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The molecular mobility of chemically cross-linked and thermoplastic elastomers is of utmost importance in establishing physical properties for uses ranging from automotive tires and shoe soles to more sophisticated aerospace and biomedical applications. In many of these applications, network chain dynamics under external stresses/strains are critical for determining ultimate performance. It is well established that T g (or T R , the dynamic T g ) and the breadth of the segmental (R) relaxation time distribution increase with increasing cross-link density for chemically cross-linked polymers. For elastomers, there are considered to be two contributions important in determining the change in T R upon uniaxial extension: a reduction in conformational entropy, encouraging an increase in T R , and a volume increase on uniaxial deformation, leading to a reduction in T R (nonideal networks are compressible to some degree). 1,2Considering the importance of mechanical deformation in elastomer applications, there have been relatively few previous investigations of mechanical strain on chain dynamics, e.g., 3,4 even fewer using broadband dielectric spectroscopy, and none on thermoplastic elastomers. [5][6][7][8] This is despite the fact that modern broadband dielectric spectroscopy is a powerful tool for the investigation of material dynamics over very wide frequency and temperature ranges.In this Communication, we report the findings of our initial dielectric spectroscopy investigation of the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene and segmented polyurethane elastomers. A Novocontrol GmbH Concept 40 broadband dielectric spectrometer was used to measure dielectric permittivity and loss over a broad range of temperatures and frequencies.Synthetic polyisoprene [PI, NIPOL-IR2200 (Zeon Chemicals), 98.5% cis-1,4 content] was cross-linked with dicumyl peroxide (Sigma-Aldrich) at 180°C for 4 min in an aerated oven. Crosslink density was determined by measuring the swelling ratio in toluene and the Flory-Rehner equation.9 A poly(tetramethylene oxide) (PTMO, MW ∼ 1000) soft segment polyurethane (PU) with 32.5 wt % of hard segments [4,4 0 -diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO)] was supplied by AorTech Biomaterials (Scoresby, QLD, Australia) in film form.For the uniaxial deformation experiments, the stretching rig shown in Figure 1 was developed in order to allow convenient loading of deformed specimens directly into the dielectric spectrometer. This rig has three essential parts: sample clamper, electrode, and stretching knob. A sample strip (10 mm  80 mm) was placed along the groove on the rig and drawn to a specific extension ratio (λ). An electrode was positioned in the middle of the strained sample and clamped. The strained sample-electrode assembly was then separated from the rig and dried in vacuum for 24 h before measurement.The influence of uniaxial extension on the dynamics of the cross-linked PI (cross-link density = 6.1  10 -5 mol/cm 3 ) is displayed in Fi...

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