Kinetic and equilibrium phenomena in the system: Acetone vapor and polycarbonate film

Amorphous films of Lexan polycarbonate have been exposed to acetone vapor at controlled temperatures and partial pressures in order to study sorption kinetics and thermodynamics and polymer crystallization behavior. Sorption isotherms show a discontinuity is slope at or near the depressed glass transition, which itself was identified by torsion pendulum measurements. Crystallization abruptly begins to occur at partial pressures equal to or slightly above that of the solubility transition and is manifested by delayed desorption and whitening phenomena. In this process 20% crystallinity is usually developed, as measured by calorimetry which, however, produces a 40% drop in acetone solubility. Although the depressed glass temperature is near 0°C. in saturated atmospheres—a drop of 145°C.—the melting point is only depressed 60 or 70°C. Such disparity probably accounts for the enhanced polycarbonate crystallization rate in acetone over that in the dry bulk polymer above the normal T_g.     

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     

Brillouin scattering from amorphous bisphenol-A polycarbonate

Brillouin scattering has been studied from amorphous bisphenol-A polycarbonate in the temperature interval 60–240°C. Both longitudinal and transverse Brillouin peaks are observed over the entire range. The behavior of both types of Brillouin splittings, Δω_l and Δω_t, in the region of the glass–rubber relaxation is typical of an amorphous polymer. Equilibrium values of Δω_l and Δω_t were obtained 20°C below the glass-transition temperature T_g determined at cooling rates of 20°C/hr. Comparison of the present results with previous ultrasonic data reveals a considerable dispersion in the longitudinal phonon velocity below T_g. The origin of the large transverse Brillouin intensities is related to the structure of polycarbonate.     

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     

Polymer thin film properties as a function of temperature and pressure

We report on evanescent wave optical measurements of the glass transition temperature, T_g, of spin-cast PMMA films as a function of film thickness and molecular weight. It was found that for films of high molecular weight PMMA (M_n > 100,000 g mol⁻¹) a strong T_g depression occurs for films that are thinner than 100 nm in case they are deposited on hydrophobic substrates. This strong T_g depression of up to 25°C decreases if similarly thick films of PMMA of low molecular weights are investigated and vanishes completely for PMMA with M_n < 12,000 g mol⁻¹. For films made of these materials T_g is found to be identical to that of the bulk even for films as thin as 5 nm. The results might be interpreted in terms of free volume considerations. To check this assumption we also designed and built a pressure cell that can be used together with the evanscent wave optical techniques for similar measurement, but with the additional option to do the measurements at different pressures up to ca. 100 MPa to further vary the free volume of these polymer films in constrained geometry. Some first results obtained with this setup are also described.     

Photochemistry in polymer solids-7. Photochromic reaction of spiropyran in polycarbonate film

The formation of coloured merocyanine form of spirobenzopyran molecularly dispersed in bisphenol-A polycarbonate by a single laser pulse irradiation at 337 nm and the following rate of decoloration by monitoring light irradiation at 560 nm were measured over the temperature range 80–453 K. The coloration of spiropyran in polycarbonate film proceeds within several nanoseconds and even at temperatures below T₇ of the matrix polymer (−120°C). The photo-induced decoloration of merocyanine form in the film proceeds exponentially for T >T_g (150°C), but deviates from single exponential type kinetics for T < T_g probably due to the inhomogeneous distribution of free volume in the matrix polymer. Arrhenius plots of the apparent rate coefficients for the decoloration showed breaks at T_g, T_β and T_γ of the matrix polycarbonate. A brilliant red fluorescence was observed from the merocyanine form of spiropyran.     

Physical aging of polycarbonate investigated by dynamic viscoelasticity

The physical aging of polycarbonate was investigated with dynamic viscoelastic measurements. Physical aging was observed for samples aged at 110 °C (QA) and room temperature (QP) after being quenched from the molten state. The shapes of the temperature dispersion curves of the dynamic viscoelastic functions (E′, E″, and tan δ) of the QA and QP samples changed with aging time in a temperature range below the glass‐transition temperature (T_g). However, at temperatures close to but below T_g, the curves for the aged samples merged into the curve of the quenched sample at a temperature denoted T_H. T_H increased with aging time. The experimental results suggest that the aged sample has a memory of having been quenched and that as the sample approaches the equilibrium state, this memory is lost. Differential scanning calorimetry thermograms showed an endothermic peak below T_g for the QA samples. The peak temperature (T_p) also increased with aging time. T_H and T_p of the QA samples were approximately the same. The increase of both T_H and T_p with aging time indicates that the structure of the polymeric chain in the glassy state relaxes over larger segment scale lengths because the scale of the movable segments is related to temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 337–341, 2001     

Thermally stimulated discharge current studies on the effect of thermal treatment on the strength of polycarbonate

It is well known that polycarbonate annealed at 80–130°C undergoes gradual changes in mechanical properties. Annealing below T_g (ca. 150°C) results in a decrease in impact resistance and an increase in strength. Polycarbonate has three single relaxation processes and some distributed relaxation processes in the temperature range between 100 and 250°K (the β transition region). The effect of thermal pretreatment on the relaxation has been investigated by the thermally stimulated discharge current technique. Partial heating, peak cleaning, and theoretical fitting have also been performed and the activation parameters associated with the relaxation processes have also been calculated to assist in the analysis of the relationship between effects of annealing and structural motions in polycarbonate.     

Glass transition of polymer/single-walled carbon nanotube composite films

The glass-transition temperatures (T_g's) of nanocomposites of polystyrene (PS) and single-walled carbon nanotubes were measured in the bulk and in thin films with differential scanning calorimetry and spectroscopic ellipsometry, respectively. The bulk T_g of the nanocomposites increased by approximately 3 °C and became much broader than that of PS. For the nanocomposite films thinner than 45 nm, T_g decreased with decreasing film thickness [i.e., ΔT_g(nano) < 0]. This phenomenon also occurred in thin PS films, the magnitude of the depression in PS [ΔT_g(PS)] being somewhat larger. The film thickness dependence and the differences in the magnitude of ΔT_g in the two systems were examined in light of current theory, and a quantitative comparison was made. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3339–3345, 2003     

Acceleration of the cationic polymerization of an epoxy with hexanediol

Thin films of 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexane carboxylate were UV irradiated (1.1 J cm^-2) under isothermal conditions ranging from 0 to 50°C. Under these conditions the polymerization advanced quickly but only to a conversion level of less than 10% before the reaction rate slowed by more than an order of magnitude. This drop off in rate was not caused by the glass transition temperature, T _g, reaching or exceeding the reaction temperature, T _rxn, since the epoxide's T _g remained at least 40°C below T _rxn. Raising the sample temperature above 60°C caused a sharp increase in the conversion level. At 100°C conversion exceeds 80% and the ultimate T _g approaches 190°C. The addition of 10 mass% 1,6-hexanediol, HD, to the epoxy caused the conversion at room temperature to quintuple over the level obtained without the alcohol present. The heat liberated from this alcohol epoxy blend during cure on a UV conveyor belt system caused the sample's temperature to increase by about 100°C above ambient whereas the epoxy alone under these conditions only experienced a modest temperature rise of about 26°C. If the amount of HD in the blend is increased above 10% the heat of reaction at 23°C decreases due to HD being trapped in a nonreactive crystalline phase. Boosting reaction temperatures above 50°C melts the HD crystals and yields significantly improved conversion ratios. As the level of alcohol blended with the epoxy is raised its ultimate T _g is lowered and when the concentration of alcohol in the blend nears 30 mass%T _g drops below room temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electron paramagnetic resonance spectroscopic studies of thermally generated free radicals in PMR-15 polyimide resins

PMR-15 is a high-performance thermoset polyimide resin that is used in many high-temperature applications. Postcured PMR-15 produces room-temperature electron paramagnetic resonance (EPR) spectra from stable freeradical species that are formed during the postcuring stages. The variable-temperature EPR spectral intensities show a minimum at T_min in the range ?60 to ?40°C, and a maximum at T_max in the range 80–120°C. The EPR intensities follow the inverse temperature dependence of Curie's law below T_min and are due to a stable free radical. The intensities then increase with increasing temperature between T_min and T_max. The free radical with such temperature dependence is not present below T_min and is undetectable by EPR at temperatures above T_max. These free radicals are generated during the postcuring process at elevated temperature above 310°C. The thermo-oxidative degradation involves free radicals generated during the postcuring process in the presence of oxygen gas. © 1993 John Wiley & Sons, Inc.     

Synthesis and characterization of perfluoro‐3‐methylene‐2,4‐dioxabicyclo[3,3,0] octane: Homo‐ and copolymerization with fluorovinyl monomers

The synthesis of perfluoro‐3‐methylene‐2,4‐dioxabicyclo[3,3,0] octane (D), its radical homopolymerization, and copolymerization with fluoroolefins are presented. Fluorodioxolane (D) was synthesized through direct fluorination of the corresponding hydrocarbon precursor in a fluorinated solvent by F₂/N₂ gas. It was polymerized in bulk using perfluorodibenzoyl peroxide as the initiator. The resulting homopolymer had a limited solubility in fluorinated solvents, and its glass transition temperature (T_g) was in the range of 180–190 °C. The polymeric films prepared by casting from hot hexafluorobenzene (HFB) solution were transparent with low refractive index (1.329 at 633 nm). These films were thermally stable (T_d > 350 °C), and were hard and brittle. The copolymers of monomer (D) were prepared with fluorovinyl monomers such as chlorotrifluoroethylene (CTFE), perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, and vinylidene fluoride. The kinetics of radical copolymerization of monomer (D) with CTFE led to the assessment of the reactivity ratios of both comonomers: r_D = 3.635 and r_CTFE = 0.737 at 74 °C, respectively. The copolymers obtained were soluble in HFB and perfluoro‐2‐butyltetrahydrofuran, with T_g in the range of 84–145 °C depending on the copolymer composition. The films of the copolymers were flexible and clear with a low refractive index (1.3350–1.3770 at 532 nm). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6571–6578, 2009     

Influence of local chain dynamics on diffusion of gases in polymers as determined by pulsed field gradient NMR

Diffusion of gases in polymers below the glass transition temperature, T_g, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon‐13 PFG NMR measurements of [¹³C]O₂ diffusion in polymer films at 298 K are performed. Data obtained in polymers with T_g above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below T_g is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below T_g requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231–235, 2010     

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     

Temperature dependence of the activation volume in a nonlinear optical polymer: Evidence for chromophore reorientation induced by sub-Tg relaxations

Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume of 4-(diethylamino)-4′-nitrotolane (DEANT) in poly(methyl methacrylate) (PMMA). The decay of the SHG signal from films of DEANT/PMMA was recorded at hydrostatic pressures up to 3060 atm and at different temperatures between 25°C below the glass transition temperature to 35°C above it. The activation volume, ΔV*_αβ associated with the long range α-type motion of the polymer remained constant at 213 ± 10 ų between T_g − 25°C and T_g + 10°C. At higher temperatures, ΔV*_αβ decreased linearly with increasing temperature. The activation volume, ΔV*_αβ, associated with short range secondary relaxations was constant over the entire temperature range with a value of 77 ± 10 ų. The data suggest that above T_g chromophore reorientation is coupled to both the long range and local motions of the polymer; whereas, well below T_g chromophore reorientation is closely coupled to the local relaxations of the polymer. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 901–911, 1998     

Glass‐transition temperature behavior of alumina/PMMA nanocomposites

Alumina/poly(methyl methacrylate) (PMMA) nanocomposites were synthesized by an in situ free‐radical polymerization process with 38 and 17 nm diameter γ‐alumina nanoparticles. At extremely low filler weight fractions (<1.0 wt % of 38 nm fillers or < 0.5 wt % of 17 nm fillers) the glass‐transition temperature (T_g) of the nanocomposites drops by 25 °C when compared to the neat polymer. Further additions of filler (up to 10 wt %) do not lead to additional T_g reductions. The thermal behavior is shown to vary with particle size, but this dependence can be normalized with respect to a specific surface area. The nanocomposite T_g phenomenon is hypothesized to be because of nonadhering nanoparticles that serve as templates for a porous system with many internal interfaces that break up the percolating structure of dynamically heterogeneous domains recently suggested by Long, D.; and Lequeux, F. Eur Phys J E 2001, 4, 371 to be responsible for the T_g reductions in polymer ultrathin films. The results also point to a far field effect of the nanoparticle surface on the bulk matrix. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4371–4383, 2004     

Effect of the Free Quenching on Mechanical and Thermomechanical Properties of ABS

The purpose of this work was to investigate the influence of thermal annealing and quenching on mechanical, and thermomechanical properties of acrylonitrile butadiene styrene (ABS). The free quenching was applied over a temperature range starting from a temperature slightly above T_g to temperatures below 0°C. The improvement of the impact strength is linked to the cavitation of the rubber particles close to the samples surface. The existence of a relaxation mode located around–10°C may participate to the relaxation of the residual stresses when the sample is quenched under 0°C, this phenomenon would promote cavitation.     

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (T_g) on film thickness. Sizeable reduction of T_g was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of T_g on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the T_g or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Thermal behavior of polytriazole films: A thermal analysis study

The thermal behavior of poly(1,3-phenyl-1,4-phenyl)-4-phenyl-1,2,4-triazole has been investigated using different scanning calorimetry (DSC) and thermogravimetry (TG). Processes are studied for this thermally stable polymer that take place between 200 and 500°C. While the polycondensation reaction product in powder from appeared to be partially crystalline, films prepared by casting from a formic acid solution appeared to be completely amorphous. A thermal treatment between T_g(～ 270°C) and T_m(～ 430°C) can introduce crystallinity in the films because of the polymer's ability to cold crystallize. The cold crystallization temperature T_c seems to be dependent on the preparation history of the solid polymer phase. Thermal annealing of the films just below T_g does not introduce crystallinity but inhibits subsequent cold crystallization at higher temperatures. Crystallization upon cooling from the crystalline melt has not been observed either. At temperatures just above the crystalline melting point the polymer starts to decompose in an exothermic reaction.