Dynamics of Diblock Copolymers in Dilute Solutions

We consider the dynamics of freely translating and rotating diblock (A-B), Gaussian copolymers, in dilute solutions. Using the multiple scattering technique, we have computed the diffusion and the friction coefficients D_AB and Zeta_AB, and the change Eta_AB in the viscosity of the solution as functions of x = N_A/N and t = l_B/l_A, where N_A, N are the number of segments of the A block and of the whole copolymer, respectively, and l_A, l_B are the Kuhn lengths of the A and B blocks. Specific regimes that maximize the efficiency of separation of copolymers with distinct "t" values, have been identified.Comment: 20 pages Revtex, 7 eps figures, needs epsf.tex and amssymb.sty, submitted to Macromolecule

Assessing the in vitro efficacy of biocontrol agents and oil cakes against basal rot of onion incited by Fusarium oxysporum f.sp. cepae

Onions are an important vegetable crop, which is infected by many soils and foliar pathogens. Among them, Fusarium Basal Rot (FBR) causes yield losses of up to 50 per cent in the field and 30 to 40 per cent during post-harvest storage of bulbs.  For management of basal rot of onion, the efficacy of native antagonists such as six different Trichoderma sp. (T1-T6), five different Bacillus sp. (B1-B5) and five different oil cakes was assessed against the Fusarium oxysporum f.sp. cepae under in vitro condition. Among them, T3 collected from Kulithalai recorded maximum virulence as well as dark green sporulation with conidia length of 2.68–3.25 and breadth of 2.54-3.46µ. Among the tested isolates, In the case of  Bacillus sp., isolate B4 recorded the maximum inhibition zone (66.16%), followed by B. subtilis (B5), which recorded a (59.03%) inhibition on the mycelial growth. Among the five different oil cakes, the filtrates of neem cake showed a maximum inhibition zone against F. oxysporum f.sp. cepae of 1.29 cm @ 15% concentration, followed by groundnut cake at 1.36 cm @ 30% concentration. Hence the different control measures, Trichoderma sp. showed critically acclaimed performance under in vitro than others. The combined application of Trichoderma sp, Bacillus sp and neem oilcake significantly inhibited the growth of basal rot of onion due to the presence of the antimicrobial property.   

Microphase separation in polyelectrolytic diblock copolymer melt : weak segregation limit

We present a generalized theory of microphase separation for charged-neutral diblock copolymer melt. Stability limit of the disordered phase for salt-free melt has been calculated using Random Phase Approximation (RPA) and self-consistent field theory (SCFT). Explicit analytical free energy expressions for different classical ordered microstructures (lamellar, cylinder and sphere) are presented. We demonstrate that chemical mismatch required for the onset of microphase separation ($\chi^{\star} N$) in charged-neutral diblock melt is higher and the period of ordered microstructures is lower than those for the corresponding neutral-neutral diblock system. Theoretical predictions on the period of ordered structures in terms of Coulomb electrostatic interaction strength, chain length, block length, and the chemical mismatch between blocks are presented. SCFT has been used to go beyond the stability limit, where electrostatic potential and charge distribution are calculated self-consistently. Stability limits calculated using RPA are in perfect agreement with the corresponding SCFT calculations. Limiting laws for stability limit and the period of ordered structures are presented and comparisons are made with an earlier theory. Also, transition boundaries between different morphologies have been investigated

Continuum Theory of Polymer Crystallization

We present a kinetic model of crystal growth of polymers of finite molecular weight. Experiments help to classify polymer crystallization broadly into two kinetic regimes. One is observed in melts or in high molar mass polymer solutions and is dominated by nucleation control with $G \sim \exp(1/T \Delta T)$, where $G$ is the growth rate and $\Delta T$ is the super-cooling. The other is observed in low molar mass solutions (as well as for small molecules) and is diffusion controlled with $G \sim \Delta T$, for small $\Delta T$. Our model unifies these two regimes in a single formalism. The model accounts for the accumulation of polymer chains near the growth front and invokes an entropic barrier theory to recover both limits of nucleation and diffusion control. The basic theory applies to both melts and solutions, and we numerically calculate the growth details of a single crystal in a dilute solution. The effects of molecular weight and concentration are also determined considering conventional polymer dynamics. Our theory shows that entropic considerations, in addition to the traditional energetic arguments, can capture general trends of a vast range of phenomenology. Unifying ideas on crystallization from small molecules and from flexible polymer chains emerge from our theory.Comment: 37 double-spaced pages including 8 figures, submitted to the Journal of Chemical Physic

The J_1-J_2 model revisited : Phenomenology of CuGeO_3

We present a mean field solution of the antiferromagnetic Heisenberg chain with nearest (J_1) and next to nearest neighbor (J_2) interactions. This solution provides a way to estimate the effects of frustration. We calculate the temperature-dependent spin-wave velocity, v_s(T) and discuss the possibility to determine the magnitude of frustration J_2/J_1 present in quasi 1D compounds from measurements of v_s(T). We compute the thermodynamic susceptibility at finite temperatures and compare it with the observed susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls transition.Comment: Phys. Rev.

Production and partial purification of α-amylase producing Streptomyces sp. SNAJSM6 isolated from seaweed Sargassum myriocystum J. Agardh

1245-1251The bacteria Streptomyces sp. was isolated from the seaweed Sargassum myriocystum that was collected from Kilakarai, Ramanathapuram District, Tamil Nadu. Among the isolates, the SNAJSM6 produced the maximum content of extracellular α-amylase, which was conformed by starch hydrolysis and identified as Streptomyces sp. SNAJSM6. The culture optimization studies for augmented enzyme production in the Streptomyces sp. were conducted under different physico-chemical parameters, such as pH, temperature, and carbon and nitrogen sources for α-amylase production. The maximum α-amylase production was recorded on the third day of incubation (56U/ml) by Streptomyces sp. SNAJSM6. The Streptomyces sp. optimized α-amylase enzyme production was recorded at pH 7.0 and temperature 30 °C, which was 1.86 fold higher than the unoptimized culture broth of the Streptomyces sp. The Streptomyces sp. α-amylase enzyme was partially purified and its molecular weight was found to be 44 kDa by using SDS-PAGE. Among the isolates that were screened, Streptomyces sp. SNAJSM6 is a potential candidate for maximum production of α-amylase. Further, it showed excellent antibacterial activity against selected human pathogens