A Study on Low Temperature Relaxations of Polyurethane Networks

The low temperature relaxations of amorphous polyurethane networks were investigated by measuring their dynamic mechanical properties over temperature range of -150℃ to 20℃. Two series of crosslinked polyurethanes were prepared by prepolymer technique. In one series, prepolymers based on poly(oxypropylene) glycol (PPG) and 2,4-tolylene diisocyanate (TDI) were crosslinked with p-xylylene glycol (XG) ,,1 4-butanediol(BD), trimethylol propane (TMP), or 4,4' -methylenebis (2-chloroaniline) (MOCA). In the other, prepolymers based on PPG and hexamethylene diisocyanate (HMDI) were crosslinked with XG, BD, or TMP. Three relaxations (α ,β, γ-relaxation) were observed for all samples. The α relaxation is associated with micro-Brownian motion of polymer backbones . The γ relaxation (-110℃～-150℃) is associated with local motion of polymer backbones and with a rotation of the methyl side groups of PPG. The γ relaxation of TMP-cured polyurethanes includes a rotation of the ethyl side groups of TMP. The β relaxation is associated with the motion of crosslinking site. The structure of crosslinking site and crosslinking density infuluence temperature and peak intensity of β relaxation. The relaxation temperatures are as fol lows: -TDI-XG -68℃; -TDI-BD -90℃; -TDI-TMP -70℃; -TDI-MOCA -70℃;/ -HMDI-XG -78℃; -HMDI-BD-80℃; -HMDI-TMP -75℃. The temperature and peak intensity of these relaxations were discussed in connection with the results of swelling measurement