Thermal properties influencing polymer melting

Polymer compounds vary considerably in the amount of heat required to bring them up to processing temperatures. These differences arise not so much as a result of differing processing temperatures but because of different specific heats. Crystalline polymers additionally have a latent heat of fusion of the crystalline structure which has to be taken into account. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by  

The cooling requirements will be discussed further in Section 8.2.6. What is particularly noteworthy is the considerable difference in heating requirements between polymers. For example, the data in Table 8.1 assume similar melt temperatures for polystyrene and low-density polyethylene, yet the heat requirement per cm is only 295 J for polystyrene but 543 J for LDPE. It is also noteworthy that in spite of their high processing temperatures the heat requirements per unit volume for FEP (see Chapter 13) and polyethersulphone are, on the data supplied, the lowest for the polymers listed. 

Polymer Melt temperature i°C) Mould temperature (X) SG Specific heat (Jkg- K ) Heat required to melt Heat removed on cooling  

The meli and mould temperatures and the value of the heal removed per gram on cooling are taken from the paper by Whelan and Goff. The values for the amount of heat required to raise the temperature to the melting point and the heat requirements per unit volume (both for heating and cooling) have been calculated from these data by the author. 

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Thermal properties that influence the melting of the polymer. 

An important chemical characteristic of a polymer is the chain length. Chain length strongly influences the thermal properties as illustrated in Table 2 for several aliphatic compounds of different lengths. Detailed studies have been published on the influence of chain length on melting temperatures of various polymers [5] or on Tg and Tm (melting temperature) of aromatic polymers [6]. 

The mutual repulsion between substituents may cause some displacement. As a result, the plane of symmetry is bent in the form of a helix. This occurs also in biopolymers (double-helix of deoxyribonucleic acid (DNA)). Different stereoisomers have different mechanical and thermal properties. For example, atactic polystyrene is an amorphous polymer whereas syndiotactic polystyrene is a crystalline substance. The chemical design of macromolecules determines their properties as extent of crystallization, melting point, softening (glass transition temperature), and chain flexibility which in turn strongly influence mechanical properties of the materials. 

Basic principle underlying the isothermal crystallization of PHAs will be emphasized in this chapter. Crystallization of polymer melts is often accompanied by a heat release in the system. This can be measured using differential scanning calorimeter (DSC). Following the isothermal crystallization of PHAs, the influence of thermal treatment of a semicrystalline polymer on the mechanical properties is also given. 

Both molecular structure and the thermal history of polymers can influence the morphology of the material. The morphological structure of a polymer can have considerable influence on the observed physical properties. Hence the study of the relationship between molecular structure and properties is never simple. It is not possible to explore the relationships between structure, morphology, processing history and observed properties in any significant way here. Nor is it possible to give due consideration to the melt or solution behaviour of polymers. However, it is instructive to consider very briefly some different types of mechanical behaviour observed in polymers. The behaviour of polymers in the melt and in solution should also be considered, but space does not permit it here. 

On the basis of the DSC data summarized in Table 8.3, the crystallization temperature (T ) of PLA fibers becomes lower than that of as-received PLA pellets. It is convincing that the crystallization process can be accelerated by the well-structured PLA molecular chains when tailored into the fiber-like form. By decreasing HMW PCL concentration from 15 to 9wt%/v, the glass transition temperature (T ) of PCL within PLA/PCL blends is reduced whereas the of PLA is hardly identified in that its point has overlapped the melting peak of PCL. The different thermal properties in terms of and melting temperature (T ) may be influenced by the variation of fiber diameters as well as electrospinning process for the orientation of polymer chains [32]. In comparison to HMW PCL with PLA in blends, LMW PCL gives rise to an evident increase in the of PLA, but a considerable decline in the of PCL. The solvent effect on thermal... 

Virtually every polymer can be used in film form. Most thermoplastic polymer films are prepared by conventional extrusion techniques based on calendering, casting, and blown-film, or tenter-film systems. Other polymers, which cannot be easily melted, are formed into films by solvent casting. In the selection of a film for a particular application, the properties of the poljuneric materials must be considered in view of the application. Thermal properties, molecular characteristics, and crystallinity of the polymer affect processing and film properties. Additives influence extrusion and orientation processes and improve film properties. 

Thermal properties of PET are greatly influenced by the presence of the aromatic benzyl group substituted in 1,4 (para) positions which provides an overall stiffness of the polymer repeating unit. It explains the relatively high thermal stability (for a polymer) of PET. The value of the experimental melting temperature of commercial PET (fibers) is in the range 250 - 265 °C. It is possible to calculate a theoretical value of 292 °C which can be experimentally approached if a correct annealing process is applied (8). 

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