Direct UV photocrosslinking of poly(N-vinyl-2-pyrrolidone) (PVP) to produce hydrogels

Hydrogels for biomedical purposes, made from synthetic polymers as starting materials and free of co-adjuvant molecules, have been produced almost exclusively by high-energy radiative processes. On the other hand, UV photocrosslinking of such materials has been used in conjunction of monomers and/or photoinitiators. This work was addressed to the analysis of poly(N-vinyl-2-pyrrolidone) (PVP) submitted to direct photocrosslinking in aqueous solution, using low pressure Hg lamp (λ_em=254 nm). The process efficiency was evaluated, and the properties of the hydrogel formed were determined. The product thus formed has similar micro- and macroscopic properties, as compared to hydrogels produced by high-energy radiation and presents no cytotoxicity. These results demonstrated the viability of using this method as a versatile alternative to hydrogel production, broadening the possibility of its production where high-energy radiation facilities are not available.     

Poly(N-vinyl-2-pyrrolidone) hydrogel production by ultraviolet radiation: new methodologies to accelerate crosslinking

Poly(N-vinyl-2-pyrrolidone) hydrogels produced by high-energy radiation relies on water radiolysis as a primary process leading to crosslinks. Conversely, ultraviolet direct irradiation into PVP leads to crosslinking trough pyrrolidinone moiety photolysis. However, this process showed to be rather inefficient. This work describes the crosslinking of poly(N-vinyl-2-pyrrolidone) based on hydrogen peroxide photolysis, therefore mimicking water radiolysis, using UV-C (e.g. low pressure Hg lamp) or UV-A radiation sources. The process efficiency and the properties of the hydrogel formed are discussed and compared with other methods of hydrogel production.     

Synthesis, characterization and thermodynamic properties of poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone)

Poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone) P(MHPMA-co-VP) was synthesized in 1, 4-dioxane solution using benzoyl peroxide (BPO) as initiator at 60 °C. The copolymer was characterized by ¹H ¹³C NMR, FT-IR, DSC, TGA, size exclusion chromatography analysis (SEC) and elemental analysis techniques. According to SEC, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of PMHPMA-co-VP were found to be 58,000, 481,000 g/mol and 8.26, respectively. According to TGA, carbonaceous residue value of PMHPMA-co-VP was found to be 6% at 500 °C. Also, some thermodynamic properties of PMHPMA-co-VP such as the adsorption enthalpy, ΔH_a, molar evaporation enthalpy, ΔH_v, the sorption enthalpy, , sorption free energy, , sorption entropy, , the partial molar free energy, , the partial molar heat of mixing, , at infinite dilution was determined for the interactions of PMHPMA-co-VP with selected alcohols and alkanes by inverse gas chromatography (IGC) method in the temperature range of 323-463 K. According to the specific retention volumes, , the weight fraction activity coefficients of solute probes at infinite dilution, , and Flory-Huggins interaction parameters, between PMHPMA-co-VP-solvents were determined in 413-453 K. According to and , selected alcohols and alkanes were found to be non-solvent for PMHPMA-co-VP at 413-453 K. The glass transition temperature, T_g, of the PMHPMA-co-VP found to be 370 and 363 K, respectively, by IGC and DSC techniques, respectively.     

Maghnite-H, an ecocatalyst for cationic polymerization of N-vinyl-2-pyrrolidone

The polymerization of N-vinyl-2-pyrrolidone catalyzed by the Maghnite-H⁺ (Mag-H) was investigated. Mag-H is a montmorillonite sheet silicate clay, exchanged with protons. It was found that the cationic polymerization of N-vinyl-2-pyrrolidone (NVP) is initiated by Mag-H at 30 °C in bulk and in solution. The effect of the amount of Mag-H, the temperature and the solvent was studied. The polymerization rate increased with increase in the temperature and the proportion of catalyst, and it was larger in nitrobenzene than that in toluene. These results indicated the cationic nature of the polymerization. It may be suggested that the polymerization is initiated by proton addition to monomer from Mag-H.     

Synthesis and characterization of pH-sensitivity semi-IPN hydrogel based on hydrogen bond between poly(N-vinylpyrrolidone) and poly(acrylic acid)

A novel spherically shaped semi-interpenetrating network (semi-IPN) hydrogel, which is based on hydrogen bond between chemical crosslinked poly(N-vinylpyrrolidone) (PVP) and linear poly(acrylic acid) (PAA), was prepared. The semi-IPN hydrogel was synthesized by three steps: (1) linear PAA with different molecular weights were obtained by a reaction of free radical polymerization used 2,2′-azo-bis-iso-butyronitrile (AIBN) as an initiator; (2) crosslinked PVP bead was obtained by a reaction of N-vinylpyrrolidone with AIBN used as an initiator and N,N′-methylene-bis-acrylamide (NNMBA) used as a crosslinker by the way of suspension polymerization; (3) complexation occurred between suitable amount of aqueous solution of PAA and the porous PVP bead and was stabilized by multiple frost-defrost, from this step the semi-IPN hydrogel was obtained. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) proved the presence of the hydrogen bond in the hydrogel. The swelling behaviour of the hydrogel was studied in buffer solution with different pH and NaCl aqueous solution. The results showed that the semi-IPN hydrogel had excellent pH-sensitivity in the range of pH from 2.25 to 4.00 and the small molecule salt had little influence on the swelling behaviour of the semi-IPN hydrogel over the range of concentration of NaCl aqueous solution investigated. The results were confirmed further by scanning electron microscope (SEM). The mechanism of swelling and deswelling was discussed.     

Spectroscopic studies of polymer electrolytes based on poly(N-ethylethylenimine) and poly(N-methylethylenimine)

Poly(ethylethylenimine), PEEI, was prepared from poly(ethylenimine) by reductive alkylation with acetaldehyde. Samples of PEEI and poly(methylenimine), PMEI, complexed with LiCF₃SO₃ were prepared and characterized using differential scanning calorimetry and FT-IR. Small differences in the room temperature spectra of the two complexes were noted; these differences were due to the presence of a CH₂ group in the side chain of PEEI. The predominant form of cation-anion interactions was a contact ion pair. As the samples were heated, a transition from ion pairs to “free” ions was observed, with most of the change occurring between 140 and 150 °C in both PEEI and PMEI complexes. Thermal cycling established that the transition was irreversible in the time frame of the cycling experiments. Two-dimensional correlation spectroscopy did not show any significant intensity or frequency changes in bands sensitive to cation-polymer interactions during any heating or cooling cycle.     

Photo cross-linkable poly(N-isopropylacrylamide) copolymers III: micro-fabricated temperature responsive hydrogels

Micro-fabricated temperature responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels were produced by photolithographic patterning of photo cross-linkable polymers. These polymers were synthesized by copolymerization of N-isopropylacrylamide (NIPAAm) and 2-(dimethyl maleimido)-N-ethyl-acrylamide (DMIAAm). The patterning process of polymers with 9.2 mol% DMIAAm and film thickness below 5 μm in the dry state was able to depict a lateral resolution of 4 μm with insignificant shape change. In order to increase the adhesion of the swollen hydrogels, and thus, the resolution of a particular pattern, a special adhesion promoter based on a monochlorosilane anchor group and a chromophore head group was synthesized. If a silicon wafer surface was pretreated with the adhesion promoter, the structures were stable and well adhered even at lower cross-linking densities. The hydrogels are suitable as working substances for micro-actuators because of their thermally induced volume changes. The swelling ratio of the pattern at low temperatures increased with a decreased cross-linking density. As expected from the chemical composition of the gels, the phase transition temperature (T_c) decreased with increasing DMIAAm content. The swelling of microstructures in water in comparison to macroscopic objects occured significantly faster. This behavior was attributed to the small gel dimension but it was even more pronounced because of the sponge-like nanostructure of the hydrogels characterized by high-resolution field emission scanning electron microscopy. Suitable applications of these hydrogels are adjusting limbs in fluid micro-systems such as micro-pumps and micro-valves.     

The influence of N-vinyl-2-pyrrolidinone in polymerization of holographic polymer dispersed liquid crystals (HPDLCs)

Holographic polymer dispersed liquid crystals (HPDLCs) are targeted for application in a wide range of devices as dynamically switchable transmission or reflection diffraction gratings tunable through the visible spectra. The inclusion of N-vinyl pyrrolidinone (NVP) has been shown to reduce liquid crystal (LC) droplet size in HPDLC gratings and subsequently improve HPDLC performance. In this work, the influence of NVP on HPDLC polymer/LC morphology is examined and correlated to the influence of NVP on HPDLC photopolymerization kinetics and LC phase separation. As in other photopolymer systems, NVP significantly increases the rate of polymerization in HPDLC photopolymerization. In all the HPDLC formulations studied, NVP is completely incorporated into the polymer network while less than half of acrylate double bonds react. Furthermore, as the highly cross-linked polymer network forms, the small mono-vinyl NVP appears to react preferentially with acrylate double bonds, facilitating additional conversion of pendant double bonds otherwise trapped in the polymer network. NVP also induces a delay in the onset of reaction diffusion termination and extends the range of conversions for which reaction diffusion is observed. Interestingly, NVP also impacts polymer/LC morphology by delaying LC phase separation to higher double bond conversions. Together, the influence of NVP on the polymerization kinetics and LC phase separation alters HPDLC morphology by limiting LC droplet size, consequently resulting in improved HPDLC performance.     

Interactions in miscible blends and complexes of poly(N-acryloylmorpholine) with poly(p-vinylphenol)

Poly(p-vinylphenol) (PVPh) and poly(N-acryloylmorpholine) (PAcM) form interpolymer complexes in ethanol/water (1:1) solution. However, only ordinary blends are obtained from dimethylformamide solution. Each of the complexes and ordinary blends shows one composition-dependent glass transition temperature, indicating its single-phase nature. Fourier transform infrared spectroscopy and ¹³C solid-state nuclear magnetic resonance spectroscopy reveal the existence of hydrogen-bonding interactions between the hydroxyl groups of PVPh and the carbonyl groups as well as the ether oxygen of PAcM in the blends and complexes. In addition, X-ray photoelectron spectroscopy shows that the nitrogen atoms in PAcM are also involved in hydrogen-bonding interactions. Measurements of proton spin-lattice relaxation time in the rotating frame, T_1ρ(H), reveal that each of the complexes and ordinary miscible blends has one composition-dependent T_1ρ(H), indicating an intimate mixing on a scale of about 1.5 nm. The blends show a higher degree of surface enrichment of PVPh than the complexes.     

Characterization of electrospun poly(N-isopropyl acrylamide) fibers

Poly(N-isopropyl acrylamide) (pNIPAM) is an interesting material in that it shows a thermoresponsive behavior around 32 °C in aqueous solutions. This behavior mimics that of many proteins in solution and as a result, many researchers have studied pNIPAM as a model for protein behavior. Yet, little is known about the processability of pNIPAM into three-dimensional matrices and whether such processing affects polymer conformation. In this work, 3D fibrous mats of pNIPAM were prepared by electrospinning from three different solvents and the resulting morphologies evaluated. Additionally, electrospun pNIPAM was evaluated with polarized Raman and infrared spectroscopies and compared against the spectra of the bulk material. It was found that the electrospinning process did not alter the polymer structure or morphology.     

H NMR study of temperature-induced phase transitions in D2O solutions of poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide) mixtures and random copolymers

¹H NMR spectroscopy was used to investigate temperature-induced phase transitions in D₂O solutions of poly(N-isopropylmethacrylamide) (PIPMAm)/poly(N-isopropylacrylamide) (PIPAAm) mixtures and P(IPMAm/IPAAm) random copolymers of various composition on molecular level. While two phase transitions were detected for PIPMAm/PIPAAm mixtures, only single phase transition was found for P(IPMAm/IPAAm) copolymers. The phase transition temperatures of PIPAAm component (appears at lower temperatures) are not affected by the presence of PIPMAm in the mixtures; on the other hand, the temperatures of the phase transition of PIPMAm component (appears at higher temperatures) are affected by the phase separation of the PIPAAm component and depend on concentration of the solution. For P(IPMAm/IPAAm) random copolymers, a departure from the linear dependence of the transition temperatures on the copolymer composition was found for a sample with 75 mol% of IPMAm monomeric units.     

Thermo- and pH-sensitive dendrimer derivatives with a shell of poly(N,N-dimethylaminoethyl methacrylate) and study of their controlled drug release behavior

Novel dendrimer derivatives combining the temperature- and pH-sensitivities are synthesized. At first, polyamidoamine (PAMAM) dendrimers with generations 1-5 are synthesized by the reaction of ethylenediamine with methyl acrylate, and then the dendrimers are acylated by chloroacetyl chloride to obtain PAMAM-Cl, which can act as a macroinitiator for further synthesizing functional dendrimers. For fulfilling this goal, the polymers consisting of a dendritic PAMAM core and poly(N,N-dimethylaminoethyl methacrylate) (PDMA) shell are synthesized by atom transfer radical polymerization (ATRP). Their macromolecular structures are characterized by FTIR, ¹H NMR, DSC and particle size analyses, and their aqueous solutions are inspected by UV spectroscopy for understanding their thermo- and pH-sensitivities. The results show that novel dendrimer derivatives exhibit clearly thermo- and pH-respondings in accordance with the change of the environment. Using chlorambucil (CLB) as a model drug, the behaviors of the controlled drug release from polymers with different average graft length of PDMA are studied. The results indicate that the rate of the drug release can be effectively controlled by the pH value.     

Synthesis and thermo-responsive behavior of fluorescent labeled microgel particles based on poly(N-isopropylacrylamide) and its related polymers

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) as an intramolecular fluorescent probe were prepared by emulsion polymerization. The thermo-responsive behavior of the VDP-labeled PNIPAM microgel particles dispersed in water was studied by turbidimetric and fluorescence analyses. The transition temperature of the VDP-labeled PNIPAM microgel particles in water determined by turbidimetric analysis was ca. 32.5 °C. The wavelength at the maximum fluorescence intensity of the VDP units linked directly to the microgel particles dramatically blue-shifted around the transition temperature. In addition it gradually blue-shifted even below the transition temperature where there was no change observed in the turbidity. These findings suggest that the gradual shrinking of microgel particles occurs with increasing temperature and the subsequent dramatic shrinking results in the increasing in the turbidity. The transition temperatures of VDP-labeled poly(N-n-propylacrylamide) and poly(N-isopropylmethacrylamide) microgel particles determined by turbidimetric analysis were ca. 23 and ca. 42.5 °C, respectively, and their thermo-responsive behavior was similar to that for the VDP-labeled PNIPAM system. In these three systems the microenvironments around the fluorescent probes above the transition temperatures became more hydrophobic than those below the transition temperature, and the estimated values of microenvionmental polarity around the VDP units on their collapsed states were almost the same.     

Influence of the swelling history on the swelling kinetics of stimuli-responsive poly[(N-isopropylacrylamide)-co-(methacrylic acid)] hydrogels

The influence of the swelling history on the swelling behavior of poly[(N-isopropylacrylamide)-co-(methacrylic acid)] P[(N-iPAAm)-co-(MAA)] random copolymers hydrogels synthesized by free radical polymerization in solution of N-iPAAm and MAA comonomers crosslinked with tetraethylene glycol dimethyl acrylate (TEGDMA) has been studied. The swelling behavior under pH 7 at 18, 29, 39 and 49 °C of this series of copolymers, previously soaked either at pH 2 or 7 has been investigated. The swelling kinetics of these two series of samples displays different behavior as function of the composition and temperature. However, the equilibrium swelling values only show slight dependences on the previous soaking pH and temperature. When samples are soaked at pH 7, then the swelling at pH 7 follows a first order kinetics, irrespective of the copolymer composition or the temperature at which the experiment has been carried out. In this case, the swelling process is very fast and depends only slightly on temperature. The first order rate constant increases with the MAA content in the hydrogel. Furthermore, the swelling rate of copolymer hydrogels soaked at pH 2, show strong dependence on composition and temperature. They follow an autocatalytic swelling kinetics due to the disruption of hydrogen bond arrangements. An initial slow water uptake is followed by an acceleration process, in which water molecules inside the gel help the next water molecules to come in. Two rate constants, a first-order rate constant and an autocatalytic one have been obtained from the kinetics analysis. They have revealed different temperature dependence which may be due to a balance between hydrophobic and hydrogen bond interactions. The temperature dependence of the swelling kinetics is stronger and more complex for copolymers treated under pH 2 than for copolymers soaked under pH 7.     

Degradation and geometric isomerization of poly(N-propargylamides)

The stability of several poly(N-propargylamides) was investigated in solution and in solid state on the basis of molecular weight change with time, and further their thermal stability was investigated by TGA. When the stability of poly(N-propargylamides) with varying pendent groups was compared, polymers with pendent groups of moderate size showed the highest stability in solution. Too short and too bulky pendent groups were not favorable for the stability of polymers. When poly(N-propargylheptanamide) (poly(6)) was stored in THF as solution at −20 °C in the absence of oxygen in dark, its degradation rate was the lowest. The degradation rate of poly(6) depended on the solvents used, which may be related to different solubility of oxygen in these solvents. Polymers with high cis contents degraded faster than polymers with low cis contents did. Addition of TEMPO and DPPH into the poly(6)/THF solution more or less depressed the degradation of poly(6). The degradation of polymer main chain in solution was always accompanied by the decrease of cis content, i.e. geometric isomerization from cis- to trans-structure. When the polymers were stored in the solid state at −20 °C, the polymers having alkyl pendent groups with moderate length were more stable than those with bulky pendent groups. Geometric isomerization occurred along with degradation in the solid state as well.     

Collapse kinetics for individual poly(N-isopropylmethacrylamide) chains

The kinetics for the coil-to-globule transition of linear poly(N-isopropylmethacrylamide) (PiPMA) chains has been studied by use of the fluorescence and Rayleigh scattering with a fast laser pulse infrared heating. We have observed the two-stage kinetics in the collapse transition with the characteristic relaxation times, τ_fast and τ_slow, which are attributed to the nucleation and growth of pearls on the chain and the merging and coarsening of pearls to a globule, respectively. The collapse kinetics of PiPMA is similar to that of poly(N-isopropylacrylamide) which has one less methyl in each monomeric unit, indicating that the additional methyl groups in PiPMA chains slightly influence the kinetics. In other words, the pearls are not completely coarsened to form compact globules within τ_slow.     

Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature

Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d₀, at 20 °C was found to be strongly dependent on the preparation temperature, T_prep, where d and d₀ are the diameter of gel during observation and preparation, respectively. The value of d/d₀ was about 1.2 for T_prep=20 °C, but steeply increased by approaching the phase separation temperature ≈32.0 °C. Above 32.0 °C, d/d₀ decreases stepwise to 1.46. This upturn in d/d₀ was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T_prep. Though the shrinking half-time, t_1/2, of gel was on the order of 500 min for T_prep≤20 °C, t_1/2 decreased to 2 min for T_prep≥26 °C. Hence, a rapid shrinking was attained by simply increasing T_prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.     

Thermosensitive poly(N-isopropylacrylamide) nanocapsules with controlled permeability

In this paper, novel thermosensitive poly(N-isopropylacrylamide) (PNIPAM) nanocapsules with temperature-tunable diameter and permeability are reported. Firstly, the core-shell composite microparticles were synthesized by precipitation polymerization with isothiocyanate fluorescein (FITC) entrapped SiO₂ as core and cross-linked PNIPAM as shell. Then, the SiO₂ core was etched by hydrofluoric acid at certain condition and the pre-trapped FITC molecules remained within the inner cavity. The FITC release profile and TEM studies clearly indicate that the release behavior of FITC could be controlled effectively by the external temperature. Above the LCST of PNIPAM (32 °C), the dehydrated PNIPAM shell inhibited the release of FITC from the internal cavity while below its LCST, the fluorophore could permeate the swollen shell easily.     

Synthesis of thermosensitive poly(N-alkylacrylamide) gels and core-shell type gels

When the poly(acrylic acid) (PAA) gel-1,8-diazabicyclo-[5,4,0]-7-undecene salt (DAA) was placed in N-methyl-2-pyrrolidone containing an excess of alkylamine and triphenylphosphine, selective amidation took place from the outside to give the corresponding poly(N-alkylacrylamide) gel containing a C3 alkyl chain through a DAA-poly(N-alkylacrylamide) type gel capsule consisting of a hydrophilic unreacted core part and an amidated shell layer. The amidation proceeded by a reaction mechanism similar to the unreacted-core model. Thermal properties of the resulting poly(N-alkylacrylamide) gels such as deswelling behavior and equilibrium swelling ratio in water as a function of temperature were measured. The release of methyl orange from a poly(N-alkylacrylamide) gel and the gel capsule was also examined. PAA-poly(N-alkylacrylamide) type gel capsules containing a PAA core part and thermosensitive poly(N-alkylacrylamide) shell layer, prepared by the neutralization of DAA-poly(N-alkylacrylamide) type gel capsules, showed on-off chemical release characteristics in response to stepwise temperature changes across the LCST.