Water-Uptake Kinetics in Poly(methyl methacrylate) Films with a Fluorescent Rotor Probe J. P. Goodelle, 1 R. A. Pearson, 1 M. M. Santore 2 1 Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 2 Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 Received 6 July 2000; accepted 21 December 2001 Published online 19 September 2002 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/app.10964 ABSTRACT: A fluorescence method was adapted to mea- sure moisture-uptake kinetics in films of poly(methyl methacrylate), and data were interpreted in the context of a Fickian diffusion model. The films, 2– 60 m thick, were supported on acid-etched microscope slides. They were compared with several freestanding slabs about 1 mm thick. The moisture diffusion in the slabs was Fickian with a diffusivity of 3.2  10 -9 cm 2 /s. The apparent Fickian diffu- sivity in the films decreased substantially with decreasing film thickness; however, a careful examination revealed that the initial moisture uptake was governed by a thickness- independent diffusivity for a wide range of film thicknesses. This suggested that the appearance of non-Fickian behavior originated within about a micrometer of the buried interface, possibly as a result of water accumulation beneath the film or slight thickness variations. Moisture uptake in the thick- est films was more rapid than in the slabs, most likely because of residual thermal stresses. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2463–2471, 2002 Key words: moisture diffusion; moisture sensor; residual stresses; rotor probes; supported poly(methyl methacrylate) (PMMA) films INTRODUCTION Solvent and moisture uptake by polymers is important in biomedical engineering and dentistry, for which polymers are used as adhesives and implanted com- ponents. Moisture uptake may cause expansion, com- promising function and biocompatibility. In engineer- ing adhesives, moisture or solvents can compromise joint strength. In microelectronic packages, adhesives, and underfills, moisture can facilitate corrosion or cause device failure via swelling stresses. When plastic components involve thin films or con- fined geometries, the properties in the device may differ from bulk testing specimens. One explanation is that, as a result of chemistry and curing conditions, a skin (a region of chemically different composition) may develop near the free surface. 1 In other situations, polymers may be processed in contact with a solid surface possessing a different thermal expansion coef- ficient. After the polymers cool, stresses develop that could influence adhesion or solvent permeation. 2,3 Fi- nally, there remains the ongoing dispute in the poly- mer physics community about the influence of con- fined geometries on film properties such as the mobil- ity 4 and glass-transition temperature (T g ), 5–7 which may ultimately influence moisture uptake. These complications emphasize the importance of studying real devices or employing test specimens whose properties approximate real applications. In the context of moisture uptake, this means taking care to ensure that sample chemistries match those in the real applications and that physical constraints, such as the attachment of a polymer film to a surface, are also taken into account. These restrictions limit the appli- cability of common methods such as mass evolution and motivate the development of testing methods that can, in principle, be broadly applied. Spectroscopic methods for moisture and solvents have the advantage that the signal can be generated exclusively within the relevant portion of a device, such as a polymer film on a massive substrate. A number of spectroscopic techniques, such as IR 8,9 and NMR, 10 show promise for probing buried interfaces and other inaccessible regions. Fluorescence spectros- copy also holds promise because of its high sensitivity. Probes based on fluorescence have the advantage that the instrumentation is relatively economical and por- table. This article is concerned with the use of fluorescent rotor probes as sensors of moisture and, potentially, other solvents in thin polymer films. Rotor probes are a family of fluorescent molecules, such as dialkylami- nomalonitriles, that have two major pathways to re- Correspondence to: M. M. Santore (santore@mail.pse.umass. edu). Contract grant sponsor: Semiconductor Research Corp. Contract grant sponsor: National Science Foundation; contract grant numbers: CTS-9202413 and CTS-9310932. Contract grant sponsor: EPRI; contract grant number: RP- 8019-02. Journal of Applied Polymer Science, Vol. 86, 2463–2471 (2002) © 2002 Wiley Periodicals, Inc.