Biomedical Engineering Reference
In-Depth Information
2
Techniques for the characterization
of physical gels
2.1
Introduction
Most of the techniques for physical gel characterization are well known in the physico-
chemistry of polymer networks, in colloid science or in both, and many standard texts
describe these in more detail than we can hope to include. In this chapter, we have
decided to adopt a somewhat eclectic choice, biased in some cases by the personal
interests of the authors.
Techniques (van Holde et al., 2006 ) such as nuclear magnetic resonance (NMR),
particularly including broad-line NMR,
'
'
absorption spectroscopy and chirop-
tical (dichroic) methods (the latter of which have proved invaluable in the study of, for
example, marine and microbial polymers) and mass spectrometry are not covered in this
chapter, although reference to some of them appears in a number of places, in later,
system-speci
classical
c chapters. Instead, the techniques we concentrate upon include scattering,
ultramicroscopy (cryomicroscopy and atomic force microscopy), differential scanning
calorimetry (DSC), rheology and simulations. The order in which these are discussed
follows, at least roughly, the so-called distance scale approach (Clark and Ross-Murphy,
1987 ), although most of the techniques are macromolecular or supramolecular (in
modern parlance, nano- or micro-scale) in nature.
As a
final remark, it is crucial to understand and appreciate that in physical gel systems
several parallel investigative approaches are almost always needed. No single technique
-
for example, rheology or calorimetry
should be used in isolation because of the
convolution of factors. These include (a) the several distance scales appropriate for
determining the structure, (b) the in
-
uence that thermodynamic properties have on
mechanisms of aggregation and (c) the fact that functional properties are often based
on rheological properties. In general, appropriate molecular characteristics (such as
molecular mass or composition of gel precursors) should be performed before inves-
tigating macroscopic gel properties.
2.2
Scattering techniques
Scattering techniques based on X-ray, neutron and light sources all play an important role
in studying polymer gels and colloidal suspensions. Light scattering has been used for a
long time for characterizing polymers in solution, whereas small-angle X-ray and
 
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