Natural polymers.
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| Composites Design and Manufacture (Plymouth University teaching support materials) Natural polymers. |
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CAUTION: For the purpose of the Sustainable Composites pages, the materials described are those from natural sources, without prejudice to the results of any future Quantitative Life Cycle Analysis (QLCA) which may (or may not) make the case for these materials being more environmentally-friendly than equivalent systems manufactured from man-made fibres and synthetic resins. The inclusion of any specific system here is not an endorsement of that product: potential users will need to fully consider each system in the context of their specific technical requirements.
The value of Eco-System Services
Natural polymers are generated by living things, primarily plants and animals. Polysaccharides are the most abundant organic compounds on earth with annual production estimated to approach 100 gigatonnes (1011 tonnes) each year [1]. Cellulose, synthesised mainly in plants, is the most abundant polysaccharide, followed by chitin, which is synthesised mainly in lower animals [1,2].
Chitin is a linear high molecular weight crystalline polymer, poly(β-(1,4)-N-acetyl-D-glucosamine), which differs from cellulose by having the acetamide (-NHAc) group at the C-2 position on the heterocyclic ring, whereas this position is occupied by a third hydroxyl (-OH) group in cellulose [2]. Guthrie and Honeyman [3] confirm that chitobiose, a disaccharide isolated from the hydrolysis products of chitin, has been shown to be identical with cellobiose except for an amino group on C(2) of each of the D-glucose units and that X-ray studies suggest that the unit cell has almost the same length as that of cellulose.
The most abundant form of chitin is the α-allomorph found in crab and shrimp shells (the main commercial source), various marine organisms, insect cuticle, fungi and yeast cells [2]. The rarer β-allomorph is found in squid pens, tubes synthesised by worms, some protozoa and seaweeds [2]. In the α-chitin crystal, there are two antiparallel molecules per unit cell with strong intermolecular hydrogen bonding. In the metastable β-chitin crystal, there is only one antiparallel molecule per unit cell with weaker hydrogen bonding and, hence, this can easily convert to α-chitin on dissolution or extensive swelling [1].
| a (nm) | b (nm) | c (nm) | γ (°) | crystal system | Reference | |
| α-chitin | 0.474 | 1.886 | 1.032 | 90 | orthorhombic | 4 |
| anhydrous β-chitin | 0.485 | 0.926 | 1.038 | 97.5 | monoclinic | 5 |
| chitosan | 0.807 | 0.844 | 1.034 | 90 | orthorhombic | 6 |
Chitin can be prepared by cleaning and decalcifying (removal of the calcium carbonate) the shells with hydrochloric acid at room temperature. The shells are then reduced to flakes or powder and reacted near 100°C with 1-2 mol/l NaOH to destroy the pigments and proteins (enzymes may be used for the latter). The product is an almost colourless to off-white powder [1]. The insolubility of chitin in ordinary solvents makes it difficult to process and to characterise. The high costs of extraction and purification currently limits application of this polymer to high-added-value applications (e.g. biomedical, pharmaceutical and cosmetics) [2].
When chitin is deacetylated to around 50% it becomes a soluble in aqueous acidic media. Deacetylated chitin is called chitosan (the only pseudonatural cationic polymer) [2]. Chitosan is much easier to process than chitin, but has lower stability, is biodegradable and has antibacterial and antifungal activity [2]. Chitosan can be converted to chitin by N-acetylation under mild conditions [1].
Vincent [7] expects chitin to have a higher stiffness (tensile modulus) than cellulose as the acetyl group not only provides for more hydrogen bonding but also reduces flexibility due to steric hindrance. Vogel [8] notes that our "human technology hasn't put this fine material [chitin] to use in any mechanical device" [8a]. Vogel suggests a Young's modulus >10 GPa [8b] and, for insect cuticle, a tensile strength of 100 MPa [8c] and a work of fracture of 1500 J/m2 [8d].
Liu et al [9] have prepared composites of carbon nanotubes in chitosan.
These materials have anti-bacterial properties [see e.g. 10-11] (and hence may find application as active fillers in composite materials) and can be used for the removal of dyes from aqueous solutions [12].
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