Fibre Structure Analysis Techniques: Density, Birefringence, X-ray Diffraction, SAXS & FTIR Explained

Introduction: Why Structural Analysis of Fibres Matters

The colour, luster or texture is just the tip of the iceberg when it comes to textile understanding when it comes to science. Performance of any textile fibre is described by what is very much enshrined within physical structure. It encompasses parameters like crystallinity, molecular location, empty spaces within itself, chemical activities and even nanoscale patterns.

What difference does this make? Since the behaviour of fibre under stress, heat, light and moisture is defined by the manner in which the molecules are packed, aligned or bonded within the fibre. Some fibre that feels soft could be structurally weak. Another that is hard to bend is the one that is designed to be performing highly.

As a textile engineer, as a student studying GATE or Textile committee exams, or anybody interested in spinning, dyeing or development of fibres, you need to know how these structures are learnt.

In this post, we will explore five powerful characterisation techniques used to understand the physical structure of textile fibres:

• Density Measurement
• Optical Birefringence
• X-ray Diffraction (XRD)
• Small-Angle X-ray Scattering (SAXS)
• Infrared Spectroscopy (FTIR)

Each of these methods is non-destructive and helps us visualise what cannot be seen by eye or under a normal microscope. Let’s begin with the first.

1. Textile Fibres Density Measurement

What Is Fibre Density?

The density of a fibre is determined by dividing the mass by the volume, defined as grams per cubic centimetre (g/cm 3 ). It is one of the main properties, which indicates the density of the molecules in a fibre. That is, density tells us a ballpark figure of the amount of solid matter in a volume.

Why is this important?

Crystallinity is directly associated with density in the related field of fibre technology and in polymer science. Crystalline areas are more prone and positioned, hence are heavier. Amorphous regions in contrast are disorderly and have voids or free volume. Thus a fibre with high density usually implies that there is a higher proportion of crystallinity.

This importance is used to yield density as a practical indirect measurement used to determine the internal structure of the fibres. It can also be used to determine unknown fibre on basis of reference-density values.

Common Fibre Densities:

• Cotton: ~1.54 g/cm³
• Polyester: ~1.38 g/cm³
• Wool: ~1.30 g/cm³
• Silk: ~1.34 g/cm³
• Nylon 6: ~1.14 g/cm³
• Acrylic: ~1.17 g/cm³

Even small differences in density can influence the final performance of yarn and fabric.

How Is Density Measured?

One of the most popular methods is using a density gradient column. This is a tall glass column filled with a mixture of two liquids that gradually vary in density from top to bottom. The fibre is dropped into the column, and it floats at the level where the liquid density matches its own. That level is calibrated to provide the density reading.

Other methods include:

• Liquid displacement using a pycnometer
• Archimedes’ principle: measuring weight in air and in a reference liquid
• Suspension in known density liquids (e.g., CCl₄ and methanol)

What Can We Learn?

• Crystallinity index (approximate, comparative)
• Detection of voids or hollowness in fibres (lower density = more hollow)
• Batch consistency in synthetic fibre production
• Raw material authenticity in trade or quality inspection

Limitations:

• Cannot provide orientation or size of crystals
• Hollow, textured, or modified fibres can give misleading values
• Sensitive to moisture (especially in natural fibres like wool)

In short, density measurement is a fast, inexpensive, and reasonably reliable method to estimate fibre structure and purity—especially when combined with other techniques.

2. Optical Birefringence

What Is Optical Birefringence?

Optical birefringence is the phenomena when a substance possesses two different refractive indices considering the light direction with which light propagates. In textile fibres, the phenomenon directly depends on the orientation of the polymer chains with respect to the fibre axis.

Introducing light under polarised conditions into such a fibre, it becomes in two rays, call them the ordinary and the extraordinary. The extent to which these rays vary in velocity (and consequently in optical length) is that which originates birefringence.

How does it get measured?

Birefringence can be examined with a polarising microscope. The fibre is stuck to a cross on polarisers and viewed. By visual observation of the colours and patterns of interference, it is possible to obtain qualitative information but with the help of compensator or interferometer, one can undertake detailed quantitative analysis.

Birefringence (Dn) can be calculated with:

[Delta n] = (0.18 * Retardation[nm]) / (1000 * Fibre diameter[micro m])

What Information Can It Reveal?

• Molecular orientation: The more aligned the molecules, the higher the birefringence.
• Degree of crystallinity: Though not a direct measure, highly crystalline fibres often show stronger birefringence.
• Processing effects: Drawing, heat setting, or annealing all affect orientation, and therefore birefringence.
• Fibre identification: Acetate, with low orientation, shows dull colours. Nylon or polyester, being highly oriented, show bright interference.

Applications in Textiles:

• Assessing the quality and effectiveness of fibre stretching
• Analysing orientation differences in multi-component fibres or blends
• Studying shrinkage or deformation in post-finished fabrics

Limitations:

• Only effective for optically anisotropic materials
• Surface irregularities or coatings can distort results
• Requires good sample preparation and calibration

Optical birefringence provides a simple yet powerful way to visualise the invisible orientation of molecules inside a fibre—making it invaluable in both research and quality control.

3. X-ray Diffraction (XRD)

What is XRD?

X-ray diffraction represents the method, which could be used to identify the crystalline structure of polymers, and X-ray scattering is measured through the material. Diffraction by a periodical arrangement of atoms In scattering of X-rays under a periodical concatenation of atomic planes, the beams are scattered at special angles dependent upon the distance between the atomic layers.

This pattern of scatter represents the spacing and the orientation in addition to the quantity of regional crystalline areas in a fibre sample.

The Science Behind It

XRD operates based on Bragg’s Law:

nλ = 2d sinθ

Where:

• n = order of reflection (usually 1)
• λ = wavelength of the X-ray
• d = distance between planes of atoms (interplanar spacing)
• θ = diffraction angle

Types of XRD Used:

• WAXD (Wide-Angle XRD): Reveals interatomic distances and orientation.
• Powder XRD: Used when samples are in powder form, often for non-fibrous tests.

What You Can Discover:

• Crystallinity Index (CI): Is the percentage of the crystalline and amorphous fibre.
• Crystalline orientation: The fibres that are well-oriented and aligned display sharp peaks.
• d-spacing: Shows what kind of polymer form is contained (e.g. Nylon 6 or Nylon 66)
• Crystal size: Smaller peaks, smaller crystallites; broader peaks, an amorphous zone

Examples in Fibres:

• Cotton shows a distinct change in crystallinity after mercerisation.
• Nylon 6 exhibits different crystal structures than Nylon 66, which can be identified.
• Polyester drawn at different levels will show changing orientation.

Limitations:

• Requires complex equipment and calibration
• Cannot detect amorphous content alone
• Needs pure, dry, well-prepared samples

Despite the complexity, XRD is a gold-standard method in determining the structural backbone of synthetic and natural fibres. It allows researchers to tailor mechanical and chemical properties at the molecular level.

4. Small-Angle X-ray Scattering (SAXS)

What Is SAXS?

SAXS, Short-Angle X-ray Scattering more directly relates to XRD, but measures even larger structural features which are commonly between 1 and 100 nanometres. In contrast to WAXD, which provides atomic scale crystallinity, SAXS provides details about long range order and microfibrillar structure in fibres.

How It Works

After a fibre is subjected to X-rays, the pattern of the reflected rays which are scattered to small angles tells how the density of electrons changes in the material. Such differences occur in directions of amorphous and crystalline density, holes or fibrillar assemblies.

What Can SAXS Reveal?

• Lamellar spacing: Repetitive layered structure within the fibre
• Void distribution: Pores or microvoids can affect strength and moisture absorption
• Fibril diameter and orientation: Especially relevant for drawn or annealed fibres
• Nano-structural regularity: Indicates processing quality in nanofibres or advanced textiles

Applications in Textiles:

• Analysing high-performance fibres like aramids and carbon fibres
• Studying microstructure in melt-spun or solvent-spun fibres
• R&D in fibre-reinforced composites and breathable performance fabrics

Limitations:

• Complex interpretation of data
• Requires highly sensitive detectors and sample alignment
• Often used in combination with WAXD or TEM for complete analysis

SAXS is particularly valuable in cutting-edge textile research where nano-level structure and performance are deeply linked, such as in sportswear, filtration media, and smart textiles.

5. Infrared Spectroscopy (FTIR)

What Is FTIR?

FTIR (Fourier Transform Infrared Spectroscopy) is an analysis technique that looks at the rate at which the molecules (in a fibre) absorb the infrared radiations at particular wavelengths. This absorption is matched with those of vibrations of chemical group there is a possibility of establishing the types of bond and functional groups existing in the fibre. 

How It Works

An IR light beam is passed through the fibre (or reflected from its surface using ATR mode). The resulting spectrum shows absorption peaks at characteristic wavelengths.

Each peak corresponds to a specific bond or group, such as:

• O-H (hydroxyl)
• C=O (carbonyl)
• N-H (amide)
• C-H (aliphatic/aromatic)

What FTIR Can Tell Us:

• Polymer identification: Distinguishes between nylon, polyester, cellulose acetate, etc.
• Functional finishes: Detects chemicals used in fire-retardants, antimicrobial finishes, or coatings
• Degradation analysis: Identifies oxidation or hydrolysis effects over time
• Crystallinity and orientation: Through intensity and shape of absorption bands
• Advanced Application: Orientation Factor

FTIR can be used to estimate orientation of chemical groups using Herman’s orientation factor:

f = (3cos²α - 1) / 2

Where α is the angle between the fibre axis and molecular direction.

Applications in Textiles:

• Quality control in fibre production
• Confirming recycled content (e.g., rPET vs virgin PET)
• Investigating failure in garments (e.g., breakdown of waterproof membranes)

Limitations:

• Surface-sensitive (especially in ATR mode)
• Overlapping peaks can complicate interpretation
• Not suitable for very opaque or reflective materials without special accessories

FTIR is highly valuable because it not only confirms what a fibre is made of, but also how it might have been modified, making it an indispensable tool in both research and industrial labs.

Related posts:

• AFIS and HVI in Textile Testing: Principles, Parameters, Applications, and Key Differences
• Yarn Count Explained: Linear Density Systems in Textiles 
• What is Tg, Tm, and Tc in Polymers: Explained with Definitions, Factors and Important aspects to remember