Mechanism of emulsion polymerization of styrene in soap-free systems

The formation and growth of monodisperse polystyrene latex particles in the absence of added surfactant has been studied by sampling polymerization reactions at different times and determining the surface and bulk properties of the latex. A large number of nuclei in excess of 5 × 10¹²/ml were generated during the first minute of reaction, but this fell due to coagulation until a constant number (10¹¹?10¹²/ml) was reached. The rate of polymerization per particle was then found to be proportional to the particle radius. Gel-permeation chromatography has shown that the initial particles consist mainly of material of MW 1000 with a small amount of polymer up to MW 10⁶, and the presence of this low molecular weight polymer, which in many cases can still be detected after 100% conversion, is taken as being indicative of particle formation via a micellization-type mechanism involving short-chain (MW 500) free-radical oligomers. M?_n values determined for the latex particles throughout the course of reactions show that the molecular weight increases to a maximum of about 10⁵ as the particles grow. The presence of anomalous regions within the particles has been confirmed by transmission electron microscopy, scanning electron microscopy, and gas adsorption studies. It has also been found possible to re-expose these regions within apparently homogeneous particles by stirring with styrene monomer; this is indicative of a molecular weight heterogeneity within the latex particles. The presence of sulfate, carboxyl, and hydroxyl groups upon the latex particle surfaces has been determined by conductometric titration.     

Studies on formaldehyde condensation resins. XIII. Gelation of acetoguanamine–formaldehyde resin

The gelation reaction of acetoguanamine with formaldehyde was investigated in the light of the gelation theory for tetrafunctional amino resins described in the previous paper. The gel time and extents of reaction of formaldehyde, amino groups, and imino groups varied with the molar ratio in the feed, but values of K (the ratio of the rate constant for condensation to that for addition) and k (the ratio of the rate constant for addition of the imino group to that of the amino group) were nearly constant. When the catalyst concentration was increased, the gel time, extents of reaction of each functional group, and the values of K and k varied; in particular K increased markedly. From the results of varying the molar ratio and concentration of acidic catalyst, it was found that the number of methylol groups per molecule of acetoguanamine at the gel point was influenced by the reaction conditions but the number of methylene linkages per molecular of acetoguanamine was nearly constant at about 0.6, regardless of reaction conditions. The number-average molecular weights up to the gel point varied with the reaction conditions, but at the gel point they were all nearly constant at about 385.     

Influence of size and functionality of polymeric nanoparticles on the adsorption behavior of sodium dodecyl sulfate as detected by isothermal titration calorimetry

Isothermal titration calorimetry was used to monitor the adsorption of the surfactant sodium dodecylsulfate (SDS) on different sized pure and carboxy functionalized polystyrene nanoparticles prepared by the mini-emulsion process. The ITC experiment gives, additionally to the CMC values, information about the interaction of the surfactant molecules to the particle’s surface due to the particle surface properties. The adsorption heat depends on the chemical composition of the polymer as well on the particle size. It also provides information about the surface coverage with surfactant and the number of additional adsorbed molecules per particle until full coverage by surfactant is obtained. The surfactant adsorption increases from 0.3 molecules per nm² for 50 nm to 8.5 molecules per nm² for carboxy functionalized particles with diameters larger than 160 nm. The area A _Surf-dens after the adsorption process gives information about the packing density of surfactant molecules on the particles in dependence of carboxy groups: an increasing number of carboxylic groups decreases the area occupied per SDS molecule. The adsorption process was also monitored by zeta potential measurements, where an increasing potential during the adsorption was detected.     

Dilation and plasticization of polystyrene by carbon dioxide

We have used an optical interference technique to measure the dilation of polystyrene films in the presence of carbon dioxide or helium at pressures up to 20 atm. Dilation isotherms (plots of dilation versus gas pressure at constant temperature) were obtained for three samples of polystyrene which had widely differing molecular weights. The dilation isotherms have the same general shape as sorption isotherms, which means that all of the sorbed gas molecules contribute to volume dilation and non can be thought of as occupying molecular-sized voids in the polymer. Using sorption results from the literature we show that the partial molar volume of CO₂ at 35°C is about 39 cm³ mol^?1 and appears to be independent of polystyrene molecular weight. For a polystyrene sample with M_n = 3600, the partial molar volume of sorbed CO₂ increases to 44 and 50 cm³ mol^?1 at 45 and 55°C, respectively. The sorption of CO₂ in polystyrene is shown to depress the glass transition temperature of the mixture, consistent with theoretical predictions. The shape of the dilation and sorption isotherms are consistent with the depression of the glass transition temperature.     

Conformation of polyesters adsorbed on solid surfaces

The conformation of a polyester, poly(ethylene o-phthalate), of relatively low molecular weight was studied after adsorption. The extension of the adsorbed molecule in a poor solvent on several planar metal surfaces was studied by ellipsometry and the fraction p of attached groups on colloidal silica particles in a good solvent was determined by the shift in the infrared absorption frequency between free and adsorbed carbonyl groups. In contrast to previously reported results for polystyrene, the extension normal to the surface remained constant (～70 A.) while the concentration of polymer in the adsorbed film increased during the adsorption period. The value of p (0.34 for MW = 5400) is relatively high and was independent of surface population for the range of solution concentrations measured. Differences between these results and those for polystyrene are interpreted as resulting from differences in interaction energy and chain stiffness.     

Synthesis of Telechelic Polystyrene by Radical Polymerization Using 1,4-Bis(p-tert-Butylphenylseleno-Methyl)benzene as a Photoiniferter

Abstract

1,4-Bis(p-tert-butylphenylselenomethyl) benzene was synthesized, and used as a bifunctional photoiniferter for the polymerization of styrene. Both the polymer yields and the number average of molecular weights (_n) of polymers increased with the polymerization. The polymerization of styrene by this iniferter permitted telechelic polystyrene containing arylseleno groups at both chain ends, and the degree of functionality was 1.9. The seleno groups of both chain ends of polystyrene were reduced quantitatively by tri-n-butyltin hydride. These seleno groups in polystyrene were also eliminated by treatment with hydrogen peroxide to give telechelic polystyrene with carbon-carbon double bond at both chain ends. Further, polystyrene with double bonds was converted to telechelic polystyrene carrying terminal functional groups as epoxy, hydroxy, and iodide group, respectively.

     

Polymerization of styrene by poly(bisphenol a 4,4′-azobis-4-cyanopentanoate)

A free radical initiator, poly(bisphenol A 4,4′-azobis-4-cyanopentanoate) (BPA), containing more than one azo group per molecule, was used to polymerize styrene in N,N-dimethylformamide (DMF) at 60°C. Polymerization rates were measured gravimetrically or dilatometrically, and the molecular weights of the isolated polystyrenes were determined viscometrically, both before and after hydrolysis. BPA has a relatively low initiator efficiency of 0.28. The activation energy and velocity constant at 60°C for decomposition of BPA per azo group in DMF were found spectroscopically to be 105.9 kJ/mole and 2.08 × 10^?5 sec^?1, respectively. The molecular weights of unhydrolysed polystyrenes increased with increasing conversion and a theory is developed to explain these results.     

Phase-distribution chromatography (PDC) of polystyrene

A chromatographic technique is described where the stationary phase is a layer of a very high molecular polystyrene fraction (M = 10⁷) on glass beads (ballotines). The mobile phase is cyclohexane passing the column at a constant temperature below the theta-temperature. A polystyrene sample of sufficiently low molecular weight (M ≤ 10⁶) injected as a small plug at the top of the column is fractionated because the distribution between the mobile and the stationary phase depends on the molecular weight. Since the large molecules preferentially remain in the stationary phase, the smaller molecules leave the column first. The fractionation effect is inverse to that found in GPC experiments. The separation efficiency is rather good and can be described by a simple thermodynamic theory.     

Dynamic Properties of Molecular Tweezers with a Bis(2‐hydroxyphenyl)pyrimidine Backbone

4,6‐Bis(2‐hydroxyphenyl)‐2‐alkylpyrimidines with two anthryl or 9‐ethylnylanthryl substituents at the positions para to the OH groups prefer a U‐shaped conformation supported by two intramolecular OH ??? N hydrogen bonds in the solid state and in CDCl₃ solution. The compound with a hexyl substituent on the pyrimidine group and two 9‐ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7‐trinitrofluorenone. Its association constant K_a was estimated to be 2100 M ^?1 at 298 K, which is larger than those of other molecular tweezers (K_a<1000 M ^?1). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π–π interaction between the anthryl groups and 2,4,7‐trinitrofluorenone. Addition of nBu₄NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH ??? N hydrogen bonds, formation of new O ??? HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6‐bis(2‐hydroxyphenyl)‐2‐methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ?<0.01), owing to photoinduced electron transfer of the molecule with a U‐shaped structure. However, the O‐hexylated compound exhibits emission from the anthryl groups with ?=0.39.     

Branching generation during the free radical copolymerization of p-vinyl benzene sulfonyl chloride with styrene

The branching generation during the free radical copolymerization of chain transfer monomer p-vinyl benzene sulfonyl chloride (VBSC) with styrene was investigated by a simple mathematic model. Chain transfer constant of VBSC was determined to be around 0.3 by fitting the ¹H-NMR monitored experimental results with a mathematic model. According to the theoretical analysis, the obtained poly(VBSC) and its copolymers were substantiated to have a grafting-like main chain with residual pendent sulfonyl chloride groups after consuming most of the vinyl groups. The copolymerization results of VBSC with styrene at varied feed ratios demonstrated that conversion of sulfonyl chloride groups was lower than that of the monomer, which was in agreement with the theoretical results. The glass transition temperature, number average molecular weight and distribution of those obtained polymers were primarily investigated. Comparing with other chain transfer monomers, VBSC has a chain transfer constant much closer to unity therefore a more branched polymer is expected. Additionally, the branched polystyrene with residual sulfonyl chloride groups is hopefully to be further used as ATRP macroinitiators or reactive intermediates to synthesize functional polymers with complex structure.     

The mechanical degradation of polystyrene

The catastrophic failure of a polymeric material is preceded by a number of complex, partially understood events occurring on the molecular level. These events range from the flow of ordered regions to the cleavage of primary bonds in the chain. In recent years, stress-induced bond cleavage in polymers has received increased attention, many authors nothing the presence of free radicals and/or volatile products released upon fracture; a free-radical decomposition mechanism involving up to 10³ molecules per chain rupture also has been postulated. A special tensile stress–strain and shear apparatus was developed and located inside the ion-source housing of a time-of-flight mass spectrometer to characterize the volatile products released during mechanical degradation of polystyrene. Volatile compounds evolved during stress and fracture of polystyrene were monitored either continuously or by z-axis modulated oscilloscopic display. The polystyrene was purified by two methods: vacuum outgassing and fractional reprecipitation. Large amounts of styrene evolved from both the as-received and outgassed samples; however, essentially none was observed from the reprecipitated samples. Previous reports on monomer evolution during mechanical stress of polystyrene may be the result of residual monomer and not mechanical degradation products. The product of the surface density of primary radicals and the chain length for unzipping is less than 3 × 10¹⁰ radicals/mm² indicating a maximum radical concentration of approximately 10¹⁰ radicals/mm².     

Cationic grafting of olefins from PVC: The effect of reaction conditions

The cationic grafting of isobutylene, styrene, α-methylstyrene, and β-pinene from a poly(vinyl chloride) (PVC) backbone was investigated. Grafting-from was induced by Et₂AlCl in 1,2-dichloroethane and methylene dichloride solutions from 20 to −70 °C. The effects of temperature and proton trap [2,6-di-tert-butylpyridine (DtBP)] on grafting-from efficiency (G_eff), extent of grafting, branch length (molecular weight), and number of branches per PVC molecule were determined. Reducing the temperature invariably increased the G_eff and the molecular weight of polyisobutylene, polystyrene, poly(α-methylstyrene), and poly(β-pinene) branches attached to PVC. The magnitude of the effects was different with the various olefins and depended on the reaction conditions. The effect of DtBP was examined in the 5 × 10⁻⁴–4 × 10⁻³ mol/L range. By increasing the DtBP concentration the G_eff increased; however, the number-average molecular weight of the grafted branches decreased. The lengths of the grafted branches can be controlled, and G_eff's close to 100% were obtained. The fact that the proton trap reduced the molecular weights of grafted branches suggests that besides proton scavenging, DtBP may also abstract protons from the growing carbenium ion site. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1675–1680, 2001     

Effect of the solvent upon the diffusion coefficient of polydisperse polystyrene and styrene-acrylonitrile copolymer in dilute solution

A laser homodyne spectrometer was used to obtain translational diffusion coefficients for dilute polystyrene and styrene-acrylonitrile copolymer solutions at room temperature. Data were obtained in the concentration range from 0.01 to 2.0 g polymer per 100 cm³ solution for polystyrene in benzene and in decalin; and for copolymer in dimethyl formamide, in methyl ethyl ketone, and in benzene. The samples were polydisperse polystyrenes of weight average molecular weights between 80,000 and 350,000 and polydisperse copolymers of weight average molecular weights between 200,000 and 800,000. The SAN copolymers were random copolymer samples containing 24% by weight acrylonitrile. For each of the systems investigated the concentration dependence of the diffusion coefficient was linear over the concentration range studied, and was expressed as D(c) = D₀(1+k_Dc). Values of D₀ could be explained with a modified Kirkwood-Riseman expression. Values of the parameter k_D obtained from the slopes could be interpreted using the two-parameter theory approach as suggested by Vrentas and Duda. The value of k_D is positive for high-molecular-weight polymers and negative for low-molecular-weight polymers. For a particular polymer, the molecular weight at which k_D changes sign is greater for poor solvents than for good solvents. Observed values of D₀ were 1 × 10^?7 to 7 × 10^?7 cm²/sec.     

Synthesis of polystyrene microspheres by dispersion polymerization in supercritical carbon dioxide using a poly(dimethylsiloxane)-based macroazoinitiator

The synthesis of polystyrene microspheres was achieved by the dispersion polymerization of styrene in supercritical carbon dioxide using azobisisobutylonitrile (AIBN) and a poly(dimethylsiloxane) (PDMS)-based macroazoinitiator, VPS-1001. VPS-1001 contained seven to nine molecules of the azo groups and the PDMS blocks with a molecular weight of 10,000 per molecule. The polymerization in the presence of both VPS-1001 and AIBN produced polystyrene microspheres with a diameter below 4 μm in over 85% yields, whereas the polymerization with VPS-1001 in the absence of AIBN provided a nonspecific polystyrene in only 20% yield. The particle size decreased as a result of increasing the concentration of VPS-1001. It was confirmed that the polystyrene particles were stabilized by the PDMS-block-polystyrene formed through the polymerization initiated by VPS-1001 because the polymerization using a PDMS homopolymer provided nonspecific polystyrene as a precipitate during the polymerization.     

Method of determination of dielectric relaxation characteristics of plasticized polystyrenes on the basis of calorimetric glass temperature

The dielectric loss measurements of different polystyrenes (fractions and blends) with different molecular weights (M _n 2000–125000 g/mol) were carried out in the frequency range 10^–2–10⁶ Hz and the temperature range of the glass process (60°–135°C, depending on the molecular weight). The measurements of the pure fractions showed that the half-width of the glass relaxation process of the different polystyrenes can be correlated by a straight line, if they are plotted versus the relaxation frequency maxima of the glass process, regardless of the difference in both their molecular weight and glass transition temperature. Moreover, the fine structure of the shape of the glass process of polystyrenes with different molecular weights was found to be the same when the glass process appears at the same relaxation frequency range. The addition of oligostyrenes or low molecular <10% wt additives to the high molecular weight polystyrene did not influence the shape of the glass process. The calorimetric glass transition temperature of polystyrene was found to be only dependent on the number average molecular weight as well as on the number of end groups, but not on the molecular weight distribution. The obtained experimental results were correlated to develop a method for the estimation of the dielectric relaxation characteristics (relaxation frequency as well as the shape parameters) of the glass process of plasticized polystyrenes based on the calorimetric glass transition temperature. A method for the analysis of the dielectric relaxation curves of mixtures of label and polymer is suggested.     

Degradation on freezing dilute polystyrene solutions in p-xylene

Chain scission was observed during the crystallization of p-xylene in dilute polystyrene solutions. Degradation yields were determined by gel permeation chromatography, as a function of the number of freeze-and-thaw cycles, polymer concentration, and initial polymer molecular weight (M). The rate constant for chain scission K_c increases with the polymer chain length, from 0.021%/cycle at M = 110·10³ to 4.7%/cycle at M = 8.5·10⁶. Over the two decades range of investigated molecular weights, K_c follows an empirical scaling law of the form K_c ～ (M ? M_lim)^1.17578, where M_lim is a limiting molecular weight ? 29,000 g. mol^?1 below which no degradation could be induced. Some propensity for midchain scission was detected, although this tendency was much weaker in comparison to flow-induced degradation. A chain scission model based on crack propagation failed to reproduce the experimental results. To explain the observed dependence of K_c with the square of the radius of gyration, an interfacial stress transmission mechanism between the crystallization fronts and the polymer coil has been proposed. © 1994 John Wiley & Sons, Inc.     

Kinetics of the dissociation of nonionic detergent micelles by a temperature-jump technique

The rate of dissociation of a nonionic detergent, octylphenyl polyoxyethylene ether, was studied by a temperature-jump technique. A relaxation process which is concentration-dependent was observed. A linear relation between the reciprocal relaxation time and the detergent concentration above the critical micelle concentration was obtained, from which the rate constant of dissociation k_n,n–1 of one molecule of detergent from the micelle has been determined. Values of k_n,n–1 equal to 0.4 ± 0.05 and 73 ± 5 sec^?1 at 24.8°C were obtained for detergents with 16 and 30 ethylene oxide units per molecule of surfactant, respectively, showing and increase of the rate constant of dissociation with the length of the hydrophilic-head chain.     

The special features of H-bonding in supercritical water close to the saturation curve

A complex study of H-bonding in supercritical water in the region close to the saturation curve was performed at constant pressures of 30 and 50 MPa and a constant temperature of 673 K. The topological H-bond characteristics were calculated by the molecular dynamics method with the use of the TIP4P-HB potential. The calculations included the number of H-bonds per water molecule, the degree of H-bonding, and the distributions of molecules according to the number of H-bonds (fractions and the gradients of fractions of molecules with n H-bonds). Changes in these characteristics as the temperature, density, and pressure varied were studied.     

Viscoelastic behavior of polyvinylcyclohexane

The tensile stress relaxation master curve for polyvinylcyclohexane (completely hydrogenated polystyrene) has been measured. Direct relaxation experiments were carried out at several temperatures above the glass transition temperature over the rather long time range of four orders of magnitude. This long time span was realized by calculating the modulus during the period when a constant small strain rate was applied to the sample as well as during the usual constant strain interval. A computer solution to the Boltzmann superposition equation allowed data from these two regions to be joined into a smooth curve representing E(t), a parameter indicative of an instantaneous strain experiment. The measured T_i was found to be 143°C; T_g is expected to fall within several degrees of this temperature. This result is apparently at odds with a previously reported T_g value of 120°C. More importantly, the maximum value of the negative slope of the stress relaxation master curve of polyvinylcyclohexane in the primary transition region was only slightly different from that for polystyrene. This observation clearly indicates that the molecular factors which result in the highly coupled nature of the primary transition in polystyrene are not strongly dependent upon any side-chain π–π interactions which might be present in polystyrene.     

Molecular architecture and rheological characterization of novel intramolecularly crosslinked polystyrene nanoparticles

Novel polystyrene nanoparticles were synthesized by the controlled intramolecular crosslinking of linear polymer chains to produce well‐defined single‐molecule nanoparticles of varying molecular mass, corresponding directly to the original linear precursor chain. These nanoparticles are ideal to study the relaxation dynamics/processes of high molecular mass polymer melts, as the high degree of intramolecular crosslinking potentially inhibits entanglements. Both the nanoparticles and their linear analogs were characterized by measuring their intrinsic viscosity, hydrodynamic radius (R_h), and radius of gyration (R_g). The ratio R_g/R_h was computed to characterize the molecular architecture of the nanoparticles in solution, revealing a shift toward the constant density sphere limit with increasing crosslink density and molecular mass. Further, confirming particulate behavior, Kratky plots obtained from neutron scattering data show a shift toward particle‐like nature. The rheological behavior of the particles was found to be strongly dependent on both the extent of intramolecular crosslinking and molecular mass, with a minimal viscosity change at low crosslinking levels and a gel‐like behavior evident for a large degree of crosslinking. These and other results suggest the presence of a secondary mode of polymer relaxation/movement besides reptation, which in this case, is influenced by the total number of crosslinked loops present in the nanoparticle. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1930–1947, 2006