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Characterization of carboxylate sites in surface oxidized polyethylene films by fluorescence techniques
Eur. Polym. J. Vol. 22, No. 12, pp. 949 953, 1986 Printed in Great Britain
0014-3057/86 $3.00 + 0.0C Pergamon Journals Ltd
C H A R A C T E R I Z A T I O N OF C A R B O X Y L A T E SITES IN S U R F A C E O X I D I Z E D P O L Y E T H Y L E N E FILMS BY FLUORESCENCE TECHNIQUES M. HENNECKE, I. SCHNEIDER and J. FUHRMANN Fachbereich Chemie, University of Kaiserslautern, P.B. 3049, D-6750 Kaisertautern, F.R.G.
(Received 1 May 1986)
Abstract--Oxidized reactive sites introduced by controlled chemical reactions in non-crystalline regions of semi-crystalline polyolefin films are located mainly on the film surface or in adjacent subsurface regions. The so-formed reactive sites in low density polyethylene (LDPE) blown films were labelled by fluorescent groups and investigated by various fluorescence techniques. The surface or subsurface location of the labelled site is indicated by a solvatochromic spectral shift and confirmed by the polarity dependent fluorescence lifetime of the labelled LDPE film immersed in a polar liquid. Fluorescence concentration quenching and concentration depolarization demonstrate the high density of oxidized sites near the film surface. The measurement of rotational mobility and orientation during film stretching adds further experimental tools to distinguish reactive oxidized sites in the surface or in the bulk.
l. INTRODUCTION
Polarized fluorescence measurements yield infor-
The surface properties of solid organic polymers influence many of their applications and considerable effort is made to study the chemical modification of a given polymer surface [1]. The aim of this work is to study low density polyethylene (LDPE) blown films modified at the surface and in the neighbouring bulk with reactive sites which are labelled by fluorescent derivatives. The location of these probes near the surface is verified and their mobility and orientation is investigated by using various fluorescence techniques. We define "surface" of a polymer, in agreement with other authors [2], to be that part where the functional groups interact with a reagent which would be expected to be insoluble in the bulk of polyethylene and which is dissolved in a solvent that does not swell PE. Attacking the surface of L D P E by strong oxidizing agents such as chromic acid followed by nitric acid generates primarily carbon acid groups formed after chain scission. Variation of the reaction conditions may lead to oxidation in subsurface regions up to bulk regions or may even cause extensive degradation of the amorphous polymer phase. Therefore, care must be taken for mild reaction conditions. The carboxylic acid functionality is the starting point for subsequent chemical reactions at the polymer surface [2] where a dansyl-chromophore can become chemically bound to almost any oxidized group. Alternatively, acid functionalities near the polymer surface can be created by the chemisorption of thionyl chloride [2]. The fluorescence spectrum of the dansylchromophore is strongly influenced by the polarity of the surroundings; it shows a bathochrome shift with increasing polarity. The fluorescence lifetime is likewise sensitive to the environment of the dansyl group and reflects the amount of dynamic quenching.
mation about the local concentration of chromophores and their mean rotational mobility At A~t = 0.5 (3 < cos20 > - l ) (l) where 0 means the angle of rotation during the fluorescence lifetime z. This definition implies that A~t= 1 for frozen molecules and A l = 0 for cornpletely mobile molecules. In stretched samples, the 2nd moment of the orientation distribution function (odf) or Herman's orientation parameter f2 and the 4th moment or f4 can be measured if there is no rotational motion of the chromophore during the fluorescnece lifetime. Provided some reasonable assumptions [4] hold, in the case of rotational motion, instead of f4, the mean mobility amplitude ATt of the chromophore becomes accessible in the anisotropic state. 2. MATERIALS AND METHODS 2.1 Preparation o f samples
Low density PE films (Lupolen 1810 BASF, thickness 50/~m) were soxhletted for 12hr with 2-propanol and subsequently 12hr with n-hexane. The dried films were treated with Na [3] in order to remove oxidation products. Samples of 1.5 x 4 cm in size were chemically labelled with fluorescence probes (cf. scheme) using the methods of Whitesides et al. [2], yielding surface labelled samples (sls) and samples labelled beneath the surface (bls). For comparison, reference samples were treated in the same way but without the final step of coupling the chromophore. In addition, the soxhletted films were physically doped with fluorescence probes from 10-3M solutions of dansylbutyramide in toluene at room temperature. The diffusion time was 48 hr for each sample. Subsequently, the samples were washed with toluene and dried in vacuo for 24 hr. Frozen diluted samples of dansylbutyramide in polystryrene film were solution-casted from benzene (0.01 weight % dansylbutyramide).
949
950
M. HENNECKE et al. Chemical reaction scheme PE--C H2---C HE- pE At the surface ( s L s )
Beneath the s u r f a c e [ b L s )
1. H2CrO 4 chain scission
1.SOC12
2. CH 2 N 2
2.NH 2 -
3. NH 2 -
NH 2
3. R -
chemisorption (CH2) 2 -
NH 2
S02C12
4 R - - S02C 1 .(3 PE--C///-a H-
NH - -
SO2 -
R
PE - - C,H2 -
CH 2---- PE
NH --~CH2~NH-- SO2-- R
2.2 Fluorescence measurements
Polarized fluorescence spectra were measured with a fluorescence spectrophotometer (Perkin-Elmer 512) especially equipped for measuring polarized fluorescence at polymer films [5]. To reduce the stray light from the turbid surface, the samples were immersed in glycerin between two quartz prisms. Measurements during stretching were performed with the fluorescence polarization apparatus previously described [6]. A strip of 10mm width and 10mm initial length was immersed with glycerin between two thin quartz plates and then simultaneously stretched in a two-sample-mounting machine with an unlabelled reference sample. By subtracting sample and reference fluorescence intensities during the stretching process, the non-negligible residual fluorescence of the reference PE was eliminated. Uniaxial stretching at a rate of 5 % min-~ was performed at room temperature in the winding (machine's) direction of the blown films; the initial over-all orientation in the plane of the film as determined by the birefringence method is negligible. It was verified that the immersion did not influence the measured orientation of
probes and the dependence of the probe mobility on the stretching ratio. Time resolved measurements were performed using the time correlated single photon counting method [7]. 3. RESULTS AND DISCUSSION 3. I Fluorescence spectra
As noticed by Whitesides [2], the fluorescence spectrum o f dansyl labelled PE is strongly influenced by the polarity a r o u n d the c h r o m o p h o r e . T h e fluorescence maximum (excitation wavelength 2exc = 365 n m ) shifts from 438 n m in physically d o p e d P E films to 457 n m in bls a n d to 480 n m in sis (Fig. 1). In diluted glycerin solution, the m a x i m u m lies at ~'max= 494 rim. T h e h a l f w i d t h o f the spectra in d o p e d PE, sls, a n d bls is smaller t h a n in glycerin indicating different s u r r o u n d i n g s o f the c h r o m o p h o r e . The half width in sis a n d bls turns o u t to be m u c h smaller as
1.o-
0.8
0.6
'
O.
380
420
460
I
I
500
'
I
540
i
I
580
Wovelength (nm)
Fig. 1. Quantum corrected fluorescence spectra (fluorescence of reference sample subtracted) of PE films (Lupolen 1810). 2cxc=365nm; FWHM =full width at half maximum. (a) physically doped PE "~max= 438 nm, FWHM = 96 nm; (b) (bulk labelled PE) '~max= 454 rim, FWHM = 87 nm; (c) sis (surface labelled PE) ~'max= 480 nm, FWHM 93 nm. Not shown: (d) dansylbutyramide in glycerin ~max = 494 nm, FWHM = 120 nm; (e) dansylbutyramide in polystyrene )-max= 455 nm, FWHM = 83 nm.
Characterization of carboxylate sites it would be if the glycerin and the doped PE spectra are superimposed. The red shift accompanied with nearly constant half-width in the labelled PE is obviously due to partial solvation of the labels by the glycerin immersion. Calculated from the spectral shift, the fraction of glycerin solvation amounts to 0.73 in the case of sls and to 0.27 in the case of bls. Whereas the results of the sis are in agreement with [2], the bls behaviour is not, since it reflects a distinct influence of the immersion,
3.2 Time resolved measurements Time resolved measurements confirm the influence of the polarity and the oxygen content of the surroundings on the fluorescence lifetime of the label, Because of the dynamic quenching and the oxidation of the label, in the presence of air, the fluorescence decay law becomes biexponential. Therefore, the samples were kept under N 2. The influence of the polarity of the surroundings becomes apparent when the solvent is changed from acetone (lifetime r = 18 nsec) to glycerin (r = 16 nsec) and to hexane (z = 11.5 nsec) (see Table 1). The lifetime decreases with decreasing polarity of the solvent. In the case of labelled PE, a nearly mono-exponential decay was observed under N 2. No remarkable dependence on the type of chemical bonding of the dansylderivates is noticed, The physically doped PE films shows nearly the same lifetime as dansylbutyramide in hexane. For the labelled samples, taking the fractions of glycerin solvation of labels from the spectral displacement, a lifetime around 14.8 nsec (sls) and 12.7 nsec (bls) would be expected, The drop in the lifetime of about 2 nsec indicates a certain contribution of dynamic quenching to the excited state of the label. Increasing labelling by extending the oxidation time significantly alters neither the lifetime nor the fluorescence intensity. Therefore, the predomination of static self-quenching becomes evident, Table I. Fluorescence lifetime of labelled and doped polyethylene films and solutions of dansylbutyramide (C = 10 4 mol I ~) 2 ~ = 3 6 5 n m , ),~ at > 4 1 8 n m D A B A =
dansylbutyramide
Sample sis bls PE physically doped DABA in acetone DABA in glycerine DABA in hexane
~tnsec +0.5 nsec) 12.4 I 1.4 12.0 18.0 ~6.0 11.5
951
3.3 Fluorescence polarization in the isotropic state The emission anisotropy r in the isotropic state of the film Ivy-IvH r ~-Ivv + 21VH (2) is highest in a frozen sample (polystyrene doped with dansylbutyramide) and considerably lower in labelled and probed PE (Table 2). The rotational mobility in the PE samples is further substantiated by' the decrease of r with increasing temperature. At lower temperatures, the r values of both the physically doped sample and the glycerin solution tend towards the r0-value of a frozen sample, whereas the r values of sis and bls reach a limiting value given by r o - Arc. As already indicated by the drop in the lifetimes, we have to consider a high density of oxidized sites in the surface region of the films in spite of the relatively short oxidation time (the shortest used by previous workers [2]). In accordance with the magnitude of the lifetime drop, the concentration depolarization, Ar~, is higher in bls than in sis. Usually concentration depolarization of the fluorescence can be observed at lower concentration than that where concentration quenching occurs [11].
3.4 Uniaxial stretching experiments T h e ~ values of the labels in PE blown films during stretching at room temperature are shown in Fig. 2 together with a model curve for affine orientation of rigid revolution ellipsoids. In the calculation of Ji from the polarized fluorescence intensities, the concentration depolarization was considered to be represented by the room temperature value of r instead of r 0. In the range of low orientation, r was assumed to be constant during stretching. All measured curves are corrected with respect to the partial rotational mobility of the chromophores [4a]. The curves show a similar course as the corrected vector affine model curve for prolate ellipsoids. However, choosing reasonable values for the thickness-to-length ratio, a quantitative adjusting is reasonably possible. No time dependence of the .~-values was observed after the end of the siretching experiment. The different orientation behaviour of the differently labelled PE films clearly indicates that the polymer is labelled in different morphological regions. The value of/2 confirms the supposition that labels on the surface, predominatelv immersed in glycerin, remain unoriented during stretching. In all cases, the label is mobile during the fluorescence lifetime. The sls shows a higher mobility than bls as can be seen from Fig. 3.
Table 2. Emission anisotropy r at various temperatures ),o~ = 365nm, ,;,~: weighted average over the spectrum, d~r~ remaining depolarization in immobile samples Sample.. sis bls PE physically doped DABA in polystyrene (glassy sample) DABA in glycerin* (10 4 mol,/I) *The Perrin equation [10] holds.
- 20'C
20C
100 C
Ar~
0.23 0.18 0.32
0.15 0.14 0.29
0.08 0.04 0.25
0.08 0.11 0
0.33
0.33
0.33
0
0.314
0.22
0,02
0
952
M. HENNECKEet al.
f2
during stretching is to suppose that, during stretching, the fraction of labels located in the surface immersion is increased; these labels have a greater mobility. Of course, during an affine deformation, the distance (in % of the thickness) of any chromophore relative to the surface remains unchanged. If the chromophore density is independent of the normal direction, an elementary analysis yields the following relation for the fraction f~ of chromophores lying within both surface layers of thickness s
0~6 o / ~ 012 o /o ~ ~ . , 0.08
o- / f
o.0~ ,o~/~°~ ~ 0
~ is
~.0
2sv/X
~__ ~_.~~ ~'~ z.0
f~= , 2.s
x Fig. 2 . f 2 of fluorescent dansyl chromophores with emission anisotropy r obtained by stretching at 25°C. The stretching ratio 2 is corrected for the volume fraction of the crystallites. /k = sis r = 0.25; © = bls r = 0.22; [] = physically doped r = 0.33. Continuous curve: vector affine orientation [8] corrected for the chromopbore geometry, thickness-tolength ratio t/1 = 0.78. Dotted curves fit the sis and bls data. The birefringence of the PE film in the considered 2 range is identical for the three samples,
During stretching, the rotational mobility decreases in the physically doped sample but increases in both chemically modified films. A decrease of probe mobility during stretching in PE has been reported previously using physically incorporated probe molecules of different shape [9]. A reduction of concentration depolarization, Arc, during stretching would simulate an increase of mobility; this possibility can be clearly excluded since Arc is related to f2. At the measured low orientation (see Fig. 2), r = r 0 - Arc should remain nearly constant. A recalculation of ~ using a linear relationship between r ' = r at the stretching ratio 2 = 1 and r ' = r ÷ 0.04 at 2 = 2.5 introduced no significant change in the course of )ff vs 2. The most likely interpretation for the increase of mobility in the cases of sls and bls ~.o
I z---~"--''~'~-'''''~
08
0.6
~ -
~ ~
.
L
T
0~ .0
' 1.s
' z.0
' 2.s
x Fig. 3. Change of mobility (37 = 1 for frozen molecules) of fluorescent dansyl chromophores at 25°C with emission anisotropy r during stretching. A = sis r = 0.25; O = bls r = 0.22; [] = physically doped PE r = 0.33. Mobility of DABA in glycerin 37 = 0.50. Glycerin solvated fraction at ). = Isis: 0.77, bls: 0.68.
/0
(3)
where to means the initial thickness of the film at 2 = 1. Setting s in the order of magnitude of an atom diameter, the increase off~ during stretching directly monitors the fraction of chromophores located near the film surface that are "unscreened" and accordingly are in contact with the glycerin immersion. Since s/to is very small for homogeneously labelled or physically doped samples, the observation of an influence off~ on the mean mobility (of all chromophores) during stretching is almost impossible. On the other hand, the atypical course of ~ t vs 2 in the case of sis and bls establishes directly the "unscreening" hypothesis. Within this model, it is required by the values of M" obtained at 2 = 2.5 that, in the stretched state, 90% or more of the labels possess the same mobility as in glycerin solution. Starting from the glycerin solvated fraction of ,,~0.7 in the isotropic state (cf. 3.1), this fractional part can easily be obtained by use of Eqn 3. Once again it is shown that sis and bls are preferentially labelled in near-surface regions. The verification of the "unscreening" during stretching by measuring the accompanying bathochromic spectral shift failed because of the large width of the spectrum and the superimposed noise (cf. Fig. 1). It should be further admitted that, in the case of bls, the glycerin solvated fraction as determined from the spectral shift (0.27, cf. 3.1) disagrees with the fraction as determined from the mobility (0.68, cf. Fig. 3). In the case of sis, in spite of the different physical effects, the agreement is surprisingly good (0.73, cf. 3.1, 0.77 cf. Fig. 3). CONCLUSION Using various fluorescence techniques, it becomes possible to characterize oxidized sites in PE films which are labelled with fluorescent derivatives. All results show that the prepared LDPE films are oxidized in the expected thin layer at the surface. Fluorescence lifetime and polarization measurements in the isotropic state yield information on the local density of those oxidized sites which can produce concentration quenching and concentration depolarization. The polarized fluorescence in the anisotropic state yields the mean mobility and orientation of chromophores during stretching, indicating clearly whether the chromophore is located at the surface or in the bulk. REFERENCES 1. (a) D. R. Gagnon and T. J. McCarthy, J. appl. Polym. Sci. 29, 4335 (1984). (b) S. Haridoss and M. M.
Characterization of carboxylate sites
2.
3. 4. 5.
Perlanan, J. appl. Phys. 55, 1332 (1984). (c) R, G. Nusso and G. Smolinsky, Macromolecules 17, 1013 (1984). (a) J. R. Rasmussen, E. R. Stedronsky and G . M . Whitesides, J. Am. chem. Soc. 99, 4746 (1977). (b) J.R. Rasmussen, D. E. Bergbreiter and G. M. Whitesides, J. Am. chem. Soc. 99, 4746 (1977). E. Lindner, H. Behrens and S. Birkle, J. Organomet. Chem. 15, 165 (1968). (a) M. Hennecke, J. Polym. Sci. Polym. Phys. Edn 24, l l l (1986). (b) J. P. Jarry and L. Monnerie, J. Polym. Sci., Polym. Phys. Edn A-2 16, 443 (1978). M. Hennecke, To be published.
953
6. M. Hennecke and J. Fuhrmann, Colloid Polym. Sci. 258, 219 (1980). 7. K. P. Ghiggino, A. J. Roberts and D. Phillips, Adt. Polym. Sci. 40, 69 (1981). 8. (a) O. Kratky, Kolloid Z. 64, 213 (1933). (b) S. Oka, Kolloid Z. 86, 242 (1939). 9. M. Hennecke, C. K. Yeung and L. Monnerie, 15th Europhys. Conf. Macromolecular Physics, Hamburg (1983). 10. F. Perrin, J. Phys. Rad. 7, 390 (1926). I I. A. J. Pesce, C. G. Ros~n and T. L. Pasby, Fluorescence Spectroscopy, p. 143. Dekker, New York (1971).
0014-3057/86 $3.00 + 0.0C Pergamon Journals Ltd
C H A R A C T E R I Z A T I O N OF C A R B O X Y L A T E SITES IN S U R F A C E O X I D I Z E D P O L Y E T H Y L E N E FILMS BY FLUORESCENCE TECHNIQUES M. HENNECKE, I. SCHNEIDER and J. FUHRMANN Fachbereich Chemie, University of Kaiserslautern, P.B. 3049, D-6750 Kaisertautern, F.R.G.
(Received 1 May 1986)
Abstract--Oxidized reactive sites introduced by controlled chemical reactions in non-crystalline regions of semi-crystalline polyolefin films are located mainly on the film surface or in adjacent subsurface regions. The so-formed reactive sites in low density polyethylene (LDPE) blown films were labelled by fluorescent groups and investigated by various fluorescence techniques. The surface or subsurface location of the labelled site is indicated by a solvatochromic spectral shift and confirmed by the polarity dependent fluorescence lifetime of the labelled LDPE film immersed in a polar liquid. Fluorescence concentration quenching and concentration depolarization demonstrate the high density of oxidized sites near the film surface. The measurement of rotational mobility and orientation during film stretching adds further experimental tools to distinguish reactive oxidized sites in the surface or in the bulk.
l. INTRODUCTION
Polarized fluorescence measurements yield infor-
The surface properties of solid organic polymers influence many of their applications and considerable effort is made to study the chemical modification of a given polymer surface [1]. The aim of this work is to study low density polyethylene (LDPE) blown films modified at the surface and in the neighbouring bulk with reactive sites which are labelled by fluorescent derivatives. The location of these probes near the surface is verified and their mobility and orientation is investigated by using various fluorescence techniques. We define "surface" of a polymer, in agreement with other authors [2], to be that part where the functional groups interact with a reagent which would be expected to be insoluble in the bulk of polyethylene and which is dissolved in a solvent that does not swell PE. Attacking the surface of L D P E by strong oxidizing agents such as chromic acid followed by nitric acid generates primarily carbon acid groups formed after chain scission. Variation of the reaction conditions may lead to oxidation in subsurface regions up to bulk regions or may even cause extensive degradation of the amorphous polymer phase. Therefore, care must be taken for mild reaction conditions. The carboxylic acid functionality is the starting point for subsequent chemical reactions at the polymer surface [2] where a dansyl-chromophore can become chemically bound to almost any oxidized group. Alternatively, acid functionalities near the polymer surface can be created by the chemisorption of thionyl chloride [2]. The fluorescence spectrum of the dansylchromophore is strongly influenced by the polarity of the surroundings; it shows a bathochrome shift with increasing polarity. The fluorescence lifetime is likewise sensitive to the environment of the dansyl group and reflects the amount of dynamic quenching.
mation about the local concentration of chromophores and their mean rotational mobility At A~t = 0.5 (3 < cos20 > - l ) (l) where 0 means the angle of rotation during the fluorescence lifetime z. This definition implies that A~t= 1 for frozen molecules and A l = 0 for cornpletely mobile molecules. In stretched samples, the 2nd moment of the orientation distribution function (odf) or Herman's orientation parameter f2 and the 4th moment or f4 can be measured if there is no rotational motion of the chromophore during the fluorescnece lifetime. Provided some reasonable assumptions [4] hold, in the case of rotational motion, instead of f4, the mean mobility amplitude ATt of the chromophore becomes accessible in the anisotropic state. 2. MATERIALS AND METHODS 2.1 Preparation o f samples
Low density PE films (Lupolen 1810 BASF, thickness 50/~m) were soxhletted for 12hr with 2-propanol and subsequently 12hr with n-hexane. The dried films were treated with Na [3] in order to remove oxidation products. Samples of 1.5 x 4 cm in size were chemically labelled with fluorescence probes (cf. scheme) using the methods of Whitesides et al. [2], yielding surface labelled samples (sls) and samples labelled beneath the surface (bls). For comparison, reference samples were treated in the same way but without the final step of coupling the chromophore. In addition, the soxhletted films were physically doped with fluorescence probes from 10-3M solutions of dansylbutyramide in toluene at room temperature. The diffusion time was 48 hr for each sample. Subsequently, the samples were washed with toluene and dried in vacuo for 24 hr. Frozen diluted samples of dansylbutyramide in polystryrene film were solution-casted from benzene (0.01 weight % dansylbutyramide).
949
950
M. HENNECKE et al. Chemical reaction scheme PE--C H2---C HE- pE At the surface ( s L s )
Beneath the s u r f a c e [ b L s )
1. H2CrO 4 chain scission
1.SOC12
2. CH 2 N 2
2.NH 2 -
3. NH 2 -
NH 2
3. R -
chemisorption (CH2) 2 -
NH 2
S02C12
4 R - - S02C 1 .(3 PE--C///-a H-
NH - -
SO2 -
R
PE - - C,H2 -
CH 2---- PE
NH --~CH2~NH-- SO2-- R
2.2 Fluorescence measurements
Polarized fluorescence spectra were measured with a fluorescence spectrophotometer (Perkin-Elmer 512) especially equipped for measuring polarized fluorescence at polymer films [5]. To reduce the stray light from the turbid surface, the samples were immersed in glycerin between two quartz prisms. Measurements during stretching were performed with the fluorescence polarization apparatus previously described [6]. A strip of 10mm width and 10mm initial length was immersed with glycerin between two thin quartz plates and then simultaneously stretched in a two-sample-mounting machine with an unlabelled reference sample. By subtracting sample and reference fluorescence intensities during the stretching process, the non-negligible residual fluorescence of the reference PE was eliminated. Uniaxial stretching at a rate of 5 % min-~ was performed at room temperature in the winding (machine's) direction of the blown films; the initial over-all orientation in the plane of the film as determined by the birefringence method is negligible. It was verified that the immersion did not influence the measured orientation of
probes and the dependence of the probe mobility on the stretching ratio. Time resolved measurements were performed using the time correlated single photon counting method [7]. 3. RESULTS AND DISCUSSION 3. I Fluorescence spectra
As noticed by Whitesides [2], the fluorescence spectrum o f dansyl labelled PE is strongly influenced by the polarity a r o u n d the c h r o m o p h o r e . T h e fluorescence maximum (excitation wavelength 2exc = 365 n m ) shifts from 438 n m in physically d o p e d P E films to 457 n m in bls a n d to 480 n m in sis (Fig. 1). In diluted glycerin solution, the m a x i m u m lies at ~'max= 494 rim. T h e h a l f w i d t h o f the spectra in d o p e d PE, sls, a n d bls is smaller t h a n in glycerin indicating different s u r r o u n d i n g s o f the c h r o m o p h o r e . The half width in sis a n d bls turns o u t to be m u c h smaller as
1.o-
0.8
0.6
'
O.
380
420
460
I
I
500
'
I
540
i
I
580
Wovelength (nm)
Fig. 1. Quantum corrected fluorescence spectra (fluorescence of reference sample subtracted) of PE films (Lupolen 1810). 2cxc=365nm; FWHM =full width at half maximum. (a) physically doped PE "~max= 438 nm, FWHM = 96 nm; (b) (bulk labelled PE) '~max= 454 rim, FWHM = 87 nm; (c) sis (surface labelled PE) ~'max= 480 nm, FWHM 93 nm. Not shown: (d) dansylbutyramide in glycerin ~max = 494 nm, FWHM = 120 nm; (e) dansylbutyramide in polystyrene )-max= 455 nm, FWHM = 83 nm.
Characterization of carboxylate sites it would be if the glycerin and the doped PE spectra are superimposed. The red shift accompanied with nearly constant half-width in the labelled PE is obviously due to partial solvation of the labels by the glycerin immersion. Calculated from the spectral shift, the fraction of glycerin solvation amounts to 0.73 in the case of sls and to 0.27 in the case of bls. Whereas the results of the sis are in agreement with [2], the bls behaviour is not, since it reflects a distinct influence of the immersion,
3.2 Time resolved measurements Time resolved measurements confirm the influence of the polarity and the oxygen content of the surroundings on the fluorescence lifetime of the label, Because of the dynamic quenching and the oxidation of the label, in the presence of air, the fluorescence decay law becomes biexponential. Therefore, the samples were kept under N 2. The influence of the polarity of the surroundings becomes apparent when the solvent is changed from acetone (lifetime r = 18 nsec) to glycerin (r = 16 nsec) and to hexane (z = 11.5 nsec) (see Table 1). The lifetime decreases with decreasing polarity of the solvent. In the case of labelled PE, a nearly mono-exponential decay was observed under N 2. No remarkable dependence on the type of chemical bonding of the dansylderivates is noticed, The physically doped PE films shows nearly the same lifetime as dansylbutyramide in hexane. For the labelled samples, taking the fractions of glycerin solvation of labels from the spectral displacement, a lifetime around 14.8 nsec (sls) and 12.7 nsec (bls) would be expected, The drop in the lifetime of about 2 nsec indicates a certain contribution of dynamic quenching to the excited state of the label. Increasing labelling by extending the oxidation time significantly alters neither the lifetime nor the fluorescence intensity. Therefore, the predomination of static self-quenching becomes evident, Table I. Fluorescence lifetime of labelled and doped polyethylene films and solutions of dansylbutyramide (C = 10 4 mol I ~) 2 ~ = 3 6 5 n m , ),~ at > 4 1 8 n m D A B A =
dansylbutyramide
Sample sis bls PE physically doped DABA in acetone DABA in glycerine DABA in hexane
~tnsec +0.5 nsec) 12.4 I 1.4 12.0 18.0 ~6.0 11.5
951
3.3 Fluorescence polarization in the isotropic state The emission anisotropy r in the isotropic state of the film Ivy-IvH r ~-Ivv + 21VH (2) is highest in a frozen sample (polystyrene doped with dansylbutyramide) and considerably lower in labelled and probed PE (Table 2). The rotational mobility in the PE samples is further substantiated by' the decrease of r with increasing temperature. At lower temperatures, the r values of both the physically doped sample and the glycerin solution tend towards the r0-value of a frozen sample, whereas the r values of sis and bls reach a limiting value given by r o - Arc. As already indicated by the drop in the lifetimes, we have to consider a high density of oxidized sites in the surface region of the films in spite of the relatively short oxidation time (the shortest used by previous workers [2]). In accordance with the magnitude of the lifetime drop, the concentration depolarization, Ar~, is higher in bls than in sis. Usually concentration depolarization of the fluorescence can be observed at lower concentration than that where concentration quenching occurs [11].
3.4 Uniaxial stretching experiments T h e ~ values of the labels in PE blown films during stretching at room temperature are shown in Fig. 2 together with a model curve for affine orientation of rigid revolution ellipsoids. In the calculation of Ji from the polarized fluorescence intensities, the concentration depolarization was considered to be represented by the room temperature value of r instead of r 0. In the range of low orientation, r was assumed to be constant during stretching. All measured curves are corrected with respect to the partial rotational mobility of the chromophores [4a]. The curves show a similar course as the corrected vector affine model curve for prolate ellipsoids. However, choosing reasonable values for the thickness-to-length ratio, a quantitative adjusting is reasonably possible. No time dependence of the .~-values was observed after the end of the siretching experiment. The different orientation behaviour of the differently labelled PE films clearly indicates that the polymer is labelled in different morphological regions. The value of/2 confirms the supposition that labels on the surface, predominatelv immersed in glycerin, remain unoriented during stretching. In all cases, the label is mobile during the fluorescence lifetime. The sls shows a higher mobility than bls as can be seen from Fig. 3.
Table 2. Emission anisotropy r at various temperatures ),o~ = 365nm, ,;,~: weighted average over the spectrum, d~r~ remaining depolarization in immobile samples Sample.. sis bls PE physically doped DABA in polystyrene (glassy sample) DABA in glycerin* (10 4 mol,/I) *The Perrin equation [10] holds.
- 20'C
20C
100 C
Ar~
0.23 0.18 0.32
0.15 0.14 0.29
0.08 0.04 0.25
0.08 0.11 0
0.33
0.33
0.33
0
0.314
0.22
0,02
0
952
M. HENNECKEet al.
f2
during stretching is to suppose that, during stretching, the fraction of labels located in the surface immersion is increased; these labels have a greater mobility. Of course, during an affine deformation, the distance (in % of the thickness) of any chromophore relative to the surface remains unchanged. If the chromophore density is independent of the normal direction, an elementary analysis yields the following relation for the fraction f~ of chromophores lying within both surface layers of thickness s
0~6 o / ~ 012 o /o ~ ~ . , 0.08
o- / f
o.0~ ,o~/~°~ ~ 0
~ is
~.0
2sv/X
~__ ~_.~~ ~'~ z.0
f~= , 2.s
x Fig. 2 . f 2 of fluorescent dansyl chromophores with emission anisotropy r obtained by stretching at 25°C. The stretching ratio 2 is corrected for the volume fraction of the crystallites. /k = sis r = 0.25; © = bls r = 0.22; [] = physically doped r = 0.33. Continuous curve: vector affine orientation [8] corrected for the chromopbore geometry, thickness-tolength ratio t/1 = 0.78. Dotted curves fit the sis and bls data. The birefringence of the PE film in the considered 2 range is identical for the three samples,
During stretching, the rotational mobility decreases in the physically doped sample but increases in both chemically modified films. A decrease of probe mobility during stretching in PE has been reported previously using physically incorporated probe molecules of different shape [9]. A reduction of concentration depolarization, Arc, during stretching would simulate an increase of mobility; this possibility can be clearly excluded since Arc is related to f2. At the measured low orientation (see Fig. 2), r = r 0 - Arc should remain nearly constant. A recalculation of ~ using a linear relationship between r ' = r at the stretching ratio 2 = 1 and r ' = r ÷ 0.04 at 2 = 2.5 introduced no significant change in the course of )ff vs 2. The most likely interpretation for the increase of mobility in the cases of sls and bls ~.o
I z---~"--''~'~-'''''~
08
0.6
~ -
~ ~
.
L
T
0~ .0
' 1.s
' z.0
' 2.s
x Fig. 3. Change of mobility (37 = 1 for frozen molecules) of fluorescent dansyl chromophores at 25°C with emission anisotropy r during stretching. A = sis r = 0.25; O = bls r = 0.22; [] = physically doped PE r = 0.33. Mobility of DABA in glycerin 37 = 0.50. Glycerin solvated fraction at ). = Isis: 0.77, bls: 0.68.
/0
(3)
where to means the initial thickness of the film at 2 = 1. Setting s in the order of magnitude of an atom diameter, the increase off~ during stretching directly monitors the fraction of chromophores located near the film surface that are "unscreened" and accordingly are in contact with the glycerin immersion. Since s/to is very small for homogeneously labelled or physically doped samples, the observation of an influence off~ on the mean mobility (of all chromophores) during stretching is almost impossible. On the other hand, the atypical course of ~ t vs 2 in the case of sis and bls establishes directly the "unscreening" hypothesis. Within this model, it is required by the values of M" obtained at 2 = 2.5 that, in the stretched state, 90% or more of the labels possess the same mobility as in glycerin solution. Starting from the glycerin solvated fraction of ,,~0.7 in the isotropic state (cf. 3.1), this fractional part can easily be obtained by use of Eqn 3. Once again it is shown that sis and bls are preferentially labelled in near-surface regions. The verification of the "unscreening" during stretching by measuring the accompanying bathochromic spectral shift failed because of the large width of the spectrum and the superimposed noise (cf. Fig. 1). It should be further admitted that, in the case of bls, the glycerin solvated fraction as determined from the spectral shift (0.27, cf. 3.1) disagrees with the fraction as determined from the mobility (0.68, cf. Fig. 3). In the case of sis, in spite of the different physical effects, the agreement is surprisingly good (0.73, cf. 3.1, 0.77 cf. Fig. 3). CONCLUSION Using various fluorescence techniques, it becomes possible to characterize oxidized sites in PE films which are labelled with fluorescent derivatives. All results show that the prepared LDPE films are oxidized in the expected thin layer at the surface. Fluorescence lifetime and polarization measurements in the isotropic state yield information on the local density of those oxidized sites which can produce concentration quenching and concentration depolarization. The polarized fluorescence in the anisotropic state yields the mean mobility and orientation of chromophores during stretching, indicating clearly whether the chromophore is located at the surface or in the bulk. REFERENCES 1. (a) D. R. Gagnon and T. J. McCarthy, J. appl. Polym. Sci. 29, 4335 (1984). (b) S. Haridoss and M. M.
Characterization of carboxylate sites
2.
3. 4. 5.
Perlanan, J. appl. Phys. 55, 1332 (1984). (c) R, G. Nusso and G. Smolinsky, Macromolecules 17, 1013 (1984). (a) J. R. Rasmussen, E. R. Stedronsky and G . M . Whitesides, J. Am. chem. Soc. 99, 4746 (1977). (b) J.R. Rasmussen, D. E. Bergbreiter and G. M. Whitesides, J. Am. chem. Soc. 99, 4746 (1977). E. Lindner, H. Behrens and S. Birkle, J. Organomet. Chem. 15, 165 (1968). (a) M. Hennecke, J. Polym. Sci. Polym. Phys. Edn 24, l l l (1986). (b) J. P. Jarry and L. Monnerie, J. Polym. Sci., Polym. Phys. Edn A-2 16, 443 (1978). M. Hennecke, To be published.
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6. M. Hennecke and J. Fuhrmann, Colloid Polym. Sci. 258, 219 (1980). 7. K. P. Ghiggino, A. J. Roberts and D. Phillips, Adt. Polym. Sci. 40, 69 (1981). 8. (a) O. Kratky, Kolloid Z. 64, 213 (1933). (b) S. Oka, Kolloid Z. 86, 242 (1939). 9. M. Hennecke, C. K. Yeung and L. Monnerie, 15th Europhys. Conf. Macromolecular Physics, Hamburg (1983). 10. F. Perrin, J. Phys. Rad. 7, 390 (1926). I I. A. J. Pesce, C. G. Ros~n and T. L. Pasby, Fluorescence Spectroscopy, p. 143. Dekker, New York (1971).