Incoherent Neutron Scattering as a Probe of the Dynamics in Molecularly Thin Polymer Films Christopher L. Soles,* Jack F. Douglas, and Wen-li Wu NIST Polymers Division, Gaithersburg, Maryland 20899-8541 Robert M. Dimeo NIST Center for Neutron Research, Gaithersburg, Maryland 20899-8562 Received June 17, 2002 ABSTRACT: Incoherent neutron scattering measurements were performed on polycarbonate (PC), poly- (methyl methacrylate) (PMMA), and poly(vinyl chloride) (PVC) films of variable thickness, ranging from bulklike down to 75 Å, or length scales comparable to the polymer’s radius of gyration. The temperature dependencies of the incoherent elastic scattering are analyzed in terms of a Debye-Waller factor to estimate the hydrogen-weighted mean-square atomic displacement 〈u 2 〉. We find a general reduction of 〈u 2 〉 as the polymer films become increasingly thin, especially above the calorimetric glass transition temperature, Tg. However, below Tg this reduction depends strongly upon the relative amplitude of the displacement. Specifically, if 〈u 2 〉 in the bulk glass is especially large, as seen in PC, the extensive sub-Tg motions are strongly suppressed by thin film confinement. On the contrary, glassy PVC displays comparatively small-amplitude displacements in the glass and virtually no reduction of 〈u 2 〉 upon confinement. These results are discussed in terms of a caging of the atomic motions as the degree of thin film confinement increases. Introduction Materials properties at surfaces and interfaces often deviate substantially from the bulk. For polymers, these deviations have potential ramifications in a range of thin film applications, such as photolithography, protec- tive coatings, lubricants, and adhesives. Of particular interest are surface or thin film induced changes in the polymer mobility. Deviations in a polymer’s dynamical properties could have deleterious effects on a variety of thin film applications. For example, an increase in the viscosity of an exceedingly thin hard disk lubricant film could lead to catastrophic failure of the drive. Con- versely, increased molecular mobility in a very thin photoresist film could lead to enhanced photoacid dif- fusion and significant blur of the latent lithographic image. It is of both technical and scientific interest to understand how the molecular mobility of a polymer is affected by thin film confinement. In the following, we limit our discussion to polymer films that are supported on a rigid substrate. A burgeoning body of literature addresses the issue of confinement and its effects on the mobility and glass transition of amorphous polymer thin films. To appreci- ate the wealth of research in this field, we refer to a recent review on glass transition phenomenon in thin polymer films. 1 Most experimental endeavors in this field focus on tracking the temperature dependence of some physical parameter, such as film thickness, index of refraction, etc. It is common to define a “kink” in the temperature dependence of this kind of data as the glass transition temperature, T g , of the thin film and then to infer changes in the molecular mobility from shifts of this feature. A more direct approach is to probe changes in the local atomic or molecular mobility with neutron scattering. Neutrons are scattered from the nuclei and directly reflect the displacements at the atomic level. A large increase in the amplitude of these atomic motions occurs when a rigid amorphous solid softens into a viscous liquid, analogous to the melting of a crystal. Incoherent neutron scattering provides a useful means to monitor this motion and identify this dynamic transi- tion. In principle, inelastic neutron scattering spectra contain detailed information about the time scale and geometry of the atomic motions. Typically, the intensity of this inelastic scattering is several orders of magnitude smaller than the elastic scattering, especially in the glassy state. Unfortunately, this means that in a thin polymer film, where there is limited sample mass, the inelastic scattering is comparable to the experimental noise and exceedingly difficult to observe. However, the changes in the strong elastic scattering intensity also contain information, albeit less detailed, about the sample dynamics. This is because the total scattering must be conserved, and an increase in the inelastic scattering (i.e., an increase of mobility) necessitates a decrease in the elastic scattering. For elastic incoherent scattering, the Q dependence may be approximated by the Debye-Waller factor where Within this model, based on a harmonic solid, the slope in a plot of ln(I inc,elastic (Q)) vs Q 2 is equal to 〈u 2 〉/3. While most atomic motions in soft condensed matter are admittedly anharmonic, this approximation has been useful characterizing the dynamics in both synthetic 2-6 and biological 7-11 macromolecules. Recently, we dem- onstrated that this approximation is also instructive for studying the confinement effect on the dynamics of thin polycarbonate films supported on Si substrates. 12 Here we build upon these initial measurements and examine three very different polymers to obtain a more complete understanding of how thin film confinement influences polymer dynamics. I inc,elastic (Q) ∝ exp ( - 1 3 Q 2 〈u 2 〉 ) (1) 373 Macromolecules 2003, 36, 373-379 10.1021/ma020952k This article not subject to U.S. Copyright. Published 2003 by the American Chemical Society Published on Web 01/01/2003