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Crystal orientation
July 3, 2008
Films are produced from molten and solid polymers by stretching either the melt as in blown and cast films or the solid as in biaxially oriented PET and PP. In these processes crystal alignment is observed which can affect some film properties. In general when stretching a molten polymer the molecules will be aligned by the applied force on the melt, from pulling the melt from the die and by stretching in the TD as by inflation. Here we are orienting a high viscosity liquid and the level of residual orientation will depend on the quenching rate and the applied forces. In this instance the crystals are forming in a stress field and the oriented polymer chains in the melt serve as nuclei for the growing crystals. In this case the c-axis of a crystal will tend to align in the stretching directions and the crystals can serve as reinforcing particles in the finished film giving it strength and rigidity depending on the crystal size and amount of crystals. The actual resulting crystal structure and film morphology will be dependent on several factors which I will discuss later at some length.
In solid stretching, the situation is a little different in that the crystals are already formed, as in making the cast or blown film from which the solid stretched films will be made. Here the crystalline morphology can play a roll in determining the success of the solid orientation steps. The crystals are reinforcements for the amorphous phase and resist the stretching forces. When a critical stress is reached the polymer structure collapses (seen as a neck in some polymers) and the material flows. Here the amorphous phase is deforming and in so doing it drags the crystals with it. During this process the crystals C-axis will align in the direction of stretching and the A and B crystal axis will align in the perpendicular directions, in a fairly regular manner. This is seen in x-ray diffraction studies of solid state oriented films.
The final morphology of the films will determine much of the physical properties of the different films.
Posted by Eldridge M. Mount on July 3, 2008 | Comments (1)
In response to: Crystal orientation
DaveO commented:
Is it possible for different orientations in a 2D, expanded-PET film to cause variable diffraction of beta particles of X-rays in a measurement device?