Maybe you can try equation from rubbery elasticity theory as follows:

v = G'rp/RT, where G'rp is storage modulus in rubbery plateau, R universal gas constant and T=Tg+30 C.

I hope that this will be helpful.

First, measure the equilibrium swelling of the polymer in a good solvent, normally by allowing the polymer to swell for 1-2weeks in the dark to reach equilibrium. The swollen gel is then isolated and weighed. The weights of swelling solvent and polymer are determined after removing the solvent by vacuum-drying.

Apply the following equation:

Crosslink density, network chain per gram =

-[ln(1-Vp) + (Vp) + X(Vp)^2]/ {Dp(Vo)[(Vr)^(1/3) - (Vp)/2]}

Where

Vp=Volume fraction of polymer in the swollen polymer

X= Huggins polymer-solvent interaction constant

Dp=Density of polymer (g/cm^3)

Vo=Molar volume of solvent (cm^3/mol)

Do=Density of solvent (g/cm^3)

Here,

Vp=1/(1+Q),

Where Q=[weight of solvent in the swollen polymer x Dp]/[Weight of polymer x Do]

How to measure crosslinking density?. Available from: https://www.researchgate.net/post/How_to_measure_crosslinking_density2 [accessed Jun 3, 2017].

For characterization of network, the most often used is network density and average molecular weight of rubber segments situated between two cross-links. Both these characteristics may be determined in the same way, because dependence:

is valid. ( ν is crosslink density, ρ is rubber density, Mc is molecular weight of rubber segments between two cross-links ).

Sometimes the network of vulcanizates is characterized also with number of cross-links created in the vulcanizate volume unit. It can be calculated also from network density under assumption that cross-links are created between two rubber segments:

(n is number of cross-links in the vulcanizate volume unit, ν is its cross-link density)

Several types of methods are used for determination of these characteristics of the vulcanizate network. Chemical methods may be used only in that case, when exact curing mechanism of using cross-linking system is known and either change of the curing agent or definable products of the low molecule character may be sufficiently determined in quantitative way. Number of the cross-links n can be calculated in this case on the basis of material balance of appropriate chemical equation related to curing.

Physical methods are based on evaluation of deformation properties of appropriate vulcanizates. In case, that these evaluations are performed under low deformations (

You can also calculate the effective cross-linking using young modulus obtained from stress-strain plot(from UTM).

E = 3NKBT

where N is the effective cross-linking

          E is Young modulus

         KB is Boltzmann constant 

         T is absolute temperature in Kelvin 

See the below article.

https://www.mendeley.com/research-papers/triazine-functionalized-hydroxyl-terminated-polybutadiene-polyurethane-influence-triazine-structure/

https://www.researchgate.net/publication/282333502_Triazine_functionalized_hydroxyl_terminated_polybutadiene_polyurethane_Influence_of_triazine_structure

Article Triazine functionalized hydroxyl terminated polybutadiene po...

thanks