Poly(butylene terephthalate) copolymers obtained via solid-state polymerization and melt polymerization : a study on the microstructure via 13C-NMR sequence distribution

The chemical microstructure of poly(butylene terephthalate) (PBT) copolymers obtained by incorporation of a rigid diol, 2,2-bis [4-(2-hydroxyethoxy)phenyl] propane (Dianol 220), into PBT using solid-state polymerization (SSP) and melt polymerization (MP) was analyzed. C-13 NMR sequence distribution analysis showed that Dianol was randomly incorporated in the PBT-Dianol copolymers obtained by MP. However, the used C-13 NMR sequence distribution analysis method was based on solution NMR. As a consequence, the obtained chemical microstructure reflects both the crystalline and amorphous part. For PBT-Dianol copolymers obtained by SSP, the modification only took place in the amorphous phase of PBT. Hence, knowledge of the chemical microstructure of the amorphous phase is important for tailoring the final properties of these copolymers obtained by SSP. Therefore, a calculation method was developed to adjust the solution C-13 NMR peak integral values of the dyad sequences in such a way that only the amorphous fraction, which participates in the transesterification process, was taken into consideration for calculating the degree of randomness. The semicrystalline PBT-Dianol copolymers as obtained by SSP were initially represented by the often-used two-phase (crystalline/amorphous) model. The resulting chemical microstructure of the amorphous phase gave a strong indication that part of the amorphous phase was not accessible for incorporation of Dianol by SSP. Therefore, a three-phase (crystalline, rigid amorphous and mobile amorphous) model was used to represent the morphology of the PBT-Dianol copolymers. Crystalline, mobile amorphous, and rigid amorphous fractions were determined by DSC. Using this three-phase model, the calculation method showed that only the mobile amorphous fraction was accessible for incorporation of Dianol by SSP. At the used solid-state polymerization temperature of 180 degrees C, the PBT chains in the rigid amorphous and crystalline phase are not mobile enough to participate in the transesterification reaction. Furthermore, determination of the chemical microstructure of the mobile amorphous fraction showed that Dianol was fully randomly incorporated in this fraction.