Elsevier

Polymer

Volume 45, Issue 16, 21 July 2004, Pages 5713-5718
Polymer

9-Anthryl and 1-pyrenyl groups as reporters of local environments and diffusion of two N,N-dialkylanilines in polyethylene films. Excited state lifetimes determine spatial information

https://doi.org/10.1016/j.polymer.2004.05.054Get rights and content

Abstract

Changes in the static and dynamic fluorescence from two covalently attached lumophores, 1-pyrenyl and 9-anthrylmethyl, induced by the diffusion of two N,N-dialkylanilines, N,N-dimethylaniline and N,N-dioctadecylaniline, within polyethylene films of 42% crystallinity have been examined. The longer lived excited singlet state of 1-pyrenyl is found to report on events occurring within a larger surrounding volume than the much shorter lived excited singlet state of 9-anthylmethyl. In addition, diffusion of N,N-dioctadecylaniline is found to be more rapid than expected on the basis of simple molecular volume considerations and to occur through two parallel pathways. The results suggest that lumophore lifetimes can be ‘tuned’ to interrogate different volumes of host space in polymers.

Introduction

Molecules containing 1-pyrenyl (1-Py) and 9-anthrylmethyl (9-AnCH2) lumophoric groups have been used previously to probe different aspects of anisotropic environments [1], including chain relaxation processes [2], [3], [4], [5], [6] at amorphous and interfacial sites within polyethylene (PE) films [7], [8]. Lumophores that are covalently attached to polymer chains of PE films have also been used to determine the accessibilities of those environments to small dopant molecules, such as N,N-dialkylanilines (DAA) [4], [6], [7], [8], [9], [10], that are efficient quenchers of 1-Py and 9-AnCH2 excited singlet states [11], [12]. As molecules of a DAA diffuse into a film and encounter the lumophores, the overall fluorescence intensity decreases and the temporal changes can be related to the diffusion coefficients of the DAA as well as their ability to occupy different site types. In the reverse process examined here, the diffusion of DAA molecules from a film and into a solvent bath (i.e. out-diffusion) leads to an increase in the measured intensity of the fluorescence. 1-Py (van der Waals volume=205 Å3 [13]) is expected to occupy preferentially sites in PE with somewhat larger free volumes than those occupied by 9-AnCH2 (van der Waals volume=178 Å3 [13]). In addition, the polymer chains around 1-Py groups may be somewhat more constrained than those near 9-AnCH2. By comparison, the mean free void volume of sites in the polyethylene film used in these studies, with 42% crystallinity (PE42), is calculated from positron annihilation lifetime spectroscopy (PALS) data assuming spherical site geometry to be only 143 Å3 [14]. Because DAA molecules must collide with the aromatic groups in order to quench their excited singlet states, a progressive decrease in fluorescence intensity (the monitor of diffusion) will occur only if the fraction of lumophore-occupied sites either accommodating at least one DAA molecule or separated from one by no more than one polymer chain increases with time.

The results from experiments reported here on PE42 films with attached 9-AnCH2 groups (PE42-An) are compared with those obtained previously [10] using films of the same type with covalently-attached 1-Py groups (PE42-Py). Specifically, we compare the abilities of 1-Py and 9-AnCH2 lumpohoric groups to assess the static and dynamic properties of their local environments using N,N-dimethylaniline (DMA; van der Waals volume=129 Å3 [13]) and N,N-dioctadecylaniline (DODA, van der Waals volume=684 Å3 [13]) as quenchers and the abilities of the two DAA to diffuse within the films. The results demonstrate that DODA does not diffuse through PE films as expected on the basis of simple molecular volume considerations and that the much longer-lived excited singlet states of 1-Py provide more global information about their environment than do the much shorter-lived excited singlet states of 9-AnCH2; lumophore lifetimes can be ‘tuned’ to interrogate different volumes of host space.

Section snippets

Materials

Low-density polyethylene films (PE42, density 0.918 g cm−3; 42% crystallinity by X-ray diffraction [15]) were Sclairfilm from DuPont of Canada, Mississauga, Ontario, Canada. N,N-dimethylaniline (DMA) (Fischer Scientific, reagent grade) was distilled prior to use and N,N-dioctadecylaniline (DODA), a white solid, mp 40–41.5 °C (lit. [16] mp 40–41 °C) was synthesized according to a procedure [17] modified from the literature [18]. The procedures for covalent attachment of 9-AnCH2 [4] and 1-Py [10]

Localized versus global information about diffusion; dependence on lumophore

Although the excited singlet-state lifetimes of 1-Py (ca. 200 ns [22]) and 9-AnCH2 (ca. 5 ns [4b]) are very different, both are quenched by DAA at diffusion-controlled rates in liquid media. However, as a result of the disparity in the lifetimes, dynamic fluorescence quenching of 1-Py will be the more efficient if a DAA molecule is initially at the same distance from a 1-Py and a AnCH2 group at the moment of their electronic excitation. It follows that (1) efficient dynamic fluorescence quenching

Conclusions

Two fluorophores, 1-pyrenyl and 9-anthrylmethyl groups, have been attached covalently to chains in interior positions of polyethylene films with 42% crystallinity in order to assure that the probes do not diffuse over time or as perturbations are applied to the films. Static and dynamic fluorescence measurements on the modified films have revealed information about their micro- and macro-structures, especially about the accessibility of two N,N-dialkylanilines to the sites in which the probes

Acknowledgements

We thank the National Science Foundation for its support of this research.

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