Structural relaxation of polystyrene in nanolayer confinement

The physical aging of polystyrene (PS) confined in a multilayered film arrangement was explored using differential scanning calorimetry (DSC). The multilayered films were produced via multilayer coextrusion and consisted of alternating layers of PS and polycarbonate (PC), with PS layer thicknesses ranging from 50 nm to 500 nm. A 125 μm bulk control film of pure PS was also extruded and studied for comparison. The glass transition temperatures (T_g) of the PS in multilayered films did not appear to be systematically dependent on layer thickness, and T_g values in all PS/PC films were similar to the bulk value of 104 °C. Two approaches were used to investigate the structural relaxation of PS in the layered films. In the first method, PS layers were aged isothermally at 80 °C after annealing above the T_g of PS (135 °C for 15 min) to reset the thermal history and provide a well-defined starting point for aging experiments. Recovered enthalpy data for aged films (calculated from DSC thermograms) showed that the aging rate in the PS layers decreased with decreasing layer thickness. Calculated aging rates were also compared with the fraction of interphase material (which increases significantly with decreasing layer thickness), and the decrease in aging rate for films with thinner layers was found to correlate with an increase in interphase fraction. The elevated T_g of the interphase material (compared to pure PS) was suggested as a possible reason for reduced aging rates in the thin PS layers. In the second method, PS layers were cooled from above their T_g at different rates under confinement by PC layers. After this cooling step was performed, subsequent heating thermograms revealed that the enthalpy recovered upon reheating through the T_g of PS was similar for bulk and nanolayered films.     

Dynamics of the leveling process of nanoindentation induced defects on thin polystyrene films

Using Atomic Force Microscopy (AFM) we study the effect of nanoindentation induced defects on 50 and 120 nm thick unentangled polystyrene (PS) films, spin cast on silicon (Si) substrates. Indents with residual depths of penetration less than the film thickness level upon heating above the glass transition temperature (T_g) of bulk PS. The resulting leveling process is discussed in terms of a diffusion process driven by the curvature gradient. Calculated diffusivity values are close to the self-diffusivity of bulk PS.     

Measuring Tg in ultra‐thin polymer films with an excimer fluorescence technique

An excimer fluorescence technique has been applied to the measurement of T_g of ultra‐thin polystyrene films. This technique utilizes an excimer‐forming molecule with fluorescent emission in two wavelength bands. The intensity ratio of these bands is a sensitive measure of local viscosity. This technique has been applied to five polystyrene films in the thickness range of 25 nm to 200 nm supported on quartz substrates. The observed T_g for the five ultra‐thin polymer films was similar to the bulk T_g with no observed dependence upon thickness. Additionally, the T_g determined for each film did not show any dependence upon thermal history.     

Temperature- and thickness-dependent elastic moduli of polymer thin films

The mechanical properties of polymer ultrathin films are usually different from those of their counterparts in bulk. Understanding the effect of thickness on the mechanical properties of these films is crucial for their applications. However, it is a great challenge to measure their elastic modulus experimentally with in situ heating. In this study, a thermodynamic model for temperature- (T) and thickness (h)-dependent elastic moduli of polymer thin films E _f(T,h) is developed with verification by the reported experimental data on polystyrene (PS) thin films. For the PS thin films on a passivated substrate, E _f(T,h) decreases with the decreasing film thickness, when h is less than 60 nm at ambient temperature. However, the onset thickness (h*), at which thickness E _f(T,h) deviates from the bulk value, can be modulated by T. h* becomes larger at higher T because of the depression of the quenching depth, which determines the thickness of the surface layer δ.     

Confinement, composition, and spin-coating effects on the glass transition and stress relaxation of thin films of polystyrene and styrene-containing random copolymers: Sensing by intrinsic fluorescence

The glass transition temperatures (T_gs) of polystyrene (PS) and styrene/methyl methacrylate (S/MMA) random copolymer films are characterized by intrinsic fluorescence, i.e., monomer fluorescence from an excited-state phenyl ring and excimer fluorescence from an excited-state dimer of two phenyl rings. The T_g is determined from the intersection of the rubbery- and glassy-state temperature dependences of the integrated fluorescence intensity measured upon cooling from an equilibrated state. With PS, the effects of nanoconfinement on T_g and the transition strength agree with results from studies using probe fluorescence and ellipsometry. The T_g-nanoconfinement effect is “tuned” by copolymer composition. As S-content is reduced from 100 mol% to 22 mol%, the confinement effect changes from a reduction to an enhancement of T_g relative to bulk T_g. Intrinsic fluorescence is also a powerful tool for characterizing relaxation of residual stresses. Stresses induced by spin coating affect local conformations, which in turn affect excimer and monomer fluorescence and thereby integrated intensity. The heating protocol needed to achieve apparently equilibrated local conformations is determined by equivalence in the integrated intensities obtained upon heating and subsequent cooling. While partial stress relaxation occurs upon heating in the glassy state, full relaxation of local conformations requires that a film be heated above T_g for times that are long relative to the average cooperative segmental relaxation time. For example, in thin and ultrathin films, equilibration is achieved by heating slowly (∼1 K/min) to 15-20 K above T_g. Dilute solution fluorescence of PS and S/MMA copolymers is also characterized and compared to reports in the literature.     

Mobility and glass transition temperature of polymer nanospheres

Recent studies have illustrated a decoupling between cooperative segmental mobility and the glass transition temperature (T_g) of thin polymer films and nanocomposites. Here, we use dielectric spectroscopy to probe the cooperative segmental mobility and capacitive dilatometry to determine the T_g of films of polystyrene nanospheres with diameters (d) less than 400 nm. We find that both capacitive dilatometry and calorimetry revealed nearly identical suppressions in T_g as the size of the nanospheres was reduced. While T_g was impacted by confinement, in the range 130 nm ≤ d ≤ 400 nm, in stark contrast, the cooperative segmental mobility, i.e., the peak position of the α-relaxation process was not. Furthermore, when d ≤ 200 nm, an additional molecular relaxation process, not observed in bulk, was present. We interpret these findings as evidence of a decoupling between T_g and cooperative segmental mobility in nanospheres. That is, the latter may be impacted by confinement under conditions in which the former is not.     

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

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

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

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

The effect of confinement in nanoporous polymers on the glass transition temperature

The glass transition temperature (T_g) of nanoporous polyetherimide (PEI) and PEI thin films was investigated. The T_g decreased from its bulk value in both of these confined systems. Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances in the nanoporous PEI. A quantitative analogy between the nanoporous PEI and PEI thin films is proposed through an equivalence of nearest neighbor pore-to-pore distances and thin film thickness. The effect of confinement is believed to be due to the interface regions, which possess higher chain mobility than the bulk. When these high mobility interface regions are sufficiently close together, the excess mobility at the interface region affects the dynamics of the system by restraining percolation of the slow domains resulting in the observed decrease in T_g.     

Contributions of torsional braid analysis to Tu

The accomplishments of Borden Award Winner John K. Gillham and his colleagues using Torsional Braid Analysis (TBA) and Differential Scanning Calorimetry (DSC) to investigate the T_g and T > T_g or T_u regions of anionic and thermal poly styrenes (PS) are evaluated and related to work on PS and other polymers and to controversies surrounding TBA and the Tu transition. Arguments are presented to refute the contention that T_u by TBA is an artifact produced by the braid and the contention that T_u has a relaxational nature but no thermodynamic basis. Two distinct behavior patterns are found for T_u vs log M?_n plots: quasi-static methods such as DSC on fused films show T_u to level off above M_c and approach an asymptotic value of ～435K; dynamic methods involving melt flow show that T_u increases without limit above M_c because of entanglements. A compilation is presented of 25 investigations of T_u on polybutadiene, poly(methyl methacrylate) (PMMA), PS, plasticized PS, and atactic polypropylene, involving twenty experimental techniques. The behavior of zero shear melt viscosity for PS is summarized. Gillham's work has not only led to clarification of many isolated papers in the literature but has also inspired various parallel experimental and literature studies on T_u. We conclude that T_u is a molecular level transition which, like T_g, exhibits both kinetic (relaxational) and thermodynamic aspects. It is shown that heat capacity should be a more sensitive method than dilatometry for studying the T_u transition.     

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

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

Enthalpy recovery and structural relaxation in layered glassy polymer films

Recent studies of physical aging in confined polymer glasses have revealed that aging behavior in confinement often differs from bulk behavior. This study used DSC to characterize physical aging and structural relaxation in bulk polysulfone (PSF) and co-extruded multilayered films of PSF and an olefin block copolymer (OBC) that have average PSF layer thicknesses of 640 nm, 260 nm, and 185 nm. The films were aged isothermally at 170 °C, and the recovered enthalpy upon reheating was measured over time. The films with 640 nm and 260 nm PSF layers had aging rates very similar to that of bulk PSF, while the film with 185 nm PSF layers had an aging rate slightly greater than the bulk value. The cooling rate dependence of the limiting fictive temperature (T_f′) in multilayered and bulk PSF samples was also characterized. Values of T_f′ were similar for all films at each cooling rate. The results of this work are in general agreement with our previous gas permeation aging study of multilayered PSF films aged at 35 °C, in which the effect of layer thickness on aging behavior was minimal. This stands in contrast to studies with thin, freestanding PSF films, which exhibit accelerated aging relative to bulk and have aging rates that depend strongly on film thickness.     

In situ monitoring of thermal transitions in thin polymeric films via optical interferometry

A novel, low-cost, rapid, accurate, non-invasive and high throughput method based on the principles of Optical Interferometry (OPTI method) has been developed and applied for the in situ monitoring in one simple run of first (melting) and second (glass transition) order transitions as well as of the thermally induced decomposition of various thin polymeric films spin coated on flat reflective substrates (untreated silicon wafers). The new method has been applied successfully for measuring the glass transition, melting and decomposition temperatures of six commercially available polymers [poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate), (PHEMA), poly(vinyl acetate-co-crotonic acid), (PVACA), poly(vinyl pyrrolidone) (PVP), poly(vinyl chloride-co-vinyl acetate) (PVCVA) and crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVFHP)] of known T_gs or T_ms. The recorded interferometric signals were identified and characteristic signal patterns were qualitatively correlated to specific transitions. The monitoring of first and second order transitions in thin polymeric films is based on detectable differentiations of the total energy of a fixed wavelength laser beam incident almost vertically (angle of incidence <5°) onto a thin polymeric film spin coated on a flat reflective substrate. These differentiations are caused by film thickness and/or refractive index changes of the polymeric film both resulted from the significant change of the polymer's free volume taking place on the transitions. For film thicknesses over approx. 200-250 nm, the T_g or T_m of the polymeric films measured with the OPTI method were in excellent agreement with the corresponding values of the polymer, measured by DSC. An investigation on the trends of the T_g of PHEMA and PMMA films in a wide thickness range (30-1735 nm) was also carried out. Ultra-thin (∼30 nm) films of PMMA and PHEMA showed significant increase in their T_g values by approx. 30 °C upon comparing to their corresponding bulk T_gs. This behavior was attributed to an enhanced polymer-surface interaction through hydrogen bonding and/or to changes in the tacticity of the polymer.     

Segmental mobility in the vicinity of Tg in PS/SBR blends: Nanodomain size prediction of the dispersed phase

The blends of polystyrene (PS) and styrene‐butadiene rubber (SBR) are melt‐blended at different ratios to form physical thermoplastic elastomers. This polymeric blend is expected to behave more or less similar to chemically synthesized block copolymers such as styrene‐butadiene block copolymers (SBS). In this study, mechanical and the thermomechanical properties of this blend are investigated and compared to those of SBS copolymer. As far as morphology is considered, the blend shows a two‐phase morphology with an interface, which shows very weak interactions. According to the observed morphology and the domain size of dispersed phase the blends are of good integrity. The mechanical properties of the blends confirm the integrity of the blend and effective interface stress transfer. The tensile and Izod impact properties of the blends shows improvements upon increase in SBR content of the blend. As SBR content augments the elongation at break increases, whereas tensile dissipated energy and impact resistance go through a maximum. Therefore, blend with SBR‐content in 60–75% range can be considered as preferred one. In a wide range of concentration a phase inversion was observed and T_g‐depression was detected also for the SBR phase. This T_g‐depression was correlated to the changes in dynamics of segments (segmental mobility) near the surfaces. Using the proposed relationships between T_g‐depression and the thickness of the thin films, it was tried to calculate domain size of SBR inclusions in PS matrix. A rough correlation between SBR domain sizes in SEM images and calculated thicknesses using T_g‐depression in thin films was found. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Low temperature CVD diamond deposition using halogenated precursors — deposition on low melting materials: Al,Zn and glass

Using HFCVD and MWCVD processes with 0.5–2.0% C₂H₅Cl+H₂, deposition was carried out at substrate temperatures between 650 and 1073 K onto Si, Zn, Al and glass (DESAG type D263) substrates. A coolable substrate holder allowed control of the substrate temperature independently from the CVD process. The temperature was measured using thermocouples. The melting point of Zn (692 K) and the unchanged shape of the substrate after deposition experiments are further proof of the correct temperature measurements. The deposited films were analyzed using Raman spectroscopy and SEM. On Si substrates, the adhesion of diamond deposited at low temperatures (650–700 K) is rather poor. In contrast to this result, we observed quite good adhesion on Al (melting point T_m 933 K) for films deposited at 760 K. The Raman spectrum shows the presence of diamond, with a peak located at 1332 cm⁻¹ with a broad luminescence background. The deposition onto Zn (T_m 692 K) proved to be much more difficult than on Al or glass D 263.     

In situ AFM study of near‐surface crystallization in PET and PEN

The surface crystallization behavior of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6‐naphthalate) (PEN) spin‐coated thin films was compared by means of atomic force microscopy (AFM) with an in situ heating stage. As the films were heated up stepwise, characteristic surface crystals appeared at a crystallization temperature (T_c) in the near‐surface region which is about 15 °C under the bulk T_c, and were replaced by bulk crystals when the temperature was increased to the bulk T_c. In the case of films whose thickness is less than 70 nm (PET) and 60 nm (PEN), significant increases in the bulk T_c were observed. Scanning force microscopy (SFM) force‐distance curve measurements showed that the glass transition temperature (T_g) of the near‐surface region of PET and PEN were 22.0 and 26.6 °C below their bulk T_g (obtained by DSC). After the onset of surface crystallization, edge‐on and flat‐on crystals appeared at the free surface of PET and PEN thin films, whose morphologies are very different to those of the bulk crystals. Although the same general behavior was observed for both polyesters, there are significant differences both the influence of the surface and substrate on the transition temperatures, and in morphology of the surface crystals. These phenomena are discussed in terms of the differences in the mobility of polymer chains near the surface. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44269.     

Mechanical properties of films prepared from model high‐glass‐transition‐temperature/low‐glass‐transition‐temperature latex blends

The mechanical properties of films prepared from model high‐glass‐transition‐temperature (T_g)/low‐T_g latex blends were investigated with tensile testing and dynamic mechanical analysis. Polystyrene (PS; carboxylated and noncarboxylated) and poly(n‐butyl methacrylate‐co‐n‐butyl acrylate) [P(BMA/BA); noncarboxylated] were used as the model high‐T_g and low‐T_g latexes, respectively. Carboxyl groups were incorporated into the PS latex particles to alter their surface properties. It was found that the presence of carboxyl groups on the high‐T_g latex particles enhanced the Young's moduli and the yield strength of the PS/P(BMA/BA) latex blend films but did not influence ultimate properties, such as the stress at break and maximum elongation. These phenomena could be explained by the maximum packing density of the PS latex particles, the particle–particle interfacial adhesion, and the formation of a “glassy” interphase. The dynamic mechanical properties of the latex blend films were also investigated in terms of the carboxyl group coverage on the PS latex particles; these results confirmed that the carboxyl groups significantly influenced the modulus through the mechanism of a glassy interphase formation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2788–2801, 2002     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

Synthesis of PS‐g‐POSS hybrid graft copolymer by click coupling via ”graft onto” strategy

Polystyrene (PS)‐incorporated polyhedral oligomeric silsesquioxanes (POSS) organic–inorganic hybrid graft copolymer could be achieved by click coupling reaction between alkyne groups in POSS and azido groups in PS via “graft onto” strategy. Alkyne‐functionalized POSS was synthesized via thiol‐ene facile click reaction and subsequent amidation reaction with very high yield. Azido‐multifunctionalized PS could be synthesized by chloromethylation and subsequent azido reaction. The chemical structures of PS‐(CH₂Cl)_m, PS‐(CH₂N₃)_m, and PS‐g‐POSS were determined by Fourier transform infrared and ¹H NMR characterization. PS‐g‐POSS presented a better hydrophobic property with contact angle of 113° than that of PS (85°). And PS‐g‐POSS with ≤5% of grafting degree had lower glass transition temperature (T_g) than that of PS and then it increased up to 112°C with grafting degree. An obvious aggregation of POSS phase with 10–80 nm in size was formed in PS‐g‐POSS matrix. In addition, 5 wt % of PS‐g‐POSS was added to general purpose polystyrene (GPPS) to remarkably improve its tensile strength from 45 to 57 MPa. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermal molecular motion at surface of atactic polypropylene films

Surface molecular motion in atactic polypropylene (aPP) films was studied by scanning force microscopy. Glass transition temperature at the surface, T_g^s, was determined to be 251±1 K on the basis of temperature dependence of lateral force, whereas its bulk glass transition temperature, T_g^b, by dynamic viscoelastic and differential scanning calorimetric measurements was 262 K. In general, polyolefin is easily oxidized, and thus, many kinds of additives are mixed in it for practical use. Hence, effects of oxidation and additives on surface properties of the aPP surface were examined as well. To achieve oxidation, the aPP films were annealed at 428 K for 100 min under the ambient atmosphere. After this treatment, T_g^s decreased by approximately 15 K in comparison with the intact film due to degradation of surface chains. On the contrary, in the case of the aPP containing 10 wt% antioxidant, T_g^s was almost the same as that of the intact film before and even after the oxidation treatment.