THEORY EXAMINATION (SEM–VI) 2016-17 STRUCTURE AND PROPERTIES OF FIBRE

STRUCTURE AND PROPERTIES OF FIBRE (EC011)

Section-wise Solved Answers & Notes

SECTION – A (10 × 2 = 20 Marks)

Very short & precise answers

(a) Concept of Transmission Electron Microscope (TEM)

TEM works on the principle of transmission of a high-energy electron beam through an ultra-thin sample to study internal structure at very high resolution (nanometer scale).

(b) Fibre fracture analysis for fracture mechanism

Fibre fracture behaviour helps to identify ductile or brittle failure, crack initiation, fibrillation, and micro-structural defects using fracture surface observation.

(c) Number of furnaces used in DSC

Two furnaces are used in Differential Scanning Calorimeter (DSC) to satisfy the null-balance principle (one for sample, one for reference).

(d) Molecular weight of PET suitable for fibre formation

The molecular weight of PET suitable for fibre formation is approximately 15,000–20,000 g/mol.

(e) Polydispersity of polymers

Polydispersity is the ratio of weight-average molecular weight to number-average molecular weight:

PDI=MwMnPDI = \frac{M_w}{M_n}PDI=Mn​Mw​​ 

(f) Scanning medium in SEM

The scanning medium in Scanning Electron Microscope is an electron beam.

(g) Principle of FTIR

FTIR works on the principle of absorption of infrared radiation by molecular bonds, causing vibrational transitions characteristic of chemical groups.

(h) Thermal analysing technique for melting behaviour

Differential Scanning Calorimetry (DSC) is used to scan the melting behaviour of polymers.

(i) Prime application of TGA

Thermogravimetric Analysis (TGA) is mainly used to study thermal stability and decomposition behaviour of materials.

(j) Essential features of chemicals for density gradient column

• Complete miscibility                                    • Known density range
• Chemical stability                                         • No reaction with fibre
• Uniform density gradient formation

SECTION – B (Attempt Any Five) (5 × 10 = 50 Marks)

(a) Fibre crystallinity by density gradient column

Crystallinity fraction of fibre can be determined because density increases with crystallinity.

Relation:

ρ=Xcρc+(1−Xc)ρa\rho = X_c \rho_c + (1-X_c)\rho_aρ=Xc​ρc​+(1−Xc​)ρa​

Where:
• XcX_cXc​ = crystallinity fraction
• ρc\rho_cρc​ = crystalline density
• ρa\rho_aρa​ = amorphous density

By measuring fibre density in a gradient column, crystallinity can be calculated.

(b) Best technique for precise crystallinity measurement

Among DSC, X-ray diffraction, and Density Gradient Column, X-ray diffraction (XRD) is most accurate because it directly separates crystalline and amorphous scattering peaks, giving precise crystallinity %.

(c) Role of TGA thermogram in thermal stability

TGA thermogram shows:                 • Initial decomposition temperature
• Weight loss stages                        • Residual mass

It helps identify thermal degradation mechanism and stability range of textile materials.

(d) Birefringence in textile fibres

Birefringence is the difference in refractive indices along and across fibre axis.

Δn=n∣∣−n⊥\Delta n = n_{||} - n_\perpΔn=n∣∣​−n⊥​

It measures overall molecular orientation in both amorphous and crystalline regions, hence the statement is correct.

(e) Directly & indirectly attached water molecules

• Directly attached water: Bound to polar groups (–OH, –NH)
• Indirectly attached water: Hydrogen-bonded to other water molecules

Quantitative theory explains moisture absorption using thermodynamic equilibrium concepts.

(f) Fibre structure of cotton fibre

Cotton fibre consists of:
• Cuticle                                                        • Primary wall
• Secondary wall (S1, S2, S3 layers)              • Lumen

High cellulose crystallinity gives strength and absorbency.

(g) Image formation in SEM

SEM image is formed by secondary electrons emitted from specimen surface when scanned by electron beam, giving high-resolution surface topography.

(h) Fibre fracture study using SEM

SEM reveals:
• Crack propagation                                       • Fibrillation
• Brittle or ductile fracture                              • Surface defects

It helps correlate mechanical behaviour with microstructure.

SECTION – C (Attempt Any Two) (2 × 15 = 30 Marks)

Q3. Heat of absorption in textile materials

Heat of absorption

It is the heat evolved when moisture is absorbed by textile fibres.

Differential heat of absorption

Heat absorbed per unit moisture at a given moisture content.

Integral heat of absorption

Total heat absorbed up to a given moisture content.

Relation

Integral heat=∫(Differential heat) dM\text{Integral heat} = \int (\text{Differential heat})\, dMIntegral heat=∫(Differential heat)dM

Quantitative theory of moisture absorption

Explains moisture uptake using thermodynamic and molecular interaction principles, considering fibre-water bonding.

Q4. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR identifies functional groups and chemical structure of textile materials.

Usefulness in textile characterization

• Identification of fibres (cotton, wool, polyester)
• Detection of finishes and treatments
• Study of polymer degradation
• Analysis of moisture absorption

Each fibre shows a unique IR absorption spectrum.

Q5. Fine structure of wool fibre & property correlation

Structure of wool fibre

• Cuticle (scales)
• Cortex (ortho- and para-cortex)
• Cell membrane complex

Structure–property relation

• Scales → felting property
• Cortex → crimp and elasticity
• Keratin structure → strength and resilience

Thus, wool morphology directly governs mechanical, thermal, and comfort properties.