SIS Triblock Copolymer Research Articles

Degradation of polymer blends. Part VI: Photo-oxidation of polyethylene containing SIS triblock copolymers

Two aspects of the problem have been investigated: the initiation of the degradation of the polyethylene phase by the very rapidly oxidized unsaturated block, and the deterioration of the mechanical properties of the blend which finally leads to the spontaneous fragmentation of the film. The first aspect involves the identification and the quantitative determination of the functional CO and OH groups formed in each phase as a function of the irradiation time. The molecular weight of the polyethylene phase was found to decay and to be accompanied by a narrowing of the molecular weight distribution which indicates that its degradation is initiated homogeneously. Concerning the second aspect, two types of spontaneous fragmentation of the films have been identified and related to the morphology of the blend. The first involves initiation and growth of cracks parallel to the direction of extrusion in the SIS phase when SIS is dispersed as a continuous fibrillar structure. The second concerns blends with a low SIS content dispersed as small spheres. Cracks are then initiated at the heavily degraded SIS sites but propagate isotropically in all directions through the polyethylene phase. Such isotropic fragmentation is also observed when low molecular weight photo-initiators are used instead of the unsaturated copolymer.

Phase equilibria in mixtures of block copolymer and homopolymer

Phase diagrams were constructed experimentally for mixtures of a polystyrene- block-polyisoprene- block-polystyrene (SIS) copolymer and a homopolymer polystyrene. For the study, two SIS triblock copolymers having spherical and cylindrical microdomain structures, respectively, and a series of homopolymer polystyrenes were synthesized via anionic polymerization. In constructing phase diagrams, we combined the results of dynamic viscoelastic measurement (i.e. logarithmic plots of dynamic storage modulus versus dynamic loss modulus), which allowed us to determine the boundary between the mesophase and homogeneous phase, and turbidity measurement, which allowed us to determine cloud point curves for liquid-liquid (macrophase) separation. Experimental results are compared with predictions made by the theory of Hong and Noolandi. We have found that predicted results are at variance with experimental results.

Rheological behaviour of block copolymers in the disordered state: effects of molecular weight and block length ratio

The effects of molecular weight and block length ratio on the rheological behaviour of polystyrene- block-polyisoprene- block-polystyrene (SIS) copolymers in the disordered state were investigated. For the study, SIS triblock copolymers were synthesized via anionic polymerization, and linear dynamic viscoelastic properties (i.e. dynamic storage and loss moduli, G′ and G″) of the SIS block copolymers were measured at various temperatures as functions of angular frequency (ω), using a cone-and-plate rheometer. The disordered state of the samples was determined by the threshold temperature at which the plot of log G′ versus log G″ becomes virtually independent of temperature as the temperature was increased. It has been found that when the block length ratio is kept the same, the melt elasticity of SIS block copolymers in the disordered state becomes independent of molecular weight, while the viscosity increases with molecular weight, and the block length ratio greatly influences both melt viscosity and elasticity.

The molecular theory of viscoelasticity for thermoplastic elastomer SBS (SIS) at large deformations

A microstructure model for SBS and SIS triblock copolymers with hard domains as multifunctional reinforcing fillers is proposed. Based on this model and proposed mechanism of large deformations, the probability distribution function of the end-to-end vector for each constituent chain and the free energy of deformation for the total networks was calculated by the combination of statistical thermodynamics and kinetics. A new molecular theory of non-linear visco-elasticity for SBS and SIS at large deformations is presented. It is successful in relating the viscoelastic state to molecular constitution by three important parameters (C100,C020, andC200) of the networks. The relations of stress to strain for four types of deformation, the elastic modulus and the constitutive equation for the stress relaxation were derived from this theory. It provides a theoretical foundation for studying the relationships of multiphase network structures and mechanical properties at large deformations. An excellent agreement between the theoretical relationships and experimental data from the experiments and the reference was obtained.

Molecular-weight dependence of the glass transition temperatures of rigid SIS triblock copolymers studied by DSC

Differential scanning calorimetry (DSC) measurements have been carried out on a series of ABA poly(styrene-b-isoprene) triblock copolymers with 30% polyisoprene content and various molecular weights. The DSC data show an inward shift for the glass transition temperatures (T g) of the blocks compared to the corresponding homopolymers. As a function of the molecular weight, one to three transitions were found. The additional thirdT g gives some further evidence of the existence of an interphase between the microdomains.