Study On The Rigid-rod Polyelectrolyte Molecular Composites

Since the presentation of the concept of molecular composite, a variety of polymeric composite systems with rigid-rod polymers as the reinforcing agents and flexible polymers as the matrices have been produced for evaluation. Molecular composites exhibit advantages of superior mechanical properties and processability. The major obstacle for the production of true molecular composites is to molecularly disperse the stiff, rigid-rod polymers in the flexible matrices, because of the unfavorable enthalpy of mixing even for polymers with similar chemical structure. To resolve this problem, researchers have synthesized the copolymers of the rigid and flexible polymers or introduced certain interactions, such as ionic, acid-base interaction and hydrogen bonding between the two components. In this dissertation, three rigid-rod polyelectrolytes, i.e. poly(p-sulfophenylene terephthalamide) (sPPTA) and poly(2,2’-disulfonyl-4,4’-benzidine terephthalamide) (PBDT) and a sulfonated polyschiff base (PSB) were synthesized and used to fabricate molecular composites with poly (vinyl alcohol) (PVA) and sodium alginate (SA) as the matrices. The dispersion of the rigid-rod polyelectrolytes in the flexible matrices and mechanical properties of the composites were investigated.sPPTA and PBDT were synthesized by interfacial polymerization and PVA/sPPTA, PVA/PBDT and SA/PBDT composites were prepared by a green and easy-to-scale-up water casting method. Influence of rigid-rod polyelectrolytes on the microstructure and properties of the molecular composites was systematically investigated. Wide-angle X-ray diffraction patterns and electron microscopy show that sPPTA disperses well at low content. The PVA/sPPTA composites exhibit both high strength and high ductility. When sPPTA content is 5 wt%, the PVA/sPPTA composite exhibits the best mechanical properties, with a tensile strength of 169±13 MPa,54% higher than that of neat PVA (110±10 MPa). The reinforcement factor is even superior to that of multiwalled carbon nanotubes, vapor grown carbon fibers and nanodiamonds previously reported for the reinforcement of PVA nanocomposites. However, PBDT shows self-aggregation in the matrix of PVA and SA. The tensile strength of PVA/PBDT-5 wt% is 126±8 MPa, which is 15% higher than that of neat PVA. SA/PBDT composites also exhibit high tensile strength, for example, SA/PBDT-10 wt% has the tensile strength of 94±10 MPa, which is 67% higher than neat SA.Sulfonated polyschiff base (PSB) was synthesized by interfacial polymerization and PVA/PSB molecular composites were prepared by water casting. Influence of PSB on the microstructure and properties of the PVA was systematically investigated. PSB disperses uniformly in PVA and no self-aggregation of PSB was observable at relatively low PSB content. The tensile strength of PVA/PSB-13 wt% is 154±8 MPa, which is is 40% higher than that of neat PVA film. When the content increased to 15 wt%, self-aggregation of PSB occurs can be seen and the mechanical properties begin to decrease.