Excessive Cross-linking Research Articles

Sulfonated polystyrene/polyvinyl chloride composite membranes for PEMFC applications

The sulfonated polystyrene (SPSt)/polyvinyl chloride (PVC) composite electrolyte membranes were prepared via styrene-impregnated polymerization within PVC matrix and subsequent sulfonation. The propagated polystyrene chains were interpenetrated with those of the PVC film, and hence the two components were completely compatible. The effects of some preparation parameters, including monomer solution composition, polymerization and sulfonation conditions, on membrane performance were investigated. It was found that crosslinking with DVB improved oxidative stability, while excess crosslinking might lead to decrease in the proton conductivity. Polymerization should be carried out at 80 °C for more than 8 h; otherwise the produced polystyrene had relatively low molecular weight and might leach out of the membrane. The higher concentration of chlorosulfonic acid or the higher sulfonation temperature caused side reactions, which led to decrease in proton conductivity as well as slight improvement in oxidative stability. Furthermore, repetitious polymerization under hot-pressing plates further improved proton conductivity with slight increase in membrane thickness. Preliminary data in single cell test showed that the prepared membrane had comparable performance with commercial Nafion. This study demonstrates that an inexpensive polymer electrolyte membrane suitable for low-temperature PEMFC applications can be prepared easily by styrene/DVB-impregnated polymerization within PVC substrate with subsequent sulfonation in chlorosulfonic acid.

On the effectiveness of different additives and concentrations on the re-building of the molecular structure of degraded polyethylene

Mechanical recycling is an easy and economic way to re-use plastic waste as secondary materials, but, in general, their properties are worse with respect to the reclaimed materials and the virgin polymer. The aim of this work was to study the effect of concentration and reaction kinetics of two additives, an ethylene- co-glycidyl methacrylate (Lotader) and a hydroxylamine derivative (CGX), in the re-building of a degraded polyethylene. CGX is a nitroxyl radical generator able to form branching in polyolefins while the epoxy groups of Lotader can react with the functional groups present in the recycled polyethylene. The results indicate that the CGX has a higher reaction rate than Lotader, probably due to its lower molecular weight and the different reaction path. As for the effect of concentration, as expected, a higher amount of additive accelerates and increases re-building, especially when CGX is used. The melt strength increases with the additive content and the mechanical properties show a significant reduction at the highest concentrations of both additives due to excessive cross-linking.

Selection and crosslinking modification of membrane material for FCC gasoline desulfurization

Selection and modification of membrane material is important fundamental research for fluid catalytic cracking (FCC) gasoline desulfurization by membrane process. Solubility parameters of FCC gasoline components and typical polymeric membrane materials were calculated and given out. For thiophene species, which are the primary ones in FCC gasoline, solubility parameters were about 19–21 (J/cm 3) 1/2, while about 14–15 (J/cm 3) 1/2 for most hydrocarbon species in FCC gasoline. The distinct difference in solubility parameter between the two typical species was just the key to fulfill the separation. By the study and analysis of solubility parameter theory, pervaporation (PV) performance and membrane preparation, polyethylene glycol (PEG) was determined as the promising membrane material for gasoline desulfurization. In spite of its desulfurization capacity, PEG needs modification for its easier swelling and unstable PV performance. According to the comparison on PV performance of PEG before and after modification as well as the swelling experiments, crosslinking was an effective modification way for PEG membrane applied in gasoline desulfurization. After crosslinking, not only the sulfur enrichment factor increased distinctly, but also the membrane performance was stable which was the most important point for practical application. Sulfur enrichment factor increased with the increase of amount of crosslinking agent and crosslinking time, while total permeation flux decreased. Unfavorable results occurred with exorbitant crosslinking resulted form excessive crosslinking agent and overlong crosslinking time. Since sulfur enrichment factor was for key consideration, 16% of crosslinking agent amount and 60 min of crosslinking time were more practical.

Investigation on fracture performance of aqueous polymer isocyanates in terms of energy release rate

In order to investigate the fracture performance of aqueous polymer isocyanates (APIs), an analytical expression for the energy release rate of adhesively-bonded three-point bending specimens has been derived from linear elastic fracture mechanics (LEFM) and the energy release rates of nine API samples are evaluated. Experimental results show that a proper loading of polymeric diphenylmethane-4,4′-diisocyanate (p-MDI) as cross-linker can improve the resistance of aqueous polymer latex adhesive layer to cracking. Excess cross-linker cannot maintain such an effect of strengthening and may decrease considerably the energy release rate of API polymer. When the loading of polyisocyanate is less than the critical value, the polymer cross-linked by aqueous emulsified p-MDI has a higher resistance to cracking. In contrast, polymers modified by p-MDI blended with an organic solvent have much better performance.

Investigation of Adhesion Performance of Aqueous Polymer Latex Modified by Polymeric Methylene Diisocyanate

ABSTRACT Aqueous polymer isocyanate (API), which has good adhesive properties at ambient temperature and excellent resistance to warm/boiling water, and is friendly to the environment, is widely used in the timber-processing industry. To prepare high performance API, vinyl acetate homopolymer and copolymer emulsion were respectively cross-linked by three types of polymeric methylene diisocyanate (p-MDI). The potlife, curing time, bonding strength, and water resistance of API adhesives were tested with different cross-linkers and varying loadings (5–20%). Also the effect of polyvinyl alcohol (PVOH) content of aqueous vinyl latex on the performance of API was investigated. It was shown that the potlife and curing time of API were obviously influenced by the types of cross-linker and its loading. Correct loadings of p-MDI as crosslinker can remarkably improve the adhesive performance of aqueous polymer emulsion at ambient temperature. Excess cross-linker cannot maintain such an effect of strengthening and may decrease considerably the bond properties of API. The warm- and boiling-water resistance of API improved markedly with increasing cross-linker loading, where emulsifiable isocyanate gave better cross-linking performance, and p-MDI mixed with organic solvent was the secondbest. With the increase of PVOH content, the curing time of API increased, but no statistically apparent differences in the potlife of API were found. The bonding performance of API was improved as PVOH content increases, but excess PVOH also weakenes the warm- and boiling-water resistance of the joint.

Leu309 Plays a Critical Role in the Encapsulation of Substrate Protein into the Internal Cavity of GroEL

In the crystal structure of the native GroEL.GroES.substrate protein complex from Thermus thermophilus, one GroEL subunit makes contact with two GroES subunits. One contact is through the H-I helices, and the other is through a novel GXXLE region. The side chain of Leu, in the GXXLE region, forms a hydrophobic cluster with residues of the H helix (Shimamura, T., Koike-Takeshita, A., Yokoyama, K., Masui, R., Murai, N., Yoshida, M., Taguchi, H., and Iwata, S. (2004) Structure (Camb.) 12, 1471-1480). Here, we investigated the functional role of Leu in the GXXLE region, using Escherichia coli GroEL. The results are as follows: (i) cross-linking between introduced cysteines confirmed that the GXXLE region in the E. coli GroEL.GroES complex is also in contact with GroES; (ii) when Leu was replaced by Lys (GroEL(L309K)) or other charged residues, chaperone activity was largely lost; (iii) the GroEL(L309K).substrate complex failed to bind GroES to produce a stable GroEL(L309K).GroES.substrate complex, whereas free GroEL(L309K) bound GroES normally; (iv) the GroEL(L309K).GroES.substrate complex was stabilized with BeF(x), but the substrate protein in the complex was readily digested by protease, indicating that it was not properly encapsulated into the internal cavity of the complex. Thus, conformational communication between the two GroES contact sites, the H helix and the GXXLE region (through Leu(309)), appears to play a critical role in encapsulation of the substrate.

Glucose oxidase effect on dough rheology and bread quality: A study from macroscopic to molecular level

Enzymes are used in baking to improve dough handling properties and the quality of baked products. Glucose oxidase (GO) is an enzyme with oxidizing effect due to the hydrogen peroxide released from its catalytic reaction. In this study, the macroscopic effect of increasing glucose oxidase concentrations on wheat dough rheology, fresh bread characteristics and its shelf life during storage was determined. A reinforcement or strengthening of wheat dough and an improvement of bread quality can be obtained with the addition of GO, although inverse effects were obtained when excessive enzyme levels were added. The analysis of the gluten proteins at molecular level by high performance capillary electrophoresis and at supramolecular level by cryo-scanning electron microscopy revealed that the GO treatment modified gluten proteins (gliadins and glutenins) through the formation of disulfide and non-disulfide crosslinks. The high molecular weight glutenin subunits showed to be the most susceptible glutenin fraction to the oxidation action of GO. Excessive addition of GO produced an excessive crosslinking in the gluten network, responsible of the negative effect on the breadmaking properties.

Electrical properties of nanostructured interpenetrating polymer networks based on natural rubber (NR)/polystyrene (PS)

AbstractThe electrical properties of nanostructured sequential interpenetrating polymer networks from natural rubber (NR) and polystyrene (PS) have been studied in the frequency range of 102–107 Hz. The permittivity, volume resistivity, dielectric loss factor, and dissipation factor were analyzed as a function of frequency, blend composition, crosslinking level, and initiating system. It was found that the volume resistivity and dissipation factor first increase, reach a maximum around 103–105 Hz, and then decrease gradually with the increase of frequency. As the NR content is increased from 30 to 70%, the dissipation factor (tan δ) increases. On increasing the extent of the PS phase crosslinking, the permittivity increases. However, at higher levels of PS crosslinking, the permittivity values decrease due to the agglomeration of PS phase arising from excessive crosslinking. The morphology studies using a scanning electron microscope confirmed the agglomeration of PS phase at high crosslinking level. The permittivity values are maximum at 4% of crosslinking content. The influence of initiating system on the dielectric properties was not very significant. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2017–2026, 2005

Actin cytoskeleton regulates extracellular matrix-dependent survival signals in glomerular epithelial cells

Adhesion of rat glomerular epithelial cells (GEC) to collagen activates focal adhesion kinase (FAK) and the Ras-extracellular signal-regulated kinase (ERK) pathway and supports survival (prevents apoptosis). The present study addresses the relationship between actin organization and the survival phenotype. Parental GEC (adherent to collagen) and GEC stably transfected with constitutively active mutants of mitogen-activated protein kinase kinase (R4F-MEK) or FAK (CD2-FAK) (on plastic) showed ERK activation, low levels of apoptosis, and a cortical distribution of F-actin. Parental GEC adherent to plastic showed increased apoptosis, disorganization of cortical F-actin, and formation of prominent stress fibers. Assembly of cortical F-actin was, at least in part, mediated via ERK. However, disruption of the actin cytoskeleton with cytochalasin D or latrunculin B in parental GEC (on collagen) and in GEC that express R4F-MEK or CD2-FAK (on plastic) decreased ERK activation and increased apoptosis. Expression of a constitutively active RhoA (L(63)RhoA) induced assembly of cortical F-actin, promoted ERK activation, and supplanted the requirement of collagen for survival. Adhesion of GEC to collagen increased phosphatidylinositol-4,5-bisphosphate (PIP(2)). Downregulation or sequestration of PIP(2) by transfection with an inositol 5'-phosphatase or the plextrin-homology domain of phospholipase C-delta1 decreased F-actin content and survival. Moreover, overexpression of wild-type or K256E mutant alpha-actinin-4 with increased affinity for F-actin increased apoptosis. These results demonstrate a reciprocal relationship between collagen-induced cortical F-actin assembly and collagen-dependent survival signaling, including ERK activation. Appropriate remodeling of the actin cytoskeleton may be necessary for facilitating survival, as both disassembly and excessive crosslinking affect survival adversely.

Characterization of the Protein Dimerization Domain Responsible for Assembly of Functional Selenodeiodinases

Thyroid hormone metabolism is catalyzed by a small family of selenoenzymes. Type I deiodinase (D1) is the best characterized family member and is an integral membrane protein composed of two 27-kDa subunits that assemble to a functional holoenzyme after translation. To characterize the protein domain(s) responsible for this post-translational assembly event, we used deletion/truncation analysis coupled with immune depletion assays to map the dimerization domain of D1. The results of our studies show that a highly conserved sequence of 16 amino acids in the C-terminal half of the D1 subunit, -D148FL-YI-EAH-DGW163-, serves as the dimerization domain. Based on the high conservation of this domain, we synthesized a novel bait peptide-green fluorescent protein fusion probe (DDD(GFP)) to examine holoenzyme assembly of other family members. Overexpression of either the DDD(GFP) or an inert D1 subunit (M4) into SeD2 (accession number U53505)-expressing C6 cells specifically led to the loss of >90% of the catalytic activity. Catalytically inactive D2 heterodimers composed of SeD2: DDD(GFP) subunits were rescued by specific immune precipitation with anti-SeD2 IgG, suggesting that SeD2 requires two functional subunits to assemble a catalytically active holoenzyme. These findings identify and characterize the essential dimerization domain responsible for post-translational assembly of selenodeiodinases and show that family members can intermingle through this highly conserved protein domain.

Large area UV casting using diverse polyacrylates of microchannels separated by high aspect ratio microwalls

Large area molding of long and deep microchannels separated by high aspect ratio microwalls is important for high sensitivity and high throughput microfluidic devices. Ultraviolet (UV) casting is a feasible, economical and convenient method of replication of such microstructures in plastics. It is shown that a wide variety of polyacrylates with diverse properties such as those made from epoxy (EP), polyurethane (UR), polyester (ES), poly (ethylene glycol) (EG) and poly(propylene glycol) (PG) can be used for the high aspect ratio (7-9) UV casting of such linear microstructures over a 100 mm diameter, enlarging the range of applications of the replicated microstructures. Some challenges arise. With the EG formulation, wavy microstructures were observed; this can be overcome by stress relaxation. With non-polar PG formulation, poor adhesion between the polyester substrate and resin can lead to delamination of the casting from the substrate during demolding; this can be overcome by pre-coating a partially cured same resin on the polyester substrate. An optimum UV irradiation time was important for cure at the deepest end of the microstructure without excessive crosslinking leading to much increased demolding forces. The viscosity and wetting capability of the formulations were found to affect replication fidelity.

Inhibition of Factor VIII-Specific Memory B Cell Responses by Supra-Physiological Concentrations of Factor VIII.

Inhibitory antibodies against factor VIII (FVIII) are the major complication experienced by hemophilia A patients treated with FVIII products. The most effective therapy to eradicate these antibodies is elevated doses of FVIII over a prolonged period. Despite clinical practice in using such protocols, nothing is known about the immunological mechanisms that cause the down-modulation of FVIII-specific immune responses and the induction of long-lasting immune tolerance against FVIII. Understanding the underlying mechanisms, however, would facilitate designing new therapeutic strategies.The re-stimulation of FVIII-specific memory responses after each dose of FVIII is probably the most important event in the maintenance of FVIII inhibitors in patients. Therefore, the eradication of these memory responses should be an essential step in the down-modulation of inhibitory antibodies and the induction of immune tolerance. We used a murine model of hemophilia A to answer the question whether FVIII-specific memory responses are sensitive to increasing doses of FVIII. In particular, we were interested in the differential effects of FVIII on memory-B-cell and memory-T-cell responses. For the analysis of FVIII-specific memory responses, we re-stimulated FVIII-specific memory B- and T-cells obtained from spleens of hemophilic mice treated with four doses of human FVIII or eight doses of murine FVIII as described (Sasgary et al.: Thromb Haemost 2002; 87:266–72; Hausl et al.: Blood 2004; 104:115–22).Our results show dose-dependent effects of FVIII on the re-stimulation of FVIII-specific memory B cells in vitro. Physiological concentrations of FVIII below 100 ng/ml re-stimulate memory B cells and induce their differentiation into anti-FVIII antibody-secreting plasma cells. Supra-physiological concentrations above 100 ng/ml, however, inhibit memory-B-cell re-stimulation. The inhibition of memory-B-cell re-stimulation is irreversible and seems to be due to an induction of apoptosis that is at least partly mediated by Fas-dependent mechanisms. Furthermore, the inhibition appears to be initiated by triggering the B-cell receptor (BCR) without the requirement of an excessive cross-linking of the BCR. The activation of FVIII-specific T cells is not affected by increasing doses of FVIII.We conclude that the induction of apoptosis in FVIII-specific memory B cells might be the first step in the induction of immune tolerance in hemophilia A patients with FVIII inhibitors who receive high doses of FVIII. The eradication of memory B cells would prevent their differentiation into antibody-secreting plasma cells and, moreover, might lead to a deficiency of effective antigen-presenting cells required for the re-stimulation of FVIII-specific memory T cells. The induction of regulatory T cells rather than effector T cells could be the consequence of this deficiency.

Development of novel aqueous coatings which meet the requirements of ecology-conscious society: novel cross-linking system based on the carbonyl–hydrazide reaction and its applications

This paper reviews the results of our research on the reaction between a carbonyl group and a hydrazide group, especially the cross-linking, physical characteristics of the cross-linking system, and the reverse (decomposition) reaction. Discoveries in this investigation may lead to development of various novel products which would answer the requirements of today's coatings industry. The reaction, which proceeds rapidly in aqueous media at ambient temperature, can be used in one-component cross-linkable latex paints offering many advantages, such as good water and stain resistance, without detracting the workability. Applications of this reaction to aqueous two-component emulsion paint were also examined and the possibility for practical use was envisaged. Exploitation of the physical properties of dihydrazide compounds has resulted in the development of a novel curing system, namely, a “penetrative cross-linking process”. This process, where the paint films are cured by the penetration of a cross-linker, exploits three features of dihydrazide compounds to enable both easy penetration of the cross-linker and quick removal of excess cross-linker: low molecular weight, good water-solubility, and poor compatibility with paint resins. With this cross-linking reaction the reverse reaction can take place under specific conditions. This finding leads to the development of decomposable cross-linked coatings which are desirable for the recycling of coated substrates.

Modifications to Improve the Adhesion of Crosslinked Starch Sizes to Fiber Substrates

This paper describes improvements to starch sizes, both for stability of the viscosity of the hot pastes and for adhesion to fibers. Crosslinking of starch stabilizes the viscosity of sizing paste, but excess crosslinking damages the adhesion to cotton and polyester/cotton blend fibers. The problem with decreased adhesion can be eliminated by cationic modi fication of the crosslinked starch. The effect of cationic modification is present even when the degree of substitution is as small as 0.021. Increasing the extent of cationic modifi cation enhances adhesion. Moreover, cationic modification does not deteriorate viscosity stability.

Chronological age modifies the microscopic remodeling process in viable cardiac tissue after infarction

To define the impact of age on microscopic structural remodeling after myocardial infarction, the physical properties of infarct scar tissue and viable remote zone tissues in young (3 months) and older adult (18 months) Fischer rats were quantified with the use of high-frequency (50 MHz) high-resolution acoustic microscopy 3 months after coronary artery occlusion. We observed that integrated backscatter increased by 100% in the viable zones of old animals after infarction, but remained relatively unaffected in the same regions of younger animals. Mathematical models of myocardial scattering behavior indicated that a 25% increase in stiffness of the extracellular matrix materials in viable zones likely occurred in the older animals. Alterations in gross tissue collagen content were not responsible for this increased stiffness. These observations are compatible with the hypothesis that progressive age-related changes in the quality of the collagen ( e.g., excessive age-related crosslinking) rather than its amount per se may have altered the stiffness of the extracellular matrix of remodeled viable tissue in older animals. (E-mail: smh@wuphys.wustl.edu)

Controlled destruction of residual crosslinker in a silicone elastomer for drug delivery

AbstractIn drug delivery systems that use silicone elastomers as a diffusion matrix for the active drug, it is common to crosslink the material by the hydrosilylation reaction. In this platinum‐catalyzed reaction, vinyl groups on a polymer add to the methyl siloxane hydride (MHS) groups on a low molecular mass crosslinker. With an excess of crosslinker, a fast curing is achieved and a fully crosslinked material is formed. Unreacted MHS groups were shown to remain in the elastomer after curing because of the excess crosslinker. In this work, a simple procedure was developed to eliminate the unreacted MHS groups from the final product. We found that storage of the product at +40°C and 75% relative humidity for a few weeks will effectively destroy the residual MHS groups in the elastomer. The effects of varying levels of humidity, oxygen, and temperature on this postcuring procedure were studied. The amount of MHS groups was measured with NMR and IR spectroscopy. We also found that the hardness of the material increased by approximately 25% as a consequence of this postcuring treatment. This increase is probably due to a secondary crosslinking reaction between MHS and silanol groups. Heat treatment at higher temperatures led to an even further increase in the hardness and compression modulus. Because no MHS groups remained in the elastomer when this heat treatment was started, it is apparent that another secondary crosslinking reaction is occurring, probably silanol condensation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2254–2264, 2002

Electron‐beam irradiation of poly(vinyl chloride)/epoxidized natural rubber blends in presence of trimethylolpropane triacrylate

AbstractElectron‐beam initiated crosslinking of poly(vinyl chloride)/epoxidized natural rubber blends, which contained trimethylolpropane triacrylate (TMPTA), was carried out over a range of irradiation doses (20–200 kGy) and concentrations of TMPTA (1–5 phr). The gel content increased with the irradiation dose and the TMPTA level, although the increase was marginal at higher doses and higher TMPTA levels. Blends containing 3–4 phr TMPTA achieved optimum crosslinking, which in effect caused the maximum tensile strength (TS) at a dose of 70 kGy. A further addition of TMPTA caused a decline in the TS above 40 kGy that was due to embrittlement, which is a consequence of excessive crosslinking and the breakdown of the network structure. The possible formation of a more open network as a result of the breakdown of the network structure was further confirmed by the modulus results. Dynamic mechanical analysis (tan δ curve) and scanning electron microscopy studies on samples irradiated at 0 and 200 kGy were undertaken in order to gain further evidence on the irradiation‐induced crosslinking. The plasticizing effect of TMPTA prior to irradiation and the formation of microgels upon irradiation were also discussed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1926–1935, 2001

Increases in Renal ε-(γ-Glutamyl)-Lysine Crosslinks Result from Compartment-Specific Changes in Tissue Transglutaminase in Early Experimental Diabetic Nephropathy: Pathologic Implications

Diabetic nephropathy (DN) is characterized by an early, progressive expansion and sclerosis of the glomerular mesangium leading to glomerulosclerosis. This is associated with parallel fibrosis of the renal interstitium. In experimental renal scarring, the protein cross-linking enzyme, tissue transglutaminase (tTg), is up-regulated and externalized causing an increase in its crosslink product, ε-(γ-glutamyl)-lysine, in the extracellular space. This potentially contributes to the extracellular matrix (ECM) accumulation central to tissue fibrosis by increasing deposition and inhibiting breakdown. We investigated if a similar mechanism may contribute to the ECM expansion characteristic of DN using the rat streptozotocin model over 120 days. Whole kidney ε-(γ-glutamyl)-lysine (HPLC analysis) was significantly increased from Day 90 (+337%) and peaked at Day 120 (+650%) (p < 0.05). Immunofluorescence showed this increase to be predominantly extracellular in the peritubular interstitial space, but also in individual glomeruli. Total kidney transglutaminase (Tg) was not elevated. However, using a Tg in situ activity assay, increased Tg was detected in both the extracellular interstitial space and glomeruli by Day 60, with a maximal 53% increase at Day 120 (p < 0.05). Using a specific anti-tTg antibody, immunohistochemistry showed a similar increase in extracellular enzyme in the interstitium and glomeruli. To biochemically characterize glomerular changes, glomeruli were isolated by selective sieving. In line with whole kidney measurement, there was an increase in glomerular ε-(γ-glutamyl) lysine (+361%); however, in the glomeruli this was associated with increases in Tg activity (+228%) and tTg antigen by Western blotting (+215%). Importantly, the ratio of glomerular ε-(γ-glutamyl) lysine to hydroxyproline increased by 2.2-fold. In DN, changes in the kidney result in increased translocation of tTg to the extracellular environment where high Ca2+ and low GTP levels allow its activation. In the tubulointerstitium this is independent of increased tTg production, but dependent in the glomerulus. This leads to excessive ECM cross-linking, contributing to the renal fibrosis characteristic of progressive DN.

Electric Poling and Relaxation of Thermoset Polyurethane Second-Order Nonlinear Optical Materials: Role of Cross-Linking and Monomer Rigidity

A high μβ isophorone-derived phenyltetraene chromophore (denoted CLD-5) was synthesized. The chromophore was modified with a hexyl group at the middle of the π-conjugate bridge to improve solubility and processability and was dihydroxy-functionalized for covalent incorporation into various cross-linked PU polymer systems. Its electrical poling and relaxation behavior in PU polymer thin films were studied. First, the chromophore was incorporated into conventional TDI/TEA polyurethane, and an electrooptic (EO) coefficient of 57.6 pm/V at 1.06 μm was obtained, which is 28% higher than that obtained from nonhexylated chromophore (CLD-2) in the same polyurethane system. Two new polyurethane systems, poly[(phenyl isocyanate)-co-formaldehyde] (PPIF)/triethanolamine (TEA) and PPIF/bisphenyl-1,1‘-dimethanol (BPDM), were designed to study the influence of cross-link density and monomer rigidity on electrical field poling of chromophore dipoles and relaxation behavior of poling-induced chromophore alignment. CLD-5/PPIF/TEA polymer has the highest cross-link density (3.07 mmol/g) among all the polyurethanes studied here. It gains 38 °C in thermal stability but loses 50% of EO activity as compared with the CLD-5/TDI/TEA polyurethane material, which has a cross-link density of 1.91 mmol/g. A higher EO coefficient (41 pm/V), lower optical loss of 2.56 dB/cm at 1.3 μm, and the highest dynamic stability (133 °C) were obtained for the CLD-5/PPIF/BPDM polyurethane EO material, which has the lowest cross-link density (1.27 mmol/g) and the most rigid monomers. The results indicate that excessive cross-linking deteriorates electric poling of long chromophores in a cross-linked polymer thin film. Therefore, cross-linking by itself is not necessarily a good approach to achieving high thermal stability of electrical field-induced chromophore alignment in polyurethane systems.

Protein glycosylation in diabetes mellitus: biochemical and clinical considerations

Protein glycosylation in diabetes mellitus: biochemical and clinical considerations