A Study of the Deformation-Induced Whitening Phenomenon for Cavitating and Non-cavitating Semicrystalline Polymers Laurent Farge, 1 St ephane Andre, 1 Andrzej Pawlak, 2 Christophe Baravian, 1 Sarah C. Irvine, 3,4 Adrian-Marie Philippe 1 1 LEMTA-CNRS 7563-Univ ersit e de Lorraine , 2 avenue de la fore ˆ t de Haye, 5450 4 Vand oeu vre-l e ` s-Nancy, France 2 Depa rtment of Poly mer Phys ics, Cent er of Molecular and Macromolecular Stud ies, Poli sh Acad emy of Scie nce s, Sien kiewicza 112, 90-363 Lodz, Poland 3 Paul Scherrer Inst itute , SLS, PSI, 5232 Vill igen , Switzerland 4 Sch ool of Biology and Medi cine , Universi ty of Lausann e, 1015 Lausanne, Switzerland Correspon dence to: L. Farge (E-mail: Laurent.Fa rge@univ -lorraine.f r) Received 14 November 2012 ; revised 10 Janu ary 2013; accepted 11 Janu ary 2013 ; published online DOI: 10.1002/polb.23267 ABSTRACT: In this work, we used two tec hniqu es to study the de- formation-induced whit enin g phen omen on that occurs when cer- tai n semicr yst alline pol ymers (SCPs) are sub jec ted to tensil e drawing: (1) IPLST (Incoherent Polarized Stead y Light Transpor t) was use d for cha rac ter izi ng the lig ht sca tte rers and in par tic ula r for det ermining their size. (2) SRX TM (Synchro tro n Radiation X-Ray Tomogr aphic Micros copy) was used to visua lize the inter - nal str ucture of the def ormed SCP s. In par tic ula r, wit h thi s tec h- ni qu e the possible presence of micromet ric cavi ties can be detec ted. In the early whit enin g stage of a cavit ating polypr opyl- ene (PP), the IPL ST techn iqu e was foun d to sho w tha t the size ofth e light sc at te rers is larger than 1 lm. At th e same time, th e SRX TM mea sur eme nts sho wed tha t no void lar ger than 1 lm was presen t in the material. The micrometr ic light sca tt ere rs respo nsible for the whit enin g phen omenon may thus not be sim- ple cavities. In fac t, this exp eri men tal study sugges ts tha t the y corre spond to areas where smaller objec ts (pos sibly nanovoid s) are hig hly con fin ed. At the sca le of vis ibl e wav ele ngt hs, the se regions could scatt er visib le light like indiv idual entities of micro - metric size. Th e stud y also showed that the si ze of ca vi ti es observable using SRXTM for a very deformed PP is dependent on the initial dimensions of the sph erulites. Results pre vio usl y obtai ned for a non- cavit ating high densi ty polye thyle ne are also briefly presented in thi s art icle to con fir m the the ory tha t def or- mation-induced -whi tenin g phen omenon may have vario us ori- gins for such comp lex micr ostru cturi ng. V C 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 000: 000–000, 2013 KEYWORDS: light scat teri ng; microstructure; structural charac- terization ; transparen cy; voids INTRODUCTIONThe main purpose of this paper is to analyze deformation-induced whitening phenomenon (or more simply whit enin g phe nome non) in relation to the evolut ion of the micros tructur e of semicr ystalline polymer s (SCPs) subjected to uniaxial drawing. New microstructural objects are created in the material during the deformation process. If these objects have a refractiv e index which differs from that of the surround - ing mat rix , the y can scat ter visi ble ligh t. Mor eov er , if the se objects have a micrometric size (order of magnitude of the visi- ble wave lengths ), they scatter approxima tely the same amountof ligh t for all the wave leng ths of the visibl e spec trum (Mie scattering) and this results in the whitening phenomenon. 1 In this wor k, we used IPSL T (Incoh erent Pol ar ize d Ste ady Light Transport), an original technique with which we can: •quantify the whitening phenomenon, 2–4 •def ine the size and mor pho log y of the ligh t scattering objects, 2,5 This theref ore makes IPSL T a unique tool for stud ying the whitening phenomenon. Another of our objectives was to analyze the evolution of the microstructure associated to the whitening phenomenon so, in addition to IPSLT, we also used Synchrotron Radiation X-Ray Tomographic Microscopy 6 (SRX TM) to visu alize the inte rna l structure of the SCPs for different deformation states. Undef orme d SCPs are gene rally not compl etel y tran spar entfor light. Lig ht sca tte rin g doe s not come fro m the periodic amorphous-crystalline layer structure where the thickness ofthe lamellae and amorphous layer is in the range of a few nanomete rs. Usua lly , for non-de forme d polyp ropy lene (PP) or polye thy lene, light is scattered by differen t part s of spher - ulite s or more precis ely by bulk microme tric domains with more or less the same lamellae orientation. 7,8 In the case of defor med specime ns, the cavitat ion phen om- enon is cited in almost all publis hed works to expla in the V C 2013 Wiley Peri odic als , Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2013, 000, 000–00 0 1 JOURNAL OF POLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER
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8/13/2019 Cavitating and Non-Cavitating Semicrystalline Polymers
A Study of the Deformation-Induced Whitening Phenomenon for
Cavitating and Non-cavitating Semicrystalline Polymers
Laurent Farge,1 Stephane Andre,1 Andrzej Pawlak,2 Christophe Baravian,1
Sarah C. Irvine,3,4 Adrian-Marie Philippe1
1LEMTA-CNRS 7563-Universite d e L o r ra i n e, 2 a v e nu e d e l a f o r et d e H a y e, 5 4 5 04 V a n do e uv r e - les - N an c y , F r a nc e
2D e p ar t m en t o f P o l ym e r P h y si c s , C e n te r o f M o le c u la r a n d M a c ro m ol e c ul a r S t u di e s, P o l is h A c a de m y o f S c i en c es , S i e nk i e wi c z a
1 1 2 , 9 0 - 3 6 3 L o d z, P o l an d
3P a ul S c h er r e r I n s ti t u t e, S L S , P S I , 5 2 3 2 V i l li g e n, S w i tz e r la n d
4S c ho o l o f B i o lo g y a n d M e d ic i n e, U n i ve r s it y o f L a u sa n ne , 1 0 1 5 L a u sa n n e, S w i tz e r la n d
Correspondence to: L. Farge (E-mail: Laurent.Farge@ univ-lorraine.fr)
R e c ei v e d 1 4 N o v em b er 2 0 1 2; r e v is e d 1 0 J a n ua r y 2 0 1 3; a c c ep t e d 1 1 J a n ua r y 2 0 1 3; p u bl i s he d o n li n e
DOI: 10.1002/polb.23267
ABSTRACT: I n t h is w o rk , w e u s ed t wo t e ch ni q ue s t o s t u d y t h e d e -
formation-induced whitening phenomenon that occurs when cer-
t a in s e mi c ry s ta l li n e p o ly me r s ( S CP s ) a r e s u bj e ct e d t o t e ns i le
Concluding Remarks for the Tomographic Measurement
Cavities with a size as small as 1 lm are clearly observable
by SRXTM for PP at very high strain levels. No such resolved
holes are visible for the tomographic views corresponding toHDPE or during the whitening stage for PP. For all stages of
the deformation process, the orientation of the micrometric
objects revealed by SRXTM is very similar to that of the light
scatterers. This clearly suggests that these objects (which
cannot be considered as ‘‘voids’’ or ‘‘holes’’) are responsible
for the whitening phenomenon.
SAXS-WAXS
Figures 16 and 17 show the WAXS and SAXS patterns for PP
for different true strain values in particular for the deforma-
tion states for which the tomographic measurement was
obtained. The beam crossed the whole specimen of 4 mm
thickness. If we take into account the small size of the skin
zone [200 lm, see Fig. 1(a)], it can be considered that X-ray
scattering resulted nearly exclusively from the core region.
For small strain levels (e 0.3), concentric rings are
observed on the WAXS patterns (Fig. 16). The orientation of
crystallographic planes is not observed. Spherulites are
known to become elongated along the drawing direction at
this deformation level. Intensive lamellar slip processes
occurs with rotation of some lamellae although this is not
followed by rotation of crystalline planes (see Fig. 9 in Paw-
lak and Galeski30). For higher strain levels, between e ¼ 0.3
and e ¼ 0.7, the orientation of the crystallographic planes,
that is, orientation of structure occurs. For e 0.7, the
fibrillar structure is clearly observable.
The SAXS patterns (Fig. 17) show that the nanocavitation
phenomenon begins for true strain values smaller than e ¼
0.1. The nanovoids are originally elongated perpendicularly
to the drawing direction. For the same strain level for which
a change of the WAXS shape patterns can be observed
(between e ¼ 0.3 and e ¼ 0.7), the SAXS patterns are also
modified. Such evolutions of the SAXS and WAXS patterns
have already often been observed.9,11,13,15–17,18,20,24 Two pos-sible interpretations are possible: reorientation of existing
cavities or formation of a new void population oriented
along the drawing direction. The increase of the total inten-
sity associated to this reorientation of the SAXS patterns
(between e ¼ 0.3 and e ¼ 0.7) is rather small, suggesting
that the first possibility is more probable. Also the volume
strain measurements done for this type of material did not
show any rapid increase, which should be the case if numer-
ous new voids were formed.
DISCUSSION
This discussion section includes three parts addressing three
different issues:
1. clarifying the origin of the deformation-induced whitening
phenomenon,
2. describing some aspects of the microstructural evolution
of the SCPs subjected to tensile loading,
3. highlighting and illustrating the relation between the
initial microstructure and the cavitation phenomenon.
This analysis is based upon the following experimental data:
• at the micrometric level, the quantitative results obtained
by IPSLT and SRXTM,
• at the nanometric and crystallographic levels, the qualita-
tive analysis of the SAXS/WAXS patterns.
Whitening Stage (e 0.3)
The objective of this part is to give more information about
the precise nature of the microstructural objects that scatter
the visible light and are thereby responsible for the whiten-
ing phenomenon. First, we summarize the experimental
observations that were obtained for the strain range during
which the whitening phenomenon takes place. Next we inter-
pret this experimental data.
Observations
For the two materials, the crystallinity index remains con-
stant during the deformation process: whitening caused by
additional crystallization phenomena is therefore precluded.
FIGURE 15 X - r a y S R X TM a n d I P S LT a n i so t r op y i n d ex e s .
FIGURE 16 W A X S m e a su r e me n t s f o r P P c o r re s p on d in g t o d i f fe r e nt t r u e s t r ai n l e v el s ( t h e a r r ow i n d ic a t es d e f or m a ti o n d i r e c t i on ) .
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First, let us remember that the SCPs are not completely
transparent at the undeformed state: the light scatterers are
then the different domains (roughly of micrometric size) of
the spherulites7,8 where the lamellae (at the nanometric
level) have approximately the same orientation. It shows
that the presence of micrometric light scatterers can result
from the local organization of the matter at a smaller scale.
For the two materials studied in this work, the whitening
stage mainly takes place in the strain range 0.08–0.3; the
asymptotic value for l TR being reached roughly for e ¼ 0.5
(Fig. 7). During the whitening stage, the IPSLT technique has
also shown that, either for HDPE or for PP, the light scatter-
ers have a size larger than 1 lm and are anisotropic:
elongated perpendicularly to the drawing direction.
SRXTM
For PP, the tomographic views obtained during the whitening
stage have revealed objects that have a grey level slightly
inferior (darker: less dense regions) than the surroundingmedium, indicating a lower density. In the space, these
objects look like flat cylinders (‘‘dark disks’’), elongated in
the transverse direction [Fig. 10(b)]. The diameter of these
‘‘dark disks’’ is roughly a few tens of micrometers and their
height is approximately comprised between 5 and 10 lm. As
previously mentioned, let us note that the characteristics of
these objects could correspond to the light scatterers high-
lighted by IPSLT: their smaller size is larger than 1 lm and
they have a transverse orientation with respect to the draw-
ing direction. In the case of HDPE, it is not possible to see
by naked eye comparable objects on the tomographic view
(Fig. 14). As previously mentioned, objects that are transver-
sally oriented with respect to the drawing direction can behighlighted by calculating the Fourier transform of the
images. The necessity of using this reciprocal space based
analysis for extracting this information from the tomographic
views shows that the corresponding objects are associated
to very low changes of the matter density. The link between
these objects and the whitening phenomenon was clearly
shown in a previous paper.23
It is important to note that, for the two studied SCPs, the to-
mographic measurement has shown that no hole larger than
1 lm is present in the material during the whitening stage.
SAXS/WAXS (for PP only)
SAXS : A huge increase of the intensity of the SAXS signal was
observed when the material starts to whiten (e ¼ 0.1,
Fig. 17). This observation is generally associated to the de-
velopment of nanovoids in the material: only nanoobjects
presenting a strong density difference with the surrounding
matrix are likely to scatter a significant X-ray amount. As
mentioned in the introduction section, the concomitance
between the increase of the SAXS signal and the whitening
phenomenon was often reported.15–18 The global intensity of
the SAXS pattern increases significantly during the whitening
stage (strain level , e ¼ 0, e ¼ 0.1, and e ¼ 0.3 in Fig. 17)
but remains roughly constant when l TR has reached its final
value (strain level: e ¼ 0.7, and e ¼ 1.7 in Fig. 17). Moreover,
light scatterers and SAXS detected nanovoids are both elon-
gated, perpendicularly to the drawing direction. To summa-
rize, the characteristics of the SAXS detected nanovoids and
the light scatterers are apparently very similar: they both
appear for the same strain level (e 0.1), they develop in
the same strain range e 0.1–0.3 and they have the same
initial orientation. One must keep in mind that the light scat-terers are necessarily much larger than the SAXS detected
nanovoids.
WAXS : WAXS results show that the spherulitic structure glob-
ally still exists during the whitening stage (perfectly resolved
rings remain observable for e ¼ 0.3 see Fig. 16).
Interpretation
In the case of SCPs, the nanovoiding phenomenon does not
occur in a homogeneous way within the material, depending
in fact on the local orientation of the lamellae.10,16 Galeski
et al .31 have outlined the mechanisms describing the
nanovoiding phenomenon under imposed uniaxial tension at
the spherulite scale for small strain levels. According to thisscenario, the equatorial planes are subjected to both a radial
compression and to an accentuated tensile stress. The com-
pression of the stacks of lamellae and amorphous regions
gives rise to unstable kinking of the lamellae. The accentuated
tensile stress acting on the equatorial disks expands amor-
phous material within lamellae kinks into pores. The final
result should be an array of aligned cavities in the equatorial
disks of spherulites (i.e., in the regions where the large dimen-
sion of lamellae is transversally oriented). The average
distance between the centers of the nanocavities can be of the
FIGURE 17 S A X S m e a su r e me n t s f o r P P c o r re s p on d in g t o d i f fe r e nt t r u e s t r ai n l e v e ls ( t h e a r r o w i n d ic a t es d e f or m a ti o n d i r ec t i on ) .
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• At the microscopic level (IPSLT see Fig. 8): the micrometric
light scatterers are first transversally oriented. Starting
from a strain value comprised in the range e ¼ 0.2…0.3, a
reorientation of the scatterers toward the drawing direc-
tion can be observed. For e > 1.2, the IPSLT anisotropy
index reaches its maximum value.
• At the microscopic level (SRXTM see Fig. 10): For small
strain levels [e ¼ 0.3, Fig. 10(b)], objects presenting a
transverse orientation with respect to the drawing direc-
tion are clearly observable. For intermediate strain levels
[e ¼ 0.7, Fig. 10(c) or 11], it is not easy to discern any
clear orientation on the tomographic views. For high strain
levels, the microstructure is clearly elongated along the
tensile direction [e ¼ 1.7, Fig. 10(d)].
In Figure 11 [or Fig. 10(c)], it is possible to see regions that
are much darker (dark part of the ‘‘zebra patterns’’) than the
‘‘dark disks’’ observable on the previous strain level [Fig.
10(b)]. These very dark regions could correspond to areas
where unresolved voids tend to concentrate.
The tomographic views can be related to observations made
by SEM. For example, in the case of PP, Figure 18(a) shows a
cigar shape arrays of holes imaged by SEM for a strain level
of 1.2. Holes that have a size significantly larger than one mi-
crometer are clearly observable. These holes could then be
resolved using SRXTM and it is clearly the same objects (‘‘ci-
gar shape’’ array of holes) that are imaged by SRXTM [Fig.
10(d]) or by SEM [Fig. 18(a)] for the high strain levels. The
longitudinal size of the ‘‘cigar shape’’ array of holes is
roughly the same in Figure 18(a) (SEM) and in Figure 10(d)
(SRXTM): about 60 lm.
Figure 18(b) shows a SEM picture corresponding to a
smaller strain level of about 0.8. The resolution of the SEM
images is finer than that of SRXTM. Consequently, in Figure18(b), micrometric clusters of intertwined voids and matter
are clearly discernable. Because of the coarser resolution of
the tomographic measurement, these regions could simply
appear on the tomographic view as the dark areas
corresponding to the ‘‘zebra patterns’’ that can be seen in
Figure 10(c) or 11.
Thanks to the fine resolution of the SEM measurement, it
makes no doubt that it is the same microstructural objects
that are imaged in Figure 18(a) and (b) for different strain
levels. The large voids observable in Figure 18(b) could then
not result from the expansion of a unique void but from the
agglomeration of voids of smaller size as the ones that can
be seen in Figure 18(b). Similarly, for the SRXTM measure-ment, it can be envisaged that the ‘‘zebra-patterns’’ observ-
able in Figure 10(c) or Figure 11 evolve towards the ‘‘cigar
shape’’ arrays of holes that can be seen in Figure 10(d). This
evolution could be due to a progressive coalescence of
initially unresolved voids.
Other observations can contribute to strengthen this idea:
On the tomographic view corresponding to Figure 10(d), it
is possible to discern regions (in particular near the cigar
tips) where fragments of matter and cavities are still
combined.
FIGURE 18 ( a ) ‘ ‘ C i ga r s h a pe ’’ a r r a y o f h o l es ( s e e F i g u r e 1 0 d ) i m a g e d b y S E M f o r P P ( s t r a i n l e v e l 1 . 2 ) . ( b ) P o s s i bl y a ‘‘ z e br a p a t te r n ’’
[ s e e F i g . 1 0 ( c ) o r 1 1 ] i m a g ed f o r P P w i t h t h e f i n e S E M r e s ol u t io n ( s t r a i n l e v e l 0 . 8 ) .
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