1390
ULRICH
P. S T R A U S S
AKD
value for poly-(ethyl methacrylate) is about 5 gauss2 below the estimated rigid lattice value, indicating some rotation of the ester methyl group even a t this temperature. Since it is expected that the calculated rigid lattice and methyl rotation second moments for the three monomers studied would be within 1-3 gauss2 of those for the respective polymer, it appears from the data in Fig. 8 that methyl group rotation of comparable amount is taking place in 4-methylpentene-1 a t temperatures only -1OOK. lower than those for the polymer. This is in marked contrast to the resultslO for ar-methylstyrene and its polymer where complete methyl group rotation is believed to take place a t temperatures about 100°K. lower in the monomer than for the polymer. Removal of one of the methyl groups from 4-methylpentene-1 to give pentene-1 leads to a situation where considerably more motion is taking place a t 77°K. in the monomer than in the respective polymer. This effect is even more noticeable in the data for the branched isorner of pentene-1, 3-methylbutene1. The abrupt decrease of the second moment to values near zero, exhibited by all three monomers a t temperatures 25-3OOIC. below their respective melting points indicates the onset of motion other than side chain ]rotation, possibly involving tumbling of the individual monomers. This latter result is decidedly different from that obtainedlO for the monomers methyl methacrylate, methacrylic acid and a-methylstyrene, which have second moment values of 9-10, 7 and 10 gauss,%respectively, a t teimperatures not more than 8°K.
BERNARD L.WILLI.4MS
Vol. 65
below the melting point, indicating a much more rigid crystal structure for the latter three monomers. The second moment decreases taking place a t 260'K. or above for all polymers investigated is believed to be a consequence of side chain and main chain reorientations associated with the respective glass transitions. As reported p r e v i o u ~ l y ~ ~ ~ such reorientations are also believed to be the cause of the substantial changes found in the dynamic mechanical properties in the same temperature region for each polymer. On the other hand P31, PP1, P3MB1 and P4MP1 do not appear to exhibit marked line narrowing in or near the 150°K. region where a loss peak attributed to the motion of three or four segments in the amorphous regions of the polymer has been found.g The fact that the line narrowing process near room temperature occurs for PTDP before that for polypropylene prepared under the same conditions is not understood at this time. Acknowledgments.-We wish to express our thanks to Professor R. E. Click of Florida State University for frequent discussions during the formative stages of this work, to Professor R. S. Stein of University of Massachusetts and Dr. W. P. Slichter of Bell Telephone Laboratories for bringing to our attention their work prior to publication, to Mr. R. P. Gupta and Mr. R. A. Wall for preliminary n.m..r. measurements on some of these compounds, and to Drs. F. P. Reding and R. D. Lundberg of Union Carbide Chemicals Company for their aid and interest in this study.
THE TRAXSITIOX FROM TYPICAL POLYELECTROLYTE TO POLYSOAP. 111. LIGHT SCATTERING AND VISCOSITY STUDIES OF POLY-4VIXYLPYRIDINE DERIVATIVES1 BY ULRICHP. STRAUSS AND BERNARD L. WILLIAMS^ Ralph G. Wriyht Laboratory, School of Chemistry, Rutgers, The State University, N e w Brunswick, New Jersey Recetved March 3, 1961
One polyelectrolyte and four polysoaps were prepared from poly-4-vinylpyridine by quaternising 0, 4.8, 10.3, 16.3 and 34.1% of its pyridine groups with n-dodecyl bromide and the remainder with ethyl bromide. Light scattering studies in a solution of LiBr in iisopropyl alcohol gave results typical of normal polymers. The molecular weights were considerably lower than t h a t of the parent polymer, indicating t h a t degradation had occurred during quaternization. The mean-square end-to-end distance per unit chain length a t the theta point increased with increasing dodecyl group content, which was ascribed to steric hindrance. On the other hand, light scattering results obtained in 0.05, 0.1 and 0.2 M aqueous KBr solutions, while normal for the polyelectrolyte, indicated aggregate formation of the polysoap molecules. These aggregates are fairly stable to dilution at 25'; but the degree of aggregation decoreases with dilution if the diluted solutions are heated t o 45" for a t least 24 hours before being allowed to equilibrate a t 25 . . With the latter type of procedure, one can obtain parameters characteristic of the individual polysoap moleculea a t infinite dilution. Flory's equation relating the rootmean-square end-to-end distance of random coils to the intrinsic viscosity is found to hold fairly well for all the samples except for the polysoap with the highest dodecyl group content. For the latter, only the assumption of a compact sphere model was consistent with both light scattering and viscosity results. Contrary to the behavior in isopropyl alcohol, t h e unperturbed molecular dimensions decreased with increasing dodecyl group content in the aqueous systems. Thus, in such systems, the previously postulated aggregation of dodecyl groups belonging to the same polysoap molecule is confirmed.
Introduction Several reports have appeared in recent years concerning the transitioii from polyelectrolyte to (1) This work was supported in part by the Officeof Naval Research. The paper is based on a. thesis presented by B. L. Williams in 1958 t o Rutgers University in ]partial fulfillment of the requirements for the Ph.D. degree. (2) Eestman Kodak Fellow, 1956-1957; Colgate-Palmolive Fellow, 1957-1958.
p ~ l y s o a p . ~ -The ~ studies were carried out with series of poly-4-vinylpyridine derivatives prepared by quateriiizing part (y%) of the pyridine groups with n-dodecyl bromide and the remainder with (3) U. P. Strauss and N. L. Gershfeld, J . Phys. Chem., 68, 747 (1954). (4) U. P. Strauss, X. L. Qeishfeld and E. H. Crooh, abzd., 60, 577 (1956). (5) D. \I oermann a n d 1;. T . FVall, %bid.,64, :8l (19bO).
Auguqt, 1961
1391
LIGHTSCATTERING O F POLY-4-VINYLPYRIDINE DEPTVATIVES
ethyl bromide. Based mainly on viscosimetric results in aqueous KBr or KCl solvent systems, i t was found that the derivatives with 1~ smaller than a critical value in the neighborhood of 8 had the loosely coiled chain structure typical of polyelectrolytes, while the derivatives with values of y larger than the critical value had the highly compact shape typical of polysoaps.6 In addition, all the derivatives containing dodecyl groups showed a tendency toward intermolecular association. 3 , 4 To obtain more quantitative information concerning the molecular dimensions and the aggregate formation than can be obtained from viscosity results alone, it was decided to study these phenomena using both light scattering and viscosity techniques. The results of this investigation which were carried out with poly-4-vinylpyridine derivatives of the type described above (g = 0, 4.8, 10.3, 16.3 and 34.1) are presented in this paper. Experimental Materials .-Pol~~-4-vinylpyridine (our sample No. B120) was prepared a t 50’ according to a modification of the method of I’uoss and Strauss? using benzene and azobisisobutyronitrile as the polymerization medium and catalyst, respectively. Two successive fractionations by the method of Boyes and Strausss yielded among others a sample (our KO. B-120F2B) which was used for the subsequent work. Its weight-average molecular weight, M,“, and degree of polymerization, P,, determined by light scattering in a solvent containing by weight 86% 2-butanone and 14% isopropyl alcohol were 2.0 X lo6 and 19,000, respectively. The polyelectrolyte and polysoaps w r e prepared and analyzed by a previously described method3with the modification that 2,4-dirnethyl-l,l-diouvtetrahpdrothiophene(dimethylsulfolane) was used as the solveiit for the quaternization reactions, and that the residual HBr was removed by ion exchange.9 The chemical composition of the compounds is given in Table I. The quantity ilfo in the fourth column is the molecu1,tr weight per pyridine group, and is used below to convert niolecular weights to degrees of polymerization.
design of Boedtker and Doty,l* by providing a housing for thermostating the cylindrical light-scattering cells. The latter had flat faces a t 0 and 180”. The slit system of the housing was 2.5 mm. wide and 8.5 mm. high. The small auxiliary diaphragms, which are supplied with the instrument, were used a t the end of the collimating tube and in the nosepiece of the photomultiplier housing unit. I n this way a well defined narrow beam of light was obtained which allowed measuring the scattering envelope from 25 to 135’. The optical uniformity and correct positioning of the cells was checked with the angular envelope obtained from a fluorescein solution. Several concentrations of the latter served to ascertain the linearity of the phototube response. All solvents and solutions were clarified with sintered glass filters of “ultrafine” porosity. In the case of the polysoap solutions in aqueous KBr systems, the filtering procedure caused the formation of a few fibers which would redissolve on standing. If about an hour was allowed for this redissolution, reproducible and time-stable scattering envelopes were obtained.13 The modified apparatus was calibrated with several solutions whose scattering power had been determined in the unmodified instrument in square cells. The latter was calibrated as described previ0us1y.l~ It was found that the ratio of the light scattered in the modified system to that in the unmodified system was the same for all solutions tested, which included polystyrene in toluene, poly-4-vinylpyridine in methanol and polysoaps in aqueous KBr . Refractive index differences between solutions and solvents were measured in a Brice-Phoenix differential refractometer which was calibrated by means of sucrose solutions.16 Viscosity.-Viscosities were measured a t 25’ in a Bingham viscometer as described previously.14
Results and Discussion Degree of Polymerization.-The light-scattering results for the ‘‘ 10 3%” polysoap in aqueous 0.2 M KBr are represented by a Zimm ploti6J7in Fig. 1, 20---
1
TABLE I CHEMICAL C’OMPOSITION
OF
POLY-4-VIiXYLPYRIDINE DERIVATIVES
% of pyridine groups substituted with n-
Sample”
Meg. Br-/g.
Meq. N/g.
Ma
CizHzsBr ( = y)
CzHsBr
0 99.1 4.46 224 4.42 B-1227D 4.8 94.2 230 4.29 4.33 B-1229D 10.3 89.2 239 4.16 4.19 B-1232D 86.0 254 16.3 4.00 3.93 B-1245D 65.2 270 34.1 3.69 3.71 B-1250D a In the tes , the polysoap samples will be ident,ified by y, the percentag of pyridine groups subst.ituted with dodecyl bromide; for instance, “4.8%’J polysoap, etc. All solvents used were purified and distilled according t o standard met,hods. Light Scattering.-Light-scattering measurements were performed a t 25” in a Brice-Phoenix light-scattering hotometer,’D using incident unpolarized monochromatic glue ( k 4360 A.).11 The apparatus was modified, following the (6) U. P. Strauss and E. G. ,Jackson, J . Polymer Sci., 6, 649 (1951). (7) R. hI. F’uoss and E. P. Strauss. .4nn. N . Y. Acad. Sei.. 61, 836 (1949). ( 8 ) A. G . Bores and U. P. Strauss, J . PoZj/mer Sei., 22, 403 (1956). (9) An ana1,ytical grade anion-exchange resin in the hydroxyl form contained i n a bag made of dialysis casing was immersed briefly in aqueous solutions of tt.e materials t o raise the pH t o between 5 and 6. (10) B. A. Brice, M. Ralwer a n d R. Rpeisrr, J . Optical Sac. A m . , 40, 708 (1950). (11) Results obtained with the 5460 .I. mercury line were identical indicating that flnorescence of the polyvinylpyridine derivatives was negligible.
04!-
I
i
I
i
02
L Oo?p------I-.p-0
1 ~~
I Sin2
%t
500c,.
Fig. l.--Zimm plot of “ 1 O . S ~ ~ polysoap ” in 0.2 ?il KBr, with each solution brought to equilibrium. (12) H. Boedtker and P. Doty, J . P h p . Chem., 68, 908 (1954). (13) We have satisfied ourselves that the redissolved fibers do not lea\-e any significant amount of large aggregates of polysoap molecules which would affect the scat,tering. f l 4 ) U. P. Straiiss and P. Wineman, J . Am. Chem. Soc., 80, 236b (1958). (15) C . A . Browne and F. W. Zerban, “Physical and Chemical Methods of R u m r Analysis,“ 3rd Ed., .John TViley and dons, Inc., New York, N. Y., 1941, p. 1206. (16) B. H. Zirnm, J . Chem. Phys.. 16, 1093, 1099 (1948). (17) P. Doty and J. T. Edsall, Adnnnces in Prolein Chem., 6, 35 ( 19.51).
ULRICHP. STRAUSS AND BERNARD L. WILLIAMS
1392
where R e is the reduced intensity a t the angle 6 and polysoap concentration c2 in g./ml., and K is a parameter defined in equation 2 below. Each solution had been brought to equilibrium by being maintained first at 45’ for a t least 24 hours, and then at 25’ for a t least another 24 hours. The curves are unusual in that their slopes are strongly negative. This behavior confirms previously obtained evidence that, just as with gelatin,12 one is dealing with aggregates which dissociate continuously on d i l ~ t i o n . ~All the polymers containing dodecyl groups produce similar Zimm plots in 0.05, 0.1 and 0.2 M aqueous KBr solutions.’* On the other hand the polyelectrolyte containing only ethyl side groups produces normal Zimm plots. Such a plot obtained in 0.2 M KBr is shown in Fig. 2.
_-
I
I
I
0 kG
Yol. 65
I n these equations Mj is the molecular w i g h t and cj the concentration in g./ml. of componciit j , j = 1, 2 and 3 referring to solvent, polymer and simple electrolyte, respectively. Furthermore, N A is Avogadro’s number, X is the wave length of light in vacuo, n is the refractive index of the solution, pu is the depolarization and a j k = (l/RT) X (dpj/dmk) where pj and mj are the chemical potential and molarity, respectively, of component j. In order to calculate D , the term az3/M2a33was determined from membrane equilibrium data,21 the quantity dn/bcz was measured, and bn/dca was interpolated from literature values.22 To convey an idea of the magnitude of the quantity (1 - D)-2, its values are 1.03, 1.04, 1.07 and 1.19 for the all-ethyl derivative in 0.05, 0.1, 0.2 and 0.8 M KBr, respectively, and 1.02, 1.03 and 1.04 for the “34.10j,” polysoap in 0.05, 0.1 and 0.2 M KBr, respectively. The depolarization correction was negligible in these solvents. In view of some difficulties encountered in obtaining accurate molecular weights of the polysoaps in the aqueous KBr systems, light scattering was also performed in a solvent consisting of a 0.1213 M LiBr solution in isopropyl alcohol (IPA). A Zimm plot of the “10.370” polysoap which is similar to the Zimm plots obtained with the other polysoaps in this solvent system, is shown in Fig. 3. I n the calculation of the molecular weights from
I-
-
Fig. 2.--Zimm
,plot of polyelectrolyte (sample B-1227D) in 0.2 M KBr.
In all these three-component systems the molecular weight of the polymer (component 2) is obtained from the following equations, which refer to the line extrapolatJed to 0 = 0.19020
I;ig. 3.--Ziinm
-
~
-
(18) Actually, the Zimm plots of the “4.8%” polysoap in 0.05 and 0.1 M KBr and of t,he “10.3%” polysoap in 0.05 M KBr appear to be normal. However, subsequent analysis of the results indicatea aggregation in all these cases. (19) J. 0. Kirkwood and R. J. Goldberg, J . Chem. P h w . . 18, 54 (1950). (20) W. H. Stochmayer, ibid., 18,58 (1950).
plot of “10.,70” polysoap i r i 0.1213 in IPA.
M LiBr
equations 1-4, the quantity D Tvas assumed to be negligible in this organic solvent. However, the Cabannes factor in equation 4 deviated from unity (21) D. Fraser, Ph.D. thesis, Rutgers University, 1960. (22) G. P. Bsxter, A. C. Boylston, E. Mueller, N. €1. Black and P. B. Coode, J . Am. Chem. Soc., 33, 901 (1911).
12g
by 5-8y; arid was included. Thc molecular weight results are represented in ternis of 1’w,.23 the weight-average degree of polymerization, III Table 11.
14
I
I
I ~
I
0 0672
TABLE I1 LIGHT-SCATTERING DEGREESOF POLYMERIZATION
--
II
Pw in
0.12!3 M LiBr in I P A
in
0.05 M KBr
in 0.10 M KBr
in
-
0.20 M
IO-
KBr
0.0“ .. .. 4140 4250 4.8 3’260 5210 5840 3930 10.3 3‘300 4560 4230 3070 16.3 2480 3400 3170 3520 34.1 3000 1970 2960 3870 a P, is equal to 3670 for this sample in 0.8 M KBr.
0 2995
0 I 703
J-----4
GO397
2
KC2 x IO6 R8 30-
08
0’
I
I
The values of P, for a given sample are somewhat scattered, but of the same order of magnitude. We believe that the main cause for the observed discrepancies is the uncertainty in the extrapolation of the extremely steep KCZ/ROagainst c2 curves of the polysoap-KBr-H20 systems. However, in some cases, incomplete breaking up of the aggregates in iihese systems may have been also responsible. For these reasons the values in the LiBr-IPh soh-ent syst’em are considered most reliable and will be used in subsequent calculations. It is seen that the Pw-values of all the poly-4vinylpyridine derivatives are much lower than 19,000, the ]’,-value of the parent polymer. Apparently degradation takes place during the quaternization reaction.24 The cause for this is not known. From some scattered observations that the degree of polymerization of the quaternization products is about 3500 regardless of how much higher than 3500 the chain length of the parent polymer is, one might suspect t’hat on the average about every 3500th bond in poly-4vinylpyridine is different from and weaker than the rest’, possibly through some reaction which is competitive to the normal pathway of the polymerization of vinylpyridine.Z5 The fact that the P,-values of the polysoaps do not seem to follow any trend is believed to be due to accidentd f.ractionation during the many purifications of the samples. Aggregates.--When, instead of preparing each polysoap solution in aqueous KBr separately, the solutions were prepared by dilution from a more concentrated s’olution a t 25”, the resulting Zimm plots appeared normal. In Fig. 4, such a plot is shown for the “10.3%” polysoap in 0.1 M KBr. The resulting :molecular weights were from 30 to 170% higher than those obtained with the directly prepared solutions. This difference in molecular weights is interpreted as being due to t,he formation (23) To simplify the presentation, we shall hereafter omit the Riibscript “2” in symbols designating the molecular weight, degree of polymerization and the molecular dimensions of the polymer component.
(24) The reduced specific viscosity of a 0.1% solution of the parent polymer in ethanol remained constant a t 4.25 dl./g. on heating a t 45” for 50 hours. ( 2 5 ) Such a reactjon rnieht involve oxygen, as suggested by Fuoss [.I. B. Berkowits, M. Yaniin and R. RI. Fiioss, J . Polymer Sei., 28, (iD (1958)l. .4nother possibility is free-radical formation at the pyridine nitrogen instead of at the --carbon so that the nitrogen is part of the backhone chain, with the pyridine ring being in the quinoid form.
0 6-
0
4
L Sl2Q
Fig. 4.-Zimm
1 t
4
1oooc,.
plot of “10.3%’ polysoap in 0.1 M KBr; dilution run.
of aggregates of polysoap molecules. The dilution method measures the average molecular weight of the polysoap species existing in the solution from which the dilutions were made, since at 25’ the aggregates which exist in this solution did not have enough time to dissociate significantly during the three to six hours it took to complete the experiments after the dilution process. The slowness of the dissociation was substantiated by observations indicating that in the first 24 hours after dilution the Rayleigh ratios of the diluted polysoap solutions decreased by only a few per cent. If, on the other hand, the diluted solutions were treated like the originally freshly prepared ones, Le., if they were first heated to 45’ for a t least 24 hours and then allowed to equilibrate a t 2 5 O , the lightscattering results were the same as those of a freshly prepared solution of the same concentration. A similar type of behavior has been observed in gelatin solutions. l 2 Second Virial Coefficients.-The second virial coefficients as determined from the light-scattering data by means of equation 1 are given in Table 111.
n 1211 M LiBr in
IPA
y
...
oa
4.8 10.3 16.3 34.1
TABLE I11 SECOND VIRIALCOEFFICIENTS B x 101 -Regular
0.05 .M
M
KBr
KBr
....
-1.2
16.0
+l.z
0.0
1.1 3.3
1.2 -31.6
In 0.8 M KBr, B
0.10
runs-
32.9 2.5 - 2.7
=
0.20 M KBr
14.8
-11.5
- 7.1 -14.0 - 4.4
-10.8
-14.1
-
-Dilution runs0.05 0.10 0.20 M M M KBr KBr KBr
..
..
...
6.7
2.5
0.3 -0.9
6.7 3.3
2.5 1.3
..
..
-0.2
-1.0
4.7 X
Several features are noteworthy. The negative values of B which are observed with the polysoaps in most of the aqueous KBr solutions in the regular runs reflect the decrease in the ext’ent of polysoap association with decreasing polysoap concentration.
ULRICHP. STRAUSS AND BERNARD L. WILLIAMS
1394
The values of B obtained in the dilution runs where the association is assumed to remain constant with dilution are seen to be generally more positive and reflect interactions other than the association. It is our belief that these values would be close to the correct ones for individual polysoap molecules if association did not occur. It is noteworthy that these values as well as those obtained in the LiBr-IPA solvent system are quite small. This probably is due to the compact molecular size of the polysoap molecules, which will be discussed below. In contrast, the much more extended polyelectrolyte (y = 0) is seen to have much larger second T-irial coefficients. For each of the firht three samples, the B-values decrease with increasing ionic strength, as has been observed with other polyelectrolytes in aqueous solution^.'^ However. the two samples with the most dodecyl groups seem to show somewhat irregular behavior in this respect. Molecular Dimensions.-The molecular dimensions of the Eamples were determined from the light-scattering; results in all solvent systems used and from the intrinsic viscosity results in the aqueous KBr solutions. The calculations depend to some extent on the assumed configuration and, in the case of the intrinsic viscosity, also on the molecular weight distribution of the samples. From the light-scattering results the z-average mean-square-radius of gyration @,le is obtained by the relation where I and S are the intercept and slope, respectively, of the Zimm plot cz = 0 line.16826J‘ For a random coil, the z-average mean-square endis related to by the equato-end distance (7)2 tion
Vol. 65
( d 2 ) s = (20/3)(s2)z (7 1 Flory, et a1.,28have shown that to obtain comparable x-average values from the intrinsic viscosity, one may use the relation
We estimate the quantity q by assuming the molecular weight distribution given by Zimm16 with the parameter characterizing the heterogeneity equal to unity. ?Vith this choice Mw/Mn = 2 and p = 1.95.29 The value for cp is 2.2 X lo2’ for random coils. The results are compared in Table IV. The agreement between the viscosimetric and light-scattering values is, in general, quite satisfactory considering the uncertainty underlying the assumptions made concerning the molecular weight d i s t r i b ~ t i o n . ~However, ~ there seems to be some discrepancy in the case of the “34.1%” polysoap in the 0.1 and 0.2 M KBr solutions. Since this discrepancy is greater than the combined experimental uncertainty of the light-scattering and Tiscosity methods, we believe this to be an indication that the polysoap molecules have become so compact that their shape is no longer representable by a random coil model. This interpretation is further substantiated by the observation that the decrease in molecular dimensions with increasing ionic strength, which is typical of polyelectrolytes and clearly noticeable for the other samples given in Table IV, is absent in the two “34.17,” polysoap-HzO-I
