Mechanical losses polymers

Robotic peptide synthesizers are now used to automatically repeat the coupling, washing, and deprotection steps with different amino acids. Each step occurs in high yield, and mechanical losses are minimized because the peptide intermediates are never removed from the insoluble polymer until the final step. Using this procedure, up to 25 to 30 mg of a peptide with 20 amino acids can be routinely prepared. 

FIGURE 28.12 Mechanical loss factor as a function of temperature for the ENR stocks containing various fillers. (From Sihy Varghese, J. and Karger-Kocsis, J., J. Appl. Polym. Sci., 91, 813, 2004.)... 

However, because measurements are kinetically determined, this is a less accurate form of the equation. Very often it is observed that the measured shift factors, defined for different properties, are independent of the measured property. In addition, if for every polymer system, a different reference temperature is chosen, and ap is expressed as a function of T — rj, then ap turns out to be nearly universal for all polymers. Williams, Landel and Ferry believed that the universality of the shift factor was due to a dependence of relaxation rates on free volume. Although the relationship has no free volume basis, the constants and may be given significance in terms of free volume theory (Ratner, 1987). Measurements of shift factors have been carried out on crosslinked polymer electrolyte networks by measuring mechanical loss tangents (Cheradame and Le Nest, 1987). Fig. 6.3 shows values of log ap for... 

In this contribution we present results obtained with tetra-ethyleneglycol diacrylate (TEGDA). This compound was chosen since its polymer shows an easily discernible maximum in the mechanical losses as represented by tan 5 or loss modulus E" versus temperature when it is prepared as a thin film on a metallic substrate. When photopolymerized at room temperature it forms a densely crosslinked, glassy polymer, just as required in several applications. Isothermal vitrification implies that the ultimate conversion of the reactive double bonds is restricted by the diffusion-limited character of the polymerization in the final stage of the reaction. Therefore, the ultimate conversion depends strongly on the temperature of the reaction and so does the glass transition. 

DMT A measurements were made with a Polymer Labs instrument. Samples were clamped in the single cantilever mode in a frame of 22 mm using 6 mm clamps with 0.5 mm faces. The sample length between the clamps was 8 mm. Measurements were performed at a frequency of 1 Hz, a strain amplitude of 0.063 mm and a heating rate of 5 K.min . Clamping was checked by monitoring the strain amplitude on an oscilloscope. The measurements were carried out in air. Values of the temperature of maximum mechanical loss, T (tan 5max). were reproducible to 2 K. 

The electric properties of polymers are also related to their mechanical behavior. The dielectric constant and dielectric loss factor are analogous to the elastic compliance and mechanical loss factor. Electric resistivity is analogous to viscosity. Polar polymers, such as ionomers, possess permanent dipole moments. These polar materials are capable of storing... 

Fig. 2.22. Dependence of the elastic modulus E and the mechanical loss factor 6 on temperature for various polymers. Curves 1 elastomer (statistical copolymer of ethylene and propylene) curves 2 isotactic polypropylene (semicrystalline)...

The corresponding curves for the mechanical loss factor 6 show the following characteristics The transition to the glassy state for elastomers is seen in curve 1 as a characteristic mechanical absorption . On the other hand, two absorption maxima are visible in the curve for the partially crystalline polymer d2. The first one at 10 °C indicates the glass transition, the second one at about 145 °C is coherent with the crystalline melting point. 

Any motion occurring within any polymer system leads to a change of the dynamic mechanical behaviour, in particular its mechanical loss. This makes the dynamic mechanical measurements the most appropriate technique for studying solid-state transitions. However, in order to assign the molecular motions involved in the considered transition from only the dynamic mechanical results, it is necessary to perform systematic studies on a large series of compounds with gradual modification of their chemical structure. Such an approach has been used in some cases, but it requires lots of effort in synthesising the various compounds. 

For amorphous polymers, the glass-rubber transition is usually referred as the a transition, the solid state transitions (frequently called sub-Tg or secondary transitions) being designated by /3, y, S,... A typical example is shown in Fig. 1 with the temperature dependence of the mechanical loss modulus, E", which exhibits several peaks corresponding to the various transitions. 

Thus, at room temperature, the molecular motions responsible for the mechanical losses of poly(cyclopentyl methacrylate) and poly(cycloheptyl methacrylate) are very rapid processes, having too-high a frequency to broaden the line widths of MAS CP DD 13C NMR spectra, in agreement with the absence of observable motional broadening in the spectra of these two polymers (Fig. 8a and c). 

Merrifield s idea was to attach the initial amino acid molecules to the surface of insoluble polymer beads. To isolate the product after the next amino acid has been attached, it is necessary only to collect the beads by filtration and wash them to remove any remaining reagent. Not only is the isolation procedure fast and simple, but mechanical losses are minimized. 

Figure 11. Mechanical loss vs. temperature for polymer K and its 50/50 blend with aMS/AN copolymer...

Figure 12. Shear modulus and mechanical loss vs. temperature for the 50/50 blend of polymer K with aMS/AN/MMMA terpolymer...

Some semi-crystalline polymers show a mechanical loss peak just below Tm. This T,c is the temperature at which hindered rotation of polymer chains inside the folded crystals can occur. As a general rule... 

Figure 1. Generalized mechanical loss (tan S) and modulus behavior for different types of polymer blends. Case 1 (dashed-dotted line), miscible case 2 (dashed line), limited miscibility case 3 (dotted line), microheterogeneous case 4 (solid line), heterogeneous. (Reproduced with permission from Ref. 5. Copyright 1979 Academic Press.)...

The dynamic mechanical loss spectrum of polystyrene, in common with the spectra of most polymers, shows a small number of discrete loss peaks which are best resolved by a low-frequency test, preferably at or below 1 Hz. Figure... 

Viscoelastic measurements of ionomers have been used to indirectly characterize the microstructure and to establish property structure relationships. Forming an ionomer results in three important changes in the viscoelastic properties of a polymer. First, T usually increases with increasing ionization. This is a conseqi nce of the reduced mobility of the polymer backbone as a result of the formation of physical, ionic crosslinks. Second, an extended rubber plateau is observed in the modulus above T, again as a result of the ionic network. Third, a high temperaturi mechanical loss is observed above T, which is due to motion in the ion-rich phase. The dynamic mechan cal curves for SPS ionomers shown in Fig. 9 clearly demonstrate these three characteristics. 

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. 

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