Separating polymer solutions using high pressure lower critical solution temperature (LCST) phenomena

A lower critical solution temperature (LCST) phase split can be used as an alternative to steam stripping for separating polymer solutions, By adding a supercritical fluid (SCF) additive to the polymer solution, the LCST can be lowered, thus minimizing the possibility of polymer degradation and also reducing the thermal energy requirements for the process. Experimental results for the poly(ethylene-co-propylene)-hexane-SCF ethylene system are shown as an example of the type of phase behavior observed with polymer-solvent-SCF additive solutions. Adding 20 percent (w/w) ethylene to the polymer solution lowers the temperature of the LCST by 109°C. The addition of 30 percent (w/w) ethylene to the polymer solution lowers the temperature of the LCST curve sufficiently to merge this curve with the upper critical solution temperature (UCST) curve. When the lower critical end point (LCEP) is plotted against the critical temperature of the solvent the data for poly(ethylenie-co-propylene)-solvent systems are well represented by a single curve. A more fundamental modeling approach is needed to estimate the pressure of the LCEP and the concentration of SCF additive necessary to merge the LCST and the UCST curves. Patterson's theory of corresponding states can be used for these calculations.     

Lower critical solution temperature (LCST) polymer solution for clear/cloud glazing applications

An aqueous-based clear/cloud solution was developed using poly(vinyl methyl ether) and an antifreeze mixture of ethylene and propylene glycols. The solution has a controllable cloud point from 15 to 90°C, freeze protection to ?46°C, and a density that prevents the precipitated polymer from settling and is fairly stable to UV degradation. A significantly smaller heat gain is achieved through the clear/cloud glazing when compared to a normal double-pane window. © 1994 John Wiley & Sons, Inc.     

Fractionation and characterization of linear low-density polyethylene at a lower critical solution temperature (LCST)

The composition and molecular weight distribution of linear low-density polyethylenes (LLDPE) have been determined by fractionation and characterization at a lower critical solution temperature (LCST). The fractionation in 2,4-dimethyl pentane takes advantage of the fact that the LCST is sensitive to the side-group content as indicated by the 70 K difference between the LCSTs of PE and polypropylene at the same molecular weight (MW). Temperature-rising elution fractionation (TREF) and LCST fractions are compared by IR and SEC and also by a new method based on turbidity at an LCST. It is found that the LCST fractionation is sensitive to MW and TREF largely depends on a comonomer content. Calibration of the LCST method for MW distribution and effect of the nature of the comonomer on the LCST are discussed. The LCST technique associated with IR analysis is found to be quite effective for characterizing LLDPE.     

Small-angle neutron scattering from polymer solutions: 3. Semi-dilute solutions near the lower critical solution temperature

Measurements of the radius of gyration and the screening lengths have been made for semi-dilute solutions of polystyrene near the lower critical solution temperature. From these data two regions of behaviour are clearly discerned. By analogy with similar data near the upper critical solution temperature, a ‘phase diagram’ has been drawn from the results around the lower critical solution temperature.     

The effect of graphene on the lower critical solution temperature of poly(N-isopropylacrylamide)

N-isopropylacrylamide is grafted on graphene by free radical polymerization to produce a series of graphene–poly(N-isopropylacrylamide) (PNIPAM) hybrid materials with different contents of graphene. The lower critical solution temperatures of the resulting materials have been investigated by ultraviolet–visible transmission spectroscopy. The result shows that the lower critical solution temperature of the resulting materials increases with the content of graphene increasing, and can be tuned to the human body temperature (37 °C) when the content of graphene is in the range of 10–30%, which is very important for application of the hybrid materials in biomedical field. The result is ascribed to the high surface area of graphene that could graft a large amount of PNIPAM molecules and the immobilization of one end of PNIPAM chain on the surface of graphene. To further understand the effect of graphene on the lower critical solution temperature of PNIPAM, atomic force microscopy has been used to trace the morphology change of the hybrid materials in solid state with the temperature increasing from 33 to 40 °C.     

Upper and lower critical solution temperatures in poly (ethylene glycol) solutions

Upper and lower critical solution temperatures have been determined for solutions of poly(ethylene glycol) in t-butyl acetate and water over the molecular weight range of M_η = 2.18 × 10³ to ～1020 × 10³. The phase diagram for solutions of poly(ethylene glycol) (M_η = 719 × 10³) in t-butyl acetate was expressed as the ‘hour glass’ type, while the phase diagram for solution of poly(ethylene glycol) (M_η = 2.18 × 10³ to ～2.29 × 10³) in water was expressed as the ‘closed loop’ type. The value of the pressure dependence of the lower critical solution temperature $\text{(}\text{d}\text{T}\text{d}\text{P}\text{)}{}_{\mathrm{c}}$ in the poly(ethylene glycol) (M_η = 1020 × 10³)/water system over the pressure range of 0 to ～50 atm was negligibly small and positive.     

Small-angle neutron scattering from polymer solutions: 2. The semi-concentrated region near the upper critical solution temperature

Small-angle neutron scattering experiments have been made on solutions of polystyrene in cyclohexane. From these experiments the radius of gyration has been determined as a function of polymer concentration at a fixed temperature (60°C). Additionally, the screening length has been determined as a function of temperature for a fixed polymer concentration of 36% (w/v). The results support the notion of an additional region of solution behaviour, the semi-concentrated region, predicted by mean field-type theory.     

Solvent effect on the lower critical solution temperature of biodegradable thermosensitive poly(organophosphazenes)

Summary The solvent effect on the lower critical solution temperature (LCST) of poly(organophosphazenes) with methoxy-poly(ethylene glycol) (MPEG) and amino acid esters as side groups was examined in terms of the structure of polyphosphazenes in aqueous solutions containing one of the organic solvents selected from monoalcohols, ethylene glycol derivatives, alkylamines, and other common solvents. When such a solvent was added to the aqueous solutions of the polymers, their LCST was found to be mainly dependent on the hydrophobic and hydrophilic properties of the solvents. Most of the alcohols and amines with shorter alkyl chains increased the LCST of the polymers but those with longer chains decreased the LCST. Trifluoroethanol (TFE) showed a strong LCST decreasing effect in spite of its short chain, which seems to be due to its strong hydrophobicity. Temperature-induced molecular weight fractionation of the polymer bearing MPEG350 (M _w= 350) and L-aspartic acid ethyl ester as a side group was carried out by using the LCST decreasing effect of TFE, and the fractionated samples were characterized by gel permeation chromatography (GPC) and ¹H- and ³¹P NMR spectroscopies. Thus it has been shown that a polymer may be fractionated to the higher and lower molecular weight fractions with smaller polydispersity indices (PDI): the polymer with the weight-average molecular weight (M _w) of 73,500 with PDI of 5.56 was fractionated to those of 106,000 with PDI of 4.37 and 11,000 with PDI of 1.86. Received: 8 September 2000/Revised version: 6 November 2000/Accepted: 9 November 2000     

Upper and lower critical solution temperatures in polyethylene solutions

Upper and lower critical solution temperatures have been determined for solutions of polyethylene in n-butyl acetate and n-amyl acetate over the molecular weight range of M_η = 1·36 × 10⁴ to 17·5 × 10⁴. Polyethylene solution in n-butyl acetate displays a smaller miscibility region than that of the polyethylene/n-amyl acetate system, as indicated by the relative positions of their upper and lower critical solution temperatures. Contributions of the energy and the equation of state terms to the χ1 parameter have been examined by an application of the Patterson-Delmas corresponding state theory to the experimental results of the polyethylene solutions.     

Comonomer effect: Switching the lower critical solution temperature to upper critical solution temperature in thermo-pH sensitive binary graft copolymers

Hydrophilic biocompatible surfaces can be obtained by grafting stimuli-sensitive polymers onto commercially available medical devices. Thermo and pH-responsive polymers are two of the most studied materials due to their potential application as drug delivery systems. Poly(N-vinylcaprolactam) has a lower critical solution temperature (LCST) near to physiological temperature. However, when it is grafted with pH-sensitive moieties its LCST it is affected undergoing remarkable displacements. We studied the effect of acrylic acid (AAc), 4-vinylpyridine (4VP), and 1-vinylimidazole (Vim) on the LCST of N-vinylcaprolactam (NVCL) grafted onto silicone rubber (SR), and SR-g-NVCL (32.5 °C). The binary graft copolymers were obtained by ionizing grafting radiation using the simultaneous technique; the samples were characterized by Fourier transform infrared attenuated total reflectance (FTIR-ATR), cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS ¹³C-NMR), and thermogravimetrical analysis (TGA). LCST value was dramatically affected by the comonomer content; even it was observed the switching from LCST to upper critical solution temperature (UCST) for (SR-g-NVCL)-g-AAc and (SR-g-NVCL)-g-4VP samples. The observed behavior is rarely reported for binary graft copolymers. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48170.     

Potential for desalination using lower critical solution temperature polymers: Concentration of salt solutions by pluronic PE6200

A commercially available lower critical solution temperature (LCST) polymer was demonstrated to preferentially adsorb water containing a reduced ion concentration, from CrCl₃ salt solutions and release some of this scavenged water at higher temperatures. The scavenging ability of the polymer was demonstrated to be dependent on the bulk salt concentration. The volume of water scavenged and the concentration of the remaining CrCl₃ solution increased linearly with the amount of LCST polymer (Pluronic PE 6200) used. Both the volume of water removed and the amount by which the remaining CrCl₃ solution was concentrated decreased with increasing CrCl₃ concentration. Implications for research into a novel method for sea water desalination are addressed. We also show water extraction from dilute sodium chloride and sea water, and that these results are consistent with the CrCl₃ model system. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mixtures of tetrabromobisphenol-A polycarbonate and bis (2-ethoxyethyl) ether: Examples of a lower critical solution temperature

Tetrabromobisphenol-A polycarbonate was found unexpectedly to form mixtures with the solvent bis(2-ethoxyethyl) ether that display a lower critical solution temperature at about normal room temperature and less than about 10 wt % polymer. © 1993 John Wiley & Sons, Inc.     

Hydrophobically modified polyglycidol – the control of lower critical solution temperature

Summary Polymers of glycidol (2,3-epoxypropanol-1) of different molar masses (between 2˙10⁴ and 2˙10⁵) and chain topology (linear and comb-like) were synthesised and used to obtain a series of temperature responsive water-soluble poly(glycidol-co-glycidol acetate)s. The degree of the substitution of the hydroxyl groups with the acetate groups influences the solution behaviour of the obtained copolymers, the cloud point may be controlled between +4°C and +lOO°C. Received: 24 September 2002/Revised version: 2 November 2002/ Accepted: 2 November 2002 Correspondence to Andrzej Dworak     

Interfacial tension of demixed polymer solutions near the critical temperature: polystyrene + methylcyclohexane

Interfacial tension between demixed solutions of polystyrene + methylcyclohexane has been measured near the critical temperature as a function of temperature using polystyrenes with molecular weights 9000 ～ 1.26 × 10⁶. The critical exponent for the interfacial tension was determined to be about 1.30 for the lower molecular weight systems. However, for higher molecular weights the exponent could not be obtained because the system departed from critical behaviour. Magnitudes of the interfacial tension were proportional to about N^?0.44, where N is the polymerization index. Experimental results were compared with the recently-proposed theories and found to be in qualitative agreement. The tricritical theory of polymer solutions was also compared with the experimental results.     

Miscibility and lower critical solution temperature type phase behavior of poly(ethyl acrylate)/poly(vinylidene fluoride-cohexafluoro acetone) blends

Summary The miscibility of mixtures of poly(ethyl acrylate) (PEA) with poly (vinylidene fluoride-co-hexafluoro acetone) (P(VDF-HFA)) was investigated with optical microscopy and differential scanning calorimetry (DSC). In PEA/P(VDF-HFA) blends with P(VDF-HFA) content of 50 and 70(wt%), the heterogeneous phase morphology was observed on optical micrographs at 210°C. It was found that PEA/P(VDF-HFA) blends showed the lower critical solution temperature type (LCST) phase behavior. The endothermic peak for PEA / P(VDF-HFA) blend observed on DSC thermogram near 200°C corresponded to the liquid-liquid phase transition temperature as shown in the heterogeneous phase morphology with optical microscopy. It was expected that the endothermic peak is the transition temperature from miscibility to immiscibility.     

A brillouin scattering study of a polymer blend showing upper critical solution temperature behaviour

Summary The Brillouin spectra of a polymer blend of poly(styrene-co-allyl alcohol) and poly(neopentyl glycol adipate), showing upper critical solution temperature behaviour, have been measured as a function of temperature. Conspicuous changes have been observed in the Brillouin width near the cloud point and have been interpreted as arising from composition changes in the two originally distinct low-temperature microscopically separated liquid phases towards a common high-temperature solution.