1.3 Poly(Vinyl Alcohol) (PVA)
9
The molecular weight (MW) of PVA is also considered as an important factor in
controlling solubility. In case of full-hydrolysis PVA, the solubility decreases with
increasing the molecular weight, which is associated with the increase in PVA molecular weight leading to the higher possibility of intramolecular hydrogen bonding
between hydroxyl groups in side branches [68]. As such, these types of bonds
make PVA difficult to dissolve in water. For example, Chang et al. [65] noted that
full- hydrolysis PVA with MW = 22,000 dalton (Da) was higher than that of fullhydrolysis PVA with MW = 74,800 dalton (Da). However, partial-hydrolysis PVA
demonstrates different solubility behaviour. The solubility of PVA increases with
increasing its molecular weight, which is attributed to a small number of hydroxyl
groups in partial-hydrolysis PVA, and this is insufficient to build up the intramolecular
hydrogen bonding [68]. In addition to molecular-weight effect, electrolyte impact
on hydrogen bonding in a water-soluble system has also been investigated [69]. It
is believed that the use of electrolyte can disrupt hydrogen bonding between polymeric chains with an decrease in the viscosity of polymer solutions [69]. In case
of PVA, the additional NaCl can induce the disruption of inter- and intrachains for
hydrogen bonding. That is why it is essential to enhance both PVA solubility and
viscosity in water. Nevertheless, the further addition of NaCl leads to the disruption
of solute-solvent hydrogen bonding with decreasing the solution viscosity [70].
1.4 Nanofillers for Bionanocomposites
1.4.1 Layered Silicates
Natural or synthetic mineral-layered silicates have gained great popularity as effective reinforcements for nanocomposite systems. These types of fillers comprise the
stacks of layers separated by Van der Waals interactions with gallery spacing. Each
layer is typically 1 nm thick and 30 nm to several microns long [71, 72]. Depending
on clay sources, silicate types and synthesis techniques, these layers can be bound
together with counter ions. Each layer comprises a tetrahedral sheet containing a
silicon atom surrounded by four oxygen atoms and an octahedral sheet where metals
such as aluminium or magnesium hydroxide are surrounded by eight oxygen atoms.
For instance, kaolinite 1:1 layered structure consists of a silicon tetrahedral sheet
and aluminium octahedral sheet that share a common plane of oxygen atoms [71].
In comparison to this, the crystal structure for 2:1 phyllosilicates has two tetrahedral
sheets with a central octahedral sheet of alumina (Fig. 1.4) [73, 74]. Pyrophyllite
is generated in a 2:1 crystal structure containing silicon in tetrahedral sheets and
aluminium in an octahedral sheet without any partial or complete replacement of
atoms. Pyrophyllite layers do not expand in water and possess only outer surface
areas. When the substitution takes place between the silicon in tetrahedral sheets and
aluminium in octahedral sheets, the final structure is known as mica. Any negative
hole resulting from mineral substitution is counter balanced by potassium cations
9
The molecular weight (MW) of PVA is also considered as an important factor in
controlling solubility. In case of full-hydrolysis PVA, the solubility decreases with
increasing the molecular weight, which is associated with the increase in PVA molecular weight leading to the higher possibility of intramolecular hydrogen bonding
between hydroxyl groups in side branches [68]. As such, these types of bonds
make PVA difficult to dissolve in water. For example, Chang et al. [65] noted that
full- hydrolysis PVA with MW = 22,000 dalton (Da) was higher than that of fullhydrolysis PVA with MW = 74,800 dalton (Da). However, partial-hydrolysis PVA
demonstrates different solubility behaviour. The solubility of PVA increases with
increasing its molecular weight, which is attributed to a small number of hydroxyl
groups in partial-hydrolysis PVA, and this is insufficient to build up the intramolecular
hydrogen bonding [68]. In addition to molecular-weight effect, electrolyte impact
on hydrogen bonding in a water-soluble system has also been investigated [69]. It
is believed that the use of electrolyte can disrupt hydrogen bonding between polymeric chains with an decrease in the viscosity of polymer solutions [69]. In case
of PVA, the additional NaCl can induce the disruption of inter- and intrachains for
hydrogen bonding. That is why it is essential to enhance both PVA solubility and
viscosity in water. Nevertheless, the further addition of NaCl leads to the disruption
of solute-solvent hydrogen bonding with decreasing the solution viscosity [70].
1.4 Nanofillers for Bionanocomposites
1.4.1 Layered Silicates
Natural or synthetic mineral-layered silicates have gained great popularity as effective reinforcements for nanocomposite systems. These types of fillers comprise the
stacks of layers separated by Van der Waals interactions with gallery spacing. Each
layer is typically 1 nm thick and 30 nm to several microns long [71, 72]. Depending
on clay sources, silicate types and synthesis techniques, these layers can be bound
together with counter ions. Each layer comprises a tetrahedral sheet containing a
silicon atom surrounded by four oxygen atoms and an octahedral sheet where metals
such as aluminium or magnesium hydroxide are surrounded by eight oxygen atoms.
For instance, kaolinite 1:1 layered structure consists of a silicon tetrahedral sheet
and aluminium octahedral sheet that share a common plane of oxygen atoms [71].
In comparison to this, the crystal structure for 2:1 phyllosilicates has two tetrahedral
sheets with a central octahedral sheet of alumina (Fig. 1.4) [73, 74]. Pyrophyllite
is generated in a 2:1 crystal structure containing silicon in tetrahedral sheets and
aluminium in an octahedral sheet without any partial or complete replacement of
atoms. Pyrophyllite layers do not expand in water and possess only outer surface
areas. When the substitution takes place between the silicon in tetrahedral sheets and
aluminium in octahedral sheets, the final structure is known as mica. Any negative
hole resulting from mineral substitution is counter balanced by potassium cations