Search

We have investigated the short-range order in 1,4-polybutadiene by combining fully atomistic MD simulations and neutron diffraction with polarization analysis on isotopically labeled samples. Thereby the structure factor corresponding to the fully deuterated sample and the partial structure factor of the fully protonated polymer have been obtained. Both functions show a first main peak centered at about 1.4 Å-1 and a second one at around 3 Å-1. With varying temperature, we observe opposite tendencies for the intensity of the first diffraction peak in each sample. The direct comparison between simulation and experimental data has allowed validation of our simulated cell. Exploiting the information on the simulations, we have found that CC correlations dominate the first diffraction peak for both the deuterated and the protonated samples, while the origin of the second maximum depends on the labeling. Finally, the temperature behavior of the first peaks has also been rationalized. To a large extent, density changes are driving the evolution of the peaks, and the way they affect mainly the CC correlations and the mixed correlations involving C and H determines the behavior observed in the diffraction patterns.

In this paper, we have combined molecular dynamics simulation and neutron diffraction experiments with polarization analysis, to unravel the different atomic correlations contributing to the total and partial static structure factors of polyisoprene (PI). Four different PI samples have been investigated: PId3 (methyl group deuterated and main chain protonated); PId5 (methyl group protonated and main chain deuterated); PId8 (fully deuterated); PIh8 (fully protonated). The neutron diffraction experiments with polarization analysis were carried out by means of the diffuse scattering spectrometer D7 at the Institute Laue Langevin (ILL, Grenoble, France). By means of this technique the partial static structure factors corresponding to the PIh8, PId3, and PId5 samples and the total static structure factor S(Q) (PId8) were obtained in absolute units in the wavenumber regime Q ≤ 4 Å-1. In addition, the temperature evolution of S(Q) was also measured by a neutron powder diffractometer (D20, ILL) without polarization analysis but in a wider Q range Q ≤ 13 Å-1. On the other hand, fully atomistic molecular dynamic (MD) simulations were carried out at different temperatures on a model of PI built by means of the amorphous-cell protocole. The static structure factors measured on the different samples were also calculated from the simulation data. The agreement found between simulation and measurements shows that our simulation cell is a realistic representation of the actual structure of PI. Taking advantage of the information contained in the simulation runs, we have unambiguously identified the different atomic correlations contributing to the different “peaks” of the total and partial structure factors measured. In particular, we have shown that a “prepeak” present in some of the data is not related to intermediate range order but is naturally explained by the interplay of the different partial structure factors, a result which may have some bearing also for other systems. In

Fully atomistic molecular dynamic simulations were carried out by using the Insight (Insight II 4.0.0 P version) and the Discover-3 programs from MSI with the polymer consortium force field. The model system used in these simulations was built using the Amorphous Cell module. The polymer system simulated was glassy polyisoprene (PI) as used in previous neutron scattering (NS) measurements. A first molecular dynamics at 363 K was run for 1 ns using the Discover-3 program collecting data every 0.01 ps and a subsequent one (taking the previous output sample as an input for the following dynamics) was run for 2 ns collecting data every 0.5 ps. The results of the second run agreed to those of the first run, indicating that the sample was well equilibrated at this high temperature. Starting from the obtained atomic trajectories we have calculated the partial static structure factors for NS corresponding to different PI samples with different levels of deuteration (PId3, i.e., methyl group deuterated and main chain protonated; PId5, i.e., methyl group protonated and main chain deuterated; PId8, i.e., fully deuterated and PIh8, i.e., fully protonated). The results obtained are compared to the coherent NS cross-sections measured on real samples by means of D7 spectrometer with polarization analysis (ILL, Grenoble). A good agreement is obtained between experimental and simulated data validating the simulated sample. Moreover, the dynamic evolution of these correlations has also been calculated from the simulations. With these time dependent functions, the magnitude measured in a neutron spin echo (NSE) experiment can be constructed. Here we present two examples dealing with the fully deuterated sample PId8 and a partially deuterated sample, PId5, that show how computer simulation constitutes an invaluable tool for interpreting NSE results.