Shape Memory Effect, Shock- and Energy-Absorption Capability of Critically Cross-Linked Syndiotactic Polypropylene

Abstract

Abstract Critical cross-linking, a light cross-linking right at the borderline between thermoplastic and elastomer, was applied to syndiotactic polypropylene (sPP) to gain at least a shape memory effect for storing preferably high strains. Beside the expected shape memory effect with maximum storable strains of 550%, critically cross-linked syndiotactic polypropylene (x-sPP) exhibits further amazing properties. For example, amorphous x-sPP is one of a few “cold programmable” shape memory polymers. At room temperature, it is easily deformable by hand and forms strain-induced crystals, which stabilize it at strains of up to 340%. Thermal triggering of cold-programmed x-sPP exhibits a triple-shape memory effect with an intermediate shape that depends on the heating rate applied for triggering. Thus, cold-programmed x-sPP is capable of reacting to a temperature increase by adapting its shape change according to the applied heating rate. Since a material with such property can be used e.g. to predict a system failure when used in a defined environment, we called it “predictive material”. Since amorphous x-sPP is easily deformable, transforms itself from a compliant, elastomeric into a rigid, semi-crystalline state upon stretching to large elongations and retains the latter after removing the stretching force, we explored its suitability as a shock- and energy absorber. To this end, thin safety lines of x-sPP were prepared and tested by falling weight experiments to determine characteristic physical quantities such as maximum impact force, maximum jerk (first derivative of the deceleration) and energy absorption capability. It was found that x-sPP is excellently suited for cushioning the impact force, vibration and chatter, which occur when a safety line abruptly tightens after the free fall in a falling weight experiment.