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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).

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The structure and phase transitions in poly[bis‐(2,2,3,3‐tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x‐ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis‐trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.

Polydialkoxyphosphazenes with different substituents varying from methoxy up to octoxy which contain only minor amounts (below 0,l mol-%) of various defective units in their macromolecules were synthesized. It was shown that such polyphosphazenes with propoxy, butoxy and pentoxy side groups can exist in the mesomorphic state and that a defect content as low as about 2 mol-% prevents mesophase formation. In spite of high flexibility of the main chain, the polydialkoxyphosphazenes were found to gain the ability to crystallize only if the number of C atoms in the alkoxy substituents is higher than six.