Morphology of iPP spherulites crystallized in a temperature gradient

Abstract

ABSTRACT:The influence of temperature gradient on the lamellar structure of isotactic polypropylene spherulites was studied. Polypropylene films were crystallized in a constant temperature gradient of 35 K/mm with the free surface and studied by means of atomic force microscopy and scaning electron microscopy. Samples crystallized isothermally were also prepared and examined for comparison. The results show that the temperature gradient influenced only the alignment of lamellae within spherulites. The lamellae deviated from a radial direction in a stepwise manner. The lamellae became less and less inclined and finally assumed a direction parallel to the temperature gradient. The change of the lamellae alignment in spherulites was the result of the competition of lamellae to fill the space during growth in the temperature gradient.

Nucleation and Spherulitic Growth Rates in Thin Films With Embedded Thermocouples

“…The spherulites were perfectly spherical at large distances where the temperature gradient caused by the heat conduction effect does not seem to play a role in the growth process. The anisotropy in the growth rate and morphology found within a spherulite is similar at least conceptually to that demonstrated by several authors [8][9][10] in the past. Elongated spherulites were observed in all cases with the lamellae preferentially oriented along the direction of the thermal gradient.…”

“…This disparity can be explained in terms of the nature as well as the magnitude of the imposed thermal gradient. The fundamental difference is that in all the earlier studies [8][9][10] a thermal gradient was imposed externally using a specially designed setup with the magnitude of the gradient being 308C/mm or higher. In the current experiments, a gradient lower by orders of magnitude is anticipated to be induced by the heat conduction effect of the embedded thermocouple.…”

“…In the past there has been a considerable amount of interest in studying the morphology of spherulites crystallized in thermal gradients. Experimentally induced gradients have been applied to a variety of semi-crystalline materials including poly (ethylene oxide) [8], polybutene [9], and isotactic polypropylene [10], the resulting morphology being studied using optical and electron microscopy. The most crucial observation common to all materials was that the spherulites were oriented parallel to the direction of the gradient.…”

Thermodynamics and Thermokinetics to Model Phase Transitions of Polymers over Extended Temperature and Pressure Ranges Under Various Hydrostatic Fluids

“…The viscous polymer melt partially crystallizes after undergoing a complex flow history or during flow, under temperature gradients and imposed pressure (Watanabe et al, 2003;Elmoumni & Winter, 2006) resulting into a non homogeneous final macrometric structure throughout the thickness of the processed part. The final morphologies are various sizes and shapes of more or less deformed spherulites resulting from several origins: i) isotropic spherulites by static crystallization (Ferreiro et al, 2002a;Nowacki et al, 2004), ii) highly anisotropic morphologies as oriented and row-nucleated structures (i.e., shish-kebabs) by specific shear stress (Janeschitz-Kriegl, 2006;Ogino et al, 2006), iii) transcrystalline layer (as columnar pattern in metallurgy) by surface nucleation and/or temperature gradient, and iv) teardrop--shaped spherulites or "comets" (spherulites with a quasi-parabolic outline) by temperature gradients (Ratajski & Janeschitz-Kriegl, 1996;Pawlak et al, 2002). Together with the deformation path (e.g., tension, compression), the morphology strongly influences the behaviour of polymers.…”