Antiplasticisation

There is no reason why interaction should not more than offset the spacing effect and this is consistent with descriptions of antiplasticisation which have recently found their way into a number of research publications. 

It is worth noticing that some small molecule additives, called antiplasticisers , are able to significantly decrease the amplitude of the secondary transition (the first transition appearing at a lower temperature than the glass-rubber transition) and, consequently, to affect the material properties. 

Such a question has been recently revisited for pure PET and PET blended with antiplasticiser small molecules [12,13]. 

Some antiplasticisers exist for PET [13] and it is interesting to study their effect on the /> relaxation in PET, using the same investigation tools as the ones applied to pure PET. 

It is worth noting that an amount of DMT as small as 2 wt % affects the temperature and the amplitude of the fJ> peak. Saturation seems to be approached from 10 to20% of additive. It can Ire seen that the additive suppresses the high-temperature side of the /> peak considerably more than the low-temperature side. This would appear to support the statement that the (J> peak consists of more than one relaxation process, as evidenced by NMR studies and that the antiplasticiser has suppressed the relaxation processes that occur on the high-temperature side of the /> peak. 

To identify the processes that are suppressed by the antiplasticisers, NMR studies are required [12]. The t /2 contact times in PET/TPDE blends, shown in Fig. 17, are lower than those in pure PET. They do not change with temperature until 70 °C. As t /2 is mostly sensitive to ring flips, the number of ring flips in the polymer has been determined for PET/TPDE blends (Fig. 18). Within the accuracy of the experiments, no ring flips are observed between 25 and 70 °C. 

The investigation of pure PET and PET/additive blends by combining dynamic mechanical analysis, dielectric relaxation and solid-state NMR techniques, leads to a clear attribution of the molecular processes involved in the ft relaxation of PET, as well as an understanding of the effect of an antiplasticiser additive ... 

The antiplasticiser additives do not affect the motions of the carboxyl groups, but they hinder the phenyl ring flips. 

Furthermore, the effect of miscible small molecule additives, antiplasticisers, on the secondary transition, is worth analysing in order to reach a deeper understanding of the involved molecular motions. 

Some small molecule additives, called antiplasticisers, miscible with BPA-PC, lead to an increase of both modulus and yield stress at room temperature [14, 15]. In contrast, the toughness and the elongation at break decrease. 

It has been recognised for a long time that the antiplasticisers induce a decrease of the f3 transition peak intensity [16]. 

Such an effect of small molecule antiplasticiser is not specific to BPA-PC. It seems to occur with most polymers undergoing a transition originating from motions in the main chain. In the present paper, the effects of antiplasticisers on the f3 transition of poly(ethylene tere-phthalate) and epoxy networks are analysed in Sects. 4 and 7, respectively. 

One of the most efficient antiplasticiser for BPA-PC is Aroclor 1254, which consists of a polychlorinated bi-phenyl with five chlorine substituents it will be denoted AP. The results reported here deal with this additive. 

Fig. 70 Dynamic mechanical tan 8, at 10 Hz, as a function of temperature for BPA-PC and various concentrations (wt/wt) of antiplasticiser AP (from [17])...

It appears that it is mostly the high-temperature part of the fi transition which is suppressed, leading to a downward shift of the peak maximum. As presented in Fig. 71, the strength of the fi relaxation, expressed by the area under the dynamic mechanical loss, peak, Rsec, decreases much more rapidly with increasing the AP concentration that it should according to a dilution effect. It is worth pointing out that a similar behaviour has been reported, a long time ago, on the same system [53] and is encountered in BPA-PC-antiplasticiser mixtures, irrespective of the specific nature of the antiplasticiser small molecule [16,17,53,54]. 

Finally, it is interesting to notice that small molecule antiplasticisers added to BPA-PC decrease the strength of the mechanical ft transition, mostly the high-temperature part where phenyl ring 7r-flips occur. 2H NMR analysis confirms the hindering of these n-flips, but in addition it shows quite a substantial increase of their frequency distribution. 

A very convenient system for performing this investigation consists of epoxy networks formed by reacting aromatic epoxy on aliphatic diamine. Indeed, the different nature (aromatic or aliphatic) of the units allows one to apply 13 C NMR for identifying the groups involved in the transitions and their motions. Furthermore, in the same way as for polyethylene tere-phthalate), specific small molecule additives act as antiplasticisers, which can offer an additional possibility for investigation of the molecular motions and their cooperativity. 

Table 8 Characteristics of the chemicals used for the epoxy resins and antiplasticiser...

Table 9 Code names, compositions and glass transition temperatures of pure and antiplasticised epoxy resins...

Table 10 Characteristic temperatures (T) and activation energies ( a), enthalpies (Aifa) and entropies (ASa), in the f3 relaxation region for the different systems, pure and antiplasticised (from [70])...

In a way similar to that described for polyethylene fere-phthalate (Sect. 4.2), some antiplasticiser small molecules with a specific chemical structure are able to affect the ft transition and the yield stress of epoxy resins, but they do not have any effect on the y transition. In the case of HMDA networks, an efficient antiplasticiser, EPPHAA, whose chemical structure is shown in Table 8, has been reported [69]. The investigation of such antiplasticised epoxy networks by dynamic mechanical analysis as well as solid-state NMR experiments [70] can lead to a deeper understanding of the molecular processes involved in the ft transition and of their cooperativity. 

The antiplasticiser molecule is added to the reacting agents before curing. A single glass transition has been observed in the final samples, indicating that the antiplasticiser is fully miscible with the epoxy networks. 

The code names of antiplasticised systems contain the reference APx, where x corresponds to the wt % of antiplasticiser in the system. They are gathered in Table 9. 

Figure 99 shows the temperature dependence of the loss compliance, /", at 1 Hz for the pure and antiplasticised HMDA networks. 

In the case of the HA95 quasi-linear network, J" data plotted in Fig. 100a show that the shape of the J" peak is nearly unchanged by the presence of the antiplasticiser and its amplitude is only slightly reduced. 

In the HA60/AP19 system (Fig. 100b), the effect of antiplasticiser is intermediate between the behaviours observed in the densely crosslinked network and quasi-linear system, because some residual cooperative motions still occur in the pure network and can be hindered by the presence of the antiplasticiser. 

Investigations on the pure networks (Sect. 7.1.1.3) led to the conclusion that the introduction of a mesh extender allows intramolecular cooperativity to develop. Consequently, it is interesting to determine whether the antiplasticiser prevents these intramolecular cooperative motions from occurring. 

As regards the DMHMDA60/AP69 system, in the J" plot, shown in Fig. 101b, the influence of the antiplasticiser is seen in the high-temperature range. However, its role cannot be precisely described since both inter-and intramolecular cooperativities are likely to occur in the pure homo-logue. 

In the case of antiplasticised networks, because of similarities in their chemical structure, in the solid-state 13C NMR spectrum the lines arising from the CHOH - CH2 - O sequence of the polymer matrix and antiplasticiser overlap. It is the same for the lines corresponding to the protonated aromatic carbons. 

As regards the HPE sequence (Fig. 102a), the difference in the behaviour in pure and antiplasticised networks is less pronounced, but an increase in the temperature at which the onset of mobility occurs is observed, in particular with 19 wt % of antiplasticiser. It means that the presence of antiplasticiser induces some slowing down of the HPE motions. 

Results of ( 1/2)0/( 1/2) determinations as a function of temperature for the protonated aromatic carbons are shown in Fig. 103. The higher content of antiplasticiser (19 wt %) is required to see the occurrence of the onset of mobility of the aromatic carbons at a higher temperature than in pure matrix. Such a behaviour is quite similar to that observed for HPE units, in agreement with the conclusion, reached in pure epoxy networks, of a likely correlation between the motions of the aliphatic units and the ring flips. 

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