Y. Holl

Distribution of surfactants in latex films

Abstract The distribution of four surfactants, namely sodium dodecyl sulfate (SDS), hexadecyl trimethylammonium (or cetyltrimethylammonium) bromide (HTAB), hexadecylpyridinium chloride (HPCl), ethoxylated nonyl phenol containing 10 or 25 ethoxy groups (NP10 and NP25), in poly(2-ethylhexyl methacrylate) (P2EHMA) latex films was studied by Fourier transform infrared (FTIR) spectroscopy (transmission and attenuated total reflection (ATR)) as a function of time, total surfactant concentration and film formation conditions. The distribution is established during the drying period of the latex and then evolves slowly in the dry film owing to the incompatibility of the surfactant with the polymeric medium. The interfaces, film—air and film—support, can be enriched or deficient in surfactant. In the interpretation of the results, three levels of analysis can be distinguished. The first is purely phenomenological, an intermediate level analysis goes within the film formation process and discusses the fate of the surfactant in this context, and the third is the most fundamental level, at which the importance of the polymer—surfactant interactions clearly appears. Lines of research are proposed for further improvement of the understanding of the surfactant distribution in latex films.

FTIR-ATR spectroscopic determination of the distribution of surfactants in latex films

FTIR-ATR (Fourier Transform Infra-Red-Attenuated Total Reflection) has been used to analyze the surface composition of coalesced acrylic latex films. The behavior of two anionic surfactants has been characterized. It has been found that surfactant distribution depends on the nature of the surfactant. A comparison between the normalized absorbance in transmission and in reflection has shown an enrichment of surfactants at the surfaces of films with a coalescence time of 3 days. The surfactant concentration at the film-air interface is higher than at the film substrate interface. A concentration gradient exists through the film thickness. In addition, the incompatible surfactant migrates towards the interface as coalescence proceeds.

Surface composition of coalesced acrylic latex films studied by XPS and SIMS

Abstract Surface concentrations of two anionic surfactants (sodium dodecyl sulfate (SDS) and sodium dodecyl diphenyl ether disulfonate (SDED)) in coalesced acrylic latex films (methyl methacrylate and butyl acrylate copolymer, 45 55 wt%) were studied by XPS and SIMS. For SDS, surface enrichment is very pronounced and can reach saturation in the layer analyzed by XPS (∼50 A). Film maturation for 3 months gives rise to SDS expulsion toward interfaces. For SDED, enrichment is much less pronounced and during maturation the surfactant surface concentration does not change. It has been shown that the interfaces of 3-day-old latex films were almost saturated in the layer studied by SIMS (10–15 A), whatever the nature of surfactant, the side of the film (air or substrate), and the average concentration. These XPS-SIMS results are useful in the interpretation of adhesion properties of films.

Distribution of water-soluble and surface-active low-molecular-weight species in acrylic latex films

Monodisperse core-shell latices were synthesized, differing in the acrylic acid (AA) content in the particle shell (1 or 4 wt%) and the Tg of the acrylic core (around -40 or 10 degrees C). In a first step, the drying mechanisms of the dialyzed latices were studied by confocal Raman spectroscopy. It was shown that, besides some unexpected features (briefly described in the article), drying occurred in a rather classical way, i.e., simultaneously from top to bottom and from edge to center. Then, the distributions of sulfate ion (SO4) (from sodium sulfate) and sodium dodecyl sulfate (SDS) in the dry latex films were established by confocal Raman spectroscopy and attenuated total reflectance (ATR). The two techniques were complementary. SO4 and SDS distributions were quite different, although presenting some common characteristics. In both cases, repartition of the low-molecular-weight species in the film was even less homogeneous when the AA content was lower and the particle core softer. However, SO4 showed enrichment at the film-substrate interface and depletion at the air side, whereas SDS showed concentration maxima at both interfaces. Interpretations stress the importance of desorption from the particle-water interface, transport by water, size effects, and diffusion.

Adhesion of latex films; influence of surfactants

Abstract The aim of the work presented in this article was to investigate the role of surfactants on adhesion properties of latex films. Several polymer/surfactant/support systems were used. Polymers were poly(2-ethyl hexyl methacrylate) or a methyl methacrylate/ethyl acrylate copolymer partially grafted onto a hydrophilic polyester. Surfactants were sodium dodecyl sulfate (SDS), hexadecyl pyridinium chloride (HPCI), or ethoxylated nonyl phenol containing 10 or 30 segments of ethylene oxide (NP10 or NP30). Supports were glass plates or poly(ethylene terephthalate) films. Adhesion was measured by a peel test at 180°. Loci of failure were determined by multitechnique analysis of the surfaces revealed after peeling. At medium and high peel rates, the peel energy versus surfactant concentration curves show either a maximum or a minimum, depending on the surfactant. When the peel rate is decreased, these maxima and minima flatten out and, at zero peel rate (extrapolated values), the peel energy becomes independent of the surfactant concentration. Surfaces analyses revealed that the surfactant is always present at the locus of failure. Rupture takes place in a thick (above 10 nm) (SDS), in a thin (a few nanometers) (NP30), or at the top of a surfactant layer (HPCI). The locus of failure is independent of the peel rate and of the surfactant concentration. The conclusion is that the surfactant strongly influences adhesion properties of latex films but several points remain unexplained.

Coalescence mechanisms of polymer colloids: I. Coalescence under the influence of particle-water interfacial tension

Abstract Coalescence of dense packings of spheres of model latex particles was studied in water. The model particles had a core-shell structure. The core comprised 90% of the mass of the particle, thus largely determining the mechanical properties. The shell contained 10, 15, or 25 wt% methacrylic acid, resulting in three kinds of particles with different particle-water interfacial tensions. It was shown that coalescence in water under the influence of particle-water interfacial tension was possible even at temperatures below the glass transition temperature. The kinetics of coalescence was closely related to interfacial tension. It was shown that the rate of coalescence decreased when interfacial tension decreased, i.e., when the amount of methacrylic acid increased in the shell or when sodium dodecyl sulfate was adsorbed onto the particles, or when the methacrylic acid was neutralized by sodium hydroxide. It was also established that the activation energy of coalescence was equal to the activation energy of the motions of the macromolecules at the glass transition.

Coalescence mechanisms of polymer colloids: II. Coalescence with evaporation of water

Abstract Mechanisms of coalescence of model latices were studied under standard conditions, i.e., with simultaneous evaporation of water. The model latices had core—shell particles with the same diameters and the same bulk properties but different surface characteristics. Different surface compositions were achieved by varying the amount of methacrylic acid (MAA) in the shell, by neutralizing the MAA to different extents, or by adding various amounts of surfactant to the latices. The limit temperature/relative humidity diagrams for film formation were established for several latices. They were all identical, regardless of the surface characteristics of the particles. Total coalescence times of latices and dense packings of latex particles were measured and compared with the coalescence times of the same packings of particles in water. This led to the conclusion that particle—water interfacial tension has a negligible contribution to the coalescence mechanisms under standard conditions. The mechanisms and their relationship to evaporation of water for various thermal conditions of coalescence are discussed.

Poly(ethylene terephthalate) surface dynamics in air and water studied by tensiometry and molecular modelling

Abstract The wetting characteristics of an amorphous poly(ethylene terephthalate) (PET) film by water were studied by the Wilhelmy plate technique in the dynamic and static modes. At room temperature, a dynamic hysteresis of 40° ( θ A = 80°, θ R = 40°) was found. It was interpreted in terms of water adsorption-desorption phenomena. The air-water and water-air equilibration processes of the PET surface were studied. The static mode was most useful for this purpose. It showed that it takes about 10 days to reach equilibrium at 20°C and that equilibration with water is only partially reversible. Surface restructuration was described by molecular modelling. The validity of this description was confirmed by a good quantitative agreement between experimental and calculated data. An increase of the temperature accelerates molecular rearrangements especially around the glass transition temperature of the polymer.

Surfactant distribution in waterborne acrylic films. 1. Bulk investigation.

The distribution of an anionic surfactant, sodium dodecyl sulfate (SDS), in waterborne acrylic films was investigated, focusing on the effects of particle composition and size, and pH of the latex. The observed surfactant distributions could be classified in two categories: homogeneous and heterogeneous, the latter showing SDS aggregates. The shape of the profiles was related to the stability of the latex during drying, at short interparticular distances. The stability of the latex was determined by the presence or not of fixed charges at the surface of the particles. The latices with particles carrying neutralized acrylic acid at high pH (COO(-)) led to homogeneous distributions, whereas the latices with acrylic acid at low pH (COOH) or without acrylic acid led to heterogeneous distributions. Our interpretation is that the stable latices present a narrow network of paths between particles at high polymer volume fraction, limiting the mobility of the surfactant, whereas in the less stable latices wider routes between flocs allow enough mobility for large aggregate formation. Thermal treatments of the dry films confirmed the strong confinement of the surfactant in the dense film structure obtained at high pH and the more open structure, allowing easier surfactant transport and oxygen penetration, observed at low pH. In order to account for the shapes of the profiles more quantitatively, a model was developed based on the diffusion of the surfactant and its transport by the drying front. It was found that the apparent diffusion coefficient of SDS micelles had to be lowered to a great extent (D = 10(-13)-10(-14) m(2)/s) during drying in order to explain aggregate formation. It should be even lower (D = 10(-15) m(2)/s) to interpret homogeneous surfactant profiles. These results are consistent with our hypothesis of the key importance of the surfactant mobility during drying.

Adhesion of latex films. Part III. Surfactant effects at various peel rates

In order to study the effect of surfactants on the adhesion of latex films, peel energy versus surfactant concentration curves were established at various peel rates. The main latex polymer was a methyl methacrylate (MMA)/ethyl acrylate (EA) copolymer synthesized in the presence of a hydrophilic polyester. Another polymer, less extensively studied, a styrene/butyl acrylate/methacrylic acid terpolymer, was also used for comparison purposes. The surfactants were either sodium dodecyl sulfate (SDS) or ethoxylated nonyl phenol containing 30 segments of ethylene oxide (NP30). The substrates were glass plates or poly(ethylene terephthalate) films. It was found that with SDS-containing films, whatever the substrate or the polymer, the curves went through a maximum, whereas with NP30 they went through a minimum, at medium or high peel rates. When the peel rate was decreased, the curves flattened out and at zero peel rate (extrapolated values), they became horizontal. The peel energies at zero peel rate were three...