Bowls, Vases and Goblets – The Microcrockery of Polymer and Nanocomposite Morphology Revealed by Two-Photon Optical Tomography

Abstract

On the >1µm scale the morphology of semicrystalline plastics like polyethylene or Nylon features spherulites, “shish-kebabs”, cylinddrites and other crystalline aggregates which strongly affect mechanical and other material properties. Current imaging techniques give only a 2D picture of these objects. Here we show how they can be visualized in 3D using fluorescent labels and confocal microscopy. As a result, for the first time we see spherulites in 3D, both in neat polymers and their nanocomposites, and observe how unevenly nanoparticles and other additives are distributed in the material. Images of i-polypropylene and biodegradable poly(lactic acid) reveal previously unsuspected morphologies such as “vases” and “goblets”, nonspherical “spherulites” and, unexpectedly, “shish-kebabs” grown from quiescent melt. Also surprisingly, in nanocomposite sheets spherulite nucleation is seen to be copied from one surface to another, mediated by crystallization-induced pressure drop and local melt-flow. These first results reveal unfamiliar modes of self-assembly in familiar plastics and open new perspectives on polymer microstructure.

Morphology of Shear-Induced Polymer Cylindrites Revealed by 3D Optical Imaging

“…It seems that the closer to the surface, the greater the protrusion of the cylindrite toward it. Similar to the observation of "nonspherical spherulites" observed in our recent 3D study, 45 this unusual phenomenon is the consequence of the anomalies in the local stress and flow fields induced by negative pressure ahead of the growth front. 63,64 More unsuspected results of such negative pressure are described further below.…”

“…These phenomena were found to be particularly pronounced in PLA-containing silica nanoparticles, which acted to additionally restrict chain mobility and thus hinder the refilling of cavities caused by volume contraction. As in the case of spherulites, 45 here we find that a smaller film thickness leads to greater distortion of the cylindrites. In fact, cylindrites are more prone to such a distortion than spherulites, since a negative pressure cavity is spread along a line, while with spherulites it is localized, centered around a spot; while the access for the replenishing polymer in the spherulite case will be 3D, in the case of the line cavity along a cylindrite, it will be only 2D.…”

“…In comparison with the dotted green reference circles in Figure 4i and the two parallel dotted lines in Figure 4j, it can be seen that the cylindrite is very close to circular, while its small misalignment relative to the sample surface is somewhat less expected. Prompted by our recent findings of the effects of surface proximity on the growth of spherulite, 45 we decided to investigate the effect of film thickness on the morphology of the cylindrites. Thus, cylindrites grown in a thinner film (20 μm) were then observed by 2PCM.…”

“…SCP morphology on the super-μm scale has not been reported until our recent work in which we use two-photon confocal laser scanning microscopy (2PCM) on suitably labeled isotactic polypropylene (iPP) and poly(lactic acid) (PLA). 44,45 The key step was the addition of a compatible fluorescent dye that segregated at spherulite boundaries either during crystallization or through postcrystallization penetration of the dye solution. In the 2PCM study, spherulites have been seen for the first time in 3D, both in neat polymers and in their composites with inorganic nanoparticles.…”

“…The study revealed previously unsuspected morphologies such as "vases" and "goblets" as well as nonspherical "spherulites". 45 These first results have uncovered unfamiliar modes of self-assembly in familiar SCPs and opened new perspectives on a polymer microstructure.…”