Imelda Keen

Control of the orientation of symmetric poly(styrene)-block-poly(D,L-lactide) block copolymers using statistical copolymers of dissimilar composition

The interactions of block copolymers with surfaces can be controlled by coating those surfaces with appropriate statistical copolymers. Usually, a statistical copolymer comprised of monomer units identical to those of the block copolymer is used; that is, typically a poly(styrene)-stat-poly(methyl methacrylate) (PS-stat-PMMA) is used to direct the alignment of poly(styrene)-block-poly(methyl methacrylate) (PS-block-PMMA), and poly(styrene)-stat-poly(2-vinylpyridine) (PS-stat-P2VP) has been used for poly(styrene)-block-poly(2-vinylpyridine) (PS-block-P2VP). Reports of controlling the orientation of block copolymers with statistical copolymers with a dissimilar composition are limited. Here, we demonstrate that this method can be further extended to show that PS-stat-PMMA can be used to control the wetting properties of poly(styrene)-block-poly(D,L-lactide) (PS-block-PDLA). Surfaces were modified with a series of cross-linked PS-stat-PMMA-stat-glycidyl methacrylate terpolymers, and the surface chemistries and energies were assessed using angle-dependent X-ray photoelectron spectroscopy and the two-liquid harmonic method, respectively. From these experiments, an expected neutral compositional window was identified for symmetrical PS-block-PDLA. Moreover, high-resolution SEM, AD-XPS, and grazing-incidence SAXS measurements were used to evaluate the morphology of PS-block-PDLA as a function of the surface composition of the underlying cross-linked copolymer films, and the neutral composition was found to range from 32 to 38 mol % of PS, in the bulk polymer. Ultimately, we demonstrated the determination of nonpreferential surface compositions that allow the self-assembly of lamellae with sizes in the sub-10 nm regime that are oriented perpendicular to the substrate. These findings have important implications for the use of PS-block-PDLA block copolymers in directed self-assembly, most specifically in advanced lithographic processes.

Characterization of Fibers by Raman Microprobe Spectroscopy

A set of fibers, mainly synthetic, has been examined by Raman microprobe spectroscopy. It was found that high quality spectra, requiring no sample preparation, are easy to obtain and that fibers with different molecular structure have very different Raman spectra. Fluorescence was occasionally a problem with excitation at 632.8 nm, but could be significantly reduced by using a semiconductor laser emitting at 780 nm. Fibers of the same polymer type from different manufacturers have Raman spectra which are only slightly different but which could be distinguished by the multivariate statistical technique of principal components analysis (PCA). Dyed fibers gave spectra with bands due to the polymer, but also with intense bands due to the dye, because of the high Raman cross-section of dye molecules. Extraction of the dye, followed by a spectral subtraction procedure allowed separate spectra of the polymer and the dye to be obtained.

Surface Plasma Modification of LLDPE for Biomedical Applications

Linear low density polyethylene (LLDPE) surface was modified by water plasma treatment to functionalized with oxygen-containing functional groups and to improve wettability. The LLDPE surface was treated at 10 and 20 W discharge power at various exposure times. A laboratory scale Megatherm radio frequency (RF) plasma apparatus that operates at 27 MHz was used to generate the water plasmas. Comparative studies were also made on LLDPE by using Argon plasma discharge followed by exposure to oxygen. The changes in chemical structure of the LLDPE polymeric chain upon plasma treatment were characterized by FTIR and XPS techniques. The selectivity of trifluoroacetic anhydride (TFAA) toward hydroxyl groups is used to quantify the hydroxyl groups formed on the polymer surface upon plasma treatment. The surface wettability of the samples was evaluated by measuring water contact angle of the samples before and after modification. In an attempt to understand the effect of surface modification of polymers on organopolysiloxane coating, selected samples were coated with SIGMACOTE. After exposition to the plasma discharge a decline in water contact angle were observed. FTIR and XPS measurements indicate an oxidation of degraded polymeric chains and creation of hydroxyl, carbonyl, ether, ester and carboxyl groups. Chemical derivatization with TFAA of water plasma treated polymer surfaces has shown that under the conditions employed, a very small (less than 5%) of the oxygen introduced by the water plasma treatment was present as hydroxyl group. The XPS results revealed that, under the plasma condition utilized, the surface modification of LLDPE using water plasma improves the wetting of polysiloxane onto the LLDPE surface.

EUVL compatible LER solutions using functional block copolymers

Directed self assembly (DSA) of block copolymers is an emerging technology for achieving sub-lithographic resolution. We investigate the directed self assembly of two systems, polystyrene-block-poly-DL-lactic acid (PS-b-PDLA) and PSb- poly(methyl methacrylate). For the PS-b-PDLA system we use an open source EUVL resist and a commerciallyavailable underlayer to prepare templates for DSA. We investigate the morphology of the phase separated domains and compare the LER of the resist and the PS-PDLA interface. For the PS-b-PMMA system we again use an open source resist, but the annealing conditions in this case require crosslinking of the resist prior to deposition of the block copolymer. For this system we also investigate the morphology of the phase separated domains and compare the LER of the resist and the PS-PMMA interface.

Raman and Infrared Microspectroscopic Mapping of Plasma-Treated and Grafted Polymer Surfaces

The grafting of polystyrene (PS) onto a predominantly polypropylene (PP) substrate has been followed by Raman and infrared microspectroscopic mapping. For exactly the same 50 μm × 50 μm section of polymer surface, Raman spectra were obtained at 1 μm intervals for the substrate, the surface after plasma treatment, and the surface after PS grafting. Maps of the substrate were constructed, indicating the crystallinity variation across the surface and also the distribution of the minor component ethylene-propylene rubber (EPR). After plasma treatment, the crystallinity was found to decrease slightly. Infrared microspectroscopic maps of a larger plasma-treated surface were also obtained with the technique of attenuated total reflection. The spatial resolution of these maps was 50 μm, and they showed the distribution of the hydroxy groups introduced onto the surface by the plasma treatment. PS grafting was found to be heterogeneous. Increased concentrations of grafted PS showed some correlation with positions on the surface which had higher EPR after plasma treatment.

A structural study of hybrid organosilica materials for colloid-based DNA biosensors

Organosilane hybrid materials are of interest in the development of diagnostic devices and drug-delivery applications. Here we report a spectroscopic study involving the chemical and structural modification of thiol-functionalised organosilica particles with aminosilane to produce a bifunctional silica hybrid. The aminosilane was revealed to be distributed throughout the microsphere as opposed to being surface-localised as is commonly reported for modifications of pure silica. Spectroscopic methods including NMR, XPS, Ninhydrin and gravimetric measurements were employed to investigate the surface and internal elemental composition of the particles independently. A multiplexed model bioassay is presented to demonstrate the advantage of organosilane bifunctionality, enabling separate covalent attachment strategies for both homogeneous incorporation of fluorescent dyes and surface-specific biomolecule attachment. This study represents an advance in the understanding of organosilane chemistry resulting in versatile materials with a range of functionalities for covalent attachment.

PHEMA Hydrogels Modified through the Grafting of Phosphate Groups by ATRP Support the Attachment and Growth of Human Corneal Epithelial Cells

Converting the surface of poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel into a cell-adhesive surface has been successfully achieved through a method based on atom transfer radical polymerization (ATRP) grafting. Following activation of the surface hydroxyl groups of PHEMA by bromination, surface-initiated ATRP of mono(2-methacryloyloxyethyl) phosphate (MMEP) was conducted in a methanol—water system with Cu(I)Br as catalyst at room temperature. The conversion of PHEMA hydroxyl groups to brominated isobutyryl groups and the occurrence of grafting of PMMEP were confirmed by infrared and X-ray photoelectron spectroscopies. Cell attachment experiments were conducted by culturing human corneal limbal epithelial cells on the PMMEP-grafted PHEMA, and on unmodified PHEMA and tissue culture plastic for comparison. The results showed that the grafted PMMEP was homogeneously distributed, and the phosphate groups appeared to significantly promote the attachment, spreading and growth of cells, at a level comparable to the tissue culture plastic.

Investigations into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) surface properties causing delayed osteoblast growth.

Osteoblast proliferation is sensitive to material surface properties. In this study, the proliferation of MC3T3 E1-S14 osteoblastic cells on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) films with different surface characteristics was investigated with the aim of evaluating the cause of a lag in cell growth previously observed. The solvent-cast films were prepared using three different solvents/solvent mixtures which produced PHBV films with both a rough (at the air interface) and smooth (at the glass interface) surface. Investigation of the surface roughness by scanning electron and scanning probe microscopy revealed that the surfaces had features that were different in both average lateral size and average amplitude (R a 20–200 nm). Water contact angles showed that all surfaces were hydrophobic in nature ( A in the range 69–82°). The lateral distribution of surface crystallinity of the films was evaluated by use of micro-attenuated total reflectance Fourier transform infrared (ATR–FT-IR) by determining the surface crystallinity index (CI) which was found to differ between samples. MC3T3-E1-S14 osteoblasts were cultured on the six surfaces and proliferation was determined. After 2 days, cell proliferation on all surfaces was significantly less than on the control substrate; however, after 4 days cell proliferation was optimal on three surfaces. It was concluded that the initial lag on all substrates was due to the hydrophobic nature of the substrates. The ability of the cells to recover on the materials was attributed to the degree of heterogeneity of the crystallinity and surface roughness: samples with a roughness of 80 nm were found to support cell proliferation. In addition, the lateral surface features influenced the proliferation of osteoblasts on the PHBV film surface.

Raman Microspectroscopic Mapping: A Tool for the Characterisation of Polymer Surfaces

Raman mapping by point illumination of polymer surfaces is discussed with examples taken from the plasma treatment of polypropylene (PP) and subsequent grafting of polystyrene (PS). Maps can be constructed for surface properties such as crystallinity, blend components and distribution of grafted PS. The Raman sampling volume was estimated for confocal operation using a 50x objective lens.

Degradable Hydrogels for Tissue Engineering – Part I: Synthesis by RAFT Polymerization and Characterization of PHEMA Containing Enzymatically Degradable Crosslinks

A nonapeptide, which is sensitive to enzymatic digestion by collagenase, was modified by the covalent attachment of an acrylamido group at the terminal positions. The functionalized peptide was used as a crosslinking agent during polymerization of 2-hydroxyethyl methacrylate (HEMA). Reversible addition-fragmentation chain transfer (RAFT) method was used to obtain a polymer (PHEMA) with an average theoretical molecular weight of 4000 Da, containing enzymatically labile peptide crosslinks. The functionalized peptide was analyzed in detail by 1H and 13C nuclear magnetic resonance (NMR) spectrometry. The polymerization reaction was monitored by near infrared spectrometry, while the resulting polymer was analyzed by size exclusion chromatography and solid NMR spectrometry. The peptide-crosslinked PHEMA was subjected to an in-vitro degradation assay in the presence of collagenase. At the highest concentration of enzyme used in the study, a weight loss of 35% was recorded after 60 days of incubation in the collagenolytic medium. This suggests that crosslinking with enzymatically degradable peptides is a valid method for inducing biodegradability in polymers that otherwise are not degradable.