320
13.8 Inherent Reactivity of the Siloxane Bond
Finally, this chapter will be closed with an examination of all siloxane polymers as
inherently reactive materials. It had been generally thought for many years that
siloxanes are ‘unreactive’ materials. In fact, although this view is slowly fading,
there is still a lot of literature that supports this idea, due to the high thermodynamic
stability of the siloxane bonds. However, we have previously shown that siloxane
bonds are inherently reactive to acids and bases. In retrospect, this should have been
an obvious feature of siloxanes, as they are prepared via acid/base-catalyzed ROPs.
Despite that many authors consider this reaction as a ‘weak point’ of siloxane
polymers, under appropriate circumstances, siloxane bond reactions can be
extremely beneficial. One of the first examples of using this reactivity was the
chemical modification of inorganic oxide surfaces (Krumpfer and McCarthy 2011).
This was achieved by Krumpfer and McCarthy (2010) by a simple procedure: wetting an inorganic oxide surface with silicone oil at 100 °C for 24 hours. Astonishingly,
the hydrophobic surfaces that were prepared from this simple procedure exhibited
insignificant contact angle hysteresis (θ A /θ R = 104°/103°), i.e. there was no kinetic
barrier to sliding (Gao and McCarthy 2006b). The surprise to this finding was that
most surfaces exhibit some contact angle hysteresis, and previous attempts to prepare such surfaces involved complicated procedures and low reproducibility.
The lack of a contact angle hysteresis is attributed to a ‘liquid-like’ monolayer
which is formed from grafted siloxane chains (Gao and McCarthy 2006b). Despite
being covalently attached to the interface, the high degree of flexibility of the siloxane chain creates a dynamic interface which prevents liquids from being fixed to the
surface. In fact, since this procedure, other methodologies for preparing similar
surfaces have been reported (Hozumi et al. 2011; Hastings et al. 2020). In each case,
low hysteresis surfaces are created, which suggests that this is an innate characteristic of siloxanes which had not been exploited so far.
The reaction of silicones on inorganic surfaces is attributed to the cleavage of the
siloxane bond at the interface (Fig. 13.16) (Krumpfer and McCarthy 2011). This is
probably due to hydrolysis via surface-bound water, since inorganic surfaces are
typically quite hydrated (they act as desiccants due their high surface energies) or
direct silanolysis from acid/base moieties on the inorganic substrate. This ultimately
results in the covalent attachment of these siloxane chains to the surface. We found
that this reaction is not specific to any one inorganic oxide, but can be applied to any
inorganic oxide (the initial study used silica, titania, alumina and nickel oxide to
provide a range of acidic-to-basic substrates). However, in each case, the reaction
was found to be thermally activated, requiring temperatures above ambient conditions to occur. The hydrophobic coatings were also found to be quite robust, with no
change in characteristics, even when immersed in a good solvent. In addition, the
use of increasing Mw siloxanes resulted in thicker coatings on the surface. Finally,
by using functional siloxanes, such as poly(3- aminopropylsiloxane) copolymers,
reactive groups can be added to any inorganic oxide surface from commercially
available polymers.
K. M. Ryan et al.
13.8 Inherent Reactivity of the Siloxane Bond
Finally, this chapter will be closed with an examination of all siloxane polymers as
inherently reactive materials. It had been generally thought for many years that
siloxanes are ‘unreactive’ materials. In fact, although this view is slowly fading,
there is still a lot of literature that supports this idea, due to the high thermodynamic
stability of the siloxane bonds. However, we have previously shown that siloxane
bonds are inherently reactive to acids and bases. In retrospect, this should have been
an obvious feature of siloxanes, as they are prepared via acid/base-catalyzed ROPs.
Despite that many authors consider this reaction as a ‘weak point’ of siloxane
polymers, under appropriate circumstances, siloxane bond reactions can be
extremely beneficial. One of the first examples of using this reactivity was the
chemical modification of inorganic oxide surfaces (Krumpfer and McCarthy 2011).
This was achieved by Krumpfer and McCarthy (2010) by a simple procedure: wetting an inorganic oxide surface with silicone oil at 100 °C for 24 hours. Astonishingly,
the hydrophobic surfaces that were prepared from this simple procedure exhibited
insignificant contact angle hysteresis (θ A /θ R = 104°/103°), i.e. there was no kinetic
barrier to sliding (Gao and McCarthy 2006b). The surprise to this finding was that
most surfaces exhibit some contact angle hysteresis, and previous attempts to prepare such surfaces involved complicated procedures and low reproducibility.
The lack of a contact angle hysteresis is attributed to a ‘liquid-like’ monolayer
which is formed from grafted siloxane chains (Gao and McCarthy 2006b). Despite
being covalently attached to the interface, the high degree of flexibility of the siloxane chain creates a dynamic interface which prevents liquids from being fixed to the
surface. In fact, since this procedure, other methodologies for preparing similar
surfaces have been reported (Hozumi et al. 2011; Hastings et al. 2020). In each case,
low hysteresis surfaces are created, which suggests that this is an innate characteristic of siloxanes which had not been exploited so far.
The reaction of silicones on inorganic surfaces is attributed to the cleavage of the
siloxane bond at the interface (Fig. 13.16) (Krumpfer and McCarthy 2011). This is
probably due to hydrolysis via surface-bound water, since inorganic surfaces are
typically quite hydrated (they act as desiccants due their high surface energies) or
direct silanolysis from acid/base moieties on the inorganic substrate. This ultimately
results in the covalent attachment of these siloxane chains to the surface. We found
that this reaction is not specific to any one inorganic oxide, but can be applied to any
inorganic oxide (the initial study used silica, titania, alumina and nickel oxide to
provide a range of acidic-to-basic substrates). However, in each case, the reaction
was found to be thermally activated, requiring temperatures above ambient conditions to occur. The hydrophobic coatings were also found to be quite robust, with no
change in characteristics, even when immersed in a good solvent. In addition, the
use of increasing Mw siloxanes resulted in thicker coatings on the surface. Finally,
by using functional siloxanes, such as poly(3- aminopropylsiloxane) copolymers,
reactive groups can be added to any inorganic oxide surface from commercially
available polymers.
K. M. Ryan et al.