Introduction to polymer chemistry Flashcards
The Glass Transition Temperature (T g )
the lowest temperature at which molecular motion of the polymer chain is possible
Elastomers
- crosslinked rubbery networks (above T g at room temp)
- can be stretched to high extension but recover when stress is released
- once crosslinked, it cannot be processed
- lightly crosslinked polymers which can be deformed
Thermosets
- rigid network polymers with a high crosslink density
- once formed, they cannot be processed or stretched
- rigid, inelastic structure so cannot be stretched/ deformed
Thermoplastics
- linear or branched polymers and solid at room temp
- when heated above a characteristic temperature, polymers can be processed as viscous liquids (they can be molded into shape)
- upon cooling they solidify but can be reheated and reprocessed, therefore recycled
Amorphous Thermoplastics
polymer chains are disordered even in solid state
- can be recycled like thermoplastics
Semi-crystalline thermoplastics
contains both amorphous and crystalline regions or domains
- they can only be reprocessed when heated above T m crystalline domains as above T g only the amorphous domains become mobile.
Amorphous domains
disordered chains with a glass transition temp (T g )
Crystalline domains
ordered chains with a melting point (T m )
Properties of linear/ branched polymers
soluble in some solvent
Properties of crosslinked/ network polymers
not soluble but can interact with solvent - they may swell which depends on crosslink density (thermosets with a high crosslink density may not swell at all)
Low density polyethylene (LDPE)
- comprised of chains with random branching therefore chain-packing/ crystallinity is inhibited by branching (e.g. plastic bags, food wrappers, etc)
- LPDE is less dense than HDPE so has a lower melting point and is soft and flexible
High density polyethylene (HDPE)
- comprised of chains with only a few branches therefore chains pack easily and the degree of crystallinity is high.
- HDPE has high density therefore a higher melting point and is hard, tough and rigid (e.g. plastic milk bottles)
Homopolymers
only one type of monomer
Copolymers
two or more types of monomer
- random
- alternating
- block (diblock, triblock, graft or comb block)
What are the conditions for Size Exclusion Chromatography (SEC)?
- in a good solvent for the polymer
- under ideal consitions where there are no enthalpic interaction between polymer and packing materials
What is K SEC a function of?
The loss of conformational entropy when a polymer chain enters a pore
How do SEC separate polymers?
In terms of the size of the polymer chain in solution relative to the size of the pores.
Examples of physical properties of polymers
they can be
- tough or brittle
- strong and durable
- rigid and inflexible
- flexible and rubbery
- hard or soft
(Properties vary with temperature thus allowing polymers to be processed)
Below T g
Polymer chains are effectively frozen and it is described as glassy.
Motion is restricted to short-range vibrations and rotations.
The polymer is hard, flexible and brittle.
Above T g
Chains have enough thermal energy to allow long range molecular motions so chains can move past each other.
Polymer chains are rubbery, can flow as a viscous liquid and be processed.
What determines the T g of a polymer? (stiff)
T g increases with increasing chain stiffness - stiffness is induced in polymer chains that have aromatic units in the polymer backbone or with bulky substituents.
What determines the T g of a polymer? (forces)
T g increases with increasing forces of intermolecular attraction.
T g of chain segments in a thermoset?
No T g with almost no mobility due to high crosslink density.
T g of chain segments in an elastomer?
Has a T g and retains mobility due to low crosslink density.
T g and T m in semi-crystalline polymers
T m is always higher than T g
Why don’t we get 100% crystalline polymers?
- Melted polymer chains will have a disordered, random conformation.
- As polymer melt cools to T m , it starts to crystallise which occurs at T c , the crystallisation temp.
- It takes a long time for all chains to unravel, align and form a 100% crystalline polymer.
What sort of control is crystallisation under?
Kinetic rather than thermodynamic - the degree of crystallinity obtained depends upon crystallisation conditions.
What will crystallinity be like if we rapidly reduce temperature?
Degree of crystallinity will be low as the chains have little time to rearrange into ordered crystalline regions before T g is reached and all chains become rigid in place.
What are semi-crystalline polymers used for?
Making fibres for clothing and textiles, as well as bulk plastic products.
Chain packing influence on T g
The ability of chains to pack in a regular arrangement influences the degree of crystallinity and may inhibit crystallinity altogether thus forming an amorphous polymer.
Two factors affecting chain packing
- Stereoregularity (tacticity)
- Polymer Architecture (chain branching)
Low density polyethylene (LDPE)
- comprises of chains with random branching
- chain packing / crystallinity is prohibited
- LDPE is less dense, has a lower melting point and is soft and flexible
What is LDPE used for?
- food wrappers
- shopping and rubbish bags
High density polyethylene (HDPE)
- comprises of chains with only a very few branches
- chain packing is easy
- HDPE is more dense, has a higher melting temp and is hard, tough and rigid
What is HDPE used for?
- plastic milk bottles
- containers
What is tacticity?
refers to the arrangement of side groups along a polymer chain
Isotactic
all the repeating units have the same stereochemical configuration
Syndiotactic
the repeating units have alternating stereochemical configurations
Atactic
the repeating units have no regular stereochemical configuration
What type of polymers can give rise to semi-crystalline polymers?
Isotactic and syndiotactic polymers since they have a regular orientation of side groups thus enabling regular packing
Isotactic vs Atactic Polypropylene
Atactic propylene has no commercial uses - a soft waxy polymer that becomes sticky upon heating.
Isotactic propylene is a semi-crystalline polymer that is tough and has excellent chemical resistance – used for packaging, textiles, carpets, fibres, automotive parts, etc
What can be used to measure thermal properties such as heat capacities (T g ), heats of fusion (T m ), enthalpies of reactions, etc
Differential Scanning Calorimetry (DSC)
What does DSC measure?
How much more heat (heat flow) is required to keep the sample pan at the same temperature as reference plan.
- Plot the difference vs temp
What type of reaction leads to increase in heat flow?
Endothermic (e.g. T g and T m )
Exothermic transitions result in decreased heat flow (e.g. T c )
What happens when polymers crystallise?
They give off latent heat - exothermic
What happens when polymer crystals melt (T m )?
They absorb latent heat - endothermic
What is an alternative to DSC as it is hard to detect small change in heat flow?
Dynamic Mechanical Analysis (DMA)
measures variations in mechanical properties such as
- force
- strain
- frequency
- time
- temperature
Many mechanical properties change suddenly at T g as polymer goes from a glassy solid to a rubbery liquid.
More sensitive than DSC.
What is tensile strength?
it measures how difficult it is to break a substance when stress is applied to pull it apart.
How to measure stress (σ)?
Stress (σ) = Force / Area
How to measure strain?
Elongation / Original length
What is strain?
a measure of how much the material has been stretched
What is Young’s Modulus and how do you measure it?
It is a measure of stiffness.
Young’s Modulus = Stress / Strain
Stress at the Yield point
the load is sufficient to cause polymer chains to slip past each other and yield occurs
Elongation at Yield Point
how far you can stretch the polymer before yield failure takes place (chain slip)
Stress at Failure (ultimate tensile strength)
polymer molecules can no longer maintain cohesive integrity and sample breaks
Elongation at Break
how much the sample can stretch before failure occurs
When does tensile strength generally increase?
With molar mass (chain length)
What is a crosslinked/ network polymer?
one in which all polymer chains are chemically bonded together
What happens when an elastomer absorbs and releases heat?
Shrinks when absorbs heat
Stretch when releases heat
Elastomers exhibit negative thermal expansivity.
What happens when elastomers are stretched?
the chains become more uncoiled, and more ordered thus sometimes resulting in crystallisation - exothermic (heat is released)
What happens when solvent is added to low cross-link density network of a gel?
highly swollen gel
What happens when solvent is added to high cross-link density network of a gel?
less swollen gel
What is the main distinguishing feature between elastomers and thermosets?
The crosslink density
elastomer = low crosslink density
thermoset = high crosslink density
