Q: What is the importance of polymer structure and orientation in films? Part 4 of 4
By Dr. Eldridge M. Mount
Forces of orientation
What causes molecular orientation is a force which acts on the polymer molecule to pull it in a direction and then the direction being frozen in place as by quenching from a molten state (blown and cast films) or from being heated above its glass transition temperature (Tg) as in solid-state orientation (double-bubble and tenter processes). Above (Tg), the amorphous chains are in a rubbery state and free to move while below (Tg) the amorphous chains are frozen in a glassy state. In a semicrystalline polymer, the crystals are impervious particles and act as physical reinforcements, much like carbon black fillers in rubbers, and will increase the stiffness, modulus and barrier of a sample with an increasing percentage of crystallinity.
But measuring orientation is a tricky business. There are absolute and indirect methods. Absolute methods, such as X-ray diffraction are slow and expensive but can be correlated with indirect methods, such as shrinkage and mechanical properties. While mechanical and shrinkage properties give an indirect indication of molecular orientation level, optical methods (refractive index and bi-refringence) and the sonic modulus are more direct measures of molecular orientation because they depend on molecular features of the polymer chain.
When I was first learning about the impact of molecular orientation of film properties, there were many papers to read. Many focused on crystal-orientation X-ray studies, but what was most surprising to me was that crystal orientation alone generally did not correlate with most improvements in film physical and barrier properties. Physical properties correlate best with average orientation, and as will be explained next, this is principally due to the changes in the amorphous orientation during stretching.