The Driving Force For Pnipam50 In A Coiling And Swellign Process

In response to a variety of environmental stimuli (i.e. temperature, pH, light, etc), nanogel is based on biocompatible and temperature-sensitive polymers[1,7, 8], and that can undergo reversible volume-phase transition. Such unique behaviors make the nanogel attractive for a variety of appli-cations in both biomedicine (e.g. "intelligent" drug-delivery system) and electrical engineering (e.g. photonic crystal). In particular, Poly(N-isopropylacrylamide) (PNIPAM) has become "the most popular member" of thermo-sensitive polymer, due to its LCST near human body’s temperature (305 K). It swells (globule-to-coil) at the temperature below LCST, and the conformation of polymer collapses (coil-to-globule) when the temperature increases above LCST. This property of PNIPAM plays an important role in a number of significant practical applications. For example, tuning the LCST of PNIPAM to be close to the human body temperature by copolymerization is a promising method in the controlled drug delivery technology[2,13-15]. Thus it is fundamental important to un-derstand the mechanism of the conformation transitions of PNIPAM around the special LCST in the field of polymer science.In many recent studies, (theoretical and experimental) researchers tried to find the nature of PNIPAM single chain’s phase transition around the LCST. There has been much effort devoted to simulate the phase tran-sition in coiling process (the coil-to-globule transition). For example, Deshmukh et al. find the decreasing of proximal water is an obvious change in inducing the coiling process above LCST.They also used the vibrational spectra to observe the movement of proximal water above LCST, and find the structural stability of proximal water is reduced. While there is still a lack of PNIPAM single chain sim-ulation study in detail about the swelling process. Specially, little is known about the driving force for the opposite process (the glob-ule-to-coil transition). Moreover, the atomic-scale insight to the molecu-lar mechanism of the swelling process that still remained unclear[10,11,16]. One important reason may be that it could be far slower than the coiling process. In general terms, the time of MD simulation is just nearly 10-20 ns for the coiling process. But simulation time scale of the swelling pro-cess is in a long time scale and that requires a large computational re-sources. Another considerable reason may be that the fluctuate inter-chain contact always happen before polymer have a chance to reach the fully swelling thermodynamically stable. Debashish et al. tried to study the concentration driven reentrant collapse and swelling transition of PNIPAM in aqueous methanol and demonstrate the role of the delicate interplay of the different inter-molecular interactions. However, it still cannot be got a detailed study to continuous coil-globule-coil confor-mation transition for PNIPAM around the LCST only in aqueous[17]. But the reversible conformation change is the representative characteristic for the research value of PNIPAM. For example, the elastic conformation changes have coil-globule-coil transition which could satisfy the demand of smart drug transport processes.Here, we employ long time scale MD simulations (more than 1000 ns) to understand the detailed mechanism behind the size-depended (coil-globule-coil) transition of PNIPAM below the LCST. Our work both discusses the coiling (coil-to-globule) and swelling (globule-to-coil) pro-cess for PNIPAM single chain in aqueous below and above the LCST: Oligomer of PNIPAM with 50-mers which is carried out at temperatures above the LCST, namely at 310 K, and we get a globule conformation (GC) after the simulation time of 20 ns. It should be noted that the origi-nal conformation is a completely stranded stretch single PNIPAM in aqueous before the coiling process. Then the globule conformation (GC) is carried out at temperature 295 K (below the LCST) with solution envi-ronment that tested to form a coil conformation (CC) during the swelling process. From now on, we describe a continuous coil-globule-coil process for the PNIPAM single chain in aqueous. We find that the swelling pro-cess mainly reflected in the fall of hydrophobic interactions for PNIPAM single chain. In particular, the interaction between the side-chain nonpolar groups for the interior polymer that played a key role in the both coiling and swelling process. Further, we compared the PNIPAM hydration shell structure under different temperatures (below and above the LCST) at the same time. Moreover, it showed that the influence of the number of proximal water in the side-chain nonpolar group have a considerable dif-ference.In this article, we use molecular dynamics simulation method to re-search PNIPAM50 curled up near the critical phase transition and dy-namics behavior and properties of coiling and swelling process in the molecular scale. The main contents are as follows:The first chapter is mainly introduced this topic research background, content, and the simulation method;The second chapter mainly introduces the force field of molecular dynamics simulation need and the role of general expression. In addition, will also introduce some common force field and is suitable for polymer simulation GROMOS force field;The third chapter introduces the calculation principle of molecular dynamics simulation, including: basic principle, the numerical method for solving the Newton equation of motion, the periodic boundary conditions, the integral step selection process, the molecular dynamics calculation process, the molecular dynamics calculation of initial set, each depart- ment general molecular dynamics calculation method as well as the in-troduction of GROMACS software;The fourth chapter introduces PNIPAM50 model and the main pa-rameters, including: PNIPAM50 model build, PNIPAM50 force field pa-rameters;The fifth chapter introduces PNIPAM50 near the LCST of the phase change process dynamics research, including:when the temperature is higher than the LCST PNIPAM50 curled up in the process of dynamics research, as well as PNIPAM50 at the temperature below the LCST ex-pansion process dynamics research;The sixth chapter was devoted to the research summary and outlook.