Optical sensor for measuring fluorescence anisotropy during polymer processing

Abstract

An optical sensor containing polarizing optical components measures fluorescence anisotropy of fluorescent dyes. The measurement involves the detection of vertical and horizontal components of fluorescent light. Using Glan-Taylor and Wollaston calcite polarizers, both vertical and horizontal components are collected by separate optical fibers and measured simultaneously using a two-channel photon counter. One application of this sensor is the measurement of molecular orientation during polymer processing. Two sensor head designs are described, one of which fits into the 1/2 inch instrumentation port in polymer processing machines. To carry out process monitoring, a fluorescent dye is mixed with a polymer resin at approximately 10 ppm by weight concentration. Dyes which have geometrical asymmetry in their molecular structure or are covalently bonded to the polymer molecule are usually used. The anisotropy measurement yields information about the orientation of the dye molecule in an oriented medium. Anisotropy measurements of fluorescent dyes in media other than polymers can also be obtained using this sensor.

We claim: 
     
       1. An optical sensor for sensing a fluorescence anisotropy of a sample, the optical sensor comprising: a source of linearly polarized light and means for causing the linearly polarized light to be incident on the sample in a fixed and recognizable direction of the light polarization;   polarizing means for receiving return fluorescence light from the sample and for splitting the return fluorescence light into a first linearly polarized component and a second linearly polarized component, the first and second linearly polarized components having orthogonal directions of polarization;   means for detecting amplitudes of the first and second linearly polarized components and producing an output representing the amplitudes of the first and second linearly polarized components; and   means, receiving the output, for computing the fluorescence anisotropy in accordance with the output.   
     
     
       2. An optical sensor as in claim 1, wherein the means for detecting comprises: photodetecting means for converting the first and second linearly polarized components into electrical signals;   a first optical fiber means for conveying the first linearly polarized component from the polarizing means to the photodetecting means; and   a second optical fiber means for conveying the second linearly polarized component from the polarizing means to the photodetecting means.   
     
     
       3. An optical sensor as in claim 2, wherein the photodetecting means comprises: a first detector for receiving the first linearly polarized component; and   a second detector, provided separately from the first detector, for receiving the second linearly polarized component.   
     
     
       4. An optical sensor as in claim 2, wherein the photodetecting means comprises: a single photodetector for receiving the first and second linearly polarized components; and   a chopper, receiving the first and second linearly polarized components, for allowing only one of the first and second linearly polarized components at a time to be incident on the single photodetector.   
     
     
       5. A method for sensing a fluorescence anisotropy of a sample, the method comprising: (a) producing linearly polarized light and causing the linearly polarized light to be incident on the sample from a fixed and recognizable direction of light polarization;   (b) receiving return fluorescence light from the sample and splitting the return fluorescence light into a first linearly polarized component and a second linearly polarized component, the first and second linearly polarized components having orthogonal directions of polarization;   (c) detecting amplitudes of the first and second linearly polarized components and producing an output representing the amplitudes of the first and second linearly polarized components; and   (d) computing the fluorescence anisotropy in accordance with the output.   
     
     
       6. A method as in claim 5, wherein step (c) is carried out with the following: photodetecting means for converting the first and second linearly polarized components into electrical signals;   a first optical fiber means for conveying the first linearly polarized component to the photodetecting means; and   a second optical fiber means for conveying the second linearly polarized component to the photodetecting means.   
     
     
       7. A method as in claim 6, wherein the photodetecting means comprises: a first detector for receiving the first linearly polarized component; and   a second detector, provided separately from the first detector, for receiving the second linearly polarized component.   
     
     
       8. A method as in claim 6, wherein the photodetecting means comprises: a single photodetector for receiving the first and second linearly polarized components; and   a chopper, receiving the first and second linearly polarized components, for allowing only one of the first and second linearly polarized components at a time to be incident on the single photodetector.   
     
     
       9. The optical sensor of claim 1, wherein the means for causing includes a sensor head having a body with a square cross section. 
     
     
       10. The optical sensor of claim 9, wherein the body with a square cross section fits into a receptacle having a corresponding square shape to receive the square cross section of said body. 
     
     
       11. The optical sensor of claim 1, wherein means for causing includes a sensor head having a body and wherein the body is fixedly mounted so as to cause a fixed orientation of the sensor head relative to a material being examined. 
     
     
       12. The optical sensor of claim 11, wherein the body is fixedly mounted to an istrumentation port of a polymer processing machine. 
     
     
       13. The optical sensor of claim 1, wherein only a single polarizing means is present to create the source of linearly polarized light and to receive the returned fluorescence light. 
     
     
       14. The optical sensor of claim 1, wherein the means for causing comprises six optical fibers of 200 μm core diameter. 
     
     
       15. The optical sensor of claim 14, including a fiber bundle of 13 optical fibers of 200 μm core diameter for conveying fluorescence of the sample. 
     
     
       16. The method of claim 5, wherein the linearly polarized light is from two light sources, the polarization of each light source being mutually perpendicular to the other.