Adsorption of linear polymer chains of polyacrylamides onto silica nanoparticles 

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Delaying damage initiation:  improvement of chemical gel architecture

The work led on conventional hydrogels evidenced the fact that homogeneous networks lead to stiffer and highly stretchable materials. As the load can be allocated over a larger fraction of elastically active chains, homogeneous networks display higher strength and extensibility than heterogeneous gels with the same cross-linking density.

Tetra arm gels

Following the aim of ideal network, Sakai et al. designed homogeneous structures consisting in the combination of two symmetrical tetrahedron-like structure of the same size alternately con-nected [Sakai et al., 2008]. They are composed of tetra-amine-terminated PEG (TAPEG) and tetra-NHS-terminated PEG (TNPEG) synthesized using controlled anionic polymerization in stoe-chiometric ratio either in aqueous solution (TAPEG) or in organic solvent (TNPEG). These star-shaped macromonomers possess functional groups able to react with each other at the end of each arm, providing homogeneous hydrogels networks displaying very high compressive strength and de-formability, with a maximal compressive strength up to 27 MPa for polymer concentration of 14 wt% using units of 20 kg/mol and maximum stretchability up to 1 100% for units of 40 kg/mol [Matsunaga et al., 2011]. A representative scheme of tetra PEG gel is illustrated in figure I.5.

Double network gels:  reaching mechanical reinforcement through sacrificial bonds

In 2003, Gong et al. [Gong and Osada, 2003] pioneered the domain of tough polymeric gels. They de-signed materials consisting in two covalently cross-linked interpenetrating networks, synthesized in a two-step process: first by forming a tightly cross-linked polyelectrolyte hydrogel network (PAMPS) using radical polymerization in aqueous medium then by swelling this gel in an aqueous solution of monomers with a low content of cross-linking agent before performing another polymerization as illustrated in figure I.7. The first swollen network is highly stretched, while the second neutral network (PAAm) is loose. The key-point is to find the proper combination between the rigidity and brittleness of the first network and the extensibility and ductility of the loose network. Here, the strategy relies on introducing dissipative mechanism at the molecular level by bond-breaking [Tominaga et al., 2007].

Beyond the sacrificial bond concept using reversible interactions

Another way to promote dissipative processes is to introduce sacrificial reversible interactions as transient cross-linkers. This strategy allows to overcome the challenges inherent to network ho-mogeneity. Indeed, the reversible associations grant rearrangements to polymer networks contrary to frozen covalently cross-linked polymer chains [Furukawa et al., 2003], thus delaying fracture propagation [Wu et al., 2011]. Furthermore, the breaking and re-formation of the transient bonds dissipate the energy efficiently, which greatly increases the toughness of the network. The re-versibility of such bonds leads to recovery processes, providing materials with self-healing abilities. Likewise, secondary physical interactions contribute to the enhanced strength and deformability of these polymer networks. As these weak interactions can be tuned by environmental conditions, it opens the way to the conception of responsive hydrogels.
Usually, versatile reversible interactions involved in hydrogel reinforcement are weak compared to covalent bonding (Ed, the dissociation energy of covalent carbon-carbon bond = 347 kJ/mol [eds, 2005]), comprising hydrogen bonds (Ed= 8-35 kJ/mol), crystallization, ionic bond (Ed 102 kJ/mol), Van der Waals and hydrophobic interactions (Ed=1-10 kJ/mol [Luo, 2009]) as illustrated in figure I.9.

Hydrophobic associations

Materials using hydrophobic interactions as physical cross-linkers have also been developed. Hy-drogels are mainly hydrophilic, so the driving force for hydrophobic associations is the interactions that arise to minimize the contact with water.
These associations were first explored using complex synthesis of triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) [Brown et al., 1992] or by performing hydrophobic modifications of poly(acrylic acid) (PAA) [Miquelard-Garnier et al., 2006].
A simpler method was then developed consisiting in micellar copolymerization of hydrophobic compounds solubilized in micelles in presence of an hydrophilic monomer. Okay et al. used this method to design polyacrylamides and PAA gels comprising hydrophobic associations in NaCl so-lutions possessing very interesting properties [Abdurrahmanoglu et al., 2009; Tuncaboylu et al., 2011]. The authors designed hydrogels formed through hydrophobic interactions between N -alkyl units (C18 or C22) forming bilamellar lipidic structures displaying good stiffness with a Young’s modulus ranging from 180 to 600 kPa and shape memory with a recovery ratio of 100% , along with good toughness and self-healing properties [Algi and Okay, 2014; Gulyuz and Okay, 2014]. The strong associations between the alkyl groups promote the gel structure even under chemical changes of the environment (pH, salts) while the dynamic junctions between the network allow for the self-healing properties.
Inspired by the lamellar lipidic bilayer of cell membranes that are both strong and multi responsive to their environment, Gong et al. designed anisotropic gels containing stratified lamellar bilayers composed of chemically cross-linked PAAm and dodecylglyceryl itaconate (DGI) as a surfactant. They obtained gels with laminated structure, as illustrated in figure I.12, possessing a very inter-esting combination of properties.

Our approach: using polymer adsorption as reinforcement mechanism

The idea of organic/inorganic nanocomposites has been generalized to other type of inorganic fillers, using cellulose nano-crystals [Zhou et al., 2011], enzymatically-produced calcium phosphate[Rauner et al., 2017], graphen carbon or iron oxides [Cha et al., 2014; Jaiswal et al., 2016], and silica nanopar-ticles. The latter has been widely studied since silica is a well-known, commercially available com-pound that presents several advantages such as isotropy, chemical stability and well-controlled, easily functionnalizable surface chemistry.
At a first glance, the strategy was to develop Haraguchi’s approach and with the concepts of dou-ble network and sacrificial bonds pioneered by Gong. The idea relies on using polymer adsorption on nanoparticles (NPs) to promote reversible cross-links and to combine such “physical” network with a covalently cross-linked one to control the recovery processes. Based on pioneering studies on semi-dilute solutions [Portehaut et al., 2006], hybrid assemblies have highlighted the adsorp-tion properties of poly(N -alkylacrylamides) chains as poly(N, N -dimethylacrylamide) (PDMA) or poly(N -isopropylacrylamide) (PNIPAm) on aqueous suspensions of silica nanoparticles, that were evidenced using depletion method and calorimetric studies [Petit, 2007]. The resulting physical gels remained soft, with a modulus of the order of the kPa. Likewise, Hihara et al. used silica nanopar-ticles (NPs) as a multifunctional cross-linker to design gels composed of a poly(vinylpyrrolidone) (PVP) backbone grafted with trimethoxysilyl groups. The PVP backbone can interact physically with the silica through hydrogen-bonding while the side groups are able to form covalent bonds with silica surface groups, serving as anchoring groups [Takafuji et al., 2011].
Extending the same concept, it was found that a variety of mechanical properties can be en-hanced by the introduction of silica nanoparticles into an interacting polymer matrix like poly(N,N – dimethylacrylamide) (PDMA) as shown in figure I.16. Tough gels were designed, seeking to drasti- cally increase both the concentration of silica and polymer, using in situ free radical polymerization of DMA monomers and silica nanoparticles. In such hybrid networks, the silica NPs are well-dispersed in the chemically cross-linked polymer matrix, serving as physical cross-linkers through specific adsorption of PDMA chains onto silica surface, forming a transient secondary network [Rose et al., 2013b].
Marcellan et al. explored the abilities offered by the reversible bonding between covalently cross-linked poly(N, N -dimethylacrylamide) and silica nanoparticles to induce strong bulk reinforcement on the mechanical behavior at large strain [Carlsson et al., 2010].

Hydroxylation and dehydroxylation of silica surface

The surface of silica is covered to varying degrees by hydroxyl groups or ions which play an important role in the adsorption processes. Water surrounding the particles can react with the silicon atoms of the surface engaged in siloxane bonds to create silanols groups through a condensation reaction as depicted in figure I.
A complete coverage can be achieved, meaning that the surface is fully hydroxylated. Likewise the surface can be dehydroxylated under high temperature implying that all silanols groups condensated to leave a surface composed only of siloxane groups. Hydroxylation/dehy- droxylation is a reversible process that depends on several parameters, the more important being temperature and humidity. Hydroxylation must not be confused with hydration of the surface which is the uptake of phys-ically adsorbed water at the surface [Vansant and Vrancken, 1995]. The amount of physisorbed water and hydroxylated can be assessed through Thermo Gravimetric Analysis, ATG as illustrated in figure I.24.

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Chemical grafting onto silica nanoparticles

Covalent interactions between silica and polymers have been developped to tune the properties of silica such as its hydrophilic behavior for chemical separative processes [Czaun et al., 2008] or to tune silica interactions with polymers when embedded in elastomer or gel matrix. Takafuji et al. designed hydrogels based on polymer chains containing trimethoxysilyl groups able to react both covalently and physically with precipitated silica surface [Takafuji et al., 2011]. Another example is the design of responsive materials containing silica nanoparticles. Coradin et al. developed collagen fibrils linked to modified-Stober silica network in which the polymer organization is strongly depending on the pH [Aime et al., 2012].

Reversible interactions between polymer and silica

Reversible interactions between polymers and silica can also take place as shown in the second part of this chapter with the example of PDMA/silica hybrid gels. Hybrid hydrogels have also been developed with polyacrylamide (AAm) as monomer, leading to no specific mechanical reinforcement, highlighting the key role of polymer/fillers interactions in mechanical reinforcement [Lin et al., 2011; Rose et al., 2013b]. In this case, the silica nanoparticles act as non-interacting fillers and these materials can be compared to filled elastomers [Kalfus et al., 2012].

Polymer adsorption at solid/liquid interface

Understanding adsorption mechanisms onto solid surface is of major importance so as to be able to control better the mechanical properties of hybrid hydrogels using adsorption as cross-linking mechanism.
Using the tools and knowledge developed for the understanding of adsorption of polymer chains in solution on solid surface is thus necessary to go deeper in the understanding of hydrogels behavior at solid interface, as highlighted by Joanny et al. [Joanny et al., 2001]. Adsorption of polymers have been widely studied both experimentally and theoretically and now a good picture of flexible macromolecules at solid interfaces can be drawn from experimental records.

Table of contents :

I State of the art 
1 General survey on hydrogels and reinforcement mechanisms
1.1 Synthetic polymer hydrogels: definition and applications
1.1.1 Hydrogels as smart and active materials
1.1.2 Structure-property relationship
1.1.3 Frozen inhomogeneities within the network
1.1.4 Mechanical properties of chemical hydrogels
1.2 Strategies of mechanical reinforcement for chemical hydrogels
1.2.1 Delaying damage initiation: improvement of chemical gel architecture
1.2.2 Promoting dissipative processes
1.3 Beyond the sacrificial bond concept using reversible interactions
1.3.1 Ionic hydrogels
1.3.2 Hydrophobic associations
1.3.3 Nanocomposite hydrogels
1.3.4 Our approach: using polymer adsorption as reinforcement mechanism
2 Generalities about silica solutions
2.1 Surface chemistry of silica nanoparticles
2.1.1 Silanols groups
2.1.2 Hydroxylation and dehydroxylation of silica surface
2.2 Stability of colloidal suspensions
2.3 Aggregation of silica
2.4 Ludox TM-50c : specificity and uses
2.4.1 Precipitated silica
2.4.2 Ludox TM-50c
2.5 Polymer/silica interactions
2.5.1 Chemical grafting onto silica nanoparticles
2.5.2 Reversible interactions between polymer and silica
3 Polymer adsorption at solid/liquid interface
3.1 Physics of polymer chains in solution
3.1.1 Ideal linear polymer chain in solution
3.1.2 Free energy of ideal chain
3.1.3 Describing polymer/solvent interactions
3.2 Adsorption of polymer chains
3.2.1 Conformation of adsorbed polymer chains
3.2.2 Adsorption: a low-energy but irreversible process
3.2.3 The impact of chain concentration on polymer adsorption
3.3 Assessement of polymer adsorption
3.4 Relevant parameters for polymer adsorption
3.4.1 Impact of molecular weight on adsorption
3.4.2 Role of chemical environment
3.5 Probing the dynamics of polymer adsorption
4 Towards an analogy between hybrid gels and filled elastomers
II Adsorption of linear polymer chains of polyacrylamides onto silica nanoparticles 
1 Introduction
2 Principles and techniques
2.1 Polymer synthesis
2.1.1 Conventional Free Radical Polymerization: principle and limits
2.1.2 Controlled radical polymerization
2.2 Atom Transfer Radical Polymerization (ATRP): basics
2.3 1H Nuclear Magnetic Resonance relaxation time analysis of the solvent
2.3.1 Effect of interfaces on the solvent relaxation time
2.3.2 Principle of the measurements
2.4 Total Organic Carbon Analysis
2.5 Zeta potential measurements
3 Materials and methods
3.1 Chemicals
3.2 Polymer synthesis
3.2.1 Polymer characterization
3.3 Adsorption measurements
3.4 Zeta potential measurements
3.5 1H NMR relaxation times
4 Results and Discussions
4.1 PDMA, PAAm and P(AAm-co-DMA) ATRP synthesis
4.2 Surface chemistry of silica suspensions
4.2.1 pH effect on dilution behavior of Ludox TM-50R
4.3 Adsorption isotherms of PAAm, PDMA and P(AAm-co-DMA) on silica nanoparticles using TOC depletion method
4.3.1 Adsorption isotherms on homopolymers of PAAm and PDMA .
4.3.2 Impact of molecular weight and molecular weight distribution on adsorption of PDMA and PAAm
4.3.3 Adsorption isotherms of P(AAm-co-DMA) at various AAm content
4.4 Zeta potential analysis
4.4.1 Silica and polymers measurements
4.4.2 Zeta potential of adsorbing and non adsorbing polymer onto silica .
4.4.3 Impact of molecular weight, Mn on PDMA adsorption
4.5 1H NMR solvent relaxation times to quantify adsorption
4.5.1 Silica and polymers measurements
4.5.2 Solvent relaxation times of PDMA/silica dispersions
4.5.3 Impact of non-interacting monomer
5 Conclusion
III Tuning polymer/particles interactions in hybrid hydrogels 
1 Introduction
2 Principles and techniques
2.1 Titration of extractibles
2.2 Mechanical properties
2.3 Small Angle X-ray scattering
3 Materials and methods
3.1 Chemicals
3.2 Experimental part
3.3 Analytical methods
3.3.1 Swelling experiments
3.3.2 Mechanical tests
3.3.3 Rheological measurements
3.3.4 Small Angle X-Ray Scattering
4 Results and discussion
4.1 Characterization of pure PAAm and PDMA networks
4.1.1 Tensile behavior of chemical gels
4.1.2 Young’s modulus
4.1.3 Swelling behavior
4.1.4 PAAm and PDMA hydrogels dynamics studied by Linear Rheology
4.2 Introducing silica nanoparticles in PAAm and PDMA polymer networks
4.2.1 Hybrid gel structure: uniform dispersion of nanoparticles in the gel
4.3 Impact of polymer adsorption on mechanical properties at small strain and swelling behavior
4.3.1 Swelling behavior and extractibles content: impact of non-interacting monomer
4.3.2 Small strain behavior within the viscoelastic regime
4.4 Impact of the chemical nature of the monomer at a given strain rate
4.4.1 Linear tensile modulus
4.4.2 Tensile behavior
4.4.3 Impact of non-interacting monomer on dissipation processes and recovery
4.4.4 Mechanical behavior of hybrid gels at swelling equilibrium
4.5 Time-dependence and behavior at large strain
4.5.1 Impact of strain rate on the mechanical behavior at large strain
4.5.2 Impact of strain rate on dissipation processes
4.5.3 Fracture properties at large strain
5 Identifying the nature of interactions
5.1 Tensile behavior and swelling properties
5.2 Dissipative properties
5.3 Strain rate impact and fracture properties
6 Conclusion
IV 1H NMR structural study of PDMA and PAAm hybrid hydrogels 165
1 Introduction
2 Materials and methods
2.1 Chemicals
2.2 Gel preparation and composition
2.3 Analytical methods
2.3.1 1H NMRMSEmeasurements
2.3.2 1H NMR DQmeasurements
3 Results and discussion
3.1 Adsorbed polymer
3.1.1 Measurements on dry polymers
3.1.2 Measurements on PDMA hybrid gels: impact of interacting monomer
3.1.3 Impact of chemical cross-linker content
3.1.4 Disturbing polymer-filler interactions
3.2 DQ measurements on PDMA and PAAm hydrogels
3.2.1 Normalization of the DQ signal
3.2.2 Impact of chemical cross-linking
3.2.3 Impact of silica on PDMA matrix
3.2.4 Impact of the non-interacting monomer and the silica surface chemistry
4 Conclusion
V Double networks: hybrid gels with clustered silica 
1 Introduction
2 Materials and methods
2.1 Chemicals
2.2 Experimental part
2.2.1 Gel preparation and composition
2.2.2 Swelling experiments
2.2.3 Mechanical tests
2.2.4 Rheological measurements
2.2.5 Small Angle X-Ray Scattering
3 Results and Discussion
3.1 Aggregating silica within hybrid gels network
3.1.1 Structural characterization of hybrid gels
3.1.2 Swelling behavior and extractible content
3.2 Mechanical properties
3.2.1 Mechanical properties: tensile behavior
3.2.2 Dissipation processes and recovery
3.2.3 Fracture properties
4 Exploring the non linear behavior: Large Amplitude Oscillatory Shear
4.1 General background
4.2 Performing LAOS experiments on hydrogels
4.2.1 LAOS experiments on chemical gels
4.2.2 Linear domain of PDMA hybrid gel
4.3 Behavior at large strain of hybrid gels: impact of the monomer nature
4.3.1 Amplitude strain sweep on hybrid gels
4.3.2 Behavior at large strain
4.4 LAOS on hybrid gels with aggregated silica
4.4.1 Small strain linear viscoelasticity of hybrid gels with clustered silica
4.4.2 PDMA gels with aggregated silica
4.4.3 PAAm gels with aggregated silica
5 Conclusion
General Conclusion 

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