N-isopropylacrylamide Content Research Articles

Constructing self-healing high-strength elastomer with condensed state microphase separation by solubilization and copolymerization

Self-healing is a fascinating property for materials, but it is still a challenge to simply, effectively and greenly prepare self-healing material. Here, a self-healing elastomer was fabricated by solubilization copolymerization of ethylene glycol methyl ether acrylate (MEA) and N-isopropylacrylamide (NIPAM). When NIPAM content was 40%, “strong condensed state” emerged and led to obvious microphase separation, which greatly improved stress to 3.5 MPa at break (≈650%). Meantime, because the condensed state was still weak in the copolymer, the dissociation and re-association could occur to improve self-healing performance. The copolymer had about 60% self-healing efficiency after 8h at 100 °C, and the actual stress was 2 MPa at break (≈500%). The copolymer with good solubility can be easily recycled and reprocessed. A wrinkled skin-core conductive filament also was prepared by wet spinning and stretching coating, which was conductive and stretchable. And the self-healing core can drive conductive PEDOT: PSS shell to re-touch after break. • The solubilization copolymerization preparation is more simple, effective and green. • Microphase separation structure are formed by polymerizing monomer with micellar distribution. • NIPAM condensed state can achieve dissociation and re-association to improved self-healing performance. • Copolymers have good mechanical properties, self-healing properties, recyclability and secondary processability.

NIPAm-Based Modification of Poly(L-lysine): A pH-Dependent LCST-Type Thermo-Responsive Biodegradable Polymer.

Polylysine is a biocompatible, biodegradable, water soluble polypeptide. Thanks to the pendant primary amines it bears, it is susceptible to modification reactions. In this work Poly(L-lysine) (PLL) was partially modified via the effortless free-catalysed aza-Michael addition reaction at room temperature by grafting N-isopropylacrylamide (NIPAm) moieties onto the amines. The resulting PLL-g-NIPAm exhibited LCST-type thermosensitivity. The LCST can be tuned by the NIPAm content incorporated in the macromolecules. Importantly, depending on the NIPAm content, LCST is highly dependent on pH and ionic strength due to ionization capability of the remaining free lysine residues. PLL-g-NIPAm constitutes a novel biodegradable LCST polymer that could be used as “smart” block in block copolymers and/or terpolymers, of any macromolecular architecture, to design pH/Temperature-responsive self-assemblies (nanocarriers and/or networks) for potential bio-applications.

Preparation and Drug Release Properties of a Thermo Sensitive GA Hydrogel.

A high-strength galactomannan (GA)-based hydrogel with thermal response and pH response is introduced in this paper. GA, N-isopropylacrylamide (NIPAM), N-[3-dimethylamino)propyl]methylacrylamide (DMAPMA), and montmorillonite were used to form hydrogels through a simple mixed static system. Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) were used to characterize the structure and properties of the hydrogels. The compressive strength of the the hydrogel increased from 23.9 to 105.61 kPa with the increase of GA dosage from 0 to 1.5 wt%. When the NIPAM content in the monomer increased from 75% to 95%, the lower critical solution temperature (LCST) of the hydrogel changed from 36.5 to 45.8 °C. When the monomer content was higher than 10wt%, the swelling kinetics of the sample changed from the second-order equation to the first-order equation. With the increase of the proportion of NIPAM monomer, the release rate of bovine serum album in the early stage was faster, and the cumulative release rate was close to 100%.The release rate of bovine serum albumin at 37 °C was higher than that at 25 °C. The release rate of the hydrogel containing bovine serum albumin was the fastest under the condition of pH 7.4, followed by those at pH 6.6 and pH 5.0. The results showed that this thermal-responsive hydrogel has potential applications as a drug carrier for colon delivery.

Investigation of the Effects of Different Hydrophilic and Hydrophobic Comonomers on the Volume Phase Transition Temperatures and Thermal Properties of N-Isopropylacrylamide-Based Hydrogels

In this work, a series of thermally responsive terpolymers of N-isopropylacrylamide (NIPA) with three different comonomer contents was synthesized, and their swelling behaviour was studied as a function of composition and temperature. Temperature-sensitive, random cross-linked terpolymers of NIPA were prepared from methyl methacrylate (MMA), N-tert-butylacrylamide (NTBA), and acrylic acid (AA) using a free radical polymerization method. In the synthesis of terpolymer hydrogels, N,N-methylene bisacrylamide (BIS) was used as cross-linkers and ammonium persulfate (APS) as initiator. The NIPA content of the monomer feed varied from 80 to 50 mol %, and other comonomer feed varied from 40 to 5 mol %. The swelling equilibrium of these hydrogels was studied as a function of temperature and hydrophobic and hydrophilic comonomer contents. The swelling properties of the polymers were investigated in pure water at temperatures from 10 to 80°C. All of the synthesized gels were found to be sensitive to temperature. Glass transition temperature analyses and thermal analyses of the synthesized hydrogels were studied. The volume phase transition temperature and the swelling equilibrium (r) values of NIPA-based hydrogels synthesized in different feed ratios and in varying monomer contents were found in the range of 17–52°C and 14–51 g H2O/g polymer, respectively. The glass temperature (Tg) of the NIPA/AA/(MMA or NTBA) hydrogels synthesized with feed ratios of 50/40/10 was found to be 133 or 142°C, respectively. The initial and the end degradation that were determined for hydrogels at high temperatures indicated the quite good thermal stability of hydrogels. When the mass loss of the synthesized hydrogels was 50%, the temperatures were measured between 247 and 258°C.

Thermo-Responsive Starch-g-(PAM-co-PNIPAM): Controlled Synthesis and Effect of Molecular Components on Solution Rheology.

A series of highly branched random copolymers of acrylamide (AM) and N-isopropylacrylamide (NIPAM) have been prepared from a waxy potato starch-based macroinitiator by aqueous Cu0-mediated living radical polymerization (Cu0-mediated LRP). The NIPAM intake in the copolymer was varied between 0% and 50 mol % to evaluate the influence of chain composition on the aqueous rheological properties as well as their low critical solution temperature (LCST). The viscosity of the copolymer was found to increase with the NIPAM intake and an LCST can be observed when the NIPAM content is high enough (e.g., 50 mol %). In addition, thermo-thickening behavior was observed at a low shear rate (γ ≤ 10 s−1) and higher NIPAM content was found to shift the onset of thermo-thickening behavior to a lower temperature. However, the absolute increase in viscosity values is reduced with the NIPAM intake. Besides this, an interesting significant thermo-thickening behavior was also observed on highly branched starch-g-polyacrylamide at high temperatures (>80 °C), which has not been previously reported. Rheology tests also revealed a good salt-resistant property in copolymers with low NIPAM content (e.g., <25 mol %). Considering the viscosity profile in saline as compared to that in pure water, this NIPAM intake seems to represent an optimum balance of viscosity and salt-resistance performance.

Ultrasound-responsive NIPAM-based hydrogels with tunable profile of controlled release of large molecules

Episodic release of bioactive compounds plays an important role in biological systems. “On-demand” release systems which based on polymeric materials and activated by external stimuli may provide the necessary functionality. Here we describe an ultrasound-responsive hydrogel based on N-isopropylacrylamide (NIPAM) and N,N′-methylenebisacrylamide (MBAm), which is suitable for triggered release of two large molecules: bovine serum albumin (BSA, 66kDa) and dextran (3–5kDa). It is shown that the release amount of these two large molecules increased with increasing hydrogel temperature, and the application of ultrasound further increased the release. By simply adjusting the contents of NIPAM and MBAm, the difference of BSA release between the presence and absence of ultrasound could be adjusted from 2.7 to 84 folds. There was also a positive correlation between the ultrasound intensity and release amount. These properties made the NIPAM-based hydrogel a tunable platform for focal drug delivery.

Mucoadhesive nanogels by ionotropic crosslinking of chitosan-g-oligo(NiPAam) polymeric micelles as novel drug nanocarriers.

To investigate a novel kind of mucoadhesive nanogel based on the supramolecular aggregation of chitosan-g-oligo(N-isopropylacrylamide) copolymers. Copolymers were synthesized by the graft-free radical polymerization of N-isopropylacrylamide on chitosan. The aggregation was studied by dynamic light scattering and nanoparticle tracking analysis (NTA), the nanostructure by transmission electron microscopy(TEM)/cryo-TEM, the mucoadhesiveness in vitro with mucin and the cytocompatibility in Caco2 cells. Copolymers (36-74% w/w N-isopropylacrylamide content) showed critical micellar concentration between 2.0 and 40.0 × 10(-3)% w/v and micelles were nanometric and positively charged. Physical stabilization was achieved with ionotropic crosslinking. TEM/cryo-TEM revealed multimicellar aggregates with good mucoadhesion and cytocompatibility properties. Micellar systems (1-10% w/v) increased the solubility of efavirenz up to 1249-fold. Results support the potential of these nano-drug delivery systems for improved mucosal administration of hydrophobic drugs.

PH- and temperature-responsive hydrogels of acrylic acid, N-isopropylacrylamide and a non-ionic surfmer: phase behaviour, swelling properties and drug release

Copolymer hydrogels composed of N-isopropylacrylamide (NIPAM), acrylic acid (AA) and the non-ionic surfactant monomer (surfmer) ω-methoxy poly(ethylene oxide)40undecyl-α-methacrylate (PEO-R-MA-40) were prepared and studied with regard to swelling behaviour and drug release behaviour. The gels were prepared upon γ-irradiation of the corresponding aqueous comonomer solution in a one-step reaction. Transparent, stable hydrogels were obtained. Studies of light transmission indicate a dual pH- and T-responsive behaviour, which originates from the AA and NIPAM content of the gels, respectively. Presence of large amounts of surfmer increases the phase transition temperature, but also increases the network density, which lowers the permeability of the gels. Swelling properties and release of ibuprofen (Ibu) were studied in simulated gastric fluid (SGF, pH 1) and phosphate buffer solution (PBS, pH 6.8). It was found that swelling and release are controlled by the nature and quantity of comonomers, pH, temperature and ionic strength of the aqueous phase. Swollen gels shrink in SGF and PBS, whereas dry gels exhibit a strong swelling both in SGF and PBS. Copolymer gels of AA and surfmer exhibit a strong, linear release of Ibu in SGF and PBS. If NIPAM is copolymerized in the gel, the drug release is decelerated in SGF probably due to formation of hydrogen bonds between NIPAM and Ibu at low pH. For example, a gel composed of 10 % (w/w) NIPAM, 1 % (w/w) AA and 1.5 % (w/w) surfmer exhibits a release of 10 % within 2 h in SGF and 58 % within 20 h in PBS.

Elastic Properties of a Swollen PAAm-NIPA Copolymer with Various NIPA Contents

Copolymer based on cross-linked polyacrylamide (PAAm) having N-isopropylacrylamide (NIPA) was prepared and their elastic properties were studied as a function of NIPA contents. NIPA content dependency of the shear modulus, S of the PAAm-NIPA copolymers due to volume phase transition was measured using tensile testing technique at 30°C. It was observed that its shear modulus and toughness were found to be strongly dependent on NIPA content. It is understood that the shear modulus was found to increase with NIPA contents, keeping constant temperature at 30°C. Elastic properties of the PAAm-NIPA copolymers show compositional dependence.

Influence of multiple stimuli on the lower critical solution temperature of new cationic poly(N‐acryloyl‐N′‐ethylpiperazine‐co‐N‐isopropylacrylamide) solutions

ABSTRACTNew multi‐stimuli responsive cationic copolymers based on N‐acryloyl‐N′‐ethyl piperazine (AcrNEP) and N‐isopropylacrylamide (NIPAM) were prepared by thermal free‐radical solution polymerization in dioxane at 75 °C. The chemical composition of the copolymers was determined by 1H NMR spectroscopy and was found that the copolymers were slightly rich in NIPAM content than that of AcrNEP. The reactivity of the two monomers for the copolymerization reaction was evaluated by the extended Kelen‐Tüdös method. The distribution of monomer sequence in the copolymer chain was estimated using the terminal copolymerization model. The maximum tendency to alternation (∼ 70%) was at 60 mol % of AcrNEP in the monomer feed. The copolymers were readily soluble in water at room temperature at all compositions and exhibited well‐defined lower critical solution temperature (LCST) phenomenon. The influence of various stimuli such as pH, temperature, simple inorganic salts, and surfactants on the LCST of the copolymers was studied in detail. Simple inorganic salts such as sodium chloride, sodium bromide, and sodium sulfate showed a salting‐out effect while sodium iodide showed a salting‐in effect. The salting‐out coefficient of the salts were calculated using the Sestchenow method, and the salting trend followed the order SO42− &gt; Cl− &gt; Br− &gt; I−. The divalent salt was more effective in lowering the LCST than the monovalent salts. The cationic surfactant hexadecyl trimethylammonium bromide at concentrations above the critical micelle concentration caused a gradual increase in the LCST of the copolymer solutions. The intrinsic viscosity and light scattering behavior of the copolymers in water and in sodium chloride solutions were studied in detail. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1175–1183

Synthesis and characterization of thermoresponsive copolymers for drug delivery

We report the synthesis and characterization of two nontoxic, thermogelling drug delivery systems which are liquid at room temperatures but become a gel at physiological temperature (37°C) potentially leading to release of a drug molecule. We selected temperature as the stimulus for drug release as it is physiologically invariant. A free radical polymerization of N-isopropylacrylamide (NIPAM) and N-vinylpyrrolidone (VP) was carried out under nitrogen atmosphere in double-distilled water at two different temperatures (30°C and 70°C), and the copolymers obtained were characterized by various analytical techniques. The molar ratios of the two monomers were altered with increasing NIPAM content and their cloud point temperature or least critical solution temperature (LCST) was determined. The copolymer at 9:1 ratio of NIPAM to VP resulted in the formation of nanoparticle-based gel (NG1) at 30°C; however, at 70°C, a microgel (MG1) was formed. The LCST of the nanogel and microgel was 33.5-34°C and 36.5-37°C, respectively. Thus, both the copolymers are water soluble at room temperature, but distinct phases appear at physiological temperatures. We hypothesized that these copolymers on entrapment with a drug could be used for topical application to the skin or eye for controlled drug delivery applications. Toxicological studies revealed that the copolymers are nontoxic in HeLa cells. Finally, our experiments show that a model drug [bovine serum albumin (BSA)] is released at 37°C with zero-order kinetics and confirmed using multiple well-known mathematical models.

Injectable hydrogel as stem cell scaffolds from the thermosensitive terpolymer of NIPAAm/AAc/HEMAPCL

A series of biodegradable thermosensitive copolymers was synthesized by free radical polymerization with N-isopropylacrylamide (NIPAAm), acrylic acid (AAc) and macromer 2-hydroxylethyl methacrylate-poly(ɛ-caprolactone) (HEMAPCL). The structure and composition of the obtained terpolymers were confirmed by proton nuclear magnetic resonance spectroscopy, while their molecular weight was measured using gel permeation chromatography. The copolymers were dissolved in phosphate-buffered saline (PBS) solution (pH = 7.4) with different concentrations to prepare hydrogels. The lower critical solution temperature (LCST), cloud point, and rheological property of the hydrogels were determined by differential scanning calorimetry, ultraviolet-visible spectrometry, and rotational rheometry, respectively. It was found that LCST of the hydrogel increased significantly with the increasing NIPAAm content, and hydrogel with higher AAc/HEMAPCL ratio exhibited better storage modulus, water content, and injectability. The hydrogels were formed by maintaining the copolymer solution at 37°C. The degradation experiment on the formed hydrogels was conducted in PBS solution for 2 weeks and demonstrated a less than 20% weight loss. Scanning electron microscopy was also used to study the morphology of the hydrogel. The copolymer with NIPAAm/AAc/HEMAPCL ratio of 88:9.6:2.4 was bioconjugated with type I collagen for the purpose of biocompatibility enhancement. In-vitro cytotoxicity of the hydrogels both with and without collagen was also addressed.

Temperature-sensitive hydrogel prepared by graft polymerization of N-isopropylacrylamide onto macroradical Pluronic

A new series of thermo-responsive graft copolymers were prepared via consecutive Pluronic backbone-initiated by t-butylperoxybenzoate with N-isopropylacrylamide (NIPAAm) onto Pluronic by a radical-induced graft polymerization technique. The thermal characterizations of the graft copolymers were investigated by differential scanning calorimetry (DSC). The Pluronic- g-poly(NIPAAm) copolymers exhibited sol-to-gel transitions depending on chain length of the grafted poly(NIPAAm) onto Pluronic. The graft copolymers indicated that the hydrophobic–hydrophilic balance achieved by varying contents of NIPAAm was an important parameter in controlling the transition temperature of the graft copolymers in solution and stability of the resultant gels. The data suggest that graft copolymers may be attractive substrates for tissue engineering and drug delivery applications as the combination of mechanistically independent gelation techniques with regard to gelation kinetics.

Stimuli‐responsive cationic terpolymers by RAFT polymerization: Synthesis, characterization, and protein interaction studies

AbstractThe controlled synthesis and characterization of a range of stimuli responsive cationic terpolymers containing varying amounts of N‐isopropylacrylamide (NIPAM), 3‐(methylacryloylamino)propyl trimethylammonium chloride (MAPTAC), and poly(ethylene glycol)monomethyl methacrylate (PEGMA) is presented. The terpolymers were synthesized using reversible addition‐fragmentation chain transfer (RAFT) polymerization. Compositions of the terpolymers determined using 1H NMR were in close agreement to the theoretical values determined from the monomer feed ratios. GPC‐MALLS was used to analyze the molecular weight characteristics of the polymers, which were found to have low polydispersities (Mw/Mn 1.1–1.4). The phase transitions were studied as a function of PEGMA and NIPAM content using temperature controlled 1H NMR and turbidity measurements (UV‐Vis). The relationship between thermal stability and the comonomer ratio of the polymers was measured using thermogravimetric analysis (TGA). Protein interaction studies were performed to determine the suitability of the polymers for biological applications. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4021–4029, 2008

Stimuli-responsive poly(ampholyte)s containing L-histidine residues: synthesis and protonation thermodynamics of methacrylic polymers in the free and in the cross-linked gel forms

Methacrylate-structured poly(ampholyte)s were synthesized in the homopolymer and copolymer forms starting from the N-methacryloyl-L-histidine (MHist) and the N-isopropylacrylamide (NIPAAm). They were also obtained in the cross-linked (hydrogel) form, showing a close thermodynamic behaviour as that shown by the corresponding soluble free polymer analogues. Viscometric data revealed that the minimum hydrodynamic volume of the polymer at its isoelectric point (pH 5) shifted to lower pHs as the NIPAAm content increased, and beyond a critical low MHist content the reduced viscosity decreased, even at low pHs. The phenomenon was attributed to hydrophobic forces between the isopropyl groups outweighing the repulsive electrostatic interactions of the polymer in the positively charged form. A similar behaviour was shown by the corresponding hydrogel. The latter also revealed a different phase transition phenomenon induced by external stimuli (temperature, pH, ionic strength, electric current) when compared to the acrylate-structured analogues. The polyMHist, as well as the corresponding monomer, was found for two days to be non toxic against the mouse osteoblasts (MC3T3-E1).

Botryoidal Assembly of Cholesteryl−Pullulan/Poly(N-isopropylacrylamide) Nanogels

Hybrid nanogels consisting of cholesteryl-modified pullulan (CHP) and poly(N-isopropylacrylamide) (PNIPAM) were synthesized by graft free-radical copolymerization of N-isopropylacrylamide (NIPAM) onto methacryloyl-substituted CHP nanogels (CHPMA) in water at 50 degrees C in the presence of a water-soluble free radical initiator. Depending on the initial NIPAM/CHPMA ratio, CHP-PNIPAM (CN) nanogels containing 30.8-84.8 wt % PNIPAM were obtained in the form of self-assembled nanoparticles with a hydrodynamic radius (Rh) of 69.0-116.0 nm in water kept at 20 degrees C. Hybrid nanogels of sufficiently high NIPAM content, such as the sample CN90, which contains 79.6 wt % NIPAM, exhibited a two-step response to changes in solution (3 mg/mL) temperature: a decrease in Rh from 93 to 57 nm as the temperature increased from 20 to 35 degrees C, followed by a sharp increase in Rh from 57 nm to 90 nm at 55 degrees C. Both steps in this temperature response were reversible. The multistep response to temperature of the CN nanogels was attributed to the morphology of the nanogels, which are seen as consisting of grape-like (botryoidal) clusters of associated native nanogels held together via cholesteryl cross-linking points and held together by the grafted PNIPAM chains.

Stimuli-Responsive Polymers Based on l-Phenylalanine Residues: Protonation Thermodynamics of Free Polymers and Cross-Linked Hydrogels

Vinyl polymers carrying l-phenylalanine residues were synthesized in the free and in the cross-linked hydrogel form, as homo- and copolymers with N-isopropylacrylamide (NIPAAm). The protonation reaction thermodynamics of the COO- group were studied in aqueous media, at different temperatures and at increased concentrations of sodium chloride, mainly by potentiometry and solution calorimetry. The soluble polymer, namely poly(N-acryloyl-l-phenylalanine), and its copolymers with low NIPAAm content displayed characteristic features. Their basicity constants (log K), as well as the enthalpy (−ΔH°) changes in relation to α (degree of protonation) showed an abrupt drop at α = 0.5. This was ascribed to the formation of hydrogen bonds between the protonated and the ionized neighboring COO- groups. The process was driven by the side-chain aromatic rings that improved hydrophobic interactions. The entropy (ΔS°) changes sharply increased as a result of the increased macromolecular conformational freedom and the relea...

Initiator‐fragment incorporation radical copolymerization of divinylbenzene and N‐isopropylacrylamide with dimethyl 2,2′‐azobisisobutyrate: Formation of soluble hyperbranched polymer nanoparticle

AbstractThe copolymerization of divinylbenzene (DVB) and N‐isopropylacrylamide (NIPAm) with dimethyl 2,2′‐azobisisobutyrate of a concentration as high as 0.50 mol/L proceeded homogeneously without any gelation at 80 °C in N,N‐dimethylformamide, where the concentrations of DVB and NIPAm were 0.15 and 0.50 mol/L. The copolymer yield increased with time and leveled off over 50 min. Although DVB was consumed more rapidly than NIPAm, both comonomers were completely consumed in 50 min. The homogeneous polymerization system at 80 °C involved electron spin resonance‐observable propagating polymer radicals, the total concentration of which increased with time. The resulting copolymer was soluble in tetrahydrofuran, chloroform, acetone, ethyl acetate, acetonitrile, N,N‐dimethylformamide, dimethyl sulfoxide, and methanol, but insoluble in benzene, n‐hexane, and water. The copolymer showed an upper critical solution temperature (50 °C on cooling) in a methanol–water [11:3 (v/v)] mixture. Dimethyl 2,2′‐azobisisobutyrate fragments as high as 37–45 mol % were incorporated as terminal groups in the copolymers through initiation and primary radical termination. The contents of DVB and NIPAm were 10–30 mol % and 30–50 mol %, respectively. The intrinsic viscosity of the copolymer was very low (0.09 dL/g) at 30 °C in tetrahydrofuran despite high weight‐average molecular weight (1.2 × l06 by multi‐angle laser light scattering). These results indicate that the copolymer was of hyperbranched structure. By transmission electron microscopy observation, the individual copolymer molecules were visualized as nanoparticle of 6–20 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1609–1617, 2004

Thermal effects in the synthesis of thermoresponsive hydrogels of poly(N-isopropylacrylamide-co-acrylic acid)

N-isopropylacrylamide (NIPA) has been copolymerised with acrylic acid (AA) in the presence of N,N′-methylenebisacrylamide (BIS) as crosslinker. Incorporation of an activator with the free radical initiator enabled the reaction to be conducted to high conversion at 25°C. Use of a thin mould as reaction vessel yielded optically clear hydrogels of high swelling ratio in water, whereas opaque macroporous products of reduced swellability were obtained by polymerisation in a glass vial. Exothermicity was verified by monitoring the actual (as opposed to nominal) temperature within the reaction vessel as a function of time. It was found that the heat could be dissipated adequately when the reaction vessel has a high surface to volume ratio (mould). In contrast, the heat generated in a vial was sufficient to raise the temperature of the reaction medium to the critical deswelling temperature and thereby produce an opaque macroporous gel. Separate experiments in which the contents of NIPA, AA and BIS in a vial were varied showed that the dominant role was played by NIPA.

Solution properties and phase behaviour of copolymers of acrylic acid with N-isopropylacrylamide: the importance of the intrachain hydrogen bonding

The solution properties and the phase behaviour of random copolymers of N-isopropylacrylamide (NIPAM) with acrylic acid (AA) have been investigated. The mole fraction of NIPAM in these copolymers (x) varies from 0 to 0.29. At pH=3.00, i.e. when the AA groups are in the undissociated form, the intrinsic viscosity decreases substantially as x increases. This chain shrinkage is attributed to the formation of intrachain hydrogen bonds between the two complementary groups, AA and NIPAM. The weakening of the hydrophilic character and the appearance of hydrophobic properties with increasing x is further supported by fluorescence probing studies and potentiometric measurements of the aqueous copolymer solutions. Moreover, the phase behaviour of these copolymers in salt solution changes dramatically with x. The copolymer with x=0.10 presents, like the homopolymer poly(acrylic acid), an Upper Critical Solution Temperature (UCST) behaviour. On the contrary, the copolymer with x=0.29 presents, similar to the homopolymer poly(N-isopropylacrylamide), a Lower Critical Solution Temperature (LCST) behaviour. Finally, the copolymer with the intermediate NIPAM content, x=0.17, presents both an UCST and an LCST behaviour.