Triblock Terpolymers Research Articles

Nanoporous Double-Gyroid Structure from ABC Triblock Terpolymer Thick Films

The creation of nanostructured materials with a triply periodic minimal surface (TPMS), defined as a zero mean curvature surface having periodicity in three-dimensional space, is an emerging solution to optimize transport (i.e., the ion-conductivity and hydraulic permeability) through the next-generation of electrolyte and ultrafiltration (UF) membranes. Here, we used an amphiphilic ABC-type block copolymer (BCP) (namely, polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO)) to generate symmetric thick films (~8 μm) composed entirely of a TPMS-based structure, consisting of a PS matrix with a double gyroid (DG) minimal surface and hydrophilic stimuli-responsive (P2VP/PEO) nanochannels. To produce the core/shell DG-structured monoliths, we used a process combining the nonsolvent-induced phase separation (NIPS) process with a solvent vapor annealing (SVA) treatment. From such symmetric ABC-type BCP-thick films generated by NIPS-SVA, a mean hydraulic permeability as high as 514 L h-1 m-2 bar-1 was measured. This mean value was revealed to be nearly equal to that of asymmetric PS-b-P2VP-b-PEO membranes manufactured by NIPS, which have a substructure with an implicit irregular and random distribution of the internal pore structure and a skin layer with P2VP/PEO nanopores arranged into a hexagonal array.

Multicompartment Nanostructures as Templates for Multimetallic Hybrid Materials

ABC triblock terpolymers are promising soft templates for organic/inorganic hybrids because of their ability to form nanostructures with complex shapes, multiple compartments, and precisely localized chemistry. Exemplified on multicompartment nanofibers (MCNFs) of triblock terpolymers, it is demonstrated that microdomains can be selectively loaded, thereby giving access to nanoscale multimetallic hybrid materials. MCNFs with micrometer length, homogenous diameter (90 nm), and a helix‐on‐cylinder morphology are formed from polystyrene‐block‐polybutadiene‐block‐poly(tert‐butyl methacrylate) (PS‐b‐PB‐b‐PT). After postmodification (cross‐linking/hydrolysis), selective loading with FeCl3, PdCl2, H2PtCl6, AgNO3, CuCl2, or ZnCl2 leads to a variety of hybrid MCNFs analyzed by transmission electron microscopy, scanning transmission electron microscopy, electron tomography, energy‐dispersive X‐ray spectroscopy, and atomic force microscopy. Mild sulfonation of the PS shell to polystyrene sulfonate renders the MNCFs water‐dispersible and allows the formation of mixed‐bimetallic Pt/Pd/Pt@MCNFs and trimetallic Pt/Pd/Ag@MCNFs. It is demonstrated that the order of loading is key to successfully create multimetallic nanostructures. These and other structures can become useful for energy applications as well as in photo‐ and electrocatalysis.

Improved Elastic Recovery from ABC Triblock Terpolymers.

The promise of ABC triblock terpolymers for improving the mechanical properties of thermoplastic elastomers is demonstrated by comparison with symmetric ABA/CBC analogs having similar molecular weights and volume fraction of B and A/C domains. The ABC architecture enhances elasticity (up to 98% recovery over 10 cycles) in part through essentially full chain bridging between discrete hard domains leading to the minimization of mechanically unproductive loops. In addition, the unique phase space of ABC triblocks also enables the fraction of hard-block domains to be higher (fhard ≈ 0.4) while maintaining elasticity, which is traditionally only possible with non-linear architectures or highly asymmetric ABA triblock copolymers. These advantages of ABC triblock terpolymers provide a tunable platform to create materials with practical applications while improving our fundamental understanding of chain conformation and structure-property relationships in block copolymers.

Exploring the Effect of Drug Loading on the Biological Fate of Polymer-Coated Solid Nanoparticles

Polymer-coated inorganic nanoparticles (NPs) have attracted great attention in drug delivery fields due to their unique magnetic, optical, and thermal properties. Recent studies have shown that the surface physicochemical properties of these NPs, including the size, shape, shell hydration, protein corona, and so forth, affect their biological fate. However, there has been less attention on the effect of the drug loading amount on the biological activity of inorganic NPs. Here, we used upconverting NPs (UCNPs) as an inorganic nanocarrier model, coated them with a triblock terpolymer poly(poly(ethylene glycol) methyl ether methacrylate)-block-polymethacrylic acid-block-ethylene glycol methacrylate phosphate (PPEGMEMA-b-PMAA-b-PEGMP), and loaded them with different amounts of anticancer drug doxorubicin (DOX) by electrostatic interactions. Our results show that the UCNPs with the lowest DOX loading amount (1.32%) have the best biological behavior. We find that the effect of DOX loading on the biological behavior of UCNPs is dominated by polymer shell hydration and the protein corona. The lowest drug-loading UCNPs are more hydrated and have more albumin coronas around, which leads to an increasing effect on the association with cancer cells and drug accumulation in the mouse tumor area. Our findings highlight the importance of optimizing the drug loading amounts in polymer-coated inorganic drug delivery systems as the amount and type of drug will alter the physical properties of the polymer shell.

Effect of Surfactant Selectivity on Shape and Inner Morphology of Triblock Terpolymer Microparticles.

The confined assembly of block copolymers (BCPs)has become a useful tool to prepare microparticles with controlled anisotropy and inner structure. While a solid understanding of the behavior of AB diblock copolymers exists, knowledge on the parameters affecting ABC triblock terpolymer assembly is much more limited. In this work, the effect of block-selective surfactants, sodium-4-vinylbenzenesulfonate (VBS) and sodium dodecylsulfate (SDS), is analyzed in the evaporation-induced confined assembly (EICA) of a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock terpolymer (SBM). Despite using the same terpolymer and emulsification process, SDS results in ellipsoidal microparticles with axially stacked lamellae, while VBS results in spherical microparticles with concentric lamellae or 3D spiral morphology. This change in morphology upon switching the surfactant is further substantiated by molecular simulations and enhances the understanding of terpolymer microphase separation in confinement.

Square arrays of vertically aligned nanoporous cylinders from a linear ABC triblock terpolymer

AbstractVertically oriented nanoporous cylinders, demonstrating an unprecedented alignment persistence, were produced within freestanding poly(1,1‐dimethyl silacyclobutane)‐block‐polystyrene‐block‐poly(2‐vinyl pyridine) (PDMSB‐b‐PS‐b‐P2VP) layers (~15 μm thick) blended with short PS‐b‐P2VP chains by combining the non‐solvent induced phase separation (NIPS) process with a solvent vapor annealing (SVA) treatment. Here, the NIPS step allowed for the formation of an asymmetric and porous PDMSB‐b‐PS‐b‐P2VP film having a top surface exhibiting poorly‐defined nanopores while the subsequent SVA treatment enabled to produce a symmetric layer that possesses highly‐ordered cylindrical nanodomains arranged into a 27 nm period square array. As the unblended NIPS/SVA‐made PDMSB‐b‐PS‐b‐P2VP monoliths exhibited a mixed orientation of parallel and perpendicular cylinders, a blending strategy was used to achieve tetragonally‐packed PDMSB and P2VP nanodomains having an exceptional vertical alignment persistence. Such solvent‐annealed (3 h, CHCl3) PDMSB‐b‐PS‐b‐P2VP monoliths blended with 20 wt% of PS‐b‐P2VP chains showed a water permeance close to the value measured through their parent NIPS‐made terpolymer films having poorly‐ordered nanopores.

Alternating Gyroid Stabilized by Surfactant-like Triblock Terpolymers in IS/SO/ISO Ternary Blends

Block polymers self-assembled into chiral alternating-gyroid (GA) morphologies are desirable templates for fabricating photonic materials. Recent self-consistent field theory (SCFT) calculations on a model system predicted that adding a minuscule amount of ABC triblocks into a blend of thermodynamically symmetric double-gyroid-forming AB and BC diblocks with immiscible A and C blocks stabilizes GA. Here, we use SCFT to investigate the feasibility of this method in practice by computing the phase behaviors of ternary mixtures of poly(isoprene-b-styrene) (IS), poly(styrene-b-ethylene oxide) (SO), and poly(isoprene-b-styrene-b-ethylene oxide) (ISO). Addition of ISO indeed stabilizes GA; however, the phase window is relatively narrow, due to the packing frustration induced by the mismatch in preferred domain sizes between IS and SO diblock copolymers. Designing the system to have comparable diblock domain sizes significantly enhances the relative stability of GA against the competing alternating lamellae, and this stability is also sensitive to the degree of polymerization of the ISO midblock. Moreover, the stabilization of alternating morphologies is sensitive to temperature, and less ISO is needed at lower temperatures. The effect of some other design parameters, such as diblock volume fraction, is also investigated. These computational results provide insights into possible experimental strategies for producing GA and highlight the sensitivity of network phase formation to asymmetries in the thermodynamic parameters.

Thermal and Bulk Properties of Triblock Terpolymers and Modified Derivatives towards Novel Polymer Brushes.

We report the synthesis of three (3) linear triblock terpolymers, two (2) of the ABC type and one (1) of the BAC type, where A, B and C correspond to three chemically incompatible blocks such as polystyrene (PS), poly(butadiene) of exclusively (~100% vinyl-type) -1,2 microstructure (PB1,2) and poly(dimethylsiloxane) (PDMS) respectively. Living anionic polymerization enabled the synthesis of narrowly dispersed terpolymers with low average molecular weights and different composition ratios, as verified by multiple molecular characterization techniques. To evaluate their self-assembly behavior, transmission electron microscopy and small-angle X-ray scattering experiments were conducted, indicating the effect of asymmetric compositions and interactions as well as inversed segment sequence on the adopted morphologies. Furthermore, post-polymerization chemical modification reactions such as hydroboration and oxidation were carried out on the extremely low molecular weight PB1,2 in all three terpolymer samples. To justify the successful incorporation of -OH groups in the polydiene segments and the preparation of polymeric brushes, various molecular, thermal, and surface analysis measurements were carried out. The synthesis and chemical modification reactions on such triblock terpolymers are performed for the first time to the best of our knowledge and constitute a promising route to design polymers for nanotechnology applications.

Complex Phase Behavior in Binary Blends of AB Diblock Copolymer and ABC Triblock Terpolymer

Blending block polymers with different block compositions provides an opportunity to create nanostructures that are not accessible from neat block copolymer melts. In this study, we investigate the equilibrium phase behavior in binary blends composed of an AB diblock copolymer and an ABC triblock terpolymer, using self-consistent field theory (SCFT). Blending these simple linear block polymers results in complex phase behaviors, including unusual core–shell network phases with asymmetric core and shell volume fractions and a new cylinder-in-O70 hybrid network phase. The phase diagrams with respect to segregation strength and blend composition reveal an unexpected morphological richness, including 15 ordered phases, illustrating the potential inherent in blending these relatively simple components.

Stimuli-Responsive Triblock Terpolymer Conversion into Multi-Stimuli-Responsive Micelles with Dynamic Covalent Bonds for Drug Delivery through a Quick and Controllable Post-Polymerization Reaction

Stimuli-responsive copolymers are of great interest for targeted drug delivery. This study reports on a controllable post-polymerization quaternization with 2-bromomethyl-4-fluorophenylboronic acid of the poly(4-vinyl pyridine) (P4VP) block of a common poly(styrene)-b-poly(4-vinyl pyridine)-b-poly(ethylene oxide) (SVE) triblock terpolymer in order to achieve a selective responsivity to various diols. For this purpose, a reproducible method was established for P4VP block quaternization at a defined ratio, confirming the reaction yield by 11B, 1H NMR. Then, a reproducible self-assembly protocol is designed for preparing stable micelles from functionalized stimuli-responsive triblock terpolymers, which are characterized by light scattering and by cryogenic transmission electron microscopy. In addition, UV-Vis spectroscopy is used to monitor the boron-ester bonding and hydrolysis with alizarin as a model drug and to study encapsulation and release of this drug, induced by sensing with three geminal diols: fructose, galactose and ascorbic acid. The obtained results show that only the latter, with the vicinal diol group on sp2-hybridized carbons, was efficient for alizarin release. Therefore, the post-polymerization method for triblock terpolymer functionalization presented in this study allows for preparation of specific stimuli-responsive systems with a high potential for targeted drug delivery, especially for cancer treatment.

Catalytic living ROMP: block copolymers from macro-chain transfer agents.

Vinyl ether based macro-chain transfer agents (m-CTAs) are used to produce different di or tri-block copolymers under catalytic living ROMP conditions. Polystyrene (PS) vinyl ether m-CTA and polycaprolactone (PCL) or polylactide vinyl ether (PLA) m-CTAs are synthesized straightforwardly via ATRP and ROP respectively. Regioselectivity as well as the high metathesis activity of these m-CTAs enabled us to synthesise a range of metathesis-based A-B diblock copolymers with controlled dispersities (Đ < 1.4). In this manner, PS-ROMP (here, ROMP refers to a poly(MNI-co-DHF) block), PCL-ROMP and PLA-ROMP were synthesized using substoichiometric amounts of ruthenium complex in a living fashion. Also, a more complex PEG-PCL-ROMP tri-block terpolymer was obtained catalytically. All block copolymers were characterized by SEC and DOSY NMR spectroscopy. We believe that this methodology of using macro-chain transfer agents to prepare degradable ROMP polymers under catalytic living ROMP conditions will find applications in biomedicine.

Janus nanoplates, -bowls, and -cups: controlling size and curvature via terpolymer/homopolymer blending in 3D confinement.

The synthesis and properties of Janus nanoparticles with spherical, cylindrical, and disk-like shapes are nowadays rather well understood. Other topologies such as nanorings and bowl-shaped Janus nanoparticles are believed to show distinctly different solution behavior and interaction with interfaces, but limitations in their synthesis currently prevents a proper investigation of these properties. Especially the combination of shape- and surface-anisotropy of bowl-shaped Janus nanoparticles could result in enhanced selectivity in uptake of cargo and enhanced directional diffusion. We here produce bowl-shaped Janus nanoparticles without noticeable side products through evaporation-induced confinement assembly (EICA) of triblock terpolymers blended with high molecular weight homopolymer. The triblock terpolymer phase separates from the homopolymer into spherical domes, where the terpolymer adopts a hemispherical lamella-lamella morphology (ll). Selective cross-linking, removal of the homopolymer, and disassembly of the microparticles releases the bowl-shaped Janus nanoparticles. The amount of blended homopolymer determines the size of the spherical dome, allowing to control particle curvature into flat Janus nanoplates, hemispherical Janus nanobowls, and deep Janus nanocups. The use of Shirasu Porous Glass (SPG) membranes with pore sizes in the range of dpore = 0.2-2.0 μm further provides control of particle diameter. Size and shape were analyzed with electron microscopy and the Janus character through selective surface decoration. The diffusion behavior of bowl-shaped Janus nanoparticles was investigated depending on particle curvature and anisotropy using angle-dependent dynamic light scattering.

An extensible density-biasing approach for molecular simulations of multicomponent block copolymers.

A node-density biased Monte Carlo methodology is proposed for the molecular structure generation of complex block copolymers. Within this methodology, the block copolymer is represented as bead-spring model. Using self-consistent field theory, a density field for all monomer species within the system is calculated. Block copolymers are generated by random walk configuration biased by the density fields. The proposed algorithm then modifies the generation process by taking the global structure of the polymer into account. It is then demonstrated that these global considerations can be built into the sampling procedure, specifically through functions that assign a permissible difference in density field value between relevant monomer species to each step of the random walk. In this way, the random walk may be naturally controlled to provide the most appropriate conformations. The overall viability of this approach has been demonstrated by using the resulting configurations in molecular dynamics simulations. This new methodology is demonstrated to be powerful enough to generate molecular configurations for a much wider variety of materials than the original approach. Two key examples of the new capabilities of the method are viable configurations for ABABA pentablock copolymers and ABC triblock terpolymers.

Effects of ionic strength and ion-specificity on the interface behavior of PDMAEMA-b-PLMA-b-POEGMA triblock terpolymer

Ion-specificity on the stretching degree of diblock copolymers containing poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) at the air/water interface has previously been proved to be inconsistent with the Hofmeister series. In this work, the aggregation behavior of an amphiphilic linear triblock terpolymer poly[2-(dimethylamino)ethyl methacrylate]-block-poly(lauryl methacrylate)-block-poly[oligo(ethylene glycol) methacrylate] (PDMAEMA-b-PLMA-b-POEGMA) on the aqueous subphase with different ionic strengths and ion species (salts) was studied, and the morphologies of its Langmuir−Blodgett (LB) films were explored. At the air/water interface, the terpolymer tends to form surface micelles composed of hydrophobic PLMA cores and mixed hydrophilic coronas of PDMAEMA and POEGMA. Under acidic conditions with low ionic strength, the hydrophilic PDMAEMA blocks of the terpolymer are almost completely electrostatic screened. With the increase of subphase ionic strength, the surface pressure−molecular area isotherms of the terpolymer monolayers at the air/water interface gradually move to large areas due to the increased interfacial stretching degree of POEGMA blocks resulting from the increased salting-out effect and interfacial steric hindrance of PDMAEMA blocks. The hysteresis degrees of the terpolymer monolayers in the presence of salts in the subphase are larger than that in the absence of salt. Under alkaline conditions, the isotherms of the terpolymer monolayers move to large areas with the increase of salt concentration because the introduction of salt makes the hydrophilic blocks more stretched. The experimental results show that the relationship between their interfacial stretching degrees of two kinds of hydrophilic blocks and the types of salt in solution is inconsistent with the Hofmeister series. This may be due to tethering of the corona chains on the hydrophobic core and the configurational constrains (crowding phenomena) induced within the mixed micelle corona. Under different subphase conditions, the LB films of the terpolymer show isolated circular micelles due to the mutual repulsions of the long hydrophilic PDMAEMA and POEGMA blocks.

Oxides and Nitrides with Asymmetric Pore Structure from Block Copolymer Co‐Assembly and Non‐Solvent Induced Phase Separation

AbstractMaterials combining an asymmetric pore structure with mesopores everywhere enable high surface area accessibility and fast transport, making them attractive for e.g., energy conversion and storage applications. Block copolymer (BCP)/inorganic precursor co‐assembly combined with non‐solvent induced phase separation (NIPS) provides a route to materials in which a mesoporous top surface layer merges into an asymmetric support with graded porosity along the film normal and mesopores throughout. Here, the co‐assembly and non‐solvent‐induced phase separation (CNIPS) of poly(isoprene)‐b‐poly(styrene)‐b‐poly(4‐vinylpyridine) (ISV) triblock terpolymer and titanium dioxide (TiO2) sol‐gel nanoparticlesare reported. Heat‐treatment in air results in free‐standing asymmetric porous TiO2. Further thermal processing in ammonia results in free‐standing asymmetric porous titanium nitride (TiN). processing changes alter structural membrane characteristics is demonstrated. Changing the CNIPS evaporation time results in various membrane cross‐sections ( finger‐like to sponge‐like). Oxide and nitride material composition, crystallinity, and porosity are tuned by varying thermal processing conditions. Finally, thermal processing condition effects are probed on phase‐pure asymmetric nitride membrane behavior using cyclic voltammetry to elucidate their influence, e.g., on specific capacitance. Results provide further insights into improving asymmetric and porous materials for applications including energy conversion and storage, separation, and catalysis and motivate a further expansion of CNIPS to other (in)organic materials.

Efficient Creation and Morphological Analysis of ABC Triblock Terpolymer Libraries

Multiblock copolymers with increasingly complex block sequences─for example, triblock terpolymers─offer unique opportunities to create nanostructured materials, but this potential has been hindered by a vast design space that complicates the exploration of structure–property relationships. Here, we report a versatile and scalable strategy to separate parent ABC and isomeric ACB triblock terpolymers into libraries of fractionated samples spanning a wide range of compositions. Using a combination of controlled polymerization and automated chromatography, the synthesis and separation of less than 10 ABC and ACB parent materials gave rise to over 100 purified triblock terpolymers. Separations follow systematic and predictable trends in volume fraction resulting from an adsorption-based mechanism where chains rich in non-polar blocks elute first, followed by more polar derivatives, yielding fractions with improved purity in composition and molar-mass dispersity. As evidenced by small-angle X-ray scattering, fractionation significantly enhances long-range order compared to as-synthesized parent materials and allows for the definitive identification of various nanoscale morphologies. This user-friendly separation strategy significantly increases the availability of well-defined ABC triblock terpolymer libraries to the polymer community while also improving sample quality and accelerating discovery.

Interfacial aggregation behavior of triblock terpolymers

Interfacial aggregation behavior of triblock terpolymers

Aggregation behavior of triblock terpolymer poly[n‐butyl acrylate‐block‐N‐isopropylacrylamide‐block‐2‐(dimethylamino)ethyl acrylate] at the air/water interface

AbstractLangmuir film balance technique was used to study the subphase pH and temperature effects on the aggregation behavior of a dual‐responsive triblock terpolymer poly[n‐butyl acrylate‐block‐N‐isopropylacrylamide‐block‐2‐(dimethylamino)ethyl acrylate] (PnBA‐b‐PNIPAM‐b‐PDMAEA) at the air/water interface. Atomic force microscopy was used to observe the morphologies of its Langmuir–Blodgett (LB) films. At the interface, the terpolymer tends to form isolated circular micelles composed of hydrophobic PnBA cores and hydrophilic PNIPAM‐PDMAEA coronas. The limiting areas in the surface pressure–molecular area isotherms under acidic and alkaline conditions are larger than that under neutral conditions due to the larger interfacial stretching degrees of PNIPAM blocks, which results from the strong electrostatic repulsions of the fully protonated PDMAEA blocks underwater in the acidic case. In our minds, the hysteresis degrees of the compression−expansion cycles under different subphase pH conditions should mainly depend on the initial stretching degrees of the hydrophilic blocks. Under acidic conditions, the isotherms shift toward small areas with the increase of temperature due to the increased solubility of PNIPAM blocks. However, under alkaline conditions, the isotherms significantly exhibit the lower critical solution temperature behavior of PNIPAM blocks, which is a good example to build up the relationship between the interfacial and solution properties of block copolymers.

Poly(2‐(dimethylamino) ethyl methacrylate)‐b‐poly(lauryl methacrylate)‐b‐poly(oligo ethylene glycol methacrylate) triblock terpolymer micelles as drug delivery carriers for curcumin

AbstractThis work aims to investigate the ability of poly[2‐(dimethylamino)ethyl methacrylate‐b‐lauryl methacrylate‐b‐(oligo ethylene glycol)methacrylate], PDMAEMA‐b‐PLMA‐b‐POEGMA amphiphilic triblock terpolymers, to encapsulate the model hydrophobic compound curcumin. The terpolymers were synthesized by RAFT polymerization methodologies at variable molecular weights and compositions and self‐assemble into spherical micelles with mixed PDMAEMA/POEGMA coronas and PLMA cores, when inserted in aqueous solutions. Fourier transform infrared and UV–Vis spectroscopy measurements were utilized in order to confirm and quantify the encapsulation of curcumin within the polymeric micelles respectively. Dynamic light scattering was used for the investigation of the size of the nanostructures formed and transmission electron microscopy proved that their morphology is spherical. Moreover, the colloidal stability of the systems in physiological conditions was assessed and two independent in vitro cytotoxicity assays revealed that curcumin‐loaded PDMAEMA‐b‐PLMA‐b‐POEGMA micelles were not cytotoxic.

Effect of poly(4‑tert-butylstyrene) block length on the microphase structure of poly(ethylene oxide)-b-poly(4-vinylbenzyl chloride)-b-poly(4‑tert-butylstyrene) triblock terpolymers

A series of linear poly(ethylene oxide)-b-poly(4-vinylbenzyl chloride)-b-poly(4‑tert-butylstyrene) (PEO113-b-PVBC130-b-PtBSx or E113V130Tx) triblock terpolymers with various lengths x (=20, 33, 66, 104, 215) of PtBS block were synthesized via a two-step reversible addition-fragmentation chain transfer (RAFT) polymerization. The E113V130Tx triblock terpolymers were non-crystalline because the PVBC and PtBS blocks strongly hindered the crystallization of PEO block. The effects of PtBS block length x on the phase structures of E113V130Tx triblock terpolymers were investigated by combined techniques of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It was found that with increasing x from 20 to 215, the phase structure of E113V130Tx triblock terpolymers became more ordered and changed from disordered structure, hexagonally-packed cylinder (HEX), hexagonally perforated layer (HPL), to lamellar (LAM) phase structures. Temperature-variable SAXS measurements showed that the HEX, HPL and LAM phase structures obtained for E113V130T66, E113V130T104 and E113V130T215 by thermal annealing, respectively, were thermodynamically stable in the temperature range of 30–170 °C.