Thermochromism of polypropylene gels

Abstract

It is found that polypropylene gels in some solvents of benzene-derivatives show a striking change of colour. The colour changes from blue to yellow as the temperature rises from the melting point of the solvent to around 70-80 ~ The apparent characteristic of the phenomenon resembles the thermochromism of cholesteric liquid crystals although potypropylene itself is colourless and has no asymmetric carbons in the molecules. Polypropylene flakes swelled in these solvents also show similar thermochromism. [14][15][16][17][18][19][20]. However, all these investigations have been carried using dilute and quasi-concentrated solutions.We have previously reported on the gelation of the concentrated solutions of polyethylene in cyclohexane and carbondisulfide mixed solvent [21][22][23], successively, the gelation of the concentrated solutions of polyethylene [24][25][26] and polypropylene [27] also in a single organic solvent such as benzene, toluene, xylene, decalin, and tetralin. In preparing the gels, these solvents were added to polypropylene contained in glass tubes. The glass tubes were then sealed and the temperature raised up to near boiling point of the solvents. The polymer solutions were then cooled with running water until the gels were formed.When isotactic polypropylene flakes, prepared from xylene solution by addition of methanol, were swelled with benzene-derivative solvents prior to heating, the flakes were faintly coloured. A subsequent change of the temperature of these swelled polymer flakes resulted in the colour change. The yellowish colour at higher temperatures changed to a bluish colour at lower temperatures. N 296The gels prepared with these polymer flakes and the benzene-derivative solvents mentioned above also showed a faint colouring. On cooling the gels, the colour change was not as clear as that of the swelled polymer flakes. These gels were formed by gradual cooling of the polymer solution with running water. We therefore thought that the method of cooling might affect the thermochromic characteristics of the gels. In order to determine this, the polymer solutions were rapidly cooled to about-20 ~ Some of the solutions were frozen and others were not, but gels were formed anyway. Thus formed gels all showed clear thermochromism. Photographic plates (a) to (d) in Figure 1 show the thermochromism of the polypropylene gel in benzene.In order to make this more quantitative, the wavelength dependent transparency and the spectrum of light scattered in the direction perpendicular to the incident light were also measured. These are shown in Figures 2 and 3, respectively.The colour of the gels, as shown in the photographic plates in Figure 1, may be explained on the basis of the shape of the spectra in Figure 3. The spectrum of the light scattered from the gel at 26.5 ~ has a peak near 300 nm and a tailing edge in the wavelength range of 400 to 470 nm. Therefore, the scattered light itself appears to be bluish, resulting in the photographic plate, Figure la. As the temperature of the gel...

A Review of Temperature Influence on Adsorption Mechanism and Conformation of Water Soluble Polymers on the Solid Surface

“…Although the Shultz-Flory method is approximate, it has been widely applied by many authors. [47][48][49] The theta temperatures for many polymer-solvent systems obtained from the second virial coefficient and the Shultz-Flory methods are in good agreement, but W 1 values are insignificantly lower in the case of the Shultz-Flory method.…”

Coloration of Polypropylene Gel and Temperature Conditions for Gel Formation

“…1 This phenomenon is considered to be of importance in connection with the coloration mechanism. Figure 1 shows the transmittance spectra of 2 g/v-% of polypropylene gels in o-xylene after reaching equilibrium at 25 • C that had been prepared at various temperatures after being dissolved in the solvent at 160 • C.…”

“…This coloring phenomenon has been considered to be due to selective light-scattering, as the transmitted-light spectra of the gels forms a rough reflected image of the scatteredlight spectra, which are measured in the direction perpendicular to the incident light. 1,2 We have more recently demonstrated that the development of coloration in polypropylene gels can be explained due to selective light-scattering on the basis of differences in the lightdispersive power between solvent and polypropylene, and to size-controlled polypropylene networks formed in the gel. [3][4][5][6][7][8][9][10][11] Furthermore, it has been experimentally proved that the thermochromic phenomenon in the gel can be explained consistently using the same logic.…”

Thermochromism of poly-1-butene gels

“…Previously [1] we reported the thermochromism of swelled and gelled isotactic polypropylene in some benzene-derivative solvents. It was revealed that the colouring phenomenon was due to selective scattering, but not to optical absorption.…”