By Dr. Eldridge M. Mount
A: In an earlier column, I spoke of the foremost substrate secret, i.e.: what are the polymers of the substrate? After this secret is discerned, one of the most profound secrets of the substrate is “What is the molecular orientation of the polymers in the substrate as a whole or of the various layers if a multilayer substrate?” In this column, I will focus on what is the general polymer molecular structure, and once this question is answered, in my next 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.
But to begin, we need to start with a discussion on film orientation and then the molecular structure of polymers. Orientation, in a film, is the average alignment of polymer chains in particular, defined directions in the film. In general, there are three primary directions we consider, two in the plane of the film and the third through the thickness of the film. For our purposes, we will define them as the machine direction (MD) or X, which is the direction that the films moves through the machine from start to finish. Next, there is the transverse direction (TD) or Y, which is the direction perpendicular to the machine direction, and finally there is the thickness direction (TK) or Z direction, which is perpendicular to the MD and TD directions. The MD and TD directions are sometimes called the in-plane directions. These are general definitions of the principle directions of the film and are for reference purposes.
The pasta analogy
Orientation is due to the average direction which the polymer molecular chain shows. There are two potential parts to the film orientation: the crystalline and the amorphous-phase orientation. In amorphous polymers such as atactic polystyrene (a-PS), polycarbonate (PC) or polymethylmethacrylate (PMMA), there are no polymer crystals and therefore, there can be only amorphous-phase orientation.
In semi-crystalline polymers such as PP, PE and PET, there are polymer chains in both crystals and amorphous regions. The amorphous chains are best imagined as cooked spaghetti with no apparent order while the crystals will resemble the rigid uncooked spaghetti which can be held as a tight bundle. Try to imagine short bunches of uncooked spaghetti mixed into a plate of cooked spaghetti (with the ends of the bunched spaghetti being the loose cooked spaghetti, and you get an idea of what a semi-crystalline polymer is like at the level of the molecules.
Inside the crystals, the molecules are folded and packed together, while some chain ends escape, and may enter other crystals (tie molecules), “tying” the various crystals together. As I can only highlight polymer molecular structure here, the interested reader will find a book on polymers and study the nature and molecular structure of polymers.