Cyclohexyl-methacrylate

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. 

Under viscoelastic measurements poly(cycloalkyl methacrylates) show a loss maximum (designated y), located in the very low temperature range (T <-60 °C), as illustrated in Fig. 6 in the case of poly(cyclohexyl methacrylate). Such a series of polymers has been extensively studied by Heijboer in his Ph.D. thesis [5], by performing viscoelastic studies at 1 Hz (sometimes 180 kHz) as a function of temperature and exploring quite a large number of cycloalkyls, either substituted or not. In cyclopentyl, cyclohexyl, cyclohep-tyl derivatives, the y transition was shown to occur at ca. - 185 °C (180 Hz), - 80 °C (1 Hz), - 180 °C (1 Hz), respectively. The associated activation energies, a> are 13, 47, 26kJmol 1 for the cyclopentyl, cyclohexyl, cycloheptyl derivatives, respectively. 

These y transitions were assigned to motions within the alkyl cycles. In the specific case of poly(cyclohexyl methacrylate), in order to identify the pre-... 

Fig.6 Temperature dependence of the mechanical modulus, G and loss tangent, tan 5, at 1 Hz, for poly(cyclohexyl methacrylate) (from [5])...

Later on, when high-resolution solid-state 13C NMR became available, these questions concerning motions within the rings of poly(cycloalkyl methacrylates) and the assignment of the specific motion occurring in the case of poly(cyclohexyl methacrylate) were revisited [6]. 

The spectra of poly(cyclohexyl methacrylate) and poly(cycloheptyl methacrylate) (Fig. 8b and c) are identical with those of poly(cyclopentyl methacrylate)... 

The data obtained from dynamic mechanical measurements lead to the following values of the correlation times at 25 °C r = 3 x 10 12 s for poly(cyclopentyl methacrylate), r = 3 x 10-6 s for poly(cyclohexyl methacrylate), r = 5 x 10-12 s for poly(cycloheptyl methacrylate). 

In contrast, in the case of poly(cyclohexyl methacrylate), the calculated correlation time is much longer. Its value agrees nicely with a strong line broadening of the ring carbon NMR peaks. 

In order to identify more precisely the type of motion occurring in the cyclohexyl side group, a variable-temperature MAS CP DD 13 C NMR study has been performed on solid poly(cyclohexyl methacrylate) [7]. The spectra obtained at various temperatures are shown in Fig. 9. 

This example dealing with the precise identification of the ring motions involved in the y transition of poly(cyclohexyl methacrylate) clearly illustrates how convenient and powerful the MAS CP 13C NMR is for studying solid-state transitions of polymers. It is worth noting that, owing to the very low temperature at which the y transition occurs, the involved motions are very localised. 

The a transition is not included in the considered temperature range. For CMI contents equal to or higher than 10%, two transitions are observed. At low temperatures a shoulder is present, whose extent increases with increasing CMI (y transition). Studies performed on copolymers with maleimide unit N-substituted by isopropyl or phenyl groups [79] do not show this low-temperature transition, which appears to be specific for cyclohexylmaleimide. Such a situation is analogous to the one encountered with poly(cyclohexyl methacrylate) described in Sect. 3. Consequently, this low-temperature transition is assigned to the internal motion of the cyclohexyl ring, i.e. the chair-chair inversion represented in Fig. 7. 

Heijboer [28] has reported the dynamic mechanical properties of poly(nethacrylate)s with different size of the saturated ring as side chain. The y relaxation in these polymers is attributed to a conformational transition in the saturated ring. In the case of poly(cyclohexyl methacrylate), the transition is between the two chair conformations in the cyclohexyl ring. However, this type of internal motion in hindered by rather high intramolecular barriers, which can reach about 11 kcal mol-1. 

The temperature dependence of a secondary transition can be used to determine the mechanism of the transition, by comparing it with that of other polymers. In Fig. 13.25 the for poly(cyclohexyl methacrylate), PCHMA, G and tan 8 are plotted vs. temperature for a number of frequencies varying from 10 4 to 8 x 105 FIz. Upon plotting log vmax, where vmax is the frequency at the maximum in tan 8, vs. 1/T a straight line is obtained as depicted in Fig. 13.26. It is not clear beforehand what mechanism is responsible for this transition. Therefore in Fig. 13.26 also results are shown not only for poly(cyclohexylacrylate), PCHA, i.e. PCHMA without the 0C-CH3 group, but also for the liquid cyclohexanol (two measurements at very high frequencies). It appears that the results for the three materials fall on... 

FIG. 13.25 Loss modulus G" as function of temperature Tat six frequencies for poly(cyclohexyl methacrylate) (PCHMA). From Heijboer (1972). 

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