Morphology

Morphology, languages, Filologia Angielska, linguistics

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234 MOLECULARLY IMPRINTED POLYMERS
Vol. 7
MORPHOLOGY
Introduction
Polymer morphology, literally the study of organization within solid polymers,
seeks to read the messages from the specimen’s history contained in its physical
form and hence gain understanding of the physical processes involved in its cre-
ation. In crystalline polymers the morphological record is not only extremely rich,
because long molecules are largely immobile and remain where they were placed,
but it has also provided the most effective means of establishing the fundamental
patterns of behavior of long molecules and the increasingly important materials
they compose, paving the way for quantification by appropriate techniques.
Historically, synthetic polymers are essentially twentieth-century materials,
coming into prominent use in the second half with the introduction of polyethylene
and polypropylene. The earliest textural model, the fringed-micelle, was an inter-
pretation of wide-angle X-ray scattering (WAXS) patterns containing evidence of
both small crystalline and disordered (amorphous) regions (1). According to this,
long molecules were partly entangled and meandered from one kind of region to
the other with mechanical and other properties dependent on the
degree of crys-
tallinity
, ie the proportion of crystalline phase. The scale of the fringed micelle was
believed, on the basis of the small widths of WAXS reflections, to be of the order
of 10 nm, a dimension well below what it was then possible to see. Only with the
advent of the transmission electron microscope (TEM) did such resolution become
feasible but then it became clear that organic materials were vulnerable to dam-
age by the imaging electrons, which mostly destroyed crystals, thereby tending to
distort the physical record and adding little to knowledge until the discovery of
polymer single crystals in 1957.
Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
Vol. 7
MORPHOLOGY 235
Single lamellar crystals were the second element of polymer texture to be
identified, the first being spherulites, whose presence in polyethylene was reported
in 1945 (2). Study with the polarizing microscope established that molecules lay
tangentially to polyethylene spherulites and nearly so to those of polyamides and
polyesters, systems with hydrogen bonding. Spherulites and their presumed sub-
structure of fringed micelles were hard to reconcile (3), a difficulty that disap-
peared with the discovery of polyethylene single crystals and the inference of
their internal molecular chainfolding. This is where modern polymer morphology
started and much that was new and important was achieved within a few years. At
the same time, the problem of radiation damage remained a serious obstacle to the
study of melt-crystallized polymers for 20 years more and, until it was resolved,
textural models were adopted from the limited information available and, un-
fortunately, have not always been substantiated, requiring radical revision when
actual textures were eventually made visible.
Nowadays, polymer morphologies can be observed routinely, at least in the
major polymers, to lamellar levels and, in combination with the complementary
techniques of small-angle X-ray scattering (SAXS), WAXS, differential scanning
calorimetry (DSC),
13
C nuclear magnetic resonance (NMR), neutron and Raman
scattering among others, have served to lay firm foundations for the understand-
ing of crystallization and texture-dependent properties not just in the materials
studied but across all polymers. The central, but not the only, element in this
understanding is the lamella.
Polymer Lamellae
0.01%) in xylene, separate lamellar crystals precipitated.
Their appearance closely matched that of
n
-alkane lamellae in which molecules
are transverse to lamellae. Electron diffraction showed that this was also true of
polyethylene lamellae that, given that molecular lengths were not significantly
reduced by crystallization, led Keller to propose that molecules were chainfolded
within lamellae (7). This inference has proved correct and chainfolded lamellae
have now been shown to be a general feature of synthetic crystalline polymers
including those grown from the melt.
The inference says nothing, however, about surface condition, whether fold-
ing is tight, occurring within four or so main-chain carbon atoms in polyethylene,
or loose, extending over many more. Although there may be complete folding in
isolated lamellae, in bulk material a proportion of molecules in adjacent lamellae
must pass from one lamella to the next—
tie molecules
—and not fold back at a
lamellar surface; otherwise mechanical integrity would be lost and eg the tough-
ness associated with long polyethylene molecules be replaced by the waxy behavior
of solid paraffins. Knowledge of the precise nature of fold surfaces still continues
to be refined as more techniques are introduced and experiments performed and
is currently a central theme of research. This reflects the fact that fold surfaces
∼
The Inference of Chainfolding.
The introduction of linear polyethylene
in the late 1950s led quickly to the discovery that it could crystallize with a lamel-
lar habit, visible in the optical microscope. First, surface steps were seen (4),
then three individuals, Till (5), Fischer (6), and Keller (7) found that on cool-
ing dilute solutions (
236 MORPHOLOGY
Vol. 7
are a major manifestation of the long molecular nature of polymeric solids in con-
trast to lamellar interiors whose crystal structures follow the same principles as
in nonpolymeric materials.
Chainfolded Lamellae from Solution.
Much of our basic knowledge of
polymer lamellae and their properties was established relatively early from the
study of monolayer or bilayer crystals grown from solution and examined on a sup-
porting carbon substrate (8). It is possible to study these systems with diffraction
imaging in a TEM for a short while until crystallinity is lost, then subsequently
as residual amorphous pseudomorphs, which, when on a carbon substrate, retain
the shapes of the original although they distort when embedded in a soft matrix
such as gelatine. It was some 20 years before melt-crystallized textures could be
examined in similar detail and, at the time, they were either assumed by some (9)
to be basically similar although less well organized than lamellae grown from di-
lute solution or, by others (10), to be intrinsically different because the conditions
for ordering from the melt were not present. In fact, most of the essential features
shown by solution-grown lamellae do carry over at least in some degree to those
grown from the melt although these are generally more disordered.
Crystal Habits.
The habits of solution-grown polymer crystals are many
and varied, with slow growth from the most dilute solutions giving the simplest
objects. As usual, much is known about linear polyethylene (11) not only because
this was the first system to be investigated and shows a profusion of informative
detail but also because this is the closest approach to the ideal linear chain whose
properties underlie those of all linear polymers. The basic habit of polyethylene
lamellae is the lozenge, bounded by four
{
110
}
faces. At higher temperatures,
>
faces
whose relative size increases with temperature. For crystallization from other
solvents at higher temperatures, the truncating faces become the largest and may
become slightly curved (12). Curved faces are not uncommon in other systems, but
planar facets are the norm for most polyolefines. They even persist, albeit with
very narrow widths
80
◦
C from 0.01% solution in xylene, these are truncated by two
{
100
m, for very rapid growth when dendrites form because
diffusion limits the supply of material to the growth front (11,13).
The lozenge habit characteristic of orthorhombic polyethylene is paralleled
by hexagons of polyoxymethylene (14) and i-polystyrene (15), squares of i-poly(4-
methylpentene-1) (16), and, very unusually, triangles of poly(
L
-hydroxyproline)
(17), in each case corresponding to the symmetry of the chain packing or subcell.
Twins occur, especially for lower molecular masses. For example, objects with
six arms are common for
∼
0.1
µ
twinning in low mass polyethylene because (110)
and (110) faces are inclined at 67.5
◦
, close to 60
◦
(18). Moreover, laths are prone
to develop with such a twin boundary along its center line (11,19) because of
the accelerated growth provided by the notch at its tip, which aids molecular
attachment. More exotic, three-dimensional twins are shown by
{
110
}
-polypropylene
in “quadrites” (20) and by interpenetrating layers in polyethylene (21).
Sectorization.
The long lengths of polymer chains and chainfolding in par-
ticular are not directly reflected in the lateral habits of polymer lamellae. The
first feature that does show their influence is sectorization, revealed by diffrac-
tion contrast, internal microstructure, or surface texture, possibly augmented by
decoration with gold (22) or evaporated polyethylene (23). This is the division of a
lamella into discrete regions bounded by a growth face, a phenomenon that is both
α
∼
}
Vol. 7
MORPHOLOGY 237
Fig. 1.
A truncated lozenge of linear polyethylene, crystallized from 0.01% solution in
xylene, with boundaries delineating its six sectors and a step at the outer edge where the
crystallization temperature was reduced from 90 to 76
◦
C. From Ref. 18.
a consequence and confirmation of chainfolding along the growth face; it would not
occur if, for example, molecules prefolded into blocks before adding to a lamella.
Sectorization arises because such folding transforms a long molecule into a
pleated sheet, which can extend across several successive lattice planes (24), but
essentially lies along the relevant growth surface, denoted the
fold plane
.Inso
doing it breaks the symmetry of, and slightly distorts, the subcell lattice, ie the
repetitive unit of chain packing within the lamella, excluding the surfaces. The
fold plane in a given sector differs from nominally equivalent ones along which
there is no folding and so transforms a single lamella into a multiple twin accord-
ing to the respective fold planes. A four-sided polyethylene lozenge bounded by
four
110
}
surfaces will
be parallel to the fold plane, the other not. When, at higher growth temperatures,
lozenges become six-sided with truncating
{
110
}
surfaces, then there are six cor-
responding sectors (Fig. 1). The two new ones, denoted
{
100
}
sectors, are twinned
with respect to each other but different in character to the four twinned
{
100
}
}
sectors because of the different fold plane with, for example, a lower melting point
(25).
{
110
growth surfaces is, accordingly, a fourfold twin with four equivalent
sectors in each of which one of the two nominally equivalent
{
238 MORPHOLOGY
Vol. 7
The size of a sector is governed by that of its growth face and can be very
small, eg in dendritic growth, when individual facets, each of which has its own
microsector attached, may be as little as 20 nm wide (13).
Nonplanar Geometries.
One consequence of sectorization is to make lamel-
lae nonplanar (18). This happens in two distinct ways, the first of which is a conse-
quence of subcell distortion. Folding along the growth face makes nominally equiv-
alent planes in the subcell become unequal, adopting slightly different spacings.
For the three systems on which measurements have been made: polyoxymethy-
lene, isotactic poly(4-methylpentene-1) (26), and low mass polyethylene (13), with,
respectively, hexagonal, tetragonal, and orthorhombic subcells and all with chains
normal to lamellae, the difference in spacing between otherwise equivalent planes
only one of which is the fold plane is
∼
89
◦
rather than a planar sheet, as confirmed by direct observation (14). This
deviation from planarity is sufficiently small to allow all sectors to be seen simul-
taneously in diffraction contrast from one principal reflection after a lamella has
been flattened on a substrate prior to examination. In consequence, it is possible
to make measurements of the small spacing difference of the same plane as it
crosses a sector boundary, possible using bilayers and moire patterns. Flattening
also produces characteristic streaking normal to the growth face in each sector
where slight local shearing in the fold plane has created the necessary additional
surface (14).
Such small effects are probably universal in sectored polymer lamellae to
which chains are normal. But they are overshadowed by much greater nonpla-
narity when chains are substantially inclined to lamellar normals, most notably
in polyethylene of typical molecular weights. Then the diffraction pattern, eg for
a single polyethylene lamella, is not generally single crystalline, as was initially
assumed (7), but a composite with different reflections contributed by different
sectors (27), according to the particular fold plane—showing that, in a flattened
crystal, the molecular direction changes from sector to sector. In complementary
dark-field images, only sectors with the same fold plane may appear in bright
contrast.
These effects are a consequence of the collapse of what, in suspension, are
highly nonplanar lamellae (27–29). The nonplanarity can take several forms but
arises because molecular inclination to lamellar normals increases the surface
area per end group or fold and so reduces surface congestion, as is common for the
alkane subcell. The simplest polyethylene geometries are hollow pyramids that
form, at higher temperatures, with
{
312
}
fold surfaces in
{
110
}
sectors and a com-
planes is thought to reflect the asymmetric
shape of sharp folds, both within and normal to the fold plane.
There are two other principal nonplanar habits of polyethylene lamellae:
chair and ridged crystals. Chair crystals (30) form simultaneously with, and are
related to, hollow pyramids; their shape may be described as resulting from a
hollow pyramid divided in two along the short diagonal, the two halves rotated
180
◦
around their long diagonal and then constrained to rejoin. The proportion of
chair to hollow pyramidal crystals appears to vary with crystallization conditions.
{
31
}
0.001 nm. In consequence, transverse axes
of the subcell no longer meet precisely at 60
◦
or 90
◦
. Joining six or four such
sectors, as appropriate, without dislocations (as is observed) gives, therefore, a
lamella that is not strictly planar but slightly dished into a cone of semiangle
∼
mon
c
-axis (30). The adoption of
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