Modeling of creep and shrinkage behavior of polymeric films used in magnetic tapes

For ever increasing high recording densities of magnetic tape drives, improved dimensional stability of the polymeric films used as magnetic tape substrates is required. During storage and use, creep and shrinkage occur simultaneously and it needs to be minimized. To obtain constitutive relationships for creep and shrinkage, these contributions need to be separated and modeled. A mathematical model based on Kelvin–Voigt models has been developed to characterize simultaneous creep and shrinkage behavior to obtain the constitutive relationships for creep and shrinkage. Experiments have been performed to separate out creep and shrinkage effects and this model has been used to compensate the effect of shrinkage on creep data and to get true creep data. The experimental creep and shrinkage data of various films have been modeled to obtain viscoelastic parameters. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 78–88, 2004     

Viscoelastic and shrinkage behavior of ultrathin polymeric films

Viscoelastic and shrinkage characteristics of five ultrathin polymeric films are presented. These films include poly(ethylene terephthalate) or PET, poly(ethylene naphthalate) or PEN, an aromatic polyamide (ARAMID), a polyimide (PI), and poly(benzoxazole) or PBO. PET film is currently the standard substrate used for magnetic tapes, and the other four films represent alternative substrates with improved material properties. Thicknesses of the films range from 14.4 μm for PET to 4.4 μm for ARAMID. A creep apparatus is used to measure the viscoelastic and shrinkage characteristics of the films. The largest amount of creep compliance was measured for PET followed by PI, PEN, ARAMID, and PBO. Creep velocity was highest for PET and PI, followed by ARAMID, PEN, and PI. Shrinkage measurements at 50°C for 100 h show that PEN shrinks more than all the other substrates. Time–temperature superposition is used to predict long-term creep behavior, and relationships between polymeric structure and viscoelastic behavior are also discussed. Based on their relative cost and creep behavior, PEN and ARAMID substrates appear to be suitable alternatives to PET. © 1995 John Wiley & Sons, Inc.     

Nanoindentation behavior of ultrathin polymeric films

Measurement of the mechanical properties of nanoscale polymeric films is important for the fabrication and design of nanoscale layered materials. Nanoindentation was used to study the viscoelastic deformation of low modulus, ultrathin polymeric films with thicknesses of 47, 125 and 3000 nm on a high modulus substrate. The nominal reduced contact modulus increases with the indentation load and penetration depth due to the effect of substrate, which is quantitatively in agreement with an elastic contact model. The flow of the nanoscale films subjected to constant indentation loads is shear-thinning and can be described by a linear relation between the indentation depth and time with the stress exponent of 1/2.     

Creep and stress relaxation modeling of vinyl ester nanocomposites reinforced by nanoclay and graphite platelets

This article discusses the viscoelastic behavior of a vinyl ester (Derakane 411‐350) reinforced with 1.25 and 2.5 wt % nanoclay and exfoliated graphite nanoplatelets during short‐term creep and relaxation tests with a dynamic mechanical analyzer. Linear viscoelastic models are generally composed of one or more elements such as dashpots and springs that represent the viscous and elastic properties. Stress relaxation data from the dynamic mechanical analyzer have been used to obtain the elastic parameters based on model constitutive equations. The standard linear solid model, which is a physical model, has been used for predicting the creep deformation behavior of the vinyl ester nanocomposites over a wide temperature range. Some correlations have been made with the mechanical model, such as the effect of temperature on the deformation behavior, which is well explained by the dashpot mechanism. At lower temperatures, higher creep compliance has been observed for the vinyl ester versus the nanocomposites, whereas at temperatures near the glass‐transition temperature of the vinyl ester, creep compliance in the nanocomposites is closer in magnitude to that for the vinyl ester. The creep response of the pure vinyl ester and its nanocomposites appears to be modeled reasonably well at temperatures lower than their glass‐transition temperatures. A comparison of the predictions and experimental data from the creep tests has demonstrated that this model can represent the long‐term deformation behavior of these nanoreinforced materials reasonably well. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermally stimulated creep (TSCr) study of viscoelastic behavior and physical aging of a polymeric matrix composite for spacecraft structures

Thermally Stimulated Creep (TSCr) mechanical spectroscopy has been used to analyze molecular movements in KMU‐4lcarbon/epoxy composite material around the glass transition temperature. This technique is powerful to characterize the microstructure and micromechanical properties of the epoxy matrix and their evolution upon thermal aging. Three cooperative submodes have been distinguished by resolving the fine structure of the material complex α‐retardation mode. The elementary processes constituting this mode possess activation enthalpies and preexponential factors that strongly depend on the thermal history of the sample. The activation parameters of the composite are subject to perceptible evolution due to postcuring degradation. The α‐mode associated complex spectrum shifts towards higher temperatures by 27°C as a consequence of a series of quenching in the temperature range 260 to 0°C; the material shows a rise in the fragility and a deterioration in the crack‐growth resistance qualities. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 342–350, 2002     

Tensile and dynamic mechanical properties of improved ultrathin polymeric films

Tensile and dynamic mechanical properties of improved ultrathin polymeric films for magnetic tapes are presented. These films include poly(ethylene terephthalate) or PET, poly(ethylene naphthalate) or PEN, and aromatic polyamide (ARAMID). PET film is currently the standard substrate used for magnetic tapes; thinner tensilized‐type PET, PEN, and ARAMID were recently used as alternate substrates with improved material properties. The thickness of the films ranges from 6.2 to 4.8 μm. Young's modulus of elasticity, F5 value, strain‐at‐yield, breaking strength, and strain‐at‐break were obtained at low strain rates by using a tensile machine. Storage (or elastic) modulus, E′, and the loss tangent, tan δ, which is a measurement of viscous energy dissipation, are measured by using a dynamic mechanical analyzer at temperature ranges of ?50 to 150°C (for PET), and ?50 to 210°C (for PEN and ARAMID), and at a frequency range of 0.016 to 29 Hz. Frequency–temperature superposition was used to predict the dynamic mechanical behavior of the films over a 28 decade frequency range. Results show that ARAMID and tensilized films tend to have higher strength and moduli than standard PET and PEN. The rates of decrease of storage modulus as a function of temperature are lower for PET films than those for PEN and ARAMID films. Storage modulus for PEN films are higher than that for PET films at high frequencies, but this relationship reverses at low frequencies. ARAMID has the highest modulus and strength among the films in this study. The relationship between polymeric structure and mechanical properties are also discussed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2225–2244, 2002     

Mechanical,hygroscopic, and thermal properties of ultrathin polymeric substrates for magnetic tapes

Mechanical, hygroscopic, and thermal properties of improved ultrathin polymeric films for magnetic tapes are presented. These films include poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), and aromatic polyamide (ARAMID). PET films are currently the most commonly used polymeric substrate material for magnetic tapes, followed by PEN and ARAMID. The thickness of the films ranges from 6.2 to 4.8 μm. Tensile tests were run to obtain the Young's modulus, F5 value, strain at yield, breaking strength, and strain at break. The storage modulus, E′, and the loss tangent, tan δ, were measured using a dynamic mechanical analyzer (DMA) at temperature ranges of ?50 to 150°C (for PET) and ?50 to 210°C (for PEN and ARAMID) and at a frequency range of 0.016–28 Hz. Frequency–temperature superposition was used to predict the dynamic mechanical behavior of the films over a 28‐decade frequency range. Short‐term longitudinal creep behavior of the films during 10, 30, 60, and 300 s, 7 MPa, were measured at 25 and 55°C. Long‐term longitudinal creep measurements were performed at 25, 40, and 55°C for 100 h. The Poisson's ratio and 50‐h long‐term lateral creep were measured at 25°C/15% RH, 25°C/50% RH, 25°C/80% RH, and 40°C/50% RH. The in‐plane coefficient of hygroscopic expansion (CHE) at 25°C/20–80% RH and the coefficient of thermal expansion (CTE) at 30–70°C were measured for all the samples. The properties for all films are summarized. The relationship between the polymeric structure and the mechanical and physical properties are discussed, based on the molecular structure, crystallinity, and molecular orientation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3052–3080, 2003     

Molded geometry and viscoelastic behavior of film insert molded parts

Virgin injection‐molded tensile specimens without any inserted film and four kinds of film insert molded (FIM) tensile specimens were prepared. They were annealed at 80°C to investigate the effect of residual stresses and thermal shrinkage of the inserted film on thermal deformation of tensile specimens. The FIM specimens with the unannealed film were bent after ejection in such a way that the film side was protruded and the warpage was reversed gradually during annealing and the film side was intruded. Warpage of the FIM specimen with the film annealed at 80°C for 20 days was not reversed during annealing. Processing of the FIM specimens have been modeled numerically to predict thermoviscoelastic deformation of the part and to understand the warpage reversal phenomenon (WRP). Nonisothermal three‐dimensional flow analysis was carried out for filling, packing, and cooling stages. The flow analysis results were transported to a finite element stress analysis program for prediction of deformation of the FIM part. The WRP was caused by the combined effect of thermal shrinkage of the inserted film and relaxation of residual stresses in the FIM specimen during annealing. It is expected that this study will contribute towards the improvement of the FIM product quality and prevention of large viscoelastic deformation of the molded part. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dual‐curable unsaturated polyester inorganic/organic hybrid films

An unsaturated polyester, based on maleic anhydride, 1,6‐hexanediol, and trimethylol propane, was formulated with tetraethylorthosilicate (TEOS) oligomers and a coupling agent to prepare inorganic/organic hybrid films. TEOS oligomers were prepared through the hydrolysis and condensation of TEOS with water, and 3‐(triethoxysilyl)propylisocyanate was used as the coupling agent between the organic and inorganic phases. The hybrid materials were cured by moisture via sol–gel chemistry and by the UV curing of unsaturated polyesters. To compare the properties of the moisture‐cured inorganic/organic hybrid films, a conventional 2K polyurethane system was also prepared. The tensile, adhesion, abrasion, and fracture toughness properties were investigated as functions of the coupling agent and relative amount of UV cure versus thermal cure. Although no difference could be observed in the tensile properties, the abrasion resistance, fracture toughness, and adhesion were enhanced by the incorporation of TEOS oligomers into polyurethane films. Also, the abrasion resistance, fracture toughness, and tensile properties were increased with both moisture and UV exposure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 115–126, 2006     

Correlations between creep,shrinkage, and dynamic mechanical characteristics of magnetic tape materials

Creep compliance, shrinkage, and dynamic mechanical analysis (DMA) results are presented and discussed for developmental magnetic tapes made from PEN and metalized PET (Spaltan®) substrates as well as PEN substrate samples cut from wide‐stock in the machine and transverse directions. Curve fit parameters from the Kelvin‐Voigt model are discussed to shed light on the creep‐compliance characteristics, particularly the roll‐off characteristics observed at elevated temperatures and long time periods. Characteristic peaks observed in storage and loss moduli measured using DMA that correspond with molecular movement provide information that assists with the understanding of creep‐compliance and shrinkage behavior for these materials. Such movement corresponds with dimensional instabilities that need to be understood for future generations of advanced digital magnetic tapes. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Characterization of viscoelastic behavior of shape memory epoxy systems

Viscoelastic behavior has a remarkable impact on the functional realization of shape memory polymers and their composites. Our previous work reported that a series of shape memory epoxies with varied curing agents and contents were synthesized and exhibited higher shape fixture and recovery rates. The viscoelastic behavior of the materials at different temperatures is experimentally investigated in this study. Stress–strain hysteresis under uniaxial tension, stress relaxation, and creep tests are performed. The energy dissipation factor and residual strain factor as functions of temperatures are presented in the basis of stress–strain hysteresis tests. Moreover, the effects of test temperature, curing‐agent type, and content on the viscoelastic behavior of these materials are discussed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Creep resistant polymeric nanocomposites

In the present study, one of the unique improvements in polymer nanocomposites has been detected. Only with a very low volume fraction of inorganic nanoparticles, the creep resistance of thermoplastic could be significantly improved. 21 nm-TiO₂/PA6,6 nanocomposites were compounded using a twin-screw-extruder. The final specimens were formed using an injection-moulding machine. Static tension and tensile creep tests were carried out at room and an elevated temperature (50 °C). It was found out that the nanoparticles contributed to a remarkable reduction of the creep rate under various constant loads at both temperature levels. It is assumed that the nanoparticles restrict the slippage, reorientation and motion of polymer chains. In this way, they influence the stress transfer on a nanoscale, which finally results in these improvements.     

Anisotropic character and ultrasonic stiffness changes during the natural weathering of LDPE films

Blown extruded polyethylene films without stabilizers have been exposed outdoors under severe weathering conditions in the Sahara. The chemical aspect of aging has been followed by IR spectroscopy. The mechanical aspect of aging has been monitored by means of a nondestructive method. It consists of measuring velocities and attenuations of ultrasonic waves propagating in several directions in the film plane. Stiffness constants and energy dissipation terms have been calculated. From the results obtained it is shown that stiffening of the material leads to an increase of velocities and a decrease of wave attenuation. Moreover, the stiffness constants as well as the energy dissipation terms vary with aging and show a changing anisotropic character of the films. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 559–564, 2003     

Characterization of cross‐linking depth for thin polymeric films using atomic force microscopy

Thin polymeric films made with various elastomers, like polyisoprene, and elastomer composites were prepared for characterization of cross‐linking depth in this study. Various cross‐linking methods have been applied to get mechanically stronger, more thermally stable and chemically resistant polymer coatings. However, there is no existing approach that could effectively characterize the degree or depth of cross‐linking for thin polymer films. The objective of this work is to use atomic force microscopy to characterize cross‐linking depth in a precise way. Hyperthermal hydrogen bombardment‐induced cross‐linking was employed as a cross‐linking method and the depth of cross‐linking was estimated via local change of the elastic modulus along the sample cross‐section with precise force measurement and high spatial resolution. It is found that the cross‐linking depth is closely related to the chemical composition of thin films. Understanding the depth of cross‐linking is vital for a broad range of applications. It is believed that the developed technique is also applicable for studying other cross‐linkable materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41493.     

Viscoelastic analysis applied to the determination of long‐term creep behavior for magnetic tape materials

Creep‐compliance experiments were performed for three representative magnetic tapes. Two of these tapes used a magnetic particle (MP) coating, and one used a metal‐evaporated (ME) coating. The MP tapes used the following polyester substrates: semitensilized poly(ethylene naphthalate) (PEN) and supertensilized poly(ethylene terephthalate). The ME tape used an aromatic poly(amide) or aramid substrate. Time–temperature superposition was used to make creep‐compliance predictions at 30 and 50°C reference temperatures. Comparisons were made with dimensional stability requirements based on position error signal (PES) specifications for magnetic tape drives along with in‐cartridge creep specifications based on PES measurements. Circumferential and lateral creep strains were determined that account for storage of the tapes in a reel, and creep strains were predicted for future tapes with thinner, lower compliance coatings. A rule of mixtures method was also used to extract compliance information for individual layers of MP‐PEN tapes, and stress profiles through the thickness of the tapes were determined. Additional measurements and analyses were performed to determine the creep recovery and shrinkage characteristics for the magnetic tapes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1106–1128, 2006     

Time‐Temperature Creep Behaviour of Poly(propylene) and Polar Ethylene Copolymer Blends

Polymers commonly undergo deformation under an applied stress over their lifetime; some deformations are irrecoverable once the source of stress is removed. Therefore an understanding of the response of a polymer can be achieved by investigating the viscoelastic properties using creep experiments, where the behaviour can be monitored under small deformational loads. Poly(propylene) (PP) was blended with a polar elastic, thermoplastic, poly[ethylene‐co‐(methyl acrylate)] (EMA), to toughen the matrix. EMA formed a dispersed phase in PP that maintained its strength through its crystallinity rather than crosslinking. EMA can form a compatible interface with PP through inclusion of maleated‐PP as a compatibiliser. The viscoelasticity of the PP–EMA blends, particularly the creep behaviour is an important factor if the properties of PP are to be maintained. The creep and recovery of PP–EMA blends with varying compositions were investigated under different loads and number of cycles. High EMA content provided an alternative deformation pathway due to its elastomeric properties. The experimental creep behaviour has been evaluated using the 4‐element model with some limitations evident in the viscoelastic transitional region.

     

Crystallization and morphology of ultrathin films of homopolymers and polymer blends

Ultrathin films of thicknesses below 100 nm are now considered in different areas of applications. Their behavior in term of kinetics of crystallization is very different from that of bulk samples due to the film confinement in two-dimensions, and their morphologies are unique. In this review, recent advances in the crystallization of ultrathin films of homopolymers and miscible polymer blends will be described, with an emphasis on morphologies and, in the case of blends, on mixtures made of two crystalline polymers.     

Measurements of vapor sorption/desorption properties of ultrathin polymeric films

After a brief review of various standard methods of diffusion measurements of vapors into and out of polymeric solids, a direct gravimetric method for studying the sorption/desorption process in thin polymer films is discussed. The apparatus used consisted of a sensitive electrical balance housed in a vacuum pump. The sorption and desorption characteristics of thick polystyrene films were studied for comparison with literature values. The system polystyrene/methylene chloride was chosen to calibrate the system because of its known data. Experiments were carried out using the polystyrenes PS-5 and Dow Trycite 1000 at 35 and 50 mm Hg. Results agreed well with those reported in the literature. The apparatus was also suitable for measuring the very initial uptake of vapor. This information is important in the study of very thin films where the amount of vapor absorbed is minimum and the initial rate of uptake is almost instantaneous.     

Lamellar arrangements of linear polyethylene in ultrathin films

The crystal orientations of linear polyethylene films on silicon substrates are investigated using grazing incidence X‐ray diffraction and atomic force microscopy. From diffraction analysis, we can identify the structural arrangement of PE crystals in ultrathin film. The orientation of lamellar crystal in PE films changes from edge‐on to flat‐on with the decrease of film thickness in the film thickness below ～ 100 nm. The slightly inclined lamellae relative to the substrate are found to coexist with the flat‐on lamellae in thin PE films that we have investigated. We find that the crystal orientation and structures is governed by the constraint imposed by film thickness rather than enthalpy gain as the film got thinner especially in the thickness below 200 nm. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Studies on polymer latex films: IV. Comparison of the permeability of latex and solvent-cast films

Latex and solvent-cast films have been prepared from poly(butyl methacrylate) surfactant-free latices and their permeabilities to water vapour and an aqueous organic solute (p-nitrophenol) have been studied. Latex films show considerable ageing effects and their performance is very dependent upon the conditions of film formation but their barrier properties are not necessarily inferior. A greater dependence on film orientation is evident for latex films.