Iuliana Stoica

Chain flexibility versus molecular entanglement response to rubbing deformation in designing poly(oxadiazole-naphthylimide)s as liquid crystal orientation layers

Four poly(oxadiazole-imide)s containing naphthalene rings, with different flexibility and molecular weight, are investigated with respect to their rheological properties to establish the optimal processing conditions from solution phase to film state for liquid crystal orientation purposes. The film uniformity and strength are determined by monitoring the flow behavior and chain entanglements. The solution rheological data are in agreement with film tensile testing, revealing that higher molecular weight favors chain entanglements and implicitly the film mechanical resistance. In order to analyze the suitability of these films as alignment layers their surface is patterned by rubbing with two types of velvet. Liquid crystal alignment of 4′-pentyl-4-biphenylcarbonitrile nematic is tested by polarized light microscopy. The resulting behavior is correlated with the polyimide malleability and characteristics of the textile fibers, namely their polarity, size, and mechanical features. The competitive effects between chain flexibility and entanglements, together with the interactions occurring between the polymer and velvet are analyzed in order to explain the surface regularity, which influences the uniformity of the liquid crystal alignment. The contrast between dark and bright states recorded on the liquid crystal cell indicates that some of these polynaphthalimides are promising candidates for liquid crystal display devices.

Properties of some azo-copolyimide thin films used in the formation of photoinduced surface relief gratings

Thin free standing films have been obtained by casting from dimethylacetamide solutions of some azo-copolyimides. The dynamo-mechanical and dielectric properties, and the effect of the chemical structure of polymers on the physical properties are investigated. The incorporation of substituted azobenzene groups and hexafluoroisopropylidene units in the macromolecular chain allowed the patterning of the materials under different irradiation conditions. The azo-copolyimide thin films showed high thermal stability, low dielectric constant, good dynamo-mechanical characteristics and uniform surface relief gratings.

Morphological Properties and Antibacterial Activity of Nano-Silver-Containing Cellulose Acetate Phthalate Films

Modification of the rheological properties of cellulose acetate phthalate in 2-methoxyethanol/acetone/water, at different compositions of solvent mixtures, allowed the identification of optimal composition of solvent mixtures for obtaining fibers with controlled diameters. Changing the solvent content in the casting solutions favors modification of the morphological aspects of cellulose acetate phthalate (CAP) films, as observed from atomic force microscopy images. Silver nitrate was incorporated into CAP, as a dispersion medium, and the silver-containing polymer (Ag-CAP) films thus obtained were studied for obtaining information on antimicrobial activity, using Escherichia coli ATCC 10536 and Staphylococcus aureus ATCCC 6538 microorganisms. The results were compared with the antibacterial activity of nano-silver-containing cellulose acetate (Ag-CA) films. The different inhibiting effects of CA or Ag-CA and of CAP or Ag-CAP on the tested Escherichia coli and Staphylococcus aureus bacteria are due to a different antimicrobial activity of the polymers and to the antiseptic character of nano-silver.

Statistical analysis on morphology development of some semialicyclic polyimides using atomic force microscopy

The morphological features and surface texture parameters of some polyimide films prepared from a flexible and alicyclic dianhydride, in combination with five aromatic diamines, were evaluated by atomic force microscopy (AFM) in order to determine their applicability in electronics. By means of the surface roughness, shape of the surface height distribution, and angular and radial texture, a precise description of the actual surface topographies at the interface with other materials was made. The polyimide structures led to the development of different surface morphologies (from granular to porous and from bumpy to spiky). The relief diversity was described by the entropy of morphology, which had a similar trend with the root mean square roughness, which presents low values, i.e. 0.5–1.8 nm. Three‐dimensional AFM images and the corresponding angular spectra, together with texture aspect ratio and texture direction index (close to 1), indicate that no predominant orientation exists on the investigated surfaces. The redundancy in the morphology was associated with the concept of fractals, the maximum redundancy being achieved for the polyimide with the most complex polymer chain conformation. These results provide useful insights in selecting the polyimide structure, which has the optimal morphology, roughness, orientation, bearing properties, or self‐similarity for microelectronic applications such as: substrate for display backplanes, planar technology, microelectronic packaging, etc. Microsc. Res. Tech. 76:503–513, 2013.

Origin of rheological behavior and surface/interfacial properties of some semi-alicyclic polyimides for biomedical applications

High-performance alicyclic-containing polyimides for advanced applications, derived from 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic acid anhydride or bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride and two flexible aromatic diamines, were synthesized by a classical two-step polycondensation reaction and analyzed by rheological method. The results were discussed according to the chemical structure of polyimides and their different properties, such as flexibility, hydrophobicity and surface morphology. It has been showed that the obtained parameters, controlled by the interactions occurring in the polyimide systems, can be correlated with the adhesion/cohesion of blood components and plasma proteins. Thus, the results of the work of spreading proteins on the hydrophobic polyimide surfaces indicated that albumin is not absorbed preferentially, while fibrinogen is characterized by a higher degree of adhesion on the surfaces, and also that selective adsorption of plasma proteins modifies blood compatibility. In addition, these results and the ascertained antimicrobial activity of the studied polyimides contribute to the development of new applications in the bio-technical field.

Rheological and Morphological Properties of Phosphorus-Containing Polysulfones

Polysulfones with bulky phosphorus pendant groups, obtained by chemical modification of the chloromethylated polysulfones were subjected to rheological, surface tension and atomic force microscopy investigations, to establish the processing/morphology/property relationships of these polymers. The effects of the substitution degrees, of the bulky phosphorus pendant groups bound in different positions, and also of intermolecular and intramolecular interactions, influenced the rheological results. High hydrophobicity, low work of water adhesion and low adhesion to interface, would be advantageous for some applications. Atomic force microscopy showed that increasing of the bulky phosphorus pendant groups influences the pores characteristics and roughness, confirming the poor adhesion.

Calcium carbonate microparticle templates using a PHOS-b-PMAA double hydrophilic copolymer

The crystallization characteristics of calcium carbonate microparticles grown from supersaturated aqueous solutions in the presence of a double hydrophilic block copolymer poly(p-hydroxystyrene-b-methacrylic acid), PHOS-b-PMAA, have been investigated. The studies aim to highlight both the possibilities and the limitations of CaCO3/PHOS-b-PMAA microparticle formation under different relative inorganic/polymer ratio conditions, varying the initial solution supersaturation or the polymer concentration. Scanning electron microscopy and atomic force microscopy were used to provide high-resolution images of particles and thereby information on the particle morphology, while X-ray diffraction analysis was used to determine the polymorph type and crystallite characteristics. The presence of the polymer in the composite particles was shown by thermogravimetric, particle charge density and zeta potential analysis. The polymer-induced sensitivity of the new composites to environmental pH variations has been followed by streaming potential variation.

Silver Nanoparticles in Cellulose Acetate Polymers: Rheological and Morphological Properties

Silver nitrate was incorporated in cellulose acetate with different substitution degrees as a dispersion medium. Silver-containing polymer solutions in 2-methoxyethanol were rheologically investigated, both before and after boiling, which permitted a better understanding of the morphological aspects of the corresponding cast films. The different distribution of the obtained silver nanoparticles in the polymer matrix, as well as the influence of the hydrophobic character of cellulose acetate, induced by the substitution degree, were established by atomic force microscopy and environmental scanning electron microscopy. Increasing of cellulose acetate hydrophobicity resulted in a more uniform distribution of the silver nanoparticles, with dimensions higher, in the matrix of cellulose acetate second to smaller surface roughness.

Influence of Casting Solution Characteristics on Cellulose Acetate Membranes: Rheology and Atomic Force Microscopy

Cellulose acetates with different substitution degrees were investigated as to their rheological properties and morphology. Atomic force microscopy (AFM) studies on membranes showed ordered domains, in which pores and nodules with different sizes and intensities are distributed. The substitution degree, as well as the history of the membranes formed from the solutions in acetone and water nonsolvent mixtures, influenced the aspect of surface images.

Structural and dielectric properties of some epiclon-based polyimide films

Abstract Surface and electrical properties of two flexible and cross-linked epiclon based polyimide films were investigated. Surface tension measurements indicate an increased hydrophobicity due to both isothermal treatment - which induces imidization and cross-linking - and to the chemical structure, characterized by the different number of ether linkages in the polyimide backbone. Morphology of film surfaces, modified by plasma treatment, shows high roughness and, implicitly, improved adhesion, as required in electronics. The studied polyimides exhibit low dielectric constants and dielectric losses, comparatively to conventional aromatic polyimides. The electrical conductance and resistivity characteristics of these polymers recommend them as good insulators for dielectric layers in microelectronic applications.