Chapter 11. Mechanical Properties of Polymers

1. Viscous flow (omit this section)

2. Kinetic theory of rubber elasticity

The mechanical properties of typical rubber like elastomers are:

I.                    Stretch rapidly and considerably under tension without generating much heat. (can extend to 5 to 10 folds of length)

II.                 Exhibit tensile strength and high modulus (stiffness) when fully stretched.

III.               Retract rapidly, exhibiting the phenomenon of snap or rebound

IV.              Recover their original dimensions fully on the release of stress, exhibiting the phenomena of resilience and low permanent set.

To be the elastomer, the polymer must be:

1. highly molecular weight polymer
2. Above its glass transition temperature Tg
3. Amorphous state
4. Crosslinking polymer

Thermodynamic of Rubber Elasticity

The ideal elastomer:

Stress-Strain Behavior of Elastomers

As shown in figure 11-4, the relative low slope of the curve decreases to about one-third its original value over the first hundred percent elongation, and later increases, often to quite high values at high elongations.

3. Viscoelasticity – time dependent mechanical properties of amorphous polymers

Theories of dynamic mechanical properties of polymers are not thoroughly developed and they are more depending on the models.

Stress Relaxation:

If elongation is stopped at some point, the stress decreases with time as the specimen approaches equilibrium under the imposed strain.

Usually the sample is deformed rapidly to a specified strain, and stress at this stress is observed for periods ranging from several minutes to several days or longer.

The stress-relaxation behavior of rubbery polymers are independent of strain and time. At small strains, the stress-strain function is almost linear and can be represented by a time-dependent modulus of elasticity G(t).

An example of stress-relaxation for polyisobutylene is shown in figure 11-7.

Creep: Creep is studied by subjecting a sample rapidly to a constant stress and observing the resulting time-dependent strain for relatively long periods of time, frequently for a week or even a year or more. Creep and stress relaxation are complementary aspects of plastic behavior and provide equivalent information for studies of both fundamental viscoelastic properties and performance in practical applications.

4. The glassy state and glassy transition

5. The mechanical properties of crystalline polymers

Classification of crystalline polymers

Polymers with low crystallinity including plasticized poly(vinyl chloride) and elastic polyamide behave like lightly crosslinked amorphous polymers.

At very low (<1%) elongation, at temperature well below Tm, and not too long time, polymers with intermediate degree of crystallinity behave like the low crystallinity.

Polymer with high degree of crystallinity are brittle.

Crystallinization of Stressing: Mechanical stress can cause the development of crystallization of a crystallizable polymer, either by raising Tm or by increasing the rate of crystallization.

Homework:

3. List four physical properties characteristic of typical elastomers.

4. List four molecular structure characteristics necessary for the development of typical elastomer physical properties.

11. Describe briefly (a) creep and (b) stress relaxation.