Polyethylene oxide mixtures with thymol: Unusual thermal behavior and morphology due to a crystallizable hydrogen-bonded complex

Abstract

Formation of cocrystals in polymer-containing systems attracts a lot of attention lately due to their promise in pharmaceutics, being a poorly studied phenomenon in polymer science. For the first time, we report on the cocrystallization in mixtures of polyethylene oxide and thymol and study in detail the structure and thermal behavior of the mixtures by differential scanning calorimetry, optical microscopy, wide angle X-ray scattering, attenuated total reflection Fourier-transformed infrared spectroscopy, and quantum chemical simulation. The results are summarized into the experimental phase diagram that describes well all the transformations of the systems during their heating and cooling. On the molecular level, we conclude the existence of noncovalent-bonded complexes composed of three monomeric units of polyethylene oxide and two molecules of thymol, which might become structural units of cocrystals upon cooling. This results in a growth of huge centimeter-sized spherulites in the stoichiometric mixture and to the coexistence of two types of spherulites in the mixtures of other compositions. The peculiarities of thymol release from the complexes into different media are also investigated.