Abstract
In this mini-review, we summarize the evidences of lamellar twisting in the birefringent banded polymer spherulites demonstrated by various characterization techniques, such as polarized optical microscopy, real-time atomic force microscopy, micro-focus wide angle X-ray diffraction, etc. The real-time observation of lamellar growth under atomic force microscopy unveiled the fine details of lamellar twisting and branching in the banded spherulites of poly(R-3-hydroxybutyrate-co-17 mol% R-3-hydroxyhexanoate). Organization of the twisting lamellar crystals in the banded spherulites was revealed as well. The lamellar crystals change the orientation via twisting rather than the macro screw dislocations. In fact, macro screw dislocation provides the mechanism of synchronous twisting of neighboring lamellar crystals. The driving force of lamellar twisting is attributed to the anisotropic and unbalanced surface stresses. Besides molecular chirality, variation of the growth axis and the chemical groups on lamellar surface can change the distribution of the surface stresses, and thus may invert the handedness of lamellar twisting. Thus, based on both experimental results and physical reasoning, the relation between crystal chirality and chemical molecular structures has been suggested, via the bridge of the distribution of surface stresses. The factors affecting band spacing are briefly discussed. Some remaining questions and the perspective of the topic are highlighted.
Highlights
In this mini-review, we summarize the evidences of lamellar twisting in the birefringent banded polymer spherulites demonstrated by various characterization techniques, such as polarized optical microscopy, real-time atomic force microscopy, micro-focus wide angle X-ray diffraction, etc
Combining various characterization techniques and physical modelling, we demonstrate that the birefringent banded polymer spherulites consist of twisting lamellar crystals
The fine details of lamellar twisting in the banded spherulites was revealed for the first time by real-time AFM
Summary
Polarized optical microscopy (POM) is a simple method to distinguish the change of growth direction of crystals in the film plane and to check whether there exists twisting of crystals. If the banded spherulites are consisted of wavy bending lamellar crystals (Figure 1a,b), there should be no shift of bands when tilting the sample stage under POM. Spherulites consisting of helicoidally twisted lamellar crystals will demonstrate single or double bands under POM (Figure 1c), corresponding to uniaxial and biaxial birefringent crystal, respectively [11,12,13,14]. (d) Helical twisting of lamellar crystals and pattern the corresponding of optical micrograph oftwisting; uniaxial and crystals biaxial birefringent crystal, respectively. 3. Tilting banded spherulites around an axis on the sample stageAdapted under POM twisting and PHBV with mol.
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