Macromolecules in the Molten State

As explained earlier (Sect. 1.3.1), macromolecules in a low-molecular-weight solvent prefer a coiled chain conformation (random coil). Under special conditions (theta state) the macromolecule finds itself in a force-free state and its coil assumes the unpertubed dimensions. This is also exactly the case for polymers in an amorphous melt or in the glassy state their segments cannot decide whether neighboring chain segments (which replace all the solvent molecules in the bulk phase) belong to its own chain or to another macromolecule (having an identical constitution, of course). Therefore, here too, it assumes the unperturbed ) dimensions. 

Far below the glass transition temperature (T see Sect. 2.3.4.3) the macro-Brownian motions are frozen in completely, and most of the micro-Brownian motions are frozen in as well ( glassy state ). Near Tg, the micro-Brownian motions set in and become stronger with increasing temperature. The material softens. Finally, upon further raise of temperature, the macro-Brownian motions set in as well, and the polymer can be deformed by applying an external force. 

Finally, it behaves like a liquid provided the chain length is not too long. Just around T some physical properties change distinctively such as the specific volume, the expansion coefficient, the specific heat, the elastic modulus, and the dielectric constant. Determination of the temperature dependence of these quantities can thus be used to determine Tg. 

Softening as a result of micro-Brownian motion occurs in amorphous and crystalline polymers, even if they are crosslinked. However, there are characteristic differences in the temperature-dependence of mechanical properties like hardness, elastic modulus, or mechanic strength when different classes of polymers change into the molten state. In amorphous, non-crosslinked polymers, raise of temperature to values above results in a decrease of viscosity until the material starts to flow. Parallel to this softening the elastic modulus and the strength decrease (see Fig. 1.9). 

In semicrystaUine, non-crosslinked polymers (thermoplastics) where Tg is considerably below (and below room temperature), rigidity and elastic mod- 

The flow behavior of molten macromolecular substances is generally quite different from that of low-molecular-weight compounds. This is obvious, for example, from the shape of the flow curves. Moreover, flow orientation can be observed. In ideal liquids (water, glycerol, sulfuric acid, etc.), the viscosity is a characteristic quantity which does not depend on the shear rate y ( Newtonian flow ) if the shear force x is plotted vs. y, a straight line is obtained with slope q (Newtonian viscosity). Macromolecular melts behave differently since their melt 

The rate of all these processes, of course, depends strongly on the temperature in the vicinity of Tg the polymer chains are still relatively inflexible. Thus deformation requires considerable forces, and recovery occurs very slowly. Well above Tg the melt deforms more easily, but the tendency to flow as a result of increased macro-Brownian motion is still outweighed by the elastic recovery. The temperature range for pronounced elastic behavior of the polymer melt depends on the chain structure (e.g., whether it is branched) and in particular on the molecular weight and molecular weight distribution of the polymer. 

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U. Gaur and B. Wunderlich, Additivity of the heat capacities of linear macromolecules in the molten state. Polymer Div. Am. Chem. Soc. Preprints 20, 429 (1979)... 

Poly(2,5-di-n-dodecyl-l,4-phenylene) of 73000-94000 show a single anisotropic liquid crystalline mesophase in the molten state and macromolecules with Mw 44000—73000 gave coexisting isotropic/anisotropic phases [17]. 

Ramos, J., Peristeras, L. D., and Theodorou, D. N., 2007. Monte Carlo simulation of short chain branched polyolefins in the molten state. Macromolecules,... 

Polypropylene (PP) is one of the most widely used plastics in large volume. To overcome the disadvantages of PP, such as low toughness and low service temperature, researchers have tried to improve the properties with the addition of nanoparticles that contains p>olar functional groups. An alkylammonium surfactant has been adequate to modify the clay surfaces and promote the formation of nanocomposite structure. Until now, two major methods, i.e., in-situ polymerization( Ma et al., 2001 Pirmavaia, 2000) and melt intercalation ( Manias et al.,2001) have been the techniques to prepare clay/PP nanocomposites. In the former method, the clay is used as a catalyst carrier, propylene monomer intercalates into the interlayer space of the clay and then polymerizes there. The macromolecule chains exfoliate the silicate layers and make them disperse in the polymer matrix evenly. In melt intercalation, PP and organoclay are compounded in the molten state to form nanocomposites. 

From the point of view of blend morphology in the molten state, compatibilization enhances the dispersion, increases the total apparent volume of the dispersed phase, rigidifies the interface, and increases interaction not only between the two phases, but also between the dispersed drops. Furthermore, reactive compatibilization may involve chemical bonding between the two polymer macromolecules, resulting in significant increase of the molecular weight at the interface. 

Aoki Yuji, and Tanaka Takeshi. Viscoelastic properties of miscible poly(methyl methacry-late)/poly(styrene-co-acrylonitrile) blends in the molten state. Macromolecules. 32 no. 25 (1999) 8560-8565. 

Figure 9.2 The effect of chain flexibility of macromolecules on their conformations in solution, in the molten state, or in the solid state. The flexibility of a macromolecule can be correlated to the type of crystal and physical properties of the corresponding solid polymer. (Reprinted from Samulski, Physics Today 35(5) 40. Copyright 1982, with permission from the American Institute of Physics.)...

The disordered state of a statistical coil is what is displayed by polymers in the molten and amorphous states and also in solution. To describe the conformation of a macromolecule consisting of a main chain N +l atoms, the positions of all them have to be determined. Using vectorial... 

Macromolecules may exist in the solid state, the molten state and in solutions, but not in the gaseous state. This is so because the boiling temperature of a polymer melt is proportional to the degree of polymerization N. 

These are macromolecules that can align into crystalline arrays while they are in solution lyotropic) or while in a molten state thermotropic). Such liquids exhibit anisotropic behavior [51,52]. The regions of orderliness in such liquids are called mesophases. Molecular rigidity found in rigid rod-shaped polymers, for instance, is the chief cause of their liquid crystalline behavior. It excludes more than one molecule occupying a specific volume and it is not a result of intermolecular attractive forces. Some aromatic polyesters or polyamides are good examples, like polyphenylene terephthalate ... 

Recently, diffusion of macromolecules in the bulk molten state across freshly-made Junctions between formerly separate pieces of material has been studied intensely [1-9]. Of most interest is the part of the diffusion process occuring at times shorter than the longest configurational relaxation time of the macromolecules in the melt. Transfer of material across the Junction surface on this time scale can be produced by more local configurational rearrangements of a macromolecule than those required to produce diffusion of the center-of-mass. 

Some transitions that are only known for macromolecules, however, will not be mentioned at all since they are covered elsewhere in this Encyclopedia (see, eg. Gel Point). Also we shall not be concerned here with the transformations from the molten state to the solid state of polymeric materials, since this is the subject of separate treatments (see Crystallization Kinetics Glass Transition Viscoelasticity). Unlike other materials, polymers in the solid state rarely reach full thermal equilibrium. Of course, all amorphous materials can be considered as frozen fluids (see Glass Transition) Rather perfect crystals exist for metals, oxides, semiconductors etc, whereas polymers typically are semicrystalline, where amorphous regions alternate with crystalline lamellae, and the detailed structure and properties are history-dependent (see Semicrystalline Polymers). Such out-of-equilibrium aspects are out of the scope of the present article, which rather emphasizes general facts of the statistical thermodynamics (qv) of phase transitions and their applications to polymers in fluid phases. 

T> Tg, for irradiation above the melting range, the whole sample exists in the molten (amorphous) state. Cross-linking of macromolecules by irradiation also yields a stabilization of this phase after cooling down to room temperature (i.e., any crystallization is prevented, and no lamellar structures are visible in TEM). 

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