Dharmaraj Raghavan

Characterization of starch/polyethylene and starch/polyethylene/poly(lactic acid) composites

This paper examines the microstructural aspects of vernonia oil-added starch–polyethylene and vernonia oil added starch–polyethylene–polylactic acid composite. Polymer composite films containing different percentage of additives were melt processed and acid hydrolyzed. Scanning electron microscopy of the fracture surfaces of OsO4 stained vernonia oil-added composite showed that the vernonia oil is present at the interface of the starch–polyethylene. Tapping mode atomic force microscopy (TMAFM) was used to obtain pore size of the hydrolyzed polymer composite. The progressive hydrolysis of the degradable component in the composite was studied by mass loss measurement and FTIR analyses. The quantity of water passing through the porous acid hydrolyzed composite was found to depend on the thickness of film.

Characterization of polyester degradation using tapping mode atomic force microscopy: exposure to alkaline solution at room temperature

Abstract Tapping mode atomic force microscopy (AFM) was used to examine the microstructure of polyester films before and after exposure to an alkaline solution. Phase imaging and force curves showed differences in properties between the degraded and undegraded regions. Additionally, chemical analyses of the degraded films and the immersion solutions were carried out using attenuated total reflection Fourier transform infrared spectroscopy, total carbon analysis and liquid chromatography–mass spectrometry to aid in the interpretation of AFM data. The results showed that the base-catalyzed hydrolysis of polyester was a heterogeneous process, involving the formation of pits that increase in number and size with exposure time. Information provided by this study can be used to better understand the degradation mode and mechanism of polyester coatings in alkaline media.

Biodegradation dynamics of polymer–starch composites

The dynamics of starch biodegradation in polyethylene–starch (PE–S) composites was investigated by aerobic biodegradation methods and computer simulations, with the starch fraction p above and below the percolation threshold pc. Two models for starch degradation were considered: (i) microbial invasion through the composite and (ii) macromolecular (enzyme) diffusion which results in the back-diffusion of small molecules to the surface for further assimilation by microorganisms. The microbial-invasion model was based on scanning electron microscopy (SEM) studies of PE–S composites that contained a 1–15-micron distribution of starch particles. Following exposure to soil test conditions, micrographs of thin films clearly showed the colonization of microorganisms within channels of the matrix that were initially occupied by starch. The enzymatic diffusion was based on hydrolytic experiments of PE–S composites. Following exposure of a composite to a hydrolytic test condition, small molecules were produced. The starch accessed by microbes and enzymes was computed by simulating degradation of a monodisperse and polydisperse (starch grains of 1–10-micron diameter) composite. Aerobic degradation studies in a biometer indicate that the starch accessibility. A follows a power-law dependence with time A ∼ tn, where the exponent n depends on the fractal dimension of the accessed starch clusters and pathways and approaches unity when p > pc. Microbial invasion simulations indicate that the average power-law exponent near pc is approximately 0.5 and approaches 1.0 at p > pc, whereas the enzymatic diffusion simulations indicate that the average power-law exponent near pc is about 0.25 and approaches 0.5 at p > pc. The observed exponent for the aerobic degradation study suggests that for composites with a starch fraction less than and greater than pc the starch is predominantly accessed by microbial invasion.

Characterization of Silver/Bovine Serum Albumin (Ag/BSA) nanoparticles structure: Morphological, compositional, and interaction studies

The primary objective of this study was to elucidate the structure of protein conjugated silver nanoparticles prepared by chemical reduction of AgNO(3) and bovine serum albumin (BSA) mixture. The role of BSA in the formation of Ag/BSA nanoparticles was established by UV-Vis Spectroscopy. The association of silver with BSA in Ag/BSA nanoparticles was studied by the decrease in the intensity of absorbance peak at 278 nm in UV-Vis spectra and shift in cathodic peak potential in cyclic voltammogram. The molar ratio of silver to BSA in the Ag/BSA nanoparticles is 27:1, as ascertained by thermogravimetric analysis and atomic absorption spectrometry. Based on atomic force microscopy, dynamic light scattering and transmission electron microscopy (TEM) measurements, the average particle size of nanoparticles was found to be range of 11-15 nm. TEM image showed that the nanoparticle has two distinct phases and selected area electron diffraction pattern of nanoparticles indicated that the silver phase in Ag/BSA is fcc. X-ray photo electron spectroscopy measurements of freshly prepared and argon sputtered nanoparticles provided evidence that the outer and inner region of nanoparticles are mainly composed of BSA and silver respectively. The structural and compositional findings of nanoparticles could have a strong bearing on the bioavailability and antimicrobial activity of nanoparticles.

Langmuir Adsorption Study of the Interaction of CdSe/ZnS Quantum Dots with Model Substrates: Influence of Substrate Surface Chemistry and pH

We investigate the utility of Langmuir adsorption measurements for characterizing nanoparticle-substrate interactions. Spherical CdSe/ZnS core-shell nanoparticles were chosen as representative particles because of their widespread use in biological labeling measurements and their relatively monodisperse dimensions. In particular, the quantum dots were functionalized with 11-mercaptoundecanoic acid, and we utilized an amine-terminated self-assembled monolayer (SAM) as a model substrate. SAMs with different end-groups (-CH(3) and -COOH) were also considered to contrast with the adsorption behavior on the amine-terminated SAM substrates. We followed the kinetics of nanoparticle adsorption on the aminosilane layer by quartz crystal microgravimetry (QCM) over a range of particle concentrations and determined the corresponding Langmuir adsorption isotherms. Analysis of both equilibrium adsorption and kinetic adsorption data allowed us to determine a consistent value of the Langmuir adsorption equilibrium constant for the amine-terminated SAM at room temperature (K(L) approximately 2.7 (micromol/L)(-1)), providing a useful characterization of the nanoparticle-substrate interaction. The effect of varying solution pH on Langmuir adsorption was also investigated in order to gain insight into the role of electrostatic interactions on nanoparticle adsorption. The equilibrium extent of adsorption was found to be maximum at about pH 7. These changes of nanoparticle adsorption were further quantified and validated by X-ray photoelectron spectroscopy (XPS) and confocal fluorescence microscopy measurements. We conclude that Langmuir adsorption measurements provide a promising approach for quantifying nanoparticle-substrate interactions.

Combinatorial investigation of dewetting: polystyrene thin films on gradient hydrophilic surfaces

Film stability and dewetting is important to control for applications in coatings such as photoresists, paints, adhesives, lubricants, and biomaterials. We demonstrate the use of 2D combinatorial libraries to investigate thin film dewetting. Substrate libraries with gradients in contact angle ðuÞ were prepared by immersing Si ‐ H passivated Si in a Piranha solution (H2SO4/H2O2/H2O) at a controlled rate. Libraries of thin films of polystyrene on gradient etched silicon substrates containing orthogonal continuous variation of thickness were screened for dewetting behavior using automated optical microscopy. After comparing the high-throughput screening method to conventional studies of thickness effect on dewetting, a detailed morphological phase-map of the effects of contact angle on dewetting of polystyrene film was generated. Dewetting trends were visibly apparent. The number of polygons of dewetted polymer is sensitive to surface hydrophilicity as characterized by contact angle studies. q 2002 Elsevier Science Ltd. All rights reserved.

The influence of elastomer concentration on toughness in dispersions containing preformed acrylic elastomeric particles in an epoxy matrix

The influence of toughener concentration on the fracture behavior of two-phase, rubber-toughened epoxy is studied. To vary the concentration without altering other morphological features, samples generated from dispersions of preformed rubber (acrylic) particles in liquid epoxy monomer are used. By diluting with different amounts of epoxy prior to cure, the toughener concentration can be varied over a wide range. Thermal and microscope studies support the assertion of a constant morphology. The fracture results show that the toughness increases to a maximum and then decreases as the concentration is increased. This suggests an optimum concentration of toughening. Micrographs of the initiation zone on the fracture surface at high concentrations of rubber show less deformation than the equivalent surfaces at lower concentrations. This is consistent with a toughening mechanism based on particles initiating yielding and plastic flow in the matrix.

The influence of silane coupling agent composition on the surface characterization of fiber and on fiber-matrix interfacial shear strength

It is well known that the fiber-matrix interface in many composites has a profound influence on composite performance. The objective of this study is to understand the influence of composition and concentration of coupling agent on interface strength by coating E-glass fibers with solutions containing a mixture of hydrolyzed propyl trimethoxysilane (PTMS) and n -aminopropyl trimethoxysilane (APS). The failure behavior and strength of the fiber-matrix interface were assessed by the single-fiber fragmentation test (SFFT), while the structure of silane coupling agent was studied in terms of its thickness by ellipsometry, its morphology by atomic force microscopy, its chemical composition by diffuse reflectance infrared Fourier transform (DRIFT), and its wettability by contact angle measurement. Deposition of 4.5 ‐ 10 m 3 mol/L solution of coupling agent in water resulted in a heterogeneous surface with irregular morphology. The SFFT results suggest that the amount of adhesion between the glass fiber and epoxy is dependent not only on the type of coupling agent but also on the composition of the coupling agent mixture. As the concentration of APS in the mixture increased, the extent of interfacial bonding between the fiber and matrix increased and the mode of failure changed. For the APS coated glass epoxy system, matrix cracks were formed perpendicular to the fiber axis in addition to a sheath of debonded interface region along the fiber axis.

Synthesis and characterization of collagen grafted poly(hydroxybutyrate-valerate) (PHBV) scaffold for loading of bovine serum albumin capped silver (Ag/BSA) nanoparticles in the potential use of tissue engineering application.

The objective of this study is to synthesize and characterize collagen grafted poly(3-hydroxylbutyrate-co-3-hydroxylvalerate) (PHBV) film for loading of BSA capped silver (Ag/BSA) nanoparticles. Thermal radical copolymerization and aminolysis methods were used to functionalize macroporous PHBV, followed by collagen grafting so as to formulate collagen-g-poly(hydroxyethylmethyl acrylate)-g-poly(3-hydroxylbutyrate-co-3-hydroxylvalerate) [collagen-g-PHEMA-g-PHBV] and collagen-g-aminated-poly(3-hydroxylbutyrate-co-3-hydroxylvalerate) [collagen-g-NH2-PHBV] films, respectively. Spectroscopic (FTIR, XPS), physical (SEM), and thermal (TGA) techniques were used to characterize the functionalized PHBV films. The amount of collagen present on grafted PHBV film was quantified by the Bradford method. The Ag/BSA nanoparticles were then loaded on collagen grafted and untreated PHBV films, and the nanoparticles loading were determined by atomic absorption spectrometry. The amount of nanoparticles loaded on collagen grafted PHBV film was found to be significantly greater than that on the untreated PHBV film. The nanoparticles loaded PHBV film can potentially serve as a scaffold to promote the growth of bone cells while inhibiting the bacterial growth.

Adsorption-desorption study of BSA conjugated silver nanoparticles (Ag/BSA NPs) on collagen immobilized substrates.

There has been a growing interest in the use of protein conjugated nanoparticles for applications in biomedical, sensing, and advanced imaging. The objective of this study was to understand the interaction of protein conjugated silver nanoparticles (Ag/BSA NPs) with biological substrate (collagen layer). The adsorption behavior of synthesized Ag/BSA NPs on collagen immobilized silanized surface was followed by UV-vis spectroscopy by initially studying the formation of collagen layer and subsequent adsorption of Ag/BSA NPs to the immobilized layer. Surface plasmon resonance (SPR) data provided the real time profile of adsorption of Ag/BSA NPs from solution onto collagen immobilized and control substrates as well as desorption of nanoparticles from the substrates. The retention of NPs to substrate is sensitive to chemistry of the underlying substrate and on the external environment. UV-vis and atomic absorption spectrometric analysis of Ag/BSA NPs desorption performed under different pH conditions showed more NPs retained at physiological pH than the acidic and basic conditions. Nanoparticles retention on collagen immobilized substrate at physiological pH could influence properties of biological interest such as circulation lifetime and biodistribution of nanoparticles in the body.