Semicrystalline miscible polymer blends

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). 

D IR spectroscopy has been applied extensively to studies of polymeric materials. A recent review of 2D IR spectroscopy cites numerous applications in the study of polymers by this technique [6]. In this section, some representative examples of 2D IR analysis of polymers are presented. We will start our discussion with a simple homogeneous amorphous polymer then move to more complex multiphase systems, such as semicrystalline polymers. Alloys and blends consisting of more than one polymer components are of great scientific and technical importance. Both immiscible and miscible polymer blend systems may be studied by 2D IR spectroscopy. Analysis of microphase-separated block copolymers is also possible. Finally, the possible application of 2D IR spectroscopy to the studies of natural polymers of biological origin is explored. 

SMP based on miscible blends of semicrystalline polymer/amorphous polymer was reported by the Mather research group, which included semicrystalline polymer/amorphous polymer such as polylactide (PLA)/poly vinylacetate (PVAc) blend [21,22], poly(vinylidene fluoride) (PVDF)/PVAc blend [23], and PVDF/polymethyl methacrylate (PMMA) blend [23]. These polymer blends are completely miscible at all compositions with a single, sharp glass transition temperature, while crystallization of PLA or PVDF is partially maintained and the degree of crystallinity, which controls the rubbery stiffness and the elasticity, can be tuned by the blend ratios. Tg of the blends are the critical temperatures for triggering shape recovery, while the crystalline phase of the semicrystalline PLA and PVDF serves well as a physical cross-linking site for elastic deformation above Tg, while still below T ,. 

Another miscible semicrystalline polymer/amorphous polymer blend SMP is a polyethylene oxide (PEO)/novolac-type phenolic resin blend [24]. The blend was found to be completely miscible in the amorphous phase when the phenolic content is up to 30 wt%, and the crystalline melting temperature (T,f) of the PEO phase working as a transition temperature can be tuned. 

Neutron scattering is used to study the structure of polymers including polymer chain dimensions, conformations, miscibility of blends structures of block copolymers, semicrystalline polymers and biomacromolecules. The unique property of neutrality makes a neutron an excellent probing tool for... 

A large number of polymer blends contain one or two crystallizable components. The crystallization behavior of a polymer component in a blend is expected to be altered by the presence of the second blend component, whether both are completely miscible, partially miscible, or totally immiscible. Therefore, a profound scientific understanding of the crystallization behavior and the resulting semicrystalline structure in polymer blends is necessary for effective manipulation and control of their properties. 

Neutron scattering is being used in a wide variety of applications in the study cf polymer structure. These include studies of dimensions of polymer chains in solution, conformation of chains in networks to test theories cf rubber elasticity, miscibility of blends, structure of block copolymws and semicrystalline polymers, and size and shape of bio-macromolecules. Adsorbed polymer layers can be studied by neutron reflectivity. 

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. 

PVDF is among the few semicrystalline polymers that exhibit thermodynamic compatibility with other polymers,80 in particular with acrylic or methacrylic resins.81 The morphology, properties, and performance of these blends depend on the structure and composition of the additive polymer, as well as on the particular PVDF resin. These aspects have been studied and are reported in some detail in Reference 82. For example, polyethyl acrylate is miscible with polyvinylidene fluoride, but polyisopropyl acrylate and homologues are not. Strong dipolar interactions are important to achieve miscibility with PVDF, as suggested by the observation that polyvinyl fluoride is incompatible with polyvinylidene fluoride.83... 

This review has illustrated various properties of multiphase polymer systems obtained from computer simulation. Three modeling techniques - atomistic, coarse-grained, and atomistic-continuum modeling - are applied to miscibility of homopolymer/copolymer and homopolymer/homopolymer blends, compat-ibilizing effect of block copolymers, and mechanical properties of semicrystalline polymers, respectively. 

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]. 

There are a number of important factors governing the change of the crystallization rate and semicrystalline stracture of a polymer in blend systems. Those include the degree of miscibility of the constituent polymers, their concentration, their glass-transition and melting temperamre, the phase morphology and the interface structure in the case of immiscible blends, etc. 

Polyetherimide was found to be miscible with polyetheretherketone (PEEK) exhibiting a single T. Since PEEK is a semicrystalline polymer with a T of ca. 150°C, the blend should have... 

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