Drying Latex Films Research Articles

Dynamic surfaces of latex films and their antifouling applications

Latex polymer particles have been widely used in industry and everyday life. For decades the fabrication of “smart” latex film from latex particles has been a great challenge due to the difficulty in the synthesis of the functional latex particles by traditional emulsion polymerization using small molecular surfactants. In this manuscript, a simple and environmentally-friendly approach to the fabrication of “smart” latex films with dynamic surfaces is reported. Latex particles with poly(n-butyl methacrylate) (PnBMA) in the cores and zwitterionic poly-3-[dimethyl-[2-(2-methylprop-2-enoyloxy) ethyl]azaniumyl]propane-1-sulfonate (PDMAPS) in the shells are synthesized by reversible addition-fragmentation chain transfer (RAFT) mediated surfactant-free emulsion polymerization. The kinetics for the emulsion polymerization is studied, and the latex particles are analyzed by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and dynamic light scattering (DLS). Latex films are prepared by casting aqueous solutions of the latex particles at temperatures above the glass transition temperature (Tg) of PnBMA. On the dried latex film, the hydrophobic PnBMA blocks occupy the top surface; after water treatment, the hydrophilic PDMAPS blocks migrate to the surface. A change in the surface hydrophilicity results in a change in the water contact angle of the latex film. A mechanism for the formation of the dynamic surface structure is proposed in this research. Antifouling applications of the latex films are investigated. Experimental results indicate that the water-treated latex film is able to efficiently inhibit protein adsorption and resist bacterial adhesion.

Indoxacarb-Loaded Anionic Polyurethane Blend with Sodium Alginate Improves pH Sensitivity and Ecological Security for Potential Application in Agriculture.

Traditional pesticide formulations show poor utilization and environmental safety due to their low foliage adhesion and large auxiliaries. In this study, a novel and environment-friendly indoxacarb formulation was prepared to improve the pesticide’s utilization rate, target control characteristics and ecological security. Indoxacarb-loaded waterborne polyurethane–sodium alginate (PU/SA) nanoemulsions with film forming properties, alkaline responsive release, high effectiveness against Spodoptera litura, and reduced acute contact toxicity for nontarget organisms were successfully prepared. The colloidal properties, swelling and release behaviors, leaf adhesion, degradation dynamics and bioactivity assay of the indoxacarb-loaded PU/SA nanoemulsions were determined. Results showed that the obtained indoxacarb-loaded microcapsule particulates were approximately 57 nm in diameter, electronegative −45.9 mV, and uniformly dispersed in the nanoemulsions. The dried latex films of PU/SA in the alkaline environment revealed better responsive swelling and release characteristics than those in acidic and neutral conditions. Compared with a commercial emulsifiable concentrate, the indoxacarb-loaded PU/SA nanoemulsions were useful for the targeted control of S. litura, which have alkaline gut and showed reduced acute contact toxicity to Harmonia axyridia. Furthermore, the PU/SA formulation had better foliage adhesion and indicated the property of controlled-release and a persistent effect.

Latex films with gradients in crosslink density created by small-molecule-based auto-stratification.

A suitable balance of convective and diffusive transport of small molecules contained in the liquid phase of a drying latex film leads to auto-stratification and to functionally graded films. Differing from blends of latex particles, which may also experience drying-induced segregation, small molecules retain their mobility after the particles have touched and have formed an elastically coupled network. The use of a thickener, which turns the dispersion into a weak gel and prevents the free flow of particles, is compatible with this approach (and even advantageous). A problem with small molecules is fast diffusive equilibration of concentration differences. For this reason, composition gradients along the lateral direction, where the characteristic length scale is centimeters, are more easily achieved than gradients along the vertical. Addition of a thickener slows down the diffusion, which aids the development of gradients along the vertical. The application example chosen was the crosslinking agent adipic dihydrazide, ADH, which takes part in keto-hydrazide coupling. Its heterogeneous distribution produces a spatially variable crosslink-density in the dry film as evidenced by Raman microscopy. A side aspect of the work is an inward flow of serum, which is observed for high-Tg films. An explanation for this "anti-coffee-ring effect" --based on pore collapse driven by the polymer-water interfacial energy combined with finite polymer elasticity-- is proposed.

Preparation and characterization of the acrylic latex-laminating adhesives applied in BOPP/PE composite films

Acrylic latex-laminating adhesives (ALLAs) were successfully prepared via the monomer-starved seeded semi-continuous emulsion copolymerization with butyl acrylate–methyl methacrylate–acrylamide (BA–MMA–Am) monomers, and only 1.1 wt% emulsifier contents were employed. The effect of Am on final latex, the dried latex films, and adhesive properties of ALLAs were discussed by utilizing rotational viscometer, dynamic light scattering, transmission electron microscopy, Fourier transform infrared spectrometer, contact angle, differential scanning calorimeter, thermogravimetric analysis (TG), and mechanical experiments, respectively. The results indicated that the peel strength of the biaxial-oriented polypropylene/polyethylene composite films can reach a peak (3.15 N/15 mm) at the content of 2 wt% Am in the pre-emulsion feed, which satisfies an acceptable minimum peel strength (0.8 N/15 mm) for application. Additionally, average particle size of the final latex experienced few changes, but the particle size distribution of the latex changed notably as Am contents varied from 0 to 10 wt%, while Tg of ALLAs increased gradually, and the molecular weight (Mn and Mw) increased obviously. TG results showed that the ALLAs have excellent thermal stability with a high decomposition temperature of 300 °C.

Preparation and properties of environment-friendly acrylic latex laminating adhesives applied in plastic/plastic composite films

Acrylic latex laminating adhesives (ALLAs) were successfully prepared via a monomer-starved seeded semi-continuous emulsion polymerization with butyl acrylate (BA), methyl methacrylate (MMA), styrene (St), acrylamide (Am), and methacrylate glycidyl ether (GMA) as monomers. Impacts of GMA on the final latex, the dried latex films and the adhesive properties of ALLAs were investigated, respectively. The results indicated that the increase of GMA contents in the pre-emulsion feed has no apparent effect on the final latex average particle size and size distribution, while the gel contents, glass transition temperature (Tg) and water contact angle of the ALLAs gradually increased, and the molecular weight (Mn, Mw) obviously increased. Additionally, as the amount of GMA increased from 0 to 10 wt%, the maximum peel strength of the composite films reached 3.72 N/15mm with 5 wt% GMA contents. When heated to 65 °C, the peel strength of the composite films with 5 wt% of GMA can still maintain an acceptable peel strength (2.51 N/15mm) for application, showing excellent adhesive performance and heat resistance properties.

Formation of metal (nano-)particles in drying latex films by means of a reducing plasma: a route to auto-stratification

Subjecting drying latex films, the water phase of which contains metal ions, to a reducing, low-temperature atmospheric pressure plasma results in the formation of metal (nano-)particles or a metal film. The process is demonstrated with gold and silver. A continuous metal layer forms on top of the film, when the salt concentration is high enough. At lower concentrations, particles are created inside the film. In the case of gold these are mostly nanoparticles, which are enriched at the top. The reduction drives an auto-stratification process.

Simulation of Vertical Surfactant Distributions in Drying Latex Films.

Following our previous contribution ( Gromer, A. et al. Langmuir 2015 , 31 , 10983 - 10994 ) presenting a new simulation tool devoted to particle distributions in drying latex films, this Article describes the prediction of surfactant concentration profiles in the vertical direction during the complete film formation process. The simulation is inspired by cellular automata and equations by Routh and co-workers. It includes effects that were not considered before: surfactant convection by water and surfactant desorption upon particle deformation. It is based on five parameters describing the nature of the polymer/surfactant system and on film formation conditions. In particular, the viscoelastic properties of the polymer were taken into account through the λ̅ parameter introduced by Routh and Russel. Results show the importance of convection by water and the influence of the particular deformation mechanism on the final surfactant distribution. Excesses or depletions can be predicted either on the surface or on the substrate sides, in qualitative agreement with the numerous existing experimental studies. The complex interplay between parameters governing surfactant distributions makes the results unpredictable without the help of such a simulation tool. Therefore, it should be of interest to both industrial and academic scientists.

Latex Films with In-Plane Composition Gradients Caused by Lateral Drying

Making use of a pH-sensitive dye contained in the aqueous phase of drying latex films, it was found that lateral drying can cause pH-gradients in the final film. The effect requires the presence of a thickener, which stabilizes a coupled network of latex spheres at a time, when there is still a significant amount of flow in the liquid phase. The thickener decouples the movement of the liquid from the movement of the latex spheres. If the glass temperature of the polymer is much below the drying temperature, the pH of the final film is low at the edge of the film, which can be explained with the coffee ring effect. The opposite behavior—meaning protons being carried to the center of the film—is found for films with a glass temperature close to the drying temperature and above. When there is a flow of protons toward the center of the film, one also finds the thickness to be increased at the center. Composition gradients can be exploited to generate films with spatially variable properties, examples being color (as demonstrated here), tack, etch resistance, contact angle, or swellability in solvent vapor.

Coarsening of the pore network in drying latex films upon interparticle aggregation.

The lateral drying front observed during film formation from latex dispersions with a Tg of the polymer around room temperature is composed of three three distinct lines. The lines are characterized by a decrease in turbidity, a renewed sharp increase in turbidity, and a more gradual decrease in turbidity at the end of what can be called a "halo". Microcracks with herringbone morphology develop at the first line, where the turbidity decreases. If macrocracks are present, these nucleate close to the end of the halo. At the line, where the turbidity sharply increases, one also observes an increase in stress birefringence. The substructure of the drying front is characteristically different from the structures described previously for films drying from hard particles. In particular, the renewed increase in turbidity cannot be explained as pore-opening, but rather is the consequence of a coarsening of the pore network after the particles jump into contact. A capillary instability sets in, by which the small pores collapse under the polymer/water interfacial energy, while the larger pores expand correspondingly. The instability (related to the Rayleigh instability of liquid jets) makes the films appear turbid. Also, the induced mechanical heterogeneity prevents straight macrocracks from penetrating into the halo because crack deflection and crack branching would result, which is energetically unfavorable.

Correlation between Particle Deformation Kinetics and Polymer Interdiffusion Kinetics in Drying Latex Films

Using an experimental setup which determines the turbidity of the sample and the efficiency of Förster resonance energy transfer (FRET) at the same time, we have correlated the particle deformation kinetics in a drying latex film, quantified by light scattering with the kinetics of polymer interdiffusion. Interdiffusion was quantified making use of energy transfer (FRET) between donor molecules and acceptor molecules, bound to polymer chains on different particles. When the chains cross the interparticle boundaries, the rate of energy transfer increases. The latex was prepared by miniemulsion polymerization. The amount of emulsifier employed during polymerization had a pronounced effect on the relative timing of interdiffusion and particle deformation. Increasing the amount of emulsifier delayed the onset of interdiffusion relative to the time when the film became transparent. This is mostly the consequence of a size effect, as opposed to surfactant acting as a barrier for transport.

Addition of Halloysite Nanotubes Prevents Cracking in Drying Latex Films

Investigating the process of film drying from aqueous dispersions containing a polymer latex as well as halloysite nanotubes (HNTs), we found that composite films could be formed without cracking under conditions where films of the pure polymer would always crack. Scanning electron micrographs showed that the HNTs were well dispersed and, further, that the distribution of fiber orientations was close to isotropic. The pendulum hardness of films formed from acrylate dispersions strongly increased upon addition of the inorganic phase. The pencil hardness, on the other hand, was poor, which presumably goes back to insufficient coupling between the organic and the inorganic phase. All films were white in appearance. For fiber concentrations higher than 10 vol %, the final films were porous.

On-line determination of Förster resonance energy transfer efficiency in drying latex films: Correlation of interdiffusion and particle deformation

An instrument is described, which measures the efficiency of Förster resonance energy transfer (FRET) in parallel to the sample's turbidity. The instrument was used to study the film formation from polymer latex dispersions. In this context, the FRET efficiency reflects the diffusion of polymer chains across the interparticle boundaries, while the loss of turbidity reflects the progress of particle deformation. Particle deformation causes tensile in-plane stress, while polymer interdiffusion creates cohesion and thereby helps to prevent cracking. The relative timing between the two therefore is of fundamental importance for successful film formation. The on-line determination of FRET efficiency while the film dries is complicated by the fact that the fluorescence lifetime of the donor, τ(D), depends on the water content in the vicinity of the donor. In the established procedure for data analysis, drifts in τ(D) induce corresponding artifical drifts in the values of the FRET efficiency. A novel algorithm for the analysis of fluorescence decay profiles is proposed, which makes use of the method of moments. The FRET efficiency is quantified by the upward curvature of the fluorescence decay curve in log-linear display. In the application example, interdiffusion is delayed relative to particle deformation by about 10 min. For successful film formation, this delay should be as small as possible.

Effect of film thickness and particle size on cracking stresses in drying latex films

The stress at which latex films crack during drying was investigated using beam bending. Two systems were investigated: (i) poly(methyl methacrylate/butyl acrylate) particles cast as thin films to examine the effect of film thickness on cracking film stress and (ii) polystyrene particles dried as drops to investigate the effect of particle size. Results indicated an inverse relationship between film thickness and film stress, whilst film stress was shown to be independent of the original particle size. These outcomes were in good agreement with Tirumkudulu and Russel's theoretical analysis [M.S. Tirumkudulu and W.B. Russel, Langmuir 21 (2005) 4938], albeit the measured stress values were almost twice the theoretical estimation.

Dilational Lateral Stress in Drying Latex Films

Drying latex films usually experience tensile stress due to the reduction in volume. While an unconstrained film would shrink affinely in all three dimensions, a coating can only shrink along the vertical and therefore exerts tensile stress onto the substrate. Using an instrument capable of producing maps of the stress distribution, we found that dilational stress sometimes develops as well. The in-plane stress was monitored by spreading the latex dispersion on a flexible membrane. Usually, the membrane bends upward under the tensile stress exerted by the film, but it may also bend downward. Dilational stress was only found with samples showing a strong coffee stain effect, that is, samples in which there is a significant lateral flow from the center to the edge while the film dries. During drying, particles consolidate first at the edge because of the lower height in this region. Continued evaporation from the consolidated region results in a water flow toward the edge, exerting a force onto the latex particles. At the time, when the network is formed, any single sphere must be in a force-balance condition: the network must exert an elastic force onto the sphere which just compensates the viscous drag. Pictorially speaking, a spring (an elastic network) is created while an external force acts onto it. Once the flow stops, the drag force vanishes and the internal stress, which previously compensated the drag, expands the film laterally. This phenomenon can lead to buckling. Given that lateral flow of liquid while films dry is a rather common occurrence, this mode of structure formation should be widespread. It requires lateral flow in conjunction with elastic recovery of the particle network.

Cracking of Drying Latex Films: An ESEM Experiment

In this study environmental scanning electron microscopy was used to observe the cracking of drying latex films below their glass-transition temperature. By controlling the relative humidity so that it decreases linearly with time, a critical level of humidity at which cracking occurs can be determined and this is measured as a function of film thickness. It was found that the cracking humidity decreases with increases in film thickness for thicknesses in the range of 30 to 100 mum and then remains almost unchanged. A scaling argument can be used to fit the data very well and indicates that cracking occurs as soon as the entire film is consolidated into close packing.

Lateral surface nonuniformities in drying latex films

AbstractThe length scales of film thickness nonuniformities, commonly observed in polymer colloid (i.e., latex) films, are predicted. This prediction is achieved by investigating the stability behavior of drying latex films. A linear stability analysis is performed on a base solution representing a uniformly drying latex film containing a surfactant. The analysis identifies film thickness nonuniformities over two length scales: long (millimeter) range (from lubrication theory) and short (micrometer) range (from nonlubrication theory). Evaporation and surfactant desorption into the bulk film are identified as the primary destabilizing mechanisms during drying. Experimental evidence through direct visualization and atomic force microscopy confirm the existence of nonuniformities over both length scales, which are shown to be functions of parameters such as initial particle volume fraction, surfactant amount, and desorption strength, while being independent of drying rate. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Applications of environmental scanning electron microscopy (ESEM) in the study of novel drying latex films

We have employed Environmental Scanning Electron Microscopy (ESEM) and Energy Dispersive X-ray (EDX) analysis to study the microstructural evolution of acrylic latex stabilised with a novel polysaccharide derived from agricultural waste. The analysis revealed that the micro structure of the latex in the ‘wet’ state consists of individual particles and clusters. Upon evaporation a discontinuous film is formed, with voids present within its structure, which is inconsistent with the conventional descriptions of film formation. Further ESEM examination of ‘dry’ specimens revealed that aging resulted in the formation of dendritic structures on the surfaces of the latex films, which EDX analysis confirmed to have been formed via crystallisation of salt. The experimental evidence suggests that the clusters, which are part of the structure of the latex, may act as nucleation centres that would allow the dendrites to form.

Internal Dynamics in Drying Latex Films: A Study Based on Fluorescence Anisotropy and Acoustic Dissipation

The internal dynamics of drying latex films was investigated with an instrument combining measurements of fluorescence depolarization and of acoustic dissipation. A soft latex film, containing a hydrophilic fluorescent probe, was deposited on the surface of a torsional quartz crystal resonator, thereby shifting frequency and bandwidth of the resonance. The depolarization of the fluorescence was recorded in parallel to the acquisition of the resonance parameters. As the film dried out, the fluorescence depolarization and the bandwidth of the resonator both changed in step‐wise manner. The former effect is related to the arrest of rotation on the molecular scale, whereas the acoustic technique probes the viscous compliance on a macroscopic scale. Latexes containing various amounts of sodium dodecyl sulphate (SDS) were studied. For latexes containing little or no added surfactant, the transition from a soft to hard coating (as reflected in fluorescence anisotropy and acoustic dissipation) is rather abrupt. The decrease of rotational mobility and acoustic dissipation occurs at the same time. Adding excess surfactant to the latex decreases the initial softness of the wet film and makes the transition from soft to hard more gradual.

Maps of the stress distributions in drying latex films

We report on spatially resolved measurements of the mechanical stress in drying polymer films. The technique is based on the deflection of a flexible membrane serving as the substrate. Assuming that the lateral tension of the membrane is the main source of its resistance to deformation, one can show that the local surface stress sigma f (x,y) is proportional to the vertical displacement of the membrane u z(x,y). The membrane distortion was determined by optical means. Measurements taken on drying latex dispersions revealed a maximum of film stress at the rim. The heterogeneous stress distribution often persisted after the film had become dry.

Time evolution of transition points in drying latex films

The change of minimum film formation temperature (MFFT) with time was studied utilizing a temperature gradient bar. Fitting the data to a theory which assumed that particles deform due to the action of the polymer-air surface tension, the glass transition temperature, Tg, of the latex was predicted. This Tg was compared to the value obtained by differential scanning calorimetry (DSC) and good agreement was observed between the two measurements.