Isothermal crystallization kinetics and melting behavior of poly(oxyethylene)-b-poly(oxybutylene)/poly(oxybutylene) blends

摘要

Poly(oxyethylene-b-oxybutylene) block copolymers of various chain lengths (EmBn) were blended with poly(oxybutylene) homopolymer (B-h), materials having different segregation strengths and morphologies (lamellae, cylinder, and sphere) were obtained. The isothermal crystallization kinetics and melting behavior of the blends were studied using differential scanning calorimetry (DSC) and synchrotron small-angle X-ray scattering (SAXS), respectively. Two types of crystallization are identified: confined crystallization occurring at lower temperature with Avrami exponent close to 1.0, having a strong dependence of crystallization halftime on crystallization temperature and a smooth change of q* in crystallization; breakout crystallization occurring at higher temperature with an Avrami exponent much bigger than 1.0 and a step change in q*. The relative segregation strength chi(c)/chi(ODT) has an important influence on the crystallization mechanism, and confined crystallization is observed at chi(c)/chi(ODT) > 3 whereas breakout crystallization occurs with chi(c)/chi(ODT) < 3. For the blends with chi(c)/chi(ODT) around 3 the mode of crystallization is dependent on the crystallization temperature (confined at lower temperature but breakout at higher temperature). In confined crystallization the interfacial energy a. can be determined from crystallization rate. It is found that the values of sigma(e) for the EmBn, block copolymers are slightly larger than that of PEO homopolymer. Confined crystallization can be further divided into three types based on melting behavior: partially confined crystallization for the blends with intermediate relative segregation strength having a cylinder structure in the melt, in which only part of the morphology in the melt is retained after crystallization and the other part is transformed into lamellae; "weakly",. confined crystallization with no step change of q* upon nonisothermal crystallization but remarkable change of q* during heating for the blends with strong relative segregation strength and a cylinder structure in the me and "strongly" confined crystallization showing little change of q* with temperature upon both crystallization and melting for the blends with sphere structure and strong relative segregation strength. In the range of molecular weight studied, only breakout crystallization was observed in lamellar melt morphologies. [References: 38]