Colloid & Polymer Science

Colloid & PolymerSci. 261,800-804 (1983)

Orientation of fluorescent probes and polymer segments in uniaxially drawn polycarbonate and polyvinylchloride films as revealed by dichroism, fluorescence polarization and birefringence*) H. Springer, R. Neuert, F. D. MOiler, and G. Hinrichsen Technische Universit~it Berlin, Institut f/.ir Nichtmetallische Werkstoffe, Polymerphysik, Berlin. Abstract: PVC- and PC-films doped with various fluorescent molecules were uniaxially drawn below and just above the glass transition temperature. The orientation of fluorescent probes and polymer segments is discussed, comparing Hermans' orientation factors evaluated from birefringence for the segments and from dichroism and fluorescence polarization for the probes. Moreover, the orientation of the fluorescent probes is related to that of hypothetical units, orientated according to the pseudoaffine deformation mechanism. Key words: Noncrystalline polymers, Hermans' orientation factor, dichroism, fluorescence polarization, birefringence.

1.

Introduction

The anisotropy in absorption and emission of geometrically anisotropic fluorescent molecules imbedded in polymeric materials m a y be used to measure the second and fourth m o m e n t u m of the orientation distribution function [1-3]. In a first step one gets information about the orientation of the fluorescent probes. If the relative orientation of p o l y m e r segments to the fluorescent probes is determined, one can infer in a second step the segmental orientation. Therefore it is i m p o r t a n t to k n o w the location and orientation of the fluorescent molecules with regard to the p o l y m e r segments [4-6]. M o r e detailed questions are: h o w do the probes orientate as a function of drawing p a r a m e ters, as for example of draw temperature and draw ratio, and h o w do these parameters influence the relative orientation of segments and probes ? T h e present w o r k deals with these questions for noncrystalline polymers, uniaxially drawn at temperatures below and just above glass transition t e m p e r a ture. Polyvinylchloride (PVC) and bis-phenol-Ap o l y c a r b o n a t e (PC) were chosen as suitable materials.

*) Lecture, given during the Spring Conference of the Deutsche Pbysikalische Gesellschaft, March 7-9, I983 in UIm. K 644

2.

Experimental

and

evaluation

PVC-films and PC-films were prepared, drawn and measured as described elsewhere [7, 8] in more detail. Table 1 shows the fluorescent molecules used for doping. The last column indicates the length of the probes with regard to the length of p,p'-diphenyltrans-stilbene (DPS). The probe concentration, expressed in mol fluorescent probe per mol monomer, was 10-4 and 10-3 for PVC and PC respectively. Drawing temperatures were set at 55 and 85 ~ for PVC and 60, 120 and 150 ~ for PC. The segmental orientation was characterized by means of birefringence measurement. The probe orientation was measured by UV-dichroism and fluorescence polarization. Ail the three methods render the second momentum of orientation distribution" birefringence that of the polymer segments and dichroism as well as fluorescence polarization that of the fluorescent molecules. Additionally, the fluorescence polarization method yields the fourth momentum. In this work we shall confine ourselves to the discussion of the second momentum, represented by Hermans' orientation factor. The following assumption were made in order to obtain the orientation factor of the "'microscopic" structural units from the "macroscopic" quantities : - the characteristic optical properties of the units (tensors of molecular polarizability, absorption and emission) do not depend on orientation ; - the macroscopic quantities are always determined in the same way additively from the orientation dependent contributions of isolated microscopic units; - the orientation distribution exhibits uniaxial symmetry; - the structural units are optically transverse isotropic; - the optical anisotropy of the fluorescent molecules is the same both in absorption and emission.

Springer et aL, Orientation of fluorescent probes and polymer segments

vrobe home

Gbbre: clarion

formula

L/LDpS

801

0`14

PC. DPS

120~

0,12. 1,4-d~phenyl1,3 - butadiene

DPB

1,5-dlphenyl1.3,5 hexafrlene

DPH

~C%c/C"%c ~ ~

0,58

oo8

, , i d~.y~C'%c./C%c/C%c

dtphenylshfbene

DP8

b,s[5-phenyol ~azoel ;

,

~_

MSB

1,00

0

"~"

0,01

0.02

0`03

0,04

0.05

zfn

H

~/~o

/

O,94

/ ~

if

'

0,06.

i

c%c/ c%c/ C%c/C%c

f-

methylsfyry/)

I

,5ooc /

0,81 A

i

1.8- dzpheny/DPO 3,5,,7-ocfotetroene

e

0,10-

0,91

/ ~

~J

-

Fig. 1. Hermans' orientation factor as a function of An for DPS in PC. ~ , ~ , 9 values from dichroism F1, ~, 9 values from fluorescent polarization; parameter: drawing temperature

Q96

PC , dn= 0,02

DPS = / -.i B / / /

0,06Table 1. Fluorescent molecules The assumptions may be justified by the following facts: - the probe structures are highly symmetrical and the geometrical dimensions are similar to those of cylinders; - the probe concentration is low; - the drawing process is uniaxial and the achieved orientation is not very high; - the polymers are noncrystalline (there is only a single phase) ; - the measuring temperature (room temperature) is far below glass transition temperature. The identity assumption of absorption and emission anisotropy is supported by a good correspondence of the orientation factors, evaluated from dichroism 0cff c) and fluorescence polarization 0cFp) respectively (s. fig. 1 and 2).

3. Results and discussion

~ g/"/'w DPO

0,05....

0'031

o,o:y 0

A ~ /i

/~

DPH

MSB

. . . . . 0,01 0,02 0,03 0,04 0.05 0,06 =

fH~

Fig. 2. Comparison of Hermans' orientation factors, obtained at An = 0.02 from dichroism and fluorescence polarization respectively, for fluorescent probes in PC. Parameter: drawing: 60 ~ 120 ~ 150 ~ open, divided and full symbols respectively

Comparison of the results of dichroism and fluorescence polarization measurements

Influence of probe geometry on the orientation

In figure 1 Hermans' orientation factors measured by dichroism and fluorescence polarization for DPS in PC-films are plotted against birefringence. Obviously the orientation mean values obtained from both methods coincide well and are proportional to the birefringence values. Furthermore the orientation factors are recognized to depend on drawing temperature. In figure 2 Hermans' orientation factors of all probes in PC, evaluated from dichroism and fluorescence polarization respectively, are compared at constant birefringence An =0.02, i.e. at constant segmental orientation. The measuring points scatter about the line representing the identity f f f = fffc.

As can be seen from figure 2, the fluorescent molecules vary in their orientation mean values, An being constant. An essential cause for this effect is given by the probe geometry (cf. [9, 10]). Regarding the probes as rigid rods and neglecting differences in their diameters, the length of the molecules seems to be the distinguishing parameter responsible for the variation of fffc and fF; at the constant An-value, characterizing the segmental orientation. This is shown in figure 3 for PVC and in figure 4 for PC. The orientation factor increases with growing probe length and it comes out greater, if the samples have been drawn at higher temperatures.

Colloid and Polymer Science, VoL 261. No, 10 (1983)

802

PVC, dn=O,O04 0.5

O,2

O,6

DPB~/~"'~

0'7

POPOP

0'8

o,'9 --

1,'o

L/Lnpr

Fig. 3. Hermans' orientation factors fffCof probes in PVC-samples with An = 0.004 plotted against relative probe length. Parameter: drawing temperature 05-

0.4-

85~ j 5 5 ~ PVC.DPS~ / j ~ ! ~ ~

~o

%~ 0,3-

~''~'~I'~"

0.2-

o

d~

o,L5

o,'.s

zln/dno

0.'4

Fig. 4. Hermans' orientation factors f~c for probes in PC-samples with a n = 0.02 plotted against relative probe length. Parameter: drawing temperature

The temperature dependence is more pronounced for longer probes. These facts may be interpreted as follows: the longer probe molecules are orientated more directly with the elongation of the sample. At higher temperatures especially the longer probes move preferentially into locally higher oriented sample regions, i. e. there may be some "anisotropic diffusion" during the drawing process. Otherwise the fluorescent probes may act as "orientation nuclei", depending on probe length and draw temperature, i. e. there may be a locally higher molecular orientation in the neighbourhood of the probes. Probe and segmental orientation Comparison of figures 3 and 4 shows: the orientation factors of the fluorescent molecules, which are achieved with PVC-films, are considerably greater than those obtained for the probes in PC-films. Does

this difference in the orientation simply reflect the orientation of polymer segments or does it result from a stronger orientation of the probes with regard to the PVC-segments than to the PC-segments? To answer this question, the intrinsic birefringence Ano of the polymers has to be known. Unfortunately these values are not well established. If one, in spite of this uncertainty, assumes Ano = 0.013, obtained from sound velocity measurementsl), and 0.236 according to [11] for PVC and PC respectively, Hermans' orientation factors of the probes can be displayed against An/Ano, i.e. against Hermans' orientation factor of the polymer segments. Figure 5 demonstrates this for the most orientated fluorescent probe DPS. The dotted line is valid, if the orientation factors of probes and segments are identical. Obviously the DPS orientation in PVC-samples is stronger and in PCsamples weaker than the segmental orientation. This leads to the conclusion that the intermolecular orientation correlation between probe and polymer chain in PVC is different from thatin PC. Within experimental error proportionality between the orientation factors of probes and segments is proven. Of.course, it turns out again, that the relation between probe and segment orientation depends on the drawing temperature. For the other investigated fluorescent molecules results analogous to those of DPS are obtained. In PC the probe orientation is always lower than that of the segments, in PVC the orientation of DPO is higher, while that of POPOP, DPH and DPB is nearly equal or lower than that of the PVC-segments.

~) unpublished measurements of the authors, similar An0-values are given in [13] and [14].

PC. dn=O.02

DPS DPO/....~m 150~

0,06" O,05-

DPH /

~"* 0,04-

/

9

A ~ ~

o,o3]

Zk.

9

/

E3

vg~...~

o MSB POPOP o

~t Ol

o,6

o,'7

o.'8

o.~ -

~;o

L/LDPs

Fig. 5. Hermans' orientation factor as a function of An~Ano for DPS in PVC and PC. Parameter: drawing temperature

120~ 50oC

Springer et al., Orientation of fluorescent probes and polymer segments

Probe orientation and pseudoaffine deformation The comparison between the orientation factors of fluorescent probes and polymer segments suffers from the uncertainty of the intrinsic birefringence values. Therefore it seems reasonable to compare the orientation behaviour of the fluorescent probes to that of hypothetical units, which are oriented according to a deformation model. A model, often proposed for drawing below or not far above the glass transition, is that of pseudoaffine deformation [12]. In figures 6 and 7 Hermans' orientation factor fffc and f~P of the DPS molecules, in PVC and PC respectively, are plotted against Hermans' orientation factor f.~/(~.), calculated from the corresponding draw ratio 2 according to the pseudoaffine deformation. In a good approximation PVC , DPS

0,7. 0,6'

I

,/"o/

/

I

[]

0,8

/- /

r

55oc

v

0,7

DPS PO~

9

T / . , , . , ~ 85~

% O,5 9

9

> / DPB

O`4J

DPH

~

.

~

POPOP

.45~ I/

/

PVC

O`5

/

o..

J 03.

deviation from the pseudoaffine deformation represented in figures 6 and 7 by the dotted line is greater in PC than in PVC and for drawing temperatures below Tg smaller than above Tg. The constant of proportionality can be interpreted as a measure of comparison of probe orientation with that of pseudoaffine deformation. As shown in figures 8 and 9, this measure is dependent on probe length, polymer matrix and drawing temperature.

/

Zl

Sf

r o,; /

fffc or f[P and f ~ are proportional to each other. The

/ I

803

O,2

~t 0,1

oJ / f : x 0

0,1

0,2

0 0,6

0,3

0,4

aS

.

O,6

0,7

o.'8

f# (,u

0,'7

0,'8

. L/Lops

Q'9

I,'0

Fig. 8. Probe orientation in PVC related to the orientation according to pseudoaffine deformation as a function of relative probe length. Parameter: drawing temperature

Fig. 6. Hermans' orientation factor fDC of DPS in PVC plotted against f,~ (2). Parameter: drawing temperature []

PC

0,3

0,J

20oc

PC, DPS

.o<

/ /6ooc

o\

/[]/

@

0,2-

0,08" O,06"

0,1"

o,oz,o,02.

/

/

oil

O!

O,1

0,'2

o,'~

o,'4

o.'5

. CcaJ Fig. 7. Hermans' orientation factor f~P of DPS in PC, plotted against f ~ (2.). Parameter : drawing temperature

o,6

o:7

o,'8

0,'9

1:o

- L ~Lops Fig. 9. Probe orientation in PC related to the orientation according to pseudoaffine deformation as a function of relative probe length. Parameter: drawing temperature

Colloid and Polymer Science, VoL 261. No. lO (1983)

804

Proportionality between the orientation factors f~*) and f~2) of the units of two different ensembles 1 and 2 respectively : f~l) = Afff)

(A <~ 1)

(1)

may be interpreted by various models. Two simple ones are the following: I) Only a fraction A of ensemble 1 takes part in orientation and orients in exactly the same manner as the units in ensemble 2, whereas the remaining fraction (1 - A) keeps statistically orientated. II) The units of ensemble 1 are positioned on a cone with angle a around the units of ensemble 2. Then it can be seen easily: A = - T1 (3 cos2ol -

1).

(2)

To decide, which model is more adequate, one has to compare the fourth moments. This will be done in a subsequent paper. Using model I from equation (1) follows :

= A

(3)

Acknowledgement Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

while model II implies



constants of proportionality depend on the polymer matrix, the fluorescent probe and the drawing temperature. The orientation of the fluorescent probes with regard to the orientation of the polymer segments as well as of the hypothetical units of the pseudoaffine deformation model turns out to be higher in the PVCsamples than in the PC-samples.

(4)

1 (35 cos4a - 30 cos2a + 3). with B = P4(cos o) = -~-

P4 represents the fourth order Legendre polynomial.

11. 12. 13. 14.

Nishijima, Y., Ber. Bunsenges. Physikal. Chem. 74, 778 (1970). Patterson, D., I. M. Ward, Trans. Faraday Soc. 53, 1516 (1957). Desper, C. R., I. Kimura, J. Appl. Phys. 38, 11, 4225 (1967). Nobbs, J. H., D. I. Bower, I. M. Ward, D. Patterson Polymer 15, 287 (1974). Nobbs, J. H., D. I. Bower, I. M. Ward, Polymer 17, 25 (1976). Fuhrmann, J., M. Hennecke, Makromol. Chem. 181, 1685 (1980). Springer, H., J. Kussi, H.-J. Richter, G. Hinrichsen, Coll. & Polym.Sci. 259, 911 (1981). Springer, H., R. Neuert, F. D. M/.iller, G. Hinrichsen, in preparation. Hennecke, M., thesis, Kaiserslautern (198l). Thulstrup, E.W., J. Michl, J. Am. Chem. Soc. 104, 5594 (1982). Biangardi, H. J., Habilitationsschrift, TU Berlin (1980). Kratky, O., Kolloid-Z. 64, 213 (1933). Kashiwagi, M., I. M. Ward, Polymer 13, 145 (1972). Hibi, S., M. Maeda, H. Kubota, T. Miura, Polymer 18, 137 (1977). Received April 22, 1983; accepted June 6, 1983

4. C o n c l u s i o n s

The observed Hermans' orientation factors of fluorescent molecules within PC- and PVC-films uniaxially drawn below or not far above T.g are proportional to birefringence (or segmental orientation factor) and to orientation factors, calculated according to the pseudoaffine deformation model. The

Authors' address : Dr. H. Springer Institut fiir Nichtmetallische Werkstoffe Polymerphysik TU Berlin Englische Str. 20 D-1000 Berlin 12