Hypercrosslinked Additives for Ageless Gas‐Separation Membranes

The loss of internal pores, a process known as physical aging, inhibits the long‐term use of the most promising gas‐separation polymers. Previously we reported that a porous aromatic framework (PAF‐1) could form a remarkable nanocomposite with gas‐separation polymers to stop aging. However, PAF‐1 synthesis is very onerous both from a reagent and reaction‐condition perspective, making it difficult to scale‐up. We now reveal a highly dispersible and scalable additive based on α,α′‐dichloro‐p‐xylene (p‐DCX), that inhibits aging more effectively, and crucially almost doubles gas‐transport selectivity. These synergistic effects are related to the intimately mixed nanocomposite that is formed though the high dispersibility of p‐DCX in the gas‐separation polymer. This reduces particle‐size effects and the internal free volume is almost unchanged over time. This study shows this inexpensive and scalable polymer additive delivers exceptional gas‐transport performance and selectivity.     

Metalloporphyrin‐Based Hypercrosslinked Polymers Catalyze Hetero‐Diels–Alder Reactions of Unactivated Aldehydes with Simple Dienes: A Fascinating Strategy for the Construction of Heterogeneous Catalysts

We describe a novel and intriguing strategy for the construction of efficient heterogeneous catalysts by hypercrosslinking catalyst molecules in a one‐pot Friedel–Crafts alkylation reaction. The new hypercrosslinked polymers (HCPs) as porous solid catalysts exhibit the combined advantages of homogeneous and heterogeneous catalysis, owing to their high surface area, good stability, and tailoring of catalytic centers on the frameworks. Indeed, a new class of metalloporphyrin‐based HCPs were successfully synthesized using modified iron(III) porphyrin complexes as building blocks, and the resulting networks were found to be excellent recyclable heterogeneous catalysts for the hetero‐Diels–Alder reaction of unactivated aldehydes with 1,3‐dienes. Moreover, this new strategy showed wide adaptability, and many kinds of homogeneous‐like solid‐based catalysts with high catalytic performance and excellent recyclability were also constructed.     

Hypervalent iodine‐mediated direct azidation of polystyrene and consecutive click‐type functionalization

Polystyrene was directly azidated in 1,2‐dichloroethane or chlorobenzene using a combination of trimethylsilyl azide and a hypervalent iodine (III) compound, (diacetoxyiodo)benzene. 2D NMR HMBC experiments indicated that the azide groups were attached to the polymer backbone and also possibly to the aromatic pendant groups. The amount of introduced azide groups was estimated by semi‐quantitative IR spectroscopy and elemental analysis. Approximately 1 in every 11 styrene units could be modified by using a ratio of hypervalent iodine compound to trimethylsilyl azide to styrene units of 1:2.1:1 at 0 °C for 4 h followed by heating to 50 °C for 2 h in chlorobenzene. The azidated polymers were further used as backbone precursors in the synthesis of polymeric brushes with hydrophilic side chains via a copper‐catalyzed click grafting‐onto reaction with poly(ethylene oxide) monomethyl ether 4‐pentynoate. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 966–974, 2010     

Nonlinear stress relaxation of an entangled linear polystyrene in single step‐strain flow: A quantitative theoretical investigation

The nonlinear stress relaxation of a nearly monodisperse, moderately entangled polystyrene solution (i.e., roughly seven entanglements per chain at equilibrium) in single step‐strain flow is investigated quantitatively by a detailed comparison of an existing set of experimental data with a simulation based on the tube model. The proposed simulation enables the effects of primary nonlinear relaxation mechanisms other than chain retraction to be identified more clearly and investigated individually. Two peculiar nonlinear relaxation behaviors are observed in this experiment. One is concerned with an apparent enhancement in the stress relaxation at short times, and the other is responsible for a seeming slowdown of the stress relaxation at long times. These findings are discussed within the tube model, in view of the effects of convective constraint release, partial strand extension, and nonaffine deformation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1281–1293, 2003     

Bulk and shear rheology of a symmetric three‐arm star polystyrene

The bulk and shear rheological properties of a symmetric three‐arm star polystyrene were measured using a self‐built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure–volume–temperature behavior, the pressure‐dependent glass transition temperature (T_g), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher T_gs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above T_g obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using T ? T_g scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear polystyrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecular mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short‐time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Synthesis of 1H‐quinazoline‐2,4‐diones from 2‐aminobenzonitriles by fixation of carbon dioxide with amidine moiety supported polymer at atmospheric pressure

1H‐Quinazoline‐2,4‐diones, which are key intermediates in the synthesis of medicines, were successfully synthesized from 2‐aminobenzonitriles by the fixation of CO₂ in the presence of a polystyrene derivative bearing amidine moiety [poly(amidine)]. A model reaction, that is, the reaction of 2‐aminobenzonitrile ( 1a ) with CO₂ in the presence of N‐methyltetrahydropyrimidine ( MTHP ) revealed that a catalytic amount of MTHP afforded 1H‐quinazoline‐2,4‐dione ( 2a ) quantitatively at atmospheric pressure. Several 1H‐quinazoline‐2,4‐diones ( 2a ‐ 2c ) were successfully synthesized from the corresponding 2‐aminobenzonitriles ( 1a ‐ 1c ) in the presence of poly(amidine). The poly(amidine) could easily be separated from the reaction mixture by filtration and reused in subsequent reactions owing to the heterogeneous system. These demonstrated that poly(amidine) is a useful heterogeneous polymer‐supported reagent for the synthesis of 1H‐quinazoline‐2,4‐diones from CO₂. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 653–657, 2009     

Influence of physical aging on the performance of corona‐charged amorphous polymer electrets

The influence of physical aging on the electret properties before corona charging of three amorphous polymers, polyetherimide (PEI), poly(phenylene ether) (PPE), and polystyrene (PS), as well as with blends of PPE and PS, was investigated. The degree of aging was monitored by determining the enthalpy relaxation Δh using differential scanning calorimetry (DSC). The electret performance was evaluated by isothermal potential decay (ITPD) at elevated temperatures and by thermal stimulated discharge (TSD) measurements. It was demonstrated that physical aging below the glass transition temperature substantially improves the electret performance of amorphous polymers by reducing the free volume and thus hindering charge motion. As an example, the performance of nonaged PEI was improved by physical aging at 200 °C for 4 days from 18 to 95% retained charge after 24 h at 120 °C. A similar beneficial influence of physical aging on the charge storage capability was achieved using blends of PPE with PS. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 990–997, 2010     

Relaxation of shish‐kebab precursor in isotactic polystyrene after short‐term shear flow

We studied the formation and relaxation of precursors of shish‐kebab in isotactic polystyrene after applying pulse shear flow at temperatures above the nominal melting temperatures T_m (=223 °C). It was found that the string‐like objects that were assigned to precursors in a previous article appeared in micrometer scale up to ～285 °C, which was very close to the equilibrium melting temperature T (=289 °C), and the length and the diameter showed two‐step decays consisting of the fast and slow (almost nondecaying) components below ～270 °C, whereas the slow component disappeared above ～270 °C, suggesting that some mechanism stabilizing the string‐like objects disappeared at ～270 °C. It was also found that the two‐step decay was a nature of a single string‐like object, but not an average nature of many precursors, showing heterogeneous inner structure of the precursor. We discussed a possibility that the string‐like object had a fringed micelle type structure including large crystals with a melting temperature of ～270 °C. Within the proposed picture, the highest temperature for the precursor formation (～285 °C) was explained as a melting of the large crystals in oriented melt. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Molecular motion during physical aging in polystyrene: Investigation using probe reorientation

A photobleaching method has been used to observe the reorientation of tetracene and rubrene in polystyrene during physical aging. Rotation times change more than an order of magnitude during isothermal aging after a temperature quench from above T_g. Down‐ and up‐jumps of the temperature show the expected asymmetry due to the nonlinearity of the aging process. The rotation times of tetracene and rubrene require the same amount of time to reach equilibrium after a temperature change (10³ − 10⁵ s in the range 93–99 °C). These equilibration times are the same order of magnitude as equilibration times for volume and enthalpy relaxation, but have a somewhat weaker temperature dependence. Very near equilibrium, the rates of aging are different for the two probes, with rubrene approaching equilibrium more rapidly at very long times. This may be understood if the aging process is spatially heterogeneous, that is, if aging occurs more rapidly in some small regions of the sample than in others. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 68–79, 2000     

Molecular relaxation of isotactic polystyrene: Real‐time dielectric spectroscopy and X‐ray scattering studies*

The molecular relaxation processes and structure of isotactic polystyrene (iPS) films were investigated with real‐time dielectric spectroscopy and simultaneous wide‐ and small‐angle X‐ray scattering. The purpose of this work was to explore the restrictions imposed on molecular mobility in the vicinity of the α relaxation (glass transition) for crystallized iPS. Isothermal cold crystallization at temperatures of T_c = 140 or 170 °C resulted in a sigmoidal increase of crystallinity with crystallization time. The glass‐transition temperature (T_g), determined calorimetrically, exhibited almost no increase during the first stage of crystal growth before impingement of spherulites. After impingement, the calorimetric T_g increased, suggesting that confinement effects occur in the latter stages of crystallization. For well‐crystallized samples, the radius of the cooperativity region decreased substantially as compared with the purely amorphous sample but was always smaller than the layer thickness of the mobile amorphous fraction. Dielectric experiments directly probed changes in the amorphous dipole mobility. The real‐time dielectric data were fitted to a Havriliak–Negami model, and the time dependence of the parameters describing the distribution of relaxation times and dielectric strength was obtained. The central dipolar relaxation time showed little variation before spherulite impingement but increased sharply during the second stage of crystal growth as confinement occurred. Vogel–Fulcher–Tammann analysis demonstrated that the dielectric reference temperature, corresponding to the onset of calorimetric T_g, did not vary for well‐crystallized samples. This observation agreed with a model in which constraints affect primarily the modes having longer relaxation times and thus broaden the glass‐transition relaxation process on the higher temperature side. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 777–789, 2004     

Above,below, and in‐between the two glass transitions of ultrathin free‐standing polystyrene films: Thermal expansion coefficient and physical aging

In previous work we observed two simultaneous transitions in high molecular weight (MW) free‐standing polystyrene films that were interpreted as two thickness‐dependent reduced glass transition temperatures (T_gs). The weaker lower transition agreed well with the MW‐dependent T_g(h) previously reported, while the much stronger upper transition matched the MW‐independent T_g(h) previously observed in low‐MW free‐standing films. Here, we investigate the nature of these two transitions by inspecting the temperature dependence of the films' thermal coefficient of expansion (TCE) and present physical aging measurements using ellipsometry both below and in‐between the two transitions. TCE values indicate approximately 80 to 90% of the film solidifies at the upper transition, while only 10 to 20% remains mobile to lower temperatures, freezing out at the lower transition. Physical aging is observed at a temperature below the upper transition, but above the lower transition, indicative of the upper transition being an actual glass transition associated with the α‐relaxation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 64–75     

End‐functionalization of syndiotactic polystyrene by vinylsilane inducing selective chain transfer reactions

Structurally well‐defined end‐functionalized syndiotactic polystyrene (sPS) can be prepared by conducting a simultaneous selective chain transfer reaction during the syndiospecific polymerization of styrene in the presence of vinylsilanes. The production of vinylsilane end‐capped sPS involves a unique selective chain transfer pathway via the incorporation of a terminal vinylsilane unit at the polymer chain end by 2,1‐insertion. This unusual insertion pattern situates the bulky silyl functional group at a closer β‐position from the active catalyst center, thus deactivating the propagating chain by a steric jam between the vinylsilane end group and the active catalyst. Subsequently, chain releasing by hydrogen addition (in the presence of H₂) or by β‐elimination (in the absence of H₂) can take place, which leads to the production of end‐functionalized sPS with precise controls of stereoregularity and of the location of functionality. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1690–1698, 2010     

Effect of low‐temperature physical aging on the dynamic transitions of atactic polystyrene in the glassy state

The local and cooperative dynamics of atactic PS (a‐PS) were studied by broadband dielectric relaxation spectroscopy (BDRS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR). The a‐PS has been subjected to thermal rejuvenation and subsequent quenching, short‐term aging (6 weeks), and long‐term aging (1 year) at ambient conditions. Where for the rejuvenated sample only an α‐ and a γ‐relaxation is observed, short‐term aging results in an additional β*‐relaxation that merges with the α‐relaxation at longer aging times. The γ‐relaxation is increasing in intensity and activation energy during aging. The α‐process shows no spectral changes and shift in the relaxation time upon aging. This may be attributed to a possible erasure of history of the material during the temperature‐sweep mode measurement. Fourier transform infrared spectroscopy (FTIR) results suggest that the energetically favorable trans‐trans (tt) conformers are increased in population with aging. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1394–1401     

Intramolecular screening effects in nondilute polymer solutions: An equation‐of‐state approach

In a previous article, we presented a simple modification of the traditional Flory–Huggins theory that took intramolecular screening effects (or same chain contacts) into account. In this article, we present a natural extension of that work, in which free‐volume effects are also explained with an equation‐of‐state model. The predictions of the interaction parameter, χ, for several polymer–solvent systems are presented, over the entire concentration range, in θ solvents and good solvents. A geometric mean assumption is applied to the calculation of an exchange energy interaction term. The predictions of χ are successful to various degrees when internal pressures are used, whereas the use of solubility parameters in most cases produces fairly good agreement with experimental results. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2911–2922, 2003     

Synthesis and dielectric properties of photoreactive polystyrene containing [1‐(3‐isopropenyl‐phenyl)‐1‐methyl‐ethyl]‐carbamate in the side chain

Novel polystyrene derivatives comprising [1‐(3‐isopropenyl‐phenyl)‐1‐methyl‐ethyl]‐carbamate in the side chain were synthesized as photoreactive copolymers. Poly(4‐vinylphenol) was made to react with 1‐(1‐isocyanato‐1‐methyl‐ethyl)‐3‐isopropenyl‐benzene (m‐TMI) and the unreacted hydroxyl groups were protected with acetyl chloride. The copolymers are highly sensitive to the radical photoinitiators that can be activated by irradiation of UV light (λ = 300–365 nm). FTIR spectroscopy was employed to monitor the structural changes in the copolymers exposed to UV irradiation. The dielectric properties of the copolymers were investigated by measuring the capacitance and calculating the permittivity as a function of frequency, along with the I–V characteristics. Their properties were compared with those of thermally crosslinkable poly(4‐vinylphenol) blended with poly(melamine‐co‐formaldehyde), which is frequently used as a dielectric layer in organic field‐effect transistors (OFETs). No significant dielectric dispersion was observed in the frequency range of 1 kHz–1 MHz. The dielectric constant was determined to be in the range of 4.2–6.0, which offers a potential for the application of these copolymers to OFET gate insulators. These soluble dielectrics exhibit good film uniformity and can also be patterned using a standard photolithographic technique. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1710–1718, 2008     

Temperature dependence of the conformational relaxation time of polymer molecules in elongational flow: Invariance of the molecular weight exponent

Using an elongational flow technique, we have investigated the relationship between the molecular conformational relaxation time and the molecular weight for a solvent whose quality was altered thermally from near θ to a good solvent state. The materials used were closely monodisperse samples of atactic polystyrene. The results show that the relaxation time τ varies with the molecular weight M as τ ∝ M^1.5, independent of the solvent quality, a result which apparently is at variance with the observed molecular weight dependence of intrinsic viscosity. Despite this invariance of the molecular weight exponent with solvent quality, our results also show that the coils do expand when the solvent quality was increased in agreement with the mean-field theory of Flory.     

A visible light responsive azobenzene‐functionalized polymer: Synthesis,self‐assembly,and photoresponsive properties

A novel visible light responsive random copolymer consisting of hydrophobic azobenzene‐containing acrylate units and hydrophilic acrylic acid units has been prepared. The azobenzene molecule bearing methoxy groups at all four ortho positions is readily synthesized by one‐step conversion of diazotization. The as‐prepared polymer can self‐assemble into nanoparticles in water due to its amphiphilic nature. The tetra‐o‐methoxy‐substituted azobenzene‐functionalized polymer can exhibit the trans‐to‐cis photoswitching under the irradiation with green light of 520 nm and the cis‐to‐trans photoswitching under the irradiation with blue light of 420 nm in both solution and aggregate state. The morphologies of the self‐assembled nanoparticles are revealed by TEM and DLS. The controlled release of loaded molecules from the nanoparticles can be realized by adjusting pH value since the copolymer possesses pH responsive acrylic acid groups. The fluorescence of loaded Nile Red in the nanoparticles can be tuned upon the visible light irradiation. The reversible photoswitching of the azobenzene‐functionalized polymer under visible light may endow the polymer with wide applications without using ultraviolet light at all. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2768–2775     

Synthesis and characterization of silica‐graft‐polystyrene hybrid nanoparticles: Effect of constraint on the glass‐transition temperature of spherical polymer brushes

The effect of the chain constraint on the glass‐transition temperature of polystyrene (pS) was studied in the context of polymer tethering to curved surfaces. The synthesis and characterization of silica‐graft‐polystyrene (SiO₂‐g‐pS) hybrid nanoparticles is reported. Silica nanoparticles possessing covalently bound pS chains were prepared by the atom transfer radical polymerization of styrene from functionalized colloidal surfaces. These hybrid nanoparticles serve as interesting examples of spherical polymer brushes, as a high density of grafted pS was achieved on the inorganic colloid. The confirmation of a brushlike extension of immobilized chains in a good solvent was obtained with dynamic light scattering in toluene of SiO₂‐g‐pS colloids possessing various molar masses of tethered pS. The solid‐state morphology of SiO₂‐g‐pS ultrathin films was assessed with transmission electron microscopy, and this confirmed that the silica colloids were well‐dispersed in a matrix of the tethered polymer. Differential scanning calorimetry was used to study the effects of tethering and chain immobilization on the glass‐transition temperature of pS. The measured glass‐transition temperature of annealed bulk films of the hybrid nanoparticles was elevated with respect to the value for pure bulk pS. The enhancements ranged from 13 to 2 K for SiO₂‐g‐pS brushes possessing tethered pS with number‐average molecular weights of 5230 and 32,670 g/mol, respectively. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2667–2676, 2002