Angel Romo-Uribe

Polyhedral Oligomeric Silsesquioxanes (Poss): Silicon Based Monomers and their Use in the Preparation of Hybrid Polyurethanes

Abstract : A series of Polyhedral Oligomeric Silsesquioxane (POSS) monomers bearing reactive hydroxyl functionalities, suitable for incorporation into step-growth polymers, is described. These monomers are difunctional in nature and are particularly well suited for use as chain extenders in the synthesis of polyurethanes. This work describes the synthesis of these POSS and their incorporation into a series of polyurethanes. Preliminary thermal, mechanical and rheological data for the POSS containing polyurethanes will also be discussed.

Thermally crosslinkable thermotropic copolyesters : synthesis, characterization, and processing

The synthesis, characterization, and processing of thermotropic copolymers composed of hydroxybenzoic acid (HBA), hydroxynaphthoic acid (HNA), and systematically varying amounts of hydroquinone (HQ) and crosslinkable terephthalic acid (XTA) are described. The XTA monomer contains a benzocyclobutene (BCB) group that lies dormant during synthesis and initial steps of processing, but that can be thermally activated to introduce covalent crosslinking between laterally adjacent macromolecules. The XTA-containing HBA/HNA copolymers all remain thermotropically liquid crystalline, and can be processed into oriented fibres by melt spinning. Rheological characterization reveals an increase in the viscosity and transition from liquid-like to solid-like behaviour as the crosslinking proceeds. X-ray diffraction reveals the changes in local organization with increasing XTA content. The microstructures of the XTA-containing copolymers (up to 20% XTA) in the condensed state are similar to those seen in HBA/HNA copolymers.

Spatially resolved electron diffraction and the determination of orientational order parameters in thermotropic liquid crystalline polymer

A low-dose, high-resolution, electron-diffraction technique has been used to calculate local orientational order parameters from thermotropic liquid crystal polymer (TLCP) fibers. Diffracted intensities are extracted from digital electron diffraction patterns for the orientational order parameter calculation, in a manner similar to that used with X-ray diffraction data. The resolution of local orientation is made possible by electron diffraction as opposed to other methods because of the ability to sample regions as small as 100 nm in diameter. Working within the critical radiative dose for structural damage constrains the ultimate spatial resolution. The signal-to-noise ratio (SNR) of the diffraction data collected at high spatial resolution is low due to the small volume sampled. This work demonstrates the dependence of the orientational order parameter on signal-to-noise effects and the convergence of the incoming electron beam.

3D-Hydrogel Based Polymeric Nanoreactors for Silver Nano-Antimicrobial Composites Generation

This study underscores the development of Ag hydrogel nanocomposites, as smart substrates for antibacterial uses, via innovative in situ reactive and reduction pathways. To this end, two different synthetic strategies were used. Firstly thiol-acrylate (PSA) based hydrogels were attained via thiol-ene and radical polymerization of polyethylene glycol (PEG) and polycaprolactone (PCL). As a second approach, polyurethane (PU) based hydrogels were achieved by condensation polymerization from diisocyanates and PCL and PEG diols. In fact, these syntheses rendered active three-dimensional (3D) hydrogel matrices which were used as nanoreactors for in situ reduction of AgNO3 to silver nanoparticles. A redox chemistry of stannous catalyst in PU hydrogel yielded spherical AgNPs formation, even at 4 °C in the absence of external reductant; and an appropriate thiol-functionalized polymeric network promoted spherical AgNPs well dispersed through PSA hydrogel network, after heating up the swollen hydrogel at 103 °C in the presence of citrate-reductant. Optical and swelling behaviors of both series of hydrogel nanocomposites were investigated as key factors involved in their antimicrobial efficacy over time. Lastly, in vitro antibacterial activity of Ag loaded hydrogels exposed to Pseudomona aeruginosa and Escherichia coli strains indicated a noticeable sustained inhibitory effect, especially for Ag–PU hydrogel nanocomposites with bacterial inhibition growth capabilities up to 120 h cultivation.

Molecular and Textural Ordering of Thermotropic Polymers in Shear Flow

The texture and microstructural order present in mesomorphic polymers and their relation to their macroscopic behavior has been investigated using rheological, optical and dynamic scattering (WAXS and SALS) experiments. Shear orientation is observed under constant rate-of-deformation conditions where this orientation is always parallel to the flow direction. However, the high degree of orientation suggested by optical and SALS measurements is not reflected in the degree of molecular order observed in WAXS experiments. After cessation of flow, a rapid relaxation of stress is observed, while only little microstructural relaxation is found; i.e. , the state of orientation is very stable

PDMS Nanodomains in DGEBA Epoxy Induce High Flexibility and Toughness

ABSTRACT A nanostructured, highly flexible, and optically transparent epoxy nanocomposite has been formulated reacting diglycidyl ether of bisphenol-A with amine-functionalized silyl-diglycidyl ether-terminated poly(dimethylsiloxane). The concentration of poly(dimethylsiloxane) varied up to 10 wt% and curing was performed at 90°C. Transmission electron microscopy revealed poly(dimethylsiloxane) droplets up to 180 nm in diameter well dispersed in the epoxy. Dynamic mechanical analysis exhibited a decrease in elastic modulus E′ and glass transition temperature Tg, and an increase in the α relaxation strength suggesting increased local molecular motions. Strikingly, poly(dimethylsiloxane) induced a threefold increase in strain at fracture and toughness. Furthermore, hydrophobic behavior was induced by poly(dimethylsiloxane) as the water contact angle increased from 72° (neat epoxy) up to 110° at 5 wt% poly(dimethylsiloxane) content. Fractured surfaces exhibited plastic deformation, contrary to brittleness in the neat epoxy. These nanocomposites are attractive for coatings/encapsulates with improved flexibility, toughness, optical transparency, and water resistance. GRAPHICAL ABSTRACT

Viability of HEK 293 cells on poly-β-hydroxybutyrate (PHB) biosynthesized from a mutant Azotobacter vinelandii strain. Cast film and electrospun scaffolds

Sterilization, cytotoxicity and cell viability are essential properties defining a material for medical applications and these characteristics were investigated for poly(β-hydroxybutyrate) (PHB) of 230kDa obtained by bacterial synthesis from a mutant strain of Azotobacter vinelandii. Cell viability was investigated for two types of PHB scaffolds, solution cast films and non-woven electrospun fibrous membranes, and the efficiency was compared against a culture dish. The biosynthesized PHB was sterilized by ultraviolet radiation and autoclave, it was found that the thermal properties and intrinsic viscosity remained unchanged indicating that the sterilization methods did not degrade the polymer. Sterilized scaffolds were then seeded with human embryonic kidney 293 (HEK 293) cells to evaluate the cytotoxic response. The cell viability of these cells was evaluated for up to six days, and the results showed that the cell morphology was normal, with no cytotoxic effects. The films and electrospun membranes exhibited over 95% cell viability whereas the viability in culture dishes reached only ca. 90%. The electrospun membrane, however, exhibited significantly higher cell density than the cast film suggesting that the fibrous morphology enables better nutrients transfer. The results indicate that the biosynthesized PHB stands UV and autoclave sterilization methods, it is biocompatible and non-toxic for cell growth of human cell lines. Furthermore, cell culture for up to 18 days showed that 62% and 90% of mass was lost for the film and fibrous electrospun scaffold, respectively. This is a favorable outcome for use in tissue engineering where material degradation, as tissue regenerates, is desirable.

Hot-pressed boards based on recycled high-density polyethylene tetrapack: Mechanical properties and fracture behavior

In this research, the mechanical properties and fracture behavior of composites based on recycled high-density polyethylene and recycled Tetrapack have been investigated. The matrix and filler were recovered from landfills, ground into flakes of ∼1.6 cm2 size, washed and physically mixed before putting the mixture in a cast, introduced in an oven at 250℃, and pressed applying 1 Metric Ton pressure. Mixtures with varying concentration of tetrapack flakes were prepared. Mechanical properties such as Young’s modulus, yield stress, and ultimate tensile stress were obtained from uniaxial tensile deformation tests carried out at room temperature. The results showed that the tetrapack flakes were effective reinforcers, increasing the Young’s modulus and yield stress relative to neat high-density polyethylene. However, it was also found that the filler acts as stress concentrator where mechanical failure initiates. Scanning electron microscopy showed that fracture of the composite occurred mainly by the lack of adhesion between polymer matrix and filler. Moreover, a percolation threshold was reached at about 5% g/g concentration of tetrapack, beyond which mechanical properties are severely compromised. Interestingly, this investigation also showed that the color of the high-density polyethylene flake, that is, the pigment, strongly influences the mechanical properties of the composite. Thus, boards hot-pressed from individual colors were also investigated. The results showed that pigments, like those used in gray color, favored higher degree of crystallinity, as measured by differential scanning calorimetry, and therefore higher Young’s modulus.

Polymer-based nano/composites: theory, synthesis, modifications, and properties

1Department of Physics and Astronomy, The University of Texas Rio Grande Valley, 1201 W. University Drive, Edinburg, TX 78539, USA 2Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong 3Department of Materials Science, Faculty of Engineering, Tarbiat Modares University, Tehran, Iran 4Instituto de Ciencias Fisicas, Universidad Nacional Autonoma de Mexico (UNAM), Cuernavaca, MOR, Mexico 5Centre for Advanced Materials and Performance Textiles, RMIT University, Room 8, Building 512, 25 Dawson Street, Brunswick, VIC 3056, Australia