Abstract
We revived and implemented a method developed by Kuhn in 1934, originally only published in German, that is, the so-called “freely jointed chain” model. This approach turned out to be surprisingly useful for analyzing state-of-the-art computer simulations of the thermosensitive coil–globule transition of N-Isopropylacrylamide 20-mer. Our atomistic computer simulations are orders of magnitude longer than those of previous studies and lead to a reliable description of thermodynamics and kinetics at many different temperatures. The freely jointed chain model provides a coordinate system, which allows us to construct a Markov state model of the conformational transitions. Furthermore, this guarantees a reliable reconstruction of the kinetics in back-and-forth directions. In addition, we obtain a description of the high diversity and variability of both conformational states. Thus, we gain a detailed understanding of the coil–globule transition. Surprisingly, conformational entropy turns out to play only a minor role in the thermodynamic balance of the process. Moreover, we show that the radius of gyration is an unexpectedly unsuitable coordinate to comprehend the transition kinetics because it does not capture the high conformational diversity within the different states. Consequently, the approach presented here allows for an exhaustive description and resolution of the conformational ensembles of arbitrary linear polymer chains.
Highlights
Thermosensitive polymers have been of major interest in many fields of research.[1]
We show the conformational entropy of the polymer at different temperatures within the ensembles of the two conformational states, that is, coil and globule
Having invested a substantial amount of simulation time, we were able to sample the conformational dynamics of the NIPAAM 20-mer at different temperatures with superior accuracy
Summary
Thermosensitive polymers have been of major interest in many fields of research.[1]. Besides various medical applications, for example, drug carriers and synthetic tissues, they have been proven to be applicable in gel actuators and oil refineries.[2−5] Interestingly, thermosensitive polymers undergo a phase transition with a lower critical solution temperature (LCST).[6,7] this phase transition is connected to a conformational change of the polymer chains, that is, the coil− globule transition (CGT).[8,9] Possibly, the most prominent example of such a polymer is N-Isopropylacrylamide (NIPAAM).[10,11] A schematic visualization of the CGT and the chemical formula of the NIPAAM monomer are given in the supporting information. The origins of this transitions are not fully understood yet
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