Effect of nanoscale confinement on glass transition of polystyrene domains from self-assembly of block copolymers.

The understanding of size-dependent properties is key to the implementation of nanotechnology. One controversial and unresolved topic is the influence of characteristic size on the glass transition temperature (T(g)) for ultrathin films and other nanoscale geometries. We show that T(g) does depend on size for polystyrene spherical domains with diameters from 20 to 70 nm which are formed from phase separation of diblock copolymers containing a poly(styrene-co-butadiene) soft block and a polystyrene hard block. A comparison of our data with published results on other block copolymer systems indicates that the size dependence of T(g) is a consequence of diffuse interfaces and does not reflect an intrinsic size effect. This is supported by our measurements on 27 nm polystyrene domains in a styrene-isobutylene-styrene triblock copolymer which indicate only a small T(g) depression (3 K) compared to bulk behavior. We expect no effect of size on T(g) in the limit as the solubility parameters of the hard and soft blocks diverge from each other. This strongly segregated limiting behavior agrees with published data for dry and aqueous suspensions of small polystyrene spheres but is in sharp contrast to the strong influence of film thickness on T(g) noted in the literature for free standing ultrathin polystyrene films.

Effect of structural arrest on Poisson's ratio in nanoreinforced elastomers.

We evaluate the strain amplitude (gamma) dependence of the dynamic storage and loss moduli for polybutadiene elastomers containing carbon black particles of varied size at a constant volume fraction of 0.18. Measurements of the low-strain hysteretic softening (Payne effect or unjamming transition) in both shear and uniaxial extension modes allow the gamma dependence of Poisson's ratio (nu) to be determined. For elastomers with a fully agglomerated filler network, the breakup of the jammed state with increasing strain induces a transition from nu approximately 0.35 to the limiting value for an incompressible material, nu=0.5 . This transition is more marked for samples containing smaller filler particles. The effect is similar to the change caused by temperature in the glass transition zone of amorphous polymers, reflecting the parallel between deformation in jammed systems and temperature (and the volume changes associated with it) in glasses; thus, the Payne effect resembles the glass transition in polymers and molecular liquids. We also show herein that the alleviation of structural arrest in filled rubber occurs at a constant value of strain energy, independent of the size of the reinforcing particles or the nature of the strain (shear vs tension). Contrary to the results for shear or tensile deformation, under hydrostatic pressure (volumetric strain) a value of nu=0.5 is maintained for all magnitudes of strain. Along with the absence of hysteresis in cyclic pressure-volume measurements, this demonstrates the distinct behavior of the filler network for hydrostatic compression, in which there is no relative displacement of polymer and filler.

Obstructive sleep apnea syndrome: diagnosis and management.

Increased awareness that changes in sleeping habits and daytime behaviour may be attributable to obstructive sleep apnea syndrome (OSAS) has led many patients to seek both information and definitive treatment. The purpose of this article is to provide information to dentists that will enable them to identify patients who may have OSAS and to assist these patients in making informed decisions regarding treatment options. In patients who have identifiable anatomic abnormalities of the maxilla and mandible resulting in a narrow pharyngeal airway, orthognathic surgery appears to be an excellent treatment option.