Structural analysis of dendrimers based on polyhedral oligomeric silsesquioxane and their assemblies in solution by small-angle neutron scattering: Fits to a modified two correlation lengths model

Abstract

The small-angle neutron scattering (SANS) technique has been used to investigate the structure of dendrimers based on polyhedral oligomeric silsesquioxane (POSS) in solution. Three generations and two distinct terminal groups (either vinyl or polyethylene glycol (PEG) terminal groups) as well as four concentrations for each dendrimer have been measured in deuterated tetrahydrofuran. Two main scattering regimes, one with a length scale of nm and the other with a length scale of sub-micron, are found to contribute to the SANS signals. In order to fit the SANS data, a new modified two correlation lengths model has been developed. Fit parameters include scale factors, correlation lengths, Porod exponents and “particle” elongation parameters. Our fit results show that the local structure does not correspond to single dendrimer molecules, but rather to POSS-rich domains surrounded by POSS-poor domains. The large-scale structure represents the overall morphology of multi-dendrimer aggregates. The flexible PEG outer links help to enhance the stability of inorganic POSS cages, and allow the hybrid polymers to adjust their conformation in order to respond to their surroundings. With increasing concentration, the local structure becomes more compact as evidenced by the decrease of correlation length and concurrent increase of the Porod exponent. The deformation of local structure during the compaction process is also observed. The structure becomes elongated at intermediate concentrations whereas it tends to be more globular at low and high concentrations.