Study of reorientational dynamics during real‐time crystallization of absorbable poly(p‐dioxanone) by dielectric relaxation spectroscopy

The real‐time crystallization of absorbable poly(p‐dioxanone) (PDS) was studied by dielectric relaxation spectroscopy. The dipole dynamic changes in the diminishing amorphous phase were investigated over a wide range of crystallization conditions. The location, shape, and magnitude of the α relaxation and the apparent activation energy were monitored and compared before and after the onset of crystallization. We observed no correlation between the degree of crystallinity and the location (hence, the most probable relaxation time, τ) of the α relaxation from just after the initiation up to the latest stages of the isothermal crystallization. However, an abrupt change in the intensity of the α process and the apparent activation energy allowed for the precise detection of the onset of crystallization. This was probably caused by a reorganization of dipole units occurring a few moments before the crystallization began. As crystallization proceeded, an asymmetric broadening of the α peak was observed that was directly influenced by the appearance of a new lower frequency process that originated in the highly confined amorphous portion located inside the spherulites. Finally, PDS crystallization kinetics determined from the changes of the relaxed permittivity with time are discussed and compared with calorimetric and optical microscopy data. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2436–2448, 2000     

Structure–reactivity relationships of alkyl α‐hydroxymethacrylate derivatives

A series of alkyl α‐hydroxymethacrylate derivatives with various secondary functionalities (ether, ester, carbonate, and carbamate) and terminal groups (alkyl, cyano, oxetane, cyclic carbonate, phenyl and morpholine) were synthesized to investigate the effect of intermolecular interactions, H‐bonding, π–π interactions, and dipole moment on monomer reactivity. All of the monomers except one ester and one ether derivative are novel. The polymerization rates, determined by using photo‐DSC, showed the average trend (aromatic carbamate > hydroxyl > ester > carbonate ～ aliphatic carbamate ～ ether), with several exceptions due to the differences in terminal groups. There is a correlation between the chemical shift differences of the double bond carbons, the calculated dipole moments, and the reactivities only for nonhydrogen bonded monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Newborn screening for 3‐hydroxy‐3‐methylglutaric aciduria using direct analysis in real‐time mass spectrometry

A novel method utilizing ambient thermal desorption ionization with a direct analysis in real‐time source integrated with mass spectrometry (DART‐MS) was established and applied to the rapid analysis of 3‐hydroxy‐3‐methylglutaric (3‐HMG) acid in the neonatal urine. Instrument parameter settings were optimized to obtain high sensitive and accurate determination of 3‐HMG acid. The use of helium gas heated to temperature of 400°C was observed to permit deprotonation, 3‐HMG acid producing an abundant (M‐H)^? (m/z 161) in the negative ion mode. The calibration curve was determined to be linear over the range of 0.05‐5 mg/L, with the correlation coefficient r = 0.9988 and the relative standard deviations (n = 6) in the range of 1.5‐11.8%. The limit of detection was 0.002 mg/L, and the limit of quantitation was 0.007 mg/L. The recoveries ranged from 88.0% to 123.1%. Four urine samples from patients and four simulated urine samples were investigated. The results of DART‐MS were in agreement with the values determined using established methods at the hospitals. The proposed method demonstrated significant potential in the application of the high‐throughput screening in newborn screening.     

The nearly constant loss,Johari‐Goldstein β‐relaxation,and α‐relaxation of 1,4‐polybutadiene

From high‐resolution dielectric spectroscopy measurements on 1,4‐polybutadiene (1,4‐PB), we show that in addition to the structural α‐relaxation and higher frequency secondary relaxations in the spectra, a nearly constant loss (NCL) is observed at shorter times/lower temperatures. The properties of this NCL are compared to those of another chemically similar polymer, 1,4‐polyisoprene. The secondary relaxations in 1,4‐PB include the well‐known Johari‐Goldstein (JG) β‐relaxation and two other higher‐frequency peaks. One of these, referred to as the γ‐relaxation, falls between the JG‐relaxation and the NCL. Seen previously by others, this γ‐relaxation in 1,4‐PB is not the JG‐process and bears no relation to the glass transition. At very low temperatures (<15 K), we confirm the existence of a very fast secondary relaxation, having a weak dielectric strength and an almost temperature‐invariant relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 342–348, 2007     

α‐ and β‐Relaxations in neat and antiplasticized polybutadiene

Dielectric spectroscopy was carried out to measure the α‐relaxation (local segmental motion) and the higher frequency, secondary relaxation (β‐mode) in 1,4‐polybutadiene, both neat and containing a nonpolar diluent, mineral oil. The α‐relaxation shifted to lower frequencies (antiplasticization) in the presence of the diluent, suggesting the glass temperature of the latter is higher than the T_g of the polymer (i.e., >187K). The T_g of neat mineral oil cannot be determined directly, due to crystallization. While the diluent increased the magnitude of the α‐relaxation times, it had no effect on the β‐relaxation. Moreover, neither the shape of the α‐relaxation function nor its temperature dependence was influenced by the diluent. From this we conclude that the main effect of the mineral oil was to increase the local friction, without changing the degree of intermolecular cooperativity of the molecular motions. We also find that near the glass temperature, there is rough agreement between the time scale of the secondary relaxation process and the value of a noncooperative relaxation time estimated from theory. This approximate correspondence between the two relaxation times also holds for 1,2 polybutadiene. However, the β‐process cannot be identified with the noncooperative α‐relaxation, and the relationship between them is not quantitative. © 2000 John Wiley & Sons, Inc.* J Polym Sci B: Polym Phys 38: 1841–1847, 2000     

One‐pot inimer promoted ROCP synthesis of branched copolyesters using α‐hydroxy‐γ‐butyrolactone as the branching reagent

An array of branched poly(?‐caprolactone)s was successfully synthesized using an one‐pot inimer promoted ring‐opening multibranching copolymerization (ROCP) reaction. The biorenewable, commercially available yet unexploited comonomer and initiator 2‐hydroxy‐γ‐butyrolactone was chosen as the inimer to extend the use of 5‐membered lactones to branched structures and simultaneously avoiding the typical tedious work involved in the inimer preparation. Reactions were carried out both in bulk and in solution using stannous octoate (Sn(Oct)₂) as the catalyst. Polymerizations with inimer equivalents varying from 0.01 to 0.2 were conducted which resulted in polymers with a degree of branching ranging from 0.049 to 0.124. Detailed ROCP kinetics of different inimer systems were compared to illustrate the branch formation mechanism. The resulting polymer structures were confirmed by ¹H, ¹³C, and ₁H‐₁₃C HSQC NMR and SEC (RI detector and triple detectors). The thermal properties of polymers with different degree of branching were investigated by DSC, confirming the branch formation. Through this work, we have extended the current use of the non‐homopolymerizable γ‐butyrolactone to the branched polymers and thoroughly examined its behaviors in ROCP. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1908–1918     

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     

Allyl sulfides and α‐substituted acrylates as addition–fragmentation chain transfer agents for methacrylate polymer networks

The rapid and uncontrolled nature of network formation from di(meth)acrylate monomers produces high shrinkage stress and results in polymers with oftentimes brittle mechanical properties. Methods for regulating polymerization and network formation are sought. One option is the use of addition–fragmentation chain transfer (AFCT) agents, which are well known to control molecular weight and molecular weight distribution of monofunctional (meth)acrylates. A series of novel and previously described AFCT reagents were synthesized and screened with laser flash photolysis to determine reactivity. Well‐performing AFCT reagents were then tested in polymerizations with monofunctional and difunctional methacrylates. With monofunctional monomers, the molecular weight and polydispersity of the resultant linear polymers tend to decrease with the addition of AFCT agent. In copolymerization with dimethacrylate monomers, the AFCT agents were found to substantially lower and sharpen the glass transition. Sharpness of the glass transition is here indicative of a more regular and homogenous network. After coupling of the instruments, photorheology was performed simultaneously with real‐time IR to show an increase in monomer conversion at the time of gelation, which appears to have a positive effect on reducing shrinkage stress. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 394–406     

Fast analysis of glycosides based on HKUST‐1‐coated monolith solid‐phase microextraction and direct analysis in real‐time mass spectrometry

Glycosides are a kind of highly important natural aromatic precursors in tobacco leaves. In this study, a novel HKUST‐1‐coated monolith dip‐it sampler was designed for the fast and sensitive analysis of trace glycosides using direct analysis in real‐time mass spectrometry. This device was prepared in two steps: in situ polymerization of monolith in a glass capillary of dip‐it and layer‐by‐layer growth of HKUST‐1 on the surface of monolith. Sufficient extraction was realized by immersing the tip to solution and in situ desorption was carried out by plasma direct analysis in real time. Compared with traditional solid‐phase microextraction protocols, sample desorption was not needed anymore, and only extraction conditions were needed to be optimized in this method, including the gas temperature of direct analysis in real time, extraction time, and CH₃COONH₄ additive concentration. This method enabled the simultaneous detection of six kinds of glycosides with the limits of detection of 0.02–0.05 μg/mL and the linear ranges covering two orders of magnitude with the limits of quantitation of 0.05–0.1 μg/mL. Moreover, the developed method was applied for the glycosides analysis of three tobacco samples, which only took about 2 s for every sample.     

Relationship between the α‐ and β‐relaxation processes in amorphous polymers: Insight from atomistic molecular dynamics simulations of 1,4‐polybutadiene melts and blends

Amorphous polymers exhibit a primary (glass, or α‐) relaxation process and a low‐temperature relaxation process associated with polymer backbone motion usually referred to as the β‐relaxation process. The latter process can be observed below the glass transition temperature of the polymer and usually merges with the α‐relaxation process at temperatures somewhat above the glass transition temperature. While it is widely held that both the α‐relaxation and β‐relaxation processes are engendered by localized (segmental) motions of the polymer backbone, and that there is a strong mechanistic connection between them, the molecular mechanisms of the α‐relaxation and β‐relaxation processes in amorphous polymers are not well understood. Recently, atomistic molecular dynamics simulations of melts and blends of 1,4‐polybutadiene have provided insight into the relationship between the α‐ and β‐relaxation processes in glass‐forming polymers and an improved understanding of their molecular origins. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 627–643, 2007     

Study of the dielectric β-relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters

The dielectric β-relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters with various ethylene isophthalate contents was investigated over a wide range of frequencies. Isothermal spectra and normalized plots show that the dielectric β-relaxation is characterized by the molecular structure between the aromatic rings. The equilibrium polarizability of the motional segment, which was estimated from the dielectric increment obtained by the best fit of the Havriliak-Negami equation to the Cole-Cole plot, indicates that torsional vibrations of the main chain are correlated over a length corresponding to one monomer.     

Open Probe fast GC–MS — combining ambient sampling ultra‐fast separation and in‐vacuum ionization for real‐time analysis

An Open Probe inlet was combined with a low thermal mass ultra‐fast gas chromatograph (GC), in‐vacuum electron ionization ion source and a mass spectrometer (MS) of GC–MS for obtaining real‐time analysis with separation. The Open Probe enables ambient sampling via sample vaporization in an oven that is open to room air, and the ultra‐fast GC provides ~30‐s separation, while if no separation is required, it can act as a transfer line with 2 to 3‐s sample transfer time. Sample analysis is as simple as touching the sample, pushing the sample holder into the Open Probe oven and obtaining the results in 30 s. The Open Probe fast GC was mounted on a standard Agilent 7890 GC that was coupled with an Agilent 5977A MS. Open Probe fast GC–MS provides real‐time analysis combined with GC separation and library identification, and it uses the low‐cost MS of GC–MS. The operation of Open Probe fast GC–MS is demonstrated in the 30‐s separation and 50‐s full analysis cycle time of tetrahydrocannabinol and cannabinol in Cannabis flower, sub 1‐min analysis of trace trinitrotoluene transferred from a finger onto a glass surface, vitamin E in canola oil, sterols in olive oil, polybrominated flame retardants in plastics, alprazolam in Xanax drug pill and free fatty acids and cholesterol in human blood. The extrapolated limit of detection for pyrene is <1 fg, but the concentration is too high and the software noise calculation is untrustworthy. The broad range of compounds amenable for analysis is demonstrated in the analysis of reserpine. The possible use with alternate standard GC–MS and Open Probe fast GC–MS is demonstrated in the analysis of heroin in its street drug powder. The use of Open Probe with the fast GC acting as a transfer line is demonstrated in <10‐s analysis without separation of ibuprofen and estradiol. Copyright © 2017 John Wiley & Sons, Ltd.     

Interpretation of the TSDC fractional polarization experiments on the α‐relaxation of polymers

We report on the interpretation of the thermally stimulated depolarization current (TSDC) experiments, with partial polarization methods, on the dielectric α‐relaxation. The results obtained on polyvinyl acetate are rationalized on the basis of the Boltzmann superposition principle in combination with a Kohlrausch–Williams–Watts (KWW) time decay of the polarization (with the β exponent essentially temperature independent and equal to the value determined by conventional dielectric methods at T_g). From this analysis of the global TSDC spectrum we found a complex temperature dependence of the KWW relaxation time, which is Arrhenius‐like at the lowest temperatures but crosses over to the Vogel–Fulcher behavior observed above T_g in the temperature range of the TSDC peak. On the basis of these results, we found the way of predicting the TSDC spectra measured after partial polarization procedures. We found that, the distribution of activation energies and compensation behavior deduced by following the standard way of analysis are associated to the assumption of an Arrhenius‐like temperature dependence of the α‐relaxation time in the temperature range explored by TSDC. Therefore we conclude that both the distribution of activation energies and compensation behavior obtained by following the standard way of analysis do not give a proper physical picture of the α‐relaxation of glassy polymers around the glass‐transition temperature. Our results also show that the partial polarization TSDC methods are not able to give insight about the actual existence or not of a distribution of relaxation times at the origin of the nonexponentiality of the α‐relaxation of polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2105–2113, 2000     

35Cl NQR frequency and spin lattice relaxation time in 3,4‐dichlorophenol as a function of pressure and temperature

The pressure dependences of ³⁵Cl nuclear quadrupole resonance (NQR) frequency, temperature and pressure variation of spin lattice relaxation time (T₁) were investigated in 3,4‐dichlorophenol. T₁ was measured in the temperature range 77–300 K. Furthermore, the NQR frequency and T₁ for these compounds were measured as a function of pressure up to 5 kbar at 300 K. The temperature dependence of the average torsional lifetimes of the molecules and the transition probabilities W₁ and W₂ for the Δm = ±1 and Δm = ±2 transitions were also obtained. A nonlinear variation of NQR frequency with pressure has been observed and the pressure coefficients were observed to be positive. A thermodynamic analysis of the data was carried out to determine the constant volume temperature coefficients of the NQR frequency. An attempt is made to compare the torsional frequencies evaluated from NQR data with those obtained by IR spectra. On selecting the appropriate mode from IR spectra, a good agreement with torsional frequency obtained from NQR data is observed. The previously mentioned approach is a good illustration of the supplementary nature of the data from IR studies, in relation to NQR studies of compounds in solid state. Copyright © 2012 John Wiley & Sons, Ltd.     

Energy/Hole Transfer Phenomena in Hybrid α‐Sexithiophene (α‐STH) Nanoparticle–CdTe Quantum‐Dot Nanocomposites

Considerable attention has been paid to hybrid organic–inorganic nanocomposites for designing new optical materials. Herein, we demonstrate the energy and hole transfer of hybrid hole‐transporting α‐sexithiophene (α‐STH) nanoparticle–CdTe quantum dot (QD) nanocomposites using steady‐state and time‐resolved spectroscopy. Absorption and photoluminescence studies confirm the loss of planarity of the α‐sexithiophene molecule due to the formation of polymer nanoparticles. Upon photoexcitation at 370 nm, a nonradiative energy transfer (73 %) occurs from the hole‐transporting α‐STH nanoparticles to the CdTe nanoparticles with a rate of energy transfer of 6.13×10⁹ s^?1. However, photoluminescence quenching of the CdTe QDs in the presence of the hole‐transporting α‐STH nanoparticles is observed at 490 nm excitation, which is due to both static‐quenching and hole‐transfer‐based dynamic‐quenching phenomena. The calculated hole‐transporting rate is 7.13×10⁷ s^?1 in the presence of 42×10^?8 M α‐STH nanoparticles. Our findings suggest that the interest in α‐sexithiophene (α‐STH) nanoparticle–CdTe QD hybrid nanocomposites might grow in the coming years because of various potential applications, such as solar cells, optoelectronic devices, and so on.     

Kinetics of thiol–ene and thiol–acrylate photopolymerizations with real‐time fourier transform infrared

We used real‐time Fourier transform infrared to monitor the conversion of both thiol and ene (vinyl) functional groups independently during photoinduced thiol–ene photopolymerizations. From these results, the stoichiometry of various thiol–ene and thiol–acrylate polymerizations was determined. For thiol–ene polymerizations, the conversion of ene functional groups was up to 15% greater than the conversion of thiol functional groups. For stoichiometric thiol–acrylate polymerizations, the conversion of the acrylate functional groups was roughly twice that of the thiol functional groups. With kinetic expressions for thiol–acrylate polymerizations, the acrylate propagation kinetic constant was found to be 1.5 times greater than the rate constant for hydrogen abstraction from the thiol. Conversions of thiol–acrylate systems of various initial stoichiometries were successfully predicted with this ratio of propagation and chain‐transfer kinetic constants. Thiol–acrylate systems with different initial stoichiometries exhibited diverse network properties. Thiol–ene systems were initiated with benzophenone and 2,2‐dimethoxy‐2‐phenylacetophenone as initiators and were also polymerized without a photoinitiator. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3311–3319, 2001     

Molecular dynamics of cis‐1‐(2‐hydroxy‐5‐ methylphenyl)ethanone oxime and N‐(2‐hydroxy‐4‐methylphenyl)acetamide in solution studied by NMR spectroscopy

The formation of hydrogen bonds and molecular dynamics for the molecules cis‐1‐(2‐hydroxy‐5‐methylphenyl)ethanone oxime ( I ) and N‐(2‐hydroxy‐4‐methylphenyl)acetamide ( II ) have been investigated in solution using NMR. The results confirm the formation of O? H···O, O? H···N and O···H? N type inter‐ and intramolecular hydrogen bonds. Spin‐lattice relaxation times (T₁), activation energy of molecular dynamics and energy of intramolecular hydrogen bonds have been determined. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of the alkene structure on the mechanism and kinetics of thiol–alkene photopolymerizations with real‐time infrared spectroscopy

The effect of the chemical structure on the reactivity of alkenes used in thiol–ene photopolymerizations has been investigated with real‐time infrared spectroscopy. Model studies of thiol–ene photoreactions with various monofunctional hydrocarbon alkenes and the monofunctional thiol ethyl‐3‐mercaptopropionate have been performed to identify and understand structure–reactivity relationships. The results demonstrate that terminal enes react very rapidly with thiol, achieve complete conversion, and are independent of the aliphatic hydrocarbon substituent length. Disubstitution on a single carbon of a terminal ene significantly reduces the reactivity, whereas substitution on the carbon α to the terminal ene has a minimal influence on the reactivity. Internal trans enes display reduced reactivity and a lower overall conversion and deviate from the standard thiol–ene reaction mechanism because of steric strain induced by 1,3‐interactions. The reactivity and conversion of internal trans enes decrease as the substituents on the ene become larger, reaching a minimum when the substituent size is greater than or equal to that of propyl groups. Internal cis enes react rapidly with thiol; however, they undergo a fast isomerization–elimination reaction sequence generating the trans ene, which proceeds to react at a reduced rate with thiol. The reactivity of cyclic enes is dictated by ring strain, stereoelectronic effects, and hydrogen abstractability. The reactivity trends in the model studies have been used to explain the photopolymerization mechanism and kinetics of a series of multifunctional thiol–ene systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6283–6298, 2004