Miscibility in crystalline polymer blends: Isotactic polypropylene and linear low‐density polyethylene

Abstract

AbstractThe phase behavior and the crystallization kinetics of blends composed of isotactic polypropylene (iPP) and linear low‐density polyethylene (LLDPE) were investigated by differential scanning calorimetry. The phase behavior indicates the formation of separate crystals of iPP and LLDPE at each investigated blend composition. The crystallization trace reveals that iPP crystallizes in its normal range of temperatures (i.e., at temperatures higher than that of LLDPE), when its content in the blend is higher than 25% by weight. In the blend whose iPP content is as high as 25%, at least a portion of iPP crystallizes at temperatures lower than that of LLDPE. This behavior has been proposed by Bassett to be attributed to a change in the kind of nucleation from heterogeneous to homogeneous. From the Avrami analysis of the isothermal crystallization of iPP in the presence of molten LLDPE, n values close to 2 are always obtained. According to our previously proposed interpretation of the Avrami coefficient, it can be related to the crystallite fractal dimension, through d = n + 1, which gives values close to 3, according to the spherulitic observed morphology. The kinetics parameter, i.e., the half‐time of crystallization, and the kinetic constant k show that a decrease in the overall rate of crystallization of iPP occurs on blending. Optical microscopy photographs, taken during the cooling of the samples from the melt, confirm the above results and show increasingly less resolved spherulite texture on increasing LLDPE content in the blend. The diffusion parameters evaluated for the neat polymers and for the blends in dichloromethane, which give information on the miscibility in the amorphous state, show that the diffusional behavior of the blends is governed by iPP, suggesting a two‐phase amorphous state. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3338–3346, 2003