Effect of block length on the network connectivity and temperature resistance of model, soft thermoplastic elastomers

Abstract

We discuss the connection between high-temperature mechanics, block structure, and composition of a model series of industrially relevant, soft, thermoplastic elastomers (TPEs) containing polydisperse hard blocks (HBs). The high-strain deformation behavior of these materials results from the combination of multiple dynamics in the system, i.e., the HB associations and the mobile and entangled amorphous phase. Many soft-TPEs show a reduction in toughness with increasing temperature. Molecular weight (Mw) has been shown to improve the temperature-dependent mechanics by increasing network connectivity. In this work, we investigate the possibility to increase the network connectivity by tuning block length at constant Mw and composition. The average number of HBs per chain can be used to quantify network connectivity; however, by using block statistics, we show how increasing this value is not enough to increase the high-temperature mechanics, especially in the case of polydisperse HBs. Since temperature affects the HB ability to associate with each other, only the number of associated HBs per chain determines network connectivity. The experimental results are consistent with modeling predictions, revealing how decreasing the average block length influences the crystal stability, which ultimately controls network connectivity, and how this relationship is affected by temperature.

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

“…This decrease is believed to originate from the T - and rate-dependent kinetics of the associations between HBs 20,34 and from a decreasing crystalline volume fraction with increasing T . 35…”

“…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.…”

“…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 occurs when the fraction of chains connected to the HB network falls below the percolation threshold.…”

“…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.…”

“…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. 26,35 The HB length distribution is one of the main contributing factors in determining the melting temperature of the PBT crystals in these materials, 35 which is generally lower than the one of pure PBT. Indeed, the materials tested in this paper are characterized by a melting peak temperature of B175 1C.…”