Polymer drying. II. Replicated time-studies of molecular desorption from liquid swollen poly(styrene-co-divinylbenzene) before and after transition from the gel-state to the glass-state

A total of 90 time-studies were monitored gravimetrically in order to establish the parameters that affect the reproducibility of the kinetic changes that occur sequentially when liquid-saturated poly(styrene-co-divinylbenzene) [poly(Sty-co-DVB)] particles, enmeshed in a matrix of polytetrafluroethylene (PTFE) microfibers, are allowed to evaporate to apparent dryness at constant temperature. The test liquids were toluene, chloroform, and acetone. The absorbent samples were made from six different compositions of polymer particles, the DVB mode fractions, x, of which increased from 0.01 to 0.11. The weight, W_g, of residual sorbed molecules per gram of polymer at the glass-transition composition was constant despite the large differences in x. Thereafter the weight of residual sorbed molecules was given by a linear combination of up to six exponential decay functions, which indicated that these molecules were “locked” into six different types of molecular environments (i.e., populations) created by the change from the gel-state to the glass-state. The fractions of W_g in the populations with the fastest (k₁) and the slowest (k_n) decay rates were usually less than those with intermediate decay rates. The logarithm of k_i in a given time study was a linear function of i, the numerical sequence of decreasing k.     

Polymer reactions. V. Kinetics of autoxidation of polypropylene

The rate constants for the autoxidation of polypropylene were determined by a combined ESR, volumetric, and chemical method. The values of k_i, k_p, and k_t at 110°C. are 3 × 10^?4 sec.^?1, 1.9 l./mole-sec., and 3 × 10⁶ l./mole-sec., respectively. The values of k_i and its activation energy are the same as those for the decomposition of polypropylene hydroperoxide, thus identifying the latter as the principal initiation process. The values of the temperature-independent k_t suggest that secondary peroxy radicals are the terminating species. The rate constants are compared with rate constant ratios for initiated autoxidations of squalane and other related systems.     

Polymer drying. III. Comparison of molecular desorption from poly(styrene-co-divinylbenzene) in the gel-state and in the glass-state

The results observed in 44 time-studies that monitored evaporation from liquid-saturated poly(Sty-co-DVB) particles enmeshed in poly(tetrafluoroethylene) microfibers under conditions that precluded decrease in area of the microporous composite film sample verified that after the enmeshed particles undergo transition from the gel-state to the glass-state, the number of residual adsorbed molecules per phenyl group of polymer is given by a linear combination of n < 7 exponential decay functions, the first order rate constants of which are related to one another by the equation: log k_i = log k₀ ? mi, where m is characteristic of the polymer, and k₀ is characteristic of the sorbed liquid such that k₁ is about 10⁵ times faster than k₆. The results also show that k₁ of the set of nk_i for desorption from poly(Sty-co-DVB) in the glass-state is equal to k_gel, the first-order rate constant for desorption of the test-liquid from that polymer in the gel-state. These results are interpreted to mean that i, the identification number of the ith population, reflects the number of phenyl groups in the “host” polymer with which the “guest” volatile molecule is associated simultaneously as discussed in the text.     

INTERMEDIATES IN THE PHOTOCONVERSION OF FUNCTIONAL PHYTOCHROME IN FERN SPORES OF Dryopteris–II. In vivo KINETICS OF THE DECAY OF Ii700 AND OF THE FORMATION OF Pfr STUDIED WITH A DOUBLE-LASER APPARATUS*

Abstract— Photoconversion of the red-light-absorbing form of phytochrome, P_r, to the far-red-light-absorbing form, P_fr, was investigated in vivo at 22°C with 600 or 800 ns laser pulses of high spectral purity and induction of spore germination in Dryopteris paleacea was used as indicator for the progress of photoconversion. This reaction is initiated by a saturating R-laser pulse of 648.5 nm, establishing an equilibrium of the photochromic system between P_r and the very early intermediates, Iⁱ₇₀₀ (P_rφ Iⁱ₇₀₀)- The decay of Iⁱ₇₀₀ as well as the formation of P_fr was recorded by the application of a second pulse varied between 698 and 717.5 nm, which inhibits the formation of P_lr being absorbed predominantly by Iⁱ₇₀₀or P_fr, respectively. The most effective inhibition for the second pulse is found up to 10 u.s after the first pulse and this is interpreted by photoreversion of Iⁱ₇₀₀ to P_r; thus reducing the formation of P_fr from Iⁱ₇₀₀. This early inhibition decreases between 10 μs to 100 ms after the R-laser pulse, as a result of the decay of Iⁱ_bl to a bleached species I,;. This decay can be described by three first order kinetics with the rate constants k₁₂= 16830 ± 2970 s^-1, k₁₂= 666 ± 218 s^-1,k₁₃= 9.8 ± 0.9 s^-1. A second inhibition, due to the formation of P_fr, is found for dark intervals <100 ms and can be described by two first order kinetics with the rate constants k₂₁= 2.9 ± 0.6 s^-1 and k₂₂= 0.17 s^-l.     

Pyrolytic production of Se (3P) from carbon diselenide. II. Rate of CSe2 decay

Pyrolytic decay of carbon diselenide was monitored by ultraviolet absorption spectroscopy in reflected shock waves in the temperature range of 1600–2600°K. The temperature dependence of the absorption coefficient of CSe₂ at 2308 Å was determined and was used to provide kinetic information along with a deconvolution procedure which accounted for and removed systematic distortions of the fast time-resolved absorbance profile. For temperatures of 1600–2600°K and argon densities of 1.5–7.0 × 10^?5 mol/cm³ dilute (1.0–9.0 × 10^?9 mol/cm³) CSe₂ pyrolyzed with measured first-order decay rates in the range of log₁₀ k₁ (sec^?1) = 3.0?5.7; at midrange (2100°K and 4.3 × 10^?5 mol/cm³ in Ar) k₁ ≈ 3 × 10⁴ sec^?1. The decay probably occurs via a unimolecular low-pressure process, first order in both CSe₂ and Ar, for which k₂ ± 10⁹ cm³/mol·sec at 2100°K. The deconvoluted data yield Arrhenius activation energies of 53.2 kcal/mol under second-order treatment, but the activation energy is less reliable than the general magnitude of the rate constant. A comparison of CSe₂ with other molecules which are isoelectronic in their valence shells (CO₂, CS₂, OCS, and N₂O) is made.     

Polymer drying. VII. 1H-NMR studies of evaporation from liquid saturated poly(styrene-co-divinylbenzene)

Composition-studies were carried out in which samples of films composed of poly(styrene-co-divinylbenzene) particles enmeshed by poly(tetrafluoroethylene) fibers were saturated with dichloromethane and then allowed to evaporate at 23°C to virtual dryness as the ¹H-NMR signals and residual weight of sorbed liquid were monitored. The correlations, of ¹H-NMR line-widths with the corresponding log of the number, α_t of residual sorbed molecules per monomer unit show that sharp changes occur as α_t passes through α_G the composition that exists when all the liquid not sorbed (i.e., not present within the liquid-saturated particles), has been eliminated, and again as α_t passes through α_g the composition that exists when the particles attain the rigidity characteristic of the glassy state. Correlation of the corresponding T₁ relaxation times with log α_t show that sharp changes occur when α_t becomes equal first to α′_s and then to α′_g the compositions that exist respectively when, firstly, all the sorbed liquid not immobilized by adsorption to the polymer chains within the particles has been eliminated, and then when the desorption of immobilized liquid from the chains causes the composition to begin to undergo transition from its rubbery state to its glassy state.     

Solvent effect on the radical polymerization of di-n-butyl itaconate

Solvent effect on the polymerization of di-n-butyl itaconate (DBI) with dimethyl azobisisobutyrate (MAIB) was investigated at 50 and 61°C. The solvents used were found to affect significantly the polymerization. The polymerization rate (R_p) and the molecular weight of the resulting polymer are lower in more polar solvents. The initiation rate (R_i) by MAIB, however, shows a trend of being rather higher in polar solvents. The stationary state concentration of propagating poly(DBI) radical was determined by ESR in seven solvents. The rate constants of propagation (k_p) and termination (k_t) were evaluated by using R_p, R_i, and the polymer radical concentration observed. The k_p value decreases fairly with increasing polarity of the solvent used, whereas k_t is not so influenced by the solvents. The solvent effect on k_p is explained in terms of a difference in the environment around the terminal radical center of the growing chain. Copolymerization of DBI with styrene (St) was also examined in three solvents with different physical properties. The poly(DBI) radical shows a lower reactivity toward St in a more polar solvent.     

Structure‐Reactivity Relationships as Probes for the Inhibition Mechanism of Cholesterol Esterase by Aryl Carbamates. I. Steady‐State Kinetics

For substituted phenyl‐N‐butyl carbamates (1) and 4‐nitrophenyl‐N‐substituted carbamates (2), linear relationships between values of NH proton chemical shift (δ_NH), pKa, and logk_[OH] and Hammett substituent constant (σ) or Taft substituent constant (σ*) are observed. Carbamates 1 and 2 are pseudo‐substrate inhibitors of porcine pancreatic cholesterol esterase. Thus, the mechanism of the reaction necessitates that the inhibitor molecule and the enzyme form the enzyme‐inhibitor tetrahedral species at the K_i step of the reaction and then form the carbamyl enzyme at the k_c step of the reaction. Linear relationships between the logarithms of K_i and k_c for cholesterol esterase by carbamates 1 and σ are observed, and the reaction constants (ρs) are ?3.4 and ?0.13, respectively. Therefore, the above reaction forms the negative‐charge tetrahedral species and follows the formation of the relatively neutral carbamyl enzymes. For the inhibition of cholesterol esterase by carbamates 2 except 4‐nitrophenyl‐N‐phenyl carbamate and 4‐nitrophenyl‐N‐t‐butyl carbamate, linear relationships of ‐logK_i and logk_c with σ* are observed and the ρ* values are ?0.50 and 1.03, respectively. Since the above reaction also forms the negative‐charge tetrahedral intermediate, it is possible that the K_i step of this reaction is further divided into two steps. The first K_i step is the development of the positive‐charge at the carbamate nitrogen from the protonation of the carbamate nitrogen. The second K_i step is the formation of the tetrahedral intermediate with the negative‐charge at the carbonyl oxygen. From Arrhenius plots of a series of inhibition reactions by carbamates 1 and 2, the isokinetic and isoequilibrium temperatures are different from the reaction temperature (25°C). Therefore, the observed ρ and ρ* values only depend upon the electronic effects of the substituents. Taken together, the cholesterol esterase inhibition mechanism by carbamates 1 and 2 is proposed.     

Equibiaxial deformation of poly(4-methyl-1-pentene) by a forging process

Poly(4-methyl-1-pentene) (PMP) has been uniaxially compressed by a forging (equibiaxial) process. The rheology of the process has been examined for this semicrystalline polyolefin, melting point about 235°C. The yield energy, area under the compressive stress-strain curve up to the yield point, as a function of temperature was found to consist of two linear components of different slope. These two linear relations arise from the glassy and crystalline phases of PMP. The intercept temperature (T_i) at zero yield energy for the glassy phase has been evaluated. The attainable maximum compression ratio without sample rupture (CR_max) increased steadily on increasing forging temperature above T_i, and below T_m. In this range, the crystalline relaxation temperature (T_c), evaluated from an Arrhenius plot of yield stress was 160°C. Above T_c, a CR_max of 240 was reached. This value is five times higher than that attained for isotactic polypropylene (i-PP). However, the draw efficiency evaluated by elastic recovery in the plane direction of PMP (0.76) is lower than for i-PP (0.97). Differential scanning calorimetry analyses showed that the melting peak became a complex doublet on increasing compression ratio ( > 100). The drawing and stress-strain behavior of PMP are compared with i-PP. © 1994 John Wiley & Sons, Inc.     

Nitrous Oxide as Carrier Gas in Capillary Gas-Liquid Chromatography

On use of nitrous oxide as carrier gas the retention factors of the chromatographed compounds decrease linearly with increasing average column pressure. Other retention characteristics (relative retention, retention index) change linearly. This effect was demonstrated by using a capillary column coated with nonpolar polydimethylsiloxane phase SE-30. As shown for capillary GLC the linear correlation is valid for the same column:k_i(G₁,P₁) = A k_i(G₂, P₂) + B, where k_i(G₁, P₁) and k_i(G₂,P₂) are the retention factors of compound i at average column pressures P₁ and P₂ when using carrier gases G₁ and G₂, respectively; A and B are coefficients.     

Propagation rate coefficients of isobornyl acrylate,tert‐butyl acrylate and 1‐ethoxyethyl acrylate: A high frequency PLP‐SEC study

Pulsed laser polymerization (PLP) coupled to size exclusion chromatography (SEC) is considered to be the most accurate and reliable technique for the determination of absolute propagation rate coefficients, k_p. Herein, k_p data as a function of temperature were determined via PLP‐SEC for three acrylate monomers that are of particular synthetic interest (e.g., for the generation of amphiphilic block copolymers). The high‐T_g monomer isobornyl acrylate (iBoA) as well as the precursor monomers for the synthesis of hydrophilic poly(acrylic acid), tert‐butyl acrylate (tBuA), and 1‐ethoxyethyl acrylate (EEA) were investigated with respect to their propagation rate coefficient in a wide temperature range. By application of a 500 Hz laser repetition rate, data could be obtained up to a temperature of 80 °C. To arrive at absolute values for k_p, the Mark‐Houwink parameters of the polymers have been determined via on‐line light scattering and viscosimetry measurements. These read: K = 5.00 × 10⁵ dL g⁻¹, a = 0.75 (piBoA), K = 19.7 × 10⁵ dL g⁻¹, a = 0.66 (ptBA) and K = 1.53 × 10⁵ dL g⁻¹, a = 0.85 (pEEA). The bulky iBoA monomer shows the lowest propagation rate coefficient among the three monomers, while EEA is the fastest. The activation energies and Arrhenius factors read: (iBoA): log(A/L mol⁻¹ s⁻¹) = 7.05 and E_A = 17.0 kJ mol⁻¹; (tBuA): log(A/L mol⁻¹ s⁻¹) = 7.28 and E_A = 17.5 kJ mol⁻¹ and (EEA): log(A/L mol⁻¹ s⁻¹) = 6.80 and E_A = 13.8 kJ mol⁻¹. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6641–6654, 2009     

Polymer drying. IV. A molecular interpretation of polymer drying

Further consideration of data already collected, in many time-studies that monitored gravimetrically evaporations of acetone, toluene, and chloroform from the corresponding liquid-saturated poly(styrene-co-divinylbenzene) samples, show that α_t, the residual number of sorbed molecules per phenyl group at a given level of desorption, is a linear function of α_s, the number of adsorbed molecules per accessible phenyl group in the polymer at thermodynamic equilibrium with excess test-liquid. Such linear relationships were noted at successive break-points in the kinetics of desorption (from gel-saturation to virtual dryness) that signaled sequentially (a) incipient elimination of adsorbed molecules from polymer in the rubbery state, (b) incipient transition from the rubbery state to the glassy state, (c) completion of this transition, and (d) successive depletions of five of the six populations of residual adsorbed molecules that were trapped in six different molecular environments by rigidification of the system during the transition. These results support the viewpoint that α_s is a parameter that reflects how well the molecular structure of the sorbed molecule can be accommodated by that of the monomer unit of the polymer.     

Very low-pressure pyrolysis. VIII. The decomposition of Di-t-amyl peroxide

The very low-pressure pyrolysis (VLPP) technique has been applied to the pyrolysis of di-t-amyl peroxide (DTAP) over the temperature range 523-633°K. VLPP yields a low-pressure rate constant, k_uni The conversion of k_uni to k_∞ which must be made to calculate the Arrhenius parameters, is accomplished via the RRKM theory. The transition state model used in the RRKM calculations was based on a transition state model which accurately reproduced the VLPP data for di-t-butyl peroxide for which the Arrhenius parameters are well known. For the decomposition of DTAP it was found that log k_∞(300°K) = 15.8 - 36.4/θ, where θ = 2.303RT, in kcal/mole, and the units of k_∞, are sec^?1.     

Olefin copolymerization with metallocene catalysts. II. Kinetics,cocatalyst, and additives

The kinetics of ethylene/propylene copolymerization catalyzed by (ethylene bis (indeyl)-ZrCI₂/methylaluminoxane) has been investigated. Radiolabeling found about 80% of the Zr to be catalytically active. The estimates for rate constants at 50°C are k₁₁ = 1104 (M_s)^?1, k₁₂ = 430 (M_s)^?1, k₂₂ = 396 (M_s)^?1,k₂₁ = 1020 (M_s)^?1, and k^A_tr,1 + k^A_tr.2 = 1.9 × 10^?3 s^?1. Substitution of trimethylaluminum for methylaluminoxane resulted in proportionate decrease in polymerization rate. The molecular weight of the copolymer is slightly increased by loweing the [Al]/[Zr] ratio, or addition of Lewis base modifier but at the expense of lowered catalytic activity and increase in ethylene content in the copolymer. Lowering of the polymerization temperature to 0°C resulted in a doubling of molecular weight but suffered 10-fold reduction in polymerization activity and increase of ethylene in copolymer.     

Rates and equilibria in the reaction system Br + i-C4H10 ⇌ HBr + t-C4H9. The heat of formation of the t-butyl radical

The very low pressure reactor (VLPR) technique has been used to measure the bimolecular rate constant of the title reaction at 300 K. The rate constant is given by log k₁ (1/mol s) = (11.6 ± 0.4) ? (5.9 ± 0.6)/θ the equilibrium constant has also been measured at the same temperature and is given by K₁ = (5.6 ± 1) × 10^?3 and hence log k_?1 (1/mol s) = 9.5 ± 0.1. The results show that the reaction Br + t? C₄H₉ → HBr + i? C₄H₈ is unimportant under the present experimental conditions. Assigning the entropy of t-butyl radical to be 74 ± 2 eu which is in the possible range, the value of K₁ gives ΔH (t-butyl) = 9.1 ± 0.6 kcal/mol^?1. This yields for the bond dissociation, DH° (t-butyl-H) = 93.4 ± 0.6 kcal/mol. Both of these values are found to be in good agreement with recent VLPP studies.     

Absolute propagation rate coefficients in radical polymerization from gel permeation chromatography of polymers produced by intermittent initiation

The pulsed laser polymerization technique is now a well accepted method to determine propagation rate coefficients for radical polymerization from molar mass distributions resulting from intermittent initiation. A simplified apparatus for the periodic photoinitiation is used which is much less expensive than the laser equipment. The usefulness of the simplified equipment was proved by the determination of k_p for styrene at technically relevant temperatures up to 130°C for the first time. Furthermore, careful inspection of the molar mass distribution (mmd) reveals that depending on the reaction conditions, inflection points (L_i) can not only be found at integer multiples of k_p • t_o • [M] but also at 0.5ⁱ • k_p • t_o • [M], i = 1, 2, 3, … . A rule to find the inflection points leading to correct values for k_p is proposed. It is shown that the shape of the mmd inter alia depends on the amount of primary radical termination compared to the termination reaction between growing chains. With dominant primary termination, the maxima of the distribution will give the correct k_p, whereas in the absence of primary termination the inflection points should be used. Experimental conditions like initiator concentration, light intensity etc. may influence the position of the L_i at least to some extent, and so may give a small but principal error or uncertainty in k_p. A new mathematical method for the time-dependent simulation of the resulting mmd is presented which allows the calculations being performed on a PC within an acceptable time.     

Alignment of Heptagonal Diimide and Triazine Enables Narrowband Pure-Blue Organic Light-Emitting Diodes with Low Efficiency Roll-Off

The endeavor to develop high-performance narrowband blue organic light-emitting diodes (OLEDs) with low efficiency roll-off represents an attractive challenge. Herein, we introduce a hetero-acceptor design strategy centered around the heptagonal diimide (BPI) building block to create an efficient thermally activated delayed fluorescence (TADF) sensitizer. The alignment of a twisted BPI unit and a planar diphenyltriazine (TRZ) fragment imparts remarkable exciton dynamic properties to 26tCz-TRZBPI, including a fast radiative decay rate (k_R) of 1.0×10⁷ s⁻¹ and a swift reverse intersystem crossing rate (k_RISC) of 1.8×10⁶ s⁻¹, complemented by a slow non-radiative decay rate (k_NR) of 6.0×10³ s⁻¹. Consequently, 26tCz-TRZBPI facilitates the fabrication of high-performance narrowband pure-blue TADF-sensitized fluorescence OLEDs (TSF-OLEDs) with a maximum external quantum efficiency (EQE_max) of 24.3 % and low efficiency roll-off even at a high brightness level of 10000 cd m⁻² (EQE₁₀₀₀₀: 16.8 %). This showcases a record-breaking external quantum efficiency at a high luminance level of 10000 cd m⁻² for narrowband blue TSF-OLEDs.     

Polymer drying. V. Intramolecular-rotor fluorescence studies of evaporation from liquid-saturated poly(styrene-co-divinylbenzene)

The fluorescence intensity and residual weight of poly(styrene-co-divinylbenzene) saturated with a < 0.003M solution of a intramolecular-rotor-fluorescent probe-molecule in a volatile liquid were monitored simultaneously as the system evaporated at 23°C to virtual dryness. The “breakpoints” in the pattern for fluorescence increase coincided with the “breakpoints” in the kinetics of desorption with respect to the number, α_t of residual sorbed volatile molecules per phenyl group in the polymer, showing that both time-studies reflect the same physical changes in the system that occur reproducibly as α_t decreases monotonically through α′_s and α′_g the compositions that signal respectively incipient elimination of volatile molecules immobilized by adsorption to polymer, and incipient transition of the system from the rubbery state to the glassy state. The fluorescence intensity attained its asymptotic limit before α_t became equal to α_g the composition identified earlier to be that which marks completion of the transition to the glassy state.     

Effects of solvent on the reactions of coordination complexes. part 24. kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylene‐pentamine)cobalt(III) complexes in methanol+water media: The role of substrate hydrophobicity and solvent structure

The kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)cobalt(III) complexes, [(tetren)CoO₂CR]²⁺ (R= NH₂CH₂, pyridine‐2 ,  NH₂CH₂CH₂,  NH₂CH(CH₃) (αβS isomer); R= NH₂CH(CH₃) (αβR isomer)), have been investigated in methanol–water media (0–80 vol % MeOH) at 15.0≤t°C≤40.0 (0.02 mol dm⁻³ NaOH). The second‐order rate constant at zero ionic strength, k₂°, increases nonlinearly with X_MeOH. The transfer free energy of the initial state and the transition state of the amido conjugate base ([Δ_tG (i)]_(s←w)) for the glycinato‐ and pyridine‐2–carboxylato complexes have been calculated using the solubility data of their picrate salts, pK _NH date of their N‐protonated forms, and the k₂° values in mixed solvent media. The kinetic solvent effects have been interpreted in terms of preferential solvation of the initial state, transition state, and the solvent structure. The activation enthalpies and entropies varied nonlinearly with X_MeOH displaying extrema, which is attributable to the solvent structural effects on these thermodynamic parameters. It is also evident that the mutation process, αβR→αβS isomer for the α‐alaninato complex, where this isomerisation refers to the arrangement of the tetren skeleton around the planar secondary NH is sensitive to the nature of the cosolvent molecules and solvent structure. The mutation process is generally more favorable for the five coordinate amido conjugate bases than the initial state. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 55–64, 1999