Gelation & Crosslinked network Gelation is the transition from liquid to soild by formation of crosslinked network of polymer chains Pre-requisites: Polyelectrolytes, single chain conformation Course Name: Gelation Level(UG/PG): PG Author: Moumita Sarkar Mentor: Dr Abhijit P Deshpande

Learning objectives After interacting with this Learning Object, the learner will be able to: Explain what are gels. Explain the steps of gelation. Explain the role of crosslinkers in gel formation. Define the critical crosslinker concentration. Explain the effect of crosslinker concentration on elasticity of gels

Master Layout (1) This animation consists of two parts: 1.What are gels? 2. What is gel point and how gelation occurs?

Definitions and Keywords 1Gel: Gel is a solid that can be soft or hard. It is formed due to formation of three dimensional crosslinked network within the liquid. 2Crosslinker: They are molecules with reactive ends, which are used to join two or more monomer molecules with chemical bonds. 3 Gel point: As the crosslinker connects the polymer chains, larger branched chain polymers are obtained. At a certain extent, a huge molecule spanning the whole system appears, called the infinite molecule. This is called the gel point. 4Critical crosslinker concentration: The density of cross-linker, enough for the system to form an infinite molecule and hence cause gelation is called the critical crosslinker density. 5Monomer: Small molecules that binds with other monomers chemically to form polymers

Examples of commercial gel Poly sodium acrylate is a 3D crosslinked polymer. It can absorb times its own weight of water. They can be used for decoration purpose as colorful balls. silicone silicone hydrogels, polyacrylamides are used for making contact lenses polyacrylamides Car parts are often made up of fibrereinforced plastic. They are polymers reinforced with fibre. Fibre is geneally fibreglass, carbon and polymer is epoxy PSA crystals en.wikipedia.orgDiaper time.com Poly sodium acrylate are used in diapers and sanitary napkins for their ability to absorb water. They are also known as waterlock Contact lens elec-intro.com Car parts are often made up of fibrereinforced plastic. They are polymers reinforced with fibre. Fibre is geneally fibreglass, carbon and polymer is epoxy Car seat z1auto.com Car bonnet junauto.co.jp FRPs are also used for making tanks and drums which can be used in chemical/pharmac eutical industries. Drum indiamart.com

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.The images should appear in the sequence as shown in the animation.

Role of monomer, initiator and crosslinker in gelation process monomer initiator crosslinker Only representation..a ctual molecular structure is complex

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.The images should appear in the sequence as shown in the animation. 2.The blue squiggly lines in the first box should move around. 3.The call out is for the viewer. 1During the gelation process, each of monomer, initiator and cross- linker plays separate roles. To understand their roles, let us assume three beakers in which monomer, initiator and crosslinker are kept separately. 2The microscopic view is shown for better understanding of the molecular mechanism. 3The microscopic view of the monomer shows some squiggly individual molecules moving around.

Role of monomer, initiator and crosslinker in gelation process Monomer + initiator Make the green balls flash and the blue lines move around

Role of monomer, initiator and crosslinker in gelation process Monomer + initiator Make the red ends of the blue squiggly lines flash and the blue lines along with the green balls move around Reactive end

Role of monomer, initiator and crosslinker in gelation process Monomer + initiator Make the red ends of the blue squiggly lines flash and the blue lines along with the green balls move around Reactive end

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.The images should appear in the sequence as shown in the animation. 2.The call out is for instruction and is not to be shown to the viewer. 1Now, lets see what happens when the monomer and initiator are mixed together. 2The magnified view of what happens in the molecular level is shown in the rectangular gray box. 3The initiator creates a reactive end in the monomer molecules, and help them join end to end to another monomer molecule. 4Thus the initiator makes the monomer molecules join end to end to form a polymer molecule.

Role of monomer, initiator and crosslinker in gelation process Monomer + initiator + crosslinker Make the orange lines flash

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.The images should appear in the sequence as shown in the animation. 2.The call out is for instruction and is not to be shown to the viewer. 1Then, some of the crosslinker is also added to the monomer and initiator solution. Lets see what happens. 2As shown in the molecular view, the role of the crosslinker, is to act as a link and join two separate polymer molecules. Like this slowly all the polymer molecules are interlinked and hence forms a 3D network. This leads to the formation of gel.

What is gel point and how gelation occurs? Reaction Time Gel point liquid gel Gel time Yellow lines shows the growing cross-linked network

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.The call-outs are to be shown to the viewer. 1Initially the box shows a solution which is a poly disperse mixture of polymer chains. 1After addition of crosslinker and initiator to the monomer solution, as the reaction proceeds, crosslinking increases in the monomer and initiator solution the size of the largest linked polymer chain increases and thus the network size increases. 2The gel point is where, the largest polymer molecule spans the whole system. This is called the percolation model. 3The state of the polymer solution is shown in the images shown below corresponding to the polymer molecule images shown above. Initially its in liquid state, then it becomes a gel after gel- point.

Role of crosslinker concentration in gelation Jar of crosslinkers Monomer and initiator solution DRAG AND DROP Molecular view after completion of reaction

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.This will be a drag and drop animation. The box will initially consist of unattached black worm-like lines. And the jar will consist of 15 balls. 2.As the viewer will drag and drop red balls from the jar into the box, The lines will be joined by the red balls. The largest joined line will be shown by yellow colour. 3.With increasing balls added, more and more lines will be joined and the size of the yellow line will increase. 4.After the viewer has added 12 balls, the yellow line will cover the box from one side to the other. This box will be shown as the gel point 5.If the viewer adds some of the more remaining balls, the yellow line will become bigger and occupy more space of the box and lesser black lines will be left. 6.The images in the box at different instants are shown in the next slide for help. 1Drag and drop the crosslinkers from the jar into the polymer solution. 2The crosslinkers links the polymer chains together. 3The polymer chain in yellow colour shows the largest linked polymer chain. 4As the amount of crosslinking increases, the size of the largest polymer chain increases. And so the network size also increases. 5At a certain extent the largest polymer molecule will span the whole system, forming an infinite molecule or a network, which is the onset of gel formation. The minimum concentration of crosslinker required for this infinite molecule to span the system, is called critical crosslinker concentration. 7 Addition of more cross-linker will increase the elasticity of the gel.

Crosslinker conc., size of network, Elasticity of gel liquidgel Critical crosslinker concentration Role of crosslinker concentration in gelation

Part 1,Step 1: ActionDescription of the actionAudio Narration As shown in animation 1.The call-outs are to be shown to the viewer. 1As the amount of crosslinking increases in the monomer and initiator solution, the size of the largest linked polymer chain increases and thus the network size increases. 2The state of the polymer solution is shown in the images. Initially its in liquid state, then it becomes a gel at critical cross-linker concentration. 3Addition of cross-linker above critical concentration increases the elasticity of the gel.

Questionnaire: 1 Gels can flow at steady state. (a) T (b) F 2. Chemical gelation involves vander waal forces. (a) T (b) F 3. Increasing the crosslinker above the critical crosslinker density ________ the elasticity of the gel. 4. Strike the odd one out: (a) Cheese (b) Fake snow (c) silly putty (d) jelly …needs furthur reading(Types of gels) 5.Match the following: 1. gel point a. Creates reactive ends 2. Covalent bond b. Joins polymer chains 3. initiator c. Infinite polymer molecule 4. crosslinker d. Chemical gels APPENDIX 1

Questionnaire: 1 Gels can flow at steady state. (a) T (b) F 2. Chemical gelation involves vander waal forces. (a) T (b) F 3. Increasing the crosslinker above the critical crosslinker concentration increases the elasticity of the gel. 4. Strike the odd one out: (a) Cheese (b) Fake snow (c) silly putty (d) jelly 5.Match the following: 1. gel point a. Infinite polymer molecule 2. Covalent bond b. Chemical gels 3. initiator c. Creates reactive ends 4. crosslinker d. Joins polymer chains APPENDIX 1

Links for further reading Reference websites: Books: Research papers: APPENDIX 2

Summary APPENDIX 3 1Gelation is the phase transition from liquid to solid, where the solid gel can range from soft to hard. Gelation happens as a result of formation of 3D-crosslinked network. 2 Initially the polymer solution is like a polydisperse mixture of polymer chains. Due to addition of crosslinkers the polymer chains links and longer chains are formed. The extent at which the largest polymer molecule spans the whole system, is the gel point. 3Elasticity of gel formed is directly proportional to the concentration of crosslinkers used.