Free-volume studies in polyurethane membranes by positron annihilation spectroscopy

doi:10.1016/S0969-806X(03)00234-2

Radiation Physics and Chemistry

Volume 68, Issues 3–4, October–November 2003, Pages 573-576

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Abstract

The free-volume parameters in various urethane/urea membranes obtained by varying the ratio of the structural constituents, polypropylene oxide and polybutadiene, were studied by positron lifetime and Doppler broadening measurements. On bi-soft segment membranes, a correlation was found between the composition of membranes, the normalized free volume, the radii of the holes and gas permeability. However, the correlation is not clear when PU data are also considered, indicating that other features must also play an important role in the permeation mechanism.

Introduction

Polyurethanes, extensively used in the chemical industry, usually consist of hard segments (urethane/urea groups and/or low molecular weight chain extenders) and one or more soft segments (e.g., polyether/polyesters). Depending on the processing parameters and on the type of hard and soft segments these can be totally mixed or phase-separated to different degrees (Cooper and Tobolsky, 1966; Blackwell and Gardner, 1979; Fridman and Thomas, 1980). This structure versatility enables the production of materials with a wide range of properties. One important application of urethane/urea membranes is the exploitation of their selective permeability in industrial separations, namely gas permeation and pervaporation.

The free-volume concept is extensively adopted in polymer science to explain many properties at microscopic structural level and to relate them with macroscopic properties. Positron annihilation lifetime spectroscopy is an important method for studying subnanometer size holes and for the determination of their size distribution and free-volume fractions (e.g. Jean, 1993; Dlubek et al., 2002). In a polymer matrix, thermalized positrons can form positronium (Ps) that will be localized in free-volume holes. The primary annihilation mechanism of the long-lived triplet state of positronium (ortho-positronium, o-Ps) is the pick-off annihilation with electrons of the host medium, with annihilation parameters dependent on the physico-chemical properties. The lifetime and intensity of this component can be translated into hole size and density of holes. Recently, it has been shown that the Doppler broadening method may also give an important contribution to these studies (e.g. Dlubek et al., 2000). Indeed, the W-parameter, extensively applied in solid-state physics investigations (Flores et al., 2002) seems to provide specific sensitivity to the molecular bounds and ordering of the polymer chains and for this reason the Doppler spectroscopy gains increasing importance and acceptance in polymer studies as well (e.g., Semaan et al., 2001).

In this work, urea/urethane membranes obtained through the introduction of two soft segments were studied with the aims of establishing how the free-volume hole properties are influenced by the phase separation or mixing of the polymer components and correlate them with the known permeability features.

Section snippets

Experimental

The polymer membranes were synthesized from two basic units: polypropylene oxide-based prepolymer with three isocyanate terminal groups (PU), supplied by Portuguese Hoechst, and polybutadiene diol (PBDO) from Aldrich, containing 20–30 wt% vinyl, 10–25 wt% cis-1,4 and 50–60 wt% trans-1,4 isomers. Dibutyltin dilauryate (DBTDL), supplied by Aldrich, was used as catalyst. Toluene, from Merck, was used as solvent.

Cross-linked membranes were prepared through the reaction of the isocyanate groups of PU

Results and discussion

The LT spectra were deconvoluted into three components with lifetimes τ_i and intensities I_i. Subscripts i=1–3 are ascribed to p-Ps, free e⁺ and o-Ps, respectively. The lifetime τ₃ and intensity I₃ are associated with pick-off annihilation of o-Ps in free volumes. According to the simple model of Tao and Eldrup (Tao, 1972; Eldrup et al., 1981) for an o-Ps confined in an infinitely deep spherical potential well, there is a correlation between the free-volume hole radius, R, and the long-lived

Conclusions

It is shown that hole size and overall free volume in bi-soft segment polymer membranes, as obtained by positron annihilation, are correlated to gas permeation properties. However, the changes observed in those free-volume parameters are of much lower magnitude than the changes found in permeability and it can be concluded that hole size and overall free volume cannot by themselves describe those changes. The free volume seen by the o-Ps does not need to be identical to the free volume

Acknowledgements

One of the authors, MFFM expresses thanks to the Instituto Politécnico de Coimbra for partial funds. Additional support through the European Commission HPMF-CT-2000-01038/MCFI-2000-01879 contract is highly acknowledged.

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Free-volume studies in polyurethane membranes by positron annihilation spectroscopy

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Polymer

Chem. Phys.

Polymer

Comput. Phys. Commun.

Radiat. Phys. Chem.

Polymer

Polymer

Properties of linear elastomeric polyurethanes

J. Appl. Sci.