Heat-activated radical cure or High Temperature Vulcanization (HTV)

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Stress-Strain behavior of polymers

The behavior of polymeric materials can be divided into three main types; brittle plastic, tough plastic, and elastomer. Brittle plastics have elastic moduli of a few GPa and a linear stress-strain curve up to the fracture point with a small deformation, normally 2% to 5 % elongation (Figure 1-7 (a)). Polymers showing this kind of behavior are mostly in the glassy state at room temperature or have a high glass transition temperature (Tg) such as poly(methyl methacrylate); Tg 110˚C17, polystyrene; Tg 100 ˚C17, and poly(acrylic acid); Tg 106 ˚C18.
However, some glassy polymers such as polycarbonate exhibit a tough plastic behavior, this type of behavior is generally observed for semi-crystalline polymers, with an amorphous fraction above the Tg such as polyethylene, polyethylene terephthalate, polytetrafluoroethylene, polyamide-11, polyamide-12 and isotactic polypropylene (Figure 1-7 (b)). The initial modulus or Young’s modulus of these polymers is typically a bit lower than for the brittle plastics but remains in the GPa range. The stress-strain curve shows a yield point and is followed by extensive elongation and sometimes necking.
For an elastomer behavior, the initial modulus is in the MPa range, the stress-strain curve is highly non-linear and the deformation is mostly reversible up to elongations at break that can be several hundred % (Figure 1-7 (c)) and the Tg of the polymer is below room temperature. This kind of behavior can be found in many elastomers such as natural rubber, silicone rubber, styrene-butadiene rubber, nitrile rubber etc.

Fracture mechanics in polymers

The fracture mechanics approach is based on the quantification of the energy per unit area required to propagate a crack. This approach has been developed for small deformations and linear elastic materials. The fracture toughness can be described as the value of the energy release rate (G) where the crack starts to propagate. The energy release rate is a quantity that can be calculated from the elastic properties of the material, test geometry, crack length and applied stress far from the crack. It corresponds to the elastic energy available from the sample to propagate the crack and is proportional to the square of the applied load. At the point where the crack starts to propagate, G = where is called the fracture energy. This fracture energy typically depends on the loading rate and at vanishing loading rate or high temperature is called threshold value0.
For rubbers, several approaches have been developed to quantify the fracture toughness, or the fracture energy, as a material property. Rivlin and Thomas20 and then Greensmith21 proposed a simple method to determine the strain energy release rate G in the case of single edge notched specimens with the following equation: G = 2 × n × o N × p $%.1−19.

Interpenetrating Polymer Networks (IPNs)

Interpenetrating polymer networks or IPNs are a combination of two or more polymers in the network form. In an IPNs, at least one of the networks is polymerized or crosslinked in the immediate presence of the other(s).54, 55, 56 IPNs are in essence a specific type of polymer blend. The first pioneering work was reported in 1914 by Jonas Anylswoth54, 57 who reinforced phenol-formaldehyde resin with crosslinked rubber. However the term of IPNs came about much later in 1960, introduced by Millar who developed PS/PS IPNs to be used as ion exchange resin.57 IPNs have been then extensively studied in the literature. Nowadays, IPNs are classified into 6 mains types.

Sequential IPNs

In sequential IPNs, polymer network I is synthesized first to create the base network. Then the polymer network I is immerged into the monomer bath containing monomer II, crosslinker II, and activator. The swollen polymer I is finally polymerized in situ to create IPNs54, 55. The procedure of this process is explained in Figure 1-13.

Simultaneous Interpenetrating Network (SIN)

The simultaneous interpenetrating network method or SIN is a synthesis method where IPNs are made by mixing monomers or linear polymers, crosslinkers, initiator, etc., of both polymers to form a homogeneous fluid. The two polymer components are then simultaneously polymerized by independent, noninterfering reactions as the procedure presented in Figure 1-14.
SIN create IPNs by controlling the rates of polymerization kinetics of the two polymer systems which can generally proceed in three ways55: (i) the simultaneous gelation of both polymers, (ii) a sequential polymerization of the pre-polymer mixture, and (iii) the introduction of a larger or smaller number of graft sites between the two polymers.
A major advantage of SINs over sequential IPNs is the ease of the process. For example, the mixture can be prepolymerized until just short of the gel point, followed by pumping into a mold or die, with continued polymerization. The main disadvantage of the method is however that the structure is much less well controlled and some level of macroscopic phase separation often occurs.

Room Temperature Vulcanization (RTV)

Room temperature vulcanization is a method that takes advantage of the displacement of ligands of PDMS chain ends. An organosilicon crosslinking agent is used to interact with siloxanol ends groups in the presence of a catalyst under anhydrous conditions.21 The reaction occurs at room temperature, therefore the term of RTV is related to the process of curing. The rate of the substitution reaction depends on the leaving group during the crosslinking process. In general the selected leaving groups are oximes, carboxylates and alkoxides.
RTV curing can be with a single or duo package system.21 The single package system is more convenient, since all the components are premixed together. The system usually needs the use of an excess of the multifunctional crosslinking agent to interact with the siloxanol end groups of the silicone chains in the presence of a catalyst. This one component system has the drawback of a relatively long curing time because of the requirement of diffusion of moisture to react with the residual multifunctional crosslinker. Also the curing silicone rubber from this system possess multifunctional end groups which could be hydrolyzed and condensed when exposed to the atmospheric moisture.21 The duo package system requires the mixing of two separate components. One component contains a multifunctional organosilicone crosslinker and a catalyst while another part contains hydride-terminated PDMS and filler. The system is prepared with equivalent chemistry, thus after the two components are mixed, the curing proceeds relatively fast at room temperature.

Heat-activated radical cure or High Temperature Vulcanization (HTV)

High temperature radical cure of silicones arise from peroxides. Generally, peroxides decompose at a temperature around 150°C to create the radical .21 There are two different types of radical cure commonly used; radical H• and vinyl group. In the first, peroxides are used to initiate radicals H • which are abstracted from a methyl group in PDMS and form silylmethyl radicals (SiCH2• ). The silylmethyl radicals crosslink by coupling or by addition to a small amount of pendent vinyl groups. This process provides a high efficiency, although the efficiency drops at high conversion due to the viscosity of the product.27 In addition this process produces byproducts such as phenols that needs to be removed from the elastomer. In the second method , vinyl groups which are more reactive toward radicals than alkanes (to H • subtraction), can form a crosslinked network at low temperature in the presence of less reactive “vinyl-specific” peroxide s.27 This system also creates byproducts that need to be removed either by heat stripping or by titration.

Transition metal catalyzed hydrosilylation or addition cure

Addition cure by hydosilylation is widely used in the silicone industries.28 The reaction is useful for functionalizing siloxane monomers and polymers and also is good to crosslink polysiloxane. The hydrosilylation reaction only occurs with the help of catalysts; platinum catalyst, palladium, peroxides, UV light and azidonitriles to conjugate the end group of the silicon hydride (Si-H) bond to unsaturated groups of PDMS (Si-CH=CH2 or Si-C≣CH). The reaction performs rapidly from room temperature to high temperature under mild conditions.27 One example of hydrosilylation reaction in which a vinyl-terminated PDMS reacts with a tetrafunctional hydrosilane, tetrakisdimethylsiloxy silane is shown in Figure 2-3.29 By this method, the reaction shows a high efficiency and provides a high conversion, without any byproduct 29 and is relatively free from side reactions.

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Synthesis and reaction conditions

In our synthesis, poly(dimethylsiloxane), vinyldimethylsiloxy terminated (PDMS-V) was crosslinked with 2,4,6,8-Tetramethylcyclotetrasiloxane (D4H) by the hydrosilylation reaction and poly(dimethylsiloxane) hydride terminated (PDMS-H) was used to chain extend PDMS-V. The Karstedt’s catalyst was served as the initiator to trigger the reaction between vinyl (=CH2) end group of PDMS-V and hydride (-H) of PDMS-H or D4H. The basic principle of sequential interpenetrating networks (IPNs) was described in chapter 1. In our study, silicone elastomers were investigated by using 2 different systems: a highly crosslinked network which is a rigid network and a loosely crosslinked network which controls the extensibility of the material. The stoichiometric ratio between vinyl groups of the PDMS-V chains and the -H of the D4H crosslinker (in the highly crosslinked system) or the PDMS-H (in the loosely crosslinked system) were optimized separately in order to obtain the highest modulus (for the highly crosslinked system) and the highest extensibility simultaneous increase in stiffness and extensibility. Then the silicone loosely crosslinked network was interpenetrated into the more crosslinked network by using a sequential polymerization technique. The products after this process were called silicone multiple networks.

Synthesis of the silicone highly crosslinked network

Two types of rigid networks of well crosslinked silicone (or 1st networks) were used in our study: a small mesh size and a larger mesh size (Figure 2-6). These two different 1st networks were prepared by using PDMS-V with MW = 6kg.mol-1 and PDMS-V with Mw = 17.2 kg.mol-1 respectively. The networks were synthesized by crosslinking PDMS-V with D4H crosslinker via a hydrosilylation reaction at room temperature. Toluene was added into the system in order to decrease the viscosity of the PDMS-V and also to decrease the entanglement density of silicone chains in the final product as reported by Urayama et al.11, 56, 57

Table of contents :

General introduction
Chapter 1: Introduction to polymer physics and interpenetrating polymer network
1. Basic concepts of rubber elastic
1.1 Elasticity of a single molecule
1.2 Elasticity of a three-dimensional polymer network
2. Continuum theories of rubber elasticity
2.1 Affine network model
2.2 Phantom network theory
2.3 Slip-tube model
2.4 Mooney-Rivlin model
3. Mechanical behavior of polymers
3.1 Stress-Strain behavior of polymers
3.2 Fracture mechanics in polymers
3.3 Energy dissipation
3.3.1 The Mullins effect
3.3.2 Permanent set
3.3.3 Hysteresis
3.3.4 Anisotropy
4. Interpenetrating Polymer Networks (IPNs)
4.1 Sequential IPNs
4.2 Simultaneous Interpenetrating Network (SIN)
4.3 Latex IPNs
4.4 Gradient IPNs
4.5 Thermoplastic IPNs
4.6 Semi-IPNs
5. Conclusions and objectives of the manuscript
Chapter 2: Synthesis of silicone multiple networks
1. Silicones
1.1 General features of silicones
1.2 Crosslinking of Silicones
1.1.1 Room Temperature Vulcanization (RTV)
1.1.2 Heat-activated radical cure or High Temperature Vulcanization (HTV)
1.1.3 Transition metal catalyzed hydrosilylation or addition cure
1.3 State of the art in silicone elastomer
2. Method and synthesis
2.1 Chemicals and reagents
2.2 Synthesis and reaction conditions
2.2.1 Synthesis of the silicone highly crosslinked network
2.2.2 Silicone loosely crosslinked network
2.2.3 Extraction of the uncrosslinked fraction
2.2.4 Drying process
2.3 Stoichiometry study of silicone highly crosslinked network (1st network)
2.4 Stoichiometry study of silicone prepolymer (2nd network without crosslink)
2.4.1 Curing time of the silicone prepolymer……….
2.5 Stoichiometry study of silicone loosely crosslinked network (2nd network with crosslink)
2.6 Silicone multiple networks
2.6.1 Swelling process
2.6.2 Synthesis and reaction conditions of the multiple networks
Chapter 3: Mechanical properties and characterization of silicone multiple networks
1. Materials and methods
1.1 Rheology experiments
1.1.1 Steady shear testing
1.1.2 Dynamic mechanical testing
1.2 Mechanical testing experiments
1.2.1 Tensile test
1.2.2 Step-cycle extension
1.2.3 Fracture in a single edge notch test
2. Effect of Stoichiometry on the mechanical properties of simple silicone networks
2.1 Silicone highly crosslinked networks (1st network)
2.1.1 Stoichiometry of silicone small mesh size networks, using PDMS-V6k
2.1.2 Stoichiometry of silicone large mesh size networks, using PDMS-V17k
2.1.3 Selection of the 1st networks: summary
2.2 Silicone loosely crosslinked networks (2nd network)
2.2.1 Optimization of the stoichiometry of silicone prepolymer without any crosslinker
2.2.2 Dynamic rheology of silicone prepolymer without crosslinker
2.3 Loosely crosslinked networks of silicone
3. Silicone multiple networks
3.1 Silicone multiple networks based on PDMS-V6k made in solvent
3.2 Silicone multiple networks based on PDMS-V17k synthesized in the bulk
3.3 Silicone multiple networks based on PDMS-V17k made in solvent
4. Energy dissipation characteristic of silicone multiple networks
5. Fracture mechanic of silicone multiple networks
Chapter 4: Synthesis of latex double network films
1. Latex
1.1 General feature and application of latex
1.2 Emulsion polymerization
1.3 RAFT polymerization
1.4 Block copolymers through RAFT-mediated polymerization
1.5 Surfactant free core-shell latex mediated by RAFT polymerization
1.6 Latex film formation process
1.7 State of the art and aims of this study
2. Method and synthesis
2.1 Chemicals and reagents
2.2 Latex characterization methods
2.2.1 Gravimetric analysis
2.2.2 Nuclear Magnetic Resonance (NMR)
2.2.3 Size Exclusion Chromatography (SEC)
2.2.4 Dynamic Light Scattering (DLS)
2.2.5 pH-meter
2.2.6 Transmission Electronic Microscopy (TEM)
2.3 P(BA-co-BDA) crosslinked latex synthesized by radical emulsion polymerization
2.4 PAA-b-PBA core-shell latex synthesized by RAFT emulsion polymerization
2.4.1 PAA-TCC macro-RAFT agent
2.4.2 PAA-b-PBA latex
2.4.3 PAA-b-PBA latex with crosslinked PBA-core
2.4.4 PAA-b-PBA latex with Na+ counter ion
3. Latex film formation
3.1 Drying process and conditions
3.2 PAA-b-PBA film with added PADAME
3.3 P(BA-co-BDA) and PAA-b-PBA films with added PAA homopolymer
3.4 Thermally annealed latex film
4. Latex double network films
4.1 Swelling study of latex film
4.2 PBA elastomer, serving as an interpenetrating network to DN
4.3 Latex double network synthesis and conditions
4.4 HMP consumption effect
Chapter 5: Structure and Mechanical properties of latex films and DN made from latex films
Part I: Mechanical properties of latex films
1. Mechanical properties of the PBA, serving as an interpenetrating network for DN films
2 The differences between P(BA-co-BDA) latex films and PAA-b-PBA core-shell latex films.
2.1 P(BA-co-BDA) crosslinked latex films (0.5 mol% BDA)
2.2 PBA-b-PAA core-shell latex films
2.3 Differences between crosslinked latex, P(BA-co-BDA), and core-shell latex, PAA-b- PBA
3 Mechanical results of PAA-b-PBA core-shell latex films with modified compositions
3.1 PAA-b-PBA latex film with a crosslinked PAA-Shell
3.1.1 Crosslinked PAA-shells by added PADAME
3.1.1-1 SN films of PAA-b-PBA with added PADAME
3.1.1-2 DN films based on PAA-b-PBA with added PADAME…..
3.1.2 Ionic interactions through PAA deprotonation with NaOH instead of NH4OH
3.1.2-1 SN films of PAA-b-PBA with Na+ counter ions
3.1.2-2 DN films based on PAA-b-PBA with Na+ counter ions
3.2 PAA-b-PBA latex film with a crosslinked PBA-core
3.2.1 Crosslinked PBA-core by BDA
3.2.1-1 SN films of PAA-b-PBA crosslinked by BDA
3.2.1-2 DN films based on PAA-b-PBA crosslinked by BDA
3.2.2 Crosslinked PBA-core by DVB
3.3 The effect of the Mn on the PAA (shell thickness)
3.3.1 SN films of PAA-b-PBA with different Mn of the PAA
3.3.2 DN films based on PAA-b-PBA with different Mn of the PAA
3.4 The effect of the PBA-core size on the PAA-b-PBA latex film
3.4.1 SN films of PAA-b-PBA with different Mn of the PBA
3.4.2 DN films based on PAA-b-PBA with different Mn of the PBA
3.5 PAA-b-PBA films with added PAA5k
3.5.1 Standard latex (PAA 2.5k, PBA 100k) with added PAA5k
3.5.1-1 SN films of PAA-b-PBA with added PAA5k
3.5.1-2 DN films based on PAA-b-PBA with added PAA5k
3.5.2 High Mn PBA latex, (PAA2.5k, PBA200k) with added PAA5k
3.5.2-1 SN films of (PAA2.5k, PBA200k) with added PAA5k
3.5.2-2 DN films based on (PAA2.5k, PBA200k) with added PAA5k
Part II: Summary and discussion of the mechanical properties of the different SN and DN films
1. Method of data analysis
2. Swelling equilibrium of modified latex films
3. Toughness of DN films
4. Dissipation energy in DN films
5 Fracture toughness of DN films
Chapter 6: General conclusion and outlook


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