Experimentally driven atomistic model of 1,2 polybutadiene

Abstract

Absence of annealing effect in the vibrational density of states in a glassforming polymerWe present an efficient method of combining wide angle neutron scattering data with detailed atomistic models, allowing us to perform a quantitative and qualitative mapping of the organisation of the chain conformation in both glass and liquid phases. The structural refinement method presented in this work is based on the exploitation of the intrachain features of the diffraction pattern and its intimate linkage with atomistic models by the use of internal coordinates for bond lengths, valence angles, and torsion rotations. Atomic connectivity is defined through these coordinates that are in turn assigned by pre-defined probability distributions, thus allowing for the models in question to be built stochastically. Incremental variation of these coordinates allows for the construction of models that minimise the differences between the observed and calculated structure factors. We present a series of neutron scattering data of 1,2 polybutadiene at the region 120-400 K. Analysis of the experimental data yields bond lengths for CAC and C @ C of 1.54 Å and 1.35 Å , respectively. Valence angles of the backbone were found to be at 112 and the torsion distributions are characterised by five rotational states, a three-fold trans-skew6 for the backbone and gauche6 for the vinyl group. Rotational states of the vinyl group were found to be equally populated, indicating a largely atactic chan. The two backbone torsion angles exhibit different behaviour with respect to temperature of their trans population, with one of them adopting an almost all trans sequence. Consequently, the resulting configuration leads to a rather persistent chain, something indicated by the value of the characteristic ratio extrapolated from the model. We compare our results with theoretical predictions, computer simulations, RIS models and previously reported experimental results. V C 2014 AIP Publishing LLC. [http://dx.

Controlling and Evaluating the Structure and Morphology of Polymers on Multiple Scales

“…The main principle behind this approach is the possibility to utilize computer generated models to reconstruct the scattering and compare it with the experimentally observed structure factor in a Reverse Monte Carlo approach [17]. By accurately extracting the chain conformation from the high Q scattering region (WANS) [17]- [19] we can reconstruct the polymer structure and build a coarse-grain model to account for the crystal lamellas [20] that can be driven and evaluated by the actual experimental data in a similar manner to the simple case of the amorphous polymer (Figure 15).…”

The Use of Scattering Data in the Study of the Molecular Organisation of Polymers in the Non-Crystalline State

“…In other words, the structure factor can be considered the driving force of the model (i.e., energy), and σ corresponds to the temperature. In principle, any type of statistical test for comparison of that observed with the simulated structure factor can be used, but the χ 2 is typically used due to simplicity [6,13,31]. It can be shown that for a structural system with only pairwise forces, the RMC procedure leads to an exact solution.…”

“…In Figure 3, the broad Q neutron scattering results for two of the individual components (protonated and deuterated 1,4 and 1,2 polybutadiene) and the resulting blends can be seen. Information regarding the sample preparation and experimental procedure for the individual components can be found elsewhere [6,31] and for the blends in the Supportive Information. Clearly, qualitative differences between the different structure factors exist (as expected) but the data quality and extent of Q range are visible.…”

“…In this work, the interchain peak is of interest and, in our approach, we decompose the observed structure factor into inter and intrachain components. We have developed atomistic models that match the intrachain scattering for 1,4 [6] and 1,2 polybutadiene [31], and this is subtracted from the observed structure factor yielding the scattering that arises from the intersegmental scattering alone. The partial structure factors can then be obtained by inversion of the previous expression, and the results can be seen in Figure 5.…”

“…The RMC protocol discussed previously has been used with various degrees of success in the study of the atomic structure of disordered materials like simple liquids [37], molten salts [38], glasses [39] and polymers [26,27]. The basic idea of the method, namely the comparison of a computer-generated model with experimental data following a stochastic set of pre-defined rules, has been extended to approaches that treat the intrachain contribution [6,31], the establishment of actual experimentally based force fields [40] and the role of crystallinity in the overall structure [14,41]. Extending the simulation towards coarser length scales and introducing crystals by arranging the segments in specified ways while keeping them consistent with the chain conformation have been seen to offer possibilities for an in-situ study of time-resolved crystallization [42,43], thus indicating the potential of such intimate coupling of an RMC-based procedure with experimental data [44,45].…”