Enhanced Mechanical Properties and Fracture Resistance of a Gradient E-P-E Structure for Improved Device Stability

Through simulation analysis of the three groups of models, it is observed that the maximum stress in tension for both Ecoflex and PDMS models occurred at the notch, indicating that the notch is the stress concentration point and the weak point of the structure. Conversely, the maximum stress in the gradient structure is located in the middle of the model within the PDMS material area (Fig. 2a,b), indicating that the notch is not the stress concentration point. The cross-sections of the three sets of models exhibit different stress distribution trends along the direction of the arrow (Fig. 2c). Ecoflex shows less pronounced stress changes at positions far from the notch, with stress gradually increasing as the position gets closer to the notch. PDMS exhibits a similar stress change trend to Ecoflex, but with a greater increase in stress at positions close to the notch. In the gradient structure, the stress changes as the position approaches the notch: the stress in the Ecoflex zone at the starting point is nearly unaffected by the notch and changes very little, while there is a steep increase in stress in the PDMS zone. As the position enters the Ecoflex zone again, there is a sudden drop in stress followed by an increase towards the notch. The different stress distributions at the cross-sections of the three models represent varying degrees of stress concentration, with stress concentration factors of 2.769 for pure PDMS, 2.436 for pure Ecoflex, and 1.723 for the E-P-E structure at the notched cross-section. The E-P-E structure exhibits a 37.7% reduction in stress concentration compared to PDMS, indicating that it can significantly reduce stress concentration in the substrate. Therefore, the E-P-E structure can effectively mitigate stress concentration when the substrate is damaged.

The material properties exhibited by Ecoflex and PDMS under standard tensile and tear conditions differ significantly (Fig. 2d-f). Ecoflex outperforms PDMS in terms of elongation at break (800%) and tear displacement (110 mm), but has lower tensile strength (1.05 MPa). On the other hand, PDMS exhibits higher tensile strength (2.34 MPa) but displays a large gap in elongation at break (140%) and tear displacement (12 mm) compared to Ecoflex. Under 100% strain, PDMS maintains a constant tensile stress of 1.24 MPa, while Ecoflex maintains a constant tensile stress of 0.08 MPa. This indicates that PDMS has a 15.5 times higher ability to maintain its original form within 100% strain compared to Ecoflex. The low rigidity of single Ecoflex makes it difficult to fix larger rigid electronic components, hampering device stabilization. Additionally, the modulus of elasticity of Ecoflex is much smaller than that of PDMS, and the difference in modulus of elasticity between Ecoflex and rigid components is even larger, leading to increased stress concentration and jeopardizing the device's service life.

In comparison to PDMS and Ecoflex, the E-P-E structure exhibits superior mechanical properties. The E-P-E structure demonstrates a 43% increase in elongation, a 4.75-fold increase in tear force, and an 8.69-fold increase in tear displacement compared to pure PDMS. Furthermore, it displays a 9.5-fold increase in tear force, a 9.25-fold increase in constant tensile stress at 100% strain, and a 5.85-fold increase in elastic modulus compared to single Ecoflex, with the modulus of elasticity increasing by a factor of 5.85.

Microscopic images of crack tip extension in PDMS with Ecoflex material were observed using an optical digital microscope (Fig. S1a-b). The crack tip of Ecoflex exhibits a distribution of numerous ligaments extending parallel to the load direction. As the crack opens, the ligaments develop inward, with material at the edge of the fossa pooling to form ligaments, ultimately creating ligamentous structures surrounded by fossae. The crack tip of Ecoflex displays dense and homogeneous ligaments and fossae, with the alternating growth of ligaments and fossae reducing the rate of crack propagation and inhibiting crack growth. This demonstrates Ecoflex's excellent ability to resist crack propagation [40]. In contrast, PDMS exhibits larger ligament fossae and fewer ligaments at the crack tip, indicating a poorer ability to resist crack extension.

In this study, the fracture surfaces of the specimens were observed using a scanning electron microscope, and the fracture morphology of PDMS and Ecoflex in the E-P-E structure showed significant differences, with a clear intersection line (Fig. 2g). The fracture morphology in the PDMS area exhibits large irregular fracture defects, with clear texture visible at low magnification. The fracture surface also displays numerous tiny cracks, oriented longitudinally. In the Ecoflex area, the fracture morphology appears rougher, with a large number of irregular cracks. The fracture morphology of single PDMS (Fig. 2h) exhibits a small amount of ligament rupture at the initial stage of fracture, but the fracture surface becomes smoother as the cracks propagate. The fracture morphology of single PDMS significantly differs from that of the PDMS zone in the gradient structure, indicating different fracture behaviors. The fracture behavior of single PDMS involves standard edge prefabricated crack extension leading to overall sample fracture, whereas the fracture of the PDMS zone in the gradient structure exhibits the growth of internal defects in the material, ultimately resulting in fracture. This behavior differs from the extension of the edge crack, suggesting that the fracture of the PDMS zone originates from material defects rather than crack extension of Ecoflex. The fracture morphology of single Ecoflex (Fig. 3d) shows no significant difference compared to that of the Ecoflex zone in the gradient structure, with both exhibiting a large number of irregular cracks. This is because the fracture of the internal PDMS zone transmits cracks to the Ecoflex zone. The external load generated at the moment of fracture is much larger than what the Ecoflex can bear, causing cracks in the Ecoflex zone to propagate and leading to fracture. Therefore, the fracture is caused by cracks in the PDMS zone, where zone crack extension induces fracture, similar to fracture caused by the extension of prefabricated edge cracks. Thus, it can be initially concluded that the fracture behavior of the gradient structure involves the PDMS zone reaching its elongation limit and fracturing, subsequently leading to fracture in the Ecoflex zone.