Characterization of biaxial strain of poly(l‐lactide) tubes

Abstract

Poly(L-lactide) (PLLA) in its L-form has promising mechanical properties. Being a semi-crystalline polymer, it can be subjected to strain-induced crystallization at temperatures above T g and can thereby become oriented. Following a simultaneous (SIM) biaxial strain process or a sequential (SEQ) biaxial strain process, the mechanical properties of biaxial strained tubes can be further improved. This study investigated these properties in relation to their morphology and crystal orientation. Both processes yield the same mechanical strength and modulus, yet exhibit different crystal orientation. Through further WAXS analysis it was found that the SEQ biaxial strain yields larger interplanar spacing and distorted crystals and looser packing of chains. However, this does not influence the mechanical properties negatively. A loss of orientation in SEQ biaxial strained samples at high degrees of strain was detected, but not seen for SIM biaxial strain, and did not correlate with mechanical performance in either case. However, post-annealing reduced the orientation to the same level in both cases, and the modulus and strength is decreased for both SIM and SEQ biaxial. It is therefore concluded that mechanical properties after biaxial strain are related to strain-induced amorphous orientation and the packing of crystals, rather than strain-induced crystallinity.

Mechanical properties of biaxially strained poly(l‐lactide) tubes: Strain rate and temperature dependence

“…As expected, the circumferential tensile behavior (Figure ) differs from the axial (Figure ) . When straining the circumferential test specimens above their yield point, a higher stress is required to further plastically deform the polymer (Figure ), also known as strain‐hardening.…”

“…The tubes were axially strained by 100% at processing strain rates ( of 0.1, 0.2, or 2.1 s −1 followed by cooling. Some of these tubes were exposed to an additional transverse straining before cooling by injection of air (the sequential biaxial strain) as described by Løvdal et al …”

“…]. The reproducibility of identical samples is 0.06% in crystallinity and the variation between positions of samples in the heating chamber is 2% …”

“…Our previous work has shown that it is possible to induce ∼30% crystallinity upon biaxial straining of tubes, and further orient the amorphous domain and the secondary binding between strain‐induced crystals. The process improved the mechanical properties as a function of extent of straining . The induced crystallinity adds to the strength of the materials, whereas the amorphous domain adds to the flexibility .…”

Evaluation of a Bioabsorbable Self-Expandable Vein Stent-Base Made of Poly(l-lactide) In Vitro and In Vivo

“…A PLLA stent base was made by sequential biaxially straining a hollow tube of PLLA 2003D (NatureWorks LLC, Minnetonka, MN) as previously described [ 21 ] and expanded from 5.5 to 10.5 mm. The tube was laser cut in a diamond shaped pattern (8 cells circumferentially) (Fig.…”