Hydroplasticization of polymers: model predictions and application to emulsion polymers

Synthesis of dimpled polymer particles and polymer particles with protrusions - Past, present, and future

Since the development of emulsion polymerization techniques, polymer particles have become the epitome of standard colloids due to the exceptional control over size, size distribution, and composition the synthesis methods allow reaching. The exploration of different variations of the synthesis methods has led to the discovery of more advanced techniques, enabling control over their composition and shape. Many early investigations focused on forming particles with protrusions (with one protrusion, called dumbbell particles) and particles with concavities, also called dimpled particles. This paper reviews the literature covering the synthesis, functionalization, and applications of both types of particles. The focus has been on the rationalization of the various approaches used to prepare such particles and on the discussion of the mechanisms of formation not just from the experimental viewpoint but also from the standpoint of thermodynamics. The primary motivation to combine in a single review the preparation of both types of particles has been the observation of similarities among some of the methods developed to prepare dimpled particles, which sometimes include the formation of particles with protrusions and vice versa. The most common applications of these particles have been discussed as well. By looking at the different approaches developed in the literature under one general perspective, we hope to stimulate a more ample use of these particles and promote the development of even more effective synthetic protocols.

Polymer Colloids: Current Challenges, Emerging Applications, and New Developments

Polymer colloids are complex materials that have the potential to be used in a vast array of applications. One of the main reasons for their continued growth in commercial use is the water-based emulsion polymerization process through which they are generally synthesized. This technique is not only highly efficient from an industrial point of view but also extremely versatile and permits the large-scale production of colloidal particles with controllable properties. In this perspective, we seek to highlight the central challenges in the synthesis and use of polymer colloids, with respect to both existing and emerging applications. We first address the challenges in the current production and application of polymer colloids, with a particular focus on the transition toward sustainable feedstocks and reduced environmental impact in their primary commercial applications. Later, we highlight the features that allow novel polymer colloids to be designed and applied in emerging application areas. Finally, we present recent approaches that have used the unique colloidal nature in unconventional processing techniques.

Designed incorporation of semi-crystalline domains into structured latex particles via solvent-aided emulsion polymerization

We describe an emulsion polymerization route towards the design of structured latex particles containing semi-crystalline domains with improved mechanical properties.

Surface Morphology Signature of Critical Separated Length and Glass Transition Temperature during Seeded Dispersion Polymerization

The properties of colloids are considerably affected by particles' surface morphology. In this work, for understanding the mechanism of roughness formation in polymeric core-shell (CS) particles, the surface morphology of synthesized CS particles through seeded dispersion polymerization (SDP) in the presence of poly(methyl methacrylate) seeds was investigated. The results revealed that shell polymers with higher solubility parameters (δ) and glass transition temperatures (T_g) had a rougher surface. These parameters directly affect the time needed for chain deformation, which is a critical parameter in controlling the final morphology. We suggested a relation based on these parameters to predict the surface morphology (smoothness or roughness) of CS particles synthesized through SDP in water.

A surprisingly gentle approach to cavity containing spherocylindrical microparticles from ordinary polymer dispersions in flow

Herein, we demonstrate a facile approach to fully transform spherical polymeric microparticles to elongated spherocylinders containing an internal cavity under ambient and mild stirring conditions. Critical to the process is to deform the amorphous and non-crosslinked particles under glassy conditions for an unusually long time; 120 hours for the poly(styrene-co-glycidyl methacrylate) microparticles discussed in greatest detail. Larger particles in the 5 micron and greater range were markedly more susceptible to the shear imposed by stirring the aqueous dispersion. The resulting morphology is robust and kinetically frozen yet reverts to the original spherical shape if annealed above the glass transition temperature with suitable temperature or plasticizer. The volume fraction of the internal void can be modulated by particle composition and process conditions and is irregular in shape we believe as a result of a cavitation event during plastic deformation.

Ionic Inter-Particle Complexation Effect on the Performance of Waterborne Coatings

The performance of waterborne (meth)acrylic coatings is critically affected by the film formation process, in which the individual polymer particles must join to form a continuous film. Consequently, the waterborne polymers present lower performance than their solvent-borne counter-polymers. To decrease this effect, in this work, ionic complexation between oppositely charged polymer particles was introduced and its effect on the performance of waterborne polymer films was studied. The (meth)acrylic particles were charged by the addition of a small amount of ionic monomers, such as sodium styrene sulfonate and 2-(dimethylamino)ethyl methacrylate. Density functional theory calculations showed that the interaction between the selected main charges of the respective functional monomers (sulfonate-amine) is favored against the interactions with their counter ions (sulfonate-Na and amine-H). To induce ionic complexation, the oppositely charged latexes were blended, either based on the same number of charges or the same number of particles. The performance of the ionic complexed coatings was determined by means of tensile tests and water uptake measurements. The ionic complexed films were compared with reference films obtained at pH at which the cationic charges were in neutral form. The mechanical resistance was raised slightly by ionic bonding between particles, producing much more flexible films, whereas the water penetration within the polymeric films was considerably hindered. By exploring the process of polymer chains interdiffusion using Fluorescence Resonance Energy Transfer (FRET) analysis, it was found that the ionic complexation was established between the particles, which reduced significantly the interdiffusion process of polymer chains. The presented ionic complexes of sulfonate-amine functionalized particles open a promising approach for reinforcing waterborne coatings.

Light-Activated Adhesion and Debonding of Underwater Pressure-Sensitive Adhesives

Pressure-sensitive adhesives (PSAs) such as sticky notes and labels are a ubiquitous part of modern society. PSAs with a wide range of peel adhesion strength are designed by tailoring the bulk and surface properties of the adhesive. However, designing an adhesive with strong initial adhesion but showing an on-demand decrease in adhesion has been an enduring challenge in the design of PSAs. To address this challenge, we designed alkoxyphenacyl-based polyurethane (APPU) PSAs that show a photoactivated increase and decrease in peel strength. With increasing time of light exposure, the failure mode of our PSAs shifted from cohesive to adhesive failure, providing residue-free removal with up to 83% decrease in peel strength. The APPU-PSAs also adhere to substrates submerged underwater and show a similar photoinduced decrease in adhesion strength.

Bioresponsive Hybrid Nanofibers Enable Controlled Drug Delivery through Glass Transition Switching at Physiological Temperature

To avoid excessive usage of antibiotics and antimicrobial agents, smart wound dressings permitting controlled drug release for treatment of bacterial infections are highly desired. In search of a sensitive stimulus to activate drug release under physiological conditions, we found that the glass transition temperature (T_g) of a polymer or polymer blend can be an ideal parameter because a thermal stimulus can regulate drug release at the physiological temperature of 37 °C. A well-tuned T_g for a controlled drug release from fibers at 37 °C was achieved by varying the blending ratio of Eudragit® RS 100 and poly(methyl methacrylate). Octenidine, an antimicrobial agent often used in wound treatment, was encapsulated into the polymer blend during the electrospinning process and evaluated for its controlled release based on modulation of temperature. The thermal switch of the nanofibrous membranes can be turned "on" at physiological temperature (37 °C) and "off" at room temperature (25 °C), conferring a controlled release of octenidine. It was found that octenidine can be released in an amount at least 8.5 times higher (25 mg·L^-1) during the "on" stage compared to the "off" stage after 24 h, which was regulated by the wet T_g (34.8-36.5 °C). The "on"/"off" switch for controlled drug release can moreover be repeated at least 5 times. Furthermore, the fabricated nanofibrous membranes displayed a distinctive antibacterial activity, causing a log3 reduction of the viable cells for both Gram negative and positive pathogens at 37 °C, when the thermal switch was "on". This study forms the groundwork for a treatment concept where no external stimulus is needed for the release of antimicrobials at physiological conditions, and will help reduce the overuse of antibiotics by allowing controlled drug release.

Water-Resistant Latex Coatings: Tuning of Properties by Polymerizable Surfactant, Covalent Crosslinking and Nanostructured ZnO Additive

This paper deals with the development of acrylic latexes providing high-performance water-resistant coatings. For this purpose, mutual effects of anionic surfactant type (ordinary and polymerizable), covalent intra- and/or interparticle crosslinking (introduced by allyl methacrylate copolymerization and keto-hydrazide reaction, respectively) and ionic crosslinking (provided by nanostructured ZnO additive) were investigated. The latexes were prepared by the standard emulsion polymerization of methyl methacrylate, butyl acrylate and methacrylic acid as the main monomers. The addition of surface-untreated powdered nanostructured ZnO was performed during latex synthesis, resulting in stable latexes comprising dispersed nanosized additive in the content of ca 0.9−1.0 wt.% (based on solids). The coating performance with emphasis on water resistance was evaluated. It was determined that the application of the polymerizable surfactant improved coating adhesion and water-resistance, but it wasn′t able to ensure high water-resistance of coatings. Highly water-resistant coatings were obtained provided that covalent intra- and interparticle crosslinking together with ionic crosslinking were employed in the coating composition, forming densely crosslinked latex films. Moreover, coatings comprising nanostructured ZnO additive displayed a significant antibacterial activity and improved solvent resistance.

Green Electrospinning of Polymer Latexes: A Systematic Study of the Effect of Latex Properties on Fiber Morphology

Green electrospinning is a relatively new promising technology in which a polymer (latex) can be spun from an aqueous dispersion with the help of a template polymer. This method is a green, clean and safe technology that is able to spin hydrophobic polymers using water as an electrospinning medium. In this article, a systematic study that investigates the influence of the template polymer molar mass, the total solids content of the initial dispersion and the particle/template ratio is presented. Furthermore, the influence of the surfactant used to stabilize the polymer particles, the surface functionality of the polymer particles and the use of a bimodal particle size distribution on the final fiber morphology is studied for the first time. In green electrospinning, the viscosity of the initial complex blend depends on the amount and molar mass of the template polymer but also on the total solids content of the dispersion to be spun. Thus, both parameters must be carefully taken into account in order to fine-tune the final fiber morphology. Additionally, the particle packing and the surface chemistry of the polymer particles also play an important role in the obtained nanofibers quality.

Photochemically induced cyclic morphological dynamics via degradation of autonomously produced, self-assembled polymer vesicles

Autonomous and out-of-equilibrium vesicles synthesised from small molecules in a homogeneous aqueous medium are an emerging class of dynamically self-assembled systems with considerable potential for engineering natural life mimics. Here we report on the physico-chemical mechanism behind a dynamic morphological evolution process through which self-assembled polymeric structures autonomously booted from a homogeneous mixture, evolve from micelles to giant vesicles accompanied by periodic growth and implosion cycles when exposed to oxygen under light irradiation. The system however formed nano-objects or gelation under poor oxygen conditions or when heated. We determined the cause to be photoinduced chemical degradation within hydrated polymer cores inducing osmotic water influx and the subsequent morphological dynamics. The process also led to an increase in the population of polymeric objects through system self-replication. This study offers a new path toward the design of chemically self-assembled systems and their potential application in autonomous material artificial simulation of living systems.

Crosslinking in Semi-Batch Seeded Emulsion Polymerization: Effect of Linear and Non-Linear Monomer Feeding Rate Profiles on Gel Formation

Waterborne latex is often called a product-of-process. Here, the effect of semi-batch monomer feed rate on the kinetics and gel formation in seeded emulsion polymerization was investigated for the copolymerization of n-butyl methacrylate (n-BMA) and ethylene glycol dimethacrylate (EGDMA). Strikingly, the gel fraction was observed to be significantly influenced by monomer feed rate, even while most of the experiments were performed under so-called starve-fed conditions. More flooded conditions from faster monomer feed rates, including seeded batch reactions, counterintuitively resulted in significantly higher gel fraction. Chain transfer to polymer was intentionally suppressed here via monomer selection so as to focus mechanistic insights to relate only to the influence of a divinyl monomer, as opposed to being clouded by contributions to topology from long chain branching. Simulations revealed that the dominant influence on this phenomenon was the sensitivity of primary intramolecular cyclization to the instantaneous unreacted monomer concentration, which is directly impacted by monomer feed rate. The rate constant for cyclization for these conditions was determined to be first order and 4000 s-1, approximately 4 times that typically observed for backbiting in acrylates. This concept has been explored previously for bulk and solution polymerizations, but not for emulsified reaction environments and especially for the very low mole fraction divinyl monomer. In addition, while gel fraction could be dramatically manipulated by variations in linear monomer feed rates, it could be markedly enhanced by leveraging non-linear feed profiles built from combination sequences of flooded and starved conditions. For a 2 h total feed time, a fully linear profile resulted in 30% gel while a corresponding non-linear profile with an early fast-feed segment resulted in 80% gel.

Tracking Hydroplasticization by DSC: Movement of Water Domains Bound to Poly(Meth)Acrylates during Latex Film Formation

The film formation step of latexes constitutes one of the challenges of these environmentally friendly waterborne polymers, as the high glass transition (TG) polymers needed to produce hard films to be used as coatings will not produce coherent films at low temperature. This issue has been dealt by the use of temporary plasticizers added with the objective to reduce the TG of the polymers during film formation, while being released to the atmosphere afterwards. The main problem of these temporary plasticizers is their volatile organic nature, which is not recommended for the environment. Therefore, different strategies have been proposed to overcome their massive use. One of them is the use of hydroplasticization, as water, abundant in latexes, can effectively act as plasticizer for certain types of polymers. In this work, the effect of three different grafted hydroplasticizers has been checked in a (meth)acrylate copolymer, concluding that itaconic acid showed the best performance as seen by its low minimum film-formation temperature, just slightly modified water resistance and better mechanical properties of the films containing itaconic acid. Furthermore, film formation monitoring has been carried out by Differential Scanning Calorimety (DSC), showing that itaconic acid is able to retain more strongly the water molecules during the water losing process, improving its hydroplasticization capacity.

Environmentally Friendly Water-Based Self-Crosslinking Acrylate Dispersion Containing Magnesium Nanoparticles and Their Films Exhibiting Antimicrobial Properties

A water-based polymeric acrylate dispersion (latex) containing MgO nanoparticles, which had been added at a concentration of 1.5% (with respect to the monomers) during the preparation procedure, was investigated as an environmentally friendly binder for sanitary interior paints. The properties of this new latex were compared to those of a reference system free of the magnesium nanoparticles, synthesized by the same route, i.e., by semi-continuous emulsion polymerization. Tests were made in order to ascertain the mechanical and chemical properties, flash corrosion resistance and antimicrobial effect of the latex films. The results revealed that the new latex containing magnesium nanoparticles provided solvent-resistant coating films having pronounced antimicrobial activity against all the tested bacterial and fungal strains. The desirable antimicrobial properties can be ascribed to the sharp-edged character of magnesium nanoparticles, the peroxidation of lipids and the formation of reactive oxygen species. Moreover, no flash corrosion was formed beneath coating films containing magnesium nanoparticles, which can be attributed to the alkaline action due to the dissolution of a fraction of MgO in latex medium. The results of all of the tests provided evidence of the superiority of the polymeric dispersion with the magnesium nanoparticles to the reference system containing no nanoparticles.

Wurster Fluidised Bed Coating of Microparticles: Towards Scalable Production of Oral Sustained-Release Liquid Medicines for Patients with Swallowing Difficulties

Suspension of microparticles in an easy-to-swallow liquid is one approach to develop sustained-release formulations for children and patients with swallowing difficulties. However, to date production of sustained-release microparticles at the industrial scale has proven to be challenging. The aim of this investigation was to develop an innovative concept in coating sustained-release microparticles using industrial scalable Wurster fluidised bed to produce oral liquid suspensions. Microcrystalline cellulose cores (particle size <150 μm) were coated with Eudragit® NM 30 D and Eudragit® RS/RL 30 D aqueous dispersions using a fluidised bed coater. A novel approach of periodic addition of a small quantity (0.1% w/w) of dry powder glidant, magnesium stearate, to the coating chamber via an external port was applied throughout the coating process. This method significantly increased coating production yield from less than 50% to up to 99% compared to conventional coating process without the dry powder glidant. Powder rheology tests showed that dry powder glidants increased the tapped density and decreased the cohesive index of coated microparticles. Reproducible microencapsulation of a highly water-soluble drug, metoprolol succinate, was achieved, yielding coated microparticles less than 200 μm in size with 20-h sustained drug release, suitable for use in liquid suspensions. The robust, scalable technology presented in this study offers an important solution to the long-standing challenges of formulating sustained-release dosage forms suitable for children and older people with swallowing difficulties.

Room-Temperature Synthesis of Hollow Polymer Microparticles with an Open Hole on the Surface and Their Application

Compared with hollow microparticles with a completely closed shell structure, hollow polymer microparticles with an open hole on their surface have attracted considerable attention because of the obvious importance of the open hole on their surface; however, the development of a facile method to synthesize such unique open-ended hollow particles has remained a great challenge. In this study, an easy-to-use method was developed to rapidly produce monodispersed hollow and pored microparticles in high reaction yield at room temperature. The key to achieving the unusually shaped polystyrene (PS) microparticles was the use of anisotropic PS seeds. When hollow and dimpled PS seeds prepared using a modified dispersion polymerization method were dispersed in a water-toluene mixture followed by solvent evaporation under ambient conditions, they transformed into hollow PS microparticles with an open hole on their surface. A plausible mechanism for the transformation of the PS microparticles during the swelling and drying processes was proposed on the basis of our results and observations. The structural features of the hollow and pored PS microparticles motivated us to use the particles as a catalyst support. By using modified heterophase polymer dispersion processing involving the addition of a Ag precursor, hollow and pored PS microparticles covered with Ag nanocrystals were obtained on the basis of the in situ reduction of metal precursor on the surface of polymer particles. The resulting Ag nanocrystals/PS hybrid microparticles exhibited enhanced catalytic activity at low concentrations of nanocrystals and could be reused several times without loss of activity when used as catalysts for the reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride.

Film Formation of High Tg Latex Using Hydroplasticization: Explanations from NMR Relaxometry

The film formation of acrylic latex dispersions, containing different amounts of carboxylic acid functional groups by the incorporation of methacrylic acid (MAA), was studied with GARField ¹H NMR at various relative humidities (RH). Polymer particles with glass-transition temperatures in the range from 26 to 50 °C formed films at room temperature because of hydroplasticization. It was found that with an increased drying rate due to lower RH, the evaporation flux of water was limited by the latex polymer. Only in the second stage of drying this phenomenon was more obvious with increasing MAA content. ¹H NMR relaxometry was used to study the change of hydrogen mobilities during film formation and hardening of the films. This showed that the drying rate itself had no impact on the hydrogen mobility in the latex films as measured via the T₂ relaxation time. Hydrogen mobilities of water and the mobile polymer phase only significantly decrease after most water has evaporated. This implies that the rigidity of the polymers increases with the evaporation of water that otherwise plasticizes the polymer through hydrogen bonding with the carboxylic acid groups. This hardening of the polymer phase is essential for applications in a coating. The hydrogen mobilities were affected by the MAA concentration. Densities of mobile hydrogens increase with increasing MAA content. This is expected if the mobile protons are contained in the MAA groups. The result thus confirms the role of carboxylic acid groups in hydrogen bonding and plasticization of the copolymers. Hydrogen mobilities, however, decrease with increasing MAA content, which is hypothesized to be caused by the formation of dimers of carboxylic acid groups that still hold water. They still enable short-range polymer hydrogen mobility due to hydroplasticization but limit long-range polymer mobility due to interaction between the carboxylic acid groups.

Role of the Hydrophilic Latex Particle Surface in Water Diffusion into Films from Waterborne Polymer Colloids

The diffusion mechanism and growth of large-scale domains during the immersion of latex films in water have been thoroughly investigated with scattering techniques in a combination with the gravimetric method. Latex dispersions for film formation studies had identical main monomer compositions and only differ in the hydrophilic comonomers that result in distinct "hairy" layer structures of the particles. The major effects of the presence and the structure of the surface layers were identified: (1) Introducing the hydrophilic surface layer in the binder structure results in a more uniform penetration of water and a reduction in the water domain growth. (2) The nature of the particle shell defines the rate of the formation of the first hydration layer and the beginning of the large cluster formation. Poly(acrylamide) in the particle shell promotes the formation of the homogeneously swollen film and slows down the development of water "pockets." Poly(acrylic acid) leads to a more heterogeneous material and accelerates water uptake and cluster growth. (3) The thickness of the particle hairy layer regulates the thickness of the interstitials in the dry film and the number of the chemical groups involved in H-bonding with water molecules without a cluster formation. The amount of water that was absorbed before large domains start evolving increased with the growth of the particle shell thickness.

Contribution of Surface Energy to pH-Dependent Underwater Adhesion of an Acrylic Pressure-Sensitive Adhesive

Maintaining the underwater adhesive performance over a broad range of solution pH is challenging but necessary for many biomedical applications. Therefore, understanding how environmental conditions influence the mechanisms of bonding and debonding of pressure-sensitive adhesives (PSAs) can provide guidelines for materials design. We investigate how the presence of acrylic acid as a co-monomer impacts the adhesion of a model PSA in aqueous environments of varying pH. The adhesives under investigation are poly(2-ethylhexyl acrylate), or poly(2-EHA), and poly(2-EHA) co-polymerized with 5 wt % acrylic acid, or poly(2-EHA- co-AA). We characterize bonding and debonding (adhesion) of the adhesives using probe tack measurements with a spherical hydrophobic glass probe. We analyze the performance of the two PSAs in air and in low-ionic-strength buffered aqueous solutions of pH 3- 11. We find that the presence of the acrylic acid co-monomer increases the cohesiveness of the PSA and leads to stronger adhesion under all conditions investigated. We also observe that the presence of the acrylic acid co-monomer imparts the PSA with a strong dependence of adhesion on the solution pH. Dynamic contact angle and  ζ potential measurements support the hypothesis that deprotonation of the acrylic acid groups at higher pH causes the decrease in adhesion at higher pH. Rheological measurements do not show changes in the dynamic mechanical properties of the PSAs after exposure to solutions of pH 3- 11. Our measurements allow us to isolate the effect of the solution pH on the surface and bulk properties of the PSA. In the absence of the acrylic acid co-monomer, the bulk dissipation and the surface properties of the PSA are independent of the solution's pH.

Humidity-Induced Phase Transitions of Surfactants Embedded in Latex Coatings Can Drastically Alter Their Water Barrier and Mechanical Properties

Latex coatings are environmentally friendly i.e., they are formed from aqueous polymer dispersions, are cheap to produce and provide exceptional mechanical properties. Therefore, they are ubiquitous and can be found in a wide range of different applications such as paints and varnishes, pressure-sensitive adhesives, textiles, construction materials, paper coatings and inks. However, they also have weaknesses and their surfactant content is among them. Surfactants are often needed to stabilize polymer particles in the aqueous latex dispersions. These surfactants also form part of the coatings formed from these dispersions, and it is well-known that they can lower their performance. This work further explores this aspect and focuses on the role that embedded surfactant domains play in the response of latex coatings to humid environments. For this purpose, we made use of several experimental techniques where humidity control was implemented: quartz crystal microbalance with dissipation, atomic force microscopy and differential scanning calorimetry. By means of this multimethodological approach, we report that surfactants embedded in latex coatings can undergo humidity-induced transitions towards more hydrated and softer phases, and that this results in a drastic decrease of the mechanical and water barrier properties of the whole coatings. Subsequently, this work highlights the potential of taking into account the phase behavior of surfactants when choosing which ones to use in the synthesis of latex dispersions as this would help in predicting their performance under different environmental conditions.

Swelling of latex particles-towards a solution of the riddle

The assumption that during emulsion polymerization, the monomer molecules simply diffuse through the aqueous phase into the latex particles is a commonplace. However, there are experimental hints that this might not be that easy. Here, simulation results are discussed based on Fick’s diffusion laws regarding the swelling of latex particles. The results of quantitative application of these laws for swelling of latex particles allow the conclusion that the instantaneous replenishment of the consumed monomer during emulsion polymerization requires a close contact between the monomer and the polymer particles.

Water Vapor Sorption and Diffusion in Secondary Dispersion Barrier Coatings: A Critical Comparison with Emulsion Polymers

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.

Functional polymer particles via thiol–ene and thiol–yne suspension “click” polymerization

Highly functionalized, water-borne, micron-sized polymer particles were synthesized using thermally or photochemically initiated thiol–ene and thiol–yne “click” suspension polymerizations.

Microgel colloidosomes based on pH-responsive poly(tert-butylaminoethyl methacrylate) latexes

Emulsion copolymerization of 2-(tert-butylamino)ethyl methacrylate (TBAEMA) with divinylbenzene (DVB) cross-linker in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded sterically stabilized poly[2-(tert-butylamino)ethyl methacrylate] (PTBAEMA) latexes at 10% solids at pH 9. Such particles proved to be an effective Pickering emulsifier at pH 10 for both n-dodecane and n-hexane. (1)H NMR spectroscopy was used to follow the model reaction between the secondary amine of the TBAEMA monomer and the isocyanate groups of tolylene 2,4-diisocyanate-terminated poly(propylene glycol) (PPG-TDI). Cross-linking the PTBAEMA latex particles adsorbed at the n-dodecane/water interface using this oil-soluble PPG-TDI cross-linker at around 0 (o)C led to robust colloidosomes that survived an acid challenge. This resistance to demulsification was confirmed via laser diffraction studies following an in situ switch from pH 10 to 3, since no change was observed in either the oil droplet size or concentration (compared to non-cross-linked PTBAEMA-stabilized Pickering emulsions). Such PTBAEMA colloidosomes survived removal of the internal oil phase on washing with excess ethanol. However, because ethanol is a good solvent for the PTBAEMA chains, imaging the ethanol-treated colloidosomes via electron microscopy proved rather problematic due to partial film formation. Therefore, a series of TBAEMA/styrene copolymer latexes (comprising 10, 30, 50, or 60 mol % styrene) were prepared via emulsion copolymerization at 70 °C in the presence of DVB and PEGMA. The higher glass transition temperatures exhibited by these copolymer particles (and their greater resistance to ethanol swelling) enabled better-quality electron microscopy images to be obtained. The presence of nitrogen atoms at the surface of these copolymer latex particles was confirmed via X-ray photoelectron spectroscopy studies; these secondary amine groups allow covalent cross-linking via PPG-TDI when adsorbed at the surface of n-dodecane droplets at TBAEMA comonomer contents as low as 40 mol %. After removal of the n-hexane oil phase by evaporation, fluorescence microscopy studies indicate that these colloidosomes undergo collapse in their latex form at pH 10 but regain their original spherical morphology in their cationic microgel form at pH 3.5.

Correlating particle deformation with water concentration profiles during latex film formation: reasons that softer latex films take longer to dry

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.

Near-infrared-induced heating of confined water in polymeric particles for efficient payload release

Near-infrared (NIR) light-triggered release from polymeric capsules could make a major impact on biological research by enabling remote and spatiotemporal control over the release of encapsulated cargo. The few existing mechanisms for NIR-triggered release have not been widely applied because they require custom synthesis of designer polymers, high-powered lasers to drive inefficient two-photon processes, and/or coencapsulation of bulky inorganic particles. In search of a simpler mechanism, we found that exposure to laser light resonant with the vibrational absorption of water (980 nm) in the NIR region can induce release of payloads encapsulated in particles made from inherently non-photo-responsive polymers. We hypothesize that confined water pockets present in hydrated polymer particles absorb electromagnetic energy and transfer it to the polymer matrix, inducing a thermal phase change. In this study, we show that this simple and highly universal strategy enables instantaneous and controlled release of payloads in aqueous environments as well as in living cells using both pulsed and continuous wavelength lasers without significant heating of the surrounding aqueous solution.

Stopped-flow kinetics of pH-responsive polyamine latexes: how fast is the latex-to-microgel transition?

Four poly(ethylene glycol)-stabilized polyamine latexes, namely, poly(2-vinylpyridine) (P2VP), poly(2-(tert-butylamino)ethyl methacrylate) (PTBAEMA), poly(2-(diethylamino)ethyl methacrylate) (PDEA), and poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) were prepared via emulsion copolymerization using divinylbenzene (DVB) as a cross-linker at 0.80 mol % for all formulations. According to dynamic light scattering studies, the resulting latexes were near-monodisperse and had approximately constant hydrodynamic diameters of 205-220 nm at pH 10; a latex-to-microgel transition was observed at around the respective pKa of each polyamine on addition of acid. The kinetics of swelling of each latex was investigated by the pH-jump method using a commercial stopped-flow instrument. The most rapid swelling was observed for the P2VP latex, which exhibited a characteristic swelling time (t*) of 5 ms. The corresponding t* values for PTBAEMA and PDEA were 25 and 35 ms, respectively, whereas the PDPA particles exhibited significantly slower swelling kinetics (t* = 180 ms). These t* values could not be correlated with either the latex Tg or the polyamine pKa. However, there is a positive correlation between t* and the repeat unit mass of the amine monomer, which suggests that the cationic charge density of the protonated polymer chains may influence the kinetics of swelling. Alternatively, the observed differences in swelling kinetics may simply reflect subtle differences in the DVB cross-link density, with more uniformly cross-linked latexes being capable of responding more quickly to a pH jump. The kinetics of deswelling for the corresponding microgel-to-latex transition was also briefly investigated for the PTBAEMA and P2VP particles. In both cases, much slower rates of deswelling were observed. This suggests that a latexlike "skin" is formed on the outer surface of the microgel particles during their deprotonation, which significantly retards the excretion of both salt and water.

Use of a Routh-Russel deformation map to achieve film formation of a latex with a high glass transition temperature

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 (T(g)), 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 T(g)) coatings is achieved without using coalescing aids that emit volatile organic compounds (VOCs), which is a significant technical achievement.

Importance of capillary forces in the assembly of carbon nanotubes in a polymer colloid lattice

We highlight the significance of capillary pressure in the directed assembly of nanorods in ordered arrays of colloidal particles. Specifically, we discuss mechanisms for the assembly of carbon nanotubes at the interstitial sites between latex polymer particles during composite film formation. Our study points to general design rules to be considered to optimize the ordering of nanostructures within such polymer matrices. In particular, gaining an understanding of the role of capillary forces is critical. Using a combination of electron microscopy and atomic force microscopy, we show that the capillary forces acting on the latex particles during the drying process are sufficient to bend carbon nanotubes. The extent of bending depends on the flexural rigidity of the carbon nanotubes and whether or not they are present as bundled ensembles. We also show that in order to achieve long-range ordering of the nanotubes templated by the polymer matrix, it is necessary for the polymer to be sufficiently mobile to ensure that the nanotubes are frozen into the ordered network when the film is formed and the capillary forces are no longer dominant. In our system, the polymer is plasticized by the addition of surfactant, so that it is sufficiently mobile at room temperature. Interestingly, the carbon nanotubes effectively act as localized pressure sensors, and as such, the study agrees well with previous theoretical predictions calculating the magnitude of capillary forces during latex film formation.

A fluorescence approach to investigate repartitioning of coalescing agents in acrylic polymer emulsions

Repartitioning of co-solvents between particles of latex emulsions was investigated by means of a fluorescence method based on the detection of the amount of co-solvent via the solvatochromic shift of the emission maximum of a fluorescent probe, copolymerized at a low concentration. Complete repartitioning of co-solvents between particles of latex materials with a low T_g (ca. 25 °C) occurred within minutes. For a hydrophilic latex with a T_g of 68 °C, equilibration was achieved within an hour. Repartitioning was faster for more hydrophobic co-solvents. For a hydrophobic latex of similar T_g, co-solvent repartitioning took place on the same time scale, but complete equilibration was not reached. Possibly, there is an additional slow component in the repartitioning, or the prolonged presence of co-solvent causes a structural change in the latex particles that affects the outcome of the experiment.