Chunxia Chen

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Duration in group: 2001 - 2007

Research Summary

Intermolecular Packing and Segmental Dynamics in Poly(ethylene oxide)/Poly(methyl methacrylate) Blends

Introduction
There has been an increasing interest in polymer blends during last decades due to the development of new materials with desirable properties including flexibility, processing temperature, viscosity, etc. The packing structure and dynamics are important because they control the polymer properties; however, they are not well understood so far. Our research aims to develop a molecular-level understanding of packing and short-time dynamics in polymeric materials. This work seeks to address the following two questions: What is the packing structure and short-time dynamics of the investigated materials? How is the packing related to the relaxation processes? How do the structure and dynamics of the polymer change with the addition of another polymer? Poly(ethylene oxide)/Poly(methyl methacrylate) (PEO/PMMA) blends are chosen as the system because the two polymers have weak interactions (? ? 0) and a large difference in glass transition temperatures (?Tg?180K). This large Tg difference will make it easy to identify each component’s influence from the blend dynamics.

Investigated methods
Though a lot of techniques including NMR, IR, and dielectric spectroscopy have been carried out to investigate polymer dynamics, most of the techniques can not provide the spatial information necessary to associate the relaxation processes with specific molecular motions. Molecular dynamics (M.D.) simulation and neutron scattering experimental techniques can simultaneously resolve temporal and spatial scales, and when taken together they become more powerful in probing packing structure and short times dynamics. Molecular dynamics simulations provide the exact location and identity of all atoms at all times, while neutron scattering techniques measure the exchange of energy and momentum between the neutrons and the investigated sample. Both techniques access times scales of picoseconds to nanoseconds, and length scales of a few angstroms. The observables that describe structural and dynamic properties of the given system, such as static structure factor S(q) and dynamic structure factor S(q,t) (where q is scattering vector and is related to distance r between particles as q=2?/r) can be calculated from simulation and also obtained from neutron scattering experiments. Therefore the simulation model can be verified by experimental data. On the other hand once the reliability of the simulation model has been assessed, it can be used to interpret the experimental data and predict the information that cannot be directly obtained from experiments.

Our group has employed several neutron scattering techniques including high flux backscattering [HFBS], Disk Chopper time-of-flight [DCS], neutron spin echo [NSE] and BT7 triple axis to measure the static and dynamic properties of PEO/PMMA blends, as well as their pure components. In conjunction with experiments we are also working on the molecular dynamics simulation of the polymers by united atom models [UA] (i.e. the CH3, CH2 and CH groups are considered as a single unit) and all atom model [AA] (i.e. every atom in the system is considered explicitly). Currently we finished the UA modeling on pure components. The graph below gives an example of the comparison between the experimental and simulation results, where the UA model is used. It indicates that our united atom simulation model captures the structure characteristics of simulated PEO. We are testing the AA model now. The effect of blending on the dynamics will be explored in the future using the UA and AA simulation models and neutron scattering techniques.

Figure 1. Comparison of the static structure factors of PEO from UA simulation, the UA simulation with hydrogen added afterwards and neutron diffraction experiments at the same temperature of 343K.