Amorphous Atactic Polystyrene Research Articles

Physical Ageing of Polystyrene: Does Tacticity Play a Role?

The ageing kinetics of amorphous atactic (a-PS), isotactic (i-PS), and syndiotactic (s-PS) polystyrene were studied by means of flash-differential scanning calorimetry. The specimens were aged for up to 2 h at six different ageing temperatures: the optimum ageing temperature, that is, the temperature at which the enthalpy overshoot at the glass transition is maximal for the given elapsed time, and five ageing temperatures ranging from 20 to 80 K below the optimum ageing temperature. A logarithmic increase of the enthalpy overshoot with ageing time is observed for specimens at their optimum ageing temperatures. For temperatures significantly lower than the optimum, there is a range where the enthalpy overshoot is constant, but for higher temperatures (still below the optimum), a logarithmic increase is also observed. Moreover, the ageing kinetics appear to depend on tacticity, with s-PS and i-PS exhibiting the slowest and fastest ageing kinetics, respectively, and a-PS exhibiting ageing kinetics between these two extremes.

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.

Dynamics of Water Near Oxidized Polystyrene Films

Atomistic MD simulations of water in the vicinity of oxidized amorphous atactic polystyrene are presented. The changes in the orientational and translational dynamics of water near polymer surfaces with different hydrophilicity are studied. Two main orientational relaxation processes of water molecules are distinguished: a process on a fs timescale, associated with the ballistic motion of water molecules, and a process on a ps timescale, associated with the self-diffusion of water. The fast process is not affected by the presence of the polymeric surface. The second relaxation process differs at the interface from that in the bulk in that the dynamics of water molecules is more heterogeneous in the first. The effect of the representation of polystyrene films on the water dynamics is discussed.

Self Assembling and Coordination of Water Nano-Layers On Polymer Coated Long Period Gratings: Toward New Perspectives for Cation Detection

In this work, polymeric coated Long Period Gratings (LPGs) working in transition mode have been used to monitor the coordination and self assembling of water nano-layers (typical thicknesses range in few tens of nanometers) providing new scenarios in chemical sensing applications. In particular, nano-scale layers (∼320 nm) of semicrystalline syndiotactic and amorphous atactic polystyrene (sPS and aPS), have been deposited by dip-coating onto LPGs to tune the devices at the transition point. Experimental results demonstrate the polymers capability to orient water molecules in proximity of their surfaces. The sPS and aPS interactions with water have been continuously monitored and then compared demonstrating the higher capability of the crystalline phase of sPS to orient water nano-layers. Moreover, the high sensitivity of the coated LPGs was used to monitor the effect of disorder induced on the interfacial water molecular arrangement by different cations (sodium, Na+, potassium, K+, and calcium, Ca2+, ions) depending on their size and electrical charge. Experimental results show for the first time that, thanks to the water-polymer interaction, sPS coated LPGs could be successfully employed as high sensitivity cation sensors. In fact, the monitoring of the disorder induced by cations on the coordinated water layer leads to high sensitivities, in terms of detected RI change for unitary variation of concentration (∼ for Na+ ions, ∼ for K+ ions, and ∼ for Ca2+ ions).

Rheological and conductive percolation laws for syndiotactic polystyrene composites filled with carbon nanocapsules and carbon nanotubes

Semicrystalline syndiotactic polystyrene (sPS) composites with carbon nanocapsule (CNC) and carbon nanotube (CNT) fillers were prepared and good filler dispersion confirmed by electron microscopy. Their rheological and electrical properties were investigated to reveal the effect of filler aspect ratio. Amorphous atactic polystyrene (aPS) was used to prepare composites with a CNT filler to elucidate the effect of matrix tacticity. Percolation scaling laws are applied and the threshold concentration and exponent are determined. Above a threshold, the magnitudes of storage modulus ( G′) and conductivity are related to the level of percolation network as well as the intrinsic properties of the matrix and filler. Master curves are obtained provided that an appropriate percolation function is selected. Different scaling laws are validated for the G′ and conductivity results. Composites with CNTs show a much lower threshold than those with CNCs. A lower threshold is derived from the G′ results compared to that obtained from the conductivity data regardless of the filler aspect ratio and matrix tacticity. Owing to the pronounced nucleating effects of CNT, crystalline sPS composites exhibit a four times larger conductivity threshold compared to their amorphous aPS counterparts, although their rheological thresholds are similar.

Anisotropy of the Thermal Conductivity of Stretched Amorphous Polystyrene in Supercritical Carbon Dioxide Studied by Reverse Nonequilibrium Molecular Dynamics Simulations

The thermal conductivity (lambda) of stretched amorphous atactic polystyrene (PS) swollen in supercritical carbon dioxide (sc CO(2)) has been investigated over a wide temperature, pressure, and concentration range. Nonequilibrium molecular dynamics simulations with a full atomistic force-field have been employed to calculate the thermal conductivity of neat stretched PS and of different mixtures of supercritical CO(2) with stretched PS. As the energy transport in PS parallel and perpendicular to the stretching direction differs, an anisotropy in the thermal conductivity occurs. The magnitude of lambda is enhanced with an increasing number of carbon-carbon backbone bonds oriented parallel to the direction of the heat transport. The degrees of freedom in the side chain of the polymer are rather unimportant for the thermal conductivity. To understand the conditions leading either to an equivalence or nonequivalence of the system degrees of freedom for the heat transport, we have analyzed lambda of PS, CO(2), binary PS-CO(2) mixtures and other model systems as a function of the bond constraints in the computational model. Furthermore, we have commented on differences in the thermal conductivity provided either by a vibrational energy transfer or by collisions.

Thermal Conductivity of Amorphous Polystyrene in Supercritical Carbon Dioxide Studied by Reverse Nonequilibrium Molecular Dynamics Simulations

The thermal conductivity of amorphous atactic polystyrene (PS) swollen in supercritical carbon dioxide (sc CO(2)) has been investigated over wide temperature, pressure, and concentration ranges. Nonequilibrium molecular dynamics simulations with a full atomistic force field have been used to calculate the thermal conductivity of neat PS and sc CO(2) as well as of the binary system at different compositions. An analytical interpolation formula for the thermal conductivity of the binary mixture on the basis of PS and CO(2) data has been obtained. Particular attention has been paid to the implications of the quasi-degeneracy and finite-size effects in the simulated polymer system. It has been found that, in addition to the degrees of freedom per volume, the orientation of the carbon-carbon bonds in the backbone relative to the direction of the temperature gradient is important for the heat transport in PS.

Nano-friction of polystyrene : the role of the surface chemistry

The purpose of this study is to analyse the influence of the chemical contribution on the nano-friction of polystyrene. The role of interfacial interactions is analysed by comparing friction between the polymer and hydrophobic AFM tip (methyl-terminated grafted layer) and hydrophilic tip (hydroxyl-terminated tip) as a function of sliding velocity and normal force. Nano-friction experiments are achieved by using atomic force microscopy (AFM) in torsion mode. Amorphous atactic polystyrene films are prepared by spin coating a solution onto a smooth and rigid substrate (silicon wafer). Two amorphous atactic polystyrene, varying by their different molecular weights are used. Experimental results show that the friction coefficients measured with hydroxylated tip are always larger than those obtained with non-treated tip (intermediate value) and hydrophobic tip (lower friction), indicating a relationship between nano-friction and interfacial interactions. The dependence of frictional force on velocity is consequent, with a great increase of friction with speed in the case of the hydroxylated tip. A higher friction is obtained for the higher molecular weight polystyrene in contact with hydroxylated tips. However, differences between both polymers become negligible in the case of non-treated and hydrophobic tips.

Polymeric sensing films absorbing organic guests into a nanoporous host crystalline phase

Syndiotactic polystyrene (s-PS) semicrystalline films, including the nanoporous crystalline δ phase, have been tested as sensing elements of resonant sensors for vapors of volatile organic compounds (VOCs). In particular, the response to chloroform vapor of quartz crystal microbalance (QCM) sensors, coated with films of semicrystalline δ form of s-PS have been analyzed and compared to analogous systems coated with films of amorphous atactic polystyrene (a-PS). The sensitivity of sensors based on s-PS films was found markedly higher than those based on a-PS, particularly for low chloroform pressures. The higher sensitivity of the semicrystalline s-PS films is associated with a peculiar sorption mechanism: in fact, the organic compound, rather than being dissolved only into the amorphous phase, as it is generally the case for semicrystalline polymers, is mainly absorbed into the nanoporous crystalline phase, each molecule being confined into regularly spaced crystalline nanocavities, thus leading to a clathrate structure.

Correlated Segmental Dynamics in Amorphous Atactic Polystyrene: A Molecular Dynamics Simulation Study

Molecular dynamics simulations of bulk atactic polystyrene have been performed in a temperature range from 100 to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long-time asymptotic slope of these dependencies is 0.54 at T > Tg, showing Rouse behavior. Crossover from motion in the cage to Rouse-like dynamics has been studied at T > Tg with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode-coupling theory (MCT). We studied the coherent intermediate scattering function and show the agreement with scaling predictions of MCT. Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs (Legendre polynomials of the first and second order), of both the main-chain and side-group bonds. The ACF relaxation times analysis shows strong coupling between the phenyl groups and the backbone relaxation. The relaxation times ...

Large-scale computer simulation of local segmental dynamics in amorphous atactic polystyrene

Abstract Molecular dynamics computer simulations of amorphous atactic polystyrene have been performed for chains of up to 320 monomer units in a temperature range from 100 to 600 K and in a pressure range from 0.1 to 1000 MPa. The MD-determined values of the glass transition temperature T g are in a good agreement with experimental PVT data at different values of applied pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers as a function of time. The long-time asymptotic slope of these dependencies is close to 0.6 at T > T g , showing diffusive behaviour. The cage effect when local translational motions are essentially frozen in the glassy state has been studied. Local orientational mobility is studied with the help of Legendre polynomials of the first, P 1 ( t ), and second, P 2 ( t ), order for the bonds in the main chain and along the phenyl side groups. Temperature dependencies of the orientational relaxation times at different values of pressure are well described by the Vogel–Fulcher law.

Orientation of Phenyl Rings and Methylene Bisectors at the Free Surface of Atactic Polystyrene

Atomistically detailed models of free-standing thin films and the bulk structure of amorphous atactic polystyrene have been produced by reverse mapping from equilibrated coarse-grained models. The bridging technique employed in the simulation allows the generation of a moderate sized atomistic system (six independent parent chains of C400H402, 4812 atoms) with a more reasonable computational effort than is required when all of the construction is performed on chains expressed with atomistic detail. Reverse mapping from the coarse-grained model to the atomistically detailed model is found to be straightforward, without ring piercing or concatenation. The calculated surface energy (38 ± 10 erg/cm2) is in reasonable agreement with prior experimental findings. The surface of the thin films is enriched in phenyl rings. The rings at the surface tend to be oriented so that they are pointing outward, but rings in the middle of the thin film show no preferred orientation. In contrast with the phenyl rings, the bis...

Viscoelasticity and birefringence of syndiotactic polystyrene. I. Dynamic measurement in supercooled state

Dynamic viscoelasticity and dynamic birefringence measurements were performed on a quenched amorphous sample of a syndiotactic polystyrene, sPS. The measurements were carried out at temperatures where crystallization did not occur above the glass transition temperature. The data were analyzed with a modified stress-optical rule: The complex Young's modulus was separated into two components, designated with subscripts R and G, for which the ordinary stress-optical rule held well individually. A comparison with the results for a conventional amorphous atactic polystyrene has shown that the stereoregularity of sPS enhances the stiffness of main chain and then increases the optical anisotropy of segments. This conclusion is in accord with theoretical calculation on coil dimension and optical anisotropy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 399–404, 1999

On the multiple melting behavior of polymorphic syndiotactic polystyrene and its behavior in a miscible state

Differential scanning calorimetry (DSC) and X-ray diffraction analysis were performed to study the multiple melting behavior and crystal forms of neat syndiotactic polystyrene (s-PS) in comparison with its miscible blends with amorphous atactic polystyrene (a-PS) or poly(2,6-dimethyl-p-phenylene oxide) (PPO). For neat s-PS annealed at 242 to 250°C, three melting peaks of changing intensity were observed (labeled I, II, and III from low to high temperature), while if annealed at high temperatures (254°C or higher) only two melting peaks were left. The missing peak was determined to be Peak-III for s-PS annealed at high temperatures. For the s-PS/a-PS or s-PS/PPO blends, two melting peaks were observed, and the missing peak is Peak-II. The missing Peak-II in the blend systems is attributed to the β-modification. Apparently, the condition of s-PS being compatible with a-PS or PPO is unfavorable for generating the β-modification upon melt crystallization. This study also showed that Peak-I is associated with the β-modification. Peak-III may be associated with the less stable β′ form, which can be transformed to the β form upon annealing at high temperatures. For the s-PS/a-PS or s-PS/PPO blends upon annealing at higher temperatures, Peak-III quickly decreases, leaving only a single melting peak (Peak-I) for the blends. The phenomenon of the decrease of Peak-III crystals in the blend is similar to that for neat s-PS annealed at high temperatures.

On the multiple melting behavior of polymorphic syndiotactic polystyrene and its behavior in a miscible state

Differential scanning calorimetry (DSC) and X-ray diffraction analysis were performed to study the multiple melting behavior and crystal forms of neat syndiotactic polystyrene (s-PS) in comparison with its miscible blends with amorphous atactic polystyrene (a-PS) or poly(2,6-dimethyl-p-phenylene oxide) (PPO). For neat s-PS annealed at 242 to 250°C, three melting peaks of changing intensity were observed (labeled I, II, and III from low to high temperature), while if annealed at high temperatures (254°C or higher) only two melting peaks were left. The missing peak was determined to be Peak-III for s-PS annealed at high temperatures. For the s-PS/a-PS or s-PS/PPO blends, two melting peaks were observed, and the missing peak is Peak-II. The missing Peak-II in the blend systems is attributed to the β-modification. Apparently, the condition of s-PS being compatible with a-PS or PPO is unfavorable for generating the β-modification upon melt crystallization. This study also showed that Peak-I is associated with the β-modification. Peak-III may be associated with the less stable β′ form, which can be transformed to the β form upon annealing at high temperatures. For the s-PS/a-PS or s-PS/PPO blends upon annealing at higher temperatures, Peak-III quickly decreases, leaving only a single melting peak (Peak-I) for the blends. The phenomenon of the decrease of Peak-III crystals in the blend is similar to that for neat s-PS annealed at high temperatures.

Shift-independent nuclear spin diffusion by slow magic-angle sample spinning for the exploration of solids

An improved spin-diffusion experiment for the exploration of local order in solids is described. By slow magic-angle sample spinning, it is possible to remove the dependence of the spin diffusion rate on the chemical shift difference between two interacting nuclear spins, while the informative chemical shielding anisotropy is fully retained. Slow magic-angle spinning shortens the time necessary for the spin system to reach the quasi-equilibrium state, and it allows for rapid spin diffusion also in the absence of heteronuclear dipolar interactions in contrast to static or rapidly spinning samples. The feasibility of the experiment is demonstrated for a sample of carbon-13 labelled amorphous atactic polystyrene.

Characterization of Local Order in Atactic Polystyrene Using Two-Dimensional Nuclear Magnetic Resonance and Atomistic Simulations

The local ordering of the phenyl groups in amorphous atactic polystyrene is investigated by radio-frequency-driven carbon-13 NMR polarization transfer (spin diffusion) between site-selectively carbon-13 labeled monomer units. The NMR data agree with atomistic simulations of atactic amorphous polystyrene where orientational correlations among the phenyl rings do not extend over distances larger than 5 A. This finding is at variance with earlier X-ray and neutron diffraction measurements.

Thermal interactions in oriented polymeric materials: Shrinkage, crystallization, and melting

AbstractThermal analysis of oriented polymers involves several transitional processes which may be coupled in a complex way. The disorientation process that occurs during heating is often accompanied by dimensional shrinkage which may occur in both amorphous and crystalline oriented polymers. From differential scanning calorimetry (DSC) measurements it has been determined that shrinkage is an exothermic process. In amorphous atactic polystyrene (PS), hot drawn uniaxially, the shrinkage occurs just above Tg. In triaxially oriented polyethylene (PE) and isotactic polypropylene (iPP), shrinkage and melting take place concomitantly, producing multiple melting peaks. In rolltruded (and also in biaxially oriented) poly(ethyleneterephthalate) (PET) films, shrinkage and crystallization are found to be coupled and to occur between Tg and Tm upon heating. If the enthalpy contribution due to shrinkage is ignored, it follows that crystallinities determined by DSC are incorrect. In this study, the influence of shrinkage upon transitional changes in oriented polymers is investigated in a systematic manner.

Ｘ線回折法による非晶弾性率の測定に関する研究 Ｉ Ｘ線回折法によるアタクチックポリスチレンの非晶弾性率の測定

典型的な非晶性高分子であるアタクチック・ポリスチレン (PS) を試料として取り上げ, X線回折における非晶ハローのピーク位置の応力による移動から非晶弾性率の測定を行い, 以下の結果を得た. 熱履歴の異なる4種類の試料について, 分子鎖間の凝集エネルギーに対応する非晶弾性率 (Ea) として1.9×104kg/cm2から2.3×104kg/cm2の値が得られた. 示差走査熱量計 (DSC) サーモグラム, 密度, 結晶弾性率, との関係を検討した結果, これらの値が分子鎖のパッキングの点から説明できることが明らかになった. また, 試料全体の弾性率は分子鎖間の凝集エネルギーと分子鎖骨格構造の変化に対応しているものと考えられた.