Monodisperse polymer populations

A monodisperse polymer population is an idealization. Rather, one has to consider the polymer as a multicomponent mixture of many species with identical molecular... 

Two major entry models - the diffusion-controlled and propagation-controlled models - are widely used at present. However, Liotta et al. [28] claim that the collision entry is more probable. They developed a dynamic competitive growth model to understand the particle growth process and used it to simulate the growth of two monodisperse polystyrene populations (bidisperse system) at 50 °C. Validation of the model with on-line density and on-line particle diameter measurements demonstrated that radical entry into polymer particles is more likely to occur by a collision mechanism than by either a propagation or diffusion mechanism. 

When all the polymer molecules have the same degree of polymerization or, in other words, when all polymer molecules contain equal numbers of monomers, the system is called homodisperse or monodisperse. When the degree of polymerization varies among the polymer population, it is called heterodisperse or polydis-perse. Synthetic polymers and some biopolymers (e.g., most polysaccharides) are more often than not heterodisperse, whereas other biopolymers such as proteins and nucleic acids are usually homodisperse. 

Polymer populations in which all chains have identical masses are termed monodisperse. Proteins and DNA of the same type are necessarily monodisperse, as they are coded for precise monomer sequences. In general, however, polymer populations contain a wide distribution of masses. Such populations are termed polydisperse. While most synthetically produced polymers are polydisperse, many biopolymers are also produced in polydisperse populations for example, a variety of polysaccharides, glycoproteins, etc. 

Ungar and Zeng [33] have comprehensively summarized the research on strictly monodisperse materials from their first synthesis in 1985 until 2001. From the earliest studies it became apparent that, due to the monodisper-sity of the materials, the thickness of the lamellar crystals formed is always an integer fraction of the extended chain length (allowing for any chain tilt), such that the polymers always crystallize in the extended chain form or fold exactly in half (once-folded), or in three (twice-folded), etc. This behavior means that, when the alkanes are crystallized at a particular temperature, the entire lamellar population has very closely the same thickness and stability. The use of such an ultra-pure system to study the impact of thickness on lattice parameters removes many of the problems inherent to polymers, whilst maintaining the most important characteristic of chain length. 

Molecular weight distribution of a polymer is another level of complexity, as polymers are rarely synthesised in a sharp monodisperse population. This brings in the need to study the behavioiu- of polymeric liquid in simple flows and for simple systems, with the hope that the knowledge gained can be appropriately used in a complex flow pattern. They are called viscometric as they are used to define an effective shear viscosity t] from the measurements. 

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