Cross-linked Polymer Films Research Articles

Enhanced Tg-Confinement Effect in Cross-Linked Polystyrene Compared to Its Linear Precursor: Roles of Fragility and Chain Architecture

The glass transition temperature (Tg) of cross-linked polystyrene (PS) is directly compared to that of its linear precursor polystyrene-co-vinylbenzocyclobutene (PS–VBCB) in bulk and confined states. The VBCB units incorporated into a linear PS precursor cross-link with one another upon annealing at 250 °C. Bulk Tg (Tg,bulk) is characterized by differential scanning calorimetry (DSC) and ellipsometry, with Tg,bulk and Tg breadth increasing with increased cross-linking. The Tg-confinement effect is characterized by ellipsometry in supported PS–VBCB films before and after cross-linking; Tg decreases with decreasing nanoscale thickness in both supported linear and cross-linked polymer films. The magnitude of the confinement effect is greater in cross-linked PS compared to linear precursors; e.g., with PS-VBCB containing 8.5 mol % VBCB, Tg – Tg,bulk = −2 to −3 °C for a 23 nm thick film of linear polymer whereas Tg – Tg,bulk = −8 to −9 °C after cross-linking. The larger Tg reduction upon confinement in cross-linked PS is correlated with increased bulk fragility after cross-linking as measured by DSC and ellipsometry. Neat linear PS–VBCB copolymers provide an extension to lower fragility of the correlation between the Tg-confinement effect and bulk fragility observed previously [Evans Macromolecules 2013, 46, 6091] for neat linear polymers lacking attractive polymer–substrate interactions; i.e., the strength of the Tg-confinement effect increases with increasing bulk fragility. Both cross-linked PS and doped PS films deviate from the relationship for neat linear polymers, the former being weaker and the latter stronger. These results indicate that chain architecture and dopant content modify the relationship between the strength of the Tg-confinement effect and bulk fragility observed in many neat, linear polymers.

On the Generation of Polyether‐Based Coatings through Photoinduced C,H Insertion Crosslinking

In this work, the synthesis of a photoreactive polyether and its subsequent photochemical attachment to various substrates is described. Poly(bisphenol A‐co‐epichlorohydrin) is chosen as an example for a technologically important class of polymers and modified in a polymeranalogous reaction with photoreactive benzophenone units. During brief UV activation, benzophenone forms reactive intermediates, which are able to formally insert into any aliphatic C,H bond, eventually forming a crosslinked polymer film. During the formation process the emerging polyether networks additionally bind to (practically) any organic molecule present at the substrate surface. This way in one reaction step directly a surface‐attached polymer network is obtained. The examples, which are shown, include silane‐covered oxidic surfaces and unmodified polyolefins. When appropriate masks are used, brief UV irradiation is sufficient to generate crosslinked, surface‐attached polyether microstructures on a broad spectrum of substrate materials. The described strategy allows the generation of one‐component polymer coatings from standard commodity polymers under very simple process conditions, which can be lithographically structured and covalently attached to a broad spectrum of substrates, comprising even chemically rather inert materials. image

Cross-linked nanocellular polymer films: water- and oil-repellent anti-reflection coating

Cross-linked nanocellular polymer films: water- and oil-repellent anti-reflection coating. We applied the block copolymer template assisted with supercritical carbon dioxide method to poly(methyl methacrylate-r-glycidylmethacrylate)-b-poly(perfluorooctylethyl methacrylate) (PMGMA-PFMA) to fabricate cross-linkable nanocellular thin films. The nanocellular thin films showed a very low refractive index, low reflection as an AR coating as well as good chemical robustness by chemical cross-linkage of the glycidyl groups. In addition, the film surface provides water- and oil-repellent properties due to hydrophobic and oleophobic fluorine-containing PFMA.

Reduced Protein Adsorption on Plasma Polymer Films Comprising Hydrophobic/Hydrophilic Vertical Chemical Gradients

The adsorption of albumin protein (bovine serum albumin) from aqueous solution onto different types of cross-linked plasma polymer films is studied with unexpected results. Films are plasma-polymerized from a methylated siloxane precursor with controlled oxygen addition to obtain variable oxygen/carbon ratios, which adjust the hydrophobic–hydrophilic balance. Using the same technique vertical chemical gradients are generated with a hydrophobic surface termination and gradual transition to the hydrophilic plasma polymer over a few nanometers in depth. Remarkably, protein adsorption is significantly reduced on such gradient surfaces, suggesting existence of an energetic contribution from the sub-surface region.

Cross-linking and postfunctionalization of polymer films by utilizing the orthogonal reactivity of 7,7,8,8-tetracyanoquinodimethane.

Cross-linked polymer films are prepared by the alternating copolymerization of 4-chloromethylstyrene and 7,7,8,8-tetracyanoquinodimethane comonomers, which is followed by the postfunctional surface modification with donor-acceptor chromophores or redox active components using click chemistry reactions.

Biomimetic Submicroarrayed Cross-Linked Liquid Crystal Polymer Films with Different Wettability via Colloidal Lithography.

Photoresponsive cross-linked liquid crystal polymer (CLCP) films with different surface topographies, submicropillar arrays, and submicrocone arrays were fabricated through colloidal lithography technique by modulating different types of etching masks. The prepared submicropillar arrays were uniform with an average pillar diameter of 250 nm and the cone bottom diameter of the submicrocone arrays was about 400 nm, which are much smaller than previously reported CLCP micropillars. More interestingly, these two species of films with the same chemical structure represented completely different wetting behavior of water adhesion and mimicked rose petal and lotus leaf, respectively. Both the submicropillar arrayed film and the submicrocone arrayed film exhibited superhyrophobicity with a water contact angle (CA) value of 144.0 ± 1.7° and 156.4 ± 1.2°, respectively. Meanwhile, the former demonstrated a very high sliding angle (SA) greater than 90°, and thus, the water droplet was pinned on the surface as rose petal. On the contrary, the SA of the submicrocone arrayed CLCP film consisting of micro- and nanostructure was only 3.1 ± 2.0°, which is as low as that of lotus leaf. Furthermore, the change on the wettability of the films was also investigated under alternating irradiation of visible light with two different wavelengths, blue light and green light.

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

Photoinitiated Polymerization-Induced Microphase Separation for the Preparation of Nanoporous Polymer Films.

We report on the use of photoinitiated reversible addition-fragmentation chain transfer (RAFT) polymerization for the facile fabrication of cross-linked nanoporous polymer films with three-dimensionally (3D) continuous pore structure. The photoinitiated polymerization of isobornyl acrylate (IBA) in the presence of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (CTA) and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator proceeded in a controlled manner, yet more rapidly compared to thermally initiated polymerization. When polylactide-macroCTA (PLA-CTA) was used, PLA-b-PIBA with high molar mass was obtained after several minutes of irradiation at room temperature. We confirmed that microphase separation occurs in the PLA-b-PIBA and that nanoporous PIBA can be derived from the PLA-b-PIBA precursor by selective PLA etching. To fabricate the cross-linked nanoporous polymer, IBA was copolymerized with ethylene glycol diacrylate (EGDA) in the presence of PLA-CTA to produce a cross-linked block polymer precursor consisting of bicontinuous PLA and P(IBA-co-EGDA) microdomains, via polymerization-induced microphase separation. We demonstrated that nanoporous P(IBA-co-EGDA) monoliths and films with 3D continuous pores can be readily obtained via this approach.

Submicrometric Films of Surface-Attached Polymer Network with Temperature-Responsive Properties.

Temperature-responsive properties of surface-attached poly(N-isopropylacrylamide) (PNIPAM) network films with well-controlled chemistry are investigated. The synthesis consists of cross-linking and grafting preformed ene-reactive polymer chains through thiol-ene click chemistry. The formation of surface-attached and cross-linked polymer films has the advantage of being well-controlled without any caution of no-oxygen atmosphere or addition of initiators. PNIPAM hydrogel films with same cross-link density are synthesized on a wide range of thickness, from nanometers to micrometers. The swelling-collapse transition with temperature is studied by using ellipsometry, neutron reflectivity, and atomic force microscopy as complementary surface-probing techniques. Sharp and high amplitude temperature-induced phase transition is observed for all submicrometric PNIPAM hydrogel films. For temperature above LCST, surface-attached PNIPAM hydrogels collapse similarly but without complete expulsion of water. For temperature below LCST, the swelling of PNIPAM hydrogels depends on the film thickness. It is shown that the swelling is strongly affected by the surface attachment for ultrathin films below ∼150 nm. For thicker films above 150 nm (to micrometers), surface-attached polymer networks with the same cross-link density swell equally. The density profile of the hydrogel films in the direction normal to the substrate is confronted with in-plane topography of the free surface. It results that the free interface width is much larger than the roughness of the hydrogel film, suggesting pendant chains at the free surface.

Coarse‐grained molecular‐dynamics simulations of capped crosslinked polymer films: Equilibrium structure and glass‐transition temperature

We present our recent results from constant temperature‐pressure (NPT) molecular dynamics (MD) simulations of a bead‐spring copolymer model, in which the polymer is confined between two crystalline substrates. Our goal was to study the combined effect of the polymer crosslinking density and the degree of confinement on the glass‐transition temperature and the equilibrium structure of the films. In the direction perpendicular to the substrates, the polymer chains are ordered in layers of increasing density towards the substrates, for all crosslinking densities and the degrees of confinement. In the direction parallel to the substrates, the polymer films display an amorphous structure, just like in the bulk. The glass‐transition temperature increases with confinement and crosslinking density, with the former having a large effect compared to the later. POLYM. COMPOS., 36:1012–1019, 2015. © 2015 Society of Plastics Engineers

Crosslinked poly(ethylene oxide) containing siloxanes fabricated through thiol‐ene photochemistry

ABSTRACTHomogenous amphiphilic crosslinked polymer films comprising of poly(ethylene oxide) and polysiloxane were synthesized utilizing thiol‐ene “click” photochemistry. A systematic variation in polymer composition was Carried out to obtain high quality films with varied amount of siloxane and poly(ethylene oxide). These films showed improved gas separation performance with high gas permeabilities with good CO2/N2 selectivity. Furthermore, the resulting films were also tested for its biocompatibility, as a carrier media which allow human adult mesenchymal stem cells to retain their capacity for osteoblastic differentiation after transplantation. The obtained crosslinked films were characterized using differential scanning calorimetry, dynamic mechanical analysis, thermogravimetric analysis, FTIR, Raman‐IR, and small angle X‐ray scattering. The synthesis ease and commercial availability of the starting materials suggests that these new crosslinked polymer networks could find applications in wide range of applications. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1548–1557

Benzoxazine-Based Thermosets with Autonomous Self-Healing Ability

A self-healing strategy for poly(propylene oxide)s bearing benzoxazine units (PPO-Benz) through supramolecular attractions is described. Poly(propylene oxide) bisamine (PPO) with a molecular wieght of 2000 Da was reacted with formaldehyde and bisphenol A to yield desired PPO-Benz with 12 360 Da. The cross-linked polymer films were then prepared by solvent casting of suitable compositions of PPO-Benz and carboxlic acid containing benzoxazine monomer (Carb-Benz) in chloroform followed by thermal ring-opening reaction of benzoxazine groups at 200 °C. Thermal curing and thermal stability of the film and final products were investigated. It was demonstrated that the self-healing capacity of the films was improved by employing Carb-Benz in the formulation.

Thin polymer films with embedded CdS nanocrystals

A new method for formation of the cross-linked polymer films at the solid surface as well as the thin nanocomposite films based on them with embedded CdS nanocrystals is reported in this paper. The process involves several stages, namely, (i) thin polymer film formation on glass slide by spin coating of an organic solution of reactive peroxide-containing copolymer (RC), polyethylene glycols (PEG) of various molecular weight, and cadmium acetate [Cd(Ac)2] as nanocrystal precursor; (ii) melting and cross-linking of the polymer film; and (iii) synthesis of CdS nanocrystals inside the polymer film via sol–gel technique. The X-ray diffraction analysis and UV–visible spectroscopy prove formation of the CdS nanocrystals in the thin polymer films. This method allows forming multilayered nanocomposite films with tuned properties. The obtained results reveal that size of the CdS nanocrystals and optical properties (absorption and photoluminescence spectra) of the nanocomposite films depend on the nature of polymer matrix, PEG/RC ratio, Cd(Ac)2 concentration, and the number of layers composing the films.

Photoinduced smart, self-healing polymer sealant for photovoltaics.

Polyisobutylene (PIB)-based polymer networks potentially useful as smart coatings for photovoltaic devices have been developed. Low molecular weight coumarin functional triarm star PIB was synthesized via a single step SN2 reaction of bromoallyl functional triarm star PIB with 4-methylumbelliferone or umbelliferone in the presence of sodium hydride. Quantitative end functionality was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. UVA (λmax = 365 nm) induced reversible photodimerization of the coumarin moieties resulted in cross-linked elastomeric films exhibiting self-healing behavior. The extent of photodimerization/photoscission was monitored by UV-vis spectroscopy. The low oxygen (1.9 × 10(-16) mol m m(-2) s(-1) Pa(-1)) and moisture (46 × 10(-16) mol m m(-2) s(-1) Pa(-1)) permeability of the cross-linked polymer films suggest excellent barrier properties of the cross-linked polymer films. The self-healing process was studied by atomic force microscopy (AFM). For this, mechanical cuts were introduced in the cross-linked PIB films through micromachining with an AFM tip and the rate of healing induced by UV, sunlight, or both was followed by taking AFM images of the film at different time intervals during the repair process.

Photomobile polymer materials: photoresponsive behavior of cross-linked liquid-crystalline polymers with mesomorphic diarylethenes.

Cross-linked liquid-crystalline (LC) polymers with a mesomorphic diarylethene were prepared to demonstrate a versatile strategy for cross-linked photochromic LC polymers as photomobile materials. Upon exposure to UV light to cause photocyclization of the diarylethene chromophore, the cross-linked polymer films bend toward an actinic light source. By irradiation with visible light to cause a closed-ring to open-ring isomerization, the bent films revert to the initial flat state. Without visible-light irradiation, the bent films remain bent even at 120 °C, indicating high thermal stability of the cross-linked diarylethene LC polymers.

Structure of Plasma Poly(Acrylic Acid): Influence of Pressure and Dielectric Properties

Thin poly(acrylic acid) PAA films were deposited by pulsed plasma polymerization on different organic and inorganic substrates. The structure–property relationships of the deposited acrylic acid polymers were studied in dependence on the monomer pressure by various techniques and probes. The surface and bulk properties of the plasma deposited films were investigated by X-ray photoelectron spectroscopy, attenuated total reflection infrared, and broad band dielectric spectroscopy. The experimental infrared frequencies of PAA films are compared with those predicted from quantum mechanical calculation. The concentration of the COOH groups in the film (stored in ambient air) decreased by about 15 % compared to the as-prepared sample. The plasma deposited PAA probably form a highly branched product. However, the dielectric measurements show that in addition to the hydrogen bonds, self condensation process was able to hinder the localized fluctuation as well. These processes lead to form a cross-linked network polymer film. Nevertheless, a low energy is sufficient to break these processes during heating at atmospheric pressure. Therefore, homogenized samples with free branches (functional group) were obtained after a first heating with structures close to conventional polymerized acrylic acid. Thus, a thermally stable product was obtained.

Scalable fabrication of polymer membranes with vertically aligned 1 nm pores by magnetic field directed self-assembly.

There is long-standing interest in developing membranes possessing uniform pores with dimensions in the range of 1 nm and physical continuity in the macroscopic transport direction to meet the needs of challenging small molecule and ionic separations. Here we report facile, scalabe fabrication of polymer membranes with vertically (i.e., along the through-plane direction) aligned 1 nm pores by magnetic-field alignment and subsequent cross-linking of a liquid crystalline mesophase. We utilize a wedge-shaped amphiphilic species as the building block of a thermotropic columnar mesophase with 1 nm ionic nanochannels, and leverage the magnetic anisotropy of the amphiphile to control the alignment of these pores with a magnetic field. In situ X-ray scattering and subsequent optical microscopy reveal the formation of highly ordered nanostructured mesophases and cross-linked polymer films with orientational order parameters of ca. 0.95. High-resolution transmission electron microscopy (TEM) imaging provides direct visualization of long-range persistence of vertically aligned, hexagonally packed nanopores in unprecedented detail, demonstrating high-fidelity retention of structure and alignment after photo-cross-linking. Ionic conductivity measurements on the aligned membranes show a remarkable 85-fold enhancement of conductivity over nonaligned samples. These results provide a path to achieving the large area control of morphology and related enhancement of properties required for high-performance membranes and other applications.

Neutral state colorless electrochromic polymer networks: Spacer effect on electrochromic performance

A series of carbazole-based monomers and their corresponding polymers with various spacer units were synthesized and coated onto ITO-glass surface via an electrochemical cross-linking process. All carbazole-based polymers exhibited multi-electrochromic behavior thanks to two separate oxidation processes. All materials were characterized by FT-IR, 1H NMR, GPC, DSC, UV–vis, Florescence and CV. In addition, electrochromic properties of the cross-linked polymer films were investigated via spectro-electrochemical measurements. The colorless films were converted to yellowish green and greenish blue colors at the anodic regime. The results indicate that the spacer unit between the carbazole unit and the polymer backbone has a great impact on the optical and electrochemical properties and also on the electrochromic performance of these polymers. PVBEC with benzyloxy based spacer exhibits a high contrast ratio (ΔT% = 55 at 690 nm), a faster response time of about 2.1 s, a higher coloration efficiency (331 cm2 C−1) and a better stability (retains its performance by 94.3% even after 1000 cycles).

Measuring Reduced C60Diffusion in Crosslinked Polymer Films by Optical Spectroscopy

The diffusion of fullerenes such as C60and PCBM in organic semiconductors is a key factor in controlling the efficiency of organic solar cells, though it is challenging to measure and to control. A simple optical method based on photoluminescence quenching is developed to assess the diffusion of a quencher molecule such as C60through a semiconducting polymer film, in this case made with the polymer polyfluorene. When the mobility of the polymer chains is reduced by chemical crosslinking, the diffusion coefficient of C60can be reduced by up to three orders of magnitude.

Self-healing of poly(propylene oxide)-polybenzoxazine thermosets by photoinduced coumarine dimerization

In this work, a self-healing strategy for poly(propylene oxide)s bearing coumarine-benzoxazine units (PPO-CouBenz)s based on light induced coumarine dimerization reactions is described. Four different types of poly(propylene oxide) amines with molecular weights ranging from 440 to 5000 Da were reacted with formaldehyde and 4-methyl-7-hydroxycoumarin to yield desired (PPO-CouBenz)s. The crosslinked polymer films were prepared by solvent casting of various compositions of PPO-CouBenzs in chloroform followed by thermal ring opening reaction of benzoxazine groups at 210–240 °C. Thermal curing and thermal stability of the initial PPOs and final products were investigated. Using allyl benzoxazine in the formulation, it was demonstrated that the toughness of the films was improved. Photoinduced healing of coumarin-based cured PPO-CouBenz polymer films was investigated. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2911–2918