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Effect of Polymer Composition on the Morphology of Self-AssembledDibenzylidene Sorbitol John R. Ilzhoefert and Richard J. Spontak" Department of Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907 Received November 17, 1994. In Final Form: February 16, 1995@ Three-dimensional networks resulting from self-assembled dibenzylidene sorbitol (DBS) in poly(dimethylsiloxane)(PDMS) polymers have been isolated using supercritical fluid extraction and imaged with field-emission scanning electron microscopy. In pure PDMS, DBS organizes into loosely connected fibrils and fibrillar bundles 70 nm to 2 pm thick and tens of micrometerss long. Chemical incorporation of poly(oxyethy1ene)grafts along the PDMS backbone yields layered sheets of DBS, whereas grafted poly(oxypropylene)promotes the formation of a percolation network in which connective fibrils measure 10-20 nm in diameter. Such morphological variations reflect molecular interactions between DBS and the dissimilar monomer sequences in each polymer. Addition of relatively little low-molecular-weightsolvent to a flexible, nonionic homopolymer typically results in polymer plasticization and swelling,l whereas polymer association, gelation, or precipitation may occur, depending on polymer-solvent compatibility, in the dilute, or semidilute, polymer regime.z If the homopolymer is replaced by a block or graft copolymer comprised of long contiguous sequences of chemically dissimilar monomers, a variety of randomly dispersed or periodic self-assembled structures may develop3-lZ which minimize repulsive monomer-monomer or monomer-solvent interactions. While such phenomena have been extensively studied, few have addressed the microstructure which results upon the addition of a strongly self-associating agent to a homopolymer, let alone a block or graft copolymer. An example of one such agent is dibenzylidene sorbitol (DBS) which, through hydrogen bonding, readily selfassembles a t relatively low concentrations (a few wt %) to form a three-dimensional percolation network in lowmolecular-weight organic14and inorganic15liquids. The resultant network induces physical gelation, which can be accompanied by thermal and mechanical reversibility. If mixed with a semicrystalline homopolymer (e.g., isotactic polypropylene16 or poly(ethy1ene terephthalate)17)
* To whom correspondence should be addressed. +
Present address: Harris Corporation, MS 59-060, Melbourne,
FL 32902-0883. Abstract mblished inAduance ACSAbstracts, Aueust 1.1995. (1)Flory, P: J. Principles of Polymer Chemistry; Corn& University Press: Ithaca, New York, 1953. (2) Finkelmann, H.; Jahns, E. In Polymer Association Structures; El-Nokaly, M. A,, Ed.; ACS Symp. Ser. 384; American Chemical Society: Washington, DC, 1989. (3) Kinning, D. J.; Thomas, E. L. Macromolecules 1984, 17, 1712. (4) Nojima, S.;Roe, R.-J.; Rigby, D.; Han, C. C. Macromolecules 1990, 23, 4305. ( 5 ) Cogan, K. A.; Leermakers, F. A. M.; Gast, A. P. Langmuir 1992, 8, 429. (6) Spontak, R. J.;Smith, S. D.; h h r a f , A. Macromolecules 1993,26, 956, 5118. (7) Mortensen, K.; Brown, W.; NordBn, B. Phys. Rev. Lett. 1992,19, 2340. (8) Mortensen, K.; Brown, W.; Jorgensen, E. Macromolecules 1994, 27, 5654. (9) Thomas, E. L.; Anderson, D. M.; Henkee, C. S.;Hoffman, D. Nature 1988,334,598. (10) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (11)Almdal, K.; Koppi, K. A,; Bates, F. S.; Mortensen, K. Macromolecules 1992,25, 1743. (12)Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E. L.; Fetters, L. J. Macromolecules 1994,27,4063. (13) Khan, S. A,; Zoeller, N. J.Rheol. 1993, 37, 1225. (14) Roehl, E.-L.; Tan, H.-B. U.S. Patent 4,154,816, May 15, 1979. (15)Shepard, T. A. M.M.S.E. Project Report, North Carolina State University, 1995. @
Table 1. Characteristics of the Polymers Employed in This Study" PDMS PEO PPO molwt viscosity densityb designation (wt %) (wt %) (wt %) (g/mol) (cSt) (g/cm3) PDMS PS-PO PS-EO
PS-ED0
100 27 27 20
73 73 32
48
4900OC 3000 1000 29000
1000 140 50 4000
0.970 0.987 1.030 1.033
*
Provided by OSi Specialties, Inc. Reported for 25 "C. Estimated from the PDMS data provided in ref 30. a
in the melt, DBS serves as a nucleating agent to reduce crystal size upon solidification and subsequently enhance optical transparency. In amorphous homopolymers such as polystyrene and polycarbonate, DBS has been foundla to phase-separate into aggregates which measure on the order of 0.1-0.2 pm. Recent efforts have demonstrated that addition of DBS to an amphiphilic graft terpolymer composed of poly(dimethylsiloxane) (PDMS), poly(ethy1ene oxide) (PEO), and poly(propy1ene oxide) (PPO) segments yield thermoreversible physical gels which can, at sufficiently high concentrations, exhibit hierarchical structures, as discerned by rheological measurements and microscopy.lg Preferential removal of the nonvolatile polymeric matrix, permitting morphological analysis of the percolation network formed in situ, is possible through the use of supercritical fluid extraction.20 In this work, the effect of polymer composition on network formation and, hence, DBS self-assembly is explored as a means by which to differentiate dispersive interactions between DBS and chemically dissimilar polymer segments in a single-phase multicomponent matrix. Four PDMS-based polymers, provided as liquids by OSi Specialties, Inc., and varying in composition, have been used as-received (without further purification or analysis) in this study. The molecular characteristics of the polymers, as well as material designations and limited physical property data, are provided in Table 1. One of the materials was a linear PDMS, while the three remaining polymers possessed either PEO, PPO, or PEOI PPO segments grafted onto a PDMS backbone. According to the manufacturer, the PDMS backbones were either (16) Kobayashi, T.; Hasegawa, H.; Hashimoto, T. Hihon Reoroji Gakkaishi 1989, 17, 155. (17) Mitra, D.; Misra, A. J. Appl. Polym. Sci. 1988, 36, 387. (18) Mitra, D.; Misra, A. Polymer 1988,29, 1990. (19) Ilzhoefer, J. R.; Broom, B. C.; Nepa, S. M.; Vogler, E. A,; Khan, S. A.; Spontak, R. J. J. Phys. Chem., in press. (20) Ilzhoefer, J. R.; Knowlton, V. M.; Spontak, R. J. Microsc. Res. Tech., in press.
0743-746319512411-3288$09.0010 0 1995 American Chemical Society
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A -
Figure 1. Series of SEM micrographs illustrathg the morphological characteristics of the DBS network formed in PDMS homopolymer.The low-magnificationimages in (a)and (b)reveal that the DBS self-assembles into long fibrilsand fibrillar bundles. As seen by the hairpin in (c) and the twisted fibril (arrow)in (d), the fibrils are surprisingly flexible. Note that many of the fibrils in (c) and (d) appear faceted.
hydrogen, methyl, or butyl end-capped, and chemical incorporation of poly(alky1ene oxide) grafts was achieved through hydrosilation coupling (which results in hydrolytically stable Si-C bonds). The DBS was obtained as a powder from Milliken Chemicals and was also used asreceived. Vis~oelastic~~ gels were produced by heating approximately 10 mL of polymer and 1 wt % of DBS at 185-190 "C under continuous agitation until the opaque mixture cleared, signaling DBS dissolution. Each mixture was cooled quiescently to ambient temperature, upon which gelation occurred, as signified by a dramatic increase in apparent viscosity. While the steps involvedin the sample preparation for morphological analysis have been described in detail elsewhere,20suffice it to say here that the polymer matrix of each gel was preferentially extracted with supercritical Freon 13 in a Bomar SPC 1500 critical point dryingunit maintained at 42 "C and 93.1 bar, leaving behind the intact gel network which was subsequently sputter-coated with 30 nm of AdPd and imaged with a JEOL JSM-6400F field-emission scanning electron microscope operated at 5 kV. Since removal of water can be readily achieved through freeze-drying (i.e., sublimation at low temperature and
high vacuum), most morphological of threedimensional gel networks have specifically addressed aqueous polymer gels, in which collapsed polymer chains are responsible for network formation. In the present study, however, nonvolatile polymer constitutesthe matrix and DBS contributesstructural rigidityby self-assembling into a 3-D fibrillar network. While block and graft copolymer melts are also capable of ordering into a variety of periodic morphologies if xN > (xIV)om (x denotes the Flory-Huggins interaction parameter, N is the number of monomers, and ODT refers to the order-disorder t r a n ~ i t i o n ) , ~ ~the - ~ ' three multicomponent materials investigated here are sufficiently compatible so that xN < (xIV)om. (21) Tohyama, K.; Miller, W. G. Nature 1981,289,813. (22) Muller, T.; Hakert, H.; Eckert, Th.Colloid Polym. Sci. 1989, 267, 230, and references therein. (23) Echlin, P. Low-Temperature Microscopy and Analysis;Plenum Press: New York, 1992. (24) Leibler, L. Mucromokcules 1980,13, 1602. (25) Benoit, H.; Hadziioannou, G. Macromolecules 1988,21, 1449. (26)Mayes, A. M.; Olvera de la Cruz, M. J. Chem. Phys. 1989,91, 7228. (27)Melenkevitz, J.; Muthukumar, M. Macromolecules 1991, 24, 4199.
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Figure2. Pair of electronmicrographs of self-assembled DBS in a PS-PO polymer. At low magnification (a), a DBS percolation network is evident. A higher magnification image of this highly connected fibrillar network is displayed in (b).
I
L.
f
Figure 4. Electron micrographs of self-assembled DBS in a PS-E/PO polymer in which the E0:PO ratio is 2:3.While lowmagnification images indicate that the packing density differs from the surface (a) to the interior (b) of the gel, close examination reveals that both morphologies are comprised of fibrils (arrow), as further verified by the high-magnification image in (c). Figure3. Scanningelectronmicrograph of the PS-EO polymer revealing a solid-like layered DBS morphology, in contrast to the fibrillar networks seen in Figures 2 and 3.
Micrographs obtained from the gel composed of pure PDMS are displayed in Figure 1. The low-magnification images in parts a and b of Figure 1 clearly reveal that the
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Letters network is not homogeneous, consisting of loosely connected fibrils and fibrillar bundles up to tens of micrometers in length. The diameter of the smallest observed fibrils measures on the order of 100 nm, which, upon correcting for the Au/Pd coating, translates into an actual fibril diameter of approximately 70 nm. Existence of such thin fibrils is a good indication that the supercritical extraction process used to obtain these images has not promoted large-scale collapse of unsupported fibrils. Some of the larger fibrillar bundles measure 2 pm across and, from Figure IC,consist of several fibril layers, each about 130 nm thick. Even though the fibrils impart rigidity to PDMS, they exhibit flexibility as well, as demonstrated by the hairpin seen in Figure ICand the twisted fibril in Figure Id. An interesting feature evident in these images is that most of the fibrils possess a faceted, not rounded, surface, which is reminiscent of both as-received DBS powder and pure DBS exposed to supercritical Freon 13.z8 Chemical incorporation of PPO grafts along the PDMS backbone results in a DBS-induced gel composed of a fine, highly connected fibrillar network, as is seen in the micrographs provided in Figure 2. While fibrillar bundles are abundant in the PDMS gel network (Figure la-c), they are not observed in the PS-PO network (Figure 2a), indicating that DBS interacts more favorably with PPO than with PDMS and consequently forms a network which possesses considerably higher surface area. The mean fibril diameter in images such as the one shown in Figure 2b, appearing more uniform and smaller than that in the PDMS gels, measures about 20 nm (corrected for the A d Pd coating). In addition, the fibril ends seen in Figure 2b appear to be capped by nodules, which may be indicative of the collapse of finer unsupported structure during extraction of the PS-PO matrix. If PEO grafts are substituted for the PPO grafts along the PDMS backbone, discrete DBS fibrils are not observed to form. Rather, as seen in Figure 3, a layered and inhomogeneous structure develops. Formation of this structure may be the result of an increase in the polarity of the poly(alky1ene oxide) segments, since the polar and hydrogen-bonding contributions to the cohesive energy density (in J/cm3) are estimatedz9 to be 101 and 75, (28) Pure DBS is sparingly soluble in, and virtually unaffected by, supercritical Freon 13 under the extraction conditions employed here. (29) van Krevelen, D.W. Properties of Polymers: Their Estimation and Correlation with Chemical Structure;Elsevier: Amsterdam, 1976; pp 142-155. (30) Fox, T. G.; Gratch, S.; Loshaek, S. In Rheology; Eirich, F. R., Ed.; Academic Press: New York, 1956; Vol. 1.
respectively, for PEO and 51 and 54, respectively, for PPO. Thus, DBS molecules are expected to interact more with PEO grafts than with PPO grafts or the PDMS backbone. Another difference between the PS-EO polymer and both PDMS and PS-PO polymers which may, in part, be responsible for this layered structure is that the PS-EO polymer possesses the lowest molecular weight (1000)and kinematic viscosity (50 cSt) of the four materials employed in this study. In contrast, the DBS network formed in a highmolecular-weight PS-EPO polymer (with a mass ratio of 2/3 EOPO) is displayed in Figure 4 and is reminiscent of the fibrillar network observed in the PS-PO polymer (Figure 2). In Figure 4a, the surface of the network is shown and, as reported elsewhere,lg consists of discrete fibrils collapsed and tightly packed, due presumably to surface tension effects. The existence of highly stretched fibrils in Figure 4a is consistent with this picture. It is important to recognize that this surface (“skin”)texture, also observed in DBSPS-PO gels, is considerablydifferent from the layered structure shown in Figure 3 for DBS in the PS-EO polymer. A micrograph illustrating the representative (internal) DBS percolation network is presented in Figure 3b, and an enlargement showing discrete fibrils is provided in Figure 3c. These fibrils measure about 10 nm in diameter (corrected for the A d Pd coating) and are therefore comparable to those seen in Figure 2b for the DBS network in PS-PO. In this work, the morphological characteristics of DBS gel networks in four PDMS-based polymers have been investigated. Networks composed of discrete fibrils are observed in the PDMS homopolymer and the PDMS polymer possessing PPO grafted segments, whereas a layered structure is found to develop in the presence of pure PEO grafts. These morphologicaldifferencesprovide evidence that, although these grafted polymers are unstructured (microscopically disordered), the presence of chemically dissimilar segments governs the dispersive interactions and self-assemblyof DBS and, consequently, the ultimate properties of the resultant gel.
Acknowledgment. We are indebted to Becton Dickinson, Inc., for supporting this project and C. K. Chiklis, E. A. Vogler, and T. A. Shepard for valuable discussions. We thank V. M. howlton and J. M. Mackenzie for assistance in sample preparation. LA940917+
