M. Monleón Pradas

Porous poly(2-hydroxyethyl acrylate) hydrogels

Abstract Porous hydrogels were prepared by copolymerisation of 2-hydroxyethyl acrylate and ethyleneglycol dimethacrylate (as crosslinking agent) in solution using water or ethanol as solvents. Macroscopic pores are formed due to the segregation of the solvent from the polymer network during the polymerisation process. In the dry state the polymer network had nearly the same density as the poly(2-hydroxyethyl acrylate) polymerised in bulk thus showing that the pores collapse during the drying process. When the dry samples were swollen in water the pores opened and the volume fraction of pores could be determined by weighing. The pore morphology was observed by scanning electron microscopy. The dependence of the pore size on the solvent used and on the monomer/solvent ratio in the polymerisation process is shown. The elastic modulus and loss tangent were measured as a function of temperature in the region of the main (or α) dynamic-mechanical relaxation process. These spectra were correlated with the morphology of the samples.

Molecular mobility and hydration properties of segmented polyurethanes with varying structure of soft‐ and hard‐chain segments

The molecular mobility and hydration properties of model segmented polyurethanes from either poly(propylene glycol) (PPG) or poly(butylene adipate) (PBAD), both of molecular weight 2000 (soft segments), and three different diisocyanates (all-trans 4,4′-dicyclohexylmethane diisocyanate, 100% t,t HMDI; HMDI with 20% of trans isomers, 20% t,t HMDI; and 4,4′-diphenylmethane diisocyanate, MDI) (hard segments) were investigated using differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) measurements, ac dielectric relaxation spectroscopy (ac DRS), equilibrium water-sorption isotherms (ESI), and dynamic water-sorption isotherms (DSI). No effects of the structure and of the amount of the soft segments on the overall degree of microphase separation (DMS) into microphases rich in soft and hard segments, respectively, were observed. On the contrary, DMS depends on the composition of the diisocyanates used and systematically increases in the order MDI, 20% t,t HMDI, 100% t,t HMDI as indicated by DSC, TSDC, and ac DRS. The PPG-based polyurethanes are characterized by larger values of water content at saturation, h, and smaller values of the diffusion coefficient of water, D. h increases with temperature, indicating that the sorption process is endothermic.

Polymer-water interactions in poly(hydroxyethyl acrylate) hydrogels studied by dielectric, calorimetric and sorption isotherm measurements

Abstract Poly(hydroxyethyl acrylate) hydrogels with water contents up to 0.96 (g water/g dry material) are studied with equilibrium and dynamic water sorption isotherm measurements, differential scanning calorimetry, thermally stimulated depolarization current measurements and broadband a.c. dielectric relaxation spectroscopy. Our main interest is focused on comparing results on the state of water in the hydrogel obtained with the different techniques on the same sample and on discussing in a quantitative way the relations among the different descriptions imposed by the different techniques. The results show that for water contents lower than about 0.20-0.30 the hydrogel is a homogeneous system in the temperature range down to −100 °C. At higher water contents phase separation occurs. Several critical water contents have been obtained by means of the different techniques used.

Structural relaxation of glass‐forming polymers based on an equation for configurational entropy, 4. Structural relaxation in styrene‐acrylonitrile copolymer

The structural relaxation process in styrene-acrylonitrile copolymer has been characterized by means of differential scanning calorimetry (DSC) experiments. The results in the form of heat capacity, cp(T), curves are analyzed using a model for the evolution of the configurational entropy during the process recently proposed by the authors.11,12 The model simulation allows one to determine the enthalpy (or entropy) structural relaxation times and the β parameter of the Kohlrausch-Williams-Watts equation characterizing the width of the distribution of relaxation times. This material parameters are compared with their analogues determined from the dielectric and dynamic-mechanical relaxation processes.

Forced compatibility in poly(methyl acrylate)/poly(methyl methacrylate) sequential interpenetrating polymer networks

The aim of this work is to study the miscibility of poly(methyl acrylate)/poly(methyl methacrylate), (PMA/PMMA), sequential interpenetrating networks, (IPNs), as a function of the crosslink density using dielectric and dynamic-mechanical techniques. The PMA/PMMA system is immiscible and so, for low crosslink densities, phase separation appears, as detected by the occurrence of two clearly differentiated main dynamic-mechanical relaxation processes corresponding to the two components. If crosslink density is high enough, a homogeneous IPN can be obtained, achieving a forced compatibilization of both networks. The IPN crosslinked with 10% ethyleneglycol dimethacrylate shows a single main dynamic-mechanical relaxation process. Only the α main relaxation process appears in the PMA networks within the temperature range (−60 to 200°C) of the experiments conducted in this work. The dielectric relaxation spectrum of PMMA networks shows the secondary β relaxation followed by a small α relaxation partially overlapped with it. In the IPNs, both the main relaxation processes tend to merge into a single one and the dielectric spectrum shows a single peak that mainly corresponds to the secondary relaxation of the PMMA.

Dielectric relaxation spectroscopy in PHEA hydrogels

Abstract Poly(hydroxyethyl acrylate) hydrogels with water contents varying in a wide interval are studied with differential scanning calorimetry and thermally stimulated depolarization currents (TSDC). The TSDC measurements reveal the influence of water on the secondary relaxations and suggest an additional mechanism due to reorientation of water molecules when the water content (dry basis) is higher than 0.3. The dependence of the glass transition temperature and the intensity and temperature of the main dielectric relaxation have been determined as a function of the water content.

Structural relaxation of glass-forming polymers based on an equation for configurational entropy: 3. On the states attained at infinite time in the structural relaxation process. Results on poly(ether imide)

The aim of this paper is to discuss the limit state attained at infinite time in the structural relaxation process. This state usually is identified with the equilibrium state extrapolated from the experimental data obtained at temperatures above the glass transition. The analysis is conducted with the help of a phenomenological model with fitting parameters, based on an equation for the evolution of the configurational entropy during the process. The model avoids the use of the fictive temperature, which makes it easier to introduce a different hypothesis on the limit states of the process.

Dielectric relaxations in poly(hydroxyethyl acrylate): influence of the absorbed water

Abstract Poly(hydroxyethyl acrylate) presents two relaxation zones, labelled γ and α, when it is completely dry. The temperature of the maximum of the γ relaxation, as well as its apparent activation energy, are somewhat higher than in poly(hydroxyethyl methacrylate), a fact that could be explained by higher intermolecular interactions in the series of polyacrylates than in the series of polymethacrylates. The absorption of even slight traces of water causes a new relaxation to appear, the intensity of which increases with the content of water, while at the same time the intensity of the γ relaxation decreases. This fact suggests the formation of an association of the water molecules with the side groups of the polymer. The characterization of the α relaxation is difficult because of the high d.c. conductivity component of the permittivity. Its temperature suggests the presence of hydrogen bonds which render the main chains rigid.

Combining self-assembling peptide gels with three-dimensional elastomer scaffolds.

Some of the problems raised by the combination of porous scaffolds and self-assembling peptide (SAP) gels as constructs for tissue engineering applications are addressed for the first time. Scaffolds of poly(ethyl acrylate) and the SAP gel RAD16-I were employed. The in situ gelation of the SAP gel inside the pores of the scaffolds was studied. The scaffold-cum-gel constructs were characterized morphologically, physicochemically and mechanically. The possibility of incorporating an active molecule (bovine serum albumin, taken here as a model molecule for others) in the gel within the scaffolds pores was assessed, and the kinetics of its release in phosphate-buffered saline was followed. Cell seeding and colonization of these constructs were preliminarily studied with L929 fibroblasts and subsequently checked with sheep adipose-tissue-derived stem cells intended for further preclinical studies. Static (conventional) and dynamically assisted seedings were compared for bare scaffolds and the scaffold-cum-gel constructs. The SAP gel inside the pores of the scaffold significantly improved the uniformity and density of cell colonization of the three-dimensional (3-D) structure. These constructs could be of use in different advanced tissue engineering applications, where, apart from a cell-friendly extracellular matrix -like aqueous environment, a larger-scale 3-D structure able to keep the cells in a specific place, give mechanical support and/or conduct spatially the tissue growth could be required.

Surface modification of P(EMA-co-HEA)/SiO2 nanohybrids for faster hydroxyapatite deposition in simulated body fluid?

P(EMA-co-HEA)/SiO(2) nanocomposites with 0, 15 and 30 wt% of silica were obtained by copolymerization of ethyl methacrylate, EMA, and hydroxyethyl acrylate, HEA, during the simultaneous acid-catalyzed sol-gel polymerization of tetraethoxysilane, TEOS. A surface modification treatment was applied in order to reduce the induction time for hydroxyapatite (HAp) nucleation, combining a previous NaOH attack to increase the number of surface nucleating sites, and an alternate soaking process in Ca and P solutions to form apatite precursors, prior to the immersion in a simulated body fluid (SBF). The NaOH treatment was not effective by itself in shortening the HAp induction time. It introduced sodium carboxylates in the copolymer but hydrolyzed the silica network excessively, thus reducing the surface nucleating potential of its boundary silanols. Therefore, bioactivity was only due to the surface carboxylate groups of the organic phase. Maybe a controlled dissolution extent of the silica network so as to improve bioactivity could be attained by reducing the duration of the NaOH-treatment. This would be interesting in the hybrid with 30wt% of silica, because its dense silica network is not able to hydrolyze in SBF without any previous treatment, whereas the silica network in the hybrid with 15wt% of silica hydrolyzes at the surface promoting the deposition of HAp. The CaP treatment was able to coat the surfaces of the samples with a calcium phosphate layer within minutes. This amorphous calcium phosphate acted as HAp precursor, skipping the induction period in SBF.