FREE VIBRATION ANALYSIS OF LATTICE SANDWICH BEAMS UNDER SEVERAL TYPICAL BOUNDARY CONDITIONS

Free vibration problems of lattice sandwich beams under several typical boundary conditions are investigated in the present paper. The lattice sandwich beam is transformed to an equivalent homogeneous three-layered sandwich beam. Unlike the traditional analytical model in which the rotation angles of the face sheets and the core are assumed the same, different rotation angles are considered in this paper to characterize the real response of sandwich beams. The analytical solutions of the natural frequencies for several typical boundary conditions are obtained. The effects of material properties and geometric parameters on the natural frequencies are also investigated.     

Organo-Polyoxometalate-Based Hydrogen-Bond Catalysis

Several urea-inserted organo-polyoxometalates (POMs) derived from polyoxotungstovanadate [P₂V₃W₁₅O₆₁]⁹⁻ were prepared. The insertion of the carbonyl into the polyoxometallic framework activates the urea toward Hydrogen-bond catalysis. This was shown on the Friedel-Crafts arylation of trans-β-nitrostyrene. Modelling shows that the most stable form of the organo-POMs features a cis-trans arrangement of the two N−H bonds, but that the likely catalytically active trans-trans form is accessible at room temperature. Finally, it is possible that the oxo substituents next to the vanadium atoms may help the approach of the nucleophile via H-bonding.     

Hydrophobic cryogels for DNA adsorption: Effect of embedding of monosize microbeads into cryogel network on their adsorptive performances

As alternative hydrophobic adsorbent for DNA adsorption, supermacroporous cryogel disks were synthesized via free radical polymerization. In this study, we have prepared two kinds of cryogel disks: (i) poly(2‐hydroxyethyl methacrylate‐N‐methacryloyl‐l ‐tryptophan) [p(HEMA‐MATrp)] cryogel containing specific hydrophobic ligand MATrp; and (ii) monosize p(HEMA‐MATrp) particles synthesized via suspension polymerization embedded into p(HEMA) cryogel structure to obtain p(HEMA‐MATrp)/p(HEMA) composite cryogel disks. These cryogel disks containing hydrophobic functional group were characterized via swelling studies, Fourier transform infrared spectroscopy, elemental analysis, surface area measurements and scanning electron microscopy. DNA adsorption onto both p(HEMA‐MATrp) cryogel and p(HEMA‐MATrp)/p(HEMA) composite cryogels was investigated. Maximum adsorption of DNA on p(HEMA‐MATrp) cryogel was found to be 15 mg/g polymer. Otherwise, p(HEMA‐MATrp)/p(HEMA) composite cryogels significantly increased the DNA adsorption capacity to 38 mg/g polymer. Composite cryogels could be used repeatedly without significant loss on adsorption capacity after 10 repetitive adsorption–desorption cycles. Copyright © 2013 John Wiley & Sons, Ltd.     

Acyclovir release from its composites with silica as a function of the silica matrix modification and the drug loading

Acyclovir release from its composites with unmodified silica at pH 1.6 and 7.4 follows zero order kinetics for two days. Modification of the silica matrix with phenyl groups leads to dramatic decrease in the drug release level at pH 7.4 compared with pH 1.6 as well as to heterogeneous phase state of acyclovir in the phenyl modified composite with high loading of the drug.     

Thermal properties stability of UV irradiated PVA nanohybrid composites

Zn‐Al‐salicylic nanohybrid layers have been prepared and used as fillers for polyvinyl alcohol (PVA). Nanohybrid layers of a broad absorption area in UV region were completely and uniformly dispersed in a continuous polymer matrix. PVA and PVA nanohybrid composite (NHC) films were exposed to UV irradiation. Thermal properties (diffusivity, effusivity, and conductivity) of both have been measured through photoacoustic technique before and after UV irradiation. Thermal parameters of PVA suffered from a quick deterioration with UV exposure due to reduction of the phonon mean free path as a result of molecular chain scissions. However, significant stability in such parameters of NHC has been obtained under the influence of UV irradiation. This thermal properties stability may be an important step on the way of obtaining photostable polymer NHC. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and characterization of novel elastomeric poly(D,L-lactide urethane) maleate composites for bone tissue engineering

Here, we report the synthesis and characterization of a novel 4-arm poly(lactic acid urethane)-maleate (4PLAUMA) elastomer and its composites with nano-hydroxyapatite (nHA) as potential weight-bearing composite. The 4PLAUMA/nHA ratios of the composites were 1:3, 2:5, 1:2 and 1:1. FTIR and NMR characterization showed urethane and maleate units integrated into the PLA matrix. Energy dispersion and Auger electron spectroscopy confirmed homogeneous distribution of nHA in the polymer matrix. Maximum moduli and strength of the composites of 4PLAUMA/nHA, respectively, are 1973.31 ± 298.53 MPa and 78.10 ± 3.82 MPa for compression, 3630.46 ± 528.32 MPa and 6.23 ± 1.44 MPa for tension, 1810.42 ± 86.10 MPa and 13.00 ± 0.72 for bending, and 282.46 ± 24.91 MPa and 5.20 ± 0.85 MPa for torsion. The maximum tensile strains of the polymer and composites are in the range of 5–93%, and their maximum torsional strains vary from 0.26 to 0.90. The composites exhibited very slow degradation rates in aqueous solution, from approximately 50% mass remaining for the pure polymer to 75% mass remaining for composites with high nHA contents, after a period of 8 weeks. Increase in ceramic content increased mechanical properties, but decreased maximum strain, degradation rate, and swelling of the composites. Human bone marrow stem cells and human endothelial cells adhered and proliferated on 4PLAUMA films and degradation products of the composites showed little-to-no toxicity. These results demonstrate that novel 4-arm poly(lactic acid urethane)-maleate (4PLAUMA) elastomer and its nHA composites may have potential applications in regenerative medicine.     

Development of new high‐performance visible light photoinitiators based on carbazole scaffold and their applications in 3d printing and photocomposite synthesis

In this article, new compounds based on the carbazole scaffold (DMs = DM1 and DM2, constituted by a carbazole unit connected on positions 3 and 6 to a two 4,4′‐dimethoxydiphenylamine groups and differing by the substituent present on the nitrogen heteroatom of the carbazole core) were synthesized and proposed as high‐performance visible light photoinitiators/photosensitizers for both the free‐radical polymerization of methacrylates and the cationic polymerization of epoxides upon visible light exposure using LED@405 nm. Remarkably, DM2 leads to higher final conversions than DM1. In order to study the photophysical and photochemical properties of the carbazole derivatives, different parameters were taken into account such as the light absorption, the steady‐state photolysis, and the fluorescence spectroscopy. Using different techniques such as fluorescence quenching, redox behavior, and cyclic voltammetry, we are able to discuss the photosensitization/photoinitiation reactions providing a full coherent picture of the involved chemical mechanisms. The photosensitization of the carbazole derivatives occurred predominantly via singlet excited states at the rate of the diffusion limit. Upon exposure to laser diode at 405 nm, DMs show high performance in initiating systems for 3D resins. Remarkably, DM2 can also be used in photocomposite synthesis using light‐emitting diode conveyor. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2081–2092     

Influence of multiwall carbon nanotubes on morphological and structural changes during UV irradiation of syndiotactic polypropylene films

The effect of UV irradiation on the morphology and structure of syndiotactic polypropylene (sPP) is investigated both for pure films and those filled with multiwall carbon nanotubes (MWCNTs). UV treatment causes a structural reorganization of pure sPP films. It seems that the incorporation of MWCNTs has a strong stabilizing effect, with nanotubes acting as a limiter of the UV‐induced chain breakage, especially when a good dispersion of carbon nanotubes in the polymer is achieved. The incorporation of MWCNTs introduces a high concentration of defects in the crystal structure of sPP, which limits the UV‐induced growth of crystallites, and prevents the development and propagation of cracks caused by UV irradiation. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Shear modulus determination in model hydrogels by means of elongated magnetic nanoprobes

The mechanical characterization of complex soft matter by quasi-static magnetometry using nanoscopic magnetic probes is demonstrated for model hydrogels doped with two types of elongated magnetic nanoparticles. Chemically crosslinked poly(acrylamide) (PAAm) hydrogels serve as the matrix in which nickel nanorods or weakly magnetized hematite (α-Fe₂O₃) ellipsoids are embedded as local probes. We investigated the swelling behavior of the ferrogels in order to verify that their equilibrium swelling degree in water is not influenced by the probes, shows a good correlation with the Frenkel–Flory–Rehner model. The proposed magnetomechanical method relies on a correlation between the shear modulus of the PAAm hydrogel matrix and the coercive fields of the corresponding isotropic ferrogels. By extending the Stoner–Wohlfarth model for single-domain blocked magnetic particles by a term for particle rotation in an elastic matrix, information on the shear modulus of the matrix can be obtained. Comparison of the results with the expected relation from rubber elasticity theory illustrates both the general potential as well as the limits of the approach. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012