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Langmuir 2002, 18, 1368-1373
Surfactant-Induced Mesomorphic Structures in Poly(1-vinylimidazole)-Alkanoic Acid Complexes Hua Jiao, S. H. Goh,* and S. Valiyaveettil Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Received June 29, 2001. In Final Form: November 5, 2001 Mesomorphic structures of supramolecular systems based on poly(1-vinylimidazole) (PVI) and alkanoic acids with chain lengths (n) of 10-18 carbon atoms have been studied. FTIR studies show the existence of hydrogen-bonding interaction and a low level of ionic interaction. POM measurements show that all the complexes are mesomorphic, and the isotropization temperature increases with increasing chain length of the acid. In addition, the isotropization temperature increases with decreasing acid content in the complex. DSC studies show that, besides isotropization transition, two melting transitions exist in complexes containing alkanoic acids with chain lengths n g 12. On the basis of XRD studies at room temperature and elevated temperatures, the complexes are grouped into two types: melted lamellar liquid crystal phase with interdigitating layer structure; crystallized lamellar phase with partial interdigitating layer structure. For PVI(PA)x and PVI(MA)1.0 (MA: myristic acid, n ) 14) complexes, these two types of structures are interconvertible upon heating/cooling with a change in layer thickness of about 10 Å. As shown by the present studies, the transition temperature and the thickness of layer can be tailored by varying the acid type and by changing the acid content in the complex.
Introduction The use of specific interactions for the design and preparation of self-organized materials has attracted much attention.1,2 Intensive investigations have been conducted on the use of ionic interaction,3-5 metal coordination,6 and hydrogen bonding7-13 to generate supramolecular nanostructures based on polymer/amphiphile systems. Systems based on poly(4-vinylpyridine) (P4VPy) hydrogen-bonded to amphiphiles exhibit very interesting phase structures.7-11,13 In general, when there is a good balance between association interaction and polar-nonpolar repulsion, microphase-separated mesomorphic states exist in these systems. The structure of this state usually consists of lamellar layers with polar sublayers consisting of polymer and the polar head of surfactant and nonpolar sublayers consisting of the alkyl chains of surfactant. * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kato, T.; Frechet, J. M. J. Macromol. Symp. 1995, 98, 311. (2) Lehn, J.-M. Makromol. Chem., Macromol. Symp. 1993, 69, 1. (3) MacKnight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781. (4) Antonietti, M.; Burger, C.; Thunemann, A. Trends Polym. Sci. 1997, 5, 262. (5) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17. (6) Ruokolainen, J.; Tanner, J.; ten Brinke, G.; Ikkala, O.; Torkkeli, M.; Serimaa, R. Macromolecules 1995, 28, 7779. (7) Ruokolainen, J.; Tanner, J.; Ikkala, O.; ten Brinke, G.; Thomas, E. L. Macromolecules 1998, 31, 3532. (8) Ruokolainen, J.; ten Brinke, G.; Ikkala, O.; Torkkeli, M.; Serimaa, R. Macromolecules 1996, 29, 3409. (9) Ikkala, O.; Ruokolainen, J.; Torkkeli, M.; Tanner, J.; Serimaa, R.; ten Brinke, G. Colloids and Surf., A 1999, 147, 241. (10) Luyten, M. C.; Alberda van Ekenstein, G. O. R.; ten Brinke, G.; Ruokolainen, J.; Ikkala, O.; Torkkeli, M.; Serimaa, R. Macromolecules 1999, 32, 4404. (11) Ruokolainen, J.; Torkkeli, M.; Serimaa, R.; Vahvaselka, S.; Saariaho, M.; ten Brinke, G.; Ikkala, O. Macromolecules 1996, 29, 6621. (12) Ikkala, O.; Knaapila, M.; Ruokolainen, J.; Torkkeli, M.; Serimaa, R.; Jokela, K.; Horsburgh, L.; Monkman, A.; ten Brinke, G. Adv. Mater. 1999, 11, 1206. (13) Ruokolainen, J.; Makinen, R.; Torkkeli, M.; Serimaa, R.; Makela, T.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557.
For the various nonmesogenic amphiphiles investigated so far, alkanoic acids with different chain lengths seem to be interesting because of their easy availability. Although the alkanoic acid-pyridine interaction is known to stabilize liquid crystallinity,14 in some systems it results in only partial miscibility and at levels insufficient to support liquid crystallinity.15,16 For benzoic acid derivatives,17 molecular mixing occurs up to an acid mole fraction of ca. 0.3, while for alkanoic acid derivatives molecular mixing occurs below an acid mole fraction of ca. 0.2.18 The partial miscibility of acids with polymers was ascribed to strong self-association of the acid to form dimers.17 For P4VPy-alkanoic acid systems, the lack of mesomorphic structures was believed to be the results of both macrophase separation and weak repulsive polar-nonpolar interaction.11 The phase behavior of polymer-amphiphile systems can be tailored by modifying the attractive and repulsive interactions in the systems. The latter can be realized by modifying the length of alkyl tail of the amphiphile or by adjusting the polarity of the polymer through the introduction of charges.9 Imidazole (pKb ) 7.05) is a stronger base than pyridine (pKb ) 8.75). Therefore, polymers containing imidazole groups are likely to interact strongly or even to induce proton transfer with carboxylic acids.19-21 We have recently reported that poly(1-vinylimidazole) (PVI) interacts more strongly with carboxyl-containing polysiloxanes than P4VPy does.21 Thus, it is expected that the strong interactions between PVI and alkanoic acids with long alkyl tails will improve the miscibility and also (14) Kato, T.; Fujishima, A.; Frechet, J. M. J. Chem. Lett. 1990, 919. (15) Bazuin, C. G.; Brandys, F. A. Chem. Mater. 1992, 4, 970. (16) Brandys, F. A.; Bazuin, C. G. Chem. Mater. 1996, 8, 83. (17) Alder, K. I.; Stewart, D.; Imrie, C. T. J. Mater. Chem. 1995, 5, 2225. (18) Stewart, D.; Imrie, C. T. J. Mater. Chem. 1995, 5, 223. (19) Luo, X.; Goh, S. H.; Lee, S. Y.; Huan, C. H. A. Macromol. Chem. Phys. 1999, 200, 874. (20) Luo, X.; Goh, S. H.; Lee, S. Y. Macromol. Chem. Phys. 1999, 200, 399. (21) Li, X.; Goh, S. H.; Lai, Y. H.; Wee, A. T. S. Polymer 2001, 42, 5463.
10.1021/la011001n CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002
Surfactant-Induced Mesomorphic Structures
Figure 1. FTIR spectra of palmitic acid (a), PVI(SA)1.0 (b), PVI(PA)1.0 (c), PVI(MA)1.0 (d), PVI(LA)1.0 (e), PVI(DA)1.0 (f), and PVI (g).
enhance the repulsive polar-nonpolar interaction of the systems. As a result, mesomorphic structures in these systems may be obtained. In this paper, we report on systems of PVI complexed with alkanoic acids of varying alkyl chain lengths. Experimental Section Materials. PVI (number-average molecular weight (Mn) ) 60 000) was synthesized by free radical polymerization.19,20 Palmitic acid (PA, CH3(CH2)14COOH) (>97%) was purchased from Fluka, stearic acid (SA, CH3(CH2)16COOH) (>97%) and myristic acid (MA, CH3(CH2)12COOH) (>98%) were purchased from Merck, lauric acid (LA, CH3(CH2)10COOH) (>98%) was supplied by Tokyo Kasei, Tokyo, Japan, and n-decanoic acid (DA, CH3(CH2)8COOH) (>99%) was supplied by Aldrich. Dimethylformamide (DMF) was dried using 3 Å molecular sieves. Sample Preparation. PVI was dried in vacuo at 60 °C for at least 2 days before use. Acids were used as received. PVI(acid)x complexes were prepared from DMF solutions, with x denoting the number of surfactant molecules/vinylimidazole repeat unit. The complexes were obtained by first dissolving the solid mixtures in DMF (2.5% w/v) with continuous stirring for 2 days at room temperature followed by film casting on an aluminum foil. The solution was evaporated under reduced pressure at 50 °C. The complexes were dried in vacuo at 60 °C for 24 h and then stored in a desiccator. Characterization. The thermal properties of various samples were measured with a TA Instruments 2920 differential scanning calorimeter (DSC) equipped with a cooling accessory. The heating rate was 20 °C/min. When control cooling was needed, the cooling rate was 3 °C/min. All measurements were conducted under a nitrogen atmosphere. Infrared spectra were recorded on a Bio-Rad 165 FTIR spectrophotometer; 32 scans were signal-averaged with a resolution of 2 cm-1. FTIR samples were prepared by grinding solid samples with KBr and compressing the mixtures to form disks. The mesomorphic structures of complexes were observed with an Olympus BH2-UMA polarizing optical microscope, equipped with a Leitz Wetzlar hot stage and an Olympus exposure control unit. Samples were made by melting solid samples sandwiched between a glass slide and a cover glass and followed by slow cooling to room temperature. A Siemens D5005 X-ray powder diffractometer with Cu KR (1.540 51 Å) radiation (40 kV, 40 mA) was used to analyze the room-temperature XRD patterns of the samples. For hightemperature measurements, a D8 ADVANCE X-ray diffractometer (Cu KR radiation, 40 kV and 40 mA) equipped with an Anton Paar HTK 1200 high-temperature oven camera and a scintillation detector was used. Dried film samples cast on aluminum foil were used directly for XRD measurements.
Results and Discussion Interactions in PVI(acid)x Systems. The interactions in a two-component system can be studied by FTIR where the frequency shifts of the absorption bands of functional groups provide information on the nature and intensity of intermolecular interactions. For acid-containing sys-
Langmuir, Vol. 18, No. 4, 2002 1369
Figure 2. POM micrograph for PVI(DA)1.0 at room temperature.
tems, however, the spectra are sometimes too complex to allow for even a qualitative characterization.18,22 To simplify our analysis, we mainly focus on the carbonyl group of the acids. It is well-known that most carboxylic acids exist as dimers in pure state and the carbonyl groups of saturated aliphatic carboxylic acids absorb very strongly in the region 1725-1700 cm-1. When acids interact with basic polymers, acid dimer may dissociate and various categories of hydrogen bonds form with respect to the strength of interactions:22
AH + B h A-H‚‚‚B h a b A‚‚‚H‚‚‚B h A‚‚‚H-B h A- HB+ (1) d c e
(
)
Figure 1 shows the spectra of PVI(acid)1.0 in the region of 1800-1000 cm-1. Pure acids exhibit asymmetric absorption bands at about 1700 cm-1. When they are complexed with PVI, these bands move to ca. 1720 cm-1, which can be ascribed to the “liberated” carbonyl group as a consequence of the interaction between the acid group and polymers.22 Furthermore, the bands in this region become broader and new peaks at about 1644, 1571, and 1544 cm-1 appear in this region. The characteristic antisymmetric vibration of COO- usually appears in the 1550-1600 cm-1 region. Also, it was reported23 that the stretching vibration of C-C and C-N of PVI shifted from 1500 to 1573 and 1544 cm-1 upon protonation. Thus, the bands at 1580-1540 cm-1 are contributed by both the protonated PVI and the ionized acids. Conceptually, the broad bands of PVI(acid)1.0 in the region of 1800-1540 cm-1 can be interpreted in terms of a dynamic distribution of various structures containing different hydrogen bond strengths. Qualitatively, as shown in Figure 1, the shorter the acids, the more intense the peaks at 1644, 1571, and 1544 cm-1, indicating that short-chain acid interacts more strongly with PVI. Nonetheless, FTIR shows the existence of both hydrogen-bonding and ionic interactions between PVI and various alkanoic acids. Phase Transitions in PVI(acid)1.0 Systems. PVIalkanoic acid systems though they contain no mesogenic groups, possess a mesomorphic phase observable by both polarized optical microscopy (POM) and differential scanning calorimetry (DSC). A grainlike structure is observed by POM for all equal molar complexes. Figure 2 shows a typical POM micrograph of PVI(DA)1.0 at room temperature. The isotropization temperature of PVI-acid com(22) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 954. (23) van de Grampel, H. T.; Tan, Y. Y.; Challa, G. Macromolecules 1992, 25, 1041.
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Jiao et al.
Table 1. Phase Transition Temperatures and Enthalpies of PVI(acid)1.0 Complexesa phase transn tempb (°C)/ enthalpyc (J g-1) (by DSC)
n 18
K1
54.7
16
K1
40.8
14
K1
22.4
12
K1
-6.5
K2 62.00 K2 53.14 K2 33.98 K2 16.30
isotropization temp (°C) (by POM)d
65.5
L
53.1
L
33.6
L
6.2
L
10
L
106.3 0.830 115.2 1.449 113.6 1.986 92.3 2.840 77.1 1.698
I
114
I
114
I
112
I
107
I
86
a DSC data are taken from the second heating scan after being quenched to -30 oC at the end of the first heating scan. Heating scan rate: 20 oC min-1. n: carbon number of acid. b K1, K2: solid crystal phase. L: lamellar liquid crystal phase. I: isotropic phase. c First enthalpy values are the total of K1-K2 and K2-L transitions. d Values are taken from the heating scan.
plex increases with increasing chain length of the alkanoic acid (Table 1). For all these samples the mesomorphic structures are thermally reversible and no visible change can be detected below the isotropization temperature. DSC measurements give detailed phase transition information for these systems (Table 1). The endothermic peaks at the highest temperatures in the DSC curves correspond to the isotropization temperatures as shown by POM measurements. Except for PVI(DA)1.0, all the other PVI(acid)1.0 complexes have two other endothermic firstorder transitions. For complexes involving acids with long tails, the first transition is weak and PVI(SA)1.0 gives the example in which the first peak is a weak shoulder superimposing on the strong second peak. Since no original pure acid phase is detected by FTIR and XRD, as analyzed in a later section, these transitions are not related to the original acid phase. Ujiie et al.24 studied the mesomorphic properties of mixtures consisting of polyamine and nalkanoic acids. They found that, for polyamine-acid systems with acid carbon n > 5, a lamellar liquid crystal phase forms. For those systems with n g 14 two solid phases exist. As shown in Table 1, our DSC results are comparable to those of Ujiie et al.24 The enthalpy change of liquid crystal phase to isotropic phase transition is very small, showing that, in the lamellar liquid crystal phase, the order is far less than that in solid crystals. It is wellknown that in comb copolymers only the outer parts of the hydrocarbon side chains take part in crystallization, and the inner 7-10 CH2 groups are usually found not to crystallize.25 In our studies, crystallization of the alkyl chain is not detected for PVI(DA)1.0 by DSC. For the other systems, the crystallization temperatures of alkyl chains become lower with decreasing length of the alkyl chain. The total enthalpy changes of solid crystal 1 to solid crystal 2 and solid crystal 2 to liquid lamellar crystal transitions also show the same trend. This means that the crystallinity of the short alkyl chain is less than that of longer one. Effect of Acid Content on Mesomorphic Properties. The effect of acid content on the mesomorphic properties of PVI(PA)x complexes was examined. DSC and POM measurements show that mesogenic properties exist in complexes with higher PA contents (x g 0.4). For PVI(PA)0.2, the first heating scan shows one endothermic peak at 23.7 °C and a very broad peak centered at ca. 145 °C. After quenching from 200 °C, both peaks disappear and a glass transition appears at ca. 100 °C. For samples with higher PA contents, the DSC curves were reproducible before and after quenching. Figure 3 shows the DSC curves (24) Ujiie, S.; Takagi, S.; Sato, M. High Perform. Polym. 1998, 10, 139. (25) Jordan, E. F., Jr.; Feldeisen, D. W.; Wrigley, A. N. J. Polym. Sci., Part A-1 1971, 9, 1835.
Figure 3. DSC curves of the first heating scan for various PVI(PA)x complexes: (a) x ) 1; (b) x ) 0.8; (c) x ) 0.6; (d) x ) 0.4; (e) x ) 0.2.
of various PVI(PA)x complexes in the first heating scan. It is interesting that the isotropization temperature increases as the acid content decreases. The same trends were also observed in other PVI(acid)x systems. Chen and Chang26 studied the mesomorphic properties of solid complexes of polyacrylamide and dodecylbenzenesulfonic acid (DBSA). They found that the mesophase isotropization temperature increases with increasing degree of complexation. The difference between our results and theirs is believed to be due to the different nature of the two systems: one involves ionic interaction26 while only a low level of ionic interaction is present in our systems. FTIR measurements show that interactions between PVI and acids still exist up to 150 °C. Therefore, the isotropization of the PVI(acid)x mesomorphic structures is due to the disorder of the comblike supramolecules but not the detachment of the acids from polymer chain. In a “tightly bound” polymer-surfactant supramolecular system, isotropization will occur only when the polymer backbone has enough mobility to disrupt the ordered structure induced by the repulsive polar-nonpolar interactions. The change on the surfactant content will affect not only the mobility of the polymer backbone but also the repulsive polar-nonpolar interactions. In the polyacrylamide-DBSA system,26 with increasing DBSA content, the mobility of the polymer backbone decreases (as indicated by Tgs) while the repulsive polar-nonpolar interactions increase. Both effects make the isotropization temperature increase with increasing DBSA content. In comparison, in the PVI(acid)x system, the repulsive polarnonpolar interactions between polymer backbone and alkyl chains of the acid are also expected to increase with increasing acid content. Yet the repulsive interactions is weaker than those in the polyacrylamide-DBSA system (26) Chen, H.-L.; Chang, M.-N. J. Polym. Res. 1999, 6, 231.
Surfactant-Induced Mesomorphic Structures
Langmuir, Vol. 18, No. 4, 2002 1371
Figure 4. FTIR spectra of PVI (a), SA (g), PA (f), MA (e), and complexes PVI(MA)1.0 (b), PVI(PA)1.0 (c), and PVI(SA)1.0 (d). Figure 5. XRD patterns of PVI(PA)1.0 (a) and PVI(MA)1.0 (b).
due to the hydrogen-bonding nature in the PVI(acid)x system. On the other hand, also due to the hydrogenbonding nature, the acid is expected to act as a plasticizer of PVI. A Tg lower than that of the pure PVI (167 °C) is observed for PVI(acid)x with low acid content (x e 0.2). Although Tg cannot be observed in samples with high acid contents, it is expected that the mobility of the PVI chain will increase with increasing acid content. The plasticizing effect is supported by the physical appearance of the samples: a sample with a low acid content is hard and strong while that with a high acid content is soft. We have also observed the plasticizing effect of low molecular weight acids on basic polymers.27,28 Since the repulsive interactions are weak, the influence of the acid content on the mobility of PVI chain will be dominant. As a result, the isotropization temperature decreases with increasing acid content in the PVI(acid)x systems. The critical interaction strength for surfactant-induced mesomorphic structures in polymer-surfactant systems has been studied.11 It was found that, for aliphatic acid surfactant/poly(4-vinylpyridine) systems, although a sufficiently strong hydrogen-bonding interaction induces a comblike structure in the melt, the repulsive polarnonpolar interaction is too weak to induce a mesomorphic structure. More importantly, the strong tendency toward dimerization of acid makes macrophase separation occur at room temperature in P4VPy-acid systems with long alkyl tails. The phase behavior of surfactant/polymer systems can be tailored by modifying the attraction and repulsion of the two components.9,11 In this study, PVI is a stronger base than P4VPy and the stronger PVI-acid attraction leads to a better compatibility. Furthermore, the presence of ionic interaction contributed by proton transfer makes the repulsive polar-nonpolar interactions strong enough to induce mesomorphic properties. Since only a low degree of ionic interaction is involved, the present systems are essentially different from the polyelectrolyte-surfactant salt systems. Supramolecular Structures. As discussed in the previous section, long alkyl side chains of PVI(acid)x complexes are crystalline. The crystalline structures of aliphatic compounds can be characterized by both FTIR29 and XRD.10,30 Aliphatic acids with long alkyl chains (n g 14) have two bands at 720 and 728 cm-1, as shown in Figure 4, corresponding to the orthorhombic crystal structure. For PVI(acid)1.0 complexes, only one band is visible at 721 cm-1, which demonstrates the formation of a hexagonal-packed crystalline structure. The hexagonal structure of the alkyl chains is also confirmed by the (27) Li, X. D.; Goh, S. H. J. Appl. Polym. Sci. 2001, 81, 901. (28) Li, X. D.; Goh, S. H. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 1815. (29) Chapman, D. J. Chem. Soc. 1957, 4489. (30) Antonietti, M.; Maskos, M. Macromolecules 1996, 29, 4199.
Table 2. XRD Data for PVI-Acid Complexes at Room Temperature sample name
peak posn/2θ
corresponding dist/Å
rel ratio
proposed indexing
PVI(SA)1.0
1.729 2.508 3.330 4.943 6.636 8.165 1.853 2.782 3.560 5.465 7.066 9.101 1.836 2.529 3.606 5.401 7.134 1.852 3.647 5.417 1.814 3.597 5.378 2.015 3.990 5.960 7.952 9.947 13.937 2.876 5.721 8.461 3.258 6.337 9.574
51.067 35.192 26.511 17.864 13.309 10.820 47.648 31.731 24.798 16.158 12.500 9.709 48.084 34.910 24.482 16.350 12.381 47.656 24.209 16.302 48.654 24.544 16.418 43.801 22.124 14.816 11.109 8.885 6.349 30.690 15.436 10.441 27.093 13.935 9.230
1.00 1.45 1.93 2.86 3.84 4.72 1.00 1.50 1.92 2.95 3.81 4.91 1.00 1.38 1.96 2.94 3.88 1.00 1.97 2.92 1.00 1.98 2.96 1.00 1.98 2.96 3.94 4.93 6.90 1.00 1.99 2.94 1.00 1.94 2.94
(100) a (200) a (400) (500) (100) a (200) (300) (400) (500) (100) a (200) (300) (400) (100) (200) (300) (100) (200) (300) (100) (200) (300) (400) (500) (700) (100) (200) (300) (100) (200) (300)
PVI(PA)1.0
PVI(PA)0.8
PVI(PA)0.6 PVI(PA)0.4 PVI(MA)1.0
PVI(LA)1.0 PVI(DA)1.0
a
Unknown.
appearance of a single peak between 4.1 and 4.4 Å in the X-ray diffractograms of PVI(acid)x complexes (Figure 5). Since the pure acid exhibits two main diffraction peaks at ca. 4.1 and 3.7 Å which correspond to the orthorhombic (β0) structure of alkyl chains,10 no original pure acid phase exists in all the samples as shown by FTIR studies. As shown in Figure 5, several rather sharp peaks are observed, reflecting a high degree of order of these complexes on a nanometer scale. Table 2 summarizes the locations of all the complexes and a proposed indexing of the peak sequences. The supramolecular structures of complexes at elevated temperatures were also studied by XRD. Though complicated spectra were obtained at temperatures near transition temperatures and some uncertainties were encountered when the diffraction was near the lower limit (2θ ) 1.5°), interesting results were
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Jiao et al. Scheme 1. Proposed Structural Models for PVI(PA)1.0 at Room Temperature (Structure 2) and at 65 oC (Structure 1), Where the Cocrystallized Phase of Acid Is Not Taken into Consideration
Figure 6. Dependence of the long period of the lamellae of PVI-acid complexes on the number of carbon atoms (n) in the acid. Data from Table 2 (9, b, 1) and Table 3 (2) are included. Table 3. XRD Data for PVI(PA)x and PVI(MA)1.0 at Elevated Temperatures sample name PVI(PA)1.0 at 65 °C PVI(PA)0.6 at 82 °C PVI(PA)0.4 at 90 °C PVI(MA)1.0 at 90 °C
peak posn/2θ
corresponding dist/Å
rel ratio
proposed indexing
2.275 4.623 7.091 2.393 4.936 6.987 2.366 4.881 2.705 5.478
38.811 19.098 12.455 36.891 17.888 12.641 37.306 18.088 32.636 16.119
1.00 2.03 3.12 1.00 2.06 2.92 1.00 2.06 1.00 2.02
(100) (200) (300) (100) (200) (300) (100) (200) (100) (200)
still obtained. Table 3 summarizes some of these results. It is interesting to see that, for samples in the liquid lamellar phase at certain temperatures, another welldefined lamellar structure with a smaller layer thickness appears. Figure 6 summarizes the data in Tables 2 and 3. The dependence of the thickness of the lamellae of the complexes on the number of carbon atoms (n) can be represented by two straight lines, indicating the existence of two groups of lamellar structures. It is well-known that systems of comb copolymers with crystallizable side chains crystallize with either an interdigitating or an end-to-end packing of the crystalline layer.31,32 Recent studies33 have shown that a flexible backbone favors an end-to-end close packing of the side chains, while enhanced rigidity of the backbone chain as well as the presence of bulky groups promotes interdigitating packing of the side chains. In Figure 6, group 1 consists of PVI(DA)1.0, PVI(LA)1.0, PVI(PA)x, and PVI(MA)1.0 at elevated temperatures. A line guided by PVI(DA)1.0 and PVI(LA)1.0 gives a slope of ca. 1.8 Å/CH2 unit. This value is smaller than the theoretical value of 2 × 1.27 Å/CH2 unit for all-trans conformation in a bilayer lamellar structure. As only odd orders are found in the spectra of double-layered materials,31,34 the presence of second-order peak in group 1 samples further demonstrates that the packing of the complexes is in the interdigitating form. Similar analysis is also applicable to group 2, which consists of complexes in the solid crystal state at room temperature. Line 2 guided by PVI(MA)1.0, PVI(PA)1.0, and PVI(SA)1.0 in Figure 6 has nearly the same slope as that of line 1. Since in a hexagonal structure the chains stretch perpendicularly to the end group planes,10 it can be concluded that complexes in group 2 also possess (31) Plate, N. A.; Shibaev, V. P. Comb-shaped Polymers and Liquid Crystals; Plenum Press: New York, 1987. (32) Hsieh, H. W. S.; Post, B.; Morawetz, H. J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 1241. (33) Kricheldorf, H. R.; Domschke, A. Macromolecules 1996, 29, 1337. (34) Shearer, G. Proc. R. Soc. London 1925, A108, 655.
an interdigitating structure. Yet in group 2, only partial interdigitating is present. With estimation from the results of Luyten et al.,10 the thickness of P4VPy sublayer is about 10 Å (36.5 Å for P4VP(PDP)1.0 is subtracted by 26.2 Å for pentadecylphenol (PDP)). Assuming that the PVI sublayer has the same thickness of 10 Å (the value obtained by extrapolation of line 1 to n ) 0 is about 9 Å) and the extended length of PA is ca. 23 Å (the long period of pentadecane is 21.0 Å 10), the interdigitating thickness in PVI(PA)1.0 will be 2 × 23 + 10 - 48 ) 8 Å, which corresponds to 6 CH2 units. Since the inner 7-10 CH2 units in the hydrocarbon side chains are usually found not to crystallize,25 this partial interdigitating zone will be the crystalline one in the layer. Scheme 1 gives the proposed structural models of the two groups represented by PVI(PA)1.0 in liquid crystal state and in solid crystal state, respectively. Here the cocrystallized phase of acid is not considered. Since samples for room-temperature measurements were dried at 60 °C (a temperature at which alkyl chains are all in the melt state) for 1 day and followed by a long “annealing” period (>7 days) at room temperature before XRD measurements, structure 2 seems to be the thermodynamic stable one. Thus, the structural transition as depicted in Scheme 1 is believed to be thermodynamically reversible. The “collapse” of the long period with decreasing temperature was reported by Ruokolainen et al. in the poly(4-vinylpyridine)-pentadecylphenol complex.8 They found a sudden decrease of 5 Å in the long period accompanying a structural transformation due to crystallization of the alkyl chains, a case different from ours. Their results are understandable as it was found that the crystallization might be accompanied by a partial overlap of the side chains.31,35-37 In our case, the large “expansion” of the layer thickness with decreasing temperature is also accompanied by crystallization of the alkyl chains. The driving force for this expansion could be the need of close packing of crystalline units. (35) Kaufman, H. S.; Sacher, A.; Alfrey, T., Jr.; Frankuchen, I. J. Am. Chem. Soc. 1948, 76, 6280. (36) Chen, S. A.; Ni, J. M. Macromolecules 1992, 25, 6081. (37) Hsu, W. P.; Levon, K.; Ho, K. S.; Myerson, A. S.; Kwei, T. K. Macromolecules 1993, 26, 1318.
Surfactant-Induced Mesomorphic Structures
Conclusions The phase transitions and self-assembled mesomorphic structures of supramolecular systems based on poly(1vinylimidazole) and alkanoic acids with chain lengths of 10-18 carbon atoms have been studied. FTIR studies show the existence of hydrogen-bonding interaction and a low level of ionic interaction in these systems. Both FTIR and XRD measurements cannot detect the original pure acid phase in the complexes. All systems with equal molar components show mesomorphic properties as shown by POM and DSC. Besides isotropization transition, a solidsolid transition and a solid phase to lamellar liquid crystal transition exist in systems containing alkanoic acids with chain lengths n g 12. The mesomorphic properties of PVI(PA)x complexes are retained even at low acid content (x ) 0.2), and the lower the acid content, the higher the
Langmuir, Vol. 18, No. 4, 2002 1373
isotropization temperature is. On the basis of XRD studies at room temperature and elevated temperatures, the complexes are grouped into two types: melted lamellar liquid crystal phase with interdigitating layer structure; crystallized lamellar phase with partial interdigitating layer structure. For PVI(PA)x and PVI(MA)1.0 complexes, these two types of structures are interconvertible upon heating/cooling with a change in layer thickness of ca. 10 Å. As shown by these studies, the transition temperature and the thickness of layer can be tailored by varying the acid type and by changing the acid content in the complex. Acknowledgment. We thank the National University of Singapore for its financial support of this research. LA011001N
