Mass Polystyrene Research Articles

Elongational viscosity and brittle fracture of bidisperse blends of a high and several low molar mass polystyrenes

Elongational viscosity data of four well-characterized blends consisting of 10% mass fraction of monodisperse polystyrene PS-820k (molar mass of 820 kg/mol) and 90% matrix polystyrenes with a molar mass of 8.8, 23, 34, and 73 kg/mol, respectively, as reported by Shahid et al. Macromolecules 52: 2521–2530, 2019 are analyzed by the extended interchain pressure (EIP) model including the effects of finite chain extensibility and filament rupture. Except for the linear-viscoelastic contribution of the matrix, the elongational viscosity of the blends is mainly determined by the high molar mass component PS-820k at elongation rates when no stretching of the lower molar mass matrix chains is expected. The stretching of the long chains is shown to be widely independent of the molar mass of the matrix reaching from non-entangled oligomeric styrene (8.8 kg/mol) to well-entangled polystyrene (73kg/mol). Quantitative agreement between data and model can be obtained when taking the interaction of the long chains of PS-820k with the shorter matrix chains of PS-23k, PS-34k, and PS-73k into account. The interaction of long and short chains leads to additional entanglements along the long chains of PS-820k, which slow down relaxation of the long chains, as clearly seen in the linear-viscoelastic behavior. According to the EIP model, an increased number of entanglements also lead to enhanced interchain pressure, which limits maximal stretch. The reduced maximal stretch of the long chains due to entanglements of long chains with shorter matrix chains is quantified by introducing an effective polymer fraction of the long chains, which increases with the increasing length of the matrix chains resulting in the excellent agreement of experimental data and model predictions.

Reduced graphene oxide resistance in composites with polystyrene of different molecular masses

Multi-layered reduced graphene oxide after UV-perforation (RGOp) flakes of large areas, some of them up to 1 mm in diameter, have been produced on polystyrene surface during special precipitation method of RGOp/polystyrene composites from benzene solution. For this, composites on the base of polystyrenes of different molecular masses: 4500 Da; 6000 Da; 9000 Da; and 45,000 Da were synthesized. Very low resistances at the room temperature for some RGOp flakes deposited from benzene-petroleum ether solution of composite of 9000 Da molecular mass were obtained. The absolute values of measured resistances turned out to be almost 3 orders lower than copper resistance. This result can be related to formation a high conductive phase in the reduced graphene oxide bilayer inclusions. In the case of RGOp flakes produced from our highest molecular mass polystyrene composite (45,000 Da) resistance was high and varying from semiconducting values to non-conductive state.

Particle morphology dependent superhydrophobicity in treated diatomaceous earth/polystyrene coatings

Superhydrophobic surfaces have been prepared from three different types of diatomaceous earth (DE) particles treated with 3-(heptafluoroisopropoxy)propyltrimethoxysilane (HFIP-TMS) and low molecular mass polystyrene. The untreated particles, consisting of CelTix DE (disk shape), DiaFil DE (rod shape) and EcoFlat DE (irregular), were studied using particle size analysis, bulk density, pore volume and surface area analysis (via Brunauer–Emmett–Teller, BET, methods). The treated particles were characterized with thermogravimetric analysis (TGA), contact angles, scanning electron microscopy, profilometry, and FTIR spectroscopy. The minimum amount of silane coupling agent on the DE surfaces required to obtain superhydrophobicity of the particles was determined and found to be dependent on the particle morphology. In the coatings made from different particles with 2.4wt% HFIP-TMS, the minimum amounts of treated particles (loadings) for superhydrophobicity was determined with the less dense CelTix DE requiring about 30wt%, DiaFil DE requiring about 40wt%, and EcoFlat DE each requiring about 60wt% loading of treated particles.

Development of superhydrophobicity in fluorosilane-treated diatomaceous earth polymer coatings

Superhydrophobic coatings were prepared using 3-(heptafluoroisopropoxy)- propyltrimethoxysilane (HFIP-TMS) treated diatomaceous earth (DE) particles with high molecular mass polystyrene or poly(vinyl acetate) as polymer binders. DE is a highly hydrophilic material and treatment of the DE with HFIP-TMS turned it into superhydrophobic diatomaceous earth (HFIP-DE). Thermogravimetric analysis (TGA) was used to determine the amount of grafted fluorosilane on the surface of the DE particles. The results showed that approximately 1.8% of HFIP-TMS grafted onto the surface of DE particles resulted in superhydrophobicity with contact angles as high as 164° for the particles themselves and also in coatings. Fourier transformed infrared spectroscopy (FTIR) was used to confirm the presence of HFIP-TMS on the surface of DE particles. The development of the hydrophobicity in the coatings with either polystyrene (PS) or poly(vinyl acetate) (PVAc) as binders was followed as a function of the particle loading using contact angle measurements and scanning electron microscopy. It was found that for these model DE-binder systems, the contact angles of the coatings were independent of the polymers used as long as the particle loading was greater than a minimum amount (∼40% treated DE particles). It was also found that more treated DE particles moved to the air interface as the particle loadings in the coatings increased and then levelled off.

Improved analysis of ultra-high molecular mass polystyrenes in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using DCTB matrix and caesium salts.

The ionization of polystyrenes in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is typically achieved by the use of silver salts. Since silver salts can cause severe problems, such as cluster formation, fragmentation of polymer chains and end group cleavage, their substitution by alkali salts is highly desirable. The influence of various cations (Ag(+), Cs(+) and Rb(+)) on the MALDI process of polystyrene (PS) mixtures and high mass polystyrenes was examined. The sample preparation was kept as straightforward as possible. Consequently, no recrystallization or other cleaning procedures were applied. The investigation of a polystyrene mixture showed that higher molecular polystyrenes could be more easily ionized using caesium, rather than rubidium or silver salts. In combination with the use of DCTB as matrix a high-mass polymer analysis could be achieved, which was demonstrated by the detection of a 1.1 MDa PS. A fast, simple and robust MALDI sample preparation method for the analysis of ultra-high molecular weight polystyrenes based on the use of DCTB and caesium salts has been presented. The suitability of the presented method has been validated by using different mass spectrometers and detectors.

Ultrathin polymer films by single molecule deposition

While physical vapour deposition of glass forming materials below their glass transition temperature, Tg, is an exciting route towards glasses with an extremely high packing density, strong kinetic and thermodynamic stability, vapour deposition of polymer systems is less evident and has so far not been investigated systematically.Here we have successfully prepared ultrathin films of low molecular mass polystyrene (800g/mol) by thermal evaporation from the melt into a UHV chamber at a maximum deposition rate of 1nm/h. Samples were studied in situ and in real-time by dielectric spectroscopy, chip-based ac-calorimetry and a quartz crystal microbalance, both during deposition and after reaching the final sample thickness of 5nm.During film growth well below the bulk-Tg, an initially retarded deposition kinetics was observed along with an accelerated dielectric relaxation dynamics compared to the bulk glass transition. Subsequent temperature cycling above the bulk-Tg revealed continuous changes in the (dielectric) glass transition dynamics and finally lead to desorption of the material at elevated temperatures without restoring the “dielectric” bulk glass transition dynamics.In contrast, simultaneous specific heat spectroscopy revealed bulk dynamics, a striking discrepancy that was discussed in terms of a dominant and accelerated response of PS end-groups in the dielectric spectra in combination with terminal sub-chain dynamics and some degree of end-group segregation.

Cohesion Mechanisms of Polystyrene-Based Thin Polymer Films

The cohesion mechanisms of end-functionalized high molar mass polystyrene with very low polydispersity (PS, Mn = 1.26 × 106 g mol–1, PDI = 1.06) and polylysine (PLL, 150–130 × 103 g mol–1) on silicon (Si) supported thin PS films are investigated by desorbing single polymers covalently bound to an atomic force microscope (AFM) cantilever tip. The influence of film preparation conditions and film architecture on polymer cohesion mechanisms is probed by comparing spin-coated PS films (scPS) with a thickness range of 6–52 nm and covalently surface-attached PS films (saPS) with a thickness of 15–83 nm. Annealed scPS prevents cohesion of further PS polymers unless the scPS partly dewets. In all other cases, two different cohesion mechanisms are observed: first, a previously described equilibrium desorption similar to hydrophobic solid substrate desorption, represented by a plateau of constant force in the force–extension curve, and second, a nonequilibrium mechanism with nonlinear force–extension behavior. The second requires a geometrical interlock between the tip bound single molecule and the PS film. Remarkably, this mechanism is observed below the glass transition temperature of PS films and is promoted by good solvent conditions. These findings contrast many bulk measurements assuming a glassy state of the complete polymer film, but they are consistent with fluid like boundary layers having a high mobility. Our results further underline the decisive influence of polymer film conformation and mobility close to its solvent exposed boundary layer for the cohesion of polymer coatings.

Thermoresponsive Hydrogels from Symmetrical Triblock Copolymers Poly(styrene-block-(methoxy diethylene glycol acrylate)-block-styrene)

A series of symmetrical, thermo-responsive triblock copolymers was prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and studied in aqueous solution with respect to their ability to form hydrogels. Triblock copolymers were composed of two identical, permanently hydrophobic outer blocks, made of low molar mass polystyrene, and of a hydrophilic inner block of variable length, consisting of poly(methoxy diethylene glycol acrylate) PMDEGA. The polymers exhibited a LCST-type phase transition in the range of 20-40 °C, which markedly depended on molar mass and concentration. Accordingly, the triblock copolymers behaved as amphiphiles at low temperatures, but became water-insoluble at high temperatures. The temperature dependent self-assembly of the amphiphilic block copolymers in aqueous solution was studied by turbidimetry and rheology at concentrations up to 30 wt %, to elucidate the impact of the inner thermoresponsive block on the gel properties. Additionally, small-angle X-ray scattering (SAXS) was performed to access the structural changes in the gel with temperature. For all polymers a gel phase was obtained at low temperatures, which underwent a gel-sol transition at intermediate temperatures, well below the cloud point where phase separation occurred. With increasing length of the PMDEGA inner block, the gel-sol transition shifts to markedly lower concentrations, as well as to higher transition temperatures. For the longest PMDEGA block studied (DP(n) about 450), gels had already formed at 3.5 wt % at low temperatures. The gel-sol transition of the hydrogels and the LCST-type phase transition of the hydrophilic inner block were found to be independent of each other.

Anomalous Behavior of Polystyrene Blends Using Thermally Induced Surface Wrinkling

ABSTRACTThe wrinkling of a 50:50 blend of a high molecular mass (Mn = 990 kg/mol) and low molecular mass (Mn = 1.3 kg/mol) polystyrene (PS) film is studied as a function of annealing temperature and film thickness. Both thermal and mechanical wrinkling are utilized to elucidate the apparent modulus of these PS blend films. The PS blend shows a modulus comparable to the high molecular weight PS, ≈ 3.2 GPa for mechanical wrinkling at ambient and thermal wrinkling for T ≤ 50 °C. A sharp decrease in the apparent modulus of the film occurs when thermal wrinkling occurs at 60 °C or higher. The calculated modulus in this case is 0.5 GPa, which is significantly below the modulus determined the neat PS for either Mn when thermally wrinkled at T > 60 °C. This behavior is attributed to a combination of surface segregation of the low molecular weight PS as well as the large difference in bulk glass transition temperature (Tg) of each component. During thermal wrinkling, the high Mn PS vitrifies first, while the surface containing primarily low Mn PS is rubbery; this leads to only the underlayer of PS wrinkling initially and selection of a shorter wavelength due to the effective thickness. The increased thermally induced stresses during cooling when the low Mn PS is vitrified do not change the selected wavelength and instead only leads to an increase in the wrinkle amplitude. These results illustrate a potential method to modulate the wrinkle wavelength without changing the overlayer, which could be useful for patterning applications.

Investigating the structure of a monolithic capillary column by means of hydrodynamic and size-exclusion chromatography

The porosity of a monolithic capillary column having a structure optimized for gas chromatographic analysis was investigated by means of hydrodynamic and size-exclusion chromatography. It was found that the total porosity of the column exceeded 90%, and the column had a bimodal pore structure with a micropore diameter of about 1.5 nm and a macropore diameter of about 1.2 μm. The column separated with good selectivity high molecular mass polystyrene standards with molecular masses higher than 100 kDa and low molecular mass solutes of up to 500 Da. The structure of monolithic column has to be optimized for application in hydrodynamic chromatography with an aim to provide selectivity on separation of polymers with molecular mass from 1 to 100 kDa.

Reduced sample recovery in liquid chromatography at critical adsorption point of high molar mass polystyrene

Low sample recovery may represent an important drawback in liquid chromatography at the critical adsorption point (LC-CAP) if the critical eluent is not carefully fitted to the system. So far, this problem was often overlooked and only few experimental examples can be found in literature. We showed that in the case of polystyrene (PS) in a tetrahydrofuran (THF)/ n-hexane critical mixed eluent, PS with molar masses higher than 100 kg mol −1 were not eluted from a tandem of two columns packed by bare silica gels with 30 nm and 100 nm pore size, respectively. The polymer trapped within the columns was well recovered after injection of a small volume of pure THF as demonstrated using 2D chromatography. We studied PS conformations by means of small angle neutron scattering and found that the THF/ n-hexane critical eluent is in fact a theta solvent for PS. By replacing it by a CH 2Cl 2/ n-hexane critical mixture, which is a good solvent for PS, the limits of reduced sample recovery was displaced towards far higher molar masses. Thus, thermodynamic quality of eluent – theta or good solvent – plays an important role on the phenomenon of sample recovery.

The Analysis of Polystyrene and Polystyrene Aggregates into the Mega Dalton Mass Range by Cryodetection MALDI TOF MS

Mass spectra of atactic polystyrene were collected into the mega-dalton mass range with a matrix-assisted laser desorption ionization time of flight (MALDI TOF) mass spectrometer, which incorporates a cryodetector comprised of an array of 16 superconducting tunnel junctions (STJ). The STJ cryodetector, theoretically, has no loss in signal response at any mass compared with the reduced signal found at high mass when using a conventional secondary-ionization detector. Since ion detection at high m/z is one of the fundamental limitations of mass spectrometry (MS), the cryodetector was used to explore the high m/z limit of the MALDI TOF technique for the analysis of two polymer types. Mass spectra were collected for polystyrene at Mn 170, 400, 900, and 2000 kDa and polymethyl methacrylate (PMMA) at Mn 62.6 kDa and 153.7 kDa. For polystyrene, the data showed a trend toward increased aggregation and charge state with mass. The Mn 2 MDa polystyrene data revealed a peak at m/z 2.2 MegaTh and a charge state analysis revealed that these ions were primarily polystyrene aggregates with a mass of approximately 4 MDa. This aggregate assignment was possible because the cryodetector response allows for the determination of a charge state up to about four. The contribution of each charge state for a selected peak can be determined in this fashion. This analysis revealed the preferential formation of doubly charged even-numbered aggregates over odd-numbered aggregates for high molecular mass polystyrene. A potential mechanism for the aggregation process for doubly charged species is discussed.

Blends of Poly(ε-caprolactone) and Intermediate Molar Mass Polystyrene as Langmuir Films at the Air/Water Interface

Poly(epsilon-caprolactone)/polystyrene (PCL/PS) blends, where nonamphiphilic PS is glassy in the bulk state at the experimental temperature of 22.5 degrees C, are immiscible as Langmuir films at the air/water (A/W) interface. Surface pressure-area per monomer isotherm analyses indicate that the surface concentration of amphiphilic PCL is the only factor influencing the surface pressure below the collapse transition. For PS-rich blends, Brewster angle microscopy (BAM) studies at the A/W interface and atomic force microscopy studies on Langmuir-Schaefer films reveal that PS nanoparticle aggregates formed at very low surface pressures can form networks upon further compression. The morphologies seen in PS-rich blends (networklike rings) are consistent with a recent study of a nonamphiphilic polyhedral oligomeric silsesquioxane (POSS), octaisobutyl-POSS, blended with amphiphilic poly(dimethylsiloxane), suggesting that the nonamphiphilic PS aggregates at the A/W interface produce domains with dipole densities that differ from that of pure PCL. In all composition regimes, the amphiphilic PCL phase tends to spread and form a continuous surface layer at the A/W interface, while simultaneously improving the dispersion of nonamphiphilic PS domains. During film expansion, BAM images show a gradual change in the surface morphology from highly continuous networklike structures (PS-rich blends) to broken ringlike structures (intermediate composition) to small discontinuous aggregates (PCL-rich blends). This study provides valuable information on the morphological evolution of semicrystalline PCL-based polymer blends confined in a "two-dimensional" geometry at the A/W interface and fundamental insight into the influence of microstructure (domain size, phase-separated structures, crystalline morphology, etc.) on the interfacial properties of blends as Langmuir films.

Monodisperse, micrometer-sized low molar mass polystyrene particles by two-stage dispersion polymerization

Free-radical dispersion polymerization of styrene was carried out in ethanol and in ethanol–water mixtures in the absence and presence of carbon tetrabromide (CBr4) as a chain transfer agent. When CBr4 was present at the onset of the reaction, particles with a broad size distribution were obtained. If, however, the addition of CBr4 was delayed ca 1h, so that the particle nucleation step was complete, then 1–2wt% chain transfer agent, dissolved in monomer plus solvent, could be added to the reaction without a deleterious affect on particle formation. The particle size and size distribution was essentially identical to that obtained in the absence of CBr4. When more CBr4 was added, other problems arose. These problems appeared to be due to solubility of low molar mass polymer in the reaction medium. They could be overcome by running the reaction in ethanol–water mixtures (e.g. 5wt% water) to decrease the solubility of oligo-styrene at 70°C, the reaction temperature. In this way, monodisperse particles could be prepared in the presence of 3wt% CBr4 based upon total styrene, consisting of polymers with Mn=7060, Mw/Mn=2.4.

Quantitative prediction of transient and steady-state elongational viscosity of nearly monodisperse polystyrene melts

Elongational behavior of four narrow molar mass distribution polystyrene melts of masses 50 000, 100 000, 200 000, and 390 000, g∕mol, respectively was investigated up to Hencky strains of 5. All melts show strain hardening behavior. For the two highest molar mass polystyrenes, strain hardening starts at elongation rates larger than the inverse reptation time, and the steady-state elongational viscosities decrease with increasing elongation rate according to a power law with a power-law index of approximately −1∕2 instead of −1 as predicted by the original Doi–Edwards tube model. Marrucci and Ianniruberto [Macromolecules 37, 3934 (2004)] have introduced an interchain pressure term arising from lateral forces between the chain and the tube wall into the Doi–Edwards model to account for the latter effect. Based on the molecular stress function theory allowing for a strain-dependent tube diameter, we show that the transient and steady-state elongational viscosities of the nearly monodisperse polystyrene melts can be modeled quantitatively by assuming affine chain deformation balanced by the interchain pressure term of Marrucci and Ianniruberto. The interchain pressure is governed by a tube diameter relaxation time τa, which is found to be larger than the Rouse time τR of the chain, and which is the only parameter of the model. For monodisperse polystyrene melts of sufficient low molar mass, τa is larger than the reptation time, and a maximum in the steady-state elongational viscosity is predicted.

Coil−Stretch Transition of High Molar Mass Polymers in Packed-Column Hydrodynamic Chromatography

The elution behavior of high molar mass polystyrene (PS) in packed-column hydrodynamic chromatography (HDC) was studied at different eluent flow rates by using a multiangle laser light scattering (MALLS) detector. A chromatography mode transition from HDC to slalom chromatography (SC) was observed for high molar mass PS samples with abnormal elution behaviors. The critical polymer size Rg,c of the HDC to SC transition was determined by the plateau value of the Rg−Ve curve for the earliest eluting polymer fraction. The dependence of Rg,c on flow rate was explained by coil−stretch transitions of the polymer molecules in elongational flow through the packed column. The transition of chromatography mode may provide a new method to study the coil−stretch transition of polymers in elongational flow through packed beds. For high molar mass polymer samples, HDC must be performed at flow rates sufficiently low that the elongational rate in the column is less than the critical strain rate for coil−stretch transitions to occur for all species in the sample.

Cobalt(II) octanoate and cobalt(II) perfluorooctanoate catalyzed atom transfer radical polymerization of styrene in toluene and fluorous media—A versatile route to catalyst recycling and oligomer formation

AbstractCobalt(II) perfluorooctanoate‐catalyzed atom transfer radical polymerization (ATRP) and reverse ATRP were developed to prepare oligostyrenes (Mn < 2500) with low polydispersities Mw/Mn < 1.5. Fluorous biphase catalysis was applied for effective recycling of catalyst and fluorous solvent. The homogeneous polymerization reaction was performed at 90 °C in toluene/cyclohexane/perfluorodecalin mixture (1:1:1) and fluorine‐free solvents. Temperature‐induced phase separation of this fluorous solvent mixture occurred at room temperature and proved to be the key for the very effective separation of the cobalt(II) perfluorooctanoate from the oligostyrene and fluorine‐free solvents. Both the fluorine‐tagged cobalt catalysts and the fluorous media were recycled and reused up to three times without encountering catalyst activity losses. The roles of cobalt catalysts, fluorous media, and monomer/initiator ratio were examined with respect to the polymerization kinetics. Fluorine‐containing and fluorine‐free cobalt(II) octanoate catalyzed controlled styrene oligomerization according to the ATRP mechanism. The molar mass control range was limited in fluorous biphase catalysis most likely because of precipitation of high molar mass polystyrenes in the fluorous reaction medium. To the best of our knowledge, this is the first time temperature‐induced phase separation of fluorous and fluorine‐free solvents has been successfully applied to polymerization processing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3804–3813, 2005

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Interlaboratory Comparison of Mixtures of Polystyrene with Different End Groups: Statistical Analysis of Mass Fractions and Mass Moments

A matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) interlaboratory comparison was conducted on mixtures of synthetic polymers having the same repeat unit and closely matching molecular mass distributions but with different end groups. The interlaboratory comparison was designed to see how well the results from a group of experienced laboratories would agree on the mass fraction, and molecular mass distribution, of each polymer in a series of binary mixtures. Polystyrenes of a molecular mass near 9000 u were used. Both polystyrenes were initiated with the same butyl initiator; however, one was terminated with -H (termed PSH) and the other was terminated with -CH2CH2OH (termed PSOH). End group composition of the individual polymers was checked by MALDI-TOF MS and by nuclear magnetic resonance (NMR). Five mixtures were created gravimetrically with mass ratios between 95:5 and 10:90 PSOH/PSH. Mixture compositions where measured by NMR and by Fourier transform infrared spectrometry (FT-IR). NMR and FT-IR were used to benchmark the performance of these methods in comparison to MALDI-TOF MS. Samples of these mixtures were sent to any institution requesting it. A total of 14 institutions participated. Analysis of variance was used to examine the influences of the independent parameters (participating laboratory, MALDI matrix, instrument manufacturer, TOF mass separation mode) on the measured mass fractions and molecular mass distributions for each polymer in each mixture. Two parameters, participating laboratory and instrument manufacturer, were determined to have a statistically significant influence. MALDI matrix and TOF mass separation mode (linear or reflectron) were found not to have a significant influence. Improper mass calibration, inadequate instrument optimization with respect to high signal-to-noise ratio across the entire mass range, and poor data analysis methods (e.g., baseline subtraction and peak integration) seemed to be the greatest obstacles in the correct application of MALDI-TOF MS to this problem. Each of these problems can be addressed with proper laboratory technique.

Synthesis of Chain End Functionalized Multiple Hydrogen Bonded Polystyrenes and Poly(alkyl acrylates) Using Controlled Radical Polymerization

Hydrogen bonding uracil functionalized polystyrenes and poly(alkyl acrylate)s were synthesized via stable free radical polymerization. Quantitative chain end functionalization was achieved using novel uracil containing TEMPO- and DEPN-based alkoxyamine unimolecular initiators. Polymerizations were conducted at 130 °C and yielded functionalized homopolymers with narrow molecular weight distributions (Mw/Mn ∼ 1.20) and predictable molecular weights. Polymerizations of both n-butyl acrylate and styrene using the DEPN- and TEMPO-based alkoxyamines resulted in molecular weight control over a wide range of conversions. Terminal functionalization of poly(alkyl acrylate)s with hydrogen bonding groups increased the melt viscosity at temperatures below 80 °C, which was defined as the dissociation temperature, and as expected, the viscosity approached that of the nonfunctional analogues above this temperature. The hydrogen bonding effect was also evident in thermal (DSC) analysis and 1H NMR spectroscopic investigations, and low molar mass polystyrenes exhibited glass transition temperatures that were consistent with a higher apparent molar mass. 1H NMR spectroscopy confirmed the presence of a single hydrogen bonding group at the chain terminus, which was consistent with a well-defined initiation process for two families of novel alkoxyamines.

Characterization of free volume and density gradients of polystyrene surfaces by low-energy positron lifetime measurements

Free volume and density gradient widths of the free surface of high molecular mass polystyrene was characterized by use of a pulsed low-energy positron lifetime beam. A density gradient in terms of mass density (g/cm 3 ) versus depth (nm) was obtained from the experimental beam data using a novel approach, yielding a width of the density gradient of approximately 3.5 nm. The procedure accounted for the broadening of the positron implantation probe as a function of energy as well as correcting for the increase in positron implantation depths due to the presence of a lower density at the surface region. Moreover, the spectra in the low implantation energy range were found to contain a long-lived lifetime that yielded very large estimates of the free volume effects at the surface. Accounting for this lifetime in the evaluation procedure, resulted in a much improved agreement with the results of other experimental and theoretical investigations in the literature.