polymer_excl_volume

Polymer Excluded Volume model

Parameter	Description	Units	Default value
scale	Scale factor or Volume fraction	None	1
background	Source background	cm-1	0.001
rg	Radius of Gyration	Å	60
porod_exp	Porod exponent	None	3

The returned value is scaled to units of cm^-1 sr^-1, absolute scale.

This model describes the scattering from polymer chains subject to excluded volume effects and has been used as a template for describing mass fractals.

Definition

The form factor was originally presented in the following integral form (Benoit, 1957)

$$P(Q)=2\int_0^{1}dx(1-x)exp\left[-\frac{Q^2a^2}{6}n^{2v}x^{2v}\right]$$

where $\nu$ is the excluded volume parameter (which is related to the Porod exponent $m$ as $\nu=1/m$ ), $a$ is the statistical segment length of the polymer chain, and $n$ is the degree of polymerization.

This integral was put into an almost analytical form as follows (Hammouda, 1993)

$$P(Q)=\frac{1}{\nu U^{1/2\nu}} \left\{ \gamma\left(\frac{1}{2\nu},U\right) - \frac{1}{U^{1/2\nu}}\gamma\left(\frac{1}{\nu},U\right) \right\}$$

and later recast as (for example, Hore, 2013; Hammouda & Kim, 2017)

$$P(Q)=\frac{1}{\nu U^{1/2\nu}}\gamma\left(\frac{1}{2\nu},U\right) - \frac{1}{\nu U^{1/\nu}}\gamma\left(\frac{1}{\nu},U\right)$$

where $\gamma(x,U)$ is the incomplete gamma function

$$\gamma(x,U)=\int_0^{U}dt\ \exp(-t)t^{x-1}$$

and the variable $U$ is given in terms of the scattering vector $Q$ as

$$U=\frac{Q^2a^2n^{2\nu}}{6} = \frac{Q^2R_{g}^2(2\nu+1)(2\nu+2)}{6}$$

The two analytic forms are equivalent. In the 1993 paper

$$\frac{1}{\nu U^{1/2\nu}}$$

has been factored out.

SasView implements the 1993 expression.

The square of the radius-of-gyration is defined as

$$R_{g}^2 = \frac{a^2n^{2\nu}}{(2\nu+1)(2\nu+2)}$$

Note

This model applies only in the mass fractal range (ie, $5/3<=m<=3$) and does not apply to surface fractals ($3<m<=4$). It also does not reproduce the rigid rod limit (m=1) because it assumes chain flexibility from the outset. It may cover a portion of the semi-flexible chain range ($1<m<5/3$).

A low-Q expansion yields the Guinier form and a high-Q expansion yields the Porod form which is given by

$$P(Q\rightarrow \infty) = \frac{1}{\nu U^{1/2\nu}}\Gamma\left( \frac{1}{2\nu}\right) - \frac{1}{\nu U^{1/\nu}}\Gamma\left( \frac{1}{\nu}\right)$$

Here $\Gamma(x) = \gamma(x,\infty)$ is the gamma function.

The asymptotic limit is dominated by the first term

$$P(Q\rightarrow \infty) \sim \frac{1}{\nu U^{1/2\nu}}\Gamma\left(\frac{1}{2\nu}\right) = \frac{m}{\left(QR_{g}\right)^m} \left[\frac{6}{(2\nu +1)(2\nu +2)} \right]^{m/2} \Gamma (m/2)$$

The special case when $\nu=0.5$ (or $m=1/\nu=2$ ) corresponds to Gaussian chains for which the form factor is given by the familiar Debye function.

$$P(Q) = \frac{2}{Q^4R_{g}^4} \left[\exp(-Q^2R_{g}^2) - 1 + Q^2R_{g}^2 \right]$$

For 2D data: The 2D scattering intensity is calculated in the same way as 1D, where the $q$ vector is defined as

$$q = \sqrt{q_x^2 + q_y^2}$$

Fig. 116 1D plot corresponding to the default parameters of the model.

Source

polymer_excl_volume.py

References

1. H Benoit, Comptes Rendus, 245 (1957) 2244-2247

2. B Hammouda, SANS from Homogeneous Polymer Mixtures - A Unified Overview, Advances in Polym. Sci. 106 (1993) 87-133

3. M Hore et al, Co-Nonsolvency of Poly(N-isopropylacrylamide) in Deuterated Water/Ethanol Mixtures, Macromolecules 46 (2013) 7894-7901

4. B Hammouda & M-H Kim, The empirical core-chain model, Journal of Molecular Liquids 247 (2017) 434-440

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