Dynamic mechanical properties damping

Dynamic Mechanical Tests. Plasticizer efficiency, can be measured, not only be the lowering of T , but also by temperature dependence of typical dynamic mechanical properties, such as modulus and damping. 

Earlier investigations on the dynamic mechanical properties of PPMI over a wide range of temperatures indicated the existence of two distinct relaxations at around 250 and 400 K [15,16]. The former relaxation was assigned to adsorbed water molecules in the polyimide chain and the latter was due to local relaxation modes of the backbone. Recently Ahlbom reported the mechanical relaxation of various polymers at low temperatures [17]. In the film sample of PPMI, the small relaxation at 93 K. attributed to the motion of phenyl rings was observed, although the dominant damping peak at 198 K is not yet explained. 

In many investigations dynamic-mechanical properties have been determined not so much to correlate mechanical properties as to study the influence of polymer structure on thermo-mechanical behaviour. For this purpose, complex moduli are determined as a function of temperature at a constant frequency. In every transition region (see Chap. 2) there is a certain fall of the moduli, in many cases accompanied by a definite peak of the loss tangent (Fig. 13.22). These phenomena are called dynamic transitions. The spectrum of these damping peaks is a characteristic fingerprint of a polymer. Fig. 13.23 shows this for a series of polymers. 

Dynamic mechanical property (DMP) measurements are used to evaluate the suitability of a polymer for a particular use in sound and vibration damping. Since the dynamic mechanical properties of a polyurethane are known to be affected by polymer morphology (4), it is important to establish the crystallization and melting behavior as well as the glass transition temperature of each polymer. Differential scanning calorimetry (DSC) was used to determine these properties and the data used to interpret the dynamic mechanical property results. 

Dynamic mechanical property data were obtained using Du Pont DMA 982 instrument for structural analyses and Rheology DVE instrument for measurement of damping ability. Scanning electron microscopy was performed on samples etched with strong chromic acid. The mechanical properties were measured at 20°C by tensile test. 

If a rubber-like polymer is used as the vinyl polymer, this IPN will show good damping properties at elevated temperatures. So, butyl acrylate, ethylene glycol dimethacrylate, phenolic novolac, and bisphenol A type epoxies were used as IPN components. The dynamic mechanical properties of these IPNs were examined first, because the loss tangent is very important to damping properties. Then the damping properties of IPN and commercial chloroprene rubber were measured at various temperatures. 

For this purpose, the dynamic mechanical properties of a series of 25/75 in situ SIM IPNs have been investigated (Figure 5). With 1% OcSn, the tan 8 vs temperature curve shows a classical shape, as for in situ SEQ IPNs and corroborates the kinetic results two separated transitions, broadened and damped, exist (10). The lower transition corresponding to the polyurethane phase, is shifted... 

Dynamic mechanical properties exhibit side chain or branch motions short main chain segment motions, main chain segmental motions, recrystallization, and melting. These transitions are observed as inflections in the storage modulus curve with temperature, peaks in either the loss modulus or damping factor (tan( )) curves. Figure 3.11 shows the dynamic mechanical spectroscopy (DMS) of a ZN-VLDPE at 1 Hz in tensile mode. The glass transition temperature (maxima of the loss modulus... 

A variation on PWM is pulse position modulation (PPM), also known as pulse period modulation or pulse frequency modulation (PFM). In this case, the duty-cycle pulse remains on for a fixed time while the base period is varied. The frequency of the pulses (how close together the pulses are) determines the voltage level. The neuromuscular system is an example of a pulse position modulation system. A muscle is made up of many discrete motor units. A motor unit has an all or nothing response to a nerve impulse in much the same way that a nerve impulse is a nonlinear (thresholded) all-or-nothing event. The level of sustained force output of a motor unit is dictated by the frequency of incidence of the nerve impulses, with the motor units dynamics [mechanical properties—inertial and damping properties (acts as a mechanical filter)] holding the force output smooth between incoming impulses. The motor unit is pulse frequency modulated by the nervous system. 

The effect of the fillers on the dynamic mechanical property of NR material was analysed by DMA in this work. The elastic modulus ( ") and the loss factor (tan 5) of the neat NR and NR composites were characterized as functions of temperature. Under an oscillating force, the resultant strain in specimen depends upon both elastic and viscous behaviour of materials. The storage modulus reflects the elastic modulus of the rubber materials which measures t recoverable strain energy in a deformed specimen, and the loss factor is related to the energy damped due to energy dissipation as heat. 

Absorption is a measure of the energy removed fi om a soimd wave as a result of conversion to heat as the wave travels through the poljuner. Absorption is synonymous with dissipation and is related to dynamic mechanical terms damping, loss factor, and loss tangent. Absorption is a material property, usually given the... 

Dynamic Mechanical Properties n (1) The stress-strain properties of a material when subjected to an applied sinusoidally varying stress or strain. For a perfectly elastic material the strain response is immediate and the stress and strain are in phase. For a viscous fluid, stress and strain are 90° out of phase. (2) The mechanical properties of composites as deformed under periodic forces such as dynamic modulus, loss modulus and mechanical damping or internal friction. (Sepe MP (1998) Dynamic mechanical analysis. Plastics Design Library, Norwich, New York)... 

The measurement of the dynamic-mechanical properties of polymers requires the separation of the response of the material to cyclic or transient loading into two components an elastic response and an inelastic response. Frequently, these two are best resolved experimentally as a complex response and a damping. Whilst the complex or total reponse is familiar to most experimentalists, damping frequently poses a conceptual problem. However, the effect of damping is manifested in many ways. For example (i) the phase lag between stress and strain (Le. load and displacement) (ii) the decrease with time of the amplitude of stress and strain in a freely vibrating system and (iii) the limited amplitude of a system excited at resonance. 

Rheological measurements under oscillating conditions yield the dynamic mechanical properties of polymers, i.e. the storage modulus, G, the loss modulus, G , and a mechanical damping or internal friction, tan 5. In studies of the response of a material to vibrational forces, stress, strain, frequency and temperature are the key variables. When a material is subjected to cyclical stress under conditions analogous to those encountered in the intended applications, the data reflect both... 

---