Self‐Healing Metallo‐Supramolecular Amphiphilic Polymer Conetworks

The current challenge in self‐healing materials resides in the design of materials which exhibit improved mechanical properties and self‐healing ability. The design of phase‐separated nanostructures combining hard and soft phases represents an attractive approach to overcome this limitation. Amphiphilic polymer conetworks are nanostructured materials with robust mechanical properties, which can be tailored by tuning the polymer composition and chemical functionality. This article highlights the design of phase‐separated nanostructured polymers from metallo‐supramolecular amphiphilic polymer conetworks, and their application for self‐healing surfaces. The synthesis of poly(N‐(pyridin‐4‐yl)acrylamide)‐l‐polydimethylsiloxane polymer conetworks from the poly(pentafluorophenyl acrylate)‐l‐polydimethylsiloxane activated ester is presented. Loading of ZnCl₂ salt into the phase‐separated polymer conetwork strengthens the network by cross‐linking the poly(N‐(pyridin‐4‐yl)acrylamide) phases, while offering reversible interactions needed for self‐healing ability.     

Direct Insertion Mass Spectrometric Analysis of Thermal Degradation of Poly(2‐alkyl‐2‐oxazoline)

Direct pyrolysis mass spectrometry is applied to investigate the thermal behavior of poly(2‐isopropyl‐2‐oxazoline) (PIPOX, a thermoresponsive polymer) and poly[2‐(3‐butenyl)‐2‐oxazoline] (PBOX, a “clickable” polymer). It is found that the thermal degradation of PIPOX is started by a loss of side chains. At slightly higher temperatures the degradation of the polymer backbone occurs by random chain scission processes. In the case of PBOX, vinyl polymerization of the side chains produces chains with variable thermal stabilities. The number of repeating units of the polymer has almost no effect on the thermal behavior of PIPOX, but significantly affects both thermal stability and degradation product distribution of PBOX.     

Chiral pH‐Responsive Amphiphilic Polymer Co‐networks: Preparation,Chiral Recognition,and Release Abilities

Novel amphiphilic polymer co‐networks (APCNs), poly(N‐acryloyl‐L ‐alanine)‐l‐polydimethylsiloxane (denoted as PNAA‐l‐PDMS), are prepared, and exhibit remarkable pH‐responsiveness, chiral‐recognition, and enantioselective‐release abilities. The APCNs are prepared by free‐radical copolymerization starting from N‐acryloyl‐L ‐alanine (NAA) and methacrylate‐terminated poly(dimethylsiloxane) (M‐PDMS) as co‐(macro)monomers. The APCNs show pronounced pH‐sensitivity, evidenced by a reversible swelling–deswelling transition upon a cyclicly altering pH. The chiral co‐networks are applied for enantioselective recognition and release. A maximum adsorption is achieved towards D ‐proline (61%), whereas for L ‐proline, it is only 10%. More interestingly, the release for the L ‐proline is 90%, whereas for the D ‐proline, only 70% is released.

     

pH‐Responsive Aminomethyl Functionalized Poly(p‐xylylene) Coatings by Chemical Vapor Deposition Polymerization

The preparation of stimuli‐responsive aminomethyl functionalized poly(p‐xylylene) coatings by chemical vapor deposition polymerization is reported. Modification of the paracyclophane precursor with ionizable aminomethyl groups leads to polymer coatings with pH‐responsive swelling properties. The swelling behavior is monitored in situ using spectroscopic ellipsometry and additional streaming potential measurements are performed. With decreasing pH‐value, the coating becomes increasingly charged and reversibly swells to several times its dry thickness. The swelling ratio is sensitive to the ionic strength of the solution. By using a mixture of unfunctionalized and functionalized precursors in the chemical vapor deposition process, the number of charges in the polymer layer can be tuned and with it the swelling ratio of the coating. As a proof‐of‐concept for possible applications, a commercial paper filter is coated. This results in a pH‐dependent wetting behavior and pH‐dependent transport through the capillaries of the paper.

     

pH‐dependence of the swelling capacity of poly(N‐vinylimidazole) hydrogels

The swelling capacity (S) of poly(N‐vinylimidazole) (PVI) hydrogels in aqueous acid solutions was measured as a function of the pH, the nature of the acid, and the polymer network structure. In acid solutions, imidazole groups become protonated and the swelling capacity increases to reach a maximum corresponding to a degree of protonation equivalent to the Manning limit charge density. The variable controlling the degree of protonation, and therefore the swelling capacity, is the mass of dry polymer immersed in a given volume of solution, which was called the gel effective concentration. Divalent counteranions reduce significantly S with respect to the same ionic strength and degree of protonation, obtained with monovalent anions, while non‐complexant divalent cations have no specific influence on the PVI swelling capacity. Results were discussed in terms of the effective charge density caused by counterion condensation on protonated PVI, the non‐covalent crosslink density caused by ionic interactions with divalent anions, and the non‐Gaussian character of chains between crosslinks.

Swelling capacity of two PVI samples in aqueous solutions of HCl with two different gel effective concentrations: 10⁻² M and 2.7 × 10⁻² M .     

Swelling and Mechanical Properties of Solution‐Crosslinked Poly(isobutylene) Gels

We prepared several series of poly(isobutylene) (PIB) gels starting from butyl rubber in dilute toluene solutions using sulfur monochloride as a crosslinking agent. Solution and suspension‐crosslinking techniques were used for the preparation of PIB gels in the form of rods, membranes, and beads in the size range of 1 to 3 mm. The gels were subjected to dynamic and equilibrium swelling measurements in toluene and cyclohexane as well as to the elasticity tests. Depending on the amounts of sulfur monochloride and butyl rubber in the crosslinking solution, PIB gels with different swelling capacities and elastic moduli were synthesized. The swelling ratio of the gels first increased rapidly with increasing swelling time but then decreased until an equilibrium was obtained. This unusual swelling behavior was accompanied with an increase of the elastic moduli of the gels during the swelling process. The results were explained with the post‐crosslinking reactions taking place during the swelling process of PIB gels. By using the theory of equilibrium swelling, the number of segments in the network chains and the polymer‐solvent interaction parameters were calculated for PIB gels prepared under various reaction conditions.     

Poly(methacrylic acid)‐l‐Polyisobutylene Amphiphilic Conetworks by Using an Ethoxyethyl‐Protected Comonomer: Synthesis,Protecting Group Removal in the Cross‐Linked State,and Characterization

A series of poly(methacrylic acid)‐l‐polyisobutylene anionic amphiphilic conetworks with hydrophobic polyisobutylene (PIB) content between 33% and 70% (w/w) is synthesized by the macromonomer method using a new approach by applying ethoxyethyl‐protected methacrylic acid (EEMA) comonomer. To remove the protecting group after copolymerization of EEMA with PIB‐dimethacrylate, acidic hydrolysis and thermal treatment are attempted, and the success of the deprotection is monitored by different techniques. Comparing the two protective group removal steps, it is found that, in contrast to homopolymers of EEMA, the acidic hydrolysis is more favorable than the thermal deprotection due to anhydride formation during the latter process in the conetworks. Distinct glass transitions are obtained by differential scanning calorimetry (DSC) indicating phase separation in the resulting conetworks. The swelling ability of the obtained conetworks in polar and nonpolar solvents proves the amphiphilic nature of these materials. Due to the polyacidic component of the conetworks, their water swollen gels have pH‐sensitive, intelligent swelling behavior in aqueous media. Because of the relatively high stability of EEMA, the applied protection–deprotection strategy can be easily utilized in several other designed polymer syntheses.

     

Porous Poly(styrene‐co‐divinylbenzene) Neutral Monolith: From Design and Characterization to Reversed‐Phase Capillary Electrochromatography Applications

Macroporous cross‐linked organic polymers, based on styrene and divinylbenzene (PS‐DVB), are prepared as monolithic stationary phases for capillary electrochromatographic applications. The synthetic strategy, which relies on the semi‐interpenetrating polymer network (semi‐IPN) approach, is performed through UV‐initiated free radical copolymerization of the comonomers in the presence of poly(?‐caprolactone) (PCL) within the confines of fused silica capillaries. The characterization of PS‐DVB monoliths is carried out at the different stages of the synthesis using a combination of experimental techniques, thus providing information on the chemical and porous structures. All experimental results evidence the sole role of the PCL oligomer as a porogen.     

Combining Ultra‐High Drug‐Loaded Micelles and Injectable Hydrogel Drug Depots for Prolonged Drug Release

Hydrogel‐based drug depot formulations are of great interest for therapeutic applications. While the biological activity of such drug depots is often characterized well, the influence of incorporated drug or drug‐loaded micelles on the gelation properties of the hydrogel matrix is less investigated. However, the latter is of great importance from fundamental and application points of view as it informs on the physicochemical interactions of drugs and water‐swollen polymer networks and it determines injectability, depot stability, as well as drug‐release kinetics. Here, the impact of incorporated drug, neat polymer micelles, and drug‐loaded micelles on the viscoelastic properties of a cytocompatible hydrogel is investigated systematically. To challenge the hydrogel with regard to the desired application as injectable drug depot, curcumin (CUR) is chosen as a model compound due to its very low‐water solubility and limited stability. CUR is either directly solubilized by the hydrogel or pre‐incorporated into polymer micelles. Interference of CUR with the temperature‐induced gelation process can be suppressed by pre‐incorporation into polymer micelles forming a binary drug delivery system. Drug release from a collagen matrix is studied in a trans‐well setup. Compared to direct injection of drug formulations, the hydrogel‐based systems show improved and extended drug release over 10 weeks.     

ABA and BAB Triblock Copolymers Based on 2‐Methyl‐2‐oxazoline and 2‐n‐Propyl‐2‐oxazoline: Synthesis and Thermoresponsive Behavior in Water

Inspired by the well‐known amphiphilic block copolymer platform known as Pluronics or poloxamers, a small library of ABA and BAB triblock copolymers comprising hydrophilic 2‐methyl‐2‐oxazoline (A) and thermoresponsive 2‐n‐propyl‐2‐oxazoline (B) is synthesized. These novel copolymers exhibit temperature‐induced self‐assembly in aqueous solution. The formation and size of aggregates depend on the polymer structure, temperature, and concentration. The BAB copolymers tend to agglomerate in water, with the cloud point temperature depending on the length of poly(2‐n‐propyl‐2‐oxazoline) chain. On the other hand, ABA copolymers form smaller aggregates with hydrodynamic radius from 25 to 150 nm. The dependence of viscosity and viscoelastic properties on the temperature is also studied. While several Pluronic block copolymers are known to form thermoreversible hydrogels in the concentration range 20–30 wt%, thermogelation is not observed for any of the investigated poly(2‐oxazoline)s at the investigated temperature range from 10 to 50 °C.

     

Novel biostable and biocompatible amphiphilic membranes

We determined the biostability and biocompatibility of two types of amphiphilic conetworks (APCNs): (1) hydrophilic poly(N,N-dimethyl acrylamide) (PDMAAm) and hydrophobic polydimethylsiloxane (PDMS) microdomains co-crosslinked with polymethylhydrosiloxane (PMHS) clusters (PDMAAm/PMHS/PDMS), and (2) poly(ethylene glycol) (PEG) and PDMS microdomains co-crosslinked with two specially designed small-molecule crosslinking agents SiC(6)H(5)(SiH)(2)OEt (Y) and polypentamethylhydrocyclosiloxane (PD(5)) (PEG/Y or PD(5)/PDMS). Negative standards for comparing biocompatibility and biostability were crosslinked PDMS. Biostability was assessed by quantitatively determining extractables, equilibrium water swelling, mechanical properties (stress-strain response) of polymer samples before and after implantation in rats for up to 8 weeks, and oxidative accelerated degradation test. Biocompatibility was assessed by determining body weight, fibrous tissue encapsulation, fluid accumulation, and by histological evaluation of lymphocyte infiltration, fibrous tissue accumulation and collagen deposition. According to these stringent metrics PDMAAm/PMHS/PDMS is both biostable and biocompatible, whereas PEG/Y or PD(5)/PDMS degrades in living tissue but is biocompatible. Surprisingly, the overall biocompatibility scores of these APCNs were superior to those of the PDMS negative standard.     

Synthesis and Properties of pH‐ and Temperature‐Sensitive Poly[(N‐isopropylacrylamide)‐co‐(2‐methylenebutane‐1,4‐dioic acid)] Hydrogels

Summary: Hydrogels of NIPA and MBDA were synthesized by free‐radical crosslinking copolymerization with different monomer ratios and with two concentrations of the crosslinking agent. The aim of this work was to study the swelling behavior of these gels that are both temperature and pH sensitive. PNIPA hydrogels are typical examples of thermo‐shrinking hydrogels with a LCST, T_C, around 31–34 °C. MBDA is a weakly ionizable monomer which imparts a pH sensitiveness to the copolymer hydrogels. The pH influence on the swelling behavior of the studied hydrogels was analyzed using deionized water and aqueous HCl and NaOH as swelling media. According to the results found in deionized water, the swelling processes of P(NIPA‐MBDA) hydrogels follow second‐order kinetics at 22 and 37 °C. The equilibrium water content, W_∞, and the rate constant, K, increased at greater concentrations of MBDA and decreased as the crosslinking agent concentration increased. As the MBDA content in the hydrogel increased, the collapsing of the hydrogels at higher temperatures than the LCST became of less importance. The degree of swelling of pure PNIPA hydrogels was not influenced by the pH of the swelling medium. However, this influence increased as the MBDA content increased. This was due to the fact that at low pH most of the MBDA units are in the protonated (neutral) form and at high pH in the ionized one.

Swelling isotherms of hydrogels with different copolymer compositions and with 1.5 wt.‐% of BIS at 22 °C in deionized water.     

Temperature and pH Dual‐Responsive Behavior of Dendritic Poly(N‐isopropylacrylamide) with a Polyoligomeric Silsesquioxane Core and Poly(2‐hydroxyethyl methacrylate) Shell

Novel temperature and pH dual‐responsive dendritic polyoligomeric silsesquioxane (POSS)–poly(N‐isopropylacrylamide) (PNIPAm)–poly(2‐hydroxyethyl methacrylate) (PHEMA) copolymers are prepared via atom transfer radical polymerization and click reactions. The cloud points (T_c) decrease with decreasing pH from 10.0 to 5.0 due to the weakened inter‐molecular interactions and enhanced intra‐molecular hydrogen bonding, whereas the T_c exhibits a small increase from pH 5.0 to 4.0 because of the better solvation of PHEMA at highly acidic conditions. The above findings are corroborated by the different sizes of aggregates observed by dynamic light scattering. The encapsulation of a fluorescent dye and stimulated release by temperature and pH changes are also demonstrated.     

Parameters Affecting the Lower Critical Solution Temperature of Linear and Crosslinked Poly(N‐ethylacrylamide) in Aqueous Media

A series of linear poly(N‐ethylacrylamide) (PEA) samples varying in molar mass has been prepared by free radical polymerization in the absence and presence of a chain transfer agent and a hydrogel of PEA has been prepared using N,N‐methylenebisacrylamide (BIS) as crosslinker. The lower critical solution temperatures (LCST) of the linear polymers in water and aqueous media were determined turbidimetrically as a function of molecular weight, concentration, heating/cooling rate, and concentrations of KCl and anionic surfactant sodium dodecyl sulfate (SDS) in the aqueous solution. The corresponding LCST for the hydrogel was determined from the gravimetric swelling ratios (r). In pure water the values of LCST for linear polymer and hydrogel are 73 °C and 62 °C, respectively. The LCST of linear PEA increases with decreasing molecular weight. The swelling ratio for gels and the LCST for solutions and gels increased with the inclusion of SDS into water. The opposite effects prevailed on inclusion of KCl into water or incorporation of crosslinker. Additionally, the rates of heating/cooling play a significant role in the measured value of LCST. Hence the swelling ratio of hydrogel or the LCST can be adjusted via 1) addition of either SDS or KCl into water; 2) use of different molecular weight samples; 3) incorporation of crosslinker into polymer chain.

     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

A Novel Characterization Method for Graft‐Polymer Structures Chemically Attached on Thermally Stable Polymer Films

Most polymer grafts in grafted polymer films obtained by radiation‐induced solid graft polymerization are not analyzed in detail due to difficulties in isolation of the grafts without structural decomposition. Herein, a novel structural and molecular weight characterization method is reported for polymer grafts that are chemically attached to thermally and chemically stable polymer substrates, based on a swelling‐induced graft detachment in hot water. Polymer grafts prepared by the radiation‐induced polymerization of alkyl acrylate into poly(ethylene‐co‐tetrafluoroethylene) (ETFE) followed by a sulfonation reaction are found to have a ternary copolymer structure whose monomer units contain –COOH or –SO₃H groups, or both.     

Counter‐Anion Effect on the Properties of Anion‐Conducting Polymer Electrolyte Membranes Prepared by Radiation‐Induced Graft Polymerization

Graft‐type anion‐conducting polymer electrolyte membranes (AEMs) are prepared by the radiation‐induced graft polymerization of chloromethylstyrene into poly(ethylene‐co‐tetrafluoroethylene) (ETFE) films and subsequent quaternization with trimethylamine. AEMs in the hydroxide form (AEM‐OH) are prepared by immersing the chloride form (AEM‐Cl) in 1 M potassium hydroxide (KOH) solution, followed by KOH and washing with nitrogen‐saturated water to prevent bicarbonate formation (AEM‐HCO₃). The AEM‐OH shows conductivity and water uptake four and two times higher than AEM‐Cl and ‐HCO₃ and is thermally and chemically less stable, resulting in the tendency to absorb water and to convert to the bicarbonate form.

     

Formation of Well‐Defined Polymer Particles in the Sub‐100 nm Size Range by Using Amphiphilic Block Copolymer Surfactants and a Microemulsion Approach

Here, the formation of nanoparticles based on a microemulsion approach and the use of polymer surfactants are described. Therefore, two amphiphilic poly(2‐oxazoline) block copolymers P1 and P2 with alkyne groups in their hydrophobic block have been synthesized by ring‐opening, cationic polymerization. The polymers P1 and P2 are employed in a microemulsion process to stabilize the particle core by core cross‐linking of 1,6‐hexanediol diacrylate (HDDA) using either AIBN as azo‐initiator or 2‐propanethiol as a photo‐initiator for the polymerization reaction. The results show that particle size can be controlled by sonication time, the hydrophilic–hydrophobic balance of the polymer surfactant, and the ratio of polymer surfactant versus HDDA giving access to water‐soluble nanoparticles in a size range of 10–70 nm.

     

Design and Properties of Degradable Networks Based on Free Radical Copolymerization of Poly(1,3‐dioxolane) Macromonomers with Hydrophobic or Hydrophilic Comonomers in Water

Summary: Potentially degradable amphiphilic or hydrophilic copolymeric networks were synthesized directly in water by free radical copolymerization of α, ω‐poly(1,3‐dioxolane) (PDXL) macromonomers with vinyl monomers such as methyl methacrylate, butyl methacrylate, styrene or 2‐hydroxyethyl methacrylate. Since PDXL macromonomers behave typically as amphiphiles, the rates of copolymerizations with hydrophobic comonomers are much higher than those of similar reactions carried out in organic solvents. Therefore crosslinking in water takes place rapidly. Once swollen to equilibrium in appropriate solvents, the physicochemical properties of the amphiphilic (or hydrophilic) copolymeric PDXL networks were investigated. The volume degree of equilibrium swelling and uniaxial compression modulus strongly depended upon the macromonomer molar mass (and concentration), and also the hydrophobic/hydrophilic balance. Their solid state properties were examined by DSC and compared to those of homopolymeric PDXL networks.

Amphiphilic PDXL conetworks synthesized directly in water by free radical copolymerization of bifunctional PDXL macromonomers with hydrophobic comonomers such as styrene or butyl methacrylate.     

One‐Pot Polymerization Syntheses: Incorporating Bioactives into Poly(anhydride‐esters)

To reduce raw material consumption and increase synthetic efficiency, bioactive‐based poly(anhydride‐esters) containing aliphatic dicarboxylic acid‐linkages and bioactives, salicylic and p‐hydroxybenzyl acid, are synthesized via one‐pot melt‐condensation polymerizations. One‐pot poly(anhydride‐esters) physicochemical characteristics, molecular weight, and thermal properties are analyzed and compared. One‐pot salicylic acid‐based poly(anhydride‐esters) are further evaluated against analogous polymers synthesized via established methods, possessing statistically similar polymer and thermal properties while drastically reducing reaction time and solvent usage. Interestingly, p‐hydroxybenzyl acid‐based poly(anhydride‐ester) synthesis is temperature‐dependent, as higher reaction temperatures facilitate polyester formation. Compared to their poly(anhydride‐ester) analogs, these are found to possess improved thermal properties and higher molecular weight while. Accelerated hydrolytic degradation studies confirm complete bioactive release and polymer degradation. Furthermore, polymer cytotoxicity studies using 3T3 mouse fibroblasts show all polymers to be cytocompatible above therapeutically relevant concentrations. This method demonstrates that the synthesis of high‐yielding monomers can be followed by melt‐condensation polymerization in situ for the synthesis of polyanhydrides and their derivatives.