223 research outputs found
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Water in polyurethane networks: physical and chemical ageing effects and mechanical parameters
The 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 â—¦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, H2O molecules replace part of the weak urethane H-bonds by H2O–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 H2O concentration of only cH2O ≈ (0.4 ... 0.5) cH2O,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, μrelaxed(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, μrelaxed(T) slightly grows in wet
PU. H2O 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 H2O-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 H2O molecules
have on the PU network, and they explain the consequences for the mechanical properties
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