Molecular architecture repeat unit structure

Cooling past the glass transition temperature is accompanied by a dramatic change in the mechanical properties. The elastic modulus increases by a factor of 1000 when the polymeric liquid is cooled below Tg and the modulus of the glassy polymers is relatively insensitive to changes in molar mass and repeating unit structure. The actual value of Tg is, however, very dependent on the repeating unit, the molecular architecture and the presence of low molar mass species, as is shown in section 5.2. 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

If one accepts that the number of adsorbed molecules per accessible phenyl group in poly(Sty-co-DVB) at liquid-saturation (i.e. as, hereafter in this section referred to simply as a without the subscript s) is an adsorption parameter that reflects how well the molecular structure of the adsorbed molecule is accommodated by the molecular structure of the repeat unit, then it follows that oc should vary in accordance with the electronic nature and molecular architecture of the adsorbed species. That this is indeed the case is made evident by the a data collected for homologous series of liquids ZR in which Z [a substituent that has relatively strong affinity for the phenyl groups in poly(Sty-co-DVB)] is kept constant, while R [the remaining molecular structure] is varied systematically [160-164]. 

Firestone et al. investigated the relationship between the molecular architecture of a series ofpoly(ethyleneoxide)-b-poly(propylene oxide) (PEO—PPO) di- and triblock copolymers and the nature of their interactions with lipid bilayers [213], The number of repeat units in the hydrophobic PPO block has been found to be a critical determinant for the polymer-lipid bilayer association. Further studies showed that temperature, polymer architecture and concentration also control the mode of interaction of PEO—PPO—PEO copolymers with lipid bilayers. Increasing either the number of repeat units in the PEO block or the polymer concentration promotes a greater degree of structural ordering [197],... 

To be able to understand polymer properties, we must be able to develop a physical picture of what these long molecules are really like. This is what we refer to as the secondary structure, i.e., the size and shape of an isolated single molecule. The size of the polymer is best discussed in terms of moleeular weight. The shape of the polymer molecule (molecular architecture) will be influenced naturally by the nature of the repeating unit and the manner in which these units are linked together. It is therefore convenient to consider polymer shape in two contexts ... 

The part of a macromolecule from which the macromolecule is built is called monomer unit, while the smallest part of a macromolecule that repeats periodically is called structural repeating unit. Polymers can consist of one or more kinds of monomer unit. The former are called homopolymers and the latter copolymers. Synthetic polymers are usually varied mixtures of molecules of different molar mass (M) and possibly chemical composition and/or molecular architecture that is, they are nonuniform... 

Polymers are macromolecules based on a large number of repeating units (monomers) covalently bound in a chain-like molecular architecture with a variety of compositions, structures, and properties. A large diversity of synthetic strategies and monomer combinations can be used to generate linear or nonlinear polymers, such as star-shaped, comb (brush), or branched polymers, and dendrimers (Fig. 11.1). 

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