All issues / Volume 5 (2011) / Issue 9 (September)
To modify the rheological properties of certain commercial polymers, a set of block copolymers were synthesized through oxyanionic polymerization of 2-(dimethylamino)ethyl methacrylate to the chain ends of commercial prepolymers, namely poly(ethylene oxide) (PEO), poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO), and poly(propylene oxide) (PPO). The formed block copolymers were analysed with size exclusion chromatography and nuclear magnetic resonance spectroscopy in order to confirm block formation. Thermal characterization of the resulting polymers was done with differential scanning calorimetry. Thermal transition points were also confirmed with rotational rheometry, which was primarily used to measure melt strength properties of the resulting block co-polymers. It was observed that the synthesised poly[2-(dimethylamino)ethyl methacrylate]-block (PDM) affected slightly the thermal transition points of crystalline PEO-block but the influence was stronger on amorphous PPO-blocks. Frequency sweeps measured above the melting temperatures for the materials confirmed that the pre-polymers (PEO and PEO-PPO-PEO) behave as Newtonian fluids whereas polymers with a PDM block structure exhibit clear shear thinning behaviour. In addition, the PDM block increased the melt viscosity when compared with that one of the pre-polymer. As a final result, it became obvious that pre-polymers modified with PDM were in entangled form, in the melted state as well in the solidified form.
Chemical functionalization of multi-wall carbon nanotubes (MWCNTs) is conducted by means of acid oxidation, direct silanization of the as-received MWCNTs and a sequential treatment based on oxidation and silanization. Polymer composites made from the functionalized MWCNTs and a vinyl ester resin are fabricated and tested in compression. It is found that although silanization could be achieved without the assistance of a previous oxidative treatment, oxidizing the MWCNTs by HNO3/H2O2 prior to silanization yields significantly better attachment of the silane molecules to the CNT surface and hence, better mechanical performance of the resulting composite. The limited improvements in mechanical properties found are discussed in light of the reduction of the nanotube length after MWCNT oxidation and composite processing.
Brominated isobutyl-isoprene rubber/clay nanocomposite (BIIRCN) and ethylene-propylene-diene-monomer rubber/clay nanocomposite (EPDMCN) were prepared by melt blending. The micro-structural evolution of these two kinds of rubber/clay nanocomposites (RCNs) with vulcanization process was investigated using wide-angle X-ray diffraction (WAXD) and transmission electron microscope (TEM). The WAXD results revealed that the intercalated structure of organically modified clay (OMC) changed throughout the whole curing process. The intercalated structure kept on changing beyond the vulcanization stage of T90. The interlayer space of intercalated silicate in uncured BIIRCN is larger than that in uncured EPDMCN. However, the intercalated structure for EPDMCN changed by a larger extent than that for BIIRCN during the vulcanization process, and the interlayer space of the intercalated structure is larger in the cured EPDMCN than that in the cured BIIRCN. It was found that the intercalant (i.e., octadecylamine, ODA) for OMC could shorten the scorch time of the curing reaction, and increase the curing rate, which was attributed to the further intercalation during vulcanization. TEM results indicated that the spatial distribution of OMC is much better in BIIR (a polar rubber matrix) than that in EPDM (a non-polar rubber matrix). The changes in spatial dispersion structure during vulcanization for EPDMCN and BIIRCN show different trends. In conclusion, the polarity of the rubber is the determining factor influencing the evolution of both the intercalated structure and the spatial dispersion of clay during vulcanization.
Sterilization of propylene/ethylene random copolymers: Annealing effects on crystalline structure and transparency as influenced by polymer structure and nucleation
M. Gahleitner, C. Grein, R. Blell, J. Wolfschwenger, T. Koch, E. Ingolic
Vol. 5., No.9., Pages 788-798, 2011
Vol. 5., No.9., Pages 788-798, 2011
An extensive investigation of three different series of isotactic ethylene/propylene (EP) random copolymers was performed to understand the factors influencing the change in optical properties in the steam sterilization of extrusion cast films from such materials. Different analytical methods (differential scanning calorimetry (DSC), X-ray diffraction and electron microscopy) were employed to elucidate structural changes determining film optics, and in addition to the polymer structure parameters also nucleation and processing effects were studied. The findings clearly show that a combination of homogeneously randomized comonomer distribution and nucleation can partly inhibit lamellar thickening in sterilization, thus preserving high transparency even after a heat treatment. In detail, attention has to be paid to the combined effects of primary and secondary post-crystallization, which both are affected by the chain regularity.
Intrinsic fluorescence was applied to quantitatively describe the interfacial adhesion of nanoparticles in polystyrene/poly(vinyl methyl ether) (PS/PVME) blends. Due to the aggregation of aromatic rings on PS chains, the temperature dependence of excimer fluorescence intensity (I324) showed the high sensitivity to the phase separation process. Consistent with Ginzburg thermodynamic model, it was found that the addition of spherical hydrophilic nanoparticles shifted the phase separation temperature to higher temperatures due to the aggregation of silica into PVME chains leading to the free energy reduction and slowing down the phase separation dynamics. A certain composition of polymer blend, i.e. 2/8, was focused on to shed light on the dynamic of spinodal decomposition (SD) phase separation by using decomposition reaction model. It was shown that the addition of nanoparticles to polymer blends resulted in the deviation of linear relationship between the initial SD phase separation rate (Rp0) and thermodynamic driving force (ΔfSD). Besides, for PS/PVME (2/8) with 2 vol% silica nanoparticles, the apparent activation energy of phase separation (Ea) was 196.61 kJ/mol, which was higher than that of neat PS/PVME (2/8) blend (Ea = 173.68 kJ/mol), which strongly confirmed the interfacial adhesion effect of silica nanoparticles as compatibilizers.
The influence of graphene oxides (GOs) on the cure behavior and thermal stability of a tetrafunctional tetraglycidyl-4,4’-diaminodiphenylmethane cured with 4,4’-diaminodiphenylsulfone was investigated by using dynamic differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dynamic DSC results showed that the initial reaction temperature and exothermal peak temperature decreased with the increase of GO contents. Furthermore, the addition of GO increased the enthalpy of epoxy cure reaction. Results from activation energy method showed that activation energies of GO/epoxy nanocomposites greatly decreased with the GO content in the latter stage, indicating that GOs significantly hindered the occurrence of vitrification. The oxygen functionalities, such as hydroxyl and carboxyl groups, on the surface of GOs acted as catalysts and facilitated the curing reaction and the catalytic effect increased with the GO contents. TGA results revealed that the addition of GOs decreased the thermal stability of epoxy.
Graphene/polymer films were prepared by casting water dispersion of graphene oxide (GO) in the presence of polystyrene (PS) latex particles. The samples were heated up to 180°C and exposed to an external electric voltage during their annealing. We observed that for the GO/PS films deposited before the electric field assisted thermal annealing the polymer latex was embedded in the graphene sheets, while the electric field assisted thermal annealing induces a phase separation with the enrichment of the PS phase above an underlying GO layer. For the films annealed under an external electric field we have also found that as the electric current passes through the GO film, GO could be recovered to reduced GO with decreased resistance.
A new chiral poly(ester-imide) (PEI) was prepared via direct polyesterification of N,N'-(pyromellitoyl)-bis-(Ltyrosine dimethyl ester) and N-trimellitylimido-L-methionine using tosyl chloride/pyridine/N,N'-dimethylformamide system as a condensing agent. The resulting new chiral polymer was obtained in good yield and had good thermal stability as well as good solubility in common organic solvents. After that, PEI/titanium bionanocomposites (PEI/TiO2 BNCs) were prepared using the modified nanosized TiO2 via sonochemical reaction that can accelerate hydrolysis, increase collision chance for the reactive system and improve the dispersion of the nanoparticles in polymer matrix. The scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results indicated that there is no aggregation of a large quantity of particles. Thermogravimetric analysis (TGA) confirmed that the heat stability of the BNC polymers in the temperature range of 400–800ºC was enhanced by addition of TiO2 nanoparticles. Furthermore, in vitro toxicity test was employed for assessing the sensitivity of these compounds to microbial degradation. To this purpose, polymer and PEI/TiO2 BNCs were investigated under soil burial conditions. The results of this study revealed that polymer and its BNCs are biologically active and non-toxic in the natural environment although some antimicrobial properties were found for BNCs.