Influence of strain rate and temperature on necking transition in a polydomain smectic main chain elastomer

Abstract

Smectic main-chain liquid crystalline elastomers (MCLCE) with polydomain morphology are rare examples of elastomers that can form a neck and undergo cold drawing under tension. However, not all previous studies of the mechanical behavior of smectic MCLCE reported neck formation. The mechanical response of a polydomain smectic MCLCE has therefore been characterized by elongation at varying strain rates and temperatures to identify factors favoring mechanical instability. Yielding and neck formation are increasingly favored as the strain rate increases at constant temperature, or as the temperature decreases toward T g . As cold drawing pro-ceeds, significant creep occurs continuously within the neck, in contrast to the behavior of certain linear polymers that exhibit a ''natural'' draw ratio. Thermal imaging during elongation indicates that viscous heating is not a prerequisite for neck formation. Rather, inherent softening of the material during yielding due to morphological changes leads to an enhanced rate of deformation and contraction at the neck. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 591-598, 2011

Influence of thermal history on mesoscale ordering in polydomain smectic networks

“…Figure 2 shows the results of tensile testing at a nominal elongation rate of 12.5% min −1 (strain rate 2.08 × 10 −3 s −1 ) at 30 °C. For all samples, yielding and neck formation initiated near λ ≡ L / L 0 = 1.17, preceding a drop in the nominal stress 19, 22. The formation of a neck indicates local softening of the material, most likely due to fragmentation of the original domains near a stress concentrator.…”

“…The formation of a neck indicates local softening of the material, most likely due to fragmentation of the original domains near a stress concentrator. The temperature and elongation rate were kept constant in this study; their effects on necking and cold drawing are described in a previous communication 22…”

“…While MCLCN behave like elastomers at temperatures well above their glass transition temperatures ( T g ), they can exhibit mechanical phenomena reminiscent of semicrystalline polymers at lower temperatures. For example, smectic MCLCN still possess the flexibility and extensibility of elastomers, yet exhibit cold drawing behavior at temperatures only slightly above T g 19, 22. Another similarity to semicrystalline polymers is the incomplete strain recovery after uniaxial elongation, which was noted by Ortiz et al25 and characterized as a shape‐memory effect by Rousseau et al1…”

“…In previous reports, we exclusively characterized smectic MCLCN that were cooled rapidly from the isotropic state in order to ensure repeatable mechanical response 8, 19, 22. In this report, the influence of thermal history (specifically, annealing) on mesoscale ordering is characterized systematically for the first time.…”

“…Smectic liquid crystalline elastomers, especially those with mesogenic units in the main chain, have attracted attention as shape‐memory materials,1 as soft actuators,2–4 as energy‐dissipative coatings,5 and as model anisotropic soft materials. Recent studies of smectic elastomers focused on their synthesis and phase behavior,6, 7 on the molecular factors influencing their static and dynamic mechanical response,5, 8–13 and on basic understanding of smectic ordering11, 14, 15 and rubber elasticity 1, 16–22. Smectic elastomers have also attracted interest due to the ability of globally oriented specimens to change shape in response to temperature change,18 or in the case of the S type, to exhibit electromechanical effects 3, 4, 23, 24.…”