No abstract

No abstract

The dramatic slowdown of the structural relaxation at the glass transition is one of the most puzzling features of glass dynamics. Single molecule orientational correlation times show this strong Vogel-Fulcher-Tammann temperature dependence typical for glasses. Through statistical analysis of single molecule trajectories, we can identify individual glass rearrangement events in the vicinity of a probe molecule in the glass former poly(vinyl acetate) from 8 K below to 6 K above the glass transition temperature. We find that changes in the distribution of waiting times between individual glass rearrangement events are much less dramatic with temperature, the main difference being a small, but decisive number of increasingly long waiting times at lower temperatures. We notice similar individual, local relaxation events in molecular dynamics trajectories for a variety of glassy systems further from the glass transition, leading to waiting time distributions with similar features as those observed in the single molecule experiments. We show that these rare long waiting times are responsible for the dramatic increase in correlation time upon cooling.

The emission rates of volatile organic compounds (VOCs) were measured to investigate the emission characteristics of five types of common furniture products using a 5 m3 size chamber at 25°C and 50% humidity. The results indicated that toluene and α-pinene are the most dominant components. The emission rates of individual components decreased constantly through time, approaching the equilibrium emission level. The relative ordering of their emission rates, if assessed in terms of total VOC (TVOC), can be arranged as follows: dining table > sofa > desk chair > bedside table > cabinet. If the emission rates of VOCs are examined between different chemical groups, they can also be arranged in the following order: aromatic (AR) > terpenes (TER) > carbonyl (CBN) > others > paraffin (PR) > olefin (HOL) > halogenated paraffin (HPR). In addition, if emission strengths are compared between coated and uncoated furniture, there is no significant difference in terms of emission magnitude. Our results indicate that the emission characteristics of VOC are greatly distinguished between different furniture products in terms of relative dominance between different chemicals.

Nanoparticles formed from amphiphilic block copolymers can be used as drug delivery vehicles for hydrophilic therapeutics. Poly(ethylene glycol) (PEG)-peptide copolymers were investigated for their self-assembling properties and as consequent potential delivery systems. Mono-and dihydroxy PEGs were functionalized with a pentavaline sequence bearing Fmoc end groups. The molecular weight of the PEG component was varied to evaluate copolymer size and block number. These di-and tri-block copolymers readily self-assemble in aqueous solution with critical aggregation concentrations (CACs) of 0.46-16.29 lM. At concentrations above the CAC, copolymer solutions form spherical assemblies. Dynamic light scattering studies indicate these aggregates have a broad size distribution, with average diameters between 33 and 127 nm. The copolymers are comprised b-conformations that are stable up to 80 C, as observed by circular dichroism. This peptide secondary structure is retained in solutions up to 50% MeOH as well. The triblock copolymers proved to be the most stable, with copolymers synthesized from 10 kDa PEG having the most stable particles. Loading of carboxyfluorescein at 2-5 mol % shows that these copolymers have the potential to encapsulate hydrophilic drugs for delivery applications. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 871-878, 2011