Relation between network structure and gas transport in crosslinked poly(propylene glycol diacrylate)

doi:10.1016/j.memsci.2006.06.038

Journal of Membrane Science

Volume 283, Issues 1–2, 20 October 2006, Pages 253-265

https://doi.org/10.1016/j.memsci.2006.06.038 Get rights and content

Abstract

A series of crosslinked poly(propylene oxide) rubbers was prepared by UV photopolymerization of poly(propylene glycol) diacrylate (PPGDA) in the presence of varying amounts of poly(propylene glycol) methyl ether acrylate (PPGMEA). The polar ether oxygen linkages in the resulting copolymers interact favorably with CO₂, imparting a high selectivity for CO₂ over light, non-polar gases as required for CO₂ separation applications. The introduction of mono-functional PPGMEA in the polymerization reaction mixture resulted in the insertion of short side branches along the copolymer network and a corresponding reduction in the effective crosslink density; the concentration of propylene oxide (PO) segments in the networks ranged from 60 to 85 wt.%, depending upon the initial reaction composition. The effect of PPGMEA content on the mass density, free volume, and viscoelastic relaxation properties of the polymer networks was studied, and these results were related to the gas transport performance of the rubbery films. Permeability measurements (35 °C) are reported for H₂, N₂, CH₄, CO₂, C₂H₆, and C₂H₄; solubility and diffusivity data are presented for CH₄, CO₂, C₂H₆, and C₂H₄. The physical and gas transport characteristics of the crosslinked PPGDA polymers were compared with those obtained for rubbery networks based on poly(ethylene glycol) diacrylate.

Introduction

Poly(ethylene oxide) (PEO) is known for its excellent CO₂ separation performance, in particular for the removal of CO₂ from light gases (e.g., CH₄, N₂) [1], [2]. The polar ether oxygens in the polymer exhibit favorable interaction with CO₂ that results in high solubility selectivity for CO₂/non-polar gas pairs [3]. Pure PEO is semi-crystalline and exhibits rather low gas permeability. However, Lin et al. successfully prepared amorphous crosslinked PEO networks by ultraviolet (UV) photopolymerization of poly(ethylene glycol) diacrylate (PEGDA). PEGDA was polymerized in the presence of either water diluent or mono-functional PEG acrylate in order to control crosslink density and network architecture [4], [5], [6]. In a series of networks prepared by the copolymerization of PEGDA with poly(ethylene glycol) methyl ether acrylate (PEGMEA), for example, CO₂ permeability was as high as 570 Barrer, which is almost 50 times greater than the permeability obtained in semi-crystalline PEO [1], [5]. At the same time, the polymer maintained a high selectivity for CO₂ over non-polar light gases: the selectivity at 35 °C was 40 for CO₂/N₂ and 14 for CO₂/CH₄ [5]. This balance between high CO₂ permeability and favorable CO₂/light gas selectivity makes crosslinked PEO interesting for applications such as the separation of acid gases from mixtures with non-polar gases [3].

Poly(propylene oxide) (PPO) is structurally similar to poly(ethylene oxide). Therefore, it is of interest to study the effect of an additional methyl group in the repeat unit on the gas transport properties of these crosslinked polymer networks. In this work, a series of crosslinked PPO copolymers were prepared by UV photopolymerization of poly(propylene glycol) diacrylate (PPGDA) solutions containing various amounts of poly(propylene glycol) methyl ether acrylate (PPGMEA) co-monomer. The viscoelastic properties of the resulting networks have been studied using dynamic mechanical analysis, and the relaxation characteristics of these materials have been examined as a function of network composition and effective crosslink density. The insight into network structure obtained via the dynamic mechanical studies is subsequently related to gas separation performance. Specifically, the pure gas permeability and solubility of various penetrants in crosslinked amorphous poly(propylene glycol) diacrylate (XLPPGDA) and XLPPGDA copolymers at 35 °C are reported. Diffusion coefficients estimated from the permeability and solubility data are presented as a function of the polymer fractional free volume (FFV).

Section snippets

Background

The steady-state gas permeability coefficient of a polymer membrane is defined as follows [7]: $P = \frac{N l}{p_{2} - p_{1}}$ where P is the gas permeability coefficient (cm³(STP) cm/(cm²s cmHg)), N the steady-state penetrant flux through the membrane (cm³(STP)/cm²s), l the membrane thickness (cm), p₂ the upstream pressure (cmHg), and p₁ is the downstream pressure (cmHg). When Fick's law is obeyed and the downstream pressure is much less than the upstream pressure, the permeability coefficient defined in Eq. (1) can be

Materials

Gas cylinders of methane, ethane and ethylene of chemical purity (i.e., 99%), and helium, hydrogen, nitrogen and carbon dioxide of 99.9% purity were purchased from Air Gas Southwest, Inc. (Corpus Christi, TX) and were used as received. Poly(propylene glycol) diacrylate (PPGDA), poly(propylene glycol) methyl ether acrylate (PPGMEA), toluene, and 1-hydroxylcyclohexyl phenyl ketone (HCPK) were purchased from Aldrich Chemical Company (Milwaukee, WI). All chemicals were used as received unless

PPGDA and PPGMEA molecular weight characterization

Based on the information provided by the supplier, the PPGDA crosslinker and PPGMEA monomer used in these studies have number average molecular weights of 900 and 202 g/mol, respectively. ¹H NMR and FAB-MS spectra of the crosslinker and monomer molecules were collected to confirm the molecular weight and to determine polydispersity (see Fig. 1, Fig. 2). The number average molecular weights estimated from ¹H NMR and FAB-MS were similar. According to the ¹H NMR and FAB-MS spectra, the average

Conclusions

The physical and gas transport characteristics of a series of rubbery crosslinked copolymers based on poly(propylene glycol) diacrylate have been examined. The presence of the bulky propylene oxide segment along the PPGDA crosslinker and PPGMEA branches leads to relatively large amounts of free volume in these networks that are reflected in correspondingly high values of gas permeability. Rubbery membranes based on 100% PPGDA (XLPPGDA100; FFV = 0.160), for example, showed a CO₂ permeability of 160

Acknowledgements

We gratefully acknowledge support of this work by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (grant no. DE-FG03-02ER15362). The research was also supported by the United States Department of Energy National Energy Technology Laboratory under a subcontract from Research Triangle Institute through their prime contract no. DE-AC26-99FT40675. This paper was prepared with partial support from the U.S.

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¹: Current address: Membrane Technology & Research, Inc., 1360 Willow Road, Suite 103, Menlo Park, CA 94025, United States.

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Relation between network structure and gas transport in crosslinked poly(propylene glycol diacrylate)

Abstract

Introduction

Section snippets

Background

Materials

PPGDA and PPGMEA molecular weight characterization

Conclusions

Acknowledgements

J. Membr. Sci.

J. Membr. Sci.

J. Mol. Struct.

J. Membr. Sci.

J. Membr. Sci.

Polymer

Prog. Org. Coat.

J. Non-Cryst. Solids

Polymer

J. Membr. Sci.

The effect of crosslinking on gas permeability in crosslinked poly(ethylene glycol diacrylate)

Macromolecules

High-performance polymer membranes for natural-gas sweetening

Adv. Mater.

Gas separation using polymer membranes: an overview

Polym. Adv. Technol.

Separation of gases using solubility–selective polymers

Trends Polym. Sci.

Introduction to Infrared and Raman Spectroscopy

Photopolymerization of multifunctional monomers in condensed phase

J. Appl. Polym. Sci.

Gas transport properties of poly(ether-b-amide) segmented block copolymers

J. Polym. Sci. Part B: Polym. Phys.

Performance of a versatile variable-volume permeability cell. Comparison of gas permeability measurements by the variable-volume and variable-pressure methods

J. Appl. Polym. Sci.

Relationship between structure and transport properties for polymers with aromatic backbones

Selection of barrier materials from molecular structure

Polym. Eng. Sci.