Real-time quantification of network growth of epoxy/diamine thermosets as a function of cure protocol

Traditionally, understanding of thermoset cure has been limited to the analysis of a single degree of cure value obtained via techniques such as dynamic scanning calorimetry. Such analyses limit the scope of understanding of network development during cure. The continued development of rapid cure matrix chemistries necessitates the advancement of analytical techniques capable of quantifying how thermal cure profiles influence crosslinked network architectures throughout cure. In this work, the formation of epoxy/diamine networks was studied, in real time, throughout cure with Fourier Transform Infrared Spectroscopy in the near infrared region (NIR). The NIR technique allows for direct quantification of all functional groups directly involved in the cure of aerospace matrices. This work establishes a means to view a complete picture of the development of epoxy/diamine networks throughout cure, which allows for a more complete understanding of the effect of cure protocol on final network structure

Real-Time Quantification of Etherification Reactions During Cure and Postcure of Epoxy/Diamine Networks

Traditional understanding of the progress of thermoset cure is limited to a single degree of cure value related to relative results obtained in techniques such as dynamic scanning calorimetry (DSC). In this work, the development of epoxy/diamine networks was monitored, in real time, throughout cure with Fourier Transform Infrared Spectroscopy in the near infrared region (NIR). Networks cured with difunctional (DGEBF) or tetrafunctional (TGDDM) epoxy monomers were also compared in order to determine the effect of chemical gelation on final network formation. The NIR technique allows for direct quantification of functional groups directly involved in the cure of aerospace matrices. It was determined that the etherification could be monitored through the out of step consumption of epoxide and amine functional groups. This allows for the abundance of etherification throughout cure to be correlated to cure protocol. Molar absorptivity was determined to be dependent on temperature and was adjusted during NIR analysis to allow for more representative results of functional group consumption and overall network conversion. The accuracy of this method to measure network conversion was validated by use of DSC. This work establishes a means to view a complete picture of the development of epoxy-amine networks throughout cure, which allows for a more complete understanding of the effect of cure protocol on final network structure

Analyzing the Effect of Thermal Ramp Rate On Epoxy Network Formation During Cure Using Real Time Infrared Spectroscopy

Full understanding of network formation in an epoxy/diamine matrix has previously not been achieved due to a limitation in directly studying the creation and consumption of secondary amine during cure. In this work, the development of epoxy/diamine networks was monitored, in real time, throughout cure with Fourier Transform Infrared Spectroscopy in the near infrared region (NIR) and the effect of ramp rate studied. Networks were heated at slow and rapid ramp rates and held at 180°C for 2 hours with functional group consumption monitored throughout. Networks cured with difunctional or tetrafunctional epoxy monomers were also compared in order to determine the effect of chemical gelation on final network formation. Molar absorptivity was determined to be dependent on temperature and was adjusted during analysis to allow for more representative results of functional group consumption and overall network conversion. The accuracy of this method to measure network conversion was validated using dynamic scanning calorimetry. This work establishes a means to view a complete picture of the development of epoxy-amine networks throughout cure, which allows for a more complete understanding of the effect of cure protocol on final network structure

Highly Tunable Thiol-Ene Networks via Dual Thiol Addition

Throughout the past decade, investigations of thick thermoset thiol-ene networks (TENs) have become increasingly prominent in the literature due to facile, quantitative synthesis giving rise to unique network characteristics, specifically high mechanical energy damping. This article reports the synthesis and thermomechanical properties of ternary thiol-thiol-ene systems that exhibit tunable glass transitions that maintain high, narrow tan delta values in the glass transition region. We begin with a base network of a trifunctional thiol and a trifunctional ene and then systematically substitute the trifunctional thiol with a series of difunctional thiols while maintaining stoichiometric balance between total thiol and ene content. The resultant ternary networks exhibit glass transition temperatures that follow the Fox equation. In contrast to other ternary thiol-ene networks, we observe minimal broadening of the glass transition region, which implies that we can retain the energy-absorbing capabilities of the thiol-ene system. This approach has high potential as a simple tool for scientists and researchers to tune T(g)s for select networks without detrimentally affecting other physical properties

3D Printing of Dual-Core Benzoxazine Networks

A novel 3D printing formulation based on a multifunctional benzoxazine (BOX) monomer possessing both photo and thermally polymerizable functional groups is reported. Printing formulation viscosity is readily tuned using a monofunctional-acrylate reactive diluent to enable Stereolithography (SLA) 3D printing. In the primary curing step, the printing formulation is UV-cured by SLA 3D printing to prepare accurate parts on the millimeter size scale. The 3D printed parts are then heated in the secondary curing step to activate a thermally initiated BOX ring opening polymerization. Dynamic mechanical analysis demonstrated that the 3D printed parts exhibit a single Tan δ peak after both the primary UV-cure and secondary thermal cure steps, suggesting the two polymerizations behave as one crosslinked network. The unique dual-cure strategy demonstrated in this research utilizes both photo and thermally initiated polymerizations to expand the library of materials available for 3D printing applications

Impact Properties of Thiol-Ene Networks

In this study, a series of thiol–ene networks having glass transition temperatures ranging from −30 to 60 °C were synthesized utilizing several multifunctional thiols and two trifunctional alkenes. Thermomechanical properties were determined using dynamic mechanical analysis, and impact properties were determined using pendulum impact and drop impact testing protocols. The impact behavior was found to directly correlate to the glass transition temperature, except when the temperature at which the impact event occurs overlaps with the range of temperatures corresponding to the viscoelastic dissipation regime of the polymer. Additionally, we discuss insight into the spatial limitations of energy dissipation for thiol–ene network polymers and establish a platform for predictability in similar systems

Impact Properties of Thiol–Ene Networks

In this study, a series of thiol–ene networks having glass transition temperatures ranging from −30 to 60 °C were synthesized utilizing several multifunctional thiols and two trifunctional alkenes. Thermomechanical properties were determined using dynamic mechanical analysis, and impact properties were determined using pendulum impact and drop impact testing protocols. The impact behavior was found to directly correlate to the glass transition temperature, except when the temperature at which the impact event occurs overlaps with the range of temperatures corresponding to the viscoelastic dissipation regime of the polymer. Additionally, we discuss insight into the spatial limitations of energy dissipation for thiol–ene network polymers and establish a platform for predictability in similar systems