5504 Macromolecules 1994,27, 5504-5510 The Structure of Polymeric Alkylpyridinium Monolayers Spread on Water Studied by Specular X-ray and Neutron Reflection D. A. Styrkas and R. K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K. Z. A. Adib, F. Davis, P. Hodge, and X. H. Liu Department of chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K. Received April 4, 1994’ ABSTRACT: Grazing incidence X-ray reflection has been used to examine the structure of spread monolayers of poly(N-alkyl-4-vinylpyridinium bromide)s, denoted by PC, where n is the length of the alkyl side chain, on water. Members of the series with even n from 8 to 22 were investigated, and the results for PCls are compared with those of neutron reflection on the same sample. The comparison of neutron and X-ray results on PCle makes it possible to identify those contributions to the X-ray reflectivity coming from the polymer backbone and alkyl chains for this particular polymer. On the basis of this comparison the thickness of the polymer layer has been estimated for each member of the series. The ratio of the observed thickness of the monolayer to its maximum possible value (6/1) varies from 1.3 for n = 8 through a minimum of 0.7 at n = 16 to 0.8 at n = 22. Even when allowance is made for the contribution of capillary waves to the thickness of the layer, 6/1 remains above unity for the shortest chain compounds. This is interpreted as arising from disorder in the layer. As the side chain length increases, it imposes order on the chain orientations. For small n an increase of n therefore decreases the thickness of the layer, but once the side chains are reasonably ordered the increase in chain length causes the thickness to increase once more. The X-ray reflectivity shows, independently of the neutron result, that part of the polymer layer protrudes out of the water. For small n this projection is about one-third of the thickness of the total monolayer thickness, but this proportion increases with n until eventually it has reached about half the total thickness of the layer. Comparison with similar dimensions in surfactant layers and the PC, Langmuir-Blodgett films indicates similar behavior at small n. The comparison between LB films and the precursor monolayer of the PC, shows that disorder in the latter carries over into the LB film. Introduction Monolayers of amphiphilic polymers a t the air-water interface are of interest because such systems are some of the simplest involving polymers a t interfaces and also because they may be the precursors to polymeric Lang- muir-Blodgett (LB) films. LB films have numerous potential applications in micro- and opto-electronics.’v2 This is particularly the case for polymeric LB films, as they are more robust than nonpolymeric films. One application is devices based on second harmonic generation (SHG). Films for such devices must be noncentro- symmetric and contain a highly polarizable *-electron systema3 Several all-polymeric LB films for SHG have been described recently: see, for example, refs 4-6 and references cited therein. The nonsymmetric LB films used are generally Y-type films prepared from two different polymers deposited alternately. One polymeric am- phiphile contains the “active” moiety; the other is a “passive” polymer. We recently showed that poly(N- docosyl-4-vinylpyridinium bromide) served as a particu- larly good passive p01ymer.~This is probably associated with the fact that this and some other polymers of the same general type form some of the best organized polymeric LB films so far investigated as judged by the number of Bragg peaks obtained in X-ray studies.’ In this paper we present the study of spread monolayers of poly(N-alkyl-4-vinylpyridinium bromidels, denoted by PCn, on water using specular X-ray and neutron reflection. The range of wavevectors accessible in neutron and X-ray reflection enables one to study layer thickness and structure on a molecular scale, and these techniques are therefore suitable for investigating the properties of Abstract published in Advance ACS Abstracts, August 1,1994. 0024-9297/94/2227-5504$04.50/0 Figure 1. Structure of poly(N-alkyl-4-vinylpyridine) bromide compounds. monolayers deposited on the water surfacea8p9 The struc- tural information obtained in such experiments is of particular importance for the deeper understanding of the structure of the monolayer presursors of the LB films. Experimental Details The Teflon Langmuir trough had two moving barriers with a working area which could be varied between 200 and 70 cm2. The surface pressure was controlled with fl mN/m accuracy using an induction sensor with spring and Wilhelmy plate. High purity water (Elga Ultrapure system) was used throughout as a subphase. The PC, (see Figure 1) were synthesized by quaternization of poly(viny1pyridine) (M, = 42 700, by viscometry in ethanol at 25 “C) as described previously.’ The fraction of reacted pyridine moieties (degree of quaternization = DQ) for each compound is shown in Table 1. The deuterated polymer was prepared similarly and had a DQ of 0.85. PC, solutions in chloroform (Aldrich, HPLC) of approximately 0.02 wt % concentration were used to deposit the monolayer on the water subphase. The X-ray measurements are very sensitive to vibration, and a vibration free table (Vibration Isolation System Mod-2, JRS) together with a heavy granite block was used to eliminate external vibrations of the surface. The granite block could be raised and lowered remotely, which made it possible to maintain the level of the 0 1994 American Chemical Society