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.
The first degradation step, attributed to the PU hard segments, occurs at a temperature range from 250[degrees]C to 375[degrees]C for PTMG-based SMPUs and from 275[degrees]C to 370[degrees]C for PEG-based SMPUs.
g,DSC] values of PTMG-based SMPUs are higher than [T.
The thermally activated shape memory behavior of the synthesized SMPUs can be also studied quantitatively by means of thermomechanical cycles, shown in Fig.
In relation to shape fixity, values are higher to PTMG-based SMPUs because soft segment, which is responsible for the fixation, is different and, as we have seen by DSC and DMA, PEG is more flexible than PTMG.
Simple crosslinking, by adopting a three-way or four-way linker that connects polymer chains in a random manner, resulted in a very rigid SMPU as the crosslinker content increased , .
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.
The stretching vibrational mode of the carbonyl group is also affected by dipole-dipole interactions between SMPU chains.
g]) of a soft segment of SMPU was low (under -60[degrees]C) and difficult to detect by DSC, so instead the melting temperature ([T.
Crosslink density of SMPU determined by swelling experiments.