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.

Thermoresponsive Water-in-Oil-in-Water Pickering Double Emulsions Stabilized with Biodegradable and Semicrystalline Poly(ethylene glycol)-b-poly(ε-caprolactone-co-δ-valerolactone) Diblock Copolymer Micelles for Controlled Release.

Water-in-oil-in-water (W/O/W) Pickering double emulsions are promising materials for the construction of carriers for water-soluble and oil-soluble molecules or drug delivery systems if the contradictive trade-off between their extreme stability and controlled release properties can be resolved. In this study, biodegradable and biocompatible poly(ethylene glycol)-b-poly(ε-caprolactone-co-δ-valerolactone) (PEG-b-PCVL) diblock copolymers with predesigned hydrophilic to hydrophobic block length ratios and nearly identical ε-caprolactone/δ-valerolactone molar ratio (8/2), were synthesized by ring-opening copolymerization. Then, they self-assembled to create semicrystalline micelles. The melting points of PEG-b-PCVL copolymers and their lyophilized micelles were within a physiological range of temperatures, as determined by differential scanning calorimetry. Water contact angle measurements provided evidence that the surface wettability of PEG-b-PCVL micelles could be tuned by the PCVL block mass fractions or temperature stimulus. Such PEG-b-PCVL micelles were employed as a single particulate stabilizer to develop Pickering double emulsions through a one-step emulsification technique. W/O/W Pickering double emulsions could be generated using relatively hydrophobic PEG-b-PCVL micelles with high mass fractions (exceeding about 89%) of PCVL blocks, and they displayed excellent long-term physical stabilities at room temperature. However, the Pickering double emulsions underwent a rapid microstructural transition into simple oil-in-water Pickering emulsions instead of complete demulsification at elevated temperature (37 °C), which was attributed to the hydrophilicity of micelles enhanced when the core-forming PCVL melted realized by temperature stimulus. Consequently, such W/O/W Pickering double emulsions stabilized solely with semicrystalline PEG-b-PCVL micelles exhibit thermal responsiveness, enabling them to release vitamin B12 encapsulated within the internal aqueous phase rapidly.

Microstructurally tunable pickering emulsions stabilized by poly(ethylene glycol)-b-poly(ε-caprolactone) diblock biodegradable copolymer micelles with predesigned polymer architecture.

Poly(ethylene glycol)-b-poly(ε-caprolactone) diblock copolymers (PEG-b-PCL) with predesigned hydrophilic/hydrophobic block length ratios have been synthesized and self-assembled to form micelles, then used to emulsify medium-chain triglycerides with an aqueous phase. The morphologies and sizes of PEG-b-PCL copolymer micelles have been characterized by transmission electron microscopy and dynamic light scattering. Interfacial tension testing between micellar dispersions and oil, combined with water contact angle measurements, have been performed to assess the ability of these micelles to adjust interfacial tension and micellar hydrophobicity, respectively. Relationship between the wettability of PEG-b-PCL copolymer micelles and their emulsification properties has been proved through phase diagram, optical microscopic observation, droplet sizes evolution and phase separation behavior of Pickering emulsion samples. Results show that both oil-in-water and water-in-oil Pickering emulsions, as well as water-in-oil-in-water (W/O/W) double-Pickering emulsions, may be controllably prepared through one-step homogenization. Double microstructure of W/O/W Pickering emulsion has proved to be extremely stable during long-term storage.

Thermoresponsive physical hydrogels of poly(lactic acid)/poly(ethylene glycol) stereoblock copolymers tuned by stereostructure and hydrophobic block sequence.

CBABC-type poly(lactic acid) (PLA)/poly(ethylene glycol) (PEG) pentablock copolymers composed of a central PEG block (A) and enantiomeric poly(l-lactic acid) (PLLA, B), poly(d-lactic acid) (PDLA, C) blocks were synthesized. Such pentablock copolymers form physical hydrogels at high concentrations in an aqueous solution, which stem from the aggregation and physical bridging of copolymer micelles. These gels are thermoresponsive and turn into sols upon heating. Physical gelation, gel-to-sol transition, crystalline state, microstructure, rheological behavior, biodegradation, and drug release behavior of PLA/PEG pentablock copolymers and their gels were investigated; they were also compared with PLA-PEG-PLA triblock copolymers containing the isotactic PLLA or atactic poly(d,l-lactide) (PDLLA) endblocks and PLLA-PEG-PLLA/PDLA-PEG-PDLA enantiomeric mixtures. PLA hydrophobic domains in pentablock copolymer gels changed from a homocrystalline to stereocomplexed structure as the PLLA/PDLA block length ratio approached 1/1. The gel of symmetric pentablock copolymer exhibited a wider gelation region, higher gel-to-sol transition temperature, higher hydrophobic domain crystallinity, larger intermicellar distance, higher storage modulus, and slower degradation and drug release rate compared to those of the asymmetric PLA/PEG pentablock copolymers or triblock copolymers. SAXS results indicated that the PLLA/PDLA blocks stereocomplexation in pentablock copolymers facilitated the intermicellar aggregation and bridging. Cylindrical ordered structures were observed in all the gels formed from the PLA/PEG pentablock and triblock copolymers. The stereocomplexation degree and intermicellar distance of the pentablock copolymer gels increased with heating.

In situ formation and gelation mechanism of thermoresponsive stereocomplexed hydrogels upon mixing diblock and triblock poly(lactic acid)/poly(ethylene glycol) copolymers.

A novel in situ formed gel system with potential biodegradability and biocompatibility is developed by mixing the diblock and triblock poly(lactic acid)/poly(ethylene glycol) (PLA/PEG) copolymers with opposite configurations of PLA blocks. In situ gelation of such system is extremely fast, which happens within 10 s after mixing. In situ gelation, gel-to-sol transition, crystalline structure, microstructures, and mechanical properties of PLA-PEG/PLA-PEG-PLA enantiomerically mixed gels are significantly influenced by the mixing ratio, degree of polymerization for PEG block in triblock (DPPEG,tri) and diblock copolymers (DPPEG,di). It is found that in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixture just happen at relatively smaller PLA-PEG/PLA-PEG-PLA mass ratio and larger DPPEG,tri. Hydrodynamic diameters of PLA-PEG and PLA-PEG-PLA copolymers in dilute solution increase remarkably upon mixing, indicating the formation of bridging networks. Stereocomplexed crystallites are formed for the PLA hydrophobic domains in PLA-PEG/PLA-PEG-PLA enantiomeric mixtures. As indicated by synchrotron-radiation SAXS analysis, the enantiomeric mixture changes from a compactly to loosely aggregated structure and the intermicellar distance enhances with increasing DPPEG,tri, DPPEG,di, or PLA-PEG-PLA fraction. Gelation mechanism of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is proposed, in which part of PLA-PEG-PLA chains act as the connecting bridges between star and flower-like micelles and the stereocomplexed crystallites in micelle cores act as physically cross-linked points.

Core-shell structure, biodegradation, and drug release behavior of poly(lactic acid)/poly(ethylene glycol) block copolymer micelles tuned by macromolecular stereostructure.

Poly(ethylene glycol)-b-poly(L-lactic acid)-b-poly(D-lactic acid) (PEG-b-PLLA-b-PDLA) stereoblock copolymers were synthesized by sequential ring-opening polymerization. Their micelle formation, precise micelle structure, biodegradation, and drug release behavior were systematically investigated and compared with the PEG-b-poly(lactic acid) (PEG-b-PLA) diblock copolymers with various PLA stereostructures and PEG-b-PLLA/PEG-b-PDLA enantiomeric mixture. Stereoblock copolymers having comparable PLLA and PDLA block lengths and enantiomerically-mixed copolymers assemble into the stereocomplexed core-shell micelles, while the isotactic and atactic PEG-b-PLA copolymers formed the homocrystalline and amorphous micelles, respectively. The PLA segments in stereoblock copolymer micelles show smaller crystallinity than those in the isotactic and enantiomerically-mixed ones, attributed to the short block length and presence of covalent junction between PLLA and PDLA blocks. As indicated by the synchrotron radiation small-angle X-ray scattering results, the stereoblock copolymer micelles have larger size, micellar aggregation number, core radius, smaller core density, and looser packing of core-forming segments than the isotactic and enantiomerically-mixed copolymer micelles. These unique structural characteristics cause the stereoblock copolymer micelles to possess higher drug loading content, slower degradation, and drug release rates.

Interactions between an anticancer drug and polymeric micelles based on biodegradable polyesters.

Interactions between the anticancer drug quercetin and biodegradable polyesters within micelles were investigated by DSC, WAXD, and UV analyses. For micelles based on poly(ethylene glycol) methyl ether-block-poly(epsilon-caprolactone) (MPEG-PCL), DSC analysis indicated that the interactions were between the hydrophobic core and the drug within the micelle. For micelles based on poly(ethylene glycol) methyl ether-block-poly(L-lactide) (MPEG-PLLA), the interactions were between the hydrophobic core and the drug and between hydrophilic segments and the drug. WAXD results indicated that no crystalline phase of the drug was found in either of the micelle types. Based on the DSC and WAXD results, two probable micelle structures were proposed. The UV spectra revealed the presence of hydrogen bonding as the main interaction between the drug and the polyesters. In vitro studies demonstrated that quercetin release from micelles was sustained and was affected by the polymer-drug interaction.