Tethering macromolecules to surfaces represents a versatile approach for functionalizing, protecting, and structuring both organic and inorganic materials. In this study, thin films of poly(acrylamide) (PAAm) brushes and covalently cross-linked hydrogel brushes were grown from iniferter-functionalized silicon substrates by UVLED-initiated photopolymerization and their properties subsequently studied by means of a variety of analytical methods. The employed photografting method allowed the controlled fabrication of very thick films (up to 1 μm) in an aqueous environment, over a period of less than 1 h of polymerization and in the absence of side reactions. PAAm covalently cross-linked hydrogel brushes were prepared by feeding trace amounts of the cross-linker bis(acrylamide) (up to 1.0 wt % of monomer solution) into the reaction vessel. Both bulk and interfacial properties of these polymer films were found to be strongly influenced by lateral cross-linking of the grafted polymer chains. In agreement with theoretical expectations, the decrease of polymer-brush conformational freedom with increasing cross-link density resulted in a substantial increase of film wettability with water. The swelling ratio of the hydrogel brushes, as measured by ellipsometry and atomic force microscopy (AFM), also confirmed the formation of grafted networks and was found to be directly related to the amount of cross-linker in the monomer feed. In addition, the Young’s moduli and tribological properties of PAAm brushes and hydrogel brushes were tuned by adjusting the cross-linker concentration. Because of the additional constraint given by the surface tethering of each chain end, intermolecular cross-linking generated very high mechanical stresses within the brush structure. Covalently cross-linked hydrogel brushes thus displayed higher Young’s moduli and coefficients of friction, when compared to the grafted polymer-brush analogues. These hydrogel brushes present an opportunity for readily tailoring physical properties, especially as they allow tuning of the physical characteristics of surfaces while maintaining the interfacial chemical composition nearly constant.

The development of surface coatings with well-defi ned architecture is of particular interest in several disciplines, notably in biomaterials. Bio-organic layers on different length-scales, featuring variable composition, structure and modulus have been observed in several biological systems, such as human cartilage, mammalian skin and the nacre of oyster shells. [1][2][3] These complex systems often consist of mechanically graded structures that resist and respond to external normal and shear forces, and therefore protect underlying tissues from incurring any damage. As an example, the human epidermis consists of keratinizing epithelial cells-an elastic layer with low modulus-buried under a layer of dead cells, the stratum corneum , with higher modulus. The stratifi ed epithelium thus acts as the body's major barrier against abrasion and inhospitable environments. [ 4,5 ] In order to mimic mechanically useful structures found in nature, materials scientists have been made numerous efforts to fabricate coatings with discontinuous mechanical properties at pre-determined depths in a single multilayered architecture. These materials, unlike homogenous fi lms, have been shown to successfully redistribute contact stresses at the interface and to resist failure-inducing contact deformations. [ 6,7 ] Progress has also been made in fabricating graded morphologies of organic/ inorganic nanocomposites, thereby mimicking the approach of natural counterparts, to obtain coatings with higher toughness and modulus. [ 8 ] Despite all these efforts, the fabrication of a full-organic, polymer-based coating architecture featuring nanoscale variations of properties still represents a challenging task in polymer and materials science.The development of surface-initiated polymerization (SIP) techniques [ 9 ] has provided a promising path towards the stepwise fabrication of stratifi ed polymer fi lms. To this end, controlled-radical SIP methods incorporating sequential polymerization processes can be applied. [ 10 ] With the exception of the pioneering work of Tsukruk et al., [ 11 ] which introduced prototypes of stratifi ed polymeric fi lms, few such investigations have been reported. Moreover, the full characterization of stratifi ed fi lms, i.e. the physical and chemical analysis of the fi lm properties at different depths (for each stratum), on the supporting substrate certainly represents a challenging experimental task. Although real-space analytical techniques, such as atomic force microscopy (AFM), enable access to surface properties in two dimensions, determination of physical properties in the third dimension (from surface to bulk) can be more readily achieved by reciprocal-space-based techniques, such as grazing-incidence, small-angle scattering (GISAS), or X-ray and neutron refl ectivity-these methods are, at present, among the few that could determine the density profi le of materials perpendicular to the sample surface. However, probing the vertical architecture of polymeric nanofi lms with such techniques normally requir...

Random copolymers of 4-vinylbenzyl tri(oxyethylene) and tetra(oxyethylene) ethers, as well as alternating copolymers of 4-vinylbenzyl methoxytetra(oxyethylene) ether and a series of N-substituted maleimides, were synthesised by conventional free radical polymerisation, reversible addition fragmentation chain transfer (RAFT) and atom transfer radical polymerisation (ATRP). Their thermosensitive behaviour in aqueous solution was studied by turbidimetry and dynamic light scattering. Depending on the copolymer composition, a LCST type phase transition was observed in water. The transition temperature of the obtained random as well as alternating copolymers could be varied within a broad temperature window. In the case of the random copolymers, transition temperatures could be easily fine-tuned, as they showed a linear dependence on the copolymer composition, and were additionally modified by the nature of the polymer end-groups. Alternating copolymers were extremely versatile for implementing a broad range of variations of the phase transition temperatures. Further, while alternating copolymers derived from 4-vinylbenzyl methoxytetra(oxyethylene) ether and maleimides with small hydrophobic side chains underwent macroscopic phase separation when dissolved in water and heated above their cloud point, the incorporation of maleimides bearing larger hydrophobic substituents resulted in the formation of mesoglobules above the phase transition temperature, with hydrodynamic diameters of less than 100 nm

Zwitterionic ring-opening polymerization (ZROP) of Nbutyl N-carboxyanhydrides (Bu-NCAs) has been investigated using 1,8diazabicycloundec-7-ene (DBU), a bicyclic amidine initiator. It was found that poly(N-butylglycine)s (PNBGs) with molecular weight (M n ) in the 3.5−32.4 kg mol −1 range and polydispersity index (PDI) in the 1.02−1.12 range can be readily obtained by systematically varying the initial monomer to initiator feed ratio. The polymerization exhibits characteristics of a controlled polymerization, as evidenced by the linear increase of polymer molecular weight with conversion and the successful enchainment experiments. Kinetic studies revealed that the reaction is first-order dependent on the monomer and the DBU concentration. The rate of initiation is comparable to that of the propagation. Random copolypeptoids of poly[(N-propargylglycine)-r-(N-butylglycine)]s [P-(NPgG-r-NBG)s] were also synthesized by DBU-mediated copolymerization of Bu-NCA and N-propargyl N-carboxyanhydride (Pg-NCA). Subsequent grafting with azido-terminated poly(ethylene glycol) (PEG) produces bottlebrush copolymers. Analysis of bottlebrush copolymer samples using atomic force microscopy (AFM) revealed a surface morphology of toroid-shaped nanostructures, consistent with the polypeptoid backbone having cyclic architecture. Small-angle neutron scattering (SANS) characterization of the bottlebrush polymer ensemble in solution also confirms the cyclic architecture of the polypeptoid backbones.