Retardation of rotational reorientation dynamics in polymers near the glass transition: a novel study over eleven decades in time using second-order non-linear optics

A novel protocol has been developed using second harmonic generation to quantify the rotational, reorientation dynamics of second-order non-linear optical chromophores in amorphous polymers. Dynamics are monitored over 11-12 decades in time, from microseconds onwards, allowing characterization both well above and below the glass transition temperature, T_g. The time dependence of the orientational component of the second order macroscopic susceptibility, χ⁽²⁾, can be represented by a Kohlrausch-Williams-Watts equation from which average rotational reorientational time constants, 〈τ〉, are determined. For the Disperse Red 1-poly(isobutyl methacrylate) (PIBMA) system studied here, 〈τ〉 is shown to be coupled to the α-relaxation dynamics of PIBMA. Retardation of these dynamics is investigated by physical aging (below T_g) and covalent attachment to the polymer (above and below T_g). Physical aging for several days near T_g can result in a two orders of magnitude increase in 〈τ〉, above the value obtained for the quenched state. Covalent attachment of Disperse Red 1 chromophores also increases 〈τ〉 relative to the doped-chromophore systems; this effect is shown to be due to the increase in T_g for the cases of 1 and 6 mol% attachment. The implications of these effects for the design of temporally stable SHG polymeric materials are discussed.