Jr. Ebdon et al., LUMINESCENCE STUDIES OF POLYMER MATRICES .1. PHOSPHORESCENCE OF BENZOPHENONE DISPERSED IN POLY(METHYL METHACRYLATE), Polymer, 36(8), 1995, pp. 1577-1584
The photophysical behaviour of benzophenone (BP) in poly(methyl methac
rylate) (PMMA) has been studied as a function of temperature. Time-res
olved phosphorescence data reveal that at no temperature between 77 an
d 420 K can the decay behaviour of the triplet state of BP in PMMA be
described adequately by a single exponential function. This contrasts
with the phosphorescence behaviour of BP in low temperature, low molec
ular mass, glassy matrices and that reported previously for BP dissolv
ed in PM MA at temperatures below the onset of the beta relaxation of
the polymer. The complexity of the phosphorescence decay kinetics of t
he BP/PMMA system increases at temperatures greater than that of the b
eta transition of the polymer host. This has been rationalized in term
s of the onset of enhanced energy transfer interactions between the BP
triplet excited state and the PMMA solvent, as proposed earlier. Howe
ver, the function derived previously on the basis of diffusion-control
led quenching of the triplet state of BP by the PMMA host, did not mod
el, adequately, the time-resolved phosphorescence data obtained in the
current study. In this respect, the form of the quenching influence e
xerted by the polymer matrix upon the photophysical behaviour of the B
P solute remains to be explained. Notwithstanding the absence of a com
plete rationalization of the influence of the PMMA host upon the photo
physical behaviour of the BP guest, it has been shown that both the in
tensity of phosphorescence emitted by the BP and its mean decay time p
rovide means whereby transitions within the polymer matrix can be dete
cted. In this respect the data are not equivalent: the lifetime data '
sense' the presence of the alpha' transition (at similar to 345 K) of
the PMMA. This transition is not apparent in the temperature dependenc
e of intensity data which, in common with the lifetime data, are sensi
tive to the onset of both the gamma and beta transitions of the polyme
r. The relevance of the data in respect of establishing phosphor/polym
er combinations for application as phosphorescent coding media is disc
ussed.