Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 37

Vitreous block Surface scraping Deep etching

Filaments

(a) (b) (c)

Amorphous ice

Pt-C evaporation Pt-C replica

(d) (e)

Figure 2.12

(a) A gel sample with vitrified ice; (b) scraping of the surface; (c) surface after deep etching
(c.0.5 μm), exposing the network; (d) rotary shadowing with heavy metals (Pt, Au, W), followed by
carbon evaporation under normal incidence; (e) observation of the gel replica by EM, after
chemical dissolution of the underlying organic material.

display the structure of the network, a knife is used to scrape the surface that was in
contact with the copper block, first at a small angle (see below), then at various depths

below the free surface. After scraping, the water is allowed to sublimate at

− 90°C. This

stage, called , is followed by platinum evaporation (shadowing under

small-angle incidence) to create the relief contrast of the structure, and then by carbon
evaporation normal to the surface, to create a continuous

' deep etching '

x the structure. The
underlying organic material is then removed by chemical dissolution. This is the method
used to prepare a

film and

' replica ' of the network. Figure 2.12 summarizes the various prepara-

tion steps.
The scraping of the gel surface prior to replica preparation is shown schematically in
Figure 2.13 . It is seen that additional labelling was used to mark the surface of the gel, in
order to reveal the part which was frozen at the highest rate.
In Figure 2.14 , the progressive distortion of the gel network with distance from the
surface can be observed. The scale is the same (0.5 μm) in all panels but the distance from
the surface varies from 0

28 μm from top to bottom. The development of ice
crystals of more than 1 μm in size can be seen in the bottom image. Only the upper part of
the

- 4μmto24 -

figure represents the exact structure of the gel network.

2.4.2 Atomic force microscopy (AFM)

Atomic force microscopy has rapidly emerged as a versatile imaging tool for investiga-
tions in various areas of chemistry, biology and physics (Eaton and West, 2010 ), and
it has been used by a number of workers, particularly on pre-gel systems, and in