Simultaneous birefringence, small- and wide-angle X-ray scattering to detect precursors and characterize morphology development during flow-induced crystallization of polymers

research papers

J.SynchrotronRad.

(2008).

, 185–190

doi:10.1107/S0909049508002598

185

Journal of

Synchrotron

Radiation

ISSN 0909-0495

Received 30 April 2007

Accepted 23 January 2008

#

2008 International Union of Crystallography

Simultaneous birefringence, small- and wide-angle

X-ray scattering to detect precursors and

characterize morphology development during

flow-induced crystallization of polymers

Lucia Fernandez-Ballester,

‡

Tim Gough,

Florian Meneau,

}

Wim Bras,

Fernando Ania,

Francisco Jose Balta-Calleja

and Julia A. Kornfield

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,

CA 91125, USA,

University of Bradford, Bradford BD7 1DP, UK,

Netherlands Organization for

Scientific Research (NWO), DUBBLE at ESRF, BP 220, F-38053 Grenoble CEDEX, France, and

Instituto de Estructura de la Materia, CSIC, Serrano 119, 28006 Madrid, Spain.

E-mail: jak@cheme.caltech.edu

An experimental configuration that combines the powerful capabilities of a

short-term shearing apparatus with simultaneous optical and X-ray scattering

techniques is demonstrated, connecting the earliest events that occur during

shear-induced crystallization of a polymer melt with the subsequent kinetics and

morphology development. Oriented precursors are at the heart of the great

effects that flow can produce on polymer crystallization (strongly enhanced

kinetics and formation of highly oriented crystallites), and their creation is

highly dependent on material properties and the level of stress applied. The

sensitivity of rheo-optics enables the detection of these dilute shear-induced

precursors as they form during flow, before X-ray techniques are able to reveal

them. Then, as crystallization occurs from these precursors, X-ray scattering

allows detailed quantification of the characteristics and kinetics of growth of

the crystallites nucleated by the flow-induced precursors. This simultaneous

combination of techniques allows unambiguous correlation between the early

events that occur during shear and the evolution of crystallization after flow has

stopped, eliminating uncertainties that result from the extreme sensitivity of

flow-induced crystallization to small changes in the imposed stress and the

material. Experimental data on a bimodal blend of isotactic polypropylenes are

presented.

Keywords: simultaneous; optical; SAXS; WAXD; flow-induced crystallization; polymer;

birefringence.

1. Introduction

It is well known that flows applied during polymer processing

can significantly alter the kinetics of crystallization and

structure development in semicrystalline polymers; therefore,

flow and thermal history profoundly impact on final material

properties (Haas & Maxwell, 1969; Andersen & Carr, 1978;

Fujiyama

et al.

, 1988; Kantz

et al.

, 1972; Trotignon & Verdu,

1987). Study of flow-induced crystallization of polymers poses

daunting scientific challenges because of its highly non-linear

nature. For example, small changes in the stress imposed on a

polymer melt can produce abrupt transitions in kinetics and

morphology (Kumaraswamy, Issaian & Kornfield, 1999). In

addition, flow-induced crystallization exhibits extreme sensi-

tivity to material composition (

e.g.

molar mass distribution)

(Vleeshouwers & Meijer, 1996; Seki

et al.

, 2002). Structure

evolution during and after flow involves length scales from

m

m and time scales from under 100 ms to more

than 10

s. Especially for events occurring at the fastest time

scales, comparisons between results from different techniques

(separate instruments) are difficult to synchronize. Thus, it

is beneficial to perform simultaneous measurements with

complementary experimental techniques, ensuring in this way

that the sample and imposed conditions are truly identical

(Bras & Ryan, 1998).

The application of simultaneous X-ray and optical

measurements has been well established for quiescent crys-

tallization (Wutz

et al.

, 1995). Overcoming the challenge of

‡ Current address: Netherlands Organization for Scientific Research (NWO),

DUBBLE at ESRF, BP 220, F-38053 Grenoble CEDEX, France.

}

Current address: Synchrotron SOLEIL, L’Orme des Merisiers, BP 48,

St Aubin, 91192 Gif sur Yvette, France.

simultaneously incorporating these methods into flow-induced

crystallization processes is the subject of the present paper.

Simultaneous X-ray and optical techniques have been applied

before in fiber spinning experiments (Ran

et al.

, 2003; Chu &

Hsiao, 2001). Here, we focus on an instrument that imposes

well defined stress and thermal histories which places addi-

tional constraints on the configuration of the X-ray and optical

beams. In our case, the sample is confined to a high-aspect-

ratio channel, which necessitates that light and X-rays

propagate along the same axis. Therefore, we combine an

approach described previously for studying liquid crystals

using simultaneous X-ray and birefringence measurements

(Gleeson, 1995) with the short-term shearing strategy that has

proved to be a powerful tool for understanding flow-induced

crystallization (Liedauer

et al.

, 1993; Vleeshouwers & Meijer,

1996; Kumaraswamy, Verma & Kornfield, 1999; Pogodina

al.

, 1999; Koscher & Fulchiron, 2002; Somani

et al.

, 2001;

Devaux

et al.

, 2004; Azzurri & Alfonso, 2005; Baert & Van

Puyvelde, 2006; Langouche, 2006; Heeley

et al.

, 2006).

The short-term shearing apparatus (Liedauer

et al.

, 1993;

Kumaraswamy, Verma & Kornfield, 1999) is capable of

accessing a high stress regime (which induces highly oriented

crystallization) under well defined flow conditions (level of

shear stress and duration of shear pulse). In addition, it allows

independent control of the temperature history imposed

on the polymer; a typical experiment involves raising the

temperature above the equilibrium melting point to erase the

flow history, cooling down to a shearing temperature where

flow is imposed under isothermal conditions, and then either

monitoring development of crystallization at the same

temperature or under a non-isothermal temperature program.

A range of structural probes has been utilized with the short-

term shearing method such as birefringence, turbidity, small-

angle X-ray scattering (SAXS), wide-angle X-ray diffraction

(WAXD) and optical microscopy (Liedauer

et al.

, 1993;

Kumaraswamy, Verma & Kornfield, 1999; Pogodina

et al.

1999; Koscher & Fulchiron, 2002; Somani

et al.

, 2001; Devaux

et al.

, 2004; Azzurri & Alfonso, 2005; Baert & Van Puyvelde,

2006; Langouche, 2006; Heeley

et al.

, 2006). These capabilities

have previously allowed isolation of the effects of stress,

shearing time and shearing temperature; for example, studying

the influence of shearing temperature revealed a kinetic

pathway in oriented flow-induced crystallization in isotactic

polypropylene (iPP) (Kumaraswamy

et al.

, 2002).

Oriented precursors mediate the enormous effects of flow

on polymer crystallization. If the flow conditions are strong

enough, thread-like precursors form that increase the kinetics

of crystallization by orders of magnitude and nucleate highly

oriented crystallites, thus dramatically changing the final

morphology. In spite of their importance, the mechanism of

creation of thread-like precursors and their dependence on

temperature, flow conditions and material properties remains

elusive. One reason is that it is difficult to directly detect these

precursors. An unusual upturn in the birefringence during flow

(Kumaraswamy, Issaian & Kornfield, 1999; Kumaraswamy

al.

, 2002) has been shown to be the characteristic signature for

the creation of these highly oriented precursors; however,

X-ray scattering techniques have so far been unable to detect

the bare precursors,

i.e.

while they are not yet decorated by

oriented crystallites grown on them.

To obtain a better understanding of the fundamental

processes that govern flow-induced crystallization, there is a

need to connect the events that occur during flow, which are

detectable by birefringence but not by SAXS and WAXD,

with the subsequent morphology development that can be

quantified by X-ray scattering techniques. After cessation of

flow, the growth of birefringence is limited to the time range

prior to development of substantial turbidity, and interpreta-

tion depends on independent measurements (

e.g.

ratio of

parent-to-daughter crystallites in isotactic polypropylene).

Synchrotron X-ray scattering techniques are able to provide

quantitative information about the crystallization kinetics,

degree of crystallinity, orientation distribution of the crystal-

lites, and long period. The present paper aims at achieving the

correlation between the events that occur during flow with the

progress of crystallization after flow has stopped.

2. Instrumentation

2.1. Rheo-optical apparatus to control flow and thermal

history

The instrument used was based on the short-term shearing

apparatus developed by Kumaraswamy (Kumaraswamy,

Verma & Kornfield, 1999), with dimensions modified to fit

the beamline configuration. The flow cartridge houses a slit

channel geometry with a 1:10 ratio. Two windows are

mounted flush on the channel and allow the beam to propa-

gate through the sample in the velocity gradient direction. A

conical aperture at the exit of the flow cell allows recording of

WAXD up to an angle of 35

(Fig. 1

). To obtain simultaneous

optical and X-ray scattering measurements, the laser and

X-ray beams must be collinear and the windows must be

transparent to both visible light and X-rays without introdu-

cing an excessive deterioration in the data quality. Therefore,

diamond windows were used here, in contrast to prior work

using quartz windows for optical measurements and beryllium

windows for X-ray scattering. Diamond is transparent for

HeNe laser light and it exhibits low X-ray scattering and

absorbance. In addition, the mechanical properties of

diamond allow it to withstand without deformation the

maximum pressures generated inside the flow channel (

). Windows of diamond type 2A of dimensions

4 mm diameter

0.5 mm were obtained from Harris Inter-

national and glued to the flow cell with epoxy (353ND from

Epotek). Care was taken that the diamond surface was flush

with the stainless steel in order to avoid distortion of the flow

profile. The clear aperture in the stainless steel cell has a

diameter of 1.90 mm.

2.2. X-ray configuration

Measurements were performed at the BM26b beamline

(DUBBLE) of the European Synchrotron Radiation Facility,

Grenoble, France (Bras

et al.

, 2003). The wavelength used was

research papers

186

Lucia Fernandez-Ballester

etal.

Flow-induced crystallization of polymers

J.SynchrotronRad.

(2008).

, 185–190