Exam Prep - Definitions Flashcards
Describe the definition of a polymer at molecular level and point out the most distinct feature of a polymer in contrast to metals and ceramics from a molecular perspective.
- Many units of monomers covalently bonded together to form a long chain.
- Distinct feature is its long chainlike molecules, which is different than the basic metallic and ionic atoms in metals and ceramics respectively.
Describe the difference between thermoplastic and thermosetting polymers in terms of behaviour, solid material structure, and reversibility of their formation.
- Thermoplastic polymers can turn into viscous liquid by heating and can then restore to a solid after cooling. This process is reversible and used repeatedly to shape it.
- Thermosetting polymers start as a liquid of monomers and can turn into hard solid by heating. This process is irreversible.
- Thermoplastic polymers are typically made up of linear or branched molecules without crosslinking.
- Thermosetting polymers contain cross-linked molecules.
Which type of polymers will have a chance to crystallise; thermoplastics or thermosets?
- Thermoplastic polymers may partially crystallise if their molecules have ordered molecular structure and regular spatial arrangement.
- Thermosetting polymers cannot crystallise due to cross-linked structure.
If you were tasked to improve the toughness/strength of a polymer by changing weighting of the fractions, what would you suggest and why?
Increase the fraction of high molar mass (M) as it gives better mechanical properties (e.g. strength and toughness) due to increased chain interaction and entanglement
and/or decrease the fraction of low molar mass
If you were tasked to improve the stiffness of the polymer, what would you suggest and why?
- Increase average molar mass (increased chain interaction and entanglement).
- Choose a polymer that can crystallise (isotatic or syndiotatic polystyrene).
- Incorporate reinforcing materials such as glass/carbon fibres as fibres are stiffer than the matrix.
Describe how the Young’s modulus changes in relation to their crystallinity and why?
The modulus will increase with crystallinity, there are stronger interchains, which make them more resistant to deformation.
Define glass transition temperature of a polymer and describe what type of molecular motion is enabled around glass transition temperature (Tg).
• Glass transition temperature is a characteristic temperature at which polymers behaviour changes between rigid glassy solid and rubbery material. The spatial arrangement of atoms can change by rotation around the chain. This molecular motion is not possible when the temperature is below Tg due to insufficient internal energy in the material. (amorphous molecules only).
Explain why glass transition temperature is important to polymer-based applications from stiffness perspective and what implications this has for structural design using polymers.
- Stiffness, and other physical properties, of polymers typically undergoes a significant change around Tg and may be viewed as upper limit temperature for service.
- For loading bearing design, significant stiffness drop can result in dimension instability and excessive deformation due to creep.
Explain what material model you may consider to describe stressstrain behaviour when
(a) T < Tg, (b) T = Tg, (c) T > Tg, and (d) T»_space; Tg
(a) Glassy, Linear elastic model / Hooke’s law
(b) Leathery, Viscoelastic model (e.g. Kelvin Voigt
model)
(c) Rubbery Flow, Viscoelastic model (e.g. Maxwell model)
(d) Viscous flow (e.g. Newton’s law)
State at least 3 reasons for why typical fibre reinforcement (e.g. glass, carbon) has extremely small diameters
- A lower probability of a large flaw being present (size effect) maximises the fibre strength.
- Small diameters provide large surface area for bonding the fibre with the matrix.
- Small diameters provide sufficient flexibility in fibre production
What are the processing methods for thermoplastics?
The most common options for processing thermoplastic polymers:
- Extrusion: Plastics are melted and pushed through a die to form a continuous profile
- Injection moulding: Moulding is achieved by final injection through reciprocating-screw
- Blow moulding: Film thickness and diameter are controlled through roller velocity, cooling rate and pressure inside the bubble
- Compression moulding: Involves heating the polymer in a cavity and consolidating the melt under pressure.
- 3D Printing: A form of additive manufacturing technology, in which 3D objects are created by laying down successive layers of materials.
Describe the function of silane couple agent in glass fibre coating and its effect on the ultimate tensile strength of glass fibre reinforced composites
Silane couple agent has the ability to form strong bonding with glass fibre surface and polymer matrix respectively. This creates fibre-matrix interface with sufficient interfacial shear strength so that fibres will not be easily pulled out of the polymer when composites are subject to external loading. As a result, ultimate tensile strength of glass fibre reinforced composites can be significantly improved.
Describe how the change of the length would affect the Young’s modulus of glass fibres
Unlike the strength in brittle materials, the Young’s modulus is a structural property and is not dependent on the fibre length. Thus, the Young’s modulus remains the same for both long and short fibres.
Define the degree of crystallinity (Dc) and glass transition temperature (Tg) of a polymer. Explain why polyethylene and polypropylene can potentially have high degrees of crystallinity but poly(ethylene-stat-propylene) has almost zero crystallinity.
- The degree of crystallinity measures the volume or weight percentage of the material that forms a periodic regular structure. Due to the long-chain molecules, a polymer cannot fully crystallise.
- Glass transition temperature is a characteristic temperature at which polymers behaviour changes between rigid glassy solid and rubbery material. The spatial arrangement of atoms can change by rotation around the chain. This molecular motion is not possible when the temperature is below Tg due to insufficient internal energy in the material. (amorphous molecules only).
The ability of crystallisation requires a polymer to possess certain stereo-regularity in its molecular structure. A linear long-chain with periodic order allows molecules to be packed in a regular structure leading to high degree of crystallinity. However randomly arranged chains with no stereo-regularity will have a low degree of crystallinity.
How do the failure micromechanics of a real composite differ from that described in a ductile composite? What aspect of the material needs to be optimised to provide the highest strength?
Fibre strength has a distribution rather than a constant value. Thus accommodation of fibre breaks can occur even at low stress.
Transfer of stress from broken fibre to local fibre in bundle increases the probability of failure of adjacent fibres.
Strength of fibre-matrix bond is important since debonding of fibre can occur from the matrix at the broken end. This redistributes the stress over a large volume.
