Correlation between Morphology and Mechanical Properties of Polystyrene-b-Polybutadiene-b-Poly(methyl methacrylate) in Dependency of the Polybutadiene's Block Microstructure.
In this work correlation between morphology and mechanical properties of different triblock terpolymers of polystyrene-b-polybutadiene-b-poly(methyl methacrylate), SBM, especially of those having lamellar morphology subtypes (e. g. well segregated lamellae or a transitional lamellar pattern) have been extensively investigated. Moreover, the influence of polybutadiene (B) isomers (which is mainly controlled by the solvent polarity) on the morphology and mechanical properties have been explored. Different polybutadiene isomers (from higher 1,2- to lower 1,2-B) were obtained by sequential anionic polymerization method. The morphological and mechanical investigations for different lamellar type SBMs showed that very well segregation of the lamellar domains for both 1,2- and 1,4- SBM were obtained at molar masses ranging from 80 to 170 kg/mol. Short lamellar domains were found for the molar masses ranging from 40 to 55 kg/mol. Mixed lamellae can be achieved in the polymers containing low molar mass (35-70 kg/mol) and higher contents of 1,4-B. A transitional lamellar morphology were found in the 1,2-B dominant SBMs and for the presence of significant amount of residual precursors (S homopolymer and SB diblock copolymer). The chain like oriented lamellae was obtained in a SBM existing short chain B domains where the ratio of the 1,2- and 1,4-B is 1:2. The mechanical investigation shows that the polymers with lower x(1,2)% and relatively higher 1,4-B content show moderate stress drops after yielding and an increasing stresses during elongation process. On the other hand, the polymers with higher x(1,2)% and relatively higher 1,2-B contents, higher stress drops and lower strain hardening are observed. Higher elongation was obtained if 1,4- B is predominant and the molar masses of SBMs are high. The tensile modulus and stresses at break were mainly affected by the nature of glassy domains of the polymers as well as the 1,2- B content. However, the effect of B isomers on the stresses at break is moderately low. A strong influence of the ratio of the 1,2- and 1,4-B was observed on the mechanical properties. If the molar mass of the B domains are low and the ratio of 1,2- and 1,4-B is close to 1:2, the mechanical properties are found remarkably low. The deformation patterns were studied from in-situ tensile test- SAXS evaluation. An anisotropic deformation behavior for 1,2-B content SBMs and a four point SAXS pattern for 1,4-B content SBMs were unveiled. No orientation occurs in low molar mass 1,2- SBM (1,2S31B31M3855) due to the ductile behavior. On the contrary, orientation perpendicularly to the stretching direction at large elongation occurs in high molar mass 1,2-SBM (1,2S32B31M3791). In case of low molar mass 1,4-SBM (1,4S30B29M4153), random orientation occurs at 300% elongation whereas slightly orientation perpendicular to the stretching upto 400% elongation was seen for high molar mass 1,4-SBM (1,4S34B31M3580). Significant influences of the residual precursors on the mechanical properties during tensile test were found. The results show an increasing tendency of E-modulus but a decreasing tendency of stress at break and strain at break for the presence of high amount of residual precursors.
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