Poly(butylene terephthalate) Copolyesters with Dilinoleic Derivatives: Solution Properties and Influence of Short-Chain Branching

The solution behavior of poly(butylene terephthalate) (PBT) copolyesters with dilinoleic monomer units (dilinoleic diol (LD) / dilinoleic diacid (LA)) is studied to understand the processing behavior from melt. NMR investigations show that the short-chain branches are characterized by a distribution of different alkyl chain lengths, where n-butyl (C4) and longer chains have the largest content (>55 mol%). High-temperature size exclusion chromatography with threefold detection (multi-angle light-scattering, refractive index, and viscosity detector) in 1,2,4-trichlorobenzene is performed to determine molar masses, hydrodynamic radii, viscosity radii, and Kuhn–Mark–Houwink–Sakurada (KMHS) exponents α in dependence of the molar amount of short-chain branched comonomers. Low concentrations of dilinoleic comonomers in PBT copolyesters improve the solubility of the copolyesters significantly. Higher amounts cause contraction of the polymer coils as illustrated by smaller hydrodynamic radii as well as lower KMHS exponents. The α values decrease with increasing amount of short-chain branched comonomer except for very low concentrations. The solution behavior of LA and LD copolyesters with a higher aliphatic content shows different trends of the influence of short-chain branches on the melt behavior, which reflects the influence of intramolecular versus intermolecular entanglements in solution and in melt.