STUDY OF THE RELATIONSHIP BETWEEN CRACK-TIP STRAIN AND CRACK-PROPAGATION IN POLYURETHANE FILMS USING MICRO-FTIR

In the present study, a series of polyether-urethane-ureas (PEUU) were selected for investigation of crack propagation behaviour under dynam ic loading conditions. These model polyurethanes were synthesized by t wo-stage polymerization. The hard segments were composed of 4,4'-diphe nyl methane diisocyanate (MDI) and ethylene diamine (EDA). The soft se gments were polyglycols having different chemical structures and numbe r average molecular weights of 1000 and 2000. Monitoring of the variat ion in molecular orientation at the crack tip region was accomplished using polarized FTi.r. microscopy. Molecular orientation of the four m ajor functional groups, NH-, CH-, C=O, and C=C representing the domain , matrix, and interface region were measured as a function of strain f or uncracked samples using the i.r.-dichroism technique. NH-and C=O fu nctional groups present in the urea and correlated with the hard domai ns behaviour, exhibit a generalized orientation function-strain curve which was characterized by three regions. Region was associated with a n 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 i n strain. The molecular orientation was used to determine the real str ain 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 obse rved that higher soft segment molecular weights correlated with a larg er strain at the crack tip. For the same soft segment molecular weight s, 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 t he strain data and the generally accepted deformation theory for PEUU elastomers, in all PEUUs, crack propagation occurred after the individ ual hard segments separated and oriented along the stretching directio n. (C) 1997 Elsevier Science Ltd.