Crosslinked Elastomers: Structure–Property Relationships and Stress-Optical Law

We present a combination of independent techniques in order to characterize crosslinked elastomers. We combine well-established macroscopic methods, such as rheological and mechanical experiments and equilibrium swelling measurements, a more advanced technique such as proton multiple-quantum NMR, and a new method to measure stress-induced segmental orientation by in situ tensile X-ray scattering. All of these techniques give access to the response of the elastomer network in relation to the crosslinking of the systems. Based on entropic elasticity theory, all these quantities are related to segmental orientation effects through the so-called stress-optical law. By means of the combination of these techniques, we investigate a set of unfilled sulfur-vulcanized styrene butadiene rubber elastomers with different levels of crosslinking. We validate that the results of all methods correlate very well. The relevance of this approach is that it can be applied in any elastomer materials, including materials representative of various industrial application, without prerequisite as regards, e.g., optical transparency or simplified formulation. Moreover, the approach may be used to study reinforcement effects in filled elastomers with nanoparticles.

Some Characterization Techniques Useful for Polysiloxanes

The general approach used in choosing a polymer suitable for a particular application is: . . . Polymerization ↔ Structure ↔ Properties ↔ Application . . . For example, if one wants a polymer for fire-resistant fabrics, then a polymer with good high-temperature properties is required, which implies aromatic structures, which suggest condensation polymerizations. More relevant here, however, would be that a polymer remains elastomeric at low temperatures. This requirement evokes a polymer with high flexibility (low glass transition temperature), which indicates use of the polymerization techniques used with the polysiloxanes. An example of a relevant optical property is the birefringence of a deformed polymer network. This strain-induced birefringence can be used to characterize segmental orientation, and both Gaussian and non-Gaussian elasticity. Infrared dichroism has also been helpful in this regard. In the case of the crystallizable polysiloxane elastomers, orientation is of critical importance with regard to strain-induced crystallization and the tremendous reinforcement it provides. Segmental orientation has also been characterized by fluorescence polarization, deuterium nuclear magnetic resonance (NMR), and polarized infrared spectroscopy. Infrared spectroscopy has been used to characterize the structures of silica-filled polydimethylsiloxane (PDMS). Other optical and spectroscopic techniques are also important, including positron annihilation lifetime spectroscopy, spectroscopic ellipsometry, confocal Raman spectroscopy, and photoluminescence spectroscopy. Surface-enhanced Raman spectroscopy has been made tunable using gold nanorods and strain control on elastomeric PDMS substrates. A great deal of information is now being obtained on filler dispersion and other aspects of elastomer structure and morphology through the use of scanning probe microscopy, which consists of several approaches. One approach is that of scanning tunneling microscopy (STM), in which an extremely sharp metal tip on a cantilever is passed along the surface while measuring the electric current flowing through quantum mechanical tunneling. Monitoring the current then permits maintaining the probe at a fixed height above the surface. Display of probe height as a function of surface coordinates then gives the desired topographic map. One limitation of this approach is the requirement that the sample be electrically conductive. Atomic force microscopy (AFM), on the other hand, does not require a conducting Surface.

The In Vitro Reliability of the CODA MPX30 as the Basis for a Method of Assessing the In Vivo Motion of the Subtalar Joint

Background: The considerable variation in subtalar joint structure and function shown by studies indicates the importance of developing a noninvasive in vivo technique for assessing subtalar joint movement. This article reports the in vitro testing of the CODA MPX30, an active infrared marker motion analysis system. This work represents the first stage in the development of a noninvasive in vivo method for measuring subtalar joint motion during walking. Methods: The in vitro repeatability of the CODA MPX30 system’s measurements of marker position, simple and intermarker set angles, was tested. Angular orientations of markers representing the position of the talus and the calcaneus were measured using a purpose-designed marker placement model. Results: Marker location measurements were shown to vary by less than 1.0 mm in all of the planes. The measurement of a 90° angle was also found to be repeatable in all of the planes, although measurements made in the yz plane were shown to be consistently inaccurate (mean, 92.47°). Estimation of segmental orientation was found to be repeatable. Estimations of marker set orientations were shown to increase in variability after a coordinate transform was performed (maximum SD, 1.14°). Conclusions: The CODA MPX30 was shown to produce repeatable estimations of marker position. Levels of variation in segmental orientation estimates were shown to increase subsequent to coordinate transforms. The combination of the CODA MPX30 and an appropriate marker placement model offers the basis of an in vivo measurement strategy of subtalar joint movement, an important development in the understanding of the function of the joint during gait. (J Am Podiatr Med Assoc 101(5): 400–406, 2011)