Abstract:
A novel method for estimating the upper critical
solution temperature (UCST) of N,N-diethyl-m-toluamide
(DEET)-polyethylene systems was developed. It was validated
using data for the dimethylacetamide (DMA)-alkane
systems which showed that refractive index mixing rules,
linear in volume fraction, can accurately predict mixture
composition for amide-alkane systems. Furthermore,
rescaling the composition descriptor with a single adjustable
parameter proved adequate to address any asymmetry
when modeling the DMA-alkane phase envelopes. This
allowed the translation of measured refractive index cooling
trajectories of DEET-alkane systems into phase diagrams
and facilitated the estimation of the UCST values by
fitting the data with an adjusted composition descriptor
model. For both the DEET- and DMA-alkane systems, linear
behavior of UCST values in either the Flory–Huggins critical
interaction parameter, or the alkane critical temperature,
with increasing alkane molar mass is evident. The UCST
values for polymer diluent systems were estimated by extrapolation using these two complimentary approaches.
For the DEET-polyethylene system, values of 183.4 and
180.1 °C respectively were obtained. Both estimates are
significantly higher than the melting temperature range of
polyethylene. Initial liquid–liquid phase separation is therefore likely to be responsible for the previously reported
microporous microstructure of materials formed from this
binary system.
