Metal-loaded polymer films for chemical sensing of ES&H-related pollutants Page: 8 of 74
This report is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
t, strain is accommodated inelastically by interchain motion; this is the rubbery regime,
characterized by G' 108 dyne/cm2 and G" comparable to or less than G'. Thus, when probing at
a given (angular) frequency o, the material behaves as glassy if ot 1 and rubbery if or 1. When
wt = 1, there is a peak in K" and G". Since t is strongly dependent on temperature, varying
temperature can cause this transition from elastic to viscoelastic behavior. The temperature of this
transition is the dynamic glass transition temperature T. Since this glass transition temperature is
defined by t(Tg) = 1/o, the dependence of T on measurement frequency is apparent*
There are two quasi-static phenomena associated with the glass transition that are not
observable at the high frequencies at which SAW devices operate. These are (1) an increase in
specific heat as observed in DSC, and (2) an increase in thermal expansion coefficient. The latter
has an indirect influence on SAW performance through changes in film thickness. Therefore, it is
important to identify the temperature at which these quasi-static effects occur. The expression "static
glass transition," denoted herein as T( ), is conventionally used, although it must be noted that
measurements of T(0) are done by scanning methods, and therefore should be thought of as Tg in the
limit of low frequency. Tg(0) and T can differ significantly; for example, in the PIB films used here
Tg( )= -68*C, while Tg at 100 MHz is 40 C.
Although glass transitions have reportedly been observed in the response of polymer-coated
SAW devices" (including the present authors2), this interpretation has been correctly challenged by
Grate et al.'3. Changes in the intrinsic polymer properties known to occur at T, specifically a rapid
decrease in shear storage modulus (G') and a peak in shear loss modulus (G"), have usually been
assumed to appear directly as changes in SAW velocity and attenuation2. Under this assumption,
peaks in attenuation or velocity slope changes with temperature have been used to indicate Tg.
However, it will be shown in this paper that velocity and attenuation changes track intrinsic
properties only in extremely thin films (defined below). In this limit, attenuation peaks and velocity
slope changes do coincide with Tg. For thicker films, however, the acoustic coupling to the film
varies strongly with film thickness due to film interference effects. In this case, velocity and
attenuation do not track changes in the intrinsic properties directly and, therefore, care must be used
in inferring glass transitions from SAW velocity and attenuation responses.
SAW velocity and attenuation responses arise from the mechanical interaction that occurs
Here’s what’s next.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Reference the current page of this Report.
Martin, S.J. & Frye, G.C. Metal-loaded polymer films for chemical sensing of ES&H-related pollutants, report, March 1, 1997; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc675600/m1/8/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.