David E. Kranbuehl

Monitoring phase separation and reaction advancement in situ in thermoplastic/epoxy blends

Abstract Frequency-dependent dielectric measurements have been used to monitor and characterize the phase separation process in high-performance thermoplastic–thermoset blends of 2,6-dimethyl-1,4-phenylene ether (PPE) with an epoxy diglycidylether of bisphenol A (DGEBA) and 4,4-methylene bis(3-chloro-2,6-diethylamine) (MCDEA). The systems studied are 30, 45 and 60% PPE/DGEBA–MCDEA blends. The results are compared with dynamical mechanical measurements and the developing morphology. Both dielectric and mechanical measurements are shown to be good techniques to monitor the phase-separation process and the reaction advancement. Dielectric measurements monitor the buildup in T g in both the PPE-rich continuous phase and in the epoxy-rich occluded phases. Dielectric measurements are advantageous as they can be made in situ continuously on a single sample throughout the entire cure process. The results show that the phase separation process initially occurs rapidly involving a large amount of the epoxy–amine diffusing into the occluded phase. The rate of the epoxy–amine reaction in the epoxy-rich 30% PPE mixture is approximately equal to that in the neat epoxy–amine system due to two opposing effects, a slower reaction rate due to dilution and a lower level of conversion at vitrification due to the presence of high T g PPE. In the 60% PPE mixture, the dilution effect of the PPE has a large affect on the decreasing the reaction rate and achievement of vitrification. The continuous thermoplastic-rich phase is observed to vitrify first, followed by vitrification of the thermoset as occluded particles. Finally, the results show as evidenced by the size of the occluded particles and the composition of the continuous phase that the morphology is strongly influenced by the kinetics, diffusion, and viscosity conditions during phase separation.

Is the Maxwell–Sillars–Wagner model reliable for describing the dielectric properties of a core–shell particle–epoxy system?

Abstract Dielectric measurements have been performed on neat acrylic core–shell particles, and core–shell modified-epoxy networks in order to assess the accuracy of the Maxwell–Wagner–Sillars (MWS) model in describing the dielectric properties of heterogeneous polymeric systems. Measured and predicted interfacial polarizations are compared. The temperature dependence of the frequency of maximum interfacial dielectric loss process is well described by the MWS model and from the temperature dependence of the conductivity of the two phases. However, the measured and predicted absolute values differ and a distribution in the interfacial relaxation time is observed. This is attributed to the effect of a gradient in concentration across the interface between the acrylic particles and the epoxy matrix. It is thus concluded that the nature and properties of interphases can strongly influence the dielectrical properties and interfacial polarization processes in polymer blends, and that theoretical models which do not take into account details of interfacial charge transfer might not be reliable in describing the morphology of heterogeneous polymeric systems.

Dielectric properties of the polymerization of an aromatic polyimide

Abstract Dielectric measurements made over a wide range of frequency are shown to provide a sensitive means for observing the chemistry of the polymerization process of an aromatic polyimide continuously as the monomer mixture undergoes major changes in viscosity leading to a fully cured, insoluble, crosslinked thermoset. Dynamic dielectric measurements of the polymerization of the high temperature polyimide prepared from a mixture of BTDE, MDA, NE and designated LaRC-160 have been made at a series of frequencies from 5 to 5 × 10 6 Hz. The capacitance ( C ), dissipation ( D ), and the conductivity ( G ) together with time and temperature were recorded. The complex permittivity (e∗) has been determined throughout the cure cycle. Using their frequency dependence, e′ and e″ have been separated into their ionic and dipolar components. The time and temperature dependence of the dielectric data are correlated with the previously reported imidization cure chemistry of the LaRC-160 system (ref. 1). The high temperature crosslinking reaction has also been observed dielectrically and compared with published viscosity and reaction data. Changes in the time-temperature properties of this polyimide system as a function of resin age over a period of six months have also been studied. In addition, the effect of the resins composition on the time-temperature cure properties of the polymer has been examined using dynamic dielectric measurements.

Relaxation of the aspherical shapes of random‐coil polymer chains

The conformations of random‐coil polymer chains are known to be appreciably more aspherical than might have been expected intuitively. However, this instantaneous asphericity will only affect the measured physical properties of flexible chains in solution if the relaxation of the asphericity requires a time longer than the inherent sampling time of the experiment. It is therefore important, in the analysis of phenomena which depend upon chain shape, to know the time scale over which deviations from spherical symmetry persist. In this paper we extend our previous work on the relaxation of asphericity in chains without excluded volume interactions and present corresponding results for chains with excluded volume. Autocorrelation functions for the radius of gyration squared, its principal components, and the moments of inertia are determined using a dynamical model for linear polymer chains which simulates excluded volume effects. The results suggest that the asphericity of a random‐flight chain is increased...

Continuous dielectric measurement of polymerizing systems

Abstract Frequency-dependent dielectric measurement techniques made over many decades, Hz to MHz, can provide a convenient automated technique for characterizing the polymerization reaction in thermoset polymers. The frequency dependence of the complex permittivity is used to simultaneously separate and measure the relaxation and diffusion parameters governing dipolar and ionic mobility. The quantitative relationship of the viscosity and degree of cure during polymerization to the ionic and dipolar mobility is discussed and analyzed in relation to the gel point and the Vogel-Tammann-Fulcher (VTF) equation. An interdigitated planer electrode permits measurement of the variation in the rate of polymerization in situ at various positions in the system due to exothermic and heat transfer effects.

Simulation of the relaxation of random‐coil polymer chains by lattice models with excluded volume: The effect of bead movement rules

Earlier work showed that excluded volume constraints lengthen the long relaxation times of lattice‐model polymer chains using single‐bead elementary motions by a factor proportional to about the first power of chain length. This paper reports the extension of this work to chains employing double‐bead elementary motions and to chains employing random mixtures of both kinds of motion. For pure two‐bead motion, the chain‐length dependence of the lengthening of relaxation time is essentially identical with that previously obtained for pure single‐bead motion. For models using both kinds of move, this chain‐length dependence is suppressed at shorter chain lengths. However, for chains longer than about 30 beads, contrary to the prediction of Deutch and others, the lengthening of the relaxation time by excluded volume again becomes proportional to about the first power of chain length. Other studies that have not exhibited this first power dependence have either examined relaxation from highly nonequilibrium con...

Dielectric measurements and computer simulation of the effect of confinement on the glass transition temperature

Abstract Dielectric relaxation experiments on the relaxation of five glass forming systems have been measured in porous glass with 4.0 nm diameter pores. An off lattice Monte Carlo model has been used to simulate glass transition behavior of oligomer chains in a confined space. Both the experimental and simulation results support the experimental differential scanning calorimetry results that show fixed confinement decreases the glass transition temperature. We suggest that the results show that confinement by fixed walls can reduce the cooperative length scale and corresponding vitrification, α-relaxation time.

Moisture-Curing Kinetics of Isocyanate Prepolymer Adhesives

The reaction between isocyanate-terminated prepolymers and atmospheric moisture produces urea linkages and results in a hydrogen-bonded network of linear high molecular weight polymers with adhesive properties. This study describes the synthesis of isocyanate-terminated prepolymers and investigates the use of in situ infrared spectroscopy as a technique for monitoring the chemistry of the polymerization reaction kinetics. In situ FTIR was successfully used as a means to monitor the residual isocyanate levels and the extent of the polymerization reaction. Frequency dependent dielectric sensing (FDEMS) using a thin, planar sensor has been used to monitor the reaction kinetics by monitoring changes in the mobility of ions in the reacting medium. A direct correlation of the extent of prepolymer cure was found using the normalized FTIR isocyanate absorbance spectrum and FDEMS imaginary permittivity at 500 Hz for the duration of the cure cycle. The results of this investigation demonstrate that FDEMS is an effective online method to monitor the extent of moisture cure in the bulk as well as in a coating or adhesive bondline.

Direct Measurement of the Interfacial Attractions between Functionalized Graphene and Polymers in Nanocomposites

The objective of this work is to characterize and thereby achieve a fundamental scientific understanding at the molecular level of the relationship of the inter molecular forces measured at the nano scale between an individual nano particle and a polymer system. In this paper, we developed a method to directly characterize and measure the relative strength of the interfacial attractive forces between graphene nano particles and the polymer on the nano length scale of an individual particle using AFM techniques. The method was successfully applied to study the interfacial attractive strength in graphene polymer nano‐composites. Two polymers poly(methyl methacrylate) (PMMA) and polyvinyl alcohol (PVA) were studied. The results support that this method is capable of characterizing the interfacial attractive forces directly at the nano‐scale of graphene with a macromolecule material. The method can be applied to other nano particle macromolecular systems allowing determination of the interaction strength betw...

Ion mobility time of flight measurements: effect of experimental parameters on measurements in a non-hydrogen bonded system

Abstract An in situ measurement technique that isolates the mobility of charge carriers is described and analyzed. The technique allows significant improvement over conductivity measurements to monitor changes in the physical properties and state of a material as it cures. This is essential in systems where N i , the number of charge carriers, cannot be assumed constant such as during cure of epoxies, urethanes and polyimides. Currently, there is an assumption made in the literature that the number of charge carriers present in a curing material is constant when conductivity is used as an in situ measurement technique to monitor changes in mobility (and thereby viscosity). This assumption is widely used, for example when dielectric conductivity measurements are correlated with changes in properties such as viscosity. Ion mobility, time of flight (ITOF) measurements, which are described here, are an appropriate technique to isolate and measure particle mobility due to changes in the state of the material. Furthermore, the ITOF technique, coupled with the measurement of σ , the dielectric conductivity, allows one to measure separately changes in the mobility and the number of charge carriers due to curing or changes in temperature. This is possible since conductivity is the product of the number of charge carriers and their mobility. Length of pulse, strength of applied field, sensor geometry, and temperature/viscosity are examined to determine optimum parameters of measurement for a simple, non-curing system: dimethacrylate of tetraethoxylated bisphenol A (D121). This paper seeks to show that with changes in viscosity, the pulse length and magnitude of voltage in relation to the distance between the electrodes should be varied to obtain accurate ITOF information on the changing mobility.