Conformational properties of poly(diallyldimethyl ammonium chloride) (PDDA) determined by combination of molecular dynamics, rotational isomeric states and Monte Carlo procedures

A theoretical model describing the conformational properties of poly(diallyldimethyl ammonium chloride) (PDDA) is developed with the following strategy: molecular dynamics simulations are performed on model molecules representing dimers of PDDA. The results are employed to formulate a rotational isomeric state model for these molecules in terms of short-range interactions. Furthermore, the MD trajectories permit the evaluation of conformational energies and probabilities for the 27 conformations allowed to the three bonds sequence contained in the dimers. These probabilities are then employed to generate single chains of PDDA in vacuo according to standard Monte Carlo procedures and their main squared end to end distance 〈r2〉 are computed. The procedure (MD simulations, evaluation of energies and probabilities and MC calculations) is then repeated for two more realistic systems obtained by fitting into a cubic box, having side length L and periodic boundary conditions, a forty repeating units oligomer of PDDA together with 40 Cl− as counterions and 500 water molecules (referred to as water system, L=2.95 nm, d≈1 g cm−3) or the oligomer with its counterions, 500 water molecules and 20 NaCl molecules (referred to as salt system, L=3.02 nm, d≈1 g cm−3). The behavior of the three systems is noticeably different and the value of unperturbed dimensions computed for the realistic systems are in good agreement with preliminary results of experimental measurements being carried out in our laboratory.