Temperature dependence of crystallization of nano-oriented crystals of iPP and the formation mechanism

Abstract

We have studied the melt-elongational crystallization of isotactic polypropylene. In 2010, we observed that ‘nano-oriented crystals (NOCs)’ were formed when the elongational strain rate ( ) exceeded a critical value ( ). In this study, we have clarified the formation mechanism of NOCs using the crystallization temperature (Tc) dependence of the formation of NOCs and structural studies using small angle X-ray scattering, SAXS, wide angle X-ray scattering, WAXS, and optical microscopy. We propose a ‘chain reaction model’ for the formation of NOCs; the locally oriented melt is formed by the melt elongation, which significantly accelerates nucleation. The generated nuclei accelerate the formation of the locally oriented melt, which further accelerates the nucleation. We formulate a relationship between and the nucleation rate (I) based on the above model. We have confirmed that NOCs are formed in the range of Tc=150–167 °C. We have obtained the degree of supercooling (ΔT) dependence of I from the Tc dependence of . Here, we estimated an equilibrium melting temperature in the oriented melt Tm0=220 °C from the observation of NOC formation, where the error in Tm0 was a few tens of K. The observed I was well-fitted with the well-known equation of for homogeneous nucleation from classical nucleation theory (CNT), where C is a constant. Therefore, we conclude that the formation of NOCs is primarily controlled by a homogeneous nucleation process, which confirms the chain reaction model. We studied the formation mechanism of ‘nano-oriented crystals (NOCs)’ of isotactic polypropylene in elongational crystallization through crystallization temperature dependence of NOCs formation. We proposed a ‘chain reaction model’ of NOCs formation: The local oriented melt is formed by the melt elongation, which significantly accelerates nucleation; and, generated nuclei accelerate formation of local oriented melt that accelerates nucleation again. We obtained the nucleation rate (Iobs) of NOCs against the degree of supercooling (ΔT). Here, an equilibrium melting temperature in the oriented melt Tm0=220 °C is estimated from observation of NOCs formation. Iobs was well fitted by well-known equation of for homogeneous nucleation in classical nucleation theory, where C is a constant. Therefore, we concluded that NOCs formation is mainly controlled by the homogeneous nucleation process.