spherulitic growth rate of PLA. Different spherulitic morphologies in neat PLA film, PCL/PLA, PEO/PLA and PEG/PLA double-layer films can be observed. The miscibility for these polymer pairs was investigated by using differential scanning calorimetry (DSC) and phase contrast optical microscope (PCOM), which is proposed to play the key role for the observed accelerating effects.3. Cellulose-graft-poly [poly(ethylene glycol) methyl ether acrylate](Cell-g-PPEGA) and poly[poly(ethylene glycol) methyl ether acrylate](PPEGA) comb-like copolymers were synthesized by electron transfer atom transfer radical polymerization (ARGET ATRP). PPEGA/PLA and Cell-g-PPEGA/PLA pairs are both immiscible as indicated by slightly decreasing glass transition temperatures of PLA in the blends measured by differential scanning calorimetry (DSC) and phase separation morphologies observed by phase contrast optical microscope (PCOM). Cell-g-PPEGA/PLA and PPEGA/PLA double-layer films and neat PLA film were prepared to measure the nucleation and spherulitic growth rates during isothermal crystallization at various temperatures above melting points of Cell-g-PPEGA and PPEGA layers by using polarized optical microscope (POM). Opposite to our previous results on the miscible polymer pairs, covering immiscible molten PPEGA layer can greatly accelerate the spherulitic growth rates for PLA and Cell-g-PPEGA layer shows similar but less significant effect. Nevertheless, nucleation density is much higher for Cell-g-PPEGA/PLA film than for PPEGA/PLA and neat PLA films. A significant finding is that although the whole double-layer film systems represent phase separated ones, the densely grafted PEGA chains in the comb-like copolymers can simultaneously take an action to amplify the segmental mobility of PLA chains through local contacts at interfacial layers between phase separated domains, which significantly enhances the formation of chain folding lamellae of PLA, resulting in obvious enhancements of crystallization kinetics.4. Crystallization growth rates and spherulitic morphologies of PLA are affected by layer-by-layer method. It is revealed that two types of spherulitic morphologies are observed in PCL/PLA double-layer films. One is the well defined highly birefringent spherulites, and the other one is the coarse spherulites. It is interesting to find that the spherulitic growth rates of the coarse spherulites are higher than that of the well defined spherulites. It is thought that the coarse spherulites nucleate and grow with the assistance of the interfaces between the PCL and PLA layers, and the well defined highly birefringent spherulites only nucleate and grow in the PLA layer. It is surprisingly found that there exists a speeding of PLA spherulitic growth rate for PEO/PLA double-layer films at the late stage of isothermal crystallization, which does not exist for PLA/PEO blend films and neat PLA films. The mutual diffusion between PEO and PLA layers plays the key factor to bring out the observed speeding of spherulitic growth rate. It is interesting to find that the changing trend of band space for the formed ring-banded spherulites with isothermal crystallization temperature for PEO/PLA double-layer films shows a complex relationship as compared with the case of PEO/PLA blend films. For the latter ones, the band space shows a general monotonic increase with increasing temperature in the temperature range higher than105℃. While for former ones, the ring-banded spherulites can form at the lowest temperature approaching65℃(40℃lower than that of105℃for the PLA/PEO blend films) and the band space shows slight increase in the temperature range from65to90℃, absence of band space from90to105℃, obvious decrease from105to115℃, and significant increase from115to135℃.