THE EFFECT OF DIFFERENT MOLECULAR-WEIGHT OF SOFT SEGMENTS IN POLYURETHANES ON PHOTOOXIDATIVE STABILITY

The effect of molecular weight of polyol (soft segment) and the concen tration of urethane (hard segment) in segmented polyurethane elastomer s on their photodegradation was investigated. Polyurethane elastomers have been prepared from 4,4'-dipheylmethane diisocyanate (MDI) and pol y(oxytetramethylene) glycol (PTMO) of 1000 and 2000 molecular weight a t NCO/OH ratios of 2/1 and 4/1. Purified 1,4-butanediol (BD) was used as the chain extender. Mechanical, thermal, dynamic mechanical and Fou rier Transform Infrared Spectroscopy (FTIR) measurements have been use d for mechanical and structural studies of PUR elastomers before and a fter UV irradiation, for a better understanding of the role of the con centration of hard segments (urethane) and molecular weight of soft se gments (polyol) in PUR. It was found that the molecular weight of soft segments as well as the NCO/OH ratios have an influence on the UV sta bility of the examined elastomers. The photooxidative degradation is m ore prevalent in elastomers with lower hard segment concentration and with higher soft segment molecular weight. The correlation between the compositions of the investigated elastomers and mechanical properties , dynamic mechanical properties, physical transitions, as well as inte nsities of the changes of these properties after irradiation were esta blished. The results show that with an increase of hard segment concen tration in all examined polyurethane elastomers, the glass transition temperature (T-g) of the soft segment increased, the tensile strength increased and the elongation at break decreased. In irradiated polyure thane elastomers based on PTMO (1000) with the higher hard segment con centration T-g of the soft segment decreased, the opposite effect was obtained in PUR elastomer with lower hard segment concentration. Resis tance to photooxidative degradation was enhanced with increase of the hard segment concentration in polyurethane elastomers based on PTMO (1 000). The photooxidative degradation is more prevalent in polyurethane elastomers based on polyether with higher molecular weight PTMO (2000 ), than those based on polyether of molecular weight PTMO (1000), even at higher hard segment concentration. (C) 1996 Elsevier Science Limit ed