Recent developments in the formation, characterization, and simulation of micron and nano-scale droplets of amorphous polymer blends and semi-crystalline polymers

Polymer micro- and nano-particles are fundamental to a number of modern technological applications, including polymer blends or alloys, biomaterials for drug delivery systems, electro-optic and luminescent devices, coatings, polymer powder impregnation of inorganic fibers in composites, and are also critical in polymer-supported heterogeneous catalysis. In this article, we review some of our recent progress in experimental and simulation methods for generating, characterizing, and modeling polymer micro- and nano-particles in a number of polymer and polymer blend systems. By using instrumentation developed for probing single fluorescent molecules in micron-sized liquid droplets, we have shown that polymer particles of nearly arbitrary size and composition can be made with a size dispersion that is ultimately limited by the chain length and number distribution within the droplets. Depending on the time scale for solvent evaporation—a tunable parameter in our experiments—phase separation of otherwise immiscible polymers can be avoided by confinement effects, producing homogeneous polymer blend micro- or nano-particles. These particles have tunable properties that can be controlled simply by adjusting the size of the particle, or the relative mass fractions of the polymer components in solution. Physical, optical, and mechanical properties of a variety of micro and nano-particles, differing in size and composition, have been examined using extensive classical molecular dynamics calculations in conjunction with experiments to gain deeper insights into fundamental nature of their structure, dynamics, and properties.     

Manipulating crystal growth and polymorphism by confinement in nanoscale crystallization chambers

The phase behaviors of crystalline solids embedded within nanoporous matrices have been studied for decades. Classic nucleation theory conjectures that phase stability is determined by the balance between an unfavorable surface free energy and a stabilizing volume free energy. The size constraint imposed by nanometer-scale pores during crystallization results in large ratios of surface area to volume, which are reflected in crystal properties. For example, melting points and enthalpies of fusion of nanoscale crystals can differ drastically from their bulk scale counterparts. Moreover, confinement within nanoscale pores can dramatically influence crystallization pathways and crystal polymorphism, particularly when the pore dimensions are comparable to the critical size of an emerging nucleus. At this tipping point, the surface and volume free energies are in delicate balance and polymorph stability rankings may differ from bulk. Recent investigations have demonstrated that confined crystallization can be used to screen for and control polymorphism. In the food, pharmaceutical, explosive, and dye technological sectors, this understanding and control over polymorphism is critical both for function and for regulatory compliance. This Account reviews recent studies of the polymorphic and thermotropic properties of crystalline materials embedded in the nanometer-scale pores of porous glass powders and porous block-polymer-derived plastic monoliths. The embedded nanocrystals exhibit an array of phase behaviors, including the selective formation of metastable amorphous and crystalline phases, thermodynamic stabilization of normally metastable phases, size-dependent polymorphism, formation of new polymorphs, and shifts of thermotropic relationships between polymorphs. Size confinement also permits the measurement of thermotropic properties that cannot be measured in bulk materials using conventional methods. Well-aligned cylindrical pores of the polymer monoliths also allow determination and manipulation of nanocrystal orientation. In these systems, the constraints imposed by the pore walls result in a competition between crystal nuclei that favors those with the fastest growth direction aligned with the pore axis. Collectively, the examples described in this Account provide substantial insight into crystallization at a size scale that is difficult to realize by other means. Moreover, the behaviors resulting from nanoscopic confinement are remarkably consistent for a wide range of compounds, suggesting a reliable approach to studying the phase behaviors of compounds at the nanoscale. Newly emerging classes of porous materials promise expanded explorations of crystal growth under confinement and new routes to controlling crystallization outcomes.     

One-dimensional confinement in crystallization of P(VDF-TrFE) thin films with transfer-printed metal electrode

The crystallization of thin polymer film depends on surface/interface properties, due to the fact that molecular chain motion is affected by the presence of the surface. In this work, we measured the ferroelectric properties, crystallinity, chain conformation and surface morphologies of one-dimensionally confined P(VDF-TrFE) thin films using transfer-printed Au film, annealed at elevated temperatures, from just below melting temperature up to 200 °C. Crystallization at low temperature, i.e., below melting temperature, the confinement effect has been found to be negligible. At high temperatures, however, confined crystallization has led to superior ferroelectric properties, compared to samples annealed without confinement. These observations have led to two- or three-layer model for those crystallized thin films with or without confinement, respectively. Further, the transfer-printing of metal as a confining surface has been found to be beneficial, compared to vacuum evaporation, due to deposition-induced damages on organic polymer. This confinement-induced retention of ferroelectricity in P(VDF-TrFE) thin films above its melting temperature can extend processing temperature in organic devices using the ferroelectric polymer, such as non-volatile organic memory devices.     

Multilayered confinement of iPP/TPOSS and nylon 6/APOSS blends

A series of isotactic polypropylene and nylon 6 blends with silsesquioxane (POSS) additives were produced, then layered to nanometer thicknesses to test the effects of confinement upon polymer property modification. POSS is shown to be a poor filler, lacking solubility and favorable interaction with the polymer matrices. It was initially hypothesized that under extreme confinement and orientation, such as in melt-spun fibers, or confined within 2D nanoscale layers, that POSS would undergo forced-assembly into elongated, rebar-like reinforcement structures, or even act as crosslinking molecules for the polymer chains. The current results, however, show POSS existing as large, phase separated aggregates, in order to minimize interactions with the polymer matrix; the aggregates behave as debonded hard particles upon tensile deformation. POSS has been previously shown to enhance the properties of polymer matrices in which the POSS molecules have been grafted to, or copolymerized within the chain, but this is not the case for these POSS blends. In comparison to results from the iPP/DBS/TPOSS system, in which POSS is unable to directly interact with the polymer matrix, and the nylon 6/APOSS system, in which POSS can potentially form hydrogen bonds with the polymer matrix, the results are similar and reveal that POSS blends are largely incompatible with the polymer matrix. Small improvements in blend properties can be made via functionalization of the POSS cage, in order to enhance interactions, but these improvements are quite limited.     

Combinatorial methods for polymer materials science: Phase behavior of nanocomposite blend films

Polymer materials are often mixed with inorganic materials in the bulk to enhance properties, including mechanical, electrical, thermal, and physical. Such property enhancements are induced not only by the physical presence of the filler but also significantly by the interaction of the polymer with the filler via altering the local properties of the polymer material. In this regard, recently layered silicate nanocomposites have been shown to be effective in modifying the polymer properties because of their high surface area of contact between the polymer and the high aspect ratio nanoparticle. Potential property enhancements should also occur in polymer nanocomposite thin films owing to nanoparticle orientation from film confinement effects. In this paper we investigate the effect of layered silicate nanoparticles on the phase behavior of a classic polymer blend using small angle neutron scattering and compare those results to phase diagrams obtained by high throughput combinatorial methods.     

Permeability control in polymeric systems: a review

Controlling the extent of permeant (gas/vapour/liquid) transport through a polymer is critical in packaging applications. This can be achieved by changing the chemical and physical nature of the polymer, blending the polymer with another, dispersing particulate, fibrillar or lamellar fillers, converting the microstructure of the polymer to a cellular one, or using an effective combination of all of these processes. This review critically analyses different methods of controlling polymer permeability reported in literature. It provides recommendations and fresh approaches for modification of polymer permeability when considering large scale manufacturing of packages with controlled permeability properties. In addition, the subsequent effects of these modification techniques on the mechanical properties of the polymeric system are considered.     

Confinement of a polymer chain: An entropic study by Monte Carlo method

The properties of macromolecules in presence of an interface could be considerably modified due to confinement effects. When phase separations are performed in nanoconfined domains, the concurrent presence of high‐energy interfaces and conformational entropy constraints of the macromolecules causes profound differences in polymer aggregation behavior. Here, thermodynamics of a polymer chain in solution, confined by a three‐dimensional cubic interface, is studied by means of Monte Carlo method, focusing on the chain conformational entropy penalty arising from the excluded volume effects. The presented method might become a general tool for a preliminary evaluation of the thermodynamic effects due to the confinement of a polymer system. Further, the interface effects on Thermally Induced Phase Separation (TIPS) of polymer solutions, confined by High‐Pressure Homogenization, are experimentally studied, regarding final morphologies. It is confirmed how peculiar polymer morphologies are obtained only when the TIPS develops under nanoconfinement degrees above a threshold one. © 2017 American Institute of Chemical Engineers AIChE J, 64: 416–426, 2018     

In-situ polymerization of styrene in AAO nanocavities

One of the most promising aspects of the anodic aluminium oxide (AAO) template is the ability to generate a variety of different hierarchical one-dimensional (1D) polymer morphologies with structural definition on the nanometric scale. In-situ polymerization of monomers in reduced space of porous AAO template nanocavities can give rise to the direct production of versatile polymer nanostructures. In this work, porous AAO devices of 35 nm of diameter have been obtained by a two-step electrochemical anodization process and used as a nanoreactor to study the radical polymerization kinetics of styrene (St) in confinement and the results compared to those of polymerization in bulk. SEM morphological study has been conducted to establish the final structure of obtained polymer nanostructures. Confocal Raman microscopy has been performed to study the formation of the polymer through the AAO cavities as a function of time and with this methodology it has been possible to establish the monomer conversion for styrenic polymerization in AAO devices. Polystyrene obtained in the nanoreactor was characterized by SEC, NMR, TGA and DSC and the properties compared with those of bulk polymer. It was found that both the average molecular weights and polydispersity index of nanostructured polymer are lower than those obtained for bulk polymer. NMR studies have shown that the use of a reactor with nanometric size dimensions gave the obtained polystyrene greater stereospecificity than that obtained in bulk. Thermal stability and glass transition temperature (T_g) values are higher for nanostructured than bulk polymers. Moreover, the methodology proposed in this work, using AAO nanocavities as nanoreactors for polymerization reaction, can be generalized and applied to obtain polymer nanostructures of very different chemical nature and morphology by choosing the appropriate monomer or monomer reactants and by tailoring the dimension of AAO cylindrical nanocavities, that is, diameter from 20 to 400 nm and length from a few to hundreds of microns.     

Dynamics of single-walled carbon nanotube (SWNT)/polyisoprene (PI) nanocomposites in electric and mechanical fields

Relaxation dynamics of single-walled carbon nanotube (SWNT)/polyisoprene (PI) nanocomposites were examined by dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS) over a wide range of frequency and temperature. Both functionalized (SWNT-f) and pristine (SWNT-p) nanotubes were used and their effect on dynamics compared. Functionalized (PISF) nanocomposites were characterized by an increase in the time scale of the normal mode process as a consequence of the strong surface interactions between the polymer matrix and the nanotubes. The exact opposite is seen in pristine (PISP) nanocomposites where a decrease in the time scale of the normal mode relaxation is observed and attributed to weaker surface interactions and the effect of confinement on dynamics. The segmental process in PISF or PISP is not affected by the presence of nanotubes. The temperature dependence of the average relaxation time for normal and segmental modes is of the Vogel-Fulcher-Tammann (VFT) type. A good agreement is observed in the time scale of processes measured by DRS and DMS in PISF nanocomposites. In PISP nanocomposites, however, the time scales obtained from DRS and DMS measurements are not in consistently good agreement and an explanation is offered in terms of confinement.     

Functionalization of polymer multilayer thin films for novel biomedical applications

The design of functional polymer multilayer thin films with nanometer scale control is of great interest for biomedical applications such as tissue engineering, targeted drug delivery, controlled release system, and regenerative medicine. Various functions and properties of polymer thin films can be easily programmed and realized by the layer-by-layer assembly strategy, which is a facile and versatile deposition method to prepare well-defined biomedical multilayer platforms due to its benign process to prepare films under mild conditions and the capability of incorporating bioactive materials at a desired location within the films. Particularly, the fine tuning of physicochemical and biological properties of multilayer thin films is significantly important for designing novel biomedical platforms capable of adjusting the cellular functions. In this review, we focus on the overall background of the layer-by-layer assembly as well as the tuning of multilayer film properties and the programming of biological functions into the polymer thin films with a view on the control of cellular functions. Furthermore, we highlighted the recent achievements toward the design of novel biomedical platforms based on functionalized polymer multilayer thin films.     

Melt strength of blends of linear low density polyethylene and comb polymers

The melt strength of a metallocene linear low-density polyethylene (m-LLDPE) can be enhanced significantly by blending in less than 10 wt% of long chain branched comb polymer. The extent of the enhancement could be ten-fold and depends on the architectural details of the comb polymer. Comb polymers primarily affect melt strength and have little effect on other properties such as shear thinning, melt index, melt index ratio, and intrinsic tear.Balancing melt strength properties against shear-thinning properties is important in LLDPE fabrication processes. One approach would be to augment the effect of comb polymer by blending in another component, namely an easy processing (also known as sparsely long chain branched) LLDPE. In the examples given here, the enhancements in melt strength and shear thinning properties of the base polymer were found to be additive, i.e. a simple weighted sum of component properties matched the blend properties within 10%.     

Nanostructured piezoelectric polymers

Among the wide variety of piezoelectric materials available, polymers offer an interesting solution because of their high mechanical flexibility, easy processing, and conformable features; they maintain good ferroelectric and piezoelectric properties. The most prominent examples of these are poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene difluoride–trifluoroethylene) [P(VDF–TrFE)]. An attractive prospective consists of the preparation of nanostructured polymers. It has been shown that the dimensional confinement of such macromolecules down to the nanoscale can improve their piezoelectric properties because the tailoring of the chemical structure is performed at the molecular level. In this review, we show how nanostructured polymers can be obtained and discuss reports on the ferroelectric and piezoelectric properties of nanostructured PVDF and P(VDF–TrFE) materials. In particular, we show how dimensional confinement leads to piezoelectric nanostructures with relevant performances, with a focus on the macromolecular structural arrangement that enhances their behavior. Experimental results and applications are also reported to compare the performances of different nanostructuration processes and the polymer efficiencies as piezoelectric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41667.     

Unveiling the Effects of In Situ Layer–Layer Interfacial Reaction in Multilayer Polymer Films via Multilayered Assembly: From Microlayers to Nanolayers

Multilayered polymer films are increasingly used in the daily life, but their macroscopic properties are always limited by the layer–layer interfacial compatibility. In this work, the influence of interface modification through in situ layer–layer interfacial reaction during a multilayered assembly is revealed from micro‐ to nanolayer films, based on maleated poly(vinylidene fluoride) and polyamide‐6. In the presence of interfacial reaction and confinement, layer architecture and microstructure are highly dependent on the number of layers. Specifically, for nanolayer films having smaller layer thicknesses and higher reaction extent, layer integrity is reduced with the occurrence of interfacial instabilities. Depending on the microstructural evolution from multilayer assembly, those films exhibit quantitatively different extensional rheological and dielectric properties from micro‐ to nanolayers. More importantly, dielectric spectroscopy reveals the contribution of copolymer‐rich interphases to the dielectric performance of micro/nanolayered films. Additionally, charge transport dynamics in nanolayered films also differ significantly from their microlayered counterparts. They are attributed to the strong dependence of interfacial reaction extent and resulting microstructure on the number of layers and layer thicknesses. This work clearly illustrates how the control of layer–layer interfacial reaction in micro/nanolayer assembly can tune the interfacial, microstructure, and macroscopic properties of multilayered products.     

Nanohybrid shish kebab structure and its effect on mechanical properties in poly(L‐lactide)/carbon nanotube nanocomposite fibers

In our previous work, the formation of a nanohybrid shish kebab (NHSK) structure was successfully achieved in helical polymer systems promoted by using single‐walled carbon nanotube (CNT) bundles with a unique ‘groove structure’, which is of great crystallographic interest. To further investigate the effect of surface groove structure of CNT bundles on the formation of NHSK structure in helical polymer systems, in the work reported here double‐walled carbon nanotube (DWNT) fibers with bundle structure were used as nucleating agents and orientation templates for poly(L ‐lactide) (PLLA) crystallization. A fine NHSK structure with controlled lateral size and period of kebabs was successfully obtained under various experimental conditions by using DWNT bundles. This could be due to the geometric confinement effect of the surface groove structure of the DWNT bundles, which could facilitate the orientation of PLLA chains along the DWNT axis and the lateral formation of a stable nucleus. Our work suggests an efficient method for the functionalization of CNTs with biocompatible PLLA, which may have some potential applications in biomedical areas. In addition, it is demonstrated that the formation of NHSK structure can effectively improve the physical bonding between PLLA and nanotubes, thus significantly improving the mechanical properties of PLLA/CNT nanocomposite fibers. Copyright © 2012 Society of Chemical Industry     

Nanomechanical study of polymer‐polymer thin film interface under applied service conditions

Single layer and multilayer polymer thin film coating on polymer substrate are gaining significant importance in different industries. The quantitative and qualitative estimation of interface response for thin film coating under different service conditions is significantly important from the perspective of modeling and designing novel materials. However, to characterize an interface between the soft polymer layer and soft polymer substrate is challenging because of the confinement effect, surface roughness, the viscoelastic nature of the polymers involved, and most importantly, the comparable mechanical properties of soft polymeric film and polymer substrate. Nanoindentation technique was applied in this work to find out the mechanical response of thin film PMMA (100–200 nm) and Epoxy interfaces of different interfacial strengths. Interfaces of different strengths were obtained by exposing the film‐substrate system to different service conditions. It has been observed from this study that pile‐up plays a major role in finding out the mechanical response of the interfaces of different strengths. The hardness was observed to increase as the interfacial strength reduces. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43532.     

Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam

While the current self-healing approaches such as micro-capsules, hollow fibers, thermally reversible covalent bonds, ionomers, incorporation of thermoplastic particles, etc., are very effective in self-healing micro-length scale damage, self-healing of structural scale or macro-length scale damage remains one of the grand challenges facing the self-healing community. We believe that self-healing of structural damage may need multiple steps, at least two steps: close then heal (CTH), similar to the biological healing of wounds in the skin. In a previous study [1], it has been proven that the confined shape recovery functionality of a shape memory polymer (SMP) based syntactic foam can be utilized to repair structural damage such as impact damage repeatedly, efficiently, and almost autonomously. The purpose of this study is to investigate the effect of various design parameters on the closing efficiencies of both the pure SMP and the SMP based syntactic foam. A systematic test program is implemented, including glass transition temperature (T_g) determination by dynamic mechanical analysis (DMA), isothermal compressive constitutive behavior at various temperatures, and stress-controlled uniaxial compression programming and shape recovery. During thermomechanical cycle testing, two stress levels are utilized for programming and three confinement conditions (fully confined, partially confined, and free) are investigated for shape recovery. It is found that the programming stress is restored under confined recovery conditions, which helps in fully closing the crack; the foam shifts the T_g higher and increases the stiffness at temperatures above the T_g; higher programming stresses lead to slightly higher shape fixity but lower shape recovery in free recovery cases; a higher programming stress also results in a higher peak stress for confined recovery conditions; while the peak stress recovered is controlled by thermal stress, the final stress recovered is controlled by the programming stress, which is stored and recovered using an entropic mechanism. This study lays a solid foundation for using shape memory polymer based composites to self-repair macro-length scale damage.     

The relation between the dispersion state of polymer microspheres in coatings and film properties

Coatings composed of non-aqueous polymer dispersion (NAD)/solution acrylic/melamine-formaldehyde resin give a typical two-phase structured film. The dispersion state of the polymer microspheres (NAD) in a crosslinked polymer matrix affects the mechanical and esthetic properties of the derived coatings. A major concern is how to control and characterize quantitatively the dispersion state of polymer microspheres in the films. In this study, the cross-section of thermally cured clear films containing NAD has been observed by transmission electron microscopy (TEM) and the dispersion state characterized in a statistical manner using Morishita's quadrat method.

The dispersion state of polymer microspheres can be varied from Poisson's distribution to an aggregated distribution by modulating the miscibility between the polymeric dispersant of the NAD and the polymer which constitutes the continuous phase. The dispersion state can also be varied to some extent by the composition of the NAD spheres.