Toward Strong Thermoplastic Elastomers with Asymmetric Miktoarm Block Copolymer Architectures

Abstract

Thermoplastic elastomers (TPEs) are designed by embedding discrete glassy or semicrystalline domains in an elastomeric matrix. Typical styrenic-based amorphous TPEs are made of linear ABA-type triblock copolymers, where the volume fraction f of the glassy domains A is typically less than 0.3. This limitation ultimately restricts the range of mechanical strength attainable with these materials. We had previously predicted using self-consistent field theory (SCFT) that A(BA′) n miktoarm block copolymers with an approximately 8:1 ratio of the A to A′ block molecular weights and n ≥ 3 should exhibit discrete A domains at considerably larger f and offer potential for the combination of high modulus, high recoverable elasticity, and high strength and toughness. Using transmission electron microscopy and small-angle X-ray scattering on model polystyrene-bpolyisoprene (PS−PI) miktoarm copolymers, we show that such polymers indeed possess discrete PS domains for f values considerably higher than 0.3. The hexagonal morphology with PS cylinders was achieved for f = 0.5 and n = 3. Mechanical testing indicates that these miktoarm materials are strong, tough, and elastic and thus may be potential candidates for a new generation of thermoplastic elastomers.

Thermoplastic Elastomers Based on Block, Graft, and Star Copolymers

“…Progress in self-consistent ield theory (SCFT) [92] facilitated the ability to design TPEs based on nonlinear architectures such as miktoarm star polymer with superior mechanical properties [93]. For SIS triblock copolymer, over 36 vol% of PS component leads to lamellar morphology which is unfavorable for TPE applications [94]. For A(BAʹ) 4 miktoarm star polymer with one A block and four BAʹ blocks emanating from the same core, Fredrickson [93,94] predicted a stable morphology, of cylindrical A phase hexagonally dispersed in B matrix with a volume fraction of A polymer up to 70%.…”

“…For SIS triblock copolymer, over 36 vol% of PS component leads to lamellar morphology which is unfavorable for TPE applications [94]. For A(BAʹ) 4 miktoarm star polymer with one A block and four BAʹ blocks emanating from the same core, Fredrickson [93,94] predicted a stable morphology, of cylindrical A phase hexagonally dispersed in B matrix with a volume fraction of A polymer up to 70%. As shown in Figure 7a, asymmetric miktoarm star polymer S(ISʹ) 3 contains one long PS chain and three PSʹ-PI chains connecting at the same core.…”

Mechanics of an Asymmetric Hard–Soft Lamellar Nanomaterial

“…Highly asymmetric hard-soft lamellae, with a high content (above 65%) of a hard component, are expected to manifest a promising balance between toughness and stiffness. [37][38][39][40][41][42][43] However, almost nothing has been achieved in this direction. In addition, the mechanics of lamellar structures is highly dependent on the molecular architecture.…”

“…For hard-soft nano-layers, hard blocks are expected to behave as anchoring points at both ends to provide stiffness, while the middle soft blocks supply extensibility. 37,38,40 The number of hard and soft blocks within one molecule is a key parameter to tune mechanical properties.…”

“…The PS 1 -(PI-b-PS 2 ) 3 miktoarm star block copolymers were synthesized by anionic polymerization, which was reported elsewhere. 38,39 The long PS 1 Transmission electron microscopy (TEM). After the tensile testing, the broken specimens were collected and stained in osmium tetroxide aqueous solution (2 wt%) for one week.…”