Processing temperature dependent mechanical response of a thermoplastic elastomer with low hard segment

Reduction of thrombotic and inflammatory complications of polystyrene-block-polyisoprene-block-polystyrene (SIS) with one-step electrospinning

Polystyrene-block-polyisoprene-block-polystyrene (SIS) has been used as biomaterials due to its soft and stable properties under physiological conditions. However, the thrombotic and inflammatory complications caused by SIS restrain its application as blood-contacting implant. To overcome this problem, the hydrophilic core-shell structured SIS-based microfiber with antioxidant encapsulation is fabricated with one-step reactive electrospinning. We demonstrate that the phase separation of SIS and acylated Pluronic F127 (F127-DA) components and crosslinking during electrospinning renders the microfiber blood compatible and stable under physiological condition; the encapsulation of 2-O-d-glucopyranosyl-l-ascorbic acid (AA-2G) in microfiber and subsequent release of AA-2G detoxifies the excess reactive oxygen species (ROS). The microfibers are nontoxic to cells and promote the fast growth and proliferation of human umbilical vein endothelial cells (HUVECs) in the presence of ROS; the thrombotic and inflammatory complications are effectively reduced with implant evaluation in vivo. Therefore, our work paves a new way to improve the biocompatibility of SIS, making it a promising candidate for blood contact materials.

Aliphatic Polyester Thermoplastic Elastomers Containing Hydrogen-Bonding Ureidopyrimidinone Endgroups

Polylactide- block-poly(γ-methyl-ε-caprolactone)- block-polylactide (LML) is a sustainable thermoplastic elastomer (TPE) candidate that exhibits competitive mechanical properties as compared to traditional styrenic TPEs. The relatively low glass transition temperature of the polylactide endblocks, however, results in stress relaxation and low levels of elastic recovery. We report the synthesis and characterization of poly(γ-methyl-ε-caprolactone) (PMCL) and LML end-functionalized with ureidopyrimidinone (UPy) hydrogen-bonding moieties to improve the elastic performance of these polymers. Although UPy-functionalized PMCL shows dynamical mechanical behavior that is distinct from the unfunctionalized homopolymer, it does not exhibit elastomeric behavior at room temperature. The addition of UPy endgroups to LML increases the ultimate tensile strength, elongation at break, and tensile toughness compared to unfunctionalized LML. Stress relaxation studies at a fixed strain show reduced levels of stress relaxation in LML with UPy endgroups. The stress relaxation was further reduced by including semicrystalline poly(( S, S)-lactide) as endblocks with UPy endgroups.

Highly elastic and degradable thermoset elastomers from CO2-based polycarbonates and bioderived polyesters

The first example of CO₂-based thermoset elastomers (CO₂Es) on the basis of two sustainable and degradable polymers, rigid CO₂-based polycarbonates and soft polyesters, is reported.

Mild synthesis of environment-friendly thermoplastic triblock copolymer elastomers through combination of ring-opening and RAFT polymerization

Environment-friendly thermoplastic triblock copolymer elastomers, polylactide-block-polyisoprene-block-polylactide, were synthesized by a mild ROP and RAFT method.

Strong, Resilient, and Sustainable Aliphatic Polyester Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) composed of ABA block polymers exhibit a wide variety of properties and are easily processable as they contain physical, rather than chemical, cross-links. Poly(γ-methyl-ε-caprolactone) (PγMCL) is an amorphous polymer with a low entanglement molar mass (M_e = 2.9 kg mol^-1), making it a suitable choice for tough elastomers. Incorporating PγMCL as the midblock with polylactide (PLA) end blocks (f_LA = 0.17) results in TPEs with high stresses and elongations at break (σ_B = 24 ± 2 MPa and ε_B = 1029 ± 20%, respectively) and low levels of hysteresis. The use of isotactic PLA as the end blocks (f_LLA = 0.17) increases the strength and toughness of the material (σ_B = 30 ± 4 MPa, ε_B = 988 ± 30%) due to its semicrystalline nature. This study aims to demonstrate how the outstanding properties in these sustainable materials are a result of the entanglements, glass transition temperature, segment-segment interaction parameter, and crystallinity, resulting in comparable properties to the commercially relevant styrene-based TPEs.

Evolution of unique nano-cylindrical structure in poly(styrene-b-isoprene-b-styrene) prepared under "dynamic packing injection moulding"

This work reports the evolution of ordered nano-cylindrical structures in a thermoplastic elastomer, poly(styrene-b-isoprene-b-styrene) (SIS), utilizing a newly designed processing technique, so-called "dynamic-packing injection moulding". In this injection moulding technique, controlled oscillating shears with different shear cessation times under constant pressure were applied on the moulded samples during cooling. It was found that these additional controlled oscillating shears resulted in a change of orientation in skin-core structures in these samples, compared with corresponding "reference" samples processed via traditional injection moulding (without controlled oscillating shears). For the "reference" samples, a highly oriented PS cylindrical structure combined with relatively weak lateral ordering was observed in their skin layers, whereas the lateral ordering of the PS nano-cylinders gradually disappeared when entering the core region. On the other hand, for the SIS samples obtained via "dynamic-packing injection moulding", the orientation of the PS nano-cylinders in the skin layers was similar to the case of the "reference" sample due to their extremely fast cooling rate. However, the lateral ordering of these cylinders had been extended to the core region. With an increase in the cessation time, the lateral ordering of the PS nano-cylinders was further improved and finally resulted in hexagonal lateral packing along the flow direction in the mould. Furthermore, a mixture of parallel/perpendicular orientation of the cylinders relative to the flow direction was found, particularly when the cessation time was short (such as 3 s). We speculated that this specific perpendicular orientation was a transient state for development of a final parallel orientation aligned with the flow direction with increasing cessation time, accompanied by a further enhancement of the nano-cylindrical parallel orientation. This study could provide a better understanding of the shear and relaxation effects on the structural evolution of this class of thermoplastic elastomers, enhancing supramolecular ordered cylindrical orientation in the core region, and paving a way to tune the nano-structures of block copolymers via this new processing technique to achieve desired properties.

Influence of magnetic nanoparticle size on the particle dispersion and phase separation in an ABA triblock copolymer

Oleic acid modified iron oxide nanoparticles (IONs) with different sizes were synthesized and mixed with styrene-butadiene-styrene block copolymer (SBS) with a lamellar structure. The octadecene segments on the oleic acid molecules have chemical affinity with the polybutadiene (PB) blocks, which makes IONs tend to be selectively confined in the microphase-separated PB domains. However, the dispersion state strongly depends on the ratio of the particle diameter (d) to the lamellar thickness (l) of the PB domains, which further changes the phase separation of SBS. When d/l ∼0.5, most of IONs are concentrated in the middle of the PB layers at low particle loading. Upon increasing the particle loading, part of IONs contact each other to form long strings due to their strong magnetic interactions. Away from the strings, IONs are either selectively dispersed in the middle and at the interfaces of the PB domains, or randomly distributed at some regions in which the phase separation of SBS is suppressed. The phase separation of SBS transforms from the lamellar structure to a cylinder structure when the IONs loading is higher than 16.7 wt %. As d is comparable to l, IONs aggregate to form clusters of 100 to 300 nm in size, but within the clusters IONs are still selectively dispersed in the PB domains instead of forming macroscopic phase separation. It is interpreted in terms of the relatively small conformational entropy of the middle blocks of SBS; thus, incorporation of nanoparticles does not lead to much loss of conformational entropy. Although incorporation of IONs with d/l ∼1 significantly increases the interfacial curvature and roughness, it has less influence on the phase separation structure of SBS due to the inhomogeneous dispersion. When d is larger than l, IONs are macroscopically separated from the SBS matrix to form clusters of hundreds of nanometers to several micrometers. More interestingly, the phase separation of SBS transforms from the lamellar structure to a two-phase co-continuous structure, probably due to the rearrangement of SBS molecules to cover the clusters with PB segments and the strong magnetic interaction exerting additional force on the SBS matrix during the evaporation of the solvent and the subsequent thermal annealing process.