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(eth ylene 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 measurement s,and wide-angle X-ray diffraction support the formation of a crystalline m olecular complex, comprising two ethylene oxide repeat units per one CNA. T he structure of the complex is the same for PEO homopolymer and PEG-based d iblocks. Wide-angle X-ray diffraction from uniaxially aligned samples of th e 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 w ith a structural model in which the PEO chains adopt a nominally all-trans zigzag configuration, the chains organized as (100) layers separated by lay ers of one-dimensional stacks of CNA molecules. Polarized infrared measurem ents 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 hydr ogen bonding between CNA molecules in adjacent stacks. The arrangement of t he CNA molecules in the crystalline PEO:CNA complex differs from the struct ure of bulk CNA, clearly indicating that host-guest interactions play a maj or role in chromophore alignment. Small-angle X-ray scattering from a serie s of samples reveal changes in the block copolymer microstructure as the ef fective 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 hexagonal ly packed cylinder microstructures in which cylinders of the minority PEO b lock are surrounded by PS or PEP,the lamellar microstructure is observed fo r all three block copolymers at the composition of the 2:1 complex. Consequ ently, these materials can be described as rigid crystalline molecular comp lexes 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 molecule s, a desirable feature for applications such as optoelectronics. The confor mational rigidity associated with these systems offers considerable advanta ges for the design of SHG materials as entropically driven disordering is i nhibited compared to noncrystalline polymer-chromophore materials.