The Deformation of Rigid Gels of One-Step Polyamide Networks and Their Constituent Stiff Segments

Abstract

When a gel of rigid polyamide network is prepared in solution by a single step polymerization, some of its stiff segments are straight and others are bent. The motional freedom of the majority of the segments is greatly constrained by having fully or mostly reacted multifunctional rigid branchpoints at both their ends. A minority of the segments are less constrained because they are attached to the network at one end only. We believe that a most efficient mechanism for bending of constrained straight stiff polyamide segments is by a pair of syn-anti interconversions occurring in the same segment. For polyamides these are the least energy consuming changes in which 180° rotations around the ring-to-carbonyl or ring-to-nitrogen bonds take place at an activation energy as low as 4 kcal/mol. In this way, significant translations of one branchpoint relative to the other take place without any torsions of the moving branchpoint and with the bent portions of the stiff segment defining no or almost no “cone of rotation” in space. Conversely, the most efficient straightening mechanism for constrained bent stiff segments is the reversal of the above process, i.e., two placement interconversions occurring per segment. Stiff segments attached to the network at only one end can easily deform by a single inter-conversion, of course.

During macrodeformation the gels maintain constant volume. In this deformed state, the size of the populations of straight and bent stiff segments changes from the relaxed state. In the directions where the stressed sample increases its dimensions, the population of straight segments grows at the expense of the bent segments. In the perpendicular directions where the dimensions of the gel decrease, the population of bent segments increases at the expense of the straight ones. The inability of straight stiff segments to stretch results in increases in modulus with deformation. After the stress is removed the populations of straight and bent segments revert back to their original size. From the above it is obvious that the deformation of rigid gels is not affine. Similarities exist between deformation and swelling, except for the fact that in the swollen gel the population of straight segments increases isotropically throughout the sample which is not the case in the deformed gel. The strong effects of network imperfections are discussed in the context of rigid network swelling.     

Gelling properties of kefiran, a food-grade polysaccharide obtained from kefir grain

The physicochemical and gelling properties of kefiran, a water-soluble glucogalactan with probed health-promoting properties, were investigated. Gel permeation chromatograms revealed a single distribution of molecular weight corresponding to 10⁷ Da. Intrinsic viscosity of kefiran determined using Huggins extrapolations was 6.0 dl/g and using Kramer approximations was 5.95 dl/g.Kefiran has a Newtonian behaviour in diluted solutions, which becomes pseudoplastic at higher concentrations. Rheological behaviour of the solution before and after freeze drying was evaluated by small deformation oscillatory rheological measurements. The mechanical spectrum of solution corresponded to an entangled network behaviour. After freeze–thaw treatment of the solution, a rheological behaviour transition from a liquid-like system to a gel was observed. The storage modulus (G′) in cryogels was 35 times higher than the value obtained for the solution. Rheological characteristics of the cryogel were influenced by kefiran concentration. As the polymer concentration increased, higher number of interactions was evident for the increment in both moduli (G′ and G″). The behaviour of kefiran cryogels about 37 °C determines its ability to melt at mouth temperature. These results suggest that kefiran cryogels could be an interesting alternative for its application in food formulations.     

Mass transfer enhancement due to a soft elastic boundary

Electrochemical mass transfer experiments are performed in a channel in which one wall is made of a soft polymer gel. Mass transfer is enhanced up to 25% relative to rigid walls when the ratio of viscous to elastic forces on the gel increases above a critical value. The enhancement is attributed to a hydrodynamic instability occurring at the fluid-gel interface. The results suggest that soft elastic boundaries could serve as a mechanism for improving mixing and transport in the laminar flows characteristic of microfluidic devices and other small-scale geometries.     

可动凝胶纵向非均质物理模拟研究

由于注入水的长期冲刷,储集层的孔隙结构及物理性质发生了变化,层间和层内的非均质性加剧,影响后期开发效果.可动凝胶能调整地层的非均质性,在纵向非均质模型上进行可动凝胶油藏物理模拟实验,通过布置高精度的压差传感器,了解可动凝胶体系调整非均质地层渗透率和压力场动态分布的能力,对比水驱、聚合物驱和可动凝胶封堵对采收率的影响.结果表明,可动凝胶封堵比水驱提高采收率8.7%,比聚合物驱提高采收率1.6%.由压力场变化分析可知聚合物驱确实能改变油藏内流体流动方向,提高波及范围,驱替出水驱不能波及到的低渗透油藏内的油;可动凝胶成胶后,封堵高渗区,可改变油藏内流体流动方向,进一步提高采收率.图9表1参5     

Development and in-vivo evaluation of ondansetron gels for transdermal delivery

Nausea and vomiting are some of the major side effects caused by certain drug therapies, e.g. chemotherapy, radiotherapy and general anesthesia. Because of the nature of the symptoms, oral delivery is inappropriate, while intravenous administration may be unpractical. The aim of the present study was to develop a transdermal gel (2% Klucel®) for ondansetron, a first line 5-HT3-receptor-antagonist antiemetic. The effects of the penetration enhancer camphor and isopropyl-myristate (IPM) were first investigated in-vitro using modified Franz diffusion-cells and then tested in-vivo in a rabbit model by measuring skin and plasma concentrations. Since a disadvantage of transdermal delivery is a prolonged lag-time, the effect of skin treatment with a micro-needle roller was tested. The in-vitro permeation studies through excised porcine ear skin showed that the presence of 2.5% camphor or IPM increased steady state flux by 1.2- and 2.5-fold, respectively, compared to the control gel. Ondansetron was not detectable in either skin or plasma following in-vivo application of the base-gel, whereas the camphor gel and IPM gel delivered 20 and 81?µg/cm² of ondansetron, respectively. Microporation led to an increase in plasma C_max and AUC by 10.47?±?1.68-fold and 9.31?±?4.91-fold, respectively, for the camphor gel, and by 2.31?±?0.53-fold and 1.59?±?0.38-fold, respectively for the IPM gel. In conclusion, the 2.5% IPM gel demonstrated optimal in-vivo transdermal flux. Skin pretreatment with a micro-needle roller slightly improved the delivery of the IPM gel, whereas dramatically increased the transdermal delivery of the camphor gel.     

Gels as Energy Dissipation Media for Energetic Materials Desensitization

The hypothesis that gels, which have a variety of dissipation mechanisms in response to external forces can provide efficient energy deflection media protecting sensitive energetic materials from accidental initiation is explored. Entrapment of explosives, with an emphasis on peroxide materials, in gels is demonstrated as a general method to substantially lower explosives sensitivity diverting stimuli from the sensitive materials to the gel. Friction is a more difficult stimulus to dissipate in comparison to impact. The desensitization method developed, is with minimal material manipulations. Practical considerations as mode of application, solubility, and controlling speed of gelation determine that the method of choice is a sol‐gel process using a mixture of three alkylalkoxysilanes, R_nSi(OR′)_4–n, producing a hybrid organic‐inorganic gel enabling complete triacetone triperoxide (TATP) desensitization at 20–25 % v/v concentrations.     

Influence of Nanogels on Mechanical, Dynamic Mechanical, and Thermal Properties of Elastomers

Use of sulfur crosslinked nanogels to improve various properties of virgin elastomers was investigated for the first time. Natural rubber (NR) and styrene butadiene rubber (SBR) nanogels were prepared by prevulcanization of the respective rubber lattices. These nanogels were characterized by dynamic light scattering, atomic force microscopy (AFM), solvent swelling, mechanical, and dynamic mechanical property measurements. Intermixing of gel and matrix at various ratios was carried out. Addition of NR gels greatly improved the green strength of SBR, whereas presence of SBR nanogels induced greater thermal stability in NR. For example, addition of 16 phr of NR gel increased the maximum tensile stress value of neat SBR by more than 48%. Noticeable increase in glass transition temperature of the gel filled systems was also observed. Morphology of these gel filled elastomers was studied by a combination of energy dispersive X-ray mapping, transmission electron microscopy, and AFM techniques. Particulate filler composite reinforcement models were used to understand the reinforcement mechanism of these nanogels. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.