Thermoplastic Polyurethane Cross-Linked by Functionalized Silica. Nanostructure Evolution under Mechanical Load

Composites of TPU (Elastollan 685A by BASF) and 0-0.5 wt % 3-aminopropyltriethoxysilane functionalized silica spheres (diameter 14 nm) are strained and subjected to load cycling. Tests are monitored by SAXS which is analyzed by the chord distribution function (CDF) method. Extensibility of the soft phase is limited. Macroscopic strain is accomplished by failure of single soft domains. Broken entities comprise the overstretched soft domain, sandwiched between two hard domains. Macroscopic strain at failure determines the most probable thickness of the sandwich. Failure propagates from disordered regions into well-arranged ensembles (WAE) of domains. The higher the nanosphere content, the earlier the WAR are affected. Outbursts of local failure happen at strains of 100% and 200%. They relieve WAEs. Load cycling does not induce failure of pure TPU but of the nanocomposites. Here failure spreads in the material. The fraction of material that is not affected is a function of the nanosphere content Extrapolation yields 1.25 wt % when all the material would be affected. Relaxed samples contain sandwiches.