XPS and SEM study of UV laser surface modification of polymers

Several commercial polymers—poly(ethylene) (PE), poly(propylene) (PP), poly(vinylidene fluoride) (PVF₂), poly(vinyl chloride) (PVC) and polystyrene (PS)—were treated in air with an argon-fluorine UV excimer laser (λ = 193 nm). The polymer etch rate was investigated by two methods: quartz crystal microbalance (QCM) and piercing of films. X-ray photoelectron spectroscopy (XPS) analysis was performed on the modified surfaces after laser exposure at various fluences. Samples were subsequently analysed by scanning electron microscopy (SEM). From our results, polymers may be classified into two categories concerning their reactivity towards UV laser light. — the weakly absorbing polymers (e.g. PE, PP, PVF₂) where a photothermal process (thermal degradation) dominates the interaction. — the strongly absorbing polymers (e.g. PVC, PS) where a photochemical process (photoablation) dominates the interaction.     

Surface modification of polyethylene by plasma pretreatment and UV‐induced graft polymerization for improvement of antithrombogenicity

The purpose of this study was to enhance blood compatibility of polyethylene (PE) films. Glycidyl methacrylate (GMA) was grafted onto the surface of PE by Ar plasma pretreatment and UV‐induced graft polymerization without photo‐initiator, then heparin was immobilized onto the poly (glycidyl methacrylate) segments. The surface compositions and microstructure of GMA graft polymerized PE films were studied by X‐ray photoelectron spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transfer Infrared (ATR‐FTIR) spectroscopy. It was confirmed that heparin was successfully immobilized onto the surface of PE films by XPS analysis. The antithrombogenicity of the samples was determined by the activated partial thromboplastin time (APTT), prothrombin time (PT), thrombin time (TT), and plasma recalcification time (PRT) tests and platelet adhesion experiment. Results indicated that the antithrombogenicity of modified PE was improved remarkably. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2014–2018, 2004     

Adhesion of calcium phosphate coatings on polyethylene (PE), polystyrene (PS), poly(tetrafluoroethylene) (PTFE), poly(dimethylsiloxane) (PDMS) and poly-L-lactic acid (PLLA)

Calcium phosphate (CaP) coatings can be applied to improve the biological performance of polymeric medical implants. For clinical applications, a strong adhesion of the coating to the polymeric substrate is important. Therefore, the adhesion of rf magnetron-sputter-deposited CaP coatings on five polymers was studied: polyethylene (PE), polystyrene (PS), poly(tetrafluoroethylene) (PTFE), poly-L-lactic acid (PLLA) and poly(dimethylsiloxane) (PDMS). To influence the adhesion, the interface was varied in six different ways, e. g. by a plasma or ion-beam pretreatment, or by using a Ti interlayer. The adhesion was determined by using scratch, tensile and 180^° bend tests. Especially the polymers PE and PS needed a bombardment by energetic particles prior to or during coating deposition, to enable the formation of chemical bonds between the coating and the polymer, which gave adhesion. On PLLA and PDMS, being oxygen containing polymers, it was easier to establish a strong interface. An overtreatment of the polymeric substrates gave worse adhesion, probably due to the formation of weak low molecular weight (LMW) layers on the polymer. On PTFE, the use of a Ti interlayer was necessary to prevent the PTFE from UV degradation during coating deposition, as this caused cohesive failure within the PTFE. The results showed that each polymer requires a different approach for obtaining optimal adhesion. The observed adhesion could often be explained in the terms of processes occurred during the pretreatment of the polymers or the deposition of the coating.     

Breaking polymer chains by dynamic plowing lithography

The adhesion of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films, whose surface has been previously structured by dynamic plowing lithography (DPL), has been measured by means of force-displacement curves. The different adhesion of modified and unmodified PS leads to the assumption that polymer chains are broken during DPL. After measuring the energy dissipated by the tip during DPL, in order to check that the transferred energy is sufficient to break covalent bonds, the polymer chain scission caused by the lithographic process has been definitely confirmed by size exclusion chromatography measurements of the lithographed films.     

Synthesis of biomembrane-mimic polymers with various phospholipid head groups

Lipid bilayers in biomembranes consist of diverse phospholipids, including phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) with various compositions according to the cell and tissue types. We synthesized biomembrane-mimic polymers, poly(2-methacryloyloxyethyl phosphoric acid) (PMPA), poly(2-methacryloyloxyethyl phosphorylethanolamine) (PMPE), and poly(2-methacryloyloxyethyl phosphorylserine) (PMPS), with PA, PE, and PS head groups, respectively. PA monomer was synthesized from 2-hydroxyethyl methacrylate (HEMA) and dimethyl chlorophosphate (DCP). PE and PS monomers were synthesized from N-tert-butoxycarbonyl (tBoc) protected ethanolamine and serine through the reaction with 2-chloro-2-oxo-1,3,2-dioxaphospholane (COP). Each biomembrane-mimic polymer was successfully synthesized by atom transfer radical polymerization (ATRP) from the monomer. The molecular weight distributions of PMPA, PMPE, and PMPS were analyzed by gel permeation chromatography (GPC) and in vitro cytotoxicity was also examined by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assay. The new biomembrane-mimic polymers could be used to prepare a polymeric platform that mimic a cell- or tissue-specific membrane for future applications in biomedical fields such as tissue engineering or bioimplants.     

Cured melamine systems as thick fire-retardant layers deposited by combination of plasma technology and dip-coating

Melamine and melamine resins are widely used as fire-retardants for polymer building materials. Cured melamine systems are used in heat-sensitive items, such as furniture and window frames and sills. In this work, differently cured methylated poly(melamine-co-formaldehyde) (cmPMF) resins were used as fire-retardant coverage for poly(styrene) (PS) and poly(ethylene) (PE) building materials. Such polymer layers should have several tenths of micrometers thickness to produce sufficient fire retardancy. These thick layers were produced by dip-coating. To promote sufficient adhesion of such thick coating to the polyolefin substrates, also in the case of high temperatures occurring at fire exposure, the polymer substrates were firstly coated with a few hundred nanometer thick adhesion-promoting plasma polymer layer. Such thin plasma polymer layers were deposited by low-pressure plasma polymerization of allyl alcohol (ppAAl). It was assumed that the hydroxyl groups of ppAAl interact with the melamine resin; therefore, ppAAl was well suited as adhesion promoter for thick melamine resin coatings. Chemical structure and composition of polymer films were investigated using infrared-attenuated total reflectance and X-ray photoelectron spectroscopy (XPS). Peel strengths of coatings were measured. After peeling, the peeled polymer surfaces were also investigated using optical microscopy and XPS the layers for identification of the locus of peel front propagation. Thermal properties were analyzed using TGA (thermo-gravimetric analyses). Finally, the fire-retardant properties of such thick coated polymers were evaluated by exposure to flames.     

The role of surface modification in digital printing on polymer‐coated packaging boards

Digital printing is increasingly being used for package printing. One of the major techniques of digital printing is dry‐toner electrophotography. This paper evaluates the printability of three different extrusion coatings used for packaging boards: low‐density polyethylene (PE‐LD), ethylene methyl acrylate (E/MA) and polyethylene terephthalate (PET). Extrusion coatings in general have an impervious, chemically inert, nonporous surface with low surface energies that cause them to be non‐receptive to bonding with toners. The most common methods used in improving the adhesion properties of polymer coatings are different surface treatments. These increase the surface energy and also provide the polar molecular groups necessary for good bonds between the toner and polymer molecules. The polymer coatings have been modified with electrical corona discharge treatment. The effects of corona on polymer surfaces and the correlation between surface modification and print quality have been evaluated. Results show that sufficiently high surface energy and surface‐charge uniformity are necessary for even print quality and toner adhesion. E/MA and PET have the required surface‐energy level without the corona treatment, but PE‐LD needs surface modification in order to succeed in the electrophotographic process. E/MA also has exceptional surface‐charge properties compared with PET and PE‐LD. Polym. Eng. Sci. 44:2052–2060, 2004. © 2004 Society of Plastics Engineers.     

PROBING HIDDEN POLYMERIC INTERFACES USINGIR–VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR–visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer–air, polymer–solid, and polymer–polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer–polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

Wetting and adhesion of water-borne printing inks on surface-modified polyolefins

Polyolefin films were surface-modified by different methods to improve the wetting and adhesion of water-borne printing inks. Polyethylene (PE) films were treated with corona at various energy levels. Surface-modified PE films were characterized by contact angle measurements and electron spectroscopy for chemical analysis (ESCA). Good wetting was already achieved with treatment at a lower energy level. Various degrees of adhesion were obtained at various degrees of treatment. A hydrophilic monomer, 2-hydroxyethylmethacrylate (HEMA), was polymerized onto the surfaces of polypropylene (PP) with radiation-induced grafting, which was carried out at two different radiation doses. In both cases, a thick, visible layer of polyHEMA was formed on the surface of PP, and satisfactory wetting was already achieved at lower radiation doses. Scanning electron microscopy (SEM) showed that different degrees of roughness were achieved at various radiation doses. Like the case of corona-treated PE, different degrees of adhesion were obtained at different degrees of surface treatment. This study shows that improved wetting alone is not satisfactory for good practical adhesion', regardless of the surface modification method used.     

Modifying stainless steel surfaces with responsive polymers: effect of PS-PAA and PNIPAAM on cell adhesion and oil removal

The treatment of stainless steel surfaces with responsive polymers was investigated using X-ray photoelectron spectroscopy and scanning electron microscopy. Poly(N-isopropylacrylamide) (PNIPAAM) is a temperature-sensitive polymer which is soluble in water below 32°C (Lower Critical Solubility Temperature (LCST)) but collapses and aggregates above 32°C. This polymer was adsorbed at 50°C from solutions at different concentrations, followed by rinsing and different drying procedures. Thereby, a large variety of surface structures could be obtained, from smooth films to surfaces showing marked relief features due to residues of PNIPAAM aggregates. Two poly(styrene)-b-poly(acrylic acid), PS-PAA, with different block lengths, were spin-coated on stainless steel and showed preferential exposure of PS blocks at the outermost surface. PNIPAAM-conditioning was shown to strongly reduce yeast cell adhesion and to facilitate the removal of oil soil due to its high water affinity and chain mobility below the LCST. PS-PAA coatings also reduced yeast cell adhesion; this may partly be due to a reorganization of the surface, leading to exposure of PAA chains in contact with water and thus to electrostatic repulsion of the yeast cells.     

PROBING HIDDEN POLYMERIC INTERFACES USINGIR-VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR-visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer-air, polymer-solid, and polymer-polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer-polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

Chemical modification combined with corona treatment of UHMWPE fibers and their adhesion to vinylester resin

The influence of corona treatment on the near-surface structures of treated ultra-high-molecular-weight polyethylene (UHMWPE) fibers was studied first by atomic force microscopy (AFM). AFM pictures showed that the pits on the corona-treated PE fiber surfaces had different change characteristics in depth compared with in length and breadth with variations of corona power. Then the UHMWPE fibers were subjected to chemical modification following the corona treatment, named the two-stage treatment. Surface morphologies and chemical properties of the treated fibers were analyzed by scanning electron microscopy (SEM), FT-IR–ATR spectroscopy and Raman spectroscopy. The results obtained suggested that some carbon–carbon double bonds had been introduced on the surfaces of the PE fibers after the two-stage treatment. These unsaturated groups could participate in free-radical curing of vinylester resin (VER), and this resulted in improvement of interfacial adhesion strength in the PE fiber/VER composites. In addition, the mechanical properties of the UHMWPE fibers reduced after corona treatment did not reduce further after subsequent chemical treatment with increase of corona power. In short, the two-stage treatment proved to be effective in improving the interfacial adhesion of the composites and maintaining the high mechanical properties of the PE fibers, as this treatment method did not destroy the bulk structure of the UHMWPE fibers.     

Fundamental study on the development of a surgical device for polymer‐tissue adhesion using vibration damping of polymeric materials

To develop a surgical handheld device that can be used to promote polymer‐tissue or tissue‐tissue adhesion, we designed a polymeric clamp material (PCM) that self‐heats as a result of vibration. By using the PCM, heat can be applied to the target biomaterial and the tissue simultaneously. The optimal temperature is high enough to promote adhesion but low enough to retain the native tissue's integrity. Furthermore, the PCM should not adhere to the target polymer or the native tissue. We found that the temperatures of fluorinated polymers, such as poly(tetrafluoroethylene) (PTFE) and perfluoroalkoxy (PFA), increased within 60 s to 150°C and maintained a stable temperature thereafter. The heat that was transferred to the saucer attached to the potential PCM was slightly above 100°C, a temperature that promotes adhesion but does not damage the native tissue. No deformation or melting was observed during the experiment, indicating that PTFE or PFA possess desirable PCM characteristics for use as a surgical heating device. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2532–2537, 2013     

Model filled polymers

Summary Polyphenylene vinylene (PPV) coated polystyrene (PS) beads which have moderate conductivity when doped were prepared by mixing monodisperse crosslinked PS beads, the surfaces of which had been sulfonated to render them anionic, with cationic PPV precursor polymers. Two different PPV precursor polymers, poly (p-xylylidene tetrathiophenium chloride) and poly (p-xylylene--dimethylsulphonium chloride), were employed. Monodisperse crosslinked PS beads were sulfonated in the gas phase using fuming sulfuric acid to yield the surface activated monodisperse polystyrene sulfonic acid (PSSA) beads. Chemical doping with AsF₅, of pellets prepared by pressing the coated beads resulted in conductivities as high as 10^-1S/cm. The integrity of the polymer beads was determined by Scanning Electron Microscopy (SEM) and arsenic was found throughout the samples by examing fracture surfaces of the pressed coated pellets using EDXS.     

Effect of γ‐irradiation on the physical properties and dyeability of poly(vinyl butyral) blends with polystyrene and poly(ethylene glycol)

Cast films of polymer blends essentially based on poly(vinyl butyral) (PVB) and equal ratios of polystyrene (PS) and poly(ethylene glycol) (PEG) were prepared from benzene and butyl alcohol solutions of the individual polymers. The effect of γ‐irradiation on the thermal decomposition and tensile mechanical properties was investigated. Moreover, the effect of γ‐irradiation on the dye affinity of PVB/PS and PVB/PEG for basic and acid dyestuffs was studied. The thermogravimetric analysis (TGA) study showed that the unirradiated PVB polymer films prepared in benzene displayed higher thermal stability than the same polymer films prepared in butanol. However, in all cases the thermal stability was found to increase with increasing γ‐irradiation dose. On the other hand, PVB/PS blend possesses higher thermal stability than PVB/PEG, as shown from the determination of the weight loss (%) at different heating temperatures, the temperatures of the maximum rate of reaction and the activation energy. While, pure PS films showed the stress‐strain behavior of brittle polymers, PVB/PS films showed the behavior of tough polymers with yielding properties. The results of dyeing clearly showed that the solvent type, blend composition, and irradiation dose are determining factors for the dye affinity for basic or acid dyes. For example, unirradiated PVB films prepared from butanol displayed a higher affinity for the basic and acid dyes than the same polymer prepared from the same benzene. However, PVB prepared from butanol showed higher affinity to the dyes than PS prepared from the same solvent. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Piezoelectric inkjet printing of polymers: Stem cell patterning on polymer substrates

Generating patterns of cells on surfaces is of great significance not only for fundamental studies in biomedical science but also for the creation of functional customized tissue or organs in regenerative medicine. In this paper, arbitrary, complex stem cell patterns were created using piezoelectric inkjet printing of biocompatible polymers. After a systematic study with different inkjet process variables, various poly(lactic-co-glycolic acid) (PLGA) patterns were fabricated on a polystyrene (PS) substrate. Human adipose-derived stem cells (hASCs) were isolated from subcutaneous adipose tissue and were seeded on the PLGA-patterned PS substrate which consists of areas either favorable (PLGA) or unfavorable (bare PS) to cell adhesion. The hASC stably attached, proliferated within the PLGA patterns, thus, complex and confluent hASC patterns were created. Polymer micro-patterning by inkjet printing could be an effective technique to control cell adhesion geometry, leading to arbitrary cell patterning on surfaces.     

Surface modification of ethylene copolymers moulded against different mould surfaces-1. Surface enrichment by functional groups

A polymer surface chemical composition can be changed by the influence of different environments. Results presented from this study show that the surface of the mould influences the outermost polymer surface by enriching it with specific functional groups. This was done by moulding random copolymers against polymer films with low and high surface energies. The values presented are interpreted in terms of differences in surface energy between the mould surface and the copolymer. The random copolymers used were poly(ethylene-co-vinylacetate) (EVA) and poly(ethylene-co-acrylic acid) (EAA), both with a different comonomer content. The copolymers were moulded in contact with mould surfaces made of polymer films which were perfluorinated ethylene propylene copolymer (FEP), poly(tetrafluoroethylene) (PTFE), and poly(ethylene terephthalate) (PET). The resultant surfaces were characterized by X-ray photoelectron spectroscopy (XPS or ESCA) and contact angle measurements The surface content of acrylic acid functional groups increased in the case of EAA copolymer moulded against PET, and decreased when moulded against FEP as compared to the bulk concentration. EVA copolymers were found to be enriched in acetate groups when moulded against FEP and deficient when moulded against PET. The contact angle measurements together with the XPS measurements showed significant differences between materials moulded in contact with low and high energy surfaces. A low molecular weight additive (an internal release agent), in an EVA copolymer, was found to be enriched at the moulded polymer surface when a PET film was used as mould surface. A material transfer was also found to occur from the solid polymer films to the moulded polymer surface.     

Effect of polymer structure on the morphologies and dielectric properties of nanoporous polyimide films

Low dielectric constant polyimide (PI) films have potential applications in integrated circuit. In this study, poly(methyl methacrylate), poly(ethylene oxide), and polystyrene as thermally labile materials were used as templates to generate PI films with nanopores by first mixing the polymer templates with the precursor of PI, poly(amic acid), followed by imidization of poly(amic acid) together with degradation of the polymer templates. The sizes of the formed pores, the thermal and dielectric constant of the nanofoamed PI films were studied and compared in detail. It is concluded that the dielectric constant of PI films using poly(ethylene oxide) as pore template is more stable because of the formation of uniform pores which is from the great accordance of imidization temperature of poly(amic acid) with the degradation temperature of poly(ethylene oxide). But that using poly(methyl methacrylate) as pore template is frequency dependent as the influence of inhomogeneous pores and PMMA residue from incompletely degradation of poly(methyl methacrylate). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41480.     

Proliferation rate of fibroblast cells on polyethylene surfaces with wettability gradient

Surface wettability on anchorage‐dependent cells has an important role in cell growth rate. In our previous studies, we prepared a wettability gradient on polyethylene (PE) surfaces using corona discharge treatment from a knife‐type electrode whose power increased gradually along the sample length. The PE surfaces were oxidized gradually with increasing power and characterized by Fourier transform infrared spectroscopy, contact angle goniometry, and electron spectroscopy for chemical analysis. The purpose of this study is to determine the rate of proliferation on polymer surfaces with different wettability. The behavior of cell growth for NIH/3T3 fibroblast cells attached on the polymer surfaces with different hydrophilicity was investigated using wettability gradient PE surfaces prepared by corona discharge treatment. They were investigated for the number of grown cells from 24 to 60 h in terms of surface wettability. From the slope of cell number on PE gradient surface versus culture time, the proliferation rates (number of cell/cm² · h) were calculated. It was observed that the proliferation rate was increased more on positions with moderate hydrophilicity of the wettability gradient surface than on the more hydrophobic or hydrophilic positions, i.e., 1111 (number of cell/cm² · h) of 57° of water contact angle at the 2.5‐cm position (P < 0.05). This result seems closely related to the serum protein adsorption on the surface: the serum proteins were also adsorbed more on the moderately hydrophilic surface. In conclusion, surface wettability plays an important role in cell adhesion, spreading, and proliferation on the polymer surfaces. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 599–606, 2004     

Surface energy of corona treated PP,PE and PET films,its alteration as function of storage time and the effect of various corona dosages on their bond strength after lamination

The aim of this study was to analyze how corona dosages above recommended levels affect film surface energy and hydrophobic recovery of such treated film surfaces as well as laminate bond strength of laminates made of these films. The adhesive for lamination was a polyurethane‐adhesive with a dry film thickness of ～5 µm. Polar and dispersive parts of the surface energy were measured frequently according to DIN 55660‐2 (Owens–Wendt–Rabel‐and‐Kaelble method) for up to 140 days after corona treatment. The corona dosage had a value of up to 280 W min/m². Laminate bond strength was measured according to DIN 55543‐5. The effect of corona treatment was highest for low‐density polyethylene (PE‐LD) films, mean for biaxial‐oriented polypropylene (PP‐BO) films, and lowest for biaxial‐oriented poly(ethylene terephthalate) (PET‐BO) films. With increasing storage time, surface energy decreased, as expected. The higher the effect of corona treatment, the faster the polar part of surface energy decreased. At PE‐LD, laminate bond strength increased with a higher corona dosage from 0.05 to 8.87 mN/15 mm, whereas at PET‐BO and PP‐BO laminate bond strength was so high that samples teared before delamination during bond strength testing. By our results is shown that corona dosages above recommended levels resulted in higher laminate bond strength. Only at PP‐BO a reduction of laminate bond strength due to “overtreatment” was be observed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45842.