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1. Sustainable thermoplastic elastomers produced via cationic RAFT polymerization

   https://doi.org/10.1039/D0PY01640C  

   Spring, Scott W.; Smith-Sweetser, Red O.; Rosenbloom, Stephanie I.; Sifri, Renee J.; Fors, Brett P. (March 2021, Polymer Chemistry)

   null (Ed.)

   Plastic production continually increases its share of global oil consumption. Thermoplastic elastomers (TPEs) are a necessary component of many industries, from automotive and construction to healthcare and medical devices. To reduce the environmental burden of TPE production on the world, we developed two new ABA triblock copolymers synthesized through cationic reversable addition–fragmentation chain transfer (RAFT) polymerization from renewable monomers. Using poly(isobutyl vinyl ether) (PIBVE) as the soft block and either poly( p -methoxystyrene) (PMOS) or poly(2,3-dihydrofuran) (PDHF) as the hard blocks, we produced triblock copolymers with varying volume fractions and characterized their material properties. PDHF-PIBVE-PDHF is sourced almost entirely from simple alcohols and exhibits mechanical properties comparable to those of commercial TPEs. This effort demonstrates the utility of cationic RAFT for the production of sustainable TPEs. 

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2. High Performance Star Block Aliphatic Polyester Thermoplastic Elastomers Using PDLA- b -PLLA Stereoblock Hard Domains

   https://doi.org/10.1021/acsmacrolett.3c00437  

   Liffland, Stephanie; Kumler, Margaret; Hillmyer, Marc A. (September 2023, ACS Macro Letters)

   Star block (ABC)4 terpolymers consisting of a rubbery poly(γ-methyl-ε-caprolactone) (PγMCL) (C) core and hard poly(l-lactide) (PLLA) (B) and poly(d-lactide) (PDLA) (A) end-blocks with varying PDLA to PLLA block ratios were explored as high-performance, sustainable, aliphatic polyester thermoplastic elastomers (APTPEs). The stereocomplexation of the PDLA/PLLA blocks within the hard domains provided the APTPEs with enhanced thermal stability and an increased resistance to permanent deformation compared to nonstereocomplex analogs. Variations in the PDLA:PLLA block ratio yielded tunable mechanical properties likely due to differences in the extent and location of stereocomplex crystallite formation as a result of architectural constraints. This work highlights the improvements in mechanical performance due to stereocomplexation within the hard domains of these APTPEs and the tunable nature of the hard domains to significantly impact material properties, furthering the development of sustainable materials that are competitive with current industry standard materials. 

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3. Controlling nanostructure and mechanical properties in triblock copolymer/monomer blends via reaction-induced phase transitions

   https://doi.org/10.1039/D0SM01661F  

   Torres, Vincent M.; LaNasa, Jacob A.; Vogt, Bryan D.; Hickey, Robert J. (February 2021, Soft Matter)

   Thermoplastic elastomers based on ABA triblock copolymers are typically limited in modulus and strength due to crack propagation within the brittle regions when the hard end-block composition favors morphologies that exhibit connected domains. Increasing the threshold end-block composition to achieve enhanced mechanical performance is possible by increasing the number of junctions or bridging points per chain, but these copolymer characteristics also tend to increase the complexity of the synthesis. Here, we report an in situ polymerization method to successfully increase the number of effective junctions per chain through grafting of poly(styrene) (PS) to a commercial thermoplastic elastomer, poly(styrene)–poly(butadiene)–poly(styrene) (SBS). The strategy described here transforms a linear SBS triblock copolymer–styrene mixture into a linear-comb-linear architecture in which poly(styrene) (PS) grafts from the mid-poly(butadiene) (PBD) block during the polymerization of styrene. Through systematic variation in the initial SBS/styrene content, nanostructural transitions from disordered spheres to lamellar through reaction-induced phase transitions (RIPT) were identified as the styrene content increased. Surprisingly, maximum mechanical performance (Young's modulus, tensile strength, and elongation at break) was obtained with samples exhibiting lamellar nanostructures, corresponding to overall PS contents of 61–77 wt% PS (including the original PS in SBS). The PS grafting from the PBD block increases the modulus and the strength of the thermoplastic elastomer while preventing brittle fracture due to the greater number of junctions afforded by the PS grafts. The work presented here demonstrates the use of RIPT to transform standard SBS materials into polymer systems with enhanced mechanical properties. 

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4. Effect of Polymer Composition and Morphology on Mechanochemical Activation in Nanostructured Triblock Copolymers

   https://doi.org/10.1021/acs.macromol.2c02475  

   Huo, Zijian; Arora, Swati; Kong, Victoria A.; Myrga, Brandon J.; Statt, Antonia; Laaser, Jennifer E. (March 2023, Macromolecules)

   The effect of composition and morphology on mechanochemical activation in nanostructured block copolymers was investigated in a series of poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate) (PMMA-b-PnBA-b-PMMA) triblock copolymers containing a force-responsive spiropyran unit in the center of the rubbery PnBA midblock. Triblock copolymers with identical PnBA midblocks and varying lengths of PMMA end-blocks were synthesized from a spiropyran-containing macroinitiatior via atom transfer radical polymerization, yielding polymers with volume fractions of PMMA ranging from 0.21 to 0.50. Characterization by transmission electron microscopy revealed that the polymers self-assembled into spherical and cylindrical nanostructures. Simultaneous tensile tests and optical measurements revealed that mechanochemical activation is strongly correlated to the chemical composition and morphologies of the triblock copolymers. As the glassy (PMMA) block content is increased, the overall activation increases, and the onset of activation occurs at lower strain but higher stress, which agrees with predictions from our previous computational work. These results suggest that the self-assembly of nanostructured morphologies can play an important role in controlling mechanochemical activation in polymeric materials and provide insights into how polymer composition and morphology impact molecular-scale force distributions. 

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5. Sustainable glucose-based block copolymers exhibit elastomeric and adhesive behavior

   https://doi.org/10.1039/c6py00700g  

   Nasiri, Mohammadreza; Reineke, Theresa M. (January 2016, Polymer Chemistry)

   Herein, we present the direct modification of glucose, an abundant and inexpensive sugar molecule, to produce new sustainable and functional polymers. Glucose-6-acrylate-1,2,3,4-tetraacetate (GATA) has been synthesized and shown to provide a useful glassy component for developing an innovative family of elastomeric and adhesive materials. A series of diblock and triblock copolymers of GATA and n -butyl acrylate (n-BA) were created via Reversible Addition–Fragmentation Chain Transfer (RAFT) polymerization. Initially, poly(GATA)- b -poly(n-BA) copolymers were prepared using 4-cyano-4-[(ethylsulfanylthiocarbonyl)sulfanyl] pentanoic acid (CEP) as a chain transfer agent (CTA). These diblock copolymers demonstrated decomposition temperatures of 275 °C or greater and two glass transition temperatures ( T g ) around −45 °C and 100 °C corresponding to the PnBA and PGATA domains, respectively, as measured by differential scanning calorimetry (DSC). Triblock copolymers of GATA and n-BA, with moderate dispersities ( Đ = 1.15–1.29), were successfully synthesized when S , S -dibenzyl trithiocarbonate (DTC) was employed as the CTA. Poly(GATA)- b -poly(nBA)- b -poly(GATA) copolymers with 14–58 wt% GATA were prepared and demonstrated excellent thermomechanical properties ( T d ≥ 279 °C). Two well-separated glass transitions near the values for homopolymers of n-BA and GATA (∼−45 °C and ∼100 °C, respectively) were measured by DSC. The triblock with 14% GATA exhibited peel adhesion of 2.31 N cm −1 (when mixed with 30 wt% tackifier) that is superior to many commercial pressure sensitive adhesives (PSAs). Use of 3,5-bis(2-dodecylthiocarbonothioylthio-1oxopropoxy)benzoic acid (BTCBA) as the CTA provided a more efficient route to copolymerize GATA and n-BA. Using BTCBA, poly(GATA)- b -poly(nBA)- b -poly(GATA) triblock copolymers containing 12–25 wt% GATA, with very narrow molar mass distributions ( Đ ≤ 1.08), were prepared. The latter series of triblock copolymers showed excellent thermal stability with T d ≥ 275 °C. Only the T g for the PnBA block was observed by DSC (∼−45 °C), however, phase-separation was confirmed by small-angle X-ray scattering (SAXS) for all of these triblock copolymers. The mechanical behavior of the polymers was investigated by tensile experiments and the triblock with 25% GATA content demonstrated moderate elastomeric properties, 573 kPa stress at break and 171% elongation. This study introduces a new family of glucose-based ABA-type copolymers and demonstrates functionality of a glucose-based feedstock for developing green polymeric materials. 

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