Crystalline-Amorphous Blend

4 Optical Characterization of Mesoscale Morphologies in Polymer Blends 1533 (C) 

Maltese cross (regime III) to dendritic (regime II). The blending of PCL with different amorphous polymers suppresses the growth rate and induces spherulitic patterns that are different from those in neat PCL. 

Role of Polymer Tacticity on Polymer Blend Morphologies 

The effect of tacticity (i.e., the stereochemical arrangement of the units in the main chain of a polymer) on the properties of polymers and polymer blends has long been recognized with such basic differences as in the Tg, miscibility, crystallization, and blend characterization, including their mesoscale morphologies. In general, isotactic polymers (where all substituents are located on the same side of the polymer backbone) are semicrystalline in nature, whereas atactic polymers (where all substituents are placed randomly along the backbone) are amorphous. 

Typically, Maltese cross patterns were observed for blends with equimolecular-weight components. In such blends, the kinetics of crystal (spheruHte) formation depended on the of the parent components, whereas the Tn was dependent on 

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Model C is mostly found for a blend of crystalline and amorphous polymers. In general, the miscibility for the crystalline/amorphous blends would be better in an amorphous component-rich system than that of a crystalline-rich system. For example, when the crystalline PEO composition is more than 60 wt% in PEO/amorphous PVPh, PEO in the blend showed two Tip relaxation times (Table 10.2) [34]. One of the two Tip agrees well with Tip... 

Modes of Segregation of the Amorphous Component during Crystallization in Crystalline/Amorphous Blends... 

Table 3.14. Thermal data on immiscible crystalline/amorphous blends (after Nadkarni and Jog [1991])...

Some examples of the final semicrystalline morphology in several immiscible crystalline/ amorphous blend systems have already been given in Table 3.21 for the discussion of the spherulite growth rate (Part 3.4.3.2). Some more information about this topic can be found in the articles listed in Table 3.23. 

Table 3.24. Examples of the melting behavior of the ciystaUizable matrix in some crystalline/amorphous blend systems.

For most commonly studied polymer blends, crystallization of the matrix occurs in the presence of a molten dispersed phase. The crystallization behavior of the continuous phase can be compared to that found for crystalline/amorphous blend systems in which the dispersed amorphous phase was in the molten state. 

S Melting Behavior of the Crystalline Matrix in Crystalline/ Amorphous Blends... 

Figure 6.7a depicts the IDF profile from different compositions of melt-miscible crystalline/amorphous blends of PCL/PVC (i.e., polycaprolactone (PCL)/poly(vinyl chloride). The maxima and minima in the obtained IDF profile, and their deconvolution, provides an estimate of the long period, I (negative peak) and the thickness of crystalline (Ic) and amorphous layers (y as either li and (2 or vice-versa, as shown in Figure 6.7b. The entire IDF profile has been fitted to the superimposition of three peak contributions two positive peaks (corresponding to Ic and la) and one negative peak (L), assuming their Gaussian distribution.

The morphology of polymer blends that consist of one crystallizable component and one noncrystallizable amorphous compound has been studied in detail by several groups, with attention focused on various crystalline-amorphous combinations and compositions [48-50]. A single l -value always characterizes crystalline-amorphous blends, and is typically an intermediate value between that of the pure homopolymers the exact Tg depends on the weight fraction of the two polymers, however. Flighly crystalline polymers that have been studied in crystalline-amorphous blends include poly(e-caprolactone) (PCL), poly(phenylene oxide) (PPO), poly(ethylene oxide) (PEO), poly(butyleneterephthalate) (PBT), poly(ethyleneter-ephthalate) (PET), and high- and low-density polyethylene (HOPE and LDPE, respectively), whereas poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) are examples of amorphous polymers used for blending studies (51j. 

As a rule of thumb the crystalline-amorphous blend composition is soft (the behavior is more amorphous than crystalline) when the blend contains more than 70% of the amorphous component, but becomes rigid and crystalline when the weight fraction of the crystalline component exceeds 30% of the blend. Besides composition, aging of the blend also determines the compatibility of the two polymers. For instance, the crystallization rate and induction time for crystallization is critically dependent on the concentration of the components and aging (5 2). Crystalline interactions have been shown to exist when PCL is blended with polyethylene and polypropylene, with a-relaxation in polyethylene being affected in particular. However, because this effect is interrelated to motion in the polyethylene crystallites it was elucidated by assuming that the blend might be cocrystalline in nature. 

Y. (2002) Miscibility and morphology in crystalline/amorphous blends of poly (caprolactone) /poly(4-vinylphenol) as studied by DSC, FTIR, and C solid state NMR. Polymer, 43 (4), 1357-1364. 

Table4.5 Additional Examples of Crystalline-Amorphous Blend Combinations ...

To analyze the crystallization behavior of crystalliz-able polymer blends, it is suitable to distinguish between systems containing only one crystallizable component (crystalline/amorphous blends) and those containing both crystallizable components (crystalline/crystalline blends). In all cases, the crystallization of a polymer blend will take place in a defined temperature range, between the values of the glass transition temperature, Tg, and the equilibrium melting temperature of the crys-taUisable polymer, below Tg the chain mobility is inhibited, while at temperatures near T° the crystal nucleation does not occur. 

Crystallizable immiscible blends may consist of both crystallizable polymer components (crystalUne/crystaUine blends), or of one crystalline component (crystalline/ amorphous blends), which can be present as matrix phase or as dispersed phase. The crystallization behavior and the structure-properties relationships of immiscible blends of various polymer classes have been investigated by numerous authors a comprehensive overview of the crystallization phenomena and morphological characteristics of these systems has been reported in References [19,72],... 

When the crystallization of the matrix polymer takes place in the presence of a molten dispersed phase, the crystallization behavior is comparable to that of crystalline/amorphous blends in which the amorphous component is the dispersed phase. However, a different behavior may be observed when the crystallization of the matrix occurs in the presence of a crystallized dispersed phase. Coincident crystallization of the components has been reported for blends in which the matrix phase has a crystallization temperature lower than that of the dispersed component, and this latter does not crystallize at its usual undercooling, owing to its very fine dispersion into the blend, which causes a lack of heterogeneities that is able to initiate the crystallization of the droplets at their characteristic T. In such case, as reported for PVDF/polyamide-6 (PA6) and PVDF/PBT blends... 

Figure 4.10 Phase diagram of a crystalline/ amorphous blend having Tn = 350K,...

In the Matkar-Kyu model, the crystal-amorphous interaction parameter was assumed to be inversely proportional to the absolute temperature, that is, Xca = coAHu/RT, where AH is heat of fusion of the crystal in the pure state and T is absolute temperature. However, the analytical solution for the melting point depression was not sought in their original approach [66, 67]. This deficiency has been remedied recently by Rathi et ol. [75] for a crystalline/amorphous blend by solving the combined FH/PF free energies and obtaining an analytical expression from the equilibrium conditions, viz., df/d i = 0 and/(i )) = 0 as described below ... 

For shape memory applications, crystals formed upon cooling of the semicrystalline polymer act as physical crosslinks by which a permanent or equilibrium shape can be set. In a complementary fashion, the miscible amorphous phase governs temporary shape fixing. In turn, crystallization kinetics, crystallite size, and degree of crystallinity collectively determine kinetics of permanent shape setting and shape memory performance characteristics. Thus, it is possible to tailor the shape memory properties of a system by varying the composition and the thermal history of such blends[9], leading us to the present study focused on crystallization kinetics from melt-miscible crystalline-amorphous blends. 

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