Nanostructured latexes made by a sequential multistage emulsion polymerization

A series of linear and lightly crosslinked nanostructured latices was prepared by a sequential multistage semicontinuous emulsion polymerization process alternating styrene (S) and n‐butyl acrylate (BA) monomer feeds five times, that is ten stages, and vice versa, along with several control latices. Transmission electron micrographs of the RuO₄‐stained cross sections of nanostructured and copolymer latex particles and films showed that their particle morphologies were not very different from each other, but the nanostructured latex particles were transformed into a nanocomposite film containing both polystyrene (PS) and poly(n‐butyl acrylate) (PBA) nanodomains interconnected by their diffuse polymer mixtures (i.e. interlayers). The thermal mechanical behaviors of the nanostructured latex films showed broad but single T_gs slightly higher than those of their counterpart copolymer films. These single T_gs indicated that their major component phases were the diffuse interlayers and that they behaved like pseudopolymer alloys. The minimum film formation temperatures of nanostructured latices capped with PBA and PS, respectively, were 15 °C lower than and equal to those of their counterpart copolymer latices, but their T_gs were about 10 °C higher. Consequently, nanostructured latices enabled us to combine good film formation with high strengths for adhesives and coatings applications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2826–2836, 2006     

Glass transition and film formation of VeoVa/vinyl acetate latices; role of water and co-solvents

The physical forces causing deformation of latex particles during the film formation process have been witley studied. However, the forces resisting particle deformation are still poorly characterized. It is clear that the extent of particle deformation is dependent on the viscoelastic nature of the polymer. In an emulsion, the latex particles will normally contain water, surfactants and “free” monomers which lead to plasticization of the polymer. Although this effect has been recognized, so far it has been studied only on films that had been dried and then partially or completely swollen by water. In this work, plasticization of the emulsion polymers by water and co-solvent has been quantified via differential scanning calorimetry investigation directly on the aqueous latex dispersions. More specifically, the plasticizing effect of water on VeoVa/vinyl acetate copolymer latices and its influence on minimum film-forming temperature (MFFT) has been studied. A linear correlation has been found between T_g and MFFT for the wet latices. This new direct method should help to improve our understanding of the forces resisting latex film formation. Additionally, the homogeneous distribution of the hydrophobic and hydrophilic monomers (VeoVa and vinyl acetate respectively) in the latex particles was verified via a ¹³C-NMR (nuclear magnetic resonance) study performed directly on the latices. This study confirmed that no significant core/shell type of morphology had influenced latex film formation.     

Chemical heterogeneity in emulsion copolymers of carboxylic monomers

Copolymer latices of butylacrylate (BA) with acrylic and methacrylic acid (AA and MAA) were prepared by batch type emulsion polymerization, and, for comparison, copolymers with identical monomer composition were prepared by batch type solution polymerization.The distribution of the carboxylic monomers in the latex particles and the serum was studied by density gradient and sedimentation experiments with the analytical ultracentrifuge. Dynamic mechanical measurements of films of these copolymers were used to determine the storage and loss moduli as a function of temperature. From these measurements the position and extension of the glass transition range on the temperature scale is obtained. For heterogeneous emulsion copolymers with two glass transition temperatures the distribution of the carboxylic monomer units in the different copolymer phases can be determined. Electron microscopy of ultra thin cross-sections of stained films gave further insight into the film morphology.The combination of the results obtained with the different methods gives rise to the following clues: In the BA/AA latices about 40% (by weight) of the total AA used in the recipe are found in the serum as a water soluble polymer, about 50% are found to increase the glass transition temperatureT _g of the bulk of the BA copolymer and, therefore, are thought to be incorporated into the interior of the latex particles, and the remaining 10% are, conclusively, located on the particle surface.In the BA/MAA latices no water soluble copolymer could be detected in the serum, about 90% of the MAA used is found in the bulk of the copolymer, and about 10% form a second hard phase on the surface of the latex particles.Dynamic mechanical measurements on the copolymer latex films show at least two phases with different glass transition temperatures: the bulk of the copolymer with a relatively low content of (M)AA units and a glass transition range at low temperatures, and a second (M)AA rich phase with a highT _g.The latter phase forms a honeycomb-like structure surrounding the packed latex particles. That results in a three-dimensional network of polymer with a highT _g extending throughout the latex film. In spite of the fact that this phase is built from a small fraction of the total copolymer only, it has a very pronounced influence on the performance behaviour of latex films.Dedicated to Professor Dr. R. Manecke on the occasion of his 70th birthday.     

Latex interpenetrating polymer networks: From structure to properties

Latex interpenetrating and semi-interpenetrating polymer networks (LIPNs and semi-LIPNs) combine the morphological characteristics of bulk-polymerized IPNs with the characteristics of polymers produced by emulsion polymerization; there are IPN structures within the latex particles. These LIPNs can be injection-molded using standard thermoplastic methods and machinery. A dual thermoset—thermoplastic nature characterizing the LIPN manifests itself in the mechanical and rheological behavior reflecting unique morphologies. These morphologies result from a sequential two-stage latex (TSL) polymerization and include core—shell, domain, interpenetrating polymer networks and various other combinations. Elastomeric TSL with crosslinked polyacrylates (xPA) as the first stage and crosslinked polystyrene (xPS) as the second, each stage lightly crosslinked, yield IPN-nano-domain structural particles. Upon molding, the particles become interconnected by joint PS nanodomains, introducing a particle—particle strength-forming mechanism. The intraparticle glassy PS nanodomains reinforce the soft elastomeric particles enhancing their modulus. Glassy “all-styrene” semi-LIPNs made of PS and xPS show improved mechanical performance compared to PS, while exhibiting good transparency. Volumetric crazing in these PS/xPS materials develops in tension-improving elongation and strength. The presence of xPS particles, denser and thus stiffer than the PS matrix, renders a higher modulus. Essentially xPS highly filled blends are achieved along with significant particle—matrix interactions. The ability to generate a controlled plethora of morphologies offers a wealth of potential applications, from reinforced elastomers to high impact plastics. Poly(acrylonitrile—butadiene—styrene), a semi-LIPN, is a commodity plastic, clearly demonstrating the utilization potential of the TSL procedure for generating very fine multiphase materials of scientific and technological merits.     

Ion-Free latex films as dual-phase electrolytes: Styrene–butadiene rubber and nitrile–butadiene rubber synthesized by emulsion polymerization with poly-(vinyl pyrrolidone) stabilizer

Ion-free latices of styrene-butadiene rubber (SBR) and nitrile-butadiene rubber (NBR) were synthesized by emulsion polymerization with use of poly (vinyl pyrrolidone) (PVP) stabilizer. The goal was to prepare ion-free latex films, possessing dual-phase latex particle morphology, and swell the films with liquid electrolyte to yield dual-phase polymer electrolytes (DPE). SBR/PVP latex was prepared readily, but NBR/PVP latex was sensitive to coagulation. Differential scanning calorimetric (DSC) and scanning electron microscopic (SEM) analyses of latex films provided morphological evidence concerning particle structure and phase separation. Blends of NBR/PVP and PB/PVP latices (PB = polybutadiene) were also investigated, but particle structure was not present in the blended latex film, even though particle structure was present in the individual NBR/PVP and PB/PVP latex films. After extensive swelling of SBR/PVP latex films, PVP was extracted from the films, and ionic conductivities greater than 10^?3 S/cm were achieved. © 1994-John Wiley & Sons, Inc.     

An overview of polymer latex film formation and properties

The literature on polymer latex film formation has grown enormously in recent times--driven by the need to find alternatives for solvent-based systems with their adverse environmental impacts. Although greater insight has been shown by the use of modern instrumental techniques such as small angle neutron scattering, direct non-radiative energy transfer and atomic force microscopy, the actual mechanisms involved in deforming spherical particles into void-free films are still the subject of controversy and debate. Surfactant-free homopolymer model colloid latices, favoured in academic studies, together with latices containing surfactants whose redistribution can influence film properties, and the more complex copolymer, blended, core-shell and pigmented systems needed to satisfy a full range of film properties are all considered.     

Thickening effect of dispersions of ethyl acrylate-methacrylic acid copolymer prepared by different polymerization routes

Thickening of latices (particle diameters 105, 157 and 221 nm) by model dispersions of ethyl acrylate-methacrylic acid copolymer (15 wt.% of the acid) prepared by both non-seeded and seeded semicontinuous emulsion copolymerizations was investigated. Using viscometry and dynamic and steady shear measurements, we found that the thickening effect of the dispersions strongly depends on their particle structure and the intensity of interactions between the components in the system. In weakly interacting systems (lower latex concentrations, large latex particles) the thickening effect of the dispersions is controlled by effective volume fraction of swollen particles. This leads to a higher viscosity of systems thickened by more swollen (less crosslinked) particles obtained by the non-seeded process. On the other hand, in strongly interacting systems (high latex concentrations, small latex particles) lower deformability of more crosslinked particles prepared by the seeded process causes a higher flow resistance of the systems thickened by this dispersion.     

Effect of Molecular Weight on Packing during Latex Film Formation

A UV-visible technique is used to study the evolution of transparency during film formation from latex particles. Latex particles with high and low molecular weight (HM and LM) polymethyl methacrylate (PMMA) are used to prepare films. Two sets of films with different latex content were prepared from HM and LM particles separately, by annealing PMMA particles above the glass transition temperature. Transmitted photon intensity, I(tr), from HM and LM films increased as the annealing temperature was increased. The increase in the transmitted photon intensity is attributed to the latex content (film thickness) for the annealed film samples. It is suggested that as the latex particles are packed (film thickness is increased) fewer voids or cracks are formed in the films. Positive and negative absorption coefficients are measured below and above 210 and 180 degrees C annealing temperatures for the HM and LM films. Packing coefficients are obtained for films in various latex contents. It is observed that LM particles are packed much easier than HM particles. Copyright 2001 Academic Press.     

Synthesis of ion-free latex particles composed of polybutadiene and poly (vinyl pyrrolidone) as polymer electrolyte materials

The synthesis of polybutadiene (PB) by emulsion polymerization with use of poly (vinyl pyrrolidone) (PVP) stabilizer was investigated. The goal was to prepare flexible latex films that clearly retain particle morphology in the solid state after heat treatment and contain no ionic, hydroxyl, or (primary, secondary) amino groups. The latex particle core composed of PB was nonpolar and rubbery, while the particle shell composed of PVP was polar and glassy. Average particle diameter was measured by the dynamic light scattering technique, and particles were imaged by scanning and transmission electron microscopic analyses. Dialysis of the latices resulted in successful exchange of the dispersion medium without precipitation. © 1994 John Wiley & Sons, Inc.     

Some aspects of polymer colloids I. Preparation and properties of different types of latex particles

The boundary region separating a latex particle from the surrounding medium has a great influence on the properties of latex dispersions. Four types of polystyrene and polystyrene/comonomer latices differing greatly in the structure of the boundary region were prepared. The first part of a series of papers reports on the preparation of the various latex dispersions. Mean particle sizes were obtained from simple turbidity measurements, quasi-elastic light scattering, and electron micrographs. The behavior of the particles in the centrifugal force field is a simple tool for detecting aggregation tendencies that are not directly related to salt stability. The BET-surface area agrees with the area calculated from the mean particle size when a sharp boundary and smooth surface is developed between the particle and the surrounding medium. In the case of particles with extended boundary regions (core/shell particles or particles with hairy envelopes), film formation reduces the specific surface area. Removal of soluble oligomers and polymers from the boundary region during subsequent treatments (purification and centrifugation before freeze-drying) can increase the surface area considerably.     

Preparation and characterization of polysiloxane-poly(butyl acrylate-styrene) composite latices and their film properties

Gamma ray induced seeded emulsion co-polymerization of styrene and butyl acrylate was carried out in the presence of polymerizable polysiloxane seed latex which was obtained by the ring opening co-polymerization of octamethyl cyclotetrasiloxane (D₄) and tetramethyl tetravinyl cyclotetrasiloxane (VD₄) catalyzed by dodecylbenzene sulphonic acid (DBSA). A series of polysiloxane seed latices with different molecular weight, vinyl content, and particle size were used. The conversion-time curve showed that the polymerization rate was accelerated much by the seed latex. The obtained composite latices also showed good storage stability, mechanical stability and high electrolyte resistance ability. The morphology of the composite latex particles was found to be a quite uniform fine structure by transmission electron microscope (TEM). The graft polymerization between polymerizable polysiloxane and butyl acrylate or styrene was confirmed by the Fourier transform infrared spectroscopy (FT-IR) and the graft efficiency was also studied. The influence of seed content, molecular weight, vinyl content of the polysiloxane and seed latex particles size to the mechanical performance, water absorption ratio, surface properties, transparency and UV resistance of the latex films, was also investigated.     

Vapor-induced film formation from low-Tg particles for different solvent compositions

The photon transmission method was used to study latex film formation from poly(vinyl acetate) (PVAc) particles induced by two different solvents. Films with the same latex content were prepared from PVAc particles and exposed to vapor of ethanol-water and acetone-water mixtures in various compositions. Transmitted photon intensities, Itr, from these films increased with increasing vapor exposure time. The increase in Itr is attributed to the increase in crossing density at the polymer-polymer junction. The Prager-Tirrell model was employed to obtain the back-and-forth frequency, nu, of the reptating polymer chain during film formation induced by solvent vapor. It was observed that the produced nu values increase as the solvent content is increased for both solvents. Abilities of both solvents to form films were interpreted with the solubility parameters of the solvents and the PVAc.     

The Film Formation of Polymer Particles in Drying Thin Films of Aqueous Acrylic Latices

The aim of this study is to determine the factors that contribute to the process of film formation of binder particles in drying aqueous dispersion coatings, based on acrylic polymers. It is known that concentrated latices of uniform size show iridescent, colored light patterns. These colors are caused by interparticle interference, and they are only present when the latex particles are ordered in a regular structure. The interparticle interference can be characterized by measuring the transmission as a function of wavelength of the incident light. It appeared that the changes of the interparticle interference of a drying latex film can be related to changes in the interparticle distance and displacement. It was also found that the interparticle distance becomes "negative" upon coalescence of the latex particles. This means that from this point on, the change in interparticle interference is directly related to the indentation or deformation of the latex particles. It became clear that the coalescence process differs from deformation mechanisms accepted in the literature. It seems that the deformation of the particles follows a biaxial mechanism. This means that the particles deform only in one direction, perpendicular to the film surface. Copyright 2000 Academic Press.     

Multiphase Polymer Dispersions and Nanocomposites by Catalytic/Free Radical Emulsion Polymerization

Summary: Free radical emulsion polymerization of styrene (S) or butyl acrylate (BA) in the presence of latices of linear polyethylene (PE) prepared by catalytic emulsion polymerization affords colloidally stable multiphase latices. Coagulation of a PE/PS latex affords nanocomposites composed of small PE phases dispersed in a PS matrix, as evidenced by the large supercoolings of PE crystallization (by DSC). TEM of PE/PBA latices indicates a PBA phase around the PE particles under the emulsion polymerization conditions investigated. Films formed from these dispersions exhibit homogeneously dispersed PE particles.

Multiphase latices are obtained by free radical emulsion polymerization of butyl acrylate in the presence of latices of linear polyethylene (PE) prepared by catalytic emulsion polymerization.     

Film Formation and Redispersion of Waterborne Latex Coatings

Poly(vinyl acetate-co-ethylene) latex dispersions are prepared and their films investigated with a focus on the effect of composition upon redispersion. Films of dispersions containing sufficient amounts of poly(vinyl alcohol) (PVA) can be redispersed in water. This property is lost in the presence of surfactant, a fact which suggests a procedure to control film formation. It is demonstrated that redispersion is due to a PVA-membrane which separates the particles. Loss of redispersibility in the presence of surfactant proceeds with the breakup of the membranes and a corresponding change of film properties. Experimental data is provided by light microscopy, mechanical testing, and TEM in conjunction with a staining method new to the field. The hypothesis is developed that interaction with surfactant leads to imperfect PVA-membranes that are no longer able to prevent latex polymer interdiffusion. Fluorescence correlation spectroscopy demonstrates the formation of surfactant micelles, as well as the simultaneous adsorption and aggregation of PVA onto the micelles. It is concluded that the competing surface of the surfactant micelles traps enough PVA to cause thinning and fragmentation of the membranes surrounding the particles, which enables interdiffusion of latex polymer. This effect can be used to convert the system from one forming a redispersible coating to one forming a nonredispersible (permanent) film. Copyright 2000 Academic Press.     

Preparation of epoxy-acrylic latex based on bisphenol F epoxy resin

Bisphenol F based epoxy-acrylic latex with different amount of epoxy resin was successfully prepared by semi-continuous seeded emulsion polymerization. The resulting composite latexes had a narrow size distribution of about 105 nm in diameter. The DSC result showed that the epoxy resin and polyacrylate were grafting copolymerization. The FTIR spectra showed that the epoxy group had been introduced into the epoxy acrylic latex system, and the composite latex could be crosslinked with epoxy hardener at room temperature. The crosslinked composite latex film exhibited a high Tg compared to epoxy-acrylic latexes. The surface of the films with the epoxy resin was regular, and diffused into the polyacrylate phase in the epoxy-acrylic latexes films. Since the curing reactions occurred before latex particle coalescence stage, the surfaces of the cured epoxy-acrylate latex films had a number of interface particle. Compared with the acrylic latex, the thermal stability of the epoxy-acrylate latex was increased, and the stability of the cured film increased with increasing epoxy content.     

Ion exchange in carboxylated latices — AUC studies using sedimentation and density gradient techniques

Crosslinked highly carboxylated acrylic latices with narrow particle size distributions were prepared by emulsion polymerization and characterized carefully by different AUC techniques (particle size distributions and particle density measurements). The acid form of those latices was neutralized with metal oxides like MgO, CaO, ZnO, or PbO in order to obtain the corresponding salt form of the latices which again were characterized carefully. The kinetics of the ion exchange between latex particles were studied by mixing, for example, the acid and the salt form of the latices monitoring the density distribution of the latex particles by density gradient ultracentrifugation. With all latices the hydrogen-metal ion exchange tends to be a complete one provided this process is given a sufficiently long exchange time. Theoretical models are provided which yield a qualitative explanation of the experimental data.     

Determination of the glass transition temperature of latex films: Comparison of various methods

The glass transition temperature (T_g) for two latices with different styrene/butadiene compositions was determined by the thermal SPM probe resonance frequency method. The results were compared with the T_g values obtained by differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), process rheometer (PR) and thermo-mechanical analyzer (TMA) measurements. The T_g values detected by the thermal SPM method agreed well with the T_g values obtained by DSC and calculated by the Fox–Flory equation. DMA, on the other hand, showed a significantly higher T_g value for both latices than those obtained from theoretical calculations, the thermal SPM method and DSC. The T_g obtained from the PR curve was slightly higher for the latex with a low styrene content, whereas good agreement was obtained with the thermal SPM data for the latex with a high styrene content. The glass transition temperature determined by TMA agreed fairly well with the thermal SPM data for the latex with the low styrene content, whilst the value of T_g for the second latex was much less than those obtained by the other methods. The thermal SPM method detects changes in thermal behavior (thermal diffusivity, heat capacity) during heating of the latex films rather than changes in the mechanical properties. Information about the sample history could be seen in the thermal SPM curves, which was further associated with the degree of latex film formation, especially when the roughness of the films was taken into consideration.     

Latex interpenetrating polymer networks of epoxidised natural rubber/poly(methyl methacrylate): an insight into the mechanism of epoxidation

Composite latex particles consisting of epoxidised natural rubber (ENR) and poly(methyl methacrylate) (PMMA) were synthesised to obtain interpenetrating polymer networks. Among the ENR latices having 9 to 36 mol% epoxide, prepared by in situ reaction using performic acid, the ENR latex with 25 mol% epoxide was selected for prevulcanisation by sulphur or γ-radiation system. The swelling ratios of sheets cast from the sulphur-prevulcanised ENR (SPENR) latices decreased with increasing prevulcanisation time while those cast from the γ-radiation-prevulcanised ENR latices were also inversely proportional to the irradiation dose. By applying the phase transfer/bulk polymerisation/transmission electron microscopy (TEM) technique, a homogeneous network structure in each of the SPENR particles and also a relative dense network near the surface in γ-radiation (RV) ENR particle were noticed. When 10 to 30 wt% of MMA swollen in ENR particles was polymerised, the mesh structure was observed in each particle. The dense network near the RVENR particle surface might be used as additional evidence that the degree of epoxidation and, hence, the presence of swollen n-butyl acrylate in the outer zone were higher than in the internal region.     

氟含量对含氟丙烯酸酯共混乳胶膜梯度结构和表面性能的影响

首先将制备出的平均粒径较小的含氟丙烯酸酯均聚物乳液与平均粒径较大的纯丙烯酸酯共聚物乳液按不同的比例( 1/9,2/8,3/7,4/6,5/5)共混,接着将各共混乳液在室温下(20℃)玻璃基材上干燥后,于110℃/210℃下热处理一段时间.运用接触角法,XPS、AFM、SEM-EDX等详细研究了共混乳胶膜中含氟组分含量对...