Letter pubs.acs.org/macroletters
Olefinic Thermoplastic Elastomer Gels: Combining Polymer Crystallization and Microphase Separation in a Selective Solvent Daniel P. Armstrong,† Kenneth P. Mineart,†,‡ Byeongdu Lee,§ and Richard J. Spontak*,†,∥ Departments of †Chemical and Biomolecular Engineering and ∥Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States § Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Since selectively swollen thermoplastic elastomer gels (TPEGs) afford a wide range of beneficial properties that open new doors to developing elastomer-based technologies, we examine the unique structure−property behavior of TPEGs composed of olefinic block copolymers (OBCs) in this study. Unlike their styrenic counterparts typically possessing two chemically different blocks, this class of multiblock copolymers consists of linear polyethylene hard blocks and poly(ethylene-co-α-octene) soft blocks, in which case, microphase separation between the hard and the soft blocks is accompanied by crystallization of the hard blocks. Here, we prepare olefinic TPEGs (OTPEGs) through the incorporation of a primarily aliphatic oil that selectively swells the soft block and investigate the resultant morphological features through the use of polarized light microscopy and small-/wideangle X-ray scattering. These features are correlated with thermal and mechanical property measurements from calorimetry, rheology, and extensiometry to elucidate the roles of crystallization and self-assembly on gel characteristics and establish useful structure−property relationships.
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of the copolymer molecules, the A-rich microdomains serve as physical cross-links to stabilize the B-rich molecular network.9 Independent efforts have demonstrated that the properties of TPEs can be chemically10 or physically11 modified to achieve specific properties, such as amphiphilicity.12 One approach by which to regulate the mechanical properties of TPEs is through the physical incorporation of a midblock-selective solvent. Such materials, collectively known as thermoplastic elastomer gels (TPEGs), exhibit highly tunable properties that make them attractive in adhesive,13 electroelastomeric,14 flextronic,15 microfluidic,16 and photovoltaic17 applications. Most of the TPEs employed for this purpose derive from styrenic copolymers with a polyolefin midblock selectively swollen with an aliphatic oil, although the principle has been extended to acrylic TPEs.13,18 In these previous studies, the parent TPE has been an ABA triblock copolymer that typically requires a specialized synthetic route.19,20 Such reactions are often performed under specific conditions and, in some cases, with relatively low-yield catalysts that compromise economic viability. A recent development in TPE synthetic design employs a “chain-shuttling” catalyst to yield multiblock copolymers with linear polyethylene hard blocks and poly-
lastomeric materials are becoming increasingly pervasive in both academic research and commercial technologies due mainly to their inherent durability and stretchability. Contemporary applications benefiting tremendously from the properties of such soft materials include biocompatible sensors for health monitoring,1 microfluidic platforms,2 and stretchable electronic devices.3,4 Traditional elastomers derive from chemically cross-linked, low-Tg homopolymers and exhibit well-described mechanical properties.5 Recent studies focused on the development of so-called “bottlebrush” elastomers reveal6 that the properties of such materials can be further tailored at the molecular level by independently controlling the cross-link density, the graft density along the polymer backbone, and the length of the grafts. Such materials are particularly suitable as dielectric elastomers since they actuate to ultrahigh strains (>300%) at very low electric fields (
