This model provides the form factor, $P(q)$, for a flexible cylinder where the form factor is normalized by the volume of the cylinder. **Inter-cylinder interactions are NOT provided for.**
$$ P(q) = \text{scale} \left<F^2\right>/V + \text{background}
$$
where the averaging $\left<\ldots\right>$ is applied only for the 1D calculation
The 2D scattering intensity is the same as 1D, regardless of the orientation of the q vector which is defined as
$$ q = \sqrt{q_x^2 + q_y^2}
$$
Definitions
The chain of contour length, $L$, (the total length) can be described as a chain of some number of locally stiff segments of length $l_p$, the persistence length (the length along the cylinder over which the flexible cylinder can be considered a rigid rod). The Kuhn length $(b = 2*l_p)$ is also used to describe the stiffness of a chain.
In the parameters, the sld and sld\_solvent represent the SLD of the cylinder and solvent respectively.
Our model uses the form factor calculations in reference [1] as implemented in a c-library provided by the NIST Center for Neutron Research (Kline, 2006). This states:
'Method 3 With Excluded Volume' is used. The model is a parametrization of simulations of a discrete representation of the worm-like chain model of Kratky and Porod applied in the pseudocontinuous limit. See equations (13,26-27) in the original reference for the details.
.. note::
There are several typos in the original reference that have been corrected by WRC [2]. Details of the corrections are in the reference below. Most notably
- Equation (13): the term $(1 - w(QR))$ should swap position with $w(QR)$
- Equations (23) and (24) are incorrect; WRC has entered these into Mathematica and solved analytically. The results were then converted to code.
- Equation (27) should be $q0 = max(a3/(Rg^2)^{1/2},3)$ instead of $max(a3*b(Rg^2)^{1/2},3)$
- The scattering function is negative for a range of parameter values and q-values that are experimentally accessible. A correction function has been added to give the proper behavior.
**This is a model with complex behaviour depending on the ratio of** $L/b$ **and the reader is strongly encouraged to read reference [1] before use.**
.. note::
There are several typos in the original reference that have been corrected by WRC [2]. Details of the corrections are in the reference below. Most notably
- Equation (13): the term $(1 - w(QR))$ should swap position with $w(QR)$
- Equations (23) and (24) are incorrect; WRC has entered these into Mathematica and solved analytically. The results were then converted to code.
- Equation (27) should be $q0 = max(a3/(Rg^2)^{1/2},3)$ instead of $max(a3*b(Rg^2)^{1/2},3)$
- The scattering function is negative for a range of parameter values and q-values that are experimentally accessible. A correction function has been added to give the proper behavior.
**This is a model with complex behaviour depending on the ratio of** $L/b$ **and the reader is strongly encouraged to read reference [1] before use.**
References
J S Pedersen and P Schurtenberger. *Scattering functions of semiflexible polymers with and without excluded volume effects.* Macromolecules, 29 (1996) 7602-7612
Correction of the formula can be found in
W R Chen, P D Butler and L J Magid, *Incorporating Intermicellar Interactions in the Fitting of SANS Data from Cationic Wormlike Micelles.* Langmuir, 22(15) 2006 6539-6548
Authorship and Verification
**Author:**
**Last Modified by:**
**Last Reviewed by:** Steve King **Date:** March 26, 2019
Created By | sasview |
Uploaded | Sept. 7, 2017, 3:56 p.m. |
Category | Cylinder |
Score | 0 |
Verified | Verified by SasView Team on 07 Sep 2017 |
In Library | This model is included in the SasView library by default |
Files |
flexible_cylinder.py flexible_cylinder.c |
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