Poly Secondary relaxations

A typical loss maximum of this type was observed for poly(methyl methacrylate) containing caprolactam or derivatives of cyclohexane12,13. It is noteworthy70 that in the latter case the relaxation induced by the cyclohexyl group present in the incorporated plasticizer and the secondary relaxation of poly(cyclohexyl methacrylate) or poly(cyclohexyl acrylate) are characterized by an identical temperature position, 190 K (1 Hz), and activation energy, 47.9 kJ/mol (AU = 47.7 kJ/mol is reported for the chair-chair transition of cydohexanol). Hence, it can be seen that the cyclohexyl ring inversion, which represents a specific molecular motion, is remarkably insensitive to the surrounding molecules. 

Of the diluents known to affect the dynamic relaxation behavior of polymers in the glassy state, water has so far received the greatest attention. Many polymers, which in the dry state are lacking any secondary relaxation process at temperatures from 77 to 273 K, e.g. poly(methyl methacrylate)135, polymethacrylamide136, cellulose and its derivatives137, collagen138, polysulfones139, poly(2,6-dimethylphenylene oxide)139, and others,... 

DSC and DTA. They can be used to confirm suspicious glass transitions revealed by DSC and most important, they can further quantify molecular mobility associated with sub-glass transitions. For example, DSC analysis of poly (ethylene 2,6-naphthalene dicarboxylate) (PEN) only revealed the presence of a glass transition around 112 °C (Hardy et al., 2001). DMA analysis of the same sample, however, revealed two secondary relaxations below this glass transition (Hardy et al., 2001). In the case of humic materials, it is not uncommon for DSC to fail to detect clear thermal transitions due to their heterogeneous nature, which contributes to overlap/ broadening or washout of thermal transitions. As such,TMA and DMA represent powerful, complementary tools to DSC. 

Both polymers show a strong relaxation at about 120°C and 100°C for as can be seen in Figs. 2.15 and 2.16, for (P2tBCHM) and, (P4tBCHM) as Dfaz Calleja et al. [32] have reported. Moreover P2tBCHM show a complex secondary relaxation at about -80° and a remainder of the mechanical activity at about -20°C and 30°C respectively [32] poly(4-tert butylcycloheyl methacrylate) (P4tBCHM) respectively. 

The variation of fanS with temperature at 1 kHz for the six poly(thiocarbonate)s is represented in Fig. 2.86. In all cases a prominent relaxation associated to the glass transition temperature labelled as a -relaxation is observed in Figure PT-1. A secondary relaxation which covers a range of about hundred degrees and which by comparison with the results reported for PCs is labeled as y relaxation. Between 80°C and 100°C a slightly dielectric activity is observed (f) zone) and at — 120°C another relaxation labelled as 5 relaxation for polymers 4,5 and 6. 

Kolarik, J. Secondary Relaxations in Glassy Polymers Hydrophilic Polymethacrylates and Poly-acrylates Vol. 46, pp. 119-161. 

Polymers that have bulky repeat units can have multiple secondary relaxations. If more than one secondary relaxation is found, then the slowest one has to be the JG relaxation, assuming that the latter is resolved. Excellent illustrations of this scenario are found by dielectric relaxation studies of aromatic backbone polymers such as poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) [43]. The calculated To from the parameters, n and xa, of the a-relaxation are in good agreement with the experimental value of %jq obtained either directly from the dielectric loss spectra or from the Arrhenius temperature dependence of xjg in the glassy state extrapolated to Tg. The example of PET is shown in Fig. 46. 

First is the crankshaft mechanism of Schatzki.16 It has been observed for many polymers containing linear (CH2) sequences with n = 4 or greater, that a secondary relaxation occurs at about -120 °C at 1 Hz. This seems to be true regardless of whether the CH2 sequences occur in the main chain or in the side groups. Thus both polyethylene and poly-n-butyl methacrylate exhibit this relaxation. The mechanism proposed by Schatzki is shown in Figure 5-14. 

The above values of and Ea are associated with strictly local processes due to individual mobility of sub-units, and thus involve independent molecular motion with nearly no activation entropy, according to the Eyring [71] formalism. The y high-frequency secondary relaxation in lower poly(alkyl methacrylates) is an example of such relaxation process with an activation energy around 38 kJ mof [2,72]. For the larger members, even lower activation energies are found, as is the case for poly(w-lauiyl dimethacrylate) with a value of 28 kJ mof for the y... 

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. 

However, the expected increase of the intensity of Af with temperature for secondary relaxations is not always observed as is evidenced by data relative to two epoxy resins poly[(phenyl glycidyl ether)-co-formaldehyde] (PPGE) and DGEBA, where the dielectric strength of the P process decreases with temperature [74]. The additionally detected y secondary process follows the usual pattern. In these systems the y relaxation is more intense than the P process, as was also found in n-ethyleneglycol dimethaciylate monomers with [47]. For the poly(n-alkyl... 

Xoa primitive relaxation time of the Coupling Model Johari-Goldstein secondary relaxation time Xoj primitive relaxation time of the network junction Xaf segmental relaxation time of the fast component in binary poly-... 

Dielectric data revealed secondary transitions in all of the polymers studied. Secondary relaxations were all much broader than those of the a transitions. A representative plot of e" versus temperature for the P relaxation in PHFiPA is shown in Figure 8. It is well known that poly(alkyl methacrylate)s exhibit both P and y transitions. The P transition has been interpreted as due to the hindered motion of the -COOR group about the carbon-carbon bond which links the side group to the main chain. The y transition is thought to be due to local molecular motion of alkyl groups in the side chain (5). The P transitions in PEMA and PTFEMA overlapped the a, Tg, transitions. An attempt to resolve the P transition with the PeakFit program resulted in tan(5) and e versus temperature curves which exhibited anomalous fi-equency effects. 

See Figure 9 in S. C. Kuebler, D. J. Schaefer, C. Boeffel, U. Pawelzik, and H. W. Spiess, Macromolecules 30, 6597 (1997). The secondary relaxation of poly(ethyl metacrylate) involves some motion of the main chain and is hence a JG relaxation according to [170]. 

TABLE 13.4. Glass-transition and secondary-relaxation temperatures of poly(phenylene oxides). 

Secondary-relaxation temperatures of poly(pyromellitimide-l,4-diphenyl ether)... 

---