The Crystalline Melting Point

In analogy to the Molar Glass Transition Function, defined by Eq. (6.4), we shall define the equivalent Molar Melt Transition Function by  

The function Ym (like Yg) has the dimension (K kg moF1). The group increments for this function could be derived from the available literature data on crystalline melting points of polymers, totalling nearly 800. The quantity Ym (like Yg) does not show simple linear additivity due to intra- and inter-molecular interactions between structural groups. The available group contributions and their structural corrections are summarised in Table 6.8. We shall again discuss these data step by step. 

Derivation of the group contributions to Ym 6.3.2.I. The unbranched polymethylene chain 

It is known from the literature that the melting point of pure polymethylene is 409 K. This 

Aliphatic carbon chains with small side groups (substituted polymethylene chains) 

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Because of the high melt viscosity of polyolefins, normal spinning melt temperatures are 240—310°C, which is 80—150°C above the crystalline melting point. Because of the high melt temperatures used for polyolefin fiber spinning, thermal stabilizers such as substituted hindered phenols are added. In the presence of pigments, the melt temperature must be carefully controlled to prevent color degradation and to obtain uniform color dispersion. 

In some appHcations the high heat stabiHty of the micropowder can be utilized over a reasonably wide temperature range. A maximum service temperature is normally 260°C, provided the crystalline melting point is between 320 and 335°C. Exposure above 300°C leads to degradation and possible evolution of toxic decomposition products. 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

Fig. 2. Effect of polymerization temperature on the crystalline melting point of chloroprene mbbers produced by emulsion polymerization ...

In the case of a crystalline polymer the maximum service temperature will be largely dependent on the crystalline melting point. When the polymer possesses a low degree of crystallinity the glass transition temperature will remain of paramount importance. This is the case with unplasticised PVC and the polycarbonate of bis-phenol A. 

Since the polymer is far from completely stereoregular the level of crystallinity is fairly modest, with a typical value of 25%. The crystalline melting point is about 90°C and the is -23°C. Such materials are rheologically similar to LDPE. Bottles and films are transparent. The particular features of the polymer... 

As with other crystalline polymers, the incorporation of glass fibres narrows the gap between the heat deflection temperatures and the crystalline melting point. 

As with the aliphatic polyamides, the heat deflection temperature (under 1.82 MPa load) of about 96°C is similar to the figure for the Tg. As a result there is little demand for unfilled polymer, and commercial polymers are normally filled. The inclusion of 30-50% glass fibre brings the heat deflection temperature under load into the range 217-231°C, which is very close to the crystalline melting point. This is in accord with the common observation that with many crystalline polymers the deflection temperature (1.82 MPa load) of unfilled material is close to the Tg and that of glass-filled material is close to the T. ... 

The copolymers are prepared using a mixture of dimethyl terephthalate and dimethyl naphthalate. Published data indicates a reasonably linear relationship between and copolymer composition on the lines discussed in Section 4.2, e.g. Tg for a 50 50 copolymer is about 100°C which is about mid-way between Tg figures for the two homopolymers. In line with most other copolymers there is no such linearity in the crystalline melting point (T, ). As comonomer levels are introduced drops from the values for both homopolymers and indeed crystallisation only readily occurs where one of the components is dominant, i.e. 80%. Thus commercial copolymers are usually classified into two types ... 

These adhesives differ from normal hot-melt adhesives, such as the standard ethylene vinyl acetate hot melts. Standard hot-melt adhesives like EVA have no curing mechanism. They are heated above the crystalline melting point and applied as a low-viscosity liquid in the same manner as is the curing hot melt. The bond is closed in the same manner and strength is developed upon crystallization. 

Features of chemical structure that affect the degree of molecular freedom influence both the crystalline melting point and the glass transition temperature. Moreover, such features have roughly similar effects on both properties, so that the empirical rule has been found that for many polymers ... 

The crystallinity is more persistent for the polypentena-mer over the same increments of comonomer or 1,5-cyclooctadiene introduction. The crystalline melting point is 8°C for the polypentenamer, 2°, -2°, and -8°C at 5, 10 and 15% comonomer (1,5-cyclooctadiene), respectively, from Table III and -13°C at 25% comonomer from Table II. The position of the crystallization temperature during the heating cycle moves upward from -69°C for polypentenamer, to -65°C at 10% comonomer, to -61°C at 15% and -55°C at 25% comonomer. 

The two main transitions in polymers are the glass-rubber transition (Tg) and the crystalline melting point (Tm). The Tg is the most important basic parameter of an amorphous polymer because it determines whether the material will be a hard solid or an elastomer at specific use temperature ranges and at what temperature its behavior pattern changes. 

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