Chemistry Reference
In-Depth Information
in stress after the initial extension.
128-130
The same may be said for the
“Payne Effect,” in which increase in the shear strain of a filled elastomer
causes a drop in the elastic modulus.
113,
131,
132
The tensile properties of
PDMS
133
and the origins of the failure of silica-filled PDMS elastomers
have also been elucidated from first principles, specifically using Car-
Parrinello molecular dynamics.
134
In an unusual example of biomimicry,
PDMS was molded into arrays of microlens shells that snapped from one
curvature (concave) to another curvature (convex) in a way that is remi-
niscent of the trapping mechanism of the Venus flytrap plant.
135
In some
cases, PDMS thin films showed extreme hardening due to high compres-
sive strain and confined thickness.
136
An example of a property that is less understood in molecular terms is
the scratch resistance exhibited by polysiloxane elastomers.
137
7.2 UNIMODAL MODEL ELASTOMERS
7.2.1 General Approach
Until recently, there was little reliable, quantitative information on the
relationship of stress to structure, primarily because of the uncontrolled
manner in which elastomeric networks were generally prepared.
3,
96,
124,
126,
138
Segments close together in space were linked irrespective of their
locations along the chain trajectories, resulting in a random network
structure in which the number and locations of the cross links were un-
known. New synthetic techniques are now available, however, for the
preparation of “model” polymer networks of known structure. If networks
are formed by end linking functionally terminated chains then the re-
quired structural information is determined by the cross-linking reac-
tion.
96,
139,
140
The functionality of the cross links is the same as that of the
end-linking agent, and the molecular weight distribution between cross
links matches that of the chains prior to end linking.
An example is the reaction shown in figure 7.5, in which hydroxyl-ter-
minated chains of a polymer such as poly(dimethylsiloxane) (PDMS) are
end-linked using tetraethyl orthosilicate (alternatively called tetraeth-
oxysilane, TEOS). Networks have also been formed from PDMS chains
that have either amide or urea groups at both ends.
141,
142
In these studies,
hydrogen-bonded polymerizable end groups have been of particular inter-
est. It is also possible to make networks from PDMS chains having a
known number of potential cross-linking sites placed as side chains along
the polymer backbone, as long as their distribution is known as well.
143
