Material properties and bioactivity of a resin infiltrant functionalized with polyhedral oligomeric silsesquioxanes.

OBJECTIVES: To investigate the effect of methacrylate polyhedral oligomeric silsesquioxanes (POSS-8) on various material properties and mineral precipitation potential of a resin infiltrant. METHODS: A TEGDMA-based resin infiltrant was mixed with 0.5, 1, 3, 5 or 10 wt% POSS-8 or left unchanged (control). Degree of conversion (DC), water sorption (WS), viscosity, elastic modulus (E-modulus), flexural strength (FS), Knoop microhardness (KHN) and softening ratio (SR) were assessed. Growth of calcium phosphate (Ca/P) precipitates infiltrant-treated bovine enamel and dentin specimens immersed in artificial saliva or artificial dentinal fluid, respectively, for 28 days was analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. For viscosity assessment, pure TEGDMA filled with 0-10 wt% POSS-8 was used. Statistical analyses were performed using ANOVA and Tukey's post-hoc tests (p < 0.05). RESULTS: POSS-8 did not change the flexural strength, water sorption and softening ratio. The apparent degree of conversion was increased at lower concentrations only while E-modulus remained constant in almost all groups. The particles led to a slight decrease of KHN at concentrations below 3%. The effect on viscosity is comparable to the reinforcement effect. Ca/P precipitates formed on dentin specimens treated with POSS-8-filled infiltrant after 4 weeks of immersion, but were not detected on the control infiltrant. The mineral precipitation on enamel was not improved by POSS-8. SIGNIFICANCE: POSS-8 particles did not worsen the material properties of the resin infiltrant, while the Ca/P precipitation on dentin was stimulated.

The role of the binding salt sodium salicylate in semidilute ionic cetylpyridinium chloride micellar solutions: a rheological and scattering study.

The micellar system based on cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) in brine solution is investigated on both macro- and micro-length scales through rheology and scattering measurements. The linear viscoelasticity of the system and its structural parameters are explored by systematically changing the amount of NaSal over an extremely wide range of concentrations, thus producing salt-to-surfactant molar ratios from zero to about 8.5. As a result, the well-known non-monotonic behaviour of the zero-shear rate viscosity as a function of salinity can be connected to micellar morphological changes, whose driving force is represented by the simultaneous binding and screening actions of NaSal. The viscosity behaviour can be seen as a direct consequence of consecutive lengthening/shortening of the contour length, where the micelles attempt to minimize the electrostatic charge density on their surface. Along similar lines, the scattering measurements of the semidilute solutions show that the local stiffness of the micellar chain changes with increasing salt content influencing the elasticity of the resulting network. Within this general view, the branching of the micelles can be seen as a side effect attributable to the main character of the play, namely, the binding salt NaSal, whereas the overall dynamics of the system is driven by the considerable changes in the entanglement density of the micellar network.

Advanced rheological characterization of soft colloidal model systems.

The complex flow behavior of polymer-based soft colloidal model systems was investigated using steady and oscillatory shear to prove new concepts for advanced rheological characterization. In the very dilute regime we investigated high molecular weight polybutadiene star polymers to quantify the internal relaxation time arising from the polymeric nature of these ultra-soft colloids. The observed shear-induced brush deformation is interpreted in terms of the internal Zimm time τ(z). The observed dependence of τ(z) on matrix viscosity can be explained by shrinkage of the star polymer due to an increasing incompatibility with increasing matrix molecular weight. The influence of the polymeric nature on the characteristic structural relaxation time in the concentrated regime was investigated using non-linear rheology following Wyss et al (SRFS) (2007 Phys. Rev. Lett. 98 238303). Here we used star-like block copolymer micelles to systematically tune the 'softness' of the colloids by variation of the block ratio. A master curve with proper scaling parameters could be generated independent of the degree of colloidal 'softness'. However, the obtained strain-rate independent structural relaxation time τ(0) was not observed in the linear regime. In addition, a high frequency discrepancy was clearly found in all our experimental data. Both reflect the shortcomings of the SRFS approach.

Ultrasoft colloid-polymer mixtures: structure and phase diagram.

Binary mixtures of ultrasoft colloids and linear polymer chains were investigated by small-angle neutron scattering and liquid state theory. We show that experimental data can be described by employing recently developed effective interactions between the colloid and the polymer chains, in which both components are modeled as point particles in a coarse-grained approach, in which the monomers have been traced out. Quantitative, parameter-free agreement between experiment and theory for the pair correlations, the phase behavior and the concentration dependence of the interaction length is achieved.

Anxiolytic and antidepressant effects of intracerebroventricularly administered somatostatin: behavioral and neurophysiological evidence.

Somatostatin (SST) is a cyclic polypeptide that inhibits the release of a variety of regulatory hormones (e.g. growth hormone, insulin, glucagon, thyrotropin). Moreover, SST is widely distributed within the CNS, acting both as a neurotransmitter and as a neuromodulator of other neurotransmitter systems. However, despite its extensive expression in limbic areas, and its co-localization with GABA, a neurotransmitter previously implicated in emotion, the effects of SST on anxiety and depression have not been investigated. By performing intraventricular infusions in rats we demonstrate, for the first time, that SST has anxiolytic- and antidepressant-like effects in the elevated plus-maze and forced swim test, respectively. In addition, by performing local field potential recordings of hippocampal theta activity evoked by reticular stimulation in urethane-anesthetized rats we also show that SST application suppresses the frequency of theta in a similar fashion to diazepam. This neurophysiological signature, common to all classes of anxiolytic drugs (i.e. benzodiazepines, selective 5-HT reuptake inhibitors, 5-HT1A agonists) provides strong converging evidence for the anxiolytic-like characteristics of SST. Our pharmacological antagonism experiments with bicuculline further suggest that the anxiolytic effect of SST may be attributable to the interaction of SST with GABA, whereas the antidepressant-like effect of SST may be GABA-independent. In addition to contributing to the current understanding of the role of neuropeptides in mood and emotion, these findings support a clinical role for SST (or its analogues) in the treatment of anxiety and depression.

Asymmetric poly(ethylene-alt-propylene)-poly(ethylene oxide) micelles: a system with starlike morphology and interactions.

We report on an experimental study of single particle properties and interactions of poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP-PEO) starlike micelles. The starlike regime is achieved by an extremely asymmetric block ratio (1:20) and the number of arms (functionality) is changed by varying the composition of the solvent (the interfacial tension). Small angle neutron scattering (SANS) data in the dilute regime can be modeled by assuming a constant density profile in the micellar core (compact core) and a starlike density profile in the corona (starlike shell). The starlike morphology of the corona is confirmed by a direct comparison with SANS measurements of dilute poly butadiene star solutions. Comparison of structure factors obtained by SANS measurements in the concentrated regime shows in addition that the interactions in the two systems are equivalent. Micellar structure factors at several packing fractions can be modeled by using the ultrasoft potential recently proposed for star polymers [Likos, Phys. Rev. Lett. 80, 4450 (1998)]. The experimental phase diagram of PEP-PEO micelles is quantitatively compared to theoretical expectations, finding good agreement for the location of the liquid-solid boundary and excellent agreement for the critical packing fraction where the liquid-to-bcc crystal transition takes place for f<70. The functionality, i.e., the coronal density, strongly influences the nature of the solid phase: for f<70 the system crystallizes into a bcc phase, high f>70 formation of amorphous arrested states prevents crystallization.

Starlike micelles with starlike interactions: a quantitative evaluation of structure factors and phase diagram.

Starlike PEP-PEO block copolymer micelles offer the possibility to investigate the phase behavior and interactions of regular star polymers (ultrasoft colloids). Micellar functionality f can be smoothly varied by changing solvent composition (interfacial tension). Structure factors obtained by small-angle neutron-scattering can be quantitatively described in terms of an effective potential for star polymers. The experimental phase diagram reproduces to a high level of accuracy the predicted liquid-solid transition. Whereas for intermediate f a bcc phase is observed, for high f the formation of a fcc phase is preempted by glass formation.

An in situ study of the t-butyllithium initiated polymerization of butadiene in d-heptane via small angle neutron scattering and 1H-NMR.

We present a combined 1H-NMR and small angle neutron scattering in situ study of the anionic polymerization of butadiene using t-butyllithium as the initiator. Both initiation and propagation phases were explored. This combined approach allows the structural and kinetic characteristics to be accessed and cross compared. The use of the D22 instrument (ILL Grenoble) permits the attainment of Q approximately equal to 2 x 10(-3) A. This, in turn, led to the identification of coexisting large-scale and smaller aggregates during all stages of the polymerization. The smaller aggregates contain most of the reacted monomers. Their structure changes from high functionality wormlike chains at early stages of the reaction to starlike aggregates where the crossover occurs at a degree of polymerization of approximately equal to 40. The initiation event involved these small, high functionality (approximately equal to 120) aggregates that apparently consisted of cross-associated t-butyllithium with the newly formed allylic-lithium head groups. As the initiation event progressed the initiation rate increased while the functionality of these small aggregates decreased and their size increased. Propagation, in the absence of initiation, was found to have a rate constant that was molecular weight dependent. At approximately 11 kg/mol the measured polymerization rate was found to increase while no further structural changes were seen.

Phase separation in star-polymer-colloid mixtures.

We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-polymer-colloid size ratios 0.18< or =q< or =0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The numerical results show a spinodal instability. The demixing binodals are approximately calculated and found to be consistent with experimental observations.