Spherulite boundary strengthening concept for toughening polypropylene

Spherulite Boundary Strengthening Conceptfor Toughening Polypropylene

A. LUSTIGER, C. N. MARZINSKY, R. R. MUELLER

Exxon Research and Engineering Co., Route 22 East, Annandale, New Jersey 08801

Received 9 September 1997; revised 4 March 1998; accepted 6 March 1998

ABSTRACT: During spherulitic crystallization of polymers, there is a tendency for lowmolecular weight and other less crystallizable entities to be rejected from the body ofthe spherulites. This rejection process causes a segregation of these species to thoseareas where spherulites impinge. As a result of this segregation, lamellar and spheru-lite boundaries have a tendency to become weak, often resulting in premature mechani-cal failure. The objective of this work, anthropomorphically speaking, is to develop amelt miscible blend system in which a propylene copolymer ‘‘fools’’ a polypropylenehomopolymer into rejecting the copolymer to the spherulite boundaries as an impurity.However, once the copolymer arrives at these boundaries, the copolymer subsequentlyconnects adjacent spherulites through cocrystallization of the propylene copolymer seg-ments. It was found that addition of either a random ethylene–propylene copolymeror an isotactic–atactic block copolymer was able to yield the desired effect. Cocrystalli-zation was confirmed by calorimetry, and segregation of copolymer and subsequentreinforcement at the spherulite boundaries was directly observed microscopically. Us-ing this approach, toughness was increased with little loss in stiffness. q 1998 JohnWiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2047–2056, 1998Keywords: polypropylene; spherulite; cocrystallization; lamellae

INTRODUCTION noncrystallizable material at spherulite/lamellarboundaries results in mechanical weakness inthese regions of the polymer.2–4

Semicrystalline polymers, as the name suggests,Although isotactic polypropylene (IPP) gener-contain both a liquidlike amorphous phase and

ally crystallizes from the melt in the form ofan ordered crystalline phase. When solidifiedspherulites, these structures may or may not formfrom the melt, the crystalline phase resides inin polyethylene, depending on polymer molecularplatelets called lamellae, which typically grow ra-weight, comonomer content, and quench rate.5dially from a central point to form spherulites.When spherulites do not form, the material gener-The basis for the phenomenon of spherulitically shows a random lamellar orientation.6crystallization is the tendency for the polymer to Interlamellar regions have a similar tendencyreject noncrystallizable material during their for- for separation due to the lack of connectivity be-

mation.1 As a result, the boundaries between ad- tween adjacent lamellae. Those portions of chainsjacent spherulites contain a high concentration of that connect adjacent lamellae, known as tie mol-noncrystallizable material composed of low molec- ecules, provide this connection. If there are tooular weight chains, atactic polymer, and various few tie molecules, interlamellar failure will resultimpurities. Not unexpectedly, the segregation of in failure similar to the interspherulitic failure

described above.6 Increasing molecular weightgenerally improves interspherulitic4 (as well asCorrespondence to: A. Lustigerinterlamellar2) adhesion, resulting in improved

Journal of Polymer Science: Part B: Polymer Physics, Vol. 36, 2047–2056 (1998)q 1998 John Wiley & Sons, Inc. CCC 0887-6266/98/122047-10 toughness. However, although desirable for im-

2047

9709008/ 8q64$$9008 06-15-98 13:08:12 polpa W: Poly Physics

2048 LUSTIGER, MARZINSKY, AND MUELLER

Table I. Copolymers Blended in this Studyproved mechanical properties, increasing molecu-lar weight also renders the polymer more difficult

Molecular Weightto process due to the corresponding increase inDistribution (11004)melt viscosity.

Other means of increasing interlamellar/inter-Polymer Mn Mw Mzspherulitic strength are accompanied by other

tradeoffs. For example, adding comonomer to Random ethylene–propylenepolyethylene is known to increase tie molecule copolymer 60 134 243concentration in polyethylene,7–9 but at a cost of Isotactic–atactic polypropylenelower crystallinity, resulting in lower stiffness. (IPPAPP) multiblocks 105 254 400Adding ethylene comonomer to a polypropylenebackbone has the same drawback.

The objective of this work is to develop aergy dissipation specifically through the mecha-scheme for increasing interspherulitic and inter-nisms of crazing and shear banding12 aroundlamellar strength in polypropylene (PP) whilerubber particles that are randomly distributed inmaintaining as much as possible the original crys-the polypropylene matrix. In spherulite boundarytallinity and melt flow characteristics. The ap-strengthening, this second phase is preferentiallyproach we have taken towards this objective is tolocated in the areas where the polymer is theblend isotactic polypropylene homopolymer withweakest, connecting adjacent spherulite bound-a copolymer that segregates at spherulite bound-aries through cocrystallization.aries and interlamellar regions like a noncrystal-

lizable impurity during primary crystallization,but then cocrystallizes across adjacent spheru-

EXPERIMENTALlites/lamellae during secondary crystallization.In anthropomorphic terms, the principle is to

Materials‘‘fool’’ the polypropylene homopolymer spherulitesinto rejecting a copolymer that the homopolymer Homopolymers‘‘thinks’’ is an impurity. However, after this rejec-

The homopolymer used in this study was antion, the copolymer would strengthen spheruliteexperimental high crystallinity polypropylene.and lamellar boundaries through forming newThrough poisoning those catalyst sites which pro-connections between them. Recently, we haveduce atactic PP segments, the crystallinity of IPPseen a similar approach taken to strengtheningcan be significantly increased. High crystallinitypolypropylene through the addition of stereoblockIPP with an isopentad fraction of 95% was pro-polypropylene,10 and polyamides through the ad-duced with a melt flow rate of 1.5. Moleculardition of ABA triblocks.11

weight of the material was Mn Å 87,700, MwThe principle behind the spherulite boundaryÅ 360,900, and Mz Å 945,000.strengthening concept is fundamentally different

from the presently used toughening schemes forCopolymerspolypropylene. In conventional rubber tough-

ening, the blend components must be immiscible Two types of copolymers were used in this study.in the melt; in the case of polypropylene, tough- The first was a commercially available nominallyness is normally introduced through the addition random ethylene propylene copolymer containingof ethylene–propylene rubber as this immiscible 5.2% ethylene, produced commercially by Exxoncomponent. In the spherulite boundary strength- Chemical (Baytown, TX) using conventional catal-ening concept, however, there is no need for im- ysis. The second comprised a multiblock copoly-miscibility in the melt, because phase separation mer of isotactic and atactic polypropylene. Thisof the reinforcing component takes place occurs copolymer was synthesized at 407C based on aduring crystallization rather than in the melt. catalyst and recipe developed by Chien et al.13 TheBoth the rubber toughening and spherulite copolymers are listed in Table I.boundary strengthening concepts result in the for-mation of a second phase, but the principle behind Blendingthe toughening effect of this second phase is dif-ferent. In rubber toughening, toughness is intro- Blending of the homopolymers and copolymers

took place using a CSI laboratory scale extruderduced through the introduction of enhanced en-


BOUNDARY STRENGTH CONCEPT FOR TOUGHENING POLYPROPYLENE 2049

PP films were prepared by pressing a few milli-grams of material between two glass slides. Thespecimens were isothermally crystallized over-night at 1377C under a hot stage. These films werethen carefully removed from between the glassslides, cut into a rectangular shape, and singleedge notched using a razor blade. The film wasthen loaded in the tensile stage, and pulled at1500 microns per minute while recording the fail-ure events using a video recorder. Load-elonga-tion curves are superimposed on the video imageduring the testing.

Figure 1. Video and mechanical testing subsystemson optical microscope. Mechanical Characterization

Flexural testing was accomplished using Instronwith a barrel temperature of 2007C. In specific 4502 mechanical test equipment. Specimens werecases, 1000 parts per million (PPM) of sodium molded 0.5-mm thick with a gage length of 25.4benzoate was added to these blends in order to mm. Standard flex and Izod specimens had identi-raise the crystallization temperature of the homo- cal geometries. Flex specimens were tested withpolymer. Tensile, flexural, and impact specimens a span of 51 mm deformed at 1.27 mm/min. Flex-were compression molded directly from the extru- ural data given is an average of three specimens.date at 2007C. Izod impact specimens were notched and tested

in accordance with ASTM standard D256. ImpactAnalytical Characterization data given is an average of five specimens.Fourier Transform Infrared (FTIR) Microscopy

A Nicolet 510P FTIR microscope was used to de-termine relative ethylene content at spherulite RESULTS AND DISCUSSIONboundaries vs. spherulite centers in the homo-polymer blended with the random ethylene pro-

Random Copolymer Blendspylene copolymer.

To prove the feasibility of the spherulite boundaryDifferential Scanning Calorimetry (DSC)strengthening concept, it was necessary to experi-

A Seiko 550 DSC was used to characterize virtu- mentally demonstrate that: (1) the homopoly-ally all the blends in this study. Samples were mer–copolymer blends would be melt miscible,typically heated from room temperature to 2007C or at most immiscible on the nanoscale; (2) theat 107C/min to determine melting points, and copolymer migrates to spherulite boundaries dur-cooled at the same rate to determine crystalliza- ing crystallization of these blends; (3) after thistion temperatures. migration, the copolymer forms a link between

adjacent spherulites through crystallization of theHot Stage Microscopycopolymer chains onto the previously existing ho-

A Mettler hot stage was used in conjunction with mopolymer lamellae in adjacent spherulites.a Zeiss microscope fitted with Nomarski Interfer- The initial experiments were designed to dem-ence reflection optics in order to observe spheru- onstrate that the above three conditions were metlitic structures during isothermal crystallization. in IPP blends containing the random copolymer.

Lohse14 has demonstrated that ethylene–pro-Microtensile Testing pylene random copolymers are melt miscible with

polypropylene homopolymers up to a concentra-A specially designed tensile-testing fixture wasfitted on the polarizing optical microscope to allow tion of 8% ethylene content in the copolymer. As

a result, the copolymer initially chosen for thismicroscopic observation of spherulites and spher-ulite boundaries during tensile testing. A sche- study contained the highest amount of ethylene

(5.2%) commercially available.matic of the arrangement is shown in Figure 1.



Cocrystallization

A 75–25 blend of homopolymer and random co-polymer was isothermally crystallized at 1377C,etched using potassium permanganate, placed ina constant strain fixture, and strained until closeto yield. Subsequently, the sample while still inthe fixture was placed under the FTIR microscope,and spectra were obtained at the spheruliteboundaries and spherulite centers. A composite ofthese results is shown in Figure 3. The opticalmicroscope results clearly show the presence oforiented material at the spherulite boundaries inthe strain direction. The orientation suggests thatthe material that was rejected cocrystallizedacross adjacent spherulite boundaries. If cocrys-tallization had not taken place, the rejected copol-ymer would merely reside at the spheruliteFigure 2. 10 : 90 blend of isotactic polypropylene :

ethylene–propylene copolymer crystallized at 1357C. boundaries without the orientation evident be-tween the spherulites upon the application ofstrain.

FTIR microscopy reveals a higher concentra-Copolymer Segregation

The first experiment was designed to demonstratecopolymer segregation at the spherulite bound-aries. This was done by implementing a classicexperiment first described by Keith and Padden.1

In their experiment, isotactic and atactic polypro-pylene was blended in the ratio of 10 : 90 wt %and crystallized isothermally using a hot stage.These spherulites were compared with isotactichomopolymer spherulites with no diluent grownunder similar conditions.

In their article, segregation was demonstratedby observing the nature of the spherulites formedin these blends with high diluent (i.e., atacticpolymer) concentration. While pure homopolymeryielded the familiar compact spherulites, the highdiluent blends displayed open armed, coarsespherulites. These open spherulites are producedbecause so much noncrystallizable impurity is re-jected from the growing spherulites that the for-mation of new lamellae is sacrificed in favor ofthickening of existing lamellae. In contrast, thehomopolymer contains a much lower level of im-purity, resulting in compact spherulites.

Figure 2 shows a 10 : 90 blend of IPP homopoly-mer and the random ethylene propylene copoly-mer described above crystallized isothermally at1377C, displaying a similar open spherulitic mor- Figure 3. FTIR microscope data showing relativephology. This appearance is consistent with the peak intensity at 730 cm01 for (A) spherulite centerKeith–Padden hypothesis, and thus suggests that vs. (B) spherulite boundary indicating higher ethylenethe copolymer is indeed segregated to the spheru- content at the boundary. The direction of strain is hori-

zontal.lite boundaries.



When the two components are blended togetherand crystallized isothermally, only one meltingpeak is evident, suggesting the presence of cocrys-tallization in the blend [Fig. 5(a)] [ it is importantto note that the crystallization conditions for thehomopolymer in Fig. 4(a) (melting point of1647C) was based on a cooling rate of 107C/min,while in Fig. 5, the random copolymer–homopoly-mer blend when crystallized isothermally [Fig.5(a)] melted at 1637C, and the injection-molded(quenched) blend [Fig. 5(b)] melted at 159.97C.As a result, the melting temperatures are notdirectly comparable between these three. The

Figure 4. DSC endotherms for homopolymer (A) andehtylene–propylene copolymer (B).

tion of ethylene at the spherulite boundaries thanat the spherulite centers, as evidenced by the in-creased intensity of the 730 cm01 peak at thespherulite boundaries. This finding confirms thatthe ethylene propylene copolymer indeed has atendency to segregate at these boundaries.

Kinetic Limitations

Cocrystallization is evident from the DSC data aswell. Figure 4 shows the melting endotherms forthe IPP homopolymer and EP copolymer, respec-tively. Note that the EP displays two meltingpoints, suggesting two populations with differingethylene concentration. This finding suggests thatthe copolymer, although commercially dubbed arandom copolymer, in fact contains regions of con- Figure 5. DSC endotherms for 75 : 25 blend of isotac-siderable ‘‘blockiness’’ ( i.e., nonuniform) ethylene tic PP and ethylene–propylene after (A) isothermal

crystallization at 1307C, and (B) quenching.distribution.



quenched material, resulting in thinner lamellae,would naturally display a lower melting pointthan material crystallized isothermally.]

A different picture, however, becomes evidentif the blend is injection molded, and hence, cooledunder nonisothermal conditions: in this casethrough rapid quenching. DSC of the blend [Fig.5(b)] now reveals the clear presence of threemelting peaks, corresponding to the one meltingendotherm of the homopolymer and the two of thecopolymer as shown above in Figure 4(b).

Because practical polymer processing invari-ably involves highly nonisothermal crystalliza-tion, it was necessary to find a copolymer thatcould cocrystallize with IPP even under quench

Figure 6. DSC endotherms (heating rate of 107C/conditions. To develop such a blend, it was pre-min) for various blends of isotactic PP and isotactic–sumed that the copolymer must display a muchatactic block copolymers immediately after coolinglower crystallization temperature than the IPP.from the melt at 107C/min to 0807C. Endotherm energ-In the case of the IPP–EP blends, the crystalliza-ies were calculated between 85 and 1807C.tion temperatures were found to be 1137C for the

homopolymer and 877C for the copolymer whencooled at 107C/min. As illustrated above, this dif-

pylene. This conclusion regarding the block na-ference in crystallization temperature was clearlyture of the copolymer is based primarily on thenot sufficient to provide the necessary cocrystalli-highly elastic nature of the material, a propertyzation under quench conditions.consistent with thermoplastic elastomers. ThisOne obvious way to decrease the crystallizationelasticity is attributed by the authors to an alter-temperature of the copolymer would be to increasenating assembly of hard (isotactic–crystalline)ethylene content beyond 5.2%. However, based onand soft (atactic–amorphous) segments.Lohse’s criteria for EP copolymers,14 the maxi-

The material was polymerized using the methodmum ethylene content that could be tolerated inin the Chien articles. Polymerization was accom-a melt miscible blend would be 8%. It was feltplished at 407C: the molecular weight for this ma-that the change from 5.2 to 8% ethylene would beterial is shown in Table I.too insignificant to effect a sufficient difference in

To explore the extent and nature of cocrystalli-crystallization temperature. As a result, ratherzation between IPP homopolymer and IPPAPP co-than increasing ethylene content, a new copoly-polymer, DSC was implemented on a series ofmer was needed.blends of the two components. Figure 6 is a com-posite of six melting endotherms generated at a

Isotactic–Atactic Polypropylene (IPPAPP) heating rate of 107C/ min after cooling at 107C/Copolymer Blends min with varying IPP/ IPPAPP compositions. The

blend ratios are shown on the figure, and meltingAs was evident in the Keith–Padden experimentendotherm position and intensity are compared.described above, subsequently confirmed by Lohse,15

The following observations were made:atactic polypropylene is melt miscible with isotac-tic polypropylene in all concentrations. Althoughpure atactic polypropylene does not crystallize, if 1. The pure IPPAPP block copolymer displays a

crystallization exotherm upon heating pastone were to use a multiblock copolymer of isotacticand atactic polypropylene (IPPAPP) in a blend Tg . This finding indicates that crystallization

kinetics through homogeneous nucleation arewith pure IPP, the desired cocrystallization couldresult. too slow to allow significant crystallization at

a cooling rate of 107C/min when cooling con-Such a copolymer has in fact been synthesizedby Chien et al.16,17 using single-site catalyst tech- tinues through Tg . However, when only 10%

of the IPP is blended in, this exotherm disap-nology. In these articles, polymerization condi-tions are laid out to synthesize what the authors pears, suggesting that the preexisting IPP la-

mellae act as nucleating sites for IPPAPPdescribe as crystalline–amorphous block polypro-



crystallization. The only substrates upon any trace of the IPPAPP peak above the 50%IPP concentration.which heterogeneous nucleation of the IP-

PAPP can possibly take place are the pre- 4. In contrast, the 50 and 10% blends seem toshow significant evidence of separate crystal-viously existing IPP lamellae. Because nucle-

ation by definition involves attachment of lization of the IPPAPP, because the intensityof the endotherm is approximately half andchains onto a previously existing crystalline

template, which in this case are the IPP la- 10% of the pure IPP endotherm, respectively,as would be expected by a simple rule of mix-mellae, it would therefore appear that cocrys-

tallization is itself the heterogeneous nucle- tures. Yet, at the same time, even at the 90%IPPAPP concentration the IPPAPP endo-ation mechanism.

2. The IPP melting endotherm shifts to lower therm is still barely perceptible, and the sub-tle temperature shift of the higher tempera-temperature with increasing IPPAPP concen-

tration, although much more subtly than ture endotherm continues, suggesting thatsome cocrystallization is still taking place aswould be suggested by a rule of mixtures. This

is presumably because some of the IPPAPP well. In addition, as noted in point 1 above,the lack of an IPPAPP crystallization exoth-cocrystallizes onto preexisting IPP lamellae.

The slight lowering of the melting point takes erm in the 10% blend similarly suggests thepresence of some cocrystallization.place because the IPPAPP, containing a sig-

nificant atactic PP content, is much less crys-tallizable than the IPP. Upon cocrystalliza- The DSC evidence, therefore, points to mostlytion of the IPPAPP, the lamellae themselves cocrystallization in the 90/10 and 80/20 blendsbecome more imperfect, thereby lowering the of IPP/IPPAPP, with cocrystallization along withmelting point. separate crystallization in the 50/50 and 10/90

3. As far as the intensity of the endotherm is blends.concerned, even given the inherent uncer- The Keith–Padden experiment was repeatedtainty in drawing DSC endotherm baselines, to confirm the appearance of open spherulites inthe 90 and 80% IPP blends shows greater en- a 90 : 10 IPPAPP/IPP blend and, hence, verifyergy under the primary endotherm than segregation of the IPPAPP copolymer to thewould be predicted were the endotherm sim- spherulite boundaries.ply governed by a decreasing proportion ofIPP. Thus, the area under the endotherm of Use of High Crystallinity IPP and Nucleatingthe 90/10 blend is, in fact, 95% of the original Agents to Enhance CocrystallizationIPP endotherm, while the 80/20 blend shows86% of this area, suggesting that a significant We wanted to develop means of raising the crys-

tallization temperature and enhancing the crys-portion of the IPPAPP chains have indeed co-crystallized on to the IPP lamellae. Another tallization kinetics of the IPP, so that the crystal-

lization temperature and rate differences betweenindication of cocrystallization is the lack of

Table II. Stiffness and Toughness of High Crystallinity Isotactic PP–IPPAPPBlends

Flexural Modulus Izod ImpactMaterial (MPa) (joules)

High crystallinity IPP 1322 0.147 { 0.006High crystallinity IPP/ 1000 PPM sodium benzoate 1679 0.133 { 0.006

High crystallinity IPP/ 10% IPPAPP 307C 1250 0.237 { 0.016

High crystallinity IPP/ 10% IPPAPP 307C/ 1000 PPM sodium benzoate 1301 0.289 { 0.016

High crystallinity IPP/ 10% atactic PP 1163 0.128 { 0.017



the IPP and IPPAPP copolymer would thus bemaximized. Maximizing these differences wouldobviate the kinetic limitation described above forthe random copolymers that could not cocrystal-lize upon quenching.

A means of maximizing the difference in crys-tallization temperatures is to add a nucleatingagent to the high crystallinity polypropylene. Thenucleating agent chosen was sodium benzoate, in-corporated at a concentration of 1000 parts permillion (PPM) in the blend.

Modulus and toughness results for the homo-polymers and blends are shown below in Table II.As is well known in the industry, the addition ofnucleating agent to polypropylene increases flex-ural modulus but moderately decreases tough-ness. When IPPAPP alone was added to the IPP,toughness increased but modulus decreased.However, when both sodium benzoate and IP-PAPP were added, the blend showed a simultane-ous increase in both Izod toughness values asflexural modulus. In contrast, if pure atactic PPis included as the blending component instead ofIPPAPP, a significant reduction in modulus is evi-dent, with no increase in toughness. Such an ef-fect would be expected with an atactic PP blend,because the desired cocrystallization is precluded.

The simultaneous improvement in toughnessand stiffness brought about by the addition of thenucleating agent is presumably because the addi-tive provides both enhanced crystallization thatincreases stiffness, and enhanced cocrystalliza-tion that increases toughness.

In the case of nucleated material, where thespherulites are on the order of 1 micron in diame-ter, one would expect that the spherulite bound-ary reinforcing effect might be an unimportantfactor in increasing toughness. However, it shouldbe noted that the cocrystallization effect is pre-sumably not limited to the interspherulitic re-gions, but takes place between adjacent lamellaeas well, resulting in an increase in the interlamel-lar tie molecule concentration.

Figure 7. Video sequence (top to bottom) of tensileDirect Observation of Spherulite Boundary failure at spherulite boundaries in pure IPP crystal-Strengthening lized isothermally at 1377C.

Finally, direct evidence of reinforcement of thespherulite boundaries using IPPAPP can be seenin the next two figures taken from two video se- in a hot stage overnight to generate spherulites

greater than 200 microns in diameter. This filmquences using the microtensile apparatus.In the first video sequence, high crystallinity was removed from between the glass slides and

mechanically deformed under the polarizing opti-polypropylene homopolymer was pressed into afilm that was isothermally crystallized at 1377C cal microscope with a microtensile stage while be-



from the homopolymer, the spherulite boundariesare so weak that the stress concentration affordedby the razor notch is all but ignored. Failure canreadily be seen along these boundaries. Picturestaken from this video sequence are shown in Fig-ure 7.

In the second video sequence the experiment isrepeated but with a 75 : 25 IPP/IPPAPP blend,crystallized under identical conditions and show-ing similarly sized spherulites . In stark contrastto the homopolymer, this blend displayed dra-matic ductility ahead of the razor notch and in-deed throughout the sample, providing direct evi-dence of the spherulite boundary strengtheningmechanism. Pictures taken from this sequenceare shown in Figure 8.

On the basis of the FTIR microscope evidenceand DSC evidence shown previously, one can con-clude that this reinforcement is due to the segre-gation of the copolymer to the spherulite bound-aries and the subsequent cocrystallization of theisotactic polypropylene segments of the copolymeracross adjacent spherulites.

CONCLUSIONS

Spherulite boundary and interlamellar strength-ening of isotactic polypropylene can take placethrough blending with an appropriate copolymer.The copolymer migrates to spherulite boundariesas well as interlamellar regions and cocrystal-lizes, resulting in a higher number of interspheru-litic links and tie molecules. The isotactic–atacticpolypropylene block copolymer has been shown tobe an effective spherulite boundary-strengthen-ing additive.

Toughness as measured by Izod impact testinghas been found to significantly increase throughblending with this copolymer. Perhaps more sig-nificantly, the toughness enhancement is not ac-companied by a significant strength or stiffnessFigure 8. Video sequence (top to bottom) showing re-decrease, as is generally evident in conventionalinforcement at spherulite boundaries in a 75 : 25 blendrubber-toughening schemes.of IPP and IPPAPP block copolymer crystallized iso-

thermally at 1377C.We thank the referees of this manuscript for their help-ful criticisms and suggestions.

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Spherulite boundary strengthening concept for toughening polypropylene

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