Preparation And Properties Of Shape Memory POM/TPU Blends

Polyoxymethylene (POM) is honored as the "metallic plastics"due to excellent comprehensive property, along with relative strength and stiffness close to non-ferrous metals, which has been used widespread. But because of the regular structure of POM chain, larger-size spherocrystal was easily generated during process, reduced its structural strength. In order to enhance its notch impact strength, thermoplastic polyurethane (TPU) was selected to modify POM in this paper, the POM/TPU blends was prepared by melt blending extrusion. The morphology, crystallization, fracture mechanics, melt compatibility, non-isothermal crystallization kinetics, spherulite growth and the shape memory property of the blends were studied through scanning electron microscope (SEM), polarizing light microscopy (PLM), wide angel X-ray diffraction (WXRD), differential scanning calorimeter (DSC), mechanical testing machines, dynamic rheometer and capillary rheometer, respectively.It was found that POM/TPU blends consisting of two phases, TPU particle acts as dispersion phase uniformly distributed in POM matrix with the interface vagued, which indicated TPU had partial compatibility with POM. TPU promoted the crystallinity of POM, which manifested as the degree of crystallinty increased, apparent crystalline size (ACS) largent, while the crystalline structure of POM was not changed.TPU can remarkable improved the toughness of POM, the elongation at break increased obviously, the tensile failure of POM transferred from brittle fracture to ductile fracture, the fracture mechanics of the blends were carried by./-integral, the resistance to crack initiation (Jc), the resistance to steady state crack propagation (Rp) and the resistance to crack growth at maximum load (Rmax) were estimated. A Newman model was established, TPU partial played as stress concentration, which caused crazing and shear band, could notable improve the resistance to crack fracture.The melt compatibility between POM and TPU was analyzed via dynamic rheology, the result indicates that the complex viscosity of blends was reduced by TPU, the Cole-Cole curve was semi-parabolic, along with the TPU content increased, "a small tail" semicircle appeared offside of the curve. The Han plots concentrated distribution around a principal axis, the slope was 2, but a slight divergent in low frequency range, both which indicates that TPU and POM has partial melt compatibility, and also exist phase separation after TPU content above 15wt%.The non-isothermal crystallization kinetics was analyzed through Jeziorny method and Mo’s equation, which showed that the crystallinity of POM was easily affected by cooling rate, both homogeneous nucleation and heterogeneous nucleation were existed during crystallization. TPU accelerate crystallinity, the rate of spherulitic radial growth displayed linear relation.TPU endowed POM as shape memory property, the crystalline region of POM played as immobile phase, TPU along with amorphous region of POM together acted as mobile phase. The response temperature of deformation was 165℃, closed to viscous flow temperature of POM. When the response time was set as 120s, the blends recovery rate could achieve above 95%.