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Tightly Bound PMMA on Silica Has Reduced Heat Capacities
journal contribution
posted on 2019-08-23, 16:38 authored by Bal K. Khatiwada, Frank D. BlumThe heat capacities
of very small adsorbed amounts of poly(methyl
methacrylate) on high-surface-area silica (Cab-O-Sil) were measured
using temperature-modulated differential scanning calorimetry (TMDSC)
using a quasi-isothermal method and interpreted via different models.
The composition-dependent heat capacities of the adsorbed samples
were measurably less than those predicted with a simple mixture model.
A two-state model, composed of tightly and loosely bound polymer,
fits the data better with heat capacities of the tightly bound polymer
found to be 70–80% (glassy region) and 70–94% (rubbery
region) of that of the bulk polymer at the same temperatures. The
amount of tightly bound polymer was estimated to be about 1.2 mg/m2 (about 1 nm thickness) in both the glassy and rubbery regions,
consistent with heat flow measurements. The data sets were also extensive
enough to model them with a more detailed layered gradient model,
including a nonzero heat capacity for the polymer at zero adsorbed
amount, which increased based on an exponential growth function to
bulk polymer value of the heat capacity away from the surface. More
importantly, this gradient model mimicked the experimental dependence
on adsorbed amounts in the tightly bound adsorbed amount region (approximately
1 mg/m2). This model provided, for the first time, an experimental
estimate of the heat capacity of the polymer adsorbed closest to the
surface. The fractional heat capacity of the adsorbed polymer closest
to the silica surface, relative to bulk polymer, increased with temperature
from 0.3 (well below) to 0.8 (well above the bulk Tg). It was also possible to estimate the exponential growth
parameter of the development from the initial heat capacities to the
bulk heat capacity as 0.4 to 0.6 mg/m2, identifying a distance
scale (0.3 to 0.5 nm) consistent with the notion of a transition from
tightly bound to loosely bound polymer.