The objective of this research was to improve the properties of immiscible polymer blends by developing a new ultrasonic extrusion process. The ability of ultrasonic treatment to induce recombination reactions in polymer blends was anticipated to result in fast in-situ compatibilization of immiscible blends. In order to test this hypothesis, a new ultrasonic extruder operating at a frequency of 20 kHz at amplitudes of 5, 7.5, and 10 μm was developed. Polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and wholly aromatic liquid crystalline copolyesters (LCPs) were selected to illustrate the effect of ultrasonic treatment on copolymerization of components through transesterification reactions in blends. The LCPs studied were a copolymer of hydroxybenzoic and hydroxynaphthoic acid (LCP1) and a copolymer of dioxydiphenyl, terephthalic and isophthalic acid (LCP2). PET/PEN, PET/LCP1, PEN/LCP1, and LCP1/LCP2 blends and their components were subsequently injection molded and spun into fibers.
PET underwent homopolymerization and degradation, respectively, at ultrasonic amplitudes of 7.5 μm and 10 μm, while PEN underwent degradation at all amplitudes. MALDI-TOF mass spectroscopy revealed greater amounts of hydroxyl and carboxyl terminated oligomers in ultrasonically treated PET and PEN. Transesterification (copolymer formation) was observed in PET/PEN blends, which was enhanced with ultrasonic treatment, as indicated by ¹H NMR and MALDI-TOF. Oxygen permeability of compression molded films of untreated and ultrasonically treated PET/PEN blends followed theoretical predictions for miscible blends.
Ultrasonic treatment of LCP1 at amplitudes of 7.5 and 10 μm led to improved mechanical properties of its injection moldings. On the other hand, LCP2 underwent degradation with treatment, leading to a reduction of mechanical properties of LCP2 and LCP1/LCP2 blends. However, due to enhanced fibrillation, these blends retained synergism such that moldings exhibited mechanical properties above the rule of mixtures. At the same time, mechanical properties of spun fibers followed the rule of mixtures.
Ultrasonically induced copolymer formation, further enhanced with higher residence time in the ultrasonic zone, was also detected by MALDI-TOF in PET/LCP1 and PEN/LCP1 blends. LCP fibrillation in moldings and spun fibers of these blends was controlled by the viscosity ratio of matrix polymer to LCP1. Homopolymerization of PET in PET/LCP1 blends, along with copolymer formation at 7.5 μm, improved fibrillation of LCP1 phase and interfacial adhesion. On the other hand, PET degradation and copolymer formation at 10 μm led to a competition between the reduction of LCP1 fibrillation and the improvement of interfacial adhesion. This competition dictated the mechanical properties of blends. Similar effects were observed in PEN/LCP1 blends.
The addition of transesterification catalysts (antimony trioxide and tetrabutyl orthotitanate) to PEN/LCP1 blends induced degradation of PEN without and with ultrasonic treatment, leading to reduced LCP1 fibrillation, and therefore poor mechanical properties. However, treatment of PEN/LCP1 blends at an amplitude of 10 μm in the presence of catalysts induced greater copolymerization and higher mechanical properties.
Observations were supported by rheological, thermal, and morphological analysis.