Studies of Polymer Dimensions Small Angle Scattering

1 STUDIES OF POLYMER DIMENSIONS SMALL ANGLE SCATTERING 

The most frequently encountered use of neutrons in the study of polymers is the measurement of small angle scattering (SANS). Use of neutrons with wavelengths in the range 0.5-2 nm and scattering angles between 0.1° and 10° readily allows 

In many circumstances it is possible to extend the range of measurements with neutrons to cover more than this low Q limit, and then more detailed models of the structure must be evaluated. Debye [22] has derived a result for the scattering from a Gaussian distribution of polymer segments appropriate to a random polymer coil which is of the form  

The constant C describing the absolute intensity is of importance as it permits determination of the molecular weight of polymers. By rearrangement of the constants in Equation (5), it can be expressed as  

A major boost to the application of SANS to polymers in the bulk came from the recognition that the screening of molecular interactions in bulk homopolymers could be used to extend the range of concentrations over which measurements can be made. This idea, which is known as the random phase approximation or RPA [52, 53], states that if there are no interactions, i.e. the second virial coefficient is zero, then measurements of molecular dimensions can be made at any concentration. In order to optimise count rates this may often be close to 50% blends of deuterated and protonated polymers. Several experiments have been performed to test this theory [54,55], which is now widely applied. 

---