Deuterium solid-state quadrupole-echo NMR techniques were used to
probe the dynamics
of bulk and silica-adsorbed methyl-labeled poly(methyl
acrylate)-d
3 (PMA-d
3).
For bulk PMA-d
3, collapse
of the 2H NMR powder pattern indicated a sudden increase in
segmental motion at about 50 °C. This
temperature was 40 °C above the reported glass transition temperature
(T
g) of bulk PMA and was due
to the higher frequencies to which the “NMR”
T
g was sensitive. Surface samples with
different adsorbed
amounts exhibited different behavior at the silica−polymer−air
interface and consisted of superpositions
from different components. At lower temperatures, a mobile
component not found in the bulk
PMA-d
3
was observed while, at higher temperatures, a rigid component still
existed. The mobile component was
attributed to polymer segments near the polymer−air interface and the
rigid component to segments
nearer to the polymer−silica interface. As the adsorbed amount
increased, the relative amounts of mobile
components increased. The results were consistent with an
interface graded in mobility with a broad
distribution in the adsorbed layer. The mobilities were generally
reduced at lower adsorbed amounts.
Deuterium solid-state quadrupole-echo NMR was used to
study the effect of molecular weight
on the segmental dynamics of bulk and adsorbed poly(methyl
acrylate)-d
3 (PMA-d
3).
It was found that
the bulk PMA-d
3 with lower molecular weight was
more mobile and more heterogeneous in motion than
the high molecular weight species. Surface samples with coverages
up to saturation coverage in toluene,
adsorption maximum in toluene (A
m), were
prepared by adsorption of PMA-d
3 with two
different molecular
weights onto silica from different solvent systems. After drying,
the spectra were collected from the
surface species at the solid−air interface. For the adsorbed
polymers, the low molecular weight
PMA-d
3
segments on silica were more rigid than those of the high molecular
weight PMA-d
3. For both
molecular
weights, the surface polymers exhibited a wider range of mobilities
than the bulk polymers.
The interface in composite materials containing an ultrathin layer of poly(methyl acrylate)-d(3) (PMA-d(3)) on silica was studied using deuterium NMR. PMA-d(3) was deposited from solution at saturation coverage from toluene onto silica. The samples were dried and composite samples made by hot pressing the PMA-d(3)/silica samples with hydrogenated polystyrene (PS) and high (HMW) and low (LMW) molecular weight hydrogenated poly(methyl acrylate) (PMA) as the overlayer. The interfacial layers of PMA-d(3) were studied at the air-polymer-silica and polymer-polymer-silica interfaces using deuterium solid-state quadrupole-echo NMR and the results compared to those for the bulk polymer. It was found that for samples at the air-polymer-silica interface, some of the polymer segments in the surface sample had segmental mobility higher than that of the corresponding bulk PMA-d(3) sample at the same temperature. When overcoated with unlabeled polymer, the interfacial polymer at the polymer-polymer-silica interface showed reduced mobility due to the presence of the overlayer. The adsorbed PMA-d(3), in the composite samples, decreased in mobility in the order of LMW-PMA > HMW-PMA > PS. The PS sample caused the greatest reduction in the PMA-d(3) interfacial mobility. The order was consistent with the segmental mobilities of the polymers used for the overlayers. The lower the mobility of the polymer used for the overlayer, the more restricted were the polymer segments in the adsorbed PMA-d(3) layer.
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