Polymers- More Rubber Elasticity

Question 1

Shear modulus of ideal rubber

Answer 1

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

Question 2

Shear modulus for ideal rubber in high temperature region

Answer 2

The r squares are equal in and outside the network so

G=NkT

Question 3

Entropy origin of rubber elasticity

Answer 3

Comes from the random movement of molecular segments which makes them prefer being coiled rather than extended

Question 4

Difference between natural rubber and vulcanised natural rubber

Answer 4

The vulcanised has sulfur crosslinks between the chains

Question 5

Tension of 100%extension vs percentage of crosslinking

Answer 5

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

Question 6

Variation of Young’s modulus E with temperature for polymer

Answer 6

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.

Question 7

What happens to molecular chains in an elastomer without cross-linking under stress?

Answer 7

They slip past each other into new positions in the sample and recoil

Question 8

Effect of cross-linking on Tg

Answer 8

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

Question 9

Difference between chemical and physical cross-linking

Answer 9

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

Question 10

Describe natural rubber

Answer 10

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

Question 11

How many cross-links are generally formed to improve natural rubber?

Answer 11

One cross-link per 500 to 1000 isoprene units

Question 12

Advantages and disadvantages of mixing natural rubber with carbon black

Answer 12

Advantages: mechanical reinforcement, reduce degradation by sunlight and ozone, reduce liquid absorption
Disadvantages: mechanical hysteresis, strain softening (on reloading new curve below initial)

Question 13

Stress strain with carbon black

Answer 13

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

Question 14

Stereoisomers for polyisoprene and polybutadiene

Answer 14

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

Question 15

Examples of copolymers

Answer 15

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

Question 16

Neoprene

Answer 16

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

Question 17

Butyl rubber

Answer 17

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

Question 18

Making silicon rubber

Answer 18

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

Question 19

Styrene-butadiene block copolymers

Answer 19

Have glassy domains of polystyrene at end of some chains of polybutadiene. Can be melted and reshaped

Question 20

PVC

Answer 20

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

Question 21

How is resilience measured?

Answer 21

Percentage rebound recovery (drop ball on hard floor) measured at 298K

Question 22

Most to least resilient rubbers

Answer 22

Cis-polybutadiene 
Synthetic cis polyisoprene 
Natural cis polyisoprene 
Ethylene-propylene copolymer
Styrene-butadiene copolymer (SBR)
Trans-polyisoprene
Butyl rubber

Question 23

Temperature effect on resilience

Answer 23

Resilience (weird symbol and units) vs T forms V shape graph linked to the Tg of each polymer