Bulk mechanical properties and microstructure of a model blend (homoblend), made of polystyrene-co-styrenesulfonic acid) (SPS) and poly(styrene-co-4-vinylpyridine) (SVP), were investigated by tensile measurements and modulated differential scanning calorimetry (MDSC). The mechanical properties exhibited synergism: tensile strength and toughness values of the blends were higher than those expected based on the rule of mixtures. The results were attributed to the formation of ionic cross-links due to intermolecular ion-ion interactions between pyridinium cations and sulfonate anions, which arose from proton transfer from sulfonic acid groups to pyridine groups upon blending. MDSC detected only one T-g for the SPS/SVP blends, indicating that the blends were unclustered. However, after annealing the blends at 150 degreesC for 24 h, two T-g's were detected, reflecting microphase separation (formation of a cluster phase and a matrix phase) due to aggregation of ionic groups during annealing. (C) 2003 Elsevier Ltd. All rights reserved.