Mechanical properties and transparency of injection‐molded polyacetal with branched and linear structure: Influence of crystalline morphology

Abstract

Branched and linear polyacetals prepared by cationic bulk polymerization were molded under high-injection rate and pressure, and the resultant 1-mm-thick specimens were investigated regarding the crystalline morphology, mechanical properties, and transparency. The branched polyacetal exhibited shear-induced transformation of crystalline morphology, namely, the spherulites, the elongated spherulites, and shish-kebab morphology parallel to the flow direction, with increasing shear viscosity. The degree of orientation of the branched polyacetal, calculated from the intensity distribution on the Debye ring of the (100) diffraction by WAXS, linearly and significantly increased with the increase of the logarithm of the shear viscosity. The difference of the crystalline morphology greatly influenced the mechanical properties and transparency of the branched and linear polyacetals. The branched polyacetal with the shishkebab morphology had approximately 20% higher tensile strength and modulus as compared with those with the spherulites morphology, and showed translucent with a higher light transmittance over a wide range of wavelength of incident light. The results indicate that a large number of fibrous crystals in the shish-kebab morphology result in the self-reinforcement of specimens parallel to the flow direction and diminishment of the scattering of incident light.

The crystallization morphology evolution of polyoxymethylene/poly(ethylene oxide) blend micropart prepared under microinjection molding conditions

“…14 Meanwhile, the highly oriented shish-kebab structures along the melt flow direction can be observed in the shear layers, as shown in Figure 6(d,e), respectively. This is consistent with those occurring in literature, 28 which found the shish-kebab structures of POM sample prepared by conventional injection molding. In addition, some oriented structures can also be observed in the skin layers, as shown in Figure 6(a), which were different from the shishkebab structures in the shear layers.…”

Creep rupture properties of homopolymer, copolymer, and terpolymer based on poly(oxymethylene)

“…The reason why the polyacetal terpolymers have higher activation volume than the polyacetal copolymers is not clear at present. However, it is reported that the branched polyacetal polymers has good tensile strength properties and the modulus like the shih‐kebab morphology 12. Although the polyacetal terpolymers and the polyacetal copolymers in this work have the almost same mechanical properties and the morphology, it is likely that the short branched structures present in the polyacetal terpolymers, due to the EHGE unit as the comonomer component, may provide the high yielding strength35 to the polyacetal terpolymers in the creep process as the slow strain property.…”

“…The samples for the NMR measurement were purified by dissolution‐reprecipitation method by using HFIP as a solvent and methanol as a precipitant, respectively. The DOX content in the polyacetal copolymers and the DOX and EHGE contents in the polyacetal terpolymers were determined by 1 H‐NMR spectrometry according to the same methods reported previously 11–13. The signals of the CH 2 CH 2 O group assigned to the DOX unit and the CH 2 CH 2 CH 3 group assigned to the EHGE unit were detected in the spectrum of 1 H‐NMR, and the each unit content was calculated from the area of each characteristic peak.…”

Formation of a large‐scale shish‐kebab structure of polyoxymethylene in the melt spinning and the crystalline morphology evolution after hot stretching

“…A large number of studies on the single crystals, spherulites, and whiskers of POM have been reported in the past several decades; however, the shish-kebab morphology of POM, which is common in other polymers, has not been reported substantially. [25,26] Pennings first discovered the shish-kebab structure in a polyethylene (PE) solution in 1965; the structure was then intensively investigated in other polymers, compounds, and composites. [27][28][29][30][31][32][33][34][35] For decades, the shear or flow field has been confirmed to induce a shish-kebab morphology and enhance crystallization.…”