Effect of reaction conditions on synthesis and properties of multiarm star-branched polyisobutylenes

A series of multiarm star-branched polyisobutylenes was synthesized from narrow polydispersity arms with molecular weights ranging from 12,000 to 60,000 g/mol, via living carbocationic polymerization using the cumyl chloride/TiCl₄/pyridine initiating system and divinylbenzene (DVB) as core-forming comonomer. The effect on star development of arm molecular weight, temperature, solvent composition, and DVB concentration was studied. The rate of star formation and the weight-average number of arms per star polymer, N̄_w, were found to scale inversely with arm molecular weight; N̄_w = 60 was attained for 13,100 g/mol arms, but N̄_w = 2.5 for 60,000 g/mol arms. It was established that star formation was much faster at −80°C compared to 23°C, regardless of solvent composition. For hexane : methyl chloride (MeCl) solvent compositions containing from 40 to 60 vol % MeCl, star–star coupling was observed at −80°C, but not at 23°C, even after 312 h; for the most polar 40 : 60 hexane : MeCl composition, star–star coupling was so extensive at −80°C that gelation was observed after only 44 h. The rate of star formation was found to be substantially higher in 60 : 40 hexane : MeCl compared to 60 : 40 hexane : methylene chloride (MeCl₂). Some reactions containing MeCl were immediately warmed to 23°C after DVB addition, and the MeCl thus volatilized was replaced with either MeCl₂ or hexane for the duration of the star-forming reaction. Slightly higher rates were consistently observed when MeCl₂ was the replacement solvent. The strong influence of initial MeCl content on rate of star formation was found to persist throughout the star-forming reaction, even when the solvent was immediately converted to 100% hexane. The fraction of arms that remained unlinked into stars was found to be higher at the higher temperature and at lower solvent polarity. Regardless of solvent or temperature, the residual arm fraction was approximately the same at a given stage of star development as measured by the average number of arms per star. One star sample was produced with the UV-transparent 2-chloro-2,4,4-trimethylpentane initiator; analysis showed that the residual arm fraction had approximately the same UV absorbance as the star fraction, indicating efficient crossover to DVB and the potential for approximately quantitative arm incorporation given sufficient time. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36 : 471–483, 1998     

Energy transfer and triplet exciton confinement in polymeric electrophosphorescent devices

Energy transfer and triplet exciton confinement in polymer/phosphorescent dopant systems have been investigated. Various combinations of host‐guest systems have been studied, consisting of two host polymers, poly(vinylcarbazole) (PVK) and poly[9,9‐bis(octyl)‐fluorene‐2,7‐diyl] (PF), blended with five different phosphorescent iridium complexes with different triplet energy levels. These combinations of hosts and dopants provide an ideal situation for studying the movement of triplet excitons between the host polymers and dopants. The excitons either can be confined at the dopant sites or can flow to the host polymers, subject to the relative position of the triplet energy levels of the material. For PF, because of its low triplet energy level, the exciton can flow back from the dopants to PF when the dopant has a higher triplet energy and subsequently quench the device efficiency. In contrast, efficient electrophosphorescence has been observed in doped PVK films because of the high triplet energy level of PVK. Better energy transfer from PVK to the dopants, as well as triplet exciton confinement on the dopants, leads to higher device performance than found in PF devices. Efficiencies as high as 16, 8.0, and 2.6 cd/A for green, yellow, and red emissions, respectively, can be achieved when PVK is selected as the host polymer. The results in this study show that the energy transfer and triplet exciton confinement have a pronounced influence on the device performance. In addition, this study also provides material design and selection rules for the efficient phosphorescent polymer light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2681–2690, 2003     

Effect of confinement on coil-globule transition

The equilibrium thermodynamic properties of a linear polymer chain confined to a space between two impenetrable walls (lines) at a distance D under various solvent conditions have been studied using series analysis and exact enumeration technique. We have calculated the end-to-end distance of polymer chain, which shows a nonmonotonic behavior with inter wall separation D. The density distribution profile shows a maxima at a particular value of (D=)D*. Around this D*, our results show that the collapse transition occurs at higher temperature as compared to its bulk value of 2d and 3d. The variation of theta-temperature with D shows a re-entrance behavior. We also calculate the force of compression exerted by the walls (lines) on the polymer.     

Dynamics of polymeric systems

Using dielectric and mechanical relaxation data of polymeric systems, an alternative formula is examined. The formula describes the experimental data with a high degree of accuracy, not received by the other model functions (Arrhenius and non-Arrhenius type) under investigation. The proposed formula gives the answer on questions concerning a supermolecular structure and the nature of the structural relaxation. New definition of the glass-transition temperature is proposed. It is shown how we should combine the results of different experimental methods to conduct an unambiguous analysis of polymeric systems.     

Synthesis and characterization of multiarm star-branched polyisobutylenes: Effect of arm molecular weight

A series of star-branched polyisobutylenes with varying arm molecular weights was synthesized using the 2-chloro-2,4,4-trimethylpentane/TiCl₄/pyridine initiating system and divinylbenzene (DVB) as a core-forming comonomer (linking agent). The resulting star-branched polymers were characterized with regard to the weight-average number of arms per star molecule (N̄_w) and dilute solution viscosity behavior. As the molecular weight of the arm (M̄_{w, arm}) was increased, dramatically longer star-forming reaction times were needed to produce fully developed star polymers. It was calculated that N̄_w varied from 50 to 5 as the M̄_{w, arm} was increased from 13,000 to 54,000 g/mol. The radius of gyration, R_g, of the star polymers was observed to increase as M̄_{w, arm} was increased. The solution properties of the star polymers were evaluated in heptane using dilute solution viscometry. It was determined that the stars had a much higher [η] compared to the respective linear PIB arms, but a much lower [η] compared to a hypothetical linear analog of an equivalent molecular weight. The dependence of [η] on temperature for the stars and linear arms was very small over the temperature range 25 to 75°C, with only a very slight decrease with increasing temperature. [η]_star was also determined to increase with increasing M̄_{w, arm}, but decrease with increasing M̄_{w, star}. The branching coefficient, g′, calculated for the stars at 25°C, increased as N̄_w decreased and agre ed well with literature values for other star polymer systems. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3767–3778, 1997     

Properties of star-branched polymer brushes

A simple model of a polymer brush was constructed. The star polymers with three arms were terminally attached with one arm (the stem) to an impenetrable surface with the other two arms (branches) free. The excluded volume effect was included into the model as the only interaction. Therefore, the system was studied in good solvent conditions. The simulations were carried out by means of the dynamic Monte Carlo method using the local changes of chain conformations to sample efficiently the conformational space. The influence of both the number of chains (the grafting density) and the length of chains on the static properties of the polymer brush was studied. The internal and local structure of a formed polymer layer was determined. It was shown that the size of the stems increased rapidly with the increase of the grafting density, while the size of the branches diminished. The changes of the spatial orientations of the stems and the branches for different grafting densities were shown and discussed.     

Kinetic model of star-branched polycondensation

A kinetic model was developed for the whole process of star-branched polycondensation of AB type monomers with a multifunctional core, RA_f. The evolution of molecular weight distribution and other molecular parameters during reaction were estimated in terms of the derived expressions. The molecular weight distribution first becomes broader with increasing reaction extent of B groups and the functionality of RA_f, but suddenly turns to be markedly narrower than the Schulz-Flory distribution when the polycondensation approaches completion.     

Synthesis and characterization of multi-arm star-branched polyisobutylenes

Multi-arm star-branched polyisobutylenes were synthesized by the “arm-first, core-last” method using the 2-chloro-2,4,4-trimethylpentane/pyridine/TiCl₄ initiating system and the reactive core-forming comonomers 1,3-diisopropenylbenzene (DIPB) and divinylbenzene (DVB). Star formation was confirmed by RI and UV GPC and static light-scattering analyses. It was determined that DVB was significantly superior to DIPB. Using DVB, star polymers formed more rapidly and contained a much lower amount of residual PIB arms. Increasing the concentration of the reactive comonomer from 1 to 10 times the concentration of chain ends, [CE], increased the efficiency of the star-forming reaction substantially. Modest increases in the fraction of PIB arms incorporated into the star could be obtained by increasing the duration of the star-forming reaction. The timing of addition of the reactive comonomer to the PIB arms seems to be the process parameter most critical to the star development, since early addition at excessively low IB conversion hinders star formation by causing a copolymerization between IB and the core-forming comonomer. Late addition risks loss of a significant fraction of PIB chains due to spontaneous β-proton expulsion. A fully developed multi-arm star-branched PIB was synthesized by utilizing 10:1 [DVB]:[CE], 24 h star-forming reaction time, 14,000 g/mol target arm M_n, and addition of DVB at 99% IB conversion. The resultant star polymer contained only 4% unreacted PIB arms and possessed M_w = 345,000 g/mol by light scattering. The weight-average number of arms per star polymer was calculated to be 23. © 1996 John Wiley & Sons, Inc.     

Preparation of titanium‐containing polymeric foam for inertial confinement fusion target

In the framework of the increasing need for metal‐doped polymer materials for inertial confinement fusion targets, we report the preparation of low‐density titanium‐containing materials. For this purpose, we developed a new monomer based on hydroxyl and oxime chelation: Ti₃(5‐vinylsalicylaldoximato)₂(Oi‐Pr)₈. We report here the synthesis, characterization and first results of polymerization of this new titanium‐containing monomer. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of sonication on polymeric membranes

This article studies the effect of 47 kHz ultrasonic (US) waves on polymeric membranes immersed in an aqueous bath. The membranes under study are made from three different polymers: polyethersulfone (PES), polyvinylidenefluoride (PVDF) and polyacrylonitrile (PAN) and present various molecular weight cut-off (MWCO). The evolution of the polymeric structure exposed to US was followed by the measurement of the water permeability and the A_k/Δx parameter which represents the ratio of surface porosity to thickness. Results showed that important variations occurred on certain membranes after irradiation. In addition, microscopic imaging using field emission electron scanning microscopy (FESEM) was performed on irradiated membranes in order to visualize the nature of the degradation. An image analysis method gives the evolution of the pore density, porosity and pore size distribution of a homogeneous area of this membrane before and after irradiation.It has been shown that, over the three materials tested, only the PES is affected by the ultrasonic treatment over all its surface, whereas the others present no significant change in the measured parameters except the PAN (50 kDa) and PVDF (40 kDa) membranes whose edges are affected. In conclusion, in spite of their great efficiency in enhancing filtration processes, ultrasonic waves have to be used with care as the polymeric material itself is sensitive to the ultrasonic waves at the chosen frequency.     

Grafted interpenetrating polymeric systems based on cross-linked siloxaneurethanes

A series of cross-linked polysiloxaneurethanes with different functional groups in the nodal organosilicon fragment and related grafted interpenetrating polymeric systems with poly(methyl methacrylate) were synthesized, and their structural relaxation and mechanical properties were studied. Specific features of microphase separation and the mechanism of molecular mobility in these systems were considered.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1946–1951, September, 2004.     

Oscillatory shear flow of polymeric systems

On the basis of the Leonov viscoelastic constitutive equation, oscillatory shear flow of elastic fluids in the linear and nonlinear regimes has been considered. The Fourier components and associated phase angles of the shear and normal components of the elastic strain tensor have been found as functions of frequency and deformation amplitude in the range usually employed in experiment, and are presented in a form convenient for further rheological applications. In the linear case, the results correspond to many known theories. In the nonlinear case, the theoretical results have been compared with experiments, on different polymeric systems, with very good agreement being obtained for the shear stress in polymeric solutions but only qualitative agreement for the shear stress and first normal-stress difference in polymer melts.     

Studies on the solution behaviour of self-assembling polymeric systems

Functional groups were attached to styrene-butadiene copolymers with different polymeric architectures and a low molecular weight compound. These polar stickers form binary, chelate-like contacts or extended juntion zones depending on their chemical nature. The solution properties of these amphiphilic species in toluene were studied by static light scattering.     

Effect of water-soluble polymeric additives on energy expenditure in transportation of water-oil systems in turbulent mode

New installation of the circulation type was used to perform a joint analysis of rheological and energy parameters of turbulent flows of water and water-oil media in the presence of polymeric additives of anionic copolymers of acrylamide. Copolymers with varied molecular mass were produced by ultrasonic destruction of a starting high-molecular-weight sample of the polymer under mild conditions. The effect of the molecular mass of the polymer additive on the specific energy expenditure in the case of circulation of water-oil media in the turbulent mode was analyzed.     

Synthesis and properties of tocopherol-containing polymeric vesicle systems

Two different tocopherol-containing amphiphilic monomers, [ [ (tocopheryloxy)-carbonyl ]-methyl ] [ 2-(methacryloyloxy)ethyl ]dimethyl-ammonium chloride(3a) and [[ (tocopheryloxy)penta(ethoxy)carbonyl ]methyl ]2-(methacryloyloxy)ethyl]dimethylammonium chloride(3b), were synthesized and polymerized. The formation of polymeric closed vesicles having diameters ranging from 200 to 5200 Å was confirmed by electron micrographs, entrapment of [¹⁴C]sucrose, permeability measurements and gel filtration. The polymeric vesicles showed reduced permeability and enhanced thermodynamic stability. Antioxidative activities were determined by the thiocyanate method confirming that polymeric tocopherols also exhibited significant activities.     

Synthesis of energetic polynuclear and polymeric nitroazole systems

Interaction of cyanuric chloride and its mono and dichloro derivatives with ammonium or sodium 4-nitro-1,2,3-triazolates and polymer-analogous transformations of tetrazole-containing polymers were used to synthesize polynuclear systems and macromolecular compounds with heterocyclic structures bearing explosophoric groups.     

Effect of confinement on droplet breakup in sheared emulsions

The breakup of Newtonian droplets in a Newtonian matrix during shear flow is investigated in a counterrotating parallel plate device. For bulk conditions, the critical capillary number for breakup is known to be only determined by the viscosity ratio. Here, we show that the critical capillary number is also affected by the degree of confinement: for low viscosity ratios, confinement suppresses breakup, whereas for high viscosity ratios, confinement promotes breakup. This way, above a critical value for the degree of confinement, even droplets with a viscosity ratio larger than 4, which are unbreakable by shear in a bulk situation, can be broken in a simple shear flow field.     

Diffusion of single star-branched dendrimer-like DNA

We report a single-molecule fluorescence study on the diffusion of star-branched polymer dendrimer-like DNA (DL-DNA). The DL-DNA molecules were synthesized from ligating Y-shaped DNA to the fourth generation. It was found through single particle tracking that the diffusion coefficient of DL-DNA changes in a nonmonotonic fashion with its increased concentration possibly due to arm arrest and arm retraction. The diffusion of DL-DNA in linear lambda DNA solution displayed a monotonic concentration dependence 1 order of magnitude greater than the diffusion of DL-DNA in DL-DNA solution. This difference is attributed to the different conformation of DL-DNA and lambda DNA and the entanglement of lambda DNA with a large radius of gyration. Our diffusion study facilitates DL-DNA transportation for drug delivery.