Critical Solution Temperature Behavior Research Articles

Thermodynamic Assessment of the Partitioning of Acetone between Supercritical CO2 and Polystyrene Using the Polar PC-SAFT Equation of State

Supercritical carbon dioxide (scCO2) has gained considerable attention in the process industry due to its favorable economic, environmental, and technical characteristics. Polymer processing is one of the key industrial applications where scCO2 plays an important role. In order to be able to efficiently design the polymer processing equipment, understanding the phase behavior and partition of solutes between scCO2 and polymers is necessary. This paper investigates the partitioning of acetone – a conventional polar cosolvent – between scCO2 and polystyrene – a glassy polymer. We highlight the importance of taking into account the polar interactions between acetone molecules and their role in the polymer phase behavior. The system is modeled under a wide range of temperatures and pressures (278.15–518.2 K and 1.0–20.0 MPa) using the polar version of the perturbed chain statistical associating fluid theory (polar PC-SAFT) equation of state. The results show that at relatively low pressure, the system exhibits a vapor–liquid–liquid (VLL) three-phase region bounded by two two-phase regions (VL and LL). At high pressure, VLL and VL regions disappear and only the LL region remains. The temperature effect is more interesting, showing a transition of upper critical solution temperature behavior to lower critical solution temperature behavior at 10 MPa and 398.15 K. It is found that neglecting the polar term can lead to significant changes in the description of the polymeric-system phase behavior especially at lower temperatures. No such differences are observed at higher temperatures (above 500 K) where the effect of polar interaction is considerably weaker.

Regulating Dynamics of Polyether-Based Triblock Copolymer Hydrogels by End-Block Hydrophobicity

The relaxation dynamics of poly(ethyl glycidyl ether-co-isopropyl glycidyl ether-b-ethylene oxide-b-ethyl glycidyl ether-co-isopropyl glycidyl ether) triblock copolymer hydrogels were investigated as a function of the end-block hydrophobicity and temperature primarily using the oscillatory rheometer, which is crucial to understand the unique viscoelastic behaviors including injectability and self-healing property of the self-assembled hydrogels. Lower critical solution temperature behavior of the poly(alkyl glycidyl ether) end-blocks was harnessed by random copolymerization of poly(ethyl glycidyl ether) (EGE) and poly(isopropyl glycidyl ether) (iPGE), resulting in the remarkable temperature responsiveness of the hydrogel. As the fraction of hydrophilic monomer (i.e., EGE) increases, the sol-to-gel transition occurs at higher temperature and higher polymer concentration. The hydrogel relaxation measured by the oscillatory rheometer becomes faster with decreasing temperature and increasing fraction of hydrophilic monomers. In particular, we observed that a small increment of the hydrophilic monomer fraction significantly reduces the unfavorable interaction between the end-block and aqueous media, resulting in faster hydrogel relaxation dynamics. All polyether-based hydrogels showed biocompatibility and injectability, indicating promising soft materials for biological and biomedical applications. The results are discussed in terms of the current understanding of self-assembled triblock copolymer hydrogels, and particular attention is paid to the issue of chain dynamics.

Phase equilibria and surface tension in castor oil‐based polyols‐water–methanol mixture: Thermodynamic basis

AbstractConsidering the current environmental issues with petro‐polymers, castor oil is a promising green alternative to the polyols to be used as the main raw material for polyurethane. However, the basic information regarding the processability of castor oil‐based polyol (CP) such as its miscibility in different solvents is still elusive. Herein we have investigated the phase equilibria and surface tension of various CP solutions using thermo‐optical analysis and pendent drop method, respectively. The modified double lattice (MDL) model was employed to theoretically describe those phase equilibria and surface behaviors. The phase diagrams of CPs/methanol binary systems represent upper critical solution temperature (UCST) behaviors, and CPs/methanol/water ternary systems show Treybal Type 2 behaviors. The surface tension results exhibit preferential solvent adsorption, which means affinity of the solvent to be present in surface is higher than that of the polymer. The calculated results showed good agreement with the experimental observations.

Effect of Poly (vinyl methyl ether) on the Miscibility, Crystallization and Rheology of Poly(L-lactide)/Poly (methyl methacrylate) Blends

The effect of poly (vinyl methyl ether) (PVME) on the miscibility, crystallization and rheological behavior of a 50/50 poly(L-lactide) (PLLA)/poly (methyl methacrylate) (PMMA) blend was investigated. The PLLA/PMMA blend, with compositional symmetry, was immiscible at room temperature when prepared by solution casting. However, it became miscible when heated to a high temperature, indicating an upper critical solution temperature (UCST) behavior. With the addition of PVME, whose glass transition temperature was much lower than those of PMMA and PLLA, the thermodynamic miscibility was improved for the PLLA/PMMA/PVME blend, all components being miscible at a low temperature. The crystallization kinetics of the blends was studied by observing the PLLA spherulite growth with annealing time at 100 °C via polarized optical microscopy; the addition of PVME accelerated the crystallization kinetics of the PLLA. Rheological sweep results confirmed this conclusion. The reason for this acceleration effect on the crystallization was explored by etching the PVME in the PLLA/PMMA/PVME blends. The participation of PVME in the crystallization of PLLA played a crucial role in the crystallization process.

Temperature-Dependent Phase Behavior of the Thermoresponsive Polymer Poly(N-isopropylmethacrylamide) in an Aqueous Solution

Poly(N-isopropylmethacrylamide) (PNIPMAM) is a thermoresponsive polymer, exhibiting lower critical solution temperature (LCST) behavior in an aqueous solution. We investigate the temperature-depend...

A molecular brush with thermoresponsive poly(2-ethyl-2-oxazoline) side chains: a structural investigation

The thermoresponsive behavior of a poly(2-oxazoline)-based molecular brush is investigated in aqueous solution. The molecular brush under study, PiPOx100-g-PEtOx17, has a poly(2-isopropenyl-2-oxazoline) (PiPOx) backbone grafted with thermoresponsive poly(2-ethyl-2-oxazoline) (PEtOx) side chains. Since the backbone degree of polymerization is only a factor of ~ 6 higher than the ones of the side chains, it features an architecture between a star-like polymer and a comb-like polymer. Its aqueous solution exhibits lower critical solution temperature (LCST) behavior with a cloud point temperature Tcp = 40.5 °C at 30 g L−1. The temperature-dependent structural evolution is disclosed using dynamic light scattering (DLS) and small-angle neutron scattering (SANS). An increase of the molecular brush size is found upon heating from room temperature to Tcp, which is attributed to the extension of the backbone resulting from the dehydration and collapse of the side chains. Above Tcp, the size decreases again, which indicates the collapse of the whole molecular brush. Large aggregates are found to be present in the solution in the temperature range 25–50 °C. These become more compact, as the temperature is increased across Tcp.Graphical abstractTransmission and scattering intensity in relation to temperature and momentum transfer, respectively

Insights on the Lower Critical Solution Temperature Behavior of pNIPAM in an Applied Electric Field

Poly(N-isopropylacrylamide), or pNIPAM, is a free-radical polymer that is commonly studied for uses in surface coatings, tissue engineering, energy storage, biosensing, and more, due to its temperature responsiveness. pNIPAM is known to solubilize at temperatures below its lower critical solution temperature (LCST) and agglomerate above its LCST. This behavior has been shown to be reproducible and reversible. We confirmed this reversibility and the value of the LCST by performing dynamic light scattering (DLS) with a temperature sweep (increase and decrease). However, performing the same experiment under an applied voltage from copper electrodes, we observed a decrease in the LCST of pNIPAM and irreversible aggregation. Here we present preliminary data comparing the LCST behavior of pNIPAM in the presence of applied voltage using copper, aluminum, and carbon electrodes. We present data in support of the hypothesis that a phenomenon is occurring specifically with the use of copper electrodes that is altering pNIPAM LCST behavior.

Effect of different polymer molar mass on the phase behavior of carbon dioxide + dichloromethane + ε‒caprolactone + poly(ε‒caprolactone) system

The phase behavior of the system containing carbon dioxide + dichloromethane + ε-caprolactone + poly(ε-caprolactone) was investigated in order to provide fundamental information to understand the polymerization reaction in a high-pressure system. The experiments were performed using a variable-volume view cell over the temperature range from 323.15 to 353.15 K, different mass ratios of carbon dioxide: dichloromethane: [ε-caprolactone + poly(ε-caprolactone)] (1:0.5:1, 1:1:1, and 1:2:1), different mass average molar mass of poly(ε-caprolactone) (14,000 and 138,840 g∙mol−1), and different poly(ε-caprolactone) mass fractions (reaction conversion) from 0.0 up 15.0 wt%. Phase transitions of vapor-liquid bubble point type were verified and lower critical solution temperature (LCST) behavior phase in P-T diagrams was observed. The experimental results were modeled using the PC-SAFT EoS, providing a positive representation of the experimental phase equilibrium data.

Thermo/Anion Dual-Responsive Supramolecular Organoplatinum-Crown Ether Complex.

Stimuli-responsive complexes have attracted significant interest in supramolecular chemistry and material science. In this study, a new organoplatinum-crown supramolecular complex has been successfully constructed via the coordination of benzo-21-crown-7 (B21C7) units and cis-Pt(PEt3)2(OTf)2. The resulting complex displays intriguing lower critical solution temperature (LCST) and anion-sensitive deassembly behavior. This organoplatinum-crown ether complex with dual-responsive behavior is a promising candidate for stimuli-responsive materials.

Mechanistic Investigation of the Counterion-Induced UCST Behavior of Poly(N,N-dimethylaminoethyl methacrylate) Polymer Brushes

Because of its multiresponsive behavior upon variation of pH value, temperature, and ionic strength, poly­(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) is an attractive candidate for adaptive surface coatings, especially in the field of biomedical sensors. In the presented article, PDMAEMA Guiselin brushes (5–12 nm, dry thickness) were prepared by the grafting-to approach, and the thermoresponsive UCST-type (upper critical solution temperature) behavior of the generated films, induced by [Fe­(CN)6]3–, was demonstrated for the first time. To clarify the specific mechanism of the temperature responsiveness, the molecular interactions between polymer chains and complex ions were studied by in situ infrared spectroscopy and in situ ellipsometry, showing wavenumber shifts of the ν­(CN) band due to ion pairing as well as changes of the ν­(OH) band intensity due to swelling. Conclusively, experimental data suggest that the electrostatic interaction between the two components plays a significant role in the thermoresponsive behavior of PDMAEMA films. Using this knowledge, the UCST of the PDMAEMA Guiselin brushes could be adjusted between 34.0 ± 1.2 °C at pH 8 and 40.7 ± 2.0 °C at pH 5.

Amphiphilic Super-Wetting Ionic-Liquid-Based Lower Critical Solution Temperature System: Preparation, Characterization, and Excellent Dispersion Performance for Nanostructured Materials

Lower critical solution temperature (LCST) behavior is a well-known thermo-response and materials with LCST behavior are attractive in nanostructured material applications. However, traditional LCS...

PH and thermo-responsive hybrid hydrogels based on PNIPAAM and keratin

This study investigated PNIPAAM/keratin hybrids that were prepared by free radical polymerization of NIPAMM in the presence of functionalized keratin with vinyl group. Functionalized keratin presented functionality greater than 2, therefore, it acted as a crosslinker and a pH-responsive segment in the polymer network. Preparation occurred in an aqueous medium at room temperature or 60 °C, using H2O2/ascorbic acid and ammonium persulfate redox initiators, respectively. Polymerization at 60 °C led to phase segregation due to the lower critical solution temperature (LCST) behavior of the PNIPAAM aqueous solution. However, this phenomenon had little influence on the global composition of the hybrids, and the incorporation of functionalized keratin into the hybrids was slightly higher for those prepared at room temperature. Keratin had no influence on the critical temperature of the PNIPAAM/keratin hydrogels. However, the extent of hydrogel contraction at LCST was proportional to the keratin amount and functionalization degree. The charge balance of the polymeric network determined the swelling behavior in solutions at different pH. The mechanical properties of the hydrogels, evaluated by compression tests, showed that the resistance of the hydrogels was proportional to the keratin concentration and functionalization degree. The hydrogels presented a foam-like structure with interconnected pores, a configuration that allowed the transport and adsorption of Cu2+ from aqueous solutions. These characteristics make the hybrids and their hydrogels potential candidates as cation adsorbent materials.

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.

Nonionic Water-Soluble and Cytocompatible Poly(amide acrylate)s

We report secondary amide side-chain-bearing polyacrylates with tunable aqueous solubility and cellular interaction. Redox-initiated reversible addition-fragmentation chain transfer polymerization of methylamide ethyl acrylate (MAmEA) and ethylamide ethyl acrylate (EAmEA) yielded well-defined homopolymers. P(MAmEA) and P(EAmEA) possess high glass-transition temperatures compared to the equivalent alkylated polyacrylates. P(EAmEA) showed a degree of polymerization and concentration-dependent lower critical solution temperature (LCST) behavior, which can be fine-tuned over a biologically relevant range through copolymerization of both monomers. No cytotoxicity against NIH 3T3 and RAW 264.7 cell lines was observed. Detailed cellular interaction studies revealed cell associations for P(MAmEA) similar to poly(N-acryloylmorpholine), a well-known nonionic water-soluble polymer and colocalization of both homopolymers with lysosomes. Due to the simple access of these monomers, the aqueous solubility, cytocompatibility, and cellular interaction of the homopolymers, and the possibility to combine them with a large number of acrylates, they hold great promise to become suitable building blocks for biomedical materials.

Irregular solution thermodynamics of wood pulp in the superbase ionic liquid [m-TBDH][AcO

Knowledge of solution thermodynamics is fundamental for solution control and solvent selection processes. Herein, experimentally determined thermodynamic quantities for solutions of wood pulp (hardwood dissolving pulp, i.e. cellulose) in [m-TBDH][AcO] are presented. Model-free activities (ai,j) and associated mass fraction (wi,j) activity coefficients (Ωi,j), are determined to quantify inherent solution non-ideality. Access to the Gibbs energy of mixing, Gmix, in combination with associated partial molar thermodynamic quantities, reveal strong enthalpically favourable (exothermic) interactions due to solvent-j and solute-i contact-encounters. Onset of an entropy driven phase instability appears at increased temperatures as excess entropic contributions dominate solvation character of the irregular solutions formed.

Synthesis and characterization of thermo-responsive ionic liquids (TRILs)

Normally, an ionic liquids (ILs) is classified as hydrophilic or hydrophobic depending on its composition, whether it is miscible or immiscible with water. Nevertheless, this classification is ambiguous, since the miscibility of some ILs with water is strongly depending on temperature change. There are two classifications of thermo-responsive ionic liquids (TRILs) which are lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behavior types. The aim of this study is to synthesis and characterize tetrafloroborate-based (BF4 −) anion with different alkyl chain of imidazolium- based cations ([Emim], [Bmim], [Hmim], [Omim]) and trifloroacetate (CFOO−) anion with different cations (ammonium, anilinium, pyridinium and phosphonium). The prepared ILs were mixed with water (30:70 wt %) and the resulting solutions were stirred vigorously for certain period of time. These solutions were cooled to 00C and then gradually heated to 70 0C. When the solution was found to be phase-separated upon heating, this mixture undergoes the LCST- type phase transition but its contrary with UCST behavioral. The TRILs were characterized using Fourier Transform Infrared (FTIR) to identify the functional group and Nuclear Magnetic Resonance (NMR) in confirming the structure. Due to the special property of TRILs that became phase separation by changing the temperature, it can be recovered easily and reused for many recycles in wide range of application.

Shuttling Catalyst: Facilitating C−C Bond Formation via Cross‐Couplings with a Thermoresponsive Polymeric Ligand

AbstractA poly(ethylene glycol) (PEG) linked ortho‐MeO‐phenyldicyclohexylphosphine (MeO‐WePhos) ligand has been synthesized to promote Pd‐catalyzed carbon‐carbon bond formation by cross‐couplings including Sonogashira, Heck, Hiyama and Stille reactions, providing corresponding (hetero)aryl substituted alkynes, alkenes and bi(hetero)aryls in good to excellent isolated yields with low Pd loadings. Facilitated by the lower critical solution temperature behaviour of the polymeric monophosphine ligand, the metal‐complex could rapidly shuttle between organic and water phases as regulated by temperature, enabling highly efficient catalyst recycling via a simple phase separation. The chemical structure of ligand was determined by matrix‐assisted laser desorption/ionization‐time of flight mass spectrometry, nuclear magnetic resonance spectrometry and size‐exclusion chromatography measurements. Notably, as demonstrated by the inductively coupled plasma‐atomic emission spectrometry measurement, 98% Pd was kept in the water phase after 6 cycles of catalyst recycling experiments. Given the profound impact of transition‐metal‐catalyzed covalent bond formation and the increasing demand of sustainable chemistry, this work provides an alternative method to conduct cross‐couplings with a polymeric shuttling catalyst.

Assembly Pattern of Supramolecular Hydrogel Induced by Lower Critical Solution Temperature Behavior of Low-Molecular-Weight Gelator.

Although the gelation process and lower critical solution temperature (LCST) behavior are well acknowledged in polymer systems, low-molecular-weight gelators (LMWGs) rarely display LCST behavior during supramolecular gelation. Herein, we report an LMWG system with LCST-type thermoresponsiveness and an LCST-triggered supramolecular gelation process. Temperature plays a crucial role in this system, not only affecting the LCST phase separation but also triggering the gelation process. The backbones (three-dimensional structures) of the resulting hydrogel are the hierarchical assemblies of the LMWG undergoing the LCST phase separation. Hence, the gelation of the LMWG is only realized when the gelation temperature is above the critical transition temperature (Tcloud) of the LCST behavior, which is different from many supramolecular or polymeric hydrogel systems.

Swelling of multi-responsive spherical polyelectrolyte brushes across a wide range of grafting densities

Multi-stimulus responsive weak polyelectrolyte brushes were grafted by surface-initiated atom transfer radical polymerization from spherical silica nanoparticles across a wide range of grafting densities approaching the limit of close packing of grafting sites: a regime not previously explored in the brush swelling literature. Brushes consisted of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) homopolymers or poly(2-(dimethylamino)ethyl methacrylate)-r-poly(4-metharyloyloxyazobenzene) random copolymers P (DMAEMA-r-MOAB) and were subjected to pH-, temperature-, and photoisomerization-induced swelling and deswelling. Homopolymer brushes were prepared at grafting densities ranging from 0.12 to 1.4 nm−2 and number average degrees of polymerization ranging from 300 to 1200. Two copolymer brushes with 8 mol% and 13 mol% of the nonionizable, photoresponsive MOAB units were compared with homopolymer brushes prepared at similar grafting densities and degrees of polymerization. Dynamic light scattering was used to measure the hydrodynamic radius of grafted nanoparticles, and thus brush thickness, in water. Brush swelling in response to increased DMAEMA protonation followed a single scaling behavior across the full range of degrees of polymerization and grafting densities. Incorporating azobenzene-containing MOAB comonomers introduced a modest photoresponsive control of brush swelling and decreased the brush thickness relative to the comparable PDMAEMA homopolymer brush, without affecting the scaling with respect to protonation. PDMAEMA brushes exhibited a pH-dependent lower critical solution temperature behavior, resulting in a thermal collapse of up to 50% upon heating. This work demonstrates that scaling behaviors previously tested at low grafting densities hold for extremely high grafting densities and that they are unaffected by moderate nonionizable comonomer content.

A Multifunctional Hybrid[4]arene-Based Macrocyclic Amphiphile: Self-Assembly, Tunable LCST Behavior, and Construction of Fluorescent Nanoparticles for Cell Imaging.

A novel macrocyclic amphiphile based on hybrid[4]arene containing tri(ethylene glycol) chains as the hydrophilic part and benzene rings as the hydrophobic part was synthesized. It self-assembled to produce nanoparticles and showed lower critical solution temperature behavior that was affected by its concentration and K+. Moreover, amphiphilic H can encapsulate dye G to form host-guest complexes H⊃G, accompanied by significant fluorescence enhancement. H⊃G can further self-assemble to form fluorescent nanoparticles that can be applied in cell imaging.