2010-02-02 Small Angle Light Scattering

							Time-resolved Small Angle Light
       Scattering (SALS)


                        It is in Room D47
                               Susi Su
 Introduction
 Small angle light scattering (SALS) provides an informative and
  convenient method of examining the structure in micrometer scale.
 For our SALS, a CCD camera having 512*512 pixels enables fast data
  acquisition and multiple scans to be recorded.
 The light source is a He-Ne laser. It is 20 mW, linearly polarized,
  with wavelength of 632.8 nm and a beam size at 1/e2 of 0.65mm.
 A Linkam DSC hot stage could be selected for the sample holder for
  the in-situ investigation of morphology of samples. Heating and
  cooling between temperatures of -196 to 600 oC can be carried out
  at the rate of up to 130 oC/min.
  Principle



                         Schematic representation of a light scattering experiment.


• Light scattering techniques is used to probe the structure of materials at micro scale.
• Light scattering measures the variations in dielectric properties.
• It is well known that the wavelength of visible light is 400-700 nm, in order to see
   large objects or longer length scales, we need smaller angles.
•Small Angle Light Scattering: measure size range: 1-1000 µm (in theory).
•The mathematical principle behind this can be explained in a simple way.



The scattering vector

     Bragg’s Law
Experiment setup
Experiment operation
 Turn on the power and cooler power on detector controller.
 Turn on the laser.
   !! Make sure to cover the laser pinhole.
   !! Laser needed to be warmed up for about 1 hour, when it is ready,
      the button on the detector controller will turn green.
 Prepare samples.
   !! Make sure light can pass through the sample (transparent).
 Put sample on the sample stage, remove the cover for the pinhole and
  then close the door.
 Open software ‘‘Winview/32’’.
   Experiment operation--Calibration
 Using a diffraction grating of 200 lines per millimeter (1mm/200=5 μm).




                                                     center
                                                         5μm
                                                            2.5μm
Data acquisition
 The CCD camera was operated using ‘‘Winview/32’’ software.
 The data were taken as 512*512 pixel image files.
 Exposure time was carefully chosen.




     Make sure the intensity of scattering light < 60000
 counts/pixel, otherwise the detector floods and the image
  is distorted, and excessive light may cause permanent
         damage to the active array in the detector.
Data acquisition-‘‘Winview/32’’
                                                              Click
                                                            ‘‘Acquire’’




        More information about how to use ‘‘Winview/32’’,
                 please refer to ‘help’-’contents’
Data processing
1.   Use ‘‘Winview/32’’ software to open the calibration file, then use
     cursor to find out the centre coordinates.
2.   Use ‘‘aveoff.exe’’ software to open calibration file, then input centre
     coordinates, make ‘integration step’ to be ‘1’, click ‘save ascii’, and
     then click ‘circular average’, there would be a ‘Intensity vs. Pixel’
     graph shown on ‘‘Winview/32’’ software, and also *.dat file would
     be saved in the same folder.
3.   Use ‘Origin’ to open *.dat file and make a graph (‘Intensity vs.
     Pixel’ ), then find out the coordinates for the peak.
4.   Because Pixel=a+b*q, use the peak’s coordinates to figure out the
     value for a and b.
5.   Repeat step1-3 for the sample file.
6.   Use Pixel=a+b*q again to find out the value for the value of q at
     peak, then use d=2pi/q.
Example: Shaomin’s experiment—phase separation when heating a blend PS30/PVME70 at 112 oC
                References
 Y.Ishii’s PhD thesis.
\\Ncdbackup\backup\phd\Susi\Study\useful handout
  Calibration
 Using a diffraction grating of 200 lines per millimeter
  (1mm/200=5 μm).
                             dsinθ=mλ
 d-diffraction spcaing; λ-wavelength,
 θ-scattering angle ; m-order of the principle diffraction maximum.


 Scattering pattern of a diffraction grating
                      center
                         5μm
                         2.5μm
                           1.25μm          So from this calibration, you can probe the
                                           length scale above 1.25 μm.