Studies On The Viscoelastic Behaviors Of Styrene Block Copolymers And Their Blends

As a kind of popular thermoplastic elastomers, the styrene block copolymer has been widely applied in many fields, such as automobile parts, toolholders, casings of electrical wire and cable, medical appliances, food containers, encapsulant, adhesive, coating, polymer blend materials. It is well-known that viscoelasticity is an important factor to determine processing and ultimate properties for polymeric materials, especially for block copolymers. In this dissertation, we choose four styrene block copolymers as models to investigate their linear and nonlinear viscoelastic behaviors and thermorheological behaviors. On the other hand, we also investigate the linear and nonlinear viscoelastic behavior of nylon / elastomer blends in view of their having extensive widely applications.From the dynamic strain sweep test, the critical shear strains (γ_c) of four block copolymers have been determined. The results show the following ranks as y_c(sBS791) ~ γ_c(sbs796) > γ_c(seeps) > γ_c(sebs). It is found that SEBS and SBS796 block copolymers exhibit peaks in the dynamic loss modulus (G') ~frequency (ω) curves at γ = 24 % and γ = 61 %, respectively. These phenomena are similar to that of the rubber system filled with inorganic particles. We believe that cross linking networks of block copolymers have the similar change with the networks in filled rubber systems when the dimensions of PS phase is in the same level with those of inorganic particles, which has been conformed by the TEM micrograph of SBS796. The curves of dynamic time sweep test indicate that the structure of samples destroyed by the cumulating shear strain is repairable repair gradually with increase of time.Except SEBS block copolymer, the master curves of other three block copolymers with relatively large ω ranges were obtained from the curves of dynamic ω sweep based on time-temperature principal. It is found that WLF equation and Arrhenius equation can be applied to simulate the curves of shift factors (α_T) ~ (T) temperature well, and the fluid activation energy of SEEPS and SBS block copolymers can be obtained. The examination of the effects of temperature on the microstructures ofblock copolymers based on the curves of logG' vs. logG" shows that the master curves is not a straight line but a curves with upturn in the high ? because of the micro-phase separation structures. In addition, the plateau modulus and the entanglement molecular weight were calculated from the minimum of tan8 and cross modulus (Gc). The long time relaxation curves show the complex relaxation behaviors, from which the relaxation spectrums of block copolymers were calculated. A modified BSW model can be used to predict the spectrums well and obtain the longest relaxation times.The nonlinear viscoelastic behavior of block copolymer were investigated and simulated by a nonlinear Wagner model. The results show that the linear relaxation behaviors of block copolymer can be evaluated by the Maxwell model, and the predictions give the characteristic relaxation time and corresponding modulus of different relaxation modes. Moreover, it is found that melts of block copolymer follows the time-strain separation principle and the damping functions of block copolymer calculated from nonlinear relaxation modulus can be simulated well by Wagner function, Soskey-Winter function, Laun function and Papanastasiou function. In additon, the successive start-up of shear behavior, the steady-up shear behavior and the relaxation of steady-up shear behavior are investigated, respectively. The results show that Wagner model, derived from the K-BKZ (Kearsley-Bernstein, Kearsley, Zapas) constitutive equation, can simulate the experiment data of SEEPS block copolymers well, but underpredict the nonlinear viscoealstic behaviors of SEBS block copolymer and overpredict the nonlinear viscoelastic behaviors of SBS. The reason for failure is believed to be that the damping functions obtained are not precise.The studies on the thermorheological behavior of block copolymer demonstrate that block copolymers are thermorheological complex materials. From the co corresponding to maximum of tan5 and Gc, the mean relaxation times of segmental relaxation process and terminal relaxation process were calculated. The VTF function was used to predict the curves of mean relaxation time and a satisfactory prediction was obtained. It is known from analysis of parameters in VTF function that VTF function is unsuitable to describe the segmental relaxation behaviors of blockcopolymers but can describe the terminal relaxation behavior well.The results from dynamic temperature sweep torsion test show that the glass transition temperature (T%) of PS block is lower than that PS homopolymer, and a shoulder peak appears at 115°C, which is perhaps the relaxation behavior of polystyrene macromolecular chains under the effect of interaction between hard block and soft block. The results from dynamic temperature sweep in parallel plate mode show that when the shear strain is low, the curves of viscoelastic parameters ~ T have relatively finely symmetrical, which demonstrates that shear strain don't destroy the structures of sample but induce the structures to be perfect in the process of temperature circulation. When the shear strain is high, the structure breakdowns and the obvious repaired process of structures also can be observed.Studies on the linear viscoelastic behavior of nylonl212 toughened with SEEPS (styrene-[ethylene- (ethylene-propylene)]-styrene block copolymer) elastomer were carried out. For the virgin polymers, the complex viscosity of nylonl212 increases as the (o decreases and approaches a constant at the low a, exhibiting a behavior of Newtonian fluid;while for SEEPS elastomer no Newtonian plateau appears in the (o observed. For the blends, curves of G'~a> are located between those of virgin nylon and SEEPS within o> range measured, and the G' of blends increase with the content of SEEPS increasing and show self-similarity viscoelastic behavior. Furthermore, Palierne emulsion model was used to describe the viscoelastic behaviors of blends. The results of prediction show that it is not decent to describing the viscoelastic behaviors of the double phase system toughened with elastomer, especially for the high content of elastomer.The positive deviation observed in the plot of C versus blend composition shows that the blends are immiscible. From the point of phase transition, the phase-inversion region for these blends was predicted to be in the range of 30 % ~ 50 % weight fractions of SEEPS, which agrees with the morphology analysis of nylonl212 / SEEPS blends. Furthermore, it can be found that from the curves of G*(co) ~ r/*(a>) that the blends don't exhibit stress yield behavior but SEEPS elastomer show the character of stress yield behavior. However, the cole-cole plots of modulus show thatthe microstructures of blends are unstable in the phase transition region. In addition, the weighted relaxation spectrums were calculated based on the Schwarzl and Staverman differential equation and present the particular relaxation behavior of virgin polymers and their blends.The nonlinear Wagner model can give a satisfactory prediction of nonlinear viscoelastic behaviors for virgin nylonl212 and their blends with SEEPS (90 / 10), but cannot predict the nonlinear viscoelastic behaviors of other blends with high content of SEEPS. Studies on the thermorheological behaviors of blends show that modulus is sensitive to temperature in the melting and crystal processes.