Chapter 3

MORPHOLOGY AND ORDER INCRYSTALLINE POLYMERS

•What is the arrangementof the chains in thecrystalline domains andthe morphology of thesecrystal?

Polymers yield X-ray diffraction patterns?

•In 1920s, it was found that some polymersdid yield x-ray diffraction pattern!

•Metals and inorganic salts exhibit well-defined patterns

•Polymers produced only a few broad Braggdiffraction peaks pattern

•Small, relatively perfect crystallites existedin an amorphous matrix

Bragg's Law

Schematic diagram for determining Bragg's law

n= 2dsin, n=1,2,3…:wavelength

:Angle between x-ray beam

n :Any whole number

Fringed micelle model

•X-ray deffraction showed their dimensions to beon the order of several hundred Anstroms

•The crystallites were though to serve asmechanical crosslinks and to affect the physicalproperties in much the same way as chemicalcrosslinks in vulcanized rubber

Crystallographic structures of polymers結晶學構造

•Configuration: defined in terms of its chemicalrepeat unit and a statement of moleculararchitecture

•Local conformation: refer to geometricalarrangements of neighboring groups in molecule,which can be altered only by rotation aboutprimary valence bonds

•Molecular packing: refer to the arrangement of themolecules in the crystal in terms of unit cell andits contents

•As you can also see inthe picture, a singlepolymer chain may bepartly in a crystallinelamella, and partly in theamorphous state. Somechains even start in onelamella, cross theamorphous region, andthen join another lamella.These chains are calledtie molecules.

As you can also see in the picture, a single polymerchain may be partly in a crystalline lamella, andpartly in the amorphous state. Some chains evenstart in one lamella, cross the amorphous region,and then join another lamella. These chains arecalled tie molecules.

No polymer is completely crystalline. If you'remaking plastics, this is a good thing. Crystallinitymakes a material strong, but it also makes it brittle.A completely crystalline polymer would be toobrittle to be used as plastic. The amorphous regionsgive a polymer toughness, that is, the ability to bendwithout breaking.

Amorphousness and Crystallinity

Are you wondering about something? If you look atthose pictures up there, you can see that some of thepolymer is crystalline, and some is not! Mostcrystalline polymers are not entirely crystalline. Thechains, or parts of chains, that aren't in the crystalshave no order to the arrangement of their chains.They are in the amorphous state. So a crystallinepolymer really has two components: the crystallineportion and the amorphous portion. The crystallineportion is in the lamellae, and the amorphous potionis outside the lamellae. If we look at a wide-anglepicture of what a lamella looks like, we can see howthe crystalline and amorphous portions are arranged.

Three factors that influence the degree ofcrystallinity

•Chain length

•Chain branching

•Interchain bonding

The fringed micelle model纓狀膠束模型

•The molecules passedsuccessively through anumber of these crystallineand intervening amorphousregion

•The crystallites werepictured as sheaves ofchains aligned in a parallelfashion

•X-ray diffraction patternshowed their dimensions tobe on the order of severalhundred angstroms

Arrangement of chains in unit cellof Polyethylene

• Parallelepiped(平行六面體)with axes a, b, c (T dependent)and angle , ,

• a = 7.41 Åb = 4.94 Åc = 2.55 Å(chain axis)

• = = = 90o

• Length of c-axis =crystallographic repeat unit

Packing in crystal structure of PEas viewed along the c-axis

•Top view of unit cell•The planes of chain

zigzag and the planecontaining theconsecutively foldedchain are shown asthey pass throughthe unit cell

Packing of Nylon 66 molecules intriclinic unit cell(三斜晶系)

• Its crystallographic repeatunit (結晶學重複單位)corresponds to itschemical repeat unit(化學重複單位)

• Its conformation isessentially planar zigzag(平面鋸齒型 )

•Left(-form) is the mostinteresting feature of thiscrystal structure whichdominates the propertiesof Nylon 66

Molecular conformation of polyethylene terephthalate聚對苯二甲酸乙烯酯

• Its conformation is nearlya planar zigzag with thebenzene ring in the planeof the zigzag. As shown,there is some distortionalong the axis of the chainas the –CO-(C6H6)-CO-group make a slight anglewith axis by rotation aboutthe C-O bond in order toallow for close packing

Polyethylene terephthalate

Molecular packing of polyethylene terephthalate

•Unit cell of PET•a = 4.56 Å = 90o

•b = 5.94 Å = 118o

•c = 10.75 Å = 112o

Projection of the monoclinic unit cell (單斜晶胞)

of polypropylene along the chain-axis

• Isotactic polypropylene can crystallize under normalconditions in two forms(hexagonal & monoclinic), bothwith a conformation of 3 units in one turn

Single crystals or linear PE

•All polymer singlecrystals seem to have thesame general appearanceand structure. In theirsimplest form, theyappear in the electronmicroscope as thin, flatplatelets on the order of100 to 200 Å thick andseveral microns inlateral dimensions

Folded Chains

Polymers form stacks of these folded chains. There is apicture of a stack, called a lamella, right below.

~100A

Of course, it isn't always as neat as this. Sometimes partof a chain is included in this crystal, and part of it isn't.When this happens we get the kind of mess you see below.our lamella is no longer neat and tidy, but sloppy, withchains hanging out of it everywhere!

Of course, being indecisive, the polymer chains will oftendecide they want to come back into the lamella afterwandering around outside for awhile. When this happens,we get a picture like this:

Model of fold plane illustrating chain folding withimperfections which may occur in the structure

•Electron-diffraction studies of these crystals indicatethat the polymer chains are oriented perpendicular tothe plane of the lamellae. Since the crystals are only 100Å or so thick and the polymer chains are generally onthe order of 1,000 to 10,000 Å long, the chain must befolded back and forth on themselves

Fold packing in a PE single crystal•Top view of an

idealized model of adiamond-shaped PEsingle crystal,showing its skeletalstructure as viewedalong the c-axis

•Side view shows thisfold plane

The morphology of polymerscrystallized from the melt

•Polymers crystallized from the melt seem tomaintain the two most prominent structuralfeatures of single polymer crystals:aggregates of 100 Å-thick lamellae ofdifferent degree of perfection areobserved, and the chains are orientedperpendicular to the face of the lamellaeso that chain folding must also beinherent in melt-crystallized materials

Lamella grow like the spokes(輻條) of a bicycle wheel from a centralnucleus. (Sometimes scientists like to call the “spokes”lamellar fibrils.)The fibrils grow out in three dimensions, so they really look more like

spheres than wheels. The whole assembly is called a “spherulite”球晶. In a sample of a crystalline polymer weighing only a few gramsthere are many billions of spherulites.

Spherulite(球晶)of a low density polyethylenecrystallized from a molten thin film as observed

between crossed polarizers

•The characteristic

Maltese-crosspattern馬爾它交叉results from thebirefringent nature ofthe polymer film

•Crystal growth can beobserved if a temp-controlled mountingstage is used

Sequence of growth of spherulitesin polypropylene

•The sample is firstheated above its Tmand then supercooledby 10 or 15oC

•Rate of crystallizationcan be measured fromsuch micrographs bycomparing the areaoccupied by thespherulites to the totalarea

Surface replica of a portion of a linearpolyethylene spherulite球晶表面的複製

•The spherulites are comprised of ribbonlike lamellaewhich grow radially from a central nucleus

•The lamellae are parallel at the nucleus, but as theybegin to grow outward, they diverge, twist, and branchto form an overall structure being radially symmetricspherulite

Interlamellar ties 晶片間的接合Boundary region between spherulite grown

at 95oC in a PE fraction

•The links are highlyoriented and stronglyanchored to theneighboring lamellae

•These links shouldpossess considerablestrength

Interlamellar ties

PE Crystallized under Pressure

•Fracture surface of apolyethylene spherulitecenter

•At about 5000 atm and aTc at 236 oC, the density~0.997

•All molecules crystallize,not in a folded fashion,but in paraffinlikecrystals in which thechains are fully extended

Annealing Polymer Crystals

•Annealing a formed crystalis accomplished bymaintaining the crystal at aT above its Tc

•Upon annealing, extensiveremelting andrecrystallization take placeand lead rapidly to aseveralfold increase in thefold period of lamellae

•A single crystal of linear PEcrystallized fromperchloroethylene solution,then annealed for 30 min at125oC, 10o below Tm

Thickness increase from 100 Å

to ~200 Å during annealing

Orientation and Drawing

•No preferred orientation of crystallites ormolecules when bulk polymer is crystallized inthe absence of external forces

•The crystallites and molecules become orientedif it is subjected to an external force, such asmechanical drawing

•The polymer is stronger in the draw directionthan in any other direction, and it will beweakest in directions 90 deg to the drawdirection

Effect of tensile stress on molecularconfiguration

X-ray diffraction patterns for (a) unoriented and(b) oriented polyoxymethylene

Schematic illustrating the necking processin a crystalline filament

•Many crystalline polymers,such as fibers and films, areoriented.

•Films can be biaxiallyoriented

•Orientation can beaccomplished bymechanical drawing orrolling

•Drawing is liable toincrease the crystallinity

Two suggested models for an oriented polymer

• (a)Interlamellar amorphous model for an oriented polymer(b) Fibrils in drawn crystalline polymer

• (a) is appropriate for polymers being hot drawn or annealedafter drawing at RT(cold drawn) Cold drawing by itselfcauses the original lamellae to break up into blocksaccompanied by the formation of long fibrils

Crystallinity and intermolecular forces

Intermolecular forces can be a big help for a polymer if itwants to form crystals. A good example is nylon. You cansee from the picture that the polar amide groups in thebackbone chain of nylon 6,6 are strongly attracted to eachother. They form strong hydrogen bonds. This strongbinding holds crystals together.

In fact, very few polymers can stretch out perfectlystraight, and those are ultra-high molecular weight,and aramids like Kevlar and Nomex. Most polymerscan only stretch out for a short distance beforethey fold back on themselves.

For polyethylene, the length the chains will stretchbefore they fold is about 100 angstroms.

Polymer Chains

How Much Crystallinity?

Remember we said that many polymers containlots of crystalline material and lots ofamorphous material. There's a way we can findout how much of a polymer sample isamorphous and how much is crystalline. Thismethod is called differential scanningcalorimetry(DSC)(will be discussed in nextchapter).

DSC

EX

O

For making fibers, we like our polymers to be ascrystalline as possible. This is because a fiber is reallya long crystal.Many polymers are a mix of amorphous andcrystalline regions, but some are highly crystalline andsome are highly amorphous. Here are some of thepolymers that tend toward the extremes:

PolybutadienePolyketones

PolyisopreneKevlar and Nomex

PolycarbonateNylon

Atactic polystyreneSyndiotactic polystyrene

Poly(methyl methacrylate)polypropylene

Some Highly AmorphousPolymers:Some Highly Crystalline

Polymers:

Why?So why is it that some polymers are highly crystallineand some are highly amorphous? There are twoimportant factors, polymer structure and intermolecularforces.

Crystallinity and polymer structureA polymer's structure affects crystallinity a good deal. Ifit is regular and orderly, it will pack into crystals easily.If not, it won't. It helps to look at polystyrene tounderstand how this works.

Polyesters are another example. Let's look at thepolyester : poly(ethylene terephthalate)

The polar ester groups make for strong crystals. In addition,the aromatic rings like to stack together in an orderlyfashion, making the crystal even stronger.

Polyethylene is another good example. It can be crystallineor amorphous. Linear polyethylene is nearly 100%crystalline. But the branched stuff just can't pack the waythe linear stuff can, so it is highly amorphous.

As you can see on the lists above, there are two kinds ofpolystyrene. There is atactic polystyrene, and there issyndiotactic polystyrene. One is very crystalline, and oneis very amorphous.

Syndiotactic polystyrene is very orderly, with the phenylgroups falling on alternating sides of the chain. This meansit can pack very easily into crystals.

But atactic styrene has no such order. The phenyl groupscome on any which side of the chain they please. With noorder, the chains can't pack very well. So atactic polystyreneis very amorphous.

Other atactic polymers like poly(methyl methacrylate) andpoly(vinyl chloride) are also amorphous. And as you mightexpect, stereoregular polymers like isotactic polypropyleneand polytetrafluoroethylene are highly crystalline.

Order and disorder molecular chains

Transcrystallization of iPP onPitch-based HM Carbon Fiber

SEM of iPP on Pitch based HMcarbon Fiber

Transcrystallization of iPP on PTFE substrate

Transcrystallization of iPP on anodized Alsubstrate