Biopolymers

The term “biopolymers” usually describes polymers produced by living organisms.

Microorganisms’ degradation of these biopolymers occurs under suitable temperature, moisture and oxygen conditions. It leads to the decomposition of the polymer materials with no toxic or harmful residue. Carbohydrates and proteins are the major examples of biopolymers. Among the biopolymers, polysaccharides obtained from linear carbohydrate structures are important in living organisms. Cellulose and chitosan are another important

polymers from the polysaccharide family. Biopolymers can also be produced from monomers obtained from biological resources using conventional chemical processes (poly (lactic acid)), or directly in microorganisms or genetically modified organisms (polyhydroxyalkanoates).

Fig. 1.2 describes the classification of the biopolymers.

Biopolymers are classified mainly based on the source of raw materials and their synthesis processes.

 Biopolymers are obtained directly from bioresources (plant or animal origin), such as protein (wheat gluten, gelatin, collagen etc.) and carbohydrates (starch, chitosan, cellulose, carrageenan, agar etc.).

 Biopolymers a r e obtained by chemical synthesis. Synthetic biodegradable polymer poly(lactic) acid is obtained from bio-based monomer lactic acid. Polycaprolactone and polyvinyl alcohol are obtained from petrochemicals.

 Biopolymers are produced by the fermentation of microorganisms such as polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), polyhydroxyl-valerate (PHV), bacterial cellulose, and pullulan.

Table 1.1. Biopolymers and antimicrobial agents for bio-nanocomposites Biopolymers Antimicrobial materials References Starch/TPS Quaternary ammonium salt

modified MMT-cloisite 30B/ Ag-NPs

(Abreu et al., 2015)

Copper nanoparticles (CuNPs)

(Hasanin et al., 2022)

Layered silicates/Essential oil

(Campos-Requena et al., 2017)

Chitosan nanoparticles (Babaee et al., 2022)

Chitosan Bentonite-Ag-ZnO (Motshekga et al., 2015)

ZnO (D. Li et al., 2022)

Ag/ZnO (Ali et al., 2022)

Polylactic acid ZnO (Boro et al., 2022)

TiO2 (Fonseca et al., 2015)

MgO (Swaroop & Shukla, 2018)

Polycaprolactone (PCL)

clay@ZnO (Maghfoori et al., 2022)

Cellulose acetate/Copper NPs

(El-Naggar et al., 2022)

Polyhydroxybutyrate (PHB)

Silver nanoparticles (Castro-Mayorga et al., 2018)

Poly(hydroxybutyrate -co-valerate) (PHBV)

Clay/oregano oil (da Costa et al., 2020)

Pullulan lysozyme nanofibers

(LNFs)

(Silva et al., 2018)

Fig. 1.2. Classification of the biopolymers

1.3.1 Poly (lactic acid)

Poly(lactic acid) is one of the most widely used biopolymers in the family of poly-α-hydroxy acid, a linear aliphatic polyester. It is considered a sustainable substitute for petroleum-based products due to its high mechanical strength, thermal stability, transparency, low carbon footprint, easy processability, etc., compared to other biobased polymers (Taib et al., 2022). It can be easily molded in different-shaped articles by using conventional thermoplastic processing methods such as extrusion, compression molding, blow molding, injection molding and thermoforming etc.

Furthermore, after the end of the life cycle, it degrades mainly by hydrolysis after several months of contact with moisture. The degradation of PLA occurs in two steps. Firstly, random non- enzymatic chain scissioning of ester groups of PLA occurs, which causes a reduction in molecular weight. Secondly, the decrease in molecular weight occurs until the formation of lactic acid and low molecular weight oligomers are consumed by microorganisms to produce water, CO2 and solid biomass. PLA is one of the topmost choices of biobased polymer materials to be used in different applications, including engineering and biomedical (due to its biocompatibility and biodegradability) applications. In contrast, the fields of packaging and fiber technology represent

the primary utilisation sectors. It is considered generally recognized as safe (GRAS) by the United States Food and Drug Administration (FDA) and is safe for all food packaging applications (Garlotta, 2001). Fig. 1.3 depicts the number of publications on PLA-based composites in the last 10 years. The number of papers published on PLA has followed an increasing trend over the last 10 years, showing PLA's significance in composites.

Fig. 1.3. No of paper published on PLA composites (Ranakoti et al., 2022)

1.3.2 Structure and properties of poly(lactic acid)

The biopolymer poly(lactic acid) is obtained from the polymerization of monomer lactic acid (LA). Four different groups are attached to the central carbon atom of LA. Therefore, it is a chiral molecule and exists in three forms: Levorotatory (L-), dextrorotatory (D-) and meso (combination of L- and D-). The formation of L-lactic acid, D-lactic acid or a mixture of L- and D- depends on the source and synthesis method. The lactic acid obtained from the chemical method is a racemic mixture of 50% L- and 50% D-isomers, whereas lactic acid obtained by the fermentation process consists of 99.5% of the L-isomer, and the remaining is D-isomer. The PLA synthesized by using

L-, D- and meso isomers are called poly(L-lactic acid), poly(D- lactic acid) and poly(meso-lactic acid), respectively. The final properties of poly(lactic acid), such as its crystallinity, thermal and mechanical properties, mainly depend on the ratio of L/D content (Ranakoti et al., 2022).The presence of L-lactic acid would produce semi-crystalline PLA (PLLA), while poly (DL-lactide) produces an amorphous polymer (PDLLA). PLA’s glass transition temperature (Tg) and melting point (Tm) decrease with decreasing amounts of L-isomers. Physical properties such as heat capacity, density, and mechanical and rheological properties of PLA depend on its Tg. In the case of amorphous PLA, the Tg is an important parameter due to higher changes in polymer chain mobility at and above Tg. Tg and Tm are important parameters for determining PLA behaviour for semicrystalline PLA. Fig. 1.4 shows the chemical structure of lactic acid isomers and poly(lactic acid). The physical, mechanical and thermal properties of commercial PLA grades are shown in Table 1.2.

Fig. 1.4. Chemical structure of (a) isomers of lactic acid and (b) poly(lactic acid)

Table 1.2. Physical, mechanical and thermal properties of commercial PLA grades (Farah et al., 2016)

Properties PLA PLLA PDLLA

Density (g/cm³) 1.211.25 1.241.30 1.251.27

Tensile strength (MPa) 2160 15.5150 27.650

Elastic modulus (GPa) 0.350.5 2.74.14 13.45

Ultimate strain (%) 2.56 3.010.0 2.010.0

Glass transition temperature (℃) 4560 5565 5060

Melting temperature (℃) 150162 170200 Amorphous-no melting point

1.3.3 Synthesis of poly(lactic acid)

Poly(lactic acid) was first synthesized by Carothers (at DuPont) in 1932. He could only synthesize PLA of low molecular weight by simply heating lactic acid under an inert atmosphere with the simultaneous removal of condensed water. The challenge to synthesize high molecular weight PLA was solved by employing ring-opening polymerization of the lactide. Nowadays, different processes are available to produce PLA, but none of them is easy to perform. The PLA synthesis requires proper control of synthesis conditions such as temperature, pressure and pH, choice of catalyst, polymerization time etc. PLA can be synthesized from monomer lactic acid using two different methods: (a) direct condensation process, which involves solvents under high vacuum or in a solvent-free process, a cyclic dimer intermediate called lactide is formed followed by catalytic ring opening polymerization of the cyclic lactide. The direct condensation route is an equilibrium reaction, and there are difficulties in removing water and impurities. Hence, the final product usually has low molecular weights (Mw~1-10 kDa). Because of the problems faced in the direct condensation method, the commercial production processes are based on lactide ring-opening polymerization. Fig. 1.5 shows the synthesis route of polylactic acid.

Fig. 1.5. Synthesis route of polylactic acid

Limitations of poly(lactic acid)

Although PLA possesses the prospective to compete with conventional plastics, it is important to highlight the drawbacks, which limit its usage in the packaging application as follows:

a) Highly brittle nature of PLA restricts its applications, such as flexible films.

b) PLA possesses poor thermal stability c) PLA has a slow crystallization rate.

d) PLA shows poor barrier properties (gas, moisture etc.) compared to conventional polymers.

1.4 Nanofillers/antimicrobial additives for PLA