614
J. Phys. Chem. 1994, 98, 674-684
Hydrogen/Deuterium Isotope Effects on Microphase Separation in Unstable Crystalline Mixtures of Binary *Alkanes R. G. Snyder,' V. J. P. Srivatsavoy,t D. A. Cates, and H. L. Strauss Department of Chemistry, University of California, Berkeley, California 94720
J. W. White The Research School of Chemistry, The Australian National University, Canberra, Australia
D. L. Dorset Medical Foundation of Buffalo, Buffalo, New York 14203-1 196 Received: September 24, 1993'3
We have observed large hydrogen-deuterium isotope effects on the microphase separation that occurs in unstable crystalline binary mixtures of n-alkanes rapidly quenched from the melt. These effects were studied in C30/C36 mixtures at room temperature, mainly by infrared spectroscopy and electron diffraction. The isotope effects are manifest in a number of different ways: (i) immediately after the quench, the D / H (C3OD/C3aH)mixture is significantly more demixed than the H / D mixture; (ii) the spontaneous demixing, which begins immediately after the quench to room temperature, is faster for D / H by a factor of about 3; (iii) the C3aH domains in the D / H mixture grow at a notably faster rate than those of C30Dand at a faster rate than the domains of either component in the H / D mixture. These effects can be largely accounted for in terms of molar volume differences between the hydrogenated and deuterated n-alkanes that constitute the mixtures. Similar isotope effects are found to occur in other binary n-alkane mixtures.
I. Introduction In 1980 Buckingham and Hentschell concluded that some degree of isotopic phase separation was possible in amorphous mixtures of the hydrogenated (H) and the deuterated (D) forms of a high molecular weight polymer. Such separation was subsequently reported for amorphous poly(butadiene).* In addition, significant differences in miscibility between H / H and H / D combinations of amorphous mixtures of chemically dissimilar polymers have also been r e p ~ r t e d . ~Although .~ questions of interpretation and of the universality of this phenomenon are still to be settled,4-5the existenceof an isotope effect on the degree of segregation for high molecular weight polymers mixtures is now well established. To our knowledge, however, there have been no reports of similar phenomena for mixturesof chains of low molecular weight. A priori, this would not seem surprising since the treatment of Buckingham and Hentschel predicted that mixtures of the H and D forms of a given n-alkane will not phase separate and certainly not if the chains have fewer than 100 carbons. We report here an unexpected hydrogen/deuterium effect on the phase separation in an unstable crystalline binary mixture of n-alkanes of unequal length, namely C30/c36, the chains of which are decidedly of low molecular weight. This effect became apparent during our earlier studies of microphase separation using vibrational spectroscopy,68 electron diffraction and calorimetry,6,9JO and small-angle neutron scattering.11 It was manifest in several ways, all associated with the greater degree of separation in the binary n-alkane mixture D/H (C3)D/C36H)relative to the isotopicallycomplementary mixture H/D (C#/C36D). (Henceforth, the designation 'H/D" and "D/H", otherwise unspecified, will refer to the C ~ O / mixture.) C~~ Unstable crystalline solid solutions of & / e 3 6 are formed by quickly quenching the melt to room temperature. The initial t Present address: Institut de Chimie Physique, EPFL-DC, CH-101s Lausanne, Switzerland. Abstract published in Advance ACS Abstracrs, December 15, 1993.
0022-3654/94/2098-0674S04.50/0
state of the mixture just after the quench is that of a crystalline solid solution in which the chains are all-trans, except possibly for some conformational disordering at their ends. The C30 and C36 chains are more or less randomly mixed. However, both the H / D and D/H crystals maintain lamellar structures with a orthorhombic perpendicular subcell, essentially the structure found for the pure components. While the chains in some pure n-alkanes crystals are tilted, those in the mixtures appear not to be tilted. As a result of the demixing, two kinds of domains emerge, one rich in Cpochainsand the other in C36chains. Because the growth rate of the domains at room temperature is such that it can be measured in real time by vibrational spectroscopy,lateral growth can be monitored quantitatively, provided one component is hydrogenated and the other deuterated. This condition is met for both the H/D and D/H mixtures, making the present study possible. These isotope effects occur for the c30/c36 mixture over a wide rangeof the component concentration. Similar effects have also been found to occur in other n-alkane mixtures such as C32/ c36."
In the present study we have used mainly infrared spectroscopy and electron diffraction. The results of a low-angle neutron scattering study on C30/C36 are presented in a separate paper." 11. Methods
A. Infrared Spectroscopy. 1, Experimental Procedures. Infrared spectra were measured with an evacuatable Nicolet Model 8000 FTIR spectrometer equipped with a cooled MCT infrared detector. A resolution of 2 cm-l was used. A fuller description of the instrumentation and its use is given in ref 7. Thin films of the binary mixtures suitable for infrared transmission were prepared by first placing a small amount of the melt-crystallized mixture on the surface of a horizontallyheld potassium bromide window. The window was then heated until the solid melted. The melt was maintained on the window for at least 10 min. Then, a second window, at room temperature, 0 1994 American Chemical Society
Unstable Crystalline Mixtures of Binary n-Alkanes was placed over it, and the resulting sandwich was cooled to ambient temperature, normally 23 f 1 OC. The infrared spectrum of the mixture was thereafter measured at various times. 2. Determination of Domain Size. Average domain size was determined from the infrared spectrum in the manner described in our earlier ~ t u d i e s . ~The , ~ determination is based on the magnitude of the experimentally measured frequency separation between the component bands of the crystal-split infrared-active methylene (CH2 or CD2) scissors mode. The splitting increases as the size of the domains increases. The average domain size is found from the magnitude of the splitting, which is first normalized by dividing it by the splitting observed for the pure n-alkane. The normalized splitting is converted to domain size with the use of the calibration curve reported in ref 7. Average domain size is expressed as a lateral dimension, L,given in terms of number of chains. The total number of chains in a domain is approximately L2. The time dependence of demixing is usually displayed in plots in which the log of the average lateral dimension L is plotted against the log of the time t . The value of L derived from the splitting is a measure of the degree of demixing. However, "degree of demixing" does not always lend itself to a straightforward interpretation. An increase in splitting normally reflects an increase in size of the domain, but it may also reflect an increase in lateral shape toward compactness, or a change in composition favoring the dominant ~omponent.~ These complications, if they occur, are likely to be associated with mixtures whose compositions fall in the spinodal region. B. Electron Diffraction and Calorimetry. Selected-area electron diffraction experiments were carried out at 100 kV with a JEOL JEM lOOCXII electron microscope, operating under typical low beam dose conditions to ensure that the specimens were protected against significant radiation damage.14 Electron diffraction patterns were recorded on Kodak DEF-5 X-ray film to permit a fast recording time at a useful camera length when low beam currents are used. If heating of the specimen was necessary (that is, to study the phase separation of metastable solid solutions), a GATAN 626 heating/cooling stage was used in the microscope. Binary samples examined in this study were epitaxially oriented on benzoic acid using the procedure of Wittmann, Hodge, and Lotz.IS These procedures have been as described in studies on the perhydrogenated paraffiinsl0and their perdeuterated analogues.I6 To measure DSC (differential scanning calorimeter) curves, we used a Hart 7707 calorimeter. The calorimeter sample temperature was calibrated against the phase transition temperatures for selected n-alkanes given in ref 17. 111. Experimental Evidence for the Isotope Effect
In this section we briefly indicate some experimental evidence that demonstrates for the c3o/c36 mixture that the D/H form has a significantly greater tendency to phase separate than its isotopically-interchangedcounterpart, H/D. A. Infrared Spectroscopic Measurements. The effect on demixingof switching H and D in c3O/c36can be observed directly from the splitting of the scissorsbands. These bands are displayed in Figure 1 for 1:l (molar concentration ratio) H / D and D/H mixtures about 15 min after quenching. At the top of Figure 1 are shown the CH2 scissors bands for C 3 p in H / D and C3aHin D/H; at the bottom are shown the corresponding CD2 bands. The splitting is clearly larger for the bands associated with the D/H mixture, indicating a greater degree of demixing than for H/D. B. Electron Diffraction Measurements. This techniquereveals in a spectacular way that H/D and D / H differ in their degree of microphase separation. This is illustrated for H/D and D/H in Figure 2, where the average lamellar spacing, I,, obtained from
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 675
CH, Scissors
J ..a, c: IC$ CC$
1490
1480
1470
1480
14E4
Frequency (cm-') Figure 1. Infrared spectral evidence indicating a substantial difference in the extent of demixing between the H/D and D/H mixtures of 1:l c30/c36 that were aged for about 15 min. Comparison of the infrared CHI scissors band of C30H in H/D with that of c36." in D/H (top); Comparison of the infrared CD2 scissors band of C3sDin H/D with that of C30Din D/H (bottom). Note that the "middle band" that appears between the (outside) bands of the doublet represents chains that are isotopically isolated.
thediffraction pattern is plotted as a function of C36 concentration, X(c36). These mixtures have been aged for about 1 day. For H/D, the average lamellar spacing bears a roughly linear relation to the concentration of C36. The H / D mixture thus behaves like a solid solution in that Vegard's rule is approximately followed; that is, I, is proportional to X(C36).'* For D/H, this is not the case. The plot of I, vsX(&) for this mixture shows threeaverage lamellar spacings that are approximately independent of X(C3,). The D/H mixture thus appears to be highly demixed. C. Calorimetric Measurements. The phase diagrams for the H / D and D/H mixtures are shown in Figure 3. These diagrams are based mainly on DSC curves and have been constructed as described in ref 8. The diagrams indicate that the components in the D/H mixture are substantially less miscible than those in H/D, that is, the miscibility gap or the length of the tie line at T, is greater for the D/H system than for the H/D one. The difference reflects the sizable increase in the melting point separation between the C30 and c 3 6 components in going from the H / D to the D/H mixture. The DSC heating curves in Figure 4 show that the differences in phase behavior between the H / D and the D/H 1:l mixtures, aged for 2 weeks, are much greater at temperatures above the tie-line at T, than below it. In theorthorhombiccrystalline state below T,, the differences between these mixtures are in fact relatively minor. We will return later to this point.
Snyder et al.
676 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 50
I
7"
0.0
0.2
0.4
0.6
0.8
1.0
01 +02
q+%
30
30
0.0 0.2
0.4 08
OD 02
0.8
1.0
Figure 3. Approximate phase diagrams for the C30H/C36Dand CmD/ I C3sHmixtures: L is the liquid; H is the hexagonal phase; 0 1 and 02 are
the twoorthorhombicphase; T. is the orthorhombic-hexagonal transition; T ' , and T,' are the 'onset" and 'peak" temperatures that characterize the 'mixing" endotherm shown in Figure 4.
48
n
08
xccg,
xccg,
50
0.6
0.4
46 I-
5 Y
u)
44
42
A.-0
0.0
0.2
0.4
0.6
0.8
- 1.0
I
x (Q
10
za
30
40
50
0
I
m
I
m
Temperalum ('C)
Figure 2. Observed lamellar spacing from electron diffraction mca-
surements plotted against c 3 6 concentration in 1:1 mixtures C3#/C3,5D (top) and C30D/C3,jH(bottom). The samples were aged for about 1 day. That demixing has occurred in both H / D and D/H is indicated in the DSC curve by the appearance of the "mixing" endotherm, which is marked in Figure 4. This endotherm is found only for aged, partially-demixed binary mixtures7**and is associated with thedissolution of the domains. The mixing endotherms for H / D and D/H c30/c36 have maxima near 40 OC. We note that the enthalpy represented by the endotherm is nearly the same for both 1:l mixtures. Thus, the degree of phase separation that has occurred after the two mixtures after have aged for 2 weeks is, from the calorimetric point of view, essentially the same.
IV. Results and Discussion A. Differences between H/Dand D/H.The greater stability of the H/D mixture relative to the D / H mixture results, of course, from the fact that the n-alkane components that make up H / D are more nearly alike than those of D/H. A measure of the equilibrium immiscibility of the components in the mobile phases of the mixture is provided by the difference between the melting points of the pure-component n-alkanes. Similarly, a measure of the demixing kinetics is provided by the difference in molar volume. To facilitate comparison between mixtures, both kinds
I
3 W
5. L. . , . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 za 30 40 P m m
I
m
nlnP"('C) Figure 4. DSC heatingcurvesfor 1:1 CmH/CxDand CmD/C36Hmixtur#r
that have been aged for 2 weeks. The 'mixing" endotherm is indicated by an asterisk. of differences will be converted to equivalentdifferencesin number of carbons. Conversion to number of carbons is particularly useful for estimating the rate of spontaneous demixing, since it is known that for a binary mixture Cn/Cn, the demixing rate is highly
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 671
Unstable Crystalline Mixtures of Binary n-Alkanes
TABLE 1: Isotropic Dependence of the Chain-Length Difference for the 1:l Mixture CdCw Estimated from Melting Points and Molar Volume isotopes mixture C%/C36
i/j
H/D H/H D/D D/H
ATm‘j “(“C) 6.4 10.4 10.8 14.8
Tm(C,.H)b (“C) 71.8 74.8 76.2 80.2
33.4 35.9 36.1 39.0
A& 3.4 5.9 6.1 9.0
An/ 5.82 6.00 5.97 6.15
0 Melting point difference (eq l), computed from the melting points in ref 19. (Tm= 66.0(C30H),61.6(C30D),76.4(C3sH),and 72.4(C3sD). b Melting point of C p computed from eq 2 with Tm(C30H)= 65.4 “C from ref 10. CThe chain length, in terms of number of carbons, corresponding to Tm(CdpH). Estimated by interpolation using the Tm versus n data in ref 10. Chain-length difference, in terms of number of carbons, estimated from melting points. Chain-length difference, in terms of number of CHI groups, estimated from H-D volume differences.
dependent on the chain-length difference, An (=n’- n). This dependence has been determined quantitatively for mixtures for which n = 28-30 and n’ = 36.’ 1 . Melting Point Difference. The difference in the melting points, AT,,,, between the components of the c30/c36mixture is defined
AT:] = T,(C$) - Tm(C3i)
(1)
where i a n d j represent either H or D and are associated with C30 and c36,respectively. Table 1 lists the ATmilfor the four possible isotopic combinations. The smallest value of ATmijis associated with the H / D mixture and the largest with D/H. Therefore, on this basis, D/H would be expected to be less stable than H/D; also H / H and D/D are expected to be nearly alike and intermediate between H / D and D/H. This ordering of stabilities is exactly what is observed experimentally.12 To convert the melting point difference ATmijto an equivalent , have used chain-length difference in number of carbons, A ~ Twe the experimentallydetermined relation between the melting points of the pure hydrogenated n-alkanes and their chain lengths.” For internal consistency, we will define AnT = n ” - 30 for the mixture C30H/CnJ’. The short component of this reference mixture is thus always C30H, while n”, which can assume noninteger values, defines the effective length of the long chain. With the use of Broadhurst’s melting-point/chain-lengthdata,17 the value of n” can be evaluated from the estimated melting point, T,(C,,J+), of C,P. The latter can be obtained from T,(C,,,H)
= T,(C3,H)
+ AT:]
Some values of AT,U, n”, and AnT are listed in Table 1. The chain-length difference, A ~ Tcomputed , in this way is 3.4 carbons for H/D and 9.0 for D/H. The value of An that separates thestablefromtheunstablemixturesof C30H/Cnfflis2.8carbons.lO Therefore, on the basis of melting point differences, the H / D mixture is marginally unstable, while the D / H mixture is very unstable. It is appropriate to rank the degree of miscibility of the components on the basis of melting point differences only if the system is at equilibrium. At temperatures above Tu,the mobility of the chains in the liquid and hexagonal phases is sufficient to ensureequilibrium. Thisis not the case below T,, since the mixture then has a closely packed orthorhombic structure. For example, on the basisof melting point differences, the D/H c30/c36mixture turns out to be equivalent to C#/C3gH. Indeed, at temperatures above Tu, the miscibility of the components in D/H c30/c36 is comparable to that for C30H/C3gH. Furthermore, well below Tu, for example at room temperature, where the mixtures are orthorhombic, the C30H/C39Hmixture is much more prone to demixing than D/H c30/c36.
2. Molar Volume Difference. As noted, the H/D and D/H mixtures at room temperature are not in equilibrium. The diffusion that leads to demixing is slow due to the large activation energies. These energies are sensitive to steric effects and, therefore, to changes in molar volume, and thus there are measurable differences between the H / D and the D/H mixtures in their demixing rates. The relative kinetic stabilities of H / D and D/H in the orthorhombic crystalline phase at room temperature can be estimated on the basis of the small decrease in molar volume that occurs in going from a hydrogenated to a deuterated n-alkane. Thevolumechange per methylene is small, of the order of 0.5%.19 However, the change in the volume difference (between the component chains of the mixture) that occurs when H and Dare interchanged is a significant fraction of the actual volume difference between the two chains. The values of the chainlength difference, with the isotopic differencestaken into account and the total expressed in terms of number of carbons Anv, are listed in Table 1. For the H/D and D/H mixtures, the estimated values of Anv are respectively 5.82 and 6.15 carbons. The difference between the values of AnV for D/H and H / D is thus about 0.33 carbons, sufficient to account for the magnitude of the isotope effect on the demixing rate, as will be shown in section IV.B.2.b. In considering effects of isotope interchange on demixing, we have focused on the molar volume difference between the H and D chains. There is also a difference, albeit small, in molecular polarizabilities that may affect the interchain potential energy! However, the polarizability difference generally mimics the volume difference in its effect on equilibrium propertiesand kinetic phase behavior. Therefore, at the level of the present treatment, we have not felt it necessary to consider volume and polarizability changes separately. B. Spectroscopically Determined Demixing-Behavior Differences between H/D and D/H. log/log plots showing the time evolution of the average size of the minority-componentdomains in H / D and D/H c30/c36 mixtures at concentration ratios 4:l and 1:4 are shown in Figures 5 and 6. In Figure 5, plots for the c30and the c 3 6 components are shown separately. To facilitate comparison, these curves (without their data points) are shown together in Figure 6. log/log plots for 1: 1 H/D and D/H mixtures are shown in Figure 7. Figure 8 shows corresponding log/log plots for C28H/C36Dmixtures. 1 . Stateof Demixingafter the Quench. Somedemixingoccurs in both H/D and D/H during the quench, before the mixtures reach room temperature. The average size of the domains in D / H is significantly larger than those in H/D. This may be seen clearly in the log/log plots for the 4:1 and 1:4mixtures in Figures 5 and 6. For each mixture, the average domain size exceeds that associated with random mixing (indicated on the figures). The 1:l c30/c36 mixtures behave in ways similar to the 4:l and 1:4 mixtures, as may be seen in Figure 7. However,for the 1: 1 mixture relative to the 4:1/1:4 pair, the difference between H/D and D/H in their degree of mixing is considerably less and the components are more homogeneously mixed. The degree of mixing observed for the D/H mixture just after the quench is, in fact, entirely out of line with the trend found for H / D mixtures, This may be seen in Figure 9, where the average domain sizes of the minority components are plotted against chain length, n, for a series of just-quenched 1:4 c&6 mixtures. ThesizeoftheC3odomainsintheD/H C30/C36mixture clearly does not fit in with the pattern observed for the H / D mixtures. The D/H mixture is found to be demixed to about the same degree as C29H/C36D. (1:4mixtures rather than 1:l were used for this comparison because minority-component domains are better defined than the domains in mixtures whose compositions are nearly equal and thus near the center of the spinodal regi~n.)~
Snyder et al.
678 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
-2
0 03
1
LOOTMEW '
4
El
0,8
4 0.4
1:4 01
Figure 5. log L plotted against log t for the minority component in the mixtures C3oH/C3eDand C30D/C36H,each at molar ratios of both 4:l and 1:4. L is the average lateral dimension of the domains in terms of numbcr of chains. The mixtures are at room temperature. The curves for C30 and c 3 6 are plotted separately. The estimated value of log L (=0.22) for random mixing is marked on the plot. 1
I
/ /'
/
/ "
i -. 0.4
random -2
-1
LWTSIZM)
1
1
2
Figure 6. Simplified version of Figure 5, emphasizing the differences between the H/D and D/H mixtures in their degree of demixing.
At the quench speeds used here, it appears that the mixtures remain in equilibrium during the high-temperature part of the quench. This is suggested by the fact that, for both H / D and D/H, changing the quench speed does not change the degree of demixing found just after the quench, at least for quench times longer than about 0.5 min. This was indicated from DSC measurements on D/H 1:l mixtures, one such mixture quenched
Figure 7. log L versus log 1 for 1:l CwH/CxDand C3OD/C36" at room temperature. Plots for the separate n-alkanecomponentsare shown. The estimated value of log L (=0.51)for random mixing is marked.
rapidly (
