Graphene oxide has emerged as a significant reinforcement material in polymer composites due to its unique combination of mechanical, thermal, and barrier properties. Unlike pristine graphene, graphene oxide contains oxygen functional groups such as hydroxyl, epoxy, and carboxyl groups, which enhance its dispersibility in polymer matrices. These functional groups also facilitate interfacial interactions between the filler and the polymer, leading to improved load transfer and mechanical performance.

The mechanical strength of polymer composites reinforced with graphene oxide is influenced by several factors, including the degree of oxidation, dispersion quality, and interfacial bonding. Studies have shown that even low loadings of graphene oxide, typically between 0.5 to 5 wt%, can lead to substantial improvements in tensile strength and Young's modulus. For example, in epoxy-based composites, an addition of 1 wt% graphene oxide has been reported to increase tensile strength by up to 40% and modulus by 60%. The enhancement is attributed to the high aspect ratio of graphene oxide sheets and their ability to form a strong interface with the polymer through covalent or hydrogen bonding.

Dispersion remains a critical challenge in maximizing the reinforcing efficiency of graphene oxide. Agglomeration of sheets due to van der Waals forces can lead to stress concentration points, reducing composite performance. Several techniques have been employed to achieve uniform dispersion, including solution mixing, in-situ polymerization, and melt blending. Solution mixing is particularly effective for polar polymers, where graphene oxide can be exfoliated in solvents such as water or dimethylformamide before mixing with the polymer. Ultrasonication and high-shear mixing further aid in breaking up aggregates. In-situ polymerization allows for covalent bonding between graphene oxide and the polymer matrix, improving stress transfer. Melt blending, though less effective for achieving exfoliation, is industrially preferred for its scalability.

Barrier properties of polymers are significantly enhanced by the incorporation of graphene oxide. The impermeable, high-aspect-ratio sheets create a tortuous path for gas and moisture diffusion, delaying permeation. Research indicates that a well-dispersed 2 wt% graphene oxide loading can reduce oxygen permeability by over 50% in polyvinyl alcohol films. The barrier improvement factor depends on the alignment of sheets; composites processed with techniques like layer-by-layer assembly or mechanical stretching exhibit superior performance due to increased sheet orientation parallel to the film surface.

Reduced graphene oxide, produced by the chemical or thermal reduction of graphene oxide, exhibits different reinforcement behavior. The removal of oxygen groups restores electrical conductivity and enhances intrinsic mechanical properties, but at the cost of reduced dispersibility in polar polymers. Composites with reduced graphene oxide often require functionalization or surfactant assistance to prevent aggregation. Mechanically, reduced graphene oxide can outperform graphene oxide in certain systems due to its higher stiffness and restored sp² carbon network. However, the lack of functional groups weakens interfacial adhesion, sometimes leading to lower tensile strength despite higher modulus.

Thermal reduction of graphene oxide within a polymer matrix has been explored as a means to balance dispersibility and performance. Partial reduction after composite fabrication retains some oxygen groups for dispersion while improving electrical and thermal conductivity. However, excessive reduction can lead to reaggregation, diminishing mechanical benefits.

Comparative studies between graphene oxide and reduced graphene oxide composites highlight trade-offs. While reduced graphene oxide provides superior electrical properties, graphene oxide excels in enhancing barrier performance and maintaining dispersion stability. The choice between the two depends on the target properties of the composite.

In summary, graphene oxide serves as an effective reinforcement for polymer composites, offering improvements in mechanical strength and barrier properties. Dispersion techniques play a pivotal role in determining composite performance, with solution-based methods providing the most uniform distributions. Reduced graphene oxide, though mechanically superior in some aspects, presents challenges in dispersion and interfacial adhesion. Future advancements in functionalization and processing may further optimize the use of both materials in polymer nanocomposites.