Effect of sub-Tg relaxations on chromophore reorientation in corona-poled polymers

Activation volumes for chromophore reorientation were measured for a series of guest–host polymeric materials, indicating a significant coupling between chromophore motion and the glassy α and β relaxation dynamics of the polymer host. The specific systems studied were formed by individually dissolving N,N-dimethyl-p-nitroaniline (DpNA), 4-(dimethylamino)-4′-nitrotolane (DMANT), 4-(diethylamino)-4′-nitrotolane (DEANT), and 1-((4-(dimethylamino)phenyl)ethynyl)-4-((4-nitrophenyl)ethynyl)benzene (DMAPEANT) in poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(isobutyl methacrylate) (PiBMA). In each of these systems, the isothermal, sub-T_g decay of the second-order optical susceptibility χ⁽²⁾ was monitored as a function of pressure using second harmonic generation. In each system, the observed decay of χ⁽²⁾ was represented by a stretched exponential equation from which the decay time τ₀ and decay distribution width β_KWW were determined. For each dopant molecule, the decrease in activation volume with the increasing size of the polymer host's alkyl side group and the pressure dependence of β_KWW were indicative of partial coupling between chromophore rotation and the glassy β relaxation dynamics of the polymer host. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1013–1024, 1998     

Fluorophenyl Bilirubins: Synthesis and Stereochemistry

 Analogs of bilirubin with vinyl groups replaced by symmetrically-disposed o-fluorophenyls (1, bis-exo, and 2, bis-endo) were synthesized and characterized spectroscopically. Their ¹H NMR spectra and NOE data are consistent with an intramolecularly hydrogen-bonded ridge-tile conformation where each propionic acid group embraces an opposing dipyrrinone. Like bilirubin, 1 and 2 exhibit negative chirality induced circular dichroism (ICD) Cotton effects in chloroform containing quinine. Unlike bilirubin, however, in aqueous buffer containing human serum albumin, 2 exhibits a negative exciton chirality ICD, whereas that of 1 is positive.     

Activation volumes associated with chromophore reorientation in corona poled guest–host and side-chain polymers

The decay of the second-order optical susceptibility χ⁽²⁾ as a function of temperature and pressure has been studied in a variety of corona poled guest–host and side-chain polymeric materials using second harmonic generation (SHG). The specific systems studied include the side-chain copolymer poly(disperse red 1 methacrylate-co-methyl methacrylate) (DR1-MMA) as well as the series of guest–host materials formed by individually dissolving the dyes Disperse Red 1 (DR1), Disperse Orange 3 (DO3), and N,N dimethyl-p-nitroaniline (DpNA) in poly(methyl methacrylate) (PMMA), polycarbonate (PC), and polystyrene (PS). In each of these systems, the observed relaxation of χ⁽²⁾ can be represented by a Kohlrausch-Williams-Watts stretched exponential, from which the decay time τ and decay distribution width β are determined. For pressures up to approximately 1000 atm, the natural log of the pressure shift factor is seen to vary linearly with applied pressure, yielding the activation volume for rotational reorientation of the chromophores in each system. The activation volumes are loosely correlated with dopant size in a given polymer host, but are not the same for a given dopant in different hosts. Modeling the chromophores as rotating cylinders, we show that the measured activation volumes do not correspond to the average volume swept out by the dye molecules as they reorient. On the other hand, the activation volumes for each of the three dyes dissolved in PS are seen to be in agreement with the measured activation volumes for the molecular motions associated with volume recovery in neat PS. Moreover, the activation volumes for DR1 and DpNA dissolved in PS are seen to correlate with the proposed couplings between the rotational reorientation of DR1 and the α-relaxation dynamics of PS and the slight decoupling of DpNA from the α-transition motion of PS. This correlation suggests a possible relationship between the activation volumes for chromophore reorientation and the size of the components of the host polymer or the volume swept through by the polymer components during structural reconfiguration. We demonstrate that assuming activation volumes for chromophore reorientation to be related to the size or motion of the polymer host constituents yields a consistent interpretation of the observed trends in the measured activation volumes. © 1995 John Wiley & Sons, Inc.     

Critical phenomena in discrete-time interconnection networks

We present theoretical and numerical results for the performance of a multiprocessor network modeled as a ring and as a toroidal square lattice of nodes with local processors that generate messages for output ports/buffers. The output buffers are assumed to have infinite capacity, and the service time is deterministic. Two models are considered. One assumes that every processor generates messages with rate λ per time slot and per output port/buffer. The other model considers that the generation rate of a node depends on the intensity of the flow of arriving messages. Explicit expressions for the distribution of queue lengths, the average number of messages in the buffers, the average latency, and the critical network load depending on the distance between the source and the destination are obtained. Simulation results show excellent agreement with theoretical predictions based on the assumption of independent queues.