Well-Defined Deuterated Polymers for Neutron Scattering

CNMS Macromolecular Complex System Group

In order to design novel polymers with desired properties, it is essential to understand their structures at different scales, including those in solution. The solution properties of a polymer mainly depend on its structure, such as conformation and aggregation (self-assembly). Small angle neutron scattering (SANS) is a powerful technique for revealing global and local structures of polymers in solution because it provides a direct access to the many different length scales of the systems simultaneously. At small scale size (unaggregated species), SANS can provide information about the conformation of the local structures in the system. At larger scales, SANS allows us to determine the properties of (size, shape), and the interactions between aggregates. Moreover, the dramatic difference of scattering lengths seen for protonated and deuterated molecules makes SANS extremely advantageous in studying heterogeneous systems through a contrast matching method. This method allows us to obtain the specific structures of the individual system components; the scattering from one of the components can be selectively suppressed by matching the scattering length density of that component to that of the solvent through selective deuteration.

With the completion of SNS construction and HFIR upgrades, ORNL has reemerged as a world center of neutron science. The CNMS is strategically collocated with these facilities to provide novel materials to take advantage of these powerful tools. Several CNMS user projects currently in progress seek to take advantage of this partnering. Two current user project have already seen the synthesis of well-defined deuterated polymers, including polyisoprene-d8 (CNMS2004-040) and poly(1,3-cyclohexadiene)-d8 (PCHD-d8, CNMS2003-041), and the subsequent analysis using neutron scattering techniques.

PCHDs are very interesting due to the six-member rings in the main chain. Moreover, the unsaturated double bonds offer many possibilities for post-polymerization modifications, such as sulfonation. Sulfonated PCHD (sPCHD) in solution shows a wide range of self-assembled structures with unique optical properties. However, it is very hard to elucidate the solution properties of sPCHD due to its tendency to form aggregates. We were able to overcome this difficulty by synthesis of per-deuterated sPCHD (scheme 1) and investigation of the aggregates by SANS using contrast matching approach (Figure 1). SANS results indicate that sPCHD form aggregates mainly though short range interactions of hydrophobic moiety and help us in understanding the phase behavior of sPCHD in aqueous solutions. We are extending this approach to other selectively deuterated polymers. This work was carried out within Partner User Proposal CNMS2003-041, including contributions from researchers sponsored by the BES Division of Scientific User facilities. Partner users (PI, J. M. Simonson) were supported by the BES Division of Chemical Sciences, Geosciences and Biosciences under FWP ERKCC49.

M_n ~8.7 Kg/mol (LS)
PDI ~1.06 (SEC)