Composite Polymer Nanoparticles via Transitional Phase Inversion Emulsification and Polymerisation

Summary: A new route was employed to produce composite polymer nanoparticles. First, a model polymer (a low molecular-weight polyisobutene) was dissolved in a model monomer (styrene) and then the solution was emulsified in water containing a pair of nonionic surfactants via a transitional phase inversion route. After phase inversion, which produced an oil-in-water miniemulsion, polymerisation of the vinyl monomer gave composite polymer particles. Low temperature emulsification was not practical because the inverted oil-in-water emulsions reinverted to water-in-oil emulsions upon raising the temperature to the reaction temperature. Miniemulsions prepared at the reaction temperature with low monomer content in the oil phase showed good stabilty in the course of polymerisation and produced latexes with a particle size similar to the size of drops in the initial miniemulsions.     

The effect of acrylic acid amount on the colloidal properties of polystyrene latex

Poly(styrene-co-acrylic acid) (St/AA) latices were prepared by using a batch soap-free emulsion copolymerisation in non-buffered medium. Polymerisation kinetics, followed by gravimetric method, revealed that increasing AA comonomer concentration was directly proportional to the copolymerisation rate, while adding AA comonomer caused a strong decrease of particle size of final St/AA latex particle without affecting the size distribution. Transmission electron microscopy indicated that the particles were monodispersed and spherical in shape irrespective of AA amount used in the investigated range. The colloidal stability of the latices was increased upon increasing the AA concentration; owing to the electrosteric stabilisation originated from AA-rich layer on the particle surface. In addition, electrophoretic mobility of formed particles versus polymerisation conversion exhibited the constancy of the surface charge density during the polymerisation process and was inferred for discussion of the polymerisation mechanism of this system.     

Kinetics of the surfactant-free emulsion polymerization of styrene:-The post nucleation stage

The kinetics of the surfactant-free emulsion polymerisation of styrene have been studied. Reactions were sampled at several stages throughout their course, and the samples characterized by electron microscopy, gravimetric weights analysis and gel permeation chromatography, After an initial period of particle nucleation and coagulation, the reaction proceeds at a constant number density. The theories developed for emulsion polymerisation in the presence of surfactant above its critical micelle concentration might be expected, in Interval II, to apply to the surfactant-free system if due regard is taken of the lower number densities and larger particle sizes developed. The results are in best accord with theories invoking a surface phase polymerisation mechanism. None of these theories predict a bimodel molecular weight distribution as found here and which is ascribed to polymerisation in two loci. The activation energy was found to be the same as for surfactant containing emulsion polymerisations.     

The dispersion polymerization of unsaturated monomers initiated by the macromonomeric initiator

The dispersion polymerizations of styrene (St) and methyl methacrylate (MMA) initiated by poly(oxyethylene) macroinimer (PEO-MIM) in ethanol/water were investigated at 50, 60 and 80°C. The polymerisation rate vs. conversion dependence was described by with a maxim at the beginning of polymerisation. Polymerization was faster with MMA than with St. The limiting conversion was inversely proportional to temperature and was much more pronounced with St. The rate of polymerization increased with temperature. The overall initial activation energy increased with conversion and reached value ca. 25 kJ.mol⁻¹ for MMA and 50 kJ.mol⁻¹ for styrene at ca. 60% conversion. The particle size was observed to decrease with increasing the macroinimer concentration. The polymer dispersions were unstable and a large amount of coagulum appeared during the polymerisation especially in the styrene-containing reaction system.     

Investigation of Catalyst Fragmentation in Gas‐Phase Olefin Polymerisation: A Novel Short Stop Reactor

Summary: A short stop reactor (SSR) was developed to study nascent particle morphology and reaction kinetics in the gas‐phase polymerisation of olefins on supported catalysts. It is shown that the SSR provides a useful means to look into important phenomena such as catalyst fragmentation and catalyst site activation and deactivation that take place during the very early stages of the heterogeneous polymerisation of olefins. New experimental results obtained from gas‐phase polymerisation of ethylene show that, depending on the type of catalyst system and on the reaction conditions, different kinds of morphologies can be obtained for the nascent polymer (e.g., cracks and folded chain). Experimental data also indicate that the growth of the polymer chains occur at a non‐steady state during the very early stages of the polymerisation.

SEM image showing the morphology of a polymer/catalyst particle after 2 seconds of polymerisation at 8 bars of ethylene on an MgCl₂‐supported Ziegler‐Natta catalyst.     

Translucent nanolatexes through emulsion polymerisation of ethyl acrylate

Turbid emulsion systems of ethylacrylate/sodium dodecyl sulphate/water with monomer to surfactant (M/S) ratios 10 and 40 were transformed into stable transparent/translucent nanolatexes through emulsion polymerisation using potassium persulphate as an initiator. The latex particle size was observed to be similar to that obtained by true microemulsion polymerisation where M/S ratio is one. The kinetic plots exhibited two intervals upto M/S ratios 10. AIBN initiated systems showed separation of two phases for the M/S ratio ?10. M/S ratios were varied from 1 to 54 for the comparative study of polymerisation in emulsion and microemulsion media. Gel effect dominance was observed around 40-60% conversion for the microemulsion polymerisation of ethylacrylate. Only one chain per particle was observed for microemulsion system with M/S ratio 1 and three to four chains per particle were observed for the systems with M/S ratios 10 and 40. Unlike M/S=1 system, higher dependency of polymerisation rate on initiator concentration was observed for the systems with M/S=10 and 40. A possible mechanism for such a transformation has been proposed.     

Poly(butyl cyanoacrylate) nanoparticles stabilised with poloxamer 188: particle size control and cytotoxic effects in cervical carcinoma (HeLa) cells

In this study, the preparation of poloxamer 188-coated poly(butyl cyanoacrylate) colloidal nanospheres of controlled size distribution and their physicochemical characterisation were investigated and their cytotoxic effects in cervical carcinoma (HeLa) cells evaluated. The nanoparticles were prepared by controlled emulsion polymerisation of butyl cyanoacrylate in an aqueous medium containing poloxamer 188 as an amphiphilic non-ionic colloidal stabiliser. The colloids thus obtained were characterised by scanning electron microscopy, dynamic and electrophoretic light-scattering, Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. The average size of the particles could be finely controlled within an interval between 220 nm and 290 nm by varying the concentration of the precursor and citric acid in the polymerisation medium. The particle zeta-potentials in phosphate-buffered saline were approximately ?4.5 mV. FTIR and NMR data confirmed the expected composition of nanoparticles and the complete precursor polymerisation. In-vitro studies with cervical carcinoma (HeLa) cells demonstrated the dose-dependent cytotoxicity of nanoparticles (IC₅₀ ≈ 30 εg mL^?1). Observations by phase contrast and fluorescence microscopy revealed that at cytotoxic concentrations (40 εg mL^?1) nanoparticles induced changes in cell morphology and chromatin fragmentation. The colloidal stabiliser (poloxamer 188) alone was not cytotoxic at the applied concentrations.     

Morphology of natural rubber latex particles prevulcanised by sulphur and peroxide systems

Transmission electron microscopy provided direct evidence of the morphology of sulphur- and peroxide-prevulcanised natural rubber latex particles. A mesh structure of all cross-linked rubber particles containing polystyrene, prepared using the phase-transfer/bulk polymerisation process, was found. Each peroxide-prevulcanised particle had a nonuniform network structure, whereas the rubber network in sulphur-prevulcanised particles was homogeneous, irrespective of size. The effects of maturation and sodium dodecyl sulphate on the swelling ratio of the sulphur-prevulcanised latex film were investigated.     

Phosphated polyurethane dispersions: synthesis,emulsification mechanisms and the effect of the neutralising base

Phosphate-containing polyester macroglycols with different phosphate contents were synthesised from a phosphorus-containing monomer, a dicarboxylic acid and a diol. The macroglycols were then used as a soft segment for the preparation of segmented polyurethane dispersions. Aqueous dispersions were made by phase inversion from the organic solvent after the carboxylic acid groups were neutralized. Phase inversion was found to take place in three distinct stages. The stability and particle size of the polyurethane dispersions were dependent of the amount of carboxylic acid groups present, the degree of neutralisation and the neutralising cations. Metal-neutralized polyurethane dispersions gave smaller particle sizes and the corresponding films showed higher swelling in water due to the ease of hydration. The particle size and stability of the tertiary amine-neutralised polyurethanes were found to be related to the water solubility of the amines at a given dispersion temperature. Particle size increases for the higher alkyl chain neutalising amines due to the poor hydration of the corresponding cations.     

The preparation of monodisperse polyacrylonitrile particles in the presence of AB block co-polymers of poly-2-vinylpyridine/poly tertbutylstyrene

Radical dispersion polymerisation of acrylonitrile in cyclohexane was performed in the presence of the diblock copolymer poly-2-vinylpyridine/poly tertbutylstyrene, giving polymer latex particles stabilised by surface layers of chains of poly tertbutylstyrene. A seeded polymerisation route was employed and the addition of up to a further seven feeds of reactants resulted in the particle growth and the total volume fraction of particles increased to 0.2. The effect of varying the molecular weight of the stabiliser and stabiliser concentration was also investigated. Particle diameter in the range 40–150 nm were obtained depending on the condition of the polymerisation.     

Suspension polymerisation to form polymer beads

The conventional method employed for the production of large beaded particles is suspension polymerisation. A review of suspension polymerisation is presented, with particular reference to variations in chemical composition of the component phases. Both oil-in-water and water-in-oil systems are considered, as are the relevant methods of droplet stabilisation for such systems. Factors governing droplet stability and particle size and morphology are discussed. New developments including the use of continuous type reactors are also included.     

Poly(ethylene oxide) macromonomer-grafted polymer nanoparticles synthesised by emulsifier-free emulsion polymerisation

Ultrafine polymer nanoparticles based on poly(ethylene oxide) (PEO) macromonomer-grafted polystyrene (PS) have been synthesised by emulsifier-free emulsion polymerisation. In addition to the binary copolymerisation between PEO macromonomer and styrene, ternary copolymerisations were also conducted in the presence of a cationic monomer (2-(methacryloyloxy)ethyl) trimethylammonium chloride (MATMAC) as a second comonomer. The size and charge characteristics of fine nanoparticles were characterised using both photon correlation spectroscopy and transmission electron microscopy techniques as well as colloidal titration. It was found that after PEO chains (repeat unit 9 or higher) were incorporated into the PS latex, the particle size was significantly reduced owing to the steric effect contributed from grafted PEO chains. Ternary copolymerisation using MATMAC as comonomer further reduced the particle size, leading to nanoparticles as small as 60 nm. Increasing the MATMAC feed ratio gradually reduced the final size of the nanoparticle, owing to the enhancement in electrostatic stabilisation, whereas increasing the PEO macromonomer feed ratios led to slightly larger particles but significantly inhibited the agglomeration of primary particles. The formation mechanism of the nano- or microparticles with various sizes during polymerisation is discussed in terms of nucleation, agglomeration and adsorption of primary particles.     

Thermoresponsive submicron-sized core–shell hydrogel particles with encapsulated olive oil

Thermoresponsive submicron-sized core–shell hydrogel particles with incorporated olive oil were synthesised and studied. The microspheres having poly(N-isopropylacrylamide-co-methyl methacrylate) core and poly(N-isopropylacrylamide) shell were synthesised by emulsifier-free seed polymerisation method. The morphology, particle size and distribution characteristics of the core microspheres were studied with different amount of initiator, monomer–solvent ratio and polymerisation time using scanning electron microscopy and dynamic light scattering particle size analysis. The prepared core and core–shell microspheres were regularly spherical with average size of around 190.0 and 320.0 nm respectively and nearly monodispersed size distribution. Transmission electron microscopy study revealed the core–shell structure of the microspheres. The thermoresponsive transition temperature (T _t) of the core–shell microspheres was determined as 33 °C by optical absorbance measurement, dynamic light scattering particle size analysis and differential scanning calorimetry. The release rate of olive oil from core–shell microspheres was accelerated by squeezing out the entrapped olive oil as the temperature was increased above T _t. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy study indicated the formation of copolymer.     

Production of porous suspension polymer beads with a narrow size distribution using a cross-flow membrane and a continuous tubular reactor

The work described focuses on a two-stage process for the production of large porous suspension polymer beads of defined particle size and narrow size distribution. Emulsification has been performed using purpose built cross-flow membrane equipment, which allows controlled production of large emulsion droplets with a much narrower size distribution. The work described compares the production of large emulsion droplets of monomer prepared both by agitation and using a cross-flow membrane. The effects of variations in the pore size of the membrane and flow-rates on the size of the emulsion droplets produced are also investigated. The second stage of the process is polymerisation of performed monomer emulsion droplets using a continuous tubular reactor. Samples polymerised using such a method show a narrower size distribution than similar systems polymerised as a batch.     

Some studies on the photo-initiated cationic polymerisation of epoxides

Several triarylsulphonium and diaryliodonium compounds have been synthesised. Alkylarylsulphonium compounds were obtained by alkylating diaryl sulphides with trialkyloxonium salts and by reaction of diaryl sulphides with alkyl halides in the presence of silver tetrafluoroborate. Photolysis of the sulphonium salts in methanol gave diaryl sulphides and, in the case of triarylsulphonium compounds, the appropriate aromatic hydrocarbon and its methyl ether. Diaryliodonium tetrafluoroborates and pentafluorophosphates gave aryl fluorides, biaryls and aromatic hydrocarbons. The decomposition of the salts is suggested as occurring via both free radical and ionic pathways. The ability of the compounds to sensitise the polymerisation of epoxy resins was found to be dependent upon the counterion (hexafluorophosphates being more efficient than tetrafluoroborates) and upon the structure of the cation. Sensitised cationic initiated polymerisation was also investigated and it was found that both excited singlet state and triplet state sensitisers were effective.     

Characterisation of poly(N-isopropylacrylamide) by asymmetrical flow field-flow fractionation, dynamic light scattering, and size exclusion chromatography

Asymmetrical flow field-flow fractionation (AsFIFFF) was used to determine the hydrodynamic particle sizes, molar masses, and phase transition behaviour of various poly(N-isopropylacrylamide) (PNIPAM) samples synthesised by reversible addition--fragmentation chain transfer (RAFT) and conventional free radical polymerisation processes. The results were compared with corresponding data obtained by dynamic light scattering (DLS) and size exclusion chromatography (SEC). Agreement between the three methods was good except at higher molar masses, where the molar mass averages obtained by SEC were much lower than those obtained by AsFIFFF and light scattering. The aggregation of the polymers, which are thermally sensitive, was studied by DLS and AsFIFFF at various temperatures. In deionised water there was an abrupt change in the particle size due to phase separation at approximately equal to 32-35 degrees C. The critical temperatures determined by AsFIFFF were 3-5 degrees C higher than those obtained by DLS.     

A comparative study of 9,9-bis(4-aminophenyl)fluorene polymers prepared by catalytic and non-catalytic oxidative polymerisation methods

9,9-Bis(4-aminophenyl)fluorene (APF) polymers were synthesized by two different pathways. The different synthetic routes gave polymers with different structures. FT-IR and NMR studies revealed that the catalytic oxidative polymerisation of APF in acetonitrile produced P2, whereas oxidative polymerisation of APF in aqueous alkaline media gave P3. The effects of these different techniques on spectral, thermal, optical, electrical, electrochemical, and morphological properties of resulting polymers were investigated.     

Vinyl chloride suspension polymerisation and the control of polymer properties

A number of poly(vinyl alcohols), used in the suspension polymerisation of vinyl chloride, have been fractionated and characterised. The most effective had the highest molecular weight and contained the most unsaturation. The more insoluble fractions gave the best balance of product properties. Reactor sampling experiments have been used to determine the mechanism of polymerisation. In most polymerisations, the final grain size is determined by factors that control the coalescence and break-up of the monomer droplets during the first 15% conversion. It is suggested that the structure within the grains is controlled by the formation of a continuous network of PVC primaries which retards droplet contraction.     

Population Balance Model for Emulsion Polymerisation Under Pseudo-Bulk Conditions: Combined Particle Size Distribution and Molecular Weight Distribution

A comprehensive model for emulsion polymerisation is presented, accounting for particle size distribution (PSD) and molecular weight distribution (MWD). The PSD information is incorporated through a population balance framework. A mechanistic formulation is adopted in modelling the average number of radicals/particle under pseudo-bulk compartmentalisation conditions. The method of moments is adopted to simplify the MWD equations over each discrete size class. The impact of the pseudo-bulk assumption on the PSD and MWD results is assessed. An identification of potential manipulated variables for control of PSD and MWD is done through sensitivity analysis.     

A novel preparation method for microspheres of water soluble polymers using polypropyleneglycol as the dispersion medium

Polypropyleneglycol (PPG) was used as a dispersion medium for the preparation of microspheres (MS) consisting of starch, gelatin, whey protein or dextran. Aqueous solutions of the polymers were dispersed in PPG at various initial temperatures and then the systems were cooled to 0.5 degrees C to allow water in the dispersed phase to dissolve in PPG. The particle size of the starch-MS was dependent on the initial temperature of PPG in the preparation process. There were two different processes for particle generation in the procedure. One of them was via the formation of a temporary emulsion during the early phase of dispersion of the aqueous polymer solution into PPG. The other was via the stable emulsion in which the aqueous polymer solution was dispersed in water-saturated PPG. The particle size generated in the former process was dependent on the initial temperature: a high temperature gave large particles but a low temperature gave small particles, while that in the latter process was temperature-independent. This preparation method for MS will be useful for the formulation of heat-sensitive material, such as protein-containing drugs.