The latent imaging component of a photographic film is comprised primarily of silver halide as the light-sensitive constituent dispersed in an aqueous gelatin binder. After this emulsion is coated on a polymer (or other support), the coating is allowed to dry such that the moisture content is dependent on the surrounding ambient relative humidity (RH). During this drying process, the final gelatin film thickness is reduced resulting in a compression of the silver halide grains perpendicular to the sample plane. It is desirable to correlate the strain characteristics of different silver halides as a function of humidity so that a photographic density response due to drying stress can be monitored. In this study photographic films were analyzed at various humidities in situ while collecting x-ray diffraction data.
The emulsions in this study were coated on poly(ethylene terephthalate) support. The silver halide grains had the composition AgI$_{0.03}Br_{0.97}$, with a uniform iodide distribution in the grains. The grains were a blend of 0.32 and 0.25 $\mu$ (30/70 v/v) diameter cubical grains. X-ray diffraction data were collected using a Siemens D500 q/q diffractometer equipped with a copper anode x-ray tube, diffracted beam nickel filter, and a Braun position sensitive detector. An Anton-Paar HTK temperature stage installed on the D500 was modified for {\it in-situ\/} humidity x-ray diffraction experiments. A cover was placed over the HTK stage permitting passage of the x-ray beam, as well as two ports for controlled RH air to enter and exit the chamber. To generate the RH air, a dry nitrogen (N2) gas line was divided via a Y-tube connector, with one half remaining dry N2, the other half passing through H$_{2}$O. The RH was monitored in the humidity chamber using a Fisher Scientific digital hygrometer. To insure that samples would not curl during low-humidity data collection, each sample was held onto the platinum heating strip using double-sided cellophane tape. Diffraction results for quantitative strain analysis were obtained from the (222) AgI$_{0.03}Br_{0.97}$ lattice plane. Peak position and full width at half maximum (FWHM) were determined using a Split Pearson VII Function.
An observed shift in the diffraction peak position indicates that macrostrain is present. A shift to lower 2q (larger d-spacing) would indicate the presence of tensile stress while a shift to higher 2q (smaller d-spacing) would indicate the presence of compressive stress, perpendicular to the film plane. To determine the macrostrain present at a specified humidity, the following calculation was applied:
\begin{displaymath} e (\%) = ((d - d0)/d0)x100 \end{displaymath}
where e is the silver halide strain, d0 is the d-spacing (Å) for the unstrained AgI$_{0.03}Br_{0.97}$ (222) diffraction peak (1.6702Å) and d is the observed d-spacing (Å) for the AgI$_{0.03}Br_{0.97}$ (222) diffraction peak at a specified humidity.
The effect of compressive stress on silver halide grains dispersed in gelatin is illustrated in the figure below. As the RH was lowered to 0\%, the (222) diffraction peak for AgI$_{0.03}Br_{0.97}$ is observed to shift to higher 2q indicating that additional compressive stresses are present perpendicular to the sample plane due to the further drying of the gelatin. At 0\% RH a strain level of -0.77\% is observed in this film. The diffraction peak FWHM is also observed to increase with decreasing RH consistent with the formation of dislocations in the silver halide grain. When a critical strain level is achieved, a large population of dislocations can be created in silver halides, resulting in an increase in photographic density after film processing. For the photographic film described here, this critical strain level was found to be -0.5\% which was obtained at ~10\% RH. Therefore, to avoid undesired photographic density due to drying stress, this film should not be exposed to RHs less than 10\%.
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