(209g) New Micro Viscosity Sensor – a Novel Analytical Tool for Online Monitoring of Polymerization Reactions in a Micro Reaction Plant

During the last 15 years, it has been shown that micro reactors can help to develop new and optimize conventional chemical reactions. Different micro mixers, micro heat exchangers, micro reactors for homogeneous or heterogeneous liquid-liquid reactions or for gas/liquid reactions have been developed and tested for development as well as for production. Even the handling of solids, e.g. manufacturing of particles or polymerization reactions could be shown. Very important for running micro reactors is the detection of the products formed during the process. This is necessary to optimize the reaction as well as to control the process during the production. Even though a variety of different reactions in micro reaction plants could be realized, still not many online analytical methods for micro reactors are known. Within this project it was necessary to develop a method to control the chain length of the produced polymer. During the polycondensation of the monomers the viscosity of the solution changes. The higher the chain length and the lower the concentration of monomers in the solvent, the higher becomes the viscosity of the solution. Therefore, it was decided to use this parameter as a signal to follow the reaction. With this new analytical method it is possible to monitor the reaction and control the chain length and molecular weight distribution. While the viscosity of small volumes can be measured with a classical rheometer, this analysis cannot be performed "online" during production. On the other hand, online analysis can be performed with conventional viscosity sensors. However, large flow-through volumes are required. Therefore, the project presented here combines online analysis with the handling of small volumes and flow rates as used within micro reaction plants. The sensor was developed at the Research Center caesar. The polymer solution has to flow through a stainless steel capillary (1/16") with an inner diameter of 0.75 mm, which is excited by an ac voltage through an electrical coil in a homogeneous magnetic field to perform shear waves (torsion) at a frequency of approx. 20 kHz (see fig 1a (drawing) and b (photo)). The resonant frequency of the capillary depends on the viscosity of the liquid going through it. Using the frequency and amplitude information from such a sensor, it is possible to measure the viscosity of the polymer solution. The respective calibration curve is shown in Figure 2. Higher viscosity leads to damping of the amplitude as well as to a shift in resonance frequency. Knowing the viscosity of a given concentration of a polymer in a solvent, it is possible to calculate the molecular weight of this polymer. The presentation will show the first results with this sensor used in a Suzuki reaction. The sensor allows the measurement of the viscosity of liquids over a wide range (1 ? 350 mPa s). The signals depend not only on the viscosity of the solutions but also on the density and temperature. The authors developed within a project funded by the German government a micro reaction plant which allows to produce polymers out of up to 8 monomers by using the Suzuki coupling method. Micro reactors can have a positive influence on the quality of the polymer product. Especially the mixing of the monomers, the catalyst solution, and end capper solutions have a great influence on the molecular weight distribution. With micro reactors it is possible to have a very narrow distribution which is not achievable in typical batch processes. The authors would like to thank VDI-VDE-IT and the German Ministry for Education and Research for funding this work within the POKOMI project as well as Merck (Frankfurt), IMM (Mainz), hte (Heidelberg), Jumo (Fulda) for their help within this project. Fig 1a and b. Viscosity sensor, measuring the torsion of a stainless steel capillary. Fig 2: Calibration curves for the Viscosity Sensor