Q: What is the importance of polymer structure and orientation in films? Part 2 of 4
By Dr. Eldridge M. Mount
Orientation = pulling and freezing in place
What causes molecular orientation is a force which acts on the polymer molecule to pull it in a direction and then being frozen in place, such as by quenching during stretching from a molten state. This is generally the case in film manufacturing where the polymer molecule is stretched by pulling or inflating a polymer melt, followed by rapid quenching as for cast and blown films and with solid-state orientation as in double-bubble or tentered films.
A truly unoriented film is isotropic, or of uniform molecular structure in all direction in the film. These are extremely rare, generally not easily made and prized in applications which need uniform optical properties. They can be recognized by the uniformity of their optical properties, for example, the refractive index in the X, Y and Z directions.
Films with uniform orientation in the X and Y planes of the film are orthotropic and, unless the TK properties are measured, might appear isotropic because the in-plane properties (X and Y directions) are exactly the same. Films with variable orientation in all three directions are called anisotropic films where none of the properties in any principle direction are the same. If you think about it, we need only one direction to be different to have an anisotropic film, thus orthotropic film is a special case of anisotropic orientation.
Alignment impact on film properties
So, what does all this special language mean to film people? Well, the physical, barrier and optical properties will be different in each direction due to the molecular alignment of the polymer chains. Too much orientation in one direction will weaken the film in the other direction, such as by decreasing tear strength and elongation to break. Higher in-plane orientation, MD, TD or MD-TD, will lower the TK (Z) orientation, and this improves the moisture and gas barrier of a film, which is why oriented films have better barrier properties than unoriented films.
Typically, when we orient a film, we orient both the crystals and the amorphous parts of the polymer. The crystalline orientation plays some part in changing the film properties, but increasing the amorphous orientation is more important in the overall strength and barrier properties of the film (or what we are generally most concerned about). Naturally amorphous orientation is more difficult to measure directly than crystalline orientation
Measuring orientation…and properties
Relative levels of orientation can be measured many different ways, both sophisticated (expensive) and easy (cheap) such as X-ray diffraction (expensive) and tensile properties (cheap). Optical properties also are related to orientation and can be expensive or cheap depending on the amount of sample preparation. Z-direction orientation is always hard to measure directly because the films are thin, so most orientation is measured in MD and TD directions. The simplest ways to measure average film orientation is by measuring the film’s shrinkage and tensile properties. These measurements will give relative levels of orientation and can be correlated with physical properties and the absolute measures of orientation.
Generally, if a film (of the same material) shrinks more than another film, it is oriented more, or annealed less. To tell the difference we would need the mechanical properties, and here, if the mechanical properties are better (better being higher), the film is oriented more.