We have investigated the formation of poly(γ-benzyl l-glutamate) (PBLG) crystals from semidilute thin film solutions. Crystallization was initiated by adding methanol, a nonsolvent. After drying, each PBLG crystal possessed an internal domain structure exhibiting a zigzag pattern consisting of parallel stripes of alternating orientations between domains. X-ray scattering and electron diffraction revealed within these crystals a pseudohexagonal packing of the PBLG α-helices with their axis oriented parallel to the plane of the substrate. On the basis of optical anisotropy studies, it could be shown that the orientation of the helix axis was parallel to the stripes. While forming in solution, the objects are assumed to consist of a hexagonal columnar liquid crystalline phase. Upon drying, lateral packing density of the helices increased and resulted in a net dilative strain perpendicular to the columns, which is supposed to cause the formation of zigzag patterns.
The effect of swelling with a near-Θ solvent on the thermal expansion and dewetting behavior of polystyrene films is studied. Spin-coating a film from a near-Θ solvent has been found to produce a chain conformation further from equilibrium than that by spin-coating from a good solvent. Swelling a film with a near-Θ solvent is expected to produce a similar effect. We find that there is a significant thermal contraction in the films swelled with decalina Θ-solvent of polystyrene at 16 °C. The size of this thermal contraction decreases as the sample is aged, and appears to be due to the solvent used and not the solvent quality. On the other hand, the effect of aging on the dewetting behavior is seen to depend on the solvent quality similar to that for the films spun-cast from a near-Θ solvent, with an offset attributable to the swelling conditions and initial chain conformation of the film. Our result suggests that the thermal expansion is not nearly as sensitive to the film's chain conformation as is the dewetting behavior.
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.
By combining atomic force microscopy experiments and full-atomistic computer simulations, we compared the twodimensional ordering dynamics of two variants of supramolecular polymers of bis-urea molecules which differed only by a single cisdouble bond in their side groups. At early stages of ordering, the double bonds favored kinks at the level of individual molecules, which induced transient steric constraints hindering the spontaneous formation of long supramolecular polymers. In addition, due to these kinks, molecule−substrate interactions were disturbed. At later stages, however, due to a progressively increasing number of established directional hydrogen bonds between molecules, the self-assembly process improved and thereby increased the length of the supramolecular polymers. Large domains of micrometer-long and aligned supramolecular polymers were formed, epitaxially guided by the graphite substrate and having a constant width consistent with the length of the molecule. Thus, introducing flexible (kinked) side chains can reduce the nucleation probability and slow the growth of supramolecular polymers due to incommensurablility with the crystalline substrate. Such an elementary control of nucleation and growth via the introduction of a single double bond represents a powerful pathway for the formation of large ordered domains of aligned one-dimensional supramolecular polymers.
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