浇注PBX炸药老化过程中交联密度与力学性能的关系

为分析浇注PBX炸药交联密度与力学性能的关系,在70℃时对浇注PBX炸药样品进行高温加速老化试验,对比分析了平衡溶胀法和核磁共振(NMR)法测定交联密度的差异,研究了其在常温时的抗拉强度(σm)、抗压强度(σb)、压缩率(εb)、抗剪强度(τ)、硬度(SH)与黏结剂母体凝胶分数(G)、交联密度(ve)之间的关系。用动态热机械分析仪(DMA)分析了不同老化时间下样品损耗因子tanδ和黏弹系数的变化规律。结果表明,PBX炸药样品的交联密度在老化初期增加,老化中期略有降低,老化后期又增加;NMR法因其测试简单、快捷,精确度高、试样非破坏性等优点,可以作为今后浇注PBX炸药交联密度表征方法的一个重要发展方向;样品老化过程中,浇注PBX炸药样品的交联密度与各力学性能的变化呈线性相关,其力学损耗降低的原因是黏结剂母体的G和v_e增加,浇注PBX炸药的降解和交联是由黏结剂母体结构变化引起。     

Nanoindentation of shape memory polymer networks

This work examines the small-scale deformation and thermally induced recovery behavior of shape memory polymer networks as a function of crosslinking structure. Copolymer shape memory materials based on diethylene glycol dimethacrylate and polyethylene glycol dimethacrylate with a molecular weight of 550 crosslinkers and a tert-butyl acrylate linear chain monomer were synthesized with varying weight percentages of crosslinker from 0 to 100%. Dynamic mechanical analysis is used to acquire the bulk thermomechanical properties of the polymers, including the glass transition temperature and the elastic modulus over a wide temperature range. Instrumented nanoindentation is used to examine ambient temperature deformation of the polymer networks below their glass transition temperature. The glassy modulus of the networks measured using nanoindentation is relatively constant as a function of crosslinking density, and consistent with values extracted from monotonic tensile tests. The ambient temperature hardness of the networks increases with increasing crosslinking density, while the dissipated energy during indentation decreases with increasing crosslinking density. The changes in hardness correlated with the changes in glass transition but not changes in the rubbery modulus, both of which can scale with a change in crosslink density. Temperature induced shape recovery of the indentations is studied using atomic force microscopy. For impressions placed at ambient temperature, the indent shape recovery profile shifts to higher temperatures as crosslink density and glass transition temperature increase.     

Mechanical and gas transport properties of poly-ϵ-caprolactone model networks

The glass transition temperature increases with increasing crosslink densities in model networks formed by endlinking poly-?-caprolactone with a triisocyanate crosslinking agent. In the noncrystalline networks, the gas permeability decreases with increasing crosslink density. These results are consistent with an interpretation that the crosslinks reduce the main-chain molecular motions which are important to these processes. At the lowest crosslink density, where poly-?-caprolactone networks are crystalline, the gas permeability is lower than would be expected based on the volume fraction of amorphous polymer. The excess reduction in permeability is attributed to crystallization-induced enrichment of crosslink junction points in the amorphous fraction of the network. This reduces the permeability by creating an artificially high crosslink density in those regions of the network responsible for gas transport. Since crosslinking increases the stiffness and reduces the flexibility of the network polymer chains, it affects large penetrants more strongly than small ones. Therefore, increasing the crosslink density proves to be a useful method for increasing gas separation factors.     

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.     

Thermal aging of bentonite‐filled ethylene propylene diene monomer composites

Bentonite‐filled ethylene propylene diene monomer (EPDM/Bt) composites were prepared using two roll mill compounding method and the effect of Bt loading on the thermal aging, swelling resistance and crosslink density of EPDM/Bt composites were studied. The effect of in situ addition of different silane coupling agents (SCAs) on the above properties at optimum Bt loading of EPDM/Bt composite was also investigated. Thermal aging test results show that the tensile strength and tensile modulus at 100% elongation (M100) increase initially for 2 days aged composites and decrease slightly after 4 days of aging, meanwhile the elongation at break (E_b) decrease gradually with aging period as compared to the unaged composites. Upon aging, swelling resistance increase initially indicating increased crosslink density of EPDM/Bt composite due to post‐curing and reduced after 4 days of aging due to crosslink destruction and EPDM chain scissioning. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4419–4427, 2013     

Stress–strain properties of polyurethane–polyacrylate interpenetrating polymer networks

Two-component interpenetrating polymer networks (IPN) of the SIN type (simultaneous interpenetrating networks) were prepared from two different polyurethanes (a polyester type and a polyether type) and a polyacrylate of two different crosslink densities. The linear polymers and prepolymers were combined in solution, together with crosslinking agents and catalysts, films cast, and subsequently chain extended and crosslinked in situ. In all cases, maxima in tensile strengths significantly higher than the tensile strengths of component networks occurred. This was explained by an increase in crosslink density due to interpenetration.     

Mechanical property changes and degradation during accelerated weathering of polyester-urethane coatings

Chemical changes, measured using spectrocopy, and crosslink density, measured by mechanical thermal analysis, were determined during accelerated weathering on a model polyester-urethane coating of known composition. The tensile modulus, measured above the glass transition temperature, and thus the crosslink density, decreased with exposure, as expected from the chemical changes. However, the tensile modulus, measured at room temperature, increased with exposure. Physical aging of the polymer network was found to occur concurrently with photodegradation and accounts for much of the increase in room temperature modulus. Increased hydrogen bonding in the increasingly oxidized polyester-urethane may also contribute to the increase in modulus at room temperature. Both physical and chemical changes must be determined if changes, and rates of change, in performance due to weathering are to be understood. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

Application of polyurethane/citric acid/silicone softener composite on cotton/polyester knitted fabric producing durable soft and smooth surface

Application of softeners on fabrics can usually increase the fabric pilling tendency and it is difficult to obtain a soft handle fabric without pilling during wearing. This research was conducted to use various chemicals to reduce pilling with reasonable softness on the cotton/polyester knitted fabric. Diverse composites of the water‐based polyurethane resin (PU), citric acid (CA) as a crosslinking agent and silicone‐based softener were selected and applied on the fabric through conventional pad‐dry‐cure method. The characteristics of the treated fabrics including pilling rate, pilling density, water droplet adsorption time, bending length, crease recovery angle, tensile strength, and water contact angle were examined and reported. Application of the polyurethane resin along with citric acid reduced the fabric pilling. However, co‐application of resin, CA, and softener improved the fabric crease recovery angle, bending length, and water droplet adsorption time. The preferred formulation was 20 g L^?1 CA, 25 g L^?1PU resin, and 20 g L^?1 silicone softener. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Change of network structure of natural rubber vulcanizate with thermal aging

The effect of sulfur accelerators during the curing process in natural rubber (NR) was investigated. The samples were prepared by adding the same fraction of various types of accelerators in the compound and all samples were cured at the same temperature but for two different times. Determination of the molecular mass between crosslinks immediately after vulcanization shows the crosslink density in the samples, which differs for different accelerators. After that, the same samples undergo thermal aging and the changes of the crosslink density were studied. During the first period of thermal aging, additional crosslinks were observed and then a period of deterioration of the network structure is following. From the obtained results of the molecular mass between crosslinks as well as from mechanical properties, conclusions can be drawn about the most suitable accelerator for use in certain conditions and applications. To predict the behavior of vulcanizates under various effects as heat and radiation, it is important to know more about the chemical structure of the network. The changes of crosslink density in the investigated samples were followed by equilibrium swelling and by mechanical measurements. IR spectroscopy was used to study the changes of the crosslink bridges.     

Correlation of mechanical property,crosslink density and thermogravimetric behavior of castor oil polyurethane–polystyrene divinyl benzene simultaneous IPN networks

Several simultaneous interpenetrating networks (IPN) of castor oil polyurethane (COPUN) and polystyrene divinyl benzene (PSN) were synthesized under conditions where the free radical polymerization of styrene and the crosslinking reaction of castor oil and toluene diisocyanate progress at comparable rates. Comparison of the mechanical properties and crosslink density of the COPUN and COPUN/PSN-IPNs indicates a marginal increase in tensile strength and crosslink density from COPUN to 60COPUN/40 PSN IPN. IPN samples prepared with further increased PSN content show steady decrease in the above properties. This reversal of the expected trend was attributed to the possible greater molecular interpenetration achieved due to similar gelation times with resultant extension of chains and increase in free volume between crosslinks. This was further confirmed from thermogravimetric data on the initial stages of decomposition of the IPNs.     

Sol fraction and swelling analysis of cis-1,4-polybutadiene networks produced by high-energy irradiation

Summary Soluble fraction and equilibrium swelling in toluene were investigated on cis-1,4-polybutadiene networks being crosslinked in vacuo by ⁶⁰Co-irradiation. Analysis of the sol fraction data by means of INOKUTI's equation reveals a drastic increase of the degradation-crosslink ratio for samples unpurified prior to crosslinking. Furthermore, the network crosslinking susceptibility depends on purification, although it is rather temperature insensitive for irradiation above the glass transition temperature of the rubber. The statistics of creation of crosslinks can be examined by suitable modification of INOKUTI's theory. From the present data it may be concluded that chain reaction crosslinking is negligible. The FLORY-REHNER formula for swelling equilibrium fails to reproduce the data at the cross link densities studied, but introducing a free end correction, the formula becomes applicable, with some over-correction. Hence the crosslink susceptibility could be obtained independently if the polymer-solvent heat interaction parameter were known. Additionally, the assumption about the proportionality of the crosslink density with radiation dose has been successfully tested by the swelling experiments.Financial support by DEUTSCHE FORSCHUNGSGEMEINSCHAFT and by MINISTERIUM FÜR FORSCHUNG UND TECHNOLOGIE is gratefully acknowledged. Dr. GOLDBACH from CHEMISCHE WERKE HÜLS AG we have to thank for the cis-1,4-polybutadiene.Herrn Professor Dr. K. überreiter zu seinem 70. Geburtstag gewidmet     

Fluorescence polarization and rheological studies of the poly(N-vinyl-2-pyrrolidone) hydrogels produced by UV radiation

Poly(N-vinyl-2-pyrrolidone) hydrogels produced by direct ultraviolet irradiation of PVP aqueous solution leads to crosslinking through pyrrolidinone moiety photolysis. Generally, hydrogel physical properties, like crosslinking density, pore size, swelling capacity, storage and loss moduli are obtained by swelling and rheological tests. However, relations between anisotropy obtained by fluorescence polarization studies and these properties have not been addressed for hydrogel systems. In this work we show that there is a correlation between the data obtained from anisotropy and rheological experiments, since both of them are related with crosslinking density of the hydrogels. These results reveal that fluorescence polarization spectroscopy is a promising tool for understanding the structure of hydrogels.     

Crosslinking of poly(vinyl chloride) with bismaleic compound. II. Rheological behavior and mechanical properties of crosslinked PVC

In this study the rheological behavior of crosslinking PVC was studied. We carefully probed the effects of styrene and initiators on crosslink course and further verified PVC's crosslink mechanism suggested previously. St as auxiliary crosslinker was very important for PVC crosslink, no matter whether at lower temperature or at higher temperature, in the absence of initiator or in the presence of initiator. With higher decay temperature cumvl hydroperoxide (CHP) was a very appropriate initiator for PVC crosslinking. By controlling technological conditions, the crosslinked PVC with different cross density was obtained. We found that PVC with THF-insoluble fraction of about 7% had the best tenacity and maximum breaking energy. The tensile strength would increase as the gel-fraction content increased. By citing T. Kurauchi's ROF toughening theory, we satisfactorily explained the good ductility of crosslinked PVC with low cross density and the high fragility of over-crosslinked PVC. Because the motion of segments is hindered in thoroughly crosslinked PVC, its glass transition was so weak that we hardly found out T_g.     

Influence of vinyl ester/styrene network structure on thermal and mechanical behavior

The influence of vinyl ester/styrene network structure on thermal and mechanical properties was investigated. The crosslink density of the resins was altered by changing the molecular weight of the vinyl ester oligomer and by varying the amount of styrene used during the crosslinking reaction leading to variations in both the physical network structure and the chemical composition of the polymeric networks. The glass transition temperatures of the network polymers were found to increase systematically with increasing crosslink density without the additional influence of the chemical composition as determined from both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The breadth of the glass transition regions increased with crosslink density for the DSC data, but the breadth assessed from the DMA data did not vary significantly for the network materials. A secondary relaxation was observed for the materials using DMA, and this relaxation did not appear to be significantly affected by changes in either the crosslink density or the composition of the network. Cooperativity studies involving time–temperature scaling of dynamic mechanical data in the glass formation temperature region were also conducted. The degree of segmental cooperativity at T_g appeared to be primarily influenced by the chemical composition of the networks. These issues dealing with the structure of the networks provided insight into the associated fracture properties in the glassy state (ambient temperature). Specifically, an empirically based linear correlation was found between the fracture toughness of the networks and the cooperative domain size at the glass transition temperature normalized by the crosslink density. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 917–927, 2001     

Rheological properties of silicon polymer networks: The influence of the crosslink density

This study was focused on the influence of the microstructural properties of a silicon polymer network on its rheological properties. Two commercial silicon oils were mixed in different ratios to form, by hydrosilylation, networks with different crosslink densities. The chemical compositions of the oils were determined by NMR, whereas their molecular weights and viscosities were studied with gel permeation chromatography and viscosimetry, respectively. The different networks were characterized through their crosslink densities. Afterward, the rheological properties were studied. The formulation notably influenced some characteristic values of the rheological behavior: the α‐transition temperature and the onset temperature of the caoutchoutic plateau shifted toward higher temperatures as the crosslink density increased, the storage modulus at the onset temperature of the caoutchoutic plateau increased with the crosslink density, and the amplitude of the peak associated with the α‐transition temperature decreased. These behaviors were explained as follows: as the crosslink density increased, a drastic decrease in the amount of free chains in the network occurred, and both phenomena induced a large decrease in the chain mobility, which might explain the aforementioned behavior. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1152–1160, 2003     

Thermal aging on mechanical properties and crosslinked network of natural rubber/zinc Dimethacrylate composites

This article dealt with the relationship between mechanical properties and crosslinked networks of natural rubber (NR) reinforced by zinc dimethacrylate (ZDMA) after thermal aging. After thermal aging at the present experimental conditions, the covalent crosslink density showed a decrease all the time, whereas the ionic crosslink density was stable at 80°C but decreased at a higher temperature. The decrease in the total crosslink density after aging indicates the degradation of the crosslinked network. However, an experimental phenomenon observed was that the tensile strength and tear strength increased in a certain degree after aging at 80°C or at a 100°C for a short time. In addition, the thermal stability of the NR/ZDMA composite was evaluated by thermal gravimetric analysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis and characterization of castor oil urethane triamine networks

Castor oil was polymerized with diisocyanate and crosslinked with primary triamine (Jeffamine T-403) to form networks. The effect of triamine as a crosslinking agent on rubbery castor oil urethane elastomer was determined by measuring network parameters such as average molecular weight between crosslinks (MC) number of polymer chains per unit volume (N), tensile strength, and modulus of the networks. The crosslinking density was varied by varying the ratio of NCO : NH₂ from 0.60 to 0.95. The results indicated the formation of highly crosslinked elastomers at all NCO : NH₂ ratios employed. The tensile stregth and modulus increased with increasing crosslink density up to a value of NCO : NH₂ 0.85 and after this there was no significant change, indicating the maximum limit of improvement attainble in terms of network characteristics.     

Investigation of crosslinking in the thermooxidative aging of nitrile–butadiene rubber

Chemical crosslinking is possibly the most significant factor affecting the mechanical behavior of rubbers. In this study, we investigated the evolution of network structures (the crosslinking degree and crosslinking density) during the thermooxidative aging of a nitrile–butadiene rubber (NBR) using characterization methods such as low‐dimensional NMR, solvent extraction, solvent swelling, IR spectroscopy, and mechanical property measurements. The NMR and solvent extraction results show the change of the crosslinking degree. The solvent swelling results show the change of the crosslinking density. The IR results show the chemical changes relating to crosslinking and chain scissions. Therefore, a comprehensive picture of the thermal oxidative aging of the NBR compound was drawn by the integration of various results from these methods. Crosslinking occurred throughout the aging process, whereas chain scissions took place and competed with crosslinking in the later stage. The crosslinking density increased at a nearly constant rate, whereas the increase in the crosslinking degree slowed down in the later stage. The crosslinking density was closely correlated with the hardness and Young's modulus. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41319.     

Aging behavior and mechanism of bio‐based engineering polyester elastomer nanocomposites

Bio‐based elastomers used in industry have attracted much attention. We prepared bio‐based engineering polyester elastomer (BEE) nanocomposites by mixing bio‐based engineering polyester elastomers with carbon (CB). The CB/BEE nanocomposites were exposed to an artificial weathering environment for different time periods. Both its aging behavior changes and aging mechanism were investigated in this article. The tensile strength retention rates were each above 90% after aging at 100°C and 125°C for 72 h. CB/BEE nanocomposites exhibited good anti‐aging properties. Furthermore, the chemical changes were detected by Fourier transform infrared spectroscopy and differential scanning calorimetry. The crosslink density changes during aging of BEE were determined as well. A plausible aging mechanism of BEE was proposed. It can be concluded that the thermal oxidation process gives priority to further crosslinking in the initial period of aging. As the aging time increases, chain scission becomes the dominant element in the subsequent thermal oxidation process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40862.     

Chemical crosslink densit and network structure of natural rubber vulcanizates modified with phosphorylated cardnol prepolymer

The chemical crosslink density (CLD) and network structure of natural rubber (NR) vulcanizates, modified with phosphorylated cardanol prepolymer (PCP), have been studied by equilibrium swelling and other chemical methods. The PCP-modified NR vulcanizates showed lower CLD, as compared to the unmodified NR samples, the decrease being lesser for the semiefficient vulcanization (SEV) system, as compared to the conventional (CV) and efficient (EV) vulcanization system. The superior tensile characteristics of the PCP-modified vulcanizates of the SEV system is presumed to be partly due to the presence of an entangled network structure between the aliphatic segment of PCP and the isoprene chains, as evident from X-ray diffraction studies. The critical role of Zn⁺⁺ions in the crosslinking reactions, especially at higher concentrations of PCP, was evident from the increase in CLD at higher concentrations of ZnO. The reduction in the IR absorption intensity, in the presence of ZnO, indicated the probable complex formation of Zn⁺⁺ions with the phosphate groups of PCP. © 1994 John Wiley & Sons, Inc.