A New Class of Poly(ester hydrazide) Copolymers with Liquid Crystalline Properties. Synthesis and Characterization

Abstract

ABSTRACT:A series of copolyesters (PE) of low molecular weight containing propyloxy-substituted aromatic ring and/or ethylenic sequences between aromatic units were prepared by melt polycondensation between corresponding diacids and diacetates, and used as precursor of poly(ester-co-polyhydrazide) (PEH) new copolymers. The PEs showed to have thermotropic properties, with transition temperatures which depend on the chemical composition and with a good interval of stability of the mesophase. The corresponding PEHs were obtained by high-temperature polycondensation of carboxyl-terminated PEs with terephthalic dihydrazide. The PEHs showed as well a stable mesophase in the melt, and improved thermal stability respect to polyesters. All the polymers were characterized by elemental analysis, inherent viscosity, infrared and 13 C solid state NMR spectroscopy, thermal and thermogravimetric analysis and optical microscopy. Fibers of PEHs were obtained by melt-spinning and characterized by WAXS analysis.KEY WORDS Liquid Crystallinity / Copolymers / Polyester / Polyhydrazide / Fiber / In the last decades, many efforts have been pointed on the synthesis of rigid-chains polymers with high heat resistance and good mechanical properties for extreme environment applications. [1][2][3][4][5][6] Heat resistant polymers are defined as organic materials which maintain their mechanical characteristics at T > 300 • C for several days or hours. Such temperatures can be reached during manufacturing or utilization of products. 7 Polyhydrazides and their polyoxadiazole derivatives, polyimides, polybenzoxazoles, and polybenzothiazoles are classified as heat-resistant polymers. At early, scarce attention was paid to their processability, and so to real possibilities of application. More recently, then, research on heat-resistant polymers is directed to improve processing window. In fact, the characteristics which render these polymers attractive for utilization in severe conditions are the same which make them hard to process: high melting temperatures (sometimes above decomposition temperature), high melt viscosity and low solubility.Aromatic polyhydrazides 8, 9 have recently attracted our attention. They are obtained from the reaction between aromatic diacids and dihydrazides, and are viewed as precursor of much more stable aromatic polyoxadiazoles, in the same way as polyamic acids are precursor of polyimides. We found that the insertion of mesogenic units containing short methylenic sequences lowers T m giving rise to a window of processability compatible with that of more common polyesters and polyamides, while keeping a still high T g . 10 Unfortunately, the resulting polymers did not show any stable mesophase in the melt, which would further improve their processability, as occurs in the case of liquid crystalline thermotropic polyesters.Liquid crystalline polymers are characterized by low melt viscosity and their fibers show excellent mechanical properties with stiffness close to theoretical values. A very well known class of thermo...

Biodegradable Poly(ester hydrazide)s via Enzymatic Polymerization

“…The resulting polymers are tough and stable, but often hard to process. Aliphatic PHs exhibit numerous interesting properties, such as heat and chemical resistance ( T m > 300 °C, insoluble in organic solvents), high degree of crystallinity, and scratch firmness 3–5. Poly(arylhydrazide)s are even more heat resistant than their aliphatic homologues, i.e., they do not degrade at temperatures below 550 °C 6.…”

“…This resulted in macromolecules with a better solubility and lower melting temperatures 7,8. Recently, aromatic poly(ester hydrazide)s were found to show liquid crystal behaviour in their melts 4. Heating of almost all PH derivatives under vacuum affords a polymer analogue reaction to convert the hydrazine groups into the respective oxadiazole functions.…”