Glass Transition Of PMMA Research Articles

Development of Poly(methyl methacrylate)/nano-hydroxyapatite (PMMA/nHA) Nanofibers for Tissue Engineering Regeneration Using an Electrospinning Technique.

The study explores the in vitro biocompatibility and osteoconductivity of poly(methyl methacrylate)/nano-hydroxyapatite (PMMA/nHA) composite nanofibrous scaffolds for bone tissue engineering (BTE). Electrospun scaffolds, exhibiting both low and high fiber orientation, were investigated. The inclusion of hydroxyapatite nanoparticles enhances the osteoconductivity of the scaffolds while maintaining the ease of fabrication through electrospinning. SEM analysis confirms the high-quality morphology of the scaffolds, with successful incorporation of nHA evidenced by SEM-EDS and FTIR methods. DSC analysis indicates that nHA addition increases the PMMA glass transition temperature (Tg) and reduces stress relaxation during electrospinning. Furthermore, higher fiber orientation affects PMMA Tg and stress relaxation differently. Biological studies demonstrate the composite material's non-toxicity, excellent osteoblast viability, attachment, spreading, and proliferation. Overall, PMMA/nHA composite scaffolds show promise for BTE applications.

Thermomechanical Response of Fullerene-Reinforced Polymers by Coupling MD and FEM.

The aim of the present study is to provide a computationally efficient and reliable hybrid numerical formulation capable of characterizing the thermomechanical behavior of nanocomposites, which is based on the combination of molecular dynamics (MD) and the finite element method (FEM). A polymeric material is selected as the matrix—specifically, the poly(methyl methacrylate) (PMMA) commonly known as Plexiglas due to its expanded applications. On the other hand, the fullerene C240 is adopted as a reinforcement because of its high symmetry and suitable size. The numerical approach is performed at two scales. First, an analysis is conducted at the nanoscale by utilizing an appropriate nanocomposite unit cell containing the C240 at a high mass fraction. A MD-only method is applied to accurately capture all the internal interfacial effects and accordingly its thermoelastic response. Then, a micromechanical, temperature-dependent finite element analysis takes place using a representative volume element (RVE), which incorporates the first-stage MD output, to study nanocomposites with small mass fractions, whose atomistic-only simulation would require a substantial computational effort. To demonstrate the effectiveness of the proposed scheme, numerous numerical results are presented while the investigation is performed in a temperature range that includes the PMMA glass transition temperature, Tg.

Multilayered assembly of poly(vinylidene fluoride) and poly(methyl methacrylate) for achieving multi-shape memory effects

The alternately-organized poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) multilayer materials were prepared through layer-multiplying coextrusion. With the multiplication of layers, the thickness of each layer was reduced in proportion and the layer interfaces were enriched generating a broader and more continuous thermal transition temperature (Ttrans) from PVDF to PMMA layers as mapped by in-situ thermal analysis. The low-Ttrans side originated from the glass transition of PMMA, whereas the high-Ttrans side was dominated by the melting of PVDF crystals based on the heating curves of DMA and DSC. The dielectric spectroscopy and 2D-SAXS were performed and demonstrated that the compositional diffusion not only broadened the relaxation distribution of amorphous chains, but also strengthened the interaction between amorphous and crystalline domains. Therefore, a unique multilayer network, where the crystals in PVDF layers acting as physical networks connected the neighboring amorphous layers, was fabricated and its potential application in obtaining multi-shape memory effect (MSME) was disclosed for the first time. The results exhibited that the 1024-layer specimen owned a better triple- and quadruple-shape memory capacity than conventional blend which possessed the same compositions and a similar Ttrans range. The latter one even failed to successively memorize more than two temporary shapes. A possible mechanism was proposed through polarized IR and creeping-recovery measurements. Higher phase continuity which benefited for the stress transfer was revealed to play a significant role in strengthening the shape-fixing and -recovering ability during each shape-memory progress. Accordingly, a new physically-compounding strategy was addressed to achieve outstanding MSME for meeting complex demands in smart applications.

Feasibility analysis of hot incremental sheet forming process on thermoplastics

Incremental sheet forming has been proposed as a flexible manufacturing technique to process thermoplastic resins characterized by a glassy state at room temperature. Specifically, poly(methyl methacrylate) (PMMA) sheets were formed. A controlled room was designed, and the sheets were heated, before starting the forming phase, to a temperature above the PMMA glass transition, but avoiding onset of internal stresses that may lead to significant material springback at elevated temperature. Therefore, the process parameters, such as forming temperature and punch speed rate, have to be kept in well-defined ranges to be able to optimize the forming process of thermoplastic components by ISF. Furthermore, different sheet thicknesses were formed. Indeed, this size affects the flexural strength of the processed polymer as confirmed by reported tests. Experiments were planned out for the aim to take into account various process variables, i.e. spindle speed, step depth, punch diameter and feed rate. A plan based on a design of experiments (DoE) method was applied for a robust analysis. Macroscale observations were carried out to evaluate the product soundness, highlighting influences of the monitored process variables, on the process temperature and on the accuracy error of the formed parts. Furthermore, microscopic analyses evidenced the integrity grade of the surface on the side in contact with the punch for various combinations of process parameters. The results proved the process feasibility also for thermoplastics, which need to be heated before their forming phase.

High-strength thermoplastic bonding for multi-channel, multi-layer lab-on-chip devices for ocean and environmental applications

A solvent vapour thermoplastic bonding process is reported which provides high-strength bonding of PMMA over a large area for multi-channel and multi-layer microfluidic devices with shallow high-resolution channel features. The bond process utilises a low-temperature vacuum thermal fusion step with prior exposure of the substrate to chloroform (CHCl3) vapour to reduce bond temperature to below the PMMA glass transition temperature. Peak tensile and shear bond strengths >3 MPa were achieved for a typical channel depth reduction of 25 µm. The device-equivalent bond performance was evaluated for multiple layers and high-resolution channel features using double-side and single-side exposure of the bonding pieces. A single-sided exposure process was achieved which is suited to multi-layer bonding with channel alignment at the expense of greater depth loss and a reduction in peak bond strength. However, leak and burst tests demonstrate bond integrity up to at least 10 bar channel pressure over the full substrate area of 100 mm × 100 mm. The inclusion of metal tracks within the bond resulted in no loss of performance. The vertical wall integrity between channels was found to be compromised by solvent permeation for wall thicknesses of 100 µm which has implications for high-resolution serpentine structures. Bond strength is reduced considerably for multi-layer patterned substrates where features on each layer are not aligned, despite the presence of an intermediate blank substrate. Overall a high-performance bond process has been developed that has the potential to meet the stringent specifications for lab-on-chip deployment in harsh environmental conditions for applications such as deep ocean profiling.

Solid-state poly(methyl methacrylate) (PMMA) nanofoams. Part I: Low-temperature CO2 sorption, diffusion, and the depression in PMMA glass transition

In this paper, we report on the solubility and diffusivity of CO2 in PMMA in the range −30 °C to 100 °C, and on the depression in the glass transition temperature when the polymer acquires an equilibrium mass% of CO2 in this temperature range. Solubility of CO2 in PMMA increases ten-fold from 3.9% at 100 °C to 39.3% at −30 °C. We observe a drop in ΔHs, the heat of sorption, at the transition of CO2 from vapor phase to liquid phase, leading to a weaker dependence of solubility on temperature in the liquid CO2 regime. Four distinct diffusivity regions are identified, where diffusivity may increase or decrease with the decreasing temperature, characterized by the different activation energies. Besides the two transitions (normal vitrification and retrograde vitrification) between glassy state and rubbery state observed previously in PMMA-CO2, we also find a new transition in the vicinity of CO2 vaporization temperature. The dissolution of CO2 in PMMA lowers its glass transition temperature in a non-linear fashion and the incorporation of 39.3% CO2 in PMMA decreases its Tg from 103 °C to −12.5 °C. In part II of this paper, we describe a process, based on low-temperature CO2 saturation, that creates PMMA nanofoams with 30–40 nm cells.

Influence of Functionalized Graphene Sheets on Modulus and Glass Transition of PMMA

Influence of Functionalized Graphene Sheets on Modulus and Glass Transition of PMMA

Optical and radiative properties of clear PMMA samples exposed to a radiant heat flux

Radiative transfer through clear PMMA samples subjected to a constant radiant heat flux was studied. The aims of the present study were threefold. First, optical indices of clear PMMA were determined from reflectivity and transmissivity measurements. Clear PMMA exhibits a strong non-grey behavior, quite transparent in the visible and near infrared ranges of the spectrum, and almost opaque in some bands of the mid infrared. Absorption and refraction indices were obtained using an identification method applied to the transmissivity data of several samples with different thicknesses whenever possible (i.e. in ranges with sufficiently high transmission levels), whereas elsewhere, the Lorentz oscillators model coupled with a genetic algorithm was applied to the reflectivity data. Secondly, the absorption coefficient was determined at a very fine resolution of 4 cm−1. A nineteen-band model was proposed to represent the spectral variation of the absorption coefficient in the range of [590–18999 cm−1]. Third, a reciprocal Monte Carlo simulation was performed to calculate the response of a 3-cm-thick PMMA sample to a 18 kW m−2 externally incident radiation, involving absorption and internal emission into the solid. Radiative transfer, pure or combined with conduction and convection, was considered to simulate the early stages of sample heating (before the glass transition of PMMA) from two different radiative sources, namely a cone calorimeter and a tungsten lamp. The influence of the type of the radiation source was clearly demonstrated in terms of temperature distribution inside the material and divergence of the radiative flux.All input data (optical and radiative properties) are available upon request to the corresponding author.

Self-Assembly Characteristics of a Crystalline–Amorphous Diblock Copolymer in Nanoscale Thin Films

A diblock copolymer of crystalline polyethylene (PE) and amorphous poly(methyl methacrylate) (PMMA), PE69-b-PMMA92, was synthesized; this polymer is thermally stable up to 270 °C. The morphological structures of thermally annealed nanoscale thin films of the copolymer were investigated in detail at various temperatures by using in-situ grazing incidence X-ray scattering (GIXS) with a synchrotron radiation source. Quantitative GIXS analysis found that the PE and PMMA blocks undergo phase separation to produce a vertically oriented hexagonal PE cylinder structure in the PMMA matrix that is very stable up to around 100 °C (which is the onset temperature of PE crystal melting and PMMA glass transition); over the range 100–200 °C, slight variations with temperature in the cylinders’ dimensions and orientation were observed. Furthermore, the PE block chains of the cylinder phase crystallize and undergo crystal growth along the cylinders’ long axes; however, these lamellar crystals do not stack properly because ...

Single-material-based multilayered nanostructures fabrication via reverse thermal nanoimprinting

We demonstrated a multilayered, residual-free nanostructure by stacking a one-dimensional grating in an orthogonal arrangement via reverse thermal nanoimprinting and reactive ion etching technique using a single material (PMMA). We found that a good pattern transfer with minimal residual layers was mainly determined by four important factors: the concentration of PMMA in toluene solution, the type of fluorosilane used on the mold surface, and the temperature and pressure used in reverse nanoimprinting. The first layer was reverse-nanoimprinted at the temperature of 150 °C, while the subsequent layers were achieved at the onset temperature of the glass transition of PMMA. Experimental results showed that the compression of the bottom layer was inevitable due to a change in mechanical property of the PMMA after repeated reverse nanoimprinting process.

Role of salt concentration on conductivity optimization and structural phase separation in a solid polymer electrolyte based on PMMA-LiClO4

Although a large number of ionic conductors based on poly(methyl-methacrylate) (PMMA) are reported in literature, an optimization of salt concentration with respect to conductivity and stability properties remains by and large neglected. We report, perhaps for the first time, such an optimization of salt (LiClO4) concentration on structural, morphological, electrical, and ion–polymer interaction in PMMA-based solid polymer films. The active coordination site for the cation (Li+), out of the two possible electron donating functional groups (i.e. C=O and O–CH3) in PMMA, has been ascertained on the basis of evidences recorded in Fourier transform infrared spectrum. The results suggested C=O as the only possible site in PMMA matrix for coordination with Li+ cation. The X-ray diffraction results have clearly indicated an optimum limit of salt dissolution in PMMA matrix corresponding to O/Li = 4 (i.e., ~21wt.%) above which “phase-separation” occurs distinctly. The effect of salt concentration on amorphous → crystalline phase changes in PMMA and its correlation to morphology have been clearly observed in terms of their impact on electrical properties. An optimum electrical conductivity of ~7.2 × 10−5S cm−1 has been recorded at 100°C (~PMMA glass transition). The temperature dependence of conductivity follows typical Vogel–Tamman–Fulcher behavior.

Fabrication of nanopores with subnanometer precision on poly(methyl methacrylate) nanofibers by in situ electron beam irradiation

Nanopores with subnanometer precision were fabricated and fine-tuned in freestanding poly(methyl methacrylate) (PMMA) nanofibers by in situ electron beam irradiation. The nanopores were formed and expanded by focused electron beam irradiation, which was attributed to the sputtering and decomposition of PMMA. Meanwhile, the fabricated nanopores could also be shrunk and closed by low electron intensity irradiation, which may have been caused by surface tension induced mass flow due to the glass transition of PMMA. This work provides a new method to fabricate precise nanopores. It also provides important evidence about the theory of direct nanofabrication by electron beam irradiation.

Measurement of the x-ray dose-dependent glass transition temperature of structured polymer films by x-ray diffraction

The glass transition temperature (Tg) of polymer nanostructures was measured using a technique based on synchrotron x-ray diffraction from periodic grating structures. Poly(methyl methacrylate) (PMMA) nanostructures consisting of 1:1 lines:spaces with a 100 nm period and 100 nm height were characterized to have a Tg of 118 °C, which is comparable to the Tg of PMMA in bulk systems. The Tg of the PMMA structures also was measured as a function of absorbed x-ray dose. Doses ranging from 0 to 2400 mJ/mm3 were delivered to the PMMA structures prior to the Tg measurements; the Tg of the structures was found to decrease from 118 °C to 95 °C, respectively. The dose dependence of the PMMA glass transition temperature can be attributed to changes in the polymer molecular weight under exposure to x rays.

Relaxations of Poly(methyl methacrylate) Probed by Covalently Attached Anthryl Groups

The molecular relaxations of poly(methyl methacrylate) (PMMA) were studied using three different techniques: DSC, DMTA, which are sensitive to bulk properties, and the variation of fluorescence emission with temperature, a molecular sensitive method. Because this is a nonfluorescent polymer, we synthesized copolymers of methyl methacrylate with several contents of 9-methylanthryl methacrylate. The increase in the spacing between each anthryl group brought about a gradual decrease in Tg of the copolymers, leveling out at about 30−40 PMMA mers, indicating that the minimum spacing to attain the PMMA glass transition was situated in this length range. The fluorescence emission of a copolymer with 98 MMA units interspersed between each chromophoric unit was proved to be mechanically equivalent to PMMA homopolymer and was used to probe macromolecular motions involving shorter segments: apart from the main transitions commonly found for PMMA (α and β), the other two were detected at lower temperatures (γ and δ). Apparent activation energies for all relaxations using both techniques are also reported and discussed.

Studies of host‐guest thin films of corona‐poled betaine‐type polar molecules by kelvin probe technique and atomic force microscopy

AbstractIn this work betaine‐type molecules were investigated. As a result of the asymmetry of charge distribution, molecules possess in the ground state a considerable permanent dipole moment. The decay of surface potential of poled polymer films is dependent at least on two relaxation processes. The influence of glass transition of PMMA on thermal dependence of the surface potential is shown. The transition temperature, where no changes of the surface potential appeared, is related to glass transition temperature of the host‐guest system. The topography of the film surface was obtained by AFM.

Ac conductance and capacitance of carbon black polymer composites during thermal cycling and isothermal annealing

The ac electrical properties of acetylene black composites mixed into ethylene butylacrylate copolymer (EBA) and into poly (methyl methacrylate) (PMMA) have been measured in thermal cycling and isothermal annealing experiments. The results show that changes in electrical properties are due to rearrangement of gaps between the carbon black aggregates. This has been concluded using an exponent z that relates the critical frequency ωc denoting the crossover of the conductivity from the dc-plateau to its frequency-dependent part to the dc conductivity, σdc, according to ωc ∝σdcz. Below the melting range of EBA and the glass transition of PMMA z is about one corresponding to strong variation of the conductivity and weak dependence of the permittivity on the gaps. Above the melting range of EBA z is about 1.5, indicating strong dependence of both the conductivity and the permittivity on the gaps, as predicted by percolation theory. This was not found in the PMMA composites above the glass transition. We conclude that the polymer matrix affects the nature of the gaps between carbon black aggregates, either allowing their size to vary continuously (z about 1) or letting them open and close (z about 1.5).

Cryogenic Mechanical Alloying of Poly(methyl methacrylate) with Polyisoprene and Poly(ethylene-alt-propylene)

Mechanical alloying is performed at cryogenic temperatures to incorporate polyisoprene (PI) or its hydrogenated analogue poly(ethylene-alt-propylene) (PEP) into poly(methyl methacrylate) (PMMA) as an example of high-energy solid-state blending. Morphological characterization of the blends by X-ray and electron microscopies confirms that the degree of dispersion of the constituent polymers improves with increasing milling time. Such dispersion in the PEP/PMMA blends is, however, ultimately compromised by phase coarsening when the materials are postprocessed above the PMMA glass transition temperature in the melt. Milling-induced PI cross-linking serves to suppress phase coarsening in PI/ PMMA blends, which remain relatively well-dispersed even after postprocessing. These blends are generally less fracture-resistant than the as-received PMMA due mainly to the accompanying reduction in PMMA molecular weight. Their optical transparency is observed to decrease dramatically with increasing PEP or PI concentration until they appear opaque. An overall improvement in blend properties by mechanical alloying is, however, anticipated upon judicious selection of more degradation-resistant polymers.

Secondary Ion Mass Spectrometry of Polymers: a ToF SIMS Study of Monodispersed PMMA Standards

This paper is concerned with the effects that polymer molecular weight, casting solvent, sonification (with a non-polymer solvent) and annealing have upon the secondary ion mass spectrometry (SIMS) of polymers. Films of a series of monodispersed poly(methyl methacrylate) (PMMA) standards, Mw 2965–1200000, were prepared by solution casting from four solvents (tetrahydrofuran (THF), 2-butanone, toulene and chloroform) onto clean aluminium (Al) substrates. The PMMA films were characterized by high- and unit-mass resolution time-of-flight secondary ion mass spectrometry (ToF-SIMS). To determine the significance of variations seen in ion intensities, one PMMA sample was re-analysed 24 times, and the scatter in the absolute counts and normalized and ratioed ion intensities for specific (termed ‘key’) positive and negative ions were calculated. This provided 95% confidence error bars, which were subsequently employed. The 95% confidence limits were significantly reduced by normalizing or ratioing. Molecular weight was found to have the greatest effect in the SIMS spectra obtained, with the differences being most marked between Mw=2965 and Mw=89100. This was seen in both the total negative ion counts (m/z 31–200) and the key negative ion ratios. An explanation based on the surface concentration of chain ends is presented. Negative ion ratios were shown to be very sensitive to trace amounts of residual solvents. By annealing above the PMMA glass transition temperature Tg solvent-free films were produced from three solvents and these were spectrally indistinguishable. Solvent-free films could not be produced from THF. Residual solvent was identified by high mass resolution ToF-SIMS. Sonification with hexane, a polymer non-solvent, had a considerable effect on the total negative ion counts (m/z 31–200). No concomitant chemical changes were detected, so this effect is thought to arise from a physical perturbation of the surface. © 1997 by John Wiley & Sons, Ltd.

Phase behaviour of poly(ethylene oxide)/poly(methyl methacrylate) blends containing alkali metal salts

The effect of alkali metal salts on the phase behaviour of poly(methyl methacrylate) (PMMA)/poly(ethylene oxide) (PEO) blends was studied using d.s.c. Addition of lithium trifluoromethane sulfonate (LiT) to the normally miscible PEO/PMMA blends induced phase separation with the formation of a crystalline PEO phase, a crystalline PEO/salt complex phase and an amorphous PMMA phase. When potassium thiocyanate was used instead of LiT the mixture separated into two amorphous phases. Phase separation in these blends was not merely a result of the expected PEO/salt interaction, but also of the interaction of the salt with the PMMA. The evidence for this second interaction comes from an observed increase in the PMMA glass transition temperature with the addition of salt. Weak shifts in the carbonyl peaks in the i.r. spectra of the PMMA in the PMMA/salt mixtures suggest that the oxygen atom in the carbonyl interacts with the cation in the salt.

STRUCTURAL TRANSITIONS AND FREE-VOLUME PROPERTIES OF PMMA POLYMER STUDIED BY POSITRON ANNIHILATION

Positron lifetime and Doppler broadening of annihilation radiations have been measured for PMMA polymer in temperature range from 20℃ to 150℃. The O-Ps annihilation and Doppler broadening data consistently show the structural changes in PMMA. The transitions at 48℃ and 108℃ are attributed to the β transition and glass transition of PMMA respectively. An anomalous change of o-Ps lifetime and intensity around 84℃ is found to be the result of the mergence among some freevolume voids. The results of Doppler broadening measurement also give the information about the variations of degree of structural order. The temperature dependences of size and concentration of free volume in PMMA are estimated from the experimental o-Ps annihilation parameters.