Abstract
Poly(ethylene terephthalate) (PET) films have found applications as the backsheets for photovoltaic devices. The properties of PET films are however gradually deteriorated upon prolonged use at ambient condition. To enhance the weathering resistance of PET, trifunctional monomers were introduced here to synthesize PET with small degree of branching. We attempt to understand the effect of such a branching on the crystallization, melting and morphology of PET films. It was found that branching in general led to a strong depression in crystallization kinetics of PET due to restriction in segmental mobility upon branching. The recrystallization of PET in the DSC heating was also hindered by branching, as manifested by the change of melting curves. Nevertheless, crystallization of the branched PET led to the formation of thicker lamellar crystals, which was attributed to the reduction of degree of supercooling due to depression of the equilibrium melting point. The introduction of slight branching did result in better resistance to hydrolysis, as demonstrated by the smaller reduction of crystal orientation and hydrolysis test in the biaxially oriented branched PET films as compared to the corresponding linear PET films.