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 (CH_x) 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 CH_x 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)= k_B t ^-h. The values of the initial bonding rate constant, k _B, 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 k _B 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, k _B, also changes abruptly near C₁/C₂ ≈ 1, with k _B 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.     

Effects of solvent and thermal annealing on the dielectric properties and morphology of dimethyl siloxane/α-methylstyrene block copolymers

The dielectric and morphological properties of a series of 9/1 poly(dimethyl siloxane) PDMS)/poly(α-methylstyrene) (PαMS) block copolymers and a 6/4 PDMS/PαMS block copolymer have been determined as a function of solvent casting and thermal treatment. Transmission electron microscope (TEM) results show better phase separation as a function of thermal annealing and casting from cyclohexane, a PDMS preferential solvent. Dielectric studies in the temperature region of the PDMS glass transition are consistent with the TEM results and are interpreted in terms of a most probable distribution of PDMS/PαMS mixed states. When the PDMS segment molecular weight is less than the critical molecular weight, thermal annealing of the solvent cast samples produces a phase separated sample exhibiting the T_g of PDMS as well as a mixed phase. Thermal annealing of samples with the PDMS segment molecular weight greater than the critical molecular weight produces little change in the mixed structure and the dielectric data. All samples have PαMS segment molecular weights less than the critical molecular weight. The observed changes are interpreted in terms of kinetic effects associated with polymer melt viscosity (critical molecular weight) and expected mixing of the two components.