Mechanical spectroscopy of laser deposited polymers

Pulsed laser deposition (PLD) at 248 nm in ultra high vacuum was used to produce thin poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) films. The ablation and deposition mechanisms were found to be similar in both systems. Having the same backbone, these polymers differ in the size of their polar side groups leading to changes in their dynamics. Studies of the relaxation processes were performed using mechanical torsion and bending spectroscopy by means of a double-paddle oscillator (DPO) and an in-situ plasma plume excited reed (PPXR), respectively. A strong increase of the mechanical damping was observed during annealing of the polymer films well above the glass transition temperature T _g, while in-situ X-ray measurements did not reveal any structural changes. For PEMA, the glass transition temperature T _g=335 K and the main absorption maximum appear at lower temperatures compared to PMMA (T _g=380 K), allowing one to measure the mechanical properties in a much wider range above T _g.     

Editorial: Special issue on Properties of Thin Polymer Films

The behavior of polymers in thin films or close to interfaces is far from being understood. Many observations, encompassing both structural and dynamical behavior, indicate that the properties of polymers in thin films deviate from what we know from the bulk. Questions about the density (irreversible and reversible changes after annealing even above the bulk glass transition temperature), film stability and dewetting, glass transition temperature, diffusion coefficient and chain conformation and relaxation are intensively debated. In particular, it is not yet clear how the size of such chain-like molecules (their molecular weight) comes into play, especially if the thickness of the film is less than the radius of gyration of these macromolecules. In addition, due to the high surface-to-volume ratio the influence of surface and interfacial properties becomes important if not dominant. This interfacial sensitivity highlights the importance of the properties of the near surface region in polymer films; a topic whose importance is beginning to be recognized. This special issue presents experimental and theoretical works on a variety of questions related to polymers at interfaces and in thin films, ranging from space-averaged properties like adhesion to surface ordering or dynamic molecular (segmental) motion in confining geometries. Since the first reports of anomalous dynamics in thin polymer films almost a decade ago, the subject of dynamics in thin films has gained considerable momentum. Until very recently, this body of work focused almost solely on measurements of the thermodynamic signature of the kinetic glass transition. Such measurements are, at best, a very indirect probe of the microscopic dynamics and convolute the temperature dependence, time dependence and sometimes even the thermal history into a single measured value. The articles focusing on dynamics in thin films published in this special issue illustrate an important shift in this rapidly evolving field. There is now a strong focus on many different fronts. Measurements of dynamics are more varied, ranging from indirect studies such as adhesion to direct measurement of the segmental relaxation using dielectric spectroscopy. There is a concerted effort now to draw analogies to bulk systems in order to learn about what effects may contribute to the observation made in thin films. There is also a strong effort using numerical simulations to make quite direct comparisons to measured values in thin films. Finally, in a way that signals a new maturity to this field, a significant fraction of literature being currently published concerns ideas as to why the dynamics in thin films behave the way they do. In this special issue we have aimed at capturing a cross-section of problems of high current interest. While all contributions definitely provide highly valuable insight in the behavior of polymers in thin films, many questions are still unanswered and await further in-depth-going studies. We just want to note a few of these questions, also highlighted in several commentaries published in this issue: Can polymers in experimental studies on thin films ever be fully equilibrated? What is the relaxation behavior of macromolecules in confining geometries with interacting boundaries? How does the relaxation behavior depend on the length scale over which it is measured? What are the mechanical properties of thin polymer films? We tried to assemble different approaches and opinions from various viewpoints. We hope that such a complementary presentation is helpful and stimulates further discussions in order to dissolve some of the confusion in this area, leading eventually to a clear understanding of thin-film properties of polymers. The European Physical Journal E - Soft Matter will continue to provide a forum for the discussion of such questions and a place for the publication of future work on properties of polymers in thin films for all colleagues interested in these questions. Günter Reiter (Editor) James Forrest (Guest Editor)     

Glass transition and dynamics of single and stacked thin films of poly(2-chlorostyrene)

The glass transition temperature and the dynamics of the α-process have been investigated using dielectric relaxation spectroscopy for single and stacked thin films of poly(2-chlorostyrene) (P2CS). The stacked film consists of 10 layers of single thin films with thickness of 12 nm or 18 nm. The glass transition temperature T _g of the single thin films of P2CS is found to decrease with decreasing film thickness in a similar way as observed for polystyrene thin films. The magnitude of the depression of T _g for the stacked thin films is larger than that of the single thin films with corresponding thickness. The depression of the temperature at which the dielectric loss shows a peak due to the α-process at a given frequency, T _α, is larger than that of the single thin films, although the magnitude is smaller than that of T _g . Annealing at a high temperature could cause the T _g and T _α of the stacked thin films to approach the values of the bulk system.     

Relaxation processes and structural transitions in stretched films of polyvinylidene fluoride and its copolymer with hexafluoropropylene

Relaxation processes and structural transitions in nonstretched and uniaxially stretched films of poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) and its homopolymer polyvinylidene fluoride (PVDF) for comparison were investigated with the aim of understanding the electromechanical properties of this lower-modulus ferroelectric copolymer. The mechanical and the dielectric response at the glass transition (?? _a relaxation) exhibit similar temperature dependence of the relaxation time, whereas mechanical and dielectric processes above the glass transition are not related. They represent a continuous softening process within the amorphous phase and the dielectric ?? _c relaxation, respectively. The latter is attributed to conformational changes of VDF segments in lamellae of spherulites constituting the nonpolar crystalline ?? phase. Furthermore, there is a contribution from melting of secondary crystallites formed in the amorphous phase during annealing or storage. Mechanically, this transition appears in nonstretched and stretched films as an accelerated decrease of the elastic modulus that terminates the rubber plateau. Dielectrically, this transition becomes visible as a frequency-independent loss peak only in stretched films, because stretching removes the ?? _c relaxation, which superimposes the transition in nonstretched films. Melting of secondary crystallites is shown to appear in the homopolymer, too, though less pronounced because of more complete primary crystallisation. Stretching increases the modulus above the glass transition only slightly, and it does not significantly influence the softening process. On the other hand, stretching causes a spontaneous polarisation and introduces order within the amorphous phase, rendering it more polar. Melting of secondary crystallites provides an additional contribution to the polarisation. These findings may explain the relatively high electromechanical activity of P(VDF-HFP) but also its relatively low thermal stability. Moreover, they may be important for correct procedure and analysis of temperature-dependent dielectric measurements on partially crystalline polymers, in particular on those with less favourable sterical conditions for primary crystallisation.     

Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films: Novel fluorescence measurements

Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine the effects of decreasing film thickness on glass transition temperature, T _g, the relative strength of the glass transition, and the relative rate of physical aging below T _g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine) (P2VP) differs from that in the rubbery state with a transition at T _g. Positive deviations from bulk T _g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T _g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of physical aging in films as thin as 10 nm. Received 21 August 2001     

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T _g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T _g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T _g with decreasing film thickness. The findings concerning the different thickness dependences of T _g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T _g shifts incorporates recently developed models to describe the glass transition in thin polymer films. Received 12 August 2001 and Received in final form 16 November 2001     

Activation Energy for the Glass Transition of a Confined Elastomer in HDPE/PP Blends

Blends of two highly crystalline polymers containing an elastomer were prepared to study the glass transition of the confined elastomer. The polymers chosen were high density poly ethylene (HDPE), polypropylene (PP), and two elastomers of a different nature: natural number (NR) and EPDM. The dynamic mechanical analyzer (DMA) technique was used to analyze the storage modulus of blends with elastomer content from 0% to 30% by weight, with the remainder made up of equal amounts of HDPE and PP, and blends with 10% of the elastomer, but varied ratios of polyolefins. We used the differentiation modification of the Arrhenius method in the kinetic analysis assuming an n‐order relaxation mechanism, which allowed detecting the percolation threshold of NR. Results indicate that both temperature and activation energy for glass transition (T _g ) are dependent on the types of polymers in the blend and blend composition. The T _g and E values of the unblended elastomers are higher than those in blends; this behavior is associated with the elastomer confinement and blend morphology.     

Glass transition in ultrathin polymer films: calorimetric study

The ultrasensitive differential scanning calorimetry is used to observe the glass transition in thin (1-400 nm) spin-cast films of polystyrene, poly (2-vinyl pyridine) and poly (methyl methacrylate) on a platinum surface. A pronounced glass transition is observed even at a thickness as small as 1-3 nm. Using the high heating (20-200 K/ms) and cooling (1-2 K/ms in glass transition region) rates which are typical for this technique, we do not observe appreciable dependence of the glass transition temperature over the thickness range from hundreds of nanometers down to 3 nm thick films. The evolution of calorimetric data with film thickness is discussed in terms of broadening of transition dynamics and loss of transition contrast.     

Thickness dependence of the dynamics in thin films of isotactic poly (methylmethacrylate)

The European Physical Journal E - The film thickness dependence of both the glass transition temperature (T g ) and the 1 kHz alpha relaxation were studied for thin films of isotactic Poly...     

Hypersonic absorption in amorphous polymers by light scattering

Brillouin lines have been observed in the spectrum of light scattered by poly(methylmethacrylate) and, for the first time, for poly(vinylchloride). The line widths have been measured, by a special technique, as a function of temperature. The attenuation per wavelength of thermal phonons below the glass transition temperature is almost constant over a wide frequency range (5 MHz to 10 GHz). The attentuation is ascribed to the mechanical strain field round static imperfections. Above the glass transition temperature the attenuation is associated with the well-known mechanical ∝ process. The Rayleigh light scattered by these polymers and their transparency is also discussed.     

Preparation, characterization and surface pKa values of poly(N-vinyl-2-pyrrolidone)/chitosan blend films

Blend films of poly(N-vinyl-2-pyrrolidone) (PVP) and chitosan (CTS) were prepared by casting method from acetic acid solutions. All blend films obtained are optically clear to the naked eye. The structure and physical properties of the blend films were analyzed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mechanical testing (Instron) and contact-angle measurements. The ATR-FTIR results indicated that there is no detectable band shifts at 1650 cm⁻¹. From TGA studies, it was found that the onset degradation temperature of the blends almost unchanged due to the presence of a weak interaction between PVP and CTS chains. The DSC analysis showed a single glass transition temperature for all the blends, indicating that these polymers are miscible over the entire composition range. The mechanical properties of the blends, such as breaking stress and elongation at break and Young modulus were greatly affected with increase of CTS content. In addition, we found that the blends exhibit well defined contact angle titration curves from which the surface pK_a values were determined. In conclusion, these experimental findings provide fundamental knowledge for the preparation of bioreactive surfaces of controlled reactivity on CTS based blends.     

Acoustic,dielectric, and mechanical properties of a new rigid-rod poly(heteroarylene)

The structural, relaxational, and stress-strain properties of rigid-rod poly(heteroarylene)s containing high-polarity functional groups in macrochains are studied. The groups are capable of forming strong intermolecular bonds, including those with polar low-molecular substances (residual solvent or water). The acoustic and dielectric spectroscopy methods are used. The properties of a recently obtained poly(heteroarylene)—poly(naphthoyleneimidobenzimidazole)—are studied in detail. It is shown that these rigid-rod polymers can exhibit local and large-scale molecular rearrangements at temperatures considerably lower than the glass transition temperature. The frequency vs. temperature diagram of relaxation transitions is obtained, and the activation energies for these processes are determined. The possible molecular dynamics mechanisms are discussed.     

Call for papers Special issue on "Properties of thin polymer films"

In recent years, there have been numerous reports of anomalous behavior of polymers in thin films. These observations, encompassing both structural and dynamical behavior, have attracted a great deal of interest. Measurements showing such anomalous behavior include density (irreversible and reversible changes after annealing below the bulk glass transition temperature), film stability and dewetting, glass transition temperature, diffusion coefficient and chain conformation and relaxation. The lack of a definitive M_w dependence in many of these studies means that it is not yet clear if the molecular size is a defining parameter in these observations. Alternatively, the surface to volume ratio may make such systems dependent on the (equally poorly understood) properties of polymer surfaces and interfaces. Despite a growing literature in this area, it is becoming increasingly obvious that a clear understanding of thin film properties has not yet been reached. In particular in glassy thin polymer films a possible cause for the anomalous behavior may originate from sample preparation. The widely used process of spin coating can potentially produce samples that are far from equilibrium. While most studies use thin films that have been annealed above the bulk glass transition, it is not clear if this represents sufficient annealing because the terminal relaxation times may be much larger than typical annealing times. One of the key questions is: Are these deviations from equilibrium important enough to cause any measurable effects? What are the consequences of film preparation and the resulting non-equilibrium conformations of the polymers on measurements of film stability, glass transition temperature, diffusion coefficient, etc.? Alternatively, if these non-equilibrium effects are not responsible for the anomalies observed in the structural and dynamical properties of thin polymer films, then other explanations must be considered. In the search for a unifying concept to explain all aspects simultaneously (i.e., density, film stability, glass transition temperature, ...) and in order to provide a forum for an open and possibly controversial discussion, The European Physical Journal E - Soft Matter invites regular articles and comments on this topic. In addition we invite submission of well-founded conjectures which may be published together with complementary views (solicited by the Editors). Papers to be considered for this special issue should be submitted by August 15, 2001 to the Editorial Office of the European Physical Journal E Véronique Condé, Editorial Office of The European Physical Journal E, Université Paris Sud, bâtiment 510, 91405 Orsay Cedex, France Tel.: 33 (0)1 69 15 59 76, Fax: 33 (0)1 69 15 59 75, e-mail: conde@edpsciences.org and marked clearly to the attention of either Günter Reiter (Editor) or James Forrest (Guest Editor). We look forward to receiving your submission.     

Fluorescence studies of confinement in polymer films and nanocomposites: Glass transition temperature, plasticizer effects, and sensitivity to stress relaxation and local polarity

Confinement effects in polystyrene and poly(methyl methacrylate) films and nanocomposites are studied by fluorescence. The ability to employ an intensive measurable, the excited-state fluorescence lifetime, in defining the glass transition temperature, T_g, of polymers is demonstrated and compared to the use of an extensive measurable, fluorescence intensity. In addition, intrinsic fluorescence from the phenyl groups in polystyrene is used to determine the T_g-confinement effect in films as thin as ~15 nm. The decrease in T_g with decreasing film thickness (below ∼60 nm) agrees well with results obtained by extrinsic pyrene fluorescence. Dye label fluorescence is used to quantify the enhancement in T_g observed with decreasing thickness (below ~90 nm) in poly(methyl methacrylate) films; addition of 2–4 wt% dioctyl phthalate plasticizer reduces or eliminates the T_g-confinement effect in films down to 20 nm thickness. Intrinsic polystyrene fluorescence, which is sensitive to local conformation, is used to quantify the time scales (some tens of minutes) associated with stress relaxation in thin and ultrathin spin-coated films at T_g + 10 K. Finally, the shape of the fluorescence spectrum of pyrene doped at trace levels in polystyrene films and polystyrene-silica nanocomposites is used to determine effects of confinement on microenvironment polarity.     

Effect of nanoparticle dispersion on glass transition in thin films of polymer nanocomposites

We present spectroscopic ellipsometry measurements on thin films of polymer nanocomposites consisting of gold nanoparticles embedded in poly(styrene). The temperature dependence of thickness variation is used to estimate the glass transition temperature, T _g . In these thin films we find a significant dependence of T _g on the nature of dispersion of the embedded nanoparticles. Our work thus highlights the crucial role played by the particle polymer interface morphology in determining the glass transition in particular and thermo-mechanical properties of such nanocomposite films.     

Mechanical spectrum study of glass transition by a composite method

Normalized mechanical spectra of glycerol, 1,2-propanediol carbonate and poly(vinyl chloride)/di(2-ethyl-hexyl) phthalate (PVC/DOP) blends were studied in the temperature range from 100 to 300 K by a composite method. The dynamic glass transition was observed, which exhibits a peak of temperature-dependent loss modulus. The peak moves toward higher temperature with higher measuring frequency, which accords with the relaxation feature of the dynamic glass transition. Another characteristic temperature can be marked in the mechanical spectrum by the onset of storage modulus change, which is labeled as T_gm. T_gm is found to be nearly equal to the calorimetric glass transition temperature in glycerol, 1,2-propanediol carbonate and di(2-ethyl-hexyl) phthalate. As we expected, this onset temperature in the mechanical spectrum has an intimate relation with the calorimetric glass transition of materials, and it can be regarded as a representative when the calorimetric glass transition temperature is not available. Finally, normalized mechanical spectra of PVC/DOP blends with different PVC content were obtained and mechanical glass transition temperatures T_gm were determined.     

Mechanical properties of thin confined polymer films close to the glass transition in the linear regime of deformation: Theory and simulations

Over the past twenty years experiments performed on thin polymer films deposited on substrates have shown that the glass transition temperature T(g) can either decrease or increase depending on the strength of the interactions. Over the same period, experiments have also demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. A model for the dynamics of non-polar polymers, based on percolation of slow subunits, has been proposed and developed over the past ten years. It proposes a unified mechanism regarding these two features. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers close to the glass transition in the linear regime of deformation, with a spatial resolution corresponding to the subunit size. We focus on the case of polymers confined between two substrates with non-negligible interactions between the polymer and the substrates, a situation which may be compared to filled elastomers. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, this allows to calculate the corresponding increase of glass transition temperature, up to 20 K in the considered situations. Moreover, between the bulk T(g) and T(g) + 50 K the modulus of the confined layers is found to decrease very slowly in some cases, with moduli more than ten times larger than that of the pure matrix at temperatures up to T(g) + 50 K. This is consistent with what is observed in reinforced elastomers. This slow decrease of the modulus is accompanied by huge fluctuations of the stress at the scale of a few tens of nanometers that may even be negative as compared to the solicitation, in a way that may be analogous to mechanical heterogeneities observed recently in molecular dynamics simulations. As a consequence, confinement may result not only in an increase of the glass transition temperature, but in a huge broadening of the glass transition.     

Dielectric relaxations in ultrathin isotactic PMMA films and PS-PMMA-PS trilayer films

The local and cooperative dynamics of supported ultrathin films ( L = 6.4 - 120 nm) of isotactic poly(methyl methacrylate) (i-PMMA, Mn = 118 x 10(3) g/mol) was studied using dielectric relaxation spectroscopy for a wide range of frequencies (0.1 Hz to 10(6) Hz) and temperatures (250 - 423 K). To assess the influence of the PMMA film surfaces on the glass transition dynamics, two different sample geometries were employed: a single layer PMMA film with the film surfaces in direct contact with aluminum films which act as attractive, hard boundaries; and a stacked polystyrene-PMMA-polystyrene trilayer film which contains diffuse PMMA-PS interfaces. For single layer films of i-PMMA, a decrease of the glass transition temperature T(g) by up to 10 K was observed for a film thickness L < 25 nm (comparable to R(EE)), indicated by a decrease of the peak temperature T(alpha) in the loss epsilon(")(T) at low and high frequencies and by a decrease in the temperature corresponding to the maximum in the apparent activation energy E(a)(T) of the alpha-process. In contrast, measurements of i-PMMA sandwiched between PS-layers revealed a slight (up to 5 K) increase in T(g) for PMMA film thickness values less than 30 nm. The slowing down of the glass transition dynamics for the thinnest PMMA films is consistent with an increased contribution from the less mobile PMMA-PS interdiffusion regions.     

Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene

The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d相似文献