Miscible Polymer/polymer Interdiffusion Investigated By Atomic Force Microscopy

The properties of multi-phase and multi-component polymers, such as mechanical, electronic, optical, and permeable properties, are dependent on the microstructures of domain and interface. Diffusion plays a key role on the formation of interface. However, polymer/polymer interdiffusion is much more complicated than the diffusion between small molecules which can be described by a simple Fickian model well. Therefore, theoretical and experimental methods for polymer/polymer interdiffusion are particularly important. So far, lots of theories have been used to describe polymer/polymer interdiffusion, including reptation theory, slow-mode theory, fast-mode theory and fast-slow hybrid theory. Meanwhile, many kinds of experimental methods, such as neutron reflectivity (NR), secondary ion mass spectrometry (SIMS), vibration scpectroscopies (Raman spectroscopy and infrared spectroscopy), transmission electron microscopy (TEM), have been successfully used for the study of polymer/polymer interfusion. However, to the best of our knowledge, the investigation on this interdiffusion kinetics at a level of molecular size in real space has not been reported yet.In this work, the miscibility of poly(n-butyl methacrylate)/poly(vinyl chloride) (PnBMA/PVC) blend and the interdiffusion of PnBMA/PVC laminates have been investigated by differential scanning calorimetery (DSC) and atomic force microscopy (AFM), respectively. The tensile property of PnBMA/PVC blends has been studied also.(1) A series of PnBMA/PVC blend films with different PVC mass were prepared by casting their mixture solution. DSC results showed each blend appeared only a single glass transition temperature (T_g) which increased with PVC mass, indicating that PVC and PnBMA were miscible well. These T_gs were be described by Fried equation, Fox equation and Gordon-Taylor equation well. Without the fitting parameter k, Fox equation was better than Fried equation; but with the parameter k=0.99±0.10, Gordon-Taylor equation was the best suitable one, where T_gs almost presented a linear relation with PVC mass. This blend exhibited a lower critical solution temperature (LCST) behavior with a critical composition of 90wt% PVC and a critical temperature of 192℃, respectively.(2) The tensile properties of PnBMA/PVC blend films showed that the specific modulus and yield stress increased but elongation at break decreased with the increase of PVC mass.(3) PnBMA/PVC laminates were annealed at 110℃for 0.5 h, 1 h, 4 h and at 75℃for 16 h, 37 h, 96 h, respectively. An ultra-smooth cross-section across interface was then prepared by ultramicrotoming. Combined with topography and phase images of tapping mode atomic force microscopy (TM-DFM), the relative concentration profile, interface width and the relationship between interface width and annealing time could be obtained. At 75℃, the diffusion followed a typical Case-Ⅱdiffusion behavior where the interface width was proportional to annealing time. The penetration velocity was 4.36×10^-13 m/s. However, at 110℃, the diffusion obeyed a typical Fickian diffusion behavior where the interface width was proportional to the square root of annealing time. The mutual diffusion coefficient was 2.04×10^-19 m²/s which was in good agreement with those obtained from DSC (2.0×10^-18 m²/s) and positron annihilation lifetime spectroscopy (2.5³.5×10^-17 m²/s). These results imply that AFM is a reliable and powerful tool for the investigation of polymer/polymer interdiffusion at a level of polymer chain size.