Analytic calculation of scaling dimensions: Tricritical hard squares and critical hard hexagons

The finite-size corrections, central chargesc, and scaling dimensionsx of tricritical hard squares and critical hard hexagons are calculated analytically. This is achieved by solving the special functional equation or inversion identity satisfied by the commuting row transfer matrices of these lattice models at criticality. The results are expressed in terms of Rogers dilogarithms. For tricritical hard squares we obtainc=7/10,x=3/40, 1/5, 7/8, 6/5 and for hard hexagons we obtainc=4/5,x=2/15, 4/5, 17/15, 4/3, 9/5, in accord with the predictions of conformal and modular invariance.     

Performance of a CW pumped Nd:YVO4 amplifier with kHz pulses

Until recently, simple and reliable high repetition-rate laser sources with nanosecond pulses much shorter than from conventional A-O Q-switch lasers were not available. However over the past 2 years we have developed such lasers based on proprietary fast E-O switching technology, which allows designs delivering 1 ns pulses and subnanosecond jitter for good synchronisation. The technology provides pulses with multi-kW peak power and repetition-rates to >100 kHz.Most recently, the performance of these short pulse lasers has been developed further by implementing oscillator/amplifier (master oscillator and power amplifier, MOPA) technology which increases the output to >1 W average power. Here we report on a simple model that has been used to predict the performance of the CW pumped Nd:YVO₄ amplifier used in the MOPA laser. The model is based on the well-known expressions for the saturated gain applying to laser pulses, but more usually applied to pulse-excited amplifiers. The model is shown to allow a good interpretation of the amplifier behaviour for kHz pulses and to be a useful tool for predicting the performance of the MOPA laser.     

Glass transitions in hydrogen-bonded polymer complexes

Mutual precipitates of poly (N, N-dimethyl acrylamide) and poly (4-hydroxystyrene) were collected from dioxane, methanol, or acetone. The glass transition (T_g) temperatures of the precipitates are higher than the weight-average values. Clear films cast from dimethylformamide solutions have lower T_g values. Complexation also occurred between poly (ethyl oxazoline) and poly (4-hydroxystyrene) in dioxane and between poly (vinyl pyrrolidone) and poly (4-hydroxystyrene) in methanol. Again, the glass transition temperatures of the precipitates are higher than the values for the blend films. The ΔC_p values associated with the glass transitions of the complexes are smaller than those of the blends having the same compositions. Negative excess heat capacities of mixing have been observed for several precipitates.     

FT–IR spectroscopic study on the photo- and photooxidative degradation of nylons

The progress of photo- and photooxidative degradation of nylon films were studied by FT–IR spectroscopy. The gases evolved from the photolysis of various nylons and their model amides were also analyzed. The formation of double bonds, changes of crystallinity, and the effect of amino and carboxyl end groups has been studied and discussed. The band shapes of the IR spectra pertaining to the carbonyl groups formed by thermal oxidation or photooxidation were found to be very similar, suggesting that the two oxidation mechanisms might be similar. The broadness of these bands indicates that the carbonyl groups may belong to more than one species. The photodegradation of nylons containing purposely inserted carbonyl groups formed ? CH?CH₂ groups. Carbonyl groups formed during oxidation, and present either as keto groups or part of N-acylamide units make nylons susceptible to degradation reactions entailing mainly a Norrish type II mechanism.     

Styrylpyridine-based epoxy resins: Synthesis and characterization

Polymers containing rigid aromatic structures in the chain backbone usually gave high thermal stability and good flammability resistance. Three glycidyl ethers of epoxy resins were prepared from 2,4-di(p-hydroxystyryl)pyridine (2,4-DGESP), 2,6-di-(p-hydroxystyryl)pyridine (2,6-DGESP), and 2,4,6-tri-(p-hydroxystyryl)pryidine (2,4,6-TGESP) to study the relationships of structure to polymer degradation. To prepare a highly crosslinked material, trimethoxyboroxine (TMB) was used as the curing agent. The relative char yields of the three different resins, as measured by TGA, were 2,4-DGESP ≈ 2,6-DGESP > 2,4,6-TGESP. The char yield of the cured 2,6-DGESP varied with different amounts of the TMB curing agent, and was higher than the uncured 2,6-DGESP. The oxygen index increased as a function of thermal curing time for the 2,6-DGESP epoxy resin. An intermolecular Diels–Alder reaction with 2,6-DGESP is proposed as a primary reaction during thermal curing.     

Compatibility studies of styrene and hydroxyl containing styrene copolymers with poly(ethylene oxide)

Copolymers of styrene with vinylphenyl trifluoromethyl carbinol, p-vinylphenyl trifluoromethyl carbinol, vinylphenyl hexafluorodimethyl carbinol, and p-vinylphenol are conditionally compatible with poly(ethylene oxide), depending on their composition and blending ratios, whereas copolymers of styrene and vinylphenyl methyl carbinol are much less compatible with poly(ethylene oxide), as determined by T_g criteria and differential scanning calorimetry. The crystallinity of poly(ethylene oxide) is changed in the copolymer/poly(ethylene oxide) blends, as indicated by depressions of the poly(ethylene oxide) melting point. Hydrogen-bond formation has been studied in two selected blends by infrared (IR) spectroscopy. Hydrogen bonding dissociation and reassociation as a function of temperature are reported. The conformation changes of poly(ethylene oxide) in the blends, the interaction between copolymer and poly(ethylene oxide) as well as in the reference blend, polystyrene/poly(ethylene oxide), are also investigated.