Rational design of polymer colloids

Recent advances in understanding of the fundamental mechanistic events in emulsion polymerization give the potential for rational design of new materials based on polymer colloids. It is now possible to design a new industrial process from first principles, based on well‐understood mechanistic principles. An overview of recent developments in the fundamental science of emulsion polymerization is given, with examples of the application of this knowledge to topologically‐controlled synthesis of novel materials based on natural rubber and polybutadiene seed latexes.     

The growth of polymer colloids

The traditional theoretical approach to emulsion polymerization is extended to include effects due to the size of each polymer latex particle. Specific account can thus be taken of the particle size distribution in considering the growth of the colloid. Coupled partial differential equations are derived to describe the system and shown to reduce to the conventional Smith-Ewart equations under certain limits. Solutions are presented for simple models for the emulsion polymerization of styrene and vinyl acetate.     

The cleaning of polymer colloids

The current state of knowledge of the cleaning of polymer colloids is reviewed with regard to a wide range of cleaning and characterisation techniques. The type, level and quantity of impurities involved with different polymer latex formulations varies widely. Even for similar formulations, differences in the nature and number of functional groups reported are often a consequence of sometimes subtle differences in the cleaning procedures employed. Not only may surface functionality be affected but also monomer and oligomer extraction procedures may lead to morphological changes in the particles. No single technique alone is likely to be able to remove all impurities. Care is needed to avoid the introduction of new impurities from the equipment, materials and water used as well as possible contamination from atmospheric carbon dioxide, bacteria and fungi. These factors also need to be considered in the storage of latex particle standards.     

Functional polymer colloids with ordered interior

Polymer colloids with internal ordering were synthesized using hydrolytic condensation of octadecyl-dimethyl(3-trimethoxysilylpropyl)ammonium chloride (ODMACl) and a mixture of ODMACl and the trisodium salt of the triacetic acid N-(trimethoxysilylpropyl)ethylenediamine (TANED). The structure and morphology of these colloids were studied with small-angle X-ray scattering, transmission electron microscopy, nuclear magnetic resonance, sedimentation in ultracentrifuge; and other methods. When polymer colloids are obtained from a single precursor (ODMACl), their local structure, molecular weight characteristics, and morphology strongly depend on the reaction conditions, while lamellar ordering remains nearly unaffected. Use of a mixture of cationic and anionic silanes (ODMACl and TANED) as precursors in hydrolytic condensation results in novel zwitterionic copolymer colloids with two-dimensional hexagonal packing. Interaction of the ODMACl quaternary ammonium groups with the three carboxy groups of TANED leads to replacement of sodium and chloride ions and formation of gegenions, resulting in a molar ratio ODMACl:TANED = 3:1 (each TANED molecule contains three carboxy groups). Due to their ordered interior, polyODMACl (PODMACl) and PODMACl-TANED colloids can be used as templates for controlled positioning of nanoparticles within these colloids. For example, lamellar ordering controls Pt nanoparticle formation within PODMACl colloids providing Pt nanoparticle alignment within the lamellar structure. Loading of PODMACl-TANED colloids with iron salts followed by pH increase results in the formation of iron oxide nanoparticles located within PODMACl-TANED cylinders.     

A spectrophotometric assay for solid phase primary amino groups

A rapid, sensitive, and convenient spectrophotometric assay was developed for the measurement of amino groups on solid supports. This method is based on the reaction of amino groups of solids with an excess ofo-phthaldialdehyde (OPA) and subsequent quantitative determination of unreacted OPA by reaction with glycine. Four solids possessing variable quantities of amino groups were examined. Results indicate that about 70% of the total surface amino concentration (determined by the microKjeldahl method) are available for ligand attachment. Unlike the spectrophotometric 2,4,6-trinitrobenzenesulfonic acid method, the OPA spectrophotometric assay is more rapid, sensitive, and convenient, and unlike the spectrofluorimetric OPA, it does not require sophisticated instrumentation.     

Probing polymer colloids by Xe NMR

Model aqueous dispersions of polystyrene, poly(methyl methacrylate), poly(n-butyl acrylate) and a statistical copolymer poly(n-butyl acrylate-co-methyl methacrylate) were studied using xenon NMR spectroscopy. The ¹²⁹Xe NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the ¹²⁹Xe spectral time-scale and a single ¹²⁹Xe signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the ¹²⁹Xe spectral time-scale and two ¹²⁹Xe NMR signals can be observed. If the signal of sorbed ¹²⁹Xe is observed, its chemical shift, line width and integral relative to the integral of free ¹²⁹Xe can be used for the characterization of the particle core. The line width of free ¹²⁹Xe provides the residence time of xenon outside the particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle surface.     

Electrically conducting polymer colloids and composites

The preparation, characterization and properties of aqueous dispersions of electrically conducting polymer particles are described. Three types of such particles have been prepared: polyacetylene, polypyrrole and polyaniline. Particles in the size range 50–500 nm (diameter) and with conductivities generally ? 1–10 S cm may be made. Composites of such particles, dispersed in a matrix polymer which is non-conducting, are also described.     

Microdetermination of primary amino groups in amino acids and polypeptides by gas chromatography

Summary A gas chromatographic method has been developed for the microdetermination of primary amino groups in amino acids and polypeptides, permitting the determination of 0.07–0.4 mg of -amino nitrogen with an error less than 0.5% in a period of time not exceeding 15 minutes.     

Prediction of rheological properties in polymer colloids

The rheological properties of concentrated suspensions, containing sterically stabilized polymer colloids, are reviewed. It is shown that the various rheological characteristics are strongly interrelated in such systems. They deviate from Brownian hard sphere behaviour, but these deviations are closely related to the interparticle potential and other colloidal characteristics like particle diffusivity. As a result the rheological properties can be predicted from the colloidal characteristics and vice versa. This is demonstrated by means of available data on well characterized model dispersions. In the experiments volume fraction, particle size, suspending medium and temperature have been changed. Data reduction schemes and scaling laws for these parameters are discussed.     

Coating of nanoparticles bearing amino groups on the surface with hydrophilic HPMA-based polymers

Two hydrophilic polymer systems, multivalent N-(2-hydroxypropyl)methacrylamide-based copolymers bearing thiazolidine-2-thione (TT) reactive groups randomly distributed along the polymer chain and monovalent semitelechelic pHPMA with the TT end group, were designed for surface modification of gene delivery vectors, namely, DNA polyelectrolyte complexes (PECs) and adenoviruses. In this study, the amino group-modified polystyrene latex nanoparticles were selected as a suitable model of a nanoparticulate delivery system bearing NH₂ groups on the surface. The coating process was monitored by changes in molecular weight and hydrodynamic parameters of the nanoparticles by light scattering methods. It was shown that for study of the coating process the model latex particles are more suitable than the original PEC vectors due to better chemical and physical stability of latexes. The results obtained in the model study (reaction conditions, methods of evaluation) suggest an optimal polymer structure and a method of efficient and complete coating of nanoparticle surface well applicable to the real gene delivery vectors.     

SAXS and SANS studies of polymer colloids

The analysis of latex particles by small-angle scattering (small-angle X-ray scattering, SAXS; small-angle neutron scattering, SANS) is reviewed. Small-angle scattering techniques give information on the radial structure of the particles as well as on their spatial correlation. Recent progress in instrumentation allows to extend SANS and SAXS to the q-range of light scattering. Moreover, contrast variation employed in SANS and SAXS studies may lead to an unambiguous determination of the radial scattering length density of the particles in situ, i.e. in suspension. Hence, these techniques are highly valuable for a comprehensive analysis of polymer colloids as shown by the examples discussed herein.     

Synthesis of methacrylate polymer bearing cyanate groups and its chemoselective reaction with amines

A novel reactive polymer containing cyanate groups in the side chain was prepared by free radical polymerization of a cyanate‐containing monomer, 2‐(4‐cyanatophenyl)ethyl methacrylate ( 1 ). The monomer 1 and its polymer, poly[2‐(4‐cyanatophenyl)ethyl methacrylate] (PCPMA), were stable under the air for a long period. The copolymerization of 1 and methyl methacrylate provided the corresponding copolymers with various cyanate contents. The availability of the cyanate‐containing polymers as a reactive polymer was investigated. Model reaction using 4‐cyanatotoluene revealed that a cyanate group reacted with aliphatic amines, whereas no reaction occurred in the presence of water, alcohols, and aromatic amines under mild conditions. Post‐functionalization of PCPMA was demonstrated using aliphatic amines or diamines. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 699–706     

Self-assembly of photonic crystals from polymer colloids

This article reviews recent developments in self-assembly of polymer colloids into colloidal crystals, a good candidate material for photonic crystals. Self-assembly strategy has developed as a facile and efficient method to fabricate colloidal crystals. Much research work has been focused on controlling the morphology and improving the quality, as well as finding applications of the colloidal crystals.     

Self-assembly of doublets from flattened polymer colloids

Bottom-up fabrication methods are used to assemble strong yet flexible colloidal doublets. Part of a spherical particle is flattened, increasing the effective interaction area with another particle having a flat region. In the presence of a moderate ionic strength, the flat region on one particle will preferentially "bond" to a flat region on another particle in a deep (≥10 kT) secondary energy minimum. No external field is applied during the assembly process. Under the right conditions, the flat-flat bonding strength is ≥10× that of a sphere-sphere interaction. Not only can flat-flat bonds be quite strong, but they are expected to remain freely rotatable and flexible, with negligible energy barriers for rotation because particles reside in a deep secondary energy minimum with a ~20-30 nm layer of fluid between the ~1 μm radius particles. We present a controlled technique to flatten the particles at room temperature, the modeling of the interparticle forces for flattened spheres, and the experimental data for the self-assembly of flat-flat doublets.     

One-dimensional diffusion of colloids in polymer solutions

For colloid-polymer systems confined in one-dimensional channels, the diffusion of the colloidal particles is obtained by tracking individual particles using enhanced video microscopy and digital image analysis. For short times, the diffusion is normal, of the Fickian type, with mean-squared displacement varying linearly with time. For long times, however, the mean-squared displacement is found to increase more slowly with time, being proportional to the square root of time, in agreement with the theoretical prediction for diffusion of hard rods in one dimension in which mutual crossing of the particles cannot take place. The crossover from short-time to long-time diffusion is observed and is found to depend on the colloid and polymer concentrations. Unexpectedly, for small polymer-to-colloid size ratios, it is the polymer rather than the colloid concentration which has a leading effect on the colloid diffusion.     

Latent reactive groups unveiled through equilibrium dynamics and exemplified in crosslinking during film formation from aqueous polymer colloids

The concept of using equilibrium dynamics to provide for both protection and unveiling of latent functional groups at appropriate times in aqueous polymer colloid coatings designed for crosslinking only during film formation is introduced; the new functional monomer, 4-hydroxyethylsulfonylstyrene (HESS), readily undergoes emulsion copolymerization with acrylates to form stable latexes, followed by crosslinking by loss of water during film formation.     

Functionalized polymer particles for chiral separation

The synthetic chiral polymer poly(N-acryloyl-S-phenylalanine ethyl ester) was immobilized by grafting to macroporous polymer particles of various composition and structure in a process involving copolymerization of the chiral monomer with residual double bonds present in the macroporous support particles. The support particles were prepared by suspension or micro- suspension polymerization of trimethylolpropane trimethacrylate (TRIM), divinylbenzene or by copolymerization of styrene and TRIM. The maximum amount of immobilized chiral polymer and the mechanical properties of the resulting materials varied with the swelling capacity of the parent support particles. Up to 60% (w/w) of chiral polymer could be immobilized to the pore system of highly cross-linked TRIM particles. The enantioselectivity of the chiral stationary phases increased with increase in the amount of immobilized chiral polymer. The results of studies of porosity and particle size variation during grafting form the basis for a discussion of the structure of the final materials.