Isothermal and Non-Isothermal Crystallization Studies of Long Chain Branched Polypropylene Containing Poly(ethylene-co-octene) under Quiescent and Shear Conditions.

Zinan Zhang,Fengyuan Yu,Hongbin Zhang

doi:10.3390/polym9060236

Abstract

Isothermal and non-isothermal crystallization behaviours of the blends of long chain branched polypropylene (LCB PP) and poly(ethylene-co-octene) (PEOc) with different weight ratios were studied under quiescent and shear flow using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and rheological measurements. Experimental results showed that the crystallization of the LCB PP/PEOc blends were significantly accelerated due to the existence of the long chain branches (LCBs), the blends being able to rapidly crystallize even at 146 °C. The addition of PEOc that acts as a nucleating agent, could also increase the crystallization rate of LCB PP. However, the crystallization rate of LCB PP was reduced when the PEOc concentration was more than 60 wt %, showing a retarded crystallization growth mechanism. The morphology of the binary blend was changed from a sea-island structure to a co-continuous phase structure when the PEOc concentration was increased from 40 to 60 wt %. In comparison with linear isotactic iPP/PEOc, the interfacial tension between LCB PP and PEOc was increased. In addition, flow-induced crystallization of LCB PP/PEOc blends was observed. Possible crystallization mechanisms for both LCB PP/PEOc and iPP/PEOc blends were proposed.

Highlights

Isotactic polypropylene is one of the leading and fast growing thermoplastic resins widely used in automotive, construction, and industrial applications
We demonstrated that the presence of the highly viscous elastomer, PEOc, tended to promote the nucleation of the supercooled Isotactic polypropylene (iPP) melting at the interface of two-phases due to their long relaxation behaviour in shear flow field [45,46]
We investigated the effect of shear flow at various shear rates, shear time, and shear strains on the crystallization of polyolefin melts [8,51,52,53]

Summary

Introduction

Isotactic polypropylene (iPP) is one of the leading and fast growing thermoplastic resins widely used in automotive, construction, and industrial applications. In general, the prepared iPP exhibits relatively low melt strength and poor low-temperature impact resistance because of the presence of highly linear chains and relatively narrow molecular weight (Mw ) distributions. This limits its applicability in applications such as blow molding, thermoforming, extrusion coating, foaming, and certain other engineering fields which require high impact performance. Increasing the Mw , broadening the Mw distribution [1], or introducing long chain branches (LCBs) [2,3], are some of the methods adopted to improve the melt strength. Some approaches have been developed to synthesize long chain branched polypropylene (LCB PP), such as reactive extrusion [4,5,6,7,8,9,10,11,12] and Polymers 2017, 9, 236; doi:10.3390/polym9060236 www.mdpi.com/journal/polymers

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Conclusion