Hard-elastic or springy polypropylene

Originally hard-elastic PP was made from crystalline lamellar materials and was processed via melt spinning and crystallization under stress, followed by annealing under tension. The structure of the material consists of stacked crystalline lamellae (5-40 nm thick) with fold planes normal to the fiber direction. Between the lamellae, microfibrils oriented parallel to the draw direction are located. Under load, the lamellae tend to separate and voids bridged by fibrils appear. The void volume is initially about 18% at zero strain which increases to about 65% at 15% strain. No further increase in void volume was observed above 15% deformation. 

Hard-elastic polypropylene has an interesting combination of mechanical and physical properties [2] including  

constant cross-sectional area during deformation. 

Relaxation and cyclic deformation behavior of the hard-elastic polypropylene is of particular interest [2]. If the fiber is allowed to relax, the stress needed to continue the deformation is different from the initial stress. The stress increment can be divided into a permanent and transient part. As the recovery properties of the fibers begin to deteriorate, the permanent increment changes from a negative to a positive value and the transient component decreases to zero. Stress relaxation of 

Liquid-induced stress depression was also observed for hard-elastic PP. llte specimen was stretched to 10% elongation under vacuum. After one hour relaxation, ethanol vapor was introduced leading to a rapid drop in equilibrium stress to a new level. The process is completely reversible, i.e. after removing the vapor, the stress returned to the original relaxed value as measured in vacuo [2]. 

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