Eur. Phys. J. E (2016) 39: 41
https://doi.org/10.1140/epje/i2016-16041-6

Regular Article

Spiral crack patterns observed for melt-grown spherulites of poly(L-lactic acid) upon quenching

¹ Department of Materials Science and Engineering, University of Fukui, 3-9-1 Bunkyo, 910 8507, Fukui, Japan
² Department of Frontier Fiber Technology and Science, University of Fukui, 3-9-1 Bunkyo, 910 8507, Fukui, Japan

^* e-mail: sasaki@matse.u-fukui.ac.jp

Received: 23 February 2016
Accepted: 1 March 2016
Published online: 19 April 2016

Abstract

In this paper, we demonstrate the characteristic spiral cracking that appears on the surface of melt-grown poly(L-lactic acid) (PLLA) spherulites with relatively large sizes (greater than 0.4mm in diameter). The crack occurs via thermal shrinkage upon quenching after crystallization. Although concentric cracks on polymer spherulites have been found to occur in quite a few studies, spiral crack patterns have never been reported so far. The present spiral crack was observed for thick spherulites (> 10 μm), whereas the concentric crack pattern was frequently observed for thin spherulites (typically 5μm). The present PLLA spherulites exhibited a non-banded structure with no apparent structural periodicity at least on the scale of the spiral pitch, and thus no direct correlation between the crack pattern and the spherulitic structure was suggested. The spiral was revealed to be largely Archimedean of which the spiral pitch increases with an increase in the thickness of the spherulite. This may be interpreted in terms of a classical mechanical model for a thin layer with no delamination from the substrate.