Linear and Cross-linked UCST-Type Polymers : Synthesis and Properties

Thermoresponsive polymers are well studied and applied in both scientific and industrial areas. Among them, polymers with lower critical solution temperature (LCST)-type thermoresponsivity were extensively investigated. However, in some applications polymers with upper critical solution temperature (UCST)-type thermoresponsivity are required. To have a better understanding of UCST behavior, this dissertation deals with the synthesis of polymers, that show UCST-type thermoresponsivity in water or in electrolytes, with different architectures and their properties. There are four main topics in this thesis: (1) the effect of polymer characters such as composition, chain ends and molar mass on the UCST behavior; (2) chemistry for grafting polymers with UCST-type thermoresponsivity onto metal nanoparticles; (3) UCST-type volume changes of chemically cross-linked hydrogels; and (4) photo cross-linkable polymer with UCST-type thermoresponsivity. In the Introduction, thermoresponsive polymers showing both LCST- and UCST-type thermoresponsivity in aqueous solutions were summarized as literature background of this thesis. The aims of the thesis were listed based on the challenges in the UCST field. Additionally, controlled radical polymerization methods were introduced as effective technique for synthesizing polymers with linear structures. Gold nanoparticles protected by thermoresponsive polymers were reviewed. The Synopsis introduces the main results of five publications and the connection between each work. Five publications were attached in the form of a journal article. Publication 1 and Publication 2 comprise the synthesis of poly(N-acryloylglycinamide) (PNAGA) with a linear structure, by using two controlled radical polymerization methods, namely: reversible addition fragmentation transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP). Both methods exhibited a good control of the synthesis of PNAGA, by showing a linear increase of the polymer molar mass with conversion, narrow molar mass dispersity and successful chain extension experiments. It was found that a non-ionic chain transfer agent (only for RAFT) and a non-ionic initiator were important to keep the key property of the polymers: the UCST-type thermoresponsivity in aqueous solutions. It was shown in Publication 1 that the hydrophobic dodecyl end groups caused an increasing of cloud points of polymers with lower molar mass (Mn below 10000 g/mol). Thus, in Publication 2, as consecutive work, a monomer-like initiator was chosen to synthesize linear polymer with primary amide end-groups. The prepared polymers with hydrophilic end-groups showed cloud points independent from the molar mass. In both works, the influence of molar mass, polymer end-groups and salt (NaCl and Na2SO4) concentrations on the cloud point was analyzed by means of turbidimetry measurements. In Publication 3, the previously synthesized trithiocarbonate end-functionalized PNAGA, synthesized via RAFT polymerization (shown in Publication 1), was grafted onto gold nanoparticles (AuNPs) by ligand exchange in phosphate buffered saline (PBS) solutions. The PNAGA functionalized gold nanoparticles (PNAGA@AuNPs) showed positive thermoresponsivity in PBS, additionally the UCST-type phase transition was reversible for at least nine cooling/heating cycles. It was found that the grafting process had no negative effects onto the cloud points, as the PNAGA@AuNPs showed similar phase transition behaviour as that of the free-PNAGA sample used for grafting purpose. Publication 4 presents the UCST-type thermosensitivity of chemically cross-linked PNAGA hydrogel with N,N’-methylenbis(acrylamide) (MBAAm) as cross-linker synthesized via free radical polymerization. The hydrogel showed continuously positive volume transitions in water and in electrolyte solutions, meaning the hydrogel swells at high temperature and shrinks by decreasing the temperature. The degree of hydrogel swelling was controlled by varying the contents of cross-linkers. It was found that with less amount of cross-linker, the swelling/deswelling behaviour of the hydrogel became more similar like the linear polymer in aqueous solution. The volume-change was reversible in pure water as well as in PBS, by showing at least seven cooling/heating cycles in a temperatures range between 4 °C and 40 °C. In Publication 5, a terpolymer system with UCST-type thermoresponsivity based on acrylamide (AAm), acrylonitrile (AN) and UV cross-linkable comonomer was reported. Terpolymers with linear structures synthesized via free radical and RAFT polymerization showed UCST-type thermoresponsivity in water and in electrolytes. The terpolymers showed almost no hysteresis during cooling and heating cycles. Furthermore, they are stable against hydrolysis for at least nine cycles. The cloud points of the polymer solution can be tuned by varying the AN contents in polymer. Chemically cross-linked films and nanofibers were successfully produced from terpolymers by solution casting and electrospinning followed by UV irradiation. The hydrogels showed temperature dependent positive volume-change that was utilized for design of microactuators. In Outlook, the challenges and opportunities for UCST-type polymers were discussed. So far nonionic UCST-type polymers are focusing on polymers containing amide or ureido groups. Other hydrophilic polymers with suitable content of hydrogen bonding units could theoretically display UCST property. This may enlarge the UCST-type polymer family greatly. The non-ionic hydrogel with positive thermoresponsivity could be applied for drug loading and release system. Nevertheless, it is now possible to produce cross-linked polymers from linear polymers by using UV light with different forms as demand for example by lithography.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Liu, Fangyao

Contributors dc:contributor

• Schmid, Franz Xaver

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2144/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2144