2054 Macromolecules 1994,27, 2054-2057

Effects of Three-Segment Interactions on the Temperature Dependence of the Third Virial Coefficient for Flexible Chains near the 8 Point

Takashi Norisuye' and Yo Nakamura

Department of Macromolecular Science, Osaka University, Toyonaka, Osaka 560, Japan

Received October 4, 1993; Revised Manuscript Received December 16, 19938

ABSTRACT The third virial coefficient As of long flexible chains with three-segment interactions is evaluated perturbatively to third order in the ternary segment cluster integral 8 3 to explain the recent experimental finding on polystyrene and polyisobutylene that A3 has a positive minimum around the 8 temperature (where the second virial coefficient A2 is zero) in contrast to the prediction from the two-parameter theory. The calculation indicates that A3 near the 0 point is determined by two functions. One corresponds to the two-parameter theory, but the binary cluster integral 82 is replaced by an effective binary cluster integral A, i.e., by a linear combination of the binary and ternary cluster integrals. The other is a decreasing function of be, multiplied by 83, and it reduces to 8 3 at the 8 point. The sum of these two functions is shown to give As a positive minimum around the 8 temperature, in accord with the experimental finding mentioned above. Thus, the experimentally observed increase in A3 with a decrease in temperature below 8 is due to a marked effect of three-segment interactions. I t is also shown that the second virial coefficient, which is readily derived from the calculation of A3 up to second order in 82 and 83, contains no separate term dependent on 8 3 and is a function of up to at least second order. This indicates that in practice the binary cluster approximation holds for A2 of long flexible chains near 0 even when the ternary cluster contribution is not negligible.

Introduction

The two-parameter theory1 predicts that the third virial coefficient A3 for linear flexible chains vanishes at the 8 point and becomes negative below it. In contrast to this prediction, our recent light scattering studies on poly- styrene2 and polyisobutylene3 in 0 solvents showed that A3 remains positive at T = 8 and rather increases when the temperature Tis lowered below 8. Here, the 8 point is defined as the temperature at which the second virial coefficient A2 vanishes. The apparent breakdown of the two-parameter theory for A3 near 8 is most likely due to the neglect of three-segment interactions.

In the present work, with such interactions taken into account, we carried out elementary perturbation calcula- tions on A3 of linear flexible chains to third order in the ternary segment cluster integral 83, hoping to explain the temperature dependence observed for polystyrene and polyisobutylene near the 8 temperature. The calculation was confined to infinitely long chains that obey the Gaussian statistics in the unperturbed state.

Perturbation Calculations

If both 8 2 (the binary cluster integral) and 8 3 are vanishingly small, A3 in the superposition approximation' may be expanded in powers of 8 2 and 83 as

Here, M is the molecular weight of each chain containing n segments and N A is the Avogadro constant. The leading term results from4v5

Abstract published in Advance ACS Abstracts, March 1, 1994.

0024-929719412227-2054$04.50/0

where P(O!ih3) represents the probability density that segment il in chain 1, segment j 2 in chain 2, and segment k3 in chain 3 are in contact. The coefficient J1 corresponds to the first-order term in the binary cluster approximation. I t is already known to be617

Xl(3/2ab2)3/2n3/2 (2)

with

A, = 1.664 (3)

and b the effective bond length. We evaluated 11, I2, Jz, J3, and J4 in the limit of infinite

n, with sums replaced by integrals and with a cutoff parameter a (n >> a > 0) introduced for mathematical tractability. The results read

6C1 (31 2 ~ b ~ ) ~ / ~ n ~ / ~ (4)

. . . . . . .

3Xl(4/~1/2)(3/2~b2)3n3/2 (6)

0 1994 American Chemical Society

Macromolecules, Vol. 27, No. 8, 1994 A3 for Flexible Chains near the 8 Point 2055

where C1 is given by

C , = 2.865 (9)

which is the well-known coefficientl~~ of the double-contact term of A2 in the two-parameter theory. Two points should be noted here. First, intramolecular excluded volume does not affect I l , I 2 , Jz, J 3 , and J 4 in the limit of infinite n. Second, J 2 , 53, and J 4 originally contain many terms associated with Za~mZP(Oij2~8r..) including intramolec- ular excluded-volume contributions such as n-3Z...

[P(Oij2k,,0111tZ,Oulvl) - P(Oij2k,,Oslt2)~(Oulul) I , but these terms give values of order n (see eq 11).

Substituting the above expressions for 11, ..., J 4 into eq 1, we obtain

where

and

2 = ( 3 / 2 ~ b ~ ) ~ / ~ @ , n ~ / ' (13)

with 0, an effective binary cluster integral defined by

j3, = Pz + ( 4 / ~ ' / ~ ) ( 3 / 2 ? r b ~ ) ~ / ~ 8 ~ (14)

The last term O(22) in eq 11 comes from J 2 , J 3 , and J 4 all containing n-3 Z...ZP(Oij2k,,...) and is always positive. Though, in the limit of infinite n, this term can be neglected in comparison to X1(2?rb~/3)~fl, it is important to note that H(Z) arises necessarily from the configurations with contact of three segments il, j 2 , and k3 in the three different chains and is a 2-expansion different in order of n from K(2) . The leading term of the latter [i.e., X1(2~b~/3)~Z3] with 8, = 8 2 is just the first-order term in the binary cluster approximation!J whereas j33H(Z) is completely ignored in this approximation. With regard to the effective binary cluster integral, the following remarks may be in order.

If the sums in eqs 5-8 are asymptotically evaluated, it is found that 2 d 2 is replaced by {(3/2) (=2.612), with { ( x ) being the Riemann f function of variable x . Thus, in the discrete random flight chain model, u is slightly smaller than unity. However, a value much larger than unity must

be taken for it8 when chain stiffness is incorporated with the wormlikegJ0 or helical worm1ike1lJ2 bead model. In all cases, the effective binary cluster integral very near 8 is a sum of 182 and CB3, though the constant C differs depending on the model or the mathematical approxima- tion employed. Since u does not explicitly appear in eqs 10-13, its precise value is unnecessary for the present purpose.

The present calculation also gives A2 for infinitely long chains up to second order in both PZ and (33, since the basic equation for A3 contains 4Az2M (see eq 22.5 of ref 1). The first-order terms of PZ and P3 are already kn0wn~1~J~ but the second-order calculation is new. The result obtained is written in the form

h(2) = 1 - C'Z + ... (16)

Importantly, eq 15 with eq 16 and @e = @Z is identical to the familiar expression in the two-parameter theory' at least up to O(Pe2), showing that the 8 point for sufficiently long chains is the temperature at which b e vanishes. Under this condition, eq 10 reduces to

A, = (Npn3/3M3)p3 (at the 8 point) (17)

so that a positive A3 at the 8 temperature corresponds to a positive P3. In previous w ~ r k , ~ ~ ~ we used this equation to estimate P 3 from A3 data for polystyrene and poly- isobutylene at the 8 temperature. Another point to note is that there is a close relation between H ( 2 ) and h(Z) for very small 2. In fact, n-3 C...CP(OjjakS,Osltlu2) in eq 5 becomes identical to n-, C...ZP(Oij2,0slt1,J appearing in the &P3 term of A2 (not shown here) when the sum over k3 is taken; the factor 6 inH(Z) arises from a larger number of distinguishable configurations available for 3 chains.

Discussion For convenience of our discussion, the previously

reported A3 data for different samples of polystyrene in cyclohexane (8 = 34.5 "Q2 and of polyisobutylene in isoamyl isovalerate (8 = 27 "CI3 are reproduced as functions of T i n Figures 1 and 2, respectively. For both polymers the curve of A3 vs T is nearly parabolic with a broad, positive minimum around 8 and becomes almost horizontal as M decreases. Thus A3 below 8 is strongly M-dependent, and, in particular, its rise with a decrease in T is opposite to the two-parameter theory prediction, as already mentioned in the Introduction. Our purpose here is to explain these features of the A3 vs T curves on the basis of eq 10.

If P 2 varies linearly with T1 and if 8 3 is independent of T, the effective binary cluster integral defined by eq 14 may be expressed ad4

(18)

where PO is a constant independent of T. This equation with be = P2 is the one usually assumed for data analysis within the framework of two-parameter theory. When j30 is positive (as is the case for the two polymer + solvent systems in Figures 1 and 2), Be and hence Z are negative for T < 8, so that for a positive 03, P3H(Z) in eq 10 is positive below 8 and is a sharply decreasing function of T. Equation 10 indicates that A3 near 8 is a sum of this function and X1(2?rb2/3)3Z3, corresponding to the leading term in the two-parameter theory. Since the latter is an

P, = o0[i - (e/ni

2056 Norisuye and Nakamura Macromolecules, Vol. 27, No. 8, 1994

10

10 .a 1:1-, ~ n, ~

~ 2; 0 25 30 35 40 45

T I ' C

Figure 1. Temperature dependence of A3 for polystyrene samples in cyclohexane (0 = 34.5 OC).

15

10

15 5 L

:: 0 20 25 30 35 40

TI'C Figure 2. Temperature dependence of A8 for polyisobutylene samples in isoamyl isovalerate (e = 27 "C). increasing function of T giving zero at T = 8, the sum of these two functions opposite with respect to a change in T may give A3 a positive minimum somewhere around 8. It may also be anticipated that since 14 stays smaller for a smaller n in a given T range, the temperature dependence of A3 should become weaker as M decreases. These are our interpretation of the features of the experimental A3 vs T curves in Figures 1 and 2 and are shown below to be the case in a semiquantitative way by using appropriate closed expressions for H(Z) and K(Z) .

Since H ( Z ) for small Z has a close relation to the h function appearing in A2, it may fairly well be approxi-

10 - 5 -

10 - 5 -

IS

:: 0 -

E 10':

?J =. 5 - v

10 O l 997

10 O I

5.07

1 1 207

01 25 30 35 40 45

T i *C Figure 3. Theoretical curves of AS vs T calculated from eq 10 with eqs 13,18,19, and 20 for polystyrene in cyclohexane. See the text for the parameter values used.

mated by h(6Z) in the vicinity of the 8 point. We adopt the Casassa-Markovitz h functionls in the two-parameter A2 theory, assuming, for simplicity, that the expansion factor for the chain expansion due to intramolecular excluded-volume effects is equal to unity over the ranges of T and M of interest, i.e.,

1 - exp(-12C1Z) 12c,z H ( Z ) =

This gives the correct first-order coefficient 6C1 in eq 11. Its advantage is that it has a solution for negative 2, differing from other available h functions.1J6-18 For K(Z) , we use Yamakawa's two-parameter theory' for As, with the expansion factor again assumed to be unity. We thus have

K ( Z ) = [I - e x p ( - ~ , Z ) ~ ~ l - exp(-2C,~)1~/(2C,)~ (20)

Though the Yamakawa theory overestimates A3 in good so1vents,19-21 it gives the correct coefficient A1 at small 2 and has a solution for Z < 0.

Figures 3 and 4 show the curves of As vs T computed as functions of A4 from eq 10 with eqs 13,18,19, and 20 using b = 0.74 nm, = 7.2 X lez3 cm3, and p3 = 4 X 10-45 cms for polystyrene in cyclohexane and b = 0.58 nm, j30 = 1.4 X 10-23~m3,and83 = 7 X lVcm6forpolyisobutylene in isoamyl isovalerate. The values for DO have been taken from the paper of Miyaki and Fujita22 and those for j33 from our previous papem2J It can be seen that the calculated curve for the highest molecular weight sample of either polymer has a pronounced minimum around 8 (more precisely, at a T slightly higher than 81, rising with decreasing T below 8. As M decreases, the curve becomes nearly flat. These features of the theoretical curves closely resemble those observed in Figures 1 and 2. We may therefore conclude that the experimentally observed increases in A3 with lowering T below 8 are due to the

Macromolecules, Vol. 27, No. 8, 1994 A3 for Flexible Chains near the 8 Point 2057

consideration of /33 gives a consistent explanation of an, A2, and A3 for long flexible chains. A similar conclusion was drawn recently by Yamakawa? who explicitly con- sidered effects of chain stiffness on an and A2 (see ref 25 for A3 in good solvents). He further argued that all terms associated with 83 may be neglected compared to ,i32 as far as as and A2 are concerned. This is the case for stiffchains? but the remarks made below for usual flexible chains supplement his argument for the ternary cluster contri- bution to /30 (the temperature-independent part of Be) .

Yamakawas estimated this contribution to be at most 5% for polystyrene in cyclohexane by taking account of the chain stiffness with the helical wormlike chain model.11J2 For polyisobutylene in isoamyl isovalerate,3 the corresponding contribution is estimated to be 1055, which is somewhat larger than the value for polystyrene, probably because the polyisobutylene chain is more flexible.2s These contributions are small (compared to &), as argued by Yamakawa? but they cannot be ignored compared to 82 or /3e in the vicinity of the 8 temperature, especially when /32 approaches zero at a certain temperature close to 8. The point we wish to make here is that, since the sum of the binary and ternary cluster contributions, Le., &, is the key parameter in the vicinity of the 8 point, the magnitude of the ternary cluster contribution to /3e is immaterial for properties of long flexible chains other than AB. As the present study shows, onlyA3 contains a separate function of 83, i.e., /3a(Z) in eq 10.

References and Notes (1) Yamakawa, H. Modern Theory of Polymer Solutions; Harper

(2) Nakamura, Y.; Norieuye,T.;Teramoto,A. Macromolecules 1991,

(3) A h a k a , K.: Nakamura, Y.: Norisuve, T.: Teramoto. A. Polvm.

& Row: New York, 1971.

24, 4904.

M 110'

180

O L

20 25 30 35

T I C Figure 4. Theoretical curves of AS vs T calculated from eq 10 with eqs 13, 18, 19, and 20 for polyisobutylene in isoamyl isovalerate. See the text for the parameter values used.

nontrivial contribution of /3a(Z) ignored in the binary cluster approximation. This is probably the first exemp- lification of a remarkable effect of three-segment inter- actions on dilute solution properties.

A few theoretical curves for lower molecular weights in Figures 3 and 4 continue to decline gradually with increasing T even above 8. Though this behavior differs somewhat from the experimental dependence, the dif- ference does not seem serious in terms of the present semiquantitative analysis. Actually, the declines of the theoretical curves for T above 8 are a reflection of the fact that the calculated A3 for high M has a minimum at a T slightly above 8, and thus any of the theoretical curves rises at high T where ( 2 ~ b ~ / 3 ) ~ K ( Z ) surpasses /3a(Z). The contribution of the latter to A3 indeed diminishes to less than 10% for either of the polymers when Z exceeds 0.2.

Concluding Remarks The present calculation of A3 for long flexible chains

with three-segment interactions explains at least quali- tatively the observed temperature dependence of A3 for polystyrene2 and polyisobutylene3 in 8 solvents, demon- strating that the ternary cluster interaction plays an essential role in A3 near the 8 temperature. It is thus legitimate to consider effects of 163 on other dilute-solution properties of flexible chains.

Equation 15 with eq 16 shows that A2 near 8 has no separate term associated with 03 and is determined only by the effective binary cluster integral This is also the case for the expansion factor14 of the end-to-end distance up to 0(Be2) (and probably for the radius expansion factor 4 . 2 3 These theoretical results indicate that the binary cluster approximation should be accurate enough for A2 and as of long chains provided 82 is replaced by be." This explains why three-segment interactions need not be considered explicitly for these properties. In other words,

- . . . . . . J. 1994, 26,363.

(4) Z i . B. H. J. Chem. Phvs. 1946.14.164. (5j Cherajril, B. J.; Douglas, J.F.; Freed, K: F. J. Chem. Phys. 1985,

(6) Stockmayer, W. H. Makromol. Chem. 1960,35,54. (7) Yamakawa, H. J. Chem. Phys. 1965,42,1764. (8) Yamakawa, H. Macromolecules 1993,26, 5061. (9) Kratky, 0.; Porod, G. Recl. Trau. Chim. Pays-Bas 1949, 68,

(10) Yamakawa, H.; Stockmayer, W. H. J. Chem. Phys. 1972,57,

(11) Yamakawa, H. Annu. Rev. Phys. Chem. 1984,35,23. (12) Shimada, J.; Yamakawa, H. J. Chem. Phys. 1986,85,591. (13) Yamakawa, H. J. Chem. Phys. 1966,45,2606. (14) Norisuye, T.; Nakamura, Y. Polymer 1993,34, 1440. (15) Casassa, E. F.; Markovitz, H. J. Chem. Phys. 1958,29,493. (16) Tanaka, G.; golc, K. Macromolecules 1982, 15, 791. (17) Barrett, A. J. Macromolecules 1985, 18, 196. (18) Freed, K. Renormalization Group Theory of Macromolecules;

(19) Nakamura, Y.; Norisuye, T.; Teramoto, A. J. Polym. Sci., Part

(20) Norisuye, T.; Fujita, H. ChemTracts-Macromol. Chem. 1991,

(21) Nakamura, Y.; Akasaka, K.; Katayama, K.; Norisuye, T.;

(22) Miyaki, Y.; Fujita, H. Macromolecules 1981, 14, 742. (23) The second-order calculation is yet unavailable. (24) According to Yamakawa? the constant C in the equation 8. -

82 + C& for CY, differs from that for A2 when chain Stiffness is introduced, but the difference may give no serious problem.

(25) Norisuye,T.; Nakamura, Y.; Akasaka, K. Macromolecules 1993, 26, 3791.

(26) A h , F.; Einaga, Y.; Yamakawa, H. Macromolecules 1991,24, 4423.

83, 5293.

1106.

2843.

Wiley-Interscience: New York, 1987.

B Polym. Phys. 1991,29, 153.

2, 293.

Teramoto, A. Macromolecules 1992,25,1134.