Substrate vs. free surface: Competing effects on the glass transition of polymer thin films

The glass-transition temperature (T_g) of polymer thin films can be strongly influenced by the combined effects of the supporting solid substrate and the free surface. The relative importance of these two effects, which often compete with each other, depends on the strength of the substrate–film interactions. Utilizing an atomistically informed coarse-grained model for poly(methyl methacrylate) (PMMA), here we uncover the relationship between the substrate–film interfacial energy and the spatial distribution of T_g across thin films. We find that above a critical interfacial energy, the linear dependence of film T_g on the interfacial energy breaks down and film T_g attains an asymptotic value. Analyses on the spatial variation of T_g across the thin film reveal that the short-range interface near the cohesive surface generates a long-range interphase that leads a spatially uniform appreciation of T_g throughout the film, unlike weakly cohesive surfaces that show sharp gradients along the depth of film. These findings explain recent experiments and reveal a versatile approach for tuning film T_g via engineered substrate-film interactions.     

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.     

Complex microstructures of ABC triblock copolymer thin films directed by polymer brushes based on self-consistent field theory

The morphology and the phase diagram of ABC triblock copolymer thin film directed by polymer brushes are investigated by the self-consistent field theory in three dimensions. The polymer brushes coated on the substrate can be used as a good soft template to tailor the morphology of the block copolymer thin films compared with those on the hard substrates. The polymer brush is identical with the middle block B. By continuously changing the composition of the block copolymer, the phase diagrams are constructed for three cases with the fixed film thickness and the brush density: identical interaction parameters, frustrated and non-frustrated cases. Some ordered complex morphologies are observed: parallel lamellar phase with hexagonally packed pores at surfaces (LAM₃ ^ll -HFs), perpendicular lamellar phase with cylinders at the interface (LAM^⊥-CI), and perpendicular hexagonally packed cylinders phase with rings at the interface (C₂^⊥-RI). A desired direction (perpendicular or parallel to the coated surfaces) of lamellar phases or cylindrical phases can be obtained by varying the composition and the interactions between different blocks. The phase diagram of ABC triblock copolymer thin film wetted between the polymer brush-coated surfaces is very useful in designing the directed pattern of ABC triblock copolymer thin film.     

Effects of film thickness on moisture sorption, glass transition temperature and morphology of poly(chloro-p-xylylene) film

Moisture sorption, glass transition temperature (T_g) and morphology of ultra thin poly(chloro-p-xylylene) (parylene-C) are greatly influenced by geometrical confinement effects. For film <50 nm, the equilibrium moisture saturation is a decreasing function of film thickness. However, the T_g of film <50 nm is about 10 °C higher than thicker films. The above phenomena are attributed to the effect of geometrical confinement on the thermal properties and the morphology of parylene-C film. Surface confinement results in an increased in T_g, but a decreased in crystallinity for films <50 nm. In this study, we show that the increase in moisture sorption for parylene-C films <50 nm is dominated by the crystallinity variation.     

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.     

Morphology of an asymmetric ethyleneoxide-butadiene di-block copolymer in bulk and thin films

We present experiments on the melt and crystal morphology of a asymmetric semi-crystalline poly(ethylene/butylene-b-ethyleneoxide) diblock copolymer (PB_h-b-PEO) in bulk as well as in thin films. Simultaneous small- and wide-angle X-ray scattering combined with AFM and TEM images reveal in the melt a bulk morphology of hexagonally packed cylinders of PEO in a PB_h matrix, that transforms into a hexagonal perforated lamellar phase upon crystallization. X-ray reflectivity of thin films of PB_h-b-PEO in the melt indicates wetting layers at the top and bottom interfaces, which force the cylinders in the interior to orient parallel to the substrate. Crystallization of the PEO block leads to roughening of the air/film interface and causes lateral structuring coexisting with planar lamellar layers in thinner films.     

Thermal Expansion of Thin Diblock Copolymer Films

The thermal expansion of thin films of symmetric diblock copolymers of polystyrene (PS) and poly(methyl methácrylate) (PMMA) was investigated by X-ray reflectivity. The confinement of the copolymer to the substrate, coupled with the multilayering of the copolymer where PS and PMMA layers are oriented parallel to the substrate, gives rise to unusual thermal expansion characteristics. The total thickness of the film increases as 3α_L, where α_L is the linear thermal expansion coefficient of the copolymer. Unlike homopolymer films, the thermal expansion of an ordered block copolymer film results in an excessive stretching of the copolymer chains at the interface between the PS and PMMA layers. This excess stretching is a result of the confinement of the junction points of the copolymer chains to the interfaces and the suppression of the lateral expansion of the copolymer. When the stretching of the chains becomes too high, relaxation occurs by transporting copolymer chains to the surface. This is evidenced by a reduction in the period of the multilayer. After the copolymer chains have relaxed, the change in the multilayer period with temperature closely follows α_L.     

Impacts of stereoregularity and stereocomplex formation on physicochemical, protein adsorption and cell adhesion behaviors of star-shaped 8-arms poly(ethylene glycol)-poly(lactide) block copolymer films

Biodegradable stereocomplex film exhibiting soft and stretchy character was prepared by simply blending between enantiomeric 8-arms poly(ethylene glycol)-block-poly(l-lactide) (8-arms PEG-b-PLLA) and 8-arms PEG-b-PDLA copolymers with star-shaped structure. The stereocomplex film exhibited higher T_g and PLA crystallinity than those of original copolymer films. Effects of stereoregularity and stereocomplexation on protein adsorption and L929 cells attachment/proliferation behaviors onto the films were analyzed from the viewpoint to design a new class of implantable soft biomaterial. The stereocomplex film was found to exhibit large amount of protein adsorption than original films. Furthermore, cell attachment efficiency and proliferation rate on the film were significantly enhanced by stereocomplexation. This stereocomplex material is expected to be applicable as degradable temporary scaffold for soft tissue regeneration. Consequently, it was indicated that the stereocomplex formation could be proposed to be a novel method to control the protein- and cell-adhesive properties of biodegradable matrix composed of PEG-PLA copolymer.     

Synthesis and characterization of flexible polyoxadiazole films through cyclodehydration of polyhydrazides

Aiming to produce flexible films of poly(4,4′-diphenylether-1,3,4-oxadiazole), POD-DPE, the synthesis of POD-DPE through thermal cyclodehydration of polyhydrazide in solutions of poly(phosphoric acid), PPA, and in film form is studied here. It was observed that POD-DPE polymer samples obtained when the cyclodehydration of polyhydrazide is performed in solutions of PPA are insoluble in the solvents normally used for film preparation. On the other hand, the cyclodehydration of polyhydrazide in film form can lead to brittle POD-DPE films due to the high temperatures required for reaction to occur. In order to obtain flexible POD-DPE films with high conversion through cyclodehydration of polyhydrazide films, it is shown here that it is very important to adjust the cyclodehydration temperature (T_c) as a function of the glass transition temperature (T_g) and the weight average molecular weight (M_w) of the polyhydrazide. In all cases analyzed, polyhydrazides were synthesized by low temperature solution polycondensation reactions and were characterized by intrinsic viscosity measurements, size exclusion chromatograpy, nuclear magnetic resonance, infrared spectroscopy and thermal analysis. Polyhydrazides with weight average molecular weights up to 74,100 g/mol were obtained. The POD-DPE films obtained were characterized by infrared spectroscopy and nuclear magnetic resonance, showing that the production of POD-DPE films is indeed possible from cyclodehydration of polyhydrazide films.     

Improvement of film properties of vinyl acetate based emulsion polymers by using different types of maleic acid diesters

Two types of maleic acid diesters, dibutyl maleate (DBM) and dioctyl maleate (DOM) were used as comonomers in semicontinuous emulsion copolymerization of vinyl acetate (VAc) in order to improve the film properties of poly(vinyl acetate), PVAc emulsion polymer. The effects of the comonomer type and comonomer ratio on minimum film forming temperature (MFFT), glass transition temperature (T_g), polymer structure, molecular weights, water contact angle and water resistance of PVAc latex films were examined. It was found that MFFT and T_g of the PVAc emulsion polymer decreased by the presence of the maleic acid disters in copolymer composition. This decrease was more affected by the increasing content and alkyl chain length of the comonomers. The molecular weights of the emulsion polymers were also affected by the comonomers and their ratios. Moreover, hydrophobicity and water resistance of the PVAc latex films were increased by using DBM and DOM as comonomer.     

Reversible thickness control of polymer thin films containing photoreactive coumarin derivative units

The reversible control of the thickness of polymer thin films was investigated using (meth)acrylic polymers containing photoreactive coumarin derivative units in the side chain. Coumarin derivative units underwent dimerization and the reverse-dimerization by photoirradiation and were used as a reversible cross-linking point. The homopolymer of 7-methacryloyloxy-4-methylcoumarin (T_g = 194 °C) did not cause changes in film thickness after photoreactions. The homopolymer of 7-(2′-acryloyloxyethoxy)-4-methylcoumarin (AEMC) (T_g = 89 °C) decreased 19% of film thickness by photodimerization and 73% of the decreased thickness was recovered after the reverse-dimerization and the subsequent thermal annealing at 130 °C. The reverse-dimerization of the copolymer of AEMC and n-butyl acrylate (AEMC content = 19 mol%, T_g = 11 °C) resulted in 53% of recovery from the decreased film thickness without annealing. The mobility of polymer main-chain was revealed to be essential factor to change film thickness by photoreactions. Photodimerization of coumarin derivative units in low glass transition temperature (T_g) tended to proceed faster than in high T_g polymers and resulted in larger decrease in film thickness.     

Transition behavior of PS-b-PMMA films on the balanced interfacial interactions

The thickness dependence of the order-to-disorder transition (ODT), measured by in situ grazing-incidence small-angle X-ray scattering (GISAXS), has been investigated in thin films of a symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) on a random copolymer (P(S-r-MMA)) grafted to the substrate where the interfacial interactions are balanced. With decreasing film thickness less than 25L₀, the ODT significantly decreases to 193 °C for film of 10L₀ in thickness, because the interfacial interactions by a random copolymer grafted to the substrate provide a surface-induced compatibilization toward two block components. However, a plateau of the ODT at ∼213 °C for films thicker than 25L₀ was observed above the bulk value of 200 °C. The elevation of this ODT indicates a suppression of compositional fluctuations normal to the film surface, more than likely because the dominant orientation of the lamellar microdomains was found to be parallel to the film surface.     

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.     

Heterogeneous crosslinking of waterborne two-component polyurethanes (WB 2K-PUR); stratification processes and the role of water

Stoichiometric imbalance and crosslinking conditions during film formation of waterborne two-component polyurethanes (WB 2K-PUR) play a significant role in the development of material properties. Changing isocyanate-to-hydroxyl (NCO:OH) ratios from 1.0 to 2.2 over a range of humidities significantly affects film morphology, and these studies show that while films with higher NCO:OH exhibit increased T_g and surface roughness when crosslinked at high RH, a T_g decrease is observed at elevated RH. Higher RH conditions not only result in increased urea and decreased urethane content, but also facilitate enrichment of poly(ethylene glycol) (PEG) functionality near the film-air (F-A) interface due to stratification of PEG-modified polyisocyanate crosslinkers. Reaction-induced stratification also occurs during film formation resulting in the T_g differences between F-A and F-S interfaces: namely T_g,F-A>T_g,F-S at 32 and 52%RH, whereas T_g,F-A≤T_g,F-S at 75%RH, as determined by interfacial micro thermal analysis (μTA). This behavior is attributed to concentration gradients of water during film formation and their corresponding effects on isocyanate hydrolysis reactions in conjunction with PEG stratification near the F-A interface. Furthermore, excessive stoichiometric NCO:OH imbalance results in the formation of microscopic ‘hills’ and ‘valleys’ on the F-A surface having T_g differences of 6 °C. These phenomenological processes are incorporated into a model describing WB 2K-PUR film formation as a function of crosslinking conditions and resulting morphological features.     

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.     

Methacrylate-based polymer films useful in lithographic applications exhibit different glass transition temperature-confinement effects at high and low molecular weight

The effects of confinement on polymer films are important in applications related to photoresists. To optimize resolution, methacrylate polymers used in photoresists are often low molecular weight (MW). We use ellipsometry and fluorescence to study how the glass transition temperature (T_g) is affected by confinement in silica-supported films of low and high MW poly(1-ethylcyclopentyl methacrylate) (PECPMA) and poly(methyl methacrylate) (PMMA). With decreasing nanoscale thickness, T_g is nearly invariant for high MW (M_n = 22.5, 188 and 297 kg/mol) PECPMA but decreases for low MW PECPMA, with T_g – T_g,bulk = −7 to 8 °C in a 27-nm-thick film (M_n = 4.1 kg/mol) via ellipsometry and −15 °C in a 17-nm-thick film (M_n = 4.9 kg/mol) via fluorescence. Fluorescence studies using a 20-nm-thick dye-labeled layer in multilayer, bulk PECPMA films reveal a much greater perturbation to T_g in the free-surface layer for low MW PECPMA, which propagates tens of nanometers into the film. The effect of MW in single-layer monodisperse PMMA films is even more striking; T_g increases with confinement for high MW but decreases for low MW, with T_g – T_g,bulk = 9 °C in a 12-nm-thick film (nominal MW = 509 kg/mol) and −16 °C in a 17-nm-thick film (nominal MW = 3.3 kg/mol). The strong influence of MW on confinement effects in PECPMA and PMMA is in contrast to the previously reported invariance of the effect with MW in supported polystyrene films, reconfirmed here.     

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.     

A paradigm shift in morphological architecture of PEO-b-PFOMA semi-fluorinated block copolymer thin films upon facile solvent annealing

A systematical study on the morphological transition of the micelle films of semi-fluorinated poly(ethylene oxide)-b-poly(1H,1H-dihydro perfluorooctyl methacrylate) (PEO-b-PFOMA) diblock copolymers was carried out upon perfluroalkanes (PF-5080) or α,α,α-trifluorotoluene (TFT) solvent annealing. Poorly ordered short cylindrical structures of the PEO_5k-b-PFOMA_21k micelle film underwent a phase inversion with PEO cores in the PFOMA continuous phase with a short period of PF-5080 solvent annealing. In contrast, the highly ordered morphology of PEO_10k-b-PFOMA_21k with PFOMA cores in the PEO continuous phase developed into cylindrical microdomains presumably via the fusion process. Prolonged annealing of the film transformed its morphology into inverted-spherical domains of PEO in the PFOMA continuous phase through long-range ordering by following the fission process. In order to find out a synthetic application of the morphology inversion strategy, an attempt was undertaken by adding a gold precursor to the PEO_10k-b-PFOMA_21k micelle solution, and as-cast thin films were prepared accordingly. Upon PF-5080 solvent annealing, the nanoparticles populated in self-assembled thin films resulted in inverted-spherical domains having gold nanoparticles populated in PEO cores surrounded by the PFOMA continuous phase. When the annealing solvent was changed to TFT, a highly ordered in-plane cylindrical morphology with respect to the substrate was achieved from the poorly ordered cylindrical microdomains of the PEO_5k-b-PFOMA_21k thin film, whereas an uneven cylindrical structure was produced from PEO_10k-b-PFOMA_21k.     

Efficient and color pure blue light emitting random copolymer with fluorene and fluorenylstilbene

A blue electroluminescent polymer, random copolymer of fluorenylstilbene and fluorene, was prepared by the nickel catalyzed coupling reaction. The structure and properties of the copolymer were analyzed by various spectroscopic methods. The obtained polymer had good solubility and thermal stability with high T_g. The polymer in thin film emits strong blue luminance (max=468 nm) with narrow bandwidth upon photoexcitation. PL spectrum of the polymer in the film is almost consistent with that of solution one as well as the EL spectrum, indicating that the aggregation and the excimer fluorescence are suppressed by the introduction of fluorenylstilbene comonomer. Moreover, the introduction of fluorenylstilbene comonomer lowered the oxidation potential to lead feasible hole injection, when the compared with poly(fluorene) homopolymer. The ITO/PEDOT/polymer/LiF/Al device showed the maximum brightness of 3500 cd/m² with a turn on voltage of 4.4, the maximum efficiency of 0.878 lm/W and blue emission with CIE chromaticity coordinates of ((x,y)=(0.17, 0.25)).     

Study of Phosphorus Doped Micro/Nano Crystalline Silicon Films Deposited by Filtered Cathodic Vacuum Arc Technique

Phosphorus doped micro/nano crystalline silicon thin films have been deposited by the filtered cathodic vacuum arc technique at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 ^°C. The films have been characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, secondary ion mass spectroscopy, dark conductivity ( σ _D), activation energy ( ΔE) and optical band gap (E _g). The XRD patterns show that the RT grown film is amorphous in nature but high T_s (225 and 350 ^°C) deposited films have a crystalline structure with (111) and (220) crystal orientation. The crystallite size of the higher T_s grown silicon films evaluated was between 17 to 31 nm. Raman spectra reveal the amorphous nature of the film deposited at RT whereas higher T_s deposited films show a higher crystalline nature. The crystalline volume fraction of the silicon film deposited at higher T_s was estimated as 65.7 % and 74.4 %. The values of σ _D, ΔE and E _g of the silicon films deposited at different T_s were found to be in the range of 8.84 x 10 ^?4? 0.98 ohm ^?1 cm ^?1, 0.06 - 0.31 eV and 1.31-1.93 eV, respectively. A n-type nc-Si/p-type c-Si heterojunction diode was fabricated which showed the diode ideality factor between 1.1 to 1.5.