1.Phase Transitions in Liquid Crystalline Solutions of Hydroxypropyl Cellulose under Deformation
E. V. Rusinova, S. A. Vshivkov, and M. S. Alekseeva. Polymer Science, Ser. B, 2007, Vol. 49, Nos. 1–2, pp. 26–29
During their service and fabrication, polymer systems are subjected to different deformation fields (shear, tension, compression, etc.). For example, application of directed mechanical fields is the main method for the orientation of polymers aimed to improve their mechanical properties. The molecular orientation created during deformation of solutions or melts and fixed by a phase transition has opened up opportunities for the manufacture of high-strength materials. Polymeric liquid-crystalline (LC) systems are of particular value in this respect. Owing to the ability of macromolecules to be readily oriented in external fields, these systems are widely used for fabrication of high-modulus fibers via the processing of LC solutions. In order to control these processes, it is necessary to know the phase diagrams of feedstock systems both under static and deformation conditions. Solutions of cellulose derivatives occupy a special place among LC systems [2, 3]. However, in spite of extensive research, the character of their phase diagrams has not been fully understood. Some fragments of phase diagrams were obtained for systems undisturbed by external action, although it is known that mechanical and electromagnetic fields exert an influence on the structure and phase transition temperature of liquid crystals. Data on the phase diagrams of LC systems under deformation are lacking. The aim of this investigation was to study LC phase transitions and the physical state of the hydroxypropyl cellulose (HPC)–DMAA system under static and dynamic conditions.
2.Phase Diagrams of a Hydroxypropyl Cellulose–Water System under Static Conditions and in the Shear Field
S. A. Vshivkov and E. V. Rusinova. Polymer Science, Ser. B, 2007, Vol. 49, Nos. 7–8, pp. 209–212.
Cellulose and its derivatives are semirigid polymers. The LC state of their melts and solutions was discovered and studied in the 1960s–1980s. Hydroxypropyl cellulose (HPC), which is soluble in both water and many organic solvents, is of great interest. HPC is widely used for the preparation of forms of dosage drugs and coatings. Aqueous solutions of HPC are characterized by strong electron–donor (hydrogen)
bonds. Moreover, water is a specific solvent having a very developed structure of hydrogen bonds that depends on the nature and concentration of solutes. Because of the presence of two mobile protons and two unshared electron pairs at its oxygen atom, a water molecule may play the role of both a donor and an acceptor of electrons and form four hydrogen bonds with an energy of 20 kJ/mol. Therefore, a loose open-work
structure with a large free volume is formed in water. The fraction of nonspecific interactions in water is as low as 7%. HPC–water intermolecular interactions are determined by both the hydrophilic hydration giving rise to hydrogen bonding between the polymer and the solvent and the hydrophobic hydration of water that consists in densification of water structure upon penetration of nonpolar molecules or their fragments into open-work cavities of the solvent. In the case of HPC, the nonpolar fragments are its methyl and methylene groups. As a result of hydrophobic hydration, intermolecular distances in water around the cavities shorten. This phenomenon is accompanied by the exothermic effect and negative values of dissolution entropy owing to additional structuring. The prevalence of one or another contribution depends on the concentration of the system and temperature.