Multivariate Analysis of DSC-XRD Simultaneous Measurement Data: a Study of Multistage Crystalline Structure Changes in a Linear Poly(ethylene imine) Thin Film
A multivariate analytical technique has been applied to analysis of simultaneous measurements data of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) for studying thermal changes in crystalline structure of a linear poly(ethylene imine) (LPEI). A lot of XRD patterns generated from the simultaneous measurements were subjected to an augmented alternative least-squares (ALS) regression analysis, and the XRD patterns were readily decomposed into chemically independent XRD patterns and their thermal profiles were also obtained at the same time. The decomposed XRD patterns and the profiles were useful to discuss the minute peaks in the DSC. The analytical results revealed changes of polymorphisms in detail as the followings: An LPEI film prepared by casting an aqueous solution was comprised of sesquihydrate and hemihydrate crystals. The sesquihydrate one was lost at an early stage of heating, and the film changed into an amorphous state. Once the sesquihydrate was lost by heating, it was not recovered even when it was cooled back to room temperature. When the sample was heated again, structural changes were found between the hemihydrate and the amorphous components. In this manner, the simultaneous DSC-XRD measurements combined with ALS analysis proved to be powerful for better understanding of the thermally induced changes of the crystalline structure in a polymer film.
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