Development of ultra-thin poly(L-lactic acid)-based films integrating toughness, barrier properties, and gas selectivity: Towards gas-permeation controllable green food packaging

High costs and low performance have constrained the application of bio-based materials in food packaging. Herein, a series of ultra-thin poly(L-lactic acid-iconic acid N-diol) (P(LA-NI)) copolymer films were developed using a "one-step" polycondensation process with integrated toughness, barrier properties, gas selectivity, and quality control features. The massive branched structure and gg conformers in P(LA-NI) act as "internal chain expansion" and "internal plasticization". Meanwhile, P(LA-NI) contains numerous polar groups and unique nanoscale microphase structures to realize excellent CO2, O2 barrier, CO2/O2 selectivity, anti-fogging, and UV shielding functions. The atmosphere within the package spontaneously achieves the desirable low O2 and high CO2 levels when packaging button mushrooms with high respiratory metabolism. Eventually, the shelf life of button mushrooms reached 24 days, >3-fold extended. This PLLA-based film meets "dual carbon" and "food safety" goals and has vast potential for fresh food preservation.

"Flexible-strong" polylactic acid porous membrane via tailored polymerization degree of lactic acid side-chains grafting for passive daytime radiative cooler

Aerogel possesses the advantages of high specific surface area, low density, and high porosity, which have shown great application in thermal regulation due to its efficient light scattering capability. However, traditional polymer-based aerogels have poor mechanical properties and lack ductility in outdoor applications, the cooling efficiency of the material is easily affected by damage during transportation, installation, and environmental factors. In this work, combining the porous nature of aerogels and the high ductility of membranes, a polylactic acid-based porous membrane cooler was developed by combining a regular honeycomb surface porous structure design and physical/chemical modification to enhance flexibility, using a simple non-solvent induced phase separation method. This porous membrane exhibits both super-flexibility (116 % elongation at break) and porous characteristics. It achieves a sub-ambient temperature decrease of 4-6 °C under direct sunlight. The optimized porous membrane demonstrates high solar reflectance (94 % of peak reflectivity, 90 % of average reflectivity) and strong infrared emissivity (96 % of peak emissivity, 91 % of average emissivity), it also maintains a solar peak reflectivity of 91 % under 100 % tensile strain and 1000 bending cycles, the cooler still maintains a cooling effect of 2-5 °C below ambient temperature. This work paves the way for developing mechanical flexible and strong radiative coolers for thermal regulation.

Towards ductile and high barrier poly(L-lactic acid) ultra-thin packaging film by regulating chain structures for efficient preservation of cherry tomatoes

The irreconcilable paradox between barrier performance and ductility is a "stumbling block" restricting the development of poly(L-lactic acid) (PLLA) films in the packaging industry. In this work, we reported the fabrication of an ultra-thin PLLA-based film with barrier properties and ductility by adjusting the polarity and conformational behavior of the polymer chains. Firstly, a novel unsaturated poly(L-lactic acid-co-butyrate itaconate) P(LA-BI) copolymer containing CC double bonds was synthesized using melt polycondensation. The results reveal that the addition of 60 % of P(LA-BI) enables PLLA film to achieve an elongation at a break of 83.6 % due to P(LA-BI) containing partially branched structures, which resulted in the polymer chains being arranged more in a high-energy gg conformer. Meanwhile, because of the large number of CO polar groups in P(LA-BI), PLLA/P(LA-BI)60 film show CO2 and O2 permeability coefficients (CDP and OP) of 1.8 and 0.45 × 10-8 g·m·m-2·h-1·Pa-1 respectively, which means that it has excellent gas barrier properties. Moreover, PLLA/P(LA-BI)60 film shows a 33.3 % increase in CO2/O2 ratio and an excellent ultraviolet (UV) barrier performance compared to neat PLLA. Preservation results suggested that the CO2 and O2 levels within the package could be regulated by varying the amount of P(LA-BI) added. Among them, PLLA/P(LA-BI)40 film generated a more desirable CO2 and O2 atmosphere for cherry tomatoes preservation, which was reflected by the delaying of senescence, discoloration, and decay, inhibition of oxidative cell damage through reduced malondialdehyde production, and maintenance of nutritional and flavor substances in cherry tomatoes. This PLLA-based film offers the advantages of operational simplicity, environmental friendliness, and inexpensive cost, making it great promising for food preservation and other applications requiring barrier properties and ductility.

Bio-Based Poly(lactic acid)/Poly(butylene sebacate) Blends with Improved Toughness

A series of poly(butylene sebacate) (PBSe) aliphatic polyesters were successfully synthesized by the melt polycondensation of sebacic acid (Se) and 1,4-butanediol (BDO), two monomers manufactured on an industrial scale from biomass. The number average molecular weight (M_n) in the range from 6116 to 10,779 g/mol and the glass transition temperature (T_g) of the PBSe polyesters were tuned by adjusting the feed ratio between the two monomers. Polylactic acid (PLA)/PBSe blends with PBSe concentrations between 2.5 to 20 wt% were obtained by melt compounding. For the first time, PBSe’s effect on the flexibility and toughness of PLA was studied. As shown by the torque and melt flow index (MFI) values, the addition of PBSe endowed PLA with both enhanced melt processability and flexibility. The tensile tests and thermogravimetric analysis showed that PLA/PBSe blends containing 20 wt% PBSe obtained using a BDO molar excess of 50% reached an increase in elongation at break from 2.9 to 108%, with a negligible decrease in Young’s modulus from 2186 MPa to 1843 MPa, and a slight decrease in thermal performances. These results demonstrated the plasticizing efficiency of the synthesized bio-derived polyesters in overcoming PLA’s brittleness. Moreover, the tunable properties of the resulting PBSe can be of great industrial interest in the context of circular bioeconomy.

Tailoring compatibility and toughness of microbial poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/bio-based polyester elastomer blends by epoxy-terminated hyperbranched polyester

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a type of promising bio-based thermoplastic for food packaging but find restricted industrial applications due to its brittleness and poor processability that is caused by its large spherulite sizes. In this study, for the purpose of toughening PHBV, bio-based engineering polyester elastomers (BEPE) were synthesized and blended with PHBV to prepare fully bio-based blends. In order to improve the compatibility and toughness of the BEPE/PHBV blends, epoxy-terminated hyperbranched polyesters (EHBP) were synthesized, which could be homogeneously dispersed into the PHBV/BEPE blends and improve the compatibility between the two phases of the matrix. The results showed that compared to those of the PHBV/BEPE blends, the elongation at break, impact strength and tensile toughness of the PHBV/BEPE blends with 3.0phr EHBP were enhanced by 134.2 %, 76.8 %, and 123.5 %, respectively. The crystallization study demonstrated the crystallization rate of PHBV/BEPE blends decreased due to the addition of EHBP. The reasons lied in that the addition of EHBP leads to chemical cross-linking between PHBV and BEPE. Meanwhile, the formation of hydrogen bonding, co-crystallization and chain entanglement increased the adhesion between PHBV and BEΡE, which generated the superior toughness of the blends.

Reverse atom transfer radical polymerization of dimethyl itaconate initiated by a new azo initiator: AIBME

Reverse atom transfer radical polymerization (RATRP) was used to synthesize poly(dimethyl itaconate) (PDMI) using an AIBME/CuBr₂/dNbpy system. The number average molecular weight (M _n) of PDMI was as high as M _n = 15 000 g mol^-1, the monomer conversion rate reached up to 70%, and the dispersity remained low (Đ = 1.06-1.38). The first-order kinetics of PDMI are discussed in detail. The AIBME initiator had a higher initiation efficiency than the AIBN initiator. As the ratio of initiator (AIBME) to catalyst (CuBr₂) decreased, the M _n and Đ of PDMI decreased. At 60 °C and 80 °C, the M _n of PDMI was much higher than the theoretical number average (M _n,th), and the Đ of PDMI broadened with the conversion rate. At 100 °C, the Đ of PDMI remained low, and the M _n of PDMI was closer to the M _n,th. As the ratio of monomer (DMI) to initiator (AIBME) increased, the M _n of PDMI changed little over time. These phenomena could be explained by the influence of the initiator and catalyst on polymerization kinetics.

Ultra-Tough Elastomers from Stereochemistry-Directed Hydrogen Bonding in Isosorbide-Based Polymers

The remarkable elasticity and tensile strength found in natural elastomers are challenging to mimic. Synthetic elastomers typically feature covalently cross-linked networks (rubbers), but this hinders their reprocessability. Physical cross-linking via hydrogen bonding or ordered crystallite domains can afford reprocessable elastomers, but often at the cost of performance. Herein, we report the synthesis of ultra-tough, reprocessable elastomers based on linear alternating polymers. The incorporation of a rigid isohexide adjacent to urethane moieties affords elastomers with exceptional strain hardening, strain rate dependent behavior, and high optical clarity. Distinct differences were observed between isomannide and isosorbide-based elastomers where the latter displays superior tensile strength and strain recovery. These phenomena are attributed to the regiochemical irregularities in the polymers arising from their distinct stereochemistry and respective inter-chain hydrogen bonding.

Synthesis and characterization of biobased thermoplastic polyester elastomers containing Poly(butylene 2,5-furandicarboxylate)

A series of sustainable and reprocessible thermoplastic polyester elastomers P(BF-PBSS)s were synthesized using dimethyl-2,5-furandicarboxylate, 1,4-butanediol, and synthetic low-molecular-weight biobased polyester (PBSS). The P(BF-PBSS)s contain poly(butylene 2,5-furandicarboxylate) (PBF) as their hard segment and PBSS as their soft segment. The microstructures of the P(BF-PBSS)s were confirmed by nuclear magnetic resonance, demonstrating that a higher content of the soft segment was incorporated into P(BF-PBSS)s with higher PBSS content. Interestingly, dynamic mechanical analysis indicated that P(BF-PBSS)s comprised two domains: crystalline PBF and a mixture of amorphous PBF and PBSS. Consequently, the microphase separations of P(BF-PBSS)s were mainly induced by the crystallization of their PBF segments. More importantly, the thermal, crystallization, and mechanical properties could be tailored by tuning the PBSS content. Our results indicate that the as-prepared P(BF-PBSS)s are renewable, thermally stable, and nontoxic, and have good tensile properties, indicating that they could be potentially applied in biomedical materials.

A series of sustainable and reprocessible thermoplastic polyester elastomers P(BF-PBSS)s were synthesized using dimethyl-2,5-furandicarboxylate, 1,4-butanediol, and synthetic low-molecular-weight biobased polyester (PBSS).

Green chemical engineering in China

In China, the rapid development greatly promotes the national economic power and living standard but also inevitably brings a series of environmental problems. In order to resolve these problems fundamentally, Chinese scientists have been undertaking research in the area of green chemical engineering (GCE) for many years and achieved great progresses. In this paper, we reviewed the research progresses related to GCE in China and screened four typical topics related to the Chinese resources characteristics and environmental requirements, i.e. ionic liquids and their applications, biomass utilization and bio-based materials/products, green solvent-mediated extraction technologies, and cold plasmas for coal conversion. Afterwards, the perspectives and development tendencies of GCE were proposed, and the challenges which will be faced while developing available industrial technologies in China were mentioned.

Preparation and Properties of Novel Thermoplastic Vulcanizate Based on Bio-Based Polyester/Polylactic Acid, and Its Application in 3D Printing

Thermoplastic vulcanizate (TPV) combines the high elasticity of elastomers and excellent processability of thermoplastics. Novel bio-based TPV based on poly (lactide) (PLA) and poly (1,4-butanediol/2,3-butanediol/succinate/itaconic acid) (PBBSI) were prepared in this research. PBBSI copolyesters were synthesized by melting polycondensation, and the molecular weights, chemical structures and compositions of the copolyesters were characterized by GPC, NMR and FTIR. Bio-based 2,3-butanediol was successfully incorporated to depress the crystallization behavior of the PBBSI copolyester. With an increase of 2,3-butanediol content, the PBBSI copolyester transformed from a rigid plastic to a soft elastomer. Furthermore, the obtained TPV has good elasticity and rheological properties, which means it can be applied as a 3D-printing material.

Thermally-Induced Triple-Shape Hydrogels: Soft Materials Enabling Complex Movements

Shape-memory hydrogels enable directed movements of a specimen in response to temperature, whereby crystallizable switching segments incorporated as side chains resulted in constant degrees of swelling during the shape-memory cycle. Here we report about hydrogels exhibiting a thermally induced triple-shape effect that allows complex movements of soft materials with two almost independent shape changes. Potential applications for those soft triple-shape materials are two-step self-unfolding devices or temperature-sensitive hydrogel actuators, for example, smart valves for flow rate control in aqueous media. Series of hydrogels with two different hydrophobic crystallizable switching segments were prepared. The degrees of swelling of the triple-shape hydrogels were not affected for different shapes or temperatures, which avoided in this way interferences on the shape shifts. During the two-step programming procedure, two distinct shapes can be implemented as reflected by shape fixity ratios of generally >50%. Structural analysis of the switching domains during the triple-shape cycle by means of X-ray scattering indicates that longer side chains gain lower orientation after deformation and that shorter side chains orient perpendicular to the hydrophilic main chain. Furthermore, it is observed that increased orientation of the switching domains is not a key requirement for adequate shape fixity and recovery ratios of the triple-shape effect in hydrogels, thus longer side chains can be utilized as switching segments in other shape-memory hydrogels.

Fully biobased thermoplastic elastomers: synthesis and characterization of poly(l-lactide)-b-polymyrcene-b-poly(l-lactide) triblock copolymers

Fully biobased thermoplastic elastomers poly(l-lactide)-b-polymyrcene-b-poly(l-lactide) triblock copolymers with PLLA as hard block and polymyrcene as soft block were synthesized and evaluated.