Lower critical solution temperature surface

The preparation of monoliths with polyNIPAAm chains grafted to the internal pore surface was discussed previously. The extended solvated polyNIPAAm-chains that are present below the lower critical solution temperature of this particular polymer are more hydrophilic, while the collapsed chains that prevail above the lower critical solution temperature are more hydrophobic. In contrast to isothermal separations in which the surface polarity remains constant throughout the run [ 136], HIC separation of proteins can be achieved at constant salt concentrations (isocratically) while utilizing the hydrophobic-hydrophilic... 

Using this approach, hydrophilic (neutral or ionic) comonomers, such as end-captured short polyethylene oxide (PEO) chains (macromonomer), l-vinyl-2-pyrrolidone (VP), acrylic acid (AA) and N,N-dimethylacrylamide (DMA), can be grafted and inserted on the thermally sensitive chain backbone by free radical copolymerization in aqueous solutions at different reaction temperatures higher or lower than its lower critical solution temperature (LCST). When the reaction temperature is higher than the LOST, the chain backbone becomes hydrophobic and collapses into a globular form during the polymerization, which acts as a template so that most of the hydrophilic comonomers are attached on its surface to form a core-shell structure. The dissolution of such a core-shell nanostructure leads to a protein-like heterogeneous distribution of hydrophilic comonomers on the chain backbone. 

Affinity chromatography of streptavidin was performed on a PET chip. The microchannel was first filled with the dual-modified latex beads (as shown in Figure 6.3). The biotinylated beads were surface-modified with a temperature-sensitive polymer, poly(N-isopropylacrylamide (PNIPAAm, 11 kDa). When the temperature was raised above the lower critical solution temperature (LCST) of PNIPAAm, the beads aggregated and adhered to the channel wall, because of a hydrophilic-to-hydrophobic phase transition. Then streptavidin from a sample solution was captured by these adhered biotinylated beads. Thereafter, when the temperature was reduced below the LCST, the beads dissociated and eluted from the channel wall together with the captured streptavidin [203],... 

Figure 16.7 Sequence of insulin release from pH/temperature-sensitive polymer matrix. Both glucose oxidase and insulin are loaded inside the matrix. The decrease in pH by gluconic acid results in ionization of the polymer, which in turn increases the lower critical solution temperature. This makes the polymer water-soluble, and erosion of the polymer matrix at the surface releases the loaded insulin...

Fujimatsu, H. Ogasawara, S. Kuroiwa, S., "Lower Critical Solution Temperature (LCST) and Theta Temperature of Aqueous Solutions of Nonionic Surface Active Agents of Various Polyoxyethylene Chain Lenghts," Coll. Polym. Sci., 266, 594 (1988). 

Poly(amino acid)s (PAAs) have also been used in drug delivery PEO-(l-aspartic acid) block copolymer nano-associates , formed by dialysis from a dimethyl acetamide solution against water, could be loaded with vasopressin. PLA-(L-lysine) block copolymer microcapsules loaded with fluorescently labelled (FITC) dextran showed release profiles dependent on amino acid content. In a nice study, poly(glutamate(OMe)-sarcosine) block copolymer particles were surface-grafted with poly(A-isopropyl acrylamide) (PNIPAAm) to produce a thermally responsive delivery system FITC-dextran release was faster below the lower critical solution temperature (LCST) than above it. PAAs are prepared by ring-opening polymerisation of A-carboxyanhydride amino acid derivatives, as shown in Scheme 1. 

For the hydrocarbon--CO2 systems studied here, at pressures above the critical pressure (7.383 MPa) and above the critical temperature (304.21 K) of C02 the isobaric x,T coexistence plots of liquid and vapor phases form simple closed loops. The minimum occurs at the lower consolute point or the Lower Critical Solution Temperature (LCST). Since pressure is usually uniform in the vicinity of a heat transfer surface, such diagrams serve to display the equilibrium states possible in a heat transfer experiment. 

The lattice fluid equation-of-state theory for polymers, polymer solutions, and polymer mixtures is a useful tool which can provide information on equa-tion-of-state properties, and also allows prediction of surface tension of polymers, phase stability of polymer blends, etc. [17-20]. The theory uses empty lattice sites to account for free volume, and therefore one may treat volume changes upon mixing, which are not possible in the Flory-Huggins theory. As a result, lower critical solution temperature (LCST) behaviors can, in principle, be described in polymer systems which interact chiefly through dispersion forces [17]. The equation-of-state theory involves characteristic parameters, p, v, and T, which have to be determined from experimental data. The least-squares fitting of density data as a function of temperature and pressure yields a set of parameters which best represent the data over the temperature and pressure ranges considered [21]. The method,however,requires tedious experiments to deter-... 

The lower critical solution temperature (LCST) phase behavior exhibited by the nanocrystals is often found for low molecular weight solutes in supercritical fluids (25,26) and also for polymers dissolved in SCFs, and results from compressibility differences between the polymer and the solvent (15). As the temperature increases or the pressure decreases, the solvent prefers to leave the solute to increase its volume and entropy. The same mechanism that governs phase separation in supercritical fluids also drives flocculation of two surfaces with steric stabilizers, as has been shown with theory (22) and simulation (23). 

Figure 21 (a) Calculated reactivity ratios for DMAEMA and PEGMA monomers in RAF polymerization, (b) Dependence of lower critical solution temperature (LOST) on PEGMA polymer composition and pH. (c) Calculated surface energies of copolymer thin films. Adapted from Fournier, D. Hoogenboom, R. Thijs, H. M. L. etal. Macromolecules 2007, 40(4), 915-920, and reprinted with permission from the American Chemical Society. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

JHO Jhon, Y.K., Bhat, R.R., Jeong, C., Rojas, O.J., Szlerfer, I., and Genzer, J., Salt-induced depression of lower critical solution temperature in a surface-grafted neutral thermoresponsive polymer, Macromol. Rapid Commun., 27,697,2006. 

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