Preparation of pH sensitive poly(vinilydenefluoride) porous membranes by grafting of acrylic acid assisted by supercritical carbon dioxide

Dedicated to Professor Gerd Brunner on the occasion of his 70th birthday.
https://doi.org/10.1016/j.supflu.2011.09.004Get rights and content

Abstract

Free radical grafting of acrylic acid (AA) on poly(vinilydenefluoride) (PVDF) porous membranes was studied at 65 °C using supercritical carbon dioxide (scCO2) as a solvent and delivery agent. The process was initiated by the thermal decomposition of benzoylperoxide (BPO).

Spectroscopic analyses confirmed the presence of poly(AA) chains linked to treated membranes. The mass fraction of grafted AA increased with grafting time and BPO concentration while it decreased when the density of the fluid phase was enhanced. A not-monotonic trend was obtained when the effect of the initial AA concentration was studied. The grafting process was accompanied by a reduction of the crystallinity of the PVDF matrix.

Flow measurements and caffeine release experiments performed using PVDF membranes with proper grafting degree exhibited a pH dependent behaviour.

Highlights

Free radical grafting of acrylic acid on poly(vinylidenefluoride) membranes in supercritical carbon dioxide (scCO2). ► Grafting level and modification of polymer crystallinity well controlled by adjusting the density of scCO2. ► Significant pH dependent permeability of the membranes obtained at intermediate degree of grafting. ► pH sensitive dissolution rate of caffeine encapsulated in grafted membranes.

Introduction

Poly(vinylidenefluoride) (PVDF) is known for its excellent mechanical and physicochemical properties. Moreover, because of its chemical inertness and good processability, PVDF has been extensively investigated for the preparation of membranes [1], [2], [3]. However their utilization for biomedical applications, such as selective permeation and controlled drug release and delivery, is limited to some extent by the hydrophobic nature of the polymer. Moreover, for these applications, it could be important to impart the matrix a stimuli-responsive behaviour allowing reversible changes in their properties in response to some environmental stimuli, such as temperature and pH [4], [5], [6], [7], [8], [9], [10], [11].

Several approaches have been developed to endow the conventional hydrophobic membranes with hydrophilic properties and pH-sensitivity [12], [13], [14], [15], [16], [17], [18]. An interesting method to reach this goal was based on the preactivation of the fluoropolymer, dissolved in N-methylpyrrolidone (NMP) or N,N-dimethylformamide, by ozone treatment. After precipitation and drying, the PVDF matrix containing peroxide groups is redissolved in the solvent and reacted in homogeneous phase with acrylic acid (AA) or 4-vinylpyridine [15], [16], [17]. A similar approach in the presence of 1-phenylethyl dithiobenzoate as chain transfer agent was also used to graft living poly(acrylic acid) on PVDF [18]. These methods are anyway based on the utilization of low-volatile toxic solvents that must be thoroughly removed from the polymer before its utilization in biomedical applications. A different tested approach was the activation of PVDF by ionizing or UV–vis radiation and subsequent modification of the matrix by heterogeneous grafting of AA in bulk or from aqueous solutions [19], [20], [21], [22], [23], [24], [25], [26], [27]. By this route purification concerns have been decreased and are mainly related to the removal of unreacted monomer, anyway non-uniform grafting levels can be obtained.

Supercritical carbon dioxide (scCO2) has been already adopted to perform modification of polymers by grafting of several vinyl monomers [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. By this choice one would take benefit of scCO2 peculiar properties, i.e. biocompatibility, easy recovery from the processed polymer by depressurization, inertness to chain transfer reaction and good swelling capability towards amorphous polymers that imparts fast mass transfer rate of low molecular weight species inside the CO2 plasticized polymer [40], [41], [42], [43]. Moreover, initiator efficiency is improved in scCO2 because solvent-solute cage effects are negligible relative to liquid solvents [44], [45], [46]. Furthermore, CO2 can be used also for the purification of the matrix at the end of the grafting process by supercritical extraction of unreacted species [47].

Last but not least it must be underlined that the modified PVDF matrix is a specialty polymer for a market segment that is characterized by small volume of production and high added value of the product thus creating favourable conditions for the industrial adoption of an unconventional technology such that based on the utilization of dense gases.

In this work we have studied the preparation of hydrophilic and pH-sensitive PVDF membranes by free radical grafting of the AA monomer using scCO2 as solvent and swelling agent and benzoyl peroxide as thermal initiator at mild temperature condition. The effect of selected operative parameters such as the grafting time, the initial concentration of monomer and initiator and the density of the fluid phase was investigated. The modified membranes (PVDF-g-PAA) were also characterized in terms of their flow behaviour at different pH levels and for what concern their ability to modify the dissolution rate of caffeine, adopted as a model bioactive compound, in aqueous phase in response to modifications of environmental pH.

Section snippets

Materials

Durapore PVDF hydrophobic membranes were used as model substrates. They have an average pore size of 0.22 μm, a diameter of 47 mm, a thickness of about 100 μm and 75% (vol/vol) porosity. Acrylic acid (AA) 99% anhydrous (with 200 ppm of monomethyl ether hydroquinone as inhibitor) and ethanol (ACS, 99.8%) were purchased from Aldrich; CO2 was Air Liquide 99.998 pure. All these chemicals were used without further purification. Benzoyl peroxide (BPO), purchased from Aldrich (75% remainder water, USP

Phase behaviour of the CO2/AA/BPO system

Solubility of acrylic acid in supercritical carbon dioxide is quite high. Up to 40% (w/w) concentration of the vinyl monomer can be dissolved in CO2 at 65 °C and pressure higher than 13 MPa [49].

The phase behaviour of the CO2/AA/BPO mixture was investigated at 0.6 M concentration of AA and with the highest BPO concentration used to perform grafting reactions, at the lowest value of investigated CO2 density, i.e. 0.5 g/mL, by visual observation in a view cell. We observed that, at temperature higher

Conclusions

Free radical grafting of AA on PVDF membranes was successfully performed in scCO2 using the initiator benzoyl peroxide as source of radicals to initiate the process at mild temperature condition. Results collected from 1H NMR, 19F NMR and HATR-FTIR analyses of treated membranes thoroughly extracted with hot ethanol indicated the presence of PAA chains with no evidence of dehydrofluorination of the PVDF chains.

The results collected studying the influence of selected operative parameters

Acknowledgements

The financial support of Università di Palermo (Programma di Ricerca Innovativo di Ateneo 2007) is gratefully acknowledged.

Authors are grateful to Emma Barchiesi and Roberto Biancardi of Solvay Solexis for NMR analyses.

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