Porosimetrie gequollener makroretikularer Ionenaustauscher

The relative micro and macro pore volumes and the macro pore size distribution were used to characterize the pore structure of ion exchangers. The evaluation of these from density measurements and mercury penetration measurements was described for a sulphonated macro reticular ion exchange resin in the swollen state with different contents of swelling agent. On taking up water the dry volume of the exchange resin increased (swelling) until a water content of about 33 wt.-% was reached. Additional water filled up the macro pores to a final content of 58 wt.-%. With the specific volume of the exchange resin in the swollen state in addition to the specific volume of the gel phase, the relative macro pore volume of the fully swollen state was evaluated. The results of the mercury penetration method agreed with those of the density measurements taking into account the compressibility of the mercury itself and those of the swelling agent and the matrix. Corresponding correction factors could be evaluated from measurements on micro reticular ion exchange resins. The mercury penetration measurements provided additional information on the macro pore size distribution of the swollen state. For the examined ion exchange resins a logarithmic standard distribution with an arithmetically weighed mean pore radius of about 1 100 Å was evaluated. The relative over-all pore volume of 0,65 was not identical with the value of 0,61 for the partition coefficient of ethylene glycol from dilute aqueous solutions, as only that part of the water, which is bound to the sulphonic groups (hydrated water), could be exchanged by ethylene glycol. Pure ethylene glycol substituted the hydrated water completely. The specific volume of the exchange resin thereby decreased in comparison to that of the water-swollen state.     

Effects of curing program on mechanical behavior and water absorption of DGEBA/TETa epoxy network

Water uptake in organic coatings remains an interesting challenge for fundamental and applied researches because chemical, physical, and mechanical properties are concerned. The polymer network, which is affected by the curing program, is a key factor for water absorption. In this work, an epoxy network based on diglycidyl ether of bisphenol A and a hardener triethylentetramine was cured at different temperatures: below T_g (protocol 1) and above T_g (protocol 2). DMA, Differential Scanning Calorimetry (DSC), and FT‐IR measurements showed that both protocols allow to obtain totally cured networks. However, DSC and DMA results revealed that both cured networks present different levels of homogeneity, depending on the different curing conditions, which affect the free volume and the activation volume associated with visco‐elastic properties. The mechanical properties of free films and water sorption behaviors were investigated as function of cured conditions. It was found that protocol 1‐cured networks present higher mechanical properties and was less affected by water ingress than protocol 2‐cured systems, leading to better barrier properties. These results highlight the influence of the curing program onto the heterogeneous distribution of the epoxy network. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Physical aging of polydimethylsiloxane-modified epoxy resin

The thermal expansion coefficient and physical aging behavior of cured epoxy resins modified with amino-terminated polydimethylsiloxane were investigated. The epoxy resin was tetramethyl biphenol diglycidyl ether. Two factors influenced the thermal expansion behavior and the free volume relaxation of the polysiloxane-modified epoxy systems. One was the miscibility between the epoxy resin and the polysiloxane and the flexibility of the chains in the network system. The intrinsic thermal expansion of the network chains and the volume relaxation increased as a result of building polysiloxane molecules into the network structure. The other factor was the size and concentration of the dispersed polysiloxane particles. The increased local free volume at the interface between the epoxy matrix and the polysiloxane particle resulted in a higher thermal expansion coefficient and led to a large driving force for the volume relaxation during annealing. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1291–1300, 1998     

Effects of cross-linked networks on dielectric properties of epoxy resins based on molecular dynamics

In this paper, three epoxy resin systems commonly used in power equipment are prepared to obtain cross-linked networks with structural differences. The relationship between microscopic structures and dielectric properties of epoxy resins is investigated and discussed. Experimental results show that the polarization and conductance losses are inhibited in the anhydride-cured systems with methyl groups. The molecular dynamics (MD) simulation shows that rigid anhydride structures (such as methyl groups) play a major role in reducing the local segment mobility and increasing the free volume at cross-linking points. The decrease in local segment mobility has been confirmed by the decrease of mean square displacement (MSD) at the cross-linking points, which is consistent with the change of measured polarization and conductance loss. In the glassy state, the dielectric properties of different anhydride-cured systems can be reflected by local MSD at the cross-linking points. At high temperatures near the glass transition temperature, both the free volume and network mobility increase significantly, which reflects the increased dielectric relaxation strength and conductance loss. The understanding of the structure–property relationships could provide a theoretical foundation for epoxy modification in a controlled manner for power equipment applications.     

Network structure and properties of imidazole-cured epoxy novolac adhesives

The structural characteristics of four epoxy adhesives, obtained by crosslinking an epoxy novolac with various levels of a substituted imidazole curing agent, were investigated and correlated with thermal and mechanical properties. Variations in network structure were characterized by measuring crosslink densities and by qualitatively assessing glassy state free volume from densities and coefficients of thermal expansion. Differential scanning calorimetry was used to obtain glass transition temperatures, and dynamic mechanical thermal analysis was used to follow primary (alpha) and secondary (beta) transitions. Bulk behavior was characterized by tensile modulus, strength, and toughness, together with compressive modulus and yield strength. The effect of sub-T_g aging on compressive yield strength was investigated as well. As the level of imidazole increased, crosslink density, and hence network packing efficiency and free volume, decreased. For fully cured networks, both the glass and the alpha transition temperatures increased with crosslink density. Calculated activation enthalpies and entropies indicated significant degrees of network cooperativity in the alpha transitions, particularly for the more highly crosslinked systems. Beta transition temperatures, however, were found to be independent of crosslink density. Bulk properties generally showed a dependence both on crosslink density and free volume. Yield stress, for example, was highest for the network with lowest crosslink density and free volume. Volume relaxation associated with physical aging also caused yield stress to increase.     

Additives for improving the strength,stiffness, and ductility of epoxy resins

A range of additives at 7 to 20 wt% loading can increase the stiffness and strength of crosslinked epoxy resins by up to 60%, yet the tensile specimens also fail in a ductile fashion. In additive formulations where little chemical bonding occurs between the additive and the epoxy matrix (e.g., the reaction product of 4-hydroxyacetanilide and 1,2-epoxy-3-phenoxypropane), the increase in modulus is directly related to the decrease in free volume available for segmental mobility. The increase in strength results from a combination of the increased modulus and an increased fracture energy, A ductile mode of failure occurs because the cured plastic exhibits a large increase in free volume on straining (low Poinsson's ratio). The strain-induced increase in free volume effectively takes the sample through the glass to rubber transition. In more highly reactive formulations (e.g., the reaction product of 4-hydroxyacetanilide and vinyl cyclohexene dioxide), chemical bonding effects complicate this simple free volume interpretation, but the occurrence of a ductile failure mode is again related to the free volume increase with strain.     

环氧树脂水性化反应中新型溶剂的研究

从分子设计的角度出发,选用E-44型环氧树脂为母体,2-丙烯酰胺基-2-甲基丙磺酸(AMPS)为改性剂,通过自由基接枝聚合反应在环氧树脂分子中引入亲水性的分子链段,制得了具有自乳化功能的环氧树脂体系;研究了乙二醇丁醚、正丁醇、丙酮、乙醇、甲醇和蒸馏水,以及不同配比的正丁醇和丙酮的混和溶剂分别作溶剂对反应效果的影响;并用红外光谱和激光粒度分析仪对改性产物的结构和乳液粒径进行了表征。结果表明,以丙酮与正丁醇的混和溶剂溶解环氧树脂可以使环氧树脂得到良好水溶性的同时保留相当数量的环氧基,产物收率最高可达到98%,完全符合分子设计的预期结果。     

Effect of grafting on phase volume fraction,composition, and mechanical behavior: Epoxy/poly (n-butyl acrylate) simultaneous interpenetrating networks

Grafted simultaneous interpenetrating polymer networks (SINs) were prepared from Epon 828 epoxy resin and n-butyl acrylate monomer. The amount of grafting monomer (glycidyl methacrylate) was found to effect profound changes in the morphology and mechanical behavior of these materials. While the size of the dispersed rubbery phase increased from approximately 2 microns to 20 microns, the number of domains decreased, with increasing amounts of grafting agent. The total dispersed phase volume decreased with increased grafting. At the highest level of grafting, the two-phase morphology disappeared, and only one phase was observed. With increased grafting, dynamic mechanical spectroscopy showed a movement of the loss modulus peaks toward each other, confirming an increase in compatibility in the system and showing that the compositions in each phase were becoming more alike. The SIN with the most glycidyl methacrylate (3.0 percent) showed only one peak in the loss modulus curve, supporting the single phase morphology found through microscopy. At the point of compatibility between the two networks, the SIN supermolecular structure may be visualized as becoming one complex network, where the number of grafts between the two polymer chains outnumbers the number of homopolymer crosslinks. The chemical grafts were also shown to significantly alter the free energy of mixing of the two polymers. A grafting level-composition phase diagram showed that at well defined levels of grafting the free energy of mixing goes from a positive value to a negative value.     

Effects of chain ends and molecular weight on specific volumes of anionic polystyrene melts

Specific volumes of anionic polystyrenes prepared by butyl lithium initiation have been measured at temperatures between 110° and 237°C. Molecular weights of the fractions studied ranged from 2500 to 700 000. Plots of isothermal specific volume against reciprocal molecular weight (M^?1) are fitted by two straight lines intersecting at a molecular weight near 10 000. This parallels the behaviour of thermally initiated polystyrene fractions, which have different end-groups. Volumes attributable to end-groups are greater for the anionic polymers. The specific volume-molecular weight-temperature relations in the M ≤ 10 000 region are explained quite well by the equation of Francois and coworkers, in terms of end-group volumes and a uniform volume contribution from internal repeating units. Restraints on the motion of the butyl end-groups in these polymers by the attached polystyrene chain appear to be temperature dependent in terms of this model. Specific volumes of polymers with M ≥ 10 000 are affected by chain ends and by variation of the volumes of non-end repeating units with molecular weight. The present results parallel those which have been reported for partial specific volumes of polystyrenes in dilute solution and suggest that the same factors operate in both cases.     

Nanohole volume dependence on the cure schedule in epoxy thermosetting networks: A PALS study

A systematic study on the dependence of the volumes at nanoscale in epoxy systems cured with two selected aminic hardeners at different pre-cure temperatures is presented. Nanohole volumes were measured by positron annihilation lifetime spectroscopy. Additional information regarding the structure of the thermosets was obtained using dynamic mechanical analysis. Volume results obtained are discussed in terms of the cure schedule applied to the epoxy systems, their characteristic glass transition temperatures and their crosslink density. The pre-cure temperature and the structure of the hardeners govern the packing of the molecular chains of the epoxy network. Using together positron and mechanical experimental techniques allows to conclude that a strong change in the volume and number density of the nanoholes takes place when the pre-cure temperature crosses the glass transition temperature of the systems.     

A study by Raman, near-infrared and dynamic-mechanical spectroscopies on the curing behaviour, molecular structure and viscoelastic properties of epoxy/anhydride networks

The curing characteristics of a TGDDM/HHPA formulation have been investigated by Raman spectroscopy, which allowed us to monitor the evolution of the different reactive species (i.e. epoxy, anhydride and ester groups) participating in the curing process. The curing mechanism and, in particular, the role of side processes, were elucidated. NIR spectroscopy was employed to investigate the post-curing process, in view of the superior sensitivity of this technique for monitoring polar groups. Quantitative methods were developed to measure residual concentration of epoxy groups in the high conversion regimes (≥98%). Dynamic-mechanical measurements were performed to gather information on the molecular structure and viscoelastic properties of the investigated networks. For formulation rich in epoxy resin, clear evidence of an inhomogeneous phase structure was found. A viscoelastic analysis in terms of the WLF approach demonstrated that both the free volume and the thermal expansion coefficient of the networks decrease by enhancing the anhydride/epoxy molar ratio.     

Application of positron annihilation: Study of pervaporation dense membranes

Separation of liquid mixtures, particularly azeotropic mixtures, is being tried out by pervaporation process, which utilizes dense membranes. Such membranes, although are of non-porous type, still requires determination of free volume sizes which is crucial to the understanding of the process. Positron annihilation technique has been developed into a powerful characterization tool for the study of free volume and free volume fraction in polymers. In the present work, an attempt has been made to estimate the free volume sizes of commercial and laboratory made pervaporation membranes with the application of positron annihilation lifetime spectroscopy. The positron lifetime spectra were analyzed, assuming 3 or 4 exponential components. Long-lived components (lifetimes: in the range of 1.4-3 ns) were found, which were attributed to ortho-positronium (o-Ps) pick-off annihilations in free volumes. Accordingly, free volume size determinations were carried out, considering shapes for free volumes as spherical as well as cylindrical. Further, utilizing such free volume data, results of hydrazine hydrate separation by pervaporation were interpreted leading to a better understanding of the process.     

环氧树脂/纳米SiO2复合材料摩擦学性能与正电子湮没谱的研究

利用超声波和偶联剂处理的方法，将不同比例的纳米SiO2与环氧树脂相复合，制备了环氧树脂／纳米SiO2复合材料并测定了其显微硬度。用摩擦磨损试验机评价了试样在干摩擦条件下的摩擦学性能，用正电子湮没寿命技术(PALT)测试了试样的微观结构。结果表明，纳米SiO2可有效地改善环氧树脂的摩擦性能；摩擦后试样的自由体积孔穴尺寸比摩擦前大，随着SiO2含量的增加，自由体积孔穴尺寸呈增大趋势，而自由体积孔穴浓度呈减少趋势。     

Effect of plasticizer on the thermal,mechanical, and anticorrosion properties of an epoxy primer

The influence of the glass transition temperature and the mechanical and anticorrosion properties of factors such as the amount of plasticizer added to an epoxy primer were investigated by DSC (differential scanning calorimeter), DMA (dynamic mechanical analysis), stress-strain tests, salt fog spray tests, accelerated tests, and electrochemical tests. The addition of plasticizer results in a decrease in the glass transition temperature and a change in the mechanical properties. Different tests were carried out to the optimum percentage of plasticizer content (1.5–3% weight ratio to epoxy resin) required to obtain the maximum anticorrosion performance of the epoxy primer. These changes are explained by the structural-kinetic effect exerted by the plasticizer on the chemical crosslinking in the course of the epoxy network synthesis and the increase in the excess free volume.     

Epoxy and hyperbranched polymer blends: Morphology and free volume

In this work, the phase separation of an epoxy‐functionalized hyperbranched polymer (HBP) in a blend with a conventional epoxy resin is examined. Morphology development with the advancement of curing reaction was investigated by hot‐stage polarized optical microscope, where it was found that HBP is miscible in epoxy resin solvent at 120°C and undergoes phase separation during the curing reaction, leading to a two‐phase microstructure which maintains a dispersed morphology up to 20 wt % HBP. The degree of phase separation and morphology were also investigated using differential scanning calorimetry, and the resultant microstructure was confirmed by atomic force microscopy. The epoxy/HBP blends were characterized by positron annihilation lifetime spectroscopy for their free volume characteristics where behavior typical of miscible blends was seen, likely due to chemical bonding between the two phases. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structure-property relations of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies

The relations between the chemical and physical network structure, the deformation and failure processes and the tensile mechanical properties of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies are reported for a series of epoxy glasses prepared from a range of polyethertriamine concentrations. Near-infra-red spectroscopy indicates that these glasses form exclusively from epoxide-amine addition reactions. Their T_g exhibits a maximum and swell ratio a minimum at the highest crosslink density. Stress-birefringence studies reveal that these highly crosslinked glasses are ductile and undergo necking and plastic deformation. The plastic deformation initially occurs homogeneously but ultimately becomes inhomogeneous and shear bands develop. Tensile failure occurs in the high strain shear band region. The ultimate tensile strain of these epoxies attains a maximum of 15% for the highest crosslinked glass. Off stoichiometric networks fail at lower strains because such networks inherently contain more defects in the form of unreacted ends. The density, yield stress, tensile strength, and modulus of these glasses all decrease with increasing polyethertriamine concentration as a result of increasing free volume because of the poor packing ability of the amine molecule. A slight minimum is superimposed on this downtrend in density and modulus with increasing amine content at the highest crosslink density because of geometric constraints imposed on segmental packing by the network crosslinks. The ability of these crosslinked glasses to undergo deformation is discussed in terms of the free volume and the crosslinked network topography. Network failure is considered in terms of stress-induced chain scission which is determined by the concentration ad extensibility of the least extensible network segments.     

Solvent‐free acetylation of lignocellulosic fibers at room temperature: Effect on fiber structure and surface properties

Acetylation is one of the most interesting chemical treatments to improve the affinity of lignocellulosic fibers with polymeric matrices for the elaboration of several types of composites. In this paper, the acetylation of flax and wood pulp (bleached softwood Kraft pulp and thermomechanical pulp) fibers was carried out at room temperature in a solvent‐free system with acetic anhydride in the presence of sulfuric acid as catalyst. The effect of acetylation on the fine structure of fibers was investigated by spectroscopic methods, while the extent of acetylation was quantified by weight percent gain. The effect of reaction time on fiber morphology was studied at macro‐ and microscale using scanning electron microscopy, optical microscopy, and fiber quality analysis. The evolution of the hydrophobic/hydrophilic character of fibers was determined by contact angle measurements. The wettability of fibers by liquid epoxy resin was also evaluated to confirm the improvement of the affinity of acetylated fibers with the epoxy matrix. It was found that the hydrophilic character of fibers decrease with increasing reaction time, whereas the trend was less pronounced beyond specific reaction times. Acetylated fibers can therefore be potential candidates for replacing nonbiodegradable reinforcing materials in composite applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42247.     

Selection of barrier materials from molecular structure

A prediction technique for gas permeability from polymer structure has been developed on the basis of a specific free volume diffusion theory. In this theory, the free volume available per unit mass in a polymer structure controls the rate of gas diffusion and, hence, its rate of permeation. The smaller this specific free volume is, the more difficult the gas diffusion and, thus, the better its barrier to gases becomes. Specifically, the theory predicts a linear relationship between log (permeability) and (?1/specific volume). A number of existing polymers covering six orders of magnitude in CO₂ permeability and O₂ permeability were found to follow this correlation. The specific free volume in a polymer was obtained from group contribution calculations. As a result, the gas permeabilities become predictable from the specific volume in a polymer which, in turn, varies with its molecular structure. The advent of this specific free volume theory for gas permeation simplifies greatly the selection of barrier materials for packaging applications. For a given barrier application, a critical specific free volume is first defined from its gas barrier requirement. The polymer structures having specific free volumes smaller than the critical value are then identified. These are the polymers that would have the necessary barrier performance. By this theory, molecular structures, with string polar-to-polar interactions and hydrogen-bonding forces are found to be good barriers to CO₂ and O₂.     

Effect of adding feldspar on free volume properties of crosslinked polyester studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PAL) was applied to study the feldspar effect on the free volume properties of crosslinked polyester based on neopentyl glycol, succinic acid, phthalic anhydride, and maleic anhydride. The measurements have been carried on the polyester resin samples cured with three crosslinking agents namely styrene (SS) or styrene/methyl methacrylate (SM) or styrene/acrylonitrile (SA) comonomers mixtures in the ratios of 2 : 1 and loaded with different concentrations of feldspar in the range from 0 to 80 wt %. The free volume parameters (size and fractions of holes) depend on type of the crosslinking agent as well as the feldspar content added to the polyester. The results are supported by a significant variation in the nanoscale free volume hole size distributions. Moreover, the correlation between positron annihilation parameters and electrical parameters was discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012