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
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
Step growth polymerisation
Chain growth polymerisation
