The microenvironment in polymer solids with added plasticizers. An attempt to measure the size and distribution of free volume in poly(methyl methacrylate) solids

Various amounts of n-alkylbenzenes (Cn: C6H5–CnH2n+1 (n = 3–16)) were doped into poly(methyl methacrylate) (PMMA) films, and the emission and thermal properties of each film were measured in detail together with their solid-state 13C NMR spectra. The aim of the present work was to estimate the size distribution of free volume in amorphous regions of polymer solids heavily doped with plasticizers by using Cn as models of a plasticizer. The excimer fluorescence yields of Cn in PMMA were found to depend on both the amount of Cn and the length of the alkyl chains of Cn, although the fluorescence spectra of all of the Cns in dilute fluid solution were almost the same. The quantitative analysis showed: (1) Cn with n ⩾ 12 induces a phase separation in PMMA, in which almost all of the Cn molecules are in a separated phase, and thus they cannot penetrate regions in which PMMA chains are aggregated. This means that Cn with n ⩾ 12 cannot enlarge the space between PMMA chains. (2) Smaller Cn (n < 11) can enter free volumes between PMMA chains that correspond to their molecular size, but they can enlarge them only to a limited extent. Thus, the amount by which plasticization can increase the free volume of PMMA is limited by the size of the dopant and the inherent free volume of the polymer matrix. (3) The efficiency of excimer formation was found to reveal the maximum amount of Cn that could fit in the free volume of PMMA. Thus our fluorescence measurements showed that PMMA solids that were plasticized to their limit had a free volume that was larger than the volume occupied by all the conformers of C5 and smaller than the volume occupied by almost all the conformers of C12. In conclusion, we were able to obtain information on plasticization and to demonstrate a method of monitoring microenvironments in polymer solids after they have been doped with plasticizers.