Prediction of Surface Segregation of Fluorinated Additives from Measurement of Wettability Properties of Polymer-Based Blends

The aim of this work is to describe the segregation phenomenon of random perfluorinated copolymers toward the surface of a polystyrene (PS) matrix. Three random perfluorinated copolymers, named POISE-a (Polymer prOcessing Interface StabilizEr), synthesized with different amounts of chemically bound fluorine, were mixed with a commercial PS matrix by a solvent casting process. Their effect on the wettability properties of PS as a function of their concentration was measured first under static conditions with different liquids (water, formamide, diethylene glycol) and then under dynamic conditions with water. The evolution of the cosine of the static and dynamic advancing contact angles as a function of the fluorine content by weight in the bulk was predicted using an original physical law, in which the different parameters were related to the morphology of the additivated polymers as revealed by experimental X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy-energy dispersive X-ray (SEM-EDX) measurements. This law has been successfully used to predict the wetting properties of other polymer blends and has contributed to a better understanding of the phase segregation mechanism of additives migrating to the PS surface, at the origin of the increase in hydrophobicity.

Distribution of block copolymers in drying polymer films

HYPOTHESIS

Block copolymers (BCP) consisting of a polar block and a surface active apolar block are widely used for surface functionalization of polymer films. The characteristics of the copolymer blocks determine whether surface segregation and/or phase separation occurs, for a given bulk mixture. This data can be used to find the optimal BCP composition where high surface enrichment is obtained without accumulation of phase separated BCP in the bulk.

METHODS

The distribution of poly(ethylene oxide)-polydimethylsiloxane (PEO-PDMS) BCP in a polymer formulation relevant for coating applications is systematically investigated. The surface segregation is studied in liquid formulations with surface tension measurements and dried films with X-ray photoelectron spectroscopy (XPS), whereas phase separation is quantified using turbidity measurements. The results are compared with Scheutjens-Fleer self-consistent field (SF-SCF) computations, which are also applied to determine the effect of film drying on BCP phase stability and surface segregation.

FINDINGS

Longer PDMS blocks result in lower interfacial tension of the liquid polymer mixture, whereas for the cured films, the largest PDMS concentration at the interface was obtained for intermediate PDMS block lengths. This is explained by the observation that phase separation already occurs at very low BCP concentrations for long PDMS blocks. The SCF predictions qualitatively agree with the experimental results and reveal that the BCP distribution changes significantly during film drying.

Unraveling the surface properties of PMMA/azobenzene blends as coating films with photoreversible surface polarity

Various reports demonstrated that azobenzene derivatives are the chromophore of choice in photoresponsive surfaces showing reversible surface polarity. Hitherto the surface study of coating films based on polymer/azobenzene blends using contact angle measurements remained unexplored. To provide insight into the surface polarity of polymer/dye blend films, poly(methyl methacrylate) (PMMA) blends containing photoresponsive 4-hydroxy-4′-methylazobenzene (AZO1) and 4,4′-dimethylazobenzene (AZO2) as coating films on clear glass substrates are investigated in this work. Contact angle measurements were carried out to unravel the role of substituents in the surface polarity and the orientation of chromophores in the coating matrices before and after UV light (λ_max = 365 nm) irradiation. Changes in water contact angles measured on the PMMA/azobenzene coating films indicated that the surface polarity is reversible as the chromophores underwent reversible trans–cis isomerisation. It has been revealed that the repeated trans–cis isomerisation led to the random reorientation and arrangement of chromophores in PMMA/AZO1 coating films. Then, to indicate the possibility of the disruption of interfacial interactions due to the repeated trans–cis isomerisation processes, as a proof of concept experiment, it is shown that the commercial acrylic-based pressure-sensitive sticker which adhered strongly to the PMMA/AZO1(13) coating film is peeled off from the coating surface after being subjected to a cycle of UV light irradiation for 12 hours, followed by dark conditions for another 12 hours within 14 days. The proof of concept study will lead to more development of smart photoresponsive coating films using simple polymer/dye blends.

A repeated trans–cis isomerisation led to the random reorientation and arrangement of chromophores in PMMA/azobenzene blends as coating films.

Investigating the effect of grafting density on the surface properties for sequence-determined fluoropolymer films

Six sequence-determined fluoropolymers were synthesized and their surface properties were affected by their grafting densities. The reason can be attributed to the assembled structure of the perfluoroalkyl chains at the surface.

Surface-Modifying Polymers for Blood-Contacting Polymeric Biomaterials

Bulk blending is considered as one of the most effective and straightforward ways to improve the hemo-compatibility of blood-contacting polymeric biomaterials among many surface modification methods. Zwitterionic structure-, glycocalyx-like structure-, and heparin-like structure-based oligomers have been synthesized as additives and blended with base polymers to improve the blood compatibility of base polymers. Fluorinated end- and side-functionalized oligomers could promote the migration of functionalized groups to the surface of biomedical polymers without changing their bulk properties, and it highly depends on the number and concentration of functional groups. Moreover, oligomers having both zwitterion and fluorine are receiving considerable attention due to their desirable phase separation, which can avoid undesired protein adsorption and platelet adhesion. The surface analysis of the surface-modified materials is usually investigated by analytical tools such as contact angle measurement, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Blood compatibility is mainly evaluated via platelet adhesion and protein adsorption test, and the result showed a significant decrease in the amount of undesirable adsorption. These analyses indicated that surface modification using bulk blending technique effectively improves blood compatibility of polymeric biomaterials.

Rheological and thermal degradation properties of hyperbranched polyisoprene prepared by anionic polymerization

Hyperbranched polyisoprene was prepared by anionic copolymerization under high vacuum condition. Size exclusion chromatography was used to characterize the molecular weight and branching nature of these polymers. The characterization by differential scanning calorimetry and melt rheology indicated lower Tg and complex viscosity in the branched polymers as compared with the linear polymer. Degradation kinetics of these polymers was explored using thermogravimetric analysis via non-isothermal techniques. The polymers were heated under nitrogen from ambient temperature to 600°C using heating rates from 2 to 15°C min-1. Three kinetics methods namely Friedman, Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose were used to evaluate the dependence of activation energy (Ea ) on conversion (α). The hyperbranched polyisoprene decomposed via multistep mechanism as manifested by the nonlinear relationship between α and Ea while the linear polymer exhibited a decline in Ea at higher conversions. The average Ea values range from 258 to 330 kJ mol-1 for the linear, and from 260 to 320 kJ mol-1 for the branched polymers. The thermal degradation of the polymers studied involved one-dimensional diffusion mechanism as determined by Coats-Redfern method. This study may help in understanding the effect of branching on the rheological and decomposition kinetics of polyisoprene.

Synthesis and characterisation of highly branched polyisoprene: exploiting the "Strathclyde route" in anionic polymerisation

This work aimed at developing a synthetic route towards highly branched poly(isoprene) from commercially available raw materials, in good yield and devoid of microgelation, i.e., to prepare a completely soluble polymer via the versatile technique anionic polymerisation. The polymerisations were conducted under high vacuum conditions using sec-butyllithium as initiator at 50 °C in toluene. Toluene served both as a solvent and as a chain-transfer agent. The polar modifier used was tetramethylethylenediamine (TMEDA), and a commercial mixture of divinylbenzene (DVB) was employed as the branching agent for the "living" poly(isoprenyl)lithium anions. The nature of the reaction was studied on the TMEDA/Li ratio as well as the DVB/Li ratio. The obtained branched polymers were characterised by triple detection size exclusion chromatography (SEC), proton nuclear magnetic resonance spectroscopy (1H NMR), differential scanning calorimetry (DSC) and melt rheology. Broad molecular weight distributions have been obtained for the highly branched polymer products. 1H NMR spectroscopy reveals the dominance of 3,4-polyisoprene microstructure. It was found that the complex viscosities and dynamic moduli of the branched samples were much lower compared to their linear counterparts. The results conform with earlier findings by the "Strathclyde team" for radical polymerisation systems. This methodology has the potential of providing soluble branched vinyl polymers at low cost using the readily available raw materials.

Time-resolved surface rearrangements of polymer films monitored by dynamic recovery contact angle (DRCA)

The time-frames involved in surface rearrangements of polymer films are investigated by Dynamic Recovery Contact Angle (DRCA) through a simple, non-invasive and reconstructive approach.

Polystyrene with hydrophobic end groups: synthesis, kinetics, interfacial activity, and self-assemblies templated by breath figures

Polystyrenes with hydrophobic end groups are synthesized from a series of alkyl or fluorinated ATRP initiators to fine-tune the surface morphologies of honeycomb films prepared by the breath figure method.

Surface modification of polyethylene with multi-end-functional polyethylene additives

We have prepared and characterized a series of multifluorocarbon end-functional polyethylene additives, which when blended with polyethylene matrices increase surface hydrophobicity and lipophobicity. Water contact angles of >112° were observed on spin-cast blended film surfaces containing less than 1% fluorocarbon in the bulk, compared to ~98° in the absence of any additive. Crystallinity in these films gives rise to surface roughness that is an order of magnitude greater than is typical for amorphous spin-cast films but is too little to give rise to superhydrophobicity. X-ray photoelectron spectroscopy (XPS) confirms the enrichment of the multifluorocarbon additives at the air surface by up to 80 times the bulk concentration. Ion beam analysis was used to quantify the surface excess of the additives as a function of composition, functionality, and molecular weight of either blend component. In some cases, an excess of the additives was also found at the substrate interface, indicating phase separation into self-stratified layers. The combination of neutron reflectometry and ion beam analysis allowed the surface excess to be quantified above and below the melting point of the blended films. In these films, where the melting temperatures of the additive and matrix components are relatively similar (within 15 °C), the surface excess is almost independent of whether the blended film is semicrystalline or molten, suggesting that the additive undergoes cocrystallization with the matrix when the blended films are allowed to cool below the melting point.