LINEAR-CHAIN EXTENSION THROUGH ASSOCIATIVE TERMINI

Poly(tetramethylene oxide) (PTHF) oligomers, M(n)BAR = 650-2000, have been capped with phthalate ester (2) and with phthalic half-ester (3). Properties of the latter are strongly influenced by hydrogen-bonding association of carboxylic acid termini. DSC revealed new, high-temperature transitions in the carboxyl-terminated oligomers (3) which decreased in temperature as PTHF molecular weight increased. Ester-terminated oligomers 2 have the rheological properties of simple liquids. Oligomers 3 exhibit melt viscosities and dynamic moduli which decrease sharply at the highest DSC transition temperature but nevertheless exceed those of 2 over the entire range studied. These carboxyl-terminated oligomers exhibit elasticity-dominated responses to small strains with a linear viscoelastic region and good recovery to large strain amplitude oscillatory shear. All observations are consistent with formation of large crystalline aggregates of carboxyl-terminated chain ends below the high-temperature DSC transition. Solid-state C-13-NMR suggests the presence of phthalic half-ester end groups in both crystalline and amorphous environments.