Effect Of Crosslinked Structure Research Articles

Changes in crosslinking structure of thermo‐oxidative aging chloroprene rubber and its effect on strain‐induced crystallization

AbstractChloroprene rubber (CR) is frequently quoted as an example of strain‐induced crystallizing elastomer. Many researchers have focused on studying the aging mechanism and mechanical properties of aged CR, little has been done on the effect of crosslinking structure on strain‐induced crystallization (SIC). The present paper related systematic investigations combining the changes of crosslinking structure and its influence on SIC. It was firstly investigated based on FTIR, crosslinking density test and GPC. The results showed that the molecular chains were oxidized in the early stage of aging, but the crosslinking reaction still dominated. At the same time, sol content and polydispersity decreased. In the later stage of aging, CR was oxidized, resulted in severe molecular chains fracture and an increase in the free sol content and polydispersity. The impact of aging on SIC under tensile condition was further investigated by the tensile tests. An obvious stress‐hardening was found for unaged CR, but not in aged CR. The crystallinity and microcrystalline region orientation tests showed that oxidation, crosslinking by double bonds and dechlorination during aging were not conducive to the orientation of molecular chains during stretching. It is the aging process that has an adverse effect on the SIC of CR.

The effect of cross-linker structure on interfacial interactions, polymer dynamics and network composition in an epoxy-amine resin

Understanding interactions at the polymer / metal oxide interface is central to improving the performance lifetime of corrosion resistant coatings, where network polymers commonly form via step growth mechanisms in the presence of pigments. Here we employ a holistic analytical approach encompassing ATR-FTIR, DSC and molecular dynamics simulations to consider how crosslinker structure affects adsorption and incorporation into the network, using a stoichiometric mixture of diglycidylether of bisphenol-A (DGEBA) with m-xylylenediamine (MXDA) cured in the presence of hematite (Fe2O3) and goethite (FeOOH) powders. We find that the rigid MXDA molecule has two distinct binding modes on both hematite and goethite, and that synergistic hydrogen bonding modes observed on goethite limit interconversion between the two. Moreover, we find that binding persists in fully cured composite samples, determining the levels of residual amine. In contrast to previously reported results using triethylenetetramine (TETA) crosslinkers, however, we find that the Tg of composite specimens is independent of added hematite and goethite volumes. Molecular dynamics simulations demonstrate this is due to electrostatic binding between the cationic Fe sites and electronegative heteroatoms in MXDA. This renders both amine functionalities unavailable for incorporation into the network and hence, unlike TETA, MXDA adsorption does not determine polymer dynamics.

High sulfur content polymers: The effect of crosslinker structure on inverse vulcanization.

The discovery of inverse vulcanization has allowed polymers to be made using elemental sulfur as the major component. However, until now, there has been little discussion of why seemingly similar crosslinkers result in polymers with radically different properties. Combining synthesis, spectroscopy, and modeling, this study reveals the structure-property relationships of sulfur polymers and reports a new system using 5-ethylidene-2-norbornene as a crosslinker that can stabilize up to 90 wt% of elemental sulfur.

Effect of crosslinking agents on the dispersive behaviour of polymer particles in seed swelling polymerisation

Abstract This study focused on the effect of crosslinker structure between PMMA on the dispersity of crosslinked particles in seed swelling polymerisation. We used five different crosslinking agents in an assessment of the dispersity of crosslinking monodisperse particles using PMMA seed particles in seed swelling polymerisation. These results show that the structure of the crosslinking agent affects the dispersity during the swelling stage. According to the solubility parameter theory of computation, the departure of similar solubility parameter causes the swollen particles to segregate into more dispersity.

The effect of crosslinker structure upon the rate of hydroperoxide formation in dried, crosslinked poly(vinylpyrrolidone)

AbstractThe hydroperoxide levels of two commercial types of crosslinked poly(vinylpolypyrrolidinone) (or crospovidone) disintegrants were studied as a function of time and temperature using the titanyl sulfate titration method. It was noted that the two crospovidone types exhibited significantly different rates of hydroperoxide buildup and that this difference increases with increasing temperature. While the chemical and physical structures of these commercial types are essentially the same, they are known to be different with respect to their crosslinkers which represent nominally 2% of the total mass. These crosslinkers are 3‐ethylidene‐N‐vinyl‐2‐pyrrolidinone and N,N′‐divinylimidazolidinone. While both contain lactam type rings, the latter possesses two nitrogen atoms versus one in the former. It was hypothesized that this difference in the number of heteroatoms results in a greater susceptibility to oxidative attack at carbon positions vicinal to these nitrogen atoms. To test this hypothesis, model compounds representing the two crosslinker ring structures were subjected to oxidative attack and analyzed for hydroperoxide buildup over time and at several temperatures. The results of the model compound study were consistent with the observations made for hydroperoxide buildup in the corresponding crospovidone types thereby supporting this hypothesis. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Effect of crosslinking structure on sorption of CO2 in photocrosslinked PVCA film

The sorption behavior of CO2 gas in photocrosslinked poly(vinyl cinnamate) film was examined under atmospheric pressure. The sorption isotherm was well described by the Langmuir equation, suggesting that sorption of CO2 is mainly governed by adsorption in the microvoids. The amount of sorbed CO2 was significantly affected by the degree of crosslinking. The CO2 sorption was enhanced at a lower degree of crosslinking but was decreased at a higher degree of crosslinking. The unexpected increase in the amount of adsorbed CO2 correlated with the increase in the number of microvoids that occurred as a result of the crosslinking reaction. However, further crosslinking led to a decrease in the mean size of the microvoids. The smaller microvoids, in comparison to CO2 molecules, did not act as adsorption sites, so that the amount of sorbed CO2 decreased. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1744–1750, 2000

Effect of Crosslink Structure on Properties of Natural Rubber

Abstract Natural rubber-sulfur-diphenylguanidine vulcanizates were allowed to react with triphenylphosphine under nitrogen to convert polysulfidic crosslinks principally to di- and monosulfidic crosslinks and to assess the effect of this change in the chemical nature of the crosslinks on tensile strength, flex life, and oxygen absorption characteristics. The vulcanizates were analyzed for organically combined sulfur and polysulfidic sulfur. Data obtained by this approach were treated statistically and showed that polysulfidic crosslinks are not essential for attaining high tensile strength or superior flex life. In another approach, polysulfidic crosslinks were introduced into dicumyl peroxide-cured vulcanizates containing carbon-carbon crosslinks to yield two-stage vulcanizates. Data for these vulcanizates fitted the curve of tensile strength as a function of 300% modulus for the precursor samples. It was further shown that the reduced failure envelope for natural rubber gum vulcanizates is independent of the type and relative concentration of chemical crosslinks. An interesting observation was made that the tensile strengths of dicumyl peroxide-cured samples were enhanced on extraction with acetone. Vulcanizates which had lost 95–99 per cent of their polysulfidic sulfur due to reaction with triphenylphosphine exhibited significantly lower rates of oxygen absorption at 100° C as compared to the untreated vulcanizates. This suggests that polysulfidic structures in the original vulcanizates act as oxidation initiators.

The Effect of Crosslink Structure on the Tensile Strength and Oxygen Absorption Characteristics of Natural Rubber Vulcanizates

Abstract The chemical nature of crosslinks in natural rubber—sulfur—diphenylguanidine vulcanizates was modified by reaction with triphenylphosphine under nitrogen to determine the effect of this change on tensile strength and oxygen absorption characteristics of the vulcanizates. The vulcanizates were characterized by organically combined sulfur and polysulfidic sulfur. Polysulfidic crosslinks, i.e., crosslinks containing three or more sulfur atoms in the crosslink, were found not to be essential for the attainment of high tensile strength in these vulcanizates. Data for the samples which had lost significant amounts of polysulfidic crosslinks by reaction with triphenylphosphine fitted the tensile strength versus 300% modulus curve for the control samples. Vulcanizates which had been reacted with triphenylphosphine for 16 days at 80° C, had lost 95–99.6% of their polysulfidic sulfur. These triphenylphosphine-reacted vulcanizates exhibited significantly lower rates of oxygen absorption at 100° C as compared to the untreated vulcanizates. This suggests that polysulfidic structures in the original vulcanizates act as oxidation initiators.