Amorphous polymers, thermal properties

KE Ikeda, M., Suga, H., and Seki, S., Thermodynamic studies of solid polyethers 5. Crystahine-amorphous interfacial thermal properties, Polymer, 16, 634, 1975. 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Some representative backbone stmctures of PQs and PPQs and their T data are given in Table 1. As in other amorphous polymers, the Ts of PQs and PPQs are controlled essentially by the chemical stmcture, molecular weight, and thermal history. Several synthetic routes have been investigated to increase the T and also to improve the processibiUty of PPQ (71). Some properties of PPQ based on 2,3-di(3,4-diaminophenyl)quinoxaline and those of l,l-dichloro-2,2-bis(3,4-diaminophenyl)ethylene are summarized in Table 2. 

Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat and other properties to change dramatically. 

The transition between crystalline and amorphous polymers is characterized by the so-called glass transition temperature, Tg. This important quantity is defined as the temperature above which the polymer chains have acquired sufficient thermal energy for rotational or torsional oscillations to occur about the majority of bonds in the chain. Below 7"g, the polymer chain has a more or less fixed conformation. On heating through the temperature Tg, there is an abrupt change of the coefficient of thermal expansion (or), compressibility, specific heat, diffusion coefficient, solubility of gases, refractive index, and many other properties including the chemical reactivity. 

The properties of a rubbery amorphous polymer form a continuum ivith those of the polymer in its molten state. Rubbery amorphous polymers exhibit the same range of motions as molten polymers, but they happen much slower, due to reduced thermal motion and the associated decrease in free volume. 

Using Differential Scanning Calorimetric and Roentgen-phase analyses methods it has been established that synthesized polymers are amorphous systems. Thermal (phase) transformation temperatures of synthesized polymers have been determined. Thermooxidation stability of the synthesized polymers has been studied. There was shown that their thermooxidation stability exceeded the analogical characteristic of polyorganocarbo-siloxanes. Using synthesized diallylsilazanes modification of the properties of some important industrial polymer composites based on phenolformaldehide resins has been carried out. Preliminary investigations showed that synthesized polymers in combination... 

The challenges involved in the material properties of PPC relate to its thermal features, i.e., its thermal decomposition, and the glass transition temperature (Tg) of about body temperature of the otherwise amorphous polymer. These have implications for processing and application of the material. This review will discuss consecutively the thermal, viscoelastic, and mechanical properties of PPC and the experiences in processing PPC and its composites. The properties of solutions of PPC will also be presented, and the biodegradabUity and biocompatibility discussed. Spectroscopic properties will not be discussed. Further information on NMR data can be found in the following references [2, 10-12]. A t3 pical spectrum is shown in Fig. 2 [13]. 

The thermal properties of polymers include their behavior during heating from the solid amorphous (glassy) or crystalline to the liquid (molten) state, but also their chemical and mechanical stability in the entire range of application. 

Thermal Properties. A typical dsc thermogram of an HPL/PVA blend (Fig. 4) shows a single Tg and Tm (10). Differences in the shape of the melting endotherms of PVA(96), (88), and (75) can be attributed to different degrees of crystallinity in the three polymers. Changes in crystalline structure of polymer blends usually result from polymer-polymer interactions in the amorphous phase. Such interactions result in a reduction of crystallinity, thereby reducing the enthalphy of the phase change (16,17). The observed reductions in melt endotherm area of HPL blends with PVA (> 0) may therefore indicate the existence of polymer-polymer interactions between the two types of macromolecules. 

Based on their thermal properties, polymeric and glassy molecular PR materials can be classified as high-7 and low-7), materials. The Tg values of polymer composites can be lowered by doping small molecules as plasticizer. Introduction of a long alkyl side chain can effectively reduce the Tg of fully functionalized polymers. The glassy molecular materials usually possess low Tg if they are amorphous. For the high-7 and low-7), materials, the figures of merit of the NLO chromophores are defined by Eqs. (35) and (40), respectively. The latter are obviously enhanced by contributions from the polarizability anisotropy of the NLO chromophore. 

Structure and Thermal Properties Amorphous Polymers and the Glass Transition Crystallinity and Melting Density... 

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