Phase transition of aqueous solutions of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide)

Longer time kinetics of collapse transition of polymer-surfactant complexes at interfaces near to collapse temperatures

The later stages of the collapse transition kinetics of fractionated poly(N-isopropylacrylamide) (PNIPAM) chains at interfaces in the presence of the surfactant sodium dodecyl sulfate (SDS) were studied for molecular weights ranging from 4.5x10(5) to 1.6x10(6). The interfacial PNIPAM-SDS complexes were quickly heated to the temperatures that were near to the collapse transition temperatures. The longer time collapse of the PNIPAM-SDS complexes at interfaces was found to proceed through two stages. The collapse kinetics at the later stages was interpreted in terms of a "globule growth" model. It was found that under the experimental conditions, the fast relaxation time was independent of the molecular weight, whereas the slow relaxation time was weakly dependent on the molecular weight.

On the temperature-induced coil to globule transition of poly-N-isopropylacrylamide in dilute aqueous solutions

Poly-N-isopropylacrylamide (PNIPAM) is a chemical isomer of poly-leucine, having the polar peptide group in the side-chain rather than in the backbone. It has been demonstrated experimentally that PNIPAM dissolved in aqueous solution undergoes a collapse transition from coil to globule on increasing temperature above the θ-point. By a careful reviewing of existing experimental data, we emphasize that such coil to globule collapse has to be considered an intramolecular first-order transition, analogous to the cold renaturation of small globular proteins. The main theoretical approaches to the coil to globule collapse in homopolymers are discussed briefly, and a critical comparison between the existing models is performed. We point out that, as a general result, the coil to globule collapse is expected to be a first-order transition for rigid and semi-rigid macromolecules. Finally, taking advantage of the analogy between the coil to globule collapse of PNIPAM and the cold renaturation of small globular proteins, we try to clarify some important and intriguing aspects of protein thermodynamics. This leads to the conclusion that the amphiphilic nature of polypeptide chain plays the fundamental role for the existence of two temperature-induced conformational transitions.

Temperature-Responsiveness of A-B-A Block Telomers at Solid-Liquid Interfaces

Macroinitiators were prepared by coupling disuccinimidyl ester of 4,4'-azobis(cyanovaleric acid) with poly(N-isopropylacrylamide) (PIPA), which had an amino group at its end. Styrene was telomerized with the initiators in THF. When the styrene content in the A-B-A block telomer obtained (PIPA-b-PSt-b-PIPA) was high, the telomer formed an irreversible aggregation resulting in microspheres, whereas the telomer with a much shorter styrene block could be dispersed monomolecularly. The telomers dispersed in water were aggregated by raising the temperature above 32 degrees C due to a coil-globule transition of PIPA moieties. The PIPA-b-PSt-b-PIPA could be strongly adsorbed to polystyrene (PSt) solid surfaces to form a layer, and the PSt blocks might lay on the PSt surface and the PIPA blocks might direct to the solution phase. The contact angle of air bubbles on the surface of telomer-coated PSt in the air-in-water system was dependent on temperature; that is, with the increase in temperature the contact angle of air bubbles largely decreased and leveled off above the coil-globule transition temperature (Tc). Correspondingly, the amount of protein Concanavalin A adsorbed to the telomer layer deposited on the PSt surface increased gradually with an increase in temperature and leveled off above the Tc. These phenomena were attributed to the changes in hydrophobicity of the telomer layer below and above the Tc. The usefulness of macroinitiators in preparing various kinds of block telomers which have responsiveness to external stimuli was strongly suggested. Copyright 1999 Academic Press.

Development of tissue-simulating optical phantoms: poly-N-isopropylacrylamide solution entrapped inside a hydrogel

The average turbid optical properties of the N-isopropylacrylamide (NIPA) polymer solution entrapped inside a polyacrylamide hydrogel (called an NIPA/PAAM gel system) were studied using a multiwavelength oblique-incidence reflectometer. The turbidity of such a system can be drastically changed by simply switching the temperature from below the low critical solution temperature of the NIPA, around 33 degrees C, to above. The absorption coefficient and the reduced scattering coefficient were obtained as a function of wavelength for samples with selected NIPA and blue dextran concentrations. It is found that the scattering of the optical phantom comes from the NIPA polymer chains and the absorption from the blue dextran. The turbid optical properties of an NIPA/PAAM gel system can be tuned to simulate biological tissues at a specific wavelength by varying compositions of NIPA and blue dextran and further modified by controlling the temperature.

Effect of protein and cell behavior on pattern-grafted thermoresponsive polymer

A thermoresponsive copolymer, poly(Nisopropylacrylamide-co-acrylic acid), was coupled with azidoaniline. The azidophenyl-derivatized copolymer was grafted in a specific pattern on a polystyrene matrix by photolithography. The surface micropattern appeared and disappeared interchangeably, as observed under a phase-contrast microscope, by varying the temperature between 10 degrees C and 37 degrees C. The copolymer-grafted polystyrene surface was hydrophobic at 37 degrees C and hydrophilic at 10 degrees C. Albumin and fibronectin adsorption on the matrix was investigated using the fluorescent-labeling method. Fibronectin adsorbed onto both the grafted and nongrafted regions, while albumin adsorbed more onto the nongrafted regions than the grafted regions. Protein adsorption did not affect surface wettability. Mouse fibroblast STO cells were cultured on tissue culture plates pattern-grafted with the thermoresponsive copolymer. Fibronectin adsorption enhanced cell spreading, while albumin reduced it. When the temperature was lowered, the cells selectively detached from the surface areas grafted with the thermoresponsive copolymer when cultured in serum-free medium; the cells partially detached from these areas when cultured in serum-containing medium. The effect of serum proteins on cell detachment was similar to that caused by a mixture of albumin and fibronectin. Albumin adsorption did not affect the detachment of cells, while fibronectin adsorption inhibited it. The results of the present study indicate that a pattern-grafted, thermoresponsive, azidophenyl-derivatized copolymer can effectively facilitate selective cell detachment under some conditions such as serum-free culture or preadsorption of albumin. The pattern-grafting technique will be useful for qualitative microscopic comparison of surfaces prepared differently on one chip under the same conditions.

Phase transition parameters of potential thermosensitive drug release systems based on polymers of N-alkylmethacrylamides

The phase separation and its thermohysteresis in dilute aqueous solutions of polymeric components of potential drug release systems (homopolymers and copolymers of N-isopropylacrylamide, N-isopropylmethacrylamide, N-propylmethacrylamide, N-sec-butylmethacrylamide, and N-(2-hydroxypropyl)methacrylamide) was studied, both on heating and cooling. Plots of light transmittance vs temperature were constructed and the parameters characterizing them were correlated with polymer structures. Qualitative information was obtained on the rate of formation of the concentrated phase on heating and its disappearance on cooling. Attention has been drawn to the improper identification of the cloud-point temperature, measured at an arbitrary concentration, with the lower critical solution temperature (LCST) as is frequently found in papers dealing with biomedical applications of thermosensitive polymers.

Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials

Chemical sensors respond to the presence of a specific analyte in a variety of ways. One of the most convenient is a change in optical properties, and in particular a visually perceptible colour change. Here we report the preparation of a material that changes colour in response to a chemical signal by means of a change in diffraction (rather than absorption) properties. Our material is a crystalline colloidal array of polymer spheres (roughly 100 nm diameter) polymerized within a hydrogel that swells and shrinks reversibly in the presence of certain analytes (here metal ions and glucose). The crystalline colloidal array diffracts light at (visible) wavelengths determined by the lattice spacing, which gives rise to an intense colour. The hydrogel contains either a molecular-recognition group that binds the analyte selectively (crown ethers for metal ions), or a molecular-recognition agent that reacts with the analyte selectively. These recognition events cause the gel to swell owing to an increased osmotic pressure, which increases the mean separation between the colloidal spheres and so shifts the Bragg peak of the diffracted light to longer wavelengths. We anticipate that this strategy can be used to prepare 'intelligent' materials responsive to a wide range of analytes, including viruses.

Mechanism of the interaction of hydrophobically-modified poly-(N-isopropylacrylamides) with liposomes

Interactions of hydrophobically-modified poly-(N-isopropylacrylamides) (HM PNIPAM) with phospholipid liposomes were studied as a function of the lipid type, the lipid bilayer fluidity, and the polymer conformation. Fluorescence experiments monitoring non-radiative energy transfer (NRET), between naphthalene attached to the HM PNIPAM and 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated into the lipid bilayer, confirmed the direct penetration of hydrophobic anchor groups linked to the polymer into the liposome hydrophobic core. Contraction of the polymer backbone above the lower critical solution temperature (LCST) resulted in a partial withdrawal of the anchor groups from the lipid bilayer. Analysis of polymer/lipid mixtures by centrifugation and quasi-elastic light scattering (QELS) revealed the polymer-induced fission of liposomes in the liquid-crystalline state, resulting in the formation of vesicles 150-230 nm in diameter. The process is reversible and upon transition of the bilayer into the gel state these vesicles are converted into larger aggregates. According to the results of gel-filtration experiments the HM PNIPAM is in dynamic exchange between the liquid-crystalline lipid bilayer and the water solution, while the binding to the bilayer in the gel state is more static in nature. The binding constant for mixture of HM PNIPAM with DMPC liposomes, evaluated from the centrifugation experiments, was found to be 120 M(-1).