Solution‐Processable Donor‐Acceptor‐Donor Oligomers with Cross‐Linkable Functionality

Electron‐acceptor units, combined with bithiophene substituted with flexible chains end‐functionalized with cross‐linkable moieties, provide soluble donor‐acceptor‐donor (DAD) π‐conjugated oligomer‐type molecules with cross‐linking ability and broad absorption in the visible spectrum. A study on the cross‐linking conditions of the new oligomers to yield insoluble polymer networks is presented, including conditions for obtaining polymer films over poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate‐covered substrates. The combination of the DAD molecular design and cross‐linking functionality opens prospects for applications in solution‐processed small‐molecule solar cells with morphologically‐stable organic layers.

     

Photoinitiator‐Free,Open‐Air Acceptor/Donor Copolymerization of Bismaleimides: Simple Polymerization Conditions for New Thermoplastic Elastomer Production

A photoinitiator‐free acceptor–donor copolymerization between a mixture of bismaleimides containing oligo(propyleneglycol)s as acceptor monomers and divinyl ether as the donor monomer under UV is described. The copolymerization without any photoinitiator is faster than homopolymerization of the sole maleimide, thanks to an acceptor–donor charge‐transfer complex. The thermal and mechanical properties of the cured materials depend essentially on the cross‐linking densities and on the number of repeated units in the bismaleimide backbone. Most copolymerizations produce elastomers with remarkable rubbery properties on a large temperature scale. The simple chemistry presented here can easily compete with acrylate photochemistry since neither photoinitiator nor oxygen removal is needed.

     

Microwave Assisted Synthesis of Thiol Modified Polymethacrylic acid and Its Cross‐Linking with Allyl Modified Polymethacrylic acid via Thiol‐ene “Click” Reaction

In this paper, we describe the thiol‐ene “click” reaction with modified poly methacrylic acid in water. The thiol group was implemented by a polymer analogous condensation reaction of polymethacrylic acid ( 1 ) and cysteamine ( 2 ), which was carried out in bulk by use of microwave. The allyl modified polymethacrylic acid ( 5 ) was obtained by DCC‐coupling of the allyl amine onto the polymethacrylic acid backbone. By combination of cysteamine ( 3 ) and allyl modified polymethacrylic acid ( 5 ) the thiol‐ene click reaction could be started with a redox initiator in aqueous solution at room temperature. The kinetics of this reaction were determined by ¹H NMR spectroscopy and the resulting hydrogels were analyzed by rheological measurements and differential scanning calorimetry (DSC).

     

3D Network Binder via In Situ Cross‐Linking on Silicon Anodes with Improved Stability for Lithium‐Ion Batteries

Silicon (Si) has attracted intensive academic and commercial attention due to its extremely high theoretical capacity. However, it is still far away from practical application because of its fast capacity fading, which is caused by the huge volume change. Here, a novel network polymer binder is synthesized through in situ thermal crosslinking of water‐soluble carboxymethyl cellulose (CMC) and maleic anhydride (MAH). The as‐obtained polymer binder network can effectively restrict the huge volume change of Si anodes upon lithiation. Because of the significantly enhanced structural stability, the Si anodes with network polymer binder deliver enhanced electrochemical performance, with a capacity of 996 mAh g^?1 at a high current density of 1 A g^?1 after 120 cycles under high mass loading. Most importantly, a high average Coulomb efficiency (CE) of 99.4% is obtained, which is superior over the average CE (98.7%) of Si only using CMC as binder. It is considered that this novel 3D network cross‐linking binder can be used for high‐capacity anode materials in next‐generation Li‐ion batteries.     

Investigation on Naphthalene and Its Derivatives‐Based Microporous Organic Hyper‐Cross‐Linked Polymers via Different Methodologies

A series of cost‐effective hyper‐cross‐linked polymers (HCLPs) are synthesized from naphthalene via the external cross‐linker (ECL) knitting method, the solvent knitting method, and the Scholl coupling reaction, respectively. According to multiple characterizations, the resulting polymers are thermally stable and fluorescent with large specific surface areas (SSAs) and narrow pore distributions. In particular the HCLP synthesized using dimethoxymethane as the ECL exhibits SSAs up to 2870 m² g^?1 and shows a great potential in gas adsorption applications. Naphthol and 1‐methylnaphthalene are used as monomers to synthesize HCLPs by the above three methods to investigate whether introducing functional groups to naphthalene would improve properties of the resulting polymers. Moreover, HCLPs feature high SSAs, outstanding thermal and fluorescent performances, and facile synthesis, making them promising candidates for industrial applications.     

Tensile Behavior of a Substituted Poly(m‐,p‐phenylene) versus Its Parent Counterpart and Synthesis of Related Polyarylenes

In a recent report, a poly(m ,p‐phenylene) with acid‐cleavable trialkylsilyl side chains is synthesized by Suzuki polycondensation in a molecular weight of M _w = 300 kDa determined by gel permeation chromatography (GPC). Such a high value suggests this polymer as a potentially attractive material for strong films and fibers both in the presence and absence of the side chains. After confirming these molecular weights by static light scattering, the polymer is processed into thin films for tensile stress measurements. Above the glass transition, thus >180 °C, the material shows an extended plasticity, which is used to obtain elongated samples of draw ratios ranging from 1.5 to 6. The elongated samples show a Young's modulus of up to 5 GPa and a yield strength of 140 MPa. Upon removal of the solubilizing side chains, these values further increase to a Young's modulus of 7.5 GPa in the direction of drawing and a maximum strength of 300 MPa. Motivated by these findings, the scope of this polymer class is broadened by synthesizing two new high molecular weight poly(m ,p‐phenylene)s. The structural variation allows to tune the glass transition temperatures between 125 and 240 °C depending on the amount of the meta connected phenylene units.

     

Synthesis of Dual‐Sensitive Core Cross‐Linked Mixed Micelles through Thiol–Ene Addition and Subsequent Drug Release Behavior

Herein the synthesis of core cross‐linked (CCL) mixed micelles through a UV‐promoted thiol–ene addition onto lipid core unsaturations and the subsequent release of an encapsulated drug depending on the external conditions (pH/temperature) are reported. The thiol–ene addition proceeds even in the absence of a photoinitiator to reach a complete conversion in a few minutes in bulk. The cross‐linking reaction is applied in aqueous media onto lipid‐b‐poly(acrylic acid) (lipid‐b‐PAA) only and then a mixture of pH‐sensitive lipid‐b‐PAA and thermo‐sensitive lipid‐block‐poly(2‐isopropyl‐2‐oxazoline) copolymers. Structurally CCL micelles, preserved in any conditions, with a stimuli‐sensitive corona whose swelling depends on the external pH or/and temperature, are successfully prepared. The release of vitamin K1 (VK1) is then investigated from all the previous systems and demonstrates a strong dependency to the external conditions. Indeed, the dual‐sensitive CCL micelles release VK1 only when two triggers (pH 10 and T = 38 °C) are simultaneously activated.

     

Synthesis and Characterization of PNIPAm Core Cross‐Linked Star Polymers and Their Functionalization with Cyclodextrin

Poly(N‐isopropylacrylamide) (PNIPAm) core cross‐linked star (CCS) polymers (s‐(PNIPAm)_n) are synthesized by arm‐first ATRP method. The related synthesis conditions are investigated and optimized. By varying cross‐linker N,N‐methylenebisacrylamide (BIS) concentration, PNIPAm CCS polymers with about 47, 86, and 211 arms are synthesized. Then, under ATRP condition, the “living” sites at the core of s‐(PNIPAm)_n reacting with a monovinyl β‐cyclodextrin (β‐CD) monomer afford β‐CD functionalized s‐(PNIPAm)_n (CDF‐SPNIPAm). The structures of the star polymers are characterized. The results indicate that in CDF‐SPNIPAm, the ratio of β‐CD units to PNIPAm arm numbers could be up to 0.6:1. The fluorescence spectra of star polymer/ANS (8‐­anilino‐1‐naphthalenesulfonic acid ammonium salt hydrate) systems prove that the β‐CD moieties of CDF‐SPNIPAm are available for including guest molecules. By using pH‐sensitive adamantyl (Ada)‐terminated poly(4‐vinylpyridine) (Ada‐P4VP) (synthesized by ATRP strategy) as a model guest macromolecule, the host–guest complexation between β‐CD units of CDF‐SPNIPAm and adamantyl groups of Ada‐P4VP is confirmed via 2D NOESY ¹H NMR and DLS measurements. The results indicate that the presence of the Ada‐P4VP arms provides temperature‐responsive star polymers with pH sensitivity. Therefore, the β‐CD‐functionalized star PNIPAm could provide host macromolecular platform for constructing novel miktoarm star polymers.

     

Poly(ε‐caprolactone) and Pluronic Diol‐Containing Segmented Polyurethanes for Shape Memory Performance

A series of novel segmented linear and crosslinked polyurethanes (PUs) are synthesized from poly(ε‐caprolactone) (PCL) (25 kg mol^?1), methylene diphenyl diisocyanate (MDI), and various polyether diols (Pluronic (PLU) and polyethylene glycol (PEG)). The basic structures of the highly deformable PUs are PLU/PEG–MDI–PCL–MDI–PLU/PEG and PLU–MDI–PCL–MDI–PLU, respectively. The linear and crosslinked PUs are characterized. Changes in the tensile behavior are attributed to the effects of compositional variables and alterations in the crosslink density. Additional information on the morphology of the segmented PUs is deduced from differential scanning calorimetry, as well as transmission and scanning electron microscopy investigations. Both the linear and the crosslinked PUs exhibit a broad rubbery plateau above the melting temperature of the crystalline PCL phase, which is highly beneficial for shape memory function. This work highlights that the chemical build‐up of soft segments containing high‐molecular‐weight crystallizable chain units is a proper tool to tailor the morphology and mechanical properties of PUs, and thus also their shape memory properties.

     

Cross‐Linked Poly(ε‐caprolactone/D,L‐lactide) Copolymers with Elastic Properties

Cross‐linked ε‐caprolactone (CL) and D ,L ‐lactide (DLLA) copolymers with elastic properties were synthesized in three steps. First, the monomers were copolymerized in ring‐opening polymerization to obtain telechelic star‐shaped oligomers with almost completely random monomer distribution. The oligomers were methacrylated with methacrylic anhydride in the second step and cured in a third. Molar CL/DLLA compositions of 30/70, 50/50, 70/30, 90/10, and 100/0 were used to obtain elastic structures with a wide range of properties. The effect of the average length of the copolymer block on the properties of the networks was evaluated with three different co‐initiator contents (0.5, 1.0, and 2.0/100) in the oligomer synthesis. The oligomers were characterized by ¹³C NMR spectroscopy, size‐exclusion chromatography (SEC), and differential‐scanning calorimetry (DSC). The formation of elastic networks was confirmed by the absence of a flow region in dynamic mechanical analysis (DMA), the increase in T_g in DSC, and the full recovery of the sample dimensions after tensile testing. In addition, gel contents were high and the samples swelled in CH₂Cl₂. The networks possessed break stresses from 0.7–9.7 MPa with elongations from 80–350%. Networks with 100 or 90% of ε‐caprolactone retained their form in vitro for 12 weeks, but an increase in lactide content made the networks more vulnerable to hydrolysis.

Water absorption of the polymers during hydrolysis.     

Control of In‐Plane and Out‐Of‐Plane Reorientation in Photo‐Cross‐Linkable Copolymer Liquid‐Crystal Films by Irradiation with Linearly Polarized Ultraviolet Light and Annealing

This paper describes thermally enhanced in‐plane and out‐of‐plane reorientation of mesogenic groups in copolymer liquid‐crystal (CPLC) films that have a photo‐cross‐linkable 4‐(4′‐methoxycinnamoyl)biphenyl (MCB) side group and a 4‐methoxybiphenyl (4MB) side group. The side groups are cross‐linked by irradiation with linearly polarized ultraviolet (LPUV) light and subsequent annealing. Due to axis‐selective photo‐cross‐linking, a small negative optical anisotropy is observed for all CPLCs after exposure to LPUV light. When the composition of the 4MB comonomer is less than 45 mol‐%, thermal treatment in the liquid‐crystalline temperature range causes in‐plane reorientation of the film and results in an in‐plane order parameter of 0.71. In contrast, when the composition of the 4MB is increased, out‐of‐plane reorientation is generated. Three‐dimensional molecular reorientation and inclined out‐of‐plane reorientation are achieved by adjusting the exposure angle.

Angular dependence of UV absorption spectrum of film 3 . The inset shows a conoscope observation of the film. The film was annealed at 130 °C for 10 min.     

Exploring the Scope of the Baylis–Hillman Reaction for the Synthesis of Side‐Chain Functional Polyesters

The Baylis–Hillman reaction provides a protective‐group‐free strategy to heterobifunctional polyesters, which can be further modified via various post‐polymerization modification reactions. This contribution explores the Baylis–Hillman polymerization of 1,3‐butanediol diacrylate with two readily available dialdehyde monomers, iso‐ and terephthaldehyde. The influence of a variety of catalysts and additives on the polymerization of these two pairs of monomers and in particular the molecular weight of the resulting polymers was investigated. The presence of the orthogonally reactive vinyl and hydroxyl groups makes the polymers interesting, e.g., as precursors for dual‐cure coatings.     

Ageing of Silicone‐Based Dielectric Elastomers Prepared with Varying Stoichiometric Imbalance: Changes in Network Structure,Mechanical, and Electrical Properties

Silicone‐based dielectric elastomers are promising dielectric electroactive polymers (DEAPs) applicable to various actuator applications. However, the lack of information concerning their long‐term performance still limits their industrial use. Here, the time‐dependent behavior of silicon‐based DEAPs under electromechanical cycling is investigated. A series of thin silicone films prepared with different stoichiometric imbalances are coated with compliant silver nanowire electrodes and then electromechanically cycled under alternating voltage (V_pp = 2 kV, V_DC = 1 kV) over 10⁵ cycles. Afterward, changes in cross‐linking density, elastic modulus, permittivity, and breakdown behavior are examined. With increasing load cycles, electrically induced cross‐linking of the hydrosilane groups occurs, which leads to increased cross‐linking density of the material. Concomitantly, increase in elastic modulus and decrease in permittivity are observed, resulting in a significant deterioration of actuation performance. The measured breakdown strength, which is related to an extrinsic breakdown strength based on electromechanical instability, increases too.

     

Synthesis and Characterisation of Side‐Chain Liquid Crystalline Poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane]

Summary: A new series of side‐chain liquid‐crystal polymers, the poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane], has been synthesized in which the spacer length is varied from 2 to 8 methylene units. The synthesis used for the chemical modification of polyepichlorohydrin (PECH) involved the phase transfer catalyzed etherification of the chloromethyl groups with sodium 4‐cyano‐4′‐biphenoxide and lithium n‐(4‐cyano‐4′‐oxybiphenyl)‐alkanoates. All the resulting polymers (except polymers 7 and 8), including that without spacer, characterized by ¹H NMR, and IR, exhibit thermotropic liquid crystalline mesomophism. The thermal behavior of the polymers has been characterized using differential scanning calorimetry (DSC) and polarized light microscopy (POM) equipped with hot‐stage. A more pronounced odd‐even effect in the clearing temperatures is observed on increasing the spacer length in which the odd members display slightly higher values, which were also dependent on the substitution degree of PECH. The flexible PECH chain assists nematic LC formation compared with other more rigid backbone polymers where a smectic phase is often encountered with the same mesogen. A comparison of the thermal properties of the cyanobiphenyl based series reported here. Polyacrylate (PA) and poly(methacrylate)‐based (PMA) SCLCPs containing 4‐cyanobiphenyl as the mesogenic unit are consistent with the general rule that increasing backbone flexibility for a given mesogenic unit and spacer length enhance the clearing temperature. This was not found from the PECH‐based series, which show the lower clearing temperature than the PMA and PA series, even though they have more flexible backbone than PA and PMA series. So the clearing temperature is not solely determined by the flexibility of the backbone.

Schlieren textures of PECHOC2‐B taken at 110 °C (cooling from isotropic phase after annealing 1 h).     

Effect of a Polymeric Additive on the Pore‐Size Distribution and Shrinking Process of a Hydrogel Network

A systematic comparison of the effect of architectural modifications to the network structure on the internal microstructure of N‐isopropylacrylamide (NIPA) based hydrogels showed that the addition of a second component to the network significantly increased the proportion of macropores in the network. The second components considered were short poly(N‐isopropylacrylamide) (PNIPAM) chains grafted to the network backbone, high‐molecular‐weight polyacrylamide (PAM) chains, or microsphere particles of PNIPAM. Structures are proposed for each of the modified gel networks taking into account the new structural information. Through a combination of the pore size and network structure data reported here, and with the shrinking data obtained previously, shrinking mechanisms are proposed for each of the modified network structures. In all cases, the enhanced shrinking rates were directly caused by the presence of the second component, which acted as nuclei for shrinking (graft‐PNIPAM and PNIPAM microspheres) or as water‐release channels (PAM gel), and indirectly caused by the second components via their affect on the network microstructure.

Proposed structures for the architecturally modified gels based on the pore‐size information. Graft‐PNIPAM gel. The freely mobile graft chains prevent chains from meeting resulting in larger pores.     

Aggregation‐Induced Emission of Tetraphenylethylene in Styrene‐Based Polymers

In the present work, the preparation of different styrene‐based polymer films containing small amounts of TPE and the evaluation of their photoluminescent behaviour is reported. When TPE is dispersed in a poor solvent or in a glassy PS matrix, the arrested intramolecular rotations of its aryls favour the strong emission of light centred at about 455–460 nm. Conversely, TPE fluorescence significantly weakens to a faint signal when good solvents or viscous but not glassy polymer matrices are used. Near‐field optical microscopy correlates the fluorescence behaviour with the different matrix morphologies. These results should be able to be used for developing a new tool for polymer traceability.

     

Non‐Isothermal Crystallization of Hyperbranched Poly(ε‐caprolactone)s and Their Linear Counterpart

Summary: Three hyperbranched poly(ε‐caprolactone)s were prepared with the architectural variation in the length of linear backbone segments consisting of 5, 10, and 20 ε‐caprolactone units (accordingly given the names HPCL–5, –10, and –20, respectively) and in the number of branching points as characterized by ¹H NMR end group analyses. The non‐isothermal crystallizations of HPCLs and LPCL were performed using DSC at various cooling rates and the kinetic study was further performed by using both Ozawa and Kissinger methods. All the kinetic parameters such as the cooling functions and the apparent activation energy of crystallization indicated that HPCLs with longer linear segments and fewer number of branching points showed faster crystallization rates, whereas LPCL exhibited an intermediate rate between HPCL–10 and HPCL–20, i.e., HPCL–5 < HPCL–10 < LPCL < HPCL–20. The decrease in the crystallization rate is attributed to the presence of heterogeneous branching points in HPCLs with shorter segments, which hinders the regular chain packing to crystallize. In addition, the faster crystallization of HPCL–20 compared to LPCL was associated with the higher cooperative chain mobility in the melt.

Schematic illustrations for HPCL and LPCL.     

Reversible Photo‐Mechanical Switching Behavior of Azobenzene‐Containing Semi‐Interpenetrating Network under UV and Visible Light Irradiation

Summary: Light‐induced reversible changes in elasticity of semi‐interpenetrating network (semi‐IPN) films bearing azobenzene moieties were achieved under both ultraviolet (UV) and visible light irradiation. The semi‐IPN film was prepared by a cationic copolymerization of azobenzene‐containing vinyl ethers in a linear polycarbonate (PC) film as a matrix. When the irradiation was switched on and off, the semi‐IPN film showed rapid reversible deformation with the same behavior occurring over a range of wavelengths, including both the UV and visible regions. The observed reversible deformation of the film was attributed to the decrease in the film's elasticity, which was assumed to be caused by the frequent trans‐cis cycling isomerization of azobenzene moieties taking place during the UV and visible light irradiation. This cycling makes it difficult for the azobenzene groups to aggregate, thus hindering their ability to function as pseudo‐crosslinking points.

     

Enhanced Mechanical Properties of PET‐Based Thermoplastic Elastomers: Brittle–Ductile Transition via Micro Cross‐linking Technology

A novel micro cross‐linking technology is proposed to enhance the mechanical properties of poly(ethylene terephthalate) (PET)‐based thermoplasctic elastomers (TPEEs) whose molecular weights are difficultly increased via traditional synthetic technologies. It is found that the brittle fractures of TPEEs, usually observed in polymers with low molecular weights, can be converted into ductile form with the introduction of a certain fraction of 1,2,3,4‐butane tetracarboxylic acid as a cross‐linking agent. Unlike the traditional cross‐linking technologies, which usually cause the elastomers to suffer the deficiencies of extremely high stiffness, inherent weakness, and the inability to be efficiently reused, TPEEs still maintain their elastomeric characteristics well with the incorporation of micro cross‐linking structure and are endowed with a well‐defined microphase‐separated structure, excellent tensile properties, and the abilities to be melt‐processed and reused. This work offers a new guideline to improve the mechanical properties for polymers with relatively low molecular weights.     

Synthesis and Gas Permeability of Chemically Cross‐Linked Polynorbornene Dicarboximides Bearing Fluorinated Moieties

This work reports on the synthesis of a novel bifunctional norbornene dicarboximide monomer (HFDA) based on 4,4′‐(hexafluoroisopropylidene)bis(p‐phenyleneoxy)dianiline and its application as a cross‐linking agent in the ring‐opening metathesis polymerization (ROMP) with N‐3‐trifluoromethylphenyl‐exo,endo‐norbornene‐5,6‐dicarboximide (mCF₃) employing the Grubbs 2nd generation catalyst (I) and cis‐1,4‐diacetoxy‐2‐butene as a chain transfer agent (CTA) to yield a series of soluble nonlinear highly branched chains polymers with increasing degree of cross‐linking. A comparative study of gas transport in membranes based on these cross‐linked polynorbornene dicarboximides is performed and the gases studied are hydrogen, oxygen, nitrogen, carbon dioxide, methane, ethylene, and propylene. It is found that cross‐linking increases the gas permeability, leads to the highest separation factor reported to date for the H₂/C₃H₆ mixture in this kind of polymers, and also enhances the CO₂ plasticization resistance up to 14 atm upstream pressure. The chemical cross‐linking approach employed in this research is an effective tool to enhance gas transport properties for dense polynorbornene dicarboximide membranes.