Study On The Conformational Transition Of Poly (N-isopropylacrylamide) By Computer Simulation

The coil-to-globule transition of macromolecule is one of the most important fundamental problems both in theory and applications. The theories related to this topic are mainly concentrated in the description of the change of the configuration and size of the polymer chains thermodynamically and kinetically during the last 40 years. However, it is insufficient for understanding well the transition mechanism of polymer chain including the solvation effects during the process of conformational transition in molecular level and quantificational description.Poly(N-isopropylacrylamide) (PNiPA), which consists of both hydrophilic and hydrophobic group in each repeat unit, shows LCST in its solution, and is a very famous water soluble thermo-sensitive polymer. The coil-to-globule transition can take place within several degrees around LCST, and the hydrodynamic volume of polymer chain will decrease several hundred times. In this thesis, both quantum chemical methods and molecular dynamics simulations are employed to study the thermal phase transitions of PNiPA from the electronic and molecular structures. Various effects, including structural factor and solvent components, on the conformational transition have been investigated, respectively. Some interesting results have been got as follow:(1) Molecular dynamics simulations with PCFF have been adopted to study the conformation state of PNiPA below and above LCST, respectively. The molecular interactions among solutes and solvents:polymer segment…solvent and polymer segment…segment interactions have been investigated carefully. Below LCST, the former interaction is dominant, and PNiPA chain is solvated well and shows a random coil conformation; while temperature is raised above LCST, the chains collapse into a compacted globule gradually. Further investigations indicate that desolvation process is occurred on both (C) O and (N) H group of NiPA with the increase of temperature, and polymer segments begin to aggregate together, which illuminate that polymer segment…segment interactions are enhanced. The simulation results are qualitatively consistent with those from experiments.(2) Poly(N-alkylacrylamide) bears its LCST around 32℃in aqueous solution. However, it is found that various substituting groups on its main or side chain would greatly affect its phase transition behaviors and result in different levels of LCST. Based on a structural formula of a repeat unit of CH3CH(R1)(CONHR2), where R1 is H or CH3 and R2 is isopropyl or n-propyl, there exist three kinds of thermo-sensitive polymers with various structures:poly(N-isopropylacrylamide) (PNiPA), poly(N-n-propylacrylamide) (PNnPA), and poly(N-isopropylmethacrylamide) (PNiPMA), respectively, as shown in Figure 1. These polymers exhibit different levels of LCST in aqueous solutions. In this thesis, the monomers of these polymers are selected to demonstrate the cause of different LCST levels in aqueous solutions for various molecular structures using molecular dynamics simulations and quantum mechanics calculations. The monomers have functional groups of differing steric volume that greatly affect the conformational transition of chains and LCST levels of the polymers. A branched chain of N-propyl group in N-isopropylacrylamide and an additional methyl group at a-carbon in N-isopropylmethacrylamide both increased the steric effect, making it more difficult for monomers to draw closer and resulting in higher LCST levels of the polymers. (3) In addition to structure factor, the solvent component could affect the conformational transition of PNiPA also. It is reported that water and methanol are both good solvent for PNiPA at room temperature, but their mixture, with mole fractions of methanol (xmethanol) from 0.25 to 0.80, is bad one. With the increase of xmethanol, the coil-to-globule-to-coil transition of PNiPA would take place. The reentrant transition behaviors including solvation effects during conformational transition process in water/methanol mixtures are investigated by molecular dynamics simulations from monomer of PNiPA. The results indicate that NIPAM-solvent interactions are weakened with the increase of xmethanol from 0.25 to 0.80, and the total hydrogen bonding number between solute and solvent decreases, rationalizing the reentrant coil-to-globule-to-coil transition behaviors of PNiPA in the mixed solvents, as shown in Figure 2. Interestingly, hydrogen-bonded water-methanol clusters are abundant in binary mixed solvents, leading to the decrement of NIPAM-solvent interactions. To better understand the intermolecular interactions in the water-methanol complex clusters, the structures of pure water and methanol clusters are also studied, respectively. Although the amount of water clusters decreases with the increase of xmethanol, the structure of water clusters still keeps stable, and hydrogen-bonded networks are not essentially disrupted. As for methanol molecules, they prefer to form short non-branched chain-like hydrogen-bonded clusters. However, most of the chain-like hydrogen-bonded methanol clusters are broken in water-rich solutions, leaving the little probability of the formation of dimeric and trimeric methanol clusters.