Thermal Stability and Phase Composition of Stratifying Polymer Solutions in Small-Volume Droplets
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 4, July, 2020
THERMAL STABILITY AND PHASE COMPOSITION OF STRATIFYING POLYMER SOLUTIONS IN SMALL-VOLUME DROPLETS A. V. Shishulin and V. B. Fedoseev
UDC 678.01:53
Within the framework of the thermodynamic approach the characteristic features of the phase equilibria and of thermal stability of liquid polymer solutions in small-volume systems are considered. Using as an example a stratifying solution of oligomer fractions of polybutadiene and polystyrene in coarsely disperse droplets of different volumes with core–shell configuration, the regions of thermal stability of various heterogeneous states of the core– shell structure that differ by the compositions of core phases and of homogeneous state are obtained . It is shown that these regions of temperatures depend substantially on the droplet volume, with the compositions of the coexisting phases in different states of the core–shell structure being different. A decrease in the droplet volume is accompanied by a substantial change of the mutual solubilities of components, as well as by a decrease of the upper critical temperature of solubility and by the expansion of the temperature region, in which solutions of any composition up to the equimolar one are dynamically stable. Keywords: disperse systems, dimensional effect, solutions of polymers, phase transformations, solution stratification, metastable states. Introduction. The physicochemical properties of disperse systems containing droplets of liquid solutions as a disperse phase are the object of increased inertest among researchers [1, 2] and represent the subject of recently intensively developing micro- and nanofluidics [3]. The considerable interest in such systems is motivated by the intense development of liquid-phase technologies of obtaining nanomaterials [4–6] including a complex of spray-technologies [7, 8], additive technologies with participation of a liquid phase [9, 10], as well as by a wide collection of technical applications of heterogeneous structures containing droplets of liquid solutions in micro- and nanodimensional pores of a solid matrix — from highly efficient heatinsulating materials [11], materials-models of biological tissues [12], and membrane micro- and nanoreactors [13] to armored packets with elevated energy-absorption capacity [14] and materials of gigahertzian photonics [15]. Description of the properties of liquid solutions in small-volume systems with phase transformations is complicated by a number of characteristic features. In isolated droplets or particles these features manifest themselves in a substantial change in the number, composition, volume, and thermal stability of phases coexisting in the system, as well as of temperatures of phase transitions depending on the system′s volume [16–21], form of all interfaces [22–26], and on a number of other factors, whereas in ensembles of droplets or particles that form a disperse phase, they manifest themselves in the formation of size distributions of a complex st
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