Tham khảo tài liệu 'fundamentals of polymer engineering part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 11 Polymer Crystallization INTRODUCTION Low-molecular-weight materials such as metals typically exist as crystals in the solid state. The driving force behind the formation of crystals which are structures having a long-range periodic order is the lowering in free energy that accompanies the process of crystallization. Thus if we were to plot the Gibbs free energy per unit volume G of a material in both the solid crystalline and molten forms as a function of temperature we would get a plot of the type shown in Figure the decrease in free energy with increasing temperature for both phases is due to the relative increase in the temperature-entropy term. The point of intersection of the two curves is the equilibrium melting point tM whereas the vertical difference between them represents the free-energy change DGv between the two states at any temperature. Note that many materials such as iron exhibit polymorphism that is they exist in more than one crystalline form. In such a case each form has its own G versus temperature curve. Long-chain molecules can also crystallize and they do so for the same energetic reasons as short-chain molecules. However for a polymer to be crystallizable its chemical structure should be regular enough that the polymer molecule can arrange itself into a crystal lattice. Thus isotactic and syndiotactic polypropylenes crystallize easily but atactic polypropylene does not. For the same reason the presence of bulky side groups as in polystyrene hinders crystallization but the possibility of hydrogen-bonding as in polyamides 437 Copyright 2003 Marcel Dekker Inc. 438 Chapter 11 Figure Variation with temperature of the Gibbs free energy per unit volume. promotes the process. Figure shows a schematic representation of nylon 66 1 . A unit cell contains a total of one chemical repeat unit and the molecules are in the fully extended zigzag conformation. The polymer chain direction is generally labeled the c axis of the cell. The
