Study of network composition in interpenetrating polymer networks of poly(N isopropylacrylamide) microgels: The role of poly(acrylic acid)

Abstract

Hypothesis:The peculiar swelling behaviour of poly(N-isopropylacrylamide) (PNIPAM)-based responsive microgels provides the possibility to tune both softness and volume fraction with temperature, making these systems of great interest for technological applications and theoretical implications. Their intriguing phase diagram can be even more complex if poly(acrylic acid) (PAAc) is interpenetrated within PNIPAM network to form Interpenetrating Polymer Network (IPN) 1 microgels that exhibit an additional pH-sensitivity. The effect of the PAAc/PNIPAM polymeric ratio on both swelling capability and dynamics is still matter of investigation.Experiments: Here we investigate the role of PAAc in the behaviour of IPN microgels across the volume phase transition through dynamic light scattering (DLS) 2 , transmission electron microscopy (TEM) 3 and electrophoretic measurements as a function of microgel concentration and pH.Findings: Our results highlight that aggregation is favored at increasing weight concentration, PAAc content and pH and that a crossover PAAc content C * PAAc 4 exists above which the ionic charges on the microgel become relevant. Moreover we show that the softness of IPN microgels can be tuned ad hoc by changing the PAAc/PNIPAM ratio. These findings provide new insights into the possibility * Corresponding authors. C PAAc : weight content (%) of the PAAc network within each IPN microgel to control experimentally aggregation properties, charge and softness of IPN microgels by varying PAAc content. Introduction The novel class of responsive microgels has recently become very popular since their smart responsivity to external stimuli makes them very attractive for industrial applications [1-6] and excellent model systems for exploring the exotic behaviours emerging in soft colloids due to their softness [7-9]. Their interparticle potential and their effective volume fraction can be easily managed through unusual control parameters such as temperature, pH or solvent, allowing to explore unusual phase-behaviours [10-15], significantly far from those of conventional hard colloids [16-25].The deep investigation in the last years has shown how responsive microgels based on poly(Nisopropylacrylamide) (PNIPAM) undergo a reversible Volume Phase Transition (VPT) 3 at about 305 K that drives the system from a swollen hydrated state to a shrunken dehydrated one, as a consequence of the coil-to-globule transition of NIPAM chains [26]. It has been shown that the driving force for swelling can be estimated from the properties of linear PNIPAM solutions, while the microgel elasticity opposing swelling is mainly due to the network topology dependent on the cross-linker concentration [27][28][29]. The typical swelling/shrinking behaviour of any PNIPAM-based microgel leads to intriguing phase diagrams which may be even more complex if other polymers, sensitive to different external stimuli, are copolymerized or interpenetrated to obtain multi-responsive microgels. In particular PNIPAM microgels containing poly(acrilic acid) ...

Relaxation Dynamics, Softness, and Fragility of Microgels with Interpenetrated Polymer Networks

“…This can be related to the increased interactions due to COOH groups belonging to PAAc chains as deeply discussed in Ref. (46). Our data are well described by the super-Arrhenius expression:…”

“…Particles with higher softness can shrink more, providing a larger value of α that decreases with increasing PAAc content as reported in Ref. (46).…”

Chemical-Physical Behaviour of Microgels Made of Interpenetrating Polymer Networks of PNIPAM and Poly(acrylic Acid)

“…It is worth noting that eventual differences among samples reported in the paper can be attributed to slight differences in the pH condition in the range of between pH 4.5 and 5.5. We can thus conclude that interpenetration of PAAc network introduces additional control parameters for the interaction between PNIPAM and water molecules: polymer/solvent interaction can be therefore controlled through PAAc content [73], by varying pH or by changing solvent [64].…”

Accounting for Cooperativity in the Thermotropic Volume Phase Transition of Smart Microgels

“…However, in this approach in most cases the virial coefficients do not have a physically meaningful interpretation. Such series expansions were successfully used to fit the swelling curves of different microgels (e.g., [ 19 , 44 , 45 , 46 , 47 ]).…”