Biomedical Engineering Reference
In-Depth Information
2.1.2
SMA “H-Bond Grafting” for Stable Blend and Surface Modification
The general strategy for SMA H-bond grafting is explicated in Fig. 1. The
PEU hard blocks are transplanted into the SMA-MPEO structure serving
as coupling units (“C”-block in Fig. 1) connecting both PEG spacers (“B”-
block in Fig. 1). SMA-MPEO is mixed with PEU chains in a co-solution sys-
tem. The elastomeric MPEO-PEU blends are achieved by co-solution casting
and subsequent solvent evaporation. During the exclusion of solvent, PEU
hard blocks gradually crosslink together via H-bond “conjugation”, simultan-
eously microphase separation also occurs by the incorporation of H-bonded
hard blocks. Since the “C”-blocks of SMA-MPEO are exactly the copies of
PEU hard blocks, what happens to PEU hard blocks also equally happens to
the SMA “C”-blocks. Hence, the “C”-blocks, together with the overall com-
pound of MPEO, are physically grafted onto PEU hard block domains via
H-bond connection. That's how MDI-based MPEO “C”-blocks function as lo-
calizing anchors for SMA immobilization [76, 77].
The H-bond grafting mechanism is certified by infrared spectroscopic
evidence and also verified by blending stability evidence. A special MPEO
that is endowed with stearic (18-Carbon fatty alkyl) endgroups (“A”-blocks),
named “MSPEO”, was applied as the model SMA; accordingly, a mono-stearic
polyethylene glycol with equivalent PEG chain-length (molecular weight, Mw,
∼
2000 Da), abbreviated “SPEO”, was employed as the control. The only dif-
ference between MSPEO and SPEO is that SPEO does not possess the MDI
anchors.
Infrared Spectroscopic Evidence
Among the regular means of chemical analysis, infrared [IR] spectroscopy
specially targets the functional/structural groups of the compounds of in-
terest as information units. Attenuated total reflection Fourier transform
infrared spectroscopy [ATR-FT-IR] has the talent of revealing molecular in-
formation in the surface layers of materials with an effective detecting depth
of 400 nm [140, 141]. Both regular transmission IR and ATR-FT-IR were em-
ployed to recruit the information respectively from material bulk and surface
layer. To investigate H-bond structures in PEU systems, the IR-targeting unit
is the ester-carbonyl groups localized on PEU hard blocks, which, as men-
tioned above in Sect. 2.1.1, are major H-bond “chromophores”. The IR ab-
sorbance band of non-H-bonded, or “free” carbonyl groups, namely “- CO -
(a)”, is centered at 1730 cm
-1
; while the band of H-bonded (with urethane
imine) carbonyl groups, namely “- CO - (b)”, is centered at 1700 cm
-1
[142,
143]. The quantification follows Lambert-Beer's Law. The ratio of IR ab-
sorbance, - CO - (b) to - CO - (a), reflects the relative quantity of H-bonded
structures to their non-H-bonded counterparts among all the urethane-
containing structures in PEU systems. As shown in Fig. 2, the IR absorbance
