Crystal structures, polymers thermal factors

Polymers and foamed polymers. Temperature, pressure, density of polymer, orientation of chain segments, crystal structures, degree of crystallinity, and many other factors may significantly affect thermal conductivity of polymers [9—18]. Therefore the thermal conductivity values can be varied in literatures for same polymers. In addition, discrepancies also occur for thermal conductivity values obtained using different test methods [19]. The data in the tables in this chapter may be the representative values of thermal conductivity and are not necessarily the absolute ones. 

The mechanical anisotropy of oriented polymers is determined by the following factors, which will be discussed in turn (1) the structure of the molecular chain and, where the polymer crystallizes, the crystal structure (2) the molecular orientation and, in a crystalline polymer, the morphology (3) thermally activated relaxation processes in both the crystalline and non-crystalline regions. 

Vast natural and synthetic polymers are semicrystalline, and such a structural character dominates their physical performance, such as mechanical, optical, and thermal properties. Therefore, it is necessary to understand the process of polymer crystallization. Still, polymer crystallization appears as complicated. It can be influenced by many factors, such as chemical structures of polymer chains, compositions, temperatures, thermal history, spatial confinements, and pressures. In order to gain a better understanding about these factors in polymer crystallization, theoretical and simulation approaches are often concerted with experimental approaches. 

Morales et al. [323] prepared bionanocomposites of PEA (derived from glycohc acid and 6-aminohexanoic add by in situ polymerization) reinforced with OMMTs. The most dispersed structure was obtained by addition of C25A organoclay. Evaluation of thermal stability and crystallization behavior of these samples showed significant differences between the neat polymer and its nanocomposite with C25A. Isothermal and nonisothermal calorimetric analyses of the polymerization reaction revealed that the kinetics was highly influenced by the presence of the silicate particles. Crystallization of the polymer was observed to occur when the process was isothermally conducted at temperatures lower than 145 °C. In this case, dynamic FTIR spectra and WAXD profiles obtained with synchrotron radiation were essential to study the polymerization kinetics. Clay particles seemed to reduce chain mobility and the Arrhenius preexponential factor. 

The presence of nanomaterial influences the crystaltization process as it acts as additional heterogeneity creating more interfaces [29]. In some cases, the presence of a nanofiller alters the crystaltization temperature and thus the thermal properties. In the dispersed phase, the presence of nanoparticles increases the occurrence of fractionated crystaltization [17,23,27,29,53-55,61-63,93-95]. Also, the extent of compatibilization effect from the nanomaterial can alter the whole process of crystaltization in the polymer blend. When a nanofiller is organically modified, using compounds with a similar chemical structure to one or both polymers present in the blend, the influence of the nanofillers here on the crystallization behavior is complex. This is because several factors have to be taken in consideration, such as the nucleating effect of the matrix on the dispersed phase or vice-versa, the nucleating effect of the nanofiller compatibitizer on both phases. There are reported cases where modified nanofillers have impeded the crystaltization process and thus retarded crystaltization in the overall material. 

In the field of polymer science, the crystallization behavior of polymer blends represents a key issue for the analysis of structure-properties relationships of macro-molecular systems. The presence of the second polymer component, either in the melt or in the solid state, can infiuence the whole crystallization process of the polymer phases, thus the morphology, phase behavior, and physical/mechanical properties. The crystallization processes are controlled by several factors, which are related to equilibrium thermodynamics, kinetic aspects, thermal conditions, melt rheology, as well as chain structure and polymer/polymer interactions. In the present chapter, an overview of the thermodynamic conditions, accompanied by a description of main morphological features of blends containing one or both crystallizable components, is reported. 

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