Tg and Tm

In a perfectly crystalline polymer, all the chains would be contained in regions of three dimensional order, called crystallites, and no glass transition would be observed, because of the absence of disordered chains in the sample. A perfectly crystalline polymer, on heating, would thus follow curve G-F-A, melting at to become a viscous liquid. 

Both Tg and Tm are important parameters that serve to characterize a given polymer. While Tg sets an upper temperature limit for the use of amorphous thermoplastics like poly(methyl methacrylate) or polystyrene and a lower temperature limit for rubbery behavior of an elastomer-like SBR rubber or 1,4-cij-polybutadiene, Tm or the onset of the melting 

The onset of softening is usually measured as the temperature required for a particular polymer to deform a given amount under a specified load. These values are known as heat deflection temperatures. Such data do not have any direct relation with Tm, but they are widely used in designing with plastics. 

Problem 2.17 Polycaprolactone has Tg = — 60°C and = + 60°C, while poly(ethylene terephthalate) has Tg = + 60°C and T = 250°C. Which polymer would be more suitable in a study of biodegradability at 25°C for which it is desirable to vary the crystallinity holding all other variables constant  

Crystallization can take place whenever the polymer is at a T above T. At 25° C, polycaprolactone is above Tg and so it may crystallize. On the other hand, polyfethylene terephthalate) can be quenched to 25°C at various rates resulting in various degrees of crystallinity which will not change as long as the temperature does not exceed Tg. So this polymer will be more suitable for the biodegradabOity study. 

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Substantially crystalline plastics in the range between Tg and Tm are referred to as leathery, because they are made up of a combination of rubbery noncrystalline regions and stiff crystalline regions. The result is that such plastics as PE and PP are still useful at the higher temperatures. 

The diamide unit acts as a nucleation initiator for the polyester segments.24 The presence of these diamide units at low concentrations results in an increased crystallization temperature. With increasing diamide concentrations in the polyester, the Tg and Tm also increase. 

An important chemical characteristic of a polymer is the chain length. Chain length strongly influences the thermal properties as illustrated in Table 2 for several aliphatic compounds of different lengths. Detailed studies have been published on the influence of chain length on melting temperatures of various polymers [5] or on Tg and Tm (melting temperature) of aromatic polymers [6]. 

Gonzalez et al. [106] copolymerized PDO and DPG with DMT to modify the polymer chain stiffness between the aromatic rings. It is interesting to note that the addition of 13 % DPG suppressed the copolymer Tg and Tm to 44 and 220 °C, respectively, and the copolymer was still crystalline with 33 % DPG content and had a 172 °C melting point, whereas PET/PTT copolymers with 33 % PET content would be non-crystallizable. 

Chuah et al. [107] prepared a series of PTT/poly(trimethylene napthalate) (PTN) copolyesters by copolymerizing PDO with dimethyl terephthalate and dimethyl naphthalate. The PTN homopolymer has a 7 g of 75 °C and a Tm of 245 °C. Despite the more rigid napthalate moiety, the PTN Tg and Tm were much lower than the Tg of poly(ethylene naphthalate) (PEN), indicating the strong influence of the flexible trimethylene units. 

The testing of materials can be based on whether the tested material is chemically changed or is left unchanged. Nondestructive tests are those that result in no chemical change in the material which may include many electrical property determinations, most spectro-analyses, simple phase change tests (Tg and Tm), density, color, and most mechanical property determinations. Destructive tests result in a change in the chemical structure of at least a... 

The power factor is the energy required for the rotation of the dipoles of a polymer in an applied electrostatic field of increasing frequency. Typical values vary from 1.5 x 10 for polystyrene to 5 x 10 for plasticized cellulose acetate. Values increase at Tg and because of the increased chain mobility gained so that Tg and Tm have been measured using differences in the power factor as temperature is increased. 

FIGURE 18.12 Idealized formation of order, crystals, as a function of temperature. The raised curve between the Tg and Tm ranges illustrate an idealized rate of crystal formation. This figure is sometimes referred to as the Molders Diagram of Crystallization. ... 

Fig. 18 Thermal properties (Tg and Tm) of the P(EtOx)-itot-(SoyOx) copolymer before and after UV-curing. (Reprinted with permission from [89]. Copyright (2007) John Wiley Sons, Inc.)...

The values of Tg and Tm for a polymer affect its mechanical properties at any particular temperature and determine the temperature range in which that polymer can be employed. The T and Tm values for some of the common polymers are shown in Table 1-3 [Brandrup et al 1999 Mark, 1999]. (These are the values at 1 atm pressure.) Consider the manner in... 

The rigidity of polymer chains is especially high when there are cyclic structures in the main polymer chains. Polymers such as cellulose have high Ts and Tm values. On the other hand, the highly flexible polysiloxane chain (a consequence of the large size of Si) results in very low values of Tg and Tm. 

With increasing n the -CO-NH- groups in the main chain are more and more diluted in the -CH2- sequences their effect decreases and Tg and Tm approach to the values of PE. The zig-zag effect is a matter of parity the formation of hydrogen bridges is only possible if the groups are in the proper position. 

Only Tg and are present in an amorphous thermoplastic and in a non-vulcanised rubber. A semi-crystalline thermoplast with short chains only shows Tg and Tm- In a (very exceptional) fully crystalline polymer Tg is not present... 

Selected factors affecting crystallinity regarding Tg and Tm are described in Chapter 2. Here we discuss the influence of crystallinity on the mechanical properties of polymers. For thermoplastics the relation between the degree of crystallinity and the physical nature is shown in Table 5.1. The general lack of difference in physical nature shown by largely crystalline polymers at... 

Most general purpose linear polymers, such as polyolefins, PS, PVC, and polymethyl methacrylate (PMMA), are not suitable for use at temperatures above 100 °C. PMMA and other polymers of 1,1-substituted vinyl monomers, such as poly-a-methylstyrene, decompose almost quantitatively to their monomers at elevated temperatures. However, the Tg and Tm values of these polymers are greater than those of polymers from 1-substituted vinyl monomers. For example, the Tg values of polymethyl acrylate (PMA) and PMMA are 276 and 381 K, respectively. 

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. 

Temperature is an important parameter in radiation degradation, yet remarkably little experimental work has been reported for irradiation at other than ambient temperature. The transition temperatures, Tg and Tm, of the polymer are as important in the radiolysis as they are for the properties. The irradiation temperature can be deliberately varied to enhance the radiation-induced modifications of polymers and should not be regarded as just an environmental property. Similarly, once the sensitization to, or protection from, radiation degradation by small molecule additives is understood — as a function... 

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. 

Recently, various techniques that produce highly oriented linear polyethylene with a ultra high modulus (hereafter, referred to as UHMPE) have been developed. In this section, we will examine the structure of the UHMPE that was prepared by highly drawing a dried gel [69]. Even if bulk polyethylene is uniaxially highly drawn by a normal method at a temperature between the Tg and Tm, the phase structure is essentially similar to the undrawn sample. That is, it involves three phases of the crystalline and two noncrystalline phases, although the mass fraction and detailed content of each phase are somewhat different. However, UHMPE samples may have a particular phase structure. 

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