Study On The Nonlinear Rheological Behavior And Crystallization Of Olefin Multiblock Copolymers

Thermoplastic elastomer has been widely used in more and more fields due to excellent performance,efficient processing and recycling ability.Multiblock copolymer with alternating soft segments and hard segments is one of the most widely used thermoplastic elastomers,including olefin block copolymers,styrene block copolymers and polyurethane block copolymers,etc.Block copolymer can undergo microphase separation because of the thermodynamic incompatibility between soft segments and hard segments.The microphase separated morphology always determines its mechanical properties.For crystallizable block copolymer,the microphase separation and crystallization may coexist,and the interplay between the microphase separation and crystallization will affect the final properties in application.The research on structure,microphase separation and crystallization of block copolymer is the basis for designing and controlling the performance of block copolymer.Most previous studies on the microphase separation and crystallization have focused on diblock copolymers or triblock copolymers.So far,the microphase separation and crystallization of multiblock copolymers,especially under the flow condition,remain far unrevealed.What’s more,the understanding of relationship between the flow and phase transitions plays an important role in the processing of thermoplastic elastomers.In this thesis,several olefin block copolymers(OBCs)are chosen to investigate their rheological behaviors during the melt phase transitions and the crystallization,and the correlation between the flow and phase transitions had been established.The main contents and conclusions in this thesis are summarized as follows: 1.New nonlinear rheological characterization for microphase separation of olefin block copolymerIn this work,the nonlinear response of olefin multiblock copolymers melts has been investigated under medium amplitude oscillatory shear(MAOS).Based on Fourier-Transform rheology,we had defined a series of rheological functions,and proposed the rheological experiment method for accurate measurement.The degree of the deviation from the reference of homogeneous state in intrinsic nonlinearity(Q3,0)is more significant than that in linear dynamic moduli(i.e.11,0G￠ and11,0G￠)for all microphase-separated samples.However,the extent of failure of time-temperature superposition(TTS)is similar in both linear dynamic moduli and intrinsic nonlinearity.What’s more,33,0G￠ and33,0G￠ are very sensitive parameters to justify the TTS principle.It is believed that such sensitivity is ascribed to the phase angle.Thus,we suggested a new method,the v GP3 plot,i.e.,the intrinsic phase angle at third harmonic versus the intrinsic complex moduli at third harmonic.Using the v GP3 plot,the weaker microphase separation can be successfully distinguished from the homogeneous state.The high sensitivity of v GP3 to the failure of TTS makes it a new candidate in studying the thermorheological behaviors of complex fluids.Finally,the intrinsic nonlinearity of OBCs under medium amplitude oscillatory shear was modelled by combining the molecular stress function(MSF)model and emulsion model.The predictions of the combined model agree quite well with the frequency dependence of the experimental data for both linear storage moduli and intrinsic Q factor(Q3,0).The correlation between nonlinear viscoelasticity and the size of the domain was established,which is the theoretical basis for characterizing microphase separation by the medium amplitude oscillatory shear.2.Strain accelerated effect on the microphase separation process of olefin multiblock copolymerBased on the research of monodispersed di-or tri-block copolymers,it is generally believed that microphase separation of block copolymer can form equilibrium structure.However,we had found that polydispersed multiblock copolymer is difficult to form the equilibrium structure,and flow can significantly accelerate the process of microphase separation.Such phenomenas are in contrast to the monodispersed di-or tri-block copolymers,but somewhat similar to polymer blends.In this work,the nonlinear relaxation behavior of olefin multiblock copolymers subjected to step shear strains has been investigated.Due to the microphase separation,failure of timetemperature superposition and time-strain separability as the molecular weight or hard block fraction increases are found using the nonlinear relaxation moduli.Compared to homogeneous polymer,a two-step relaxation behavior was identified in olefin multiblock copolymers,namely a fast relaxation reflecting the chain relaxation and a slower one for the domain relaxation.In order to further study the relaxation behavior of domains,the nonlinear relaxation modulus of heterogeneous OBCs was separated into two parts,namely(7),(8)chainG t g and(7),(8)domainG t g.(7),(8)chainG t g in the heterogeneous regime can be obtained from the nonlinear relaxation modulus in the homogeneous state using time-temperature superposition or from the nonlinear relaxation modulus of the low molecular weight sample using time-molecular weight superposition.Finally,the effect of flow on the microphase separation of olefin multiblock copolymers has been investigated.The relaxation process of olefin multiblock copolymers is independent of the annealing time under quiescent condition,and also the relaxation process is not affected by the annealing time in homogeneous regime even under large step strains.However,Large strain can accelerate microphase separation of olefin multiblock copolymer,resulting in increases of the relaxation time due to the coarsening of domain as well as the increase of the volume fraction of domains.What’s more,as the strain increase,the acceleration effect is more significant.3.In situ Raman spectroscopic investigation on the crystallization and melting of olefin multiblock copolymersIn this work,the non-isothermal crystallization behaviors of olefin multiblock copolymers were investigated by Raman spectroscopy.In polyethylene,three-phase morphological structure have been revealed: the orthorhombic crystalline phase,the amorphous phase,and the interfacial phase,which is located between the crystalline and amorphous phases.By comparison,we have proposed the integrated intensity of the symmetric C-C stretching vibration from 1110 cm-1 to 1160 cm-1 centered at 1130 cm-1 is used to measure the crystallinity of the sample.In contrast to previous analysis method of Raman spectrum,the new approach can fulfill the three phases analysis with no requirement on the Raman spectrum in melt state.The crystallinity obtained from such approach is well consistent with the method using melt Raman spectrum as a reference.The Raman total crystalline fraction is consistently correlated with the crystallinities from DSC.It is found that the increment in Raman orthorhombic crystalline fraction as the hard block content increases in OBCs is not as much as that in the total crystalline fraction,which might be ascribed to the multiblock chain structure of OBCs.Non-isothermal experiments on melting and crystallization illustrates that Raman spectrum and DSC can give identical transition temperatures.However,Raman spectrum analysis shows two distinct features during crystallization and melting of OBCs,namely the disappearance of all-trans noncrystalline phase at temperature much higher than the melting point and the dynamic balance of the interfacial phase fraction.4.Shear-induced crystallization of olefin multiblock copolymersIn this work,the isothermal crystallization behaviors of olefin multiblock copolymers were investigated by combining the preshear and oscillation multiwave.Based on the suspension theory,the dynamic modulus master curve during isothermal crystallization had been obtained by horizontal shift factor and vertical shift factor.The crystallization fractions with the isothermal crystallization time would be obtained by the vertical shift factor.This method successfully overcome the problem of angular frequency dependence.Then,the effect of shear on crystallization for different OBCs had been investigated.Compared with the strong phase separation system of OBCs,the crystallization for weak phase separation system of OBCs are more sensitive to shear,which is very different from the effect of molecular weight on shear-induced srystallization of homopolymer or the effect of composition on shear-induced srystallization of polymer blend.What’s more,the moduli at a crystallization time can be decomposed into two parts,namely the aggregation contributions and the hydrodynamic contributions.Based on the modulus decomposion,the effect of shear on crystallines aggregation for different OBCs had been investigated.The aggregation contributions to the dynamic moduli for the weak phase separation system of OBCs are higher than that for the strong phase separation system of OBCs.What’s more,shear did not change the crystallines aggregation during OBCs crystallization,indicating the crystallization mechanism of OBCs is not changed.