Molecular engineering of designer surfaces by controlled radical polymerizations: brushes, hedges and hybrid grafts
The aim of the work described in this Thesis was to develop new fabrication techniques based on surface-initiated controlled polymerization (SIP) for the preparation of functional polymeric platforms across the lengthscales. Several synthetic processes based on controlled radical SIP were employed for the preparation of polymer brush platforms which presented tunable characteristics and, in some instances, a stimulus-responsive behavior. The SIP methods employed were furthermore coupled with atomic force microscopy (AFM)-assisted nanolithographic approaches in order to accomplish the preparation of polymer grafts constituted by a limited number of macromolecules grown on pre-determined positions on surfaces.
Chapter 2 describes a broad overview of the most advanced methods for the structuring of surfaces using SIP. Representative examples of surface fabrication using SIP were reported highlighting the cases when the prepared grafted films presented tunable properties.
In Chapter 3 the SIP method based on initiator-transfer-terminator (iniferter) was introduced focusing on its kinetic aspects. Photografting of poly(methacrylic acid) (PMAA) using mixed initiating systems based on SAMs was investigated, demonstrating how the fine tuning of the composition of the starting layers (initiator surface coverage) could influence the morphology and the properties of the grafted films.
The growth rate of the PMAA films did not show a significant dependence on the composition of the starting self-assembled monolayers (SAMs), implying that radical recombination reactions played a substantial role in the surface grafting process. Comparing the extreme cases of PMAA grafts from full SAMs of initiators and from highly diluted SAMs, important morphological differences, different swelling and mechanical properties were found. In addition, in situ quartz crystal microbalance with dissipation (QCM-D) measurements confirmed the different grafting kinetics for the two samples. The differences observed in morphology, swelling and mechanical properties were attributed to different graft structure originating from the reaction kinetics. Comparing the initial film mass growth and the viscoelastic properties for diluted and concentrated initiating systems more chains are initially grown in the case of concentrated SAMs of initiators. Nevertheless, due to termination reactions, this potential grafting efficiency did not turn into a higher thickening rate during the course of the polymerization with respect to films grown from diluted SAMs.
An example of the use of iniferter-based SIP for the synthesis of tunable brush platforms was given in Chapter 4. In this case DTCA photo-iniferter was used for the controlled surface grafting of micro-patterned temperature-responsive Poly-N(isopropylacrylamide) (PNIPAM) brushes. The brush height was easily controlled by the irradiation time through multiple subsequent irradiation stages. Temperature-induced changes in surface morphology and adhesion properties of the prepared PNIPAM brushes were monitored, for the first time, by in-situ AFM measurements. In addition, the inherent features of this SIP method were demonstrated to allow control of chain end functionalities as it was proven by a radical exchange test.
A further application of iniferter-mediated SIP was described in Chapter 5. Here the synthesis of RGD-functionalized PMAA brush films with variable structural characteristics was reported. PMAA brushes were first grafted from SAMs of initiators on gold surfaces and subsequently functionalized with the RGD cell-adhesive peptide sequence. These peptides were covalently immobilized both at the top of the brushes and within the brush layers following extension of the polymer chains by further polymerization step. After the preparation and characterization of the films, MG63 osteoblast cells were used to evaluate the effect of RGD positioning within the brush on cell adhesion. The results obtained demonstrated a good spread of cells with marked focal adhesion points at the periphery of the cytoplasm on samples with RGD motifs coupled on the surface, whereas in the case of the samples where RGD was buried, cells were found to adopt a rounded morphology and focal adhesions concentrated toward the internal part of the cell. A direct correlation between the vertical position of the RGD motif inside the brush architecture and cell morphology was consequently found.
Atom transfer radical polymerization (ATRP) was introduced in the following Chapters.
Chapter 6 describes the preparation of pH-responsive PHEMA-based brush-gels by surface-initiated ATRP and their successful application as matrixes for the controlled synthesis of silver nano-particles (Ag-NPs) was reported.
The swelling, mechanical and morphological properties of these brush films were investigated by AFM and they were demonstrated to be dependent on the amount of crosslinking agent used during their preparation. The solvent uptake at equilibrium markedly decreased for brush-gels if compared to the corresponding free brushes confirming the presence of a brush-network structure. In addition, AFM compression analysis showed enhanced mechanical resistance towards compression by the brush-gels and also the surface morphology of the films was found to be influenced by the presence of crosslinks.
The pH responsive behavior of the prepared layers was furthermore studied with AFM showing that the brush structures undergo a sharp transition around pH 8.
In the second part of this Chapter the use PHEMA-based brushes and brush-gels as supports for the controlled preparation of Ag NPs was described. From the results of AFM and UV-absorption spectroscopy a direct correlation between the characteristics of the Ag colloids and the type of brush architecture used was found. In particular, Ag NPs synthesized inside the brush-gels were found smaller, narrower distributed and with a more isotropic shape if compared to the case when the freely grafted brush was used as support.
In Chapter 7 the fabrication of grafted polymeric nano-structures (“hedges” and “dot” brushes) by coupling AFM-assisted lithography and controlled SIP was reported.
The iniferter-mediated preparation of PMAA “hedge” brushes grafted from Au nano-wires fabricated by dip-pen nanolithography-assisted reduction of Au salts was described. Following this procedure Au nano-structures with lateral size down to 20 nm were successfully fabricated. After functionalization of the metallic nano-wires with DTCA-initiator controlled grafting of limited number of PMAA chains was achieved. In the second part of the Chapter the preparation of pH-responsive PHEMA-based “hedge” and “dot” brushes by scanning-probe oxidation lithography (SPOL) and SIP was reported. Silicon oxide linear and dot-like nano-structures were fabricated through SPOL and served as site-specific anchoring platforms for the immobilization of initiators and the subsequent ATRP-based SIP of HEMA. Post-functionalization of the PHEMA grafts turned the grafted polymers into pH-responsive “hedge” and “dot” brushes with tunable swelling properties. The proposed step-wise fabrication technique was successfully used for the synthesis of stimulus-responsive polymeric grafts reaching sub-40 nanometers in dimensions.
Both these examples demonstrated the possibility of attaching few tens macromolecules placed in preselected positions at surfaces. The results reported inferred that further technical improvements of AFM lithography coupled with controlled SIP could lead to controlled grafting of stimuli-responsive single macromolecules. This achievement would eventually allow to scale down to the molecular level the dimensions of bio-sensors and stimuli-responsive systems based on polymer grafts.
The results presented in this Thesis illustrate that the fabrication of polymeric platforms using SIP-based methods represents a versatile and powerful surface engineering technique. Several examples of the preparation of chemically structured polymeric coatings aimed at a wide range of applications were given. Spanning from the synthesis of bio-interfaces to the preparation of polymer/metal hybrid surfaces, surface modification using grafted polymers has been revealed as a key enabling approach in materials science. In addition, our new approaches for the immobilization of macromolecules in predetermined positions at surfaces highlighted the high potential of the techniques used for surface engineering at the single molecular level. Several further developments of the reported studies could be carried out opening new possible routes for the fabrication of multifunctional polymeric films. In addition in depth studies of the phenomena described in some sections of this Thesis could allow an enhanced understanding of the physical behavior of brush platforms in certain conditions.