Fundamentals of Polymer Science and technology University of Shanghai for Science and Technology Hua Zou hua.zou@usst.edu.cn

Table of Contents Topic Classes Introduction to Polymer Science 3 Polymer Synthesis 18 Solid-State Properties 9 Viscoelasticity Polymer Degradation and Environment Polymer Solutions Miscellaneous 6 In Total 45

Lecture 5: Polymer Synthesis 5.1 Polymerization Techniques (Review) 5.2 Polymerization Techniques: Emulsion Polymerization

5.1 Polymerization Techniques (Review)

5.1.1 Bulk Polymerization Simplest – just monomer + initiator (or catalyst) Highest-purity polymer: only monomer, a monomer-soluble initiator, (a chain-transfer agent)

5.1.1 Bulk Polymerization: Advantages and Limitations high yield per reactor volume, easy polymer recovery casting the polymerization mixture into final product form (i.e., cast polymerization). Limitations: Difficulty of removing residual traces of monomer Dissipating heat produced during the polymerization.

5.1.1 Bulk Polymerization: Heat Problem Free-radical polymerizations are highly exothermic (typically 42 to 88 kJ mol-1), while the thermal conductivity of organic monomers and polymers are low. An increase in temperature will increase the polymerization rate and, therefore, generate additional heat to dissipate.

5.1.1 Bulk Polymerization: Heat Problem Heat removal becomes particularly difficult near the end of the polymerization when viscosity is high. This is because high viscosity limits the diffusion of long-chain radicals required for termination. This means that radical concentration will increase and, therefore, the rate of polymerization also will increase.

5.1.1 Bulk Polymerization: Heat Problem Autoacceleration process (gel effect): The diffusion of small monomer molecules to the propagation sites is less restricted. This means that the termination rate decreases more rapidly than the propagation rate, and the overall polymerization rate, therefore, increases with accompanying additional heat production. Heat dissipation can be improved by providing special baffles (隔板) for improved heat transfer or by performing the bulk polymerization in separate steps of low-to-moderate conversion.

5.1.1 Bulk Polymerization: Examples Bulk-polymerization processes can be used for many free-radical polymerizations and some step-growth (condensation) polymerization. (less exothermic). Important examples of polymers usually polymerized by free-radical bulk polymerization include PS and PMMA for which cast polymerization accounts for about half of the total production. Low-density PE and some ethylene copolymers are sometimes produced by bulk free-radical polymerizations.

5.1.2 Solution Polymerization Heat removal during polymerization can be facilitated by conducting the polymerization in an organic solvent or preferably water: cost and handling advantages as well as high thermal conductivity. Solvent acts as a diluent and dissipates the heat of polymerization. Lower viscosity, so it is easier to stir the reaction solution.

5.1.2 Solution Polymerization: Solvent Choice Both the initiator and monomer can be soluble in it. Have acceptable chain-transfer characteristics. Suitable melting and boiling points. Other factors: flash point, cost, and toxicity. Suitable organic solvents: aliphatic and aromatic hydrocarbons, esters, ethers, and alcohols. Often, solvent reflux to maximize heat removal. Reactors: usually stainless steel or glass-lined.

5.1.2 Solution Polymerization: Disadvantages Small yield per reactor volume, a solvent-recovery step Can get chain-transfer of radicals to solvent (Mn decreases) Have to remove solvent (toxic, expensive) to isolate polymer

5.1.2 Solution Polymerization: Examples Water soluble polymers: poly(acrylic acid), polyacrylamide, poly(vinyl alcohol), and poly(N-vinylpyrrolidinone). Vinyl acetate and acrylic esters polymerized by free-radical chain polymerization in various organic solvents; acrylic acid polymerized in alcohol/water mixtures. Most ‘living’ anionic polymerizations.

5.1.3 Suspension Polymerization Improved heat transfer can also be obtained by utilizing the high thermal conductivity of water through either suspension or emulsion polymerization. Droplets of monomer containing the initiator and chain-transfer agent are formed. These are typically between 50 and 200 m in diameter and serve as miniature reactors for the polymerization. Coalescence of these "sticky" droplets is prevented by the addition of a protective colloid, and by constant agitation of the polymerization mixture.

5.1.3 Suspension Polymerization 60 oC M,I M = monomer, I = initiator P = polymer Suitable monomers include vinyl chloride, vinyl acetate and methyl methacrylate Formulation: water-immiscible monomer, water-insoluble initiator, water-soluble polymeric stabiliser, water Stabilizer can be poly(vinyl alcohol) or cellulosic derivatives Efficient stirring is important to obtain polydisperse polymer beads (50 μm to 5 mm)

5.1.3 Suspension Polymerization Near the end of the polymerization, the particles harden and can then be recovered by filtration, which is followed by a final washing step. Although solvent cost and recovery operations are minimal in comparison with solution polymerization, polymer purity is low due to the presence of suspending and other stabilizing additives that are difficult to completely remove. In addition, reactor capital costs are typically higher than for solution polymerization.

5.1.3 Suspension Polymerization: Examples styrenic ion-exchange resins, extrusion and injection-molding grades of poly(vinyl chloride), poly(styrene-co-acrylonitrile) (SAN), extrusion-grade poly(vinylidene chloride-co-vinyl chloride).

5.1.3 Suspension Polymerization PVAc beads PMMA particles Water is cheap, non-flammable and has a high heat capacity Low solution viscosity (since ‘beads’ not soluble polymer) Some industrial utility

5.1.4 Dispersion Polymerization Precipitation polymerization in solvent polymerization Precipitation polymerization in presence of a suitable polymeric stabilizer e.g. poly(vinyl alcohol) or poly(N-vinyl pyrrolidone) (i) Get good control over heat dissipation and viscosity; (ii) Can be carried out in aqueous or non-aqueous media Stabilizer adsorbs onto precipitating polymer nuclei and prevents further growth via steric stabilisation Latex Particle Adsorbed solvated stabiliser layer

5.1.4 Dispersion Polymerization Get microscopic particles of polymer suspended in solvent that do not aggregate further – a.k.a. ‘latexes’ Typical particle size range: 0.1 to 10 µm (human hair width = 80 µm) Can get very high solids ~ 50-60 % but with low viscosity (since particles) Widely used to make solvent-based (non-aqueous) latexes for ‘gloss’ paints Other applications: Coatings for textiles, paper, photographic film Lacquers, varnishes, floor polish etc. Both high tonnage, high volume products and also speciality (value-added) products industrially important!

5.1.4 Dispersion Polymerization AIBN PNVP, methanol, 60 oC 1 - 2 m PS Latex Particle PNVP Styrene PNVP = poly(N-vinylpyrrolidone) Styrene monomer, AIBN initiator, PNVP stabiliser are all soluble in methanol Polystyrene [PS] latex particles can be readily prepared by free radical dispersion polymerisation in alcoholic solvent.

5.1.4 Dispersion Polymerization Scanning electron micrograph of PNVP-stabilised polystyrene latex particles prepared by alcoholic dispersion polymerization.

5.2 Emulsion Polymerization

Contents 5.2.1 Overview 5.2.2 Process of Emulsion Polymerization 5.2.3 Kinetics of Emulsion Polymerization 5.2.4 Advantages and Applications of Emulsion Polymerization 5.2.5 Other Emulsion Polymerization Techniques

5.2.1 Emulsion Polymerization: Overview Preferred method for (co)polymerizing many vinyl monomers such as vinyl acetate, acrylics, vinyl chloride, methyl methacrylate, styrene etc. Formulation: water-immiscible monomer, water-soluble initiator, surfactant, water Surfactant ( ) forms loose colloidal structures called micelles:

5.2.1 Emulsion Polymerization: Overview Monomer has low solubility in aqueous phase Initiator is not soluble in monomer droplets So where is the locus of the polymerisation? 99 % of the polymerization occurs inside the monomer-swollen micelles! At any given time during emulsion polymerization, the polymerizing micelles contain either one (growing) or none (dormant) polymer radicals.

5.2.1 Emulsion Polymerization: Overview nonionic anionic cationic Surfactant classification according to the composition of their head amphoteric Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (tails) and hydrophilic groups (heads). Therefore, a surfactant contains both a water insoluble (or oil soluble) component and a water soluble component.

5.2.1 Emulsion Polymerization: Overview Surfactants will diffuse in water and adsorb at interfaces between air and water or at the interface between oil and water, when water is mixed with oil. The (hydrophobic) monomer molecules form large droplets (液滴). These are stabilized by the surfactant molecules whose hydrophilic ends point outward and whose hydrophobic (aliphatic) ends point inward toward the monomer droplet. The size of monomer droplets depends upon the polymerization temperature and the rate of agitation.

5.2.1 Emulsion Polymerization: Overview A micelle (胶束) Above a certain surfactant concentration, the critical micelle concentration (临界胶束浓度), residual surfactant molecules can align to form micelles (胶束).

5.2.1 Emulsion Polymerization: Overview Type of Polymerization Solvent Type Monomer Solubility Initiator Solubility Stabiliser Particle Size Range Suspension Water Immiscible with water Soluble in monomer Polymer e.g. PVA 20 μm to 5 mm Dispersion Water or org. solvents Miscible with solvent solvent e.g. PNVP 100 nm to 10 μm Emulsion Soluble in water Surfactant e.g. SDS 100 to 500 nm

. styrene monomer Polymerizing mixture contains surfactant/emulsifier Monomer-swollen micelle Water-soluble Initiator (eg. (NH4)S2O8) Free Surfactant (eg. SDS) Macroscopic monomer droplet radical D ~ 10-50 nm: 1018 /mL D ~ 3 mm: 1010 /mL

5.2.2 Process of Emulsion Polymerization Monomer molecules that have a small but significant water solubility can migrate from the monomer droplets through the water medium to the center of the micelles.

5.2.2 Process of Emulsion Polymerization Polymerization is initiated when the water-soluble initiating radical enters a monomer-containing micelle. Due to the very high concentration of micelles, typically 1018 per mL, compared to that of the monomer droplets (1010 to 1011 per mL), the initiator is statistically more likely to enter a micelle than a monomer droplet.

5.2.2 Process of Emulsion Polymerization As the polymerization proceeds, additional monomer molecules are transferred from the droplets to the growing micelles. At 50% to 80% monomer conversion, the monomer droplets are depleted and the swollen micelles are transformed to relatively large polymer particles, typically between 0.05 and 0.2 μm in diameter.

5.2.2 Process of Emulsion Polymerization K2S2O8 2 SO4 - . Δ (1) Chemical: . H 2 C CH n Styrene monomer Growing polymer chain has a radical at one end and an anionic initiator fragment at the other end K2S2O8 initiator (R ) 60 - 90 oC Polymerization occurs solely within micelles SO4 - (2) Physical: PS - 100 to 500 nm diameter Surface charge is due to surfactant and/or initiator fragment 60 - 90 oC Monomer-swollen surfactant micelles and styrene-in-water emulsion nM . surfactant-stabilized polymer latex particles K2S2O8 initiator, R

5.2.2 Process of Emulsion Polymerization 400 nm Highly monodisperse, spherical polystyrene particles of around 220 nm diameter SEM Image of PS Latex by Emulsion Polymerization

5.2.3 Kinetics of Emulsion Polymerization The free-radical kinetics of emulsion polymerization is different from the usual free radical kinetics of bulk, solution, or suspension polymerization. Smith and Ewart assumed that the monomer-swollen micelles are sufficiently small that, on the average, only one propagating chain or one terminated chain can exist inside a particle at any time. This means that the radical concentration, [IMx•], is simply equal to one- half of the particle concentration, N (units of particle number per mL), which in turn is determined by the surfactant and initiator concentrations.

5.2.3 Kinetics of Emulsion Polymerization The polymerization rate: where [M] is the concentration of monomer inside the swollen polymer particles.

5.2.3 Kinetics of Emulsion Polymerization The polymerization rate: This expression should be compared to the usual steady-state rate expression for free-radical polymerization. While both rates are proportional to monomer concentration, the rate of emulsion polymerization is no longer proportional to the square root of initiator concentration but follows a more complicated dependence on initiator concentration through its dependence on N.

5.2.4 Advantages and Applications of Emulsion Polymerization Compartmentalization of polymerization within micelles changes kinetics dramatically! In bulk, solution, suspension or dispersion polymerization: But with emulsion polymerisation can get high molecular weight polymer formed at high reaction rates due to Smith-Ewart kinetics. This is a major advantage of emulsion polymerisation! Usually mol. wt.  1 Rate of polym. i.e. difficult to generate high MW polymer efficiently

5.2.4 Advantages and Applications of Emulsion Polymerization with good thermal, viscosity control very high (> 99.9 %) monomer conversions so minimal organic waste water is a cheap, non-toxic, non-flammable, high heat capacity solvent with low VOC’s Extremely environmentally-friendly process

5.2.4 Advantages and Applications of Emulsion Polymerization Emulsion polymers are very widely used as: aqueous latex paints, additives for concrete and cement, wood coatings, tile adhesives, latex gloves, bottle label adhesives, cosmetics, medical diagnostics, rubber-toughening additives etc.

5.2.4 Advantages and Applications of Emulsion Polymerization Chemical companies : ICI BASF AkzoNobel Rohm and Haas …

5.2.5 Other Emulsion Polymerization Techniques 5.2.6.1 Surfactant-Free Emulsion Polymerization 5.2.6.2 Core-Shell Emulsion Polymerization 5.5.6.3 Inverse Emulsion Polymerization 5.2.6.4 Miniemulsion Polymerization 5.2.6.5 Microemulsion Polymerization

5.2.5.1 Surfactant-Free Emulsion Polymerization The presence of surfactant is a disadvantage for certain applications of emulsion polymers. The presence of adsorbed surfactant gives rise to somewhat variable properties since the amount of adsorbed surfactant can vary with the polymerization and application conditions. Removal of the surfactant, either directly or by desorption, can lead to coagulation or flocculation of the destabilized latex. Surfactant-free emulsion polymerization is a useful approach to solving this problem.

5.2.5.1 Surfactant-Free Emulsion Polymerization The process uses an initiator yielding initiator radicals that impart surface- active properties to the polymer particles. Persulfate is a useful initiator for this purpose. Latexes prepared by the surfactant-free technique are stabilized by chemically bound sulfate groups of the SO4- initiating species derived from persulfate ion. Since the surface-active groups are chemically bound, the latexes can be purified (freed of unreacted monomer, initiator, etc.) without loss of stability, and their stability is retained over a wider range of use conditions than the corresponding latices produced using surfactants.

5.2.5.1 Surfactant-Free Emulsion Polymerization A characteristic of surfactant-free emulsion polymerization is that the particle number is generally lower by up to about 2 orders of magnitude compared to the typical emulsion polymerization, typically 1012 versus 1014 particles per milliliter. This is a consequence of the lower total particle surface area that can be stabilized by the sulfate groups alone relative to that when added surfactant is present.

5.2.5.2 Core-Shell Emulsion Polymerization polymerisation of first monomer of second monomer ‘seed’ latex core-shell latex Core-shell latex morphologies are also possible via emulsion polymerization.

5.2.5.3 Inverse Emulsion Polymerization In the conventional emulsion polymerization, a hydrophobic monomer is emulsified in water and polymerization initiated with a water-soluble initiator. Emulsion polymerization can also be carried out as an inverse emulsion polymerization. Here, an aqueous solution of a hydrophilic monomer is emulsified in a nonpolar organic solvent such as xylene or paraffin and polymerization initiated with an oil-soluble initiator. The two types of emulsion polymerizations are referred to as oil-in-water (o/w) and water-in-oil (w/o) emulsions, respectively.

5.2.5.3 Inverse Emulsion Polymerization Inverse emulsion polymerization is used in various commerical polymerizations and copolymerizations of acrylamide as well as other water-soluble monomers. The end use of the reverse latices often involves their addition to water at the point of application. The polymer dissolves readily in water, and the aqueous solution is used in applications such as secondary oil recovery and flocculation (clarification of wastewater, metal recovery).

5.2.5.4 Miniemulsion Polymerization Micelles are usually not present because surfactant concentrations are generally below CMC. Miniemulsion polymerization involves systems with monomer droplets in water with much smaller droplets than in emulsion polymerization, about 50–1000 nm compared to 1–100 mm in diameter. Water-insoluble costabilizers such as hexadecane and cetyl alcohol are present along with the surfactant to stabilize the monomer droplets against coagulation.

5.2.5.4 Miniemulsion Polymerization The droplet size depends not only on the amount of surfactant and costabilizer but also on the amount of energy used in the homogenization process. The final polymer particle size is similar to the monomer droplet size. Both water-soluble and oil-soluble initiators have been used in miniemulsion polymerization. Miniemulsion polymerizations are useful for producing high-solids-content latexes.

5.2.5.5 Microemulsion Polymerization Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10–100 nm compared to 1–100 mm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10–50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same.

5.2.5.5 Microemulsion Polymerization Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with oil-soluble initiators. Nucleation occurs over a larger portion of the process in microemulsion polymerization because of the large amount of surfactant present. Nucleation may extend over most of the process.

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