Vibrational Spectra of Proximal Water in a Thermo-Sensitive Polymer Undergoing Conformational Transition Across the Lower Critical Solution Temperature (original) (raw)
Related papers
Polymer, 2012
Molecular dynamics simulations (MD) are used to calculate the vibrational spectra of a thermo-sensitive oligomer, namely, poly(N-isopropylacrylamide) (PNIPAM) and a non-thermo-sensitive oligomer, namely, poly(acrylamide) (PAAM) and characterize the atomic scale conformations. Despite the structural similarity between the two polymers, the response of PNIPAM and PAAM to a thermal stimulus is widely different; a coil-to-globule transition is observed for PNIPAM above a lower critical solution temperature (LCST) of 305 K whereas the same is absent in PAAM. Simulations in both the cases are performed above and below the LCST of PNIPAM, namely at 278 K and 310 K, to evaluate the effect of temperature on the polymer conformations. The vibrational spectra of bonds involving atoms from the polymer backbone and the various side-groups (amide I, amide II, and isopropyl group of PNIPAM and amide I and amide II group of PAAM) of the polymers were analyzed to study the conformational changes in the polymer. The differences in the vibrational spectra are used to understand the dynamics of conformational transitions in the two polymers and identify the changes in the relative interactions between various atoms in the backbone and in the side groups of the polymer with water at two different temperatures, namely at 278 K and 310 K. The systematic trends in the observed peak intensities and frequency shifts at the low, medium, and high frequency end of the spectrum for the various atoms in the two polymers are rationalized on the basis of bond-lengths, local coordination, strength of hydrogen bonding, and neighboring solvation environment. The analysis of the vibrational spectra for amide I and amide II regions of PNIPAM suggests a coil-to-globule transition in going from 278 K to 310 K. The differences are evaluated in terms of the strength, stability, and structure of the hydrogen-bond network between polymer and polymer and between polymer and water. Comparison of the vibrational spectra of isopropyl groups in PNIPAM at 278 K and 310 K suggests dehydration of the isopropyl moieties at 310 K. In the case of PNIPAM, we observe that polymer-water interactions are dominant below the LCST whereas polymerepolymer interactions dominate above the LCST. On the other hand, the vibrational spectra of amide I and amide II group of PAAM, at 278 K and 310 K, do not show any significant difference in terms of the interactions between polymer and polymer and interactions between polymer and water. Analysis of the peak intensities, of the amide II stretching band, observed in the frequency range 3500e3700 cm À1 suggests that the fraction of bonded and non-bonded hydrogen atoms are similar at both 278 K and 310 K. This indicates that the interactions between polymer and polymer and between polymer and water are similar at both the temperatures. The interactions between PAAM and its surrounding environment are found to be unaffected as the temperature is raised from 278 K to 310 K. Comparisons with experimental studies are made where possible. Our study provides useful insights into the nature of various inter-molecular interactions and their role in influencing the atomic scale conformational dynamics in oligomers.
Structural Dynamics of Neighboring Water Molecules of N-Isopropylacrylamide Pentamer
ACS Omega
Poly(N-isopropylacrylamide) (PNIPAM) is a popular polymer widely used in smart hydrogel synthesis due to its thermo-responsive behavior in aqueous medium. Aqueous PNIPAM hydrogels can reversibly swell and collapse below and above their lower critical solution temperature (LCST), respectively. The present work used molecular dynamics simulations to explore the behavior of water molecules surrounding the side chains of a NIPAM pentamer in response to temperature changes (273−353 K range) near its experimental LCST (305 K). Results suggest a strong inverse correlation of temperature with water density and hydrophobic hydration character of the first coordination shell around the isopropyl groups. Integrity of the first and second coordination shells is further characterized by polygon ring analysis. Predominant occurrence of pentagons suggests clathrate-like behavior of both shells at lower temperatures. This predominance is eventually overtaken by 4membered rings as temperature is increased beyond 303 and 293 K for the first and second coordination shells, respectively, losing their clathrate-like property. It is surmised that this temperature-dependent stability of the coordination shells is one of the important factors that controls the reversible swell-collapse mechanism of PNIPAM hydrogels.
The Journal of Physical Chemistry B, 2009
Poly(N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize dynamic light scattering, H-NMR, and steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAM's hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in interamide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal CdO hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a monoexponential collapse with τ ∼ 360 ((85) ns. The collapse rate indicates a persistence length of n ∼ 10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas at lengths longer than the persistence length the polymer backbone conformation forms a random coil. On the basis of these results, we propose the following mechanism for the PNIPAM volume phase transition. At low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. As the temperature increases an increasing entropic penalty occurs for the water molecules situated at the surface of the hydrophobic isopropyl groups. A cooperative transition occurs where hydrophobic collapse minimizes the exposed hydrophobic surface area. The polymer structural change forces the amide carbonyl and N-H to invaginate and the water clusters cease to be stabilized and are expelled. In this compact state, PNIPAM forms small hydrophobic nanopockets where the (i, i + 3) isopropyl groups make hydrophobic contacts. A persistent length of n ∼ 10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM.
BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI. Secția Matematica. Mecanică Teoretică. Fizică
Molecular dynamics (MD) simulations are intensively used to obtain information on the microscopic behavior of a system and to evaluate certain properties which they determine at macroscopic level. But, as well as setting up an experiment in a “wet” laboratory, also setting up a simulation in a virtual enviroment implies many variables that influence the results. Two of the variables are the starting structure of the system and the water model in which it is solvated. The present work aims to identify an optimal combination of this two variables able to reveal best the transition from an extended to a collapsed conformation for poly(n-isopropylacrylamide) (PNIPAM) thermoresponsive polymer.
Molecular dynamics simulation of a model oligomer for poly(N-isopropylamide) in water
Chemical Physics Letters, 2004
Molecular dynamics (MD) has been used to simulate a dilute aqueous solution of a 50-units oligomer model for the thermoresponsive polymer poly(N-isopropylacrylamide) at 300 and 310 K, i.e., below and above its lower critical solution temperature (LCST) in water. Statistical analyses of the system trajectories show that at 310 K the oligomer exhibits a more compact conformation than at 300 K, in qualitative agreement with experiments, and that it is surrounded by a smaller number of first-hydrationshell water molecules.
The Journal of …, 2008
We have performed 75-ns molecular dynamics (MD) simulations of aqueous solutions of a 26-unit NIPAAm oligomer at two temperatures, 302 and 315 K, below and above the experimentally determined lower critical solution temperature (LCST) of p(NIPAAm). We have been able to show that at 315 K the oligomer assumes a compact form, while it keeps a more extended form at 302 K. A similar behavior has been demonstrated for a similar NIPAAm oligomer, where two units had been substituted by methacryloyl-l-valine (MAVA) comonomers, one of them being charged and one neutral. For another analogous oligomer, where the same units had been substituted by methacryloyl-l-leucine (MALEU) comonomers, no transition from the extended to the more compact conformation has been found within the same simulation time. Statistical analysis of the trajectories indicates that this transition is related to the dynamics of the oligomer backbone, and to the formation of intramolecular hydrogen bonds and water-bridges between distant units of the solute. In the MAVA case, we have also evidenced an important role of the neutral MAVA comonomer in stabilizing the compact coiled structure. In the MALEU case, the corresponding comonomer is not equally efficacious and, possibly, is even hindering the readjustment of the oligomer backbone. Finally the self-diffusion coefficient of water molecules surrounding the oligomers at the two temperatures for selected relevant times is observed to characteristically depend on the distance from the solute molecules.
The role of polymer structure on water confinement in poly(N-isopropylacrylamide) dispersions
Journal of Molecular Liquids, 2022
Poly(N-isopropylacrylamide) (PNIPAM) is a synthetic polymer that is widely studied for its thermoresponsive character. However, recent works also reported evidence of a low temperature (protein-like) dynamical transition around 225 K in concentrated PNIPAM suspensions, independently of the polymer architecture, i.e., both for linear chains and for microgels. In this work, we investigate
The Journal of Physical Chemistry B, 2012
Dielectric relaxation spectroscopy has been utilized for studying the molecular dynamics of polymer solutions. 1À6 In the case of polymer solutions composed of polar solvents in a solvent-rich region, relaxation processes due to the reorientation of dipoles of solvents and polymer chains are observed separately at higher and lower frequencies, respectively. 3À5,7,8 Typically, the relaxation process observed at frequencies on the order of 10 GHz is associated with the molecular motion of solvent molecules, while the relaxation process observed at kHz-MHz frequencies is attributed to the relaxation modes of polymer chains. The relaxation process owing to solvent molecules is affected by the addition of polymers. 3,4,9À11 This implies that the dynamical structures of solvent molecules are related to the polymers through interactions at the molecular level. The relaxation process that arises from the polymer chains should also be affected by the solvent molecules. This interdependence of polymer chains and solvent molecules can be analyzed by investigating the dielectric relaxation spectrum as a function of concentration and/or temperature. Dielectric relaxation spectra can be described by, for example, the relaxation time, the relaxation strength, and the shape parameter characterizing the distribution of the relaxation process. Therefore, the relaxation parameters obtained by the variation of polymer concentration or temperature, as well as the solvent species, can provide important information leading to greater understanding of molecular interactions. Recently, the relaxation processes of polymer chains and solvent molecules have been studied systematically for the poly(vinylpyrrolidone) [PVP] system in various polar and nonpolar solvents in broad temperature and frequency ranges. 3À5 It has been revealed that the cooperation between segmental motion and the reorientation of solvent molecules provides intrinsic information about the molecular dynamics of polymer solutions. In this study, we report the experimental results of dielectric relaxation behavior for the systems of poly(N-isopropylacrylamide) (PNiPAM) in protic and aprotic solvents as a function of PNiPAM concentration studied by dielectric relaxation spectroscopy. An aqueous solution of PNiPAM has a Θ-temperature of 30.6°C and undergoes a coilÀglobule transition upon heating. 12À15 The transition of PNiPAM chains in water is also observed upon the addition of a second water-miscible solvent, such as methanol,