# Flexible Cylinder

## Description:

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:**