Synthesis, characterization and thermoreversible behaviours of poly(dimethyl siloxane)/poly(N-isopropyl acrylamide) semi-interpenetrating networks

Simultaneous and sequential poly(N-isopropyl acrylamide) (PNIPAAm)/poly(dimethyl siloxane) (PDMS) semi-interpenetrating polymer networks (IPNs) with different linear PDMS contents were prepared by free radical polymerization method. Their phase morphologies have been characterized by FTIR, DSC and SEM. The simultaneous semi-IPNs exhibited phase transition temperatures (T_pt) shifted higher temperature from glass transition temperatures (T_g) of their respective homopolymers, suggesting a heterophase morphology and only physical entanglement between the PNIPAAm network and linear PDMS with high molecular weight (Mn≈9000 g/mol). For sequential semi-IPNs, the shift of T_pts towards lower temperature suggested that the chemical interaction between the constituents of the IPNs increased with increasing PDMS content in the network. In addition, these semi-IPNs were characterized for their thermo-sensitive behaviour by equilibrium swelling studies. The results showed that incorporation of hydrophobic PDMS polymer into the thermo- and pH-sensitive PNIPAAm and P(NIPAAm-co-IA) (itaconic acid) hydrogels by semi-IPN formation decreased swelling degrees of IPNs without affecting their LCSTs whereas addition of acrylated PDMS (Tegomer V-Si 2250) as crosslinker instead of N,N^′-methylenebisacrylamide (BIS) into the structures of these hydrogels changed their LCSTs along with their swelling degrees.     

Observations on the swelling of cross-linked poly(dimethylsiloxane) networks by solvents

The Flory Huggins Solvent parameter (χ) previously published for a range of solvents and a cross-linked silicone polymer, have been recalculated using the original swelling data, but including a term representing the loss of configurational entropy consequent on crosslinking. From the Shore hardness of the polymer, the Young’s modulus E was calculated. E = 6(C₁ + C₂), where C₁ and C₂ are the parameters from the Mooney Rivlin equation for the elastic deformation of an elastomer. Since C₁ is related to M_c, the average molecular weight between crosslinks, revised χ values could be calculated for various values of C₂/C₁. These showed that for good solvents for the silicone polymer, the values published previously were too high.     

Effect of filler networking on the response of thermosensitive composite hydrogels

Several composite hydrogels of poly(N-isopropylacrylamide) (pNIPAAm) with sodium montmorillonite (NaMM) have been synthesized using a fixed polymer/NaMM ratio (4:1 wt./wt.), but various monomer concentrations, in order to obtain hydrogels with different degrees of swelling, and thus different clay contents in the swollen state. For comparison, unfilled pNIPAAm gels have been also prepared at the same concentrations. The equilibrium swelling behaviour of the gels has been studied both in the swollen and in the shrunk state. In the swollen state, the polymer volume fraction increases with the initial monomer concentration C₀. In the shrunk state, the polymer fraction in pNIPAAm hydrogels is dependent on the specimen size and on C₀, whereas in the composite gels a constant polymer content is observed. When subjected to stepwise heating from 25 to 45 °C, unfilled gels undergo only poor deswelling. By contrast, complete deswelling takes place in composite gels. The latter show half-shrinking times varying over two orders of magnitude, depending on the monomer concentration and on the procedure followed to disperse NaMM, which determine the overall dispersion state of the filler, as evidenced by transmission electron microscopy (TEM). In particular, TEM observations show clay networking above a percolation threshold near 2.5 wt.% of NaMM. The effect of the incorporation of clay on the response to thermal stimuli is discussed in terms of the ability of NaMM to hinder the hydrophobic association of pNIPAAm segments and in terms of its dispersion state. It is suggested that, above the percolation threshold, NaMM forms a hydrophilic, physical network, through which water can flow also above the volume transition temperature, where pNIPAAm acquires a hydrophobic character.     

Single-center vs. multi-center post-metallocene catalysts for propylene polymerization

The structural characteristics of polypropylene samples prepared with two post-metallocene catalysts based on complexes bis-{M-(3,5-di-tert-butyl-salicylidene)-4-[bis-(5-methyl-2-furyl)methyl]aniline}titanium dichloride and [(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetra(pentafluorophenyl)-1,3-dioxalan-4,4-dimethanol)titanium dichloride are investigated by GPC, ¹³C NMR, IR, DSC, and XRD methods. A combination of the first complex and MAO forms a single-center catalyst which polymerizes propylene to a nearly perfectly atactic polymer. A combination of the second complex and MAO forms a multi-center catalyst system producing polymer mixtures with broad molecular weight distributions containing five to six Flory components with different average molecular weights. Relative contents of the Flory components strongly depend on the type of solvent in the polymerization reactions. Some of the active centers produce high molecular weight, highly isotactic crystalline material with the melting point over 154 °C. The nature of steric errors in these polymer fractions (determined by ¹³C NMR) can be explained by a variant of stereocontrol similar to that exerted by metallocene catalysts of the C₁ symmetry.     

Some properties of networks produced by the Diels-Alder reaction between poly(styrene-co-furfuryl methacrylate) and bismaleimide

Poly (styrene-co-furfuryl methacrylate) networks were prepared by the Diels-Alder (D-A) reaction in solution at 25 °C between the linear copolymer and bismaleimide (BM). The resultant crosslinked polymers were swollen to equilibrium in toluene at 25 °C and swelling properties were studied by gravimetric and dimensional measurements. The swelling behaviour of these organogels depended on the composition of the copolymer and the concentration of BM used. Shear and Young’s moduli and the effective crosslinking densities (ν_e) were determined by compression (stress)-strain measurements. The theoretical crosslinking density was higher than the ν_e for all the crosslinked copolymers. An endothermic peak without T_g was observed on the DSC curve on heating the dry crosslinked polymer. On reheating a T_g at ≈98 °C was found, which is attributed to presence of linear copolymer produced by the retro D-A reaction in the first heating. The thermal stability of a crosslinked copolymer under nitrogen and air showed differences with the stability of the linear copolymer. The increase in viscosity of the solution during the D-A reaction was followed with time, for initial linear copolymers of different molecular weights. It was found that onset of gelation increased to longer reaction times the lower the molecular weight of copolymer.     

Microgels containing methacrylic acid: effects of composition on pH-triggered swelling and gelation behaviours

pH-responsive microgels are cross-linked polymer colloids that swell when the pH approaches the pK _a of the particles. In this work, we present a comprehensive investigation of pH-triggered particle swelling and gel formation for a range of microgels containing methacrylic acid (MAA). The microgels investigated have the general composition poly(A/MAA/X), where A and X are the primary co-monomer and cross-linking monomer, respectively. The primary co-monomers were methyl methacrylate (MMA), ethyl acrylate (EA) or butyl methacrylate. The cross-linking monomers were either butanediol diacrylate (BDDA) or ethyleneglycol dimethacrylate (EGDMA). The microgels were studied using scanning electron microscopy, photon correlation spectroscopy (PCS) and dynamic rheology measurements. Gel phase diagrams were also constructed. The particles swelled significantly at pH values greater than approximately 6.0. It was shown that poly(EA/MAA/X) microgels swelled more strongly than poly(MMA/MAA/X) microgels. Furthermore, greater swelling occurred for particles prepared using EGDMA than BDDA. Concentrated dispersions of all the microgels studied exhibited pH-triggered gel formation. It was found that the fluid-to-gel transitions for the majority of the six microgel dispersions investigated could be explained using PCS data. In those cases, gelation was attributed to a colloidal glass transition. Interestingly, the microgels that were considered to have the highest hydrophobic content gelation occurred under conditions where little particle swelling was evident from PCS. The data presented show that gelled poly(EA/MAA/BDDA) and poly(MMA/MAA/EGDMA) microgel dispersions have the strongest elasticities at pH = 7.     

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%.     

Swelling studies of crosslinked poly(p-phenylene terephthalamide) copolymers in sulfuric acid

In an attempt to improve the mechanical properties of extended chain polymers such as poly(p-phenylene terephthalamide) (PPTA), a crosslinkable terephthalic acid derivative (XTA) has been developed which can be incorporated into copolymers in various concentrations and activated after polymerization. The crosslinking of PPTA-co-XTA copolymer particles was investigated through a series of swelling experiments in concentrated H₂SO₄. The data show a systematic decrease in equilibrium swelling with increasing XTA content, indicating the XTA units are in fact acting as crosslink sites. Values for crosslink density were calculated from the Flory-Rehner theory of polymer swelling and compared with previous findings on crosslinked rigid polymer network systems. The effective number of crosslinks per XTA unit (efficiency) predicted by the Flory-Rehner theory increases and then decreases with % XTA. The decrease in crosslinking efficiency at high XTA concentrations is consistent with differential scanning calorimetry data which show the enthalpy of XTA reaction decreasing slightly with % XTA. The deviations at low % XTA may represent a failure of the Flory-Rehner theory to properly describe the rubbery elasticity of extended chain polymers. © 1994 John Wiley & Sons, Inc.     

Solid-state NMR and EPR study of fluorinated carbon nanofibers

Carbon nanofibers were fluorinated in two manners, in pure fluorine gas (direct fluorination) and with a fluorinating agent (TbF₄ during the so-called controlled fluorination). The resulting fluorinated nanofibers have been investigated by solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). This underlines that the fluorination mechanisms differ since a (CF)_n structural type is obtained, whatever the temperature, with the controlled reaction, whereas, during the direct process, a (C₂F)_n type is formed over a wide temperature range. Through a careful characterization of the products, i.e. density of dangling bonds (as internal paramagnetic centers), structural type (acting on molecular motion) and specific surface area (related to the amount of physisorbed O₂), the effect of atmospheric oxygen molecules on the spin-lattice nuclear relaxation has been underlined.     

Deuterium diffusion in vanadium deuterides (VDx; 0.4?x?0.6) studied by H NMR

Sites and dynamics of deuterium in VD_0.50 as well as in VD_0.40 and VD_0.57 have been studied by means of X-ray diffraction, differential scanning calorimetry and ²H NMR. VD_0.40 consists of the body-centered-cubic (bcc) α_D and body-centered-tetragonal (bct) β_D phases at room temperature and only the octahedral (O) sites are occupied in the β_D phase. On the other hand, in VD_0.50, consisting of the bctβ_D phase at room temperature, the O site is dominantly occupied with a small occupancy of the tetrahedral (T) sites in the β_D phase. VD_0.57 is composed of the bccα′_D (a high-deuterium-concentration phase of the α_D phase) and bctβ_D phases at room temperature, and the occupancy of the T sites is observed. The mean activation energy (E_D) of the deuterium diffusion is much larger for the O sites than for the T sites, and the diffusion is also much slower in the O sites. The mean E_D values for the O sites are almost the same in the β_D phase of VD_0.40 and VD_0.50, although the distribution of the mean residence time is much larger in VD_0.50 than in VD_0.40.     

pH-thermosensitive behavior of N,N-dimethylaminoethyl methacrylate (Co)polymers with N-vinylcaprolactam

The effect of acidity of a medium on the phase separation temperature and the intensity of light scattering for dispersions produced by heating of aqueous solutions of N,N-dimethylaminoethyl methacrylate, N-vinylcaprolactam, and their copolymers has been studied. It has been demonstrated that the phase separation temperature and the turbidity of polyvinylcaprolactam (and vinylcaprolactam-enriched copolymers) solutions are pH-independent. Poly(N,N-dimethylaminoethyl methacrylate) (and N,N-dimethylaminoethyl methacrylateenriched copolymers) exhibits temperature sensitivity only in the alkaline region, and the phase separation temperature and turbidity versus pH plots are described by curves with maxima. The addition of sodium dodecyl sulfate to polymer solutions in the general case causes an increase in the phase separation temperature. However, if positive charges occur on macromolecules (in initial solutions of poly(N,N-dimethylaminoethyl methacrylate) or acidified solutions of polyvinylcaprolactam), the increase in the phase separation temperature is preceded by its decrease owing to the electrostatic interaction of surfactant anions with cationic centers. As acid is introduced into the H₂O-sodium dodecyl sulfate-polyvinylcaprolactam ternary system, the phase separation temperature of the polyvinylcaprolactam-dodecyl sulfate complex is decreased.     

Ionic transport studies on (PEO)6:NaPO3 polymer electrolyte plasticized with PEG400

Conduction characteristics of the poly(ethylene oxide) based new polymer electrolyte (PEO)₆:NaPO₃, plasticized with poly(ethylene glycol) are investigated. Free standing flexible electrolyte films of composition (PEO)₆:NaPO₃ + x wt.% PEG₄₀₀ (30 ? x ? 70) are prepared by solution casting method. A combination of X-ray diffraction (XRD), optical microscopy and differential scanning calorimetry (DSC) studies have indicated enhancement in the amorphous phase of polymer due to the addition of plasticizer. Further, a reduction in the glass transition temperature observed from the DSC result has inferred increase in the flexibility of the polymer chains. The cationic transport number (t_Na⁺) of 0.42 determined through combined ac-dc technique has confirmed ionic nature of conducting species. Ionic conductivity studies are carried out as a function of composition and temperature using complex impedance spectroscopy. The electrolyte with maximum PEG₄₀₀ content has exhibited an enhancement in the conductivity of about two orders of magnitude compared to the host polymer electrolyte. The complex impedance data is analyzed in conductivity, permittivity and electric modulus formalism in order to throw light on transport mechanism. A solid state electrochemical cell based on the above polymer electrolyte with a configuration Na|SPE|(I₂ + acetylene black + PEO) has exhibited an open circuit voltage of 2.94 V. The discharge characteristics are found to be satisfactory as a laboratory cell.     

pH-responsive microgels containing hydrophilic crosslinking co-monomers: shell-exploding microgels through design

pH-responsive microgels are crosslinked polymer colloids that swell when the pH approaches the pK _a of the particles. They have potential application for injectable gels for tissue repair and drug delivery systems. This study focuses on the pH-triggered gelation behaviour of a series of poly (EA/MAA/X) microgels. EA and MAA are ethylacrylate and methacrylic acid. Here, we investigate the effect of crosslinking monomer type (X) on microgel properties. The crosslinking monomers used were poly (ethyleneglycol) dimethacrylate (PEGD), ethyleneglycol dimethacrylate (EGD) and butanediol diacrylate (BDD). The microgel containing PEGD (m-PEGD) is a new system. The microgel containing BDD (m-BDD) was used as a control system. The concentrated microgel dispersions formed physical gels when the pH was increased to 5.3?C6.7, and the polymer volume fractions (? _p ) were above about 0.05. Evidence from photon correlation spectroscopy (PCS) and dynamic rheology was presented for abrupt pH-triggered increases, and then decreases of the hydrodynamic diameters for m-PEGD and the microgel prepared using EGD (m-EGD). This appears to be tuneable through crosslinker structure. An unexpected gelation behaviour, which may involve a new gel state for microgels, was found for m-PEGD dispersions. Uniquely, those dispersions formed gels at pH values less than the microgel's pK _a . This behaviour was linked to an outer-shell electrostatic repulsive interaction. The data point to a phenomenon, whereby the m-PEGD shells appear to explode at pH values above 7.0. The control microgel prepared, using BDD (m-BDD), did not show any evidence of shell fragmentation at any pH. That microgel has potential as a model pH-responsive microgel system in that the properties measured by PCS and rheology agreed well. To probe that system in more detail, the rheological data for m-BDD was analysed using scaling theory. The variation of the storage modulus (G') with ? _p gave a scaling exponent of 2.0.     

MR Study of Water Distribution in a Beech (Fagus sylvatica) Branch Using Relaxometry Methods

Wood is a widely used material because it is environmentally sustainable, renewable and relatively inexpensive. Due to the hygroscopic nature of wood, its physical and mechanical properties as well as the susceptibility to fungal decay are strongly influenced by its moisture content, constantly changing in the course of everyday use. Therefore, the understanding of the water state (free or bound) and its distribution at different moisture contents is of great importance. In this study, changes of the water state and its distribution in a beech sample while drying from the green (fresh cut) to the absolutely dry state were monitored by 1D and 2D ¹H NMR relaxometry as well as by spatial mapping of the relaxation times T₁ and T₂. The relaxometry results are consistent with the model of homogeneously emptying pores in the bioporous system with connected pores. This was also confirmed by the relaxation time mapping results which revealed the moisture transport in the course of drying from an axially oriented early- and latewood system to radial rays through which it evaporates from the branch. The results of this study confirmed that MRI is an efficient tool to study the pathways of water transport in wood in the course of drying and is capable of determining the state of water and its distribution in wood.     

Further EPR studies of molecular motions in polymer/plasticizer mixtures

Molecular motions in mixtures of the side chain polymer—poly(vinyl acetate) and dibuthyl phthalate were studied as a functionof polymer concentration and temperature using the technique of paramagnetic resonance (EPR). When the small spherical probe tempol (TPL) was used, we were able to approximate the observed EPR spectrum with a simulation using a single rotational correlation time τ. The peviously developed Grest–Cohen all-temperature model matched the Arrhenius polts. The EPR spectra from a cigar-shaped cholestane (COL) probe could not be adequately matched by single τ simulation when the polymer was at temperatures somewhat above the glass to rubber transition temperature (T_g). Points corresponding to these temperatures were left of the Arrhenius plot and a discontinuity was observed where the gap in the data occurred. As the concentration of plasticizer was increased, we found that the discontinuity became less steep, but the τ at which the gap occurs was always ≈ 10^?8. The spectra observed at the temperature region of the gap were approximately 50–50 composites of experimental spectra observed at ± K. In both the TPL and COL cases, there was evidence of the existence of multiple correlation times. Preliminary studies of other polymers, both with and without side chains, also indicated the existence of the gap when COL is used as the probe. © 1993 John Wiley & Sons, Inc.     

Influence of electron beam irradiation on physicochemical properties of poly(trimethylene carbonate)

Electron beam (EB) irradiation of poly(trimethylene carbonate) (PTMC), an amorphous, biodegradable polymer used in the field of biomaterials, results in predominant cross-linking and finally in the formation of gel fraction, thus enabling modification of physicochemical properties of this material without significant changes in its chemical structure. PTMC films (M_w: 167-553 kg mol⁻¹) were irradiated with different doses using an electron accelerator. Irradiation with a standard sterilization dose of 25 kGy caused neither significant changes in the chemical composition of the polymer nor significant deterioration of its mechanical properties. Changes in viscosity-, number-, weight-, and z-average molecular weights of PTMC for doses lower than the gelation dose (D_g) as well as gel-sol analysis and swelling tests for doses above D_g indicate domination of cross-linking over degradation. EB irradiation can be considered as an effective tool for increasing the average molecular weight of PTMC and sterilization of PTMC-based biomaterials.     

Dispersion of magnetic nanoparticles in a polymorphic liquid crystal

In order to enhance the phase stability of dispersions of magnetic nanoparticles (NPs) in a polymorphic liquid crystal, new ligands have been designed consisting of a terphenyl-based liquid crystalline core. The most stable dispersions were obtained with 7 nm super-paramagnetic Fe₃O₄ NPs decorated with the new ligands in place of 10 nm ferromagnetic CoFe₂O₄ spherical NPs.     

Photo Polymerized Interpenetrating Polymer Network of Poly(antimony acrylate) and Poly(arsenic acrylate): Synthesis and Characterization

A series of IPN based on poly(antimony acrylate) and poly(arsenic acrylate) have been synthesized by a sequential mode of synthesis. Formation of complex based on “polymer solvent” method reflects the contraction of the polymer coils by determining the value of mutual interaction constant (k_AB) in different solvents such as dimethylsulphoxide (DMSO) (k_AB=0.60); dimethylformamide (DMF) (k_AB=0.42); dioxane (k_AB=0.26) predicting weak Vander Waal interaction. The scanning electron microscopy reveals dual phase morphology of both metal acrylates. The infrared spectrum indicates characteristic frequencies of (>C?O) at 1730 cm^?1,thus giving structural evidence for IPN. The properties namely percentage swelling, average molecular weight between crosslinks(M_c),Young's modulus, increases with concentrations of linear polymer(polyantimony acrylate) and initiator (benzoyl peroxide). However, it decreases with concentrations of monomer (arsenic acrylate) and crosslinker (divinyl benzene).The value of activation energy calculated from thermo gravimetric analysis is 15 KJ/mol.     

Unusual Lower Critical Solution Temperature Phase Behavior of Poly(benzyl methacrylate) in a Pyrrolidinium-Based Ionic Liquid

Polymer/ionic liquid systems are being increasingly explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Poly(benzyl methacrylate) in certain ionic liquids constitute unusual LCST systems, in that the second virial coefficient (A₂) in dilute solutions has recently been shown to be positive, indicative of good solvent behavior, even above phase separation temperatures, where A₂ < 0 is expected. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide for three different molecular weights (32, 63, and 76 kg/mol) in concentrated solutions (5–40% by weight). Turbidimetry measurements reveal a strong concentration dependence to the phase boundaries, yet the molecular weight is shown to have no influence. The critical compositions of these systems are not accessed, and must therefore lie above 40 wt% polymer, far from the values (ca. 10%) anticipated by Flory-Huggins theory. The proximity of the experimental cloud point to the coexistence curve (binodal) and the thermo-reversibility of the phase transitions, are also confirmed at various heating and cooling rates.     

Comparative Study on the Collapse Transition of Poly(N-isopropylacrylamide) Gels and Magnetic Nanoparticles Loaded Poly(N-isopropylacrylamide) Gels

A novel polymer gel exhibiting simultaneous temperature and magnetic field sensitivity has been prepared and studied. Poly(N-isopropylacrylamide) (PNIPA) and magnetic nanoparticles (magnetite, Fe₃O₄) loaded PNIPA gel beads with mm size and monolith gels with cm size were prepared. The dependence of swelling degree on the temperature has been studied. The effects of cross-linking density and the presence of magnetic nanoparticles on the equilibrium swelling degree as well as on the collapse transition have been investigated. Swelling kinetic measurements were also made. By comparing the equilibrium swelling properties of PNIPA and magnetite loaded PNIPA gels it was found that the built in magnetic nanoparticles do not modify the temperature sensitivity of these gels. Within the experimental accuracy the temperature of the collapse transition was not sensitive to the presence of magnetic particles. We have compared the swelling behaviour of mm size gel beads to the cm size monolith gels in order to study the influence of surface skin layer on the swelling equilibrium. It was established that the extent of surface skin formation was decreased by the presence of magnetic particles.