8694
Ind. Eng. Chem. Res. 2005, 44, 8694-8698
Long-Channel Cavity and Gas Diffusion in Liquid-Crystalline Polyester with n-Alkyl Side Chains Masanori Matsui, Yuji Yamane, Shigeki Kuroki, and Isao Ando* Department of Chemistry and Materials Science, International Research Center of Macromolecular Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan
Kai Fu and Junji Watanabe Department of Organic and Polymeric Materials, International Research Center of Macromolecular Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan
The diffusion coefficients of probe ethane molecules charged into long-channel cavities in the hexagonal columnar phase of oriented poly(p-biphenylene terephthalate) with long n-dodecyl side chains have been successfully measured in the directions parallel and perpendicular to the long channel axis by means of a pulsed-field-gradient spin-echo (PFGSE) 1H NMR method, to elucidate the nature of the cylindrical channel cavities. On the basis of these experimental results, the structural characterization of the long-channel cavities has been made. Furthermore, it is shown that ethane molecules are diffusing to neighboring channels through defects in the wall of the channel cavity, in addition to diffusion parallel to the channel axis. Introduction In our previous work,1 it has been shown that probe methane molecules diffuse in the cylindrical channel cavities formed in the hexagonal columnar phase of oriented poly(p-biphenylene terephthalate) (PBpT-O12) (Figure 1) with n-dodecyl side chains, which forms a thermotropic liquid-crystalline phase2,3 via interactions between the rigid rodlike main chains and flexible side chains above any specified temperature. The polyester forms the columnar phase, the nematic phase, and the isotropic phase with temperature elevation via interactions between the rigid rodlike main chains and flexible side chains.4 In the columnar phase, the honeycombed network is formed and then has long-channel cavities with a diameter of ∼3 nm, as viewed from the top (Figure 2). The polyesters that have such cavities may be expected to have application as smart membrane materials. For this purpose, clarification of the property and function of long-channel cavities in the columnar phase of PBpT-O12 is necessary. Probe molecules diffusing in long-channel cavities are strongly affected by interactions with the channel cavities. Therefore, the structure and dynamics of the PBpT-O12 forming longchannel cavities and the diffusion process of probe molecules with different sizes in the long-channel cavities must be elucidated. In the previous work, structural changes of PBpTO12 via phase transition have been clarified by the solid-state 13C NMR method.4 It has been shown that n-dodecyl chains in the cylindrical channel cavities of oriented PBpT-O12 in the hexagonal columnar phase take two types of regions at room temperature: one is the crystalline region with the all-trans zigzag form, and another is the mobile region in which fast exchange between the trans and gauche conformations exists. Furthermore, a pulsed-field-gradient spin-echo (PFGSE) * To whom correspondence should be addressed. Tel: +81-467-47-0366. Fax: +81-467-47-0366. E-mail addresses:
[email protected];
[email protected].
Figure 1. Structure of poly(p-biphenylene terephthalate) (PBpT-O12) with long n-alkyl side chains attached to the terephthalate moiety.
Figure 2. Schematic diagram for the space in the long-channel cavity in the columnar phase of PBpT-O12, as viewed from the top. Closed ellipsoids represent the main chain, and curved strings linked with the closed ellipsoid represent the n-alkyl side chains. 1H NMR method has shown that methane molecules with small size in the channel cavities are diffusing in the direction parallel and perpendicular to the channel cavity axis, and the diffusion of methane molecules in the direction perpendicular to the channel cavity axis comes from the existence of defects in the walls of the channels.4 In addition, it is very important to clarify the size of the defects, using gaseous probe molecules with large size, to characterize the nature of the channels in the polyester. From such background, we intend to measure the diffusion coefficient (D) of ethane molecules with a somewhat larger size, compared to methane, in the longchannel cavity by means of PFGSE 1H NMR method to
10.1021/ie048785v CCC: $30.25 © 2005 American Chemical Society Published on Web 07/08/2005
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8695
Figure 3. Optical micrograph of PBpT-O12 fibers.
elucidate the defects and the nature of the inside of the long-channel cavities. Experimental Section Materials. The poly(p-biphenylene terephthalate) with n-dodecyl side chains (PBpT-O12) (see Figure 1) used in this work has been synthesized via the polycondensation of 4,4′-dihydroxybiphenyl with 2,5-bis(dodecyoloxy)terephthaloyl chloride in dry pyridine at 60 °C, as reported previously.3 PBpT-O12 in the nematic liquid-crystalline phase at 160 °C has been mechanically stretched to obtain highly oriented fibers and then subjected to cooling at room temperature. A photograph of PBpT-O12 fibers, as observed via optical microscopy, is shown in Figure 3. The averaged diameter of the PBpT-O12 fibers is ∼0.6 mm. Furthermore, the channel cavity axis is oriented to the direction parallel to the oriented PBpT-O12 fiber axis. The diameter of the fiber is 0.6 mm. The PBpT-O12 fibers have been cut into pieces with a length of ∼4 mm, and then the rod pieces were placed in an NMR tube with the outside diameter of 5 mm in the direction parallel or perpendicular to the NMR tube. The schematic diagram for an NMR tube with the PBpT-O12 fibers is shown in Figure 4. After an NMR tube with oriented PBpT-O12 fibers was degassed under vacuum for 12 h, ethane gas was charged into the NMR tube at a pressure of ca. 1 atm. Ethane gas then was charged into cylindrical channel cavities. Finally, the NMR tube was sealed off. Measurements. The diffusion coefficient measurements for ethane molecules in the cylindrical channel cavities of the PBpT-O12 fibers were performed with a Bruker DSX-300 spectrometer operating at 300 MHz for 1H, using a standard PFGSE pulse sequence (π/2 pulseτ-π pulse) based on the Hahn echo sequence, where τ is the pulse interval, and then a field-gradient pulse was added between the π/2 and π pulses and after the π pulse.5-13 The field gradient is applied to the direction parallel to an NMR tube. The diffusion coefficient of ethane molecules diffusing in the direction parallel to the channel (D|) is determined using an NMR tube with ca. 40 pieces of PBpT-O12 fiber placed in the direction parallel to the tube, and that of ethane molecules diffusing through defects of the wall of the channels in direction perpendicular to the channels (D⊥) is determined using an NMR tube with 50 pieces of PBpT-O12 fiber placed in the direction perpendicular to the tube.
Figure 4. Schematic diagram for an NMR tube with 50 PBpTO12 fibers.
The D values were determined using the following relationship between signal intensity and field-gradient parameters at 25 °C:
ln
[ ]
( )
A(G) δ ) -γ2G2Dδ2 ∆ 3 A(0)
(1)
where A(G) and A(0) are the signal-integrated intensities at t ) 2τ with and without the field-gradient pulse, respectively. δ is the field-gradient pulse length, γ the gyromagnetic ratio of 1H, and ∆ the field-gradient pulse interval. (∆ is also called the diffusion time and the observation time.) The signal intensity of ethane molecules was measured as a function of G. The appropriate δ, ∆, and G values for measuring precise diffusion coefficients of pure ethane gas and ethane molecules in the channel cavities of PBpT-O12 fiber must be chosen, because their diffusion coefficients are considerably different from each other. For example, for absolutely large D measurements, the absolutely small δ and G values must be used. For pure ethane gas, δ ) 0.16 ms, ∆ ) 6 ms, and G ) 0-30 G/cm, and those values for ethane molecules in the channel cavities were δ ) 1 and 3 ms, ∆ ) 6 ms, and G ) 0-1100 G/cm. The plots of ln[A(G)/A(0)] against γ2G2δ2(∆ - δ/3) give a straight line with a slope of -D. Therefore, the D value can be determined from the slope of a straight line. The experimental errors are
