Structure and Crystallization Behavior of Poly(ethylene oxide) (PEO

Article pubs.acs.org/Macromolecules

Structure and Crystallization Behavior of Poly(ethylene oxide) (PEO) Chains in Core−Shell Brush Copolymers with Poly(propylene oxide)block-poly(ethylene oxide) Side Chains Sotiria Kripotou,† Christina Psylla,‡ Konstantinos Kyriakos,‡ Konstantinos N. Raftopoulos,‡ Junpeng Zhao,∥ Guangzhao Zhang,∥ Stergios Pispas,§ Christine M. Papadakis,‡ and Apostolos Kyritsis*,† †

Physics Department, National Technical University of Athens, Iroon Polytechneiou 9, Zografou Campus, Athens 15780, Greece Physik-Department, Fachgebiet Physik weicher Materie, Technische Universität München, James-Franck-Str. 1, Garching 85748, Germany § Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Ave., Athens 11635, Greece ∥ Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, P. R. China ‡

S Supporting Information *

ABSTRACT: Core−shell brush copolymers featuring a poly(p-hydroxystyrene) (PHOS) backbone and PPO-b-PEO (PPO and PEO stand for poly(propylene oxide) and poly(ethylene oxide)) side chains with different molecular compositions and exhibiting two inverse molecular architectures in regard to the side chains were investigated. Differential scanning calorimetry (DSC) and temperature-resolved wide- and small-angle X-ray scattering (WAXS/SAXS) were used to characterize the thermal and structural behavior. For the sample with the crystallizable PEO block linked directly to the backbone and a high PEO fraction (84.9 wt %), our results reveal a PEO crystallization/melting behavior similar to the one of bulk PEO. Surprisingly, the crystalline order, as determined by WAXS, persists up to 30 K above the melting point determined by DSC (Tm = 54 °C). For the samples where the PPO block is directly linked to PHOS backbone and the PEO chains are dangling, our results indicate that the side arm architecture has remarkable effects on the thermal and structural behavior. With decreasing PEO fraction in the side arms, the calorimetric crystallization temperature, Tc, and the melting point, Tm, of the PEO domains are strongly suppressed, reaching values as low as −45 °C and −8 °C, respectively. Furthermore, PEO crystallizes in an asymmetric lamellar phase with a distorted PEO crystalline phase. Above Tm the morphology changes from microphase-separated symmetric lamellae to hexagonally perforated lamellae with PEO domains immersed within a PHOS/PPO matrix with decreasing PEO fraction. Our results suggest that this specific brush copolymer architecture allows for tuning the ability of PEO blocks to crystallize.

1. INTRODUCTION

increasing attention in recent years due to their unique core− shell nanoscopic molecular structures and their potential application in core−shell nanomaterials.2,3,20−22 Compared to block copolymers, more variables of polymer brushes, particularly the inherent brush chains structure grafted along the backbone and many tethered side chain ends, can affect both the molecular conformation and condensed morphology.5,23−29 Recently, we reported on the synthesis of core−shell brush copolymers with PPO-b-PEO side chains.3 Poly(p-hydroxystyrene) (PHOS) with narrow molecular weight distribution was utilized as the backbone polymer, and two different sequences of the monomer addition were conducted in order to achieve the two correspondingly inverse molecular architectures in regard to the side chains; i.e., either PPO or PEO was

Graft copolymers with densely grafted side chains (called brush copolymers or bottle-brush polymers) have gained significant academic interest due to their inherent properties related to the multibranched molecular structures.1−3 The architecture of polymer brushes can greatly vary, depending on a number of factors, such as the grafting density, the flexibility or stiffness of the side chains and the backbone, and the degree of complexity of the macromolecules which may be homopolymers or copolymers.4−6 A number of computer simulations have addressed the conformational behavior of bottle brushes in solution.7−11 Polymeric bottle brushes with two types of side chains12,13 and macromolecules being block copolymers consisting of a linear chain and a bottle brush have been synthesized and investigated.14 Experimental work has addressed bottle brushes in solution using light scattering and small-angle neutron scattering,15,16 in thin films,17,18 and also as additives in thin films.19 Of all brush copolymers, those consisting of diblock copolymer side chains have attracted © 2016 American Chemical Society

Received: April 27, 2016 Revised: July 18, 2016 Published: August 10, 2016 5963

DOI: 10.1021/acs.macromol.6b00879 Macromolecules 2016, 49, 5963−5977

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Macromolecules

crystallization behavior of PEO segments in brush copolymers has also been studied. The results indicate that the crystallization temperature, Tc, decreases, and the degree of supercooling depends strongly on the length of the side chains, with Tc ranging from slightly below room temperature to about 65 °C.20,23,25,36 In this work, we study novel core−shell brush copolymers featuring a poly(p-hydroxystyrene) (PHOS) main chain and PPO-b-PEO side chains with different molecular compositions and exhibiting two inverse molecular topologies with respect to the side chains.3 The grafting density is virtually 100%. The crystallization behavior of PEO blocks, regarding both dynamic crystallization temperatures and crystallization kinetics, was studied employing differential scanning calorimetry (DSC). Temperature-resolved WAXS and SAXS measurements provide information on the crystalline structure in the block copolymers side chains and on the nanodomain morphology due to microphase separation of the block copolymers.

linked directly to the backbone. The morphology/structure, thermal properties, and the parameters that affect microphase separation in block copolymers based on poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) have been studied extensively.30−35 Graft copolymers containing both PEO and PPO, however, have been reported scarcely regarding both their synthesis and physical properties. Inomata et al. studied the morphology and packing manner of graft copolymers featuring rigid-rod-like poly(γ-benzyl L-glutamate) (PBLG) main chains with grafted diblock copolymers of amorphous poly(propylene glycol) (PPG) and crystalline poly(ethylene glycol) (PEG).36 Small-angle X-ray scattering showed that the graft copolymers form repeated layers consisting of segregated PBLG, PPG, and PEG layers. Self-assembly in the melt of block copolymers is induced by microphase separation, creating ordered structures with periodicities on the micro- or nanometer scale. The microdomain structures are influenced by the composition and the segregation strength between the blocks. In addition, if one of the blocks is semicrystalline and the other amorphous, a competition between the driving force of crystallization and phase segregation will define the final morphology.37−40 It is generally accepted that the changes of state as a function of temperature can determine the final morphology according to three key transition temperatures: the order−disorder transition (ODT) temperature, TODT, the crystallization temperature, Tc, of the crystallizable block, and the glass-transition temperature, Tg, of the amorphous block. When a crystallizable polymer is confined to isolated phases (i.e., droplets in immiscible blends) or to the microdomain morphology of block copolymers (cylinders or spheres), a higher supercooling may be observed than that encountered when crystallizing the same polymer in the bulk. In differential scanning calorimetry (DSC) nonisothermal experiments the crystallization exothermic peak may occur within a broad temperature range: from a few degrees below the crystallization temperature for the bulk polymer down to temperatures close to the Tg of the polymer (maximum possible supercooling).39 PEO has been widely used as a crystallizable block copolymer component, and there is also a wide range of data on PEO droplets produced in a wide range of sizes, leading to a great diversity of literature reports on its crystallization temperature as a function of morphology,38,41−45 usually far below the PEO equilibrium melting temperature Tm = 76 °C.30 The dynamic crystallization of homogeneously nucleated PEO nanodroplets has been reported in the range from −30 to −45 °C (peak crystallization temperatures during cooling from the melt), i.e., quite close to Tg for small PEO microdomains,38 whereas for bulk PEO confined in self-ordered nanoporous aluminum oxide (AAO) with a pore diameter of 25 nm, the homogeneous nucleation crystallization temperature has been observed at a temperature as low as −38.8 °C.44 Interestingly, suppression of PEO crystallization has also been observed in very asymmetric PEO-b-poly(ε-caprolactone) (PCL) diblock copolymers, the crystallization of PEO being shifted to −14.6 °C, under the confinement of PEO chains imposed by PCL crystals.45 Strong PEO supercooling effects have usually been interpreted as manifestations of homogeneous nucleation process for PEO chains. It is well established, however, that the nucleation process can and should be studied with respect to both the crystallization temperature, Tc, and the features of isothermal crystallization kinetics. The latter are of first, or even lower, order in the case of homogeneous nucleation. 43 The

2. EXPERIMENTAL SECTION 2.1. Materials. Synthesis and characterization of the brush copolymers in this study have been described previously.3 The PHOS forming the backbone has a weight-average molecular weight (Mw) of 10 500 g/mol. The molecular characteristics of the side chains are given in Table 1. The Flory−Huggins segment−segment

Table 1. Molecular Characteristics of the Side Chains in the Brush Copolymers3 gEP1 gPE1 gPE2 gPE3

Mw,arm [g/mol]

wPEO [%]

Mw,PEOa [g/mol]

NEOb

Nc

16300 17500 13000 7400

84.9 67.5 40.8 23.4

13840 11810 5300 1730

314 268 120 39

377 412 314 182

a Mw,PEO = Mw,arm. × wPEO (weight-average molecular weight of PEO in each side chain). bNumber of EO monomer units per side chain, estimated by NEO = Mw,arm × wPEO/(44 g/mol). cDegree of polymerization used for calculation of χN. The same expression as in ref 32 was used for the calculation of N.

interaction parameter of PEO/PPO has been reported to be χ = 20.2/T + 0.0221.32 The core−shell brush copolymers are termed as gPE, when PO was polymerized first, so that the PPO segment is linked directly to the backbone, and gEP, when EO was polymerized first and the PEO segment is linked directly to the backbone (Scheme 1).

Scheme 1. Structural Formula of the Brush Copolymers Used in This Study

2.2. Methods. Differential Scanning Calorimetry. Thermal properties of the materials were investigated in the temperature range from −150 to 150 °C using a TA Q200 series DSC instrument with Tzero functionality. Helium was used as a purge gas (25 mL min−1). Indium was used for temperature and enthalpy calibration. Samples of ∼6 mg were sealed in standard Tzero aluminum pans. Typical cooling and heating rates were 10 °C/min. For fast cooling experiments, cooling rates of approximately 60 °C/min were achieved. 5964


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Macromolecules In conventional DSC experiments and in order to erase the thermal history of the samples, a first heating scan up to 150 °C was performed just prior to the cooling scan (thus, cooling started always from 150 °C). Small- and Wide-Angle X-ray Scattering (SAXS/WAXS). SAXS/ WAXS measurements were carried out using a Ganesha 300XL instrument (SAXSLAB ApS, Copenhagen/Denmark) equipped with a GENIX 3D microfocus Cu X-ray source operated at 50 kV/0.6 mA (λ = 1.542 Å), and optics, together with a three (scatterless)-slit collimation system. The samples of thickness ∼2 mm were mounted in a heatable/coolable Linkam cell between mica windows of thickness 5−7 μm. Both the sample chamber and the beam path were under vacuum. A two-dimensional Pilatus 300 K detector was used, which can be moved to the desired sample-to-detector distance (SDD), in the present case 101.4 mm (WAXS) and 401.4 mm (SAXS), resulting in a q-range of 0.16−25.5 nm−1. q = 4π sin(θ/2)/λ is the momentum transfer where θ is the scattering angle. A pin diode was used to measure the transmission of each sample. All images were corrected for cosmic background and parasitic scattering. The obtained 2D images were azimuthally averaged, and the background from the mica windows was subtracted. The samples were mounted at room temperature. They were heated to 80 °C to melt the PEO microdomains. After 10 min at 80 °C, they were cooled down to −80 °C at a rate of 10 °C/min. For measurements, the temperature was increased at a rate of 10 °C/ min in steps of 5 or 10 °C, and measurements were carried out after 5 min equilibration time. The measuring times at each temperature were between 600 and 1200 s. The peaks in the WAXS data were analyzed by fitting a sum of Lorentz functions: N

I(q) = I0 +

∑ i=1

2Ai wi π wi 2 + 4(q − qi)2

3. RESULTS AND DISCUSSION 3.1. Thermal Analysis Using DSC. In Figure 1, we show DSC thermograms obtained during cooling on all the systems

Figure 1. DSC cooling thermograms obtained at 10 °C/min from 150 to −150 °C for the samples indicated in the graph. The arrow indicates the weak exothermic peak observed for gPE2 at −45 °C.

(1)

I0 is a constant background, Ai the area, wi the width, and qi the position of the reflection i. The aim was to characterize the positions of the strong (120) and (032) reflections of the PEO lattice as well as to approximate the areas under the Bragg reflections and under the amorphous halo. Moreover, the software simDiffraction46 was used to calculate X-ray diffractograms from PEO. At this, the known lattice parameters and angles of PEO and the atomic positions in the unit cell were applied: a = 8.05 Å, b = 13.04 Å, c = 19.48 Å, α = 90°, β = 125.4°, and γ = 90° were used together with the atomic coordinates of the C and the O atoms of PEO.47 A peak width of 0.03 Å−1 was assumed, and the Lorentz correction for powders was applied. For samples gPE1 and gPE2, a and b were varied to match the measured peak positions. The SAXS data were fitted by the expression I(q) = I0 +

⎛ ⎛ q ⎞2 ⎞ A exp⎜ − 2⎜ ⎟ ⎟ + w π /2 ⎝ ⎝w⎠ ⎠

N

∑ i=1

Figure 2. DSC heating thermograms obtained at 10 °C/min from −150 to 150 °C for the samples indicated in the graph.

under investigation, following a conventional thermal protocol, i.e., cooling from 150 °C down to −150 °C. We observe that for gEP1 the crystallization peak appears at 36 °C, exhibiting a supercooling of ΔT = 40 K (assuming that Tm° for unconstrained bulk PEO is 76 °C),30,49 an effect usually observed when PEO crystallizes under various constraints.30,32,45,50 Moving to the gPE type of core−shell brush copolymers, we observe remarkably stronger supercooling effects. Indeed, gPE1 (67.5 wt % PEO) crystallizes at Tc = −27 °C, while gPE2 (40.8 wt % PEO) exhibits a weak exothermic peak at Tc = −45 °C. The gPE3 sample, with only 23.4 wt % PEO in the side chains, does not show any crystallization events during cooling. Figure 2 shows the subsequent heating DSC curves. We observe that the gEP1 sample melts in the temperature region where usually bulk PEO melts (Tm = 54 °C for gEP1). From the endothermic melting peak, we estimate the degree of PEO crystallinity, taking into account that the heat fusion for bulk PEO is ΔHm = 197 J/g.51 All estimated values are listed in Table 2. For gEP1, a degree of crystallinity of about 60% has been estimated. (This is the crystalline fraction of PEO; thus, the sample consists of 51% crystalline PEO and 34% amorphous PEO with respect to the total mass.) The most striking result in Figure 2 is the remarkable reduction of melting temperatures for the gPE-type brush copolymers. More

2Ai wi π wi 2 + 4(q − qi)2 (2)

where I0 is a constant background. The second term is a Gaussian centered at q = 0 having the area A and the width w and describes a decaying background. The Lorentzians having the areas Ai, the widths wi, and the positions qi describe the Bragg reflections. The repeat distance was calculated from the peak position q1 using Bragg’s law:

d=

2π q1

(3)

SAXS data from sample gEP1 (where a clear second-order Bragg reflection is present) were additionally modeled using the software Scatter.48 A Lorentzian peak shape was assumed. The model includes, among others, the lamellar thickness, the disk height, which is equivalent to the thickness of one part of the lamellae, the disk radius, and the domain size along the lamellar normal. 5965


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Macromolecules Table 2. Crystallization/Melting Temperatures, Corresponding Enthalpies, and Degree of PEO Crystallinity sample

wPEO [%]

cooling Tc [°C]

cooling ΔHc [J/g]

gEP1 gPE1 gPE2 gPE3

84.9 67.5 40.8 23.4

36 −27 −45

104 41 3

a

heating Tcc [°C]

−51

heating ΔHcc [J/g]

heating Tm [°C]

heating ΔHm [J/g]

XcPEOa [%]

15

54 11 −8

101 44 19

60 33 24

With respect to the PEO fraction in the copolymers.

Table 3. Crystallization and Melting Temperatures and Estimated Thicknesses of Lamellae in the Brush Copolymers Where PEO Crystallizes as Well as the Estimated Values for the Thickening Factor b crystallization

gEP1 gPE1 gPE2 a

melting

Tc [°C]

ΔT [K]

Lc* [nm]

Tm [°C]

ΔT [K]

Lc [nm]

ba

36 −27 −45

40 103 121

4.7 1.8 1.5

54 11 −8

22 65 84

8.4 2.9 2.2

1.8 1.5 1.5

b = Lc/Lc*.

Table 4. Glass Transition Temperatures and Δcp during Heating and Coolinga heating sample

wPPO [%]

Tg [°C] ± 1 °C

gEP1 gPE1 gPE2 gPE3

15.1 32.5 59.2 76.6

−66 −71 −74 −75

cooling

Δcp [J/(g K)]

Tg [°C] ± 1 °C

± ± ± ±

−72 −73 −74 −75

0.25 0.45 0.84 0.67

Figure 4. DSC thermograms of gPE2 during heating from −150 up to −35 °C (solid black line) and up to −40 °C (dashed red line). The heating was interrupted, and the sample was cooled down to −150 °C at a rate of 10 °C/min. The second heating scan up to 20 °C for each experiment is also shown (solid black line for the sample heated previously up to −40 °C; dashed red line for the sample heated up to −35 °C).

0.01 0.02 0.05 0.01

Δcp [J/(g K)] 0.26 0.40 0.66 0.60

± ± ± ±

0.03 0.03 0.05 0.01

specifically, gPE1 melts at 11 °C, exhibiting a degree of crystallinity of about 33%, whereas gPE2 shows the endothermic peak related to PEO melting at even lower temperatures, namely at Tm = −8 °C. Interestingly, the gPE2 curve shows a very pronounced cold crystallization peak at Tcc = −51 °C. For gPE2, a degree of crystallinity (actually crystallized via cold crystallization at −50 °C) about 24% was estimated from the melting peak. In contrast, gPE3 shows neither crystallization nor melting events during heating. In order to check whether slower cooling rate would provide the adequate conditions for gPE3 to crystallize, we performed DSC experiment with a cooling rate of 1 °C/min (10 times slower than the conventional one). The obtained thermograms are shown in Figure SI-1 of the Supporting Information and reveal no crystallization effects under this slow cooling rate either. Therefore, according to our DSC results, PEO chains with molar mass of 1730 g/mol in the periphery of the brush macromolecules are not able to crystallize. 3.1.1. Crystallization Behavior of PEO Chains. The strong suppression of crystallization and melting temperatures observed in some of our brush copolymers can be related to the thickness of the initially formed crystals at Tc and/or to the thickness of the crystals which melt at Tm (due to thickening effects the two thickness values may be (and usually are) different). More specifically, the melting temperatures are converted to lamellar thicknesses, Lc, by the Gibbs−Thomson equation:52

For heating scans the fictive temperature method has been applied, whereas for cooling scans the half-extrapolated tangents method was used. a

Figure 3. Heat capacity increment at glass transition, Δcp, as a function of PPO weight fraction in the side chains of the brush copolymers. Experimental values are represented by the red squares while the black circles represent values calculated by assuming fully contribution of the amorphous parts of PPO and PEO chains according to their weight fraction and the corresponding values of pure components (eq 7). The lines assume contributions from solely PPO (eq 5) and from the total PEO and PPO components (eq 6). The triangles (in rectangular) represent calculated (black) and experimental (red) values of Δcp related to DSC experiments on sample gPE2 with specific thermal histories (full red triangle: after annealing; open red triangle: after cold crystallization). For details see the text in section 3.1.3.

Lc =

2σeTm° ΔT ΔH °ρc

(4)

where Tm° is the equilibrium melting temperature, σe the surface free energy of crystalline lamellae in the final stabilized 5966


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Macromolecules Table 5. Values of Δcp and Tg from the Endothermic Step at the Glass Transition Temperature Region of Heating Thermograms of the Brush Copolymer gPE2 Exposed to Various Thermal Histories prior thermal history of the specimen

Tg [°C]

Δcp [J/(g K)]

typical cooling from 40 to −150 °C at 10 °C/min fast cooling from 40 to −150 °C (60 °C/min) typical cooling from 150 to −150 °C at 10 °C/min cold crystallization and heating up to −40 °C and then cooling with 10 °C/min down to −150 °C cold crystallization and heating up to −35 °C, then fast cooling down to −150 °C fast cooling from 40 to −55 °C, annealing for 30 min, and cooling to −150 °C with 10 °C/min fast cooling from 40 to −50 °C, annealing for 20 min, and cooling to −150 °C with 10 °C/min

−75 −76 −74 −74 −73 −74 −73

0.86 0.89 0.84 0.52 0.54 0.60 0.61

The data in Table 3 reveal that in the gPE1 and gPE2 brush copolymers the PEO blocks crystallize at remarkably low temperatures, forming very thin crystalline lamellae. Summarizing, our conventional DSC measurements reveal that in gEP1 brush copolymer the PEO blocks, which are directly linked to PHOS backbone, crystallize like PEO bulk chains do. On the contrary, gPE1 and, mainly, gPE2 show particular PEO crystallization behavior (strong supercooling effects, cold crystallization effects, shift of the melting temperature to very low values, very thin “virgin” and final crystals, as well). Therefore, DSC results suggest that PEO crystallization in the gPE brush copolymers is hindered, probably by frustration in packaging of PEO segments due to their particular architecture. In order to get insight into the particular crystallization process in those brush copolymers, and focusing on the behavior of the gPE2 sample, we performed additional DSC experiments which will be discussed in section 3.1.3. 3.1.2. Glass Transition of the Copolymers. The endothermic step at the low temperature range of the thermograms in Figure 2 is related to the glass transition of the brush copolymers. The estimated glass transition temperature, Tg, and the heat capacity increment at the transition, Δcp, are listed in Table 4. Aging effects contribute to the heating curves of gPE3 and gPE2 and are visible as an overshooting on the glass transition step. Therefore, the glass transition temperature, Tg, was determined by the fictive temperature method in Universal Analysis 2000 software (TA Instruments) for all the brush copolymers. For reasons of comparison, we estimated Tg also from the cooling scans shown in Figure 1 by employing the half-extrapolated tangents method. We can see the rather good agreement among the values estimated from the heating and cooling scans. Regarding the dependence of Tg on the composition of the side chains, we observe that for the gPE series Tg decreases with increasing PPO fraction in the side chains, approaching the value of Tg for bulk PPO, −75 °C.51 Concerning sample gEP1, we observe in Figure 2 a quite broad endothermic step at Tg, the glass transition breadth being ΔT = 13 K, while for the gPE series, the width does not exceed the 3−5 K (depending slightly on the degree of crystallization). For gEP1, a value of Tg = −66 °C can be estimated. This value is slightly lower than the expected one for the PEO amorphous phase (−60 to −50 °C38,39,51). In addition, the measured Δcp value (0.24 J/(g K)) normalized to the amorphous PEO fraction in the sample at Tg, i.e., 40%, Δcp,norm = 0.60 J/(g K)) is rather low compared to the value of 0.97 J/(g K) that has been suggested for fully amorphous PEO.51 This result may be explained by invoking the concept of a rigid amorphous fraction (RAF) of PEO in the vicinity of the crystals53 that does not participate in the glass transition. Regarding, however, the measured heat capacity increment, Δcp, at the glass transition for the copolymers, the picture becomes more complex if we

Figure 5. Heat flow as a function of time for gPE2, isothermally crystallized at −55 and −50 °C. The inset shows the relative volume fraction of crystalline material as a function of time, as calculated from the integration of the peaks shown in the main graph.

Figure 6. Calculation of Avrami equation parameters for gPE2, crystallized at −55 and −50 °C. In the main graph, the results from the linearization of the Avrami equation (red lines) and, in the inset, the results from fitting of the Avrami equation to the data of vc as a function of time (red lines) are shown.

state, ΔH° the specific bulk heat of fusion, and ρc the density of crystalline phase. ΔT = Tm° − Tm quantifies the suppression of melting temperature. The following values were applied for a stabilized PEO crystal: Tm° = 349 K, σe = 0.065 J/m2, ΔH° = 197 J/g, and ρc = 1.239 g/cm3.30,31,44 The estimated values of Lc are listed in Table 3. Equation 4 is usually employed also for the estimation of the thickness, Lc*, of the initially formed crystals during cooling, their thickness being then controlled by the degree of supercooling, ΔT = Tm° − Tc. The estimated values of Lc* are also listed in Table 3 together with the values of the factor b, which relates the final thickness of the crystal with the “virgin” crystal thickness, Lc*, Lc = bLc*, quantifying, thus, the thickening effect.52 5967


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Figure 7. (a) Representative SAXS curves of gEP1. The temperatures are indicated in the graph. The vertical arrows indicate the peak position expected at 2q1 at −30 °C. (b−d) Resulting parameters from model fitting in dependence on temperature. The vertical dashed line indicates Tm from DSC and the dash-dotted line the ODT temperature from SAXS. Different symbols indicate different runs. Slightly different values may be due to small differences in the thermal history. (b) Repeat distance; (c) ratio of peak positions q2/q1. (d) Width of the first-order reflection.

to glass transition. The line has been calculated using the equation

take into consideration that an additional contribution of the (amorphous) PPO chains to the measured Δcp may exist. In order to reveal the possible distinct contributions of PEO and PPO chains to the measured Δcp for each copolymer, we plot the temperature derivative of the heat flow as a function of temperature for each heating scan. In Figure SI-2 of the Supporting Information, we show such plots on the basis of the data presented in Figure 2, and it is readily observed that only for sample gEP1 the peak has a double structure: a main peak around −69 °C and a shoulder around −60 °C. This could indicate the existence of two amorphous phasesa PPO rich phase (−69 °C) and an amorphous PEO phase (−60 °C) within a semicrystalline PEO structure. For the gPE brush copolymers, it is obvious that the steps are narrower, and the main contribution comes from the PPO rich domains. For gPE3, the PPO dynamics dominates, shifting the peak to the lowest temperature, Tmax = −75 °C, whereas only for the gPE1 curve (the copolymer of the gPE architecture with the highest content of PEO), we can observe traces of contribution at temperatures higher than −70 °C. It is interesting at this point to comment on the features of the glass transition in the brush copolymers. As has been discussed previously, our measurements reveal a single endothermic step for each brush copolymer, its increment, Δcp, and Tg, however, not being dictated by a single mixing rule. In Figure 3, we show the measured Δcp values (red squares) as a function of the weight fraction of PPO in the side chains of the brush copolymers. In the plot, we also include two lines representing two extreme situations: The line with the positive slope in the lower part of the figure represents the expected dependence of Δcp for the case that only PPO chains contribute

Δcp = wPPO × 0.552 [J/(g K)]

(5)

ΔcpPPO

51

where we considered equal to 0.552 J/(g K). The line with the negative slope in the upper part of the figure has been calculated assuming that PPO and PEO (total mass) contribute analogous to their weight fraction and that ΔcpPPO is equal to 0.552 J/(g K) and ΔcpPEO equals 0.973 J/(g K):51 Δcp = wPPO × 0.552 + wPEO × 0.973 [J/(g K)]

(6)

Furthermore, in Figure 3, we represent with black circles Δcp values which have been calculated assuming that PPO and amorphous fraction of PEO contribute fully to the total Δcp according to the equation Δcp = wPPO × 0.552 + (1 − XcPEO) × wPEO × 0.973 [J/(g K)] (7)

where XcPEO represents the crystalline fraction of the PEO phase (in the temperature region of the glass transition). We observe that for high PPO fractions, i.e., for gPE2 and gPE3 samples with practically no crystalline PEO fraction, the experimental data are in rather good agreement with the values calculated by assuming that both amorphous blocks contribute to the endothermic step. (We note here that for sample gPE2 a crystallinity of PEO at Tg of about 3% has been taken into account, on the basis of a small exothermic crystallization peak recorded during cooling (Figure 1). The value of 24% for the crystallinity of gPE2 shown in Table 2 has been estimated by the PEO melting peak after the cold crystallization of PEO 5968


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the pronounced cold crystallization during subsequent heating implies a close relationship between the spatiotemporal heterogeneities associated with the liquid-to-glass temperature and the nucleation/crystallization process. Thus, the study of the crystallization behavior of gPE2 offers an ideal opportunity to investigate in more detail the effects of PEO crystallization on the glass transition of the amorphous part of the brush copolymer. In Figure 4, we show heating thermograms obtained on copolymer gPE2, grouped in pairs and following distinct protocols: The samples were cooled from 40 °C down to −150 °C with a cooling rate of 10 °C/min (no crystallization traces were detected in the cooling scans) and were then subsequently heated with a rate of 10 °C/min. The two recorded heating thermograms are shown in Figure 4, revealing the large endothermic step of the glass transition together with overshooting features, being followed by a pronounced cold crystallization peak at each scan. The heating process is interrupted at Tx = −35 and −40 °C, respectively, in two independent experiments, in order to avoid the melting of PEO crystals that have been formed during the whole process (initial cooling and heating up to Tx). The specimen was then cooled down to −150 °C at 10 °C/min, and a new curve, second of each experiment, is recorded during the second heating scan, which now runs from −150 until 20 °C and is shown in Figure 4 as well. Obviously, in the second scans there are no cold crystallization effects, indicating that the PEO crystallization process has been completed during the previous stage. Each of the second thermogram includes the endothermic step, with no overshooting effects, related to the glass transition, now within a semicrystalline structure for the copolymers, and two endothermic peaks related to the PEO melting at elevated temperatures. Concerning the melting behavior of PEO, each heating thermogram shows, apart from the main melting peak at −8 °C, a new weak endothermic peak with Tm located about 5 K higher than the Tx value of the previous stage of each experiment. We note here that endothermic peaks with features similar to those of the previously mentioned weak peaks appeared also in the heating scans obtained after annealing of gPE2 at −55 °C for 30 min and at −50 °C for 20 min (Supporting Information, Figure SI-3). These endothermic peaks may be considered as melting endotherms appearing just above the annealing temperature (commonly called “annealing peaks”).54 Consequently, we attribute their origin to less ordered crystals which are formed during the time interval when the specimen remains in the temperature region close to Tx. The remarkable sensitivity of the features of this peak, e.g. peak temperature and height, on Tx implies the high instability of PEO crystals in the brush copolymer gPE2 in the temperature region below Tm = −8 °C.54 On the other hand, the main melting peak at −8 °C is practically unaffected, indicating that allowing the PEO chains to crystallize for longer times than in the conventional DSC experiments (e.g., during annealing at Tc) has no impact on the melting behavior of sample gPE2. Similar annealing experiments performed on the sample gPE1 (not shown here) lead to the same conclusions. Therefore, our data indicate that the strong suppression of crystallization/melting temperatures of samples of gPE series originates in existing structural constraints. Concerning the glass transition endothermic steps, the analysis of the recorded curves provides information with respect to both Δcp and the glass transition temperature Tg. The estimated values are listed in Table 5 together with the corresponding values estimated from DSC experiments with

Figure 8. (a) Representative WAXS curves of gEP1. The temperatures are indicated in the figure. The inset shows shifted WAXS curves at (from above) 50, 60, 80, and 90 °C. (b) Peak positions from model fitting in dependence on temperature. (c) Area fraction of the (120) and (032) Bragg reflections with respect to the total area. In (b) and (c), different symbols indicate different runs, and the vertical dashed lines indicate Tm from DSC and the dash-dotted lines TODT (see text).

during heating above Tg.) This is a quite interesting result since it suggests that for the copolymers with high PPO fraction and no crystalline PEO domains PEO segments contribute to the glass transition with all their configurational modes, even at temperatures 20−30 deg lower than the Tg of bulk PEO (i.e., amorphous PEO chains in the copolymers exhibit accelerated segmental mobility with respect to the one in semicrystalline bulk PEO). For the copolymers with low PPO fractions, the measured Δcp values are smaller than the calculated ones, being however always higher than the values calculated by assuming solely contribution of PPO chains. We conclude, thus, that a small fraction of amorphous PEO chains (being constrained among PEO crystallites) contribute also to the low-Tg glass transition of these copolymers. Our results suggest that for these compositions the RAF may represent a significant fraction of PEO chains. 3.1.3. Crystallization Effects in the Glass Transition of gPE2 Sample. The very weak crystallization of PEO chains during cooling in the gPE2 brush copolymers combined with 5969


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Figure 9. (a) Representative SAXS curves of gPE1. The temperatures are indicated in the graph. The vertical arrows indicate the peak positions expected at 2q1 and at 3q1 at −70 and 100 °C. (b−d) Resulting parameters from model fitting in dependence on temperature. The vertical dashed line indicates Tm from DSC; see above. (b) Repeat distance. (c) Ratio of peak positions; closed symbols: q2/q1; open symbols: q3/q1. (d) Width of the first-order reflection.

triangle represents the expected value for Δcp, as is calculated by eq 7, whereas the red triangles represent the experimental values after the annealing process (full symbol) and after the cold crystallization process (open symbol). These findings imply that the reorganization of amorphous chains and their kinetics related with the crystal growth process may have some distinct differences depending on the crystallization procedure (crystallization during cooling or via cold crystallization process, i.e., crystal growth on preformed nuclei). 3.1.4. Crystallization Kinetics of PEO Blocks in gPE2. In order to study further the particular crystallization behavior of PEO in the brush copolymer gPE2, we performed isothermal crystallization experiments at two slightly different temperatures, namely −50 and −55 °C, which lie in the temperature region of the cold crystallization peak. These experiments allow for the study of the crystallization kinetics at these two temperatures. In Figure 5, the time evolution of the heat flow during the isothermal crystallization at −50 and −55 °C is shown. It is seen that PEO crystallizes faster at the higher temperature, while the crystallization enthalpy is slightly higher at −50 °C than at −55 °C. More specifically, the time at which the degree of crystallization reaches 50%, t1/2, is 4 and 6 min, and the estimated enthalpy is 16 and 14 J/g, respectively. We analyze these data by employing the Avrami formalism.52 Thus, assuming a constant nucleation rate and constant linear growth, the overall isothermal crystallization kinetics can be described by

different thermal protocols: typical cooling rate of 10 °C/min or fast cooling (approximately 60 °C/min), different starting temperatures for the cooling scans, and heating scans obtained after special thermal treatments indicated in the table. Several comments can be made. First, the brush copolymer gPE2 shows marginal crystallization events during cooling, independently from the cooling rate (10 °C/min or about 60 °C/min) and from the initial temperature (40 or 150 °C, much higher than any critical temperature of the copolymer). This fact is reflected also in the values of Δcp and Tg, as estimated from the subsequent heating scans, which are similar in all these cases (around the values of Δcp = 0.86 J/(g K) and Tg = −75 °C). In addition, the appearance of the cold crystallization peak in the heating scans suggests that active nuclei and probably small crystals are formed during cooling which can now be developed into crystals that melt around −8 °C. Second, the partial crystallization of PEO and the semicrystalline structure, formed by the application of two different thermal treatments, i.e., crystallization during cooling or via cold crystallization, affect the glass transition by reducing, mainly, the endothermic step, Δcp, while the glass transition temperature Tg is slightly increased (1 or 2 K). Furthermore, the data in Table 5 suggest that the decrease in Δcp is slightly stronger after the cold crystallization process as compared to the reduction in Δcp after crystallization during the annealing process (experiment described in the Supporting Information). Taking average values for the enthalpy of crystallization, ΔHc, and the heat fusion for bulk PEO equal to 197 J/g, we estimate a similar degree of crystallinity, XcPEO = 18%, for the two different procedures. (We note that in conventional DSC experiments a degree of crystallinity XcPEO ∼ 24% is achieved (Table 2).) The experimental values for Δcp obtained by performing these two different experiments together with the expected one are included in Figure 3 (represented by triangles). The black

vc = 1 − ekt

n

(8)

where n is the Avrami index, a parameter that indicates the dimensionality of growth and the nucleation type (taking the value of 1, 2, or 3 for athermal nucleation in 1, 2, or 3 dimensions, respectively, increased by 1 in the case of thermal nucleation process), k is the overall relative transformation rate 5970


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3.2. Structural Analysis Using SAXS/WAXS. In this section, we present the temperature-dependent mesoscopic structures obtained by SAXS and the crystalline structure obtained simultaneously by WAXS for each sample. In SAXS/WAXS, the thermal history of the samples was chosen to be similar to the one in conventional DSC measurements: Before the measurements, the sample was heated to 80 °C and afterward cooled to −80 °C with a rate of 10 °C/min. Then, the temperature was increased with a rate of 10 °C/min in steps of 5 or 10 K. After each step, measurements were conducted after an equilibration time of 5 min. Considering that the measuring time at each temperature was between 10 and 20 min, we assume that finally SAXS/WAXS measurements were performed on thermally equilibrated samples. The measurement at the lowest temperature of each run was discarded because it may be not in equilibrium yet. 3.2.1. Sample gEP1. The sample gEP1 has a high fraction of PEO (weight fraction 84.9%), with PEO being the inner block tethered to the backbone. As shown by DSC (see above), the glass transition of gEP1 is −66 °C (Table 4) and the melting point of PEO blocks 54 °C (Table 3). The sample was studied between −70 and 110 °C in two runs, covering a χN range of 37.4 to 19.9 (see Experimental Section). The fully extended length of the crystalline PEO block as calculated from its molar mass, the degree of crystallinity (Table 2), and the PEO crystal structure47 amounts to 52.43 nm. Between −80 and 50 °C, the SAXS curves feature a strong diffraction peak at ∼0.31−0.33 nm−1, which increases in intensity with temperature (Figure 7a). A second-order reflection is present at (2.0−2.2) × q1 (Figure 7c), pointing to an asymmetric lamellar structure with a repeat distance 18.8−19.7 nm (Figure 7b). Between 60 and 80 °C, the peak moves to smaller q values (0.25 nm−1), and forward scattering appears, which may be due to large domains, possibly due to a restructuring. The second-order peak moves to values of 2.5q1; the morphology cannot be determined in this temperature range. At 90 °C and above, the peak becomes much weaker and broader (Figure 7d) and moves to much higher q values (0.42 nm−1). No higher order peak is observed. It seems that the samples becomes disordered at these temperatures; i.e., the order-to-disorder transition is located at TODT = 85 °C, which corresponds to χN = 21.2. Detailed modeling of the lamellar structure was attempted and is described in the Supporting Information (Figure SI-4). These results demonstrate that below Tm = 54 °C the lamellae have an overall thickness of 16.7−17.4 nm, in consistency with the values from fitting Lorentz functions (Figure 7b). They consist of a layer having a thickness of ∼6.8−8.1 nm (twice the disk height), which is consistent with the value of Lc of 8.4 nm found from the melting point using DSC (Table 3) and thus seems to be the crystalline layer. The two values for the thickness of the crystalline layer point to a high degre of folding of PEO, since its fully stretched length is 6−8 times higher. The thickness of the amorphous (PPO and amorphous PEO fraction) layer is the difference of the overall lamellar thickness and the thickness of the crystalline layer and amounts to 10.0− 11.3 nm in this temperature range. Up to Tm, the domain size along the film normal is 50−60 nm; i.e., only a few lamellae are correlated which may point to the structure formation by nucleation and growth. Between Tm and TODT, the domain size decreases to 25 nm, i.e., a value similar to the lamellar thickness which hampers a reliable determination of the lamellar thickness and the thickness of the crystalline layer in this

Figure 10. (a) Representative WAXS curves of gPE1. The temperatures are indicated in the graph. (b) Peak positions from model fitting in dependence on temperature. (c) Area fraction of the (120) and (032) Bragg reflections with respect to the total area. The vertical dashed lines in (b) and (c) indicate Tm from DSC (see above).

constant (its value includes contributions from both nucleation and growth), and vc is the relative volume crystalline fraction (its value is within the range from 0 to 1). In Figure 6, we present the linearization of the Avrami equation (eq 8), a stage of the analysis of the isothermal crystallization data which leads to an estimation of the exponent n. The analysis reveals that the Avrami exponent takes the value n = 3.1 for crystallization at Tc = −55 °C and n = 3.3 for Tc = −50 °C. These rather high values for the Avrami exponent n indicate that for the gPE brush copolymers the strong suppression of PEO crystallization is not related with a homogeneous nucleation process. These values suggest rather that the crystallization process consists of a heterogeneous nucleation process and crystal growth on the interfaces between PEO and PPO blocks. Additionally, the sigmoidal crystallization kinetics observed in Figures 5 and 6 points to breakout crystallization process, as has been observed also in diblocks and blends with PEO chains being the crystallizable component.55,56 5971


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Figure 11. (a) Representative SAXS curves of gPE2. The temperatures are indicated in the graph. The vertical arrows indicate the peak positions expected at 2q1 at −30 °C (navy) and at √3q1, √4q1, and √7q1 at 120 °C (red). (b−d) Resulting parameters from model fitting in dependence on temperature. The vertical dashed lines indicate Tm from DSC (see above). Different symbols indicate different runs. (b) Repeat distance. (c) Ratio of peak positions; closed symbols: q2/q1; open symbols: q3/q1. (d) Width of the first-order reflection.

3.2.2. Sample gPE1. The sample gPE1 has a lower fraction of PEO (weight fraction 67.5%) than gEP1, and PEO is the outer block. The glass transition of gPE1 is −73 °C (Table 4) and the melting point of PEO blocks 11 °C (Table 3). It was studied using SAXS and WAXS between −70 and 100 °C, which corresponds to a χN range of 40.9 to 22.3. The fully extended length of the crystalline PEO block as calculated from its molar mass, the degree of crystallinity (Table 2), and the PEO crystal structure47 amounts to 24.61 nm. The SAXS curves feature a strong diffraction peak which moves from 0.43 nm−1 at −70 °C to 0.38 nm−1 at 0 °C (Figure 9a). In this temperature range, the main peak has a shoulder, which is a weak second-order reflection, at ∼2q1 (Figure 9d). The morphology is thus probably lamellar with asymmetric lamellae. Between 0 and 20 °C, the main reflection becomes weaker and narrower; it moves to 0.52 nm−1 and further to 0.54 nm−1 at 100 °C. It has a higher-order reflection at ∼3q1; this points to the formation of symmetric lamellae, where the second-order reflection is extinct. The lamellar morphology is thus assumed both in the crystalline and the amorphous state of PEO, even though the volume fraction of PEO is far from 50%. The results from fitting a Gaussian background and two Lorentz functions describing the reflections (eq 2, the Gaussian contribution was only needed above −10 °C) are given in Figure 9b−d. The repeat distance increases from 14.6 nm at −70 °C to 16.3 nm at 0 °C. These low values point again to a high folding state of PEO. Between 0 and 20 °C, the repeat distance decreases from 16.3 to 12.1 nm, as expected for a transition from stretched PEO stems to a coil conformation in the amorphous state.57 The width of the first-order reflection also decreases between 0 and 20 °C (Figure 9d). This is consistent with a transition from a morphology, which is a result of crystallization, where the correlation between the layers is usually poor, and an amorphous microphase-separated

temperature range. In this temperature range, the effects of both melting of the PEO crystallites and microphase separation seem to compete. The changes between 50 and 60 °C reflect the melting of the PEO domains observed in DSC at Tm = 54 °C. In the WAXS measurements, which were carried out quasi simultaneously with the SAXS measurements, two strong Bragg reflections are observed up to Tm at 13.6 and 16.5 nm−1 (Figure 8a,b) which are close to the (120) and (032) reflections at 13.59 and 16.48 nm−1.47 (Modeling of a diffractogram using the PEO bulk lattice parameters is shown in Figure SI-5 of the Supporting Information.) Interestingly, the diffraction peaks do not vanish at Tm, but only become slightly weaker up to 80 °C (inset of Figure 8a). Only at 90 °C, the reflections become very weak, and at 100 °C, they vanish completely (inset of Figure 8a, Figure 8c). Thus, the WAXS experiments reveal that even 30 K above the melting point determined by DSC, crystalline order persists. Trying to study further this apparent contradiction, we performed polarizing optical microscopy (POM) measurements in order to investigate the superstructure of the crystalline PEO domains in sample gEP1. POM micrographs of gEP1 are shown in the Supporting Information. The data show that the spherulites disappear at 55 °C (Figure SI-6), in agreement with the DSC results. However, specific POM experiments (Figure SI-7 and discussion in Supporting Information) suggest that in the temperature range between 55 and 85 °C the crystalline structure is not completely destroyed, and unmolten crystals may serve as self-nuclei upon subsequent cooling. We speculate that the local crystalline order is maintained up to 80−85 °C due to the dense grafting of the PEO blocks to the PHOS backbone. Further experimental studies are needed for clarifying the origin of the particular melting behavior of gEP1. 5972


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Figure 13. Phase diagram showing the four bottle brush copolymers investigated using DSC, SAXS, and WAXS. Results are plotted into the experimental phase diagram for PI-b-PEO.60 The black full lines denote the phase transitions measured in PI-b-PEO. Red lines and symbols refer to the bottle brush copolymers: The vertical lines denote the χN ranges studied, the closed circles the melting points from DSC, and the open circle the order-to-disorder transition. Lc, Lam, HPL, and Dis stand for crystalline lamellae, amorphous lamellae, hexagonally perforated lamellae, and disordered, respectively. Results are plotted into the experimental phase diagram for PI-b-PEO.60 Adapted with permission from ref 60.

goes to 0 above −20 °C, which is again consistent with the Tm value from DSC. Thus, two regimes are discerned in gPE1: (i) up to 10 °C, a lamellar phase where PEO is crystalline with a slightly distorted PEO crystal structure and (ii) a microphase-separated, close to lamellar morphology with symmetric lamellae above. Melting of the PEO domains leads to a strong shrinkage of the repeat distance, which points to breakout crystallization. 3.2.3. Sample gPE2. Sample gPE2 has a relatively low fraction of PEO (weight fraction 40.8%), and PEO forms the outer block. As described in sections 3.1.1 and 3.1.2, the glass transition temperature of gPE2 is −74 °C (Table 4) and the melting point of PEO blocks −8 °C (Table 3). It was studied using SAXS and WAXS between −65 and 125 °C, which corresponds to a χN range of 30.5 to 15.9. The fully extended length of the crystalline PEO block as calculated from its molar mass, the degree of crystallinity (Table 2), and the PEO crystal structure47 amounts to 8.01 nm. The SAXS curves (Figure 11a) feature a strong diffraction peak which, up to −15 °C, is at ∼0.41−0.42 nm−1 and moves to 0.51 nm−1 as the temperature is increased to 125 °C. Up to −15 °C, the corresponding repeat distance is 15.0−15.2 nm (Figure 11b); above this temperature, it decreases rapidly to 14.2 nm at 0 °C and then linearly to 12.3 nm at 125 °C. The small decrease of the corresponding repeat distance, ∼0.8 nm, after the melting of PEO crystals is in agreement with the DSC results that indicate the formation of thin lamellae with thicknes about 2.2 nm (Table 3). Above −15 °C, the intensity of the first-order reflection decreases (Figure 11a). Up to −10 °C, a second-order peak is observed at ∼2q1 (Figure 11c), consistent with the lamellar morphology. Above this tempeature, a very weak second-order and a less weak third-order reflection are observed at ∼2q1 and ∼2.6 q1 (Figure 11c). These may be due to coexisting lamellar and hexagonal domains or the hexagonally perforated lamellar (HPL) structure. These would result in higher order reflections at positions √3q*1 ≈ 1.73q1*, √4q1* = 2q1*, and √7q1* ≈ 2.65q1*. The HPL morphology has previously been observed in diblock

Figure 12. (a) Representative WAXS curves of gPE2. The temperatures are indicated in the graph. (b) Peak positions from model fitting in dependence on temperature. (c) Area fraction of the (120) and (032) Bragg reflections with respect to the total area. The vertical dashed lines in (b) and (c) indicate Tm from DSC (see above). Different symbols indicate different runs.

morphology, which is due to thermodynamic interactions between the blocks and usually has a well-defined repeat distance. The temperature range 0−20 °C where d and the width decrease discontinuously coincides with the melting temperature Tm = 11 °C from DSC. The (120) and (032) Bragg reflections from the crystalline PEO lattice are observed up to 10 °C at 13.44−13.39 and 16.38−16.29 nm−1 (Figure 10). Most of the other reflections are too weak to be observed. The positions are slightly lower than the expected ones (13.58 and 16.60 nm−1). These values can be reproduced by a and b values of the PEO lattice which are 1.5% higher than the literature values.47 A calculated diffractogram which matches the observed peak positions is shown in Figure SI-5. The deviation of the lattice parameters from the values for bulk PEO may be an indication of the constraints imposed by the side arm architecture of gPE brush copolymers to the necessary reconfiguration of the whole side chain for the formation of the crystalline phase.27 The area fraction of the reflections decreases with increasing temperature (Figure 10c), starting from 0.14 at −70 °C, and 5973


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Macromolecules copolymers of similar composition.58,59 Possibly, PEO forms domains that are immersed in a continuous PHOS/PPO matrix, this way forming the HPL morphology. The width of the first-order reflection is constant up to −15 °C (Figure 11d); above, it decreases strongly up to 0 °C and then increases again. The latter increase is expected for an amorphous block copolymer melt in the ordered state. The (120) and (032) Bragg reflections from the crystalline PEO lattice are observed up to −10 °C at 13.44−13.39 and 16.38−16.29 nm−1 (Figure 12). As in gPE1, the positions are slightly lower than the expected ones (13.58 and 16.60 nm−1), which may be due to a and b values of the PEO lattice which are 1.5% higher than the literature values. The area fraction of the (120) and (032) reflections decreases, starting from 0.14 at −65 °C and going to 0 above −20 °C, in consistency with the Tm value from DSC of −8 °C. The area fraction is lower than in gPE1, as expected from the lower weight fraction of PEO. Thus, two regimes are discerned in gPE2: (i) up to −10 °C, a lamellar phase where PEO is crystalline, and (ii) a microphaseseparated morphology above, which is possibly the HPL morphology. The degree of crystallinity and the melting point of PEO are lower than in gPE1, in consistency with the lower weight fraction of PEO in gPE2. The repeat distance decreases slightly when PEO melts but much less (∼0.8 nm) than in gPE1 where the change amounts to ∼4 nm. Also in this sample, the crystallization occurs presumably via breakout crystallization. 3.3. Resulting Phase Diagram. We have investigated four bottle brush copolymers with PPO-b-PEO side arms with a focus on the effect of the side arm architecture on the thermal and structural behavior, using differential scanning calorimetry and temperature-resolved wide- and small-angle X-ray scattering. A sample in which the crystallizable PEO block has a high weight fraction and is linked directly to the PHOS backbone shows a melting point of PEO in the usual range, namely at 54 °C. The crystalline state for PEO persists to more than 30 K above this temperature, as found using WAXS, which showed a large number of Bragg reflections. The mesoscopic morphology in the crystalline state is lamellar, as expected. Between 54 and 80 °C, an ordered morphology is encountered, and between 80 and 90 °C, an order-to-disorder transition takes place. In the three other samples of type gPE, the PPO block is directly linked to the PHOS backbone and the PEO block is dangling. In these samples, the melting points are significantly lower than in bulk PEO and in gEP1 samples: For PEO weight fractions of 67.5 and 40.8 wt %, the melting points are 11 and −8 °C, respectively, whereas a sample with 23.4 wt % does not show crystallization or melting in DSC. In the two former samples, the Bragg reflections from the PEO crystallites are shifted to lower q-values compared to the ones in bulk PEO. The sample with a PEO weight fraction of 67.5 wt % features asymmetric lamellae which become symmetric when the PEO crystallites melt. The sample with 40.8% PEO shows a transition from asymmetric lamellae below the melting point to another morphology which we tentatively attribute to hexagonally perforated lamellae with PEO domains immersed within a PHOS/PPO matrix. The morphologies identified in the brush copolymers using SAXS and WAXS are summarized in Figure 13. The samples are plotted into the experimental phase diagram identified by Floudas et al. for polyisoprene-b-poly(ethylene oxide) (PI-bPEO) diblock copolymers according to the volume fractions of

the PEO block in the side arms of the bottle brushes and the temperature range studied.60 It is seen that the bottle brushes investigated by us show the same morphologies as these diblock copolymers. However, the χN values of the ODTs of the bottle brushes are lower than the ones of the diblock copolymers. Thus, the bottle brush architecture seems to stabilize the microphase-separated state. The melting points of the PI-bPEO diblock copolymers are located between 38 and 71 °C (the PEO block molar masses are between 1200 and 21 000 g/ mol). They are thus higher than or similar to the Tm values in gEP1, but consistently higher than the ones of gPE1 and gPE2, which confirms the important role of the bottle brush architecture for the crystallization behavior. Interestingly, whereas diblock copolymers with a PEO block molar mass as low as 1200 g/mol crystallize, this is not the case in the bottle brush gPE3 where the PEO block molar mass is 1730 g/mol but cannot crystallize. Only a few investigations have addressed block copolymers containing PEO and PPO.32,34 Fairclough et al. investigated diblock and triblock copolymers from poly(oxyethylene) and poly(oxypropylene) in the melt.32 The Flory−Huggins parameter was determined and the microphase separation behavior was studied. At temperatures below 63 or 68 °C, a lamellar phase with crystalline PEO layers was observed for two diblock copolymers having PEO blocks with NPEO = 58 or 69, i.e., lower than in gEP1, gPE1, and gPE2. These Tm values are close to the bulk value of PEO and higher than the ones observed in the bottle brushes. At the melting point, the morphology changed from lamellar to another, unknown one, before the order-to-disorder transition was crossed. Upon cooling, these processes were reversible. PEO-b-PPO-b-PEO triblock copolymers were investigated by Zhang and Stühn.34 In their study, the melting temperatures were found between 20.3 and 46.2 °C for PEO blocks with molar masses between 580 and 3000 g/mol. These values are higher than in the bottle brushes gPE1 and gPE2. One study addressed the morphology and packing manner of graft copolymers featuring rigid-rod-like poly(γ-benzyl Lglutamate) (PBLG) main chains with grafted diblock copolymers of amorphous poly(propylene glycol) (PPG) and crystalline poly(ethylene glycol) (PEG).36 Using SAXS, it was found that the graft copolymers form repeated layers consisting of segregated PBLG, PPG, and PEG layers. In contrast to our study, the molar fraction of side chains was very low (between 2.4 and 7.0%); i.e., the grafting density was significantly lower, and the side arms are shorter.

4. SUMMARY AND CONCLUSION We have investigated the thermal and structural behavior of a series of brush copolymers featuring densely packed side arms which are diblock copolymers from amorphous poly(propylene oxide) and crystallizable poly(ethylene oxide) with the backbone consisting of polystyrene. Our results suggest that the chain architecture, i.e., the sequence of the crystallizable and the amorphous block in the side arms, affects drastically the crystallization of PEO blocks and the structure of the microphase-separated states. For the sample with the crystallizable PEO block linked directly to the backbone and a high PEO fraction (84.9 wt %), our results reveal a PEO crystallization/melting behavior similar to the one of bulk semicrystalline PEO. SAXS shows that the morphology in the crystalline state is lamellar with highly asymmetric lamellae, as expected. Our results suggest that this 5974


Macromolecules specific architecture favors crystallization dictating an enhanced packing ability of the PEO segments. We speculate that this particular feature of PEO segments is the origin of the crystalline order, as determined by WAXS and supported by POM, that persists up to 30 K above the DSC melting point. For the samples where the PPO block is directly linked to PHOS backbone and the PEO chains are dangling, our DSC and X-ray scattering results indicate that crystallization is hindered by frustration in packing of the PEO segments and by reduced molecular mobility of side arms due to the tethered end at the rigid backbone. Thermal analysis reveals a strong suppression of melting temperatures Tm of the PEO domains: For PEO weight fractions of 67.5 and 40.8 wt %, the melting points are 11 and −8 °C, respectively, whereas for the sample with 23.4 wt % PEO and featuring PEO blocks with molar mass of 1730 g/mol, no crystallization/melting events have been observed. Furthermore, the fact that the sample with PEO fraction of 40.8 wt %, gPE2 sample, shows marginal crystallization ability during cooling and exhibits strong cold crystallization process supports the concept of strong constraints in the mobility of side arms. Taking into consideration that our DSC results imply that the segmental mobility of amorphous PEO chains is not retarded in the brush copolymers as compared to bulk PEO, on the contrary, it has been found that it is accelerated, we conclude that it is mainly the constraints to the motions of the whole side arm that hinder the crystallization of the PEO domains. In the gPE samples that crystallize, an asymmetric lamellar phase with a distorted PEO crystalline phase is formed. Above Tm, a microphase-separated symmetric lamellar morphology is adopted by the sample with the higher PEO fraction, whereas for the other sample our results suggest hexagonally perforated lamellae with PEO domains immersed within a PHOS/PPO matrix. Polymeric bottle brushes with high grafting density and with amorphous−crystalline diblock copolymers thus feature structure formation at a multitude of length scales and complex thermal behavior. Compared to homologous diblock copolymers, side arms in brush copolymers may exhibit different thermal and structural behavior depending on the chain architecture. Our results imply that the bottle brush architecture stabilize the microphase-separated state of the diblocks and allow for tuning the ability of PEO blocks to form crystalline structures.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge Deutscher Akademischer Austauschdienst for the travel support within the program “Hochschulpartnerschaften mit Griechenland” (ResComp). Prof. Bernhard Rieger provided access to the polarizing microscope of the WACKER-Chair of Macromolecular Chemistry at Technische Universität München, and Katia Rodewald assisted us with the experiments. Their contribution is gratefully acknowledged.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00879. Additional DSC experiments; model fitting of SAXS curves; modeling of the WAXS curves; study of gEP1 melting behavior by polarized optical microscopy (PDF)

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*(A.K.) Phone +30 210 772 3053, Fax +30 210 772 2932, email [email protected]. Notes

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Structure and Crystallization Behavior of Poly(ethylene oxide) (PEO

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