Viscoelastic Behavior of Poly(methyl methacrylate) Networks with Different Cross-Linking Degrees

Mediating the slow dynamics of polyacrylates by small molecule-bridged hydrogen bonds

This report studied the changes in the slow dynamics of polyacrylate by adding a hindered phenol (CA) capable of forming three intermolecular hydrogen bonds (inter-HBs) per molecule with the polymer chain. The CA molecule apparently diminishes the slow modes (with lower peak temperatures and peak heights) of the polyacrylate, although it has a higher glass transition temperature (T_g) than the acrylic matrix, and the rigid CA-bridged HB network significantly amplifies the α-relaxation near T_g (with higher peak temperatures and peak heights). Consequently, the mixtures exhibit a diminishing slow mode that gradually merges with the prominent α-peak with increasing CA loadings. The anomalous dynamics concerning the opposite behaviors of the slow mode and α-relaxation was further rationalized in terms of dissociation of inter-HBs when the temperature is higher than T_g, together with the small molecule-alleviated macromolecular connectivity. This work provides essential insights into the slow dynamics of such HB-driven hybrids, and paves the way for tailoring the viscous flow properties of the hybrid material from a molecular level perspective.

High temperature shape memory poly(amide-imide)s with strong mechanical robustness

Shape memory poly(amide-imide)s with strong mechanical robustness, outstanding heat resistance and low water uptake were fabricated.

New method to differentiate surface damping behavior and stress absorption capacities of common CAD/CAM restorative materials

OBJECTIVES

To assess energy dissipation capacities and surface damping abilities of different CAD/CAM restorative materials (CRMs) to characterize stress resistance during load peaks.

METHODS

Using instrumented indentation testing (IIT), Martens hardness (HM) together with its elastic (η_IT) and plastic index (η_ITdis) and Leeb hardness (HLD) together with its deduced energy dissipation (HLD_dis) were determined for eight ceramic, eight composite, and four polymer-based materials as well as three metals. The results were compared to those of bovine enamel. Ten indentations per material were performed at room temperature (23 ± 1 °C) on two separate specimens (12.0 × 12.0 × 3.5 mm³) after water storage (24 h; 37.0 ± 1.0 °C). Hardness parameters were recorded, and data were analyzed with one-way MANOVA (Games-Howell post hoc tests, α = 0.05). Correlations between different parameters were tested (Pearson, α = 0.05).

RESULTS

Independently determined HLD_dis, and η_ITdis values substantiated different energy dissipation characteristics of CRM, whereby a strong correlation was observed for the two datasets (r = 0.956, p = 0.011). Ceramics had the significantly lowest values (p < 0.001) while both parameters revealed the highest surface damping effects for metals (p < 0.001), followed in both cases by bovine enamel. Energy dissipation of polymer and composite CRM was in between ceramics and bovine enamel (p < 0.001), whereas only for HLD_dis did both show no significant difference (p > 0.05).

SIGNIFICANCE

Promising new HLD_dis and η_ITdis data allow a reliable differentiation of energy dissipation and surface damping capacities of CRMs. Previously published rankings of edge chipping and loss tangent results were perfectly reproduced, especially by HLD_dis.

Bioinspired multilayer membranes as potential adhesive patches for skin wound healing

Bioinspired and adhesive multilayer membranes are produced using the layer-by-layer (LbL) assembly of chitosan (CHT), alginate (ALG) and hyaluronic acid modified with dopamine (HA-DN). Freestanding multilayer membranes without DN are also produced as a control. The success of the synthesis of HA-DN was confirmed using UV-visible spectroscopy. Scanning electron microscopy images indicate that the surface of the DN-containing membranes is more porous than the control ones; they also present a higher average thickness value for the same number of CHT/ALG/CHT/HA(-DN) tetralayers (n = 100). Also, water uptake, mechanical strength and adhesion are enhanced with the introduction of DN moieties along the nano-layers. Besides, human dermal fibroblast viability, enhanced adhesion and proliferation were confirmed by immunofluorescence assays and by measuring both the metabolic activity and DNA content. Moreover, in vivo assays with such kinds of DN-containing multilayer membranes were performed; the application of these membranes in the treatment of dermal wounds induced in Wistar rats results in the highest decrease of inflammation of rat skin, compared with the control conditions. Overall, this investigation suggests that these mussel-inspired freestanding multilayer membranes may enhance either their mechanical performance or cellular adhesion and proliferation, leading to an improved wound healing process, being a promising material to restore the structural and functional properties of wounded skin.

Adhesive free-standing multilayer films containing sulfated levan for biomedical applications

This work is the first reporting the use of layer-by-layer to produce adhesive free-standing (FS) films fully produced using natural-based macromolecules: chitosan (CHI), alginate (ALG) and sulfated levan (L-S). The deposition conditions of the natural polymers were studied through zeta potential measurements and quartz crystal microbalance with dissipation monitoring analysis. The properties of the FS films were evaluated and compared with the control ones composed of only CHI and ALG in order to assess the influence of levan polysaccharide introduced in the multilayers. Tensile tests, dynamic mechanical analysis and single lap shear strength tests were performed to evaluate the mechanical properties of the prepared FS films. The presence of L-S conferred both higher tensile strength and shear strength to the developed FS membranes. The results showed an adhesion strength 4 times higher than the control (CHI/ALG) FS films demonstrating the adhesive character of the FS films containing L-S. Morphological and topography studies were carried out revealing that the crosslinking reaction granted the L-S based FS film with a higher roughness and surface homogeneity. Preliminary biological assays were performed by cultivating myoblasts cells on the surface of the produced FS films. Both crosslinked and uncrosslinked FS films containing L-S were cytocompatible and myoconductive.

STATEMENT OF SIGNIFICANCE

Sutures remain as the "gold standard" for wound closure and bleeding control; however they still have limitations such as, high infection rate, inconvenience in handling, and concern over possible transmission of blood-borne disease through the use of needles. One of the challenges of tissue engineering consist on the design and development of biocompatible tissue adhesives and sealants with high adhesion properties to repair or attach devices to tissues. In this work, the introduction of sulfated levan (L-S) on multilayered free-standing membranes was proposed to confer adhesive properties. Moreover, the films were myoconductive even in the absence of crosslinking just by the presence of L-S. This study provides a promising strategy to develop biological adhesives and for cardiac tissue engineering applications.

Control of Cell Alignment and Morphology by Redesigning ECM-Mimetic Nanotopography on Multilayer Membranes

Inspired by native extracellular matrix (ECM) together with the multilevel architecture observed in nature, a material which topography recapitulates topographic features of the ECM and the internal architecture mimics the biological materials organization is engineered. The nanopatterned design along the XY plane is combined with a nanostructured organization along the Z axis on freestanding membranes prepared by layer-by-layer deposition of chitosan and chondroitin sulfate. Cellular behavior is monitored using two different mammalian cell lines, fibroblasts (L929) and myoblasts (C2C12), in order to perceive the response to topography. Viability, proliferation, and morphology of L929 are sensitively controlled by topography; also differentiation of C2C12 into myotubes is influenced by the presence of nanogrooves. This kind of nanopatterned structure has also been associated with strong cellular alignment. To the best of the knowledge, it is the first time that such a straightforward and inexpensive strategy is proposed to produce nanopatterned freestanding multilayer membranes. Controlling cellular alignment plays a critical role in many human tissues, such as muscles, nerves, or blood vessels, so these membranes can be potentially useful in specific tissue regeneration strategies.

Nanomechanics of layer-by-layer polyelectrolyte complexes: a manifestation of ionic cross-links and fixed charges

This study investigates the roles of two distinct features of ionically cross-linked polyelectrolyte networks - ionic cross-links and fixed charges - in determining their nanomechanical properties. The layer-by-layer assembled poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) network is used as the model material. The densities of ionic cross-links and fixed charges are modulated through solution pH and ionic strength (IS), and the swelling ratio, elastic and viscoelastic properties are quantified via an array of atomic force microscopy (AFM)-based nanomechanical tools. The roles of ionic cross-links are underscored by the distinctive elastic and viscoelastic nanomechanical characters observed here. First, as ionic cross-links are highly sensitive to solution conditions, the instantaneous modulus, E0, exhibits orders-of-magnitude changes upon pH- and IS-governed swelling, distinctive from the rubber elasticity prediction based on permanent covalent cross-links. Second, ionic cross-links can break and self-re-form, and this mechanism dominates force relaxation of PAH/PAA under a constant indentation depth. In most states, the degree of relaxation is >90%, independent of ionic cross-link density. The importance of fixed charges is highlighted by the unexpectedly more elastic nature of the network despite low ionic cross-link density at pH 2.0, IS 0.01 M. Here, the complex is a net charged, loosely cross-linked, where the degree of relaxation is attenuated to ≈50% due to increased elastic contribution arising from fixed charge-induced Donnan osmotic pressure. In addition, this study develops a new method for quantifying the thickness of highly swollen polymer hydrogel films. It also underscores important technical considerations when performing nanomechanical tests on highly rate-dependent polymer hydrogel networks. These results provide new insights into the nanomechanical characters of ionic polyelectrolyte complexes, and lay the ground for further investigation of their unique time-dependent properties.

Myoconductive and osteoinductive free-standing polysaccharide membranes

Free-standing (FS) membranes have increasing applications in the biomedical field as drug delivery systems for wound healing and tissue engineering. Here, we studied the potential of free-standing membranes made by the layer-by-layer assembly of chitosan and alginate to be used as a simple biomimetic system of the periosteum. The design of a periosteum-like membrane implies the elaboration of a thick membrane suitable for both muscle and bone formation. Our aim was to produce well-defined ∼50 μm thick polysaccharide membranes that could be easily manipulated, were mechanically resistant, and would enable both myogenesis and osteogenesis in vitro and in vivo. The membranes were chemically crosslinked to improve their mechanical properties. Crosslinking chemistry was followed via Fourier transform infrared spectroscopy and the mechanical properties of the membranes were assessed using dynamic mechanical analysis. The loading and release of the potent osteoinductive growth factor bone morphogenetic protein 2 (BMP-2) inside and outside of the FS membrane was followed by fluorescence spectroscopy in a physiological buffer over 1 month. The myogenic and osteogenic potentials of the membranes in vitro were assessed using BMP-2-responsive skeletal myoblasts. Finally, their osteoinductive properties in vivo were studied in a preliminary experiment using a mouse ectopic model. Our results showed that the more crosslinked FS membranes enabled a more efficient myoblast differentiation in myotubes. In addition, we showed that a tunable amount of BMP-2 can be loaded into and subsequently released from the membranes, depending on the crosslinking degree and the initial BMP-2 concentration in solution. Only the more crosslinked membranes were found to be osteoinductive in vivo. These polysaccharide-based membranes have strong potential as a periosteum-mimetic scaffold for bone tissue regeneration.

Swelling equilibrium of dentin adhesive polymers formed on the water-adhesive phase boundary: experiments and micromechanical model

During their application to the wet, oral environment, dentin adhesives can experience phase separation and composition change, which can compromise the quality of the hybrid layer formed at the dentin-adhesive interface. The chemical composition of polymer phases formed in the hybrid layer can be represented using a ternary water-adhesive phase diagram. In this paper, these polymer phases are characterized using a suite of mechanical tests and swelling experiments. The experimental results were evaluated using a granular micromechanics-based model incorporating poro-mechanical effects and polymer-solvent thermodynamics. The variation in the model parameters and model-predicted polymer properties was studied as a function of composition along the phase boundary. The resulting structure-property correlations provide insight into interactions occurring at the molecular level in the saturated polymer system. These correlations can be used for modeling the mechanical behavior of the hybrid layer, and are expected to aid in the design and improvement of water-compatible dentin adhesive polymers.

Chitosan scaffolds with a shape memory effect induced by hydration

Chitosan-based porous scaffolds exhibit a shape memory effect triggered by hydration, and they are candidates for applications in minimally invasive surgery.

The effect of an organophosphonite antioxidant on poly(ether ether ketone)/poly(ether imide) blend as studied by quasi-isothermal modulated differential scanning calorimetry and dynamic mechanical thermal analysis methods

In the present study, the effect of P-EPQ, an organophosphonite antioxidant, on the glass transition and the viscoelastic behavior of optimized poly(ether ether ketone) (PEEK)/poly(ether imide) (PEI; PEEK/PEI: 50/50) blend has been reported. For this, P-EPQ was added in various proportions to the PEEK/PEI blend and was studied by quasi-isothermal modulated differential scanning calorimetry (Qi-MDSC) and dynamic mechanical thermal analysis (DMTA) methods. The Qi-MDSC results clearly indicate a significant decrease in the glass transition temperature of the blend up to 1 wt% P-EPQ addition. The DMTA measurements revealed a decrease in tan δ peak temperature upon antioxidant addition in agreement with calorimetric results. The shift factors obtained using the time–temperature superposition of DMTA data exhibit Williams–Landel–Ferry behavior, and the fractional free volume of the blend increases with increase in antioxidant addition (up to 1 wt%). The “master curve” of storage modulus predicts high modulus for the blend with 1 wt% antioxidant, revealing its suitability for high-impact applications.

Polyphenol control of cell spreading on glycoprotein substrata

Cell-surface contacts are vital for many eukaryotic cells. The surface provides anchorage (facilitating spreading and proliferation), is involved in sensation, i.e., via mechano-, osmo- and chemoreceptors, and in addition nutrients may also be supplied via vessels adjacent to the basal lamina. Hence, the ability to manipulate the surface characteristics provides a mechanism for directly influencing cell behaviour. Applications such as medical implants and tissue engineering require biocompatible, stable surfaces for controlling cell behaviour. Mucin-coated surfaces inhibit cell spreading compared with poly(L-lysine) in vitro; here, we show that a composite layer assembled from mucin-EGCg aggregates counters the inhibition. Although the anti-spreading effects of the glycoprotein substratum on cell behaviour are similar to those observed for pure polysaccharide surfaces, the reversal of cell spreading inhibition by the admixture of polyphenol/glycoprotein substrata is remarkable and unexpected. Possible applications for a composite glycoprotein-polyphenol layer include medical devices, in particular for those operating at mucosal interfaces such as the oral, tracheal or gastrointestinal tract cavities, wound healing, cancer control and the controlled growth of therapeutic cell cultures.

Viscoelastic Properties of Chitosan with Different Hydration Degrees as Studied by Dynamic Mechanical Analysis

Dynamic mechanical analysis, DMA, is an adequate technique for characterizing the mechanical features of biomaterials, as one can use test conditions that can more closely simulate the physiological environments in which they are going to be applied. In this work it was possible to perform different tests on chitosan membranes using low/moderate hydration levels, as well in completely wet conditions. In the first case the data obtained at different relative humidity environments were rationalized under a time-humidity superposition principle, where a master curve for the storage modulus could be obtained along a wide range of frequencies. The temperature dependence of the shift factors exhibited a curvature opposite to that expected by the WLF equation, and is consistent with relaxation dynamics behavior below the glass transition. Temperature scans above room temperature in both dry and wet conditions did not reveal strong variations in the viscoelastic properties. It was possible to follow in real time the water uptake in an initially-dry membrane. During the initial strong and fast decrease of the storage modulus the loss factor exhibited a peak that should correspond to the occurrence of the glass transition resulting from the plasticization effect of water. Upon equilibration the loss factor reached similar values as for the dry material (tandelta approximately equal to 0.5). The viscoelastic characterization reported in this work for chitosan may be useful in the use of such material for a variety of biomedical applications.

The kinetics of the structural relaxation process in PHEMA-silica nanocomposites based on an equation for the configurational entropy

The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate) (PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, Tg, and the temperature interval of the glass transition, DeltaTg , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used for determining the temperature dependence of the relaxation times during the process. The results show an increase of the fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution of relaxation times characterized by the beta parameter of the stretched exponential distribution. On another hand the silica content increase produces a significant change of the relaxation times in the glassy state.

Dielectric relaxation spectrum of poly (epsilon-caprolactone) networks hydrophilized by copolymerization with 2-hydroxyethyl acrylate

The dielectric relaxation spectrum of polycaprolactone (PCL) networks hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) is investigated. PCL is a semicrystalline polyester with a complex relaxation spectrum that includes the main alpha relaxation and two secondary modes (beta, gamma) at lower temperatures. The overlapping of the different relaxational modes was split by using several Havriliak-Negami functions. Crosslinking the material modifies the dynamics of the main relaxation process as reflected by the parameters that characterize the Vogel behavior of the process and the dynamic fragility. The incorporation of HEA units in the network results in a material with microphase separation: two alpha processes are detected, the one corresponding to the PCL chains and the new one associated to nanometric regions that contain different amount of both comonomers. The incorporation of the HEA units in the system involves the presence of a new beta(sw) relaxation due to the link of two side chains by water molecules through hydrogen bonding.