Carbon black-filled poly(ethylene-co-alkyl acrylate) composites: Calorimetric studies

Abstract

The calorimetric characteristics of carbon black (CB)/poly(ethylene-co-alkyl acrylate) composites depend on both the CB and acrylate contents. An increase of the acrylate content in the pure copolymers tends to decrease all the crystalline characteristics: Tc,n, the nonisothermal crystallization temperature; Tm, the melting temperature, and ΔHm, the melting enthalpy. CB modifies the crystallization kinetics of poly(ethylene-co-ethyl acrylate) (EEA) alone and in blends with poly(ethylene-co-24% w/w methyl acrylate) (24EMA) and poly(ethylene-co-35% w/w methyl acrylate) (35EMA). In the presence of CB, Tc,n, the nonisothermal crystallization temperature of EEA, increases and t1/2, the half-crystallization time, decreases for a given isothermal crystallization temperature, Tc,i. The thermograms obtained during the melting of EEA after isothermal crystallization show multiple endotherms, suggesting that crystalline-phase segregation has occurred. The existence of different crystalline species can be explained by the presence of fractions of different acrylate content in the copolymers as shown by SEC. Therefore, CB does not seem to have much effect on the subsequent melting temperature of EEA, Tm,s. CB also induces a lower melting enthalpy, Δ Hm, in the blends. This decrease of ΔHm appears to be constant whatever the compound, but when reported to the melting enthalpy of the polymer without CB, δΔHm/ΔHm increases with the acrylate content. A slight increase of the amorphous phase stiffness after CB introduction is noticed: The Tg of EEA/24EMA and EEA/35EMA blends increases by several degrees. Therefore, plotting ΔHm versus ΔCp shows that for the same ΔHm the ΔCp is lower in CB-filled samples, suggesting there is some kind of rigid amorphous phase not contributing to the glass transition. We propose to explain the CB activity during the crystallization process by the existence of molecular interactions between CB and acrylate groups rather than by a pure nucleating effect. Thus, the increase of Tc,n and the decrease of ΔHm could be explained by the fact that CB separates acrylate-rich chains from the crystallization medium, accelerating the crystallization of the acrylate-poor chains. During such a crystallization process, CB may be preferentially localized in the more polar amorphous phase and scattered between the two crystalline phases of EEA and EXA. These blends of poly(ethylene-co-alkyl acrylate) copolymers with CB provide interesting materials with adjustable properties depending on the acrylate and CB contents and on the thermomechanical treatments. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 779–793, 2001