Elsevier

Journal of Membrane Science

Volume 561, 1 September 2018, Pages 26-38
Journal of Membrane Science

Synthesis and oxygen permeation of novel well-defined homopoly(phenylacetylene)s with different sizes and shapes of oligosiloxanyl side groups

https://doi.org/10.1016/j.memsci.2018.04.031Get rights and content

Highlights

  • New poly(phenylacetylene)s with oligodimethylsiloxanes (ODMS) have been synthesized.

  • The polymers had high molecular weights and gave self-standing membranes.

  • Introduction of the branched spherical ODMS increases PO2 with no drop of PO2/PN2.

  • Introduction of the linear ODMS increases PO2 largely exceeding PDMS.

  • Oxygen permeation could be tuned by changing sizes and shapes of the ODMS.

Abstract

Six new silicon-containing phenylacetylenes having short oligosiloxane substituents with different sizes and shapes have been synthesized. Homopolymerization of the monomers yielded the corresponding homopoly(substituted phenylacetylene)s with high molecular weights and good solubility. These comb-shaped homopolymers gave self-standing membranes whose PO2 and PO2/PN2 could be measured. We discuss the effect of the chemical structures on their membrane performances together with the other seven related polymers we synthesized before and commercially available cross-linked polydimethylsiloxane (PDMS). While the homopolymers with linear oligodimethylsiloxanes (ODMS) show very high PO2 values which are higher than PDMS, the homopolymers with branched ODMS show high PO2/PN2 values which are close to that of poly(phenylacetylene) (poly(PA)) and, in addition, had ten times higher PO2 than poly(PA). Introduction of the branched spherical ODMS substituents increases PO2 without any drop of PO2/PN2 and introduction of the linear ODMS substituents increases PO2 largely which is higher than that of PDMS. It was found that the chemical structures, i.e., the shapes of the ODMS substituents were more important than that of composition for determining their performance as oxygen permselective membrane materials. In conclusion, oxygen permeation could be tuned by changing sizes and shapes of the ODMS side groups.

Introduction

Many gas permselective membranes have been reported on the past three decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] as energy saving separation process. Some of them have been applied for practical use to solve environmental and energy problems. Among them, membranes which oxygen permeates selectively than nitrogen through are very important and interesting in view of scientific and industrial aspects because oxygen and nitrogen have similar sizes and properties, and oxygen enrichment is important for medical and industrial needs [13].

There are three requirements for oxygen permselective membrane materials. First, a polymer can be fabricated to a self-standing membrane and its mechanical strength is high. Second, the membrane shows high oxygen permeability coefficient (PO2: barrer), and third, the membrane shows high oxygen permselectivity (PO2/PN2). In general, while polymer membranes from rigid macromolecules tend to show brittle nature, high PO2/PN2 and low PO2, polymer membranes from flexible macromolecules tend to show waxy nature, low PO2/PN2 and high PO2. Trade-off relationships between PO2/PN2 and PO2 have been often observed in many papers[14], [15], [16].

Syntheses of many poly(substituted acetylene)s and their application to oxygen permselective membrane materials have been reported by Masuda et al. [17], [18], [19], [20], [21], [22], [23]. Because in general poly(substituted acetylene)s have good membrane forming abilities and relatively high PO2/PN2, they are very suitable as oxygen permselective membrane materials [17], [18], [19], [20], [21], [22], [23]. On the other hand, it is well-known that polydimethylsiloxane (PDMS) shows many unique characters such as very high gas permeability [24], a low glass transition temperature, and high solubility in ordinary solvents. However, since pure linear PDMS has no self-membrane forming ability, to apply it for separation membranes some chemical modification is needed. The use of short oligosiloxane (ODMS) chains such as disiloxanyl and trisiloxanyl groups was an effective method to use advantages of PDMS as new polymer membrane materials [25], [26], [27], [28], [29], [30], [31], [32], [33].

Therefore, in this study we selected the combination of poly(substituted acetylene)s and PDMS (or ODMS) to obtain good oxygen permselective membrane materials, and designed, synthesized and polymerized new substituted acetylenes with a short ODMS, i.e., an oligosiloxane. Although we reported synthesis and oxygen permeabilities of some poly(substituted phenylacetylene)s having trimethylsilyl groups [34] and fluorine containing groups [35] and cis-cisoidal poly(phenylacetylenes) [36], no oxygen permeation of poly(substituted acetylenes) having ODMS groups have been studied and systematic study was not enough to investigate relationships between chemical structures of polymers and membrane performances from these polymers. Therefore, in this study, we synthesized novel six ODMS-containing phenylacetylenes and polymerized them to obtain the corresponding comb-shped homopolymers with high molecular weights and good solubility. And we fabricated the comb-shaped homopolymers to self-standing membranes and measured their oxygen permselectivity. We discuss relationships between their chemical structures and oxygen permeabilities.

Section snippets

Materials

All the solvents used for monomer synthesis and polymerization were distilled as usual. 1,4-dibromobenzene used as starting materials, was purchased from Aldrich Chemical Co., Inc. The polymerization initiators, [Rh(nbd)Cl]2 (nbd = 2,5-norbornadiene), and tungsten(VI) chloride (WCl6) purchased from Aldrich Chemical Co., Inc., were used as received.

Measurement of oxygen and nitrogen permeability

Oxygen and nitrogen permeability coefficients (PO2 and PN2: cm3(STP) ·cm·cm−2·s−1·cmHg−1) and the oxygen separation factor (α = PO2/PN2) were

Syntheses of seven new phenylacetylene monomers having different lengths and shapes of short oligodimethylsiloxanyl (ODMS) groups (Chart 1)

As shown in Chart 1 nos.7–13, seven new phenylacetylene monomers having different lengths and shapes of a short oligodimethylsiloxanyl (ODMS) group were designed and synthesized. At the same time, some phenylacetylenes having trimethylsily groups at different positions (nos.4–6) and having no silicon atoms (nos.1–3) have been synthesized as a reference. All the monomers were purified by vacuum distillation.

Conclusions

We synthesized six new phenylacetylenes having short oligodimethylsiloxane (ODMS) substituents with different sizes and shapes. Homopolymerization of the silicon-containing monomers yielded the corresponding homopoly(substituted phenylacetylene)s with high molecular weights and good solubility. The comb-shaped polymers were able to be fabricated by solvent casting method to self-standing membranes which had strength enough to resist one atom pressure difference in permeation measurements, even

Acknowledgments

This work was supported by the National Natural Science Foundation of China [21404064]; the Natural Science Foundation of Heilongjiang Province of China [LC2016022]; the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province [UNPYSCT-2016089]; Scientific Research Foundation for the Returned Overseas Chinese Scholars, Human Resources and Social Security Department of Heilongjiang province of China [[2017] No.490]; the JSPS KAKENHI Grant Number [16H04153];

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