Retardation of Rotational Reorientation Dynamics in Polymers near the Glass Transition: a Novel Study over Eleven Decades in Time using Second-Order Nonlinear 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, Tg. 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 Tg) and covalent attachment to the polymer (above and below Tg). Physical aging for several days near Tg 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 Tg 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.