(32h) Vapor-Liquid Equilibria of VOC-Loaded Shape-Memory Natural Rubber

The high importance of natural rubber (NR) for mankind was identified already in the beginning of the 19^th century. After the discovery of the vulcanization process, NR became one of the most significant renewable materials with an annual production of about 11 billion tons at present. Recently, the so-called shape-memory natural rubber (SMNR) was discovered [1]. SMNR is a lightly cross-linked NR, which can be used to store energy and very high strains. The shape-memory properties are received due to strain-induced crystallization, which occurs by stretching the polymer during a programming procedure. Programmed SMNR samples are thus semi-crystalline and can be triggered by mechanical force, heat and even through contact with VOC (volatile organic compound) vapors.

Quitmann et al. [2] showed that constrained, lightly cross-linked SMNR even generates a reversible stress response upon exposure to VOC vapors, which depends on VOC type and concentration. This can be applied e.g. as vapor detector, as long as the mechanical reaction of the SMNR can be related to a certain VOC type and concentration. This requires the vapor-liquid-equilibrium (VLE) data of constrained SMNR/VOC systems.

In this work, VLEs of constrained and unconstrained SMNR/VOC systems were investigated at 293.15 K using a magnetic suspension balance. The measurements provide VOC equilibrium concentrations and diffusivity data. The different strains correspond also to different initial polymer crystallinities, which show a notable effect on the VLEs.

VLE modeling was performed using the Perturbed-Chain Statistical Associating Fluid Theory (PC‑SAFT). To account for the influence of constant strain on the VLE, a Helmholtz-energy contribution was used that accounts for network elasticity. This contribution is based on the affine network theory with a correction for the finite extensibility of the polymer chains [3].

The solubility of the VOCs in the amorphous part of the semi-crystalline SMNR was modeled based on the assumption that the crystalline phase does not absorb any solvent. The crystallinity of SMNR samples for each strain investigated was estimated based on literature data. Using the modeling approach suggested by Minelli and De Angelis [4], it was possible to correctly describe the experimental VLE data. For that purpose it was assumed that the crystallites exert an isotropic stress on the amorphous phase, which leads to reduced solubility in comparison to a non-stretched (fully amorphous) sample. This stress is quantified as an additional thermodynamic pressure inside the polymer phase and called constraint pressure p_c. p_c was considered as adjustable parameter. This allowed for quantitatively describing VLEs of SMNR/VOC systems over a broad range of strains and VOC concentrations.

References

[1]           F. Katzenberg, B. Heuwers, J. C. Tiller, Adv. Mater., 2011, 23, 1909-1911.

[2]           D. Quitmann, N. Gushterov, G. Sadowski, F. Katzenberg, J. C. Tiller, ACS Appl. Mater. Interfaces, 2013, 5, 3504-3507.

[3]           B. Miao, T. A. Vilgis, S. Poggendorf, G. Sadowski, Macromol. Theory Simul., 2010, 19, 414-420.

[4]           M. Minelli, M. G. De Angelis, Fluid Phase Equilibria, 2014, 367, 173-181.