Applied Photophysics Chirascan LD, Руководство пользователя

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Document 4207Q258 V3.00 January 2018 Page 9 of 30
Chirascan Couette Cell LD Accessory User Manual
1 INTRODUCTION
The Chirascan High Shear Couette Cell LD Accessory is designed to be used with the Chirascan V100
Spectrometer. The accessory is shown in Figure
1.1 .
Figure 1.1: The Applied Photophysics Couette Cell LD accessory
The accessory is usually used for linear dichroism (LD) measurements. Linear dichroism is the difference in
the absorbance of light polarized parallel and perpendicular to an orientation axis (Equation 1.1)

A = A
||
- A

1.1
where A is the absorbance of the linearly polarized light, and the subscripts || and

indicate parallel and
perpendicular respectively. For a sample to have non-zero linear dichroism it must be anisotropic; the
anisotropy may be intrinsic, as in liquid crystals for example, or it may be induced, for example by stretching a
polymer film, by the application of an electrical field or by alignment of deformation in a shear field.
The accessory uses a Couette cell to generate a shear field. The sample is contained in the annular gap
between two concentric quartz cylinders, the outer of which, the rotor, is rotated about its cylindrical axis, while
the inner, the stator, is stationary. This arrangement is named after its originator, Maurice Couette.
A shearing motion is shown in two dimensions in Figure 1.2 , where the blue rectangle, of height y, deforms to
the yellow parallelogram, with the top edge of the rectangle moving by a distance

x in the direction of the
arrow. The shear strain,

, is then defined as

x/y, and the shear rate,
γ̇
, as d

/dt, in the limit as y goes to zero
(the overdot is Isaac Newton’s original symbol for the time derivative, and is still universally used by
rheologists). The strain is therefore dimensionless, and the shear rate has dimensions of reciprocal time (s
-1
in the S.I. system)