Pressure-induced structural change of intermediate-range order in poly(4-methyl-1-pentene) melt

High-pressure in situ x-ray diffraction and specific-volume measurements on isotactic poly(4-methyl-1-pentene) melt have uncovered abrupt changes in the pressure dependence of microscopic structure as well as that of macroscopic density. The first sharp diffraction peak of the polymer melt, which is related to the intermediate-range order and is explained as resulting from the correlations between main chains, is suppressed at pressures less than 1 kbar. These changes in intermediate-range order show similarities to those seen in liquid-liquid or amorphous-amorphous transitions in simpler small molecule based systems, suggesting that this kind of phenomenon may occur in a wide range of materials.

Figure 1

Phase diagram of P4MP1. The results of specific-volume measurements are shown by the colored contours. The break in these measurements that indicates melting is drawn as a solid black line. The low- and high-density crystalline phases are indicated with I and IV, respectively [19]. The black circles show the abrupt changes seen in the pressure dependence of x-ray diffraction profiles [Figs. 4a and 4b] and the dashed-dotted line shows those seen in specific-volume data [Fig. 4c].

Figure 2

Schematic illustrations of the cross section of the diamond piston-cylinder high-pressure cell. (a) Whole picture of the cell. (b) Enlarged picture of the central part of the cell. Inner piston and cylinder were made of synthetic Ib single-crystal diamond and heat resistant tungsten carbide alloy. SrB${}_4$O${}_7$:Sm${}^{2+}$ pressure maker and $K$-type (chromel-alumel) thermocouple were used.

Figure 3

X-ray diffraction patterns of P4MP1 melt. An example of the two-dimensional diffraction pattern (raw data at 0.1 kbar) is shown in the inset. The peak (i) shows the FSDP, (ii) the second peak, (iii) a diffraction line from a graphite gasket, and (iv) Compton scattering from diamond pistons.

Figure 4

Pressure dependence of FSDP and specific volume of P4MP1 melt. (a) The ratio of the intensities of the FSDP ($I_1$) to that of the second peak ($I_2$) and (b) the position of the FSDP. Filled symbols show the data taken under increasing pressure and open symbols under decreasing pressure. (c) The normalized stress ($F$)–finite strain ($f$) plot for P4MP1 melt extracted from the specific-volume measurements. The arrows show abrupt changes in the slopes, corresponding to the boundary between the low-density melt (LDM) and the high-density melt (HDM). The same plot for polyethylene melt [47] is shown for comparison. Lines are guides for the eyes.

Figure 5

Schematic pictures of possible structures of P4MP1 melt. (a) The low-density melt, LDM, and (b) the high-density melt, HDM. Small black circles indicate carbon atoms, and black thick and red thin lines indicate main and side chains, respectively. Blue arrows indicate correlations between backbones (main chains) and gray large circles indicate void spaces, both of which would cause the intermediate range order.