Polymer Interdiffusion near an Attractive Solid Substrate Eric K. Lin,* Wen-li Wu, and Sushil K. Satija † Polymers Division and Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received April 1, 1997; Revised Manuscript Received August 9, 1997 X ABSTRACT: Neutron reflectometry is used to study interdiffusion in bilayers of hydrogenated and deuterated poly(methyl methacrylate) (PMMA) on silicon substrates where the polymer-substrate interaction energy is strongly attractive. The effect of the polymer-substrate interaction energy on the interdiffusion rate as a function of distance from the surface is investigated. Samples are prepared with lower layer thicknesses of the deuterated PMMA ranging from 35 to 335 Å or 0.4 to 3.6 radii of gyration (Rg) of the bulk polymer chain. The rate of interdiffusion is found to be strongly dependent upon the lower film size. Polymers in films less than Rg in thickness have effective diffusion constants 2 orders of magnitude smaller than polymers in the thickest films. The effective range of the substrate on the interdiffusion dynamics is found to be between 300 and 400 Å or 3 Rg and 4 Rg. The observed dynamics are attributed to the different polymer chain conformations at varying distances from the solid surface. 1. Introduction The properties of polymer chains near an impen- etrable surface have received much recent attention and are important in applications such as electronics pack- aging, composite materials, coatings, and adhesion. 1 Oftentimes, the performance of the materials and devices is dependent upon knowledge about the struc- ture and dynamics of the interfacial polymer. Knowl- edge of the bulk polymer properties is not useful in these areas because the equilibrium structure and dynamics of the interfacial polymer chains are quite different from those of the bulk polymer. Geometric constraints from the surface alter the conformation of the polymer chains, and the interaction energy between the polymer and the surface can provide additional enthalpic contributions to the interfacial polymer properties. Several recent experiments have shown that both chain confinement and the interaction of the polymer with the substrate strongly affect the properties of interfacial polymers. Orts et al. 2 and Keddie et al. 3 observed that ultrathin polystyrene films on silicon oxide surfaces (with a weakly attractive interaction energy) exhibit a lower glass transition temperature, T g , than the bulk polymer. Additionally, Reiter 4 ob- served that polystyrene films dewetted silicon oxide surfaces at temperatures below the bulk polymer T g , providing evidence of enhanced chain mobility. How- ever, for ultrathin polymer films with strongly attractive interactions between the polymer and the substrate, the T g increased by as much as 30 °C and the thermal expansion coefficients decreased relative to those of the bulk polymer. 5-7 From these studies, it was estimated that the influence of the substrate reached several hundred ångstroms, or several times the length scale of the unperturbed polymer chain. 5 The changing thermal properties observed in these experiments reflect differences in the polymer chain dynamics. Increases in T g imply that the polymer dynamics have slowed down near an attractive interface, and decreases in T g suggest that the polymer chains have greater mobility than in the bulk polymer. Recent measurements of T g in freely standing thin polystyrene films show large decreases in the observed T g , demonstrating the large effects the free surface and the substrate have on the polymer properties. 8 In many applications, information about the proper- ties of interfacial polymers in contact with the bulk polymer are needed. Only limited experimental infor- mation about these interfacial polymer properties is available because of the challenge of probing buried interfaces and distinguishing the interfacial polymer from the bulk polymer. Progress has been made by several groups using techniques such as dynamic sec- ondary ion mass spectrometry (SIMS) and neutron reflectometry to study buried interfaces with deuterated components; providing a convenient way to selectively label the interfacial polymer. Zheng et al. 9 used SIMS and van Alsten et al. 10 used neutron reflectometry to study the bulk rate of diffusion from a solid surface of polystyrene on different substrates and of poly(methyl methacrylate) on a silicon substrate, respectively. In both studies, decreases in the effective diffusion con- stants or chain mobilities for polymer chains near the surface are observed. It was suggested that the de- creased chain mobility arises from an increased friction factor due to the attractive interaction energy between the polymer segments and the substrate. 9 This argu- ment suggests that at distances greater than ≈ R g from the surface, the polymer dynamics should resemble bulk behavior. Computer simulations studying the center of mass diffusion of oligomer chains near surfaces have also shown that the diffusion of chains from the surface decreases relative to bulk values with an effective range of influence of about R g . Interestingly, the diffusion parallel to a neutral surface was enhanced because the chains are oriented parallel to the surface. 11-14 Longer ranged perturbations from bulk chain behavior were observed for supercooled melts between neutral walls. 14,15 No computer simulations to date have been able to examine polymer chain melt dynamics above the en- tanglement molecular weight. In this work, we are interested in measuring chain mobility in polymeric thin films. We use neutron reflectometry to measure the movement of polymer chains near an attractive substrate from bilayer inter- diffusion experiments. Information about the polymer chain mobility can be obtained from the measurement of the rate of interdiffusion between polymer layers. For * To whom correspondence should be addressed at the Polymers Division. † Reactor Radiation Division. X Abstract published in Advance ACS Abstracts, October 15, 1997. 7224 Macromolecules 1997, 30, 7224-7231 S0024-9297(97)00442-7 CCC: $14.00 © 1997 American Chemical Society