M. Arellano

Semi-continuous Heterophase Polymerization of n-Butyl Methacrylate: Effect of Monomer Feeding Rate

The effect of monomer feeding rate on particle size, molar masses, glass transition and tacticity of poly(n-butyl methacrylate) (PBMA) nanoparticles synthesized by semi-continuous heterophase polymerization under monomer-starved conditions is reported. Three feed rates were examined. Highly monomer-starved conditions at the two slower addition rates were confirmed by the low amount of residual monomer throughout the reaction and by the fact that the instantaneous polymerization and feeding rates became similar at later stages of the reaction. Under these conditions, polymer particles in the nanometer range (30 to 35 nm) were obtained. Glass transition temperatures are substantially higher than those reported for commercial PBMA. Polymers tacticity was determined by 13C-NMR spectroscopy. NMR measurements confirm that the syndiotactic content of the PBMA synthesized here is larger than those of the commercial ones made by free-radical polymerization. Molar masses are much lower than those expected from termination by chain transfer to monomer, which is the typical termination mechanism in microemulsion polymerization.

Semicontinuous Heterophase Polymerization of Methyl and Hexyl Methacrylates to Produce Latexes with High Nanoparticles Content

The semi-continuous heterophase polymerization (SHP) under monomer-starved conditions of methyl methacrylate and hexyl methacrylate is reported, at various monomer addition rates to elucidate the effect of monomer solubility on the mechanism of particle nucleation and growth. Polymer particles in the nanometer range from both monomers were obtained, which decreased in size as monomer addition rate was decreased. Molar masses of both polymers also diminished as monomer addition rate was slowed down. The propagation rate constant dictates how fast the starved condition is achieved rather than the monomer solubility in the aqueous phase. A slow particle rate of growth helps to obtain a narrower PSD.

Poly(hexyl methacrylate) Nanoparticles Templating in Nanoemulsions-Made by Phase Inversion Temperature

Nanoemulsions of water, Brij 56 and hexyl methacrylate (with a small amount of squalene as hydrophobe or costabilizer) were made by the phase inversion temperature (PIT) method and then polymerized. In the absence of squalene, the nanoemulsions destabilized within minutes; however with squalene, the nanoemulsions were kinetically stable for at least several hours, which is long enough to carry out the reactions, with diameters of narrow size distribution in the range from 32 to 44 nm, depending on the surfactant concentration. Polymerizations, carried out a 20°C using a par redox, were extremely fast (ca. 100% conversion in less than 5 min) yielding polymer particles of ca. 40 nm, which were similar to the original nanoemulsion droplets (ca. 35 nm), indicating that the nanoemulsion droplets act as templates, and that squalene diminishes substantially monomer diffusion between reacting and non-reacting monomer droplets. Molar masses were high and alike to those produced by microemulsion polymerization of this monomer, suggesting that chain transfer to monomer is the main termination mechanism.

Effect of Particle Size on the Swelling and Compression Modulus of Nanostructured Polyacrylamide Hydrogels

The effects of concentration and size of polyacrylamide (PAM) nanoparticles on the swelling behavior and compression modulus of nanostructured polyacrylamide hydrogels are examined here. These hydrogels are made by free radical polymerization in an aqueous solution of acrylamide and a crosslinking agent containing dispersed crosslinked PAM nanoparticles of different sizes, previously made by inverse emulsion or microemulsion polymerization. Faster swelling rate and larger equilibrium swelling are observed as particle content increases or the nanoparticle size diminishes. The compression modulus depends on the concentration and size of the disperse nanoparticles: it is larger for hydrogels containing smaller particles at similar concentrations, and it increases as the nanoparticle concentration augments. Moreover, the nanostructured hydrogels have larger compression modulus than those of the conventional ones having similar swelling.

Temperature and pH-Responsive Polyacrylamide/Poly(Acrylic Acid) Interpenetrating Polymer Network Nanoparticles

ABSTRACT The synthesis, by two sequential inverse microemulsion polymerizations, of interpenetrating polymer networks (IPN) formed by polyacrylamide (PAM) and poly(acrylic acid) (PAA) and their response to changes in pH and temperature are reported here. The temperature and pH responses of the IPN nanoparticles are compared with those of polyacrylamide and random copolymers of polyacrylamide and poly(acrylic acid) P(AM-co-AA) nanoparticles also made by inverse microemulsion polymerization. We found that only the IPN nanogels exhibited a sharp swelling increase with temperature associated with its Upper Consolute Solution Temperature, driven by hydrogen bonding interactions, and with pH, driven by electrostatic repulsions of the PAA carboxylic groups, especially at pHs larger than the pKa of the PAA. The ς-potentials of the PAM, P(AM-co-AA) and IPN nanogels were measured as a function of pH and temperature, to determine the effects of these two variables, which in turn, affected the swelling of the nanogels. Field emission scanning electron microscopy revealed that the IPN nanogels were spheroidal with sizes similar to those determined by dynamic light scattering.

Polymerization of Hexyl Methacrylate in Nanoemulsions Made by Low and High Energy Methods

The synthesis of poly(hexyl methacrylate) nanoparticles in nanoemulsions containing squalane as hydrophobe is reported here. A comparison of the polymerization kinetics of nanoemulsions prepared by Phase Inversion Temperature (low energy method) and microfluidization (high energy method), as well as polymer characteristics are presented. Nanoemulsion polymerizations carried out a 20°C were extremely fast using a par redox especially for the low-energy nanoemulsions. The particles obtained were only slightly larger than the original nanoemulsion droplets, indicating that the droplets acted as templates, and that squalane diminished substantially monomer diffusion between reacting and non-reacting monomer droplets. Molar masses and glass transition temperatures of the poly(hexyl methacrylate) obtained here were practically independent of conversion and surfactant concentration, as well as of the nanoemulsification method used.

Synthesis, Characterization, and Drug Delivery from pH- and Thermoresponsive Poly(N-Isopropylacrylamide)/Chitosan Core/Shell Nanocomposites Made by Semicontinuous Heterophase Polymerization

Temperature- and pH-responsive core/shell nanoparticles were prepared by semicontinuous heterophase polymerization of N-isopropylacrylamide (NIPA) in the presence of chitosan micelles for drug delivery purposes. Micelles of chitosan, formed in an acetic acid aqueous solution at 70°C containing potassium persulfate, were fed with N-isopropylacrylamide (NIPA) at a controlled rate, to produce PNIPA/chitosan core/shell nanoparticles of about 350źnm. Then, the crosslinking agent, glutaraldehyde, was added to crosslink the nanoparticles. These nanocomposites were temperature- and pH-responsive, which make them suitable as controlled drug releasing agents. The nanoparticles exhibit thermoreversibility to heating-and-cooling cycles and show different responses depending on the releasing mediumźs pH. Drug delivery tests were performed, employing as a model drug, doxycycline hyclate.

Dispersive Mixing of Surfactant-Modified Titanium Dioxide Agglomerates into High Density Polyethylene

ABSTRACT The description of the dispersion process during the manufacture of composites is an important factor in designing polymer processing machines. In this work, the mixing of titanium dioxide agglomerates into high density polyethylene (HDPE) has been studied. The surface of the titanium dioxide particles was treated with an anionic surfactant to compatibilize this polar powder with non-polar polymers, such as HDPE. The mixing process was carried out in an internal mixer equipped with roller blades at different operational conditions and using various compounding sequences. The rheological properties for these filler-polymer systems were measured using a dynamic rheometer. The energy requirement for dispersive mixing was obtained from the torque-time curves. The surface modification of the titania particles improved the dispersibility of the powder into this non-polar hydrophobic polymer.