Structure and Dynamics of Confined Chain Molecules

We simultaneously studied the surface forces and dynamics of chain molecules confined between either low energy or high energy surfaces. The surfaces were separated distances on order of the molecular diameter. The surface forces were determined using the Surface Forces Apparatus (SFA) while the dynamics, the relative amounts of elasticity and dissipation, were determined using the surface rheometer. The chain molecules were of alkane, siloxane or perfluorinated polyether chemistry. The alkanes were relatively short and some had methyl side-branches. All were liquids at room temperature. All were dominated by dissipative dynamics in the bulk. The work described focuses on two specific areas: (i) the structure and dynamics of confined long polymeric chains, and (ii) the structure and dynamics of short alkanes. The confining surfaces were either high energy bare mica surfaces or low energy alkane surfaces. The low energy surfaces were necessary in order to distinguish the effects of geometry from surface attraction. To form the low energy surfaces mica had to be modified with self-assembled monolayers of condensed octadecyltriethoxysilane. The modified surfaces were carefully characterized in an effort to prove that the surfaces would meet the strict requirements necessary for SFA experiments. Typically, when surface forces were measurable for confined polymers, so were the dynamics of the thin films. Polymers confined between low energy surfaces could be squeezed more easily to thicknesses smaller than their radius of gyration than if confined between high energy surfaces. Dynamics measurements typically showed elasticity dominating over dissipation. The polymer molecules confined between low energy surfaces were in equilibrium with the chains in the reservoir. Those between high energy surfaces can be in equilibrium with the molecules in the reservoir, provided the rate of surface approach is slow enough for the intrinsic dyanamics of the polymers to dissipate squeezing forces. Shorter alkanes showed layering if they have no methyl side-branches. As the amount of branching increased, the layering was disrupted so that no oscillatory surface forces were measured. On the other hand, molecules with low degree of branching showed a weakly ordered structure, yet layering persisted. Curiously, all of these molecules could show dynamics where the elasticity was larger than dissipation, although both types of forces were measured at levels many orders of magnitude larger than the bulk.