Macro- and Microphase Separation in Block Copolymer Supramolecular Assemblies Induced by Solvent Annealing

We fabricated block copolymer (BCP) supramolecules by hydrogen bonding various carboxyl- and phenol-containing azo compounds to the poly­(4-vinylpyridine) blocks of polystyrene-<i>block</i>-poly­(4-vinylpyridine) (PS-<i>b</i>-P4VP). Thin films of the BCP supramolecules were prepared by spin-coating. Optical microscopy showed that all films of BCP supramolecules are macroscopically homogeneous immediately after spin-casting. To induce phase separation, all films were exposed to 1,4-dioxane vapor at room temperature. This solvent annealing caused always microphase separation between PS and P4VP-azo phases and sometimes also macrophase separation, i.e., azo compounds crystallized out of BCP matrices. The problem of macrophase separation in the BCP supramolecules is observed already at low concentrations of carboxyl-containing azo compounds. But phenol-containing azo compounds do not macrophase separate up to a molar ratio of azo compounds to repeat units of P4VP as large as 0.5. We conclude that self-associated hydrogen bonds of carboxylic groups and π–π stacking of azo chromophores are driving forces for macrophase separation

From Single Chains to Aggregates, How Conjugated Polymers Behave in Dilute Solutions

Conjugated polymers offer unique combination of easily tailored mechanical, electrical and optical properties that makes them perfect materials for the preparation of various devices such as light-emitting diodes, photovoltaic cells or field-effect transistors. However, the design and fabrication of such devices in a controlled and reproducible way are possible only if the behavior and the properties of individual polymer chains are well understood. One major problem in this respect is that aggregation often occurs even in dilute solutions and prevents the single polymer chain studies. To address this issue, in this work we employed fluorescence correlation spectroscopy (FCS) to study the behavior of a model conjugated polymer, poly­(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in several commonly used solvents. The very high sensitivity of FCS allowed measurements in ultradilute solutions and thus unambiguous determination of the hydrodynamic radius of single polymer chains. The solvent quality for MEH-PPV was then quantitatively evaluated from the measured logarithmic scaling of the single chain hydrodynamic radius versus the polymer molecular weight. Scaling exponents of 0.40, 0.41, and 0.43 were found in toluene, chloroform and 1,2-dichlorobenzene, respectively. These values are well below the θ-condition, emphasizing poor solvent quality for MEH-PPV, despite the fact that all studied solvents are commonly regarded as “good” solvents. In addition, by investigating the aggregation behavior of MEH-PPV at higher polymer concentrations, we found a clear relation between aggregates size and solvatochromism that indicates more extended chain conformation in larger aggregates.