The swelling and phase transition behavior upon increasing temperature of a doubly thermoresponsive diblock copolymer thin film in steps above the characteristic cloud points (CPs) of the blocks is ...studied. An upper critical solution temperature (UCST)-type zwitterionic poly(sulfobetaine), poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate) (PSPP, CPUCST = 31.5 °C), is combined with a lower critical solution temperature (LCST)-type nonionic poly(N-isopropyl-/methacrylamide) (PNIPMAM, CPLCST = 49.5 °C) block. Using time-of-flight neutron reflectivity (ToF-NR), we observe the swelling in D2O vapor at a constant temperature of 20 °C, followed by two subsequent temperature jumps, from 20 to 40 °C (above CPUCST) and from 40 to 60 °C (above CPLCST). The observed response of the diblock copolymer films deviates from the aqueous solution behavior, which is mainly attributed to the increased polymer concentration. Temperature-induced changes in the thin-film nanostructure are investigated with ToF grazing-incidence small-angle neutron scattering (GISANS). Alterations in the chain conformation and hydrogen bonding are probed by Fourier transform infrared (FTIR) spectroscopy. The ionic SO3 – groups (in the PSPP block) and the nonionic hydrophilic amide groups (in both blocks) are found to affect the mechanisms of D2O uptake and release significantly.
The thermosensitive aggregation behavior in an aqueous solution of a library of amphiphilic BAB* copolymers is studied, where “A” represents a long permanently hydrophilic ...poly(N,N-dimethylacrylamide) (pDMAm) block, “B” represents a permanently hydrophobic end with an n-dodecyl chain, and “B*” represents a thermoresponsive (TR) block featuring a lower critical solution temperature (LCST). Four polyacrylamides are employed for B*, namely, poly(N-n-propylacrylamide) (pNPAm), poly(N-isopropylacrylamide) (pNiPAm), poly(N,N-diethylacrylamide) (pDEAm), and poly(N-acryloylpyrrolidine) (pNAP), which differ with respect to the hydrophilicity of their amide side chains and LCST behavior. While blocks A and B were kept constant, the lengths of the TR blocks were varied systematically. These amphiphilic copolymers were studied as a function of concentration and temperature via light and neutron scattering (static light scattering (SLS), dynamic light scattering (DLS), small-angle neutron scattering (SANS)). For sufficiently long pNiPAM and pDEAm blocks (DP n > 40), a pronounced hydrophobic effect at temperatures above the LCST transition results in well-structured, ordered aggregates. Thus, the aggregation can be controlled by the choice and length of the TR block, thereby elucidating a so far hardly explored class of temperature-sensitive polymeric amphiphiles.
Temperature control of rheological properties of aqueous solutions can be achieved by the addition of amphiphilic polymers that show temperature-dependent self-assembly. For this purpose, we explored ...three sets of acrylamide-based block copolymers with BAB*-, B2AB*-, and B(AB*)2-type architectures, where “B” represents a permanently hydrophobic unit, “A” is a permanently hydrophilic block, and “B*” is a thermoswitchable block, which undergoes a phase transition of the lower critical solution temperature (LCST) type. Depending on the specific polymer architecture and choice of the thermoresponsive block, the viscosity of their aqueous solutions can augment substantially with increasing temperature. The macroscopic rheological changes were correlated with the results of static and dynamic light scattering (SLS, DLS) and small-angle neutron scattering (SANS) experiments, showing a clear correlation with the mesoscopic organization of the respective systems. Complementary studies with the fluorescence probe Prodan also revealed a clear correlation of the enhanced viscosity to the formation of hydrophobic domains of the thermoresponsive block. Accordingly, the appropriate design of such “smart” copolymer thickeners enables the tuning of the viscoelastic properties of aqueous solutions.
A 2-fold thermoresponsive diblock copolymer PSPP430-b-PNIPAM200 consisting of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block is prepared by ...successive RAFT polymerizations. In aqueous solution, the corresponding homopolymers PSPP and PNIPAM feature both upper and lower critical solution temperature (UCST and LCST) behavior, respectively. The diblock copolymer exhibits thermally induced “schizophrenic” aggregation behavior in aqueous solutions. Moreover, the ion sensitivity of the cloud point of the zwitterionic PSPP block to both the ionic strength and the nature of the salt offers the possibility to create switchable systems which respond sensitively to changes of the temperature and of the electrolyte type and concentration. The diblock copolymer solutions in D2O are investigated by means of turbidimetry and small-angle neutron scattering (SANS) with respect to the phase behavior and the self-assembled structures in dependence on temperature and electrolyte content. Marked differences of the aggregation below the UCST-type and above the LCST-type transition are observed. The addition of a small amount of NaBr (0.004 M) does not affect the overall behavior, and only the UCST-type transition and aggregate structures are slightly altered, reflecting the well-known ion sensitivity of the zwitterionic PSPP block.
The swelling and solvation of 100–200 nm thin films of a diblock copolymer consisting of a short poly(methyl methacrylate) (PMMA) block and a long poly(N-isopropylacrylamide) (PNIPAM) block are ...investigated in mixed water/methanol vapors. The processes are followed in real time using spectral reflectance (SR), time-of-flight neutron reflectometry (ToF-NR), and Fourier transform infrared (FT-IR) spectroscopy, applying two neutron scattering contrast variation sequences. After hydration in pure water vapor, the vapor composition (relative to a flow rate of 1 L/min ≙ 100%) is changed to 70% water (D2O/H2O) and 30% methanol (CH3OH/CD3OH). Upon the mixed vapor stimulus, a two-step response is found, in which an initially enhanced swelling of the films is followed by a contraction. Differences in the solvent exchange kinetics found in ToF-NR experiments coincide with characteristic changes in the FT-IR spectra. While the initially enhanced swelling of the films is driven by the absorption of methanol, the film contraction is related to the release of both solvents, with almost no further change in solvent composition. In analogy to the coil-to-globule transition encountered in the polymer solution, these film response characteristics are attributed to the cononsolvency behavior of PNIPAM in water/methanol mixtures.
Dual responsive inverse opal hydrogels were designed as autonomous sensor systems for (bio)macromolecules, exploiting the analyte‐induced modulation of the opal’s structural color. The systems that ...are based on oligo(ethylene glycol) macromonomers additionally incorporate comonomers with various recognition units. They combine a coil‐to‐globule collapse transition of the LCST type with sensitivity of the transition temperature toward molecular recognition processes. This enables the specific detection of macromolecular analytes, such as glycopolymers and proteins, by simple optical methods. While the inverse opal structure assists the effective diffusion even of large analytes into the photonic crystal, the stimulus responsiveness gives rise to strong shifts of the optical Bragg peak of more than 100 nm upon analyte binding at a given temperature. The systems’ design provides a versatile platform for the development of easy‐to‐use, fast, and low‐cost sensors for pathogens.
A successful marriage: The combination of smart hydrogels and inverse opal structures unites simplicity with efficacy for sensing macromolecules. While the inverse opal structure provides structural color and a large accessible interface for binding, the induced phase transition of the analyte‐responsive hydrogel produces strong optical effects. The resulting spectral shifts can surpass 100 nm and are easily detected.
The hydrolytic stability of polymers to be used for coatings in aqueous environments, for example, to confer anti-fouling properties, is crucial. However, long-term exposure studies on such polymers ...are virtually missing. In this context, we synthesized a set of nine polymers that are typically used for low-fouling coatings, comprising the well-established poly(oligoethylene glycol methylether methacrylate), poly(3-(
-2-methacryloylethyl-
-dimethyl) ammoniopropanesulfonate) ("sulfobetaine methacrylate"), and poly(3-(
-3-methacryamidopropyl-
-dimethyl)ammoniopropanesulfonate) ("sulfobetaine methacrylamide") as well as a series of hitherto rarely studied polysulfabetaines, which had been suggested to be particularly hydrolysis-stable. Hydrolysis resistance upon extended storage in aqueous solution is followed by ¹H NMR at ambient temperature in various pH regimes. Whereas the monomers suffered slow (in PBS) to very fast hydrolysis (in 1 M NaOH), the polymers, including the polymethacrylates, proved to be highly stable. No degradation of the carboxyl ester or amide was observed after one year in PBS, 1 M HCl, or in sodium carbonate buffer of pH 10. This demonstrates their basic suitability for anti-fouling applications. Poly(sulfobetaine methacrylamide) proved even to be stable for one year in 1 M NaOH without any signs of degradation. The stability is ascribed to a steric shielding effect. The hemisulfate group in the polysulfabetaines, however, was found to be partially labile.
The KCl-modulated swelling of double thermoresponsive diblock copolymer (DBC) thin films in D2O atmosphere and their subsequent responsive behavior are investigated via spectral reflectance (SR), in ...situ time-of-flight neutron reflectometry (ToF-NR), and Fourier-transform infrared (FT-IR) spectroscopy. The copolymer consists of a short zwitterionic (poly(4-(N-(3-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate)) (PSBP) block and a long non-ionic poly(N-isopropylmethacrylamide) (PNIPMAM) block. DBC thin films are prepared via spin-coating from solutions containing 5 mM or devoid of KCl. The addition of KCl to the DBC thin films leads to a higher swelling ratio and D2O content during vapor treatment with D2O. Upon the subsequent heating of the hydrated DBC films, the films swell further and reach a maximum thickness before contracting. Two separate volume phase transition temperatures (VPTTs) are observed, namely where a further swelling plateau is reached (VPTTfs), and where contraction starts (VPTTc). Based on complementary SR studies of the effect of KCl on the swelling behavior of the respective PSBP and PNIPMAM homopolymer thin films, we conclude that the “further-swelling” period is mainly a consequence of the UCST-type phase transition of PSBP, whereas the “film contraction” period is due to the LCST-type phase transition of PNIPMAM in thin-film geometry. We observe that KCl reduces the VPTTc of the PNIPMAM blocks. Moreover, the salt migrates or aggregates inside the thin film upon heating, thereby forming a KCl enrichment layer in the intermediate section of the film. Furthermore, the observations by FT-IR prove that macroscopic and mesoscopic D2O absorption and desorption are correlated with the effect of KCl on hydration and de-hydration of the hydrophilic groups.
Poly(N-isopropylmethacrylamide) (PNIPMAM) is a stimuli-responsive polymer, which in thin film geometry exhibits a volume-phase transition upon temperature increase in water vapor. The swelling ...behavior of PNIPMAM thin films containing magnesium salts in water vapor is investigated in view of their potential application as nanodevices. Both the extent and the kinetics of the swelling ratio as well as the water content are probed with in situ time-of-flight neutron reflectometry. Additionally, in situ Fourier-transform infrared (FTIR) spectroscopy provides information about the local solvation of the specific functional groups, while two-dimensional FTIR correlation analysis further elucidates the temporal sequence of solvation events. The addition of Mg(ClO4)2 or Mg(NO3)2 enhances the sensitivity of the polymer and therefore the responsiveness of switches and sensors based on PNIPMAM thin films. It is found that Mg(NO3)2 leads to a higher relative water uptake and therefore achieves the highest thickness gain in the swollen state.
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