Influence Of Molecular Structure On The Phase Behavior Of Styrene-butadiene Copolymer

The rheology of multiphase/multi-component polymer systems, such as polymer blends and block copolymers, has become the attractive interest because of the challenging physical problems and the increasing technological applications behind. In this thesis, the relationship between dynamic rheological response and the morphology for the styrene-butadiene copolymers with different molecular structures is studied mainly by using dynamic rheological measurements and differential scanning calorimetry (DSC).The existence of oxygen was unavoidable during the dynamic rheological test in high temperature. The thermo-oxidation of SBS triblock copolymer was investigated by means of dynamic rheological test. The results revealed that dynamic rheological fuctions such as dynamic storage modulues (G' ) and dynamic loss modulues (G" ) at low frequency (ω) region could sensitively reflect the changes of structure of SBS. The antioxidant-added SBS can confine the thermo-oxidation.Two kinds of styrene-butadiene copolymers with different molecule sequence, random copolymer of styrene and butadiene (SBR) and styrene-butadiene-styrene triblock copolymer (SBS) were used. The DSC test reveals that there two glass transition temperatures (T_g) for SBS, one is at -90 ℃, the other is at 70 ℃, while there is only one T_g at about -90 ℃ for SBR. The heating history that samples exposed to had significant effect on SBS phase structure, especially for PS phase with a small mass proportion in SBS. There is an order-disorder transition with temperature increasing, and the transition temperature is considered to be related to the size of PS domain. It is found through the TEM observation that there are cocontinuous phases in the SBS and the dimension of each phase was confirmed. The dynamic rheological measurements were conducted to evaluate the viscoelastic relaxation behavior of SBS triblock copolymer at different temperatures. Three relaxation peaks in the plot of loss tangent (tanδ) vs temperature at different frequency (ω) can be observed, which oweto the relaxation of various motorial unit of SBS. When a is O.lrad/s, the small peak at about 110°C (above Tg of PS phase^a the plot of tanS versus temperature (7), which is due to the order-disorder transition of SBS. However, there is no cocontinuous structure in SBR, so there is also no corresponding peak on the plot of tand versus T which means that there was no order-disorder transition in the temperature range tested. With the increase of temperature, SBS still appears high elastic modulus because of the interaction between PS and PB phases, and the entanglement of PB chains. Moreover, an obvious "second plateau" as shown in the dynamic frequency sweep curves is involved in the order-disorder transition when the temperature is over 102 °C. In master curve of time-temperature superposition, it's not so well in the low temperature region in which the SBS was in microphase separated while it can be superposition well in high temperatures in which the SBS was in disorder state. The plateau modulus and entanglement molecule weight was calculated.Two kinds of SBS with different topological stucture, linear styrene-butadiene-styrene triblock copolymer and star styrene-butadiene-styrene triblock copolymer were investigated. The star-SBS with four-arms had the structure similar with "island-sea" , which PS island phase diffused in PB continuous phase with blurry interface different from linear SBS's cocontinuous structure which is like fingerprint with certain direction. During the dynamic temperature sweep test, star-SBS exhibits higher order-disorder transition temperature than that of linear SBS, which owes to the more restriction come from the center of star-SBS.The dynamic rheological behaviors of SBS with different S/B ratio are also investigated. Tg of PS block (7^) and the order-disorder transition temperature (Tod) for various SBS are obtained from the plot of tand vs T. The results exhibit that both Tgvs and Tod increase with the ratio of styrene increase. When the content of styrene is low, such as 20%, order-disorder transition peak is not so obvious, which reveals PS phase may play a dominative role in the order-disorder transition. The PS phase was smaller when the S/B ratio reduced. In spite of different S/B ratio, all of linear SBS possess wonderfully similar cocontinuous structure at room temperature obtainedfiomTEM.In addition, the effect of small molecule (oil) and particles (SiC>2 particle) on the rheological behavior of star-SBS is also investigated owing to its application background. The results show that Tgps and Tod would lower when oil was added, while Tgps and To& increase as 3wt% S1O2 particles is added. It is very interesting that there is a wider and bigger order-disorder transition peak for star-SBS filled with 3wt% SiO2 particles than that of virgin star-SBS, in which the interaction between star-SBS and Si(>2 particles confined the movement and increased the movement active energy of each block of star-SBS.