Investigation of Temperature-Dependent Relaxation Processes in Polyethylene Films by Covalently Attached Anthryl Probe

Abstract

The dependence of several aspects of the fluorescence from 9-anthryl groups, covalently attached by tethers of differing lengths to interior sites of five polyolefinic films whose crystallinities range from 0 to 74%, has been explored between 40 and 400 K. The data are employed to determine microscopically the onsets of various relaxation processes of the polymers, the sensitivity of the probes to changes in their local morphologies, and the distribution of the groups between amorphous and interfacial sites. The results are compared with those from noncovalently attached (doped) 9-methylanthracene molecules. Specifically, the relationship between the lengths of tethers for the 9-anthryl groups, as well as the chemical method by which each tether is attached to the polymer chains, and the ability of the fluorescence from the lumophores to detect the onsets of host relaxation processes are explored. The two attachment methods lead to different distributions of lumophores in the amorphous and interfacial regions of the polymers. Furthermore, very short (one atom; methylene) and very long (12 atoms) tethers allow the 9-anthryl groups to sense changes in the local environments more acutely than a tether of intermediate length (three atoms). The advantages of covalent attachment for this sort of study (specifically, the inability of the probe to diffuse within a film from site-to-site with time) and the limitations of the utility of the data are discussed.

Synthesis and photophysical properties of a poly(methyl methacrylate) polymer with carbazolyl side groups

“…The vibronic structure in the emission spectrum of the film is also similar to that of the solution, although the former is broader due to both inhomogeneous broadening (usually observed in the solid state) and conformational disorder (always present in polymer chains of amorphous solids). 34,35 Care was taken to maintain the same instrumental parameters and same position for each PMMA-Cz film throughout the collection of a data set (i.e., over the whole temperature range) to allow comparison of emission intensities (between 330 and 500 nm; λ exc = 295 nm) from steady-state fluorescence spectra recorded at different temperatures (from 30 to 410 K at intervals of 10 degree) ( Figure 4). The information available from this experiment is illustrated by comparing spectra at 80 and 300 K. Although the overall spectral shape at 300 K is very similar to that in solution (Figure 3), the 0-0 band at 347 nm is followed by other vibronic bands at 367 and 385 nm and a red-edge tail from 390 nm to 480 nm.…”

“…Moreover, because the luminescent groups are attached to polymer chains, they will be more sensitive to motions of the polymer matrix that are in the immediate vicinity than to those that are far from it. [34][35][36][37] Furthermore, the persistence of a part of the 440 nm emission at temperatures higher than 300 K suggests that some of the ground-state aggregates do not reside in places within the PMMA matrix where their association is affected by even local chain motions.…”

“…The link between the relaxation processes of a polymer matrix and the rate constants associated with excited state species has been attributed to electron-phonon coupling. 23,[33][34][35][36][37] In this regard, the onsets of two relaxation phenomena of PMMA are known to occur at 130-140 K and at 300-310 K. 35,36,[42][43][44][45]50 The slope change at 130-140 K is attributed to the onset/cessation of γ-relaxations, involving rotation of short segments of the polymer chains and of α-methyl groups bound to the main chains. 36,45,[51][52][53] In addition, another relaxation process, associated with the onset/cessation of rotations of the ester groups (β-relaxation), is also observed for PMMA around 300 K. At still higher temperatures (370-390 K), the glass transition occurs.…”

“…[20][21][22][23][24][25][26][27][28][29][30][31][32] The eventual emission depends on complex equilibria involving several carbazolyl groups, which, in turn, depends on temperature as it influences the equilibria, the relative importance of intrinsic radiative and non-radiative processes, and extrinsic polymer relaxation processes. 20,22,23,[33][34][35][36][37] Here, we describe the synthesis and the chemical, photophysical and thermal characterization of a carbazolylbased PMMA copolymer (PMMA-Cz) which has been prepared by exhaustive esterification of a commercially available copolymer, poly(methyl methacrylate-comethacrylic acid) (PMMA-co-PMAA). The photophysical properties of the PMMA-Cz are compared with those of the corresponding model compound, 9H-carbazole-9-ethanol (9HCz).…”

Photochemical and Photophysical Studies of and in Bulk Polymers

“…104 The sensitivity of the lumophores to a particular relaxation process depends on how strongly the lumophores interact with the local chain motions of the polymer and how similar are the time constants for fl uorescence decay and chain relaxation. Thus, fl uorescence effi ciency from covalently attached anthryl groups (with shorter -lived excited singlet states) is more sensitive to very rapid motions, 105 while covalently attached pyrenyl groups (with longer -lived excited -singlet states) are more sensitive to slower motions from larger segments of PE chains. The onset of relaxation processes in PE fi lms has been determined from discontinuities in the emission intensities (as total integrated spectral areas), intensities of wavelength maxima, and the full widths at half maxima (FWHM) of 0,0 emission bands versus temperature (e.g., Fig.…”