The influence of average molar mass and shear on the crystallization of isotactic polypropylene (iPP) has been studied. The Weissenberg number, a dimensionless parameter consisting of the product of the shear rate and a characteristic relaxation time (γ̇0λr), provides a criterion to locate the morphological transition from spherulitic to extended growth. The Weissenberg criterion applies to polymers of high and of low molecular weight. This is attributed to the importance of the initial nucleation process. Comparison of materials of different molar masses and various shear histories is allowed by the introduction of the normalized crystallization time τ, which is the ratio of the experimental time and a characteristic crystallization time λVvmax that needs to be defined from the several possible choices. Rheological and rheooptical experiments furnish the material characteristic relaxation and crystallization time scales, respectively. The time λVvmax, defined with light scattering as the moment at maximum density fluctuation invariant Q η, is chosen here as a characteristic crystallization time scale since it is especially sensitive to the early stages of crystal growth that are responsible for the liquid-to-solid transition. DSC and WAXD have been used to further analyze the effect of shear on the populations of crystalline fractions present in the cooled iPP samples.

The in-situ polymerization at different polymerization temperatures, and the crystallization behavior of cyclic poly(butylene terephthalate) oligomers, using various process-suitable catalysts, are investigated in this paper. The choice of the catalyst has a large effect on the polymer conversion and the time required for polymerization, as studied by light transmittance measurements. The resulting polymer is characterized by GPC for molecular weight and polydispersity and by WAXD and DSC for crystal morphology. SALS is used for in-situ observation of the melt crystallization during polymerization.

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Summary: Two well‐characterized metallocene isotactic poly(propylene) (iPP) samples, one of high and one of low molecular weight, were blended together for studying the effects of a second component on quiescent and shear‐induced crystallization. A small amount of added high molecular weight (HMW) (up to 10 wt.‐%) polymer increases the quiescent crystallization rate. This was observed as a decrease in characteristic halftime of transmitted light intensity. The crystallization halftime increases again when adding more than 10 wt.‐% of HMW polymer. The crystallization halftime of pre‐sheared samples decreases with increasing HMW fraction and is lowest for the HMW sample by itself. For the specific shearing conditions (γ = 600, Tx = 145 °C), wide‐angle X‐ray diffraction (WAXD) images show the presence of the gamma (γ) crystalline phase of iPP for samples containing 25 wt.‐% of HMW and higher. DSC thermograms demonstrated higher crystalline fractions with increasing HMW fraction in pre‐sheared samples.Optical micrograph of an iPP sample after quiescent crystallization at 145 °C.magnified imageOptical micrograph of an iPP sample after quiescent crystallization at 145 °C.