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cured polymers with the lowest elongation-at-break but higher storage
modulus, thermal stability, and T
g
than their less epoxidized, lower oxirane
content counterparts. Due to its rich linolenic content, ELO possesses the
highest oxirane content among common EVOs. Thus, cured ELO thermoset
polymers generally have higher cross-link densities, T
g
values and moduli.
113
A series of epoxy resins with different structures, oxirane contents and
contents of saturated fatty acids was synthesized by Wang and Schuman
57
in
order to examine their structure-property relationships. Both anhydride-cured
ESO and ELO showed broader T
g
regions, indicating a broader distribution of
chain environments and more heterogeneous polymer structures due to the
lower reactivity of internal oxiranes and the presence of saturated fatty acids.
The T
g
values of either MHHPA or BF
3
-amine cationically cured thermoset
polymers were observed to increase fairly linearly with oxirane value
(Scheme 9.14). Linseed-oil-based epoxies, such as ELO and EGL, had much
higher T
g
values compared to their respective ESO or EGS counterparts of
lower oxirane content. Removal of saturated components greatly increased the
T
g
. A 30 and 20 1CincreaseinpolymerT
g
was observed for MHHPA-cured EGS
and EGL compared to EGS-S and EGL-S, EGS and EGL monomer resin without
removal of saturated fatty acids, respectively. Such trends were also observed
in cross-link density measurements, as a significant increase of cross-link
density upon removal of saturated components. Due to the loss of glycerol as a
cross-linking site and liberated saturated fatty acid esters, the addition of an
unreactive functional group to the ester end, e.g., allyl (EAS) or methyl group
(EMS), generated even lower T
g
values and polymer cross-link densities,
though the oxirane values were similar to those of ESO. The results reiterated
the reported, very low T
g
value of an epoxidized bio-diesel polymer.
114
Scheme 9.14 Measured T
g
as a function of oxirane content where -P: partially
epoxidized and -S: monomer where the saturated fatty ester content
was not removed. NOTE: the vertical 'error bars' indicate the breadth
of glass transition and polymer heterogeneity.
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