On the Origin of the ‘‘Core-Free’’ Morphology in ... · On the Origin of the...

On the Origin of the ‘‘Core-Free’’ Morphology in Microinjection-Molded

HDPE

Julien Giboz,1,2 Anne B. Spoelstra,3 Giuseppe Portale,4,5 Thierry Copponnex,6

Han E. H. Meijer,7 Gerrit W. M. Peters,7 Patrice Mele1

1LEPMI, UMR 5279, CNRS—Grenoble INP-Universite de Savoie—Universite J. Fourier, LMOPS—Bat. IUT,

Campus de Savoie Technolac, F-73376 Le Bourget du Lac Cedex, France

2Haute Ecole Arc, domaine Ingenierie, Rue de la Serre 7, CH-2610 St-Imier, Switzerland

3Laboratory of Polymer Technology, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology,

P.O. Box 513, 5600 MB, Eindhoven, The Netherlands

4Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The Netherlands

5DUBBLE, CRG/ESRF, Netherlands Organization for Scientific Research (NWO), c/o ESRF BP 220, F-38043 Grenoble Cedex, France

6Cendres & Metaux SA, Rue de Boujean 122, CH-2501 Biel/Bienne, Switzerland

7Materials Technology, Eindhoven University of Technology, PO Box 513, WH 4.146, 5600 MB Eindhoven, The Netherlands

Correspondence to: J. Giboz (E-mail: [email protected])

Received 27 May 2011; revised 11 July 2011; accepted 13 July 2011; Published online

DOI: 10.1002/polb.22332

ABSTRACT: This study investigates the morphology of a high-

density polyethylene processed with microinjection molding.

Previous work pointed out that a ‘‘core-free’’ morphology exists

for a micropart (150-lm thick), contrasting with the well-known

‘‘skin-core’’ morphology of a conventional part (1.5-mm thick).

Local analyses are now conducted in every structural layer of

these samples. Transmission electron microscopy observations

reveal highly oriented crystalline lamellae perpendicular to the

flow direction in the micropart. Image analysis also shows that

lamellae are thinner. Wide-angle X-ray diffraction measure-

ments using a microfocused beam highlight that highly ori-

ented shish–kebab morphologies are found through the

micropart thickness, with corresponding orientation function

close to 0.8. For the macropart, quiescent crystallized morphol-

ogies are found with few oriented structures. Finally, the mor-

phology within the micropart is more homogeneous, but the

crystalline structures created are disturbed due to the com-

bined effects of flow-induced crystallization and thermal crys-

tallization during processing. VC 2011 Wiley Periodicals, Inc. J

Polym Sci Part B: Polym Phys 000: 000–000, 2011

KEYWORDS: flow-induced crystallization; high-density polyethyl-

ene; local morphology; microinjection molding; orientation;

polyethylene; WAXS

INTRODUCTION Microinjection molding is regarded as aniche market for the plastic industry, and a growing rangeof applications are found in producing microparts (micro-mechanical sector) or very precise microfeatured parts(telecommunication or pharmaceutical sectors). This contin-uous growth is explained by the needs of integrating morefunctions in smaller spaces, especially in microelectrome-chanical systems. Polymeric materials are good candidate asthey give the good compromise between ease of processingand properties. However, their molding conditions are farfrom conventional injection molding. These technologicalaspects, previously reported in ref. 1, will not be addressedin this article.

Studies of microinjection molding were focused on process-ing rather than on relations between process, resultingstructures, and final properties. Indeed, these relations

greatly depend on the thermomechanical history. The shearand/or elongational rates in injection molding result inorientation and stretching of macromolecules, and the lowthermal conductivity of polymers generates considerablethermal gradients within the thickness. For semicrystallinepolymers, a heterogeneous morphology results through thethickness, called ‘‘skin-core.’’ Four layers are generally dis-tinguished: skin layer, shear layer, fine-grained layer, andcore, where different crystalline and orientation statesexist.2–4

Details of the structure are related to the combined effectsof high shear rates and severe quenching conditions that aretraditionally used in injection molding process. Shear andelongation flow fields modify the crystallization process viaflow-induced crystallization (FIC).5 Flow enhances the nucle-ation density by decades increasing the overall crystallization

VC 2011 Wiley Periodicals, Inc.

WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2011, 000, 000–000 1

WWW.POLYMERPHYSICS.ORG FULL PAPER

kinetics.5,6 Even for relatively low flow conditions, the nucle-ation density increases. This involves closely spaced growingentities, which development is hindered by mutual impinge-ment.7 Moreover, an oriented morphology can develop,depending on the deformation amplitude:

i. When shear rates range between few tens of s�1 and afew thousands of strain units, the highest molecularweight chains align in flow direction (FD) and first formfibril-like structures, so-called ‘‘shish.’’8 Next, epitaxialgrowth of twisted lamellae occurs, in the direction per-pendicular to flow, leading to the formation of ‘‘kebabs.’’9

The final morphology is then composed by fibrillarshishes and twisted lamellae and is generally called ‘‘rowstructure.’’

ii. When shear rates are higher than 100 s�1, a largernumber of macromolecules are oriented in the FD. Theresulting morphology is then made up of shishes andkebabs with straight lamellae, whose chains are orientedin FD.9

For microinjection molding, few studies deal with the influ-ence of the process on the final morphology and propertiesof products. In a previous work, some discrepancies wereobserved between the global morphologies found in a mac-ropart (1.5-mm thick) and a micropart (0.15-mm thick), bothmade from a high-density polyethylene (HDPE). The micro-part presents a ‘‘core-free’’ morphology that contrasts withthe well-known ‘‘skin-core’’ morphology. Two main originswere proposed to explain these differences, that is, the pro-cess itself or the difference in geometries. This articleendeavors to define clearly the local morphology found inthe micro and the macro sample to understand better theprocess–structures relationships. Experimental techniques,like transmission electron microscopy (TEM) and microfo-cused wide-angle X-ray diffraction (WAXD), are used to eval-uate the local orientation state and crystallinity through thesamples thickness. The influence of processing conditionsand geometrical conditions can then be separated fromquantified morphological features.

EXPERIMENTAL

SamplingTwo different geometries were used with thicknesses differingby one order of magnitude, with 1.5 mm for the macropartand 0.15 mm for the micropart. The macropart geometryis classical for an injection-molded part, as it is a 40 � 40 �1.5 mm3 square plate. A scheme is proposed in Figure 1. It isinjection molded on a Babyplast 6/10V

R

injection moldingmachine, the 6 � 0.6 mm2 rectangular gate is lateral. Theinjection speed is 50 mm/s, the injection pressure is 80 MPa,and the cooling time is 10 s. Finally, the melt and moldtemperatures are, respectively, set to 230 and 30 �C.

The micropart is inspired from a part geometry found intowatch mechanisms, as depicted in Figure 2. Its thickness is150 lm, and its weight is 0.62 mg. It is injection molded ona Sesame Nano-molderV

R

machine from Lawton Machinery. Aspecific cold runner system is needed to feed the cavity. Itintegrates three capillary ‘‘pin-point’’ gates (Ø 200 lm) sizedusing MoldflowVR simulations. More details about the feedingsystem are accessible elsewhere.10 The melt and mold tem-peratures are, respectively, 230 and 80 �C, the injectionspeed is 50 mm/s, the injection pressure is 235 MPa, thepacking pressure applied is 188 MPa during 3 s, and thecooling time is 10 s.

The polymer used is a commercial injection-molding gradeof HDPE, commercialized by the Borealis company under thereference MG9641.

Polarized Light MicroscopyHDPE was observed by using 5-lm-thick cryomicrotomedsections. The sampling zones are situated 15 mm far fromthe gate for macropart and at �1 mm from the gate for themicropart, where the flow is unidirectional in both cases(Figs. 1 and 2), as observed from Moldex simulations (notreported here). The microtome used was a Leica RM2265with a liquid nitrogen freezing attachment, and cuts weremade with glass knives. Polarized light microscopy (PLM)observations in the TD direction are performed with a

FIGURE 1 Illustration of the macropart, with the sampling used

for PLM and TEM analyses (1.5-mm thick). X ¼ 5 lm for PLM

and X ¼ 60 nm for TEM. Flow direction (FD) and transverse

direction (TD) are reported.

FIGURE 2 Illustration of the micropart, with the sampling used

for PLM and TEM analyses for the micropart (150-lm thick).

X ¼ 5 lm for PLM and X ¼ 60 nm for TEM. Flow direction (FD)

and transverse direction (TD) are reported.

FULL PAPER WWW.POLYMERPHYSICS.ORG

2 JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2011, 000, 000–000

DM-LM Leica microscope with 90� cross-polarized light. Thisallows identifying the structural layers.

Transmission Electron MicroscopyTEM observations were conducted within the structurallayers to visualize their lamellar morphology. The sampleswere trimmed at low temperature (�120 �C) using a Dia-tome trimming tool and subsequently stained for 24 h witha RuO4 solution prepared according to Montezinos et al.11

Ultrathin sections (60 nm) were obtained at room tempera-ture using a Leica Ultracut S microtome equipped with aDiatome 35� knife (Figs. 1 and 2). The sections were put ona 200-mesh copper grid with a carbon support layer. Thesections were examined in TD, in a FEITM Tecnai 20 trans-mission electron microscope, operated at 200 kV. The trans-mission electron micrographs were analyzed using ImageJsoftware.12 The orientation of lamellae is qualified using therepresentation of the TEM images in the Fourier domain,using a fast Fourier transform (FFT). The lamellae thick-nesses lc is also quantified. Image processing was done bybinarizing the eight-bit images, using segmentation and theOtsu’s method for thresholding.13 The intensity profilesalong the direction perpendicular to lamellae were then usedto estimate their thickness. More details about this method-ology are given in ref. 14. The values reported were averagedfrom 40 measurements, where only values higher than 4 nmwere taken into account. The representation chosen for theresults is box plots, where the lower quartile, the median,the upper quartile, the average, and the extrema are given,as proposed in ref. 15.

Microfocused Wide-Angle X-Ray DiffractionLocal WAXD analyses were performed at the beamlineBM26B/DUBBLE of the European Synchrotron RadiationFacilities (ESRF, Grenoble, France).16 The beam wavelengthwas k ¼ 1.03 Å and the energy 12 keV. The beam was micro-focused thanks to an experimental setup using double-focus-ing mirrors that allowed focusing the beam size down toabout 20 � 20 lm2.17 The sample-to-detector distance wasset to 15.1 cm to access the (110) and (200) reflections ofHDPE, since the intensity of the (020) reflection is weak. 2D-WAXD images were collected using a 2048 � 2048 pixelsfast-readout, low-noise CCD FReLoN camera, with pixel sizeof 48.82 � 48.82 lm2. The macropart thickness was ana-lyzed with steps of 10 lm from the edge until 200 lm to-ward the thickness and then every 50 lm until the center.The micropart thickness was analyzed with steps of 10 lm.The diffraction pattern of HDPE depends on the morphologyand the incident beam direction. In our case, different layersare observed over the sample thickness, and these are ana-lyzed in TD. The FD is reported on the 2D scattering images,which corresponds to the meridional direction of the pattern.Data processing was made using the software Datasqueeze.18

RESULTS

Local Observations at a Submicron ScaleThe optical micrograph of the half thickness of the macro-part is given in Figure 3 (to the left), where the differentstructural layers are observed. The local morphology is given

accordingly on the transmission electron micrographsdepicted in the center. The respective Fourier transformgiven on the right gives a qualitative appreciation on thelamellae orientation. From the surface to the center, it can benoted:

i. For the core [Fig. 3(a)], the lamellae are in the plane ofthe microtome section, with no specific arrangement. TheFourier transform shows a round and diffuse intensity,suggesting a random orientation of lamellae, correspond-ing to a spherulitic morphology. This is consistent withthe results reported for usual injection-molded parts. Thesmooth zones where no lamella is observed are due tothe tilt of lamellae compared to the plane of the micro-tome section.

ii. The fine-grained layer exhibits fewer lamellae in theplane of the microtome section. These are slightly alignedperpendicularly to FD [Fig. 3(b)], as suggested by theFourier transform showing a stronger intensity along FD.

iii. The shear layer exhibits some lamellae slightly alignedperpendicularly to FD [Fig. 3(c)], as suggested by thestronger intensity along FD on the Fourier transform.The orientation state is not as strong as evidenced byMendoza et al.19

FIGURE 3 TEM micrographs of the macropart observed in TD

(middle) with the image Fourier transform (right), referring to

cross sections viewed in TD (left). The results are reported for

the core layer (a), the fine-grained layer (b), the shear layer (c),

and the skin layer (d). The arrows at the bottom right-end cor-

ner of micrographs indicate FD.



iv. For the skin layer [Fig. 3(d)], the lamellae are hereclearly observed, whereas small crystallites were revealedwith PLM observations. The associated Fourier transformshows a homogeneous halo, suggesting that a randomorientation of crystalline lamellae exists.

The same analysis is performed for the micropart (Fig. 4):

i. The central layer of the micropart shows most of thelamellae in the plane of the microtome section [Fig. 4(a)].The lamellae are highly oriented in a direction perpendic-ular to flow. Accordingly, the Fourier transform shows astreak of intensity in the FD, related to the periodicity oflamellae along FD. Two main lamellae arrangements canbe observed:• most lamellae are aligned perpendicularly to the FDand are not twisted.

• some of them gradually disappear, as they are tiltedcompared to the plane of the microtome section. Thisphenomenon can be related to the twist of lamellae.20

• The morphology corresponds most probably to low-oriented row structures.20 However, additional measure-ments are required to know more about this specificcrystalline architecture.

ii. The skin layer reveals crystalline lamellae that are allviewed in the plane [Fig. 4(b)] and oriented in a directionperpendicular to flow. The FFT representation gives clearevidence with a streak of intensity in FD. On a largerscale, some straight lines parallel to flow can be observedand could be related to the presence of extended chainbundles forming ‘‘shishes.’’ The lamellae have grownepitaxially from these shishes, giving structurescalled ‘‘kebabs.’’ These observations are in accordance

with the work of Schrauwen et al.20 The lengths of thelamellae are shorter than in the central layer. Here, thelamellae impinge on each other most probably due to animportant nucleation density. According to the work ofOdell et al., it can be concluded that the shish–kebabs areinterlocked. This specific morphology appears under thecombined influences of high strains and high coolingrates.21

From the PLM observations, the micropart layer seems to bemade of two layers: a skin layer and a central layer. TEMreveals an additional layer (called ‘‘transition layer’’) halfwaybetween the central and the skin layer (Fig. 5). Lamellae arefound in the plane of the microtome section, with a diffusedtransition between lamellae. All of them are oriented in adirection perpendicular to flow. The FFT representation con-firms this assumption.

The transmission electron micrographs were used toestimate the distributions in lamellae thickness for eachstructural layer of macropart and micropart [Fig. 6(a,b),respectively]. The distributions found for the micropart arereported in both plots, with dashed lines.

From a global view, the lamellae are thinner for the micro-part, compared to the macropart. This corroborates our for-mer results evidenced with differential scanning calorimetry(DSC), where a lower melting point has been found for theHDPE coming from the micropart.10 This suggests that lamel-lae growth was limited in the micropart. When comparingthe lamellae thickness distributions (given by the boxes),these are also tighter for the micropart, which confirms ahigher homogeneity.

FIGURE 4 TEM micrographs of the micropart observed in TD (middle) with the image Fourier transform (right), referring to cross

sections viewed in TD (left). The results are reported for the central layer (a) and the skin layer (b). The arrows at the bottom right-end

corner of micrographs indicate FD. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]



To complete these investigations, microfocused WAXDmeasurements are performed. The WAXD images fromscanning over the morphological layers of the macropartare given in Figure 7. The two Debye rings correspond tothe reflections of the (110) and the (200) planes, situated,respectively, at 2y(110) ¼ 14.4� (d(110) ¼ 4.12 Å) and2y(200) ¼ 15.91� (d(200) ¼ 3.72 Å). Each reflection wasisolated, and the azimuthal distributions of intensities areplotted in order to evaluate the crystal orientation withinsamples.

Starting from the core [Fig. 7(a)], it can be seen that:

• the evolution of the scattered intensity from the planes(110) and (200) is homogeneously distributed along thediffraction ring. A random orientation exists and confirmsthat the core of macropart is mainly composed ofspherulites.

• weak variations of intensities along the azimuthal angle uare seen for the fine-grained layer [Fig. 7(b)], for bothreflections, but no typical intensity maximum is detected.The slight intensity variation with u can be related to anorientation of few crystal planes, but the main morphologyis again spherulitic.

• For the shear layer [Fig. 7(c)] and the skin layer[Fig. 7(d)], the diffraction patterns exhibit four-arc maximaoffset from the equator for the (110) reflection and ameridional maximum for the (200) reflection, with thehighest intensity on the azimuthal positions 0 and 180�.Following Keller et al. observations with respect to FD,this typical scattering suggests that the b-axis (related to(110)) is oriented in the transverse direction (TD), withthe a- and c-axis rotating around it (related to (200)).9

These observations suggest that row structures are pres-ent in these layers.

FIGURE 5 TEM micrograph of the micropart, observed in TD (middle) with the image Fourier transform (right), referring to cross

sections viewed in TD (left). The results are reported for the transition layer (c). The arrows at the bottom right-end corner of

micrographs indicate FD. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 6 Comparison of the lamellae thickness of the different layers of (a) macropart and (b) micropart. Data are extracted from

TEM micrographs and are represented with box plots (lower quartile, median and upper quartile), with the average (l) and the

extrema (þ) of the distributions.



One should note that the evolution of the intensity withinthese layers is gradual, and not as sharp as implied by therepresentation chosen. The detailed analysis is availableelsewhere.14

The results obtained for scanning over the thickness of themicropart are given in Figure 8.

The WAXD patterns show the scattering fingerprint of theorthorhombic PE, with nonconstant intensities alongthe Debye rings [Fig. 8(a–e)]. For all the positions analyzed,the intensity maxima of the (110) and the (200) reflectionsare found at the same positions, namely �90� and 90�. Fromthe azimuthal distribution of the (200) plane, it can be con-cluded that the a-axis is normal to FD for all the position inthe sample thickness. Accordingly, the b-axis (related to the(110) reflection) is also normal to FD, and the c-axis (thatcorresponds to chains axis) is therefore in FD.

The scattering observed for the micropart (Fig. 8) indicates apreferred orientation of the c-axis in FD that corresponds toa shish–kebab morphology. Therefore, the term ‘‘core’’ cannotbe applied here, as a specific oriented morphology exists.The result also suggests the existence of a more homogene-ous morphology throughout the micropart thickness.

The local morphology could be more accurately defined withthe pole figures of the samples, as shown by Mendozaet al.19 This gives the orientation of the crystallographicaxis in space. The experimental device used in this studywas not adapted for these analyses, and the uniqueness ofthe different layers would have been affected by the samplerotation. The microfocused WAXD analyses evidence chang-ing morphologies of different orientation states betweensamples. A rotational symmetry along FD exists forthese morphologies, and at the risk of oversimplification,the measured 2D WAXD patterns are used to determine the

FIGURE 7 Selected WAXD images (middle) from the scanning over the half thickness of the macropart, referring to cross sections

viewed in TD (left). The azimuthal distributions of the intensities are reported for the (110) and the (200) reflections (plots, on the

right). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]



crystalline orientation functions and the crystallinity.20 First,the degree of orientation of the crystal’s axis compared to areference axis can be quantified with the Herman’s orienta-tion factor (FH), defined as:

FH ¼ ð3 cos2 ui

� �� 1Þ=2 (1)

with ui the angle between a reference axis and the axis ofinterest (i).

In our case, the reference is the c-axis, corresponding to thechains axis compared to FD. As the pure reflection from thec-axis is not accessible for HDPE, the Wilchinsky’s method is

applied to calculate its orientation from the (110) and the(200) reflections, given by (2):20

cos2 uc

� � ¼ 1� 0:565 cos2 u200

� �� 1:435 cos2 u110

� �(2)

The terms hcos2u110i and hcos2u200i are defined as:

cos2 ujkl

� � ¼R p=20 IðujklÞ � cos2 ujkl � sinujkl � dujkl

R p=20 IðujklÞ � sinujkl � dujkl

(3)

The Herman’s orientation factor of the c-axis compared toFD is then equal to:

FIGURE 8 Selected WAXD images (middle) from the scanning over the thickness of micropart, referring to cross section viewed in

TD (left). The azimuthal distributions of the intensities are reported for each reflection (plots, on the right). [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]



FH ¼ ð3 cos2 uc

� �� 1Þ=2 (4)

The c-axis of the crystals is parallel to the FD when FH ¼ 1,randomly oriented when FH ¼ 0, and orthogonal to the FDwhen FH ¼ �0.5. Upon calculation from the azimuthal distri-bution, the Herman’s orientation factor FH is reportedthrough the normalized half-thickness of samples, where 0corresponds to the center and edge is 1.

The evolution of FH through the normalized sample thicknessis depicted in Figure 9, accounting for experimentaluncertainties.

The values found for the micropart clearly lie above those ofthe macropart, ranging from 0.4 to 0.75 against 0 to 0.06.For the external layers (normalized thickness close to 1), FHis about 0.75 for the micropart and 0.05 for the macropart.Thus, a high orientation is revealed for the micropart, com-ing from a former orientation involved by flow and notrelaxed during processing. The maximum of orientation of0.8 is found for the so-called ‘‘transition layer’’ of the micro-part, meaning that additional flow strength occurs in thisregion. This has disturbed the lamellae formation andexplains the diffused transition between lamellae revealed inthe TEM micrographs (Fig. 5). The minimum value is foundat the center of the parts: FH ¼ 0 for the macropart, whichis typical for a spherulitic morphology, while for the micro-part, the value FH ¼ 0.4 means that orientation still exists inthe central layer. The values of the orientation factor clearlyshow that macromolecular chains tend to be oriented in FD,meaning that FIC occurs in the micropart.

To complete these findings, the degree of crystallinity wascalculated from the microfocused WAXD patterns. To do so,the intensity was first integrated over the azimuthal angle(one-half of the pattern was considered) and plotted againstthe scattering angle 2y. The intensity profiles were normalizedsuch that the area underneath the curves equals to unity. Thescattering of the amorphous halo was modeled thanks to the

association of two Lorentzian functions. The scattering of thecrystalline phase was calculated by subtracting the amorphoushalo from the global scattering of samples. Then, the WAXDdegree of crystallinity (vc) was calculated with the followingexpression, assuming that the contribution of the (020) reflec-tion to the overall scattered signal is negligible.

vc ¼Crystalline area

ðCrystallineþ amorphous haloÞ areas� 100 (5)

The values of vc through the normalized samples thicknessare plotted in Figure 10.

For the macropart, vc gradually increases from 60 to 67%going from the edge to the center and then stays constantuntil the center (core). For the micropart, the vc increasesfrom the edge (�57%) to the central layer (64%). It can beremarked that vc values found for the micropart lie underthe ones of the macropart, and this tendency is in agreementwith previous DSC experiments, reported with the dashedlines in Figure 10.10 Therefore, the thermomechanical condi-tions during micropart molding promote defects within thecrystalline morphology. One should note that the values of vcare similar for the center of the micropart and the externallayers of the macropart (<65%), both situated at an absolutedistance around 70 lm from the samples edge. Remindingthat orientation is higher for the micropart, the comparabledegrees of crystallinity found in the shallow layers suggestthat the rise in crystallinity in this region is thermally con-trolled, in agreement with by Van Der Beek et al.’s results.22

Finally, important morphological differences are observedbetween the macropart and the micropart. Orientation ishigher in the micropart, due to a higher initial orientation ofmacromolecular chains within the feeding system and in thecavity, but also caused by a limited relaxation, induced bythe high cooling rates throughout the sample thickness. Thisalso tends to create defects within the crystals, which limitsthe crystallinity. Similar effects occur in the surface layers(few microns) of conventional parts and microparts, but notwithin the other layers. Finally, the reduction of the parts

FIGURE 9 Herman’s orientation factor FH over half the thick-

ness of the micropart (n) and the macropart (*). The thick-

nesses are normalized, 0 ¼ center and 1 ¼ edge.

FIGURE 10 Crystallinity over half the thickness of the micropart

(n) and the macropart (*). The thicknesses are normalized,

0 ¼ center and 1 ¼ edge. The lines drawn report the degrees

of crystallinity values found with DSC.10



thickness limits the gradients of cooling rates within thethickness, promoting a more homogeneous morphology.However, the flow is stronger and favors the orientation ofmacromolecules in the FD. This, combined with high coolingrates, is responsible for the specific morphology observed inthe micropart. An illustration of the morphology created bythe microinjection molding process would be a more homo-geneous and more disturbed crystalline morphology.

CONCLUSIONS

The local morphology of injection-molded and microinjec-tion-molded HDPE samples was investigated by opticalmicroscopy, TEM, and microfocused X-ray diffraction techni-ques. The results give evidence to the existence of a ‘‘core-free’’ morphology for the micropart, which contrasts with thewell-known ‘‘skin-core’’ morphology found for the macropart.Local observations with TEM evidence highly oriented lamel-lae in a direction perpendicular to flow, in all layers of themicropart, whereas no particular arrangement is observed inthe macropart. As clear differences are observed, microfo-cused WAXD analyses were conducted to define themorphology throughout the sample’s thickness. Randomlyoriented spherulites dominate the macropart, with few loworiented row structures in the shallow layers. In comparison,a highly oriented shish–kebab morphology was identifiedfrom the typical scattering in the micropart. The averagelevel of orientation was calculated from the 2D diffractionpatterns using the Hermans’ orientation function. Valuesranging from 0.4 to 0.8 are found for the micropart, meaningthat polymer chains are preferably oriented in FD. Thispoints out that FIC is the main process driving the micropartmorphology. This is not so extensive in the macropart wherethe orientation factor is close to 0. Moreover, the thinnerlamellae and the lower crystallinity observed in the micro-part have been attributed to thermal effects. Therefore,microinjection molding of thin parts involves antagonistphysical effects. The reduced cavity thickness limits the ther-mal and mechanical gradients, but the strain and coolingrates increase accordingly. The final morphology of semicrys-talline polymers is greatly affected by FIC, whose effects aresupported by high cooling rates.

ACKNOWLEDGMENTS

The authors acknowledge Bernard Lotz (ICS Strasbourg) andGilles Regnier (Arts & Metiers Paristech) for the fruitful discus-sions about the TEM andWAXD results. They also thank PaulineSchmidt for her support with microtomy (TUE), Dario Cavallo(TUE) for the support provided during microfocused WAXDanalyses, and Luigi Balzano (TUE) for the discussions aboutdata processing. The Netherlands Organization for ScientificResearch (NWO) is acknowledged for making the beamtimeavailable.

REFERENCES AND NOTES

1 Giboz, J.; Copponnex, T.; Mele, P. J. Micromech. Microeng.2007, 17, R96–R109.

2 Pantani, R.; Speranza, V.; Coccorullo, I.; Titomanlio, G.

Macromol. Symp. 2002, 185, 309–326.

3 Kantz, M. R.; Newman, H. D., Jr.; Stigale, F. H. J. Appl.Polym. Sci. 1972, 16, 1249–1260.

4 Jerschow, P.; Janeschitz-Kriegl, H. Rheol. Acta 1996, 35,127–133.

5 (a) Tribout, C.; Monasse, B.; Haudin, J. M. Colloid Polym. Sci.1996, 274, 197–208; (b) Swartjes, F. H. W. Stress induced crys-

tallization in elongational flow. PhD Thesis, Technische Univer-

siteit Eindhoven, Netherland, November 2001.

6 (a) Janeschitz-Kriegl, H.; Ratajski, E.; Stadlbauer, M. Rheol.Acta 2003, 42, 355–364; (b) Stadlbauer, M.; Janeschitz-Kriegl,

H.; Eder, G.; Ratajski, E. J. Rheol. 2004, 48, 631–639; (c) Ma, Z.;

Steenbakkers, R. J. A.; Giboz, J.; Peters, G. W. M. Rheol. Acta2010, 1; (d) Binsbergen, F. L. Nature 1966, 211, 516–517; (e)

Eder, G.; Janeschitz-Kriegl, H. In Materials Science and

Technology; Meijer, H. E. H., Ed.; Wiley-VCH: New York, 1997;

Vol. 18, p 269; (f) Kumaraswamy, G. Polym. Rev. 2005, 45,375–397.

7 Hosier, I. L.; Bassett, D. C.; Moneva, I. T. Polymer 1995, 36,4197–4202.

8 Pople, J. A.; Mitchell, G. R.; Sutton, S. J.; Vaughan, A. S.;

Chai, C. K. Polymer 1999, 40, 2769–2777.

9 Keller, A.; Kolnaar, H. W. H. In Materials Science and

Technology; Meijer, H. E. H., Ed.; Wiley-VCH: New York, 1997;

Vol. 18, p 189.

10 Giboz, J.; Copponnex, T.; Mele, P. J. Micromech. Microeng.2009, 19, 025023.

11 Montezinos, D.; Wells, B. G.; Burns, J. L. J. Polym. Sci.Polym. Lett. Ed. 1985, 23, 421–425.

12 ImageJ software website. Available at: http://rsbweb.nih.

gov/ij/. Accessed May 15, 2011.

13 Sezgin, M.; Sankur, B. J. Electron. Imaging 2004, 13,146–165.

14 Giboz, J. De l’injection traditionnelle a la microinjection

de pieces en polymeres thermoplastiques: Divergences et simi-

litudes. PhD thesis, Universite de Savoie, France, October,

2009.

15 Garnier, G.; Brechet, Y.; Flandin, L. J. Mater. Sci. 2009, 44,4692–4699.

16 Bras, W.; Dolbnya, I. P.; Detollenaere, D.; van Tol, R.;

Malfois, M.; Greaves, G. N.; Ryan, A. J.; Heeley, E. J. Appl.Cryst. 2003, 36, 791–794.

17 Portale, G.; et al., manuscript in preparation.

18 Heiney, P. Datasqueeze software website. Available at:

www.datasqueezesoftware.com. Accessed May 15, 2011.

19 Mendoza, R.; Regnier, G.; Seiler, W.; Lebrun, J. L. Polymer2003, 44, 3363–3373.

20 Schrauwen, B. A. G.; van Breemen, L. C. A.; Spoelstra, A.

B.; Govaert, L. E.; Peters, G. W. M.; Meijer, H. E. H. Macromole-cules 2004, 37, 8618–8633.

21 Odell, J.; Grubb, D.; Keller, A. Polymer 1978, 19, 617–626.

22 Van der Beek, M. H. E.; Peters, G. W. M.; Meijer, H. E. H.

Macromol. Mater. Eng. 2005, 290, 443–455.



On the Origin of the ‘‘Core-Free’’ Morphology in ... · On the Origin of the...

Documents

Transcript of On the Origin of the ‘‘Core-Free’’ Morphology in ... · On the Origin of the...