Why Study Polymer Science and Processing?Employment Opportunities: 135,000,000 tons of plastics alone are produced annually an estimated one in three research dollars in North America is invested in polymer science. Scientific Interest: structure-property relationships of polymers and polymer compounds chemical modification of polymers f advanced applications h i l difi ti f l for d d li ti polymer blending and compatiblization techniques U.S. Polymer Production (billions of pounds) 1993 PLASTICS Thermosetting Resins Phenol resins Urea resins Polyesters (unsaturated) Epoxies Melamine resins Thermoplastic Resins Low-density polyethylene PVC and copolymers High-density polyethylene Polystyrene Polypropylene Total 1992 FIBERS Cellulosics Rayon Acetate Noncellulosics Polyester Nylon Olefin Acrylic Total SYNTHETIC RUBBER Styrene-butadiene rubber Polybutadiene Ethylene-propylene rubber Nitrile rubber (NBR) Other Total TOTAL PRODUCTION 1993 1992 3.08 1.74 1.26 0.51 0 51 0.27 2.92 1.55 1.18 0.46 0 46 0.23 0.28 0.23 0.28 0.22 Engineering Design Challenges: life-cycle analyses y y polymer compound development design/optimization of polymer processing methodologies polymer synthesis Introduction CHEE 490 1.1 12.04 10.26 9.91 5.37 5 37 8.61 53.06 11.92 9.99 9.81 5.10 5 10 8.42 51.57 3.56 2.66 2.14 0.43 9.30 3.58 2.56 2.00 0.44 9.07 1.89 1.03 0.58 0.14 1.37 5.00 67.35 1.92 1.02 0.58 0.13 1.42 5.07 65.71 1.2 Chemical and Engineering News, April 11, 1994. News 11 1994 Introduction CHEE 490 Polymer Science and Processing Technology Successful product design requires a knowledge of: the requirements of the final product the behaviour of polymeric materials commercial polymer processing technology relevant cost and market factors. Classification of Polymer Applications 1. Elastomers static uses: gaskets, hoses dynamic uses: tires sports equipment tires, 2. Adhesives structural: epoxy resins non-structural: pressure-sensitive tapes, hot-melt adhesives 3. Coatings 3 C ti lacquers, paints 4. Plastics semi-crystalline: automobile exterior amorphous: p p packaging films, p g g plexi-glass g 5. Fibres natural/modified: cotton, rayon synthetic: carpeting, apparel 1.3 Introduction CHEE 490 1.4 At the heart of polymer science and technology is molecular structure. It gy dictates not only final product properties, but polymer synthesis and processing methods. Introduction CHEE 490 Emphasis of Course Material (Weeks 1-6) Each of the five applications will be examined from the following perspectives: Industrial requirements for end-use and processing end use » Basic testing methods Polymer compound formulations » essential polymer properties » compound additives Relevant engineering science » Elastomers: origin of elasticity, crosslinking, reinforcement » Adhesives: surface energy » Coatings: viscosity » Plastics: mechanical properties, p y p p polymer composites p » Fibres: crystallization Emphasis of Course Material (Weeks 7-12) Each of the five applications will be examined from the following perspectives: Industrial polymer processing techniques » extrusion » injection molding » fibre spinning » compression molding » polymer/additive blending Key processing variables » polymer compound rheology » fluid mechanics Assessment of processing variables p g Introduction CHEE 490 1.5 Introduction CHEE 490 1.6 Design Project Develop a polymer compound and processing method for a component of your choice. 1. Define engineering and aesthetic qualities. 2. 2 Propose a compounding recipe that will satisfy these requirements. 3. Recommend appropriate processing t h i 3 R d i t i techniques f for manufacturing the product. Examples? Contact lens, medical catheter, biodegradable packaging, artificial joint, high-performance tire tread... j g p Physical Properties of Polymer Compounds The materials selection component of a part design demands careful consideration of all required properties. Consider the following case studies: electric drill casing automobile bumper aircraft tire What properties must a given material provide for each of these components? As engineers, you must be able to translate qualitative terms (strong, flexible) into engineering terms for which quantitative data is available. We will survey various physical testing methods that are used industrially, and highlight important behavioural y g g p characteristics of polymers Throughout the course we will refer to these testing methods as we examine adhesive, elastomer, plastic, fiber and coatings applications. 1.7 Polymer Properties CHEE 490 1.8 Introduction CHEE 490 Polymer Material Selection - Key Questions When developing a polymer compound for a given application, you may ask yourself the following questions: What are the maximum and minimum temperatures the compound will experience throughout its lifetime? » includes manufacturing as well as product use To what loads will the material be subjected, and what is the frequency of load application? f fl d li ti ? » engine mounts, fishing line Is the part transparent, translucent or opaque? Colouring? Is flame resistance necessary, and to what environmental y conditions will the product be exposed? » solvent resistance, oxidative degradation Static Testing of Polymers and Polymer Compounds Stress-strain analysis is the most widely used mechanical test. However, it is only a rough guide as to how a material will behave in a given application application. Test specimens are prepared in the form of “dog bones” whose dimensions are known accurately: A static test involves deformation of the sample at a steady rate, usually with one end fixed and the other pulled at a constant rate of elongation (tensile testing). The retractive force of the material is recorded as a function of the elongation, and the engineering g g g stress, σ, is calculated as a function of the engineering strain, ε. σ= 1.9 Polymer Properties F : Pa Ao CHEE 490 ε= ΔL Lo 1.10 Polymer Properties CHEE 490 Static Testing of Polymers and Polymer Compounds We will soon see that observed polymer properties are strongly dependent on temperature and the applied rate of deformation. Under some conditions, an elastomer can behave like a brittle conditions plastic, and vice-versa. A Three typical behaviours are illustrated here. A B Compression and Shear vs. Tensile Tests Stress-strain curves are very dependent on the test method. A modulus determined under compression is generally higher than one derived from a tensile experiment as shown below for experiment, polystyrene. Tensile testing is most sensitive to material flaws and microscopic cracks. Compression tests tend to be characteristic of the polymer, while tension tests are more characteristic of sample flaws. Note also that flexural and shear test modes are commonly employed. Often cited sample properties: A: Ultimate tensile stress (Pa) and elongation at break (%) B: B Yield tensile stress Pa stress, Toughness: Area under σ − ε curve. curve Polymer Properties CHEE 490 1.11 A Polymer Properties CHEE 490 1.12 5 1-2 5 100-350 2-10 25 400-700 Temperature Sensitivity of Polymer Properties Crystallinity and crosslinking of polymer chains influence the modulus of a polymer as shown below. while nylon-6.1-0. At room temperature. Copper Polystyrene Soft Rubber Polymer Properties E (Pa) 1.0*109 2. glass transitions and melt viscosity. poly(ethylene) is above Tg but below temperature Tm.6 2. What differences in mechanical properties might you expect? Poly(ethylene):low-density 0 1-0 3 0. E: σ=Eε which relates strain to retractive stress over the linear region.0 10 3.2*1011 3.4-2.8 2 4-2 8 3.0*106 CHEE 490 1.Static Testing of Polymers and Polymer Compounds Shown is a representative stress-strain curve for a polymer undergoing brittle failure. An often quoted material property is the tensile (or Young’s) modulus.9-14 6 9-14 23 --10-14 48-62 24-38 38-55 14-28 48-69 800 10-17 200-600 1-2.16 .6 is below both Tg and Tm.41 2.0-1.15 Polymer Properties CHEE 490 1.8-3.8 6.5 2 8-3 5 0.13 Mechanical Properties of Representative Polymers Elastic Modulus ( (GPa) ) Poly(propylene) Poly(styrene) Poly(tetrafluoroethylene) Poly(methylmethacrylate) Nylon Yield Ultimate Elongation Strength Strength to Break ( (MPa) ) ( (MPa) ) (%) 1. Polymer Properties CHEE 490 1. We ill W will see th t b h i that behaviour i hi hl i fl is highly influenced b t d by temperature t when we examine factors such as degree of crystallinity.3 Note that these values depend on temperature and strain rate. strain. Impact strength units vary. Creep behaviour arises from the viscoelastic properties of polymers and their compounds. σo. but notched tests are defined in terms of energy per unit length of notch: kJ/m. Izod and Charpy (shown to right) impact tests use a weighted pendulum to measure the loss of kinetic energy associated with specimen fracture. ε. Polymer Properties Transient Testing: Creep Tests Creep tests can be made under all load conditions. poly(chloroprene). D: Cellulose acetate (Plasticized) 25 psi load. poly(methyl methacrylate). and provide data needed to design products that sustain loads for long periods A constant stress. is applied with the strain ε varying with stress applied.18 Transient Testing: Impact Resistance Impact tests are high-speed fracture tests that measure the energy required to break a specimen specimen. The HDT is defined as the temperature at which the elongation becomes 2%. A: Rigid poly(vinyl chloride) 50 psi load.19 Transient Testing: Resilience of Cured Elastomers Resilience tests reflect the ability of an elastomeric compound to store and return energy at a given frequency and temperature. poly(isobutylene). time. notching and size.20 . Agreement between different methods can be poor. Polymer Properties CHEE 490 1.17 Polymer Properties CHEE 490 1. Polymer Properties CHEE 490 1. but dependent on sample geometry. Change of rebound resilience (h/ho) with temperature T for: 1. 4. and results are not material constants. B: Low-density poly(ethylene) Low density 50 psi load.Heat Distortion Temperature The maximum temperature at which a polymer can be used in rigid material applications is called the softening or heat distortion temperature (HDT). C: Poly(styrene-co-acrylonitrile) 25 psi load load. A: Elastic B: Viscous C: Viscoelastic CHEE 490 1. cis-poly(isoprene). Above are ill t t d th response of diff Ab illustrated the f different idealized materials t t id li d t i l to step changes in applied stress. 3. and heating at a rate of 2oC per min. p y( p ) 2. (HDT) A typical test (plastic sheeting) involves application of a static load. is subjected to steady simple extension at a rate ε starting at a time=0. meaning that they exhibit shear thinning behavior Flow Characteristics – Rheology of Polymer Melts Extensional thickening effects are observed when tracking the extensional viscosity as a function of time. ε) − σ 22 ( t. careful consideration of thermal expansion/contraction must be made. defined as: coefficient & η+ ( t. ε) ≡ E & & σ11 ( t. state. Changes in density must be taken into account when designing the mold. the tensile stress growth coefficient.Flow Characteristics – Rheology of Polymer Melts Polymer melts and solutions are pseudoplastic.ε) CHEE 490 . If a sample.22 Thermal Expansion If a part is to be produced within a close dimensional tolerance.21 Polymer Properties ηE+(t. initially at its rest . Polymer Properties CHEE 490 1. ε) & ε shows the onset of strain hardening effects depend on the applied shear rate.23 . Parts are produced in the melt state. Polymer Properties CHEE 490 1. 1. and solidify to amorphous or semi-crystalline states. Chain architecture has a dramatic effect on properties such as viscosity. from the polymer backbone. Polystyrene. but varies with chain stiffness. there is just one polymer chain of infinite molecular weight. Phenol-formaldehyde resins. elasticity. Crosslinked: A continuous C li k d ti network of polymer chains is a crosslinked condition. melamine paints p permanent adhesives. toughness) and chemical properties (solubility. Thermoset: polymers whose individual chains have been chemically crosslinked by covalent bonds and therefore resist heat softening.1 Polymer Classification: Thermoplastic/Thermoset One of the most practical and useful classification of polymer compounds is based on their ability to be refabricated.3 Polymer Classifications CHEE 490 2. Polystyrene polyethylene recyclable food containers Polymer Classification: Chain Architecture Linear: A linear polymer chain is one without branches. Differences in composition.Polymer Classification What distinguishes polymers from other organic compounds is molecular weight and dimension.4 . architecture and molecular weight give rise to differences in mechanical properties (strength. Polymer Classifications CHEE 490 2. Thermoplastic: polymers that can be heat-softened in order to process into a desired form. Branched: Chains with an appreciable number of side-chains are classified as branched These side chains may differ in composition branched. coatings g Polymer Classifications CHEE 490 2. Its actual conformation may not be “line-like”. elasticity and temperature stability. In effect. crystallinity and applied stresses stresses. creep and solvent attack. aging). poly(ethylene) poly(butadiene) Random copolymers: two monomers randomly distributed in chain. again favoured by chemists is based on differences between the polymer and constituent monomer(s).nylon 6 Urethanes: carbamate linkages through reaction of diisocyanates and diols.e. Distributions CHEE 490 2.Dacron Polyamides: P l id poly(caprolactam) . poly(ethylene).Polymer Classification: Chemical Microstructure Homopolymers: polymers derived from a single monomer (can be linear. but exhibit different processing characteristics i h t i ti and physical properties. Molecular Weight Averages Suppose we have a mixture of four different sized ball bearings.5 Polymer Classification: Chemical Class A popular classification scheme amongst chemists is based on polymer functionality. What meaningful averages can be calculated? In addition to molecular weight. These materials share the same chemical composition (-CH2-).8 .6 Molecular Weight and Composition Distributions While small molecules are defined uniquely by molecular weight and atomic connectivity. poly(butadiene). monomer(s) Condensation polymers: synthesis involves elimination of some small molecule (H2O in the preparation of nylon) Addition polymer: formed without loss of a small molecule i e i. polymers are not. ethylene polymerization to generate poly(ethylene) Polymer Classifications CHEE 490 2. chemical structure differs between polymers and even between chains within a given sample. or the polymer microstructure has an enormous impact on engineering properties. Polyesters: P l t poly(ethylene terephthalate) . and run them across the trough shown below. branched or crosslinked).7 Distributions CHEE 490 2. O C O H O N C H O N C O Another (!) classification scheme. Chemical composition distribution. AABAAABBABAABBA poly(acrylonitrile-ran-butadiene) Alternating copolymers: two monomers incorporated sequentially ABABABABABABABAB poly(styrene-alt-maleic anhydride) p y g g g Block copolymers: linear arrangement of blocks of high mol weight AAAAAAAAAAABBBBBBBBBBBBBBBAAAAAAAA polystyrene-block-polybutadiene-block-polystyrene or poly(styrene b butadiene b styrene) poly(styrene-b-butadiene-b-styrene) Graft copolymers: differing backbone and side-chain monomers poly(isobutylene-graft-butadiene) Polymer Classifications CHEE 490 2. Consider the highdensity polyethylene samples whose distribution of molecular weights are very different. 12 . Mn.10 Molecular Weight Distribution Given a measure of the continuous molecular weight distribution. ways The number average. unique average. molecular weight averages are calculated by integration of the number of chains of each molecular weight N(M): Number Average: ∞ Mn = ∫ N(M) • M dM 0 ∞ ∫ N(M) dM 0 Weight Average: ∞ Mw = ∫ N(M) M 0 ∞ 2 dM ∫ N(M) 0 M dM Distributions CHEE 490 2. average. considers the number of molecules of each size. in the sample: Mn = ∑ niMi ∑ ni The weight average Mw. but defined through a number of different ways. Mi.Molecular Weight Distribution The distribution of molecular weights within a polymer is characterized not by a single. considers the mass of molecules of each size within the sample: ∑ w iMi Mw = ∑ wi Distributions CHEE 490 2. Distributions CHEE 490 2. The ratio of Mw to Mn. but includes boiling point elevation and freezing point depression Weight Average Molecular Weight Light scattering » Use the distribution of scattered light intensity created by a dissolved g y y polymer sample as an absolute measure of weight-average MW Viscosity Average Molecular Weight Viscometry » the viscosity of an infinitely dilute polymer solution relative to the solvent relates to molecular dimension and weight.Methods of Molecular Weight Determination Number Average Molecular Weight End-group analysis » determine the number of end-groups in a sample of known mass Polydispersity By virtue of its definition.14 Polydispersity Molecular Weight Influence on Physical Properties In addition to phase transition temperatures.13 Distributions CHEE 490 2. Mw cannot be less than Mn. MW is therefore expected to affect several amorphous phase properties.15 . higher tensile strength and better toughness in g polyethylene. which g is a measure of the energy imparted to the material that is stored elastically. viscosity. Distributions CHEE 490 2.16 Dependence of melt viscosity on shear rate for two polyethylenes of different molecular weight distribution. heat distortion and. i l di h ti including modulus. f fG C ) Distributions CHEE 490 2. molecular weight distribution alters the physical properties of the bulk state the properties of an amorphous phase when above Tg are dictated largely by molecular entanglement and weak chain association forces. G’ is the storage modulus. Molecular Weight Distribution Gel permeation chromatography » fractionation on the basis of chain aggregate dimension in solution (See slide 1 for an example of GPC output). defines the polydispersity of a molecular weight distribution. elasticity. Colligative Properties C lli ti P ti » most commonly osmotic pressure. It is influenced by the high molecular weight fraction of the material to a greater degree than Mn. Low polydispersities (PD=Mw/Mn ≈ 2) generate higher melt viscosity. as shown to the right. isotactic Polypropylene and Poly(ethylene-copropylene) their sequence distribution within polymer chains.19 Distributions CHEE 490 2. Incorporating different monomers in each polymer chain is often the only means of generating “mixture” behaviour. Shown h Sh here i a generic is i phase diagram for a system of “semi-crystalline” monomers. TEM of a suspension-prepared ABS.17 A wide range of Tm. Bridging across the phase boundary improves interfacial adhesion and physical properties Segmented Polyurethanes Schematic morphology of unstretched semicrystalline polyurethane copolymer (segmented block copolymer) copolymer). the results of blending can be disappointing.Composition Distribution Inclusion of two or more monomers in a material has a remarkable effect on processing and end-use properties. blending of polymers usually results i a di ti bl di f l ll lt in dispersion of i f one material in the other. A: hard nylon fibre B: bicomponent nylonp y spandex C: mechanical stretch nylon D: spandex E: extruded latex Distributions CHEE 490 2. Tg. “Tailored” polymers can be developed through consideration of: the character of incorporated monomers and. Compare: » Polyacrylonitrile. » random chain composition » alternating » block sequencing » graft structure Note that most polymers are immiscible. mechanical blending of different materials leads to phase separated mixtures. degree of crystallinity as well as both chemical and physical properties can be adjusted by varying the content of each monomer. Depending on interfacial adhesion and the properties of the continuous phase. Polybutadiene and Poly(acrylonitrile-co-butadiene) » Polyethylene. » polarity of main chain or pendant functionality » potential for crystallization Random Copolymers Materials comprised of a random distribution of different monomers are the most widely employed industrial copolymers. Distributions CHEE 490 5. Most block copolymer systems exhibit microphase-separated structures with the minor component dispersed in a matrix of the majority phase. While small molecules can be combined to generate single-phase mixtures with unique properties. Typically rubber domains in suspension-derived polymer contain substantial amounts of occluded copolymer of styrene and acrylonitrile acrylonitrile.20 .18 Block Copolymers Given that very few polymers are miscible. Distributions CHEE 490 2. 21 .and weight-average molecular weights number weight average weights. What is the polydispersity of this sample? Distributions CHEE 490 2. calculate the number.Test Your Knowledge A polymer is fractionated and is found to have the molecular weight distribution shown below. For this continuous distribution. The lowest-energy conformation of polymer chains depends on composition .4 . a highly organized g y g chain conformation is the most stable state for the polymer. Most products use polymers in their bulk (solid. For these applications. you should be able to identify amorphous and crystalline states. Tt.hydrogen bonding. condensed) state.2 Phase Transitions CHEE 490 Crystallinity in Nylon-6. To clarify key concepts we will handle a few different polymers: concepts. By the end of this lecture topic. Heat Capacity p y Hydrogen bonding between amide groups of Nylon-6.1 Phase Transitions CHEE 490 3.1-4. some polymers can be cooled from a melt condition can generate an imperfect crystal structure. Phase Transitions CHEE 490 3. Nonadjacent reentry Regular adjacent reentry Irregular adjacent reentry Crystallization/melting of polymer crystallites is a classical phase transition. result. Below the melting p g point of the material. cis and trans.3 Phase Transitions CHEE 490 3. The basic units of crystalline polymer morphology are crystalline lamellae.6 Identifying the Crystalline Melting Temperature A transition in which the first derivatives of the molar Gibbs energy are discontinuous is defined as a first order phase transition. consisting of folded chains. poly(methylmethacrylate) high density poly(ethylene) low d l density poly(ethylene-co-hexene) it l ( th l h ) poly(tetrafluoroethylene) poly(isoprene). relate these to mechanical p y properties and predict how each material will behave with respect to temperature changes. 3. van der Waals interactions.3 Crystalline State Under appropriate conditions. identical to that of small molecules. the physical properties detailed in lecture 2 are strongly dependent on phase morphology and as a result on and.6 gives rise to strong interchain associations with a preferred i i ih f d orientation. the unit cell of which is shown here. temperature. The result is a defined crystal structure.Phase Transitions in Polymer Systems Fried 4. first-order transition The chemical potential of the material changes abruptly at the transition point. Phase Transitions CHEE 490 3. Solid circles: crystallized at 130°C for 40 days.Identifying the Crystalline Melting Temperature Dilatometry studies involve confining the polymer by a wellwell characterized. A DSC trace is a plot of energy (ΔH=Hsample-Href) as a function of T. inert liquid and recording the change in volume as the temperature is varied. then cooled to 25°C prior to fusion.6 Identifying the Crystalline Melting Temperature A Differential Scanning Calorimeter (DSC) controls the energy input to a sample and reference so they remain at the same T throughout a programmed temperature rise. Phase Transitions CHEE 490 3. i d Linear polyethylene.7 . Li l th l Open circles: cooled rapidly from melt to 25°C b f before f i fusion. Factors Influencing Crystallinity Chain architecture and composition distribution determines whether a polymer exists in a semi-crystalline or completely amorphous state. Branching and molecular mass: packing efficiency deteriorates with i ith increasing b i branching and th relative number of f hi d the l ti b f free chain h i ends. Chain symmetry: symmetrical structures that permit close p packing of chains favour crystallinity. g y y atactic poly(propylene) versus isotactic (polypropylene) poly(tetrafluoroethylene)? 2. therefore: Tm = ΔHm ΔSm Polymers in which ΔHm is relatively large (strong intermolecular attraction) and ΔSm relatively small (minimal ordering from melt to crystalline state). isotactic(polypropylene) Phase Transitions CHEE 490 3. Tm. Phase Transitions CHEE 490 3.T ΔSm where ΔHm and ΔSm represent the enthalpy and entropy of fusion per repeat unit. At the equilibrium temperature. respectively. ΔGm= 0. the temperature of melting is high.9 Factors Influencing Tm The fundamental equation of thermodynamics for a closed system states: ΔGm = ΔHm . state 1. Intermolecular forces: hydrogen bonding and attractive van der Waals forces promote crystallization atactic-poly(vinyl atactic poly(vinyl alcohol) 3.11 . but the second derivatives are discontinuous is. Tg. Note that the thermal expansion coefficient changes at Tg. Cool natural rubber below -73 °C and it becomes a brittle. The transition from a glassy to a rubbery state in amorphous materials is called the glass transition temperature. Phase Transitions CHEE 490 3. Physical properties derived from an amorphous phase are strongly dependent on temperature.poly(methyl methacrylate) Natural rubber . Plexiglass . and a g g discontinuity is observed at the glass transition point. Tg is taken at the temperature at which half the increase in heat capacity has occurred. by definition. where chain orientation is not present on a large scale.Amorphous Bulk State An amorphous state is one of relative disorder. there is insufficient thermal energy to allow significant chain mobility or even chain segmental motion. Consider.cis poly(isoprene) cis-poly(isoprene) Both exist in an amorphous phase under conditions of common use. quenched. it becomes rubbery.13 Identifying the Glass Transition Temperature A transition in which the first derivatives of the molar Gibbs energy are continuous. The width of the transition is indicated by ΔT. side groups as well as atomic vibrations. Dynamic mechanical testing Specific volume determinations Differential Scanning Calorimetry Identifying the Glass Transition Temperature DSC trace of poly(ethylene terephthalate-co-p-oxbenzoate). Shown here is the specific volume vs. Below Tg.15 Phase Transitions CHEE 490 3. and reheated for measurement at I0°K/min. transition Molar Volume Phase Transitions CHEE 490 3. Phase Transitions CHEE 490 3. cooled at 0.16 . a second-order second order phase transition. but exhibit very different mechanical properties. If Plexiglass is heated above 105°C. T plot for l l tf poly(vinyl acetate). reheated.5°K/min through the glass transition. rigid material. Only cooperative motion of a few atoms of the main chain or side-groups is present.14 Identifying the Glass Transition Temperature Transition from a glass amorphous state to a rubbery amorphous state can be detected by a number of methods. Factors Influencing Tg Polymers whose structures are flexible. Chain length .rotational freedom along the chain as influenced by side chains. 1 Free volume . p y y Illustrated here is the specific effect of molecular weight on amorphous polymers (right) and semi crystalline semi-crystalline materials (below). do not provide for strong intermolecular attraction. Tm We discussed factors that influence phase transition temperatures in polymer systems in Lecture 3. g g Phase Transitions CHEE 490 3. Distributions CHEE 490 2.20 . Tg’s Four factors are generally accepted to affect Tg: 1.volume of the material that is not occupied by polymer molecules 2.18 Molecular Weight Influence on Tg. Internal chain mobility . Attractive f 2 Att ti forces . di l association dipole i ti 3.shorter chains have greater relative free volume. and do not “pack” well are those with relatively Tg s.h d hydrogen b di bonding. 4. 2 °C/min. What advantage(s) do you think it might have? Phase Transitions CHEE 490 3. Sketch how the observed Tg may vary with heating rate.21 . Most plastic soda bottles are made from poly(ethylene terephthalate) (PET).Test Your Knowledge A series of DSC runs is made on an amorphous polymer starting at room temperature but using different heating rates (1 °C/min. One manufacturer is pushing a terpolymer in which some of the ethylene glycol is replaced by a cyclohexanedimethanol for that application. 5 °C/min). . At 100oC and 1 atm pressure.4 . Phase Transitions II CHEE 490 4. one can readily determine the degree of crystallinity (φ) in a sample by DSC: ρ − ρa φ= ρc − ρa φ= ΔH f . water exists as At -5oC and 1 atm pressure.2 Extent of Crystallinity: Density Measurements Measurements of specific volume (cm3/g) or density (g/cm3) can reflect the degree of crystallinity of a material.Polymer Phase Transition Temperatures Meta-stable. Phase Transitions II CHEE 490 4. . water exists as 5 pressure . if a knowledge of the densities of the amorphous and crystalline phases are known: Extent of Crystallinity: Calorimetry (DSC) Given independent knowledge of the heat of crystallization for a given polymer.sample (J/g) is recorded from the melting endotherm and ΔHf (J/g) sample is the heat of crystallization for a perfect crystal of the material. where ρ is the density of the sample (as determined by a gradient column or dilatometry). .1 Phase Transitions II CHEE 490 4. ρa is the density of the amorphous phase. water exists as At 80oC and 1 atm pressure. and ρc represents the density of the crystalline phase.sample ΔH f . d t th d it f th t lli h where ΔHff. high density polyethylene (Tm=130oC. 130 Tg=-100oC) exists as .3 Phase Transitions II CHEE 490 4. Multi-phase Systems At 150oC and 1 atm pressure. Tm=265°C) as a function of temperature.8 . Crystallization occurs below Tm Tm.7 Isothermal Crystallization Poly(ether-ether-ketone) Tm = 334oC. Product Morphology 2 P d tM h l Crystallization at low temperature nucleates a great number of spherulites which grow slowly High temperature crystallization results in rapid growth of relatively few spherulites 3.Models of Semi-crystalline Polymer Structure Many materials crystallize from the melt into organized structures called spherulites. but segmental mobility of chains is required. Phase Transitions II CHEE 490 4. The extent of crystallization depends on the rate of crystallization for the material and the time during which a melt temperature is maintained. shown below for poly(ethylene oxide): Crystals grow out radially to create aggregates that can reach a few millimetres in diameter. Tg = 143oC O O O C 308°C 312°C 315°C 164°C 160°C Phase Transitions II CHEE 490 4.5 Shown is the linear growth rate of p y( y poly(ethylene terephthalate) p ) (Tg=69°C. Crystallization Kinetics Even the most easily crystallized polymers contain amorphous defect regions. Ultimate degree of crystallinity The maximum degree of crystallinity depends on the impingement of spherulites as well as polymer chain mobility Phase Transitions II CHEE 490 4.6 Influence of Crystallization Temperature The temperature at which a material undergoes crystallization (during injection molding. Rate of crystallization The growth of the crystalline phase requires a driving force for driving-force crystal nucleation as well as chain mobility 2. the crystallization rate is zero (metastable condition) Note that semi-crystalline polymers are comprised of crystalline lamellae y and amorphous regions that “bind” crystallites together. for example) influences the product and process in several ways: 1. Below Tg. Phase Transitions II CHEE 490 4. the compound is a crystalline solid. What differences in mechanical properties might you expect? Glassy Leathery Polystyrene Rubbery Viscous Phase Transitions II CHEE 490 Stress applied at x and removed at y 4. Avrami and other have used this conceptual model to develop an empirical equation for crystallization kinetics: Tg/Tm Demo: Glucosepentaacetate Glucosepentaacetate is not a polymer. Given that crystallinity is seldom complete. and the points of impact are the crystallite nuclei. The drops may fall sporadically or all at once. Heating to 110°C melts the solid to generate an amorphous phase of liquid like viscosity. 4. The expanding circles of waves. brittle solid. but they must strike the puddle surface at random points.11 Phase Transitions II CHEE 490 4. At room temperature. At room temperature. the Avrami equation is commonly modified by the ultimate degree of crystallinity.9 Phase Transitions II CHEE 490 X = 1 − e − kt n .10 Modulus vs. X∞: − t X = 1 − e X∞ X∞ Phase Transitions II CHEE 490 O O O CH3 CH3 k n Why does it crumble where semicrystalline polymers do not? CH3 . These produce expanding circles of waves which intersect and cover the whole surface. poly(ethylene) is above Tg but below temperature Tm. are the growth fronts of the spherulites. liquid-like Rapid chilling in ice water creates a glassy. of course.Simple Crystallization Kinetics: Avrami Equation Crystallization kinetics have been modeled using a framework analogous to raindrops falling in a puddle. O CH3 O O O 4. while nylon-6.12 . Warming results in a glass to leather transformation O Continued working of the sample O promotes further crystallization CH3 O O until a solid powder is observed. but it does exhibit glass transition and crystallization phenomena in a manner that is consistent with polymeric systems systems.6 is below both Tg and Tm. where k is a rate constant (sec-1) and n is a dimensionless parameter that ( relates to the type of phase nucleation. Melting point = 110°C Tg = not well defined but approx 5°C.Temperature: Amorphous PS Modulus vs. Temperature: Semi-Crystalline Polymers Crystallinity and crosslinking of polymer chains influence the modulus of a polymer as shown below. and the sample with Mn=143.MW Requirements of Industrial Polymers Elastomers amorphous materials operating above Tg physical props derived from chain entanglement crosslinking entanglement. ethylene glycol (-O-CH2-CH2-O. Which would you choose for a polyester sample to generate: transparency? the highest possible Tg? This is a plot of the crystal growth rate for poly(tetramethylene p-phenylene) t f l (t t th l h l ) siloxanes of different Mn as a function of crystallization temperature.repeat units) and bisphenol-A (-O-PhC(Me)2-Ph-O. The melting temperature of this polymer g . with the Mn=8. varies with molecular weight. and why a maximum crystallization rate is observed at approximately 70°C i t l 70°C.repeat units) are commercially available at low cost. Explain why the rate of crystallization is effectively zero at -10°C and at 130°C.14 .13 Test your knowledge Two diols. Phase Transitions II CHEE 490 4.700 material having a Tm = 140°C. Use your knowledge of the factors that affect melting temperature to f t th t ff t lti t t t describe the origin of this difference in Tm. Adhesives range from elastomeric (p g (pressure sensitive) to semi) crystalline (hot melt) to glassy (epoxy resins) Plastics broad class of materials whose properties are derived from an amorphous phase and often from a crystalline phase Fibres highly crystalline materials p y physical p p properties derived from degree of crystallinity g y y Coatings must be applied as a low viscosity medium and “cure” to y produce satisfactory properties Distributions CHEE 490 4.000 having a Tm=155°C. poly(ethylene) and poly(propylene) do not mix even at high temperature. Solvent vehicle various evaporates.8.4. A-B solution mA grams polymer A + ΔGmix > 0 ΔGmix (Joules/gram) is defined by: ΔGmix = ΔHmix -T ΔSmix where ΔHmix = HAB .4 .(wAHA + wBHB) ΔSmix = SAB .1 Polymer Solubility CHEE 490 5. appliances Oil-based paints. wB are the weight fractions of each material. tough.2 Thermodynamics of Mixing Whether the mixing of two compounds generates a homogeneous solution or a blend depends on the Gibbs energy change of mixing. While ΔSmix is always negative (p y g (promoting solubility). hard coatings Water It is therefore interesting that polymeric analogues of these compounds. b t when combined produce a di hi h t t but h bi d d dispersion of one i f material in the other. Polymer Solubility CHEE 490 5. n Polyurethanes. systems lowers pumping costs Varnishes.(wASA + wBSB) immiscible blend mB grams material B ΔGmix < 0 Entropy of Mixing Consider the two-dimensional lattice representation of a solvent (open circles) and its solute (solid circles): small molecule solute polymeric solute and wA. shellac and adhesives Lower polymer Tg. Polymer Solubility CHEE 490 5. its g y) magnitude is less for polymeric systems than for solutions of small molecules When dealing with polymer solubility. we (not surprisingly) generate a homogeneous solution: Industrial Relevance of Polymer Solubility Polymer Diblock copolymers Poly(ethylene oxide) Solvent Motor Oil Effect Colloidal suspensions dissolve at high T T. evaporates leaving film for glues Dibutyl phthalate Poly(phenylene oxide) Triglyceride oils Plasticizes polymer Mutual solution. the enthalpic contribution ΔHmix to the Gibbs energy of mixing is critical. raising viscosity Reduces turbulent flow Application Multiviscosity motor oil (10W40) Heat exchange systems.6.Polymer Solubility When two hydrocarbons such as dodecane and 2.10pentamethyldodecane are combined.3 Mixing of small molecules results in a greater number of possible molecular arrangements than the mixing of a polymeric solute with a solvent. cellulose esters Poly(vinyl chloride) Polystyrene Polystyrene Esters. making “vinyl” vinyl Impact resistant objects. alcohols. toughens polystyrene Phase separates upon oil polymerization n Polymer Solubility CHEE 490 5. If the enthalpy of mixing is greater than TΔSmix.2 = internal energy change of mixing per unit volume. Originally developed to guide solvent selection in the paint and coatings industry it is widely used in spite of its industry. d h ti t In general.2 ≈ ΔU1.( AHA + wBHB) = 0 (w for f an ideal mixture id l i t In general. Polymer Solubility CHEE 490 5. solubility parameters are frequently used as a rough estimator. δi = solubility parameter of component i: (cal/cm3)1/2 Note that this formula always predicts ΔHmix > 0. Polymer Solubility CHEE 490 5. then ΔHmix < 0 (solution state is lower in energy) An ideal solution is defined as one in which the interactions between all components are equivalent. ΔHmix can be estimated through: ΔH1. δ reflects the cohesive energy density of a material. then we know that the lower Gibbs energy condition is the unmixed state. For regular solutions in which intermolecular attractions are minimal minimal.2 = φ1 φ 2 (δ1 − δ 2 )2 cal / cm3 l where ΔU1. limitations. δi. the exceptional cases being those in which significant hydrogen bonding between components is possible. most polymer-solvent interactions produce ΔHmix > 0. which holds only for regular solutions. φi = volume fraction of component i in the proposed mixture. ΔHmix = HAB . Predicting solubility in p y g y polymer systems often amounts to y considering the magnitude of ΔHmix > 0.Enthalpy of Mixing ΔHmix can be a positive or negative quantity If A-A and B-B interactions are stronger than A-B interactions.5 ΔHmix and the Solubility Parameter The most popular predictor of polymer solubility is the solubility parameter. While a precise prediction of solubility requires an exact knowledge of the Gibbs energy of mixing.6 Solubility Parameter The aforementioned solubility parameter is defined as: δ = (ΔEv / ν)1/2 where ΔEv = molar change of internal energy on vapourization ν = molar volume of the material As defined.8 . Polymer Solubility CHEE 490 5. As a result. or the energy of vapourization per unit volume. a polymer will dissolve in a given solvent if the absolute value of the difference in δ between the materials is less than 1 (cal/cm3)1/2. then ΔHmix > 0 (unmixed state is lower in energy) i If A-B interactions are stronger than pure component interactions. 10 Solubility Parameters of Select Materials Hansen Solubility Parameter Contribution Model A more elaborate treatment of the solubility parameter recognizes contributions from fluctuating dipole moments. and observing the δ at which swelling is maximized.4 3 1/2 Polymer Solubility CHEE 490 5.9 Polymer Solubility CHEE 490 5.1 6.5 12.9 9. permanent dipole moments and hydrogen-bonding interactions moments.8 3 1/2 Material Poly(butadiene) Poly(ethylene) Poly(methylmethacrylate) Poly(tetrafluoroethylene) Poly(isobutylene) Poly(styrene) Cellulose triacetate Nylon 6. the total cohesion energy for a solvent is: Etotal = Edisp + Epolar + EH-bond With the overall solubility parameter defined as δ2 = ΔE / ν.6 10. The solubility parameter of a polymer is therefore determined b exposing d t i d by i it to different solvents. we get: or.9 23. or dispersion forces.85 9.6 13.4 7. Polymer Solubility CHEE 490 5. δ2 = Edisp / ν + Epolar / ν + EH-bond / ν δ2 = δdisp2 + δpolar2 + δH-bond2 Tabulated values of component p p parameters for a wide range of g solvents have been generated by fitting of internal energy of vapourization data.10 13. Thus.6 8.2 62 7.2 9.6 86 6. as solvent penetrates the material.45 6.Determining the Solubility Parameter The conditions of greatest polymer solubility exist when the solubility parameters of polymer and solvent match. Solubility Parameters of Select Materials Material Acetone Benzene Tetrahydrofuran Carbon tetrachloride n-Decane Dibutyl amine Mineral spirits Methanol Toluene Water Xylene δ (cal/cm ) 9.5 8.9 14.11 Polymer Solubility 5.12 .5 8. hydrogen bonding interactions.4 8. If the polymer is crosslinked it cannot dissolve but only swell crosslinked.6 Poly(vinyl chloride) Poly(acrylonitrile) δ (cal/cm ) 8.9 9. 6 7.1 30.0 5.1 6.3 19.9 14.5 23 5 15.0 16.6 7.6 17.8 19.8 20.1 2-Nitropropane p p Diethyl ketone 4.6 12.4 = 10.5 26.9 15.0 13.4 Dioxane 7.9 δ p = 0.1 14.8 14.4 17.6 11.8 16.5 4.4 6.1 5.6 19.0 0.0 15.3 6.5 7.0 16.2 7.1 4.8 0.3 21. 2 + φ3δ d .1 22.0 00 0.2 /MPa1/2 δp δh 12.6 4.6 18.7 9.0 17.7 Nitrobenzene Acetophenone 3.0 18.0 16.9 27.8 16.0 19.0 0.5 17.6 0.9 4.6 16 6 15.3 Dipropylene glycol Water 20.0 17.5 4.9 16 9 16.0 00 0.8 16.4 15.4 17.8 15.Diethyl benzene Halohydrocarbons Chloro methane Methylene chloride 18.6 20.6 25.1 16.0 11.8 17.4 42.3 23 3 20.2 7.2 9.9 20 9 19.1 8.7 10.8 2.7 20.7 23.1 16.0 18.4 16.7 4.3 14 3 Glycerol Propylene glycol Diethylene glycol 36.2 16.3 5.9 17 9 22.5 5..8 17.0 18.9 21.4 Isobutyl isobutyrate Ketones Nitrogen Compounds Acetone 20.7 57 7.0 15.6 26.1 Dichloroethane Trichloroethylene Carbon tetrachloride Chlorobenzene o-Dichlorobenzene 1.8 15.3 3.7 18.6 15.7 3.0 0.7 13.8 16. Liquids whose parameters lie within the p volume are active solvents.7 Ethanolamine Methyl isobutyl ketone 4. thermoplastic Poly(isobutylene) Poly(ethylmethacrylate) Poly(methyl methacrylate) Polystyrene Poly(vinyl acetate) Poly(vinyl butyral) Poly(vinyl chloride) Polymer Solubility Polar Component 29.0 19 0 17.1 δ h = 0.7 10.3 16.0 17.3 10.0 14.8 16.3 7.3 16.9 5.5 31.5 16.0 0.4 4.0 29.7 Triethylene glycol 22.4 16.8 15.0 17.0 0.8 -1.2 17.1 29.8 14.5 9.1 7.5 18.2 92 10.5 23.8 17.6 6.i δ h = ∑ φiδ h .1 3.4 5.8 15.5 22.5 19.6 2.3 δ d = 0.4 17.1 8.1 7.8 15.1 17.0 5.4 R 7.0 14.8 4.5 20.4 18.8 19.3 δ d = φ1δ d .0 ∂d ∂p ∂h 1.3 33 16.0 5.8 19.2 3.1 61 7. For a 50:50 vol:vol mixture of toluene and ethanol: 24.7 47.2 14. δh) and its radius of interaction (R).4 19.2 6.0 18.0 26.4 15.6 18.6 18.3 15.9 22.0 1.3 14 3 16.8 18.9 21.0 17.4 4.0 15.1 41 The Hansen volume of solubility for a polymer is located within a 3-D model by giving the coordinates of the center of a solubility sphere (δd.0 0.8 18.2 22.7 3.6 17.3 9.7 12.4 17.7 3.0 18.4 20.9 12.0 7.5 55 29.3 Hansen Solubility Parameter Contribution Model The real power of the Hansen system stems from the fact that a simple mixing rule can be applied according to the following equations to derive the solubility parameters of a solvent blend: 22.3 16.8 15.2 Tetrahydrofuran 19.0 4.13 Methylcyclohexane Aromatic Hydrocarbons Isobutyl acetate 2-Ethoxyethyl acetate Diethyl ether 5.9 3.1.1 5.3 15.9 17.0 0.6 12.2 20.5 18.1 2.3 16.6 15.0 20.0 3.3 6.8 14.4 9.1 18.0 0.0 0.4 12.5 20.5 ⋅ 8.6 8.4 17 4 15.0 19.3 5.6 18.0 22.4 + 0.3 20.7 8.3 14.0 2.9 2.0 5.3 3.6 10.2 6.1 51 12.1 Isoamyl acetate Dibenzyl ether 7.NDimethylformamide Sulfur Compounds p Carbon disulfide Dimethylsulphoxide Ethanethiol Alcohols Methanol Ethanol Allyl l h l All l alcohol 1-Propanol 2-Propanol 1-B utanol 2-Butanol Isobutanol Benzyl alcohol Cyclohexanol 21.3 + .2 12.1 21.6 4.0 0.0 16 0 10.0 11.3 19.9 18.4 2.8 20..5 ⋅18.2 9.7 5.Hansen Solubility Parameter Contribution Model Solvent ∂/MPa ½ ∂t Alkanes n-Butane n-Pentane n-Hexane n-Heptane n-Octane Isooctane I 14.0 0.8 16.6 8.1 Ethylene diem me Methyl isoamyl ketone 4.1 12.5 ⋅ 2.1 7.2 20.8 8.5 19.4 14.0 26.4 0.i where φi = volume fraction of the ith solvent in the mixture.3 14 3 16. δp.4 15.0 10.3 16.2 18.8 10 8 6.1 + φ3δ d .5 14.1 22.1 22.4 16 4 11.1 4.4 -0.2 112 Trichlorotrifluoroethane Ethers 18.1 3.6 13.9 16.6 20.6 24.0 1.5 16.5 9.8 16.6 19.8 19.7 14 7 17.9 17.0 00 Isophorone Di-(isobutyl) Di (isobutyl) ketone Esters Ethylene carbonate Methyl acetate Ethyl formate Propylene 1.0 0.1 9.2 carbonate Ethyl acetate Diethyl b Di h l carbonate Diethyl sulfate n-Butyl acetate 19.5 25.0 18.6 22 6 22.1 8.9 5.0 15.2 18.8 18.0 δ p = ∑ φiδ p .6 2.0 7.4 13.7 5.7 Polymer Solubility 5.2 13.0 0.1 14.0 16 0 8.5 ⋅1.2 30 2 29.5 5.0 + 0.7 4.5 25.4 74 3.6 21.8 16 8 16.3 16.3 3.2 6.7 14 7 6.5 15.1 1.0 6.7 1.1 12.6 20.5 ⋅15.0 15.7 6.0 1.0 6.6 18.6 3.7 11.0 17.8 23.5 8.2 10 7.9 8.6 20.6 8.7 9.4 1.8 12.6 0.9 Dispersion Component n-Dodecane Cyclohexane δd 18.2 6.3 11.8 22.5 12.6 18.3 Diacetone alcohol Ethylene glycol monoethyl ether Diethylene glycol monomethyl ether Diethylene glycol monoethyl ether h l h Ethylene glycol monobutyl ether Diethylene glycol y monobutyl ether 20.5 Benzoic acid Oleic acid Stearic id St i acid Phenols Phenol Resorcinol m-Cresol Methyl salicylate Polyhydric Alcohols Ethylene glycol 24.5 4.5 14.6 9.1 Nitroethane Cyclohexanone 5.8 15.0 16.0 10.9 18.9 6.6 22.8 5.5 15.0 5.7 7.5 17.3 18.2 16 2 16.1 21.4 19 4 18.5 23.0 1.4 16.0 0.2 1 -Decanol Acids Formic acid Acetic acid 20.6 16.0 17.4 16.5 ⋅19.7 8.6 10.7 13.7 37 7.1 41 Pyridine Benzene Toluene Napthalene Styrene o-Xylene Ethyl benzene y p.1 15.5 18.3 0.8 = 16.8 19.3 0.6 12.2 16.3 0.0 16.4 5.3 15.4 21.6 16 4.4 94 14.9 8.3 8.1 21.0 17. 20.2 16 2 8.1 41 5.8 10.0 19.7 0.0 0.7 18.0 .0 16.8 16 8 17.6 18.0 19.2 31.5 14.3 1.8 = 5.8 14.9 15.0 36.1 Dichloroethylene Ethylene dichloride Chloroform 1.8 16.0 Nitromethane Methyl ethyl ketone 5.6 7.0 16.0 3. Polymer Cellulose acetate Chlorinated polypropylene Isoprene elastomer Cellulose nitrate C ll l i Polyamide.8 16.7 25 7 24.6 17 6 14.2 3.7 18.4 16.0 2.16 32.1 Morpholine Analine N-Methyl-2-pyrrolidone Cyclohexylamine Quinoline Formamide N.8 17.0 0.8 6.3 17.8 19.6 15.1 0.5 14.4 15.0 19.7 21.0 + 0.7 10.0 14.2 14.1 31 14.2 7.7 27.4 5.3 18.3 4.8 7.3 15.5 14.0 5.0 7.3 23.0 0. ) If the distance (D(s-p)) is less than the radius of interaction for the polymer. polymer Calculate whether the distance of the solvent from the center of the polymer solubility sphere is less than the radius of interaction for the polymer: D(S-P) = [4(∂ds . Co po e t parameters o Component pa a ete s for a polymer of interest can be compared to potential solvents.∂hp)2]½ where D(S-P) = Di t Distance b t between solvent and center of polymer l t d t f l solubility sphere ∂xs=Hansen component parameter for solvent ∂xp=Hansen component parameter for polymer (The figure "4" in the first term doubles the dispersion component scale to create a nearly spherical volume of solubility. twodimensional plots of δH versus δp are commonly used for solvent selection purposes. Consider Einstein’s equation: η η( η=ηs(1+2.Hansen Solubility Parameter Contribution Model A polymer is probably soluble in a solvent (or solvent blend) if the Hansen parameters for the solvent lie within the solubility sphere for the polymer.∂pp)2 + (∂hs . 5.20 Polymer Solubility . The i Th viscosity of a polymer it f l solution is therefore dependent on solvent strength.19 Polymer Solubility CHEE 490 5. Dilute Solution Viscosity The “strength” of a solvent for a given polymer not only effects solubility. A polymer dissolved in a “poor” solvent tends to aggregate poor while a “good” solvent interacts with the polymer chain to create an expanded conformation. but the conformation of chains in solution. Increasing temperature has a similar effect to solvent strength.5φ) φ) where η is the viscosity ηs is the solvent viscosity and φ is the volume fraction of dispersed spheres. with the likelihood of solubility increasing with improved matching of individual component parameters parameters. Polymer Solubility 5. the-solvent would be expected to dissolve the polymer.∂dp)2 + (∂ps .17 Hansen Solubility Parameter Contribution Model Since the dispersive component parameters are similar for most solvents. Poly(vinyl chloride) is a rigid material (pipes house siding) (pipes. polymethyl methacrylate. In this method the material is dissolved in a minimum amount of a good solvent (solvent 1) to produce a homogeneous solution The solution. polystyrene and cellulose acetate. Concentrated Solutions . This is another method of determining the solubility g y parameter of a given polymer. but is transformed into a leathery material upon addition of a few percent of dioctylphthate.Determining the Solubility Parameter Shown below is the intrinsic viscosity of A: Poly(isobutylene) and B: Poly(styrene) as a function of solubility parameter. Plasticizers are small molecules that dissolve within a polymeric matrix to greatly alter the material’s viscosity. a common plasticizer. resulting solution is added dropwise to an excess of a second solvent (solvent 2). Solvent 2 is chosen for its inability to dissolve the polymer. siding). Should they be “good” solvents i a th Sh ld th b “ d” l t in thermodynamic sense or d i relatively “poor” solvents? On what basis would you choose a plasticizing agent? What process would you use to mix the agent with the polymer? Polymer Solubility CHEE 490 5. f ti i t d d resulting in a maximum viscosity. When δ for the solvent matches that of the polymer. the polymer precipitates from the solvent 1 / solvent 2 mixture. and its ability to maintains a miscible condition with polymer solvent 1. Therefore. the chain conformation is most expanded.21 Polymer Solubility CHEE 490 5.23 .Plasticizers Important commercial products are solutions where the polymer is the principal component. Polymer Solubility CHEE 490 5. Use U your k knowledge of solubility parameters t propose l d f l bilit t to solvent 1 / solvent 2 combinations to purify polyisobutylene.22 Test your knowledge Polymers can be purified by dissolution/precipitiation. We will start with an inflated balloon Diffusion in Polymer Systems CHEE 490 6. and semicrystalline polymers. gas purification Controlled drug delivery – transdermal patches for morphine. nicotine d li i ti delivery Home brewing of high quality bitter ales – carbon dioxide flux through plastic bottles We will discuss the definition and measurement of permeability. glassy. is the molar flux of permeant through the polymer (NA) normalized by the film thickness (L) and the difference between upstream (p2) and downstream (p1) partial pressures: P= N AL p 2 − p1 The units used in the permeability literature vary with the nature of the permeant (gas phase. and the differences in rubber.3 . liquid) and the source of data. P.1 Permeability The permeability coefficient of a permeant / polymer system. solvent removal from polymer cements Membrane separations – desalination.Permeability in Polymer Systems The transport of small molecules through polymeric materials is relevant to a wide range of engineering applications. the solution-diffusion model commonly used to describe the y phenomenon. Food packaging – oxygen and water transport Coatings and adhesives – drying of polymer films Polymer synthesis – gas-phase polymerizations. Diffusion in Polymer Systems CHEE 490 6. 5 Diffusion in Polymer Systems CHEE 490 6.6 . Sorb into the polymer from a high activity external gas or liquid phase. 3 3. in which penetrant molecules: 1. Why do oxygen permeabilities vary by seven orders of magnitude on going from PVA to PDMS? Why is the water permeability of PVA so hi h while it oxygen permeability high.Permeability Here is a table of permeability values for oxygen and water vapour through a range of polymeric materials. While these three general steps of the solution-diffusion mechanism are agreed upon. the th specifics of sorption i t ifi f ti into and out of the polymer phase and diffusion across it are still active areas of research. Diffuse across the polymer driven by a chemical potential gradient. and which may be useful for contact lens applications? Solution Diffusion Model of Permeability Small molecule transport through nonporous polymers is generally believed to proceed by the solution diffusion mechanism. Diffusion in Polymer Systems CHEE 490 6. hil its bilit is very low? Which material is best suited for food packaging. Desorb from the polymer to a low activity g or liquid external p p y y gas q phase. 2. Permeant Diffusivity The diffusion coefficient is a kinetic parameter that measures the overall mobility of the penetrant molecules through the polymer matrix.2 = 0 Which component will experience the higher concentration gradient? Can you tell which component will have the higher permeability? X=L CHEE 490 6. (2) the mobility of the polymer chains.1 pA.Solute Transport in Amorphous Rubbery Polymers Polymer molecules do not occupy the entire volume of a sample.1 = pB. Diffusion in Polymer Systems CHEE 490 6.2 = cB. Because of packing inefficiencies and molecular motion.11 .1 cA.9 Diffusion in Polymer Systems CHEE 490 6. and (3) the free-volume size and distribution of the polymer p y Diffusion coefficients for penetrants in natural rubber and rigid poly(vinyl chloride) at 30°C. the permeability (P) of a component across the membrane is a product of diffusivity (D) and solubility coefficient (S): Permeability = Diffusivity * Solubility Coefficient Consider the diffusion of an equimolar mixture of two gases through a rubbery membrane. The rate-limiting step for penetrant diffusion is the creation of transient “gaps” in the polymer matrix via random.1 cB 1 B. including (1) th size and shape of the i d h f molecules. one being much more soluble than the other.2 = 0 X=0 Diffusion in Polymer Systems pA. thermally-stimulated motion of polymer segments. some of the volume in the polymer matrix is empty or “f ” and this so-called f “free” d thi ll d free volume is redistributed continuously. cA.10 Permeant Solubility On the basis of the solution-diffusion model. The diffusivity depends on various factors.2 = pB. 6. l l in its i i it These higher concentrations can occur i cases where f in h favorable bl polymer–penetrant interactions promote solubility.14 Influence of Temperature The temperature dependence of permeability is usually modeled by an Arrhenius expression: Solute Transport in Amorphous Glassy Polymers In the glassy state. and EVOH 27 is ethylene vinyl alcohol copolymer containing 27 mol% ethylene. C =k p A Henry A in which case: S= CA = k Henry pA Simple equations describing the partitioning of liquid phase permeants are not available. as illustrated h i d ti ill t t d here. Permeabilities of various gases in silicone rubber at 35°C.13 Diffusion in Polymer Systems CHEE 490 6. AN is an acrylonitrile–styrene copolymer. This is common for organic vapors and even for molecules such as CO2. albeit to a smaller extent. Diffusion coefficients for penetrants in natural rubber and rigid poly(vinyl chloride) at 30°C. while rigid PVC has a Tg of 81°C C. The impact of polymer chain mobility on the diffusion coefficient is dramatic. C A. Diffusion in Polymer Systems CHEE 490 Effect of temperature on oxygen permeability at 75% RH.polymer S= C A. Note that the selectivity of permeation for different penetrants is greater for glassy polymers. The concentration of gases in rubbery polymers (above Tg) relative to their partial pressures in the contacting gas phase can be using Henry’s L H ’ Law.fluid Rubbery Polymers: Plasticization Effects Transport plasticization is defined as a significant increase in the diffusivity f diff i it of a penetrant b t t because polymer segmental motion is enhanced by another penetrant molecule i it vicinity . but Flory-Huggins treatments of this problem have been developed. Diffusion in Polymer Systems CHEE 490 6. y p y thereby increasing permeability Solubility is also affected.16 .Permeant Solubility The solubility or partition coefficient (S) is a thermodynamic parameter that relates penetrant concentration in the polymer to that in the contacting gas/liquid phase phase. Natural rubber has a Tg of -73°C. PVDC is vinylidene chloride–vinyl chloride copolymer. H2S and SO2. ⎛ E P = Po exp ⎜ − p ⎜ ⎝ RT ⎞ ⎟ ⎟ ⎠ As evident from the log plot provided t th right. significant id d to the i ht i ifi t increases in permeability are observed for amporphous polymers on either side of Tg.15 Diffusion in Polymer Systems CHEE 490 6. Heightened molecular mobility improves diffusivity. a p fact that is exploited in some membrane applications. PET is poly(ethylene terephthalate). molecular mobility is limited to severely hindered torsional motions motions. and a chain i h i immobilization f t β.Semi-Crystalline Polymers: Solubility Coefficient Crystalline domains are impenetrable by even tiny gas molecules. and inflate it to a radius of 10 cm. Diffusion in Polymer Systems CHEE 490 6. and develop an equation that can be used to calculate the film’s permeability coefficient.6 0.0E+00 0 10 Hours 20 30 6. Consider this typical test cell. Derive an equation for the balloon radius as a function of time. This twofold effect has been treated in terms of a tortuosity factor τ .0E‐02 5. increasing the path length for transport and restricting chain mobility.000 0.0E‐02 4.025 0.0E‐02 3.8 1.0E 02 9 0E‐02 7.000 15.18 Test Your Knowledge You are given a spherical balloon with a radius of 1 cm. D= Da τβ where Samorph is the solubility coefficient of the amorphous phase.0 Volume fraction of amorphous phase The parameters D and Da are the diffusion coefficients in the actual sample and in a totally amorphous sample.17 Diffusion in Polymer Systems CHEE 490 6.5E-4 m 0.000 0.000 35 000 30.20 .000 0 10 Hours 20 30 CHEE 490 0 Diffusion in Polymer Systems B Balloon Thick kness (m) 8. p y both of which reduce permeant diffusivity.0E‐02 6.000 20.075 Ballo oon Radius (m m) Ball loon Pressure e (Pa) Test Your Knowledge Isostatic methods of measuring permeability use a continuous flow on both sides of the polymer film to provide constant penetrant concentrations. respectively. and Φamorph is the p phase volume fraction.2 0.0E‐02 Diffusion in Polymer Systems CHEE 490 6. b th bili ti factor both of which increase with increasing crystalline fraction. 9.0 0.0E‐02 2. amorphous p 0. The proportional relationship between S and the volume fraction of the amorphous phase supports a simple scaling expression: S = Samorph Φ amorph Polyethylene at 25°C Solubility coefficie CA/pA ent Semi-Crystalline Polymers: Diffusion Coefficient Impermeable crystallites act as physical barriers to diffusion.100 35.4 0. resulting in a drop in solubility coefficient for semi-crystalline materials such as polyethylene polyethylene.0E‐02 1.19 25.050 0.0E‐02 0.000 10 000 5.000 10. Relevant data: Permeability = 2E-11 m3 m / 2 h P bilit 2E 11 /m hour P Pa Modulus of cured rubber = 350 Pa Unstretched balloon thickness = 2. Derive an equation for the steady-state flux of penetrant through the film.