Topic I: Controlled polymer synthesis and development of characterization methodology
Polypeptide-based hybrid block copolymers prepared by ROP offer a possibility to synergistically integrate the properties and functions of biomacromolecules and synthetic polymers in a single hybrid material, providing new and unique applications in biomedicine. Recently, we have developed a completely new approach for the preparation of well-defined synthetic polypeptide-based block copolymers by ROP of α-amino acid N-carboxyanhydrides (NCA) initiated by the hydroxyl-functionalyzed (macro)initiators instead of normally used initiators bearing primary amine group (ACS Macro Letters 2017, 6, 637). To overcome the issue of slow initiation by the hydroxyl group, we completely separated slow initiation from the fast chain propagation, since such a combination leads to the poorly defined products. Instead, they were performed in a successive manner by using a combination of organic catalysts. This unprecedented approach in polymer synthesis is challenging the well-established rule that for controlled polymerization initiation rate has to be much faster than the chain propagation rate. Importantly, the method significantly simplifies the synthesis of polypeptide-based hybrid block copolymers and even enables their synthesis in a one-pot manner from the cyclic monomers differing not only in the reactivity but also in the type of propagating group (Polymer Chemistry 2018, 9, 4764).
In another fundamental study, we synthesized semi-interpenetrating polymer networks (semi-IPNs), which served us as the precursors for the preparation of porous monoliths (Macromolecules 2019, 52, 819). Semi-IPNs were prepared in situ by simultaneous orthogonal polymerizations, where linear poly(ε-caprolactone) (PCL) was synthesized by organocatalyzed ROP of ε-caprolactone (CL) cyclic ester and poly(styrene-co-divinylbenzene) (PS) network by free-radical polymerization of styrene/divinylbenzene. To obtain porous PS monoliths, PCL domains were selectively removed by hydrolysis under basic conditions. By changing the amount of organocatalyst for ROP of CL, the relative polymerization kinetics of both monomers was varied, which was found to have a profound effect on the morphology of thus-obtained PS frameworks. This work demonstrates importance of the kinetics of simultaneous and orthogonal polymerizations since it governs the time order of system gelation and phase separation, which in turn strongly dictates the morphology of PS monoliths.
To prepare fully degradable polyester-based macroporous scaffolds we developed a method, where organocatalyzed ROP of CL and bis-lactone crosslinker was performed within the oil-in-oil high internal phase emulsions (HIPEs) (Macromolecules 2019, 52, 9291). In this way, degradable crosslinked PCL polymerized HIPE (polyHIPE) foams were synthesized, which are distinguished by high porosity with interconnected macroporous structure and low skeletal density. Thermo-mechanical properties of polyHIPEs were found to strongly depend on a degree of PCL crosslinking, which controls the melting and crystallization temperatures as well as the degree of crystallinity of PCL polyHIPE foams. Semi-crystalline polyHIPEs therefore demonstrate thermal shape memory behavior with excellent shape fixity and shape recovery. At appropriate degree of PCL crosslinking, the polyHIPE temporary shape can be fixed at room temperature, while a transition to the permanent shape occurs at 40 °C.
Achievements in the field of materials with permanent porosity include development of a novel method for the preparation of emulsion-templated macroporous polypeptide scaffolds suitable for tissue engineering purposes (Polym. Chem. 2020, 11, 4260). The scaffolds were prepared by ring-opening polymerization of N-carboxyanhydrides directly in oil-in-oil HIPE, where we successfully solved several problems associated with the synthesis (phase separation, foaming). The scaffolds were found to be excellent for cell proliferation.
Biodegradable macroporous stimuli-responsive polypeptide hydrogels were prepared for tissue engineering purposes by deprotecting the corresponding organogels (Macromolecules 2021, 54, 8321). The organogels show the typical interconnected macroporous polyHIPE morphology, which is completely preserved in the hydrogels after removal of the protecting groups. The hydrogels show a pH-dependent behavior, which we were able to modulate together with their mechanical properties by changing the chemical composition of the polypeptides.
In another study, ROP was used to prepare sustainable, (bio)degradable and functionalized aliphatic polyesters, which is one of the key trends in modern polymer science since conventional aliphatic polyesters lack functionality, making their broader application limited. For this reason, we developed a synthetic pathway for the preparation of novel, biomass-derived dicyano-substituted CL cyclic ester monomer, which is currently in the process of being patented. Using aluminum based catalyst we were able to prepare the nitrile-polyester homopolymers as well as copolymers with CL, which exhibited significantly higher glass transition temperatures compared to the unsubstituted PCL. Furthermore, the cyano groups of the nitrilePCL homopolymers were transformed into the amide groups, resulting in the water-soluble amide-functionalized polyesters (Eur Polym J, 2019, 119).
Good results on polymer synthesis are closely related to accurate and correct characterization of complex polymers, which simultaneously show distribution in several properties (chemical composition, molecular weight, functionality, architecture). Therefore, special attention is paid to continuous development of characterization methodology of complex macromolecules as well as supramolecular structures by using modern instrumental analytical techniques; i.e., various liquid chromatographic techniques and asymmetric flow field-flow fractionation (AF4) coupled to a multi-detection system (UV-MALS-DLS -RI), and MALDI-TOF MS. In addition to polymer characterization, we have developed a method for size-characterization and quantification of large supramolecular soft particles; i.e. large unilamellar vesicles and lipid droplets as well as natural extracellular vesicles (Anal Chem 2015, 87, 9225; J Chromatogr A 2015, 1418, 185) by using AF4-MALS.
With the development of new techniques capable of size-separation of large particles with sizes above 100 nm it is critical to interpret the data obtained by static and dynamic light-scattering correctly. Namely, we have shown that hydrodynamic size of large particles is underestimated at high longitudinal flow rates, which in turn results in incorrect assessment of particle shape from the so-called form factor. Moreover, we have demonstrated that the shape of size-separated, large particles can be assessed solely from the static light-scattering data from the angular dependence of scattered light (Anal Chem 2017, 89, 11744).
Topic II: Highly porous polymers and nanocomposites
The research carried out on porous polymers has been focused on the synthesis of functionalized polyHIPE polymers and their nanocomposites. We have developed a highly efficient pre-polymerization functionalization method for the preparation of hydrogel polyHIPEs. By functionalizing the glycidyl methacrylate (GMA) with different multifunctional amines and their subsequent polymerization in an oil-in-water HIPEs, we were able to prepare the highly porous hydrogel polyHIPEs, which are distinguished by considerably higher water uptakes and specific surface areas as compared to the corresponding values of polyHIPEs prepared by post-polymerization modification of GMA-based polyHIPEs by amines (Polym Chem 2016, 7, 5132).
In another work, we continued the study on polyHIPE-MOF hybrid materials. Previously, we developed a method for direct metal–organic framework (MOF) incorporation onto the surface of polyHIPE pores by using MOFs to stabilize the Pickering HIPE emulsions. Because this method results in partial clogging of MOF micropores, we have developed more sophisticated method, wherein MOFs were in situ generated from the metal oxides through the secondary recrystallization in the polyHIPE foam by using suitable ligands (J Mater Chem A 2017, 5, 1967 ). The MOF phase within the hybrid polyHIPE exhibits superior micropore accessibility and hydrostability.
In another study, zeolite nanocrystals were hybridized with the electrically-conducting 3D-carbon foam to produce hierarchically porous zeolite@carbon monolithic nanocomposite adsorbent for CO2 capture applications. Specifically, zeolite nanocrystals were embedded within the walls of the carbon foam macropores by polymerization of the zeolite-stabilized Pickering followed by a high-temperature carbonization treatment of the obtained zelite@polyHIPE composites. Hierarchically porous system shows enhanced CO2 capture performance compared to neat zeolites, as evidenced by excellent CO2/N2 selectivity and multi-cycle performance under humid conditions (ChemSusChem, 2020, /10.1002/cssc.201903116).
In another fundamental study, microcellular π-conjugated polyHIPEs with solar light activity were prepared using a combination of HIPE templating and C-C coupling reactions in HIPE. As-synthesized poly(p-phenylene ethynylene)-based polyHIPE foams possess intriguing properties such as 3D-interconnected macroporous morphology, tunable HOMO-LUMO band gaps, and high specific surface area, making them efficient photocatalysts for degradation of water-dissolved endocrine disrupting compound Bisphenol A under visible light (Catalysis Today 2021, 361, 146).
In addition, π-conjugated poly(arylenecyanovinylene) polyHIPE beads, which are photoactive, were synthesized by a Knoevenagel condensation reaction in an oil-in-oil-in-oil double emulsion as a template. The beads were used to catalyze the sulphoxidation of thioanisole in the presence of air and under visible light, achieving near quantitative conversion and chemoselectivity (ACS Macro Lett. 2021, 10, 1248−1253).
We prepared also amphoteric polyelectrolyte monoliths for water treatment applications. The co- and mixed-structured amphoteric polyelectrolytes show anti-polyelectrolyte behavior, while the one with a bilayer structure behaves like a typical polyelectrolyte (J. Colloid Interface Sci. 2020, 575, 480).
In the field of conventional nanocomposites, we studied the reinforcing effect of different nanofillers, which were incorporated in various thermoplastic polymeric matrices. Research on utilization of cellulose nanocrystals (CNC) as a renewable resource is focused on our innovative technology of CNC isolation by using a polyol method. In subsequent studies, the surface of CNC was modified in different ways to improve CNC compatibility with hydrophobic matrices, like poly(methyl methacrylate) (PMMA) (Cellulose 2016, 23, 505) and linear low-density polyethylene (LLDPE), since unmodified CNC showed detrimental effect on composites’ mechanical properties due to high degree of CNC agglomeration. Mechanical testing showed that PMMA modified CNCs have a moderate potential in reinforcing the PMMA matrix, while the reinforcing effect of alkyl modified CNCs on the LLDPE stiffness is high already at low nanofiller loadings. Research on the use of CNC as a reinforcement in polymer matrices has been carried out in close cooperation with the companies Postojna Forest Enterprise and Isokon (S4 project MARTINA: Materials and technologies for new applications), and in the field of CNC application in paper coatings with Radeče and Vipap industrial partners (S4 project NMP: Advanced materials and products from cellulose fibers and paper).
The use of nanocelluse to prepare polymer-based composites with improved properties involves the surface modification of cellulose nanocrystals (CNCs) with a silane modifier to achieve better compatibility of the CNCs with linear low-density poly(ethylene) (LLDPE) matrix. Addition of 1-2 wt% of CNCs to LLDPE resulted in a 20% increase in elastic modulus and a 30% increase in tensile strength of the composite compared to neat LLDPE (Cellulose 2020, 27, 5785). We also surface-modified nanocellulose with benzoic anhydride to achieve better compatibility with the hydrophobic polystyrene matrix. Measurements of the mechanical properties of nanocomposites showed an improvement in tensile strength (up to 30%) and Young's modulus (up to 23%), which was attributed to the intense interaction of the nanocellulose with the polymer chains due to strong π-π interactions (Cellulose 2021, 28, 7813-7827).
Topic III: Lowering the environmental footprint of polymers
In the field of lowering the environmental footprint of polymers we have been participating in four international projects as a coordinator or partner. The DeFishGear project (coordinator) was a groundbreaking project that made a concerted effort to evaluate for the first time the state of marine litter, derelict fishing gear and microplastics in the Adriatic. The major result of the project is a harmonized protocol for microplastics analysis and a collection system for derelict fishing gear. The BioComPack-CE project (coordinator) addresses the challenge that multimaterial (paper and plastic) packaging represents in a circular economy and seeks to find solutions using expertise available in the Central and East European (CEE) region. A number of pilot actions with companies explored real-life actions and enabled the establishment of an expert network capable of assisting the transition to more sustainable packaging in the post-project period. The project developed a strategy for innovative paper-bioplastics packaging value chain development in CEE. The BIOECO-R.D.I. project (partner) acts as a connecting force for linking basic and applied research with knowledge transfer and consulting towards industrial applications. We have offered exchange and transfer of knowledge on managing and using different biomass in order to give prerequisites for bioeconomy products and promoting new technological advances in several Slovenian companies and abroad. By common research actions, testing, sharing the results and transferring them from our research into companies, the project is encouraging innovative marketable products such as CNC, which thus support the development of sustainable Bio-Economy.
In the field of polymer recycling, we developed a rapid, robust, and convenient method for the quantitative determination of PA6 and PA66 polyamides (PA) in plastic waste (ACS Sustainable Chem. Eng. 2020, 8, 11818). Information on the content of a particular PA type in plastic waste is important as it determines the maximum recovery of PA constituent monomer(s) by chemical recycling and consequently the suitability of the waste as a raw material. An efficient (several minutes) microwave-assisted chemical recycling process of aliphatic polyamides is reported in ACS Sustainable Chem. Eng. 2020, 8, 16274. It results in a complete and straightforward conversion of PA into the constituent monomers, simplifying the isolation and purification of monomers and reinforcing additives in the case of PA-based composites. The secondary raw materials thus obtained are of comparable quality to commercially available chemicals.
Microwave-assisted acidolysis of polyurethane foams (PUFs) using adipic acid was performed in dependence of experimental conditions; i.e., reaction temperature, time, and amount of degradation reagent. The structural and molecular weight characteristics of the recycled polyols, the presence of side products, and the degree of degradation of the remaining PUF hard segments were studied. The recycled polyols were used for the synthesis of flexible PUFs. The morphology and mechanical properties of the PUFs show that the degree of functionalization of the polyol by the amino end groups, originating from incomplete urethane group degradation, and particularly carboxyl end groups, originating from esterification side reaction, significantly affect the quality and performance of the flexible PUFs prepared from the recycled polyols (ACS Sustainable Chemistry & Engineering, 2022, 10, 1323).