Melting and glass transitions

Small amounts (usually <10%) of plasticizer could be used in the blending system to improve the processing properties of the material by lowering the melting and glass-transition temperatures. The addition of liquid plasticizer also makes the material soft but at the same time, the strength and toughness of the material decreases. 

Based on this simplified description of the melting and glass transitions, it is possible to propose six major types of mesophases. Figure 2 shows these mesophases in relation to the glass, crystal, and melt. First, it is possible to keep orientational order, but lose positional order. These positionally disordered crystals or orientationally ordered liquids 7) are widely known as liquid crystals 15). The name liquid crystal was given because of the obvious, liquid-like flow of these materials. By now it is too late to try to change the nomenclature, especially when the possible new names would be cumbersome 7). 

Melting and glass transition temperature for random ethylene-propylene copolymers. 

Roos, Y. (1993). Melting and glass transitions of low molecular weight carbohydrates. Carbohydr. Res., 238, 39-48. 

A simple attempt to correlate the two critical parameters of melt and glass transition temperatures of extruded raw materials with the structure of the final extrudate has been made. (Strahm et al. 2000). They measured glass transition temperatures (Tg) and melt temperatures T on a breakfast cereal formulation by rheometry. Tg s were observed as broad transition commencing at 67°C at 9.7% moisture and 14.6 C at 20% moisture. r , s for the same formulations were 146 and 54.5 , respectively. While these temperatures are obviously relevant to mass flow, it is not obvious how they relate to molecular events. Comparable measurements by DSC and NMR on the same samples would be extremely illuminating. 

Melting and glass transition temperatures determined by DSC Glass transition determined by DMA... 

General conditions 0.02-0.35 mmol Ln total volume 1-60 mL Melting and glass transition temperatures determined by DSC Glass transition determined by DMA... 

As expected, the melting temperature decreased with decreasing water content. The intersection of the extrapolated ice melting and glass-transition curves would be expected to occur at Tg (Slade and Levine, 1991). Other authors (Karel et al., 1994) consider that this intersection would occur at T[n, which corresponds to the lowest equilibrium melting point within the unfrozen matrix. Values of T[n were also included in Figure 58.3. 

Many properties of pure polymers (and of polymer solutions) can be estimated with group contributions (GC). Examples of properties for which (GC) methods have been developed are the density, the solubility parameter, the melting and glass transition temperatures, as well as the surface tension. Phase equilibria for polymer solutions and blends can also be estimated with GC methods, as we discuss in Section 16.4 and 16.5. Here we review the GC principle, and in the following sections we discuss estimation methods for the density and the solubility parameter. These two properties are relevant for many thermodynamic models used for polymers, e.g., the Hansen and Flory-Hug-gins models discussed in Section 16.3 and the free-volume activity coefficient models discussed in Section 16.4. 

In the range between the melting and glass transition temperatures, the material is usually referred to as a supercooled liquid. 

The melting and glass transitions in commercial Nylons and both homo-and copolyamides prepared by interfacial polycondensation have been studied by DTA by Ke and Sisko (171). The ETTA curves for a number of the polyadipamides and polysebacamides are given in Figure 7.54. 

Below the melting point the situation is more complicated. As mentioned above the dynamics now depends on the thermal history. Quenched samples exhibit relatively fast chain motions conqiarable to those in the nematic melt. For annealed samples two components are observed, which we assign to a LC and a crystalline phase. Decomposition of various relaxation curves into two components yields a crystallinity of 55% 5y practically independent of ten ierature [10, 96, 175]. A similar heterogeneity is observed for ordinary polymers, which exhibit amorphous and crystalline phases [176]. Accordingly, melt and glass transitions are detected in the DSC thermograms of both types of polymers [96, 177]. 

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