Attenuation data from polymer

Ultrasonic A-, B-, and C-scans (23) can be obtained from polymers and PMC. Isotropic or anisotropic elastic constants, attenuation, and phase transitions are detected in detailed A-scan data analysis (127,128). Ultrasonics is also used for process monitoring in manufacturing of polymeric or PMC parts (129). 

Value p was determined for a great nmnber of systems (see ref. 6). The main methods used here are IR and NMR spectroscopies. The data from these methods allow one to make some conclusions about the conformations of adsorbed chains. The NMR method is especially informative because the fraction of bound segments can be measured directly in the adsorption system without removing adsorbent. The method of attenuated total inner reflection (ATIR) gives the best possibility to evaluate the concentration of polymer in adsorption layers of various thicknesses. Having no aim to give the full analysis of the results of many authors who use these methods (see ref 75), we would like to consider the results of one study. ... 

Plasma-polymerized materials differ significantly from those polymerized by conventional methods in their surface properties, and surface tension values do not correspond. This difference may be due to the highly cross-linked nature of plasma polymers or to the incorporation of other entities from the carrier gas. These effects are more important than the intrinsic differences in backbone fiexibility. Wrobel (88) presents ATR-IR (attenuated total reflection infrared) spectroscopic data indicating that silazanes and silanes cross-link more readily than do siloxanes under plasma conditions. Wrobel and his co-workers (89) have also used contact angles to study the thermal decomposition of plasma-polymerized organosilicon polymers. 

IR-transmitting optical fibres are evanescent wave sensors using a mathematical deconvolution technique to extract the absorbances and follow the concentrations of the components as they occur in both laboratory scale and process production. The fibre-optic probe used can be placed at specific locations within the samples or at the surface. The specificity of the technique, the speed of data acquisition and the portability of equipment make this method ideal as a tool to fundamentally probe polymer reactions and processes. Chalcogenide optical fibres are used to direct IR radiation from an FUR spectrometer through an attenuated total reflection (ATR) probe immersed in a reactor and back to the spectrometer. 

The attenuated reflectance technique presents an excellent example of how radiation at sample surface can enhance signal-to-noise ratio. Details of general optics and reflectance techniques can be found in the classic text (10). This technique is used extensively to determine differences between the structure of polymers in surface and bulk phases. Commercial accessories make these spectroscopic experiments easy to perform, although quantitative analysis of the data remains difficult. Examples of ATR applications include chemical composition analysis of polymers, surface orientation resulting from various processing methods, and chemical or thermal degradation of polymers. For samples such as powders or poorly defined surfaces, the diffuse reflectance technique can be used (11). In addition, the photoacoustic technique has been used to probe surface structure and multilayer structure commonly found in polymer laminates (17). In all these cases, optical effects can complicate analysis of infrared spectra. Nevertheless, these data have proven very useful in analytical applications. 

For studies of dilute polymer solutions, measurements of high precision are necessary to obtain the small differences between the properties of solutions and solvent. Apparatus for pulse propagation in such solutions at 20 MHz has been described by Miyahara, Wada, and Hassler," and for variable path interferometry by Cerf, and for standing waves by Miyahara. jjj jjjg latter method, the frequency can be varied continuously from 1 to 20 MHz. At lower frequencies (10 to 700 kHz), the free decay of waves in a spherical vessel can be measured. In such measurements, the data are ordinarily left in terms of M (or simply velocity and attenuation, or the acoustic absorption coefficient identified in Chapter 18) with no attempt to convert them to K. ... 

A more recent application of ultrasonics has been the characterization of the extent of polymerization in a condensation or radical process. The first observations were made by Sokolov, and subsequent reports of measurements on polystyrene, poly(vinyl chloride) and poly(vinyl acetate) " have confirmed the utility of the method. It is clear that this type of study is still in its infancy however, certain facts emerge which demonstrate the importance of this method. The compressibility of a solution containing monomer and polymer is directly related to the proportion of each component present. It is therefore possible to quantitatively estimate the extent of conversion from the observed velocity of sound. In a suspension polymerization, the glass transition of the polymer forming the bead is itself a function of the extent to which unreacted monomer is retained in the system. In this case, observation of the attenuation can indicate the extent to which polymerization has occurred in the system. Unfortunately the data are not sufficiently extensive to estimate the general validity of the method for the monitoring of polymerization in reactors, although the potential has been clearly demonstrated. 

Infrared spectroscopy (IR) is used extensively in the analysis of plastics. It is a very good, relatively quick technique for the determination of the polymer present in a plastic product. Unlike rabbers, where the majority of compounds contain additives such as carbon black and plasticisers that can interfere with the infrared data obtained, it is normally possible to record infrared spectra directly from the plastic sample using either a surface technique (e.g., attenuated total reflectance (ATR) spectroscopy) or in transmission through the sample from a film produced by pressing a specimen above its glass transition or melting point. 

---