Low‐temperature structural relaxation in poly(alkyl methacrylate)s and related polymers probed by photochemical hole burning

摘要

AbstractExcursion temperature dependences of spectral hole profiles in photochemical hole burning for free‐base systems porphyrin/poly(alkyl methacrylate)s are investigated to see the relationship between relaxation properties and chemical structure of the polymers. The irreversible change in hole width is a characteristic of each polymer and is very similar to the relaxation behavior measured mechanically and dielectrically. Poly(methyl methacrylate) (PMMA) shows the highest thermal stability of hole profiles among the polymers presently studied with no marked appearance of local relaxation at 20–100 K. Poly(ethyl methacrylate) (PEMA) and poly(isopropyl methacrylate) (PiPMA) show an increase in hole width even at 20 K with a plateau region at 50–70 K, which is attributed to the rotation of the ethyl or isopropyl group around the CO bond. Polymers which contain more than two serial methylene groups in the side chains (poly(propyl methacrylate), poly(butyl methacrylate), poly(isobutyl methacrylate)) show a similar relaxation behavior around 50–60 K, suggesting the onset of the rotation of the terminal ethyl or isopropyl group in the ester side chain around the CC bond. Low‐density polyethylene (LDPE) shows a similar excursion‐temperature dependence probably due to the existence of branched oligomethylene side chains. The relaxation behavior is independent of the nature of chromophores, which indicates that irreversible hole broadening is controlled mainly by the local motion of th