Dynamic mechanical methods

These may be carried out in extension, torsion or flexure, and are concerned with alternating strains, normally over a range of frequencies and temperatures. Such methods are particularly suited to the study of molecular relaxation processes and have not been applied very extensively to oriented materials, where the studies to date have been more concerned with the static arrangement of structural elements in the materials. However, Takayanagi, and Stachurski and Ward have used dynamic tensile and torsion modulus to study the anisotropy of relaxations in oriented polyethylene. 

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Sauer, J. A., and A. E. Woodward Transitions in polymers by nuclear magnetic resonance and dynamic mechanical methods. Rev. Mod. Phys. 32, 88-101 (1960). 

Crystallinity can be measured using the same thermal and dynamic mechanical methods described for measuring T however, the melting transition is much sharper than the Tg because it is a first-order transition compared with second-order for the T. 

The dynamic mechanical method, however, suffers from two important limitations. Firstly, it is not sensitive for blends having less than 10 wt.- % of one component, as the response is dependent on the fraction of the stress born by each phase. Secondly, instruments have poor resolution for blends of materials with close T s. Different machines obviously have different sensitivities in both these respects. 

Experimentally, the glass transition has also manifested itself by a sharp increase in relative rigidity (measured by dynamic-mechanical methods) and a simultaneous drastic decrease in the rate constant of the autocatalytic epoxy-amine reaction The mobility or rigidity of the system is a function of reaction conversion a in the pre-gel region it can be characterized by dynamic viscosity which is proportional to M of the reacting system. Beyond the gel point, still in the rubbery region but not close to the gel point, the dynamic modulus, G, is at low frequencies proportional to " (m a 1)... 

The influence of the cyclohexane ring on the glass transition temperature and sub-Tg transitions has been studied by dynamic mechanical methods [10,16,42]. These comparative studies have focused on the influence of the cyclohexyl substituent on observed thermal transitions. A systematic study of cycloalkyl substituents on alternating carbons of a polyethylene backbone was performed to study the influence of these substituents on the dynamic mechanical spectra of the polymers [42], These workers also prepared materials in which the cycloalkyl substituents were spaced away from the polymer backbone by successively longer methylene chains, testing the influence of ring proximity... 

Malcolm RK, McCullagh S, Woolfson AD, et al. A dynamic mechanical method for determining the silicone elastomer solubility of drugs and pharmaceutical excipients in silicone intravaginal drug delivery rings. Biomaterials 2002 23 3589-3594. 

Tg can be measured by dilatometry, refractive index, differential scanning calorimetry, dynamic mechanical methods and by dielectric relaxation techniques. 

DYNAMIC MECHANICAL METHODS FOR THE ANALYSIS OF THE VISCOELASTIC PROPERTIES OF PHARMACEUTICAL AND BIOMEDICAL MATERIALS... 

Based on this, it is no surprise that dynamic mechanical methods may be employed to quantify the effects of plasticizers on the glass transition temperature and the compatibility of such plasticizers with the polymeric components. Such agents are common components of polymeric systems and are included to reduce the glass transition temperature of the polymer, frequently to temperatures below... 

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

The glass transition temperature, Tg, can also be calculated from the stress-temperature profile. It can be taken as the intersection of two lines drawn from the linear regions of the glassy and rubbery regimes, in a manner similar to Tg determination from TMA. Here, however, the measured Tg corresponds more with those derived from dynamic mechanical methods because of the strong dependence of stress on the modulus transition between the rubbery and glassy states. Other ways to measure Tg would be to take the derivative of the a(T) curve to obtain Ea(T) the Tg could then be determined as either the inflection point or the midway point in the transition between glassy Ea and rubbery Ea. 

The crosslinking of polydimethylsiloxane and polydimethyldiphenyl-siloxane was studied as a function of temperature by dynamic mechanical methods [366]. A reduction in crosslinking rate was observed in N20 and air compared with vacuum [367]. Marked discolouration of the samples occurred in the presence of air or nitrous oxide. 

In contrast to other Tg methods, dynamic measurements easily detect glassy state relaxations and have been extensively applied to their study. These include dynamic mechanical methods, dielectric relaxation, and nuclear magnetic resonance (NMR). Since we are primarily concerned with viscoelastic response at this point, we shall confine the discussion to the dynamic mechanical technique and delay our consideration of dielectric and NMR methods until Chapter 7. 

Of the many other ways of detecting phase transitions, including cloud points for systems with mixed phases, particular mention should be made of the use of dynamic-mechanical methods, and specifically measurement of the shear compliance to 1 kbar. When applied to columnar crystals of poly-diethylsiloxane) such a method was able to detect the melting point in conditions where the volume change was close to zero. 

The forced non-resonance techniquet l is one of the simpler dynamic mechanical methods. In the... 

The nature (e.g., the softening range) of a hard occlusion in the interior of a rubber particle can never be determined by dynamic mechanical methods. Any dispersed phase can only be characterized in the region of higher modulus. Its low modulus or higher temperature properties are completely lost. Thus, the relaxation spectrum of a composite is generally not a superposition of the component spectra. 

The experimental value for p quoted above (505 x 10 N m" ) comes from the work of Davidse et The method was based on extrapolating the modulus of semi-crystalline polyethylene to 100 crystallinity. A more recent estimate based on a dynamic mechanical method at — 190°C gave a value of 13 x 10 Nm". ... 

The onset of deformation mechanisms at particular temperatures indicated that it was appropriate to investigate viscoelastic relaxation processes by dynamic mechanical methods. Some preliminary studies were made in 1956 and 1957 by Hellwege et who demonstrated... 

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