Melvin D. Croucher

The preparation of micron-size polymer particles in nonpolar media

Abstract The dispersion polymerization of methyl methacrylate has been investigated using mixtures of carbon tetrachloride and 2,2,4-trimethylpentane as the dispersion medium and polyisobutylene as the steric stabilizer. It was found that as the concentration of carbon tetrachloride in the dispersion medium increased, the particle size increased with monodisperse particles of up to 13 μm in diameter being produced. However, the molecular weight of the resulting polymer particles and the polymerization kinetics decreased with increasing carbon tetrachloride concentration. It is suggested that the locus of the particle polymerization changes as the solvency of the dispersion medium for the resulting polymer is varied with a concomitant change in the size of the final particle.

Stabilization of dispersion polymerization by poly(styrene-b-ethylene oxide) copolymers

Abstract A series of poly(styrene-b-ethylene oxide) copolymers were investigated as stabilizers for the dispersion polymerization of styrene in methanol. The effects of stabilizer molecular weight, stabilizer/anchor ratio and stabilizer concentration on particle diameter and polydispersity were examined. Large (2 μm) monodisperse polystyrene particles were formed when a low molecular weight stabilizer was used, while broad bimodal distributions were found for polystyrene particles stabilized by higher molecular weight copolymeric stabilizers.

Energy transfer studies on polymer membrane films. Materials with variable apparent dimensionality

Abstract Polymer films composed of poly(vinyl acetate) (PVAc, 91.7 mol%) and poly(2-ethylhexyl methacrylate) (PEHMA, 8.3 mol%) (all the PEHMA picient as a graft copolymer) were prepared by solvent casting. The PEHMA phase was labeled with a low concentration (0.7 mol%/PEHMA) of naphthalene groups. Exposure of the films to a pentane solution of 9-anthrylmethyl pivalate (AMP) transferred the AMP molecules exclusively to the PEHMA phase of the material. Direct energy transfer studies showed that the energy transfer rate was proportional to the AMP concentration, and that the process was characterized by an effective dimensionality d = 1.4. This result can be explained in terms of a crossover phenomenon if the PEHMA is present in the film in the form of interconnected long, thin (≈ 50 A diameter) cylinders. Supporting this idea is our finding that swelling agents for PEHMA such as hexadecane are also taken up by the film from pentane solution. Experiments on these materials give d values that increase with increasing hexadecane until d becomes equal to 3.0.

Phosphorescence from covalently labeled nonaqueous dispersions: Insights into the swelling of microdomains: 2. Luminescence from polymer colloids

Abstract Nonaqueous dispersions of polymer colloids were prepared containing 2 and 10 mol% naphthalene groups covalently attached to the poly(methyl methacrylate) core. The dispersions were sterically stabilized by cillia of polyisobutylene chains that were terminally attached to the PMMA. The phosphorescence spectra and phosphorescence decay times of these particles were studied in various media at 77°K. The freeze-dried powder, suspensions of this powder in methanol, and its colloidally stable dispersions in methylcyclohexane were examined. Two models are considered to explain the variation of the triplet lifetimes with the concentration of naphthalene groups. A mass diffusion model considers diffusion as the origin of phosphorescence quenching. A static quenching model cites particle swelling by solvent. The former model yields unrealistic values of the medium viscosity, whereas the lifetimes permit one to calculate the degree of particle swelling in the presence of different solvents.

Luminescence studies on sterically stabilized polymer colloid particles: pyrene excimer formation

Abstract Detailed information about the conformation and dynamics of fluorescently labelled polymer chains free in solution and bound to colloidal sized particles can be obtained using luminescence techniques. Copolymers of pyrenealkyl methacrylate and 2-ethylhexyl methacrylate (EHMA) were used as stabilizers (St-Py) in the dispersion polymerization of vinyl acetate. In this way, poly(vinyl acetate) particles (0.3 μm) labelled in the stabilizer phase with pyrene (SLP) were prepared. Results from steady-state and time-resolved fluorescence studies on solutions of St-Py and dispersions of SLP are compared. In these systems, pyrene excimers are formed with a distribution of rate constants. We find that the mobility and dimensions of particle-bound stabilizer chains are less than the corresponding free stabilizer chains in solution, a consequence of the mechanism of particle synthesis. When the spacer separating pyrene from the stabilizer backbone is increased, excimer formation is facilitated, inferring an increase in stabilizer chain mobility relative to the systems with short spacer units. Studies on particles prepared with a mixture of labelled and unlabelled PEHMA showed that inter-chain interactions in the stabilizer phase are negligible.

Selective flocculation in heterosterically stabilised nonaqueous dispersions

Abstract The incipicient flocculation behaviour of poly(vinyl acetate) particles stabilised by polystyrene and of polyacrylonitrile particles stabilised by polyisobutylene has been investigated as a function of temperature using cyclopentane as the dispersion medium. It was found that the polystyrene stabilished particles flocculated at both an upper and lower critical temperature while the polyisobutylene stabilised particles flocculated only at an upper critical temperature. These critical flocculation temperatures correlated qualitatively with the relevant theta temperature of polysbutyrene and of polyisobutylene when disolved in cyclopentane. When the homosterically stabilised dispersions were mixed together in cyclopentane to give a heterasterically stabilised nonaqueous dispersion it was found that selective flocculation of the latices took place as a function of temperature. This indicates that polymer stabilised particles can distinguish “self” from “non-self” and behave independently under certain conditions. These results are discussed within the theoretical framework that has been developed for heterosterically stabilised nonaqueous dispersions.

Study of polystyrene-poly(ethylene oxide) block copolymer micelles in aqueous solution by size-exclusion chromatography

Abstract Size-exclusion chromatography experiments are reported for aqueous solutions of polystyrene-poly-(ethylene oxide) (PS-PEO) diblock and triblock copolymers. These samples show two peaks: the first peak elutes due to polymer micelles (characterized independently by light scattering), and the second due to individual polymer molecules. Sharp micelle peaks imply slow dynamic exchange. By collecting fractions and examining them by Fourier transform infrared spectrometry we were able to establish that the single molecule peak contained more PEO and less PS than the micelle.

Temperature dependence of excess enthalpies for systems containing normal hexadecane

Excess enthalpies, and heat capacities derived therefrom, have been obtained between 25 and 65 or 75°C at a constant concentration for cyclohexane and octamethylcyclotetrasiloxane mixed with normal hexadecane and with a highly branched C16 isomer, 2,2,4,4,6,8,8-heptamethylnonane, and also forcis-andtrans-decalin mixed withn-C16. ThehE values withn-C16 are positive and much larger than with the branched-C16. They decrease rapidly withT so thatcpE is large and negative. These results imply the presence of orientational order in then-C16, which is destroyed on mixing with the other component and which decreases withT. ThehE fortrans-decalin+n-C16 is much smaller than forcis-decalin+n-C16, and becomes negative with increase ofT. This change of sign, which is unexplained by current theory, is interpreted as due to an interference of the flat, plateliketrans-decalin molecule with the molecular motion of then-C16 chain.

Viscoelastic properties of sterically stabilized nonaqueous dispersions

The viscoelastic properties of a dispersion of poly(vinyl acetate) particles sterically stabilized by poly(2-ethyl hexyl methacrylate) and dispersed in Isopar G have been measured as a function of particle concentration and frequency at ambient temperature.At low particle concentrations, it was found that the loss modulus was larger than the storage modulus, while at high particle concentrations, i.e.,w>0.40, the storage modulus was found to be larger than the loss modulus. This inversion from a viscous to an elastic response as a function of particle concentration is attributable to the change in the configurational entropy of the steric barrier as a result of increasing the concentration of particles in the dispersion.

Effect of free volume on the steric stabilization of nonaqueous latex dispersions

Abstract It has been observed that polyacrylonitrile particles that are sterically stabilized by polyisobutylene flocculate on heating, but not on cooling, when dispersed in a series of normal, branched, and cyclic alkanes. The UCFT values were found to be considerably higher for cyclic alkanes than for the n -alkane isomer, while values for the branched alkanes were found to lie on either side of the UCFT values for the corresponding n -alkane. These UCFT values were found to correlate with the free-volume dissimilarity between the steric stabilizer and the dispersion medium. There was also a qualitative correlation between the UCFT and the 0 temperature associated with the lower critical solution temperature of the stabilizing polymer in solution. The observed correlation between the UCFT and the free volume, and the thermodynamics of sterically stabilized nonaqueous dispersions, is discussed in terms of corresponding states theories of polymer solutions. It is shown that the free-volume dissimilarity contribution to the free energy of interpenetration of these particles is almost entirely an entropic effect. Therefore, both the mechanism of stabilization, i.e., the combinatorial entropy, and the mechanism causing flocculation at the UCFT, i.e., the free-volume dissimilarity, are entropic in origin.