317
PDMS could not be modified using PVOH, based on their results. However, Yan
et al. (2018) realized that the morphology of PVOH on silicone substrates depends
largely on the coating conditions and the nature of the silicone used, and also noted
that the PDMS layer thickness increases, the resulting PVOH layer becomes unstable
and noncontinuous coatings are formed. They were also able to increase the PVOH
layer stability by a light oxygen plasma treatment of the silicone before the coating
procedures. This is potentially due to the covalent bonding of PVOH with the resulting silanol species present after reacting with oxygen, although this is never explicitly determined. The resulting films did not show the typical ‘hydrophobic recovery’
associated with silicones, and therefore, were effectively hydrophilic.
A final approach that has had some promising initial applications is the use of
silicone macroinitiators. One of the previously reported attempts to make a siloxanebased macroinitiator was carried out by Baysal et al. (1996) through end-chain
functionalization using a functional urethane-based initiating group. This was later
expanded by Taskiran (2006) for the copolymerization of polystyrene particles.
Taskiran (2006) found that the PDMS macroinitiator was preferentially directed to
the particle interface, providing an in-situ coating method of the particles. More
recently, there is some work on the preparation of siloxane-based atom transfer radical polymerization (ATRP) initiators for controlled polymerization techniques.
Semsarzadeh and Amiri (2013) reported the development of an ATRP silicone-macroinitiator for the copolymerization of styrene and vinyl acetate to make pentablock
copolymers. Although the concept of silicone macroinitiators is promising, more
work is still needed in this field.
13.7 Silicone Surfactants
One of the most important applications of siloxane polymers is as surfactants
(Yilgör and Yilgör 2014). The most common siloxane surfactants are by far PDMSpolyether surfactants, such as the PDMS-b-PEG copolymer shown in Fig. 13.7.
Given the biocompatibility and non-toxicity of both polymers, it is not surprising
that they are used in a wide range of applications. Another extremely important
class of surfactants are oligo(ethylene oxide)trisiloxanes (Wagner et al.
1999a; Wagner et al. 1999b; Kanner et al. 1967) (Fig. 13.15), which are used as
‘superspreaders’ (Sankaran et al. 2019), and which reduce the surface tension of
water to values between 20–25 mN/m (as compared to organic based surfactants,
which reduce the surface tension of water to 30–45 mN/m) (Kendrick et al. 1967).
These surfactants show extremely fast spreading dynamics across many substrates.
For this reason, these surfactants are commonly used in the spraying of pesticides in
agriculture, as they disperse pesticides quickly and evenly without damaging the
environment.
Given the importance of oligosiloxane surfactants, many works have been carried out to control and improve their surface activities. Recently, investigations on
gemini surfactants and their performances have been undertaken. Lin et al. (2017)
13 Synthetic Methods and Applications of Functional and Reactive Silicone Polymers
PDMS could not be modified using PVOH, based on their results. However, Yan
et al. (2018) realized that the morphology of PVOH on silicone substrates depends
largely on the coating conditions and the nature of the silicone used, and also noted
that the PDMS layer thickness increases, the resulting PVOH layer becomes unstable
and noncontinuous coatings are formed. They were also able to increase the PVOH
layer stability by a light oxygen plasma treatment of the silicone before the coating
procedures. This is potentially due to the covalent bonding of PVOH with the resulting silanol species present after reacting with oxygen, although this is never explicitly determined. The resulting films did not show the typical ‘hydrophobic recovery’
associated with silicones, and therefore, were effectively hydrophilic.
A final approach that has had some promising initial applications is the use of
silicone macroinitiators. One of the previously reported attempts to make a siloxanebased macroinitiator was carried out by Baysal et al. (1996) through end-chain
functionalization using a functional urethane-based initiating group. This was later
expanded by Taskiran (2006) for the copolymerization of polystyrene particles.
Taskiran (2006) found that the PDMS macroinitiator was preferentially directed to
the particle interface, providing an in-situ coating method of the particles. More
recently, there is some work on the preparation of siloxane-based atom transfer radical polymerization (ATRP) initiators for controlled polymerization techniques.
Semsarzadeh and Amiri (2013) reported the development of an ATRP silicone-macroinitiator for the copolymerization of styrene and vinyl acetate to make pentablock
copolymers. Although the concept of silicone macroinitiators is promising, more
work is still needed in this field.
13.7 Silicone Surfactants
One of the most important applications of siloxane polymers is as surfactants
(Yilgör and Yilgör 2014). The most common siloxane surfactants are by far PDMSpolyether surfactants, such as the PDMS-b-PEG copolymer shown in Fig. 13.7.
Given the biocompatibility and non-toxicity of both polymers, it is not surprising
that they are used in a wide range of applications. Another extremely important
class of surfactants are oligo(ethylene oxide)trisiloxanes (Wagner et al.
1999a; Wagner et al. 1999b; Kanner et al. 1967) (Fig. 13.15), which are used as
‘superspreaders’ (Sankaran et al. 2019), and which reduce the surface tension of
water to values between 20–25 mN/m (as compared to organic based surfactants,
which reduce the surface tension of water to 30–45 mN/m) (Kendrick et al. 1967).
These surfactants show extremely fast spreading dynamics across many substrates.
For this reason, these surfactants are commonly used in the spraying of pesticides in
agriculture, as they disperse pesticides quickly and evenly without damaging the
environment.
Given the importance of oligosiloxane surfactants, many works have been carried out to control and improve their surface activities. Recently, investigations on
gemini surfactants and their performances have been undertaken. Lin et al. (2017)
13 Synthetic Methods and Applications of Functional and Reactive Silicone Polymers