Polymer Physics

Chapter 10

Polymer Crystallization

10.1 Thermodynamics of Polymer Crystallization

The phase transition from disordered states of polymer melt or solutions to ordered
crystals is calledcrystallization; while the opposite process is calledmelting.
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 crystal-
line 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 materialsCPand the scanning rateq,
as given by

dQ

dt

¼

dQ

dT



dT

dt

¼Cpq (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 peakTccorresponding to the crystallization, while the
heating curve shows an endothermic peakTmcorresponding 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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