Summary of the research

Polymer is components of materials such as elastomer and plastics. Furthermore, many biomaterials, for example, DNA and proteins, are also polymers. Properties of polymers are featured by composition of atoms and the dynamics of polymer chains. At atomic scale, quantum chemistry is useful and statistical physics is effective for polymer chain dynamics in nm –μm (mesoscopic) scales. In the study of polymers, it is impossible to reveal the properties when all the information is considered because degree of freedom becomes enormous and the systems are very complicated. Thus, based on polymer chain dynamics, I focused on qualitative properties and the universal behavior of polymers, rather than the detailed polymer composition. Final goal is to reveal the structures and properties at molecular level, leading to the design of materials and understanding of the functions in biomaterials. It is noted that practical materials and biomolecules should be considered, which means that we should solve inverse problem but not direct problem. (i) Mechanical properties of polymeric materials and (ii) conformational transition of DNA and bio-membrane are studied. Moreover, (iii) other materials are studied based on quantum chemical simulations.

(i) Fracture processes of polymers

For the safety and resource saving, one of the severe problems is fracture of materials by stretching and shock. The toughness against the stretching and shock is evaluated by relevance of mechanical properties on macro scale via experimental method; however, the understanding the deformation and fracture processes of polymers in mesoscopic scale based on molecular theory is also important to improve mechanical properties. Thus, fracture process of polymers have been studied by coarse-grained molecular dynamics simulation using beads-spring model.

In the previous simulations studies, the lamellar structure was not constructed, which is the basic structure of crystalline polymers, and also could not show the stress-strain curve observed in the experiments. I have established the molecular technology to reveal the mechanical properties of polymers at the molecular level. Then, the lamellar structure of polyethylene, a kind of crystalline polymers, is successfully constructed and the stress-strain curve is in agreement with the experiment. The generation of voids in amorphous layers, its growth, bucking of crystalline layers, and fragmentation of crystalline layers are found. It is revealed that void is generated by movement of chains ends and they act as defect.

The molecular technology is applied to other polymeric materials. The fracture process of double-network (DN) gels consisting of highly and slightly cross-linked networks, which exhibit good mechanical properties, complicated deformation behavior, and fracture processes owing to the existence of a large number of influential parameters, is successfully revealed at the molecular level. The wear of polymer brushes which are good friction properties is also successfully revealed at the molecular level.

(ii) Conformational transition of biomaterials

The conformation and its dynamics of DNA and bio-membrane are related to biology functions and therefore important factors. Many ions and proteins exist in a cell and the structure of DNA and bio-membrane is complicated. Then, to reveal the conformation of DNA and bio-membrane in the complex system, findings of the essential factors are needed using by statistical approach. In mesoscopic scale, DNA is treated as long polymer chain which has stiffness. Bio-membrane can be simply modeled by bilayer consisting of hydrophilic head bead followed by two hydrophobic tail beads. Based on this background, conformation transition of DNA has been studied by coarse-grained simulation.

・The effect of chain length on conformational transition of DNA

Since DNA is very long polymer chain, the effect of chain length on the conformational transition is important. It is unraveled that the transition point changes with an increase in chain length. New structure appears in long chain case and its physical property is revealed.

・Conformational transition of DNA in a small cell

In vivo, DNA is confined in a small cell and therefore conformational transition in a small space is important. It is found that transition point changes to poor solvent because of repulsive force in flexible polymer. In contrast, the transition point of semiflexible polymer changes to good solvent because of a decrease in entropy. The relevance of conformational transition in vivo model is meaningful in biology and the understanding of the phase transition of polymer closed in small space is meaningful in physics.

・Torsional effect on nucleosome

The basic unit of DNA in eukaryote is nucleosome, where DNA wraps around a cationic protein. In this structure, it is predicted that torsional force caused by double stranded structure affects its wrapping.

・Dynamics of bio-membrane

Phase separation and conformation of lipid bilayer is studied. The effect of charge on them is successfully revealed and the pore formation is observed.

(iii) Material design based on quantum chemical calculations

Recently, the manufacturing size of materials becomes atomic scale. The problems of manufacturing process are “chemical reaction dynamics with impact, friction, and stress” and “influence of micro scale on macro scale”. By using quantum chemical molecular dynamics method, crystal growth process, etching process, friction process, and degradation of polymers have been revealed. The influence of micro scale on macro scale in the fracture process of ceramics and crystal growth process has been revealed. These relevance are based on statistical physics and quantum chemistry. This combination method is unique and effective.