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

Formation of surface-attached responsive gel layers via electrochemically induced free-radical polymerization

We report on the formation of hydrogel layers on conducting substrates via a simple electrochemical route. Free-radical polymerization is initiated by an electron transfer from the substrate to a redox-active initiator. Gels of the thermally responsive material poly-N-isopropylacrylamide (p-NIPAM) with a thickness between 25 and 250 nm were produced and characterized. The gels adhere well to the substrate. They show the characteristic swelling transition at 32 degrees C. Although the films appear homogeneous in optical microscopy, AFM images reveal a slightly heterogeneous, globular structure. The gels are permeable to small ions as evidenced by electrochemical experiments with gel-covered electrodes.

Thermosensitivity of Optically Active Hydrogels Constructed withN-(L)-(1-hydroxymethyl)propylmethacrylamide

We previously reported on the water soluble, optically active polymer, poly[N-(L)-(1-hydroxymethyl)propylmethacrylamide] (P(L-HMPMA)), which has lower critical solution temperatures of approximately 30 and 21 degrees C upon heating and cooling, respectively. The phase separation behavior of the P(L-HMPMA)-water binary system has a reversible and clear hysteresis during heating and cooling cycles. The present study describes the thermosensitive properties of the optically active polymer and its hydrogel. Circular dichroism and microcalorimetric measurements of the polymer in water supported the hypothesis that the soluble polymer chains might be compactly folded with an interaction between optically active side chains. In addition, these measurements showed the polymer chains in a state of relatively low hydration compared to that of P(D,L-HMPMA), which was free-radically synthesized from racemates of monomers. The solution properties reflected the swollen-shrunken behavior of corresponding hydrogels in response to temperature changes. The microscopic observation of aqueous polymer solutions and hydrogels also confirmed that the optically active properties of polymer chains affect their structure and thermosensitivity. SEM micrograph of the surfaces of the crosslinked P(L-HMPMA).

Two phase transitions of poly(N-isopropylacrylamide) brushes bound to gold nanoparticles

The thermally induced phase transition of the poly(N-isopropylacrylamide) (PNIPAM) brush covalently bound to the surface of the gold nanoparticles was studied using high-sensitivity microcalorimetry. Two types of PNIPAM monolayer protected clusters (MPCs) of gold nanoparticles were employed, denoted as the cumyl- and the cpa-PNIPAM MPCs, bearing either a phenylpropyl end group or a carboxyl end group on each PNIPAM chain, respectively. The PNIPAM chains of both MPCs exhibit two separate transition endotherms; i.e., the first transition with a sharp and narrow endothermic peak occurs at lower temperature, while the second one with a broader peak occurs at higher temperature. With increase of the MPC concentration, the transition temperature corresponding to the first peak only slightly changes but the second transition temperature strongly shifts to lower temperature. The calorimetric enthalpy change in the first transition is much smaller than that in the second transition. The ratio of the calorimetric enthalpy change to the van't Hoff enthalpy change indicates that in the first transition PNIPAM segments show much higher cooperativity than in the second one. The investigation of pH dependence of two-phase transitions further indicates the PNIPAM brush reveals two separate transitions even with a change in interchain/interparticle association. The observations are tentatively rationalized by assuming that the PNIPAM brush can be subdivided into two zones, the inner zone and the outer zone. In the inner zone, the PNIPAM segments are close to the gold surface, densely packed, less hydrated, and undergo the first transition. In the outer zone, on the other hand, the PNIPAM segments are looser and more hydrated, adopt a restricted random coil conformation, and show a phase transition, which is dependent on both concentration of MPC and the chemical nature of the end groups of the PNIPAM chains. Aggregation of the particles, which may also affect the phase transition, is briefly discussed.

Model for the hydration of nonpolar compounds and polymers

We introduce an exactly solvable statistical-mechanical model of the hydration of nonpolar compounds, based on grouping water molecules in clusters where hydrogen bonds and isotropic interactions occur; interactions between clusters are neglected. Analytical results show that an effective strengthening of hydrogen bonds in the presence of the solute, together with a geometric reorganization of water molecules, are enough to yield hydrophobic behavior. We extend our model to describe a nonpolar homopolymer in aqueous solution, obtaining a clear evidence of both "cold" and "warm" swelling transitions. This suggests that our model could be relevant to describe some features of protein folding.

Interactions between Local Anaesthetic Agents and Poly(N-isopropyl acrylamide) through Phase Behavior, Surface Tension, and Adsorption Measurement

The interaction between the local anaesthetic agents prilocaine and lidocaine, on one hand, and poly(N-isopropyl acrylamide) (pNIPAM), on the other, is investigated through studies of the polymer phase behavior and through surface tension and adsorption measurements. In particular, the cloud points (CP) for pNIPAM in the presence of lidocaine and prilocaine under different conditions were compared to the effects of electrolytes and alcohols. It was found that the electrolytes affect the CP of pNIPAM in a lyotropic manner, whereas alcohols depress the CP of pNIPAM in an alkyl chain length dependent way; i.e., the longer the chain, the larger the decrease in CP. Lidocaine and prilocaine affect the CP of pNIPAM in a pH-dependent manner. Below the pK(a) of lidocaine and prilocaine, these cosolutes do not substantially affect the CP in the concentration range investigated, but rather behave analogous to simpler electrolytes. Above the pK(a), on the other hand, they strongly depress the CP already at low concentrations. In parallel, at low pH, the surface tension reduction due to lidocaine or prilocaine is marginal, whereas at high pH the surface tension is reduced considerably. Thus, the poor solubility of prilocaine and lidocaine at high pH causes these to become more surface active and simultaneously interact in a more pronounced way with pNIPAM. Furthermore, it was found from ellipsometry that an adsorbed pNIPAM layer contracts when lidocaine is added, presumably due to a lidocaine-pNIPAM interaction similar to that causing pNIPAM to phase separate. Analogous to this, it was demonstrated that an adsorbed pNIPAM layer shrinks and swells reversibly when the temperature is cycled above and beneath the CP. Copyright 2001 Academic Press.