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.

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The x-ray fluorescence and absorption of highly oriented pyrolytic graphite have been measured using monochromatic synchrotron radiation. The spectra can be separated into contributions from~and oband components by measuring at different angles of incidence and at different emission angles. The shape of the x-ray fluorescence spectra varies dramatically with excitation energy near the C E edge.This dependence on excitation energy can be interpreted within a resonant-inelastic-scattering formalism. The results are compared with previously published band-structure calculations and photoemission results, and demonstrate the potential for using x-ray fluorescence to obtain symmetry-resolved band information.

Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), are presented. Single shot measurements were performed at 780 eV x-ray photon energy using apertures containing double pinholes in "diffract and destroy" mode. We determined a coherence length of 17 µm in the vertical direction, which is approximately the size of the focused LCLS beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.6 fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.

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