Network Formation In Free-radical Copolymerization Research Articles

Kinetics of polyelectrolyte network formation in free-radical copolymerization of acrylic acid and bisacrylamide

Recently, a research/development program has been initiated to investigate the kinetics of synthesis, characterization and applications of polyelectrolyte networks. The research on crosslinking involves both theoretical development and experimentation. Herein, is provided a summary of this work. In the experimental polymerization done to date, acrylic acid (AA)/N.N'-methylenebisacrylamide (BAM) was studied in considerable detail. The polymerization conditions were: temperature, 50°C; initial monomer concentration, 5 wt%, of which 1.0 mol% is BAM; K2S2O8(KPS) as the initiator, 10−3 mol/L; pH range, 1 − 13; sodium chloride concentrations up to 3.1 mol/L. Measurements included: monomer conversion, polymer composition, sol/gel fraction, swelling ratio, and the densities of primary cyclization, secondary cyclization and crosslinking. It was found that the effect of polymerization parameters on the resulting polymer network microstructure was dramatic, and in particular, the pH and ionic strength of the reaction medium were important parameters. In the theoretical studies, the Tobita-Hamielec kinetic gelation model was extended to incorporate the concept of ion pair interaction and the divinyl loop formation. The system was treated as a multi-component polymerization of acrylic acid, acrylate ion, acrylate ion pair and bisacrylamide. The model permits one to investigate the development of the crosslinking density distribution among primary polymer chains during the course of polymerization as a function of pH and ionic strength.

Computer simulation of network formation in free-radical crosslinking copolymerization

Abstract A new Monte Carlo simulation model for the kinetics of network formation in free-radical copolymerization of vinyl and divinyl monomers is used to investigate various important property changes during polymerization, such as the whole molecular weight distribution profile, the distribution of crosslink points among various polymer molecules, weight fraction of gel and sol as well as the spatial distribution of the crosslinked polymer chains. This simulation model does not assume a finite reaction system as an approximation for the infinite system, but employs a sampling technique in which polymers are sampled from an infinite number of polymer molecules in the reaction system. The structure of each crosslinked polymer is simulated based on the crosslinking density distribution of the primary polymer molecules that results from a kinetically controlled network formation. The proposed simulation method is quite general and can be applied to various complex polymerization reactions that involve crosslinking and branching in a kinetically controlled system irrespective of reactor types used.

Kinetics of network formation in free-radical crosslinking copolymerization

A new kinetic theory for network formation in free-radical copolymerization of vinyl and divinyl monomers is used to investigate various important property changes in the postgelation period, such as the weight fraction of gel, the molecular weight distribution, the cross-linking density within sol and gel fractions, the partition of the unreacted pendant double bonds between sol and gel, etc. This theory is based on the cross-linking density distribution among primary polymer molecules, and the history of the generated network structure is fully accounted for. It is shown clearly that the deviation from the Mory-Stockmayer theory becomes significant as the variance of the cross-linking density distribution increases

Influence of cross-link density distribution on network formation in free-radical copolymerization of vinyl/divinyl monomers

The influence of the cross-link density distribution on polymer network formation in free-radical copolymerization of vinyl/divinyl monomers was investigated using the Tobita-Hamielec kinetic model coupled with the Macosko-Miller statistical gelation model. The properties investigated were molecular weight development of sol, the gelation point, the sol/gel fraction, and properties of primary polymer chains. Errors involved in neglecting the cross-link density distribution (or assuming a zero variance for the distribution) were correlated with an index of distribution variance in the gelation equations

Crosslinking kinetics in polyacrylamide networks

Kinetics of network formation in free-radical copolymerization of acrylamide and N, N′-methylenebis-acrylamide in aqueous solution (56.6 g comonomer per litre) at 25°C have been studied. It was found that as high as 80% of pendant double bonds are consumed immediately on polymerization and are wasted in primary cyclization. Primary cyclization is responsible for the delay in the onset of gelation at low mole fractions of divinyl monomer. The effects of decreased reactivity of pendant double bonds and secondary cyclization become significant as the mole fraction of divinyl monomer increases. The decreased reaction rate between huge molecules due to steric hindrance in the pre-gelation period contributes to microgel formation, and consequently to formation of spatially inhomogeneous networks.