Atomistic Molecular Dynamics Simulations of the Lower Critical Solution Temperature Transition of Poly(N-vinylcaprolactam) in Aqueous Solutions

Abstract

Poly­(N-vinylcaprolactam) (PVCL) is a thermo-responsive polymer, which exhibits a lower critical solution temperature (LCST) in an aqueous solution. The LCST of this hydrophilic-to-hydrophobic transition is found to be strongly dependent on the salt-type and salt-concentration as well as on the molecular weight and concentration of the polymer. Here, atomistic molecular dynamics simulations have been successfully conducted for the first time to investigate the LCST transition of a 100 degree of polymerization PVCL chain in water, 1 M NaCl, 3.5 M NaCl, and 0.5 M CaCl2 solutions. Our results show that steric hindrance resulting from the bulky 7-member ring on the PVCL chain plays a critical role in the conformational transition. Moreover, the degrees of hydration and dehydration below or above the transition temperature are highly dependent on the specific solution condition and temperature. Water molecules are found to be trapped inside the collapsed polymer chains leading to the varying degrees of hydration and dehydration of the polymer chain in different solutions. Calculated water diffusion coefficients for both trapped and free water molecules agree very well with experimental measurements.

Cumulative Submillisecond All-Atom Simulations of the Temperature-Induced Coil-to-Globule Transition of Poly(N-vinylcaprolactam) in Aqueous Solution

“…The computed atomic partial charges are similar between respective atoms of the repeating units of PVP and PNVCL (Figure ). The geometry optimization at the ab initio-level yielded an envelope conformer of the pyrrolidine ring and a chair conformer of the azepane ring, which is in agreement with other quantum-mechanical calculations as well as experimental studies on vinylpyrrolidone derivatives, such as N -methylpyrrolidone and vinylcaprolactam. , The SEM of the atomic partial charges across the set of eight low-energy conformers of PNVCL is small (<0.02 e for 28 atoms (90%) and <0.04 e for the remaining three atoms; Figure S6) and within the range of deviations found among other published parameterizations. , The largest SEM values are found for the backbone carbon atom connected to the nitrogen, the nitrogen itself, and the C α atom. The partial charges and positions for all atoms of the PNVCL and PVP repeating units are listed in Tables S5 and S6, respectively.…”

“…97,98 The SEM of the atomic partial charges across the set of eight low-energy conformers of PNVCL is small (<0.02e for 28 atoms (90%) and <0.04e for the remaining three atoms; Figure S6) and within the range of deviations found among other published parameterizations. 35,96 The largest SEM values are found for the backbone carbon atom connected to the nitrogen, the nitrogen itself, and the C α atom. The partial charges and positions for all atoms of the PNVCL and PVP repeating units are listed in Tables S5 and S6, respectively.…”

“…Attempts to investigate the LCST behavior of PNVCL by all-atom MD simulations have been comparatively sparse, − and evaluations of the underlying thermodynamics, which is of great importance to fully understand the coil-to-globule transition, have been even rarer . Moreover, to our knowledge, there is no study in which, in a comparative manner, the behavior of PVP was probed under identical simulation conditions.…”

“…8,35−39 These include MD simulations at the coarse-grained, 39 unitedatom, 36 and atomistic levels. 8,35 While indicating that modern MD simulations are suitable to investigate the coil-to-globule transition in PNVCL at the LCST, the majority of the published simulation data 35,36,40−42 for thermoresponsive polymers describe the collapse at the LCST as a seemingly irreversible process because often no transition from the globule back to the coil is observed during the simulation time, which contradicts experimental observations; 34,43 notable exceptions exist. 38,44,45 Furthermore, a systematic assessment of the influence of polymer length, tacticity, and concentration on the thermoresponsiveness observed in MD simulations is rare.…”

“…First studies on the thermoresponsiveness of PNVCL and the effect of the salt concentration and types on the LCST using MD simulations have been reported recently. ,− These include MD simulations at the coarse-grained, united-atom, and atomistic levels. , While indicating that modern MD simulations are suitable to investigate the coil-to-globule transition in PNVCL at the LCST, the majority of the published simulation data ,,− for thermoresponsive polymers describe the collapse at the LCST as a seemingly irreversible process because often no transition from the globule back to the coil is observed during the simulation time, which contradicts experimental observations; , notable exceptions exist. ,, Furthermore, a systematic assessment of the influence of polymer length, tacticity, and concentration on the thermoresponsiveness observed in MD simulations is rare. Finally, while most MD simulation studies evaluate structural parameters of the coil-to-globule transition, energetic determinants of the transition have only been computed for interactions between two isolated NVCL ( N -vinylcaprolactam) monomers so far …”