Synthesis and characterization of polyelectrolytes and aqueous latices

Abstract

<p>Methods for molecular weight characterization of polyelectrolytes, poly(acrylamide-co-sodium acrylate) by viscometry and gel permeation chromatography have been developed based on the use of fractionated polyacrylamide with known average molecular weights. A set of Mark-Houwink equations for the copolymer with various acrylate content has been established. With the viscosity measurements, these equations can be used to calculate the molecular weights of the copolymer with known composition, and to estimate the acrylate content when the weight average molecular weight of the polymer is known. The universal calibration method has been proven valid for poly(acrylamide-co-sodium acrylate) using nonionic polyacrylamide standards. The correlation of elution volume shift with the content of ionic groups has been found, from which the molecular weight of the copolymer can be estimated by gel permeation chromatography. Heterogeneous free-radical polymerization of N-isopropylacrylamide (NIPAM) and NIPAM with N,N'-methylene bisacrylamide (BAM) has been conducted in aqueous media in the presence of low levels of sodium dodecyl sulfate (SDS). The influence of temperature, BAM level, SDS level, pH and ionic strength on the polymerization rate, particle size and size distribution, and particle concentration has been investigated. Studies on the formation mechanism of latex particles of poly(NIPAM) and poly(NIPAM/BAM) have been carried out by measuring the kinetics of polymerization and the micro-structure of the particles. A mechanism of homogeneous nucleation (dispersant-limited agglomeration) and dispersion polymerization has been proposed. The microstructure of the latex particles has been studied from the swelling extent of the particles at various conversion. It is found that the particles have lowest swelling at early stage and have higher swelling at later stage. This has been interpreted as that the cross-linking density changes along the radius of the particles. In addition to the heterogeneity within the particles, the density distribution among the particles has been observed from the comparison of the particle size distribution by dynamic light scattering and that by disk centrifuge. The interaction of NIPAM polymers with SDS has been investigated by conductometry and turbidimetry. It has been found that the critical micelle concentration (CMC) of SDS increases with the concentration of NIPAM polymers, and that the cloud point of aqueous solutions of NIPAM polymers is elevated in the presence of SDS.</p>