Viscoelastic Polymer Nanocomposites for Strain Sensing Applications

Abstract

Strain sensors are a technology that have developed in parallel with the major milestones in material science over the last century. Most recently, the field of 2D materials has proven to have found a wide range of applications with strain sensors being no exception. With the field having entered adolescence through the development of scalable means of production, the prospect of the next generation of nanomaterial based strain gauges for emerging areas of technology such as wearables and flexible electronics is being looked at in earnest. In this work we examine polymer nanocomposite materials as a candidate for the next generation of low-cost, highly sensitive strain sensing technology. Initially, we look at graphene embedded in a soft, viscoelastic polymer matrix which has previously been demonstrated to display high gauge factors - though the exact relationship between matrix properties and the high observed gauge factors are not yet known. We have developed a scalable method of synthesising the graphene-polysilicon composite which enables us to control the gauge factor of the sensing material as a function of the molecular weight and degree of cross-linking in the polysilicon as well as the curing reaction time - allowing for the development of device specific sensing materials. We identify the processing conditions required to produce a material with gauge factors >100 and establish a well defined relationship between composite viscosity and sensitivity which shows maximised gauge factors for viscosities ca. 4×10^5 Pa·s. In addition to this we examine the relationship between conductivity and gauge factor which displays an inverse power law dependence consistent with literature.

While high gauge factors and reasonable conductivity are the standard metrics for determining the viability of new strain sensing devices, little consideration has been given to application specific, real-world performance related metrics. In this regard, we examine our material under a number of additional headings which look to characterise the material's performance in terms of its sensing range, degree of mechanical and electrical hysteresis and rate/frequency dependence of sensitivity. These additional headings are particularly relevant for conductive, composite based strain sensors as the mechanical properties of the composite are intimately linked to its embedded conductive filler phase, and by extension, its sensitivity. We characterised our viscoelastic composite material under these additional headings as a means of determining its viability as a practical, real-world strain sensor. Here it was found that owing to the weak, non-covalent crosslinks present in the matrix phase of the composite, the material was highly susceptible to permanent deformation at relatively low strains. The mobile nature of the flakes within the soft polymer matrix resulted in an accurate sensing range of < 1% and high degrees of electrical hysteresis were present during cycling. In addition to this, a time dependent sensitivity was observed due to the continuous relaxation of the graphene network within the soft polymer as well as the material being highly strain rate dependent - demonstrating a 20 fold increase in sensitivity over a 5 fold increase in strain rate. This highlights the need for further characterisation of sensing materials simply outside of gauge factor.

To address these issues, polymer nanocomposite based inks were developed using the viscoelastic graphene silicone composite as a base. The ink was printed into patterned thin films using three different methods of deposition (i) Spraying (ii) Screen Printing (iii) Aerosol-jet deposition onto thin, flexible PDMS substrates. A partial graphene-polymer phase segregation occurs during printing which results in a ×10^6 increase in thin film conductivity when compared to the bulk material base. The resultant mechanical properties are dominated by the elastomeric substrate which largely suppresses hysteresis and completely removes the previously observed strain rate and frequency dependence. This demonstrates the potential of printed polymer nanocomposite based materials to be used in high-performance strain sensing applications. Three applications are investigated focusing on the use of printed wearable sensors for pulse measurement, dysphagia diagnosis and low-signal vibration sensing.