Modeling of stress relaxation of a semi-crystalline multiblock copolymer and its deformation behavior


Stress relaxation can strongly influence the shape-memory capability of polymers. Recently a modified Maxwell-Wiechert model comprising two Maxwell units and single spring unit in parallel has been introduced to successfully describe the shape recovery characteristics of amorphous polyether urethanes. In this work we explored whether such a modified Maxwell-Wiechert model is capable to describe the stress relaxation behavior of a semi-crystalline multiblock copolymer named PCL-PIBMD, which consists of crystallizable poly(ε-caprolactone) (PCL) segments and crystallizable poly(3S-isobutylmorpholine-2,5-dione) (PIBMD) segments. The stress relaxation behavior of PCL-PIBMD was explored after uniaxial deformation to different strains ranging from 50 to 900% with various strain rates of 1 or 10 or 50 mm·min−1. The modeling results indicated that under the assumption that in PCL-PIBMD both PCL and PIBMD blocks have narrow molecular weight distributions and are arranged in sequence, the two relaxation processes can be related to the amorphous PCL and PIBMD domains and the spring element can be associated to the PIBMD crystalline domains. The first Maxwell unit representing the faster relaxation process characterized by the modulus E1 and the relaxation time τ1 is related to the amorphous PCL domains (which are in the rubbery state), while the second Maxwell unit (E2 ;τ2) represents the behavior of the amorphous PIBMD domains, which are in the glassy state at 50°C. Increasing strain rates resulted in an increase of E1 and a significantly reduction in τ1, whereas the elastic modulus as well as the relaxation time related to the amorphous PIBMD domains remained almost constant. When a higher deformation was applied (ε ≥ 200%) lower values for the elastic moduli of the three model elements were obtained. In general the applied model was also capable to describe the relaxation behavior of PCL-PIBMD at a deformation temperature of 20°C, where additional crystalline PCL domains are existent. The presented approach using a modified Maxwell-Wiechert model to analyze the stress relaxation behavior can be useful to understand the changes in structure-function relation of amorphous as well as semi-crystalline polymers occurring during its uniaxial deformation.
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