Triggering Mesophase Order in Melts of Metastable, Ultrathin Diblock Copolymer Films through Microstretching: Effect of Melt Film Thickness

thin films of poly(isoprene-b-ethylene oxide) diblock copolymers on mica: an atomic force microscopy study

The structural behavior of three amphiphilic semicrystalline poly(isoprene-b-ethylene oxide) block copolymers (PI-b-PEO) with different PEO volume fraction (f(PEO) = 0.32, 0.49, and 0.66), spin-coated on freshly cleaved mica surfaces from aqueous solutions, was investigated by atomic force microscopy. We focus on the dependence of the resulting thin film nanostructures on the molecular characteristics (f(PEO) and molecular weight) and the adsorbed amount. The nanostructures obtained immediately after spin-coating were robust and remained unchanged after annealing and/or aging. The PEO affinity for the highly hydrophilic mica and the tendency of the hydrophobic and low surface energy PI to dewet and be at the free interface caused the soft PI-b-PEO micelles to collapse leading to the formation of 2D dendritic networks over mica. We show that, for all three polymers, the dendritic monolayer thickness can be predicted by a model consisting of a PEO crystallized layer (directly on top of mica) of the same thickness in all cases and a PI brush layer on top. In thicker areas, polymer material self-assembled into conelike multilamellar bilayers on top of the monolayer and oriented parallel to the substrate for both symmetric and asymmetric diblock copolymers with the lowest f(PEO). We compare the lateral morphology of the films and discuss the thickness heterogeneity, which results from the coupling and competition of crystallization kinetics, phase separation, and wetting/dewetting phenomena highlighting the role of the two blocks to inhibit or enhance certain morphologies. We show that the deviation of the f(PEO) = 0.32 thin film from its bulk phase structure (cylinders in hexagonal lattice) continues for several lamellar bilayers away from the substrate. For the asymmetric PI-b-PEO polymer with the higher PEO volume fraction (f(PEO) = 0.66) and higher APT, laterally extensive stacks of flat-on lamellar crystallites formed on the surface demonstrating the crucial role of the PEO crystallization.

Rationalized approach to the determination of contact point in force-distance curves: application to polymer brushes in salt solutions and in water

In this work, we present two methods to determine the contact point in force-distance curves obtained with the atomic force microscope. These procedures are compared with the typical determination of contact point by a visual assessment of the data. One method, based on the assumption that the sample shows linear elastic behavior, provides results similar to those obtained by a visual assessment of the data, and will be suitable for determining the contact point in cases where ionic repulsion is not significant. The second method is based on a series of measurements in which the sample deformation is measured at increasing values of applied load; the contact point is determined by extrapolation to zero load. Because this method is based on extrapolation of measurements made in the contact regime, it is not subject to long-range repulsion. It is thus suitable for the analyses of the contact point even in cases where ionic repulsions will affect the point at which individual force curves deviate from the baseline or zero-force regime. The methods described here are demonstrated with a glycopolymer brush compressed with a colloidal silica particle on the tip of the AFM cantilever.