236
energy barriers for the rotation around Si-O axis (2.5 kJ mol
-1
in dimethylsiloxane)
and the linearization of Si-O-Si angle (1.3 kJ mol
-1
) explain the unusual flexibility
of the polysiloxane chain. The Si-O-Si angle (140–180°) is much wider than the
tetrahedral angle, while the siloxane bond is one of the most stable bonds formed by
silicon, being estimated in the range 422–494 kJ mol
-1
(Voronkov et al. 1978;
Chojnowski and Cypryk 2000). The extreme flexibility of the siloxane backbone is
reflected in the low glass transition temperature (T g ) value which is around -125 °C
in poly(dimethylsiloxane) (PDMS), the best-known representative of this class of
materials. By modifying the organic groups attached to the silicon atoms or by
replacing them with bulkier or more polar ones, the T g of polysiloxanes can be
increased as follows: C 3 H 7 -120 °C, C 3 H 4 F 3 -70 °C, C 6 H 5 -90…-75 °C. Poly(diphenylsiloxane)s (PDPhS) show T g between 50–100 °C depending
on the molecular mass (Cazacu 2008). In PDMS, the weak intermolecular interactions and the methyl groups oriented toward the air interface lead to a significantly
lower surface tension than in organic polymers (Özçam et al. 2014) and a marked
hydrophobic character. In addition, silicones have excellent release properties and
dielectric strength, good resistance to UV radiation and performance at extreme
(high/low) temperatures, high thermal stability and oxygen permeability, minimum
temperature effect on the properties and physiological inertness (Kuo 1999;
Cazacu 2008).
The chemical versatility of the siloxane precursors allows modifications with
functional and reactive groups, thus extending the area of properties and applications. Therefore, the chemistry of organic silicon compounds is one of the fastest
growing fields in synthetic organic chemistry. The development of new methodologies continues to be actively investigated (Panek 1991). The differently substituted
polysiloxanes on the silicon atoms can be obtained by direct synthesis from suitable
monomers (co-condensation or copolymerization) or by subsequent chemical modification (post-functionalization).
Polysiloxanes containing Si-vinyl and Si-H groups constitute valuable substrates
for functionalization, allowing a wide range of organic groups or functions such as
alcohols, alkyls, amines, haloalkyls, thiols, etc. to be linked by hydrosilylation,
which is the most accepted approach towards functional silicones, while the thiolene addition has been studied extensively to introduce thioether functions (Shankar
et al. 2013). In fact, almost any other synthetic approach from organic chemistry can
be easily adapted to modify the organic group attached to the silicon atom resulting
in new compounds.
End-functionalized polysiloxanes are very useful in the preparation of block and
segmented copolymers by joining PDMS as a flexible moiety with organic polymeric sequences such as polyamides, polycarbonates, polyimides, polysulphones,
polyureas, polyurethanes (PUs), etc. in various combinations and sequences within
the backbone, which can induce a wide range of properties (Cazacu 2008). These
systems generally exhibit biphasic morphology, due to the pronounced incompatibility between polysiloxanes and most organic polymers (Yilgör and McGrath
1988; Mark 2000; Racles et al. 2004; Bronnikov et al. 2005; Zha et al. 2016).
Elastomeric networks, functional telechelic oligomers, hyperbranched compounds,
C. Racles et al.
energy barriers for the rotation around Si-O axis (2.5 kJ mol
-1
in dimethylsiloxane)
and the linearization of Si-O-Si angle (1.3 kJ mol
-1
) explain the unusual flexibility
of the polysiloxane chain. The Si-O-Si angle (140–180°) is much wider than the
tetrahedral angle, while the siloxane bond is one of the most stable bonds formed by
silicon, being estimated in the range 422–494 kJ mol
-1
(Voronkov et al. 1978;
Chojnowski and Cypryk 2000). The extreme flexibility of the siloxane backbone is
reflected in the low glass transition temperature (T g ) value which is around -125 °C
in poly(dimethylsiloxane) (PDMS), the best-known representative of this class of
materials. By modifying the organic groups attached to the silicon atoms or by
replacing them with bulkier or more polar ones, the T g of polysiloxanes can be
increased as follows: C 3 H 7 -120 °C, C 3 H 4 F 3 -70 °C, C 6 H 5 -90…-75 °C. Poly(diphenylsiloxane)s (PDPhS) show T g between 50–100 °C depending
on the molecular mass (Cazacu 2008). In PDMS, the weak intermolecular interactions and the methyl groups oriented toward the air interface lead to a significantly
lower surface tension than in organic polymers (Özçam et al. 2014) and a marked
hydrophobic character. In addition, silicones have excellent release properties and
dielectric strength, good resistance to UV radiation and performance at extreme
(high/low) temperatures, high thermal stability and oxygen permeability, minimum
temperature effect on the properties and physiological inertness (Kuo 1999;
Cazacu 2008).
The chemical versatility of the siloxane precursors allows modifications with
functional and reactive groups, thus extending the area of properties and applications. Therefore, the chemistry of organic silicon compounds is one of the fastest
growing fields in synthetic organic chemistry. The development of new methodologies continues to be actively investigated (Panek 1991). The differently substituted
polysiloxanes on the silicon atoms can be obtained by direct synthesis from suitable
monomers (co-condensation or copolymerization) or by subsequent chemical modification (post-functionalization).
Polysiloxanes containing Si-vinyl and Si-H groups constitute valuable substrates
for functionalization, allowing a wide range of organic groups or functions such as
alcohols, alkyls, amines, haloalkyls, thiols, etc. to be linked by hydrosilylation,
which is the most accepted approach towards functional silicones, while the thiolene addition has been studied extensively to introduce thioether functions (Shankar
et al. 2013). In fact, almost any other synthetic approach from organic chemistry can
be easily adapted to modify the organic group attached to the silicon atom resulting
in new compounds.
End-functionalized polysiloxanes are very useful in the preparation of block and
segmented copolymers by joining PDMS as a flexible moiety with organic polymeric sequences such as polyamides, polycarbonates, polyimides, polysulphones,
polyureas, polyurethanes (PUs), etc. in various combinations and sequences within
the backbone, which can induce a wide range of properties (Cazacu 2008). These
systems generally exhibit biphasic morphology, due to the pronounced incompatibility between polysiloxanes and most organic polymers (Yilgör and McGrath
1988; Mark 2000; Racles et al. 2004; Bronnikov et al. 2005; Zha et al. 2016).
Elastomeric networks, functional telechelic oligomers, hyperbranched compounds,
C. Racles et al.