An investigation of spatially heterogeneous mobility domains for liquids in the glass transition region
Abstract
Recent experimental evidence has shown that amorphous polymers are motionally heterogeneous in the glassy state and the glass transition region. This has raised the possibility that amorphous polymers are also spatially heterogeneous on the sub 1000A length scale. If true, this hypothesis would have profound implications about current models of amorphous polymer relaxation, which generally assume that amorphous polymers are homogeneous and thermorheologically simple. In order to test this hypothesis we have performed solid state nuclear magnetic resonance (NMR) spin diffusion experiments from 70$\sp\circ$C to 130$\sp\circ$C on phenyl ring deuterated polystyrene ($T\sb{g}$ = 105$\sp\circ$C), in which only 1-D intramolecular diffusion along the backbone occurs. The results indicate that polystyrene is indeed phase separated in the glass transition region with the motionally faster region having a length scale of approximately 30A. In addition a stochastic model of volume relaxation has been developed which explicitly takes into account the existence of spatial heterogeneity. The model assumes that an amorphous material consists of an ensemble of isotropic, isothermal, and non-interacting regions of characteristic volume $V\sb{r}$. The time development of the specific volume of each region is assumed to obey a stochastic single relaxation time version of the well known KAHR model. This inherently thermorheologically complex model is shown to be successful in qualitatively describing all of the phenomena associated with volume relaxation. Specifically the model qualitatively describes (i) volume relaxation which is nonlinear with respect to the magnitude and sign of the volume deviation, (ii) the expansion gap in the effective relaxation time for up-jumps in temperature, (iii) the memory effect in which a maximum in volume occurs after an up-jump to a temperature intermediate to an annealing temperature and the initial temperature, (iv) the shape of the volume recovery curve following the final up-jump in a two step thermal history in which the final and initial temperatures are the same, and (v) the complex time development of the microscopic mean square volume fluctuation $\langle \delta V\sp2\rangle = \langle (V - \langle V\rangle)\sp2\rangle$ after step changes in temperature which has been recently observed in some inorganic glass formers by light scattering experiments.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering|Materials science
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