Conformational transitions in thermo-sensitive polymers are critical in determining their functional properties. The atomistic origin of polymer collapse at the lower critical solution temperature (LCST) remains a fundamental and challenging problem in polymer science. Here, molecular dynamics simulations are used to establish the role of solvation dynamics and local ordering of water in inducing conformational transitions in isotactic-rich poly(N-isopropylacrylamide) (PNIPAM) oligomers when the temperature is changed through the LCST. Simulated atomic trajectories are used to identify stable conformations of the water-molecule network in the vicinity of polymer segments, as a function of the polymer chain length. The dynamics of the conformational evolution of the polymer chain within its surrounding water molecules is evaluated using various structural and dynamical correlation functions. Around the polymer, water forms cage-like structures with hydrogen bonds. Such structures form at temperatures both below and above the LCST. The structures formed at temperatures above LCST, however, are significantly different from those formed below LCST. Short oligomers consisting of 3, 5, and 10 monomer units (3-, 5-, and 10-mer), are characterized by significantly higher hydration level (water per monomer ~ 16). Increasing the temperature from 278 to 310 K does not perturb the structure of water around the short oligomers. In the case of 3-, 5-, and 10-mer, a distinct coil-to-globule transition was not observed when the temperature was raised from 278 to 310 K. For a PNIPAM polymer chain consisting of 30 monomeric units (30-mer), however, there exist significantly different conformations corresponding to two distinct temperature regimes. Below LCST, the water molecules in the first hydration layer (~12) around hydrophilic groups arrange themselves in a specific ordered manner by forming a hydrogen-bonded network with the polymer, resulting in a solvated polymer acting as hydrophilic. Above LCST, this arrangement of water is no longer stable, and the hydrophobic interactions become dominant, which contributes to the collapse of the polymer. Thus, this study provides atomic-scale insights into the role of solvation dynamics in inducing coil-to-globule phase transitions through the LCST for thermo-sensitive polymers like PNIPAM.

No abstract

Type III deep eutectic solvents (DES) have attracted significant interest as both environmentally friendly and functional solvents that are, in some ways, advantageous to traditional aqueous systems. While these solvents continue to produce remarkable thin films and nanoparticle assemblies, their interactions with metallic surfaces are complex and difficult to manipulate. In this study, the near-surface region (2–600 nm) of a carbon surface is investigated immediately following silver nanoparticle nucleation and growth. This is accomplished, in situ, using a novel grazing transmission small-angle X-ray scattering approach with simultaneous voltammetry and electrochemical impedance spectroscopy. With this physical and electrochemical approach, the time evolution of three distinct surface interaction phenomena is observed: aggregation and coalescence of Ag nanoparticles, multilayer perturbations induced by nonaggregated Ag nanoparticles, and a stepwise transport of dissolved Ag species from the carbon surface. The multilayer perturbations contain charge-separated regions of positively charged choline-ethylene and negatively charged Ag and Cl species. Both aggregation-coalescence and the stepwise decrease in Ag precursor near the surface are observed to be very slow (∼2 h) processes, as both ion and particle transport are significantly impeded in a DES as compared to aqueous electrolytes. Altogether, this study shows how the unique chemistry of the DES changes near the surface and in the presence of nanoparticles that adsorb the constituent species.

Soft-polymer based microparticles are currently being applied in many biomedical applications, ranging from bioimaging and bioassays to drug delivery carriers. As one class of soft-polymers, hydrogels are materials, which can be used for delivering drug cargoes and can be fabricated in controlled sizes. Among the various hydrogel-forming polymers, poly(ethylene glycol) (PEG) based hydrogel systems are widely used due to their negligible toxicity and limited immunogenic recognition. Physical and chemical properties of particles (i.e., particle size, shape, surface charge, and hydrophobicity) are known to play an important role in cell-particle recognition and response. To understand the role of physicochemical properties of PEG-based hydrogel structures on cells, it is important to have geometrically precise and uniform hydrogel structures. To fabricate geometrically uniform structures, we have employed electron beam lithography (EBL) and ultraviolet optical lithography (UVL) using PEG or PEG diacrylate polymers. These hydrogel structures have been characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), optical microscopy, and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) confirming control of chemistry, size, and shape.

Molecular dynamics simulations in conjunction with finite element calculations are used to explore the conformational dynamics of a thermo-sensitive oligomer, namely poly(N-isopropylacrylamide) (PNIPAM), subjected to an ultra-fast heating-cooling cycle. Finite element (FE) calculations were used to predict the temperature profile resulting from laser-induced heating of the polymer-aqueous system. The heating rate (∼0.6 K ps(-1)) deduced from FE calculations was used to heat an aqueous solution of PNIPAM consisting of 30 monomeric units (30-mer) from 285 K to 315 K. Non-equilibrium effects arising from the ultra-fast heating-cooling cycle results in a hysteresis during the coil-to-globule transition. The corresponding atomic scale conformations were characterized by monitoring the changes in the vibrational spectra, which provided a reliable metric to study the coil-to-globule transition in PNIPAM and vice-versa across the LCST. The vibrational spectra of bonds involving atoms from the oligomer backbone and the various side-groups (amide I, amide II, and the isopropyl group of PNIPAM) of the oligomers were analyzed to study the conformational changes in the oligomer corresponding to the observed hysteresis. The differences in the vibrational spectra calculated at various temperatures during heating and cooling cycles were used to understand the coil-to-globule and globule-to-coil transitions in the PNIPAM oligomer and identify the changes in the relative interactions between various atoms in the backbone and in the side groups of the oligomer with water. The shifts in the computed vibrational spectral peaks and the changes in the intensity of peaks for the different regions of PNIPAM, seen across the LCST during the heating cycle, are in good agreement with previous experimental studies. The changes in the radius of gyration (Rg) and vibrational spectra for amide I and amide II regions of PNIPAM suggest a clear coil-to-globule transition at ∼301 K during the heating cycle from 285 K to 315 K. During the heating cycle, a comparison of the vibrational spectra of isopropyl groups in PNIPAM at 285 K and 315 K suggests dehydration of the isopropyl moieties at 315 K. This implies that the oligomer-water interactions are dominant below the LCST whereas oligomer-oligomer interactions pre-dominate above the LCST. On the other hand, during the cooling cycle minor changes in the Rg and vibrational spectra of the PNIPAM oligomer in going from 315 K to 285 K indicate that the interactions between oligomer-oligomer and between the oligomer and water are less perturbed during the cooling cycle. Our simulations suggest that the observed hysteresis is a consequence of ultrafast heating-cooling kinetics, which allows insufficient relaxation times for the solvated oligomer.