Thermally crosslinked rigid…rod aramids, 1. Synthesis of a new monomer and its polymerization

With 2,2,6,6-tetraoxo-1,3,5,7-tetrahydro-2,6-dithia-s-indacene-4,8-diamine (DSDA) ( 8 ), a new monomer for the synthesis of thermally crosslinkable rigid-rod aramids was developed. DSDA was copolymerized with p-phenylenediamine and terephthaloyl dichloride by a low-temperature solution polycondensation to give high-molecular-weight aramids (inherent viscosity η_inh > 2,2 dL/g). The fraction of DSDA could be varied from 0 to 100 mol-% of the diamine used in the condensation. All polymers showed a weight loss due to thermal elimination of SO₂. Thermal crosslinking could be effected by curing the samples at high temperatures (280–320°C). The crosslinking reaction was investigated by means of thermogravimetry (TG), thermogravimetry/mass spectrometry (TG-MS), Fourier transform infrared (FT-IR), ¹³C cross polarization-magic angle spinning nuclear magnetic resonance (¹³C CP-MAS NMR) and electron paramagnetic resonance (EPR). The activation energy for this reaction was found to be between 150 and 170 kJ/mol. Curing of the polymer samples led to the generation of stable, matrix-isolated radicals.     

Ordering of liquid crystalline solutions of rigid-rod aramids using mechanical shearing and electric-field poling

2′,5′-Diamino-4-(dimethylamino)-4′-nitrostilbene was polymerized with terephthaloyl dichloride and 2,6-difluoroterephthaloyl dichloride in a low-temperature solution polycondensation to give two novel, fully aromatic rigid-rod polyamides. Nematic solutions of these polymers were processed into fibers and films that were characterized by wide-angle X-ray diffraction measurements. A post-spin annealing process was employed to enhance the chain orientation in the fibers. The dominating crystal structure was found to be similar to “modification II” of the fibers from poly(p-phenyleneterephthalamide), but the fibers do not suffer a comparable structural transformation upon heat treatment. A corona-discharge poling process gave rise to a remarkable gain in average chain orientation in the films. Again the crystal structure was found to be similar to “modification II”. The aramids investigated in this work represent a new approach to the design of liquid-crystalline rigid-rod polymers, where different mechanisms of orientation can be combined. In the nematic phase, the rigid-rod molecules form highly oriented domains that can be oriented using mechanical processes such as shearing. In addition the stilbene units that are fixed in the polymer backbone with their dipole moments oriented transverse to the main chain can be oriented in an electric field. The combination of both orientation mechanisms seems to cause a synergistic effect, probably since each affects different levels of the polymer microstructure in the solid.     

MALDI-TOF mass spectrometry in polymer analytics, 2. Molecular weight analysis of rigid-rod polymers

Molecular weight analysis of rigid-rod polymers by gel permeation chromatography (GPC) is known to give overestimated values if polystyrene is used for calibration. In this work matrix-assisted laser desorption/ionisation time of flight (MALDI-TOF) mass spectrometry was applied for the first time toward direct molecular weight determination of a rigid-rod type tetrahydropyrene (THP) polymer. The obtained results were supported by investigation of a homologous series of GPC-separated oligomers with the respective structure. As opposed to common MALDI-TOF mass spectra, monodisperse THP oligomers reveal two unusual facts: (i) Beside the formation of molecular ions fragmentation also takes place. (ii) The radical ions are detected instead of cationised species. The fully characterised THP oligomers were also used to calibrate the GPC measurements of the THP polymers. The number-average molecular weight (M̄_n), weight-average molecular weight (M̄_w) and polydispersity (M̄_w/M̄_n) data obtained from the oligomer-calibrated GPC compare well to those values measured by MALDI-TOF mass spectrometry.     

In vitro properties of crosslinked, reconstituted collagen sheets

Reconstituted, 100-microns-thick collagen sheets were crosslinked with either UV light, chromium, or cysteine for use as a burn covering. The sheets were also exposed to a "surface agent" (hydroxyproline, fibronectin, or soluble basement membrane matrix containing Type IV collagen) as a preliminary step in planned adherence studies. Since some chemicals render the collagen toxic, the modified sheets were tested for cytotoxicity using human keratinocytes and fibroblasts. Autoradiography and 3H-thymidine incorporation were used to quantitate the proliferative rate of these cells in vitro. There was a universal depression of keratinocyte incorporation of 3H-thymidine following a 1-day exposure to any collagen sheet when compared to cells not exposed to any collagen. This effect had lessened by 5 days' exposure to the collagen. Conversely, the fibroblasts the collagen. Conversely, the fibroblasts showed an enhancement in rate of incorporation after 1-day exposure, especially for cells exposed to collagen sheets cross-linked by UV light. This effect had also lessened by 5 days' exposure. Autoradiography showed few significant variations for any of the cells exposed for either time period. Chromium leaching was determined, with no values greater than 30% of the allowable maximum set by both the British and American Pharmacopeia.     

Structure and mechanical properties of films of rigid-rod polymers having flexible side chains

A series of rigid-rod polyesters with flexible alkoxy side chains of different length has been studied in order to determine the structure and properties of the polymers in various states (solid state, mesophase, melt). Various supermolecular structures can be obtained depending on the length of the side chains and on the preparation conditions of the samples. The relaxation behavior of the polymers has been investigated by dynamic mechanical measurements. Complex relaxation spectra have been observed, and types of molecular motions and structural rearrangements have been assigned to the observed mechanical relaxation transitions. The rheological behavior was studied in the mesophase and in the melt. In some polymers typical liquid-crystalline properties have been observed, as for example the flow-thinning effect. Large deformation of polymer films as well as the structure and mechanical strength of oriented samples have also been studied.     

Mechanical properties of substituted,rigid‐rod aramids in the highly‐ordered solid state

Fibers of 21 rigid‐rod aromatic polyamides with different substitution patterns at their aromatic rings, produced by polycondensation of functionalized p‐phenylenediamine and functionalized terephthaloyl dichloride and spun from nematic solutions as described in the accompanying paper (B. H. Glomm, M. C. Grob, P. Neuenschwander, and U. W. Suter, Macromol. Chem. Phys.), were characterized by the mechanical properties most relevant for compressive failure. In particular, the torsional moduli G₀ and the axial compressive strength σ_C were determined for each fiber sample before and after employing a post‐spinning heat treatment optimized to improve the degrees of orientation and the crystallinity of the fibers. The dependence of the measured values on the structural parameters of the respective polymers was studied, leading to the result that the volume of the side‐chains of the studied aramids seems to influence the extent of the mechanical “anisotropy” of the fibers, probably through an effect on the interchain interactions. The relationship between the torsional modulus and the axial compressive strength was scrutinized in the light of the theoretical approach of DeTeresa, Allen, and Farris, and Allen, which suggests the existence of a proportionality between G₀ and σ_C. In general, the results provided by our experiments are consist with this theoretical approach.     

Tensile and tribological properties of high-crystallinity radiation crosslinked UHMWPE

Osteolysis due to particulate wear debris associated with ultrahigh molecular weight polyethylene (UHMWPE) components of total joint replacement prostheses has been a major factor determining their in vivo lifetime. In recent years, radiation crosslinking has been employed to decrease wear rates in PE components, especially in acetabular cups of total hip replacement prostheses. A drawback of radiation crosslinking is that it leads to a crosslinked PE (or XPE) with lower mechanical properties compared with uncrosslinked PE. In contrast, high-crystallinity PEs are known to have several mechanical properties higher than conventional PE. In this study, we hypothesized that increasing the crystallinity of radiation crosslinked and remelted XPE would result in an increase in tensile properties without compromising wear resistance. High-pressure crystallization was performed on PE and XPE and analyzed for the resulting morphological alterations using differential scanning calorimeter, low voltage scanning electron microscopy, and ultrasmall angle X-ray scattering. Uniaxial tensile tests showed that high-pressure crystallization increased the tensile modulus and yield stress in both PE and XPE, decreased the ultimate strain and ultimate stress in PE but had no significant effect on ultimate strain or ultimate stress in XPE. Multidirectional wear tests demonstrated that high-pressure crystallization decreased the wear resistance of PE but had no effect on the wear resistance of XPE. In conclusion, this study shows that high-pressure crystallization can be effectively used to increase the crystallinity and modulus of XPE without compromising its superior wear resistance compared with PE.     

Thermally induced time dependence of mechanical properties in biomedical grade polyurethanes.

The time dependence of the mechanical properties of segmented urethanes as well as urethane-urea systems were monitored after the materials had been given a short thermal treatment followed by rapid cooling. Both linear and crosslinked materials were studied but the major focus was on many of the common biomedical grade urethanes. As had been noted in earlier studies on nonmedical segmented urethanes from this laboratory, many of the biomedical grade materials also showed time-dependent changes in mechanical properties that can be directly related to time-dependent changes in the degree of domain structure (microphase separation) that may occur in these segmented copolymers. Interestingly, those systems possessing significant amounts of urea linkage show little or no significant time-dependent changes in structure or properties following thermal treatment. The effect of chemical cross-linking can also influence the domain formation process and its thermal stability. The ramifications of these time dependent effects may have bearing on the biomaterial applications of segmented urethane polymers.     

Lightly crosslinked liquid crystalline epoxy resins: The effect of rigid-rod length and applied stress on the state of order of the cured thermoset

Lightly crosslinked liquid crystalline epoxy resins were synthesized by reacting rigid rod epoxy-terminated molecules with decanedioic acid (SA). The obtained networks exhibited a smectic phase even in the case of 2,6-bis(2,3-epoxypropoxy)naphthalene, whose aspect ratio is very low. Upon application of uniaxial stress, a smectic phase was surprisingly obtained also for the system obtained by reacting p-bis(2,3-epoxypropoxy)-benzene (QH), whose aspect ratio is practically equal to one, and SA. Stretching of the smectic samples above the glass transition temperature determines orientation of polymer chains, yielding considerably high values of the order parameter S. Thermomechanical analysis showed, for one of these systems, that stretching induces a shift of the isotropization temperature to higher values.     

Friction, wear, and tensile properties of vacuum hot pressing crosslinked UHMWPE/nano-HAP composites

Ultra high molecular weight polyethylene (UHMWPE) is a thermoplastic engineering plastic with excellent mechanical properties. In this study, nonirradiated and irradiated UHMWPE/nano-hydroxyapatite (nano-HAP) composites were prepared by vacuum hot-pressing method, and then friction, wear, and tensile properties were investigated. To explore mechanisms of these properties, differential scanning calorimetry, infrared spectrum, and scanning electron microscopy with energy dispersive spectrometry analysis were carried out on the samples. The results in this study indicated that reduced friction coefficients and wear rate could be obtained when nonirradiated and irradiated UHMWPE were filled with 7% nano-HAP. The irradiated UHMWPE/7% nano-HAP also had a synergistic function of wear reduction as compared with irradiated UHMWPE and nonirradiated UHMWPE/7% nano-HAP. Samples filled with 7% nano-HAP showed a brittle fracture behavior, and a linear relationship between modulus and crystallinity for a nonirradiated and irradiated sample was found in this study.     

Composite fiber of poly D,L-lactic acid/hydroxyapatite produced by melt spinning technology

In this paper, preparation technologies for the compound fibers of poly D,L-lactic acid (PDLLA)/hydroxyapatite (HA) were investigated. Starting with PDLLA of weight average molecular mass 94,200-1,130,000 and HA powders of diameter 4-10 microm, the compound fibers of PDLLA/HA were obtained through a two stage process: first the adsorption of HA particles on the surface of PDLLA flakes using the liquid-phase adsorption method then melt-extrusion, and second the spinning collection. Experimental result was showed that the high performance composite fibers of PDLLA/HA with diameter of 15-30 micrometer could be produced by the PDLLA of weight average molecular mass 150,000-300,000, the HA powders content 5 wt%, the melt extrusion temperature below 160 degrees C and the screw rotating speed of 10-15 r/min, the spinning collection speed of 2-5 m/min.     

Fiber density in congenital muscle fiber type disproportion. I. Congenital myopathies

In 15 cases with congenital muscle fiber type disproportion, concentric needle EMG (CNEMG) and single fiber EMG (SFEMG) with fiber density (FD) estimation was performed. The aim of the work was to establish the nature of congenital muscle fiber type disproportion. In 11 cases CNEMG revealed interference pattern on maximal effort and low, polyphasic motor unit action potentials (MUAPs) of short duration indicating a myopathic lesion. In four other cases electromyographic changes were less evident. The FD values were slightly increased in 9 cases. This finding could be explained by pronounced muscle fiber splitting with subsequent ephaptic transmission or even secondary denervation and reinnervation. The most important conclusion from our pilot study is the confirmation of the fact that reinnervation with excessive sprouting is not the only mechanism responsible for muscle fiber type disproportion.     

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.     

Ionically crosslinked alginate hydrogels as scaffolds for tissue engineering: part 1. Structure, gelation rate and mechanical properties

Alginate gels have been used in both drug delivery and cell encapsulation applications in the bead form usually produced by dripping alginate solution into a CaCl2 bath. The major disadvantages to these systems are that the gelation rate is hard to control; the resulting structure is not uniform; and mechanically strong and complex-shaped 3-D structures are difficult to achieve. In this work controlled gelation rate was achieved with CaCO3-GDL and CaSO4-CaCO3-GDL systems, and homogeneous alginate gels were formulated as scaffolds with defined dimensions for tissue engineering applications. Gelation rate increased with increasing total calcium content, increasing proportion of CaSO4, increasing temperature and decreasing alginate concentration. Mechanical properties of the alginate gels were controlled by the compositional variables. Slower gelation systems generate more uniform and mechanically stronger gels than faster gelation systems. The compressive modulus and strength increased with alginate concentration, total calcium content, molecular weight and guluronic acid (G) content of the alginate. MC3T3-E1 osteoblastic cells were uniformly incorporated in the alginate gels and cultured in vitro. These results demonstrated how alginate gel and gel/cell systems could be formulated with controlled structure, gelation rate, and mechanical properties for tissue engineering and other biomedical applications.     

Thermoelastic and photoelastic properties of crosslinked liquid-crystalline side chain polymers

Measurements of stress and birefringence of crosslinked liquid-crystalline side chain polymers are reported. The results are compared with those obtained for conventional elastomers. It is found that the thermoelastic behaviour in the nematic state completely differs from that observed for common polymer networks due to the anistropic ordering of the mesogenic side chains. The retractive force in the nematic state is found to be independent of path, indicating that the nematic phase is a homogeneous phase. Photoelastic investigations prove the propotionality of stress and birefringence above the clearing temperature T_c which is similar to common elastomers. The stress-optical coefficient, however, strongly depends on temperature in the pretransformation region. X-ray measurements show a uniform orientation of the mesogenic side groups in the nematic state. The diffraction patterns observed contain many more reflexes than observed for common nematics.     

Gel permeation chromatography with crosslinked polystyrene gels and poor and theta solvents for polystyrene, 2. Separation mechanism

For separations performed with crosslinked polystyrene gels, an interpretation is proposed for the displacement to high retention volume of plots of log hydrodynamic volume versus retention volume for polystyrene (PS) in poor and theta solvents, e.g. cyclohexane, trans-decalin, and N,N-dimethylformamide, with respect to plots for other polymers for which these same eluents are good solvents. PS separates by steric exclusion and by solute-gel interaction effects giving rise to partition and adsorption mechanisms. The retention volume V_R is given by where V₀ is the interstitial volume, V_i is the solvent volume within the gel, K_D is the distribution coefficient for steric exclusion, and K_p is the distribution coefficient for solute-gel interaction effects. For polymers in good solvent media separating solely by steric exclusion, K_p is unity. For PS in poor and theta solvents, network-limited partition and network-limited adsorption mechanisms are proposed in which K_p is greater than unity. The hydrodynamic volume universal calibration method is applicable when K_p is greater than unity, provided the retention parameter is changed from V_R to (V_R–V₀)/K_p. From the network-limited adsorption mechanism, a plot of log[(V_R–V₀)/K_p] versus polymer hydrodynamic radius yields the average pore size of the crosslinked polystyrene gel. Experiments are reported showing the dependence of K_p on gel porosity and on solvent power by varying the temperature of the eluent.     

Highly crosslinked, PEG-containing copolymers for sustained solute delivery.

Novel ionizable polymer networks were prepared from oligo(ethylene glycol) (OEG) multiacrylates and acrylic acid (AA) employing bulk radical photopolymerization techniques. The properties of these materials exhibited a complex dependence on the network structure and composition, and the materials were therefore used in the design of controlled release devices with precisely controlled properties. The release kinetics of model solute proxyphylline exhibited a strong compositional dependence, with measured diffusion coefficients varying over several orders of magnitude, depending on the polymer network structure and the pH of the release medium. Varying the OEG chain length provided a means of coarsely adjusting the proxyphylline release rate, while varying the AA content and the pH offered a more precise measure of control.     

Fiber density in congenital muscle fiber type disproportion. II. Congenital muscle hypotonia and hip dislocation

Our previous paper presenting electromyographic findings in patients with congenital fiber type disproportion myopathy, confirmed the myogenic character of the disease process. That group of patients was however fairly heterogenous regarding both the clinical features and the morphological changes in muscle fibers (e.g. cases with central cores). In the present study we have examined 13 children with hypotonia and muscle fiber type disproportion operated on in childhood for congenital hip dislocation. In all cases CNEMG and SFEMG with FD estimation was performed in biceps brachii and quadriceps femoris muscles. In all muscles examined either slight EMG changes indicative of myopathy or a normal EMG pattern was found. None of the patients demonstrated an evident increase in FD values. Normal FD and the recruitment pattern proportional to the force of contractures indicate that the normal number of motoneurons is preserved. Accordingly, our present findings confirm the conclusions of our previous paper.     

Structural characterization and physical properties of P2O5-CaO-Na2O-TiO2 glasses by Fourier transform infrared, Raman and solid-state magic angle spinning nuclear magnetic resonance spectroscopies

Phosphate-based glasses have been investigated for tissue engineering applications. This study details the properties and structural characterization of titanium ultra-phosphate glasses in the 55(P₂O₅)-30(CaO)-(25 − x)(Na₂O)-x(TiO₂) (0 ? x ? 5) system, which have been prepared via melt-quenching techniques. Structural characterization was achieved by a combination of X-ray diffraction (XRD), and solid-state nuclear magnetic resonance, Raman and Fourier transform infrared spectroscopies. Physical properties were also investigated using density, degradation and ion release studies; additionally, differential thermal analysis was used for thermal analysis of these glasses. The results show that with the addition of TiO₂ the density and glass transition temperature increased whereas the degradation and ion release properties are decreased. From XRD data, TiP₂O₇ and CaP₂O₆ were detected in 3 and 5 mol.% TiO₂-containing glasses. Magic angle spinning nuclear magnetic resonance results confirmed that as TiO₂ is incorporated into the glass; the amount of Q³ increases as the amount of Q² consequently decreases, indicating increasing polymerization of the phosphate network. Spectroscopy results also showed that the local structure of glasses changes with increasing TiO₂ content. As TiO₂ is incorporated into the glass, the phosphate connectivity increases, indicating that the addition of TiO₂ content correlates unequivocally with an increase in glass stability.