ME 330 Engineering Materials Lecture 20 Polymers Polymer Structure Mer Structure Molecular Structure and Shape Copolymers Please read Chapter 14

Learning Objectives (Ch. 14) After careful study of this chapter you should be able to do the following: Describe a typical polymer molecule in terms of its chain structure, and, in addition, how the molecule may be generated by repeating mer units. Draw mer structures for polyethylene, polyvinyl chloride, polytetrafluoroethylene, polypropylene, and polystyrene. Calculate number-average and weight-average molecular weights, and number-average and weight-average degrees of polymerization for a specified polymer. Name and briefly describe: four general types of polymer molecular structures; three types of stereoisomers; two kinds of geometrical isomers; four types of copolymers. Briefly describe the crystalline state in polymeric materials. Briefly describe/diagram the spherulitic structure for a semicrystalline polymer.

Polycarbonate (LEXAN) Natural vs. Synthetic Natural Synthetic Rubber Cellulose Starch Silk Spider Web Cotton Wool Leather Wood Proteins PVC PMMA (Plexiglas) HDPE LDPE ABS Nylon Polycarbonate (LEXAN) Epoxy Polyester Acrylic Interesting Website : http://www.pslc.ws/macrog/index.htm

Organic Materials Most natural and synthetic polymers are organic Carbon based compounds Made of most abundant elements in the periodic table Valence - how many additional bonds needed to fill outer shell 4 - C, Si 3 - N 2 - S, O 1 - H, F, Cl, Br, I Most polymers involve carbon backbone, focus on this ...

Polymer Materials Poly-mer [many - meros (Greek, parts)] – long molecule consisting of many smaller monomers chemically joined Built from a basic structure (monomer) joined in the same way Often chain-like Hydrocarbon molecule most common backbone Strong primary bonds in chain (Covalent) Weak secondary bonds between chains (Van der Waals) Polyethylene C H Ethylene: C2H4 C H

Hydrocarbon Review Covalent bond between carbon and hydrogen atoms Carbon has 4 electrons to share Hydrogen has 1 electron to share Saturated hydrocarbon - carbon bonded to 4 other atoms Unsaturated hydrocarbon - carbon not bonded to 4 other atoms Secondary bonds hold chains together Low melting temperatures that increase with molecular weight H C H C C H C H

From Hydrocarbons to Polymers Saturated  all bonds are single; molecule not reactive If unsaturated, double bond present, and catalyst (R) will react: Often indicate polymers by: Basic repeating structure n > 1000! C H R* + H H + C H C R C C* * +…  Polyethylene H H Ethylene mer mer Polymer C H n

A Few Definitions Homopolymer - all mers are of the same type Copolymers - two or more mer units of different structure Bifunctional - mer units with two active sites available for covalent bonding Trifunctional - mer units with three active sites available for covalent bonding Aromatic ring - H C

Molecular Weight Molecular weight indicates the size of a polymer molecule Measure polymer size distribution Number-average Weight-average Count the number of chains that are a certain length (molecular weight) Weigh the number of chains that are a certain length (molecular weight) Molecular weight (103 g/mol) 0.1 0.2 0.3 Weight fraction 10 20 30 40 w1 M1 Molecular weight (103 g/mol) 0.1 0.2 0.3 Number fraction 10 20 30 40 x1 M1

Degree of Polymerization Average number of mers per chain is the mer molecular weight Number average degree of polymerization Weight average degree of polymerization

Problem 14.4 (Callister) (a) Compute the repeat unit molecular weight of polystyrene. (b) Compute the number-average molecular weight for a polystyrene for which the degree of polymerization is 25,000.

Solution (a) The repeat unit molecular weight of polystyrene can be calculated as follows. For polystyrene, from Table 14.3, each repeat unit has eight carbons and eight hydrogens. Thus,   m = 8(AC) + 8(AH) = (8)(12.01 g/mol) + (8)(1.008 g/mol) = 104.14 g/mol

Solutions (cont) We are now asked to compute the number-average molecular weight. Since the degree of polymerization is 25,000, using Equation 14.6

Prob. 14.6 in Callister Molecular weight data for some polymer are tabulated here. Compute: Compute the number-average molecular weight. Compute the weight-average molecular weight. (c) If it is known that this material's degree of polymerization is 710, which one of the polymers listed in Table 14.3 is this polymer? Why?

Solution (a) From the tabulated data, we are asked to compute the number-average molecular weight. Molecular wt. Range Mean Mi xi xiMi 15,000-30,000 22,500 0.04 900 30,000-45,000 37,500 0.07 2625 45,000-60,000 52,500 0.16 8400 60,000-75,000 67,500 0.26 17,550 75,000-90,000 82,500 0.24 19,800 90,000-105,000 97,500 0.12 11,700 105,000-120,000 112,500 0.08 9000 120,000-135,000 127,500 0.03 3825 _________________________

(b) From the tabulated data, we are asked to compute the weight-average molecular weight. Molecular wt. Range Mean Mi wi wiMi 15,000-30,000 22,500 0.01 225 30,000-45,000 37,500 0.04 1500 45,000-60,000 52,500 0.11 5775 60,000-75,000 67,500 0.24 16,200 75,000-90,000 82,500 0.27 22,275 90,000-105,000 97,500 0.16 15,600 105,000-120,000 112,500 0.12 13,500 120,000-135,000 127,500 0.05 6375 _________________________

We are now asked if the degree of polymerization is 710, which of the polymers in Table 14.3 is this material? It is necessary to compute m in Equation 14.6 as Answer: Polysterene.

Molecular Structure Linear Tangled covalently bonded chains Secondary interchain bonds Branched Tangled covalently bonded chains with side branches Crosslinked Tangled covalently bonded chains Covalent and secondary interchain bonds Networked Highly crosslinked polymer

Polymer Classifications I Thermosetting polymers: Become permanently hard Will not soften before burning Heavily networked or crosslinked. Hard, strong, brittle Thermoplastic polymers: Soften when heated, harden when cooled. Will degrade if heat is sufficient to break primary bonds along chains. Reversible changes in secondary bonding between chains. Generally soft and ductile. Elastomers: Can be classified as either lightly crosslinked thermoset or thermoplastic Main property: returns to reference state when load is released. Can deform to VERY large strains.

Copolymers Random: Alternating: Block Graft copolymer: Mer #1 Mer #2

Polymer Classifications II Crystalline: Periodic 3-D, repeating array of molecules Amorphous: Literally “without structure” No repeating array Semi-crystalline: Structure containing regions of both crystalline and amorphous molecular arrangements

Polymer Crystallization Akin to grain formation in metals Involves entire molecules instead of individual atoms or ions Consider a polymer cooling from melt at three different rates Note inflection points at: Melting temperature (Tm) Glass transition temperature (Tg) Specific Volume Tg Tm Temperature

Polymer Crystallization Fast cooling ( ) Molecules do not have time to reorient into ordered grouping (diffusion) Change in volume TG  amorphous structure Slow cooling ( ) Molecules have time to reorient during solidification Change in volume at melting temperature - More ordered upon solidificaiton, Takes up less space No kink at TG  crystalline structure Moderate cooling ( ) Both crystalline and amorphous structure Kink at Tm & Tg due to crystalline and amorphous, respectively Degree of crystallinity:

Crystallographic Models

Grain Formation (Spherulites)

ME 330 Engineering Materials Lecture 21 Polymers Mechanical Properties Deformation and Strengthening Thermal Properties Viscoelasticity Polymerization Processing Techniques Read Chapter 15

Learning Objectives (Ch 15) Make schematic plots of the three characteristic stress–strain behaviors observed for polymeric materials. Describe/sketch the various stages in the plastic deformation of a semicrystalline (spherulitic) polymer. Discuss the influence of the following factors on polymer tensile modulus and/or strength: molecular weight degree of crystallinity predeformation heat treating of undeformed materials. List four characteristics or structural components of a polymer that affect both its melting and glass-transition temperatures. Cite the differences in behavior and molecular structure for thermoplastic and thermosetting polymers. Describe the molecular mechanism by which elastomeric polymers deform elastically. Briefly describe addition and condensation polymerization mechanisms. Name five types of polymer additives and, for each, indicate how it modifies the properties. Cite the seven different polymer application types and, for each, note its general characteristics. Name and briefly describe five fabrication techniques used for plastic polymers.

Mechanical Properties Mechanical properties are dictated by microstructure Microstructure depends on processing Microstructural features to engineer: Mer structure Molecular weight Degree of crosslinking/branching Degree of crystallinity 60 x Brittle Stress (MPa) 40 x Plastic x 20 Elastomer 2 4 6 8 Strain

Comparing Properties to Metals Much more temperature sensitive near room temperature Increasing temperature will Reduce E Reduce UTS Increase %EL May demonstrate ductile-to-brittle transition near 25 ºC Much more strain rate sensitivity Slower rate similar to increasing temperature Generally, polymers have: Lower elastic modulus Lower tensile strengths Higher ductility Lower melting temperatures Higher thermal expansion

Comparing Deformation to Metals (Stable Neck Formation)

Deforming Semicrystalline Polymers Elastic Behavior: Higher crystallinity increases elastic modulus; Elastic deformation via stretching and bending of primary (covalent) bonds Plastic Behavior: Higher crystallinity greatly increases strength; Deformation via complicated process Produces highly oriented structure Crystalline Amorphous

Stiffening Mechanisms Raise elastic modulus (E) Colder temperatures Higher strain rate More crosslinking More secondary bonding Higher crystallinity Aligned polymer chains

Strengthening Mechanisms Anything that hinders deformation Colder temperatures Higher strain rate More crosslinking More secondary bonding Higher crystallinity Longer chains (higher molecular weight)

Strengthening Mechanisms Drawing of polymers Analogous to cold work in metals, but more dramatic. Strength and modulus can have 3x increase in drawing direction Heat treatment On un-drawn materials, May lead to higher modulus, strength, and reduced ductility. On drawn materials, Will undo strengthening by undoing chain orientation and strain-induced crystallinity

Thermal Characteristics Melting point increases with: Chain stiffness Double bonds Aromatic groups Bulky side groups Polar side groups (more intermolecular bonding) Longer chains (reason why there is a range of melting temperatures) Increased linearity (fewer branches) Glass transition temperature increases with: Chain stiffness Double bonds Aromatic groups Bulky side groups Polar side groups Longer chains Increase crosslinking Generally, Tg= (0.5-0.8)*Tm Units of Kelvin

Glass Transition Temperatures Temperature at which polymers transitions from rubbery to rigid (glassy) behavior Rubbery above this temperature Occurs only in amorphous regions of polymers Tg -110 -100 -20 50 90 100 150 Tm 115 330 175 265 212 240 Temperature Specific Volume Tg Amorphous Crystalline Semi-Crystalline Polyethylene PTFE Polypropylene Nylon PVC Polystyrene Polycarbonate

Creep Properties  t  t  t  t Due to low melting temperatures, creep and stress relaxation are important at room temperatures.  t  t Relaxation modulus:  t  t Creep modulus: As silly putty shows, similar viscous-elastic transition takes place by changing time taken for loading As a viscous fluid, the behavior of a thermoplastic depends on both temperature and strain rate.

Viscoelasticity Polymer behavior depends on temperature At high temperatures & slow strain rates, flow like a viscous fluid At low temperatures & high strain rates, solid-like elastic deformation Intermediate temperatures … combination of both … viscoelastic! stress time Instantaneous elasticity followed by stress relaxation =E / strain time

Below Tg stress stress time strain strain Linear elastic behavior time

Above Tg Creep Stress relaxation stress time strain time strain time

Material Models liquids viscoelastic liquids viscoelastic solids Maxwell Kelvin - Voigt

Maxwell Model (Stress Relaxation) t time constant 1 0.8 0.6 0.4 stress time 0.2 1 2 3 4

Kelvin – Voigt Model (Creep) strain time 1 2 3 4

Thermoplastic Modulus 10-4 10-3 10-2 10-1 1 10 102 103 104 Relaxation Modulus, MPa 80 100 120 140 160 180 200 Temperature, °C Glassy Leathery Rubbery Viscous Flow Tg

Effect of Crystallinity 10-4 10-3 10-2 10-1 1 10 102 103 104 Relaxation Modulus, MPa 80 100 120 140 160 180 200 Temperature, °C Glassy Leathery Rubbery Viscous Flow Crystalline Elastomer Amorphous

Fracture, Fatigue & Impact Thermoset fracture: brittle. Thermoplastic fracture: brittle or ductile. Craze in a thermoplastic Fatigue: Polymers fatigue failure is similar to metals, but very rate sensitive Impact: Some polymers have brittle to ductile transition, and then decreasing impact strength as thermal softening occurs

Overview of Processing Mixing (thermoset) or melting/softening (thermoplastic) Form polymer from individual mer units Disperse additives in polymer Achieve uniform shapable state Shaping Extrusion Continuous, constant cross section Generally not for thermosets Molding processes Batch process 3-D products molded and stabilized Near net shape - often do not need secondary operations Stabilizing Solidifying such that it retains shape

Addition Polymerization “Chain-reaction” or “free radical” polymerization Three stages Initiation Propagation Termination Extremely exothermic reaction C H R°  C ° R + C C +…= polyethylene mer polymer

Condensation Polymerization Joining of two small molecules to give big molecule and biproduct Often release water during condensation Usually much shorter chains (lower MW) than addition CH2 C R OH R’ O + H20 + = ethylene glycol adipic acid polyester water

Polymer Additives Usually added in melt stage Fillers: Increase strength, abrasion resistance, toughness, etc. Particles of wood, glass, clay, etc. Plasticizers: low molecular weight liquids. Molecules sit between polymer molecules and “screen out” the secondary bonding, reducing glass transition temperature. Softens material and increases room temperature ductility. Stabilizers: Counteract degradation due to UV light (which breaks primary bonds) Retarders: slows reaction Allows more uniform chain length and more uniform properties Slows heat build-up during addition polymerization Colorants Flame Retardants

Elastomers An elastomer is a lightly cross-linked polymer Goodyear achieved this in poly-isoprene by adding sulfur to cross-link … vulcanization Crosslinking forces chains to return to original position even after large deformation Stronger, stiffer, similar ductility S S S CH2 CH CH2 CH2 S CH2 CH2 CH2 CH2 S S S + S S S CH2 CH2 CH2 CH2 CH2 CH CH2 CH S S

Shaping Techniques Many, many possible techniques Must consider Geometry of finished product Stability of material being formed Thermoplastic vs. Thermosetting Thermoplastics require processing at temperature above softening Thermosets require curing using a chemical additive or catalyst Often inject two different liquids Hardener Catalyst

Compression Molding Polymer mix or charge placed in heated mold Mold closes and applies pressure Plastic becomes viscous Conforms to mold shape Thermoset and thermoplastics

Extrusion Injection molding through open ended die As molten liquid passes through cold die Takes form of die Cools very quickly Incredibly fast process Constant cross section Sheets Rods Pipe/Tube Gutters

Injection Molding Pellets melted Viscous melt injected into mold Usually in screw chamber that also pushes melt into mold Sometimes heating chamber and hydraulic ram Viscous melt injected into mold Quick solidification in mold Thermoplastics ~30 sec Thermoset longer for cure Widely used for thermoplastics

Specialty methods Blow molding Casting Fiber drawing or spinning Force extruded tube walls to conform to mold with air injection Often used to make bottles Casting Similar to metal casting Pour liquid polymer into mold and allow to freeze Fiber drawing or spinning

New Concepts Deformation and strengthening mechanisms of polymers Viscoelasticity Temperature effects vs. rate effects Creep and relaxation modulus