Structural transitions and free-volume properties of PMMA polymer studied by positron annihilation

Positron lifetime and Doppler broadening of annihilation radiations have been measured for PMMA polymer in temperature range from 20 C deg to 150 C deg. The O-Ps annihilation and Doppler broadening data consistently show the structural changes in PMMA. The transitions at 48 C deg and 108 C deg are attributed to the β transition and glass transition of PMMA respectively. An anomalous change of o-Ps lifetime and intensity around 84 C deg is found to be the result of the emergence among some free volume voids. The results of Doppler broadening measurement also give the information about the variations of degree of structural order. The temperature dependences of size and concentration of free volume in PMMA are estimated from the experimental o-Ps annihilation parameters