Polymers dynamic mechanical spectra

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... 

Spin relaxation in dilute solution has been employed to characterize local chain motion in several polymers with aromatic backbone units. The two general types examined so far are polyphenylene oxides (1-2) and aromatic polycarbonates (3-5) and these two types are the most common high impact resistant engineering plastics. The polymer considered in this report is an aromatic polyformal (see Figure 1) where the aromatic unit is identical to that of one of the polycarbonates. This polymer has a similar dynamic mechanical spectrum to the impact resistant polycarbonates (6 ) and is therefore an interesting system for comparison of chain dynamics. 

FIGURE 13.1. Idealized representation of the dynamic mechanical spectrum for an amorphous polymer illustrating temperature assignments for the a Tg), j3, and y relaxations. 

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). 

PVF2), polypivalolactone (PPVL), polypropylene (PP), polybutene (PB), and PET. In all cases (except PET) the ultraquenched samples have shown an extra peak in the dynamic mechanical spectra, below the normal Tg peak. These polymers can be divided into two classes, those which crystallize at the new, lower peak and those which crystallize at tlw higher, normal Tg. For convenience we will label these two peaks and T, Representative results are considered below, considering first those which crystallize at the new, low dynamical mechanical spectrum peak. For some of the samples (PP, PVF2, and PB) ca. 0.2 mm thick samples were used for the dynamic mechanical and DSC studies. 

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. 

Figure 1. Dynamic mechanical spectroscopy of formula B This material, containing 70% EA comonomer between polymer networks I and II, displays a mechanical spectrum only slightly broader than would be expected of the corresponding random copolymer.

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. 

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... 

Damping is an engineering material property and the observed response is much more sensitive to the polymer constitution than in step-function experiments. Oscillatory experiments (also referred to as dynamic mechanical experiments) thus offer a powerful technique to study molecular structure and morphology. A significant feature is the breadth of the time-scale spectrum available with these methods, e.g., 10 -10 cycles/sec. 

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] ... 

The T of the H-H polyacrylates were measured by DSC, (Table 5), by the determination of the thermal mechanical spectrum and by dynamic mechanical measurements. In general, the T of the polyacrylates increased with increasing bulkiness of the substituent the H-H polymers have higher T than the H-T polymers the individual values ranged from 27° to about 40°C. higher for the H-H polymer than for the H-T polymers. Small differences have been noted between the measurements of the T by DSC as compared to TMS. ... 

Similarly to the didectric analysis, the modification of the a-rdaxation spectrum during crystallization of a semirigid chain polymer can also be monitored by dynamic mechanical thermal analysis (DMTA) In this case, due to a more restricted frequency... 

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). 

Fig. 8. Dynamic mechanical damping spectrum for poly(dicyclooctyl itaconate). ("Reproduced from J.M.G. Cowie, R. Ferguson, and I.J. McEwen, Polymer, 23, 605 (1982), by permission of the publishers, Butterworth and Co. (Publishers) Ltd. "). 

The use of techniques such as torsional braid analysis (TEA), which provides an inert support for the polymer samples, allows the exploration of the dynamic mechanical response of a polymer above the glass transition. In certain systems this has revealed further features in the damping spectrum which appear in the rubber-liquid region. While this type of observation has engendered some controversy in the literature as to whether these damping peaks represent real transitions or are merely artifacts of the technique, the ability to explore this super Tg region is extremely useful, particularly if complementary measurements are also available. 

Dynamic mechanical techniques are important in the characterization of the rheological properties of polymers. In dynamic mechanical analysis, a small-amplitude oscillatory strain is applied to a sample, and the resulting dynamic stress is measured as a function of time. The dynamic mechanical technique allows the simultaneous measurement of both the elastic and the viscous components of the stress response. Typically, the temperature and deformation frequencies are changed in order to determine the mechanical relaxation spectrum of the system. The molecular basis of changes in the dynamic mechanical properties can be investigated using dynamic IR dichroism. 

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