The kinetics of the attachment of polymer chains to reactive latex particles and the resulting latex stabilization

Poly(vinyl benzyl chloride) latex was reacted with ethanolamine and with amine terminated chain molecules containing up to 33 ethylene oxide residues. The kinetics of these reactions were essentially independent of the length of the chain carrying the amine group. An increasing length of the chains bound to the surface of the latex particles produced an increasing latex stabilization.     

Interdiffusion of polymer chains during latex coating

The fluorescence method was used to study the interdiffusion of polymer chains during annealing of latex above its glass transition temperature (T_g). The latex film was prepared from mixture of naphthalene (N) (donor) and pyrene (P) (acceptor)-labeled poly(methyl methacrylate) (PMMA) latex particles. Heptane was used as the coalescent agent. A steady-state fluorescence technique was employed to measure the amount of direct nonradiative energy transfer from N to P during the interdiffusion of polymer chains across the particle–particle junction. Various latex films with different latex contents were used to measure the percentage critical occupation for the reliable steady-state fluorescence measurements. Diffusions activation energies in these latex films were measured and found to be around 30 kcal/mol, which was attributed to the backbone motion of PMMA chains.     

Structure,conformation and dynamics of polymer chains at solid melt interfaces

We have performed molecular dynamics, and lattice Monte Carlo simulations of polymeric melts in the vicinity of solid surfaces. The structural features of the solid-melt interface were very simple. The interfacial width was comparable to the segment size. Inside this narrow interface the segment density profile was oscillatory. The density oscillations were much less pronounced than those present at solid-atomic liquid interfaces. On a scale much larger than the segment size, chain conformations were found to be identical with those of ideal chains next to a reflective barrier. In particular, the number of surface-segment contacts scaled like the square root of the molecular weight. Extensive molecular dynamics simulations showed that chain desorption times increase with molecular weight but at a rate much slower than the longest relaxation time of Rouse chains. Therefore, sufficiently long chains desorbed almost freely from the surface despite the presence of attractive surface-segment interactions. A study of chain relaxation dynamics confirmed that the Rouse modes constitute an appropriate set of normal coordinates for chains in the melt interacting with a solid surface. The effect of the surface on mode relaxation was significant. All relaxation processes of chains located within a couple of radii of gyration from the surface were slowed down considerably. This effect, however was approximately the same for fast and slow modes and independent of molecular weight for sufficiently long chains.     

Effects of grafted polymer chains on lamellar membranes

We have investigated the effects of grafted polymer chains [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] on the bending modulus and the intermembrane interactions of lamellar membranes (C(12)E(5) water) by means of a neutron spin-echo and a small-angle x-ray scattering technique. In this study the hydrophilic chain takes the mushroom configuration on the membrane. The bending modulus of the polymer-grafted membranes increases in proportion to the square of the end to end distance of the polymer chain, which agrees well with the theoretical prediction of Hiergeist and Lipowsky [J. Phys. II 6, 1465 (1996)]. From the interlamellar interaction point of view, the mushroom layer is renormalized to the membrane thickness, which enhances the repulsive Helfrich interaction. When the size of the decorated polymer chain increases to the interlamellar distance, however, the mushroom is squeezed so as to optimize the interlamellar potential. Further increase of the grafted polymer size brings a lamellar-lamellar phase separation, where the grafted polymer chains are localized in the dilute lamellar phase and the concentrated lamellar phase forms the onionlike texture.     

Radical entry in emulsion polymerization: propagation at latex particles/water interfaces

In emulsion polymerization, complete entry of an initiator-derived, surface-active radical may involve its adsorption onto latex particles/water interfaces and subsequently its propagation with one more monomer molecule therein. However, all publications to date have defined this propagation step as a three-dimensional bulk reaction between a surface-active entry radical and a monomer molecule. This is incorrect conceptually. It is proposed that the rate of the propagation of surface-active entry radicals with monomer at latex particles/water interfaces be expressed as [Formula: see text] . In this equation, A is the interfacial area between water and latex particles; [M](P) and [Formula: see text] are the mean concentrations of monomer in the particle phase and entry radicals in the aqueous phase, respectively; k(I) is the radical propagation constant at the interfaces, and may be estimated via transition state theory. For seeded styrene polymerization by Hawkett et al. (J. Chem. Soc. Faraday Trans. 1 76 (1980) 1323), k(I) approximately approximately 4.2x10(-9)k(p) (mol(-1)dm(4)s(-1)) is estimated. Here k(p) is the propagation rate coefficient in bulk polymerization. This alternative approach should be useful for one to simulate radical entry rate in emulsion polymerization where the propagation step may be rate-determining, such as under monomer-starved conditions.     

Synthesis of a new polymer surface bearing grafted azo polymer chains

Azobenzene, a photosensitive chromophore that undergoes photoinduced and thermal cis–trans isomerization, can be applied in a nonlinear optical field. {4′‐[(Hydroxy)ethyl]amino}‐4‐nitroazobenzene (disperse red 1) corresponds to one of these azo compounds, which can be grafted to a polymer chain as a part of the main chain, as a dangling group, or onto the polymer surface. In the last case, disperse red 1 is transformed into an acrylic monomer and then grafted onto a polypropylene surface modified with a cold carbon dioxide‐plasma treatment. A method is proposed for quantifying the radicals formed during the plasma treatment and, consequently, for optimizing the grafting reaction. The best conditions (the nature of the solvent, temperature, monomer concentration, and duration) are given. Both IR and Raman spectroscopies were used as efficient techniques for grafting characterization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3052–3061, 2001     

Thiol-terminated hydroxy-functional polymer as a transtab toward polymer latex particles

Hydroxy-functional macrodisulfides have been synthesized by atom transfer radical polymerization of 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate in 2-propanol. Mean degrees of polymerization of the polymer chains beside the disulfide were fixed at 30, 60, and 90; since ATRP has reasonably good living character, the molecular weight distribution is relatively narrow. Furthermore, the macrodisulfides were reduced to synthesize corresponding thiol-terminated polymers with relatively narrow molecular weight distributions. ¹H nuclear magnetic resonance and gel permeation chromatography were used to characterize the macrodisulfides and thiol-terminated polymers in terms of their chemical structure, molecular weight, and polydispersity, respectively. Dispersion polymerizations of styrene using the thiol-terminated hydroxy-functional polymers as a transtab (chain transfer agent + colloidal stabilizer) in ethanol resulted in colloidally stable submicrometer-sized polystyrene latex particles. Scanning electron microscopy, Fourier transform infrared spectroscopy, and elemental microanalysis were used to characterize the particles in terms of their morphologies, particle sizes and their distributions, and chemical compositions.     

Preparation of micro- and nanopatterns of polymer chains grafted onto flexible polymer substrates

In this work a simple novel method for preparing micro- and nanoscale patterns of polymer chains grafted onto flexible polymer substrates is described. A combination of the two techniques of radiation grafting and "grafting-from" has been made. This combination makes it possible to prepare grafted structures having micro- or nanoscale lateral dimensions that are determined by the electron beam or X-ray irradiation patterns used. The height of the grafted features can be controlled by the irradiation dose or such grafting reaction conditions as time, temperature, or monomer concentration. Our first results for nanopatterned samples demonstrate resolution comparable to those of other polymer-based lithography processes.     

Brownian dynamics of grafted polymer chains: time dependent properties

Results of computer simulations of polymer layers consisting of chains grafted by one end on an unpenetrable plane are presented. Characteristics of translational and rotational motion of different chain segments and correlation functions of chain radii were calculated both for single layers at different grafting densities s and for two interacting layers at different distances D between parallel grafting planes. Two values of grafting density were used in the latter case. The behavior of different correlation times as function of s and D and the interplay between the interpenetration of the brushes and rotational and translational motion are discussed. Both relaxation functions and mean square displacements are discussed in terms of stretched exponentials, and the behavior of the resulting “Kohlrausch exponents” γ is presented in detail.     

Molecular electrocatalysis at soft interfaces

The fundamental aspects of electrochemistry at liquid-liquid interfaces are introduced to present the concept of molecular electrocatalysis. Here, a molecular catalyst is adsorbed at the interface to promote a proton coupled electron transfer reaction such as hydrogen evolution or oxygen reduction using lipophilic electron donors.     

Dewetting at soft viscoelastic interfaces

The dewetting transition of thin liquid films (approximately 100 nm) at soft viscoelastic interfaces is analyzed theoretically. It is shown that viscoelastic losses in the soft material can drastically increase the time to complete the dewetting. Thus, the influence of the thinning of the liquid film, due to the hydrodynamic drainage caused by the external applied pressure, has to be considered too. The squeezing pressure coupled with the hydrodynamic drainage may slow down the dewetting to almost zero growth rate of the dry zone; in this case a trapped rim of fluid should be observed.     

Janus particles at liquid-liquid interfaces

Following recent theoretical predictions, we report on the first experiments on the interfacial activity of so-called Janus nanoparticles (i.e., bifacial particles consisting of a gold and an iron oxide moiety). Using pendant drop tensiometry, we show that the amphiphilicity derived from the Janus character of the particles leads to a significantly higher interfacial activity compared to that of the respective homogeneous particles of the same size. The self-assembly of Janus particles at the hexane-water interface results in a significant decrease in the interfacial tension. Furthermore, we demonstrate control over the interfacial activity by tuning the particles' amphiphilicity via ligand-exchange reactions.     

Behavior of nanocelluloses at interfaces

Despite being non-surface active, nanocelluloses position efficiently at interfaces, already at very low concentration. This behavior has lately triggered a strong interest in the cellulose and colloids communities. This review reports the recent developments on the use of nanocelluloses at interfaces and highlights the fundamental principles governing the high efficiency observed in reinforcing the boundary between two phases. The use of nanocelluloses as emulsifier and emulsion stabilizer is first discussed, and the structural properties of nanocelluloses such as aspect ratio and surface properties are correlated with the high efficiency in forming colloidally-stable multiphase systems. Then, the behavior at the air/water interface is presented and the most recent advances are reviewed with focus on the surface free energy of nanocelluloses and their role in the interfacial self-assembly process.     

Preparation and characterization of crosslinked PMMA latex particles stabilized by grafted copolymer

 The preparation of polymethyl methacrylate lattices stabilized by polyhydroxystearic acid and crosslinked with ethylene glycol dimethylmethacrylate (EGDM) has been studied. Crosslinking is a new development in the synthesis of PMMA latex. The particles are monodisperse when the concentration of EGDM ranges from 0.33 to 1.44%. The lattices are stable in aromatic and aliphatic solvents. Swelling occurs due to penetration of solvent molecules into the latex. The degree of swelling is calculated by viscosity and by dynamic light scattering measurements. Received: 30 January 1997 Accepted: 2 June 1997     

Thermally-responsive surfaces comprising grafted poly(N-isopropylacrylamide) chains: Surface characterisation and reversible capture of dispersed polymer particles

In this study we investigate thermally-responsive surfaces prepared by grafting PNIPAm from a cationic macroinitiator (MI) that was adsorbed onto a range of anionic substrates. The substrates used were mica, glass, quartz and high surface area carbon foam. The carbon foam was rendered thermally responsive by first coating it with a layer of calcined laponite particles. PNIPAm brushes were grown from the substrates using surface-initiated atom transfer radical polymerisation. The thermally-responsive PNIPAm layers were characterised in detail at room temperature and 50 °C using atomic force microscopy (AFM) and contact angle measurements. The surfaces changed from being non-adhesive to adhesive when the temperature was increased to 50 °C. Young’s modulus values and adhesive force values are reported. Particle capture experiments involving dispersed polystyrene or poly(BD/MAA) (butadiene and methacrylic acid) particles were conducted. High extents of particle capture were observed. It was shown that the highest extents of thermally-triggered particle capture at 50 °C occurred for surfaces that exhibited the largest increases in contact angle upon increasing the temperature. Importantly, thermally-triggered capture for both anionic polystyrene and poly(BD/MAA) particles was shown to be partially reversible with up to 30% of the captured particles released during cooling. This is the first time that significant reversibility of thermally-triggered capture of polymer particles has been reported.     

Uniform nonspherical latex particles as model interpenetrating polymer networks

Polystyrene/polystyrene latex interpenetrating polymer networks (IPNs) were prepared by seeded emulsion polymerization of styrene–divinylbenzene mixtures in crosslinked monodisperse polystyrene particles. The resulting latexes comprised uniform nonspherical particles, e.g., ellipsodal and egg-like singlets, symmetry and asymmetric doublets, and ice cream cone-like and popcorn-like multiplets. The nonspherical particles, which were formed by separation of the second-stage monomer from the crosslinked seed network during swelling and polymerization, are excellent models for studying phase separation in IPN's. The degree of phase separation increased with increasing degree of crosslinking of the seed particles, monomer/polymer swelling ratio, polymerization temperature, and seed particle size, and with decreasing divinylbenzene concentration in the swelling monomer. The results were consistent with a thermodynamic analysis based on the elastic-retractile force of the polymer network, the monomer/polymer mixing force, and interfacial tension force.     

Aqueous particulate foams stabilized solely with polymer latex particles

In this article, a wide range of latexes are evaluated as possible foam stabilizers. These include near-monodisperse, poly(N-vinyl pyrrolidone)-stabilized polystyrene [PNVP-PS] latexes with diameters ranging from 170 nm to 1.62 microm, submicrometer-sized poly(ethylene glycol)-stabilized polystyrene [PEGMA-PS] latex particles, a PNVP-stabilized poly(4-bromostyrene) [PNVP-PBrS] latex with a mean diameter of 870 nm, two PNVP-stabilized poly(methyl methacrylate) [PNVP-PMMA] latexes with mean diameters of 730 nm and 1.20 microm, a PNVP-stabilized poly(2-hydroxypropyl methacrylate) [PNVP-PHPMA] latex with a mean diameter of 630 nm, and a charge-stabilized anionic PS latex of 220 nm diameter. The effect of varying the particle size, latex concentration, and latex surface composition on foam stability were studied in detail. The larger PNVP-PS latexes, the PNVP-PBrS, and the two PNVP-PMMA latexes gave highly stable foams, whereas PEGMA-PS, PNVP-PHPMA, and the charge-stabilized PS latex produced either no foams or foams with inferior long-term stabilities. Scanning electron microscopy studies revealed hexagonally close-packed latex arrays in the walls of the dried foam, which leads to localized moiré patterns being observed by optical microscopy. Moreover, these dried foams are highly iridescent in bright transmitted light.