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Polymer 52 (2011) 5513e5520

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Polymer

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Structural studies of poly(butyl acrylate) e poly(ethylene oxide) miktoarm starpolymers

Monika Makrocka-Rydzyk a,b, Aleksandra Wypych a, Kamil Szpotkowski a, Maciej Kozak a,Stefan Jurga a,b,*, Haifeng Gao c, Hong Yul Cho c, Krzysztof Matyjaszewski c

aDepartment of Macromolecular Physics, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Pozna�n, PolandbNanoBioMedical Centre, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Pozna�n, PolandcDepartment of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 1521, USA

a r t i c l e i n f o

Article history:Received 14 July 2011Received in revised form12 September 2011Accepted 12 September 2011Available online 28 September 2011

Keywords:Miktoarm star polymerDSCFTIR

* Corresponding author. Department of MacromPhysics, Adam Mickiewicz University, UmultowskaFax: þ48 (61) 829 5216.

E-mail address: zfmak@amu.edu.pl (S. Jurga).

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.09.020

a b s t r a c t

Structural behavior of miktoarm star polymers comprising poly(butyl acrylate) (PBA) and poly(ethyleneoxide) (PEO) arms was studied by means of Differential Scanning Calorimetry (DSC), Wide Angle X-RayScattering (WAXS), Polarized Optical Microscopy (POM) and Fourier Transform Infrared Spectroscopy(FTIR) methods. The aim of this study was to correlate changes in the composition of the arms of the PBA/PEO miktoarm star polymers with their structures. As a consequence of increasing PBA content, thedecrease in crystallinity of the studied PBA/PEO heteroarm star copolymers was observed. Regardless ofthe copolymer composition, fraction of oxyethylene units in the crystalline PEO phase was similar in allinvestigated systems. The POM images showed spherulitic morphology of the materials having low PBAcontent, while an increase in PBA arms fraction leads to the formation of less ordered structures. Theanalysis of FTIR vibrational spectrum indicates helical conformation of PEO chains in the crystallinephase. Isothermal crystallization studies carried out using the FTIR technique suggest the existence ofisolated domains in the nanoscopic scale of investigated materials.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Star polymer molecules consist of several linear chains (arms)joined covalently to a centrally located nanoscopic core [1e3]. Dueto the compact structure of star polymers these materials combinethe properties of linear polymer chains with those of hard spherecolloids (e.g. lower viscosity compared to their linear analogs) [4].Miktoarm star polymers (also called heteroarm star copolymers)represent a special type of star shape polymers, containing two ormore arm species with different chemical compositions and/ormolecular weights attached to a central core. Progress in thedevelopment of atom transfer radical polymerization (ATRP)methods [5e7] enabled synthesis of the miktoarm star polymersystemswith controlledmolecular weights, low polydispersity, andwell defined architectures [8]. The multiple functionality of thesematerials may be adjusted by choosing the chemical structure ofarms, depending on the required application. The miktoarm star

olecular Physics, Faculty of85, 61-614 Pozna�n, Poland.

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polymers, may show, similarly to the block copolymers, the self-ordering features which lead to the separation of microphasesresulting in various morphologies like spherical, cylindrical,lamellar or other more complex architectures [9]. The hierarchicalstructures formed by this class of particles can be useful for variousapplications in nanotechnology, biomedicine and pharmacy (e.g.for drug-delivery systems) [10,11]. Star-shaped polymers withpoly(ethylene oxide) (PEO) components represent a particularlypromising class of these materials [12] since polyethylene oxide(PEO) is a nonionic and biocompatible polymer having uniqueproperties such as chemical stability, water solubility, non-toxicity,and resistance to recognition by the immune system. The structuralfeatures of PEO have been extensively reported in the literature forthe homopolymer [13,14] and for more complex systems like itsblends [15,16] as well as for block copolymers containing PEOchains [17e23]. Studies on the polymer structure are theoreticallyand practically important because the physical and mechanicalproperties of polymers are strongly dependent on the morphologyand on the extent of crystallinity. Although there are a lot of studieson PEO structure, little so far has been reported for miktoarm starpolymers having PEO arms. This is the reason for our interest in thenewly synthesized miktoarm star polymers, built of hydrophobicpoly(n-butyl acrylate) (PBA) arms and hydrophilic poly(ethylene

Table 1Molecular characteristics of miktoarm star copolymers comprising poly(butylacrylate) PBA (B) and poly(ethylene oxide) (E) arms. The data were taken from Ref.[8].

sample Mw(kg/mol) [GPC] nB/nE wB/wE N NB NE PI

B2E8 202.6 0.16/0.84 0.36/0.64 66 10 56 1.17B5E5 230.1 0.46/0.54 0.71/0.29 57 26 31 1.13B8E2 210.9 0.76/0.24 0.90/0.10 42 32 10 1.15

nB, nE e average molar fraction of PBA and PEO arms in the miktoarm star copoly-mers wB, wE e average weight fraction of PBA and PEO arms in the miktoarm starcopolymers NB, NE, N e number-average value of the number of PBA, PEO and allarms, respectively, per star molecule, PI e polydispersity index.

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e55205514

oxide) (PEO) arms. The study on structural behavior of thesesystems allow to address the question of how the increase in PBAarms number influences the crystallization behavior of PEO andthus modifies the thermal properties and morphology of thesystem.

In this article, we describe the investigations of phase behavior,crystallization and nano-phase structure of amphiphilic PBA/PEOmiktoarm star polymers, with various molar ratios of the compo-nents [8]. To study their structure and thermal properties (e.g.degree of crystallinity Xc, and melting Tm and crystallization Tctemperatures) we make use of Differential Scanning Calorimetry(DSC), Fourier Transform Infrared (FTIR) spectroscopy, PolarizedOptical Microscopy (POM) and Wide Angle X-ray Scattering(WAXS).

2. Experimental

2.1. Materials

Miktoarm star polymers of different compositions of PEO andPBA arms with the structure shown in Fig. 1 and characteristicsgiven in Table 1 were synthesized using the ATRP method by Gaoet al. [8]. The samples with molar percentage of PEO arms of 24%,54% and 84% are denoted with the acronyms B8E2, B5E5, and B2E8,respectively. The samples have uniform structure, as evidenced bya low value of the polydispersity index. The length of the PEO armsis the same (with approximate molecular mass of 2230 g/mol) forall copolymers under study. The linear low molecular mass PEO(PEG2000) of 2000 g/mol average molecular weight was purchasedfrom SigmaeAldrich and used in the present studies for compar-ison with the studied miktoarm star polymers.

2.2. Differential Scanning Calorimetry (DSC)

The thermal behavior of PEO/PBA miktoarm star polymers wasstudied with DSC (Q2000 TA Instruments). All samples wereweighed (between 5 and 8 � 0.2 mg) and enclosed in an aluminumpan. An empty aluminum pan was also used as a reference. Thesamples were first heated to the temperature of about 35 K abovethe respective melting temperature at the heating rate of 10 K/min.To ensure complete melting they were kept at this temperature for5 min. After this period, each sample was cooled to 173 K at thecooling rate of 10 K/min. The procedure was repeated, while theappropriate calculations were made from the second scan.

2.3. Wide angle X-ray scattering (WAXS)

The wide angle X-ray scattering (WAXS) data were collectedusing a HZG-4 diffraction system. The polymer samples weremounted on the polycarbonate sample holder. X-ray tube witha copper anode (l ¼ 1.54178 Å) operating with the accelerating

Fig. 1. Schematic structure of miktoarm star polymers.

voltage of 40 kV, and the tube current of 35 mA, was used as thesource of X-ray radiation. All WAXSmeasurements were performedat room temperature within the 2q angle ranging from 4� to 60�, at0.3�/min scanning rate. The diffraction peaks were deconvolutedusing ORIGIN software (www.microcal.com).

2.4. Polarized optical microscopy (POM)

The optical microscopy micrographs were obtained betweencrossed polarizers at 100� magnification using a PolarizingMicroscope (Leica, Germany) at room temperature (296 K).

2.5. Fourier Transform Infrared (FTIR) spectroscopy

FTIR spectra of samples were collected on a BRUKER IFS 66 FTIR-RAMAN Spectrometer equipped with a SPECAC Variable Tempera-ture Cell, from 600 to 4000 cm�1 with the resolution of 1 cm�1. Thesamples of 150 mg KBr and 1 mg of miktoarm star polymer weremixed manually in an agate mortar, which resulted in theirmacroscopically phase-separated blend. The measurements wereperformed in the temperature range from 133 to 370 K and thesample was incubated for 15 min before data acquisition at eachtemperature. The following procedure was applied for the study ofthe kinetics of crystallization: initially the samplewas incubated for15 min at the temperature of 333 K (well above themelting point ofthe studied copolymer having the highest PEO content), then it wasquenched to liquid nitrogen temperature and heated to therequired temperature, which was higher than the observed crys-tallization temperature (Tc). The isothermal FTIR measurements forthe B2E8 copolymer were carried out at the temperature of 253 Kand the spectra were collected every 5 min. Differential spectrawere used to distinguish the bands sensitive to the change ofcrystallinity of the sample. Each of the spectra used for analysis wasobtained by subtracting the first one, recorded in the temperatureat which the crystallization process was observed. Since themelting point of KBr (1007 K) is much higher than the abovementioned incubation temperature of the sample, the investigatedKBr/miktoarm polymer system remains macroscopically separated.

3. Results and discussion

3.1. DSC

The DSC thermograms registered on heating for the studiedsamples, presented in Fig. 2, show one endothermic peak, whichappears at 316.8 K, 308.7 K, 300.1 K, and 325.0 K for B2E8, B5E5,B8E2, and linear PEO homopolymer, respectively (Table 2). Thetemperature of melting determined for the investigated PEO ofaverage molecular mass of 2000 g/mol (PEO2000) is close to thatreported in the literature [21]. The observed peaks in the DSCthermograms of the investigated miktoarm PBA/PEO copolymerswere attributed to the melting of the PEO crystalline phase taking

200 250 300 350

-2

-1

0

1

2

Temperature [K]

eg

zo B2E8 I heating

I cooling II heating II cooling

heating

cooling

Heat F

lo

w [W

/g]

T

heating

200 250 300

-1

0

1

Temperature [K]

eg

zo B5E5 I heating

I cooling II heating II cooling

heating

cooling

Heat F

low

[W

/g

]

T

heating

200 250 300

-0,5

0,0

0,5

Temperature [K]

eg

zo B8E2

I heating I cooling II heating II cooling

heating

cooling

Heat F

low

[W

/g]

T

heating

200 250 300 350

-6

-4

-2

0

2

4

6

8

Temperature [K]

eg

zo PEO

I heating I cooling II heating II cooling

heating

cooling

Heat F

low

[W

/g]

b

c d

a

Fig. 2. DSC thermograms obtained on heating and cooling for linear PEO and the studied miktoarm star copolymers at the first (.) and second (�) runs. The scanning rate was 10 K/min and the heat of fusion and the heat of crystallization were determined from the second run. The glass transition processes in miktoarm star copolymers were indicated in theinserts of the thermograms.

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e5520 5515

also into account the amorphous structure of the neat PBA [24]. Onemay notice the decrease of the melting point with increase of thePBA fraction in the studied star polymers. In turn, the length of PEOchain (Mn ¼ 2000) and that of PEO arm in studied copolymers(Mn ¼ 2230) have very close values, while the melting temperaturefor the crystalline structures of PEO in the copolymers is lower thanthat observed for PEO2000. These observations indicate the sup-pressing of crystallization of the PEO segments by adjacent PBAphase as was found also for PEO segments in PEO/PBA block

Table 2Melting temperature Tm, crystallization temperature Tc melting enthalpy DHobs anddegree of crystallinity (xc) determined from DSC and WAXS measurements(normalized to the weight fraction of PEO).

Sample Tm (K) Tc (K) Tg (K) DHobs

(J/g)DHnorm

(J/g)xc

a (%) xnorm ac(%)

xcb (%)

Linear PEO 325.0 304.9 e 173.2 e 84 84 e

B2E8 316.8 283.5 233.0 62.4 97.0 30 47 39B5E5 308.6 249.2 229.6 30.7 106.7 15 52 11B8E2 300.1 227.3 227.0 8.8 90 4 44 <5Linear PBA e e 224.0c e e e e e

a As determined from DSC measurements (DHnorm and xnorm ac e denote melting

enthalpy and degree of crystallinity, normalized to the weight fraction of PEO,respectively).

b As determined from WAXS measurementsc As determined by Fioretto et al. [24].

copolymers [25]. Similar observations have been reported by Myaet al. [21], who studying crystallization behavior of star shapedpoly(ethylene oxide) with cubic silesquioxane (CSSQ) core,observed a reduction in the crystallization temperature of PEOchains attached to CSSQ as compared with its linear counterpart.

The single glass transition process appears as a step transition inthe DSC thermogram (inserts in Fig. 2) at 233.0 K, 229.6 K, and227.0 K for the samples B2E8, B5E5, and B8E2, respectively(Table 2). This process is barely visible for B2E8 and not detectablein the case of linear PEO, which is caused by a high content ofcrystalline phase in these systems. However, analysis of ourbroadband dielectric measurements corroborates well the findingsof the glass transition process in the studied miktoarm polymers,which is the subject of publication being in preparation. In turn, theobserved single glass transition process in the studied copolymers,with the glass temperature decreasing slightly with a PBA contentincrease, may result either from the random grafting of PEO andPBA arms to the central core and/or from the similar values of Tg forPEO and PBA homopolymers which constitute the arms of mik-toarm star molecules.

The thermograms, recorded during cooling runs, contain PEOcrystallization exotherms, which appear at 304.9 K, 283.5 K,249.2 K, 227.3 K for the linear PEO homopolymer, and for thesamples B2E8, B5E5, and B8E2, respectively. It is worth noticing thedifference between the melting and crystallization temperatures(i.e. melting hysteresis) which is the lowest for linear PEO (20.1 K)

5 10 15 20 25 30

0

200

400

600

Int [a.u.]

B5E5

B8E2

B8E2

θ [o

]

B2E8

Fig. 3. The WAXS data recorded for B2E8, B5E5 and B8E2 samples.

6 8 10 12 14

0

100

200

300

400

500

600

In

te

nsity

[a

.u

.]

θ [ o

]

Fig. 4. The deconvolution of the B2E8 X-ray diffraction data: amorphic halo (gray line)and crystalline phase (black line).

Table 3X-ray scattering angles q and corresponding d-spacing obtained fromWAXS data forstudied miktoarm star polymers.

Peak 1 2 3 4 5

B2E8q (�) 9.53 11.59 13.15 15.49 17.25d (nm) 0.466 0.384 0.339 0.289 0.260B5E5q (�) 9.58 11.62 13.29 e e

d (nm) 0.463 0.383 0.335 e e

B8E2q (�) 9.6 e e e e

d (nm) 0.461 e e e e

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e55205516

and it increases with the growth of PBA fraction in the copolymers,reaching a maximum value of 72.8 K for B8E2. A similar effect hasalso been observed for PEO by Taden et al. [26] confined in mini-emulsion droplets, which was interpreted as resulting from poly-mer crystallization in restricted geometries. Moreover, the authorsstated that the structure of a polymer confined in crystallizeddroplets was topologically controlled. Taking into account thefindings mentioned above one may suppose that the meltinghysteresis, which is observed for the studied copolymers, may beassigned to the crystallization of PEO chains confined within a PBAmatrix. Additionally, a very broad range of melting, which isrevealed in the DSC thermogram for the B8E2 copolymer, charac-terized by the lowest PEO fraction, may suggest the frustration ofthe crystallization process of PEO chains due to the influence ofamorphous poly(butyl acrylate) surroundings. Similar observationshave been made by Chen et al. [27], who found a highly frustratedPEO crystal growth in the nanoscaled microdomains of poly(-ethylene oxide)-block-poly(1,4-butadiene) both in the cylindricaland in the spherical morphology.

The quantitative information on the degree of crystallinity of theinvestigated system may be obtained by analysis of the enthalpy ofmelting, calculated from the area of the endothermic peak. Theweight fraction of the crystalline PEO phase was estimated usingthe following equation:

xc ¼ DHnorm

DHPEO (1)

where DHPEOn is the enthalpy of fusion for perfectly crystalline PEO

(206.2 J/g [28]), while DHnorm denotes a normalized enthalpy ofmelting.

Yao et al. [25] suggest that in the case of copolymers containingone crystallizable component, it is necessary to take into consid-eration two kinds of crystallinity: (1) substantial crystallinityrelating strictly to the crystallizable component alone and (2)apparent crystallinity referring to the entire sample. The calculatedsubstantial crystallinity of the PEO fractions is 47%, 52% and 44% forcopolymers B2E8, B5E5, and B8E2, respectively. It means thatroughly half of the oxyethylene units of the PEO chain is located inthe non-crystalline state. Whereas, the apparent crystallinity ofmaterials under study decreases linearly with the decrease in thePEO content, taking the value of 30%, 15%, and 4% for B2E8, B5E5,and B8E2, respectively (Table 2).

3.2. WAXS

The copolymers were examined by wide-angle X-ray scattering(WAXS) to define their microstructure and the degree of crystal-linity. The WAXS data are presented in Fig. 3. The information fromthe WAXS patterns clearly indicates that separated crystallinemicro-domains of PEO exist within the B2E8 and B5E5 samples. Forthe B2E8 sample 5 characteristic reflections of crystalline PEO wereobserved and are in good agreement with those observed by Burbaet al. [29]. The strongest peaks (q ¼ 9.53, 11.59 and 13.15�) of eachsample, which correspond to the PEO chains in a helical confor-mation (monoclinic form) [26], were deconvoluted (Fig. 4). Thecalculated peak parameters are summarized in Table 3. On the basisof calculated areas of crystalline peaks (Ac) and of amorphous halo(Aa) the degree of crystallinity (Xc) for the B2E8 and B5E5 sampleswas calculated according to the formula:

Xc ¼ Ac

ðAc þ AaÞ � 100% (2)

The sample with the highest content of PEO (B2E8) is charac-terized by 39.2% crystallinity and assuming the PEO arms fraction of

0.8 in the sample this value is in agreement with the degree ofcrystallinity characterizing pure unoriented PEO [29e31]. For theB5E5 sample the crystallinity degree (Xc) calculated fromX-ray datawas 10.8%. However, the X-ray scattering pattern for B8E2demonstrates amorphous halo at wide angles and only one veryweak diffraction peak appears at q ¼ 9.6�. Furthermore, the esti-mated crystallinity characterizing the sample is lower than 5%. Thecalculated Xc for the studied miktoarm star polymers are collectedin Table 2. It worth pointing out that a planar zig-zag crystal

Fig. 5. POM microphotographs for miktoarm copolymers with varied composition ofPEO and PBA, namely: (a) B2E8, (b) B5E5, (c) B8E2.

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e5520 5517

structure of PEO is marked by strong scattering at q equal to 10.25,12.2, and 12.7 as found by Takahashi et al. [32] for high molecularweight PEO held under tension. Thus, the zig-zag crystal structureof PEO in the studied miktoarm star polymers can be ruled outbecause of the lack of the above mentioned scattering peaks in ourWAXS data.

3.3. POM

A polarized optical microscopywas used to reveal the crystallinemorphology and to track the crystallization process for the studiedmiktoarm star polymers B2E8, B5E5, and B8E2. The polarizationphotomicrographs obtained for fully crystallized samples at roomtemperature are presented in Fig. 5. The PEO spherulites consistingof lamellae radiating from the spherulite center are visible both forB2E8 and for B5E5 illuminated with polarized light [33]. For B2E8the PEO spherulites of regular shape with well defined polygonalboundaries are clearly discernible. For the B5E5 sample thespherulites have anisotropic shape with jagged edges. They remainrelatively isolated and are much smaller than the ones occurring inB2E8 system, even for a fully crystallized sample. The estimatedsize of the spherulites in the B2E8 sample is about 200 mm, whilethe largest ones observed in B5E5 are almost three times smaller.The existence of white small dots in the micrograph of B8E2 mayindicate the initial form of spherulites, but the spheruliticmorphology was not observed at all for this material.

3.4. FTIR

To characterize the phase structure of the studied miktoarm starpolymers, their melting, and crystallization processes the infraredspectroscopy is a very useful method [34]. The wave number of thespecific band is sensitive to the H-bond strength, which, in turn, isresponsive to the environment. This feature gives opportunity todistinguish between different conformations and molecularsurroundings of a polymer chain, which is characteristic of anindividual phase state of polymer. Thermodynamically stable forPEO homopolymer is 72 helical conformation, containing sevenchemical units (CH2eCH2eO) turns twice in the identity period of19.3 Å, in monoclinic crystal form [35]. This type of structure alsoforms PEO arms in the triarm star block copolymer of polystyrene,poly(ethylene oxide), and poly( 3-caprolactone) as reported byFloudas et al. [36].

Analysis of the FTIR spectrum of the investigated linear PEO(shown in Fig. 6) reveals the presence of crystalline-sensitive bands,which, according to the calculations of normal vibrations of PEOchain having 72 helix conformation made by Yoshihara et al. [35],can be assigned to either the A2 (transition moment parallel to thesymmetry axis) or E (transition moment perpendicular to thehelical axis) symmetry species (see Table 4). However, it is worthmentioning, that some of the crystalline-sensitive bands are over-lapped by the bands associated with the molecular vibrations in anamorphous phase. The bands characteristic of the crystalline phaseof PEO homopolymer have also been found for all miktoarm starpolymers, though crystalline-sensitive bands are barely visible forcopolymer B8E2 of the lowest PEO content. In addition to thesebands, which are characteristic of PEO polymer, the bands typical ofPBA [37] at: 1455 cm�1, 1380 cm�1, 1245 cm�1 and 1165 cm�1 alsoappear (see Fig. 6.). However, some of them are overlapped withPEO bands (e.g. 1245 cm�1, 1165 cm�1), but they are clearly visibleat high PBA content.

The observation of the evolution of IR spectrum providesinformation on the melting and crystallization processes of PEOsegments in the studied copolymers. The FTIR spectra in function oftemperature were recorded for PEO and for all miktoarm

copolymers in the temperature range from 123 K to 373 K. Theevolution of IR spectrumwith the temperature increase for B2E8 ispresented in Fig. 7. The melting of the material is disclosed in the IRspectrum as a shift and/or broadening of the crystalline-sensitivebands, which reflects the changes in the populations of trans and

Fig. 7. Temperature dependence of FTIR spectra for the B2E8 in the range from 123 Kto 373 K.

70080090010001100120013001400

B8E2

B5E5

10611152

1343

1279

1242

966 844

PEO

B2E8

Wavenumber [cm-1

]

Fig. 6. FTIR spectra of the linear PEO and miktoarm copolymers: B2E8, B5E5, B8E2recorded at 143 K. The arrows indicate the bands characteristic of crystalline PEO(graphs are shifted vertically for better visualization).

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e55205518

gauche conformers [38]. In the IR spectrum of PEO (data not pre-sented) it was found that the transition from the crystalline state tothe melt does not change the positions of bands assigned to thevibrations in the amorphous phase. The same behavior was foundfor all studied star polymers.

The isothermal FTIR measurements have been performed tostudy the PEO crystallization behavior of miktoarm star polymerwith the highest PEO content (B2E8). The studies have been per-formed for a sample which was initially warmed up above itsmelting temperature, and subsequently quenched to liquidnitrogen temperature and finally heated to the temperature of253 K (lower than the crystallization temperature Tc determined

Table 4Assignments of Infrared Active Bands of PEO in the Amorphous Phase (A) and in theCrystalline Phase (C). The observed wavenumbers are determined from differentialspectra of B2E8 miktoarm copolymer. Literature values were taken from Yoshiharaet al. [35]* and Dissanayake et al. [39]**.

Assignmenta Polarizationb Wavenumbers(cm�1)

Change duringcrystallization

Literaturevalues

Observed

C A

ws(CH2) þ n(CC) t 1358 1360 [

1350 1355 Y

was(CH2) ǁ 1342 1341 [

1325 1304 Y

tas(CH2) þ ts(CH2) t 1278 1279 [

1250 1254 Y

tas(CH2) ǁ 1240 1241 [

n(CC) � nas(COC) t 1147 1151 [

1142 1134 Y

ns (COC) þ nas (COC) t 1116 1118 [

1110 1090 Y

nas(COC) þ rs(CH2) 1062 1078 Y

ras(CH2) ǁ 958 964 [

948 953 Y

ras(CH2) t 844 842 [

*Reprinted with permission. Copyright (1964) American Institute of Physics.**Reprinted with permission. Copyright (1995) American Chemical Society.

a ne stretching, te twisting, w- wagging, re rocking, the suffixes s and as denotethe symmetric and antisymmetric vibrational modes, respectively.

b symbols t and ǁ direction of transition dipole moments perpendicular andparallel to the helical axis of PEO, respectively.

from DSC measurements), at which the in-situ spectra wererecorded every 5 min. The differential IR spectra were obtained tobetter distinguish the changes in the spectrum due to crystalliza-tion. The analysis of the spectrum evolution shows an increase in IRbands, which can be assigned to molecular vibrations in the PEOcrystalline phase, at the expense of bands corresponding to those inthe PEO amorphous phase. The assignment of the IR absorptionbands for PEO, summarized in Table 4, is consistent with that re-ported by Dissanayake et al. [39], who have studied phase transi-tions in PEO and its salts by use of IR spectroscopy.

The analysis of crystallization behavior was made for theselected crystalline-sensitive bands i.e. 1343 cm�1, 1279 cm�1,964 cm�1 and 843 cm�1, after the normalization of their intensities.The time dependence of crystallized fraction of polymer x(t) isdescribed by classic Avrami equation, which may be expressed inthe following form [18]:

xðtÞ ¼ 1� expð�ktnÞ; (3)

where k is the overall relative transformation rate constant and n isthe Avrami exponent characteristic of the crystal growth geometryand nucleation mechanism. Parameter n represents the number ofthe growth dimensionality of the crystal, with typical values of 1, 2and 3 in the case of instantaneous nucleation (predeterminednumber of nuclei) for one-, two- and three-dimensional growth,respectively. When a sporadic nucleation occurs (number of nucleiincreasing linearly with time), then Avrami index n is greater by 1for all crystal geometries mentioned above (taking the value of 2, 3and 4, respectively). However, also non-integer values of parametern are observed experimentally when the nucleation is somewherein between sporadic and instantaneous [18].

Time dependence of intensity of selected IR bands for theinvestigated sample (shown in Fig. 8b) at the crystallizationtemperature of 253 K was fitted according to the followingequation:

lnð � lnð1� xðtÞÞÞ ¼ nlnðtÞ þ lnðkÞ (4)

to obtain the Avrami parameter n. The slope n obtained for the B2E8copolymer was close to 0.8 for each of the studied bands. In theliterature one finds a wide range of Avrami index describing crys-tallization kinetics of PEO, taking the values from 1 to 3.2

1400 1200 1000 800

Ab

so

rb

an

ce [

a.u

.]

Wavenumber [cm-1

]

00101

10-2

10-1

1340 1280 966 840

inte

nsity

[a.u

.]

time [min]

a

b

Fig. 8. Crystallization kinetics of miktoarm copolymer B2E8 as seen by isothermal FTIRat 253 K; (a) differential IR spectra with the absorption bands of crystalline PEO, (b)Avrami plot, representing evolution of peak height as a function of annealing time. Thesolid line represent the best fit to the experimental data.

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e5520 5519

[23,27,35,36,40e44]. Avrami index value lower than 1 (n ¼ 0.5) asa result of homogenous nucleation for PEO in polyethylene oxide/polystyrene block copolymer at large supercoolings, has been re-ported by Lotz and Kovacs [45]. Also Mihut et al. [44] have foundAvrami index of value below 1 (n ¼ 0.7) for homogenous PEOcrystallization in poly(ethylene oxide)-block-polybutadienesystem, which was dissolved in n-heptane and then wasquenched and crystallized at the liquid nitrogen temperature. Itwas concluded that the observed value of Avrami index is related toconfined crystallization. Balsamo et al. [46] have found the value ofAvrami index of 0.5 for confined crystallization of polyethylenewithin semicrystalline triblock copolymers in contrast to the valuestypically fluctuating between 1.5 and 2.5 for polyethylene andethylene/a-olefin random copolymers. Moreover, it was noticedthat as the degree of PE confinement increases the Avrami indexdecreases. They claimed that Avrami index takes non-integervalues when polymer nucleation is not purely instantaneous orpurely sporadic but ranges between the two extremes. Followingthe idea of Frensch [18] this statementmay be explained as follows:when the number of droplets of crystallizable component,dispersed in matrix formed by the second component, is greaterthan the number of heterogeneities present in the system then thefractionated crystallization takes place. In such conditions there isa possibility of the homogenous nucleation i.e. formation ofhomogenous nuclei through spontaneous chain aggregation. When

the crystallization is isolated within individual microdomains itsrate is proportional to the fraction of material not yet crystallizedi.e. the crystal growth is essentially instantaneous and homoge-neous nucleation is the rate-determining step of the crystallization.

Taking into account the observed low value of Avrami exponent,reflecting rate-determining homogenous crystallization of PEOdispersed in PBA matrix for the studied miktoarm star polymers aswell as the fact that PEO/PBA blends and copolymers are known asstrongly segregated systems, it may be concluded that PEOmicrodomains in the nanoscopic scale are formed in the PBA/PEOmiktoarm star copolymers.

Theoretically, the influence of KBr on crystallization processshould be considered. We are, however, convinced that in thisstudy, the presence of KBr in the sample (prepared for FTIRexperiment as described earlier) does not influence the crystalli-zation kinetics due to macroscopically separated KBr/miktoarmpolymer system and because of the fact that our experiments wereperformed under supercooling conditions. The presence of KBrmayinfluence the crystallization process of PEO, according to Geng et al.[14]; however, for samples specifically prepared, namely with verysmall KBr grains (of 2 mm), well dispersed in PEO matrix andcrystallizing at relatively high temperatures. It should be empha-sized that our experimental conditions differ significantly fromthosementioned above. Moreover, the isolated nature of PEO phasein B2E8 (separated also from KBr) is confirmed by the low value ofthe Avrami index of 0.8, as determined from FTIR data. It is worthnoting that this value is close to the Avrami index characterizingcrystallization of PEO segments in PEO/PB diblock copolymer at lowtemperatures [44]. To verify the correctness of the Avrami indexdeterminationwe have carried out the dielectric measurements forneat B2E8 (unpublished data) which corroborate the discussedvalue.

4. Conclusion

The aim of our studies was to determine the effect of changes inthe composition of the arms in the PEO/PBA miktoarm star poly-mers on their structure and thermal properties. The DSCmeasurements have shown an increase of both the meltingtemperature and the value of heat of fusion due to increase in PEOcontent in the studiedmiktoarm star polymers. The estimated fromcalorimetric studies degree of crystallinity in the investigatedsamples is in accordance with that calculated on the basis of WAXSdata. However, it was found that there is a similar fraction ofethylene oxide groups forming the crystalline phase (in relation toall ethylene oxide groups) namely 47%, 52% and 44% for the samplesB2E8, B5E5 and B8E2, respectively. The observed melting hyster-esis, having the strongest effect for the sample with the lowest PEOcontent, may be interpreted as a result of confinement of PEOchains within a PBA matrix. Moreover, observed broad range ofmelting suggests the frustration of the crystallization process ofPEO chains due to the influence of amorphous poly(butyl acrylate)surroundings. The POM studies show the inhibition of spheruliticstructures formation as a result of an increase in PBA fraction in thematerials under study. Isothermal crystallization kinetics of mik-toarm star polymers was monitored by FTIR spectroscopy. The timedependence of an intensity of particular FTIR bandswas analyzed interms of the classical Avrami model for nucleation. The obtainedvalues of Avrami exponent suggest that crystallization processoccurs via homonucleation of PEO chains.

Acknowledgments

This work was supported by research grant No. N N202 128536(Poland) and NSF DMR 0969301 (USA). The authors gratefully

M. Makrocka-Rydzyk et al. / Polymer 52 (2011) 5513e55205520

acknowledge the support of Faculty Structural Research Laboratoryat Physics Department in DSC measurements.

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