Chapter 14 Polymers CHAPTER 7 POLYMERIC MATERIALS. Ancient Polymer History. Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark

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1 Chapter 14 Polymers What is a polymer? Polymers are organic materials made of very large molecules containing hundreds of thousands of unit molecules called mers linked in a chain-like structure (repeated pattern) CAPTER 7 many Poly mer repeat unit POLYMERIC MATERIALS repeat unit C C C C C C Polyethylene (PE) repeat unit C C C C C C Cl Cl Cl Polyvinyl chloride (PVC) repeat unit C C C C C C C 3 C 3 C 3 Polypropylene (PP) Adapted from Fig. 14.2, Callister 7e. 2 Ancient Polymer istory Originally natural polymers were used Wood Cotton Leather Oldest known uses Silk Rubber Wool Rubber balls used by Incas Noah used pitch (a natural polymer) for the ark Polymers are characterized by: Low density materials (replace metals such as steel, aluminium etc) Versatility in synthesis processing properties relationship Raw materials and processing are cost-effective Recycling is possible and practical 3

Bottles extrusion blow molding Applications of

(RUBBER) SYNTETIC TERMOPLASTICS PE, PVC, PP, PS

Polymers when compared to metals and ceramics

density Low thermal conductivity Electrical

disadvantage Ease of processing/ lower useful

lower strength Lightweight products/ low structural

poorly Good electrical insulation/ do not conduct

colors Waste can be burned/ may cause fume or fire

2 Bottles extrusion blow molding Applications of Polymers Classification of Polymers POLYMERS PP Pplycabonate roof NATURAL e.g; ; wood, cotton, leather, skin, hair ELASTOMER (RUBBER) SYNTETIC TERMOPLASTICS PE, PVC, PP, PS TERMOSETS Epoxy, phenolic resins Characteristics of Polymers when compared to metals and ceramics Characteristic Low melting point igh elongation Low density Low thermal conductivity Electrical resistance Easily colored Flammable Advantage / disadvantage Ease of processing/ lower useful temperature range Low brittleness/ high creep and lower strength Lightweight products/ low structural strength Good thermal insulation/ dissipates heat poorly Good electrical insulation/ do not conduct electricity Use without painting/ difficult to match colors Waste can be burned/ may cause fume or fire hazard Polymer Most polymers are hydrocarbons i.e. made up of and C Saturated hydrocarbons Each carbon bonded to four other atoms C C C n 2n+2 8

3 Unsaturated ydrocarbons Double & triple bonds relatively reactive can form new bonds Double bond ethylene or ethene - C n 2n C C ❿4-bonds, but only 3 atoms bound to C s Triple bond acetylene or ethyne - C n 2n-2 C C 9 10 Isomerism Isomerism two compounds with same chemical formula can have quite different structures Bulk or Commodity Polymers Ex: C 8 18 n-octane C C C C C C C C = 3C C 2 C 2 C 2 C 2 C 2 C 2 C 3 3C ( C2 ) 6 C3 2-methyl-4-ethyl pentane (isooctane) C3 3C C C 2 C C 3 C2 C

4 13 14 Chemical Composition of Polymers Polymers are classified into: 1. omopolymers Only 1 type of repeat unit Chemical Composition of Polymers 2. Copolymers At least 2 types of repeat unit

Copolymers two or more monomers polymerized together random A and B randomly vary in chain alternating A and B alternate in polymer chain block large blocks of A alternate with large blocks of B

5 Copolymers two or more monomers polymerized together random A and B randomly vary in chain alternating A and B alternate in polymer chain block large blocks of A alternate with large blocks of B graft chains of B grafted on to A backbone random alternating block Adapted from Fig. 14.9, Callister 7e. Thermoplastics: PVC, acrylic, polyethylene Rubbers Polymer Architecture polyethylene Thermostetting polymers A B graft 17 PVC Nylon POLYMER FORMATION The properties and processing of polymers depend on their structure ure and chemical composition Polypropylene Linear polymers Polycarbonate (PC) Complex polymers Polymers are formed by causing small units (monomers) to chemically bond together and form very long molecules (polymers) The process used to cause this bonding is called polymerisation Polymerisation reactions can be: 1. Chain- Growth Polymerisation (addition( polymerisation) No by-product formation 2. Step-Growth Polymerisation (condensation( polymerisation) Formation of a by-product

1. Chain - Growth Polymerisation Applies to monomers that have double bonds Proceeds by several sequential steps 1.

Propagation step: : linear growth of molecule as monomer units become attached to one another producing chain molecule 3.

Step Growth Polymerisation of 6,6 nylon Chain polymerisation of polyethylene Basic Properties 1.

That is, the polymer molecules (chains) are usually different molecular weights.

6 1. Chain - Growth Polymerisation Applies to monomers that have double bonds Proceeds by several sequential steps 1. Initiation step: : active initiator (peroxide) interact with monomer double bond 2. Propagation step: : linear growth of molecule as monomer units become attached to one another producing chain molecule 3. Termination step: : chain will eventually stop when the active end of two propagating chains react or link together to form a non-reactive molecule 2. Step Growth Polymerisation of 6,6 nylon Chain polymerisation of polyethylene Basic Properties 1. Molecular Weight: In all real polymer systems the nature of the polymerisation process results in chains with many different lengths. That is, the polymer molecules (chains) are usually different molecular weights. Molecular weight is defined as the average of the weight of each species or size of the molecule present. Molecular weight is most frequently characterised by: Weight average, M w Based on the weight fraction of each specie or size present in the t molecule Basic Properties Number average, M n Based on the sum of the number of fractions of the weight of each specie or size of the molecule present 2. Degree of Polymerisation (DP)

Polymers with high molecular weight are tougher and chemically resistant: this is because long chains are easily entangled (anchored).

Fig. 14.10, Callister 7e. The melting point (plastics) also increases with increasing M w Adapted from Fig. 14.12, Callister 7e.

26 Polymer Crystallinity Polymer Crystal Forms Polymers rarely 100% crystalline Too difficult to get all those chains aligned crystalline region % Crystallinity: % of material that is crystalline.

7 Polymers with high molecular weight are tougher and chemically resistant: this is because long chains are easily entangled (anchored). Polymers with low molecular weight are weak and more brittle Polymer Crystallinity Ex: polyethylene unit cell Crystals must contain the polymer chains in some way Chain folded structure Adapted from Fig , Callister 7e. The melting point (plastics) also increases with increasing M w Adapted from Fig , Callister 7e. 10 nm Effect of molecular weight and degree of polymerization on the strength and viscosity of polymers. 26 Polymer Crystallinity Polymer Crystal Forms Polymers rarely 100% crystalline Too difficult to get all those chains aligned crystalline region % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. amorphous region Single crystals only if slow careful growth Adapted from Fig , Callister 6e. (Fig is from.w. ayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.) 27 Adapted from Fig , Callister 7e. 28

Polymer Crystal Forms Spherulites fast growth forms lamellar (layered) structures Spherulites crossed polarizers A transmission photomicrograph showing the spherulite structure of ployethylene.

Nucleation site Spherulite surface Adapted from Fig. 14.13, Callister 7e. 29 Adapted from Fig. 14.14, Callister 7e.

Polymers with high T g (above service temperature) are strong, stiff and brittle Polymers with low T g (below service temperature) are weak, less rigid and ductile T g is due to a reduction in motion

8 Polymer Crystal Forms Spherulites fast growth forms lamellar (layered) structures Spherulites crossed polarizers A transmission photomicrograph showing the spherulite structure of ployethylene. Maltese cross Linear boundaries form between adjacent spherulites, and within each spherulite appears a Maltese cross (Courtesy, FP Price, General Electric company). Nucleation site Spherulite surface Adapted from Fig , Callister 7e. 29 Adapted from Fig , Callister 7e. 30 (Amorphous) semi-crystalline Mechanical behavior of polymers (amorphous or semi-crystalline) is strongly dependent on the glass transition temperature,t g (state where, liquid transform to glass) Polymers with high T g (above service temperature) are strong, stiff and brittle Polymers with low T g (below service temperature) are weak, less rigid and ductile T g is due to a reduction in motion of large segments of molecular chains with decreasing temperature At T g the polymer changes from rubbery to rigid state Factors Affecting T g 1. Melting temperature of polymer (T m ): as T m increases T g increases 2. Chain stiffness or flexibility: as stiffness increases (or flexibility decreases), T g increases 3. Molecular weight: T g increases with increasing molecular weight 4. Degree of branching or cross- linking (restrict chain movement) increases T g

9 POLYMER PROPERTIES Both time and temperature affect the mechanical properties of polymers: they are visco-elastic materials (have both viscous and elastic behaviour) Polymer properties also depend whether the material is: amorphous, semi-crystalline or rubber 3 different types of stress strain behaviour: 1. Curve A: brittle polymer; fracture while deforming elastically 2. Curve B: plastic polymer: similar to metallic materials behaviour (e.g( e.g; ; nylon) 3. Curve C: totally elastic; typical for elastomer (rubber) (e.g( e.g; ; polyethylene) Mechanical Properties i.e. stress-strain behavior of polymers brittle polymer σ FS of polymer ca. 10% that of metals elastic modulus less than metal plastic Strains deformations > 1000% possible (for metals, maximum strain ca. 10% or less) elastomer Adapted from Fig. 15.1, Callister 7e. 35 Near Failure aligned, crosslinked case Tensile Response: Brittle & Plastic Initial networked case σ (MPa) x brittle failure onset of necking semicrystalline case x unload/reload amorphous regions elongate plastic failure ε crystalline regions align crystalline regions slide fibrillar structure near failure Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice- all, Inc., 1974, pp ) 36

10 Tensile Response: Elastomer Case σ(mpa) x initial: amorphous chains are kinked, cross-linked. brittle failure plastic failure x elastomer Deformation is reversible! Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) ε x final: chains are straight, still cross-linked Effect of Temperature on Stress-Strain Strain Behaviour 37 Viscoelastic Deformation of Polymers Viscoelastic stress relaxation An amorphous polymer may behave like a: 1. Glass at low temperatures Elastic deformation (conform to ooke s law) 2. Rubbery solid at intermediate temperatures (above T g ) Exhibit the combined characteristics of the low and high temperatures This condition is called Viscoelasticity 3. Viscous liquid at higher temperatures Viscous or liquid-like like behaviour

CHAPTER 14: POLYMER STRUCTURES

CHAPTER 14: POLYMER STRUCTURES CAPTER 14: POLYMER STRUCTURES ISSUES TO ADDRESS... What are the basic microstructural features? ow are polymer properties effected by molecular weight? ow do polymeric crystals accommodate the polymer