Polymer Structure Flashcards
Aromatic
Cyclic compound derived from benzene
Aliphatic
Non-aromatic group with the main chain of carbon and hydrogen atoms that are either linear or cyclic. Derived from methane
Saturated vs Unsaturated
Saturated only have single bonds. Unsaturated having double of tripple bonds
Radical
Highly reactive species with an unpaired elctron eg H-
Functional Group
Specific group of atoms within molecule that determine chemical properties
Make up of polymers
Polymers are large molecules made of repeating strucural units called monomers, which are chemically bonded in long chains
Isomerism
Hydrocarbons taking on different atomic arrangements (different structure, same composition). Changes properties
Two polymerisation processes
Chain addition:
Monomers add to a growing polymer chain through free radicals. The chain grows rapidly until two chains combine or the radical reacts with another species.
Step addition:
Monomers with functional groups react in steps, releasing small by-products like water or methanol. This slow process forms a 3D network (common in thermosets), with properties depending on the degree of polymerization (DP), typically becoming useful around DP ≈ 50.
Thermoset
Polymers that undergo permanent cross-linking during curing making them hard and rigid. Once set, they cannot be melted or reshaped, making them highly heat resistant and mechanically strong.
Elastomer
Polymers with loosley cross-linked structure allowing them to be highly stretchable and flexible. They return to their original shape once the force is removed. moderate heat resistance, resistant to wear and fatigue.
Thermoplastics
Long linear or branched polymer chains held together by van der Waals force or hydrogen bonds. Does not have cross-linking. This allows chains to slide past each other when heated, making them remoldable and recyclable. Has Tg and Tm.
Curing reaction
A curing reaction is a chemical process that hardens or solidifies materials, typically resins or polymers, by cross-linking their molecular structure. This process is often triggered by heat, light, or the addition of a curing agent, resulting in improved mechanical properties and stability.
TTT diagrams
First line is gelation point where system is transformed from a viscous liquid into a gel (many of the chains become interconnected)
Second line is transition from rubbery region to glassy/rigid polymers
Macromolecular structure (3 Cs)
Composition:
Atoms in side groups, end groups and bonding types
Configuration:
Permanent spatial arrangement of atoms or groups which can only be changed by breaking and reforming bonds
Conformation:
Spatial arrangement of polymer chains that can be changed without breaking any covalent bonds.
Primary bonding of polymers
Covalent bonding (atoms share electrons to become stable)
Secondary bonding (lowest to highest strength)
Dispersion (London forces): Electrons in a molecule may not be evenly distributed in the molecule, creating a temporary instantaneous dipole. Very weak, helps to explain bonding of non-polar polymers
Induction Forces: Permanent dipoles induce temporary dipoles in neighbour
Dipole-Dipole: Caused by differences in electronegativity leading to permanent dipole.
Hydrogen bonding: Occurs when H bonded to O, N or F (high electronegativity). H atoms become positively charged and are strongly attracted to other electronegative atoms in adjacent polymet chains.
Tacticity (positions of side groups on backbone)
Isotactic: All groups on the same side of the polymer backbone. Results in a more crystalline structure and higher melting points.
Syndiotactic: The side groups alternate sides of the backbone in a regular pattern
Atactic: Sidegroups arranged randomly. Lower crystallinity making them more amorphous and flexible but less strong compared to isotactic or syndiotactic polymers.
Conformation
Dictated by repulsion between side groups/atoms. When side groups are too close together, repulsive forces cause polyer chain to adjust conformation to increase distance. For repulsion centre-to-centre distance of side groups less than 2x VDW radius.
Macromolecular chain structures
Linear: Long unbranched cahin of monomers
Branched: Polymer chains with side branches attached to the main chain
Crosslinked: All chains covalently bonded
Network polymers: Highly crosslinked where polymer forms continuous 3D network making material rigid and thermoset
Molecular weight affect of properties
High MW: High tensile strength, high toughness high impact resistsnce
Low MW: Weaker mechanical properties, low Tg and Tm. Less viscous.
Entaglements
No chemical bonds, chains are mechanically tangled together. Thermoplastics
Amorphus polymer structure.
Amorphous structures lack order or repeatabilty (non-crystalline). When heating Energy is absorbed to break crystal structure (ENDOTHERMIC). When cooled energy is released as the polymer cyrstallizes (EXOTHERMIC).
As crystallinity increases, density increases as polymer becomes more ordered so shrinks.
Nucleation types
Nuclei first form on cooling below the Tm (undercooling).
Homogeneous Nucleation:
Occours sponaneously and uniformly throughout the parent phase without preferential sites (requires greater under cooling).
Heterogenous Nucleation:
occours at prefential sites (surfaces, interfaces, impurities (less undercooling required as sites lower nucleation energy barrier.
Optimal Growth range;
Tg + 30 < Toptimal < Tm - 10
Semi-crystalline materials
Have crystalline and amorphous regions. Crystalline regions are connected by tie molecules.
Lamellar fibres
Thin crystalline regions formed by folded polymer chains.
Spherulites
Radial, semi-crystalline structures that form when polymers crystallize from a melt, consisting of alternating crystalline and amorphous regions. Lamellae grow out from nucleation site and provide hardness while amorphus regions provide elasticity
Viscous behaviour
Material takes time to respond to applied force
Elastic behaviour
Material deforms instantly under applied stress
Viscoelastic behaviour
Material exhibits combination of both elastic and viscous behaviours. At short time scale and rapid loading behaves more elastic. Long time scale and slow loading more viscous.
Effect of crystal size on mechanical properties
Decreasing spheralite size improves strain till failure and toughness by reducing crack initiation sites. Larger spherilites can increase strength but make mure brittle.
Tm generaly uneffected by spheralite size
slow cooling forms large crystalites
Effect of crystallinity on mechanical properties
As crystallinity increases intermolecular forces increase. This increases tensile strength stiffness and Tm. However becomes more brittle
Crystalisation occurs between Tg and Tm
Cooling rates effect on crystallinity
Slow cooling: allows for more time for crystalline structures to form, leading to increased crystallinity, thicker lamellae, less defects but cavitation more likely under stress
Fast cooling: Rapid solidification prevents polymer chains from fully organizing into a crystalline structure, resulting in lower crystallinity, thinner lamellae, and more defects. This can enhance slip mechanisms, allowing for higher strain to failure, making the material more ductile but potentially weaker.
Stretching deformation in crystals
Involves the breaking of van der Waals bonds between chains.
iIf slip cannot occur stresses cannot be relieved in the amorphus phase and cavitation occcurs
Free volume
Empty space within the polymer structure that is not occupied by the polymer itself. Allows for movement of polymer chains. As temp increased above Tg, free volume increases dramatically compared to occupied volume.
Higher Mw = Higher Tg = Less free volume
Increased bulkiness of side groups increases free volume
Plasticisation
Used to increase mobility/flexibility of polymer chains. separates polymer chains, increasing the distance between them. Decreasing viscosity and increasing flexibility.
Thermal Transitions
Tbeta:
transition occours in the glassy region. It is when rotation of side groups in possible. Under this transition polymer completely ridgid and extremely brittle.
Tg/Talpha:
When molecular motion of the main chain is possible. Polymer goes into rubbery state where modulus rapidly decreases.
Intermolecular vs Intramolecular
Intermolecular are chemical and physical bonds between macromolecules whereas intramolecular is within macromolecules
Short vs long range order
Short range order refers to the local arangement of polyermers over a short distance. Long range is the repeating arangment of polymer chains over large distance.
amorphus polymers rely completely on short range order for mechanical properties while semi-crystalline rely on short range below Tg.
Rate of crystallisation for polymer structures
Linear chains are able to crystallise to large extent. As branched chains get longer crystallisation become harder as molecular packing in inhibited.
Cross linked and network structures are unable to crystallise at all due to covalent bonding. They remain completely amorphous.
