Density-Branching Relationships for LLDPE

Most commercially available LLDPE's are heterogeneous in the solid state, consisting of separate crystalline and amorphous phases. The crystalline phase may be considered as a solid solution of polyethylene itself and a miscible copolymer. This miscible copolymer can contain as little as 4+ branches/1000 C, depending on the comonomer. At higher comonomer contents, phase separation occurs. A semi-empirical relationship to predict densities of miscible copolymers relates the specific volume (reciprocal density) linearly with the degree of branching and the square root of the side chain molecular weight. It appears valid for comonomers from propylene to octene-1. At higher comonomer incorporations, there is no discernible comonomer effect; specific volume varies linearly with branchiness only. This relationship appears valid for comonomers from propylene to octadecene-1 (and their mixtures). It extrapolates to the EPR region. This model is consistent with previously reported studies of polyethylene crystallinity and copolymer phase separation.