Crystalline polymers secondary transitions

As is commonly the case with crystalline polymers the glass transition temperature is of only secondary significance with the aliphatic polyamide homopolymers. There is even considerable uncertainty as to the numerical values. Rigorously dried polymers appear to have TgS of about 50°C, these figures dropping towards 0°C as water is absorbed. At room temperature nylon 66 containing the usual amounts of absorbed water appears to be slightly above the T ... 

For a semi-crystalline polymer the E-modulus shows between Tg and (in which region it is already lower than below Tg), a rather strong decrease at increasing T, whereas with amorphous polymers, which are used below Tg, the stiffness is not much temperature dependent (apart from possible secondary transitions). The time dependency, or the creep, shows a similar behaviour. 

Higher values can be reached for semi-crystalline polymers below Tg the crystalline phase is stiffer than the glassy amorphous phase (e.g. PEEK, E 4 GPa). Semicrystalline polymers above Tg have, however, a much lower E-value, such as PE (0.15 to 1.4 GPa) and PP 1.3 GPa) E is, in these cases, strongly dependent on the degree of crystallinity and on the distance to Tg. Sometimes a low modulus is also found for semi-crystalline polymers below Tg, due to the effect of one or more secondary transitions a strong example is PTFE (E = 0.6 GPa ). 

Semi-crystalline polymers, such as PE an PP, are tough at temperatures above Tg, though for PP (Tg -15 °C) the critical temperature limit is about room temperature here also the time-temperature equivalence plays a role. Below Tg, semi-crystalline polymers have a low impact strength (unless secondary transitions occur). 

Brittleness is found with semi-crystalline polymers below their glass-rubber transition Tg. An example is PP, which becomes brittle at about T -10 °C. PE retains its ductile nature down to very low temperatures. Other polymers have a Tg of some tens of °C above room temperature, such as polyamides and thermoplastic polyesters. Various mechanisms are responsible for a reasonable impact strength at room temperature for polyamides this is, for instance, the absorption of water also secondary transitions in the glassy region may play a role. 

We have seen already (Sect. 13.4.7) that every amorphous material (including that in semi-crystalline polymers) becomes brittle when cooled below the first secondary transition temperature (Tp) and becomes ductile when heated above the glass transition point (Tg). Between these two temperatures the behaviour - brittle or ductile - is mainly determined by the combination of temperature and rate of deformation. 

In the glass transition region, the storage modulus of an amorphous polymer drops by a factor of I000, and tan 6 is generally one or more. (The tan S in Fig. 11-18a is less than this because the polymer is oriented and partially crystalline.) In addition to Tg, minor transitions are often observed at lower temperatures, where the modulus may decrease by a factor of 2 and tan S has maxima of 0.1 or less. These so-called secondary transitions arise from the motions of side groups or segments of the main chain that are smaller than those involved in the displacements associated with Tg. Secondary transitions increase in temperature... 

The transition from crystalline to melt state, which is normal for crystalline polymers, is not observed with cellulose under normal conditions. It appears that the secondary bonds giving rise to the crystalline state are too strong and too numerous to be broken by a rise in temperature. Thermal degradation (beginning at ca. 180 °C) precedes melting under atmospheric pressure conditions. Nevertheless, a plastic deformation interpreted as melting has recently been reported for cellulose fibers exposed to laser radiation in a highly confined (pressurized) space [43]. The fracture surface of a thermoplastically deformed cellulose disc is shown in e Fig. 10. 

The higher crystalline, cold compressed sample shows a so-called crystalline phase (a) transition at about 130°C, a (weak) glass-rubber (S) transition at about 50°C and a secondary, amorphous phase (y) transition at -75°C. This weak glass-rubber transition effect is typical for a semicrystalline polymer. It indicated already that it would be difficult to detect this effect by DSC. 

Dynamic mechanical tests provide useful information about the viscoelastie nature of a polymer. It is a versatile tool for studying the effects of molecular structure on polymer properties. It is a sensitive test for studying glass transitions and secondary transitions in polymer and the morphology of crystalline polymers. 

Plasticizers are frequently incorporated to improve the workability of polymers, but often transform a rigid plastomer into a soft and ductile material. From early times, camphor was used to plasticize nitrocellulose and yield celluloid. Today plasticizers are most common in soft PVC, while unplasticized rigid PVC is also extensively used. Plasticizers may consist of liquids or solids (oils, esters or prepolymers). They are characterized by an extremely low glass-transition temperature, weakening the secondary bond strength in low-crystalline polymers with which they form a solid solution phase. 

Linear crystalline polymers always contain a fraction of amorphous material. For this reason they are usually considered biphasic systems. They show the typical transitions of amorphous polymers (glass and secondary) but also the common transitions of crystalline polymers (polymorphic, order-disorder, melting). Mechanical and physical properties of this category of polymers depend on morphology and amorphous/crystalline ratio, but also on the molecular mobility of the amorphous phase. 

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