Experimental and self-consistent-field theoretical study of styrene block copolymer self-adhesive materials

This work addresses thermodynamic and mechanical properties of styrene block copolymer (SBC) self-adhesive materials. Mixtures of polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymers with resin and their blends with polystyrene-block-polyisoprene (SI diblock) copolymers were investigated experimentally and theoretically. The experiments involved small-angle X-ray scattering, atomic force microscopy, and tensile measurements, while the theoretical analysis invoked a mesoscopic representation of SBC materials combined with a continuum, 3D real-space self-consistent-field (SCF) approach. The calculations predict that the SBC systems should become microphase separated, with PS-rich spherical domains forming a bcc lattice in SIS triblock/SI diblock blends and in SIS triblock/resin systems with low resin content (20 wt %), while at high (60 wt %) resin content a disordered structure of PS-rich domains is expected. Extensive comparisons of SCF predictions concerning the type and the geometric characteristics of the morphologies obtained, the composition profiles, and the effect of diblock and resin content on the long-range space ordering of the PS-rich domains are performed against the experimental data. SCF theory predicts that approximately 79% of SIS molecules are connecting different PS-rich domains (i.e., are forming bridges) in SIS/SI blends, almost independently of the SI diblock content. The SCF results are combined with a slip tube model of rubber elasticity to predict the elastic behavior of SBC materials. In particular, the bridging properties are used to estimate the PS-rich domain cross-linking contribution, G(c), to the total shear modulus. The entanglement contribution, G(e), is evaluated from experimental and theoretical results reported in the literature concerning the properties of entanglements. The predicted stress-strain curves are in good qualitative agreement with the tensile experimental data for small values of strain.