Showing papers in "Macromolecules in 1993"
TL;DR: In this paper, free radical polymerization was used to obtain polystyrene and poly(styrene-co-butadiene) with narrow polydispersity (1.19-1.36) in the presence of 2,2,6, 6,6-tetramethyl-1-piperidinyloxy using benzoyl peroxide as initiator
Abstract: Polystyrene and poly(styrene-co-butadiene) with narrow polydispersity (1.19-1.36) could be obtained by free radical polymerization in solution, bulk or suspension in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy using benzoyl peroxide as initiator
1,863 citations
TL;DR: The amphiphilic nature of the macromers causes them to assume a micellar conformation, which enables them to undergo rapid photopolymerization, resulting in the formation of crosslinked gels.
Abstract: Macromers having a poly(ethylene glycol) central block, extended with oligomers of α-hydroxy acids such as oligo(dl-lactic acid) or oligo(glycolic acid) and terminated with acrylate groups, were synthesized and characterized with the goal of obtaining a bioerodible hydrogel that could be formed in direct contact with tissues or proteins by photopolymerization of aqueous solutions of the macromer. The amphiphilic nature of the macromers causes them to assume a micellar conformation, which enables them to undergo rapid photopolymerization, resulting in the formation of crosslinked gels
1,262 citations
TL;DR: In this paper, the authors investigated the mechanism of temperature-induced phase separation and the effect of comonomer content, hydrophilicity, and charge on the lower critical solution temperature.
Abstract: DSC was performed on aqueous solutions of poly(N-isopropylecrylamide-co-butyl methacrylate-co-X), with X being hydrophilic, hydrophobic, cationic, or anionic comonomers, to elucidate the mechanism of temperature-induced phase separation and the effect of comonomer content, hydrophilicity, and charge on the lower critical solution temperature (LCST). The endothermic heat of phase separation, which is related to the breaking of hydrogen bonds between water molecules surrounding hydrophobic moieties on the polymer, was a linear, decreasing function of the LCST. This suggests that the hydrophobic interactions between polymer side groups, which are the major driving force for phase separation, ere enhanced at elevated temperatures due to a decrease in the structuring of water around hydrophobic side groups
1,004 citations
TL;DR: The phase behavior of the triblock copolymers dissolved in water has been studied using SANS as discussed by the authors, and the structural properties have been studied as a function of polymer concentration and temperature.
Abstract: The phase behavior of the triblock copolymers dissolved in water has been studied using SANS. The structural properties have been studied as a function of polymer concentration and temperature. At low temperature (T ≤ 15°C) and low polymer concentrations the unimers are fully dissolved Gaussian chains with radius R g =17 A. Close to ambient temperature, the hydrophobic nature of PPO causes aggregation of the polymers into spherical micelles with core sizes of the order of 40-50 A, somewhat temperature dependent. The concentration of micelles increased roughly linearly with temperature, until either a saturation is reached,where all the polymers are part of a micelle, or the volume density of micelles is so high that they lock into a crystalline structure of hard spheres
669 citations
TL;DR: In this article, the authors studied the aggregation properties of hydrophobized pullulan containing 1.6 cholesterol groups per 100 glucose units (CHP-55-1.6).
Abstract: Solution properties in water of hydrophobized pullulan containing 1.6 cholesterol groups per 100 glucose units (CHP-55-1.6) were studied. SEC measurements show that CHP (1.0 mg/mL, 0.10 wt %) intermolecularly aggregates and provides relatively monodispersive particles upon ultrasonication. Spherical particles with relatively uniform size (the diameter, 25±5 nm) were observed in the negatively stained electron microscopy of the aqueous CHP solution. The hydrodynamic radius of the CHP self-aggregate was approximately 13 nm and the aggregation number was approximately 13.
553 citations
TL;DR: In this article, critical micelle concentrations (cmc's) were measured for a range of block copolyelectrolytes based on styrene (the insoluble block) and sodium acrylate.
Abstract: Critical micelle concentrations (cmc's) were measured for a range of block copolyelectrolytes based on styrene (the insoluble block) and sodium acrylate. The lengths of the styrene blocks ranged from 6 to 110, while those of the polyelectrolyte ranged from ca. 300 to ca. 1400. cmc results were interpolated for a constant polyelectrolyte block length of 1000 units. It was found that changing the insoluble block length from 6 to 110 lowered the cmc from 1.6×10 -5 to 5×10 -8 M. By contrast, changing the soluble blocks length from 300 to 1400 typically changed the cmc values by less than a factor of 2
552 citations
TL;DR: In this paper, the linear viscoelastic behavior of two-phase polymer blends in the melt has been analyzed by an emulsion model, with takes into account the visco-elasticity of the phases.
Abstract: The linear viscoelastic behavior of two-phase polymer blends in the melt has been analyzed by an emulsion model, with takes into account the viscoelasticity of the phases. In this paper, we have studied two types of blends: PS/PMMA and PDMS/POE-DO. For PS/PMMA blends, the model leads to values of relaxation times and secondary plateau modulus in accordance with experimental data. This supports the assumption that long-time relaxation mechanisms are due to geometrical relaxation of the droplets of the dispersed phase. For a quantitative comparison, further experiments have been carried out on PDMS/POE-DO blends for which the distribution of size of the dispersed POE-DO inclusions may be easily determined. The data confirm the validity of the model and show that dynamic shear measurements can be used as a method to determine the interfacial tension between two polymer melts. © 1993, American Chemical Society. All rights reserved.
513 citations
TL;DR: In this article, the authors introduced a new method of creating ultrathin films based on the electrostatic attraction between opposite charges and showed that alternating adsorption of anionic and cationic polyelectrolytes leads to the formation of multilayer assemblies.
Abstract: We have recently introduced a new method of creating ultrathin films based on the electrostatic attraction between opposite charges. Consecutively alternating adsorption of anionic and cationic polyelectrolytes leads to the formation of multilayer assemblies. Here, we extend our concept to the procedure of ultrathin (100-500-A) film growth with alternation of DNA and poly(allylamine) molecular layers. Multilayer buildup was monitored by small-angle X-ray reflectivity; the step of growth for layer pairs of DNA/poly-(allylamine) was 40 A. The relationship between this growth mechanism and the process of natural complex formation between DNA and polyamines (spermine, spermidine, etc.) may be of interest
497 citations
TL;DR: In this article, it has been demonstrated that repeated dipping of the substrate into solutions of polycations and polyanions in an alternating sequence leads to the deposition of continuous molecular layers that form a polymer film with a well-defined supramolecular structure.
Abstract: Ultrathin polymer films were physisorbed to surface-modified Si wafers by electrostatic deposition of polyelectrolytes from aqueous solutions. For the first it has been demonstrated that the preparation procedure, which involves repeated dipping of the substrate into solutions of polycations and polyanions in an alternating sequence, leads to the deposition of continuous molecular layers that form a polymer film with a well-defined supramolecular structure
452 citations
TL;DR: α-Cyclodextrin was found to form inclusion complexes with poly(ethylene glycol) (PEG) of various molecular weights to give stoichiometric compounds in a crystalline state in high yields.
Abstract: α-Cyclodextrin (α-CD) was found to form inclusion complexes with poly(ethylene glycol) (PEG) of various molecular weights to give stoichiometric compounds in a crystalline state in high yields. α-CD does not form complexes with the low molecular weight ananlogs, ethylene glycol, diethylene glucoln and triethylene glycol. The rate of the complex formation depends on the molecular weight of PEG, PEG of molecular weight 100 forms complexes most rapidly
405 citations
TL;DR: In this article, the Wittig reaction was used to obtain the polymer [O(CH 2 ) 8 Ph(OMe) 2 CH=CHPhCH=CH Ph(O 2 ) 2 ] n, which was found to emit blue light with a maximum peak in the electroluminescent spectrum.
Abstract: The polymer [O(CH 2 ) 8 Ph(OMe) 2 CH=CHPhCH=CHPh(OMe) 2 ] n was prepared by a Wittig reaction. It forms homogeneous solutions in THF and CHCl 3 from which transparent films can be cast. The films have good mechanical properties and are slightly yellow in thick section. This polymer was found to emit blue light with a maximum peak in the electroluminescent spectrum
TL;DR: Stereocomplexation of PDLA and PLLA from the melt state was studied using DSC and polarizing microscopy in this paper, showing that homocrystallization was most prevalent upon annealing the polymer mixtures when the molecular weight of both poly(D-lactic acid) (PDLA) was below 6×10 3 and the PDLA content (X D ) was between 0.4 and 0.6.
Abstract: Stereocomplexation (racemic crystallization) of poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) from the melt state was studied using DSC and polarizing microscopy. Complexation predominantly occurred upon annealing the polymer mixtures when the molecular weight of both PDLA and PLLA was below 6×10 3 and the PDLA content (X D ) was between 0.4 and 0.6. In contrast, homocrystallization of PDLA or PLLA prevailed when X D was around 0 or 1 or the molecular weight of PDLA and PLLA was higher than 1×10 6
TL;DR: In this article, three methods of synthesis in aqueous media have been investigated: (i) a ''micellar'' process in which the presence of a surfactant ensures the solubilization of the hydrophobic monomer; (ii) a «homogeneous» process wherein a miscible cosolvent is used; (iii) a "heterogeneous" process, without additive to solubiliize the insoluble monomer.
Abstract: Hydrophobically modified water-soluble polymers have been prepared by radical copolymerization of acrylamide and ethylphenylacrylamide as the hydrophobic comonomer. Three methods of synthesis in aqueous media have been investigated: (i) a «micellar» process in which the presence of a surfactant ensures the solubilization of the hydrophobic monomer; (ii) a «homogeneous» process wherein a miscible cosolvent is used; (iii) a «heterogeneous» process, without additive to solubilize the insoluble monomer. The properties of the copolymers in dilute and semidilute aqueous solutions strongly depend on the conditions of the synthesis
TL;DR: In this paper, a nonionic poly(N-isopropylacrylamide) hydrogel, which exhibits a temperature-dependent volume phase transition behavior in aqueous solution, demonstrates an unusual NACl-induced phase transition phenomenon.
Abstract: A nonionic poly(N-isopropylacrylamide) hydrogel, which exhibits a temperature-dependent volume phase transition (lower critical solution temperature, LCST) behavior in aqueous solution, demonstrates an unusual NACl-induced phase transition phenomenon as well. The gel collapses sharply at critical NaCl concentration, which depends on the temperature. Other salts tested show no such behavior, and typical salting-out behaviors (the gradual gel collapse with increasing their concentration) were observed. It appear that chloride ions play a major role in this phase transition
TL;DR: Carbosilane dendrimers with 64 and 128 surface Si-Cl bonds were used as coupling reagents for monodisperse poly(butadienyl)lithium as discussed by the authors.
Abstract: Carbosilane dendrimers with 64 and 128 surface Si-Cl bonds were used as coupling reagents for monodisperse poly(butadienyl)lithium. Two series of regular star polybutadienes with 64 and 128 arms were prepared. The arm molecular weight was varied between 6400 and 72 000. The dilute-solution properties of the stars were determined in a good solvent (cyclohexane) and in a θ-solvent (dioxane) for polybutadiene. Measurements of R G . A 2 , D 0 , and [η] indicate that the isolated stars behave as hard spheres. The ratio of the hydrodynamic radius over the radius of gyration is slightly larger than (5/3) 1/2
TL;DR: In this paper, the rheological properties of star-shaped polyisoprenes having a wide range of arm numbers and arm molecular weights are reported, showing that the viscosity of 3-arm stars is approximately 20% lower.
Abstract: The rheological properties of star-shaped polyisoprenes having a wide range of arm numbers and arm molecular weights are reported. In contrast to linear polymers, stars have a broad relation spectrum and a viscosity that increases exponentially with arm molecular weight. A comparison of eight pairs of samples having 3 and 4 arms and identical arm molecular weights showed that the viscosity of 3-arm stars is approximately 20% lower. For higher degrees of functionality, 4 ≤ f ≤ 33, the effect of functionality saturates and the viscosity is determined by arm molecular weight only
TL;DR: In this paper, a molecular dynamics simulation of polymeric brushes in solvents of varying quality is presented, and the resulting equilibrium structures are compared with those predicted by self-consistent field (SCF) analysis.
Abstract: A molecular dynamics simulation of polymeric brushes in solvents of varying quality is presented. The resulting equilibrium structures are compared with those predicted by self-consistent field (SCF) analysis. The scaling of the brush height is found to agree with the SCF predictions at temperatures T>T θ , where T θ is the θ temperature of the solvent. The agreement at T
TL;DR: Squioxane copolymers have high solubilities, excellent high long-term shelf life and exhibit 10% weight loss temperatures of 420-510°C under nitrogen as mentioned in this paper. But they also function as highly processible preceramics.
Abstract: Silsesquioxane-siloxane copolymers are synthesized by polycondensation of a polyhedral silsesquioxane with α,ω-diamino- or α,ω-dichloro-silanes or -oligosiloxanes. The resulting polymers have high solubilities, excellent high long-term shelf life and exhibit 10% weight loss temperatures of 420-510°C under nitrogen. They also function as highly processible preceramics. Heating an uncross-linked sample of siloxane-silsesquioxane copolymer at 900°C under nitrogen results in its conversion into a glassy black ceramic material
TL;DR: In this paper, the effect of chemical structure on the segmental relaxation behavior of polymers was examined for a wide range of different polymers, including smooth, compact, symmetrical chain backbones and lees flexible backbones.
Abstract: The effect of chemical structure on the segmental relaxation behavior is examined for a wide range of polymers. Both the time and temperature dependence of the glass transition dispersion in the dielectric loss spectrum are shown to be correlated with the degree to which local structure engenders steric constraints on the relaxation from neighboring nonbonded segments. The polymers studied exhibit a range of segmental relaxation behavior. For polymers with smooth, compact, symmetrical chain backbones, segmental relaxation times have a near Arrhenius temperature dependence, representing the limiting-type behavior associated with relatively unconstrained ('Debye") relaxation. Polymers having lees flexible backbones andl or sterically-hindering pendant groups exhibit broad segmental dispersions and temperature dependencies near the extreme fragile edge; these characteristics reflect segmental relaxation that is strongly intermolecularly cooperative. The data for polymers parallel observations made on small-molecule glass-forming liquids, whose chemical structure similarly governs the strength of the intermolecular coupling, and thus the time and temperature dependence of segmental relaxation.
TL;DR: In this article, stress-strain curves of different thermoplastics in tension were used to evaluate a theoretical equation derived from a model in which the strain hardening effect is related to the Gaussian network theory of rubber elasticity.
Abstract: Published stress-strain curves of different thermoplastics in tension are used to evaluate a theoretical equation derived from a model in which the strain hardening effect is related to the Gaussian network theory of rubber elasticity
TL;DR: In this article, the volume fraction profiles of spherical and cylindrical polymers are analyzed, with special attention being paid to spherical polymers in athermal solvents.
Abstract: The polymer adsorption theory of Scheutjens and Fleer is used to describe polymer brushes at spherical and cylindrical surfaces that are immersed in a low molecular weight solvent. The volume fraction profiles of such brushes is analyzed, special attention being paid to spherical brushes in athermal solvents. These are shown to generally consist of two parts: a power law-like part and a part that is consistent with a parabolic potential energy profile of the polymer segments
TL;DR: In this article, a modified version of the chain-end free volume theory was derived to account for the large number of chain ends in these unusual structures, and the experimental variation in T with molecular weight was found to correlate well with theoretical predictions.
Abstract: The variation of glass transition temperature with molecular weight and chain-end composition for dendritic polyethers and polyesters was studied. A new and modified version of the chain-end free volume theory was derived to account for the large number of chain ends in these unusual structures. For both homopolymers and a variety of novel block copolymers, the experimental variation in T with molecular weight was found to correlate well with theoretical predictions, thus demonstrating the wide applicability of the chain-end free volume theory. The glass transition was found to be greatly affected by the nature of the chain ends and internal monomer units.
TL;DR: In this paper, small-angle X-ray scattering is used to determine the structure of nanoporous organic aerogels made by the base-catalyzed polymerization of resorcinol and formaldehyde.
Abstract: Small-angle X-ray scattering is used to determine the structure of nanoporous organic aerogels made by the base-catalyzed polymerization of resorcinol and formaldehyde. This study probes the effect of several variables on the morphology of the supercritically dried aerogels including catalyst concentration, monomer concentration, acid aging, and pyrolysis. In addition, the development of structure during polymerization and aging of the precursor aquagels is reported. The data are consistent with a smooth-surfaced morphology with no indication of fractal clusters or rough surfaces in the length-scale regime from 10 to 600 [angstrom]. These results, taken together, indicate that microphase separation is the dominant process underlying the morphology of these unique materials.
TL;DR: In this article, surface modification of poly(tetrafluoroethylene) (PTFE) films by radio frequency argon plasma treatment as well as by graft copolymerization of the plasma-pretreated films with acrylamide (AAm) has been carried out.
Abstract: Surface modification of poly(tetrafluoroethylene) (PTFE) films by radio frequency argon plasma treatment as well as by graft copolymerization of the plasma-pretreated films with acrylamide (AAm) has been carried out. XPS results show that mild plasma treatment is sufficient to cause substantial surface defluorination end oxidation. The oxygen functionalities incorporated greatly facilitate subsequent graft copolymerization in the presence of near-UV radiation. Strong plasma treatment, on the contrary, results in considerable cross-linking of polymers on the surface and thus has an adverse effect on the graft copolymerizetion
TL;DR: In this article, the phase behavior of a series of poly(ethylene oxide)-poly(propylene oxide), poly(methylene oxide), E m P n E m, triblock copolymers dissolved in water has been studied using SANS and dynamic light scattering.
Abstract: The phase behavior of a series of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), E m P n E m , triblock copolymers dissolved in water has been studied using SANS and dynamic light scattering. All copolymers show the characteristics of fully dissolved polymers at low temperatures, whereas aggregates are formed at elevated temperatures due to the hydrophilic-to-hydrophobic transition in PPO.
TL;DR: In this article, a theoretical analysis is presented to describe the stiffening of flexible linear polymer chains due to complexation with oligomeric surfactants, which might be functionalized protonic acids or possess other types of end groups that exhibit strong, specific interactions with the flexible polymer backbone.
Abstract: A theoretical analysis is presented to describe the stiffening of flexible, linear polymer chains due to complexation with oligomeric surfactants. These surfactants might be functionalized protonic acids or possess other types of end groups that exhibit strong, specific interactions with the flexible polymer backbone. The enhancement of the persistence length due to steric interactions between the bound surfactant tails is considered and the possibility of lyotropic behavior at finite polymer concentration is investigated. The results are discussed in the context of recent experiments by Cao and Smith, who reported liquid crystallinity in solutions of polyaniline complexed with camphorsulfonic acid
TL;DR: Monte Carlo calculations on a set of diamond-lattice model dendrimers are reported in this article, where equilibrium results through nine generations have been obtained, where the spacer in each generation is a seven-step walk on the lattice.
Abstract: Monte Carlo calculations on a set of diamond-lattice model dendrimers are reported. Equilibrium results through nine generations have been obtained, where the spacer in each generation is a seven-step walk on the lattice. We report data on radius of gyration, principal moments of inertia, chain extension, radial density profiles, and molecular scattering structure factors. We find that terminal groups are dispersed throughout the molecule, that spacers near the core are extended, and that the later generation dendrimers exhibit some hollowness
TL;DR: In this article, all-aromatic hyperbranched polysters with phenol and acetate and groups were prepared by the melt condensation of the A 2 B monomers 3,5-bis(trimethylsiloxy)benzoyl chloride (2) and 3,6-diacetoxybenzoic acid (3).
Abstract: All-aromatic hyperbranched polysters with phenol and acetate and groups were prepared by the melt condensation of the A 2 B monomers 3,5-bis(trimethylsiloxy)benzoyl chloride (2) and 3,6-diacetoxybenzoic acid (3). The trimethylsilyl groups of the polyesters from monomer 2 are hydrolyzed during workup, resulting in polymers with phenol terminal groups. Although the acetate groups of polymers prepared from 3 are quite stable and remain in the polymer, conditions were found where they could be hydrolyzed to give a phenolic polymer similar to that obtained from 2
TL;DR: In this article, the morphology of a series of ethylene-(ethylene-alt-propylene) diblock copolymere (E/EP), in which microphase separation is driven by crystallization, has been investigated.
Abstract: The morphology of a series of ethylene-(ethylene-alt-propylene) diblock copolymere (E/EP), in which microphase separation is driven by crystallization, has been investigated. The E/EP diblock were prepared by catalytic hydrogenation of narrow-distribution butadiene-isoprene diblocks. All the diblocks crystallized from single-phase melts so that the morphology and domain size were not restricted by «freezing» of a microphase-separated melt. The E/EP diblock adopted a lamellar morphology over the full range of compositions studied (E block fractions ranging from 0.12 to 0.56), and also exhibited spherulitic superstructure
TL;DR: Two intermediate phases have been observed upon heating an asymmetric poly(ethylenepropylene)-poly(ethylethylene) diblock copolymer between lamellar and hexagonal cylinder phases near the order-disorder transition.
Abstract: Two intermediate phases have been observed upon heating an asymmetric poly(ethylenepropylene)-poly(ethylethylene) diblock copolymer between lamellar and hexagonal cylinder phases near the order-disorder transition. Phase transitions betwen ordered morphologies are indicated by the temperature dependence of the dynamic shear moduli, and SANS experiments are used to identify these structures as hexagonally modulated lamellae and layered hexagonal packed channels. The observed wavevectors associated with the interlayer scattering for these structures are incommensurate, leading to an a periodic structure in which the long range translational order of the layers is destroyed