Lower Critical Solution Temperature Phase Behavior Research Articles

Unraveling the interplay of temperature with molecular aggregation and miscibility in TEA-water mixtures.

In the phase diagram of binary liquid mixtures, a miscibility gap is found with the concomitant liquid-liquid phase separation, wherein temperature is a key parameter in modulating the phase behavior. This includes critical temperatures such as the lower critical solution temperature (LCST) and upper critical solution temperature (UCST). Using a comprehensive approach including molecular dynamics (MD) simulation, graph theoretical analysis and spatial inhomogeneity measurement in an LCST-type mixture, we attempt to establish the relationship between the molecular aggregation pattern and phase behavior in TEA-water mixtures. At lower temperatures of binary liquid mixtures, TEA molecules tend to aggregate while simultaneously interacting with water forming a homogeneous solution. As the temperature increases, these TEA aggregates tend to self-associate by minimizing the interaction with water, which facilitates formation of two distinct liquid phases in the binary liquid. The spatial distribution analysis also reveals that the TEA aggregates compatible with water promote uniform distribution of water molecules, maintaining a homogeneous solution, while the water-incompatible ones generate isolation of water H-bond aggregates, leading to liquid-liquid phase separation in the binary system. This current study on temperature-induced molecular aggregation behavior is anticipated to contribute to a critical understanding of the phase behavior in binary liquid mixtures, including UCST, LCST, and reentrant phase behavior.

Unusual Lower Critical Solution Temperature Phase Behavior of Poly(benzyl methacrylate) in a Pyrrolidinium-Based Ionic Liquid.

Polymer/ionic liquid systems are being increasingly explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Poly(benzyl methacrylate) in certain ionic liquids constitute unusual LCST systems, in that the second virial coefficient (A2) in dilute solutions has recently been shown to be positive, indicative of good solvent behavior, even above phase separation temperatures, where A2 < 0 is expected. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide for three different molecular weights (32, 63, and 76 kg/mol) in concentrated solutions (5–40% by weight). Turbidimetry measurements reveal a strong concentration dependence to the phase boundaries, yet the molecular weight is shown to have no influence. The critical compositions of these systems are not accessed, and must therefore lie above 40 wt% polymer, far from the values (ca. 10%) anticipated by Flory-Huggins theory. The proximity of the experimental cloud point to the coexistence curve (binodal) and the thermo-reversibility of the phase transitions, are also confirmed at various heating and cooling rates.

Controlling the Chain Folding for the Synthesis of Single-Chain Polymer Nanoparticles Using Thermoresponsive Polymers

Synthetic polymer single-chain nanoparticles (SCNPs) are an emerging new class of nanomaterials that possess similar folded structures as natural proteins. However, most SCNPs reported so far are p...

UV-Triggered On-Demand Temperature-Responsive Reversible and Irreversible Gelation of Cellulose Nanocrystals.

We show ionically cross-linked, temperature-responsive reversible or irreversible hydrogels of anionic cellulose nanocrystals (CNCs) and methacrylate terpolymers by mixing them homogeneously in the initially charge-neutral state of the polymer, which was subsequently switched to be cationic by cleaving side groups by UV irradiation. The polymer is a random terpolymer poly(di(ethylene glycol) methyl ether methacrylate)-rnd-poly(oligo(ethylene glycol) methyl ether methacrylate)-rnd-poly(2-((2-nitrobenzyl)oxycarbonyl)aminoethyl methacrylate), that is, PDEGMA-rnd-POEGMA-rnd-PNBOCAEMA. The PDEGMA and POEGMA repeating units lead to a lower critical solution temperature (LCST) behavior. Initially, homogeneous aqueous mixtures are obtained with CNCs, and no gelation is observed even upon heating to 60 °C. However, upon UV irradiation, the NBOCAEMAs are transformed to cationic 2-aminoethyl methacrylate (AEMA) groups, as 2-nitrobenzaldehyde moieties are cleaved. The resulting mixtures of anionic CNC and cationic PDEGMA-rnd-POEGMA-rnd-PAEMA show gelation for sufficiently high polymer fractions upon heating to 60 °C due to the interplay of ionic interactions and LCST. For short heating times, the gelation is thermoreversible, whereas for long enough heating times, irreversible gels can be obtained, indicating importance of kinetic aspects. The ionic nature of the cross-linking is directly shown by adding NaCl, which leads to gel melting. In conclusion, the optical triggering of the polymer ionic interactions in combination with its LCST phase behavior allows a new way for ionic nanocellulose hydrogel assemblies.

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

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.

The effect of light penetration depth on the LCST phase behavior of a thermo‐ and photoresponsive statistical copolymer in an ionic liquid

ABSTRACTA reflection cloud point technique allows for rapid screening of light‐dependent phase separation temperatures of thermo‐ and photoresponsive polymer/ionic liquid solutions as a function of sample thickness, molecular weight, and copolymer composition. We systematically investigate the lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate‐stat‐(4‐phenylazophenyl methacrylate)). Under UV light, the photoresponsive azobenzene‐based repeat unit becomes more polar as the cis form dominates, increasing its solubility in the ionic liquids 1‐ethyl‐3‐methyl imidazolium and 1‐butyl‐3‐methyl imidazolium bis(trifluoromethanesulfonyl)imide. This light‐dependent polarity change leads to two phase separation temperatures, depending on the illumination wavelength. Under visible light, which drives the azobenzene moiety into the trans ground state, the LCST shows no sample thickness dependence. Under UV light, however, sample thickness plays a significant role. Samples of around 1 mm thickness show no apparent difference under UV and visible light, whereas thinner samples show an increasing difference between the phase separation temperatures with decreasing sample thickness. Neither phase separation temperature exhibits a significant dependence on molecular weight. Increasing the photoresponsive monomer content did not lead to an increase in the difference between the phase separation temperatures at fixed thickness, due to a concomitant increase in UV light absorbed at the sample surface. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 281–287

Thermosensitive Phase Behavior of Benzo-21-crown-7 and Its Derivatives.

For designing water-soluble responsive materials, utilizing crown ethers as main building blocks has been rarely explored in contrast to their linear poly(ethylene glycol) counterparts. In the current study, we report the robust thermoresponsive properties of the benzo-21-crown-7 (B21C7) family with lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behavior. Different substituent groups on the benzene ring exhibit significant effects on water solubility and thermoresponsiveness. B21C7 and its cyano derivative display LCST phenomena, while B21C7-based carboxylic acid derivative presents UCST followed by LCST phase behavior. Supramolecular interactions with KCl provide an additional tuning approach for this crown ether system. These results demonstrate that B21C7s can serve as an easily accessible toolbox to develop new thermosensitive systems and prepare thermally responsive materials.

Phase Behavior and Self-Assembly of a New Family of Stimulus Responsive Peptide Polymers

Elastin like polypeptides (ELPs), composed of repeats of VPGXG pentapeptides that recur in all tropoelastin sequences, are the best studied class of peptide polymers that exhibit lower critical solution temperature (LCST) phase behavior in water, and these polymers have enabled innovative approaches to nanoparticle self-assembly, cancer therapy, regenerative medicine and protein purification These prototypical ELPs, show aqueous coacervation behavior above their cloud point, leading to the formation of a polymer rich phase and a second, largely aqueous phase. To investigate the origins of coacervation in these systems, we asked the question, how is LCST phase behavior encoded in peptide polymers at their amino acid —primary— sequence level? To understand the sequence determinants of LCST phase behavior, we used bioinformatics to guide the synthesis of a large family of peptide polymers that are predicted to exhibit LCST phase behavior. Analysis of the LCST phase behavior of these polymers provides sequence heuristics to encode LCST phase behavior in intrinsically disordered peptide polymers and enables the design of polymers that encode two orthogonal functions —phase behavior and bioactivity— seamlessly at the primary sequence level. These studies also identified polymers that display tunable degrees of thermal hysteresis in their LCST phase behavior, a property that is exploited for the design of thermal “shape memory” nanoparticles.

Phase Behavior and Self-Assembly of Perfectly Sequence-Defined and Monodisperse Multiblock Copolypeptides.

This paper investigates how the properties of multiblock copolypeptides can be tuned by their block architecture, defined by the size and distribution of blocks along the polymer chain. These parameters were explored by the precise, genetically encoded synthesis of recombinant elastin-like polypeptides (ELPs). A family of ELPs was synthesized in which the composition and length were conserved while the block length and distribution were varied, thus creating 11 ELPs with unique block architectures. To our knowledge, these polymers are unprecedented in their intricately and precisely varied architectures. ELPs exhibit lower critical solution temperature (LCST) behavior and micellar self-assembly, both of which impart easily measured physicochemical properties to the copolymers, providing insight into polymer hydrophobicity and self-assembly into higher order structures, as a function of solution temperature. Even subtle variation in block architecture changed the LCST phase behavior and morphology of these ELPs, measured by their temperature-triggered phase transition and nanoscale self-assembly. Size and morphology of polypeptide micelles could be tuned solely by controlling the block architecture, thus demonstrating that when sequence can be precisely controlled, nanoscale self-assembly of polypeptides can be modulated by block architecture.

From Macromolecular to Small‐Molecular Triggers: Facile Method toward Photoinduced LCST Phase Behavior of Thermoresponsive Polymers in Mixed Ionic Liquids Containing an Azobenzene Moiety

Instead of the reported photoinduced lower critical solution temperature (LCST) phase transition behavior in ionic liquids (ILs) achieved by photofunctional polymers, this study reports the facile photoinduced LCST phase behavior of nonfunctionalized polymers (poly(benzyl methacrylate) (PBnMA) and poly(2-phenylethyl methacrylate) (PPhEtMA)) in mixed ILs (1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)amide; [C1 mim][NTf2 ] and a newly designed functionalized IL containing an azobenzene moiety (1-butyl-3-(4-phenylazobenzyl)imidazolium bis(trifluoromethanesulfonyl)amide; [Azo][NTf2 ])) as a small-molecular photo trigger. Interestingly, the length of the alkyl spacer between the ester and aryl groups, which is the only structural difference between the two polymers, leads to two different photoresponsive LCST phase transition behaviors. On the basis of spectroscopic studies, the different phase transition behaviors of PBnMA and PPhEtMA may attribute to the different cooperative interactions between the polymers and [C1 mim][NTf2 ].

Kinetics of liquid-liquid phase separation in protein solutions exhibiting LCST phase behavior studied by time-resolved USAXS and VSANS.

We study the kinetics of the liquid-liquid phase separation (LLPS) and its arrest in protein solutions exhibiting a lower critical solution temperature (LCST) phase behavior using the combination of ultra-small angle X-ray scattering (USAXS) and very-small angle neutron scattering (VSANS). We employ a previously established model system consisting of bovine serum albumin (BSA) solutions with YCl3. We follow the phase transition from sub-second to 104 s upon an off-critical temperature jump. After a temperature jump, the USAXS profiles exhibit a peak that grows in intensity and shifts to lower q values with time. Below 45 °C, the characteristic length scale (ξ) obtained from this scattering peak increases with time with a power of about 1/3 for different sample compositions. This is in good agreement with the theoretical prediction for the intermediate stage of spinodal decomposition where the growth is driven by interface tension. Above 45 °C, ξ follows initially the 1/3 power law growth, then undergoes a significant slowdown, and an arrested state is reached below the denaturation temperature of the protein. This growth kinetics may indicate that the final composition of the protein-rich phase is located close to the high density branch of the LLPS binodal when a kinetically arrested state is reached.

Molecular description of the LCST behavior of an elastin-like polypeptide.

Elastin-like polypeptides (ELPs) with the repeat sequence of VPGVG are widely used as a model system for investigation of lower critical solution temperature (LCST) transition behavior. In this paper, the effect of temperature on the structure, dynamics and association of (VPGVG)18 in aqueous solution is investigated using atomistic molecular dynamics simulations. Our simulations show that as the temperature increases the ELP backbones undergo gradual conformational changes, which are attributed to the formation of more ordered secondary structures such as β-strands. In addition, increasing temperature changes the hydrophobicity of the ELP by exposure of hydrophobic valine-side chains to the solvent and hiding of proline residues. Based on our simulations, we conclude that the transition behavior of (VPGVG)18 can be attributed to a combination of thermal disruption of the water network that surrounds the polypeptide, reduction of solvent accessible surface area of the polypeptide, and increase in its hydrophobicity. Simulations of the association of two (VPGVG)18 molecules demonstrated that the observed gradual changes in the structural properties of the single polypeptide chain are enough to cause the aggregation of polypeptides above the LCST. These results lead us to propose that the LCST phase behavior of poly(VPGVG) is a collective phenomenon that originates from the correlated gradual changes in single polypeptide structure and the abrupt change in properties of hydration water around the peptide and is a result of a competition between peptide-peptide and peptide-water interactions. This is a computational study of an important intrinsically disordered peptide system that provides an atomic-level description of structural features and interactions that are relevant in the LCST phase behavior.

Controlling Electrochemical Properties with the LCST Phase Behavior of Homopolymers in Ionic Liquids

In recent years, remarkable progress in the development of batteries, supercapacitors and Li-ion technology has led to devices that deliver new records in power and energy density. The thermal instability in batteries, particularly lithium ion, creates a major roadblock to implementing these devices into large scale systems to support emerging, intermittent energy sources such as wind and solar. Efforts to mitigate safety issues such as flammability, reactivity, and thermal runaway typically involve the implementation of low conductivity solid-state materials or irreversible safety devices, which prove problematic for efficient, large-scale energy systems. Before high performance devices can be applied to large format energy storage applications, thermal safety must be improved. “Smart materials” or stimuli-responsive materials are those with properties responding in a desired manner to an external stimulus or environmental change. While several classes of responsive materials exist, soft materials, particularly synthetic polymers, represent an opportunity to design systems with unique functionality to achieve a wide variety of chemical and physical properties. Here we present two systems utilizing thermally-responsive polymers capable of reducing lithium conductivity or lithium charge transfer ability in non-aqueous electrolytes with increasing temperature. These proposed systems would create self-limiting lithium reactions within a battery should the temperature increase beyond a target, thus inhibiting thermal runaway. Li-ion systems utilizing polymer gel electrolytes are commercially available and ionic liquids (IL) have gained attention as potential replacements for traditional electrolytes (low volatility, non-flammability, high conductivity, and a wide potential window). Poly(ethylene oxide) (PEO) gels have long been combined with lithium salts to create polymeric electrolytes, which while safe, possess diminished conductivity. These PEO systems have recently been combined with ILs to boost performance while retaining the safety and mechanical stability of conventional polymer gel electrolytes. In our first proposed system, we combine current research efforts utilizing polymer gel electrolytes and ILs, both of which provide improved thermal stability, while making use of the recently discovered lower critical solution temperature (LCST) between PEO and certain ILs, to modulate electrolyte conductivity with temperature [1,2]. We design the polymer electrolyte to decrease conductivity during a phase transition at elevated temperatures. As the temperature cools, the initial conductivity is reversibly restored as the electrolyte returns to its initial state. Additionally, we show how the PEO/IL composition and the addition of lithium salts affect both the magnitude of change in conductivity and the LCST in these thermally-responsive polymer electrolyte systems. In our second proposed system, we utilize similar LCST phase behavior in IL systems by modifying electrode surfaces with responsive polymer brushes consisting of poly(methacrylates) [3-5]. Electrode surfaces were modified using the grafting from approach so that the polymer collapses on the surface of the electrode during a phase transition at elevated temperatures. In this collapsed form, the electrode surface is coated with an electrically and ionically insulating layer that prevents double layer formation in supercapacitors and charge transfer (or redox behavior) in Faradaic systems. As in the previous system, the initial electrochemical activity is reversibly restored as the temperature cools and the responsive brushes return to their solvated state. The experiments performed to date show tremendous promise for controlling battery or supercapacitor operation using temperature. Further development of responsive electrolytes and electrode surfaces will lead to numerous opportunities associated with thermal safety and “smart” electrochemical systems.H.-N. Lee and T. P. Lodge, J. Phys. Chem. Lett., 1, 1962 (2010).H.-N. Lee, N. Newell, Z. Bai and T. P. Lodge, Macromolecules, 45, 3627 (2012).T. Ueki and M. Watanabe, Langmuir, 23, 988 (2006).K. Kodama et al., Langmuir, 25, 3820 (2009).H.-N. Lee and T. P. Lodge, J. Phys. Chem. B, 115, 1971 (2011).

Thermoreversible Nanogel Shuttle between Ionic Liquid and Aqueous Phases

We describe a nanogel that can reversibly shuttle between a hydrophobic ionic liquid (IL) phase and an aqueous phase in response to temperature changes. A thermosensitive diblock copolymer, consisting of poly(ethylene oxide) (PEO) as the first segment and a random copolymer of N-isopropylacrylamide (NIPAm) and N-acryloyloxysuccinimide (NAS) as the second segment, was prepared as a nanogel precursor using anionic ring-opening polymerization of EO followed by reversible addition-fragmentation chain-transfer (RAFT) polymerization of NIPAm and NAS. After the micellization of the diblock copolymer in an aqueous solution upon heating to temperatures higher than the lower critical solution temperature (LCST) of the second segment, a coupling reaction of the NAS group of the P(NIPAm-r-NAS) core with ethylenediamine gave a nanogel with a well-solvated PEO corona. The nanogel exhibited contrasting thermosensitivities in the aqueous and IL phases. Dynamic light scattering measurements revealed that the nanogel exhibited LCST phase behavior (low-temperature-swollen/high-temperature-shrunken) in the aqueous phase and the opposite upper critical solution temperature (UCST) phase behavior (high-temperature-swollen/low-temperature-shrunken) in hydrophobic ILs. The nanogel favored the aqueous phase at low temperatures and the IL phase at high temperatures because of the solubility changes in the PEO corona. Upon increasing the temperature, the nanogel underwent a swollen-to-shrunken phase change in the aqueous phase, a transfer from the aqueous phase to the IL phase, and a shrunken-to-swollen phase change in the IL phase. These processes were thermally reversible, which made the round-trip shuttling of the nanogel between the aqueous and IL phases possible.

UCST and LCST phase behavior of poly(trimethylene ether) glycol in water

AbstractWe describe the initial studies of the complex aqueous phase behavior of poly(trimethylene ether) glycol (PO3G), a renewably sourced polyether glycol. Cloud point measurement revealed that a low molecular weight PO3G exhibits both lower critical solution temperature (LCST) and upper critical solution temperature (UCST) in water in the temperature range between 30°C and 80°C. At low concentrations of PO3G, the polymer solutions exhibit LCST‐type phase behavior. In the intermediate concentration ranges, PO3G and water are immiscible. However, at higher concentrations of PO3G, the solutions show UCST‐type phase behavior. In addition, both the LCSTs and UCSTs can be easily tuned over a wide range by varying the amount of alcohol co‐solvents. These findings have potential applications in the design of personal care applications and in the development of thermosensitive “smart” materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Structural aspects of the LCST phase behavior of poly(benzyl methacrylate) in room-temperature ionic liquid

The structure and lower critical solution temperature (LCST) phase behavior of well-defined poly(benzyl methacrylate) (PBnMA) solution using 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C2mim][NTf2] ionic liquid (IL) as a solvent have been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) at various temperatures. The SANS profiles observed for fully deuterated IL ([C2mim]-d11[NTf2]) containing PBnMA were kept practically unchanged in the temperature range between 298 and 363 K, while they suddenly changed at 363 K. This indicates that the LCST behavior of PBnMA–IL solution is a first-order phase transition, which is consistent with the DLS results. The SANS profiles below 363 K were well represented by the theoretical Debye scattering function with inter-molecular interaction and the radius of gyration, Rg was estimated to be almost constant, i.e., ∼45 Å. The SANS result obtained here was compared with those in aqueous PNIPAm solutions as a typical LCST system, and some differences between IL and aqueous solution systems are pointed out. It is found that thermodynamic quantities (ΔHdemix, ΔSdemix and ΔGdemix) from the homogeneous solution to the phase separation states strongly depend on the solvation of the PBnMA polymer by the IL ([C2mim] cation and [NTf2] anion). We propose an LCST phase separation mechanism in the polymer-IL solution.

Structural effects of polyethers and ionic liquids in their binary mixtures on lower critical solution temperature liquid-liquid phase separation

The solubility and phase behavior of five polyethers (poly(ethylene oxide), poly(glycidyl methyl ether), poly(ethyl glycidyl ether), poly(ethoxyethyl glycidyl ether) and poly(propylene oxide)) in 14 different room-temperature ionic liquids (ILs) were studied by changing the structures of polyethers and the cations and anions in the ILs. Certain combinations of a polyether and an IL binary mixture exhibited lower critical solution temperature (LCST) phase behavior. For ILs containing the same anions, the polyethers were highly soluble in imidazolium- or pyridinium-based ILs, whereas they were insoluble in ammonium- or phosphonium-based ILs. An increase in length of the alkyl chain in the imidazolium cation and an increase in polarity of the polyethers resulted in a higher LCST phase separation temperature, whereas substitution of the hydrogen atoms on the imidazolium ring by methyl groups resulted in a lower LCST phase separation temperature. The hydrogen bonding interaction between the oxygen atoms in the polyethers and the aromatic hydrogen atoms on the cations in the ILs had an important role in the LCST phase behavior of the mixtures. Miscibility of the mixtures was also affected by the Lewis basicity of the anions in the ILs. The solubility and phase behavior of five polyethers in 14 different room-temperature ionic liquids (ILs) were studied by changing the structures of polyethers and the cations and anions in the ILs. Certain combinations of a polyether and an IL binary mixture exhibited lower critical solution temperature (LCST) phase behavior. The phase separation temperature (cloud point) was revealed to be sensitive to the aromaticity and alkyl chain length of the IL cations, the anionic Lewis basicities of the ILs and the polarity of the polyethers. PEGE, poly(ethyl glycidyl ether).

Thermodynamic study on phase transitions of poly(benzyl methacrylate) in ionic liquid solvents

Abstract The lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate) (PBnMA) in room-temperature ionic liquids (ILs) was studied by considering the effect of the chemical structure of ILs on the phase separation temperature (T c). It was found that the LCST behavior of PBnMA was observed in ILs containing [NTf2] anions and PF6 anions. T c changed significantly with a small change in the chemical structures of the cations. High-sensitivity differential scanning calorimetry (DSC) was successfully performed for studying the LCST phase separation of PBnMA in two different imidazolium-based ILs. Endothermic peaks corresponding to the phase separation of PBnMA from the ILs were clearly observed at ca. 100 °C in the DSC thermograms. It was experimentally verified for the first time that the negative enthalpy and entropy change of mixing of PBnMA in ILs caused the LCST phase separations. The absolute values of the thermodynamic parameters for the phase transition of PBnMA in ILs obtained in this study were much lower than those reported in previous studies for aqueous polymer solutions that exhibit LCST phase behavior, such as poly(N-isopropylarylamide) and poly(vinyl methyl ether). Small changes in the thermodynamic parameters resulted in a large change in the phase separation temperature even by small changes in the chemical structure of the ILs and polymers. The microscopic desolvation process detected from the DSC measurements was inferred to have occurred before the macroscopic phase separation detected from turbidity measurements. The dependence of the endothermic peak temperatures on the DSC scan rate was observed even at slow scan rates. These results indicate that the phase separation of PBnMA from ILs is characterized by extremely slow kinetics.

Lower Critical Solution Temperature Phase Behavior of Natural Rubber/Polybutadiene Blend

The lower critical solution temperature (LCST) phase behavior of the blend of natural rubber (NR) with polybutadiene (PB) having 32.3% 1,2 unit was investigated by measuring T g through DSC after annealing the blend. NR was separated with toluene into branched polymer and crosslinked one as a soluble fraction (NR-sol) and gel fraction (NR-gel), respectively, followed by mixing with PB. Coexistence composition of NR-sol/PB and NR-gel/PB blends was determined by a method proposed in this work using DSC. The LCST of the NR-sol/PB blend was 328 K, being identical to the reported LCST of synthetic cis-1,4-polyisoprene/PB blend in our previous paper. The LCST of NR-gel/PB blend was quite similar to that of NR-sol/PB blend despite that NR-gel was crosslinked rubber (v = 1.5 x 10 -6 ). Molecular weight was proved to produce no effect on the LCST phase behavior for high molecular weight polymer-polymer blend, i.e., NR/PB blend.

LCST phase behavior according to the Simha-Somcynsky theory: application to the n-hexane polyethylene system

Expressions for the free energy of mixingδG, including equation of state contributions, have been previously derived by Jain and Simha. However, the chemical potentials and compositional derivatives required for the description of phase behavior were approximated by polynomial fits to the theoreticalδG for a particular case. Exact and general equations for a binary system are now derived. This opens the way for future analyses of phase equilibria, based on the Simha-Somcynsky (S-S) theory. The present application involves asimulation of the system n-hexane/polyethylene, previously treated by Jain and Simha. The analysis is extended now to a wide range of molar masses from 8 to 103 kg/mole and binodals as well as spinodals and critical conditions are computed over a wider range of pressures than previously. In the simulation the polymer samples are assumed to be monodispers. The phase boundaries differ significantly from the previous ones, as one should expect from the higher derivatives of polynomial fits. However, previous conclusions concerning an iso hole or free volume fraction difference between the constituents as a critical condition for the appearance of a lower critical solution temperature (LCST) stand. Moreover, and as an alternative, this condition can be expressed in terms of the excess free volume fraction hexc of the mixture. Master curves for binodals and spinodals can be generated for different pressures over a reasonable temperature distance from the critical point. Similarly, hexc at the binodal and spinodal condition can be represented by universal curves. Finally, we discuss the variation of the critical coordinates with chain length and conclude with an evaluation of the x-function for different compositions, temperature, and pressure.