Polymer blending as a surface modification technique

Tissue engineering and drug delivery

The resemblance of electrospun mats with the natural ECM makes them a natural fit as a substrate for tissue engineering (Agarwal, Wendorff, & Greiner, 2008; Zeng et al., 2003).
The natural ECM is composed, among many other components, of hydrophilic carbohydrate polymers. Hydrophilicity increases cell affinity so that hydrophobic polymers used in tissue engineering such as PLA, PHAs or PCL need to undergo surface modification (Liang, Hsiao, & Chu, 2007). By incorporating minerals in PCL (Araujo et al., 2008) or PHBV (Ito et al., 2005) mats to mimic the ECM found in bone structures, for instance, the resulting hydrophilic mats presented increased cell attachment and proliferation. Another promising biomedical application where the wettability is important is drug delivery. Drugs can be incorporated on the surface of electrospun fibers in different ways , or they can be embedded in the matrix with different morphologies (Sill & von Recum, 2008). The wetting behavior of the polymer matrix must be tailored to enable a better drug encapsulation (Zeng et al., 2003). Amphiphilic block-copolymers, for example, can form micelles used to encapsulate hydrophobic drugs and at the same time increase the dispersions inside a hydrophilic matrix (Rösler, Vandermeulen, & Klok, 2012), or they can be used to tune drug release rate (K. Kim et al., 2004).

Electrospinning process

Electrospinning is a technique in which polymer fibers are formed, generally from a solution, under an electrical field. In the most basic apparatus, the solution is placed inside a syringe and directed through a charged syringe needle. Raised to a high potential, the drop formed at the needle tip is elongated until a thin jet erupts and travels towards a grounded collector.
The jet experiences bending instabilities and solvent evaporation before the solid fibers reach the collector deposited as a nonwoven mat (Reneker & Yarin, 2008; Rutledge & Fridrikh, 2007). The charged drop deformation phenomenon and jet eruption was described by Zeleny (1917) (Zeleny, 1917). Later, the theory was further developed by Taylor (1964), who described the conditions for the droplet instability in high fields that leads to the formation of a conic shape before the eruption. Taylor hypothesized that the electric field is locally greater at the vertex of the cone, as an explanation for this region to be the first to accelerate, and for the fact that is possible to generate fibers thinner than the capillary from which the solution ejects (Taylor, 1964). However, it was not until the mid-nineties that this technology experienced a resurgence, with the work of Doshi and Reneker (1996) as an example that showed the formation of polymer fibers from different solution, with different diameters and cross-sections, which had the potential for many applications(Doshi, 1995). After that, the number of publications on electrospinning increased dramatically. Although the phenomena involved is a complex interplay of solution characteristics such as rheological properties, surface tension, conductivity and evaporation rate, with the processing parameters like applied voltage, flow rate, distance to the collector, temperature and humidity, the basic apparatus is easy to assemble in a laboratory and the technique has proven to be remarkably versatile.
A non-woven mat composed of randomly aligned fibers is the most common type of structure obtained by electrospinning. Alternatively, rotating or air-gap collectors can produce aligned fibers and a processing variation called near-field electrospinning with a collector with controlled movement enables the design of mats with more precise geometry (D. Li, Wang, & Xia, 2003; Persano et al., 2013; D. Sun, Chang, Li, & Lin, 2006).

Wettability of electrospun mats

When a droplet of water is placed on a flat surface a contact angle θE is formed, representing the equilibrium of all the interfacial energies involved and indicating the affinity of the surface with water. If θE is lower than 90° the surface is hydrophilic; if greater than 90°, hydrophobic. The introduction of roughness tends to amplify the original characteristic of the surface and so rough surfaces of hydrophobic materials have higher values of static contact angle when compared to the flat film. By increasing the roughness, the static contact angle increases as the droplet fills the whole surface area (Wenzel state) up to a critical value in which the energy associated with the formation of air pockets under the droplet becomes lower than the energy related to following the whole solid surface(Callies & Quere, 2005). Air is trapped underneath and the droplet is deposited on a composite surface of air and solid (Cassie and Baxter state) with even higher values of contact angle. In the hydrophilic case, the contact angle decreases while the droplet fills the roughness and the critical value marks the start of water impregnation by capillary action. The surface becomes wet ahead of the contact line and the droplet is deposited on a composite surface of liquid and solid (J. Bico, Tordeux, & Quéré, 2001).

Post-treatments

Plasma :Plasma is a partially ionized gas formed by the same number density of negative and positive charge carriers (Strobel, Lyons, & Mittal, 1994), which can be created by heating the gas to extremely high temperatures or subjecting it to strong electromagnetic fields. Plasmas present collective behavior and are electrically conductive, responding to the presence of electromagnetic fields. Plasma processes were originally used to either etch a surface (removing material) or for the deposition of thin films. Surface treatment using plasmas can also modify only a few molecular layers with no significant amount of deposition or etching (Strobel et al., 1994).
Plasmas can be divided in thermal/high-temperature/hot plasma and non-thermal/low temperature/cold plasma. The former is characterized by very high temperatures and therefore is not suitable for temperature-sensitive materials like polymers (Morent, De Geyter, Desmet, Dubruel, & Leys, 2011). The advent of nonthermal/low-temperature/cold plasma processes, in which the gas molecules and ions are closer to the room temperature allowed the use of plasma treatments for polymers, reducing the risk of thermal degradation.
Glow discharge is a source of nonthermal plasma formed by the passage of an electric current between two electrodes through a low-pressure gas. This type of plasma was originally used for etching and creating patterns on surfaces or to deposit thin films, usually operated in direct-current powering mode, which requires an electrically conductive sample to act as the negatively charged electrode (cathode).

One-step surface modification

Nanoparticles :The incorporation of nanoparticles to the polymer solution before electrospinning represents a practical alternative to alter the wettability of the fibers, provided that the particles are homogeneously located at the surface.The homogeneity of the treatment will depend on the final dispersion of nanoparticles after solvent evaporation. The location of the nanoparticles will depend on their compatibility with the matrix, which is related to the surface energy of the components involved. Also, the incorporation of nanoparticles normally increases the dielectric constant of the solution and its ability to be electrospun and produce thinner fibers. If they are not well-dispersed, the mat can present nonuniform fiber morphology and surface treatment. In a study to produce antibacterial membranes, thermoplastic polyurethane (TPU) and tourmaline nanocomposite mats were prepared by dispersing the nanoparticles in a solution by ultrasonication and mixing with a TPU solution prior to electrospinning. Particle agglomeration was observed in the composite with higher tourmaline content (5 wt%) together with spiderweb-like nanofibers of much smaller diameter between thicker fibers .

Table des matières

INTRODUCTION 
0.1 Research Hypothesis
0.2 Objectives
CHAPTER 1 SURFACE MODIFICATION TO CONTROL THE WETTABILITY OF ELECTROSPUN MATS 
1.1 Introduction
1.1.1 Applications of mats with controlled wetting behavior
1.1.1.1 Membranes for separation/filtration
1.1.1.2 Sensing applications
1.1.1.3 Self-cleaning surfaces and antifouling membranes
1.1.1.4 Tissue engineering and drug delivery
1.2 Basic principles
1.2.1 Electrospinning process
1.2.2 Wettability of electrospun mats
1.3 Surface modification methods
1.3.1 Post-treatments
1.3.1.1 Plasma
1.3.1.2 Wet chemistry
1.3.1.3 Grafting
1.3.1.4 Coating
1.3.2 One-step surface modification
1.3.2.1 Nanoparticles
1.3.2.2 Blending
1.4 Future prospects and conclusions
CHAPTER 2 ARTICLES ORGANIZATION 
CHAPTER 3 BLENDING AND MORPHOLOGY CONTROL TO TURN HIGHLY HYDROPHOBIC SEBS ELECTROSPUN MATS SUPERHYDROPHILIC 
3.1 Introduction
3.2 Experimental
3.3 Results and discussion
3.4 Conclusions
3.5 Acknowledgements
CHAPTER 4 WETTING OF HYDROPHILIC ELECTROSPUN MATS PRODUCED BY BLENDING SEBS WITH PEO-PPO-PEO COPOLYMERS OF DIFFERENT MOLECULAR WEIGHT 
4.1 Introduction
4.2 Experimental
4.3 Results and discussion
4.4 Conclusions
4.5 Acknowledgements
CHAPTER 5 SURFACE PROPERTIES EVOLUTION IN ELECTROSPUN POLYMER BLENDS BY SEGREGATION OF HYDROPHILIC OR AMPHIPHILIC MOLECULES 
5.1 Introduction
5.2 Experimental
5.3 Results and discussion
5.4 Conclusions
5.5 Acknowledgements
CHAPTER 6 DISCUSSION 
6.1 Wetting of electrospun mats
6.2 Electrospinning of SEBS and SEBS/PEO-PPO-PEO blends
6.3 Hydrophilization of SEBS with PEO-PPO-PEO
6.4 Morphology evolution of electrospun blends
CONCLUSIONS AND RECOMMENDATIONS 
7.1 Main findings and conclusions
7.1.1 Summary and conclusions of Chapter 3
7.1.2 Summary and conclusions of Chapter 4
7.1.3 Summary and conclusions of Chapter 5
7.1.4 Main Conclusion
7.2 Recommendations

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