Effects of thermal treatment of biaxially oriented poly(ethylene terephthalate)

Two distinguishable effects of thermal exposure of biaxially oriented poly(ethylene terephthalate) (PET) have been observed in the temperature range from room temperature to 140°C. Upon heating above the glass transition temperature T_g of the film an irreversible shrinkage of a few percent occurred with a concomitant decrease in the rate of creep. Some loss of orientation in the noncrystalline phase with an attendant slight increase in density is believed to be responsible. Since the film was anisotropic in its plane, different amounts and rates of shrinkage were observed along with differing thermal expansion coefficients in various directions relative to the primary optic axis. Upon cooling the 50% crystalline PET from above T_g to lower temperatures, reversible “physical aging” was observed. Creep rates were found to decrease with the residence time below T_g. As with purely amorphous polymers, the effects of the aging are removed by heating the specimen above T_g where the density of the amorphous phase achieves equilibrium values.     

Impact of chain architecture (branching) on the thermal and mechanical behavior of polystyrene thin films

The modulus and glass transition temperature (T_g) of ultrathin films of polystyrene (PS) with different branching architectures are examined via surface wrinkling and the discontinuity in the thermal expansion as determined from spectroscopic ellipsometry, respectively. Branching of the PS is systematically varied using multifunctional monomers to create comb, centipede, and star architectures with similar molecular masses. The bulk‐like (thick film) T_g for these polymers is 103 ± 2 °C and independent of branching and all films thinner than 40 nm exhibit reductions in T_g. There are subtle differences between the architectures with reductions in T_g for linear (25 °C), centipede (40 °C), comb (9 °C), and 4 armed star (9 °C) PS for ≈ 5 nm films. Interestingly, the room temperature modulus of the thick films is dependent upon the chain architecture with the star and comb polymers being the most compliant (≈2 GPa) whereas the centipede PS is most rigid (≈4 GPa). The comb PS exhibits no thickness dependence in moduli, whereas all other PS architectures examined show a decrease in modulus as the film thickness is decreased below ～40 nm. We hypothesize that the chain conformation leads to the apparent susceptibility of the polymer to reductions in moduli in thin films. These results provide insight into potential origins for thickness dependent properties of polymer thin films. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Water absorption in EVOH films and its influence on glass transition temperature

Moisture sorption kinetics of nonoriented ethylene vinyl alcohol copolymer (EVOH) film (EF‐E15) were studied at 25, 35, and 45°C. Anomalous diffusion was observed for the polymeric film at high relative humidities (RH) and higher temperatures. Diffusion and solubility coefficients of water were found to be concentration dependent. The moisture sorption isotherms of three types of EVOH films (EF‐E15, EF‐F15, and EF‐XL15) determined at 25, 35, and 45°C, were well described using the GAB equation. Glass transition temperatures (T_g) of the EVOH films, as influenced by RH, were measured using differential scanning calorimetry. T_g values decreased with increasing RH due to the plasticization effect of water, and were found to be dependent on ethylene content and orientation of the EVOH films. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 691–699, 1999     

Morphology of polylactic acid crystallized during annealing after uniaxial deformation

Uniaxial deformation of amorphous L -polylactic acid films was performed at two different temperatures at which thermal degradation was minimal, 70 °C or T_g + 10 and 90 °C or T_g + 30. Samples were annealed postdeformation for long times (either 15 or 45 min) to approach equilibrium conditions. Samples deformed and annealed at 70 °C showed low crystallinity and poor crystalline order or crystal size, as determined by wide-angle X-ray diffraction. At 90 °C, high crystallinity and order parameters were observed. In addition, once the oriented chains had crystallized at this temperature, nonoriented chains also underwent crystallization, and a small fraction of nonordered crystal phase was therefore observed after long annealing times. These observations are explained on the basis of different morphologies in samples drawn at the two temperatures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Orientation of uniaxially stretched poly(ethylene naphthalene 2,6‐dicarboxylate) films by polarized infrared spectroscopy

Poly(ethylene naphthalene‐2,6‐dicarboxylate) has been uniaxially stretched at different draw ratios and at two different temperatures below and above its glass transition (T_g ～ 120 °C) respectively, at 100 and 160 °C. Crystallinity has been evaluated from calorimetric analyses and compared to the values deduced by FTIR spectroscopic data. As expected, the obtained results are quite similar and show that films stretched at lower temperature (100 °C) are more crystalline than those stretched at 160 °C. Optical anisotropy associated with orientation has been evaluated by birefringence and show that films stretched at 100 °C are more birefringent than those stretched at 160 °C as a result of a higher chain relaxation above T_g. Polarized FTIR was also performed to evaluate the individual orientation of amorphous and crystalline phases by calculating dichroic ratios R and orientation functions 〈P₂(cos θ)〉 and also show that amorphous and crystalline phases are more oriented in the case of films stretched below T_g. Nevertheless, the orientation of the amorphous phase is always weaker than that of the crystalline phase. Films stretched at 100 °C show a rapid increase in orientation (and crystallinity) with draw ratio and 〈P₂(cos θ)〉 reaches a limit value when draw ratio becomes higher than 3.5. Films drawn at 160 °C are less oriented and their orientation is increasing progressively with draw ratio without showing a plateau. A careful measurement of the IR absorbance was necessary to evaluate the structural angles of the transition moments to the molecular chain axis. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1950–1958, 2007     

The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

Thermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from T_g + 25°C to T_g + 80°C. Between T_g and T_g + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time-scale shift factors a_T different from those of the terminal (rubberlike to steady-state) dispersion. The a_T values calculated from zero-shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (T_g ? 35°C). The a_T shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The a_T obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some a_Ts which cannot be represented by the usual Doolittle free-volume expression.     

Determination of the glass transition temperature in thin polymeric films used for microelectronic packaging by temperature-dependent spectroscopic ellipsometry

We characterized the glass transition temperature T_g of thin polyimide films by temperature-dependent spectroscopic ellipsometry and compared the results to DSC measurements of the bulk polymer. The effect of the curing temperature on T_g and the thermal expansion α(T) was analyzed. An improved ellipsometric data evaluation was used to get most precise and reliable T_g data. T_g increased with increasing curing temperature, while the bulk T_g was considerably lower than the thin film T_g. Both observations are attributed to the temperature sensitive release of the imidization by-product 2-hydroxyethyl methacrylate (HEMA) and crosslinker components as well as decomposition products from the material. Variation in the curing temperatures of 230–380 °C led to an increase in the T_g of 34 °C.     

Effects of POSS nanoparticles on glass transition temperatures of ultrathin poly(t‐butyl acrylate) films and bulk blends

As a model system, thin films of trisilanolphenyl‐POSS (TPP) and two different number average molar mass (5 and 23 kg mol^?1) poly(t‐butyl acrylate) (PtBA) were prepared as blends by Langmuir–Blodgett film deposition. Films were characterized by ellipsometry. For comparison, bulk blends are prepared by solution casting and the samples are characterized via differential scanning calorimetry. The increase in T_g as a function of TPP content for bulk high and low molar mass samples are in the order of ～10 °C. Whereas bulk T_g shows comparable increases for both molar masses (～10 °C), the increase in surface T_g for higher molar mass PtBA is greater than for low molar mass (～22 °C vs. ～10 °C). Nonetheless, the total enhancement of T_g is complete by the time 20 wt % TPP is added without further benefit at higher nanofiller loads. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 175–182     

Comparative Studies on Thermomechanical Behavior of Veriflex®, a Shape Memory Polymer,for a Low Strain (ϵm = 70%): Laser Experiments

An interesting comparative case study on thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE was done using Veriflex® at 62°C (T < T_g close to and below 5°C of T_g) and also at 72°C (T > T_g, close to and above 5°C of T_g) for slightly low strains (?_m = 70%) and the recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using both 70% strains at 62°C (T < T_g), as well as at 72°C (T > T_g). Recovery ratios for 70% strains at 62°C (T < T_g), as well as for 72°C (T > T_g) were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE, in particular, during the first thermomechanical cycles. A LSCM (Laser Scanning Confocal Microscopic) study shows very little change in surface structure which evolved during cycling up to 70% strains at 72°C (T > T_g).     

Drawing of poly(p-phenylene sulfide) by solid-state coextrusion

The effects of drawing temperature on the physical and mechanical properties of poly(p-phenylene sulfide) have been studied. A melt-quenched film was drawn by solid-state coextrusion both below (75°C) and above (95 and 110°C) the glass transition temperature T_g (85°C) of PPS. The maximum extrusion draw ratio (EDR_max) increased from 3.4 to 5.6 with increasing extrusion temperature T_e from 75 to 110°C. It was found that extrusion drawing just above the T_g of PPS (95°C) produced more stress-induced crystals. A high efficiency of draw in the amorphous region was achieved by extrusion at T_e-75°C. The tensile modulus at EDR_max decreased from 5.1 to 3.5 GPa with increasing T_e from 75 to 110°C. The low efficiency of draw for the samples extruded at 110°C is explained in terms of disentanglement and chain slippage during drawing due to a less effective network.     

The state of high global chain orientation but nearly random segmental orientation of amorphous polymers

In the rubbery state of amorphous polymers under uni-axial drawing the global chain orientation will relax in orders of magnitude slower than the relaxation of local segmental orientation. When this state of hot drawn sample is frozen at temperatures lower than its glass transition, T_g, an amorphous state with high global chain orientation but nearly random segmental orientation (GOLR) could result. Experimentally the GOLR state of amorphous polymers is easily realized by uni-axial drawing the polymer at temperatures 20–30°C above its T_g. The characteristic features of a GOLR state are i) large recovery of the elastic deformation of global chain on being heated to temperatures above T_g, ii) very small birefringence and IR dichroism, iii) nearly isotropic sonic and ultrasonic velocity of propagation, iv) nearly isotropic WAXD pattern, while it shows v) pronounced anisotropy in stress-strain behavior for large deformations and vi) appreciable anisotropy of thermal conductivity and of microwave dielectric properties. Concrete examples with detailed experimental results will be reviewed.     

Tearing energy study of “oriented and relaxed” polystyrene in the glassy state

To study the effect of processing history, molecular weight/molecular weight distribution, and thermal history on solid state properties (in particular fracture properties and orientation), carefully characterized polydisperse and monodisperse polystyrene samples were drawn above T_g and the orientation frozen in. The objective was to simulate the incidental orientation of polymer chains after processing, molding, and so forth (e.g., injection or compression, blow molding) as a result of melt flow. A series of polystyrene samples was produced by hot drawing at temperatures of 113 and 148 °C, followed by a relaxation period, and then a quench to below T_g. The level of segmental orientation imposed in the samples was determined by birefringence measurements. The tear energy of the sheets was measured at 20 °C by tearing along the draw direction, ultimately giving a value for the fracture energy, G_3C. Samples of high draw ratio and low segmental orientation were unexpectedly found to have highly anisotropic fracture properties despite the low level of optical anisotropy. The fracture properties also depended significantly on whether the samples were drawn with or without lateral constraint. The results are compared with measurements of isotropic samples and the findings of a previous investigation utilizing SANS and birefringence. Modeling the drawing conditions at the chain level using a recent nonlinear tube theory explains how birefringence alone is an inadequate measure of molecular orientation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 377–394, 2007     

Effects of thermal treatment on biaxially oriented poly (ethylene terephthalate). III. Creep behavior following various thermal histories

Elongational creep measurements were carried out on a biaxially oriented poly (ethylene terephthalate) (PET) film parallel to, orthogonal to, and at 45° to the principal optic axis. Measurements made after various thermal treatments which were intended to stabilize the physical state of the PET were shown to be ineffective. Samples were annealed at 140°C for 12 days and aged at 95°C for over 24 days before measurement without success. Thermal cycling between 41 and 91°C which was also employed to stabilize the mechanical response also failed. Significant deceleration of the creep rate caused by densification of amorphous regions of the samples during storage below the glass temperature T_g is illustrated. Because of physical aging below T_g and morphological changes occurring above T_g during the various thermal treatments and histories, time-scale shift factors were found to be not unique.     

The glass transition temperature of polystyrene

A round robin test was performed to determine the reliability of values for the glass transition temperatureT _g as determined by DTA on polymers. Ten different instruments were involved. The test material was high molecular weight polystyrene. Values forT _g (midpoint) were reported in the range 107°C±2 K. The respective heat flow curves differed considerably in shape. In the literature aT _g of 100°C is often given for polystyrene. The discrepancy between this value and the value of 107°C found in the round robin test is due to three differences: the thermal history of the sample, the evaluation of the heat flow curves, and the effect of finite sample size.     

Effect of anisotropic deformation on the differential scanning calorimetry response of polymer glasses

Large anisotropic deformation affects the physical state of a polymer glass, where the changes in the state of material are revealed by performing a differential scanning calorimetry (DSC) experiment. Previously, the deformation was applied to polymers well below their glass transition temperatures, and it was found that uniaxial compressive loading–unloading resulted in a broad exothermic peak on the DSC trace. Here we report on the effect on the subsequent DSC response of a deformation experiment performed in uniaxial extension on a ductile 50:50 co-polymer poly(BMA-co-MMA) (PBMA/MMA). The deformation of up to 80% strain was applied at T_g − 30°C and T_g − 40°C, that is, closer to T_g than in the previous work. Unlike in the well below T_g deformation case, the DSC trace contains an endothermic peak followed by an exothermic peak. The magnitude of the endothermic peak as well as the asymptotic glassy heat capacity increase with the amount of mechanical work performed during the deformation cycle.     

Visualization of strain-induced structural rearrangements in amorphous poly(ethylene terephthalate)

A direct microscopic observation procedure is applied to study the deformation of amorphous PET decorated with a thin metal layer when stretching is performed at different draw rates and at temperatures below and above the glass transition temperature T _g. Analysis of the formed microrelief allows stress fields responsible for the deformation of the polymer to be visualized and characterized. When tensile drawing is performed at temperatures above T _g, inhomogeneity of stress fields increases with the increasing draw rate; at high draw rates, the stress-induced crystallization of PET takes place. In the case of drawing the polymer at temperatures below T _g, direct microscopic observations make it possible to visualize the development of shear bands that appear in the unoriented part of the polymer specimen adjacent to the neck. The shear bands are oriented at an angle of about 45° with respect to the draw direction. When necking involves the unoriented part of the polymer, shear bands abruptly change their orientation and become aligned practically parallel to the draw axis.     

Determination of ultra-low glass transition temperature via differential scanning calorimetry

A novel method was developed to determine the ultra-low glass transition temperature (T_g) of materials through physical blending via differential scanning calorimetry. According to the Fox equation for polymer blends, a blend of two fully compatible polymers has only one T_g. The single T_g is a function of the T_gs of the two simple polymers. Thus, the ultra-low T_g of one material can be obtained from the T_gs of another polymer and their blends. The error of T_g measurements depends on the measurement error of the T_gs for the blends and another polymer. The method was successfully applied to determine the T_gs of acetyl tributyl citrate (ATBC), tributyl citrate (TBC) and poly(ethylene glycol)s (PEG)s with different molecular weights. The T_gs for ATBC, TBC, PEG-4000 and PEG-800 were ?57.0 °C, ?62.7 °C, ?76.6 °C and ?83.1 °C, respectively. For all the samples, the standard deviation of measurements was less than 3.3 °C, and the absolute error of measurements was theoretically not more than 5.3 °C. These results indicate that this method has acceptable precision and accuracy.     

Structure-property behavior of films containing disk spherulites: A model study

Thin films of a polyester of lactic and glycolic acid were prepared to give controlled amounts of disk spherulites. The spherulite contents ranged from zero to 100% and were accurately measured. The stress-strain properties of the films were then determined at 60°C, i.e., about 20°C above the glass transition temperature T_g. The mechanical behavior varied quite systematically with spherulite content and displayed little dependence on spherulite size. It was found that much of the mechanical data could be reasonably well described by a simple composite model. In addition, the yield strain as well as the strain to break could be principally coupled to the deformation of only the amorphous phase. SEM and optical microscopy studies supported the above conclusion, also demonstrating that the isolated spherulites adhered well to the amorphous matrix and behaved as stress concentrators in the system when the deformation temperature was above T_g.     

Plastic deformation of polypropylene. IV. Wide-angle x-ray scattering in the neck region

Wide-angle x-ray scattering (WAXS) patterns of two polypropylene samples, a quenched sample drawn at 21°C and an annealed sample drawn at 100°C, were investigated in a range of values of draw ratio λ very closely spaced through the neck region. In both cases, a range of small λ where deformation occurred by spherulite deformation was followed by one of higher λ where microfibrils were formed. The contribution to the WAXS pattern of microfibrils could be clearly distinguished from that of deformed spherulites because of the better orientation parallel to the draw direction of the former as compared to the latter. Additionally, for a drawing temperature of 21°C, microfibrils crystallize in the “smectic” phase as compared to the monoclinic phase for the initial sample and deformed spherulites. At this temperature, plastic deformation proceeds through the spherulite deformation mechanism up to λ = 1.4 accompanied by an increase in chain orientation with increasing λ. For λ > 1.4 plastic deformation appears to occur exclusively through microfibril formation. For drawing at 100°C, spherulite deformation is accompanied by very little change in chain orientation up to λ = 2, where microfibril formation begins. For λ > 2 (T_d = 100°C) plastic deformation is accompanied by both microfibril formation and some spherulite deformation as reflected by changes in both orientation and crystallite size. At this temperature the lateral crystallite size in the microfibrils is related to the long period according to the “equilibrium crystallite shape” previously found for annealed polypropylene.     

Heat Dissipation of Flying Wax Moths (Galleria Mellonella) Measured by Means of Direct Calorimetry

Heat production rates and flight speed of adult wax moths (Galleria mellonella) were investigated by means of direct calorimetry at T_A=20 and 30°C. Specific heat production rates were not significantly different between males and females at T_A=20°C (p_TH=747±123.7 mW g^-1, n=5 for males and p_TH=791±169 mW g^-1, n=5 for females) even with females having a higher body mass (M_B=83.8±21.6 mg, n=9 for males and M_B=146.4±25.7 mg, n=11 for females) and wing load. In females, heat production rates were dependent on temperature with higher heat production rates at T_A=20°C (p_TH=791±169 mW g^-1, n=5) than at T_A=30°C (p_TH=441±74 mW g^-1, n=6). Flight speed was also clearly correlated with T_A. Both males and females flew more slowly at T_A=20 than at 30°C. This revised version was published online in August 2006 with corrections to the Cover Date.