Rhythmic Growth-Induced Ring-Banded Spherulites with Radial Periodic Variation of Thicknesses Grown from Poly(ε-caprolactone) Solution with Constant Concentration

Abstract

Birefringent ring-banded spherulites with radial periodic variation of thicknesses were grown from poly(ε-caprolactone) (PCL) solutions under conditions for which the solution concentration was held constant during the whole development of the morphology. The as-grown ring-banded spherulites were investigated by optical (OM) and atomic force (AFM) microscopies, by transmission electron microscopy (TEM) of samples sectioned parallel to the plane of film, and also by electron diffraction (ED) and grazing incidence X-ray diffraction (GIXD) techniques. The results indicate that the concentric rings in the birefringent ring-banded spherulites, as well as those in the nonbirefringent ring-banded spherulites, are a manifestation of periodic variation of thicknesses along the radius. The development of the ring-banded spherulites with radial periodic variation of thicknesses is due to periodic diffusion-induced rhythmic growth associated with periodical change in the concentration gradient in polymer solution with constant concentration. The morphological features reflect the competition between the diffusion flux of polymer chains in solution, J, and spherulitic growth with radial growth velocity, V, which can be characterized by the parameter δ ) J/V. The effects of crystallization conditions, including polymer molecular weight, initial solution concentration, and solvent evaporation rate, on the formation of ring-banded spherulites with radial periodic variation of thicknesses were studied.

Effect of Film Formation Method and Annealing on Crystallinity of Poly(L-Lactic Acid) Films

“…During the casting process crystallization from solution can occur if the solvent evaporation rate is low enough for polymer macromolecules to diffuse to the crystal growth front and overcome the energy barrier of deposition. Category 1 spherulites, defined by Norton and Keller [6], can be commonly observed in this case [3,7,8]. Spin coating generates polymer films with thinner and more uniform thickness.…”

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

“…1.50 × 10 −4 mL·h −1 at 20 • C [44,45]. Both the non-birefringent and birefringent concentric ringed morphologies, as exhibited in Figure 4, were observed by changing the initial concentration of the casting solution.…”

“…The lower initial solution concentration leads to the thinner film, in which the spherulites are composed of almost uniform flat-on lamellae because of the combination of a strong polymer-substrate interplay and the slow spherulite radial growth, so a non-birefringent pattern is observable in the uniaxial PCL spherulites with the radial and tangential directions along the fixed crystallographic b-and a-axis, respectively (Figure 5a) [44]. As the films thicken with increasing initial solution concentration, the lamellae orientations change from the uniform flat-on case near the substrate to become gradually tilted and, eventually, edge-on situations toward the top layers of the ridges (Figure 5b) [45]. As a result, the tangential directions alter gradually from the a-axis on the valleys to even the c-axis upon the peak of ridges, leading to the appearance of the continuously-enhanced birefringence with the increase of ridge height [46].…”

“…The occurrence of non-traditional concentric rings in PCL spherulites is first dependent on the average solvent evaporation rate. They can merely be developed upon slow evaporation, and rapid drying, such as free evaporation, gives rise to the emergence of the usual Maltese Cross spherulites (MCS) with continued packing of randomly oriented, radial lamellae [45][46][47]. By tuning a gradually accelerated solvent withdrawal that drives the successive increasing in crystal growth rate, Li et al have observed that the concentric rings became gradually narrowed and eventually disappeared, leading to a band-to-non-band transition and, consequently, the emergence of a novel PCL crystal pattern, as depicted in Figure 6 [48].…”

“…Hence, rhythmic crystallization is predicted to occur more easily. Even so, non-traditionally concentric-ringed spherulites can also be found merely under slow evaporation [44][45][46][47]. Obviously, when polymers crystalize from solutions, the supersaturation is the basic requirement.…”