Bio and Natural Polymers

An aqueous solution of methyl cellulose and hydroxylpropyl cellulose will gel upon heating [55]. The viscosity of the solution reaches a maximum with the introduction of several alkyl side chains with 6, 12, and 16 carbons onto hydroxyethyl cellulose. The optimum concentration of the alkyl side chain decreases as the length increases [56]. 

This is considered to be due to crosslink formation by micelles as a consequence of the aggregation of hydrophobic side chains (see Fig. 5 (e)). Here, the driving force of hydrophobic bonding is the positive entropy change accompanying the release of structural regular water molecules that are restricted by the hydrophobic alkyl side chains. Therefore, this is a unique crosslink structure for hydrogels. 

Examples of gelling polymers General methods Formation of crosslinks 

Polysaccharides Starch Cooling Microcrystal formation by hydrogen bonding 

Gellan Arginic acid Pectinic acid Addition of Crosslinking by coordination bonding 

---

Globular proteins, such as albumen, soy protein and casein, partially denature while maintaining their globular shape. When they are heated or disulfide bonds are broken, these complexes form network structures [58]. Hard proteins, such as fibrin, elastin, and keratin, possess covalent crosslinks and are difiBcult to dissolve in water. However, they swell in acid or base solutions to some extent. Table 3 summarizes the examples of bio and natural polymers that gel via intermolecular physical bonding. 

S. Kalia, B.S. Kaith, 1. Kaur, Cellulose Fibers Bio- and Nano-Polymer Composites Green Chemistry and Technology, Polyolefin Based Natural Fiber Composite (Springer, Heidelberg, Dordrecht, London, New York, 2011)... 

Shen, L., Haufe, J. and Patel, M. (2009) Product Overview and Market Projection cf Emerging Bio-Based Plastics, downloadable from http //www.epnoe.eu/research/Life-Cycle-Analysis (accessed 8 luly 2013). Lemstra, P. (2008) Introduction - Synthetic versus natural polymers. European polymer Federation workshop on Bioplastics Crossing the border between synthetic and natural polymers - May 30-31, Paris. Kim, S. (2004) Global potential bioethanol production from wasted crops and crop residues. Biomass and Bioenergy, April, 361-375. 

Njuguna J, Wambua P, Piehchowski K, Kayvantash K. Natural fibre-reinforced polymer composites and nanocomposites for automotive applications, cellulose fibers bio- and nano-polymer composites. In Kaha S, Kaith BS, Kaur 1, editors. Cellulose fibres bio- and nano-polymer composites. Berlin, Heidelberg Springer 2011. 

Njuguna, J., Wambua, P., Pielichowski, K., and Kayvantash, K. (2011) Natural fiber-reinforced polymer composites and nanocomposites for automotive applications, in Cellulose Fibers Bio-and Nano-Polymer Composites, Springer. 

Thomas S, Paul SA, Pothan LA, Deepa B (2011) In Kalia S, Kaith BS, Kaur I (eds) Natural fibres structure, properties and applications in cellulose fibers bio- and nano-polymer composites. Springer, Berlin, pp 3-42... 

Fig. 1.1 Number of publications per year related to bionanohybrid materials. Data collected from the ISI Web of Knowledge [v3.0]-Web of Science. Keywords for search (biopolymer AN D nanocomposite ) OR (natural polymer AND nanocomposite ) OR (bio-nanohybrid ) OR (biohybrid AND nano ).

Besides the previously mentioned collagen, a wide variety of natural polymers have been involved in the synthesis of bio-nanohybrid materials with potential application in bone repair and dental prostheses. For instance, some recent examples refer to bionanocomposites based on the combination of HAP with alginate [96,97], chitosan [98,99], bovine serum albumin (BSA) [100], sodium caseinate [101], hyaluronic acid [102], silk fibroin [103,104], silk sericin [105], or polylactic add (PLA) [106,107]. These examples illustrate the increasing interest in the subject of HAP-based biohybrid materials, which has led to almost 400 articles appeared in scientific journals in 2006 alone. 

A variety of water-soluble and water-insoluble polymers of both synthetic and natural origin ° have been studied as bio adhesives. 

On the more commercial front, the photooxidation of polymers continues to attract attention with a continued special interest in natural materials. Bio- and photodegradable plastics are important for agricultural usage. The same applies to polymer stabilisation where commercial applications dominate significantly with much emphasis on the synergistic behaviour of stabilisers. For dyes and pigments stability continues to be of major concern. 

Over the past decade or so, these remarkable achievements by nature have been recognized by the polymer science community. This has led to an increased interest in the use of biological concepts to synthesize polymers or to control the structure and properties of synthetic polymers. Of particular interest are peptide hybrid polymers. Combining peptide and synthetic polymer segments into a single macromolecule offers interesting possibilities to synergize the properties of the individual components and to compatibilize bio- and synthetic systems. 

Polymers derived from renewable resources (biopolymers) are broadly classified according to the method of production (1) Polymers directly extracted/ removed from natural materials (mainly plants) (e.g. polysaccharides such as starch and cellulose and proteins such as casein and wheat gluten), (2) polymers produced by "classical" chemical synthesis from renewable bio-derived monomers [e.g. poly(lactic acid), poly(glycolic acid) and their biopolyesters polymerized from lactic/glycolic acid monomers, which are produced by fermentation of carbohydrate feedstock] and (3) polymers produced by microorganisms or genetically transformed bacteria [e.g. the polyhydroxyalkanoates, mainly poly(hydroxybutyrates) and copolymers of hydroxybutyrate (HB) and hydroxyvalerate (HV)] [4]. 

Also, the use of renewable resonrces to synthesize polymers results in the development of new scientific challenges, snch as redaction chemistry, because biomass is highly oxygenated (whereas earlier, petrochemistry was based on the oxidation in air of the heavily rednced fossil resonrces) depolymerization of natural polymers with high molar masses polycondensation in that the bio-sonrced bnilding blocks are polyfunctional and may not necessarily have polymerizable double bonds and nse of LCA as a decision-snpport tool to identity pathways with the least enviromnental impact. 

---