On the application of the “rule of mixture” to microhardness of complex polymer systems containing a soft component and/or phase

The main goal of this work is to find a reasonable explanation for the frequently reported drastic deviations from the “rule of mixture” applied for calculation of the overall microhardness, H, of complex polymer systems comprising a soft, (with a glass transition, T _g, or melting, T _m, temperatures below room temperature) component and/or phase. According to the common practice, the contribution to H of the soft component and/or phase, H ^s, is considered as H ^s = 0, which results in extremely large differences between the measured and calculated H values for systems comprising more than 20–25 wt% soft component and/or phase. For such systems a different deformation mechanism during indentation process is postulated, namely “floating” of the solid particles in the soft component and/or phase, in addition to their plastic deformation. The contribution of the “floating effect” to the overall H is accounted for by the empirically derived relationship H = 1.97 T _g−571. Using the reported data on H and T _g for homopolymers, blockcopolymers and blends, the H values are recalculated and a good agreement with the experimentally measured values is found. A modified additivity law is suggested, which contains a term accounting for the contribution of the soft component and/or phase to the overall microhardness via the relationship between H and T _g; its application results in much smaller differences between the measured and calculated H values. On leave from Laboratory on Polymers, University of Sofia, 1126 Sofia, Bulgaria.     

Glass transition and fragility of telluro-vanadate glasses containing antimony oxide

The activation energy (ΔH ^*) of the glass transition and the heating-rate dependence of the glass transition temperature (T _g) of V₂O₅–Sb₂O₃–TeO₂ glasses were determined using differential scanning calorimetry technique. Non-isothermal measurements were performed at different heating rates φ (=3, 6, 9, 10, 13 K/min). The heating rate dependence of T _g was used to investigate the applicability of different theoretical models describing the glass transition. The application of Moynihan and Kissinger et al. models to the present data led to different values of (ΔH ^*) at each different heating-rate regions. This behavior was attributed to the strong heating rate dependence of the activation energy of the process. The fragility parameter (m = ΔH ^*/RT _g) were ≲90, suggesting that these glasses may be classified as strong glasses. The viscosity, η, calculated at a few selected temperatures near the glass transition region increased with increasing Sb₂O₃ content at any given temperature, which is also expected. Also the compositional dependence of T _g and ΔH ^* was investigated.     

Crystallization behavior of PVDF/PMMA blends prepared by in situ polymerization from DMF and ethanol

The crystallization behavior of poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends was investigated at different PMMA content and from two solvents N,N-dimethylformamide (DMF) and ethanol. The PVDF/PMMA blends were obtained by in situ polymerization of methyl methacrylate (MMA) in the solution of PVDF in DMF. The crystalline phases of PVDF were dependent on crystalline solvents and independent on PMMA content. For the in situ PVDF/PMMA blends, β phase of PVDF was predominant when they were crystallized from their good solvent DMF, while PVDF exhibited well-defined α and β phases from non-solvent ethanol. However, the relative fraction of β phase of PVDF in blends crystallized from ethanol varied with PMMA content. The crystallization morphology was related to crystallization solvent and PMMA content. The in situ blends crystallized from DMF and ethanol presented spherulites morphology and numerous minute particle structures, respectively. The addition of PMMA could reduce the spherulite size of PVDF. Thermal properties of in situ blends were also dominated by crystallization solvent and PMMA content. For the blends crystallized from DMF, their peak melting temperatures and lamellar thickness calculated by WAXD showed a first increasing and then decreasing tendency. At the same PMMA content, the blends crystallized from ethanol had a higher degree of crystallinity (X _c) of PVDF compared with those from DMF. In addition, the X _c calculated by DSC increased noticeably at PMMA content of 1.0 wt% and afterward it decreased with PMMA content, regardless of the kind of crystallization solvent. Besides, the hydrophilicity of the PVDF/PMMA blends was improved with PMMA content based on contact angle measurements.     

Tensile behaviour of blends of poly(vinylidene fluoride) with poly(methyl methacrylate)

Blends of poly(vinylidene fluoride) (PVF₂) and poly(methyl methacrylate) (PMMA) were prepared over a wide concentration range and tested in tension at the same relative temperature below the glass transition. Testing was performed at strain rates ranging from 10 to 0.01 min^–1 at test temperatures fromT _g-40 toT _g-10. By normalizing the test temperature to fixed increments belowT _g, blends and homopolymers can be compared on the basis of PVF₂ and PMMA composition and crystallinity. In nearly all blends, under conditions favouring disentanglement, (decrease in strain rate, or increase in test temperature), the yield stress and drawing stress decreased while the breaking strain increased. For materials with about the same degree of crystallinity, those with a higher proportion of amorphous PVF₂ exhibited brittle-like behaviour as a result of interlamellar tie molecules. In the semicrystalline blends, yield stress remains high as the test temperature approachesT _g, whereas in the amorphous blends the yield stress falls to zero nearT _g. Results of physical ageing support the role of interlamellar ties which cause semicrystalline blends to exhibit ageing at temperatures aboveT _g.     

Influence of oxidized polyethylene wax (OPW) on the mechanical,thermal, morphological and biodegradation properties of PHB/LDPE blends

Blends of poly(β-hydroxybutyrate) (PHB) and low density polyethylene (LDPE) were prepared in proportions of 100/0, 75/25, 50/50, 27/75 and 0/100 (PHB/LDPE wt.%), with and without oxidized polyethylene wax (OPW, 5 wt.%), and the mechanical, thermal (differential scanning calorimetry and melting flow index, morphological (scanning electron microscopy) and biodegradation (aging in simulated soil) properties were evaluated. The addition of OPW increased the tensile strength and Young’s modulus but decreased the elongation at break of the blends. Similarly, OPW increased the T _g of the pure LDPE and enhanced the melt flow index. Scanning electron microscopy showed that OPW reduced the phase separation of LDPE and increased the biodegradation during aging in simulated soil.     

Synthesis and mechanical properties of cardanol-formaldehyde (CF) resins and CF-poly(methylmethacrylate) semi-interpenetrating polymer networks

Cardanol-formaldehyde (CF) resins (both novolac and resol) and CF-poly(methylmethacrylate) (PMMA) semi-interpenetrating polymer networks were synthesized and their mechanical properties and thermal transitions were evaluated. The lower tensile strength of CF resins compared to phenol-formaldehyde (PF) resins may be understood on the basis of the structure of the C₁₅ side chain imparting steric hindrance and reduction in intermolecular interactions. Interpenetration of CF with PMMA increased the mechanical properties only marginally. Scanning electron micrographs of the semi-IPNs showed two distinct phases. Thermomechanical analysis gave two glass transition temperatures,T _g, for the IPNs, the lowerT _g corresponding to the PMMA phase and the higherT _g to the CF phase. However, the unusual increase inT _g of the CF from 128°C to 144°C suggests restrictions in the segmental motion of the CF phase brought about by mixing with another rigid polymer such as PMMA.     

Recovery processes in amorphous polymers

The release of stored energy and the recovery of dimension and volume of cold compressed polystyrene (PS), polymethyl methacrylate (PMMA) and polycarbonate (PC) were studied. The release of stored energy has two peaks, one broad peak at 70° C (PS), 55° C (PMMA) and 85° (PC) and one sharp peak at 113° C (PS,T _g = 105° C), 123° C (PMMA,T _g = 117° C) and 157°C (PC,T _g = 152° C). The recovery of dimension has only one peak at 114° C (PS), 124° C (PMMA) and 157° C (PC). This peak agrees very well with the second peak in the release of stored energy. The recovery of volume has a broad peak similar to the first peak in stored energy. Thus there are probably two recovery processes in these polymers: one releases energy and recovers volume belowT _g and the other releases energy and recovers dimension aboveT _g. These two processes are consistent with the two-stage formation mechanism of shear bands in amorphous polymers.[/p]     

Mutual diffusion studies of polystyrene and poly(xylenyl ether) using Rutherford backscattering spectrometry

The concentration versus depth profile in diffusion couples initially consisting of layers of pure polystyrene (PS) and poly(xylenyl ether) (PXE) were measured. To reveal the profile, the PXE molecules (after diffusion) were preferentially stained in the glass by exposing the couple to a solution of 2 mol % Br in methanol; the covalently bound Br nuclei serve as heavy nuclear tags which permit a sensitive determination of the PXE depth profile by Rutherford backscattering spectrometry (RBS). The samples were annealed at temperatures, T, ranging from 177–210 °C that are above the glass transition temperature, T _g, of the pure PS (105 °C) but below the T _g of pure PXE (216 °C). The measured concentration versus depth profile is markedly asymmetric, with a low slope at low values of the volume fraction of PXE, _PXE, but with a much greater slope at high values of _PXE. These results are in qualitative agreement with the variation in the mutual diffusion coefficient, D, expected as _PXE increases, causing the T _g of the blend to increase towards, and finally exceed, the diffusion temperature. Values of D extracted from these concentration versus depth profiles at low values of _PXE using the Boltzmann-Matano analysis are in good agreement with those measured by forward recoil spectrometry (FRES) in deuterated-PS: PXE couples with only small composition differences.     

Electrical conductivity of modified poly(vinyl chloride)

Chemical modification of poly(vinyl chloride) (PVC) by dehydrochlorination with ethanolic KOH is found to yield modified PVC with conjugated polyene sequence. The semiconducting nature of ethoxide-modified PVC is illustrated with temperature dependence of conductivity (σ). The relative ratios (r) of conductivity,σ _modifiedpvc /σ _{unmodifiedpvc} , are greater than unity in the temperature range 50° to 180°C,r being maximum in the vicinity of glass-transition temperature (T _g).T _g inferred from conductivity-temperature profiles is found to be greater for modified PVC relative to unmodified PVC, which is explicable in terms of restricted free rotation limiting segmental motion. For comparison with the conductivity andT _g of ethoxide-modified PVC, LiCl-modified PVC and (aniline + S₂O ₈ ²⁻ )-modified PVC have also been studied.     

Thermal effects on fracture of biaxial-oriented poly(ethylene terephthalate) (BOPET) film

The effect of temperature on the fracture behaviour of biaxial-oriented poly(ethylene terephthalate) (BOPET) film was studied using the Essential Work of Fracture (EWF) approach. Fracture tests were performed over the temperature range +25 to +160 °C at the speed of 5 mm/min using double edge notched tension (DENT) specimens. The length of the specimens was either along the machine direction (MD) (0°), transverse direction (TD) (90°) or at 45° to either MD or TD. Ductile tearing of the ligament region was noted over the entire temperature range in all three directions. A linear relationship was found between the specific total work of fracture and the ligament length at all test temperatures. Values of the specific essential work of fracture (w _e) in the MD and TD were similar and smaller than in the 45° direction. Within temperature range 25–140 °C, w _e showed little variation if any with respect to temperature. As expected, the Specific Non-Essential Work of Fracture (βw _p) was temperature dependent. This parameter increased with increasing temperature and reached a maximum around the glass transition temperature of BOPET (T _g ≈ 80 °C). The values of the maxima are respectively 16.15, 20.38 and 17.8 MJm⁻³ for the 0°, 45° and 90°.     

Microhardness study of (1-x-y)(B2O3)-x(Li2O)-y(MCI2), (M=Cd, Zn) glasses

Results of microhardness measurements on (1-x-y)(B₂O₃)-x(Li₂O)-y(MCI₂), (M=Cd, Zn) glasses, in the applied load range 25–500 g, are presented. The microhardness was found to decrease with increase in load up to 50 g, then it increased and finally attained a practically constant value with increase in load. The effects of composition of the glasses on microhardness are discussed.     

Dielectric properties of doped polystyrene and polymethylmethacrylate

About 1 μm thick films of polystyrene (PS) and polymethylmethacrylate (PMMA) doped with diphenylsulfoxide (DS) up to 40 wt.% were prepared from solutions using spin-coating method. Glass transition temperature (T _g) of doped polymer films was determined by DSC technique. The depth profile and surface concentration of DS dopant were measured by RBS and XPS methods, respectively. The temperature dependence of relative permittivity of the films was determined from capacitance measurement. The dependence of polarization (P) on electric field (E) was measured using a standard Sawyer–Tower circuit. The glass transition temperature T _g of both composites was found to be decreasing function of the DS concentration. The DS doping leads to an increase of relative permittivity of the PS and PMMA films. RBS and XPS measurements reveal an outward diffusion of DS dopant in PS/DS films at elevated temperature. No such effect was observed in PMMA/DS films. PMMA/DS layers were found to be more thermally stable comparing to PS/DS.     

Toughening and reinforcement of poly(vinylidene fluoride) nanocomposites with “bud-branched” nanotubes

Bud-branched nanotubes, fabricated by growing metal particles on the surface of multi-wall carbon nanotubes (MWCNTs), were used to prepare poly(vinylidene fluoride) (PVDF) based nanocomposites. The results of differential scanning calorimetry (DSC) showed that the introduction of the MWCNTs and bud-branched nanotubes both increased the crystallization temperature, while no significant variation of T_m (melting temperature), ΔH_c (melting enthalpy) and ΔH_m (crystallization enthalpy) occurred. The results of wide angle X-ray diffraction (WAXD) tests showed that α-phase was the dominated phase for both pure PVDF and its nanocomposites, indicating the addition of the MWCNTs and bud-branched nanotubes did not alter the crystal structures. Dynamic mechanical analysis (DMA) tests showed that bud-branched nanotubes were much more efficient in increasing storage modulus than the smooth MWCNTs. In addition, no significant variation of the T_g (glass transition temperature) was observed with the addition of MWCNTs and bud-branched nanotubes. Tensile tests showed that the introduction of MWCNTs and bud-branched nanotubes increased the modulus. However, a dramatic decrease in the fracture toughness was observed for PVDF/MWCNTs nanocomposites. For PVDF/bud-branched nanotubes nanocomposites, a significant improvement in the fracture toughness was observed compared with PVDF/MWCNTs nanocomposites.     

Specific heat and enthalpy of lattice disordering of LaF3 superionic crystals

Specific heat (C _p ) data obtained near the phase transition of LaF₃ are used to estimate the activation enthalpy for anion disordering, H _d. At the critical point T _c = 263 K, the concentration of disordered fluoride ions is n _c = 2.86 × 10²⁰ g⁻¹. In the dielectric phase of LaF₃ (T < T _c), H _d is 0.24–0.26 eV. In the superionic phase (T ≥ T _c), where the concentration of disordered fluoride ions exceeds n _c, the enthalpy of disordering drops to H _d ≃ 0.04 eV.     

Refractive index of polymethylmethacrylate oriented by fluid temperature under electrical field

We prepared micron and submicron polymethylmethacrylate (PMMA) layers by the spin-coating method. We investigated the possibility to orientate polymer dipoles in electric field in the glass transition area (T _g) and the fluid temperature of PMMA with the aim to increase its refractive index (n) after the layer is cooled below T _g. We have studied the effect of electric field (up to 12 kV cm⁻¹) on change of surface morphology of the layer, dependence of n and contact angle (surface wettability) on the field and dependence of layers orientation on orientation of electric field. The surface morphology was examined using atomic force microscopy (AFM), contact angles were measured by goniometer, film thickness was measured by profilometer, refractive index of films was determined using refractometer. The change of refractive index as dependent on the PMMA layer orientation in electric field depends on temperature and electric field. The highest change in n was found for electric field 11 kV cm⁻¹. The change in contact angle (wettability) on surface of an orientated PMMA layer confirms the dipoles orientation in electric field unambiguously. The orientation of layers causes a “slight” change in their morphology and a “slight” increase of surface roughness only for one direction of field effect. Change in colour for oriented layers does not depend on orientation of electric field.     

Effect of poly(<Emphasis Type="SmallCaps">L</Emphasis>-lactide) surface topography on the morphology of in vitro cultured human articular chondrocytes

Human articular chondrocytes were cultured in vitro on poly(L-lactic) acid, PLLA, substrates. Influence of the surface topography on cell morphology was found. Different surface microtopographies were obtained on PLLA by crystallizing at 120 °C after nucleation treatments that include isothermal stages at temperatures just below (55 °C) and just above (75 °C) the glass transition temperature (T _g = 65 °C). Isothermal crystallization from the melt gave rise to big spherulites (approx. 50 μm diameter) with approx. 1 μm depth. Crystallization after nucleation treatments results in smaller (approx. 5 μm)—difficult to distinguish—spherulites. Cell viability was excellent and not affected by the surface roughness. Cell population on the nucleated samples resembles the result of culture on the reference tissue culture polystyrene (TCPS). However, cells cultured on big spherulites (PLLA isothermally crystallized without nucleation treatment) show a peculiar morphology, with a more isolated disposition and growth oriented in a characteristic direction.     

Detailed Physical Characterizations of Cu-rich and Ru-poor (Ru0.9Cu0.1)Sr2YCu2O7.9

The optimized nominal composition, (Ru_0.9Cu_0.1) Sr₂YCu₂O_7.9 sample, has been prepared through high-pressure and high-temperature solid-state densification method. The obtained material has been studied by X-ray (laboratory) diffraction powder technique, magnetization and detailed magneto-transport measurements. The title compound indicates bulk magneto-superconducting properties under field strengths of H=10, 100, 500 and 1000 Oe. It shows diamagnetic transition at T _d=54, 38, 20 and 8 K for H=10, 100, 500 and 1000 Oe, respectively, in the zero-field-cooled susceptibility measurements. The high-field (H=5 and 10 kOe) molar susceptibility measurements show sharp ferromagnetic transition at ∼150 K with reduced molar susceptibility values. The various field dependence of magnetization, M(H), isotherm curves recorded at constant temperatures (5, 10, 25, 50, 100 and 150 K) indicate ferromagnetic saturation, whereas the MH curves measured at 200 and 300 K conditions reveal the paramagnetic state of the compound. Though the sample showed onset transition temperature, T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}, at ∼34 K under different field strengths (H=0, 10, 30, 50, 70 and 90 kOe), no T_c^R=0T_{\mathrm{c}}^{R=0} is seen down to 2 K. Even under relatively low applied field (ΔH=10 kOe) the title compound shows large negative magnetoresistance (MR) of about 68% at 2 K and increases with increasing the field strength up to ΔH=90 kOe (MR=77% at 2 K). This value is amazing and probably higher than other 1212 type ruthenocuprates. The title compound which shows little negative MR (about 1%) in the high temperature regions (125–300 K) is not affected much by different field strengths. Among the different fixed temperature MR(H) isotherms, the MR(H) curve measured at 5 K shows maximum negative MR of about 47% at 90 kOe compared to other four (T=50, 100, 200 and 300 K) MR(H) curves.     

Quasi-static and dynamic compressive behaviour of poly(methyl methacrylate) and polystyrene at temperatures from 293 K to 363 K

Flow stress, Young’s Modulus, energy and strain of fracture of poly(methyl methacrylate) (PMMA) and polystyrene (PS) were studied under compressive loading at strain rates of 10⁻⁴–10 s⁻¹ and temperatures from 293 K to temperatures ∼20 K below T _g. It was found that the energy of fracture shows an increase in the quasi-static strain rate (10⁻⁴–10⁻³ s⁻¹) region and becomes constant in the low strain rate (10⁻²–10 s⁻¹) region, while the strain of fracture shows a slow decrease with rate over the strain rate range tested. The activation energies and volumes of PMMA and PS at yield stress, 20% and 30% strain were evaluated using Eyring’s theory of viscous flow. ΔG was found to be constant for all strain rates and strains for both PMMA and PS. The activation volume for both materials increased as a function of strain.     

Dielectric response of polymer relaxors

Dielectric response of vinylidene fluoride type ferroelectric polymers is dominated by that of segmental motions in the amorphous phase in temperature range 200–300 K and contributions related to the local mode and ferroelectric–paraelectric transition in the crystalline phase of the polymer at higher temperatures. Diffuse and frequency-dependent dielectric anomaly observed in fast electron irradiated polyvinylidene fluoride-trifluoroethylene P(VDF/TrFE) has been related to relaxor-like behavior induced in the semicrystalline ferroelectric copolymers. As random field and the response of polar nanosize clusters determine the relaxor behavior the effects of disorder and fast electron irradiation (below and above T _C) on the three contributions to the dielectric response of PVDF, P(VDF/TrFE)(75/25) and P(VDF/TrFE)(50/50) are shown. The processes involved in radiation-induced functionalization of PVDF-type polymers are discussed on the basis of results of ESR, IR and Raman spectroscopy studies.     

Surface characteristics and blood compatibility of PVDF/PMMA membranes

Poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) membranes have important applications as biomaterials. In this study, PVDF/PMMA with varying ratios is blown into membranes, the surface property and blood compatibility of which are thoroughly investigated. Membrane surface characteristics including composition and topography exert considerable influence on the blood compatibility. Introduction of PMMA disturbs PVDF crystallization, however, favors the β phase crystal formation. PVDF content, crystallization ability, and surface enrichment have decisive effects on the membrane surface composition. Meanwhile, with increased PMMA fraction, the membrane surface roughness is also increased, and subsequently results in decreased hemocompatibility. While the membranes with PMMA content lower than 30 wt% show good blood compatibility, those with higher PMMA fraction exhibit obvious platelet adhesion to the surface. Thermal annealing promotes the formation of β phase PVDF and generates much smoother surface, thus endowing the membranes with greatly enhanced blood compatibility. These results show the prospect for optimization of processability, surface property, and blood compatibility of PVDF/PMMA membranes through facile modulation of PMMA content and fabrication process.