Effects of functionality and temperature on gel points in random polymerisation

First, the relationship between gel point and average functionality for an RA₂+R′B_f type polymerisation has been investigated by measuring the product of extents of reaction at gelation (α_c) for sebacoyl chloride (SC)/polyoxyproplyene (POP)diol/POP triol mixtures reacting at various initial dilutions in diglyme as solvent at 60°C. Intramolecular reaction always delays gelation and a generalisation of the Ahmad-Stepto gel point expression has been used to interpret the gelation data in terms of the ring-forming parameter λ′_ab. From the variation of λ′_ab with initial dilution, values of b, the effective bond length of the chain forming the smallest ring structure, have been derived. It is found that b decreases with average polyol functionality (f_w). This decrease apparently compensates for the increase in λ′_ab with f_w, so that ring formation is less sensitive to functionality than may have been expected. It is found that the Ahmad-Stepto expression does not predict a consistent relationship between λ′_ab, f_w and the gel dilution of reactive groups. Second, the effect of temperature on the gel point has been studied by measuring α_c for SC/POP triol mixtures reacting at various initial dilutions in diglyme as solvent at 27° C, 40° C and 60° C. At a given temperature, b decreases as triol molar mass increases, indicating that the POP residue of the chain forming the smallest ring structure is more flexible than the SC residue. This result is in keeping with those from previous investigations,^1,2 For a given triol, α_c decreases as temperature increases, indicating an increase in chain stiffness with temperature. Values of d In <r²>/dT are derived and found to be larger than those for linear chains.     

The gel point and network formation – theory and experiment

Summary Gel points, accounting for intramolecular reaction, are predicted using Ahmed-Rolfes-Stepto (ARS) theory. They are compared with experimental gel points for polyester (PES)-forming, polyurethane (PU)-forming and poly(dimethylsiloxane) (PDMS) polymerisations. The PES and PU polymerisations were from stoichiometric reaction mixtures at different initial dilutions and the PDMS ones were from critical-ratio experiments at different fixed dilutions of one reactant. The ARS predictions use realistic chain statistics to define intramolecular reaction probabilities and employ no arbitrary parameters. Universal plots of excess reaction at gelation versus ring-forming parameter are devised to enable the experimental data and theoretical predictions to be compared critically. Significant deviations between experiment and theory are found for the PU systems and the PDMS systems with higher molar-mass reactants. However, ARS theory fits well the results for the PES systems and the PDMS systems having lower molar-mass reactants. Possible reasons for these differences in behaviour are discussed in terms of the PU-forming reaction mechanism and the effects of entanglements in PDMS. Whilst ARS theory provides a good basis for gel-point predictions and can be applied to many types of polymerisation, more experimental systems at different initial dilutions and ratios of reactants still need to be studied and the various methods used for detecting gel points still need to be compared to enable a definitive assessment of its performance to be made.     

Precise determination of the point of sol-gel transition in carbon gel synthesis using a microwave heating method

A simple and precise method for determining the point of gelation in microwave-assisted synthesis of organic gels is reported. This method provide a relevant information in the carbon gels field, as it can be applied to stop the gelation process at the precise instant for using the material in further processes where it is necessary to have the material with a specific viscosity (i.e., point of gelation).     

Rheology of epoxy networks near the gel point

Summary Curing and gelation of diglycidyl ether of Bisphenol A with poly(oxypropylene)-diamine (Jeffamine D 400) was followed using dynamic mechanical measurements. The gel point was determined rheologically and the characteristic value of the loss factor tan at the gel point, (tan)_gel, was found to depend on the structure of the polymer. A negative normal force is generated in the shear dynamic mechanical measurement at the gel point.Dedicated to Dr. Karel Duek on the occasion of his 60th birthday for his important contribution to polymer science     

NMR based determination of minute acid functionality: end-groups in PET

Conditions are determined for the carbodiimide-mediated room temperature esterification of the carboxylic acid end-groups of poly(ethylene terephthalate) with hexafluoroisopropanol. The hexafluoroisopropyl ester is quantified with ¹⁹F NMR, using α,α,α-trifluorotoluene as a secondary standard. This provides a technique for accurate determination of minute amounts of carboxylic acid functionality in small samples of polymers, and potentially in animal and plant based foods.     

Automatic melting point determination of fats

The Mettler FP3 automatic dropping point apparatus was used for the determination of melting points of a variety of edible fat products. The instrument was particularly suitable for this purpose because of the availability-of different heating rates. Advantages were the fully automatic heating, the automatic and objective endpoint determination, the application for melted as well as solidified samples (shortening, margarine, butter), and good reproducibility of results. A standard laboratory method for melted fat samples has been suggested.     

Relaxation patterns of endlinking polydimethylsiloxane near the gel point

While the rheological behavior at the gel point (GP) itself is well known, many questions remain about the approach to the gel point (from either side). For studying this phenomenon, rheological experiments were performed on crosslinked polydimethylsiloxanes (PDMS) and characteristic patterns in the relaxation behavior were abstracted into a phenomenological model for the relaxation time spectrum. The well known power law with slope-n and cutoff at the longest relaxation time λ _max governs the relaxation near the gel point. Beyond the gel point, an additional box-like contribution appears in the spectrum. The relaxation exponent n decreases with increasing extent of crosslinking. The longest relaxation time λ _max and equilibrium modulus G_e show power-law scaling with the distance from the gel point, |p − p_c|. For samples with imbalanced stoichiometric ratios r ≥ 1, the function n (p − p_c) is independent of r. Samples with strong crosslinker deficiency (r = 0.5) exhibit a different functional form for n (p − p_c) from those with r ≥ 1. The power law relaxation with decreasing n in the terminal zone was found previously for a different crosslinking system. It seems to be a common pattern for partially crosslinked materials from polymeric precursors. Received: 11 December 1997/Revised version: 18 December 1997/Accepted: 18 December 1997     

Rheology of diallyl phthalate polymers below the gel point

The analysis of molding operations for thermosetting polymers requires information on the reaction rates and rheology of the materials. The purpose of this research was to measure kinetic and rheological data on diallyl phthalate resins and to develop models describing the flow behavior. The rheological data were measured with a mechanical spectrometer equipped with eccentric rotating discs. For the region below the gel point of the polymer, the elastic modulus and viscosity were correlated with molecular weight, temperature and shear rate through fairly simple models. By combining kinetic and rheological correlations, the viscosity of a reacting thermoset can be predicted as a function of time, temperature and shear rate.     

Critical point measurements on nearly polydisperse fluids

The critical temperatures, pressures and volumes of several mixtures containing CO₂, C₂H₆, C₃H₈, and C₄H₁₀ have been measured using a heavy walled, variable volume, cylindrical glass vessel. In each mixturre the relative proportions of the three hydrocarbon solutes to one another were changed; total solute mole fraction never exceeded 0.1. A detailed study of the mixture CO₂ + C₃H₈ shows that the critical temperature exhibits a minimum at a C₃H₈ mole fraction of 0.0265. Our mixture data are analyzed using a polydisperse model of dilute solutions.     

NMR measurements of self diffusion in polymer gel electrolytes

The pulsed field gradient NMR spin-echo technique has been used to measure the self diffusion coefficients of solvent molecules, Li⁺ cations and CF₃SO₃⁻¹ anions in polyvinylidene fluoride (PVDF) gel electrolytes incorporating liquid electrolytes based on either N,N-dimethylformamide (DMF) or tetraglyme (TG) and lithium triflate (LiT). Using the Nernst–Einstein equation with the NMR diffusivities, calculated values for the ionic conductivity were obtained and compared with those determined by ac conductivity measurements to provide estimates of ionic association. These results for the gel electrolytes are compared with those for the corresponding liquid electrolytes, over a range of salt concentrations and temperatures. In general terms, the comparisons support the simple conclusion that regions of liquid electrolyte exist within the gel electrolytes, consistent with limited measurements of the transverse relaxation time T₂ on these systems, which are also reported. There are, however, some small but significant differences between the behaviours of the gel electrolytes and the liquid electrolytes and these are discussed.     

The effect of shear rate on the molecular weight determination of acrylamide polymers from intrinsic viscosity measurements

The rheological response of dilute solutions of high molecular weight polyacrylamides at low shear rates has been measured using a capillary viscometer that provided for a fivefold variation in shear rate at each concentration. The non-Newtonian effects were found to be significant for polyacrylamides with number-average molecular weights exceeding 10⁶. The molecular weight average–intrinsic viscosity relationship most widely used in the literature, [η] = 6.80 × 10^?4M , was found to be valid when [η] was measured at high shear rates where the polymer solutions approached Newtonian behavior. A new relationship was developed relating M _n to the intrinsic viscosity extrapolated to zero shear rate.     

Formation of polyurethane networks studied by the gel point method

Summary Determination of critical stoichiometric ratio necessary for gelation in polyurethane systems composed of poly(oxypropylene)triol and diol, trimethylolpropane and 4,4-diphenylmethane diisocyanate revealed the effects of a different functionality, reactivity of functional groups, tendency to cyclization and the course of the crosslinking process.Presented at the 7th IUPAC Discussion Conference Polymer Networks, Karlovy Vary, CSSR, September 15–19, 1980     

Effect of laser beam focusing point on AFM measurements

The optical beam deflection method, which is used in AFM to obtain surface images, may distort the resulting image. The flexible and long cantilever is easily overdamped by the laser radiation pressure, resulting in steady deflection of the cantilever (<1 nm). This deflective force distorts the image and influences the force-distance (F-D) curve. The present study investigated the effect of laser radiation pressure on image distortion. As a proof-of-concept test, two grating samples (with step heights of 150 and 18 nm for TGX01 and TGZ01, respectively) were examined with an NSC36 series cantilever in air and water media.     

Experimental determination of interface widths in binary polymer blends from free volume measurements

Interfaces in binary polymer blends play a pivotal role in moulding their physical properties. We observe a diffused interface in immiscible and partially miscible binary polymer blends for which the methods of preparation characterize the interface widths. The interface widths for three binary blends of PVC/PS, PS/PMMA, and PVC/SAN are in the maximum range of 2.15, 5.04 and 6.24 nm respectively. We establish a correlation between hydrodynamic interaction and the Flory–Huggins interaction parameter which is consistent with self-consistent mean field theory and the usual constructed density profile across the interface. Our approach is based on the hydrodynamic interaction which is derived from free volume data measured by positron lifetime spectroscopy.     

Electric field-associated deformation of polyelectrolyte gel near a phase transition point

The influence of de electric fields on sodium acrylate—acrylamide copolymer gel near a phase transition point in acetone—water mixtures containing NaOH was studied. In the mixtures free of NaOH, the shrunken-state gel near a phase transition point was not influenced by dc electric fields, while the swollen-state gel near it shrank from its part facing to the positive electrode in dc electric fields. In the presence of NaOH, unprecedented swelling of the shrunken-state gel has been observed. The swollen-state gel showed swelling or shrinking in the field. The type of deformation was determined by the concentration of NaOH. The drift of mobile ions played a role in the deformation. The shrinking occurred through the conformational change of polymer network caused by the drift. The swelling was observed by the change of the osmotic pressure based upon the ion concentration difference between the inside and the outside of a gel.     

Apparatus for determination of the wiley melting point

Summary A mechanically stirred Wiley Melting Point bath apparatus is described, which makes the Wiley Melting Point more accurate and reproducible, permits more determinations to be run at one time, and is less time-consuming than the hand-agitation procedure of the A.O.C.S. Method.     

凝胶色谱法测试PVC树脂平均分子质量及分子质量分布

叙述了凝胶色谱法的测试原理,介绍了用凝胶色谱测试PVC树脂平均分子质量及分子质量分布的方法。