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|Title: ||Neue Charakterisierungsmöglichkeiten von supramolekularen Strukturen in Biopolymerlösungen mit rheo-optischen Methoden|
|Authors: ||Clasen, Christian|
|Issue Date: ||2001 |
|Publisher: ||Shaker Verlag|
|Abstract: ||Solutions of biopolymers are often not homogeneously solved, but show overstructures in the form of associates and aggregates that strongly influence the flow behaviour. For the detection of the properties of the solution, rheo-mechanical measurements are not enough. Especially the behaviour on a molecular level in a flow field can only be measured by correlating rheo-optical and rheo-mechanical data. Using flow birefringence, flow dichroism and its orientations, detailed description of the deformation behaviour, the stability and the size of these overstructures can be given. The goal of this work was therefore the characterisation of the behaviour in the flow field of selected associating biopolymer solutions using a newly developed rheo-optical method.
In the first part, the developement and the integration of the new rheo-optical device for measuring flow birfringence, flow dichroism and its orientations into a commercial rheometer with a synchronized recording of the mechanical and the optical data is described. For that a prototype of a laser device using three diode lasers with different planes of polarization working in an alternating pulsed mode that are aimed in a single optical path through a measuring cell was build. The heart of the laser device is a collocation of thin-film-polarizers and polarizing and non-polarizing beam splitting prisms that allow for a interferenceless combination of the differently polarized laser beams in a single optical path.
For the pulsed control of the laser as well as the intensity measuring unit after the rheological measuring cell, an electronical control device was conceived that also functioned as an interface to the analyzing computer. For the analysis of the high frequency signal an algorithm was developed using the graphical programming language LabView which allowed for a seperation of the output signal and a correlation to the exciting input frequency.
For the mathematical analysis of the output signal, the Mueller-matrices of the polarized lasers after going through the measuring cell were calculated and an approximation method was developed that allowed the calculation of the flow birefringence and dichroism from the phase difference of the lasers. The calculation of the optical material functions were then integrated into the LabView program, so that for the first time an online measurement with a graphical presentation of the rheo-optical material functions after the new measuring method was possible. For the verification of the capability of the new device and a comparison with the old rheo-optical methods, rheo-optical measurements with a hydroxypropylcellulose(HPC)/water system were made. The solution of the birefringence of the new device is a decade lower than the old methods, but allows for a synchronized presentation of rheo-mechanical and rheo-optical data. Furthermore it is possible with the new method to show the optical material functions online while measuring. The new device is also smaller, less expensive and shortens the needed calibration and measuring time up to 90%.
In the second part of this work the structure-property-relationships of pseudo plastic flow for the polymer system (1,3)(1,4)--glucane/water was derived. Since the (1,3)(1,4)--glucanes are subject to a spontaneous gelation at room temperature, the measurement of a -M-relationship at room temperature was not possible. The dependence of the critical flow curve parameters was therefore derived directly in dependence of the intrinsic viscosity . To the already known 0--c-relationship the subsequent 0--c- and n--c-relationships were derived. Together with the modified Carreau model the --c-g-relationship could be established:
For the rheo-optical material functions, starting from the stress optical rule, a new model for the correlation of the birefringence and its orientation with the shear state in an ideal homogeneous polymer solution was derived, that can also describe the behaviour in the non-Newtonian flow region and explain it with disentanglement. Following from that, two new structure-property-relationships could be derived, which with the before mentioned 0--c- and n--c-relationships, allowed for a model of the rheo-optical material functions in dependence of the concentration and the intrinsic viscosity.
The new theory was verified through measurements of the ideal polymer standard system polystyrene/toluene. Especially from the rheo-optical structure-property-relationships two new normalization methods for the Newtonian flow region were derived. For the birefringence and its orientation reduced values could be derived that depend only on the shear viscosity:
For the non-Newtonian region it was possible to quantitatively specify the decline of the slope of the birefringence curve from the by dissentenglement caused negative slope n of the flow curve, as well as the substancially lower increase of the orientation angle.
In the third part of this work, the flow behaviour of associates and aggregates was examined with the help of rheo-optical measurements for different biopolymer solutions. The detection of associated overstructures was first done using the (1,3)(1,4)--glucane/water system. By measuring the elastic part of the shear flow (storage modulus G‘ and first normal stress difference N1) and the resulting main stresses, it was possible to show that at high shear rates the intrinsic birefringence is dominant. Using the stress optical rule, the normal stresses can be derived from the intrinsic birefringence. The from aggregation resulting form parts were distinguished from the intrinsic parts of the non-aggregated polymer segments using the above mentioned rheo-optical structure-property-relationship. The stress optical coefficient for the (1,3)(1,4)--glucane/water system was determined from the intrinsic parts at .
For the form part of the birefringence, a from the shear viscosity dependend formation of a plateau regions could be detected, which can be attributed to a finit deformability of local aggregate structures in the sol state. Using the normalization of the birefringence n‘/0 after the newly developed rheo-optical structure-property-relationship, a change in the aggregate structure of the (1,3)(1,4)--glucane/water system below a critical concentration c+ could be detected, that corresponds with the transition from spontaneous to shear induced gelation.
The dichroism, like the birefringence showed a pronounced plateau region in dependence of the shear rate, that was caused by the form part of the dichroism. In contrast to the birefringence, the plateau region could be detected at high shear rates. With the help of the on the concentration normalized dichroism, it could be shown that the aggregates below a critical concentration c+ are considerably larger, but the concentration of the aggregates proportionally to the segment density declines. This decline in the aggregate concentration is a possible explaination for the up to now unexplainable suspension of the spontaneous gelation below c+.
By measuring the orientation angle of the birefringence and the dichroism, it was possible to separate the early orientation of the aggregates in the shear field from the orientation of the segments. By depicting the orientation of the birefringence versus the reduced shear rate (= f(g/gkrit)) it could be shown that below c+ the orientation of the aggregates is independend of the concentration of the aggregates, but dependend of the surrounding polymer segments. The orientation angle of the dichroism shows a rise at high shear rates above c+, which is possibly caused by the appearance of an intrinsic part of the dichroism. This presumption is backed up by a rise of the dichroism at high shear rates above c+.
Other biopolymer/water systems with a tendency towards association, such as mixtures of locust bean gum with -carrageenan or locus bean gum with potatoe starch, were taken into account as a comparison. Depicting the reduced birefringence n‘/0 allowed for a possibility of a quantitatively estimation of the intermolecular interaction in the mixtures since the absolute polymer concentration was held constant. For the stress optical coefficient of the mixture of locust bean gum with potatoe starch a linear dependence from the ratio of the composition of the mixture could be shown, which can be attributed soley to the birefringend properties of the polymer segment part of the solution. The form part shows for both mixtures plateau regions, which point to finitly deformable aggregate structures. A maximum interaction results from the position of the flow region of the reduced depiction of the birefringence for the locust bean gum/-carrageenan at a ratio of 1:4. For locus bean gum with potatoe starch a maximum in the interaction was found at a ration between 1:4 and 2:3.
A temperatur induced aggregation that leads to phase separation, was found for the system HPC/water. Whereas for these solutions at room temperature a behaviour was detected that coincides with the intrinsic parts of the new rheo-optical structure-property-relationship, measurement taken into account as a comparison at a temperature of 39°C show, that before the onset of the phase separation a form birefringence could be detected. In contrast to the (1,3)(1,4)--glucane solutions the level of the form part increases with the concentration for the HPC. Thus in this case with an increasing segment density an enlargement of the aggregates, i.e. an intensification of the deformability with increasing segment density can be assumed.
|Publication status: ||published|
|KU Leuven publication type: ||ABa|
|Appears in Collections:||Soft Matter, Rheology and Technology Section|
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