Semicrystalline polymers first-order transitions

While Tm is a first order transition, Tg is a second order transition and this precludes the possibility of a simple relation between them. There is, however, a crude relation between Tm and Tg. Boyer [25] and Beamen [265] inspected data for a large number of semicrystalline polymers, some of which are shown in Table 2.4. They found that the ratio TgjTm ranged from 0.5 to 0.75 when the temperatures are expressed in degrees Kelvin. The ratio is closer to 0.5 for symmetrical polymers such as polyethylene and polybutadiene, but closer to 0.75 for unsymmterical polymers, such as polystyrene and polyisoprene. The difference in these values may be related to the fact that in unsymmterical chains with repeat units of the type -(CH2-CHX-)- an additional restriction to rotation is imposed by steric effects causing Tg to increase, and conversely, an increase in symmtery lowers Tg. 

The simplest case is the first-order phase transition of semicrystalline polymers in the presence of a chemically inert fluid as a pressure-transmitting medium. Mercury was preferred due to its chemical inertness and due to its convenient and well-known thermomechanical coefficients ( p= 1.80X10 " K" and k j- = 0.40X10 MPa ). The polymer sample was always in intimate contact with the pressure-transmitting fluid. 

In a semicrystalline polymer, the molar volume V (and hence also the density p), and not just a, may manifest discontinuous changes at the melting temperature Tm of the crystalline phase. The enthalpy and entropy also typically manifest discontinuities at Tm. Such discontinuities observed at Tm in the first derivatives of the Gibbs free energy signify that melting is a first-order phase transition . 

While each of these second-order transitions has a frequency dependence, the corresponding first-order melting transition for semicrystalline polymers does not. 

The investigation of a classic first-order phase transition is illustrated by the melting/crystallization of a semicrystalline polymer like MDPE. Chemically inert Hg was used as pressure-transmitting fluid, the polymer sample being completely surrounded by the fluid inside the detecting calorimetric zone in fact, the polymer sample was simply floating on the Hg. 

The transformation of crystalline zones of a semicrystalline polymer into a viscous liquid state a first-order thermodynamic transition. 

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