The structure changes accompanying phase transitions in poly(diethylsiloxane) (PDES) have been studied by WAXS and SAXS techniques using oriented and isotropic samples. PDES may exist in two low‐temperature modifications (the monoclinic α1‐form and presumably the “tetragonal” β1‐form) and two high‐temperature modifications (the monoclinic α2‐form and the “tetragonal” β2‐form). In linear PDES the crystal ‐ crystal transitions α1–α2 and β1–β2 occur near 214 and 206 K, respectively. At higher temperatures α2 (280 K) and β2 (290 K) forms transform into the mesomorphic phase αm that gradually melts at 280–300 K giving an amorphous phase. According to x‐ray and density data, αm phase is also characterized by monoclinic structure slightly different from hexagonal packing.

No abstract

Comparative CD and X-ray diffraction studies of DNA compact particules which were obtained in PEG-containing water-salt solutions, have been carried out. Compact particles, formed from native DNA, produce a psi CD spectrum (characterized by a negative band at lambda-270 nm) and a small-angle X-ray diffraction pattern, which shows two reflections: I at 34-40 A and II at 80-90 A (together with its second-order reflection). Compact particules, formed from DNA molecules with partially disordered secondary structure, do not produce the psi CD spectrum and the reflection I, while the reflection II remains unchanged. It is suggested that the spacing of 34-40 A is associated with a side-by-side packing of DNA fragments in "microcrystallization' regions in compact particules and that such "microcrystallization' accounts for the generation of the psi CD spectrum.

An unusual transition from the mesomorphic state to the amorphous state has been found on cooling some poly(dialkoxyphosphazenes). This transition is proved by data obtained from thermomechanical analysis and optical and X-ray investigations. A generalized phase diagram is constructed for these polymers showing the influence of the alkoxy side group length on their glass transition temperatures, crystallization, and ability to form mesophases. The competition of interactions of the polar inorganic main chains and the side methylene chains has been proposed to be an important factor which controls the phase behavior of poly(dialkoxyphosphazenes).

No abstract