Spherulites, Cylindirtes and Other Morphologies of Semicrystalline Polymers Revealed in 3D by Optical Tomography

Morphology of semicrystalline polymers on the super-µm scale, spherulites, shish-kebabs, and cylindrites, strongly affects the mechanical and other material properties. Current imaging techniques give only a 2D picture of these objects, whose features in the 3rd dimension have been assumed rather than observed. Here we show how they can be visualized in 3D using two-photon confocal laser-scanning microscopy on suitably labelled polymers such as i-polypropylene or poly(lactic acid). As a result, we see spherulites and shear-induced cylindrites in 3D. The images also reveal unsuspected morphologies such as "vases" and "goblets", nonspherical "spherulites" and, unexpectedly, "shish-kebabs" grown from static melt. For shear-induced cylindrites, 3D images show that their circular cross-section is deformed into elliptical extended toward film-substrate interface. Unexpectedly, in the case of fiber pull induced cylindrites, a row of spherulites is nucleated at polymer-substrate interface, aligned along the fiber axis. All these intriguing morphologies are attributed to negative pressure buildup peaking at the growth front and at the nearby polymer surface caused by crystallization-induced volume contraction. This interpretation is supported by finite element simulation. These results reveal unfamiliar modes of self-assembly in familiar plastics and open new perspectives on polymer microstructure.