Statistical mechanics is about systems with a very large number of particles. As the number of particles becomes very large, discrete particles (atoms, monomers, molecules, etc.) begin to take on what appears to be a continuous character. Hence, if one were able to count the number of particles possessing a certain state of the system at some specific time, they would find that the number of particles having this state to within some range of tolerance is roughly equal to the product of the total number of possible configurations times the probability.
The value of in (3) represents a measurable thermodynamic state variable. For the state , there will be polymers with a rms end-to-end separation-distance of . For a large ensemble, . We let
and we perturb the system such that the parameters in (3) reflect the character of distorted chain ( hence . Because of the inverse relationships between and , the overall character of (3) remains unchanged (one can check this by substitution). We approximate the number of molecules having this state as and use these relationships in the Maxwell-Boltzmann expression
Now, we take the logarithm of both sides and apply Stirling's approximation at zeroth order yielding
where we have further simplified by using . Using the definition for and , we have
To find the elastic FE for swelling or contracting the coil, we must integrate over all the microstates of . Changing the summation to integration, the Helmholtz FE as a function of for the elastic contribution to the FE becomes
and the result simplifies to
Now, differentiating this expression yields the optimal value for