Polycarbonate Secondary relaxation

In the case of polycarbonates, it has been observed that by adding miscible low molecular weight additives, with specific chemical structures, it is possible to increase the yield stress of the polymer, as well as to reduce the local molecular motions that are responsible for the secondary relaxation processes... 

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

In addition to Tg (amorphous phases) and the melting temperature Tm (crystalline phases), polymers also manifest secondary relaxations at temperatures below those of major relaxations (Tg or Tm, which will collectively be referred to as T. The main secondary relaxation temperature wil be designated generically as Tp, although it may be labeled differently in the literature on specific polymers. For example, it is commonly labeled as Ty for bisphenol-A polycarbonate where Ty is for a relaxation of higher intensity than Tp, and occurring at a lower temperature, which is the main secondary relaxation of this particular polymer. 

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. 

Intermediate - Temperature Relaxations. Secondary relaxations in the glassy state at temperatures intermediate between those of the a- and P- relaxations have been reported, but workers disagree as to their nature, location and origin. Confusion arises in part from a failure to recognize the existence of two separate processes. Krum and MUller [19] observed an intermediate relaxation only for injection-moulded or cold-drawn polycarbonate samples. Since the magnitude was diminished by annealing and the loss was not detected in fully annealed samples, they concluded that the intermediate process is a non-equilibrium effect associated with residual stresses. 

TABLE 13.2. Glass-transition and secondary-relaxation temperatures of polycarbonates. 

Polycaproiactam (nylon 6). This polymer has a crystallinity 50 and shows u scries of relaxations as the specimen is heated. The major relaxations are at 50 C (glass-to-rubber relaxation of the amorphous fraction), and crystal moiling point at 220 C. Other less intense relaxations of the amorphous fraction occur below 0 C. The difference between the temperature characteristics of amorphous and crystalline polymers illustrated in Fig. 4.21 is most marked and entirely characteristic (compare also Figs. 4.12 and 4.11). Glas.sy polymers (4.N.4) have one dominant relaxation (glass-to-rubber) and, sometimes, a smaller secondary relaxation (as in PMMA, PVC, and polycarbonate ). Crystalline polymers usually have several relaxations. 

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. 

The first secondary transition below Tg, the so called fj-relaxation, is practically important. This became evident after Struik s (1978) finding that polymers are brittle below Tp and establish creep and ductile fracture between Tp and Tg. The p-relaxation is characteristic for each individual polymer, since it is connected with the start of free movements of special short sections of the polymer chain. In view of more recent data of Tp Boyer s relation, Eq. (6.29), is very approximate and fails completely for amorphous polymers with high Tg s (e.g. aromatic polycarbonates and polysulphones). Some rules of thumb may be given for a closer approximation. 

Other secondary factors that could influence the enthalpy-relaxation process are substantial subsidiary modes of motion and structure-forming capability. Enthalpy-relaxation rates for blsphenol-A polycarbonate (PC), which has substantial mcdn-chcdn motion in the glassy states (8.10-17), and for poly(ethylene terephthalate) (PET), which in addition to having subsidiary modes of motion has significant crystal-ordering tendency (8,9)> were studied in detail, and the results are reported here. 

Figure 5 presents the correlation of log(D) and E at 35 C. The correlation between log(D) and E was similar to the correlation between log(D) and CED. These results suggest that gas diffusivity is influenced by motion of not only the side chain but also that of the backbone chain in the polymer because E, which is nearly equal to E in our study, includes effects of both primary dispersion based on microbrownian motion and secondary dispersion based on local relaxation modes or side chain motion. Gas diffusivity may increase as E decreases due to an increase in segmental motion. Elasticity depends on the polymer structure, and E is useful for the estimation and analysis of gas diffusion. Koros et al.(77), Yee et al.(32) and Muruganandam et al.(33) have studied the rdation between diffusivities and results of viscoelastic measurements in polycarbonates. Their values of D and E (E E ) agree well with our results in Figure 5. These results suggest that gas diffusivities may be strongly influenced by total motion of segments in many families of polymers, including polyimide.

SBCOndary Transitions. Most acoustic studies of polymer transitions have been of the glass transition, but some work has been done on secondary transitions, ie, those occurring below the glass transition. One transition that has received a fair amount of study is the p relaxation in polycarbonate (65). The transition has an activation energy of 40 kJ/mol and arises from a combination of phenylene and heteroatom motion. This value, one-half to one-third the value for a glass transition, is t q)ical of secondary transitions which involve smaller segments of the polymer. This study also found a correlation that often, but not always, exists between acoustic and dielectric measurements. The two sets of data... 

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