Achille M. Bivigou-Koumba

Examining the UV-vis absorption of RAFT chain transfer agents and their use for polymer analysis

The absorption characteristics of a large set of thiocarbonyl based chain transfer agents (CTAs) were studied by UV-vis spectroscopy in order to identify appropriate conditions for exploiting their absorbance bands in end-group analysis of polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerisation. Substitution pattern and solvent polarity were found to affect notably the wavelengths and intensities of the π–π*- and n–π*-transition of the thiocarbonyl bond of dithioester and trithiocarbonate RAFT agents. Therefore, it is advisable to refer in end group analysis to the spectral parameters of low molar mass analogues of the active polymer chain ends, rather than to rely on the specific RAFT agent engaged in the polymerisation. When using appropriate conditions, the quantification of the thiocarbonyl end-groupsvia the π–π* band of the thiocarbonyl moiety around 300–310 nm allows a facile, sensitive and surprisingly precise estimation of the number average molar mass of the polymers produced, without the need of particular end group labels. Moreover, when additional methods for absolute molar mass determination can be applied, the quantification of the thiocarbonyl end-groups by UV-spectroscopy provides a good estimate of the degree of active end group for a given polymer sample.

Kinetics of Collapse Transition and Cluster Formation in a Thermoresponsive Micellar Solution of P(S-b-NIPAM-b-S) Induced by a Temperature Jump

Structural changes at the intra- as well as intermicellar level were induced by the LCST-type collapse transition of poly(N-isopropyl acrylamide) in ABA triblock copolymer micelles in water. The distinct process kinetics was followed in situ and in real-time using time-resolved small-angle neutron scattering (SANS), while a micellar solution of a triblock copolymer, consisting of two short deuterated polystyrene endblocks and a long thermoresponsive poly(N-isopropyl acrylamide) middle block, was heated rapidly above its cloud point. A very fast collapse together with a multistep aggregation behavior is observed. The findings of the transition occurring at several size and time levels may have implications for the design and application of such thermoresponsive self-assembled systems.

Switching kinetics of thin thermo-responsive hydrogel films of poly(monomethoxy-diethyleneglycol-acrylate) probed with in situ neutron reflectivity

The switching kinetics of thin thermo-responsive hydrogel films of poly(monomethoxy-diethyleneglycol-acrylate) (PMDEGA) are investigated. Homogeneous and smooth PMDEGA films with a thickness of 35.9 nm are prepared on silicon substrates by spin coating. As probed with white light interferometry, PMDEGA films with a thickness of 35.9 nm exhibit a phase transition temperature of the lower critical solution temperature (LCST) type of 40 °C. In situ neutron reflectivity is performed to investigate the thermo-responsive behavior of these PMDEGA hydrogel films in response to a sudden thermal stimulus in deuterated water vapor atmosphere. The collapse transition proceeds in a complex way which can be seen as three steps. The first step is the shrinkage of the initially swollen film by a release of water. In the second step the thickness remains constant with water molecules embedded in the film. In the third step, perhaps due to a conformational rearrangement of the collapsed PMDEGA chains, water is reabsorbed from the vapor atmosphere, thereby giving rise to a relaxation process. Both the shrinkage and relaxation processes can be described by a simple model of hydrogel deswelling.

Kinetics of aggregation in micellar solutions of thermoresponsive triblock copolymers – influence of concentration, start and target temperatures

In aqueous solution, symmetric triblock copolymers with a thermoresponsive middle block and hydrophobic end blocks form flower-like core–shell micelles which collapse and aggregate upon heating through the cloud point (CP). The collapse of the micellar shell and the intermicellar aggregation are followed in situ and in real-time using time-resolved small-angle neutron scattering (SANS), while heating micellar solutions of a poly((styrene-d8)-b-(N-isopropyl acrylamide)-b-(styrene-d8)) triblock copolymer in D2O rapidly through their CP. The influence of polymer concentration as well as of the start and target temperatures is addressed. In all cases, the micellar collapse is very fast. The collapsed micelles immediately form small clusters which contain voids. They densify which slows down or even stops their growth. For low concentrations and target temperatures just above the CP, i.e. shallow temperature jumps, the subsequent growth of the clusters is described by diffusion-limited aggregation. In contrast, for higher concentrations and/or higher target temperatures, i.e. deep temperature jumps, intermicellar bridges dominate the growth. Eventually, in all cases, the clusters coagulate which results in macroscopic phase separation. For shallow temperature jumps, the cluster surfaces stay rough; whereas for deep temperature jumps, a concentration gradient develops at late stages. These results are important for the development of conditions for thermal switching in applications, e.g. for the use of thermoresponsive micellar systems for transport and delivery purposes.

In-operando study of swelling and switching of thermo-responsive polymer films

Thermo-responsive polymers can exhibit a demixing transition which is of the lower critical solution temperature (LCST) type. The collapse transition of polymers with such LCST behavior is of great interest for applications where a strong change of volume is desired even for small changes of an external stimulus such as temperature. Examples of applications are valves in micro-fluidics, the release of drugs in the body or sensors. A polymer which is a promising candidate in this context is poly(N-isopropylacrylamide), PNIPAM. It exhibits a LCST of about 32 °C that is attributed to alterations in the hydrogen-bonding interactions of the amide group. A polymer with a higher transition temperature as compared to PNIPAM is the thermosensitive acrylate poly(methoxydiethylene glycol acrylate) or PMDEGA. In thin film geometry, such thermoresponsive polymers are of particular interest for use as thermosensitive surfaces, artificial pumps and muscles, light modulation systems and optical switches. The strong volume change translates into a change of the film thickness due to the thin film geometry. We investigate the kinetics of chain collapse of thin thermosensitive films as a function of quench depth. Homopolymer films and amphiphilic block copolymer films are compared. With time resolved neutron reflectivity (NR) we follow the kinetics of the transition from a swollen to a collapsed thermoresponsive film. Within 15 seconds a full NR curve is probed, which allows to determine the evolution of film thickness and of the water content in real time. Thus, we investigate in-operando the switching behavior caused by a thermal trigger. The observed complex three-step switching of the films is discussed.