Microstructure And Cold Deformation Shape Memory Effect Of Poly(L-Lactide) And Its Copolymers

Poly-L-lactide(PLLA)/Poly-ε-caprolactone(PCL) random and di-blockcopolymer initiated by stannous octoate Sn(Oct)2 were synthesized by ringopeningpolymerization. The film and filament of the copolymers were fabricatedby means of solvent evaporation and melting extrusion technique respectively.The spinning feasibility, microstructure, mechanical properties and colddeformation shape memory effect of the copolymer were investigated by meansof rheological testing, differential scanning calorimetry (DSC), Raman spectrumanalysis, X-ray diffraction(XRD) and tensile testing. The relationship between themicrostructure and cold deformation shape memory properties were illuminated.The mechanical behavior of random copolymers in glass state shows thatlarge strain after yielding and high elastic strain characteristics are observed andexhibition good cold deformation shape memory effect in which the PLLA crystaland the PCLA amorphous serve as fixed phase and reversible phase, respectively.The composition, deformation strain and number of cycles have an obviouseffects on the cold deformation shape memory effect. With the content ofε-CLincreasing, the shape retention ratio is decreased, and the shape recovery stress isincreased firstly and then decreased. It should be noticed that the maximum shaperecovery stress is 9.3Mpa, which is twice that of the stress induced by thermalshape memory effect. With the increase of deformation strain, the shape retentionratio is increased, but the shape recovery ratio is decreased. And the shaperecovery stress is firstly increased and then decreased. Noting that when thedeformation strain is up to 100% the stress is maximum. With the number ofcycles increasing, the shape recovery ratio is decreased and then keeps constant.Annealing time has an obvious effects on the shape retention ratio of therandom copolymers. With the annealing time increasing, the shape retention ratiois increased but the shape recovery ratio decreases slightly.The di-block copolymer has the better cold deformation shape memory effectthan random copolymer. The PLLA crystal serves as fixed phase and the PCLchain acts as reversible phase. With the increase of content of PCL, the shape retention ratio is decreased. When the content of PCL is up to 20%, the shaperecovery ratio is maximum(100%), and the shape recovery stress is decreasedinitially and then increased result in that the highest shape recovery stress is12.5MPa.The result of rheological properties of PCLA copolymer shows goodspinning feasibility. There is only PLLA crystal phase in random copolymerfilament as the same of the film, but the degree of crystallization is higher thanthat of the film. With the content ofε-CL increasing, the tensile stress and theshape retention ratio is decreased, while the shape recovery ratio is increased.With the deformation strain increasing, the shape recovery stress is increasedobviously initially then increased slowly. Both the shape retention ratio and theshape recovery ratio of the filament are higher than that of the film, but the theshape recovery stress is lower.