Fabricating Microcracks in SBS-g-MAH/CB Composites to Improve Conductivity and Small Strain-Sensing Sensitivity

Microcracks were generated by a freezing–thawing process, by which maleic anhydride-grafted styrene–butadiene–styrene copolymer/carbon black (SBS-g-MAH/CB) composites prepared by melt-blending were repeatedly frozen in liquid nitrogen and thawed in ethyl alcohol. The effect of freezing–thawing on the conductivity and strain-sensing behaviors of the composites was investigated. Scanning electron microscopy (SEM) results showed that freezing–thawing caused microcracks to form between CB aggregates and the matrix due to asymmetric shrinkage and expansion. The microcracks decreased the strength of the interaction between the CB and SBS-g-MAH of the composites, decreasing the glass transition temperature. The effect of the number of freezing–thawing cycles on the conductivity of SBS-g-MAH/CB composites was investigated. The results showed that freezing–thawing improved the conductivity and that the conductivity increased as the number of cycles increased. After 20 repeated freezing–thawing cycles, the conductivity of the composites with 15 wt.% CB increased by 50 times. In strain and dynamic strain tests, freezing–thawing improved the strain sensitivity of the composites. For composites with 25 wt.% CB, the sensitivity increased by 10 times after freezing–thawing. A simple application showed that composites that have undergone freezing–thawing can be applied as sensors to detect the extension and flexion of the index finger.