Vitrification Phenomena Research Articles

Simultaneous measurements of enthalpy and heat capacity of a thermosetting polymer during the curing process

We report a new microcalorimetric technique which allows simultaneous measurements of the heat capacity and the rate of enthalpy release during the chemical reaction in a substance under quasi-isothermal conditions. The basic features of the instrument are presented together with brief experimental results on the polymerization of diglycidyl ether of bisphenol A mixed with ethylene diamine. The instrumentation is an advance for studies of both synthetic and biological formation of macromolecules, for now it becomes possible to measure the heat capacity of a time-variant system and to characterize its vitrification transition occurring at constant temperature, as the growth of the macromolecule by chemical reaction increases the sample’s viscosity. The abrupt decrease observed in the heat capacity curve corresponds to the vitrification transition, which cannot be observed in the corresponding enthalpy release curve. The present calorimeter has an important use in both technological assessment and in understanding the fundamental aspects of the vitrification phenomenon.

Retrograde vitrification of polymers with compressed fluid diluents: experimental confirmation

Experimental measurements of the T g for the poly(methyl methacrylate)/CO 2 system clearly establish the existence of the previously predicted retrograde vitrification phenomenon

Vitrification and further structural relaxation in the isothermal curing of an epoxy resin

AbstractIsothermal curing of an epoxy resin based on diglycidyl ether of bisphenol A, using a hardener derived from phthalic anhydride, has been performed at curing temperatures between 30 and 130°C. Samples were cured isothermally at various intervals of time and analyzed by differential scanning calorimetry (DSC), the glass transition temperature Tg, and the conversion degree being determined by the residual enthalpy technique. The vitrification phenomenon and a further structural relaxation process, occurring at curing temperatures (Tc) lower than the maximum Tg (109°C), at which Tg equalizes Tc, have been studied at curing temperatures between 30 and 100°C. The structural relaxation process is analyzed by the endothermic peak that appears superposed on Tg in dynamic DSC scans. The area of this peak (Q) is a measure of the recovery enthalpy, and thus of the extent of the relaxation process. This process begins at higher curing times (tc) when Tc decreases because the vitrification of the system starts later. Both the enthalpy recovery (Q) and the temperature of the endothermic peak (Tm) increase with the annealing time (ta), calculated as the difference between tc and the time in which vitrification occurs, and tend to have a limiting value due to the fact that the system loses mobility when the free volume decreases during its asymptotic approach toward the metastable equilibrium state. Furthermore, the dependence of Q and Tm on ta at different Tc shows that the relaxation process in partially cured resins depends on the conversion degree of the system and consequently on the crosslinking density of the network.

Vitrification and physical ageing on isothermal curing of an epoxy resin

Vitrification phenomena and further structural relaxation processes or physical ageing occurring in the isothermal curing reaction of an epoxy resin are studied by Differential Scanning Calorimetry (DSC). The vitrification time,tv, the limiting conversion degree and the limiting glass transition temperature (Tg) are evaluated at curing temperatures (Tc) between 30 and 100‡C. The dependence of limitingTg withTc permits the determination of the maximumTg of the resin (109‡C). The physical ageing, which appears as the the last step of curing reaction for curing times above to vitrification, is analyzed through the endothermic peak superposed to the glass transition temperature. The results obtained in partially reticulated resin show the kinetics of the physical ageing to slow down asTc increases, as a consequence that the segmental mobility is reduced.

Gelation and vitrification of epoxy resins monitored by ultrasonic measurements

AbstractUltrasonics measurements were performed during the cure of epoxy resins. The results show some different comportments if there is only the gelation process which passes through the frequency window of the study or if there is also a vitrification phenomenon. The last case occurs with a high glass transition temperature epoxy resin like DGEBA‐DDS. with which the increasing glass transition temperature of the resin is rapidly higher than the study temperature. The former case happens for low Tg epoxy resin like BDGE‐HMDA. But in any case the method is very sensitive to the mechanical properties evolution of the material.

Molecular mobility and fracture processes in ultimately drawn high-density polyethylene

Abstract The reasons for cessation of drawing and for the local fracture processes occurring in linear polyethylene near ultimate draw ratios, λmax, have been studied by broad-line proton NMR and scanning electron microscopy. As λmax was reached during drawing, the sample whitened and kink bands similar to those observed at deformation of low molecular solids were formed. The kink bands were clearly seen in the micrographs. As evidenced by NMR, the segmental mobility of chains in the amorphous regions is almost completely suppressed because of a sharp increase of the orientation stress near λmax (though the temperature of drawing is close to the melting temperature). This phenomenon of mechanical vitrification is regarded as a fundamental reason for cessation of drawing and for transition to the “solid-state” fracture mechanism. Quantitative estimates are given for formation of submicrocracks arising at boundaries of kink bands.

Current status and future prospects of large scale micropropagation in commercial plant production

Abstract Currently 180–200 million plants are produced worldwide per year by tissue and cell culture techniques. Conventional plant production has been replaced by large scale micropropagation, highest in ornamentals, followed by fruit trees and small fruits, and to a smaller extent in vegetables and agricultural plants. The objectives are higher productivity, elimination of pathogens, rapid multiplication of new genotypes, selection of somaclonal and induced variants, preservation of germplasm and long‐term storage. The profitable application of in vitro methods requires the maintenance of high regeneration rates, but this can be limited by the decline of the organogenic competence, genetic instability, especially in callus cultures, and by vitrification phenomena. Regulation of organ differentiation on a cellular level is not fully explored. Some histological features and biochemical markers, such as specific proteins, indicate early steps of organogenesis. Optimally controlled bioreactors for somatic e...

Isothermal and temperature programmed kinetic studies of thermosets

AbstractDifferential Scanning Calorimetry (DSC) is a popular method for the characterization of the cure kinetics of thermosetting materials. The experiments may be carried out under isothermal or temperature program modes. Several authors have pointed out the fact that the kinetic rate expressions for non‐isothermal experiments must be different from the isothermal ones; however, this fact has been neglected in practice. In this work, we present a kinetic characterization of a thermosetting system using a phenomenological model for the reaction rate expression and DSC data from isothermal and non‐isothermal experiments. In general, thermosetting materials exhibit the vitrification phenomenon which stops the reaction before complete conversion is achieved. Vitrification is taken into account in both Isothermal and temperature programmed experiments.

The process of spontaneous vitrification evaluated by calorimetry

The Cr 40Ti 60 and Cr 28Fe 8Ti 64 high temperature b.c.c. solid solutions, if properly quenched to room temperature, transform into an amorphous structure on subsequent annealing. Here we report on calorimetric studies of this vitrification process. The specific heat was measured during temperature cycling between 50 °C and 600 °C. In the first heating cycle, the crystal phase undergoes quick stress relaxation and subsequently exhibits the thermal and elastic behaviour of the vitrified state. However, X-ray diffraction still shows predominantly crystalline features which then progressively disappear in the following heating cycles. We conclude that vitrification—with respect to different system variables — evolves on different time scales. A possible mechanism for the vitrification phenomenon is proposed on the basis of statistical mechanics.

Étude rhéologique de la réticulation du système tétraglycidyldiaminodiphénylméthane-diaminodiphénylsulfone—I. Influence de la concentration

The kinetics of crosslinking of the TGDDM-DDS system were studied at 180° with concentrations covering a wide range around stoichiometric. The variations of the viscoelastic modulus, determined by sinusoidal shearing between parallel plates, show clearly gelation and vitrification phenomena. For low concentrations of amines, the gelation involves a small decrease of the loss modulus, before it increases again. The results show that a concentration near 100 g TGDDM for 33 g DDS (somewhat below stoichiometric) leads to the best storage properties for the material at the end of cure. This value is very near the concentration used industrially, and shows that the epoxide-hydroxyl and epoxide-epoxide reactions are very important in the crosslinking of the resin.

Award address formation and properties of network polymeric materials

AbstractA general model for thermosetting materials which relates polymerization to properties is discussed. In particular, a phase diagram relates the four states of matter encountered (i.e., liquid, rubber, ungelled glass and gelled glass) to gelation and vitrification phenomena. The model is extended to rubber‐modified thermosetting systems to show that, by control of the time to gelation, a given chemical formulation can provide different morphologies and hence different material behavior. The principal experimental approach uses a torsional braid analyze (TBA), an automated version of which is described. Use of the technique to measure gelation and vitrification times is discussed.