In situ measurement of the thermal expansion behavior of benzocyclobutene films

In situ measurement techniques suitable for determination of the coefficient of thermal expansion (CTE) in thin, spin‐cast polymer films in both the in‐plane and through‐plane directions are presented. An examination of the thermal expansion behavior of cyclotene thin films has been performed. In particular, the effect of film thickness on the in‐plane and through‐plane CTE and in‐plane Young's modulus of spin‐coated cyclotene films was examined. It is shown that the mechanical response of in situ cyclotene films can be adequately described by isotropic film properties. It was also demonstrated that there is no thickness dependence on the free‐standing mechanical properties or on the resulting through‐plane thermal strain in an in situ film. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 311–321, 1999     

Relaxing nonequilibrated polymers in thin films at temperatures slightly above the glass transition

We have studied the dewetting process of thin polystyrene films on nonwettable substrates in the viscoelastic regime slightly above the glass transition temperature. The evolution of the shape of the dewetting rim for varying film thickness, molecular weights and dewetting temperatures allowed us to determine the relaxation rates of residual stresses, which originated from nonequilibrated polymer chain conformations formed during film preparation by spin‐coating. For long chain polymers, we found rates notably faster than the longest bulk relaxation processes, highly independent of molecular weight and temperature. Our study demonstrates that dewetting is a powerful tool for sensitive characterization of nonequilibrium properties of thin polymer films. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 515–523     

Dual capacitor technique for measurement of through‐plane modulus of thin polymer films

Polymer thin films are widely used as coatings and interlevel dielectrics in microelectronic applications. In thin‐film structures, stresses are generated due to interaction with adjacent layers and film shrinkage due to solvent evaporation or curing. This causes polymer chain orientation resulting in anisotropic (direction dependent) film properties. The dual capacitor technique has been developed to measure in situ, the through‐plane (z) stress‐strain behavior of thin polymer films. A parallel plate capacitor device and an interdigitated electrode structure were used as sensors to detect changes in dielectric permittivity and thickness of thin polymer films under compression. The analytical and finite element models used to interpret the capacitance measurements have been presented. The Clausius–Mossotti equation was used to determine the volume change in the film from the permittivity measurements. Results have been reported for 10–14 μm thick, Cyclotene 4026‐46 benzocyclobutene films and 10–12 μm thick films of polyimide PI‐2611. The Cyclotene 4026‐46 films were found to be mechanically isotropic, whereas the PI‐2611 films were highly anisotropic. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1634–1644, 2000     

Experimental evidence and theoretical analysis of physical aging in thin and thick amorphous glassy polymer films

Through time‐dependent gas transport properties, we have investigated the physical aging process of amorphous glassy polymer films made from a polynorbornene. By combining the concepts of free volume and the kinetic theory of glass stabilization, it was found that the time dependence of the gas permeability could be rationalized through the thickness dependence of the glass transition temperature. A mathematical relationship was developed that directly relates polymer physical aging (tracked by the gas permeability decay) and sample thickness. It was confirmed by permeation measurements with nitrogen and helium that the aging process is accelerated for thin glassy polymer films (about 8000 Å). The theoretical results show that accelerated aging for thin films compared to thick films can be qualitatively predicted, based on the decrease in the glass transition temperature when the film thickness decreases. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2239–2251, 1999     

Discrepancies in the characterization of the glass transition in thin films of hyperbranched polyesters

The dynamic glass transition and the dilatometric glass transition temperature are simultaneously characterized in thin films of hyperbranched aromatic polyesters by broadband dielectric spectroscopy and capacitive scanning dilatometry. A diverging thickness dependence is detected: while the temperature position of the alpha relaxation peak T_α decreases by ∼30 K, the dilatometric T_g increases by ∼10 K with decreasing film thickness. This emphasizes the subtle character of the glass transition phenomenon—as manifested in the molecular dynamics and in the (structural) thermal expansion—and proves that, in contrast to the bulk, different experimental techniques do not necessarily deliver similar results in confinement. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3006–3010, 2006     

Impact of chain architecture (branching) on the thermal and mechanical behavior of polystyrene thin films

The modulus and glass transition temperature (T_g) of ultrathin films of polystyrene (PS) with different branching architectures are examined via surface wrinkling and the discontinuity in the thermal expansion as determined from spectroscopic ellipsometry, respectively. Branching of the PS is systematically varied using multifunctional monomers to create comb, centipede, and star architectures with similar molecular masses. The bulk‐like (thick film) T_g for these polymers is 103 ± 2 °C and independent of branching and all films thinner than 40 nm exhibit reductions in T_g. There are subtle differences between the architectures with reductions in T_g for linear (25 °C), centipede (40 °C), comb (9 °C), and 4 armed star (9 °C) PS for ≈ 5 nm films. Interestingly, the room temperature modulus of the thick films is dependent upon the chain architecture with the star and comb polymers being the most compliant (≈2 GPa) whereas the centipede PS is most rigid (≈4 GPa). The comb PS exhibits no thickness dependence in moduli, whereas all other PS architectures examined show a decrease in modulus as the film thickness is decreased below ～40 nm. We hypothesize that the chain conformation leads to the apparent susceptibility of the polymer to reductions in moduli in thin films. These results provide insight into potential origins for thickness dependent properties of polymer thin films. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Focused laser-induced marangoni dewetting for patterning polymer thin films

Highly-localized focused laser spike (FLaSk) heating of polymer thin films is a resist- and developer-free alternative to two-dimensional (2D) laser direct write for creating patterns on the single micron or, by exploiting overlap effects, submicron scale. The massive temporal and spatial thermal gradients and resulting thermal Marangoni stresses generated by FLaSk are an effective means for the directed dewetting and patterning of such films. Here, the general applicability of this technique to glassy amorphous polymer thin film systems is investigated through systematic investigation of film thickness, glass transition temperature, and polymer mobility. The results reveal that the important parameters are the film thickness (coupled to the optical heating effects through anti-reflection coating effects) and the high-temperature polymer melt mobility, allowing for generation of single features with linewidths of down to ∼1 μm. Further, the introduction of spatial mobility variations by using polymer brushes, bilayers, and microphase separated block copolymers leads to additional profile manipulation effects (i.e. spontaneous 2D pattern generation and flattened top profiles). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 225–236     

High‐pressure sorption of carbon dioxide and methane in all‐aromatic poly(etherimide)‐based membranes

The sorption of compressed carbon dioxide and methane in a series of all‐aromatic poly(etherimide) (PEI) thin films is presented. The polymer films are derived from the reactions between an arylether diamine (P1) and four different dianhydrides [3,3′,4,4′‐oxydiphthalic dianhydride (ODPA), 3,3′,4,4′ biphenyltetra‐carboxylic dianhydride (BPDA), 3,3′,4,4′‐benzo‐phenonetetracarboxylic dianhydride (BTDA), and pyromellitic dianhydride (PMDA)] that have been selected to systematically change the flexibility of the polymer backbone, the segmental mobility, and the nonequilibrium excess free volume (EFV) of the polymer. The EFV, gas sorption capacities, and sorption‐ and temperature‐induced dynamic changes in film thickness and refractive index have been investigated by spectroscopic ellipsometry. The sorption capacity depends to a great extent on the PEI backbone composition. PMDA‐P1 shows the highest carbon dioxide sorption, combined with the lowest sorption selectivity because of the predominant sorption of methane in the EFV. For ODPA‐P1, the highest sorption selectivity is obtained, while it shows little long‐term relaxations at carbon dioxide pressures up to 25 bar. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 986–993     

Impact of low‐molecular mass components (oligomers) on the glass transition in thin films of poly(methyl methacrylate)

Commercial polydisperse atactic poly(methyl methacrylate) (PMMA) exhibits a decreased glass transition temperature (T_g) when the film thickness is less than ～60 nm, whereas more model atactic PMMA shows an increased T_g in thin films supported on clean silicon wafers. NMR indicates no difference in tacticity, so the divergent thin film behavior appears related to the relative distribution of molecular mass. Extraction of some low molecular weight PMMA components from the commercial sample results in a significant modification of the thin film T_g compared with the initial PMMA fraction. The extracted sample exhibits initially a slight decrease in T_g as the film thickness is reduced below ～60 nm, but then T_g appears to increase for films thinner than 20 nm. These results illustrate the sensitivity of polymer thin film properties to low‐molecular mass components and could explain some of the contradictory reports on the T_g of polymer thin films that exist in the literature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Creep behavior of ultra‐thin polymer films

A novel microbubble inflation method has been used to determine the creep compliance of poly(vinyl acetate) and polystyrene ultra‐thin films (13–300 nm thick) at temperatures from below to above the glass temperature. We present results that suggest that time‐temperature and time‐thickness superposition hold in the glassy relaxation regime. Although time‐temperature superposition is found for the entire response curve for each thickness, we also find that time‐thickness superposition fails as the long‐time compliance is approached. This effect occurs because of a strong stiffening as the film thickness decreases. We also show first evidence of stiffening in the glassy regime of free standing films of polystyrene. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1952–1965, 2008     

A novel temperature‐step method to determine the glass transition temperature of ultrathin polymer films by liquid dewetting

A novel temperature‐step experimental method that extends the Bodiguel‐Fretigny liquid dewetting method of investigating polymer thin films is described and results presented from an investigation of thickness effects on the glass transition temperature (T_g) of ultrathin polystyrene (PS) films. Unlike most other methods of thin film investigation, this procedure promises a rapid screening tool to determine the overall profile of T_g versus film thickness for ultrathin polymer films using a limited number of samples. Similar to our prior observations and other literature data, with this new method obvious T_g depression was observed for PS thin films dewetting on both glycerol and an ionic liquid. The results for PS dewetting on the two different liquids are similar indicating only modest effects of the substrate on the T_g‐film thickness relationship. In both instances, the T_g depression is somewhat less than for similar PSs supported on silicon substrates reported in the literature. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1343–1349     

Roughness correlation and interdiffusion in thin films of polymer chains

The surface morphology of thin bilayer polymer films on top of glass substrates was investigated. The bilayer consists of a blend film of protonated and deuterated polystyrene and an underlying deuterated polystyrene film. Choosing the thickness of the top film larger than 8 times and smaller than 2 times the radius of gyration of the chains enables the determination of film thickness and confinement effects. With diffuse neutron scattering at grazing incidence in the region of total external reflection, a depth sensitivity and a contrast even at the internal polymer–polymer interface was achieved. The underlying film is conformal to the substrate, and depending on the thickness of the top film two different types of roughness correlations are observed. Thin confined films nestle to the underlying polymer films, while the stiffness of thicker bulky films provides an independent morphology. In both cases, annealing above the glass-transition temperature yields an interdiffusion at the internal polymer–polymer interface, and the polymer–air surface remains essentially unchanged with respect to roughness correlations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2862–2874, 1999     

Thickness dependence of structural relaxation in spin‐cast polynorbornene films with high glass transition temperatures (>613 K)

The isothermal structural relaxation (densification) of a family of glassy polynorbornene films with high glass transition temperatures (T_g > 613 K) is assessed via spectroscopic ellipsometry. Three polymers were examined: poly(butylnorbornene) (BuNB), poly(hydroxyhexafluoroisopropyl norbornene) (HFANB), and their random copolymer, BuNB‐r‐HFANB. The effective aging rate, β(T), of thick (∼1.2 μm) spun cast films of BuNB‐r‐HFANB is approximately 10⁻³ over a wide temperature window (0.49 < T/T_g < 0.68). At higher temperatures, these polymers undergo reactions that more dramatically decrease the film thickness, which prohibits erasing the process history by annealing above T_g. The aging rate for thick BuNB‐r‐HFANB films is independent of the casting solvent, which infers that rapid aging is not associated with residual solvent. β (at 373 K) decreases for films thinner than ∼500 nm. However, the isothermal structural relaxation of thin films of BuNB‐r‐HFANB exhibits nonmonotonic temporal evolution in thickness for films thinner than 115 nm film. The thickness after 18 h of aging at 373 K can be greater than the initial thickness. The rapid aging of these polynorbornene films is attributed to the unusual rapid local dynamics of this class of polymers and demonstrates the potential for unexpected structural relaxations in membranes and thin films of high‐T_g polymers that could impact their performance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 53–61     

Crystal growth rate in ultrathin films of poly(ethylene oxide)

Many dynamical properties of polymers, including segmental relaxation and chain diffusion, exhibit anomalies in thin‐film samples. We extend the studies of thin‐film dynamics to the case of semicrystalline polymers and present a study of the crystal growth rate for thin films of poly(ethylene oxide). We used optical microscopy and quartz crystal microbalance techniques to characterize the kinetics of crystallization for films with thicknesses from 40 to 1000 nm for a range of temperatures near the melting point. A remarkable slowing down of the crystal growth is observed at all temperatures studied for films with a thickness of less than ～100 nm. The results can be used to suggest reductions of the mobility of chains at the crystal/amorphous interface. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2615–2621, 2001     

Surface and bulk ordering in thin films of a symmetrical diblock copolymer

Amphiphilic diblock copolymers have the ability to adapt their surface's molecular composition to the hydrophilicity of their environment. In the case of about equal volume fractions of the two polymer blocks, the bulk of these polymers is known to develop a laminar ordering. We report here our investigation of the relationship between bulk ordering and surface morphology/chemical composition in thin films of such an amphiphilic diblock copolymer. Upon annealing in vacuum, the expected lamella ordering in the bulk of the film is observed and we find the morphology of the film surface to be defined by the thickness of the as‐deposited film: If the as‐deposited thickness matches the height of a lamella stack, then the film exhibits a smooth surface. Otherwise, an incomplete lamella forms at the film surface. We show that the coverage of this incomplete layer can be quantified by X‐ray reflectivity. To establish the lamella ordering in the bulk, the film needs to be annealed above the glass temperature of the two blocks. Molecular segregation at the film surface, however, is already occurring at temperatures well below the glass temperature of the two blocks. This indicates that below the glass temperature of the blocks the bulk of the thin film is “frozen,” whereas the polymer chains composing the surface lamella have an increased mobility. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013 , 51, 1282–1287     

Molecular probe techniques for studying diffusion and relaxation in thin and ultrathin polymer films

Two optically based, molecular probe techniques are employed to study relaxation and small-molecule translational diffusion in thin and ultrathin (thicknesses < ∼200 nm) polymer films. Second harmonic generation (SHG) is used to study the reorientational dynamics of a nonlinear optical chromophore, Disperse Red 1 (DR1) (previously shown to be an effective probe of α-relaxation dynamics) either covalently attached or freely doped in polymer films. Our studies on films ranging in thickness from 7 nm to 1 μm show little change in T_g with film thickness; however, a substantial broadening of the relaxation distribution is observed as film thickness decreases below approximately 150 nm. Experimental guidelines are given for using fluorescence nonradiative energy transfer (NRET) to study translational diffusion in ultrathin polymer films. Appropriate choice of a fluorescence donor species is important along with ensuring that diffusion is slow enough to be measured appropriately. Initial results on the diffusion of a small-molecule probe, lophine, in poly(isobutyl methacrylate) indicates that there is little change in probe diffusion coefficients in films as thin as 90 nm as compared to bulk films. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2795–2802, 1997     

WHAT ARE THE DIFFERENCES OF POLYMER SURFACE RELAXATION FROM THE BULK?

The polymer surface relaxation in thin films has been a long debating issue.We report a new method on studying surface relaxation behaviors of polymer thin films on a solid substrate.This method involved utilizing a rubbed polyimide surface with a pretilting angle in a liquid crystalline cell.Due to the surface alignment,the liquid crystals were aligned along the rubbing direction.During heating the liquid crystalline cell,we continuously monitored the change of orientation of the liquid crystals.It is u...     

Crystalline morphology evolution in PCL thin films

The transition of crystalline morphology is revealed in poly(?‐caprolactone) (PCL) thin films as the polymer film thickness changes from hundreds of nanometers to several nanometers. The PCL can crystallize into spherulites, dense‐branching morphology (DBM), or dendrites, depending on the polymer film thickness. It was found that when the polymer film thickness approaches 2Rg (radius of gyration of polymer), there is a remarkable change in crystalline morphology. Under this condition, the polymer crystallization is a diffusion‐controlled process. When the value of polymer film thickness closes to Rg, PCL cannot crystallize, and a dewetting phenomenon will take place. Moreover, polymer morphology can be controlled by varying supercooling. The effect of molecular weight on polymer morphology has been investigated. The main factors that affected pattern formation in nonequilibrium crystallization are also discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1303–1309, 2005     

Model experiments for a molecular understanding of polymer crystallization

Time‐resolved real‐space observations of morphology and pattern formation resulting from crystallization of ultrathin films of low‐molecular‐weight poly(ethylene oxide) (PEO) or diblock copolymers containing PEO shed light on the mechanisms of how polymer crystals are formed. We used simple but restricted geometries like thin films of controlled thickness or confinement resulting from block copolymer mesotructures. Under such conditions, we were able to relate the observed morphology and its temporal evolution directly to molecular processes and the kinetics of crystal growth. We demonstrate that changes in the morphology with time are due to different thermal histories and are the consequence of the mestable nature of polymer crystals. Information about the nucleation process was obtained by examining crystal formation in 12‐nm small spherical cells of a block copolymer mesostructure. We discuss the advantages of thin‐film studies for a better understanding of polymer crystallization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1869–1877, 2003     

Effect of sample preparation on the glass‐transition of thin polystyrene films

We have investigated the effect of sample preparation on the glass‐transition temperature (T_g) of thin films of polystyrene (PS). By preparing and measuring the glass‐transition temperature T_g of multilayered polymer films, we are able to assess the contribution of the spincoating process to the reduced T_g values often reported for thin PS films. We find that it is possible to determine a T_g even on the first heating cycle, and that by the third heating cycle (a total annealing time of 15 min at T = 393 K) the T_g value has reached a steady state. By comparing multilayered versus single layered films we find that the whole T_g depends only on the total film thickness, and not on the thickness of the individual layers. These results strongly suggest that the spincasting process does not contribute significantly to T_g reductions in thin polymer films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4503–4507, 2004