ABSTRACT
The electrical and friction properties of ω-(trans-4-stilbene)alkylthiol self-assembled monolayers (SAMs) on Au(111) were investigated using atomic force microscopy (AFM) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The sample surface was uniformly covered with a molecular film consisting of very small grains. Well-ordered and flat monolayer islands were formed after the sample was heated in nitrogen at 120 °C for 1 h. While lattice resolved AFM images revealed a crystalline phase in the islands, the area between islands showed no order. The islands exhibit substantial reduction (50%) in friction, supporting the existence of good ordering. NEXAFS measurements revealed an average upright molecular orientation in the film, both before and after heating, with a narrower tilt-angle distribution for the heated fim. Conductance-AFM measurements revealed a 2 orders of magnitude higher conductivity on the ordered islands than on the disordered phase. We propose that the conductance enhancement is a result of a better π-π stacking between the trans-stilbene molecular units as a result of improved ordering in islands.
Subject(s)
Gold/chemistry , Membranes, Artificial , Sulfhydryl Compounds/chemistry , Electrons , Particle Size , Sulfhydryl Compounds/chemical synthesis , Surface PropertiesABSTRACT
The gradual rigidification of a single dendronized chain upon increasing the size and density of the dendron units attached to it is studied using a Monte Carlo simulation. The dependence of the backbone flexibility on the size and density of the dendrons is used as an input to study the self-assembly of dendronized polymers using a real space, self-consistent field theoretic method. These calculations predict different phases in melts and solutions, ranging from lamellar to gyroid phases, depending upon various physical (temperature, concentration) and architectural (relative volume fraction of the backbone and the dendron units) parameters.
ABSTRACT
The molecular architecture of dendronized polymers can be tuned to obtain nanoscale objects with desired properties. In this paper, we bring together experiments and computer simulations to study the thermodynamic and dynamic properties of a single dendronized polymer chain. We find that, upon changing certain architectural features, dynamic correlations characterizing backbone conformational fluctuations of a dendronized polymer exhibit dynamics akin to glass-forming bulk liquids. Thus, a dendronized polymer chain is a novel macromolecule that is a single-molecule glass. Over a range of conditions that lead to glassy dynamics, there does not appear to be any thermodynamic singularities. We discuss how a dendronized polymer is a molecular system that can directly test different models of glassy dynamics. We also show that defect densities characteristic of typical synthesis conditions do not alter the material properties of dendronized polymers.
