Diffusion of polymers at interfaces: a secondary ion mass spectroscopy study

Electronic Interaction of Epoxy Resin with Copper at the Adhered Interface

A better understanding of the aggregation states of adhesive molecules in the interfacial region with an adherend is crucial for controlling the adhesion strength and is of great inherent academic interest. The adhesion mechanism has been described through four theories: adsorption, mechanical, diffusion, and electronic. While interfacial characterization techniques have been developed to validate the aforementioned theories, that related to the electronic theory has not yet been thoroughly studied. We here directly detected the electronic interaction between a commonly used thermosetting adhesive, cured epoxy of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenylmethane (DDM), and copper (Cu). This study used a combination of density functional theory (DFT) calculations and femtosecond transient absorption spectroscopic (TAS) measurements as this epoxy adhesive-Cu pairing is extensively used in electronic device packaging. The DFT calculations predicted that π electrons in a DDM molecule adsorbed onto the Cu surface flowed out onto the Cu surface, resulting in a positive charge on the DDM. TAS measurements for the Cu/epoxy multilayer film, a model sample containing many metal/adhesive interfaces, revealed that the electronic states of excited DDM moieties at the Cu interface were different from those in the bulk region. These results were in good accordance with the prediction by DFT calculations. Thus, it can be concluded that TAS is applicable to characterize the electronic interaction of adhesives with metal adherends in a nondestructive manner.

The glass transition and enthalpy recovery of polystyrene nanorods using Flash differential scanning calorimetry

The glass transition (Tg) behavior and enthalpy recovery of polystyrene nanorods within an anodic aluminum oxide (AAO) template (supported nanorods) and after removal from AAO (unsupported nanorods) is studied using Flash differential scanning calorimetry. Tg is found to be depressed relative to the bulk by 20 ± 2 K for 20 nm-diameter unsupported polystyrene (PS) nanorods at the slowest cooling rate and by 9 ± 1 K for 55 nm-diameter rods. On the other hand, bulk-like behavior is observed in the case of unsupported 350 nm-diameter nanorods and for all supported rods in AAO. The size-dependent Tg behavior of the PS unsupported nanorods compares well with results for ultrathin films when scaled using the volume/surface ratio. Enthalpy recovery was also studied for the 20 and 350 nm unsupported nanorods with evolution toward equilibrium found to be linear with logarithmic time. The rate of enthalpy recovery for the 350 nm rods was similar to that for the bulk, whereas the rate of recovery was enhanced for the 20 nm rods for down-jump sizes larger than 17 K. A relaxation map summarizes the behavior of the nanorods relative to the bulk and relative to that for the 20 nm-thick ultrathin film. Interestingly, the fragility of the 20 nm-diameter nanorod and the 20 nm ultrathin film are identical within the error of measurements, and when plotted vs departure from Tg (i.e., T - Tg), the relaxation maps of the two samples are identical in spite of the fact that the Tg is depressed 8 K more in the nanorod sample.

Segmental dynamics of polystyrene near polymer-polymer interfaces

This study investigated the segmental dynamics of polymers near polymer-polymer interfaces by probing the rotation of polymer-tethered fluorescent molecules using imaging rotational fluorescence correlation microscopy. Multilayered films were utilized to provide spatial selectivity relative to different polymer-polymer interfaces. In the experimental setup, for the overlayer polymer, polystyrene (PS) was employed and a 15 nm-thick probe-containing layer was placed ≈25 nm apart from different underlayer polymers with glass transition temperatures (Tg) either lower or higher than that of PS. The underlayer of poly-n-butyl methacrylate had 72 K lower Tg than that of PS, whereas polymethyl methacrylate and polysulfone had 22 and 81 K higher Tg, respectively, than that of PS. Two key dynamic features of the glass transition, the non-Arrhenius temperature dependence and stretched relaxation, were examined to study the influence of soft and hard confinements on the segmental dynamics of the overlayer polymer near the polymer-polymer interfaces. Although complications exist in the probing location owing to the diffusion of the polymer-tethered probe during the annealing protocol to consolidate the multilayers, the results suggest that either the segmental dynamics of the polymer near the polymer-polymer interface do not change owing to the soft and hard confinements or the interfacial perturbation is very short ranged.

Chain Movements at the Topmost Surface of Poly(methyl methacrylate) and Polystyrene Films Directly Evaluated by In Situ High-Temperature Atomic Force Microscopy

The surfaces of polymeric materials are thermodynamically unstable, and the glass-transition temperature (Tg) is significantly lower than that in the bulk material. However, the mobility of the chains at the top of the surface has never been directly evaluated. In this study, the movements of the topmost chains of poly(methyl methacrylate) (PMMA) and polystyrene (PS) bulk films were observed in situ at high temperatures with atomic force microscopy in tapping mode. PMMA and PS chains started moving at ∼97 and ∼50 °C, respectively, which were slightly and significantly below the values of their bulk Tg (PMMA, 108 °C; PS, 104 °C), respectively. The activation energies of the apparent diffusion constants of PMMA and PS, derived by particle image velocimetry analysis, were 193 and 151 kJ mol-1, respectively, and reasonable for the glass transition. Movements of isolated PMMA chains deposited on a PMMA film by the Langmuir-Blodgett technique were also observed and confirmed to be essentially the same as those on the PMMA film surface.

Strength evolution laws in curing of solvent-welded polymers

This paper reports the scaling laws to describe the time-evolution behavior of solvent-mediated strength at the interface between two identical thermoplastic polymers below the glass-transition temperature. Our results suggest that the evolution scales as sqrt[t], where t is the curing time. It depends on the time evolution of interfacial stiffness and toughness, each of which scales as sqrt[t]. Employing a combination of experiments and continuum scale simulations, we show that the evolution of strength, stiffness, and toughness is controlled by pure diffusion. It can therefore be treated as a Gaussian process. While the "saturation of strength," which describes the transition of strength evolution into a steady state, does not strictly follow any power-law type behavior, a simple exponential law accurately characterizes both evolution and saturation of strength. This suggests that the longer timescale nonlinear processes (that are overdetermined by the power-law type scaling laws) diminish rapidly in approaching a steady state. Furthermore, the kinetics of the evolution processes is well captured by the dissolution of polymer particles. While dissolution involves a different timescale, it strongly correlates with the solvent-welding process upon normalization. The correlation highlights the equivalence of the dissolution and solvent-joining processes and offers an easier route to determining strength at arbitrary curing times. Additionally, the dissolution rate of polymer particles is shape dependent and governed by the surface-to-volume ratio.

Direct observation of mobility of thin polymer layers via asymmetric interdiffusion using neutron reflectivity measurements

In this study, we investigated the diffusion dynamics at the interface between deuterated poly(methyl methacrylate) (d-PMMA) and protonated poly(methyl methacrylate) (h-PMMA) in two-layered thin films of d- and h-PMMA layers via neutron reflectivity (NR) measurements during isothermal annealing above the glass transition temperature T_g. When T_g of d-PMMA was higher than that of h-PMMA, the d-PMMA layer thickness increased with increasing annealing time t_a and, simultaneously, the h-PMMA layer thickness decreased. However, the opposite t_a dependence of the layer thicknesses was observed, if the T_g of d-PMMA was decreased by the increase in the fraction of the low-molecular weight d-PMMA: With increasing t_a, the d-PMMA layer thickness decreased and the h-PMMA layer thickness increased when T_g of d-PMMA was lower than that of h-PMMA. This change in the t_a dependence of the layer thickness was related to the change in the mobility of the d-PMMA layer accompanied by the change in the T_g value of d-PMMA. With the decrease in the d-PMMA layer thickness from 49 nm to 13 nm, when the h-PMMA layer thickness was maintained, the t_a dependence of the layer thickness changed and the mobility of the d-PMMA layer dramatically increased. These results suggest that the mobility of thin polymer films can be determined by the observation of interfacial dynamics via NR measurements.

Fracture Mechanism Change at a Heterogeneous Polymer-Polymer Interface Reinforced with in Situ Graft Copolymers

Dynamic secondary-ion mass spectroscopy (DSIMS) was used to investigate the change in the failure mechanism at a heterogeneous polymer-polymer interface (polystyrene (PS)/polyamide (nylon 6, Ny6)) reinforced with in situ graft copolymers produced by the reaction between Ny6 molecules and poly(styrene- co-maleic anhydride) at the interface. The variation in fracture toughness with bonding time and temperature has been explained by two different failure mechanisms: adhesive failure at the interface for short bonding times and when the bonding temperature is low and cohesive failure between chains at the interface and bulk PS for longer bonding times and when the bonding temperature is high. DSIMS results provide the direct experimental evidence that the nonreactive molecules (PS) diffuse away from the high-potential interface, which induces the cohesive failure in the bulk of the nonreactive molecules (PS) after long annealing times. The change in the adhesion strength with temperature could also cause a change in the failure mechanism. Common features of the fracture mechanisms at heterogeneous interfaces reinforced by the in situ graft copolymers are outlined, which are independent of the polymer crystallinity.

Interfacial interaction and glassy dynamics in stacked thin films of poly(methyl methacrylate)

Neutron reflectivity and dielectric permittivity of alternately stacked thin films of protonated and deuterated poly(methyl methacrylate) were measured to elucidate a correlation between the time evolution of the interfacial structure and the segmental dynamics in the stacked thin polymer films during isothermal annealing above the glass transition temperature. The roughness at the interface between two thin layers increases with the annealing time, whereas the relaxation rate and strength of the α-process decrease with an increase in the annealing time. A strong correlation between the time evolution of the interfacial structure and the dynamics of the α-process during annealing could be observed using neutron reflectivity and dielectric relaxation measurements.

Reaching the ideal glass transition by aging polymer films

By aging, we draw glassy polymer films to a thermodynamic state, the ideal glass, with the entropy of the crystal.

Subdiffraction-Resolution Optical Measurements of Molecular Transport in Thin Polymer Films

The measurement of molecular transport within polymer films yields information about the internal structural organization of the films and is useful in applications such as the design of polymeric capsules for drug delivery. Layer-by-layer assembly of polyelectrolyte multilayer films has been widely used in such applications where the multilayer structure often exhibits anisotropic transport resulting in different diffusivities in the lateral (parallel to the film) and transverse (normal to the film) directions. Although lateral transport can be probed using techniques such as fluorescence recovery after photobleaching (FRAP), it cannot be applied to probing transverse diffusivity in polymer films smaller than the diffraction limit of light. Here we present a technique to probe the transport of molecules tagged with fluorphores in polymer films thinner than the optical diffraction limit using the modulation of fluorescence emission depending on the distance of the tagged molecules from a metal surface. We have used this technique to probe the diffusion of proteins biotin and bovine serum albumin (BSA) in polyelectrolyte multilayer films. We also studied the interdiffusion of chains in multilayer films using this technique. We observed a 3 order of magnitude increase in interdiffusion as a function of the ionic strength of the medium. This technique, along with FRAP, will be useful in studying anisotropic transport in polymer films, even those thinner than the diffraction limit, because the signal in this technique arises only from transverse and not lateral transport. Finally, this technique is also applicable to studying the diffusion of chromophore-labeled species within a polymer film. We demonstrate this aspect by measuring the transverse diffusion of methylene blue in the PAH-PAA multilayer system.

Dielectric relaxation of thin films of polyamide random copolymers

We investigate the relaxation behavior of thin films of a polyamide random copolymer, PA66/6I, with various film thicknesses using dielectric relaxation spectroscopy. Two dielectric signals are observed at high temperatures, the α process and the relaxation process due to electrode polarization (the EP process). The relaxation time of the EP process has a Vogel-Fulcher-Tammann type of temperature dependence, and the glass transition temperature, T(g), evaluated from the EP process agrees very well with the T(g) determined from the thermal measurements. The fragility index derived from the EP process increases with decreasing film thickness. The relaxation time and the dielectric relaxation strength of the EP process are described by a linear function of the film thickness d for large values of d, which can be regarded as experimental evidence for the validity of attributing the observed signal to the EP process. Furthermore, there is distinct deviation from this linear law for thicknesses smaller than a critical value. This deviation observed in thinner films is associated with an increase in the mobility and/or diffusion constant of the charge carriers responsible for the EP process. The α process is located in a higher-frequency region than the EP process at high temperatures but merges with the EP process at lower temperatures near the glass transition region. The thickness dependence of the relaxation time of the α process is different from that of the EP process. This suggests that there is decoupling between the segmental motion of the polymers and the translational motion of the charge carriers in confinement.

Atomic Force Microscopy Nanomechanics Visualizes Molecular Diffusion and Microstructure at an Interface

Here we demonstrate a simple, yet powerful method, atomic force microscopy (AFM) nanomechanical mapping, to directly visualize the interdiffusion and microstructure at the interface between two polymers. Nanomechanical measurements on the interface between poly(vinyl chloride) (PVC) and poly(caprolactone) (PCL) allow quantification of diffusion kinetics, observation of microstructure, and evaluation of mechanical properties of the interdiffusion regions. These results suggest that nanomechanical mapping of interdiffusion enables the quantification of diffusion with high resolution over large distances without the need of labeling and the assessment of mechanical property changes resulting from the interdiffusion.

Glass transition dynamics of stacked thin polymer films

The glass transition dynamics of stacked thin films of polystyrene and poly(2-chlorostyrene) were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The glass transition temperature T(g) of as-stacked thin polystyrene films has a strong depression from that of the bulk samples. However, after annealing at high temperatures above T(g), the stacked thin films exhibit glass transition at a temperature almost equal to the T(g) of the bulk system. The α-process dynamics of stacked thin films of poly(2-chlorostyrene) show a time evolution from single-thin-film-like dynamics to bulk-like dynamics during the isothermal annealing process. The relaxation rate of the α process becomes smaller with increase in the annealing time. The time scale for the evolution of the α dynamics during the annealing process is very long compared with that for the reptation dynamics. At the same time, the temperature dependence of the relaxation time for the α process changes from Arrhenius-like to Vogel-Fulcher-Tammann dependence with increase of the annealing time. The fragility index increases and the distribution of the α-relaxation times becomes smaller with increase in the annealing time for isothermal annealing. The observed change in the α process is discussed with respect to the interfacial interaction between the thin layers of stacked thin polymer films.

Cluster secondary ion mass spectrometry of polymers and related materials

Cluster secondary ion mass spectrometry (cluster SIMS) has played a critical role in the characterization of polymeric materials over the last decade, allowing for the ability to obtain spatially resolved surface and in-depth molecular information from many polymer systems. With the advent of new molecular sources such as C(60)(+), Au(3)(+), SF(5)(+), and Bi(3)(+), there are considerable increases in secondary ion signal as compared to more conventional atomic beams (Ar(+), Cs(+), or Ga(+)). In addition, compositional depth profiling in organic and polymeric systems is now feasible, without the rapid signal decay that is typically observed under atomic bombardment. The premise behind the success of cluster SIMS is that compared to atomic beams, polyatomic beams tend to cause surface-localized damage with rapid sputter removal rates, resulting in a system at equilibrium, where the damage created is rapidly removed before it can accumulate. Though this may be partly true, there are actually much more complex chemistries occurring under polyatomic bombardment of organic and polymeric materials, which need to be considered and discussed to better understand and define the important parameters for successful depth profiling. The following presents a review of the current literature on polymer analysis using cluster beams. This review will focus on the surface and in-depth characterization of polymer samples with cluster sources, but will also discuss the characterization of other relevant organic materials, and basic polymer radiation chemistry.

Self-healing materials: a review

The ability of materials to self-heal from mechanical and thermally induced damage is explored in this paper and has significance in the field of fracture and fatigue. The history and evolution of several self-repair systems is examined including nano-beam healing elements, passive self-healing, autonomic self-healing and ballistic self-repair. Self-healing mechanisms utilized in the design of these unusual materials draw much information from the related field of polymer-polymer interfaces and crack healing. The relationship of material damage to material healing is examined in a manner to provide an understanding of the kinetics and damage reversal processes necessary to impart self-healing characteristics. In self-healing systems, there are transitions from hard-to-soft matter in ballistic impact and solvent bonding and conversely, soft-to-hard matter transitions in high rate yielding materials and shear-thickening fluids. These transitions are examined in terms of a new theory of the glass transition and yielding, viz., the twinkling fractal theory of the hard-to-soft matter transition. Success in the design of self-healing materials has important consequences for material safety, product performance and enhanced fatigue lifetime.