Brownian state and flexibility of the atactic polystyrene chain

2 Brownian state and flexibility of the atactic polystyrene chain 

Can we consider that a chain in solution at temperature TF, or in the melt, is Brownian on a small scale The answer is not simple, and the purpose of this section is to warn the reader against possible errors. 

To answer this question it is sufficient to set the polystyrene chain in the Brownian state and to measure its form function in the asymptotic domain q2 1. Indeed, the form function H(q), for a given q, is a revealer of correlations at distances of order q 1. Observation of the behaviour of H(q) when q increases should in principle show the crossover from Brownian to rigid conformation, However, the application of this method raises all sorts of practical difficulties which we are now going to examine. 

At first sight, the result seems to indicate that the chain remains Brownian down to scale lengths as small as 0.15 nm Indeed, the quantity defined above tends to a constant value when q increases, which is characteristic of the Brownian chain. 

We can reproduce the result by calculation, assuming that the chain is Brownian and performing the adequate polydispersion corrections (with the dispersion law of Schulz and Zimm). In this way it is possible to obtain a function °Hz(q) compatible with the experiment, and this leads to acceptable values of °Ro,z and of p  

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