Preirradiation graft polymerization of styrene in a poly(tetrafluoroethylene) (PTFE) film was examined by time-resolved small-angle neutron scattering (SANS). A crosslinked PTFE film, thickness of which is about 50
Preirradiation polymerization, using ionizing radiation, was intensively explored in the middle of twentieth century [
In this paper, we investigate Preirradiation polymerization of styrene onto poly(tetrafluoroethylene) (PTFE) film, motivated by an excellent picture (Figure 1 in [
(a) A preparation procedure of polymer electrolyte membranes based on poly(tetrafluoroethylene), which is composed of three steps of (1) crosslinking, (2) grafting by Preirradiation and (3) sulfonation. (b) A schematic view of Preirradiation graft polymerization with a crystalline domain and its interface. Long living radical is accumulated in a crystalline domain, whereas monomers access from an amorphous domain.
In a course of history of radiation chemistry, it was experimentally confirmed that free radicals induced by ionizing irradiation are stabilized in a solid phase of crystal, in which a mobility of polymer chains is strongly restricted so that a recombination reaction between radicals is difficult to occur (see Figure
This scenario, describing Preirradiation polymerization in a crystalline film, involves interdisciplinary scientific topics, such as (i) production, stabilization, and diffusion of free radicals in a crystalline PTFE domain, (ii) radical polymerization, (iii) phase separation between PTFE and PS chains, and (iv) deformation and breakage of a film by swelling monomer or grafted polymers. These individual processes are coupled with each other to control a radical polymerization behavior. Especially, the microstructure in a film specimen plays an important role. Thus, knowledge based on radiation chemistry, physical chemistry, and polymer chemistry is necessary to be combined to understand the Preirradiation polymerization.
To elucidate Preirradiation polymerization of PTFE, an in situ and real-time structural analysis during polymerization is necessary. For that purpose, we employed a time-resolved small-angle neutron scattering (SANS) method, which is suitable to monitor
Our method to prepare a polymer electrolyte from PTFE involves three steps as indicated in Figure
A crosslinked PTFE film, as referred to c-PTFE, was provided by Hitachi Cable co. ltd., which corresponds to step (1) by electron irradiation, in Figure
A picture of sample cell specially designed for in situ and real-time small-angle neutron scattering in order to observe Preirradiation graft polymerization on PTFE films.
Graft polymerization was also monitored by weighting a film during polymerization to determine a graft polymerization ratio as a function of polymerization time (
To perform time-resolved SANS observation on graft polymerization of the c-PTFE film, we prepared a special sample container, as shown in Figure
SANS measurements were performed using focusing and polarized neutron ultra-small-angle scattering spectrometer (SANS-J-II) at research reactor JRR3, at JAEA, Toki, Japan [
As schematically shown in Figure
A schematic view showing a PTFE film during Preirradiation graft polymerization.
Figure
Graft polymerization ratio
The crystalline structures in the films of original PTFE without crosslink and crosslinked PTFE (c-PTFE) were examined by SANS. In Figure
SANS obtained for the c-PTFE film exhibits a scattering maximum at
SANS obtained for crosslinked (c-PTFE) and original PTFE without crosslink (PTFE) films (solid lines are guides for eye). A crossover
Figure
Graft polymerization of step (2) was monitored by time-resolved SANS. Time-dependent SANS
In high
Time-resolved SANS obtained for Preirradiation graft polymerization in crosslinked (c-PTFE) (step (2)). Scattering curves indicated by open circles exhibit that for c-PTFE before polymerization and at 1000 min after starting polymerization. Solid lines indicate scattering curves obtained during polymerization with a measurement step of 10 min. A Porod
A power law exponent
A schematic view showing an ellipsoidal PTFE microcrystal covered by PS (gray and block parts correspond to crystalline and grafted PS regions, cross-sections for which are
In order to quantitatively discuss time-resolved SANS, we employ a scattering model considering a core/shell structure for a grafted crystalline domain; a core and shell correspond to crystalline PTFE domain and grafted PS thin layer. In a PTFE film specimen, ellipsoidal crystalline domains are dispersed in an amorphous matrix. The cross-section of crystalline domain is denoted as
After starting graft polymerization, a crystalline domain starts to be covered with a thin layer of grafted PS (a black region in Figure
Small-angle neutron scattering intensity
In our analysis, we postulate that
At the beginning of graft polymerization with
According to an illustration in Figure
By using (
On a basis of time-resolved SANS observation and its model analysis, we illustrate a mechanism of graft polymerization in the c-PTFE film. We consider two processes (I) before
In process (II), it is supposed that the grafted PS chains (or thin layer) form a bridge between crystalline PTFE domains. The bridges, growing toward a direction along a normal vector of the interface, percolate among crystalline PTFE domains. A power law
To largely swell a PTFE film during Preirradiation graft polymerization, deformation and breakage of a PTFE film are necessary. When PTFE is irradiated by ionizing radiation below a melting temperature
Due to strong segregation between grafted PS and PTFE, a thin layer of grafted PS has a sharp interface, which results in small-angle scattering of
We investigated Preirradiation graft polymerization of styrene in a crosslinked poly(tetrafluoroethylene) (c-PTFE) film by time-resolved SANS. It was elucidated that graft polymerization of PS proceeds by two steps: first, as process (I), graft polymerization occurs at an interface of PTFT microcrystal and a thin layer of pure PS chains is formed to surround the PTFE microcrystal. Second, as process (II), grafted PS chains or thin layer starts to bridge between crystalline domains. At the end of process (II), 40% of total microcrystal is covered by the PS thin layer.