Biomedical Engineering Reference
In-Depth Information
in-bulk method only serves for generating a stable blend. The achievement of
PEU surface modification relies on spontaneous SMA migration and surface
enrichment, which equivalently enables the SMA auto-makeup procedure for
SMA's surface reoccupation. The mechanism of SMA spontaneous migration
and surface enrichment is explicated as follows.
Preconditions
(i) The typical elastomeric characteristic of the PEU matrix allows the mi-
gration of invading environmental small molecules and even SMA macro-
molecules within PEU bulk; simultaneously it also allows the compliant
motion of PEU chain segments towed by SMA via the H-bond connec-
tion. (ii) The dissimilarity of mingling compatibility and interface energy
between SMA components and the PEU major phase provides both thermo-
dynamic motivation and orientation for SMA migration. (iii) The H-bond
grafting conjugation assures the stability of SMA immobilization with PEU
matrix.
Driven Force and Orientation
The driven force for SMA-MSPEO migration in PEU bulk originates from the
spontaneous micro-phase separation between the SMA-MSPEO components
that consist of PEG spacers with stearic endgroups and the major phase of
PEU matrix that is constructed by PTMG soft blocks. This micro-phase sep-
aration is caused by their unfavorable mutual compatibility. In air, the SMA
migration is oriented by the lowest-surface-energy tendency of the stearic
endgroups. The air usually behaves as a non-polar hydrophobic phase, hence
it has good affinity with non-polar fatty alkyls. The surface occupation of
stearic groups helps to minimize the surface energy at the material-air inter-
face.Inwater,theelastomericPEUmaterialisslightlyswollenbytheinvasion
of water molecules especially in the surface-adjacent area, by which a reduc-
ing gradient of water content is formed from the superficial surface to the
deep bulk. Given the extremely hydrophilic property of the SMA-PEG spacers,
a migration of SMA-MSPEO automatically occurs along with the increasing
water-content gradient from the bulk to the PEU-water interface until satu-
ration of the spatial containing capacity is reached in the surface layer. On
the water interface, the surface enrichment of hydrophilic PEG-containing
SMA also reduces the surface energy, which provides its thermodynamic
spontaneity. As mentioned previously in Sect. 2.1.2, the phenomenon of the
automaticSMA-surface-reoccupationwhenlossoccursisalsoenabledbythe
same mechanism [144-148].
As discussed above, thermodynamic factors dominate practical SMA be-
havior, while some kinetic factors also influence the final outcome. For ex-
ample, kinetic factors such as the molecular size-related steric hindrance and
polymeric chain twining are capable of assisting SMA immobilization but
retarding SMA migration. Because of the H-bond grafting, whenever SMA
