Relationship between the volume phase transition and the surface area of poly (N-isopropylacrylamide) and poly (N-isopropylacrylamide-co-sodium methacrylate) hydrogels

The volume phase transition of poly(N-isopropylacrylamide) (PNIPAAm) and poly(N-isopropylacrylamide-co-sodium methacrylate) (P(NIPAAm-co-SMA)) hydrogels was investigated with consideration of the pore characteristics. The volume phase transition temperature of the hydrogel increased by incorporating sodium methacrylate (SMA). Based on the BET equation, the surface area was evaluated by a gas adsorption method. The surface area of PNIPAAm and P(NIPAAm-co-SMA) hydrogels decreased with increasing temperature, resulting from the fact that the depth and the size of pores decreased significantly in the course of the volume phase transition. Hence, it is suggested that the change of surface area has a close relationship with the volume phase transition of PNIPAAm and P(NIPAAm-co-SMA) hydrogels.     

Preparation and properties of poly(N‐isopropylacrylamide)/poly(N‐isopropylacrylamide) interpenetrating polymer networks for drug delivery

A novel poly(N‐isopropylacrylamide) (PNIPA)/PNIPA interpenetrating polymer network (IPN) was synthesized and characterized. In comparison with conventional PNIPA hydrogels, the shrinking rate of the IPN hydrogel increased when gels, swollen at 20 °C, were immersed in 50 °C water. The phase‐transition temperature of the IPN gel remained unchangeable because of the same chemical constituent in the PNIPA gel. The reswelling kinetics were slower than those of the PNIPA hydrogel because of the higher crosslinking density of the IPN hydrogel. The IPN hydrogel had better mechanical strength because of its higher crosslinking density and polymer volume fraction. The release behavior of 5‐fluorouracil (5‐Fu) from the IPN hydrogel showed that, at a lower temperature, the release of 5‐Fu was controlled by the diffusion of water molecules in the gel network. At a higher temperature, 5‐Fu inside the gel could not diffuse into the medium after a burst release caused by the release of the drug on the surface of the gel. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1249–1254, 2004     

Preparation and characterization of poly(N-isopropylacrylamide)-modified poly(2-hydroxyethyl acrylate) hydrogels by interpenetrating polymer networks for sustained drug release

In order to investigate the influence of hydrophobic moieties formed by poly(N-isopropylacrylamide) (PNIPAm) in a hydrogel matrix on the release behavior of the hydrogel, a series of poly(N-isopropylacrylamide) (PNIPAm)-modified poly(2-hydroxyethyl acrylate-co-2-hydroxyethyl 2-hydroxyethyl methacrylate) (P(HEA-co-HEMA)) via interpenetrating polymer networks (IPNs) were prepared by a sequential UV solution polymerization. Interestingly, it was found that P(HEA-co-HEMA)/PNIPAm IPN indicated a single glass transition temperature (T(g)) and the T(g)s of the IPNs increased with an increase in the PNIPAm component. This phenomenon may be attributed to hydrogen bonding between the two polymer networks, but the hydrogen bonding exerts less influence on the swelling behavior of the IPNs, due to the fact that IPNs can respond to changes in temperature like PNIPAm. Using 2-[(diphenylmethyl)sulphiny]acetamide (modafinil, MOD) and p-hydroxybenzoic acid (HBA) as model drug compounds, the release behavior of the IPNs was studied at body temperature, and it was found that the presence of PNIPAm could retard drug release regardless of the solubility of the drugs. Release profiles of HBA from the IPNs and their component samples as a function of time at 37 degrees C.     

Semi‐interpenetrating polymer networks composed of poly(N‐isopropyl acrylamide) and polyacrylamide hydrogels

Three series of semi‐interpenetrating polymer networks, based on crosslinked poly(N‐isopropyl acrylamide) (PNIPA) and 1 wt % nonionic or ionic (cationic and anionic) linear polyacrylamide (PAAm), were synthesized to improve the mechanical properties of PNIPA gels. The effect of the incorporation of linear polymers into responsive networks on the temperature‐induced transition, swelling behavior, and mechanical properties was studied. Polymer networks with four different crosslinking densities were prepared with various molar ratios (25:1 to 100:1) of the monomer (N‐isopropyl acrylamide) to the crosslinker (methylenebisacrylamide). The hydrogels were characterized by the determination of the equilibrium degree of swelling at 25 °C, the compression modulus, and the effective crosslinking density, as well as the ultimate hydrogel properties, such as the tensile strength and elongation at break. The introduction of cationic and anionic linear hydrophilic PAAm into PNIPA networks increased the rate of swelling, whereas the presence of nonionic PAAm diminished it. Transition temperatures were significantly affected by both the crosslinking density and the presence of linear PAAm in the hydrogel networks. Although anionic PAAm had the greatest influence on increasing the transition temperature, the presence of nonionic PAAm caused the highest dimensional change. Semi‐interpenetrating polymer networks reinforced with cationic and nonionic PAAm exhibited higher tensile strengths and elongations at break than PNIPA hydrogels, whereas the presence of anionic PAAm caused a reduction in the mechanical properties. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3987–3999, 2004     

The role of water in structural changes of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide) studied by FTIR, Raman spectroscopy and quantum chemical calculations

Temperature-induced phase transition in water solutions of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamide) (PNIPMAM) have been studied by ATR FTIR and Raman spectroscopy in combination with quantum chemical calculations. The presence or absence of the α-methyl group has a strong effect on the physical structure of water solutions. Although the hydrophobic interactions for PNIPMAM and PNIPAM are very similar, PNIPMAM with additional methyl group exhibits significantly weaker intermolecular interactions between the amide groups. That effect is the cause of the higher transition temperature T_t by about 8 °C for PNIPMAM compared to PNIPAM due to the formation of larger compact structures. The presence of the methyl group is significant for the reversibility of the temperature transition during the backward cooling as the dissolution of more stable compact PNIPMAM requires overcoming of a higher energy barrier and shows a strong hysteresis.     

Photocrosslinked hydrogels based on copolymers of poly(ethylene glycol) and lysine

A group of new, water-soluble poly(ether-urethane)s, derived from poly(ethylene glycol) and the amino acid L -lysine, provide pendent carboxylic acid groups along the polymer backbone at regular intervals. The carboxylic acid groups were utilized for the attachment of acrylate and methacrylate pendent chains (hydroxyethyl acrylate, hydroxyethyl methacrylate, aminoethyl methacrylate, and aminoethyl methacrylamide), leading to functionalized polymers. The pendent chains were attached via ester and/or amide bonds having different degrees of hydrolytic stability. The attachment reactions proceeded with high yields (up to 95%). The functionalized polymers were subsequently photopolymerized (UV irradiation) to obtain crosslinked hydrogels. Crosslinked membranes with the highest degree of mechanical strength were obtained when the crosslinking reaction was performed in dioxane with benzoin methyl ether (0.1 wt %) as the initiator. the crystallinity, thermomechanical properties, and hydrolytic stability of the crosslinked membranes were studied. All membranes were transparent and highly swellable (equilibrium water content: 64–88%). The tensile strength in the swollen state ranged from 0.15 to 1.09 MPa. Under physiological conditions (phosphate buffered water, 0.1M, pH 7.4, 37°C) the hydrolytic stability of the hydrogels varied depending on the bonds used in the attachment of the acrylate pendent chains: Hydrogels with hydroxyethyl acrylate pendent chains dissolved within 30 days, while hydrogels containing aminoethyl methacrylamide pendent chains remained unchanged throughout a 30 day period. Using high molecular weight FITC-dextrans as model compounds, complete release from the swollen hydrogels required between 60 and 150 h. Overall, the evaluation of poly(ethylene glycol)-lysine derived, photocrosslinked hydrogels indicated that these materials provide a range of potentially useful properties. © 1994 John Wiley & Sons, Inc.     

MMTCA recognition by molecular imprinting in interpenetrating polymer network hydrogels based on poly(acrylic acid) and poly(vinyl alcohol)

A novel IPN hydrogel designed to recognize MMTCA is prepared by applying the molecular-imprinting method. The IPN is characterized by FT-IR, DSC, and SEM. Langmuir analysis shows that an equal class of adsorption is formed in the hydrogel. The adsorption equilibrium constant and the maximum adsorption capacity are evaluated, and the effect of the pH on MMTCA adsorption is discussed. The selectivity of the imprinted polymer for MMTCA is studied in aqueous solutions of MMTCA/aspirin/riboflavin. The results suggest that the MMTCA-imprinted polymer shows superior selectivity for MMTCA as compared to riboflavin and aspirin. The reproducibility of the imprinted polymer to MMTCA is also studied.     

Factors driving the protonation of poly(N-vinylimidazole) hydrogels

Poly(N-vinylimidazole) hydrogels immersed in aqueous acid solutions produce an increment in the pH of the bath because of proton uptake by basic imidazole moieties, leading to hydrogel protonation. Both kinetic and equilibrium measurements of the pH of the bath have been performed under a variety of conditions and with different hydrogel samples. The kinetics of the xerogel protonation process (which includes solvent and titrant diffusion, the true protonation reaction or ion–dipole association, and the polymer relaxation to a new conformation) are mostly driven by the size of the hydrogel sample, whereas other magnitudes, such as the initial pH, the effective polymer concentration, and the network structure, governed by the crosslinker ratio and total comonomer concentration in the feeding, have a minor influence. pK_a changes with the degree of protonation (α), delimitating two different regions: (1) a broad α range in which pK_a decreases with increasing α but less pronouncedly with increasing ionic strength and (2) an α range close to α = 1 in which pK_a decreases abruptly, more markedly with sulfate than with chloride counteranions and with larger ionic strengths. In the first region, pK_a is determined by repulsive electrostatic interactions and so is larger for titration with H₂SO₄ than with HCl and increases as the effective polymer concentration and ionic strength increase. Two steps (i.e., two protonation sites) can be observed in the titration curves, the second one corresponding to abrupt changes in the basicity of the second pK_a-versus-α region. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2294–2307, 2004     

交联密度对聚氨酯／聚苯乙烯互穿聚合物网络动态力学性能的影响

采用同步法合成了聚氨酯／聚苯乙烯（ＰＵ／ＰＳ）互穿聚合物网络（ＩＰＮ）．通过改变ＮＣＯ／ＯＨ比、三元醇／二元醇（３ＯＨ／２ＯＨ）比、异氰酸酯类型、多元醇分子量、二乙烯基苯含量及丙烯酸β－羟乙酯的含量，研究了单网及网间交联密度对ＰＵ／ＰＳＩＰＮ动态力学性能影响的规律．结果表明：对相容性差的ＰＵ／ＰＳＩＰＮ体系，采用增加交联密度、加快网络固化速度导致“强迫互容”，能提高两网的互穿与缠结，增进组份的互容．但单网固化速度过快相容性反而下降．     

Bistable photoswitching in poly(N-isopropylacrylamide) with spironaphthoxazine hydrogel for optical data storage

A bistable switching photochromic poly(N-isopropylacrylamide) with spironaphthoxazine hydrogel copolymer (PNIPA-SPO-BIS) has been designed and synthesized by radical polymerization. The PNIPA-SPO-BIS copolymer is identified by ¹H NMR spectroscopy, FIT-IR spectroscopy and gel permeation chromatography (GPC). The morphology of the internal microstructures of the PNIPA-SPO-BIS hydrogels was observed by scanning electron microscopy (SEM). The PNIPA-SPO-BIS copolymer showed excellent photochromic behavior in water solution and in gel state. In addition, erasable and rewritable (EARW) photoimaging on the PNIPA-SPO-BIS hydrogel was successfully demonstrated. A novel optical data storage materials based on photochromic hydrogel was developed. These developments are crucial for fundamental studies and eventual technical application for all-photo mode high-density optical data storage.     

New interpenetrating polymer networks based on uralkyd/poly(glycidyl methacrylate)

Semi- and full-interpenetrating polymer networks (IPNs) based on uralkyd resin (UA)/poly(glycidyl methacrylate) were synthesized in the laboratory by the sequential technique. Infrared spectra of the homopolymers and the IPNs were studied. A study of the mechanical properties viz. tensile strength and elongation percentage was carried out. The apparent densities of the homopolymers and their IPNs were determined and compared. Glass transition studies of the IPNs were conducted with the aid of differential scanning calorimetry (DSC). Phase morphology of the IPNs was observed using scanning electron microscopy (SEM). DSC results revealed a single glass transition temperature (T_g) for both the semi- as well as the full-IPNs suggesting good interpenetration in them. The SEM micrographs as well as the IR-spectra gave an indication that apart form the interpenetration phenomena, grafting reaction between the -NCO groups of UA and the epoxy group of glycidyl methacrylate might have occurred to some extent.     

Novel poly(N‐isopropylacrylamide)/clay/poly(acrylamide) IPN hydrogels with the response rate and drug release controlled by clay content

Novel interpenetrating network (IPN) hydrogels (PNIPAAm/clay/PAAm hydrogels) based on poly(N‐isopropylacrylamide) (PNIPAAm) crosslinked by inorganic clay and poly(acrylamide) (PAAm) crosslinked by organic crosslinker were prepared in situ by ultraviolet (UV) irradiation polymerization. The effects of clay content on temperature dependence of equilibrium swelling ratio, deswelling behavior, thermal behavior, and the interior morphology of resultant IPN hydrogels were investigated with the help of Fourier transform infrared spectroscopy, differential scanning calorimeter (DSC), scanning electron microscope (SEM). Study on temperature dependence of equilibrium swelling ratio showed that all IPN hydrogels exhibited temperature‐sensitivity. DSC further revealed that the temperature‐sensitivity was weakened with increasing amount of clay. Study on deswelling behavior revealed that IPN hydrogels had much faster response rate when comparing with PNIPAAm/clay hydrogels, and the response rate of IPN hydrogels could be controlled by clay content. SEM revealed that there existed difference in the interior morphology of IPN hydrogels between 20 [below lower critical solution temperature (LCST)] and 50 °C (above LCST), and this difference would become obvious with a decrease in clay content. For the standpoint of applications, oscillating swelling/deswelling behavior was investigated to determine whether properties of IPN hydrogels would be stable for potential applications. Bovine serum albumin (BSA) was used as model drug for in vitro experiment, the release data suggested that the controlled drug release could be achieved by modulating clay content. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 96–106, 2009     

Investigation of swelling,drug release and diffusion behaviors of poly(N‐isopropylacrylamide)/poly (N‐vinylpyrrolidone) full‐IPN hydrogels

The synthesis of sequential full interpenetrating polymer networks (IPNs) based on poly (N‐isopropylacrylamide) (PNIPAAm) and negatively charged poly(N‐vinyl‐2‐pyrrolidone) (PNVP) was described and their swelling, drug release, and diffusion studies were investigated. PNIPAAm was used as a host network. According to swelling experiments, IPNs gave relatively lower swelling ratios compared to PNIPAAm hydrogel due to the higher cross‐linking density. Lidocaine (LD) was used as a model drug for the investigation of drug release behavior of IPNs. LD uptake of the IPNs were found to increase from 24 to 166 (mg LD / g dry gel) with increasing amount of PNIPAAm and AMPS contents in the IPN structure. It was observed that the specific interaction between drug and AMPS co‐monomer influenced the drug release profile. In the diffusion transport mechanism study in water, the results indicated that the swelling exponents n for all IPNs are in the range from 0.50 to 0.72. This implies that the swelling transport mechanism was transferred from Fickian to non‐Fickian transport, with increasing AMPS content and NIPAAm character in the IPN structure. In addition, diffusion of LD within the IPNs showed similar trend. The incorporation of AMPS leads to an increase in electrostatic interaction between charge sites on carboxylate ions and cationic LD molecules. Therefore, the highest diffusion coefficient (D) of drug was found for IPN2 sample. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of poly(ethylene oxide) on the release behaviors of poly(epsilon-caprolactone) microcapsules containing erythromycin

The biodegradable poly(epsilon-caprolactone) (PCL)/poly(ethylene oxide) (PEO) microcapsules and the analyzing of form and features for the manufacturing conditions were investigated in a prospective drug delivery systems (DDS) through drug release. The effects of emulsifier, emulsifier concentration, and stirring rate on the diameter and form of the microcapsules were examined using image analyzer (IA) and scanning electron microscope (SEM). The role of interfacial adhesion between PCL/PEO and drug was determined by contact angle measurements, and the drug release rate of the microcapsules was characterized by UV-vis spectroscopy. As a result, the microcapsules were made in spherical forms with a mean particle size of 170 nm approximately 68 microm. And the work of adhesion between water and PCL/PEO was increased with increasing the PEO content, which is due to higher hydrophilicity of PEO. The drug release rate of the microcapsules was significantly increased as the PEO content increased, which could be attributed to the increasing of the hydrophilic groups or the degree of adhesion at the interfaces.     

In vitro and in vivo degradation behaviors of thermosensitive poly(organophosphazene) hydrogels

Biodegradable and thermosensitive poly(organophosphazenes) with various substituents were synthesized and their hydrolytic degradation properties were investigated in vitro and in vivo. The aqueous solutions of all polymers showed a sol-gel phase transition behavior depending on temperature changes. The side groups of polymers significantly affected the polymer degradation and accelerated hydrolysis of polymers in the order of carboxylic acid > depsipeptide > without carboxylic acid and depsipeptide. The increased gel strength led to the decreased hydrolysis rate. The polymer hydrogels with 750 Da of α-amino-ω-methoxy poly(ethylene glycol) were rapidly decreased by dissolution. The polymer degradation was also influenced by pH and temperature. The in vivo behaviors of mass decrease of the polymer hydrogels were similar with the in vitro results. These results suggest that the biodegradable and thermosensitive poly(organophosphazenes) hold great potentials as an injectable and biodegradable hydrogel for biomedical applications with controllable degradation rate.     

Characterization of poly(vinyl alcohol) hydrogel for prosthetic intervertebral disc nucleus

In the recent years, development of intervertebral disc prosthesis has been of great concern to the world of medicine and science. Substitution of the spinal disc or its part being displaced or damaged due to trauma or a disease process for the artificial structure well imitating high tensile properties and elasticity of the real disc would highly improve the existing treatment techniques. In this work, the attempt to develop the PVA-based hydrogel material for artificial spinal disc has been made. The polymer was initially processed with the use of formaldehyde solution as a crosslinking agent and sulfuric acid as a catalyst. Then properties of the material have been altered by saturating the already existing PVA hydrogel with a mixture of hydrophilic and hydrophobic monomers (2-hydroxyethyl methacrylate and methyl methacrylate) and a subsequent exposure to ionizing radiation (⁶⁰Co source). In this way, interpenetrating polymer network has been built on the crosslinked PVA scaffold. Resulting structures were tested for their mechanical behavior at different loads. Series of measurements leading to the determination of the physicochemical properties of created gels including crosslink density and swelling abilities were also performed.     

3D diamond‐containing nanocomposites based on hybrid polyurethane–poly(2‐hydroxyethyl methacrylate) semi‐IPNs: Composition‐nanostructure‐segmental dynamics‐elastic properties relationships

Nanostructure, glass transition dynamics and elastic properties were studied in the 3D nanodiamond‐containing composites based on polyurethane‐poly(2‐hydroxyethyl methacrylate) semi‐interpenetrating polymer networks (PU‐PHEMA semi‐IPNs), neat PU or PHEMA matrices. Nanodiamond (ND) content in the nanocomposites varied from 0.25 to 3 wt %. Combined differential scanning calorimetry/ laser‐interferometric creep rate spectroscopy/atomic force microscopy approach was utilized. A large impact of small 3D ND additives on PU‐PHEMA networks' dynamics and properties was revealed under conditions when an average inter‐particle distance L exceeds by far gyration radius R_g. The pronounced heterogeneity of glass transitions' dynamics and two opposite effects were observed. The main effect was a strong suppression of PHEMA glass transition dynamics at 90–180 °C, with the enhancement of creep resistance and threefold to sixfold increasing modulus of elasticity. The peculiarly crosslinked structure of nanocomposites, due to double covalent hybridization, resulted in low rheological percolation threshold, and a synergistic effect in dynamics was observed. Less pronounced effect of accelerating dynamics in the temperature region between β‐ and α‐transitions in PHEMA was associated with dynamics in domains with loosened molecular packing. The distinct physical limit for “anomalous” decreasing T_g is predicted in terms of the notion of the common segmental nature of α‐ and β‐relaxations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1696–1712, 2008     

Preparation and characterization of polyfuran/poly(2‐fluoroaniline) conducting composites

This article deals with the chemical synthesis and characterization of poly(2‐fluoroaniline) (P2FAn) and polyfuran (PFu) homopolymers and PFu/P2FAn and P2FAn/PFu composites. P2FAn and PFu homopolymers were synthesized using ammonium persulfate and antimony (III) chloride as catalyst, respectively. These homopolymers and composites were studied in the doped state using Fourier transform infrared spectroscopy and ultraviolet–visible absorption spectroscopy, thermogravimetric analysis, scanning electron microscopy, four‐probe conductivity technique, and Gouy Scale measurements. PFu/P2FAn and P2FAn/PFu composites were found to possess different thermal, conductivity, electronic, and morphological properties from each other when synthesis order of guest and host polymers was varied. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3359–3367, 2004     

Preparation and characterization of macroporous poly(N‐isopropylacrylamide) hydrogels for the controlled release of proteins

Macroporous, temperature‐sensitive poly(N‐isopropylacrylamide) (PNIPAAm) hydrogels were synthesized with poly(ethylene glycol)s (PEGs; molecular weight = 2000–6000) as the pore‐forming agents. The influence of the molecular weight and PEG content on the responsive kinetics of these macroporous hydrogels was investigated. The PEG‐modified PNIPAAm hydrogels were characterized by the swelling ratio, deswelling–reswelling kinetics, Fourier transform infrared, and differential scanning calorimetry. The morphology of these hydrogels was analyzed with scanning electron microscopy. The prepared macroporous hydrogels exhibited some unique properties in comparison with the gels with low molecular weight PEGs (molecular weight < 2000) as the pore‐forming agents. In addition, a preliminary study on the controlled release of bovine serum albumin from these macroporous hydrogels was carried out. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 152–159, 2003     

Interpenetrating polymer network (IPN) nanogels based on gelatin and poly(acrylic acid) by inverse miniemulsion technique: synthesis and characterization

Novel interpenetrating polymer network (IPN) nanogels composed of poly(acrylic acid) and gelatin were synthesised by one pot inverse miniemulsion (IME) technique. This is based on the concept of nanoreactor and cross-checked from template polymerization technique. Acrylic acid (AA) monomer stabilized around the gelatin macromolecules in each droplet was polymerized using ammonium persulfate (APS) and tetramethyl ethylene diamine (TEMED) in 1:5 molar ratio and cross-linked with N,N-methylene bisacrylamide (BIS) to form semi-IPN (sIPN) nanogels, which were sequentially cross-linked using glutaraldehyde (Glu) to form IPNs. Span 20, an FDA approved surfactant was employed for the formation of homopolymer, sIPN and IPN nanogels. Formation of stable gelatin-AA droplets were observed at 2% surfactant concentration. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) studies of purified nanogels showed small, spherical IPN nanogels with an average diameter of 255 nm. In contrast, sIPN prepared using the same method gave nanogels of larger size. Fourier-transform infrared (FT-IR) spectroscopy, SEM, DLS, X-ray photoelectron spectroscopy (XPS) and zeta potential studies confirm the interpenetration of the two networks. Leaching of free PAA chains in sIPN upon dialysis against distilled water leads to porous nanogels. The non-uniform surface of IPN nanogels seen in transmission electron microscopy (TEM) images suggests the phase separation of two polymer networks. An increase of N/C ratio from 0.07 to 0.17 (from PAA gel to IPN) and O/C ratio from 0.22 to 0.37 (from gelatin gel to IPN) of the nanogels by XPS measurements showed that both polymer components at the nanogel surface are interpenetrated. These nanogels have tailoring properties in order to use them as high potential drug delivery vehicles for cancer targeting.