Miscible polymer blends crystal growth rate

The spherulitic growth rate of the miscible polymer blends crystallized isothermally from melt that consists of a crystallizable and a non-crystallizable component can be measured using a polarizing optical microscope with a hot stage. The crystallization temperature T was determined with the hot stage. The experimental procedure is the same as that for the homo-polymer mentioned above ... 

Incorporating the concepts discussed above, the equation describing the crystal growth rate in a miscible polymer blend can be expressed as ... 

Table 3.4 refers to a number of crystaUizable miscible polymer blends for which the sphemhte growth rate as a function of the crystallization temperature has been investigated. For most blends, only a part of the bell-shaped curve could be measured. In Fig. 3.8, the complete bell-shaped spherulitic growth rate curve of iPS in iPS/PS blends containing 0,15, and 30 wt% PS is shown. Due to the addition of impurity (e.g., the amorphous PS), a suppression of the growth rate is observed, which is greater than the concentration of the impurity added. Important parameters of the impurity added to the crystaUizable component are the type, concentration, and molecular weight (Keith and Padden 1964). 

Growth Rate of Miscible Polymer Blend Spherulites Crystallized Isothermally from the Melt Measured by Polarizing Optical Microscopy [74]... 

The chapter is divided into five sections. Section 10.2 deals with the thermodynamics of polymer blends the general principles and the main theories on the phase behavior of polymer mixtures are briefly presented. Section 10.3 deals with the properties of miscible blends with crystallizable components. The phase morphology, crystal growth rate, overall crystallization kinetics, and melting behavior of miscible blends are analyzed. The crystallization phenomena in blends with miscibility gap are also described. Then, examples of miscible systems comprising one or two crystallizable components are reported with particular attention to the thermodynamic and kinetic aspects of the crystallization process. 

This is due to the effects on nucleation and growth rates. Thus, blending method may have serious effects on crystaHizability and crystal size. Experimentally, the presence of a miscible, amorphous polymer in the blend usually slows down, or it even prevents, crystallization of the semicrystalline resin. The enhancement of crystallinity and increase in T on blending have also been reported [Harris and Robeson, 1987 Dumoulin et ai, 1987]. As a result, the T method is far from being fool-proof and the obtained values of Xi2 should be confirmed by other techniques [Utracki, 1989 Groeninckx et al., 1998]. 

Part of the reason for the difficulty in making hard and fast rules in these systems is that both nucleation and growth contribute to overall crystallization rate and blending has the potential to be affective either in subtle and possibly independent ways. This is illustrated by the work of Van Ende et al. [164] who found that a semicrystalline polymer formed a miscible blend with a flexible EP and this resulted in increased nucleation density and growth rate whilst the addition of a more rigid PLC of similar chemical structure but only partial miscibility was... 

In blends composed of immiscible polymers, amorphous polymer does not affect the crystallization of ciystallizable polymer, but if two polymers are miscible, amorphous polymer acts as diluent and affects crystallization of the second polymer. Poly( -caprolactone) is a ciystallizable component of the blend with poly(vi-nyl butyral), which is studied in compositions containing carbon black. Typically, blends of these two polymers form very large spherulites, and it is interesting to find out how carbon black affects crystallization and other properties of the blend as well as the distribution of carbon black in relationship to the spherulites. Figure 16.6 shows that spherulite growth rate is independent of carbon black presence (points of carbon black filled and carbon black free blend follow the same relationship). Additional data show that crystallization rate decreases with the amount of PVB increasing. Carbon black aggregates are mainly found in spherulites. 

For miscible blends, the spherulitic growth rate equation can be employed to predict the crystallization kinetics. This equation, commonly employed for unblended crystalline polymers, is ... 

In a few cases, the addition of minor amounts of immiscible or miscible polymers results in the nucleation of a crystalline polymer. The nucleation of PP by PE and polyamides (e.g., PAl 1) (immiscible) as well as the addition of PP to poly(butene-l) (miscible) has been noted in the literature [138-141 ]. The addition of LDPE to PP showed a reduction in the spherulite size of PP, attributed to an increase in nucleation density of the a-crystalline form along with an increase in the rate of growth of the -crystalline form [141]. The nucleation of polycarbonate by the zinc salt of sulfonated polystyrene ionomers was noted to occur with both miscible and phase separated blends [142]. Nanometer sized ionic aggregates appeared to contribute to the polycarbonate nucleation. A liquid crystalline copolyesteramide (Vectra-B950 ) was shown to accelerate the crystallization of poly(phenylene sulfide) [143]. This effect was not concentration dependent and did not change the level of crystallinity. 

For a miscible blend containing a crystallizable component, following the treatment previously described for the spherulite growth rate in a polymer-diluent system (Eq. 10.13), the temperature dependence of the kinetic constant for the overall crystallization rate, under isothermal conditions becomes [37] ... 

The miscibility of the components also affects the primary nucleation behavior of the crystallizing polymer. In iPP/iPB blends the presence of iPB influences both heterogeneous and homogeneous nucleation of iPP spherulites. The rates of homogeneous nucleation and spherulite growth of iPP were related to the variation of the average spherulite radius as a function of the crystallization temperature [61]. 

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