Dispersity within Brushes Plays a Major Role in Determining Their Interfacial Properties: The Case of Oligoxazoline-Based Graft Polymers.

Many synthetic polymers used to form polymer-brush films feature a main backbone with functional, oligomeric side chains. While the structure of such graft polymers mimics biomacromolecules to an extent, it lacks the monodispersity and structural purity present in nature. Here we demonstrate that side-chain heterogeneity within graft polymers significantly influences hydration and the occurrence of hydrophobic interactions in the subsequently formed brushes and consequently impacts fundamental interfacial properties. This is demonstrated for the case of poly(methacrylate)s (PMAs) presenting oligomeric side chains of different length (n) and dispersity. A precise tuning of brush structure was achieved by first synthesizing oligo(2-ethyl-2-oxazoline) methacrylates (OEOXMAs) by cationic ring-opening polymerization (CROP), subsequently purifying them into discrete macromonomers with distinct values of n by column chromatography, and finally obtaining poly[oligo(2-ethyl-2-oxazoline) methacrylate]s (POEOXMAs) by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Assembly of POEOXMA on Au surfaces yielded graft polymer brushes with different side-chain dispersities and lengths, whose properties were thoroughly investigated by a combination of variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCMD), and atomic force microscopy (AFM) methods. Side-chain dispersity, or dispersity within brushes, leads to assemblies that are more hydrated, less adhesive, and more lubricious and biopassive compared to analogous films obtained from graft polymers characterized by a homogeneous structure.

Ethyl-2-(tosylmethyl)acrylate: A promising chain transfer agent for the development of low-shrinkage dental composites.

OBJECTIVE: To evaluate the potential of ethyl-2-(tosylmethyl)acrylate (ASEE) as chain transfer agent for the development of low-shrinkage photopolymerizable dental composites. METHODS: Composites containing 10, 20 and 30 mol% of ASEE in their organic matrix were formulated. Camphorquinone (CQ)/ethyl 4-(dimethylamino)benzoate (EDAB) (0.33 wt%/0.60 wt%), CQ/EDAB/Ivocerin® (0.33 wt%/0.60 wt%/0.10, 0.25 or 0.50 wt%), CQ/EDAB/SpeedCure 938 (SC-938) (0.33 wt%/0.60 wt%/0.30, 0.50 or 1.00 wt%) and Ivocerin® (0.50 wt%) were used as photoinitiator systems. The glass transition temperature (Tg) and the crosslink density were determined by DMTA measurements. The flexural strength/modulus and ambient light working time were assessed according to ISO 4049. The shrinkage force was evaluated using a universal testing machine. The double bond conversion (DBC) was determined by NIR spectroscopy. DBC, flexural strength and modulus were measured after the storage of the specimens in deionized water at 37 °C for 24 h. The DBC, flexural strength and modulus data were analyzed by one-way ANOVA with p = 0.05 as significance level. RESULTS: ASEE-based composites containing the classical initiator system CQ/EDAB exhibited low mechanical properties (flexural strength/modulus) and DBC. The screening of various photoinitiator systems showed that composites based on CQ/EDAB/Ivocerin® (0.33 wt%/0.60 wt%/0.50 wt%), Ivocerin® (0.50 wt%) or CQ/EDAB/SC-938 (0.33 wt%/0.60 wt%/1.00 wt%) were particularly attractive. Indeed, the use of these photoinitiator systems enabled the formulation of composites containing up to 30 mol% ASEE exhibiting excellent mechanical properties, high DBC, good network homogeneity and low shrinkage force values. Interestingly, the addition of SC-938 did not impair the ambient light working time of the uncured composites, whereas the incorporation of 0.50 wt% Ivocerin® resulted in a strong decrease of this value. SIGNIFICANCE: The addition of the allyl sulfone ASEE in combination with the initiator system CQ/EDAB/SC-938 (0.33 wt%/ 0.60 wt%/ 1.00 wt%) is a promising strategy to develop low-shrinkage dental composites which exhibit excellent mechanical properties, low shrinkage force, high DBC and suitable ambient light working time.

Acylthiourea oligomers as promising reducing agents for dimethacrylate-based two-component dental materials.

OBJECTIVES: Currently used thiourea-based two-component dental materials may release bitter compounds if they are not properly cured. To address this issue, the objective of this study was to evaluate the potential of acylthiourea oligomers as reducing agents for the development of self-cure composites. METHODS: Acylthiourea oligomers ATUO1-3 were synthesized via cotelomerization of the acylthiourea methacrylate ATU1 with butyl methacrylate. They were characterized by 1H NMR spectroscopy and size exclusion chromatography. Self-cure composites based on the redox initiator system cumene hydroperoxide/acylthiourea oligomer/copper(II) acetylacetonate were formulated. The flexural strength and modulus were measured using a three-point bending setup. The double bond conversions were determined using NIR spectroscopy. The working time of each self-cure composite was measured using an oscillating rheometer. Leaching experiments using light-cure composites were performed in DMSO-d6. RESULTS: Acylthiourea oligomers ATUO1-3 were successfully synthesized in good yields. Both the oligomer molecular weight and the amount of thiourea groups were varied. Self-cure composites containing ATUO1 or ATUO2 as reducing agents exhibited excellent mechanical properties and high double-bond conversions. The amounts of reducing agent, cumene hydroperoxide and copper(II) acetylacetonate were shown to have a significant impact on the working time. Moreover, a correlation between flexural modulus and the amount of metal salt was clearly established. Self-cure composites containing the oligomer ATUO1 exhibited a longer working time than materials containing ATU1 or acetylthiourea. Contrary to acetylthiourea, ATUO1 was not able to leach out of light-cured composites. SIGNIFICANCE: Acylthiourea oligomers are promising reducing agents for the formulation of two-component dental materials that do not induce a bitter taste in mouth.

Functional Nanoassemblies of Cyclic Polymers Show Amplified Responsiveness and Enhanced Protein-Binding Ability.

The physicochemical properties of cyclic polymer adsorbates are significantly influenced by the steric and conformational constraints introduced during their cyclization. These translate into a marked difference in interfacial properties between cyclic polymers and their linear counterparts when they are grafted onto surfaces yielding nanoassemblies or polymer brushes. This difference is particularly clear in the case of cyclic polymer brushes that are designed to chemically interact with the surrounding environment, for instance, by associating with biological components present in the medium, or, alternatively, through a response to a chemical stimulus by a significant change in their properties. The intrinsic architecture characterizing cyclic poly(2-oxazoline)-based polyacid brushes leads to a broad variation in swelling and nanomechanical properties in response to pH change, in comparison with their linear analogues of identical composition and molecular weight. In addition, cyclic glycopolymer brushes derived from polyacids reveal an enhanced exposure of galactose units at the surface, due to their expanded topology, and thus display an increased lectin-binding ability with respect to their linear counterparts. This combination of amplified responsiveness and augmented protein-binding capacity renders cyclic brushes invaluable building blocks for the design of "smart" materials and functional biointerfaces.