Polymer cure dynamics

Rheological Cure Transformation Diagrams for Evaluating Polymer Cure Dynamics... 

The dynamic mechanical properties of elastomers have been extensively studied since the mid-1940s by rubber physicists [1], Elastomers appear to exhibit extremely complex behavior, having time-temperature- and strain-history-dependent hyperelastic properties [1]. As in polymer cures, DMA can estimate the point of critical entanglement or the gel point. 

Dynamic mechanical methods may be also used to characterize time-dependent changes in the elastic modulus during polymer curing and cross-linking, from which... 

Figure 4 shows the TPA results for the bisphenol-A and the bisphenol-S linked polymers cured at 280°C for six days. Both the dynamic shear modulus and the mechanical loss factor are given as a function of temperature from -150°C to about +300°C. During a TPA run, a temperature scan covering the complete glass-to-rubber transition could not be achieved because the sample softened as the glass transition temperature, Tg, was approached. 

Dynamic mechanical analysis DMA Frequency Phase changes, polymer curing... 

Dynamic mechanical analysis DMA Moduli Phase changes, glass transitions, polymer cure... 

The cured and the liquid polymers degrade essentially by the same mechanism (see Equation 6.1). The kinetic analysis of the isothermal and dynamic thermogravimetric data of the liquid polysulfide polymer cured with ammonium dichromate is explained by a kinetic model based on random initiation, followed by rapid termination by disproportionation. The average overall activation energy obtained by different methods for the decomposition is 145.3 kj/mole ... 

John A. Pojman is a professor in the Macromolecular Science Division of the Department of Chemistry at Louisiana State University. He earned his doctorate in chemical physics from the Univereity of Texas at Austin. Dr. Pojman was a faculty member at the University of Southern Mississippi for ISyeare. He has published over 100 peer-reviewed publications and coedited three monographs and coauthored another one. His interests include nonlinear dynamics with polymers, cure-on-demand polymerizations, nonequilibrium thermodynamics of miscible fluids, microgravity research, and the aquatic salamandei of Louisiana. He also claims the world s largest collection of pocket protectors. 

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

Some of the terpolymers containing high levels of AGE give superior sour gasoline and ozone resistance, particularly dynamic ozone resistance. Since the unsaturation is not in the polymer backbone, it can be, and apparentiy is, sacrificed under sour gasoline or ozone aging. This protection scheme is limited with the peroxide and sulfur cure systems as they involve the aHyl functionaUty of the polymer. The protection is maximized when a dinucleophilic curative, such as trithiocyanurate, is used. 

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

Papke N. and Kargar-Kocsis J., Thermoplastic elastomer based on compatibilised poly(ethyleneterphtha-late) blend Effect of rubber type and dynamic curing. Polymer, 42, 1109, 2001. 

Kumar, R.C., Fuhrmann, I., and Kocsis, J.K., LDPE-based thermoplastic elastomers containing ground tire rubber with and without dynamic curing, Polym. Degrad. Stab., 76, 137, 2002. 

Bisphthalonitrile monomers were cured neat, with nucleophilic and redox co-reactants, or in combination with a reactive diluent. Dynamic mechanical measurements on the resulting polymers from -150 to +300°C turn up several differences attributable to differences in network structure. Rheovibron results were supplemented with solvent extraction, differential scanning calorimetry (DSC), vapor pressure osmometry, and infrared spectroscopy to characterize the state of cure. 

R Musto, M. Abbate, G. Ragosta and G. Scarinzi, A smdy by Raman, near-infrared and dynamic-mechanical spectroscopies on the curing behaviour, molecular structure and viscoelastic properties of epoxy/anhydride networks, Polymer, 48, 3703-3716 (2007). 

Other characterization methods that are of value are dynamic scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). A sample DSC is shown in the middle of Figure 15.2. Most cure reactions are exothermic, and the heat generated by cure can cause excessive heat to build up in the polymer if control is not exercised. DSC measures the generation of heat as a function of time and temperature. This can be used to predict the temperature at which the laminate will begin to cure (the onset of the peak in Fig. 15.2) and the temperature or time at which cure will be complete, further improving the selection of cure cycles to try. 

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