On the Origin of Heterogeneous Diffusion in Glassy Poly(alkyl methacrylates)

Abstract

The diffusion of small molecules through amorphous polymers proceeds by the thermally activated jumps whose rate changes from one region to another due to polymer heterogeneity. Little is known at present about the origin of diffusion heterogeneity. The oxidation of triplet nitrene and quenching of phenanthrene phosphorescence have been used to study the movement of molecular oxygen on a nanometer length scale in glassy poly(ethyl methacrylate) and poly(n‐butyl methacrylate) far below the glass transition temperature. It has been found that, as temperature increases, the jump rates in all regions increase by the same factor. This finding points to the equality of the activation energies of oxygen jumps in different regions of the polymers. We conclude that the correlated elastic displacements of polymer chains inside regions about several nanometers in size provide molecules jumps. Due to the large size of these regions, the activation energy of jump does not depend on local polymer structure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1097–1104

Oxygen Diffusion in Glassy Poly(ethyl methacrylate): Spatial Correlation of Jump Rates

“…MODELING THE KINETICS OF PHENANTHRENE DEACTIVATION The decay of phenanthrene phosphorescence in the oxygen-free polymer is exponential with a time constant τ 0 . As temperature increases from 105 to 155 K, τ 0 decreases linearly from 3.52 to 3.32 s. 23 In the presence of oxygen, the deactivation probability, P(r), is expressed as follows 46,47…”

“…The activation energy of the oxygen diffusion in PEMA was found to be constant in the temperature range from 90 to 330 K. 23,35 Therefore, we can safely assume that the diffusion mechanism is not changed in this range, and the suggested model can be applied also at room temperature when considering the oxygen diffusion.…”

“…22,23 Recently, we established the equality of the activation energies of oxygen jumps in different regions of glassy poly(ethyl methacrylate) (PEMA). 23 On this basis, we assumed that the size of the region to be deformed reaches several nanometers. In that case, the deformation energy is concentrated in a large enough region and, as a consequence, structural disorder is averaged over a large region.…”

“…Because of the dynamical heterogeneity of the glassy polymer, − the probabilities of molecule jumping differ strongly from one region of the polymer matrix to another. , Recently, we established the equality of the activation energies of oxygen jumps in different regions of glassy poly­(ethyl methacrylate) (PEMA) . On this basis, we assumed that the size of the region to be deformed reaches several nanometers.…”

“…Figure 7. Symbols denote the quenching curve of phenanthrene phosphorescence in PEMA at an oxygen concentration of 6 × 10 19 cm −3 and 135.8 K 23. Solid and dashed lines are the model curves simulated using σ = 4.5 kJ/mol (⟨F n ⟩ = −8.24 kJ/mol) and σ = 0 (⟨F n ⟩ = −14.46 kJ/mol), respectively.…”