The phase separation behavior of poly(ethylene oxide) (PEO) in ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was investigated by rheological, optical microscopy, FT-IR and DSC measurements. It is demonstrated that specific interactions, particularly the hydrogen bonding between PEO and the ionic liquids as evidenced by FT-IR, in which a subtle but apparent absorption peak shift near the phase transition appears, account for the unusual low critical solution temperature (LCST) phase separation. Unlike the typical trend in which the storage modulus G' simply increases with temperature near the phase boundary for polymer blends without specific interaction, in our study, a novel "V-shaped" rheological response is observed, namely a dip in G' followed by an upturn, especially at low PEO concentration (<50 wt%). The magnitude of the "V" dip has heating rate and frequency dependences, while Tr (the phase transition temperature) is almost unchanged with heating rate and frequency. Upon increasing the alkyl chain length on the imidazolium ring from an ethyl to a butyl, the "V-shape" becomes more prominent and shifts to higher temperature, which is consistent with the results of FT-IR and DSC, evidently due to the stronger hydrogen bonding interaction between PEO and [BMIM][BF4] than [EMIM][BF4]. This unusual "V" dip might be tentatively ascribed to the coupling effects of the breaking of the "hydrogen bonding cage" formed between PEO chains and IL molecules and dissolution of the heterogeneous clusters as verified by FT-IR and TEM, respectively, and the following upturn is dominated by the interface formation upon phase separation.
In traditional polymer blends or solutions, the phase separation behavior could be successfully detected by scattering technique, [5b,7] rheological measurements, [8] and optical microscopy (OM). [9] In some polymer aqueous solutions or polymer/ILs mixtures where strong specific interactions are dominated, a number of experimental studies have proved that differential scanning calorimetry (DSC) is a useful tool to clarify the phase transitions. For instance, in poly(N-isopropylacrylamide) (PNIPAM) aqueous solution with a LCST phase diagram, [10] an apparent endothermic peak was observed when heating to the phase transition temperature due to the hydrogen bonds disruption. [11] Addition of various ILs with different hydrophilic and/or hydrophobic forces in PNIPAM aqueous solution could change the endothermic thermogram. [12] For polymer/IL blends, i.e., PEO/[EMIM] [BF 4 ] as reported in the previous investigations, [13,14] the similar phenomenon was observed upon phase separation and interpreted by hydrogen bonds breaking. However, in these limited publications, it was only demonstrated that the hydrogen bonding as a main driving force contributes to the particular phase separation behavior in polymer/IL mixtures, but little attention has been paid on the recovery or reversibility behavior during phase transitions. This in our opinion has great significance from either theoretical or application viewpoint, because the in-depth study could not only help to elucidate the physical origin of the unusual phase transition behavior in PEO/ILs system, but also provide useful guidance for other specific interaction dominated systems. In these systems, the phase state is a key point to control the microstructure evolution and physical properties of materials like iongels in batteries and membranes for drug delivery.Inspired by the heat response of swell and de-swell processes in gels [15] and phase transition in polymer blends or solutions with strong specific interactions, [10] in this work, the phase separation and recovery behaviors in PEO/[EMIM] [BF 4 ] were investigated by DSC, optical microscopy, and Fourier-transform infrared (FT-IR) measurements. The interesting endothermic and exothermic responses corresponding to, respectively, the phase separation and recovery were observed for the first time in polymer/IL mixtures. Three heating-cooling treatment cycles near the phase boundary were conducted. It is found that the first cycle is partially recoverable, whereas the next two (second and third) cycles are fully recoverable. Below the critical concentration (60 wt% PEO), the distinct irreversible phase transition behavior with a large heat loss
Morphology and crystallization behavior of poly(E-caprolactone) (PCL) in its 80/20 blends with poly(styrene-co-acrylonitrile) (SAN) containing hydrophobic or hydrophilic nanosilica was investigated. It was found that hydrophilic nanosilica displayed a more significant refinement effect on co-continuous morphology of PCL/SAN blends than hydrophobic nanosilica for its selective distribution within the PCL matrix but closer to the two-phase interface. Ring-banded spherulites were observed in both kinds of nanosilica-filled blends, the periodic distance of which decreased with increasing nanosilica content. Hydrophilic nanosilica reduced the dependence of the periodic distance of ring-banded spherulites on the crystallization temperature more efficiently than hydrophobic nanosilica. Furthermore, crystallization process of PCL/SAN blends filled with hydrophobic nanosilica was suppressed as the restriction effect of nanosilica on the crystal growth always outweighed their heterogeneous nucleation effect. In contrast, low content of hydrophilic nanosilica ( 1 wt %) were more likely to exhibit growth restriction effect rather than nucleation effect, whereas heterogeneous nucleation effect of higher content of hydrophilic nanosilica (>1 wt %) dominated over growth restriction effect on facilitating the crystallization behavior.
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