Thermal stability of poly(methyl methacrylate)

Summary The average solid-state molecular structures of end-groups generated through chain termination reactions in the polymerization of methyl methacrylate have been derived from published crystallographic data. Evidence is provided for the reduced stability of the head-head chain-termination configuration and in support of the postulate that it is a preferred site of chain scission. Comparable evidence for the unsaturated end group has not been found.     

Thermal degradation and flammability of nanocomposites composed of silica cross-linked to poly(methyl methacrylate)

The effect of polymer cross-linkages on thermal degradation of silica/poly (methyl methacrylate) (PMMA) nanocomposites is investigated using a single novel nanoparticle. Nanosilica surface treated with KH570, an organic surface treatment capable of free-radical polymerisation, was used to cross-link PMMA via an in situ method. Scanning electron microscopy was used to characterise nanosilica before use, while X-ray diffraction confirmed silica was well dispersed in PMMA. Thermogravimetric analysis (TGA) results showed that thermal degradation of silica cross-linked nanocomposites was significantly stabilised compared to PMMA, with a 30% reduction in the peak mass loss rate. Kinetic studies revealed the degradation of nanocomposites in this work abide by first-order kinetics, with an increase in the degradation activation energy of approximately 100?kJ?mol^?1. This is nearly double the improvement compared to conventional PMMA-silica nanocomposites in literature, showing dramatic enhancements to thermal stability. Analysis of high-temperature residuals from TGA tests suggest that cross-linked silica have increased char yields when compared with both PMMA and traditional silica nanocomposites. Cone Calorimetry results showed the materials in this work have reduced heat release rates compared to PMMA and traditional silica-PMMA nanocomposites.     

Thermal and photolytic degradation of plates of poly(methyl methacrylate) containing monomer

Specimens of 1.5 mm thick absorber-free poly(methyl methacrylate) (PMMA) containing ～0.6% monomer but no absorber have been photolytically degraded in air at 50°, 85° and 115°C and thermally degraded in air at 115° and 125°C. Specimens were exposed to a simulated solar spectral range. Degradation was followed by gel permeation chromatographic determinations of molecular weight as a function of depth in the specimens. The results show increased photodegradation at the plate faces (back and front) over that occurring in the centres, and a rapidly attained constant amount of degradation for thermal degradation. Degradation mechanisms are proposed. The thermal degradation is ascribed to weak links and unspent initiator. Photolytic initiation is ascribed, at least in part, to degradation of the ester group by wavelengths in the range 300 to 330 nm. The effect of oxygen is to convert alkyl radicals into peroxyl radicals, some of which form alkoxyl radicals which tne undergo β-scission to give in-chain ruptures. Where the oxygen concentration is low, monomer changes non-tertiary alkyl radical sinto tertiary radicals by addition to the monomer double bond. After their peroxidation by molecular oxygen, tertiary radicals react with one another to give alkoxyl radicals and subsequent chain scission rather than undergoing the Russell termination reaction with no chain scission characteristic of non-tertiary peroxyl radicals. The effect of temperature is mostly to decrease the importance of the cage effect and to allow the initial radicals formed to diffuse away from one another. The products of photo-oxidation absorb the shorter (300 to 330 nm) radiation significantly and progressively shield the remainder of the plate as degradation proceeds.     

Ultrasonic degradation of poly(methyl methacrylate

Ultrasonic degradation studies on a variety of molecular weights and molecular weight distributions of poly(methyl methacrylate) are reported. The extent of degradation was measured using gel permeation chromatography. Polydispersity decreased as a function of irradiation time for polymers with initial broad distributions. In contrast, polymers with an initial narrow distribution increased in polydispersity, passed through a maximum and then gradually decreased in polydispersity. Results appear to show no limiting degree of polymerization for poly(methyl methacrylate).     

Thermal expansion of irradiated poly(methyl methacrylate)

The thermal expansion coefficient of irradiated poly(methyl methacrylate) is measured in the temperature range 80–350K using a three-terminal capacitance technique. The samples are irradiated with gamma-rays from a ⁶⁰Co source in air at room temperature. The infra-red spectra are taken to indicate radiation-induced changes. The thermal expansion coefficient of poly(methyl methacrylate) is found to increase with radiation dose, the increase being larger at higher temperatures. This has been explained as due to the relative increase in the van der Waals interaction caused by radiation-induced degradation.     

Hydrolytic degradation of poly(l‐lactic acid)/poly(methyl methacrylate) blends

The hydrolytic degradation of poly(l ‐lactic acid)/poly(methyl methacrylate) (PLLA/PMMA) blends was carried out by the immersion of thin films in buffer solutions (pH = 7.24) in a shaking water bath at 60 °C for 38 days. The PLA/PMMA blends (0/100; 30/70; 50/50; 70/30; 100/0) were obtained by melt blending using a Brabender internal mixer and shaped into thin films of about 150 µm in thickness. Considering that PMMA does not undergo hydrolytic degradation, that of PLLA was followed via evolution of PLA molecular weight (recorded by size exclusion chromatography), thermal parameters (differential scanning calorimetry (DSC)) and morphology of the films (scanning transmission electron microscopy). The results reveal a completely different degradation pathway of the blends depending on the polymethacrylate/polyester weight ratio. DSC data suggest that, during hydrolysis at higher PMMA content, the polyester amorphous chains, more sensitive to water, are degraded before being able to crystallize, while at higher PLLA content, the crystallization is favoured leading to a sample more resistant to hydrolysis. In other words, and quite unexpectedly, increasing the content of water‐sensitive PLLA in the PLLA/PMMA blends does not mean de facto faster hydrolytic degradation of the resulting materials. © 2018 Society of Chemical Industry     

Thermal degradation of polymer mixtures. I. Degradation of polystyrene–poly(methyl methacrylate) mixtures and a comparison with the degradation of styrene–methyl methacrylate copolymers

The degradation behavior of mixtures of polystyrene and poly(methyl methacrylate) in the form of thin films cast from a solution containing both polymers has been compared with that of the individual polymers and of copolymers of the same monomer pair by means of thermal volatilization analysis (TVA) together with product analysis by gas-liquid chromatography determinations of the molecular weight of the solid polymer residues. The results indicate no interaction between the polymers when degraded together. The polymer mixtures are readily distinguished from copolymers of the same overall composition by TVA.     

Thermal Decomposition of Energetic Materials 69. Analysis of the kinetics of nitrocellulose at 50 °C–500 °C

Kinetic constants for decomposition of nitrocellulose in the 50 °C to 500°C range are analyzed. At T < 100°C, three processes (depolymerization, peroxide formation, and hydrolysis) are consistent with the reported kinetics. For T = 100°C–200°C, 28 of 30 previously reported kinetic measurements can be organized clearly into two categories by the use of the kinetic compensation effect. These two groups fit the first-order and autocatalytic processes. Conflicting interpretations are reconciled by this approach. At T > 200°C, the kinetics are consistent with the existence of the first-order step and desorption of the products as two parallel processes which, together, control the rate. Time-to-exotherm and mass burning rate kinetics are compared as temperature-dependent reaction-desorption events.     

Miscibility of poly(methoxymethyl methacrylate) and poly(methylthiomethyl methacrylate) with poly(methyl methacrylate)

Summary Poly(n-propyl methacrylate) is known to be immiscible with poly(methyl methacrylate) (PMMA). However, we have found that poly(methoxymethyl methacrylate) is miscible with PMMA, indicating the importance of ether oxygen atoms in achieving miscibility. On the other hand, poly(methylthiomethyl methacrylate) is immiscible with PMMA.     

Continuous distribution kinetics for ultrasonic degradation of poly(methyl methacrylate)

Ultrasonic degradation of poly(methyl methacrylate) (PMMA) was carried out in several solvents and some mixtures of solvents. The time evolution of molecular weight distribution (MWD), determined by gel permeation chromatography, is analysed by continuous distribution kinetics. The rate coefficients for polymer degradation are determined for each solvent. The variation of rate coefficients is correlated with the vapour pressure of the solvent, kinematic viscosity of the solution and solvent–polymer interaction parameters. The vapour pressure and the kinematic viscosity of the solution are found to be more critical than other parameters (such as the Huggins and Flory–Huggins constants) in determining the degradation rates. © 2001 Society of Chemical Industry     

Thermal shock behavior of Ti2AlC from 200°C to 1400°C

The thermal shock behavior of Ti₂AlC synthesized by means of self‐propagating high‐temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti₂AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al₂O₃. Although the flexural strength decreases quickly in the temperature range of 300°C‐500°C, no discontinuous decrease in the retained strength is observed in Ti₂AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti₂AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared with the first quench. Quenching at 300°C and above, voids after the pullout of grains and cracks are present, which however are absent in the un‐quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock.     

Natural rubber‐g‐methyl methacrylate/poly(methyl methacrylate) blends

The grafting of the methyl methacrylate (MMA) monomer onto natural rubber using potassium persulfate as an initiator was carried out by emulsion polymerization. The rubber macroradicals reacted with MMA to form graft copolymers. The morphology of grafted natural rubber (GNR) was determined by transmission electron microscopy and it was confirmed that the graft copolymerization was a surface‐controlled process. The effects of the initiator concentration, reaction temperature, monomer concentration, and reaction time on the monomer conversion and grafting efficiency were investigated. The grafting efficiency of the GNR was determined by a solvent‐extraction technique. The natural rubber‐g‐methyl methacrylate/poly(methyl methacrylate) (NR‐g‐MMA/PMMA) blends were prepared by a melt‐mixing system. The mechanical properties and the fracture behavior of GNR/PMMA blends were evaluated as a function of the graft copolymer composition and the blend ratio. The tensile strength, tear strength, and hardness increased with an increase in PMMA content. The tensile fracture surface examined by scanning electron microscopy disclosed that the graft copolymer acted as an interfacial agent and gave a good adhesion between the two phases of the compatibilized blend. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 428–439, 2001     

The transport of methyl methacrylate monomer in poly(methyl methacrylate)

The absorption kinetics and equlibria of methyl methacrylate monomer into poly(methyl methacrylate) were studied over a range of penetrant activities. The interval sorption kinetics at elevated activities were determined, compared, and contrasted with the integral sorption experiments in previously unpenetrated film samples. The sorption kinetics in previously unpenetrated films were predominantly case II or relaxation controlled at high activities. A Fickian contribution to the overall kinetics was apparent at lower activities. In contrast, interval sorption, at elevated activities in previously equilibrated and plasticized samples, followed Fickian kinetics rather closely, whereas resorption, over an activity range which involved a traversal of the effective T_g, was characterized by more complicated kinetics involving a super case II mechanism at long times. These composite results reinforce the notion that the kinetics describing penetration of a single penetrant into a single polymer are extremely sensitive to the boundary condition imposed upon the polymeric sorbent.     

Stabilization of thermal degradation of poly(methyl methacrylate) by polysulfide polymers

The thermal degradation of poly(methyl methacrylate) (PMMA) in the presence of polysulfide polymers, namely, poly(styrene disulfide) (PSD) and poly(styrene tetrasulfide) (PST) was studied using thermogravimetry (TG) and direct pyrolysis-mass spectrometric (DP-MS) analysis. Both PSD and PST were found to stabilize the PMMA degradation, which was explained by both radical recombination and a chain-transfer mechanism. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2149–2156, 1997     

Preparation and characterization of poly(methyl methacrylate)‐intercalated graphite oxide/poly(methyl methacrylate) nanocomposite

A poly(methyl methacrylate)‐intercalated graphite oxide/poly(methyl methacrylate) nanocomposite was prepared by emulsion polymerization of methyl methacrylate in the presence of graphite oxide (GO). GO was synthesized by the oxidization of natural graphite powder with KMnO₄ in concentrated sulfuric acid. The functional groups and microstructure of the oxidized graphite and the composite were carefully characterized by use of X‐ray diffraction, infrared, transmission electron microscopy, and elemental analysis. The electrical conductivity and mechanical properties were also measured. Polym. Eng. Sci. 44:2335–2339, 2004. © 2004 Society of Plastics Engineers.     

Dynamic mechanical properties of titanium dioxide-filled poly(vinyl acetate) at 0–40°C.

The dynamic mechanical properties of titanium dioxide-filled poly(vinyl acetate) have been studied at filler concentrations of 0, 10, 20, 30, and 40 wt.-% TiO₂ by using a torsional pendulum. The damping factor was found to increase with higher temperatures. At 40°C., the damping factors for the different TiO₂ concentrations were estimated to be the same. Damping factors above 40°C. were difficult to obtain due to the rubbery nature of the TiO₂–poly(vinyl acetate) systems. From 24 to 35°C., 10 wt.-% TiO₂-poly(vinyl acetate) was closer in damping factor increase to unfilled poly(vinyl acetate) than to the higher TiO₂-content polymers. At all temperatures, damping factors decreased with higher TiO₂ concentration. As the temperature decreased to 0°C., damping factors for the filled systems approached a common value. Potential energy of filled systems as indicated by shear modulus values is increased by higher TiO₂ concentrations and lower temperature. Kinetic energy for the filled systems, as shown by the out-of-phase modulus, is actually increased by larger filler concentration and higher temperature. A model is proposed where introduction of TiO₂ filler acts to increase general long-range polymer chain stiffening and at the same time enables short-range chain mobility to rise, possibly through greater side-chain motion.     

Thermo-oxidative stability of poly(methyl methacrylate)/poly(methyl methacrylate)-grafted SiO2 nanocomposites

Thermo-oxidative stability of PMMA-grafted SiO₂ and PMMA/PMMA-grafted SiO₂ nanocomposites was investigated by conventional non-isothermal gravimetric technique. It was interesting to find that PMMA-grafted SiO₂ nanoparticles exhibited higher thermo-oxidative stability than that of PMMA. The apparent activation energy of PMMA-grafted SiO₂ nanoparticles increased with the grafting ratio of PMMA from SiO₂, which was estimated by Kissinger method. This indicates that the strong interactions existing between the grafted chains are responsible for the enhanced thermo-oxidative stability of PMMA-grafted SiO₂ nanoparticles. However, the grafting ratio of PMMA from SiO₂ in nanoparticles has only limited effect on the thermo-oxidative stability of PMMA/PMMA-grafted SiO₂ nanocomposites due to a much lower content of grafted PMMA in the nanoparticles relative to PMMA. The increased thermo-oxidative stability of PMMA/PMMA-grafted SiO₂ nanocomposites is possibly resulted from the increased SiO₂ content in the nanocomposites, in which the grafting ratio of PMMA in PMMA-grafted SiO₂ nanoparticles is kept almost as a constant. The glass transition temperature (T _g) of PMMA/PMMA-grafted SiO₂ nanocomposites is about 25 °C and is higher than that of PMMA. The grafting ratio of PMMA from SiO₂ in the nanoparticles has no qualitative effects on the T _g of the nanocomposites.     

Thermal behavior and properties of polystyrene/poly(methyl methacrylate) blends

The thermal behavior and properties of immiscible blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA) with and without PS‐b‐PMMA diblock copolymer at different melt blending times were investigated by use of a differential scanning calorimeter. The weight fraction of PS in the blends ranged from 0.1 to 0.9. From the measured glass transition temperature (T_g) and specific heat increment (ΔC_p) at the T_g, the PMMA appeared to dissolve more in the PS phase than did the PS in the PMMA phase. The addition of a PS‐b‐PMMA diblock copolymer in the PS/PMMA blends slightly promoted the solubility of the PMMA in the PS and increased the interfacial adhesion between PS and PMMA phases during processing. The thermogravimetric analysis (TGA) showed that the presence of the PS‐b‐PMMA diblock copolymer in the PS/PMMA blends afforded protection against thermal degradation and improved their thermal stability. Also, it was found that the PS was more stable against thermal degradation than that of the PMMA over the entire heating range. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 609–620, 2004