Diffusion, Swelling and Mechanical Properties of Polymers

Abstract: Polymers capability to withstand harmful interactions withdifferent environments can be determined by looking at thetransport and mechanical properties of the material exposed tothe medium. The diffusion of a penetrant in a polymer and itsswelling characteristics can be verified by a simplesorption-desorption experiment followed by methodical analysisof the data. Three different systems have been investigated andreported on using the sorption-desorption technique, tensiletesting, compression testing, stress-relaxation and curvefitting routines in Matlab.Fluoropolymers of different repeating unit structure andcrystallinities were exposed to tetrachloroethylene (TCE),water, hydrochloric acid (35%) and hydrobromic acid (47%) at70°C yielding solubility and diffusivity data. Thetransport properties were mostly controlled by the polarity ofthe polymer and to a less degree by the polymer crystallinity.Low solubilities were observed for the aqueous solutes andtheir diffusivities were best fitted using a dual sorption modeassuming no concentration dependent diffusivities. Thepolarisable non-polar TCE showed the highest solubility, andthe diffusivity was solute-concentration dependent. The rate atwhich the surface-concentration approached the saturation levelwas proportional to the product of the Young's modulus, thesquare of the dry polymer thickness and the logarithm of thesolute diffusivity. Data for water-hyperbranched polymer andlimonene-polyethylene confirmed the relationship.Low and high-density poly-ethylene (LDPE, HDPE), crosslinkedethylene vinyltrimetoxy silane (PEX), natural rubber (NR) andacrylonitrile-butadiene rubber were exposed to crude oilcomponents at 25 and 30°C. Solubility data indicated thatthe accessibility of the interfacial components decreased inthe order: cyclohexane, n-hexane/2,2-dimethylbutane andn-decane/n-tetradecane. The free-volume model describeddesorption data better than the semi-empirical exponentialmodel, but the numerical differences were for most practicalapplications negligibly small. The decrease in tensile modulus,yield stress and relaxation modulus for dry HDPE and PEXcompared to n-hexane, n-heptane, cyclohexane andtoluene-swollen samples was clearly caused by thepenetrant-induced plasticisation effect.LDPE and NR-sheets were exposed to limonene at 25°C.The limonene-NR displayed saddle-shape during sorption andcup-shape during desorption were most likely a consequence oflocal differences in limonene concentration that affected thestress state across the sheet thickness. The ratio in bulkmodulus between LDPE and NR was significantly smaller than thecorresponding ratio in tensile modulus and close to the ratioin the degree of anisotropy for the same polymers.Consequently, the bulk modulus is more accurate than thetensile modulus to use when predicting the degree of swellinganisotropy.