Decoding Elasticity Build-Up and Network Topology in Free-Radical Cross-Linking Polymerization: A Combined Experimental and Atomistic Approach

peer reviewedThe range of applications involving free-radical cross-linking processes has grown impressively over the last decades. However, in numerous fields where tightly cross-linked materials are required, the network development and its relation to the elastic properties are still a gray zone, as it is particularly challenging to design experiments that would allow validating the predictions from (often phenomenological) theoretical models or numerical simulations. Here, we report on a successful attempt to align time-resolved infrared-rheology measurements with fully atomistic simulations over the whole conversion range of an important acrylic free-radical cross-linking polymerization, unveiling the different regimes behind the elasticity build-up upon double bond conversion. Our combined experimental-theoretical approach provides an original insight into the various stages of the polymer network growth, from the earliest initiation up to final conversion. More specifically, we show that the pregel and gel formation stages are driven by the formation of branched polyfunctional polymers, which link together toward a sample-spanning network at the gel point. This regime proceeds in the postgel stage until the spatial heterogeneity in the cross-link density vanishes, leaving dangling ends as residual structural defects that then gradually connect to close the network. Following a steep transition at ultimate conversion, the elastic modulus of the network reaches the value predicted by the rubber elasticity theory in the affine limit

Modeling the formation and thermomechanical properties of polybenzoxazine thermosets

peer reviewe

Modeling the Formation and Thermomechanical Properties of Polybenzoxazine Thermosets

peer reviewe

Do carbon nanotubes improve the thermomechanical properties of benzoxazine thermosets

peer reviewe

Tuning the Piezoresistive Behavior of Graphene-Polybenzoxazine Nanocomposites: Toward High-Performance Materials for Pressure Sensing Applications

peer reviewedFlexible piezoresistive pressure sensors are key components in wearable technologies for health monitoring, digital healthcare, human-machine interfaces, and robotics. Among active materials for pressure sensing, graphene-based materials are extremely promising because of their outstanding physical characteristics. Currently, a key challenge in pressure sensing is the sensitivity enhancement through the fine tuning of the active material’s electro-mechanical properties. Here, we describe a novel versatile approach to modulating the sensitivity of graphene-based piezoresistive pressure sensors by combining chemically reduced graphene oxide (rGO) with a thermally responsive material, namely, a novel trifunctional polybenzoxazine thermoset precursor based on tris(3-aminopropyl)amine and phenol reagents (PtPA). The integration of rGO in a polybenzoxazine thermoresist matrix results in an electrically conductive nanocomposite where the thermally triggered resist’s polymerization modulates the active material rigidity and consequently the piezoresistive response to pressure. Pressure sensors comprising the rGO-PtPA blend exhibit sensitivities ranging from 10-2 to 1 kPa-1, which can be modulated by controlling the rGO:PtPA ratio or the curing temperature. Our rGO-PtPA blend represents a proof-of-concept graphene-based nanocomposite with on-demand piezoresistive behavior. Combined with solution processability and a thermal curing process compatible with large-area coatings technologies on flexible supports, this method holds great potential for applications in pressure sensing for health monitoring

Positive effect of functional side groups on the structure and properties of benzoxazine networks and nanocomposites

peer reviewe

Robust and Direct Route for the Development of Elastomeric Benzoxazine Resins by Copolymerization with Amines

The development of soft pressure sensors, in particular electronic skin, is fundamental to the interfacing between the human body and the outside world, namely, in prosthetics and biomedical applications. In this context, hybrid composite materials incorporating electrically conducting 2D flakes in an insulating matrix show attractive tunable piezoresistive properties suitable for wide-range pressure sensing applications. Here, we report on the design of novel trifunctional benzoxazine precursors for this polymer matrix based on tris(3-aminopropyl)amine and phenol reagents. These precursors have been successfully synthesized and copolymerized with polyetheramines of different lengths to tune the thermomechanical properties of the resulting networks. Extensive molecular dynamics simulations unambiguously relate the changes in glass transition temperature with chemical composition to the variations in the cross-link density and provide Tg values in excellent agreement with the experimental data. With the longest polyetheramine (2000 g mol–1), we achieve the synthesis of an elastomeric benzoxazine exhibiting remarkably low Tg of −41 °C, a modulus in compression of 50 kPa, and a shear strain modulus of 300 Pa, with high potential for low-pressure sensing applications

Do Carbon Nanotubes Improve the Thermomechanical Properties of Benzoxazine Thermosets?

Fillers are widely used to improve the thermomechanical response of polymer matrices, yet often in an unpredictable manner because the relationships between the mechanical properties of the composite material and the primary (chemical) structure of its molecular components have remained elusive so far. Here, we report on a combined theoretical and experimental study of the structural and thermomechanical properties of carbon nanotube (CNT)–reinforced polybenzoxazine resins, as prepared from two monomers that only differ by the presence of two ethyl side groups. Remarkably, while addition of CNT is found to have no impact on the glass-transition temperature (<i>T</i>_g) of the ethyl-decorated resin, the corresponding ethyl-free composite features a surge by ∼47 °C (50 °C) in <i>T</i>_g, from molecular dynamics simulations (dynamic mechanical analysis measurements), as compared to the neat resin. Through a detailed theoretical analysis, we propose a microscopic picture for the differences in the thermomechanical properties of the resins, which sheds light on the relative importance of network topology, cross-link and hydrogen-bond density, chain mobility, and free volume

Do Carbon Nanotubes Improve the Thermomechanical Properties of Benzoxazine Thermosets?

Fillers are widely used to improve the thermomechanical response of polymer matrices, yet often in an unpredictable manner because the relationships between the mechanical properties of the composite material and the primary (chemical) structure of its molecular components have remained elusive so far. Here, we report on a combined theoretical and experimental study of the structural and thermomechanical properties of carbon nanotube (CNT)–reinforced polybenzoxazine resins, as prepared from two monomers that only differ by the presence of two ethyl side groups. Remarkably, while addition of CNT is found to have no impact on the glass-transition temperature (<i>T</i>_g) of the ethyl-decorated resin, the corresponding ethyl-free composite features a surge by ∼47 °C (50 °C) in <i>T</i>_g, from molecular dynamics simulations (dynamic mechanical analysis measurements), as compared to the neat resin. Through a detailed theoretical analysis, we propose a microscopic picture for the differences in the thermomechanical properties of the resins, which sheds light on the relative importance of network topology, cross-link and hydrogen-bond density, chain mobility, and free volume

Tuning the Piezoresistive Behavior of Graphene-Polybenzoxazine Nanocomposites: Toward High-Performance Materials for Pressure Sensing Applications

Flexible piezoresistive pressure sensors are key components in wearable technologies for health monitoring, digital healthcare, human–machine interfaces, and robotics. Among active materials for pressure sensing, graphene-based materials are extremely promising because of their outstanding physical characteristics. Currently, a key challenge in pressure sensing is the sensitivity enhancement through the fine tuning of the active material’s electro-mechanical properties. Here, we describe a novel versatile approach to modulating the sensitivity of graphene-based piezoresistive pressure sensors by combining chemically reduced graphene oxide (rGO) with a thermally responsive material, namely, a novel trifunctional polybenzoxazine thermoset precursor based on tris(3-aminopropyl)amine and phenol reagents (PtPA). The integration of rGO in a polybenzoxazine thermoresist matrix results in an electrically conductive nanocomposite where the thermally triggered resist’s polymerization modulates the active material rigidity and consequently the piezoresistive response to pressure. Pressure sensors comprising the rGO-PtPA blend exhibit sensitivities ranging from 10–2 to 1 kPa–1, which can be modulated by controlling the rGO:PtPA ratio or the curing temperature. Our rGO-PtPA blend represents a proof-of-concept graphene-based nanocomposite with on-demand piezoresistive behavior. Combined with solution processability and a thermal curing process compatible with large-area coatings technologies on flexible supports, this method holds great potential for applications in pressure sensing for health monitoring