Form factor of polymer melts: Computational modeling of poly(isobutylene) for comparision with neutron scattering experiments – FqSimPIB

A cornerstone of modern polymer physics is Flory's famous `ideality hypothesis' which states that a chain in a melt behaves as if all intrachain excluded volume interactions were absent. However, recent theoretical and computational results for models of long and flexible polymer chains indicate that there exist deviations from ideal behavior which impact the conformational properties of polymer melts. Conformational properties can be measured experimentally by an analysis of the form factor F(q), i.e. the scattered intensity at wave vector q resulting from intramolecular interferences of a chain. For F(q), theory predicts and simulation finds a depression of the so-called `Kratky-plateau', characteristic of ideal chains, which only depends on the monomer density and so, is (fairly) universal. Obviously, it is important to correlate the presence of these corrections to chain ideality to the specific polymer structure of real systems in order to delineate the conditions where the corrections must be considered in understanding the molecular level physics of polymer chains. To do this, we are in collaboration with Prof. M. Dadmun (UT Knoxville, USA), a physicochemist and an expert in neutron scattering of polymer systems, who spent six months in 2006 in our group. Several findings indicated that poly(isobutylene) (PIB) might be a promising candidate to observe the abovementioned deviations from ideality in F(q). Therefore, fully protonated and deuterated samples of PIB as well as samples that are selectively deuterated only along the backbone of the chain, have been recently synthesized and the scattering experiments will start in the near future. We propose to accompany these experiments by large-scale molecular simulations of a chemically realistic model for PIB, the computational challenge being to obtain equilibrated systems of long-chain polymer melts. In this proposal, we request financial support for a PhD student to complete this task, the result of which, in conjunction with the neutron scattering experiments, hold the promise to play a critical role in understanding the fundamental physics of long-chain polymer melts.