Conformation of Intramolecularly Cross-Linked Polymer Nanoparticles on Solid Substrates Tiffany E. Dukette and Michael E. Mackay* Chemical Engineering and Materials Science, Michigan State UniVersity, East Lansing, Michigan 48824 Brooke Van Horn and Karen L. Wooley Department of Chemistry, Washington UniVersitysSt. Louis, St. Louis, Missouri 63130 Eric Drockenmuller, Michael Malkoch, and Craig J. Hawker Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106 Received May 19, 2005; Revised Manuscript Received July 7, 2005 ABSTRACT The conformation of cross-linked, monomolecular, polystyrene nanoparticles on a solid substrate is considered as a function of cross-linking degree and substrate surface free energy. It is found that an extreme amount of cross-linking is necessary for the ca. 5-10 nm diameter nanoparticles to retain their original spherical shape, regardless of surface free energy. A lesser amount of cross-linking produces a nanoparticle that collapses to a pancake-like conformation on a high-energy substrate yet remains spherical on a low-energy surface. A simple model is developed to reveal the relationship between nanoparticle modulus and surface free energy to define the nanoparticle conformation. A recent advance in data storage technology is the Millipede 1 used to store information by pushing a hole into a polymer film with a heated atomic force microscopy (AFM) tip generating, in effect, two-dimensional ticker tape. Here we study a potential technology that is essentially the geometrical inverse of the Millipede. Polymer nanoparticles, arranged on a solid substrate, are individually addressed through plastic deformation with an AFM probe. Each nanoparticle repre- sents a bit of information; i.e., “1” when deformed and a “0” in its initial state. We recognize the initial conformation of nanoparticles on solid substrates, prior to deformation, is dictated by their intrinsic rigidity and interaction with a substrate which represents a critical aspect of this technology. To delineate the effect of rigidity and nanoparticle- substrate interaction, we study unique polystyrene nanopar- ticles synthesized by intramolecular cross-linking. 2 The rigidity is varied by the degree of cross-linking to produce extremely robust nanoparticles approximately 5-10 nm in size. Further, the substrate surface free energy is changed by choosing a high free energy surface, freshly cleaved mica, and a low free energy surface, silanized silicon wafer. Both substrates are smooth at the angstro ¨m level thereby minimiz- ing roughness effects on the measurements. It should be noted that the conformation of larger organic nanoparticles on solid substrates has been studied before by determining how they change size upon heating. 3 Also, in 1965, prior to present day atomic force microscopy, Rich- ardson used scanning electron microscopy (SEM) to deter- mine the molecular weight of individual molecules. 4 Poly- styrene solution was sprayed onto a carbon-backed mica substrate and shadowed with deposits of gold and palladium. The shadow length for each molecule could then be determined through SEM images. To calculate the molecular weight of the individual molecules, it was assumed that the particles were spherical and that the diameter of the particles was equal to their height. Due to limitations, there are two major difficulties in using this technique: defining the shadow limit cast by the spheres and the assumption of a spherical conformation on the substrate. Richardson used a precipitant/solvent solution to prevent molecules from col- lapsing, but he did not take into account surface free energy effects on the molecules. According to our results, un-cross- linked polymer molecules deform substantially even on a low-energy substrate, at least with our preparation procedure. Polystyrene nanoparticles were synthesized according to a previously described procedure 2 demonstrated in Figure 1. Briefly a linear copolymer consisting of styrene monomer * Corresponding author: mackay@msu.edu; www.nanoeverything.com. NANO LETTERS 2005 Vol. 5, No. 9 1704-1709 10.1021/nl050941f CCC: $30.25 © 2005 American Chemical Society Published on Web 08/02/2005