THE APPLICATION OF SIDE-CHAIN LCPS AS SEPARATION MEMBRANES

Non-porous polymeric membranes are important for the separation of mixtures. For the practical application of non-porous membranes, it is desirable to have a high selectivity combined with a maximum permeability. Semicrystalline or glassy polymeric membranes often show good selectivity owing to their ordered or stiff structures, but low permeability.

On the other hand, membranes in the liquid state exhibit high permeability but poor selec- tivity. The liquid-crystalline polymers are expected to have high chain mobility in the LC states. This provides an enhanced transport velocity of gas molecules through LC mem- branes. However, in the LC states the anisotropic LC structures reduce the amplitude of polymer-chain mobility compared with the isotropic state, which should lead to an enhanced selectivity between small and bulky gas molecules.

To date, investigations of transport properties of membranes in the LC state have been made by seveal research groups.98-“6 Kajiyama and co-workers9*-lo1 have studied gas transport properties through low-molar-mass liquid crystal/polymer composite membranes.

THE APPLICATION OF SIDE-CHAIN LIQUID-CRYSTALLINE POLYMERS 855

Paul and collegues 104-108 reported the gas permeation through main-chain LCP membranes.

Loth and Euschen’@ investigated the transport behavior of salicylic acid through a liquid- crystalline side-chain elastomer membrane. Candia et al. ‘lo reported the transport behavior of dichloromethane through a smectic side-chain LCP. Finkelmann and co-workers’11-“3 have measured the transport behavior of hydrocarbon gases through side-chain LCP elastomer membranes,. Our group’t4-’ l6 has also studied the gas transport properties of side-chain LCPs/

porous polypropylene composite membranes. A representative example of gas permeation through a nematic side-chain LC polysiloxane-based membrane is presented below.

A side-chain LC polysiloxane containing 4-methoxyphenyl-4allyloxybenzoate side groups (Fig. 25) is used. The polymer exhibits a glassy-to-nematic transition at 15°C and a nematic-yo-isotropic transition at 75°C. During the measurements of gas permeation, two porous pokypropylene films were used as supporting layers to give the liquid-crystalline polymer membrane sufficient mechanical strength. This is necessary because the side-chain LCP will become a viscous liquid when it is heated to the liquid-crystalline and isotropic states. The permeation characteristics of the porous polypropylene film were measured, and it showed very high gas permeability with almost no selectivity for all gases used in this study. increases approximately five-fold when the temperature increases from 10°C (glassy state) to

Table 3. The temperature dependence of diffusion coefficients (D) and solubility coefficients (5) for Hr.

856 CHAIN-SHU HSU

A H2 cl co2 0 02

0 CH4 v N2

isotropic ! nematic i glassy

2.5 2.7 2.9 3.1 3.3 3.5 3.7 3

I/T x lo3(‘K-’ )

Fig. 26. Arrhenius plots of permeability P in units of cm3 (STP) cm/cm2 s cmHg for

various gases in the laminated side-chain liquid-crystalline polysiloxane LCP-6 mem-

brane: Hz (A); CO* (0); 02 (0); CH4 (+) and N2 (V).

25°C (nematic liquid-crystalline state). This result may arise from the high segmental motion in the mesophase of the side-chain LCP. On the other hand, when the temperature increases from the nematic liquid-crystalline state of the side-chain LCP to its isotropic state, no abrupt jump of the permeability was observed. Table 4 summarizes the activation energies for permeation of the side-chain LCP obtained at three different states for five gases. As can be seen from the table, the activation energy for permeation shows the highest value when the side-chain LCP is in the liquid-crystalline state.

Figure 27 shows a plot of the separation factor, PoZ /PN2, versus PO, for the laminated side- chain LCP membrane. An increase in PO, on the abscissa corresponds to a rise of the temperature of measurement. In general, the magnitude of the separation factor decreases with increasing permeability. However, the opposite tendency was observed for the lami- nated side-chain LCP membrane when the temperature rises and the phase changes from its glassy state to its liquid-crystalline state: i.e. the separation factor increased. It seems without

THE APPLICATION OF SIDE-CHAIN LIQUID-CRYSTALLINE POLYMERS 857

Table 4. The temperature dependence of the permeability coefficients and separation factors, Po2/PN, and P,-o,/PcH,, for Hz, 02, Nz, CH4 and CO* in the laminated side-chain LCP

Thermal state of the T (“C) PH, PO2 PN, PCH, Pco* PO, IPCO, side-chain LCP

(cm3 (STF) cm/cm* s cmHg) x 10 ¹⁰ _{PN, PC&}

Glassy 0 2.50 0.228 0.076 0.079 1.15 3.79 14.6

5 3.07 0.330 0.110 0.102 1.46 3.00 14.3

10 4.76 0.375 0.126 0.137 2.07 2.98 15.1

Nematic 25 21.8 1.75 0.380 0.655 11.1 4.60 16.9

35 33.7 3.06 0.706 1.18 19.8 4.33 16.8

45 55.4 6.37 1.63 2.64 39.6 3.90 15.0

60 96.5 18.6 4.95 7.42 74.2 3.75 10.0

Isotropic 75 132 46.8 15.6 21.8 110 3.00 5.04

82 182 62.7 22.4 30.4 152 2.80 5.00

90 230 87.1 31.5 39.0 200 2.76 5.13

5.0

4.5

4.0

3.0

2.5

25°C

IO-‘” lo-’

PO, (cm3(STP)cm/cm2~seccmHg)

Fig. 27. Plot of the separation factor, Po,/PN2, versus the permeability of oxygen, PO2 in units of cm3 (STP) cm/cm* s cmHg, for the laminated side-chain liquid-crystalline poly-

siloxane LCP-6 membrane and different temperatures as indicated.

858 CHAIN-SHU HSU

Table 5. Activation energies for permeation at three different states of the side-chain LCP Gas T < Ta (glassy) T, < T < TN1 (nematic) T > TN1 (isotropic) mental motions of the mesogenic side groups are restricted and the frozen liquid-crystalline domains can be considered as barriers for the gases. Therefore, the gas permeation process is predominantly controlled by the amorphous main-chain domains. However, when the tem- perature rises above Tg, the segments of the main chains are free to move and the segmental motions of the side chains become larger. The gases permeate not only through the main- chain domains but also through the ordered mesogenic side-chain domains. The separation factor Po2/PN2 is mainly controlled by the mesogenic side-chain domains. And its value increases abruptly, because these domains have an ordered supermolecular arrangement.

It is well known that the permeation coefficient can be expressed as a function of the diffusion and the solubility coefficients. According to the time-lag method, the diffusion coefficients for all five gases through the laminated side-chain LCP membrane were esti- nematic mesophase. Although the polymer backbone of the side-chain LCP is polysiloxane and the side-chain LCP is supposed to have a very high segmental motion in the liquid- crystalline state, the laminated side-chain LCP membrane showed a fairly low permeation coefficient for oxygen. The permeability of oxygen for this membrane was much smaller than those for natural rubber and poly(dimethylsiloxane) and was slightly higher than that for high-density polyethylene. This membrane, however, shows the highest selectivity for oxy- gen and nitrogen as shown in Table 6. All these results could be due to the ordered super- molecular structure of the liquid-crystalline state exhibited by the side-chain LCP.

In conclusion, the permeation properties in the laminated side-chain LCP were found to depend strongly on the different states of the side-chain LCP. In the liquid-crystalline state of

THE APPLICATION OF SIDE-CHAIN LIQUID-CRYSTALLINE POLYMERS 859

Table 6. Comparison of gas transport properties at 25°C of the side-chain liquid-crystalline polysiloxane with those of some common polymers

*LCP = side-chain liquid-crystalline polysiloxane; NR = natural rubber; PDMS = poly(dimethylsi- loxane) (vulcanized, 10% filled); HDPE = polyethylene (density = 0.964); LDPE = polyethylene molecular arrangement of the liquid-crystalline state of the side-chain LCP. The results demonstrate the possibility to achieve a highly permselective membrane simply based on a side-chain :LCP.

6. THE APPLICATION OF SIDE-CHAIN LCPS AS SOLID