Triclinic —> hexagonal transition

This is, for instance, the case of PTFE, which at atmospheric pressure presents two reversible first-order transitions at 19 °C and 30 °C [67], In the transition at 19 °C the molecular conformation changes slightly, from a 13/6 to a 15/7 helix and the molecular packing changes from an ordered structure with a triclinic unit cell (corresponding to a positioning of the chain axes nearly hexagonal) toward a partially disordered structure (partial intermolecular rotational disorder) with a... 

Many aliphatic poly(amides), more commonly known as nylons, exhibit an unusual phase transition below their melting point. First noted by Brill [122], the phenomenon has been studied extensively [128-131]. It is observed for instance in nylon (6,6) at a temperature of about 210°C, when the stable low-temperature triclinic oc phase of the crystalline polymer changes to a pseudo-hexagonal phase. 

PTFE resins exhibit a first-order transition at 19°C (66°F) due to a change of crystalline structure from triclinic to hexagonal unit cell (see Chapter 3, Section 3.2.1.3). A volume change of approximately 1% is associated with this transition (Figure 4.3). Another consequence is that the resin has a better powder flow below 19°C but responds more poorly to preform pressure. Billets prepared below this transition are weaker and tend to crack during sintering. For this reason, the resin should... 

Transition Temperature - This is a temperature (19°C for 100% homopolymer of tetrafluoro-ethylene) at which the unit crystalline cell of polytetrafluoroethylene changes from triclinic to hexagonal. 

Since transition through an optically isotropic phase, cubic, is excluded by x-ray data (12), the 57°C. extinction of rotation indicates the formation of a phase with disorder normally associated with a melt. This indicates that the chain separation and dislocation during the transition from the perpendicularly oriented q l hexagonal structure to the obliquely oriented 13l triclinic structure, Figure 2, are larger than has previously been described or implied. 

In 1953, Corbridge and Lowe confirmed that at normal temperatures white phosphorus is cubic with a = 18.51 A, and has a density of 1.83 g/cc (a form). It appears as glistening colourless polyhe-dra of various kinds if grown by slow sublimation in sealed tubes [26]. At -77°C the cubic form transforms to a hexagonal (later reported as triclinic) p form with a density of 1.88 g/cc. The transition point is raised to 64°C under a pressure of 11,600 atmospheres. The existence of a metastable low-temperature (monoclinic) y form, with density 1.94 g/cc, has also been claimed as well as other high-pressure forms [27,27a,28,29]. It appears not unlikely that quite a number of white varieties, each built from discrete P4 units, may be capable of existence. 

Crystal-Crystal Transition The transformation of one crystal structure of a polymer to a different crystal structure, for example, below 19 °C Teflon is triclinic and above 19 °C the packing is hexagonal. 

At temperatures above 180 °C, an additional melt peak forms besides the original double peaks recognizable in DSC [776], [777], Figure 5.206. This phenomenon is credited to Brill transitions. The usual crystal shape of polyamides is triclinic the result of Brill transition is a pseudo-hexagonal shape induced by post-crystallization at temperatures above 180 °C [778]. 

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