Identifying polymers



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King Saud University

Faculty of Science

Chemistry Department

Physical Chemistry of Polymers
330 Chem.
1432-1433 AH


LIST OF EXPERIMENTS (330 Chem)


Week

Date

Experiment

1




Identifying polymers

2




Making a plastic from potato starch

3



PVA polymer slime


4




Bulk polymerization of Styrene

5







6







7







8







9







10







11







Grading:
Grading in this course will be based on your total accumulated points. The table below summarizes the graded activities that affect your grade.



Graded Activity

Score

2 Quizzes

10

Reports

10

Evaluations

10


EXPERIMENT (1)

Identifying polymers

Identifying polymers

Samples of several polymers found as everyday plastics are placed into a range of liquids of known density and observed, to see whether they float or sink. From a table of known polymer densities, it is possible to identify each polymer.


Apparatus and equipment:

Each working group will require:

  • Eye protection, goggles.

  • 6 Boiling tubes.

  • Test-tube rack.

  • Glass stirring rod.

  • Samples of plastics for testing .

  • Scissors capable of cutting plastic samples .

  • Paper towels.

  • Method for labelling boiling tubes.

  • Access to liquids of known density, (1 – 6) in the table.


Technical notes:

  • Large, 150 x 25 mm, test-tubes.

  • Samples of polymers: plastics for testing. Everyday items will be a useful source; most of these will carry the recycling symbol with identification number and code as shown below:



Code

Types.



• Polyethylene terephthalate, PET – most plastic bottles for fizzy drinks, ovenproof food trays and roasting bags, audio and videotape.



• High density polyethylene, HDPE – plastic bottles for milk, fruit juices, household cleaners and chemicals. Motor oil containers, some carrier bags and most aerosol caps.



• Polyvinyl chloride, PVC – plastic bottles for mineral water, fruit squash, cooking oil and shampoo. Sandwich and cake packs, food packaging trays, DIY blister packs, baby care product containers, cling film, ring-binder covers, records and watch straps.



• Low density polyethylene, LDPE – 'Jif' lemon juice container. Some squeezy containers for sauces, cosmetics and plastic films (shrink wrap), sacks, freezer bags, carrier bags that are not crinkly, disposable pipettes, some aerosol caps, some plant pots and ink-tubes in ball-point pens.



• Polypropylene, PP – plastic straws, containers for soft cheeses and fats, some margarine tubs, microwaveable food tubs and trays, film bags for crisps, biscuits and snacks, ketchup bottles and bottle caps.



• Polystyrene, PS – yoghurt pots, margarine tubs, clear egg boxes, food packaging trays, plastic cutlery and cups, clear plastic glasses, ball-point pen cases, cassette boxes and plastic coat-hangers.

No code necessary.

• Expanded polystyrene, EPS – fast food packaging, meat packaging trays and egg boxes.

Each working group will need 6 samples about 5 mm square of each plastic, one to be added to each of the six solutions. Each plastic can either be provided as a larger flat, thin sheet with scissors to cut the samples, or as ready cut samples.



liquids of known density:

different liquids with different density such as dichloromethane, water, DMF, butanol, cyclohexane and petroleum ether. Details for solutions of known density are as follows. Each working group will need approximately 15 cm3 of each liquid.



Liquid

Density in g/cm3

liquids

1

1.3

dichloromethane

2

1

distilled water

3

0.94

DMF

4

0.8

butanol

5

0.7

cyclohexane

6

0.6

petroleum ether

  • you will need to be labelled with the name of the container they come from, or with a code letter or number.

  • note that product containers and their lids are not always made out of the same material so it is important to check their identities.

  • liquids should be labelled with their densities.

The densities of the seven polymers are somewhat variable. The table below gives the expected normal range of variation for each:

Abbreviation

Common name

Chemical name for polymer

Density range/g cm-3

EPS

expanded polystyrene

poly(phenylethene)

0.02 – 0.06

PP

polypropylene

poly(propene)

0.89 – 0.91

LDPE

low density polyethylene

poly(ethene)

0.91 – 0.93

HDPE

high density polyethylene

poly(ethene)

0.94 – 0.96

PS

polystyrene

poly(phenylethene)

1.04 – 1.11

PVC

polyvinyl chloride

poly(chloroethene)

1.20 – 1.55

PET

polyethylene terephthalate

poly(ethenediyl-1,4-benzenedicarboxylate)

1.38 – 1.40

Procedure:

  • Add about 15 cm3 of each of liquids 1 – 6 to a different boiling tube in a test-tube rack. Label each tube with the relevant solution number .

  • Prepare 6 samples of each of the plastics to be tested. Each sample should be a square of approximately 5x 5 mm. You may organise your samples by laying them out in separate piles on a paper towel. Make sure you know which sample is which, either by labelling each with a code letter, or by noting the colour. Record these in a suitable table such as:

    Sample Code

    Observations For Floating/ Sinking in solution number

    Identity of plastic

    1

    2

    3

    4

    5

    6

































































































  • Add samples of each plastic to each of the 6 solutions, so that each tube has samples of six different plastics.

  • Observe whether the plastics float or sink. A sample will sink if its density is greater than the density of the solution. For samples that sink, write the letter S in the appropriate rows and columns of the results table. You may wish as well to write the letter F for those that float, so that you can check you have noted every sample tested.

  • In discussion with the teacher and the rest of the class, you should be able to identify the polymer from which each plastic is made. When you have decided what a sample is made of, write the name in the last column of your table. As an example, the identity of polystyrene will be confirmed if samples float in a solution of density 1.15 g cm–3 but sink in a solution of density 1.00 g cm–3. An ideal table of results will look like.

Air bubbles adhering to samples can give serious problems as they may be difficult to dislodge, causing ‘sinkers’ to remain afloat. This is the purpose of stirring with the glass rod. Students can be asked why the stirring is done after they have added the samples to test-tubes and given a first stir. The discussion should then result in students checking for any remaining air bubbles, and if necessary stirring again to dislodge them.

  • Note that materials made of polymers may also contain other substances as fillers, plasticisers, stabilisers, etc., which may make the density of a particular sample fall outside the ranges indicated; note the wide range of PVC densities in the table above.





EXPERIMENT (2)

Making a plastic from potato starch

Making a plastic from potato starch:

In this activity you will make a plastic from potato starch and investigate the effect of adding a "plasticiser" has on the properties of the polymer that they make.


Apparatus and chemicals


Each working group requires:


  • Eye protection.

  • Beakers (250 cm3),

  • Measuring cylinder (10 cm3)

  • Large watch glass.

  • Petri dish .

  • Hot plate.

  • Stirring rod.

  • Universal indicator paper.

Access to:

  • Balance.

  • Starch.

  • Food coloring.

  • Propane-1,2,3-triol (glycerol).

  • Dilute hydrochloric acid, 0.1 mol dm-3

  • Dilute sodium hydroxide, 0.1 mol dm-3.

Procedure

Making the plastic film

  • Put 25 cm3 of water into the beaker and add 2.5 g of the starch, 3 cm3 of hydrochloric acid and 2 cm3 of propane-1,2,3-triol.

  • Put the watch glass on the beaker and heat the mixture using the Hot plate. Bring it carefully to the boil and then boil it gently for 15 mins. Do not boil it dry. If it looks like it might, stop heating.

  • Dip the glass rod into the mixture and dot it onto the indicator paper to measure the pH. Add enough sodium hydroxide solution to neutralise the mixture, testing after each addition with indicator paper. You will probably need to add about the same amount as you did of acid at the beginning (3 cm3).

  • You can then add a drop of food colouring and mix thoroughly.

  • Pour the mixture onto a labelled petri dish or white tile and push it around with the glass rod so that there is an even covering.

  • Repeat the process, but leave out the propane-1,2,3-triol.

  • Label the mixtures and leave them to dry out. It takes about one day on a radiator or sunny windowsill, or two days at room temperature. Alternatively, use a drying cabinet. It takes about 90 mins at 100 °C.


Teaching notes

This activity can be used simply as a practical to enhance the teaching of polymers or plastics. It can be used to introduce further work on biopolymers and bioplastics and/or it can be used as an example of the effects of plasticisers. A similar process is used in industry to extract starch, which is then used in a number of products including food and packaging.

If you have a drying cabinet, the mixture should dry in about 90 mins at 100 °C.


  • You shouldn’t let the mixture boil dry, or it ‘pops’ and has a tendency to jump out of the beaker. For this reason, students should wear eye protection at all stages.

  • While using food colouring is optional, it does enhance the product and the colour it gives makes the plastic film look more like plastic. Only one drop is needed or the film is too dark.

  • If you use too much water then their polymer won’t solidify and remains a liquid.

  • Starch is made of long chains of glucose molecules joined together. Strictly it contains two polymers: amylose which is straight-chained and amylopectin which is branched. When starch is dried from an aqueous solution it forms a film due to hydrogen bonding between the chains. However, the amylopectin inhibits the formation of the film. Reacting the starch with hydrochloric acid breaks down the amylopectin, forming more satisfactory film. This is the product that students make without propane-1,2,3-triol. The straight chains of the starch (amylose) can line up together and although this makes a good film, it is brittle because the chains are too good at lining up. Areas of the film can become crystalline, which causes the brittleness.


Section of a starch molecule (amylose and amylopectin)
you should be able to see a difference in the two films that they make. The one without the propane-1,2,3-triol is far more brittle, the one with it shows more plastic properties. Adding propane-1,2,3-triol makes a difference due to its hydroscopic (water attracting) properties. Water bound to the propane-1,2,3-triol gets in amongst the starch chains and stops the crystalline areas from forming, preventing the brittleness and resulting in more ‘plastic’ properties, thus acting as a plasticiser. This can be explained to students without mentioning water – just that the propane-1,2,3-triol acts as a plasticiser.


EXPERIMENT (3)

Making rayon

Making rayon

Cellulose, in the form of cotton wool or filter paper, is dissolved in a solution containing tetraamminecopper(II) ions to produce a viscous blue liquid. This liquid is injected into sulfuric acid with a syringe to form rayon fibres.


Apparatus and chemicals

  • Eye protection.

  • Beakers (250 cm3), 2 Beaker (1 dm3).

  • Glass stirring rod.

  • Plastic syringe (10 cm3 or 20 cm3) fitted with a hypodermic needle.

  • Access to a fume cupboard.

  • Access to a magnetic stirrer (optional).

  • Basic copper carbonate (CuCO3.Cu(OH)2.H2O) (Harmful), 10 g
    Concentrated 880 ammonia solution (Toxic, Corrosive, Danger to the environment), 100 cm3 Sulfuric acid, about 1 mol dm–3 (Irritant at this concentration), 500 cm3 Cotton wool (about 2 cotton balls), 2 g


Technical notes:

  • If a hypodermic needle is to be used, this must be kept well supervised e.g. in a locked cupboard, until needed and after use. Alternatively, a hypodermic needle need not be used, however the fibre produced will be less fine.

  • It is important that the cotton wool is pure cotton and does not contain any synthetic fibres. Two grams of filter paper, or even newspaper, can be used as an alternative. However, the best results are obtained with cotton wool.

  • Glassware can be cleaned with dilute ammonia solution.


Procedure

You can prepare a solution of cellulose before the demonstration to save time.



  • Weigh 10 g of basic copper carbonate into one of the 250 cm3 beakers, and, working in a fume cupboard, add 100 cm3 of 880 ammonia solution.

  • Stir (with a magnetic stirrer, if available) for two minutes, allow to settle and then decant the resulting deep blue solution – which contains tetraamminecopper(II) ions – into the second 250 cm3 beaker.

  • Add bits of the finely shredded cotton wool, slowly and with stirring, until the solution has the consistency of shower gel. This uses about 1–1.5 g of the wool.

  • Stir until there are no lumps, but avoid trapping any air bubbles in the liquid. Complete dissolution may take up to an hour.

The demonstration

  • Withdraw a few cm3 of this viscous solution – which is called ‘viscose’ – into the plastic syringe, avoiding taking up any remaining lumps.

  • Fit a hypodermic needle to the syringe and inject a stream of viscose under the surface of about 500 cm3 of the sulfuric acid in the 1 dm3 beaker.

  • A thin blue rayon fibre will forms. This slowly turns white as the acid neutralises the alkaline tetraamminecopper(II) solution, and destroys the complex.

  • After a few minutes, remove the rayon fibre carefully and wash under a stream of tap water and leave to dry on a filter paper. The fibre is likely to be relatively weak.


Teaching notes

You may not only prefer to make the viscose solution beforehand, but also form some rayon fibres using the syringe to have these in reserve in case the demonstration does not go according to plan.

Rayon is a so-called ‘regenerated fibre’ which was once called artificial silk. The polymer contains about 270 glucose units per molecule compared with cotton, which contains between 2000 and 10,000.

The first step in the demonstration is a reaction of basic copper carbonate with aqueous ammonia to form tetraamminecopper(II) ions:

CuCO3.Cu(OH)2.H2O(s) + 4NH3(aq) → CuCO3(s) + [Cu(NH3)4]2+(aq) + 2OH(aq) + H2O(l)

When the insoluble cellulose is added to this solution it is converted to a soluble complex compound. This in turn is converted into insoluble rayon once the pH is reduced to the acidic value found in molar sulfuric acid. Accordingly, rayon precipitates out when extruded into the acid. The blue colour quickly fades away after the copper(II) ions diffuse into the solution.

On an industrial level, the blue solution is passed through spinnerets and regenerated in a hardening bath that neutralises the product and removes the copper and ammonia.

‘Cuprammonium rayon’ is usually made in fine filaments that are used in blouses, lightweight summer dresses and in combination with cotton for textured fabrics.

The ‘cuprammonium process’ duplicated in this activity is one of the earliest methods used for producing rayon, but is less cost-effective now than some other more modern methods.

Rayon is used to manufacture carpets, tyre cords and surgical materials as well as clothing.



EXPERIMENT (4)

Synthesis of Nylon 6, 10 from Interfacial Polymerization

Synthesis of Nylon 6, 10 from Interfacial Polymerization:

The goal of this lab is to synthesize Nylon 6, 10 membranes through interfacial polymerization of sebacoyl chloride (SC) and hexamethylene diamine (HMDA). Each team will conduct the synthesis using different compositions of SC and HMDA in their corresponding solutions and qualitatively evaluate their effect on the quality of the resulting membranes. Following literatures can be consulted for the experimental procedure:

N. C. Rose, J. Chem. Ed. 1967, 44, 283.

P. W. Morgan, S. L. Kwolek, J. Chem. Ed. 1959, 36, 182.

P. W. Morgan, S. L. Kwolek, J. Chem. Ed. 1959, 36, 530.


Aqueous phase

Organic phase

2.2 g HMDA, 4.0g Na2CO3, 50 mL H2O

1.5 mL SC, 50 mL CH2Cl2

1.1 g HMDA, 4.0g Na2CO3, 50 mL H2O

0.75 mL SC, 50 mL CH2Cl2

0.82 g HMDA, 4.0g Na2CO3, 50 mL H2O

1.5 mL SC, 50 mL CH2Cl2

SC: FW 239.14, d 1.119 g/mL at 20 °C, 1.121 g/mL at 25 °C

HMDA: FW 116.20, d 0.89 g/mL at 25 °C

Na2CO3: FW 105.99

Questions:

  1. Draw a balanced chemical equation for the synthesis of Nylon 6, 10.



  1. Is stoichiometric balance important for the success of interfacial polymerization? why?

Interfacial polymerization kinetic is a complicated process and there is still research conducted in this area in attempt to model the reaction kinetics, MW dependence on reaction conditions.* The stoichiometric balance at the reaction zone is important for high MW polyamide formation. But the balance in reaction zone may not necessarily correlate with the stoichiometric balance in the two bulk phases. It also depends on the diffusion of the molecule towards the interfacial reaction zone. It was shown that for the non-stirred interfacial polymerization of nylon (6,10) the reaction zone lies towards the organic phase, and the amine diffuses through the polymer films to react with the acyl halide in the organic phase. Some modeling work has supported the experimental findings that SC bulk concentration affects the MW of the forming polymers. Higher MW is formed when SC and HMDA content is more balanced at the interfacial reaction zone.

* S. K. Karode, S. S. Kulkarni, A. K. Suresh, R. A. Mashelkar, Chem. Eng. Sci. 1997, 52, 3243.


Name a couple of industrially important polymers that are prepared from interfacial polymerization. Examples: nylon 6, 10; biosphenol-A polycarbonate

EXPERIMENT (5)

PVA polymer slime



PVA polymer slime


A solution of polyvinyl alcohol (PVA) can be made into a slime by adding borax solution, which creates crosslinks between polymer chains. In this activity, some interesting properties of the slime are investigated. you are guaranteed to enjoy the activities involved.

This experiment is easy to set up and should take no more than about 30 mins.


Apparatus and chemicals


Each working group requires:

  • Goggles.

  • Beaker (100 cm3)

  • Measuring cylinder (50 cm3).

  • Disposable plastic cup.

  • Metal spatula.

  • Petri dish (or watch glass).

  • Water-based felt-tipped pen.

  • Spirit-based felt-tipped pen.

  • Disposable plastic gloves.



  • Access to:

  • Polyvinyl alcohol, (-[CH2CH(OH)]n-), 4% (or 8%)aqueous solution, 40 cm3 .

  • Borax, hydrated sodium tetraborate (Na2B4O7.10H2O), 4% (or 8%) aqueous solution, 10 cm3.

  • Food colour or fluorescein.

  • Hydrochloric acid, about 0.5 mol dm-3, 20 cm3.

  • Sodium hydroxide, about 0.5 mol dm-3, 20 cm3.

Technical notes


  • Polyvinyl alcohol (PVA) can be high MW (about 120 000) or low MW (about 15 000). If high MW PVA is used, prepare a 4% solution by placing 960 cm3 of water into a tall 1 dm3 beaker. Measure out 40 g of high MW PVA and add this slowly to the beaker of water, with stirring. If low MW PVA is used, prepare an 8% solution by placing 920 cm3 of water into a tall 1 dm3 beaker. Measure out 80 g of low MW PVA and add this slowly to the beaker of water, with stirring.

  • If each case, heat the mixture gently, stirring occasionally, until the solution clears. Avoid boiling the solution. After cooling, this solution can be poured into suitable smaller containers, which can then be sealed and stored indefinitely.

  • If a 4% aqueous solution of PVA is used a 4% aqueous solution of borax will be required. If an 8% aqueous solution of PVA is used an 8% aqueous solution of borax will be required.

  • The hydrochloric acid and aqueous sodium hydroxide are best supplied in small glass bottles fitted with teat pipettes.


Procedure


  • Place 40 cm3 of the polyvinyl alcohol solution in the plastic cup.

  • If supplied, add one drop of food colour or fluorescein dye to the solution. Stir well.

  • Measure out 10 cm3 of borax solution into the beaker and add this to the polyvinyl alcohol solution, stirring vigorously until gelling is complete. This gel is sometimes known as a ‘slime’.

  • Wearing disposable gloves, remove the slime from the cup and knead it thoroughly to mix the contents completely. Roll the slime around in your hand, gently squeezing the material to remove air bubbles at the same time. Alternatively, place the slime in a plastic bag and mix and squeeze the mixture from outside the bag.

Tests

Test the properties of your slime in the following ways.

  1. Pull the slime apart slowly. What happens?

  2. Pull the slime apart sharply and quickly. What happens?

  3. Roll the slime into a ball and drop it on to the bench. What happens?

  4. Place a small bit of slime on the bench and hit it hard with your hand. What happens?

  5. Write your name on a piece of paper with a water-based felt-tipped pen. Place the slime on top, press firmly, and then lift up the slime. What has happened to the writing and to the slime? Try the same again, this time using a spirit-based pen. Does this show the same effect?

Tests 6–8 below are optional.

  1. Place a very small piece of slime in a Petri dish. Add the dilute hydrochloric acid dropwise, stirring well after each drop. When you notice a change record the number of drops added and your observations.

  2. Now add dilute sodium hydroxide solution to the same sample used above in 6, stirring after each drop. When you notice a change record the number of drops added and your observations.

  3. Can tests 6 and 7 be repeated time and time again to give the same results?

Teaching notes:


  • keep the slime away from clothes as it can produce permanent stains.

  • The slime can be stored in an air-tight container, such as a plastic bag with a twist-tie. It is advisable to dip the slime in some water before storing, to keep it from drying out.

  • Slime gets dirty from handling and may become mouldy after several days. When this happens you should throw it away.

  • Do not put it down the sink because it clogs the drain.

  • Slime-type materials are available under a variety of different brand names, and can be found in many toy stores.

  • Slime is sometimes described as a reversible cross-linking gel. The cross-linking between the polymer chains of polyvinyl alcohol occurs by adding borax, Na2B4O7.10H2O (sodium tetraborate).

  • PVA glue contains the polymer polyvinyl alcohol (also called polyethenol) and has the structure:

Borax forms the borate ion when in solution. This ion has the structure:



The borate ion can make weak bonds with the OH groups in the polymer chains so it can link the chains together as shown below. This is called cross-linking.





  • Slime is a non-Newtonian fluid that is dilatant – ie under stress, the material dilates or expands. Other well known stress-thickening materials are quicksand, wet sand on the beach, some printer’s inks, starch solutions and ‘Silly Putty’. Dilatant materials tend to have some unusual properties.

• Under low stress, such as slowly pulling on the material, it will flow and stretch. If careful, you can form a thin film.

• Pull sharply (high stress) and the material breaks.

• Pour the material from its container then tip the container upwards slightly, the gel self siphons.

• Put a small amount of the material on a table top and hit it with your hand, there is no splashing or splattering.

• Throw a small piece onto a hard surface; it will bounce slightly.

Adding acid to the slime breaks the crosslinking producing a liquid with lower viscosity. Adding alkali reverses the process and the slime should be regenerated.



Various types of slime have been manufactured. In this investigation you use the polymer polyvinyl alcohol, which is reasonably cheap and is readily available from suppliers because it is widely used as a thickener, stabiliser and binder in cosmetics, paper cloth, films, cements and mortars. In ethanol solution polyvinyl alcohol solution dries to leave a thin plastic film that is useful in packaging materials, especially as it is biodegradable.

EXPERIMENT (6)

Urea-formaldehyde polymerisation



Urea-formaldehyde polymerisation:


Urea is dissolved in aqueous formaldehyde in a throw-away container. Acidification of this solution initiates condensation polymerisation, and a hard, white, thermosetting polymer is formed within a few minutes. After washing, the properties of this substance can be investigated. If a mould is used, the experiment can be extended to show the formation of plastic articles made in a mould by condensation polymerisation. This is a teacher demonstration, taking about 5 minutes for one polymerisation experiment. Extensions to show production of moulded articles will take longer. It should be performed in a fume cupboard or hood to avoid exposure to formaldehyde vapour.

Apparatus and Chemicals


Each working group requires:


  • Eye protection, goggles.

  • Disposable nitrile gloves (for handling the polymer).

  • Access to a fume cupboard visible to the class.

  • Measuring cylinder, 100 cm3.

  • Throw-away containers with secure lids, at least 2

  • Dropping pipette

  • Glass stirring rod.

  • Object of simple shape for making suitable mould.

  • Bunsen burner.

  • Tongs.

Technical notes


  • The container used should be transparent, disposable, and preferably should have a secure lid. Used 100 g coffee jars with screw tops are ideal, but any similar size glass or plastic container will do.

  • A mould for casting copies of an object can be made by pressing the object into Plasticene. The object selected should have a simple external shape. Enough Plasticene should be provided to press the shape into, forming a sufficiently substantial mould to retain its shape when handled.

  • Formaldehyde solution (also known as formaldehyde solution, or formalin) should be in good condition. Because the vapour is unpleasant as well as toxic, the supply of this solution for this demonstration should be kept in the fume cupboard, in a stoppered bottle.

  • The acid used should be sulfuric acid, and NOT hydrochloric acid, because of the possibility of forming the carcinogen, bis-chloromethylether.

  • After the demonstration, dispose of the sealed container with the polymer inside as solid waste.

Procedure:


  • Dissolve 10 g of urea in 20 cm3 of formaldehyde solution in the disposable container.

  • Add about 1 cm3 of concentrated sulfuric acid a drop at a time, using a dropping pipette, and stir steadily. Within a minute the solution begins to go milky and eventually a hard, white solid is formed which is difficult to remove from the container. A lot of heat is evolved.

  • Show that the polymer is hard by poking the material with a spatula.

  • Wash the polymer thoroughly before passing around the class, as it is likely to be contaminated with unreacted starting materials. Alternatively, pass it round in the container with the lid screwed on.

  • Make a plasticene mould from the simple shape and line it with aluminium foil.

  • Make another urea-formaldehyde solution as above, but, immediately after adding the acid, pour some of the solution into the mould and allow it to polymerise. Remove from the mould when solid.

  • Hold a sample of the polymer (from the mould) with tongs and heat in a Bunsen flame. It will char but not melt, showing that it is a thermosetting polymer.


Teaching notes:


The reaction is a condensation polymerisation in which water is eliminated as the hydrogen atoms from the ends of one amino-group from each of two urea molecules combine with the oxygen atom from a formaldehyde molecule. The remaining –CH2– group from the formaldehyde molecule then forms a bridge between two neighbouring urea molecules, as shown below. This process, repeated many thousands of times, forms long chains of urea and formaldehyde molecules linked in this way.

Sometimes the second hydrogen atom on an amino-group will also react with a formaldehyde molecule, producing a branch in the chain, and chains may even become cross-linked to each other. Eventually a random three-dimensional network of cross-linked chains is formed, giving a rigid structure and thus a hard, inflexible material.




The product has many cross links:



Because the tangle of cross-linked chains is almost impossible to separate, the material does not melt on heating, although it will eventually break down at high temperature, decomposing and giving off small molecules such as steam and nitrogen, leaving a charred mass which is largely carbon.





Bulk polymerization of Styrene

Ingredients:

  • 1- gm Styrene.

  • 35.2 mg AIBN.

  • Toluene (l) (to dissolve the polystyrene) ~30ml

  • Propanol (l) ( to precipitate the polystyrene ) ~ 100ml

  • Hot plate and a water bath


Procedure:

  • Place water in the bath and heat to ~80°C (You need enough water in bath to submerge the contents of the test tube in hot water

  • Add 4gm of styrene in a test tube.

  • Add 35 mg (excess) of AIBN to the beaker. Stir to dissolve the AIBN in styrene with a glass rod. AIBN is used to initiate the styrene on heating above 60 C. The excess AIBN reacts with the stabilizer BHT which is a free radical scavenger present in styrene. You can also distill the styrene to remove BHT for a cleaner reaction.

  • Place the test tube in the hot water bath for about 20-25 mins.

  • You will notice the solution getting thicker indicating the formation of polystyrene.

  • Remove the beaker from the bath and dip the beaker in cold water for a few minutes (to quench). It will be ideal to use ice for quenching, but cold water from tap works.

  • The quenching process will either solidify or really thicken the polystyrene.

  • Add toluene to the test tube and transfer to a beaker to add the remaining toluene to a total of 30 ml to dissolve the polystyrene. Stir well. Toluene is a good solvent for the monomer and polymer.

  • Once the polystyrene is dissolved, add excess propanol while stirring. Propanol is a non-solvent for styrene but has some miscibility with monomer and toluene. You will immediately notice the white precipitate of polystyrene. Continue stirring at this time.

  • After stirring let the contents in the beaker settle down for a couple of minutes and add more propanol. You will see the polystyrene (white chunk) form in the beaker. Decant the toluene/propanol solution and wash the polystyrene precipitate with toluene a few more times. The polystyrene can be removed and dried on filter paper.


Precautions:

  • Styrene vapors are potentially carcinogenic (still under debate, check Wiki page of Styrene). You may cover the beaker while heating to reduce the release of styrene vapors. For small amounts this procedure can be conducted with no hood as long as the room is relatively well ventilated.

  • AIBN should be handled with gloves and avoid contact with AIBN, dust or vapors.

Second method for polymerization of Polystyrene
a) In a fume hood to 10 mL of toluene and 3.64g (4 mL) of styrene in a large test tube, 0.3g benzoyl peroxide.  [Care: do not CRUSH this solid].    
b) Place the test tube in a beaker of boiling water for about 1 hour.
c) Cool the tube, and note the viscosity of the solution. 
d) Pour the solution slowly and with continuous stirring into a beaker containing 100 mL of methanol.  
e) Collect the white precipitate of polystyrene on a Buchner funnel (vacuum), and wash the precipitate twice with 25 mL of methanol. 
f) Spread the polymer on a paper towel to dry. 
g) Melt a little in a small test tube, and draw a fibre from it by means of a glass rod or matchstick brought in contact with the melt and withdraw slowly.   Describe the fibre (brittle?).

  1. Methyl Methacrylate Polymer
    a) To 10 mL of freshly distilled methyl methacrylate, add 1 drop of N,N-dimethylaniline and 0.1g of benzoyl peroxide. 
    b) Set 5 mL of the mixture aside as control, and place a test tube containing the rest of the mixture in a boiling water bath.  
    c) At 2-minute intervals, transfer one drop of the hot reaction mixture to a test tube containing 10 mL of methanol, and note whether it dissolves.  Observe the change in solubility as the molecular weight of the polymer increases during the reaction. 
    d) Collect the precipitated polymer on a Buchner funnel (vacuum), dry and examine it. 
    e) Now, examine the control, by adding one or two drops to 10 mL of methanol.  Note whether any polymer precipitates.








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