Dynamic mechanical loss spectra

Similar procedures have been used by several workers (Halasa et al., 1982) to hydrogenate poly(l,4-butadiene-co-1,2-butadiene) diblocks (Halasa, 1985) and poly(l,4-butadiene-co-l,4-isoprene-co-1,4-butadiene) triblocks. Hydrogenation of these diblock and triblock copolymers forms thermoplastic elastomers with crystalline and amorphous segments. All these materials exhibit crystallinity, glass transition, solubility, and dynamic mechanical loss spectra different from those of their unsaturated counterparts. 

In the dynamic mechanical loss spectra of polymers, the transitions that are related to different molecular motions within polymers are called a, 3 and y transitions. 

The viscoelastic properties and time-temperature-rate dependency of properties are key elements of the materials science of ethylene-co-styrene polymers. Dynamic mechanical loss spectra are presented in Figures 4a and 4b for selected semicrystalline copolymers, and low ciystallinity and amorphous copolymers, respectively. 

Fig. 4. Dynamic mechanical loss spectra of (a) Three semicrystalline ethylene-co-styrene polymers having different wt% comonomer compositions, (b) Amorphous or low crystallinity ethylene-co-styrene polymers. (The label ES refers to a copolymer having ...

Fig. 53 Dynamic mechanical loss spectrum of BPA-PC. The solid line is the result of simulation using the phenyl ring motion characteristics (from [34])...

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... 

Figure 11.7 shows the dynamic mechanical spectrum reported by Celli and Scandola [42] for PLLA after heating the sample at 200°C in order to erase the thermal history. The solid line refers to a sample quenched in a water-ice mixture after extrusion, while the broken line depicts an immediate rerun on the same sample, after cooling from 160°C. Below room temperature, no relaxation process is apparent in either curves, that is, the dynamic mechanical loss tangent is as low as 10 over the range —150-20°C [42]. The absence of any loss phenomena below Tg capable of mechanical energy dissipation is likely the reason for the observed brittleness of glassy PLLA and induces failure of lower molecular... 

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. 

This chapter discusses the dynamic mechanical properties of polystyrene, styrene copolymers, rubber-modified polystyrene and rubber-modified styrene copolymers. In polystyrene, the experimental relaxation spectrum and its probable molecular origins are reviewed further the effects on the relaxations caused by polymer structure (e.g. tacticity, molecular weight, substituents and crosslinking) and additives (e.g. plasticizers, antioxidants, UV stabilizers, flame retardants and colorants) are assessed. The main relaxation behaviour of styrene copolymers is presented and some of the effects of random copolymerization on secondary mechanical relaxation processes are illustrated on styrene-co-acrylonitrile and styrene-co-methacrylic acid. Finally, in rubber-modified polystyrene and styrene copolymers, it is shown how dynamic mechanical spectroscopy can help in the characterization of rubber phase morphology through the analysis of its main relaxation loss peak. 

Figure 5. Dynamic mechanical spectrum (torsion pendulum) of a cured film of Epon 828/PACM-20. Both the shear modulus G ( Z ) and its approximation O (0) are plotted on the upper curve the lower three curves are loss modulus G" (CT), logarithmic decrement A ( ), and damping coefficient a (+j-...

However, with dynamic mechanical relaxation domains of about 100 A can be detected. The dynamic mechanical spectrum of a polymer prepared by system 3 ("Figure 3 ) indeed displays two separate loss peaks one at about -62 °C owing to the glass transition of polyol... 

Figure L The low-temperature dynamic mechanical spectrum of Halthane 73-14 is typical of the 73-series polyurethane adhesives. Two secondary relaxations, Tp and Ty, are shown as peaks in the loss modulus at —100° and —150°C. The soft segment glass transition, Tg(SS), occurs at about —50°C. The frequency of oscillation was held constant during the measurement at 0.1 Hz.

Figure 8. The high-temperature dynamic mechanical spectrum of Halthane 88-2 shows that some further curing is occurring above 100°C because both storage and loss modulus increase over a...

At acoustic frequencies, the attenuation goes through a maximum determined by the spectrum of relaxation times in the polymer hence dynamical mechanical analysis can be performed by scanning over a wide frequency range, typically 10 —lO Hz. An example of the technique is sonic DMA of PVC [54] which shows that the shear modulus increase monotonically with frequency, while the longitudinal or extensional modulus displays the transition associated with Tp. The ratio of the loss and storage moduli, or tan delta obtained via DMA can be related to the absorption coefficient through the equation [2] ... 

A possible occurrence of an additional mechanical transition (loss peak) in the dynamic mechanical spectrum of polymer blends was ascribed to the geometrical arrangement of phases, rather than to a molecular relaxation process within the interfacial area (235). 

Figure 19 (a) Dependence of storage (S ) and loss (S") modulus of a bottle brush with polymethacrylate backbone with DP = 3500 and poly(nBA) SC with DP = 30. (b) Dynamic mechanical spectra of a cross-linked sample of polymer shown in spectrum (a). 

The unique information about molecular mobility preparing a- and b-relaxations in the frequency range, where relaxation type of spectrum and the resonant tend to co-exists, may be extracted from analysis of far-infra-red spectra. This enabled one to assign a low-temperature d-relaxation (d-loss peak observed in dynamic mechanical measurements in the temperature range 20-70K) to the small-angle torsional vibration (libration) of some molecular unit close in size to a repeat unit of macromolecules (156). 

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