Local structure of syndiotactic poly(methyl methacrylate). A combined study by neutron diffraction with polarization analysis and atomistic molecular dynamics simulations

The local structure of syndiotactic poly(methyl methacrylate) (PMMA) has been investigated by combining neutron scattering and fully atomistic molecular dynamics simulations. Selectively deuterating parts of the PMMA monomer, we have accessed five different partial structure factors in the glassy state by neutrons, and polarization analysis has allowed isolating the coherent contribution to the total scattering. In addition, the temperature dependence of the static structure factor has been determined on the fully deuterated sample. The different measured partial structure factors show qualitatively different features with respect to peak positions and heights and provide a very critical check to validate the simulated structure. To gain deep insight into the structure, we have grouped the simulation results in terms of three molecular substructures: the main chain, the α-methyl group, and the ester side group. The study of the resulting partial structure functions has revealed the origin of the diffraction peaks, including those in the X-rays pattern reported in the literature. In addition, a real-space evaluation of the characteristic radial distribution functions has allowed separating intra- and interchain contributions to the total correlation functions. We have found that (i) PMMA exhibits a strong local order with an average main-chain distance of ≈8.6 Å, (ii) this is the only average interchain distance and thus no precursor effect of a layered structure is found, (iii) the main chain shows a persisting all-trans structure, and (iv) a strong anticorrelation between the main chain and the ester side groups, together with an interdigitation of the side groups, suggests a marked separation between the backbone and the side-group spatial arrangements.
This research project has been supported by the European Commission NoE SoftComp, Contract NMP3-CT-2004-502235. A.A., F.A., and J.C. acknowledge support from the projects MAT2004-01017 and 9/UPV00206.215-13568/2001. “Donostia International Physics Center” is also acknowledged. The European Commission, NMI3 Contract RII3-CT-2003-505925, supported the experiments at the Forschungsreaktor in Jülich