We have measured the incident angle dependence of the infrared (IR) absorption of thin rubbed polyimide films on CaF2 and Si substrates. The molecular orientation of the films was determined by fitting the experimental result with a theoretical calculation. The incident angle dependence for the rubbed film on CaF2 was very different from that for the rubbed film on Si. On the other hand, the actual molecular orientation determined by the fitting procedure is similar in both samples. Thus this result clearly shows that the incident angle dependence is strongly dependent on the substrate. This effect arises from the interference between the forward and backward propagating light in the film. This experiment demonstrates that the interference effect must be taken into account in determining the molecular orientation of thin films on solid substrates. We have also calculated the incident angle dependence for rubbed films with different inclination angles of polyimide chains on CaF2 and Si2 substrates. We found that CaF is a more suitable substrate than Si for determining the molecular orientation of rubbed films with a relatively high inclination angle of polyimide chains.
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