Branching and Molar Mass

Molar mass can also alter T. Chain ends are relatively free to move, and if the number of chain ends is increased by reducing the molar mass, then is lowered because of the decrease in energy required to stimulate chain motion and melting. For example, polypropylene, with M = 2000 g moF, has = 387 K, whereas a sample with M = 30,000 g moF has = 443 K. 

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While the basic chemical structure of a synthetic polymer is usually well understood, many physical properties depend on such characteristics as chain length, degree of chain branching, and molar mass, which are not easy to specify exactly in terms of a molecular formula. Moreover the macromolecules in a given sample are seldom uniform in chain length or molar mass (which for a linear polymer is proportional to chain length) thus the nature of the distribution of molar masses is another important characteristic. 

These can be dealt with under the following general headings symmetry, intermo-lecular bonding, tacticity, branching, and molar mass. 

For linear and branched molecules having the same hydrodynamic volume or elution volume, it follows that the products of their intrinsic viscosities and molar masses can be equated ... 

Size exclusion chromatography is the premier polymer characterization method for determining molar mass distributions. In SEC, the separation mechanism is based on molecular hydrodynamic volume. For homopolymers, condensation polymers and strictly alternating copolymers, there is a correspondence between elution volume and molar mass. Thus, chemically similar polymer standards of known molar mass can be used for calibration. However, for SEC of random and block copolymers and branched polymers, no simple correspondence exists between elution volume and molar mass because of the possible compositional heterogeneity of these materials. As a result, molar mass calibration with polymer standards can introduce a considerable amount of error. To address this problem, selective detection techniques have to be combined with SEC separation. 

Liquid-liquid demixing in solutions of polymers in low molar mass solvents is not a rare phenomenon. Dembcing depends on concentration, temperature, pressure, molar mass and molar mass distribution function of the polymer, chain branching and end groups of the polymer, the chemical nature of the solvent, isotope substitution in solvents or polymers, chemical composition of copolymers and its distributions, and other variables. Phase diagrams of polymer solutions can therefore show a quite complicated behavior when they have to be considered in detail (see Ref la). 

Thus, intermolecular chain transfer to polymer leads to premature termination of the growth of one propagating chain and the reactivation of a dead chain which then grows a long-chain branch. As a consequence, the molar mass distribution of the polymer broadens. The changes in skeletal structure and molar mass distribution inevitably have major effects upon bulk polymer properties. 

The flowability of a plastic melt depends essentially on the mobility of the molecular segments (and therefore on the temperature), their form, their degree of entanglement and branching, their molar mass and their distribution. 

Fig. 7. 3D surface representation of a complex star-branched copolymer consisting of 16 species varying in butadiene content and molar mass. 

Specific and intrinsic viscosity, molar masses and molar mass distribution based on Benoit s nniversal cahbration hydrodynamic radius, branching, polymer structure, and size distribution... 

Homopolymer characterization absolnte molar masses and molar mass distribntion radius of gyration (averages and distribntion), polymer structure, branching... 

Starch, a well-known hydrocoUoid biopolymer is a low cost polysaccharide, plentifully available and is one of the cheapest biodegradable polymers. It is produced by agricultural plants in the form of hydrophilic granules. Starch is mainly extracted from potatoes, wheat, com, and rice. It is composed of amylose (poly-a-1,4-D-glucopyranoside), a Hnear and crystalline polymer, and amylopectin (poly-a-1,4-D-glucopyranoside and a-l,6-D-glucopyranoside), a branched and amorphous polymer. The relative amounts and molar masses of amylose and amylopectin vary... 

Graft copolymers are produced by chain copolymerization of the macromonomer with another ethylenic monomer. This enables more well-defined graft copolymers to be prepared, especially with regard to the molar mass and molar mass distribution of the branches since these properties are defined by those of the macromonomer. 

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