Preparation and characterization of sodium carboxymethylcellulose/poly(N-isopropylacrylamide)/clay semi-IPN nanocomposite hydrogels

A new kind of pH-/temperature-responsive semi-interpenetrating polymer network hydrogels based on linear sodium carboxymethylcellulose (CMC) and poly(N-isopropylacrylamide) (PNIPA) cross-linked by inorganic clay (CMC/PNIPA/Clay hydrogel) was prepared. The temperature- and pH-responsive behaviors, the mechanical properties of these hydrogels were investigated. The CMC/PNIPA/Clay hydrogels exhibited a volume phase transition temperature around 32 °C with no significant deviation from the conventional PNIPA hydrogels. The swelling ratio of the CMC/PNIPA/Clay hydrogels gradually decreased with increasing the contents of clay. The influence of pH value on swelling behaviors showed that there is a maximum swelling ratio at pH 5.9. Moreover, the CMC/PNIPA/Clay hydrogels exhibited excellent mechanical properties with high tensile stress and elongation at break in excess of 1200%.     

Nanocomposite Gels: New Advanced Functional Soft Materials

Summary: Nanocomposite gels (NC gels) consisting of poly(N-alkylacrylamides) and exfoliated inorganic clay were prepared by in-situ, free-radical polymerization at high yield under mild conditions (near ambient temperature, without stirring). Various shapes and different surface forms of NC gels were readily obtained using corresponding vessels and templates, since NC gels were always mechanically tough. The view that polymer/clay networks are formed in NC gels was supported by dynamic viscoelastic and swelling measurements. The entirely different mechanical properties of NC gels, compared with conventional, chemically-crosslinked hydrogels (OR gels), are discussed on the basis of their network structure. In addition to functions previously reported, NC gels exhibit further interesting characteristics, such as inherent incombustibility, good thermal conductivity, large heat capacity, they can be readily colored and fabricated as foams. NC gels can be utilized as environmentally-friendly, soft materials from the viewpoints of resources and waste, as their primary component is water.     

Preparation and characterization of pH- and temperature-responsive semi-IPN hydrogels of carboxymethyl chitosan with poly (<Emphasis Type="Italic">N</Emphasis>-isopropyl acrylamide) crosslinked by clay

A new kind of pH- and temperature-responsive semi-interpenetrating polymer network hydrogel based on linear carboxymethylchitosan (CMCS) and poly (N-isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The pH-and temperature-responsive behaviors, the deswelling kinetics, and the mechanical properties of the hydrogel were investigated. The hydrogels exhibited a volume phase transition temperature around 33 °C with no significant deviation from the conventional PNIPA hydrogels. The results of the influence of pH value on the swelling behaviors showed that the minimum swelling ratios of the hydrogels appeared near the isoelectric point (IEP) of CMCS, and when pH deviated from the IEP, the hydrogels behaved as polycations or polyanions. The novel hydrogels had much higher response rate than the conventional CMCS/PNIPA hydrogels. Moreover, the semi-IPN hydrogels crosslinked by clay could be elongated to more than 800% and the elongation could be recovered almost completely and instantaneously.     

Thermal analyses of poly(N-isopropylacrylamide) in aqueous solutions and in nanocomposite gels

The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPA) prepared by free-radical redox polymerization in aqueous solutions and its nanocomposite (NC) gels were investigated by differential scanning calorimetery. The lower critical solution temperatures (LCST) of aqueous solutions of PNIPA of different molecular weights were not significantly affected by molecular weight (M _w: 0.19?×?10⁶?4.29?×?10⁶?g?×?mol^?1) or polymer concentration (1?10?wt%), although the enthalpy of transition increased with molecular weight, at M _w (<1.2?×?10⁶ g?×?mol^?1). The glass-transition temperature of PNIPA in the dried state also remained constant (138?°C), regardless of molecular weight. On the other hand, the enthalpy of the coil-to-globule transition of PNIPA in NC gels consisting of a PNIPA/clay network decreased with increasing clay concentration (C _clay), while the onset temperature (≡LCST) was almost constant, regardless of C _clay. The PNIPA chains in NC gels could be classified into the following three types: P-1, which exhibits a normal LCST transition, similar to that of linear PNIPA; P-2, exhibiting restricted transition at higher temperatures as a result of interactions with the clay; and P-3, which does not undergo that transition because of stronger restrictions. It was found that the proportion of P-3 increases with increasing C _clay. However, some P-1 and P-2 was still observed, even in NC gels with high C _clay. That the transition to the hydrophobic globular state was restricted by interactions with the clay was confirmed by measurements on PNIPA after removal of the clay from NC gels.     

Molecular Characteristics of Poly(N‐isopropylacrylamide) Separated from Nanocomposite Gels by Removal of Clay from the Polymer/Clay Network

The extraordinary mechanical and swelling/deswelling properties of nanocomposite (NC) gels are attributed to their unique organic (polymer)/inorganic (clay) network structure. In this study, poly(N‐isopropylacrylamide) (PNIPA) was successfully separated from an NC gel network by decomposing the clay (hectorite) using hydrofluoric acid (HF). A very low HF concentration (0.2 wt.‐%) was adequate for the decomposition of the clay without causing any damage to PNIPA. The separated PNIPA had a high (=5.5 × 10⁶ g · mol⁻¹). Also, was almost constant regardless of the clay concentration (C_clay = 1–25 × 10⁻² mol · l⁻¹), even though the properties of the NC gel varied widely over this C_clay range. Comparisons of NC gels, PNIPA, and SiO₂‐NC gels indicated that the clay platelets specifically play an important role in NC gels.

     

羧甲基纤维素钠/聚(N-异丙基丙烯酰胺)纳米复合水凝胶的制备及结构性能表征

以无机粘土为交联剂制备了具有温度、pH双重敏感特性的羧甲基纤维素钠/聚(N-异丙基丙烯酰胺)/粘土半互穿网络纳米复合水凝胶(CMC/PNIPA/Clay semi-IPN),并通过红外和透射电镜对其结构和形态进行了表征。在酸性(pH=1.2)和20℃条件下,分别研究了温度和不同pH缓冲液对该凝胶溶胀度的影响,测定了复合水凝胶的力学性能。结果表明:水凝胶中的粘土被剥离成单片层,且均匀分散在凝胶网络中,起交联剂的作用,而CMC以线性大分子的形态存在;CMC/PNIPA/Clay具有良好的温度、pH双重敏感特性;凝胶的断裂伸长率>1 000%。     

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     

Noncovalent binding interactions of polyacrylamide and clay in nanocomposite hydrogels

The interactions between organic and inorganic components in pregel solution for polyacrylamide (PAAm)/clay nanocomposite hydrogels (NC gels) and in prepared NC gels are investigated. Besides, a kind of self‐crosslinked PAAm gels with excellent mechanical properties is fabricated in the absence of any cross‐linking agents, the hydrogen bonding interactions among PAAm chains are acted as the cross‐linking force. It is revealed that the binding interactions of PAAm and clay in NC gels are owing to the noncovalent interactions between amide groups on PAAm chains and clay platelets, which afford the cross‐linking force for NC gels network formation. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

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     

Synthesis and characterization of Poly(N-isopropylacrylamide)/Poly(acrylic acid) semi-IPN nanocomposite microgels

A novel pH- and temperature-sensitive nanocomposite microgel based on linear Poly(acrylic acid) (PAAc) and Poly(N-isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was synthesized by a two-step method. First, PNIPA microgel was prepared via surfactant-free emulsion polymerization by using inorganic clay as a crosslinker, and then AAc monomer was polymerized within the PNIPA microgel. The structure and morphology of the microgel were confirmed by FTIR, WXRD and TEM. The results indicated that the exfoliated clay platelets were dispersed homogeneously in the PNIPA microgels and acted as a multifunctional crosslinker, while the linear PAAc polymer chains incorporated in the PNIPA microgel network to form a semi-interpenetrating polymer network (semi-IPN) structure. The hydrodynamic diameters of the semi-IPN microgels ranged from 360 to 400 nm, which was much smaller than that of the conventional microgel prepared by using N,N′-methylenebis(acrylamide) (MBA) as a chemical crosslinker, the later was about 740 nm. The semi-IPN microgels exhibited good pH- and temperature-sensitivity, which could respond independently to both pH and temperature changes.     

A novel sodium carboxymethylcellulose/poly(N‐isopropylacrylamide)/Clay semi‐IPN nanocomposite hydrogel with improved response rate and mechanical properties

A novel semi‐IPN nanocomposite hydrogel (CMC/PNIPA/Clay hydrogel) based on linear sodium carboxymethylcellulose (CMC) and poly(N‐isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The structure and morphology of these hydrogels were investigated and their swelling and deswelling kinetics were studied in detail. TEM images showed that the clay was substantially exfoliated to form nano‐dimension platelets dispersed homogeneously in the hydrogels and acted as a multifunctional crosslinker. The CMC/PNIPA/Clay hydrogels swell faster than the corresponding PNIPA/Clay hydrogels at pH 7.4, whereas they swell slower than the PNIPA/Clay hydrogels at pH 1.2. The CMC/PNIPA/Clay nanocomposite hydrogels showed much higher deswelling rates, which was ascribed to more passway formed in these hydrogels for water to diffuse in and out. The deswelling process of the hydrogels could be approximately described by the first‐order kinetic equation and the deswelling rate decreased with increasing clay content. The mechanical properties of the CMC/PNIPA/Clay nanocomposite hydrogels were analyzed based on the theory of rubber elasticity. It was found that with increasing clay content, the effective crosslink chain density, v_e, increased whereas the molecular weight of the chains between crosslinks M_c decreased. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1546–1555, 2008     

APPLICATION OF NONIONIC TEMPERATURE SENSITIVE HYDROGEL FOR CONCENTRATION OF PROTEIN AQUEOUS SOLUTION

Six different N-alkyl substituted acrylarnide nonionic hydrogels were prepared and their swelling characteristics were measured. Poly N-isopropyl acrylamide (PNIPA) and poly N-n-propyl-acrylamide (PNNPA) temperature sensitive hydrogels were chosen as the nonionic temperature sensitive hydrogels for concentration of very dilute aqueous protein solution. The separation properties of PNIPA and PNNPA hydr0gels with different network dimensions were studied and the modification of the hydrogels was surveyed in order to decrease their surface adsorption of protein molecules. The experimental results of the concentration of BSA (Bovin serum albumin) dilute aqueous solution by hydroxylpropyl methacrylate (HPMA) copolymerized PNIPA hydrogel were given. The value and the limitation of concentration of dilute aqueous protein solution by this method was evaluated.     

Temperature-sensitive polyamidoamine dendrimer/poly(N-isopropylacrylamide) hydrogels with improved responsive properties

Novel temperature-sensitive poly(N-isopropylacrylamide)/amine-terminated polyamidoamine dendrimer G6-NH2 hydrogels with fast responsive properties were synthesized by forming semi-interpenetrating polymeric networks. In contrast to the conventional PNIPA hydrogel, these new gels showed rapid shrinking rate at the temperature above lower critical solution temperature (LCST), and exhibited higher equilibrium swelling ratio at room temperature. All these properties might be attributed to the incorporation of polyamidoamine dendrimer G6-NH2, which forms water-releasing channels and increases the hydrophilicity of PNIPA network. The novel hydrogels have potential applications in drug and gene delivery.     

醇溶剂交换对聚合物/黏土纳米复合水凝胶的性能改进及结构表征

采用原位自由基聚合,制备了聚N,N-二甲基丙烯酰胺(PDMAA)/黏土(clay)纳米复合水凝胶(D-NCgel),黏土在体系结构中充当多官能团交联点的作用.考察了D-NC gel中溶剂水被交换为醇溶剂时,凝胶结构稳定性,溶胀特性,以及机械性能的变化.D-NC gel在醇溶剂中仍能保持完整的三维网络结构,体系没有瓦解.而且,D-NC gel在醇溶剂中表现出依赖于醇溶剂种类的溶剂交换和溶胀行为.在甲醇中,凝胶溶胀度呈现单调增长,但是在其它醇溶剂,如乙醇、1-丙醇或1-丁醇中,凝胶表现出先收缩后溶胀的特殊溶胀行为.通过在醇溶剂中先溶胀后干燥的方法,制备具有优异机械性能的醇溶剂纳米复合凝胶.与D-NC5 gel相比,D-NC5甲醇凝胶其拉伸力学强度提高了67%(从155 kPa增加到259 kPa),拉伸模量提高了49%(从7.5 kPa增加到11.2kPa).基于凝胶在醇水溶剂中结构可逆性讨论的基础上,探讨了醇溶剂对D-NC水凝胶的改性机理.     

Novel polymer nanocomposite hydrogel with natural clay nanotubes

Polymer nanocomposite gels (NC gels), a kind of typical soft materials, can be synthesized through free-radical polymerization of water-soluble monomers in the presence of nanoclay in aqueous system. Here, novel natural tube-like nanoparticles, halloysite nanotubes (HNTs), are firstly used as multifunctional cross-linkers for polyacrylamide (PAAm) to form a new type of organic/inorganic hybrid hydrogels. Significant improvements in mechanical properties of the PAAm-HNTs NC gels are found by the addition of HNTs as shown by the static mechanical testing and dynamic viscoelasticity measurement. HNTs are uniformly dispersed in the NC gels from the morphological result. HNTs can be intercalated by PAAm chains as observed by the X-ray diffraction result. Hydrogen bonding interactions between HNTs and PAAm are confirmed by the infrared spectroscopy and X-ray photoelectron spectroscopy. The maximum equilibrium degree of swelling (EDS) for the NC gel is 4000% and the EDS decreases with the concentration of clay nanotubes. The present work provides a novel routine for preparing NC gels using “green” one-dimensional nanoparticle. The prepared NC gels have promising application in biomedical areas due to the superior mechanical properties of the gels and good biocompatibility of HNTs.     

Novel Interpenetrating Networks (IPNs) Hydrogels Prepared In Situ by Liquid-Phase Photopolymerization

Novel interpenetrating networks (IPNs) hydrogels responsive to temperature were prepared in situ by liquid-phase photopolymerization. The first network of the IPNs (poly isopropyl acrylamide) were formed with a special kind of hectorite (Laponite XLS) modified by tetrasodium pyrophosphate as cross-linker and 2-oxogultaric acid as photoinitiator. The samples were subsequently immersed in an acrylamide (AAm) aqueous solution for at least one day for preparing IPNs hydrogels, in which acrylamide aqueous solution containing N,N′-Dimetyl acrylamide (MBAA) as cross-linker and 2-oxogultaric acid as photoinitiator. Then the second networks were in situ formed by introducing ultraviolet light irradiated PNIPAAm gels. The swelling/deswelling behaviors of IPNs hydrogels were measured. Compared with the corresponding nanocomposite PNIPAAm hydroges(NC hydrogels), chemically cross-linked PNIPAAm and PAAm IPNs hydrogels, the results indicate that the new IPN hydrogel has a faster deswelling rate above its LCST (≈32 °C). The effect was explained as being an additional contribution of the PAAm chains in IPN hydrogels, which may act as a water-releasing channel when the hydrophobic aggregation of PNIPA takes place.     

SUPER TOUGH GELS WITH A DOUBLE NETWORK STRUCTURE

 Living tissues work with fantastic functions in soft and wet gel-like state. Thus, hydrogels have attracted much attention as excellent soft & wet materials, suitable for making artificial organs for medical treatments.However, conventional hydrogels are mechanically too weak for practical uses. We have created double network (DN) hydrogels with extremely high mechanical strength in order to overcome this problem. DN gels are interpenetrating network (IPN) hydrogels consisting of rigid polyelectrolyte and soft neutral polymer. Their excellent mechanical properties cannot be explained by the standard fracture theories. In this paper, we discuss about the toughening mechanism of DN gels in accordance with their characteristic behavior, such as large hysteresis and necking phenomenon. We also describe the results on tissue engineering application of DN gels.     

Smart gel–glass based on the responsive properties of polymer gels

Polymer gels are unique smart materials in the sense that they can respond to many different stimuli. In this paper we report how poly(N‐isopropylacrylamide) (abbreviated as PNIPA) and other polymer hydrogels can be used to construct an intelligent gel‐glass which can moderate the amount of light and radiated heat. This environmental sensitive glass, which is a smart hydrogel layer placed between two glass or plastic sheets, becomes opaque when the temperature exceeds a critical value. It becomes transparent again if it is cooled down. The adaptive properties of gel‐glasses make them a promising materials to protect from strong sunlight and heat radiation. Copyright © 2001 John Wiley & Sons, Ltd.     

Thermoresponsive poly(N-isopropylacrylamide) nanogels/poly(acrylamide) nanostructured hydrogels

Here we report the preparation and characterization of nanostructured thermo-responsive poly(acrylamide) (PAM)-based hydrogels. The addition of slightly crosslinked poly(N-isopropylacrylamide) (PNIPA) nanogels to AM reactive aqueous solution produces nanostructured hydrogels that exhibit a volume phase transition temperature (T_VPT). Their swelling kinetics, T_VPT's and mechanical properties at the equilibrium-swollen state (H_eq) are investigated as a function of the concentration of PNIPA nanogels in the nanostructured hydrogels. Nanostructured hydrogels with PNIPA nanogels/AM mass ratios of 20/80 and above exhibit higher H_eq and longer time to reach the equilibrium swelling than those of the conventional PAM hydrogels. However, the PNIPA nanogels possess thermo-responsive character missing in conventional PAM hydrogels. The T_VPT of nanostructured hydrogels depends on PNIPA nanogel content but their elastic and Young moduli are larger than those of conventional hydrogels at similar swelling ratios. Swelling kinetics, T_VPT, and mechanical properties are explained in terms of the controlled in-homogeneities introduced by the PNIPA nanogels during the polymerization.