Novel flow apparatus for investigating shear-enhanced crystallization
and structure development in semicrystalline polymers
Guruswamy Kumaraswamy, Ravi K. Verma, and Julia A. Kornfield
a)
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125
͑Received 6 October 1998; accepted for publication 12 January 1999͒
An instrument to study the effects of shearing on the crystallization process in semicrystalline
polymers is described. It can impose transient stresses similar to those encountered in polymer
processing and provides in situ monitoring of microstructure development during and after cessation
of flow. Box-like wall shear stress profiles ͑rise and fall times under 50 ms with maximum wall
shear stress on the order of 0.1 MPa͒ can be applied for controlled durations. A unique feature of
our device is that it accommodates a wide variety of real-time probes of structure such as visible and
infrared polarimetry and light and x-ray scattering measurements. The design also allows us to
retrieve the sample for ex situ optical and electron microscopy. Data are acquired with millisecond
resolution enabling us to record the extent of shear deformation of the polymer melt during the
pressure pulse. Our device works with small sample quantities ͑as little as 5 g; each experiment
takes ϳ500mg) as opposed to the kilogram quantities required by previous instruments capable of
imposing comparable deformations. This orders-of-magnitude reduction in the sample size allows
us to study model polymers and new developmental resins, both of which are typically available
only in gram-scale quantities. The compact design of the shear cell makes it possible to transport it
to synchrotron light sources for in situ x-ray scattering studies of the evolution of the crystalline
structure. Thus, our device is a valuable new tool that can be used to evaluate the crystallization
characteristics of resins with experimental compositions or molecular architectures when subjected
to processing-like flow conditions. We demonstrate some of the features of this device by presenting
selected results on isotactic polypropylenes. © 1999 American Institute of Physics.
͓S0034-6748͑99͒04504-9͔
I. INTRODUCTION
Macromolecules having chemical and structural regular-
ity can, under certain conditions, arrange themselves into a
semicrystalline state. Complete crystallization is kinetically
frustrated and a significant amount of noncrystalline material
is trapped, interspersed among the crystallites. A composite
structure spontaneously forms, with the crystalline phase
conferring strength and the noncrystalline fraction providing
toughness and flexibility. The macroscopic properties of the
material depend strongly on the fraction of crystalline mate-
rial ͑called the degree of crystallinity͒ and on the morphol-
ogy ͑the spatial arrangement, size and orientation distribu-
tion of the crystallites͒. The desirable balance of properties
and the extent to which structure and material properties can
be controlled through processing lead to a wide range of
applications of semicrystalline polymers, including packag-
ing and nonpackaging films, coatings, sheets, wires and
cables and injection and blow molded objects. Worldwide
use of polyolefins alone exceeded 125 billion pounds in
1997.
1
The process of crystallization in polymer melts occurs
by nucleation and growth; crystallites grow by reorganizing
random-coil chains into chain-folded, platelet-like crystalline
lamellae ͑typically ϳ10 nm thick͒ separated by regions of
noncrystalline material. During crystallization from a quies-
cent melt, the crystalline lamellae typically arrange to form
sheaf-like stacks ͑consisting of a few lamellae, typically 50–
100 nm thick͒. These stacks radiate outwards to form spheru-
lites which can range in size from submicron to millimeters.
2
The range of morphological length scales necessitates sev-
eral measurement techniques for characterization.
The microstructure of a semicrystalline polymer depends
strongly on processing history.
3
Imposing flow fields on a
crystallizing polymer melt can accelerate the rate of crystal-
lization and result in the formation of oriented crystallites.
4
Commercial polymer processing operations ͑such as extru-
sion, injection molding, film blowing or fiber spinning͒ sub-
ject a polymer melt to intense flow fields ͑shear, elongational
or mixed͒ and crystallization occurs from the distorted
melt.
4,5
The effect of processing on semicrystalline structure
is of interest because it has a profound effect on material
properties, such as mechanical strength and gas perme-
ability.
6–9
For example, the development of highly aniso-
tropic crystallites during spinning gives certain polymeric
aramide and polyethylene fibers a Young’s modulus between
80 and 130 GPa in the direction of orientation ͑compared to
1–3 GPa for typical semicrystalline polymers or 200 GPa for
steel͒.
3
Thus, there is a strong motivation to observe structure
development in real time during processing.
In spite of intensive research over the past three decades,
the fundamental basis of the effects of processing on struc-
a͒
Author to whom correspondence should be addressed; electronic mail:
jak@cheme.caltech.edu
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 4 APRIL 1999
20970034-6748/99/70(4)/2097/8/$15.00 © 1999 American Institute of Physics