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Macromolecules: Synthesis, Order and Advanced Properties.
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- Macromolecules: Synthesis, Order and Advanced Properties | Springer
Poly styrene is composed only of styrene monomer residues, and is classified as a homopolymer. Ethylene-vinyl acetate contains more than one variety of repeat unit and is a copolymer. Some biological polymers are composed of a variety of different but structurally related monomer residues; for example, polynucleotides such as DNA are composed of four types of nucleotide subunits.
A polymer molecule containing ionizable subunits is known as a polyelectrolyte or ionomer. The microstructure of a polymer sometimes called configuration relates to the physical arrangement of monomer residues along the backbone of the chain. Structure has a strong influence on the other properties of a polymer. For example, two samples of natural rubber may exhibit different durability, even though their molecules comprise the same monomers. An important microstructural feature of a polymer is its architecture and shape, which relates to the way branch points lead to a deviation from a simple linear chain.
Types of branched polymers include star polymers , comb polymers , polymer brushes , dendronized polymers , ladder polymers , and dendrimers. A polymer's architecture affects many of its physical properties including, but not limited to, solution viscosity, melt viscosity, solubility in various solvents, glass transition temperature and the size of individual polymer coils in solution. A variety of techniques may be employed for the synthesis of a polymeric material with a range of architectures, for example living polymerization.
A common means of expressing the length of a chain is the degree of polymerization , which quantifies the number of monomers incorporated into the chain. Since synthetic polymerization techniques typically yield a statistical distribution of chain lengths, the molecular weight is expressed in terms of weighted averages.
The number-average molecular weight M n and weight-average molecular weight M w are most commonly reported. The physical properties  of polymer strongly depend on the length or equivalently, the molecular weight of the polymer chain. In the latter case, increasing the polymer chain length fold would increase the viscosity over times. Monomers within a copolymer may be organized along the backbone in a variety of ways.
A copolymer containing a controlled arrangement of monomers is called a sequence-controlled polymer. Tacticity describes the relative stereochemistry of chiral centers in neighboring structural units within a macromolecule. There are three types of tacticity: isotactic all substituents on the same side , atactic random placement of substituents , and syndiotactic alternating placement of substituents.
Polymer morphology generally describes the arrangement and microscale ordering of polymer chains in space. When applied to polymers, the term crystalline has a somewhat ambiguous usage. In some cases, the term crystalline finds identical usage to that used in conventional crystallography. For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for x-ray crystallography , may be defined in terms of a conventional unit cell composed of one or more polymer molecules with cell dimensions of hundreds of angstroms or more.
Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline. Polymers with microcrystalline regions are generally tougher can be bent more without breaking and more impact-resistant than totally amorphous polymers. For many polymers, reduced crystallinity may also be associated with increased transparency.
The space occupied by a polymer molecule is generally expressed in terms of radius of gyration , which is an average distance from the center of mass of the chain to the chain itself. Alternatively, it may be expressed in terms of pervaded volume , which is the volume of solution spanned by the polymer chain and scales with the cube of the radius of gyration.
The bulk properties of a polymer are those most often of end-use interest. These are the properties that dictate how the polymer actually behaves on a macroscopic scale. The tensile strength of a material quantifies how much elongating stress the material will endure before failure. For example, a rubber band with a higher tensile strength will hold a greater weight before snapping.
In general, tensile strength increases with polymer chain length and crosslinking of polymer chains. Young's modulus quantifies the elasticity of the polymer. It is defined, for small strains , as the ratio of rate of change of stress to strain. Like tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands.
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The modulus is strongly dependent on temperature. Viscoelasticity describes a complex time-dependent elastic response, which will exhibit hysteresis in the stress-strain curve when the load is removed. Dynamic mechanical analysis or DMA measures this complex modulus by oscillating the load and measuring the resulting strain as a function of time.
Transport properties such as diffusivity describe how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes. The movement of individual macromolecules occurs by a process called reptation in which each chain molecule is constrained by entanglements with neighboring chains to move within a virtual tube. The theory of reptation can explain polymer molecule dynamics and viscoelasticity.
Depending on their chemical structures, polymers may be either semi-crystalline or amorphous. Semi-crystalline polymers can undergo crystallization and melting transitions , whereas amorphous polymers do not. In polymers, crystallization and melting do not suggest solid-liquid phase transitions, as in the case of water or other molecular fluids. Instead, crystallization and melting refer to the phase transitions between two solid states i.
Crystallization occurs above the glass transition temperature T g and below the melting temperature T m. All polymers amorphous or semi-crystalline go through glass transitions. The glass transition temperature T g is a crucial physical parameter for polymer manufacturing, processing, and use. Below T g , molecular motions are frozen and polymers are brittle and glassy. Above T g , molecular motions are activated and polymers are rubbery and viscous.
The glass transition temperature may be engineered by altering the degree of branching or crosslinking in the polymer or by the addition of plasticizers. Whereas crystallization and melting are first-order phase transitions , the glass transition is not. In general, polymeric mixtures are far less miscible than mixtures of small molecule materials.
This effect results from the fact that the driving force for mixing is usually entropy , not interaction energy. In other words, miscible materials usually form a solution not because their interaction with each other is more favorable than their self-interaction, but because of an increase in entropy and hence free energy associated with increasing the amount of volume available to each component.
This increase in entropy scales with the number of particles or moles being mixed. Since polymeric molecules are much larger and hence generally have much higher specific volumes than small molecules, the number of molecules involved in a polymeric mixture is far smaller than the number in a small molecule mixture of equal volume.
The energetics of mixing, on the other hand, is comparable on a per volume basis for polymeric and small molecule mixtures. This tends to increase the free energy of mixing for polymer solutions and thereby making solvation less favorable, and thereby making the availability of concentrated solutions of polymers far rarer than those of small molecules. Furthermore, the phase behavior of polymer solutions and mixtures is more complex than that of small molecule mixtures.
Whereas most small molecule solutions exhibit only an upper critical solution temperature phase transition, at which phase separation occurs with cooling, polymer mixtures commonly exhibit a lower critical solution temperature phase transition, at which phase separation occurs with heating. In dilute solution, the properties of the polymer are characterized by the interaction between the solvent and the polymer.
In a good solvent, the polymer appears swollen and occupies a large volume. In this scenario, intermolecular forces between the solvent and monomer subunits dominate over intramolecular interactions.
Macromolecules: Synthesis, Order and Advanced Properties | Springer
In a bad solvent or poor solvent, intramolecular forces dominate and the chain contracts. In the theta solvent , or the state of the polymer solution where the value of the second virial coefficient becomes 0, the intermolecular polymer-solvent repulsion balances exactly the intramolecular monomer-monomer attraction. Under the theta condition also called the Flory condition , the polymer behaves like an ideal random coil. The transition between the states is known as a coil—globule transition.
Inclusion of plasticizers tends to lower T g and increase polymer flexibility. Plasticizers are generally small molecules that are chemically similar to the polymer and create gaps between polymer chains for greater mobility and reduced interchain interactions.
A good example of the action of plasticizers is related to polyvinylchlorides or PVCs. A uPVC, or unplasticized polyvinylchloride, is used for things such as pipes. A pipe has no plasticizers in it, because it needs to remain strong and heat-resistant.