Humaira M. Siddiqi

Thermodynamic analysis of reaction-induced phase separation in epoxy-based polymer dispersed liquid crystals (PDLC)

The Flory-Huggins theory for the free energy of mixing in isotropic phases, in conjunction with the Maier-Saupe theory for phase transition of a nematic liquid crystal, was applied to a diepoxide-diamine-liquid crystal blend in pre- and post-gel stages. The analysis was performed taking the distribution of polymeric species into account at any conversion level. In the post-gel stage, the elastic energy contribution was included in the free energy expression. Cloud-point and shadow curves, characteristic of isotropic-isotropic and nematic-isotropic equilibria, were generated in temperature versus composition coordinates covering the whole conversion range. Experimental results for the isotropic-nematic transition could be reproduced by numerical simulation, using an interaction parameter inversely proportional to temperature and decreasing with conversion. The thermodynamic analysis can be used to control the morphologies of polymer dispersed liquid crystals.

Exploring resin viscosity effects in solventless processing of nano-SiO2/epoxy polymer hybrids

Bisphenol A diglycidylether (DGEBA) based low viscosity, liquid epoxy resins are widely used as basis for advanced polymers and nanocomposites, adhesives, protective coatings and encapsulation. We present “green” synthesis of nano-SiO2/epoxy polymer hybrids by a two-step chronological polymerization of inorganic and organic monomers without the use of diluents (solvents). Two types of liquid epoxy resins, D.E.R. 330 and D.E.R. 332, are used to demonstrate the influence of resin viscosity on microstructure, tensile strength and thermal stability of resulting hybrids. Obviously, differences in viscosity of two epoxy resins originate from variations in respective chain lengths, i.e. molar mass, which affect the overall crosslink density and properties of hybrids. In addition, grafting of nano-SiO2 phases with organosilane is performed to achieve inorganic–organic (IO) phase interlinking and to investigate its consequences. Nano-SiO2/epoxy hybrids are characterized by FTIR spectroscopy and XPS. AFM is used to study microstructure and surface properties of hybrids. AFM images show good distribution of nano-SiO2 phases within epoxy polymer. It is observed that the size of nano-SiO2 grows significantly, if resin viscosity is increased or if covalent IO phase interlinks are not present. Tensile measurements show considerable improvement in strength and modulus of nano-SiO2/epoxy polymer hybrids as compared to neat epoxy polymers. DSC and TGA also demonstrate an increase in glass transition temperature (Tg) and thermal stability. We observe that viscosity effects are evenly pronounced in solventless processing of nano-SiO2/epoxy polymer hybrids, and small changes in resin viscosity influence the miscibility of IO phases, the dispersion of SiO2 and the performance of resulting hybrids.

Preparation, Characterization, and Enhanced Thermal and Mechanical Properties of Epoxy-Titania Composites

This paper presents the synthesis and thermal and mechanical properties of epoxy-titania composites. First, submicron titania particles are prepared via surfactant-free sol-gel method using TiCl4 as precursor. These particles are subsequently used as inorganic fillers (or reinforcement) for thermally cured epoxy polymers. Epoxy-titania composites are prepared via mechanical mixing of titania particles with liquid epoxy resin and subsequently curing the mixture with an aliphatic diamine. The amount of titania particles integrated into epoxy matrix is varied between 2.5 and 10.0 wt.% to investigate the effect of sub-micron titania particles on thermal and mechanical properties of epoxy-titania composites. These composites are characterized by X-ray photoelectron (XPS) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric (TG), and mechanical analyses. It is found that sub-micron titania particles significantly enhance the glass transition temperature (>6.7%), thermal oxidative stability (>12.0%), tensile strength (>21.8%), and Youngs modulus (>16.8%) of epoxy polymers. Epoxy-titania composites with 5.0 wt.% sub-micron titania particles perform best at elevated temperatures as well as under high stress.

Enhancing the dielectric properties of highly compatible new polyimide/γ-ray irradiated MWCNT nanocomposites

Novel polyimide/γ-ray irradiated MWCNT (PI/γ-MWCNT) nanocomposites with improved dielectric properties were fabricated by casting and curing processes. The interfacial interactions between the two domains, i.e. PI and MWCNTs, were enhanced by hydrogen bonding between the hydroxyl groups present on PI and modified CNTs. A PI matrix having pendant phenolic hydroxyl groups was derived from pyromellitic dianhydride (PMDA) and diamine monomer 4,4′-diamino-4′′-hydroxytriphenylmethane. MWCNTs (5–20 wt%) were dispersed in the synthesized PI matrix. Before addition to PI, the surface of MWCNTs was equipped with hydroxyl and carboxylic groups by irradiating with γ-rays under a dry oxygen environment. Surface examination of PI/γ-MWCNTs composite films by scanning electron microscopy (SEM) revealed that MWCNTs are uniformly dispersed and completely wrapped by the PI matrix, most likely due to the hydrogen bonding. The influence of greater adhesion of MWCNTs with PI matrix on the dielectric, visco-elastic, and mechanical properties of final PI/γ-MWCNTs nanocomposites was explored using appropriate analytical techniques. The composite films exhibited high dielectric constant, a 7.6 fold improvement as compared to pristine PI. The storage modulus (E′) and glass transition temperature (Tg) demonstrated an improvement of 1.4 and 1.2 fold, respectively. Similarly, mechanical and thermal properties were also found to be improved remarkably. We believe that significant property enhancement of PI/γ-MWCNTs nanocomposites is the direct consequence of increased interface compatibility via hydrogen bonding between the polymer matrix and the carbon nano-filler.

Synthesis and characterization of processable aromatic polyimides and their initial evaluation as promising biomaterials

Aromatic polyimides are one of the successful classes of synthetic polymers featuring high thermal stability, thermooxidative stability, low dielectric constants, excellent mechanical properties and chemical and solvent resistance, which are some of the reasons for the immense scientific and commercial interest in aromatic polyimides [1, 2]. Due to these superior properties, polyimides enjoy a wide range of application in different fields such as biomaterials, adhesives, gas separation membranes, composite matrices, coating and foams [3–6]. The prospects of synthesis and studies of extended rod-like or rigid aromatic polyimides have been extended due to their superior qualities. These include thermal stability, high modulus and high strength fibers, with low thermal expansion coefficient for packing in microelectronic applications [7]. However, the rigid, rod-like polyimides cannot be used as engineering materials as their poor solubility and processability hinders them to react below their decomposition temperature [8]. Various research efforts have been carried out to synthesize soluble polyimides in fully imidized form in such a way that their excellent properties are not compromised [9–11]. One of these investigations was the introduction of flexible segments [e.g. -O-, -SO2-, -CH2-, -C(CF3)2and –NHCO-] into the polymer chain. These groups provide kinks between the rigid phenyl rings in the backbone, and these lead to enhanced solubility of the polymer [12–17]. The replacement of symmetrical aromatic rings by unsymmetrical ones led to the reduction in crystallinity [18], while introduction of bulky lateral substituents (such as t-butyl, phenyl and adamantly groups) decreases the close-packing in the polymer backbone thus enhancing solubility of the polymer [19–22]. Besides the above applications, another striking reason for particular attention towards aromatic polyimides is that their thermal properties bear a close resemblance to a class of copolyimides called poly(amide-imide)s. Furthermore, the inclusion of an amide group into the polyimide backbone increases its processability, solubility and moldability [23]. In addition to this, poly(amide-imide)s have shown outstanding hydrolytic stability, excellent resistance to high temperature and promising balance of other physical and chemical properties [24]. Aromatic polymers with arylsulfone linkages are generally amorphous, have higher chain flexibility, lower glass transition temperatures and better tractability as compared to their corresponding polymers without these groups in the repeat units. The enhanced solubility and lower glass transition temperatures are credited to the flexible linkages that provide a polymer chain with a lower energy for internal rotation. Different disciplines such as sciences, medicine, material sciences and engineering functioned together to form the field of biomaterials [25]. The use of polymers as biomaterial is also known to the world. Biomedical polymers need to be biocompatible to avoid adverse biological effects to the nearby tissues and degradation of the ionic biological environment if exposed for a longer period [26]. G. Waris :H. M. Siddiqi (*) : Z. Akhtar Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan e-mail: humaira_siddiqi@yahoo.com

Pyridine-thiourea based high performance polymers: Synthesis and characterization

A novel diamine monomer, pyridine-2, 6-bis((4-aminophenyl)thioureido)carbonyl (PATC) was synthesized efficiently and polymerized with various aromatic dianhydrides. Consequently, poly(pyridine thiourea-imide)s (PPTIs) with good thermal properties and flame retardancy were fabricated. The structures of PATC and PPTIs were characterized by FTIR, 1H-NMR, 13C-NMR spectroscopy along with elemental analysis, crystallinity, organosolubility, inherent viscosity and gel permeation chromatographic measurements. PPTIs containing C=S, CONH and meta substituted pyridine moieties in the polymer backbone showed amorphous nature and were readily soluble in highly polar organic solvents and even in less polar solvents such as tetrahydrofuran (THF). Polymers had inherent viscosities in the range of 0.91–1.16 dL/g and molecular weight was found between 68000–77000 g/mol. The electrical properties of the PPTIs were estimated in terms of dielectric constant over a range of frequencies. Their thermal stability was determined by 10% weight loss temperature found in the range of 519–563 °C under inert atmosphere. The glass transition temperature of the polyimides varied between 247 °C and 267 °C. The flame retardant properties of PPTIs were investigated in terms of limiting oxygen index (LOI) which was found in the range of 38.26–39.95. Introduction of thiourea in the polymer backbone is an effective way to improve the thermal stability and flame retardancy. Thus PATC can be considered as an excellent candidate for the synthesis of high performance polymers.

Synthesis and characterization of novel coatable polyimide-silica nanocomposites

We report synthesis of a novel diamine 1,2-bis(4-(Hydrazonomethyl)phenoxy)ethane (bis- HPE) and a derived novel polyimide. The diamine was reacted with PMDA and ODA to synthesize copolyimide. Unmodified and modified silica particles were dispersed in the polyimide to prepare polyimide-silica hybrids: (a) unmodified (PSH-UM), and (b) modified (PSH-M). The PSH-UM were prepared by generating silica particles in situ in PI. In PSH-M, structural group identical to PI, 2,6- bis(3-(triethoxysilyl)propyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone was introduced into silica nano-particles. The structural similarity enhanced compatibility between organic–inorganic components by like-like chemical interactions as both contain flexible alkyl groups. PSH-M have shown improved surface smoothness, hydrophobicity and thermal stability. Such properties are mandatory for stable coatings. The structure of silica and PI was affirmed by FTIR, EDX, and solid-state 29Si NMR spectroscopy. Morphological and thermal properties of the prepared PI-SiO2 nano-composites were investigated by field emission scanning electron microscopy, atomic force microscopy, contact angle measurement and thermogravimetric analysis.

Development of novel coatable compatibilized polyimide-modified silica nanocomposites

A series of novel coatable polyimide silica (PI-SiO2) nanocomposites have been synthesized. A new PI matrix, containing pendant hydroxyl groups, was prepared reacting diamine monomers (4,4’-diamino-4”-hydroxytriphenylmethane, and 4,4’-oxydianiline) and pyromellitic dianhydride (PMDA). Whereas, silica reinforcement was generated using TEOS. A coupling oligomeric species 2,6-bis(3-(triethoxysilyl)propyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (APA) was used to furnish silica nanoparticles with imide linkages and hydroxyl groups. As these groups are already present in PI matrix, so their presence in nanoparticles brought structural similarity, and hence enhanced phase connectivity among two phases. The resulting PI-SiO2 hybrids, with improved interfacial interactions through hydrogen bonding and like-like chemical interactions, displayed much enhanced morphological, thermomechanical, and thermal properties. The properties of resulting hybrids were studied by various advanced techniques and compared with PI-SiO2 hybrid system which was prepared from same polyimide and unmodified silica network.

N-(3-Nitro­benzyl­idene)aniline

In the title compound, C13H10N2O2, a Schiff base derivative, the dihedral angle between the two aromatic rings is 31.58 (3)°. The C=N double bond is essentially coplanar with the nitrophenyl ring. The torsion angle of the imine double bond is 175.97 (13)°, indicating that the C=N double bond is in a trans configuration. The crystal structure is stabilized by C—H⋯O contacts and π–π interactions (centroid–centroid distances of 3.807 and 3.808Å).

Phase separation kinetics of an SA liquid crystal in a reactive epoxy-amine PDLC system : an X-ray diffraction study

A Polymer Dispersed Liqued Crystal composite based upon a smective liquid crystal and reactive crosslinkable epoxide-amine monomers was studies. The composite material is obtained by a polymerization induced phase separation process. The polymerization and the phase separation kinetics are with monitored. The polymerization induced phase separation is detected and quantiatively monitored by X-ray diffraction. The normalized X-ray intensity is used to calculate a phase separation rate and to define the phase separation kinetics which is comppared to the polymerization rate and to the polymerization kinetics. The rate difference accounts for changes in morphology.