Influence of the pore size on optical and mechanical properties of ecoflex sponges

Silicone polymers have various applications such as sensors, energy harvesters, soft robotics, prosthetics and implants. Ecoflex has become one of the most remarkable silicone polymers due to its special mechanical properties which include extreme stretchability and biocompatibility. In recent years, interest in porous silicone elastomers has increased in order to improve the absorption, flexibility and chemical activity of the material by increasing the surface area. In this study, porous Ecoflex 00-30 silicone elastomer material was prepared by using the low-cost sugar molding technique and its mechanical, optical and surface properties were investigated. In addition, we report on the influence of pore size on these properties of Ecoflex. Our results demonstrate that the Ecoflex material, which has a porous structure, has a more flexible structure. We have observed that the Ecoflex’s stretchability increased with pore size, especially in the 420–850 μm range.


Introduction
Polymers have become indispensable in many areas of our daily lives.In recent years, with the development of flexible and wearable electronics, the need for flexible, lightweight, biocompatible polymer materials that can be adapted to the human body has also increased [1].Elastomers such as PDMS, polyurethane, Ecoflex and styrenebutadiene-styrene, known for their high flexibility, stretchability and resistance to significant deformation, are often chosen as a substrate or encapsulation material for wearable and flexible devices intended for use on the human body [2].Porous polymer materials have also attracted attention due to their large surface area, high absorption capacity, low weight and high chemical interaction with active materials [3].Direct templating technique (sugar templating etc), emulsion templating technique, gas forming technique, phase separation technique, and 3D printing can be used in the production of porous elastomers [4].Porous polymer materials find a place in many applications such as sensors [5], oil/gas adsorption [6,7], separation membranes [8], gas separation systems [9], electrochemical energy storage [10].
Ecoflex is a commercially available platinum-catalyzed silicone elastomer with various Shore hardnesses from 00-10 to 00-50 [11].Because Ecoflex is a material with a low modulus of elasticity and high stretchability, it can return to its original shape without plastic deformation, even when subjected to high stresses.Thanks to these properties, Ecoflex is used in many fields, such as epidermal electronic systems, human implants, stretchable strain sensors, tissue biomechanics and soft robotics [12][13][14][15][16]. Porous Ecoflex is used as a filling material in oil absorption applications [17] due to its high oil absorption capacity, in high-efficiency triboelectric generators [18] due to its stretchability and large surface area, and in flexible thermoelectric generators [19] due to its low thermal conductivity and flexibility.
Ecoflex can be produced in many sizes and shapes for different applications by using different molds.Ultrahigh sensitivity capacitive pressure sensors have been developed in the literature using dielectric layers of porous Ecoflex elastomer composites with conductive materials [20].In another study, the effect of the pore size of the porous Ecoflex used for the dielectric layer of the pressure sensor on the sensitivity of this sensor was investigated [5].They found that the sensors they obtained with a pore size of about 150-200 μm had a high sensitivity of about 1.096 kPa −1 in the pressure range of 0-60 Pa.Lu et al fabricated 3D porous Ecoflex with different diameters by paraffin microsphere templating and conducted optimization studies to increase the oil absorption capacity [17].It was found that the obtained porous structure exhibited strain values in the range of 50%-70% at low stress values.Zhao et al developed a triboelectric nanogenerator that can provide energy for self-powered sensors using a 180°bendable and deformation-resistant electrode composite material, including highly flexible Ecoflex and highly conductive porous carbon [21].They stated that the Ecoflex/porous carbon material they used for the generator had an elastic modulus of 0.06 MPa.Koshi et al fabricated a flexible device with high efficiency by keeping the temperature difference between the thermoelectric legs high in thermoelectric devices using ultra-stretchable, soft, porous Ecoflex with very low thermal conductivity and a low elastic modulus of 0.01 MPa [19].
In previous studies, to our knowledge, the authors have only focused on the manufacturing process of porous Ecoflex and its applications.There is a need to evaluate the optical, mechanical and thermal properties of this soft, stretchable material, which is used in many applications.Therefore, this study investigated the effect of the pore size of Ecoflex on the optical, mechanical and structural properties.Porous Ecoflex elastomers with different dimensions were produced by the simple and low-cost sugar templating method.

Materials
Ecoflex 00-30, a silicone elastomer with a Shore hardness of 00-30, was purchased from Smooth-on for experimental processes.Commercially available sugar granules were used as template.By sieve analysis, sugar granules were separated into three different ranges, which are 150-350 μm, 350-420 μm, 420-850 μm.

Preparation of ecoflex elastomers 2.2.1. Preparation of pristine ecoflex
Ecoflex was prepared by mixing part A (base) and part B (catalyst) of the Ecoflex precursors in a weight ratio of 1:1.The mixture was then poured into a mold for curing.Before the curing process, degassing is done in a vacuum oven to remove the air bubbles.The curing process was carried out for four hours at room temperature.The Ecoflex, once formed, was removed from the mold.

Preparation of porous ecoflex
To obtain porous Ecoflex, the low-cost sugar templating process was used, which is mostly used in the literature for the production of porous PDMS [22].The production steps for porous Ecoflex are shown in figure 1.First, parts A and B of Ecoflex (Ecoflex 00-30) were mixed in equal amounts.Then sugar granules up to four times the amount of Ecoflex were added and mixed.Sugar granules with sizes in the range of 150-350 μm, 350-420 μm and 420-850 μm were used to investigate the effect of pore size on the properties of the elastomer.The sugar granules were then poured onto a silicone mold and degassed in a vacuum for five minutes to remove the air bubbles in the mixture.The mixture was then cured for four hours at room temperature.
After the cured mixture was removed from the mold, it was immersed in deionized water for half an hour to dissolve the sugar trapped in it using a homogenizer with a probe.The sample was then dried for two hours at 60 °C in a vacuum oven.

Characterization
The morphology and cross-sections of the porous Ecoflex with different pore sizes were examined using a scanning electron microscope (SEM, Philips XL 30 SFEG) at an accelerating voltage of 5 kV.To characterize the absorbance and transmittance of pristine and porous Ecoflex samples, optical measurements were performed in the wavelength range of 200-1100 nm using a Shimadsu 3600i plus UV-Vis-NIR spectrophotometer.The Instron 5569 universal testing machine was used to carry out the compression test to determine the mechanical properties of the material.X-ray photoelectron spectroscopy was used to further investigate the surface composition of Ecoflex.DTA/TGA analysis were performed to evaluate the thermal properties using a differential thermal analyzer/thermogravimetric analyzer (Netzsch STA449 F3).Thermal analysis of pristine Ecoflex was made in the range of 50 °C-600 °C, at a rate of 5 °C min −1 .FTIR was used to determine molecular bonding and functional groups of Ecoflex sample.Analysis was performed between 400 cm −1 and 4000 cm −1 frequency ranges.

Results and discussion
3.1.Characterization of pristine ecoflex 3.1.1.Chemical analysis of ecoflex XPS was used to quantify the elemental composition of the surface of pristine Ecoflex. Figure 2 shows the XPS spectrum of Ecoflex 00-30.The wide survey scans for Ecoflex show the presence of Si, C and O elements.The elemental composition of the surface is shown in table 1.
Fourier-Transform Infrared (FTIR) Spectroscopy was employed to detect functional groups and decipher the backbone structure.The spectra is shown in figure 3. The absorption peak at around 2962 cm −1 is typically associated with the stretching vibrations of C-H bonds in CH 3 (methyl) groups.Weak peak at around 1400 cm −1 is belong to asymmetric Si-CH 3 stretching.The strong peaks at 787 cm −1 and 1257 cm −1 were attributed to the Si-C stretching vibrations of Si-(CH 3 ) 2 and Si-CH 3 , respectively.The peaks at 468 cm −1 , 692 cm −1 and 1010 cm −1 belong to Si-O-Si stretching in Ecoflex backbone.All peaks represent the characteristic features of the Ecoflex silicon elastomer, confirming our fabrication process successfully synthesized Ecoflex.

Thermal analysis of ecoflex
DTA/TGA analysis is very useful in determining phase transition temperature during decomposition of the sample.Herein, DTA/TGA was performed from 50 °C to 600 °C.The plots for Ecoflex show that thermal decomposition occurs in the range of 350 °C-420 °C.The initial weight loss at low temperatures is attributed to the evaporation of adsorbed water on the surface of the material.It also indicates that at a temperature higher than 520 °C, the Ecoflex is mostly decomposed.Figure 4 shows that Ecoflex 00-30 is thermally stable up to over 300 °C.

Mechanical analysis of ecoflex
Ecoflex is a highly soft and stretchable silicon-based elastomer.Thus, the samples could potentially slip from the grippers during measurement.Before measurement, both ends of samples are attached between two sandpapers   Stress-strain curve of Ecoflex was shown in figure 5(a).In the stress-strain graph of Ecoflex, we observe three distinct regions.

Initial linear region
The Ecoflex 00-30 silicone rubber, known for its flexibility, shows a high elongation at break without a clear yield point.A 'yield-like' behavior is approximated where the material transitions from linear to non-linear elasticity, around a strain value of 0.8 to 1.0.

Non-linear elastic region
With increasing strain, the material continues to stretch beyond the initial linear phase, indicating sustained elasticity.The rubber is capable of returning to its original shape unless the stress is maintained beyond this region.

Strain hardening region
Approaching failure, at approximately a strain value of 2.3, the curve's slope slightly alters, suggesting strain hardening, a characteristic of some silicones nearing their tensile limits.Subsequent to this change, the graph sharply declines, illustrating the point of material failure without displaying necking, consistent with the brittle fracture behavior often found in thermoset materials.
Young's modulus of pristine Ecoflex was measured about 0.1 MPa by linear fitting in the slope of the initial section of the curve.

Characterization of porous ecoflex 3.2.1. Optical properties of porous ecoflex 3.2.1.1. UV-Vis-NIR
Although Ecoflex is normally a material with high transparency, the material becomes opaque when the structure is made porous.Compared to pristine Ecoflex, a decrease in the transmittance of the material and an increase in the absorption of the material was observed due to the porous structure.The presence of air-filled pores in the polymer leads to a deviation in the refractive index.This deviation can lead to a scattering of light and thus to a reduction in transmittance.Porous Ecoflex, where the pore size was the largest (420-850 μm), showed the highest transmittance among the porous structures (figure 6).However, its transmittance was still significantly lower than that of the non-porous samples.The UV-Vis absorption spectrum of Ecoflex elastomers clearly shows that as the number of pores increases, the π-π * transitions shift to shorter wavelengths (higher energy), from 217 nm to 210 nm.This indicates a lower conjugation (table 2).
The small absorption peaks observed in the 900 nm to 1100 nm range could possibly be attributed to the overtones of the O-H or C-H stretching vibrations.Considering that the absorption peak around 910 nm is more pronounced in porous materials, it is plausible that this enhancement is due to the surface adsorption of moisture from the air.The porous structure of the material could facilitate the retention of moisture, leading to an increased intensity of O-H overtone absorption in this wavelength range.

Band gap calculation
Using the transmittance data, the optical bandgap energy E g of the samples can be determined by plotting the Tauc equation and extrapolating the straight-line part of the curve.Equation (1) shows the Tauc relationship: In the Tauc equation, α stands for the absorption coefficient, h for the Planck constant, A for the proportionality constant and ν for the frequency of the light.The value of n depends on the type of electronic transition of the material.
Figure 7 shows the Tauc plot of Ecoflex surfaces with different pore sizes.It can be seen from the curve that the optical band gap of the pristine Ecoflex surface is 4.90 eV.When the band gaps of Ecoflex with different pore sizes are examined, it is found that the band gap becomes smaller as the pore size decreases.The reduction of the band gap may be related to surface changes influenced by the size and morphology of the pores, leading to defects in the polymer chains.Table 2 shows the bandgap values for materials with different pore sizes.

Microstructure of the porous ecoflex
The Ecoflex materials were examined using a scanning electron microscope (SEM).Figure 8 shows the microstructural image of a cross-section of pristine Ecoflex (a) and porous Ecoflex, with pore sizes of approximately (b) 420-850 μm, (c) 350-420 μm and (d) 150-350 μm.The pores mainly have an irregular polygonal shape with numerous interconnected macropores.The pore size was found to vary depending on the size of the sugar granules used.The SEM images also show that as the size of the sugar granules decreases, the pore size decreases accordingly and the number of pores increases.
Using our SEM images, the pore size distribution in the structures was determined using open-source image analysis software (ImageJ).The pore sizes were measured, counting 25 pores to calculate the average pore diameter, and the pore size distributions were plotted (figure 9).We found that the average pore sizes were consistent with the sizes of the sugar granules used during the sugar templating process.

Mechanical properties of porous ecoflex
Cylindrical specimens with a diameter of 12 mm and a length of 20 mm were used for the compression tests.
Figure 10 shows that the strains of the Ecoflex sponge with a large pore size reach about 70%-80% even at very low stress values.The modulus of elasticity of the elastomers was determined by calculating the slope of the stress-strain curve from the strain value at 10%.The modulus of elasticity of the porous Ecoflex materials was calculated to be approximately 0.002 MPa, 0.006 MPa, 0.02 MPa and 0.1 MPa for Ecoflex materials with a pore size of approximately 420-850 μm, 350-420 μm, 150-350 μm and pristine Ecoflex, respectively (table 3).As the compressive stress increases, the pore spaces decrease and the material gradually begins to harden.When compared to pristine Ecoflex, it can be seen that a softer material can be achieved by producing a porous structure, as the pores facilitate deformation.Therefore, the Ecoflex material with large pores is a very good choice, especially for wearable and stretchable electronics that require high flexibility and softness.

Porosity calculation with Archimedes
The porosity of porous Ecoflex materials with different pore sizes is measured with the water immersion technique based on the Archimedean principle using the ASTM D792 test standard.Figure 11 shows the measurement mechanism.Rectangular porous Ecoflex materials with a size of 16 mm × 12 mm × 3.5 mm (length × width × thickness) were used for this measurement.The measurements were carried out on an electronic balance with an accuracy of ±0.0001 g.The liquid used was ethanol with a low density (0.789 g cm −3 ) instead of water, as our porous materials are lighter than water.The percentage porosity (%P) of the Ecoflex materials is calculated according to equation (2):  4), which means that the percentage of pore volume is linearly proportional to the pore size.

Conclusion
3D porous Ecoflex sponges have been successfully produced using a fast, cost-effective and environmentally friendly sugar template method.In all Ecoflex sponges with different pore sizes, the pores were interconnected.Therefore, we can predict that all sugar granules were dissolved during washing with deionized water.The porous Ecoflex sponge with the largest pores had the highest porosity level (61.26%) and showed the highest flexibility in the mechanical tests.These excellent mechanical properties of these sponges ensure their durability.Overall, these Ecoflex sponges are promising materials, especially for applications in soft electronics.

Figure 5 .
Figure 5. (a) Stress-Strain curve of Ecoflex (b) Image of Ecoflex at the initial length and stretched to about 250%.

Figure 6 .
Figure 6.(a) Absorbance and (b) Transmittance characteristics of pristine and porous Ecoflex with different pore sizes.

Figure 7 .
Figure 7. Tauc's plot to determine the optical band gap for Ecoflex with different pore sizes.
s r indicate the density of the porous material and the pristine material, respectively.Our measurements showed that the p r decreased with increasing pore size from 150-350 μm to 420-850 μm (table

Figure 10 .
Figure 10.Compressive strain stress curves for Ecoflex 00-30 with different pore sizes and measurement image.

Table 1 .
Elemantal compositions by XPS analysis of Ecoflex.
in case they slide out of the grippers.For uniaxial type tensile tests, three rectangular specimens with 80 mm × 16 mm × 0.7 mm (length × width × thickness) were used.

Table 2 .
Maximum absorption wavelengths and band gaps of porous Ecoflex with different pore sizes.

Table 3 .
Mechanical properties of porous Ecoflex materials with different pore sizes.