Experimental validation of a mathematical model for the evolution of the particle morphology of waterborne polymer-polymer hybrids: paving the way to the design and implementation of optimal polymerization strategies

Polymer-polymer composite nanoparticles allow both the improvement of the performance in stablished applications of waterborne polymer dispersions and targeting new applications that are out of reach of currently available products. The performance of these materials is determined by the particle morphology. To open the way to process optimization and on-line control of the particle morphology, the capability of the recently developed model to predict the evolution of the particle morphology during seeded semibatch emulsion polymerization process was evaluated. Structured polymer particles were synthesized by copolymerization of styrene and butyl acrylate (St-BA) on methyl methacrylate and butyl acrylate (MMA–BA) copolymer seeds of different Tgs. The model captured well the effect of process variables on the evolution of the particle morphology, opening the way to the design and implementation of optimal strategies.The financial support of the RECOBA project (funding from European Framework Horizon 2020, No. 636820) is gratefully acknowledged

Optimal control of particle size distribution in semi-batch emulsion polymerisation

Imperial Users onl

In-situ control of the morphology of multiphase latex/clay nanocomposites

The key objective of this thesis is the morphology control of latex/clay nanocomposites (LCN) which are of particular interest to water-based coating and adhesive applications. Indeed, the incorporation of inorganic fillers into a polymer matrix generally leads to better performing materials. However, a good dispersion and an alignment of the clay layers as single platelets into the polymer matrix are the prerequisites for the largest property enhancement. Such requirements have been the driving force for the development of many LCN synthetic routes. The inorganic encapsulation technique, using conventional emulsion polymerization was employed in this thesis. The natural occurring montmorrilonite clay particles were used as the inorganic fillers. The primary goal was to make a start in fine tuning the dispersion and orientation of the clay into the polymer matrix by controlling the morphology of the clay-encapsulated latex particles. We realized that with so many parameters involved the potentials of high-throughput experimentation (HTE) and on-line Raman spectroscopy should be explored, so some first attempts in this direction were made. Furthermore this thesis investigated the influence of clay on the morphology of multiphase latex particles. Clay particles are hydrophilic inorganic compounds and must be rendered (partially) hydrophobic in order to be more compatible with the in-situ synthesized polymer. In this thesis, the organic modification of clay was performed using two kinds of titanate coupling agents, titanium IV, (2-propanolato)tris(2-propenoata-O), 2-(2-methoxy-ethoxy) ethanol(KR39DS) and titanium IV, 2-propanolato, tris isooctadecanoato-0 (KRTTS), where the former is unsaturated and the second contains saturated alkyl groups. In Chapter 3, a study of the hydrolytic stability of the organoclays thus synthesized showed that the titanate modifiers used were highly susceptible to hydrolysis in the emulsion polymerization conditions. From the results obtained, it was concluded that successful clay encapsulation does not require the use of unsaturated organic modifiers as previously believed. Furthermore, emulsion polymerizations carried out with pristine clay also led to successful clay encapsulation showing that the modification step could be completely omitted. In addition to the organic modification, the influence of monomer feed composition, i.e. monomer mixtures consisting of different weight ratios of methyl methacrylate/ butyl acrylate, and the process type on the morphology of two-phase LCN was studied. It was shown that the monomer feed composition added under starved conditions strongly influenced the morphology of the LCN. Indeed, when the Tg of the encapsulating (co)polymer was well above the reaction temperature (hard polymer), anisotropic latex particles containing single clay platelets were mainly observed. In contrast, the use of a soft encapsulating polymer led to armored-like latex particles. Furthermore, only starved-feed monomer addition led to successful encapsulation: a batch process generated larger aggregates of clay particles and only a few armored-like latex particles. A heat treatment of the encapsulated clay particles showed that the clay encapsulation process was mainly kinetically controlled: after the heat treatment the clay was again on the outside of the latex particles. A mechanism of encapsulation was proposed, where the clay particles act as seed in the process (polymerization carried out from the surface of the inorganic particle). A systematic study of the effects of clay loading, surfactant concentration and surfactant type on the clay/polymer interaction was performed via a design of experiments. All three parameters were found to have significant effects on the clay/polymer interaction. In Chapter 4, three-phase PMMA/PS/MMT latex particles were synthesized from clay-containing PMMA seeds via (semi-)batch emulsion polymerization of styrene. For the interpretations of the morphologies obtained, the established theories to understand the morphology development of two-phase latex particles could be applied. An interesting observation was that clay platelets could act as physical barriers preventing diffusion of second stage polymers within the seed latex particles. A methodology to successfully conduct batch emulsion copolymerization using a parallel stirred automated synthesizer is described in Chapter 5. The most challenging step for such automated reactions was sampling. Sampling operations and inhibition were found to be the main source of errors in the determination of the partial monomer conversion-time histories. A comparison of the conversion rates of the automated reactions and the analyses of the particle size distributions and the molar mass distributions of the latexes synthesized clearly showed that the automated reactions were highly reproducible. A methodology to successfully conduct batch emulsion copolymerization using an automated parallel synthesizer is described in Chapter 5. The most challenging step for such automated reactions was sampling. Sampling operations and inhibition were found to be the main source of errors in the determination of the partial monomer conversion-time histories. A comparison of the conversion rates of the automated reactions and the analyses of the particle size distributions and the molar mass distributions of the latexes synthesized clearly showed that the automated reactions were highly reproducible. Chapter 5 also clearly demonstrated the potential of a low-cost-low-resolution portable Raman spectrometer to monitor emulsion homopolymerizations. The Raman spectroscopy technique in combination with partial least regression methods requires extensive calibration steps in order to gather large and representative data sets. Low-cost low-resolution portable Raman can be used as a conversion monitoring device and might be easy to integrate in the robotic platform

Analysis of optimal operation of a fed-batch emulsion copolymerization reactor used for production of particles with core-shell morphology

In this paper dynamic optimization of a lab-scale semi-batch emulsion copolymerization reactor for styrene and butyl acrylate in the presence of a chain transfer agent (CTA) is studied. The mathematical model of the process, previously developed and experimentally validated, is used to predict the glass transition temperature of produced polymer, the number and weight average molecular weights, the monomers global conversion, the particle size distribution, and the amount of residual monomers. The model is implemented within gPROMS environment for modeling and optimization. It is desired to compute feed rate profiles of pre-emulsioned monomers, inhibitor and CTA that will allow the production of polymer particles with prescribed core-shell morphology with high productivity. The results obtained for different operating conditions and various additional product specifications are presented. The resulting feeding profiles are analyzed from the perspective of the nature of emulsion polymerization process and some interesting conclusions are drawn

Role of grafting on particle morphology and film properties of polyurethane-polyacrylate hybrid dispersions.

297 p.Typically, the PU/acrylic hybrid nanoparticle dispersions are prepared in multistep procedure, composed of synthesis of PU in organic solvents, dispersion of PU/solvents into water, solvent removal, addition of acrylic monomers into the dispersion and their free radical polymerization. Besides being complex and expensive, this method has negative implications for sustainability of the synthesis.In this work, solvent and emulsifier-free PU/acrylic hybrid dispersions were synthesized using (meth)acrylic monomers as solvent in the PU synthesis and grafted hybrids were prepared with different functional monomers. The effect of grafting and the impact of macromolecular architecture on the particle and film morphology and physical properties of PU/(meth)acrylic hybrid films is investigated. It was found that grafting improved the compatibility of PU and (meth)acrylics and resulted in films with better mechanical properties and water sensitivity. The grafting of PU and acrylic chains can result in a huge variety of macromolecular structures, depending on the nature of both the PU and the acrylic parts of the formulation.Polyma

Precise characterization and modeling of particle morphology development in emulsion polymerization.

256 p.This PhD aimed at paving the way to process optimization and on-line control of particle morphology in emulsion polymerization process. The bottleneck in achieving this goal is the lack of proper device for on-line monitoring of particle morphology. Therefore, the alternative is using a mathematical model as a soft sensor in on-line monitoring. . In this regard, the model developed by Hamzehlou et al.1 is the most promising possibility. The model needs to be capable of describing the evolution of the particle morphology during the polymerization as well as being sensitive to detect the effect of process variables on morphology changes. To evaluate the capacity of mathematical model on prediction of particle morphology development, composite polymer-polymer particle latexes were synthesized in a two-step seeded semi-batch emulsion polymerization by polymerization of more hydrophilic co-monomers (styrene/n-butyl acrylate) in the second stage of polymerization using a more hydrophilic seed of poly (methyl methacrylate-co-n-butyl acrylate). According to thermodynamics, the equilibrium morphology for the studied cases was ¿inverted core-shell¿ while in all synthesized cases in this thesis; kinetically meta-stable morphologies were achieved due to determining effect of radical concentration profile on the development of the particle morphology. The presented model was modified to account for radical concentration profile and the effect of different reaction variables to alter the movement of synthesized clusters at the exterior zone of the particles toward to the equilibrium position in the center of the particles was studied. It was recognized that although the combination of different characterization techniques can provide reliable knowledge about the particle morphology development, it was difficult to reach the conclusion on the effect of process variables on the morphology changes in some cases. To overcome this limit, a method for the precise quantitative 3D characterization of polymer-polymer composite waterborne particles based on high angular dark field -scanning transmission electron microscopy (HAADF-STEM) coupled with image reconstruction is presented. The information about the morphology gathered by this technique revealed mechanistic features on the development of the particle morphology that could not be captured by the conventional TEM images and hence it allowed upgrading the mathematical model. All overall, the upgraded model provides a better prediction of the effect of process variables on the morphology of composite polymer particles and this opens the way to use the model in optimization and on-line control strategies. (1) Hamzehlou, S.; Leiza, J. R.; Asua, J. M. A New Approach for Mathematical Modeling of the Dynamic Development of Particle Morphology. Chem. Eng. J. 2016, 304, 655-666.Polyma

Manipulation of Latex Particle Surfaces and Morphology During Emulsion Polymerization and Some Potential Applications

This dissertation contains two portions: modification of latex particle surfaces with polymerizable surfactants during emulsion polymerization for latex and coating application; synthesis of latex particles of multilayer morphology for epoxy toughening applications. The main focus of first portion of this dissertation research is to evaluate improvements in poly (n-butyl methacrylate) – PBMA - latex and film properties resulting from the use of a polymerizable surfactant HITENOL KH-10 compared with its non-polymerizable control LA-12 during the latex synthesis via emulsion polymerization, and to investigate the underlying mechanism of those improvements. Latexes prepared with KH-10 exhibited 240% higher stability against CaCl2 addition, and resulted in films with suppressed water-sensitivity and surfactant migration. Mechanism accounting for these improvements of the PBMA latex and film properties is a significant difference in surfactant distribution/incorporation into different loci in latex system (including in aqueous phase, on latex particle surfaces and inside latex particles) between KH-10 and LA-12. 66% of KH-10 was anchored on latex particles surfaces compared with only 21% for LA-12. Further study found the increase of surface-anchored polymerizable surfactants causes a 300% increase of particle coalescence enthalpy during film formation, increasing the energy barrier for dried particles to heal and form a coherent film. The second portion of this dissertation research focuses on development of a novel emulsion polymerization technique for synthesizing silica/PBA/PMMA multilayer core-shell composite latex particles with single cores of silica nanoparticles (avg. diam. 22 nm), because these multilayer particles were proposed to be promising toughening agents for epoxy. Colloidal silica nanoparticles were surface-treated with silane (3-methacryloxypropyl trimethoxysilane) before sequential emulsion polymerization of n-butyl acrylate (BA) and methyl methacrylate (MMA). Precise control of a series of parameters including surfactant concentration and monomer feed rate is critical for successful synthesis of multilayer particles with single silica cores. Synthesized multilayer nanoparticles were extracted from latex and utilized as epoxy toughening agents for diglycidyl ether of bisphenol A (DGEBA) compared with two other toughening agents—commercial core-shell poly(styrene-butadiene) rubber (CSR) particles and hybrid toughening agents containing mixture of CSR and silica nanoparticles. Multilayer particles exceeded the other two in toughening ability at low volume fractions in epoxy (\u3c2.5%) but exhibited a decreasing toughening ability as volume fraction increased, a trend contrary to the other two. Particle dispersion morphology and fracture surface morphology were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was observed that multilayer particles formed small clusters throughout the matrix for all volume fractions studied (1.25% - 7.5%), while CSR and silica nanoparticles were uniformly and individually dispersed inside epoxy matrix. SEM images of fracture surfaces showed that matrix void growth might be the primary toughening mechanism for multilayer-particle-toughened epoxy but void growth became less prominent as volume fraction of particles increased, corresponding to the trend that epoxy toughness decreased as volume fraction of multilayer particles increased

Hierarchical assemblies of polymer particles through tailored interfaces and controllable interfacial interactions

Hierarchical assembly architectures of functional polymer particles are promising because of their physicochemical and surface properties for multi-labeling and sensing to catalysis and biomedical applications. While polymer nanoparticles' interior is mainly made up of the cross-linked network, their surface can be tailored with soft, flexible, and responsive molecules and macromolecules as potential support for the controlled particulate assemblies. Molecular surfactants and polyelectrolytes as interfacial agents improve the stability of the nanoparticles whereas swellable and soft shell-like cross-linked polymeric layer at the interface can significantly enhance the uptake of guest nano-constituents during assemblies. Besides, layer-by-layer surface-functionalization holds the ability to provide a high variability in assembly architectures of different interfacial properties. Considering these aspects, various assembly architectures of polymer nanoparticles of tunable size, shapes, morphology, and tailored interfaces together with controllable interfacial interactions are constructed here. The microfluidic-mediated platform has been used for the synthesis of constituents polymer nanoparticles of various structural and interfacial properties, and their assemblies are conducted in batch or flow conditions. The assemblies presented in this progress report is divided into three main categories: cross-linked polymeric network's fusion-based self-assembly, electrostatic-driven assemblies, and assembly formed by encapsulating smaller nanoparticles into larger microparticles

The role of solid particles in emulsion polymerization – synthesis and kinetic studies

Control of particle morphology and chemical functionality in polymer dispersions have been of growing interest in the scientific community for the past decades. The possibility of regulating asymmetry, in both shape and chemical composition, has been sought as a way of creating complex advanced materials. In these materials the mechanical and physico-chemical properties of different phases are combined, in a synergistic way, in an attempt of mimicking nature’s behaviour. This thesis particularly deals with nanocomposite materials: materials where at least one of the different phases has two or three dimensions of less than 100 nanometres. Among the plethora of synthetic pathways developed for the controlled synthesis of nanocomposite colloids, this work focuses on a process called Pickering emulsion polymerization; a seeded emulsion polymerization reaction where a polymer phase is formed in-situ in the presence of a stabilizing nano-sized colloid formed ex-situ. The product of the reaction typically is that of a polymeric particle surrounded by a dense shell of stabilizing agent. The main advantages are the ease of operation, absence of high shear homogenization steps and of molecular surfactants. The latter is of key importance for instance in coating applications where surfactant migration during and after film formation can be detrimental for the final film properties. In this work, initially Pickering emulsion polymerization is thoroughly explored from a kinetic and mechanistic viewpoint using a model system consisting of SiO2 nanoparticles and styrene or methyl methacrylate as monomers. These relatively well-known systems are used to draw more conclusive theories on the mechanism governing particle formation and specifically the mode of stabilizer adsorption at the polymer interface. Once assessed the main processes influencing the fate of the reaction, a first step towards the implementation of added complexity in the system is taken by moving towards polymeric block copolymer stabilizers, where the chemical composition can be tailored by changing the type of monomer used. Both dispersion and emulsion polymerization approaches are discussed, with a particular focus on the development of protocols which do not contain added coloration or malodorous compounds. This increases the complexity of the system as the adopted chain-transfer agents require to operate in monomer starve fed conditions in order to allow control over chain-growth. This was found not to be compatible with dispersion polymerization, or polymerization induced selfassembly, reaction conditions. Nevertheless, a solution to the problem is proposed which yielded polymer self-assemblies of various morphologies. Finally, nanometric polymeric stabilizers (i.e. crosslinked block copolymer micelles, or nanogels) produced by the more successful emulsion polymerization approaches are adopted in Pickering emulsion polymerization reactions as sole stabilizers. The controlled destabilization of the nanogels by pH adjustment and background electrolyte addition led to polymer colloids of Janus, patchy or armoured morphology. Such particles are characterized by a given number of surface protrusions, with the same chemical composition as the nanogels adopted in the protocol

CONTROL OF KEY POLYMER PROPERTIES VIA REVERSIBLE ADDITION-FRAGMENTATION CHAIN TRANSFER IN EMULSION POLYMERIZATION

Free radical emulsion polymerization (FRP) is widely adopted in industry due to its applicability to a wide range of monomers. Despite its many benefits and wide spread use, the fast chain growth and the presence of rapid irreversible termination impose limitations with respect to the degree of control in FRP. Furthermore, producing block copolymers and polymers with complex structures via FRP is not feasible. Closer control of macromolecular chain structure and molar mass, using novel polymerization techniques, is required to synthesize and optimize many new polymer products. Reversible addition fragmentation chain transfer (RAFT)-mediated polymerization is a novel controlled living free radical technique used to impart living characters in free radical polymerization. In combination with emulsion polymerization, the process is industrially promising and attractive for the production of tailored polymeric products. It allows for the production of particles with specially-tailored properties, including size, composition, morphology, and molecular weights. The mechanism of RAFT process and the effect of participating groups were discussed with reviews on the previous work on rate retardation. A mathematical model accounting for the effect of concentrations of propagating, intermediate, dormant and dead chains was developed based on their reaction pathways. The model was combined with a chain-length dependent termination model in order to account for the decreased termination rate. The model was validated against experimental data for solution and bulk polymerizations of styrene. The role of the intermediate radical and the effect of RAFT agent on the chain length dependent termination rate were addressed theoretically. The developed kinetic model was used with validated kinetic parameters to assess the observed retardation in solution polymerization of styrene with high active RAFT agent (cumyl dithiobenzoate). The fragmentation rate coefficient was used as a model parameter, and a value equal to 6×104 s-1 was found to provide a good agreement with the experimental data. The model predictions indicated that the observed retardation could be attributed to the cross termination of the intermediate radical and, to some extent, to the RAFT effect on increasing the average termination rate coefficient. The model predictions showed that to preserve the living nature of RAFT polymerization, a low initiator concentration is recommended. In line with the experimental data, model simulations revealed that the intermediate radical prefers fragmentation in the direction of the reactant. The application of RAFT process has also been extended to emulsion polymerization of styrene. A comprehensive dynamic model for batch and semi-batch emulsion polymerizations with a reversible addition-fragmentation chain transfer process was developed. To account for the integration of the RAFT process, new modifications were added to the kinetics of zero-one emulsion polymerization. The developed model was designed to predict key polymer properties such as: average particle size, conversion, particle size distribution (PSD), and molecular weight distribution (MWD) and its averages. The model was checked for emulsion polymerization processes of styrene with O-ethylxanthyl ethyl propionate as a RAFT based transfer agent. By using the model to investigate the effect of RAFT agent on the polymerization attributes, it was found that the rate of polymerization and the average size of the latex particles decreased with increasing amount of RAFT agent. It was also found that the molecular weight distribution could be controlled, as it is strongly influenced by the presence of the RAFT based transfer agent. The effects of RAFT agent, surfactant (SDS), initiator (KPS) and temperature were further investigated under semi-batch conditions. Monomer conversion, MWD and PSD were found to be strongly affected by monomer feed rate. With semi-batch mode, Mn and increased with increasing monomer flow rate. Initiator concentration had a significant effect on PSD. The results suggest that living polymerization can be approached by operating under semi-batch conditions where a linear growth of polymer molecular weight with conversion was obtained. The lack of online instrumentation was the main reason for developing our calorimetry-based soft-sensor. The rate of polymerization, which is proportional to the heat of reaction, was estimated and integrated to obtain the overall monomer conversion. The calorimetric model developed was found to be capable of estimating polymer molecular weight via simultaneous estimation of monomer and RAFT agent concentrations. The model was validated with batch and semi-batch emulsion polymerization of styrene with and without RAFT agent. The results show good agreement between measured conversion profiles by calorimetry with those measured by the gravimetric technique. Additionally, the number average molecular weight results measured by SEC (GPC) with double detections compare well with those calculated by the calorimetric model. Application of the offline dynamic optimisation to the emulsion polymerization process of styrene was investigated for the PSD, MWD and monomer conversion. The optimal profiles obtained were then validated experimentally and a good agreement was obtained. The gained knowledge has been further applied to produce polymeric particles containing block copolymers. First, methyl acrylate, butyl acrylate and styrene were polymerized separately to produce the first block. Subsequently, the produced homopolymer attached with xanthate was chain-extended with another monomer to produce block copolymer under batch conditions. Due to the formation of new particles during the second stage batch polymerization, homopolymer was formed and the block copolymer produced was not of high purity. The process was further optimized by operating under semi-batch conditions. The choice of block sequence was found to be important in reducing the influence of terminated chains on the distributions of polymer obtained. It has been found that polymerizing styrene first followed by the high active acrylate monomers resulted in purer block copolymer with low polydispersity confirmed by GPC and H-NMR analysis