Dynamic mechanical experiments

Characterization Methods. Stress-strain experiments were carried out with an Instron model 1122. Dogbone samples of 10mm in length were used, and the initial strain rate was 2 mm/min. Dynamic mechanical data were obtained utilizing a DDV-IIC Rheovibron Dynamic Viscoelastometer. Most samples were tested within the temperature range of -100°C to 220°C with a heating rate of 2-3°C/min. A frequency of 11 Hz was selected for all the dynamic mechanical experiments. 

The dynamic mechanical experiment has another advantage which was recognized a long time ago [10] each of the moduli G and G" independently contains all the information about the relaxation time distribution. However, the information is weighted differently in the two moduli. This helps in detecting systematic errors in dynamic mechanical data (by means of the Kramers-Kronig relation [54]) and allows an easy conversion from the frequency to the time domain [8,116]. 

The stress depends on the extent of reaction, p(tf), which progresses with time. However, it is not enough to enter the instantaneous value of p(t ). Needed is some integral over the crosslinking history. The solution of the mutation problem would require a constitutive model for the fading memory functional Gf Zflt, t p(t") which is not yet available. This restricts the applicability of dynamic mechanical experiments to slowly crosslinking systems. 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Dynamic mechanical experiments yield both the elastic modulus of the material and its mechanical damping, or energy dissipation, characteristics. These properties can be determined as a function of frequency (time) and temperature. Application of the time-temperature equivalence principle [1-3] yields master curves like those in Fig. 23.2. The five regions described in the curve are typical of polymer viscoelastic behavior. 

Fig. 23.3 Sinusoidally varying stress and strain in a dynamic mechanical experiment.

Dynamic mechanical experiments, where the material is periodically strained, are common methods to characterize the visco-elastic behavior of elastomers by measuring the storage modulus G and loss modulus G". G is a measure for the maximal, reversibly stored energy for a periodical deformation and G" is proportional to the dissipated energy for the oscillation cycle. It is obvious to investigate, whether the l.c. state of the l.c. elastomers influences the dynamic mechanical properties and whether different modes of linking the mesogenic moieties to the backbone can be detected. 

The tan 8 loss curves obtained at 1 Hz for the PET blends with the DMT and TPDE additives [13] are shown in Fig. 23. In contrast to what happens in the dynamic mechanical experiments, the additives lead to only a small shift of the curves relative to the case of pure PET and to the same peak amplitude as for pure PET. Furthermore, the activation energies derived for the p peak obtained from dielectric measurements are the same as the ones for pure PET (Table 1) and the activation entropies are in the same range (Table 2). 

In the glassy state, these Ar-Al-PA exhibit local chain dynamics which are largely controlled by the chemical structure. Recently, the local motions that may occur in the glassy state and might take part in secondary transitions, have been investigated on a series of Ar-Al-PA of various chemical structures by using dielectric relaxation, 13C and 2H solid-state NMR and dynamic mechanical experiments [57-60]. 

The combined investigations of a series of aryl-aliphatic copolyamides (xTyl -y and MT) by dielectric relaxation, solid-state 13C and 2H NMR, and dynamic mechanical experiments demonstrate the existence of three secondary transitions y, ft and co, in order of increasing temperature. 

The stiffness modulus is, in most cases, measured in dynamical - mechanical experiments, for instance with a torsion pendulum, on a time scale of a few seconds. This experiment results in the shear modulus, G (which is related to Young s modulus, E), while the damping shows a strong maximum at Tg. 

The loss factor, tan 8, can be measured with the aid of dynamic-mechanical experiments (such as the torsion pendulum). The deformation in such a test varies as indicated in Figure 7.13 the damping follows from the logarithmic decrement , A, it can be easily shown that... 

Tg can be determined by dynamic mechanical experiments from the log E-T diagram, but also from the maximum in the tan 8 - T curve. Another possibility is differential scanning calorimetry (DSC). 

In proton T2 experiments conducted by Litvinov [18] a strong dynamic heterogeneity in the fraction of EPDM bound to carbon black could also be detected an immobilised EPDM layer covering the carbon black surfaces and a mobile EPDM part outside of this interfacial layer. In dynamic-mechanical experiments by Haidar [19], it was found that the polymer layer on the filler surface had glass-like mechanical properties. 

Three diblock copolymers of cis-1,4 polyisoprene (IR) and 1,4-polybutadiene (BR) have been studied in dynamic mechanical experiments, transmission electron microscopy, and thermomechanical analysis. The block copolymers had molar ratios of 1/2, 1/1, and 2/1 for the isoprene and butadiene blocks. Homopolymers of polybutadiene and polyisoprene with various diene microstructures also were examined using similar experimental methods. Results indicate that in all three copolymers, the polybutadiene and polyisoprene blocks are essentially compatible whereas blends of homopolymers of similar molecular weights and microstructures were incompatible. 

The Cp of PVC (lower curve in Figure 9) increases linearly with temperature from — 60° to 80°C. Between 85° and 93°C, the glass transition is manifested by a discontinuous increase in Cp and corresponds to the a transition observed in the dielectric and dynamic mechanical experiments. Above the glass-transition temperature, Cp increases smoothly to 120°C where the experiment was terminated. 

The ideal viscous element can be represented by a dashpot filled with a Newtonian fluid, whose deformation is linear with time while the stress is applied, and is completely irrecoverable (Newton element). In a dynamic mechanical experiment the stress is exactly 90° out of phase with the strain [Pg.412]

The temperature dependences of the storage modulus G are given in Figure 12.13 for unfilled and filled EPDM. The dynamic mechanical experiments were conducted from -100 to 150°C at a frequency of 1 Hz in tensile mode using a DMA (TA 2980). Heating rate was 5°C/min. 

Numerous efforts have focused upon the nature of moisture transport of epoxy systems. Previous-sorption desorption work demonstrated that equilibrium moisture levels In an epoxy system can be related to thermodynamic states (1,2,3). Transient and equilibrium dynamic mechanical experiments are performed In this work with two epoxy systems TGEBA-TETA and N-5208. These experiments provide Insight Into the nature and extent that network changes have on the dynamic mechanical properties as a result of hygrothermal cycling. 

Two different types of dynamic mechanical experiments were performed. First, the temperature dependence of "equilibrium" dynamic mechanical properties for all epoxy samples were obtained... 

The second type of dynamic mechanical experiment involved collection of "transient" -Rheovibron data at a fix frequency as a function of time after a change in either the moisture or thermal environment. The experimental apparatus designed for this purpose is depicted in Figure 1. 

There are two basic types of transient dynamic mechanical experiments which were performed In this study. The first type Involves Isothermal cycling of an epoxy sample between a dry and wet environment. The second type of experiment Involves cycling the epoxy sample between two different temperatures under a liquid water environment. In each case, the transient and equilibrium values of dynamic mechanical properties change In a unique manner. 

Isothermal Moisture Transients Isothermal experiments will first be discussed for the DGEBA-TETA systems. Film samples from this system were equilibrated In a desiccated environment and subsequently exposed to liquid water environment In the Rheovlbron apparatus. Figure 5 summarizes typical results of these dynamic mechanical experiment. In addition to the... 

State epoxy network. This does not appear to be the case. Previous diffusion work with DGEBA-TETA Indicates that a moisture altered sample raised beyond the T apparently recovers the previous dry state characterlstlcs (l). This result Is also coincident with Information derived from dynamic mechanical experiments (3). 

Activation energies are derived from the dynamic mechanical experiments assuming that the kinetic mechanism does not change with temperature. The thus arrived at activation energy represents a lumped kinetic parameter for the reactions which have led to the point in time of the dynamics mechanical dispersion being measured. Activation energies for the first dispersion by DSA... 

A complete description of the viscoelastic properties of a material requires information over very long times. Creep and stress relaxation measurements are limited by inertial and experimental limitations at short times and by the patience of the investigator and structural changes in the lest material at very long times. To supplement these methods, the stress or the strain can be varied sinusoidally in a dynamic mechanical experiment. The frequency of this alternation is u cycles/s or m(= 27ri ) rad/s. An alternating experiment at frequency w is qualitatively equivalent to a creep or stress relaxation measurement at a time t = (I /w) sec. 

The real and imaginary parts of the complex numbers used here have no physical significance. This is simply a convenient way to represent the component vectors of stress and strain in a dynamic mechanical experiment. 

Dynamic mechanical experiments were performed on compression molded bars which had been molded from powders. Compression molding of the samples produced specimens adequate for solid state evaluation but not sufficient for physical property evaluation. Samples were molded using a standard Nabash press and a molding temperature of 300 C. 

The network structure of W-cured acrylates was analyzed by dynamic mechanical experiments, H T2 NMR relaxation, and C NMR. ... 

The authors would like to express their gratitude to Mr. T. K. Tran for synthesizing the materials and to Mr. M. J. Castille, Jr. for conducting the dynamic mechanical experiments. 

---