Rheologically determined phase diagram of poly(ε‐caprolactone)/poly(styrene‐co‐acrylonitrile) blends: Role of ramp rate in dynamic measurements

Abstract

ABSTRACTThe phase behavior of poly(ε‐caprolactone) (PCL)/poly(styrene‐co‐acrylonitrile) (SAN) blends, with a lower critical solution temperature (LCST)‐type phase diagram over a virtual upper critical solution temperature (UCST) one, was investigated through thermal analysis and dynamic rheological measurements as a function of ramp rate. The LCST phase diagram was detected rheologically from the observed slope changes in the dynamic temperature ramps of storage modulus (G′). The determined phase transition points along with the spinodal temperatures, which are estimated based on the theoretical approach of Ajji and Choplin's mean field theory, shift to higher temperatures by reducing the ramp rate. The detected shifts show a composition dependency. Far away from the critical point, the phase transition temperatures of PCL/SAN blends change more noticeably, which originates from the smaller magnitude of concentration fluctuations in the metastable region and the stronger competition between the phase dissolution and cooling rate. The Flory–Huggins interaction parameter (χ) was appropriately adjusted into the LCST phase boundary as a function of temperature and composition. The results confirmed that the adjusted χ has higher temperature dependency at larger ramp rate of the dynamic measurements. The experimentally combined LCST and UCST phase behavior were also verified by the compressible regular solution model. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47750.