Thermally sensitive block copolymer hydrogels in bulk and with decreased dimensions

Antti Nykänen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

The research on stimuli-responsive polymers has increased rapidly during the last two decades. Poly(N-isopropylacrylamide) (PNIPAM) is one of the most studied thermally responsive polymer because its lower critical solution temperature (LCST) 32 ℃ is close to the ambient conditions. Below 32 ℃, PNIPAM is water soluble, but at temperatures above 32 ℃ the polymer phase separates from water. In this thesis, amphiphilic triblock copolymer polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene (PS-b-PNIPAM-b-PS) was used to prepare thermally-responsive hydrogels. We studied self-assembly and phase behavior of PNIPAM rich compositions of PSb-PNIPAM-b-PS in bulk as well as selected blends with low molecular weight PNIPAM homopolymers. In aqueous solutions, the glassy PS domains act as physical cross-links, and therefore hydrogels are formed. We observed, that the bulk block copolymer morphology has a strong effect on the degree of swelling in aqueous solutions below the LCST. Bulk compositions swelled in water up to 58 times by weight, whereas all the samples shrunk close to their dry state, after heating above 32 ℃ as a result of their thermally sensitive character. Hydrogel swelling and shrinking are diffusion based processes. The response time is directly proportional to the square of the size of the hydrogel, which makes bulk hydrogels unattractive for applications, which require fast response. To address this problem, we prepared thin films, submicron sized fibres, and spherical particles, whose morphology and behavior was characterized in water. It was found out that in thin films, the PS domains undergo extensive plastic deformation upon the gel swelling process, although temperature is always maintained well below the bulk PS glass transition temperature. In electrospun fibres and aerosol particles we also noticed, that the molecular architecture and molecular weight can cause stability issues, and the limits of the concept of physical cross-linking were reached. Finally, the applicability of these thermally sensitive gels was demonstrated by preparing thermally switchable molecular filters, by studying thermo-sensitive wetting transition and by evaluating thermally controlled release of an organic dye.
Translated title of the contributionLämpöherkät lohkopolymeerihydrogeelit tarkasteltuna sekä makroskooppisina että nanomateriaaleina
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Ruokolainen, Janne, Supervising Professor
  • Ruokolainen, Janne, Thesis Advisor
Publisher
Print ISBNs978-952-60-5208-3
Electronic ISBNs978-952-60-5209-0
Publication statusPublished - 2013
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • stimuli-responsive hydrogels
  • poly(N-isopropylacrylamide)
  • block copolymers

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