First-order transition temperature

The melting point is the temperature range in which total or whole polymer chain mobility occurs. The melting point (T ) is called a first-order transition temperature, and Tg is sometimes referred to as a second-order transition. The values for T are usually 33%-100% greater than for Tg. Symmetrical polymers like HDPE exhibit the greatest difference between T and Tg. The Tg values are low for elastomers and flexible polymers such as PE... 

The curvature correction can be important when K is small, R is small, and/or a is large. With regard to this aspect, Hirotsu has recently found strong size-dependence of the first-order transition temperature and proposed that a can be much larger in ionized gels than in neutral gels due to the presence of an electric double layer at the interface [31]. 

Another unique feature of ionized NIPA gel has been found recently both of the equilibrium swelling ratio a and the first-order transition temperature T0 depend strongly on the shape of samples [31]. The measurement of equilibrium a has been made on ionized NIPA gel rods of various diameters, and also on plates and cubes. The gel contained 680 mM NIPA, 20 mM acrylic acid (AA), and 8.6 mM BIS. All samples were prepared from the same pregel solution at the same time so as to guarantee that the composition and the structure of all samples were the same. 

First order transition temperature in crystalline polymers. J. Appl. Phys. 34, 2442—2445 (1963). 

FIG. 1. The increment of the first-order transition temperature T, as a function of the pressure, The calculated T, at zero pressure is 270 K. A linear fitting through the calculated points gives dT,/recent experimental results of 11.7 and 10.4 KAbar IlOl. 

In these equations T, T2. .. represent equilibrium first order transition temperatures and AH, AH-. .. the corresponding equilibrium transition enthalpies. is the enthalpy of the material analyzed at 0 K. 

Fig. 67. Order parameter profiles m(z)/mt, associated with surface-induced disorder. The coordinate z measures the distance from the surface (z = 0). is the bulk correlation length and mu the bulk order parameter. If ease (a) persists up to the first-order transition temperature Tc, this means the surface stays ordered up to 7 c, while case (b) shows surface induced disordering a layer of thickness L gets disordered already at T <71, and as T — Tc the (delocalized) interface at mean position z = L from the surface advances into the bulk, fJ(7 ) -+ oo as T - Tc, and the surface order parameter mj = m(z = 0) then vanishes continuously, mj a (1 - T/Tc). From Dasch et al. (1988).

Figure 32. Herringbone order parameter for the anisotropic-planar-rotor model (2.5) as a function of the reduced temperature T = TIK. Circles Monte Carlo results [244]. Dotted line mean-field approximation [62, 141]. Solid line triangular cluster-variational method [62]. Arrow first-order transition temperature obtained from a real-space renormalization group treatment of a planar quadrupolar six-state model [345]. (Adapted from Fig. 2 of Ref. 345.)...

The melting temperatures of the crystallites are designated by Tm, which is the first-order transition temperature. The melting points are often not as sharp as the melting points of ordinary crystals and may melt over a range. Table 1.2 shows some typical values of Tm-... 

Explain what is the first-order transition temperature. 

Choose a suitable standard material from those listed in Table 4.2, which has a first-order transition temperature nearest to that of the substance investigated, and measure the transition heat under the same conditions as mentioned above. 

As a rule, the phase transition from the isotropic phase into the nematic phase is a weak first-order transition [6] with a small jump in the order parameter 5 (Fig. 1.3 [7]) and other thermodynamic properties. The so-called clearing point corresponds to this first-order transition temperature Tni. At the same time, in the pretransitional region of the isotropic phase we can observe the temperature divergence in some physical parameters, such as heat capacity, dielectric permittivity, etc., according to the power law (T — T i) where T j is the other, virtual, second-order phase transition point, (Tni — T 0.1 K) and t) is an exponent, depending on the physical property under consideration. 

Fig. 5.25. The order parameter of an elastomeric liquid-crystalline polymer versus temperature in the absence (bold solid line, Maier-Saupe-like theory) and presence of external stress (dotted line and thin line). As the stress is increased, the first-order transition temperature (I d) increases (dotted line) above some critical stress (thin line) there is a continuous increase in S as the temperature is lowered.

Two very important temperatures are indicated in Fig. 3.15 and are the melt temperature (or first order transition temperature), T j and the glass transition (or second order transition temperature) Tg. The T and Tg can only be determined approximately from isochronous modulus-temperature data similar to that given in Fig. 3.15. Often, manufacturers specification... 

First-Order Transition Temperature m The temperature at which a polymer freezes or melts. 

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