Impact of polymer structure and confinement on the kinetics of Zdol 4000 bonding to amorphous-hydrogenated carbon

Abstract

The bonding of molecularly-thin (10 Å) Zdol 4000 films to amorphous, hydrogenated carbon (CHx) was investigated as a function of the Zdol structure, i.e., the ratio of the perfluoromethylene oxide (C1) to perfluoroethylene oxide (C2) monomer units in the backbone. The influence of the C1/C2 ratio on the intrinsic mobility of the Zdol polymer was also investigated by computing the energetic barriers to internal rotation about the C-O and C-C bonds in model compounds by both ab initio and molecular mechanics methods. The calculations indicate that increasing the C1/C2 ratio increases the relative flexibility of the Zdol polymer. The kinetic results demonstrate that the rate at which submonolayer Zdol films bond to CHx is non-classical (time-dependent) regardless of the Zdol chain stiffness. The Zdol bonding rate can best be described by a kinetic equation of the form, dB/dt = k(t)A, where the rate coefficient, k(t) can be expressed as a power function in time: k(t) = kBt-h. The values of the initial bonding rate constant, kB, and the functional form of the time dependence, t-h, are both strongly dependent on the Zdol backbone flexibility. The magnitude of the initial bonding rate constants generally increase with increasing Zdol chain mobility. A discontinuous change in both the magnitude of kB and the functional form of the time dependence is, however, observed at 64°C when the C1/C2 ratio is increased from 0.97 to 1.08. The bonding rate coefficient scales as t-0.5 for the relatively rigid Zdol backbone structures with C1/C2 < 1, while a t-1.0 time-dependent bonding rate is observed for the more flexible Zdol backbones with C1/C2 > 1. The initial rate constant, kB, also changes abruptly near C1/C2 ≈ 1, with kB of the flexible Zdol chains (samples with C1/C2 > 1) being approximately an order of magnitude greater than the more rigid chains (C1/C2 < 1). These results indicate that the physical state of the confined Zdol film can be either liquidlike or solidlike depending upon the molecular stiffness of the backbone employed. The t-0.5 time-dependent bonding rate is shown to be consistent with a one-dimensional, diffusion-limited reaction from a solidlike Zdol structure, whereas the t-1.0 bonding rate results when bonding occurs from a liquidlike Zdol film structure. The temperature dependence of the Zdol 4000 bonding rate coefficient for the Zdol backbone characterized by C1/C2 = 0.97 (solidlike at T = 64°C) was found to undergo a transition from a t-0.5 time dependence for T < 150°C, to a t-1.0 time dependence at T > 180. This transition occurs over relatively narrow temperature range (150 < T < 180°C) and is attributed to a 2D melting of the confined Zdol film.