Secondary relaxation loss peak

The occurrence of a secondary relaxation loss peak by itself is not a necessary and sufficient condition for improved impact strength (99,100). Additional... 

Thus the occurrence of a secondary relaxation loss peak by itself does not ensure improved impact strength (Menges and Boden 1986 Boyer 1968). Additional information such as actual impact test data at temperatures above and below the relaxation, should be used to verify whether a given relaxation process has a significant influence on the impact behavior. 

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. 

It was further shown that the structural a- relaxation peak shifts to the lower frequencies as the reaction proceeds, while the frequeney of maximum secondary relaxation loss remains constant, and only the signifieant inerease in amplitude of this relaxation process was observed. Such a behavior of y-relaxation confirms that in fact the secondary process takes its souree from intramolecular motions occurring within the monosaceharide unit. 

By combining the results of several methods (dynamic mechanical, dielectric, NMR, etc.), it is usually possible to determine quite reliably the structural units whose motions give rise to secondary relaxations. If dynamic mechanical measurements alone are employed, the usual procedure is that the chemical constitution is systematically altered and correlated with the dynamic mechanical response spectra, i.e. with the temperature-dependence of the G" and G moduli. If the presence of a certain group in polymers is marked by the formation of a loss peak characterized by a certain temperature position, size and shape etc., then the conclusion may be drawn that the motional units responsible for the secondary relaxation are identical or related with that group. Naturally, the relations obtained in this way are empirical and qualitative. 

Dynamic mechanical response spectra of elastin145 (insoluble protein of vessels and ligaments), poly(ethylene terephthalate)141 and polycarbonate based on Bisphenol A (4,4 -dihydroxydiphenylmethane)141 show that incorporated water brings about enlargement of the existing secondary loss peak and its displacement toward lower temperatures. In conformity with the latter result, the activation energy of the relaxation process of elastin decreases. So far, no detailed data on this type of relaxation have been collected so that the copartidpation of water in the molecular motion cannot be specified more accurately. 

Figure 6. Master curve of the dielectric loss data of l,l -bis(p-methoxyphenyl)cyclohexane (BMPC). The spectra measured under pressure were shifted on the frequency scale to superpose with the a-loss peak at T = 248 K and ambient pressure. The secondary relaxation of BMPC is not a JG relaxation (its loss peak frequency is pressure-insensitive), and it is not temperature-pressure-superposable along with the a-loss peak.

There are also glass-formers that have a resolved secondary relaxation that is not the JG relaxation according to the established criteria [38], but lack an apparent JG peak in their loss spectra at ambient pressure. These glass-formers include BMPC [75], dibutyl phthalate (DBP) [77], diethyl phthalate (DEP) [76], 2PG, 3PG [101,102], m-fluroaniline (m-FA) [44], and bis-5-hydroxypentylphthalate (BHPP) [228,229]. One criterion is the lack of a pressure dependence of their relaxation times, as shown for BMPC in Fig. 30. NMR measurements of molecular motion in BMPC had shown [230] that the... 

In the previous subsection, we have provided conceptually the rationale and experimentally some data to justify the expectation that the primitive relaxation time To of the CM should correspond to the characteristic relaxation time of the Johari-Go Id stein (JG) secondary relaxation Xjg- Furthermore, it is clear from the CM relation, Ta = ( "to)1 1- , given before by Eq. 6 that To mimics Ta in behavior or vice versa. Thus, the same is expected to hold between Xjg and Ta. This expectation is confirmed in Section V from the properties of tjg- The JG relaxation exists in many glass-formers and hence there are plenty of experimental data to test the prediction, xjG T,P) xo(T,P). Broadband dielectric relaxation data collected over many decades of frequencies are best for carrying out the test. The fit of the a-loss peak by the one-sided Fourier transform of a Kohlrausch function [Eq. (1)] determines n and Ta, and together with tc 2 ps, To is calculated from Eq. 6... 

If the first scenario were real, the slower secondary relaxation should express its presence as an excess wing on high frequency side of the a- relaxation peak. To check this we superimposed dielectric loss spectra of octa-O-acetyl-lactose measured above and below Tg to that obtained at T=353 K. Next we fitted a master curve constructed in this way to the Kohlrausch-Williams-Watts function... 

However, it is difficult to interpret this in such a way. One can see that the coupling parameter n estimated for the a- relaxation peak of octa-O-acetyl-lactose is significant (it means that the distribution of a-relaxation times is quite broad). It implies that the separation between maxima of dielectric loss of the expected secondary relaxation and main structural relaxation peaks should be significant. Consequently, the p- mode should be clearly visible in the dielectric loss spectra. However, in the case of octa-O-acetyl-lactose the y- relaxation... 

However, it has to be pointed out that it is only an approximate calculation, because we did not consider intermolecular hydrogen bonds which surely also make the structure of lactose more rigid. In such a case the activation energy of the slower secondary relaxation should be comparable to that for the y-relaxation (Ea=44 kJ/mol). This may imply that the slower secondary relaxation seen in lactose may be undetectable in the case of acethyl derivative of this disaccharide, because maxima of both secondary relaxations can be too close to each other. In fact, the inspection of the dielectric loss spectra obtained for octa-0-acetyI-Iactose below its glass transition temperature (see Fig. 2) showed that there is only one secondary relaxation peak. However, a detailed analysis of the y- loss peak revealed that probably two secondary processes contribute to it. 

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.

The dielectric loss peak of secondary relaxations is extremely broad due to the variety of molecular environments (structural heterogeneity) of the relaxing unit, and, consequently, a variety of energy barriers, being more or less symmetrical. 

The dielectric strength. As, which is proportional to the area under the loss peak, is much lower for the secondary processes, relative to the a relaxation analysed in the next section. This is a common pattern foimd in both polymer materials and glass formers. The P secondary process is even more depleted in linear polymers that contain the dipole moment rigidly attached to the m chmn, such as polycarbonate [78-80] and poly(vinyl chloride) (the behaviour of this polymer was revisited in ref [81] where the secondary relaxation motions are considered as precursors of the a-relaxation motions). Polymers with flexible polar side-groups, like poly(n-alkyl methacrylate)s, constitute a special class where the P relaxation is rather intense due to some coupling vnth main chain motions. 

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