Interlayer Diffusion in Langmuir-Blodgett Multilayers
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INTERLAYER DIFFUSION IN LANGMUIR-BLODGEfI MULTILAYERS Mark D. Foster*, Timothy R. Vierheller*, Albert Schmidt**, Klemens Mathauer**, Wolfgang Knoll**, Gerhard Wegner**, Sushil Satija*** and Charles Majkrzak*** * Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 ** Max-Planck-Institut fdr Polymerforschung, D-6500 Mainz 1, Germany *** National Institute of Standards and Technology, Gaithersburg, MD 20899. ABSTRACT Interlayer diffusion in Langmuir-Blodgett multilayers containing Cadmium Arachidate (CdA), polyglutamate (PG) copolymers, or both, was investigated using neutron and x-ray reflectometry. Measurements from multilayers comprised only of CdA or PG revealed that little interdiffusion occurred upon annealing at 70'C or 84'C. However, some relaxation of the intralayer microstructure in the PG samples was observed at both temperatures. In contrast, the neutron reflectivity of a multilayer sandwich containing a bilayer of PG between two multilayers of CdA showed gradual and progressive change with annealing at both temperatures for annealing times of up to several hours. This change is surprising as PG is expected to act as a barrier to interdiffusion of the CdA molecules at these temperatures.
INTRODUCTION
Polymeric thin films or multilayers are of interest for their potential use in nanoscale devices such as chemical sensors and optoelectronics [1,2]. Such applications exploit the precise layer thickness and highly ordered layer structure which can be engineered using the Langmuir-Blodgett (LB) technique. Interdiffusion between layers of the multilayer component molecules themselves is an important issue in device stability. However, little experimental data on this phenomenon exists, with only four reported studies known to the authors [3-6]. In the present work the complementary techniques of neutron reflectometry (NR) and x-ray reflectometry (XR) are used to investigate interdiffusion in multilayers containing Cadmium
Arachidate and/or a novel copolymer. Fatty acids and their salts, such as Cadmium Arachidate(CdA), are the most widely studied LB systems. In particular, the behavior of CdA has also been studied over a wide range of temperatures [7], making it an excellent candidate material for the investigation of interdiffusion. CdA is known to undergo an order-disorder transition in the bulk at 110'C and thus ready diffusion would be expected in multilayers at temperatures above I 10°C where the lattice of Cd++ heads becomes disordered. However important structural changes occur even below I 10°C. Some pretransitional disordering begins at about 65°C and conformational changes of bonds in the hydrocarbon chains from trans to gauche states begin at 80'C [8]. Measurements made by others to date have suggested that significant interdiffusion may occur at temperatures below TODT [4] and also during deposition when the multilayer is submerged [5].
While CdA is a well characterized material, it is not aptly suited to optical applications due to the scattering loss caused by grain b
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