Phase behaviour of temperature responsive microgels

: Development of tunable, temperature responsive gelation systems

Abstract

Poly(N-isopropylacrylamide) (pNIPAM) is a temperature responsive polymer which can be synthesised to form cross-linked polymer particles, known as microgels. These microgels swell in the presence of water, and at elevated temperatures collapse, expelling water from their interior and thus decreasing in diameter. The temperature at which pNIPAM microgels collapse is known as the volume phase transition temperature (VPTT). pNIPAM is a member of a family of acrylamide polymers that are temperature responsive, however, due to pNIPAM having a collapse temperature close to physiological conditions, research into the potential biomedical applications for pNIPAM is extensive. In this thesis, we investigate the formation of temperature responsive gels made from pNIPAM microgels. These gels form reversibly at elevated temperatures. We characterise the phase behaviour of the responsive gels, using a combination of scattering and imaging techniques. We also investigate how the properties of the gels are dependent on the conditions of gelation and the solution composition.

We investigate mixtures of a polyethyleneoxide-polypropyleneoxide-polyethyleneoxide (PEO-PPO-PEO) type triblock-copolymer surfactant and pNIPAM. These mixtures were found to form temperature responsive gels at elevated temperatures. The mechanism for gelation in these systems was found to be polymer association, due to hydrogen bonding between the polymers becoming favourable at temperatures above the VPTT. The association of these polymers was investigated using scattering techniques including dynamic light scattering and small angle neutron scattering. The association was also imaged using confocal microscopy. It was also identified that mixtures of pNIPAM and triblock-copolymer have a complex response to temperature changes. The heating rate was found to effect the structures that formed and the mechanism for microgel aggregation. At temperatures above the VPTT, association resulted in gelation, however, at intermediate temperatures and high triblock-copolymer concentrations, depletion resulted in the formation of liquid droplets rich in pNIPAM.

The effects of triblock-copolymer ageing on the properties of pNIPAM mixtures was also investigated. It was identified that the degradation of triblock-copolymer by ageing or temperature resulted in the formation of micelles with significantly different shapes and sizes compared to unaged samples. The ageing of triblock-copolymer resulted in large mass losses to the polymer, along with oxidation, which increased the size of the triblock-copolymer micelles. There was also evidence of cylindrical and planar aggregates with increased degradation times. These larger aggregates formed temperature responsive gels in the presence of pNIPAM at much lower concentrations, highlighting the need to consider polymer ageing when anticipating shelf-life and the properties of polymer formulations.

Finally, we looked at alternative formulations to the mixture of triblock-copolymer and pNIPAM. We replaced triblock-copolymer with salt, carboxylic acids and polyethylene glycol (PEG)and found the resulting gel properties can be tuned and predicted, depending on the additive and the concentration. We also investigate microgels to see if control over the gelation temperature can be achieved, and determine whether the properties of the gels are universal. It was identified that the temperature gelation occurred corresponded directly to the VPTT of the microgel, however the properties of the resultant gels were not consistent. The most striking difference being that in specific microgel samples containing poly(N-isopropylmethacrylamide), syneresis was suppressed.

In summary, this thesis highlights the broad range of additive species that can be used to induce the temperature responsive gelation of microgels. The gelation properties are heavily tunable, where the temperature gelation occurs can be predicted. Several phases can be accessed, including microgel rich liquid droplets, highlighting the wide potential for applications of these highly versatile systems. the gelation properties of alternative acrylamide

Date of Award	24 Jun 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Annela M Seddon (Supervisor), Jeroen Van Duijneveldt (Supervisor) & C. Patrick Royall (Supervisor)