Miscible polymer blends melting behavior

A group of new, fully miscible, polymer blends consisting of various styrene-maleic anhydride terpolymers blended with styrene-acrylonitrile copolymer and rubber-modified versions of these materials have been prepared and investigated. In particular the effects of chemical composition of the components on heat resistance and the miscibility behavior of the blends have been elucidated. Toughness and response to elevated temperature air aging are also examined. Appropriate combinations of the components may be melt blended to provide an enhanced balance of heat resistance, chemical resistance, and toughness. 

This chapter, related to the crystallization, morphological structure and melting of polymer blends has been divided into two main parts. The first part (section 3.1) deals with the crystallization kinetics, semicrystalline morphology and melting behavior of miscible polymer blends. The crystallization, morphological strucmre and melting properties of immiscible polymer blends are described in the second part of this chapter (section 3.4). 

Table 3.12. Examples of crystaUizable miscible polymer blends exhibiting a complex melting behavior...

The PPE/PS mixtures are considered classical examples of miscible polymer blends. Within the accessible range of temperatures, single phase melts have been observed with the size of homogeneity below 20 nm. Dynamic flow behavior of PPE/PS blends, with the molecular weight ratio MW(PS)/MW(PPE) = 1, was studied in a wide range of temperatures and compositions [Prest and Porter, 1972]. The authors assumed additivity... 

Crystallization, Morphological Structure, and Melting Behavior of Miscible Polymer Blends... 

The miscibility of a polymer blend depends on temperature and blend composition, making the investigation of the dynamic behavior of miscible polymer blends very challenging. Associated with the dynamics of miscible polymer blends is the mutual diffusion that, as in determining the self-diffusion coefficient in polymer melts and solutions presented in Chapter 4, can be discussed using molecular theory. Thermodynamic interactions and free-volume effects determine the mutual diffusivity in miscible polymer blends. 

By modifying the functional groups they can be used,for example, as crosslinkers in high solid or powder coatings and in thermosets. Because of their good miscibility and low melt viscosity, they find applications as melt modifiers and as blend components. Modified hyperbranched polymers, like alkyl chain substituted poiy(ether)s and po-ly(ester)s sometimes exhibit amphiphilic behavior.They can, therefore, be used as carriers for smaller molecules,for example, dyestuff into polypropylene. 

With regard to miscible polymers, simple blending allows one to develop materials that frequently reveal an intermediate behavior between those of the individual blend components [2, 3], Such systems can be easily exploited for fine-tuning the foam-ability, for example by controlling important foaming parameters such as the melt rheology or the gas solubility. 

When a block copolymer is blended with a homopolymer that differs in composition from either block the usual result is a three-phase structure. Miscibility of the various components is not necessarily desirable. Thus styrene-butadiene-styrene block copolymers are recommended for blending with high density polyethylene to produce mixtures that combine the relative high melting behavior of the polyolefin with the good low temperature properties of the elastomeric midsections of the block polymers. 

The other possibility for what is occurring in the melt is that the addition of component (2) destroys the domain structure of the copolymer. It has been proposed (21) that the domain structure of liquid crystal polymers is responsible for the shear-thinning behavior which is experimentally observed. Since the blends are less shear-thinning than the copolymer, it is suggested that the domain structure which is responsible for this effect has been broken up. This suggestion would indicate a certain miscibility in the melt which remains in the solid state. 

The formation of miscible rubber blends slows the rate of crystallization (Runt and Martynowicz, 1985 Keith and Padden, 1964) when one of the components is crystallizable. This phenomenon accounts for data that show lower heats of fusion that correlate to the extent of phase homogeneity (Ghijsels, 1977) in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point (Nishi and Wang, 1975 Rim and Runt, 1520). This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

Zhu et al. studied a biocompatible shape memory polymer blend based on poly(e-caprolactone) (PCL) and polymethylvinylsiloxane (PVMS). Pure PCL was subject to scission rather than cross-linking under irradiahon. In the presence of a small amoimt of PVMS (<20 wt%), both polymers are miscible in the amorphous phase and the radiation cross-linking of PCL is enhanced. Mechanical properties were improved, and a strong shape memory behavior was achieved. Above the melhng point of PCL, the blend exhibited a rubber-like state and could be deformed. The switch temperature was the melting temperature of PCL. With 5 to 15 wt% of PVMS and under 100 kGy y-irradiation, the deformation fixation ratio and the deformation recovery ratio were 100%. 

In order to investigate miscibility and phase behavior of polymer blends, differential scanning calorimetry (DSC) has been frequently used for determination of glass transition temperature, crystalline melting temperature and other thermal properties. 

The former discussion deals with liquid-liquid phase behavior however, one or both components of the blend can sometimes crystallize. For a polymer pair that is miscible in the melt, cooling well below the melting point of pure ciystallizable component leads to a pure crystalline phase of that component. Far below the melting point, the free energy of crystallization is considerably larger than that of mixing- Because polymers never become 100% crystalline, the pin-e crystals coexist with a mixed amorphous phase consisting of the material that did not crystalhze (6,7). 

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