The discovery of SMPUs in 1990 by Mitsubishi Heavy Industries Ltd.
In this scenario, this work deals with the synthesis, thermomechanical and shape memory characterization of poly(oxytetramethylene) glycol (PTMG) based SMPUs and poly (ethylene glycol) (PEG)-based SMPUs.
All the SMPUs were synthesized in a similar way as previously described  by a two-step method varying either the hard-segment content or the soft segment, the polyol.
The analysis of all SMPUs by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) was similarly carried out as described in our previous work .
ATR-FTIR was used to assess the formation of polyurethanes [32, 33], Chemical structure of the resultant SMPUs was fully confirmed and results were reported in a previous work .
The representative TGA and DTG curves obtained from PTMG-based and PEG-based SMPUs with different molar ratio are shown in Fig.
Schematics of the resulting SMPU structures are shown in Fig.
Composition of flexibly linked SMPU. Reactant (mmol) Sample code MDI-1 PTMG-2000 BD MDI-2 PEG-200 C1-1 30 -- -- C1-2 30 5 2.5 C1-3 50 20 30 10 5 C1-4 30 15 7.5 C1-5 30 20 10 C2-1 20 -- -- C2-2 20 5.0 2.5 C2-3 50 30 20 10 5.0 C2-4 20 15 7.5 C2-5 20 20 10 Crosslink Density
A Fourier-transform infrared (FTTR) spectrophotometer (JASCO 300E) equipped with attenuated total reflectance capability was used to take infrared spectra of the SMPU samples.
The crosslinking method was successful in improving the mechanical and shape memory properties of SMPU, but certain weakness such as low strain and better shape recovery must still be remedied.
The advantages of allophanate linking were as follows: (1) glycerol was not included in SMPU for crosslinking, (2) a titration step for quantification of the presence of free hydroxyl groups was not necessary, (3) a more reactive amino group was used as a linking site instead of the hydroxyl group of glycerol, and (4) the carbamate group was far more evenly distributed along SMPU chain than the limited glycerol hydroxyl groups.
The structural differences between linear SMPU and flex-linked SMPUs can be observed in Fig.