Solvent-cast Thin Films Research Articles

Development of a three-dimensional tomography holder for in situ tensile deformation for soft materials.

An in situ straining holder capable of tensile deformation and high-angle tilt for electron tomography was developed for polymeric materials. The holder has a dedicated sample cartridge, on which a variety of polymeric materials, such as microtomed thin sections of bulk specimens and solvent-cast thin films, can be mounted. Fine, stable control of the deformation process with nanoscale magnification was achieved. The holder allows large tensile deformation (≃800 μm) with a large field of view (800 × 200 μm before the deformation), and a high tilt angle (±75°) during in situ observations. With the large tensile deformation, the strain on the specimen can be as large as 26, at least one order of magnitude larger than the holder's predecessor. We expect that meso- and microscopic insights into the dynamic mechanical deformation and fracture processes of polymeric materials can be obtained by combining the holder with a transmission electron microscope equipped with an energy filter. The filter allows zero-loss imaging to improve the resolution and image contrast for thick specimens. We used this technique to study the deformation process in a silica nanoparticle-filled isoprene rubber.

An Easily Accessible Self-Healing Transparent Film Based on a 2D Supramolecular Network of Hydrogen-Bonding Interactions between Polymeric Chains.

Self-healing polymers hold great promise for the future, enhancing in particular the longevity of polymeric materials. We describe a self-healing covalent polymer, presenting an extensive array of hydrogen-bonding sites based on the combination of urea, urethane, and bis-acyl-hydrazine units. Solvent-cast thin-films prepared by polycondensation of a commercially available dihydrazide and a diisocyanate prepolymer exhibited excellent room temperature autonomous healing with almost full recovery of mechanical properties when two parts of a cut film were overlapped and gently pressed together. This autonomous healing upon damage may be attributed to the supramolecular dynamics of multiple lateral inter-chain hydrogen-bonding interactions between the polymer chains. The solid-state structure of a model compound incorporating the same structural backbone corroborates the existence of an extensive two-dimensional supramolecular hydrogen-bonding network.

Synthesis of a polyurea from a glucose- or mannose-containingN-alkyl urea peptoid oligomer

N-alkyl urea peptoid oligomers containing glucose or mannose have been synthesized and characterized. The oligomers were subsequently polymerized using a step-growth polymerization with hexamethylene diisocyanate. Equal moles of both monomers were used to guarantee high-molecular weight polymers. The polymers were characterized by gel permeation chromatography, nuclear magnetic resonance, and Fourier-transform infrared spectroscopy, and contact angle measurements of solvent cast thin films. Sulfation of the final polymers was achieved using a SO3/pyridine complex in pyridine to afford the heparin biomimetics. The average degree of sulfation was calculated to be 3.5 sulfates per saccharide as measured by elemental analysis. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 5230–5238

Crystallisation and fractionation of selected polyhydroxyalkanoates produced from mixed cultures

Poly[R-3-hydroxybutyrate-co-(R-3-hydroxyvalerate)] (PHBV) copolymers were produced from mixed cultures of biomass (activated sludge) fed with acetic acid (HAc) and propionic acid (HPr). Feeding was performed in such a way as to produce materials with a wide range of monomer compositions and microstructures. Solvent-cast thin films of these materials have recently been shown to exhibit a narrow range of mechanical properties similar to those of the homopolymer poly(R-3-hydroxybutyrate) (PHB) [1]. In this work, more detailed analyses of the thermal and crystallisation properties of these mixed-culture polyesters have revealed that they like comprise complex blends with broad compositional distribution of random and/or blocky copolymers of very different 3-hydroxyvalerate (3HV) contents and melting temperatures and thus have very different respective crystallisation kinetics. This blend complexity was confirmed by solvent fractionation of selected samples. The findings support the hypothesis that overall mechanical properties of these complex copolymer blend materials will be strongly influenced by the more rapidly crystallising components that form the matrix within which the slower crystallising components exist as microdomains. New opportunities in the material development of PHAs are likely to be found in establishing and exploiting such structure-function relationships.

Improved Lignin Polyurethane Properties with Lewis Acid Treatment

Chemical modification strategies to improve the mechanical properties of lignin-based polyurethanes are presented. We hypothesized that treatment of lignin with Lewis acids would increase the concentration of hydroxyl groups available to react with diisocyanate monomers. Under the conditions used, hydrogen bromide-catalyzed modification resulted in a 28% increase in hydroxyl group content. Associated increases in hydrophilicity of solvent-cast thin films were also recorded as evidenced by decreases in water contact angle. Polyurethanes were then prepared by first preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or modified lignin, then polymerization was completed through addition of polyethylene glycol (PEG), resulting in mass ratios of TDI:lignin:PEG of 43:17:40 in the compositions investigated here. The mixture of TDI and unmodified lignin resulted in a lignin powder at the bottom of the liquid, suggesting it did not react directly with TDI. However, a homogeneous solution resulted when TDI and the hydrogen bromide-treated lignin were mixed, suggesting demethylation indeed increased reactivity and resulted in better integration of lignin into the urethane network. Significant improvements in mechanical properties of modified lignin polyurethanes were observed, with a 6.5-fold increase in modulus, which were attributed to better integration of the modified lignin into the covalent polymer network due to the higher concentration of hydroxyl groups. This research indicates that chemical modification strategies can lead to significant improvements in the properties of lignin-based polymeric materials using a higher fraction of an inexpensive lignin monomer from renewable resources and a lower fraction an expensive, petroleum-derived isocyanate monomer to achieve the required material properties.

PZT/P(VDF-HFP) 0–3 composites as solvent-cast thin films: preparation, structure and piezoelectric properties

Composite films of lead zirconate titanate (PZT) and poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) were prepared as 100 µm thin films by solvent casting. Within the 0–3 composites, the ceramic-volume fraction was varied between 0.19 and 0.65, which yielded films with different structural and dielectric properties. These influenced the piezoelectric properties of the composite films found after electric poling, which was performed here at room temperature. The piezoelectric activity, with a maximum piezoelectric coefficient of 11 pC N−1 in the film-thickness direction, originates from the polarization of the embedded ceramic particles as proved by poling experiments in corona discharges as well as in direct contact.

The surface properties of PS/PMMA blends nanostructured polymeric layers

Solvent cast thin films of blends polystyrene (PS) and poly (methylmethacrylate) (PMMA) with nominal compositions ranging from 25/75 wt.%/v% (w/v) up to 75/25 w/v PS/PMMA with toluene as the mutual solvent on crystalline Si (100) and silica substrates has been studied. Films of PS and PMMA blends have been examined by atomic force microscopy (AFM) and ellipsometry. The blend films with less than 50% PMMA bulk concentration generally exhibit pitted surfaces; the pit size varies with film thickness and bulk composition. When the PMMA bulk concentration is greater than 50%, the film surface can be described as island-like phase-separated structure. The surface segregation and morphology are explained in terms of solubility of the two polymers in the solvent and rewetting of PMMA relative to PS.

Phase-Separated Morphologies in Poly(Psuedo Amino Acid)-PEG Blends and Copolymers for Tissue-Scaffolding Applications

Abstract Central to the successful advance of tissue engineering (e.g. 1, 2) is the development of highly engineered materials which trigger particular body responses to regenerate specific tissue or sets of tissue. Such properties as hydrophilicity, hydrolytic susceptibility, and protein-recognition behavior are critical to the success of tissue scaffolds. Poly(ethylene glycol) [PEG] is often introduced into scaffold polymers to tune hydrolytically controlled properties. Relatively little is known about the local morphology and nature of phase separation in these systems, however. This research studies the development of phase-separated morphology in blends and randommultiblock copolymers of a tyrosine-based poly(psuedo amino acid) [poly(DTE carbonate)] and PEG 1000 (fig. 1). This system exhibits attractive biocompatability, strength and modulus, and resorption behavior which can be controlled by main-chain and pendant-chain chemistries (3, 4). The nature of phase separation was studied by bright-field TEM (Philips CM30 ST; CM20 FEG TEM/STEM) using solvent-cast thin films.

Surface composition and morphology of polyimidesiloxane copolymers with short polydimethylsiloxane segments studied by electron spectroscopy for chemical analysis and time-of-flight secondary ion mass spectrometry

A series of polyimidesiloxane (SIM) copolymers, based on α,ω-aminopropylpoly- (dimethylsiloxane) (PDMS), 2,2-bis(4-[4-aminophenoxyl]phenyl)propane and 4,4′- oxydiphthalicanhydride was synthesized in our laboratories. The effects of siloxane segment length (dimethylsiloxane repeat units=1, 2, 3, 4, 5, and 9) and PDMS bulk content (1–30 wt %) on the surface composition and free surface morphology of the copolymers were investigated. Experimentally, angle-dependent electron spectroscopy for chemical analysis (ESCA) was used to acquire composition information of the copolymer surface at various sampling depths. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to describe ion formation mechanism of bulk SIM copolymers with variation of bulk composition or structure. In all cases, PDMS was segregated to the topmost 100 Å of the air (free) surface of ∼75 μm thick film, even with short siloxane segments and low PDMS contents. Angle-dependent ESCA measurements show that the dominant factor in defining the surface composition of these copolymers is not bulk composition, but in fact siloxane segment length. With the same PDMS content, a longer siloxane segment gives a surface richer in siloxane. For a given siloxane segment length, varying PDMS content does not significantly change the surface composition. TOF-SIMS measurements of bulk copolymers simulated by solvent-cast thin films show that a longer siloxane segment gives higher relative ion yields of fragment ions from siloxane versus imide segments even with the same 10 wt % PDMS content in the copolymers. For a given average siloxane segment length of 9 dimethylsiloxane repeat units, a higher PDMS content in the copolymers yields higher relative yields of fragment ions formed from siloxane versus imide segments.

Controlled synthesis of amphiphilic block copolymers with pendant glucose residues by living cationic polymerization

Amphiphilic block copolymers of vinyl ethers (VEs) of the type —[CH2CH(OCH2CH2OR)]m—[CH2CH(OiBu)]n—were synthesized by living cationic polymerization, where R is a D-glucose residue, and m and n are the degrees of polymerization (m = 20–50; n = 11–89). To obtain them, sequential living block copolymerization of isobutyl vinyl ether (IBVE) and the vinyl ether carrying 1,2:5,6-diisopropylidene-D-glucose residue was conducted by using the HCl adduct of IBVE, CH3CH(OiBu)Cl, as initiator in conjunction with zinc iodide. These precursor block copolymers had a narrow molecular weight distribution (M̄w/M̄n ∼ 1.1) and a controlled composition. Treatment of them with a trifluoroacetic acid/water mixture led to the target amphiphiles. The solubility of the amphiphilic block copolymers in various solvents depended strongly on composition or the m/n ratio. Their solvent-cast thin films were observed, under a transmission electron microscope, to exhibit various microphase-separated surface morphologies such as spheres, cylinders, and lamellae, depending on composition. © 1997 John Wiley & Sons, Inc.

Cross-sectional tem analysis of solvent-cast SBS thin films

Block copolymers can assume a range of microphase-separated morphologies, and the dependence of morphology on temperature and composition is an area of active research. Our work has been studying the morphology of solvent-cast thin films of polystyrene-polybutadiene-polystyrene (SBS) as a function of solvent evaporation rate and post-specimen annealing. This paper describes the analysis of thin film cross-sections to distinguish between possible morphologies. ∼50 nm-0.5 μm thick films were cast from a 0.1 wt% solution of SBS (30 wt%, Mw=105) in toluene. GPC analysis indicates PS/PB diblock and PS homopolymer is present (<5wt% total). 50 μl of solution was deposited on water-polished salt. The solvent evaporation rate was roughly controlled between 200 nl/sec and 0.2 nl/sec. After evaporation, the film was cut, floated from the salt, collected on grids, and exposed to OsO4 vapor (20 min) to stain the PB. After plan view study, some specimens were cross-sectioned. A carbon layer was evaporated on both sides prior to embedding in epoxy and ultramicrotomy(−130°C). Microscopy used a Philips CM3 0ST TEM.

Effects of Polypeptide Conformation and Surface Bonding on Static Secondary Ion Mass Spectra

For the first time, the α-helical homopolypeptide poly(γ-benzyl-L-glutamate) (PBLG) was synthesized by solid-phase methods. A 10 residue PBLG sample (10-mer) was prepared which exhibited a β-sheet configuration, and a 15 repeat unit PBLG sample (15-mer) was made which displayed partial α-helical character. Solvent-cast thin films of these peptides were deposited on both gold and silver and analyzed by time-of-flight static secondary ion mass spectrometry (ToF-SSIMS). The PBLG 10-mer spectra consisted of a strong molecular ion (cationized with Na or Ag) and less-pronounced fragments spaced by the benzyl glutamate mass (n-mers) attributed to SSIMS fragmentation. Contrary to this, the PBLG 15-mer did not provide an intact molecular ion, and only low-massn-mer species were observed. Thesen-mers are believed to be a product of SSIMS-induced molecular fragmentation that is enhanced by the peptide α-helical conformation. A disulfide moiety (SS) was also coupled to the 10-mer and 15-mer N-termini (10-merSS and 15-merSS) to provide a functional group for self-assembly on gold. Molecular ions were observed from solvent-cast films of these samples, verifying the presence of the SS cap. However, self-assembled monolayers of these peptides on gold generated weak SSIMS signals which primarily reflect the strong binding of the SS group to the substrate.

Surface properties of solvent-cast thin films of synthetic polypeptides

The surface energy of solid polypeptide films — benzyloxycarbonyl derivatives of basic poly(α-amino acid), benzyl esters of acidic poly(α-amino acid) and poly(α-amino acid)'s consisting of nonpolar side chains — was determined from contact angle measurements. The London-dispersive component of surface free energy, γ D S may reflect the hydrophobicity of the side chain of α-helical polypeptide. The nondispersive component of the solid/water interfacial free energy, I P SW, which corresponds to the interaction energy of polypeptide solid with water, was compared with the transition pressure * tr, of the monolayer at the air/water interface. Irrespective of the difference in film nature, the comparison between contact angle measurement and monolayer method indicates a correlation in the interaction of polypeptide with water. For the polypeptides consisting of nonpolar side chains, the I P SW value of β-form PLVal is larger than that of α-helical PLLeu.

Some observations on the micromorphology of deformed ABS and HIPS rubber-modified materials

The distribution and micromorphology of crazes in deformed bulk and solvent cast thin films of ABS and HIPS have been characterized. A good correlation was shown to exist between the deformation structure in the two types of specimen. Some characteristic features of the micromorphology of the deformed matrix and rubber inclusions which are apparent in the permanently strained cast films are described, and some simple electron microscope studies with strained cast films which could lead to further insight of toughening mechanisms in rubber-modified polymers are proposed.

Microstructure of crazes in solvent-crazed polycarbonate thin films

Crazes have been produced in polycarbonate by atomizing acetone over uniaxial stressed solvent cast thin films. Transmission electron microscopy shows that two types of craze orientation exist. Crazes are formed perpendicular to the applied stress direction and at approximately 55° to the applied stress direction. The craze structures observed at 55° to the applied stress direction are suggested to result from cavitation of pre-existing deformation bands. The internal morphology of each type of craze is similar, and the fibre diameter, void size and craze width of each type depends on the amount of craze development. The structure of the craze-matrix interface is, however, different for each type of craze. Necking and work-hardening of the fibres occurs at the normal craze-matrix interface, whereas a rough undrawn interface exists for the angular crazes.

A comparison of the morphology of crazes formed in thin films and in bulk specimens of polystyrene

The microstructure of crazes formed in solvent-cast thin films have been studied by transmission electron microscopy. The results have been compared with the microstructure of crazes determined from the examination of replicas obtained from the fracture surfaces of bulk specimens. The structure of crazes formed in thin films and in the bulk have been shown to be similar with the exception that much finer fibril structures are observed in thin films at large deformations. A model of the microstructure of a craze is presented.

Wettability and Multiple Attenuated Internal Reflection Infrared Spectroscopy of Solvent-Cast Thin Films of Polyamides

Wettability and Multiple Attenuated Internal Reflection Infrared Spectroscopy of Solvent-Cast Thin Films of Polyamides