Hydrogen Bonding Sequence Directed Coil-Globule Transition in Water Soluble Thermoresponsive Polymers

Abstract

The origin of the coil-globule transition for water-soluble thermoresponsive polymers frequently used in nanomaterials remains elusive. Using polypropylene oxide as an example we demonstrate by means of atomistic molecular dynamics simulations that temperature-induced increase in the sequence length of monomers that are not hydrogen bonded to water drives the coilglobule transition. Longer chains statistically exhibit longer sequences which serve as nucleation sites for hydrophobic cluster formation facilitating chain collapse at lower temperature in agreement with experimental data.

Molecular Structure and Co-solvent Distribution in PPO–PEO and Pluronic Micelles

“…41,42 The force fields for PPO and PEO are the same as in our earlier papers on the behavior of corresponding polymers in aqueous solutions. 27,38 The standard OPLS force fields for isobutyric acid (IBA) and glycine have been used, while for alcohols (ethanol, butanol, hexanol), refined force fields 43,44 were employed. The polymer micelles were preassembled from monodisperse copolymers using Packmol 45,46 and had the following aggregation numbers based on experimentally reported values: 67 for P 29 E 26 and 12 for P 14 E 24 P 14 , reverse Pluronic 17R4, and several aggregation numbers: 40, 50, and 76 for E 13 P 30 E 13 , Pluronic L64.…”

“…We performed atomistic molecular dynamics simulations of micelles in aqueous solution. The simulations were performed using the GPU-enabled version of GROMACS 2020.4 on the Extreme Science and Engineering Discovery Environment (XSEDE) cluster using the OPLS force field with SPC/E water model. , The force fields for PPO and PEO are the same as in our earlier papers on the behavior of corresponding polymers in aqueous solutions. , The standard OPLS force fields for isobutyric acid (IBA) and glycine have been used, while for alcohols (ethanol, butanol, hexanol), refined force fields , were employed. The polymer micelles were pre-assembled from monodisperse copolymers using Packmol , and had the following aggregation numbers based on experimentally reported values: 67 for P 29 E 26 and 12 for P 14 E 24 P 14 , reverse Pluronic 17R4, and several aggregation numbers: 40, 50, and 76 for E 13 P 30 E 13 , Pluronic L64. ,,,− , The details of micelle simulations and methodology of co-solvent addition are discussed in the Supporting Information.…”

“…Chu et al demonstrated that while L64 and 17R4 Pluronics both form micelles at 42.5 and 40 °C, respectively, L64 has a lower critical micelle concentration and noticeably higher aggregation number (88) compared to 17R4 (aggregation number 10). Reverse Pluronics exhibit a weaker aggregation capability, not only because of the longer PEO block (and therefore better solubility) but primarily because of the shorter PPO block, for which the solubility is highly sensitive to temperature. ,, As these examples demonstrate, for these polymers, not only is the total hydrophobic/hydrophilic balance important but also the specific distribution of PEO and PPO monomers along the chain, which in turn influences block packing in the micelle and solvent distribution. Computer simulations can provide significant insights into these properties.…”

Coil–Globule Transition of a Water-Soluble Polymer

“…The final collapsed globule is composed of stacked beads of different sizes but is not a uniform, compact globule. These new findings are quite different from the traditional theory where unwound segments aggregated into only one uniform globule. − Under the effect of both salt and thermal stimuli, PNIPAM undergoes the same collapse process, which can be regarded as an inherent property (due to the presence of both hydrophobic and hydrogen-bonding interactions). We name this collapse process composed of nucleation, adjacent mergence, and stacking as the NAS transition.…”

“…At low temperatures and in good solvents, PNIPAM has an extended coil conformation, while at high temperatures and in poor solvents PNIPAM exists as a collapsed globule. The current theory of coil–globule transition for PNIPAM consists of a random-size nucleation process, in which the structures were not identified, and a coarsening process, in which nuclei larger than the critical size continued to swallow in its connected adjacent unwound segments to ultimately form a single uniform globule, whereas nuclei smaller than the critical size dissolved into unwound segments. − However, some recent research shows the radius of gyration that is the main basis of the present theory is an unsuitable coordinate to comprehend the transition kinetics because it does not capture the high conformational diversity within different states . How a single polymer chain transforms dynamically remains unknown because there is no experimental evidence for the actual structures that form during nucleation and coarsening or in the final globule.…”