What are polymers (Что такое полимеры). Учебное пособие

.

) 4 (4

) ) ) )), ,

):

'

) . . )

, ) , ):

, ) , 4

4 ) : «three-dimensional ordering … usually arising from…», «regularly repeating material portion contained in…» 1.

2. ! . * - ! ! "() % ! ! % *" # -"! +:

4

) )

)

)

7 )

) )

21

)

)

7 )

3. "!! . * ! "() % ! ! % - ! *" # -"! +.

arrangement of macromolecules degree of crystallinity

folding of chains crystallite crystalline region

three-dimensional ordering constituent macromolecule imperfect regularity macromolecular length scale repeating material portion parallel displacement

to display a perfection

4."!! % -* ! ! ! "' 1.

5.! ' !. "! ! %-* ! ! !.

( )

– ,

22

. .

) ). '

) ) ) 4

). +

) , , ,

7 )

.

. (" – 6

. ' - 6 ,

8 )

. ( (" )

) ) .

–RHC-N-H hydrogen bond

–RHC-C=O 11111 H-N-CHR–

O=C-CHR–

& ) ) (",

) )

.

6. * "!! -* ! !.

The attractive forces between polymer chains play a large part in determining polymer’s properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Different side groups on the polymer can lend the polymer to ionic bonding or hydrogen bonding between its own chains. These stronger forces typically result in higher tensile strength and melting points.

The attractive forces between polyethylene chains arise from weak Van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer

23

chain. Van der Waals forces are quite weak, however, so polyethylene can have a lower melting temperature compared to other polymers.

7. * ! "# ' ' (.

Branching of polymer chains also affect the supramolecular properties of polymers. Long chain branches may increase polymer strength and toughness, due to an increase in the number of entanglements per chain.

Short side chains may likewise reduce crystallinity due to disruption of the crystal structure. Reduced crystallinity may also be associated with increased transparency due to light scattering by small crystalline regions.

A good example of this effect is related to the range of physical attributes of polyethylene. High density polyethylene (HDPE) has a very low degree of branching, is quite stiff, and is used in applications such as milk jugs. Low density polyethylene (LDPE), on the other hand, has significant numbers of short branches, is quite flexible, and is used in applications such as plastic films.

The branching index of the polymer is a parameter that characterizes the effect of long-chain branches on the size of a branched macromolecule in solution.

8. ' "!! -* ' !+%.

1.Tie molecule: a molecule that connects at least two different crystals.

2.Stem: a crystallized, sticklike portion of a polymer chain connected to non-sticklike portions, or chain ends.

3.Chain folding: the conformational feature in which a loop connects two parallel stems belonging to the same crystal.

4.Globular-chain crystal: a type of crystal comprised of macromolecules having globular conformation.

5.Dendrite: a crystalline body produced by skeletal growth, leading to a ‘tree-like’ supramolecular structure.

6.Fibrous crystal: a type of crystal significantly longer in one dimension than in either of the other two.

7.Shish-kebab structure: a polycrystalline structure consisting of fibrous crystals surrounded by lamellar crystals, the stems of which are parallel to the fiber axis.

8.Spherulite: a polycrystalline, roughly spherical morphology consisting of lath, fibrous or lamellar crystals emanating from a common centre.

24

9.Lath crystal: a lamellar crystal prevailingly extended along one lateral dimension.

10.Extended-chain crystal: a polymer crystal in which the chains are in an essentially fully extended conformation.

9. ' ! -* ' !+%.

1.( 4 4

, 4

, 4 )

).

2.3 ) ) 7 ) .

3.) )

) ) ).

4." ) )

) .

5. . , ) ) ) ),

) , 8

) ).

6.) ) ) ) ) )

, , )

.

7.(" ) )

- .

8.+ )

), ),

) ) ) ) ).

9. .6 )

6 )

.

10.) ) )

) ) ) .

10. ! ! ! "# *! % %'.

crystallite — — intramolecular folding — ) — constituent macromolecule — fourth order structure

— — parallel displacement — )

— arrangement of molecules — 4 —

25

— macromolecular length scale — -

— ) ) — stacking of adjacent chains — volume fraction — ) — crystalline region —

— imperfect regularity — ) ) — three-dimensional ordering — ) ) —

— unit cell — ) — tertiary structure

— degree of crystallinity — repeating material portion —

) — complex network — supramolecular level —

—

11. / + * ' !*.

1.What scale of polymer structure and properties is called supramolecular?

2.Describe two main types of organization in supramolecular polymer aggregates.

3.What is a crystalline phase?

4.Which one, HDPE or LDPE, is more amorphous?

5.Are polymer crystals more regular than the low molecular weight ones?

12.! ! ! " "!! %

! % -* #.

26

Unit 4

PHASE BEHAVIOR OF POLYMERS

1. ! ! "#.

Consider a polymer which is slowly cooling down. At a certain temperature, the average kinetic energy of macromolecules no longer exceeds the binding energy between neighbouring molecules and the growth of organized solid crystal begins. Formation of an ordered system takes a certain amount of time since the molecules must move from their current location to energetically preferred points at crystal nodes.

As temperature falls, molecular motion slows down further and, if the cooling rate is fast enough, molecules never reach their destination - the substance enters dynamic arrest and a disordered, glassy solid (or supercooled liquid) forms. Such arrest apparently takes place at a certain temperature, which is called the glass transition temperature (Tg).

Above Tg, the secondary, non-covalent bonds between the polymer chains become weak in comparison to thermal motion, and the polymer becomes rubbery and capable of elastic or plastic deformation without fracture.

Polymer macromolecules start to reform due to thermal motion. Application of a stress favors some conformations over others, so the molecules of the polymer will gradually «flow» into the favored conformations over time. This state of a polymer is called a viscous flow state.

Therefore, the phase behavior of polymer materials is more complex in comparison with low molecular weight substances. Although crystalline fragments have a melting point, the amorphous structures have a glass transition temperature separating glass and rubber states.

27

The plot of deformation vs. temperature (thermomechanical curve) for amorphous polymer

Deformation

28

$%% ! ! ! !. $"

" & '

5, 4 4, ) )

), . ' 6

) )

, 6 ) )

. , 6 )

),

4 ). 3:

, ) binding, cooling, melting

1.

2. ! . * - ! ! "() % ! ! % *" # -"! +.

) 4

6

6

)

)

6

29

3. "!! . * ! "() % ! ! % - ! *" # -"! +.

binding energy neighbouring molecules organized solid crystal ordered system

energetically preferred points crystal node

glassy solid

glass transition temperature thermal motion

cooling rate

to flow into conformation favored conformation melting point

elastic deformation

4."!! % -* ! ! ! "' 1.

30