Mechanisms of the Complex Thermo-Mechanical Behavior of Polymer Glass Across a Wide Range of Temperature Variations

Enhancing Polymethyl Methacrylate Prostheses for Cranioplasty with Ti mesh Inlays

Biocompatible polymers such as polymethyl methacrylate (PMMA), despite fulfilling biomedical aspects, lack the mechanical strength needed for hard-tissue implant applications. This gap can be closed by using composites with metallic reinforcements, as their adaptable mechanical properties can overcome this problem. Keeping this in mind, novel Ti-mesh-reinforced PMMA composites were developed. The influence of the orientation and volume fraction of the mesh on the mechanical properties of the composites was investigated. The composites were prepared by adding Ti meshes between PMMA layers, cured by hot-pressing above the glass transition temperature of PMMA, where the interdiffusion of PMMA through the spaces in the Ti mesh provided sufficient mechanical clamping and adhesion between the layers. The increase in the volume fraction of Ti led to a tremendous improvement in the mechanical properties of the composites. A significant anisotropic behaviour was analysed depending on the direction of the mesh. Furthermore, the shaping possibilities of these composites were investigated via four-point bending tests. High shaping possibility was found for these composites when they were shaped at elevated temperature. These promising results show the potential of these materials to be used for patient-specific implant applications.

Prolonged Thermal Relaxation of the Thermosetting Polymers

The rigidity of structures made of polymer composite materials, operated at elevated temperatures, is mainly determined by the residual rigidity of the polymer binder (which is very sensitive to elevated temperatures); therefore, the study of ways to increase the rigidity of polymer materials under heating (including prolonged heating) is relevant. In the previous research, cured thermosetting polymer structure's non-stability, especially under heating, is determined by its supra-molecular structure domain's conglomerate character and the high entropy of such structures. The polymer elasticity modeling proved the significance of the entropy factor and layer (EPL) model application. The prolonged heating makes it possible to release adsorptive inter-layer bonds and volatile groups. As a result, the polymer structure is changing, and inner stress relaxation occurs due to this thermo-process, called thermo-relaxation. The present study suggests researching thermo-relaxation's influence on polymers' deformability under load and heating. The research results prove the significant polymer structure modification due to thermo-relaxation, with the polymer entropy parameter decreasing, the glassing onset temperature point (Tg) increasing by 1.3-1.7 times, and the modulus of elasticity under heating increasing by 1.5-2 times.

Raman spectroscopic insights into the glass transition of poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) is a very versatile polymer which is used as a glass substitute or as an economical alternative to polycarbonate for many types of important applications, due to its particular physical properties. In this study we deal with the Raman spectroscopic characterization of the glass transition of PMMA, the value of the glass transition temperature being generally a decisive parameter for determining the application of polymers. The information obtained by two-dimensional correlation spectroscopy (2DCOS) analysis and perturbation-correlation moving-windows spectroscopy (PCMW2D) analysis of the temperature dependent depolarized Raman spectra enabled us to recognize that the glass transition of PMMA is ruled by intermolecular interactions which influence the vibrational modes of the molecular groups associated with ν(C[double bond, length as m-dash]O), δa(C-H) of α-CH3 and/or O-CH3, ν(C-O-C), ν(C-COO), and ν(C-C-O). This information was employed for the temperature dependent study of the Raman shift and of the full width at half maximum of the Raman peaks obtained through anisotropic and isotropic Raman spectra, of the depolarization ratio, of the Raman spectroscopic noncoincidence effect, and of the Raman peak intensities represented by Arrhenius-type plots, all results supporting the outcomes of this work. The comparison with results obtained by differential scanning calorimetry and with published results in molecular dynamics studies was also part of this work. As the main result, one can highlight the peak associated with the ν(C-O-C) stretching mode at around 812 cm-1 as the one which presents the better outcome for explaining the glass transition from the molecular point of view.

Elastic modulus evolution of rocks under heating-cooling cycles

Rocks decay significantly during or after heating-cooling cycles, which can in turn lead to hazards such as landslide and stone building collapse. Nevertheless, the deterioration mechanisms are unclear. This paper presents a simple and reliable method to explore the mechanical property evolutions of representative sandstones during heating-cooling cycles. It was found that rock decay takes place in both heating and cooling processes, and dramatic modulus changes occurred near the α - β phase transition temperature of quartz. Our analysis also revealed that the rock decay is mainly attributed to the internal cracking. The underlying mechanism is the heterogeneous thermal deformation of mineral grains and the α - β phase transition of quartz.

Dynamic Viscosity and Transverse Ultrasonic Attenuation of Engineering Materials

In this paper, ultrasonic attenuation of the transverse waves of engineering materials is evaluated, covering metals, ceramics, polymers, fiber-reinforced plastics, and rocks. After verifying experimental methods, 273 measurements are conducted and their results are tabulated. Fifty of the tests are for the longitudinal mode. Attenuation behavior is determined over broadband spectra. The attenuation spectra are characterized in four patterns, with 2/3 of the tests showing linear frequency dependence and another ¼ linear spectrum plus Rayleigh scattering (Mason-McSkimin relation). The longitudinal and transverse damping factors varied from 0.004 to 0.065, which are 1/3 to 5 times those of polymethyl methacrylate, suggesting that almost all the engineering materials tested may be viscoelastic. The present test results make the term dynamic viscosity more appropriate for exploring the underlying processes. The observed results were compared between the longitudinal and transverse modes and among similar material types. In more than a half of the tests, the transverse attenuation coefficients were higher than the corresponding longitudinal attenuation coefficients by 1.5× or more. Some material groups had similar attenuation coefficients for the two modes. Since the physical basis for viscous damping is hardly understood, especially in hard solids, further studies from new angles are keenly desired. This collection of new attenuation data will be of value for such works. Practically, this will assist in materials selection and in designing structural health monitoring and non-destructive inspection protocols.

Thermal⁻Mechanical Coupling Behavior of Directional Polymethylmethacrylate under Tension and Compression

In this work, the quasi-static and dynamic mechanical behavior of directional polymethylmethacrylate is investigated under conditions of uniaxial compression and tension. The main purpose of this investigation is to discuss the effect of strain rate and temperature on the deformation characteristics and failure of such material. Research was carried out with the use of an electric universal testing machine and split Hopkinson bars, which were equipped with high- and low-temperature control systems. The experimental methods for studying the tensile and compressive response of polymer materials under different testing conditions were validated by one-dimensional stress wave theory and digital-image correlation technique. The finite deformation stress⁻strain behaviors of the samples under different loading condition were obtained with a constant temperature ranging from 218 to 373 K. The experimental results showed that the uniaxial tensile and compressive behaviors of directional polymethylmethacrylate under finite deformation are strongly dependent on temperature, decreased tensile and compressive stress of the material under different strain levels, and increased temperature. Meanwhile, the dynamic tensile and compressive stresses of the material are much higher than the quasi-static stresses, showing the strain-rate strengthening effect. Moreover, the tensile and compressive mechanical behavior of directional polymethylmethacrylate has significant asymmetry. Finally, a visco-hyperelastic model is established to predict the rate-dependence mechanical behavior of directional polymethylmethacrylate at different temperatures.