ORIGINAL CONTRIBUTION

Competitive growth of α- and β-crystals in isotacticpolypropylene with versatile nucleating agentsunder shear flow

Hui Liu & Hong Huo

Received: 3 December 2012 /Revised: 5 February 2013 /Accepted: 5 February 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Sorbitol derivatives, the conventional α-nucleatingagents of isotactic polypropylene (iPP), are discovered toinduceβ-phase iPP under normal crystalline conditions. Com-bined effects of shear flow and sorbitol derivatives on thecrystallization of iPP were investigated by using differentialscanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering. In the nucleation stage, sorbitol de-rivatives induce both α- and β-nuclei, while shear flow andthe interactions between shear and sorbitol derivatives enhancethe amount of α-nuclei. In the growth stage, the epitaxialgrowth of β-crystals on shear-induced α-row nuclei occurs.As the shear rate increases, more epitaxial β-crystals form dueto the increase of α-row nuclei, further increasing the contentof β-crystals. Under high shear rate, the presence of sorbitolderivatives and shear flow exhibit a synergistic interaction onincreasing the content of β-crystals. Moreover, α-nuclei,which arise from the interaction between shear and sorbitolderivatives, emerge earlier than shear-induced α-row nuclei.

Keywords Sorbitol derivatives . Versatile nucleating agent .

Shear . iPP .β-Crystals

Introduction

It is well known that isotactic polypropylene (iPP) exhibitsfour crystalline forms: monoclinic (α), trigonal (β), triclinic(γ), and smectic (δ) [1–3]. According to different crystallineforms, nucleating agents are classified into α- and β-nucleating agent. In recent years, some special nucleating

agents which could simultaneously induce α and β forms ofiPP under certain circumstance were found and were desig-nated as “versatile nucleating agent” [4–6]. It can simplify theexperimental procedures to obtain iPP with desired crystalforms by using versatile nucleating agents. Yu et al. discov-ered that a conventionalα-nucleating agent, sodium benzoate,could induce β-form iPP and that sodium benzoate showedpeculiar nucleating characteristics on inducing iPP crystalli-zation by comparing with the commercial β-nucleating agent[4]. Cortazar et al. found that a kind of liquid crystal polymernamedVectra A950 could induce bothα andβ forms of iPP atlow crystallization temperature [5]. Guo et al. reported thathalloysite nanotubes could induce both α- and β-phase iPPwithout weakening anyone of them under a high level ofundercooling [6]. In summary, versatile nucleating agentshave the ability to induce both α- and β-phase iPP at the sametime. Sorbitol derivatives are common α-nucleating agents ofiPP, such as 1,2,3,4-dibenzylidene sorbitol (Millad 3905),1,2,3,4-bis(p-methylbenzylidene sorbitol) (MBDS, Millad3940), and 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol(DMDBS, Millad 3988) [7–11]. Owing to the presence oftwo hydroxyl functional groups, sorbitol derivatives canself-assemble into nanofibrils and 3D networks in iPP [12,13]. The crystallization of iPP is restrained by the networks sothat the spherulite dimension of iPP is small and the transpar-ency of iPP is promoted [13–22]. Furthermore, it has beenreported that sorbitol derivatives could induce γ-form crystalsduring the crystallization of iPP under special circumstance[10, 23]. This is another valuable topic about sorbitol deriva-tives. In this work, we find that sorbitol derivatives, theconventional α-form nucleating agents, can also induce β-form iPP under normal crystallization conditions.

Besides the addition of β-nucleating agent and versatilenucleating agent, shear flow also plays an important role informing β-form iPP [24–30]. Leugering et al. first observedthat shear flow could initiate the formation of β-crystals [31].

H. Liu :H. Huo (*)Beijing Key Laboratory of Energy Conversion and StorageMaterials,College of Chemistry, Beijing Normal University,Beijing 100875, People’s Republic of Chinae-mail: hhuo@bnu.edu.cn

Colloid Polym SciDOI 10.1007/s00396-013-2922-0

Subsequently, Varga et al. found that shear flow could induceα-row nuclei and β-crystals epitaxial grew on the surface of α-row nuclei [32]. Hsiao et al. further investigated the shear-induced crystallization of iPP by using in situ synchrotronwide-angle X-ray diffraction. They observed that oriented α-crystals occurred immediately after the application of shearflow, and subsequently, β-crystals grew on the surface oforiented α-crystals [33]. Based on the above phenomena, it isinterested to investigate the crystallization of β-form iPP underthe coexistence of shear flow and β-nucleating agents. Varga etal. surprisingly found that the high shear rate restrained theformation of β-crystals [34]. We also studied the crystallizationof β-nucleated iPP under shear flow and found that the contentof β-crystals decreased with the increase of the shear rate [35].Hsiao et al. further studied the reason of the counteraction effectbetween shear and β-nucleating agent on the content of β-crystals of iPP. They found that the addition of the β-nucleating agent increased the amount of β-nuclei, while theapplication of shear flow and the interaction between shear andnucleating agent enhanced the amount of α-nuclei. As a result,α- and β-crystals grew competitively, causing simultaneouslyincrements of α- and β-crystals. The existence of abundant α-crystals decreased the content of β-crystals in the limited grow-ing space. As the shear rate increased, more α-nuclei formed,further decreasing the amount of β-crystals [36].

The versatile nucleating agent can also induce β-phase iPP.How does the content of β-crystals develop when versatile-nucleated iPP isothermally crystallizes after the application ofshear flow? As far as we know, no one studied the crystalliza-tion of iPP under the coexistence of shear flow and versatilenucleating agent. In this work, sorbitol derivatives, 1,3:2,4-bis(p-methylbenzylidene) sorbitol (MDBS, Millad 3940) and1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS, Millad3988), were used as versatile nucleating agents of iPP. Thecrystallization behaviors of iPP/MDBS and iPP/DMDBSblends after step shear were characterized by synchrotronwide-angle X-ray diffraction (WAXD) and small-angle X-rayscattering (SAXS) techniques. The shear-induced polymor-phism of iPP has been intensively investigated, which enablesus to better understand the competitive growth of α- and β-crystals in versatile-nucleated iPP.

Experimental section

Materials

iPP used in this work was provided by Yanshan PetroleumChemical Co., China, with a melt flow rate of 2.1 g/10 min(230 °C, 21.6 N); molecular weight, Mw, of 4.5×10

5g/mol;and a polydispersity, Mw/Mn, of 5.9. MDBS (Millad 3940)and DMDBS (Millad 3988) were obtained from MillikenChemicals (Gent, Belgium) and used as received.

Sample preparation

The versatile nucleating agent was first mixed with iPP in atwin-screw extruder at 195 °C for 10 min to form a masterbatch containing 5.0 wt% nucleating agent. The masterbatch was further melt compounded with pure iPP to pro-duce samples having the desired nucleating agent concen-tration. iPP nucleated by the versatile nucleating agent wasdesignated as NPP, in order to make a distinction with pureiPP. NPPs having 0.1, 0.2, 0.4 and 1 wt% MDBS weredesignated as M01, M02, M04 and M1, respectively. NPPshaving 0.1, 0.2, 0.4 and 1 wt% DMDBS were named asDM01, DM02, DM04, and DM1, respectively. The pure iPPpellets without nucleating agents were also prepared underthe same processing conditions for comparison purposesand were designated as PP0. The obtained materials werecompression molded into films of 1-mm thickness for facil-itating the processing. The compression molding tempera-ture was 220 °C and the molding time was 3 min. Theresulting film was quenched to room temperature and cutin disk-like samples.

Differential scanning calorimeter measurements

Thermal analysis was carried out using a differential scan-ning calorimeter (DSC 1 produced by Mettler-Toledo, CH)to study the melting and crystallization behaviors of iPPwith and without nucleating agents. Indium and tin wereemployed for the temperature calibration, and nitrogen gaspurge was used to prevent oxidative degradation of thesamples. Samples of approximately 5 mg were placed intoaluminum pans for the DSC analysis. In order to obtain thecrystallization temperature (Tc) and melting temperature(Tm), we designed temperature procedures as follows: (a)holding the samples at 25 °C for 2 min to stabilize themachine; (b) heating the samples from 25 to 195 °C andholding them at 195 °C for 5 min to erase earlierthermomechanical histories; (c) cooling the samples from195 to 25 °C and an exothermic peak formed in the crystal-lization curve (the peak temperature is Tc of the polymer);(d) reheating the samples from 25 to 195 °C and the endo-thermic peaks formed in the melting curve (the peak tem-perature is Tm of the polymer). Both the heating rate and thecooling rate during the measurements were 10 °C/min.

X-ray measurements

2D-WAXD and SAXS measurements were carried out at thesynchrotron X-ray beamline 4B9A of the Beijing SynchrotronRadiation Facility, Institute of High Energy Physics (ChineseAcademy of Sciences, Beijing, China). AMar165 CCDX-raydetector was employed for detection of 2D-WAXD and SAXSimages. The data acquisition time was 60 s for the WAXD

Colloid Polym Sci

pattern and 200 s for the SAXS pattern. The sample-detectordistance was 204 and 1,960 mm for WAXD and SAXS,respectively. The wavelength of incident X-ray was 1.54 Å.All WAXD and SAXS data were treated with Fit2D software.The 2D-SAXS image was integrated into 180° in order to getazimuthal intensity profiles.

Linear WAXD profiles were obtained from circular inte-gration of intensities from 2D-WAXD images. The intensitywas plotted as a function of the scattering vector, q, where

qj j ¼ 4p sin θλ , with l being the wavelength of the incident

beam and θ being the scattering angle. Subsequently, bydevonvoluting the peaks in linear WAXD profiles, the over-all crystallinity Xall is calculated by

Xall ¼P

AcrystPAcryst þ

PAamorp

� 100% ð1Þ

where Acryst and Aamorp stand for the fitted areas of crystaland amorphous peaks, respectively. The peak fitting methodwas described elsewhere [35]. The amount of β-crystals Xβ

is evaluated by

Xb ¼Ab 300ð Þ

Aa 110ð Þ þ Aa 040ð Þ þ Aa 130ð Þ þ Ab 300ð Þ� Xall ð2Þ

where Aβ(300) is the area of the (300) reflection peak andAα(110), Aα(040), and Aα(130) are the areas of (110), (040), and(130) reflection peaks, respectively. The amount of α-crystalsXα is calculated by

Xa ¼ Xall � Xb ð3Þ

Experimental procedures

A Linkam CSS-450 high-temperature shear stage was used toprecisely control thermal history and shear flow of the polymersamples. The experimental temperature profiles of isothermalcrystallization of iPP/MDBS shown in Fig. 1 were set asfollows: (1) heating at a rate of 30 °C/min from room temper-ature to 195 °C; (2) holding the temperature at 195 °C for 5 minto eliminate thermal history; (3) cooling at a rate of 30 °C/mindown to 134 °C; (4) when the temperature just reached 134 °C,shear flow was imposed with the desired shear rate, and theshear time was 5 s; (5) holding the temperature at 134 °C forenough time until the crystallization finished.

Results

Effects of versatile nucleating agents on structureand morphology of iPP under quiescent conditions

Figure 2 shows DSC cooling thermograms of iPP with differ-ent MDBS contents. For PP0, the exothermic crystallization

peak is located around 118 °C. However, the addition ofMDBS in iPP does not produce a marked shift of Tc towardhigher temperatures. M04 has the highest Tc at about 122 °C,only 4 °C higher than that of PP0. The small increment of Tc ofiPP loading with MDBS is rather unexpected. It is known thatsorbitol derivatives are common nucleating agents with highnucleation efficiency (NE) of inducingα-phase iPP [9]. In thiswork, the small increment of Tc indicates that MDBS is not anefficient nucleating agent. To understand the nucleation effi-ciency of MDBS, NE is estimated based on a methodologydeveloped by Fillon [37]:

NE% ¼ 100%TcNA�Tc1

Tc2max � Tc1ð4Þ

where TcNA, Tc1, and Tc2max are peak crystallization tempera-tures of the nucleated, non-nucleated, and self-nucleated poly-mer, respectively. There is an assumption for Fillon's method,namely, the self-nucleation procedure allows obtaining thehighest achievable crystallization temperature. Although astrict way has been developed to determine the Tc2max value,here, as a first approximation, we took 140 °C as the Tc2max ofthe self-nucleated PP [38]. Taking into account this approxi-mation, NE of iPP nucleated with MDBS is obtained andshown in Fig. 3. NE first increases with MDBS content andreaches the highest value of 18.3 %, then dramatically de-creases with the further increase of MDBS.

Fig. 1 Schematic of the thermal history and shear conditions as afunction of time for isothermal crystallization of PP0 and NPP

80 90 100 110 120 130 140 150 160

Temperature/

PP0M01M02M04M1

Exo

Fig. 2 DSC cooling scans for the neat polymer PP0 and blends M01,M02, M04 and M1

Colloid Polym Sci

DSC was used to characterize the melting behavior of iPPnucleated with MDBS, and the result is shown in Fig. 4. Themelting temperature is 165 °C for PP0, and there were lessincreases of iPP nucleated with MDBS because of the lowNE of MDBS. These melting peaks belong to the melting ofα-crystals. To our surprise, in M01, M02, and M04, thereare other melting peaks at about 150 °C, which are attributedto β-phase iPP. As known to all, MDBS is a commercial α-nucleating agent. In this work, MDBS can also induce β-form iPP and shows obviously versatile nucleating activity.Though the versatile nucleating phenomenon has beenreported before, few of them found that MDBS had theversatile nucleating ability. Figure 4 shows that there is onlyone α-phase melting peak of M1, indicating that MDBSloses the ability to induce β-form crystals when its concen-tration is higher than 1.0 wt%. Namely, in such condition,MDBS can only induce α-phase iPP. This phenomenon wasalso observed in other versatile nucleating agents [4].

Synchrotron 2D-WAXDmeasurements were carried out tostudy the effects of both the nucleating agent and shear flowon the crystallization of iPP. Linear WAXD profiles wereobtained from circular integration of intensities from 2D-WAXD images. The intensity was plotted as a function of

the scattering vector, q, where qj j ¼ 4p sin θλ , with l being the

wavelength of the incident beam and θ being the scatteringangle. In this paper, 1D linear WAXD profiles are all obtainedfrom 2D-WAXD patterns. Figure 5 shows linear WAXD

profiles of iPP/MDBS blends with different MDBS concen-trations. Both the characteristic diffraction peaks of α- and β-forms are discovered in M01, M02 and M04, indicatingMDBS has β-nucleating ability in spite of its being a well-known α-nucleating agent. At high concentration, 1.0 wt%,there are no β-phase iPP forms, MDBS loses the ability ofinducing β-crystals. It indicates that MDBS is a versatilenucleating agent; it can induce β-form iPP under appropriateconcentrations. The similar result was also obtained by Yu etal. who found that sodium benzoate showed obviously versa-tile nucleating activity under certain conditions [4].

In order to further understand the versatile nucleatingcharacteristics of sorbitol derivatives, the crystallization ofiPP nucleated with DMDBS was studied by using WAXD.Figure 6 shows WAXD profiles of iPP/DMDBS blends withdifferent DMDBS concentrations crystallized at 134 °C atroom temperature. β characteristic diffraction peaks at 2θ=16.08° are found in DM01, DM02, and DM04, implyingDMDBS has the versatile nucleating ability though it is acommercial α-nucleating agent. Like MDBS, DMDBSloses the ability to induce β-form iPP at a high concentra-tion of 1.0 wt%. Based on the above results, it confirms thatMDBS and DMDBS are versatile nucleating agents of iPPunder certain crystallization conditions.

0.0 0.2 0.4 0.6 0.8 1.02

4

6

8

10

12

14

16

18

20

NE

/%

MDBS Concentration/%

Fig. 3 NE of MDBS in M01, M02, M04 and M1

M1

M04

M02

M01

PP0

End

o

120 130 140 150 160 170 180 190Temperature/

Fig. 4 DSC heating thermograms of iPP/MDBS blends with differentMDBS concentrations. The small melting peaks of β-phase iPP werecircled in the ellipses

12 14 16 18 20 22 24 26 28

2θ / °

M1

M04

M02

M01

PP0

Fig. 5 WAXD profiles of iPP/MDBS blends with different MDBSconcentrations crystallized at 134 °C under quiescent condition

12 14 16 18 20 22 24 26 28

DM1

DM04

DM02

DM01

PP0

2θ / °

Fig. 6 WAXD profiles of iPP/DMDBS blends with different DMDBSconcentrations crystallized at 134 °C under quiescent condition

Colloid Polym Sci

Isothermal crystallization of PP0 and NPP under shear flow

A representative series of 2D-WAXD patterns of PP0 andiPP/MDBS blends with different MDBS content whichcompletely crystallized at 134 °C with and without shearflow are shown in Fig. 7. PP0-0 represents pure iPP crys-tallized under quiescent conditions. Five Debye rings of(110), (040), (130), (111) and (−131) reflections of the α-crystal lattice planes from inside to outside of the 2D-WAXD image are shown. As shown in PP0-0, there is noorientation of the crystallite. In pure iPP, the content of β-crystals is rather low. There is nearly no appearance of (300)characteristic lattice planes of β-crystals in 2D-WAXD im-age. Many groups have studied the shear-induced iPP crys-tallization and found that oriented α-row nuclei formed andthen the oriented α-row nuclei induced the growth of β-crystals, even though the shear rate was as low as 10 s−1

[33]. In this work, it is unexpected that the arc-like diffrac-tion feature of α-crystal lattice planes is clearly observeduntil the shear rate is as high as 40 s−1, such as shown inPP0-40. Arc-like (300) reflection from β-crystals is further

identified in PP0-40, indicating the form of oriented β-crystals.

For iPP samples nucleated with MDBS under quiescentisothermal crystallization, such as M01-0 and M02-0, a faint(300) reflection from β-crystals can be observed, identifyingMDBS can induce β-crystals. After the application of shearflow, the arc-like diffraction of α-crystal lattice planes (040)and (130) develops, indicating the growth of oriented α-crystal. The azimuthal breadths in the intensity of α-crystalreflections narrow with the increase of the shear rate, indi-cating that the orientation of α-crystals strengthens at highshear rate. In addition, the intensity of (300) reflection in-creases with the increase of the shear rate, as shown in M01,M02 and M04, indicating that the content of β-crystalsincreases with the shear rate. Under the same shear rate,the orientation of MDBS-nucleated iPP is higher than that ofPP0, and the orientation of iPP strengthens with the increaseof the MDBS concentration.

In order to further understand the development of differ-ent crystal forms as a function of the shear rate, linearWAXD profiles are analyzed, and the results are shown in

Fig. 7 Selected 2D-WAXDpatterns of PP0 and iPP/MDBSblends with different MDBScontent completely crystallizedat 134 °C with different shearrates. For “sample name—i,” irepresents the shear rate and thesame in the following figures

Colloid Polym Sci

Figs. 8 and 9. Generally, in iPP, a large number of α-crystalsgenerate spontaneously, a small amount of β-crystals areinduced by residual industrial additives, and impurities andcan be ignored. Figure 8a shows the WAXD profiles ofisothermally crystallized PP0 at 134 °C with and withoutshear flow. Under quiescent condition, the (300) diffractionpeak of β-phase is too small to be taken into consideration.After the application of shear, β-phase iPP forms. The α-phase crystallinity Xα, the β-phase crystallinity Xβ, and thesummation of α and β crystals Xall are calculated, and theresults are shown in Fig. 8b. The content of β-crystalsslightly increases with the shear rate when the shear rate isbelow 20 s−1, then dramatically increases when the shearrate reaches 40 s−1. Hsiao et al. considered that oriented α-row nuclei were responsible for the growth of β-crystals inpure iPP melt under shear flow field [33]. Oriented α-rownuclei arose by the application of shear, and the density ofα-row nuclei increased with the shear time and the shearrate. In this work, the shear time is fixed and the shear ratechanges from 1 to 40 s−1. With the increase of the shear rate,the content of α-row nuclei increases, so Xβ increases. InFig. 7, it shows that the orientation of α-crystals is faintwhen the shear rate is below 20 s−1, indicating the content of

α-row nuclei is low. In Fig. 8b, it is clear seen that thecontent of β-crystals increases little with the shear rate whenthe shear rate is below 20 s−1. When the shear rate reaches40 s−1, the β-phase crystallinity dramatically increases. Fororientation-induced crystallization, a certain degree of mo-lecular extension must be achieved to induce formation ofstable primary nuclei. At low shear rate, only the longerchains having long relaxation times will be oriented; as theshear rate increases, the population of polymer moleculeswith lower molecular weights and with lower relaxationtimes also become extended, thus increasing the content ofα-row nuclei and β-crystals. In this work, the variation ofthe content of α-row nuclei and the β-phase crystallinitywith the shear rate is related to the distribution of themolecular weight of iPP.

Figure 9 shows the development of α- and β-crystals ofM02 on the change of the shear rate. For M02 under quies-cent isothermal condition, because of the versatile nucle-ation ability of MDBS, β-crystals form in spite of theconcentration of MDBS being as low as 0.2 wt%. Uponthe application of shear flow, the content of β-crystals in-creases with the shear rate, such as shown in Fig. 9b. To ourknowledge, there is no report related to the phenomena thatunder shear flow, the β-nucleated iPP forms higher fraction

12 14 16 18 20 22 24 26 28

2θ / °

PP0-40

PP0-20

PP0-10

PP0-5

PP0-2

PP0-1

PP0-0

a

0 5 10 15 20 25 30 35 400

5

10

15

40

50

60

70

80

Xβ

Xα

Xall

crys

talli

nity

/%

Shear Rate/s-1

b

Fig. 8 aWAXD profiles of PP0 crystallized under different shear ratesat 134 °C; b the α-phase crystallinity Xα, the β-phase crystallinity Xβ

and the summation of α and β crystals Xall in PP0 as a function of theshear rate

12 14 16 18 20 22 24 26 28

a

2θ / °

M02-40

M02-20

M02-10

M02-5

M02-2

M02-1

M02-0

0 5 10 15 20 25 30 35 400

5

10

50

55

60

65

70

75

b

Xβ

Xα

XallCry

stal

linit

y/%

Shear Rate/s-1

Fig. 9 aWAXD profiles of M02 under different shear rates at 134 °C; bThe α-phase crystallinity Xα, the β-phase crystallinity Xβ and the sum-mation of α and β crystals Xall in M02 as a function of the shear rate

Colloid Polym Sci

β-crystals. In other words, the combination of shear flowand β-nucleation should not synergistically induce more β-crystals. In the last decades, many groups studied the effectof both shear and β-nucleating agents on the formation of β-crystals in iPP and found a common phenomenon that thecontent of β-crystals decreased with the increase of theshear rate, namely, there was a counteraction effect betweenshear flow and β-nucleating agent for the formation of β-crystals. In order to further confirm the synergy increase ofβ-crystals induced by both shear flow and MDBS, westudied the crystallization of M01, M04 and M1 under shearflow. The results are shown in Fig. 10, and the summary ofthe data is shown in Table 1. MDBS is a versatile nucleatingagent in M01 and M04. Like M02, Xβ in M01 and M04synergistically increases with the increase of shear rate. Butfor M1, MDBS only acts as α-nucleating agent, and no β-crystals form until the shear rate reaches 20 s−1. How do weunderstand the underlying mechanism of synergisticallyinducing β-crystals by shear flow and the versatile nucleat-ing agent? This will be discussed later.

Naturally, it is easy to consider that shear may destroy thestructure of MDBS, causing it to increase or decrease theability to induce β-crystals. To verify this, we designed anexperiment to reheat the sheared M02 samples (shear rateswere 2 and 20 s−1, respectively). The samples were firstreheated to 195 °C, held at this temperature for 5 min toerase the history of the first crystallization, and then cooledto 134 °C at 30 °C/min to quiescently crystallize. Thecrystallinities of sheared M02 before and after reheatingare shown in Fig. 11. It is clearly shown that Xα, Xβ andXall of the reheated samples are close to that of M02 iso-thermal crystallized at 134 °C under quiescent condition.Therefore, it can be logically concluded that the reason forthe change of Xβ in sheared, versatile-nucleated iPP is notdue to the variation of the structure of MDBS.

Similarly, we studied the crystallization of DMDBS-nucleated iPP under shear flow in order to further under-stand the effect of shear flow on the crystallization behaviorof iPP nucleated with the versatile nucleating agent. Arepresentative series of 2D-WAXD patterns of DM02 and

DM1 completely crystallized at 134 °Cwith and without shearflow are shown in Fig. 12, and the 2D-WAXD patterns of PP0are also shown for comparison. In DM02-0, a faint (300)reflection from β-crystals can be observed, identifyingDMDBS can induce β-crystals. After the application of shearflow, the arc-like diffraction of α-crystal lattice planes (040)and (130) develops, and the azimuthal breadths in the intensityof α-crystal reflections narrow with the increase of the shearrate, indicating that the orientation of the α-crystals increaseswith the shear rate. In addition, the intensity of (300) reflectionincreases with the increase of the shear rate, indicating thecontent ofβ-crystals increases with the shear rate. The changeof the orientation ofα-crystals and the content ofβ-crystals inDMDBS-nucleated iPP are in accordance with those inMDBS-nucleated iPP. For DM1, the orientation of α-crystalobviously increases with the increase of shear rate, but no β-crystals form under shear flow. In Fig. 12, it also shows thatunder the same shear rate, the orientation of DM1 is higherthan that of DM02, indicating the orientation of α-crystals ofDMDBS-nucleated iPP increases with the increase of theDMDBS concentration.

Xβ of DMDBS-nucleated iPP is calculated from WAXDdata, and parts of the results are shown in Table 2. It showsthat Xβ in DM01, DM02 and DM04 increases with the shearrate. The development of α- and β-crystals of DMDBS-nucleated iPP under shear flow is in accordance with that ofMDBS-nucleated iPP, indicating that there is synergistic

0 5 10 15 20 25 30 35 40

0

2

4

6

8

10

12

14

16

18

Xβ

/%

Shear Rate/s-1

PP0M01M02M04M1

Fig. 10 The β-phase crystallinity Xβ in PP0 and iPP/MDBS blendswith different MDBS concentrations changes with the shear rate

Table 1 The β-form crystallinity Xβ (in percent) of iPP/MDBS blendswith different MDBS contents changes with the shear rate

Shear rate/s−1 PP0 M01 M02 M04 M1

0 0.8 3.4 4.9 1.6 0

1 0.5 4.2 5.2 1.7 0

2 1.1 4.2 7.5 2.5 0

5 1.5 5.2 8.7 2.3 0

10 1.5 8.2 8.9 3.2 0

20 2.0 11.1 10.3 3.6 1.3

40 18.2 10.3 10.4 4.8 2.5

2

4

6

8

50

55

60

XβXαXall

Cry

stal

linit

y/%

0 2-remelt 20-remelt

Fig. 11 Comparisons of the α-phase crystallinity Xα, the β-phasecrystallinity Xβ and the summation of α- and β-crystals Xall in shearedM02 (shear rates were 2 and 20 s−1) before and after reheating

Colloid Polym Sci

interaction between shear and versatile nucleating agent onincreasing the content of β-crystals.

Different lamellar arrangements in α- and β-crystals ofPP0 and NPP can be detected by SAXS measurement.Selected 2D-SAXS patterns of PP0, M02, and M1, whichcompletely crystallized at 134 °C under shear flow, areshown in Fig. 13. All the samples show isotropic scatteringunder quiescent crystallization. After the application ofshear flow, the strong scattering maxima on the meridianare clearly seen in the samples with and without MDBS,indicating the lamellae are arranged perpendicularly to theshear direction. The scattering intensity becomes strongerwith the increase of the shear rate in the samples, indicatingthat the orientation increases with the enhancement of shearflow. What's more, we do not observe the equatorial streakin iPP samples with and without MDBS under shear flow.Indeed, the shish kebab scenario does not apply to thepresent range of mild shear values used.

Azimuthal intensity profiles of PP0, M02 and M1 werecalculated from the Lorentz-corrected SAXS patterns, andthe results are shown in Fig. 14. The azimuthal angle isabout 90° along the flow direction, and the peak representsthe meridian maximum of 2D-SAXS patterns shown inFig. 13. It is known that the azimuthal intensity stands for

the orientation degree. Figure 14a shows that the orientationdegree increases with the increase of MDSB content from0 to 1 wt%. It indicates that the crystal orientation is en-hanced by the addition of MDBS under shear flow. It is inaccordance with 2D-WAXD results, such as shown inFig. 7. This phenomenon resulted from the fibrillar structureof MDBS. Sorbitol derivatives under a desired concentra-tion can crystallize into nanofibrils, and a large amount ofnanofibrils can build a 3D network under supercooling.While under the shear flow field, iPP molecular chains andMDBS with the 3D network structure are inclined to alignparallel along the flow direction, and then, oriented polymerchains are absorbed on the surface of MDBS rather thanrelaxing, where the motion of extended iPP chains is re-stricted to form shish. For pure iPP at a given shear rate,only those molecules having a chain length above a criticalorientation molecular weight can form stable oriented rownuclei, and the molecules with short chain length relax fast.For iPP nucleated with MDBS, the short polymer molecularchains may be absorbed on the surface of MDBS to formstable nuclei, thus increasing the number of oriented nuclei,then increasing the orientation degree of the samples. As aconsequence, MDBS displays an amplification effect ofshear on the orientation of iPP crystals, increases the relax-ation time of iPP molecular chains, and makes the orientedstructures keep longer. The high orientation degree meansthat there is a high content of oriented α-row nuclei whichcan induce more β-crystals after the application of shearflow in iPP. From Table 1, it shows that at the same shearrate, Xβ of the MDBS-nucleated iPP such as M01, M02, andM04 are all higher than the Xβ of PP0. But for M1, there areno β-crystals when the shear rate is below 20 s−1. At highshear rate, the Xβ of M1 is obviously lower than the Xβ of

Fig. 12 Selected 2D-WAXDpatterns of PP0 and iPP/DMDBS blends with differentDMDBS content completelycrystallized at 134 °C withdifferent shear rates

Table 2 The β-form crystallinity Xβ (in percent) of iPP/DMDBSblends with different DMDBS contents changes with the shear rate

Shear rate/s−1 PP0 DM01 DM02 DM04 DM1

0 0.8 3.5 4.2 5.4 0

5 1.5 4.2 5.2 7.3 0

40 18.2 7.7 8.2 15.8 0

Colloid Polym Sci

PP0 in spite of the orientation degree in M1 being higherthan that in PP0. What is the reason for this phenomenon?This will be further discussed later.

Selected 2D-SAXS patterns of PP0, DM02 and DM1,which were obtained under the same condition as M02 andM1, are shown in Fig. 15. All the samples show isotropicscattering under quiescent crystallization. After the applica-tion of shear flow, the strong scattering maxima on themeridian are clearly seen in the samples with and withoutDMDBS, and the scattering intensity become stronger withthe increase of the shear rate. It indicates that the orientationdegree increases with the increase of shear rate and theconcentration of DMDBS, which is in accordance with theresult of MDBS-nucleated iPP.

Discussion

Nucleating agents are commonly used in industrial practice incombination with crystallizable thermoplastic polymers toreduce processing cycle times or to improve physicochemicalcharacteristics. Sorbitol derivatives are conventional and ef-fective α-nucleating agents of iPP. Interestingly, in this work,we found that sorbitol derivatives could also induce β-phaseiPP under conventional crystallization conditions, namely,sorbitol derivatives are versatile nucleating agents of iPP.However, the nucleation efficiency of sorbitol derivatives inthis work is unexpectedly low. The mechanism of the versatilenucleating effect and the low nucleation efficiency of sorbitolderivatives will be further studied in the other work.

It has been well established that shear flow can acceleratethe crystallization process of iPP, and the application ofshear flow can trigger a different polymorphism in iPP, suchas β-crystals. The mechanism of shear inducing the forma-tion of β-crystals is that shear flow causes the developmentof α-row nuclei, and the surface of α-row nuclei induces the

Fig. 13 Selected 2D-SAXSpatterns of PP0, M02 and M1completely crystallized at134 °C with different shearrates. The shear directionis vertical

20 40 60 80 100 120 140 160

Azimuthal Angle/°

PP0-20M02-20M1-20

20 40 60 80 100 120 140 160

Azimuthal Angle/ °

M02-0M02-1M02-2M02-5M02-10M02-20M02-40

a

b

Fig. 14 a Azimuthal intensity profiles of PP0, M02 and M1 complete-ly crystallized at 134 °C at a shear rate of 20 s−1; b azimuthal intensityprofiles of M02 completely crystallized at 134 °C with different shearrates

Colloid Polym Sci

growth of β-crystals in iPP [33]. It is well known that the β-nucleating agent can induce a high proportion of β-crystalsdue to strong heterogeneous nucleation [1]. We havereported that there was a counteraction effect between shearflow and β-nucleating agent, namely, shear flow restrainedthe formation of β-crystals [35]. The reason of this counter-action mechanism is the competitive growth between α-(mostly induced by shear flow and the interaction betweenshear and β-nucleating agent) and β-crystals (mostly in-duced by β-nucleating agent) [36]. In this work, MDBSand DMDBS can induce β-crystals because of their versatilenucleating ability. After application of shear flow, the con-tent of β-crystals increases with the increase of shear rate inNPP. The possible reason of the damage of the nucleatingagent structure is removed, as shown in Fig. 11.

For iPP isothermal crystallized at the desired crystalliza-tion temperature Tcry, the following contributions to nucle-ation density should be considered [39]: Nhom(Tcry, t),homogeneous nuclei generated in the system by thermallyactivated local density fluctuations whenever the materialwas supercooled; N0

het (Tcry), unavoidable nuclei from im-

purities (catalyst residues, dust, etc.); NA;ahet (Tcry), heteroge-

neous α nuclei that originated from the addition of the

versatile nucleating substances; NA;bhet (Tcry), heterogeneous

β-nuclei that originated from the addition of the versatilenucleating substances; Ns(Tcry, Ts, ts _g , M), shear-inducednuclei can be regarded as homogeneous nuclei due to thedirect effect of shear on the polymer itself; andΔNA

s , nucleithat arose from the interaction between shear flow and thenucleating agent.

In quiescent crystallization of PP0 at Tcry, Nhom and N 0het

are the main α-nuclei for the growth of α-crystals. In thecase of isothermal crystallization of sheared PP0 at Tcry, Ns

is the dominant source for α-nuclei in the nucleation stage,which is responsible for α-crystal growth. While in quies-

cent crystallization of NPP at 134 °C, Nhom, N 0het and NA;a

het

provide the α-nucleation sites for α-crystal growth; andNA;bhet

provides the β-nucleation sites for β-crystal growth. When

shear flow is applied, Ns, Nhom,N0het,N

A;ahet andN

A;bhet coexist; in

the meantime,ΔNAs arises and can be believed to provide α-

nuclei because the interaction between shear and nucleatingagent greatly lowers the entropy of the system, which favorsthe formation of α-nuclei, compared to the sole applicationof shear. Byelov et al. calculated ΔNA

s of iPP with differentα-nucleating agents after the application of mild step shear[39]. They found that if the nucleation efficiency was toohigh, any effect of shear on iPP itself was overwhelmed bythe strong nucleation due to the additive that was hardlyinfluenced by the mild step shear. The effect of flow onlymanifested itself if its contribution to the nucleation densitywas of the same order of magnitude as that under quiescentconditions.ΔNA

s increased with the nucleating agent contentunder the same shear condition. Hsiao et al. used the samecalculation method to study the competitive growth of α-and β-crystals in β-nucleated iPP under shear flow [36].They found that Ns and ΔNA

s for α-crystal growth and NAhet

for β-crystal growth coexisted in iPP melt and were in thesame order of magnitude; thus, competitive growth of α-and β-crystals occurred. The growth of α-crystals naturallydecreased the content of β-crystals in the limited growingspace. It was found that ΔNA

s increased with the increase ofshear rate, causing more reduction of β-crystals. In a sum-mary of these two literatures, the import factor of the highcontribution of shear flow to the nucleation density in iPPwith nucleating agent is that the NE of the nucleating agent

Fig. 15 Selected 2D-SAXSpatterns of PP0, DM02 andDM1 completely crystallized at134 °C with different shearrates. The shear direction isvertical

Colloid Polym Sci

is not very high, namely, NAhet is low or in the same order of

magnitude of Ns. In addition, ΔNAs increases with the

increase of the nucleating agent concentration and the shearrate.

In this work, the NE of sorbitol derivatives is low, so theeffect of shear on the crystallization of iPP is strong. α- andβ-crystals show competitive growth in the limited growingspace. ΔNA

s emerges due to the interaction between shear

and sorbitol derivatives. Compared to the quiescent isother-

mal crystallization of NPP, under shear, the growth of α-

crystals (mostly induced by Ns andΔNAs ) naturally decreases

the content of β-crystals which grow from β-nuclei inducedby the versatile nucleating agent. On the other hand, theepitaxial growth of β-crystals on shear-induced α-row nu-clei occurs after the application of shear. Ns increase with theincrease of the shear rate, resulting in the increase of thecontents of α-row nuclei and epitaxial β-crystals with theshear rate. From 2D-SAXS images, it is shown that theorientation degree of NPP increases with the shear rate dueto the fibrillar structure of the sorbitol derivatives. Under theshear field, sorbitol derivatives with fibrillar structure arelikely to align parallel along the flow direction and form anoriented network, where the motion of extended iPP chainsis restricted. Sorbitol derivatives exhibit a synergistic effectwith shear on promoting the orientation of iPP crystals. Therestrictive effect of nucleating agents preserves the self-orientation nuclei and makes them survive more time toinduce β-crystals, so the content of β-crystals induced byα-row nuclei increases with the increase of the shear rate. Inthe case of isothermal crystallization of sheared NPP at Tcry,

the β-crystals are the sum of those induced by NA;bhet (sorbitol

derivatives) and α-row nuclei. As shown in Fig. 10, Table 1and Table 2, Xβ increases with the increase of the shear rate.Ns and ΔNA

s increase with the increase of the shear rate,resulting in the formation of lots of α-crystals and restrictionin the growth of β-crystals. Moreover, the content of epi-taxial β-crystals dramatically increases due to the increaseof shear-induced α-row nuclei. Because of the low NE ofthe versatile nucleating agents, the content of nucleatingagent-induced β-crystals is low, and the influence of thedecrement of these β-crystals on Xβ is limited. Because ofthe favorable temperature (134 °C) for β-crystal growth, thegrowth rate of β-crystals is faster than that of α-crystals. Asa result, the less restrictive effect on nucleating agent-induced β-crystals and a great deal of the increase of β-crystals induced by α-row nuclei make the content of β-crystals increase with the shear rate. At high shear rate, NPPmelt could form a higher fraction of β-crystals. So, undershear flow, either the synergy or the counteraction effectbetween the shear flow and versatile nucleating agent (or β-nucleating agent) for the formation of β-crystals depends onthe combination of the decrease of nucleating agent-induced

β-crystals and the increase of α-row nuclei-induced β-crystals. If the decrement of nucleating agent-induced β-crystals is less than the increment of α-row nuclei-inducedβ-crystals, there is the synergy effect between shear flowand versatile nucleating agent for the formation of β-crystals.

In the SAXS result, the orientation degree increases withthe sorbitol derivative concentration. This is due to theinitial formation of highly oriented fibrillar sorbitol deriva-tives under shear flow. There is no β-crystal form whenshear rate is low in sheared M1. This is another interestingexperimental phenomenon in this work. For M1, MDBSacts as α-nucleating agent to induce α-crystals. This maydecrease the relative content of β-crystals to some extent.But no β-crystals were generated at low shear rate in spite ofthe existence of oriented nuclei as well as the suitable temper-ature for β-crystal growth. Hsiao et al. observed that β-crystals were absent in sheared CNTs/iPP nanocomposites incontrast to the normal observation that α-row nuclei inducedby shear-generatedβ-crystals [40]. They considered that therewere two reasons to explain this result: (1) CNTs have thestrong α-nucleation efficiency and (2) the extra nuclei whichwere aroused by the interaction between shear and CNTsencouraged the formation of α-crystals, resulting in completeinexistence ofβ-crystals. In this work, the nucleation efficien-cy of MDBS is only 4.5 % in M1, indicating that the α-nucleation efficiency of MDBS is faint. By the way, β-crystals form again when shear rate is above 20 s−1 in M1.So, we speculate that the reason of the absence of β-crystalsunder low shear rate is that the induction time of Ns is longerthan that ofΔNA

s in NPP. After the application of shear, a large

amount of ΔNAs forms due to the high content of MDBS in

M1.When shear rate is low,ΔNAs first emerges, andα-crystals

induced byΔNAs grow quickly at 134 °C. In limited growing

space, the fast growth of these α-crystals makes the later-formed Ns lose the space to induce β-crystals, causing theabsence of β-crystals in M1. The induction time of Ns

shortens with the increase of the shear rate; finally, a smallamount of epitaxial β-crystals form when the shear ratereaches 20 s−1 although the number of ΔNA

s also increaseswith the shear rate. It indicates that sorbitol derivatives have amore pronounced effect on the acceleration of crystallizationunder shear flow.

Conclusions

Sorbitol derivatives show apparently versatile nucleatingcharacteristics in the progress of iPP crystallization. Thenucleation ability of sorbitol derivatives is low. When shearis solely subjected to pure iPP melt, the content of β-crystals(induced by α-row nuclei) increases little with the shear rate

Colloid Polym Sci

when the shear rate is below 20 s−1; then, the content of β-crystals increases dramatically with further increase of theshear rate. When both shear and versatile nucleating agentcoexist, the content of β-crystals of NPP increases withincreasing shear rate. The effect of shear flow and versatilenucleating agent on the formation of β-crystals is dependenton the combination of the decrease of the content of nucle-ating agent-induced β-crystals and the increase of α-rownuclei-induced β-crystals. If the decrement of nucleatingagent-induced β-crystals is less than the increment of α-row nuclei-induced β-crystals, there is the synergy effectbetween shear flow and the versatile nucleating agent for theformation of β-crystals. Sorbitol derivatives present an am-plification effect of shear on the orientation of iPP crystals,namely, the orientation degree increases with the nucleatingagent concentration. In M1, the absence of β-crystals at lowshear rate verifies that α-nuclei which arise from the inter-action between shear and nucleating agent (ΔNA

s ) emergeearlier than α-nuclei induced by shear flow (Ns). The resultsfrom this study offer helpful guidance to control the crys-talline structure and the content of β-crystals in iPP/sorbitolderivative blends.

Acknowledgments This work is supported by the National NaturalScience Foundation of China programs (20804005) and the Funda-mental Research Funds for the Central Universities.

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