Polymer blends macro’ scale

In this chapter, we have discussed polymer stmcture at the level of the repeating unit. The repeating unit not only has a specific chemical structure but also acts as the source of interaction at the molecular scale. With respect to this, now we will be introducing the concept of biodegradable polymer blends. A polymer blend is classically defined as a physical mixture of two or more polymers, which occurs on a macro scale. We are now going to also consider the science at molecular/nanometre scales, ie, taking polymer repeating unit into account rather than entire polymer chains. For the purposes of this book, the definition of a polymer blend is as follows ... 

Evstatiev M, Fakirov S and Friedrich K (2005) Manufacturing and characterization of microfibrillar reinforced composites from poljmier blends, in Polymer Composites Prom Nano-to-Macro-Scale (Eds. Friedrich K, Fakirov S and Zhang Z) Springer, New York, NY, pp. 149-167. 

Matyjaszewski et al. [2] patented a novel and flexible method for the preparation of CNTs with predetermined morphology. Phase-separated copolymers/stabilized blends of polymers can be pyrolyzed to form the carbon tubular morphology. These materials are referred to as precursor materials. One of the comonomers that form the copolymers can be acrylonitrile, for example. Another material added along with the precursor material is called the sacrificial material. The sacrificial material is used to control the morphology, self-assembly, and distribution of the precursor phase. The primary source of carbon in the product is the precursor. The polymer blocks in the copolymers are immiscible at the micro scale. Free energy and entropic considerations can be used to derive the conditions for phase separation. Lower critical solution temperatures and upper critical solution temperatures (LCST and UCST) are also important considerations in the phase separation of polymers. But the polymers are covalently attached, thus preventing separation at the macro scale. Phase separation is limited to the nanoscale. The nanoscale dimensions typical of these structures range from 5-100 nm. The precursor phase pyrolyzes to form carbon nanostructures. The sacrificial phase is removed after pyrolysis. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

An important factor to consider on the blending of polymeric materials is that most polymers are incompatible with polypropylene on the molecular scale. This might cause many problems, such as macro-phase separation during blending, low interface adhesion, low tensile transfer rate, and low physical properties, which may be even lower than the unmodified polymer. To sustain good fiber properties, controlling the phase structure and interface adhesion is a necessity. 

F., Girard, F., and Marais, S. (2010). Transport mechanism of small molecules through bacteria polyesterfilms. 2nd International Conference on Natural Polymers, Bio-Polymers, Bio-Materials, their Composites, Blends, IPNs, Polyelectrolytes and Gels Macro to Nano Scales, Kottayam, Kerala, India. 

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