Rheology is a well-established area of study for a wide range of materials. Such analytical technique of choice in studying polymers can correlate the absolute flow and deformation characteristics of a specific product, in a relation to its behavior towards a particular application or processing step. This science is defined as the study of flow behavior. It is normally applied to fluid materials (or materials that exhibit a time-dependent response to stress). The measurement of rheological properties is applicable to all materials from fluids such as dilute solutions of polymers and surfactants through to concentrated protein formulations to semi-solids such as pastes and creams, to molten or solid polymers as well as asphalt. Rheological properties can be measured from bulk sample deformation using a mechanical rheometer, or on a micro-scale by using a microcapillary viscometer or an optical technique such as microrheology. Several types of flow behavior are generally recognized:
- The simplest is Newtonian behavior, with a linear relationship between stress rate and zero stress at zero strain rate. This is the ideal fluid behavior, analogous to Hookean behavior in a solid;
- Many fluids show plastic behavior (also called Bingham): the flow initiates above some level of stress (called the yield stress);
- Another common behavior is pseudoplastic (or shear thinning), in which viscosity decreases as strain rate increases.
Rheology is an integral method, so it is only able to provide data on the investigated samples bulk. It can not offer insight into what is going on at the molecular level at a given processing step. And here Raman spectroscopy comes to the rescue. Such spectroscopy has a proven track record as an effective, powerful and non-invasive means of chemical analysis. The combination of rheology and Raman systems which based on Raman spectroscopy allows for real-time, synchronized measurement of both physical and chemical material properties. In addition to this, the rheo-Raman approach reduces sample consumption and reduces analysis time (complete rheological information and full Raman spectra are collected in unison).
Linking a Raman system and a rheometer allows acquiring direct information on the molecular structure and on the mechanical properties of a sample. This combination is useful for the study of the crystallization behavior of polymer melts during processing as well as providing a range of information for in-situ characterization and monitoring. When measurements are performed on separate instruments, they can be difficult to correlate because there may be variations in factors such as processing history, temperature control, and the samples themselves. The Raman system can highlight an appearance and increase in crystallinity, and this correlates with an increase in the complex modulus close to the crossover point.
The benefit of simultaneous measurements is clear: many soft materials are sensitive to temperature and flow history, so simultaneous measurements minimize experimental variations.
Optromix Raman fiber optic probes are miniaturized without compromising its performance, which is enabled by the technology of direct deposition of the dielectric filters at the fiber end faces. In results in a small, cost-effective Raman probe for different Raman systems and, for example, for endoscopic and other applications.
The fiber optic Raman probe is produced for multi-wave excitation in the range 690-785 nm and 1000-1064 nm, e.g. @785 nm – “Fingerprint” spectral range with fluorescence reduction, and @690 nm – “High wavelength” spectral range for conventional Raman spectrometers.
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