Tm and crystallinity

Table 1. Melting temperature, Tm, and crystalline relaxation temperature, Tctc> of fatty acid monolayers on the water surface. 

Tabie 2. Melting enthalpy AH melting peak temperature Tm, and crystallinity aefor samples across the wall of molded part... 

Figures 7-17 and 7-18 provides examples of modulus vs. Tg for amorphous and crystalline plastics. Temperature can help explain some of the differences observed in plastics. For example at room temperature polystyrene and acrylic are below their respective Tg values, we observe these materials in their glassy stage. In contrast, at room temperature natural rubber is above its Tg [Tg = —75°C (—103°F) Tm = 30°C(86°F)], with the result that it is very flexible. When it...

The first-order transition or melting point (Tm) is energywise larger than the Eg. Entirely crystalline polymers will have only a whereas a totally amorphous polymer will have only a Tg. Since most polymers are a combination of amorphous and crystalline regions, they have both a Tg and a T -... 

As the temperature is increased there is available sufficient energy to melt the crystalline polymer, the Tm, and before this for the amorphous polymer sufficient energy so that in both cases ready wholesale movement of polymer chains occurs. The entire polymer now behaves as a viscous liquid such as molasses. For the cross-linked material wholesale mobility is not possible, so it remains in the rubbery region until the temperature is sufficient to degrade the material. 

Some polymers undego other thermal transitions in addition to Ts and Tm. These include crystal-crystal transitions (i.e., transition from one crystalline form to another and crystalline-liquid crystal transitions. 

Nylons 6/6 and 6 comprise more than 90% of the polyamide market. The two have similar properties but nylon 6 has a lower Tm (223°C). Small amounts of nylons 6/9, 6/10, 6/12, 11, 12, 12/12, and 4/6 are produced as specialty materials. Those with more methylene groups than nylons 6/6 and 6 have better moisture resistance, dimensional stability, and electrical properties, but the degree of crystallinity, Tm, and mechanical properties are lower. Specialty nylons made from dimerized fatty acids find applications as hot-melt adhesives, crosslinking agents for epoxy resins, and thermographic inks. 

Linear amorphous polymers are glasslike at low temperatures and become leathery at temperatures slightly higher than the glass transition temperature (Tg). These leathery polymers become rubbery at slightly higher temperatures, and crystalline polymers melt at the melting point (Tm). 

The Tgs and in some cases, the Tms of several poly(arylene ether-1,3,4-oxadiazolejs are reported in Table 12. The last five polymers in Table 12 display the same Tg trend as seen for other polymers, namely phenylphosphine oxide > sulfone > carbonyl > terephthaloyl > isophthaloyl. The terephthaloyl polymer could be heated above the Tm, and subsequently quenched to the amorphous form, and then annealed at 330 °C to induce crystallinity. Once the Tms of the carbonyl and isophthaloyl polymers were exceeded, crystallinity could not be reintroduced by annealing at 300 to 330 °C for several hours. The Tg and Tm of the isophthaloyl polymer are abnormally close. 

Figure 4. Crystalline melting points (Tm) and glass transition temperatures (T0) of plasticized polycarbonate as a function of plasticizer content...

Since successful commercialization of Kapton by Du Pont Company in the 1960s (10), numerous compositions of polyimide and various new methods of syntheses have been described in the literature (1—5). A successful result for each method depends on the nature of the chemical components involved in the system, including monomers, intermediates, solvents, and the polyimide products, as well as on physical conditions during the synthesis. Properties such as monomer reactivity and solubility, and the glass-transition temperature,T, crystallinity, Tm> and melt viscosity of the polyimide products ultimately determine the effectiveness of each process. Accordingly, proper selection of synthetic method is often critical for preparation of polyimides of a given chemical composition. 

The chemical state of dissolved silica (if Si02 is supplied as silica sol) must influence the nucleation and growth of crystalline silicates. Cary et al. (213) dissolved isotopically enriched silica in H20/D20 and, using 29Si NMR, concluded that tetrahedral dimers corresponding to pyrosilicic acid, H6Si207, built of two Q1 units were present in addition to monomers (Q°). The former species resonated at —9.26 ppm from TMS and accounted for up to 6 % of total spectral intensity. 

As noted in Fig. 14.1 (a), commercial fibers of semicrystallme polymers are always cold-drawn after spinning to achieve further structuring through further macromolecular orientation and crystalline morphological changes, many of which are retained because of the low temperature of the cold-drawing processes. A typical stress-strain curve for a polycrystalline polymer at a temperature Tg < T < Tm appears in Fig. 14.6. 

The lamellar habit adopted by crystalline polymers adds surface terms to the specific Gibbs function (chemical potential), most importantly the fold surface free energy, ae, which contributes 2ae/Xg for a lamella of thickness k and crystalline density q. In consequence melting points are lowered from T, for infinite thickness, to Tm according to the Hoffman-Weeks equation... 

Jong K. Lee, Interatomic Potentials and Crystalline Defects, Proceedings of a symposium sponsored by the TMS-AIME Chemistry and Physics of Metals Committee and the MSD-ASM Computer Simulation Activity at the fall meeting of the Metallurgical Society of AIME, Pittsburgh, PA, October 6-7, 1980, Metallurgical Society AIME, Warrendale, PA, 1981. 

The adaptations that thwart inappropriate phase changes have been presented as theories of homeophasic adaptation (McElhaney, 1984) and dynamic phase behavior (Hazel, 1995). In essence, both theories stress the same primary points the lipid composition of the bilayer must be modified in the face of temperature change to conserve the appropriate phase structure. The liquid-crystalline phase must be conserved at low temperatures. At higher temperatures the propensity to form the hexagonal II phase must not become too great. Thus, Tm and Th must be adjusted during adaptation. 

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