Kinetics of N-isopropylacrylamide polymerizations in supercritical carbon dioxide fluids

https://doi.org/10.1016/j.supflu.2014.04.002Get rights and content

Abstract

Free-radical polymerization kinetics of N-isopropylacrylamide (NIPAAm) using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator in supercritical carbon dioxide (scCO2) was investigated. A high-pressure differential scanning calorimeter revealed that the melting temperatures of NIPAAm and AIBN were both decreased with increasing CO2 pressure in a linear fashion and the polymerization could occur in CO2 at 55 °C. The polymerization kinetics of NIPAAm in scCO2 was compared with that in methanol (MeOH). At 55 °C, the induction periods of polymerizations in scCO2 of 27.6 MPa were much longer (up to 6 h) than those in MeOH (about 30 min) for similar feed concentrations. The monomer conversions reached and molecular weights produced were much higher in scCO2 than in MeOH. The reaction orders for initial monomer and initiator concentrations, [NIPAAm]0 and [AIBN]0, in initial stage of polymerizations were respectively 1.48 and 0.79 in scCO2 and 1.27 and 0.51 in MeOH.

Introduction

Poly(N-isopropylacrylamide) (PNIPAAm) is a temperature-responsive polymer with a lower critical solution temperature (LCST) of around 32 °C that can be synthesized from NIPAAm which is commercially available [1]. When heated in water above 32 °C, PNIPAAm undergoes a reversible LCST phase transition from a swollen hydrated state to a shrunken dehydrated state, losing about 90% of its volume, due to the temperature dependence of the interactions of polymer chains with water molecules. In dilute solution, PNIPAAm undergoes a corresponding coil-to-globule transition at similar conditions [2]. PNIPAAm forms a three-dimensional hydrogel after crosslinking reactions. Since PNIPAAm expels its liquid contents at a temperature near that of the human body, PNIPAAm has been investigated by many researchers for possible applications in tissue engineering and controlled drug delivery [3], [4].

PNIPAAm was usually synthesized by free radical initiation in organic media such as methanol, benzene, tetrahydrofuran, dioxane, and chloroform using organic medium soluble initiators such as 2,2′-azobis(isobutyronitrile) (AIBN) and benzoyl peroxides. Temperatures varied from near 50 °C to as high as 75 °C [1]. PNIPAAm was also frequently synthesized by redox initiation in aqueous solutions using water soluble initiators such as ammonium persulfate or potassium persulfate via free radical polymerizations at room temperature. Supercritical CO2 (scCO2) fluid, an environmentally friendly medium, has been reportedly an alternative for organic media [5], [6], [7], [8], [9], [10], [11], [12], [13] to address the increasing concern about the negative environmental impact of volatile organic media and aqueous waste containing organic compounds. In a communication, Cao and coworkers [14] reported precipitation copolymerization of NIPAAm and acrylic acid in scCO2 using AIBN as an initiator at 70 °C for 10 h. The molecular weights of the copolymers produced were found to decrease with increasing initiator concentration and reaction pressure. The copolymers were white particles approximately 50 nm in size and demonstrated to be thermosensitive and pH-sensitive materials. Except the above communication, the polymerizations of NIPAAm in scCO2 were rarely investigated. The polymerization kinetics of NIPAAm in scCO2 has not been reported so far in literature to our knowledge. We thus investigate here the polymerization kinetics of NIPAAm in scCO2 and compare with that in methanol (MeOH) using AIBN as an initiator at a relatively low reaction temperature of 55 °C.

Section snippets

Melting temperatures of NIPAAm and AIBN in CO2 and in N2 in HP DSC

The monomer NIPAAm was obtained from Tokyo Kasei and the free radical initiator AIBN was obtained from Aldrich. To investigate how CO2 affects meltings of NIPAAm and AIBN, melting temperatures of NIPAAm and AIBN in CO2 and N2 of different pressures were measured using a high pressure differential scanning calorimeter (HP DSC-Q20, TA Instruments). For NIPAAm, HP DSC was heated at 10 °C/min from room temperature to 80 °C and, for AIBN, HP DSC was heated at 10 °C/min from room temperature to 150 °C in

CO2-assisted melting of NIPAAm and AIBN

Fig. 1A and B shows HP DSC heating curves for NIPAAm in CO2 and N2 of different pressures, respectively. As can be seen in Fig. 1A, the endothermic peaks shifted from about 65 °C in the absence of CO2 to lower temperatures accompanying with smaller endothermic peak areas (i.e., enthalpies) in the presence of CO2 of higher pressures, an indication that the melting temperatures of NIPAAm were depressed by CO2. The depression of melting temperature was in a near linear fashion with increasing CO2

Conclusions

HP DSC data revealed that the melting temperatures of NIPAAm and AIBN were both decreased with increasing CO2 pressure in a linear fashion and the polymerization could occur in CO2 at 55 °C. The polymerization in scCO2 of 27.6 MPa at 55 °C had an induction period up to 6 h depending on the monomer and initiator concentrations used whereas the induction period for polymerization with similar initial concentrations in MeOH was much shorter being about 30 min. The monomer-to-polymer conversions reached

Acknowledgment

We gratefully acknowledge the financial support of the National Science Council of Taiwan under the NSC 98-2221-E-390-002-MY3 contract.

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