Melting temperature crystallisation

A study by Yuan and co-workers [26] discusses the influence of molecular weight on the thermal and rheological behaviour of polyarylether ketone. Melt temperature, crystallisation phenomena and rheological properties were compared for polymers with different molecular weights. 

Isoeugenol, when cooled to a very low temperature, crystallises in fine needles, which melt at 34°, but it usually exists in a state of superfusion. 

Due to both kinds of branching leading to chain irregularities, the crystallisation of radical chain-polymerised polyethylene is strongly hindered. Its maximum degree of crystallinity is limited to about 50%, its melting temperature ranges from 80°C to 115°C and its density remains low ( 0.92). From this latter property, it received the name of low-density polyethylene (LDPE). 

The heat flow into (endothermic) or out (exothermic) of a sample as a function of temperature and time is measured using the technique of DSC. In particular, it is used to study and determine the temperature of thermal transitions. For polymers, these include Tg, the glass transition temperature, Tc, the (exothermic) temperature of crystallisation for polymers that can crystallise, and Tm, the (endothermic) melting temperature. A DSC measurement requires only a small amount of sample 2-20 mg of a film, powder, fibre or liquid samples can be analysed in a DSC pan. 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Physical properties to be controlled by these molecular factors include melting point, Tm (only the crystalline part shows a melting point) crystallisation temperature, glass transition temperature, Tg strength,... 

Physical Properties.—Arsenic tribromide is a solid at ordinary temperatures, crystallising in beautiful colourless rhombic prisms6 which possess a feebly aromatic odour 7 and are stable in dry air. In the presence of moisture slight fuming occurs. The crystals melt sharply at 31° C.8 The density 9 at 15° C. is 3-66 after fusion and resolidification, the product has density 3-54 at 25° C.10 The density of the liquid at various temperatures may be obtained from the expression 11... 

With regard to the crystallisation, polymers stand in sharp contrast to other materials such as metals that crystallise completely at the melting temperature, Tm the topological... 

Most pure substances have a definite melting temperature below which the change from a random liquid structure to a well ordered, periodic crystalline structure can occur this transformation is called crystallisation the reverse process is called melting. 

Those which do crystallise invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases of such polymers are characterised by their melting temperature (TJ. Many thermoplastics are, however, completely amorphous and incapable of crystallisation, even upon annealing. Amorphous polymers (and amorphous phases of semi-crystalline polymers) are characterised by their glass transition temperature (T), the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft). This transition corresponds to the onset of chain motion below T the polymer chains are unable to move and are frozen in position. Both T and T increase with increasing chain stiffness and increasing forces of intermolecular attraction. 

Caffrey and Bilderback have made a similar study for natural rubber. Using a Vidicon camera they concluded that the amorphous halo disappears while the preferentially oriented powder pattern appears at the same time. Holl et all., have studied the reversibility of this process in more detail. Thus in Fig. 51 the variation of the modulus and the draw ration are compared with selected Vidicon patterns. corresponds to the onset of the crystallization, 3- to the maximum in crystallisation and 3-i to the melting of the last crystallites upon relaxation. Note that and Xj, occurr, at different draw ratios. This is obviously due to the nucleation process which demands a certain overdrawing while the melting occurs at the equilibrium melting temperature. 

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