Long-term behavior and side chain crystallization of poly(3-alkyl thiophenes)

Abstract

The influence of slow side chain crystallization and densification processes during physical aging below T_g on relaxation behavior, calorimetric properties and structure is studied for a series of regio-random poly(3-alkyl thiophenes) (P3ATs) containing C = 6–12 alkyl carbons per side chain. Shear measurements on these samples show typical features known from other nanophase-separated side chain polymers containing long alkyl groups. In particular, there is strong evidence for the existence of self-assembled alkyl nanodomains in non-crystalline samples with an independent dynamics at low temperatures indicated by an additional α_PE process. Results for regio-random poly(3-dodecyl thiophene) show that the intensity of this process decreases on logarithmic time scales if methylene units crystallize within the alkyl nanodomains. This decrease is accompanied by an increase in the plateau modulus and a slowing down of the conventional α relaxation at higher temperatures. Similar but less pronounced changes are observed for poly(3-octyl thiophene) during physical aging slightly below T_g. This can be understood as precursor of side chain crystallization. The results show that densification processes driven by the crystallization tendency of the alkyl groups lead to significant changes in the properties. Interestingly, there are three different polymorphic states in which the methylene units can crystallize in poly(3-dodecyl thiophene) depending on the crystallization conditions. The influence of polymorphism on average interlayer distance d_nps and overall structure is shown. A hypothetical picture describing the observed changes is proposed. The implications of the presented results for existing structural models for regio-regular P3ATs and the optimization of application relevant P3ATs containing thiophene rings and attached alkyl groups are discussed.