Bootstrap Unit‐Root Tests: Comparison and Extensions

Abstract. In this article, we study and compare the properties of several bootstrap unit‐root tests recently proposed in the literature. The tests are Dickey–Fuller (DF) or Augmented DF, based either on residuals from an autoregression and the use of the block bootstrap or on first‐differenced data and the use of the stationary bootstrap or sieve bootstrap. We extend the analysis by interchanging the data transformations (differences vs. residuals), the types of bootstrap and the presence or absence of a correction for autocorrelation in the tests. We show that two sieve bootstrap tests based on residuals remain asymptotically valid. In contrast to the literature which focuses on a comparison of the bootstrap tests with an asymptotic test, we compare the bootstrap tests among themselves using response surfaces for their size and power in a simulation study. This study leads to the following conclusions: (i) augmented DF tests are always preferred to standard DF tests; (ii) the sieve bootstrap performs better than the block bootstrap; (iii) difference‐based tests appear to have slightly better size properties, but residual‐based tests appear more powerful.     

Adsorption behavior and effectiveness of poly(N,N‐dimethylacrylamide‐co‐Ca 2‐acrylamido‐2‐methylpropanesulfonate) as cement fluid loss additive in the presence of acetone‐formaldehyde‐sulfite dispersant

A copolymer of N,N‐dimethylacrylamide and Calcium 2‐acrylamido‐2‐methylpropanesulfonate was synthesized by free‐radical copolymerization. Its performance as anionic fluid loss additive (FLA) was studied by measuring static filtration properties of oil well cement slurries at 27°C and 70 bar pressure, respectively. It was found that cement filter cake permeability and API fluid loss decrease with increasing FLA dosage. Filtrate analysis revealed a linear correlation between fluid loss and the amount of FLA adsorbed on the cement surface. FLA adsorption on cement was determined by total organic carbon (TOC) analysis in cement filtrate and confirmed by ζ‐potential measurement. According to environmental scanning electron microscopy (ESEM) investigations, FLA does not alter the filter cake structure. In the presence of an anionic acetone–formaldehyde–sulfite (AFS) polycondensate dispersant, fluid loss control from FLA decreased and cement filter cake permeability increased because AFS reduces the amount of FLA adsorbed. In comparison to FLA, AFS shows stronger adsorption on the cement surface and succeeds in the competition with FLA. The different adsorption behavior of the two polymers is the reason for limited compatibility of this admixture com bination. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4341–4347, 2006     

Regioselective Palladium(0)‐Catalyzed Cross‐Coupling Reactions and Metal‐Halide Exchange Reactions of Tetrabromothiophene: Optimization,Scope and Limitations

The Suzuki reaction of tetrabromothiophene with arylboronic acids provides a regioselective approach to various 5‐aryl‐2,3,4‐tribromothiophenes, symmetrical 2,5‐diaryl‐3,4‐dibromothiophenes, and tetraarylthiophenes. Unsymmetrical 2,5‐diaryl‐3,4‐dibromothiophenes are prepared by Suzuki reaction of 5‐aryl‐2,3,4‐tribromothiophenes. Tetraarylthiophenes containing two different types of aryl groups are obtained by Suzuki reactions of 2,5‐diaryl‐3,4‐dibromothiophenes. During the optimization of the conditions of each individual reaction, the solvent, the catalyst and the temperature play an important role. In several cases, classical conditions [use of tetrakis(triphenylphosphane)palladium(0), Pd(PPh₃)₄, as the catalyst] gave excellent yields. The yields of those transformations which failed or proceeded sluggishly could be significantly improved by application of a new biarylmonophosphine ligand developed by Buchwald and co‐workers. Regioselective metal‐halide exchange reactions of tetrabromothiophene provide a convenient approach to various 2,5‐disubstituted 3,4‐dibromothiophenes. 5‐Alkyl‐2‐trimethylsilyl‐3,4‐dibromothiophenes could be prepared in one pot by sequential addition of trimethylchlorosilane and alkyl bromides. The reaction of tetrabromothiophene with methyl chloroformate and subsequent Suzuki reactions afforded 3,4‐diaryl‐2,5‐bis(methoxycarbonyl)thiophenes.     

Polycondensation of bis(cyanoacetate) and a,10bdihydrobenzofuro[2,3‐b]benzofuran‐2,9‐dicarbaldehyde via knoevenagel reaction: Synthesis of donor–acceptor polymers containing shoulder‐to‐shoulder main chains

Compound 5 a ,10 b ‐dihydrobenzofuro[2,3‐ b ]benzofuran‐2,9‐dicarbaldehyde ( II ) was prepared by two‐step reactions from p‐cresol and glyoxal. The bis(cyanoacetate) monomers ( III ) were prepared in a high yield by reacting ethyl cyanoacetate with the appropriate diol in the presence of tetra‐n‐butyl titanate. The polymers from II and III were synthesized by Knoevenagel polycondensation that was first carried out in anhydrous THF and followed by a solid‐state polycondensation, and main‐chain polymers with good glass transition temperatures in high yield were obtained. The polymerization of II and III afforded polymers IV , which exhibited good solubility in most organic solvents. The structure of all the monomers and polymers were characterized by conventional spectroscopic methods. The synthesized polymers contain acceptor groups (cyanide and carbonyl) and a donor group (benzodihydrofuran) in their main chain. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 505–511, 2001     

(nBuCp)2ZrCl2‐catalyzed ethylene‐4M1P copolymerization: Copolymer backbone structure,melt behavior,and crystallization

The judicious design of methylaluminoxane (MAO) anions expands the scope for developing industrial metallocene catalysts. Therefore, the effects of MAO anion design on the backbone structure, melt behavior, and crystallization of ethylene?4‐methyl‐1‐pentene (E?4M1P) copolymer were investigated. Ethylene was homopolymerized, as well as copolymerized with 4M1P, using (1) MAO anion A (unsupported [MAOCl₂]^?) premixed with dehydroxylated silica, (ⁿBuCp)₂ZrCl₂, and Me₂SiCl₂; and (2) MAO anion B (Si?O?Me₂Si?[MAOCl₂]^?) supported with (ⁿBuCp)₂ZrCl₂ on Me₂SiCl₂‐functionalized silica. Unsupported Me₂SiCl₂, opposite to the supported analogue, acted as a co‐chain transfer agent with 4M1P. The modeling of polyethylene melting and crystallization kinetics, including critical crystallite stability, produced insightful results. This study especially illustrates how branched polyethylene can be prepared from ethylene alone using particularly one metallocene‐MAO ion pair, and how a compound, that functionalizes silica as well as terminates the chain, can synthesize ethylene?α‐olefin copolymers with novel structures. Hence, it unfolds prospective future research niches in polyethyne systhesis. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1688–1706, 2016     

High Na‐ion conducting Na1+x[SnxGe2−x(PO4)3] glass‐ceramic electrolytes: Structural and electrochemical impedance studies

Na‐ion conducting Na_1+x[Sn_xGe_2?x(PO₄)₃] (x = 0, 0.25, 0.5, and 0.75 mol%) glass samples with NASICON‐type phase were synthesized by the melt quenching method and glass‐ceramics were formed by heat treating the precursor glasses at their crystallization temperatures. XRD traces exhibit formation of most stable crystalline phase NaGe₂(PO₄)₃ (ICSD‐164019) with trigonal structure. Structural illustration of sodium germanium phosphate [NaGe₂(PO₄)₃] displays that each germanium is surrounded by 6 oxygen atom showing octahedral symmetry (GeO₆) and phosphorous with 4 oxygen atoms showing tetrahedral symmetry (PO₄). The highest bulk Na⁺ ion conductivities and lowest activation energy for conduction were achieved to be 8.39 × 10^?05 S/cm and 0.52 eV for the optimum substitution levels (x = 0.5 mol%, Na_1.5[Sn_0.5Ge_1.5(PO₄)₃]) of tetrahedral Ge⁴⁺ ions by Sn⁴⁺ on Na–Ge–P network. CV studies of the best conducting Na_1.5[Sn_0.5Ge_1.5(PO₄)₃] glass‐ceramic electrolyte possesses a wide electrochemical window of 6 V. The structural and EIS studies of these glass‐ceramic electrolyte samples were monitored in light of the substitution of Ge by its larger homologue Sn.     

Triplex‐Forming Twisted Intercalating Nucleic Acids (TINAs): Design Rules,Stabilization of Antiparallel DNA Triplexes and Inhibition of G‐Quartet‐Dependent Self‐Association

The majority of studies on DNA triple helices have been focused on pH‐sensitive parallel triplexes with Hoogsteen CT‐containing third strands that require protonation of cytosines. Reverse Hoogsteen GT/GA‐containing antiparallel triplex‐forming oligonucleotides (TFOs) do not require an acidic pH but their applicability in triplex technology is limited because of their tendency to form undesired highly stable aggregates such as G‐quadruplexes. In this study, G‐rich oligonucleotides containing 2–4 insertions of twisted intercalating nucleic acid (TINA) monomers are demonstrated to disrupt the formation of G‐quadruplexes and form stable antiparallel triplexes with target DNA duplexes. The structure of TINA‐incorporated oligonucleotides was optimized, the rules of their design were established and the optimal triplex‐forming oligonucleotides were selected. These oligonucleotides show high affinity towards a 16 bp homopurine model sequence from the HIV‐1 genome; dissociation constants as low as 160 nM are observed whereas the unmodified TFO does not show any triplex formation and instead forms an intermolecular G‐quadruplex with T_m exceeding 90 °C in the presence of 50 mM NaCl. Here we present a set of rules that help to reach the full potential of TINA‐TFOs and demonstrate the effect of TINA on the formation and stability of triple helical DNA.     

Biodegradable polymers based on renewable resources. V. Synthesis and biodegradation behavior of poly(ester amide)s composed of 1,4:3,6‐dianhydro‐D‐glucitol, α‐amino acid,and aliphatic dicarboxylic acid units

A series of poly(ester amide)s were synthesized by solution polycondensations of various combinations of p‐toluenesulfonic acid salts of O,O′‐bis(α‐aminoacyl)‐1,4:3,6‐dianhydro‐D ‐glucitol and bis(p‐nitrophenyl) esters of aliphatic dicarboxylic acids with the methylene chain lengths of 4–10. The p‐toluenesulfonic acid salts were obtained by the reactions of 1,4:3,6‐dianhydro‐D ‐glucitol with alanine, glycine, and glycylglycine, respectively, in the presence of p‐toluenesulfonic acid. The polycondensations were carried out in N‐methylpyrrolidone at 40°C in the presence of triethylamine, giving poly(ester amide)s having number‐average molecular weights up to 3.8 × 10⁴. Their structures were confirmed by FTIR, ¹H‐NMR, and ¹³C‐NMR spectroscopy. Most of these poly(ester amide)s are amorphous, except those containing sebacic acid and glycine or glycylglycine units, which are semicrystalline. All these poly(ester amide)s are soluble in a variety of polar solvents such as dimethyl sulfoxide, N,N‐dimethylformamide, 2,2,2‐trifluoroethanol, m‐cresol, pyridine, and trifluoroacetic acid. Soil burial degradation tests, BOD measurements in an activated sludge, and enzymatic degradation tests using Porcine pancreas lipase and papain indicated that these poly(ester amide)s are biodegradable, and that their biodegradability markedly depends on the molecular structure. The poly(ester amide)s were, in general, degraded more slowly than the corresponding polyesters having the same aliphatic dicarboxylic acid units, both in composted soil and in an activated sludge. In the enzymatic degradation, some poly(ester amide)s containing dicarboxylic acid components with shorter methylene chain lengths were degraded more readily than the corresponding polyesters with Porcine pancreas lipase, whereas most of the poly(ester amide)s were degraded more rapidly than the corresponding polyesters with papain. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2721–2734, 2001     

Carbon/carbon composites derived from poly(ethylene oxide)‐modified novolac‐type phenolic resin: Microstructure and physical,and morphological properties

A poly(ethylene oxide) (PEO) novolac‐type phenolic resin blend was prepared by the physical blending method. The modified novolac‐type phenolic resin with various PEO contents was used as a matrix precursor to fabricate carbon/carbon composites. The effect of the PEO/phenolic resin mixing ratio on the change of the density and of the porosity was studied. The flexural strength and interlaminar shear strength of the PEO/phenolic resin blend‐derived carbon/carbon composites were also investigated. The results show that the density of the PEO/phenolic resin blend‐derived carbon/carbon composites decreases with the PEO content. The X‐ray diffraction and Raman spectra studies showed that the carbon fiber in the samples will affect the growth of the ordered carbon structure. From SEM morphological observation, it is shown that the fracture surface of specimens is smooth. Also, there is less fiber pull‐out and fiber breakage on the fracture surface. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1609–1619, 2002; DOI 10.1002/app.10407     

REVO4‐Based Nanomaterials (RE = Y,La, Gd,and Lu) as Hosts for Yb3+/Ho3+, Yb3+/Er3+, and Yb3+/Tm3+ Ions: Structural and Up‐Conversion Luminescence Studies

Rare‐earth vanadates of the form REVO₄ (RE = Y, La, Gd, and Lu) doped by Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺_, or Yb³⁺/Tm³⁺ lanthanide ions were successfully synthesized using the sol–gel method and annealing at 600°C in an air atmosphere. The structure and morphology of the prepared nanocrystals were investigated by X‐ray diffraction, thermogravimetric analysis, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. All prepared materials were homogenous and had nanosized dimensions. Their elemental compositions were confirmed by optical emission spectrometry. Spectroscopic analysis of the materials was carried out by measuring excitation and emission spectra, luminescence decays, and dependence between the intensity of the luminescence and the laser energy. Following effective excitation by NIR radiation, Ln³⁺ co‐doped vanadate matrices exhibited a strong up‐conversion (UC) luminescence. Differences in spectroscopic properties between monoclinic LaVO₄ and tetragonal YVO₄, GdVO₄, or LuVO₄ doped by Ln³⁺ ions were observed, indicating the influence of the crystal structure on the UC emission. Drawing conclusions from these spectroscopic investigations, the UC mechanisms were proposed, including energy‐transfer processes between Yb³⁺ ions and emitting ions.