Coil−globule transition of poly(N-isopropylacrylamide) (PIPA), followed by intermolecular association in H2O and D2O, was investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of the solutions were measured as a function of temperature, and spectral changes induced by the transition were observed. The intensities of the difference IR bands due to the vibrational modes of isopropyl and amide groups critically increased at the lower critical solution temperature (LCST). Heating of the PIPA solutions above the LCST led shifts of the amide II, C−H-stretching, and C−H-bending bands to lower wavenumbers during a shift of the amide I band to a higher wavenumber. The amide I band of the PIPA observed below the LCST could be fitted with a single component centered at 1625 cm-1, whereas two components (1625 and 1650 cm-1) were necessary to fit the band above the LCST. These components may be assigned to the CO group which is bound to water molecules as the solvent (1625 cm-1) and to the N−H in ...

Change in Hydration State during the Coil−Globule Transition of Aqueous Solutions of Poly(N-isopropylacrylamide) as Evidenced by FTIR Spectroscopy†

A series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 103−3 × 105. For molecular weights below 4 × 104, conditions were found so that polydispersity remained below 1.4 up to high conversion (72%). Molecular weight distributions of polymer obtained using GPC in THF and MALDI were in accord for the low molecular weight range (typically 105). Particular care is required in GPC sample preparation:  it is necessary to ensure that trace amounts of water are initially present when drying a polymer sample prior to dissolution in THF, to avoid irreversible chain aggregation. The log/log plot of intrinsic viscosity against molecular weight for polyNIPAM was found to be linear for molecular weights <105, after which the hydrodynamic volume seems to be independent of molecular weight. The Mark−Houwink parameters obtained from the lower molecular weight data are K = 10-(4.24±0.42) dL g-1...

Molecular weight characterization of poly(N-isopropylacrylamide) prepared by living free-radical polymerization

Phase transitions of poly(N-n-propylacrylamide) (PnPA), poly(N-isopropylacrylamide) (PiPA), and poly(N-cyclopropylacrylamide) (PcPA) in H2O and D2O were investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of these solutions were measured as a function of temperature (T) both in the heating and cooling processes. Subtraction of a spectrum at starting temperature (T0) from a spectrum at T gives an IR difference spectrum (ΔAT-T0). The magnitudes of ΔAT-T0 for IR absorption bands attributable to the alkyl and amide groups of these polymers critically increased at their lower critical solution temperatures (LCSTs). Redshifts of the amide II, C−H stretching, and C−H deformation bands were observed during the coil-to-globule transitions of these polymers. The amide I‘ bands of N-deuterated amide groups (−COND−) in PnPA and PiPA measured below their LCSTs could be fitted with single Gaussian components centered at 1630 and 1625 cm-1, respectively. In contrast, two components were observed i...

Changes in the Hydration States of Poly(N-alkylacrylamide)s during Their Phase Transitions in Water Observed by FTIR Spectroscopy†

Aqueous polymeric sensors based on temperature-induced polymer phase transitions and solvatochromic dyes

Poly(N-isopropylacrylamide) (PNIPAAm) hydrogel is one of the most extensively studied thermosensitive hydrogels, it displays a lower critical solution temperature (LCST) at around 33 °C in aqueous solution and undergoes an abrupt thermoreversible change in volume as the external temperature cycles around this critical temperature. The fast response rate of hydrogels is critically important in some applications, such as artificial organs, actuators, and on–off switches. In this article, we review different strategies, including physical and chemical strategies, for improving the response kinetics of PNIPAAm-based hydrogels. Based on the numerous strategies, the factors that are essential to achieve the fast response rate are identified.

Strategies to improve the response rate of thermosensitive PNIPAAm hydrogels

The EPR spectra of solutions of a nitroxide-labeled poly(N-isopropylacrylamide) (PNIPAM-T/880, M v 2.9×10 6 ; labeled poly- 4-amino-2,2,6,6-tetramethylpiperidine 1-oxide (TEMPO); label content, 1×10 -5 mol g -1 ) were recorded in mixtures of water and THF in order to investigate the phenomenon of cononsolvency exhibited by the PNIPAM/water/THF ternary system. A detailed line-shape analyse of the EPR signals was carried out for spectra recorded at constant temperature as a function of solvent composition and as a function of temperature (-10 to +95 o C) for several water/THF mixtures

Cononsolvency of poly(N-isopropylacrylamide): a look at spin-labeled polymers in mixtures of water and tetrahydrofuran

The electron paramagnetic resonance (EPR) spectra of solutions of a nitroxide-labeled poly- (N-isopropylacrylamide) (PNIPAM-T/880, M, 2.9 X 106; label, 4-amino-2,2,6,6-tetramethylpiperidine l-oxide (TEMPO); label content, 1 X 1W mol g-'1 were recorded in mixtures of water and tetrahydrofuran (THF) in order to investigate the phenomenon of cononsolvency exhibited by the PNIPAM/water/THF ternary system. A detailed line-shape analysis of the EPR signals was carried out for spectra recorded at constant temperature as a function of solvent composition and as a function of temperature (-10 to +35 "C) for several water/THF mixtures. Changes in the rotational correlation time and in the isotropic hyperfine coupling constant of the label were correlated to the macroscopic polymer phase-separation process. The data support a model involving preferential water adsorption along the polymer chains in mixed water/THF solutions. This situation was compared to that offered by the PNIPAMIwaterlmethanol system, another ternary mixture which exhibita the phenomenon of cononsolvency. Differences between the two systems are described. They are interpreted in terms of the relative hydrogen-bonding abilities of the two organic cononsolventa.

Spin-Labeled Polymers in Mixtures of Water and Tetrahydrofuran

A spin label (4-amino-2,2,6,6-tetramethylpiperidine 1-oxide, TEMPO) and a fluorescence dye (1-pyrenulbutyl,Py) were attached randomly along the same poly(N-isopropylacrylamide) chain (PNIPAM-Py-T, M v 2.3×10 6 , Py content 1.9×10 -5 mol g -1 ), and their interactions in solutions of the polymer were studied using electron paramagnetic resonance (EPR), 13 C nuclear magnetic resonance ( 13 C NMR), and fluorescence spectroscopy. The thermoreversible phase transition of aqueous solutions of the polymer was examined by these three techniques

Phase Separation of Poly (N-isopropylacrylamide) in Water: A Spectroscopic Study of a Polymer Tagged with a Fluorescent Dye and a Spin Label

Electron spin resonance (ESR)l is a spectroscopic technique used to study the resonant transitions between energy levels of the magnetic dipoles due to electronic angular momentum. The usual ESR spectrum is obtained by taking the derivative of the absorption spectrum of a paramagnetic species under microwave excitation in the presence of a ramped magnetic field. The rotational mobility of the paramagnetic species strongly affects the shape of an ESR spectrum since it can affect the relaxation process of the excited state. Techniques for line shape analysis2 are well established and give information on the mobility distribution of the paramagnetic species in a window of time scales around that for the resonant transitions. ESR has been employed successfully to study the conformational details of polymer ad~orpt ion .~-~ A three-component modeP8 has been used to deconvolute spectra from adsorbed, spin-labeled polymers. The experimental spectra are reproduced by summing three components: (a) a sharply peaked component from labels with a rotational correlation time I O(10-1 ns), denoted “freely mobile” and presumed identical to the spectrum of the labeled polymer in solution; (b) a moderately broadened component from labels with a rotational correlation time O(lOo ns), having “intermediate” mobility and presumed identical to the spectrum of the labeled polymer in the melt state; (c) a “powder pattern” component from labels with a rotational correlation time L O(10’ ns), denoted “motion restricted” and presumed identical to the spectrum of the labeled polymer solution at cryogenic temperatures. The basic assumption in all previous ESR work on adsorbed polymers is that the freely mobile, intermediate, and motion-restricted labels correspond to the fractions of segments in long loops and tails, short loops, and trains, respectively. Based on this composite model, previous studies have determined the apparent conformation of adsorbed macromolecules. For example, Kobayashi et uZ6,799J0 measured the apparent equilibrium fraction of segments in trains, f&n, as a function of surface coverage and silanol density for spin-labeled polymers on nonporous silica. With labeled polystyrene (PS), they found that the apparent ftrain decreases with increasing surface coverage, while the fraction of long loops and tails simultaneously increases. Similar behavior was ob-

Wenseng Pan

Papers

Cononsolvency of poly(N-isopropylacrylamide): a look at spin-labeled polymers in mixtures of water and tetrahydrofuran

Spin-Labeled Polymers in Mixtures of Water and Tetrahydrofuran

Phase Separation of Poly (N-isopropylacrylamide) in Water: A Spectroscopic Study of a Polymer Tagged with a Fluorescent Dye and a Spin Label

Mobility Redistribution of Spin Labels on Adsorbed Polystyrene