Synthesis and Properties of Highly-Charged Cationic Polyelectrolytes

Polyelectrolytes (PEL) and PEL complexes formed by electrostatic interaction are recognized as compounds with a remarkable potential for a wide field of applications. However, comprehensive knowledge about electrostatic and structural influences on the polymerization process as well as on the complex formation and complex properties is still lacking. Thus, basic research under well-defined conditions is crucial for the improvement of production processes, which yield defined PEL suitable as high-performance process additives and components of innovative PEL complexes. The double-charged monomers, 1,3-bis(N,N,N-trimethylammonium)-2-propylmethacrylate dichloride (di-M) and 1,3-bis(N,N,N-trimethylammonium)-2-propylacrylate dichloride (di-A) served to study the free radical polymerization in the presence of strong electrostatic interactions over a wide range of conditions. PEL complexes of poly(di-M) were electrostatically formed with a series of polyanions in diluted and concentrated solutions and characterized concerning their physical properties. Polymerization. Comparing the homopolymerization of di-M and di-A with the homopolymerization of the mono-charged acryloyloxyethyltrimethylammonium chloride (Q9), more pronounced non-ideal behaviour was identified for the two double-charged monomers. This is reflected by autoacceleration from less than 10% conversion onwards observed and confirmed for di-M and di-A but not for Q9, and by monomer reaction orders much higher for di-M and di-A than for Q9. Autoacceleration was experimentally confirmed by the unusual increase of the degree of polymerization with the conversion and a mathematical model, which considers electrostatic effects, in particular the decreasing ionic strengths when the fully dissociated double-charged monomer polymerizes to polymer chains, on which more than 80% of the counterions are condensed. Such counterion condensation causes an additional viscosity increase. Polymerizations with and without addition of salt confirmed this interpretation. The extension of the initial monomer concentration to low concentrations, [di-M]0< 0.6 mol.L-1, revealed a transition to usual polymerization behaviour but also a negative activation energy for polymerizations performed between 323-338 K. A ceiling temperature of 339 K could be estimated. Pulsed laser-initiated polymerization combined with size-exclusion chromatography (PLP-SEC) was used to determine the propagation rate coefficient kp. Almost no temperature influence on kp was observed at high di-M concentration while a remarkable temperature influence seems to exist when approaching lower di-M concentrations. PEL complex formation. Electrostatic complex formation of the polycation (PC) poly(di-M) with a series of structurally different polyanions (PA) yielded novel PEL complexes. In diluted PEL solutions, a moderate influence of the molar mass of poly(di-M) on the complex stoichiometry (PC/PA) was found. At low molar mass, the stoichiometry was somewhat smaller or almost 1, while at higher molar mass, the stoichiometry approached values up to 1.2. Comparing PA of different molar masses, the stoichiometry had always a higher value for the lower molar mass of the PA. Independent of the stoichiometry, for all isolated PEL complexes (PEC) a positive zeta potential was measured. The stability of the PEC particles was moderate to excellent depending on the chemical structure of the PA and correlating with the zeta potential. Morphology and properties of PEC can be tuned varying the composition. In concentrated PEL solutions, upon mixing of the salt containing PC and PA solutions, strong sedimentation force was applied to obtain compact polyelectrolyte complexes (CoPEC). The properties of CoPEC composed of poly(di-M) and poly(sodium styrene sulfonate) (PSS) were modulated by the molar mass and the mixing ratio of the two components and the ionic strength of the medium. Uniaxial compression testing was used to obtain stress/strain data for the evaluation of the mechanical complex properties. The best properties were obtained when mixing poly(di-M) and PSS with molar masses of 3.5×105 and 2×105 g.mol-1, respectively, in NaCl 2.5 mol.L-1 using a mixing ratio PSS/poly(di-M) of 3. These samples were stable for at least one month. The relaxation results of the poly(di-M)/PSS CoPEC were fit to a “standard solid-type I” model.