Glass transition temperature semicrystalline

Plastics can be divided according to their character into amorphous and crystalline. Crystallization is never complete and the so-called crystalline polymers are virtually semicrystalline ones. Examples of amorphous plastics are polystyrene, acrylonitrile-butadiene—styrene copolymers, styrene—acrylonitrile copolymers, polymethylmethacrylate, poly(vinyl chloride), cellulose acetates, phenylene oxide-based resins, polycarbonates, etc. Amorphous polymers are characterized by their glass transition temperature, semicrystalline polymers by both melting and glass transition temperatures. 

As shown in Table 3, the glass-transition temperatures of the amorphous straight-chain alkyl vinyl ether homopolymers decrease with increasing length of the side chain. Also, the melting points of the semicrystalline poly(alkyl vinyl ether)s increase with increasing side-chain branching. 

In a semicrystalline polymer, the crystals are embedded in a matrix of amorphous polymer whose properties depend on the ambient temperature relative to its glass transition temperature. Thus, the overall elastic properties of the semicrystalline polymer can be predicted by treating the polymer as a composite material... 

The polymers can either be amorphous or semicrystalline, witli increased glass transition temperatures by increasing the phenyl units in the backbone. 

In contrast to the mature instrumental techniques discussed above, a hitherto nonexistent class of techniques will require substantial development effort. The new instruments will be capable of measuring the thermal (e.g., glass transition temperatures for amorphous or semicrystalline polymers and melting temperatures for materials in the crystalline phase), chemical, and mechanical (e.g., viscoelastic) properties of nanoscale films in confined geometries, and their creation will require rethinking of conventional methods that are used for bulk measurements. 

The isomeric bibenzoic acids (BBs), would appear to share similar structural features with naphthalene dicarboxylic acid. Like the PET-naphthalate copolymers, PET-bibenzoates have been demonstrated to possess moduli and glass transitions temperatures which increase with increasing levels of rigid comonomer [37-39], Unlike the PET/PEN copolymers, when the symmetrical 4,4 - I f I f monomer is substituted into a PET backbone, virtually every composition of PET-BB is semicrystalline the 2,4- and 3,4- isomers of BB, when... 

Resin Tensile Modulus, MPa Impact Strength, 1/cm Melting Temperature (U °C (semicrystalline) Glass Transition Temperature (g, °C (amorphous) Applications... 

Although polymers exhibit both viscous and elastic responses at all temperatures, the elastic response is particularly strong at temperatures less than 50°C above the glass transition temperature, particularly for polymers well above their critical molecular weight. Polymers are often considered to have dominant viscous rheological responses if they are stressed at temperatures over 100 °C above the glass transition temperature for amorphous polymers or 100°C above the crystalline melting point for semicrystalline resins. 

Fig. 2.18. Schematic DSC thermogram of a semicrystalline polymer = glass transition temperature = crystallization temperature = melting temperature = decomposition temperature...

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

The model PBZT/ABPBI molecular composite system is limited since the rod and the matrix do not possess glass transition temperatures for subsequent post form consolidation. In an effort to improve the processability for molecular composites, thermoplastics were used as the host matrix. Processing from acidic solvents requires the thermoplastic host to be soluble and stable in meth-anesulfonic add. Thermoplastic matrices were investigated including both amorphous and semicrystalline nylons [71,72], polyphenylquinoxaline (PPQ) [73] and polyetheretherketone (PEEK) [74], Table 5 shows the mechanical properties obtained for various processed PBZT thermoplastic molecular composite systems. As an example, the PBZT/Nylon systems showed 50-300% improvement over uniaxially aligned chopped fiber composite of comparable compositions. However, the thermally-induced phase separation during consol-... 

Amorphous polymers characteristically possess excellent optical properties. Unlike all the other commercially available fluoropolymers, which are semicrystalline, Teflon AF is quite clear and has optical transmission greater than 90% throughout most of the UV, visible, and near-IR spectrum. A spectrum of a 2.77-mm-thick slab of AF-1600 is shown in Figure 2.5. Note the absence of any absorption peak. Thin films of Teflon AF have UV transmission greater Ilian 95% at 200 mm and are unaffected by radiation from UV lasers. The refractive indexes of Teflon AF copolymers are shown in Figure 2.6 and decrease with increasing FDD content. These are the lowest refractive indexes of any polymer family. It should be noted that the abscissa could also be labeled as glass transition temperature, Tg, since Tg is a function of the FDD content of the AF copolymer. Abbe numbers are low 92 and 113 for AF-1600 and AF-2400. 

The mechanical properties of Teflon AF differ from those of the semicrystalline Teflon . Below the glass transition temperature the tensile modulus is higher (1.5 GPa) and elongation to break lower (5-50%). Similarly, below the Tg, creep is generally less than that normally observed for PTFE and shows much less variation with temperature. 

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