Polymer Crystallization
Crystalline states offer hardness and toughness necessary to polymer materials. The liquid crystalline state sometimes occurs as a stable or metastable mesophase during phase transitions. The mean-field lattice theory predicts the properties of equilibriu
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Polymer Crystallization
10.1
Thermodynamics of Polymer Crystallization
The phase transition from disordered states of polymer melt or solutions to ordered crystals is called crystallization; while the opposite process is called melting. Nowadays, more than two thirds of the global product volumes of synthetic polymer materials are crystallizable, mainly constituted by those large species, such as high density polyethylene (HDPE), isotactic polypropylene (iPP), linear low density polyethylene (LLDPE), PET and Nylon. Natural polymers such as cellulose, starch, silks and chitins are also semi-crystalline materials. The crystalline state of polymers provides the necessary mechanical strength to the materials, and thus in nature it not only props up the towering trees, but also protects fragile lives. Therefore, polymer crystallization is a physical process of phase transition with important practical relevance. It controls the assembly of ordered crystalline structures from polymer chains, which determines the basic physical properties of crystalline polymer materials. The crystallization and melting behaviors of polymers are conventionally measured by the method of differential scanning calorimetry (DSC). One can obtain the heat flow or compensation power dQ/dt as a function of temperature, which is in principle proportional to the heat capacity of materials CP and the scanning rate q, as given by dQ dQ dT ¼ ¼ Cp q dt dT dt
(10.1)
At a constant heating rate, we will observe a curve containing a pronounced peak for the first-order phase transition, as illustrated in Fig. 10.1a. The cooling curve exhibits an exothermic peak Tc corresponding to the crystallization, while the heating curve shows an endothermic peak Tm corresponding to the melting. For small molecules, the onset temperature of the melting peak is normally taken as the melting point, but for polymers, due to the existence of a broader melting range, the
W. Hu, Polymer Physics, DOI 10.1007/978-3-7091-0670-9_10, # Springer-Verlag Wien 2013
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Polymer Crystallization
Fig. 10.1 Illustration of (a) DSC curves corresponding to crystallization Tc and melting Tm of polymers upon cooling and heating processes, respectively; (b) free energy curves of amorphous and crystalline states of polymers, with the equilibrium melting point given by the crossover of two curves. The arrows indicate the phenomenon of supercooling
peak temperature is taken as the melting point Tm. In principle, when the crystal and the melt are at thermodynamic equilibrium, Tc ¼ Tm
(10.2)
As illustrated by the crossover point of the curves in Fig. 10.1b, the isobaric free energy change of the polymer bulk system at the melting point appears as DFm ¼ DHm Tm DSm ¼ 0
(10.3)
Therefore, Tm ¼
DHm DSm
(10.4)
One can see that, as illustrated in Fig. 10.1a, the practical Tc is always lower than Tm. The volume-temperature curves for crystallization/melting are roughly the same results. Such a hysteresis loop is an important feature of first-order phase transitions. If we make
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