Enhanced Tear Resistance of Gradient Structures: Combining PDMS and Ecoflex for Superior Fracture Properties

The maximum strain at fracture of single PDMS in the presence of a notch was only 26 percent, whereas the maximum strain at fracture of single Ecoflex was as high as 234 percent. The maximum strain at fracture of the gradient structure was increased by a factor of 6.5 compared to single PDMS. The maximum fracture strain of the gradient-structured specimen is concentrated around 190%. Under this strain condition, the prefabricated cracks in the Ecoflex material only form a ring at the tip, and crack extension has not yet occurred. Based on the morphological changes of the fracture surface, it can be determined that the fracture process of the E-P-E specimen involves PDMS reaching its maximum elongation rate and fracturing first, followed by fracture in Ecoflex. However, the maximum fracture strains of the gradient structure exceed the maximum elongation of PDMS under the standard tensile test (140%, Fig. 2d). This is because the Ecoflex in the gradient structure encapsulates the edge of the PDMS, reducing microcracks at the edge of PDMS and increasing its maximum elongation.

PDMS exhibits a weak ability to inhibit crack extension and has the lowest fracture energy during fracture, while Ecoflex benefits from an ultra-high fracture strain (234%), which exceeds that of PDMS (Fig. 3d). The maximum fracture energy of the gradient structure is significantly higher than that of single PDMS or Ecoflex, reaching a maximum value 74.5 times that of PDMS and 18.6 times that of Ecoflex, demonstrating excellent tear resistance. In the gradient structure, the fracture energy decreases as the percentage of PDMS volume decreases. The PDMS inner through-hole is subjected to 86% higher forces and 54% higher tensile strains in the presence of Ecoflex coating compared to the non-Ecoflex-coated condition. This can be attributed to the reduction of microcracking at the edges of the PDMS inner through-hole by Ecoflex.

Single PDMS tears at a very small displacement (11.5 mm), while single Ecoflex exhibits a lower tear force than PDMS but a greater tear displacement (Fig. 3b). Compared to single PDMS, the gradient structure shows higher tear strength (Fig. 3e), reaching a maximum value of 16.4 KN/m. This is a 7-fold increase in tear strength compared to PDMS and a 22-fold increase compared to Ecoflex. The prefabricated cracks are located in the external Ecoflex zone, and in the initial stage of tearing, no crack extension occurs in the Ecoflex zone, while in the PDMS zone, no cracks are produced and there is significant resistance to external loads. Applying a greater force is necessary to increase tear displacement and open up the cracks. As tear displacement increases, the crack gradually expands, and the force applied to the tear gradually increases. When the displacement reaches the maximum tear displacement of the Ecoflex material, the crack extends to the PDMS material, resulting in the fracture of the specimen as a whole. As the proportion of PDMS material decreases, the force and tear strength during tearing decrease, while the tear displacement does not change significantly. This is because the inhibition of cracking primarily depends on the Ecoflex material, and once the crack extends to the PDMS region, the specimen immediately fractures.