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충남산업대학교 공업화학과

Nonequilibrium Liquid-Liquid Phase Separation in Solutions

Containing a Crystallizable Polymer

Hwan Kwang Lee

Department of Industrial Chemistry, Chungnam Sanup University

INTRODUCTION

Liquid-liquid (L-L) phase separation and crystallization are major phenomena which control the morphology of polymeric materials. In general there is a possibility that the L-L phase separation process is coupled with the crystallization process. The schematic phase diagrams of semicrystalline polymer-solvent mixtures are shown in Figure 1 with different thermal conditions which can be applied to obtain the competitive phase separations. The upper critical solution temperature (UCST) is located above the equilibrium liquid-solid (L-S) transition in a poor solvent (Figure 1(a)) and below the L-S transition in a good solvent (Figure 1(b)). The relation between L-L phase separation and L-S phase transition can be altered since the binodal curve is remarkably influenced by solvent quality while the crystal melting transition remains nearly constant.1,2 When the binodal exists below the melting point, the L-L phase separation has no equilibrium significance, but it may significantly affect the final morphology in processing. L-L phase separation would be able to precede crystallization upon rapid quenching due to a high nucleation barrier to polymer crystallization.

This presentation deals with the relation between L-L phase separation and crystallization in isotactic and syndiotactic polypropylene solutions in dialkyl phthalate. Since the tacticity of polypropylene influences both crystallographic packing3 and □-temperature,4 the phase behavior of polypropylene solution in a given solvent is expected to be dependent on the stereospecificity of polymer. An isotactic polypropylene (i-PP) and a syndiotactic polypropylene (s-PP) of high stereoregularity and of similar molar mass were used with dialkyl phthalate as a solvent. The variation of the number of carbon atoms in the alkyl substituent of phthalate was employed to control systematically the interaction between polymer and solvent.

EXPERIMENTAL

The i-PP used was highly crystalline with Mw of 1.2x105 and Mw/Mn of 2.5, supplied from Himont R&D Center. The s-PP with Mw of 1.5x105 and Mw/Mn of 2.3 was supplied from Mitsui Toatsu Chemicals, Inc. The syndiotacticity was evaluated to be greater than 92% racemic pentad configuration by 13C NMR. The solvents used were series of 1,2-dialkyl phthalate with the different number of carbon atoms in the alkyl chains, designated as C6 (hexyl), C7 (heptyl), C8 (2-ethylhexyl), C9 (nonyl), and C10 (decyl).

Dried thin films of i-PP and s-PP were prepared by casting from hot decalin solution according to the procedure described elsewhere.5 Mixtures of polymer film and solvent were weighed carefully to obtain a known concentration in concavity microslides. A cover glass was placed over the sample. Phase transition temperatures were measured by optical microscopy with a Mettler hot-stage (FP82) and controller (FP80) under nitrogen purge. The samples were slowly heated to 10℃ above the higher value of either melting point (Tm) or L-L phase separation temperature (TL-L). The heating was continued for 5∼10 min to ensure a homogeneous state. The value of TL-L obtained by phase-contrast microscopy was assigned to the temperature at which fine droplet structures start to appear on cooling at a rate of 10℃/min. The crystallization temperature (Tcry) observed by polarized light microscopy was considered as being the point at which a birefringent entity starts to appear on cooling at the same rate. The value of Tm by polarized light microscopy was the temperature at which the last traces of crystallinity disappear between crossed polarizers on the reheating cycle at a rate of 10℃/min. Evaporation of solvents was checked by weighing the samples after running and was found to be negligible in the experimental conditions.

RESULTS AND DISCUSSION

The experimental phase diagrams for i-PP and s-PP solutions in the same solvent of C6 phthalate showing L-L phase separation, melting, and crystallization as a function of polymer concentration are represented in Figure 2. The observed transitions are by no means the equilibrium transitions since nonequilibrium phenomena may be involved under the thermal conditions applied. The effects of cooling and heating rates on the coupled phase transitions in the system of i-PP and dialkyl phthalate were reported in the previous study.2 Tm and Tcry of s-PP solutions are lower than those of i-PP solutions in Figure 2, which is expected from the Tm values of the pure i-PP and s-PP in the literature. On the other hand the TL-L values of s-PP solutions are interestingly higher than those of isotactic counterparts. Consequently liquid demixing of s-PP solution occurs above its Tm but that of i-PP solution below its Tm. Thus a nonequilibrium L-L phase separation was obtained in i-PP solutions. The results should be attributed to the difference in polymer-solvent interaction related to the stereospecificity of polymer chain since both polymers used are in a similar range of molecular mass and polydispersity.

It was found that the primary reason of the reduced solubility of s-PP in phthalate was an enhanced hydrophobicity arising from alternate positioning of methyl groups along the polymer chain although the results could be partly related to the molecular conformation in solution.5 The s-PP chains are stabilized with the ttgg conformation in solutions6 leading to dimensions greater than i-PP chains but smaller than a-PP chains.4 Here t denotes the trans form and g the gauche form. While polypropylene is considered to be slightly polar compared to polyethylene, alternate positioning of methyl groups in s-PP would give rise to more evenly distributed electron density donated from methyl groups along the skeletal C-C bonds than in i-PP. As the interaction between polymer and solvent became more favorable, TL-L of s-PP solutions decreased systematically, which can be seen in Figure 3. Tm and Tcry decreased slightly with increasing solvent power as expected. In the system of s-PP and C10 phthalate TL-L could not be determined precisely since liquid demixing and crystallization occurred competitively on cooling at a rate 10℃/min. In i-PP systems liquid demixing started to couple with crystallization in C7 phthalate.

The effects of the stereospecificity of polypropylene and solvent quality for 5 wt% polymer concentration on the relation of Tm and TL-L are summarized in Figure 3. In the solution of i-PP and C6 phthalate the TL-L is located 9℃ below its Tm. When the stereospecificity of polypropylene is changed to syndiotacticity in the same solvent system, the TL-L rises 35℃ above its Tm due to reduced melting point and elevated liquid demixing temperature. As the number of carbon atoms in the alkyl chain of phthalate increases, the TL-L decreases significantly but the Tm does slightly. When C9 phthalate is used as a solvent, the TL-L of the s-PP solution shifts again below its Tm due to the enhanced favorable interaction.

It seems to be logical that TL-L of i-PP and s-PP solutions with stronger interaction may exist at certain temperatures as indicated by arrows in Figure 3 if crystallization is suppressed completely. This point will be discussed in detail. Dependence of the relation between L-L phase separation and melting transition on stereospecificity may provide an opportunity to control the morphology of polymer solutions in combination with selection of solvent and applying various thermal conditions of quench depth and cooling rate.

REFERENCES

1. W. R. Burghardt, Macromolecules, 22, 2482-2486 (1989)

2. H. K. Lee, A. S. Myerson, and K. Levon, Macromolecules 25, 4002-4010 (1992)

3. G. Natta and P. Corradini, J. Polym. Sci., 39, 29-46 (1959)

4. H. Inagaki, T. Miyamoto, and S. Ohta, J. Phys. Chem. 70, 3420-3431 (1966)

5. H. K. Lee and S. C. Kim, J. Appl. Polym. Sci., in press.

6. M Kobayashi, T. Nakaoki, and N. Ishihara, Macromolecules, 23, 78-83 (1990)

Figure 1. Two possible types of phase diagrams for semicrystalline polymer-solvent mixtures with different thermal conditions to obtain the competitive phase separations in a poor solvent (a) and in a good solvent (b).

Figure 2. TL-L, Tm, and Tcry as a          Figure 3. TL-L and Tm of i-PP and

function of polymer concentration          s-PP solutions for 5 wt% polymer

for i-PP and s-PP solutions in C6         concentration as a function of solvent phthalate.                               quality.