The promise of ABC triblock terpolymers for improving the mechanical properties of thermoplastic elastomers is demonstrated by comparison with symmetric ABA/CBC analogs having similar molecular weights and volume fraction of B and A/C domains. It is shown that the ABC architecture enhances elasticity (up to 98% recovery over 10 cycles) in part through essentially full chain bridging between discrete hard domains leading to minimization of mechanically unproductive loops. In addition, the unique phase space of ABC triblocks also enables the fraction of hard-block domains to be higher (f_hard ≈ 0.4) while maintaining elasticity, which is traditionally only possible with non-linear architectures or highly asymmetric ABA triblock copolymers. These advantages of ABC triblock terpolymers provide a tunable platform to create materials with practical applications while improving our fundamental understanding of chain conformation and structure–property relationships in block copolymers.

¹H nuclear magnetic resonance spectroscopy

Solution state ¹H nuclear magnetic resonance (NMR) spectra were recorded on a Varian VNMRS 600 MHz spectrometer. Chemical shifts (δ) are reported in ppm relative to residual protio-solvent in CDCl₃ (7.26 ppm).

Size-exclusion chromatography instrumentation

Size-exclusion chromatography (SEC) was performed on a Waters instrument using a differential refractive index detector and two Tosoh columns (TSKgel SuperHZM-N, 3 μm polymer, 150 × 4.6 mm) with chloroform containing 0.25% TEA at 35 °C as the mobile phase. Molar masses and molar mass dispersity (Đ) were determined against narrow PS standards (Agilent). 

Mechanical analysis

Frequency sweeps and isochronal temperature sweeps were obtained on a TA Instruments ARES G2 in oscillatory shear mode with a 25 mm parallel-plate geometry and a nitrogen-purged forced–convection oven. For all experiments, oscillatory shear was applied within the linear viscoelastic regime as verified from isochronal strain sweeps at a fixed temperature. Isochronal (ω = 1 rad/s) temperature ramps employing small strain amplitudes, typically γ ≤ 1%, and low heating rates (2 °C/min) were used to measure the order–disorder transition temperature (T_ODT). Differential Scanning Calorimetry (DSC) was performed using a TA Instruments DSC Q2000 at a heating/cooling rate of 10 °C/min using 3–5 mg of sample in a sealed aluminum pan. Samples for mechanical testing were heat pressed using a Carver press (Wabash, IN) into a 0.5 mm thick steel rectangular mold with 5000 lbf at 180 °C for 10 min and then rapidly cooled to room temperature to suppress crystalline break out. All samples were aged at room temperature for 16 – 18 h prior to uniaxial tensile to ensure adequate crystallization of the poly(L-lactide) and consistency throughout samples. Temperature–dependent modulus data were obtained on a TA Instruments DMA 850 with a film clamp geometry and a nitrogen–purged oven using a strain of 0.05% at a frequency of 1 Hz and a temperature ramp rate of 5 °C/min. Samples were approximately 3 mm by 8 mm by 1.5 mm. For uniaxial extension tensile tests, a dog bone cutting die was used to punch out samples of the correct geometry (gauge depth = 0.5 mm, gauge width = 1.5 mm, gauge length = 10 mm, transition zone radius = 2.5 mm). Tests were performed using a custom-built setup with a vertical TwinRail positioning table (Lintech, CA) and a 10 N load cell (LSB2000 Miniature S-Beam, FUTEK, CA). A deformation rate of 10 mm min⁻¹ was used for all tests (strain rate = 1 min⁻¹). Cyclic testing of was performed on a TA.XTplusC texture analyzer equipped with A/MTG tensile grips. Ten cycles were performed for each experiment with a deformation rate of 10 mm min⁻¹, an unloading rate of 10 mm min⁻¹, and force tracking at a rate of 10 mm min⁻¹ with a 10 min dwell after each cycle to ensure complete relaxation of the thermoplastic elastomer. All samples for mechanical testing were conducted on samples from the same molded sheets and thus exhibit the same thermal history.

Small–angle X-ray Scattering (SAXS)

SAXS experiments were conducted using a custom-built SAXS diffractometer at the Materials Research Laboratory (MRL) X-ray facility (University of California, Santa Barbara). For these experiments, 1.54 Ǻ Cu Kα X-rays were generated using a Genix 50 W X-ray microsource (50 μm micro-focus) equipped with FOX2D collimating multilayer optics (Xenocs, France) and high efficiency scatterless single crystal/metal hybrid slits. A sample-to-detector distance of 1721.6 mm was employed. Triblock thermoplastic elastomer dog bone specimen was directly mounted to the sample holder for scattering. Dog bone samples were heat pressed using a Carver press into a 0.5 mm thick steel rectangular mold with 5000 lbf at 180 °C (180 °C > T_m of PLLA < T_ODT of the triblock) for 10 min and then rapidly cooled to room temperature to suppress crystalline break out. Diblock copolymers were annealed at 180 °C for 16 h, slowly cooled to room temperature before and mounted on a metal washer with a Kapton tape backing.