Measuring Polymer Film Thickness by Interference Panerns The experiment reported by Allpress, Cowell, and Herd' that measures both the thickness and the vinyl acetate content of packaging film js a useful applications experiment for undergraduate students. The authors noted that the fdm thickness measured using interference fringe patterns of infrared radiation differed from that measured using a micrometer. The reported ratio of the spectroscopidly determined thickness to the micrometer-measured thickness is 1.3-1.5. This is probably very elme to the refractive index of the polymer film. In fact, a more precise equation relating thickness ( d )and interferenee patterns should he usedz
d=
rn Zn(i7, - F2)
where rn is the number of peak maxima between measured interference peaks; n is the refractive index; and i7 is the wavenumber. When chwsing two adjacent peak maxima, m = 1, and the equation becomes 1
.
d=-
Zn(A3 where d is in centimeters. An incorrect equation far this relationship was given in the ieported experiment. I have found that using the refractive index of the monomer or an average of the comonomer refractive indices obtained from standard tables aives a reasonable approximation to the film thickness for the polymer in question and compares reliably with that obtained using a micrometer K. N., Cowell, B. J., and Herd, A.'c., J. CHEM.EDUC.,58,741 (1981). Hannah, R. W., and Swinehart, J. S., "Experiments in Techniques of Infrared Spectroscopy," Perkin-Elmer, Nonvalk, CT, 1974, p. 3-1. Benjamin F. Plummer Trinity University Sen Antonio, TX 78284 1 Allpress,
Volume 61
Number 5
May 1984
439
