Roll-to-Roll Process Optimization for Development of Magnetic Field Assisted Large-Area Anisotropic Piezoresistive Pressure Sensors

Processing strategies are fundamental to the development of advanced functional polymers, serving as the link between their microstructure and macroscopic performance. Within the framework of the structure–processing–property paradigm, external force fields such as thermal, mechanical, magnetic, electrical, or optical stimuli can be incorporated during the processing of polymers/polymer composites to fine-tune their morphology and achieve targeted functionality within the material system. This work investigates two such strategies: (i) magnetic field-assisted alignment, and (ii) uniaxial mechanical deformation, to understand the underlying physical mechanisms that govern structural evolution and performance in two distinct polymer systems.

In the first study, an optimized processing window was developed for roll-to-roll processing of magnetic field assisted alignment of Ni particles in the Z-direction of silicone elastomer films. Since the films experience an evolving temperature and magnetic fields along the web path (MD), offline characterization techniques were used to quantify particle alignment kinetics and viscosity behavior as a function of Ni fractions and temperatures. Real-time visualization during in-plane (X-Y plane) alignment and out-of-plane light transmission experiments informed that higher magnetic field strengths produce well-defined and through-thickness columns with no long-ranged lateral connectivity within 20 sec. Isothermal gelation kinetic parameters obtained via Arrhenius model indicated higher Ni fraction results accelerated curing reactions. Non-isothermal rheokinetics revealed an initial reduction in viscosity prior to gel-point which enhances particle mobility and promotes column formation. To highlight the role of residence time, the effect of line speed on final column morphology is investigated where faster speeds (and/or lower temperatures) produce backward-tilted columns, while slower speeds (and/or higher temperatures) promote forward tilt. Finally, to demonstrate the scalability of this magnetic field alignment process, a 17-ft. long and 6-in. wide piezoresistive film was produced with uniform column distribution.

The second study utilizes the anisotropic film fabricated via roll-to-roll processing to demonstrate its ability as a piezoresistive sensor. The tunability of the film over a broad stress range (up to 1.2 MPa) was evaluated through systematic electromechanical testing under uniaxial compression mode. Strain rate-dependent measurements confirmed responsive piezoresistive performance under dynamic conditions of varying loading-unloading rates, while long term stability and mechanical robustness was demonstrated with cyclic loading over 100 cycles. Competitive benchmarking demonstrated that the Z-aligned sensors provide a broad operational range spanning 3 orders of magnitude, with response time of less than 100ms. To demonstrate high-resolution sensing capabilities, a 15 × 15 cm sensor with an array of 100 discrete sensing points on one contiguous large-area film was fabricated with real-time spatial mapping capability. The inherently anisotropic micro-columnar morphology was leveraged to effectively suppress cross-talk and provide highly localized pressure detection on one unitary film.

In the third chapter, we shift focus towards a different class of polymer. Here, semicrystalline polyvinyl alcohol (PVA) films with high degree of H-bonding (98% DH) were produced using solution casting technique. Drying-induced morphological changes in the solution-cast films were monitored by tracking the real-time drying behavior at various controlled process parameters. Higher air flow speed, lower air temperatures, lower thickness, low solids concentration, and higher molecular weight of PVA resulted in a higher out-of-plane birefringence, signifying a higher degree of optical anisotropy in the films. Processing induced structural evolution was systematically investigated by real-time mechano-optical tests at various temperatures by stretching films in uniaxial tensile mode. Birefringence vs. true stress showed a characteristic 3-regime behavior (photoelastic, regime I, and regime II). A linear dependance of strain-optical behavior was observed with increasing temperatures signifying an efficient conversion of macroscopic deformation into microscopic strain as measured by birefringence. SAXS and WAXD characterization tools were utilized to reveal that the crystalline lamella break-up accompanied by minimal orientation when the films were stretched at solid-state temperatures. Stretching at elevated temperatures below melting resulted in higher elongation to break and higher crystalline orientation levels due to a balance in the weakening of H-bonds and enhanced segmental mobility of the chains. This behavior changes when the stretching temperature is within the partially molten region, where long-range interconnectivity is further weakened by partially melting smaller crystals. This also accompanies thermal re-crystallization during deformation. Partial melting, in combination of weakening of H-bonds, results in smaller elongation levels.