Water in polyurethane networks: physical and chemical ageing effects and mechanical parameters

Abstract

AbstractThe chemical structure, polymer mobility and mechanical properties are studied for a cross-linked amorphous poly(ether urethane) (PU) from glass transition to rubber elasticity for juvenile dry samples and for water-saturated states after exposure to humid air (r.h. = 29, 67, 95, 100%) at $$60~^\circ \hbox {C}$$ 60 ∘ C during 1 y of ageing. For saturated samples, network chain cleavage is the chemical ageing mechanism, but it is too weak and slow to affect on the physical properties significantly within 1 y. Water acts primarily in a physical manner. Within 1 d, $$\hbox {H}_{{2}}\hbox {O}$$ H 2 O molecules replace part of the weak urethane H-bonds by $$\hbox {H}_{{2}}\hbox {O}$$ H 2 O –urethane H-bonds and reduce all other physical interactions between network chains by solvating hydrophilic segments. Thus, the cooperative polymer mobility strongly amplifies: The gain of specific conformational entropy doubles across the caloric glass transition, which shifts by −17 K. A $$\hbox {H}_{{2}}\hbox {O}$$ H 2 O concentration of only $$\hbox {c}_{{\mathrm{H}_2\mathrm{O}}}~\approx ~(0.4~\ldots ~0.5)~\hbox {c}_{\mathrm{\mathrm{H}_2\mathrm{O},max}}$$ c H 2 O ≈ ( 0.4 … 0.5 ) c H 2 O , max suffices for the major part of these fast rearrangements. Some part of the water slowly forms (during 3–4 months) a finely dispersed water-rich mixed phase with the PU chains. Except the new phase, these molecular processes of physical ageing strongly affect the mechanical properties at damage-free deformation. For dry PU in the glass transition, the shear modulus, $$\mu _{\mathrm{relaxed}}$$ μ relaxed (T), after viscoelastic stress relaxation only depends on the deformation-induced entropy change—like in the rubber elastic state. Within one month, water drastically decreases the viscoelastic response, as expected for plasticisation. However, $$\mu _{\mathrm{relaxed}}$$ μ relaxed (T) slightly grows in wet PU. $$\hbox {H}_{{2}}\hbox {O}$$ H 2 O molecules cause these opposite trends by boosting the cooperative mobility (i.e. extension of the accessible conformational space and entropy by reduction in energy barriers) and by occupation of free volume compartments. Water quickly reduces the fracture parameters by about 50%. We explain that embrittlement by the $$\hbox {H}_{{2}}\hbox {O}$$ H 2 O -induced facilitation of cooperative network chain motions, which let fracture proceed with less energy. In summary, our findings provide a detailed conception of the molecular effects the $$\hbox {H}_{2}\hbox {O}$$ H 2 O molecules have on the PU network, and they explain the consequences for the mechanical properties.