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Polymer 53 (2012) 5856e5863

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Polymer

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Isothermal crystallization of poly(b-propiolactone) blocks starting from lamellarmicrodomain structures of double crystalline poly(b-propiolactone)-block-polyethylene copolymers

Liang Huang, Gaito Kiyofuji, Jun Matsumoto, Yasumasa Fukagawa, Cheng Gong 1, Shuichi Nojima*

Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, H-125, 2-12-1 Ookayama, Meguro-Ku,Tokyo 152-8552, Japan

a r t i c l e i n f o

Article history:Received 24 July 2012Received in revised form5 October 2012Accepted 6 October 2012Available online 16 October 2012

Keywords:Double crystalline diblock copolymerLamellar microdomain structureIsothermal crystallization behavior

* Corresponding author. Tel.: þ81 3 5734 2132; faxE-mail address: snojima@polymer.titech.ac.jp (S. N

1 Present address: Department of Chemical EnginBeijing 100084, PR China.

0032-3861/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2012.10.013

a b s t r a c t

The isothermal crystallization behavior of poly(b-propiolactone) (PPL) blocks starting from lamellarmicrodomain structures (LMS) formed in molten PPL-block-polyethylene (PPL-b-PE) copolymers hasbeen examined using time-resolved synchrotron small-angle X-ray scattering and Fourier transforminfrared spectroscopy as a function of the crystallinity of PE blocks cPE. For getting PPL-b-PE copolymerswith varying cPE, the amount of ethyl branches in PE blocks was controlled during polymerization. Thecrystallizable temperature of PE blocks was slightly higher than that of PPL blocks and the crystallizationrate of PE blocks was extremely fast, so the crystallization of PE blocks occurred first on quenching toform the crystallized LMS, in which the PPL block crystallized. The time evolution for the crystallinity ofPPL blocks cPPL showed an exponential-type increase with no induction time. In contrast, LMS wasreplaced with the crystallized lamellar morphology and cPPL showed a sigmoidal time evolution in PPL-b-PE with a higher amount of ethyl branches, where the PE block did not crystallize (i.e., cPE ¼ 0) prior tothe crystallization of PPL blocks. It was concluded that the prompt crystallization of PE blocks just afterquenching reinforced the existing LMS to make it stable against subsequent crystallization, yielding theconfined crystallization of PPL blocks.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The crystallization of double crystalline diblock copolymers isextremely complicated owing to two kinds of crystallizationcoupled withmicrodomain structures existing in themelt [1,2]. Thecrystallization behavior of such AeB copolymers can be conve-niently classified into two groups using the crystallizable temper-atures of block A (Tc,A) and block B (Tc,B).

When Tc,A >> Tc,B, the crystallization proceeds in two separatesteps. That is, the crystallization of block A occurs first onquenching to yield the crystallized lamellar morphology followedby the crystallization of block B within this morphology. Suchcrystallization process was observed in several diblock copolymers[3e11]. For example, we investigated the crystallization behavior ofpoly(ε-caprolactone)-block-polyethylene (PCL-b-PE) copolymerswhen they were quenched from weakly segregated melts [3,5,7,8],

: þ81 3 5734 2888.ojima).eering, Tsinghua University,

All rights reserved.

where the PE block crystallized first to form a PE-crystallizedlamellar morphology and then the PCL block crystallized withinit. It was concluded that the crystallization behavior of PCL blocksdepended significantly on both crystallization temperature andconfinement size.

When Tc,A w Tc,B, on the other hand, the simultaneous crystal-lization of both blocks occurs to result in peculiar crystallizationbehaviors [12e30]. PCL-block-poly(ethylene oxide) (PCL-b-PEO) iswell known as a double crystalline block copolymer with closecrystallizable temperatures [12e26], where interesting crystalli-zation behaviors, such as concentric double spherulites, areobserved when PCL-b-PEO is quenched from the microphase-separated or homogeneous melt. In addition, it is reported thatthe crystallization of block A influences that of block B [25e29].Myers and Register [29], for example, observed the crystallizationprocess in a block copolymer consisting of linear polyethylene andhydrogenated polynorbornene, and found interactive crystalliza-tion on deeper quench.

We have recently investigated the non-isothermal crystalliza-tion of strongly segregated poly(b-propiolactone) (PPL) and poly-ethylene (PE) blocks in PPL-b-PE copolymers during cooling at

L. Huang et al. / Polymer 53 (2012) 5856e5863 5857

constant rates (�1 w �10 �C/min) [30]. The crystallizable temper-ature of PPL blocks was slightly lower than that of PE blocks, so thatthe crystallization process of PPL blocks was partially overlappedwith the late stage crystallization of PE blocks at an extremely slowcooling rate (w�1 �C/min), and eventually the crystallization of oneblock (i.e., PE or PPL blocks) affected that of the other block.Furthermore, we qualitatively examined the isothermal crystalli-zation behavior of PPL blocks, and found it was significantlydifferent from the crystallization behavior usually observed inweakly segregated crystalline-amorphous diblocks [30].

In this study, we investigated the isothermal crystallization ofPPL blocks as a function of the crystallinity of PE blocks cPE. This isbecause the nature of spatial confinement (hard or soft) by lamellarmicrodomains, which is mainly controlled by cPE, will affect thesubsequent crystallization of PPL blocks. For this purpose, wesynthesized several PPL-b-PE copolymers with different mole % ofethyl branches in PE blocks so as to obtain different cPE. The crys-tallization process of PPL blocks was pursued using time-resolvedsynchrotron small-angle X-ray scattering (SR-SAXS) and indepen-dently using Fourier transform infrared spectroscopy (FTIR). Fromthese results, we try to find the relationship between the crystal-lization behavior of PPL blocks and the confined nature of lamellarmicrodomains existing in the melt.

2. Experimental

2.1. Samples and sample preparation

The crystallineecrystalline diblock copolymers used in thisstudy are poly(b-propiolactone)-block-low density polyethylene (orhydrogenated polybutadiene) (PPL-b-PE) with various molecularcharacteristics. The crystallizable temperature of PPL blocks isslightly lower than that of PE blocks, and interactive crystallizationhas been observed during cooling at a slow rate (w�1 �C/min) [30].

The synthesis route of PPL-b-PE was already shown in ourprevious paper [30], and here we briefly summarize the synthesismethod. The butadienemonomer in toluenewas first polymerized atroom temperature for 24 hwith n-butyllithium as an initiator, wheresomeamounts of 1,2-dipiperidinethane (DPE)were added to increasethe mole % for 1,2-linkage of butadiene monomers in polybutadiene(PB) blocks [31,32], which is defined here as j1e2. The reaction wasterminated by adding carbon dioxide to obtain carboxyl-terminatedpolybutadiene (PBeCOOH). Next, b-propiolactone in tetrahydro-furan (THF) was polymerized at 40 �C for 24e48 hwith PBeCOOH asan initiator. The reaction was stopped by adding a large amount ofacetic acid to obtain PPL-b-PB. Finally, PPL-b-PB in THF was hydro-genated with PtO2 at 5 MPa and 70 �C for 1 week. The PPL-b-PEcopolymers were purified several times by precipitating the hot THFsolution intomethanol to remove impurities included. Themolecularweight (Mn) and compositionwere evaluated using 1H NMR, and the

Table 1Molecular characteristics of samples used in this study.

Sample code Mn of PPLblocksa (g/mol)

Mn of PE (or PB)blocksa (g/mol)

Mol % of1,2-linkage (j1e

PL-PB(10) 8300 4800 10PL-PB(15) 12,000 6200 15PL-PB(20) 14,400 7700 20PL-PB(51) 9800 6000 51PL-PE(10) 8300 5000 10PL-PE(15) 12,000 6400 15PL-PE(20) 14,400 8000 20PL-PE(51) 9800 6200 51

a Determined by 1H NMR.b Determined by GPC.c Calculated from the specific volumes of PPL and PE (or PB) homopolymers.

polydispersity (Mw/Mn) was obtained using gel permeation chro-matography (GPC) with polystyrene standards.

We prepared four PPL-b-PE copolymers. Table 1 shows themolecular characteristics of these copolymers, together with PPL-b-PB copolymers, where PL-PB and PL-PE in the sample code repre-sent PPL-b-PB and PPL-b-PE, respectively, and the numeral in theparenthesis stands for j1e2 in each copolymer. The volume % of PPLblocks is close to 50% for every copolymer, and accordingly lamellarmicrodomain structures are expected in the melt. Because the PPLblock showed thermal degradation at high temperatures (�120 �C),i.e., the molecular weight decreased gradually, the samples werealways treated at temperatures under 100 �C.

2.2. Differential scanning calorimetry (DSC) measurements

A Perkin Elmer DSC Diamond was used with a heating rate of10 �C/min to observe the melting behavior of PPL-b-PE and PPL-b-PB crystallized at various temperatures. The melting of PPL blocksalways showed two distinct endothermic peaks, and the crystal-linity of PPL blocks cPPL was calculated from these peak areasassuming that the heat of fusion for perfect PPL crystals was 119 J/g[33]. Unfortunately, the melting peak of PE crystals was extremelysmall and indistinct when j1e2 was larger (�15%), and the reliablecrystallinity of PE blocks as well as melting temperature could notbe obtained.

2.3. Synchrotron small-angle X-ray scattering (SR-SAXS)measurements

The morphology formed in the melt and after the crystallizationof PE and PPL blocks was examined using static small-angle X-rayscattering with synchrotron radiation (SR-SAXS), and theisothermal crystallization process was also pursued using time-resolved SR-SAXS. The SR-SAXS measurements were performed atPhoton Factory in High Energy Acceleration Research Organization,Tsukuba Japan, with a small-angle X-ray equipment for solution(SAXES) installed at beam line BL-10C. Details of the equipment andinstrumentation were already described [34,35]. The scatteredintensity was detected with a one-dimensional position-sensitiveproportional counter (PSPC) with the accumulation time of 300 sfor static measurements and 10 s for time-resolved measurements.The SR-SAXS curves were corrected for background scattering andabsorption by the samples, and finally obtained as a function ofwave number s defined as s ¼ (2/l)sin q, where 2q is scatteringangle and l is X-ray wave length (¼0.1488 nm).

2.4. Fourier transform infrared spectroscopy (FTIR) measurements

FTIR spectrawere recorded during the isothermal crystallizationof PPL-b-PE and PPL-b-PB using JASCO FT-IR 6200 spectrometer

2)aTotal Mn Mw/Mn

b PPL:PE(or PB)wt.%

Pp :PE(or PB)c

vol.% at 100

13,100 1.17 63:37 54:4618,200 1.24 66:34 56:4422,100 1.17 65:35 55:4515,800 1.04 62:38 52:4813,300 1.17 62:38 52:4818,400 1.24 65:35 55:4522,400 1.17 64:36 54:4616,000 1.04 61:39 51:49

90

80

70

m,P

PL

(o C)

a

L. Huang et al. / Polymer 53 (2012) 5856e58635858

with a spectral resolution of 4 cm�1, and the normalized crystal-linity of PPL blocks, c0PPLðtÞð¼ cPPLðtÞ=cPPLðNÞÞ, was evaluated asa function of crystallization time t. The samplewas first dissolved inchloroform, and thin films (ca. 100 mm in thickness) were preparedby the solution-casting method on a silicone plate. The absorptionband at 1734 cm�1 for PPL blocks, which resulted from C]Ostretching motion in PPL crystals [36], was mainly analyzed toevaluate cPPL(t).

60

50

1.0

0.8

0.6

0.4

0.2

020100 50

PP

LT

601-2 (%)

b

χ

ψ

3. Results and discussion

3.1. Melting behavior measured by DSC

The melting behavior of PPL-b-PE and PPL-b-PB crystallized at55 �C for 30 min was observed using DSC during heating at 10 �C/min, and results are shown in Fig. 1. Two melting endothermicpeaks of PPL blocks (indicated by white arrows) are clearlyobserved in the temperature range between 60 �C and 80 �C, andthe area under two peaks changes significantly with changing j1e2;the higher melting area increases with increasing j1e2. This doublemelting behavior is usually observed in PPL crystals and is ascribedto the difference in crystal structure [36]. The endothermic peak ofPE blocks appears at 95 �C for PL-PE(10) (indicated by a blackarrow), but it is indistinct with increasing j1e2 and eventually wecannot detect it at all for PL-PE(20) and PL-PE(51). Therefore, it isimpossible to evaluate the crystallinity of PE blocks cPE from DSCmeasurements. However, we find qualitatively from FTIR resultsdescribed later that cPE decreases steadily with increasing j1e2 tobe zero for PL-PE(51).

Fig. 2 shows the melting temperature Tm,PPL and crystallinitycPPL of PPL blocks plotted against j1e2 for PPL-b-PE and PPL-b-PB.

Temperature (oC)10090807060

Exo

ther

mE

xoth

erm

Exo

ther

mE

xoth

erm

a

b

d

c

PL-PE(51)

PL-PE(10)

PL-PE(15)

PL-PE(20)

Fig. 1. DSC curves during heating at 10 �C/min for PL-PE(10) (a), PL-PE(15) (b), PL-PE(20) (c), and PL-PE(51) (d) isothermally crystallized at 55 �C for 30 min. The ordi-nate is not the same for each panel.

Fig. 2. The melting temperature Tm,PPL (a) and crystallinity cPPL (b) of PPL blocks inPPL-b-PE (open circles) and PPL-b-PB (closed circles) plotted against the mole % ofethyl branches j1e2 existing in PE or PB blocks.

Two melting temperatures of PPL blocks are almost independent ofj1e2 (Fig. 2-a), whereas cPPL depends intimately on j1e2; cPPL inPPL-b-PE is significantly smaller than that in PPL-b-PB at j1e

2 � 20%, whereas it is larger at j1e2 ¼ 51%. The reduction of cPPLin PPL-b-PE at j1e2 � 20% will be related to spatial confinement forPPL blocks by the morphology existing just before the crystalliza-tion of PPL blocks. This morphology is formed by the crystallizationof PE blocks based on lamellar microdomains existing in the melt.Therefore, we examine the morphologies formed at selectedtemperatures using SAXS techniques.

3.2. Morphology in the melt and crystallized state

The morphology at 100 �C (where both blocks are amorphous)and 30 �C (both blocks crystallized) was examined using static SR-SAXS, and results are shown in Fig. 3 for PL-PE(10) (a), PL-PE(15) (b),PL-PE(20) (c), and PL-PE(51) (d). We have several SAXS peaks at100 �C, the angular positions of which exactly correspond to a ratioof 1:(2):3:(4):5, indicating that the lamellar microdomain structureis formed in molten samples. The largely reduced intensity of even-number peaks is ascribed to equal amounts of amorphous PE andPPL blocks existing in the system (Table 1) [37,38]. It is worth notingthat the primary peak is located at extremely low angles, meaningthat the long period of lamellar microdomains is very large(w42 nm) when it is compared with that of weakly segregateddiblock copolymers with similar molecular weight. For example,the long period of microdomain structures formed in PCL-b-PEwithMn w 11,000 is 26 nm [3,5], suggesting that the PPL-b-PE copoly-mers form strongly segregated lamellar microdomains in the melt.

Fig. 3. Static SAXS curves measured at 100 �C (upper curves) and 30 �C (lower curves) for PL-PE(10) (a), PL-PE(15) (b), PL-PE(20) (c), and PL-PE(51) (d). The dotted line in (d)indicates the significant shift of the primary peak position.

L. Huang et al. / Polymer 53 (2012) 5856e5863 5859

In fact, the segregation strength parameter cN (c: FloryeHugginsinteraction parameter and N: total degree of polymerization) forpoly(L-lactide)-block-PE (PLL-b-PE) with similar N is roughly esti-mated to be larger than 350 (>>10.5) [28], where PLL-b-PE has ananalogous chemical structure with PPL-b-PE.

When both blocks are crystallized at 30 �C, the SAXS curves forPL-PE(10), PL-PE(15), and PL-PE(20) are slightly changed, that is,the second scattering peak disappears and third becomes broader.However, we cannot find any significant shift of peak positions bythe crystallization of PPL blocks, suggesting that both blocks crys-tallize within lamellar microdomains existing in the melt. That is,we speculate from Fig. 3-awc that when the sample is quenchedfrom themicrophase-separatedmelt, PE blocks crystallize first witha rapid crystallization rate to yield crystallized lamellar micro-domains, and subsequently PPL blocks crystallize within it. Thepreservation of lamellar microdomains is ascribed to the rapidcrystallization of PE blocks coupled with large incompatibilitybetween PPL and PE blocks. This point is extremely different fromweakly segregated PCL-b-PE copolymers, where the crystallizationof PE blocks induces morphological transition [3,5,7], because themicrodomain structure is not stable enough as compared with thatin PPL-b-PE.

It is found from Fig. 3-d that the primary SAXS peak of PL-PE(51)shifts slightly to lower angles by the crystallization of PPL blocks(indicated by a dotted line). Because PE blocks do not crystallize atall in PL-PE(51), the crystallization of PPL blocks may inducemorphological transition from the soft lamellar microdomainstructure into the crystallized lamellar morphology. Similarchanges of SAXS curves were observed in every PPL-b-PB copoly-mers, where only PPL blocks crystallized.

We can easily understand from Fig. 3 that the confinement effectof existing lamellar microdomains on the crystallization of PPLblocks depends on j1e2; for PL-PE(10)wPL-PE(20), PE blocks crys-tallize promptly on quenching to reinforce the existing micro-domain structure, which is stable enough against subsequentcrystallization of PPL blocks. In PL-PE(51) (and PL-PB(51)), on theother hand, PE blocks do not crystallize and existing lamellarmicrodomains are not rigid, leading that the crystallization of PPLblocks destroys lamellar microdomains to yield the PPL-crystallizedlamellar morphology. To our knowledge, it is the first time to showthat the reinforcement of lamellar microdomains by the crystalli-zation of PE blocks critically affects the subsequent crystallizationof PPL blocks. Therefore, it is reasonable to suppose that

the crystallization process of PPL blocks will be extremelydifferent between two cases, and we examine it using time-resolved SR-SAXS.

3.3. Isothermal crystallization process measured by time-resolvedSR-SAXS

Fig. 4 shows time-resolved SR-SAXS curves for PL-PE(10) (a), PL-PE(51) (b), and PL-PB(51) (c) during isothermal crystallization at60 �C after quenching from themicrophase-separatedmelt. The SR-SAXS curves for PL-PE(10) show a monotonous increase in theprimary peak intensity with increasing crystallization time t exceptat the initial period (<50 s), where PE blocks promptly crystallize totemporarily decrease the electron density contrast between PE andPPL layers. Similar monotonous increase of the peak intensity wasobserved for PL-PE(15) and PL-PE(20) at every crystallizationtemperature. The t dependence of these SR-SAXS curves issuccessfully explained by considering that PE blocks crystallizerapidly on quenching to form crystallized lamellar microdomains,which are rigid and stable against subsequent crystallization, andthe crystallization of PPL blocks proceeds gradually within thismicrodomain. The electron density contrast between PE and PPLlayers increases in this process to yield the gradual increase in peakintensity, as shown in Fig. 4-a.

Time-resolved SR-SAXS curves for PL-PE(51) and PL-PB(51) havea distinct intensity minimum (indicated by arrows in Fig. 4-b and-c) during isothermal crystallization. In addition, the crystallizationtime to give such intensity minimum depends significantly oncrystallization temperature Tc; the time is longer as Tc is higher. Thechange in the primary peak intensity is reminiscent of morpho-logical transition occurring in weakly segregated crystalline-amorphous diblocks such as PCL-block-polybutadiene (PCL-b-PB)[39], where the microdomain structure is completely replaced withthe crystallized lamellar morphology by the crystallization of PCLblocks. That is, the peak intensity arising from the microdomainstructure steadily decreases and simultaneously that from thecrystallized lamellar morphology increases with increasing t toyield the peak intensity minimum. It should be noted that thediscontinuous shift of peak positions is observed during morpho-logical transition in PCL-b-PB [39], but it is not clearly observed inFig. 4-b and -c because the angular position of SAXS peaks isextremely low. However, we find a moderate shift of the peakposition by quantitatively analyzing SR-SAXS curves (Fig. 5).

Fig. 4. Time-resolved SR-SAXS curves during isothermal crystallization at 60 �C for PL-PE(10) (a), PL-PE(51) (b), and PL-PB(51) (c). The arrow represents the minimum of primaryintensity peaks during crystallization.

L. Huang et al. / Polymer 53 (2012) 5856e58635860

Fig. 5 shows the t dependence of the primary peak intensity(circle) and long period (triangle) for PL-PE(10) (left panels), PL-PE(51) (middle), and PL-PB(51) (right) when isothermally crystal-lized at 55 �C (upper) and 60 �C (lower). For PL-PE(10), the longperiod is almost constant (41e42 nm) irrespective of t, indicatingthat lamellar microdomains are preserved during the crystalliza-tion of PPL blocks. This fact supports again that the crystallization ofPE blocks makes the microdomain structure stable against thesubsequent crystallization of PPL blocks. In addition, the primarypeak intensity increases monotonously with increasing t at small t

Fig. 5. Long period and primary peak intensity plotted against crystallization time for PL-PEpanels) and 60 �C (lower panels).

(<230 s at 55 �C and <600 s at 60 �C). Similar t dependence of theprimary peak intensity and long period was obtained for PL-PE(15)and PL-PE(20) crystallized at 55 and 60 �C.

The long period of PL-PE(51) and PL-PB(51) increases signifi-cantly during the crystallization of PPL blocks. That is, it increasesfrom 42 nm to 47 nm for PL-PE(51) and from 36 nm to 40 nm for PL-PB(51), as shown in Fig. 5-b and -c. It is interesting to find that thecrystallization time when the long period sharply increases corre-sponds to the peak intensity minimum for both PL-PE(51) and PL-PB(51) at 55 and 60 �C. We can successfully explain this fact by

(10) (a, d), PL-PE(51) (b, e), and PL-PB(51) (c, f) isothermally crystallized at 55 �C (upper

L. Huang et al. / Polymer 53 (2012) 5856e5863 5861

considering that lamellar microdomains are replaced with thecrystallized lamellar morphology by the crystallization of PPLblocks. In the next section, we show the quantitative results for thetime evolution of cPPL(t) obtained using FTIR.

3.4. Isothermal crystallization process measured by FTIR

The FTIR results for the non-isothermal crystallization of PPLblocks in PPL-b-PE and PPL-b-PB during cooling were previouslyreported [30]. Here, we describe the FTIR results for the isothermalcrystallization of PPL blocks crystallized at selected temperatures.

The typical FTIR curves of PPL-b-PE copolymers are alreadyshown in our previous report (Fig. 6 in ref. [30]). Here, we mainlyanalyzed the absorption band of PPL blocks at 1734 cm�1 to eval-uate cPPL(t), as described in Experimental Section. Fig. 6 shows thetime evolution of cPPL(t) for (a) PL-PE(10), PL-PE(15), and PL-PE(20)and (b) PL-PE(51) and PL-PB(51) isothermally crystallized at 60 �C.In Fig. 6-a, where PE blocks crystallize in all samples just before thecrystallization of PPL blocks, cPPL(t) increases exponentially with noinduction time. This time evolution of cPPL(t) is similar to that ofcrystalline blocks and homopolymers confined in isolated nano-domains [40e42], suggesting that homogeneous nucleationcontrols the crystallization process of PPL blocks. The time evolu-tion of cPPL(t) for PL-PE(51) and PL-PB(51) (Fig. 6-b) is, on the otherhand, extremely different from that shown in Fig. 6-a. That is,cPPL(t) increases sigmoidally with an appreciable induction time. Inaddition, cPPL(N) for PL-PE(51) is significantly higher than that forPL-PE(10)wPL-PE(20), as found in Fig. 2-b. Therefore, Fig. 6 clearlyindicates that the crystallization behavior is significantly differentbetween two groups to result in a large difference in the finalcrystallinity of PPL blocks.

0.5

0.4

0.3

0.2

0.1

01.0

0.8

0.6

0.4

0.2

010008006004002000

Time (s)

PP

L(t

)P

PL(t

)

a

b

PL-PE(51)

PL-PB(51)

PL-PE(20)

PL-PE(10), PL-PE(15)cc

Fig. 6. (a) Time evolution of cPPL(t) for PL-PE(10) (circle), PL-PE(15) (square), and PL-PE(20) (triangle) when crystallized at 60 �C. (b) Time evolution of PPL crystallinitycPPL(t) for PL-PE(51) (circle) and PL-PB(51) (square) when crystallized at 60 �C.

It is well known that the crystallization rate at time t fora nucleation-controlled process is proportional to the volumefraction of un-crystallized polymers remaining at t. Therefore, theplot of lnf1� c0PPLðtÞg vs. t (c0PPLðtÞ: normalized crystallinity at t)should give a straight line, and the slope is a measure for thecrystallization rate of this process [43]. It should be noted that thisplot exactly corresponds to n ¼ 1 in the Avrami equation usuallyused for the analysis of early stage crystallization for homopoly-mers. Fig. 7 shows the plots of lnf1� c0PPLðtÞg against t for PL-PE(10), PL-PE(15), and PL-PE(20), where they all show straightlines, indicating that this crystallization process is successfullydescribed by a nucleation-controlled kinetics. Of course, we cannotobtain straight lines in the plot of lnf1� c0PPLðtÞg vs. t for PL-PE(51)and PL-PB(51).

Fig. 8 shows the schematic illustration for morphology forma-tion occurring in PPL-b-PE copolymers with different cPE. For PL-PE(10)wPL-PE(20), the prompt crystallization of PE blocks onquenching reinforces the existing lamellar microdomain structureto make it stable against the subsequent crystallization of PPLblocks. Consequently, PPL blocks crystallize within the crystallizedlamellar microdomains to form double crystallized lamellarmicrodomains, in which PE and PPL blocks crystallize in each layer.For PL-PE(51), on the other hand, no crystallization occurs beforethe crystallization of PPL blocks, and accordingly the lamellarmicrodomain structure is not reinforced. As a result, morphologicaltransition takes place when PPL blocks crystallize to yield thecrystallized lamellar morphology. This lamellar morphology issolely controlled by the crystallization of PPL blocks, so that it isinherently different from crystallized microdomains.

3.5. Comparison with other experimental results

In PL-PE(51), PE blocks do not crystallize at all, and PPL blocksare allowed to crystallize from soft lamellar microdomains. In thiscase, opposite two factors, the driving force to form the crystallizedlamellar morphology and the stability of lamellar microdomains,work simultaneously to determine the crystallization behavior ofPPL blocks. The former is slightly stronger than the latter in PL-PE(51) to result in morphological transition (or break-out crystal-lization), as usually observed in the crystallization of many weaklysegregated crystalline-amorphous diblock copolymers [39,44e46].It is well known for break-out crystallization that the crystallizationbehavior is controlled by heterogeneous nucleation and subse-quent crystal growth, and the crystallinity shows the sigmoidaltime evolution with n ¼ 3 [44,45]. This is completely consistent

-3

-2

-1

0

120100806040200Time (s)

ln(1

-’ P

PL(t

))

PL-PE(10)

PL-PE(15)

PL-PE(20)

Tc = 60 oC

c

Fig. 7. lnð1� c0PPLðtÞÞ plotted against crystallization time t for PL-PE(10) (circle), PL-PE(15) (square), and PL-PE(20) (triangle).

Morphological Transition

Crystallization of PE Blocks Crystallization of PPL Blocks

LMS (Crystallized)LMS (melted)

LMS (Crystallized) LMS (Double Crystallized)

LMS (Double Crystallized)

Crystallized Lamellar Morphology

PL-PE(10)

PL-PE(20)

PL-PE(51)

Fig. 8. Schematic illustration showing the difference in morphology formation between PL-PE(10), PL-PE(20), and PL-PE(51). LMS stands for lamellar microdomain structures.

L. Huang et al. / Polymer 53 (2012) 5856e58635862

with our results for the crystallization behavior of PPL blocks in PL-PE(51). That is, judging from the time-resolved SR-SAXS curvesshown in Fig. 5-b and -e, morphological transition certainly occursin PL-PE(51) by the crystallization of PPL blocks. However, the longperiod of lamellar microdomain structures is extremely large(w41e42 nm) owing to strong segregation between PPL and PEblocks to shift the SAXS peak to extremely low angles. Though thecrystallized lamellar morphology usually gives a moderately largerlong period, it is difficult for PE-PL(51) to clearly observe thecoexistence of two morphologies during crystallization, whichappears to be different from break-out crystallization with smallerlong periods [39].

In PPL-PE(10)wPPL-PE(20), the crystallization of PPL blocksstarts from crystallized lamellar microdomains. This is similar tothe crystallization in glassy lamellar microdomains [47,48], wheremany results show the heterogeneous nucleation (i.e., n¼ 3) for thecrystallization of constituent blocks. However, we have somereports showing the homogeneous nucleation (n¼ 1). For example,Ho et al. [49] examined the crystallization behavior of PCL blocks instrongly segregated lamellar microdomain structures of PCL-block-poly(4-vinylpridine) (PCL-b-P4VP) as a function of confinementsize d, and found the homogeneous crystallization at smaller d.Furthermore, Register et al. [50] found the homogeneous nucle-ation (n ¼ 1) of polyethylene (PE) blocks confined in vitrifiedlamellar microdomains of PE-block-poly(vinylcyclohexane) atlower crystallization temperatures. It is also suggested that thestretching of crystalline blocks confined in the crystallized lamellarmorphology induces the moderate acceleration of nucleation ratesto change the crystallization behavior [51]. These reports indicatethat the crystallization behavior of crystalline blocks confined innano-scale lamellar morphologies will be affected by many factorssuch as confinement nature (soft or hard), block conformation, and

accelerated nucleation at the interface. Therefore, the crystalliza-tion of such confined blocks is very complicated as compared withthat of crystalline homopolymers without any spatial confinement.It is necessary to accumulate many experimental results regardingthe crystallization of confined blocks in different circumstances inorder to understand the characteristics of confined crystallization.

4. Conclusions

We investigated the crystallization behavior of poly(b-propio-lactone) (PPL) blocks in PPL-block-polyethylene (PPL-b-PE) copol-ymers with varying crystallinities of PE blocks cPE. The amount ofethyl branches in PE blocks was controlled during polymerizationto get PPL-b-PE copolymers with different cPE. The PPL-b-PEcopolymers formed strongly segregated lamellar microdomainstructures in the melt, and PE blocks had an extremely fast crys-tallization rate and slightly higher crystallizable temperature thanPPL blocks. Therefore, PE blocks crystallized first on quenching toform the crystallized lamellar microdomains, from which PPLblocks crystallized. Consequently, the crystallization of PPL blockswas completely confined in crystallized lamellar microdomains,and the time evolution for the crystallinity of PPL blocks cPPL(t)showed an exponential-type increase with no induction time. Incontrast, morphological transition occurred from the lamellarmicrodomain structure into the crystallized lamellar morphologyby the crystallization of PPL blocks in PPL-b-PE with cPE ¼ 0, wherecPPL(t) changed sigmoidally with an appreciable induction time.Weconcluded from these results that the prompt crystallization of PEblocks coupled with strong block incompatibility reinforced thelamellar microdomain structure and made it stable against subse-quent crystallization of PPL blocks to result in double crystallizedlamellar microdomains.

L. Huang et al. / Polymer 53 (2012) 5856e5863 5863

Acknowledgments

The SR-SAXS measurements have been performed under theapproval of Photon Factory Advisory Committee (No. 2010G014).

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