The Molecular Foundation
Raw cotton is naturally engineered: roughly 90–95% cellulose, bound with trace waxes, pectin, organic acids, and inorganic salts. After routine scouring and bleaching, technical cotton exceeds 99% pure cellulose.
Empirical Formula: (C6H10O5)n | Degree of Polymerization (DP): n ≈ 9,000–15,000
1. Monomer Anatomy
Cotton is a linear homopolysaccharide built of D-glucopyranose units joined by rigid β-(1→4)-glycosidic linkages. Each subsequent glucose ring flips 180° along the axis, creating the cellobiose repeating unit.
2. The Hydrogen-Bond Network
Three free hydroxyl groups (–OH) sit at the C-2, C-3, and C-6 positions. They build vast networks of intra-chain (O3–H…O5) and inter-chain (O6–H…O3) hydrogen bonds, forming tightly packed, insoluble microfibrils.
3. Supramolecular Crystalline Order
Native cotton arranges predominantly as Cellulose Iβ in a parallel monoclinic unit cell. The fiber is semi-crystalline (approx. 70% crystalline and 30% amorphous), delivering tensile strength while retaining moisture permeability.
Solid-State ¹³C CP/MAS NMR Spectroscopy
Because crystalline cotton resists solvent dissolution, Cross-Polarization Magic-Angle Spinning (CP/MAS) NMR provides direct structural elucidation in the solid state:
Complementary Analytical Methods
- X-Ray Diffraction (XRD): Resolves the (1-10), (110), and (200) crystalline planes. Segal’s formula measures crystallinity:
CrI = (I₂₀₀ - Iₐₘ) / I₂₀₀ × 100. - FTIR Spectroscopy: Tracks the broad –OH vibration (3300–3400 cm⁻¹) and the O’Connor Lateral Order Index ratio (A₁₄₂₉ / A₈₉₇) to monitor changes during caustic mercerization.
- Thermogravimetric Analysis (TGA): Quantifies initial water loss (50–110°C, ~6%), followed by rapid main-chain thermal degradation into levoglucosan (300–380°C).
Primary Academic References
1. Atalla, R. H., & VanderHart, D. L. (1991). Studies on the structure of cellulose using solid-state carbon-13 NMR. Macromolecules.
Definitive spectroscopic identification of the Iα and Iβ allomorphs in native celluloses.
DOI: 10.1021/ma00021a0382. Segal, L., et al. (1959). An empirical method for estimating degree of crystallinity using X-ray diffractometer. Text. Res. J.
Established the classical empirical peak-height method for fiber crystallinity indices.
DOI: 10.1177/0040517559029010033. Nishiyama, Y., et al. (2002). Crystal structure and hydrogen-bonding system in cellulose Iβ. J. Am. Chem. Soc.
Synchrotron X-ray and neutron diffraction resolving atomic coordinates and H-bonding pathways.
DOI: 10.1021/ja0257319
