Fluorescence of poly(di-n-alkylsilane)s in room-temperature solution

A comprehensive study of the electronic absorption and emission spectra and the fluorescence quantum yield and lifetime of seven poly(di-n-alkylsilane)s and of three isotopically labelled poly(di-n-hexylsilane)s in hydrocarbon solution at room temperature is reported. Also reported are fluorescence polarization and carbon tetrachloride quenching of fluorescence of poly(di-n-hexylsilane). The observed fluorescence spectra, quantum yield, and polarization depend on the selected excitation energy in a very characteristic fashion, whereas the fluorescence lifetime does not; however, it depends on the selected emission energy. These characteristic dependencies are qualitatively accounted for by the previously proposed segment distribution model if one assumes that the photophysical behavior at higher excitation energies is strongly affected by the presence of a low-lying weakly allowed state in short-segment chromophores and the behavior at lower excitation energies is dictated by the selective excitation of emitting long-segment chromophores.     

Fluorescence quenching of poly(methylphenylsilane) in solution

The solution, electron-transfer fluorescence quenching of a typical aromatic polysilane[poly(methylphenylsilane)] by a series of electronic deficient aromatic monomers is described. The rate of fluorescence quenching is a function of the reduction potential of the quencher, and only very fast processes can be observed, due to the short polymer fluorescence lifetime. The measurement of quenching rate constants, which are considerably larger than diffusion control, suggests that extensive energy migration occurs in the polymer. Although the fluorescence quenching at low quencher concentrations follows Stern-Volmer kinetics, at high concentrations, contributions from static quenching are apparent. Strong fluorescence quenching can either accelerate or inhibit photodegradation, depending on the structure of the quencher.     

Photolysis of poly(benzyl methacrylate)s and poly(benzyl acrylate)s in solution and films

Methacrylate and acrylate copolymers containing benzyl or 1‐phenylethyl groups and their monomeric model compounds were irradiated with a 254‐nm light in CH₂Cl₂ and solid films. Low molecular weight and polymeric products were analyzed by gas chromatography (GC) and NMR spectroscopy, respectively, and main‐chain scission efficiencies were determined by gel permeation chromatography (GPC). The results indicate that the ester bond cleavage in the side chain produces alkyl radicals in the main chain, leading to main‐chain scission and crosslinking. The higher stability of tertiary alkyl radicals formed in methacrylate polymers lead to the predominant main‐chain scission in solution. On the other hand, acrylate polymers were less susceptible to photodegradation. The degradabilities of the polymer films reflected those of the polymer solutions, although crosslinking preferentially occurred. The distinct effect of oxygen on the degradation was also observed in solution and films. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2227–2236, 2001     

Dean’s flow of aqueous solution of poly(ethylene oxide)

In this research the flow of Newtonian and drag reducing fluids through a helical tube,i.e.. Dean’s flow, was experimentally studied. The primary concern was to investigate the effect of added polymer on the secondary motion caused by the centrifugal force. The poiymer chosen in this study was poly(ethylene oxide) with the molecular weights of 300,000, 900,000 and 4,000,000 and the solvent was distilled water. The concentration range was 0 to 100 wppm. The Dean number investigated was in the range of 10 to 20,000. In the case of distilled water, the experimental data were in good agreement with the literature. In the case of polymer solutions, it was found that the secondary motion was supressed as the concentration and molecular weight increased. However, if the molecular weight or concentration exceeded certain values, the effects were saturated. The results were also analyzed from the view point of drag reduction phenomena.     

Supertough poly(lactide)s

Summary Semi-crystalline and amorphous copolymers of lactide and glycolide were rubber modified with degradable rubbers based on -caprolactone. The influence of crystallinity of the matrix, type of rubber and chain architecture on the impact resistance of the resulting materials was investigated. With a poly(l-lactide-co--caprolactone) rubber semi-crystalline poly(lactide)s could be impact modified to a greater extent than amorphous non-crystallizable lactide matrices. Poly(trimethylene carbonate-co--caprolactone) was used in blends and linear and star-shaped block copolymers which yield supertough materials that do not break in Izod notched impact testing. Rubber content appears critical around 20 weight percent, where a sharp transition is observed.     

Macromolecular assemblies of segmented poly(ester urethane)s and poly(ether urethane)s

Polyurethanes containing different soft and hard segments were investigated by fluorescence and scanning electron microscopy. The polarity dependence of the vibrational structure of the pyrene emission spectrum indicated the formation of aggregates at concentrations, which are significantly below the critical concentrations, which define the separation of dilute‐semidilute domains. Unlike the samples with 4,4′‐methylene diphenylene diisocyanate, the samples with 2,4‐tolylene diisocyanate in hard segments give the fluorescence spectra in which the pyrene excimer appears. The supermolecular structures associated with the form of spherulites or of spherical micelles were detected by scanning electron microscopy. The results were compared with previous reports concerning viscometric data. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal degradation kinetics of para‐substituted poly (styrene peroxide)s in solution

The thermal degradation of three polymeric peroxides of styrene monomers with substituents in the para position was studied at various temperatures (65, 75, 85, and 95°C). A continuous distribution model was used to evaluate the rate coefficients for the random‐chain and chain‐end scission degradation from the evolution of molecular weight distributions with time. The activation energy determined from the temperature dependence of the rate coefficients was in the range 18–22 kcal mol^?1. This result suggests that the thermal degradation of polyperoxide is controlled by the dissociation of the O—O bonds in the polymer backbone. The thermal stability for poly(p‐methylstyrene peroxide) lies in between that of poly(p‐tert‐butylstyrene peroxide) (highest) and poly(p‐bromostyrene peroxide) (lowest). © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 957–961, 2002     

Self-diffusion of poly(p-phenyleneethynylene)s in dilute solution determined by pulsed-field-gradient NMR

Self-diffusion of poly[(2,5-didodecyl-p-phenylene)ethynylene]s (didodecyl-PPE) was measured in dilute CDCl₃ by pulsed-field-gradient NMR spectroscopy. The reduced versions of these didodecyl-PPEs, poly[2,5-didodecyl-p-xylylene]s (didodecyl-PPX), were also examined as random-coil analogs of equivalent chain lengths (14-120 repeat units). The coefficients for infinite-dilution self-diffusion (D₀) and their concentration dependence (k_F) were determined from plots of the self-diffusion coefficients versus concentration. These quantities were fit to power laws with molecular weight (M₀) and their scaling constants determined from double-logarithmic plots. For didodecyl-PPX, D₀ scales with M₀ by −0.50±0.03 (ν) and k_F scales with M₀ by 0.51±0.08 (α), consistent with a random-coil conformation for this polymer in solution. For didodecyl-PPE, D₀ scales with M₀ by −0.71±0.06 (ν) and k_F scales with M₀ by 0.96±0.06 (α), indicative of a semiflexible rod-like macromolecule.     

Intramolecular triplet energy migration in poly(2-vinylnaphthalene) and poly(naphthylalkyl methacrylate)s in solid solution: Effects of side chain length

Summary The effect of side chain length on intramolecular triplet energy migration of naphthalene containing polymers was investigated by a delayed fluorescence (DF) spectroscopy for the solid solution of the polymers. The degree of triplet energy migration depends strongly on whether the chromophores are directly attached to the main chain or not.     

Autoxidation of poly(alkylene glycols) in solution

Data reported for the autoxidation of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) in solution at relatively long reaction times were analyzed. A relatively simple and a more general kinetic scheme with corresponding rate expressions were used. It was found that the more general scheme gave somewhat more satisfactory agreement between calculated and observed values of several reaction variables. Limitations in the applications of both schemes to PEG and PPG autoxidations are mentioned.     

Preparation of substituted poly(vinylthiophene)s via lithiation of poly(vinylthiophene)

2-Vinylthiophene was homopolymerized using benzoyl peroxide as initiator. The polymer was lithiated using n-butyllithium. The ¹H-n.m.r. spectrum of the deuterated product obtained by quenching the lithiated polymer with deuterium oxide indicated that 93% lithiation had occurred, and that it had occurred very largely, if not entirely, at the α-position of the thiophene ring. The lithiated polymer was reacted with a range of electrophiles to give products containing new bonds to carbon (in various oxidation levels), sulphur, iodine, boron, phosphorus, silicon, or mercury.     

Conformational properties of poly(alkene sulphone)s in solution: 2. A rotational isomeric state study of the poly(cyclohexene sulphone) chain

The rotational isomeric states model, coupled with the Flory matrix method, was applied to the calculation of the unperturbed mean-square end-to-end distance in poly(cyclohexene sulphone) as a function of several parameters. The calculations were performed for atactic, isotactic and syndiotactic chains; the tacticity arises from the two possible ways, D and L, in which the rings can be attached to the main chain, assuming that the CC bonds are all in the t conformation, as indicated by dielectric measurements. One of the three conformations about each CS bond is strongly hindered and was given zero statistical weight whereas the other two were assigned weights determined by second-order effects, operating over three or more consecutive bonds, and dependent on the chirality of the rings and the coulombic interactions between adjacent dipoles.The calculations for the atactic model, in which a Monte Carlo procedure was used to generate the chain, were found to agree with experiment only if the parameter α was given a value of 5 or greater. This parameter is determined by the relative orientation of successive sulphone dipoles and was expected to have a value less than unity. It appears that specific solvation effects help to stabilize (t, g) and (g, t) CSC bond pairs relative to t, t. Other parameters are not so important.The calculations for the isotactic and syndiotactic models show interesting features, particularly for certain limiting conditions when cyclic or helical structures are generated. For these models there is a greater sensitivity to the various parameters than in the atactic case.     

Biodegradability of poly(ethylene terephthalate) copolymers with poly(ethylene glycol)s and poly(tetramethylene glycol)

Poly(ethylene terephthalate) copolymers were prepared by melt polycondensation of dimethyl terephthalate and excess ethylene glycol with 10–40mol% (in feed) of poly(ethylene glycol) (E) and poly(tetramethylene glycol) (B), with molecular weight (MW) of E and B 200–7500 and 1000, respectively. The reduced specific viscosity of copolymers increased with increasing MW and content of polyglycol comonomer. The temperature of melting (T_m), cold crystallization and glass transition (T_g) decreased with the copolymerization. T_m depression of copolymers suggested that the E series copolymers are the block type at higher content of the comonomer. T_g was decreased below room temperature by the copolymerization, which affected the crystallinity and the density of copolymer films. Water absorption increased with increasing content of comonomer, and the increase was much higher for E1000 series films than B1000 series films. The biodegradability was estimated by weight loss of copolymer films in buffer solution with and without a lipase at 37°C. The weight loss was enhanced a little by the presence of a lipase, and increased abruptly at higher comonomer content, which was correlated to the water absorption and the concentration of ester linkages between PET and PEG segments. The weight loss of B series films was much lower than that of E series films. The abrupt increase of the weight loss by alkaline hydrolysis is almost consistent with that by biodegradation.     

Fluorescence from poly(N-vinylcarbazole) in uniaxially stretched polymer films

Steady-state and time-resolved fluorescence properties of poly(N-vinylcarbazole) (PVCz) dispersed in a polystyrene (PS) cast film were studied under tensile loadings at room temperature. The excited monomer emission of PVCz located around 350 nm decreased with increasing applied tensile strain from 0 to 0.8%. The strain enhanced the emission which was ascribed to the partial-overlap excimer of PVCz in a 360–430 nm region. The emission due to the full-overlap excimer of PVCz between 430 and 500 nm was unchanged by the action of the tensile loadings. The ratio of fluorescence intensities at 375 nm and 345 nm I₃₇₅/I₃₄₅ was proportional to the applied strain. The time-resolved fluorescence study indicated that the lifetimes of the excited monomer and of the partial-overlap excimer were not affected by the strain. The obtained results mean that the strain applied to the PS matrix increases the partial-overlap conformation of two adjacent carbazolyl chromophores in a PVCz chain and suggest that PVCz is a useful probe for detecting residual strains in polymer matrices. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1569–1573, 1997     

Electrochemical behavior of poly(3-hexylthiophene). Controlling factors of electric current in electrochemical oxidation of poly(3-hexylthiophene)s in a solution

Electrochemical response of regio-random and regio-controlled poly(3-hexylthiophene), P3HexTh, was investigated by cyclic voltammetry. P3HexTh underwent electrochemical oxidation at about 0.4 V vs. Ag⁺/Ag in a THF solution, and the peak anode electric current, i_pa, was proportional to the sweeping rate v; i_pa=const×v^1/2. These data indicated that diffusion of the P3HexTh molecule in the solution was important to determine i_pa. Application of a Matsuda's equation with assumptions gave a diffusion coefficient, D, of about 1×10⁻⁷ cm² s⁻¹ at molecular weight of about 5000, and the D value steeply decreased with increase in the molecular weight.     

Electroluminescence of poly(phthalidylidene arylene)s

Electroluminescence of thin films of poly(3,3-phthalidylidene-4,4′-biphenylylene) (PPB) and poly(3,3-phthalidylidene-2,7-fluorenylylene) (PPF) was studied. In both cases, electroluminescene was found to be of a threshold origin. Emission maxima and quantum efficiencies for PPB and PPF are 490 and 525 nm, 0.01 and 0.1%, respectively. The electroluminescent characteristics were found to be strongly dependent on the polymer used.     

Crosslinking of poly(arylene disulfide)s and poly(arylene sulfane)s derived from cyclic(arylene disulfide) oligomers

Cyclic(arylene disulfide) and polycyclic(arylene sulfide) oligomers were synthesized by catalytic oxidation of arylenedithiols or arylenetrithiols with oxygen in the presence of a copper‐amine catalyst. These cyclic(arylene sulfide) oligomers can undergo free radical ring‐opening polymerization at an elevated temperature in the melt or solution. Polycyclic(arylene sulfide) oligomers can be used to crosslink poly(arylene disulfide)s and poly(arylene sulfane)s derived from cyclic(arylene disulfide) oligomers and elemental sulfur. The crosslinking reactions were investigated by differential scanning calorimetry and by solubility of the cured products. The minimum concentrations of polycyclic(arylene sulfide) oligomers for producing a well crosslinked poly(arylene disulfide) and poly(arylene sulfane) are 7 wt % and 10 wt %, respectively. The crosslinking reactions between poly(arylene disulfide)s, poly(arylene sulfane)s, and triallyl‐1,3,5‐triazine‐2,4,6(1H,3H,5H)‐trione (TTT) were also investigated. TTT is more efficient for crosslinking than the synthesized polycyclic(arylene disulfide) oligomers. The crosslinkable poly(arylene disulfide)s and poly(arylene sulfane)s could be potentially used as high temperature coatings, sealants, adhesives, and as matrices for high performance thermoset composites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3069–3077, 1999