Polymer Materials

Lecture 17: Polymer Materials

Objectives

• Introduce the different types of polymer materials.
• Understand the properties of polymers and their uses.
• Consider the bonding between polymer molecules and its influence on material properties.
• Consider the influence of temperature on polymer properties.

Polymer Materials

• Polymer materials usually consist of large molecules. These molecules often form a chain of carbon atoms, with many thousands of atoms covalently bonded together.
• Some polymers use oxygen (O), nitrogen (N), or silicon (Si) atoms in the chain as well.
• The chains can rotate around the axis of the C-C bond, enabling them to twist into a variety of positions.
• Example: Polyethylene
  • Valency of C = 4
  • Bonds around C arranged in a tetrahedral shape.
  • C-C
  • H-H
  • H-H
  • H-H

Polymer Side Groups

• Polymer molecules can have one or more side groups that change the structure of the chain and its properties.
• The chain can also have elements other than C in the backbone.
• Example:
  • C-C
  • H-R
  • H-R
  • H-R
  • H-R
  • e.g. R = H, Cl, CH3, C6H5 etc.

Mer Units

• The mer unit is the basic repeat unit that the chain is formed from.
• Example: Polypropylene
  • - C - C -
  • H H
  • H CH3
  • Polypropylene Polymer n Mer repeat unit
  • C C H
  • H H
  • CH3 C C H
  • H H
  • CH3 C C H
  • H H
  • CH3 C C H
  • H H
  • CH3

Addition Polymerisation

• Small molecules, known as monomers, react together to form the polymer structure.
• Addition polymerisation is one common method of reaction.
  • Used to form polyethylene (polythene).
  • C C H H H H - - C - C - H H H H - - C - C - C - C - C - C - C - H H H H H H H H H H H H H H
  • Ethene Monomer
  • Polythene Polymer
  • Reaction Vessel containing catalyst

Condensation Polymerisation

• Condensation polymerisation usually combines two different monomers into the chain.
• Produces a small molecule by-product, e.g., water or acetic acid.
• Many thermoset polymers are formed by condensation polymerisation, but some thermoplastics, e.g., Nylon 6,6, are also produced using this process.
• Hexamethylene diamine
• Adipic acid
• Nylon 6,6
• N C H H C C H H C C H H C N H H H H H H H H H H C C H O C C H H C C H H H H H O OH HO
• N C H H C C H H C C H H C N H H H H H H H H C C H O C C H H C C H H H H H O n

Polymer Material Types

• The polymer chains are linked to other polymer chains in the structure; the type of bonds linking them determines the type of polymer and its material properties.
• Main types of polymer:
  • Thermoplastics: Only secondary bonds between the polymer chains.
  • Thermosets: Many covalent bonds (cross-links) between the polymer chains.
  • Elastomers: Rubbery materials; can be thermoset or thermoplastic versions.

Secondary Bonding Reminder

*Van der Waals forces

*   Very weak electrostatic interactions between atoms and molecules.

• Hydrogen Bonding
  • Weak forces, but stronger than other van der Waals forces.
  • H H O H H O H H O H H O H
  • \delta+ \delta+ \delta− + _ _ _ + _ _ _ _ _ _ _ _
  • Atom 1 Atom 2
  • \delta+ \delta- \delta+ \delta- \delta+ \delta- +_ _ _ _ _ _ _ _ _ _
  • Permanent dipole attraction
  • Induced dipole attraction

Thermoplastic Polymers

• Consist of many molecules that are linked to each other ONLY by secondary bonds and entanglement – there are no covalent bonds between the chains.
• The absence of primary bonds between the chains means that they can slide past each other when loaded at high temperature.
• The number and quality of the secondary bonds determines many of the thermoplastic polymer properties
  • Good alignment of chains leads to many secondary bonds – get stronger material with higher glass transition temperature.
  • The type of side groups (R) influences the strength of the van der Waals or hydrogen bonding forces. They also affect the ability of the molecules to twist and align – affects crystallinity and stiffness.
  • C C H
  • R H
  • R C C H
  • R H
  • R C C H
  • R H
  • R C C H
  • R H
  • R e.g. R = H, Cl, CH3 etc. Polymer with side groups (R)

Degree of Polymerisation

• The polymerisation process does not form molecules of exactly the same length (number of mer units).
  • The average chain length can be influenced by the process conditions.
• The degree of polymerisation, DP, represents the average number of mer units in the chain and affects the material properties.
• This is related to the molecular weight; a high DP means that the chains are long and therefore the molecular weight of the chain must be high.
  • Example: High molecular weight polyethylene (HMWPE) has DP around 18000.

Branched Molecules

• Polymer molecules may be linear. This can allow the chains to pack more effectively and increases the secondary bonding between them.
• Branched polymers: often during the polymerisation process, the chains may become branched. This can affect the ability of the chains to pack together.
  • High density material
  • Low density material

Copolymers

• Copolymers consist of chains with two different mer units that can be regularly arranged, randomly arranged, or formed into blocks (block copolymer).
• Chains of different mer units can also be attached – grafted on.
  • regular
  • random
  • block
  • graft

Thermoplastic Polymer Structures

• Thermoplastic polymer chains created by addition or condensation polymerisation processes.
• Heating can turn them into viscous liquids, enabling them to be formed and moulded, e.g., injection moulding.
• Material can be formed and reformed many times and is therefore recyclable.
• Depending on the molecules and process conditions, the thermoplastics can be amorphous or semi-crystalline.
  • Amorphous structure
  • Semi-crystalline structure

Semi-crystalline Thermoplastics

• Some thermoplastics are completely amorphous.
• In some polymers, it is possible for the long chains to fold and align so that regions with a regular crystalline structure are formed (surrounded by amorphous material).
• The driving force for this is the maximisation of the secondary bonding between the chains.
• Molecules form lamellae
  • Cooling rate determines the level of crystallinity.
  • Lamellar structure
  • Fast cooling – less crystallinity
  • Slow cooling – more crystallinity

Spherulites

• The lamellae start from a central nucleation site and grow into spherical arrangements called spherulites.
  • Lamellae structure
  • Single spherulite
  • Multiple spherulites

Effect of Chain Structure on Crystallinity

• The arrangement of the side groups on the chains can be regular or random, which influences the way in which the molecules can pack to form lamellae.
  • Atactic molecules do not normally form crystalline structures.
• Large side groups (e.g., -C6H5 on polystyrene) can also reduce chain mobility, making crystal formation difficult.
  • No side groups - polythene
  • isotactic
  • syndiotactic
  • atactic
  • Side group
  • C H C H C H C H C H C H C H H H H
  • C H C H C H C H H H H H H H H H
  • C H C H C H C H C H C H C H H H H
  • C H C H C H C H H H
  • C H C H C H C H C H C H C H H H HC H C H C H C H H H
  • C H C H C H C H C H C H C H H H H
  • C H C H C H C H H H

Semi-crystalline Thermoplastics

• The increased amount of secondary bonding between molecules enhances the material properties; the amount of crystallinity can be controlled by the solidification rate.
• Higher crystallinity leads to:
  • Higher melting point.
  • Increased solvent resistance.
  • Reduced permeation of gases.
  • Light scattering – get opaque materials if crystals are large.
  • Increased stiffness.

Volume Change with Temperature of Thermoplastics

• Tg and Tm
  • a) Perfect crystallisation, e.g., pure metal (volume change at melting point)
  • b) Amorphous polymer (gradual volume change with temperature)
  • c) Semi-crystalline polymer (volume change at melting point and glass transition temperature)

Temperature Effects on Thermoplastics

• In a material that solidifies into a perfect crystal structure, e.g., a metal, there is a clear change in volume at the melting temperature (following undercooling).
• The volume changes that take place during the solidification of a thermoplastic polymer depend on the degree of crystallinity.
• In an amorphous polymer there is no distinct solidification or crystallisation.
  • On cooling there is reformation of the secondary bonds which limits the motion of the molecules - the material turns from a viscous liquid to a rubbery and leathery material.
  • Well below T_g the molecules are unable to move as there is insufficient energy for the secondary bonds to be broken – the material becomes brittle and glassy.
• A semi-crystalline polymer shows intermediate behaviour.
  • At T_m there is a change in volume as some of the chains form regular lamellae.
  • With further cooling, the amorphous regions between the lamellae cease movement, until around T_g they become glassy.

Effect of Temperature on Thermoplastics

• Glass transition temperature, T_g, is the temperature around which polymers transform from a brittle glassy material into a leathery plastic material.
• As the temperature increases above T_g the elastic modulus decreases, as does the yield strength. However, the ductility increases.

Thermoplastic Modulus of Elasticity

• Below T_g there is little energy available to break secondary bonds.
• Around T_g the amorphous material transforms from a glassy material to a leathery material - secondary bonds are breaking and molecules can slide over each other under the action of an external stress.
• As T increases further, the amorphous material becomes rubbery, and eventually many of the secondary bonds break so that the material resembles a viscous liquid.
• Effect of crystallinity and temperature on the Young’s modulus (modulus of elasticity) of thermoplastic polymers.

Tensile Tests on Polyethylene, Polypropylene and Polystyrene

*Continues

Tensile Testing of Thermoplastics Above T_g (Leathery)

• For a thermoplastic above T_g in the leathery condition:
  • Initially, the polymer displays typical elastic properties.
  • At A, a neck forms in the test piece - molecules orient themselves in the direction of stress and the material strengthens locally. Further extension occurs by the propagation of the neck along the test piece B to C.
  • Yield strength is defined as here for a polymer.
  • Load at break is defined as the tensile strength for a polymer.

Polymer Fibres

• By drawing the polymer into a fibre, the alignment of the chains can be used to strengthen the material.
• The load is carried mostly by the strong covalent bonds as the secondary bonds have allowed the chains to slide and straighten out, often forming crystalline regions.

Viscosity and Viscoelastic Behaviour

• Viscosity is a measure of the resistance to flow.
• Defined as the ratio of the applied shear stress to the shear strain rate (typical units of Pa.s).
  • Water at 20oC: approx. 0.001 Pa.s
  • Engine oil: approx. 0.1 to 1 Pa.s
  • LDPE at 160oC: approx. 100 Pa.s
  • Soda-lime glass at 600oC: approx. 1 x 10^9 Pa.s
• Increasing temperature usually reduces viscosity.
• Viscoelastic materials exhibit elastic behaviour when loaded over a short time scale but will flow, i.e., appear viscous when loaded over a longer time scale.

Creep of Polymers

• For many polymers, creep can occur at room temperature or just above.
• In thermoplastic polymers the chains can slide over each other under the action of an external load.
• Material can stretch or deform over time if subjected to a constant stress.
• To maintain a constant strain, the amount of stress required in the polymer materials will reduce over time.
• The deflection temperature of a polymer is the temperature at which a given deformation of a beam occurs for a standard load and indicates the likelihood of creep.

Example Thermoplastic Polymers 1

• Polyethylene (PE)
  • Low cost, tough, flexible, corrosion-resistant
  • Used in applications such as bags, electrical insulation, bottles
  • Low-density polyethylene (LDPE) – many side branches – reduced packing of chains – only 50% crystalline
  • HDPE – mostly linear molecules – 90% crystalline
    • Harder and stronger than LDPE – lower diffusion rate of O_2
  • - C - C - H H H H n
• Polypropylene (PP)
  • Low cost, tough, flexible
  • Stronger than PE and better at high temperature
  • Used in applications such as packaging, pipes and fittings, chairs
  • - C - C - H H H CH3 n

Example Thermoplastic Polymers 2

• Polyvinylchloride (PVC)
  • Hard and brittle; used in ducting and pipework, window frames
  • With the addition of plasticisers, can be soft and rubbery; used in protective gloves and clothing, hoses, cable protection
  • - C - C - H H H Cl n
• Polytetrafluoroethylene (PTFE)
  • Very low surface energy (non-stick)
  • Used in applications such as gaskets, inert lab equipment, non-stick coatings, electrical insulation
  • - C - C - F F F F n
• Polystyrene (PS)
  • Glassy, transparent material
  • Usually used in an amorphous form
  • Used in applications such as moulded containers (e.g. CD cases), foamed to make packaging
  • - C - C - H H H C6H5 n

Example Thermoplastic Polymers 3

• Polyamide (includes Nylon)
  • Strong, hard, tough material with low coefficient of friction
  • Tends to absorb moisture which lowers strength and causes change in dimension
  • Used in applications such as clothing, gears, bearings, fibres (Kevlar)
• Polylactic acid (PLA)
  • A thermoplastic polyester often made by condensation polymerisation
  • Can be biodegradable and compostable under industrial conditions
  • Used in applications such as 3D printer filament, food packaging
  • PLA
  • Nylon 6,6
  • N C H H C C H H C C H H C N H H H H H H H H C C H O C C H H C C H H H H H O n
  • C CH3 C O H O n

Example Thermoplastic Polymers 4

• Polyethylene Terephthalate (PET)
  • A thermoplastic polyester
  • Used in applications such as plastic drinks bottles, fabrics, food packaging
• Acrylic
  • Includes Polymethylmethacrylate (PMMA) – hard, glassy transparent material used in windows, lenses
  • PMMA
  • PET
  • - C - C - H CH3 H C - O - CH3 n
  • O C C H H O O C O C H H n

Thermoset Polymers

• In thermoset materials the long polymer chains are linked to other chains by many strong covalent bonds
  • High degree of cross-linking (typically 10 to 50% of mer units are cross-linked)
• They are often formed by combining two components, a resin, and a hardener with heat, pressure, UV light, or mixing required to initiate the reaction.
• As the chains cannot rotate or slide past each other, they have high strength, stiffness, and hardness – little or no ductility.
• Once formed into a structure, the material cannot be reformed – cannot be recycled.
• Good chemical resistance and higher operating temperatures than thermoplastics
  • Many covalent cross-links

Example Thermoset Polymers 1

• Phenol-formaldehyde
  • Phenol and limited amount of formaldehyde react to form long chains
  • Material is thermoplastic
  • Material ground into powder and heated during moulding
  • Form many cross-links
  • Hard and rigid thermoset material
  • Good electrical resistance
  • Used in applications such as electric plugs, door knobs
  • Heated during moulding
  • Chains – thermoplastic
  • Crosslinked – solid form
  • Schematic diagram of the reaction for illustration

Example Thermoset Polymers 2

• Urea-formaldehyde
  • Used in applications such as adhesives, electric light fittings
• Melamine-formaldehyde
  • Harder and more heat resistant
  • Used in applications such as building panels, trays
• Thermoset polyesters
  • Cross-linking form of polyester
  • Combined with glass fibre to make small boats, car body panels
• Polyurethanes
  • Used in applications such as adhesives, paints and foamed for upholstery
• Epoxies
  • Used in applications such as with glass fibre for Printed Circuit Boards, as adhesives (e.g. Araldite)

Effect of Temperature on Thermoset Polymer

• As the thermoset molecules are linked together by strong covalent bonds, the material does not show the many variations in properties with temperature as thermoplastics do.
• Very little softening of the material occurs as the temperature is raised, until sufficient energy is provided to break the covalent cross-links, at which point the material begins to degrade.
• Thermosets often display a T_g, but this is usually at a relatively high temperature. The material will become more compliant but will not “flow” in the way a thermoplastic does.

Thermoset Elastomers

• Only a few cross-links joining the polymer chains together.
• Presence of these few covalent bonds means that when a stress is applied the chains can start to uncoil. This causes some ordering of the material (decrease in entropy).
  • Must be above T_g to be flexible
• The chains are prevented from sliding past each other and being permanently displaced by the few cross-links.
• On removal of the stress, the chains recoil / become disordered to increase the entropy of the system again.

Tensile Testing of Elastomers

*Engineering Stress

Silicones

• Long chains, but made up of Si – O rather than C.
• e.g.
  • - Si - O - CH3 CH3 n
  • Polydimethylsiloxane
• Generally made as thermoset materials and often form at room temperature.
• Very flexible even at low temperature and resistant to weathering and oils.
• Often used as sealants and greases.

Thermoplastic Elastomers

• These materials do not contain covalent cross-links but rely on glassy regions (that are below T_g) where the chains form lots of secondary bonds.
• The glassy regions act in a similar manner to cross-links by preventing the permanent displacement of the chains relative to each other.
• Typically formed by block copolymers e.g. styrene - butadiene - styrene (SBS)
  • The styrene groups form relatively strong secondary bonds with each other, so that at room temperature these regions are glassy, while the butadiene regions are still rubbery.

Polymer Additives 1

• Most commercial polymers are not simply pure materials; many additives are needed to alter the material properties, reduce cost, or meet safety requirements.
  • Can substantially change the mechanical properties.
• Fillers - added to improve tensile and compressive strength, abrasion resistance, toughness, and dimensional stability
  • Can be sand, glass, clay. E.g., carbon black in tyres to improve strength and wear resistance
  • Often low cost fillers are added to reduce the amount of expensive polymer required
• Plasticisers - small molecules added to the polymer
  • Obstruct secondary bonding between molecules
  • Reduce hardness and stiffness

Polymer Additives 2

• Stabilisers - prevent deterioration of the polymer in service due to exposure to light and atmosphere
  • Exposure can cause colour change, deterioration of mechanical properties, cracking, and surface crazing
  • UV light can break bonds
  • Stabiliser can absorb UV or stop further bond breakage
• Flame retardants - added to reduce the flammability of the organic materials.
  • Typically Cl, Br, or P species introduced to inhibit spread of fire
• Pigments – added to provide colour to the material
• Foaming / blowing agents – create foamed structures e.g. polystyrene packaging
• Anti-static agents - added to overcome the natural electrical insulation properties of polymers
  • Encourage thin film of moisture to cover surface

Comparison of T_g for different polymers

• Thermoplastics, Thermosets, Elastomers

Comparison of Young’s modulus for different polymers

Elastomers, Polymers, Low carbon steel

Conclusion / Key Points

• Polymers consist of long chain molecules.
• Polymer materials can be thermoplastic or thermoset, depending on the bonding between chains.
• Thermoplastics have only secondary bonds between chains; the number of bonds and their quality determine many material properties.
• Thermoplastics can be amorphous or semi-crystalline depending on their molecular structure.
• Mechanical properties of thermoplastics vary with temperature: below Tg glassy and brittle; above Tg leathery, rubbery, or viscous liquid.
• Thermosets cannot be reshaped due to extensive covalent cross-linking.
• Elastomers must be above T_g to be flexible, and there are thermoset and thermoplastic types.
• Additives are included in commercial polymers to modify their properties.