利用正電子湮滅時間光譜技術探討半結晶性之聚(3-羥基丁酸酯－共－3-羥基戊酸酯)薄膜的結構與自由體積特性

Abstract

Free volume properties in the amorphous region of a series of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) membranes, which were prepared by cold- and melt-crystallization processes, were investigated using positron annihilation lifetime (PAL) spectroscopy in this study. From the lifetime parameters, the temperature dependence of free volume size, amount, free volume size distribution, and fractional free volume, and the thermal expansion of free volume were discussed. Furthermore, the knee temperature was first observed in the melting process of the crystallized PHBV membranes. It indicated that there was structural transition of polymer chains during melting as the corresponding results observed with in situ FTIR measurement. A model which assumed that amorphous phase was subdivided into mobile and rigid amorphous fractions (MAF and RAF) in the semi-crystalline polymer was considered to interpret the temperature dependence of those free volume properties. The difference of free volume properties among various PHBV membranes was created according to the crystalline structure of the polymer from different thermal history. The polymer crystallized at higher temperatures resulted in higher crystallinities, less free volume amounts and lower fractional free volumes. Based on the crystallization conditions, the effect of the crystallization rate of PHBV polymer was first proposed to explain the thermal expansion coefficients of free volume size. The faster crystallization rate is, the higher thermal expansion coefficient and the larger free volume size at higher measuring temperatures are. Morphological observation of the semi-crystalline polymer by small-angle X-ray scattering (SAXS) indicated that the cold-crystallized membranes showed a much thinner thickness of the repeating lamellar/amorphous layers and most likely higher amount of RAF, which restrained the chain motion, than the melt-crystallized membranes. Larger dispersion of the free volume size of melt-crystallized membranes was observed as a result of the bimodal distribution of the lamellar periodicity and less amount of RAF than that of the cold-crystallized membranes. In conclusion, the relationship between the crystalline structure and the free volume properties within the amorphous phase of PHBV polymer was well established. Free volume size and thermal expansion of the free volume in PHBV membranes were affected by the kinetic of crystallization; comparatively, the total amount of free volume and fractional free volume were determined by the final crystallinity. The size distribution of free volume was associated with the crystalline lamellar structure which was dominated by the crystallization conditions.