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Showing papers in "Macromolecules in 2000"


Journal ArticleDOI
TL;DR: A detailed study of the role that solution pH plays in the layer-by-layer processing of the weak polyelectrolytes poly(acrylic acid) and poly(allylamine hydrochoride) was carried out as discussed by the authors.
Abstract: A detailed study of the role that solution pH plays in the layer-by-layer processing of the weak polyelectrolytes poly(acrylic acid) and poly(allylamine hydrochoride) was carried out. It was found that dramatically different polymer adsorption behavior is observed as one systematically increases (or decreases) the charge density of a weak polyelectrolyte including transitions from very thick adsorbed layers (ca. 80 A) to very thin adsorbed layers (ca. 4 A) over a very narrow pH range. By controlling pH, it is possible to vary the thickness of an adsorbed polycation or polyanion layer from 5 to 80 A. In addition, control over the bulk and surface composition of the resultant multilayer thin films is readily achieved via simple pH adjustments. These studies have provided new insights into the polyelectrolyte sequential adsorption process and have already opened up some interesting technological applications.

1,545 citations


Journal ArticleDOI
Heinz Schmid1, Bruno Michel1
TL;DR: In this article, the authors report the formulation of siloxane polymers for high-resolution, high-accuracy stamps for soft lithography, using models of rubber-like elasticity as guidelines.
Abstract: We report the formulation of siloxane polymers for high-resolution, high-accuracy stamps for soft lithography. With this technique, a molecular, polymeric, or liquid ink is applied to the surface of a stamp and then transferred by conformal contact to a substrate. Stamps for this technique are usually made of a commercial siloxane elastomer with appropriate mechanical properties to achieve conformal contact but are incapable of printing accurate, submicrometer patterns. To formulate better stamp polymers, we used models of rubber-like elasticity as guidelines. Poly(dimethylsiloxane) networks were prepared from vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers, with varying mass between cross-links and junction functionality. The polymer formulations were characterized by strain at break as well as compression modulus and surface hardness measurements. This resulted in the identification of bimodal polymer networks having mechanical properties that allow the replication of high...

716 citations


Journal ArticleDOI
TL;DR: The structure and morphology of polyvinyl alcohol (PVA) gels prepared by repeated cycles of 8 h freezing at −20 °C and 4 h thawing at +25 °C were examined in this paper.
Abstract: The structure and morphology of poly(vinyl alcohol) (PVA) gels prepared by repeated cycles of 8 h freezing at −20 °C and 4 h thawing at +25 °C were examined. Long-term morphological changes of such gels were determined upon swelling in water at 37 °C for 6 months. The preparation conditions were examined by varying such parameters as the number of freezing and thawing cycles, the concentration of aqueous solution, and the PVA molecular weight. The overall structure and stability were examined in terms of water content, fractional PVA dissolution, degree of crystallinity, and crystal size distribution. An increase in the number of freezing and thawing cycles served to reinforce existing crystals within the structure. Increased initial concentrations of aqueous PVA solutions resulted in hydrogels that contained initially higher crystallinity and added stability upon swelling. An increase in the PVA molecular weight resulted in crystals of higher lamellar thickness and a broadening of the crystal size distri...

674 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. And the results show that as the number and number of ortho substituents increase, the polymer molecular weight, turnover frequency and extent of branching in homopolyethylenes all increase.
Abstract: Detailed investigations of the polymerization of ethylene by (α-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were [{(2,6-RR‘C6H3)−NC(Nap)−C(Nap)N−2,6-RR‘C6H3)}NiBr2] (Nap = 1,8-naphthdiyl) (4a, R = CF3, R‘ = H; 4b, R = CF3, R‘ = CH3; 4c, R = C6F5, R‘ = H, 4c, R = C6F5, R‘ = H; 4d, R = C6F5, R‘ = CH3; 4e, R = CH3, R‘ = H, 4f, R = R‘ = CH3; 4g, R = R‘ = CH(CH3)2), [{(2,6-C6H3(i-Pr)2)−NC(CH2CH2CH2CH2)CN−(2,6-C6H3(i-Pr)2)}NiBr2] (5), and [{(2,6-C6H3(i-Pr)2)−NC(Et)C(Me)N−(2,6-C6H3(i-Pr)2)}NiBr2] (6). Active polymerization catalysts were formed in situ by combination of 4−6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polyme...

664 citations


Journal ArticleDOI
TL;DR: In this paper, the composites were conditioned at various moisture contents in order to evaluate the effect of amylopectin on the composite structure, and the resulting films were characterized using scanning electron microscopy, differential scanning calorimetry, water absorption experiments, and wide-angle X-ray scattering.
Abstract: Nanocomposite materials were obtained using glycerol plasticized starch as the matrix and a colloidal suspension of cellulose whiskers as the reinforcing phase. The cellulose whiskers, prepared from tunicin, consisted of slender parallelepiped rods with a high aspect ratio. After mixing the raw materials and gelatinization of starch, the resulting suspension was cast and evaporated under vacuum. The composites were conditioned at various moisture contents in order to evaluate the effect of this parameter on the composite structure. The resulting films were characterized using scanning electron microscopy, differential scanning calorimetry, water absorption experiments, and wide-angle X-ray scattering. An accumulation of plasticizer in the cellulose/amylopectin interfacial zones was evidenced. The specific behavior of amylopectin chains located near the interface in the presence of cellulose probably led to a transcrystallization phenomenon of amylopectin on cellulose whiskers surface.

622 citations


Journal ArticleDOI
TL;DR: In this paper, the melt-state linear viscoelastic properties for a series of intercalated nanocomposites are examined and the linear dynamic oscillatory moduli and the stress relaxation moduli are in quantitative agreement and suggest that at short times the relaxation of the nanocom composites is essentially unaffected by the presence of layered-silicate.
Abstract: The melt-state linear viscoelastic properties for a series of intercalated nanocomposites are examined. The nanocomposites are based on a short disordered polystyrene−polyisoprene diblock copolymer and varying amounts of dimethyldioctadecylammonium modified montmorillonite. The linear dynamic oscillatory moduli and the stress relaxation moduli are in quantitative agreement and suggest that at short times the relaxation of the nanocomposites is essentially unaffected by the presence of the layered-silicate. However, at long times (or equivalently low frequency), the hybrids exhibit dramatically altered viscoelastic behavior. Hybrids with silicate loadings in excess of 6.7 wt % exhibit pseudo-solidlike behavior, similar to that observed in previous studies of exfoliated end-tethered nanocomposites. On the basis of simple phenomenological arguments, the long time behavior is attributed to the presence of anisotropic stacks of silicate sheets randomly oriented and forming a percolated network structure that i...

566 citations


Journal ArticleDOI
TL;DR: In this paper, a self-concentration φ is estimated by assigning a length scale (or volume) to a particular dynamic mode, and then the relevant self-consistency φs can be estimated.
Abstract: In a miscible polymer blend the local environment of a monomer of type A will, on average, be rich in A compared to the bulk composition, φ, and similarly for B; this is a direct consequence of chain connectivity. As a result, the local dynamics of the two chains may exhibit different dependences on temperature and overall composition. By assigning a length scale (or volume) to particular dynamic mode, the relevant “self-concentration” φs can be estimated. For example, we associate the Kuhn length of the chain, lK, with the monomeric friction factor, ζ, and thus the composition and temperature dependences of ζ should be influenced by φs calculated for a volume V ∼ lK3. An effective local composition, φeff, can then be calculated from φs and φ. As lower Tg polymers are generally more flexible, the associated φ s is larger, and the local dynamics in the mixture may be quite similar to the pure material. The higher Tg component, on the other hand, may have a smaller φs, and thus its dynamics in the mixture w...

522 citations


Journal ArticleDOI
TL;DR: In this article, the synthesis and characterization of temperature and pH responsive hydrogel particles (microgels) with core−shell morphologies was reported, and a multistep volume phase transition appeared when the AAc component becomes hi...
Abstract: We report the synthesis and characterization of temperature and pH responsive hydrogel particles (microgels) with core−shell morphologies. Core particles composed of cross-linked poly(N-isopropylacrylamide) (p-NIPAm) or poly(NIPAm-co-acrylic acid) (p-NIPAm-AAc) were synthesized via precipitation polymerization and then used as nuclei for subsequent polymerization of p-NIPAm-AAc and p-NIPAm, respectively. The presence of a core−shell morphology was confirmed by transmission electron microscopy (TEM). Thermally initiated volume phase transitions were interrogated via temperature-programmed photon correlation spectroscopy (TP-PCS) as a function of solution pH. The p-NIPAm-AAc core hydrogel displays both a strong temperature and pH dependence on swelling. However, both p-NIPAm-AAc (core)/p-NIPAm (shell) and p-NIPAm (core)/p-NIPAm-AAc (shell) particles display a more complex pH dependence than the homogeneous particles. Specifically, a multistep volume phase transition appears when the AAc component becomes hi...

491 citations


Journal ArticleDOI
TL;DR: In this paper, a sequential UV polymerization method was used to construct an interpenetrating polymeric network (IPN) composed of the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) and the pH sensitive poly(methacrylic acid) (PMAA), which exhibited a combined pH and temperature-sensitivity at a temperature range of 31−32 °C and a pH value of approximately 5.5.
Abstract: Hydrogels of an interpenetrating polymeric network (IPN) composed of the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) and the pH-sensitive poly(methacrylic acid) (PMAA) were prepared by a sequential UV polymerization method. The IPN hydrogels were characterized for their temperature- and pH-responsive behavior by equilibrium swelling studies, oscillatory swelling studies, and differential scanning calorimetry. The permeability of these IPNs has been investigated under various pH and temperature conditions. The results showed that these hydrogels exhibited a combined pH- and temperature-sensitivity at a temperature range of 31−32 °C and a pH value of approximately 5.5. Permeation study results indicate a significant size exclusion behavior while model drugs with different sizes permeate through the IPN membranes. The permeability of the IPN membrane can be significantly affected by varying the pH and temperature conditions.

478 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of weight-average molecular weight (MW) and short and long chain branching on the linear viscoelastic behavior of polyethylene (and ethylene−α-olefin copolymers) are described.
Abstract: The effects of weight-average molecular weight (MW) and short and long chain branching on the linear viscoelastic behavior of polyethylene (and ethylene−α-olefin copolymers) are described. Short chain branching had no effect up to a comonomer (butene) content of 21.2 wt %. The zero shear viscosity of the linear polyethylenes scaled in the expected manner with MW. Using a high molecular weight, narrow molecular weight distribution (MWD), linear polyethylene, an estimate of the plateau modulus and molecular weight between entanglements (Me) was obtained. A solution property based technique for quantifying levels of long chain branching well below 1 LCB/104C in polyethylene is presented. Also, the applicability of 13C NMR for measuring such LCB levels is demonstrated. For metallocene polyethylene, long chain branching (LCB) increased the zero shear viscosity as compared to that of a linear material of the same molecular weight. LCB also broadened the relaxation spectrum by adding a long time relaxation mode ...

471 citations


Journal ArticleDOI
TL;DR: In this paper, small-angle X-ray scattering (SAXS) was used to follow orientation-induced crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear under isothermal conditions.
Abstract: In-situ synchrotron small-angle X-ray scattering (SAXS) was used to follow orientation-induced crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear under isothermal conditions. The melt was subjected to a shear strain of 1428% at three different shear rates (10, 57, and 102 s-1) using a modified Linkam shear stage. The SAXS patterns showed strong meridional reflections due to the rapid development of oriented polymer crystallites within the melt. On the basis of the SAXS data, a schematic representation of nucleation and growth in orientation-induced crystallization of i-PP is proposed. During flow, orientation causes alignment of chain segments of polymer molecules and results in the formation of primary nuclei in the flow direction. These nuclei facilitate the growth of oriented crystal lamellae that align perpendicular to the flow direction. The half-time of crystallization was calculated from the time evolution profiles of the total scattered intensity. Th...


Journal ArticleDOI
TL;DR: In this paper, it was shown that Sn(II)2−ethylhexanoate (SnOct2) was reacted with 2 equiv of benzyl alcohol at 20 °C, and a liberation of octanoic acid in a rapid equilibration was found.
Abstract: Sn(II)2−ethylhexanoate (SnOct2) was reacted with 2 equiv of benzyl alcohol at 20 °C, and a liberation of octanoic acid in a rapid equilibration was found. When the temperature was raised to 180 °C in steps of 40 °C, esterification of benzyl alcohol and octanoic acid was observed up to a conversion of 90%. This esterification was catalyzed by Sn(II) and not by the protons of the free octanoic acid. The esterification liberated Sn(OH)2, which finally precipitated in the form of SnO. This precipitate proved to be a good initiator for the polymerization of lactide above 120 °C. Analogous results were obtained with 1-decanol, triethylene glycol monomethyl ether, and neopentane diol. When SnOct2 was reacted with methyl lactate at 20 °C, a chelate complex of one Sn with two lactate ligands was formed, liberating almost all octanoic acid. At higher temperatures, esterification of octanoic acid with methyl lactate and transesterification of the methyl group (yielding methyl octoate) were observed. The latter ester...

Journal ArticleDOI
TL;DR: In this paper, it is shown that a small number of dead chains are produced by radical-radicaltermination of a trithio-carbonate moiety, and that the active functionality should be located in the center of the chain.
Abstract: (calcd) is basedon the assumption that all chains contain one trithio-carbonate moiety, it is important to realize that a smallnumber of dead chains are produced by radical-radicaltermination.Polymers prepared with symmetrical trithiocarbon-ates should have the active functionality located in thecenter, e.g.,

Journal ArticleDOI
TL;DR: In this article, it was shown that L,L-dilactide/sn(Oct)2 does not differ mechanistically from the CL/Sn(Oct)-2 system.
Abstract: Following our previous papers on the mechanism of cyclic esters polymerization induced by tin(II) octoate (Sn(Oct) 2 ) and particularly papers on ∈-caprolactone (CL), the present work shows that L,L-dilactide/Sn(Oct)2 does not differ mechanistically from the CL/Sn(Oct) 2 system. Sn atoms bonded through alkoxide groups to macromolecules were also observed by MALDI-TOF mass spectrometry. Formation of the actual initiator from Sn(Oct) 2 and a hydroxy group-containing compound (ROH) was envisaged by kinetic arguments. The appropriate experiments were carried out to show that some mechanisms put forward during the past few decades by several research groups were not sufficiently substantiated. Eventually, we conclude that L,L-dilactide/Sn(Oct)2 polymerization proceeds by simple monomer insertion into the ...-Sn-OR bond, reversibly formed in the reaction ...-SnOct + ROH ⇄ ...-Sn-OR + OctH, where ROH is either the low molar mass co-initiator (an alcohol, hydroxy acid, or H 2 O) or a macromolecule fitted with a hydroxy end group. These interconversions take place throughout the whole polymerization process. Sn(Oct) 2 itself does not play an active role in the polymerization.

Journal ArticleDOI
TL;DR: Detailed image analysis of nonwoven textile fabrics produced from elastin-mimetic fibers revealed that the distribution of single fiber orientation was isotropic with an associated unimodal distribution of protein fiber diameter.
Abstract: Elastin-mimetic peptide polymers have been synthesized, and the morphological properties of fabricated small diameter fibers and nonwoven fabrics have been characterized. An 81 kDa recombinant protein based upon the repeating elastomeric peptide sequence of elastin (Val-Pro-Gly-Val-Gly)4(Val-Pro-Gly-Lys-Gly) was obtained through bacterial expression of an oligomerized gene coding for tandem repeats of the monomer. The protein was processed into fibers by an electrospinning technique and morphology defined by SEM and TEM. The choice of processing parameters influenced both fiber diameter and morphology with diameters varying between 200 and 3000 nm and three morphological patterns noted: beaded fibers, thin filaments, and broad ribbonlike structures. Detailed image analysis of nonwoven textile fabrics produced from elastin-mimetic fibers revealed that the distribution of single fiber orientation was isotropic with an associated unimodal distribution of protein fiber diameter. In a dry state, the ultimate ...

Journal ArticleDOI
TL;DR: In this paper, the optimization of homopolymers from n-butyl acrylate, styrene, and methyl methacrylate using copper-based atom transfer radical polymerization (ATRP) with tris(2-dimethylaminoethyl)amine (Me6-TREN) as a ligand is reported.
Abstract: The optimization of the synthesis of homopolymers from n-butyl acrylate (BA), styrene (Sty), and methyl methacrylate (MMA), using copper-based atom transfer radical polymerization (ATRP) with tris(2-dimethylaminoethyl)amine (Me6-TREN) as a ligand is reported. Catalyst concentration (copper bromide complexed with 1 equiv of Me6-TREN) as low as 1% relative to initiator (50 ppm in the reaction mixture) was sufficient to successfully prepare well-defined poly(n-butyl acrylate) in bulk. With 50% catalyst to initiator, styrene was polymerized in bulk. The deactivator species had poor solubility in nonpolar solvents. Finally, the polymerization of methyl methacrylate with CuBr/Me6-TREN required the use of a high concentration of catalyst and the addition of Cu(II)Cl2 at the beginning of the reaction; the resulting polymers were of higher molecular weight than predicted by theory and had relatively broad molecular weight distributions.

Journal ArticleDOI
TL;DR: In this paper, the formation of gold colloids upon reduction of a gold salt precursor serves as a model reaction to study the influence of reaction conditions and dendrimer generation on the resulting nanostructures.
Abstract: Charged poly(amidoamine) (PAMAM) dendrimers are used to create organic−inorganic hybrid colloids in aqueous solution. The formation of gold colloids upon reduction of a gold salt precursor serves as a model reaction to study the influence of reaction conditions and dendrimer generation on the resulting nanostructures. Characterization by transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) show that the gold particles are formed inside the dendrimer and located offset from the center. Although lower generation dendrimers aggregate when stabilizing the metal particles formed, dendrimers of generation 6−9 can template one gold colloid per dendrimer molecule, the size of which is well-controlled by the number of gold atoms added per dendrimer. For generation 10, multiple smaller gold particles per dendrimer are observed. The effectiveness of PAMAM dendrimers as templates in the host−guest nanoscale synthesis is confirmed for different chemical...

Journal ArticleDOI
TL;DR: In this paper, the tin-containing macromolecules were identified based on the agreement between the observed m/z and the calculated molar mass values but also on the particular isotopic distribution provided by the tin atom.
Abstract: Polymerization of e-caprolactone (CL) initiated with tin(II) octoate (tin(II) 2-ethylhexanoate, (Sn(Oct)2)) in the presence of butyl alcohol (BuOH) or water and conducted in tetrahydrofuran (THF) as a solvent at 80 °C was studied using MALDI−TOF mass spectrometry. Formation of the following populations of macromolecules was revealed: Bu[O(O)C(CH2)5]nOSnOct, Bu[O(O)C(CH2)5]nOct, Bu[O(O)C(CH2)5]nOH, H[O(O)C(CH2)5]nOct, H[O(O)C(CH2)5]nOH, macrocyclics [O(O)C(CH2)5]n, and macrocyclics with incorporated tin(II) alkoxide moieties [O(O)C(CH2)5]nOSn. Thus, the most rewarding has been a direct observation of species with a tin atom covalently bonded with the polyester chain, at least for two populations of macromolecules (i.e., Bu[O(O)C(CH2)5]nOSnOct and [O(O)C(CH2)5]nOSn cyclics). Identification of the tin-containing macromolecules was based not only on the agreement between the observed m/z and the calculated molar mass values but also on the particular isotopic distribution provided by the tin atom. This resul...

Journal ArticleDOI
TL;DR: In this paper, the use of atom transfer radical polymerization (ATRP) to amplify initiators patterned on films of gold into polymer brushes of poly(methyl methacrylate) was reported.
Abstract: We report the use of atom transfer radical polymerization (ATRP) to amplify initiators patterned on films of gold into polymer brushes of poly(methyl methacrylate) (PMMA), poly(hydroxyethyl methacrylate) (PHEMMA), poly(tert-butyl methacrylate) (PTBA), poly(isobornyl methacrylate) (PIBMA), and poly((dimethylamino)ethylmethyl acrylate) (PDMAEMA). Pattern transfer into gold substrates underlying polymer brushes was achieved by using the patterned brushes as barriers to wet chemical etchants of gold. The surface-confined initiators for ATRP were prepared by the self-assembly of (BrC(CH3)2COO(CH2) 10S)2 (I) on films of gold. These monolayers were assembled from solutions of hexadecane at 60 °C so as to prevent their thermal desorption during ATRP (also performed at 60 °C). By measuring the resistance offered by these brushes to etching of underlying films of gold by aqueous solutions of KI/I2, KCN/K3Fe(CN)6, and 50 vol % aqua regia (70 vol % HNO3 and 30 vol % HCl), we conclude that both the thickness and chemi...

Journal ArticleDOI
TL;DR: In this article, the effect of the thermodynamic selectivity of a solvent on the self-assembly of a styrene−isoprene (SI) diblock copolymer with block molecular weights of 1.1 × 104 and 2.1× 104 g/mol is investigated.
Abstract: The effect of the thermodynamic selectivity of a solvent on the self-assembly of a styrene−isoprene (SI) diblock copolymer with block molecular weights of 1.1 × 104 and 2.1 × 104 g/mol is investigated. We explore the phase behavior from the melt state to dilute solution in solvents that are of varying selectivities for the two blocks. Bis(2-ethylhexyl) phthalate (DOP) is a neutral good solvent for SI. Di-n-butyl phthalate (DBP) and diethyl phthalate (DEP) are good solvents for PS but marginal and poor, respectively, for PI. Tetradecane (C14) is utilized as a complementary solvent that is good for PI but poor for PS. Small-angle X-ray scattering (SAXS) and static birefringence are used to locate and identify order−order (OOT) and order−disorder transitions (ODT). Dynamic and static light scattering were used to characterize the dilute solution micellar behavior. The neat polymer forms the gyroid (G1) morphology at low temperature, an OOT to hexagonal-packed cylinders (C1) occurs at 185 °C, and the ODT is l...

Journal ArticleDOI
TL;DR: In this paper, the glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film.
Abstract: The glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitored as the temperature of the probe was ramped from ambient temperature to temperatures as high as 200 °C. The glass transition temperature was determined from a change in slope in the heat loss versus temperature plot. The Tg of polystyrene on silicon oxide decreased by as much as 25 °C below the bulk value for films 13 nm thick. The same trend in the glass transition temperature was observed for polystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The Tg of poly(methyl methacrylate) on silicon oxide increased by up to 7 °C above the bulk value for films 18 nm thick. For poly(methyl methacrylate) on silicon oxide treated with HMDS, the Tg decreased by 10 °C below the bulk value for films 21 n...

Journal ArticleDOI
TL;DR: In this paper, the facile atom transfer radical polymerization (ATRP) of a commercially available hydrophilic monomer, methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA), under remarkably mild conditions was reported.
Abstract: We report the facile atom transfer radical polymerization (ATRP) of a commercially available hydrophilic monomer, methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA), under remarkably mild conditions. Various bromide-based initiators, in conjunction with a copper-based catalyst, allow rapid homopolymerization of OEGMA in water at 20 °C. Good living character was achieved with two ligands, namely 2,2‘-bipyridine (bpy) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA). Kinetic studies confirm that, for bpy, 90% conversion is typically achieved within 20 min, and the polymerization is first order even up to very high conversions. Molecular weight increases linearly with conversion and is close to the target molecular weight in all cases. Polydispersities remain narrow (Mw/Mn = 1.15−1.30) throughout the polymerization. It is possible to lower the copper catalyst by a factor of 10 (i.e., [Cu(I)]/[initiator] = 0.10) without significant loss of control over the polymerization, which is good eviden...

Journal ArticleDOI
TL;DR: In this article, small-angle X-ray scattering and rheology were used to determine relations between chemical composition and concentrations of the alginate and the elasticity and structure of the gels.
Abstract: Ca−alginate gels were studied by small-angle X-ray scattering and rheology to determine relations between chemical composition and concentrations of the alginate and the elasticity and structure of the gels. The gels were prepared by in situ release of Ca2+ from either Ca−EGTA or CaCO3 with total Ca2+ concentration in the range 5−30 mM. Alginates with low (39%), intermediate (50%), and high (68%) fractions of α-l-GulA originating from the brown algae Ascophyllum nodosum, Laminaria hyperborea leaf, and Laminaria hyperborea stipe, respectively, were employed. Two to three different degrees of polymerization for each chemical composition were used in the experiments. The excess small-angle X-ray scattering for the alginates in solution yielded linear cross-sectional Guinier plots, and the cross-sectional radius of gyration, Rg,c, was determined to be 3.1−4.6 A. The SAXS profiles of the alginate gels depended on the alginate concentration, Ca2+ concentration, and the alginate composition. The SAXS data sugges...

Journal ArticleDOI
TL;DR: In this paper, the authors examined the partitioning coefficients of hydrophobic solutes in aqueous dispersions of Pluronic block copolymers (poly(ethylene oxide), block-poly(propylene oxide)-block-polypoly(poly(methylene oxide)blockpoly(pluronic oxide)-poly (ethylene dioxide)poly( methylene oxide)).
Abstract: Using pyrene and homologous alkyl derivatives of fluorescein as fluorescent probes, this work examines the partitioning coefficients of hydrophobic solutes in aqueous dispersions of Pluronic block copolymers (poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide)). An incremental approach is developed, allowing measurement of the free energy of transfer of a methylene group from aqueous media into the micelles. Effects of variation of length of the ethylene oxide (EO) and the propylene oxide (PO) blocks in Pluronic molecules on the partitioning characteristics of the solutes are established. A simple reciprocal relationship between partitioning coefficients of the solute and critical micellization concentration is demonstrated.

Journal ArticleDOI
TL;DR: In this paper, a series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 103−3 × 105.
Abstract: A series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 103−3 × 105. For molecular weights below 4 × 104, conditions were found so that polydispersity remained below 1.4 up to high conversion (72%). Molecular weight distributions of polymer obtained using GPC in THF and MALDI were in accord for the low molecular weight range (typically 105). Particular care is required in GPC sample preparation: it is necessary to ensure that trace amounts of water are initially present when drying a polymer sample prior to dissolution in THF, to avoid irreversible chain aggregation. The log/log plot of intrinsic viscosity against molecular weight for polyNIPAM was found to be linear for molecular weights <105, after which the hydrodynamic volume seems to be independent of molecular weight. The Mark−Houwink parameters obtained from the lower molecular weight data are K = 10-(4.24±0.42) dL g-1...

Journal ArticleDOI
TL;DR: In this paper, the atom transfer radical polymerization (ATRP) of tert-butyl acrylate was performed using an alkyl bromide as the initiator.
Abstract: Atom transfer radical polymerization (ATRP) of tert-butyl acrylate is reported. Controlled polymerizations were performed using a CuBr/N,N,N‘,N‘ ‘,N‘ ‘-pentamethyldiethylenetriamine catalyst system in conjunction with an alkyl bromide as the initiator. Low molecular weight polymers with narrow molecular weight distributions were obtained by the addition of a solvent to create a homogeneous catalytic system. The addition of the solvent was necessary to decrease the polymerization rate and afford low polydispersity materials. This differs from the ATRP of methyl or n-butyl acrylate using this catalytic system, which do not require the addition of a solvent to obtain well-defined polymers. Subsequent hydrolysis of the polymer in refluxing dioxane with addition of HCl afforded poly(acrylic acid). Characterization using 1H NMR and FT-IR confirmed complete hydrolysis of the ester group. Further use of poly(tert-butyl acrylate) as a macroinitiator for block copolymerizations followed by hydrolysis of the ester g...

Journal ArticleDOI
TL;DR: In this paper, a novel sequential ultraviolet (UV)-induced living graft polymerization method was designed and investigated to modify polymeric materials, which consists of two steps, in the first step...
Abstract: A novel sequential ultraviolet (UV)-induced living graft polymerization method has been designed and investigated to modify polymeric materials. This method consists of two steps. In the first step...

Journal ArticleDOI
TL;DR: In this article, the authors investigated the phenomenon of counterion condensation in a solution of highly charged rigid polyelectrolytes within the cell model and proposed a method based on the charge distribution function to identify both the fraction of condensed ions and the radial extension of the condensed layer.
Abstract: We investigate the phenomenon of counterion condensation in a solution of highly charged rigid polyelectrolytes within the cell model. A method is proposed whichbased on the charge distribution functionidentifies both the fraction of condensed ions and the radial extension of the condensed layer. Within salt-free Poisson−Boltzmann (PB) theory it reproduces the well-known fraction 1−1/ξ of condensed ions for a Manning parameter ξ > 1. Furthermore, it predicts a weak salt dependence of this fraction and a breakdown of the concept of counterion condensation in the high-salt limit. We complement our theoretical investigations with molecular dynamics simulations of a cell-like model, which constantly yield a stronger condensation than predicted by PB theory. While the agreement between theory and simulation is excellent in the monovalent, weakly charged case, it deteriorates with increasing electrostatic interaction strength and, in particular, increasing valence. For instance, at a high concentration of dival...

Journal ArticleDOI
TL;DR: In this article, the effects of the electronic structure of polymer/polymer interfaces on the electroluminescence efficiency and tunable multicolor emission of polymer heterojunction light-emitting diodes were explored by a series of 16 n-type conjugated polymers with varying electron affinities and ionization potentials in conjunction with poly(p-phenylenevinylene).
Abstract: Effects of the electronic structure of polymer/polymer interfaces on the electroluminescence efficiency and tunable multicolor emission of polymer heterojunction light-emitting diodes were explored by a series of 16 n-type conjugated polymers with varying electron affinities and ionization potentials in conjunction with poly(p-phenylenevinylene). Efficiency and luminance of diodes of the type indium−tin oxide/poly(p-phenylenevinylene)/n-type polymer/aluminum were maximized and were as high as 3% photons/electron and 820 cd/m2, respectively, when the energetics at the polymer/polymer interface favored electron transfer while disfavoring hole transfer. Energetic barrier to electron transfer at the polymer/polymer interface was more important to electroluminescence efficiency and diode luminance than injection barrier at the cathode/polymer interface. By a judicious choice of the relative layer thicknesses and the components of the bilayer heterojunctions, the rate of both electron and hole transfer across t...