Biomedical Engineering Reference
In-Depth Information
T
melt
=265-278
8
C; polyvinilidenefluoride (PVF) with T
melt
=171-180
8
C;
polyvinyl alcohol
(PVA) with T
g
=85
8
C; polyformaldehyde
(PFA),
polymethylmetacrylate (PMA) with T
g
=100-105
8
C; polystyrene (PS)
with T
g
=98-102
C. In addition, copolymers of styrene (ST) with methyl
metacrylate (MMA) (ST content 80, 60 and 40 mol/%), DR with average
molecular mass equal to 106 and 2.5 polydispersity were used. All of the
polymers used in this work were fine powders and, as an antioxidant,
Irganox
®
1010 and NG-2246 in 5% weight ratio versus the polymeric binder
were used.
8
9.3
Superconducting (SC) properties of polymer-
ceramic nanocomposites manufactured by hot
pressing
Polymer-ceramic composites with the various binders used exhibit no SC
properties immediately after hot pressing [20, 22, 24] at 200
C for 30min.
The Meissner effect, for example, is missing. The absence of SC properties
can be explained by the depletion of oxygen from the orthorhombic SC
phase of the ceramic Y
1
Ba
2
Cu
3
O
7
x
after pressing at 200
8
C. The released
oxygen interacts irreversibly with the polymeric binder, causing its thermo-
oxidative destruction. Investigations carried out on composites with
superhigh molecular polyethylene using a MOM 1500 derivatographic
instrument show that, at 160
8
C and higher temperatures, weight loss took
place, which indicates that macromolecules of the binder decompose under
thermo-oxidative conditions. Oxygen participates in the thermo-oxidative
destruction of the binder by desorbing and diffusing into the polymeric
phase from the nucleus of the ceramic grains. It seems that free oxygen
dislocated on the surface of the grains of the oxide ceramic reacts with the
polymeric phase. Indirect poof of this assumption comes from restoration of
the SC properties of the composites (see Table 9.1) under an atmosphere of
dry oxygen at the
8
transition temperatures of the polymeric binders. The
characteristic curve of the SC transition (Y
1
Ba
2
Cu
3
O
7
x
+superhigh
molecular polyethylene) obtained for the samples after restoration is
shown in Fig. 9.1. Further proof can be seen in the results of the
experiments carried out when Irganox
®
1010 (a polymeric antioxidant) was
added to the initial mixture. It is known that antioxidant additives in a
polymeric matrix substantially reduce the rate of oxidative destruction of
the polymers [29-31].
Consequently, it can be deduced that the introduction of minute
quantities of antioxidant (0.5m.Kt% of binder) in the polymer-ceramic
composite decreases the rate of the oxidation reactions. This then retards the
depletion of oxygen from the surface of the ceramic grains. Directly after
α
