Aliphatic sequences

The CP MAS DD 13C NMR spectrum at 23 °C is shown in Fig. 81. The 166.9, 136.8, 127.3 and 17.4 ppm lines correspond to the C = 0 carbons, unprotonated and protonated aromatic carbons, and methyl carbons, respectively. The set of lines at 46.3, 40.7, 32.3 and 27.4 ppm correspond to aliphatic CH and CH2 carbons. The temperature dependence of the t /2 values of these aliphatic carbons is shown in Fig. 82. The carbons associated with the 40.7 and 43.6 ppm lines, whose onset of motions occurs above 100 °C, should correspond to the CH2 carbons in a position with respect to the amide group. Consequently, the 27.4 and 32.3 ppm lines, associated with carbon atoms which undergo motions at temperature equal or higher than 20 °C, should correspond to the central CH2 carbons of the aliphatic sequence. 

In the case of epoxy networks with a secondary diamine, like DMHMDA, the network architecture is such that flexible aliphatic sequences are present as chain extenders between the crosslink points. In such architectures, the motions of the HPE units can develop towards other HPE sequences (either along the chain or spatially neighbouring) without involving the crosslink points in their cooperativity. Thus, with these systems a different nature of cooperativity exists compared to the other network architectures. The introduction of an antiplasticiser in such a local packing does not affect the cooperativity as much as with the densely crosslinked architecture, for the crosslinks are not so much involved. Once more, it is important to point out that the flexible nature of the aliphatic amines does not matter since the same behaviours are observed for fully aromatic systems with identical architecture [68]. 

The y transition occurring around - 150 °C at 1 Hz originates from motions of aliphatic sequences of at least four methylene units within the amine moiety. 

The aliphatic sequences between ether linkages confer chemical resistance and flexibility. 

Other polymers with similar, aliphatic, carbon sequences will also be susceptible to oxidation. Thus, the aliphatic polyamides such as nylon 6 and nylon 6,6 will oxidize readily. The oxidation of the nylon polymers is characterized by a drop in molecular weight and by discoloration. Oxidation proceeds in a manner similar to that outlined for polypropylene, with the CHj unit nearest the amide nitrogen most susceptible to attack [32]. Oxidation is a serious problem for polyamides as mentioned earlier, nylon 6,6 will embrittle in two years [1] at 70°C. Almost all commercial polyamide formulations therefore include antioxidants, usually based on copper compounds. Similarly, the aliphatic sequences in PET are readily attacked by oxygen and a drop in molecular weight is again observed [36]. Some gel formation is also observed at higher temperatures. 

The HPX polymer of Fig. 5.149 has a rather flexible aromatic sequence, and, indeed, the polymer behaves like a typical semicrystalline polymer, just that the aromatic and aliphatic sequences cannot interdigitate in a satisfactory crystal structure... 

The above considerations are not limited to concepts of chemical modification because even when lignins are considered as additives in the preparation of polymer blends or composites, their hydroxyl functions represent a key structural element in terms of polar contributions and sources of hydrogen bonds which will affect the quality of the interfacial interactions of the ensuing materials, just as the less-polar (ether groups, aromatic rings, etc.) and non-polar (aliphatic sequences) moieties will, in terms of hydrophobic interactions. 

A different strategy providing access to linear polyesters derived from fatty acids in which their aliphatic sequences dangle from macromolecular chains (i.e., a type of structure very different to that of linear PE) has been described recently [73]. Various saturated fatty acid methyl esters were malonated to the corresponding methyl diesters, which were polymerised by transesterification with 1,6-hexanediol (Scheme 4.17). 

Fig. 1. Examples of three types of liquid crystal polymer molecule I, chain made up from rigid aromatic units joined by relatively stiff ester groups II, chain containing rigid units separated by flexible aliphatic sequences DI, chain with a flexible backbone but rigid side-groups.

Diisocyanates containing an aliphatic sequence with phenylisocyanate end groups [111], and diisocyanates containing preformed imide rings [112], have been recently synthesized and used as monomers against aromatic dianhydrides. 

The typical feature of the epoxy networks is the occurrence of strong sub-Fg relaxations. On the dynamic mechanical traces (Figs. 10 and 25), the P-relaxation process shows up over a broad temperature region, typically from 150 to 300 K with a maximum at about 200 K at the frequency of 1 Hz these spectra differ only slightly. The tendency to appear at the lower-temperature y-relaxation is also observed for the networks with sufficiently long aliphatic sequences (HMDA, HA) (Fig. 25b). 

The interpretation of P-relaxation in these epoxy-amine networks with the extraordinarily complicated chemical structure still remains disputable despite many experimental (NMR [137-139] and other) results obtained. Different authors have attributed this relaxation to motion of the hydroxypropylether (HPE) units, the DGEBA ring flips, or their combined motions, as well as to motions in the long enough flexible aliphatic sequences. The y-relaxation is unambiguously related to localized motion in the aliphatic sequences. 

Analysis of the CRS data, in combination with DMA and NMR data, obtained for the same epoxy networks, allowed the tentative assignments to major CR peaks, that is, to follow up the relations between the CR spectra and molecular mobility [20]. Thus, the basic P-relaxation processes occurred in the temperature range of about 160-260 K where all peaks are supposed to be the reflection of both localized motion of the HPE units and of ring flips of the bisphenol A units. Both rigid and flexible networks exhibited the CR peak at 270 K, which was screened by the constrained P-motion. At last, low-temperature CR peak corresponded to localized y- or y -relaxations in aliphatic sequences, or motion of the DGEBA moieties, respectively [20]. 

R,R = any aromatic or aliphatic sequence Figure 4 The cure of epoxy resins by primary amines. 

Lotz B, Wittmann JC. Structural relationships in blends of isotactic polypropylene and polymers with aliphatic sequences. J Polym Sci B 1986 24 1559-1575. 

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