Designed to Degrade: Tailoring Polyesters for Circularity.

A powerful toolbox is needed to turn the linear plastic economy into circular. Development of materials designed for mechanical recycling, chemical recycling, and/or biodegradation in targeted end-of-life environment are all necessary puzzle pieces in this process. Polyesters, with reversible ester bonds, are already forerunners in plastic circularity: poly(ethylene terephthalate) (PET) is the most recycled plastic material suitable for mechanical and chemical recycling, while common aliphatic polyesters are biodegradable under favorable conditions, such as industrial compost. However, this circular design needs to be further tailored for different end-of-life options to enable chemical recycling under greener conditions and/or rapid enough biodegradation even under less favorable environmental conditions. Here, we discuss molecular design of the polyester chain targeting enhancement of circularity by incorporation of more easily hydrolyzable ester bonds, additional dynamic bonds, or degradation catalyzing functional groups as part of the polyester chain. The utilization of polyester circularity to design replacement materials for current volume plastics is also reviewed as well as embedment of green catalysts, such as enzymes in biodegradable polyester matrices to facilitate the degradation process.

Triggering Degradation of Cellulose Acetate by Embedded Enzymes: Accelerated Enzymatic Degradation and Biodegradation under Simulated Composting Conditions.

A green strategy that significantly accelerates the biodegradation rate of cellulose acetate (CA) by triggering deacetylation was demonstrated. Lipase isolated from Candida rugosa was immobilized on CA particles (immobilized lipase (IL)) by a physical entrapment method and further incorporated in CA films. After 40 days of aging in contact with external enzymes (lipase and cellulase), the number-average molecular weight (Mn) of CA/IL 5% decreased by 88%, while the Mn of CA only exhibited a 48% reduction. Fourier transform infrared and nuclear magnetic resonance spectroscopy of CA/IL 5% indicated significant deacetylation, which was further supported by the decrease of the water contact angle from 59 to 16°. These drastic changes were not observed for CA. Similar differences in the degradation rate were observed during aging under simulated composting conditions. After 180 days of simulated composting, traces of CA/IL 5% were barely observable, while large pieces of CA still remained. This could open the door to modified lignocellulose materials with retained biodegradability, also reducing the requirements for the degradation environment as the process is initiated from inside of the material.

Degradation of Cellulose Derivatives in Laboratory, Man-Made, and Natural Environments.

Biodegradable polymers complement recyclable materials in battling plastic waste because some products are difficult to recycle and some will end up in the environment either because of their application or due to wear of the products. Natural biopolymers, such as cellulose, are inherently biodegradable, but chemical modification typically required for the obtainment of thermoplastic properties, solubility, or other desired material properties can hinder or even prevent the biodegradation process. This Review summarizes current knowledge on the degradation of common cellulose derivatives in different laboratory, natural, and man-made environments. Depending on the environment, the degradation can be solely biodegradation or a combination of several processes, such as chemical and enzymatic hydrolysis, photodegradation, and oxidation. It is clear that the type of modification and especially the degree of substitution are important factors controlling the degradation process of cellulose derivatives in combination with the degradation environment. The big variation of conditions in different environments is also briefly considered as well as the importance of the proper testing environment, characterization of the degradation process, and confirmation of biodegradability. To ensure full sustainability of the new cellulose derivatives under development, the expected end-of-life scenario, whether material recycling or "biological" recycling, should be included as an important design parameter.

Designed from Biobased Materials for Recycling: Imine-Based Covalent Adaptable Networks.

Turning thermosets into fully sustainable materials requires utilization of biobased raw materials and design for easy recyclability. Here, dynamic covalent chemistry for fabrication of covalent adaptable networks (CANs) could be an enabling tool. CAN thermosets ideally combine the positive material properties of thermosets with thermal recyclability of linear thermoplastics. Among the dynamic covalent bonds, imine bond, also called Schiff base, can participate in both dissociative and associative pathways. This induces potential for chemical recyclability, thermal reprocessability and self-healing. This review presents an overview of the current research front of biobased thermosets fabricated by Schiff base chemistry. The discussed materials are categorized on the basis of the employed biobased components. The chemical approaches for the synthesis and curing of the resins, as well as the resulting properties and recyclability of the obtained thermosets are described and discussed. Finally, challenges and future perspectives are briefly summarized.

"Like Recycles Like": Selective Ring-Closing Depolymerization of Poly(L-Lactic Acid) to L-Lactide.

Chemical recycling of poly(L-lactic acid) to the cyclic monomer L-lactide is hampered by low selectivity and by epimerization and elimination reactions, impeding its use on a large scale. The high number of side reactions originates from the high ceiling temperature (Tc ) of L-lactide, which necessitates high temperatures or multistep reactions to achieve recycling to L-lactide. To circumvent this issue, we utilized the impact of solvent interactions on the monomer-polymer equilibrium to decrease the Tc of L-lactide. Analyzing the observed Tc in different solvents in relation to their Hildebrand solubility parameter revealed a "like recycles like" relationship. The decreased Tc , obtained by selecting solvents that interact strongly with the monomer (dimethyl formamide or the green solvent γ-valerolactone), allowed chemical recycling of high-molecular-weight poly(L-lactic acid) directly to L-lactide, within 1-4 h at 140 °C, with >95 % conversion and 98-99 % selectivity. Recycled L-lactide was isolated and repolymerized with high control over molecular weight and dispersity, closing the polymer loop.

Carbon Dot-Triggered Photocatalytic Degradation of Cellulose Acetate.

Chemical modification of biopolymers, before use in thermoplastic applications, can reduce the susceptibility to open environment degradation. We demonstrate carbon dots (CDs) as green photocatalytic triggers that can render the common cellulose derivative, cellulose acetate (CA), degradable under open environment relevant conditions. CD-modified cellulose acetate (CA + CD) films were subjected to UV-A irradiation in air and simulated sea water, and the degradation process was mapped by multiple spectroscopic, chromatographic, and microscopy techniques. The addition of CDs effectively catalyzed the deacetylation reaction, the bottleneck preventing biodegradation of CA. The photocatalytically activated degradation process led to significant weight loss, release of small molecules, and regeneration of cellulose fibers. The weight loss of CA + CD after 30 days of UV-A irradiation in air or simulated sea water was 53 and 43%, respectively, while the corresponding values for plain CA films were 12 and 4%. At the same time the weight average molar mass of CA + CD decreased from 62,000 to 11,000 g/mol and 15,000 g/mol during UV-A irradiation in air and simulated sea water, respectively, and the degree of substitution (DS) decreased from 2.2 to 1.6 both in air and in water. The aging in water alone did not affect the weight average molar mass, but the DS was decreased to 1.9. Control experiments confirmed the generation of hydrogen peroxide when aqueous CD dispersion was subjected to UV-A irradiation, indicating a free radical mechanism. These results are promising for the development of products, such as mulching films, with photocatalytically triggered environmental degradation processes.

Thermoplastic "All-Cellulose" Composites with Covalently Attached Carbonized Cellulose.

Thermoplastic "all-cellulose" composites were synthesized by covalent functionalization of cellulose acetate (CA) with oxidized carbonized cellulose (OCC). The OCC were manufactured via microwave-assisted hydrothermal carbonization (HTC) of cellulose followed by oxidation and dialysis. The OCC were of micrometer-size, had plane morphology and contained a variety of oxygen functionalities, enabling transformation into acyl chlorinated OCC under moderate reaction conditions. The synthesis of OCC-modified CA composites and neat CA were performed in the recyclable ionic liquid 1-allyl-3-methylimidazolium chloride. The degree of acetylation and amount of OCC were varied to establish their influence on thermal and physical properties of the composites. The OCC-modified CA composites displayed a notably enhanced film-forming ability, which led to improved optical and mechanical properties compared to neat CA. In addition, it was shown that OCC-modified CA composites can be synthesized from waste products, such as paper tissues. The OCC-modification was demonstrated to be a promising route to transparent and strong thermoplastic "all-cellulose" composites with moderate flexibility.

Cellulose-Derived Nanographene Oxide Reinforced Macroporous Scaffolds of High Internal Phase Emulsion-Templated Cross-Linked Poly(ε-caprolactone).

Cellulose-derived nanographene oxide (nGO)-type carbon dot reinforced porous scaffolds of poly(ε-caprolactone) (PCL) were developed as templates from high internal phase emulsions (HIPE). The mechanical strength, structural integrity, and reusability of the scaffolds were enhanced via in situ cross-linking. An oil-in-oil (o/o) HIPE of ε-caprolactone monomer (CL) was made for this purpose, and the ring-opening polymerization of a continuous phase comprised of CL, catalyst (Sn(Oct)2), and cross-linker (bis(caprolactone-4-yl)) (BCY) was carried out. The functionalization of scaffolds with nGO was assessed along with its role as an effective Pickering stabilizer of the HIPEs. The pore size and porosity of the scaffolds were governed by HIPE morphology, which in turn was controlled by the amount of nGO and the volume fraction of the dispersed phase. The nGO-functionalized scaffolds of cross-linked PCL thus prepared were characterized for their morphological structure, mechanical strength, and oil sorption capacity. Enhanced oil adsorption of nGO-functionalized scaffolds proved them to be of higher potency compared to those made of neat PCL. Superior compressive strength and reusability of scaffolds for oil adsorption up to 40 times while maintaining the structural integrity for ≥25 sorption-desorption cycles added extra value to such scaffolds. The scaffolds also had excellent cell viability as evaluated by MG63 osteoblast-like cells and some bioactivity in the form of calcium phosphate mineralization on the surface of the scaffolds.

Cellulose-Derived Nanographene Oxide Surface-Functionalized Three-Dimensional Scaffolds with Drug Delivery Capability.

Multifunctional three-dimensional (3D) scaffolds were targeted by surface grafting cellulose-derived nanographene oxide (nGO) on the surface of porous poly(ε-caprolactone) (PCL) scaffolds. nGO was derived from cellulose by microwave-assisted carbonization process and covalently grafted onto aminolyzed PCL scaffolds through an aqueous solution process. Fourier transform infrared spectroscopy and thermogravimetric analysis both verified the successful attachment of nGO and scanning electron microscopy depicted a homogeneous dispersion of nGO over the scaffold surface. Mechanical tests were performed and demonstrated a significant increase in compressive strength for the nGO grafted scaffolds. Grafting of nGO was also shown to induce mineralization with the formation of calcium phosphate precipitates on the surface of the scaffolds with the size increasing with higher nGO content. The potential of surface-grafted nGO as a nanocarrier of an antibiotic drug was also explored. The secondary interactions between nGO and ciprofloxacin, a broad-spectrum antibiotic used in the treatment of osteomyelitis, were optimized by controlling the solution pH. Ciprofloxacin was found to be adsorbed most strongly in its cationic form at pH 5, in which π-π electron donor-acceptor interactions predominate and the adsorbed drug content increased with increasing nGO amount. Further, the release kinetics of the drug were investigated during 8 days. In conclusion, the proposed simple fabrication process led to a scaffold with multifunctionality in the form of improved mechanical strength, ability to induce mineralization, as well as drug loading and delivery capability.

Recyclable Fully Biobased Chitosan Adsorbents Spray-Dried in One Pot to Microscopic Size and Enhanced Adsorption Capacity.

A facile one-pot spray-drying process was developed for fabrication and in situ crosslinking of chitosan microspheres to improve the adsorption capacity by microscopic design. A fully biobased nature was achieved by utilizing genipin (GP) as a crosslinking agent and chitosan-derived nanographene oxide (nGO) as a property tuner. The produced chitosan microspheres were further proven as powerful adsorbents for common wastewater contaminants such as anionic dyes and pharmaceutical contaminants, here modeled by methyl orange (MO) and diclofenac sodium (DCF). By regulating the amount of GP and nGO, as well as by controlling the process parameters including the spray-drying inlet temperature and postheat treatment, the surface morphology, size, zeta potential, and adsorption efficiency of the microspheres could be tuned accordingly. The adsorption efficiency for MO and DCF reached 98.9 and 100%, respectively. The microspheres retained high DCF adsorption efficiency after six adsorption and desorption cycles, and the recyclability was improved by the incorporated nGO. The fabricated microspheres, thus, have great potential as reusable and eco-friendly adsorbents.

Construction of Bioactive and Reinforced Bioresorbable Nanocomposites by Reduced Nano-Graphene Oxide Carbon Dots.

Bioactive and reinforced poly(ε-caprolactone) (PCL) films were constructed by incorporation of cellulose derived reduced nanographene oxide (r-nGO) carbon nanodots. Two different microwave-assisted reduction routes in superheated water were utilized to obtain r-nGO and r-nGO-CA. For the latter, a green reducing agent caffeic acid (CA), was incorporated in the reduction process. The materials were extruded and compression molded to obtain proper dispersion of the carbon nanodots in the polymer matrix. FTIR results revealed favorable interactions between r-nGO-CA and PCL that improved the dispersion of r-nGO-CA. r-nGO, and r-nGO-CA endorsed PCL with several advantageous functionalities including improved storage modulus and creep resistance. The considerable increase in storage modulus demonstrated that the carbon nanodots had a significant reinforcing effect on PCL. The PCL films with r-nGO-CA were also evaluated for their osteobioactivity and cytocompatibility. Bioactivity was demonstrated by formation of hydroxyapatite (HA) minerals on the surface of r-nGO-CA loaded nanocomposites. At the same time, the good cytocompatibility of PCL was retained as illustrated by the good cell viability to MG63 osteoblast-like cells giving promise for bone tissue engineering applications.

Isosorbide as Core Component for Tailoring Biobased Unsaturated Polyester Thermosets for a Wide Structure-Property Window.

Biobased unsaturated polyester thermosets as potential replacements for petroleum-based thermosets were designed. The target of incorporating rigid units, to yield thermosets with high thermal and mechanical performance, both in the biobased unsaturated polyester (UP) and reactive diluent (RD) while retaining miscibility was successfully achieved. The biobased unsaturated polyester thermosets were prepared by varying the content of isosorbide, 1,4-butanediol, maleic anhydride, and succinic anhydride in combination with the reactive diluent isosorbide-methacrylate (IM). Isosorbide was chosen as the main component in both the UP and the RD to enhance the rigidity of the formed thermosets, to overcome solubility issues commonly associated with biobased UPs and RDs and volatility and toxicity associated with styrene as RD. All UPs had good solubility in the RD and the viscosity of the mixtures was primarily tuned by the feed ratio of isosorbide but also by the amount of maleic anhydride. The flexural modulus and storage modulus were tailorable by altering the monomer composition The fabricated thermosets had superior thermal and mechanical properties compared to most biobased UP thermosets with thermal stability up to about 250 °C and a storage modulus at 25 °C varying between 0.5 and 3.0 GPa. These values are close to commercial petroleum-based UP thermosets. The designed tailorable biobased thermosets are, thus, promising candidates to replace their petroleum analogs.

Starch-Derived Nanographene Oxide Paves the Way for Electrospinnable and Bioactive Starch Scaffolds for Bone Tissue Engineering.

A straightforward process that enabled electrospinning of bioactive starch-based nanofiber scaffolds was developed by utilizing starch derived nano graphene oxide (nGO) as a property enhancer and formic acid as a solvent and esterification reagent. The reaction mechanism and process were followed by detailed spectroscopic investigation. Furthermore, the incorporation of nGO as a "green bioactive additive" endorsed starch nanofibrous scaffolds several advantageous functionalities including improved electrospinnability and thermal stability, good cytocompatibility, osteo-bioactivity, and retained biodegradability. The biodegradable starch/nGO nanofibers underwent simultaneous degradation and mineralization process during 1 week of cell culture and mineralization test, thus, mimicking the structure and function of extracellular matrices (ECMs) and indicating promise for bone tissue engineering applications.

Supramolecular Assembly of Biobased Graphene Oxide Quantum Dots Controls the Morphology of and Induces Mineralization on Poly(ε-caprolactone) Films.

Biobased 2D graphene oxide quantum dots (GOQDs) were synthesized from waste paper via carbon nanosphere intermediates and evaluated as property-enhancing additives for poly(ε-caprolactone) (PCL). The morphology of PCL films was controlled by supramolecular assembly of the small, 2D GOQDs in the polymer matrix. Phase behavior studies of PCL-GOQD in the solid state indicated concentration-dependent self-association of GOQD sheets, which was confirmed by SEM observations. Depending on the GOQD concentration, the formation of, e.g., spheres and stacked sheets was observed. GOQDs also induced mineralization on the surface of the films. A calcium phosphate (CaP) mineralization test revealed that the density of growing CaP crystals was controlled by the type of GOQD aggregates formed. Thus, utilization of the aggregation behavior of small GOQD sheets in polymeric matrices paves the way for tuning the morphology and properties of nanocomposites.

Conformational Footprint in Hydrolysis-Induced Nanofibrillation and Crystallization of Poly(lactic acid).

The origin of hydrolysis-induced nanofibrillation and crystallization, at the molecular level, was revealed by mapping the conformational ordering during long-term hydrolytic degradation of initially amorphous poly(lactic acid) (PLA), a representative model for degradable aliphatic polyesters generally displaying strong interplay between crystallization and hydrolytic erosion. The conformational regularization of chain segments was essentially the main driving force for the morphological evolution of PLA during hydrolytic degradation. For hydrolysis at 37 °C, no significant structural variations were observed due to the immobilization of "frozen" PLA chains. In contrast, conformational ordering in PLA was immediately triggered during hydrolysis at 60 °C and was responsible for the transition from random coils to disordered trans and, further, to quasi-crystalline nanospheres. On the surfaces, the head-by-head absorption and joining of neighboring nanospheres led to nanofibrillar assemblies following a "gluttonous snake"-like manner. The length and density of nanofibers formed were in close relation to the hydrolytic evolution, both of which showed a direct rise in the initial 60 days and then a gradual decline. In the interior, presumably the high surface energy of the nanospheres allowed for the preferential anchoring and packing of conformationally ordered chains into lamellae. In accordance with the well-established hypothesis, the amorphous regions were attacked prior to the erosion of crystalline entities, causing a rapid increase of crystallinity during the initial 30 days, followed by a gradual fall until 90 days. In addition to adequate illustration of hydrolysis-induced variations of crystallinity, our proposed model elucidates the formation of spherulitic nuclei featuring an extremely wide distribution of diameters ranging from several nanometers to over 5 µm, as well as the inferior resistance to hydrolysis observed for the primary nuclei. Our work fuels the interest in controlling nanofibrillation mechanism during hydrolysis of PLA, opening up possibilities for straightforward nanofiber formation.

Structural Hierarchy and Polymorphic Transformation in Shear-Induced Shish-Kebab of Stereocomplex Poly(Lactic Acid).

The realization of hierarchical shish-kebab structures for stereocomplex poly(lactic acid) (PLA) is achieved by the application of a shear flow (100 s(-1) for 1 s) mimicking what can be expected during polymer processing. Compared to the normal shearing scenarios, this transient and strong shear flow enables the creation of dense shish precursors in time- and energy-saving manner. The distribution of crystal form associated with the hierarchical structure is revealed by 2D Fourier transform infrared spectroscopy imaging, creating a unique visualization for both spatial resolution and polymorphism identification. Interestingly, in the shear stereocomplex chains are preferentially extended and crystallized as stable central cores with weak temperature dependence, whereas the development of lateral kebabs is defined by the distinct relation to the crystallization temperature. Below the melting point of homocrystals, both homo and stereocomplex crystallization are engaged in lamellar packing. Above that, exclusive stereocomplex crystals are organized into ordered lamellae. Combining the direct observations at multiscale, the ordered alignment of stereocomplex chains is recognized as the molecular origin of fibrillar extended chain bundles that constitute the central row-nuclei. The proposed hypothesis affords elucidation of shish-kebab formation and unique polymorphism in sheared stereocomplex PLA, which generates opportunities for engendering hierarchically structured PLA with improved performance.

Macromol. Rapid Commun. 9/2016.

Back Cover: The shish-kebab superstructure of poly(lactic acid) stereocomplex was developed in a transient and strong shear flow. The shear-aligned stereocomplex chains were preferentially assembled into the central shish, while the crystal form of the lateral kebabs showed strong dependency on the crystallization temperature. Further details can be found in the article by L. Xie, H. Xu, Z.-M. Li, and M. Hakkarainen* on page 745.

Structural basis for unique hierarchical cylindrites induced by ultrahigh shear gradient in single natural fiber reinforced poly(lactic acid) green composites.

A local shear flow field was feasibly generated by pulling the ramie fiber in single fiber reinforced poly(lactic acid) (PLA) composites. This was featured by an ultrahigh shear gradient with a maximum shear rate up to 1500 s(-1), a level comparable to that frequently occurring during the practical polymer processing. To distinguish shear-induced self-nucleation and ramie fiber-induced heterogeneous nucleation, the shear history was classified by pulling the fiber for 5 s (pulled sample) and pulling out the fiber during 10 s (pulled-out sample), while the static fiber-induced crystallization was carried out as the counterpart. As a result of the ultrahigh shear gradient, the combination of primary shear-induced nucleation in the central region and secondary nucleation in the outer layer assembled the unique hierarchical superstructures. By comparing the architectural configurations of interphases formed in the static, pulled, and pulled-out samples, it was shown that the hierarchical cylindrites underwent the process of self-nucleation driven by the applied shear flow, very different from the formation of fiber-induced transcrystallinity (TC) triggered by the heterogeneous nucleating sites at the static fiber surface. The twisting of transcrystallized lamellae may take place due to the spatial hindrance induced by the incredibly dense nuclei under the intense shearing flow, as observed in the synchrotron X-ray diffraction patterns. The influence of chain characteristics on the crystalline morphology was further explored by adding a small amount of poly(ethylene glycol) (PEG) to enhance the molecular mobility of PLA. It was of interest to find that the existence of PEG not only facilitated the growth rates of TC and cylindrites but also improved the preferential orientation of PLA chains and thus expanded the ordered regions. We unearthed lamellar units that were composed of rich fibrillar extended chain crystals (diameter of 50-80 nm). These results are of importance to shed light on tailoring crystalline morphology for natural fibers reinforced green composite materials. Of immense practical significance, too, is the crystalline evolution that has been tracked in the simple model penetrated with an ultrahigh shear gradient, which researchers have so far been unable to replicate during the practical melt processing, such as extrusion and injection molding.

Reprocessable, Highly Transparent Ionic Conductive Elastomers Based on ß-Amino Ester Chemistry for Sensing Devices.

Ionic conductive elastomers (ICEs) exhibit a compelling combination of ionic conductivity and elastic properties, rendering them excellent candidates for stretchable electronics, particularly in applications like sensing devices. Despite their appeal, a significant challenge lies in the reprocessing of ICEs without compromising their performance. To address this issue, we propose a strategy that leverages covalent adaptable networks (CANs) for the preparation of ICEs. Specifically, ß-amino ester bonds as dynamic motifs are incorporated into a poly(ethylene oxide) network containing lithium bis(trifluoromethane) sulfonimide (LiTFSI) salt. LiTFSI-containing ß-amino ester networks (LBAEs) exhibit superb transparency (94%), thermal stability (>280 °C), and modest conductivity (0.00576 mS·cm-1 at 20 °C), and some LBAEs maintain operational capability across a wide temperature range (-20 to 100 °C). By regulating the lithium salt content, the mechanical properties, conductivities, and viscoelastic behaviors can be tailored. Benefiting from these features, LBAEs have been successfully applied in sensing devices for monitoring human motion (e.g., finger bending, swallowing, and clenching). Notably, even after four reprocessing cycles, LBAEs demonstrate structural integrity and maintain their operational capability. This novel approach represents a promising solution to the reprocessing challenges associated with flexible conductive devices, demonstrating the successful integration of CANs and ICEs.

Tunable Fungal Monofilaments from Food Waste for Textile Applications.

A fungal biorefinery is presented to valorize food waste to fungal monofilaments with tunable properties for different textile applications. Rhizopus delemar is successfully grown on bread waste and the fibrous cell wall is isolated. A spinnable hydrogel is produced from cell wall by protonation of amino groups of chitosan followed by homogenization and concentration. Fungal hydrogel is wet spun to form fungal monofilaments which underwent post-treatments to tune the properties. The highest tensile strength of untreated monofilaments is 65 MPa (and 4% elongation at break). The overall highest tensile strength of 140.9 MPa, is achieved by water post-treatment. Moreover, post-treatment with 3% glycerol resulted in the highest elongation % at break, i.e., 14%. The uniformity of the monofilaments also increased after the post-treatments. The obtained monofilaments are compared with commercial fibers using Ashby's plots and potential applications are discussed. The wet spun monofilaments are located in the category of natural fibers in Ashby's plots. After water and glycerol treatments, the properties shifted toward metals and elastomers, respectively. The compatibility of the monofilaments with human skin cells is supported by a biocompatibility assay. These findings demonstrate fungal monofilaments with tunable properties fitting a wide range of sustainable textiles applications.