华南理工大学高分子物理课件之聚合物溶液( Polymer solutions 4).ppt

Polymer solutions and its blendsLecture 4 May 6, 2010 4 parameters χ α A2 v Good solvent 1/2 1 0 0 Theta solvent 1/2 1 =0 =0 Poor solvent 1/2 1 0 0 Coil-globule transiton If the solvent quality is reduced further, the net attractions between monomers can lead to a collapse of the coil into a compact ‘globule’ conformation. Phase separation Two types of phase separation, spinodal/binodal. Mechanism of the phase separation for some polymer/polymer system depends on the volume fraction and temperature. Nishi, Wang, Kwei, Thermally Induced Phase Separation Behavior of Compatible Polymer Mixtures , Macromolecules, 8, 227, 1975 Scaling Relationship In good solvent, r21/2~N3/5 in theta solvent, r21/2~N1/2 in poor solvent, r21/2~N1/3 The above eqs tell us how r21/2 scales with N: If we double N, then r21/2 increases by a factor 23/5, 21/2, 21/3, As we can see, scaling is a property of power law dependencies, and the scaling factor follows from the exponent. Scaling laws always have the mathematical form of a power law. The continuous straight line corresponds to the scaling law r21/2~N3/5 PS in toluene Concentration regimes In a dilute solution, the molecules are well separated on average and do not interact with each other. Each molecule can therefore be considered as an isolated chain. As the concentration is increased, a point is reached where coils start to overlap. This is called the coil overlap concentration, c?. c=c*=? Concentrated solution, the chains entangle together. C* At c*, c=N/V ≈n/r23/2 r21/2~Nv c? ～ N1?3v In a good solvent, c? ～ N-4/5, whereas in a theta solvent or a poor one, c? ～ n? Semidilute (indeed , ) For a typical polymer, say with N≈104, interpenetration of chains already begins at volume fractions below 0.001(!) The solution in which 1) polymer volume fraction1 (~10?3–10?1); 2) coils are overlapping and strongly entangled. Such solutions are called semidilute. The s