Study of the relationship between crack tip strain and crack propagation in polyurethane films using micro- FTi.r

In the present study, a series of polyether-urethane-ureas (PEUU) were selected for investigation of crack propagation behaviour under dynamic loading conditions. These model polyurethanes were synthesized by two-stage polymerization. The hard segments were composed of 4,4′-diphenyl methane diisocyanate (MDI) and ethylene diamine (EDA). The soft segments were polyglycols having different chemical structures and number average molecular weights of 1000 and 2000. Monitoring of the variation in molecular orientation at the crack tip region was accomplished using polarized FTi.r. microscopy. Molecular orientation of the four major functional groups, NH, CH-, C=O, and C=C representing the domain, matrix, and interface region were measured as a function of strain for uncracked samples using the i.r.-dichroism technique. NH- and C=O functional groups present in the urea and correlated with the hard domains behaviour, exhibit a generalized orientation function-strain curve which was characterized by three regions. Region 1 was associated with an initial decrease in the orientation function at low strains followed by region 2, which is the minimum obtainable orientation, and region 3 a subsequent increase in the orientation function with an increase in strain. The molecular orientation was used to determine the real strain at the crack tip. The strains at the crack tip for the pure (PEUU) were between 4 to 7 times higher than the applied strain. It was observed that higher soft segment molecular weights correlated with a larger strain at the crack tip. For the same soft segment molecular weights, polypropylene glycol (PPG) based PEUU showed higher strains at the crack tip. Therefore, the strain at the crack tip depends on chemical structure and the molecular weight of the soft segment. According to the strain data and the generally accepted deformation theory for PEUU elastomers, in all PEUUs, crack propagation occurred after the individual hard segments separated and oriented along the stretching direction.