Beyond the Microscale: Early Nanoscale Changes in Experimental Resin Composites Exposed to Acidic and Alkaline Environments

The study investigates the effects of acidic, neutral, and alkaline environments on the micro- and nanoscale properties of experimental BisGMA/TEGDMA resin composites with varied filler loading. Experimental composites based on BisGMA:TEGDMA at 75:25 wt% with three silica-glass filler loadings (75, 40, and 0 wt%) were stored at 37 °C in phosphate-buffered saline (PBS; pH 7.2), citric acid (pH 3.8), or sodium hydroxide (NaOH; pH 11) for up to 28 days. Microscale changes were assessed using sorption, Vickers hardness, and scanning electron microscopy (SEM), while atomic force microscopy (AFM) was used to evaluate nanoscale surface roughness and Young’s modulus. Storage medium did not significantly affect percentage mass increase (p = 0.092) or Vickers hardness (p = 0.098). Higher filler loading was associated with lower mass increase (ρ = −0.902, p < 0.001) and greater hardness (r = 0.660, p < 0.0001). SEM revealed surface degradation, particularly at the filler–matrix interface following NaOH exposure. Nanoscale surface roughness was negatively associated with filler loading (ρ = −0.368, p = 0.001), with changes observed predominantly in unfilled materials after 28 days. AFM measurements demonstrated a shift towards lower Young’s modulus values after only one day of exposure to citric acid and NaOH, whereas the reduction in PBS occurred later. Acidic and alkaline exposure produced limited alterations in conventional microscale measures but more changes in surface morphology and nanoscale properties. These exploratory findings suggest that AFM-derived Young’s modulus may detect early softening following acidic and alkaline exposure, suggesting its potential utility for characterizing early changes associated with chemical ageing than conventional microscale measurements.