Increase In Polymer Molecular Weight Research Articles

Radiation‐induced seeded copolymerization of tetrafluoroethylene with propylene. I. Effects of agitation and dose rate

AbstractSeeded copolymerization of teterafluoroethylene with propylene by radiation was studied by semibatch experiment at a constant pressure of 25 kg/cm2, a temperature of 40°C, and at various dose rates and monomer compositions in polymer particles. The polymerization rate and polymer molecular weight are in the ranges of 5.6–58.7 g/h.L‐H2O and 4.6 × 104–1.6 × 105, respectively. The polymerization rate increases with agitation speed up to 300 rpm and slightly decreases above 500 rpm. The polymer molecular weight is the highest at 300 rpm. The polymerization rate and polymer molecular weight increase with tetrafluoroethylene fraction. At lower tetrafluoroethylene fraction, the polymerization rate is proportional to the 0.7–0.9 power of the dose rate and the polymer molecular weight is almost independent. The dose rate effects are explained by considering that first‐order termination by degradative chain transfer to propylene is predominant at a lower tetrafluoroethylene fraction. Decrease in the dose rate dependence of the polymerization rate and increase in that of the polymer molecular weight with the tetrafluoroethylene fraction are due to the increase in second‐order termination by recombination.

Internal friction and high-frequency behaviour of dynamic viscosity in polymer solutions

The molecular weight dependence of the high-frequency limit of viscosity [ ν] ∞ was calculated for various models of internal friction in polymer chains. A study was made of internal friction due to short-range action of segments and internal friction due to long-range action. It was shown that the independence observed experimentallly of the value of [ ν] ∞ of polymer molecular weight may only apply in the case of “short-range action” of internal friction. “Long-range acting” internal friction is described by a matrix, the coefficients of which decrease at a very low rate so that [ ν] ∞ increases logarithmically (or more rapidly) with an increase in polymer molecular weight. It was shown what rate is required for coefficients of the internal friction matrix to decrease, in order that “long-range acting” internal friction also produced a value of [ ν] ∞, independent of polymer molecular weight.

Emulsifier‐free emulsion polymerization of tetrafluoroethylene by radiation. I. Effects of reaction conditions on the polymerization rate and polymer molecular weight

AbstractThe radiation‐induced emulsifier‐free emulsion polymerization of tetrafluoroethylene was carried out at an initial pressure of 2–25 kg/cm2, temperature of 30–110°C, and under a dose rate of 0.57 × 104−3.0 × 104 rad/hr. The rate of polymerization was shown to be proportional to 1.0 and 1.3 powers of the dose rate and initial pressure, respectively, and is maximal at about 70°C. The molecular weight of polytetrafluoroethylene (PTFE) lies in the range of 105−106, increases with reaction time in the early stage of polymerization, and is maximal at 70°C but is almost independent of the dose rate. An interesting discovery is that PTFE, a hydrophobic polymer, forms as a stable latex in the absence of emulsifier. When PTFE latex coagulates during polymerization under certain conditions, the polymerization rate decreases, probably because polymerization proceeds mainly on the polymer particle surface. The observed rate acceleration and successive increase in polymer molecular weight may be due to slow termination of propagating radicals in the rigid PTFE particles.

The Characterization of Linear Polyethylene SRM 1475. VII. Differential Refractive Index of Polyethylene Solutions.

The value of dn/dc for polyethylene in 1-chloronaphthalene at 135 °C, required for the determination of molecular weight by light scattering, was found to vary with molecular weight. Similar changes were found in 1,2,4-trichlorobenzene, the gel permeation chromatograph solvent. The absolute value of dn/dc decreases by about 2 percent as the polymer molecular weight increases from 12,000 to 110,000.

Polymer adsorption studies at the solid-liquid interface using gel permeation chromatography: I. Molecular weight distribution along the adsorption isotherm

Poly(vinyl chloride) was adsorbed onto CaCO 3 from dilute chlorobenzene solution. An isotherm was completed which was consistent with the Langmuir theory based on monolayer coverage. A technique employing gel permeation chromatography was described and used to observe the molecular weight distribution (MWD) both in solution directly and in the adsorbed layer indirectly. With the use of this technique, the MWD of polymer in the adsorbed layer was shown to reach a point of equilibrium with the solution polymer within 1 hr. High molecular weight polymer was shown to be preferentially adsorbed. The MWD in the adsorbed layer was narrower than the MWD in the initial solution. The MWD in the adsorbed layer was also shown to be independent of solution concentration on the isotherm limiting plateau and at concentrations considerably below the limiting plateau. Two separate molecular weight functions appeared to be involved in the adsorption process. One function involved an increase in the amount of polymer adsorbed with increase in polymer molecular weight from a polymer molecular weight near 10,000. Beyond a certain critical molecular weight the amount adsorbed appeared to be almost independent of molecular weight. The molecular weight dependence is explained in terms of polymer attachments to the surface.

Lactone polymers. III. Polymerization of ε‐caprolactone

AbstractPolymerization of ε‐caprolactone catalyzed by dibutylzine and triisobutylaluminum has been examined. Monomer conversion and polymer molecular weight increase simultaneously during the polymerization suggesting a living polymer system. Also, molecular weight is proportional to the reciprocal of catalyst concentration. However, the polymer molecular weights are three to five times that which would be calculated from the catalyst concentration assuming a living polymer system. In addition, fractionation of poly‐ε‐caprolactone prepared with dibutylzinc revealed that the distribution is considerably broader than expected for a Poisson distribution. While no mechanistic explanation for the broad molecular weight distribution observed has been defined, examination of the metal alkyl‐catalyzed polylactones shows that the molecular weight distributions can and do change with time. This change is due to an ester interchange process occurring subsequent to polymerization. This phenomenon can be used to change the molecular weight distribution from very broad to the most probable distribution.

The effect of tacticity on polypropylene fractionation

AbstractThe basis for gradient‐elution fractionation is a regular decrease in solubility with an increase in polymer molecular weight. tacticity, however, can be an overlapping effect in the solvent separation process. In this study, the effect of tacticity in the fractionation of polypropylene has been studied using representative “atactic” and “isotactic” polymers, defined by heptane solubility. Typical and published column techniques have been used. A high resolution proton magnetic resonance analysis of protons in the chain methylene region was used for evaluation of tactic placement. A new tacticity correlation was developed which is based on data obtained here and on PMR results on polypropylene reported by SATOH and KATO. This correlation has been compared to published spectra of STEHLING and of WOODBREY. Changes of tacticity with molecular weight are most prominently observed in the first gradient‐elution fractions from both “atactic” and “isotactic” polypropylenes. The implication of these results concerning apparent molecular weights and distributions is discussed.