The influence of poly(acrylic acid) molar mass and concentration on the properties of polyalkenoate cements Part II Young''s modulus and flexural strength

The Young's modulus and flexural strength were determined for glass polyalkenoate cements as a function of poly(acrylic acid), PAA molar mass, concentration, glass volume fraction and cement ageing time. The Young's modulus was independent of PAA molar mass. The Young's modulus increased dramatically with the PAA concentration of the cement until concentrations greater than 50% m/m were reached. The modulus increased with time for nearly all the cements investigated consistent with a continuing ionic cross-linking process in the cement matrix. The modulus increased with an increase in the volume fraction of the higher modulus glass phase. Increasing the glass volume fraction provides more surface area for acid attack resulting in a more cross-linked polysalt matrix, as well as increasing the volume fraction of residual glass particles. Flexural strength was highly dependent on molar mass of the PAA and its concentration. The molar mass dependence of the flexural strength was greatest at higher PAA concentrations.     

The influence of poly(acrylic acid) molar mass and concentration on the properties of polyalkenoate cements Part III Fracture toughness and toughness

The influence of poly(acrylic acid), PAA molar mass and concentration on fracture toughness and toughness of glass polyalkenoate cements was investigated. Fracture toughness and toughness increased with both the molar mass of the PAA and its concentration. The fracture toughness and toughness increased dramatically with concentration for the highest molar mass PAA studied. However the increase in fracture toughness and toughness with PAA concentration was small for the lowest molar mass PAA. The influence of molar mass was greatest at the highest PAA concentration studied and least for the lowest PAA concentration. The toughness results were analysed with a reptation chain pull-out model. The greater dependance of toughness on PAA concentration for high molar mass cements can be explained by the critical molar mass for chain entanglements to form (M _e) being concentration dependant and M _e decreasing with increasing PAA concentration.     

Influence of poly(acrylic acid) molar mass on the fracture properties of glass polyalkenoate cements

The failure behaviour of glass polyalkenoate cements was investigated using a linear elastic fracture mechanics (LEFM) approach. Cements were based on four model glasses with varying reactivity and four poly(acrylic acid)s (PAA)s with number average molar masses (Mn) ranging from 3.25 × 104 to 1.08 × 105. Cement properties were studied at time intervals of one, seven and twenty eight days. Compressive strengths (c) of the cements increased with increasing fluorine content of the glass, with increased molar mass of the PAA and with ageing time. The Young's moduli increased with time, but were lower for cements based on the fluorine free glass. Moduli values were independant of PAA molar mass. The un-notched fracture strength (f) of the cement increased with the molar mass of the PAA and with ageing time. Glass composition did not appreciably influence the un-notched fracture strength. The fracture toughness (KIC) increased with the molar mass of the PAA and with ageing time, but reduced with increasing fluorine content of the glass. The toughness (GIC) was dependant on molar mass. The influence of molar mass was not as great as predicted by the reptation chain pull-out model for fracture. The molar mass dependence of toughness was greatest with the lower fluorine content glasses. The plastic zone size at the crack tip increased with the molar mass of the PAA. However the plastic zone size decreased with ageing time for all the cements studied and was smaller for the more reactive higher fluorine content glasses.     

The influence of poly(acrylic acid) molar mass on the properties of polyalkenoate cements formed from zinc oxide/apatite mixtures

The influence of poly(acrylic acid) molar mass was investigated on cements formed from zinc oxide-apatite mixtures at three aging times; one, seven and 28 days. Cements based on both hydroxyapatite and fluorapatite were investigated. The compressive strength, un-notched fracture strength and fracture toughness increased markedly with poly(acrylic acid) molar mass. The fracture toughness and un-notched fracture strength increased with aging time for the two highest molar mass cements, but decreased with time for the two lowest molar mass cements. The greater chain entanglement density present in the higher molar mass cements is thought to contribute significantly to the cement stability in addition to the crosslinking of the polyacrylate chains by calcium and zinc ions. Substitution of hydroxyapatite by fluorapatite had no significant influence on the mechanical properties of the cements at aging times longer than one day.     

Influence of poly(acrylic acid) molar mass on the fracture properties of glass polyalkenoate cements based on waste gasifier slags

The failure behaviour of glass polyalkenoate cements was investigated using a linear elastic fracture mechanics (LEFM) approach. Cements were based on Drayton gasifier slag and four poly(acrylic acid)s with number average molar masses ranging from 3.03 × 10³ to 6.44 × 10⁴. Cement properties were studied at time intervals of one, seven and twenty eight days. Compressive and flexural strengths of the cements increased with increasing molar mass of the poly(acrylic acid)s and time. The Young's modulii increased with time and were independent of poly(acrylic acid) molar mass. Fracture toughness increased with increasing molar mass of the poly(acrylic acid)s. Fracture toughness increases over an ageing time of one week and subsequently decreased over one month. Toughness increased with poly(acrylic acid) molar mass, these increases being most pronounced at higher molar mass. The toughness values decreased with time for the higher molar mass cements, which is consistent with increased crosslinking of the poly(acrylic acid) chains and reducing molecular flow at the crack tip. Plastic zone size increased with poly(acrylic acid) molar mass and decreased with time for lower molar mass cements, remained constant for intermediate molar mass cements and increased with high molar mass cements.     

Influence of poly(acrylic acid) content on the fracture behaviour of glass polyalkenoate cements

A linear elastic fracture mechanics approach (LEFM) was used to study glass polyalkenoate cements as a function of the poly(acrylic acid) content. Cement specimens were tested at three time intervals after mixing; one, seven and twenty eight days. Two series of cements were investigated one with a glass volume fraction of 0.4 and the other with a glass volume fraction of 0.5. The fracture toughness, toughness, Young's modulus and un-notched fracture strength increased significantly with the percentage polyacid content. The Young's modulus increased with time for all the cement samples studied. In many cases the moduli values at twenty eight days were twice the values at one day. This is consistent with increased ionic crosslinking of the polyacrylate matrix. The toughness increased with the polyacid content as predicted by the chain pull-out model for fracture and did not change significantly on increasing the glass volume fraction from 0.4 to 0.5. Fracture toughness and Young's modulus increased significantly with glass volume fraction consistent with the residual glass particles acting as a reinforcing filler.     

The influence of poly(acrylic acid) molecular weight on the fracture toughness of glass-ionomer cements

A linear elastic fracture mechanics approach has been used to characterize failure in glass-ionomer cements. The toughness, fracture toughness, flexural strength and inherent flaw size increase with the molecular weight of the poly(acrylic acid), whilst the Young's modulus remains approximately constant. The dependence of toughness on poly(acrylic acid) molecular weight is not as large as predicted by a reptation/chain pull out model for fracture and this is thought to be a result of the weak ionic crosslinks formed during the cement reaction.     

The influence of hydroxyapatite: Zinc oxide ratio on the setting behavior and mechanical properties of polyalkenoate cements

The influence of hydroxyapatite (HA) content on the setting behavior and mechanical properties of hydroxyapatite-zinc oxide-poly(acrylic acid) (HA-ZnO-PAA) composite cements were investigated as a function of HA content. The working time increased with HA content up to 45 wt % HA and then decreased whilst the setting time increased with increasing HA content. Mechanical properties were determined after storage in water at 37 degrees C for 1, 7 and 28 days. Young's moduli and compressive strength go through a maximum at approximately 30 and 45 wt % HA. Young's modulus increases with time, which is consistent with an ongoing crosslinking reaction.     

The strength of acrylic bone cements and acrylic cement-stainless steel interfaces

The strength and fracture toughness of a surgical acrylic bone cement have been evaluated in air, saline solution, and a blood serum, and little effect of environment was observed. Second phase dispersions of either barium sulphate or glass spheres were found to have significant effects upon the strength and fracture surface energy. In the first composite, a decrease in strength and fracture surface energy may be explained in terms of the formation of voids around the barium sulphate particles and tearing of material between them; in the second composite, an increase in strength and fracture surface energy occurred through a crack front-glass sphere interaction effect. The occurrence of subcritical crack growth in the acrylic bone cement composites was investigated, and crack velocity-stress intensity factor diagrams were constructed for the purposes of fracturesafe design.     

The influence of triethylcitrate on the biological properties of poly (L-lactic-co-glycolic acid) membranes

Biodegradable polymers have a variety of uses in basic and clinical research, as well as important therapeutic applications. The most commonly used are poly (lactic acid), poly (glycolic acid) and their copolymer, poly (L-lactic-co-glycolic acid) or PLGA. The incorporation of a plasticizer into a polymer can be used to obtain a product with specific properties. In this work, we examined the influence of a plasticizer (triethylcitrate) on the properties of PLGA membrane implants for human clinical uses. Membranes with and without plasticizer were dense and compact and contained no pores. The incorporation of 7% plasticizer enhanced the degradation the polymer when compared to polymer without plasticizer. In membranes without plasticizer, the initiation of degradation was very slow and was seen only 60 days after implantation, should allow the use of this material in the repair of damage tissue. In both cases, macroscopic analysis showed that there was no adhesion of the membrane to capsule fibrous, and this adversely affected preservation of the polymer. With time, the adherence of the polymer to surrounding tissue increased. Overall there was little degradation of membranes without plasticizer compared to those containing plasticizer.     

The processing,mechanical properties and bioactivity of strontium based glass polyalkenoate cements

The suitability of zinc-based glass polyalkenoate cements (GPCs) for use in orthopaedics can be improved by the substitution of strontium into the glass phase which should impart improved radiopacity and bone forming properties to the cements without retarding strength. The purpose of this research was to produce novel GPCs based on calcium–strontium–zinc-silicate glasses and to evaluate their mechanical properties and biocompatibility with the ultimate objective of developing a new range of cements for skeletal applications. Three glass compositions, based on incremental substitutions of strontium for calcium, were synthesized; BT100 (0.16CaO, 0.36ZnO, 0.48SiO₂), BT101 (0.04SrO, 0.12CaO, 0.36ZnO, 0.48SiO₂) and BT102 (0.08SrO 0.08CaO, 0.36ZnO, 0.48SiO₂). Each glass was then mixed with varying concentrations and molecular weights of polyacrylic acids in order to determine the working times, setting times, compressive strengths and biaxial flexural strengths of the novel cements. The maximum working time and setting time achieved was 29 and 110 s respectively; which, at present is inadequate for current clinical procedures. However, the optimum compressive and biaxial flexural strengths were up to 75 and 34 MPa respectively indicating that these formulations have potential in load bearing applications. Importantly, the substitution of Ca with Sr in the glasses did not have a deleterious effect on strengths or working times. Finally, the bioactivity of the best performing cements was determined in vitro using simulated body fluid. It was found that all cements facilitate the formation of an amorphous calcium phosphate at their surface which increases in density and coverage with time, indicating that these cement will bond directly to bone in vivo.     

The actuation of a biomimetic poly(vinyl alcohol)poly(acrylic acid) gel

Active polymer gels expand and contract in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as biomimetic actuators and "artificial muscles". In previous work, a thermodynamically consistent finite-elastic constitutive model has been developed to describe the mechanical and actuation behaviours of active polymer gels. The mechanical properties were characterized by a free-energy function, and the model uses an evolving internal variable to describe the actuation state. In this work, an evolution law for the internal variable is determined from free actuation experiments on a poly(vinyl alcohol)poly(acrylic acid) (PVAPAA) gel. The complete finite-elastic/evolution law constitutive model is then used to predict the response of the PVA-PAA gel to isotonic and isometric loading and actuation. The model is shown to give relatively good agreement with experimental results.     

Nanoclays reinforced glass ionomer cements: dispersion and interaction of polymer grade (PG) montmorillonite with poly(acrylic acid)

Montmorillonite nanoclays (PGV and PGN) were dispersed in poly(acrylic acid) (PAA) for utilization as reinforcing filler in glass ionomer cements (GICs). Chemical and physical interaction of PAA and nanoclay (PGV and PGN) was studied. PAA–PGV and PAA–PGN solutions were prepared in different weight percent loadings of PGV and PGN nanoclay (0.5–8.0 wt%) via exfoliation-adsorption method. Characterization was carried out by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and fourier transform infrared (FTIR) spectroscopy. XRD results of PAA–PGN demonstrated that the interlayer space expanded from 12.83 to 16.03 Å indicating intercalation whereas the absence of the peak at d ₀₀₁ in PAA–PGV indicated exfoliation. XPS scans of PGV and PGN nanoclays depicted the main peak of O 1s photoelectron due to Si–O–M (M = Mg, Al, Fe) whereas, Si–O–Al linkages were identified by Si 2p or Si 2s and Al 2p or Al 2s peaks. The disappearance of the Na peak confirmed that PAA molecules exchanged sodium ions present on surface of silicate layers and significantly reduced the electrostatic van-der-Waals forces between silicate plates resulting in intercalation or exfoliation. FTIR spectra of PAA–nanoclay suspensions demonstrated the presence of a new peak at 1,019 cm^?1 associated with Si–O– stretching vibrations which increased with increasing nanoclays concentration. Information concerning the dispersion of nanoclay in PAA aqueous solutions, chemical reaction and increase interlayer space in montmorillonite nanoclay is particularly useful regarding dispersion and reinforcement of nanoclay in PAA.     

Influence of gallium on the surface properties of zinc based glass polyalkenoate cements

This study investigates the effect of gallium (Ga) additions, substituting for zinc (Zn), on the physio-chemical surface properties of aluminium-free glass polyalkenoate cements (GPCs). Substituting Zn with Ga resulted in a significant increase in hydrophilicity and thusly wettability, as shown by a decrease in water contact angle. Increasing Ga resulted in increased Zn release, irrespective of decreasing Zn content of the starting glass. This resulted in increased antibacterial efficiency, against Escherichia coli, but not Staphylococcus epidermidis. Ga was shown to have no effect on antibacterial efficiency.     

Electrokinetic properties of Nd:YAG nanopowder and a high concentration slurry with ammonium poly(acrylic acid) as dispersant

The electrokinetic properties of Nd:YAG nanopowder with particles of about 40 nm in diameter were investigated by measuring the zeta potential of a stable YAG (Y₃Al₅O₁₂) aqueous slurry. Ammonium poly(acrylic acid) polyelectrolyte was used as dispersant to adjust the electrokinetic properties of the Nd:YAG slurry. The effect of the pH of the slurry and of the polyelectrolyte concentration on the stability of the suspension are discussed in this study. The optimal pH value and the amount of dispersant needed to obtain a stable Nd:YAG nanoparticle slurry were determined. Highly consistent Nd:YAG nanoparticle slurries with optimal pH and dispersant concentration were prepared by ball milling. The rheological behavior of Nd:YAG slip with different solid loading (60–70 wt%) has been studied by measuring the viscosity and shear stress as a function of shear rate. Slip with solid loadings of 65 wt% shows near-Newtonian behavior but becomes non-Newtonian with typical shear-thinning behavior above this solid loading value. The density and microstructure of the cast product bears a direct relationship to the state of the slip induced by alternation of the pH and the concentration of the dispersant as well as the solid loading.