Aggregation-Induced Expansion of Poly-(N-isopropyl acrylamide) Solutions Observed Directly by the Transient Grating Imaging Technique

Abstract

The anomalous volume expansion of poly-( N -isopropyl acrylamide) (PNIPAM) solutions was observed during the thermally induced polymer phase transition of aqueous solutions having concentrations in the 3–7 wt % range. The process occurred on a millisecond time scale, and a laser temperature-jump time-resolved technique was used to bring about the process. After heating a solution with a pulse laser exploiting light absorption by dyes added to the solution itself, a phase transition was observed to take place, and the temporal changes associated with it were visualized through the transient grating imaging technique, whereby the solution was heated with a stripe pattern. We found several processes occurring on a millisecond time scale, all of which clearly took place after each PNIPAM molecule had collapsed structurally from a coiled to a globular conformation. During the so-called demixing process, the globular polymers aggregated with each other within 10 ms, and suddenly the polymer phase expanded as aggregation progressed further. After this process, the individual globular polymers reverted to their coiled conformation via hydration during the remixing process. We proposed that solution expansion was caused by the mutual entangling of multiple globular PNIPAM molecules, instead each globule polymer was separated.

Entropic effect implication for change in polymer coils swelling state in the demixing enthalpy recovery of aqueous poly(vinyl methyl ether) solutions

“…Furthermore, the results for the 90 wt % water solution reveal that with increasing water concentration, the polymer globules are relatively stable even under the phase‐separation temperature. A similar aggregation and dissolution phenomenon in another PNIPAM dilute aqueous solution with LCST behavior has also been observed using the microcalorimetric method and transient grating imaging technique, respectively. Another plausible interpretation to the change in demixing enthalpy with concentration is that in a semidilute or dilute aqueous solution, the polymer chain will take the form of a much looser globule than in a concentrated solution.…”

Quantitative determination of the spring entropy effect and its indication of the conformational change of polymer coils with varying concentration in aqueous poly(N-isopropylamide) solutions

“…Although ultrafast DSC (the so-called ash DSC) can be utilized to investigate the inuence of fast cooling and heating on the dynamic phenomena in homopolymer and polymer blends, 71 this stage it can only be used to test solid samples. In contrast, laser temperature-jump time-resolved technique combined with other microscopic probing method can obtain the demixing information in a millisecond scale, 73 which could help us in obtaining more comprehensive results on the aggregation behavior of polymer above LCST temperature.…”

Hydrophilic Antireflection and Antidust Silica Coatings

“…A silica thin film has the properties to show simultaneously high optical transparency and low refractive index values, suitable to be used as optical films from the near-ultraviolet to the near-infrared spectral range. − This silicon oxide layer suppresses any parasitic light absorption in the visible range due to its suitable optical band gap and transmission range. − Also, low refractive index values allow these layers to be coupled with high-refractive-index materials for antireflection coating applications by minimizing the total light reflectance. − In addition, due to their high optical transparency, high hydrophobicity property, and the uniformity of their surface, silica layers are extensively used in various industrial and general-purpose applications including flexible displays, optics, bioengineering, ophthalmic, and energy generation. − ,− …”

“…Many techniques have been already employed to grow silica films, including electron beam evaporation, ion-assisted deposition, ion beam sputtering, magnetron sputtering (MS), sol–gel, and atomic layer deposition. − Among all of these techniques, magnetron sputtering (MS) demonstrated the best deposition competency due to the fine control over the growth parameters, film-uniformity, homogeneity, the low density of pinholes to resist defects, and stable optical properties. ,− Hence, nowadays, MS has become a state-of-the-art technique to grow pristine silica films even with large-scale capabilities. Moreover, having the ability to perform the reactive growth of these films under oxygen or any other reactive atmosphere help in controlling the film’s stoichiometry.…”