Control of hierarchical order in crystalline composites of diblock copolymers and a molecular chromophore

Addition of 2-chloro-4-nitroaniline (CNA) to diblock copolymers of poly(ethylene oxide) (PEO) and polystyrene (PS), poly(ethylethylene) (PEE), or poly(ethylenepropylene) (PEP) results in selective partitioning of CNA into the polar PEO domains. Calorimetry, infrared spectrosocpy, density measurements,and wide-angle X-ray diffraction support the formation of a crystalline molecular complex, comprising two ethylene oxide repeat units per one CNA. The structure of the complex is the same for PEO homopolymer and PEG-based diblocks. Wide-angle X-ray diffraction from uniaxially aligned samples of the PEO:CNA suggests a triclinic unit cell for the complex with alpha = 9.08 Angstrom, b = 10.48 Angstrom, c = 7.01 Angstrom; and alpha = 90.98 degrees, beta = 88.38 degrees,and gamma = 116.72 degrees. The data are consistent with a structural model in which the PEO chains adopt a nominally all-trans zigzag configuration, the chains organized as (100) layers separated by layers of one-dimensional stacks of CNA molecules. Polarized infrared measurements indicate that the molecular planes of the CNA molecules are nominally perpendicular to the PEO chains; The metrics associated with the zigzag PEO configuration appear to allow for optimal hydrogen bonding between the PEO oxygen atoms and the amine protons of the CNA chromophores as well as hydrogen bonding between CNA molecules in adjacent stacks. The arrangement of the CNA molecules in the crystalline PEO:CNA complex differs from the structure of bulk CNA, clearly indicating that host-guest interactions play a major role in chromophore alignment. Small-angle X-ray scattering from a series of samples reveal changes in the block copolymer microstructure as the effective volume of the PEO block is altered by the inclusion of CNA. Whereas the SAXS data for PS;PEO and PEP-PEO copolymers used here reveal hexagonally packed cylinder microstructures in which cylinders of the minority PEO block are surrounded by PS or PEP,the lamellar microstructure is observed for all three block copolymers at the composition of the 2:1 complex. Consequently, these materials can be described as rigid crystalline molecular complexes embedded in robust, ordered polymer microstructures. This control of hierarchical order over length scales, spanning several orders of magnitude, suggests a route to permanent macroscopic ordering of functional molecules, a desirable feature for applications such as optoelectronics. The conformational rigidity associated with these systems offers considerable advantages for the design of SHG materials as entropically driven disordering is inhibited compared to noncrystalline polymer-chromophore materials.