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1. Characterization of the Slow Molecular Dynamics of Poly( l ‐Lactic Acid) in α and α′ Phases, in a Glassy State, and in a Complex with Poly( d ‐Lactic Acid) by Solid‐State NMR

   https://doi.org/10.1002/macp.201700451  

   Chen, Wei; Zhou, Wenxuan; Makita, Yuta; Wang, Shijun; Yuan, Shichen; Konishi, Takashi; Miyoshi, Toshikazu (January 2018, Macromolecular Chemistry and Physics)

   Abstract Recently, slow molecular dynamics of poly(l‐lactic acid) (PLLA) by using 1D and 2D exchange NMR are investigated. In this work, slow molecular dynamics of PLLA chains in the α′, a stereocomplex (SC) with poly(d‐lactic acid), and glassy states are investigated in terms of centerband‐only detection of exchange (CODEX) NMR. The mixing‐time dependence of the CODEX data demonstrates that the molecular dynamics of stems become slower in the order of α′, α, and SC. The temperature dependence of the correlation time 〈τ_c〉 of the helical jump motions in the α and SC phases simply exhibits Arrhenius behaviors, with activation energy,E_a, values of 91 ± 1 and 97 ± 1 kJ mol⁻¹, respectively. In contrast, the temperature dependence of 〈τ_c〉 in the α′ sample exhibits two Arrhenius lines with substantially differentE_avalues of 273 ± 12 and 16 ± 14 kJ mol⁻¹at temperatures below and above 84 °C. The obtained kinetics of molecular dynamics not only establish the relationship between packing structure and dynamics in PLLA polymorphs and in the SC, but also allow for an understanding of the coupled dynamics between the crystalline and amorphous regions at approximatelyT_g. 

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2. Fabrication of 2D Block Copolymer Brushes via a Polymer‐Single‐Crystal‐Assisted‐Grafting‐to Method

   https://doi.org/10.1002/marc.202000228  

   Mei, Shan; Wilk, Jeffrey T.; Chancellor, Andrew J.; Zhao, Bin; Li, Christopher Y. (July 2020, Macromolecular Rapid Communications)

   Abstract Block copolymer brushes are of great interest due to their rich phase behavior and value‐added properties compared to homopolymer brushes. Traditional synthesis involves grafting‐to and grafting‐from methods. In this work, a recently developed “polymer‐single‐crystal‐assisted‐grafting‐to” method is applied for the preparation of block copolymer brushes on flat glass surfaces. Triblock copolymer poly(ethylene oxide)‐b‐poly(l‐lactide)‐b‐poly(3‐(triethoxysilyl)propyl methacrylate) (PEO‐b‐PLLA‐b‐PTESPMA) is synthesized with PLLA as the brush morphology‐directing component and PTESPMA as the anchoring block. PEO‐b‐PLLA block copolymer brushes are obtained by chemical grafting of the triblock copolymer single crystals onto a glass surface. The tethering point and overall brush pattern are determined by the single crystal morphology. The grafting density is calculated to be ≈0.36 nm⁻²from the atomic force microscopy results and is consistent with the theoretic calculation based on the PLLA crystalline lattice. This work provides a new strategy to synthesize well‐defined block copolymer brushes. 

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3. 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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4. 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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5. 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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