Molecularly Bridged Gradient Interphases via Hyperbranched-Polyamide- Functionalized Porous CNT Networks in Glass-Fiber/Epoxy Composites

Technical Summary: Molecularly Bridged Gradient Interphases via Hyperbranched-Polyamide-Functionalized Porous CNT Networks

Problem Statement
The mechanical performance and service reliability of fiber-reinforced composites (FRCs) are critically limited by the intrinsic modulus mismatch between rigid fibers and compliant resin matrices. This mismatch generates steep stress gradients within the interphase, leading to interfacial debonding, microcrack initiation, and compromised load-transfer efficiency. Conventional surface modification strategies, such as silane coupling or plasma treatment, often improve adhesion only at the immediate fiber surface, failing to establish a continuous mechanical transition across the interphase. Furthermore, while the deposition of nanofillers like carbon nanotubes (CNTs) can redistribute stress, purely physical interactions or abrupt transitions between the nanofiller layer and the matrix can still serve as preferential sites for damage initiation, particularly under cyclic or severe loading.

Methodology
This study proposes a multiscale interfacial engineering strategy to construct a continuous gradient interphase on glass fibers (GF). The methodology involves a sequential three-step fabrication process:

1. Surface Activation and Silanization: Glass fibers were treated with piranha solution to activate hydroxyl groups, followed by modification with a silane coupling agent (KH550) to introduce amino functional groups.
2. Porous CNT Network Construction: A porous carbon nanotube (pCNT) network was deposited on the fiber surface using a sacrificial template method (PS microsphere/CNT hybrid coating followed by etching). This created a 3D scaffold with abundant carboxyl groups and mechanical interlocking sites.
3. HBP Functionalization: An amino-terminated hyperbranched polyamide (HBP) was covalently grafted onto the pCNT network via EDC/NHS-mediated amination. In this step, the amino groups of the HBP reacted with the carboxyl groups of the CNTs to form stable amide bonds, while the flexible HBP branches and remaining reactive groups extended toward the epoxy matrix.

The resulting modified fibers (GF-pCNT-H) were incorporated into epoxy composites. The study employed a combination of experimental characterization and molecular dynamics (MD) simulations:

• Characterization: Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) for morphology and modulus mapping, X-ray Photoelectron Spectroscopy (XPS) and FT-IR for chemical analysis, and Thermogravimetric Analysis (TGA) for grafting quantification.
• Mechanical Testing: Single-fiber tensile tests with Weibull analysis, microdroplet debonding tests for Interfacial Shear Strength (IFSS), and Transverse Tensile Tests (TFBT) for macroscopic composite performance.
• Simulation: MD simulations using LAMMPS and the PCFF force field were conducted to analyze interfacial component overlap, fractional free volume (FFV), binding energies, and tensile failure evolution.

Key Contributions and Results
The study successfully demonstrated that the integration of a rigid porous CNT scaffold with a flexible, reactive HBP creates a chemically coupled and mechanically continuous gradient interphase. Key findings include:

• Structural and Chemical Evolution: The modification increased surface roughness by 42.2% relative to the pCNT-coated fibers (reaching 21.9 nm). XPS and FT-IR confirmed the covalent anchoring of HBP to the CNT network via amide bonds, while retaining reactive terminal groups for interaction with the epoxy matrix.
• Modulus Gradient Broadening: AFM modulus mapping revealed that the interphase transition zone thickness increased from 0.787 µm (unmodified) to 2.763 µm (GF-pCNT-H). This indicates a significantly smoother modulus transition, mitigating the abrupt mechanical jump between fiber and matrix.
• Mechanical Performance Improvements:
  • Single-Fiber Strength: The tensile strength of the modified fibers increased by 66.1% (from ~1136 MPa to ~1886 MPa), with improved strain at break and mechanical reliability (higher Weibull modulus).
  • Interfacial Shear Strength (IFSS): The IFSS increased by 97.1% compared to unmodified fibers, reaching 69.60 MPa.
  • Transverse Mechanical Properties: The transverse tensile strength of the composites improved by 73.6%, while toughness increased from 12.8 to 39.3 MJ m⁻³.
• Failure Mechanism Shift: Fracture morphologies indicated a transition from brittle interfacial debonding (in unmodified fibers) to mixed-mode failure involving matrix tearing and crack deflection in the modified composites, signifying enhanced energy dissipation.
• Molecular Insights: MD simulations revealed that the GF-pCNT-H system exhibited the largest interfacial component overlap and the strongest interfacial binding energy (dominated by electrostatic interactions). The HBP branches facilitated molecular interpenetration and chain entanglement with the epoxy network, delaying tensile failure and allowing for greater deformation (failure strain increased by 91.4% in simulations).

Significance
The paper claims that this work provides a scalable molecular-bridging strategy for designing high-performance FRCs. By transforming a nanofiller interlayer governed mainly by physical interactions into a chemically coupled gradient interphase, the study achieves simultaneous improvements in load-bearing capability and damage tolerance. The synergistic combination of rigid CNT support (for mechanical interlocking and stress redistribution) and flexible HBP bridging (for stress relaxation and molecular continuity) effectively alleviates the fiber/matrix modulus mismatch. This approach offers a practical pathway to overcome the limitations of traditional interfacial engineering, enabling the development of composites with superior strength, deformability, and long-term reliability.