Chemistry Reference
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sented by the following reaction [17]:
M
+
X
-
(solid)
+ C
(solid)
( X = F or Cl)
M
(gas)
+ X-C
(solid)
→
Scheme 1. Proposed reaction scheme for reactive metallization.
Consequently, as long as the fate of the carbon containing fragment (C
(solid)
) is the
same for a common metal thermally evaporated onto different halogen-containing
polymers (e.g. PVC and PTFE), it should be possible to rationalize differences in
metal-polymer reactivity from the thermodynamics associated with the bond breaking
(C-F or C-Cl) and forming (M-F or M-Cl) reactions that accompany reactive metalli-
zation. Scheme 1 illustrates that if we assume that the bond breaking and forming re-
actions are constant within a particular polymer, the heat of reaction associated with
reactive metallization will depend on the difference between the bond dissociation
energy (BDE) of M-X and C-X (
∆
H
BDE
= BDE(M-X)-BDE(C-X)). Table 1 shows
that
H
BDE
does indeed provide a useful diagnostic for a metal's overall reactivity
with PTFE and PVC, with a high degree of reactivity favored when
∆
∆
H
BDE
is large
and positive. Conversely, in less reactive metal/polymer systems,
H
BDE
is small or
negative. Table 1 also illustrates that for a common polymer, the reactivity trend
within a series of metals scales with the magnitude of BDE(M-X). For example, Fig-
ure 1 illustrates that although both Fe and Au are reactive with PVC, the extent of re-
action is much more pronounced for Fe (BDE(Fe-Cl) = 400 kJ mol
-1
) compared to
Au (BDE (Au-Cl) = 289 kJ mol
-1
) (Figure 1).
The thermodynamic argument derived from Scheme 1 can also rationalize the
fact that in situations where different halides (e.g. FeCl
2
vs FeCl
3
) can be formed,
the halide with the highest BDE(M-Cl) value is produced. Similarly, Table 1
shows that upon moving between PVC and PTFE the change in BDE(C-X) (X =
Cl, F) is always greater than the corresponding change in BDE(M-X). This is re-
sponsible for the generally lower reactivity of metals during thermal evaporation
on PTFE compared to PVC; for example, the formation Au and Cu halides on
PVC only (Table 1).
In a more general sense, the success of
∆
H
BDE
in predicting reactivity trends
implies that the extent of reaction is sensitive to the heat released during reaction.
This effect could be a consequence of the large increase in surface temperature at
the polymer surface during evaporation anticipated in situations when
∆
H
BDE
is
large. An increased surface temperature is expected to correlate with greater mor-
phological changes at the polymer surface that are, in turn, expected to enhance
metal diffusion, leading to a greater extent of reaction. This is supported by the
fact that in related studies, morphological changes on PVC and PTFE as evi-
denced by AFM, were generally observed to be more pronounced during metalli-
zation with more reactive metals (Ti and Fe) compared to more noble metals (Ni,
Cu, and Au) [16, 19, 23].
∆
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