Re-examination of Some Properties of Polymer Solutions
|School||University of Science and Technology of China|
|Course||Physical and chemical|
|Keywords||high-vacuum anionic polymerization laser light scattering (LLS) semidilute internal motion|
In this thesis, some properties of polymer solutions are investigated by a combination of laser light scattering and anionic polymerization. First, we have studied the association of polystyryllithium （PSt-Li+） in hydrocarbon with a specially prepared scattering cell. Second, we have prepared dust-free semidilute solutions of polystyrene （PS）, respectively, in benzene and cyclohexane by anionic polymerization and reexamined effects of solvent, concentration and temperature on the solution dynamics. Third, we have revisited the temperature dependence of the conformation and internal motions of long narrowly distributed PS chains in cyclohexane at temperatures around the 9 temperature. The results and conclusions are as follows.（1） PSt-Li+ can associate with each other to form dimers in solutions. When the PSt-Li+ chain is short, an additional slow mode can be observed in the intensity-intensity time correlation function, and its contribution to the scattering intensity decrease as the chain length increases. When the average molar mass of the chains is～104 g/mol, the slow mode disappears. Our experiments demonstrate that the slow mode is not due to large aggregates with C-Li skeletal bonds but long range density fluctuation or loose aggregates induced by weak electrostatic dipole-dipole interaction among ionic pairs at the ends of short PSt-Li+ chains.（2） We have reexamined semidilute solutions of linear PS in benzene and cyclohexane. For a PS semidilute solution in an athermal solvent （benzene）, only one fast mode can be observed in the intensity-intensity time correlation function, whose thermodynamics can be well described by the blob model and the scaling laws. For a PS semidilute solution in aθsolvent, there exists an additional slow mode in the intensity-intensity time correlation function due to stronger segment-segment interaction. When the polymer concentration （C） is higher than the overlap concentration （C*）, as C/C* increases, the scaling exponent （αs） inτs∝q-αs whereτs and q are the relaxation time for the slow mode and thescattering vector, respectively, decreases from 3 to 0.αs and the intensity contribution of the slow mode also depend on the solution temperature. At C～C* , the chains just start to touch each other, the slow mode can only be attributed to segment-segment interaction. When C/C* >> 1, each chain is confined inside a "tube" made of its surrounding chains. In an athermal solvent, the segment-segment interaction is fully excluded, so that the tube is homogenous and only one fast relaxation mode is observed, which is related to the diffusion of the center of gravity of a small segment （"blob"） inside the tube. In a less good solvent, the segment-segment interaction near the entanglement points makes the tube inhomogenous, so that the additional slow mode appears. The slow mode becomes more detectable at a large scattering angle, indicating its associated linear dimension is relatively small, presumably, it is related to the tube diameter.Our model can be visualized as follows. In semidilute solutions, only a small segment of the chain can be excited by the thermal energy （kBT）. So that its center of mass can undergo a random Brownian motion inside a volume of rb3, where rb is the diameter of the "tube". The homogeneity of the tube depends on the solvent quality. For a tube without any interaction with the chain inside （an athermal condition）, each blob experiences the same confinement so that there is only one fast relaxation mode. When sections of the tube interact with the chain, different blobs are confined in two different kinds of tubes with the same size. In the interacting sections, the blobs move slower so that an additional slow mode in the intensity-intensity time correlation function appears. The present study confirms that the slow mode is real, not due to some artifacts and reveals the nature of the slow mode in semidilute solution. Our study has solved this classic slow-mode problem once and for all.（3） The studies of the temperature dependence of the conformation and internal motions of long PS chains in cyclohexane around theθtemperature （Tθ） reveal that internal motions are enhenced when the solution temperatures is lower than Tθ. Even in the region qRg < 1, where Rg is the radius of gyration of the chain, internal motions are still visible. This might indicate that the conformation of the chains is not a Guassion distribution when the chains are in the crumpled globule state. Namely, the chain conformation cannot fully relax within the measurement time.