Abstract
The overall structure and thermodynamic interactions that take place in low-polarity ionomer solutions have been investigated via the technique of small-angle neutron scattering for two Na-neutralized carboxy-telechelic polystyrene model ionomer systems that differ in the length of the polymer backbone. The strong interchain association that is evidenced in the viscometric measurements of the ionomers compared to the ester forms is apparent in the scattering profiles of the ionomers that show a characteristic upturn that extends in the low-q regime and increases with increasing polymer concentration. The apparent molecular weight and size of the aggregated particles show a significant increase over a small concentration range, while the ionic content seems to not affect the degree of association in the concentration regime studied. The supramolecular structure of the ionomer solutions is revealed by fitting the scattering data to a scaling law in the intermediate qR g regime that supports a percolation type of behavior for the ionomer clusters for both systems studied. The fractal dimension of the aggregates is 1.76 for both ionomer systems which provides evidence for a rather extended configuration and high polydispersity of the clusters in solution. The molecular weight of the associating particles is observed to rapidly approach infinity with increasing polymer concentration, indicating the approach to a gel transition. The scattering data of the ionomer solutions were fitted to the de Gennes model based on the random-phase approximation theory. The Flory-Huggins interaction parameter of the ionomer solutions determined using this model shows a strong concentration dependence with a quite unexpected increase with decreasing polymer concentration. The thermodynamic interactions in the ionomer solutions were changed dramatically by the association as revealed by a significant decrease of the second virial coefficient.
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