Co-reporter:Jennifer Lesage de la Haye, Ignacio Martin-Fabiani, Malin Schulz, Joseph L. Keddie, Franck D’Agosto, and Muriel Lansalot
Macromolecules December 12, 2017 Volume 50(Issue 23) pp:9315-9315
Publication Date(Web):November 17, 2017
DOI:10.1021/acs.macromol.7b01885
A major drawback of conventional emulsion polymers arises from the presence of migrating low molar mass surfactants that contribute to poor water barrier properties and low adhesion to substrates. In this paper, we demonstrate how living polymer chains obtained by reversible addition–fragmentation chain transfer (RAFT) can be used as an efficient stabilizer in emulsion polymerization, leading to the production of surfactant-free latexes, which then form cross-linked films with beneficial properties. Hydrophilic poly(methacrylic acid) (PMAA) chains obtained by RAFT performed in water are used to mediate emulsion polymerization and produce film-forming latex particles from mixtures of methyl methacrylate, n-butyl acrylate, and styrene. Stable dispersions of particles with sizes between 100 and 200 nm are obtained, with very low amounts of coagulum (<0.5 wt %). The particles are stabilized by the PMAA segment of amphiphilic block copolymers formed during the polymerization. Remarkably, low amounts of PMAA chains (from 1.5 down to 0.75 wt %) are enough to ensure particle stabilization. Only traces of residual PMAA macroRAFT agents are detected in the final latexes, showing that most of them are successfully chain extended and anchored on the particle surface. The glass transition temperature of the final material is adjusted by the composition of the hydrophobic monomer mixture so that film formation occurs at room temperature. Conventional cross-linking strategies using additional hydrophobic comonomers, such as 1,3-butanediol diacrylate (BuDA), diacetone acrylamide (DAAm), and (2-acetoacetoxy)ethyl methacrylate (AAEM), are successfully applied to these formulations as attested by gel fractions of 100%. When particles are internally cross-linked with BuDA, chain interdiffusion between particles is restricted, and a weak and brittle film is formed. In contrast, when DAAm-containing chains undergoes cross-linking during film formation, full coalescence is achieved along with the creation of a cross-linked network. The resulting film has a higher Young’s modulus and tensile strength as a result of cross-linking. This synthetic strategy advantageously yields a surfactant-free latex that can be formed into a film at room temperature with mechanical properties that can be tuned via the cross-linking density.
Co-reporter:D. K. Makepeace;A. Fortini;A. Markov;P. Locatelli;C. Lindsay;S. Moorhouse;R. Lind;R. P. Sear;J. L. Keddie
Soft Matter (2005-Present) 2017 vol. 13(Issue 39) pp:6969-6980
Publication Date(Web):2017/10/11
DOI:10.1039/C7SM01267E
When films are deposited from mixtures of colloidal particles of two different sizes, a diverse range of functional structures can result. One structure of particular interest is a stratified film in which the top surface layer has a composition different than in the interior. Here, we explore the conditions under which a stratified layer of small particles develops spontaneously in a colloidal film that is cast from a binary mixture of small and large polymer particles that are suspended in water. A recent model, which considers the cross-interaction between the large and small particles (Zhou et al., Phys. Rev. Lett., 2017, 118, 108002), predicts that stratification will develop from dilute binary mixtures when the particle size ratio (α), initial volume fraction of small particles (ϕS), and Péclet number are high. In experiments and Langevin dynamics simulations, we systematically vary α and ϕS in both dilute and concentrated suspensions. We find that stratified films develop when ϕS is increased, which is in agreement with the model. In dilute suspensions, there is reasonable agreement between the experiments and the Zhou et al. model. In concentrated suspensions, stratification occurs in experiments only for the higher size ratio α = 7. Simulations using a high Péclet number, additionally find stratification with α = 2, when ϕS is high enough. Our results provide a quantitative understanding of the conditions under which stratified colloidal films assemble. Our research has relevance for the design of coatings with targeted optical and mechanical properties at their surface.
Co-reporter:Ignacio Martín-Fabiani, Andrea Fortini, Jennifer Lesage de la Haye, Ming Liang Koh, Spencer E. Taylor, Elodie Bourgeat-LamiMuriel Lansalot, Franck D’Agosto, Richard P. Sear, Joseph L. Keddie
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 50) pp:
Publication Date(Web):November 30, 2016
DOI:10.1021/acsami.6b12015
Stratified coatings are used to provide properties at a surface, such as hardness or refractive index, which are different from underlying layers. Although time-savings are offered by self-assembly approaches, there have been no methods yet reported to offer stratification on demand. Here, we demonstrate a strategy to create self-assembled stratified coatings, which can be switched to homogeneous structures when required. We use blends of large and small colloidal polymer particle dispersions in water that self-assemble during drying because of an osmotic pressure gradient that leads to a downward velocity of larger particles. Our confocal fluorescent microscopy images reveal a distinct surface layer created by the small particles. When the pH of the initial dispersion is raised, the hydrophilic shells of the small particles swell substantially, and the stratification is switched off. Brownian dynamics simulations explain the suppression of stratification when the small particles are swollen as a result of reduced particle mobility, a drop in the pressure gradient, and less time available before particle jamming. Our strategy paves the way for applications in antireflection films and protective coatings in which the required surface composition can be achieved on demand, simply by adjusting the pH prior to deposition.Keywords: Brownian dynamics simulations; functional coatings; polymerization-induced self-assembly (PISA); stimuli-responsive; stratification;
Co-reporter:André Utgenannt, Ross Maspero, Andrea Fortini, Rebecca Turner, Marian Florescu, Christopher Jeynes, Antonios G. Kanaras, Otto L. Muskens, Richard P. Sear, and Joseph L. Keddie
ACS Nano 2016 Volume 10(Issue 2) pp:2232
Publication Date(Web):January 14, 2016
DOI:10.1021/acsnano.5b06886
When fabricating photonic crystals from suspensions in volatile liquids using the horizontal deposition method, the conventional approach is to evaporate slowly to increase the time for particles to settle in an ordered, periodic close-packed structure. Here, we show that the greatest ordering of 10 nm aqueous gold nanoparticles (AuNPs) in a template of larger spherical polymer particles (mean diameter of 338 nm) is achieved with very fast water evaporation rates obtained with near-infrared radiative heating. Fabrication of arrays over areas of a few cm2 takes only 7 min. The assembly process requires that the evaporation rate is fast relative to the particles’ Brownian diffusion. Then a two-dimensional colloidal crystal forms at the falling surface, which acts as a sieve through which the AuNPs pass, according to our Langevin dynamics computer simulations. With sufficiently fast evaporation rates, we create a hybrid structure consisting of a two-dimensional AuNP nanoarray (or “nanogrid”) on top of a three-dimensional polymer opal. The process is simple, fast, and one-step. The interplay between the optical response of the plasmonic Au nanoarray and the microstructuring of the photonic opal results in unusual optical spectra with two extinction peaks, which are analyzed via finite-difference time-domain method simulations. Comparison between experimental and modeling results reveals a strong interplay of plasmonic modes and collective photonic effects, including the formation of a high-order stopband and slow-light-enhanced plasmonic absorption. The structures, and hence their optical signatures, are tuned by adjusting the evaporation rate via the infrared power density.Keywords: coupling; evaporation; gold nanoparticles; inverse opal; IR heating; photonics; plasmonics; self-assembly;
Co-reporter:Yang Liu;Agata M. Gajewicz;Victor Rodin;Willem-Jan Soer;Jurgen Scheerder;Guru Satgurunathan;Peter J. McDonald
Journal of Polymer Science Part B: Polymer Physics 2016 Volume 54( Issue 16) pp:1658-1674
Publication Date(Web):
DOI:10.1002/polb.24070
ABSTRACT
The loss of optical transparency when polymer films are immersed in water, which is called “water whitening,” severely limits their use as clear barrier coatings. It is found that this problem is particularly acute in films deposited from polymers synthesized via emulsion polymerization using surfactants. Water whitening is less severe in secondary dispersion (SD) polymers, which are made by dispersing solution polymers in water without the use of surfactants. NMR relaxometry in combination with optical transmission analysis and electron microscopy reveal that some of the water sorbed in emulsion polymer films is contained within nanosized “pockets” or bubbles that scatter light. In contrast, the water in SD polymer films is mainly confined at particle interfaces, where it scatters light less strongly and its molecular mobility is reduced. The addition of surfactant to a SD creates a periodic structure that displays a stop band in the optical transmission. The total amount of sorbed water is not a good indicator of polymers prone to water whitening. Instead, the particular locations of the water within the film must be considered. Both the amount of water and the size of the local water regions (as are probed by NMR relaxometry) are found to determine water whitening. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1658–1674
Co-reporter:Yang Liu, Willem-Jan Soer, Jürgen Scheerder, Guru Satgurunathan, and Joseph L. Keddie
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 22) pp:12147
Publication Date(Web):May 18, 2015
DOI:10.1021/acsami.5b02446
The conventional method for synthesizing waterborne polymer colloids is emulsion polymerization using surfactants. An emerging method is the use of secondary dispersions (SD) of polymers in water, which avoids the addition of any surfactant. Although there are numerous studies of the water barrier properties (sorption, diffusion, and permeability) of waterborne emulsion (Em) polymer coatings, the properties of SD coatings, in comparison, have not been thoroughly investigated. Here, dynamic water vapor sorption analysis is used to compare the equilibrium sorption isotherms of the two forms of styrene–acrylate copolymers (Em and SD) with the same monomer composition. From an analysis of the kinetics of vapor sorption, the diffusion coefficient of water in the polymer coatings is determined. The combined effects of particle boundaries and surfactant addition were investigated through a comparison of the properties of SD and Em coatings to those of (1) solvent-cast polymer coatings (of the same monomer composition), (2) Em polymers that underwent dialysis to partially remove the water-soluble species, and (3) SD polymers with added surfactants. The results reveal that both the particle boundaries and the surfactants increase vapor sorption. The diffusion coefficients of water are comparable in magnitude in all of the polymer systems but are inversely related to water activity because of molecular clustering. Compared to all of the other waterborne polymer systems, the SD barrier coatings show the lowest equilibrium vapor sorption and permeability coefficients at high relative humidities as well as the lowest water diffusion coefficient at low humidities. These barrier properties make SD coatings an attractive alternative to conventional emulsion polymer coatings.Keywords: barrier coatings; emulsion polymers; latex films; secondary dispersion; sorption isotherms; surfactant-free; water diffusion coefficients;
Co-reporter:Robert S. Gurney, Andrew Morse, Elodie Siband, Damien Dupin, Steven P. Armes, Joseph L. Keddie
Journal of Colloid and Interface Science 2015 Volume 448() pp:8-16
Publication Date(Web):15 June 2015
DOI:10.1016/j.jcis.2015.01.074
Copolymerizing an acrylic acid comonomer is often beneficial for the adhesive properties of waterborne pressure-sensitive adhesives (PSAs). Here, we demonstrate a new strategy in which poly(acrylic acid) (PAA) is distributed as a percolating network within a PSA film formed from a polymer colloid. A diblock copolymer composed of PAA and poly(n-butyl acrylate) (PBA) blocks was synthesized using reversible addition–fragmentation chain transfer (RAFT) polymerization and adsorbed onto soft acrylic latex particles prior to their film formation. The thin adsorbed shells on the particles create a percolating network that raises the elastic modulus, creep resistance and tensile strength of the final film. When the film formation occurs at pH 10, ionomeric crosslinking occurs, and high tack adhesion is obtained in combination with high creep resistance. The results show that the addition of an amphiphilic PAA-b-PBA diblock copolymer (2.0 wt.%) to a soft latex provides a simple yet effective means of adjusting the mechanical and adhesive properties of the resulting composite film.
Co-reporter:Ben S. Cooper;Robert S. Gurney;Elodie Sib;Damien Dupin
Macromolecular Chemistry and Physics 2014 Volume 215( Issue 10) pp:998-1003
Publication Date(Web):
DOI:10.1002/macp.201400011
Co-reporter:Farai T. Carter, Radoslaw M. Kowalczyk, Ian Millichamp, Malcolm Chainey, and Joseph L. Keddie
Langmuir 2014 Volume 30(Issue 32) pp:9672-9681
Publication Date(Web):2017-2-22
DOI:10.1021/la5023505
During the past two decades, an improved understanding of the operative particle deformation mechanisms during latex film formation has been gained. For a particular colloidal dispersion, the Routh–Russel deformation maps predict the dominant mechanism for particle deformation under a particular set of conditions (evaporation rate, temperature, and initial film thickness). Although qualitative tests of the Routh–Russel model have been reported previously, a systematic study of the relationship between the film-formation conditions and the resulting water concentration profiles is lacking. Here, the water distributions during the film formation of a series of acrylic copolymer latexes with varying glass-transition temperatures, Tg (values of −22, −11, 4, and 19 °C), have been obtained using GARField nuclear magnetic resonance profiling. A significant reduction in the rate of water loss from the latex copolymer with the lowest Tg was found, which is explained by its relatively low polymer viscosity enabling the growth of a coalesced skin layer. The set of processing parameters where the drying first becomes impeded occurs at the boundary between the capillary deformation and the wet sintering regimes of the Routh–Russel model, which provides strong confirmation of the model’s validity. An inverse correlation between the model’s dimensionless control parameter and the dimensionless drying time is discovered, which is useful for the design of fast-drying waterborne films.
Co-reporter:Robert S. Gurney, Damien Dupin, Elodie Siband, Keltoum Ouzineb, and Joseph L. Keddie
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 6) pp:2137
Publication Date(Web):February 21, 2013
DOI:10.1021/am303184k
We present a simple technique to switch off the tack adhesion in selected areas of a colloidal nanocomposite adhesive. It is made from a blend of soft colloidal polymer particles and hard copolymer nanoparticles. In regions that are exposed to IR radiation, the nanoparticles sinter together to form a percolating skeleton, which hardens and stiffens the adhesive. The tack adhesion is lost locally. Masks can be made from silicone-coated disks, such as coins. Under the masks, adhesive island regions are defined with the surrounding regions being a nontacky coating. When optimizing the nanocomposite’s adhesive properties, the addition of the hard nanoparticles raises the elastic modulus of the adhesive significantly, but adhesion is not lost because the yield point remains relatively low. During probe–tack testing, the soft polymer phases yield and enable fibrillation. After heating under IR radiation, the storage modulus increases by a factor of 5, and the yield point increases nearly by a factor of 6, such that yielding and fibrillation do not occur in the probe–tack testing. Hence, the adhesion is lost. Loading and unloading experiments indicate that a rigid skeleton is created when the nanoparticles sinter together, and it fractures under moderate strains. This patterning method is relatively simple and fast to execute. It is widely applicable to other blends of thermoplastic hard nanoparticles and larger soft particles.Keywords: infrared radiation; latex; nanoparticles; patterning; pressure-sensitive adhesives; tack;
Co-reporter:A. Georgiadis, F.N. Muhamad, A. Utgenannt, J.L. Keddie
Progress in Organic Coatings 2013 Volume 76(Issue 12) pp:1786-1791
Publication Date(Web):December 2013
DOI:10.1016/j.porgcoat.2013.05.017
•A new, simple and inexpensive patterning technique has been developed.•This process combines IR particle sintering with the concept of evaporative lithography.•Under optimized conditions a hard textured coating can be obtained within 5 min without the addition of volatile organic compounds, such as coalescing aids.•The process was used to create novel, textured waterborne coatings to decorate glass bottles.•The process can be applied on nearly any substrate, and it is suitable for batch processing.Polymer coatings with periodic topographic patterns, repeating over millimetre length scales, can be created from lateral flows in an aqueous dispersion of colloidal particles. The flow is driven by differences in evaporation rate across the wet film surface created by IR radiative heating through a shadow mask. This new process, which we call IR radiation-assisted evaporative lithography (IRAEL), combines IR particle sintering with the concept of evaporative lithography. Here, a series of experiments has been conducted in which the mass of the latex is measured as a function of the exposure time under infrared radiation through a mask. The water evaporation rates and the minimum exposure time required for a dry film are estimated as a function of the power density of the IR emitter. The temperature of the wet film is monitored to avoid overheating and boiling of the water, which will otherwise cause defects. It is demonstrated that textured films can be created on a variety of substrates (plastics, metals, paper and glass), and processing times can be as short as 5 min. We use IRAEL to decorate household goods with an aesthetic coating with the desired texture.
Co-reporter:Nadia Akram;Robert S. Gurney;Mohammad Zuber;Muhammad Ishaq
Macromolecular Reaction Engineering 2013 Volume 7( Issue 10) pp:493-503
Publication Date(Web):
DOI:10.1002/mren.201300109
Abstract
A requirement for optimum performance in a pressure-sensitive adhesive (PSA) is the right balance between viscous and elastic properties, achieved by controlling the molecular architecture. Here, waterborne polyurethane PSAs are synthesized using a blend of polyether and polyalkene-based polyols. The effects of the polyol type and molecular weight on the adhesive and thermomechanical properties are explored to optimize them for PSA applications. A linear polyurethane is synthesized by the reaction of an aliphatic diisocyanate with a diol blend of polypropylene glycol (PPG) and hydroxyl-terminated polybutadiene (HTPB). With increasing concentrations of flexible HTPB segments and the associated increased viscous dissipation a favorable increase in the tack adhesion energy and peel strength is obtained. Adhesive properties are improved with increasing PPG molecular weight because chain entanglements become possible in the soft segments and raise the storage modulus.
Co-reporter:Edurne Gonzalez, María Paulis, María Jesús Barandiaran, and Joseph L. Keddie
Langmuir 2013 Volume 29(Issue 6) pp:2044-2053
Publication Date(Web):January 17, 2013
DOI:10.1021/la3049967
In the film formation of latex, particle deformation can occur by processes of wet sintering, dry sintering, or capillary action. When latex films dry nonuniformly and when particles deform and coalesce while the film is still wet, a detrimental skin layer will develop at the film surface. In their process model, Routh and Russel proposed that the operative particle deformation mechanism can be determined by the values of control parameters on a deformation map. Here, the film formation processes of three methyl methacrylate/butyl acrylate copolymer latexes with high glass transition temperatures (Tg), ranging from 45 to 64 °C, have been studied when heated by infrared radiation. Adjusting the infrared (IR) power density enables the film temperature, polymer viscosity, and evaporation rate during latex film formation to be controlled precisely. Different polymer particle deformation mechanisms have been demonstrated for the same latex under a variety of film formation process conditions. When the temperature is too high, a skin layer develops. On the other hand, when the temperature is too low, particles deform by dry sintering, and the process requires extended time periods. The deduced mechanisms can be interpreted and explained by the Routh–Russel deformation maps. Film formation of hard (high Tg) coatings is achieved without using coalescing aids that emit volatile organic compounds (VOCs), which is a significant technical achievement.
Co-reporter:Robert S. Gurney, Damien Dupin, Juliana S. Nunes, Keltoum Ouzineb, Elodie Siband, José M. Asua, Steven P. Armes, and Joseph L. Keddie
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 10) pp:5442
Publication Date(Web):September 13, 2012
DOI:10.1021/am3013642
Soft adhesives require an optimum balance of viscous and elastic properties. Adhesion is poor when the material is either too solidlike or too liquidlike. The ability to switch tack adhesion off at a desired time has many applications, such as in recycling, disassembly of electronics, and painless removal of wound dressings. Here, we describe a new strategy to switch off the tack adhesion in a model nanocomposite adhesive in which temperature is the trigger. The nanocomposite comprises hard methacrylic nanoparticles blended with a colloidal dispersion of soft copolymer particles. At relatively low volume fractions, the nanoparticles (50 nm diameter) accumulate near the film surface, where they pack around the larger soft particles (270 nm). The viscoelasticity of the nanocomposite is adjusted via the nanoparticle concentration. When the nanocomposite is heated above the glass transition temperature of the nanoparticles (Tg = 130 °C), they sinter together to create a rigid network that raises the elastic modulus at room temperature. The tackiness is switched off. Intense infrared radiation is used to heat the nanocomposites, leading to a fast temperature rise. Tack adhesion is switched off within 30 s in optimized compositions. These one-way switchable adhesives have the potential to be patterned through localized heating.Keywords: adhesion; latex; nanoparticles; pressure-sensitive adhesive; self-assembly; sintering;
Co-reporter:Argyrios Georgiadis, Alexander F. Routh, Martin W. Murray and Joseph L. Keddie
Soft Matter 2011 vol. 7(Issue 23) pp:11098-11102
Publication Date(Web):13 Oct 2011
DOI:10.1039/C1SM06527K
Polymer coatings with periodic topographic patterns, repeating over millimetre length scales, are created from lateral flows in an aqueous dispersion of colloidal particles. The flow is driven by differences in evaporation rate across the wet film surface created by IR radiative heating through a shadow mask. This new process, which we call IR radiation-assisted evaporative lithography (IRAEL), combines IR particle sintering with the concept of evaporative lithography. We show that the height of the surface features increases with an increase in several key parameters: the initial thickness of the film, the volume fraction of particles, and the pitch of the pattern. The results are interpreted by using models of geometry and particle transport. The patterned coatings can function as “paintable” microlens arrays, applicable to nearly any surface. Compared with existing methods for creating textured coatings, IRAEL is simpler, inexpensive, able to create a wide variety of bespoke surfaces, and applicable to nearly any substrate without prior preparation.
Co-reporter:Akarin Intaniwet, Joseph L. Keddie, Maxim Shkunov, Paul J. Sellin
Organic Electronics 2011 Volume 12(Issue 11) pp:1903-1908
Publication Date(Web):November 2011
DOI:10.1016/j.orgel.2011.08.003
A new class of X-ray sensor – in which there is a blend of poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene in the active layer of a diode structure – has been developed. The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the sensor. The first time-of-flight charge-carrier mobility measurement of this blend is reported. The mobility of PTAA and TIPS-pentacene in a 1:25 molar ratio was found to be 2.2 × 10−5 cm2 V−1 s−1 (averaged for field strengths between 3 × 104 and 4 × 105 V cm−1), which is about 17 times higher than that obtained in PTAA over the same range of field strengths. This higher mobility is correlated with a fourfold increase in the X-ray detection sensitivity in the PTAA:TIPS-pentacene devices.Graphical abstractHighlights► New X-ray sensors are made from blends of poly(triarylamine) and TIPS pentacene. ► The charge carrier mobility is 17 times higher when TIPS pentacene is added. ► The sensor’s sensitivity increases by a factor of four. ► The organic device has an advantage of being “tissue equivalent”.
Co-reporter:Aitziber Lopez, Elise Degrandi, Elisabetta Canetta, Joseph L. Keddie, Costantino Creton, José M. Asua
Polymer 2011 Volume 52(Issue 14) pp:3021-3030
Publication Date(Web):22 June 2011
DOI:10.1016/j.polymer.2011.04.053
Water-borne polyurethane/acrylic hybrid latexes for their application as pressure-sensitive adhesives were prepared by high solids simultaneous free radical and addition miniemulsion polymerization. In these polymerizations, the polymer network was formed by polyurethane chains that were linked to acrylic chains by the joint reaction of a hydroxyl functional methacrylate and an isocyanate functional polyurethane prepolymer, and by acrylic chains linked among themselves. Under the working conditions, the modification of the polymer architecture was possible by altering the acrylic chains, the polyurethane chains and the links between the polyurethane and the acrylic chains. In the present work, the polymer microstructure was modified by the addition of different diols (polyurethane chain extenders) to the formulation. The effect of the nature of the diol on polymerization kinetics, polymer microstructure and adhesive performance of water-borne PSAs was studied. Adhesive test results demonstrate clearly for the first time that the long-term resistance to shear of an acrylic PSA does not depend only on its gel content but very much depends on the detailed microstructure of the gel inside each particle.
Co-reporter:Argyrios Georgiadis, Peter A. Bryant, Martin Murray, Philip Beharrell, and Joseph L. Keddie
Langmuir 2011 Volume 27(Issue 6) pp:2176-2180
Publication Date(Web):February 22, 2011
DOI:10.1021/la200429j
The film formation of an acrylate latex with a glass-transition temperature of 38 °C has been achieved through the use of near-infrared (NIR) radiative heating. A hard, crack-free coating was obtained without the addition of plasticizers. Sintering of acrylate particles was confirmed through measurements using atomic force microscopy. The addition of an NIR-absorbing polymer increased the rate of particle deformation such that it was significantly greater than obtained in a convection oven at 60 °C. The results are consistent with a lower polymer viscosity under infrared radiation, according to a simple analysis using a standard model of sintering.
Co-reporter:Aitziber Lopez, Elise Degrandi-Contraires, Elisabetta Canetta, Costantino Creton, Joseph L. Keddie, and José M. Asua
Langmuir 2011 Volume 27(Issue 7) pp:3878-3888
Publication Date(Web):March 11, 2011
DOI:10.1021/la104830u
Waterborne polyurethane−acrylic hybrid nanoparticles for application as pressure-sensitive adhesives (PSAs) were prepared by one-step miniemulsion polymerization. The addition of polyurethane to a standard waterborne acrylic formulation results in a large increase in the cohesive strength and hence a much higher shear holding time (greater than seven weeks at room temperature), which is a very desirable characteristic for PSAs. However, with the increase in cohesion, there is a decrease in the relative viscous component, and hence there is a decrease in the tack energy. The presence of a small concentration of methyl methacrylate (MMA) in the acrylic copolymer led to phase separation within the particles and created a hemispherical morphology. The tack energy was particularly low in the hybrid containing MMA because of the effects of lower energy dissipation and greater cross-linking. These results highlight the great sensitivity of the viscoelastic and adhesive properties to the details of the polymer network architecture and hence to the precise composition and synthesis conditions.
Co-reporter:Akarin Intaniwet, Christopher A. Mills, Paul J. Sellin, Maxim Shkunov and Joseph L. Keddie
ACS Applied Materials & Interfaces 2010 Volume 2(Issue 6) pp:1692
Publication Date(Web):May 24, 2010
DOI:10.1021/am100220y
Existing inorganic materials for radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, lack of tissue-equivalence, toxicity, and mechanical inflexibility. As an alternative to inorganics, poly(triarylamine) (PTAA) diodes have been evaluated for their suitability for detecting radiation via the direct creation of X-ray induced photocurrents. A single layer of PTAA is deposited on indium tin oxide (ITO) substrates, with top electrodes selected from Al, Au, Ni, and Pd. The choice of metal electrode has a pronounced effect on the performance of the device; there is a direct correlation between the diode rectification factor and the metal-PTAA barrier height. A diode with an Al contact shows the highest quality of rectifying junction, and it produces a high X-ray photocurrent (several nA) that is stable during continuous exposure to 50 kV Mo Kα X-radiation over long time scales, combined with a high signal-to-noise ratio with fast response times of less than 0.25 s. Diodes with a low band gap, ‘Ohmic’ contact, such as ITO/PTAA/Au, show a slow transient response. This result can be explained by the build-up of space charge at the metal-PTAA interface, caused by a high level of charge injection due to X-ray-induced carriers. These data provide new insights into the optimum selection of metals for Schottky contacts on organic materials, with wider applications in light sensors and photovoltaic devices.Keywords: charge transport; conjugated polymer; organic electronics; poly(triarylamine) (PTAA); rectification; sensor
Co-reporter:Dan Liu, Che Azurahanim Che Abdullah, Richard P. Sear and Joseph L. Keddie
Soft Matter 2010 vol. 6(Issue 21) pp:5408-5416
Publication Date(Web):09 Sep 2010
DOI:10.1039/C0SM00201A
The coating of substrates with an extracellular matrix (ECM) protein, such as fibronectin (FN), is often employed to increase cell adhesion and growth. Here, we examine the influence of the size scale and geometry of novel FN nanopatterns on the adhesion and spreading of Chinese Hamster Ovary (CHO) cells. The FN is patterned on the surface of templates created through the self-assembly of polystyrene-block-polyisoprene (PS-b-PI) diblock copolymers. Both ring-like and stripe-like FN nanopatterns are created through the preferential adsorption of FN on PS blocks, as confirmed through the complementary use of atomic force microscopy and secondary ion mass spectrometry. The ring-like FN nanopattern substrate increases the cells' adhesion compared with the cells on homogeneous FN surfaces and the stripe-like FN nanopatterns. Cell adhesion is high when the FN ring size is greater than 50 nm and when the surface coverage of FN is less than ca. 85%. We suggest that the ring-like nanopatterns of FN may be aiding cell adhesion by increasing the clustering of the proteins (integrins) with which cells bind to the nanopatterned substrate. This clustering is required for cell adhesion. In comparison to lithographic techniques, the FN templating method, presented here, provides a simple, convenient and economical way of coating substrates for tissue cultures and should be applicable to tissue engineering.
Co-reporter:Tao Wang, Carolina de las Heras Alarcón, Monika Goikoetxea, Itxaso Beristain, Maria Paulis, Maria J. Barandiaran, José M. Asua and Joseph L. Keddie
Langmuir 2010 Volume 26(Issue 17) pp:14323-14333
Publication Date(Web):August 12, 2010
DOI:10.1021/la102392x
Hybrids made from an alkyd resin and an acrylic copolymer can potentially combine the desired properties of each component. Alkyd/acrylic hybrid latex particles were synthesized via miniemulsion polymerization and used to create films at room temperature. Comparisons of the alkyd auto-oxidative cross-linking rates and the associated network development are made between two alkyd resins (with differing levels of hydrophilicity as measured by their acid numbers). The effects of increasing the compatibilization between the alkyd and the acrylic phase via functionalization with glycidyl methacrylate (GMA) are investigated. Magnetic resonance profiling and microindentation measurements reveal that film hardening occurs much faster in a GMA-functionalized alkyd hybrid than in the standard hybrid. The film’s hardness increases by a factor of 4 over a 5-day period. The rate of cross-linking is significantly slower in nonfunctionalized alkyd hybrid films and when the more hydrophilic alkyd resin is used. Tensile deformation of the hybrid latex films reveals the effects of GMA functionalization and drier concentration in creating a denser cross-linked network. Modeling of the tensile deformation behavior of the hybrid films used a combination of the upper convected Maxwell model (to describe the viscoelastic component) and the Gent model (to describe the elastic component). The modeling provides a correlation between the cross-linked network formation and the resulting mechanical properties.
Co-reporter:Tao Wang, Joseph L. Keddie
Advances in Colloid and Interface Science 2009 Volumes 147–148() pp:319-332
Publication Date(Web):March–June 2009
DOI:10.1016/j.cis.2008.06.002
Abstract
It is well established that colloidal polymer particles can be used to create organised structures by methods of horizontal deposition, vertical deposition, spin-casting, and surface pattern-assisted deposition. Each particle acts as a building block in the structure. This paper reviews how two-phase (or hybrid) polymer colloids can offer an attractive method to create nanocomposites. Structure in the composite can be controlled at the nanoscale by using such particles. Methods to create armored particles, such as via methods of hetero-flocculation and Pickering polymerization, are of particular interest here. Polymer colloids can also be blended with other types of nanoparticles, e.g. nanotubes and clay platelets, to create nanocomposites. Structure can be controlled over length scales approaching the macroscopic through the assembly of hybrid particles or particle blends via any of the various deposition methods. Colloidal nanocomposites can offer unprecedented long-range 2D or 3D order that provides a periodic modulation of physical properties. They can also be employed as porous templates for further nanomaterial fabrication. Challenges in the design and control of the macroscopic properties, especially mechanical, are considered. The importance of the internal interfacial structure (e.g. between inorganic and polymer particles) is highlighted.
Co-reporter:Tao Wang, Elisabetta Canetta, Tecla G. Weerakkody and Joseph L. Keddie, Urko Rivas
ACS Applied Materials & Interfaces 2009 Volume 1(Issue 3) pp:631
Publication Date(Web):March 10, 2009
DOI:10.1021/am800179y
Polymer colloids are often copolymerized with acrylic acid monomers in order to impart colloidal stability. Here, the effects of the pH on the nanoscale and macroscopic adhesive properties of waterborne poly(butyl acrylate-co-acrylic acid) films are reported. In films cast from acidic colloidal dispersions, hydrogen bonding between carboxylic acid groups dominates the particle−particle interactions, whereas ionic dipolar interactions are dominant in films cast from basic dispersions. Force spectroscopy using an atomic force microscope and macroscale mechanical measurements show that latex films with hydrogen-bonding interactions have lower elastic moduli and are more deformable. They yield higher adhesion energies. On the other hand, in basic latex, ionic dipolar interactions increase the moduli of the dried films. These materials are stiffer and less deformable and, consequently, exhibit lower adhesion energies. The rate of water loss from acidic latex is slower, perhaps because of hydrogen bonding with the water. Therefore, although acid latex offers greater adhesion, there is a limitation in the film formation.Keywords: acrylic acid; adhesion; adhesives; film formation; latex; pH
Co-reporter:Tao Wang, Patrick J. Colver, Stefan A. F. Bon and Joseph L. Keddie
Soft Matter 2009 vol. 5(Issue 20) pp:3842-3849
Publication Date(Web):01 Jul 2009
DOI:10.1039/B904740A
Numerous synthesis routes toward nanostructured polymer particles have emerged, but few examples demonstrate the essential need for such complex particle structures to achieve any added benefit in a target application. Here, polymer particles having Laponite clay armor were prepared by the Pickering miniemulsion polymerization of n-lauryl acrylate. The resulting “soft–hard” poly(lauryl acrylate) (PLA)–Laponite hybrid particles were blended at various low concentrations with a standard poly(butyl acrylate-co-acrylic acid) (PBA) latex for application as a waterborne pressure-sensitive adhesive (PSA). The tack adhesion properties of the resulting nanocomposite films were compared with the performance of the PBA when blended with either a conventional non-armored PLA latex, with Laponite RD nanosized clay discs, or a mixture of both. A true synergistic effect was discovered showing that the clay-armored supracolloidal structure of the hybrid particles was essential to achieve a superior balance of viscoelastic properties. The addition of small amounts, e.g. 2.7 wt%, of the “soft–hard” clay-armored PLA particles increased the tack adhesion energy considerably more than found for the two individual components or for the sum of their individual contributions. The soft PLA core ensures that the adhesives are not stiffened too much by the nanosized Laponite clay. Slippage at the interface between the nanoclay platelets and the PBA matrix introduces an additional energy dissipation mechanism during deformation. Through the synergistic effect of the clay and PLA in the supracolloidal armored latex structure, the tack adhesion energy is increased by 45 J m−2, which is about 70% greater than found for the PBA adhesive alone.
Co-reporter:Dan Liu, Tao Wang and Joseph L. Keddie
Langmuir 2009 Volume 25(Issue 8) pp:4526-4534
Publication Date(Web):March 10, 2009
DOI:10.1021/la8038957
Templated surfaces can be used to create patterns of proteins for applications in cell biology, biosensors, and tissue engineering. A diblock copolymer template, which contains a pair of hydrophobic blocks, has been developed. The template is created from well-ordered, nonequilibrium surface structures of poly(styrene-b-isoprene) (PS-b-PI) diblock copolymers, which are achieved in ultrathin films having a thickness of less than one domain period. Adsorption and nanopatterning of bovine serum albumin (BSA) on these thin films were studied. After incubation of the copolymer templates in BSA solutions (500 μg/mL) for a period of 1 h, BSA molecules formed either a striped or a dense, ringlike structure, closely resembling the underlying polymer templates. In this “hard-soft” PS-b-PI system, BSA molecules were preferentially adsorbed on the hard PS domains, rather than on the soft PI domains. Secondary ion mass spectroscopy (SIMS) and contact angle analysis revealed that, with more PI localized at the free surface, fewer BSA molecules were adsorbed. SIMS analysis confirmed that BSA molecules were adsorbed selectively on the PS blocks. This is the first example of two hydrophobic blocks of a diblock copolymer being used as a protein patterning template. Previously reported diblock copolymer templates used hydrophilic and hydrophobic pairs. A potentially useful characteristic of this template is that it is effective at high protein solution concentrations (up to 1 mg/mL) and for long incubation times (up to 2 h), which broadens its range of applicability in various uses.
Co-reporter:Elisabetta Canetta, Jeanne Marchal, Chun-Hong Lei, Fanny Deplace, Alexander M. König, Costantino Creton, Keltoum Ouzineb and Joseph L. Keddie
Langmuir 2009 Volume 25(Issue 18) pp:11021-11031
Publication Date(Web):June 9, 2009
DOI:10.1021/la901324n
Tackifying resins (TRs) are often added to pressure-sensitive adhesive films to increase their peel strength and adhesion energy. In waterborne adhesives, the TR is dispersed in water using surfactants and then blended with colloidal polymers in water (i.e., latex). In such waterborne systems, there are problems with the colloidal stability and difficulty in applying coatings of the particle blends; the films are often hydrophilic and subject to water uptake. Here, an alternative method of making waterborne, tackified adhesives is demonstrated. The TR is incorporated within the core of colloidal polymer particles via miniemulsion polymerization. Atomic force microscopy (AFM) combined with force spectroscopy analysis reveals there is heterogeneity in the distribution of the TR in films made from particle blends and also in films made from miniemulsion polymers. Two populations, corresponding to TR-rich and acrylic-rich components, were identified through analysis of the AFM force-displacement curves. The nanoscale maximum adhesion force and adhesion energy were found to be higher in a miniemulsion film containing 12 wt % tackifying resin in comparison to an equivalent blended film. The macroscale tack and viscoelasticity are interpreted by consideration of the nanoscale structure and properties. The incorporation of tackifying resin through a miniemulsion polymerization process not only offers clear benefits in the processing of the adhesive, but it also leads to enhanced adhesion properties.
Co-reporter:T. Wang;C.-H. Lei;D. Liu;M. Manea;J. M. Asua;C. Creton;A. B. Dalton;J. L. Keddie
Advanced Materials 2008 Volume 20( Issue 1) pp:90-94
Publication Date(Web):
DOI:10.1002/adma.200700821
Co-reporter:Tao Wang, Alan B. Dalton and Joseph L. Keddie
Macromolecules 2008 Volume 41(Issue 20) pp:7656-7661
Publication Date(Web):September 30, 2008
DOI:10.1021/ma800868z
The effects of physisorbed polymer molecules on carbon nanotubes dispersed in a soft polymer matrix on the resulting mechanical strength of the nanocomposite are reported. From measurements of the large-strain deformation of the nanocomposites, the shear strength, τ, of the nanotube/matrix interface was determined as a function of the interfacial polymer chain length and the chain density, Σ. The results show that the value of τ (per chain) increases with increasing chain length. τ likewise increases with Σ but then levels off above a critical value. These results are explained by the molecular friction of the adsorbed polymer chains sliding along the rubbery polymer matrix. The results can be used to guide the interfacial design of polymer nanocomposites to obtain ultimate macroscopic mechanical control. In particular, the monomeric friction coefficient, ξ1, could be used to adjust the macroscopic properties of this type of nanocomposite.
Co-reporter:Alexander M. König, Tecla G. Weerakkody, Joseph L. Keddie and Diethelm Johannsmann
Langmuir 2008 Volume 24(Issue 14) pp:7580-7589
Publication Date(Web):June 19, 2008
DOI:10.1021/la800525n
Using magnetic resonance profiling coupled with dynamic light scattering, we have investigated the mechanisms leading to the formation of a partly coalesced surface layer, or “open skin”, during film formation from waterborne polymer dispersions. We present the first use of the skewness of the distribution of free water as a model-free indicator of the spatial nonuniformity of drying. The skewness reaches a maximum at the same time at which a strong, static component, presumably originating from a skin at the film/air interface, appears in the light scattering data. Addition of salt to the dispersion increases both the skewness of the distribution of free water and the propensity for skin formation. Surprisingly, the drying is influenced not only by the concentration and valency of the ions in the salt but also by the particular ion. At intermediate particle densities, added salt strongly lowers the cooperative diffusion coefficient, Dcoop. When the particles reach close packing, Dcoop sharply increases. If the particles readily coalesce, the effects of the increased diffusivity will be counteracted, thereby inducing the formation of a skin. A modified Peclet number, Pe, using Dcoop, is proposed, so that the presence of salt is explicitly considered. This modified Pe is able to predict the nonuniformity in drying that leads to skin formation.
Co-reporter:C.H. Lei, K. Ouzineb, O. Dupont, J.L. Keddie
Journal of Colloid and Interface Science 2007 Volume 307(Issue 1) pp:56-63
Publication Date(Web):1 March 2007
DOI:10.1016/j.jcis.2006.11.036
There is a need to know the nanostructure of pressure-sensitive adhesive (PSA) films obtained from waterborne polymer colloids so that it can be correlated with properties. Intermittent-contact atomic force microscopy (AFM) of an acrylic waterborne PSA film identifies two components, which can be attributed to the polymer and the solids in the serum (mainly surfactant). It is found that when the average AFM tapping force, FavFav, is relatively low, the polymer particles appear to be concave. But when FavFav is higher, the particles appear to have a convex shape. This observation is explained by a height artefact caused by differences in the indentation depths into the two components that vary with the tapping amplitude and FavFav. To achieve the maximum contrast between the polymer and serum components, FavFav should be set such that the indentation depths are as different as possible. Unlike what is found for the height images, the phase contrast images of the PSA do not show a reversal in contrast over the range of tapping conditions applied. The phase images are thus reliable in distinguishing the two components of the PSA according to their viscoelastic properties. At the surface of films dried at room temperature, the serum component is found in localized regions within permanent depression into the film.Whether particles in an adhesive latex film appear convex or concave in atomic force microscopy depends on the differences in indentation depths between the polymer and serum phases, which are shown to vary with the tapping force. The corresponding phase contrast images (not shown) do not undergo a reversal in contrast.
Co-reporter:A. B. Dalton;C. Creton;Y. Lin;J. M. Asua;K. A. S. Ferno;Y.-P. Sun;M. Manea;J. L. Keddie;C.-H. Lei;T. Wang
Advanced Materials 2006 Volume 18(Issue 20) pp:2730-2734
Publication Date(Web):15 SEP 2006
DOI:10.1002/adma.200601335
Transparent and conductive pressure-sensitive adhesives are cast from aqueous colloidal dispersions of poly(butyl acrylate) (P(BuA)) and functionalized carbon nanotubes (CNTs). At the percolation threshold for network formation (at only 0.3 wt % functionalized CNT), the nanotubes remarkably double the amount of strain at adhesive failure and increase the adhesion energy by 85 % (see figure). The tack properties are explained by current models of adhesive debonding.
Co-reporter:P. Vandervorst, C.-H. Lei, Y. Lin, O. Dupont, A.B. Dalton, Y.-P. Sun, J.L. Keddie
Progress in Organic Coatings 2006 Volume 57(Issue 2) pp:91-97
Publication Date(Web):2 October 2006
DOI:10.1016/j.porgcoat.2006.07.005
Nanocomposites of a polymer and carbon nanotubes exhibit high electrical and thermal conductivity and enhanced mechanical properties in comparison to the polymer alone. Film formation from latex dispersions is an ideal way to create nanocomposite coatings with the advantages of solvent-free processing and a high uniformity of dispersion. It is shown here that carbon nanotubes functionalised with poly(vinyl alcohol) (PVA) can be blended with two types of acrylic latex to create stable colloidal dispersions without the need for added surfactant or emulsifier. Waterborne nanocomposite films with optical transparency can be formed. Microscopic analysis shows that the PVA-functionalized nanotubes are finely and uniformly dispersed in the polymer matrix.
Co-reporter:Z. Tabatabaian;T. R. E. Simpson;C. Jeynes;B. Parbhoo;J. L. Keddie
Journal of Polymer Science Part A: Polymer Chemistry 2004 Volume 42(Issue 6) pp:1421-1431
Publication Date(Web):28 JAN 2004
DOI:10.1002/pola.20006
We have determined with infrared spectroscopic ellipsometry how the nature of the interface between a thin poly(dimethyl siloxane) (PDMS) coating and its substrate affects the rate of PDMS crosslinking reactions. Reactions between vinyl (CHCH2) end groups on PDMS and silyl (SiH) groups in a crosslinker (hydrosilylation) and between SiH groups and silanol (SiOH) groups, during the so-called postcure crosslinking stage, have been probed in situ. The overall consumption of SiH follows first-order reaction kinetics. The first-order reaction coefficient (k1) for the hydrosilylation crosslinking reaction is the same for coatings on three different substrates: native oxide on silicon (SiO2/Si), polystyrene (PS), and poly(ethylene terephthalate). For the slower postcure reactions, however, the rate of SiH consumption depends on the substrate. In 2.5-μm PDMS coatings on PS, k1 is about seven times greater than k1 in the same coating on SiO2/Si. In PDMS coatings on a PDMS substrate, when the effect of the interface is thus minimal, k1 is 16 times higher than on SiO2/Si. The dependence of k1 on the type of interface is probably the result of the interfacial segregation and complexation of the Pt catalyst for the postcure reactions. We propose that polar surfaces more strongly attract Pt and form complexes that inhibit the postcure reactions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1421–1431, 2004
