Cracks in Polymer Spherulites: Phenomenological Mechanisms in Correlation with Ring Bands

Abstract

Abstract:This article reviews possible mechanisms of various crack forms and their likely correlations with interior crystal lamellae and discontinuous interfaces in spherulites. Complex yet periodically repetitive patterns of cracks in spherulites are beyond attributions via differences in thermal expansion coefficients, which would cause random and irregular cracks in the contract direction only. Cracks in brittle polymers such as poly(L-lactic acid) (PLLA), or poly(4-hydroxyl butyrate) (PHB), or more ductile polymers such as poly(trimethylene terephthalate) (PTT) are examined and illustrated, although for focus and demonstration, more discussions are spent on PLLA. The cracks can take many shapes that bear extremely striking similarity to the ring-band or lamellar patterns in the same spherulites. Crack patterns may differ significantly between the ring-banded and ringless spherulites, suggesting that the cracks may be partially shaped and governed by interfaces of lamellae and how the lamellar crystals assemble themselves in spherulites. Similarly, with some exceptions, most of the cracks patterns in PHB or PTT are also highly guided by the lamellar assembly in either ring-banded spherulites or ringless spherulites. Some exceptions of cracks in spherulites deviating from the apparent crystal birefringence patterns do exist; nevertheless, discontinuous interfaces in the initial lamellae neat the nuclei center might be hidden by top crystal over-layers of the spherulites, which might govern crack propagation.Keywords: crack patterns; PLLA; ring-banded spherulites PrefaceTo sum up as a preface, the crack types in polymer spherulites may be grouped into three categories:(A) Tri-branch star (or sometimes four-branch star) short cracks that usually occur in nuclei centers.Such crack pattern (tri-branch) is caused by parallel aligned sheaf-like lamellae orienting perpendicularly to substrates. The tri-branch cracks (cleaved usually when exposed to solvent or upon etching) are parallel to the shish crystal lamellae in nuclei centers. (B) Radial short cracks that are parallel to the lamellae bundles radially-grown in spherulites.Such short radial cracks are indications of interfaces between the radially oriented lamellae in spherulites. These radial short cracks are usually segregated periodically by the circumferential cracks in the same spherulites. (C) Circumferential long cracks that may assume four major patterns: (1) circular and concentric;(2) clockwise or counterclockwise spirals (similar to Archimedean spirals); (3) hexagonal shapes; or (4) hexagonal in inner portion, but circular in outer portion. Such long and circumferential cracks are often coinciding with the ring patterns commonly seen in spherulites.Conventionally, cracks are dealt by thermal stress and contraction upon cooling from T c to ambient (∆T) owing to directional differences in thermal expansion (CTE) in the spherulites. Such models took account of only the directional differences in radial vs. tangential directions of spherulites, but did n...

In situ multiscale study of deformation heterogeneities in polylactide‐based materials upon drawing: Influence of initial crystallinity and plasticization

“…Despite having observed several specimens and numerous areas, it was not possible to visualize the spherulitic morphology of aPLA with a higher precision by AFM. It is thought that the defects in aPLA are generated from initial tribranch cracks present at the nuclei centers, as previously noted for PLA although the crystallization procedure differed from our study . It is explained that such initial cracks are induced by thermal stress and contraction during the cooling procedure from the crystallization temperature to room temperature engendering directional differences in thermal expansion (CTE) in the PLA spherulites .…”

“…When submitting PLA to an annealing procedure, its crystallinity increased from 4.6 to 28.1 wt % (Table ). Based on previous work, crystallization of PLA can engender initial tribranch cracks present at the nuclei centers . Drawing of such a material is expected to open the initial cracks without formation of craze since deformation is ensured by the opening of cracks before the sample breaking.…”

“…It is thought that the defects in aPLA are generated from initial tribranch cracks present at the nuclei centers, as previously noted for PLA although the crystallization procedure differed from our study. 38,39 It is explained that such initial cracks are induced by thermal stress and contraction during the cooling procedure from the crystallization temperature to room temperature engendering directional differences in thermal expansion (CTE) in the PLA spherulites. 38 With drawing, these initial tribranch cracks may simply open preferentially along direction perpendicular to tensile direction.…”

“…38,39 It is explained that such initial cracks are induced by thermal stress and contraction during the cooling procedure from the crystallization temperature to room temperature engendering directional differences in thermal expansion (CTE) in the PLA spherulites. 38 With drawing, these initial tribranch cracks may simply open preferentially along direction perpendicular to tensile direction. In the case of semicrystalline poly(4-methyl-1-pentene) (P4M1P) upon drawing below its T g , it was reported that large crazes formed first in the amorphous regions between spherulites and then small crazes formed within the spherulites, this phenomenon being called two step cavitation.…”

Morphological Control of Polymer Spherulites via Manipulating Radial Lamellar Organization upon Evaporative Crystallization: A Mini Review

“…Woo and coworkers have performed a series of excellent works upon this topic, and the majority have been summarized in a recent review published in 2015 [9]. Moreover, the subsequently intensive studies are still in progress [18][19][20][21][22][23][24]. Thin films with thickness ranges from dozens of nanometers to several micrometers are often employed to explore spherulites or spherulitic crystals of crystalline polymers [6,7,[25][26][27].…”