Streamlined ellipsometry procedure for characterizing physical aging rates of thin polymer films

Existing studies in the research literature showing conflicting changes in physical aging rates with decreasing film thickness in nanoconfined polymer films highlight the need for a single experimental technique to efficiently characterize physical aging rates in thin polymer films of varying chemical structure. To that end, we have developed a streamlined ellipsometry procedure to measure the structural relaxation of thin glassy polymer films. We evaluate different methods of calculating a physical aging rate β from the measured thickness h(t) and index of refraction n(t) data. We present extensive measurements of β as a function of aging temperature and aging time for polystyrene (PS) films supported on silicon, and determine that the physical aging rate β can be easily and reliably determined from β = −1/h₀ dh/d(log t), where h₀ is the initial measure of the film thickness at an aging time of 10 min. We have also carried out oxygen permeation studies on poly(methyl methacrylate) (PMMA) films from 800 μm down to 190 nm in thickness, and find no change in the permeability with film thickness or physical aging at room temperature for up to 65 days, which suggests that gas permeation may be insensitive to physical aging in such low free volume polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2509–2519, 2009     

Relaxation of polymer thin films in isothermal temperature‐jump measurements

The dynamic behavior of thin polymer films is of interest in the fabrication of microelectronics and optoelectronics and in the coatings industry. It is known that polymer relaxation is affected by film thickness and the particular substrate/polymer pair. We previously used a spectroscopic ellipsometer to investigate the glass transition in thin films. In addition to information on the modification of thermal transitions such as the glass‐transition temperature, the speed of data acquisition in an automated, spectroscopic ellipsometer, operated at a single wavelength of 780 nm, allows for the direct observation of the isothermal dimensions of a thin polymer film as a function of time after a rapid temperature change. In this article, we discuss recent results from the observation of the time dependence of film‐normal thickness and normalized, in‐plane, lateral dimension as well as simple fits to this relaxation behavior in terms of a normalized viscosity and relaxation time. The results support a highly asymmetric initial thermal expansion normal to the film followed by close to isotropic relaxation and anisotropic “flow” (the flow in response to the vanishingly small shears of thermal expansion). These features may clarify issues involving the observation of chain confinement in thin polymer films in terms of potential differences between equilibrium and dynamic measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2929–2936, 2000     

Interpenetrating polymer networks of poly(dimethyl siloxane–urethane) and poly(methyl methacrylate)

Simultaneous IPNs of poly(dimethyl siloxane-urethane) (PDMSU)/poly(methyl methacrylate) (PMMA) and related isomers have been prepared by using new oligomers of bis(β-hydroxyethoxymethyl)poly(dimethyl siloxane)s (PDMS diols) and new crosslinkers biuret triisocyanate (BTI) and tris(β-hydroxylethoxymethyl dimethylsiloxy) phenylsilane (Si-triol). Their phase morphology have been characterized by DSC and SEM. The SEM phase domain size is decreased by increasing crosslink density of the PDMSU network. A single phase IPN of PDMSU/PMMA can be made at an M_c = 1000 and 80 wt % of PDMSU. All of the pseudo- or semi-IPNs and blends of PDMSU and PMMA were phase separated with phase domain sizes ranging from 0.2 to several micrometers. The full IPNs of PDMSU/PMMA have better thermal resistance compared to the blends of linear PDMSU and linear PMMA. © 1993 John Wiley & Sons, Inc.     

Rational design of polyhedral oligomeric silsesquioxane fillers for simultaneous improvements of thermomechanical properties and lowering refractive indices of polymer films

We describe here the design and synthesis of the polyhedral oligomeric silsesquioxane (POSS)‐based dual‐functional molecular fillers for simultaneously lowering refractive indices and improving thermomechanical properties of conventional polymers. We prepared the composite films with poly(methyl methacrylate) and polystyrene containing the series of the POSS derivatives with the single functional unit for interacting with polymer chains and heptacyclopentyl substituents for creating exclusive volumes around the POSS core. From the measurements of refractive indices of polymer composites, it was revealed that all POSS fillers can lower the refractive index of the films. In addition, thermal stabilities and mechanical properties were enhanced by adding POSS fillers. The filler effect on the thermal properties can be explained by the structural features of POSS: The highly symmetrical structure of the silica cube should suppress thermal motions, resulting in the large enhancement of thermomechanical properties of polymer matrices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3583–3589     

Three‐dimensional thermal annealing: An unconventional method to fabricate monodisperse polymer nanoparticles from polymer films

We develop a simple and feasible method to fabricate polymer nanoparticles by annealing polymer films in a uniform environment. Different from the conventional methods, no extra additive or emulsifier is needed in the preparation processes. Poly(methyl methacrylate) (PMMA) films are used as a model system and annealed at elevated temperatures in ethylene glycol, which provides a uniform three‐dimensional annealing environment and acts as stabilizers once the nanoparticles are formed. After the annealing process, PMMA nanoparticles with monodisperse diameters are formed. By examining the remaining films after the annealing process, the formation mechanism, which involves surface undulation and detachment of polymer nanoparticles, is proposed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2471–2475     

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     

Interface characterization in latex blend films by fluorescence energy transfer

Immiscible polymer blend films were formed by air drying aqueous dispersions containing mixtures of a high-T_g latex, poly(methyl methacrylate), and a film-forming low-T_g latex, poly(butyl methacrylate-co-butyl acrylate). Fluorescence energy transfer experiments were used to characterize the interfaces in these films, in which one component was labeled with a donor dye and the other with an acceptor. The quantum efficiency of energy transfer (Φ_ET) between the donors and acceptors is influenced by the interfacial contact area between the two polymer phases. As the amount of soft component in the blend is increased, Φ_ET approaches an asymptotic value, consistent with complete coverage of the hard polymer surface with soft polymer. This limiting extent of energy transfer is very sensitive to the total surface area in the film, with correspondingly more energy transfer at constant volume fraction for small hard particles. Some of the details of the energy transfer are revealed through a fluorescence lifetime distribution analysis. The presence of ionic surfactant (sodium dodecyl sulfate) in the dispersion from which the latex blend film is prepared reduces the cross-boundary energy transfer by 30%, which implies that in these films the surfactant decreases the interfacial contact. After annealing the surfactant-free blends above 100°C, we observe an increase in energy transfer, consistent with a broader interface between the two polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1115–1128, 1998     

Effect of temperature and solvent on polymer tacticity in the free‐radical polymerization of styrene and methyl methacrylate

The effect of temperature and solvent on polymer tacticity in free‐radical polymerization of styrene and methyl methacrylate was studied by ¹³C and ¹H NMR, respectively. Polystyrene shows a mild syndiotactic tendency (P_m = 0.36 ± 0.02) that is independent of temperature over a wide range (?10 to 120 °C), while poly(methyl methacrylate) shows a stronger syndiotactic tendency (P_m = 0.17 ± 0.01 at 30 °C) that decreases as temperature is increased (P_m = 0.22 ± 0.02 at 80 °C). None of the polymerization solvents studied (bulk, THF, DMF, DMSO, acetonitrile, and acetone) had a significant effect on polymer tacticity in either system. The triad fractions of both polymers showed deviations from the Bernoulli model, implying that the antepenultimate unit affects the propagation reaction. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3351–3358     

Polymer diffusion in latex films at ambient temperature

Polymer diffusion across interfaces at room temperature (21°C) was analyzed by direct nonradiative energy transfer (DET) in labeled latex films. Two modellatex polymers were examined: poly(butyl methacrylate) [PBMA, M_w = 3.5 × 10⁴, T_g (dry) = 21°C] and a copolymer of 2-ethylhexyl methacrylate with 10 wt % (acetoacetoxy)-ethyl methacrylate [P(EHMA-co-AAEM), M_w = 4.8 × 10⁴, T_g (dry) = −7°C]. Little energy transfer due to polymer diffusion was detected for the P(EHMA-co-AAEM) latex samples in the dispersed state or dried to solids content below ca. 90%, but above 90% solids, diffusion occurs among particles. For PBMA, diffusion occurs only after the film is dried (>97% solids) and aged. In the dry PBMA films, it requires 4–5 days at 21°C to reach a significant extent of mixing (f_m = 0.3–0.4). This corresponds to an estimated penetration depth d_app of 30–40 nm and a mean apparent diffusion coefficient (D_app) of 5 × 10⁻⁴ nm²/s. The corresponding D_app value for the dry P(EHMA-co-AAEM) sample is 5 × 10⁻² nm²/s, and it takes about 25–40 min for this polymer to reach f_m of 0.3–0.4 with d_app of 20–30 nm. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1129–1139, 1998     

Synthesis of carbosilane block copolymers by means of a living anionic polymerization of 1,1-diethylsilacyclobutane

Block polymerization of 1,1-diethylsilacyclobutane with styrene derivatives and methacrylate derivatives was investigated. Sequential addition of styrene to a living poly(1,1-diethylsilabutane), which was prepared from phenyllithium and 1,1-diethylsilacyclobutane in THF–hexane at −48°C, gave poly(1,1-diethylsilabutane)-b-polystyrene. Similarly, addition of 4-(tert-butyldimethylsiloxy)styrene to the living poly(1,1-diethylsilabutane) provided poly(1,1-diethylsilabutane)-b-poly(4-(tert-butyldimethylsiloxy)styrene). Poly(1,1-diethylsilabutane)-b-poly(methyl methacrylate) was obtained by treatment of living poly(1,1-diethylsilabutane) with 1,1-diphenylethylene followed by an addition of methyl methacrylate. Poly(1,1-diethylsilabutane)-b-poly(2-(tert-butyldimethylsiloxy)ethyl methacrylate) was also synthesized by adding 2-(tert-butyldimethylsiloxy)ethyl methacrylate to the living poly(1,1-diethylsilabutane) which was end-capped with 1,1-diphenylethylene in the presence of lithium chloride. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2699–2706, 1998     

Synthesis of Polystyrene and Polystyrene/Poly(methyl methacrylate) Nanoparticles

Summary: Polystyrene nanosize particles have been synthesized by a differential microemulsion polymerization process involving the use of a small amount of poly(methyl methacrylate) as the seeds. Sodium dodecyl sulfate and ammonium persulfate were used as the surfactant and initiator, respectively. The effects of various reaction conditions on the particle size have been investigated. Particle sizes of less than 20 nm have been achieved at milder conditions than those previously reported in the literature.

An HRTEM image of PS/PMMA nanoparticles.     

Grafting by in situ coupling of epoxy groups of a living copolymer with an anionic living polymer

A tetrahydrofuran (THF) solution of the living random copolymer of methyl methacrylate (MMA) and glycidyl methacrylate (GMA) was prepared by the living anionic copolymerization of the two monomers, using 1,1‐diphenylhexyllithium (DPHLi) as initiator, in the presence of LiCl ([LiCl]/[DPHLi]₀ = 3), at −50°C. The copolymer thus obtained has a controlled composition and molecular weight and a narrow molecular weight distribution. By introduction of an anionic living polystyrene (poly(St)) or anionic living polyisoprene (poly(Is)) solution into the above system at −30°C, a coupling reaction took place and a graft copolymer with a polar backbone and nonpolar side chains was produced. The solvent used in the preparation of the living poly(St) or poly(Is) affects the coupling reaction. When benzene was the solvent, a graft copolymer of high purity, controlled graft number and molecular weight, and narrow molecular weight distribution (M_w/M_n = 1.11–1.21) was obtained. In the coupling reaction, the living poly(St) reacted only with the epoxy groups and not with the carbonyls of the backbone polymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 105–112, 1999     

Systematic study of the fluorescence decays of amino‐coumarin dyes in polymer matrices

We measured the fluorescence decays of seven different amino‐coumarin dyes in polymer films of poly(methyl methacrylate) (PMMA), poly(styrene) (PS), and ethylene‐butene rubber (EBR); as well as in the small molecule analogs ethyl acetate and toluene. Many of the dye‐solvent and dye‐polymer combinations exhibited single exponential decays with lifetimes ranging from 2.3 to 3.9 ns. Small deviations from single exponential behavior occurred for most of the dyes in EBR. Significant deviations from single exponential behavior occurred for 7‐(diethylamino)‐2‐oxo‐2H‐1‐benzopyran‐3‐carboxylic acid (coumarin‐3) in ethyl acetate and in all polymer matrices and 2,3,6,7‐tetrahydro‐11‐oxo‐1H,5H,11H‐[1]benzopyrano[6,7,8‐ij]quinolizin‐10‐carboxylic acid (coumarin‐343) in all of the polymer matrices. Time‐resolved fluorescence spectra indicated the presence of two different excited states for coumarin‐3 and coumarin‐343 in PMMA; these spectra were qualitatively different from the time‐resolved spectra of coumarin‐3 in ethyl acetate. We rationalize these results in terms of the chemical functionalities of the various dyes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2333–2343, 2007     

Polyepichlorohydrin polyurethane/poly(methyl methacrylate) interpenetrating polymer networks

Polyurethane (PU) based on polyepichlorohydrin/poly(methyl methacrylate) (PECH/PMMA) interpenetrating polymer networks (IPNs) was synthesized by a simultaneous method. The effects of composition, hydroxyl group number of PECH, NCO/OH ratio and crosslinking agent content in IPNs were investigated in detail. Some other glycols, such as poly(ethylene glycol), poly(propylene glycol) and hydroxyl-terminated polybutadiene, were also used to obtain PU/PMMA IPNs. The interpenetrating and fracture behaviors of the IPNs are explained briefly.     

Poly(ethylene oxide)/poly(methyl methacrylate) (semi-)interpenetrating polymer networks: Synthesis and phase diagrams

Interpenetrating polymer networks (IPNs) of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) were prepared by simultaneous network formation. The PEO network was produced by acid-catlayzed self-condensation of α,ω-bis(triethoxysilane)-terminated PEO in the presence of small amounts of water. The PMMA network was formed by free radical polymerization of MAA in the presence of divinylbenzene as crosslinker. The reaction conditions were adjusted to obtain similar crosslinking kinetics for both reactions. An attempt was made to construct a phase diagram of the IPNs by measuring the composition of the IPNs at the moment of the appearance of the phase separation, as indicated by the onset of turbidity. This composition could be determined because the siloxane crosslinks of the PEO network could be hydrolyzed in aqueous NaOH with the formation of linear, soluble PEO chains. The phase diagram was compared with phase diagrams of blends of linear polymers and of semi-IPNs (crosslinked PMMA and linear PEO), obtained under similar conditions, i.e. polymerization of MMA in the presence of varying amounts of PEO. It was observed that the form of the phase diagrams of the linear polymers is similar to that of the IPNs, but is quite different from that of the semi-IPNs. Thus, homogeneous transparent materials containing up to 60% of PEO could be prepared in the blends and the IPNs, but in the semi-IPNs, phase separation occurred with PEO contents as low as 10%.     

Synthesis and characterization of pyrene bearing amphiphilic miktoarm star polymer and its noncovalent interactions with multiwalled carbon nanotubes

A novel amphiphilic miktoarm star polymer, polystyrene‐poly(ethylene glycol)‐poly(methyl methacrylate), bearing a pyrene group at the end of PS arm (Pyrene‐PS‐PEG‐PMMA) was successfully synthesized via combination of atom transfer radical polymerization and click chemistry. The structure and composition of the amphiphilic miktoarm star polymer were characterized by gel permeation chromatography and ¹H NMR. The functionalization of multiwalled carbon nanotubes (MWCNTs) via “π–π” stacking interactions with pyrene‐PS‐PEG‐PMMA miktoarm star polymer was accomplished and the resulting polymer‐MWCNTs hybrid was analyzed by using ¹H NMR, UV–vis, fluorescence spectroscopy, and thermal gravimetric analysis. The high‐resolution transmission electron microscopy and analytical techniques aforementioned confirmed that the noncovalent functionalization of MWCNT's with the amphiphilic miktoarm star polymer was successfully achieved. The MWCNT/pyrene‐PS‐PEG‐PMMA exhibited significant dispersion stability in common organic solvents such as dimethyl formamide, chloroform, and tetrahydrofuran. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Preparation of ultrathin polypyrrole films using an adhesion promoter

Adhesive ultrathin polypyrrole films were deposited on Si/SiO₂ substrates modified with the new adhesion promoter 11-(Pyrrol-1-yl Undecyl) TrichloroSilane (PUTS). The oxidation potential of PUTS in solution was determined electrochemically by cyclic voltammetry. Self-assembled monolayers of PUTS were investigated by cyclic voltammetry, contact angle measurements, ellipsometry, and X-ray photoelectron spectroscopy. Several oxidants for the deposition of pyrrole on adhesion promoter modified substrates were tested and a strong dependence on the obtained film morphology was found. It was possible to deposit chemically ultrathin polypyrrole films on insulating substrates.     

Nanomechanical properties in ultrathin polymer films: Measurement on rectangular versus circular bubbles

Prior studies of inflation of circular membranes of ultrathin polystyrene (PS) films have evidenced a reduced glass transition temperature (T_g) and rubbery stiffening, whose origins remain unclear. Here, we describe results from inflation of rectangular, ultrathin films of the same PS material. The bubble shapes obtained from the experiment are consistent with finite element (FE) simulations. The accuracy of three approximate solutions for modulus obtained from the inflation of the thin, rectangular films was evaluated by comparison with FE analysis. The best among the three solutions was used to determine the creep compliance and rubbery stiffness of the thin films. It is found that the reduction of T_g and the rubbery stiffening for rectangular bubbles are consistent with results obtained using circular bubbles, although there is some indication that the rectangular bubbles give somewhat greater rubbery stiffening. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Influence of the substrate electronic structure on metallic quantum well state dispersions in ultrathin metal films

We have studied ultrathin Cu films grown on three related ferromagnetic (FM) layers to clarify the role of the substrate in determining the two-dimensional dispersion of metallic quantum well (MQW) states in the overlayer. The dispersions with parallel momentum of Cu MQW states above the Fermi level E_F were measured in the Cu/fccFe/Cu(1 0 0) and Cu/fccCo/Cu(1 0 0) systems using inverse photoemission, and below E_F in the Cu/fccNi/Cu(1 0 0) system using angle resolved photoemission spectroscopy. This study focussed on the direction of the two-dimensional Brillouin zone. For states away from the projected band gaps of the FM layer, the identity of the FM layer (i.e., Fe or Co) has little influence on Cu MQW states and their dispersions closely resemble those of a free standing Cu film. In contrast, all three systems exhibit nondispersing MQW states when the Cu bands overlap the minority spin projected band gaps of the FM metal. A phase accumulation approach gives a very simple explanation of this behavior, showing that the flat dispersion occurs because the phase shift upon reflection from the FM layer has a strong energy dependence in the projected gap.