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Biomonomers for Green Polymers: Introduction

P. M. Visakh

Department of Physical Electronics, TUSUR University, Tomsk, Russia

1.1 Processing Methods for Bionanocomposites

The new generation of hybrid nanostructured materials has two crucial properties: biocompatibility and biodegradability [1,2]. Exploitation of various biopolymers such as proteins, nucleic acids, polysaccharides, etc. for preparation of nanocomposites has been done in last few decades [3]. Processing methods for matrix and filler are sometimes the same. However, some matrices are prepared using combinations of techniques to achieve the desired quality of bionanocomposites, therefore we will discuss the processing methods for bionanocomposites with suitable examples. Bionanocomposites of polysaccharide matrices are mainly prepared by solvent intercalation or melt processing and not through in situ polymerization where nature of the polysaccharide directly influences the route of preparation. Some polysaccharides with nanostructure fillers are discussed as examples. Most of the cellulose whiskers-reinforced poly(lactic acid) (PLA) nanocomposites are prepared by melt extrusion to avoid agglomeration and aggregation during drying [4]. Porous networks and thickened cellulose ribbons in gelatin/nanocellulose composites are prepared using an enzymatically modified form of gelatin [5]. Cellulose nanocomposites based on nanoparticles, such as clay [6-14], carbon nanotubes (CNTs) [15], graphene, layered double hydroxide (LDH) [16], and silica [17] have been prepared.

Starch is another abundant, inexpensive, naturally renewable and biodegradable polysaccharide, produced by most green plants as an energy store. It is the most common carbohydrate in human diets and animal feeds. Starch nanocomposites are mixtures of starch-based biopolymers with nanofillers (solid layered clays, synthetic polymer nanofibers, cellulose nanowhiskers, CNTs, and other metal nanostructures). Environmentally friendly starch nanocomposites exhibit significant improvements in mechanical properties, dimensional stability, transparency, improved processability, and solvent or gas resistance. Chitosan (CS)/chitin, the second most abundant natural biopolymer, also can be integrated with clay, graphene, and carbon nanostructures to prepare bionanocomposites [18-21]. Due to its high content of amino (-NH2) and hydroxyl (-OH) groups, chitosan and its derivatives are excellent adsorbents for the removal of heavy metal ions, fluoride, and organic dyes. Films of spin-coated chitosan-alginate nanocomposite have potential uses in bioapplications. Lignin-based nanocomposite films have been prepared using CNCs (carbon nanocomposites) and used in various applications such as medical, biological, optical and sensors, and electronic [22]. They are also used as adhesives, stabilizing agents, and precursors for many aromatic chemicals. Modified lignins, such as lignosulfates, kraft lignin, and acetylated lignin, contain CNCs or commercial derivatives or nanocellulosic polysaccharides. Polyethylene terephthalate (PET) film coated with graphene oxide (GO)/pullulan nanocomposite can be used in food/pharmaceutical applications [23]. Bionanocomposites with enriched properties based on two microbial polysaccharides, pullulan and bacterial cellulose (BC), were prepared by Trovatti et al. for possible application in organic electronics, dry food packaging and the biomedical field [24]. Pullulan composites with many materials, including chitosan [25], caseinate [26], starch nanocrystals [27], collagen [28], poly (vinyl alcohol) [29], and hydrogel with methacrylate [30], have excellent compatibility.

Their biodegradability, low cost, and surfaced modification with active functional groups for catching targeting molecules make these matrices feasible candidates for applications in the pharmaceutical industry [31]. Electrospun collagen-chitosan nanofibers were stabilized by glutaraldehyde vapor via crosslinking, which afforded a biomimetic extracellular matrix (ECM) for cell growth [32]. Collagen is regarded as one of the most useful biomaterials, exhibiting a number of biological advantages. The outstanding performance and biomedical application of this protein biomaterial have induced researcher interests in synthetic composite material fabrication. Soy protein isolate (SPI) has been extensively studied for bioderived packaging materials. Several recent studies have investigated the improvement of mechanical and barrier properties of nanocomposite films after incorporating nanoclays such as montmorillonite (MMT) [33-41]. Further, these nanocomposite films have also been reported for decreased water vapor and oxygen permeability, and increased elastic modulus and tensile strength, which makes them suitable for packaging industry. Recent studies have also reported that the SPI-based nanocomposite bioplastics with highly exfoliated MMT have significantly improved mechanical strength and thermal stability [42]. Thus, bio-based polycaprolactone-SPI is not only ecofriendly but intercalated nanocomposites with enhanced tensile and dynamic mechanical properties when produced by the melt compounding method [43].

In the case of biocomposites, the properties of the composites produced are dependent on the inter-phase interaction of the reinforced material and matrix. Filler is also a value-added material, but wise selection of processing methodology, optimum conditions, and compatible phase components is needed. Polymer/metal nanocomposites consisting of polymer as matrix and metal nanoparticles as nanofiller commonly exhibit several attractive advantages, such as electrical, mechanical, and optical characteristics [44]. Metal nanocomposites with protein, nucleic acid, and polysaccharides have shown potential applications in drug delivery, tissue engineering, bioimaging, wound healing, biomedicine, energy production and storage, and electronic devices such as biosensors, affinity materials, etc. [45]. Bottom-up methods are found to be promising for controlling the properties and specific orientation of nanomaterials. Thermal evaporation and sputtering techniques have been considered as facile, simple, low-cost, and high-yield methods for synthesis of high-quality nanomaterials/nanostructures [46,47]. Various immobilization methods, including entrapment, adsorption, crosslinking, electro-polymerization, and encapsulation, have been used for capturing biological moieties in the matrix. This is one of the main processes employed in the manufacturing of nanobiocomposites (NBCs) [48]. There are two main types of extrusion: reactive extrusion and extrusion cooking. Reactive extrusion uses chemical modification via crosslinking [49]. Generally, extrusion technology used in the food industry is referred to as extrusion cooking and results in different physical and chemical properties of the extrudates depending on the raw materials and extrusion conditions used [50]. Various starch nanocomposite varieties have been prepared and reported by many researchers for biodegradable packaging applications in food industry. Moigne et al. developed a continuous CO2 assisted extrusion process to prepare poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/clays NBC foams with better homogeneity and high porosity [51]. Inventor Torkelson has successfully produced a well-dispersed graphite-polymer nanocomposite [52]. Taking advantage of near-ambient-temperature processing, solid-state shear pulverization (SSSP) was recently used to produce biodegradable polymer matrix composites with starch [53], rice husk ash [54], and eggshell filler [55,56]. This technique that has proven to effectively disperse nanoscale structural entities to achieve compatibilized polymer blends and exfoliated polymer nanocomposites.

This physical method uses extrusion of the polymer solution with reinforcement of nanomaterials and biological entity for the preparation of NBCs. Polymers, molten at high temperature, can also be made into nanofibers by electrically charging a suspended droplet of polymer melt or solution [57-63]. Instead of a solution, the polymer melt is introduced into the capillary tube. The major difference is that a compound spinneret with two (or more) components can be fed through different coaxial capillary channels [64]. Wet-dry electrospinning and wet-wet electrospinning techniques are used for volatile and non-volatile solvents respectively. Both techniques offer the possibility of producing nanofibers with controlled fiber diameter to make film or membrane or an oriental controlled fiber. Such fibrous scaffolds are ideal for the purpose of tissue regeneration because their dimensions are similar to the components of the extracellular matrix and mimic its fibrillar structure, providing essential signals for cellular assembly and proliferation. Core-shell structured nanofibers where collagen as the shell and...