Reorientation of aliphatic tails during the photopolymerization of a diacetylenic lipid

The structures of polymerized and unpolymerized monolayers of a diacetylenic lipid prepared at different temperatures were studied using X-ray reflection and diffraction and electron diffraction techniques. Two different monoclinic lattices exist with the polymer axis parallel to one crystallographic axis.

The different polymeric films have slightly different lattice parameters; the main difference is the aliphatic chain tilt. During polymerization the tilt angle between the aliphatic chain and the surface normal changes from about 30° to near 0° for the low temperature structures.Although the lipid is achiral, two chiral structures are observed; these are ascribed to the different orientation of the tilt direction projected on the surface and the headgroup with respect to the polymer axis.

The data indicate that the lipid headgroups are uniformly oriented and bridge the polymer axis to form a two-dimensional network.