Solvent Crazing of Polymers

413 Pages·1995·38.443 MB·English

by  A.L. Volynskii and N.F. Bakeev (Eds.)

Studies in Polymer Science
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Studies in Polymer Science 13
by
A.L. Volynskii and N.F. Bakeev
Moscow State University, Lenin Hills, 118899 Moscow, Russia
1995
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V
ACKNOWLEDGMENT
We would like to express our sincere gratitude to the Soros
International Scientific Foundation and Russian Fund of Fundamental
Research for their financial support of our studies on solvent crazing. We
would also like to thank Prof. V. A. Kabanov, the Head of Polymer
Department, Moscow State University, whose personal knowledge and
assistance has always been so generously imparted. We must note our
indebtedness to the people involved in the studies on solvent crazing, who
worked tirelessly and with abounding enthusiasm: Dr. G. M. Lukovkin, A.
E. Sinevich, A. V. Efimov, L. M. Yarysheva, A. V. Volkov, N. I.
Nikonorova, T. E. Grokhovskaya, M. A. Moskvina, post-graduate
students and other people working at Faculty of Chemistry, Moscow State
University. Our special word of gratitude to Prof. J.C.J. Bart for his
invaluable aid in supporting this project when it was no more than an idea.
Finally our thanks to Drs. O. V. Arzhakova and I. V. Chernov, who not
only participated in all scientific activities in this specific area but also
helped a lot in preparing this manuscript. We think that without them our
task would have been immeasurably more difficult, perhaps even
impossible.
ALEKSANDR L. VOLYNSKII
NIKOLAI F. BAKEEV
Vll
INTRODUCTION
The problems related to crazing in polymers are of special interest
to polymer scientists. Only polymers are able to experience this universal
phenomenon and no analogues are available for low-molecular-mass
compounds. Obviously, the important problems of solvent crazing and
development of universal description of polymer structure and properties
have attracted much attention of many leading scientists. Among them are
A. S. Argon, , E. Baer, H. R. Brown, N. Brown, L. L. Berger, A. M.
Donald, M. Dettenmaier, R. P. Kambour, H. H. Kausch E. J. Kramer, J.
Narisawa, etc. Nevertheless, some aspects of polymer crazing are still
unclear, and scientific activities in this direction are in progress.
This work sets out to provide an up-to-date account of scientific
advances in the area of solvent crazing. Our approach involved several
directions. First, we put forth an explanation why crazing is characteristic
only of polymers. Evidently, ability to experience crazing as well as many
other specific polymer properties are intimately related to the chain
structure and flexibility of polymer molecules. Chapter 1 provides only
a qualitative description of this problem, which takes into account polymer
nature of crazing and provides a plausible explanation of stress-induced
polymer fibrillation. Just polymer fibrillation, that is, polymer self-
dispersion into colloidal aggregates under the action of mechanical stress,
is responsible for realization of polymer crazing. Our approach allowed us
to conclude that such universal phenomenon as polymer cold drawing may
be considered as a specific surface phenomenon.
According to this approach, crazing and necking were considered
not as two independent modes of polymer deformation but as a manifestation
of one and the same phenomenon of orientational drawing under different
external conditions. Furthermore, polymer crazing should not be treated as
a pre-cursor of polymer fracture.
Such approach allowed us to consider all phenomena related to
crazing from a universal standpoint. As a result of such consideration, a
decisive role of surface phenomena in the development of structure and
viii
specific properties of the solvent-crazed polymers seems to be quite evident
To provide a detailed description of specific properties of the solvent-
crazed polymers, many ideas and theories of colloidal chemistry were
invoked. For example, to explain polymer cold drawing, polymer
shrinkage, thermal behavior of the solvent-crazed polymers, such terms as
coagulation, peptization, adsorption, ultrafiltration, syneresis were widely
used.
All the above problems were considered in the first four chapters of
this book. Correlation between the growth of individual crazes and their
inner fine structure, correlation between polymer mechanical response and
craze structure as well as correlation between the craze growth and the
final properties of the solvent-crazed polymers were described. Taking into
account the surface phenomena accompanying polymer plastic
deformation, a universal description of solvent crazing was advanced.
Even though only our readers will decide how successful this approach
has been, we believe that discussing the correlation between various
aspects of polymer crazing is necessary.
Chapter 5 is devoted to the description of a new mode of solvent
crazing, that is, delocalized crazing. This phenomenon is shown to be
characteristic of crystalline polymers. As compared with glassy amorphous
polymers, crystalline polymers are able to experience both modes of
solvent crazing: classical and delocalized crazing. In contrast to classical
crazing, delocalized solvent crazing involves no well-defined stages of
craze nucleation, craze tip advance, craze thickening, and collapse.
In Chapter 6, structure and properties of the low-molecular-mass
compounds introduced into the solvent-crazed polymers are discussed.
Being introduced into the craze structure, low-molecular-mass compounds
show a number of specific properties, and their structure is quite different
from that in a free state.
Chapter 7 is focused on the applied aspects of solvent crazing.
However, the advantages of solvent crazing are too wide to be discussed
within the scope of this chapter, but a general approach to this problem is
outlined. It seems quite evident that in the nearest future the phenomenon
of solvent crazing will be used for the solution of many applied problems.
In addition to our main goal to generalize the up-to-date
knowledge concerning solvent crazing, we also wanted to provide more
information concerning the activities of Russian scientists in this area,
ix
which has escaped the attention of our colleagues because of the language
barrier.
1
CHAPTER 1
STRUCTURAL ASPECTS OF POLYMER COLD DRAWING
Crazing is one particular mode of plastic deformation of solid
polymers. At least, two other modes of plastic deformation of solid
polymers are known, i.e., necking and shear banding. All the above modes
of plastic deformation of solid polymers are well recognized. Several
excellent reviews providing a detailed description of all three modes of
plastic deformation of solid polymers and correlation between them are
available [1-4]. Each mode of polymer plastic deformation has specific
features of its own. However, to gain a deeper insight into the mechanism
of polymer plastic deformation, one should consider one universal
phenomenon, that is, polymer fibrillation. This phenomenon is common for
all three modes of polymer plastic deformation. Polymer fibrillation implies
a stress-induced disintegration of initial bulk polymer into fine aggregates
(of colloidal dimensions) of oriented macromolecules. The diameter of
such aggregates are estimated to be tens or hundreds of angstroms,
whereas their length is as high as several microns and even more. At the
present time, this is a commonplace, that neck, shear bands, and crazes are
characterized by fibrillar structure. We believe that to provide an adequate
description of polymer fibrillation in crazes as well as to understand the
mechanism of crazing, such a general problem as polymer fibrillation
should be discussed, because crazing is nothing else but one particular
mode of polymer plastic deformation.
2 Chapter 1
1.1. Conditions Providing Development of Polymer Fibrillar
Structure
Actually, polymer fibrillation is a universal phenomenon
characteristic of almost all modes of nonelastic polymer orientation.
Disintegration of polymer matter into fine aggregates of colloidal
dimensions was observed even for deformation of single crystals of
semicrystalline polymers such as polyethylene, polypropylene,
polyoxymethylene [5-7].
Fig. 1. TEM micrograph of oriented PS film.
In the case of polymer cold drawing, fibrillation phenomenon is
most well pronounced. Figure 1 shows electron microscopic micrograph of
a thin film of PS oriented at room temperature [8]. One may easily see that
polymer orientation is accompanied by the appearance of a local zone of
plastically deformed material composed of fibrillar elements with
dimensions of 20-50 nm, which are oriented along the direction of the
applied stress.
Let us consider principal features of polymer fibrillar structure
produced via cold drawing. This structure constitutes densely packed
aggregates of fibrillar elements with dimensions from few to tens
nanometers. Despite their dense packing, all fibrillar elements show well-
Conditions Providing Development of Polymer Fibriliar Structure 3
defined interfacial boundaries as evidenced by electron microscopy [9, 10]
and X-ray analysis [11]. Mechanical properties of oriented polymers are
known to be controlled by the existence of real physical boundaries
between fibrils. According to Peterlin [12], the principal stress resistance is
associated with quasiviscous friction forces acting on highly developed
surfaces of shifting fibrils. Strength properties of both amorphous and
semicrystalline polymers are also known to be controlled by fibriliar
morphology [13, 14]. The universal character of fibrillation phenomenon
was well described in a comprehensive review by Sikorskii [9], in which a
vast body of experimental evidence on fibriliar morphology for a variety of
fibers based on natural and synthetic polymers was summarized. The final
conclusion is that all oriented polymers are characterized by fibriliar
morphology, and the dimensions of fibrils vary from a few to dozens
nanometers.
Polymer molecular orientation can be achieved by two different
ways. The first way involves orientation of polymer melts, polymer
solutions, or rubbery polymers and subsequent fixation of the oriented
structure. The second way is associated with orientation of polymers with
stable structure via polymer cold drawing. Fibriliar morphology is typical
of the second mode of polymer orientation. In the case of orientation of
polymer melts, even at very high rates of spinning associated with high
molecular orientation, only spherulitic structure is produced. [15]. The
similar behavior is observed for crystallization of oriented rubbers [16]. To
obtain fibriliar structure in polymer films and fibers, orientational drawing
of solid polymers is necessary. Fibrillation in polymers was studied by
Frenkel' [17]. He concluded, that for both crystalline and amorphous
polymers, orientational drawing is associated with fibrillation.
Let us also emphasize that mode of polymer orientation is of
primary importance, because polymers oriented via two different ways
show a marked difference not only in their morphology. Burham and
Keller [18] carried out a comprehensive comparative analysis of oriented
polymer structures in PE. They concluded that at all levels, from
microscopic to macroscopic, a fundamental difference between the
structure of oriented polymers prepared via direct crystallization or
orientation of solid crystalline materials is realized. The polymers were
characterized in terms of such a fundamental property of oriented
polymers as thermally induced polymer shrinkage. Polymers oriented via
cold drawing show a marked shrinkage at temperatures far below glass