Molecular toughening mechanism, mechanoluminescence as a probe for bond breaking

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Polymerization Conditions and Environment

To avoid side reactions due to the presence of oxygen, polymerization steps were performed in a glove box under nitrogen atmosphere, with less than 3 ppm of oxygen (MBraun Unilab). Before being introduced into the glove box, every reagent and solvent was bubbled with nitrogen for 45 min to assure a low level of dissolved oxygen. Reactive species were stored in a freezer at -20 °C to avoid self initiated polymerization once in the glove box. Polymerizations were carried out under UV irradiation at low power (10 μW/cm²) measured with a radiometer VLX-3W from Vilber Lourmat. UV light was produced by a Vilbert Lourmat lamp (model VL-215.L), centered on 365 nm. The emitted light was filtered by PET sheets to decrease the UV intensity. Inspired from Gong’s procedure to prepare DN hydrogels1, we decided to proceed to a low power UV irradiation in order to decrease the number of chains growing simultaneously and to decrease the number of termination reactions, letting the network grow slowly.
Samples synthesized by the polymerization of a liquid solution were prepared in a glass mold composed of two glass plates with iron spacers and a Teflon tubing which sealed the mold. The whole setup was tightened between metal frames to assure a good contact of the glass plates with the spacers and the tubing. Spacers were used to keep a good parallelism between the plates and a fixed spacing between them, fixing the sample’s thickness at 1 mm. The Teflon tubing had anexternal diameter around 1.8 mm, it was compressed and acted as an O-ring seal.

General path to multiple networks elastomers

The synthesis of multiple networks elastomers is a multiple sequential step process. Starting from the monomer, a first network is prepared, followed by one or two sequences of swelling and polymerization. The general procedure is presented in this part, as well as prepared samples and their compositions.

First polymerization (Simple networks)

Single networks of PEA were prepared by free radical polymerization of a solution of monomer, BDA as crosslinker and HMP as UV initiator. The reactants were dissolved in anhydrous toluene as solvent at 50 wt %. A synthesis in solvent conditions is known to decrease the entanglements in the final system as reported by Urayama et al.3,4, and makes it more swellable. All the reactants initially deoxygenated and stored in the glove box were mixed for a short time and poured in a glass mold. The reaction was initiated by UV and left to proceed for 90 minutes. The sample was then taken out of the glove box and out of the mold, weighed and immersed in solvent baths for a week to remove every soluble components, unreacted species or free chains. The solvent was changed every day. Samples were finally dried under vacuum at 80 °C overnight and then weighed and stored at room temperature for later use. Those samples will be referred as simple networks (SN).

Second polymerization (Double networks)

Starting from a first network, double networks were prepared following a swelling and polymerization sequence. A piece of first network was swollen in a bath composed of the second monomer, BDA as crosslinker and HMP as UV initiator. The swelling step was performed in a sealed plastic box in the glove box with oxygen-free reactants, the sample being deposited on a Teflon and glass basket to help the recovery of the swollen sample. Once swollen at equilibrium, after two hours, the sample was carefully extracted from the monomer bath, gently wiped to eliminate the excess of monomer, and placed between siliconized PET sheets and glass plates. The mold was then exposed to the UV for two hours to initiate and complete the polymerization. The mold was then taken out of the glove box, and the sample was removed, weighed and dried under vacuum at 80 °C for a night. It was finally stored at room temperature until later use. Those samples will be referred as double networks (DN).

Theoretical background on DMA

The Dynamic Mechanical Analysis (DMA) experiment is a powerful tool to investigate the viscoelastic properties of a polymer as a function of temperature (generally at one fixed frequency of 1 rad/s). The DMA consist of loading a small strip of material with a sinusoidal strain 􀔖, at a certain frequency f.
The resulting stress σ associated to this strain has the following expression where δ is the out-ofphase angle between strain and stress.

Table of contents :

Chapter 1 –Physics of polymer networks and networks design
1- General concepts on rubber elasticity
1-1- Ideal chain model
1-2- Entropy and free energy of an ideal chain
1-3- Polymer networks
1-3-1- Affine network model
1-4- Crosslinks and entanglements
1-4-1- At small strain
1-4-2- At intermediate strain
Mooney-Rivlin model
Rubinstein and Panyukov model
1-5- High strain region
2- Fracture mechanics of rubbers
2-1- Model of Lake and Thomas
2-2- Experimental evaluation of the fracture toughness
3- Network design
3-1- Bimodal networks
3-2- Interpenetrated networks
3-2-1- Double networks hydrogels
Synthesis and structure
Mechanical properties and toughening mechanism
Model for the fracture of DN hydrogels
Recent developments on DN gel structure
3-2-2- Interpenetrated elastomers
3-3- Introduction of prestretched chains in a polymer network
Conclusions and objectives of the manuscript
Chapter 2 – Introduction of isotropically stretched chains in a polymer network 
1- Synthesis
1-1- Chemicals
1-2- Polymerization Conditions and Environment
1-3- General path to multiple networks elastomers
1-3-1- First polymerization (Simple networks)
Samples compositions
Solvent for deswelling
Shrinking and digitations
1-3-2- Second polymerization (Double networks)
Samples compositions
1-3-3- Third polymerization (Triple networks)
1-4- Variations on the second network
1-5- Solvent free simple networks
1-5-1- Second/Third Network alone
1-5-2- Simple networks of EA
2- Extractable and conversion
2-1- First networks
2-2- Bulk samples
3- Structural properties by thermomechanical analysis
3-1- Theoretical background on DMA
3-2- Results and discussion
3-2-1- Simple networks
3-2-2- Multiple networks with a single monomer type
3-2-3- Multiple networks with a contrast in monomer
Miscible Multiple networks
Immiscible Multiple networks
Chapter 3 – Mechanical signature of isotropically prestretched chains: a macroscopic investigation
1- Material and methods
1-1- Mechanical testing experiments
1-2- Tensile tests
1-3- Step-Cycle extension
1-4- Fracture in single edge notch test
2- Simple networks alone: weak elastomers
2-1- Effect of the crosslinker concentration in simple networks
2-2- First network: brittle and tunable
Crosslinker concentration
Nature of the monomer
2-3- Fracture properties of simple networks
3- Multiple networks: stretching the prestretched
3-1- General behavior
3-2- Origin of the initial modulus
3-3- The origin of stiffening and softening
3-4- Less extensible second network
4- Variations of monomers
4-1- Changes in the second network
4-2- Changes in the composition of the first network
5- Cyclic extension: from pure elasticity to bulk dissipation
5-1- Viscoelastic behavior in second networks alone
5-2- Perfect reversibility of the elasticity in double networks
5-3- Mullins effect in Triple networks
5-4- Energy dissipation and bond breaking mechanism
6- Fracture properties
6-1- Experimental results and discussion
6-2- Models
Lake and Thomas
Brown and Tanaka models for DN hydrogels
Chapter 4 – A molecular insight in prestreched chains conformation
1- Small Angle Neutron Scattering to probe polymeric chains conformations.
1-1- Generalities
1-2- Scattering from polymer melts
1-3- Scattering and anisotropy
1-4- Experimental setup
1-5- Data treatment
2- Labeled multiple networks elastomers: from monomer synthesis to mechanical properties
2-1- Synthesis of deuterated monomers
2-1-1- Procedure
2-1-2- Analysis
2-2- Labeled multiple networks
2-3- Check of the mechanical properties
3- Chains of the first network: undeformed samples
3-1- Partially deuterated multiple networks of pure poly(ethyl acrylate)
3-2- Comparison between EA and MA as second/third monomers
4- Stretching the prestretched chains
4-1- Scattering pattern of stretched multiple networks
4-2- Scattering along the principal directions
4-2-1- In double networks
4-2-2- In triple networks
4-2-3- Sacrificial bonds and scattering
Summary of SANS results
Chapter 5 – Molecular toughening mechanism, mechanoluminescence as a probe for bond breaking
1- Colors and light in polymers under stress
2- Mechanoluminescent crosslinker in multiple network elastomers
2-1- Mechanoluminescent crosslinker
2-2- Synthesis of dyed networks
2-3- Comments
Self polymerization
Successful incorporation in networks
3- Dioxetane as a probe of a bond breaking mechanism
3-1- Experimental conditions and data treatment
3-2- Results
3-2-1- Mechanical properties
3-2-2- Luminescence signal
3-3- Luminescence and mechanical properties
3-4- Stress or Strain sensor?
4- Fracture and luminescence
4-1- Experimental conditions and data treatment
4-1-1- Mechanical part of the experiment
4-1-2- Optical part of the experiment
4-2- Results
4-2-1- Fracture toughness
4-2-2- Before crack propagation
4-2-3- Crack propagation
Simple network EA0.5m
Double and Triple networks
Quantitative analysis
Crack velocity
4-3- Molecular description of the dissipation mechanism
Chapter 6 – From heterogeneous to Homogeneous first network .
1- Homogeneous first network in multiple networks elastomers
1-1- Chain synthesis by Atom Transfer Radical Polymerization
1-1-1- ATRP Principle
1-1-2- Initiator synthesis
1-1-3- Polymerization conditions
1-2- End functionalization
1-3- Perfect first network
1-4- Synthesis of DN and TN
1-5- Comments on other methods to end-functionalize the chains
Atom Transfer Radical Coupling
Click chemistry: alkyne /azide reaction
2- Properties of ‘perfect’ multiple networks
2-1- Not so ‘Perfect’ first network
2-2- ‘Perfect’ multiple networks under deformation
2-1- Fracture of ‘perfect’ multiple networks

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