Nonlinear stress relaxation in palladium(II) complexes with triblock copolymers of styrene and butadiene

Abstract

Above 200% strain, the mechanical response of triblock copolymers which contain styrene and butadiene is modified significantly by complexation with dichlorobis(acetonitrile)palladium(II). This pseudosquare-planar transition metal salt forms -complexes with, and catalyzes the dimerization of, alkene groups in the main chain and the side group of Kraton's butadiene midblock. Between 10 and 100% strain, the plastic flow regime is similar for undiluted Kraton™ and its Pd 2ϩ complexes, but the level of engineering stress is approximately twofold larger for the complex that contains 4 mol % palladium(II) [Pd(II)]. Nonlinear stress relaxation measurements in the plastic flow regime (i.e., beyond the yield point but before the large upturn in stress) are analyzed at several different levels of strain. Transient relaxation moduli were modeled by a three-parameter biexponential decay with two viscoelastic time constants. The longer relaxation time for Kraton™ increases at higher strain, and in the presence of 4 mol % palladium chloride. A phenomenological model is proposed to describe the effect of strain on relaxation times. This model is consistent with the fact that greater length scales are required for cooperative segmental reorganization at larger strain. The resistance ⍀ to conformational reorganization during stress relaxation is estimated via integration of the normalized relaxation modulus versus time data. This resistance increases at higher initial jump strain because conformational rearrangements are influenced strongly by knots and entanglements at larger strain. The effect of strain on ⍀ is analyzed in terms of time-strain separability of the relaxation modulus. Linear behavior is observed for ⍀ versus inverse strain (i.e., 1/), and the magnitude of the slope [i.e., Ϫd⍀/d(1/)] is threefold larger in the absence of PdCl 2 (CH 3 CN) 2 .