Polymorphism of POP and SOS. I. occurrence and polymorphic transformation

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Abstract
The polymorphic modifications of POP and SOS were identified with X‐ray diffraction (XRD), DSC and Raman spectroscopy by using pure samples (99.9%). In POP, six polymorphs, α,γ, pseudo‐β′2, pseudo‐β′1, β′2 and β′1, were obtained, whereas five polymorphs, α, γ, pseudo‐β′, β2 and β1, were isolated in SOS. Thermodynamic stability increased from α to β1 straightforwardly both in POP and SOS, because the polymorphic transformation went monotropically in the order described above. Additionally, the 99.2% sample of POP crystallized another form, δ, but the 99.9% sample did not, implying subtle influences of the impurity. The four forms, α, γ, β2 and β1, of POP, revealed XRD and DSC patterns identical to the four forms of SOS designated by the same symbols. The chain length structure was double inα and triple in the other three forms in both POP and SOS. Peculiarity of POP was revealed partly in the chain length structure of pseudo‐β′2 and pseudo‐β′1 which were double, whereas pseudo‐β′ of SOS was triple. This apparently showed contrast to the facts that the three forms revealed rather similar XRD short spacing patterns. Another peculiarity of POP was revealed in enthalpy value of the melt crystallization of α: ΔHc (α) = 68.1 kJ/mol which was much larger than that of SOS (47.7 kJ/mol), and also than AOA and BOB. These peculiarities mean that the double chain length structures of POP are more stabilized than the others. Raman bands of CH2 scissoring mode of SOS indicated parallel packing in γ, β2 and β1, and orthorhombic perpendicular packing in pseudo‐β′. The polymorphic transformation mechanisms were discussed based on the proposed polymorphic structure models.