Atactic Polystyrene Research Articles

Exact solutions for oscillatory shear sweep behaviors of complex fluids from the Oldroyd 8-constant framework

In this paper, we provide a new exact framework for analyzing the most commonly measured behaviors in large-amplitude oscillatory shear flow (LAOS), a popular flow for studying the nonlinear physics of complex fluids. Specifically, the strain rate sweep (also called the strain sweep) is used routinely to identify the onset of nonlinearity. By the strain rate sweep, we mean a sequence of LAOS experiments conducted at the same frequency, performed one after another, with increasing shear rate amplitude. In this paper, we give exact expressions for the nonlinear complex viscosity and the corresponding nonlinear complex normal stress coefficients, for the Oldroyd 8-constant framework for oscillatory shear sweeps. We choose the Oldroyd 8-constant framework for its rich diversity of popular special cases (we list 18 of these). We evaluate the Fourier integrals of our previous exact solution to get exact expressions for the real and imaginary parts of the complex viscosity, and for the complex normal stress coefficients, as functions of both test frequency and shear rate amplitude. We explore the role of infinite shear rate viscosity on strain rate sweep responses for the special case of the corotational Jeffreys fluid. We find that raising η∞ raises the real part of the complex viscosity and lowers the imaginary. In our worked examples, we thus first use the corotational Jeffreys fluid, and then, for greater accuracy, we use the Johnson-Segalman fluid, to describe the strain rate sweep response of molten atactic polystyrene. For our comparisons with data, we use the Spriggs relations to generalize the Oldroyd 8-constant framework to multimode. Our generalization yields unequivocally, a longest fluid relaxation time, used to assign Weissenberg and Deborah numbers to each oscillatory shear flow experiment. We then locate each experiment in the Pipkin space.

Coil-Globule Collapse of Polystyrene Chains in Tetrahydrofuran-Water Mixtures.

We study the coil and globule states of a single polymer chain in solution by performing molecular dynamics simulations with a united atom model. Specifically, we characterize the structural properties of atactic polystyrene chains with N = 20-150 monomers in tetrahydrofuran-water mixtures at varying mixing ratios. We find that the hydrophobic polymers form rather open coils when the mole fraction of water, XW, is roughly below 0.25, whereas the chains collapse into globules when XW ≳ 0.75. We confirm the theoretically expected scaling laws for the radius of gyration, Rg, in these regimes, i.e., Rg ∝ N3/5 and Rg ∝ N1/3 for good and poor solvent conditions, respectively. For poor solvent conditions with XW = 0.75, we find a sizable fraction of residual tetrahydrofuran trapped inside the collapsed polymer chains with an excess amount located at the globule surface, acting as a protective layer between the hydrophobic polystyrene and the surrounding water-rich mixture. These findings have important implications for nanoparticle fabrication techniques where solvent exchange is exploited to drive polymer aggregation, since residual solvent can significantly influence the physical properties of the precipitated nanoparticles.

Using Atomic Force Microscopy to Probe Crystallization in Atactic Polystyrenes

AbstractThe use of atomic force microscopy phase imaging to study crystallization in near oligomeric atactic polystyrenes is demonstrated. By considering different thermal histories, the distribution of polymerization index in the samples can be changed, and how this effects the resulting crystallinity can be measured. Different thermal treatments in the nucleation of crystals and the effect on the resulting measured crystal formation are also examined.

Crystallization of low molecular weight atactic polystyrene.

We observe and characterize the crystallization of atactic polystyrenes (PS) of nearly oligomeric Mw using atomic force microscopy. We find that the low Mw polystyrene exhibits observable crystals on the surface. The crystals appear to be a few nm thick and nm to microns wide. These crystals grow at all temperatures less than ∼290 K. Melting of crystals was probed over an extended temperature range, and some fraction of the crystals start to melt at 302 K, but some fraction persist to higher temperatures and do not exhibit complete melting until 343 K. The tacticity of the molecules is tested with NMR spectroscopy and found to be atactic. We suggest that the crystals form due simply to the distribution of isomerism along the molecule which necessarily leaves some fraction of the molecules with uniform stereoregularity. This natural crystallinity may be related to previously observed and not definitively explained gel formation in atactic PS (a-PS), as well as cluster formation. The measurements are compared with the theory by Semenov (Macromolecules, 2009, 42, 6761) and together suggest that such crystallinity is possible over a wide range of polymerization index (N), and is limited only by the vanishingly small volume fractions and sluggish growth.

Single-Ended Long Period Fiber Grating Coated With Polystyrene Thin Film for Butane Gas Sensing

A safe detection of hydrocarbons is a major concern in industrial and public environment, due to their high volatility and flammability. In this work, we report the development and characterization of an in-fiber sensor for the detection of aliphatic hydrocarbon gas operating at room temperature, focusing the attention on the butane (C 4H10). A long period grating (LPG) was used as optical transducer, integrated with a nanoscale high refractive index sensitive overlay of atactic polystyrene (aPS). Both LPG and aPS thin film were carefully designed to work in mode transition, in order to enhance the final device sensitivity. Moreover, a practical single ended probe was developed by integrating a mirror on the fiber tip. The sensor was tested with butane vapor concentrations up to 1.0 vol%, obtaining a maximum sensitivity of − 2.2 nm/vol%, and demonstrating the possibility to detect concentrations as low as one tenth of the lower explosive limit (LEL) of 1.8 vol%.

Residual orientation in injection‐moulded plaques of atactic‐polystyrene II: The effect of molecular weight

Injection moulding of polymers is a well‐established technique for the production of increasingly complex articles, and residual molecular orientation is a matter of considerable interest. A single matrix of commercial atactic polystyrene was systematically blended with deuterated polystyrene chains of five different weight fractions and the orientation investigated using small angle neutron scattering (SANS), optical birefringence and thermal shrinkage. The strengths and limitations of the experimental techniques used to measure residual orientation are explored and highlighted the unique contribution of SANS to observe ensemble averaged molecular orientation using deuterated polymers. The residual molecular orientation was found to vary with the molecular weight of the deuterated chains with an increase with increasing molecular weight. The observed correlations lead to a better understanding of the processing parameters that influence the degree of residual orientation on the molecular scale. Such information is valuable in the selection of the polymer matrix, mould design, and processing conditions. POLYM. ENG. SCI., 58:1332–1341, 2018. © 2017 Society of Plastics Engineers

Temperature influence on the structure and dynamics of polymers at the interface: Atomistic molecular dynamics simulation of atactic polystyrene nanoconfined between graphene surfaces

Detailed atomistic molecular dynamics simulations are done on the polystyrene oligomers nanoconfined between parallel graphene surfaces at high (400K) and low temperatures (300K) and at 101kPa using a new method, to quantify the surface effect and the temperature influence on a number of structural and dynamical properties of nanoconfined oligomers. Our findings indicate that the surface effect on the alteration in the chain structure at the interface (compared to the structure of the unperturbed chain) and the interphase thickness, determined in this regards, are temperature insensitive. The surface influence on the dynamic deceleration (compared to the dynamics of the unperturbed chain) and the interphase thickness, determined in terms of alteration in chain dynamics, are temperature dependent. The magnitude of temperature-dependency of the dynamic deceleration at the interface depends on the time scale of the dynamical property of interest. Stronger temperature-dependencies are observed for longer-time dynamical properties.

Group Contribution Coarse-Grained Molecular Simulations of Polystyrene Melts and Polystyrene Solutions in Alkanes Using the SAFT-γ Force Field

A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linear-chain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an “aromatic-like” nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature–composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction ene...

Time‐resolved Raman spectroscopy of polystyrene under laser driven shock compression

The microscopic and dynamic response of the atactic polystyrene molecules under shock compression at laser power density of 4.9 GW/cm2 was studied using time‐resolved Raman spectroscopy. The shock pressure estimated in the polystyrene sample was 2 GPa, and the temporal and wavenumber resolutions in these experiments were 3 ns and 3 cm−1, respectively. The shocked polystyrene showed a blue shift of 5 and 7 cm−1 for the 1004 cm−1 (C―C―C ring bending mode ν12) and 1606 cm−1 (C―C ring stretching mode . , respectively. The changes in ν12 and modes are observed with respect to shock wave propagation. It is observed that the intensities of the Raman peaks decrease and the FWHM of the modes increase due to formation of additional new peaks with delay. The reason behind this indicates the mechanical processes associated with the uniaxial nature of the shock compression. The shock velocity inside the polystyrene was calculated as 2.9±0.12 km/s using the evolution of intensity ratio of peaks with time. This is in good agreement with the shock velocity (3.0 km/s) deduced from the 2D simulations, performed using three temperature radiation hydrodynamic code, THRD. Copyright © 2017 John Wiley & Sons, Ltd.

Interfacial and topological effects on the glass transition in free-standing polystyrene films.

United-atom molecular-dynamics computer simulations of atactic polystyrene (PS) were performed for the bulk and free-standing films of 2 nm-20 nm thickness, for both linear and cyclic polymers comprised of 80 monomers. Simulated volumetric glass-transition temperatures (Tg) show a strong dependence on the film thickness below 10 nm. The glass-transition temperature of linear PS is 13% lower than that of the bulk for 2.5 nm-thick films, as compared to less than 1% lower for 20 nm films. Our studies reveal that the fraction of the chain-end groups is larger in the interfacial layer with its outermost region approximately 1 nm below the surface than it is in the bulk. The enhanced population of the end groups is expected to result in a more mobile interfacial layer and the consequent dependence of Tg on the film thickness. In addition, the simulations show an enrichment of backbone aliphatic carbons and concomitant deficit of phenyl aromatic carbons in the interfacial film layer. This deficit would weaken the strong phenyl-phenyl aromatic (π-π) interactions and, hence, lead to a lower film-averaged Tg in thin films, as compared to the bulk sample. To investigate the relative importance of the two possible mechanisms (increased chain ends at the surface or weakened π-π interactions in the interfacial region), the data for linear PS are compared with those for cyclic PS. For the cyclic PS, the reduction of the glass-transition temperature is also significant in thin films, albeit not as much as for linear PS. Moreover, the deficit of phenyl carbons in the film interface is comparable to that observed for linear PS. Therefore, chain-end effects alone cannot explain the observed pronounced Tg dependence on the thickness of thin PS films; the weakened phenyl-phenyl interactions in the interfacial region seems to be an important cause as well.

Fluorescence behavior of syndiotactic polystyrene and its derivative: Formation of a ground-state dimer in the solid state

The fluorescence behavior of syndiotactic polystyrene (SPS) and its derivative, syndiotactic poly(4-tert-butylstyrene) (SPTBS), in solution, gel and solid-state forms is examined in detail. The behavior of these polymers is also compared to isotactic and atactic polystyrene. The main fluorescence of SPS and SPTBS in the solid state is discovered to be the result of a ground-state dimer (GD), even in ultrathin films, where side-chain phenyl groups that fail to form a GD mainly exhibit intermolecular excimer formation. The structures of all racemo diads promote contact between the side-chain phenyl groups of a polymer chain and other polymer chain. This might be one reason for the fast crystallization of SPS. It is important that the main fluorescence of SPS is a result of a GD because SPS is expected to be a host polymer capable of orientating clathrate opto-functional guest molecules.

Electromigration properties of capillary columns with polystyrene coating as a stationary phase

Relationships in variation of the surface energy of polystyrene coatings on melted quartz at varied polymer concentration in solution were refined by studying the functional chemical composition and surface structure of model coatings of atactic polystyrene on quartz plates and the electromigration properties of capillary columns with polystyrene coatings. The optimum concentration and temperature-kinetic conditions for preparing polystyrene coatings on quartz capillaries with controllable electrokinetic properties were determined.

Raman spectroscopy of laser shocked polystyrene

We have carried out Raman spectroscopic measurements on atactic polystyrene (aPS) under shock compression up to 2 GPa. Observed wavenumber shifts of Raman active modes have been used to determine their Grüneisen parameters and bond anharmonicities. Raman mode at 798 cm−1 displays maximum blue shift of 4.95 cm−1/GPa. Our observed wavenumber shifts under shock compression are found to be consistent with the earlier results obtained under hydrostatic pressure. We have also evaluated pressure through 1D radiation hydrodynamic simulations, and the results are found to be in good agreement with the experimental values. The pressure induced wavenumber shifts and line width changes of the present study can be utilized as a gauge for pressure measurement because of laser induced shock front. Copyright © 2016 John Wiley & Sons, Ltd.

Statistics of strength distribution upon the start of adhesion between glassy polymers

For the first time, the strength distribution upon the bonding onset between the contacting glassy polymer samples has been analysed by employing the Weibull statistics. It has been found that the distribution of the lap-shear strength (σ) developed between the contacting pieces of such representative amorphous polymers as atactic polystyrene (PS) and poly(methyl methacrylate) (PMMA) at the time-temperature healing conditions at which the interface bonding starts follows the Weibull distribution function. It has been shown that the σ distributions for the compatible PS-PS and PMMA-PMMA and incompatible PS-PMMA interfaces are not affected by the factor of thermodynamic compatibility of the PS and PMMA segments. The estimated values of the Weibull modulus m and of the scale parameter σ0 for the weak polymer-polymer interfaces investigated have been compared with those estimated for the tensile strengths of the PS and PMMA monolithic bulk samples which were used for the interface healing experiments and of the ultra-high-strength inorganic quasi-brittle materials reported in the literature.

Molecular modeling and simulation of atactic polystyrene/amorphous silica nanocomposites

The local structure, segmental dynamics, topological analysis of entanglement networks and mechanical properties of atactic polystyrene - amorphous silica nanocomposites are studied via molecular simulations using two interconnected levels of representation: (a) A coarse - grained level. Equilibration at all length scales at this level is achieved via connectivity - altering Monte Carlo simulations. (b) An atomistic level. Initial configurations for atomistic Molecular Dynamics (MD) simulations are obtained by reverse mapping well- equilibrated coarse-grained configurations. By analyzing atomistic MD trajectories, the polymer density profile is found to exhibit layering in the vicinity of the nanoparticle surface. The dynamics of polystyrene (in neat and filled melt systems) is characterized in terms of bond orientation. Well-equilibrated coarse-grained long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm. Atomistic simulation results for the mechanical properties are compared to the experimental measurements and other computational works.

Highly efficient and selective pressure-assisted photon-induced polymerization of styrene

The polymerization process of condensed styrene to produce polystyrene as an industrially important polymeric material was investigated using a novel approach by combining external compression with ultraviolet radiation. The reaction evolution was monitored as a function of time and the reaction products were characterized by in situ Fourier transform infrared spectroscopy. By optimizing the loading pressures, we observed highly efficient and selective production of polystyrene of different tacticities. Specifically, at relatively low loading pressures, infrared spectra suggest that styrene monomers transform to amorphous atactic polystyrene (APS) with minor crystalline isotactic polystyrene. In contrast, APS was found to be the sole product when polymerization occurs at relatively higher loading pressures. The time-dependent reaction profiles allow the examination of the polymerization kinetics by analyzing the rate constant and activation volume as a function of pressure. As a result, an optimized pressure condition, which allows a barrierless reaction to proceed, was identified and attributed to the very desirable reaction yield and kinetics. Finally, the photoinitiated reaction mechanism and the growth geometry of the polymer chains were investigated from the energy diagram of styrene and by the topology analysis of the crystal styrene. This study shows strong promise to produce functional polymeric materials in a highly efficient and controlled manner.

Investigation of interphase effects in silica-polystyrene nanocomposites based on a hybrid molecular-dynamics-finite-element simulation framework.

A recently developed hybrid method is employed to study the mechanical behavior of silica-polystyrene nanocomposites (NCs) under uniaxial elongation. The hybrid method couples a particle domain to a continuum domain. The region of physical interest, i.e., the interphase around a nanoparticle (NP), is treated at molecular resolution, while the surrounding elastic continuum is handled with a finite-element approach. In the present paper we analyze the polymer behavior in the neighborhood of one or two nanoparticle(s) at molecular resolution. The coarse-grained hybrid method allows us to simulate a large polymer matrix region surrounding the nanoparticles. We consider NCs with dilute concentration of NPs embedded in an atactic polystyrene matrix formed by 300 chains with 200 monomer beads. The overall orientation of polymer segments relative to the deformation direction is determined in the neighborhood of the nanoparticle to investigate the polymer response to this perturbation. Calculations of strainlike quantities give insight into the deformation behavior of a system with two NPs and show that the applied strain and the nanoparticle distance have significant influence on the deformation behavior. Finally, we investigate to what extent a continuum-based description may account for the specific effects occurring in the interphase between the polymer matrix and the NPs.

Effects of three-segment interactions on the second virial coefficient of ring polymers in the Θ state

The first-order perturbation calculation is carried out of the second virial coefficient A2 of the phantom Gaussian and Kratky–Porod (KP) wormlike rings without inter- and intramolecular topological constraints with consideration of the ternary-cluster integral β3 in addition to the binary-cluster integral β2. The behavior of the residual contribution of β3 to A2 of the KP rings is examined as a function of the reduced total contour length λL as defined as the total contour length L divided by the stiffness parameter λ−1. From a comparison of the present theoretical result with experimental data, it is found that the residual contribution of β3 to A2 is negligibly small for ring atactic polystyrene in cyclohexane at Θ in the range of the molecular weight from 1 × 104 to 6 × 105. The first-order perturbation calculation is carried out of the second virial coefficient A2 of the phantom Gaussian and Kratky–Porod wormlike rings without inter- and intramolecular topological constraints with consideration of the ternary-cluster integral β3 in addition to the binary-cluster integral β2. From a comparison of the present theoretical result with experimental data, the residual contribution of β3 to A2 is shown to be negligibly small for ring atactic polystyrene in cyclohexane at Θ in the range of the molecular weight M from 1 × 104 to 6 × 105.

On the molecular mechanism of self-healing of glassy polymers

The chain ends concentrations in a near-surface nanometre-thick layer of such representative amorphous polymers as atactic polystyrene (PS), poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and poly(methyl methacrylate) (PMMA) have been estimated. It has been shown that the number of the chain ends located in that layer is sufficient to give rise to the development of the intermolecular van-der-Waals bonds via the interdiffusion of the chain ends across the interface between the two contacting polymer samples, even at the healing temperatures that are lower by some tens or even a hundred degrees of Kelvin with respect to the bulk glass transition temperatures, making the interface capable to bear the mechanical load. The validity of the molecular mechanism of self-healing of amorphous polymers with glassy bulk proposed earlier has been confirmed.

N-Heterocyclic Carbene-Ytterbium Amide as a Recyclable Homogeneous Precatalyst for Hydrophosphination of Alkenes and Alkynes.

The N-heterocyclic carbene-ytterbium(II) amides (NHC)2Yb[N(SiMe3)2]2 (1: NHC: 1,3,4,5-tetramethylimidazo-2-ylidene (IMe4); 2: NHC: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr)) and the NHC-stabilized rare-earth phosphide (IMe4)3Yb(PPh2)2 (3) have been synthesized and fully characterized. Complexes 1-3 are active precatalysts for the hydrophosphination of alkenes, alkynes, and dienes and exhibited much superior catalytic activity to that of the NHC-free amide (THF)2Yb[N(SiMe)2]2. Complex 1 is the most active precursor among the three complexes. In particular, complex 1 can be recycled and recovered from the reaction media after the catalytic reactions. Furthermore, it was found that complex 3 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights. To the best of our knowledge, complex 1 represents the first rare-earth complex that can be recovered after catalytic reactions.