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

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) 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.