Photooxidation of polymers: Relating material properties to chemical changes

This paper is devoted to a comprehensive study of the photo-oxidation of polymeric materials with the goal of correlating modifications of the polymer properties at the molecular and macroscopic levels. Several techniques were used to characterise the modifications of the chemical properties and mechanical behaviour over time under UV light. The methodology was developed on materials used as organic coatings; initially, a well-characterised phenoxy resin (PKHJ^®) was chosen as a model and then the approach was applied to an acrylate-melamine thermoset currently used as a topcoat in the automotive industry. Analysis of degraded samples by IR spectroscopy allowed us to propose a photo-oxidation mechanism. This mechanism suggested that chain scission occurred under photo-oxidation. To entirely understand the degradation of the polymers, gel fraction, thermoporosimetry, DMA, AFM nanoindentation and micro-hardness determinations were performed. The results showed that crosslinking reactions occurred in competition with chain scission and explained for the first time why crosslinking reactions were quite prevalent. Based on the obtained results, quantitative correlations were made between the various criteria of degradation, thus relating the chemical structure changes to the mechanical property modifications.     

Enhanced mechanical stability of microtubules polymerized with a slowly hydrolyzable nucleotide analogue

Atomic force microscopy (AFM) has been used to investigate the local mechanical and structural properties of microtubules polymerized using guanylyl-alpha-beta-methylene diphosphonate (GMPCPP), a slowly hydrolyzable analogue of guanosine triphosphate. Using a combination of AFM imaging and local force spectroscopy, GMPCPP-polymerized microtubules have been qualitatively and quantitatively compared to paclitaxel-stabilized microtubules. GMPCPP-polymerized microtubules qualitatively display a greater resistance to destruction by the AFM probe tip during imaging and during deformation measurements and maintain structural details after indentation. In addition, using force spectroscopy taken during the indentation and collapse of individual microtubules with the AFM probe tip, an effective spring constant of the microtubule wall (kMT) for both types of microtubules was determined. The average kMT of GMPCPP-polymerized microtubules, 0.172 N/m, is more than twice that of paclitaxel-stabilized microtubules. These results complement previously reported measurements of bending experiments on GMPCPP-polymerized and paclitaxel-stabilized microtubules.     

Photo-oxidation of acrylic-urethane thermoset networks. Relating materials properties to changes of chemical structure

Acrylic-urethane thermoset photo-ageing was investigated by various techniques including IR spectroscopy, dynamic-mechanical thermal analysis (DMTA), and micro-hardness testing. These results were used to identify the sites at which the chain oxidation reaction is initiated and the main pathways through which the degradation reaction proceeds. The chemical modifications induced by photo-ageing were qualitatively and quantitatively correlated with the modifications observed in the architecture and mechanical properties of the thermoset network. The results of this work also allow for the development of a quantitative kinetic model based on the identified mechanisms and a multi-scale approach from the molecular to the macroscopic level, which highlights the effect of the changes of the chemical structure on the modification of the macromolecule arrangement and thus on the mechanical properties. Finally, the impact of stabilisers on material ageing was studied.     

Photo-oxidation of block copoly(ether-ester) thermoplastic elastomers: Part 2—Origins of the photo-yellowing

The photo-oxidation of copoly(ether-ester) leads to yellowing of the exposed samples. The origins of this change are studied and comparison with the photo-chemical behaviour of poly(butyleneterephthalate) shows that the ester sequences are responsible for the yellowing. The photo-oxidation involves photo-products easily detected by ir spectroscopy. The yellowing can be associated with the oxidation products by varying the excitation wavelength. The longest wavelengths favour a photo-induced oxidation which would be detrimental to the mechanical properties, while the shortest favour the coloration of exposed samples.     

Surface modification of high heat resistant UV cured polyurethane dispersions

To modify the surface of UV cured polyurethane dispersion (UV-PUD), fluorinated PU called surface modifying agent (SMA) was blended with base PU prior to dispersion in water. X-ray photoelectron spectroscopy (XPS) and contact angle measurements showed that surfaces of dispersion cast film are significantly enriched with fluorine groups. Atomic force microscopy (AFM) showed that surface was roughened with the addition of SMA, which provided possible mechanism of increased water resistance as well as low friction coefficient of the film. On the other hand, hardness, mechanical and dynamic mechanical properties indicated that the bulk properties are marginally altered by the additive amount of SMA.     

Measuring the size dependence of Young's modulus using force modulation atomic force microscopy

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.     

Computer-aided cooling curve thermal analysis and microstructural evolution of Mg–5Zn–xY cast alloys

The organo-montmorillonite (MT), combined with a DOPO-based oligomer (PDAP), was used to improve the flame retardancy of epoxy thermoset. The thermal stabilities and flame-retardant properties of thermosets were investigated by thermogravimetric analysis, limiting oxygen index (LOI) and UL-94 tests. The synergistic effect of MT and PDAP was studied by Py–GC/MS, Fourier transform infrared spectroscopy, elemental analysis, laser Raman spectroscopy and scanning electron microscope. Results revealed that 0.5 mass% MT combined with 4 mass% PDAP showed obvious synergistic effect on enhancing the flame retardancy of thermoset. The corresponding thermoset achieved an LOI value of 35.5% and V-0 rating in UL-94 test, which was attributed to the intense blowing-out effect during combustion. The synergistic mechanism was probably ascribed to the formation of silicoaluminophosphate (SAPO) originating from the reaction between MT and PDAP. The SAPO serving as a solid acidic catalyst, coupled with the acid sites from the decomposition of organomodifier in MT, could promote the charring process. With the further increase in MT content, the charring process was strongly promoted and more phosphorus element was retained in the condensed phase, which inevitably resulted in the remarkable decrease of the amount of pyrolytic gases containing phosphorus-based radicals and nonflammable gases. These factors were responsible for the diminished blowing-out effect during combustion, which was adverse to the further improvement of flame retardancy.     

A review on plant fiber reinforced thermoset polymers for structural and frictional composites

In recent past years, utilization of synthetic materials has become a matter of immense concern due to increasing environmental awareness in terms of safety, sustainability and maintaining ecological balance. A substantial amount of work has been carried out on various aspects of plant based natural fiber reinforced thermoset polymer composite materials due to their numerous inherent properties like high specific strength, low cost and degradability. Current issues and challenges associated with mechanical and tribological properties of only plant based natural fiber reinforced thermoset composites have been highlighted in the present study. Various factors influencing mechanical and tribological characteristics have been discussed keeping the focus on plant fiber reinforced thermoset composites. A detailed discussion on mechanical (tensile, compressive, flexural, impact strength) and tribological properties (friction and specific wear rate) have been reported. Interfacial adhesion was found to be a dominating factor with respect to mechanical and tribological properties. Wear and frictional characteristics of plant fiber based thermoset composites can be controlled using suitable fillers and reinforcement orientation. A discussion on interfacial adhesion and its effect on composite performance have also been included.     

Local mechanical properties of plasma treated polystyrene surfaces

We study how a local air plasma treatment affects the mechanical properties of polystyrene by performing indentation measurements on the polymer in the elastic and plastic regime. The local exposure to plasma was obtained by placing a shadow-mask with quadratic holes of 45 x 45 microm(2) on top of the polymer substrate, providing uncovered (exposed to the plasma) and covered (protected from the plasma) areas. We have analyzed quantitatively the topography and the elastic-plastic properties of such a sample with atomic force microscopy (AFM) measurements, both before and after plasma treatment. To enhance the differences between covered and uncovered areas, the sample has been exposed to solvent vapor. This generates regions which are differently swollen. The quantitative investigation of the mechanical properties of the swollen sample for different solvent exposure times gives further insight into the changes of polystyrene mechanical properties caused by the plasma.     

Crosslinking chemistry and network structure in organic coatings. I. Cure of melamine formaldehyde/acrylic copolymer films

Crosslinking chemistry and network formation in hydroxy and carboxy functional acrylic copolymer resins cured with representative melamine-formaldehyde crosslinking agents have been studied by infrared spectroscopy. Network formation in these systems is dominated by two reactions, the condensation of the hydroxy (or carboxy) functionality of the acrylic resin with melamine alkoxy groups to form acrylic-melamine crosslinks, and the condensation of melamine hydroxy groups to form melamine-melamine crosslinks. The extents of these reactions have been studied as functions of acrylic resin composition, melamine type and concentration, and cure time and temperature. For melamines with just methoxy functionality, the extent of formation of acrylic-melamine crosslinks increased steadily with cure temperature. For melamines with substantial hydroxy functionality, the extent of formation of acrylic-melamine crosslinks increased rapidly then leveled off with increasing cure temperature. The formation of melamine-melamine crosslinks increased slowly with increasing cure temperature. From these data and a statistical model, effective crosslink densities were calculated. The crosslink densities correlated well with solvent resistance.     

Impedance spectroscopy of reactive polymers. 3. Correlations between dielectric,spectroscopic, and rheological properties during cure of a trifunctional epoxy resin

An investigation was carried out of correlations between dielectric, vibrational spectroscopic, and rheological properties during cure of a thermoset formulation composed of trifunctional epoxy resin and tetrafunctional amine. Experimental techniques utilized include impedance spectroscopy, near-infrared spectroscopy, steady shear, and dynamic mechanical measurements. Reaction kinetics obtained from dielectric and spectroscopic results were in excellent agreement. Gelation and vitrification times determined by dielectric and rheological measurements were also found to agree very well, despite the empirical nature of such correlations. A characteristic pattern in plots of imaginary impedance as a function of reaction time was reported for the first time in the open literature, and it was suggested that it could be used to identify gelation and vitrification during the network formation. A realization of the full potential of dielectric impedance spectroscopy in monitoring the progress of chemophysical changes in reactive polymers, however, hinges upon a development of fundamental scientific correlations between dielectric and chemorheological phenomena during cure.     

Magic-angle 13C-NMR of cured and degraded acrylic copolymer/melamine formaldehyde coatings

Magic-angle carbon-13 NMR has been used to probe the structure and dynamics of acrylic copolymer melamine formaldehyde crosslinked coatings. Changes in chemical composition that occur in conventional accelerated weathering tests were found to be dominated by hydrolysis of acrylic-melamine crosslinks and subsequent formation of melamine-melamine crosslinks. Evidence for photo-oxidation was also observed. These results are in substantial agreement with infrared measurements also made on these coatings. Gated high-power decoupling experiments were used to determine relative mobilities of the different carbon resonances as a function of acrylic copolymer composition and as a function of degradation. It was found that motion of the side-chain carbons on the acrylic copolymer was not sensitive either to the chemical composition of the coating or to the extent of degradation. Mobilities of the main-chain carbons of the acrylic copolymer decrease with increasing glass transition temperature of the acrylic copolymer. For acrylic copolymers containing styrene, it is found that the main-chain carbon mobilities decrease with weathering. The melamine triazine ring becomes very rigid after degradation consistent with the formation of short melamine-melamine crosslinks.     

基于原子力显微镜的单分子力谱在生物研究中的应用

原子力显微镜被广泛应用于生物研究领域,基于原子力显微镜的单分子力谱可以在单分子、单细胞水平上研究生物分子内和分子间的相互作用。 本文介绍了原子力显微镜单分子力谱在生物分子间相互作用、蛋白质去折叠、细胞表面生物分子、细胞力学性质和基于单分子力谱成像等研究中的最新进展。     

Mechanical and chemical analysis of plasma and ultraviolet-ozone surface treatments for thermal bonding of polymeric microfluidic devices

Here we have demonstrated that radio frequency plasma and ultraviolet-ozone (UVO) surface modifications are effective treatments for enabling the thermal bonding of polymeric microfluidic chips at temperatures below the T(g) (glass transition temperature) of the polymer. The effects of UVO and plasma treatments on the surface properties of a cyclic polyolefin and polystyrene were examined with X-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM) surface roughness measurements and surface adhesion measurements with AFM force-distance data. Three-point bending tests using a dynamic mechanical analyzer (DMA) were used to characterize the bond strength of thermally sealed polymer parts and the cross-sections of the bonded microchannels were evaluated with scanning electron microscopy (SEM). The experimental results demonstrated that plasma and UVO surface treatments cause changes in the chemical and physical characteristics of the polymer surfaces, resulting in a decrease in T(g) at the surface, and thus allowing the microfluidic chips to be effectively bonded at temperatures lower than the T(g) of the bulk polymer without losing the intended channel geometry.     

Characterization of Thin Polymer Films on the Nanometer Scale with Confocal Raman AFM

The combination of an atomic force microscope (AFM) with a Confocal Raman Microscope (CRM) has been used to study the composition of various thin films of polymer blends. The high spatial resolution of the AFM enables the morphological characterization of the polymer blends on the nanometer scale. Furthermore, when operating the AFM in Digital Pulsed Force Mode (DPFM), topographic information and local stiffness can be simultaneously recorded. This allows the material-sensitive characterization of heterogeneous materials. Thin films where PMMA (at room temperature a glassy polymer) is blended with two different styrene-butadiene rubbers are investigated. The presence of PMMA in both phase-separated thin films allows the comparison of the mechanical properties of the two different rubber phases using DPFM-AFM. When PMMA is blended with PET due to their similar mechanical properties (both are in the glassy state at room temperature) the assignment of the two phases to the corresponding polymers by AFM is rather difficult. Here, Raman spectroscopy provides additional information on the chemical composition of materials. In combination with a confocal microscope, the spatial distribution of the various phases can be determined with a resolution down to 200 nm. Therefore, the topographically different structures observed in AFM images can be associated to the chemical composition by using the Confocal Raman Microscope (CRM).     

Poly(urethane‐co‐benzoxazine)s via reaction of phenol terminated urethane prepolymers and benzoxazine monomer and investigation of their properties

In the present work, a new method was developed for the combination of polyurethanes (PUs) and polybenzoxazine (PBz) to obtain novel thermoset poly(urethane‐co‐benzoxazine)s with good thermal, mechanical, and electrical properties as well as low temperature curing profile. Knowing the catalytic effect of compounds possessing free phenolic groups on ring opening polymerization of benzoxazine monomers, preparation of phenol terminated urethane oligomers (PTPU) as the macroinitiator for a benzoxazine monomer (Ba) was considered. Firstly, NCO‐terminated urethane prepolymers were prepared from the reaction of poly(tetramethyleneether glycol), and 2,4‐tolylene diisocyanate, and then end functionalized with bisphenol‐A under proper condition. DSC, DMTA, and gel content measurements were applied to find optimum ring opening polymerization condition (170°C for 1 hr and 200°C for 15 min). Various kinds of thermoset polymers were prepared by the reaction of PTPU at different molecular weights with variable contents of Ba. All of monomeric and polymeric materials were characterized by conventional spectroscopic methods and their thermal, mechanical, viscoelastic, and electrical properties were measured and properties were correlated to their structure. Due to the interesting properties of these new materials, the possibility of using them as electrical insulators with higher service temperature in comparison to common PUs were examined and their potential applicability was confirmed. Copyright © 2010 John Wiley & Sons, Ltd.     

Combined AFM and two-focus SFCS study of raft-exhibiting model membranes.

Dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/SM/cholesterol) model membranes exhibit liquid-liquid phase separation and therefore provide a physical model for the putative liquid-ordered domains present in cells. Here we present a combination of atomic force microscopy (AFM) imaging, force measurements, confocal fluorescence imaging and two-focus scanning fluorescence correlation spectroscopy (two-focus SFCS) to obtain structural and dynamical information about this model membrane system. Partition coefficients and diffusion coefficients in the different phases were measured with two-focus SFCS for numerous fluorescent lipid analogues and proteins, while being directly related to the lateral organization of the membrane and its mechanical properties probed by AFM. Moreover we show how the combination of these different approaches is effective in reducing artifacts resulting from the use of a single technique.     

Mechanical Properties of Poly(dimethylsiloxane)-block-poly(2-methyloxazoline) Polymersomes Probed by Atomic Force Microscopy

Poly(dimethylsiloxane)-block-poly(2-methyloxazoline) (PDMS-b-PMOXA) vesicles were characterized by a combination of dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and atomic force microscopy imaging and force spectroscopy (AFM). From DLS data, a hydrodynamic radius of ～150 nm was determined, and cryo-TEM micrographs revealed a bilayer thickness of ～16 nm. In AFM experiments on a silicon wafer substrate, adsorption led to a stable spherical caplike conformation of the polymersomes, whereas on mica, adsorption resulted also in vesicle fusion and formation of bilayer patches or multilayer stacks. This indicates a delicate balance between the mechanical stability of PDMS-b-PMOXA polymersomes on one hand and the driving forces for spreading on the other. A Young's modulus of 17 ± 11 MPa and a bending modulus of 7 ± 5 × 10(-18) J were derived from AFM force spectroscopy measurements. Therefore, the elastic response of the PDMS-b-PMOXA polymersomes to external stimuli is much closer to that of lipid vesicles compared to other types of polymersomes, such as polystyrene-block-poly(acrylic acid) (PS-b-PAA).     

EFFECTS OF ORGANIC COLORANTS ON PHOTO-INITIATED CROSSLINKING AND PHOTO-OXIDATION DEGRADATION OF POLYETHYLENE AND RELATED MECHANISM

The effects of three organic colorants on photo-initiated crosslinking and photo-oxidation degradation of polyethylene (PE) samples irradiated by microwave excited (MWE) UV lamp in the melt and the related mechanism have been studied by gel content and thermal extension rate determinations,X-ray photoelectron spectroscopy (XPS),mechanical property tests,UV spectroscopy,and light microscope.The data from the gel content and thermal extension rate determinations of photo-crosslinked polyethylene (XLPE) sam...     

Spatially resolved force spectroscopy of bacterial surfaces using force-volume imaging

Force spectroscopy using the atomic force microscope (AFM) is a powerful technique for measuring physical properties and interaction forces at microbial cell surfaces. Typically for such a study, the point at which a force measurement will be made is located by first imaging the cell using AFM in contact mode. In this study, we image the bacterial cell Shewanella putrefaciens for subsequent force measurements using AFM in force-volume mode and compare this to contact-mode images. It is known that contact-mode imaging does not accurately locate the apical surface and periphery of the cell since, in contact mode, a component of the applied load laterally deforms the cell during the raster scan. Here, we illustrate that contact-mode imaging does not accurately locate the apical surface and periphery of the cell since, in contact mode, a component of the applied load laterally deforms the cell during the raster scan. This is an artifact due to the deformability and high degree of curvature of bacterial cells. We further show that force-volume mode imaging avoids the artifacts associated with contact-mode imaging due to surface deformation since it involves the measurement of a grid of individual force profiles. The topographic image is subsequently reconstructed from the zero-force height (the contact distance between the AFM tip and the surface) at each point on the cell surface. We also show how force-volume measurements yield applied load versus indentation data from which mechanical properties of the cell such as Young's modulus, cell turgor pressure and elastic and plastic energies can be extracted.