Glass Transition and Melting Temperatures

The extruder temperature profile for a single-screw extruder is set such that the functions of the process convert the polymer from a solid to a fluid. These two words are in quotation marks because for noncrystalline glassy (or amorphous) 

As discussed above, many polymers contain some crystalline structures when they are solidified. These polymers are referred to as semicrystalline resins. These crystalline structures can be observed using microscopy as shown in Fig. 2.12 for PP and sPS resins. As shown schematically in Fig. 2.13 and discussed above, not all portions of the polymer chains are incorporated into the crystalline structure. Instead, the portions of the chains that are not crystallized make up the amorphous phase. Solid density is the most commonly used method for measuring the 

Amorphous material often produces tie chains that connect two or more different crystals. These tie chains increase the properties of the solid resin by forming a temporary three-dimensional crosslinked system. As the resin is melted in an extruder, the crystals and the tie chains are destroyed, and the polymer acts like a 

The level of short-chain (SCB) and long-chain (LCB) branches control the solid resin density of a PE resin. For example, the level of SCB is controlled by the amount of alpha olefin comonomer incorporated into LLDPE resin as a pendant group. The random positioning of the pendant groups disrupts the crystailization process when the polymer is cooled from the molten state, causing the level of crystallinity to decrease with increasing amounts of alpha olefin comonomer. 

The modulus of PE resins increases with increasing solid density. Thus, a HDPE resin has a higher modulus than an LDPE resin, as shown by the data in Table 2.3. In general, resins with low solid densities feel soft to the touch while resins with high densities feel hard. The and Tg for selected semicrystalline and amorphous materials are given in Table 2.3. 

Crystal Structure Crystal system Chain conformation Unit cell parameters  

The I m value is proportional to the molecular weight according to the Flory equation  

In addition, the relationship between the Tg and the concentration of D-lactide can be predicted with a rational function  

The PDLA, PLLA or high d- or L-lactide copolymers have regular structures. The polylactides are either amorphous or semicrystalline at room temperature, depending on the molecular weight and content of l, d or meso-lactide in the main chain. PLA can be totally amorphous or up to 40% crystalline. PLA resins containing more than 93% L-lactic acid can crystallize. However, high molecular weight can reduce the crystallization rate, and therefore the 

For polypropylene (as well as any polymer), the attainment of 100% crystallinity is not possible. Therefore, in Fignre 15.17, the vertical axis is scaled as normalized fraction crystallized. A valne of 1.0 for this parameter corresponds to the highest level of crystallization that is achieved dnring the tests, which, in reality, is less than complete crystallization. 

As Section 15.8 notes, polymerie materials are responsive to heat treatments that produce structural and property alterations. An increase in lamellar thiekness may be induced by annealing just below the melting temperature. Annealing also raises the melting temperature by decreasing the vacancies and other imperfeetions in polymer crystals and increasing crystallite thickness. 

Melting and glass transition temperatures are important parameters relative to in-service applications of polymers. They define, respectively, the upper and lower temperatnre limits for numerous applications, especially for semicrystalline polymers. The glass transition temperature may also define the upper use temperature for glassy amorphous materials. Furthermore, T and Tg also influence the fabrication and processing procednres for polymers and polymer-matrix composites. These issnes are discnssed in sncceeding sections of this chapter. 

Melting and Glass Transition Temperatures for Some of the More Common Polymeric Materials 

Material Glass Transition Temperature Melting Temperature rCCF)] 

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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. 

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

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... 

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. 

Figure 2.4 Dehydrochlorination of Xylyl Chloride Table 2.2 Melting and Glass Transition Temperatures of Substituted...

FIGURE 12.16 Melting and glass transition temperatures of polyacrylonitrile gels plotted against the volume fraction of polymer. (From Krigbaum, W.R. and Tokita, N., J. Polym. Sci. 43, 467, 1960.)... 

Differential scanning calorimetry (DSC) analyzes thermal transitions occurring in polymer samples when they are cooled down or heated up under inert atmosphere. Melting and glass transition temperatures can be determined as well as the various transitions in liquid crystalline mesophases. In a typical DSC experiment, two pans are placed on a pair of identically positioned platforms connected to a furnace by a common heat flow path. One pan contains the polymer, the other one is empty (reference pan). Then the two pans are heated up at a specific rate (approx. 10 K min ). The computer guarantees that the two pans heat at exactly the same rate -despite the fact that one pan contains polymer and the other one is empty. Since the polymer sample is extra material, it will take more heat to keep the temperature of the sample pan increasing at the same rate as the reference pan. A plot is created where the difference in heat flow between the sample and reference is plotted as a function of temperature. When there is no phase transition in the polymer, the plot parallels the x-axis, and the heat flow is given in units of heat, q, supplied per unit time, t ... 

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. 

Polymers may possess other thermal transitions besides the melt and glass-transition temperatures. Not all of these transitions can be observed... 

Figure 3.3 a) Melting and glass transition temperature versus molecular weight of PLLA, content. ... 

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