Structural Characterization Analysis of Carbon Nanotubes After Oxidation for Acrylic Resin Composite

Oxidation treatment is a critical modification step that enhances the dispersion and interfacial bonding of carbon nanotubes (CNTs) in acrylic resin matrices. This article provides a comprehensive overview of the structural characterization techniques—including FTIR, Raman spectroscopy, XPS, SEM, TEM, TGA, and XRD—used to verify successful oxidation and evaluate the properties of oxidized CNT-acrylic resin composites.

Technical Overview

Carbon nanotubes (CNTs) are widely incorporated into acrylic resin systems to impart electrical conductivity, mechanical reinforcement, and barrier protection in coatings, adhesives, and films. However, pristine CNTs possess chemically inert, hydrophobic graphite-like surfaces dominated by strong van der Waals forces and π-π stacking interactions, which cause severe aggregation and poor compatibility with polar polymer matrices such as acrylic resin.

Oxidation treatment—typically performed using concentrated nitric acid, mixed sulfuric/nitric acid, hydrogen peroxide, ozone, or plasma—introduces oxygen-containing functional groups (carboxyl –COOH, hydroxyl –OH, and carbonyl C=O) onto the CNT sidewalls and tube ends. These functional groups improve wettability, enable chemical interaction with the ester and carboxyl groups of acrylic resin, and significantly enhance dispersion stability. Structural characterization after oxidation is essential to confirm successful functionalization, quantify defect generation, assess structural integrity, and verify compatibility with the acrylic resin matrix.

Testing Samples

Pristine Carbon Nanotubes

Untreated multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) serve as the reference baseline. These samples retain their original graphite crystalline structure, closed end caps, and minimal surface oxygen content, allowing direct comparison of spectral and morphological changes induced by oxidation.

Oxidized Carbon Nanotubes

CNTs treated under controlled oxidation conditions, such as reflux in concentrated nitric acid (e.g., 68% HNO₃ at 120°C for 2–8 hours) or mixed acid (3:1 H₂SO₄/HNO₃), are the primary analytical target. Samples prepared under different oxidation durations, temperatures, and oxidant concentrations may be compared to establish structure-property relationships.

Acrylic Resin/CNT Nanocomposites

Composite films, coatings, or cast specimens containing 0.1–5 wt% oxidized CNTs dispersed in acrylic resin are characterized to evaluate dispersion quality, interfacial adhesion, and the influence of functionalization on the final composite performance.

Testing Items

A complete post-oxidation characterization protocol typically includes the following analytical objectives:

• Identification and quantification of oxygen-containing surface functional groups introduced during oxidation
• Evaluation of defect density and graphitic crystallinity changes in the CNT lattice
• Determination of elemental composition, surface oxygen content, and chemical state distribution
• Observation of morphological changes, including tube shortening, end-cap opening, and amorphous carbon removal
• Assessment of thermal stability and functional group decomposition behavior
• Verification of dispersion uniformity and interfacial bonding within the acrylic resin matrix
• Measurement of changes in hydrophilicity and surface energy relevant to polymer compatibility

Testing Methods

Fourier Transform Infrared Spectroscopy (FTIR)

FTIR directly confirms the introduction of functional groups. Oxidized CNTs exhibit characteristic absorption peaks at approximately 1720–1740 cm⁻¹ (C=O stretching of carboxyl groups), 3400–3450 cm⁻¹ (O–H stretching), 1050–1200 cm⁻¹ (C–O stretching), and 1580–1620 cm⁻¹ (C=C stretching of the graphitic backbone). The appearance and intensification of carbonyl and hydroxyl bands relative to pristine CNTs provide direct evidence of successful oxidation.

Raman Spectroscopy

Raman analysis quantifies structural disorder using the D band (~1350 cm⁻¹, associated with defects and sp³-hybridized carbon) and the G band (~1580 cm⁻¹, associated with ordered sp² graphitic carbon). The intensity ratio (ID/IG) increases after oxidation—typically from approximately 0.8–1.0 for pristine CNTs to 1.1–1.5 or higher—indicating the generation of defect sites where functional groups attach. Excessively high ID/IG ratios signal over-oxidation and excessive damage to the tubular structure.

X-Ray Photoelectron Spectroscopy (XPS)

XPS provides quantitative surface chemical information. The O/C atomic ratio increases markedly after oxidation, often from below 3% for pristine CNTs to 8–20% for oxidized samples. High-resolution C1s spectra deconvoluted into components at approximately 284.6 eV (C–C), 285.5–286.5 eV (C–O), 287–288 eV (C=O), and 288.5–289.5 eV (O–C=O) confirm the specific bonding configurations and relative abundance of each functional group.

Scanning and Transmission Electron Microscopy (SEM/TEM)

Electron microscopy reveals morphological evolution. TEM images of oxidized CNTs frequently show opened end caps, shortened tube lengths, roughened sidewalls, and cleaned surfaces resulting from the removal of amorphous carbon and metal catalyst residues. SEM examination of acrylic resin fracture surfaces reveals whether oxidized CNTs are uniformly dispersed and embedded within the matrix or remain as aggregates, providing visual evidence of improved interfacial compatibility.

Thermogravimetric Analysis (TGA)

TGA evaluates thermal stability and functional group loading. Oxidized CNTs typically show an initial weight loss below 150°C (adsorbed moisture) and a distinct decomposition step between 200–500°C corresponding to the loss of carboxyl and hydroxyl groups. The main oxidation degradation temperature of the graphitic framework is often slightly reduced compared with pristine CNTs, and the magnitude of the low-temperature weight loss correlates with the degree of functionalization.

X-Ray Diffraction (XRD)

XRD verifies the preservation of the graphitic crystal structure. Both pristine and oxidized CNTs display the characteristic graphite (002) diffraction peak near 2θ ≈ 26° and the (100) peak near 2θ ≈ 43°. Retention of these peaks after oxidation, with only minor changes in intensity and peak width, confirms that mild oxidation modifies the surface without destroying the intrinsic crystalline core of the nanotubes.

Applications

Structurally characterized oxidized CNT-acrylic resin composites enable a wide range of high-performance applications:

• Conductive and antistatic acrylic coatings for electronics packaging, cleanroom facilities, and explosion-proof environments
• Anticorrosion protective coatings where uniformly dispersed CNTs create tortuous diffusion pathways and enhance barrier performance
• Electromagnetic interference (EMI) shielding coatings for communication equipment and aerospace components
• Mechanically reinforced acrylic adhesives, sealants, and structural films with improved tensile strength and toughness
• UV-curable and waterborne acrylic systems benefiting from the enhanced colloidal stability of functionalized CNTs
• Flexible sensors and smart coatings exploiting the piezoresistive response of well-dispersed CNT networks

FAQ

Does oxidation damage the structure of carbon nanotubes?

Mild, controlled oxidation primarily attacks defect sites and tube ends, introducing functional groups while preserving the graphitic core, as confirmed by retained XRD peaks and moderate ID/IG increases. Excessive oxidation, however, can severely shorten tubes, fragment sidewalls, and reduce mechanical and electrical performance, so oxidation conditions must be optimized and verified through characterization.

Why does oxidation improve CNT dispersion in acrylic resin?

The carboxyl and hydroxyl groups introduced during oxidation reduce surface hydrophobicity and weaken inter-tube van der Waals attraction. These polar groups also interact favorably with the ester, carboxyl, and hydroxyl functionalities in acrylic resin, promoting hydrogen bonding and stronger interfacial adhesion.

What ID/IG ratio indicates successful but not excessive oxidation?

An ID/IG ratio between approximately 1.1 and 1.3 after oxidation generally indicates sufficient functionalization with acceptable structural integrity. Ratios exceeding 1.5–2.0 typically suggest over-oxidation and significant degradation of the conjugated structure.

Which characterization method best quantifies the degree of functionalization?

XPS is the most quantitative method because it directly measures the surface O/C atomic ratio and resolves individual chemical states. TGA complements XPS by quantifying the thermally labile functional group content through low-temperature weight loss.

How much oxidized CNT loading is typically used in acrylic resin?

Most acrylic resin applications utilize loadings between 0.1 and 3 wt%. Because oxidized CNTs disperse more effectively, percolation thresholds for electrical conductivity are often achieved at lower loadings than with pristine CNTs, frequently below 0.5 wt%.