Lec.15 Crystallization, Morphological Structure & Melting of PB…….…..Eng.Auda Jabbar Ms.C.
Lec-15
Crystallization ,Morphological Structure ,and Melting
of Polymer Blends
Introduction:
The concept of a crystal-amorphous (also order-disorder) interface was
first proposed by Flory [1962] for binary semi-crystalline/amorphous
blends. The order-disorder interphase was defined as the region of loss
of crystalline order.
Kumar and Yoon [1991] examined this interface and found that in
blends the thickness of this transition zone was essentially independent of
the interaction parameter between the two polymers (when χ12 varied
from -1 to -0.005).
Following the theoretical predictions the thickness of this region
increases only slightly when stiffer chains are considered. Due to the
higher degree of order of segments of the crystallizable component in this
zone, the penetration of the amorphous component is limited.
The compositional interphase, however, is influenced by the stiffness of
both chains and by the interaction parameter (the interfacial thickness
varies with the reciprocal of |χ12|1/2) .
When the melt of a crystalline polymer is cooled to a temperature
between the glass-transition and the equilibrium melting point, the
thermodynamic requirement for crystallization is fulfilled.
The crystallization of miscible and immiscible polymer blends can differ
remarkably from that of the neat crystallizable component(s).
The overall crystallization kinetics of blends can often be described by
the Avrami equation:
Where:
: is the weight fraction of crystallinity at time t,
n: is the Avrami index depending on the type of nucleation and the
crystal growth geometry
k: is the Avrami constant related to the crystallization rate:
Where :
tn1/2 : is the half time of crystallization (the time for half the
crystallinity to develop), which is often used as a measure for the
overall rate of crystallization.
Lec.15 Crystallization, Morphological Structure & Melting of PB…….…..Eng.Auda Jabbar Ms.C.
Avrami equation can be rewritten as:
Plotting the left part of this equation against log t should result in a
straight line, from which both Avrami parameters, n (slope) and k
(intercept),can be obtained.
In table bellow, some literature data on the Avrami constants and the
half time of crystallization are presented.
1. Crystallization, Morphological Structure, and Melting Behavior
of Miscible Polymer Blends
When crystallized from the melt, most polymers show a spherulitic
texture (Figure 1). The spherulites then consist of lamellar stacks of
alternating crystalline and amorphous layers, radiating from the center
(the primary nucleus).
Fig. 1: Schematic representation of the spherulitic texture of a
semicrystalline polymer
Lec.15 Crystallization, Morphological Structure & Melting of PB…….…..Eng.Auda Jabbar Ms.C.
A- Modes of Segregation of the Amorphous Component during
Crystallization in Crystalline/Amorphous Blends
In blends of a crystallizable polymer with an amorphous one, the
morphology is largely determined by the type of segregation of the
amorphous component.
Crystallization in a miscible blend involves two types of polymer
transport:
1-Diffusion of the crystallizable component towards the
crystallization front, and
2-Simultaneous rejection of the amorphous component.
This latter phenomenon is called segregation; it can take place at three
different levels(Figure 2):
A- Interspherulitic
B- Interfibrillar
C- Interlamellar
Fig.2 Schematic representation of the different types of segregation of the
amorphous component in crystallizable miscible polymer blends (full
lines: crystallizable component, dotted lines: amorphous component).
Interspherulitic segregation , in which the spherulites are imbedded in
an amorphous matrix, can be distinguished from the other two types using
optical microscopy.
In the case of intraspherulitic segregation a volume-filling texture is
observed; the amorphous components can be located either between the
lamellae (interlamellar) or between stacks of lamellae (interfibrillar).
To find out whether or not interlamellar segregation occurs, small angle
X-ray scattering (SAXS) can be used. The increase of the long spacing,
which is the sum of the average thickness of the crystalline and
amorphous layers, as well as the increase of the thickness of the
amorphous layers between the crystalline lamellae, with increasing
Lec.15 Crystallization, Morphological Structure & Melting of PB…….…..Eng.Auda Jabbar Ms.C.
concentration of the amorphous component are parameters that are often
used as indications for interlamellar segregation
►In a crystallizable miscible blend, the presence of an amorphous
component can either increase or decrease the tendency to crystallize
depending on the effect of the composition of the blend on its glasstransition and on the equilibrium melting point of the crystallizable
component.
►The type of segregation of the amorphous component, influenced by
parameters such as :
1-Crystallization conditions
2-Chain microstructure
3-Molecular weight
4-Blend composition
B- Modes of Crystallization in Crystalline/ Crystalline Blends
When dealing with miscible blends containing two crystalline
components, several modes of crystallization are possible:
1-Separate crystallization:
A spherulitic crystallization was observed for the neat components
as well as for blends with small amounts of one component, and the
crystals of the minor component were included within the spherulites of
the major component, which results in a coarsening of the spherulitic
texture.
2-Concurrent crystallization:
A simultaneous (or concurrent) crystallization can only occur when the
crystallization temperature ranges overlap and if the crystallizability of
both blend components is similar.
3-Co-crystallization:
Co-crystallization requires chemical compatibility, close matching of
the chain conformations, lattice symmetry and comparable lattice
dimensions.
Spherulite Growth of the Crystallizable Component
1. Spherulite Growth Rate in Homopolymers
In the case of homopolymers, the growth rate of a lamellar crystal is
controlled by two processes:
1- By the ability of forming a surface nucleus (as determined by the
degree of undercooling, ΔT = Tm° - Tg),
2- By the ability of diffusion of the chain molecules towards the
crystal growth front (determined by the difference between the
crystallization temperature, Tc and the glass-transition temperature,
Tg).
Lec.15 Crystallization, Morphological Structure & Melting of PB…….…..Eng.Auda Jabbar Ms.C.
Both processes are inversely dependent on temperature, a maximum
rate of crystal growth is usually observed at temperatures close to Tmax
≈ (Tg + Tm)/2.
2. Spherulite Growth Rate in Miscible Polymer Blends
When dealing with crystallizable miscible polymer blends containing a
non-crystallizable component, some refinements had to be made.
Some modifications were proposed for blends in which the amorphous
component is segregated into the interlamellar region :
1-The chemical potential of the liquid phase might be altered by the
specific interactions, that are often responsible for the miscibility of
polymers . Such interactions may change the free energy required to form
a critical nucleus as well as the mobility of both the crystalline and
amorphous components.
2-The non-crystallizable component has to diffuse away from the
crystal growth front into the interlamellar region.
Thus the rate at which the growth front progresses depends on the
competition between the inherent capability of the crystal to grow and on
the rate of rejection (segregation) of the amorphous component.
The kinetics of crystal growth will ultimately be determined by the
slower of these two phenomena. A direct consequence of this
consideration is the dependence of the crystal growth rate on the
molecular weight of both components.
3- The concentration of the crystallizable component at the growth front
will decrease crystallization.
4- The glass-transition temperature and the melting temperature can be
influenced by addition of an amorphous polymer.
Melting Behavior of Crystallizable Miscible Blends
The equilibrium temperature of a polymer (blend) can experimentally be
determined by a Hoffman-Weeks plot, which is a plot of the
experimental melting point versus the crystallization temperature (Tm
vs. Tc) as presented in figure bellow.
Extrapolation from experimental data to the Tm = Tc line results in the
value of Tm°.
Lec.15 Crystallization, Morphological Structure & Melting of PB…….…..Eng.Auda Jabbar Ms.C.
Fig. ( ). Hoffman-Weeks plot for PCL rich PCL/PC blends
The influence of Tc on Tm is due to morphological contributions such as
degree of perfection and the finite size of crystals.
If the crystals are perfect and of finite size and no recrystallization
takes place during the melting, the Tm vs. Tc data can be described by:
Where:
Tm° and Tm are the equilibrium and observed melting point
φ is the stability parameter that depends on the crystal thickness,
and assumes values between 0 and 1
The value of φ = 0 implies Tm = Tm° for all Tc ,whereas φ = 1 implies Tm =
Tc. Therefore, the crystals are most stable at φ = 0 and inherently unstable
at φ = 1.
Melting Point Depression:
Melting point depression data are often used to determine the HugginsFlory interaction parameter, χ12 , that is a measure for the miscibility of
the blend, i.e., χ12 is negative for a miscible blend. A lack of melting point
depression means that χ12 is zero.