CHROMOPHORE-FUNCTIONALIZED GLASSY-POLYMERS WITH LARGE 2ND-ORDER NONLINEAR-OPTICAL RESPONSES - TRANSIENT DYNAMICS AND LOCAL MICROSTRUCTURE OF POLY(P-HYDROXYSTYRENES) AS ASSESSED BY IN-SITU 2ND-HARMONIC GENERATION TECHNIQUES
Ma. Firestone et al., CHROMOPHORE-FUNCTIONALIZED GLASSY-POLYMERS WITH LARGE 2ND-ORDER NONLINEAR-OPTICAL RESPONSES - TRANSIENT DYNAMICS AND LOCAL MICROSTRUCTURE OF POLY(P-HYDROXYSTYRENES) AS ASSESSED BY IN-SITU 2ND-HARMONIC GENERATION TECHNIQUES, Macromolecules, 28(7), 1995, pp. 2260-2269
Microstructural relaxation of thin films of the poled, chromophore-fun
ctionalized amorphous polymer 4-nitrophenyl)-(S)-prolinoxypoly(p-hydro
xystyrene) has been studied by in-situ second harmonic generation (SHG
) techniques. The temporal and temperature dependence of the SHG inten
sity decay has been analyzed as a function of poling and processing pa
rameters within the framework of the Kohlrausch-Williams-Watts (KWW ''
stretched exponential'') model. The average SHG relaxation time, tau,
increases rapidly upon reduction of the applied poling field strength,
with increasing poling time (physical aging), and with decreasing fil
m temperature. The other KWW parameter, beta, which reflects the distr
ibution of relaxation times, decreases (the distribution broadens) mod
erately with increases in the applied electric field strength and stro
ngly with increases in poling time (physical aging). The observed valu
e of beta increases (the distribution narrows) with increasing film te
mperature. These trends and the variation in KWW parameters yield info
rmation regarding reorientation dynamics of the tethered chromophore m
olecules within the polymer matrix and thus on the nature of the syste
m subspace which is explored during relaxation. Both parameters reveal
a strikingly narrower distribution of relaxation times/reduced rotati
onal mobility versus chromophore-doped, ''guest-host'' systems and cla
ssify the present materials as Angell ''intermediate'' glasses. The te
mperature dependence of the second harmonic signal decay after poling
field cessation can be divided into two distinct regions: (i) above T-
g, where the dynamics are characteristically nonlinear and best descri
bed by the Williams-Landel-Ferry (WLF) equation; (ii) below T-g, where
the behavior is linear and modeled adequately by the Arrhenius equati
on. Analysis of the growth of the second harmonic signal as the poling
field is applied yields a similar picture; however, limiting SHG valu
es at temperatures significantly above T-g appear to be influenced by
both thermal disruption of chromophore alignment and ion conduction/sp
ace charge effects.