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Sets of the nonionic polymers poly(N-vinyl isobutyramide) (pNVIBAm) and poly(N-isopropyl methacrylamide) (pNIPMAm) are synthesized by radical polymerization covering the molar mass range from about 20,000 to 150,000 kg mol−1, and their thermoresponsive and solvent-responsive behaviors in aqueous solution are studied. Both polymers feature a lower critical solution temperature (LCST) apparently of the rare so-called type II, as characteristic for their well-studied analogue poly(N-isopropyl acrylamide) (pNIPAm). Moreover, in analogy to pNIPAm, both polymers exhibit co-nonsolvency behavior in mixtures of water with several co-solvents, including short-chain alcohols as well as a range of polar aprotic solvents. While the cloud points of the aqueous solutions are a few degrees higher than those for pNIPAm and increase in the order pNIPAm pNVIBAm > pNIPMAm. Exceptionally, pNIPMAm does not show co-nonsolvency in mixtures of water and N,N-dimethylformamide. Graphical Abstract

The thermoresponsive behavior of a poly(2-oxazoline)-based molecular brush is investigated in aqueous solution. The molecular brush under study, PiPOx100-g-PEtOx17, has a poly(2-isopropenyl-2-oxazoline) (PiPOx) backbone grafted with thermoresponsive poly(2-ethyl-2-oxazoline) (PEtOx) side chains. Since the backbone degree of polymerization is only a factor of ~ 6 higher than the ones of the side chains, it features an architecture between a star-like polymer and a comb-like polymer. Its aqueous solution exhibits lower critical solution temperature (LCST) behavior with a cloud point temperature Tcp = 40.5 °C at 30 g L−1. The temperature-dependent structural evolution is disclosed using dynamic light scattering (DLS) and small-angle neutron scattering (SANS). An increase of the molecular brush size is found upon heating from room temperature to Tcp, which is attributed to the extension of the backbone resulting from the dehydration and collapse of the side chains. Above Tcp, the size decreases again, which indicates the collapse of the whole molecular brush. Large aggregates are found to be present in the solution in the temperature range 25–50 °C. These become more compact, as the temperature is increased across Tcp.

Multiresponsive polymers offer a wealth of possibilities to design switchable materials which respond to more than one stimulus. We describe first the response of polymers to a single external stimulus, namely to temperature, light, pH value, redox changes, and low molecular weight species, and discuss the influence of these stimuli on the polymer chain polarity. Then, we review multiresponsive homopolymers and statistical and block co-polymers. Finally, we discuss the similarity of multiresponsive synthetic polymers to biopolymers. As a conclusion, multiresponsiveness opens up a broad area for combining different properties in one system, enabling numerous possible applications.

In the present paper, we address the question to which extent the critical micellar concentration (cmc) value can be varied for clinically related drug delivery carriers if all crucial parameters needed for successful drug delivery are already preset. For this purpose, the cmc values of an amphiphilic polymer carrier based on N-(2-hydroxypropyl) methacrylamide (HPMA) with cholesterol side groups, differing in a variety of parameters, such as molar mass, dispersity, architecture (statistical vs. diblock copolymer), and spacer structure, were studied in detail. At this, we chose physiological conditions (phosphate buffered saline at pH 7.4) and investigated the formation of micelles using fluorescence correlation spectroscopy in dependence on polymer concentration. Moreover, conjugates where the cholesterol moieties are attached to the HPMA backbone by a pH-cleavable hydrazone bond were investigated in dependence on time with respect to their long-term stability at pH 7.4, and it was found that the nanoparticles change size in the course of a few weeks. After a pH change from 7.4 to 5.0 (typical for tumor cells), the nanoparticles grow during the first few hours, then shrink. A weak correlation between the cmc value and the content of the hydrophobic group was found for HPMA-based polymers, regardless their architecture, polydispersity, and spacer structure. A much stronger correlation was observed between the cmc value and the dispersity. This finding could be explained by the higher absolute number of hydrophobic groups located on the higher molar mass fractions of the HPMA copolymers, which initiate the micelle formation.

Fluorescence correlation spectroscopy has been increasingly used in polymer science. In the present perspective, the principles of the method are briefly reviewed, and the temporal and spatial resolutions are critically discussed. Examples of recent findings are summarized, focusing on polymer solutions, environmental parameters, combination with other techniques, near-interface measurements, simulations, and modeling. Finally, desirable new developments are discussed.

We have investigated the collapse transition of aqueous solutions of gradient copolymers from poly(iso-propyl-2oxazoline)s (PiPrOx), which contain few hydrophobic moieties (n-nonyl-2-oxazoline (NOx) monomers). We extend our previous investigations (Salzinger et al., Colloid Polym Sci 290:385–400, 2012), where, for the gradient copolymers, an intermediate regime right above the cloud point was identified where small aggregatesare predominant.Largeaggregatesare present in significant numbers only at higher temperatures. To investigate the stability of the intermediate regime, we performed time-resolved small-angle neutron scattering (SANS) experiments during temperature jumps starting below the cloud point and ending in the intermediate regime or in the high-temperature regime. We found that the intermediate regime is stable during the time investigated (∼1 h). Moreover, the collapse of the small aggregates and the surface structure of the large aggregates are related to the number of hydrophobic moieties and the quench depth. The present results elucidate the structural evolution of these polymers and relate them to their final state as well as to their macroscopic behavior.

Thermoresponsive block copolymers comprising long, hydrophilic, nonionic poly(methoxy diethylene glycol acrylate) (PMDEGA) blocks and short hydrophobic polystyrene (PS) blocks are investigated in aqueous solution. Various architectures, namely diblock, triblock, and starblock copolymers are studied as well as a PMDEGA homopolymer as reference, over a wide concentration range. For specific characterization methods, polymers were labeled, either by partial deuteration (for neutron scattering studies) or by fluorophores. Using fluorescence correlation spectroscopy, critical micellization concentrations are identified and the hydrodynamic radii of the micelles, r , are determined. Using dynamic light scattering, the behavior of r in dependence on temperature and the cloud points are measured. Small-angle neutron scattering enabled the detailed structural investigation of the micelles and their aggregates below and above the cloud point. Viscosity measurements are carried out to determine the activation energies in dependence on the molecular architecture. Differential scanning calorimetry at high polymer concentration reveals the glass transition of the polymers, the fraction of uncrystallized water and effects of the phase transition at the cloud point. Dielectric relaxation spectroscopy shows that the polarization changes reversibly at the cloud point, which reflects the formation of large aggregates upon heating through the cloud point and their redissolution upon cooling.

Thick poly(styrene-b-monomethoxydiethylenglycol-acrylate-b-styrene) [P(S-b-MDEGA-b-S)] films (thickness 5 μm) are prepared from different solvents on flexible substrates by solution casting and investigated with small-angle X-ray scattering. As the solvents are either PS- or PMDEGA-selective, micelles with different core-shell micellar structures are formed. In PMDEGA-selective solvents, the PS block is the core and PMDEGA is the shell, whereas in PS-selective solvents, the order is reversed. After exposing the films to liquid D2O, the micellar structure inside the films prepared from PMDEGA-selective solvents remains unchanged and only the PMDEGA (shell part) swells. On the contrary, in the films prepared from PS-selective solvents, the micelles revert the core and the shell. This reversal causes more entanglements of the PMDEGA chains between the micelles. Moreover, the thermal collapse transition of the PMDEGA block in liquid D2O is significantly broadened. Irrespective of the solvent used for film preparation, the swollen PMDEGA shell does not show a prominent shrinkage when passing the phase transition, and the transition process occurs via compaction. The collapsed micelles have a tendency to densely pack above the transition temperature.

We have investigated thick films from polybutadiene on gold-coated glass using surface plasmon resonance / leaky optical waveguide spectroscopy with the aim of investigating differences in bulk and interphase properties in a single measurement. A broad range of molar masses was studied. Drying under ambient conditions leads to an exponential decay of the film thickness. Subsequent vacuum drying does not result in any further changes in the bulk part of the film but at the polymer-solid interface, indicating the absence of residual solvent. For all molar masses studied, the surface plasmon resonance is observed at angles which are incompatible with the properties of the bulk part of the film. A polymer interphase is thus present next to the gold layer which has a refractive index lower than in the bulk. Using transversal magnetic- and transversal electric polarized light, an optical anisotropy is found in the interphase which is attributed to segment alignment along the interface with gold.

We have studied different thermo-responsive poly(2-oxazoline)s with iso-propyl (iPrOx) and n-propyl (nPrOx) pendant groups in aqueous solutions, where they exhibit lower critical solution temperature behavior. This paper focuses on the effect of the degree of polymerization, n, the concentration, c, in the dilute regime, and the presence of hydrophobic moieties. The cloud points were investigated as a function of the degree of polymerization, n, and of the polymer concentration, c. The aggregation behavior near the cloud point was studied by temperature-resolved small-angle neutron scattering and fluorescence correlation spectroscopy, i.e., a combination of ensemble and single molecule methods. We found that at the cloud points, large aggregates are formed and that the cloud points depend strongly on both, n and c. Diblock copolymers from iPrOx and nPrOx form large aggregates already at the cloud point of PnPrOx, and, unexpectedly, no micelles are observed between the cloud points of the two blocks. Gradient copolymers from iPrOx and n-nonyl-2-oxazoline (NOx) display a complex aggregation behavior resulting from the interplay between intra- and intermolecular association mediated by the hydrophobic NOx blocks. Above the cloud point, an intermediate temperature regime with a width of a few Kelvin was found with small but stable polymer aggregates. Only at higher temperatures, larger aggregates are found in significant number.

The thermal behavior of poly(methoxydiethylenglycol acrylate) (PMDEGA) is studied in thin hydrogel films on solid supports and is compared with the behavior in aqueous solution. The PMDEGA hydrogel film thickness is varied from 2 to 422 nm. Initially, these films are homogenous, as measured with optical microscopy, atomic force microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering (GISAXS). However, they tend to de-wet when stored under ambient conditions. Along the surface normal, no long-ranged correlations between substrate and film surface are detected with GISAXS, due to the high mobility of the polymer at room temperature. The swelling of the hydrogel films as a function of the water vapor pressure and the temperature are probed for saturated water vapor pressures between 2,380 and 3,170 Pa. While the swelling capability is found to increase with water vapor pressure, swelling in dependence on the temperature revealed a collapse phase transition of a lower critical solution temperature type. The transition temperature decreases from 40.6 A degrees C to 36.6 A degrees C with increasing film thickness, but is independent of the thickness for very thin films below a thickness of 40 nm. The observed transition temperature range compares well with the cloud points observed in dilute (0.1 wt.%) and semi-dilute (5 wt.%) solution which decrease from 45 A degrees C to 39 A degrees C with increasing concentration.

The hydrodynamic radii of micelles formed by amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution determined using fluorescence correlation spectroscopy (FCS) depend on the nature of the fluorescent tracer used. We have compared the values of the hydrodynamic radii of the unimers and the micelles as well as the critical micelle concentrations (CMC), using as tracers (1) the identical diblock copolymers being fluorescence-labeled at the hydrophilic or the hydrophobic block terminus [Bonne et al. Colloid Polym Sci (2004) 282:833–843], and (2) a low molar mass fluorescence dye, rhodamine 6G. Whereas similar values for the CMC were found for both probes, the hydrodynamic radius of micelles is significantly underestimated using a free dye as a tracer in FCS, especially near the CMC. We attribute this discrepancy to the fast exchange of the dye between micelles and solution.

The diffusional behavior of amphiphilic poly(2-oxazoline) diblock copolymers in aqueous solution is studied using photon correlation spectroscopy (PCS) and fluorescence correlation spectroscopy (FCS). The polymers were synthesized by living cationic polymerization and were fluorescence-labeled with tetramethyl rhodamine isothiocyanate either at the end of the hydrophilic or the hydrophobic block. Temperature-resolved PCS showed that, at room temperature, large metastable aggregates are present along with unimers and micelles. An annealing above ~40 °C resulted in stable equilibrium micellar solutions. By means of FCS, the hydrodynamic radii of the unimers and the micelles were measured simultaneously in a broad concentration range, and the critical micelle concentration could be determined. Comparison of the results from conventional PCS measurements with this first FCS study showed excellent agreement and the high potential of the FCS technique.

Broadband dielectric spectroscopy (1–106 Hz, 183–423 K) and differential scanning calorimetry are employed to analyze the inter- and intramolecular dynamics of a series of random copolymers based on poly(ethylene terephthalate) and poly(1,4-cyclohexylene dimethylene terephthalate). In addition to an interfacial relaxation (α*-process), three dielectric relaxation processes are observed: The α-relaxation (“dynamic glass transition”) and two secondary relaxations (β- and β*-relaxations). The α-relaxation depends sensitively on the composition of the copolymer and shows a rapid slowing down with increasing content of cyclohexylene dimethylene (CHDM) linkages. Besides the β-relaxation, attributed to local motion of the ester group, an additional process (β*-relaxation) is observed on introducing the CHDM linkages. Increasing the content of the latter reduces the strength of the β-relaxation strongly and increases its activation energy by more than 30%. This proves that owing to interactions between the cylohexylene rings and the ester group the β-relaxation no longer has local character only.

A homologous series of nine symmetric poly(styrene)-block-poly(butadieme) diblock copolymers was synthesized and characterized. The scaling of the lamellar thickness,D, with the chain lengthN(DαN δ ) was investigated with small-angle X-ray scattering. Different sample preparation methods were used: annealing, solvent-casting and shear alignment. We find that the lamellar thickness and thus the value of the exponent δ depends on the preparation method used,i.e. care must be taken to ensure that the samples investigated are in thermodynamic equilibrium.

We describe a facile and direct method for the functionalization of single-walled carbon nanotubes with 4’-substituted phenyls and biphenyls. By means of Raman spectroscopy and thermogravimetric analysis we demonstrate that a simple protocol of a direct chemical grafting in acetonitrile solution of the corresponding diazonium salts at room temperature results in a formation of stable aryl monolayers on carbon nanotubes.

We have found very slow crystallization in thin films of cylinder-forming poly(isoprene- b -ethyleneoxide) (P(I- b -EO)) diblock copolymers with PEO being the minority block. The film was crystallized at room temperature after melting at 62 ° C . Imaging methods were combined with X-ray reflectometry and grazing-incidence small-angle X-ray scattering and diffraction. Initially, hexagonally packed, amorphous PEO cylinders lie in the film plane. After 148 days, crystallized, finger-like terraces were observed over the entire film surface. The terrace height is 20% higher than the repeat distance in the as-prepared film. Thus, at the film surface, the cylinders have been destroyed by crystalline lamellae lying in the film plane. The PEO chain stems are perpendicular to the substrate surface and are once-folded and fully interdigitated. The substrate-near layers still consist of the hexagonally packed, amorphous PEO cylinders within the PI matrix.