Processing, and Measurement Methods

As manufactured, PTFE is of two principal types dispersion polymer, made by suspension polymerization followed by coagulation, and granular PTFE, polymerized and generally comminuted to a desirable particle size. Some details are given by Sperati. We have observed cast films of an aqueous colloidal dispersion and see that it consists of peanut-shaped particles, approximately 0.25 pm in size, which are composed of even finer particles. Electron micrographs of as-polymerized granular particles show three structures bands arranged in parallel, striated humps, and fibrils, some of which have the shish-kebab structure.  

We have done most of our studies on granular PTFE by compacting it into a preform and sintering to make a shape matching ASTM D-1708-66 for tensile testing. Orientation measurements by IR dichroism require thinner specimens, and for this purpose we used dispersion-grade material, commercially cast and processed into film (Dilectrix Corp., Farmingdale, NY). These films were used as received without further heat treatment.  

It is clear that the quenched material has lamellae that are both thinner in the chain direction and smaller in lateral extent. Both these parameters increase with slower rates of cooling. So, too, does density. The microstructure of PTFE prepared this way may be likened to a pack of rods (the helical polymer chains), which in turn comprise lamellar crystals. Consider a hexagonal poker chip as a macroscopic analogue of one lamella. The diameter of die poker chip is the lamellar breadth and its thin dimension is the lamellar thickness. The fully extended chain length of a 10-million-MW PTFE macromolecule would be ca. 26 pm while our thickest observed lamellae are ca. 0.5 pm thick. It therefore 

Cooling rate deg C/min. Mean lamellar thickness (A) Standard deviation (A) Standard error of the mean (A) Density g/cc Percent crystallinity 

---