Polymer 3 Flashcards
Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC)
• DSC is sensitive to enthalpy changes in sample, hence may be used for measurements/changes of heat capacities (Tg), heats of fusion (Tm), enthalpies of reaction, etc.
how does DSC work
DSC thermogram exothermic and endothermic transitions
Polymers heat capacity (DSC)
Polymers have a higher heat capacity above the glass transition temperature than they do below it, but no latent heat given off/absorbed.
• Hence Tg is a step rather than a peak – not a Mpt!!
Polymers at Tc and Tm (DSC)
In contrast when polymers crystallise at Tc they
give off latent heat - exothermic
• When the polymer crystals melt (Tm), they must absorb latent heat - endothermic
Dynamic Mechanical Analysis DMA, what does it measure and how does it work
Glass transition involves a small change in heat flow – sometimes hard to detect by DSC
DMA is a useful alternative to DSC
DMA measures variations in mechanical properties as a function of
- force
- strain
- frequency
- time
- temperature
- Applying a constant force and varying temperature allows Tg to be measured
- At Tg mechanical properties change dramatically as polymer goes from glassy, solid to rubbery liquid
- Much more sensitive that DSC.
Tensile strength
Tensile strength measures how difficult it is to break a substance when stress is applied to pull it apart.
tensilometer
A tensilometer grips a sample, pulls it apart and creates a plot of the force exerted on the sample (y-axis) and the elongation of the sample (x axis).
stress, strain and YM
identify yield point and faliure ( sample break)
Tensile strength and molecular weight trend
Tensile strength generally increases with molecular weight
Amorphous polymers below Tg
Amorphous polymers are glassy below Tg, and therefore hard, stiff, and brittle. This brittle nature means they fracture easily and the material is not tough.
amorphous polymers above Tg
Above Tg, amorphous polymers are rubbery, and often too soft to provide useful properties, (unless they are chemically cross-linked).
properties of 100% crystalline polymer
100% crystalline polymer would be hard, stiff and brittle, since the chains are held firmly in place, i.e. similar to a glassy polymer.
Semi-crystalline polymers properties
Semi-crystalline polymers have desirable properties due to the interplay between the amorphous and crystalline regions.
The crystallites provide good strength, stiffness, hardness, abrasion and temperature and chemical resistance, whilst the amorphous regions impart some flexibility into the material, making it tough and less brittle.
Tensile strength curve
A crosslinked/network polymer
A crosslinked/network polymer is one in which all polymer chains are chemically bonded together.
The degree of crosslinking strongly affects the properties of the polymer, so a very high degree of crosslinking……… and a lightly crosslinked…………….
The degree of crosslinking strongly affects the properties of the polymer.
A very high degree of crosslinking (many crosslinks per chain) results in very rigid, inelastic structure. The high crosslink density prevents individual chain segments from moving.
A lightly crosslinked material will be more flexible/elastic since the chains retain some mobility
However ALL network polymers are crosslinked and whilst individual chains may retain some mobility they cannot flow. Crosslinked polymers cannot be processed!
Elastomers (network polymers) properties and examples
Elastomers are crosslinked amorphous polymers with a low crosslink density.
They can be stretched easily by 3 to 10 times their original length, and then return to their original shape when the applied stress is removed – think elastic band!!!
The crosslinks help the chains return to their original positions when the applied stress is removed.
Examples include rubber, elastane, neoprene and lycra.
A polymer will only show elastomeric
behaviour if it is above it T g glass transition
The elastic band experiment
When an elastomer is stretched, work is transformed reversibly into heat.
Shrink (Absorbs Heat) « Stretch (Releases Heat)
So when you heat elastomers, they shrink. Elastomers are one of the very few materials to do this. They exhibit negative thermal expansitivity.
In the unstretched state, the polymer chains exist in many random conformations – crosslinked random coils.
When stretched, the chains become more uncoiled, and more ordered, (in fact partially crystallisation often occurs),
Crystallization is exothermic and so heat is given out.
Natural Rubber
natural rubber properties
What is thermoplastics and elastomers
Thermoplastics are polymers that can be molded into shape usually upon heating/cooling.
Elastomers are lightly crosslinked polymers which can be deformed (stretched) but not processed.
Thermosets
Thermosets – “A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing can be induced by the action of heat or suitable radiation, or both. A cured thermosetting polymer is called a thermoset”
Thermosets have a very high crosslink density and include epoxy resins and phenolic resins.
distinguishing elastomers and thermosets
The main distinguishing feature between elastomers and thermosets is crosslink density,
e.g. vulcanised rubber!
• 0.5 – 5% of sulphur – gives an elastomer (elastic
bands to car tyres)
• 30% sulphur –> Ebonite – a thermoset
Polymer Synthesis
Polymer Synthesis - Prologue
There are two general approaches to make polymers
- Step growth polymerisation
- Chain growth polymerisation
Step growth polymerisation
Chain growth polymerisation
