Molecular Weight Polystyrene Research Articles

Dramatic Tunability of the Glass Transition Temperature and Fragility of Low Molecular Weight Polystyrene by Initiator Fragments Located at Chain Ends

Important, yet unexplored effects of chemically distinct initiator fragments incorporated at chain ends in linear polymer are investigated in depth. Polystyrene (PS) samples of a wide range of molecular weight (MW) were synthesized by conventional free radical polymerization and controlled radical polymerization using seven different initiators and compared with anionically polymerized PS. The initiator fragments incorporated during polymerization have major consequences on the glass transition temperature (Tg) and dynamic fragility of low MW PS. For example, with ∼4 kg/mol PS, the Tg onset value and fragility can be tuned from ∼334 K and ∼65, respectively, with dodecanethiol and hydrogen atom chain ends to ∼367 K and ∼130, respectively, with cyanopentanoic acid chain ends. A similar high Tg and high fragility were measured with isobutyric acid/SG1 nitroxide chain ends. These remarkable effects, with a greater than 30 K difference in Tg and a factor of 2 difference in fragility, indicate that chain ends i...

Stress relaxation of bi-disperse polystyrene melts

We present start-up of uniaxial extension followed by stress relaxation experiments of a bi-disperse 50 % by weight blend of 95k and 545k molecular weight polystyrene. We also show, for comparison, stress relaxation measurements of the polystyrene melts with molecular weight 95k and 545k, which are the components of the bi-disperse melt. The measurements show three separated relaxation regimes: a fast regime, a transition regime, and a slow regime. In the fast regime, the orientation of the long chains is frozen and the stress relaxation is due to stretch relaxation of the short chains primarily. Conversely in the slow regime, the long chains have retracted and undergo relaxation of orientation in fully relaxed short chains.

Numerical Simulation of Phase Separation Kinetic of Polymer Solutions Using the Spectral Discrete Cosine Transform Method

ABSTRACTA spectral discrete cosine transform (DCT) method was used to solve numerically the spatio-temporal nonlinear Cahn-Hilliard equation for a temperature-induced phase separation process of a polymer solution. The properties of a high molecular weight polystyrene solution were used in the model to reflect the behavior of a real world polymer system. Based on the value of the initial concentration, three different final morphologies of the phase separated system, granular, interconnected, and microcellular, were identified. It was shown that the initial concentration is the main factor to determine the high rate of the phase separation when the phase separation time is the primary desired parameter. The degree of phase separation, as a quantitative measure of the phase separation process, indicates the rate and amount of separated polymer material for practical applications. The model is capable of providing quantitative information of morphology evolution during phase separation processes for microstructure control purposes. The structure factor profile was extracted as a theoretical output that enables comparison with scattering experiments observations as the finger-print of phase separation morphological evolution. It is shown that the DCT method is a very suitable and feasible method for solution of the nonlinear partial differential equation of phase separation kinetics.

Manipulating Polymer Molecular Weight Distribution

A typical polymer sample contains a variety of chain sizes. The average molecular weight of these chains, as well as the breadth and shape of their distribution, can influence bulk polymer properties. Adjusting how quickly a polymerization initiator is added can help tune that distribution, report Dillon T. Gentekos, Lauren N. Dupuis, and Brett P. Fors of Cornell University (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.5b13565). Working with polymerization of styrene initiated by an alkyl nitroxide, the team added the initiator in various ways. Compared with adding the initiator all at once, adding it steadily over as long as two-and-a-half hours broadened the polystyrene molecular weight distribution: The width increased for longer addition times, but the average chain length stayed the same. Adding the initiator at an increasing rate shifted the shape of the distribution curve, indicating a greater proportion of chains with lower molecular weight. And adding the initiator

Fabrication of cage-like particles via unstable seeded dispersion polymerization: A new concept in the polymerization-induced self-assembly

ABSTRACTThe formation mechanism of hollow micron-sized polystyrene (PS) particles having numerous dents on the surface, so-called cage-like particles, obtained from seeded dispersion polymerization (SDP) of 2-ethylhexyl methacrylate (EHMA) with low molecular weight (MW) PS particles stabilized by poly(vinyl alcohol) (PVA) in the presence of hexadecane droplets was investigated. It was found that association of poly(2-ethylhexyl methacrylate) (PEHMA)/hexadecane phases which occurs due to the instability of the obtained composite particles followed by a diffusion of PS ellipsoidal particles into each other is the main process responsible for the production of such unique morphology. Time course monitoring of the SDP showed that diffusion of hexadecane and/or PS and/or PEHMA phase into PS/PEHMA/hexadecane composite particles through PS shell which happens based on Ostwald ripening is the main phenomenon which results in the formation of the dents on the surface of final particles. Moreover, the experimental results revealed that in this reaction system, the polymerization develops in a faster manner rather than the SDP employing seed particles having higher MWs. Furthermore, it was observed that particles with different surface morphologies can be produced by using different hydrocarbons. The elimination of small particles which are produced in addition to the cage-like ones via decreasing the concentration of the stabilizer was another interesting finding of this research. The acquired results showed that unstable SDP is expected to be a new concept in polymerization-induced self-assembly (PISA) which employs instability of a dispersion for self-assembly of polymeric particles, and therefore, production of polymeric unique objects.

Impact of Polystyrene Oligomer Side Chains on Naphthalene Diimide–Bithiophene Polymers as n‐Type Semiconductors for Organic Field‐Effect Transistors

A series of naphthalene diimide‐based conjugated polymers are prepared with various molar percentage of low molecular weight polystyrene (PS) oligomer of narrow polydispersity as the side chain. The PS side chains are incorporated through preparation of a macromonomer by chain termination of living anionic polymerization. The effects of the PS side chains amount (0–20 mol%) versus overall sidechain on the electrical properties of the resulting polymers as n‐type polymer semiconductors in field‐effect transistors are investigated. We observe that all the studied polymers show similarly high electron mobility (≈0.2 cm2 V−1 s−1). Importantly, the polymers with high PS side chain content (20 mol%) show a significantly improved device stability under ambient conditions, when compared to the polymers at lower PS content (0–10 mol%). By comparing this observation to the physical blending of the conjugated polymer with PS, we attribute the improved stability to the covalently attached PS side chains potentially serving as a molecular encapsulating layer around the conjugated polymer backbone, rendering it less susceptible to electron traps such as oxygen and water molecules.

Thermal Stability and Crystallinity Study of Polystyrene/SiO<sub>2</sub> Nano-Composites Synthesis via Microwave-Assisted <i>In Situ</i> Polymerization

Serials of polystyrene/SiO2 Nano composites (PS/SiO2) with different content of inorganic fillers were successfully prepared by the in situ bulk radical polymerization of styrene under microwave irradiation. The effect of the amount of Nano SiO2 on the properties of the PS/SiO2 Nanocomposites along with the average relative molecular masses (Mn, Mz and Mw) was investigated by thermal analysis and X-Ray Diffraction (XRD). Their structural model was proposed on the basis of the Optical Microscopy, FTIR (Fourier Transform Infrared) analysis, differential scanning calorimetry (DSC), gel permeation chromatography (GPC) and X-Ray Diffraction (XRD). The dispersion of nanoparticles in Polystyrene is observed in the magnified image. The effect of microwave irradiation power on molecular weight of polystyrene was also studied. It was found that, the microwave assisted reaction needs less time as compare to conventional polymerization and found to be in between 10 to 15 min.

A synthetic route to ultra-high molecular weight polystyrene (>106) with narrow molecular weight distribution by emulsifier-free, emulsion organotellurium-mediated living radical polymerization (emulsion TERP)

We propose a route to synthesizing ultra-high molecular weight (>106) polystyrene (PS) having a narrow molecular weight distribution by controlled/living radical polymerization.

A perspective on experimental findings and theoretical explanations of novel dynamics at free surface and in freestanding thin films of polystyrene

Most studies of the dynamics at the surface and in thin films of polystyrene (PS) are focused on the change of glass transition temperature from the bulk value. In this perspective, we highlight three new developments in research on the dynamics of PS in high molecular weight (MW) freestanding PS thin films and at the surface of low MW PS. Novel findings from these developments require consideration of viscoelastic mechanisms with length scales longer than the segmental α-relaxation. The first development is the creep compliance measurements of high-MW PS thin films, probing not only the segmental α-relaxation, but also the polymer chain modes at higher compliance levels, including the sub-Rouse modes and the Rouse modes. The compliance data indicate the relaxation times of the sub-Rouse modes are reduced in thinner films like that of the segmental α-relaxation but to a much less extent. The second development is the novel observation of two glass transitions in freestanding polystyrene thin films by ellipsometry. The upper and lower glass transitions occurs, respectively, at temperatures and , both are below the bulk glass transition temperature. While the lower transition at is associated with the segmental α-relaxation, the only viable explanation of the origin of the upper transition at is from another intrinsic viscoelastic mechanism of PS, and not at different location of the film. Supported by various experiments on PS and other polymers, we show that the sub-Rouse modes are cooperative and coupled to density, and hence giving rise to the upper glass transition in freestanding PS films. The sub-Rouse relaxation times will increase on physical ageing, and bring along an increase in density of the freestanding film due to the coupling. This prediction can be checked by performing ageing experiment. The third development is the reduction of viscosity at the free surface of low MW PS. Since viscous flow of low MW PS is definitely carried out by the sub-Rouse modes and not by the segmental α-relaxation, the experimental finding is direct evidence of enhancement of mobility of the sub-Rouse modes by the mitigation of intermolecular coupling at the surface, consistent with the explanations given for the findings in the two other developments. The enhancement of mobility of the sub-Rouse modes occurs simultaneously with the same effect on the segmental α-relaxation. Notwithstanding, at the surface, the observed reduction of viscosity from the sub-Rouse modes is significantly smaller than the reduction of the segmental relaxation time, which is explained semiquantitatively. Altogether, the three recent advances in the study of dynamics of polymer thin films and at the free surface have shown not only the change of the glass transition temperature effected by the segmental α-relaxation is interesting, but also that of the sub-Rouse modes.

Permeability of Rubbery and Glassy Membranes of Ionic Liquid Filled Polymersome Nanoreactors in Water.

Nanoemulsion-like polymer vesicles (polymersomes) having ionic liquid interiors dispersed in water are attractive for nanoreactor applications. In a previous study, we demonstrated that small molecules could pass through rubbery polybutadiene membranes on a time scale of seconds, which is practical for chemical transformations. It is of interest to determine how sensitive the rate of transport is to temperature, particularly for membranes in the vicinity of the glass transition (Tg). In this work, the molecular exchange rate of 1-butylimidazole through glassy polystyrene (PS) bilayer membranes is investigated via pulsed field gradient nuclear magnetic resonance (PFG-NMR) over the temperature range from 25 to 70 °C. The vesicles were prepared by the cosolvent method in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]), and four different polystyrene-b-poly(ethylene oxide) (PS-PEO) diblock polymers with varying PS molecular weights were examined. The vesicles were transferred from the ionic liquid to water at room temperature to form nanoemulsion solutions of polymer vesicles in water. The exchange rate of 1-butylimidazole added to the aqueous solutions was observed under equilibrium conditions at each temperature. The exchange rate decreased as the membrane thickness increased, and the exchange rate through the glassy membranes was three to four times slower than through the rubbery polybutadiene membranes under the same experimental conditions. These results demonstrate that the permeability through nanosized membranes depends on both the dimension and chemistry of membrane-forming blocks. Furthermore, the exchange rate was investigated as a function of temperature in the vicinity of the Tg of PS-PEO membranes. The exchange rate, however, is not a strong function of the temperature in the vicinity of the membrane Tg, due to a combination of the nanoscopic dimension of the membrane, and some degree of solvent plasticization.

Poly(3-hexylthiophene)/polystyrene (P3HT/PS) blends based organic field-effect transistor ammonia gas sensor

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(3-hexylthiophene) (P3HT) blended with polystyrene (PS) as a semiconductor layer were fabricated. An optimized composition of 1.6wt% P3HT in PS matrix exhibited the best performance to various concentrations of NH3, which is comparable to that of pure P3HT (8wt%). The results showed the percentage responses of saturation current were 52% and 16% under 50ppm and 5ppm NH3, respectively. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH3 gas. By analyzing the morphologies of blend films and the electrical characteristics of OFET sensors, it was found that the film of P3HT blended with PS has more interface to interact with NH3, resulting in more efficient detection to NH3 even in the range of low concentration. Besides, the PS matrix would prevent the gas from diffusing into the semiconductor/dielectric interface directly, which was beneficial to the selectivity of P3HT/PS blend OFET sensor. Moreover, the sensing property was related to the solvents and molecular weight of PS. In addition, the environmental stability of OFET sensors was measured after storing the sensors under ambient atmosphere for 40 days, and the device with the blend semiconducting layer exhibited the superior stability.

Molecular based prediction of the extensional rheology of high molecular weight polystyrene dilute solutions: A hi-fidelity Brownian dynamics approach

A highly efficient Brownian dynamics algorithm for simulation of bead-spring chain micromechanical models that utilizes the Krylov framework and the semi-implicit predictor-corrector scheme is used to study the behavior of dilute solutions of high molecular weight polystyrene in uniaxial extensional flow. The influence of key parameters, namely, appropriate inclusion of hydrodynamic interactions (HI) and excluded volume (EV), the level of fine-graining, and the flow strength on the observed extensional hardening of the dilute solutions over a broad molecular weight range is considered. Specifically, it is demonstrated that the combination of HI and successive fine-graining results in very good predictions of rheological properties of solutions containing 1.95, 3.9, and 10.2 × 106 molecular weight macromolecules. However, for the highest molecular weight system, namely, 20 × 106, some level of discrepancy between experiment and the simulation results is observed. Finally, the incorporation of EV in order to predict material functions that are more consistent with the measurements for the highest molecular weight macromolecule solution is discussed.

Fracture of molecular glasses under tension and increasing their fracture resistance with polymer additives

The fracture of single-component molecular glasses has been characterized under tension. In-plane tension is created by cooling a liquid film below its glass transition temperature constrained on a substrate that is less thermally expansive. The fracture produces a network structure whose cell size is comparable to film thickness. For each system studied (indomethacin, o-terphenyl, and sucrose benzoate), the condition of fracture is well described by the fracture models for supported films and a characteristic energy release rate Gc. Agreement is found for a wide range of film thickness (10–250μm) in both open and sandwich geometries. For the molecular glasses studied, Gc≈1J/m2. o-Terphenyl glasses become significantly more resistant to fracture with the addition of polystyrene; at 10wt.%, Gc increases with the molecular weight of polystyrene, by a factor of 5 at 1milliong/mol. The molecular-weight dependence is consistent with an increase of fracture surface area as the crack tip goes around the pervaded volume of each polymer chain encountered (sphere defined by the radius of gyration).

Polystyrene Glasses under Compression: Ductile and Brittle Responses.

Polystyrene of different molecular weights and their binary mixtures are studied in terms of their various mechanical responses to uniaxial compression at different temperatures. PS of Mw = 25 kg/mol is completely brittle until it is above its glass transition temperature Tg. In contrast, upon incorporation of a high molecular weight component, PS mixtures turn from barely ductile a few degrees below its Tg to ductile over 40° below Tg. In the upper limit, a PS of Mw = 319 kg/mol yields and undergoes plastic flow, even at T = -70 °C. The observed dependence of mechanical responses on molecular weight and molecular weight distribution can be adequately rationalized by the idea that yielding and plastic compression are caused by chain networking.

Activators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization of Styrene in the Presence of Hydrophobically Modified Silica Aerogel Nanoparticles

Abstract Pristine hydrophilic silica aerogel nanoparticles surface was functionalized with hexamethyldisilazane (HMDS). Then, the resultant modified nanoparticles were used for in situ polymerization of styrene by activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP) to synthesize well-defined polystyrene nanocomposites. Success surface modification of hydrophilic silica aerogel nanoparticles with HMDS is demonstrated by FTIR spectroscopy and TGA. Nitrogen adsorption/desorption isotherm is applied to examine surface area and structural characteristics of the synthesized hydrophobic silica aerogel nanoparticles. Evaluation of size distribution and morphological studies were also performed by SEM and TEM. Conversion and molecular weight determinations were carried out using gas and size exclusion chromatography respectively. Adding of hydrophobic silica aerogel nanoparticles by 3 wt % results in a decrease of conversion from 89 to 64%. Molecular weight polystyrene chains decreases from 53991 to 42701 g · mol –1 by addition of 3 wt % hydrophobic silica aerogel nanoparticles; however, polydispersity index values increases from 1.22 to 1.6. A peak around 4.15 ppm which originates from hydrogen atom of terminal units of polystyrene chains in proton nuclear magnetic resonance spectra can appropriately demonstrate the living nature of the polymerization. Increasing thermal stability of the nanocomposites is demonstrated by TGA. Differential scanning calorimetry shows a decrease in glass transition temperature from 80.3 to 68.9 ℃ by addition of 3 wt % hydrophobic silica aerogel nanoparticles.

Farther study on the reactivity of cationic dinuclear platinum trihydrido compounds with unsaturated hydrocarbons: Synthesis of μ-hydrido μ-alkylidene complexes, NMR characterization and mechanism insights

The reactivity of a series of well definite dinuclear platinum complexes [Pt2(L–L)2(H)3][X] (L–L = dppe, X = BF4, 1a; L–L = dppp, X = BF4, 1b; L–L = dppb, X = BF4, 1c; L–L = dpase, X = BF4, 1d; L–L = dcype, X = Cl, 1e) with unsaturated hydrocarbons is investigated. Compounds 1a, 1c and 1e react with phenylacetylene under mild experimental conditions affording the corresponding μ-hydrido μ-alkylidene complexes [(L–L)Pt(μ-H) (μ-CHCH2Ph)][X] 2a, 2c and 2e, independently of the steric and electronic properties of the chelating ligand. Otherwise, the reaction of trihydrides 1 with olefins is more sensitive to the ancillary ligand. However, the new compounds [(dpase)2Pt2(μ-H) (μ-CHCH2Ph)][BF4], 2d, and [(dppp)2Pt2(μ-H) (μ-CHCH3)][BF4], 3b, are obtained in satisfactory yields by reaction of 1d and 1b with styrene and ethylene, respectively. Compound 1e reacts only in the presence of a very large excess of styrene, and it gives a complex mixture of platinum compounds containing 2e and high molecular weight polystyrene as main product. Complexes 2 and 3 are discussed on the basis of multinuclear NMR spectra in solution. 1H and 31P{1H} NMR data show that compound 2d can exist in three different forms with different cis-trans- disposition of phosphorous and arsenic atoms in respect of the bridging ligands. Possible mechanisms for the formation of compounds 2 from the reactions with phenylacetylene or styrene are discussed on the ground of experiments with deuterium labelled reagents.

Room-temperature electrical bonding technique based on copper/polystyrene core/shell nanowire surface fastener

At millimeter dimensions or less, the conventional bonding technology tends to suffer from severe performance and reliability degradation. Moreover, the high heating temperature is usually needed. Here, we report a room-temperature electrical surface fastener based on copper/polystyrene core/shell nanowire (NW) arrays. Uniquely, this electrical surface fastener exhibits high macroscopic adhesion strength (∼44.42N/cm2) and low electrical resistance (∼0.75×10−2Ωcm2). Furthermore, it was found that the adhesion strength of this surface fastener can be mediated by the shell thickness and the molecular weight of polystyrene. Finally, the contact mechanics theory was used to explain the adhesion mechanism.

Effect of microdomain structure on the mechanical behavior of binary blends

The relationship between microphase structure and mechanical response of the binary blends consisting of polystyrene-block-polyisoprene-block-polystyrene copolymer and low molecular weight polystyrene has been investigated. Low molecular weight polystyrene was chosen to obtain uniformly solubilized nano-blends without macrophase separation. The specimens were solution-cast by adding different amounts of homo-polystyrene to acquire different microphase structures. Small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and atom force microscopy (AFM) have been used to study the microdomain and grain structure. It is observed that the structural changes in d-spacing and grain size on account of different amounts of polystyrene alter the mechanical behavior in both monotonic tensile and cyclic tests. The elastic and the Mullins effects are strongly sensitive to the changes in d-spacing and grain sizes. Moreover, the sample with bi-continuous structure shows the largest tensile strength and Mullins effect. In addition, the Mooneye-Rivlin phenomenological model was used to evaluate and explore the relationship between the polymer topological networks and the rubber elasticity of these styrenic nano-blends.

Creating Extremely Asymmetric Lamellar Structures via Fluctuation-Assisted Unbinding of Miktoarm Star Block Copolymer Alloys.

We report the creation of highly asymmetric lamellar structures with a well-designed miktoarm star block copolymer of the S(IS')3 type, where S and S' are polystyrenes of different lengths and I is poly(isoprene). The domain spacing can be tuned continuously from 37 nm to over 300 nm when the miktoarm star block copolymer is blended with suitable molecular weight polystyrene homopolymers. Beyond the unbinding transition of the lamellar phase, extremely asymmetric lamellar structures were obtained with up to 97 wt % polystyrene, remarkably leaving the poly(isoprene) layers intact at only 3 wt %!

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.