From Computational Materials Science of Polymers

The melting point is determined as the temperature at which a polymer transits from the crystalline state into the viscous flow state. In contrast to low-molecular substances, in which this process proceeds in a jump-like manner, in the case of polymers melting is observed in a temperature range. This happens due to polydispersity of polymeric chains, their branching and imperfection of crystallites formed. The equilibrium and experimental melting points are distinguished. The equilibrium melting point = ?H m/ ?S m, where ?H m is the melting enthalpy, ?S m is the melting entropy. The equilibrium melting point is determined by the point of the phase equilibrium between a monocrystal of the polymer and its melt. Since perfect monocrystals are difficult to obtain from the polymer, the equilibrium melting point is determined by methods of extrapolation, for example, by extrapolation of the dependence of the experimental melting point on the size of crystallites or on the molecular mass of the polymer.

The melting point T m is a physical characteristic and is most difficult to calculate. The case in point is calculation based on the chemical structure of the repeat unit of the polymer. Let us discuss two approaches to solution of this problem. One of them is based on estimation of the relation between the glass transition temperature T g and the melting point T m. It should be noted that, according to the Beaman rule [132], T

Copyright Cambridge International Science Publishing 2003 under license agreement with Books24x7

Products & Services
Industrial Gases
Industrial gases are pure elements, molecular compounds or mixtures that are gaseous or vaporous at room temperature and pressure.
Solder
Solders include low melting point metal alloys usually in wire, powder, preform or paste forms. Solders are metal alloys with low melting points that are used to join metals together.
Synthetic Fibers and Fabrics
Synthetic fibers and synthetic fabrics consist of bulk fibers, yarns, woven cloth or other textile products manufactured from polymer-based materials such as polyamide (nylon), polyester, aramid, or other spun thermoplastics.
Lead, Tin, and Low Melting Alloys
Lead, tin, and low melting alloys are non-ferrous alloys that are easily meltable with relatively low melting temperatures. They are used in the manufacture of solders, semiconductors, batteries, optical, and decorative products.

Topics of Interest

Overview Let us consider the characteristic of thermal stability of polymers, determined with the help of thermogravimetric analysis. The temperature dependences of the substance mass at continuously...

14.1 Theory Crystallinity is a state of molecular structure referring to a long-range periodic geometric pattern of atomic spacings. In semi-crystalline polymers, such as polyethylene (PE), the...

In the table the following symbols are used: The glass transition temperature is listed as T g; ?C p is the change of the heat capacity at T g for the fully amorphous sample; T m is the equilibrium...

11.1 Introduction Measurements of the depression of the melting point [1] of a sample are often used to determine its purity [2]. Melting endotherms, recorded using differential scanning calorimetry...

M. Manninen, K. Manninen and A. Rytk nen, Department of Physics, University of Jyv skyl , Finland E-mail: matti.manninen@phys.jyu.fi We have used classical and ab initio molecular dynamics to study...