Chemistry Reference
In-Depth Information
experimental cross-link density of the co-polymers (n
e
), namely the average
number of cross-links per unit volume, can be determined from the rubbery
modulus using the following equation:
23,24,28
E
0
¼
3n
e
RT
¼
3dRT
M
c
(4)
where E
0
, R, T, d and M
c
represent the storage modulus of the cross-linked
co-polymer in the rubbery region, the gas constant (8.314 J K
1
mol
1
), the
absolute temperature, the polymer density, and the effective molar mass be-
tween cross-links, respectively. The E
0
values used for the calculations were
taken at approximately 50 1CaboveT
g
. The density of the TOPERMA/styrene
resins ranged from 1.11 to 1.13 g cm
3
. As shown in Table 8.2, the n
e
values of
these bio-polymers were 1483-2145 mol m
3
, which correspond to M
c
values
of 522-755 g mol
1
.Then
e
values for 20 to 40 wt% styrene concentration were
higher than those of SOPERMA/styrene polymers (1050-1550 mol m
3
), but
lower than those of COPERMA/styrene polymers (1700-4500 mol m
3
).
24
A
possible reason for this is that the fatty acid chains in SOPERMA barely
participate in the free-radical co-polymerization with styrene, while the fatty
acid chains in COPERMA are functionalized with MA and can readily co-
polymerize with styrene. Also, the fatty acid chains in TOPERMA still have
some conjugated trienes remaining (although some of them are consumed by
the Diels-Alder reaction), which may participate in the co-polymerization.
The temperature dependence of the loss factor (tan d)isshownin
Figure 8.12(b), in which the tan d curves also exhibit very broad peaks. The
curves for TOPERMA80-ST20 and TOPERMA70-ST30 show two glass transi-
tions due to phase separation. This revealed two T
g
values at the tan d peaks:
one was for the TOPERMA-rich region at about 65 1C; the other was for the
styrene-rich region in the range of 107-144 1C. This two-glass-transition phe-
nomenon was similar to previous reported results.
1,5,6
As the styrene content
increased, the lower transition peak decreased in intensity relative to the
higher transition peak, which indicates a content decrease of the TOPERMA-
rich phase and a content increase of the styrene-rich phase. The possible effect
for this decrease can be attributed to the increase of styrene content producing
more effective solubilization for TOPERMA in styrene, which leads to the
formation of a less heterogeneous system. tan d is a sensitive indication of
cross-linking. As the cross-link density increases, the tan d maximum shifts to
higher temperatures, the peak broadens and the tan d values decrease.
24
As a
result, the TOPERMA67-ST33 polymer with a lower cross-link density had a
lower T
g
value than TOPERMA70-ST30 and TOPERMA60-ST40.
8.3.2.3 Effect of Styrene Concentration on Mechanical
Properties
To find the appropriate styrene concentration, it is necessary to investigate
the effect of styrene concentration on the mechanical properties of the
