Polymers- More Rubber Elasticity Flashcards
Shear modulus of ideal rubber
G=NkT(i/o)
N is number of sub-molecules per unit volume
T is temperature
The r squares are mean square end to end distance of sub-molecules in and outside of the network
k is Boltzmann constant
Shear modulus for ideal rubber in high temperature region
The r squares are equal in and outside the network so
G=NkT
Entropy origin of rubber elasticity
Comes from the random movement of molecular segments which makes them prefer being coiled rather than extended
Difference between natural rubber and vulcanised natural rubber
The vulcanised has sulfur crosslinks between the chains
Tension of 100%extension vs percentage of crosslinking
Theory is linear through the origin. Experimental is through origin but above theory then curves to straight crossing theory. This represents the dependence of the modulus on the degree of cross-linking. Only looking at up to 3% cross-linking
Variation of Young’s modulus E with temperature for polymer
Log(E) vs T. Starts glass and is high above 9 and is horizontal. Then curves down to steep through Tg to rubber and flattens. If cross linked stays flat and curves up very slightly. If linear curves down more.
What happens to molecular chains in an elastomer without cross-linking under stress?
They slip past each other into new positions in the sample and recoil
Effect of cross-linking on Tg
More cross linking increases Tg. Graph is G on log scale vs T. The steep decline is at Tg and shifts right and the end of it is higher
Difference between chemical and physical cross-linking
Chemical links by valence bonds on the main chain to form sulfur bridges (vulcanisation). Physical is chain entanglement, existence of crystalline domains (can remove by heating) or existence of glass domains
Describe natural rubber
Monomer is cis-polyisoprene. Has C=C in the main chain and two methyl groups at the bottom side have the wings, one methyl at top along with one H. Mechanically weak, deficient in elastic recovery, greatly swollen by organic liquids
How many cross-links are generally formed to improve natural rubber?
One cross-link per 500 to 1000 isoprene units
Advantages and disadvantages of mixing natural rubber with carbon black
Advantages: mechanical reinforcement, reduce degradation by sunlight and ozone, reduce liquid absorption
Disadvantages: mechanical hysteresis, strain softening (on reloading new curve below initial)
Stress strain with carbon black
Natural is very shallow. With carbon black higher (steeper, curve less, inflection to curve up more up to 2 strain, unload steep down then curve to near flat down to almost 0 strain). 2nd load same shape but always lower and starts where 1st ended on x axis
Stereoisomers for polyisoprene and polybutadiene
Polyisoprene has cis and trans where wings are on same or opposite sides (from methyl like groups).
Polybutadiene is like polyisoprene but without extra methyl group (no wing) so has cis and trans but also another where C=C not on backbone
Examples of copolymers
Styrene-butadiene rubbers (SBR) and butadiene-acrylonitrile rubbers, nitrile rubbers (NBR)
Have different numbers of butadiene groups and the other groups in the repeating unit depending on purpose
Neoprene
Basically polybutadiene with a Cl replacing one H atom (cis for wings). First high cost speciality rubber. Hydrocarbon resistant, heat resistant, low permeability to gases, resistant to ozone oxidation
Butyl rubber
C and CH2 on backbone with wings (single bond). ThenC has two extra methyl groups. This is repeating unit. Has relatively few double bonds so resistant to oxidation. Have nigh mechanical hysteresis at room temperature. Low permeability to gases. Tyre inner tubes and liners
Making silicon rubber
Start with Si with R and R’ above and below and Cl either side. Add water so OH replaces Cls. One OH goes and one H goes leaving wings from Si and O. Has thermal stability from Si-O backbone so constant properties over wide T range 100-250 degrees. High gas permeability
Styrene-butadiene block copolymers
Have glassy domains of polystyrene at end of some chains of polybutadiene. Can be melted and reshaped
PVC
Repeating unit chloroethane. Unplasticised stiff and glassy. Plasticised mixe with low weight liquid so: low tensile strength and tear resistance, good electrical insulator, inexpensive, easy to process, used for hose, clothing, cable insulation
How is resilience measured?
Percentage rebound recovery (drop ball on hard floor) measured at 298K
Most to least resilient rubbers
Cis-polybutadiene Synthetic cis polyisoprene Natural cis polyisoprene Ethylene-propylene copolymer Styrene-butadiene copolymer (SBR) Trans-polyisoprene Butyl rubber
Temperature effect on resilience
Resilience (weird symbol and units) vs T forms V shape graph linked to the Tg of each polymer
