Q: What is the importance of polymer structure and orientation in films? Part 3 of 4

By Dr. Eldridge M. Mount

In this installment, I will discuss relative levels of orientation as in cast, blown and tenter films and how to interpret the various orientation measures we can make easily. In general, there are three primary directions we consider, two in the plane of the film and the third through the film. The MD and TD directions are sometimes called the in-plane directions. These are general definitions of the principal directions of the film and are for reference purposes.

Having introduced you to crystal and amorphous orientation in Parts 1-2, I thought I would expand the topic with a discussion on film orientation. As in Part 1, orientation is the average direction of polymer-chain alignment in particular directions in the film. A truly unoriented film is isotropic or has uniform properties in all directions in the film.

There are two potential parts to the film orientation – the crystalline and the amorphous phase orientation. In a slow-quenched amorphous polymer (PS, PMMA, PC), the polymer chains show no preferred direction for the chains and would be isotropic. In a slow-quenched semicrystalline polymer (HDPE, PP, PET), the polymer crystals are imbedded in amorphous polymer and may grow into a spherulitic structure with no particular preferred direction for the chains in the crystals. Slow-quenched semicrystalline polymers may not show perfect isotropic properties even if unoriented due to crystal orientation variations in the spherulitic structure.