High-density percolation: Exact solution on a Bethe lattice

A new percolation problem is posed where the sites on a lattice are randomly occupied but where only those occupied sites with at least a given numberm of occupied neighbors are included in the clusters. This problem, which has applications in magnetic and other systems, is solved exactly on a Bethe lattice. The classical percolation critical exponents=gg=1 are found. The percolation thresholds vary between the ordinary percolation thresholdp _c (m=1)=l/(z – 1) andp _c(m=z) =[l/(z – 1)]¹/(z–1). The cluster size distribution asymptotically decays exponentially withn, for largen, p p _c .Supported in part by National Science Foundation grant DMR78-10813.     

ELASTIC BEHAVIOR OF PROTEIN-LIKE SINGLE CHAIN

The conformational properties and elastic behaviors of protein-like single chains in the process of tensile elongation were investigated by means of Monte Carlo method. The sequences of protein-like single chains contain two types of residues: hydrophobic (H) and hydrophilic (P). The average conformations and thermodynamics statistical properties of protein-like single chains with various elongation ratio λ were calculated. It was found that the mean-square end-to-end distance r increases with elongation ratio,λ. The tensor eigenvalues ratio of : decreases with elongation ratio λ for short (HP)x protein-like polymers, however, the ratio of : increases with elongation ratioλ,especially for long (H)x sequence. Average energy per bond increases with elongation ratioλ, especially for(H)x protein-like single chains. Helmholtz free energy per bond also increases with elongation ratioλ. Elastic force (f), energy contribution to force (fU) and entropy contribution to force (fs) for different protein-like single chains were also calculated.These investigations may provide some insights into elastic behaviors of proteins.     

Thermal behavior of polyacrylonitrile polymers synthesized under different conditions and comonomer compositions

Polyacrylonitrile (PAN) polymers are used as precursors for carbon fiber production. This process requires an oxidative stabilization step, which can be studied by differential scanning calorimetry (DSC). In this sense, thermal behavior of PAN based terpolymers by different polymerization processes, compositions and itaconic acid concentrations in the reaction media were investigated. The obtained results showed that the addition of itaconic acid and methyl acrylate as comonomers resulted a lower heat flow during the process comparing to the PAN homopolymer. It suggested that these comonomers aid the oxidative stabilization stage for all studied process. The redox system polymerization at 40°C resulted in a lower heat flow. Itaconic acid decreases slightly initial and peak temperatures of the terpolymer and heat flow until concentration of 3y. The cyclization temperature decreases when MAis incorporated into the terpolymer compared to the MMA terpolymer and increases when MAA is the acidic monomer. Among terpolymers the AN/MA/AA polymer showed the best thermal behavior for carbon fiber producing.     

Poly(butylene terephthalate)/poly(ε-caprolactone) blends: Influence of PCL molecular mass on PBT melting and crystallization behavior

The isothermal crystallization kinetics and melting behavior of poly(butylene terephthalate) (PBT) in binary blends with poly(ε-caprolactone) (PCL) was investigated as a function of PCL molecular mass by differential scanning calorimetry and optical microscopy. The components are miscible in the melt when oligomeric PCL (M_w = 1250) is blended with PBT, whereas only partial miscibility was found in mixtures with higher molecular mass (M_w = 10,000 and 50,000). The equilibrium melting point of PBT in the homopolymer and in blends with PCL was determined through a non-linear extrapolation of the T_m = f(T_c) curve. The PBT spherulitic growth rate and bulk crystallization rate were found to increase with respect to plain PBT in blends with PCL1250 and PCL10000, whereas addition of PCL50000 causes a reduction of PBT solidification rate. The crystallization induction times were determined by differential scanning calorimetry for all the mixtures through a blank subtraction procedure that allows precise estimation of the crystallization kinetics of fast crystallizing polymers. The results have been discussed on the basis of the Hoffman-Lauritzen crystallization theory and considerations on both the transport of chains towards the crystalline growth front and the energy barrier for the formation of critical nuclei in miscible and partially miscible PBT/PCL mixtures are widely debated.     

界面改性剂对刚性粒子增韧尼龙6熔体流变行为的影响

研究了界面改性剂对高岭土增韧尼龙６熔体流变行为的影响,并与硅烷偶联剂ＫＨ５５０进行了比较．结果表明,ＫＨ５５０的加入明显降低了高岭土填充尼龙６熔体的粘度和弹性,而界面改性剂显著地增强了它的粘度和弹性．这一差别应归因于同ＫＨ５５０相比,界面改性剂更有效地增强了高岭土与尼龙６基体之间的界面结合和其自身的回弹性能．同时,界面改性剂的用量对高岭土填充尼龙６流变行为具有很大影响．当界面改性剂的用量为高岭土和尼龙６总量的２％时,界面改性剂在高岭土表面上趋于“饱和”,再增加界面改性剂的用量,对流变行为的影响不大．     

Self-aggregation and phase behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solution

The phase behavior and aggregation properties of block copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronics, poloxamers) in aqueous solution have recently attracted much attention. Both experimental and theoretical studies are reviewed, not comprehensively, but with the focus on studies, partly cooperative, partly independent, performed by groups in Uppsala (light scattering and fluorescence), Roskilde (rheology and calorimetry), Risø (SANS), Graz (x-ray and speed of sound), and Lund (theoretical model calculations).The phase behavior of these copolymers is similar in many respects to that of conventional nonionic surfactants, with the appearance of hexagonal, cubic, and lamellar liquid crystalline phases at high concentrations. In the isotropic solution phase the critical concentration for micelle formation is strongly temperature dependent, and at a given concentration the monomer to micelle transition occurs gradually over a broad temperature range, partly due to the broad size polydispersity of both the PO- and EO-blocks. For some Pluronic copolymers a transition from globular to long rod-like micelles occurs above a transition temperature, resulting in a strong and sudden increase of viscosity and viscoelasticity of the solution.Size and aggregation numbers have been determined for the globular micelles in some cases, and also the rod-like micelles have been characterized. NMR and fluorescence measurements have provided further information on the properties of the micellar core and mantle. In combination, results from different measurements on the same Pluronics material indicate that the aggregation number of the micelles increases with the temperature, whereas the hydrodynmic radius varies much less. The PEO-mantle of the micelles seems to contract with increasing temperature. The core appears to contain appreciable amounts of PEO in addition to PPO (and also some water). The segregation between core and mantle is not as distinct as in normal micelles, a conclusion which is in line with the predictions from the model calculations.     

Asymptotic Behavior of the t Expansion

The asymptotic behavior of a system's ground-state energy from the t expansion of Horn and Weinstein has been suggested to have the form E ₁(t)=E ₁+exp(–a _n t+b _n ). In the limit of very large t, this becomes E ₁(t)=E ₁+exp(–a ₁ t+b ₁). A simple analysis shows that the parameters are a ₁=E ₂–E ₁ and b ₁=ln[(E ₂–E ₁)|c ₂|²/|c ₁|²]. Functions are introduced which allow determination of a ₁, b ₁ and lower bounds to E ₁.     

Mesoscopic simulation of the aggregation behavior of fluorinated surfactant in aqueous solution

The aggregation behavior of fluorinated surfactant in aqueous solution was investigated using dissipative particle dynamics (DPD) simulation method. Simulation results show that fluorinated surfactants behave mainly as their hydrocarbon analogues, having similar sequences of phases and aggregate structures, which are capable of building micelle, hexagonal phase and lamellar phase. But fluorinated surfactants also show interesting differences from hydrocarbon analogues, which can easily form hexagonal and lamellar structures with comparative little curvature. They can also form ellipsoid or rod-like micelles even in very low concentrations instead of spheroid ones. The dynamic aggregation behavior of fluorinated surfactants, as well as the comparison with hydrogenated ones, was also investigated.     

Effects of Nonionic, Cationic, and Mixed Nonionic-Cationic Surfactants on the Acid-Base Behavior of Phenyl Salicylate

The effects of Brij 35 micelles, CTABr micelles, and mixed Brij 35–CTABr micelles on the acid–base behavior of phenyl salicylate (PST) have been studied in aqueous solution containing 2% v/v acetonitrile. The apparent pK_b (pK^app_b) of PST is decreased by 1.5 pK units with the increase in [Brij 35] from 0 to 0.02 M which is attributed to micellar medium effect. The values of pK^app_b remain almost independent of [CTABr] within its range 0.01–0.03 M. The increase in [CTABr] from 0 to 0.03 M in aqueous solution containing 0.02 M Brij 35 has not resulted in a change in pK^app_b. This shows that the characeristic structural features of nonionic Brij 35 micelles remain essentially unchanged on addition of CTABr under the present experimental conditions.     

载药微乳液相行为的研究

提出了一种基于相图研究实现纳米药物载体可控制备的方法.采用微乳液控温相图绘制装置绘制了硬脂酸聚烃氧(40)酯(S-40)/聚氧乙烯聚氧丙烯醚嵌段共聚物(F-68)/单硬脂酸甘油酯(GMS)/水体系的拟三元相图,基于电导率测定值确定了微乳液的结构(W/O、双连续相和O/W),该体系同时存在液晶区域.乳化剂S-40/F-68的质量比为7:3.研究了脂溶性药物维甲酸(RA)对微乳液相行为的影响，结果表明RA的加入对微乳液的相行为影响较小.基于相图研究结果制备了维甲酸固体脂质纳米粒(RA-SLN),亚微米粒度分析仪(PCS)测定的平均粒径和透射电镜测试都表明RA-SLN为10 nm左右的球状粒子.