Synthetic thermally reversible gel systems. VIII. Poly(vinyl chloride)–dioxane

When interpreted by network theory, equilibrium swelling measurements on poly-(vinyl chloride) (PVC) film in dioxane and moduli measurements on the equilibrium swelled films yield values for the approximate molecular weight between thermally reversible crosslinks and for the number of these crosslinks per polymer chain. These values are in reasonable agreement with the thermodynamic analysis of PVC–dioxane gels by Takahashi, Nakamura, and Kagawa and with the premise that three-dimensional network formation in these gels occurs by crystallization of a very limited number of syndiotactic sequences per chain having a sequence length of between 8 and 10. Failure to observe fusion endotherms by DTA on PVC–dibutyl phthalate gels supports the view that PVC gels have a low crystalline crosslink density and a low heat of crosslinking. The heat of crosslinking obtained by the method of Eldridge and Ferry shows only moderate agreement with expectations based on the heat of fusion of PVC and the number of repeating units per PVC chain passing through a crystalline crosslink in a PVC–dioxane gel.     

Dichlorotetrafluoroacetone as a derivatisation reagent in the analysis of mandelic acids in human urine

Dichlorotetrafluoroacetone has been used to prepare 4-substituted 2-bis (chlorodifluoromethyl)-1,3-dioxolan-5-one derivatives of mandelic acids which were found to be suitable for the analysis of these compounds by gas chromatography-negative-ion chemical ionisation mass spectrometry (GC-NICIMS). The high specificity of the derivatising agent facilitates the identification and quantitation of small amounts of mandelic acids in complex biological matrices. The derivatisation procedure was used to determine the concentrations of m- and p-hydroxymandelic acids and vanillylmandelic acid in to the urine of normal subjects. The method may also have application in the determination of isomeric phenylethylene glycols, the corresponding products of reductive metabolism of biogenic amines.     

Synthetic thermally reversible gel systems. VI

Forming and conditioning thermally reversible aqueous gels of polyacrylyglycinamide (PAG) at various temperatures has little effect on either the melting point (T_m) of the gels or the heat of crosslinking (ΔH_c) except at temperatures where partial hydrolysis can occur. This is added evidence that unlike with gelatin, crystallite formation does not play a role in gel formation. For unfractioned PAG, the linear relationship between the logarithm of molecular weight and 1/T_m predicted and observed for gelatin gels, does not hold. With mixed gelatin-PAG gels, a gelatin/PAG ratio of ≥4 completely inhibits the formation of a PAG gel network. At lower gelatin/PAG ratios, the PAG network forms, and if gelatin is considered as an inert diluent, normal values for the melting points and ΔH_c for PAG gels are observed. At a gelatin/PAG ratio of 4, the presence of PAG reduces the ΔH_c for the gelatin gel by inhibiting the formation of as large or as ordered crystallite crosslinks. To reconcile the problem of aggregation preceding gelation one can assume that M?_w of an aggregate is a linear function of C². If this is done, the same relationship which normally relates C with T_m is obtained. The equilibrium swelling of PAG films in water at 25°C is markedly molecular weight-de-pendent and can vary from below 5 to about 40 wt-% polymer at equilibrium. It has been found that long-term dark storage of dry samples of PAG under ambient temperature conditions results in pronounced decreases in the intrinsic viscosities of their aqueous solutions. It is speculated that this results from weak links, perhaps peroxy, in the polymer backbone. The possible relationship of this phenomenon to the slow stage of the viscosity deterioration of aqueous polyacrylamide solutions is pointed out. The higher viscosity of low DP PAG in 2M NaCNS compared to H₂O and the larger percentage increse of [η] with increasing temperature in the latter, verify the greater solvent power of 2M aqueous thiocyanate for PAG. At a concentration level of 3%, aqueous PAG solutions are almost Newtonian whereas at higher concentrations (5%), the viscosity decreases appreciably with increasing rates of shear. The copolymerization of AG with isopropylacrylamide has been studied and the somewhat unusual results discussed. Copolymers containing an AG mole fraction greater than 0.40 do not exhibit a cloud point up to 100°C. If the isopropylacrylamide mole fraction approaches 0.60, the solutions do not gel down to 0°C. This ability to prepare copolymers over a narrow composition range that neither gel or undergo phase separation in the temperature range 0–100°C is probably related to the random distribution of monomer units in the copolymer backbone.