This process is experimental and the keywords may be updated as the learning algorithm improves. These keywords were added by machine and not by the authors. Here we will be concerned with amorphous polymers in which the X-ray scattering is observed in Laue photographs as a diffuse amorphous halo similar in general features to that obtained from other amorphous structures, such as gases, liquids and inorganic glasses. Miller) however, crystalline polymers are beyond the scope of this review. The application of X-ray diffraction methods to polymers has been the subject of a textbook 3 which deals with both theoretical interpretations and experimental methods and contains a listing of the crystallographic data available for about 270 polymers (up to 1968 in an Appendix compiled by R. A very thorough treatment was given by Hosemann and Bagchi 2 with a preface which contains an interesting short critical review of the historical development of X-ray diffraction studies. The general theory of X-ray diffraction by matter has been treated in many texts and an up-to-date account is that of Warren 1 which has sections dealing with the application of X-ray methods to amorphous materials. However, since phase separation has occurred, the solid nanosuspensions would be expected to exhibit a greater tendency for physical instability under a given stress, that is, crystallization, than would a miscible system.The purpose of the present chapter is to review the contribution of X-ray diffraction studies to the elucidation of the structure of glassy amorphous polymers. Such systems would be expected to have properties intermediate to those observed for miscible and macroscopically phase separated amorphous dispersions. Since DSC can not detect two T(g) values when phase separation produces amorphous domains with sizes less than approximately 30 nm, it is concluded that the trehalose-dextran system is a phase separated mixture with a structure equivalent to a solid nanosuspension having nanosize domains. In the case of the trehalose-dextran mixture, where only one T(g) value was detected, however, PDF analysis clearly revealed phase separation. From the PDF analysis, indomethacin-PVP was shown to be completely miscible in agreement with the single T(g) value measured for the mixture. In agreement with DSC measurements that detected two independent T(g) values for the dextran-PVP mixture, the PDF profiles of the mixture matched very well indicating a phase separated system. A lack of agreement of the PDF profiles indicates that the mixture with a unique PDF is miscible. Immiscibility is detected when the PDF profiles of each individual component taken in proportion to their compositions in the mixture agree with the PDF of the mixture, indicating phase separation into independent amorphous phases. The mixtures chosen were: dextran-poly(vinylpyrrolidone) (PVP) and trehalose-dextran, both prepared by lyophilization and indomethacin-PVP, prepared by evaporation from organic solvent. Recognizing limitations with the standard method of determining whether an amorphous API-polymer mixture is miscible based on the number of glass transition temperatures (T(g)) using differential scanning calorimetry (DSC) measurements, we have developed an X-ray powder diffraction (XRPD) method coupled with computation of pair distribution functions (PDF), to more fully assess miscibility in such systems.
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