Towards understanding the increase in strength of thermotropic polyesters with heat treatment

Abstract

Thermotropic copolyester fibers of oxynaphthoate and oxybenzoate have been subjected to conditions that promote solid‐state polymerization as well as annealing. The annealing process causes the crystals to perfect with a simultaneous increase in heat of fusion and melting temperature. Solid‐state polymerization, a reaction rate‐controlled process, causes the polymer viscosity average molecular weight to increase by chain extension from about 14,000 g/mole to more than 87,000 g/mole with a simultaneous impressive increase in tenacity from about 10 g/d (1.2 GPa) to almost 30 g/d (3.7 GPa). To understand the changes in mechanical properties, we have modeled the fiber structure as short rod‐like molecules poorly bonded to a continuous matrix of parallel molecules. Lengthening of the reinforcing molecules facilitates better transfer of load from matrix to molecules, resulting in higher tenacity fibers. © 1994 John Wiley & Sons, Inc.

Flexible all‐aromatic polyesterimide films with high glass transition temperatures

“…The complex viscosity of the 7‐IM/3‐HBA 365 film (thermally treated from 25 °C to 365 °C) is 8.8 × 10 5 Pa s at 1 Hz, which is an increase by a factor of 10 as compared to that of the as‐pressed film. This observation reflects that the molecular weight of the film has indeed increased by the thermal treatment method used . Of course, the removal of small molecules or byproducts also contributes to the increase in complex viscosity.…”

High-performance liquid-crystalline polymer films for monolithic “composites”

“…This significant increase of tensile strength has been attributed to increasing molecular weight resulting from the post-condensation reaction during heat-treatment [21,22]. Monoliths attained a slightly lower longitudinal tensile strength of 1.7 ± 0.1 GPa, again likely due to some misalignment of the constituent films.…”

“…For Vectran Ò fibers it is well known that their strength can be improved by a solid-state heat-treatment at elevated temperatures, most likely due to a post-curing reaction that increases the molecular weight [21,22]. The same heat-treatment was applied to the as-extruded films, which were co-wound with glass fiber mats onto bobbins to allow for water removal during the solidstate post-condensation.…”

“…It was reported that these UD tapes show promising mechanical properties, combining a Young's modulus of 70 GPa and tensile strength of up to 2 GPa. In essence, the mechanical performance of these tapes matches that of the commercially available Vectran Ò fibers produced by conventional melt-spinning of the same material [20], often followed by a heat-treatment step to boost the tensile strength [21,22]. The spintrusion process allows for the production of continuous films in a wide range of thicknesses: from >20 lm to 4 lm or less.…”

Processing and performance of aromatic-aliphatic thermotropic polyesters based on vanillic acid

“…Generally, the increase in tensile modulus upon heat-treatment is attributed to the reorganization of the monomer sequences resulting in the formation of bigger and more perfect crystals. The improved tenacity of the fibers after heat-treatment is attributed to the occurrence of a post-condensation reaction, resulting in an increase of the molecular weight which facilitates a better stress transfer along the polymer chains [9].…”