Molecular-Level Changes of Aqueous Poly(N-isopropylacrylamide) in Phase Transition

3D Printable Active Hydrogels with Supramolecular Additive-Driven Adaptiveness

Smart hydrogels are a promising candidate for the development of next-generation soft materials due to their stimuli-responsiveness, deformability, and biocompatibility. However, it remains challenging to enable hydrogels to actively adapt to various environmental conditions like living organisms. In this work, supramolecular additives are introduced to the hydrogel matrix to confer environmental adaptiveness. Specifically, their microstructures, swelling behaviors, mechanical properties, and transparency can adapt to external environmental conditions. Moreover, the presence of hydrogen bonding provides the hydrogel with applicable rheological properties for 3D extrusion printing, thus allowing for the facile preparation of thickness-dependent camouflage and multistimuli responsive complex. The environmentally adaptive hydrogel developed in this study offers new approaches for manipulating supramolecular interactions and broadens the capability of smart hydrogels in information security and multifunctional integrated actuation.

Hydrogen-bond network distortion of water in the soft confinement of Nafion membrane

A Compton spectroscopy investigation is carried out in hydrated Nafion membranes, enabling identification of distortions in the hydrogen-bond distribution of the polymer hydrating water by means of the subtle changes reflected by the Compton profiles. Indeed, deformations of the Compton profiles are observed when varying hydration, and two different bonding kinds are associated with the water molecules: at low hydration, water surrounds the sulfonic groups, while on increasing hydration, water molecules occupy the interstitial cavities formed upon swelling of the membrane. The analysis is proposed in terms of averaged OH bond length variation. A sizable contraction of the OH distance is observed at low hydration (∼0.09 Å), while at higher hydration levels, the contraction is smaller (∼0.02 Å) and the OH bond length is closer to bulk water. An evaluation of the electron kinetic energy indicates that the spatial changes associated with the water distribution correspond to a consistent binding energy increase. Distinct temperature dependences of each water population are observed, which can be straightly related to water desorption into ice on cooling below the freezing point.

Bottom-Up Approach to Assess the Molecular Structure of Aqueous Poly(N-Isopropylacrylamide) at Room Temperature via Infrared Spectroscopy

The structure of poly(N-isopropylacrylamide) (PNIPAM) in solution is still an unresolved topic. Here, the PNIPAM structure in water was investigated using a bottom-up approach, involving the monomer, dimer, and trimer, and a combination of infrared (IR) spectroscopies as well as molecular dynamics simulations. The experiments show that the monomer and oligomers exhibit a broad and asymmetric amide I band with two underlying transitions, while PNIPAM presents the same major transitions and a minor one. Analysis of the 2D IR spectra and theoretical modeling of the amide I band indicates that the two transitions of the monomer do not have the same molecular origin as the oligomers and the polymer. In the monomer, the two bands originate from the ultrafast rotation of its ethyl group, which leads to different solvation structures for the various rotational conformers. In the case of the oligomers, the asymmetry and splitting of the amide I band is caused by the vibrational coupling among adjacent amide side chains. Moreover, it is deduced from the simulations that the oligomers have three distinct backbone conformations for neighboring amides. In particular, two of the backbone conformations have a closed and compact structure, while in the third, the backbone is open and elongated. The bottom-up approach allowed us to infer that such backbone conformations exist in PNIPAM as well. Consequently, the two major amide I transitions of the polymer are also assigned to split amide I transitions resulting from the vibrationally coupled nearest-neighboring amides. In contrast, the additional minor transition observed in PNIPAM is assigned to unsolvated amide units of the polymer. The proposed molecular model successfully describes that PNIPAM amide I band changes with temperature in terms of its molecular structure. This new model strongly suggests that PNIPAM does not have a completely random backbone structure, but has distinct backbone conformers between neighboring amides.

Coil-Globule Transition Thermodynamics of Poly(N-isopropylacrylamide)

Thermosensitive polymers such as poly(N-isopropylacrylamide) (PNIPAM) undergo a phase transition in aqueous solution from a random-coil structural ensemble to a globule structural ensemble at the lower critical solution temperature (LCST). Above this temperature, PNIPAM agglomerates and becomes insoluble, whereas it is soluble below the temperature. Thus, thermosensitive polymers represent essential targets for several applications, e.g., in drug delivery. Although their ability to change structure in response to a temperature alteration is highly relevant for industrial processes, their thermodynamic properties are mostly qualitatively understood, and the quantitative thermodynamic picture is still elusive. In this study, we used a combined atomistic molecular dynamics and well-tempered metadynamics simulation approach to estimate coil-globule transition thermodynamics. An isotactic 30-mer of PNIPAM was investigated over a broad temperature range between 200 and 360 K. The transition from the globule to the random-coil structure was observed with well-tempered metadynamics. For the first time, the free energy surface of PNIPAM was estimated and it is shown that the simulation results are in line with the experimentally observed thermosensitive behavior. Below the LCST, the random-coil ensemble represents the global energy minimum and is thermodynamically favored by 21 ± 9 kJ/mol compared to the globule ensemble; both are separated by a barrier of 49 ± 14 kJ/mol. In contrast, above the LCST, the globule ensemble is thermodynamically favored by 21 ± 8 kJ/mol over the random-coil ensemble. The barrier from random-coil to globule is 17 ± 10 kJ/mol.

On the molecular origin of the cooperative coil-to-globule transition of poly(N-isopropylacrylamide) in water

The cooperativity of PNIPAM coil-to-globule transition in water arises from the structuring of solvent in proximity to hydrophobic groups.

By means of atomistic molecular dynamics simulations we investigate the behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures below and above the lower critical solution temperature (LCST), including the undercooled regime. The transition between water soluble and insoluble states at the LCST is described as a cooperative process involving an intramolecular coil-to-globule transition preceding the aggregation of chains and the polymer precipitation. In this work we investigate the molecular origin of such cooperativity and the evolution of the hydration pattern in the undercooled polymer solution. The solution behaviour of an atactic 30-mer at high dilution is studied in the temperature interval from 243 to 323 K with a favourable comparison to available experimental data. In the water soluble states of PNIPAM we detect a correlation between polymer segmental dynamics and diffusion motion of bound water, occurring with the same activation energy. Simulation results show that below the coil-to-globule transition temperature PNIPAM is surrounded by a network of hydrogen bonded water molecules and that the cooperativity arises from the structuring of water clusters in proximity to hydrophobic groups. Differently, the perturbation of the hydrogen bond pattern involving water and amide groups occurs above the transition temperature. Altogether these findings reveal that even above the LCST PNIPAM remains largely hydrated and that the coil-to-globule transition is related with a significant rearrangement of the solvent in the proximity of the surface of the polymer. The comparison between the hydrogen bonding of water in the surrounding of PNIPAM isopropyl groups and in the bulk displays a decreased structuring of solvent at the hydrophobic polymer–water interface across the transition temperature, as expected because of the topological extension along the chain of such interface. No evidence of an upper critical solution temperature behaviour, postulated in theoretical and thermodynamics studies of PNIPAM aqueous solution, is observed in the low temperature domain.

Influence of additives on thermoresponsive polymers in aqueous media: a case study of poly(N-isopropylacrylamide)

Thermoresponsive polymers (TRPs) in different solvent media have been studied over a long period and are important from both scientific and technical points of view.

Temperature dependence of the hydrogen bond network in trimethylamine N-oxide and guanidine hydrochloride–water solutions

We present an X-ray Compton scattering study on aqueous trimethylamine N-oxide (TMAO) and guanidine hydrochloride solutions (GdnHCl) as a function of temperature.

Influence of Tacticity on Hydrophobicity of Poly(N-isopropylacrylamide): A Single Chain Molecular Dynamics Simulation Study

Several pieces of experimental evidence show that the water affinity of poly(N-isopropylacrylamide), PNIPAM, decreases with the increase of the degree of isotacticity. To give a rationale to such effect we carried out atomistic molecular dynamics simulations of PNIPAM 30-mers with content of meso dyads, m, of 45% and 59%, assuming a Bernoullian dyad distribution. The single chain behavior of these stereoisomers in water was modeled at 283 and 323 K, i.e., below and above the lower critical solution temperature, LCST. Simulation results show that the dyad composition has influence on size and conformation of the oligomer below the LCST, the isotactic-rich stereoisomer preferring conformations with lower radius of gyration. With reference to the cooperative hydration model of PNIPAM, we analyzed the organization and the dynamics of water in the surroundings of the polymer. Below the LCST the number of hydrogen bonds per water molecule in the first hydration shell and the water surface concentration around PNIPAM are affected by the meso/racemo dyad ratio, showing the higher hydrophobicity of the isotactic-rich system. Above the LCST the subtle effects leading to the modulation of conformation and hydration by means of tacticity are overtaken, and the chain collapse is observed for both systems up to a similar globular state. The comparison of simulation findings of the m 45% stereoisomer with available experimental results of atactic PNIPAM highlights additional molecular details of this macromolecule in aqueous environment. The characteristic time for motion of water molecules in the PNIPAM first hydration shell at 283 K is about 34 ps, in agreement with the experimental value.

Intramolecular structure and energetics in supercooled water down to 255 K

We studied the structure and energetics of supercooled water by means of X-ray Raman and Compton scattering. Under supercooled conditions down to 255 K, the oxygen K-edge measured by X-ray Raman scattering suggests an increase of tetrahedral order similar to the conventional temperature effect observed in non-supercooled water. Compton profile differences indicate contributions beyond the theoretically predicted temperature effect and provide a deeper insight into local structural changes. These contributions suggest a decrease of the electron mean kinetic energy by 3.3 ± 0.7 kJ (mol K)(-1) that cannot be modeled within established water models. Our surprising results emphasize the need for water models that capture in detail the intramolecular structural changes and quantum effects to explain this complex liquid.

Selective adsorption and efficient regeneration via smart adsorbents possessing thermo-controlled molecular switches

A new generation of adsorbents possessing thermo-controlled molecular switches was fabricated and consequently realized selective adsorption and efficient desorption simultaneously.

A miniature closed-circle flow cell for high photon flux X-ray scattering experiments

A closed-circle miniature flow cell for high X-ray photon flux experiments on radiation-sensitive liquid samples is presented. The compact cell is made from highly inert material and the flow is induced by a rotating magnetic stir bar, which acts as a centrifugal pump inside the cell. The cell is ideal for radiation-sensitive yet precious or hazardous liquid samples, such as concentrated acids or bases. As a demonstration of the cell's capabilities, X-ray Raman scattering spectroscopy data on the oxygen K-edge of liquid water under ambient conditions are presented.

Intra- and intermolecular effects on the Compton profile of the ionic liquid 1,3-dimethylimidazolium chloride

We present a comprehensive simulation study on the solid-liquid phase transition of the ionic liquid 1,3-dimethylimidazolium chloride in terms of the changes in the atomic structure and their effect on the Compton profile. The structures were obtained by using ab initio molecular dynamics simulations. Chosen radial distribution functions of the liquid structure are presented and found generally to be in good agreement with previous ab initio molecular dynamics and neutron scattering studies. The main contributions to the predicted difference Compton profile are found to arise from intermolecular changes in the phase transition. This prediction can be used for interpreting future experiments.