Linear viscoelastic material behaviour of semicrystalline interlayers. Ethylene vinyl acetate (EVA) and ionoplastic interlayers are increasingly used in laminated glass. Unlike polyvinyl butyral based interlayers, however, EVA and ionoplastic interlayers have partially crystalline regions that melt when the melt temperature is reached. When the melt is cooled, the crystallites are formed again. The melting and crystallization processes differ considerably from each other. This article investigates the semi‐crystalline structure of an ionoplastic interlayer using differential calorimetry measurements at different heating and cooling rates. Using dynamic mechanical thermal analysis, it is shown that the viscoelastic material behavior is influenced by the degree of crystallization.