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.     

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.     

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.     

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.     

Miscibility and viscoelastic properties of acrylic polyhedral oligomeric silsesquioxane-poly(methyl methacrylate) blends

We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d≈2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to ?=0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature T_g and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings ?<0.10. At a loading ?=0.05, the unmodified acrylic POSS induces a 4.9 °C decrease in the T_g of PMMA, far less than the 17.4 °C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS-PMMA blend compared with the DOP-PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume f₀=0.046 for PMMA at T₀=170 °C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA f₀=0.057, which corresponds to an addition of 0.47 nm³ per added POSS molecule at ?=0.05. The degree of dispersion was characterized using both wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at ?=0.20 and higher, but no significant phase separation is evident at ?=0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings ?≥0.10, while no evidence of phase separation is present in the ?=0.05 blend in DMA.     

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.     

The effect of solid content on latex coalescence and film formation: Steady-state energy transfer study with fluorescence labeled polymers

The steady-state fluorescence technique was used to examine the healing and interdiffusion of polymer molecules as a function of solid content during annealing of latex films above the glass transition (T_g). Films were prepared from a mixture of naphthalene (N)- and pyrene (P)-labeled poly(methy methacrylate) (PMMA) latex particles. Above T_g, interdiffusion of polymer chains was observed by detecting the steady-state energy transfer from excited naphthalene to pyrene molecules. Various latex films with different latex content were used to measure the critical occupation percent for the reliable steady-state fluorescence measurements. Diffusion activation energies in these latex films were measured and found to be around 30 kcal/mol, which was attributed to the backbone motion of PMMA chains. © 1996 John Wiley & Sons, Inc.     

Dewetting kinetics of thin polymer bilayers: Role of under layer

A combined experimental and computational study is presented to uncover the dewetting kinetics of the PS/PMMA system by changing the film thickness of the PMMA under layer. On the low M_w PMMA (M_w = 15 kg/mol) layer, the dewetting velocity of PS film firstly rapidly decreases (regime I), and then becomes almost invariant (regime II) with the increase of the film thickness of the liquid lower layer. Experiments suggest that the transition from regime I to regime II is correlative with the property of the solid substrates. The linear stability analysis of thin bilayers uncovers a bimodal behaviour of the instability under the experimental conditions and changeover of dominant mode of instability from one interface to the other is the major reason behind the switching of regimes. The nonlinear simulations closely mimic the experimental interfacial morphologies and suggest two different pathways of hole growth for regimes I and II under experimental condition. The simulations also indicate that the rapid reduction in the dewetting velocity is because of the increased excursion or penetration of the upper layer into the lower layer near the three phase contact line as the viscous resistance at the more viscous PMMA layer reduces with its increasing thickness. A qualitative match is thus found between the experimental and theoretical trends of the dewetting velocities. In addition, the experiments show that on a high M_w PMMA (M_w = 365 kg/mol) layer, the kinetics of hole growth of the PS layer is not affected by the PMMA layer thickness.     

Dispersion of polymer-grafted magnetic nanoparticles in homopolymers and block copolymers

The dispersion of magnetic nanoparticles (NPs) in homopolymer poly(methyl methacrylate) (PMMA) and block copolymer poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films is investigated by TEM and AFM. The magnetite (Fe₃O₄) NPs are grafted with PMMA brushes with molecular weights from M = 2.7 to 35.7 kg/mol. Whereas a uniform dispersion of NPs with the longest brush is obtained in a PMMA matrix (P = 37 and 77 kg/mol), NPs with shorter brushes are found to aggregate. This behavior is attributed to wet and dry brush theory, respectively. Upon mixing NPs with the shortest brush in PS-b-PMMA, as-cast and annealed films show a uniform dispersion at 1 wt%. However, at 10 wt%, PS-b-PMMA remains disordered upon annealing and the NPs aggregate into 22 nm domains, which is greater than the domain size of the PMMA lamellae, 18 nm. For the longest brush length, the NPs aggregate into domains that are much larger than the lamellae and are encapsulated by PS-b-PMMA which form an onion-ring morphology. Using a multi-component Flory-Huggins theory, the concentrations at which the NPs are expected to phase separate in solution are calculated and found to be in good agreement with experimental observations of aggregation.     

Role of a small amount of comonomer on the physical aging of poly(methyl methacrylate) copolymer investigated by dynamic viscoelasticity

The physical aging of a poly(methyl methacrylate) (PMMA) copolymer having a small amount of ethyl methacrylate was investigated below its glass transition temperature (T_g) using dynamic viscoelastic measurements. The results were compared with those from homo-PMMAs to investigate how the comonomer in the copolymer affects the physical aging of the glass. Annealing temperatures were classified into two categories by the glass transition temperature of a poly(ethyl methacrylate) (T_gPEMA; ca. 65°C). The relaxation rate of the copolymer was faster than those of the homo-PMMAs when the samples were aged below T_gPEMA, but the rates were equal when aged at above T_gPEMA. The experimental results are discussed in terms of the segmental motion.     

Thickness and composition dependence of the glass transition temperature in thin random copolymer films

Glass transition temperatures (T_g) of thin poly(styrene-co-methyl methacrylate) and poly(2-vinyl pyridine-co-styrene) films coated on a native oxide surface of Si wafer (100) were measured by ellipsometry. The thickness dependence of T_g can be properly fitted by previously suggested equation developed for homopolymers, based upon a continuous multi-layer model, although one component in thin random copolymer films demonstrates a slightly favorable interaction between a substrate and thin film, and another demonstrates a strongly favorable interaction. Surface and interface have a strong influence on T_g of thin film coated on substrate: the surface has the effect of reducing T_g, whereas the interface increases the T_g according to the degree of interaction between a substrate and thin film. This degree of interaction can be quantified as an interaction parameter (k), and is dependent on the composition of random copolymers. For the estimation of k values of thin random copolymer films, we proposed a parallel type additive function (1/k_ran=w₁/k₁+w₂/k₂) where w is a weight fraction of component.     

Morphology development and exclusion of noncrystalline polymer during crystallization in PVDF/PMMA blends

Morphology development during the crystallization of poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blends was investigated at various crystallization temperatures (T_C) by means of time-resolved light scattering measurements and atomic force microscopy (AFM). A coarse spherulite obtained at a high T_C of 162 °C was found to be developed with a two-step crystallization process. The ordering in the spherulites (Pr) increased with time at the early stages and then decreased at the later stages. The rate of spherulite growth started to decrease when Pr started to decrease. In contrast, in the compact spherulite obtained at a low T_C of 148 °C, Pr decreased monotonously with time while the growth rate was constant. AFM observation revealed that such characteristic crystallization behavior is attributed to the exclusion of PMMA from the crystal growth during the crystallization; i.e., the amount of excluded PMMA becomes larger as the distance from the spherulite center increases and the crystallization temperature rises.     

Synthesis and characterization of trifluoromethyl substituted styrene polymers and copolymers with methacrylates: Effects of trifluoromethyl substituent on styrene

2-Trifluoromethyl styrene (2TFMS), 2,5-bis(trifluoromethyl) styrene (25BTFMS), and 3,5-bis(trifluoromethyl) styrene (35BTFMS) were synthesized. These styrenes were readily polymerized in bulk and also in solution using AIBN as a free radical initiator. The polymerization rate of these trifluoromethyl substituted styrenes and other monomers such as styrene (St), pentafluorostyrene (PFS) and 4-trifluoromethyl-tetrafluorostyrene (TFMTFS) were measured in benzene and dioxane by monitoring the ¹H NMR spectra of the double bond hydrogen. The order of polymerization rates was TFMTFS > 35BTFMS > 25BTFMS > PFS > 2TFMS > St. T_gs of styrene polymers with CF₃ substituted on the ortho position of the phenyl ring were much higher than those of the meta and para substituted styrenes due to the steric hindrance of the bulky CF₃ group close to the polymer main chain, which resulted in a decrease in the segment mobility of the polymer chains and an increasing T_g of the polymers. The copolymers of 2TFMS with methyl methacrylate (MMA) and also 25BTFMS with trifluoroethyl methacrylate (TFEMA) were prepared. T_gs of the copolymers were in the range of 120-145 °C and the copolymers were transparent and thermally stable. The copolymer films were flexible and exhibited high transmittance as the homopolymers of MMA and TFEMA. Thus, these copolymers may be utilized as novel optical materials.     

Plasma etching durability of poly(methyl methacrylate)

The decomposition of poly(methyl methacrylate) (PMMA), as a positive resist, in CF₄/O₂ plasma etching has been studied in the thin film state in order to clarify the factors influencing the dry etching durability of resists. It becomes clear that the major PMMA decomposition in CF₄/O₂ plasma etching proceeds by the mechanism of random chain scission because very small kinetic chain lengths are estimated from the gel permeation chromatography data. The Arrhenius plots for the plasma etching rate of PMMA bend above about the glass transition temperature (T_g), where rapid increase of the etching rate and remarkable pattern deformation are observed. Activation energy of the PMMA etching rate which is changed by oxygen concentration and rf power indicates various values from 3.1 to 6.5 kcal/mol below about T_g. The result of molecular weight variation in the cross section of the film suggests that the active species permeate into the film with fairly large speed and the PMMA decomposition occurs not only at the film surface but also at deep layer of the film.     

Optical and thermal properties of methyl methacrylate and pentafluorophenyl methacrylate copolymer: Design of copolymers for low-loss optical fibers for gigabit in-home communications

As a novel base material for low-loss graded index plastic optical fibers (GI POFs) in gigabit home networks, a copolymer of methyl methacrylate (MMA) and pentafluorophenyl methacrylate (PFPMA) was prepared and its thermal and optical properties were investigated. When the PFPMA content in the monomer feed was 0-50 mol%, both the glass transition temperature (T_g) and the decomposition temperature of the copolymer were higher than that for PMMA, which is the base material for many commercially available POFs. The transmittance of the copolymer was also found to be higher than that of PMMA since it has fewer C-H bonds per unit volume. As the core material of GI POFs, MMA-co-PFPMA (65/35 mol%), which had the highest T_g of 118 °C was utilized. A low-loss GI POF with an attenuation of 172-185 dB/km at the emission wavelengths of a high-speed light source (670-680 nm) was successfully obtained for the first time.     

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.     

Phase segregation of poly(3-dodecylthiophene)-block-poly(methyl methacrylate) copolymers

We investigated, via small angle X-ray scattering (SAXS), the ordered-to-disordered transition temperature (T_ODT) of symmetric poly(3-dodecylthiophene)-block-poly(methyl methacrylate) copolymers (P3DDT-b-PMMA) with different molecular weights synthesized by anionic coupling reaction. When the molecular weight of P3DDT-b-PMMA was properly chosen, the T_ODT was observed within experimentally accessible temperature range (higher than the glass transition and melting temperature for PMMA and P3DDT, respectively, but lower than the thermal degradation temperature). We also measured the temperature dependence of Flory–Huggins interaction parameter (χ) between P3DDT and PMMA as χ = 0.1109 + 76.63/T, in which T is the absolute temperature.     

Crystal growth pattern changes in low molecular weight poly(ethylene oxide) ultrathin films

A low molecular weight (MW) poly(ethylene oxide) (PEO) crystallized in ultrathin films displays various crystal growth patterns in a crystallization temperature (T_x) range from 20.0 °C to 50.0 °C. In succession, the following patterns are found: nearly one-dimensional (1D) dendrite-like crystal patterns at T_x ≤ 38.0 °C, two-dimensional (2D) seaweed-like patterns between 39.0 °C ≤ T_x ≤ 42.0 °C and again, nearly 1D dendrite-like patterns at T_x ≥ 43.0 °C. These transitions result from a complex interplay of varying growth rates along different growth directions and preservation of growth planes. Structural analysis carried out via electron diffraction indicates that the dendrite-like crystals formed at the low and high T_x values differ by their fast growth directions: along the {120} normal at the low T_x values and along the (100) and (010) normal at the high T_x values. In the later case however, the major growth faces are still the {120}, this time tilted at 45° and indicating the a∗ and b axes growth tips. In the intermediate T_x range (39.0 °C-42.0 °C), three growth directions coexist giving rise to the seaweed morphology. The crystal growth rates at the low and high T_x values are constant versus time. For the seaweed, a square-root dependence is obtained. These differences are probably due to 1D and 2D growth in the ultrathin films and are associated with different growth patterns of the dendrites and the seaweed, respectively.