Poly(N,N‐dimethylacrylamide)‐block‐Poly(L‐lysine) Hybrid Block Copolymers: Synthesis and Aqueous Solution Characterization

Four poly(N,N‐dimethylacrylamide)‐block‐poly(L ‐lysine) (PDMAM‐block‐PLL) hybrid diblock copolymers and two PLL homo‐polypeptides are prepared via ROP of ε‐trifluoroacetyl‐L ‐lysine N‐carboxyanhydride initiated by primary amino‐terminated PDMAM and n‐hexylamine respectively. The PLL blocks render the copolymers a multi‐responsive behavior in aqueous solution due to their conformational transitions from random coil to α‐helix with increasing pH, and from α‐helix to β‐sheet upon heating. The random coil‐to‐α‐helix transition is found to depend on the PLL length: the longer the peptide segment, the more readily the transition occurred. The same trend was observed for the α‐helix‐to‐β‐sheet transition, which was found to be inhibited for short polypeptides unless conjugated with the PDMAM block.

     

Thermo‐Reversible Hydrogels Based on Poly(N,N‐diethylacrylamide)‐block‐poly(acrylic acid)‐block‐poly(N,N‐diethylacrylamide) Double Hydrophilic Triblock Copolymer

Summary: A novel double hydrophilic poly(N,N‐diethylacrylamide)‐poly(acrylic acid)‐poly(N,N‐diethylacrylamide) (PDEAAm‐PAA‐PDEAAm) triblock copolymer was synthesized by sequential anionic polymerization and modification of the poly(tert‐butyl acrylate) middle block by selective hydrolysis and neutralization to its ionic functions. Due to the pH‐sensitivity of the PAA central block and the thermo‐sensitivity of the PDEAAm end‐blocks, it exhibits responsive behavior in aqueous media. At low temperatures and high pH, it is molecularly dissolved while at temperatures above the LCST of the PDEAAm end‐blocks, a sol–gel transition was observed which should be ascribed to the formation of a three‐dimensional transient network comprising PDEAAm hydrophobic physical crosslinks interconnected by PAA negatively charged elastic chains. The sol–gel transition and the rheological properties of the physical gel are strongly influenced by the presence of salt.

Temperature induced sol–gel transition in PDEAAm‐PAA‐PDEAAm aqueous solutions.     

Rapid Phase Separation in Aqueous Solution of Temperature‐Sensitive Poly(N,N‐diethylacrylamide)

The dynamic phase separation behavior in aqueous solutions of poly(N,N‐diethylacrylamide) (PDEA), which is a representative thermoresponsive polymer, is studied using a laser temperature‐jump technique combined with transient photometry, with which the time constant for phase separation can be determined. Two components of the time constant (τ_fast and τ_slow) are clearly observed irrespective of the polymer concentration or molecular weight of the PDEA. Such a fast phase separation component (τ_fast) has never been observed in poly(N‐isopropylacrylamide) (PNIPAM) systems. Furthermore, the slow phase separation process of PDEA is faster than that in a PNIPAM system. It is shown that the fast phase separation process for PDEA originates from polymer aggregates formed even below the cloud point. The phase separation mechanism of PDEA is similar to that of poly (alkyl vinyl ether), rather than PNIPAM.

     

Miscibility and Intermolecular Specific Interactions in Blends of Poly(hydroxyether sulfone) and Poly(N‐vinylpyrrolidone)

Summary: The blends of poly(hydroxyether sulfone) (PHES) with poly(N‐vinylpyrrolidone) (PVPy) were investigated by means of differential scanning calorimetry (DSC) and FTIR spectroscopy. The miscibility of the blend system was established on the basis of the thermal analysis results. DSC showed that the PHES/PVPy blends prepared by casting from N,N‐dimethylformamide (DMF) possessed single, composition‐dependent glass transition temperatures, indicating that the blends are miscible in the entire composition. The experimental glass transition temperatures have higher values than those calculated on the basis of additive behavior; the variation of the glass transition temperatures of the blends was accounted for by the Kwei equation. FTIR studies indicate that competitive hydrogen bonding interactions exist upon addition of PVPy to the system, which were involved in the self‐ and cross‐association, i.e., ? OH···O?S, ? OH···OH of PHES and ? OH···O?C< of PVPy. The FTIR spectra in the range of the sulfonyl stretching vibrations showed that the hydroxyl‐associated sulfonyl groups are partially “set free” upon addition of PVPy to the system. The IR spectroscopic investigation of both the model compounds and the PHES/PVPy blends suggests that the strength of the hydrogen bonding interactions in the blend system increases in the following order: ? OH···O?S, ? OH···OH and ? OH···O?C<.

Plot of glass transition temperature for PHES/PVPy blends as a function of weight fraction of PVPy. The prediction of the Kwei equation yields the values of k = 1 and q = 122.     

Thermo‐Responsive Organic/Inorganic Hybrid Hydrogels based on Poly(N‐vinylcaprolactam)

A new type of ‘intelligent’ hydrogels has been developed in the form of organic/inorganic hybrid materials by making use of the sol‐gel technology. Poly(N‐vinylcaprolactam) (PVCL) has been incorporated in these materials for its thermo‐responsive properties. The synthesis of the hybrid hydrogels was achieved by the in situ formation of an inorganic silica phase in the presence of an aqueous solution of high molecular weight PVCL. This methodology results in the preparation of micro‐heterogeneous systems in which silica particles of nanometer dimensions act as physical cross‐links for the PVCL molecules. Hydrogen bonds between the remaining non‐condensed silanol groups and the PVCL carbonyl functions, together with physical entanglements, are responsible for the strong interactions between the organic and inorganic phases. Stress‐strain tests on highly swollen materials demonstrated that the unique structure of these thermo‐responsive hybrid hydrogels improves the mechanical stability to a great extent as compared to conventional hydrogels. Transmission measurements demonstrate that the presence of the inorganic phase does not influence the cloud point temperatures of PVCL significantly. On the other hand, the response of the reinforced hybrid hydrogels to temperature becomes less pronounced for increasing silica fractions. The reversibility of the swelling/deswelling process has been demonstrated by swelling experiments as a function of temperature.

PVCL/SiO₂ hybrid hydrogels.     

Conductive and Tough Smart Poly(N‐isopropylacrylamide) Hydrogels Hybridized by Green Deep Eutectic Solvent

Conductive smart hydrogels with several virtues such as similar characters to biological tissues, sensitive response to ambient variations, have shown their excellent talents in the field of flexible electrical sensors, biomedical devices and directional transportation. However, complex preparing approaches or the instable inner structures have not only been time‐consuming, but also broken up the performance and reliability of the smart hydrogel‐based devices. In this work, a facile one‐step method is put forward to synthesize a kind of conductive poly(N‐isopropylacrylamide) (PNIPAM) hydrogel doped by a new green solvent of deep eutectic solvent (DES) containing choline chloride (ChCl) and acrylic acid (AA). Through the copolymerization of AA and NIPAM, the mechanical strength of the DES‐doped PNIPAM hydrogels is drastically improved compared to the pure PNIPAM gels, and some doped hydrogels lost the typical phase transition temperature of PNIPAM. Moreover, due to the ionic property of DES, the hybrid hydrogels also present the thermal‐depending conductivity as well as sensitive deformation response, which can be used as a smart switch in a circuit or a sensing element with environmental response ability. The cost‐effective preparation and the attractive performance of the DES‐doped hydrogels offer a new avenue to construct multi‐functional materials.     

Poly(N‐isopropylacrylamide) Brush Fabricated by Surface‐Initiated Photopolymerization and its Response to Temperature

A well‐defined, high‐density poly(N‐isopropylacrylamide) (PNIPAM) brush was fabricated through a novel and reliable strategy by the combination of the self‐assembly of a monolayer of dendritic photoinitiator and surface‐initiated photopolymerization. The whole fabrication process of the PNIPAM brush was followed by water contact angles, X‐ray photoelectron spectroscopy, and atomic force microscopy. Characterization of the PNIPAM brush, such as molecular weight and thickness determination, were measured by gel permeation chromatography, and ellipsometry, and the graft density was estimated. The temperature response of the PNIPAM brush was further investigated and the result verified the coil‐to‐globule transition of the PNIPAM chains in water from low to high temperature.

     

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.     

Temperature‐Responsive Electrophoretic Deposition of Sulfone‐Containing Nonionic Poly(N‐isopropylacrylamide)

Recently, it has been found that nonionic aliphatic and aromatic poly(ester sulfone)s show anode selective electrophoretic behavior, and it is shown that the electrophoresis is induced by a partial charge separation of the protic solvent at the dispersion interface. In this paper, the first example of temperature‐responsive electrophoretic deposition (EPD) is reported. Electrophoresis of a nonionic sulfone‐containing poly(N‐isopropylacrylamide) [poly(NIPAM)] is performed above the lower critical solution temperature. The poly(NIPAM) is prepared via reversible addition–fragmentation chain transfer radical copolymerization of NIPAM with a sulfone‐containing methacrylate. After EPD, adhesion of human umbilical vein endothelial cells on the deposited surfaces is also demonstrated, aiming at the subsequent temperature‐sensitive detachment.     

The Effect of NaCl on the Conformational Behavior of Acenaphthylene Labeled Poly(N,N‐diethylacrylamide) in Dilute Aqueous Solution

Summary: The effect of salts on the conformational behavior of acenaphthylene labeled poly(N,N‐diethylacrylamide) (PDEA/ACE) has been investigated in dilute aqueous solution using UV‐vis and fluorescence spectroscopy. It is demonstrated here that the effectiveness of various salts in changing the lower critical solution temperature (T_LCS) of the PDEA/ACE solution followed the Hoffmeister series. The addition of NaCl linearly lowered the T_LCS. In dilute aqueous solution, the PDEA/ACE adopted a loose coil conformation below its T_LCS. The PDEA/ACE adopted a compact globule conformation above the temperature at which the coil to globule transition (CGT) was completed. A fluorescence anisotropy investigation suggested that a molten globule conformation existed during the CGT of the PDEA/ACE solution. In all of the above three conformations, the PDEA/ACE was more compact in the presence of NaCl than in the absence of any salt.

Temperature and NaCl concentration dependence of fluorescence anisotropy of a PDEA/ACE solution. Inset: thermodynamically stable states of PDEA/ACE.     

Correlation Between Physicomechanical and Swelling Properties of Weakly Basic Copolymer Hydrogels Based on (Meth)acrylate Polycations as New Smart Materials

2‐Hydroxyethyl methacrylate (HEMA) is incorporated to N,N‐dimethylaminoethyl methacrylate (DMAEMA) fabricating a series of stimuli‐responsive copolymer hydrogels by free radical copolymerization. The increasing comonomer content via adding more HEMA results in a higher network density and, therefore, a lower equilibrium swelling ratio. The uniaxial compressive testing results showed that increasing HEMA in the feed improved the mechanical strength of P(DMAEMA‐co‐HEMA) hydrogels. As HEMA increased, the elastic modulus as well as the effective crosslinked chain density increased. The work strives to provide method to tune mechanical and physical properties for weakly basic copolymer hydrogels based on (meth)acrylate polycations, which is hopefully to guide the design of hydrogels with desirable properties.

     

Poly(N‐isopropylacrylamide)‐Modified Styrene‐Butadiene Rubber as Thermoresponsive Material

Narrowly distributed (N‐isopropylacrylamide) (NIPAM) polymers are prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization. After successful cleavage of the trithiocarbonate end groups (thiol generation), they can be grafted to styrene‐butadiene rubber (SBR) by a radical thiol‐ene reaction leading to various grafted SBR‐copolymers. During the grafting reaction, no crosslinking or branching of the SBR can be observed. Measurements of the contact angle of water show that the lower critical solution temperature (LCST) properties of the PNIPAM fraction affect the SBR. Films of the graft‐copolymer exhibit a distinct hydrophilicity below the LCST, while they show hydrophobic behavior above the LCST. Rheological measurements reveal a physical crosslinking of the functionalized SBR due to nanophase separation of the PNIPAM chains (hard phase) in the unpolar SBR. Compared with blends of SBR and PNIPAM, the PNIPAM‐grafted SBR possesses a much finer distribution of the PNIPAM domains (10–30 nm) within the matrix. In addition, two novel difunctional chain‐transfer agents are used, leading to difunctional PNIPAM, enabling a covalent crosslinking of SBR.

     

Microwave‐Assisted Modification of Poly(vinylimidazolium salts) via N,N‐Dimethylformamide Decomposition

The modification of poly(1‐propyl‐3‐vinylimidazolium bromide) ( 3a ), poly(1‐hexyl‐3‐vinylimidazolium bromide) ( 3b ), poly(1‐isopropyl‐3‐vinylimidazolium bromide) ( 3c ), poly(1‐benzyl‐3‐vinylimidazolium bromide) ( 3d ), poly(1‐(1‐phenylethyl)‐3‐vinylimidazolium bromide) ( 3e ) has been conducted under microwave irradiation using N,N‐dimethylformamide as solvent and reagent. The products were formed by the decomposition of N,N‐dimethylformamide into dimethylamine and carbon monoxide. The proposed structure of the obtained copolymer contains mainly units of 1‐vinylimidazole and some units of amines, formamides, and alkenes. The reaction mechanism was established by use of model compounds 1‐propyl‐3‐methylimidazolium bromide ( 5a ), 1‐hexyl‐3‐methylimidazolium bromide ( 5b ), 1‐isopropyl‐3‐methylimidazolium bromide ( 5c ), 1‐benzyl‐3‐methylimidazolium bromide ( 5d ), 1‐(1‐phenylethyl)‐3‐methylimidazolium bromide ( 5e ), 1‐hexadecyl‐3‐methylimidazolium bromide ( 5f ).

     

Characterization and Properties of Poly[N‐(2‐cyanoethyl)pyrrole]

PNCPy was prepared by anodic polymerization and its properties in both doped and undoped state were characterized. The doping level of the oxidized material has been found to be larger than that of other conducting polymers; the more relevant electrochemical properties of the doped material were retained after undoping. SEM and AFM data are consistent with a lumpy surface and a multidirectional growing of the polymer chains. Finally, PNCPy has been combined with PEDOT to prepare three‐layer systems with enhanced electroactivity and electrostability. Results suggest that PNCPy is a potential candidate for the fabrication of electric circuit components that are able to block the current flow below a given potential.

     

Physical Gelation of Amphiphilic Poly(N‐isopropylacrylamide): Influence of the Hydrophobic Groups

The self‐assembling properties of hydrophobically modified (N‐isopropylacrylamide) have been investigated by dynamic light scattering and rheological measurements. Size of the globules and transmission of the solutions vary strongly in the same range of temperature. The presence of hydrophobic groups leads to contraction of the globules' size compared to poly(N‐isopropylacrylamide) (PNIPAM). The viscoelastic properties of the samples in aqueous solution have been investigated as a function of copolymer concentration, structure of the hydrophobic group (dodecyl or adamantyl group), substitution level (1–5%) and over a temperature range covering the lower critical solution temperature (LCST). At high concentration and high level of adamantyl substitution, gelation is observed several degrees before the phase transition. A physical network is formed due to the strong hydrophobic interactions, and this physical gel undergoes phase transition without macroscopic phase separation.

     

Poly(methylenepyridinium)s by Poly(N‐alkylation) of Substituted Pyridines: Structural and Kinetic Study

The solution polymerization of 4‐bromomethylpyridine ( M1 ) and 3‐bromomethyl pyridine hydrobromides ( M2 ) was studied by NMR spectroscopy. A mechanism involving a series of bimolecular reactions of the monomer, dimer, and higher oligomers closely fits with the experimental variations of bromomethyl end group concentrations with time. M1 presents a higher reactivity than M2 and an unusual behavior, since the oligomers are more reactive than the monomer. An explanation based on a mesomeric phenomenon is proposed. The influence of the anion on the solubility and thermal stability of the poly(methylenepyridinium)s were studied after various anion exchanges. Bis(trifluoromethylsulfonyl)imide anion ( Tf ₂ N ) yielded the more stable and the more organosoluble polymers.

     

Aggregation and Crosslinking of Poly(N,N‐dimethylacrylamide)‐b‐pullulan Double Hydrophilic Block Copolymers

The self‐assembly of polymers is a major topic in current polymer chemistry. In here, the self‐assembly of a pullulan based double hydrophilic block copolymer, namely pullulan‐b‐poly(N,N‐dimethylacrylamide)‐co‐poly(diacetone acrylamide) (Pull‐b‐(PDMA‐co‐PDAAM)) is described. The hydrophilic block copolymer induces phase separation at high concentration in aqueous solution. Additionally, the block copolymer displays aggregates at lower concentration, which show a size dependence on concentration. In order to stabilize the aggregates, crosslinking via oxime formation is described, which enables preservation of aggregates at high dilution, in dialysis and in organic solvents. With adequate stability by crosslinking, double hydrophilic block copolymer (DHBC) aggregates open pathways for potential biomedical applications in the future.     

Thermo‐ and pH‐Sensitivities of Thiosemicarbazone‐Incorporated,Fluorescent and Amphiphilic Poly(N‐isopropylacrylamide)

Summary: Novel hydrophobic comonomer (p‐methacrylamido)acetophenone thiosemicarbazone was synthesized and polymerized with N‐isopropylacrylamide to get a series of amphiphilic copolymers. The self‐aggregation behavior and thermo‐sensitive character of the (co)polymers were confirmed by TEM observation, fluorescence spectra, and cloud point measurement. Fluorescence emission of copolymer was significantly strengthened or switched off at an excitation wavelength of 320 nm upon the addition of acid or base, respectively. Thermo‐sensitivity, pH‐sensitivity, and pharmacologically versatile thiosemicarbazone groups were integrated into these novel fluorescent and amphiphilic copolymers, which will develop the novel applications of amphiphilic copolymer and environment‐responsive materials.

Fluorescence intensity at 393 nm of polymer 2 in water at 18 °C excited at 320 nm upon addition of acid and base.     

Thermo‐Responsive Assembly of Chemically Reduced Graphene and Poly(N‐isopropylacrylamide)

A stable aqueous solution of reduced graphene is prepared by chemical reduction of GO in the presence of PNIPAM in water through the hydrophobic interaction between graphene plates and PNIPAM backbones. Thermosensitive solubility switching of the graphene/PNIPAM assembly was successfully demonstrated below or above the LCST, which is higher (up to 42 °C) than the LCST of pure PNIPAM (36 °C) because interacting PNIPAM chains on graphene plates are less mobile than free PNIPAM chains. The success of PNIPAM as stabilizer for reduced graphene solution can be explained by both efficient hydrophobic interaction with graphene surface and matching of surface tension of water (72.8 mJ · m^?2) with “graphene‐friendly surface tension” (40–50 mJ · m^?2) by the presence of PNIPAM in water (41.8 mJ · m^?2).

     

Films of Highly Disperse Electrodeposited Poly(N‐vinylcarbazole)–Graphene Oxide Nanocomposites

The electrodeposition of polymer nanocomposite thin films of PVK–GO is demonstrated. Highly exfoliated and stable graphene oxide (GO) solutions are prepared by incorporating poly(N‐vinylcarbazole) (PVK) through mixing. Enhanced stability up to 30 d is observed in both aqueous and organic solvents. TGA, XRD, FTIR, and UV‐vis measurements confirm nanocomposite formation. CV enables electrodeposition of the films. The presence of GO on the PVK–GO surface is confirmed by the appearance of the C=O and OH stretching vibrations, attributed to the carboxylic and hydroxyl groups of GO. AFM measurements show homogeneous and well‐defined film morphology.