Diffusion Of Polymer Chains Research Articles

On-line determination of Förster resonance energy transfer efficiency in drying latex films: Correlation of interdiffusion and particle deformation

An instrument is described, which measures the efficiency of Förster resonance energy transfer (FRET) in parallel to the sample's turbidity. The instrument was used to study the film formation from polymer latex dispersions. In this context, the FRET efficiency reflects the diffusion of polymer chains across the interparticle boundaries, while the loss of turbidity reflects the progress of particle deformation. Particle deformation causes tensile in-plane stress, while polymer interdiffusion creates cohesion and thereby helps to prevent cracking. The relative timing between the two therefore is of fundamental importance for successful film formation. The on-line determination of FRET efficiency while the film dries is complicated by the fact that the fluorescence lifetime of the donor, τ(D), depends on the water content in the vicinity of the donor. In the established procedure for data analysis, drifts in τ(D) induce corresponding artifical drifts in the values of the FRET efficiency. A novel algorithm for the analysis of fluorescence decay profiles is proposed, which makes use of the method of moments. The FRET efficiency is quantified by the upward curvature of the fluorescence decay curve in log-linear display. In the application example, interdiffusion is delayed relative to particle deformation by about 10 min. For successful film formation, this delay should be as small as possible.

Influence of Thermo-degradation on the Crystallization Kinetics of Isotactic Polypropylene with a β-Nucleating Agent

Isotactic polypropylene (i-PP) with a β-nucleating agent (β-NA) was degraded isothermally at 190°C for periods up to 60 min, causing a reduction in molecular weight and a buildup of carbonyl groups. The effect of the thermo-degradation on the kinetics of subsequent crystallization was investigated using a polarizing optical microscope (POM). Kinetic studies revealed that the rates of nucleation and spherulite growth were affected differently by the thermo-degradation. No difference was found in the spherulite growth rate between pristine polypropylene and somewhat degraded samples, while spherulite growth was slightly depressed in the highly degraded sample. However, the thermo-oxidation of the polypropylene caused a gradual reduction in nucleation probability. It is suggested the extra carbonyl groups are mainly concentrated in the oligomeric products of chain scission, which are then excluded during the solidification process. Consequently, the impurity-rich phase acts as a barrier and hinders the diffusion of polymer chains, led to an increase of free energy for formation of a critical nucleus. The decrease of nucleation probability leaded to lower overall crystallization rate at a fixed temperature of 130°C, but the Avrami exponent n (n = 2.5 ± 1) was independent of the thermo-degradation, which was consistent with a three-dimensional growth habit by heterogeneous nucleation.

Interfacial Free Energy Driven Nanophase Separation in Poly(3-hexylthiophene)/[6,6]-Phenyl-C61-butyric Acid Methyl Ester Thin Films

The nanostructure of thermally annealed thin films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends on hydrophobic and hydrophilic substrates was studied to unravel the relationship between the substrate properties and the phase structure of polymer blends in confined geometry. Indeed, the nature of the employed substrates was found to affect the extent of phase separation, the PCBM aggregation state and the texture of the whole system. In particular, annealing below the melting temperature of the polymer yielded the formation of PCBM nanometric crystallites on the hydrophobic substrates, while mostly amorphous microscopic aggregates were formed on the hydrophilic ones. Moreover, while an enhanced in-plane orientation of P3HT lamellae was promoted on hydrophobic substrates, a markedly tilted geometry was produced on the hydrophilic ones. The observed effects were interpreted in terms of a simple model connecting the interface free energy for the blend films to the different polymer chain mobility and diffusion velocity of PCBM molecules on the different substrates.

Nanoconfined Self-Diffusion of Poly(isobutyl methacrylate) in Films with a Thickness-Independent Glass Transition

Even though the physics of nanoconfined polymers have been extensively studied for years, diffusion of polymer chains along confining interfaces has not been widely studied, likely because there ar...

Single molecule diffusion and its heterogeneity during the bulk radical polymerization of styrene and methyl methacrylate

The diffusion of perylene diimide dye molecules of clearly distinct size was observed during the thermally initiated polymerization of styrene and methyl methacrylate (MMA) at room temperature. A combination of fluorescence correlation spectroscopy and widefield fluorescence microscopy allowed us to cover the entire conversion range from free diffusion in the monomer solution to immobilization in the produced polymer glass. The evolution of diffusion coefficients differs significantly in both systems. The diffusion coefficient of both dyes decreases clearly more rapidly during the polymerization of MMA, i.e. at the same conversion, diffusion coefficients in the polymerizing MMA solution are clearly lower compared to the polymerizing styrene solution. Extrapolation of the diffusion of the dyes to the diffusion of polymer chains can explain the more pronounced Trommsdorff effect during the polymerization of MMA. In contrast to other methods, single molecule fluorescence microscopy allows for a direct visualization of heterogeneities in single molecule motion. Our measurements point to the fact that the reason for the distinct behaviour of both polymerization systems lies in structural heterogeneities evolving in the PMMA/MMA system already at rather low conversion. In contrast, during the polymerization of styrene, only minor heterogeneities were observed.

Mechanism analysis of ultrasonic treatment on SU-8 swelling in UV-LIGA technology

In SU-8 UV-LIGA technology, the precision and controllability of electroformed metal structure are not satisfied because of SU-8 photoresist swelling during electroforming process. To reduce SU-8 swelling and further improve the dimensional precision of electroformed microstructure, the authors originally introduce ultrasonic treatment in the fabrication process of SU-8 mould. The experimental results indicate that the swelling property of cross-linked cured SU-8 varies with ultrasonic time. SU-8 swelling is diminished when it is treated less than 15 min by ultrasonic, but the swelling is increased if the ultrasonic time is extended to 20 min. To explore ultrasonic treatment mechanism of SU-8 swelling, the contact angle between SU-8 and electroforming solutions in different ultrasonic time was measured, and the result presents that SU-8 surface hydrophilicity is affected by ultrasonic treatment. Besides, the contact angle trend is consistent with the curve of SU-8 swelling removal ratio in different ultrasonic time. Subsequently, ultrasonic treatment mechanism of SU-8 swelling is proposed based on ultrasonic mechanical scission of polymer chain and moisture diffusion mechanism.

The lateral diffusion of polymer single chain at oil-water interfaces and hydrophobic interaction

The lateral diffusion of individual polymer chain of polyethylene oxide (PEO) at oil-water interfaces was studied using fluorescence correlation spectroscopy (FCS). The diffusion coefficient of PEO was systematically under variable salt concentration of NaCl in the aqueous phase and it was found that the diffusion coefficient of the adsorbed polymer chain was very much determined by the viscosity of the oil phase. This fact indicates an enhanced hydrophobic interaction of the PEO chain with the oil phase.

Diffusion relaxation of photoinduced gratings in polyvinyl acetate latex films

The features of the postexposure relaxation of holographic gratings recorded in inhomogeneous polyvinyl acetate latex films with photosensitive agents (photochromic molecules of fulgide dyes and phenanthrenequinone) have been considered. The diffusion coefficients and rms displacements of izomerized probe in polymer latex particles and aqueous environment are determined within the model of two diffusion states. The effective diffusion coefficient of the molecular probe, which is responsible for the relaxation of gratings, increases with an increase in their period in wet films, whereas in dry films, this parameter is independent of the grating period. In the films subjected to high-temperature treatment the effective diffusion coefficient decreases with an increase in the grating period. The successive stages of grating relaxation in latex films with phenanthrenequinone are related to the diffusion of free molecules, radicals, and polymer chains, as well as to the local displacement of macromolecular segments at distances of 5–25 nm.

Real-time studies during coating and post-deposition annealing in organic semiconductors

In this contribution we explore the use of real time spectroscopic probes to gain useful insights into the kinetics of semiconductor polymer chains during thin film formation and upon post-deposition annealing treatments. In-situ ellipsometry is employed to monitor the deposition of thin films of the workhorse material system for organic photovoltaics (soluble derivative fullerene blended with poly(3-hexylthiophene)), when processed from solution using an analogous dip coating deposition method. This allows for detailed time investigation of the dynamics of film formation. Moreover, we applied spectroscopic photometry to study the in-situ crystallization and diffusion of polymer chains during post-deposition solvent annealing.

Molecular Dynamics Simulation of the Adsorption and Diffusion of a Single Hydrophobic Polymer Chain on a Hydrophobic Surface

The adsorption and diffusion of polyethylene (PE) with different degrees of polymerization (N) on a silicon (111) surface were studied by molecular dynamics simulations. The relative dielectric constant was selected to be 1 and 78 to mimic a vacuum and a solution environment, respectively. The chains were all present as two-dimensional (2D) adsorption conformation on the surface but different conformations and dynamic properties were found in the two absolutely different environments. This shows that the solvent plays an obvious role in the chain adsorption and diffusion processes on a hydrophobic surface. The relationship between the adsorption energy and the degree of polymerization follows a linear function and the average adsorption energy per segment is -0.38 kJ . mol(-1). In addition, the diffusion coefficient (D) of these chains scales with the degree of polymerization as N(-3/2).

A spectroscopic approach for structural characterization of polypropylene/clay nanocomposite

This study focuses on the degree of dispersion and structural development of organomodified MMT clay (OMMT) during processing of polypropylene clay nanocomposites using both conventional and nonconventional characterization techniques. PP-g-MA and Cloisite 15A were melt blended with three different grades of PP separately in a micro-twin screw compounder at selected screw speed and temperature. The clay was modified with fluorescent dyes and the adsorbed dye content in the clay gallery was estimated by using UV-spectrophotometric method. The effects of residence time and molecular weight of the PP matrix on the clay dispersion were studied. The extent of dispersion and exfoliation of the clay in polymer matrix determined from the torque versus time data obtained from microcompounder. It was further supported by XRD, SEM, TEM, and DSC analysis. Offline dielectric and fluorescence spectrophotometric studies were also carried out. Changes in dielectric constant and dielectric loss with both frequency and temperature yielded quantitative information about the extent of clay exfoliation and intercalation in the polymer matrix. It was observed that with an increase in MFI (decrease in molecular weight) and mixing time, the extent of clay dispersion and exfoliation were also improved due to easy diffusion of polymer chains inside clay gallery. POLYM. COMPOS., 31:2007–2016, 2010. © 2010 Society of Plastics Engineers

Study of the photoinduced surface relief modulation in photopolymers caused byillumination with a Gaussian beam of light

The formation of surface relief profile in photopolymerizable systems when illuminated witha focused beam of light is simulated numerically using a two-way diffusion model that takesaccount both for monomer and short polymer chain diffusion. The concentration andspatial distribution dynamics of monomer and short and long polymer chains arecalculated. The surface profile is obtained from the calculated component concentrationsconsidering different densities of monomer and polymer. The influence of the illuminationtime, intensity and spot diameter on the surface profile dynamics is discussed. A goodagreement between the calculated and the experimentally measured profiles is observed,thus demonstrating the successful application of the two-way diffusion model to thissystem.

Polymer chain scission, oligomer production and diffusion: A two-scale model for degradation of bioresorbable polyesters

This paper presents a computer model for the biodegradation of polyesters such as poly(lactic acid), poly(glycolic acid) and their copolymers. The model can take polymer details such as molecular weight distribution, different end and random scission rates and copolymer ratio as input data. A multi-scale approach is developed: polymer chain scission and oligomer production which occur at the molecular scale are modelled using a kinetic Monte Carlo scheme, oligomer diffusion which occurs at the device scale is modelled using a diffusion equation, and the two are connected at the finite difference nodes of the diffusion equation. The two-scale model can be used to predict the temporal evolution and spatial distribution of molecular weight distribution in a device as well as the weight loss as a function of time. It is shown that the kinetic Monte Carlo scheme can accurately predict the effect of copolymer ratio on the degradation rate. Grizzi and co-workers observed in their experiments that a PLA film 0.3 mm thick degrades much more slowly than one that is 2 mm thick. The numerical study shows that the conceptional reaction diffusion model suggested by Grizzi et al. needs to be extended in order to explain the size effect fully.

Role of matrix crystallinity in carbon nanotube dispersion and electrical conductivity of iPP-based nanocomposites

A homopolymer iPP and a series of propylene-ethylene random copolymers with a content of ethylene from 7 to 21 mol % were used as matrices to prepare single-walled carbon nanotube (SWCNT) nanocomposites in a range of SWCNT concentration from 0.15 to 1 wt %. The solution blending and melt- compression molding procedures were kept identical for all nanocomposites. The poly(propylenes) have crystallinities ranging from 70 to 10%, and serve to test the role of SWCNTs acting as nucleants to preserve in the nanocomposites the uniform dispersion of SWCNTs after sonication. The major role of polymer crystallinity is to mediate toward a more open and more connected SWCNT network structure. Fast nucleation and growth of high crystalline matrices on multiple sites along the surface of the nanotubes prevents SWCNT clustering, and entraps the SWCNT network between the semicrystalline structure reducing the driving force of nanotubes to curl and twist. Depletion of crystallites in the less crystalline matrices (<35% crystallinity) leads to curled and poorly connected nanotubes. A consequence of the gradual loss of SWCNT connectivity is a decreased electrical conductivity; however, the change with crystallinity is not linear. Conductivity decreases sharply with decreasing crystallinity for SWCNT contents near the percolation region, while for contents approaching the plateau region the electrical conductivity is less sensitive to matrix crystallinity. The percolation threshold decreases rapidly for polymers with <∼30% crystallinity and slowly levels off at crystallinities >∼40%. At SWCNT concentrations of 0.15 wt %, SEM images of nanocomposites with the highest crystallinity matrix indicate debundled and interconnected nanotubes, whereas more disconnected and curled SWCNTs remain in the lowest crystallinity nanocomposites. Electrical conductivity in the former is relatively high, whereas the latter are insulators. Also discussed is the nucleating effect of nanotubes and restrictions of the filler to polymer chain diffusion in the crystallization of the polymers. SEM images and Raman spectra in the radial breathing modes region (100–400 cm−1) are complementary tools to extract the quality and details of the SWCNT dispersion in the nanocomposites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2084–2096, 2010

Comparison Study of Morphology and Crystallization Behavior of Polyethylene and Poly(ethylene oxide) on Single-Walled Carbon Nanotubes

In this work, we provided a comparison study of morphology and crystallization behavior of polyethylene (PE) and poly(ethylene oxide) (PEO) on single-walled carbon nanotubes (SWNTs) with assistance of supercritical CO(2). The resulting polymer/SWNT nanohybrids were characterized by transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectra, wide-angle X-ray diffraction, and differential scanning calorimetry. SWNT small bundles were decorated by PE lamellar crystals, forming nanohybrid "shish-kebab" (NHSK) structure, whereas SWNTs were only wrapped by a thin amorphous polymer coating in the case of PEO. The varying morphologies of the nanohybrids were found to depend on the molecular conformation and the interactions between polymer chains and SWNTs. Nonisothermal experiments showed that SWNTs provided heterogeneous nucleation sites for PE crystallization, while the NHSK structure hindered polymer chain diffusion and crystal growth. Also, SWNTs played antinucleation effect on PEO. In addition, the formation mechanism analysis indicated that PE chains preferred to form a homogeneous coating along the tube axis before proceeding to kebab crystal growth. The purpose of this work is to enlarge the area of theoretical understanding of introducing precisely hierarchical structures on carbon nanotubes, which are important for functional design in nanodevice applications.

An investigation into the role of mucus thickness on mucoadhesion in the gastrointestinal tract of pig

Mucoadhesion in the gastrointestinal tract is a complex phenomenon and both formulation and physiological features need to be well understood and considered. Mucus thickness has been inferred to play a role in this process; however no definitive influence has been established. This study aimed to investigate the influence of mucus thickness on the mucoadhesion process, using a large animal (pig) as a model to closely resemble the human physiological features. The mucus thickness of different regions of the gastrointestinal tract of pig was fully measured by means of a histochemical method (hematoxilin/eosin) employing cryostat sections. Mucoadhesion was evaluated ex vivo on porcine mucosa by tensiometry using a polyacrylic acid polymer (Carbopol 974P NF) as a mucoadhesive model material, both in a dry and swollen state. Mucus was thickest in the stomach (body 67.9 ± 54.7 μm) and mucus thickness increased from proximal to distal segments in both the small intestine (duodenum 25.9 ± 11.8 μm, ileum 31.0 ± 15.7 μm) and large intestine (caecum 19.4 ± 8.7 μm, ascending colon 31.9 ± 17.2 μm, descending colon 35.1 ± 16.0 μm and rectum 40.8 ± 12.5 μm). Swollen polymer exhibited lower mucoadhesion than the dry form in all sections analysed. Mucus thickness plays a role on the mucoadhesion, as thicker mucus provides deeper polymer chain diffusion and entanglements; however, other factors are also involved in this complex process.

Anomalous chain diffusion in polymer nanocomposites for varying polymer-filler interaction strengths

Anomalous diffusion of polymer chains in a polymer nanocomposite melt is investigated for different polymer-nanoparticle interaction strengths using stochastic molecular dynamics simulations. For spherical nanoparticles dispersed in a polymer matrix the results indicate that the chain motion exhibits three distinct regions of diffusion, the Rouse-like motion, an intermediate subdiffusive regime followed by a normal Fickian diffusion. The motion of the chain end monomers shows a scaling that can be attributed to the formation of strong "networklike" structures, which have been seen in a variety of polymer nanocomposite systems. Irrespective of the polymer-particle interaction strengths, these three regimes seem to be present with small deviations. Further investigation on dynamic structure factor shows that the deviations simply exist due to the presence of strong enthalpic interactions between the monomers with the nanoparticles, albeit preserving the anomaly in the chain diffusion. The time-temperature superposition principle is also tested for this system and shows a striking resemblance with systems near glass transition and biological systems with molecular crowding. The universality class of the problem can be enormously important in understanding materials with strong affinity to form either a glass, a gel or networklike structures.

Macromolecular dynamics in crowded environments

The structural and dynamical properties of macromolecules in confining or crowded environments are different from those in simple bulk liquids. In this paper, both the conformational and diffusional dynamics of globular polymers are studied in solutions containing fixed spherical obstacles. These properties are studied as a function of obstacle volume fraction and size, as well as polymer chain length. The results are obtained using a hybrid scheme that combines multiparticle collision dynamics of the solvent with molecular dynamics that includes the interactions among the polymer monomers and between the polymer beads and obstacles and solvent molecules. The dynamics accounts for hydrodynamic interactions among the polymer beads and intermolecular forces with the solvent molecules. We consider polymers in poor solvents where the polymer chain adopts a compact globular structure in solution. Our results show that the collapse of the polymer chain to a compact globular state is strongly influenced by the obstacle array. A nonmonotonic variation in the radius of gyration with time is observed and the collapse time scale is much longer than that in simple solutions without obstacles. Hydrodynamic interactions are important at low obstacle volume fractions but are screened at high volume fractions. The diffusion of the globular polymer chain among the obstacles is subdiffusive in character on intermediate time scales where the dynamics explores the local structure of the heterogeneous environment. For large polymer chains in systems with high obstacle volume fractions, the chain adopts bloblike conformations that arise from trapping of portions of the chain in voids among the obstacles.

Mill processing and properties of rubber–clay nanocomposites

The dispersion of nanoscale composites in elastomers, which generally have higher molecular weight and viscosity as compared to plastics, is a challenge. Several techniques have been proposed for improvement of the dispersion of nanofillers in the polymers [1]. For example, the interaction of natural layered silicates can be improved by ion-exchange of hydrated cation with organic cations such as introducing bulky alkylammoniums to obtain larger interlayer spacing and provide the galleries for the polymer chain diffusion. The resultant swollen nanoclay was dried and dispersed in the polymer matrix by means of high shear mixers [2–3]. In this paper we describe the results from a new method of incorporating nanofillers into solid rubber by use of a conventional two-roll mill, which we call the modified mill method. The properties of the resultant material are compared with that of the material prepared by a latex method. We also test processability parameters, tensile behavior and crosslink density of carbon black composites prepared by the same two methods to provide a comparison between the nanocomposites. The rubber–clay nanocomposites prepared by the mill method are shown to have a fine dispersed phase structure and good reinforcement properties.

Conformational relaxation of single polymer chains confined in a two-dimensional plane studied by scanning near-field optical microscopy

The conformational rearrangement of poly(methyl methacrylate) (PMMA) chains confined in a two-dimensional monolayer to a three-dimensional random coil was studied by the shape and segment density analysis of an individual PMMA chain, and was directly observed by scanning near-field optical microscopy. A perylene-labeled PMMA chain was dispersed in the unlabeled PMMA monolayer, after which it was placed on a thick film of unlabeled PMMA and annealed above the glass transition temperature. Although the polymer chain of the monolayer penetrated into the bulk substrate with thermal activation, the radius of gyration in the lateral direction was maintained throughout the annealing process. The diffusion rate in the depth direction was much faster than that of the translational diffusion of the entire polymer chains, suggesting that an unstable two-dimensional conformation of the polymer chain relaxed into a three-dimensional one. This indicates that the polymer chains confined in a monolayer interlaced with the surrounding chains and expanded mainly toward the depth direction in the relaxation process, while maintaining the dimensions of the averaged radius of gyration in the lateral direction.