FILLER AND PERCOLATION BEHAVIOR OF IONIC AGGREGATES IN STYRENE SODIUMMETHACRYLATE IONOMERS

The dynamic mechanical properties of poly(styrene-co-sodium methacryla te) ionomers were reexamined in detail and the data interpreted in ter ms of filler and percolation concepts using the Eisenberg-Hird-Moore ( EHM) multiplet/cluster model of random ionomers. For this study, sampl es were synthesized over a wider range of ion concentrations as well a s more frequent intervals than in the previous studies. Deconvolutions were performed on the loss tangent data. The glassy moduli of the ion omers were found to be independent of the ion content. There were disc ontinuities in the plots of the slopes of the storage moduli, peak hei ghts, peak positions, and widths at the half-height of the loss tangen t peaks as well as of the activation energies for the glass transition s as a function of ion content. These discontinuities suggest that two morphological changes are involved in the present ionomer system. The first, at 4-6 mol % of ions, involves the formation of a dominant or a continuous phase of the ion-rich or cluster regions and is interpret ed as being associated with the percolation threshold (see below). The second, at approximately 12-14 mol % of ions, possibly involves the d isappearance of continuity of the unclustered ''phase''; between 4 and 12 mol %, two cocontinuous ''phases' are probably encountered. Differ ential scanning calorimetry (DSC) thermograms for the ionomers between 8 and 14 mol % of ions show two glass transition temperatures (T(g)). A linear relation is observed between these two T(g)s and the positio ns of the E'' peaks at 0.3 Hz obtained from dynamic mechanical thermal analysis (DMTA) measurements. In terms of filler concepts, the Guth e quation is applicable in the range of low volume fractions of clusters (<0.3), but a minor modification extends the applicability to 0.45. A pplication of the Halpin-Tsai equation for regular systems to the pres ent ionomer at low ion content suggests that the system consists of mo re or less spherical (at opposed to linear or lamellar) clusters dispe rsed in the matrix phase as filler particles. The mechanical propertie s of ionomers were also interpreted in terms of percolation concepts. At the percolation threshold, i.e., at approximately 5 mol % of ions, the critical exponent and the critical volume fraction of clusters wer e found to be 1.31 and 0.64, respectively. The value of the critical e xponent is in the range of ''universal'' values for conductivity perco lation, but the critical volume fraction is much higher than those for most other systems. This is explained in terms of the difference betw een the nonuniformity of properties of the clusters of the present sys tem and the uniformity of properties in usual percolating species. Fin ally, a novel approach is used to estimate the size of a sodium carbox ylate ion pair, which is found to be 45 X 10(-3) nm3.