The relation between morphological change, induced by copolymerized benzene rings or by kneading, and the dielectric strength of ethylene-styrene random copolymer was studied. Random distribution of styrene in the copolymer was confirmed by 13C Fourier-transform nuclear magnetic resonance. With increasing the styrene content of the unkneaded copolymer, the crystallinity, the size of spherulite and the lamellar thickness decrease, while the lattice constant increases. However, in kneaded samples, the lamellar thickness and the lattice constant remain unchanged up to the styrene content of 3.10 wt%. Styrene content of the sample showing the highest dielectric strength increases as the temperature decreases. It is considered that the effect of benzene rings, which hinder the acceleration of electrons, becomes pronounced in the low temperature region where the breakdown process is electronic. Kneaded samples, especially the kneaded sample with the styrene content of 3.10 wt%, have higher dielectric strength than unkneaded samples of the same styrene content. It is considered that benzene rings are moved to the interfacial region between lamellar and amorphous parts by kneading and hinder the acceleration of electrons along the lamellae more effectively.
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