Unconventional Non‐birefringent or Birefringent Concentric Ring‐Banded Spherulites in Poly(L‐lactic acid) Thin Films

Abstract

Unconventional concentric up-and-down bands, differing from the conventional brightextinction or blue/orange bands in other polymers, in poly( L -lactic acid) are found via controlled thin fi lms. Non-birefringent and birefringent concentric banded spherulites are obtained from 350-400 nm and 1800-2000 nm fi lm thicknesses, respectively. Non-birefringent concentric ring-banded spherulites are arranged only by fl at-on lamellae; in contrast, complex combinations of fl at-on, edge-on, and round-shaped lamellae in four transitional zones are present within a single band in the thicker fi lms of birefringent, concentric, ring-banded spherulites. Interband is an optically extinction crevice that segregates two neighboring bands. Unique lamellar arrangements at specifi c T c and thickness confi nement are two factors for the unconventional concentric bands.

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

“…Meanwhile, PLLA has quite a slow spherulite growth. As expected, the similar periodic patterns were reported recently in low molecular weight PLLA thin films both from melt-crystallization and vapor-assisted crystallization [27,41,42]. It can, therefore, be concluded from the above findings that it is the crystallization condition rather than the structural feature that is important to determine the stacking mode of radial lamellae in polymer spherulites.…”

“…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]. Based on the viewpoint of lamellar assembly, one can simply divide the radial lamellar organization of two dimensional (2D) polymer spherulites produced in thin films into two situations, the distortion or orientation and the stacking or packing mode of lamellae.…”

“…The possible radial lamellar organizations and corresponding optical features of the simplest case, i.e., the uniaxial crystal growing along an invariant crystallographic axis, are demonstrated in Figure 2, and four spherulites can exhibit the intriguingly ring-banded morphologies [28]. Taking multilevel ordering structures of polymer crystals (as depicted in Figure 1), various growth environments and crystallization media into account, it can then be predicted 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]. Based on the viewpoint of lamellar assembly, one can simply divide the radial lamellar organization of two dimensional (2D) polymer spherulites produced in thin films into two situations, the distortion or orientation and the stacking or packing mode of lamellae.…”

Origins of periodic bands in polymer spherulites

“…More strikingly, the ring patterns (shape and inter-ring spacing, etc.) differ in a systematic trend according to variation of film thickness [45]. The shape of concentric rings changes from hexagon to circular shape as thickness is increased from 400 nm to 800 nm, and birefringence color becomes more apparent with film thickness at 2000 nm (2 µm) or more.…”

“…In addition to the traditional aliphatic polyesters or aryl polyesters, which are already well known to exhibit ring-banded spherulites, many newer biodegradable polymers, such as poly(L-lactic acid) (PLLA), poly (3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co-HV), etc. [43][44][45][46], have been shown to exhibit ring-banded spherulites. The chemical structures of polymers play some roles in determining whether or not ring-banded spherulites can be formed in the polymers; yet, there are no general rules -not even the classical odd-even rule.…”