J. Phys. Chem. 1985, 89, 2652-2657
2652
An InvestJgation of Solvent-Vapor-Induced Crystallization of Soluble Vanadyl Phthalocyanine Dyes in a Polymer Matrix Kock-Yee Law Xerox Webster Research Center, Webster, New York 14580 (Received: August 13, 1984; In Final Form: January 30, 1985)
Dye-in-polymer films consisting of 20 wt % tert-butyl-substituted vanadyl phthalocyanine dye, namely (t-Bu),,,VOPc, in vinyl chloride-vinyl acetate (83/17) copolymer have been prepared by a spin-coating technique. Our results show that ( ~ - B U ) ~ , ~ Vprimarily OPC precipitates as a glassy solid, phase 1, inside the polymer during film formation. Upon exposure of these dye-in-polymer films to solvent vapors, crystallization of (t-Bu)l,4VOPcto a more stable form, phase 11, occurs. Parameters influencing this crystallization process were studied by exposing dye-in-polymer films to a large variety of solvent vapors. The progress of the crystallization was monitored by absorption spectroscopy. Our results indicate that thermodynamic (dispersion) interactions between solvent molecules, the dye, and the polymer chain are critical driving forces for the initiation of the crystallization process. They provide a medium for the solubilization-crystallization process of the dye and, at the same time, plasticize the polymer chains and make all the dye migration motion feasible. This cooperative contribution was demonstrated by a mixed vapor experiment. Finally, the completion of the crystallization bfocess was found to be inversely dependent of the density of the swelling solvent in the high-density regime ( p k 1 g/cm3) for thermodynamically favorable systems. This may be attributed either to the strong solubilization effect of these solvents, where the dye may have a greater tendency to remain in a disordered "dissolved" state, or to the density effect on the mobility of the polymer chain, which affects the dye migration process, in vapor-swollen polymer matrices.
to emphasize again that metallophthalocyanines can exist in different morphological forms in the solid state and the electrical properties of each morphological form varies.' X-ray crystallographic data of the phase I1 of VOPc showed that the phthalocyanine ring of VOPc interacts strongly with its nearest neighbors. Interplanar distances between phthalocyanine rings are reported to be in the range of 3.2-3.4 A.8 The key question now is whether we can improve the solubility of VOPc via substitution and retain the phase I1 packing at the same time. Very little information is available in the literature regarding the effect of solubilizing groups on the solid-state properties of metallophthalocyanines. We have recently shown that introduction of tert-butyl groups improves the solubility of VOPc in organic solvents (e.g., methylene chloride) by a factor of >lo4, but tert-butyl groups affect the molecular packing of the resulting solid. For (t-Bu),VOPc, due to the strong steric repulsion between these molecules, close interplanar interaction analogous to the phase I1 of VOPc is not attainable. The solid of (t-Bu),VOPc is glassy and exhibits no near-IR absorption. At an average of 1.4 tert-butyl group per VOPc ring, the resulting material, ( t B U )4VOPc, ~ consists primarily ( ~ - B U ) ~ V Oand P C (t-Bu),VOPc. Due to the decrease in steric repulsion, close interactions between phthalocyanine rings are feasible. This results in cocrystallization of (t-Bu),VOPc and (t-Bu),VOPc molecules in the solid of ( t B U ) ,VOPc. ~ By comparison with the solid-state absorption spectrum and the X-ray data of the phase I1 of VOPc, we previously concluded that (t-Bu), 4VOPc also exists as phase I1 in the solid state.9 Owing to the high solubility of (t-Bu), ,VOPc in organic solvents, dye molecules dissolve rather than disperse (like VOPc) in the coating solution. It is thus uncertain whether or not ( t Bu), ,VOPc molecules can crystallize as phase I1 during the device fabrication. In this report, we show that, under spin-coating conditions, (t-Bu), ,VOPc precipitates primarily as a non-IR absorbing glassy solid (phase I) in vinyl chloride-vinyl acetate (83/17) copolymer. This phase I however, can be crystallized to an IR-absorbing phase I1 inside the polymer matrix by exposing the spin-coated dye-in-polymer film to an appropriate solvent vapor. Factors which control or influence this crystallization
Background Metal-free phthalocyanine and many of its metal derivatives are known to possess useful photoconductive and semiconductive properties. Among various phthalocyanines, vanadyl phthalocyanine (VOPc) has been shown to be useful in electrophoretic' and xerographic2 imaging and more recently ablative optical recording3 Because of the slight difference in intermolecular interactions between phthalocyanine molecules in various polymorphs, phthalocyanines can exist in several different morphological forms in the solid states4 Without exception such a polymorphoric behavior was also observed in the case of VOPc. Griffiths, Walker, and Goldsteid have shown that VOPc exists in three different phases in the solid state. When VOPc is freshly precipitated from a concentrated sulfuric acid solution in ice water, the resulting solid only exhibits visible absorption and is amorphous as judged by X-ray diffraction data. This glassy solid is defined as phase I. When the phase I is heated to >200 O C , a crystalline -840 nm), material, which shows absorption in the near-IR (A, is obtained. This second phase, which is thermodynamically more stable, is defined as phase 11. When VOPc is heated to above 620 "C, another crystalline material, phase 111, which does not show any I R absorption, is formed. Although it remains to be established that the phase I1 of VOPc is the most photoactive form of VOPc, its IR absorptivity, which is compatible with the low-cost GaAs diode laser, makes it a necessity for various laser-addressed marking technologies. While the synthesis of crude VOPc is relatively simple, the conversion of crude VOPc to electronic grade material is more complicated. Conventional purification by recrystallization is precluded because of its low solubility in organic solvents. Purification is achieved via a tedious procedure, which involves dissolution of VOPc in concentrated sulfuric acid and reprecipitation of VOPc by discharging the resulting sulfuric acid solution into ice water.6 It would, therefore, be advantageous from the viewpoint of obtaining electronic grade material to synthesize soluble VOPc derivatives which can be purified by conventional means. We have U S . Patents 3 825 422, 4032 339, and 4076 527. Grammatica, S.; Mort, J. Appl. Phys. Lett. 1981, 38, 445. Kivits, P.; deBout, R.; VanDerVeen, J. Appl. Phys. 1981, A26, 101. For examples, see: Sharp, J. H.; Lardon, M. J . Phys. Chem. 1968, 72, Kobayashi, T . ; Fujioshi, Y.; Iwatsu, F.; Uyeda, N . Acta Crystallogr., A37, 692 and references cited therein. (5) Griffiths, C. H.; Walker, M. S.; Goldstein, P. Mol. Cryst. Liq. Cryst. 1976, 33, 149. (6) Germano, N. J.; Nealey, R. H. Xerox Disclosure J. 1977, 3, 377.
(7) For examples, see: Wihksne, K.; Newkirk, A. E. J. Chem. SOC.1961, 34, 2184. Sakai, Y.; Sadaska, Y.; Yokouchi, H. Bull. Chem. SOC.Jpn. 1974, 47, 1886. Ukei, K. J. Phys. Chem. SOC.Jpn. 1976, 40, 140. (8) Ziolo, R. F.; Griffiths, C.H.; Troup, J. M . J. Chem. SOC.,Dalton Trans. 1980, 2300. (9) Law, K . Y. Inorg. Chem., in press,
0 1985 American Chemical Society 0022-3654185 12089-2652%01.50/0 - . -, ~1
I
,
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The Journal of Physical Chemistry, Vol. 89, No. 12, I985
Vanadyl Phthalocyanine Dyes
2653
a
WAVELENGTH @m)
WAVELENGTH (nm)
Figure 2. (a) Absorption spectrum of spin-coated films of (~-Bu),,~VOPC in vinyl chloride-vinyl acetate (83/17) copolymer; (b) solid-state absorption spectrum of (t-Bu),,,VOPc obtained from acid pasting.
I
600
700 800 WAVELENGTH (nm)
900
Figure 1. Absorption spectra of (t-Bu),,VOPc (a) in chloroform solution and (b) in solution cast solid film).
monitored by absorption spectroscopy with a Cary 17 spectrophotometer.
process have been studied by exposing dye-in-polymer films to a large variety of solvent vapors of varying solvent-dye and solvent-polymer compatibilities. The mechanistic aspects of this crystallization process are the subject of this work.
Results and Discussion
Experimental Section Materials. (t-Bu) ,,,VOPc was prepared and purified as previously described.g Vinyl chloride-vinyl acetate (83/17) copolymer was from Polysciences Inc. Solvents were used as received and they were obtained from the following sources: methylene chloride, chloroform, l,l,l-trichloroethane, benzene, chlorobenzene, toluene, xylene, 2-butanone, acetone, ethyl acetate, and tetrahydrofuran were analyzed reagent from Baker; 1,l ,Ztrichloroethane (technical), 1,2-dichloroethane, and 3-pentanone (practical) were from Eastman; 1,l -dichloroethane was from Aldrich; 1,4-dioxane, methanol, and 2-propanol were certified grade from Fisher. Glass substrates (2 in. X 2 in.) (7059) were bought from Corning and were cleaned as previously described.lo Preparation of Dye-in-Polymer Films. The coating solution was prepared by dissolving ultrasonically 30 mg of (t-Bu),,,VOPc and 120 mg of vinyl chloride vinyl acetate (83/17) copolymer in 5 mL of l,l,Ztrichloroethane in a volumetric flask. It was filtered through a 1-pm Millipore filter and then spin-coated onto a precleaned 2 in. X 2 in. glass substrate at a spinning rate of 1500 rpm. The resulting polymer film, which was vacuum-dried overnight, is 100-nm thick and has an optical density of -0.22 a t 700 nm. Soluent Vapor Treatment. Dye-in-polymer films for solvent vapor treatment were first stuck onto a piece of 10 in. X 10 in. cover glass and were then exposed to solvent vapor by covering the cover glass over a crystallization dish which was partly filled with solvent (the distance between the sample and the surface of the solvent was - 5 cm). The progress of the treatment was
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(10) Law, K. Y . Polymer 1982, 23, 1627.
-
Absorption Spectroscopy and Assignment of Dye Aggregates. Parts a and b of Figure 1 show the solution and the solid-state (from solution cast) absorption spectra of (t-Bu),,.,VOPc, respectively. In chloroform solution, (t-Bu), ,VOPc exhibits an 694.7 nm, an absorption shoulder intense absorption band at ,A, at -665 nm, and a less intense absorption band at A- 626.5 nm. The solution absorption of (d-Bu),.,VOPc is identical with those of VOPc5 and tetra-tert-butyl vanadyl phthalocyanine, (tBU),VOPC.~In the solid state, (t-Bu), ,VOPc shows an intense -830 nm in addition to the expected near-IR absorption at A, visible absorption. As noted in the introductory section, by comparison with the solid-state absorption spectrum of VOPc phase 11, the spectrum in Figure l b is assigned to the phase I1 of (t-Bu), ~ V O P C . Figure 2a shows a typical absorption spectrum of a spin-coated dye-in-polymer film of (t-Bu), ,VOPc dispersed in vinyl chloride-vinyl acetate (83/17) copolymer (at 20 wt dye loading). Since the absorption spectrum in Figure 2a is distinctly different from any monomeric (Figure l a ) or dimeric species of VOPc dyes,” we attribute it to the absorption spectrum of a solid form of (t-Bu), ,VOPc. This deduction is supported by transmission electron microscopy results where dye particles (-0.1 pm in size) are clearly seen in the polymer matrix.I2 Similar to VOPc, (t-Bu),,VOPc can also form phase I, a non-’IR absorbing glass, when it is reprecipitated in ice water from a concentrated sulfuric acid solution. The absorption spectrum of phase I of (t-Bu), ,VOPc (Figure 2b) is found to be identical with that of VOPC.~By comparison of the spectra in Figure 2a,b, we conclude that the solid form in polymer matrix is the phase I of (t-Bu), ,VOPc. Since the concentration of (t-Bu), ,VOPc in (1 1) Monahan, A. R.; Brado, J. A.; DeLuca, A. F. J . Phys. Chem. 1972, 76, 1994. (12) Law, K. Y., unpublished observation.
2654
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
Law
0.10 Ohr-
2 0.05
0 500
600
700
800
WAVELENGTH I n n )
Absorption spectra of spin-coated polymer film of ( t Bu),,.,VOPc/vinyl chloride-vinyl acetate (83/17) copolymer as a function of CH2C12vapor exposure time. Figure 3.
the coating solution (6 mg/mL of solvent) is well below its saturated concentration (12 mg/mL of solvents), the phase I of ( ~ - B U ) ~ , ~ V must O P C be formed in the film formation step. A small absorption shoulder is observed at wavelengths >800 nm (Figure 2a). The intensity of this near-IR absorption shoulder is very sensitive to coating conditions. For example, more intense near-IR absorption is gen'erally observed in samples where the rate of solvent evaporation is much slower. Since IR absorption is an unique characteristic of the phase I1 of ( ~ - B U ) ~ , ~ V Owe PC, attribute the near-IR absorption shoulder in Figure 2a to the phase I1 of (~-Bu),,~VOPC. The formation of phase I1 is not surprising since it is a thermodynamically more stable, crystalline form.* The interesting feature of the results is the greater tendency for (tB U ) ~ , ~ V Omolecules PC to precipitate as a glassy phase, I, rather than the more stable crystalline phase, 11, during the film formation. This observation is however consistent with our previous results where it was shown that thermodynamically less stable dye aggregates can be formed under fast drying (or spin-coating) conditions. l o Solvent Vapor Treatment of (t-Bu)l,4VOPc/VinylChlorideVinyl Acetate (83/17) Films. Fundamental understanding on the vapor-induced phase I I1 crystallization of the (tB ~ ) ~ , ~ V O P c / v ichloride-vinyl nyl acetate (83/17) system is expected to be of value because this will allow us to tailor the molecular packing, the particle size, the crystallite size, etc. of the dye aggregates inside the polymer matrix, rendering its capability for various technological applications. In this section, the effect of solvent vapor on the crystallization process is reported. Dye-in-polymer films were exposed to a large variety of solvent vapors of varying solventdye and solvent-polymer compatibilities. The results are summarized as follows. (i) Chlorinated Hydrocarbons. Six chlorinated hydrocarbons, namely methylene chloride, chloroform, 1,l-dichloroethane, 1,2dichloroethane, 1,1,2-trichloroethane, and l,l,l-trichloroethane have been studied. Figure 3 shows a typical example on the change in optical absorption as a function of (methylene chloride) vapor exposure time. The near-IR absorption (Amx -820 nm) increases as the exposure time increases and levels off after -20 h. This is accompanied by the gradual appearance of diffraction lines in the X-ray powder diffraction pattern of these films.I3 We thus conclude that the phase I of (~-Bu),,~VOPC crystallizes to phase I1 inside the polymer by the action of solvent vapor. The occurrence of this process is simply driven by thermodynamics because the net result of the crystallization is the stabilization of (t-Bu)l,4VOPcsolid from a nonequilibrated glassy phase I to an
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(13) The X-ray diffraction pattern of a fully crystallized film is comparable, but is broader and leas intense compared with the parent VOPc/polymer system; the diffraction line corresponding to the interplanar distance of VOPc rings (d spacing 3.2 A) is clearly seen. Law, K.Y.,unpublished results.
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TIME (hr)
Figure 4. Plot of OD at 820 nm as a function of vapor exposure time: (a) 1,I-dichloroethane;(b) 1,2-trichloroethane;(c) methylene chloride; (d) chloroform; (e) 1,1,2-trichloroethane;(f) l,l,l-trichloroethane.
t
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.,
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Figure 5. Plot of OD at 820 nm as a function of vapor exposure time: (a) chlorobenzene; (b) toluene; (c) benzene, (d) xylene.
equilibrated crystalline phase 11, where extensive intermolecular phthalocyanine-ring interactions can be attained. Because of the unique near-IR absorption of the phase 11, the progress of the crystallization process could be conveniently monitored by absorption spectroscopy. Figure 4 gives a plot of t4e optical density (OD) at 820 nm as a function of exposure time for all the chlorinated hydrocarbons studied. It is interesting to note that both initial Crystallization rate and the final O D are dependent on the solvent vapor applied. The initial crystallization rate is presumably a complex function of the vapor pressure, the permeability, and the interactions between the solvent, the dye, and t~hepolymer. The kinetic aspects of the crystallization process a r e beyond the scope of the present work. Since all the OD's presented in Figure 4 are normalized for any thickness variations, the variation of final O D indicates that there is a profound vapor effect on the completion of the crystallization process. (ii) Aromatic Soluents. Four aromatic solvents, namely benzene, toluene, xylene, and chlorobenzene, were studied. Similar to that shown in Figure 3, the near-IR absorption increases as the vapor exposure time increases. The plots of the OD's at 820 nm as a function of vapor exposure time are presented in Figure 5. Except in the case of chlorobenzene vapor, where the O D increases smoothly as the vapor exposure time increases, the crystallizations induced by benzene, taluene, and xylene are sluggish and seem to occur in a stepwise fashion. (iii) Other Solvents. Dye-in-polymers films have also been exposed to several other vapors, such as acetone, 2-butanone, 3-pentanone, ethyl acetate, tetrahydrofuran, 1,4-dioxane, 2-
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2655
Vanadyl Phthalocyanine Dyes
E L
I
0 S&ellbfa bJB aye acd polvmer Non-sdventsfa tdh dye and polvmer ASciw?ntsfadyecr+y O ~ e n t s t a ~ W
0.10
a
d
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I
I
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 TIME (hr)
Figure 6. Plot of OD at 820 nm as a function of vapor exposure time: (a) tetrahydrofuran; (b) 2-butanone;(c) ethyl acetate; (d) 3-pentanone; (e) 1,4-dioxane;( f ) acetone; (g) methanol and 2-propanol.
Sd (%)
Figure 7. Solubility plots for (~-Bu),,~VOPC and vinyl chloride-vinyl
acetate (83/17) copolymer: area of solubility for (t-Bu),,,VOPc, -; area propanol, and methanol, in order to gain further insight on the of solubility for vinyl chloride-vinyl acetate copolymer, ---. vapor-induced crystallization process. Again, the progress of the crystallization was followed by absorption spectroscopy. The vapor is defined as a vapor having a strong plasticizing effect on results, which are presented in the OD vs. exposure time plot shown both the dye and the polymer such that the plasticizing effect on in Figure 6 , show that vapors of 2-butanone, 3-pentanone, ethyl the dye must be stronger than that produced by acetone vapor acetate, and tetrahydrofuran are active for the crystallization and at the same time having a plasticizing effect on the polymer process, and that vapors of alcoholic solvents are completely instronger than that produced by xylene vapor. active. Vapors of acetone and 1,4-dioxane are only partially active, We have pursued this subject further by carrying out a and relatively low final O D values are observed in the near-IR three-component (namely dispersion force (&), dipolar interaction region. (6,,), and H-bonding (6h)) solubility parameter a n a 1 y ~ i s . l ~ ~ ~ ~ On the Mechanism of the Phase Z II Crystallization. (i) Assuming acetone and xylene are marginally favorable solvents Mechanism. As indicated earlier, (~-Bu),,~VOPC primarily forms for the dye and the polymer, respectively, our three-component phase I during the film formation. Upon exposure to an approsolubility parameter analysis results (Figure 7) show that favorable priate solvent vapor, the dye crystallizes to form phase 11. Since solvents for the dye and the polymer lie inside the indicated heating a t 140-200 OC has been shown to be effective in the boundary, and that the main contributor in both cases is the crystallization of VOPc (phase I to 11),5 we have also attempted dispersion force. All the solvents in the overlapped area are active to induce the crystallization of some of these dye-in-polymer films for the vapor-induced crystallization process. This is in agreement by heating at -120 OC for over 48 h and we observed no cryswith the conclusion reached from the solubility test results. The tallization. Thermal-induced crystallization is also not observed solubility parameter analysis further reveals that the main conwhen the phase I of (~-Bu),,~VOPC is heated (140-200 "C).This tributor to the crystallization process is the dispersion interaction suggests that the glass transition temperature of the phase I of between the solvent, the dye, and the polymer. Since crystallization VOPc is above 200 OC. The higher stability of the glassy state of dyes in polymers inevitably involves some kind of dye migration of (~-Bu),,~VOPC as compared with that of VOPc is presumably or molecular rotation inside the polymer, solvent molecules appear due to the steric effect on the crystallization process. Very similar to play a dual role in the crystallization process. They not only steric stabilization of less stable polymorph was also observed for provide a medium for the solubilization-crystallization process the a-form of copper phthalocyanine lately.14 These results thus but also plasticize the polymer chain and make the necessary imply that the vapor-induced crystallization process must involve migration and molecular motions possible. some kind of plasticization of the glassy state of ( ~ - B U ) ~ , ~ V O P C The lack of crystallization induced by vapors of alcoholic so that its Tgis reduced to (or below) room temperature, or that solvents is readily attributable to the lack of solubilization effect it is simply a solubilization-crystallization process. of these solvents toward the dye as well as the inability of the The relative plasticization effect of various solvents toward the solvent to plasticize the polymer. For solvent vapors where only dye and the polymer are estimated by their relative solubility. The one of these two functions is met, incomplete crystallization which results are summarized as follows. Solubility series of (t-Bu)l.4 results in low final O D ( chloroform According to the above solubility series, acetone is highly com1,l ,Ztrichloroethane 1,l-dichloroethane, 1,2-dichloroethane patible with the polymer, but not with the dye. Under such a > tetrahydrofuran chlorobenzene benzene > toluene circumstance, the polymer matrix is highly swollen by acetone xylene 3-pentanone 2-butanone > l,l,l-trichloroethane > vapor; the partial crystallization for films exposed to acetone vapor ethyl acetate > 1,Cdioxane > acetone >> 2-propanol and methmust be due to the poor solubilization effect of acetone toward anol. Solubility series of vinyl chloride-vinyl acetate (83/17) the dye. On the other hand, the interactions of l,l,l-trichlorocopolymer in organic solvents: 2-butanone > methylene chloride ethane and xylene with the dye are quite favorable, at least relative > chloroform tetrahydrofuran 1,2-dichloroethane > acetone to ethyl acetate, a good solvent for the crystallization process. The > 3-pentanone > l11,2-trichloroethane chlorobenzene ethyl partial crystallization observed when these two vapors are used acetate > 1,4-dioxane > 1,l-dichloroethane benzene > toluene can only be attributed to the low polymer chain mobility which > xylene >> l,l,l-trichloroethane >> 2-propanol and methanol. restricts the dye migration in the crystallization process. The low By comparison of the above two solubility series and the results in Figures 4-6, one notices that crystallization can only proceed (15) Solubility parameters are taken from: Barton, A. F. M. Chem. Rev. 1975, 75, 731, and references cited therein. smoothly when the film is exposed to suitable vapors. A suitable
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(14) US.Patent 4 289 698.
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(16) For three-component solubility parameter analysis, see: Crowley, J. D.; Teague, G. S.; Lowe, J. W. J . Paint Technol. 1966, 38, 269. Crowley, J. D.; Teague, G. S.; Lowe, J. W. Ibid. 1967, 39, 20.
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
2656
Law
O."
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TIME (hr)
Figure 8. Plot of OD at 820 nm as a function of vapor exposure time.
polymer chain mobility in these two systems is not unexpected because of the poor interaction between the polymer and the solvent according to the above solubility series. A low final OD in the near-IR region is also observed when films are exposed to 1,4-dioxane. According to Figure 7, the result is readily attributable to the poor solubilization effect of 1,4-dioxane toward the dye as well as the low polymer chain mobility due to the poor solvent-polymer interaction. Very interesting crystallization results are observed when dye-in-polymer films are exposed to aromatic hydrocarbon vapors (Figure 5). Since X-ray powder diffraction results suggest that all the intermediate stages are due to incomplete crystallization, the results in Figure 5 suggest that the crystallization proceeds in a stepwise fashion when dye-in-polymer films are exposed to vapors of benzene and toluene.'' Stepwise (re)crystallization appears to be quite general for phthalocyanines. Analogous stepwise crystallization of VOPc (phase I 11) was observed when VOPc heated to different temperatures ranging from 165 to 540 O C 5 It was also reported that copper phthalocyanine'8 and zinc phthal~cyanine'~ can undergo stepwise recrystallization under certain conditions. In the present work, the stepwise crystallization appears to be in parallel with the ability of solvent molecules to plasticize the polymer chain, and we attribute the observation to the restricted polymer chain mobility imposed on the dye crystallization process. The conclusion reached here is supported by the smooth crystallization observed when the phase I of (tB U ) ~ , ~ V Ois P Ccrystallized to phase I1 in polystyrene by benzene vapor where the mobility of the polymer chain in benzene swollen polystyrene matrix is high.12 (ii) Mixed Vapor Experiment. In order to further demonstrate the participation of both dye solubilization and polymer chain plasticization in the crystallization process, we have exposed dye-in-polymer films to a mixed vapor of acetone and xylene where they by themselves are poor vapors for the crystallization process. The results are shown in Figure 8. The results of the acetoneand the xylene-treated films are also shown for comparison. Our results clearly show that better crystallization is obtained when the mixed vapor is used. As discussed earlier, the incomplete crystallization observed in films treated by acetone and xylene vapors are attributed to the poor solubilization effect on the dye and the low polymer chain mobility in these two systems, re-
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(17) The OD's of the benzene- and the toluene-treated samples increase rapidly in the first hour and remain basically unchanged in the next 2 h. Data points between I - and 3-h vapor treatment time have been omitted for clarity of the figure. As seen in Figure 5 and 6, the crystallizations of the xylene-, the acetone- and the 1,4-dioxane-treated films are terminated at an OD (at 820 nm) of -0.048, -0.055, and -0.065, respectively. It appears that, after the initial crystallization in the first hour, the crystallization induced by benzene or toluene vapor stays in one of these two stages for the next 2 h before further crystallization. (18) Suito, E.; Uyeda, N. Kolloid Zh. 1963, 193. 97. (19) Kobayashi, T.; Uyeda, N.; Suito, E. J . Phys. Chem. 1968, 72,2446.
1.0
a9 1.5
I
2.0
p (gmlcc)
Figure 9. Plot of final OD at 820 nm as a function of the density of the swelling solvent.
spectively. The better crystallization observed in the mixed vapor experiment illustrates that these two factors are complementary to each other in the crystallization process. The involvement of cooperative interactions between the solvent, the dye, and the polymer in the vapor-induced crystallization process is securely established. (iii) Density Effect. Careful examination of the results of films treated by various favorable vapors, e.g., chlorinated solvent vapors, indicates that there is a variation of final OD of each film. Since all the OD results are normalized, the variation of OD suggests that not all of the favorable solvent vapors can induce a complete crystallization. We have attempted to correlate the final OD with various thermodynamic and kinetic parameters of the solvent, such as solubility parameter, volatility, viscosity, dielectric constant, etc., and we find no relationship. However, when the OD is plotted as a function of the density of the solvent ( p ) (Figure 9), we find that the OD is independent of p in the low-density regime ( p 6 1 g/cm3) and decreases as the density increases in the high-density regime ( p 5 g/cm3). As noted earlier in our three-component solubility parameter analysis, the main contributor to the crystallization is the dispersion force, which is roughly proportional to the density of the solvent. It is possible that, due to the strong solubilization effect of the dye by solvent molecules of high density ( p 5 1 g/cm3) solvents, dye molecules may tend to remain in a disordered "dissolved" state in the swollen matrix, resulting in the incomplete crystallization. We have recently studied the properties of various vapor swollen polymers by fluorescence probing technique.20 Our results suggest that there is a density effect on the properties of the swollen polymer; the polymer chain mobility decreases as the density of the swelling solvent increases in the high-density regime ( p 2 1 g/cm3). Since polymer chain mobility is known to have a controlling effect on the crystallization process, the observed density effect may also be attributed to the variation of polymer chain mobility for films swollen by solvents of various densities. The data present in this paper however do not allow us to distinguish between the above two possibilities. Conclusion The work described here shows that, under spin-coating (or fast drying) conditions, (~-Bu),,~VOPC precipitates primarily as a glassy solid, phase I, in vinyl chloride-vinyl acetate (83/ 17) copolymers during the film formation. Upon exposure of the dye-in-polymer film to solvent vapor, crystallization of phase I to a more stable phase I1 occurs inside the polymer. Thermodynamic (dispersion) interactions between solvent molecules, dye molecules, and polymer chains are critical driving forces for the initiation of the crystallization. Solvent molecules play a dual role in the crystallization process. They provide a medium for the solubilization-crystal(20) Law, K. Y. Pofymer 1984, 25, 399.
J. Phys. Chem. 1985,89, 2657-2661 lization process as well as plastize the polymer chain, which have made all the molecular motions p i b l e . Moreover, the completion of the crystallization is found to be related to the density of the swelling solvent. This is probably due to the strong solubilization effect of the solvent toward the dye or to the density effect on the mobility of the polymer chain, which affects the dye motions in the crystallization step, in polymer matrices swollen by highdensity (p 5 1 g/cm3) solvents.
Acknowledgment. I thank Dr. W. Prest for suggestion of the three-component solubility parameter analysis, Dr. D. J. Williams
2657
for comments on the manuscripts, and Professor Uyeda for bringing our attention to references on the subject of stepwise recrystallization. Registry No. (r-bu)VOPC, 96164-79-3;(t-bu),VOPC, 95552-15-1; vinyl chloride-vinyl acetate copolymer, 9003-22-9; methylene chloride, 75-09-2; chloroform, 67-66-3; 1,l-dichloroethane,75-34-3; 1,2-dichloroethane, 107-06-2; 1,1,2-trichloroethane,79-00-5; 1,l,l-trichloroethane, 71-55-6; benzene, 71-43-2; toluene, 108-88-3;xylene, 1330-20-7; chlorobenzene, 108-90-7;acetone, 67-64-1;2-butanone, 78-93-3; 3-pentanone, 96-22-0;ethyl acetate, 141-78-6;tetrahydrofuran, 109-99-9; 1,Cdioxane, 123-91-1; 2-propanol, 67-63-0; methanol, 67-56-1.
Binding Interactions of Tetrahedral Ions in Aqueous Surfactant Solution Keith Radley and Alan S. Tracey* Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A IS6 (Received: August 29, 1984; In Final Form: December 1 1 , 1984)
The tetrahedral ions NH4+,N(CH3)4+,Clod-, and BF, contained in lyotropic liquid crystalline medium have been investigated by NMR techniques. The liquid crystalline systems employed were those prepared from mixed detergent systems of either potassium dcdecanoate/alkyltrimethylammoniumbromide or potassium N-hexadecanoyl-I-prolinate/alkyltrimethylammonium bromide. Information concerning the binding of these ions to the amphiphileswas obtained. It was found that in the dcdecanoate mesophase the ammonium ion was held in the micellar interface by binding to three amphiphiles, and the results also indicated that ammonium was held about as strongly as alkali-metal ions. Tetramethylammonium ion was held only weakly by its binding interactions, and the results were consistent with diffuse binding of this ion. Perchlorate was found to bind to three tetradecyltrimethylammonium amphiphiles in the dodecanoate system but to only two in the prolinate system. Chloride was found to behave similarly. It was suggested that this latter behavior occurred because of the large increase in salt concentration (about a factor of 10) on going from the dodecanoate to the prolinate system. Like perchlorate, the boron tetrafluoride ion was found to bind to three trimethylammonium head groups under conditions corresponding to the dodecanoate/alkyltrimethylammoniummixed mesophase system.
Introduction The interactions between monatomic ions and amphiphilic materials have been the subject of many the~reticall-~ and experimental investigations.k8 Relatively little work has been reported concerning the binding interactions of tetrahedral polynuclear species although a study of the interactions of ammonium and tetramethylammonium ions in the anisotropic aqueous ammonium octanoate systems has been reported9 as has binding in polyelectrolyte s o l ~ t i o n . ~Of major importance in understanding the binding of ions is a knowledge of the number of amphiphiles that a counterion is associated with in its bound state and whether only one type of binding does.occur. Information concerning the nature of the binding interactions is of predominate interest as is that concerning ion competition for binding sites. It has been shown that considerable information concerning ion behavior can be obtained by observing the quadrupole splittings (1) Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1983,87, 5025-5032. (2) Beunen, J. A.; Ruckenstein, E. J . Colloid Interface Sci. 1983, 96, 469-487. (3) WennerstrBm, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, 97-1 03. (4) Delville, A.; Laszlo, P. Biophys. Chem. 1983, 17, 119-124. ( 5 ) Stilbs, P.; Lindman, B. J. Magn. Reson. 1982, 48, 132-137. (6) Larsen, J. W.; Magid, L. J. J . Am. Chem. SOC.1974,96, 5774-5782. (7) Tracey, A. S.; Boivin, T. L. J . Phys. Chem. 1984, 88, 1017-1023. (8) Tracey, A. S . Can. J . Chem. 1984, 62, 2161-2167. (9) Persson, N.-0.; Lindman, B. Mol. Cryst. Liq. Cryst. 1977, 38, 321-344.
0022-3654/85/2089-2657$OlSO/O
from ions with a quadrupolar nucleus when those ions are contained in an anisotropic medium. Particularly suitable materials for such studies are the nematic lyotropic liquid crystalline materials prepared from the potassium dodecanoate/alkyltrimethylammonium bromide/decanol/electrolyte/water mixed detergent system. The advantage of this system lies mainly in the property which allows the two detergents to be mixed in various proportions so that the surface charge which the micelle carries can be readily varied from fully positive to fully negative while at the same time maintaining the other components, electrolyte and water, in constant proportion to the total detergent. Only decanol content varies significantly throughout the range of mesophases. The quadrupole splitting from a quadrupolar nucleus is determined by the quadrupole coupling constant, Q, and the degree of alignment, S, of the electric field gradients with which the quadrupole moment interacts. The splitting, Av,is then given by eq 1 where it is assumed that the asymmetry in the electric field
Av =
3Qs 2 I ( 2 I - 1)
gradient is negligibly small. For ions of tetrahedral or higher symmetry the splitting is zero unless the ion is distorted from its high symmetry. Distortion can be caused by binding interactions3 or by polarization of the ion.I0 Should more than one type of interaction occur such as having free and bound ions in equilibrium (10) Bailey, D.; Buckingham, A. D.; Fujiwara, Reson. 1915, 18, 344-357.
0 1985 American Chemical Society
F.; Reeves, L. W. J . Map.
