An X‐ray crystallinity index method with application to poly(ethylene terephthalate)

Wakelin's method of correlation analysis of x‐ray diffraction patterns was adapted for fibers, films, and polymer samples of poly(ethylene terephthalate). This method gives a number called a crystallinity index, which is a measure of how far an unknown sample is between the extremes of the most amorphous and most crystalline samples obtainable for a polymer species. By being relative, this index eliminates the controversial assumptions of absolute crystallinity determinations. It is especially suited for routine studies since the calculations are readily adapted to the use of a computer and since the total time needed to obtain an index is short. A study of the indices of a variety of poly(ethylene terephthalate) materials produced the following conclusions: ( 1 ) Maximum crystallization is achieved only by starting with an unoriented amorphous film. This is contrary to the view that orientation helps crystallization. Also, this shows that the act of spinning a fiber sets up an apparently strained state which will not crystallize to the same high level as a cast film. ( 2 ) Time of heating can be exchanged for temperature of heating, since the rate of crystallization is slow for this polymer. Thus, a longer time at a lower temperature is the equivalent of a shorter time at a higher temperature in producing equivalent crystallinities. ( 3 ) The crystallinity measured by density is not the same as the crystallinity measured by x‐rays. ( 4 ) A preordered state must exist in fibers in which segments of molecules have lost their complete randomness and are densely packed, yet the segments are not perfectly enough placed to produce x‐ray diffraction. Or, it can be concluded that spinning or drawing disrupts the fiber's perfect lattice so that less x‐ray diffraction is produced than the high density would indicate.