Arborescent graft polystyrene particle size and shape under confinement

Arborescent graft polymers (AGPs) are highly branched polymers synthesized by multiple grafting reactions of macromolecular precursors on a central core. They can have characteristic length scales on the order of tens of nanometers or more, depending on the molecular weight of the precursor chains used in the grafting reaction. They possess many of the unique and attractive structural, thermodynamic, and rheological properties characteristic of dendrimers and hyperbranched polymers, but do so on a length scale roughly one order of magnitude higher. This research studies the structure and self-assembling behavior of third (G3) generation arborescent graft polystyrenes synthesized either from 5k or 30k Mw anionic polystyrene precursor chains. Quantitative phase-contrast imaging by transmission electron holography of individual AGP particles on continuous carbon support films shows that these adopt a flattened shape which maximizes particle-substrate contact. This is consistent with previous AFM measurements of spin-cast particles. Monolayer membranes, approximately 1 micron in diameter, are formed by casting dilute solutions of polymer in toluene onto holey-carbon films. The holey carbon provides a circumferential constraint. Spatially resolved electron energy-loss spectroscopy shows that the AGP particles adopt an ellipsoidal shape to maximize particle-particle contact under these conditions. TEM imaging shows that torn films display craze-like features, suggesting the existence of particle-particle entanglements. Within monolayer films, the arborescent polymers assemble with varying degrees of order which can be quantified by digital Fourier methods.