Abstract
On the >1 µm 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, 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 fresh perspectives on polymer microstructure.
Highlights
On the >1 μm 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
We were inspired by the work of Calvert et al.[24–26] who used fluorescence microscopy (FM) to view the distribution of a UV absorber in isotactic polypropylene (iPP) and observed a lowered concentration of the additive within the spherulites
In Fig. 1(a2, a4) we show FM images recorded during the same crystallization run and from the same area as in the polarized optical microscopy (POM) images in Fig. 1(a1, a3)
Summary
On the >1 μm 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. Spherulites, shish-kebabs, cylindrites, and other morphological features of bulk semicrystalline polymers (SCP) have so far been studied using methods such as polarized optical microscopy (POM)[1], transmission electron (TEM)[2], or atomic force microscopy (AFM)[3]. For these studies either thin films or thin sections were used, giving 2D but not 3D pictures. More exotic 3D imaging techniques have been tried, such as using X-rays based on the minute difference in X-ray refractive index in polymer blends[12] and foams[13] Despite some success, these methods suffer from low contrast and require highly specialized and scarce coherent synchrotron X-ray beamlines. Our preliminary 2D studies using labeled NPs have shown that, contrary to theoretical predictions, large NPs are to a considerable extent pushed ahead of growing spherulites and deposited at their boundaries[16] with potentially serious consequences for material integrity
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