Crystal morphology and thermodynamics of poly(ethylene-4,4 '-biphenyl dicarboxylate) and related copolymers with ethylene-2,6-naphthalene dicarboxylate

We present a detailed experimental and modeling investigation of poly(ethyl ene-4,4'-biphenyl dicarboxylate) (PEBB) and the related random copolymers p oly(ethylene-4,4'-biphenyldicarboxylate-co-2,6-naphthalene dicarboxylate) ( PEBB/EN). We find that different polymorphs exist for the PEBB homopolymer and suggest a second crystal unit cell, different from that which is discus sed in the Literature. PEBB/EN copolymers, which are shown here to be rando m copolymers, are found to be semicrystalline for all naphthanoate concentr ations. By combination of DSC, X-ray, and solid-state NMR experiments, unif orm comonomer inclusion is proposed for copolymers containing a high amount of EN; for low EN concentrations, these comonomers are rejected from the c rystals. By comparison of the comonomer inclusion Gibbs energy, which was e stimated from experiments, to those obtained from thermodynamic integration calculations, it was found that PEBB/EN copolymers are able to cocrystalli ze by aggregation of like and by segregation of unlike repeat units. Once t hese comonomer inclusions have reached a considerable size, the average def ect Gibbs energy reduces to the thermal energy.