The cohesive energy density of polymers and its relationship to surface tension, bulk thermodynamic properties, and chain structure

Abstract

The cohesive energy density (CED) and the internal pressure (Pi) have similar values for most liquids at low pressures. For most polymers CED cannot be directly measured, unlike Pi which is readily obtained in the melt from PVT or other related data. Directly measured CED are available for the oligomer linear alkane series over a moderate molecular weight (MW) range. Scaling CED with measured surface tensions (ɣ) leads to separation into two universal scaling curves, which provides a measure of surface configurational entropy, as does the MW dependence of experimental surface tensions for two polymer series. A novel polymer scaling curve based on CED is shown to provide improved accuracy in the prediction of ɣ for any polymer or solvent. For polymers that interact predominately through Van der Waals forces, correlations between the average interchain separation distances and the magnitude of CED (or Pi), are shown to give a universal derivation of CED. In contrast, polymers like P4VP having strong polar interactions provide dramatic evidence of the different physical basis of CED and Pi. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44431.

Effect of chain stiffness on the entropic segregation of chain ends to the surface of a polymer melt

“…3. Interestingly, there are recent experiments 18 that show a crossover from one N −1 power-law at large N to another at small N. However, the convergence of our SCFT results in Fig. 5 to Eq.…”

“…Figure 5 This confirms the N −1 dependence observed in experiments for high molecular-weight polymers. [12][13][14][15][16][17][18] Just as in experiments, the decrease in tension becomes more gradual for shorter polymers. In fact, the empirical fit to N −2/3 obtained by experiments [14][15][16][17]49,50 for oligomers is accurately reproduced by our shortest four polymers, as illustrated in Fig.…”

“…(11) and (20) into Eq. (18). In all cases, the tension is smaller than that of flexible Gaussian chains of equal segment length.…”

“…It also implies that the surface tension should be independent of molecular weight or rather the degree of polymerization, N. However, simulations, [2][3][4][5][6] selfconsistent field theory (SCFT), [7][8][9][10] and experiments 11 all find an excess of chain ends at the surface. Furthermore, SCFT [7][8][9][10] and experiments [12][13][14][15][16][17][18] report an N-dependence in the surface tension. Other deviations from Silberberg's hypothesis have also been detected by simulations.…”

Entropic segregation of short polymers to the surface of a polydisperse melt

“…This has a wide range of implications in regards to surface tension [15,16], wall slip [17,18], the glass transition of thin films [19,20], and the effective force between polymer surfaces [21], to name a few. The phenomenon also has similar implications for mixtures of chemically identical molecules of different architecture [22][23][24][25][26].…”