Polymer structure Flashcards
Aliphatic vs Aromatic polymer
aliphatic is derived from methane, aromatic benzene
Make up of polymers
Hydrocarbon molecules join to form backbone chain of polymer. Different hydrocarbons join to form monomer
Isomerisum
Individual hydrocarbons with different arrangement and properties
Two polymerisation processes
Chain addition:
Uses free radicals to form new bond on end of chain. RAPID chain growth stops when combines with another chain or when free radical bonds with free electron
Step reaction:
using fuctional groups polymerisation occours in chemical reaction (often small molecules like water or methanol) generaly producing a by product. SLOW and creates 3D network structures. common in THERMOSETS. Properties dependent on extent of polymerisation, only becomes useful when DP approx 50
Thermosets
Polymerise with step addition. Heating may decrease stiffness but will not melt.
Chemical reaction occours between monomers if > 2 reactive groups. Curing agent contains radicals to initiate polymerisation process.
Uncured agent/monomer is called sol while gel is cured part with crosslinking. Gel point is where material becomes unworkable
Polymer Structure factors
Composition:
bonding types, end groups, atoms in sidegroups
Configuration:
spacial arangment of atoms
Conformation:
spacial arangment of polymer chains
Primary bond in polymers
Covalent bond:
atoms share electrons to reach stable state
Secondary bond types
Dispersion (London) forces:
caused by momentary dipole formation
Induction Forces:
permanat dipoles induce temparary dipoles in neighbour
Dipole-Dipole forces:
attaction due to diffence in electronegativty of atoms
Hydrogen Bonding:
due to high difference in electronegativity between hydrogen and Nitrogen, Oxygen and Florine.
(listed in increasing bond strength)
Possible polymer arangement
Atactic:
random placement of side groups
Isotactic:
all side groups on same side
(highest degree of crystallinity due to symmetry)
Sydiotactic:
Side groups regularly alternate
Properties of polymer arangment
Greater symetery = denser packing. This increases intermolecular bond strenght and crystallinity as polymer chains more easily aligned.
Molecular Structures
Linear:
High density packing leading to strong secondary bonding
branched:
lower packing effeciency so weaker secondary bonding. (longer branch = lower cystallinity and therefore lower Tm, and stength but higher ductility
Crosslinked:
All chains are covalently bonded together. (lightly crosslinked polymers can retain some thermoplastic behaviour)
3D Network polymers:
Monomer units each have extensive covalent crosslinking. (All thermosets are 3D network polymers due to permenant crosslinking)
relationship between chain length and entanglement
Chains must reach a specific length before entanglement occours effectivly. At Mc chain length is long enough for rate of entanglement to increase
Amorphus Polymer Structure
Amorphus polymers are made up of randomly aranged chains. 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 srinks.
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
Contain crystallite and amorphous phase. Amorphous phase between crystals give toughness by absorbing energy of crack tip
Effect of crystal size on mechanical properties
Decreasing spheralite size generaly improves strain till failure and toughness. As spheralites get bigger more boundry defects from where cracks can initiate - more brittle but stronger.
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
Factors for crystallisation rate
Polymers that can crystalise extensivly (regular repeating simple structures) generaly have higher growth rates.
Nucleating agents speed up crystalisation rate. Benifitial in injection molding when fast cycle time required
Stronger monomer bonding e.g. hydrogen bonds lead to faster growth rate and level of crystalinty
Deformation Behaviour in semi-crystalline polymers
Streching of polymers introduce anisotropic (directional dependance on mechanical properties) behaviour.
For slip/deformation to occur lamellae must rotate (van der waal bonds broken).
If crystals cannot easily deform cavitation occours (generaly larger, slow cooled crystals with less ductility). Stress concentrations form around crystal-amorphous boundaries due to difference in ductility. This seperation causes cavities
Post processing crystallisation
Crystallisation of the amorphous phase can continue weeks to months after production. Bad as leads to shrinking and loss of toughness as density and brittleness increase with crystal size.
This is because polymer has not completly crystalised. Crystalisation can be improved by:
-slower cooling rate, more time for crystals to grow
-use additives to accelerate nucleation
-cooler tool surface, increases number of nuclei (greater undercooling)
Plasticisers/Additives
Plasticiser are designed to increase flexibility.
External Plasticiser:
physically mixed with polymer. They reduce secondary bonding strength which increase free volume leading to greater chain mobility and ductility
Internal plasticiser:
Chemically bound to molecular groups. This introduces irregualitys that reduce degree of crystallinity. Lower crystallinity decreases secondary bonding forces therefore increasing ductility.
Free Volume
Free volume is unoccupied space within the polymer. Free volume increases above Tg as chains can easily relocate.
Higher free volume give polymer chains greater mobility at lower temp, therefore reducing Tg
At Tg, thermal expansion coeffcient increases as polymer volume increases faster
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.
Lamella thickness effect on Tm
As lamella thickness increases surface to volume ratio decreases which reduces the effect of surface free energy. This makes the crystal more stable and require higher Tm
