In this work, we explored the electrical resistance-pressure sensitivity of multiwall carbon nanotube (MWNT) networks/poly(dimethylsiloxane) (PDMS) composites and proposed a deformation-induced property transition mechanism of the nanotubes to explain this behavior. The thermoelectric coefficients of the MWNT networks/PDMS composites and MWNT mat under pressure were also measured and discussed to support our proposition. Our results revealed that the relative resistances of MWNT networks/PDMS composites with lower MWNT loadings are more sensitive on the applied pressure. Furthermore, the I-V characteristic of the MWNT networks/PDMS composites modulated with pressure on one side has shown a well rectified behavior.
Summary: Polyoxymethylene (POM)/elastomer/filler ternary composites were prepared, in which thermoplastic polyurethane(TPU) and an inorganic filler, CaCO3, were used to achieve balanced mechanical properties of POM. A two‐step processing method, in which the elastomer and the filler were mixed to a masterbatch first and then the masterbatch was melt‐blended with pure POM, was used to obtain a core‐shell microstructure with CaCO3 covered by TPU. A brittle‐ductile transition phenomenon was observed with increasing TPU content for this ternary system. To better understand the toughening mechanism, we investigated the fractured surface, interparticle distance, and the spherulite size of POM as function of the TPU and CaCO3 content. The critical TPU content depended on not only the content of CaCO3, but also the size of CaCO3 particles. The observed brittle‐ductile transition was discussed based on the crystallinity and spherulite size of POM as well as Wu's critical interparticle distance theory. The results showed that the impact strength of POM/TPU/CaCO3 ternary system depends on a critical, interparticle distance, which varies from one system to another. The dependence of the impact strength on the spherulite size was considered for the first time, and a single curve was constructed. A critical spherulite size of 40 micron was found, at which brittle‐ductile transition occurs, regardless of the TPU and CaCO3 content or the size of CaCO3 particles. Our results indicate that the spherulite size of POM indeed plays a role in determining the toughness, and must be considered when discussing the toughening mechanism.Izod impact strength vs. the crystal size for POM/TPU blends and POM/TPU/CaCO3 ternary composites.magnified imageIzod impact strength vs. the crystal size for POM/TPU blends and POM/TPU/CaCO3 ternary composites.
ABSTRACT:The aliphatic waterborne polyurethaneureas (WPU) with different isocyanate index (R) were synthesized by isophorone diisocyanate (IPDI) and polyester glycol via prepolymer mixing method. The effects of the R value on the structure and the properties of the resultant polymers have been analyzed. The polymer with a certain R value (R ¼ 1.44) exhibited excellent mechanical properties with both high tensile strength (55 MPa) and high elongation at break (1400%). The water resistance and the thermal stabilities of WPUs have been improved with the increase of R. The results were interpreted in terms of the structures associated with the micro-phase separation. According to differential scanning calorimeter (DSC) analysis, the microphase separation degree was elevated by increasing R value. FTIR spectrum showed the more R value, the more hydrogen bonded urea groups. No crystallinity was found in X-ray diffraction (XRD) patterns showing the typical amorphous morphologies of IPDI-based WPUs. Thus the primary driving force for microphase separation was the hydrogen bonding between urethane and urea groups, making significant influence on the properties of WPUs, as shown in dynamic mechanical analyzes.
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