Photophysical Approaches to Characterization of Guest Sites and

Chapter 25

Photophysical Approaches to Characterization of Guest Sites and Measurement of Diffusion Rates to and from Them in Unstretched and Stretched Low-Density Polyethylene Films Downloaded by GEORGETOWN UNIV on August 21, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch025

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Jawad Naciri , Zhiqiang Hé , Roseann M. Costantino , Liangde L u , George S. Hammond, and Richard G . Weiss 4

Department of Chemistry, Georgetown University, Washington, D C 20057-2222

Various luminescence and photochemical techniques have been developed to probe the natures and accessibilities of the sites available to guest molecules in low-density polyethylenefilms.The methods, which include covalent modification of interior sites with fluorophores, are described. The results indicate that a distribution of site types, each with its characteristic shape, free-volume, and accessibility, is present. Furthermore, this distribution is changed drastically when a film is stretched. Activation energies for diffusion of a series of Ν,Ν-dialkylanilines in unstretched and stretched, modified and "native" films are reported, compared, and analyzed. Polyethylene is the name given to literally thousands of polymer formulations, derived in some cases from mixtures of monomers which include 1-alkenes in addition to ethylene. Molecular weight average and distribution, degree of chain branching, mode of processing, degree of crystallinity, types of amorphous regions, density etc. differentiate the various polyethylenes(i). A combination of density, melting point, and degree of crystallinity are used to classify and distinguish low density polyethylene (LDPE) samples(7). Two general site types in L D P E -those near points of chain-branching and those along the interfaces between amorphous and crystalline regions ~ have been suggested for guest (dopant) molecules(2). Recent X e N M R studies with 129

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Current address: Naval Research Laboratory, 4555 Overlook Avenue, Code 6090, Washington, DC 20375 Current address: King Industries, Science Road, Norwalk, CT 06852 Current address: Office of Premarket Approval, Chemistry Review Branch, Food and Drug Administration, 200 C Street, SW, HFS-247, Washington, DC 20204 Corresponding author

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0097~6156/95/0598-04l5$12.25/0 © 1995 American Chemical Society Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

426

MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS

Downloaded by GEORGETOWN UNIV on August 21, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch025

polyethylene support this hypothesis^?). Guest molecules are unable to enter the crystalline portions of L D P E below the melting transrtion(2e). To characterize further the microscopic environments provided by L D P E to guest molecules, we have probed the shapes and free volumes of the sites and the dynamics of diffusion to and from them in several different ways(4). The results obtained thus far indicate that there is a distribution of site types which can be changed drastically by macroscopic stretching of polymer sheets. A review is presented of our efforts to develop new methods for the measurement of diffusional rates to and from guest sites in L D P E and to gain insights into the natures of the sites(5) at which guest molecules reside. Materials and Procedures 3

Films of Sclairfilm (76 nut thick, 0.92 g/cm , M 112600, from DuPont of Canada) have been employed throughout our investigations. This allows results from different experiments to be compared directly. The films were soaked in chloroform before being used in order to remove plasticizers, antioxidants, and other additives. From the heat of the melting transition (measured by differential scarining calorimetry), Sclairfilm is ca. 50% crystalline(o). Reagents, solvents, and methods are as described in our cited publications. Cold-stretching to 4-6X the original film length was accomplished with a device designed for this purpose or by hand; experiments employing the two methods led to indistinguishable results. w

Preparation of Modified L D P E Films There are many recipes for preparation of polymeric films containing low concentrations of lumophoric or other "reporter" groups. The vast majority of these involve either copolymerization of two (or more) different monomers, one of which is initially in large excess with respect to the other(s), or the physical rnixing of tagged and untagged chains. Such films have two important problems if they are to be used to probe the physical or microscopic properties of their unmodified analogues: (1) the reporter groups are located at surfaces as well as at interior positions of the films; (2) the natures of the sites at which the reporters reside is deterrnined in large part by them rather than by the intrinsic nature of an unmodified film. The modified films whose preparations are described below diminish greatly the seriousness of both problems: (1) the reporter groups can be excluded almost completely from surface-accessible sites; (2) it is the reporter group which must adapt to the demands of the sites offered by the native film. Py-LDPE(7). Films strips were soaked in chloroform containing 0 . 2 4 M pyrene until an appropriate concentration of the lumophore (ca. 10' M usually, as measured by UV/vis absorption spectroscopy) had been embibed. After standing in air for 15-30 min (to allow chloroform to evaporate) and being washed with methanol (a non-swelling solvent which removes surface pyrene), doped films were irradiated with the pyrex-filtered output of a 450 W medium pressure Hg 2

Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

25. NACIRI ET AL.

Guest Sites in Stretched LDPE Films

427

lamp (Hanovia) for various periods (usually 1 h) which were adjusted empirically to provide the desired concentrations of 1-pyrenyl groups covalently attached to the polymethylene chains. The amount actually attached was determined by washing the irradiated films exhaustively in baths of chloroform and them measuring the UV/vis absorption spectra. We have sought to produce films with ca. ΙΟ'ΜΟ" M pyrenyl groups (designated Py-LDPE). The emission spectra of a Pv-LDPE film and an L D P E film doped with pyrene are presented in Figure 1. The absorption and emission spectra of Py-LDPE are typical of 1-pyrenyl groups(8). Evidence that the pyrenyl groups of Py-LDPE are isolated from film surfaces was provided by the inability of 0.95 M 2-(dimethylamino)ethanol in methanol to quench more than 6% of the fluorescence intensity from even stretched films. The same quencher decreased the fluorescence intensity of pyrene in methanol by >90%(7).


4

A n - L D P E (9). After doping a film by immersing it in an ether solution of 9anthryldiazomethane(iΟ), the solvent was removed by evaporation in air, the film surfaces were washed with methanol, and the dopant concentration (commonly (1-5)χ10 M ) was deterrnined as described above. Each film was sealed in a glass tube and immersed in boiling methanol for 1 h. The unreacted 9anthryldiazomethane was removed from the films by exhaustive washing in ether baths. A spectroscopic history of the preparation of an anthryl-modified film, A n - L D P E . is shown is Figure 2. Typical concentrations of covalently attached anthryl groups were l û M O M . 3

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Spectroscopic Investigations Michl and Thulstrup(ii) and Yogev etal.(4d),especially, have shown that doped, stretched films of L D P E can be used to align guest molecules according to a laboratory frame of axes. Linear dichroic spectra of the dopant molecules provide information concerning the directions of transition dipoles of the electronic transitions. Fluorescence spectra of both Py-LDPE and A n - L D P E films gave no evidence for excimer emission(7,P). Thus, we assume that virtually all of the occupied sites have no more than one lumophoric group. Much higher concentrations of non-covalently linked pyrene in the films did provide emission spectra with detectable excimer emission (Figure 1). Since it has been conjectured that dopant molecules tend to translocate from one site type to another when L D P E films are stretched(2), we sought to devise an experiment in which the spectroscopic properties of dopant molecules which are forced to remain at their original locations, regardless of the applied stress on the material, can be compared with the properties of similar species which are free to migrate. To accomplish this goal, an A n - L D P E film was stretched. For comparison purposes, another piece of film was doped with an equal concentration of 9-methylanthracene ( M A . a non-covalently attached guest) and stretched by the same amount.



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360.00

455.00

550.00

Wavelength (nm) Figure 1. Room temperature emission spectra in air (a) from an L D P E film after immersion in a 0.24 M pyrene in chloroform solution for ca. 12 h (Py/LDPE) and (b) from a film, doped as above, after being irradiated for 1 h and exhaustively extracted with chloroform (PyL D P E ) . The spectra are not normalized; Xe 343 nm(7û). X


25. NACIRI ET AL.

1


429


1

1

1

1

1

1

1

(H—'— — — — — — — — —I 300

400

500

O.D.

O.D.

Figure 2. Representative UV/vis absorption spectra: (a) an unstretched L D P E film doped with 9-anthryldiazomethane before heating; (b) film in (a) after heating and exhaustive extraction with ether and chloroform (An-LDPE): (c) an unstretched L D P E film doped with 9methylanthracene. A l l spectra were recorded with an undoped film as reference.


430


The emission anisotropy of an unstretched An-LDPE film is ca. 0.15that the anthryl groups are not able to move rapidly within a film. When swelled by ca. 17% by cyclohexane, the films become more tolerant of anthryl motions and the emission anisotropy decreased to ca. onethird of its original value(Pû). Unstretched An-LDPE films and M A / L D P E films showed no linear dichroism (d): d =OD^/OD was unity at 260 and 390 nm, wavelengths at which anthryl transition dipoles are polarized almost exclusively along the long (Z) and short (Y) in-plane axes. However, the same films, after being stretched to 5 X their original lengths, displayed non-unity dichroic ratios from which orientation factors (O =df/[d +2] where f =X,Y,Zare the principal axes of anthracene) can be calculated. As shown in Table I, the values calculated for M A differ somewhat from those reported by Michl and Thulstrup(Z2). The disparities can be attributed to different degrees of stretching, different L D P E sources, and different concentrations of M A in the two experiments. More importantly, an internal comparison between our results from An-LDPE and M A / L D P E . with about equal chromophore concentrations, reveals that the non-covalently and covalently attached anthryl groups reside in nonequivalent site types after film stretching. Although these results do not confirm the hypothesis that guest molecules translocate when films are stretched(2), they are at least consistent with it.

0.18(PÛ), indicating

±


f

f

Distributions of Guest Site Sizes Information presented thus far indicates that different site types may exist in L D P E . but does not address quantitatively their free volumes. Since pyrene excimer emission can be detected from L D P E films which were swollen by chloroform at the time of doping(7), some of the sites must have a minimum of — 645 À (i.e., the sum of the van der Waals volumes of two pyrene molecules(ii)) of available free volume. However, this volume may not be representative of sites in L D P E which has not been swelled during doping. In other experiments, Py-LDPE (and An-LDPE) films were placed in methanolic solutions of Ν,Ν-dialkylanilines (PAA) until the pyrenyl (or anthryl) fluorescence intensity no longer changed. Since methanol is a non-swelling solvent for L D P E . P A A molecules had to enter unswelled films and occupy sites more like those of the native polymer. The concentrations of P A A in the films were ascertained from the film volumes and the quantity of P A A which could be extracted from them. Some of the pertinent data for Py-LDPE are presented in Table 11(14). F , the fraction of film fluorescence quenched by a P A A homologue, is expressed in equation 1(7) where I and 1^ are the fluorescence intensities observed in the absence of P A A and after equilibration with it. Since P A A are known to be very efficient (diffusion controlled) quenchers of pyrenyl and 3

0


25. NACIRI ET AL.

431


Table I. Dichroic ratios and orientation factors from anthryl groups in stretched LDPE films(Pa)


dopant

d

MA

x

0.77

2.8

MA' An-LDPE

0.89

1.94

a) Data from Michl and Thulstrup(i2). Οχ = 1 - (Oy + Ο ζ ) ( ϋ ) .

O

Οχ"

Ov

0.14

0.28

0.58

0.20

0.29

0.51

0.21

0.30

0.49

z

b) Calculated assuming

Table Π. Data related tofluorescencequenching in Pv-LDPE films by P A A homologues at 25 °C(14)

PAA

van der Waals volume, Â

3

unstretched fPAAIfa. M F

F M"

film M

stretched film ίΡΑΑ^ F M F M" M

1

1

PMA

128.7

0.19

0.35

L6

0.14

0.15

0.9

ΡΕΑ

162.8

0.12

0.25

1.7

0.09

0.10

0.9

PPA

196.9

0.09

0.19

1.6

0.08

0.04

0.3

DBA

231.0

0.07

0.16

1.6

0.09

Photophysical Approaches to Characterization of Guest Sites and

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