Chemistry Reference
In-Depth Information
crystallization temperatures were observed for benzene, which presuma-
bly reflect the pore sizes present.
Work on the mechanical properties of trimodal elastomers is lim-
ited. The problem is the large number of variables involved, three mo-
lecular weights and two independent composition variables (mol
fractions). For this reason, early mechanical property experiments in-
volved arbitrarily chosen molecular weights and compositions.
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Not
surprisingly, only modest improvements in mechanical properties were
obtained over the bimodal materials. Recent results on both bimodal
and trimodal PDMS elastomers showed significant improvement over
unimodal PDMS elastomers with regard to some mechanical properties
such as toughness, but less pronounced increases in fracture energy.
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In summary, trimodal elastomers do show some improvements over the
bimodal ones.
There has been some NMR work on trimodal PDMS elastomers, with
regard to thermal degradation
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and the presence of network heterogene-
ities.
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The idea of employing trimodality has also been employed in some
triblock copolymers.
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Recent computational studies
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indicate that it is possible to identify
those molecular weights and compositions that should maximize im-
provement in mechanical properties. Such simulations are being extended
to search for optimum properties of trimodal networks, specifically (i) the
elastic modulus, (ii) maximum extensibility, (iii) tensile strength, and (iv)
segmental orientability. Results
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suggest that a trimodal network pre-
pared by incorporating small numbers of very long chains into a bimodal
network of long and short chains could significantly improve ultimate
properties.
The interpretation of the attractive mechanical properties of bimodal
networks has been in terms of the “delegation of responsibilities,” with
the short chains serving in one role and the long chains in another. If
this is picture is true, then it would be interesting to study networks
having extraordinarily broad molecular weight distributions, in that
there would be network chains of all conceivable lengths, available for
any possible mechanism that would improve properties.
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Polymer pre-
pared from a single polymerization would not have a broad enough dis-
tribution, but the combination of a series of samples of gradually
differing average molecular weights could yield the desired distribution
(figure 7.27). An elastomer of this type might have unusually attractive
mechanical properties.
Trimodal distributions have also been studied in nonpolymeric sys-
tems (e.g., in aluminum metal-matrix composites).
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