Menas S. Vratsanos

Effects of compatibilizing agents in poly(n-butyl acrylate)/poly(methyl methacrylate) composite latexes

Graft copolymers with poly(n-butyl acrylate) (PBA) backbones and poly(methyl methacrylate) (PMMA) macromonomer side chains are used as compatibilizing agents for PBA/PMMA composite latexes. The composite latexes are prepared by seeded emulsion polymerization of methyl methacrylate (MMA) in the presence of PBA particles. Graft copolymers were already incorporated into the PBA particles prior to using these particles as seed via miniemulsion (co)polymerization of n-butyl acrylate (BA) in the presence of the macromonomers. Comparison between size averages of composite and seed particles indicates no secondary nucleation of MMA during seeded emulsion polymerization. Transmission electron microscopy (TEM) observations of composite particles show the dependence of particle morphologies with the amount of macromonomer (i.e., mole ratio of macromonomer to BA and molecular weight of macromonomer) in seed latex. The more uniform coverage with the higher amount of macromonomer suggests that graft copolymers decrease the interfacial tension between core and shell layers in the composite particles. Dynamic mechanical analysis of composite latex films indicates the existence of an interphase region between PBA and PMMA. The dynamic mechanical properties of these films are related to the morphology of the composite particles, the arrangement of phases in the films, and the volume of the interphase polymer.

Mechanical characterization of vinyl acetate based emulsion polymer blends

Emulsion blends comprise an important commercial area of polymer blend utility. Surprisingly, the fundamental study of emulsion blends is rarely noted in the literature. This study investigates emulsion blends of poly(vinyl acetate) (PVAc) and vinyl acetate-ethylene copolymers (VAE), where both components employ poly(vinyl alcohol) (PVOH) as the protective colloid. PVOH comprises the continuous phase in the emulsion cast films for both the individual components and the blends. This provides an example whereby excellent adhesion can be expected between the particles comprising the blend. The combination of low Tg/high Tg emulsion blends has been noted to be of interest, and the PVAc/VAE emulsion blends noted here offer an excellent model to study. The PVAc/VAE blends protected with PVOH exhibit poor mechanical compatibility even though there is good adhesion. Conventional theory based on polymer/filler combinations predicts a rapid loss in elongation as filler content increases if excellent adhesion is observed. The PVAc/VAE blends (where PVAc is the filler) also exhibit similar behavior. This result implies excellent adhesion may not be desired where a compliance mismatch occurs for emulsion blends. The polymer/filler theories do not properly predict PVAc/VAE blend tensile strength results. A newer approach termed the equivalent box model (EBM) employing percolation theory agrees well with experimental results. Melt mixing of the low/high compliance PVAc/VAE emulsion blends yields a significant improvement in mechanical compatibility. This indicates that a heterogeneous mixture of the same components yields better mechanical results than an array of particles with excellent adhesion between the particles.

Model acrylate-terminated urethane blends in toughened epoxies: a morphology and stress relaxation study

Improved toughness of heat-cured epoxies is readily achieved through chemical incorporation of a flexible moiety that phase-separates from the epoxy matrix during thermal cure. Many commercial flexibilizers are inherently polydisperse, which yields dispersions composed of flexibilizer chains of variable length. In this work, we examine the effect of flexibilizer polydispersity on the morphology and stress relaxation behavior of a commercial epoxy. This is achieved by systematically varying the composition of binary and ternary flexibilizer blends of monodisperse acrylate-terminated urethanes differing in molecular weight. Field-emission scanning electron micrographs of fracture surfaces permit quantification of dispersion sizes as a function of blend composition. Tensile stress relaxation data from three-point bend tests performed under isothermal conditions are analyzed in terms of a biexponential model to discern fast and slow characteristic relaxation times.

Phase-separation studies of heat-cured ATU-flexibilized epoxies

Abstract Chemical incorporation of a flexible molecule into a heat-cured epoxy offers an important route by which to toughen the epoxy and tailor its ultimate material properties. Acrylate-terminated urethane (ATU) flexibilizers containing polypropylene glycol have been employed in the present work to modify epoxies composed of a diglycidyl ether of bisphenol-A and bis(4-aminocyclohexyl) methane. Differential scanning calorimetry, dynamic mechanical analysis and electron microscopy reveal that the flexibilizer and epoxy undergo phase separation during cure when the flexibilizer loading or molecular weight is sufficiently high. Partial phase miscibility, deduced from both glass transition temperature shifts and stress—relaxation behaviour, is interpreted here in light of thermodynamic and free-volume considerations.

Novel ionomers based on blends of ethylene-acrylic acid copolymers with poly(vinyl amine)

The polymerization of N-vinyl formamide followed by hydrolysis yields a linear, water-soluble poly(vinyl amine). The high concentration of pendant primary amine groups leads to a polymer with an interesting set of properties. Complexation with water-soluble anionic polyelectrolytes in water solutions leads to a highly water-insoluble material. The study described herein investigated the phase behavior/properties of melt blends of poly(vinyl amine) with ethylene-acrylic acid (EAA) copolymers of less than 10 wt % acrylic acid. The calorimetric and dynamic mechanical analyses of the resultant blends show that the vinyl amine groups are accessible to the acrylic acid groups of the copolymers and the major property changes occur up to the stoichiometric addition of vinyl amine/acrylic acid. At higher levels of vinyl amine (vinyl amine/acrylic acid mol ratio > 4), additional poly(vinyl amine) forms a separate phase. The mechanical, dynamic mechanical, and calorimetric properties of these blends below the stoichiometric ratio show analogous trends as with typical alkali/alkaline metal neutralization. These characteristics relative to the base EAA include improved transparency, lower melting and crystallization temperature, lower level of crystallinity, and increased modulus and strength. The emergence of the β transition in dynamic mechanical testing is pronounced with these blends (as with alkali/alkaline metal neutralization), indicative of microphase separation of the amorphous phase into ionic-rich and ionic-depleted regions. A rubbery modulus plateau for the blends exists above the polyethylene melting point, demonstrating ionic crosslinking. Above 150°C exposure, further modulus increases occur presumably due to amide formation. This study demonstrates that the highly polar poly(vinyl amine) can interact with acrylic acid units in an EAA copolymer comprised predominately of polyethylene (>90 wt %). The thermodynamic driving force favoring ionic association overrides the highly unfavorable difference in composition.

Effects of colloidal stabilizer on vinyl acetate-ethylene copolymer emulsions and films

For vinyl acetate-ethylene (VAE) emulsion polymerization, a comparison of stabilization mechanisms and their effect upon latex and film properties was made in which the different stabilizers were poly(vinyl alcohol) (PVOH), alkylphenol ethoxylate (APE) surfactants, and a urethane linked poly(ethylene glycol) (PEG) based polymer. PVOH stabilized VAE emulsions possessed high viscosity and no freeze-thaw stability while the films were actually continuous in PVOH, much of which was due to the high content of PVOH in the continuous aqueous phase. APE stabilized VAE latexes were also of higher viscosity and the surfactants were miscible in the VAE continuous films. PEG-based stabilization resulted in emulsions and films which were largely affected by the hydrophobicity of the latex (amount of ethylene incorporated into the copolymer).