- W. Chassé, K. Saalwächter, J.-U. Sommer.
Thermodynamics of swollen Networks As reflected in segmental orientation correlations.
Macromolecules 45, 5513-5523 (2012).
We show that segmental orientation correlations, arising from network constraints and reflected in
NMR observables such as the apparent proton residual dipolar coupling constant (RDCC),
are sensitive to the thermodynamic state of a swollen network. The RDCC is proportional to a
dynamic tensor order parameter of the network chain, which depends upon the fluctuation statistics of a
network chain subject to swelling-induced stretching and excluded-volume interactions. The correlation of its
value at swelling equilibrium with the corresponding degree of swelling for different networks directly reflects the
value of the thermodynamic interaction parameter. A description of the phenomenon based on scaling theory allows for
conclusions on the limited validity of the classical Flory–Rehner (FR) model for the good-solvent case.
Our study is supplemented by temperature-dependent swelling experiments of different PDMS networks in various solvents,
analyzed via a novel construction of solvent-independent swelling master curves, which also emphasize deviations
from the FR model for good solvents and further confirm previous reports on concentration- or cross-linking-dependent
interaction parameters. We rationalize the failure of the FR model for the good-solvent case in terms of the ratio
of the network correlation length and the thermal blob size.