Manganese Dioxide Nanoparticles Modify Optical Band Gap in Polyacrylamide Blends

Researchers investigating polymer-based functional materials have demonstrated that incorporating manganese dioxide nanoparticles into a blended matrix of polyacrylamide and polyvinyl alcohol systematically alters the composite’s optical absorption, optical band gap, and refractive index while preserving the host polymer’s structural integrity. By introducing nanofiller loadings of 0.1, 0.5, and 1.0 weight percent through a multi-stage solution casting route, the investigators established that even minor additions of transition metal oxide nanoparticles effectively modulate photon harvesting and electronic transition profiles. Laboratory evaluations confirmed that the semi-crystalline character of the co-polymer matrix remained intact, while strong interfacial interactions between the polymer chains and the dispersed inorganic surfaces mediated noticeable modifications in local packing, energy band transitions, and surface morphology.

The study utilized specific commercial polymer and nanofiller precursors to construct the flexible composite films under strictly controlled chemical formulations. The host matrix relied on polyacrylamide of an analytical, medium anionic grade with an average molecular weight of 19 million grams per mole, paired with polyvinyl alcohol exhibiting an average molecular weight of 35,000 grams per mole, a degree of hydrolysis spanning 85 to 89 percent, and a solution viscosity between 35 and 50 centipoise at 20 degrees Celsius. The inorganic filler consisted of manganese dioxide with 99.9 percent chemical purity, utilized without further purification steps. By uniting polyacrylamide and polyvinyl alcohol, the system combined the film-forming capability, optical transparency, and chemical stability of the individual polymers, leveraging their polar functional groups to facilitate coordination with the transition metal oxide particles.

Fabrication of the thin films proceeded through an aqueous solution casting protocol designed to achieve homogeneous blending and controlled crosslinking. Separate aqueous solutions of five percent polyvinyl alcohol and one percent polyacrylamide were prepared in distilled water and subsequently combined in equal five-milliliter volumes under magnetic stirring for approximately 30 minutes. To optimize mechanical flexibility and network stability, approximately 0.15 milliliters of glycerol was introduced as a plasticizer followed by ten minutes of mixing, after which 0.5 milliliters of a one weight percent citric acid solution was added as a crosslinking agent with an additional ten minutes of stirring. For the doped samples, manganese dioxide nanoparticles at 0.1, 0.5, and 1.0 weight percent were initially dispersed into the polyacrylamide solution using magnetic agitation and ten minutes of bath ultrasonication at 170 watts to suppress early clustering before blending with the polyvinyl alcohol phase.

Thermal processing and film solidification were completed in Petri dishes inside a temperature-regulated drying oven, where casting mixtures underwent an initial 30-minute drying stage at 50 degrees Celsius before the temperature was ramped to 120 degrees Celsius and held for one hour. Physical measurements using a precision micrometer demonstrated that the pristine polyacrylamide and polyvinyl alcohol blend yielded a uniform film thickness of 230 micrometers. Incorporating the manganese dioxide nanoparticles produced incremental increases in film thickness, recording 240 micrometers for the 0.1 weight percent composition, 242 micrometers for the 0.5 weight percent sample, and 245 micrometers for the 1.0 weight percent nanocomposite film, establishing a consistent dimensional baseline across the experimental series.

Structural evaluation conducted via coupled X-ray diffraction across a diffraction angle range of 5 to 80 degrees using copper K-alpha radiation with a wavelength of 1.54060 angstroms verified the semi-crystalline and predominantly amorphous nature of the polymer host. In the pristine blend, the diffraction profile featured a characteristic broad halo centered near 19.8 degrees two-theta, corresponding to an interplanar d-spacing of approximately 4.48 angstroms, accompanied by a secondary broad reflection between 43 and 45 degrees. These features stem from short-range ordering of hydrogen-bonded polyvinyl alcohol crystallites situated within the amorphous polyacrylamide network. Peak broadening analysis utilizing the Scherrer equation, with a shape factor of 0.94, estimated that the incorporated manganese dioxide phase formed nanoscale crystallites with average dimensions ranging from 8 to 25 nanometers embedded throughout the matrix.

Systematic changes in the X-ray diffraction patterns emerged as the manganese dioxide loading increased, indicating subtle rearrangements in the local chain packing of the polymer network. Diffraction intensity centered around 20 degrees two-theta rose progressively from the 0.1 weight percent loading up to the 1.0 weight percent composition, where the sample exhibited the highest diffraction intensity without developing sharp crystalline matrix peaks. Concurrently, weak and broadened diffraction peaks corresponding to the crystalline manganese dioxide phase became discernible alongside the primary halo. The retention of the broad baseline profile confirmed that while the embedded nanoparticles influenced local structural organization and short-range chain alignment through surface interactions, they did not disrupt the fundamentally semi-crystalline and amorphous matrix balance.

Vibrational characteristics and interfacial bonding were monitored using Fourier transform infrared spectroscopy across the wavenumber range of 4000 to 450 inverse centimeters. Pristine blend spectra exhibited a broad absorption band centered between 3300 and 3304 inverse centimeters attributed to overlapping hydroxyl and amine stretching vibrations, confirming extensive intermolecular hydrogen bonding between the two polymers. Characteristic bands included aliphatic carbon-hydrogen stretching at 2999 to 3002 inverse centimeters, carbonyl stretching from polyacrylamide amide groups at 1717 inverse centimeters, amide II vibrations between 1506 and 1511 inverse centimeters, methylene bending modes from 1416 to 1391 inverse centimeters, carbon-nitrogen stretching between 1298 and 1242 inverse centimeters, and carbon-oxygen stretching modes from polyvinyl alcohol between 1157 and 1060 inverse centimeters.

Following the introduction of manganese dioxide nanoparticles, subtle shifts in peak positions and variations in band intensities were recorded across the hydroxyl, amine, and carbon-oxygen vibrational regions. These spectral modifications, accompanied by characteristic metal-oxygen lattice vibrations at lower wavenumbers, confirmed successful filler incorporation and pointed to interfacial interactions such as hydrogen bonding and coordination between the nanoparticle surfaces and polymer functional groups. The authors noted that while these infrared shifts reflect changes in the local coordination environment of the polymer chains, vibrational spectroscopy alone does not provide direct evidence for the creation of new covalent bonds or direct alterations of the underlying electronic band structure.

Optical absorption behavior examined by ultraviolet-visible spectroscopy over the 200 to 800 nanometer spectral range revealed substantial enhancements in absorbance intensity across both ultraviolet and visible regions following nanoparticle doping. The pristine blend displayed low visible absorption with a steep absorption edge confined to the ultraviolet spectrum, typical of wide-bandgap transparent polymers. As manganese dioxide loading rose to 1.0 weight percent, the absorption edge shifted progressively toward longer wavelengths. This red shift reflects enhanced light-matter interactions and photon harvesting attributed to the introduction of localized electronic energy states within the forbidden band gap of the composite system, which facilitate electronic transitions under lower-energy photon irradiation.

Quantitative band gap evaluation utilizing Tauc’s direct allowed transition model, where the square of the absorption coefficient multiplied by photon energy is plotted against incident photon energy, provided detailed insight into the absorption edge shifts. In the broader optical analysis, the overall optical band gap narrowed from 4.27 electron volts in the pristine film down to 4.00 electron volts at 1.0 weight percent manganese dioxide content. Detailed transition analysis identified dual optical transition thresholds in the pristine matrix at approximately 3.74 and 3.17 electron volts; upon introducing 0.1 weight percent nanofiller, these values initially shifted to 4.60 and 3.85 electron volts, before the higher-energy transition systematically decreased to 4.53 electron volts at 0.5 weight percent and 4.44 electron volts at 1.0 weight percent. Applying the Moss relation indicated a corresponding rise in refractive index, reaching peak values of 2.28 and 2.31 at the highest loading.

Morphological and elemental examinations via scanning electron microscopy at an accelerating voltage of 20 kilovolts and energy-dispersive X-ray spectroscopy confirmed the spatial distribution of the filler across the polymer films. Micrographs of the 0.1 weight percent sample showed a smooth, continuous, sheet-like surface with sparsely embedded nanoparticles, whereas the 0.5 weight percent film displayed slightly increased roughness with uniformly dispersed granular features and no extensive clustering. At 1.0 weight percent, visible micro-clusters and surface agglomerates appeared, reflecting reduced dispersion efficiency at higher loading. Energy-dispersive X-ray measurements on the 1.0 weight percent films verified manganese content between 1.0 and 2.1 weight percent, corresponding to 0.3 to 0.6 atomic percent, aligning with the observed morphological shift from isolated dispersion to localized aggregation.

Subject of Research: Structural, morphological, and optical characterization of polyacrylamide and polyvinyl alcohol polymer blend films incorporated with manganese dioxide nanoparticles

Article Title: Structure, morphology, and optical properties of polyacrylamide/poly (vinyl alcohol) (PAM/PVA) polymer blend containing MnO₂ nanoparticles at different concentrations

Article References: Ezzat, D., Mustapha, N., Hussein, R. K., Deghady, A. M., & Tayel, A. (2026). Structure, morphology, and optical properties of polyacrylamide/poly (vinyl alcohol) (PAM/PVA) polymer blend containing MnO₂ nanoparticles at different concentrations. BMC Chemistry, 20(1), Article 187. https://doi.org/10.1186/s13065-026-01910-1

Image Credits: AI Generated

DOI: 10.1186/s13065-026-01910-1

Keywords: Manganese dioxide nanoparticles, Polyacrylamide, Polyvinyl alcohol, PAM PVA polymer blend, Solution casting, Optical band gap, Refractive index, UV-Vis absorption spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Scanning electron microscopy, Energy-dispersive X-ray spectroscopy, Polymer nanocomposites, Citric acid crosslinking, Tauc relation, Moss relation, Scherrer equation, Nanoparticle agglomeration