ABSTRACT
Pathways to fabricate self-organized nanostructures have been identified exploiting the instabilities of ultrathin (<100 nm) polystyrene (PS) film on the polydimethylsiloxane (PDMS) substrates loaded with discrete and closely packed gold nanoparticles (AuNPs). The AuNPs were deposited on the PDMS substrates by chemical treatment, and the size and periodicity of the AuNPs were varied before coating the PS films. The study unveils that the physicochemical heterogeneity created by the AuNPs on the PDMS surface could guide the hole-formation, influence the average spacing between the holes formed at the initial dewetting stage, and affects the spacing and periodicity of the droplets formed at the end of the dewetting phase. The size and spacing of the holes and the droplets could be tuned by varying the nanoparticle loading on the PDMS substrate. Interestingly, as compared to the dewetting of PS films on the homogeneous PDMS surfaces, the AuNP guided dewetted patterns show ten-fold miniaturization, leading to the formation of the micro-holes and nanodroplets. The spacing between the droplets could also see a ten-fold reduction resulting in high-density random patterns on the PDMS substrate. Further, the use of a physicochemical substrate with varying density of physicochemical heterogeneities could impose a long-range order to the dewetted patterns to develop a gradient surface. The reported results can be of significance in the fabrication of high-density nanostructures exploiting the self-organized instabilities of thin polymers films.
ABSTRACT
Focusing on the phase-coexistence region in Langmuir films of poly(l-lactide), we investigated changes in nonequilibrated morphologies and the corresponding features of the isotherms induced by different experimental pathways of lateral compression and expansion. In this coexistence region, the surface pressure Π was larger than the expected equilibrium value and was found to increase upon compression, i.e., exhibited a nonhorizontal plateau. As shown earlier by using microscopic techniques [Langmuir 2019, 35, 6129-6136], in this plateau region, well-ordered mesoscopic clusters coexisted with a surrounding matrix phase. We succeeded in reducing Π either by slowing down the rate of compression or through increasing the waiting time after stopping the movement of the barriers, which allowed for relaxations in the coexistence region. Intriguingly, the most significant pressure reduction was observed when recompressing a film that had already been compressed and expanded, if the recompression was started from an area value smaller than the one anticipated for the onset of the coexistence region. This observation suggests a "self-seeding" behavior, i.e., pre-existing nuclei allowed to circumvent the nucleation step. The decrease in Π was accompanied by a transformation of the initially formed metastable mesoscopic clusters into a thermodynamically favored filamentary morphology. Our results demonstrate that it is practically impossible to obtain fully equilibrated coexisting phases in a Langmuir polymer film, neither under conditions of extremely slow continuous compression nor for long waiting times at a constant area in the coexistence region which allow for reorganization.
ABSTRACT
We studied morphological changes in a quasi-two-dimensional Langmuir film of low molar mass poly(l-lactide) upon increasing the surface density, starting from randomly distributed molecules to a homogeneous monolayer of closely packed molecules, followed by nucleation and growth of mesoscopic, three-dimensional clusters from an overcompressed monolayer. The corresponding nucleation density of mesoscopic clusters within the monolayer can be tailored through variation of the rate of compression. For a given surface density and temperature, the nucleation probability was found to increase linearly with the rate of compression, allowing to adjust the density of mesoscopic clusters over nearly 2 orders of magnitude.