Preparation and characterization of some side chain liquid crystalline polymer graft copolymers of high density polyethylene and isotactic polypropylene

The monomers, p-benzophenoneoxycarbonylphenyl acrylate (BPOCPA) and p-benzophenoneoxycarbonylphenyl methacrylate (BPOCPMA) were synthesized by the reaction of p-acryloyloxybenzoyl chloride (ABC) and p-methacryloyloxybenzoyl chloride (MBC) with p-hydroxybenzophenone, respectively, in xylene in the presence of pyridine. ABC and MBC were prepared by refluxing p-acryloxyloxybenzoic acid and p-methacryloyloxybenzoic acid, formerly prepared by reaction of corresponding acid chlorides with p-hydroxybenzoic acid in alkaline medium, in thionyl chloride. The graft copolymerization of BPOCPMA onto high density polyethylene (HDPE) and of BPOCPA and BPOCPMA onto isotactic polypropylene (IPP) were carried out via bulk polymerization method by using dicumyl peroxide initiator in vacuum. The variation of grafting with the mixing ratio of the monomer (BPOCPA or BPOCPMA) and the matrix polymer (HDPE or IPP) in the reaction mixtures was investigated with six different monomer/matrix polymer ratios. While the content of graft units in the products consistently increased with the percentage of the monomers in the reaction mixtures, the percent graftings reached the maximum values at relatively lower monomer concentrations, which were followed by dramatic decreases. The grafted products were characterized by FTIR, DSC, WAX, PALS, SEM and mechanical testing. Noteworthy and significant increases were recorded in the crystalline melting temperatures and percent crystallinities of matrix polymers. The graft copolymerization of BPOCPMA gave rise to significant enlargements in the lateral dimensions of unit cells (a and b parameters) of the orthorhombic structure of HDPE, in consistence with the graft content. A constant expansion in c parameter, the unit cell axis parallel with the chain axis of HDPE molecular segments, however, was recorded in all products. The grafting also resulted in increases in the particle size of HDPE crystals. The lateral dimensions of the monoclinic unit cell of IPP matrix were also found to be considerably affected by poly(BPOCPA) and poly(BPOCPMA) content in the products. Both a and b parameters increased significantly at low percentages of the graft units. The effect was more prominent in the dimension a. At relatively high contents, the grafting led to decreases in dimensions. The parameter c decreased in all products in consistence with the content. The graft copolymerization onto HDPE, while leading to relatively smaller increase in the free volume size at low percentage of poly(BPOCPMA), resulted in reduced volume with the increase of the content in the products. Moreover, the grafting resulted in dramatic decreases in the free volume fractions with the percentage, especially in the copolymers. The graft copolymerization onto HDPE resulted in improvements in the mechanical properties of the products, especially in tensile strength and modulus. The improvements were observed to be accompanied by reductions in free volume size and fraction. On the other hand, while the graft copolymerization of BPOCPA onto IPP lead to some improvements in the mechanical properties of the products at low percentages of poly(BPOCPA). The graft copolymerization of BPOCPMA gave rise to diminishes in all tensile and impact behaviors in consistence with the content of poly(BPOCPMA). Both grafting imparted brittleness, increasing consistently with the contents to the material. SEM analysis of tensile and impact fractured surface of poly(BPOCPMA)-g-HDPE, poly(BPOCPA)-g-IPP and poly(BPOCPMA)-g-IPP products revealed that the structures of the products were completely homogeneous. No phase separation was recorded at all. The products exhibited brittle nature with some ductility.