Biomedical Engineering Reference
In-Depth Information
blend of poly(vinyl alcohol) (PVA) with cellulose and compared them to chemi-
cally crosslinked gels of the same composition [
91
]. The physical gels were pre-
pared by repeat freeze-thaw cycles, whereas the chemical gels were prepared
by addition of epichlorohydrin to crosslink their hydroxy groups. The hydrogels
were characterized with a view to their morphology, structure, and properties.
With the help of differential scanning calorimetry (DSC), Zhang and colleagues
could show that the melting point and the heat of melting of the physical gels
both increase with their PVA content, as shown in Fig.
3
a; this result indicates
enhancement of the thermal stability of the gels. The covalent hydrogels exhibit
an amorphous character, which is attributable to breakdown of the crystalline
structure of PVA due to chemical crosslinking; this can also be seen by the degree
of crystallinity in these gels, which is 10 % lower than in the physical hydrogels.
To get more insight into the structure of these hydrogels, wide angle X-ray dif-
fraction was measured. These experiments reveal that the physical hydrogels
partly crystallize in the crystal plane of cellulose II, whereas the chemical hydro-
gels do not exhibit any diffraction pattern, as shown in Fig.
3
b. These two results
suggest that the structure of the physical hydrogels exhibits a dense packing with
crystalline domains of cellulose, whereas the chemical gels exhibit an amorphous
structure with no residual structural characteristics of the precursor polymers.
Scanning electron micrographs show regular pore structures in the physical
hydrogels. The pore size and density can be tuned by the number of freeze-thaw
Fig. 3
Investigation of the melting and structure of gels based on PVA and cellulose by
a
dif-
ferential scanning calorimetry (
DSC
) and
b
wide angle X-ray scattering (
WAXD
).
a
DSC ther-
mograms of physical (
a
-
c
) and chemical (
d
-
f
) hydrogels. The PVA content decreases from
a
to
c
and from
d
to
f
, respectively.
b
WAXD patterns of cellulose-PVA hydrogels: (
a
) PVA; (
b
-
d
)
physical hydrogels; (
e
-
g
) chemical hydrogels; (
h
) cellulose. The PVA content increases from
b
to
d
and from
e
to
g
, respectively. Reprinted from Zhang et al. [
91
]. Copyright 2008 Wiley VCH
Search WWH ::
Custom Search