HIROYASU UTIYAMA
4138
Physicochemical Studies on Isotactic Polystyrene
by Hiroyasu Utiyama' Institute for Chemical Research, Kgoto University, Takatsuki, O s a h f u , Japan
(Received June 10, 1966)
An isotactic polystyrene sample prepared by using a Ziegler-type catalyst has been fractionated by a stepwise separation of crystalline solid precipitate from isotactic polystyrenemonochlorobenzene-cyclohexanol a t 8". Measurement of crystallinity of the films of the fractionated samples by an infrared spectroscopic method has shown that the fractions differ in their speed and degree of crystallization. Light-scattering and viscosity measurements have been carried out a t 25.3" on the fractionated samples using monochlorobenzene as solvent and 2,4,6-trimethylphenol as antioxidant. One of the fractions (F-4), the films of which readily crystallize a t relatively low temperature, has shown a negative initial slope and a minimum in the plot of the conventional reciprocal scattered intensity function us. sin2 (8/2). From theoretical considerations, it has been concluded that this anomaly is due to a large optical anisotropy, 40 times as large as that for atactic polystyrene. This conclusion has been confirmed by the separate measurement of vertical and horizontal components of scattered light using vertically or horizontally polarized light. It has also been found that optical anisotropy of highly crystallizable samples of isotactic polystyrene fractions decreases with increase in temperature, while that of the conventional atactic polystyrene is very small and increases slightly with temperature. The second virial coefficient corrected for the effect of the optical anisotropy of sample F-4 is exceptionally small. While definitive estimations of the unperturbed chain dimension A and of the interaction parameter B were not possible, consideration of the data suggests that, as the degree of isotactic stereoregularity increases, the short-range parameter A increases and the long-range interaction parameter B decreases. A consideration of the estimated value of the optical anisotropy of the styrene monomer as a function of the mode of rotation of the benzene ring has led to the conclusion that the optical anisotropy is a good measure of the degree of the isotactic stereoregularity of polystyrene. The optical anisotropy increases as the degree of isotactic stereoregularity increases. This increase is due to the hindered rot,ation around the C-C bond connecting the benzene ring to the main chain. The hindrance is due to the larger interaction between neighboring benzene rings, which simultaneously decreases the flexibility of the chain.
Introduction Since the first success of Natta and his co-workers in the production of crystalline high polymers of cy olefins and the identification of their local stereochemical configurations, the physical chemical properties of these polymers in solution have been the subject of extensive investigation. The hindrance, due to internal rot,ation around each G C bond along the main chain, may be affected by stereochemical configuration of the asymmetric carbon atoms. Thus, dimensions and the electric dipole moments of isotactic polymers in dilute solutions will differ to some 21s
The Journal of Ph&zl
Chemistry
extent from those of atactic polymers. Furthermore, particular attention has been paid to establishing reliable techniques for estimation of the stereoregularity. Such techniques should, if possible, be based on meaaurements of quantities reflecting directly the structure of
(1) On leave of absence at the Department of Chemistry, Harvard University, Cambridge, Mass. (2) (a) G. Natta, P. Pino, P. Corradini, I". Danusso, E. Mantica, G. Maazanti, and F. Mornglio, J . Am. Chem. Soc., 77, 1708 (1955); (b) G. Natta, J. Polymer Sci., 16, 143 (1955). (3) G. Natta, Makromol. Chem., 16, 77, 213 (1955).
PI~YSICOCHEMICAL STUDIESON ISOTACTIC POLYSTYRENE
individual molecules and being free from any influence of secondary e f f e ~ t s . ~ J Numerous studies have been carried out on the solution properties of stereoregular polynicrs, particularly of isotactic polystyrene. Among these studies is that of Natta, Danusso, and Moraglio,6 who concluded, on the basis of the results of viscosity and osmotic pressure data, that the relation between molecular weight and limiting viscosity number was the same for both isotactic and atactic polystyrene. This conclusion WAS confirmed repeatedly by subsequent investigator^.^-'^ Danusso and Moraglio7fractionated isotactic and atactic polystyrene simultaneously by fractionating precipitates from a solution which contained an equal amount of both polymers. They made osmotic pressure measurements on the fractionated samples and found that the second virial coefficient and its molecular weight dependence for isotactic polyslyrene +ere both smaller than those of atactic polystyrene. On the basis of light-scattering data for fractionated samples with the use of a semioctagonal cell, Trossarelli, Campi, and Saini'O found that no differences exist between the molecular dimension of isotactic and atactic polystyrene in toluene. However they also found that the ratios A 2 M , / [ v ] of isotactic polymer fractions were smaller than those of atactic polystyrene and essentially independent of molecular weight. Krigbaum, Carpenter, and Newman" made viscosity, osmotic pressure, and light-scattering measurements on two fractionated samples of isotactic polystyrene in p-chlorotoluene and in o-dichlorobenzene. They concluded that the unperturbed dimension of isotactic polystyrene was 25-30% larger than that of its atactic counterpart. All of these authors state that, of the second virial coefficient and the molecular dimension, only the former was affected by stereoregularity. If so, the properties of dilute solutions of isotactic polystyrene may not be explained within the frame of the twoparameter theory without making any additional assumptions. It should be remarked, in this connection that the analysis of the experimental data by Krigbaum, et aZ.,llis somewhat ambiguous in that their treatment inevitably results in assuming the dependence of interaction parameter B on molecular weight. The present study has been performed in an attempt to resolve this ambiguity and to clarify the characteristic properties of isotactic polystyrene in solution by examining them with a different experimental approach. Now, in order to answer whether the physical properties of an isotactic polymer are affected by the stereoregularity, it is necessary before making any physicochemical measurement (i) to employ a solvent that will
4139
dissolve the highly crystalline portion of the polymer, (ii) to develop a method to separate out a highly stereoregular fraction, and (iii) to estimate its stereoregularity. Since an isotactic polystyrene sample is a mixture of molecules with various degrees of stereoregularity and of polymerization, the use of a solvent which does not dissolve the highly crystalline fraction of the polymer will usually result in extracting a part of the original sample with less than the maximum obtainable isotacticity. Moreover, under such conditions, the fractions of different molecular weight may not necessarily be samples of the same degree of stereoregularity, so that it is not always safe to use such fractions to study the dependence of various solution properties on molecular weight. Except in the work of Krigbaum, et al.," the isotactic polystyrene samples investigated heretofore were either the toluenesoluble fractions6l8-lo or the fraction extracted with b e n ~ e n e . ~Although it was not always mentioned specifically, the amount of the polymer extracted from the original material by the use of boiling toluene seems not to have exceeded 50%, and the amount seems to have varied from case to case from 6%1° UP. We have found that isotactic polystyrene could be completely dissolved in monochlorobenzene by a specific heat treatment. The use of a process of separation of crystalline solid particles from a system consisting of isotactic polystyrene-monochlorobenzenecyclohexanol has been shown to be effective in separating out highly isotactic material. The increase in stereoregularity of B sample has been observed to be accompanied by the increase in optical anisotropy, which can be estimated by light-scattering measurement with polarized incident light and by analyzing the data according to a theory developed previously by the author.12 The increase in the optical anisotropy has been explained in terms of the larger interaction between the benzene rings of polystyrene in the isotactic configuration. Finally, we have found that (4) V. N. Tsvetkov, 5. Y. Magarik, N. N. Boitsova, and M. G. Okuneva, J . PolymeT Sci., 54, 635 (1961). (5) F. A. Bovey and G. V. D. Tiers, Fortschr. Hochpolyner. Forsch., 3, 139 (1963). (6) G. Natta, F. Danusso, and G. Moraglio, Makromol. Chem., 20, 37 (1956). (7) F. Danusso and G. Moraglio, J . Polymer Sci., 24, 161 (1957). (8) F. Ang, ibid., 25, 126 (1957). (9) F. Ang and H. Mark, Monatsh. Chem., 88, 427 (1957). (10) L. Trossarelli, E. Campi, and G. Saki, J . P o l p e r Sci., 35, 205 (1959). (11) W. R. Krigbaum, D. K. Carpenter, and S. Newman, J . Phys. Chem., 62, 1586 (1958). (12) H. Utiyama and M. Kurata, Bull. Inst. Chem. Res. Kyoto Uniu., 42, 128 (1964); H. Utiyama, to be published.
Volume 69,Number 1% December 1966
4 140
the experimental results of light-scattering and viscosity measurements on the fractionated samples can be put into the frame of the two-parameter theory if we assume that, as the degree of isotacticity increases the unperturbed dimension of polystyrene increases, and the interaction parameter B decreases. Therefore, the polymer dimension or the limiting viscosity number of isotactic polystyrene may be either larger or smaller than those of its atactic counterpart, depending upon its molecular weight and the degree of stereoregularity.
Experimental Procedures Synthesis of Isotactic Polystyrene. A Ziegler-type catalyst used for polymerization of styrene was prepared by reacting titanium tetrachloride in heptane solution with triethylaluminum in a 1 : 4 molar ratio in the same solvent. The reaction mixture for the polymerization contained 2 moles of styrene, 0.4 mole of triethylaluminum (20% monomer), 0.16 mole of titanium tetrachloride (40% triethylaluminum), and 170 ml. of hexane. It was kept at 80" for 11.5 hr. and then at room temperature for 16 hr. The reaction was stopped by the addition of a large excess of methanol. The precipitate from the mixture was recovered and was washed repeatedly with methanol and dried under vacuum at room temperature. Sixtyseven per cent of the monomer was converted to polymer. Choice of Solvent and Puri$cation of the Sanaple and Solvent. The solubility of the crude polymer product was tested in benzene and toluene, but the polymer was found to be practically insoluble in either of the solvents. I n the course of testing the solvent power of other solvents, monochlorobenzene was found to be satisfactory although a small quantity of polymer was undissolved and remained in the swollen state until the mixture was heated. I n order t o obtain a clear and stable solution, it was necessary first to keep it at room temperature for 48 hr. and then to heat it to the boiling temperature of monochlorobenzene at 130" for about 2 hr. The monochlorobenzene solution of isotactic polystyrene prepared in this way was very stable, and it did not bring about any detectable change in the limiting viscosity number at 20" even after the solution was kept at temperatures as low as 8". Therefore we employed monochlorobenzene as solvent. Reagent grade monochlorobenzene was washed five times with equal volumes of concentrated sulfuric acid. To remove the acid, it was then washed with water and with an aqueous solution of potassium bicarbonate several times each and dried over phosphorus pentoxide overnight, Finally, it was distilled The Journal of Physical Chemistry
HIROYASU uIIIYAMA
before use. The refractive index of the purified monochlorobenzene was ri,25~1.52154 (lit. 1.52212). The crude polymer product was then dissolved in the purified monochlorobenzene, by the procedure mentioned above, to make a solution of about 2%. The solution was centrifuged to produce a force 20,000 times that of gravity for 1 hr. The supernatant was poured into an excess of methanol, and solid pieces of decomposed catalyst and a very small amount of remaining undissolved polymer gel were discarded. The polymer precipitate was washed carefully with methanol and then dried under vacuum at room temperature. The amount obtained was 138 g. The purified product was then annealed in boiling n-heptane for 12 hr. to increase the degree of crystallization. Finally, it was washed with methyl ethyl ketone to extract the soluble fraction which was presumably nonisotactic material. Fractionation. The next step wtts the separation of highly stereoregular fraction and the comparison of its solution properties with those of conventional atactic polystyrene of the same molecular weight. Here, we briefly explain the principle of fractionation employed in this work. As is well known, it is frequently observed that, on standing, crystalline solid particles separate out from a metastable solution of crystalline polymer, such as polyvinyl chloride and polyvinyl alcohol. This process is a kind of cooperative phenomenon, and processes of both nucleation and growth of nuclei are probably regulated not only by the stereoregularity but also by the molecular size as well. For example, the nucleation rate will be largest for the molecules of moderate size and of highest stereoregularity. The larger molecules will need a longer time to orient into a ordered conformation, and the smaller molecules may not form a stable nucleus which will grow at an appreciable rate. Therefore, if we successively separate the liquid phase from the crystalline solid precipitate, which is brought about by the addition of an appropriate nonsolvent to a dilute solution and by standing long enough a t an appropriate temperature, the fractions may have different molecular weight. However, the possibility of obtaining a fraction with large stereoregularity is substantial. Among the nonsolvents tested, cyclohexanol was most suitable for the present purpose because a sufficiently large amount can be added to the monochIorobenzene solution of isotactic polystyrene without instantaneous formation of threadlike solid particles. On the contrary, when we used n-heptane, methanol, butanol, and diisopropyl ether as nonsolvents, the addition of even a very small amount of any one of them into the solution resulted in an appearance of threadlike crystalline particles. The solid threads could not be redis-
PHYSICOCHEMICAL STUDIESON ISOTACTIC POLYSTYRENE
solved even after complete mixing of the solution and heating to a temperature as high as 70". Purified isotactic polystyrene (25 g.) was dissolved in 2.5 1. of monochlorobenzene by the procedure stated previously. An equal volume of cyclohexanol was added to the clear monochlorobenzene solution at room temperature, and the mixture was kept overnight a t 8". Although the mixture remained transparent in the beginning, the formation of a white, gellilce precipitate proceeded within the next 24 hr. to about 3 days. The mixture was then centrifuged at 20,000 times the force of gravity for 30 min., and the precipitate was separated out by decantation from the supernatant, which did not become turbid on further standing. The supernatant portion was concentrated by vacuum distillation, and a proper amount of monochlorobenzene was added. The solution was centrifuged to eliminate dust and precipitated by addition to a large excess of methanol. If the amount of the precipitate was appropriate, say between 1.5 and 3 g., it mas recovered as a fraction. When the amount was less than this, it was added to the subsequent supernatant portion; if greater, it was subjected to further fractionation under milder conditions. The precipitate portion, the white, gellike material, was easily dissolved in monochlorobenzene by heating the mixture to about 80" for 30 min. This solution was subjected to the same procedure: the addition of cyclohexanol, standing for up to 3 days, arid the centrifugal separation of precipitates from the solution. This procedure was repeated until finidly the amount of the precipitate became too small for further fractionation. In this way, the original isotactic polystyrene sample was divided into ten fractions arid one extraneous fraction (assigned as F-0) which was no longer soluble in monochlorobenzene by our procedure. Recovery of 85.6% of the polymer was made. A schematic diagram of the fractionation process is shown in Figure 1. The thick lines in the figure represent the precipitated fractions, and the number in the brackets shows the weight of each fraction. It may be seen that the fractions from F-3 through F-9 were fractionated essentially by the extraction of' nonprecipitating material. Preparation of Solutions. Before the measurements of light scattering and viscosity were made, it was necessary to check whether the polymeric material was dissolved in a molecularly dispersed state. In the case of usual amorphous polymers, the presence of aggregated particles may be easily checked if one measures the molecular weight of the polymeric solute in two solvents which diEer appreciably in their solvent power. On the other hand, crystalline polymers cannot, in general, be dissolved in poor solvents, and this
4141
4 ' (2,60)(2.03) (0.90)(4.09) (1.4 5) i-
1I
(1.36)
(1.52)
(2.67)
Figure 1. Schematic representation of the fractionation procedure.
type of test is not applicable. The dispersed state of crystalline polymer molecules in solution, however, depends largely on the temperature at which the polymer is dissolved and also on the subsequent heat treatment. The heat treatment a t higher temperature will of course give more dispersed solutions, but the thermal and oxidative degradation of the polymer molecule will take place to a greater extent." I n order to find an optimal condition of heat treatment (temperature and duration) for dissolving the polymer in a molecularly dispersed state with minimal degradation, a simple test was made by measuring viscosity on solutions of the unfractionated sample pretreated with heat under various conditions. The test solutions were made as follows. The original solution of about 1 g./dl. concentration was divided into several w a y tubes, and each of them was sealed after replacing the air by nitrogen. Unless otherwise stated, 10 mg. of 2,4,6-trirnethylphenol was added to 100 ml. of solution as an antioxidant. Figure 2 represents the variation of the limiting viscosity number with the time of heating. It is seen that the decrease in the limiting viscosity number is greatest for the sample which was heated at 130" in the absence of the antioxidant and that all three samples give practically the same limiting viscosity number unless the heating time exceeds 2 hr. If only the melting of aggregated particles takes place, the limiting viscosity number will decrease rapidly at short heating times and then level off. The final equilibrium value will depend on temperature of heat treatment. The present results indicate that the state of molecular dispersion is not affected by heat treatment longer than 2 hr. at temperatures higher than 120", a t which the thermal and oxidative degradation of molecules becomes noticeable. Hence, we employ as the standard condition of heat treatment: 120", 2 hr. Volume 69,Number 12 December 1966
HIROYASU UTIYAMA
4142
c Il.100 (gJhl.)
I
e
2
" v
2.2 0
10
5
15
-
2.0
2
-
n
,-.
"
0.358
o- 0 . 2 2 4
A
u
0.100
a
20
Time (hours)
Figure 2. The variation of the limiting viscosity number of the unfractionated isotactic polystyrene sample in monochlorobenzene a t 25" with time of heat treatment at 120' and a t 130" in the presence and absence of antioxidant, 2,4,6-trimethylphenol.
Viscosity Measurements. In order to study the shear rate dependence of the reduced viscosity of monochlorobenzene solution of isotactic polystyrene, viscosity measurements were made using a tilting-type Ostwald viscometer.13 Figure 3 represents an example of the experimental results on fractionated sample F-3. It may be seen that isotactic polystyrenp solutions do not exhibit an exceptiona!ly large dependence of viscosity on the rate of shear. Therefore further measurements of specific viscosity were made by using OstwaldFenslce viscometers. The constants of the apparatus were as follows: the radius of Capillary, 0.0296 cm.; the capillary length, 14.3 cm.; volume of the bulb, 1.98 ml. ; average head, 2.13 cm. ; the maximum rate of shear, 3.12 X lo2 set.-'. The correction for the kinetic energy was negligible. Refractive Index Increment. Specific refractive index increments were determined by using a differential refractometer of the Debye typeI4 (Shimadzu Seisakusho Co., Kyoto, Japan). Temperature was maintained to within *0.05". The distance of the vertical displacement of the image of the horizontal narrow slit on the focal plane of the telcscope was determined .photoelectrically. For the calibration of the refractometer, reference was made to the data reported by I2)/ lOn(a1
+ + a2
(3)
Equation 2 readily leads to the relation
i/z(e)- i / . ~-( e:) ~=
1.0
+
+
I
5 10 (1 3 .I.co s30)/(1+ co 5% )
0
MK(II - ~ A ~ M C 1 )-{ sin2 (012)] (4) Therefore, we can estimate the quantity MK(1 4A2Mc) from the slope of the plot l/.Z(e) - l/Z(n 0) vs. 1 - 2 sin2 (Of2). Once the quantity MK(1 4AzMc) is determined by use of eq. 4, then a plot of l/Z(O) M K ( 1 - 4A2Mc) sin2 (O/2) against (13 cos2e)/(l cos2e) enables one to estimate both M(1 2A2Mc) and M from the intercept and the slope of the plot, respectively. The latter plot will be a flat straight line for the solution of a polymer consisting of isotropic segments since 6 0 :forsuch a polymer. The analysis of experimental data of Z(0) a t various solute concentrations by the method illustrated above enables us to make a separate estimation of the quantities M, A,, 6, and (S2). By way of illustration, light-scattering data for F-5 were analyzed following the above procedure and plotted in Figures i‘ and 8. It appears that the anomalous light-scattering data can be explained quan-
I
I
15
+
Figure 8. Plots of l/Z(e) MK(1 - 4A2Mc)sin2 (e/2) a t various solute concentrations as a function of (13 cos2 e)/( 1 cos2e ) for the fractionated sample F-8. The original data are taken from Figure 6.
+
+
+ titatively in the present theory. The experimental data of F-2, F-3, F-4, and F-5 were re-examined and corrected for the effect of anisotropy. The lightscattering data of other samples are normal and ant+ lysed by the usual method. The results are listed in Table I. The optical anisotropies of the above four samples are, in fact, all larger than that of atactic polystyrene (see the next section) as shown in the last column of Table I. Furthermore, it should be mentioned that the optical anisotropy of F-4 is extraordinarily large, while the second virial coefficient is very small compared with those of other samples of comparable molecular weights. The molecular weight Volume 69,lvz4mber 1.@ December 1966
HIROYASU UTIYAMA
4146
of F-4 is rather small in spite of the fact that the fraction was obtained in the final stage of extraction. Results of Light-Scattering Measurements with Polarized Incident Light. When a polymer molecule consists of optically anisotropic segments, the theoretical expressions of the reduced intensity of light scattered from the solution for the V, and N, coiiiponents (vertical and horizontal components of the scattered light when incident light is vertically polarized) and for the Vh and Hh components are given by12
R,,(e)
=
+
KcMP(e) - ~ A , M Q ( ~ ) c46
RVh(6)= RH,@)= 36KcM R H h ( 6 ) = KcM[P(O) -
+
~ A ~ M Q ( B ) C61 COS^ e
5.0
6
-c
a= h
01
3 ;
4.0
h Q ' -K
'a
-, 3.0 a Ci ai
(5)
(6)
+ 36
2.0
(7)
where P(6) is the well-known particle scattering factor and &(e) is the intermolecular correlation function.23 These equations show (1) that the Hh component a t 6 = n/2 coincides with the Vih (=H,)component, (2) that the VI, component is independent of the angle of observation and proportional to the optical anisotropy, and (3) that the anisotropy e&ct in the V, component can be eliminated by using another component a t 0 = n/2. In order to compare these formal properties with experimental results and to estimate 6 directly, four components of the scattered light were measured separately. First, the accuracy of the measurement was tested by using a pure liquid such as benzene or carbon tetrachloride, in which the V.,, H,, and V h components of the scattered light are constant irrespective of the angle, and the H hcomponent, which agrees with the H., and Vh components at 6 = n/2, is symmetrical about 6 = n/2. Typical experimental results for four components of the reduced scattered intensity of benzene are shown in Figure 9. The reduced scattered intensity of the Vu component and the depolarization ratio pu are estimated from the data and listed in Table 11. It is shown that the above requirements are fulfilled with sufficient accuracy and that the agreement of pu values estimated in the present experiment with those reported in the literature are satisfactory. The experimental results of similar measurements on an isotactic polystyrene sample (a sample recovered from the fraction F-4 after the first light-scattering and viscosity measurements were completed and designated as F-4') are shown in Figure 10. The H , and J'h components of the reduced scattered intensity agree well with each other, but they show a tendency to increase slightly with the angle of observation. An explanation of this discrepancy has not been obtained The Journal of Physical Chemistry
c
2
1.0
-
0
0.2
0.4
0.6
0.8
1.0
sin'(~2)
Figure 9. The angular variation of four components of the reduced scattered intensity of benzene a t 30.0'. The wave length of incident light in vacuo, XO, was 436 mp. V , and H, represent the vertical and horizontal components of the scattered light when the incident light is vertically polarized. v h and Hh have a corresponding significance for horizontally polarized light.
;.' Vh
0
0.2
0.4
0.6 s ina(@/2 )
0.8
Figure 10. Angular variations of four components of the reduced scattered intensity of F-4' in monochlorobenzene a t 25.3"; c = 0.107 g./dl.
as yet. However, the value of the H , or v h component extrapolated to the zero angle of observation appears to be in good agreement with that of the Hh component at 6 = a / 2 and was therefore used for estimating the (23)
B.H . Zimm, J . Chem. Phys., 16,1093 (1948).
4147
PHYSICOCHEMICAL STUDIES ON ISOTACTIC POLYSTYRENE
Table I1 : Experimental Results of Depolarization Ratio and Reduced Scattered Intensity for Pure Liquids
OC.
Refractive index (436 mr)
g.m.
-Ruu Exptl.
25 30 35 25.3 40.8 61.1 82.2 36 35 40
1.5225 1.4709 1.5107 1.5334 1.5250 1.5142 1.5031 1.4019 1.4233 1.3769
0,8735 1.575 0.8509 1.102 1.084 1.062 1.038 0.8689 0.7648 0.7839
48.5 16.2 58.1 76.3 78.2 79.4 83.2 17.4 18.2 15.4
Temp., Solvent
Benzene Carbon tetrachloride Toluene Monochlorobenzene
%-Butyl chloride Cyclohexane Methyl ethyl ketone a
8,
x
+
"2 x
PU
Lit.
48.0" 15.8" 56. 3"sb
... ...
... ... ...
...
...
J. P. Kratovil, G. J. Dezelic, M. Kerker, and E. Matijevic, J . Polymer Sci., 57, 59 (1962).
optical anisotropy parameter 6. Then the Vv component was corrected for the anisotropy effect with the use of this 6 value. The reciprocal scattered intensity function of the corrected Vv component is plotted in Figure 11 as a function of sin2 (0/2) together with a conventional plot of the function K c ( 1 cos2 e)/ 2RU,(0) us. sin2 (0/2). The latter gives an anomalous curve similar to that shown in Figure 4 , while the plotted points of the former function fall strictly on a straight line, as the theory predicts, with a positive slope which is proportional to the mean-square radius of gyration (S2). The temperature dependence of the optical anisotropy parameter 6 was estimated in the same way from the light-scattering measurement on F-4' over the temperature range from 25.3 to 82.2'. The result is shown in Figure 12, which also includes, for comparison, a result on a polystyrene sample obtained by the thermal bulk polymerization and fractionating procedure ( M , = 51.2 X lo4). The optical anisotropy parameter was multiplied by the molecular weight in order to compensate for the difference in molecular weight.24 It can be seen in this figure that there is a remarkable and significant difference in the values of the optical anisotropy parameter M6 of isotactic and atactic polystyrene. The value of M6 of isotactic polystyrene is almost 40 times as large as that of the atactic sample and markedly decreases with temperature, while that of atactic polystyrene slightly increases. A reason for this difference between the two will be discussed in a later section. Crystallinity of Fractionated Samples. The degrees of crystallization XIR of fractionated samples, estimated from infrared spectroscopic measurement, are shown in Figure 13 as a function of the time of annealing a t 160". Before proceeding further, however, several
10Exptl.
Lit.
0.43 0.05 0.52 0.64 0.61 0.58 0.55 0.23 0.06 0.23
0.43" 0.05" 0.49"'*
... 0.60
... ...
... ...
...
Values a t 25'.
5.0
C
.-+ C LL
>r
4.0
I
'2 L
c
-2
M
:
3.0
.-
V
LL
2.0
0
0.2
0.4
,
0.6
0.8
s i n' ( &/2) Figure 11. Reciprocal reduced intensity plots for F-4' in monochlorobenzene a t 25.3'; c = 0.107 g./dl. The lower plotted points are the conventional reciprocal light-scattering intensity function and the upper points show the function corrected for the effect of optical anisotropy.
remarks must be made. For the quantitative discussion of the crystalline state of a polymer film, such procedures as melting and quenching of the film may be necessary before the process of annealing can be carried out, and the annealing itself should be done a t several different temperatures. Such experiments were impossible because of the lack of material. Our aim here, therefore, is not to investigate the solid properties by themselves but to look for some qualitative cor(24) H. Benoit and G. Weill, "Proceedings of the International Symposium on Macromolecular Chemistry," Prague, Czechoslovakia, Sept. 1957,p. 35.
Volume 69, Number 1.2 December 1966
4148
HIROYASU UTIYAWA
200
180 160 140 I
-
0
12
x -a
=
10
8 6 4
I 20
60
40
Temxrature
80
('C)
Figure 12. Optical anisotropy us. temperature relationships for the isotactic polystyrene fraction F-4' (open circles) and an atactic polystyrene sample ( M , = 51.2 X lo4, closed circles) in monochlorobenzene. The half-filled circles are for the atactic polystyrene in benzene.
50
40
-s
can crystallize during the film formation, during which the temperature was not greater than 60". Fraction F-1 also shows a high XIRvalue at t = 0, but the initial rate of crystallization is rather small, and the value of XIRafter 2-hr. annealing is still lower than the equilibrium value. The reason for this may perhaps be the high molecular weight of F-1, which can also be the origin of the separation of crystalline solid particles from the solution. By the same token, because of the small molecular weight, the solution of F-4 is stable a t room temperature, and the measurement of solution properties wm possible. Now, the large tendency of sample F-4 to crystallize at a low temperature is probably due to its high degree of stereoregularity. If we consider the previous result, that this sample showed exceptionally large optical anisotropy, the present results on measurements of crystallinity may confirm our view that the optical anisotropy is an intrinsic property which is closely connected with the isotactic configuration of the asymmetric carbon atoms along the main chain of the isotactic polymer. The equilibrium values of XIR of the fractionated samples, which are listed in Table 111, are not of the order of the fraction number as was expected. However, it is interesting to note that samples F-9 and F-10, which are readily soluble in monochlorobenzene a t room temperature, have a lower degree of crystallinity than the other samples.
h
Table I11 : Derived Results of Unperturbed Dimension K , Optical Anisotropy MS, and Crystallinity XIRfor Fractionated Samples of Isotactic Polystyrene
530
X
K x
20
10'
h1
F-1'
0.68 0.96 0.93 1.04 0.93 0.63 0.85 0.67 0.76 0.82
...
F-2 10
F-3 F-4 F-5 F-6 F-7 F-8 F-9 F-10
0
0
1
2
3
4
5
T i me ( hours)
Figure 13. Crystallinity of films of isotactic polystyrene fractions estimated on the basis of infrared absorption measurements as a function of duration of annealing a t 160'.
relations between the properties of isotactic polystyrene in the solid and dissolved states. The most interesting point in this figure is that the fraction F-4 exhibits an extraordinarily high degree of crystallinity at t = 0. I n other words, fraction F-4 The Journal of Physical Chemistry
I,
AIM,/
Sample
117 136 22.5 85 188 132 183 123 145
M6 X 10-2
6.4" 7.2 18.0 182.0 18.2 5.0" 3.3" 4.3" 2.5" 3.7"
-
XIR, % (1
5)
(49. 7)b 49.7 44.9 47.9
... ...
XIR,% (1 = 0)
( 10.2)b
2.5 0 31.9
... ...
45.0 40.7 18.6
1.0 0 0
26.9
0
Estimated values based on the result for atactic polystyrene. Experimental data on F-1.
Discussion Unperturbed Dimension A of Isotactic Polystyrene. Our problem here is to estimate the effect of stereoregularity on the unperturbed dimension A and the
PHYSICOCHEMICAL STUDIES ON ISOTACTIC POLYSTYRENE
interaction parameter B of polystyrene in dilute solution. A and B are defined by
4149
2.0 n
A = a/Mo1'2 B = @ / M 0 2 (8) where a is the effective bond length, iM0 the molecular weight of a segment, and /3 the binary cluster integral for a pair of segments. The short-range parameter A is determined directly and unambiguously only by the measurement of the molecular weight and the dimension of the polymer in a &solvent in which /3 vanishes. However, this cannot be done for an isotactic polymer because of the absence of an appropriate 8-solvent. At the moment, the most promising method of estimating the unperturbed dimension from a system with positive 0 may be by means of the relation between the limiting viscosity number and molecular weight of polynier. The excluded volume effect on the polymer dimension is a problem yet to be solved, but several useful approximate theories have been developed by Ptitsyn, 26 by Kurata, Stockmayer, and RoigZ6and by Fixman.n Of the three theories, Ptitsyn's equation appears to best agree with experimental results, but the functional form is not convenient for estimating the parameters A arid B from intrinsic viscosity results.21s22 Kurata and StockmayerZ8analyzed the dependence of intrinsic viscosity on molecular weight for a large number of pairs of solvent and polymer, showing that. their relation is quite useful in obtaining the two parameters separately from viscosity and molecular weight measurements. Later, Stockmayer and Fixman showed that Fixman's equation, when applied to viscosity, is different in funct,ional form from that of Kurata, et al., but the two equations are essentially e q u i ~ a l e n t . ~ ~ Because of the simpler functional form, we employ the latter, which reads
[77]/Mw1"= qK
+ 0.51~J3Mw"'
(9) where a0is the Flory constant under the 8-condition, K = %A3, and q is the correction factor for heterogeneity in molecular weight. From the intercept of the plot of [77]/MW'/'us. MW1I2, parameter A may be estimated by assuming a0as constant a0 = 2.5 X lOZ3.3O The results obtained are shown in Figure 14. The solid line in the figure expresses the correlation line for atactic polystyrene corresponding to q = 0.93.217a1 Since the points are widely scattered, they cannot be extrapolated properly. The fractionated samples, however, should not be taken as samples of the same nature differing only in the molecular weight. Since the samples show different optical anisotropy, different solubilities, and different degrees of crystallinity in the solid state, they probably represent different
E?
2" X
s
1.5
P-
e
1.0
0.5
'
I
0
I 2
I
I 4
I
I
6 Mr"' X 1 0-'
I
I 8
I
I 10
Figure 14. [ 7 ] / M , * 2 us. MW**for isotactic polystyrene fractions in monochlorobenzene a t 25.3". The straight line is for atactic polystyrene with q = 0.93, and the dashed line is drawn through the experimental point of the sample F-4with +OB = 0.11 X 10-6. The closed circles represent the fractions with negligible optical anisotropy, and the open circles are for the samples of large optical anisotropy.
molecular species. Hence, it may not be appropriate to draw a line best fitting the experimental points in order to estimate the K value. It is interesting to note, however, that the plotted points for the samples of small optical anisotropy fall close to the correlation line (closed circles), but those of large anisotropy are located well above the line (open circles). For the estimation of the unperturbed dimension of each fraction, we need an alternative means which provides information about the expansion factor a,, ( = [ T J ] / [ ~but ] ~ no ) , possible method is left except the use of the relation A2Mw/[77]us. aq2 - l.32Experi(25) 0. B. Ptitsyn, Vysokomolekul. Soedin., 1, 715 (1959).
(26) M.Kurata, 33, 151 (1960).
W.H. Stockmayer, and A. Roig, J. C h . Phys.,
(27) M.Fixman, ibid., 36, 3123 (1962). (28) M. Kurata and W. H. Stockmayer, Fortschr. Hochpolymer. Forsch., 3,196 (1963). (29) (a) W. H.Stockmayer and M. Fixman, J. Polyter Sci., C1, 137 (1963); (b) Various theoretical relations between the expansion factor and molecular weight were examined recently by G. C. Berry and T. G. Fox, J . Am. Chem. SOC.,86, 3540 (1964). (30) Burchard had made use of the same relation before it was worked out in an experimental study of solvent effects on the unperturbed dimension. It should be mentioned that the second term in the right-hand side of eq. 9 is simply the first term of series expansion of [ V I about [71e. See: W. Burchard, Makromol. Chem., 50, 20 (1961). (31) The polydispersity of the isotactic polystyrene fraction is not known, but it must be large considering the broad molecular weight distribution of the crude sample. (See, for example, K. Kawahara and R. Okada, J. P o l y t e ~Sci., 56, 97 (1962).) We assume for the present samples M w / M n = 3, but the assignment is not important for the subsequent discussion.
Volume 69, Number 18 December 1966
4150
mental values of A,M,/[7] of atactic polystyrene samplesz1are plotted in Figure 15 against [ q ] / [ q I e- 1 (filled circles), together with those of isotactic polystyrene (half-filled circles). The numerical values of [qI/[rl]e - 1 for the latter were estimated by assuming that K is the same as for atactic polystyrene. The solid line was drawn so as to best fit the experimental points of atactic polystyrene. The plotted points of the fractions F-2 through F-5 (shown by the arrows in the figure) deviate again appreciably from the correlation line, while the other experimental points appear to be in rough agreement. This result may suggest that the second virial coefficient of isotactic polystyrene is smaller and/or the unperturbed dimension is larger than that of the conventional atactic polymer. The results shown in Figures 14 and 15 indicate that, as Iong as we assume the same unperturbed dimension for both isotactic and atactic polymers, it may be concluded that the isotactic polystyrene is characterized by the larger intramolecular interaction (the larger value of B or the steeper correlation line in Figure 14) and abnormally smaller intermolecular interaction (smaller value of A%) than the atactic counterparts. However, it may be unrealistic to assume different interaction parameters for intra- and intermolecular interaction since there is no theoretical or experimental evidence indicating that the crystalline polymer solutions do not obey the two-parameter theory. Alternatively, we may tentatively assume the same B values for both polymers. Each experimental point in Figure 14 is extrapolated with the same slope as that of atactic polystyrene to obtain a gK value from the intercept on the ordinate axis. Table I11 lists the K values obtained by this procedure making use of the assumed g value. The experimental points of A&U,/[q] replotted using these K values fall near the universal correlation line as shown by open circles in Figure 15. This agreement seems too good to be taken as fortuitous. It may be said that the samples of larger optical anisotropy are characterized by larger unperturbed dimensions. Furthermore, in view of the smaller second virial coefficient, B values of isotactic polymer should be smaller, and the K value must therefore be larger to some extent than the figures listed in Table 111. I n fact, the plotted point of F-4 Figure 15 falls on the universal correlation line if we employ 0.11 X lom5 for a&, which is about one-third that of atactic polystyrene, and estimate the K value as 1.76 X (the dashed line in Figure 14). Molecular Conformation of Isotactic Polystyrene in Dilute Solution. I n summarizing the experimental results obtained in the present investigation and some The Journal of Physical Chemistry
HIROYASU UTIYAMA
0
1
2
([VI 1171,)
3
-1
Figure 15. A2Mw/[7]as a function of [ q ] / [ s ] e - 1 for isotactic (half-filled circles and open circles) and atactic polystyrene fractions (filled circles). See text.
conclusions from the previous discussion, it has been shown (1) that we can separate by the present method of fractionation a fraction which has a large optical anisotropy, high crystallizability, and rapid rate of crystallization at low temperature, (2) that the solubility and crystallizability do not always parallel each other, (3) that consideration of the data suggests that, with increasing optical anisotropy, the unperturbed dimension of isotactic polystyrene increases and the interaction parameter decreases, and (4) that the optical anisotropy of isotactic polystyrene decreases rapidly with increase of temperature, while atactic polystyrene exhibits only a negligibly small optical anisotropy which slightly increases with temperature. H,ere we may briefly discuss the relation between optical anisotropy, and the stereoregularity of polystyrene. According to the calculation by Stein and Tobolsky, 3 3 the optical anisotropy of styrene monomer depends significantly upon the mode of the orientation of the benzene ring. The difference between the principal polarizabilities along and perpendicular to the contour of the C-C bond is (1) -4.9 X loe2' (32) W. H. Stookmayer, Makromol. Chem., 35, 54 (1960). (33) R. S. Stein and A. V. Tobolsky, J . Polymer Sci., 11, 285 (1953).
PHYSICOCHEMICAL STUDIESON ISOTACTIC POLYSTYRENE
~ m if . all~ single bonds within the monomer are freely ~ m if . the ~ benzene ring rotating, (2) -6.2 X may rotate freely about the bond which connects the benzene ring to the chain, and (3) -62.8 X loAz2 cm.3 if the benzene ring is oriented within the plane perpendicular to the axis of the chain. In addition, the similar value corresponding to the optical segment34 may be genera.lly greater than that of a monomer unit owing to the restricted internal bond rotation. When the asymmetric carbon atoms in a polystyrene chain assume the isotactic configuration, the rotating motion of each benzene ring around the bond which connects the benzene ring to the chain may be more restricted by the larger steric hindrance between the neighboring benzene rings than in the atactic configuration. Therefore, if we extend the above discussion, the large optical anisotropy exhibited by the highly crystallizable fraction might be due to hindrance of benzene ring rotation, which in turn might reflect the degree of stereoregularity. Therefore, the optical anisotropy could be a measure for determining stereoregularity a t least in the case of polystyrene. I n addition, the increase in the stiffness of the isotactic polystyrene chain, as shown by the larger unperturbed dimension, supports the view that the larger interaction between the benzene rings is the main cause of the increase in optical anisotropy. If this statement is true, then optical anisotropy is very important in characterizing stereoregularity because it is an in-
4151
trinsic property which is not influenced by secondary effects. It is interesting to note that Tsvetkov and his cow o r k e r ~observed ~~ in the study of flow birefringence that isotactic polymethyl methacrylate was characterized by a large optical anisotropy. The 40-fold increase in the optical anisotropy of the fraction F-4’ may be due to the restriction of benzene ring rotation caused by the stereoregular structure, which is preserved even in solution. As the temperature is raised, the freedom of rotation of the benzene ring and the freedom of internal bond rotation will increase owing to the violent thermal motion, which would cause optical anisotropy to decrease.
Acknowledgments. The author is indebted to Professor Michio Kurata for discussion and criticism of this work. The technical assistance of Mr. J. Komiyama, Mr. H. Itani, and Mr. M. Tsuji is gratefully acknowledged. This work was supported in part by a grant-in-aid of research from the Ministry of Education of Japan. (34) The size of the optical segment is not necessarily equal to the size of the statistical segment. See, for example, M. V. Volkenstein, “Configurational Statistics of Polymer Chains,” translated by 9.N. Timasheff and M. J. Timasheff from the Russian edition, Interscience Publishers, Inc., New York, N. Y.,1963. (35) V. N. Tsvetkov, S. Y . Magarik, N. N. Boitsova, and M. G. Okuneva, J. Polyner Sci., 54, 635 (1961).
Volume 69, Number 12 December 1966
