Handbook of Solvents - George Wypych - ChemTech - Ventech!

582 S. Bistac, M. Brogly
ture is generally observed for the cast films, except for the ‘chloroform’ sample. An increase
of the β-transition temperature indicates a lower mobility of the acrylate groups compared to
the bulk polymer. However, for chloroform sample, this mobility is increased.
Two different effects appear and two groups of solvents can then be distinguished: the
first group, with chloroform, which induces a higher mobility of both the main chain and the
acrylate group (compared to the bulk polymer), and the second group, including acetone, toluene
and THF which induce also an increase of the mobility of the main chain, but a decrease
of the mobility of the lateral acrylate groups.
The influence of the nature of the solvent on the relaxation temperature can be explained
by analyzing the acid-base properties of the polymer and the solvents. PMMA,
which is a basic polymer can exchange strong acid-base interactions with an acidic solvent,
such as, chloroform. In the solid cast films, acid-base interactions between acrylate groups
and residual chloroform occur: some acrylate groups interact with chloroform molecules
and PMMA/PMMA self-associations partially disappear, leading to a lower value of the
β-transition temperature. The mobility of the lateral acrylate groups interacting with chloroform
molecules is then higher than the mobility of acrylate groups interacting with other
acrylate groups as in bulk PMMA.
Toluene, THF and acetone are described as basic solvents. The residual solvent molecules
can therefore only weakly interact with the acrylate groups. Interactions between
chains are then favored and self-associations between PMMA chains appear. The rotation
of the acrylate groups becomes more difficult compared to the bulk PMMA, probably due to
self-aggregation of some PMMA chains (even if the studied polymer is not stereoregular).
To resume, residual solvent molecules present in solid conventional PMMA films are
able to significantly modify the polymer relaxation properties. The effect of residual solvent
depends strongly on the nature of the solvent, specially its acid-base character.
10.2.4 CONCLUDING REMARKS
These works have shown that acid-basic character of solvent is able to have a major influ-
ence on polymer film properties. Acid-base interactions between stereoregular PMMA and
some solvents can lead to the formation of aggregates, which modify the solution proper-
ties. Elsewhere, residual solvent molecules trapped in solid conventional PMMA films have
an effect on the polymer chains mobility, depending on the acid-base character of the sol-
vent. Both solvent effects, due to acid-base interactions, are able to modify the wetting and
the adhesion properties of the films, but also the mechanical and the durability behavior of
the final film. It is then necessary to take into account, in the choice of a solvent, its
acid-base properties, in addition to its solubility parameter, especially for PMMA.
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