Ola Karlsson

Estimating diffusion coefficients for small molecules in polymers and polymer solutions

The diffusion coefficient for small molecules (solvent or monomer) through polymer solutions in the vicinity of the glass transition are known to change by as much as six orders of magnitude with only a small change in polymer concentration. Experimental measurements are difficult in this region and consequently there are data for only a limited number of systems. A rather simple method to estimate these diffusion coefficients for the rubbery, glass transition, and glassy regions as a function of polymer concentration and application temperature is presented. While the method is empirical in nature, it is based on carefully executed experimental studies, sound scaling laws, and agrees extremely well with free volume theories in the rubbery region. The method only requires a knowledge of the pure polymer glass transition temperature in order to estimate the diffusivity of molecules like styrenic and acrylic monomers (molecular weight of approximately 100 g/mol) at any polymer concentration and for temperatures above and below the polymer glass point.

Non-equilibrium particle morphology development in seeded emulsion polymerization. 1: penetration of monomer and radicals as a function of monomer feed rate during second stage polymerization

Abstract Starve feeding of monomers is often used in an attempt to control latex particle morphology, especially when non-equilibrium structures are desired. For the case of a polar seed polymer and a non-polar second stage polymer, we have analyzed the relative probabilities of reaction and diffusion of polymer radicals and monomers as they penetrate the seed particle. The resultant penetration ratios (for polymer radicals and monomers) and fractional penetration values (depth of penetration) correlate well with a number of different non-equilibrium morphologies obtained from a wide variety of experimental reaction conditions. We conclude that the lack of polymer radical penetration is responsible for non-equilibrium core-shell structures for the glassy PMMA seed/PS system, while the styrene monomer easily penetrates the entire particle, even at very slow monomer feed rates. When the polar, low T g PMA is substituted for the PMMA seed, the polymer radicals cannot be excluded from the particle center and an inverted core-shell equilibrium structure is obtained at all monomer feed rates.

Morphology of poly(isoprene‐co‐styrene‐co‐methacrylic acid) latex prepared by two‐stage seeded emulsion polymerization

Heterogeneous latexes were prepared by a two-stage seeded emulsion polymerization process at 80°C using potassium persulfate as the initiator and sodium dodecyl sulfate as the emulsifier. Poly(styrene-co-methacrylic acid) latexes containing varying amounts of methacrylic acid (MAA) were used as seeds. The second-stage polymer was poly(isoprene-co-styrene-co-methacrylic acid). By using different methods for the addition of the MAA and by varying the amount of MAA, the hydrophilicity of the polymer phases could be controlled. The morphologies and size distributions of the latex particles were examined by transmission electron microscopy. The latexes were in all cases unimodal, and had narrow particle size distributions. The particles displayed different morphologies depending on the polymerization conditions and monomer composition. The hydrophilic properties of the two phases in combination with the internal particle viscosity and crosslinking of the second phase during polymerization were found to be the major factors influencing the particle morphology.

Influence of seed polymer molecular weight on polymerization kinetics and particle morphology of structured styrene–butadiene latexes

Heterogeneous film-forming latexes were prepared using two-stage, seeded emulsion polymerization. The polymerization was performed in a calorimetric reactor with a control unit that monitored the reaction rate and controlled the charging rate of the monomers. Three types of styrene seed latexes were prepared at 70°C. The first was an unmodified polystyrene (PS) latex. The second had the molecular weight lowered by the use of carbon tetrachloride (CCl4) as a chain-transfer agent, added at the start of the polymerization. For the third one, divinylbenzene (DVB) was used as a comonomer. DVB was added under starved conditions near the end of the polymerization to achieve crosslinked particle shells and to introduce double bonds as possible grafting sites. The second polymerization step was performed at 80°C as a batch operation in a 200-mL calorimeter reactor. The second-stage polymer was poly(styrene-co-butadiene-co-methacrylic acid) (S/B/MAA). A fixed S/B ratio was used together with varying small amounts of MAA. Particle morphology and particle-size distributions were examined after the second stage using TEM after staining with osmium tetroxide. The particle morphology was found to depend on both the seed composition and the amount of MAA used in the second stage. Molecular weight and crosslinking of the DVB-containing seed influenced the internal particle viscosity, which gave differences in the polymerization rate and the particle morphology. Crosslinking of the second-stage polymer decreased the monomer concentration in the particles, which could be detected as a change in the slope the pressure/conversion curve. This phenomenon was used to indicate the critical conversion for crosslinking of the second-stage polymer.

Viscoelastic properties and film morphology of heterogeneous styrene–butadiene latexes

Heterogeneous carboxylated styrene–butadiene (S/Bu) latexes were prepared by a twostage emulsion polymerization process, using three PS seeds with different molecular weights. The second-stage polymer was a copolymer with a fixed S/Bu ratio of 1 : 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. Morphological studies by transmission electron microscopy (TEM) as well as studies of the viscoelastic properties by mechanical spectroscopy have been performed on films prepared from the latexes. The studies showed that the glass transition temperature, Tg, of the second-stage polymer was considerably affected by copolymerization with MAA. An increase in the MAA content in the second-stage polymer increased the Tg of this phase significantly. Addition of DVB as a crosslinking agent in the preparation of the PS seed phase substantially increased the rubbery moduli of the films, whereas the glass transition temperature of the second-stage polymer was unaffected. On the other hand, the presence of a chain transfer agent reduced the glass transition of the second-stage copolymer containing 1 wt % MAA dramatically, whereas the rubbery modulus was unaffected. When the MAA content was increased to 10 wt % the influence of the MAA monomer had a dominating effect on Tg. Latexes containing 10 wt % MAA had Tg values close to each other, regardless of chain transfer agent present in the second-stage polymerization. It was found that the morphology of the latex particles influenced the rubbery modulus of the films. The presence of irregularly shaped seed particles in samples prepared from a crosslinked PS seed had a considerable reinforcing effect on the films, whereas spherical seed particles originating from core–shell particles had a less reinforcing effect.

Heterogeneous latices as binders in porous particle structures

Heterogeneous carboxylated styrene-butadiene (S/Bu) latices were prepared by a two-stage polymerization process, using three seeds of polystyrene with different molecular weights. The second-stage polymer was a copolymer with a fixed S/Bu-ratio of 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. It has been found that the morphology of the films made from these latices influenced the modulus in the rubbery region of these films. The heterogeneous latices were used as binders in porous structures based on micron-sized kaolin particles. Such structures are typically employed as paper coatings. Polyester substrates were coated with aqueous suspensions containing the kaolin particles and the heterogeneous latex. The coatings were dried at room temperature, which corresponds to the rubbery region of the latex films. It was found that a higher modulus (which is determined here by the morphology of the latex film) in the rubbery region of the films was associated with coating layers with higher porosity, greater light scattering ability, and higher coating gloss. This is interpreted as being the result of a retarded shrinkage of the coating layers during the drying of these structures due to the increase in modulus of the latex films.

Swelling of poly(styrene‐co‐methacrylic acid) latex by isoprene

Carboxylated polystyrene latex was used as seed and isoprene as the second-stage monomer in an inhibited, seeded emulsion polymerization recipe for studies of monomer swelling kinetics at 80°C during interval III of an emulsion polymerization. The isoprene was added to the reactor in small portions using a syringe, and changes in the reactor pressure were continuously measured. Isoprene was added until a free liquid monomer phase was formed; that was, interval II was reached, as indicated by no further pressure increase upon the addition of more monomer. When the observed pressure increment, Opi, per unit isoprene added was plotted as a function of the volume fraction of polymer in the latex particles, vp, the graph could be divided into 3 domains. The break points in the Opi curve could, in an analogous emulsion polymerization, be identified as the glass transition temperature for the polymer, the so-called gel point in interval III and the onset of interval III. In the second domain, where the vp was between the glass transition temperature, Tg, for the seed polymer and the gel point, the value of Opi decreased significantly with increasing monomer concentration in the latex particles. This was due to the entropy of mixing and the monomer acting as a plasticizer in the seed polymer. The rate of sorption of monomer to the latex particles was low at high values of vp. It then increased rapidly with increasing monomer concentrations in the latex particles, [M]p, and a maximum was observed in domain 2. At lower values of vp the sorption rate decreased in domain 3 and finally became zero as the free liquid monomer phase started to form. Results from batch polymerization suggested that the rate of diffusion of adsorbed monomer and oligo radicals into the particles was retarded. A simplified form of the Vanzo equation was used to estimate the monomer partitioning. It predicted too high a value of [M]p, especially in domain 2 of the swelling process.