Ultrasonic monitoring of polymer collapse in 5% (w/w) PNIPAM solution 
An example of the application of the temperature ramp regime is in the analysis of heat transition in an aqueous solution of poly(N-isopropylacrylamide), a polymer whose applications include thermoresponsive gels. As temperature rises, the polymer coil collapses into a compact globule, and aggregates are formed. Ultrasonic velocity decreases during the process, reflecting the dehydration of the polymer, and a contribution of intrinsic elasticity of the globules and aggregates. Attenuation rises as the aggregates cause increased scattering of the ultrasonic waves. As illustrated above, ultrasonic spectrometry can be used to pinpoint the temperature and width of the phase transition and analyse the transformations in the polymer structure, both of which are illustrated by the changes in velocity. The differences in attenuation mean the structure of the aggregates can be characterized. return to top of the page Thermal transitions in aqueous carrageenan solution 
Carrageenans are the name given to a family of linear sulphated food grade polysaccharides obtained from red seaweeds. They have the unique ability to form an almost infinite variety of gels at room temperature, rigid or compliant, tough or tender with high or low melting point. Carrageenan solutions will thicken, suspend and stabilize particulates as well as colloidal dispersions and water/oil emulsions. They are used extensively in the food and other industries today, for example as a secondary stabilizer in ice cream, in the preparation of evaporated milk, dairy desserts, chocolate milk and in meat coating. The above figure illustrates the multi-frequency ultrasonic analysis of the melting of a carrageenan gel using HR-US 102 spectrometer. Below 30 and above 48ºC, ultrasonic velocity exhibits linear temperature dependence, which is a result of the normal (for most materials) decrease in storage modulus with temperature. Between these two temperatures, a transition is shown clearly. The increase in ultrasonic velocity was caused by an increase in the hydration level of the atomic groups of the carrageenan, as a result of the melting of its helical structure and breaking the intermolecular connections between the carrageenan molecules. The drop in ultrasonic attenuation in the transition temperature interval results from the loss of friction between the frozen (unmovable at ultrasonic frequencies) polymer network and the moving solvent in a course of compressions and decompressions in the ultrasonic wave. The frequency dependence of velocity and attenuation (storage and loss moduli subsequently) provide additional information on the dynamic behaviour of the gel network. In this system, high resolution ultrasonic spectroscopy allows detection of the gelation point and interval as well as the analysis of the transformations in the polymer's helical structure (ultrasonic velocity) and characterises the structure of the gel network (ultrasonic attenuation). return to top of the page | 
