Composites based on carbon fibers and liquid crystalline epoxy resins, 1 Monomer synthesis and matrix curing

The synthesis and physical characterization of a novel liquid crystalline epoxy resin, used as a matrix for carbon fiber‐reinforced composites is presented in this paper. The curing reaction was monitored by means of calorimetric and rheological measurements. Calorimetric analysis indicates that the presence of carbon fibers does not affect the reaction rate. A conventional isotropic epoxy resin is used as a model compound in the rheological analysis. According to the patent literature, two different formulations of the model compound were used, characterized by a stoichiometric ratio of epoxy and an epoxy excess, respectively, with respect to the curing agent.     

Composites based on carbon fibers and liquid crystalline epoxy resins, 2 Dynamic‐mechanical analysis and fracture toughness behavior

The dynamic‐mechanical characterization and fracture resistance of composites, prepared by using a liquid crystalline epoxy resin and continuous carbon fibers, are presented and compared with conventional composites. In the case of the LC matrix, a higher T_g was found; similarly, a superior fracture resistance, higher elastic modulus and higher T_g were found for the LC‐based composites. These results allow to consider these materials as potential candidates for advanced composites.     

The effect of curing modes on polymerization contraction stress of a dual cured composite

Although a lower curing rate is often cited as the reason why a chemical cured (CC) dental composite produces lower polymerization contraction stress (PCS) than a light cured (LC) composite, the exact mechanism is still unclear. In addition, the comparison is often made by using different brands of composites. The comparison's fairness is questionable because the two composites have different compositions and preparation procedures. The goal of the present work was to determine if the curing mode alone can produce different PCS. We formulated a dual cured composite and prepared it the same way for both CC and LC modes. We measured PCS by a strain gauge method, shrinkage by a video-imagining technique, degree of conversion (DC) by infrared spectroscopy, and flexural modulus by the three-point bending test. The CC specimens showed lower PCS and lower flexural modulus than the LC specimens, although both possessed an identical chemical composition and physical texture before cure. This finding indicates that the curing mode alone can affect PCS. Because the CC and LC specimens produced a similar shrinkage and DC, the lower modulus is considered to be one of the reasons for the lower stress. Using a structural inhomogeneity model, we explained how a resin composite with an identical DC can have different physical properties such as the modulus.     

γ-ray polymerization of mesogenic diacrylates in the crystalline,liquid crystalline and liquid state — a kinetic study

A kinetic study of the γ-ray polymerization of three mesogenic diacrylates, 4,4′-bis(3-acryloyloxypropoxy)biphenyl (DAPB), 4,4′-bis(6-acryloyloxyhexyloxy)biphenyl (DAHB) and 4,4′-bis(11-acryloyloxyundecyloxy)biphenyl (DAUB), is presented. All compounds were studied on their polymerizability in the crystalline state. It was found that they can be completely polymerized though the reaction rates are very different and increase with increasing length of the alkylene spacer unit. Up to moderate conversion, solid-state polymerization is comparable with that of acrylamide and follows a t² rate law. DAPB was chosen for a detailed kinetic study of the in-source and post-polymerization in the crystalline, liquid crystalline and liquid phase. In-source polymerization is complete in the various phases though reaction rates and curve shapes of dose vs. conversion curves are very different. Possible origins such as enhanced molecular diffusion at elevated temperatures are discussed. X-ray studies indicate that a well-ordered polymer can be obtained from polymerization in a smectic phase, while polymers obtained from a crystaline or melt phase are considerably less ordered. Post-polymerization was studied as a function of the radiation dose and the temperature of the subsequent annealing process. It was found that the limiting conversion increases with increasing radiation dose and temperature of the annealing process, but the polymerization is always incomplete. Even post-polymerization in the melt phase does not exceed 55 percent conversion. The post-polymerization behaviour is comparable with that of acrylamide and can be described by a linear relation between conversion and annealing time t.     

γ-ray polymerization of a mesogenic methacrylate in the crystalline,liquid crystalline and liquid state: kinetics,molecular weight distribution and conversion-dependent phase behaviour

The γ-ray polymerization of mesogenic 4-methoxy-4′-(6-methacryloyloxyhexyloxy)biphenyl ( 1 ) was investigated. 1 can be polymerized in the crystalline, in the liquid crystalline and in the molten state. The rate of polymerization increases from the crystalline to the melt phase. In order to reach 90% conversion to polymer in melt, smectic and crystalline phase, radiation doses of 10, 40 and more than 120 kGy are necessary. Melt polymerization leads to a weight-average molecular weight of about 1 × 10⁶ independent of the conversion to polymer. In the smectic and crystalline phase the corresponding molecular weights are about 3.8 × 10⁵ and 4.1 × 10⁴ at low conversion, and shifted to 2.6 × 10⁵ and 3.9 × 10⁴ at high conversion. The polydispersity generally increases with the γ-ray dose and the reaction temperature. The origin of the different reaction rates and molecular weight obtained in the various phases is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length. Differential scanning calorimetry (DSC) of partially polymerized samples was used to describe the conversion-dependent phase behaviour. Optical microscopy showed that the melt polymerization at 78.5°C proceeds under phase separation into a liquid, monomer-rich and a solid, polymer-rich phase. As a consequence, liquid monomer samples solidify upon γ-irradiation once a higher conversion to polymer is reached.     

The effect of stress state and surface condition on the corrosion behavior of surgical implant alloys

Annals of Biomedical Engineering -     

Thermal phase behavior of homologous liquid crystalline side group polyesters in dependence on the polymethylene main chain spacer length

With the aim to investigate the influence of a systematic variation of the length of flexible non-mesogenic main chain segments on the mesomorphic properties a series of new liquid crystalline polyesters with 4-(4-nitrobenzyloxy)biphenyl moieties as mesogenic side groups was prepared. The mesomorphic and dynamic behavior were investigated by means of differential scanning calorimetry (DSC), X-ray measurements, polarization microscopy and dielectric spectroscopy. All polyesters exhibit nematic and different smectic phases. Some of the homologues show “re-entrant” nematic phases. The clearing point decreases at first with increasing main chain spacer length and passes through a minimum. From the results it was possible to propose packing models of the smectic phases with low order.     

The order effect: reflections on the validity of multiple test presentations.

The effect of order has been previously addressed in test-retest studies but its effect on the presentation of more than one test, in the same time, is often underestimated. This study has shown that when two rating scales were given sequentially, each scored lower if received second in the pair, and that this particular form of order effect (occasion effect) was enough to affect markedly agreement on caseness when utilizing standard cut-off scores. It is therefore important that research workers not only adopt design strategies such as counterbalanced presentation, but also analyse their data for the effects of test order and position within such designs.     

Rate effects on the microindentation-based mechanical properties of oxidized, crosslinked, and highly crystalline ultrahigh-molecular-weight polyethylene

The surface micromechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE) are critical in determining the wear, deformation, and fracture in the surface region. These properties have not been accessible to simple mechanical testing on a spatial scale relevant to these mechanical processes until recently. The structural factors associated with surface mechanical properties (crosslinking, oxidation state, local orientation of polymer, crystallinity, etc.) can be highly variable and localized and may vary on micron spatial scales or smaller. Furthermore, time/frequency-dependent behavior of the surface may have an important role in the overall surface mechanical behavior. Recent work has shown the utility of depth sensing microindentation/nanoindentation testing to interrogate local surface mechanical properties. The goal of the present study was to measure the effect of loading rate on the depth-sensing microindentation testing of UHMWPE. Three different UHMWPE materials (Hylamer, a large crystal material; GUR 1020, a standard medical-grade material; and Marathon, a crosslinked material) were tested using a microindentation method at loading rates ranging from 0.01 to 1 Hz. Similarly, a gamma-irradiated in air and 15-year shelf-aged tibial component was tested through its cross-section to assess the variations in mechanical properties with location and to compare the micromechanical profile with the oxidation profile. It was found that rate of testing affected the microhardness of each material, however, only GUR 1020 and Hylamer showed rate-dependent behavior for modulus and energy dissipation factor. Micromechanical profiles through oxidized regions of the tibial component showed a high correlation with the oxidation profile. Increases in modulus, hardness, and energy dissipation factor were seen with increasing oxidation and each property was loading-rate dependent. These results show that depth-sensing microindentation/nanoindentation testing on the micron scale provides highly consistent and reproducible measurements of surface mechanical properties. This scale of testing minimizes the potential variations caused by local heterogeneity in crystallinity, surface orientation, and other submicron structural features.     

Curing reactions of a liquid crystalline epoxy resin based on the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene

A glycidyl-terminated liquid crytalline (LC) monomer is cured with aromatic amines. A nematic structure suddenly develops during the curing reaction and is related to the molecular weight increase of the developing network. An unusual exothermal double peak is observed in the isothermal curves obtained in a defined temperature range. The calorimetric analysis is used to evaluate the kinetic parameters and is related to the initial formation of a linear prepolymer and to the subsequent development of the tridimensional network.     

The effect of chain length on the solid-state polymerization of N-alkylmaleimides

N-alkylmaleimides from the N-methyl to the N-octadecyl derivative were polymerized in the solid state by γ-radiation. Polymerization during irradiation generally showed sigmoidal conversion curves with high rates and complete conversion 10°C below the melting point. Post-irradiation rates were lower and showed the typical approach to a “limiting” conversion. The polymerization rates increased with temperature and radiation dose and with the length of the alkyl chain. N-methylmaleimide had a high rate of in-source polymerization which was attributed to the volatility of the monomer.     

The influence of the length of alkoxy side groups on the phase state of polydialkoxyphosphazenes

Polydialkoxyphosphazenes with different substituents varying from methoxy up to octoxy which contain only minor amounts (below 0,1 mol-%) of various defective units in their macromolecules were synthesized. It was shown that such polyphosphazenes with propoxy, butoxy and pentoxy side groups can exist in the mesomorphic state and that a defect content as low as about 2 mol-% prevents mesophase formation. In spite of high flexibility of the main chain, the polydialkoxyphosphazenes were found to gain the ability to crystallize only if the number of C atoms in the alkoxy substituents is higher than six.     

The effect of age and telomere length on immune function in the horse

Telomeres, specialized structures present at the ends of linear eukaryotic chromosomes, function to maintain chromosome stability and integrity. Telomeres shorten with each cell division eventually leading to replicative senescence, a process thought to be associated with age-related decline in immune function. We hypothesized that shortened PBMC telomere length is a factor contributing to immunosenescence of the aged horse. Telomere length was assessed in 19 horses ranging in age from 1 to 25 years. Mitogen-induced (3)H-thymidine incorporation, total serum IgG, and pro-inflammatory cytokine expression was also determined for each horse. Relative telomere length (RTL) was highly correlated with overall age. RTL was positively correlated with (3)H-thymidine incorporation and total IgG. Expression of pro-inflammatory cytokines was negatively correlated with RTL. These measures were also correlated with age, as expected. However, RTL was not correlated with immunosenescence and inflammaging in the oldest horse.     

The effect of gamma-irradiation on collagen molecules, isolated alpha-chains, and crosslinked native fibers

This study shows how collagen molecules are readily damaged by gamma-radiation at dosages commonly used for sterilizing biomedical products. At 1 Mrad, while the reported effectiveness of the radiation at such a low dosage to completely sterilize a material is questionable, less damage was caused to the collagen peptide backbone. Above such dosage, however, significant damage was clearly demonstrated with collagen alone and collagen in a chemically crosslinked tissue matrix. The enzyme digestion study showed that the material exposed to a very high dosage of radiation resisted degradation by pronase. However, molecular weight analysis showed a significant number of peptide bonds being cleaved by the radiation which could cause considerable changes in the long-term characteristics of the material. Therefore, tissues exposed to high dosages of gamma-radiation should be tested for long term functional changes. We want to caution against the usage of the enzyme degradation assay as a universal test for all bioprosthetic derived from biological tissues.