•Synthesis of linear random polyglycerol copolymers.•Adjustable hydroxyl functionality.•Detailed rheological study.We introduce a two-step strategy for the synthesis of linear polyglycerols (linPG-OHx/OMey) with an adjustable degree of methylation (y=DM100). Ethoxy ethyl glycidyl ether (EEGE) and glycidyl methyl ether (GME) were copolymerized via the “activated monomer” polymerization technique, using tetraoctylammonium bromide (NOct4Br) as an initiator and triisobutylaluminum (i-Bu3Al) as a catalyst under mild conditions. Subsequent acidic cleavage of the acetal protective groups generates linear polyglycerols with different degree of methylation (DM) by varying the GME comonomer content between 10 and 91%. Size exclusion chromatography (SEC) evidenced good control over molecular weight and narrow to moderate polydispersity (PDI = 1.2–1.8). 1H NMR spectroscopy confirmed ideally random copolymerization of EEGE and GME (in situ 1H-NMR kinetics) and provided perfect agreement of the comonomer content with the targeted values. Thermoresponsive behavior in solution and lowering of cloud points with increasing degree of methylation was observed. Furthermore, the differently methylated polyglycerols were investigated with respect to their rheological properties in the melt. Comparison with the fully hydroxylated and permethylated polyglycerol provides new insights into the dynamic behavior of functional polyethers. A tremendous influence of DM on zero-shear viscosity and differences of up to 5 decades were observable at the same reference temperature (273 K). The trend of glass transition temperature and zero-shear viscosity in dependency of degree of methylation was described by mixing rules. To understand the changes in zero-shear viscosity, the “sticky” Rouse model was applied and led to an estimated association lifetime of stickers, i.e. hydroxyl groups of τs=4.9±1.8μs.Download high-res image (131KB)Download full-size image

Melt rheology and thermal phase transition of a series of hyperbranched polyglycerol samples (hbPG) (DB ≈ 60%) in a broad molecular weight range (Mn = 600–440 000 g/mol) were investigated and correlated to both molecular weight and nature of the end group (hydroxyl vs permethylated and trimethylsilylated). The well-characterized and defined flexible polyethers are particularly suitable to shed light on the linear viscoelastic behavior with respect to (i) hyperbranched topology and (ii) hydrogen bond interactions, particularly in comparison to the perfectly linear polyglycerol counterparts studied recently [Osterwinter, C.; Macromolecules 2015, 48, 119−130]. We present a detailed examination of differences found in the characteristic moduli as a consequence of functionality and topology leading to an estimation of both a stickiness parameter and a connectivity parameter of hyperbranched molecules. The appearance of a plateau region of the dynamic moduli indicates entanglement behavior, although the determined apparent entanglement molecular weight Me ≈ 6000 g/mol is significantly higher than the molecular weight between two branching points (Mx ≪ Me). Zero shear viscosities and terminal relaxation times show a unique scaling behavior with respect to the nature of the multiple end groups of the hyperbranched topology, suggesting entanglement transitions with a critical molecular weight around 55 000 g/mol. Most striking and in pronounced contrast to linear PG as the perfect linear analogue, the derived structure–property relationships depend on the functionality of the repeating unit of the polymer.

Viscoelastic properties of linear, hydroxyl-functional polymers are only little understood with respect to the effect of functional group interactions. Melt rheology and thermal phase transitions of linear polyethers (polyglycerol, linPG-OH) and their methylated analogues (linPG-OMe) in a broad molecular weight range (Mn = 1–100 kg/mol) with low polydispersities (PDI) have been investigated as a general model for hydroxyl-functional polymers with respect to their functionality and hydrogen bond interactions. We provide detailed insight into the rheodynamics of nonentangled and well-entangled polyethers bearing one functional group per monomer unit. Booij–Palmen plots (BBP) revealed failure of the time–temperature superposition principle (TTS) for both types of polymers in the segmental relaxation region, while TTS holds in the terminal relaxation region. The characteristic modulus of linPG-OMe derived from the BBP clearly reflects the transition from the nonentangled to the fully entangled state with increasing molecular weight. Quantitative analysis of these data allows for different estimates of the entanglement molecular weight, which is approximately 14 kg/mol. In case of linPG-OH a lower apparent entanglement molecular weight (8 kg/mol) leads to estimated 36 entanglement interactions in a cube of 10 nm edge length together with 47 association sites in the same volume. This can be determined from the molecular-weight-independent plateau modulus only, which is significantly lower than for linPG-OMe. This is explained as a consequence of the overlay of an entanglement network and an association network created by hydrogen bonding of the OH groups with themselves and with the ether linkages.

Herein we report on the characterization of rheological, morphological, and electrical properties of graphene/polystyrene nanocomposites as a function of graphene functionalization. Hydrophobic modified graphite oxide was grafted with polystyrene chains (PS-g-FG), thus enabling a high degree of compatibility with the polystyrene matrix. While nongrafted and noncompatibilized graphene showed a particle network in accordance with the time–temperature superposition principle, the grafted PS-g-FG revealed a structural development with time, which is the consequence of superior but unstable dispersion of the latter. This results in an increasing elasticity of the graphene network as well as in an enhanced electrical conductivity of the composite material. Kinetic investigations showed a multistep process being the basis of this structural evolution. While extensive shear (large amplitude oscillatory shear, LAOS) led to a destruction of the network, small shear impact (small amplitude oscillatory shear; SAOS) in combination with elevated temperature was crucial for an effective buildup of the graphene particle network. In addition, the rheological and electrical percolation threshold was determined simultaneously by a combined rheodielectric setup. The structure development was successfully monitored by electron microscopy, showing an increase in number and size of interconnected graphene domains, accompanied by an exfoliation of aggregated graphene stacks. Thus, the present investigation is a rare example of comparing compatibilization of graphene with detailed characterization of melt rheology, electrical properties, and computer-aided morphological analysis.

We present studies of molecular dynamics and interactions in azolium azolate energetic ionic liquids (ILs) by utilizing a variety of physical methods. We observe peculiar rheological behavior for these ILs, which deviates from the one expected for molecular glass-formers. Major peculiarities include high elasticity in the low-frequency zone, peculiar van Gurp plots, and failure of time temperature superposition. We attribute these peculiarities to specific interactions in the nitrogen-rich planar rings of azolate ILs. X-ray scattering measurements reveal a “nanometer” ordered organization in azolium azolates. High values of Kamlet–Taft polarity parameters indicate high probability and strength of hydrogen bond interactions in azolate ILs. This conclusion is also supported by ab initio calculations. Our next effort is to break/enhance existing interactions in nitrogen-rich ILs by adding a “H-acceptor”. This will allow better understanding of the nature of interactions in azolium azolates and eventually their control.Keywords: azolate anion; interactions; ion pair; ionic liquid; nanoscale heterogeneity;

The occurrence and development of the edge fracture phenomenon in oscillatory flow was investigated. Large-amplitude oscillatory shear experiments were performed with polystyrene melts of different molecular weights at 170 °C in parallel-plate geometry. Based on Tanner and Keentok’s criterion for the formation of the edge fracture in steady-state rotational flow, an equation was derived for the oscillatory flow. This equation can be used to predict the onset conditions in terms of the critical deformation amplitude γc and the critical frequency ωc. A very good agreement with the experimental observation was found. Within the critical parameter range, the kinetics of the fracture front propagation was studied by detailed visual observations and by analysing the effect of the fracture process on the measured rheological data. An empirical equation was found that allows the description of the time development of the fracture front, to be characterised by a single parameter b. This finding was also theoretically confirmed, by assuming a linear relationship between the driving force for the fracture formation and the fracture front propagation rate.

Theoretical predictions for the dynamic moduli of long, linear, flexible, monodisperse polymers are summarized and compared with experimental observations. Surprisingly, the predicted 1/2 power scaling of the long-time modes of the relaxation spectrum is not found in the experiments. Instead, scaling with a power of about 1/4 extends all the way up to the longest relaxation times near τ/τmax = 1. This is expressed in the empirical relaxation time spectrum of Baumgaertel-Schausberger-Winter, denoted as “BSW spectrum,” and justifies a closer look at the properties of the BSW spectrum. Working with the BSW spectrum, however, is made difficult by the fact that hypergeometric functions occur naturally in BSW-based rheological material functions. BSW provides no explicit solutions for the dynamic moduli, G′(ω), G″(ω), or the relaxation modulus G(t). To overcome this problem, close approximations of simple analytical form are shown for these moduli. With these approximations, analysis of linear viscoelastic data allows the direct determination of BSW parameters.