Dan-Thuy Van-Pham

Polymer materials with spatially graded morphologies: preparation, characterization and utilization

Experimental studies on polymer materials with spatially graded structures are reviewed in this paper. A wide variety of principles and experimental methods utilized to prepare and control these specific structures of polymer materials are summarized and discussed. In particular, the method of using light to generate and control these gradient morphologies in the micrometer scales is summarized with great detail for binary polymer mixtures. Finally, recent studies on copolymers with various gradient compositions at nanometer length scales are also summarized in this review.

Phase separation kinetics and morphology induced by photopolymerization of 2-hydroxyehyl methacrylate (HEMA) in poly(ethyl acrylate)/HEMA mixtures

Morphology and phase separation kinetics induced by polymerization of 2-hydroxyethyl methacrylate (HEMA) in a HEMA/poly(ethyl acrylate) (PEA) mixture were observed by laser scanning confocal microscope in the presence of lucirin TPO used as an initiator. The results were analyzed by 2D-Fourier transform (2D-FFT). The photopolymerization is driven by irradiation with visible light λ = 405 nm. The mixture exhibits the Trommsdorff–Norrish effect which is responsible for a drastic increase in the reaction rate during the irradiation process. The concentration fluctuations and the increase in the viscosity of the medium play an important role in promoting the reaction yield. PHEMA droplets were found to develop in the rhodamine-B-labeled poly(ethyl acrylate) (PEA-Rh) continuous matrix. The characteristic length of the morphology increases with increasing irradiation intensity, revealing the tool to control the morphology by varying the light intensity.

THE ROLES OF REACTION INHOMOGENEITY IN PHASE SEPARATION KINETICS AND MORPHOLOGY OF REACTIVE POLYMER BLENDS

The roles of reaction inhomogeneity in phase separation of polymer mixtures were described and summarized via two examples: photocross-link of polymer mixtures in the bulk state and photopolymerization of monomer in the liquid state. The reaction kinetics, the reaction-induced elastic strain and the phase separation kinetics were monitored respectively by UV-Vis spectroscopy, Mach-Zehnder interferometry and laser-scanning confocal microscopy. It was found that phase separation in the bulk state was strongly influenced by the elastic strain associated with the intrinsic inhomogeneity of the reaction, whereas the autocatalytic behavior of the polymerization plays an important role in the resulting morphology in the liquid state. These experimental results are discussed in conjunction with the morphology control of polymer mixtures by using chemical reactions.

Design and morphology control of polymer nanocomposites using light-driven phase separation phenomena

Phase separation of polymer mixtures is induced and controlled by photo-cross-link and photopolymerization using ultraviolet (UV) light. By taking advantage of the competition between phase separation and chemical reactions, a variety of morphologies such as co-continuous, spatially graded co-continuous and periodic structures with controllable periods, and hexagonal structures, etc, are obtained experimentally. The reaction kinetics (photo-cross-link or photopolymerization), reaction-induced elastic strain and phase separation kinetics are monitored, respectively, by UV–Vis spectroscopy, FTIR, Mach–Zehnder interferometry (MZI), light scattering (LS) and laser-scanning confocal microscopy (LSCM). Spatial modulation of light intensity generated by computer-assisted irradiation (CAI) is also used to induce phase separation of polymer blends. The correlation between the reaction-induced phase separation of polymer mixtures and the competing interactions is discussed with some perspectives on designing polymer materials with high performance.

Phase separation of polymer mixtures induced by light and heat: a comparative study by light scattering

Phase separation of binary blends composed of a polystyrene derivative (PS) and poly (vinyl methyl ether) (PVME) with a lower critical solution temperature (LCST) was experimentally induced by two different methods: heating and UV light irradiation. Using laser light scattering combined with the temperature jump (T-jump) technique, it was demonstrated that in the case of heating, the mixture undergoes phase separation via the nucleation-and-growth (NG) and the spinodal decomposition (SN) processes under shallow and deep quenches, respectively. Particularly, the crossover from the spinodal decomposition to the nucleation-and-growth process was observed at long time under a deep T-jump by light-scattering experiments. On the other hand, in the photo-crosslink case, the PS/PVME blends undergo a nucleation-and-growth process upon irradiation with weak light intensity, whereas the mixture exhibits the spinodal decomposition under irradiation with strong light intensity. From the analysis of the light-scattering data obtained for the blends under the photo-crosslink, the kinetic data reveal the suppression of morphologies having large characteristic length scales. This feature clearly differs from the phase separation induced by heating where no mode-suppression process was observed. It was also found that distribution of the characteristic length scales (the regularity) of the morphology becomes narrow as the phase separation proceeds for reacting blends, whereas it becomes broader as the phase separation proceeds by heating, revealing the important roles of reaction in the suppression of fluctuations with long wavelengths. These experimental results establish a method to control the length scales and the regularity of the morphology of polymer blends by chemical reaction.

Applications of Mach-Zehnder Interferometry to Studies on Local Deformation of Polymers Under Photocuring

A Mach‐Zehnder interferometer (MZI) was built and modified to in situ monitor the deformation of polymers during the photocuring process. In this review, the working principle and method of operation of this MZI were explained together with the method of data analysis. As the examples for the utilization of this modified MZI, measurements of the deformation induced by photopolymerization was demonstrated for three different types of samples: homopolymer in the bulk state, miscible polymer blends and phase‐ separated polymer blends. Finally, a concluding remark is provided for the usage of MZI in polymer research.