Polyurethane Elastomers From Morphology to Mechanical Aspects
A comprehensive account of the physical / mechanical behaviour of polyurethanes (PU´s) elastomers, films and blends of variable crystallinity. Aspects covered include the elasticity and inelasticity of amorphous to crystalline PUs, in re
- PDF / 1,685,103 Bytes
- 41 Pages / 439.37 x 666.142 pts Page_size
- 102 Downloads / 342 Views
Thermal behaviour of polyurethane elastomers
3.1 DMA experiments If an experiment works, something has gone wrong. (Murphy’s quote)
While I agree it is always good to be realistic, I never like to be pessimistic. Certainly the wellknown Murphy’s quote does not apply to the experiments that are described in this book. There is no study described here that would count on the success of a single experiment Along with differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) provides methods for probing the microphase separation characteristics through changes in the glass transitions of the components. DMA experiments measure the ability of a viscoelastic material to store and dissipate mechanical energy. They are based on the differences in the load response of the viscous and elastic components to a small sinusoidally applied strain. The resulting stress that is measured using an appropriate sensor, lags behind the applied strain by a phase angle δ. For a perfectly elastic material that stores all the input energy, the phase lag is 0◦ and for a perfectly viscous material is 90◦ out of phase. Thus the stress and strain are in phase for an elastic material. The complex Young modulus (E ∗ ) consisting of contributions of a storage modulus (E ), and of a loss modulus (E ), is measured (eq. 3.1). The complex modulus reflects the inherent viscoelastic nature of the polymer, where stress and strains will be out of phase with one another. E ∗ = E + iE
(3.1)
The E storage modulus quantifies the energy stored elastically by the material upon deformation, while the loss modulus E is a measure of the energy which is dissipated as heat during deformation. From the ratio between the loss and storage moduli, a third parameter, the loss factor (tan δ) can be calculated. This translates to a measurement of the ratio of the energy absorbed by the sample as heat to the energy used by the sample to return to its original shape. The loss modulus and loss
C. Prisacariu, Polyurethane Elastomers, DOI 10.1007/978-3-7091-0514-6 3, c Springer-Verlag/Wien 2011
61
62
3 Thermal behaviour of polyurethane elastomers
factor are indicators of the polymer ability to internally dissipate the energy under the test conditions, which is why peaks in the tan δ plots are often referred to as damping peaks. The DMA technique is sensitive to the various transitions which a polymer undergoes as a function of changing the temperature. Information on the PUs different flexible and rigid blocks can be obtained by means of DMA spectra in which the various segments are respectively observed to melt in sequence as the temperature is raised. The storage modulus E , provides information regarding the stiffness of the material, while the loss factor tan δ measures the degree of molecular motion. The glass transition temperature (TG ) of the SS is defined as the α transition in the tan δ curve. Significant features of a tan δ plot include the location of the low temperature maximum, where smaller areas are associated with bet
Data Loading...
