Cover: 2nd US-Mexico Symposium on Advances in Polymer Science — MACROMEX 2011 was held in Cancun, Mexico from December 7–10, 2011. The cover is selected from the article Guillermo J. Cruz et. al.
Cover: 2nd US-Mexico Symposium on Advances in Polymer Science — MACROMEX 2011 was held in Cancun, Mexico from December 7–10, 2011. The cover is selected from the article Guillermo J. Cruz et. al.
Cyclometalated complex, cis-[Ru(phpy)(phen)(MeCN)2]PF6, bearing strongly bound ligands (phpy = 2-phenylpyridine, phen = 1, 10- phenantroline) and relatively labile (MeCN) ligands, in conjunction with ethyl 2-bromoisobutyrate (EBiB), was used for radical polymerization of three hydrophobic, styrene (St), methyl methacrylate (MMA), n-butyl acrylate (BA), and one hydrophilic, 2-hydroxyethyl methacrylate (HEMA), monomers. The polymerizations were fast and reasonably controlled; the molecular weights increased with conversion and were close to the calculated values. The best control was achieved in polymerization of St. The complex was able to mediate this polymerization with acceptable rate and level of control even at a monomer/catalyst ratio of 2000. The living character of the polymerizations was confirmed by chain extension experiments; the degree of polymer “livingness” was less for PMMA than for PSt. Because of ionic nature, the complex was well soluble in MeOH and thus catalyzed polymerization of HEMA under homogeneous conditions at 50 °C.
The bulk ring-opening polymerization (ROP) of ε-caprolactone (CL), catalyzed by molybdenum trioxide (α-MoO3), was investigated under nitrogen atmosphere at 150°C. Effect of monomer/initiator ([CL]0/[I]0, I = MoO3) and monomer/initiator/co-initiator ([CL]0/[I]0/[CoI]0, CoI = ROH and H2O) ratios on monomer conversion and molecular weight were determined. Bulk polymerization of CL was completed within 20 hours at 150°C for [CL]0/[I]0 = 100. Propagation was first order in CL concentration and initiator at this temperature, being the rate constant of propagation kp equal to 2.27 × 10−4 mol−1 l s−1. Polymerization reaction was accelerated by the addition of small amounts of water and n-octanol. In the presence of water (up to [CL]0/[H2O]0 = 120), polymerization rate increases and was completed within 6 hours at 150°C; for higher amounts of water (150 to 180), lower conversions are observed. However, control of number-average molecular weight was only efficient for [CL]0/[H2O]0 = 20 and 30. In the presence of n-octanol, degradation of polyester occurs at early stages of polymerization. Kinetic data for polymerization (obtained by 1H-NMR) were fitted to 14 different model reaction functions. It was found that a linear model represents better the conversion versus time plots for bulk polymerization, in agreement with the pseudo living nature of polymerization.
A new high-throughput experimental technique, namely automated parallel freeze-evacuate-thaw degassing method, is applied for the facile, rapid and reliable determination of reactivity ratios in copolymerization systems. The approach allowed carrying out a detailed kinetic investigation of the reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl acrylate (MA) with vinyl acetate (VAc) monomers in 2-methyl-2-butanol at 60 °C. Thereafter, the reactivity ratios of this copolymerization system were estimated utilizing the obtained kinetic data and a computer program based on the visualization of the sum of residual space method. The obtained reactivity ratios of the system (rMA = 8.2 and rVAc = 0.17) are comparable to values previously reported in the literature, which demonstrates the utility of this approach.
This study involves the synthesis, characterization and study of self-aggregation of PEG-based amphiphilic triblock copolymers. PEG based difunctional macro RAFT agents were synthesized and characterized. PS-b-PEG-b-PS block copolymers were synthesized via the RAFT controlled free radical polymerization using the synthesized PEG dual macro RAFT agents. The content of polystyrene blocks on the triblocks was varied from 12% to 36% (based on molecular weight increase), adjusting the synthetic conditions. The triblocks self-aggregate in Dioxane-Water mixtures in sizes depending on the water content.
Preparation of PNVCL controlling its molecular weight and maintaining low dispersities by using a trithiocarbonate RAFT agent was achieved. Typical RAFT kinetics were not observed, nevertheless the prepared polymers were reactivated to grow a second hydrophobic poly(hexyl acrylate) block. A series of statistical copolymers of NVCL with N-vinyl pyrrolidone, n-hexyl acrylate, methacrylic acid, 2-(diethylamino)ethyl methacrylate and 2-(methacryloyloxy)benzoic acid with controlled dispersity were also prepared. By this copolymerization method the cloud point temperature of PNVCL was varied from 27 °C to 42 °C depending on comonomer type, content and pH of the aqueous solution.
In this work we studied the copolymerization of ethylene and different alpha-olefins (propylene, 1-butene, 1-hexene, and 1-octene) using aluminohydride zirconocene systems supported on modified silica. The copolymerization reactions were carried out in hexanes, at different Al/Zr ratios, using two different comonomer concentrations. The effect of comonomer type on catalyst activity and polymer molecular weight and chemical composition distributions (MWD and CCD) was compared. Average comonomer incorporation was determined by 13C NMR and the CCD by crystallization elution fractionation (CEF). Most of the copolymerizations showed very high catalytic activity (up to 4000 Kg PE/mol Zr), and the molecular weight was higher for the copolymers containing 1-octene, demonstrating that aluminohydride zirconocene systems can copolymerize ethylene and alpha-olefins effectively.
A new general anionic functionalization method (GFM) for preparing α-chain-end functionalized polymers utilizes the initiator formed from sec-butyllithium and dimethyl[4-(1-phenylvinyl)-phenyl]silane (DPESiH) followed by hydrosilation with substituted alkenes. α-(4-dimethylsilyl hydride)phenyl-functionalized polystyrene and poly(methyl methacrylate) were synthesized in quantitative yield by living anionic polymerization. These α-silyl hydride-functionalized polymers were further functionalized by reaction with tridecafluorooctene in the present of Karstedt's hydrosilation catalyst. These tridecafluorooctane-functionalized polymers were characterized by SEC, FTIR, 1H-, 13C- and 19F-NMR spectroscopy.
In the present study we carried out nitroxide mediated radical polymerizations (NMRP) of styrene in emulsion using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) by stepwise and semibatch processes obtaining polystyrene (PS) with a dispersity (Đ) lower than 1.5 and average particle diameter (Dp) between 180 and 520 nm when methanol was used. Otherwise, when the polymerization was carried out in the absence of methanol, a bimodal particle size distribution (PSD) was obtained. This is attributed to the super-swelling effect, which causes the particles to grow considerably and also to lower the nitroxide efficiency reducing chain formation and thus drastically reducing the particle nucleation. These polymerizations did not require a separate macroinitiator synthesis step, which represents an important simplification for further applications.
This work presents a study about the effect of gamma irradiation in plasma polymers derived from pyrrole. Gamma rays modify the whole energetic equilibrium between the nucleus and the electron clouds which results in rearrangements of atoms, molecules and in modifications in the properties of irradiated materials. Two types of interactions were analyzed, in polypyrroles synthesized from irradiated monomers, and in polypyrroles irradiated after the synthesis. The doses studied were 18 and 40 kGy. The polymers were synthesized by plasma at 13.56 MHz and 100 W. The results indicated that the exposure to gammas promoted dehydrogenation and creation of groups with double and triple bonds. The multiple bonding was reflected in the electrical conductivity which increased up to 9 orders of magnitude, from 10−11 in non irradiated polymers to 10−2 S/m in the polymers synthesized with monomers irradiated at 40 kGy. Another effect of gammas was the increase in the roughness of polymers which affects the hydrophilicity, this property was evaluated by measuring the contact angle of polymers with a Krebs-Ringer solution. The angles reached up to 72° in surfaces with the greatest pores.
This work presents a study about the synthesis by plasma of organometallic compounds of titanium oxides (TiO) obtained from titanium tetraisopropoxide (TTIP) and water on polyethylene (PE) to form TiO-PE superficial composites. The objective is the synthesis of TiO nano and meso particles attached on PE supports. The synthesis was carried out by plasma in a vacuum tubular glass reactor at 13.56 MHz, 10−1 mbar and 100–150 W. The accelerated particles in the plasma have the function of separating the organic and inorganic segments of TTIP bonding the TiO particles with PE. The resulting TiO-PE superficial compounds have agglomerates of spherical and oval organometallic particles with diameter between 50 and 3000 nm as a function of the time of synthesis. The agglomerates contain Ti and C oxides with a tendency to change from film to particles as a function of the energy applied in the synthesis. The contact angles of the superficial composites varied from 40° to 76°.
This work presents a study about the hydrophilicity and conductivity of plasma polypyrroles (PPy) in contact with solutions of salt concentration similar to human tissues. PPy is a conjugated biocompatible polymer in which the electronic distribution produces partial relocation in contact with different compounds. PPy and PPy doped with iodine (PPy/I) particles of different sizes are being studied as implants in the central nervous system to restore the lost neuronal communication after severe injuries. Particle size, hydrophilicity and conductivity of these polymers are essential in this task. In this work, plasma PPy and PPy/I were synthesized as thin films using low-pressure, rf, resistive, electrical glow discharges and grounded up to obtain microparticles in the range of neuronal nuclei. The electrical conductivity of particles was studied with the addition of human-like ionic solutions, particularly with sodium and potassium ions which are part in the ionic transference of electric charges in neuronal membranes.
Molecular dynamics simulations have been performed on seven aromatic polyamides to understand the structure-property relationships on their densities. The aromatic polyamides studied in this work are based on poly(hexafluoroisopropylidene) isophthalamide, HFAISO. The aim of this research is to investigate the effect of the side groups attached to the diacid monomer on the density of the aromatic polyamides. Densities were obtained via NVT and NPT molecular dynamics simulation using the COMPASS forcefield. Reliability and accuracy of the simulations were compared with the experimental density data from HFAISO and HFATERT aromatic polyamides, which are in good agreement.
Summary: A new lumping-based approach for calculating the molecular weight distribution (MWD) in polymerization systems is proposed. In order to illustrate its applicability the proposed methodology is used for modelling the kinetics of a 1,3-diene polymerization using a binary neodymium-based catalytic system. The model is capable of reproducing experimental data after fitting for the evolution of molecular weight and polydispersity. The results of this preliminary simulation are used as a guide to design kinetics experiments that may lead to improve the kinetic model and to deepen the knowledge of the system under study.
An experimental and theoretical study to functionalize the surface of titanium dioxide nanoparticles (NPTiO2) with 2,2,6,6,tetramethylpiperidine-N-oxyl (TEMPO) in one step of synthesis using oxoammonium salts is presented. A general reaction mechanism to obtain functionalized nanoparticles (NPTiO2-g-TEMPO) is proposed, in which a peroxide bond (Ti-O-O-N) between (Ti-OH) and TEMPO is formed. Furthermore, Density Functional Theory (DFT) calculations in combination with the 6–31 + G(d) basis set, performed on the NPTiO2-g-TEMPO surface interaction, suggest the formation and stability of the new peroxide bond. Thus, the (O-O) bond dissociation energy was calculated in 28.18 kcal/mol. Also, the maximum amount of TEMPO molecules bonded on the titanium surface was estimated by Molecular Mechanics (MM +) in 4 molecules/nm2. TGA and FTIR support our findings
The chemical surface modification of graphene and graphene oxide materials is a burgeoning area of investigation. We report the preparation of new ionic forms of graphene oxide by wet chemical modification and metathesis reactions. Graphene oxide (GO) was prepared by a modified Hummers method and functionalized to form cationic and anionic species. The former were prepared via nucleophilic substitution of alkyl amines at the epoxy and hydroxyl groups on the GO while the latter, consisting of sulfonated GO, was prepared via an aryl diazonium salt. The functionalized graphene oxide sheets consist of quaternary ammonium ion substituted GO, GO+, with Cl- or I- as counter ion and sulfonated GO, GO-, with H+, Ba2+ or tetraphenylphosphonium (TPP+) ion as counter ions. The addition reaction between GO+ and GO- in aqueous solution by metathesis to form a black [GO+][GO-] precipitate was also investigated. Materials were characterized by XRD, FTIR, Raman, STEM and TEM/HRTEM. Chemically modified graphene and graphene oxides are expected to facilitate further understanding of the chemistry and physics of graphene materials in general and contribute to the formation and further understanding of advanced hybrid graphene and graphene oxide polymer nanocomposites.1,2
A series of ABS/ZnO nanocomposites with different nanoparticles content were synthesized by mass-mass and mass-suspension processes. Because nanocomposites obtained through mass-suspension process had a greater impact resistance than those derived from mass-mass process, they were subjected to twin-screw extrusion. The extrusion led to a dramatic morphological change and increase in impact resistance, higher than 100% in most of the cases. On the other hand, the higher the ZnO content, the higher the UV blocking (>95% for 1 and 3% of ZnO) for both materials, before and after extrusion.
Polymeric electrolyte composites based on polymer poly(sulfobutylbetaines) (PMBS-4) and 1%, 3% and 5% montmorillonite (MMT) modified with organic ions, dodecyldimethyl(3-sulfo-propyl) ammonium hydroxide (C12SB) were synthesized. The structural modification and electrochemical properties of samples were investigated to understand the effects of organic MMT in a polymer matrix on ionic conductivity. The modification of the clay with organic ions resulted in a larger d-spacing which makes easier the penetration of PMBS-4 into the interlayer galleries. The values of Tg obtained from differential scanning calorimetry are 18 °C, 20 °C, 20 °C and 27 °C for PMBS-4, PMBS-4/C12SB-MMT 1%, PMBS-4/C12SB-MMT 3% and PMBS-4/C12SB-MMT 5% respectively. Since PMBS-4 and PMBS/C12SB-MMT were examined in the absence of Li salt, the conductivity of samples was attributed to the motion of dipoles in chains of zwitterionic PMBS-4. The increase in conductivity with increasing temperature was explained as a local structural relaxation and segmental motion of polymer. The values of σ, at 25 °C, for: PMBS-4, PMBS-4/C12SB-MMT 1%, PMBS-4/C12SB-MMT 3% and PMBS-4/C12SB-MMT 5% were 6.2 × 10−6 S/cm, 1.2 × 10−6 S/cm, 1.7 × 10−6 S/cm and 9.1 × 10−6 S/cm respectively.
In analyzing morphology of modified surfaces it is important to distinguish between features attributable to the matrix and those for the modifier. This study focused on an examination of the surface morphology of closely related linear and network polyurethanes intended for use as bulk coating constituents. A process dependent morphology is found for the crosslinked polyurethane matrix. Microscale gelation was found when reagents were added simultaneously, while nanoscale gelation occurs when the cross-linker is added last. Furthermore, the coating formed by sequential addition had 50% less extractables. Future work on modified networks will focus on the matrix with nanoscale gelation as it presents a “cleaner” canvas for surface modification.
High Impact Polystyrene (HIPS) was synthesized by free radical polymerization through the bulk-bulk technique. Polybutadiene (PB) and/or poly(butadiene-b-styrene) copolymer (S:B = 30:70) (at 8 wt-%) were used as impact modifiers in the presence of 0, 5 and 7.5 wt-% of polystyrene (PS) and styrene monomer. The interval of phase inversion (PI) was determined through the evolution of melt flow index (MFI) and transmission electron microscopy (TEM), the conversion (X) and grafting degree (GD) were gravimetrically determined and impact strength (IS) tests were performed. When PS was added from the beginning of the reaction in the presence of SB rubber, the PI shifted to lower values of polymerization time (from 270 min in the reference HIPS to 190 min when 5 and 7.5 wt-% of PS was added, respectively). The morphology of the reference HIPS was of the core shell type and in the case of adding PS the morphology at the PI changed to a quasi-salami type, due to a poor stability of the system as a consequence of the low GD. The final size of the disperse phase increased 292 and 51% in the presence of PS and the volume fraction increased 100% yielding HIPS with an IS from 20 (reference HIPS) to 30J/m for 7.5 wt-% of added PS.
In the system with PB as the precursor rubber the salami structure in the reference HIPS, increased its size from 0.5 µm to 2.1 and 2.6 µm for 5 and 7.5 wt-% of added PS, but instead of that, the IS remained without significant changes.
A method is proposed to characterize the elastic behavior of polyolefin melts, by calculating the first normal stress difference, N1 as a function of shear rate. The method is predicated on Cogswell's abrupt contraction flow analysis.1 These results are compared with experimental measurements and calculated values with a variety of methods, such as: i) Laun's rule2 for obtaining viscometric functions from linear viscoelastic data, ii) methods proposed by A. Khalik, et. al.3,4 and Wagner,5 for calculating N1 from viscosity data, and iii) N1 values calculated with the Johnson & Segalman constitutive equation.6 These comparisons yield acceptable agreement.
Summary: The anionic and Ziegler-Natta polymerizations of 1,3-butadiene initiated with n-BuLi and NdV/TIBA/DEAC respectively were carried out in presence of ionic liquids and cyclohexane as solvent in both cases. Polybutadienes obtained were characterized by SEC, NMR and DSC techniques and results were compared with polybutadienes synthesized under similar conditions but in absence of ionic liquids. In all cases, the presence of ionic liquids caused a decrease in the reaction exothermal and also a diminution in the molecular weight distribution. The ionic liquids increased the microstructure cis-1,4 in both kind of polymerizations, and promotes a shift towards lower temperatures in the glass-transition temperature.
High temperature semibatch free radical copolymerizations of 2-hydroxyethyl methacrylate (HEMA) and styrene (ST) were carried out in xylene and DMF solution. Experimental free monomer and polymer molecular weight (MW) profiles at different temperatures and solvent concentrations are compared to those for ST polymerized with butyl methacrylate (BMA). Solvent choice, through its influence on hydrogen bonding of HEMA monomer, has a significant impact on the polymerization that is not seen for the corresponding ST/BMA reactions. The weight-average polymer MWs of ST/HEMA are unusually high (>40 kg/mol), a result of branching reactions involving dimethacrylate impurity contained in HEMA monomer. The importance of these branching reactions is also influenced by solvent choice.
Homopolypropylene was modified in the molten state using a new cyclic multifunctional peroxide as the initiator, the Diethyl Ketone Triperoxide (DEKTP), in the presence of different branching/crosslinking co-agents: Trimethylolpropane Triacrylate (TM), Trimethylolpropane Triacrylate Propoxylate (TMPP), Pentaerythritol Tetraacrylate (PETA) and N′N′-1,3-Phenylene Dimaleimida (FDM). Experiments were carried out in an internal mixer at 180 °C, using two different concentrations of co-agent/initiator; [0.250/0.025] and [0.50/0.05], molar rate = [co-agent]/[initiator] constant equal to 10. The modified PP′s were evaluated by GPC, FTIR, DSC, WAXD, and MOP. Torque and MFI were also evaluated. The results showed the presence of branches in the backbone of PP when it was modified with TM and PETA.