Origins of periodic bands in polymer spherulites

Abstract

Historically, in the past 60years, continuous spiral twisting of polymer lamellae, assumed as tens of thousands of single crystal plates simultaneously radiating from a common center of spherulites, had been interpreted as a plausible mechanism for common optical ring-banding behavior in polymer spherulites. Furthermore, chain folding and congestion of such folds, unique only in polymer crystals, had been advocated as a cause for lamellae surface stresses, which were believed to contribute to continuous spiral twisting and ring bands in spherulites. With progress of powerful advanced instrumentation and newer discoveries of diversified patterns (e.g., birefringent vs. non-birefringent; circular vs. hedral/hexagonal; concentric vs. helicoidal, etc.) of ring bands in increasing numbers of polymers under different environment, more and more work reported behavior that contradicted interpretations based on such idealized models. Debates have gone on perpetually, with fully clarified settlement seeming to be still far away.This feature article reviews and surveys evidence accumulated in past years by numerous investigators to point out that polymer spherulites are made of complex hierarchical polycrystals with multiple branches during primary and secondary growth that are supported by clear experimental observations; thus, assumption of single-crystal lamellae plates undergoing continuous spiral from center to periphery of spherulite may need critical assessment for validity. Secondly, numerous investigators since 100years ago have amply reported (even far before discoveries of ring bands in polymer spherulites) that many small-molecule compounds, apparently without chain-folding induced surface stresses, could display exactly same orderly ring-band patterns in their crystals as those in polymer spherulites. The facts suggest that chain folding and stresses caused by congestion of folds in lamellae, may not be a necessary condition or responsible factor for ring bands in spherulites. Thirdly, crystal lamellae, being in either polymers or small-molecule compounds, do twist or scroll in responding to various external stresses; however, critical experimental evidence for matching between the twist pitches and optical inter-ring spacing, for some unknown reasons, has been lacking in past 60years. Fourthly, it takes synchronized twists of tens of thousands lamellae that all have to be identically shaped single crystals in order to build orderly ring bands in a spherulites as viewed in optical graphs. This synchronizing identity of all single crystals might be a dauntingly impossible task for polymer crystals in spherulites by considering that even simple water molecules cannot pack into two snowflakes that are alike (though the basic hexagonal prisms may look similar in appearance but it is very unlikely for two larger complex snow crystals to be identical in appearance). Finally, conventional analyses were usually based on characterization on the top-surfaces of thin films (several micrometers, with top surfaces etched or unetched) of crystallized polymers; or alternatively, polymers were cast to ultra-thin films (nanometers) for viewing on the morphology of single crystals.This article points out with demonstrated cases that etching with improper solvents may cause artificial morphology deformation owing to external stresses, or even chemical alterations, of crystals in spherulites to be characterized leading to erroneous interpretations. Banding patterns are generated from the crystal arrays of spherulites in the entire thickness optical path. However, the top-surface crystals in polymer films usually account for only a minor fraction of the whole thickness of polymer samples, and the assembly of top-surface crystals may differ from that of the interior crystals. Thus, analyses based only on the top surfaces of thin polymer films, a common approach taken by most investigators in past 60years, might yield results that were not in full agreement with truth of entire spherulites. Additionally, observations on nanometer ultra-thin film samples or thin-solution cast single crystals, though allowing development of morphology approaching single-crystal lamellae, may yield morphologies that are far different from those in true ring-banded spherulites. Although lamellae scrolling and twisting may be proven in single crystals of some polymers owing to chain-fold induced stresses or other external forces, extrapolation of possible correlations of scroll/twist in single-crystalline lamellae to the ring-banded patterns in polycrystalline spherulites is still not yet established.Major keys of cracking the mysteries are thus that steps must be put forward in alternative directions. One should also know the constraints of analyses based on results derived from single crystals, which are far different from the complex branched crystals in actual polymer spherulites. Lamellae twist (or bend, scroll, dislocation, etc.) and periodic ring bands are two facts in many spherulites; yet, they have to be dealt in entirely different perspectives for probing the origins. By surveying recent studies oriented in alternative approaches in comparison with conventional studies in the literature, this work demonstrates a new horizon of probing the origins of periodic banding behavior in polymer spherulites. This article is just an initiative. It takes courage to raise skepticism on unsettled interpretations – and strong evidence accumulated based on novel approaches – to steer in right directions.