Marcos R. Guilherme

Hydrogels based on PAAm network with PNIPAAm included: hydrophilic–hydrophobic transition measured by the partition of Orange II and Methylene Blue in water

Abstract This work describes the transition from hydrophilic to hydrophobic in hydrogels of Polyacrylamide (PAAm) having Poly(N-isopropylacrylamide) (PNIPAAm) included. The transition was measured through the partition coefficient, K, Orange II and Methylene Blue dyes at several temperatures, using different amount of Acrylamide (AAm), Methylene-bis-acrylamide (MBAAm), as cross-linking agent, and PNIPAAm. The dye concentration in the gel and the solution were measured using UV–Vis spectroscopy. Values of partition coefficient, K, were determined as the ratio of the dye concentration in hydrogel relative to water. Methylene Blue, the less hydrophilic dye, showed higher K values when compared to Orange II, more hydrophilic one. Value of K depends on the temperature and on the PNIPAAm content. The PNIPAAm chains are solvated by water and randomly distributed at temperatures below 32 °C, the LCST of PNIPAAm, but collapse near or above the LCST. The collapsed PNIPAAm chains induce a more hydrophobic environment that increases the solubility of Methylene Blue and decreases the solubility of Orange II in the hydrogels of PAAm with PNIPAAm included. These hydrogels show potential application concerning the separation processes, where the temperature and/or the hydrophilicity control the diffusion.

Reaction of Glycidyl Methacrylate at the Hydroxyl and Carboxylic Groups of Poly(vinyl alcohol) and Poly(acrylic acid): Is This Reaction Mechanism Still Unclear?

Transesterification and epoxide ring-opening reactions are two mechanism routes that explain chemical modifications of macromolecules by glycidyl methacrylate (GMA). Although the coupling reaction of the GMA with macromolecules has widely been investigated, there are still mechanisms that remain to be explained when GMA is processed in an aqueous solution at different pH conditions. To this end, reaction mechanisms of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAAc) by GMA in water in acidic and basic conditions were investigated thoroughly. The presence of hydroxyl groups in PVA and carboxyl groups in PAAc allowed for a better evaluation of the reaction mechanisms. The analysis of the (1)H and (13)C NMR spectra clearly demonstrated that the chemical reactions of GMA with carboxyl groups and alcohols of the macromolecules in an aqueous solution are dependent on pH conditions. At pH 3.5, the GMA reacts with both the carboxylic and the hydroxyl groups through an epoxide ring-opening mechanism. At pH 10.5, the GMA undergoes a hydrolysis process and reacts with hydroxyl groups by way of both the transesterification and the epoxide ring-opening mechanisms, whereas the ring-opening reaction is the preferential pathway.

Synthesis and water absorption transport mechanism of a pH-sensitive polymer network structured on vinyl-functionalized pectin.

Polysaccharide-structured copolymer hydrogel having excellent pH-sensitivity was developed from N,N-dimethylacrylamide (DMAc) and vinyl-functionalized Pectin (Pec). The Pec was vinyl-functionalized by way of chemical reaction with glycidyl metacrylate (GMA) in water under acidic and thermal stimuli. 13C NMR, 1H NMR, and FT-IR spectra revealed that the vinyl groups coming from the GMA were attached onto backbone of the polysaccharide. The hydrogels were obtained by polymerization of the Pec-vinyl with the DMAc. 13C-CP/MAS NMR and FTIR spectra confirmed that the gelling process occurred by way of the vinyl groups attached on Pec-vinyl backbone. The values of apparent swelling rate constant (k) decreased appreciably for pH greater than 6, demonstrating the swelling process of the hydrogel becomes slower at more alkaline conditions. There was an increase of diffusional exponent (n) with increasing pH of the surrounding liquid. This means the water absorption profile becomes more dependent on the polymer relaxation in basified swelling media. In this condition, a longer water absorption half-time (t1/2) was verified, suggesting the polymer relaxation mechanism of the hydrogel would have a considerable effect on the t1/2.

Synthesis of Hollow-Structured Nano- and Microspheres from Pectin in a Nanodroplet Emulsion

Hollow-structured nano- and microspheres with diameters ranging from 24 microm to 160 nm were successfully produced from chemically modified pectin (Ma-Pec) through a two-step synthesis. In a first step, the Pec was modified with glycidyl methacrylate (GMA) in a heterogeneous phase system, indeed consisting of water-soluble Pec and water-insoluble GMA, via an interfacial reaction at the interface of the GMA-water phase system after 12 h under continuous stirring of 1000 rpm at 60 degrees C. In a second step, the spheres were prepared in a water-in-benzyl alcohol nanodroplet emulsion at 12000 rpm under a bubbling stream of nitrogen in the presence of sodium persulfate, as initiator, and TEMED, as catalytic agent. FT-IR spectra revealed that the vinyl groups (CC) coming from the GMA were attached onto backbone of the polysaccharide. 13C-CP/MAS NMR spectra demonstrated that the spheres were formed via carbon-carbon pi-bonds on Ma-Pec in the water phase, for the duration of the dispersion stage. The dark center (an empty core) and edge of the hollow spheres could be easily identified by SEM micrographs. This type of polymer structure represents a class of unique material with particular importance in terms of state-of the-art applications in both nano- and microencapsulation of drugs, for example, protection shields of biologically active agents.

Albumin release from a brain-resembling superabsorbent magnetic hydrogel based on starch

Brain-resembling superabsorbent hydrogel composites were developed via UV-induced copolymerization-crosslinking of vinyl-modified starch with acrylic acid (AAc) and N,N-dimethylacrylamide (DMAAm) in the presence of Fe3O4 particles. The iodine test revealed the actual 3D network structure of starch within the swollen hydrogel composite. FTIR spectra of the hydrogel composites indicated interaction between the carboxyl groups from a hydrogel and the iron ions from Fe3O4. The formation of the hydrogel composites also was evidenced by wide-angle X-ray diffraction (WAXD) and energy dispersive X-ray (EDS) spectroscopies. Scanning electron microscopy (SEM) images revealed a homogeneous material since no phase separation between the hydrogel and the Fe3O4 phases could be observed. Minimal fragments of the swollen composite allowed the study of its actual morphology by TEM imaging. Stick-type structures were observed in the composite, as a result of a water-equilibrated structural configuration (obtained in a swollen state) of carboxyl groups of AAc coordinated to iron ions of Fe3O4. The albumin release mechanism of the hydrogel without Fe3O4 is governed by macromolecular relaxation. In the hydrogel composites, the albumin release was driven by macromolecular relaxation, but with a strong tendency to anomalous transport, because of both the tortuosity effect and attenuation of the anion–anion electrostatic repulsion forces. On the other hand, the albumin release became more dependent on anomalous transport with an applied magnetic field, which intensified the tortuosity effect. The proposed hydrogels are more appropriate for use as oral drug delivery systems because they provide a better sustention of the drug over the in vitro release experiment.

Covalent TiO2/pectin microspheres with Fe3O4 nanoparticles for magnetic field-modulated drug delivery

Covalent TiO(2)-co-pectin microspheres containing Fe(3)O(4) nanoparticles were developed through an ultrasound-induced crosslinking/polymerization reaction between the glycidyl methacrylate from vinyl groups in TiO(2) and in pectin. ζ-potentials became less negative in the nanostructured microspheres, caused by the presence of both inorganic particles in the negatively charged pectin. The nanostructured pectin microspheres showed an amoxicillin release rate slower than that of pure pectin microspheres. The proposed microspheres were found to be a sustained release system of amoxicillin in the acid medium. Furthermore, the antibiotic release may be modulated by exposition of the microspheres to a remote magnetic field. In practical terms, the nanostructured microspheres could deliver a larger proportion of their initial load to specific site of action. The cytotoxic concentrations for 50% of VERO cells (CC(50)), calculated as the concentration required to reduce cell viability by 50% after 72h of incubation, for pectin-only microspheres and nanostructured pectin microspheres were 217.7±6.5 and 121.5±4.9μgmL(-1), respectively. The obtained CC(50) values indicated acceptable cytotoxic levels for an incubation period of 72h, showing that the pectin microspheres have a great pharmacological potential for uses in biological environments, even after the introduction of both Fe(3)O(4) and TiO(2).

Smart hollow microspheres of chondroitin sulfate conjugates and magnetite nanoparticles for magnetic vector

Smart hollow microspheres composed of vinyled-chondroitin sulfate conjugates (CSπ) and magnetite nanoparticles were obtained by the intermediate of a multiple emulsion in absence of a surfactant, attributable to stabilizing properties of the CS. It was formed an oil-water multiple emulsion in which the CS played a role as an anionic stabilizer for magnetite nanoparticles via complexation. Iron oxides were bonded to the microspheres by the formation of a complex of Fe(3+) ions on the crystalline phase with oxygen atoms at the carboxyl groups without their magnetic properties being affected. The average crystal size of embedded magnetite nanoparticles was approximately 16.5nm, indicative of a good dispersion in microspheres. Furthermore, the introduction of iron oxides resulted in microspheres with a higher diameter and a narrower particle size distribution.

Synthesis and characterization of a pH-responsive poly(ethylene glycol)-based hydrogel: acid degradation, equilibrium swelling, and absorption kinetic characteristics

We developed a pH-responsive, hydrogel based on poly(ethylene glycol) (PEG) covalently cross-linked with acrylic acid and N′,N′-dimethylacrylamide along with acid-labile groups. In the hydrogel, PEG plays a role as the key constituent. If its chains break, the polymer networks are destroyed. The pharmacological potential of these hydrogels were demonstrated by determining their water transport profile, modulus of elasticity, and cytotoxicity assay. The hydrogels showed a pseudo-Fickian behavior, a transport mechanism that occurs when the diffusion coefficient changes with the time and the swelling equilibrium is never fully reached. At pH 2, the PEG-richer hydrogels degraded and the scanning electron microscopy (SEM) images illustrated less-defined shapes than at pH 7 and 10. This morphological characteristic results of the hydrogel deconstruction owing to cleavage of ether bonds of the PEG chains unmaking its 3D polymer network. The proposed hydrogels were shown to be compatible to cells, indicating acceptable biocompatibility and an appropriate level of security for use in the biological environments. Furthermore, they showed structural changes in their polymer network in response to pH, which is an important characteristic for stimuli-triggered release of guest molecules.

Solid-state radical grafting reaction of glycidyl methacrylate and poly(4-methyl-1-pentene) in supercritical carbon dioxide: Surface morphology and adhesion

Solid-state radical grafting of glycidyl methacrylate (GMA) onto poly(4-methyl-1-pentene) (PMP) was performed using supercritical carbon dioxide (scCO(2)) impregnation technology. The polymer films were firstly impregnated in the scCO(2) phase with the GMA using benzoyl peroxide as thermal initiator. The grafting degree and surface morphology of the samples may be controlled by the following factors: time, temperature, and pressure of impregnation. A 2(3) factorial design to evaluate the main and interaction effects of such factors on the grafting of the PMP by GMA (grafting response) was elaborated from FTIR data. The superior and inferior limits of the levels were defined on basis of a P-x-y diagram for binary system CO(2)+GMA that provided the location of the transition curves of such a system. Better grafting response was obtained for pressure of 130 bar, temperature of 70°C and time of 7h. The PMP-g-GMA films exhibited a thermal profile similar to that of the unmodified polymer. Adhesion characteristics of polymer films are dependent on grafting degree of GMA.

Drug release profile and reduction in the in vitro burst release from pectin/HEMA hydrogel nanocomposites crosslinked with titania

This work describes the drug release profile and the initial burst release from covalent hydrogel nanocomposites composed of pectin, hydroxyethyl methacrylate (HEMA) and titania (TiO2). Vitamin B12 (Vit-B12), a highly water-soluble substance, was used as a model drug. We studied the water transport profiles over a wide pH range, the moduli of elasticity (E), the morphological properties and the Vit-B12 release kinetics from these hydrogels. The initial release burst was reduced by crosslinking titania with vinylated pectin and HEMA. A reduction of up to ca. 60% was observed when compared with pure pectin/HEMA hydrogel. To gain insight into the burst release phenomenon, the experimental data were adjusted to diffusive-based models that include a rate constant of release (k). A decrease in the values of k was related to a reduction in the burst effect. The release mechanism of Vit-B12 from the pure hydrogels was governed by both Fickian diffusion and macromolecular relaxation, which are the driving forces for release. Upon addition of titania, the contribution of macromolecular relaxation to the release was minimized, suggesting a tendency towards Fickian diffusion. Furthermore, titania played a significant role in improving mechanical properties. Hydrogel nanocomposites showed a marked increase in E compared with pure hydrogels. This increase was found to be the result of an apparent increment in the cross-linking density, owing to chemical bonds of titania with the hydrogel. The proposed materials were demonstrated to be biocompatible with cells, showing good pharmacological potential.