Molecular weight effects on the stress-relaxation behavior of soft thermoplastic elastomer by means of temperature scanning stress relaxation (TSSR)

Abstract

The mechanical properties of multiblock copolymer thermoplastic elastomers (TPEs) are governed by the interplay of different reversible dynamics [e.g., hard block (HB) association and chain entanglements]. Understanding how these physical processes influence the high-temperature deformation behavior is relevant as many TPEs lose toughness with increasing temperature. Increasing molecular weight (Mw) improves their temperature resistance that is attributed to an increase in network connectivity. Indeed, longer chains are characterized by more HBs per chain and by a longer lifetime of the entanglements in the amorphous phase. Both the associating HB and disentanglement dynamics are temperature and rate dependent. To further understand the interconnected role of Mw, temperature and rate dependencies on the mechanical properties, we perform Temperature Scanning Stress Relaxation (TSSR) tests. The method consists of measuring the stress relaxation of the materials as the temperature monotonically increases, allowing us to probe the stress response as the HBs progressively disassociate due to the increase in temperature. The results show that increasing Mw improves the high-temperature relaxation behavior, allowing the material to retain more stress than its low Mw counterpart as the temperature increases. This distinction does not show itself when performing standard small strain dynamic mechanical thermal analyses. Depending on the deformation experienced before the TSSR is performed, different relaxation behaviors are observed illustrating the importance of the current microstructure in determining the mechanical properties. The TSSR approach is well-suited to benchmark the high-temperature stress-bearing properties of network-based polymers whose morphology and, hence, properties are strongly deformation dependent.

Effect of temperature, rate, and molecular weight on the failure behavior of soft block copoly(ether–ester) thermoplastic elastomers

“…The effect of T on ε f is stronger for the low- M w sample, consistent with previous observations. 20,34,36–38 This behavior has been rationalized by Aime et al 34 by treating the material as a transient network and considering the rate and T dependence of the kinetics of the association between the HBs. 34 With increasing T , the HB disassociation rate increases, leading to a faster loss of network connectivity.…”

“…Longer chains have more HBs and entanglements per chains, which leads to a lower probability of disconnecting chains from the network and thus to an increased lifetime of the stress-bearing segments and associated larger high-strain elasticity. 20,36…”

“…Despite being extremely tough at lower temperatures, many soft multiblock copolymer TPEs show a decrease in the energy required for fracture in continuous tensile loading with increasing temperature (T). 20,[34][35][36][37][38] This behavior limits the range of applications where TPEs can be used as well as their suitability for some processing technologies at T 4 room temperature (RT). This decrease is believed to originate from the T-and ratedependent kinetics of the associations between HBs 20,34 and from a decreasing crystalline volume fraction with increasing T. 35 The failure mechanism of these copoly(ether-ester) multiblock copolymers was first addressed by Aime et al, 34 who described failure in terms of connectivity loss of the associated hard block network.…”

“…This work uses industrially relevant model copoly(etherester) (TPEEs) with 30 wt% of hard blocks (HB) whose mechanical properties have been extensively characterized. 20,21,[34][35][36]43 They have a room temperature (RT) modulus of B25 MPa 20 with high extensibility and elasticity due to the crystallized HBs acting as physical crosslinks. As the HBs in these systems are characterized on average by only a few PTB monomeric units (B4.5 20 ), crystallization is not characterized by folding of the HBs into crystalline lamellae, but rather by parallel stacking of the crystallizing HBs forming eventually long ''ribbon-like'' crystals of height and thickness comparable to the HB length.…”

Conductive Elastic Composite Electrode and Its Application in Electrocardiogram Monitoring Clothing

“…The SCP fibers were continuously prepared by wet spinning (Figure S1). SEBS is composed of soft segments of polybutadiene to provide elasticity, a polystyrene hard segment as a chain entanglement network. − Compared to other rubber materials, SEBS can exhibit a high elasticity of vulcanized rubber without vulcanization and has good aging resistance and biocompatibility. − Therefore, SEBS was selected as a flexible base material. Carbon-based materials are widely used in composite conductive polymers owing to their good electrical conductivity.…”