Chemistry Reference
In-Depth Information
Figure 2.
Qualitative comparison of the crystallinity determined by XRD for copolymers with different percentages of
ethylene.
are smaller because the increase of irregu-
larities in the macromolecular chains.
From DSC results, values for the enthal-
py of this second melting of the copolymers
are obtained. It is possible to determine the
percentages of crystallinity by relating
such values to those for the corresponding
homopolymers (291.6 J/g for polyethy-
lene
[26]
and 207.1 J/g for polypropylene
[27]
).
The calculated values are listed in Table 2
and corroborate how the copolymers with
close to 50/50 ethylene/propylene ratio are
amorphous materials, in agreement with
the characterization by XRD.
From the experimental triad values for
the copolymers determined by
13
C-NMR
,
it
is possible to calculate the parameters n
E
and n
P
, which represent the relative
average number of ethylene and propylene
units in the copolymer chain, respectively,
and they give information about the distri-
bution of comonomers along the
co
polym
e
r
chain. The values calculated for n
E
and n
P
are also listed in Table 2 and they can be
related directly to the crystallinity obtained
by DSC analysis. It can be concluded that,
in the heterogeneous copolymers in the
conditions of the reactions, when a chain
contains 5 units of the same consecutive
monomer, it crystallizes.
Another set of experimental properties
related to crystallinity is the elution tem-
perature (T
e
) and the percentage of soluble
fraction (SF%) determined by TREF.
