**2.4 General properties of PHA**

PHA properties are very indistinguishable from that of conventional plastics since it has great chemical diversity of radicals [17]. The ranges of these polymers vary from rigid and brittle thermoplastics to elastomers, rubbers, and adhesives which is totally based on their composition. Depending on the kinds of aromatic monomers used, aromatic PHAs exhibit a variety of properties. A lot of research has been done on the thermal characteristics of aromatic PHAs, which show behavior that is particular to the structure. Due to the increase in chain length and increase in the number of comonomers in a copolymer, its elasticity increases, and thus, PHAs have different properties according to their monomeric composition.

The physical properties of PHAs are as follows:


**Figure 2.**

*Biosynthesis pathway of PHB and P(3HB-co-3 HV) copolymer. Adopted from [30].*

*Cyanobacteria as a Source of Biodegradable Plastics DOI: http://dx.doi.org/10.5772/intechopen.110376*

Commercial suitability of molecular mass and molecular weight distribution of a polymer plays a vital role in characterization., and polymers with molecular mass less than 4 <sup>10</sup><sup>4</sup> Da have their mechanical properties deteriorated.

The molecular weight of the compounds differs from 2 <sup>10</sup><sup>5</sup> to 3 <sup>10</sup><sup>5</sup> Da which depends on the type of microbial species used and growth conditions like pH, cultivation modes, and type and concentration of the carbon source. The properties of the PHA depend on the size of the polymer chains, whose structural rearrangements may depend on the degree of polymerization [31].

In addition to defining some mechanical characteristics of a material at ambient temperature, a polymer's thermal properties, such as its melting and glass transition temperatures as well as crystallinity and crystallization time, also serve as useful factors for the thermal processing of materials [32]. PHAs have melting points between 50 and 180°C and crystallinities between 30 and 70%, depending on the polymer's composition. PHAs are categorized as stiff if their crystallinity is between 60 and 80%. Medium (30–40%) and short (30%) polymer lengths characterize flexible and more elastic PHAs, respectively [31]. PHA's industrial applications are expanded thanks to its reduced degree of crystallinity, which also enhances its processing properties [32].

Semicrystalline polymers, the most popular type of PHA, are more brittle and less solvent-resistant but have tensile qualities that are comparable to those of polypropylene and polyethylene. PHB and its copolymers, which are made by cyanobacteria, have physical characteristics that can be linked to those of synthetic polymers like polypropylene and high-density polyethylene [33]. The creation of polymers with the appropriate properties will be aided by a good understanding of the connections between the PHA crystallinity and the polymer composition.

PHA is a suitable substitute for synthetic polymers due to its natural origin, biodegradability, biocompatibility, piezoelectricity, optical purity, and thermoplasticity [34]. Additionally, they are thermoplastic and/or elastomeric, nontoxic, and have a very high purity inside the cell. They are also hydrophobic, insoluble in water, inert, and indefinitely stable in the air [35]. PHA is less solvent resistant than polypropylene but has a substantially higher resilience to ultraviolet (UV) radiation degradation [36].

Numerous microorganisms in distinct situations have the ability to break down PHAs. PHA breakdown generates carbon dioxide and water under aerobic settings, whereas it generates carbon dioxide and methane under anaerobic ones [37]. The degradation time depends on a number of variables, including surface area, microbial activity of the environment, pH, temperature, humidity, the presence of other nutrients, and the properties of the polymer, such as composition and crystallinity, and can range from months (anaerobic digestion) to years (marine environment), among others.

Due to their high density, PHAs do not float in aquatic settings; as a result, after being dumped there, they sink and are biogeochemically destroyed on the surface of the sediments [37]. The two main processes involved in the biodegradation of polymeric heterocomposites, such as cellulose, starch, and aliphatic polyesters, of which PHAs are typical, are biotic or abiotic hydrolysis followed by bio-assimilation (hydrobiodegradation), and the second is peroxidation followed by the bio assimilation of low molecular mass (oxybiodegradation) products, which is applied in particular. Despite their quick biodegradability, PHAs are exceedingly stable in the air and do not decay when stored normally.

#### *2.4.1 Appearance*

Depending on the types of integrated monomers, aromatic PHAs have a variety of physical appearances. PHAs made only of phenoxy or phenyl monomers (P(3H5PhV)) are sticky and supple. When the content of 3H5PhV was increased in the instance of P(3HA-3H5PhV), the polymer softened. P(3HA-3-hydroxyphenylalkanoate) [P(3HA-3HPhA)] changed from water-soluble to glue-like as the provided acyl chain length of phenylalkanoic acid was lengthened. PHAs with methylphenoxy groups are brittle, whitish substances [38]. PHAs that contain the 3H4BzB unit are similarly difficult. PHAs with thiophenoxygroups, however, are cream in color and elastomeric. The majority of PHAs that include the difluorophenoxy monomer is also cream-colored. Even with the addition of a small number of nitrophenyl units (1.2–6.9%), the physical properties of PHAs containing the nitrophenyl group diverged significantly from those of mcl-PHA [39].

#### *2.4.2 Mechanical properties*

The P(3-hydroxydodecanoate-3H5PhV) [P(3HDD-3H5PhV)] with varied 3H5PhV contents have different mechanical characteristics. The yield strength, maximum tension strength, and elongation at the break all decreased as a result of the addition of the 3H5PhV unit to P(3HDD). It is interesting to note that P(3HDD-18.70 mol% 3H5PhV) displayed a larger elongation at break than P(3HDD). On the other hand, except for P(3HDD-31.97 mol% 3H5PhV), Young's modulus increased above that of P (3HDD). These findings suggest a nonlinear relationship between the mechanical characteristics and the content of 3H5PhV [38].

#### *2.4.3 Surface properties*

Two fluorine atoms were added to P(3H5opFPxV), and its surface characteristics were assessed. This polymer has a surface contact angle of 104°, compared to 50° for PHAs having phenoxy or alkyl groups (C3 and C5) in the side chain [38]. A surface contact angle of more than 100 is typically insufficient to use the polymer as a nonwetting material. This difluorinated PHA thus possessed water-shedding qualities [39].

#### *2.4.4 Degradability*

The capacity of aromatic PHAs to degrade has also been investigated. One crucial quality of using PHAs as biodegradable materials is degradability. The stability of PHAs at physiological pH and the safety of the substance produced during hydrolysis should be assessed for medical applications such as medication delivery systems by analyzing the chemical degradation and microorganism-mediated degradation [38].

#### *2.4.5 Chemical degradation*

According to the literature, the P(3H6PhHx) homopolymer's chemical degradation was investigated. Around pH 7, this polymer is remarkably stable. It could therefore be utilized as a medication carrier to induce a delayed release of the active ingredient [39]. Additionally, the hydrolytic products of P(3H6PhHx) may have significant pharmacological effects that could enhance or expand the therapeutic effects of the drug that is encapsulated. These hydrolytic products can be oxidized in vivo to

#### *Cyanobacteria as a Source of Biodegradable Plastics DOI: http://dx.doi.org/10.5772/intechopen.110376*

phenylbutyric acid, phenylacetic acid, or trans-cinnamic acid. The antibacterial activity of (R)-3-hydroxy-phenylalkanoates (C5-C8), a hydrolytic product of PHAs bearing a phenyl group, is established. The relevant study showed that all (R)-3-hydroxyphenylalkanoates inhibited the growth of Listeria species, attributed only (or mainly) to the phenyl group. Olivera et al. created polymeric microspheres of P(3H6PhHx) [38].

#### *2.4.6 Solubility*

Bacterial PHA copolymers often display a wide range of comonomer compositions, which may result from modifications in the bacterial metabolism during PHA production. The biosynthesized aromatic PHAs are not always formed as a copolymer, but rather occasionally as a combination of two distinct PHAs. These aromatic polymers were isolated by solvent fractionation in several investigations [38].

#### *2.4.7 Thermal properties*

PHAs are polymers that are only partly crystalline. Therefore, the Tg and Tm of the amorphous and crystalline phases are typically used to express the thermal characteristics of these materials. The results of several studies show that the properties of aromatic PHAs differ significantly from those of mcl-PHAs, which are elastomers with Tgs between 53 and 28 C and a Tm between 45 and 69 C, where the values change depending on the types of aromatic monomers used [38].

#### *2.4.8 Extraction of PHA*

Treatment of cellular disruption and/or instability, recovery, and purification of biopolymers are the steps involved in the PHA extraction process. These procedures allow for the use of chemical, physical, biological, or even a mix of these technologies to provide a product with high purity and preserved physical and thermal characteristics.

The first step in the PHA extraction method is to centrifugate the solid material, which is made up of cells containing intracellular biopolymer, from the culture broth. Additionally, the microbial cell wall may be punctured or disturbed through biological, physical, or chemical means [40]. A suspension of bio-polymer, cells containing biopolymer (cells that destabilize but do not break cell walls), and cell debris form upon rupture or instability of the cell wall (mixture of proteins, nucleic acids, lipids, and cell-wall fragments). The next stage is to recover the biopolymer, which can be done in a variety of ways, including chemically, biologically, physically, or utilizing a combination of approaches like physical and chemical, biologically and chemical, among others [16].

#### *2.4.9 Chemical methods*

Isolated or coupled solvents are used in the chemical processes of removing PHAs from the cells of the microorganisms [40]. Chloroform, acetone, methyl isobutyl ketone, methylene chloride, propylene carbonate, ethyl acetate, and isoamyl alcohol are the most often used solvents. It is vital to assess the contact time and heating temperature of the polymer with the solvent to gauge the efficacy of the extraction process and the quality of the resulting product [16].

#### *2.4.10 Physical methods*

The use of homogenizer mills and ultrasound is among the most popular physical techniques used in PHA extraction. These methods are typically used at the beginning of the extraction procedure to disrupt and weaken the microorganisms' cell membranes. When compared to chemical extraction techniques, mechanical extraction yields polymers with higher thermal characteristics while also being more cost-effective and less hazardous. If an appropriate chemical method is used in conjunction with the mechanical method to extract biopolymers, which allows for high PHA recovery without significantly altering its features, the possibility for recovery will be increased [16].

#### *2.4.11 Biological methods*

The biological method of microbial PHA extraction is a complicated procedure that relies on the use of enzymes including lysozymes, nucleases, and proteases to recover the biopolymer. The culture broth is supplemented with enzymes to hydrolyze PHAcontaining cells [16]. The gentle operating conditions, great selectivity of the enzymes in hydrolyzing the microorganisms' cell wall proteins without affecting the breakdown of the polymer, and high quality of the recovered polymer make this technology appealing [40].
