Crystallization, melting and spherulitic structure of Β-nucleated random propylene copolymers

Abstract

The characteristics of crystallization, melting and spherulitie growth of a random propylene eopolymer (PRC) containing small amount of ethylene were studied in the presence of a selective 13-nucleating agent (calcium pimelate). It was established that the products of isothermal and non-isothermal crystallization are very rich in 13-modification but have mixed polymorphic composition. The formation of ct-modifieation may be attributed to 13or-transition on the surface of growing ~-spherulites resulting in cx~-twin-spherulites. During melting of PRC of [3-modification, the characteristics observed with 13-nucleated propylene homopolymers, namely, a 13c~-recrystallization of recooled samples and separated melting of non-reeooled samples (i.e. the melting memory effect), as well as a [3~3-reerystallization leading to a perfection of the structure within the 13-modification, are also demonstrated. The disturbance of regularity of the polymer chain highly reduees the tendency to 13-crystaltization. In contrast to the observations with propylene homopolymers, the growth rate of c~-modification (G,~) is higher than that of 13-modification (C~) and no critical crossover temperature can be found (T(13ot)=413 K) below which G~,< C_~. The experimental results ~how that a partial disturbance of chain regularity by incorporation of eomonomer units considerably reduces the tendency to [3-crystallization.

Crystallization behavior and mechanical properties of nano-CaCO3/β-nucleated ethylene-propylene random copolymer composites

“…for the given random copolymer and even no crossover temperature T (!#) was found below which G ! would be higher than G # [21]. Further studies show that the T (!#) of random copolymer PP shifts to lower temperatures [23].…”

“…Evidently, the rate of crystal growth of PPR increases with the addition of nano-CaCO 3 , but decreases with the addition of !-nucleating agent or !-nucleating agent/nanoCaCO 3 . So far as we know, for the given random copolymer in a wide temperature range, growth rate of !-phase crystal is lower than that of #-phase [15,21,23]. Therefore, the decrease of crystal growth rate of !-nucleating agent/PPR might be caused by generation of !-phase crystal.…”

“…It was reported that when nanoCaCO 3 particles were added to iPP, not only modulus and toughness of the matrix were significantly improved [16], but also !-crystals were formed [17]. Although most available documents deal with !-nucleatd i-PP modification, and few have paid attention to random polypropylene copolymer [18][19][20][21][22][23][24][25], the results are meaningful. Considering that nanoCaCO 3 have applications in PPR products, and gaining enlightenment from the studies on filled !-nucleated iPP [26][27][28][29][30], we plan to add !-nucleating agent and CaCO 3 nanoparticles into PPR together.…”

The influence of chain defects on the crystallisation behaviour of isotactic polypropylene

“…From the literature it is known that only negligible amounts of the "-phase forms in random (ethylene/ propylene) copolymers with low (1.8-2.5%) ethylene content [12]. Based on these observations, it was suggested that the tendency to "-crystallization is suppressed by the disturbance of the chain regularity [13,34].…”

“…mediocre toughness, can in principle be improved by increasing the amount of the "-polymorph. Varga and Schulek-Tóth [12] found that the formation of the "-phase was strongly restricted in copolymers of propylene and ethylene. Growth rates of the "-phase in these copolymers were found to be lower for all temperatures studied [13] as compared to pure homopolymers.…”