Heterogeneity elastomers

Heterogeneities in Elastomers. Heterogeneities in elastomers materials are related to the defects induced in the process of preparation. The processes that lead to the defects are as follows. 

Unlike incompatible heterogeneous blends of elastomer-elastomer, elastomer-plastic, and plastic-plastic, the reactively processed heterogeneous blends are expected to develop a variable extent of chemical interaction. For this reason the material properties, interfacial properties, and phase morphology of reactively processed blends would differ significantly from heterogeneous mixtures. 

Heterogeneous compatible blends of preformed elastomers and brittle plastics are also an important route for the development of blends of enhanced performance with respect to crack or impact resistance. Polycarbonate blends with preformed rubber particles of different sizes have been used to provide an insight into the impact properties and the fracture modes of these toughened materials. Izod impact strength of the blends having 5-7.5 wt% of rubber particles exhibits best overall product performance over a wide range temperature (RT to -40°C) [151-154]. 

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

The properties of a surface are influenced by the surface groups to a very great extent. Knowledge of their existence and of their chemistry is important for many technological processes. Apart from heterogenous catalysis, surface chemistry is important in lubrication, in re-enforcement of rubber and other elastomers, in flotation, in the behavior of pigments in laquers, printing inks, and textile additives, and in many other applications. 

Improvement of the mechanical properties of elastomers is usually reached by their reinforcement with fillers. Traditionally, carbon black, silica, metal oxides, some salts and rigid polymers are used. The elastic modulus, tensile strength, and swelling resistence are well increased by such reinforcement. A new approach is based on block copolymerization yielding thermoelastoplastics, i.e. block copolymers with soft (rubbery) and hard (plastic) blocks. The mutual feature of filled rubbers and the thermoelastoplastics is their heterogeneous structure u0). 

Elastomer blends consisting of two immiscible components are heterogeneous rubberlike materials both components of which are in the rubbery state. Such blends consist usually of either a matrix and a discrete phase or two interpenetrating continuous phases (interpenetrating networks). At homogeneous deformations of such blends, the contribution of either component to the thermomechanical behaviour of the material is determined by the content of the component and the individual characteristics of its chains. 

Aqueous systems have been studied by a very large number of investigators. Economy, safety, convenience and quality of product have combined to make this the method of choice for commercial production of copolymers. The industrial importance of such end products as elastomers and acrylic fibers has been a special incentive to related fundamental studies. Furthermore, the relatively high solubility of acrylonitrile monomer in water coupled with insolubility of the polymer make it a convenient test monomer for studies of initiation by redox systems (6, 25, 102). Large numbers of homogeneous chemical initiators and some heterogeneous initiators have been studied as well as initiation by photochemical means, by ultrasonics and by ionizing radiation. It will not be possible here to review the enormous world literature. Several publications (/, 92, 117) refer in some detail to the older papers, and we shall restrict our comments to recent interpretations that have received support from several quarters. 

In a heterogeneous polyblend of two dissimilar elastomers, such as chlorinated butyl rubber and polybutadiene, a certain interfacial bonding... 

Tfrom thermodynamic considerations (2) supported by microscopic ob-servations (5), two different high molecular weight polymers when blended exist in a heterogeneous state. In the case of elastomers capable of crosslinking these separate phases may crosslink in the presence of one another. However, the question arises, does bonding exist across the interfaces ... 

Heterogeneities in technical elastomers arise in different stages of elastomer production and product use. Even in a perfectly prepared homogeneous elastomer product, unavoidable ageing processes induce space-dependent defects. Examples for sources of defects are ... 

Mechanical properties of crosslinked elastomers are influenced not only by the volume-average crosslink density but also by network heterogeneity. The influence of structural defects (such as residual sol, dangling chains, chain loops and the heterogeneity of the junction distribution) on the viscoelastic properties and the equilibrium swelling data is still under discussion. Local methods which probe molecular properties are very suitable for the determination of the degree of network heterogeneity [11]. 

Distinct T2 relaxation components with widely differing mean decay times suggest molecular or macroscopic heterogeneity of the material. In such cases the submolecule concept can be used to describe the relaxation behaviour [20]. In a simplified interpretation, the overall T2 relaxation decay of a heterogeneous elastomer is the weighted sum of the decays originating from the submolecules, which are defined as the network... 

A quantitative analysis of the shape of the decay curve is not always straightforward due to the complex origin of the relaxation function itself [20, 36, 63-66] and the structural heterogeneity of the long chain molecules. Nevertheless, several examples of the detection of structural heterogeneity by T2 experiments have been published, for example the analysis of the gel/sol content in cured [65, 67] and filled elastomers [61, 62], the estimation of the fraction of chain-end blocks in linear and network elastomers [66, 68, 69], and the determination of a distribution function for the molecular mass of network chains in crosslinked elastomers [70, 71]. 

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