Synthetic hydrogels 2. Polymerization induced phase separation in acrylamide systems
Introduction
Polymerization induced phase separation (PIPS) is a process in which an initially homogeneous solution of monomer and solvent becomes phase separated during the course of its polymerization. In hydrogel synthesis, PIPS can be induced by a number of factors: continuous increase in the fraction of molecules with high molecular weight, the unfavourable interactions between the polymer and other species in the reaction mixture, or the elasticity of the resultant polymeric network [1], [2]. Depending on the relative rates of the phase separation and the polymerization processes, PIPS can occur by the mechanism of nucleation-growth in the metastable region, or by spinodal decomposition in the multiphase coexisting region of the phase diagram [3], [4].
PIPS is widely employed to prepare various polymeric materials, for example porous thermosetting polymers [5], macroporous materials for chromatography [6], and liquid crystal dispersed polymer films [7]. Recently, Righetti and his co-workers [8] have synthesized macroporous acrylamide hydrogels by the addition of hydrophilic polymers into the monomer mixtures. It was postulated that competition between the gelation process and the phase separation process is responsible for the formation of pores that are much larger than those obtained in the absence of the hydrophilic polymer.
In our first paper of this series [9], we investigated the effects of solvent on the synthesis of acrylamide hydrogel; in this paper, we present results relating to a number of systems where there is the possibility of PIPS. The concept of conversion-phase diagrams is first introduced and later used in a discussion on the influences of the polymerization medium on PIPS.
Section snippets
Materials
Acrylamide (AAm) (electrophoresis grade, >99.9%) was obtained from ICN Biochemicals (Aurora, OH, USA). N,N′-methylenebisacrylamide (Bis) (99%), N,N,N′N′-tetramethylethylenediamine (TEMED) (>99.5%), ammonium persulfate (APS) (>99.5%), tri(ethylene glycol) (>99%), tri(propylene glycol) (>97%), poly(ethylene glycol) (PEG, MW 400, MW 4000), poly(propylene glycol) (PPG, MW 425), and 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt (TMSPA-Na) (99+%)were purchased from Aldrich Fine Chemicals
Results and discussion
In a simplified formation process of acrylamide hydrogel by the free radical co-polymerization of acrylamide (AAm) and a divinyl crosslinker, linear polymers are first formed in the solution during the fast propagation step, and later crosslinked with other molecules in close proximity by reaction through their pendent double bonds and additional monomer units [10]. This work therefore uses conversion-phase diagrams of linear polyacrylamide (pAAm) as a means to visualize the process of PIPS
Conclusion
In this work we have introduced the concept of conversion-phase diagram and used such diagrams of linear pAAm to understand PIPS in the synthesis of acrylamide hydrogels. PIPS was observed during hydrogel synthesis in the presence of a non-solvent for pAAm; the different degrees of opaqueness of the resultant hydrogels were explained by a discussion on the concept of frozen concentration fluctuation and the development of average molecular weight in the pre-gel solution.
Conversion-phase
Acknowledgements
We thank Prof. F. Separovic, Department of Chemistry, University of Melbourne, for technical assistance and valuable discussions on the NMR experiments. We would also like to acknowledge Gradipore Ltd, and the Australian Research Council for their financial support.
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