Structures and properties of polymers Flashcards

Question

What are the disadvantages of solution polymerisation?

Answer 1

 Potential toxicity, flammability and environmental pollution of solvents  Polymer product contains solvent impurities  Yield is significantly lower than in bulk polymerisation  Expensive due to additional solvent cost.

Answer 2

 Liquid or dissolved monomer suspended in liquid phase like water. The size of monomer droplets is 50-200 μm in diameter.  The dispersion is maintained by continuous agitation and the droplets are prevented to coalesce (unite or merge) by adding small quantity of stabilizers.  The polymerisation takes place inside the droplet & product formed being insoluble in water.  The product separated out in the form of spherical pearls or beads of polymer.

Answer 3

 Comparatively cheap: only water instead of solvents.  Viscosity increase is negligible.  Agitation & temperature control is easy.  Product isolation is easy since the product is insoluble in water.

Answer 4

 Only for water insoluble monomers.  Difficult to control polymer size.  Polymer purity is low due to the presence of suspending & stabilizing additives which are difficult to remove completely.  Highly agitation sensitive.  The method cannot be used for tacky polymers such as elastomers because of the tendency for agglomeration of polymer particles.

Answer 5

The system consists of:  water insoluble monomer,  dispersion medium (water)  emulsifying agents or surfactants (soaps and detergents) and  a water soluble initiator (e.g. potassium persulphate).  The monomer is dispersed in the aqueous phase, not as a discrete droplets, but as a uniform emulsion.  The size of monomer droplet is around 0.5 to 10 μm in diameter.  The monomer in water is stabilized by a surfactant.  When it is put into a water, the surfactant molecules gather together into aggregates called micelles.  The monomer molecules diffuse from monomer droplets to water & from water to the hydrocarbon centre of micelles.  The technique is used for the production of large number of commercial plastics & elastomers.

Answer 6

 More environmentally safe  Reaction occurs in the small droplets (or micelles), leading to produce higher molecular weight of polymers  Role of water is that of a heat sink, viscosity remains close to that of water and is not dependent on molecular weight.  Product is marketed as polymer-water emulsions (e.g. water-borne paints, adhesives (white glue), etc.)  Difficult to remove the surfactant impurity in polymer  Can not be used for condensation, ionic or Ziegler- Natta polymerisation.

Answer 7

The mase of a mole of chains

Answer 8

Resistance to stress in tensile, flexural and shear modes, toughness, creep resistance, and environmental stress-cracking.

Answer 9

Can have either high or low molecular weight and distinguishes them from metals and ceramics. The length varies in terms of mean molecular weight.

Answer 10

As the molecular weight increases (g/mol) then the tensile strength increases.

Answer 11

 The number average molecular weight (Mn) - total weight of polymer/ number of polymer chains and  The weight average molecular weight (Mw) - total number of polymer chains*molecular weight^2/polymer chains*molecular weight. Takes into account each molecules weight.

Answer 12

Through the polydispersity index (PDI) which is the ratio of the weight average molecular weight to the number of average molecular weight. Mw/Mn. Must be greater than 1. Can be bimodal, unimodal narrow or broad on a graph.

Answer 13

Chain polymerisation is much higher in molecular weight but narrower. Condensation polymer is a broader distribution so much harder to control. It at a lower molecular weight. Monomer is the lowest molecular weight and is 1 line.

Answer 14

a single backbone with no branches.

Answer 15

a main chain with one or more substituent side chains or branches. The branching of polymer chains affects their ability to slide past one another by altering intermolecular forces, in turn affecting bulk physical polymer properties.

Answer 16

Linear, branched, cross linked, network

Answer 17

Long chain branches may increase polymer strength, toughness and the glass transition temperature due to an increase in the number of entanglements per chain.  Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organisation and may likewise reduce the crystallinity of the polymer.

Answer 18

form long chains, either branched or linear, that can form covalent bonds between the polymer molecules Because cross-linked polymers form covalent bonds that are much stronger than the intermolecular forces that attract other polymer chains, resulting in a stronger and more stable material.  Natural rubber is vulcanized by heating the sample for the crosslink of the sulphur molecules in the rubber polymer chains formed covalent bonds with each other.

Answer 19

A polymer molecule with a high degree of cross-linking

Answer 20

More makes it more insoluble, infusible, thermosetting, thermosetting resin and thermosets. Less is soluble, fusible, thermoplastic

Answer 21

LDPE has the most excessive branching and has a density of 0.910-0.925 g/cm3 . HDPE has minimal branching of its’ polymer chains and has a density of 0.941-0.965 g/cm3 .. Because it is denser it is more rigid and less permeable then the LDPE.

Answer 22

Co polymers are two or more monomers polymerised together:  random – A and B randomly positioned along chain  alternating – A and B alternate in polymer chain  block – large blocks of A units alternate with large blocks of B units  graft – chains of B units grafted onto A backbone

Answer 23

 Amorphous materials undergo a “glass transition”, Tg Crystalline materials have a “melt transition”, Tm semi-crystalline can display both transition types

Answer 24

 Both Tm and Tg increase with increasing chain stiffness  Chain stiffness increased by presence of  Bulky side-groups  Polar groups or sidegroups  Chain double bonds and aromatic chain groups  Regularity of repeat unit arrangements – affects Tm only Both transition types originate from intermolecular forces.

Answer 25

It arises from the decoupling of the vibrational and translational motions.

Answer 26

The temperature at which the polymer undergoes the transformation from a rubber to a glass

Answer 27

long-range segmental motion.

Answer 28

The changes in conformation that occur above Tg require more volume, so plotting a graph of specific volume or thermal expansion coefficient against temperature will give a value for Tg. The actual volume of the molecules stays the same through Tg, but the free volume (the volume through which they can move) increases.

Answer 29

Below glass transition temperature: hard, brittle solid as a crystalline or amorphous. Between Tg and Tm, the moduluse decreases bu the crystalline is a tough solid and the amorphous becomes rubbery. Above Tm, they both viscously melt

Answer 30

(1) glassy region; (2) the glass transition region; (3) the rubber plateau region; (4) the rubber flow region and (5) the liquid flow region.

Answer 31

 In glassy region, the polymer is glassy and frequently brittle (e.g. PS).  Young’s modulus for glassy polymer just below the glass transition temperature (Tg) is surprisingly constant over a range of polymers, having the value of ~3Ga.  In the glassy state, molecular motions are largely restricted to vibrations and short-range rotational motions.

Answer 32

The polymers are leathery but T changes will affect the stiffness of the leather. Modulus drops by 1000... For quasi-static measurements, the glass transition temperature, Tg, is often taken at the maximum rate of turndown of the modulus at the elbow, where E ~ 0.1GPa.  A glass transition temperature is often defined as the temperature where the thermal expansion coefficient undergoes a discontinuity (Also have other definitions of Tg : Enthalpic and dynamic etc) .  Qualitatively, the glass transition region can be interpreted as the onset of long-range, coordinated molecular motion, only 1-4 chain atoms are involved in motions below Tg and 10-50 chain atoms attain sufficient thermal energy to move in coordinated manner in Tg region.

Answer 33

After sharp drop of the modulus taking place in the glass transition region, the modulus becomes almost constant again in the rubbery plateau region with typical values of ~2 MPa.  In the rubbery plateau region, polymers exhibit long-range rubber elasticity

Answer 34

1. The polymer is linear: the modulus will drop off slowly and the width of the plateau is governed primarily by the molecular weight of the polymer; the higher the molecular weight, the longer is the plateau. 2. The polymer is crystalline (long dotted line): the modulus will drop to some extent but remain the same after Tg due to the crystals restricted the mobility of the chains. The modulus will drop greatly after melting. 3. The polymer is cross-linked (short dotted line): improved rubber elasticity is observed with the creep portion suppressed.  The dotted line follows the equation E = 3nRT, where n is the number of active chain segments in the network and RT represents the gas constant times the temperature. Tg≈ 1/2 - 2/3 of Tm

Answer 35

In this region, the polymer is marked by both rubber elasticity and flow properties, depending on the time scale of the experiment.  For short time scale experiment, the physical entanglements are not able to relax and the material still behaves rubbery;  For longer times, the increased molecular motion imparted by the increased temperature permits assemblies of chains to move in a coordinated matter (depending on the molecular weight) and hence to flow. This region does not occur for cross linked polymers.

Answer 36

In this region, the polymer flows readily, often behaving like molasses.  The increased energy allotted to the chains permits them to be unentangled rapidly and flow as individual molecular chains.  For semicrystalline polymers, the modulus depends on the degree of crystallinity. The amorphous portions go through the glass transition, but the crystalline portion remains hard, resulting in a composite modulus.  At the melting temperature the modulus drops rapidly to that of the corresponding amorphous materials in the liquid flow region.

Answer 37

Crystalline materials have their molecules arranged in repeating patterns. As such, they all tend to have highly ordered and regular structures.  Amorphous materials, by contrast, have their molecules arranged randomly and in long chains which twist and curve around one-another, making large regions of highly structured morphology unlikely.  The morphology of most polymers is semi-crystalline: a combination with the tangled and disordered regions surrounding the crystalline areas.

Answer 38

Crystallisation = nucleation + crystal growth Nucleating agents can be added to semi-crystalline polymers for faster processing

Answer 39

In pure materials this can take a long time and it is possible to take a material well below its Tm before crystallisation occurs

Answer 40

If impurities/inclusions are present the energy barrier to nucleation is much lower and crystallisation occurs much faster

Answer 41

Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene  Single crystals – only for slow and carefully controlled growth rates Crystalline regions: thin platelets with chain folds at faces to form Chain folded structure

Answer 42

Some semi-crystalline polymers form spherulite structures  Alternating chain-folded crystallites and amorphous regions  Spherulite structure for relatively rapid growth rates

Answer 43

Cross polarized light is used The bands or rings in the spherulite image result from twisting of the lamellar crystals as they extend like ribbons from the centre.

Answer 44

Screw dislocation, branch, vacancy, chain ends, edge dislocation, dangling or loose chain etc

Answer 45

 Polymers rarely 100% crystalline- Difficult for all regions of all chains to become aligned Degree of crystallinity expressed as % crystallinity. Heat treating causes crystalline regions to grow and increase.

Answer 46

Areas in polymer where chains packed in regular way.  Both amorphous and crystalline areas in same polymer.  Crystalline - regular chain structure - no bulky side groups.  More crystalline polymer - stronger and less flexible.

Answer 47

Solutions, solidification from the melt, stretching

Answer 48

This process depends on the degree of dilution:  in dilute solutions, the molecular chains have no connection with each other and exist as a separate polymer coils in the solution.  Increase in concentration which can occur via solvent evaporation, induces interaction between molecular chains.  Crystallisation from solution may result in the highest degree of polymer crystallinity.

Answer 49

Nucleation starts with small, nanometer-sized areas where as a result of heat motion some chains or their segments occur parallel.  Those seeds can either dissociate, if thermal motion destroys the molecular order, or grow further, if the grain size exceeds a certain critical value.  Apart from the thermal mechanism, nucleation is strongly affected by impurities, dyes, plasticizers, fillers and other additives in the polymer. This is also referred to as heterogeneous nucleation.

Answer 50

The mechanism considers crystallization from the melt, which is important for injection moulding of plastic components. Another type of crystallization occurs upon extrusion used in making fibres and films.  In this process, the polymer is forced through, e.g., a nozzle that creates tensile stress which partially aligns its molecules. Such alignment can be considered as crystallization and it affects the material properties. The strength of the fibre increases so the anisotropy is enhanced with fillers added. This increases polymer strength through extrusion and blow moulding. Very good for amorphous elastomers that have rapid crystallisation on stretching.

Answer 51

Structural regularity, bond flexibility, close packing ability and inter-chain attraction, will affect the crystallinity and Tm of the polymers. Close packing capability, rigid molecules, strong inter chain attraction, cross linking.

Answer 52

If the structure of the polymer is regular and orderly, it will pack into crystals easily. If it's not, it won't.  Atactic polystyrene – amorphous  Syndiotactic polystyrene – crystalline.

Answer 53

the polar amide groups in the backbone chain of nylon 6,6 are strongly attracted to each other. They form strong hydrogen bonds. This strong binding holds crystals together. This raises the melting point compared to polymers without strong intermolecular interactions, like polyethylene.

Answer 54

 Degree of polymerisation: polymer containing long chains are more difficult to crystallise.  Complexity of the polymer: crystallisation is easiest for the polymers with simple structures such as PE (& PS.  Cooling rate: slow cooling, which permits more time for the chains to become aligned, encourage crystallisation.  Annealing: heating an amorphous structure just below the melting temperature provides the thermal activation that permits crystals to nucleate and grow.  Deformation: slow deformation of the polymer between the melting and glass transition temperatures may promote crystallisation by straightening the chains, thus permits them to align and close together.

Answer 55

Density of a crystalline polymer will be greater than an amorphous one of the same material and molecular weigh, as the chains are more closed packed together for crystalline structure. It is directly proportional.

Answer 56

% crystallinity = [ρc(ρs – ρa) / ρs (ρc – ρa )] x 100 Where ρc is the density of totally amorphous polymer and ρs is the density for which precentage crystallinity is determined and ρa is the density of the totally amorphous polymer.

Answer 57

Processing requires heating above the melting point. Processing at high temperature possesses the danger of degradation, especially if traces of water are present.  For an amorphous polymer, the polymer is in the rubbery or “soft” state above Tg. This limits its service temperature range.  For a crystalline polymer the crystalline form retains dimensional stability almost up to the melting point. Thus, crystalline polymers can be used at higher service temperatures.

Answer 58

The polymer chains are chemically bonded to each other via crosslinks.  Crosslinking raises the Tg and increase melt viscosity above Tg. The higher of the degree of crosslinks, the higher of the modulus (the more brittle) of the polymers

Answer 59

Amorphous – no short range order Crystalline – short range order Rubber is lightly crosslinked

Answer 60

Thermoplastic Use melt in liquid shaping stage Harden by freezing melt Liquid-solid reversible Scrap recovery possible Ceiling service temperature Processing usually orient chains Thermoset* Use lower Mw or rubbery polymer at shaping stage Harden by cross-linking using temperature or catalyst Liquid goes irreversibly to solid Scrap cannot be recovered directly Can withstand high T Process with low orientation

Answer 61

High speed and High shear rate

Answer 62

Rapid cooling during processing does not allow enough time for complete relaxation, leaving a substantial amount of orientation and resultant frozen-in stress in the part. The higher the molecular weight, the longer it takes for orientated molecules to relax at any given temperature.

Answer 63

 The frozen-in stress can be removed by heating above the material’s softening temperature.  The greater the shrinkage and distortion on heating, the greater the frozen-in stress. Generally, the smaller the change on heating, the less likely is the part to fail.

Answer 64

physical mixing of two or more different polymers. Tends to be quite unlikely polymers will mix well as they are thermodynamically incompatible.

Answer 65

Compatibilisers can be used to improve mixing A compatibiliser can be for example a block copolymer of thetwo polymer we want to blend