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SynopsisThe melting phenomena of aqueous polymer solutions and gels have been investigated by differential scanning calorimetry (DSC). The polymers used were synthetic polyacrylamide and poly(viny1 alcohol) samples as well as guar and xanthan gums. By using an empirical relation, the energy measured from the area under the melting peak yielded heats of mixing and sorption, when fitted by an association factor computed from the data. This factor (independent of the concentration) is a measure of the water fraction associated with the polymer and has a definite and characteristic value for a given polymer in water. When a crosslinking agent (potassium pyroantimoniate or chromic nitrate) was added to the water-polymer system, the association factor varied with the polymer concentration ; the macromolecular chains thus become less accessible to penetrating water. If a branched gel was obtained owing to the formation of chemical ci-osslinks, a hump appeared on the melting peak.

Dimethyl sulfoxide (DMSO), a polar liquid of low volatility, was used as solvent in the synthesis of branched glycidyl azide polymer (GAP) which is a potential energetic binder for rocket propellants. Branched GAP product was purified by an extraction method using dichloromethane to create an organic phase and a mixture of methanol−brine (50:50 in weight) as the extracting solution. The extraction was carried out in four steps, leaving only less than 1% DMSO in the purified branched GAP. The DMSO concentration remaining in the organic phase was deduced from the 1H NMR spectra taken after each extraction step. Finally, the physical performance of branched GAP was evaluated through the tensile properties, the glass transition temperature, and the thermal degradation of the energetic polyurethanes obtained after curing branched GAP with different isocyanate compounds. The effect of residual DMSO in branched GAP on the tensile properties of polyurethanes was also investigated.

SynopsisThe fusion of hydrogels containing ammonium nitrate (AN) has been investigated by differential scanning calorimetry (DSC). The polymers used were guar and xanthan gums as well as synthetic polyacrylamide polymers. Water in hydrogels could be classified into three types labeled as ordinary water (hump on the melting peak), intermediate water (broad component of the peak), and bound nonfreezing water (without any phase transition). The temperature of fusion of intermediate water was about 10" to 35OC lower than that of ordinary water. Intermediate and bound water was found in all the gels studied, whereas ordinary water existed mainly in mixtures with total water content higher than 62%. The presence, type, and concentration of a crosslinker had no effect on the amount of bound water in hydrogels containing AN. In such mixtures the amount of nonfreezing water increased with the polymer concentration as well with the AN proportion relative to water and represented in some cases up to 27% of the gel. Cold-crystallization was observed in all cases (except xanthan) and was probably initiated by AN or the crosslinking agent.

synopsisThe development of gel permeation chromatography (GPC) has provided a convenient tool for the rapid determination of molecular weight distribution. The question has arisen as to the suitability of the method for specification purposes. The present work, suggested by the Naval Air Systems Command, represents an attempt to assess the precision of the method through a series of tests carried out by a number of laboratories using identical procedures on the same samples. Ten laboratories agreed to take part. Naval Ordnance Station, Indian Head, worked out standard conditions for operation of the chromatograph, for calibration of the columns, and for analysis of the GPC curves. Two samples of polystyrene were used by the various organizations for calibration of their instruments. Number-average molecular weight, heterogeneity index, and cumulative molecular weight distribution curves were determined on four samples of carboxylterminated polybutadiene (CTPB) and two samples of hydroxyl-terminated polybutadiene (HTPB), all unidentified except by letter code. All laboratories used identical directions for setting up CTPB and HTPB calibration curves which were based on curves determined from vapor-pressure osmometer molecular weights and GPC count numbers of fractionated material. Variation among the different laboratories was 0.15 in heterogeneity index, and a maximum of 1200 in molecular weight provided one aberrant set of values was eliminated. The six samples had heterogeneity indices from 1.15 to 1.54, while molecular weight varied from approximately 3000 to 6000. The average coefficient of variation of the molecular weight values was 6.2 f 0.7%, which is quite acceptable. Variation in heterogeneity index was too great for specification purposes when considered among the different laboratories, but may be sufficiently good when measured by any one laboratory.