L Bert Klumperman

Controlling electrical percolation in multicomponent carbon nanotube dispersions

Carbon nanotube reinforced polymeric composites can have favourable electrical properties, which make them useful for applications such as flat-panel displays and photovoltaic devices. However, using aqueous dispersions to fabricate composites with specific physical properties requires that the processing of the nanotube dispersion be understood and controlled while in the liquid phase. Here, using a combination of experiment and theory, we study the electrical percolation of carbon nanotubes introduced into a polymer matrix, and show that the percolation threshold can be substantially lowered by adding small quantities of a conductive polymer latex. Mixing colloidal particles of different sizes and shapes (in this case, spherical latex particles and rod-like nanotubes) introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic percolation, depending on the ease of charge transport between the various conductive components.

Lowering the percolation threshold of single-walled carbon nanotubes using polystyrene/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) blends

Single-walled carbon nanotubes (SWCNTs) are introduced into a polymer matrix via a latex-based route resulting in a conductive composite. The percolation threshold for a polystyrene (PS)-based composite prepared with SWCNTs dispersed in water using a conventional surfactant like sodium dodecyl sulfate (SDS) is approximately 0.4 wt%. In this study, SDS is substituted by a conductive polymer latex, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) also known as PEDOT:PSS. This latex can effectively stabilize individual SWCNTs in water and composites prepared with these dispersions show a lower percolation threshold value of 0.2 wt%. The percolation of PEDOT:PSS in PS in a binary polymer blend without SWCNTs is also investigated, and found to occur at a remarkably low loading of 2.2 wt% of the conductive latex. The morphology of the final polymer-filled blend is further investigated and the findings provide an explanation as to why PEDOT:PSS lowers the percolation threshold of the SWCNTs, and in fact has such a low threshold itself without the presence of the nanotubes.

Synthesis of Polyolefin Block and Graft Copolymers

4. METALLOCENE-CATALYZED COPOLYMERIZATION OF OLEFINS WITH FUNCTIONAL MONOMERS ............................................... 170 4.1. Catalysis by Organolanthanide(III) Complexes......................................... 171 4.2. Group VIII Transition Metals .................................................................... 171 4.3. Group IV Metallocene Catalysts................................................................ 172 4.4. Ring-Opening Metathesis Polymerization ................................................. 175 4.5. Functional Polyolefins by Using Metallocene Catalysts and Borane Reagents...................................................................................................... 176

Atom‐transfer radical polymerization of methyl methacrylate (MMA) using CuSCN as the catalyst

The effect of CuSCN as a catalyst in atom-transfer radical polymerization (ATRP) was investigated. CuSCN can successfully be used for the ATRP of MMA. Substituted bipyridines as well as imines can be used to stabilize the copper complex in solution. CuSCN induces faster polymerization compared to CuBr and CuCl when tosylchloride is used as the initiator. However, the polydispersity is larger than that obtained in the cases of CuCl and CuBr.

Olefin copolymerization via reversible addition-fragmentation chain transfer

Successful statistical copolymerization of an alpha-olefin (1-octene) with an acrylate (butyl acrylate, BA) and with a methacrylate (methyl methacrylate, MMA), employing reversible addition-fragmentation chain transfer (RAFT) mediated polymerization has been accomplished

Randomly branched bisphenol A polycarbonates. I. Molecular weight distribution modeling, interfacial synthesis, and characterization

Randomly branched bisphenol A polycarbonates (PCs) were prepared by interfacial polymerization methods to explore the limits of gel-free compositions available by the adjustment of various composition and process variables. A molecular weight distribution (MWD) model was devised to predict the MWD, G, and weight-average molecular weight per arm (Mw /arm) values based on the composition variables. The amounts of the monomer, branching agent, and chain terminator must be adjusted such that the weight-average functionality of the phenolic monomers (FOH ) was less than 2 to preclude gel formation in both the long- and short-chain branched (SCB) PCs. Several series of SCB and long-chain branched PCs were prepared, and those lacking gels showed molecular weights measured by gel permeation chromatography–UV and gel permeation chromatography–LS consistent with model calculations. In SCB PCs, the minimum Mw /arm that could be realized without gel formation depended on both composition (molecular weight, terminator type) and process (terminator addition point, coupling catalyst) variables. The minimum Mw /arm achieved in the low molecular weight series studied ranged from ∼3300 to ∼1000. The use of long chain alkyl phenol terminators gave branched PCs with lower glass-transition temperatures but a higher gel-free minimum Mw /arm. SCB PCs where Mw /arm was less than ∼Mc spontaneously cracked after compression molding, a result attributed to their lack of polymer chain entanglements.

Synthesis of Poly(ethylene‐co‐butylene)‐block‐Poly(methyl methacrylate) by Atom Transfer Radical Polymerization: Determination of the Macroinitiator Conversion

The synthesis of poly(ethylene-co-butylene)-block-poly(methyl methacrylate) via atom transfer radical polymerization (ATRP) of methyl methacrylate onto a poly(ethylene-co-butylene) macroinitiator is described. Copper bromide (CuBr), copper chloride (CuCl) and copper thiocyanate (CuSCN) were applied as active ATRP catalysts with N-pentyl-2-pyridylmethanimine as ligand, in order to study the effect of the copper counter ion on the rate of activation of the bromine-functional polyolefin macroinitiator. The reaction products were subjected to normal phase gradient polymer elution chromatography (NP-GPEC) in order to separate residual macroinitiator from the block copolymers. The amount of residual macroinitiator was monitored for each catalyst system by evaporative light scattering detection, where the chromatograms were normalized on a PS standard, which was added to the reaction mixture before polymerization. The rate of activation of the macroinitiator is strongly affected by the copper counter ion and increases in the order CuSCN < CuBr < CuCl. The high reproducibility of this observation is in contrast with the significant effect of polymerization conditions like e.xa0g. catalyst solubility, presence of water and trace amounts of oxygen, on the overall rate of polymerization.

Molecular modelling of chain end effects in separating oligomers by reversed-phase gradient polymer elution chromatography; adsorption transition as revealed by a self-consistent-field theory for polymer adsorption

HPLC experiments to separate butyl-terminated polystyrene (B-PS) oligomers have been mimicked by equilibrium self-consistent-field calculations based upon the Scheutjens Fleer formalism for polymers at interfaces. The adsorption-desorption transition as a function of the fraction of good solvent in a non-solvent (water)-solvent (tetrahydrofuran) mixture has been analysed and correlated to corresponding experiments. Much attention is paid to keeping the modelling as realistic as possible; for example, the effects of the solvent mixture on the C18-alkyl tails that are grafted on the silica surface are retained in the calculations. It is shown that the butyl end groups affect the elution properties up to chains with approximately 30 styrene units. Excellent semi-quantitative comparison is found with experiments for a realistic set of interaction parameters. Molecular-level information is available for the adsorption layer as a function of the solvent quality. Going from poor to good solvent, it is typical to find that the B-PS is fully absorbed inside the alkyl brush, then adsorbed on top of it, and finally depleted from it. The depletion effect in good solvents increases with increasing molecular mass.

Chromatographic Behaviour of Low Molar-Mass Polyesters in Normal-Phase High-Performance Liquid Chromatography

SummaryThe normal-phase chromatographic retention behaviour of polyesters on bare silica and on a polymer-based polyamine (PA) column has been studied with a variely of binary mobile phases under isocratic conditions. The dependence of experimental retention data on the degree of polymerization (p) and on mobile phase composition (φ) was characterized by to an approach developed by Jandera et al. The bulky repeating unit and the relatively highly polar end groups of the polyesters both had a large influence on retention behaviour. The two effects in combination explain the molar-mass-independent retention observed experimentally at a particular mobile phase composition for all the mobile phase—stationary phase combinations investigated. These conditions were found to be independent of the type of end group. End group separation on a silica column improves when the polarity of the less polar solvent is increased. End group separation is better on the PA column because of a greater difference between the adsorption energy of the alcohol and acid end groups. Better prediction of retention data on the PA column was achieved by use of an approach which assumes two different types of adsorption site. Results enabled further understanding of retention behaviour in normalphase gradient polymer-elution chromatography (NPGPEC) and explained both the dependence of the order of elution onp and differences between the end-group selectivity of different systems.