Heat capacity polymer thermodynamics

THERMODYNAMICS OF VINYL SERIES MONOMERS AND POLYMERS. II. HEAT CAPACITIES AND THERMODYNAMIC FUNCTIONS OF ACRYLONITRILE AND POLYACRYLONITRILE. 

The thermodynamic constants of THF polymerization have been investigated by a number of authors. A variety of experimental techniques have been utilized including determinations of conversion to polymer, combustion, heat capacities eind vapor pressure. Comparison of our results with some previously published data shows that our results are within the range of the values reported (Table 3). 

The most common applications of DSC are to the melting process which, in principle, contains information on both the quality (temperature) and the quantity (peak area) of crystallinity in a polymer [3]. The property changes at Tm are often far more dramatic than those at Tg, particularly if the polymer is highly crystalline. These changes are characteristic of a thermodynamic first-order transition and include a heat of fusion and discontinuous changes in heat capacity, volume or density, refractive index, birefringence, and transparency [3,8], All of these may be used to determine Tm [8],... 

A new type of rotational degrees of freedom parameter will be defined for the backbones and side groups of polymers, and correlations for the heat capacity and related thermodynamic functions (enthalpy, entropy and Gibbs free energy) will be developed utilizing both the connectivity indices and the rotational degrees of freedom, in Chapter 4. 

Each atom has 3 thermodynamic degrees of freedom, so that the maximum possible number of modes of motion of any material is 3/atom. The observed heat capacities of polymers usually correspond to a much smaller effective number of degrees of freedom because of the constraints imposed on possible motions. In other words, these constraints restrict the amplitudes of the vibrations within the maximum number of three modes per atom, resulting in the observed lower effective number of degrees of freedom. For example [6], the heat capacities of polymers at room temperature typically correspond to roughly one degree of freedom per atom. 

If this parameter is assumed to be the same for all vibrations, one can obtain a bulk thermodynamic definition for y. The bulk Griineisen parameter is found to be about 4 for polymers from the effect of pressure on the velocity of sound. The data suggest that for the heat capacity only the interchain contribution should be taken into account. With this assumption, an order of magnitude calculation shows that the bulk Griineisen parameter for proteins is of the same order of magnitude as that of polymers. This suggests that the thermal expansion and the compressibility of proteins reflect primarily the movement between the secondary structures. These movements are reflected in the low frequency part of the... 

Gaur U, Lau S-F, Wunderlich B (1983) Heat capacities and other thermodynamic properties of linear macromolecules IX. Aromatic and Inorganic polymers. J. Phys. Chem. Ref. Data, 12 91... 

The transition from a glass to a rubberlike state is accompanied by marked changes in the specific volume, the modulus, the heat capacity, the refractive index, and other physical properties of the polymer. The glass transition is not a first-order transition, in the thermodynamic sense, as no discontinuities are observed when the entropy or volume of the polymer is measured as a function of temperature (Figure 12.2). If the first derivative of the property-temperature curve is measured, a change in the vicinity of is found for this reason, it is sometimes called a second-order transition (Figure 12.2). Thus, whereas the change in a physical property can be used to locate Tg, the transition bears many of the characteristics of a relaxation process, and the precise value of can depend on the method used and the rate of the measurement. 

When we look for a thermodynamic confirmation of the landscape height, however, we encounter a problem. Instead of finding that the jump in heat capacity at Tg is large in proportion to how fragile the polymer is, we find that it is much the same for all polymers (despite wide variation in fragility (27,28), and usually rather small compared with the polymer glass heat capacity at Tg. If this value is adopted for calculations using equation (3), all... 

Thermodynamics. (21) This is to determine the vdume and entropy as functions of T and P by using equation 8. It involves the eos and the temperature dependent conOgimitional heat capacity Cn at atmospheric pressure. The polymer of choice was Bisphend-A-Polycaibonate. The eos and positron data are available. (22,23) The total heat capacity Cp of the glass is linear in T. (24) The assumption then is a corresponding expression for Cp (config.). 

Thermodynamics, both classical [Appendix 3.A] and statistical [Appendix 2A], have been applied to many topics in polymer science. The results have provided insights into the origin of rubber elasticity, the nature of polymer crystalline, polymeric heat capacities and the miscibility of polyblends. 

Molecular dynamics has proved to be a powerful method for simulating and/or predicting several features of polymer systems. Properties on either side of the glass transition temperature (see Section 1.5) have been successfully simulated, as has the solid-to-liquid transition, and provided descriptions of the dynamics (segmental motions, chain diffusion, conformational transitions, etc.) that are in accord with relaxation measurements and such bulk properties as shear viscosities and elastic moduli. The method may also provide a good description of the variation in heat capacity and other thermodynamic fimctions across a phase transition. Several collections of these investigations have recently been published. ... 

The first step in the analysis must thus be to establish the crystallinity dependence of the heat capacity. In Fig. 2.44 the heat capacity of polyethylene, the most analyzed polymer, is plotted as a function of crystallinity at 250 K, close to the glass transition temperature (T = 237 K). The fact that polyethylene, [(CH2-)xl, is semicrystalline implies that the sample is metastable, i.e., it is not in equilibrium. Thermodynamics... 

There are six different thermodynamic contributions to the apparent heat capacity in the melting and crystallisation region of the analysed polymer [24]. The first three can be truly reversible, and the second three are increasingly irreversible. 

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