9918
J. Phys. Chem. 1996, 100, 9918-9928
Polymer Swelling. 18. Sorption of Geminal and Terminal Polyalkoxy-Substituted Alkanes by Poly(styrene-co-divinylbenzene) L. A. Errede* and George V. D. Tiers 3M Corporate Research Laboratories, 3M Center, 201-2N-22, St. Paul, Minnesota 55144 ReceiVed: February 27, 1996X
The adsorption parameters (R) of polyalkoxy alkanes CH4-zZz, CH3-zZz(CH2)nH, and Z(CH2)nCH3-zZz, in which Z is either CH3O or CH3CH2O, were established experimentally in the usual way.1-10 In the cases of those homologous series that comprise the structural classifications CH3-zZz(CH2)nH or Z(CH2)nCH3-zZz, the Rn-values for only two or three of the lower members (n < 5) in a given series were determined experimentally; the rest were established by extrapolation or interpolation using the appropriate log Rn Vs n linear relationship, as described in the text. The results obtained thereby support the conclusions derived earlier from similar studies6 in which Z was either a chloro or a bromo substituent, namely, that liaison between the adsorbed molecule and the polymer at liquid-saturation is monodentate rather than polydentate. The other Z-substituents on the carbon atoms adjacent to the adsorption site affect R in a manner that reflects not only the net influence of electronic and steric contributions at this site but also the affinity of these substituents for the mobile sorbed-but-not-adsorbed molecules in the liquid-saturated system. Hence, systematic incrementation of these Z-substituents to the allowable limit at constant n does not afford a linear log Rz Vs z relationship but rather reflects one that exhibits a maximum at z ) 2 or 3, depending upon the series. On the other hand, similar systematic incrementations of Z on carbon atoms further removed from the adsorption site do exhibit log Rz Vs z linear relationships at constant n in the manner previously noted for log Rn Vs n relationships at constant z.
Introduction Earlier publications from this laboratory reported1-8 that it is possible to establish the number of adsorbed molecules per accessible phenyl group of poly(styrene-co-divinylbenzene) [hereinafter referred to as poly(Sty-co-DVB) or (Sty)1-x(DVB)x] at liquid-saturation by measuring the sorption capacity of a set of (Sty)1-x(DVB)x samples having known values of x, as described briefly in the Experimental Section. This value (R) is defined as the adsorption parameter of the sorbed liquid with respect to polystyrene. It is reproducible to (1 in the third significant figure, and it reflects very sensitively the molecular structure of the sorbed liquid, ZCR1R2R3, where Z is a substituent (Cl, Br, I or phenyl) having strong affinity for the phenyl groups of the polymer and R1, R2, and R3 are H or alkyl substituents. We noted that R varies directly with the affinity of Z for the polymer and inversely with the bulkiness of CR1R2R3. It was suggested6 that the mode of adsorption involves liaisons between substituent Z of the liquid and the phenyl groups of the polymer at liquid-saturation in the manner illustrated schematically in Figure 1a; i.e., liaison is presumed to form Via a pair of nonbonded electrons of substituent Z and the π-electrons of the phenyl group in the polymer, while the rest of the molecule extends away from the adsorption site, owing to dynamic associative interactions with the mobile sorbed-butnot-adsorbed molecules of its own kind in that liquid-saturated system. In the case where ZCR1R2R3 represents sets of homologous series having the general molecular structure (GMS) ZCH2-q(CH3)q(CH2)nH (Figure 1b), the logarithm of Rz,q,n decreases linearly with n (from 0 to 7 when Z and q are constant) and also with q (from 0 to 2 when Z and n are constant), as expressed by eq 1 X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00592-8 CCC: $12.00
Figure 1. Schematic representation of association between R1R2R3CZ and the pendent phenyl group of poly(Sty-co-DVB) at liquidsaturation showing the assumed liaison Via a pair of nonbonded electrons of substituent Z (chloro, bromo, iodo, or alkoxyl) and the π electrons of the phenyl group (a) when the R-groups on R1R2R3CZ are either hydrogen or methyl substituents, and (b) when R3 is R′(CH2)n and R′, R1, & R2 are either hydrogen or methyl substitutents, or optionally R′ ) Z.
log Rz,f ) log Rz,i - D(Nf - Ni)
(1)
Here Rz,f and Rz,i are the R-values for the final and the initial members of a given homologous series, Nf and Ni are the corresponding total numbers of methylene mass units (n + q + 1) in the alkyl portions of the molecule, and D represents the values for the decrementation constants (∆ log R/∆N) which are unique to the homologous series, i.e., ∆ log R per unit increase in q (when Z and n are constant) and ∆ log R per unit increase in n (when Z and q are constant), such that the two sets of parallel log R Vs N linear relationships form a rigidly interconnected grid system characteristic of Z, as shown in Figure 2. The intersections of these two sets of parallel lines © 1996 American Chemical Society
Polymer Swelling
Figure 2. log R Vs N linear relationships exhibited by ZCH2-q(CH3)q(CH2)nH liquids [here N is the total number (n+q+1) of aliphatic carbon atoms in the molecule], showing the effect on log R caused by incremental changes in q from 0 to 2 while n is kept constant at 1-7, and the effect on log R caused by incremental changes in n from 1 to 7 while q is kept constant at 0 to 2 (see Figures 2-13 and Tables 1-10 in ref 10).
identify the R-values for all the members that comprise the above GMS classification. Later we reported9,10 that a similar rigidly interconnected grid system of log R Vs N linear relationships is exhibited by the linear aliphatic ethers having the GMS H(CH2)mO(CH2)nH, as shown in Figure 3. The value of the decrementation constant, for the set of log R Vs N linear relationships in which n is incremented at constant m from m to 8, is DR1 ) 0.0505, that for those in which m is incremented at constant n from 1 to n is DR2 ) 0.1447, and the value for those in which m and n are incremented symmetrically is DR3 ) 0.0976. The intersections of these three sets of parallel reference lines identify the R-values for all the members that comprise the above GMS classification, which means that ∆ log R in going from any initial point (log Ri:Ni) to any final point (log Rf:Nf) in this grid network is independent of the pathways chosen for doing so. It should be appreciated that each log R Vs N linear relationship is in fact a vector in multidimensional space and that Figure 3 represents the projection of that network onto the two-dimensional plane of the paper, such that the recorded decrementation constants represent the vertical component of the respective vectors. On the basis of the above results it was concluded that in these molecules the site actually immobilized by adsorption to the phenyl groups in the polymer is the oxygen atom and that the two alkyl groups attached thereto do not extend symmetrically above the adsorption site, as shown in Figure 4a. Instead the adsorbed ether (R′OR) is tilted on its side such that the alkyl group (R′) having the smaller number (m) of methylene groups lies close to the plane of adsorption, while the alkyl group (R) having the larger number (n) of methylene groups extends above this plane, as shown schematically in Figure 4b. Hence, the symbols R′ and R represent respectively the adsorbed and the nonadsorbed portions of the adsorbed ether molecule, and therefore R′O is equivalent to Z, the symbol used to identify
J. Phys. Chem., Vol. 100, No. 23, 1996 9919
Figure 3. log R Vs N linear relationships for sorption of H(CH2)mO(CH2)nH liquids by poly(Sty-co-DVB) to saturation in the cases that (a) m (equal to or less than n) in the adsorbed portion is kept constant at 1-8 while n in the nonadsorbed portion is varied from m to 8 [series R1 and R2,2 to R1,8], (b) n is held constant at 1-8 and m is varied from 1 to n (series R3 and R3,2 to R3,8], and (c) both m and n are incremented concomitantly to (m +p) and (n +p), such that neither sum exceeds 8 (series R2 and R2,2-R2,8).
Figure 4. Schematic representation of the two possible modes for adsorption of H(CH2)mO(CH2)nH to the pendant phenyl group of poly(Sty-co-DVB) at liquid-saturation: (a) symmetrical and (b) non symmetrical.
the adsorbed portion of the earlier four sets of homologous series representing non-ether liquids. It was shown (see Figure 10 of ref 10) that each set of log R Vs N linear relationships that represent the ethers having GMS H(CH2)mOCH2-q(CH3)q(CH2)nH, in which n is incremented from 0 to its allowable limit while m and q are kept constant, and those for the crossover relationships, in which q is incremented from 0 to 2 while m and n are kept constant, produce rigidly interconnected grid systems, which resemble those exhibited by the ZCH2-q(CH3)q(CH2)nH series, in which Z is Cl, Br, I, or phenyl, shown in Figure 2. Moreover each of these planes of log R Vs N linear relationships for the branched ethers at constant m intersects the plane that contains the linear ethers (Figure 3), such that the lines of intersection of these two planes are represented by the log R Vs N linear relationships for the H(CH2)mO(CH2)nH series having the corresponding values of m, as shown in Figure 10 of ref 10.
9920 J. Phys. Chem., Vol. 100, No. 23, 1996 It was also shown10 that it is possible to extend these considerations to the appropriate log R Vs N linear relationships for the homologous series having the GMS H(CH2)m(CH3)q′CH2-q′OCH2-q(CH3)q(CH2)nH, in which one of the four variables (i.e., m, n, q′, and q) is incremented systematically (within the allowable limits for the variables as described above) while the other three are kept constant, to produce multidimensional rigidily interconnected systems that identify the R-values for all the possible permutations in molecular structure within the above GMS classification. Thus, that study showed that after only a relatively few R-values for key simple ether structures have been established experimentally, the R-values for the rest can be estimated within experimental error by interpolation or extrapolation, as shown in Figure 9-14 in ref 10. The data-set obtained thereby can now be used as a reference base for comparison with other related structural classifications, as will be described for sorption of polyalkoxy-substituted alkanes and in future reports involving adsorbed liquids bearing a variety of functional substituents and combinations thereof. In contrast to the linear variation of log R for ZCR1R2R3 with N, i.e., systematic incrementation of methylene units in the CR1R2R3 portion of the sorbed molecule as described above, our analogous studies of sorption of polyhalo-substituted liquids, such as CH4-zZz, CH3-zZz(CH2)nH, Z(CH2)nZ, and Z(CH2)nCH3-zZz in which Z is a chloro or bromo substitutent, have shown6 that the value of R does not always vary monotonically with the number (z) of halogen atoms in the molecule, owing to the complex mode in which Z affects simultaneously two or more of the factors that define the conditions at the adsorption site and also the affinity of the adsorbed molecule for the sorbedbut-not-adsorbed polyhalogen molecules in the polymer-liquid system at saturation. It was inferred from these observations that adsorption to the phenyl groups of the polymer at liquidsaturation involves primarily monodentate liaison rather than polydentate, i.e., the R-values for CH4-zZz and related molecules do not decrease monotonically with z (specifically, proportional to 1/z). Instead R Vs z relationships exhibit maxima at z ) 2 or 3, which reflects the net contributions to the steric and electronic influences at the adsorption site, as shown in Figure 2 of ref 6. In the case of terminally substituted polyalkanes,such as Z(CH2)nZ, log R decreases linearly with n, as defined by eq 1, but the decrementation constants (Dz2) for the disubstituted series are measurably smaller that those (Dz1) for the corresponding monosubstituted series, Z(CH2)nH, as shown in Figure 7 of ref 6. Since the electronic interaction between terminal Z substituents is insignificant for n greater than 2, this decresae in ∆(log R/∆n) was attributed to affinity for the sorbed-but-notadsorbed polyhalogen molecules in the polymer-liquid system at saturation when the terminal Z substituent is halogen rather than hydrogen. It was also noted that in the case of Z(CH2)nCH3-zZz liquids, the Z substituent actually making liaison with the adsorption site is the one attached to the carbon atom carrying the single Z substituent. This was rationalized on the basis that this molecular orientation serves to minimize steric hindrance at the adsorption site and to maximize dynamic associative interaction with the mobile molecules, which favors a more vertical orientation of the (CH2)nR′ chain above the absorption site, as depicted in Figure 1b. The strong “pull” on the adsorbed Z(CH2)nR′ molecules by the nonadsorbed molecules causes the height (h) of the imaginary inverted cone of effective occupied space (Figure 1b) to increase and its diameter (d) to decrease, and since the decrementation constant (D; eq 1) is proportional to d/h, the value of this constant [i.e., D ) ∆(log R/∆n)] becomes accordingly less negative.
Errede and Tiers This rationalization is applicable to the observation noted for sorption of the ethers (R′OR),10 namely, that the segment having the lesser number (m) of methylene units becomes the adsorbed portion R′ of the immobilized ether and the segment having the greater number (n) of methylene units becomes the nonadsorbed portion R, as shown here in Figure 3 and in Figures 7, 11, and 12 of ref 10. Although adsorption of multifunctional molecules to poly(Sty-co-DVB) at liquid saturation is virtually all in the monodentate mode, multidentate adsorption becomes increasingly more important as the liquid-saturated system is evaporated to dryness. This change is especially pronounced as the system undergoes an evaporation-induced transition from its rubbery to a glassy state, which affects accordingly the macromolecular architecture of the glassy product and consequently the physical properties of that product. This phenomenon is discussed more fully in a manuscript submitted for publication.11 It was of interest, therefore, to undertake similar sorption studies using polyalkoxy-substituted alkanes as the test liquids. The purpose of this publication is to report the results observed in such studies using geminal and terminal polyalkoxyalkanes, such as CH4-x(OCH3)x and CH3O(CH2)nOCH3, respectively, and to compare these results with those already reported for the corresponding polyhaloalkane liquids6 and also with those observed for the reference set of ethers that comprise the GMS classification represented by H(CH2)mOCH2-q(CH3)q(CH2)nH.9,10 Experimental Section The set of six composite film samples, employed as the polymeric sorbent in all our previous studies of (Sty)1-x(DVB)x swelling-to-saturation in a test liquid,1-10 were used again in the present studies of swelling in polyalkoxy-substituted alkanes. The procedure for making these composite film samples, comprised of (Sty)1-x(DVB)x particles (>80% by weight) enmeshed in PTFE microfibers, and the distribution of these particles in the microporous composite films produced thereby (see Figures 1, 6, 7, and 20 of ref 7) are described in considerable detail elsewhere.12,13 This set of samples, each containg particles having a known value of x (i.e., x ) 0.01, 0.02, 0.03, 0.04, 0.08, and 0.11), were allowed to swell to saturation in the test liquid at 23 ( 1 °C. Reagent grade samples of most of these liquids were obtained from commercial sources, and they were used as such without further purification. Exceptions are the samples of 1,4-dimethoxybutane and 1,6dimethoxyhexane, which were synthesized by one of us and purified by fractional distillation, as described elsewhere.14 The volumes (S) of sorbed liquid, per gram of enmeshed particles, in these samples were determined gravimetrically in the usual way.1-9,12,13 The slope of the straight line obtained when the S-values are plotted as a function of the corresponding cube root of the average number (λ) of backbone carbon atoms between cross-link junctions in the respective samples indicates the relative swelling power (C, in mL of adsorbed liquid per gram of particles) of the sorbed test liquid in accordance with eq 2.
S ) C(λ1/3 - λ1/3 o )
(2)
Here λ0 is the value of λ extrapolated to S ) 0. The corresponding adsorption parameter (R) was calculated from the observed C-values by means of eq 3
R ) 104Cd/M (3) The letters d and M refer to the density and formula weight respectively of the test liquid.
Polymer Swelling
J. Phys. Chem., Vol. 100, No. 23, 1996 9921
TABLE 1: Sorption of Polyalkoxy Alkane Liquids by Poly(Sty-co-DVB) adsorption datad identification of the liquid ether series 1 CH3OCH3-q(OCH3)q q)0 q)1 q)2 q)3 series 2 CH3OCH(OCH3)(CH2)nH n)0 n)1 series 3 CH3OC(OCH3)H2-q(CH3)q q)0 q)1 q)2 series 4 CH3OC(OCH3)2(CH2)nH n)0 n)1 n)3 n)4 series 5 CH3OCH2-q(OCH3)qCH3 q)0 q)1 q)2 series 6 CH3OC(CH3)3-q(OCH3)q q)0 q)1 q)2 q)3 series 7 CH4-q(OCH2CH3)q q ) 1b q ) 2b q ) 3b series 8 CH3CH2OCH(OCH2CH3)(CH2)nH n)0 n)1 n)2 series 9 CH3CH2OC(OCH2CH3)2(CH2)nH n)0 n)1 n)2 series 10 CH3O(CH2)nOCH3 n)1 n)2 n)4 n)6 miscellaneous polymethoxyethanes CH3OCH2CH(OCH3)2c (CH3O)2CHCH2CH(OCH3)2
E
d
λ1/3 o
C
R
χ1
1 2 3 4
0.860 0.970 1.023
1.71 1.79 1.73
1.18 1.48 1.38
[1.18]a 1.39 1.41 1.08
0.78 0.60 0.66
2 5
0.852
1.70
1.27
1.39 1.25
0.73
0.89 0.56 0.55 0.54
2 5 6
0.847
1.72
1.18
1.39 1.25 1.00
3 7 8 9
0.944 0.926 0.941
1.72 1.88 1.90
1.54 1.56 1.57
1.41 1.26 1.01 0.91
10 5 7
[1.05]a 1.25 1.26
11 6 5 4
0.61a 1.00 1.25 1.08
10 12 13
0.839 0.891
1.69 1.81
1.03 1.10
[1.05]a 0.86 0.69
0.87 0.83
12 14 15
0.831 0.851
1.83 2.01
1.03 1.02
0.86 0.76 0.66
0.87 0.88
13 16 17
0.885 0.876
2.01 2.20
0.90 0.77
0.69 0.51 0.40
0.96 1.03
2 18 19 20
0.863 0.855 0.852
1.70 1.86 1.93
1.51 1.62 1.66
1.39 1.50 1.22 1.01
0.58 0.51 0.49
21 22
0.932 0.997
1.71 1.81
1.49 1.52
1.20 0.96
0.59 0.57
a These data were taken from Table 1 of ref 9. b When q for CH 4-q(OCH2CH3)q is 1, the adsorbed portion of that ether is CH3O, but when q is 2, 3, or 4 the adsorbed portion is OCH2CH3. c The methoxy group on the carbon atom having the lesser number of CH3O substituents is believed to be the adsorbed portion of this molecule for reasons analogous to those already discussed for ClCH2CHCl2 in part 12 of this series.5 d E is the identification number of the ether liquid; d is the density of the liquid; λ0 is the value of λ extrapolated to S ) 0, as defined in eq 2. C is the relative swelling power of the liquid, as defined in eq 2. R is the adsorption parameter of the liquid, as defined in eq 3. χ1 is the Flory-Huggins interaction parameter (χV at V ) 1), which was calculated from C, using eqs 4 and 5.
The Flory-Huggins interaction parameter (χV) is also calculated from C by means of eq 4, as described elsewhere.8
χV ) 0.49 + 1.01V - 0.61VC
(4)
Here V is the volume fraction of polymer in the polystyreneliquid system. Since it was noted that χV is most sensitive to the molecular structure of the sorbed species at V ) 1 (see Figure 4 of ref 8), only the χ1-values are reported in Table 1. The χV-values at any other value of V can be calculated by the interested reader using eq 5.
χV ) 0.49 + V(χ1 - 0.49)
(5)
The R-values for the members that comprise a given homologous series of the type R′OCH2-q(OR)q(CH2)nH [in which R′ (the “adsorbed” portion of the adsorbed ether) and R (on the “non-adsorbed” portion of the adsorbed ether) are either
CH3 or CH3CH2, q is 1 or 2, and n is 0-8] can be calculated by means of eq 1, provided that R-values at two or more values of n in that series have been established experimentally. In such cases, the experimental values observed for a given homologous series (i.e., R′, R, and q kept constant) are used to calculate the decrementation constant (D) for eq 1 of that series. This value of D in turn is used to calculate the Rn-values for all the other members of that series from n ) 0 to 8 by means of a simplified form of eq 1, i.e., log Rn ) log R0 - Dn, where R0 is the R-value for the member having n ) 0 and D is given by the slope of the best straight line through the subset of experimentally determined data points. Results and Discussion Accumulation of Relevant r-Values for Polyalkoxy Alkanes. The data observed thus far for sorption of polyalkoxy-
9922 J. Phys. Chem., Vol. 100, No. 23, 1996
Figure 5. Comparison of the R-values for ethers that comprise homologous series of monoalkoxyalkanes with those for the corresponding polyalkoxyalkanes.
alkanes by poly(Sty-co-DVB) are collected in Table 1, the data being grouped into 11 classifications organized according to “general molecular structure” (GMS). Each GMS is written such that the “adsorbed” portion (R′O) of the adsorbed ether (R′OR) is at the left side of the GMS, and the nonadsorbed portion (R) is represented by the rest of the molecule to the right of the adsorbed ether oxygen atom. Each member of a given homologous series is identified by a number (E, from 1 to 22) in the sequence in which they were first recorded in Table 1. The same E (i.e., the same liquid) may belong to several series. Only in the cases of series nos. 1, 3, 5, and 6 [i.e., those having respectively the GMSs CH3OCH3-q(OCH3)q at q ) 0-3; CH3OC(OCH3)H2-q(CH3)q at q ) 0-2; CH3OCH2-q(OCH3)qCH3 at q ) 0-2, and CH3OC(CH3)3-q(OCH3)q at q ) 0-3] was it possible to establish experimentally the R-values for all the members that comprise a given GMS classification. In the cases of series nos. 2, 4, 8, 9, and 10 [i.e., respectively CH3OCH(OCH3)(CH2)nH at n ) 0, 1; CH3OC(OCH3)2(CH2)nH at n ) 0, 1, 3, and 4; CH3CH2OCH(OCH2CH3)(CH2)nH at n ) 0, 1, and 2; CH3CH2OC(OCH2CH3)2(CH2)nH at n ) 0, 1, and 2; and CH3O(CH2)nOCH3 at n ) 2, 4, and 6], it was possible to establish experimentally the Rn-values at only two to four values of n in a given series. It was shown earlier,9,10 however, that in such cases it is possible to calculate with reasonable confidence the Rn-values for the remaining members of that series by means of eq 1, as described in the Experimental Section, i.e., after first establishing the characteristic decrementation constant (D; eq 1) for that series on the basis of the best straight line through the set of two or more Rn-data points determined experimentally. Numerous examples of such log Rn Vs n linear relationships established in this manner are reported in Tables 1-14 of ref 10. The completed sets of Rn-values for series nos. 3, 4, 8, 9, and 10 in this study are recorded here in Table 2, together with the respective constants R0 and D that define the log Rn Vs n linear relationship (eq 1) used to establish the rest of the Rn-
Errede and Tiers values for the members that comprise the homologous series cited therein. A modifying small-case letter following the identification number (1-22) indicates that the R-value cited was not obtained experimentally. It was obtained instead either by extrapolation or interpolation of n using eq 1, m and q being held constant while n was incremented from its initial value to its allowable limit. The calculated Rn-values for these structures are placed in brackets in Table 2 to emphasize that they were not established experimentally. The modifying letter indicates the (alphabetical) sequence following the initially stated numerical identification. Interpretation of the Data for Methoxy-Substituted Alkanes. The log Rn-values for the ethers that comprise series nos. 2, 4, 8, and 9 are plotted in Figure 5 as a function of the total number (N) of structural CH2 and CH3 groups plus ether oxygen atoms. The filled circles represent the data that were established experimentally, whereas the empty circles represent those that were established by extrapolation or interpolation. The log R Vs N relationships for the gem-methoxy-substituted alkanes (series nos. 2 and 4; Table 2) are compared to those reported earlier for the eleven subseries of GMS CH3OCH2-q(CH3)q(CH2)n+1H (bold straight lines in Figure 5; see also Figure 9 of ref 10], namely, the three parallel relationships (DR1 ) 0.0505 ( 0.0001), in which q is 0, 1, or 2 while n is varied from 1 to its allowable limit, and the set of eight parallel crossover relationships (D ) 0.118 ( 0.001), in which q is varied from 0 to 2 at each value of n from 0 to 7. Three important observations are made obvious by this comparison: (1) The log R Vs N relationships for the two series having GMS CH3OCH2-q(OCH3)q(CH2)n+1H, in which q is constant at 1 and 2 while n is varied from 0 to 7, are uniformly above that for the common line of reference, which represents the log R Vs N relationship for ethers having GMS CH3O(CH2)n+2H [n ) 0-6; DR1 ) 0.0505] in Figures 3 and 5; in contrast, those for the corresponding two series having GMS CH3OCH2-q(CH3)q(CH2)n+1H, in which n is incremented from 0 to 7 while q is kept constant at 1 or 2 (bold straight lines in Figure 5), are uniformly below this reference line, despite that the bulkiness of the OCH3 group is greater than that of the CH3 group. (2) The decrementation constants for the above two series of gem-methoxy-substituted ethers (Dq)1 ) 0.0464 and Dq)2 ) 0.0476, respectively) are significantly less than the average value (DR1 ) 0.0505 ( 0.0001) for the corresponding pair of gem-methyl-substituted ethers (bold straight lines in Figure 5 that are parallel to the reference line). (3) Unlike the reference base, which shows that the crossover relationships for the CH3OCH2-q(CH3)q(CH2)n+1H series (i.e., those in which q is increased from 0 to 2 at a given value of n) are linear and parallel to that at n ) 0 (D ) 0.118 ( 0.001), the crossover relationships for the homologous series of ethers CH3OCH2-q(COH3)q(CH2)n+1H [i.e., those in which q is varied from 0 to 2 at constant n] are not linear, nor are they even curvilinear with respect to the nonlinear relationship observed at n ) 0. These observations show clearly that the effect on R of progressive replacement of hydrogen atoms by alkoxy groups is markedly different from that observed for the corresponding progressive replacement by methyl groups. Our earlier publications1-10 have shown that there are at least three factors that affect the number (R) of adsorbed molecules per accessible phenyl group in the polymer at liquid-saturation, namely: (1) The electronic nature of a functional group of the sorbed liquid molecule, which can affect not only the affinity of the adsorbed molecule for the adsorption site on the polymer but also the angle at which that molecule is attached with respect to the plane of the adsorption site. In those cases for which
Polymer Swelling
J. Phys. Chem., Vol. 100, No. 23, 1996 9923
TABLE 2: r-Values for Polyalkoxy Alkanes That Were Calculated by Means of Eq 1 Using the Experimentally Established Data Recorded in Table 1 log Rf ) log R0 - D(Nf - N0) identification of the liquid ether series 2 CH3OCH(OCH3)(CH2)nH n)0 n)1 n)2 n)3 n)4 n)5 n)6 n)7 series 4 CH3OC(OCH3)2(CH2)nH n)0 n)1 n)2 n)3 n)4 n)5 n)6 n)7 series 8 CH3CH2OCH(OCH2CH3)(CH2)nH n)0 n)1 n)2 n)3 n)4 n)5 n)6 n)7 series 9 CH3CH2OC(OCH2CH3)2(CH2)nH n)0 n)1 n)2 n)3 n)4 n)5 n)6 n)7 series 10 CH3O(CH2)nOCH3 n)2 n)3 n)4 n)5 n)6 n)7 n)8 n)9 n ) 10 n ) 11 n ) 12 series 11 CH3OCH2CH3-q(OCH3)q q)1 q)2 q)3
E
R0
D
Nf
Rf
2 5 5a 5b 5c 5d 5e 5f
1.39
0.0464
5 6 7 8 9 10 11 12
1.39 1.25 [1.12] [1.01] [0.91] [0.82] [0.74] [0.66]
3 7 7a 8 9 9a 9b 9c
1.41
0.0476
7 8 9 10 11 12 13 14
1.41 1.26 [1.13] 1.01 0.91 [0.81] [0.73] [0.65]
12 14 15 15a 15b 15c 15d 15e
0.86
0.0609
7 8 9 10 11 12 13 14
0.86 0.76 0.66 [0.56] [0.49] [0.42] [0.37] [0.32]
13 16 17 17a 17b 17c 17d 17e
0.69
0.1165
10 11 12 13 14 15 16 17
0.69 0.51 0.40 [0.31] [0.24] [0.18] [0.14] [0.11]
18 18a 19 19a 20 20a 20b 20c 20d 20e 20f
1.50
0.0429
6 7 8 9 10 11 12 13 14 15 16
1.50 [1.36] 1.22 [1.12] 1.01 [0.92] [0.83] [0.75] [0.68] [0.62] [0.56]
18 21 21a
1.50
0.0969
6 8 10
1.50 1.20 [0.96]
there are two or more such functional groups (only one of which is adsorbed at liquid-saturation) on the carbon atom adjacent to the adsorption site, the dipole moment of that complex grouping can alter the angle of adsorption considerably, which affects R accordingly. (2) Steric hindrance to adsorption owing to the bulkiness of the nonadsorbed substituents. The magnitude of this negative influence on R varies inversely with the distance from the adsorption site. (3) The affinity of these nonadsorbed functional substituents for the mobile sorbed-but-not-adsorbed molecules in the polymer-liquid system at saturation,6,9,10 which affects the verticality of the non-adsorbed portion of the adsorbed molecule such that the “projection” over the adsorption site is altered accordingly, thereby changing the number of such molecules that can be accommodated at that site. The data collected in Figure 5 show that the effect on R, caused by progressive replacement of the hydrogen atoms by
methoxy substituents on the carbon atom adjacent to the adsorbed ether atom, cannot be represented by a log R Vs N linear relationship, presumably because a change in q and/or n may alter simultaneously two or more of the three factors so that the net effect on R can vary nonuniformly as q and/or n is incremented from 0 to its allowable limit. That this is indeed the case can be illustrated by considering first the manner in which R varies with q at n ) 0 in three similar series, namely, CH3OCH3-q(CH3)q, CH3OCH3-q(OCH3)q, and CH3OC(CH3)3-q(OCH3)q, as shown in Figure 6, and then considering the analogous correlations of R with q for the ethers having GMS CH3OCH2-q(OCH3)q(CH2)n+1H at each value of n from 0 to 6, as shown in Figure 7. The pattern of change in R with q for the CH3OCH3-q(CH3)q series (no. 1 in Table 1 of ref 10) is such that R decreases as q increases from 0 to 3 (filled circles connected by bold straight
9924 J. Phys. Chem., Vol. 100, No. 23, 1996
Figure 6. R Vs q plots that show the effect on R caused by systematic incrementation of q from 0 to 3 in series having the GMSs CH3OCH3-q(CH3)q, CH3OC(CH3)3-q(OCH3)q, and CH4-q′(OR)q′, in which R is CH3 or CH3CH2 and q′ is q + 1.
Figure 7. R Vs q plots that show the effect on R caused by systematic incrementation of q from 0 to 2 for series having the GMS CH3OCH2-q(OCH3)q(CH2)nH and for which n is kept constant at 0-8.
lines in Figure 6). This pattern is interpreted to mean that the combined negative influences on R owing to the steric and electronic (in this case electron-donating) factors is greater than the positive influence owing to the associative factor and that the difference in these net opposing influences increases as q is incremented from 0 to 3, to produce the observed monotonic decrease in R shown in Figure 6.
Errede and Tiers In contrast to the negative slope noted in the above reference line, the pattern of change in R with q for the CH3OCH3-q(OCH3)q series (no. 1 in Table 1; ether nos. 1-4) is such that R increases from 1.18 at q ) 0 to a maximal value of 1.41 at q ) 2 and then falls to a value of 1.08 at q ) 3, as depicted in Figure 6 by the set of four empty circles connected by thin straight lines. This pattern is interpreted to mean that the electronic factor (in this case dipolar and electron-withdrawing) is operating in the same sense as the associative factor to provide two positive influences on R, the sum of which is greater than the negative influence of the steric factor. Molecular models of the three CH3OCH3-q(OCH3)q ethers, made by using Harvard CPK atomic models, show how the volume of effective occupied space above the CH3O group (immobilized by liaison of the ether oxygen atom with the adsorption site) increases with q, which should affect accordingly steric hindrance to adsorption of additional ether molecules at that site. This is especially true for CH3OC(OCH3)3 [ether no. 4], which has a relatively crowded molecular structure above the immobilized portion (CH3O) of the adsorbed ether, such that the nonimmobilized portion [C(OCH3)3] of that ether is forced to project over the adsorption site much beyond the area occupied by the immobilized CH3O groups, thus decreasing R. In contrast the nonimmobilized portions in the cases of the CH3OCH2OCH3 and CH3OCH(OCH3)2 molecules appear to occupy the space directly above the adsorption site without projecting significantly beyond the area already occupied by the immobilized CH3O group. This effect should be enhanced by the affinity of the nonadsorbed CH3O groups for the mobile sorbed-but-notadsorbed molecules at liquid-saturation. Thus the expected magnitude of steric hindrance exhibited by the ethers having the GMS CH3OCH3-q(OCH3)q are in the order q ) 3 . q ) 2 > q ) 1, which is consistent with their relative R-values, as noted above. Subsequent computer modeling studies of these molecules by Dr. John Stevens of the Computational Science Center of 3M Corporate Technical Planning and Coordination, who utilized a Tripos Associates SYBYL 6.0 program run on a Silicon Graphics 340D computer, verified that the aboveassigned configurations are the energetically most probable of the set of possible configurations assuming that one CH3O group is immobilized by adsorption and that the above-stated conclusions follow logically therefrom. In his calculations, the lowest energy molecular conformation in free space was used as the starting point in these considerations. Only the twist angle (from the central carbon atom to the ether oxygen atom that makes liaison with the adsorption site) was distorted slightly to accommodate the hypothesis of immobilization by adsorption of the CH3O group common to the set of three ethers. The pattern of change in R with q for the CH3OC(CH3)3-q(OCH3)q series [i.e., no. 6 in Table 1] is such that R increases very sharply from 0.61 at q ) 0 to a maximal value of 1.26 at q ) 2 and then falls to a value of 1.08 at q ) 3. This pattern (empty stars connected by straight lines in Figure 6) is qualitatively similar to that (empty circles connected by straight lines in Figure 6) noted for the CH3OCH3-q(OCH3)q series because the three factors that affect R in these two series change sequentially with q in the same qualitative sense. The marked quantitative differences between the two patterns as q is incremented from 0 to 3 reflect the differences in the relative negative influences owing to the steric factor as q is incremented from 0 to 3. In the former case, each incrementation involves replacement of a H atom by a OCH3 group, which causes a relatively large increase in steric hindrance, whereas in the latter
Polymer Swelling case it involves replacement of a CH3 group by a OCH3 group, which causes a relatively smaller increase (or even a decrease) in steric hindrance. Consequently, the magnitude of positive ∆R/∆q from q ) 0 to 2 is much greater and the magnitude of negative ∆R/∆q from q ) 2 to 3 is much less in the latter case than are the respective ∆R/∆q noted in the former case. The patterns for change in R, caused by incremental replacement of H by CH3 at a given q, are also consistent with theory based on the expected increase in steric hindrance associated with such replacements. Thus, the R-values for the ethers (CH3O)2CH2-m(CH3)m and (CH3O)3CH1-m(CH3)m decrease with m, as shown by the second and third columns of data points at q ) 1 and 2 scaled on the abscissa of Figure 6. The patterns for the R Vs q relationships for the CH3OCH2-q(OCH3)q(CH2)nH ethers at n ) 1-7 (Figure 7 and series nos. 2 and 4 in Table 2) are similar (but not identical) to those (series nos. 1 and 6) observed at n ) 0 (Figure 6), which indicates that the rationale used to explain qualitatively the observed changes in R with q in Figure 6 also applies to the patterns recorded in Figure 7. The latter show the additional influence of n, which is greater in the change from q ) 0 to 1 than it is in the change from q ) 1 to 2; i.e., the initial increase in q from 0 to 1 produces a relatively large positive ∆ log R given by (0.12 + 0.0057n), but the corresponding subsequent change produces a much smaller ∆ log R given by (0.006 0.0022n), which states that ∆ log R from q ) 1-2 is actually negative for all values of n greater than zero. The above results (Figure 7) show the marked positive effects on R caused by incremental replacement of the hydrogen atoms by methoxy groups on the carbon atom adjacent to the adsorbed ether oxygen atom. The observed positive ∆R/∆q values at constant n (Figure 7) are attributed to the net influence of the change in electronic and associative effects owing to the corresponding changes in molecular architecture, as described above. Comparison of the log R Vs N relationship for the CH3O(CH2)nOCH3 ethers (bold straight line through the filled circles in Figure 8; series no. 10 in Table 2) with those observed for the (CH3O)2CH(CH2)nH ethers (bold straight line in Figure 8; series no. 2 in Table 2) and the reference series CH3O(CH2)nH (bold straight line in Figure 8; series no. R1 in Figure 3) shows that the uniform displacement above the reference line is even more pronounced in the case of series CH3O(CH2)nOCH3 than it is in the case of (CH3O)2CH(CH2)nH. This is interpreted to mean that the positive influence, owing to the expected decrease in steric hindrance effect (associated with the change in location of the methoxy group from the R to the ω position on the nonadsorbed portion of the adsorbed ether), is significantly greater than the negative influence from the expected decrease in the electronic contribution at the adsorption site. This comparison (Figure 8) also illustrates that the identification of the first member of a given homologous series cannot always be made solely on the basis of its GMS. This criterion per se indicates that (CH3O)2CH2 (alternatively written CH3OCH2OCH3) can be taken to be the first member of each of the homologous series CH3O(CH2)nOCH3 and (CH3O)2CH(CH2)nH. The log R Vs N linear relationships determined experimentally for these two series (Figure 8), however, show that the former (log R ) log 1.50 - 0.0429n; n ) 2-12) lies uniformly above that for the latter (log R ) log 1.39 - 0.0464n; n ) 0-8). Since the R-value for (CH3O)2CH2 is only 1.39, it is concluded that this ether is properly the first member of the series (CH3O)2CH(CH2)nH and that CH3O(CH2)2OCH3, the R-value of which is 1.50, is the true first member of series CH3O(CH2)nOCH3.
J. Phys. Chem., Vol. 100, No. 23, 1996 9925
Figure 8. Comparison of the R-values for ethers that comprise homologous series of terminally-substituted polyalkoxyalkanes with those for the corresponding gem-substituted polyalkoxyalkanes.
Comparison of the log R Vs N linear relationships for these two series with that [log R ) log 1.18 - 0.0505(n - 1); n ) 1-8] determined experimentally for the reference series CH3O(CH2)nH (bold straight line in Figure 8) shows that replacement of a H atom on the nonadsorbed portion of the adsorbed ether by a CH3O group affects not only R0 but also D (i.e., ∆R/∆n; eq 1) and that these two constants are also affected significantly by whether such replacements are on the nonadsorbed portion of the adsorbed ether, i.e., either on the carbon atom adjacent to the adsorption site or on the one farthest away from this site. It is inferred from these results that a homologous series of this type is characterized not only by the electronic, steric, and associative factors that remain constant at the carbon atoms adjacent to the adsorption site, but also by the steric and associative factors that remain roughly constant at a carbon atom farther away from this site. In theory, it should be possible to establish the R-values for the ethers that comprise the series having GMS CH3O(CH2)n+1CH3-q(OCH3)q for q ) 2 and 3, in the manner described for series no. 10, with q kept constant at 1 while n is incremented from 0 to 11 (Table 2). In order to do so, however, the R-values at two or more values of n must be established experimentally for each of the series having q > 1. This would enable one to establish the corresponding log R Vs N linear relationships at a given q, from which one could then establish quantitatively how the R Vs q relationships (from q ) 1-3 at a given n) vary with n. Thus far, we have only been able to establish experimentally the R-value (1.20; E 21 in Table 1) for the first member of series CH3O(CH2)n+1CH(OCH3)2. Nevertheless, we can still establish the approximate log R Vs N linear relationships for this series and even that for series CH3O(CH2)n+1C(OCH3)3 if we utilize the data established for CH3O(CH2)n+1CH2OCH3, i.e., series no. 10 in Table 2, as described in the following paragraph. Computer modeling studies of the three ethers CH3OCH2CH3-q(OCH3)q, carried out by Dr. John Stevens in the manner described previously, verified that the magnitude of steric
9926 J. Phys. Chem., Vol. 100, No. 23, 1996 hindrance to adsorption of additional ether molecules should increase with q much more gradually than that noted earlier for the CH3OCH3-q(OCH3)q ethers, which exhibited a very sharp increase in steric hindrance from that at q ) 2 to that at q ) 3, i.e., R in the CH3OCH2CH3-q(OCH3)q series should decrease much more gradually with q, perhaps Via an almost linear log R Vs q relationship, i.e., log Rq ) log Rq)1 - [(log Rq)1 - log Rq)2)/(2 - 1)]/(q - 1). If for the purpose of illustration we tentatively accept this to be the case, then R for CH3OCH2C(OCH3)3 should be about 0.96 (as indicated by the thin straight line in Figure 8 through the log R-values for the three members of this series from q ) 1 to q ) 3). Thus, the R-values for the first members of the three series having GMSs CH3O(CH2)n+1CH3-q(OCH3)q, in which n is varied from 0 to 11 at q ) 1, 2, and 3, are respectively R0 ) 1.5, 1.2, and 0.96, and since the log R Vs n linear relationships for the two series in which q is 2 and 3 should be approximately parallel to that for the q ) 1 series (i.e., D ) 0.043; series 10 in Table 2), the loci for the log R Vs N linear relationships at q ) 2 and 3 should lie between that for the CH3O(CH2)n+1OCH3 series [n ) 0 to 11] and that for the (CH3O)2CH(CH2)nH series (n ) 0-8), as shown approximately by the corresponding dashed lines in Figure 8. In like fashion, it is reasonable to expect that the line for the log R Vs N relationship for the (CH3O)2CH(CH2)nCH(OCH3)2 series (n ) 0-12) should lie slightly above and almost parallel to that for the CH3O(CH2)n+2OCH3 ethers, as indicated in Figure 8 by the dashed straight line that passes through the point Rn+1 ) 0.97 at N ) 11, which had been established experimentally for the second member of the series having GMS (CH3O)2CH(CH2)nCH(OCH3)2, i.e., E 22 in Table 2. Reasoning by analogy with the observed negative influence on R caused by increased steric hindrance when the last hydrogen on a given carbon atom is replaced by a CH3O group (see Figures 6 and 7), it is also reasonable to expect that the line for the log R Vs N relationship for the series (CH3O)2CH(CH2)nC(OCH3)3, in which n ) 0-12, should be uniformally slightly below that deduced for the (CH3O)2CH(CH2)nCH(OCH3)2 series and that the loci of the data points that comprise the log R Vs N linear relationship for the corresponding (CH3O)3CH(CH2)nC(OCH3)3 series in turn should be below that for the (CH3O)2CH(CH2)nCH(OCH3)3 series, as indicated in Figure 8. Interpretation of the Data for Ethoxy-Substituted Alkanes. Our earlier studies involving sorption of H(CH2)mOCH2-q(CH3)q(CH2)nH have shown9 that the negative effect on R per added methylene group increases markedly with m. It is reasonable to assume, therefore, that the same should be true with the polyalkoxyalkanes, i.e., the R-values for polyethoxyalkanes should be markedly less than those for the corresponding polymethoxyalkanes. This is found to be the case. The R-values established for the monoethoxy-substituted alkanes having the GMS CH3CH2OCH2-q(CH3)q(CH2)nH are reported in Table 5 of ref 10. The decrementation constants (D; eq 1) for the log R Vs N lines for those series having q ) 0, 1, and 2 are 0.0505 ( 0.0001, and the respective R0-values at n ) 0 are 0.75, 0.50, and 0.33; the decrementation constants for the associated cross-over relationships [i.e., q varied from 0 to 2 at constant n (from 0 to 6)] are 0.1781 ( 0.0003. Although these plots are not shown in Figure 5, out of concern for clarity of presentation, the above data for the ethoxy ethers indicate that these curves would be located in an area of Figure 5 parallel to and just below the log R Vs N linear relationship for the ethers CH3OCH(CH3)(CH2)nH from n ) 0-7. In contrast, the log R Vs N linear plots for the polyethoxyalkanes CH3CH2OCH2-q(OCH2CH3)q(CH2)nH, in which n is incremented from 0 to 7 at
Errede and Tiers
Figure 9. R Vs q plots that show the effect on R caused by systematic incrementation of q from 0 to 2 for series having the GMS CH3CH2OCH3-q(OCH2CH3)q(CH2)nH and for which n is kept constant at 0-8.
q ) 1 and 2 (series 8 and 9 respectively in Table 2), are located in the area of Figure 5 between that for the corresponding polymethoxyalkanes and that for the reference base (bold straight lines in Figure 5). In addition to the positive displacement in ∆R, owing to replacement of H by OCH2CH3, it is noted that the decrementation constants (D; eq 1) for the log R Vs N lines that represent the two polyethoxyalkane series at q ) 1 and 2 (i.e., D ) 0.0609 and 0.1165, respectively, in Table 2) are progressively greater than that (D ) 0.0505) for the reference series. This trend is opposite to that noted earlier for the polymethoxy-substituted alkanes. These two observations show that the electronic, steric, and associative factors interact to affect the value of R for the polyethoxy-substituted alkanes in a somewhat more complex way than that for the corresponding polymethoxy-substituted alkanes. The manner in which R is affected by the interplay of these factors with incremental changes in q can be understood more easily when one considers the R Vs q relationships for the CH3CH2OCH2-q(OCH2CH3)q(CH2)nH ethers at constant n, as shown in Figures 6 and 9. If one considers first the R Vs q relationship at n ) 0 [i.e., progressive replacement of H by (OCH2CH3) in CH4-q(OCH2CH3)q, which is series 7 in Table 1], the R-values for the first three members (empty diamonds in Figure 6) appear to fall on a line that is uniformly below that which represents the R Vs q relationship for the ethers having the GMS CH3OCH3-q(CH3)q [filled circles connected by bold straight lines in Figure 6]. This result presumably is fortuitous because our earlier studies9 have shown that the adsorbed portion of the immobilized ether when q is 1 (i.e., CH3OCH2CH3) is the CH3O group, whereas it is the CH3CH2O group when q is 2, 3, or 4. The observed ∆R, produced by incrementation of q from 1 to 2 in the series having GMS CH4-q(OCH2CH3)q, must reflect the net result not only of the stated incrementation in q but also a change in molecular
Polymer Swelling structure of the adsorbed portion from CH3O to CH3CH2O. From the standpoint of adsorption, this means that the member having q ) 1 is not properly in the same series as that comprised of the three members having q ) 2, 3, and 4. The latter subseries, therefore, should be represented instead by CH3CH2OCH3-q(OCH2CH3)q in which q is now 1, 2, and 3. Consequently, the line connecting the R-values for CH3OCH2CH3 and CH3CH2OCH2OCH2CH3 is dashed in Figure 6 to emphasize that CH3OCH2CH3 is not a true member of this adsorption series, despite that it is a member of the union set of four having GMS CH4-q(OCH2CH3)q. From the standpoint of adsorption, the difference in R from that (0.75) for CH3CH2OCH2CH3 to that (0.86) for CH3CH2OCH2OCH2CH3 (also represented by a dashed straight line in Figure 6) has more meaningful physical significance than the difference discussed above, because it shows the positive ∆R caused by replacement of a CH3 group by the OCH2CH3 group, despite the increase in mass that normally causes a negative change in R. This is interpreted to mean that the combined positive contributions of the electronic and associative factors is again greater than the negative contribution owing to increased steric hindrance caused by changes in the molecular architecture in the nonadsorbed portion of the adsorbed ether. The magnitude of this difference, however, is less than that noted earlier for the corresponding changes in the case of the methoxy ethers. This net difference in the positive influence on R observed for these two series therefore is attributed to the expected increase in steric hindrance owing to the greater bulk of an ethoxy group relative to that of a methoxy group. Molecular modeling considerations of the three members that do comprise the series represented by CH3CH2OCH3-q(OCH2CH3)q confirm that the magnitude of steric hindrance should be greater than that observed for the corresponding methoxy ethers, and they also indicate that the magnitude of steric hindrance in the case of the ethoxy ethers should increase monotonically with q from 1 to 3 in such a way that the R-values for these ethers could fall on a line that is almost straight, as indicated by the dashed extension of the line that extends from the R-value for the first (q ) 1) to the second (q ) 2) member of this series. If this is indeed the case then the R-value for the ether having the molecular structure C(OCH2CH3)4 in series 7 (Table 1) should be between 0.55 and 0.45. The R Vs q relationships for the CH3CH2OCH2-q(OCH2CH3)q(CH2)nH ethers at n ) 0-7 are shown in Figure 9. Again, the data established experimentally are represented by filled circles, whereas those established by interpolation or extrapolation are represented by empty circles. The data point for the ether having q ) 0 at n ) 0 is omitted for reasons discussed above. The line that connects the data points for the ethers having q ) 1 and 2 at n ) 0 permits proper orientation of Figure 9 with Figure 6, which considers the R Vs q relationships for all of the alkoxy ethers in this category having n ) 0. It is noticed that the R-values for the ethoxy ethers ranged from ca. 0.8 to ca. 0.1 (Figure 9), whereas those for the corresponding methoxy ethers range from ca. 1.4 to ca. 0.6 (Figure 7). This uniform difference reflects the relative “bulkiness” of the adsorbed portion (R′O) of the adsorbed ether (R′OR), i.e., CH3CH2O Vs CH3O. Comparison of Figure 9 with Figure 7 also shows clearly the opposing effects accompanying progressive replacement of H atoms by alkoxy groups in the nonadsorbed portion of the ether adjacent to the adsorption site. In the cases of the polymethoxy-substituted alkanes (Figure 7), the positive contributions owing to the electronic and associative factors dominate over the negative steric factor, such that the R-values
J. Phys. Chem., Vol. 100, No. 23, 1996 9927 for ethers at constant n having q > 0 are greater than that having q ) 0, whereas in the cases of the polyethoxy substituted alkanes (Figure 9) the negative steric factor dominates over the positive contributions from the electronic and associative factors, such that the R-values for ethers at constant n having q > 0 are usully less than that having q ) 0. Again the magnitude of these opposing effects varies with n in the nonadsorbed portion of the ether CH3CH2OCH2-q(OCH2CH3)q(CH2)nH. Thus, the ∆(log R)/∆N Vs n relationship for the change in q from 0 to 1 is given by ∆(log R)/∆N ) 0.030 - 0.015n, and that for the change in q from 1 to 2 is given by ∆(log R)/∆N ) -0.12 - 0.051n. By way of contrast, those deduced for the corresponding polymethoxy-substituted alkanes (Figure 7) are respectively (0.12 + 0.0057n) and (0.006 0.0022n). It is reasonable to expect, therefore, that the dominance of the steric factor should increase with the mass of the alkoxy substituent replacing the hydrogen atoms on the carbon atom of the nonadsorbed portion adjacent to the adsorption site; i.e., the order should be O(CH2)n>3H > O(CH2)3H > O(CH2)2H > OCH3. Unfortunately, examples of such ethers having n > 2 are not yet available to us to permit a test of this expectation. Comparison of the R-value for (CH3CH2O)2CHCH3 (R ) 0.76; series 8, ether 14 in Table 1) with that (1.02) for CH3CH2OCH2CH2OCH2CH3 (series 11; ether 21 in Table 1) again shows that a rearrangement in molecular structure from a gemalkoxy to a terminal alkoxy architecture without change in mass (i.e., ∆N ) 0) causes a marked increase in R, as noted earlier with the corresponding polymethoxy-substituted alkanes. The magnitude of the increase in the ethoxy case (∆R ) 0.26) is about the same as that (∆R ) 0.25) observed for the corresponding structures in the methoxy case [i.e., R ) 1.25 Vs R ) 1.50 respectively for the ethers (CH3O)2CHCH3 and CH3OCH2CH2OCH3 (i.e., ethers 5 and 18 in Table 1)], which suggests that the effect on R caused by such changes in molecular architecture without change in mass (i.e., ∆N ) 0) may be characteristic of that architectural change. Summary and Conclusions The above results support the conclusions deduced from our earlier studies involving swelling of poly(Sty-co-DVB) to saturation in polyhaloalkane liquids having the GMSs ZCH3-zZz and ZCH2-zZz(CH2)nH in which Z is a phenyl or halogen atom (see Figures 2, 4, and 5 of ref 6), namely, that the mode of adsorption of such polyfunctional sorbed molecules to the phenyl groups of the sorbent polymer at liquid-saturation is monodentate rather than polydentate; i.e., only one of the total number of Z substituents in a given molecule is immobilized by adsorption; the rest are involved in dynamic associative interactions with the sorbed-but-not-adsorbed molecules of its own kind contained in the liquid-saturated polymer system. The primary factor that determines the value of Rz in a given homologous series is the electronic character and bulkiness of the adsorbed substituent Z. This is affected in turn by the electronic, steric, and associative contributions of the components that comprise the molecular structure of the “nonadsorbed” portion of the sorbed molecule. Thus, systematic incremental replacements of hydrogen atoms by Z-substituents at the carbon atom adjacent to the adsorption site need not afford linear log Rz Vs z relationships. Such homologous series exhibit instead a maximum at z ) 2 or 3 depending on the rest of the molecular architecture, as recorded in Figures 5-9. Linear log Rz Vs z relationships at constant n, as well as log Rn Vs n relationships at constant z, are exhibited, however, by series such as Z(CH2)nCH3-zZz. In these cases, the Z-substituent
9928 J. Phys. Chem., Vol. 100, No. 23, 1996 immobilized by adsorption is the one at the less substituted end. This serves to minimize steric hindrance and to maximize “verticality”, owing to improved affinity for the mobile sorbedbut-not-adsorbed molecules of its own kind in the polymerliquid system at gel saturation. In this respect the log Rn Vs n lines for ZCH2-zZz(CH2)nH liquids lie uniformly below those for the corresponding Z(CH2)nCH3-zZz liquids. This reflects the net change in steric, electronic, and associative influences caused by the shift in Z-substituents from the R to the ω position. Thus, the plots in Figure 8 of log Rn Vs N for (CH3O)2CH(CH2)nH (series 2) and for CH3O(CH2)nOCH3 (series 10) show that the former is uniformly below the latter. This comparison shows clearly that from the standpoint of adsorption, the first member of series 2 (i.e., CH3OCH2OCH3) should not be a member of series 10, despite that from the standpoint of structure it might be so included. Figure 8 shows, furthermore, that the first member of series 10 must be CH3O(CH2)2OCH3, because the methoxy group is no longer located on the carbon atom adjacent to the adsorption site where it can affect the electronic and steric character at that site. The data accumulated in this investigation can now be used as a reference base to help deduce the log Rn Vs N linear relationships for the more complicated sets of homologous series that comprise the dialkoxy-terminated telomers of poly(ethylene oxide), i.e., those having the GMSs H(CH2)mO(CH2CH2O)pCH2-q(CH3)q(CH2)nH, in which one of the four variables (m, n, p, and q) is incremented systematically while the other three are held constant. This work is in progress.
Errede and Tiers Acknowledgment. We thank Dr. John Stevens of the Computational Science Center of 3M for performing computational modeling studies regarding the geometry of adsorption of several key ether molecules, as described in the text. References and Notes (1) Errede, L. A. J. Phys. Chem. 1989, 93, 2668. (2) Errede, L. A. J. Phys. Chem. 1990, 94, 466. (3) Errede, L. A. J. Phys. Chem. 1990, 94, 3851. (4) Errede, L. A. J. Phys. Chem. 1990, 94, 4338. (5) Errede, L. A. J. Phys. Chem. 1991, 95, 1836. (6) Errede, L. A. J. Phys. Chem. 1992, 96, 3537. (7) Errede, L. A. Molecular Interpretations of Sorption in Polymers. In AdVances in Polymer Science; Saegusa, T., Ed.; Springer Verlag: BerlinHeidelberg, 1991; Vol. 99. (8) Errede, L. A. J. Appl. Pol. Sci. 1992, 45, 619. (9) Errede, L. A. J. Phys. Chem. 1994, 98, 8580. (10) Errede, L. A. Polymer Swelling. Part 17: Systemization of steric effects on adsorption parameter as exemplified by swelling to saturation using linear and non-linear acyclic aliphatic ethers. In AdVances in Colloid and Interface Science; Berg, J., Eds.; Elsevier Science: Amsterdam, 1995; Vol. 60, pp 119-198. (11) Errede, L. A.; Henrich, P. J. Polymer Drying, Part 11: Desorption studies using poly(styrene-co-divinylbenzene) that had been swelled in chloromethanes. In AdVances in Colloid and Interface Science; Berg, J., Ed.; Elsevier Science: Amsterdam; Vol. 60, in press. (12) Errede, L. A.; Stoesz, J. D.; Sirvio, L. M. J. Appl. Pol. Sci. 1986, 31, 2721. (13) Errede, L. A. J. Appl. Polym. Sci. 1986, 31, 1749. (14) Tiers, G. V. D., to be submitted for publication.
JP9605921
