Gelation through phase transformation in synthetic and natural polymers - Physical Gels from Biological and Synthetic Polymers - page 250

(c)

(b)

(a)

10 20 40

2 θ (deg)

30 50 60

X-ray powder diffraction profiles of PVA gel after N c = 9 cycles, in as-formed (a), dried (b) and
rehydrated (c) states. The X-ray powder diffraction profile of solvent (liquid D 2 O) (dashed curves)
and of amorphous PVA (dash-dotted curves) are also shown. Adapted with permission from
Ricciardi et al.( 2004 ) © 2004 American Chemical Society.

Figure 8.20

degree of crystallinity and the size of the crystals increased with the number of freeze/
thaw cycles and the ageing time (in agreement with previous investigations). For
instance, the apparent size of the crystallites increased by 3

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4 nm as the number of
cycles went from N c = 1 to 10. WAXS patterns of melt-crystallized PVA films were also

reported, and exhibited sharp crystalline re ections, typical of crystalline PVA with a

degree of crystallinity around 64% (a surprisingly high value for a polymer which lacks
stereo-regularity, but well established in the case of atactic PVA). Xerogels also exhibit a
high degree of crystallinity (up to 50% after N c = 9 cycles).
Comparison of the diffraction patters for as-formed, dried and rehydrated gels is
shown in Figure 8.20 . Whereas in the hydrogels the main contribution to the diffraction
pattern arises from the free water molecules ( Figures 8.20a and c ), in the dry state
( Figure 8.20b ) for rehydrated gels, sharp diffraction peaks are clearly observed. This

culties in studying hydrated PVA gels, or in general any
network of physical gels, compared to the completely amorphous (sol) or dry state.
In a more recent publication the same authors (Ricciardi et al., 2005 ) propose a more
detailed scheme for the organization of phases inside the polymer-rich domains. They
provide detailed information in the medium-range (mesoscopic) length scales by using
small-angle neutron scattering (SANS), but this does not alter previous conclusions.
They suggest that during the

figure well illustrates the dif

first freeze/thaw cycle the homogeneous solution undergoes
phase separation, leading to the formation of a polymer-rich and a polymer-poor phase,
accompanied by crystallization of PVA in the polymer-rich phase. The polymer-rich
phase was modelled as a collection of polydisperse and homogeneous spherical particles,
interacting via hard-sphere potentials. The SANS intensity of the hydrogel is dominated
by particle

particle interference. The volume fraction of hard spheres (particles) roughly
corresponds to the volume fraction of crystallites in the gel (from X-ray diffraction). The

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