Time‐resolved crystallization study of absorbable polymers by synchrotron small‐angle X‐ray scattering

Changes in the lamellar morphology that occurred during the quiescent isothermal crystallization of absorbable poly(p‐dioxanone) (PDS) and PDS/poly(glycolide) block copolymer were studied by synchrotron small‐angle X‐ray scattering. Important morphological parameters such as the lamellar long period, the thicknesses of the crystal and amorphous phases, and the scattering invariant were estimated as a function of time, and trends observed over a wide range of experimental conditions are discussed. Thicker but more perfect lamellae were detected at higher crystallization temperatures. The breadth of the normalized semilog Lorentz‐corrected intensity peak systematically decreased with increasing temperature. In addition, the values of the crystallization half‐time and the Avrami exponent (n = 2.5), determined from the real‐time changes in the lamellar development, showed superb agreement with the bulk crystallinity data generated from other experimental techniques, such as calorimetry and dielectric relaxation spectroscopy. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 153–167, 2001     

Time‐resolved method for the measurement of volume changes during polymerization

A new laser scanning dilatometer, based on a simplified capillary‐type dilatometer using a laser scan micrometer for the detection of changes of the sample dimension, is described. The method can be applied to time‐resolved measurements of volume changes for transparent and nontransparent samples. The time resolution of the setup is below 1 s, the absolute error in determining the volume change is below 0.0004 cm³, and the accuracy of the measured shrinkage is better than 0.2 vol. %. The operation temperature ranges from room temperature to 160 °C. The setup has been applied to the investigation of the volume shrinkage during the curing of epoxy resins [diglycidyl ether of bisphenol A (DGEBA)] without and with fillers. Furthermore, the effect of a phase transformation on the volume has been demonstrated by the melting of a crystalline phase (succinic anhydride) dispersed in a DGEBA matrix due to the heat of reaction. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2314–2325, 2005     

Time‐of‐flight mass spectrometry for time‐resolved measurements: Some developments and applications

In this paper, the time resolution for kinetic studies of reactions with mass spectrometric detection is characterized in detail, and it is shown how this allows faster kinetic processes to be determined. The time‐resolved technique used pulsed laser photolysis to initiate reaction and a time‐of‐flight mass spectrometer (TOFMS) to monitor progress, where the reactant gas was sampled by a sampling orifice and photoionized using pulsed, laser vacuum ultraviolet light before being analyzed by the TOFMS. Characterization of this setup has been carried out to identify the parameters that affect the time for “sampling,” which limits the fastest reactions that can be measured. A simple mathematical equation has been developed to correct for “sampling” delays (k_sampling～25, 000 s^?1), which extends the range of rate coefficients to be measured in a kinetic mass spectrometry reactor to k′ < 7000 s^?1. This method could be applied to any other kinetic mass spectrometry system where k_sampling can be measured; an important advantage since it allows the study of reactions over a wider range of conditions (e.g., larger concentrations of reagents/products can be used to minimize the contribution from wall losses). The system can produce reliable kinetic data whether monitoring reactant decay or product growth even when the reaction and sampling processes are occurring on a similar timescale (k′ < 7000 s^?1). Reproducible and reliable kinetic data have been obtained for the following reactions: SO + NO₂ → products (R1), ClSO + NO₂ → products (R2), where SO and ClSO were monitored under pseudo‐first‐order conditions, and HCO + O₂ → CO + HO₂ (R3), where CO was monitored by a [1+1] resonance enhanced ionization multiphoton ionization (REMPI) scheme with HCO reacting under pseudo–first‐order conditions. The limitations and potential developments of this setup are described. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 532–545, 2012     

Morphology of polyanhydride copolymers: Time‐resolved small‐angle X‐ray scattering studies of crystallization

Synchrotron small‐angle X‐ray scattering (SAXS) was used to study the isothermal crystallization kinetics of a family of polyanhydride copolymers consisting of 1,6‐bis(p‐carboxyphenoxy)hexane and sebacic acid monomers. In situ SAXS experiments permitted the direct observation of the crystallization kinetics. The structural parameters (the long period, lamellar thickness, and degree of crystallinity) were obtained from Lorentz‐corrected intensity profiles, one‐dimensional correlation functions, and interface distribution functions to form a comprehensive picture of the crystal morphology. The combination of these three analyses provided information not only on the lamellar dimensions but also on the polydispersity (nonuniformity) of these dimensions. Where possible, the crystallization kinetics were interpreted with a modified version of the Avrami equation. The results can be used to perform the rational design of controlled‐drug‐release formulations because crystallinity affects drug‐release kinetics. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 463–477, 2005     

Time‐resolved photoacoustic calorimetry as a tool to determine rate constants of hydrogen‐abstraction reactions

The hydrogen‐abstraction reaction from phenol by tert‐butoxyl radical was used to test time‐resolved photoacoustic calorimetry (TR‐PAC) as a method to obtain kinetic data. Absolute rate constants for this reaction were determined in benzene and in the 298–312 K temperature range, using a temperature‐controlled photoacoustic calorimeter, yielding an average value of 3.5 × 10⁸ M^?1 s^?1. This is in good agreement with the literature results obtained by laser flash photolysis (LFP). Kinetic solvent effects (KSE) in the same reaction were also studied using acetonitrile, carbon tetrachloride, and ethyl acetate as solvents. The results obtained by TR‐PAC are close to those derived by LFP (using the cumyloxyl radical as the abstracting species) and follow the expected trend based on the KSE. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 357–363, 2006     

Time‐resolved dynamic light scattering as a method to monitor compaction during protein folding

The mechanisms and the dynamics of protein folding are subject of a still increasing number of theoretical and experimental studies. While spectroscopic methods are already used for many years to measure the folding rates and to monitor the formation of secondary and tertiary structure, kinetic measurements of the compactness are only beginning to emerge. Time‐resolved dynamic light scattering (DLS) is a useful tool to follow the compaction during protein folding by measuring the hydrodynamic Stokes radius R_S. Additionally, changes in the state of association can be detected by simultaneous measurements of the scattering intensity. The usefulness of different techniques for time‐resolved DLS measurements and the general limits for kinetic DLS experiments are discussed first. Then we describe the adaptation of a stopped‐flow system (SFM‐3) to a DLS apparatus, the particular data acquisition schemes, and the experimentally attainable limits. The feasibility of stopped‐flow DLS is demonstrated by the results of folding investigations with ribonuclease A, phosphoglycerate kinase, and bovine α‐lactalbumin. Refolding was initiated by denaturant dilution jumps, which were repeated up to 100 times in order to obtain a reasonable signal‐to‐noise ratio. Kinetic DLS experiments can be performed fairly with a time resolution of one second. The time resolution of 100ms is probably the attainable limit. The capabilities of time‐resolved DLS and time‐resolved small‐angle X‐ray scattering are compared.     

Tracking Pathogen Infections by Time‐Resolved Chemical Proteomics

Studying the dynamic interaction between host cells and pathogen is vital but remains technically challenging. We describe herein a time‐resolved chemical proteomics strategy enabling host and pathogen temporal interaction profiling (HAPTIP) for tracking the entry of a pathogen into the host cell. A novel multifunctional chemical proteomics probe was introduced to label living bacteria followed by in vivo crosslinking of bacteria proteins to their interacting host‐cell proteins at different time points initiated by UV for label‐free quantitative proteomics analysis. We observed over 400 specific interacting proteins crosslinked with the probe during the formation of Salmonella‐containing vacuole (SCV). This novel chemical proteomics approach provides a temporal interaction profile of host and pathogen in high throughput and would facilitate better understanding of the infection process at the molecular level.     

Time‐resolved wide‐angle X‐ray scattering measurements during the isothermal crystallization and ferroelectric phase‐transition processes of a vinylidene fluoride/trifluoroethylene copolymer

The time‐resolved measurement of wide‐angle X‐ray scattering was performed with a synchrotron radiation source during the processes of the isothermal crystallization and ferroelectric phase transition of a vinylidene fluoride/trifluoroethylene copolymer with 73 mol % vinylidene fluoride. When the sample was cooled rapidly from the melt to the temperature region of the paraelectric high‐temperature phase, the peak position of the 200/110 reflection shifted toward the higher angle side and the half‐width became narrower. This indicated an increase in the crystallite size with a more compact chain‐packing mode. Even when the temperature jump was made from the melt into the region of the ferroelectric or low‐temperature phase, the crystallization of the high‐temperature phase was first observed before the appearance of the low‐temperature phase. This was consistent with a prediction based on the so‐called Ostwald state rule: the thermodynamically unstable but kinetically preferable high‐temperature phase can appear first even when the thermodynamically more stable low‐temperature phase should be created. The time‐dependent intensity changes were analyzed with the Avrami kinetic equation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4175–4181, 2004     

Ultrafast Dynamics Probed by the Time‐Resolved Anisotropy Measurements

In this paper, we shall show that in order to effectively observe the quantum beat due to vibrational coherence by the anisotropy measurement, it is necessary to perform the frequency‐resolved measurement in the transient absorption. It is found that the quantum beat due to vibrational coherence cannot be observed if the ultrafast processes under observation do not involve an electronic transition. In order to effectively observe the quantum beat due to vibronic coherence, it is necessary to perform the frequency‐integrated time‐resolved anisotropy measurement. We also show that it is usually difficult to obtain accurate dynamics information from the time‐resolved anisotropy measurement.     

Time‐resolved Fourier transform infrared/attenuated total reflection spectroscopy for the measurement of molecular diffusion in polymers

Over the past 2 decades, the use of time‐resolved Fourier transform infrared/attenuated total reflection (ATR) spectroscopy for the measurement of diffusion in polymers has grown. ATR is a powerful technique for the measurement of diffusion in polymers because it is an in situ technique that is relatively inexpensive, provides reliable short‐time data, and provides a wealth of information at the molecular level. This article highlights the technique and its application to numerous studies, ranging from the diffusion of drugs in human skin to chemical warfare agents in barrier materials. In addition to these topics, recent studies with ATR to quantify and model molecular interactions during the diffusion process are reviewed. In the future, the ATR technique may have an impact on a variety of emerging fields in which diffusion in polymers plays an important role, such as fuel cells, membrane separation, sensors, and drug delivery. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2794–2807, 2003     

Ultrafast Dynamics of o‐Bromofluorobenzene Studied by Time‐Resolved Photoelectron Imaging

Photodissociation dynamics and rotational wave packet coherences of o‐bromofluorobenzene are studied by femtosecond time‐resolved photoelectron imaging (see figure). The decay of different photoelectron rings shows the population decay of states from which the lifetimes of different states are determined. The variation of photoelectron angular distributions reflects the evolution of rotational coherences.

     

Identification of position isomers by energy‐resolved mass spectrometry

This study reports an energy‐resolved mass spectrometric (ERMS) strategy for the characterization of position isomers derived from the reaction of hydroxyl radicals (^●OH) with diphenhydramine (DPH) that are usually hard to differentiate by other methods. The isomer analogues formed by ^●OH attack on the side chain of DPH are identified with the help of a specific fragment ion peak (m/z 88) in the collision‐induced dissociation (CID) spectrum of the protonated molecule. In the negative ion mode, the breakdown curves of the deprotonated molecules show an order of stability (supported by density functional theory (DFT) calculations) ortho > meta > para of the positional isomers formed by the hydroxylation of the aromatic ring. The gas phase stability of the deprotonated molecules [M ? H]^? towards the benzylic cleavage depends mainly on the formation of intramolecular hydrogen bonds and of the mesomeric effect of the phenol hydroxyl. The [M ? H]^? molecules of ortho and meta isomers result a peak at m/z 183 with notably different intensities because of the presence/absence of an intramolecular hydrogen bonding between the OH group and C9 protons. The ERMS approach discussed in this report might be an effective replacement for the conventional methods that requires very costly and time‐consuming separation/purification methods along with the use of multi‐spectroscopic methods. Copyright © 2015 John Wiley & Sons, Ltd.     

Integrating Time‐Resolved Imaging Information by Single‐Luminophore Dual Thermally Activated Delayed Fluorescence

The fact that the lifetime of photoluminescence is often difficult to access because of the weakness of the emission signals, seriously limits the possibility to gain local bioimaging information in time‐resolved luminescence probing. We aim to provide a solution to this problem by creating a general photophysical strategy based on the use of molecular probes designed for single‐luminophore dual thermally activated delayed fluorescence (TADF). The structural and conformational design makes the dual TADF strong in both diluted solution and in an aggregated state, thereby reducing sensitivity to oxygen quenching and enabling a unique dual‐channel time‐resolved imaging capability. As the two TADF signals show mutual complementarity during probing, a dual‐channel means that lifetime mapping is established to reduce the time‐resolved imaging distortion by 30–40 %. Consequently, the leading intracellular local imaging information is serialized and integrated, which allows comparison to any single time‐resolved signal, and leads to a significant improvement of the probing capacity.     

Time‐resolved spectroscopy study of donor–acceptor‐type copolymers in a monodisperse system: The effect of ratio between the acceptor and the donor

The photoexcitation processes of two donor–acceptor‐type copolymers PCFBT with different ratios between the donor and the acceptor ( PCFBT_0.5 and PCFBT_0.1 ) in the solution system are systematically studied. If the number of the donor is equal to that of the acceptor in one repeat unit (such as PCFBT_0.5 ), intrachain charge transfer (ICT) can occur and participate in the relaxation of the excited state after photoexcitation. When the number of donors is much larger than that of acceptors (such as PCFBT_0.1 ) in one repeat unit, the ICT character can disappear, and the localized exciton decay process is dominant in the relaxation of the copolymer, which also involves an excitation intensity‐independent vibrational thermal relaxation process at the initial time. The results further the understanding of the basic structure‐property relationship. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 992–996     

Conformational Relaxation of p‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence

Two p‐phenylenevinylene (PV) trimers, containing 3′‐methylbutyloxyl (in MBOPV3) and 2′‐ethylhexyloxyl (in EHOPV3) side chains, are used as model compounds of PV‐based conjugated polymers (PPV) with the purpose of clarifying the origin of fast (picosecond time) components observed in the fluorescence decays of poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylenevinylene] (MEH‐PPV). The fluorescence decays of MBOPV3 and EHOPV3 reveal the presence of similar fast components, which are assigned to excited‐state conformational relaxation of the initial population of non‐planar trimer conformers to lower‐energy, more planar conformers. The rate constant of conformational relaxation k_CR is dependent on solvent viscosity and temperature, according to the empirical relationship k_CR=aη_o^?α?exp(?αE_η/RT), where aη_o^?α is the frequency factor, η_o is the pre‐exponential coefficient of viscosity, E_η is the activation energy of viscous flow. The empirical parameter α, relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction (α=1), depends on the side chain. The fast component in the fluorescence decays of MEH‐PPV polymers (PPVs), is assigned to resonance energy transfer from short to longer polymer segments. The present results call for revising this assignment/interpretation to account for the occurrence of conformational relaxation, concurrently with energy transfer, in PPVs.