Large area crystal surfaces of precisely sidebranched polyethylenes: Preparation, surface order and functionalization

Upon crystallization of polymers containing precisely spaced side-branches, the side-branches are excluded from the crystalline core of the lamellar crystal. Thus, the surfaces of these crystals arecovered by side-branches. By using carboxyl groups as side-branches, which can be ionized inaqueous environments, self-stabilized nanocrystal dispersions can be prepared. In principle, the carboxyl groups also allow for the covalent attachment of functional molecules to the lamellar crystals using well established strategies for amide bond formation. However, the fold surfaces of the crystals need to be highly ordered so that the carboxyl groups are accessible for functionalization.

From x-ray scattering in semicrystalline bulk, we found that precisely spaced side-groups do not predefine the thickness of the crystalline core under all crystallization conditions. In addition, the part of the fold layer containing carboxyl groups had a considerable thickness, indicating that there is a substantial disorder in the fold surfaces of lamellar crystals of polymers containing precisely spaced side-branches. However, upon heating, the crystals underwent a first order transition to a rotator phase known from n-alkanes, which allowed for inclusion of the carboxyl groups and reordering of the fold surface.

Having the intention to use precisely carboxyl side-branched polymers as substrates for thin film organic electronics, we pursued two different approaches to cover large areas with single lamellar crystals. By assembling nanocrystals on a Langmuir trough at high subphase pH, large areas could be covered by monolayers consisting of randomly arranged nanocrystals. The formation of ordered packings was impeded by the rather broad size distribution of the nanocrystals. Alternatively, we used a method based on local supersaturation to grow large area single  crystals of the precisely side-branched polymer from solution.

AFM height measurements showed that the surfaces of both nanocrystals and large area single crystals were covered by layers of amorphous folds. We found that the thickness of this layer was comparable to the thickness of the amorphous fold layers on single crystals of linear polyethylene. However, upon annealing of large area single crystals in the rotator phase, lamellar thickening was induced and the fold surface was reordered. Fluorescence spectroscopy after chemical functionalization indicated that the areal density of attached semiconducting molecules was high. In contrast, the density of semiconducting molecules on surfaces of annealed nanocrystals was low. We concluded that annealing of nanocrystals induced Ostwald ripening, which impeded the formation of an ordered fold surface.

To confirm the findings from fluorescence spectroscopy, we investigated the morphology of the semiconducting layer on annealed large area single crystals. As a function of the duration of the grafting process, the morphology of the resulting layer of semiconducting molecules changed from patchy to compact. The compact layer consisted of smaller domains, which we attributed to the strong directional interactions of the semiconducting perylene molecules.