Phase Composition of Block Copoly(ether ester) Thermoplastic Elastomers Studied by Solid-State NMR Techniques

The microphase composition of poly(butylene terephthalate)- block -poly(tetramethylene oxide) multiblock (PBT- block -PTMO) copolymers with varying content of PTMO soft block and varying block length was studied by 1 H NMR transverse magnetization relaxation ( T 2 relaxation) and 1 H 2D DQ NMR experiments under fast MAS. The 2D NMR method showed the existence of the molecular scale mixing of PBT and PTMO blocks. The 1 H and previously obtained 13 C NMR relaxation data provided detailed information on the microphase composition. The content of soft domains (i.e., viscoelastic material which is composed of the soft block and mixed PBT) and hard domains (i.e., crystalline PBT and immobilized PBT chain portions at the crystal−amorphous PBT interface) depends largely on temperature. It was shown that about 15−25% of PBT in the samples is soft due to molecular mixing with PTMO. The results suggest that at 40 °C the mixed phase is composed of approximately one PBT chain unit per eight PTMO chain units. The PBT crystallinity in the copolymers slightly increases with increasing length of the soft block, since this causes increase of the average length of PBT blocks. The content of crystalline PBT is nearly constant for samples with hard blocks containing more than five monomer units and decreases for shorter PBT block lengths. An increase in the content of PTMO with the same block length causes a significant increase in chain mobility in the soft domains. Apparently, the content of the mixed phase and the density of physical/topological junctions at the PBT/PTMO interphase largely affect the molecular mobility in the soft domains.