Questions from class - Lecture 12

Question 1

How does the force needed to stretch a rubber vary with the elongation?

Answer 1

Scales with (lambda - 1/lambda^2), that is approximately linearly with lambda.

Question 2

How is the plateau modulus related to the molar mass between crosslinks?

Answer 2

The plateau modulus scales with 1/M_c.

Question 3

Which structures are observed in crystalline polymers on different length scales?

Answer 3

At small length scales: lamellae

At bigger length scales: spherulites

Question 4

In which temperature range is crystallization possible?

Answer 4

From approx. T_g + 30K - T_m - 10K

Question 5

Which molecular characteristics favor crystallization?

Answer 5

Highly packed arrangement of chains must be possible in 3D. Energy loss upon crystallization.

Symmetric, linear chains crystallize best.
vdW forces between chains. Polar groups can result in rigid and well-oriented arrangement. Isotacticity. Low degree of branching. High molar mass.

Question 6

What can one learn from single crystals?

Answer 6

You can get the enthalpy of crystallization by measuring the melting temperature for a given thickness. Extrapolating this to L_c -> ∞ gives T_m^0 (equilibrium melting temperature).