Properties of Polymer Blends

The properties of polymer blends depend on the physical and chemical properties of the participating polymers and on the state of the phase (homogeneous or heterogeneous). Phase morphology (particle shape and size as well as size 

Soft matrix + soft dispersed Elastomer blends 

Hard matrix + hard dispersed Polystyrene + polyphenyleneether (homogeneous) 

Apart from binary polymer blends, also blends consisting of three different polymers have found technical applications. Belonging to this group are mixtures of polypropylene, polyethylene, and ethylene/propylene elastomers. 

Even the outstanding mechanical properties of natural products are based on the principle of polymer blends, like, for example, wood, which is composed in a complicated way of cellulose and lignin. 

The mechanical properties of polymeric materials including blends are reported in detail in commercial product literature and provide a basis of comparison of the engineering properties of materials for various end-use applications. The specific mechanical properties of interest include the modulus (tensile, flexural or bulk), strength (tensile, flexural or compressive), impact strength, ductility, creep resistance as well as the thermomechanical properties (e.g., heat distortion temperature). The mechanical property profile can be employed to determine the compatibility of the blend by comparison with the unblended constituents. Compatibi-lization methods can be evaluated easily by comparison of the mechanical property profile with and without compatibihzation. 

The viscoelastic properties of polymer blends determined by dynamic mechanical analysis to yield E, E and tand has been reviewed in Section 5.2. The modulus-temperature behavior of polymer blends is a strong function of the phase behavior. In Fig. 6.2, the generalized modulus-temperature behavior of miscible versus immiscible blends is compared for the case of two amorphous polymers with different glass transition temperatures. The phase separated blend exhibits a modulus plateau between the TgS of the components with the plateau position dependent upon the composition. The miscible blends show single Tg behavior, with the Tg position dependent upon the composition. 

Blends of amorphous polymers with crystaUine polymers (Fig. 6.3) also show differences between miscible and immiscible components. In this comparison, the crystaUine polymer has a lower Tg than the amorphous polymer and exhibits a crystalline modulus plateau between the Tg and the Tm- The phase separated blend shows both TgS, with a modulus plateau between 

35 TTTTTTTTT A-HOPE B-LLDPE LDPE TTTTTTTTTTTTTTTTTT C - Thermoplastic elastomer crosslinked rubber  

The parallel and series models represent the limits for phase separated blends  

Mixing two or more polymers to produce blends or alloys is a well-established route to achieve a certain level of physical properties, without the need to synthesize specialized polymer systems. For example, an amorphous and brittle polymer, such as polystyrene, can increase its toughness when blending with polyethylene and a compatibilizer. 

In most cases, melt mixing two polymers results in blends that are weak and brittle. The incorporation of a dispersed phase into a matrix mostly leads to the presence of stress concentrations and weak interfaces, arising from poor mechanical coupling between phases. 

The mechanical properties and end-use performance of a blend have been improved by compatibilization. From a practical point of view, a blend is often considered to be compatible if a certain set of mechanical properties is achieved. 

The well-known examples of blends are impact modified, toughened polymers, where polymers with different glass transition temperatures are blended, such as a rubber with a thermoplastic. Many other blends are known, such as barrier polymers for packaging, where specific polar or nonpolar polymers improve the properties of polymer blends, combined to increase the resistance against transport of water and a certain gas (oxygen, carbon dioxide, etc.), such as PA (barrier to oxygen) with a polyolefin (barrier to water vapor). 

The incorporation of rubber particles within the matrix of brittle plastics may enormously improve their impact resistance. When a force is applied to a blend, several deformation mechanisms of the major phase and of cracks that are formed in the blend are important. Their relative importance may depend on the polymer and on the nature of the loading. The best-known effect from compatibilization is the reduction of the interfacial tension in the melt. This causes an emulsifying effect and leads to an extremely fine dispersion of one phase into the other. A second effect is the increase of the adhesion at phase boundaries giving 

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Blends of PET and HDPE have been suggested to exploit the availabiUty of these clean recycled polymers. The blends could combine the inherent chemical resistance of HDPE with the processiag characteristics of PET. Siace the two polymers are mutually immiscible, about 5% compatihilizer must be added to the molten mixture (41). The properties of polymer blends containing 80—90% PET/20—10% HDPE have been reported (42). Use of 5—15% compatbiLizer produces polymers more suitable for extmsion blow mol ding than pure PET. 

It was the objective of this work to examine the contribution hydrox-ypropyl lignin can make to the properties of polymer blends with several commercial thermoplastics. This has been the topic of three earlier publications (9-11). 

Compared to binary mixtures of low molecular fluids, the critical behavior of polymer blends has been much less explored so far. However, a number of interesting static and dynamic critical phenomena in polymer blends attract increasing attention [4, 5], Neutron, X-ray, and static light scattering experiments belong to the major techniques for characterizing the static properties of polymer blends. Photon correlation spectroscopy (PCS) has traditionally been the method of choice for the investigation of the dynamics of critical [6-9] and noncritical [10-12] polymer blends. 

The macroscopic properties such as mechanical behavior of block copolymers or polymer blends depend directly on the relative concentrations of different constituents and their meso-structures. How to predict the exact macroscopic properties of polymer blends or block copolymers with meso-phase separation structures from pure component properties remains a big challenge. Some theoretical efforts have been explored. For example, Buxton et al. found that the deformations and fractures of polymer blends can be described by the... 

Liu, J., Jean, Y.C., Yang, H. (1995) Free volume properties of polymer blends by positron annihilation spectroscopy Miscibility . Macromolecules. 28, 5774. 

Polymer blends have become a very important subject for scientific investigation in recent years because of their growing commercial acceptance. Copolymerizalion and blending are alternative routes for modilications of properties of polymers. Blending is the less expensive method. It does not always provide a satisfactory alternative to copolymerization, of course, but polymer blends have been successfully used in an increasing number of applications in recent years. Such successes encourage more attempts to apply this technique to a wider range of problems in polymer-related industries. 

In most of experimental reports, the molecular properties of polymer blend components (such as choice of segments, size, architecture, or composition) have been varied while the external interface, confining the polymer blend, was fixed. It is, however, conceivable to tune the segregation properties by the proper modification of the external interface which bounds the polymer mixture. Initial works confirming this possibility [ 116,163,279] were published. 

There has been increasing interest in recent years in using incoherent electronic excitation transport as a probe of molecular interactions in solid state polymer systems. The macroscopic properties of such systems arise from the microscopic interaction of the individual polymer chains. The bulk properties of polymer blends are critically dependent on the mixing of blend components on a molecular level. Through the careful adjustment of the composition of blends technological advances in the engineering of polymer materials have been made. In order to understand these systems more fully, it is desirable to investigate the interactions... 

Macroscopic properties of polymer blends are influenced by the degree of mixing between component polymers [4, 5]. Miscibility is a term based on thermodynamics, and a miscible state means a homogeneous single phase on... 

High-strain properties of polymer blends, such as strength, tensile elongation, and impact strength, benefit from compatibilization. These... 

The spinodal, binodal and critical point Equations derived on the basis of this theory will be discussed later. When the theory has been tested it has been found to describe the properties of polymer blends much better than the classical lattice theories 17 1B). It is more successful in interpreting the excess properties of mixtures with dispersion or weak attraction forces. In the case of mixtures with a strong specific interaction it suffers from the results of the random mixing assumption. The excess volumes observed by Shih and Flory, 9, for C6H6-PDMS mixtures are considerably different from those predicted by the theory and this cannot be resolved by reasonable alterations of any adjustable parameter. Hamada et al.20), however, have shown that the theory of Flory and his co-workers can be largely improved by using the number of external degrees of freedom for the mixture as ... 

Finally, we will briefly discuss the properties of polymer blends under shear flow. In small molecule mixtures, shear flow is known to produce an anisotropy of critical fluctuations and anisotropic spinodal decomposition [244, 245], In polymer mixtures, the shear has the additional effect of orienting and stretching the coils, thus making the single-chain structure factor anisotropic. In the framework of the Rouse model these effects have been incorporated into the RPA description of polymer blends [246, 247]. Assuming a velocity field v = yyex, where x, y, z are cartesian coordinates, y the shear rate, and ex is a unit vector in x direction, the single chain structure factor becomes [246, 247]... 

The free volume model has been also incorporated into thermodynamic theories of liquids and solutions [Prigogine et al., 1957] and it is an integral part of theories used for the interpretation of thermodynamic properties of polymer blends [Utracki, 1989a]. In particular, it is a part of the most successful equation of state (EoS) derived for liquids and glasses [Simha and Someynsky,... 

Performance of polymer blends depends on the properties of polymeric components, as well as how they are arranged in space. The spatial arrangement is controlled by the thermodynamics and flow-imposed morphology. The word thermodynamics invariably brings to mind miscibility. However, thermodynamics has a broader use for the practitioners of polymer science and technology than predicting miscibility. The aim of this chapter is to describe how to measure, interpret, and predict the thermodynamic properties of polymer blends, as well as where to find the required information and/or the numerical values. 

Polymer blends must provide a variety of performance parameters. Usually it is a set of performance criteria that determines if the material can be used or not. For specific application more weight can be given to one or another material property. The most important properties of polymer blends are mechanical. Two type of tests have been used the low rate of deformation — tensile, compressive or bending and the high speed impact. Immiscibility affects primarily the maximum elongation at break, and the yield stress. 

Table 11.64. Improvement of flexural properties of polymer blends by addition of a crosslinking agent and gamma irradiation [Numata and Fujii, 1995]...

The low-speed mechanical properties of polymer blends have been frequently used to discriminate between different formulations or methods of preparation. These tests have been often described in the literature. Examples of the results can be found in the references listed in Table 12.9. Measurements of tensile stress-strain behavior of polymer blends is essential [Borders et al., 1946 Satake, 1970 Holden et al., 1969 Charrier and Ranchouse, 1971]. The mbber-modified polymer absorbs considerably more energy, thus higher extension to break can be achieved. By contrast, an addition of rigid resin to ductile polymer enhances the modulus and the heat deflection temperature. These effects are best determined measuring the stress-strain dependence. 

It is well known that the mechanical properties of polymer blends strongly depend on the raw materials and on their final morphologies, which are controlled by interfacial adhesion, properties of the neat materials, and processing conditions, among others [2, 37-39],... 

The particulate properties of polymers, blends, niasterbatches. etc, are important from processability aspects for their influence on... 

A blend of low-density polyethylene (LDPE) polyethylene (LDPE) with the terpolymer ethylene-propylene-diene monomer rubber (EPDM) exhibits a synergistic effect on tensile strength if EPDM is partially crystalline, but a nonsynergistic effect if the EPDM is amorphous [65]. This example shows the dramatic effect that morphology can have on properties of polymer blends. The synergism apparently arises from a tendency for crystallites in the LDPE to nucleate crystalli2ation of ethylene segments in the EPDM. 

It is possible to apply a similar procedure to investigate other properties of polymer blends, using a reinforcement function defined as e=P/P- -1, where P is a given polymer property. 

Various physical properties of polymer blends are particularly interesting as being related to the conqposition, interaction between components, phase structure as well as to the conditions of processing. 

The description of properties of polymer blends seems to be a major problem which needed consideration, and therefore, this issue has been carefully discussed with special attention to mechanical and electrical properties. 

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