Relaxation of Single Polymer Chain in Poly(methyl methacrylate) Films under Uniaxial Extension Observed by Scanning Near-Field Optical Microscopy

Single chain conformations during the stress relaxation process in uniaxially stretched poly(methyl methacrylate) (PMMA) films were observed in real space by scanning near-field optical microscopy (SNOM). The extension ratio at the molecular level was directly evaluated from the SNOM images and compared with the macroscopic stress relaxation. At the early stage of the relaxation process, the whole single chain retained the stretched conformation in spite of the decrease in, stress. This suggests that the fast relaxation in stress is caused by the local conformational change at the length scale much smaller than the entire chain length. The experimental data were successfully explained by the contraction of the primitive chain contour in the Doi-Edwards model. Furthermore, the distribution function of the chain dimension was found to retain its shape during the relaxation process, indicating that the conformational relaxation of polymer chain is rather homogeneous in the well-entangled system.