J. Phys. Chem. 1990, 94, 3851-3855 produce a change in the decay profile for the fluorescence. Acknowledgment. This research was supported by a grant from the United States Department of Agriculture, 87-FSTY-9-0256. The authors thank Dr. Rujiang Tian (Department of Chemistry,
3851
Bowling Green State University, Bowling Green, OH) for assistance with the measurements of the fluorescence lifetimes. Registry No. DMSO, 67-68-5; (2R,3R)-[(-)-epicatechin], 490-46-0; (2R,3S)-[(+)-catechin], 154-23-4.
Polymer Swelling. 9. Sorption of 1-Chloroalkanes by Poly(styrene-co-divinylbenzene) L. A. Errede 3M Corporate Research Laboratories, Building 201 -2N-22, 3M Center, St. Paul, Minnesota 551 33 (Received: July 17, 1989; In Final Form: November 14, 1989)
The relative swelling power, C,, and adsorptivity, a,, with respect to poly(styrene-co-divinylbenzene)were established for the first 12 liquids in the homologous series CI(CH2),H. The results obtained thereby were compared with those obtained earlier for two analogous series in which the substituent in the 1-position is phenyl or iodo. These results confirm that the molecular nature of the adsorption process is given better by a, than by C,, which is the product of a, and the molar volume of the sorbed liquid. They also confirm that a, is determined by three parameters: the inherent dynamic packing efficiency, ao, when steric hindrance owing to (CH,),H is not a factor; the incremental decrease in packing efficiency, A, per added methylene group owing to the corresponding increase in molecular bulkiness; and the added steric hindrance factor to adsorption, B, caused by self-association in such molecules when n > 6. Relationships that correlate log anwith the corresponding Hildebrand solubility parameter, 6,, were established, and the significance of such correlations is discussed.
Introduction The results obtained in earlier studies’-4 of liquid sorption by poly(styrene-co-divinylbenzene) [hereafter referred to as either poly(Sty-co-DVB) or (Sty),-,(DVB),] have shown that the number, a,of adsorbed molecules per accessible phenyl group of polymer at gel saturation reflects how well the molecular structure of the adsorbed species is accommodated by that of the styrene unit. Since these adsorbed molecules are in exchange equilibrium with the rest of the sorbed molecules in the gel, a is considered to be the dynamic packing efficiency with respect to the finite adsorption area available on or around a phenyl group. Sets of a data for homologous series of ZR molecules, in which Z (a substituent with a relatively strong affinity for the phenyl group) is kept constant and R is modified systematically, show that a varies directly with the affinity of Z for the phenyl group and inversely with the bulkiness of the molecule ZR owing to space limitations imposed by the finite adsorption area. Thus, for liquids that comprise an homologous series of the type Z(CH2),H [hereafter referred to in abbreviated symbolic notation Z,H], a, decreases with n in accordance with eq 1, where a. is the inherent log a, = log a. - An (1) packing efficiency characteristic of Z when n = 0, Le., when the contribution to steric hindrance owing to the added bulkiness of the polymethylene chain is not a factor, and A is the incremental decrease in log a, with each additional methylene group. Negative deviation from the linearity expressed by eq 1 first appears when n becomes n’, some number greater than 6 (usually at about n’ = 7 or 8), and the magnitude, A(log a,,), of this deviation increases with n - n’thereafter. I showed that A(log a,,) observed in these studies of liquid sorption by Z,H parallels deviations from the norm observed by others, who studied excess molar volume of mixing,s light scattering6 calorimetry,’ and surface tension8 of liquids that comprise the homologous series ( 1 ) Errede, L. A. J. Appl. Polym. Sci. 1986, 31, 1749. (2) Errede, L. A. Macromolecules 1986, 19, 1522. (3) Errede, L. A. J. Phys. Chem. 1989, 93, 2668.
(4) Errede, L. A. J. Phys. Chem. 1990, 94, 466. (5) Orwoll, R. A.; Flory, P. J. J. Am. Chem. SOC.1967, 89, 6822. (6) Botherel, P. J. Colloid Sci. 1968, 27, 529. (7) Tancrede, P.; Patterson, D.; Botherel, P. J. Chem. Sor., Faraday Trans. 2 1977, 73, 29.
0022-3654/90/2094-385l$02.50/0
H(CH2),H. They concluded that their observed deviations are caused by self-association of the polymethylene chain owing to the cumulative force (F,) of correlated molecular orientation, which first becomes significant in liquids H(CH2),H when n becomes 6 and increases thereaftersq9 in accordance with eq 2. F,, = 1.15(n - 6)1/2- 0.81
(2)
I noted that A(log a,) for liquids Z,H with n > n’is a linear function of F, as given by eq 3, where F,,‘ is the force F, corresponding to n‘, and B is a proportionality constant (characteristic of substituent Z), which is normalized to 1, Le., the value of B when Z is H. A(l0g a,) = B(F, - F,,’)
(3)
Because the earlier studies of polymer swelling3q4in Z,H liquids had shown that both constants a. (eq 1) and B (eq 2) are affected markedly by the nature of substituent Z, I was encouraged to undertake more studies of this type in the hope of gaining insight into association between molecules, which appears to be of fundamental importance not only for polymer swelling and ultimately solution but also in the understanding of solvent effects in reaction kinetics, separations by chromatography, and permeation through polymeric films. With this in mind, I chose to investigate the sorption of CI(CH2),H by poly(Sty-co-DVB) and to compare the results obtained thereby with those reported earlier.
Experimental Section The set of six microporous composite film swatch samples, consisting of (Sty),,(DVB), particles enmeshed in poly(tetrafluoroethylene) (PTFE) microfibers, used in our earlier studiesI4 was used again in this study of polymer swelling by I-chloroalkanes. Each of these films contained particles with a known value of x, namely, 0.01, 0.02, 0.03, 0.04, 0.08, and 0.1 1. The physical dimensions, weights, and porosities of the swatch samples are collected in Table 1 of ref 1. The protocol for swelling the composite film samples to gel saturation in a given test liquid, and subsequent calculation of the adsorption number, a, from the observed volume, S, of sorbed (8) Fowkes, F. W. J. Phys. Chem. 1980,84, 510. (9) Errede, L. A. Macromolecules 1986, 19, 1525.
0 1990 American Chemical Society
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Errede
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
TABLE I: Correlation of Calculated Solubility Parameters, 8, and Adsorptivitiy Parameters, a, with the Corresponding Swelling Power, C,,, of Liquids CI(CH,).H for Poly(Sty-co-DVB) ri C. XnllJa db 6. (10.0 - 6.V a. I [1.70]C [O.9l6Ic 10.30 0.09 [3.21] 2 [1.54]' [0.898]' 9.70 0.09 [2.23] 3 I ,63 1.69 0.892 9.17 0.69 1.94 4 1.42 1.70 0.886 8.98 1.04 1 .s5 5 1.50 1.71 0.882 8.83 1.36 I .29 6 1.37 1.70 0.880 8.72 1.64 1.04 ? 1.42 1.90 0.881 8.65 1.82 0.96 8 1.27 1.99 0.874 8.55 2.10 0.78 9 1.03 1.71 0.872 8.48 2.31 0.57 IO 0.8.5 2.01 0.871 8.43 2.46 0.44 II [0.60Id 0.869 8.40 2.59 [0.29] 12 0.40 1.72 0.868 8.35 2.72 0.18 is as defined in eq I . b d is the density of the liquid. 'Estimated by extrapolation of data collected in Table 8 of ref 3. dEstimatcd from Figure 2.
liquid per gram of polymer, is described in earlier publications.I4 After each sorption study using a given test liquid, the set of six liquid-saturated film samples were "cleaned" by extraction in acetone and then dried (always to within fO.l mg of the original weight) in preparation for swelling in the next test liquid. To date, this set of six composite samples have undergone more than 1000 such swelling and drying cycles, and they continue to give reproducible results even after 7 years of service. There are two caveats that must be observed to ensure such longevity of service and reliability: (1) Avoid liquids that react irreversibly with the enmeshed particles; swelling data obtained thereafter would be characteristic of the chemically modified particles rather than that of the original composition. (2) Use the same source of absorbent paper to remove excess surface liquid from the liquid-saturated microporous film samples before gravimetric determination of total sorbed liquid. Constant good sorptivity properties in such papers is an important requirement to ensure reproducibility of the small volume correction that must be applied because of the residual liquid retained in the interstices of the composite film, which is characteristic of the macroporous structure produced during preparation of that film.2J0 For this purpose, I chose "Premier" paper toweling (Scott Paper Co.) as it was stocked in our laboratory. It is one of many types of suitable absorbent sheets. The test liquids in this study were obtained from commercial sources, mostly Aldrich Chemical Co., and they were used without further purification. The volume, S, of liquid sorbed by 1 g of (Sty),-,(DVB), at saturation with test liquid at 23 f 0.5 "C was and the data were plotted as a determined in the usual function of All3 (Le., [( 1 + x ) / x ] ' / ~ ] .The relative swelling power, C, as defined in eq 1, was calculated from the slope of the line obtained thereby. The corresponding number, a,of molecules of test liquid immobilized by adsorption to polymer at gel saturation per accessible phenyl group of that polymer was calculated from the observed C from the relationship a = 104Cd/M, where 104 is the molecular weight of a styrene repeat unit, and d and M are respectively the density and molecular weight of the test liquid. The Hildebrand solubility parameters, 6, of the test liquids were calculated by the method of additive component contributions to the cohesive energy density, 62 (Le., Ecoh/v, using the cohesive for CI, CH2 and CH3given in Table 7 of ref 1 1. The energy, Em,,, molar volumes, V = M/d, of these liquids were calculated with use of the formula weight and the corresponding density reported in standard reference tables. Results and Discussion ( A ) Relative Swelling Power. The test liquids used in this study of sorption by poly(Sty-co-DVB) particles enmeshed in PTFE microfibers are listed in Table I. The volumes, S (mL), of sorbed ( I O ) Errede, L. A.; Van Bogart, J. W. C.J . Polym. Sci., A 1989,27,2015. ( I I ) Van Krevelen, D. W.; Hoftyzer, P.J. Properties of Polymers; Elsevier: N e w York, 1976; Chapter 7 .
1
2
3
4
5
Figure 1 . Volume, S (mL),of sorbed liquid per gram of (Sty),,(DVB), as a function of A l l 3 , where X is ( 1 + x ) / x a t six levels of x .
liquid per gram of particles are recorded as a function of in Figure 1, which shows the best straight line through each set of six data points. The data points for CI(CH2),H where n = 10 (closed circles) and n = 12 (open circles) are shown because in these cases the correlation coefficient for the best straight lines were less than 0.98. For the liquids with n = 2-9, however, the correlation coefficients were in every case greater than 0.99; the data points, therefore, need no such designation, since they fall (within the precision of the graph) exactly on the intersections of these linear plots with the vertical lines identifying the XIIS for the sorbent polymer particles enmeshed in the PTFE matrix. All nine of these linear relationships are expressed by
s = c(~1/3 - xoi/3)
(4)
where X is the average number of backbone carbon atoms in the polystyrene segments between cross-link junctions [i.e., X = (1 x)/x], l / X o is the critical cross-link density above which S is not measurable (by the present analytical method), and the difference ( A l l 3 - Xo1I3)[hereafter referred to as A], is thus a dimensionless number that reflects the macrostructural "looseness" of the polymer network. The ratio S/A is therefore the relative swelling power, C (mL/g), of the test liquid with respect to poly(Sty-co-DVB). The values for C, deduced from the straight are lines shown in Figure 1, along with the corresponding b1l3, observed in this study of recorded in Table I. The average poly(Sty-co-DVB) swelling is 1.78 f 0.14, which is within experimental error of the corresponding averages reported for Z,H series in which Z is phenyl3or i0d0.~Unlike the two earlier studies, however, in which the relative swelling powers, C,,,for a given Z,H series of liquids decreased monotonically with n, the corresponding C, observed in the present study (Table I) do not show a monotonic decrementation. The plot of these data as a function of n (Figure 2) exhibits a pronounced odd-even alternation for n < 8; those for liquids with odd n C 8 fall on a line given by C, = 1.76 - 0.049n ( 5 , odd)
+
whereas those with even n
8, then the odd-even alternation extends over the full range of data reported here for C1(CH2),H liquids (Figure 2). If this is indeed the case, then there is a curious “crossover” at n = 8; Le., for n < 9, the line drawn through the points for the liquids with odd n is uniformly above the line drawn through the points for the liquids with even n, but for liquids with n > 8, the data for even n are uniformly above the curved line (Figure 2), whereas those for odd n appear to be below that line. In any case, the odd-even alternation exhibited by liquids with n C 9 is considerably more pronounced than that exhibited by the higher members of this homologous series. Although it is not yet clear why these changes occur as noted above, it is suspected that the cause is somehow related to self-association in CI(CH2),H liquids owing to correlated molecular orientation of London dispersion forces, which first becomes significant in this homologous series when n = 8. ( B ) Adsorptiuity Parameter, a. Since the relative swelling power, C, is related to the product of two factors, namely the number ( a ) of adsorbed molecules per accessible phenyl group in the polymer at gel saturation and the molecular volume of these adsorbed m o l e c ~ l e sthe , ~ molecular nature of the swelling phenomenon is indicated more clearly by a than it is by C. Accordingly the a, for the first 12 members that comprise the homologous series CI(CH2)H were calculated from the corresponding C,, values as described in the Experimental Section, and they are collected in Table I along with the C,, from which they were derived. Since the values of a, for liquids with n = 1, 2, and 11 were derived from the corresponding C,, data that were obtained either by extrapolation (Figure 2; open circles) or by interpolation (Figure 2; open rectangle), these values are also placed in brackets to emphasize that they were not calculated from C,, data that were obtained by direct experimental measurement. Unlike the set of C, data observed for this series, which do not decrease monotonically with n, the corresponding a,,data do indeed decrease monotonically as expected and, in fact, do so logarithmically. The odd-even alternation for liquids with n < 8, however, is still apparent (Figure 3) such that there is an almost parallel displacement between the lines that pass through the sets of logarithm data points for liquids with odd n and even n. Because these are logarithmic relationships, however, the parallel displacement noted in Figure 3 is much smaller than that noted in Figure 2, and therefore, the entire set of data from n = 1 to 7 can be represented approximately by the line passing midway between these two parallel lines. This midway line is given by (14) Lange, H.; Schwager, M . J. Kolloid Z.Z.Polym. 1968, 223, 145. ( 1 5 ) Mukerjee, P. Kolloid Z.Z . Polym. 1970, 236, 7 6 .
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The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
r
Errede 40
35
?
20 ’5
cy,,, 10
: -
85
1
2
3
4
5
6
7
8
9
10
11
12
13
n
Figure 3. Correlation of an,the number of adsorbed CI(CH2),H molecules per accessible phenyl group in the polymer, with n of the adsorbate.
TABLE 11: Constants amA , and B of Equations 1 and 3 z (Y” A B ref H 0 14 8, 9 CI 3.3 0.08 1 0.16 1 3.9 0.085 0.14 4 2.5 0.083 0.08 2 C6H5 B for Z = H has the value of 1 by definition in accordance with eqs 2 and 3 .
eq I , the constants a. and A for which are 3.35 and 0.0805, respectively. Deviation from the linearity expressed by eq 1 begins at about n = 8 (Figure 3). and the magnitude of this deviation, A(log a,,), increases with n > 8, i.e., linearly with F, as expressed by eq 3, where the value of B is 0.16. The constants, cyo, A , and B, observed in this study of polymer swelling in liquids CI(CH2),,H, are compared in Table I1 with the corresponding three constants observed earlier for swelling in the sets of Z,H liquids in which the substituent Z, in the 1 position, is C6H5, I, or H. These data show that the “inherent dynamic packing efficiencies” for such homologous series are in the order I > C1 > C6H5>> H, as might be expected on the basis that a. reflects the net result of electronic attraction and ”bulkiness” of the substituent. Bulkiness must be viewed as relative to the space limitation imposed by the finite area available at the adsorption site. The incremental decrease, A , in log a, per additional methylene group in the chain attached to substituent Z appears to be fairly constant at 0.083 f 0.002 for the three Z,H series reported thus far, but the magnitude of negative deviation from linearity per additional methylene group (Le., the constant B; Table II), owing to self-association in the liquid when n > 6, varies inversely with the molar volume of substituent Z located at the 1 position. This is consistent with the point of view that the magnitude of self-association is greatest in the n-alkanes, Le., the Z,H series in which Z is H. In accordance with eq 2, the value of the constant B for this series is 1, as noted in Table 11; Le., it is the standard of reference. The values for constants a. and A for the Z = H series, however, are too small to be measurable by the method used to determine C (eq 5 ) as described in the Experimental Section. (0Correlation of a with the Hildebrand Solubility Parameter.’6,’7 The preceding paper in this series4 reports that the
50
95
100
105
110
115
Figure 4. Correlation of log ali with the corresponding solubility parameter, 6,iq, for liquids classifie3as CI(CH2),H or as mono-substituted benzenes, C6H5R.
correlation of log a for simple mono- and di-substituted benzenes with the respective solubility parameters, hIiq,produces an inverted parabolic curve. The apex of this relationship is by definition the solubility parameter, 6,i, of the polymer;” but, I have s h ~ w n ~ ~ ~ J * that this assigned value varies with the functionality of the class of test liquids employed, which means that 61, for liquids Z,H will vary with Z. Identification of in a given Z,H series of liquids, however, is relatively imprecise because such correlations generate only about half of the expected parabolic relationship, thus not identifying precisely where a,, is maximal. Since a, varies inversely is equal to or greater than a I . with n in such series, and amax can only be estimated on the basis of symmetry in the expected parabolic relationship, as indicated in Figure 4 by the dashed line extension of the data for C1(CH2),H liquids beginning at 6, = 9.1. The most probable “correct” amaxfor this set of liquids was then adjudicated more precisely by successive iterative approximations of on the basis of the best fit of the data generated thereby to the straight line expressed by eq 6. log a, = log a,,, - E(6,] - a,)*
(6)
Nevertheless, it is obvious from the data shown in Figure 4 that the 6, where a,, is maximal for CI(CH2),H liquids is greater than that where a, is maximal for Ph(CH2),H liquids. The above approach indicated that it is close to that where a, is maximal for I(CH2),H liquids (see Figure 5 of ref 3). In the case of the chloroalkanes, however, estimation of amaxis made even more difficult by the pronounced odd-even alternation with n for those liquids with n < 8. Added thereto is the uncertainty of the validity of a I and a*,obtained by extrapolation (Figure 2). Consequently the assignment of 6,] = 10.0, where amax = 3.4, is only tentative and may require correction at a later date, when and if appropriate chloroalkanes with solubility parameters greater than 10.0 become available for swelling studies. Meanwhile the linear relationship (eq 6), established with this tentative value for 6,], was used to deduce the corresponding approximate effective solubility parameters, 6,,’, for those liquids (16) Hildebrand, J. H.; Scott, R. L. The Solubility of Non-electrolytes, 3rd ed.; Reinhold: New York, 1950. (17) Barton, A. F. F. Handbook of Solubility Parameters and Other
Cohesion Parameters; CRC: Boca Raton, FL, 1983. (1 8) Errede, L. A. Proceedings, ACS Division of Polymeric Materials Science and Engineering; American Chemical Society: Washington, DC, 1986; Vol. 54, p 561.
The Journal of Physical Chemistry, Vol. 94, No.’9, 1990 3855
Sorption of 1Xhloroalkanes by Poly(Sty-co-DVB)
TABLE 111: Effective Solubility Parameters, ,6 for Liquids in the Homologous Series Z(CH,),H Owing to Self-Association When n > 6 Z
6,,B 10.0 10.1 9.5
Ea
c1
0.260 0.289 0.454
I C6H5
87‘
8.75 8.32
88’
69’
8.53 8.17
8.35 8.33 8.09
810’ 8.23 8.15 7.98
8Il1 8.03 7.96
61; 7.86 7.83
“ E is the constant as defined in eq 7. bb,l is the solubility parameter of the polymer with respect to the homologous series with substituent Z in the I-position.
3.0
0.31
0.2 0.15
,
, 1
n.12
,
-----,-
I 2
3
I
4
\\?\,, 5
,
, 6
,
, 7
(d,,, - d1J Figure 5. Correlation of log cqiqwith (bpi - 61iq)2for liquids classified as CI(CH2)”H or as C6H5R.
in the CI(CH2),H series with n > 6 (Figure 5 ) as described in 6, was calculated by the preceding paper of this ~ e r i e s .Since ~ the method of additive contribution to the cohesive energy of the molec~le,~’ which ignores entropic effects such as self-association, deviation from linearity occurs as expected (Figure 5 ) when these effects become too great to be ignored. In this Z,H series, it occurs when n > 7. The effective solubility parameters for these liquids are collected in Table I11 along with the corresponding data deduced for the Z,H liquids in which Z is phenyl or iodo. The difference 6,‘ - 6, for each set of liquids increases with n > 6, and it appears to reflect the corresponding increase in the effective molar volume (V,’ - V,) owing to self-association when n > 6 as discussed in the Introduction. More data is needed, however, before one can evaluate quantitatively how these changes are affected by the nature of substituent Z.
Summary and Conclusions The results observed in this part of our ongoing study of Z(CH2),H sorption by poly(Sty-co-DVB) are consistent with the
molecular model proposed for liquid-saturated gels (see Figure 1 of ref 19), namely, that there are two kinds of sorbed molecules: those that are immobilized by adsorption and those that are not. It is suggested that such immobilization involves liaison of the Z substituent with a phenyl group in the polymer and the rest of the molecule extends away from the binding site, where it is in dynamic association with the adjacent nonadsorbed molecules. This model is also consistent with the observations of SchragZ0 and Lodge:’ who studied oscillatory flow birefringence of polymer solutions in high-viscosity solvents and showed thereby that addition of only small amounts of polymer affect abnormally the solvent viscosity, owing to an induced ordering of the surrounding solvent molecules that apparently extends even beyond the first layer of sorbed molecules. The results reported here confirm that the number of adsorbed Z(CH2),H molecules per accessible phenyl group in a liquidsaturated gel is a function of at least three parameters as defined in eqs 1 and 3: “the inherent dynamic packing efficiency” (ao), when steric hindrance owing to the attached (CH2),H is not a factor (Le., when n = 0); the incremental decrease (A; eq 1) in log a, per additional methylene group, which reflects the corresponding increase in steric hindrance to packing on or around the finite available adsorption area; and the added steric hindrance factor ( E ; eq 3) to adsorption owing to self-association in Z,H liquids with n > 6. The data collected thus far (Table 11) show that a. varies with the nature of substituent Z, whereas A appears to be relatively independent of Z. log a, for liquids Z,H correlate linearly (Figure 5) with the - 6,J2. When 6, is calculated by the method of difference component contribution to the cohesive energy of the test liquid, which ignores entropy effects such as self-association, deviation occurs when these effects become too great to be ignored. This usually occurs when n becomes greater than 7. In such cases, the linear relationship established with the first six members of a given Z,H series can be used to deduce the effective solubility parameters, &,,I, for those liquids of that series with n > 6 . Registry No. (Sty)(DVB) (copolymer), 9003-70-7; CICH3, 74-87-3; CI(CH2)2H, 75-00-3; CI(CH)2H, 540-54-5; CI(CH*),H, 109-69-3; C1(CH2)SH. 543-59-9; Cl(CH,),H, 544-10-5; CI(CH&H, 629-06-1; C1(CHJSH, 1 1 1-85-3; CI(CH2)9H, 2473-01-0; CI(CH2)loH, 1002-69-3; CI(CHJIIH, 2473-03-2; CI(CH2)12HI 112-52-7. (19) Errede, L. A.: Kueker. M. J.; Tiers, G. V. D.; Van Bogart, J. W.C. J . Polym. Sci., A 1988, 26, 3375. (20) Minnick, M. G.; Schrag, J. L. Mucromolecules 1980, 13, 1690. (21) Morris, R. L.; Amelar, S.; Lodge, T. P. J . Chem. Phys. 1988, 89, 6523.
