Quantitative mapping of surface elastic moduli in silica-reinforced rubbers and rubber blends across the length scales by AFM

Abstract

The surface elastic moduli of silica-reinforced rubbers and rubber blends were investigated by atomic force microscopy (AFM)-based HarmoniX material mapping. Styrene-butadiene rubbers (SBR) and ethylene-propylenediene rubbers (EPDM) and SBR/EPDM rubber blends with varying concentrations of silica nanoparticles (0, 5, 10, 20, 50 parts per hundred rubber, phr) were prepared to investigate the effect of different composition on the resulting morphology, filler distribution and elastic moduli of a specific rubber or rubber blend sample. For SBR, the elastic modulus values varied from 0.5 MPa for unfilled SBR to 5 MPa for 50 phr reinforced SBR with the increase in the concentration of filler. For EPDM, the corresponding values increased from 1.4 MPa for unfilled EPDM to 4.5 MPa for 50 phr reinforced EPDM. Local stiff and soft domains in silica-reinforced SBR and EPDM rubbers and rubber blends were identified by HarmoniX AFM imaging. While the stiff silica particles show modulus values as high as 2 GPa, the rubber matrix reveals modulus values in the range of ca. 30 MPa for the rubber blends to ca. 300 MPa for the unfilled rubbers. The lower value of elastic modulus of the EPDM phase in the blend, compared to the blank EPDM compound can be attributed to the presence of Sunpar oil in the compound which has a very good affinity with EPDM and decreases the rubber modulus. The elastic moduli maps revealed an increase of the areal fraction of silica particles showing an intrinsic surface modulus value with rising silica content in the compound preparation mixture. HarmoniX AFM measurements revealed the formation of larger silica aggregates in EPDM in contrast to SBR where isolated silica particles were observed. For silica-reinforced rubber blends a phase separation into a soft (ca. 40 MPa) and a significantly harder phase could be observed (ca. 500 MPa-1.5 GPa) indicating the incorporation of silica particles in the SBR phase. Using HarmoniX AFM imaging significantly higher surface elastic moduli were observed compared to those obtained by bulk tensile testing. Possible reasons for the observed differences between bulk modulus values and those measured by AFM are discussed in detail, including the aspect of different averaging procedures like inherent to surface probing by AFM versus bulk tensile testing, different filler distributions in SBR and EPDM and the AFM modulus calibration procedures.

Existence of a tribo-modified surface layer of BR/S-SBR elastomers reinforced with silica or carbon black

“…Isolated particle morphologies are easily detectable. AFM nano-indentation results show a higher modulus of elasticity for isolated particles than for the rubber matrix, as seen elsewhere [40][41][42]. Moreover, the mechanical response of the isolated particles did not show any significant difference when changing the loading rate during indentation, suggesting that the isolated particles are not viscoelastic.…”

Structural, mechanical, and electrical properties of carbon nanoparticles synthesized from diesel

“…6(c) that the size of a single CNP is approximately 3.8 nm. The elastic property of a material is important for the stability of materials and its device fabrication, especially for nanomaterials with potential application, because the elastic property of a material determines the stability and reliability of the devices (39,40). In this context we measured structural morphology of carbon nanoparticle thin film and its corresponding elastic modulus (DMT modulus) simultaneously.…”

Morphological and local mechanical surface characterization of ballistic fibers via AFM

“…In addition to (and in conjunction with) the caveats noted above, the uncertainty is also affected by the ca. 30 percent error inherently involved in the AFM technique …”