Entangled polymer dynamics and polymer crystallization

6 problems

• Why entangled melts and solutions differ in fast extensional flow

  Contested

  When an entangled polymer melt is stretched fast, its resistance to stretching drops, while a solution of the same polymer with the same number of entanglements becomes stiffer. Tube theory predicts the same behaviour for both, and why they differ is disputed.

• What selects lamellar thickness in polymer crystals

  Contested

  When long polymers crystallize they fold back and forth into thin plates called lamellae, about 100 Angstrom thick. Which process fixes this thickness and the growth speed is disputed among competing theories.

• How unconcatenated ring polymers relax stress in a melt

  Contested

  Ring-shaped polymers have no ends, so they cannot slide out of entanglements end-first as linear chains do. How a melt of rings flows and relaxes stress is still unclear.

• Do well-entangled polymer fluids form steady shear bands

  Contested

  Some theories predict that a strongly entangled polymer fluid, when sheared, splits into bands flowing at different speeds. Experiments disagree on whether this really happens or is caused by slip at the walls and edge failure.

• Why the glass transition temperature drops in thin polymer films

  Contested

  Very thin polymer films soften at temperatures tens of kelvin lower than the bulk material. Why, and why the effect depends on chain length in free-standing films, is debated.

• Does melt viscosity cross over to pure reptation scaling

  Contested

  The viscosity of melts of long linear chains grows as chain length to the power 3.4, while the basic sliding-in-a-tube picture (reptation) predicts power 3. Corrections explain the gap, but it is unconfirmed whether very long chains finally reach power 3.