Showing papers in "Macromolecules in 2001"
997 citations
TL;DR: In this article, a donor-acceptor conjugated copolymer and a corresponding oligomer were synthesized and their solution and solid-state photophysics were investigated, and their optical band gaps were 2.35−2.64 eV.
Abstract: Alternating carbazole−quinoline and phenothiazine−quinoline donor−acceptor conjugated copolymers and a corresponding oligomer were synthesized, and their solution and solid-state photophysics were investigated. The new copolymers, poly(2,2‘-9-methyl-3,6-carbazolylene-6,6‘-bis(4-phenylquinoline)) and poly(2,2‘-10-methyl-3,7-phenothiazylene-6,6‘-bis(4-phenylquinoline)), had intrinsic viscosities of 11.2−22.0 dL/g, indicating very high molecular weights. The optical band gaps of the new copolymers were 2.35−2.64 eV, which are significantly smaller than the corresponding homopolymers. The absorption and emission spectra of the related donor−acceptor oligomers, 3,6-[bis(4-phenyl-2-quinolyl)]-9-methylcarbazole and 3,7-[bis(4-phenyl-2-quinolyl)]-10-methylphenothiazine, in solvents of varying polarity showed positive solvatochromism. An unusual dual fluorescence, with a blue emission band at 454 nm and an orange emission band at 584 nm, was observed in solid films of the carbazole-linked oligomer and related to i...
720 citations
TL;DR: In this paper, a block copolymer of (ar-vinylbenzyl)trimethylammonium chloride with N,N-dimethylvinyl benzylbenzylamine has been synthesized, using the same methodology.
Abstract: Homopolymers of sodium 4-styrenesulfonate have been synthesized directly in aqueous media by reversible addition−fragmentation chain transfer polymerization (RAFT). The resulting homopolymers have narrow molecular weight distributions, with polydispersity indices in the range 1.12−1.25, as determined by aqueous size exclusion chromatography. Using a dithioester-capped sodium 4-styrenesulfonate homopolymer as a macro chain transfer agent, a block copolymer with sodium 4-vinylbenzoate has been prepared in aqueous media. Additionally, a block copolymer of (ar-vinylbenzyl)trimethylammonium chloride with N,N-dimethylvinylbenzylamine has been synthesized, using the same methodology. We believe these represent the first examples of AB diblock copolymers prepared directly in aqueous media via the RAFT process. Both block copolymers are stimuli-responsive and undergo reversible pH-induced micellization in aqueous solution. Micelles with hydrodynamic diameters in the range 18−38 nm were observed by dynamic light sc...
680 citations
TL;DR: The melt-state linear and nonlinear shear rheological properties of hybrid materials of polypropylene and amine-exchanged montmorillonite were studied in this article.
Abstract: The melt-state linear and nonlinear shear rheological properties of hybrid materials of polypropylene and amine-exchanged montmorillonite were studied. The materials were prepared by melt mixing with maleic anhydride functionalized polypropylene as the compatibilizer. The clay interlayer spacing (as determined by wide-angle X-ray scattering) increased upon melt mixing; however, the short-range ordering of the clay layers was preserved. Above inorganic loadings of 2.0 wt % the hybrid materials exhibited apparent low-frequency plateaus in the linear viscoelastic moduli. The hybrid storage modulus was sensitive to the chemistry of the amine exchanged into the clay. The amount of stress overshoot observed in flow reversal experiments was found to be a function of the rest time allowed between the reversal. The transient stress in start-up of steady shear scaled with the applied strain. These observations allow features of the polypropylene/montmorillonite hybrid structure to be deduced. The transient nonlinea...
649 citations
TL;DR: In this paper, a free-standing anisotropic side chain liquid crystalline elastomer films have been prepared using mesogens with laterally affixed polymerizable side chains and the calculated cross-linking density, as determined from the Young's modulus, was in the 10 -5 mol/cm3 range.
Abstract: Free-standing anisotropic side chain liquid crystalline elastomer films have been prepared using mesogens with laterally affixed polymerizable side chains. We present data on two networks: one containing the monomer of 4‘-acryloyloxybutyl 2,5-(4‘-butyloxybenzoyloxy)benzoate and another from a 50/50 mol % mixture of the above with 4‘-acryloyloxybutyl 2,5-di(4‘-pentylcyclohexyloyloxy)benzoate. The cross-linking was achieved using 10 mol % of 1,6-hexanediol diacrylate. The calculated cross-linking density, as determined from the Young's modulus, was in the 10 -5 mol/cm3 range. Thermoelastic responses show strain changes through the nematic−isotropic phase transition to be 30−45%. The order parameters of the oriented films were determined from the dichroic ratio of IR absorption at 3343 cm-1 to the in-plane aromatic stretching overtone of the LC mesogen core. The variation of the order parameter with temperature scales similar to the strain changes at constant stress. Isostrain studies, conducted through the...
600 citations
TL;DR: In this article, the authors present an experimental investigation on the creep behavior of molten polypropylene organically modified clay nanocomposites, which were prepared by melt intercalation in an extruder in the presence or absence of a compatibilizer.
Abstract: We present an experimental investigation on the creep behavior of molten polypropylene organically modified clay nanocomposites. The nanocomposite hybrids were prepared by melt intercalation in an extruder in the presence or absence of a compatibilizer. They were subsequently annealed and simultaneously characterized using high-temperature wide-angle X-ray diffraction and controlled stress rheometry. The creep resistance of compatibilized hybrids was significantly higher than that of uncompatibilized hybrids and also increased with annealing time. The microstructure of the nanocomposites as investigated by TEM and high-temperature WAXD showed the presence of clay crystallites dispersed within the polymer matrix. The creep data together with the microstructural investigation are probably indicative of a small amount of exfoliation from the edges of the clay crystallites during extrusion and annealing. The zero shear viscosity of the compatibilized nanocomposites containing greater than 3 wt % clay was at l...
587 citations
TL;DR: In this paper, a mechanism for distributing excess polymer surface charge is used to model the growth of multilayers of strongly charged polyelectrolytes, and it is shown that only modest levels of polymer charge overcompensation are required to account for large increments in polymer thickness, realized at high salt concentration, since the excess charge is distributed over several layers.
Abstract: A mechanism for distributing excess polymer surface charge is used to model the growth of multilayers of strongly charged polyelectrolytes. Two parameters are required for the semiempirical analysis: the surface, or unrestricted, charge overcompensation level, φ, which is assumed to decrease exponentially from the film surface to bulk, and the characteristic length for this decay, lcp, which is termed the charge penetration length. Modeling of the data reveals that only modest levels of polymer charge overcompensation are required to account for large increments in polymer thickness, realized at high salt concentration, since the excess charge is distributed over several “layers”. Experimentally, φ appears to be roughly independent of salt concentration. The thickness increment is primarily controlled by lcp, which is about 2.5 nominal layers for the system studied. Whereas the growth conditions and polyelectrolyte type lead to the formation of intrinsically compensated multilayers in this work, conditio...
544 citations
TL;DR: In this paper, a new synthetic method to synthesize regioregular, head-to-tail coupled poly(3-alkylthiophenes) using magnesium-halogen exchange (Grignard metathesis) called the GRIM method is described.
Abstract: An investigation of the new synthetic method to synthesize regioregular, head-to-tail coupled poly(3-alkylthiophenes) using magnesium-halogen exchange (Grignard metathesis) called the GRIM method is described. Treatment of 2,5-dibromo-3-alkylthiophenes with a variety of alkyl and vinyl Grignard reagents resulted in two metalated, regiochemical isomers, namely, 2-bromo-3-alkyl-5-bromomagnesio- thiophene and 2-bromomagnesio-3-alkyl-5-bromothiophene in an 85:15 ratio. This ratio appears to be independent of reaction time, temperature, and Grignard reagent employed. Introduction of a catalytic amount of Ni(dppp)Cl2 to this isomeric mixture afforded poly(3-alkylthiophene) that contained greater than 95% HT-HT couplings (typically 98% HT couplings were seen). The high degree of regioregularity found in the polymer can be explained by a combination of kinetic and thermodynamic effects arising from steric and electronic effects found in the catalytic reaction. A series of reaction investigations led to a general explanation of the origin of regioregularity in polythiophene polymerization reactions. These reactions included kinetic studies and competition experiments.
489 citations
TL;DR: In this paper, the glass transition temperatures of polystyrene and poly(methyl methacrylate) (PMMA) were measured on surfaces with interfacial energies (γSL) ranging from 0.50 to 6.48 mJ/m2.
Abstract: The glass transition temperatures (Tg's) of ultrathin films (thickness 80−18 nm) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measured on surfaces with interfacial energies (γSL) ranging from 0.50 to 6.48 mJ/m2. The surfaces consisted of self-assembled films of octadecyltrichlorosilane (OTS) that were exposed to X-rays in the presence of air. Exposure to X-ray radiation systematically modified the OTS by incorporating oxygen-containing groups on the surface. The interfacial energy for PS and PMMA on the OTS surface was quantified as a function of X-ray dose using the Fowkes−van Oss−Chaudhury−Good model of surface tension. The Tg values of the films were characterized by three complementary techniques: local thermal analysis, ellipsometry, and X-ray reflectivity. Within the resolution of the techniques, the results were in agreement. At low values of γSL, the Tg values of PS and PMMA films were below the respective bulk values of the polymers. At high values of γSL, the Tg values of PS an...
486 citations
TL;DR: In this article, the authors developed a theory of "sticky reptation" to model the dynamics of entangled solutions of associating polymers with many stickers per chain, which predicts a very strong concentration dependence of viscosity in good agr...
Abstract: The process of making and breaking reversible bonds between associating groups (stickers) controls the dynamics of associating polymers. We develop a theory of “sticky reptation” to model the dynamics of entangled solutions of associating polymers with many stickers per chain. At a high degree of association, there are very few unassociated stickers. It is therefore very difficult for a sticker to find a new partner to associate with after breaking the bond with an old one. Typically a sticker returns to its old partner following an unsuccessful search for a new one, prolonging the effective lifetime of reversible bonds. In the sticky reptation model, the search for a new partner is restricted to a part of the tube confining the entangled chain. Another important effect is the increase of the fraction of the interchain associations at the expense of the intrachain ones with increasing polymer concentration. The sticky reptation model predicts a very strong concentration dependence of viscosity in good agr...
440 citations
TL;DR: In this paper, shape memory polyurethane (PU) block copolymers composed of 4,4‘-methylenebis(phenylisocyanate), poly(tetramethylene glycol), and 1,4-butanediol as a chain extender were synthesized by a two-step process.
Abstract: Shape memory polyurethane (PU) block copolymers composed of 4,4‘-methylenebis(phenylisocyanate), poly(tetramethylene glycol), and 1,4-butanediol as a chain extender were synthesized by a two-step process. FT-IR spectra showed that carbonyl peak appearing at 1700 cm-1 increased with higher hard segment content, whereas another carbonyl peak at 1730 cm-1 decreased. It suggests that hard segments get more aggregated to form domains in the PU block copolymer as hard segment content increases. Such domain formation has a significant influence on the mechanical and thermomechanical properties of PU, such as maximum stress, tensile modulus, and elongation at break. Especially, maximum stress, tensile modulus, and elongation at break increased significantly at 30 wt % of hard segment content, and the highest loss tangent was observed at the same composition. Heat of crystallization as measured by differential scanning calorimetry is also dependent on the hard segment content. Finally, 80−95% of shape recovery was...
TL;DR: In this article, a colloidal suspension of tunicinan animal cellulose was used to obtain a nanocomposite material using glycerol plasticized starch as the matrix.
Abstract: In a previous work [Macromolecules 2000, 33, 8344], nanocomposite materials were obtained using glycerol plasticized starch as the matrix and a colloidal suspension of tunicinan animal cellulosewhi...
TL;DR: In this article, atom transfer radical polymerization (ATRP) was applied to the synthesis of amphiphilic cylindrical polymer brushes by using the "grafting from" technique.
Abstract: Atom transfer radical polymerization (ATRP) was applied to the synthesis of amphiphilic cylindrical polymer brushes by using the “grafting from” technique The procedure included the following steps: (1) ATRP of 2-hydroxyethyl methacrylate (HEMA) gave well-defined poly(HEMA), (2) subsequent esterification of the pendant hydroxy groups of poly(HEMA) with 2-bromoisobutyryl bromide yielded a polyinitiator, poly(2-(2-bromoisobutyryloxy)ethyl methacrylate (PBIEM), (3) ATRP of various monomers (tert-butyl acrylate, or styrene) using PBIEM as polyinitiator yielded cylindrical brushes with homopolymer side chains, (4) addition of a second monomer (styrene or tert-butyl acrylate) formed the cylindrical brushes with diblock copolymer side chains (core−shell cylinders), and (5) hydrolysis of the poly(tert-butyl acrylate) (PtBA) block of the side chains to poly(acrylic acid) (PAA) formed amphiphilic core−shell polymer brushes By using this technique, well-defined core−shell cylindrical polymer brushes with polystyr
TL;DR: In this article, a PMMA-layered silicate nanocomposites were prepared by in-situ suspension polymerization and emulsion polymerization, and wide-angle X-ray diffraction (WAXD) and atom force microscopy (AFM) were used to characterize the structures of the nanocompositionites.
Abstract: PMMA-layered silicate nanocomposites were prepared by in-situ suspension polymerization and emulsion polymerization. For the suspension polymerization, the silicate layers were dispersed individually in water, and we speculate that they are adsorbed on the surface of monomer droplets. For the emulsion polymerization, the nanocomposite was obtained by adding an aqueous dispersion of layered silicate into the polymer emulsion. Wide-angle X-ray diffraction (WAXD) and atom force microscopy (AFM) were used to characterize the structures of the nanocomposites. Analysis of samples from emulsion and suspension polymerization was consistent with an exfoliated structure; after melt pressing, WAXD analysis indicated that both intercalated and exfoliated structures were observed. The exfoliated structure was preserved when organic modifiers that produced tethered polymer chains were used. Compared to a PMMA macrocomposite, the nanocomposites prepared by these methods display glass transition temperatures that are up ...
TL;DR: In this article, the growth of multilayers made from a combination of a weak polyacid and a strongly dissociated polycation is studied as a function of salt concentration and molecular weight.
Abstract: The growth of multilayers made from a combination of a weak polyacid and a strongly dissociated polycation is studied as a function of salt concentration and molecular weight. Film thickness reaches a maximum at around 0.3 M salt and then decreases quickly. Preformed multilayers are shown to decompose rapidly and, for high molecular weights, completely when exposed to aqueous solutions of NaCl of concentration >0.6 M. The apparent dissociation of multilayer polyelectrolyte complexes is due to competition for polymer/polymer ion pairs by external salt ions. Similar experiments aimed at decomposing multilayers by protonating the weak acid, thus decreasing polymer/polymer interactions, lead to incomplete loss of polymer, probably due to additional hydrogen bonding from the protonated weak acid. A model based on ion exchange/swelling of multilayers is used to explain their stability and permeability as well as the dependence of film thickness on salt concentration and type.
TL;DR: In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear as discussed by the authors.
Abstract: In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s-1) using a parallel-plate shear apparatus. WAXD results were used to determine the type (α- and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented α-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented α-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65−70% at high shear rates (57 and 102 s-1) and low (20%) at low shear rates (10 s-1), which was attributed to the different amount of surface area of oriented α-crystal cylindrites generate...
TL;DR: In this article, the molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection, confirming the synthesis of well-defined copolymer brushes.
Abstract: Brush macromolecules having poly(n-butyl acrylate-block-styrene) and poly(styrene-block-n-butyl acrylate) side chains have been synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection. The block copolymer side chains were cleaved from the backbone and analyzed by GPC, confirming the synthesis of well-defined copolymer brushes. Visualization of individual molecules by atomic force microscopy (AFM) enabled analysis of the conformation and microstructure of the brush macromolecules on mica surface. The brushes with the pnBuA core were almost fully stretched, while the inverted structure with the pS core exhibited longitudinal contraction compared to the contour length of the main chain. In addition, the poly(n-butyl acrylate-block-styrene) brushes demonstrated a characteristic necklace morphology which...
TL;DR: In this paper, the interplay between phase separation in polyfluorene blends which show photoinduced charge transfer and photovoltaic performance in photodiodes has been investigated, and the structure of the blend films was investigated using fluorescence microscopy, fluorescence scanning near-field optical microscopy and atomic force microscopy.
Abstract: The interplay between phase separation in polyfluorene blends which show photoinduced charge transfer and photovoltaic performance in photodiodes has been investigated. Phase separation length scales have been varied from several microns to tens of nanometers by limiting the time allowed for solvent-enhanced self-organization through several different processing routes. Concurrent with the decrease in feature size, an increase in maximum photovoltaic efficiency of nearly 1 order of magnitude was observed in photodiodes incorporating the phase-separated blends as the active layer. The structure of the blend films was investigated using fluorescence microscopy, fluorescence scanning near-field optical microscopy, and atomic force microscopy. In some cases, a hierarchy of micron- and nanometer-scale phase separation was observed which may explain the unexpectedly high photoresponse in devices with up to micron-scale phase separation structure. This result along with in situ fluorescence microscopy studies of...
TL;DR: In this paper, the compatibility of the initiator and monomer with the clay surface was found to profoundly affect the clay dispersion, and the dimension stability of nanocomposites was studied and related to both nanoscale (layer separation) and mesoscale dispersion.
Abstract: Poly(methyl methacrylate) (PMMA) and polystyrene (PS)/clay nanocomposites were prepared via in-situ bulk polymerization. The compatibility of the initiator and monomer with the clay surface was found to profoundly affect the clay dispersion. By using a surfactant containing a polymerizable group to modify the clay surface, exfoliated PMMA and PS/clay nanocomposites were synthesized. The clay dispersion was quantified by both X-ray diffraction (XRD) and transmission electron microscopy (TEM). The dimension stability of nanocomposites was studied and related to both nanoscale (layer separation) and mesoscale (long-range distribution) clay dispersion.
TL;DR: In this article, arylpropargyl ether was used to obtain thermally stable polybenzoxazines by the thermal cure of the monomers, and the properties of the resulting polymers were compared with typical benzoxazine monomers without propargyl groups.
Abstract: Novel benzoxazine monomers containing arylpropargyl ether were prepared, and highly thermally stable polybenzoxazines were obtained by the thermal cure of the monomers. One monomer is a monofunctional benzoxazine, 4-propargyloxyphenyl- 3,4-dihydro-2H-1,3-benzoxazine (P-appe), and the other is a bifunctional benzoxazine, bis(4-propargyloxyphenyl-3,4-dihydro-2H-1,3-benzoxazinyl)isopropane (B-appe). The chemical structures of these novel monomers were confirmed by IR and 1H NMR. The cure behavior of the monomers, P-appe and B-appe, and the properties of the resulting polymers were studied in comparison with 4-phenyl-3,4-dihydro-2H-1,3-benzoxazine (P-a) and bis(4-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl)isopropane (B-a) as typical benzoxazine monomers without propargyl groups. DSC cure of both P-appe and B-appe showed a single exotherm corresponding to the ring-opening polymerization of oxazine ring and cross-linking of arylpropargyl ether group at almost the same temperature range as P-a and B-a. The Tg values...
TL;DR: In this paper, a series of poly(ethylene oxide) (PEO)/organoclay nanocomposites have been prepared via a solvent casting method, and the d spacings of both the pure PEO and intercalated organoclay were examined via X-ray diffraction analysis.
Abstract: A series of poly(ethylene oxide) (PEO)/organoclay nanocomposites have been prepared via a solvent casting method. Using three different organoclays modified with the alkylammonium salts, the effect of surfactants on organoclay surfaces in polymer/organoclay nanocomposites was investigated by focusing on two major aspects: internal structure analysis and rheological measurement of the nanocomposites. The d spacings of both the pure PEO and intercalated organoclay were examined via X-ray diffraction analysis, and the microstructure of these nanocomposites was examined by transmission electron microscopy. Rheological properties of these nanocomposites exhibited different behavior with different modifier concentrations and surfactant sizes (chain lengths). To analyze the non-Newtonian flow behavior, we fitted shear viscosity data via the Carreau model, showing that steady shear viscosity and power-law behavior increase with organoclay content. Hysteresis phenomenon was also enhanced with organoclay content, ...
TL;DR: In this paper, the authors used polyisobutylene polycarbonate (PIB) as the comb material for biomedical polyurethane urea segmented block copolymers.
Abstract: Poly(urethane urea) segmented block copolymers [PUU] are used in a variety of biomedical applications,1 most prominently as blood sacs in ventricular assist devices and total artificial hearts. However, one of the principal drawbacks of these elastomers (particularly when used in completely implantable devices) is their relatively high permeability to air and water vapor. Conventional biomedical PUUs are comprised of ca. 80 wt % poly(tetramethylene oxide) [PTMO] soft segments, and it is the penetrant diffusion through this continuous polyether soft phase that gives rise to the high permeability. Several chemical approaches have been taken in an attempt to reduce PUU permeability, while maintaining the desirable biocompatibility and mechanical properties. One such approach has been to replace the PTMO with aliphatic polycarbonate soft segments.2 Alternatively, we recently described the synthesis and characterization of a series of polymers that have backbones chemically identical to the multiblock PUU, but which also possess polymeric combs of a material with superior barrier properties.3 Polyisobutylene (PIB) was chosen as the comb material due to the combination of its good barrier properties and its controllable (living cationic) polymerization. PIB incorporation varied between 2 and 30 wt %, with comb lengths ranging from ∼3000 to 29 000 g/mol. Water vapor and oxygen permeability were reduced by about a factor of 2 at the highest PIB contents,4 in line with what is expected from models assuming dispersed, spherical PIB microphases.5 The lack of precise control of the morphology of the PIB domains of these comb polymers limits further significant improvement in the permeability. In the past 5 years there has been intense interest in dispersing organically modified layered silicates (OMS) in polymers, particularly as mechanical reinforcement at low silicate loading levels.6-8 As a result of their high aspect ratio (often approaching 1000), the introduction of intercalated or exfoliated OMS inorganic layers in the polymer matrix can dramatically enhance the barrier properties at very low concentrations.9-11 In a natural extension of our earlier work on polyether-based PUUs, we have applied this nanocomposite approach to biomedical poly(urethane urea)s. In this note we describe our initial experiments on polymer/inorganic hybrids based on a generic PUU and an alkylammoniummodified montmorillonite, a biocompatible layered filler frequently used in cosmetics, food supplements, and stomach acid-reducing medicines.
TL;DR: In this paper, the mechanism and kinetics of RAFT-mediated graft polymerization of styrene initiated on a silica particle were studied, where RAFT is the reversible addition−fragmentation chain transfer (RAFT) process.
Abstract: The mechanism and kinetics of RAFT-mediated graft polymerization of styrene initiated on a silica particle were studied, where RAFT is the reversible addition−fragmentation chain transfer (RAFT) process. To prepare probe graft polymers with a dithiobenzoyl (X) group at the chain end, oligomeric polystyrene (PS) was grafted on a silica particle by the surface-initiated atom transfer radical polymerization (ATRP) technique, and then the terminal halogen atom of the graft polymer was converted to the X group by the reaction with 1-phenylethyl dithiobenzoate in the presence of CuBr/dHbipy complex in toluene. The graft polymerization of styrene on the PS−X-grafted silica particle was carried out in the presence of a free RAFT agent. After the polymerization, the graft chain was cleaved from the silica particle by treatment with HF and characterized. The GPC analysis revealed that the graft polymers, while their Mn increased linearly with increasing monomer conversion, gave a GPC curve with a prominent shoulder...
TL;DR: In this article, the polymerization rate of styrene and methyl methacrylate in the presence of model polymer−dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied.
Abstract: The bulk polymerizations of styrene and methyl methacrylate in the presence of model polymer−dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied. The polymerization rate, and hence the concentration of polymer radical P•, was proportional to [BPO]1/2. The pseudo-first-order activation rate constants kact were determined by the GPC peak-resolution and the polydispersity-analysis methods. The results showed that kact is directly proportional to [P•], indicating that reversible addition−fragmentation chain transfer (RAFT) is the only important mechanism of activation. The magnitude of the exchange rate constant kex (= kact/[P•]) was strongly dependent on both the structures of the dithiocarbonate group and the polymer. The kex values for the three RAFT systems examined in this work were all very large, which explains why these systems can provide low-polydispersity polymers from an early stage of polymerization. The activation energy of kex for...
TL;DR: In this paper, the authors examined the crystallization behavior of polyethylene-b-poly(vinylcyclohexane) diblock copolymers, E/VCH, using a combination of transmission electron microscopy (TEM), dilatometry, and time-resolved small-angle X-ray scattering (SAXS).
Abstract: We examine the crystallization behavior of polyethylene-b-poly(vinylcyclohexane) diblock copolymers, E/VCH, using a combination of transmission electron microscopy (TEM), dilatometry, and time-resolved small-angle X-ray scattering (SAXS). The glassy VCH matrix effectively restricts E crystallization to within the spheres, cylinders, gyroid channels, or lamellae formed by microphase separation in the melt. The VCH matrix can contract in response to crystallization of the E microdomains, so crystallization proceeds without cavitation. The crystallization kinetics strongly reflect the connectivity of the E microdomains: homogeneous nucleation and first-order crystallization kinetics for spheres or cylinders of E; conventional sigmoidal kinetics for the highly interconnected gyroid structure. Lamellar materials show an interesting two-step crystallization behavior: at higher temperature, heterogeneous nucleation permits the crystallization of lamellae interconnected through grain boundaries or defects, and ...
TL;DR: In this paper, a simulation and data fitting procedure for the reversible addition fragmentation chain transfer (RAFT) process using the PREDICI simulation program has been extended to cumyl phenyldithioacetate mediated styrene and methyl methacrylate (MMA) bulk homopolymerizations.
Abstract: A previously published simulation and data fitting procedure for the reversible addition fragmentation chain transfer (RAFT) process using the PREDICI simulation program has been extended to cumyl phenyldithioacetate mediated styrene and methyl methacrylate (MMA) bulk homopolymerizations. The experimentally obtained molecular weight distributions (MWDs) for the styrene system are narrow and unimodal and shift linearly with monomer conversion to higher molecular weights. The MMA system displays a hybrid of conventional chain transfer and living behavior, leading to bimodal MWDs. The styrene system has been subjected to a combined experimental and modeling study at 60 °C, yielding a rate coefficient for the addition reaction of free macroradicals to polymeric RAFT agent, kβ, of approximately 5.6 × 105 L mol-1 s-1 and a decomposition rate coefficient for macroradical RAFT species, k-β, of about 2.7 × 10-1 s-1. The transfer rate coefficient to cumyl phenyldithioacetate is found to be close to 2.2 × 105 L mol-...
TL;DR: In this article, the PPV fibers interspersed within the photo-cross-linked liquid crystal polymer matrices were synthesized within the pores of ananoporous membrane.
Abstract: Al-though significant fluorescence enhancement wasobserved, the final products were composite materialsconsisting of the PPV fibers interspersed within thephoto-cross-linked liquid crystal polymer matrices. Moregenerally, conjugated polymer nanofibers and nanotubeshave also been synthesized within the pores of ananoporous membrane
TL;DR: In this article, an organic/inorganic hybrid copolymers incorporating a norbornylene-substituted polyhedral oligomeric silsesquioxane (POSS) macromonomer have been prepared using a metallocene/methylaluminoxane (MAO) cocatalyst system.
Abstract: Novel organic/inorganic hybrid copolymers have been prepared using single site catalysis. Ethylene copolymers incorporating a norbornylene-substituted polyhedral oligomeric silsesquioxane (POSS) macromonomer have been prepared using a metallocene/methylaluminoxane (MAO) cocatalyst system. Isotactic polypropylene-containing POSS nanoparticles were also synthesized for the first time using a similar approach utilizing a C2 symmetric ansa-metallocene. A wide range of POSS concentrations were obtained in these polyolefin POSS copolymers under mild conditions, up to 56 wt % for PE-POSS copolymers and 73 wt % for PP-POSS copolymers. Initial findings point to improved thermooxidative stability for these nanocomposite polyolefins containing the “molecular silica” side groups relative to their homopolymer analogues. Thermogravimetric analysis of the PE-POSS copolymers under air shows a 90 °C improvement, relative to a polyethylene control sample of similar molecular weight, in the onset of decomposition temperatur...