Atomic Force Microscopy of Langmuir-Blodgett Films Polymerized as a

Chapter 14

Downloaded by UNIV OF CINCINNATI on November 12, 2014 | http://pubs.acs.org Publication Date: July 7, 1998 | doi: 10.1021/bk-1998-0694.ch014

Atomic Force Microscopy of Langmuir-Blodgett Films Polymerized as a Floating Monolayer Jouko Peltonen and Tapani Viitala Department of Physical Chemistry,ÅboAkademi University, Porthaninkatu 3-5, FIN-20500 Turku, Finland These topographical studies demonstrate that linoleic acid LangmuirBlodgettfilmspolymerized as floating monolayers are not ideallyflatand defect-free. Both the monomeric and UV-polymerized films contain hole -defects.As a result of the polymerization part of the film folds up from the film plane. This effect is strongly surface pressure dependent and can be inhibited by polymerizing the floating monolayer at a pressure lower than 10 mN/m.

In order to improve the mechanical, thermal and chemical stability of organic LangmuirBlodgett films, polymeric materials have been widely applied (1-51). Either preformed polymers may be used (1-27) or the monolayer can be formed of amphiphiles with reactive functional groups available for polymerization after the film formation (28-51). Several different types of surface-active polymers have been employed, one of the most typical being poly(methacrylate) (or poly(acrylate)) and its numerous derivatives (3-10). The influence of the tacticity on the film forming properties has been studied (4-5) and the rheological behaviour has been compared with other types of polymers, e.g. poly(vinyl acetate) (10) which represents another class of widely studied vinyl-based polymers (10-13). Homogeneous multilayer structures of alternating anionic and cationic polyelectrolytes have been successfully formed by using e.g. polyvinyl sulfate) and poly(allylamine) (13) or poly(styrenesulfonate) and poly(allylamine) (14), respectively. So called "hairy-rod" or "rigid-rod" polymers introduced by Wegner represent a novel class of polymeric materials that, as a result of deposition onto a solid substrate, mimic properties of liquid crystalline (e.g. nematic) order and as such are potential candidates for L B film applications as sensors and optical devices (15-20). The use of mechanically, thermally and chemically stable polyamides has attracted attention, not least due to the fact that after deposition, the polymer can be converted to the polyimide form by chemically or through heat treatment removing the hydrocarbon chains (21-27).

231 In Scanning Probe Microscopy of Polymers; Ratner, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


232 Since polymeric monolayers are occasionally difficult to process due to their high viscosity and inhomogeneous distribution at the interface, an alternative approach has been to study reactive monomers which are spread at the gas/liquid interface and the so formed monolayer is polymerized either prior to or after the deposition onto a solid substrate (28-47, 49-51). Diacetylenes and their various substituted derivatives with conjugated triple bonds responsible for the reactivity are the most commonly studied in this class (28-43). In ideal conditions, topochemical polymerization (41) takes place, i.e. the crystal structure remains unchanged and no area contraction occurs during the reaction. In reality, however, the resulting film is not a 2-D single crystal but consists of domains and thus also domain boundaries, representing discontinuities. The diacetylene monolayer may be polymerized as a floating monolayer or after deposition, even as a multilayer structure; the main prerequisite is that the monomers within a monolayer are packed in a solid state and in a suitable orientation for the neighbouring triple bonds to crosslink during UV-curing. Recently, however, it has been reported that the polymerization of diacetylenes may also be carried out in the liquid-crystalline state (42). The diacetylenes are interesting not only because of their ability to create stable polymeric monolayers but also due to their optical activity which can be further tuned by the choice of the side groups (28, 29, 32-35, 38, 39). Acyl chains incorporating diacetylenic groups have also been applied in lipid systems in order to create bilayer structures with enhanced stability (30, 31, 43, 44). The reaction in these systems is, however, no more a pure topochemical one and furthermore the polymerized film restricts using the system to mimic a natural biomembrane e.g. with respect to transport phenomena through such a rigid crosslinked structure. The real time reaction of a Langmuir monolayer may also be initiated by a catalyst introduced into the liquid subphase. The group of Duran has published results on the polymerization of e.g. aniline monolayers where the reaction has been activated by an oxidizing agent (46, 47). The present work concentrates on the topographical characteristics of linoleic acid films, both as a monomer and a crosslinked polymer. The reasons for the use of this particular acid are discussed together with a short review of the recent results on in situ polymerization and the related monolayer phase behaviour (49-54). Tsukruk and Reneker have recently reviewed the use of scanning probe microscopy (SPM) in the characterization of different types of thin films (48). Zasadzinski has reviewed the latest developments in SPM instrumentation and its practical applications (55). Here, by implementing tapping mode SPM, it was possible not only to reveal new information about the phase structure of the films, but also to image unstable samples, e.g. monolayers deposited at pH's corresponding to highly metastable structures or ones only weakly adsorbed to the solid substrate (or underlying monolayer). This set of data nicely supports both the surface balance data and the spectroscopic measurements recently carried out for the same film structures (51-53). The Surfactant and Related Substances Z,Z-octadecadi-9,12-enoic (linoleic) acid (LA, reagent grade 99% purity), n-hexane (PA grade, >99.5% purity) and TbCl » 6H 0 (98% purity) were obtained from F L U K A and used without further purification. L A was dissolved in n-hexane to form a solution 3

2

In Scanning Probe Microscopy of Polymers; Ratner, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


233 with a concentration of 1 mg/mL. The details of the monolayer formation and LB-film deposition are given elsewhere (51). It is well known that the introduction of cis-type double bonds into the acyl chain increases the minimal molecular cross-sectional area and makes it difficult to obtain a condensed monolayer at the air/water interface, as compared with e.g. stearic acid. The problem is not only due to the steric hindrance when compressing molecules with nonlinear chain, but also one of energetics; the double bonds have a rather strong attractive interaction with the water subphase. The presence of only one double bond can decrease the interfacial energy of octadecane with water from 52 to 19 mJ/m, whilst the surface energy changes only slightly (56). It is well known that the degree of unsaturation together with the position and configuration (cis/trans) of the double bonds affects the melting point and the equilibrium spreading pressure (57). Thus, special attention has been paid to the condensation of the initially expanded linoleic acid monomer film through pH adjustment and suitable choice of composition of the liquid subphase (51,52). The use of TbCl subphase at elevated pH enabled the generation of a phase transition from a liquid-expanded (LE) to a solid-expanded (SE) state monolayer (Figure 1). The acid molecules were fully ionized at pH 5, but the complete condensation took place over a very narrow pH-range of 6.8-6.9, where the conversion of Tb to its monohydroxy complex reached a maximum concentration, just prior to precipitation (51). FTIR-measurements have confirmed that the soap formation really concerns the (ionized acid) - Tb(OH) complexation (51,52), as schematically illustrated in Figure 2. It is worth mentioning that on a pure water subphase or on a subphase containing any divalent metal ions, the L A monolayer remains in a liquidexpanded (LE) state, throughout the compression and at all pH values. As may be seen in Figure 1, the extrapolated molecular area of the SE-phase still deviates from that of the tightly packed stearic acid monolayer, but the density was enough to reach an * activation area' for the polymerization reaction to be successfully initiated. In fact, the polymerization of L A in the SE-phase has been found to be more effective than that of e.g. trans-unsaturated petroselaidic (trans-6-octadecenoic acid) or elaidic (trans-9-octadecenoic) acid, which can be compressed to a crystalline state (49,50,58). The utilization of the SE-phase instead of a fully crystalline state may provide an important advantage as compared with other polymerizable monolayers; the semicrystalline SE-phase with liquid or amorphous characteristics enables not only the monomers to reorganize during the crosslinking process but also e.g. functional molecules to distribute evenly within this monomer matrix by diffusion. Subsequently the system can be stabilized by crosslinking the reactive monomers, consequently freezing the whole matrix and inhibiting the diffusion and phase separation e.g. during the deposition of the monolayer. 3

2+

2

The Polymerization Technique The polymerization of the monolayers was initiated by exposure to UV-light from a 30 W low-pressure Hg lamp, as schematically shown in Figure 3. The reaction was carried out at a predetermined constant surface pressure (typically in the range 5-20 mN/m) and irradiation-induced changes in the mean molecular area and barrier speed were detected. This set of data enabled the modelling of the reaction kinetics in a similar way as

In Scanning Probe Microscopy of Polymers; Ratner, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

234


50 4/9x10-4 M T b C l , 3

45

pH: 4.8 Linoleic acid, 40-

Ion exchanged water,

pH:3 35

Linoleic acid, 10-4MTbCl , 3 pH:6.9

30 I I

a

i

25

CA

• t %

Atomic Force Microscopy of Langmuir-Blodgett Films Polymerized as a

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