Langmuir 1990,6, 307-311
tion. It was not possible to obtain information in the x direction with the equipment used, but it seems probable that the lattice spacing in this direction is distorted as the lattice planes slide across each other. Thirdly, at high shear rates the high-order diffraction spots disappear. This can be associated with shear-induced "melting" of the crystals with the necessary energy being obtained from the shear field. It is necessary to explain the observation that good "single-crystal" patterns are not always obtained on filling the cell with similar samples. However, we do have some observations that particles of diameter 2050 A at the volume fraction used in these experiments may form a "glassy" state in which the long-time self-diffusion coefficient may be very This effect is best observed by observation of particle dynamics, for example, using photon correlation spectroscopy rather than structural studies. Disordered or faulted structures may be kinetically stabilized in samples at rest. The reorientation under shear which can induce melting may permit the later establishment of better ordered structures at rest. Thus we suggest that depending upon the conditions used both disordering and ordering under shear are possible. As the initial state is not necessarily a true equilibrium state, the sample history can be important. It is interesting to compare the present work with that of previous workers. As with the work of Pieranski'* and
307
Ackerson and Clark," who used optical methods, the phenomena of shear melting has been confirmed. In the only other study by small-angle neutron scattering, Ackerson et using polystyrene particles with a diameter of 1090 A and a solids fraction of 0.14, found very similar effects, but they occurred at very much lower rates of shear. At rest, their system was highly ordered, significant changes occurred at a shear rate of 160 s-l, and the sample at 400 s-' had an amorphous structure with a pattern similar to that observed in our case at 10000 s-'. They concluded, as we do, that as the applied stress increased the intrinsic elastic limit of the crystal was exceeded as it began to flow. In an earlier study by neutron scattering of latices containing particles of diameter 310 A at a volume fraction of 0.14, we observed' that although the system was highly organized it did not crystallize. It is clear from our previous work on sheared systems2' that Brownian motion plays an important role in disordering structures, but this is almost certainly not the only effect, and further investigations of the effect of particle size are required.
Acknowledgment. Our thanks are due to S.E.R.C. and I.C.I., PLC, for support of this work and to the Institut Laue Langevin, Grenoble, France, for the use of neutron facilities. Registry No. Polystyrene, 9003-53-6.
Silylation of a Tubular Aluminosilicate Polymer (Imogolite) by Reaction with Hydrolyzed (y -Aminopropyl)triethoxysilane? Leighta M. Johnson and Thomas J. Pinnavaia' Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824 Received December 29, 1988. In Final Form: July 24, 1989
The reaction between imogolite, a novel tubular aluminosilicate with an outer molecular diameter of -23 A and an inner diameter of -8.2 A, and (yaminopropy1)triethoxysilane (APS) in aqueous acetic acid (pH 3.6) has been investigated by FTIR and 29SiMAS NMR spectroscopy. For products formed over the formal APS:AI range 2.51 to 0.51, the imogolite tube structure is retained, and the surfacebound APS is in the form of a protonated siloxane polymer. Little or no change in 29SiNMR chemical shift is observed for the silylated products, suggesting that the APS binds mainly at the external AlOH surface of the tube wall, not at the internal Si-OH surface. Both partially cross-linked RSiOH(OSi), and fully cross-linked RSi(OSi), sites are observed for the APS-functionalized imogolite. The degree of APS cross-linking is lower in the surface-boundstate than in the pure polymer, but the APS-functionalized imogolite is more stable toward hydrolysis than the pure polymer when dialyzed against distilled water. Nevertheless, nearly complete loss of APS from the imogolite surface occurs after only 3 days of dialysis time, indicating that the APS-imogolite interface is quite labile.
Introduction Chemical modification of surfaces is an area of intense interest from both fundamental and practical points of view.'-' A variety of analytical techniques have been
recently developed which enable the study of surface chemical interactions and the identification of chemical spe-
icaisociety, Los Angeles, Sept 25-30, 1988.
73.
0743-7463/90/2406-0307$02.50/0
(1) Plueddemann, E.P.Silone Coupling Agents; Plenum: New York,
0 1990 American Chemical Society
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Figure 1. Cross sectional view of the tubular aluminosilicate imogolite (adapted from ref 22). The inner and outer tube diameters are -8.2 and 23 A, respectively. Tube lengths of the syn-
thetic derivative used in the present work are typically from 250 to 350 nm. The empirical formula, as read from outer to inner atomic planes, is (HO),Al,O,Si(OH).
cies present on a ~ u r f a c e . ~This - ~ molecular-level information is essential for an understanding of the macroscopic properties of a surface, such as adhesion, resistance to chemical attack, and fracture. The tubular aluminosilicate polymer imogolite, (HO),Al,O,Si(OH), has been targeted for surface modification in the present study. The imogolite structure is shown in cross section in Figure 1. The molecular size of the tubular unit (-23-A diameter, -8.2-A internal channel, and 250-350-nm length) gives rise to a large surface area of 900 m2/g.'' The hydroxylated external surface imparts water solubility to the tubes at low concentration (C0.4wt %) and acidic pH. Treatment of the hydroxylated surfaces with organosilanes may allow for a more hydrophobic surface, permitting extraction of the tubular units into organic solvents. The ability to control the solubility properties of imogolite by using surface modification techniques could lead to broader applications of this novel inorganic polymer in materials science, especially in the design of composite structures. The reaction of imogolite with hydrolyzed (y-aminopropy1)triethoxysilae (APS) was investigated as a means of achieving the desired surface modification. APS was selected among all silanes as the preferred silylating agent, in part, because of its proven utility for the organofunctionalization of inorganic oxide surfaces",l2 in aqueous suspension.
Experimental Section Imogolite was synthesized according to the method described by Farmer and Fraser13and purified by dialysis against deionized water. The hydrolyzed APS-imogolite products were prepared by adding a 2% aqueous APS solution to a solution of purified imogolite and allowing the mixture to stir overnight. The 2% hydrolyzed APS solution was prepared by reaction of APS (Petrarch Systems) with water over a period of 0.5 h in the presence of enough acetic acid to achieve a pH of 3.6. A low pH is desirable for the coupling reaction because under these conditions imogolite remains soluble and APS undergoes rapid (4) Furukawa, T.; Eib, N. K.; Mittal, K. L.; Anderson, H. R., Jr. J. Colloid Interface Sci. 1983, 96, 322. (5) Alexander, J. D.; Gent, A. N.; Henriksen, P. N. J. Chem. Phys. 1985,83,5981. (6) Garbassi, E.; Occhiello, E.; Bastioli, C.; Romano, G. J. Colloid Interface Sci. 1987, 117,258. (7) Symposium on Adhesion Aspects of Polymeric Coatings; Mittal, K. L., Ed.; Plenum: New York, 1983. (8) Leary, H. J., Jr.; Campbell, D. S. Org. Coat. Appl. Polym. Sci. Proc. 1983,46, 433. (9) Furukawa, T.; Eib, N. K.; Mittal, K. L.; Anderson, H. R., Jr. Surf. Interface Anal. 1982,4, 240. (10) Egashira, K.; Aomine, S. Clay Sci. 1974, 4, 231. (11) Silanes, Surfaces and Interfaces; Leyden, D. E., Ed.; Gordon and Breach New York, 1985. (12) Silylated Surfaces; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach New York, 1978. (13) Farmer, V. C.; Fraser, A. R. Deu. Sedimentol. 1978,27,547.
Johnson and Pinnavaia and complete hydrolysis. The amount of AF'S incorporated in the reaction was based on the amount of aluminum used in the imogolite synthesis. Since the synthesis of imogolite is not quantitative, typically formed in
