Morphology and Melt Crystallization of PCL-PEG Diblock Copolymers

Full Paper

1836

Morphology and Melt Crystallization ofPCL-PEG Diblock Copolymers

Ying Xu, Yong He, Jia Wei, Zhongyong Fan, Suming Li*

ROP of PCL was realized in the presence of mPEG with Mn ¼5 000, using Zn(La)2 as a catalyst.The resulting diblock copolymers with molar ratios of the CL/EO repeat units from 0.2–5.0were characterized by DSC, WAXD, SEC, and 1H NMR. Melt crystallization was studied andanalyzed with the Avrami equation. The spherulitegrowth rate G was determined at different crystalliza-tion temperatures. The G values were found to rangebetween those of mPEG and PCL homopolymers. Themorphology of an isothermally crystallized samplewith CL/EO¼ 0.5 was examined. Spherulites withPCL embedded in PEG were observed, in contrast toconcentric spherulites reported in the literature.

Introduction

Poly(e-caprolactone) (PCL) is a biocompatible and biode-

gradable polyester which presents great interest for

temporary therapeutic applications such as osteosynthetic

devices and sustained drug delivery devices.[1–3] However,

the potential applications of PCL are considerably re-

strained by the high hydrophobicity.[4] It degrades very

slowly by simple hydrolysis under the human body

conditions. The hydrophilicity and biodegradability can

be improved if hydrophilic poly(ethylene glycol) (PEG)

block is attached to PCL backbone.[5] PCL-PEG block

copolymers have been prepared by ring-opening poly-

merization of e-caprolactone (e-CL) using PEG as a

macroinitiator.[6–13] Various catalysts have been used,

including stannous octoate, zinc metal, zinc lactate,

Y. Xu, Y. He, J. Wei, Z. Fan, S. LiDepartment of Materials Science, Fudan University, Shanghai200433, ChinaE-mail: [email protected]. LiMax Mousseron Institute on Biomolecules (UMR CNRS 5247),University Montpellier I/II, 34060 Montpellier, France

Macromol. Chem. Phys. 2008, 209, 1836–1844

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

etc.[14,15] The resulting copolymers present interesting

properties such as biocompatibility, amphiphilicity, self-

assembly, permeability, and controllable biodegradabil-

ity.[16]

Block copolymers can be divided into two groups,

crystalline/amorphous and crystalline/crystalline. When a

melted block copolymer is quenched below the melting

temperature, simultaneous microphase separation and

crystallization will occur. Competition between these two

processes determines the final morphology.[17] In the case

of crystalline/amorphous copolymers, weakly segregated

microphase separation structure can be destroyed by

crystallization, while strongly segregated microphase

separation structure is maintained. In contrast, block

copolymers containing only crystalline components are

subjected to a competition in crystallization between

different blocks.[18] Bogdanov et al.[19,20] characterized the

thermal properties of three PCL-PEG diblock copolymers

with PCL weight fractions ranging from 68–85 wt.-%. It

was concluded that the PCL blocks crystallize first, which

fixes the total copolymer structure and leads to imperfect

crystallization of PEG blocks. Shiomi et al.[18] observed the

morphology of spherulites of PCL-PEG-PCL triblock copoly-

mers with different block lengths. Copolymers with PCL

DOI: 10.1002/macp.200800137

Morphology and Melt Crystallization of PCL-PEG Diblock Copolymers

contents of 60 and 66 wt.-% exhibited a unique morphol-

ogy of concentric spherulites whose central and outer

sections were composed of PCL and PEG, respectively. Only

PCL spherulites were obtained for the copolymer with a

PCL content of 83 wt.-%, whereas the copolymer with a PCL

content of 34 wt.-% showed only PEG spherulites. He

et al.[21,22] reported a series of PCL-PEG diblock copolymers

with PCL weight fraction from 29 to 86%. Concentric

spherulites were observed for the diblock copolymer with

PCL content of 50%. The authors proposed that once the

new birefringence reached the edge of the single PCL

spherulite, a new outer PEG spherulite grew concentrically

from the existing front of the inner spherulite and formed

the outer portion of the concentric spherulite. Takeshita

et al.[23] studied the lamellar structure formed by

competition during crystallization of PCL-PEG diblock

copolymers with PCL weight fraction from 34 to 67%. It

was concluded that PCL crystallizes first, followed by the

crystallization of PEG chains between preformed PCL

crystal lamellae, resulting in the formation of ordinary

single-circled spherulite having alternating crystal lamel-

lae structures of PEG and PCL. The Tm and crystallinity of

PEG in the diblock copolymers are lowered especially for

those with low PEG contents, whereas the Tm of PCL

remains approximately constant for all block copolymers.

However, the crystallization kinetics have not been

studied in detail, so far. And the interactions between

the two blocks, which rely on the chain lengths and affect

the crystallization process and morphological character-

istics, have not been fully clarified.

The aim of this work was to better understand

the crystallization of PCL-PEG diblock copolymers, in

particular the influence of PCL block length, by combining

various techniques. PCL-PEG diblock copolymers with

CL/EO molar ratios ranging from 0.2 to 5.0 were

synthesized by ring-opening polymerization of e-CL in

the presence of mPEG with Mn ¼ 5 000, using zinc lactate

as a catalyst. The morphology and crystallization kine-

tics were investigated under various conditions including

WAXD, DSC, and POM. The Avrami equation was used to

evaluate the Avrami exponent and the constant of

crystallization rate (Kn), whereas the spherulite growth

rate (G) was obtained by using POM with hot stage. The

results are reported herein in comparison with literature

data.

Experimental Part

Materials

e-CL and mPEG with Mn ¼ 5 000 were purchased from Fluka. Zinc

lactate [Zn(La)2] was supplied by Sigma. All organic solvents were

of analytic grade.



Polymerization

Predetermined amounts of e-CL, mPEG, and Zn(La)2 (0.1 wt.-%)

were introduced in a polymerization flask, the CL/EO repeat unit

molar ratios ranging from 0.20 to 5.0. After degassing, the flask

was sealed under vacuum. Polymerization was allowed to proceed

at 120 8C for 7 d. The product was recovered by dissolution in

dichloromethane and precipitation with diethyl ether (CL/EO< 1),

mixture of diethyl ether and ethanol (CL/EO¼1) or ethanol (CL/

EO> 1), followed by vacuum drying up to constant weight. PCL

homopolymer was prepared by using the same procedure, except

using ethylene glycol as an initiator instead of mPEG.

Sample Preparation

Samples for POM observation were prepared by casting two drops

of 1 wt.-% chloroform solution of the polymer on a cover glass,

followed by air drying for 1 d, and vacuum drying at room

temperature for 3 d.

Measurements

Proton nuclear magnetic resonance (1H NMR) spectra of the

polymers were recorded in CDCl3 by using a Bruker 500 MHz

spectrometer. Chemical shifts were given in ppm using tetra-

methylsilane as internal standard.

Size-exclusion chromatography (SEC) measurements were

performed on a Waters 410 apparatus equipped with a refractive

index (RI) detector. 20 mL of solution (1.0%, w/v) was injected for

each analysis with tetrahydrofuran as mobile phase at a flow rate

of 1.0 mL �min�1. Calibration was accomplished with polystyrene

standards (Polysciences).

Wide-angle X-ray diffraction (WAXD) was carried out with a

Philips diffractometer with Cu Ka radiation (l¼ 1.54 A) at room

temperature. The scanning range is from u¼ 2 to 20 8.Differential scanning calorimetry (DSC) was carried out on a

DSC 10 cell (TA) calibrated with indium. 5 (�0.1) mg of samples

were scanned under N2 atmosphere after various thermal

treatments. Each sample was heated from 20 to 100 8C at

10 8C �min�1, which allows determining the melting temperature

(Tm1) and the melting enthalpy (DHm1). After rapid cooling to

�100 8C, the sample was heated again to 100 8C at 10 8C �min�1 to

evaluate the glass transition temperature (Tg), the cold crystal-

lization temperature (Tcc), and the second melting temperature

(Tm2). The melt crystallization temperature (Tmc) was obtained by

cooling at 10 8C �min�1 after isothermal melting for 3 min

at 100 8C.

The morphology of crystals was observed by using an Olympus

BH-2 polarized optical microscope (POM). Copolymer films were

melted at 100 8C for 3 min, followed by isothermal crystallization

at various temperatures for 60 min.

The spherulite growth rate was evaluated by using Olympus BX

51 POM equipped with Instec HSC 601 hot stage. The sample was

melted at 100 8C, kept for 3 min, and then subjected to different

thermal treatments. The radius of growing spherulites was

monitored during isothermal melt crystallization with a video

camera system taking photographs automatically at appropriate

www.mcp-journal.de 1837

Y. Xu, Y. He, J. Wei, Z. Fan, S. Li

1838

time intervals in the range of 30–46 8C. Dry nitrogen was applied

throughout the hot stage during microscopic observation of the

samples.

Figure 1. WAXD spectra of PCL-PEG diblock copolymers (data inparentheses correspond to CL/EO values).

Results and Discussion

Sample Characterization

Five PCL-PEG diblock copolymers were synthesized in this

work which are composed of a PEG block with Mn ¼ 5 000

and a PCL block of various Mn or chain lengths (Table 1).

The CL/EO molar ratio of the copolymers was calculated by

using the integrals of the 1H NMR resonances at 2.35 ppm

for PCL and at 3.66 ppm for PEG. It is noted that the CL/EO

ratio in the copolymers is close to the feed ratio in all cases,

indicating a good conversion of e-CL monomer. The higher

the CL/EO ratio, the longer the PCL block length and the

higher the overall Mn. The Mn values calculated from SEC

were lower than those from NMR spectra (Table 1). This

finding could be assigned to changes of hydrodynamic

volume of PCL-PEG copolymers bearing both hydrophilic

PEG and hydrophobic PCL segments as compared with

the parent homopolymers, as previously reported.[14] The

molecular weight distribution of the copolymers becomes

larger for copolymers with higher Mn. PCL homopolymer

withMn ¼ 4 600 was synthesized for the sake of comparison.

DSC and WAXD Analyses

The crystalline structures of the polymers were examined

by using WXRD (Figure 1). PEG blocks exhibit two

diffraction peaks at u¼ 9.60 and 11.65 8, and PCL blocks

at u¼ 10.7 and 11.85 8. In the case of the copolymers,

samples A and B present a summation of both PEG and PCL

diffraction peaks, indicating that both blocks are able to

crystallize and form separate crystal phases. However,

diffraction peaks corresponding to PEG blocks are hardly

detectable in sample C, and disappear in samples D, E, and

F, indicating that only the PCL crystal phase exists.

Table 1. Molecular and structural characteristics of PCL-PEG copolym

Sample CL/EO (feed) CL/EO (product)a)

A 0.2 0.17

B 0.5 0.46

C 1.0 1.36

D 2.0 2.37

E 3.0 3.0

F 5.0 4.87

a)Determined by 1H NMR; b)determined by SEC.



Generally, with increasing CL/EO ratio, the crystallization

ability of PCL block is enhanced while that of PEG block

diminished. It is also noted that the crystallizability of PEG

and PCL blocks appears almost equal in sample B with

equivalent PEG (Mn ¼ 5 000) and PCL (Mn ¼ 5 900) block

length. These findings indicate that in PCL-PEG diblock

copolymers, the crystallization ability of PEG block is

restricted by the PCL block length.

The thermal properties of the polymers were deter-

mined with DSC. Two heating runs were realized: a first

run from 20 to 100 8C at 10 8C �min�1, followed by a second

run after rapid cooling to �100 8C. Figure 2 shows the DSC

curves of the copolymers. The Tm1 values of PEG and PCL

homopolymers are rather close, i.e., 64.2 and 69.0 8C,

respectively. In contrast, a large difference is detected

between the DHm values: DHm,PCL ¼ 91.2, DHm,PEG ¼178.1 J � g�1 (Table 2). Samples A and B with CL/EO< 1

exhibit a double melting peak probably due to the melting

ers.

Mn PCLa) Mn

a) Mnb) Mw=Mn

b)

2 200 7 200 6 300 1.5

5 900 10 900 9 100 1.7

17 700 22 700 13 200 1.9

30 800 35 800 18 800 1.8

39 000 44 000 19 100 2.0

63 300 68 300 24 200 2.3

DOI: 10.1002/macp.200800137


Figure 2.DSC curves of diblock copolymers at 10 8C �min�1: (a) firstheating from 20 to 100 8C; (b) second heating from �100 to100 8C.

Table 2. Thermal properties of PCL-PEG copolymers determinedby DSC.

Sample Tgb) Tmcc) Tccb) Tm1

a) DHm1a) Tm2

b)

-C -C -C -C J � gS1 -C

A S66.9 33.4 S59.3 61.9 103.6 57.9

B S67.2 31.6 S57.0 60.9 103.0 56.9

C S67.7 32.4 S59.0 62.9 77.2 57.2

D S68.1 35.5 S58.7 65.6 75.7 58.0

E S66.7 29.3 S57.6 67.0 74.9 59.0

F S65.1 28.7 S56.0 70.8 78.7 59.2

mPEG5000 64.2 178.1 62.9

PCL4500 S65.0 S55.9 69.0 91.2 58.4

a)Obtained by first heating from 20 to 100 -C at 10 -C �minS1;b)Obtained by second heating fromS100 toR100 -C at 10 -C �minS1;c)Obtained by cooling at 10 -C �minS1 from melt at 100 -C.

Figure 3. DSC curves of sample B after isothermal crystallizationfor 5, 7, 10, 30, 60, and 120 min at 45 8C.

of both PEG and PCL crystallites, in agreement with WAXD

data. Piao et al.[5] reported double melting peaks for PCL-

PCL-PEG triblock copolymers which are ascribed to PEG and

PCL blocks. The Tm1 of samples A and B is detected at 61.9

and 60.9 8C, respectively, i.e., lower than that of PEG. In

contrast, Tm1 of copolymers with CL/EO> 1 increases with

increase in the CL/EO ratio, in agreement with the PCL-type

crystallites. Insofar as the melting enthalpy is concerned,

DHm1 of samples A and B are much higher than those of the

other samples. This is because samples A and B are mainly

composed of PEG crystallites.

The second run thermograms allow to determine Tg, Tcc,

and Tm2. Tg of PCL was detected at �65.0 8C, and that of PEG

was not detected due to its extremely high crystallization

rate. However, Tg of PEG is estimated to be around �65 8Cin literature. All the copolymers exhibit a glass transition

in the scope of �68.1 to �65.1 8C, i.e., close to the Tg values



of PCL and PEG homopolymers. Tcc is observed from �56.0

to �59.3 8C. Tm2 is slightly lower than Tm1 because lamellar

growth is not favored at a heating rate of 10 8C �min�1. Tmc

of the copolymers ranges from 28.7 to 35.5 8C. All the DSC

data are listed in Table 2.

Isothermal Melt Crystallization andCrystallization Kinetics

Sample B with CL/EO molar ratio¼ 0.5 was selected to

investigate the time-dependent isothermal melt crystal-

lization because PEG and PCL blocks are both crystallizable.

Figure 3 shows the DSC curves of sample B after various



Figure 4. Heat flow versus time curves of the copolymers duringisothermal crystallization at 45 8C.

1840

isothermal melt crystallization time periods at 45 8C. Both

the DHm and Tm remain almost constant during the

isothermal time period up to 120 min, with DHm ¼106.0 J � g�1 and Tm ¼ 58.0 8C. This finding suggests that the

melt crystallization is almost finished in 10–15 min due to

the high crystallization ability of the dominant PCL block.

After isothermal crystallization for 10 min, a very small

melting peak was detected at Tm¼ 42.6 8C which became

more obvious for a longer crystallization time, indicating

that crystallization of the PEG block starts after that of PCL.

All the copolymers and homopolymers were used to

investigate the influence of CL/EO ratio on the isothermal

melt crystallization and crystallization kinetics. After

1 min isothermal equilibrium at 100 8C, the samples were

cooled down to 45 8C at 50 8C �min�1 and allowed to

crystallize for 60 min (Figure 4). The isothermal crystal-

lization curves of samples A and B appear irregular because

both PEG and PCL blocks are crystallizable. These two

curves represent, in fact, the summation of the isothermal

crystallization of both blocks. Peak time (tp) taken at the

peak of enthalpy is used to evaluate the beginning of

crystallization. Peak time decreases from A to E and then

increases for F, indicating that sample E crystallizes the

most rapidly. A heating scan was then performed to 100 8Cto obtain Tm and DHm of the fully crystallized samples. All

the data are given in Table 3. The Tm values show the same

tendency as the first heating curves in Figure 2: sample B

exhibits the lowest Tm, and Tm increases with increase in

the CL/EO ratio for samples C–F. However, a large DHm

difference was detected: DHm decreases with increase in

the CL/EO ratio, as shown in Figure 5 (CL %¼ 0 for mPEG; CL

%¼ 100 for PCL). This can be assigned to the fact that under

the isothermal melt crystallization conditions, both PEG

and PCL blocks are able to crystallize, DHm of PEG being

much higher than that of PCL.

Table 3. Isothermal crystallization of PCL-PEG copolymers at 45 8C fo

Sample Tma) DHma) tpa

-C J � gS1 min

PEG 64.5 178.1

A 60.2 144.2 24.2

B 59.4 115.9 18.1

C 61.0 87.9 10.6

D 61.3 76.7 7.3

E 61.5 73.3 4.2

F 61.9 70.7 14.0

PCL 60.9 65.7 14.2

a)Detected by DSC, heated isothermal crystallized samples to 100 -C a



The relative crystallinity can be calculated from the

melting enthalpy, and the isothermal crystallization can

be described by the Avrami equation:[24,25]

r 60 m

)

t 10 -C

XrðtÞ ¼ 1 � expð�KntnÞ (1)

where Xr is the time-dependent crystallinity, Kn the

constant of crystallization rate, n the Avrami exponent

related to nucleation and spherulitic growth. Figure 6

shows Xr versus time plots of the copolymers. The half

crystallization time is given by t1/2 ¼ (ln 2/Kn)1/n with

Xr ¼ 0.5. The values of n and Kn can be obtained from the

slope and intercept of lg[�ln(1�Xr)] versus lg(t) plot,

respectively, as shown in Figure 7. With the increase in the

CL/EO ratio, the Avrami exponent n and the crystallization

in.

t1/2 Kn n

min 1 �minS1

14.05 3.8T 10S7 4.24

11.95 8.1T 10S5 3.41

8.03 2.4T 10S4 3.37

5.43 2.0T 10S3 2.87

2.83 6.1T 10S2 2.78

12.22 6.6T 10S5 3.37

14.30 9.8T 10S6 4.22

�minS1.

DOI: 10.1002/macp.200800137


Figure 5. DHm variation of isothermally crystallized samples as afunction of CL molar content. Figure 7. lg[�ln(1�Xr)]� lg(t) plots of the copolymers derived

from data in Figure 6.

halftime t1/2 decreases at first to reach a minimum for

sample E, and then increases. The constant of crystal-

lization rate Kn exhibits an inverse tendency with a

maximum for sample E (Table 3). During isothermal

crystallization, n¼ 3 or 4 corresponds to heterogeneous or

homogeneous nucleation with three-dimensional growth.

The Avrami exponent n of sample A with CL/EO¼ 0.2 is

about 4, in agreement with the homogeneous nucleation

of dominant PEG blocks. With increase in the CL/EO ratio,

the crystallization behavior becomes more complex

because of the competition between both blocks. The

Avrami exponent n of PCL homopolymer is close to 4. The

thermodynamic parameters could not be determined for

PEG homopolymer because of its extremely high crystal-

lization rate.

Figure 6. Time-dependent crystallinity changes of the copolymersas a function of crystallization time.



POM was used to examine the spherulitic structure of

samples isothermally crystallized at 35–50 8C for 1 h after

melting at 100 8C for 3 min. Figure 8 shows the POM

images of samples A and C isothermally crystallized at

35 8C for 1 h. The spherulites appear clearly negative in

view of the characteristic colors of the Maltese cross.

Crystals of sample A are composed of small PCL spherulites

embedded in PEG ones with a size of ca. 400 mm. Only PCL

spherulites of about 50 mm are detected for sample C. In

Figure 8. POM pictures: (a) sample A, (b) sample C after isother-mal crystallization for 60 min at 35 8C.



1842

fact, PCL spherulites are observed in all samples even with

dominant PEG content like sample A, whereas PEG ones

are detected only in samples A and B (Figure 1). The size of

PCL spherulites hardly grows with isothermal time in the

case of sample A due to the confinement of PEG blocks.

Figure 9 shows the POM images of sample B isother-

mally crystallized at 30 8C for 7, 20, and 27 s. It appears that

nucleation of PCL and PEG spherulites occurs separately

and the growth is almost simultaneous. When PEG

Figure 9. POM pictures of sample B after isothermal crystalliza-tion at 30 8C for 7 s (a), 20 s (b), and 27 s (c).



spherulites come in contact with PCL ones, they can pass

through PCL spherulites and continue to grow. The PEG

radial distance varies in different directions due to the

inclusion of PCL spherulites in PEG ones. PCL spherulites

are surrounded by PEG ones as the growth rate of PEG

spherulites is much larger. Therefore, the growth of PEG

spherulites seems independent of PCL ones with CL/

EO� 0.5. This finding contradicts with the results reported

by He et al.[21,22] who observed concentric spherulites for

PEG5000PCL5000: a new outer PEG-like spherulite grew

concentrically from the existing front of the inner PCL

spherulite, thus forming the outer portion of the concentric

spherulite. They claimed that in the concentric spherulites,

once the brighter sector reached the center of the single

spherulite, the inward PCL growth was replaced by

outward PEG growth from the single spherulite center.

According to our findings, the brighter nucleus of PCL

blocks formed first and started to grow, i.e., the nucleation

of PCL block occurs before that of PEG block. Later, once PEG

nucleus formed in the clearing near PCL spherulites, PEG

spherulites began to grow based on PEG nucleus. In the

meantime, the growth of PCL spherulites continued, and

the growth rates of both blocks were influenced mutually.

Figure 10. POM pictures of sample B after isothermal crystal-lization for 60 min at 42 8C: (a) and (b).

DOI: 10.1002/macp.200800137


Figure 11. Variation of spherulite radius growth rate (G) as afunction of crystallization temperature (Tc) of sample B, C, D,E, and PCL (G determined by using POM with hot stage).

In addition, banded spherulites are observed in sample B

with Tc ¼ 36–42 8C (Figure 10), which could be assigned to

alternative and competitive crystallization of PEG and PCL

blocks. It is noteworthy that twist in the unit cell

orientation has been used to explain the appearance of

banded spherulites.[26–29]

Further isothermal crystallization studies were per-

formed by using POM with hot stage at various isothermal

temperatures ranging from 30 to 44 8C. Figure 11 shows

the radius growth rate (G) of spherulites as a function of

crystallization temperature (Tc). Comparison was done

with mPEG and PCL homopolymers. mPEG exhibits the

highest growth rate with G¼ 68.25 mm � s�1 at 30 8C,

followed by sample A (G¼ 20.91 mm � s�1 at 30 8C), PCL

showing the lowest growth rate (G¼ 0.58 mm � s�1 at 30 8C).

In the case of samples B–E, G decreases with increase in the

CL/EO ratio. All the polymers present a steady G decrease

with increase in the crystallization temperature (Figure 11).

The growth retardation of the constituent which crystallizes

next (PEG blocks) is attributable to the PCL blocks which

are covalently attached to the PEG blocks and restrict the

mobility of the total copolymer structure for crystal-

lization. Thus, the G of sample A is much lower than that of

PEG blocks, and the competition of PCL and PEG blocks

contributes to fact that the G values of the copolymers

range between mPEG and PCL homopolymers. Therefore,

the crystallization rate of PCL-PEG copolymers can be

adjusted by varying the crystallization temperature or

copolymer composition.

Conclusion

The thermal properties of PCL-PEG block copolymers

depend on the composition or block length. The presence



of PCL disfavors the crystallization of PEG, and vice versa.

PEG block is able to crystallize for CL/EO¼ 0.2 and 0.5,

while PCL block can crystallize for all copolymers with CL/

EO from 0.2 to 5.0. The Avrami exponent n values of the

copolymers range from 2.78 to 4.24, in agreement with

heterogeneous and/or homogeneous nucleation with

three-dimensional growth. The spherulite growth rate

(G) of the diblock copolymers is intermediate between

those of mPEG and PCL homopolymers. In the case of PCL-

PEG with CL/EO¼ 0.5, PCL spherulites are embedded in PEG

ones, in contrast to concentric spherulites reported in

literature for similar copolymers.

Acknowledgements: The authors acknowledge financial supportfrom the Shanghai Leading Academic Discipline Project (no. B113)and the National Basic Research Program of China (973 Programno. 2007CB935801).

Received: March 7, 2008; Revised: May 5, 2008; Accepted: May 8,2008; DOI: 10.1002/macp.200800137

Keywords: crystallization; morphology; poly(e-caprolactone);poly(ethylene glycol)

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DOI: 10.1002/macp.200800137

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