X. Jing*, and X. Chen
State Key Lab of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun, Jilin, 130022, China
Poly(e-caprolactone) (PCL) is a well known polymer with melting temperature (Tm) around 60ºC. It is biodegradable, at least in the presence of certain microorganism. Starting from low molecular weight PCL, we prepared polycaprolactone polyurethanes (PCLU) and examined their shape-memory properties.
e-Caprolactone was polymerized into a polymer with two –OH end-groups, i.e., HO-PCL-OH. This was achieved by using stannous octoate (Sn(Oct)2) as catalyst and ethylene glycol as initiator. Their molecular weight was adjusted over the range of 500~10,000.
These polyester precursors were reacted with dicyanates in the presence of ethylene glycol in toluene solution at 65ºC and catalyzed by Sn(Oct)2 to get corresponding PCLU.
The shape-memory effects were observed for tensile deformation, compressive deformation and other complicated deformations (See Fig. 1 for an example). The recovery percentage was 94-100% for tensile elongation or compression of 300%. The recoverable deformation xould be obtained at a temperature was in the range of 30-60ºC. The lowest shape-recovery temperature (LRT) determined by thermal mechanical analysis (TMA) was much lower than Tm of PCLU. By changing Mw of PCL and hard/soft ratio, the LRT could be adjusted to body-temperature, e.g., 37-42ºC. It meant potential medical usage of PCLU as shape-memory devices.
To examine the driving force for shape memory, the contracting force in tensile specimen or the expanding force in compressed specimen was measured as temperature arose. The results showed that strong inner stress existing in the specimens due to the post-deformation and fixation was the driving force for shape recovery and pre-melting occurring in deformed specimens.
(a) (b) (c)
Fig. 1 A real shape-memory circle. (a) Original cylinder shape; (b) Flower shape at 20ºC; (c) recovered shape at 40ºC.
