Microstructure determination of AOT + phenol organogels utilizing small-angle X-ray scattering and atomic force microscopy

Dry reverse micelles of the anionic twin-tailed surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) dissolved in nonpolar solvents spontaneously form an organogel when p-chlorophenol is added in a 1:1 AOT:phenol molar ratio. The solvents used were benzene, toluene, m-xylene, 2,2,4-trimethylpentane (isooctane), decane, dodecane, tetradecane, hexadecane, and 2,6,10,14-tetramethylpentadecane (TMPD). The proposed microstructure of the gel is based on strands of stacked phenols linked to AOT through hydrogen bonding. Small-angle X-ray scattering (SAXS) spectra of the organogels suggest a characteristic length scale for these phenol-AOT strands that is independent of concentration but dependent on the chemical nature of the nonpolar solvent used. Correlation lengths determined from the SAXS spectra indicate that the strands self-assemble into fibers. Direct visualization of the gel in its native state is accomplished by using tapping mode atomic force microscopy (AFM). It is shown that these organogels consist of fiber bundle assemblies. The SAXS and AFM data reinforce the theory of a molecular architecture consisting of three length scales-AOT/phenolic strands (ca. 2 nm in diameter) that self-assemble into fibers (ca. 10 nm in diameter), which then aggregate into fiber bundles (ca. 20-100 nm in diameter) and form the organogel.

Combinatorial investigation of the structure-properties characterization of photopolymerized dimethacrylate networks

The effects of co-monomer composition and irradiation time in a model two-component dimethacrylate dental resin blend were evaluated using combinatorial methods to determine the degree of methacrylate conversion and resulting mechanical properties. 2-Dimensional gradient samples varying in monomer composition and light exposure time were fabricated. The conversion was measured using near infrared spectroscopy (NIR) and the mechanical properties (i.e., hardness and elastic modulus) were determined using nanoindentation via the continuous stiffness method. An excellent correlation was observed between the reaction conversion and mechanical properties for the cross-linked networks. The methacrylate conversion ranged from 40% to 85% and the mechanical properties increased over two orders of magnitude over this conversion range. The ultimate reaction conversion and mechanical properties depended on both the co-monomer composition and cure time.

Simultaneous measurement of polymerization stress and curing kinetics for photo-polymerized composites with high filler contents

OBJECTIVES

Photopolymerized composites are used in a broad range of applications with their performance largely directed by reaction kinetics and contraction accompanying polymerization. The present study was to demonstrate an instrument capable of simultaneously collecting multiple kinetics parameters for a wide range of photopolymerizable systems: degree of conversion (DC), reaction exotherm, and polymerization stress (PS).

METHODS

Our system consisted of a cantilever beam-based instrument (tensometer) that has been optimized to capture a large range of stress generated by lightly-filled to highly-filled composites. The sample configuration allows the tensometer to be coupled to a fast near infrared (NIR) spectrometer collecting spectra in transmission mode.

RESULTS

Using our instrument design, simultaneous measurements of PS and DC are performed, for the first time, on a commercial composite with ≈80% (by mass) silica particle fillers. The in situ NIR spectrometer collects more than 10 spectra per second, allowing for thorough characterization of reaction kinetics. With increased instrument sensitivity coupled with the ability to collect real time reaction kinetics information, we show that the external constraint imposed by the cantilever beam during polymerization could affect the rate of cure and final degree of polymerization.

SIGNIFICANCE

The present simultaneous measurement technique is expected to provide new insights into kinetics and property relationships for photopolymerized composites with high filler content such as dental restorative composites.

Systematic investigation of porogen size and content on scaffold morphometric parameters and properties

A systematic investigation of tissue engineering scaffolds prepared by salt leaching of a photopolymerized dimethacrylate was performed to determine how the scaffold structure (porosity, pore size, etc.) can be controlled and also to determine how the scaffold structure and the mechanical properties are related. Two series of scaffolds were prepared with (1) the same polymer-to-salt ratio but different salt sizes (ranging from average size of 100 to 390 microm) and (2) the same salt size but different polymer-to-salt ratios (ranging from salt mass of 70 to 90%). These scaffolds were examined to determine how the fabrication parameters affected the scaffold morphometric parameters and corresponding mechanical properties. Combined techniques of X-ray microcomputed tomography (microCT), mercury porosimetry, and gravimetric analysis were used to determine the scaffold parameters, such as porosity, pore size, and strut thickness and their size distributions, and pore interconnectivity. Scaffolds with porosities ranging from 57% to 92% (by volume) with interconnected structures could be fabricated using the current technique. The porosity and strut thickness were subsequently related to the mechanical response of the scaffolds, both of which contribute to the compression modulus of the scaffold. The current study shows that the structure and properties of the scaffold could be tailored by the size and the amount of porogen used in the fabrication of the scaffold.

Tissue engineering scaffolds based on photocured dimethacrylate polymers for in vitro optical imaging

Model tissue engineering scaffolds based on photocurable resin mixtures with sodium chloride have been prepared for optical imaging studies of cell attachment. A photoactivated ethoxylated bisphenol A dimethacrylate was mixed with sieved sodium chloride (NaCl) crystals and photocured to form a cross-linked composite. Upon soaking in water, the NaCl dissolved to leave a porous scaffold with desirable optical properties, mechanical integrity, and controlled porosity. Scaffolds were prepared with salt crystals that had been sieved to average diameters of 390, 300, 200, and 100 microm, yielding porosities of approximately 75 vol %. Scanning electron microscopy and X-ray microcomputed tomography confirmed that the pore size distribution of the scaffolds could be controlled using this photocuring technique. Compression tests showed that for scaffolds with 84% (by mass fraction) salt, the larger pore size scaffolds were more rigid, while the smaller pore size scaffolds were softer and more readily compressible. The prepared scaffolds were seeded with osteoblasts, cultured between 3 and 18 d, and examined using confocal microscopy. Because the cross-linked polymer in the scaffolds is an amorphous glass, it was possible to optically image cells that were over 400 microm beneath the surface of the sample.

Quantifying the Directional Parameter of Structural Anisotropy in Porous Media

A new method has been developed to define the directional parameter and characterize the structural anisotropy of a highly porous structure with extensive pore interconnectivity and surface area, such as scaffolds in tissue engineering. This new method called intercept segment deviation (ISD) was validated through the comparison of structural anisotropy from ISD measurements with mechanical anisotropy from finite-element stress analysis. This was carried out on a generated two-dimensional (2D) image of a two-phase material and a real three-dimensional (3D) image of a tissue scaffold. The performance of other methods for quantification of the directional parameter was also assessed. The results indicate that the structural anisotropy obtained from this new method conforms to the actual mechanical anisotropy and provides a better prediction of the material orientation than the other methods for the 2D and 3D images studied.

Material model measurements and predictions for a random pore poly(ϵ-caprolactone) scaffold

We investigated material models for a polymeric scaffold used for bone. The material was made by co-extruding poly(epsilon-caprolactone) (PCL), a biodegradable polyester, and poly(ethylene oxide) (PEO). The water soluble PEO was removed resulting in a porous scaffold. The stress-strain curve in compression was fit with a phenomenological model in hyperbolic form. This material model will be useful for designers for quasi-static analysis as it provides a simple form that can easily be used in finite element models. The ASTM D-1621 standard recommends using a secant modulus based on 10% strain. The resulting modulus has a smaller scatter in its value compared with the coefficients of the hyperbolic model, and it is therefore easier to compare differences in material processing and ensure quality of the scaffold. A prediction of the small-strain elastic modulus was constructed from images of the microstructure. Each pixel of the micrographs was represented with a brick finite element and assigned the Young's modulus of bulk PCL or a value of 0 for a pore. A compressive strain was imposed on the model and the resulting stresses were calculated. The elastic constants of the scaffold were then computed with Hooke's law for a linear-elastic isotropic material. The model was able to predict the small-strain elastic modulus measured in the experiments to within one standard deviation. Thus, by knowing the microstructure of the scaffold, its bulk properties can be predicted from the material properties of the constituents.

Real-time synchronous measurement of curing characteristics and polymerization stress in bone cements with a cantilever-beam based instrument

An instrumentation capable of simultaneously determining degree of conversion (DC), polymerization stress (PS), and polymerization exotherm (PE) in real time was introduced to self-curing bone cements. This comprises the combination of an in situ high-speed near-infrared spectrometer, a cantilever-beam instrument with compliance-variable feature, and a microprobe thermocouple. Two polymethylmethacrylate-based commercial bone cements, containing essentially the same raw materials but differ in their viscosity for orthopedic applications, were used to demonstrate the applicability of the instrumentation. The results show that for both the cements studied the final DC was marginally different, the final PS was different at the low compliance, the peak of the PE was similar, and their polymerization rates were significantly different. Systematic variation of instrumental compliance for testing reveals differences in the characteristics of PS profiles of both the cements. This emphasizes the importance of instrumental compliance in obtaining an accurate understanding of PS evaluation. Finally, the key advantage for the simultaneous measurements is that these polymerization properties can be correlated directly, thus providing higher measurement confidence and enables a more in-depth understanding of the network formation process.