Jovan Mijovic

The effect of stoichiometry and thermal history during cure on structure and properties of epoxy networks

Abstract An investigation was carried out into the effect of amine/epoxy stoichiometry and thermal history during cure on physical and mechanical properties of epoxy networks. The formulation studied consisted of a diglycidyl ether of bisphenol A epoxy resin and 4,4′-diaminodiphenyl sulfone curing agent. The experimental matrix was based upon three amine/epoxy ratios and seven different thermal histories during cure. Techniques used included dynamic mechanical and fracture analysis, and Fourier transform infra-red (FTi.r.) spectroscopy. The highest glass transition temperature (Tg) was observed in the stoichiometric formulation and the lowest in the epoxy-rich mixture. For a given stoichiometry, the value of Tg, infinity was not a function of thermal history during cure except, interestingly, in the case when the initial temperature was 180°C. The highest rubbery state modulus and the lowest average molecular weight between crosslinks were also found in the stoichiometric formulation. Our findings were rationalized in terms of the varying degrees of crosslinking in different networks. The opposite trend was observed in the glassy state at 20°C, where the lowest flexular modulus belonged to the stoichiometric formulation. An explanation for those results was offered in terms of the free volume concept. FTi.r. analysis established clearly the existence of residual epoxy groups in all formulations, even after post-cure. Etherification reaction between epoxy and hydroxyl groups takes place during post-cure, but a complete conversion of epoxy groups cannot be attained owing to the topological constraints within the three-dimensional network in the later stages of cure. This finding is of particular significance in mechanistic kinetic models based upon the absolute value of epoxy concentration at all stages of cure.

Correlation between nodular morphology and fracture properties of cured epoxy resins

Various bulk epoxy resin formulations, based on diglycidyl ether of bisphenol A (DGEBA) and cured with diethylene triamine (DETA) were studied. Methods of linear elastic fracture mechanics were employed and all systems were characterized by the corresponding values of the critical strain energy release rate for crack initiation and crack arrest. Fracture morphology was studied by scanning electron microscopy and transmission electron microscopy of carbon—platinum surface replicas. An apparent correlation between morphology and ultimate mechanical properties has been found. All fracture surfaces are shown to be characterized by distinct nodular morphology. Nodules, ranging in size from 15–45 nm, represent the sites of higher crosslink density in an inhomogeneous network structure. Fracture surfaces were further characterized by three crack propagation zones. A smooth, brittle fracture zone was preceded and followed by crack initiation and crack arrest zones, respectively. An apparent plastic flow was confined to the initiation and arrest regions. No crazing phenomenon was seen in the initiation zone; instead a step-like fracture was observed, typified by the ‘flow’ of internodular matrix during step formation. Local plastic deformation in the initiation zone and the corresponding value of critical strain energy release rate, GIc, were correlated with the nodular morphology. The size of nodules was found to vary with the curing agent concentration, thus allowing us to establish a fundamental correlation between the nodular morphology and the ultimate mechanical properties of epoxy resins.

The principles of dielectric measurements for in situ monitoring of composite processing

Abstract The fundamental concepts of dielectric behavior of polymers and the utilization of dielectric measurements for in situ monitoring of cure of polymers and composites are discussed. Information is presented on currently used dielectric sensors and the procedure for calculation of dielectric parameters from the monitored signal. The review is written to accommodate both the fundamental and the pragmatic aspects of dielectric monitoring of cure. In the final part of the review, a critical assessment is offered of the advantages and disadvantages of dielectric measurements for the in situ monitoring of processing of polymers and composites.

Correlation between dielectric and chemorheological properties during cure of epoxy-based composites

A method for the complete thermal, rheological and dielectric characterization of an epoxy matrix for advanced composites is presented. The analysis of the dielectric response suggests that ionic resistivity data can be used to develop quantitative correlations with the materials properties during thermosets processing. Ionic resistivity and degree of reaction data are correlated during the cure of the studied epoxy matrix and good agreement between the model predictions and the experimental data is observed under isothermal and non-isothermal conditions. Moreover, degree of reaction data calculated from dielectric measurements are used in a chemorheological model for viscosity calculation, obtaining very good correlation between measured and predicted viscosity.

Correlations between dynamic mechanical properties and nodular morphology of cured epoxy resins

Abstract Various epoxy resin formulations, based on the diglycidyl ether of bisphenol A (DGEBA) and cured with diethylene triamine (DETA) were studied. Dynamic mechanical measurements were used to characterize changes in mechanical properties as a function of temperature. The morphology of the cured resins was investigated by transmission electron microscopy. Correlations between dynamic mechanical properties and morphology were described and discussed by applying the concept of inhomogeneous (nodular) thermoset morphology. The elastic storage modulus in the glassy state was determined primarily by the internodular matrix, whereas the glass transition of cured resins depended upon the intranodular crosslink density.

Improved agarose gel electrophoresis method and molecular mass calculation for high molecular mass hyaluronan

The molecular mass of the polysaccharide hyaluronan (HA) is an important determinant of its biological activity and physicochemical properties. One method currently used for the analysis of the molecular mass distribution of an HA sample is gel electrophoresis. In the current work, an improved agarose gel electrophoresis method for analysis of high molecular mass HA is presented and validated. HA mobility in 0.5% agarose minigels was found to be linearly related to the logarithm of molecular mass in the range from approximately 200 to 6000 kDa. A sample load of 2.5 μg for polydisperse HA samples was employed. Densitometric scanning of stained gels allowed analysis of the range of molecular masses present in the sample as well as calculation of weight-average and number-average values. The method was validated for a polydisperse HA sample with a weight-average molecular mass of approximately 2000 kDa. Excellent agreement was found between the weight-average molecular mass determined by electrophoresis and that determined by rheological measurement of the solution viscosity. The revised method was then used to show that heating solutions of HA at 100°C, followed by various cooling procedures, had no effect on the HA molecular mass distribution.

Monitoring of reactive processing by remote mid infra-red spectroscopy

Abstract A novel experimental facility for remote mid infra-red (m.i.r.) spectroscopy has been assembled in our laboratory. We can conduct in situ real-time monitoring of practically any combination of processes and operating conditions, while, at the same time, enjoying the benefits of an unmatched wealth of molecular-level information contained in the m.i.r. range of the electromagnetic spectrum. The principal parts of our set-up are a host of optical components, gold-coated waveguides and a Fourier transform infra-red ( FT i.r.) spectrophotometer. The signal generated in the remote mode was sharp, clear and reproducible, and the results were in every aspect as good as those obtained with conventional FT i.r. spectroscopy. The applicability of our remote m.i.r. set-up to in situ real-time monitoring of processing of reactive polymers was demonstrated by analysing the kinetics of cure of a multifunctional epoxy/amine formulation composed of diglycidylether of bisphenol-F (DGEBF) and 4,4′-methylene dianiline (MDA). A particularly interesting discovery was made concerning the necessary correction of the ‘standard’ epoxy peak at 915 cm −1 , which must be considered in order to preserve the reliability of kinetic results in the later stages of cure.

In situ real-time monitoring of reactive systems by remote fibre-optic near-infra-red spectroscopy

A remote fibre-optic near-infra-red (n.i.r.) spectroscopy set-up was recently assembled in our laboratory and successfully used for in situ real-time monitoring of various reactive systems. A disposable probe was designed and employed, enabling us to study processing of thermosetting polymers. A detailed description of our experimental set-up and several selected examples are presented in this communication.

Impedance spectroscopy of reactive polymers : correlations with chemorheology during network formation

An investigation was carried out of correlations between dielectric, spectroscopic, and rheological properties during reactions in several epoxy/amine formulations of both polymer‐forming and nonpolymer‐forming nature. Dielectric results obtained from impedance spectroscopy were compared with results from near‐infrared spectroscopy, high performance liquid chromatography, and steady shear and dynamic mechanical measurements. Reaction kinetics obtained from dielectric, spectroscopic, and chromatographic results were in excellent agreement. Gelation and vitrification times of multifunctional formulations, determined by dielectric and rheological measurements, agreed surprisingly well, despite the empirical nature of such correlations. Nonetheless, a realization of the full potential of dielectric impedance spectroscopy in monitoring the progress of chemophysical changes in reactive polymers necessitates the development of fundamental scientific correlations between dielectric and chemorheological phenomena during cure.

Comparison of kinetic and rheological evaluation of gel time for an amine-epoxy system

Abstract For tetrafunctional formulations, Florys classic analysis yields the value of 58% for conversion at gel point. This study utilizes a recently developed mechanistic kinetic model for epoxy-amine reactions to calculate the time needed to reach 58% conversion. That time was then compared to the gel time determined from rheological measurements and the two were found to be in superb agreement.