Subject: Re: Polymer conformation calculations

From: [email-masked] (Wilson_Bruce)
Date: Thu, 28 Apr 1994 10:07:20 -0400

Jens Lehmann asks about calculating the preferred conformations of
the monomers in a poly(aryletherketon) ---

It has been our experience here that getting the correct relative
rotational energies for aromatic ketones/carboxylic acids (e.g.
terephthalic acid) is somewhat difficult.  The energy barrier between
conformers is very low and the energy differences are slight.  Generating
the parameters for a rotational isomeric state model is somewhat
problematic then.  You can use basically two approaches.  One is
to model pieces, using ab initio code (at least 3-21G*) to get the
rotational energies for a RIS model (I've not been satisfied when
I've tried doing this with AM1 or PM3).  The second is a periodic
cell type annealing experiment.  The trick here is to get the 
cell to match the observed density of the polymer at the desired
temperature.  Several commercial packages have the tools to be able
to do this (Biosym, Sybyl, ...).  I can't help on which would be "best"
though the Sybyl force field is qualitative, not quantitative, IMO,
which may limit the applicability to this particular problem.

		Bruce Wilson ([email-masked])