Chain extension and orientation: Fundamentals and relevance to processing and products

Abstract

Flow induced orientation, the subject of this article, is being surveyed in its wide variety of manifestations. This includes crystallisation together with the resulting orientation in the solidified product, a broad survey of past works here placed in a new perspective not presented hitherto, and novel rheological effects observed lately in melt flow containing some recent results, some to be announced here for the first time. The theme connecting the two separate subject areas is the coil → stretch transition produced by the appropriate flows allowing for only two states, the essentially random coil and the essentially fully stretched out state, with no intermediate state under steady state conditions in between. Each of the above two states leads to its own characteristic morphology on crystallisation which then characterise the final product and the resulting orientation pattern as registered by x-ray diffraction. It is being argued that the whole range of orientation patterns observed in articles crystallised under melt flow are accountable by the variable ratios of the two crystal morphologies, lamellae and fibres arising from the respective random and fully aligned chain orientations, through their various combinations, without the need for involving intermediate chain or crystal orientations at least as relating to the strength of the primary orienting influence. — The novel rheological effects are arising from the main theme, namely the random and extended chain duality, with criticalities in terms of both strain rate and molecular weight sharply delineating the two states. This, when coupled with a so far unspecified phase transition, can provide an unexpectedly sharp temperature singularity (window) in extrusion behaviour potentially highly advantageous for processing applications. With constant piston velocity this is manifest as a sharp minimum in extrusion pressure, and at fixed extrusion pressure as a sharp maximum in extrudate exit velocity, the criticality in either case displaying an inverse fourth power dependence on molecular weight. — In order to provide an underpinning for both classes of effect raised above, a brief survey is provided on the fundamentals of elongational flow induced coil → stretch transitions as explored experimentally in the Bristol laboratory on polymer solutions. This is accompanied by a discussion of the transferability of those results to melts including both the parallels between the two systems and the apparent problems such a transfer seems to raise, ending on some open ended issues as regards similarities and differences between freely flowing melts and extension of connected networks.