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

Abstract

The 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.

Emulsion polymerization of tetrafluoroethylene: effects of reaction conditions on the polymerization rate and polymer molecular weight

“…This means that the higher temperature can cause a higher degree of coagulation. This behavior is different from that found in Suwa et al18 results; in that study, emulsion polymerization of TFE by radiation showed that the rate increases with the rising reaction temperature (70°C) and then decreases. Consequently, it is shown as the behavior of the typical radical polymerization.…”

“…The apparent activation energy obtained by a slope of a plot of ln R p vs. 1/ T of Figure 16 is −0.7 kcal/mol, between 55 to 95°C. This value is lower than that obtained by Suwa et al (−5.2 kcal/mol),18 compared to the range of negative dependence of 1/ T for the rate of polymerization. This difference may be due to the initiator or agitation speed.…”

“…Instead, it is customary to estimate the molecular weight of PTFE by measuring the specific gravity15, 16 and the heat of crystallization 16, 17. The effects on molecular weight of the PTFE prepared by emulsion polymerization of TFE by radiation in connection with various polymerization conditions was investigated by Suwa et al18, 19 However, no data for the molecular weight of the PTFE prepared by chemical initiators are available.…”

Emulsion polymerization of tetrafluoroethylene: effects of reaction conditions on the polymerization rate and polymer molecular weight

“…27 This behavior may be related to the decomposition rate of initiators like APS. This behavior is different from that found in Suwa et al 18 results; in that study, emulsion polymerization of TFE by radiation showed that the rate increases with the rising reaction temperature (70°C) and then decreases. 33 Therefore, the particles formed rapidly in the nucleation period may be unstable, and hence, easy to coagulate; the surface area of the particles is large, and there is not enough surfactant to cover the nucleated particles.…”

Structure and Morphology of Fluorocarbon Films Grown by Hot Filament Chemical Vapor Deposition

“…Randomly arranged anisotropic rodlike features have been observed in thin films formed from bulk PTFE latexes . Both spherical and rodlike particles have been observed from conventional emulsion polymerization of TFE to form PTFE. ,− Studies of the formation of PTFE dispersion particles have revealed distinct nucleation and growth periods during polymerization. ,, Although the transition from the nucleation to the growth period depends on process conditions, in several cases, it has been found to occur within the first few minutes of polymerization, whether it be in an emulsion polymerization with a surfactant 21 or in an emulsifier-free radiation-induced emulsion polymerization . As growth proceeds, PTFE chains adapt a helical conformation that is highly linear, directional, and rigid with no branching or cross-linking. , Further, the development of highly extended PTFE chains favor the formation of rodlike grains because, through electron microscopy and X-ray diffraction, rodlike PTFE particles consist of extended chains aligned in the direction of the long axis of the rods. , Thus, a polymerization process that provides less nucleation would allow greater subsequent growth from these sites and would favor more extended chains with less chain ends and more rodlike grains.…”