A chain under traction

The external energy due to the force/, when the end-to-end distance is r, is simply -f-r. Thus, we may write a partition function fw the chain in the form 

Note that (r) is not linear in N at small/. This means that the tension/ is transmitted not only through the backbone (as in the ideal case) but also through contacts between certain pairs of monomers (n, m) (with n-m large). 

Consider now the limit of large tensions ( t 1). What happens here can be idealized as shown in Fig. 1.10. The chain breaks up into a series 

The number of monomers per blob, gp, is related to fp by the Flory law of real chains [eq. (1.39)], giving 

47) deserves some discussion. We see that a real chain has an elastic response which is significantly more nonlinear than an ideal chain. This appears on the plot of ip(x) shown qualitatively in Rg. 1.11. 

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