Hanneke M. L. Thijs

Tuning solution polymer properties by binary water–ethanol solvent mixtures

The solubility of polymers can be significantly altered by the use of solvent mixtures. The solvent composition also effects the self-assembly properties of amphiphilic copolymers. In addition, water-ethanol mixtures are known to exhibit abnormal physicochemical properties due to the presence of hydration shells around the ethanol molecules, while at the same time both solvents have very low toxicity. However, the solution properties of amphiphilic copolymers in water-ethanol mixtures have been scarcely studied. Here we show that the solution polymer properties of amphiphilic copoly(2-oxazoline)s can be significantly altered in binary water-ethanol mixtures resulting in increased solubility, tuneable lower critical solution temperatures as well as polymer-solvent combinations with both a LCST followed by an UCST and improved dispersion stability. Surprisingly, it was found that polymers insoluble in both ethanol and water could be dissolved in water-ethanol mixtures, opening the way to novel formulations for drug delivery or personal care applications. Our results represent a straightforward method for tuning solution polymer properties without the synthetic efforts that are generally required to change the copolymer composition and properties.

Water uptake of hydrophilic polymers determined by a thermal gravimetric analyzer with a controlled humidity chamber

The moisture uptake of several water-soluble polymers at different humidities was investigated with a thermal gravimetric analyzer equipped with a controlled humidity chamber. The water sorption of poly(acrylic acid) sodium salt, poly(ethylene glycol) and silica, which are known as super absorbers, were examined. In addition, various hydrophilic polymeric materials were selected according to their structural features. These included hydroxyl functions on the side chains (e.g. poly(2-hydroxyethyl methacrylate)), as well as acidic or basic functionalities (e.g. poly (dimethylaminoethyl methacrylate) or poly(vinylimidazole)). In addition, poly(2-methyl-2-oxazoline) (P(MeOx)) and poly(2-ethyl-2-oxazoline) (P(EtOx)), which are well-known hydrophilic polymers, were also investigated in this context. More significant weight percent changes were obtained for P(MeOx) (60% at 90% relative humidity (RH)) in comparison to P(EtOx) (35% at 90% RH) as a result of the slight difference in hydrophilicity of the structures. The effect of the chain length on the ability for water uptake was also investigated for both poly(oxazolines). Finally, thermoresponsive polymers with a lower critical solution temperature (LCST) behavior (e.g. poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate)) were also examined. The measurements for the latter polymers were performed below and above the LCST of each polymer whereby the humidities are varied from 0 to 90% with steps of 10%. Upon increasing humidity, the results revealed relatively high water uptake values (8% and 22% for P(NIPAM) and for P(DMAEMA), respectively) below the LCSTs of the polymers and, contrastingly, a small weight loss above their LCSTs. The present results allow a deeper insight into important structure–property relationships (e.g. the influence of the polymer backbone, functional groups, LCST behavior, etc. on the water-uptake properties), and will in subsequent steps permit the directed design of tailor-made polymers for selected applications.

Synthesis, Microwave‐Assisted Polymerization, and Polymer Properties of Fluorinated 2‐Phenyl‐2‐oxazolines: A Systematic Study

We present a detailed systematic study of the synthesis and ability of fluorinated 2-phenyl-2-oxazolines to undergo polymerization. The synthesis of these compounds is based on a two-step procedure that gives the desired 2-oxazolines in moderate-to-good yields. All the compounds were fully characterized by IR and NMR ((1)H, (13)C, and (19)F) spectroscopy, mass spectrometry, and elemental analysis. The 2-oxazolines were subsequently used as monomers for living cationic ring-opening polymerization (CROP) with microwave irradiation as the heat source (T=140 degrees C), nitromethane as the solvent, and methyl tosylate as the initiator. The linear first-order kinetic plots of the polymerizations accompanied by a linear increase of the molecular weight with conversion and low polydispersity index (PDI) values (generally below 1.30) indicate a living polymerization mechanism. The resulting polymerization rates reflect a strong sensitivity to the quantity of fluorine substituents in general and the presence or absence of ortho-fluoro substituents of the phenyl ring in particular. All the polymers were isolated and characterized by size-exclusion chromatography and MALDI-TOF mass spectrometry. Finally, a detailed investigation of selected polymer properties was performed by using differential scanning calorimetry, thermogravimetric analysis, and contact-angle measurements, thus resulting in structure-property relationships. Whereas the thermal properties of the polymers are mostly influenced by the presence of ortho-fluoro substituents, the surface properties are mainly determined by the presence of para- and/or meta-fluoro substituents.

Elastic moduli for a diblock copoly(2-oxazoline) library obtained by high-throughput screening

Using depth-sensing indentation, the elastic modulus E of a diblock copoly(2-oxazoline) library was investigated in order to determine structure–property relationships. The adopted experimental procedure, dropcasting of the copolymer materials and determining the elastic modulus by depth-sensing indentation, was compatible with high-throughput experimentation. The elastic modulus of the investigated materials depended strongly on the side-group. Materials containing poly(nonyloxazoline) exhibited a lower modulus than materials without any poly(nonyloxazoline) block as poly(nonyloxazoline) was at room temperature above its glass-transition temperature Tg, while the other homopolymers in this study were glassy at room temperature. The elastic modulus also depended on the relative humidity (RH) of the testing environment; the stiffness of ethyloxazoline and methyloxazoline decreased significantly due to water absorption from the air. At lower RH, hydrogen bonding or polar interactions among the polymer chains resulted in a surprisingly high modulus for the poly(methyloxazoline). In addition, as anticipated, the elastic moduli of AB diblock copolymers were bounded by those of the A and B homopolymers, both at high and at low RH. The presented results indicate how, and to what extent, for these materials the E (and the change in E) at a given (change in) humidity can be adjusted by tailoring the composition.

Correlating the mechanical and surface properties with the composition of triblock copoly(2-oxazoline)s

The elastic moduli, surface energies, and phase morphologies of poly(2-oxazoline) triblock copolymers were investigated and compared to the corresponding homopolymers and diblock copolymers, at a constant degree of polymerization. The elastic moduli of ABA triblock copolymers were bound by those of the respective AB diblock copolymers and A homopolymers. These results show that the elastic moduli of these copolymers – obtained by instrumented indentation – depended on the interplay between phase-separation, crystallization and hygroscopicity, and can be adjusted by tailoring the composition. The surface energy strongly depended on the presence of a poly(2-nonyl-2-oxazoline) block. If such a block was present, the surface energy was reduced due to segregation of nonyl side-chains to the surface. This segregation was promoted by annealing. The crystallization of nonyl side-chains at the surface promoted the development of surface texture and an increase in surface roughness, as demonstrated by atomic force microscopy topographic imaging.

Application possibilities of preparative size exclusion chromatography to analytical problems in polymer science

Abstract The application possibilities of preparative size exclusion chromatography for the detailed analysis of polymer analytes are discussed. Using the example of star-shaped polymer architecture, the possibilities of utilizing fractions obtained from preparative size exclusion chromatography and their subsequent off-line characterization with a variety of hyphenated analytic techniques are discussed and compared to one another. It was, for instance, possible to obtain an absolute SEC calibration for the investigated star-shaped polymers that showed very good agreement with theoretical expected values and values obtained by absolute molecular weight determination techniques by analyzing fractions of different molecular weights of this polymer architecture by MALDI-TOFMS in order obtain the Mp values required for SEC calibrations. Moreover, the star-shaped polymers were investigated by SEC-viscometry in order to obtain their absolute molecular weight. All analytical results are compared to each other and possibilities as well as limitations are discussed.