Elastomeric latex domain‐interpenetrating polymer networks: Physical and rheological properties

The combination of rubbery and rigid polymers in a multiphase structure using staged emulsion polymerization has yielded materials with properties ranging from reinforced elastomers to high impact plastics. The many different particle morphologies that result from a two‐stage latex (TSL) polymerization include core/shell, domain, interpenetrating polymer networks (IPN), and various combinations thereof. The sequence of polymerization, crosslinking, grafting, and composition are among the significant parameters that determine the particle morphology. Elastomeric TSL with soft polyacrylates (PA) as the seed particles and polystyrene (PS) as the second stage, with each stage lightly crosslinked, may yield IPN‐microdomain particles. The particle morphology has been elucidated through a combination of microscopy and mechanical property analyses. The significant modulus of elastomeric latex interpenetrating polymer networks (LIPN) results from reinforcement by PS intra‐particle microdomains and their significant tensile strength from a strength forming mechanism of PS inter‐particle microdomains. The increase in the PA seed crosslinking increases the crosslinked PS (xPS) level of molecular mixing with, and grafting via residual unsaturation to, the crosslinked PA (xPA) network and decreases particle deformnability. At higher xPS concentrations the formation of an xPS‐rich shell enhances xPS continuity in the molded material through the partial coalescence of the shells, diminishing the PA continuity, and yielding more PS‐like properties. The submicron lightly crosslinked latex particles with these different morphologies flow as a pseudoplastie material through a particle slippage flow mechanism exhibiting neither a Newtonian plateau nor a yield stress at low shear rates. The deformable lightly crosslinked particles with interchangeable PS ties which disintegrate at elevated temperatures retain their identity and regain their shape at the cessation of shear. The LIPN can be processed using standard thermoplastic methods and machinery, with power law constants and shear insensitive flow activation energies that are similar to those of thermoplastics at high levels of shear. Uncrosslinked PS shells around crosslinked PA seed particles, on the other hand, completely coalesce upon molding to form a continuous thermoplastic PS matrix that may essentially flow through molecular deformation.