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Macromolecules 1980,13, 1478-1483
Morphology of Crystalline Diacetylene Monolayers Polymerized a t the Gas-Water Interface David Day and J. B. Lando* Department of Macromolecular Science, Case Institute of Technology, Case Western Reserve University, Cleveland, Ohio 44106. Received March 11, 1980 ABSTRACT Polymerized diacetylene monolayers exhibit a birefringence indicative of a high degree of backbone orientation over macroscopic distances. The crystallization of diacetylene monolayers is similar to normal “three-dimensional” processes in that the resulting morphology is dependent on the conditions of crystallization. In accordance with this, large monolayer crystallites can be produced by saturating the atmosphere above the monolayer with the spreading solvent, effectively slowing down the rate of crystallization. Two monolayers sandwiched together retain their individual crystalline morphologies and act to reinforce one another. The resulting bilayer membranes are extremely strong and are able to span macroscopic holes.
Tntroduction Many attempts have been made to obtain ultrathin polymeric films through the polymerization of monolayer films. It has recently been shown’-3 that certain surfaceactive diacetylene compounds can be polymerized in the monolayer, resulting in highly ordered polymeric membranes. Although this reaction has been found to occur for a variety of diacetylene compounds, this discussion will be limited to the following surface-active compound: CHs(CHJ &EC-CEC(CH~)&~OOH Polymerization in the monolayer is the same as that of the well-characterized solid-state diacetylene reaction4 and is shown schematically in Figure 1. The free-acid monolayers were converted in some cases to the corresponding lithium salt through the addition of lithium hydroxide to the water substrate.’ Monolayers will subsequently be referred to as either free-acid or lithium salt monolayers, depending on their composition. In contrast to most other monolayers which contain an order-disorder-type packing, poly(diacety1ene) monolayers are highly crystalline with long-range two-dimensional order. The crystalline monolayer morphology can be observed between crossed polarizers in an optical microscope and will be the main topic of this paper. Experimental Section All monolayers were formed on an 8 X 10 cm Teflon trough. Deionized water was treated with potassium permanganate and then doubly distilled from an all-glass apparatus to obtain a pure-water substrate. Lithium salt monolayers were prepared through the addition of lithium hydroxide to the water substrate (approximately M).l Monolayers were spread from chloroform solutions unless otherwise stated. Concentrations of 1mg/mm were prepared and administered to the water surface with an all-glass micrometer-type syringe. Solutions were spread on the water until excess material was observed to form a “lens” on the surface. After solvent evaporation the layers were polymerized under nitrogen with a short-wavelength mercury lamp. Layers were deposited onto glass plates for observation in the optical microscope by dipping the slides with their surfaces parallel to the water surface down through the polymerized film. Two layers or bilayers were formed on porous substrates (usually electron microscope grids) by dipping twice through a polymerized layer. The technique used to form a bilayer on an electron microscope grid is shown in Figure 2. An electron microscope grid is first dropped onto the polymerized layer. A solid substrate is then brought down on top of the grid and pushed down through the water surface. The solid substrate behind the grid is important in that it traps air in the grid holes between the monolayer and itself. The trapped air acts as a supporting cushion for the monolayer while underneath the water surface. The assembly is then inverted underneath the water and drawn up through a second layer. Water initially trapped between the 0024-9297/80/2213-1478$01.00/0
two layers seeps out to the sides and the two layers come together. After drying, the grid can be picked directly off the substrate and observed in the optical or electron microscope.
Results and Discussion Optical Microscopy and Birefringence. Polymerized diacetylene monolayer removed from the water surface and then observed in an optical microscope between crossed polarizers exhibited various types of structural features. Polymerized monolayers of the free-acid monomer showed typically a mosaic block structure, where the block size or crystallite size was as large as 0.1 mm (Figure 3). Monolayers of the lithium salt were observed to have a twodimensional spherulitic-type morphology (Figure 4) with diameters as large as 1 mm. The origin of birefringence in any substance is due to crystallinity or a high degree of order over a macroscopic range. This long-range order results in an anisotropic polarizability in the material. The polarizability ( C Y ) is related to the more commonly known refractive index (n) by5
where M is the molecular weight, D is the density, and No is Avogadro’snumber. The anisotropic polarizability can lead to a rotation of polarized light passing through the substance, thus allowing it to pass a second polarizer at 90° to the first and resulting in the effect known as birefringence. A rotation of the polarized light will occur only if the incident polarized vector is at some angle other than 0 or 90’ to the axis of maximum or minimum polarizability in the material.6 If this is the case, the rotated light will only be observed if the sample is thick enough and if the difference in magnitude of the maximum and minimum polarizability is great enough (both contribute to the degree of rotation). These poly(diacety1ene) multilayers strongly absorb in the green region of the visible spectrum. Because of its extreme thinness, a monolayer, even if it were crystalline, would never be expected to exhibit any birefringence. The thinness is counteracted, however, in the poly(diacety1ene) monolayer by an extremely anisotropic polarizability of the polymer backbone. It is only anisotropic, as can be seen from their green color, in the lower end of the visible spectrum. This is not surprising, however, because this is the region of maximum absorbance of the diacetylene backbone and is exactly where a high polarizability would be expected. If polarizers are slightly uncrossed by as little as 0.2”, the colors just to either side of green in the visible spectrum are observed, depending on the angle that the polymer 0 1980 American Chemical Society
Val. 13, No. 6,November-December 1980
Crystalline Diacetylene Monolayers 1479
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Figure 1. Diacetylene monolayer reaction scheme.
usually observed. Because the color is dependent on chain orientation with respect to the polarizers, and because no optical rotation is observed where the maximum polarizability (chain axis) is either parallel or perpendicular to the polarizers, the chain orientation in the various morphologies can he easily deduced. The mosaic block structure has an obvious morphological arrangement. It consists of many blocks, each at their own random orientation, but within a given block there is total orientation with parallel polymer backbones. These individual blocks can he thought of as single monolayer crystallites and are schematically represented below.
pb.. SYWO11
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s w 0 1 1 1nf1.4
4
Figure 4. Optical micrograph of spherulitic structure.
wffnc. 01
through
and forrnolmn
lk
bilopi
blocks
rnosioc
The chain orientation in the two-dimensional spherulitic structure is not so obvious. Initially one might expect a radial- or circumferential-type chain orientation:
radial -or .notn ..tnese .. cases, one
circumferentia1 . . . . , k would expect dark regions (regions where no optical rotations occur) where the chains are parallel and perpendicular to the polarizers. If the polarizers are at 0 and go", then dark regions would be expected for both these structures also a t Ooo and goo": I .
POLARIZERS
I
II
In some cases samples show extinction at 0 and 90° to the
1480 Day and Lando
Macromolecules 5, . ,,_.., , ,.I_. ::,:.
,.. . .'i.:p:.:.
Figure 5. Optical micrograph of circumferential spherulite.
D.
Figure I. Spiral spherulite schematic.
Figure 6. Optical micrograph of spiral spherulite. polarizers (Figure 5) and from the circumferential cracks can be concluded to have the structure shown in case 11, above. It is still not certain whether the cracks originate from the contraction that occurs during polymerization' or from breaking during deposition. Other samples exhibited dark regions, not at 0 and 90" but a t +50 or -50°, both coexistent in the same sample (Figure 6). The colors in the four light quadranta are also observed to be consistent for either +50 or -50' orientations. If one looks at the edge of the monolayer in Figure 6 where it has been broken off, small fibrillar structures can be seen. It can be assumed that the polymer backbone is parallel to the fiber axis and from this it is known that backbones at +45" appear blue. If the areas of the neighboring two-dimensional spherulites are divided up into eight regions, the structures can be deduced (Figure 7A). From the yellow and blue colors, the chain directions are filled into the corresponding regions (Figure 7B). In the dark regions the chains can be either 0 or 90'. but by rotating thesample 4 5 O and checking the colors, the corre& orientations were determined and are shown in Fieure 7C. The structure has been made into a continuous one in Figure 7D, resulting in a spiral chain orientation, with both left and right spirals being present. This is confirmed by the presence of spiral cracks of opposite directions in the neighboring spherulites. Spherulites in polymers normally result from a rapid crystallization in a given crystallwaphic diredion but with limited or hindered movements, causing a radial-type growth. The long side-chain diacetylenes are simikr to polymers in that they are long, flexible, and difficult to crystallize. The fact that successive crystallites with their chain axes Y
w (1.741A
Figure 8. Polyethylene unit cell type.
in a spiral arrangement are formed indicates the direction of maximum growth rate is at an angle of 50' to the direction of polymerization in the monomeric unit cell. If a packing similar to that of orthorhombic polyethylene is assumed, this phenomenon can be explained by the fact that there is an angle of about 45' between the planar zigzag and the a axis of polyethylene (Figure 8). This means there is also an angle of about 45" between a line perpendicular to the planar zigzag (direction of highest probable growth rate) and the polyethylene b axis of 49.9 b, (most likely direction of polymerization). It is not unreasonable that the monolayer molecules would uack like polyethylene, for they are very similar. In the c&e of the circumferentiah oriented soherulites. one alwavs observes growth lines of &known oigin (Fi&e 9). Th-ese appear at +50 and -50' to the chain axis, both occurring together in the same area of the sample. The maximum growth rate for these samples occurred in a direction that would be equivalent to the a axis in polyethylene. Both the circumferential and spiral spherulites have been obtained under apparently identical conditions. It is suspected that some uncontrollable variables, possibly impurities in the spreading solution or the water substrate, play an important role in the determination of the resulting morphology. Other more controllable variables are known to affect the resulting morphology and this is described in
Vol. 13, No. 6, November-December 1980
Figure 9. Optical micrograph of growth lines in circumferential spherulite.
the next section. From electron diffraction and birefringence studies of fibers pulled out from bilayer samples, discussed in a later section, it can he concluded that the polymer has the acetylenic hackhone structure. The chain axis is along the fiber and extinction occurs at 0 and 90" to the polarizers. Monomer Monolayer Morphology. After the observation of the mosaic block and two-dimensional spherulite structures, the question arose as to whether the observed morphologies were present in the initial monomer phase or if they grew during the polymerization. If the morphologies were present in the initial monomer phase, they should he altered hy changing the conditions under which the monolayer was formed. If the morphologies arose during polymerization, they should be altered by varying the reaction conditions (UV intensity or reaction time). To answer this, UV intensity and reaction time were varied, but only the degree of polymerization (absorbance) and degree of birefringence were affected. The observed crystalline shapes in the optical microscope did not change, indicating that the crystalline morphology must already be present in the monolayer before reaction. The monomer monolayers unfortunately exhibited no birefringence in the optical microscope, hut as mentioned previously this would be expected, due to the absence of a large anisotropic polarizability [no poly(diacety1ene) chains]. The resulting polymer morphologies were observed to change radically, however, when the initial monolayer spreading conditions were altered. By changing the concentration or spreading solvent, for example, the condition and time for crystallization of the monomer molecules were altered, resulting in varied monomer morphologies. The particular morphology was retained through the reaction step and ohserved in the final polymer monolayer. Morphology Dependence. A conversion from a spherulite morphology of the lithium salt layers to a mosaic block structure was observed when the initial spreading mol of 16-8/L of solution (about 1mg/mL or 2.3 X CHC1,) was increasingly diluted with hexane, a poor monomer solvent. Dilution of the solution with ethanol, which is soluble in the water substrate, resulted in extremely poor layers. Ideally, it would he preferred to have an entire monolayer consisting of a single crystallite with the polymer chains all parallel and in a single direction. Although this has not yet been achieved, significant progress was made in increasing crystallite size and orientation. An attempt was made to grow the crystallites under a unidirectional influence in order to orient them. This was done by depositing an excess of spreading solution at one
Crystalline Diacetylene Monolayers 1481
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Figure , . 10. Optical micrograph . . .. of large . polymerized monolayer
Figure 11. Optical micrograph of large contaminated polymerized
crystallite.
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" .. ... " end of the monolayer tray wtn a smau sunace area. 'rne area was slowly increased in a direction away from the excess material by moving a barrier. .I
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IIn thin way the monolayer was allowed to form or expand out of the excess material and solvent. After polymerization, the resulting morphology appeared as row8 of large oriented crystallites (Figure 10). Since the barrier was moved by hand, the striations probably are due to the fluctuations in the barrier speed as it was being moved. Another attempt at making large crystallites was made by spreading the monolayer under a CHC13-saturated atmosphere. Because the spreading solvent was also CHCl,, it evaporated very slowly when the monolayer was spread and allowed more time for the monolayer to crystallize. Initially the vapor was saturated by dropping pure CHCl, onto the water surface in an enclosed tray until it ceased to vaporize. A saturated level was reached very quickly by this technique because as the CHCl, touches the water surface, it spreads out to a large surface area and vaporizes very quickly. The spreading solution was then dropped on the surface and very large crystallites were obtained (Figure 11). Although these sometimes were as large as 3 mm across, they often contained defects of unknown origin. Because large amounts of CHCI, must be deposited onto the water surface to reach saturation, trace impurities
1482 Day and Lando
Macromolecules
Figure 12. Optical micrograph of two large defect-free polym-
Figure 13. 01
erized crystallites.
probably accumulate and tend to contaminate the monolayer. This caused large areas with no polymer layer and possibly was the origin of the crystalline defects or contamination. Cleaner layers have been obtained, however, hy placing a container of CHC18 next to the tray in the enclosure. Because the surface is small, a much longer period is required for saturation, but large defect-free crystals can be obtained (Figure 12). Visible Crystallites. Large polymer crystallites present on the water surface are visible to the naked eye if viewed in reflected light at low angles of incidence:
”.*
I
This is presumably due to the natural polarization of light that occurs upon reflection at low angles of incidence. This polarized light (polarized horizontally or parallel to the monolayer plane) is selectively absorbed hy crystallites with polymer chains oriented perpendicular to the propagation of light and transmitted by crystallites with chains nearly parallel to the propagation direction. Observation of the crystallites on the water surface can be greatly enhanced hy passing the light through a horizontal polarizer before reflection and a vertical polarizer afterward. This geometry allows a reflective birefringence to be observed and has aided tremendously in selectively depositing areas of large crystallite size onto solid substrates. Bilayer Formation. Single-polymerized layers often broke apart along the crystalline boundaries when deposited on glass. This is presumably caused by low tensile strength in these regions from the lack of polymer chains. Coherent monolayers can be deposited onto a substrate if extreme care is taken, but it was found that by sandwiching two layers together, a much stronger membrane is formed. In such a bilayer, each layer retains its crystalline morphology independent of the second layer. This appears in the optical microscope as two superimposed structural patterns (Figure 13). Bilayers were usually made with the hydrophilic ends in the center of the membrane and the hydrophobic tails pointing outward. The polar heads act as a molecular glue to hold the layers together while the polymer chains in one layer act to hold crystalline boundaries in the other layer together. The bilayer can be thought of as a two-layer laminate or plywood-like structure. Bilayers formed in this manner result in much sturdier membranes and are able to span across macroscopic-size holes in porous substates. Bilayers
Figure 15. Electron micrograph of platinum-shadowed bilayer (magnification ZOOOX).
formed form the large crystallite layers can cover holes larger than 0.5 mm in diameter and are completely selfsupporting. The width-to-thickness ratio of such a membrane is approximately 700001. The strength of such a membrane must clearly he tremendous in order for it to be self-supportive. For ease of study, bilayers were routinely formed on electron microscope copper grids. The holes of the grids are in most cases substantially smaller than 0.5 mm, so little care had to be taken in obtaining stable bilayers. To check for successful bilayer formation, we observed the electron microscope grids in the optical microscope.
Macromolecules 1980,13, 1483-1487
Figure 16. Electron micrograDh of laree hilaver fibers (maeni-
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oratory shelves and exposed to light, however, tend to break in a period ranging from 0.5 to 1 year. This is probably caused by trace amounts of ozone in the air cleaving the polymer backbone over long periods of time. Electron Microscopy of Bilayers. Bilayers on electron microscope grids were shadowed under a high vacuum with platinum or gold at 45O to enhance any surface features. A t low magnification a thin sheet is observed spanning across the grid holes (Figure 15). Small holes are present in some places and may be caused by places in the bilayer where two crystalline boundaries in both layers coincide, resulting in low strength and breakage. The holes may also result from the bombardment of the hot metal particles during the shadowing process. Whatever the origin, films which were made from monolayers containing large crystallites exhibited relatively few holes. I Philorrn- hoc u t o d n r l tr. hraat i a oh-...Aregionwhereth,,..,,. II-I-_Y-U.lM.YI.IY”.. in Figure 16 and exhibits the very fibrous nature of the bilayer membrane. In another region of separation, fibers can be observed from each of the two layers in the bilayer forming a cross-hatched network (Figure 17). The parallel nature of the microfibers is another indication of the unidirectional polymer chain orientation within the monolayer plane in a given crystallite. As would be expected from the crvstalline nature. these lavers ~. Droduce hiehlv oriented el&& on diffraction patterns analogous tosingle-crystal diffr,action. The detailed analysis of this phenomenon is discussed in the following paper. __.._ o...--~-L ~ d I I ~ : Acknowledgment. oupporz or mis WOIK unaer XI national Science Foundation Grant DMR-77-13001-A01 is gratefully acknowledged. &
References a n d Notes
Figure 17. Electron micrograph of small bilayer fibers (magnification 17500X).
From the birefringence, the presence of the bilayer could be easily seen (Figure 14). In many cases the bilayers have holes which can be seen by lack of birefringence in some areas. Bilayers are fairly stable, especially if kept cold and in the dark. Some bilayers that have been stored on lab-
(1) Day, D. R.; Ringsdorf, H. J. Polym. Sci., Poiyrn. Lett. Ed. 1978, 16, 205. (2) Day, D. R.; Ringsdorf, H. Makromol. Chem. 1979, 180, 1059.
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~
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Microskpe;’; E. Arnold Ltd;: York, 1970. (6) Gay, P. “An Introduction to Crystal C)ptics”;Longmans, Green and Co.: New York, 1967. (7) Geil, P. H. “Polymer Single Crystal4”; Interscience: New York, ~
1963,
Structure Determina[tion of a Poly(diacety1ene) Monolayer David Day a n d J. B. LancIo* Department of Macromolecula;v Science, Case Institute of Technology, Case Western Reserve University, Cleuelan d, Ohio 44106. Received March 11, 1980
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. ..
ABSTRACT Electron diffraction of self-supporting I . . .. nom eacn or me monolayers crystalline. Two misregistered diffraction patterns were Irsuariy oDserveo, ongmamg comprising the bilayer. Weak “superstructural” diffi,acti(In intensities and uDuer laver line smearine were shown t o he the result of a systematic type of disorder and were rcproducrd th&h laser opriral diffrhion. The intensities of 13 independent reflections were collect,ed and used i n r e h e Ihe parking within a single monolayer. As a result of the partially ambiguous soslutioo, monomeric monolayer diffraction was used t o .^ .. .. . ClaIlfY tne UacKlng Symmetrv and deduce the correct pol:ymer monolayer structure.
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Introduction Recent investigations 1have i..,.,,, monolayers of certain surface-active diacetylene compounds to undergo The resulting polymeric monolayer films are rigid and consist of rather large two-dimensional crystalline domains? Two monouvAL.y,
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able to span macroscopic holes? The high degree of order and the thinness of these bilayer films make them suitable for structural investigation by electron diffraction. Due to the high structural integrity of the diacetylene lithium salt monolayers, all discussion in this paper is limited to 0 1980 American Chemical Society
