Preparation Of Branching Polystyrenes And Investigation On Their Rheological And Viscoelastic Behavior

Star and comb polymers are two kinds of special branched polymers. Their preparation and characterization methods have been extensively investigated for the decades. The relations between branching structure and the rheological/viscoelastic behavior of branching polymers have attracted more attention recently. In order to get more insight of the relations between branching structure and rheological/viscoelastic behavior of branched polymers, atom transfer radical polymerization(ATRP) was used and "core first" method was adopted to synthesize three-arm star polystyrenes with certain arm length, which were then taken as model compounds to prepare random branching polystyrenes with different branching levels and branching chain lengthes by blending with linear polystyrenes. Also, ntroxide mediated radical polymerization(NMRP) and ATRP were combinedly used to synthesize comb polystyrenes with different side chain lengthes and branching densities. A rotational rheometer was used to investigate the influence of branching structure on rheological/viscoelastic behavior in the two branching systems, repectively.For the syntheses of comb branched polystyrenes with well-defined structure by combination of NMRP and ATRP, side reactions during the preparation process should be strictly controlled. The measurements taken to eliminate side reactions included reducing side chain density along backbone in the design of comb structure, increasing solvent usage and decreasing macroinitiator concentration, controlling monomer conversion whthin lower level during comb chain propgation. These measurements could restrain interand intramolecular coupling reactions through combination of the growing side chains, and give rise to precise comb polystyrenes.In long-chain random branching polystyrene system prepared through blending of three-arm star polystyrene and linear polystyrene, because long branching chains promoted chain entanglement, with the increase of branching level, zero shear viscosity of branching PS increased gradually, while the non-Newtonian exponent kept decreasing, and the flow activation energy increased slightly. The modulus G’ both in glassy and rubbery regions, Tg, Tf and τe also greatly increase with branching level. Dynamic mechanical curves showed the terminal relaxation for backbone became stronger and the relaxation corresponding to branch disentanglements occurred at high frequencies. Furthermore, the radius of Cole-Cole curves enlarged gradually and there were upward trend at the end. In short-chain branching system, zero shear viscosity of branching PS demonstrated a tendency of increasing at the beginning, and then dropped down later with the increase of branching level. G’ both in glassy and rubbery region, Tg, Tf and τe also increased, but much slowly with branching level, compared with long-chain branching system, which could be attributed to the suppressed chain entanglement by short branching chains. The terminal relaxation for short chain branching system was much weak, and branch disentanglement relaxation could not be observed in high frequency region. The Cole-Cole curves were in half–circles as well as linear PS.Comb branched polystyrenes with different side chain lengthes and densities showed similar shear thinning behavior as in the random branching polystyrene system, but the shear thinning happened in a lower range of shear rate. The relaxation corresponding to backbone and branching chain, repectively, occurred in low frequency and intermediate frequency zone. As branching chain length increased, chain entanglement became more aggressive, and zero shear viscosity increased significantly. Non-Newton behavior was even more significant. G’ of glass state and rubber state were higher. Tg and Tf increased obviously. Due to the combined effect from the molecular weight and chain entanglement, relaxation time τe significantly increased with branching chain length, and curve slope in the terminal area changed evidently. In the short branching system, because short branches inhibited chain entanglement, with the increase of branching density, the viscosity increasing effect contributed from increase of molecular weight was partially offset by inhibited chain entanglement caused by short branching chains. The zero shear viscosity increased slowly and flow activation energy was almost unchanged. Tg, Tf,τe and the slope in terminal area all changed slightly. Cole- Cole curves of comb polystyrene upward turned at the end, which related to the end viscoelastic effect by branching chains.