4
1 Introduction to PVA-Based Bionanocomposite Films
1.2 Biopolymers
Biodegradable polymers or biopolymers are defined as those polymeric materials
that can degrade in a bioactive environment via the enzymatic action of microorganisms such as bacteria, fungi and algae. Moreover, some polymeric chains may
also undergo a scission-down by nonenzymatic processes like chemical hydrolysis. Among different types of biopolymers, polylactide or polylactic acid (PLA)
is regarded as one of the most widely produced bioplastics [46]. It is a linear thermoplastic polymer mainly derived from renewable resources such as corns or sugar
beets [46]. Fully biodegradable and non-toxic PLA is produced from lactic acid that
possesses a chiral molecule in the dextrorotatory form as l-(+)-lactic acid or else in
the levorotatory form as d-(-)-lactic acid. The insufficient properties of some PLA
polymers in relation to thermal stability, impact resistance and flexibility can be
improved by their blending with other polymers like poly(ethylene oxide) (PEO)
or by using a plasticiser such as oligomer lactic acid (OLA), citrate ester or lowmolecular-weight polyethylene glycol (PEG) [8, 47]. Moreover, it is worth noting
that the processing temperature of PLA should exceed its thermal degradation temperature taking place at 200 °C. However, the microbial degradation of PLA seems still
challenging because PLA requires the temperature level above its glass transition
temperature (T g ) in range of 55–62 °C for the onset of PLA hydrolysis, which makes
its degradation difficult at ambient temperatures. Additionally, the degradation rate
of PLA depends on its molecular weight. In general, high-molecular-weight PLA is
mechanically stronger but less susceptible to biodegradation [48].
Polyhydroxyalkanoates (PHA) as another biopolymer belongs to a family of naturally occurring hydrophobic, biocompatible and biodegradable polyesters [49, 50].
It is available in a wide variety of forms, and used for carbon or energy storage
in microorganism in form of light refracting granules inside the cells. According
to their chain length, PHA can be divided into three major types, namely short
chain length (scI PHA with a carbon-number range of C3–C5), medium chain length
(msI PHA with the corresponding range of C6–C14) and long chain length (IcI
PHA with carbon number greater than C14) [51]. Polyhydroxybutyrate (PHB) and
poly(hydroxybutyrate-cohydroxyvalerate) (PHBV) are the two most common PHAbased polymers with a wide range of microorganisms used to accumulate PHB.
However, the genetic manipulation is essential for the large-scaled production of
PHB, whereas alcaligenes eutrophus is the most common microorganism implemented in the biosynthesis of PHA [47, 49–51]. PHB is comparable to polypropylene (PP) due to several similar physical properties such as melting point, degree
of crystallinity and T g . Nevertheless, PHA exhibits higher stiffness, lower toughness, lower solvent resistance and higher natural resistance to ultraviolet radiation
when compared with PP [49]. The main disadvantage of PHA lies in its narrow
processing window since it starts to degrade under the condition of high shear and
high temperature.
1 Introduction to PVA-Based Bionanocomposite Films
1.2 Biopolymers
Biodegradable polymers or biopolymers are defined as those polymeric materials
that can degrade in a bioactive environment via the enzymatic action of microorganisms such as bacteria, fungi and algae. Moreover, some polymeric chains may
also undergo a scission-down by nonenzymatic processes like chemical hydrolysis. Among different types of biopolymers, polylactide or polylactic acid (PLA)
is regarded as one of the most widely produced bioplastics [46]. It is a linear thermoplastic polymer mainly derived from renewable resources such as corns or sugar
beets [46]. Fully biodegradable and non-toxic PLA is produced from lactic acid that
possesses a chiral molecule in the dextrorotatory form as l-(+)-lactic acid or else in
the levorotatory form as d-(-)-lactic acid. The insufficient properties of some PLA
polymers in relation to thermal stability, impact resistance and flexibility can be
improved by their blending with other polymers like poly(ethylene oxide) (PEO)
or by using a plasticiser such as oligomer lactic acid (OLA), citrate ester or lowmolecular-weight polyethylene glycol (PEG) [8, 47]. Moreover, it is worth noting
that the processing temperature of PLA should exceed its thermal degradation temperature taking place at 200 °C. However, the microbial degradation of PLA seems still
challenging because PLA requires the temperature level above its glass transition
temperature (T g ) in range of 55–62 °C for the onset of PLA hydrolysis, which makes
its degradation difficult at ambient temperatures. Additionally, the degradation rate
of PLA depends on its molecular weight. In general, high-molecular-weight PLA is
mechanically stronger but less susceptible to biodegradation [48].
Polyhydroxyalkanoates (PHA) as another biopolymer belongs to a family of naturally occurring hydrophobic, biocompatible and biodegradable polyesters [49, 50].
It is available in a wide variety of forms, and used for carbon or energy storage
in microorganism in form of light refracting granules inside the cells. According
to their chain length, PHA can be divided into three major types, namely short
chain length (scI PHA with a carbon-number range of C3–C5), medium chain length
(msI PHA with the corresponding range of C6–C14) and long chain length (IcI
PHA with carbon number greater than C14) [51]. Polyhydroxybutyrate (PHB) and
poly(hydroxybutyrate-cohydroxyvalerate) (PHBV) are the two most common PHAbased polymers with a wide range of microorganisms used to accumulate PHB.
However, the genetic manipulation is essential for the large-scaled production of
PHB, whereas alcaligenes eutrophus is the most common microorganism implemented in the biosynthesis of PHA [47, 49–51]. PHB is comparable to polypropylene (PP) due to several similar physical properties such as melting point, degree
of crystallinity and T g . Nevertheless, PHA exhibits higher stiffness, lower toughness, lower solvent resistance and higher natural resistance to ultraviolet radiation
when compared with PP [49]. The main disadvantage of PHA lies in its narrow
processing window since it starts to degrade under the condition of high shear and
high temperature.