AbstractThe addition of various nucleating agents to a polymer leads to the modification of its crystalline structure, i.e., the formation of small spherulites with consequent changes in mechanical and optical properties. The aim of our research was to determine the role of nucleating agents on the crystallization of propylene copolymers. Random and block copolymers containing propylene and low amounts of ethylene were used in formulations with nucleating agents based on derivatives of sorbitol such as dibenzylidene sorbitol (DBS) and para‐methyldibenzylidene sorbitol (MDBS). The introduction of low concentrations of DBS and MDBS leads to significant changes in the crystalline structure with respect to α and β phases, as well as in the kinetics of crystallization. Using WAXS, DSC, optical microscopy, and tensile testing, the following changes of nucleated propylene copolymers were observed: (a) an increase in the crystallization temperature; (b) an increase of the level of crystallinity; (c) a considerable decrease of the size of spherulites (i.e., an improvement of the transparency of the random copolymers); (d) a decrease of the amount of the β phase in block copolymers; (e) an increase in tensile strength at yield; and (f) an increase in secant modulus. In most cases we found that the most efficient nucleating agent was the MDBS. © 1994 John Wiley&Sons