Deformation modes were examined on strained thin films of a series of molecular composites containing ionically modified rodlike molecules of polyp-phenylene terephthalamide) (PPTA) dispersed in a polar polymer matrix. The rigid molecules were a modified form of PPTA in which the H atom of the amide group was replaced, on 30 mol % of the monomer units, by an ionic propane sulfonate group. The polar polymer matrix of these composites was the flexible-coil polymer, poly(4-vinylpyridine). Ionic interactions between the two components increased the effective entanglement strand density and produced changes in the deformation modes. The observed changes were dependent on the relative concentration of the two components and on the nature of the counterion. With K+ as the counterion, the induced deformation mode changed from pure crazing, as in the matrix polymer, to combined crazing and shear deformation at 5 wt % of the ionic polymer and to essentially pure shear deformation as the concentration increased to 15 wt %. However, when Ca2+ was the counterion, pure shear deformation developed at a concentration of only 5 wt %. This effect was attributed to a greater ionic interaction and to a higher effective strand density of the composites, when monovalent K+ was replaced by divalent Ca2+. (C) 2003 Wiley Periodicals, Inc.