Studies in solutions and gels

Analyses using the techniques described above require the gross translational and rotational motion of the molecules to occur on a timescale different from that for conformational changes. This is usually the case in dilute solutions, but not necessarily in more viscous media. 

The problem with concentrated solutions, gels, melts and sohds is that the timescale for diffusive motions can become comparable with that for the conformational change, so it is then difficult to separate these different types of motion. 

Since the dynamic behaviour of polymer molecules in solution depends critically upon concentration, it is convenient to start with dilute solutions. In very dilute solution, the polymer molecules wiU execute motions which are characteristic of isolated chains. This is the situation associated with the normal modes discussed in Chapter 8. These are the result of collective motion of large sections of the chain and cause the shear dependent viscosity of dilute solutions to be considerably greater than that of the solvent. 

The most common way of studying the motion of polymer molecules in such media is to use relaxation techniques to measure the effect of shear rate on, or the frequency dependence of, the real and imaginary parts of the modulus, the comphance or the viscosity as discussed in Chapter 8. In so doing, it is usually 

A number of experimental methods exist that allow polymer solutions to be subjected to different shear rates or to oscillatory shear. Data obtained over a given range of shear rate, or frequency, are shifted to form a universal curve (as in the use of the Williams-Landel-Ferry equation, explained in Chapter 4). This can then be compared with the predictions of various models such as those proposed by Rouse or Zimm. The former assumes that there is minimal interaction between the solvent and the polymer, and is sometimes referred to as the free draining model. In reality, there is some interaction between the solvent and the polymer chain. This is addressed in the Zimm model, where the drag introduced by the solvent influences the motion of the chains. 

---