Flexible macromolecules, definition

A special problem in heat of fusion determination is presented by linear, flexible macromolecules. They usually crystallize only partially, so that the heat of fusion measured is not the total heat of fusion and cannot be used directly for a discussion of the equilibrium entropy of fusion, for example. Similarly to the heat capacity treatment described in Fig. 5.17, one assumes that the partially crystallized samples can be described by a crystallinity, iv. . A definition of based on density is given in Fig. 5.17. Equation (1) of Fig. 5.25 shows how crystallinity can also be expressed in terms of the heat of fusion. The measured heat of fusion of the semicrystalline sample is A/7f, while Ai/f is the heat of fusion of the perfect crystal. As long as a two-phase model of semicrystalline polymers holds, the two definitions of Wf. give identical values. If perfect crystals are not available for comparison, calibration can in this case be obtained by measuring the heat of fusion of a sample of crystallinity known from some other measurement, such as density measurement. X-ray diffraction, or infrared absorption. In some cases it is possible to plot the change in heat capacity at the glass transition temperature, ACp(Tg), versus the measured heat of fusion A7/f. At the extrapolated value for ACp(Tg) = 0, A//f corresponds to the heat of fusion for the fully crystalline state, A//f. Special difficulties of this method arise from rigid amorphous fractions sometimes found in semicrystalline polymers. In this case the observed AC is lowered, as is discussed in Sect. 5.6. 

Further theoretical studies by Floryconcerned the clarification of phase transitions for the systems with a definite distribution of macromolecules over the length and also the analysis of equilibrium for a model system composed of macromolecules in which rigid blocks are separated by flexible units. An example of such systems are copolyesters exhibiting the thermotropic transition into the liquid crystalline state in the absence of a solvent... 

One of the major problems of the theory of intramolecular local mobility or kinetic flexibility lies in the establishment of the principal mechanism of mobility for the macromolecules of a given chemical structure and constitution and of local conformational microstructure. The question arises, which of the elementary acts leads to the ability of individual parts of the macromolecule for changing their shape during definite time intervals characteristic of a certain class of motions 38-148)... 

Flexible methylene chains, as any hydrocarbonic sequences, are the fragments most vulnerable for thermo-oxidation. Rigidity of the physical structure, the limitedness of molecular motions, in PAI is defined by the pyromellitimide fragment, and in PPA-1 and PPA-2 the terephthalic one. The presence of isophthalic acid (IPA) in PPA-1 also imparts a definite flexibility to the macromolecule (similar to the prviously considered LCP). To put it another way, reducing the molecular mobility of the macrochain, causes aromatic fragments to stimulate the physical mechanisms of methylene branching protection from thermo-oxidation. 

Macromolecules having a flexible side chain carrying a dipole moment are called to be of type C (Block 1979). A typical example are poly(n-alkyl methacry-late)s. This definition is only appropriate tmder the condition that the side chain can fluctuate on a shorter time scale than the segmental dynamics of the macromolecule. Otherwise the polymer is of type B. 

Figure 4 (a) Model for an idealized flexible isolated macromolecule. 4/ represent the centers of gravity of the repeating units and // the bonds, (b) Atomistic model for a macromolecule with the definitions of the bond y> and the valence angle 9. 

On the whole, it is possible to state that the currently available expmmental material does not yet permit drawing any definite conclusions concerning the roles of the chain structure of the macromolecules and the effect of the polymeric state of the liquid crystal on the elastic properties of the LC phase. The problem of the contribution of the conformational state of the main chains, their flexibility, and the length of the aliphatic spacer remains open. It is necessary to develop theoretical studies to clarify the question of the features of the elastic properties of the polym mesophases within the framework of the continuous theory of elasticity of liquid crystals. 

Finally, it should be noted that AG values regulating formation of suprasegmental structures in polymers are definitely connected with molecular characteristics of the latter. Since a polymer is a solid consisting of long chains of macromolecules, it should be expected that the most important (or at least one of the most important) property is the polymer chain flexibility, which can be expressed with the help of the... 

In 1951, The Council of the International Union of Pure and Applied Chemistry (lUPAC) adopted the following definition of a plasticizer a substance or material incorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability, or distensibility. In addition, it is noted that a plasticizer may reduce the melt viscosity, lower the temperature of a second-order transition, or lower the elastic modulus of the product, but it does not alter the chemical nature of the macromolecule. 

---