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Effect of polybutadiene structure on its refractive index increment.

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Die Angewandte Makromolekulare Chemie 23 (1972) 169-172 (Nr. 334)
From the Institute of Macromolecular Chemistry,
Czechoslovak Academy of Sciences, Prague 6, Czechoslovakia
Effect of Polybutadiene Structure on Its Refractive
Index Increment
By PAVEL
KRATOCHV~L,
DAGMAR
STRAKOV~,
and PAVEL
SCHMIDT
(Eingegangen am 24. November 1971)
SUMMARY :
The refractive index increments of polybutadiene in cyclohexane have been found
to be a linearly decreasing function of the content of 1.2-units in the polymer. The
extrapolated values at 25°C for a pure 1.4- and pure 1.2-polymer respectively are
(in ml/g) : 0.121 and 0.092 (4360 A);0.113 and 0.087 (5461 A).
ZUSAMMENFASSUNG :
Die Brechungsinkremente des Polybutadiens in Cyclohexan sind eine linear abnehmende Funktion des Gehalts an 1,2-Einheiten im Polymeren. Die extrapolierten
'Werte bei 25°C fur das reine 1,4- bzw. 1,2-Polymerebetragen (in ml/g) : 0,121 bzw.
0,092 (4360 A) oder 0,113 bzw. 0,087 (5461 A).
Cyclohexane is the most widely used solvent for dilute solution measurements
of polybutadienes. The refractive index increment, dn/dc, of this polymer-solvent system has been measured by a number of authorsl-6 and it has been
found t h a t the dn/dc value decreases with increasing content of 1.2-units in the
polymer. On the other hand, the ratio between the 1.4-trans and 1.4-cis units
does not seem t o exert any observable effect on dn/dc. I n none of the studies
published, however, the dependence of dn/dc on the 1.2-units percentage was
examined systematically; the bulk of the data refer t o polymers containing
less than 25% of 1.2-units (Fig. la). The scatter of values obtained by different
authors is unusually large. That is why we decided t o measure the refractive
index increments in cyclohexane for a series of polybutadiene samples with increasing 1.2-units content.
Experimental
The polymers were synthetized in several Czechoslovak research laboratories and
kindly supplied t o us.
169
dn
dc
Fig. 1.
170
Dependence of the refractive index increment in ml/g of polybutadiene in
cyclohexane on the percentage of 1.2-units in the polymer at 25°C.
(a)Literature values. Pipped points ( p) refer to wavelength 4360 A and
@, RIBEYROLLES
et d.20 ,
plain points (0)t o 5461 8. Data of CLELAND~
COOPERet d30, HARMON4 0 , BAHARY
et aL5 0, CURCHOD~
0.Broken
lines 1 and 2 represent the dependence obtained in the present work for
4360 and 5461 A respectively.
(b) Our measurements. refer to wavelength 4360 8 and 0 to 5461 8.
Eflect of Polybutadiene Structure on I t s Refractive Index Increment
Cyclohexane was a chemically pure product, redistilled on a 30-plate laboratory
column ; its purity was controlled chromatographically.
The refractive index increments were measured a t 25 "C with a Brice-Phoenix differential refractometer, Model BP-2000-V, using the cell for volatile solvents. The
,dn/dc values reported here (Fig. l b ) are averages from four measurements on solutions with different polymer concentrations (1 x 10-2 - 2 x 10-2 g/ml). No concenLration dependence of dn/dc was observed.
Infrared spect.roscopy was used to determine the structure (i. e. the percentage
of the individual isomers of the monomeric unit) of the polybutadiene samples. The
method recommended by HAsLAMand WILL IS^ was applied. Polybutadiene films were
made by evaporating chloroform or benzene solutions of the polymer on potassium
bromide plates under an infrared lamp. The spectra were recorded by a Zeiss UR-10
.spectrometer. The accuracy of the determination was about 5 5% of 1.2-units in
the polymer.
Results and Discussion
The refractive index increment of polybutadiene in cyclohexane is a linear
function of the 1.2-units content (Fig. lb). This is in accord with the rather
generally valid rule t h a t the refractive index increment of copolymers depends
linearly on their composition (cf. e. g.8-11). It should be noted, however, t h a t
a straight line cannot be plotted through CURCHOD'S~
three experimental points
(Fig. l a ) . The three experimental points of BAHARY
e t al.5, obtained for polymers
with 0 - 20% of 1.2-units, fall on a straight line, but its extrapolation would
yield a n improbably low dn/dc for 1.2-polybutadiene.
The dn/dc dependence on polybutadiene structure has t o be taken into consideration when evaluating the light scattering, sedimentation analysis and gel
permeation chromatography experiments with this polymer or butadiene containing copolymers. For example, neglecting this dependence in light scattering
measurements of molecular weight could, under the most unfavourable coneditions, lead t o a result false by a factor as large as 1.7.
e.
The authors feel deeply indebted t o Drs.
VYROUBALand J. CERMAK,Research Institute of Synthetic Rubber, Kralupy nad Vltavou, and t o Dr. B.
VERUOVIE,University of Chemical Technology, Prague, for the kind supply of
the polybutadiene samples.
1
3
4
R. L. CLELAND,J. Polym. Sci. 27 (1958) 349.
P. RIBEYROLLES,
A. GUYOT,and H. BENOIT,J. Chim. Phys. 56 (1959) 377.
W. COOPER, G. VAUGHAN,
D. E. EAVES,and R. W. MADDEN,
J. Polym. Sci. 50
(1961) 159
D. J. HARMON,
Polymer Preprints 5 (1964) 712; J. Polym. Sci. C 8 (1965) 243.
171
P. KRATOCHV~L,
D. S T R A K O V
and
~ , P. SCHMIDT
5
W. S. BAHARY,
D. I. SAPPER,
and J. H. LANE,Rubber Chem. Technol. 40 (1967)
1529
J. CURCHOD,
Rubber Chem. Technol. 43 (1970) 1367.
7 J. HASLAM
and H. A. WILLIS,Identification and Analysis of Plastics, -1liffe
Books Ltd., London 1965, p. 190.
8 S. KRAUSE,
J. Phys. Chem. 65 (1961) 1618.
9 H. KAMBE,
I. MITA, and Y. SHIMURA,
Aeronautical Research Institute, University of Tokyo, Report No. 381 (1964) 31.
l o Z. TUZARand P. KRATOCHV~L,
Polymer Letters 7 (1969) 825.
l1 W. MACHTLE,
Angew. makromol. Chem. 10 (1970) 1.
6
172
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effect, structure, increment, index, refractive, polybutadiene
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