Influence of laminate thickness reduction on the deformation mechanism of coextruded multilayered PC/PMMA films

The influence of the morphology of multilayered composites of poly(methyl‐methacrylate) (PMMA) and polycarbonate (PC) fabricated by layer multiplying coextrusion technique on their mechanical and especially their micromechanical deformation behavior was investigated. Electron microscopic studies revealed that the PC/PMMA multilayered composites have a well‐oriented, uniform, and continuous layered architecture. With decreasing layer thickness of each polymer in the composite, the elongation at break of the films was found to increase significantly which was correlated with a transition from a two‐component behavior (for single‐layer thickness of ≥8 μm) to an one‐component behavior (for single‐layer thickness of ≤250 nm). Rheo‐optical measurements using FTIR spectroscopy revealed that the molecular orientation during stretching of the PMMA phase remains unchanged for all the investigated films, whereas the PC orientation function decreases with decreasing layer thickness. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Isothermal physical aging of thin PMMA films near the glass transition temperature

Isothermal physical aging and the glass transition temperature (T _g) of PMMA thin films were investigated by means of differential scanning calorimetry (DSC). Freestanding thin films of different molecular weights (M _w = 120,000, 350,000, 996,000 g/mol) and film thicknesses (40–667 nm) were obtained by spin coating onto a silicon wafer substrate and then releasing the coated film using a water floating technique. The thin films were stacked in a DSC pan and isothermally aged for different aging times (t _a = 1 and 12 h) and aging temperatures (T _a = 105, 110, and 115 °C) below but near T _g. Enthalpy relaxation (ΔH _Relax), resulting from the isothermal physical aging, initially increased with increasing ΔT (T _g − T _a, driving force of aging), reached a maximum value, and then decreased with further increase in ΔT. Below ~100 nm film thickness, ΔH _Relax of samples aged near their T _g (i.e., T _a = 110 and 115 °C) decreased with decreasing film thickness, indicating the suppression of physical aging. Up to 9.9 °C depression in T _g was observed for thinner films (~40 nm), when compared to the thicker films (~660 nm) in this study. The decrease in ΔH _Relax with decreasing film thickness at a given T _a appears to be associated with the reduction in T _g.     

Study of the multilayered nanostructure and thermal stability of PMMA/PS amorphous films

The nanolayered structure of a forced-assembly of two immiscible amorphous polymers, prepared by layer-multiplying coextrusion, is analyzed for the first time by means of USAXS. The experimental long spacings for the series of PMMA/PS films studied show a good correlation with the nominal periodicity values of the stacks. In addition, the long spacings, derived from a localized area in AFM, are also in good agreement with the USAXS values averaged over much larger areas. The structural variation, after thermal treatment, of two samples with nominal periodicities of 174 and 215 nm is reported. In the range RT-140 °C, the nanolayered structure is mostly well preserved as evidenced by AFM. However, the absence of USAXS maxima after annealing at temperatures included in the above range has been tentatively explained by interfacial coarsening and spinodal dewetting occurring between the forced-assembled polymer layers. Above 140 °C, the interfacially driven break-up of the layers ends up with the final disappearance of the multilayered structure.     

Deformation-induced bonding of polymer films below the glass transition temperature

Bonding between polymers through interdiffusion of macromolecules is a well-known mechanism of polymer adhesion. A new polymer bonding mechanism in the solid state, taking place at ambient temperatures well below the glass transition value (T_g), has been recently reported; in this mechanism, bulk plastic compression of polymer films held in contact led to adhesion over timescales of the order of a fraction of a second. In this study, we prepared various blends of plasticized polymer films with desirable ductility from amorphous and semicrystalline powders of hydroxypropyl methylcellulose and polyvinyl alcohol derivatives; then, we observed the bonding of these polymers at ambient temperatures, up to 80 K below T_g, purely through mechanical deformation. The deformation-induced bonding of the polymer films studied in this work led to interfacial fracture toughnesses in the range of 1.0–21.0 J/m² when bulk plastic strains between 3% and 30% were imposed across the films. Scanning electron microscopy observation of the debonded interfaces also confirmed that bonding was caused by deformation-induced macromolecular mobilization and interpenetration. These results expand the range of applicability of sub-T_g, solid-state, deformation-induced bonding processes.     

Solubility and diffusion in latex films formed from high glass transition temperature particles

In situ steady-state fluorescence measurements were used to study the dissolution of polymer films. These films were formed from pyrene labeled poly(methyl methacrylate) (PMMA) latex particles that were sterically stabilized by polyisobutylene. Annealing was performed above the glass transition temperature at 180°C at 1-h time intervals for film formation. Desorption of pyrene labeled PMMA chains was monitored in real time by the change of pyrene fluorescence intensity. Dissolution experiments were performed in various solvents with different solubility parameters, δ, at room temperature. Diffusion coefficients, D, in various solvents were measured and found to be around 10⁻¹⁰ cm²/s. A strong relation between D and δ was observed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1493–1502, 1998     

Molecular probe technique for determining local thermal transitions: The glass transition at Silica/PMMA nanocomposite interfaces

Local glass transition temperatures (T_g) have been measured in the interfaces of solution blended silica/poly(methyl methacrylate) (PMMA) nanocomposites using florescence spectroscopy and compared with T_g measured by differential scanning calorimetry (DSC). It was found that the two types of measurements yielded significantly different information. Combinations of silanes and poly(propylene glycol)-based molecular spacers bound to fluorophores were covalently linked to the surface of the nanoparticles, allowing for variation of the fluorophore response with respect to the distance from the nanofiller surface. Increases in the bulk T_g from the neat PMMA value were found upon the addition of nanofillers, but were independent of the nanofiller concentration when the filler concentration was above 2% by weight. Furthermore, as the size of the grafted molecular spacer was increased, T_g values were found to decrease and approach T_g of the neat PMMA. Owing to variable conformations of the spacers, an effective distribution of fluorophore-silica distances exists, which influences the fluorophores’ response to the transition.     

Permanent deformation in armos fibre at a temperature below the glass transition temperature

The character of accumulation of permanent deformation of Armos fiber as a function of the temperature was established. The important change in the character of ɛ_per.(_ass.) in the temperature region of 80–100°C can be attributed to breaking of hydrogen bonds of the “polymer-water-polymer’ type. The special character of accumulation of permanent deformation is observed in dried Armos fibre. This is hypothetically explained by the effect of two mechanisms: the absence of hydrogen bonds in the initial stage of deformation and their formation in subsequent deformation as a result of convergence of the molecules. Deceased St. Petersburg State University of Technology and Design. Translated fromKhimicheskie Volokna, No. 3, pp. 36–38, May–June, 1999.     

Thickness and composition dependence of the glass transition temperature in thin random copolymer films

Glass transition temperatures (T_g) of thin poly(styrene-co-methyl methacrylate) and poly(2-vinyl pyridine-co-styrene) films coated on a native oxide surface of Si wafer (100) were measured by ellipsometry. The thickness dependence of T_g can be properly fitted by previously suggested equation developed for homopolymers, based upon a continuous multi-layer model, although one component in thin random copolymer films demonstrates a slightly favorable interaction between a substrate and thin film, and another demonstrates a strongly favorable interaction. Surface and interface have a strong influence on T_g of thin film coated on substrate: the surface has the effect of reducing T_g, whereas the interface increases the T_g according to the degree of interaction between a substrate and thin film. This degree of interaction can be quantified as an interaction parameter (k), and is dependent on the composition of random copolymers. For the estimation of k values of thin random copolymer films, we proposed a parallel type additive function (1/k_ran=w₁/k₁+w₂/k₂) where w is a weight fraction of component.     

SAXS study on the crystallization of PET under physical confinement in PET/PC multilayered films

The present work is concerned with the study of the development of the crystalline structure of poly(ethylene terephthalate) in multilayered films of poly(ethylene terephthalate)/polycarbonate (PET/PC) prepared by means of layer multiplying coextrusion. Small angle X-ray scattering patterns were recorded during isothermal crystallization experiments and evaluated by means of Ruland's interface distribution function. Thus, structural parameters describing the thickness distribution of crystalline and amorphous layers were determinated. It is shown that the crystallization of PET is delayed with increasing confinement. However when the crystallization process comes to an end, the values of the nanostructural parameters of the lamellar system are nearly the same for the confined and non-confined PET.     

The detachment behavior of polycarbonate on thin films above the glass transition temperature

When producing mono‐axially stretched films made of amorphous polycarbonate, a self‐reinforcement is generated due to the stretching process. This leads to an increase of the strength and stiffness. The mono‐axial stretching process is conducted at temperatures above the glass transition temperature, whereas better mechanical properties are obtained at higher stretching temperatures. However, the film tends to adhere to the rolls, especially at temperatures from 10°C above the glass transition temperature. The rolls of the mono‐axial stretching unit are made of an induction hardened and polished quenched and tempered steel 1.7225 – 42CrMo4. This work reports on the investigation of the detachment behavior of polycarbonate on different coatings as a function of the temperature and contact time. The main intention is to find a suitable coating on which the polycarbonate film adheres only slightly at temperatures clearly exceeding the glass transition temperature. POLYM. ENG. SCI., 56:786–797, 2016. © 2016 Society of Plastics Engineers     

Variations in the glass transition temperature of polyester with special architectures confined in thin films

Variations of the polymer dynamic of different systematically varied polyester architectures in the confinement of thin films were studied by temperature dependent spectroscopic vis-ellipsometry. The architectures were tailored in order to evaluate (a) the impact of different polymer backbones (hyperbranched, branched or linear and aromatic, aromatic-aliphatic or aliphatic), (b) the influence of functional groups (hydroxyl, benzoyl, tert-butyldimethylsilyl) and (c) the role of interfacial interactions (attractive, repulsive) with the silicon substrate. Possible reasons for the deviation of the glass transition temperature T_g in thin polymer films (10-800 nm) from the bulk value are described and compared to the literature. It was found that the functional groups of the applied polymers have the largest effect on T_g. Beside interfacial interactions, chemical and physical reactions in the polymer film are playing a significant role.     

An engineering procedure to calculate strain in glass above the glass transition temperature

A procedure based on an analysis of experimental data is proposed for determining the strain in glass above the glass transition temperature. Translated from Steklo i Keramika, No. 3, pp. 10–11, March, 1997.     

Transition of polymers from the fluid to the forced high-elastic and leathery states at temperatures above the glass transition temperature

The transition from the fluid to the forced high-elastic (rubber-like or quasi-vulkanizate) state and then to the leathery state (that is between rubbery and glassy states⁷) has been systematically studied by increasing stresses and deformation rates, using an uncured high molecular-mass 1, 2-polybutadiene characterized by a rather high glass transition temperature (?18°C). Investigation of polymers in simple shear is possible up to critical stresses corresponding to the transition of fluid polymers to FHES. Uniaxial extension offers wide possibilities for characterizing polymers in FHES and FLS since it covers the range of 4–6 decades of deformation rates over the range of simple shear. The deformability of polymers is limited by their fracture even in FHES, and the fracture process is determined only by recoverable deformation. The failure envelope covering FHES and transition to FLS has been constructed and the long-term durability in the range of 7 decades of time measured. The maximum extensibility corresponding to the transition of the polymer to FLS is determined by the ultimate extensibility of macromolecular coils. Over a wide range of stresses the polymer behaves like a linear viscoelastic body. This makes it possible to correlate the data obtained for uniaxial extension and low-amplitude sinusoidal shear deformation, which is important for the prediction of fracture phenomena by means of the low amplitude non-destructive dynamic method.     

Ratio of the glass transition temperature to the melting point in polymers

A study of the ratio of the glass temperature to the melting point of 132 polymers is described. Contrary to some other workers' observations on smaller numbers of polymers there is no sharp division between the ratios observed for symmetrical and unsymmetrical polymers. The arithmetic mean of the ratio is 0·63 and 0·69 respectively, but the combined standard deviation is 0·11. There is no correlation between the ratios and the crystalline forms of polymers.     

Study on glass transition temperature and mechanical properties of cadmium sulfide/polystyrene nanocomposites

The cadmium sulfide/polystyrene (CdS/PS) nanocomposites with concentration (0, 2, 4, 6, and 8) wt% of CdS nanoparticles were prepared by solution casting method and characterized through fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) measurements. The particle size of nanoparticles is found to be around 15 nm. Glass transition and mechanical behavior of CdS/PS nanocomposites were investigated using dynamic mechanical analyzer (DMA). The mechanical properties such as Young's modulus and tensile strength were determined at room, as well as at elevated temperatures through their stress–strain curves. The result shows that glass transition temperature (T_g) is shifted toward the higher temperature after the addition of CdS nanoparticles. The mechanical properties increased at low wt% loading of CdS nanoparticles and decreased for higher wt% loading of CdS nanoparticles. It was also found that mechanical properties decline with increase in the temperature. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Estimation of frequency from a temperature scanning rate in differential scanning calorimetry at the glass transition of polystyrene

A transform formulation of a heating rate β in differential scanning calorimetry (DSC) into frequency ω at the glass transition of polystyrene is proposed by using Duhamel’s theorem, a superposition integral based on a linear response theory.A heat capacity spectroscopy in a frequency range of eight orders of magnitudes for glass transition of polystyrene, including the results of temperature wave analysis and all the literature values, is well fitted with a Vogel-Fulcher-Tamman-Hesse (VFTH) curve. The origin of the discrepancies is discussed.     

Indentation and scratch behavior of functionalized MWCNT–PMMA composites at the micro/nanoscale

Polymethylmethacrylate (PMMA) and functionalized multiwalled carbon nanotube (F‐MWCNT) based composite films were prepared using solution casting method. Nanoindentation and scratch measurements were carried out to study the influence of F‐MWCNT as the reinforcement on the mechanical properties of the composite at the sub‐micron scale. The composites were prepared with varying weight percentages of F‐MWCNT in the PMMA matrix. The composites containing an adequate amount (0.25 wt%) of F‐MWCNT was found to demonstrate the maximum nanomechanical properties, viz. hardness, elastic modulus, recovery index. Scratch resistance measured in terms of coefficient of friction, also showed maximum value for the PMMA composite reinforced with 0.25 wt% of F‐MWCNT. POLYM. COMPOS., 35:948–955, 2014. © 2013 Society of Plastics Engineers     

Something about amber: Fictive temperature and glass transition temperature of extremely old glasses from copal to Triassic amber

In previous work we successfully used a stable Dominican amber to investigate the sub-glass transition dynamics of an organic glass with extremely low fictive temperature. The results provided an incentive to seek other stable ambers with lower values of fictive temperature. In the present work, a series of fossil resins from different locations, and ages ranging from approximately 100 years–230 million years, has been investigated using differential scanning calorimetry. The measurement results show the thermal signatures for each fossil sample: copals are unstable, and have lower glass transition temperatures than the amber samples. For the amber samples, there is not a systematic age dependence for the amber's glass transition temperature. Furthermore, even for the same sort of amber, the thermal properties can be different for different samples. The stability of the fossil resins was studied and glass transition temperatures were determined. For the stable samples, fictive temperature was also determined.     

Stress–strain behavior of SAN/glass bead composites above the glass transition temperature

Relaxation and stress–strain behavior of SAN–glass bead composites are studied above the glass transition temperature. The strain imposed on the polymeric matrix of the composite is defined as ?_p = ?_c/(1 ? ?^?). Stress relaxation data for the filled polymer which is independent of strain can be calculated by multiplying the relaxation modulus (at a certain strain) by (1 + ?_p). Stress–strain curves at constant strain rate and for different concentrations of the filler can be shifted to form a master curve independent of filler content if the tensile stress is plotted versus ?^p. The relaxation modulus increases with increasing the filler concentration and can be predicted by a modified Kerner equation at 110°C.