Relaxation Processes in Nonlinear Optical Polyimide Side-Chain Polymers

Abstract

Relaxation processes in nonlinear optical modified polyimide polymers with side-chain azo chromophores having glass transition temperatures in the range of 140 < T g < 170 °C have been studied. The relaxational mechanisms of the side-chain chromophores in these polymers have been investigated above and below the glass transition by second-harmonic decay, dielectric relaxation, and differential scanning calorimetry measurements. The nonexponential relaxation in both the time and frequency domains was modeled by the Kohlrausch−Williams−Watts (KWW) function. The nonlinear relaxational behavior of these polymers can be modeled in terms of the Tool−Narayanaswamy description of glassy state behavior. It allows for the nonlinear extension of the liquid equilibrium state behavior into and below the glass transition region with an accurate prediction of the relaxation times over more than 15 orders of magnitude in time. Time−temperature scaling of the relaxation times with (T g − T)/T as the relevant scaling parameter is observed below the glass transition.

Effect of pressure and temperature on chromophore reorientation in a side-chain nonlinear optical polymer

“…The DR1-MMA result agrees with the recent work of Kaatz et. al., 12 where they observed that, deep in the glass, chromophore reorientation is more coupled to ␣-type relaxation processes in side-chain NLO polymers than in guest-host systems.…”

“…Many researchers have performed relaxation studies [3][4][5][6][7][8][9][10][11][12] at temperatures near and above the glass transition temperature (T g ) that show a direct connection between chromophore reorientation and the long-range ␣-type motion of the polymer backbone. However, other experiments performed at temperatures below the T g show that chromophore reorientation appears to couple to secondary relaxations in guest-host systems.…”

Stretched exponential relaxation and ac universality in disordered dielectrics

“…3,5,17 Here, we utilize an expansion good for higher frequencies, which goes in inverse powers of z:…”

Second-order non-linear optical polymers

“…Heldmann et al 14,15) and Firestone et al 16) studied relaxation of poled order in rigid-like polymers with chromophores rigidly attached to the main-chain. Kaatz et al 17) , Winkelhahn et al 18) and Burland et al 10) have correlated orientational behavior above and below T g quantitatively using established theories for relaxation in glass forming materials. Other groups have thoroughly investigated the typical non-exponential relaxation processes in NLO-polymers below T g 10,19) .…”