Fumed Silica-Mediated Adhesion Evolution in Aged PDMS/Aluminum Interfaces

Silicone rubber is important in engineering elastomers because it combines the flexibility of an organic polymer with the thermal and electrical stability associated with a siloxane backbone. Poly(dimethylsiloxane), commonly known as PDMS, is used in coatings, gaskets, pressure-sensitive adhesives, sealing layers, cushioning structures, tapes, and electrically insulating components. In these applications, however, PDMS rarely functions as an isolated bulk material. It is usually placed in direct contact with a rigid substrate, often a metal or a metal oxide, where the performance of the assembled device depends not only on the properties of the rubber itself but also on the stability and strength of the buried interface. This interfacial region is important when PDMS is used in environments involving heat, pressure, mechanical loading, or long-term contact with solid supports. A joint between silicone rubber and aluminum, for example, is not simply a physical contact between a soft polymer and a hard surface. It is a chemically and structurally evolving boundary where polymer chains, surface hydroxyl groups, adsorbed water, and possible filler particles may all influence adhesion. During curing and thermal aging, these molecular arrangements can change, leading to gradual rearrangement of the interface. Understanding this process is essential because failure in PDMS/metal assemblies can originate from interfacial structural changes rather than from obvious damage to the bulk rubber. Industrial PDMS formulations are commonly reinforced with rigid fillers which is another challenge. Fumed silica is widely used to improve the mechanical strength and aging resistance of silicone rubber, but its role at PDMS/metal interfaces is less direct and less well understood. A filler added to strengthen the rubber matrix may also migrate, adsorb, or interact with the solid substrate. It may alter the balance between hydrophobic PDMS contact and hydrophilic bonding with oxide surfaces. However, without molecular-level evidence, it is difficult to determine whether fumed silica just reinforces the bulk PDMS phase or also contributes to adhesion at the buried interface.

In a recent research paper published in Colloids and Surfaces A: Physicochemical and Engineering Aspects, Dr. Yang Hu, Professor Zhaohui Xu, and Professor Xiaolin Lu from Southeast University working together with Dr. Bowen Dai and Dr. Hongbing Chen from China Academy of Engineering Physics, developed an experimentally integrated model for tracking how fumed silica reinforced PDMS forms and evolves at aluminum-related interfaces during thermal aging. The technically distinct feature is the correlation of SFG-detected buried interfacial molecular structure with SEM, EDX, XPS, lap shear, and peeling measurements on aluminum substrates. They showed that fumed silica is initially enriched at the aluminum interface, later migrates into the PDMS matrix, and still contributes to stronger adhesion through a combined hydrophobic and hydrophilic interfacial interaction.

The authors used sum frequency generation vibrational spectroscopy to monitor molecular structures at model PDMS/sapphire and fumed silica reinforced PDMS/sapphire interfaces, with sapphire serving as an α-aluminum oxide model surface. The choice of SFG mattered because the central question was not simply whether PDMS adhered more strongly after aging, but how the buried interface reorganized at the level of methyl, methylene, siloxane-related, water, and surface hydroxyl vibrations. Alongside this molecular probe, the researchers used scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy to follow morphology and elemental composition at aluminum interfaces after removal of the visible rubber layer. Lap shear and peeling tests then linked these interfacial observations to mechanical adhesion.

For unfilled PDMS on sapphire, the SFG spectra changed substantially after thermal aging. The initially weaker CH signals became stronger, with features assigned to CH2, CH3, and Si-CH3 groups becoming more prominent. At the same time, the OH region shifted in a way the study interprets as loss of interfacial water and formation of a more hydrophobic PDMS-rich environment near the sapphire surface. This point is important because it gives molecular meaning to aging: heating does not just harden or condition the rubber, but brings more PDMS chains into intimate contact with the oxide surface. The interface becomes less like a water-influenced contact and more like a polymer-occupied boundary. The filled material followed a related but not identical path. In fumed silica reinforced PDMS, the cured interface showed weak CH signals, while aging again produced stronger PDMS-related vibrational features. The OH analysis added the more distinctive observation. Although hydrogen-bonding signatures decreased with aging in both filled and unfilled systems, the relative contribution of hydrogen-bonded surface OH remained higher for the fumed silica reinforced PDMS than for the unfilled PDMS after aging. In other words, both interfaces became more hydrophobically contacted by PDMS, but the filled interface retained a stronger hydrophilic interaction component. The material feature responsible for that difference was the presence of silica particles, whose surface chemistry could interact with the aluminum oxide-related surface before and during interfacial rearrangement.

The team performed microscopy and elemental analyses  and found at the unfilled PDMS/aluminum interface, the uncovered aluminum surface gradually lost its original visible veins and defects after aging, consistent with the development of a residual PDMS layer that could not be removed by solvent treatment. At the fumed silica reinforced PDMS/aluminum interface, numerous silica particles were initially observed on the substrate side, but their visible number decreased with aging. After longer aging, the apparent particles largely disappeared from the exposed surface, while irregular wrinkles suggested that the residual interfacial layer still contained PDMS and a small number of silica particles. EDX showed that aluminum dominated the raw and early-aged surfaces, while silicon became dominant after longer aging, consistent with increasing PDMS coverage. XPS sharpened this picture: silicon and carbon increased with aging, aluminum and oxygen decreased, and the PDMS-related C-Si-O contribution became dominant in the filled system, indicating that fumed silica particles gradually migrated away from the interface into the PDMS matrix. They also found that both PDMS and fumed silica reinforced PDMS showed increasing lap shear and peeling strength with aging. For lap shear, unfilled PDMS increased from about 1.00 MPa in the cured condition to 6.42 MPa after five days of aging, while the filled PDMS increased from about 1.29 MPa to 9.60 MPa. The peeling response showed an even larger difference after aging: unfilled PDMS reached 7.57 MPa, whereas the filled material reached 26.63 MPa. The interpretation offered by the authors is careful and chemically coherent. Aging increases hydrophobic contact between PDMS and aluminum-related surfaces in both materials, while fumed silica adds a residual hydrophilic contribution through hydrogen-bonding interactions. The stronger adhesion of the filled joint therefore reflects the combined action of PDMS contact and silica-related interfacial bonding.

The engineering value of the authors’ findings is clearest for silicone rubber components that must remain bonded to aluminum or aluminum oxide surfaces during thermal exposure.  This matters for devices exposed to heat during curing, service, storage, or accelerated aging because the final interfacial strength may depend strongly on the thermal history of the assembled part. In fumed silica reinforced PDMS, silica particles are initially enriched near the aluminum surface, then migrate away from the interface during aging as PDMS increasingly covers the substrate. Even after this migration, the filled system retains stronger hydrogen-bonding contribution than unfilled PDMS. Practically, the findings support the use of fumed silica as a bulk reinforcement agent, and also as an interfacial design variable in silicone-metal assemblies. The work also suggests that formulation engineers should consider filler distribution, oxide-surface interactions, and aging conditions together.