Modification of natural rubber by grafing with hydrophilic vinyl monomers

Abstract

Polyisoprene latexes were prepared by conventional emulsion polymerization to obtain seed particles for kinetic studies of surface grafting reactions on these seeds by reaction calorimetry. The effects of the amount of monomer, surfactant concentration, initiator concentration, and reaction temperature on the latex particle size were investigated by photon correlation spectroscopy. The two-component redox-initiation system, cumene hydroperoxide (CHP) and tetraethylene pentamine (TEPA), was used to polymize dimethylaminoethyl methacrylate (DMAEMA) in the presence of synthetic polyisoprene latexes. The modified latex particles are postulated to possess a ‘hairy layer’ of surface grafted poly (DMAEMA) chains formed via an abstraction reaction between cumyloxyl radicals and the isoprene moieties present in the seed polymer. The modified latexes exhibited enhanced colloidal stability to low pH, and PCS showed that the apparent particle size was sensitive to pH. The rate of polymerization was followed by reaction calorimetry. No steady-state polymerization was observed, with a continual increase in the number of propagating chains at all initiator feed rates investigated. The data for particle size and colloidal stability, together with the calorimetric data, are consistent with radical production at the particle surface, and with abstraction near the interface being a rare event. Further, there is evidence that radical production by the redox couple is relatively slow. While this ‘topology-controlled’ reaction is responsible for the formation of the hairy layer and latex stability, the dominant polymerization process appears to be the formation of ungrafted poly(DMAEMA) in the water phase. The grafting of hydrophilic vinyl monomers DMAEMA, dimethylaminoethyl acrylate (DMAEA), and hydroxythyl methacrylate (HEMA) onto natural rubber latex was also carried out by emulsion polymerization using redox initiation. The effects were investigated arising from the type of redox initiator, and concentration of CHP/TEPA, t-butyl hydroperoxide (t-BHP)/TEPA, and K₂S₂O₈/K₂S₂O₅, monomer concentration, and reaction temperature on the conversion, grafting efficiency, water absorption and contact angles of the grafted copolymers films, and colloidal stability of the latexes at low pH. Infrared analysis confirmed that DMAEMA amd DMAEA could be grafted onto natural rubber particles. The hairy layer structure of NR-g-poly(DMAEMA) latex particles was investigated by transmission electron microscopy using positive and negative straining with OsO₄ and phosphotungstic acid, respectively. The glass transition temperature of the grafted natural rubber determined by differential scanning calorimetry was at about -64℃.