Reversible Nucleation, Growth, and Dissolution of Poly(γ-benzyl l‑glutamate) Hexagonal Columnar Liquid Crystals by Addition and Removal of a Nonsolvent

We have investigated the process of nucleation and growth and its reversal (i.e., dissolution) of ordered poly­(γ-benzyl l-glutamate) (PBLG) objects in thin film solutions containing a few percent of α-helical PBLG dissolved in chloroform. Nucleation, growth, and dissolution rate were controlled by adding and removing, respectively, defined amounts of a nonsolvent (methanol), introduced through the vapor phase by regulating its flow rate and vapor pressure. Adding methanol to the isotropic polymer solution allowed for the induction of nucleation and growth, even with polymer solutions of very low concentrations, which were significantly below the solubility limit (equilibrium volume fraction). The variation of the number density of nuclei with the supersaturation ratio was found to fit well the predictions of the classical nucleation theory, for all equilibrium concentrations. For a given supersaturation ratio, fewer objects were nucleated for lower equilibrium concentrations

Reversibly Slowing Dewetting of Conjugated Polymers by Light

Dewetting, i.e., the retraction of a fluid from a surface it “dislikes”, is a macroscopic phenomenon controlled through parameters like viscosity and surface tension on length-scales much larger than the size of the molecules. So far, dewetting was known to proceed in the same manner, independent of the dewetting film being illuminated by light or not, e.g., through an optical microscope. Here, we demonstrate that the velocity of dewetting of conjugated polymers can be reversibly tuned through appropriate exposure to light. We relate this observation to the absorption of photons of suitable energy resulting in the generation of excitons which may partially delocalize along and across polymer chains and so induce changes in polymer chain conformation. Such changes, in turn, may cause stiffening or overlap of polymer chains and thus lead to macroscopically detectable differences in behavior of an ensemble of conjugated molecules expressed via material properties like viscosity

Massive Enhancement of Photoluminescence through Nanofilm Dewetting

Due to the rather low efficiencies of conjugated polymers in solid films, their successful applications are scarce. However, recently several experiments indicated that a proper control of molecular conformations and stresses acting on the polymers may provide constructive ways to boost efficiency. Here, we report an amazingly large enhancement of photoluminescence as a consequence of strong shear forces acting on the polymer chains during nanofilm dewetting. Such sheared chains exhibited an emission probability many times higher than the nonsheared chains within a nondewetted film. This increase in emission probability was accompanied by the emergence of an additional blue-shifted emission peak, suggesting reductions in conjugation length induced by the dewetting-driven mass redistribution. Intriguingly, exciton quenching on narrow-band-gap substrates was also reduced, indicating suppression of vibronic interactions of excitons. Dewetting and related shearing processes resulting in enhanced photoluminescence efficiency are compatible with existing fabrication methods of polymer-based diodes and solar cells