Enhanced water barrier properties of surfactant-free polymer films obtained by macroRAFT-mediated emulsion polymerization

The presence of low-molar-mass surfactants in latex films results in detrimental effects on their water permeability, gloss, and adhesion. For applications such as coatings, there is a need to develop formulations that do not contain surfactants and have better water barrier properties. Having previously reported the synthesis of surfactant-free latex particles in water using low amounts ( < 2 wt %) of chains synthesized by controlled radical polymerization (Lesage de la Haye et al. Macromolecules 2017, 50, 9315-9328), here we study the water barrier properties of films made from these particles and their application in anticorrosion coatings. When films cast from aqueous dispersions of acrylate copolymer particles stabilized with poly(sodium 4-styrenesulfonate) (PSSNa) were immersed in water for 3 days, they sorbed only 4 wt % water. This uptake is only slightly higher than the value predicted for the pure copolymer, indicating that the negative effects of any particle boundaries and hydrophilic-stabilizing molecules are minimal. This sorption of liquid water is 5 times lower than what is found in films cast from particles stabilized with the same proportion of poly(methacrylic acid) (PMAA), which is more hydrophilic than PSSNa. In water vapor with 90% relative humidity, the PSSNa-based film had an equilibrium sorption of only 4 wt %. A small increase in the PMAA content has a strong and negative impact on the barrier properties. Nuclear magnetic resonance relaxometry on polymer films after immersion in water shows that water clusters have the smallest size in the films containing PSSNa. Furthermore, these films retain their optical clarity during immersion in liquid water for up to 90 min, whereas all other compositions quickly develop opacity ("water whitening") as a result of light scattering from sorbed water. This implies a remarkably complete coalescence and a very small density of defects, which yields properties matching those of some solvent-borne films. The latex stabilized with PSSNa is implemented as the binder in a paint formulation for application as an anticorrosive barrier coating on steel substrates and evaluated in accelerated weathering and corrosion tests. Our results demonstrate the potential of self-stabilized latex particles for the development of different applications, such as waterborne protective coatings and pressure-sensitive adhesives

Dynamic stratification in drying films of colloidal mixtures

In simulations and experiments, we study the drying of films containing mixtures of large and small colloidal particles in water. During drying, the mixture stratifies into a layer of the larger particles at the bottom with a layer of the smaller particles on top. We developed a model to show that a gradient in osmotic pressure, which develops dynamically during drying, is responsible for the segregation mechanism behind stratification

In situ monitoring of latex film formation by small-angle neutron scattering: Evolving distributions of hydrophilic stabilizers in drying colloidal films

The distribution of hydrophilic species, such as surfactants, in latex films is of critical importance for the performance of adhesives, coatings and inks, among others. However, the evolution of this distribution during the film formation process and in the resulting dried films remains insufficiently elucidated. Here, we present in situ (wet) and ex situ (dry) SANS experiments that follow the film formation of two types of latex particles, which differ in their stabilizer: either a covalently bonded poly(methacrylic acid) (PMAA) segment or a physically adsorbed surfactant (sodium dodecyl sulfate, SDS). By fitting the experimental SANS data and combining with gravimetry experiments, we have ascertained the hydrophilic species distribution within the drying film and followed its evolution by correlating the size and shape of stabilizer clusters with the drying time. The evolution of the SDS distribution over drying time is being driven by a reduction in the interfacial free energy. However, the PMAA-based stabilizer macromolecules are restricted by their covalent bonding to core polymer chains and hence form high surface-area disc-like phases at the common boundary between particles and PMAA micelles. Contrary to an idealized view of film formation, the PMAA does not remain in the walls of a continuous honeycomb structure. The results presented here shed new light on the nanoscale distribution of hydrophilic species in drying and ageing latex films. We provide valuable insights into the influence of the stabilizer mobility on the final structure of latex films

RAFT Polymerization of Methacrylic Acid in Water

Reversible addition–fragmentation chain transfer (RAFT) polymerization of methacrylic acid was successfully performed in water in the presence of a trithiocarbonate, the 4-cyano-4-thiothiopropylsulfanylpentanoic acid (CTPPA), as a RAFT agent. Several parameters such as the temperature, the concentration, the pH, the targeted polymerization degree, and the initiator concentration were studied. For pH value below the p<i>K</i>_a of MAA, well-defined PMAA chains with different molar mass up to 92 000 g mol^–1 exhibiting low dispersity (<i>Đ</i> < 1.19) were obtained under a broad range of synthetic conditions

Emulsion Polymerization of Vinyl Acetate in the Presence of Different Hydrophilic Polymers Obtained by RAFT/MADIX

The surfactant-free emulsion polymerization of vinyl acetate (VAc) was achieved using RAFT/MADIX-mediated polymerization-induced self-assembly (PISA) process in water. First, well-defined hydrophilic macromolecular RAFT agents (macroRAFT) bearing a xanthate chain end were synthesized by RAFT/MADIX polymerization of <i>N</i>-vinylpyrrolidone (NVP) and <i>N</i>-acryloylmorpholine (NAM) or by post-modification of commercial poly­(ethylene glycol). Chain extension of the macroRAFT with VAc in water led to the block copolymer nanoscale organization and the subsequent formation of stable and isodisperse PVAc latex nanoparticles with high solids content (35–37 wt %). The influence of various parameters, including the nature and functionality of the macroRAFT agent precursor, on the polymerization kinetics and particle morphology was also studied

Batch Emulsion Polymerization Mediated by Poly(methacrylic acid) MacroRAFT Agents: One-Pot Synthesis of Self-Stabilized Particles

The present paper describes the successful one-pot synthesis of self-stabilized particles composed of amphiphilic block copolymers based on poly­(methacrylic acid) (PMAA) obtained by polymerization-induced self-assembly. First, controlled radical polymerization of MAA is performed in water using the RAFT process by taking advantage of our recent results showing the successful RAFT polymerization of MAA in water [Chaduc Macromolecules 2012, 45, 1241−1247]. The so-formed hydrophilic macroRAFT agents are then chain-extended <i>in situ</i> with a hydrophobic monomer to form amphiphilic block copolymer chains of controlled molar mass that self-assemble into stable nanoparticles. Various parameters such as the pH, the molar mass and the concentration of the PMAA segments or the nature of the hydrophobic block have been investigated

Effect of the pH on the RAFT Polymerization of Acrylic Acid in Water. Application to the Synthesis of Poly(acrylic acid)-Stabilized Polystyrene Particles by RAFT Emulsion Polymerization

The reversible addition–fragmentation chain transfer (RAFT) polymerization of acrylic acid (AA) in water was studied in detail at different pHs using 4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA) as a control agent and 4,4′-azobis­(4-cyanopentanoic acid) (ACPA) as an initiator. Well-defined hydrophilic macromolecular RAFT agents (PAA-CTPPA) were obtained and further used directly in water for the polymerization of styrene. The corresponding polymerization-induced self-assembly (PISA) process was evaluated at different pHs and it was shown that working in acidic conditions (pH = 2.5) led to well-defined amphiphilic block copolymer particles (<i>Đ</i> < 1.4) of small size (below 50 nm). When the pH increased, the control over the growth of the polystyrene (PS) block was gradually lost. Chain extension experiments of PAA-CTPPA with <i>N</i>-acryloylmorpholine (NAM), a hydrosoluble and non-pH sensitive monomer, performed at different pHs showed that the very first addition–fragmentation steps that occurred in water were impeded when PAA was ionized leading to partial consumption of PAA-CTPPA and thus to PS molar masses higher than expected. Varying the PAA-CTPPA concentration at pH = 2.5 led in all cases to stable particles composed of well-defined block copolymers with PS segments of different molar masses

Xyloglucan-Functional Latex Particles via RAFT-Mediated Emulsion Polymerization for the Biomimetic Modification of Cellulose

Herein, we report a novel class of latex particles composed of a hemicellulose, xyloglucan (XG), and poly­(methyl methacrylate) (PMMA), specially designed to enable a biomimetic modification of cellulose. The formation of the latex particles was achieved utilizing reversible addition–fragmentation chain transfer (RAFT) mediated surfactant-free emulsion polymerization employing XG as a hydrophilic macromolecular RAFT agent (macroRAFT). In an initial step, XG was functionalized at the reducing chain end to bear a dithioester. This XG macroRAFT was subsequently utilized in water and chain extended with methyl methacrylate (MMA) as hydrophobic monomer, inspired by a polymerization-induced self-assembly (PISA) process. This yielded latex nanoparticles with a hydrophobic PMMA core stabilized by the hydrophilic XG chains at the corona. The molar mass of PMMA targeted was varied, resulting in a series of stable latex particles with hydrophobic PMMA content between 22 and 68 wt % of the total solids content (5–10%). The XG-PMMA nanoparticles were subsequently adsorbed to a neutral cellulose substrate (filter paper), and the modified surfaces were analyzed by FT-IR and SEM analyses. The adsorption of the latex particles was also investigated by quartz crystal microbalance with dissipation monitoring (QCM-D), where the nanoparticles were adsorbed to negatively charged model cellulose surfaces. The surfaces were analyzed by atomic force microscopy (AFM) and contact angle (CA) measurements. QCM-D experiments showed that more mass was adsorbed to the surfaces with increasing molar mass of the PMMA present. AFM of the surfaces after adsorption showed discrete particles, which were no longer present after annealing (160 °C, 1 h) and the roughness (<i>R</i>_q) of the surfaces had also decreased by at least half. Interestingly, after annealing, the surfaces did not all become more hydrophobic, as monitored by CA measurements, indicating that the surface roughness was an important factor to consider when evaluating the surface properties following particle adsorption. This novel class of latex nanoparticles provides an excellent platform for cellulose modification via physical adsorption. The utilization of XG as the anchoring molecule to cellulose provides a versatile methodology, as it does not rely on electrostatic interactions for the physical adsorption, enabling a wide range of cellulose substrates to be modified, including neutral sources such as cotton and bacterial nanocellulose, leading to new and advanced materials

Hydrophilic MacroRAFT-Mediated Emulsion Polymerization: Synthesis of Latexes for Cross-Linked and Surfactant-Free Films

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, <i>n</i>-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

Investigating the Reactivity of a Novel Methacrylic Transmer in Solution RAFT-SCVP and Its Impact on the Structure of the Resulting Hyperbranched Polymers

A novel methacrylic-based R-transmer, named 2-(methacryloyloxy)ethyl 4-cyano-4-(((propylthio)carbonothioyl)thio)pentanoate, was synthesized, and its reactivity (propagation versus reversible chain transfer) in reversible addition–fragmentation chain transfer self-condensing vinyl polymerization (RAFT-SCVP) with methyl methacrylate (MMA) was studied to apprehend its impact on hyperbranched polymers (HBP) structure. The resulting HBP were characterized by nuclear magnetic resonance (NMR) spectroscopy, size exclusion chromatography (SEC) and differential scanning calorimetry (DSC) to determine, respectively, their composition and degree of branching (DB), number average molecular weight (Mn) and dispersity (Đ), and glass transition temperature (Tg). Homopolymers of methacrylic-based R-transmer and copolymers with MMA were considered to study the reactivity of the methacrylic-based R-transmer with different initial molar ratios [MMA]0/[R-transmer]0 through kinetic studies and its consequences on HBP macromolecular characteristics and thermal properties. By varying the [MMA]0/[R-transmer]0 ratio from 1:1 to 100:1, HBP with increasing Mn (from 6,400 to 14,900 g mol–1) and Tg values (from 36.1 to 87.9 °C), decreasing Đ values (from 4.07 to 1.71) and DB values (from 0.30 to 0.03) were obtained