Phase Separation In Styrenic Polymer Solutions Studied By Spectroscopy

Phase separation behaviors in polymer solution have been received considerable research attention over the past years. They mainly focus on molecular weight, solvent effects, interpenetration of polymers, cyclization dynamics and etc. Specifically, the physical property of polymer solution can be totally varied with the different chain architecture. For example, highly branched polymers generally exhibit much lower solution and melt viscosities compared to those of the linear polymers with the same molar mass, and a characteristic which may help facilitate coating, extrusion, or other manufacturing processes. The high concentration of functional terminal groups, along with architectural constraints, leads to the significant changes of chemical and physical properties of highly branched structures as compared with their linear analogs. To the author's best knowledge, however, the effect of chain topologies on the phase separation behavior in polymer solution has been hardly studied.In this dissertation, intrinsic fluorescence and light scattering methods were used to study the effect of chain topologies on phase separation behavior in solution based on several model polymers (ABA copolymer and hyperbranched polymer). The results obtained have been summarized below:(1) By using elastic light scattering method, three stages can be clearly observed in the phase separation process of PE/PS/CHCl3 solution: nucleation growth, spinodal decomposition and deposition. Moreover, intrinsic fluorescence was used to describe the aggregation of PS molecular chains. The PE samples with different molecular weight were found to be the same topology by DLS and SLS method. The difference in the phase separation behavior of PE samples is explained by the chains entanglement. The phase separation temperature increased with increasing PS molecular weight. The possible reason is due to that the larger the PS radius, the more difficult the inserting process of PS into the pore space between the PE chains.(2) By using static light scattering and steady intrinsic fluorescence method, three stages can be clearly observed in the aggregation process of SEBS in methylene dichloride. By applying the Steven-Ban equation, the activation energy for excimer formation in stage II and III was determined to be E ae(II)= 22.6 kJ/mol and E ae(III)= 16.7 kJ/mol, respectively. The transient fluorescence results showed that influence of monomer emission on the excimer emission could be ignored whenλ> 320nm. By using Arrhenius equation, the decay activation energy of intramacromolecular excimer was calculated ( E aτ= -8.3 kJ/mol). The thermodynamic parameters for the micellization of SEBS in dichloride was also determined by concentration dependence of critical micellization temperature (ΔH o= -639.9 kJ/mol,ΔG o(305K) = -29.8 kJ/mol,ΔS o(305K) = -2.0 kJ/mol,ΔG o(285K) = -69.9 kJ/mol, CMC (285K) = 2.3×10-11 g/ml).(3) Both steady intrinsic fluorescence and dynamic light scattering method unequivocally showed four stages during the aggregation process of SEBS in n-hexane/cyclohexane (3/1). Interestingly, two different PS segments were found on the same SEBS chain, i.e., adjacent and nonadjacent to the PEB segments. These two PS segments showed a different conformational change behavior during the micellization of SEBS. By applying the Steven-Ban equation, the activation energies for excimer formation in stage II and IV were determined to be E ae(II)= -3.0 kJ/mol and E ae(IV)= 6.7 kJ/mol, respectively. By using Arrhenius equation, for PS segments being nonadjacent to PEB block, the decay activation energy of excimer was calculated ( E aτ= - 9.9 kJ/mol).