Novel Approach to Trigger Nanostructures in Thermosets Using

Nov 5, 2015 - Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia. Macromolecules , 2015, 48 ...
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Novel Approach to Trigger Nanostructures in Thermosets Using Competitive Hydrogen-Bonding-Induced Phase Separation (CHIPS) Nisa V. Salim,*,† Nishar Hameed,† Bronwyn L. Fox,† and Tracey L. Hanley‡ †

Carbon Nexus, Institute for Frontier Materials, Deakin University, Geelong, 3220, Australia Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia

‡

S Supporting Information *

ABSTRACT: A new route to prepare nanostructured thermosets by the utilization of intermolecular hydrogen-bonding interactions is demonstrated here. In this study, competitive hydrogen-bonding-induced microphase separation (CHIPS) in epoxy resin (ER) containing an amphiphilic block copolymer poly(ε-caprolactone)-block-poly(2-vinylpyridine) (PCL-b-P2VP) is investigated for the first time. The phase separation takes place due to the disparity in the hydrogen-bonding interactions in ER/P2VP and ER/PCL pairs leading to the formation of ordered nanostructures in the ER/block copolymer blends. SAXS and TEM results indicate that the hexagonally packed cylindrical morphology of neat PCL-b-P2VP block copolymer remains but becomes a core−shell structure at 10 wt % addition of ER, and changes to regular lamellae structures at 20−50 wt % then to disordered lamellae with 60 wt % ER. Wormlike structures are obtained in the blends with 70 wt % ER, followed by a completely homogeneous phase of ER/P2VP and ER/PCL. The formation of nanostructures and changes in morphologies depend on the relative strength of hydrogen-bonding interactions between each component block copolymer and the homopolymer. This versatile method to develop nanostructured thermosets, involving competitive hydrogen-bonding interactions, could be used for the fabrication of hierarchical and functional materials.

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interactions; “χ” describing repulsive (bad) interactions (χ > 0) and “ξ” describing attractive (good) interactions (ξ < 0). Generally, in A-b-B/C type block copolymer/homopolymer systems, two types of outcomes are possible when C is miscible with immiscible A-b-B segments. The first case is C is miscible with B but immiscible with A, i.e., with a negative χBC and positive χAB and χAC (A/B and A/C are immiscible). In such cases the immiscible A phase separates to form different ordered or disordered morphologies. Hashimoto et al. studied blending of PS-b-PI/poly(phenylene oxide) and PS-b-PB/ poly(methyl vinyl ether) (PMVE) systems, where the homopolymers exhibit negative χ parameters with polystyrene.8,9 In the second case, the homopolymer C is miscible with both the blocks of the block copolymer, i.e., A and B. For instance, P2VP-b-PEO/PVPh and PVPh-co-PMMA/PEO blends where is PVPh miscible with the other components.10,11 Here the self-assembly or microphase separation was not detected. This is because the nonselective bonding between the homopolymer and the block copolymer blocks to form a completely homogeneous system. In recent years, Guo and coworkers,12−15 and Chang and co-workers16−18 have reported a versatile method to develop self-assembled nanostructured

INTRODUCTION Many studies have taken place in regards to block copolymer modified thermosets.1 Cross-linking of the epoxy resin (ER) matrix without macroscopic phase separation of the block copolymer leads to stable well-defined nanostructures.2 All the nanostructured thermoset blends reported so far consist of a block copolymer and a highly cross-linked polymer, which can form miscible or partially miscible blends and are mainly explained by two important mechanisms, self-assembly and reaction-induced phase separation (RIPS).3,4 The requirement for the self-assembly approach is that block copolymers are selforganized into nanostructures before curing but this situation does not always occur if all the blocks of block copolymers are miscible with precursors of thermosets. RIPS nanostructures can be formed by blending selective block copolymers with a thermoset, where one block is miscible with the thermoset precursor and the other block phase separates by a reaction induced mechanism during curing.4 Although this limits the range of cross-linking agents and curing conditions that can be used. The phase behavior of a blend consisting of homopolymer C and block copolymer A-b-B (A-b-B/C type systems) can be characterized by a multitude of parameters, such as Flory− Huggins interaction parameter χ, chain lengths, and statistical segment lengths.5−7 In the random-phase approximation calculations, A-b-B/C systems were characterized by two © XXXX American Chemical Society

Received: April 2, 2015 Revised: October 5, 2015

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DOI: 10.1021/acs.macromol.5b00687 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Chemical Structures of DGEBA and MDA

thermosets” and most of them were formed either by preformed self-organized microphase separation or RIPS mechanisms as mentioned previously.3,4 In all these epoxy/ block copolymer systems, an immiscible A-b-B diblock copolymer is mixed with the resin, where the resin interacts favorably with block B, but is immiscible with A. In the present system, both the blocks were reported to be miscible with epoxy in the binary blends. Here, the nanostructure formation is not induced by the curing reaction or miscibility/ immiscibility prior to curing, instead due to the competition between the two blocks to form hydrogen bonding with epoxy. Nanostructured thermosets via CHIPS take place when the diblock copolymer PCL-b-P2VP is immiscible and the ER resin can interact with both A and B blocks, but unequally, due to the competitive hydrogen-bonding interaction between the A/C and B/C pairs. From this point of view, blend compositions were selected between an amphiphilic diblock copolymer, PCLb-P2VP, with ER knowing that (1) P2VP pyridine groups and ER hydroxyl groups can form very strong hydrogen bonds at any composition where the interassociation is much stronger than self-association in ER and (2) PCL carbonyl groups can also form hydrogen bonds with ER hydroxyl but the interassociation here is much weaker than the self-association in ER. Therefore, it is expected that by blending, selective control over the microphase separation of PCL blocks can be obtained by varying composition and meanwhile the P2VP blocks remain miscible with the ER over the entire concentration range. As a consequence, the nanostructured blends can be prepared via selective solubility. In this study, a new route to prepare nanostructured thermosets by employing CHIPS in block copolymers was investigated. The results may lead to the development of new miscible materials with highly functional properties.

block copolymer blends and complexes through competitive hydrogen-bonding interactions. The concept is based on the competition between different blocks of the block copolymer to form more than one kind of intermolecular interaction with the complementary polymer in the blend, thus leading to a highly stable blend compared to analogous systems. In the present work a simple and efficient method for the formation of nanostructured thermosets in poly(ε-caprolactone)-block-poly(2-vinylpyridine) (PCL-b-P2VP) block copolymer and ER were used. Both blocks of the block copolymer PCL and P2VP are miscible with the ER depending on the concentration of the latter. The miscibility of epoxy/PCL and epoxy/P2VP has been reported using MDA as a curing agent for epoxy.19,20 Moreover, PCL has been studied extensively as the epoxy-miscible block in many block copolymer systems for creating nanostructures of the complementary epoxy-immiscible block.21,22 The high reactivity and miscibility of P2VP with epoxy was also evident from its use as a cross-linking agent for epoxides.23 It was a delusion that miscible block copolymers in epoxy are completely homogeneous and unable to selfassemble under any chemical or physical circumstances due to more than one type of intermolecular hydrogen bonding. This study has demonstrated that the careful selection of polymer blocks and, and experimental conditions can lead to selfassembled structures in thermoset blends via CHIPS.24,25 The competitive hydrogen-bonding behavior of PCL-b-P2VP has been reported with other homopolymers such as polyvinylphenol and phenoxy; however, the ordered morphology of the PCL-b-P2VP was destroyed during the blending modification.14,15 The competitive hydrogen-bonding phenomena has been demonstrated using a model system of polyvinylphenol and poly(methyl methacrylate)-block-poly(2vinylpyridine) and compared the extent of hydrogen-bonding interactions vs composition of blends and types of nanostructures formed.25 The present system demonstrates the nanostructure formation via competitive hydrogen-bonding interactions in block copolymer/thermosets for the first time. In ER/PCL-b-P2VP, the addition of epoxy leads to a variety of highly ordered morphological transitions in the blends. Moreover, the microphase separation in ER/PCL-b-P2VP can be different as the cross-linking reactions and secondary interactions in thermoset epoxies are considered to be much more complicated. The microphase separation of epoxy/block copolymer blends and mechanisms governing this process were significantly investigated under the title “nanostructured

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EXPERIMENTAL SECTION

Materials and Preparation of Samples. The block copolymer employed in this work, poly(ε-caprolactone)-block-poly(2-vinylpyridine) (PCL-b-P2VP) with Mn(P2VP) = 20 500, Mn(PCL) = 28 000 and Mw/Mn = 1.10 was from Polymer Source, Inc. The polymers were used as received. Diglycidyl ether of bisphenol A (DGEBA) with epoxide equivalent weight of 172−176 and 4,4′-methylenedianiline (MDA) curing agent were purchased from Aldrich Co. The chemical structures of epoxy resin (ER) and the curing agent are given in Scheme 1. The value of n = 0.15 in that scheme indicates the number of times the structure within the brackets is repeated. B

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Scheme 2. Representation of Possible Hydrogen-Bonding Interaction between MDA-Cured ER and PCL-b-P2VP Diblock Copolymer

The ER precursor DGEBA and the PCL-b-P2VP diblock copolymer were mixed together, stirred and heated at 100 °C until the mixtures were homogeneous. A stoichiometric amount of the curing agent MDA was added to the mixtures with vigorous stirring until homogeneous solutions were obtained. The ternary mixture was poured into preheated molds and cured at 150 °C for 12 h and postcured at 180 °C for 2 h. Fourier-Transform Infrared (FTIR) Spectroscopy. Infrared spectra of PCL-b-P2VP/ER samples were obtained on a Bruker Vetex-70 FTIR spectrometer, and 32 scans were recorded with a resolution of 4 cm−1. The samples were mixed with KBr powder, ground well, and the KBr disks prepared. The disks were dried under vacuum in an oven at 100 °C before measurement. Differential Scanning Calorimetry (DSC). The glass transition temperatures of the blends were determined by a TA Q200 differential scanning calorimeter using 5−10 mg of the sample under nitrogen atmosphere. A heating rate of 20 °C/min was employed. All samples were first heated to 250 °C and maintained at that temperature for 3 min; they were subsequently cooled to −80 °C at 20 °C/min, held for 5 min, and heated to 200 °C. The midpoints of the second heating scan of the plot were taken as the glass transition temperatures (Tgs). Transmission Electron Microscopy (TEM). TEM analysis was carried out on a JEOL JEM-2100 transmission electron microscope operating at an acceleration voltage of 100 kV. The samples were cut into ultrathin sections of about 70 nm thickness at room temperature with a diamond knife using a Leica EM UC6 ultramicrotome machine. The ultrathin sections were collected on 400 mesh copper grids and stained with ruthenium tetroxide (RuO4) for observation. Small Angle X-ray Scattering (SAXS). SAXS experiments were conducted at the Australian Synchrotron on the small/wide-angle Xray scattering beamline, utilizing an undulator source that allowed measurement at a very high flux to moderate scattering angles and a good flux at the minimum q limit (0.012 nm−1). The intensity profiles were interpreted as the plot of scattering intensity (I) versus scattering vector, q = (4/λ) sin (θ/2) (θ = scattering angle).

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Figure 1. Hydroxyl region of ER/PCL-b-P2VP blends in the infrared spectra observed at room temperature.

hydrogen-bonding interaction between hydroxyl groups of ER and pyridine and/or carbonyl groups of the block copolymer, demonstrating the new distribution of hydrogen-bonded groups resulting from the competition between hydroxyl− hydroxyl, hydroxyl−carbonyl, and hydroxyl−pyridine specific interactions. A new band centered at 3197 cm−1 in 60 wt % or less ER complexes was observed at Figure 1. This hydroxyl stretching band corresponds to the strong intermolecular hydrogen bonding between hydroxyl groups of ER and pyridine groups of P2VP. This result can be compared with those obtained for ER/P4VP by other authors.23 The shift of peak position (Δυ) was measured and used to estimate the average strength of the intermolecular interaction between the free hydroxyl region and that of the hydrogen-bonded species.26 The Δυ values of selfassociation of pure ER (145 cm−1) ER/PCL(128 cm−1), and ER/P4VP (or P2VP) (368 cm−1) binary blends and ER/PCLb-P2VP (167 cm−1) are also given for comparison.21,27 This observation implies that the average strength of the hydrogen bond, between ER hydroxyl group and PCL-b-P2VP block copolymer, is higher than that between self-associated hydroxyls in pure ER. The analysis of the Δυ values of pure ER, ER/P2VP, and ER/PCL reflects that P2VP and PCL are both capable of making hydrogen bonds with ER, although the resulting bond strengths are unequal. The above results imply

RESULTS AND DISCUSSION

Hydrogen-Bonding Interactions. The possible hydrogenbonding interactions between the PCL-b-P2VP block copolymer with ER is schematically represented in Scheme 2. The blends were subjected to FTIR study for the confirmation of hydrogen-bonding interactions of the hydroxyl group of ER with the pyridine and carbonyl groups of P2VP and PCL. Figure 1 shows the hydroxyl stretching region in the IR spectra of ER/PCL-b-P2VP blends at room temperature. The hydroxyl region of ER shows two stretching vibrations; a broad band at 3420 cm−1 and a relatively shoulder band at 3565 cm−1 corresponding to the self-associated hydroxyl groups and nonassociated free hydroxyl groups, respectively. However, upon addition of the PCL-b-P2VP diblock copolymer, the peak corresponding to the free hydroxyl groups at 3565 cm−1 decreases in intensity and eventually disappears, whereas the hydrogen-bonded band at 3420 cm−1 shifts toward the lower wavenumber region. These changes imply the intermolecular C

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Macromolecules that the interassociation hydrogen bonds between ER and P2VP are stronger than the bonding between ER/ER and ER/ PCL. Figure 2 represents the hydrogen bonding between hydroxyl groups of ER and pyridine ring of P2VP in the region of 1600−

Figure 3. Carbonyl region of ER/PCL-b-P2VP thermosets at room temperature.

the ER concentration is above 40 wt %. This signifies that the fractions of hydrogen-bonded carbonyl group in ER/PCL-bP2VP blends are less at lower ER concentrations, mainly due to the strong hydrogen-bonding ability of P2VP with ER compared with PCL. The hydrogen-bonding interaction of PVPh and PCL was also studied at higher temperature. Figure 4 shows the FTIR spectral changes of ER/PCL-b-P2VP blends in the carbonyl stretching region at 100 °C, above the melting point of PCL. The hydrogen-bonded carbonyl band appears approximately at

Figure 2. FTIR spectra corresponding to the pyridine region of ER/ PCL-b-P2VP thermosets at room temperature.

1550 cm−1 of IR spectra. In this figure, the pyridine ring of P2VP shows intense bands at 1589 and 1570 cm−1. As the concentration of ER increases, the 1589 cm−1 band becomes broader and shifts toward a higher wavenumber region. This is due to the increase in stiffness of the pyridine ring as a result of hydrogen-bonding interactions.13 Also, the stretching vibration band of pyridine rings at 1570 cm−1 decreases in intensity and almost disappears with increase in ER which denotes that the structure of pyridine ring was destroyed due to its reaction with epoxide groups of DGEBA. It can be assumed that the hydroxyl groups of ER form hydrogen bonds with P2VP preferentially at all concentrations which is due to the favored hydrogenbonding interactions between ER/P2VP blocks. The hydrogen-bonding interaction between ER and PCL was examined by analyzing the carbonyl region ranging from 1780 to 1700 cm−1 in the IR spectra of the ER/PCL-b-P2VP in Figure 3. The IR spectra of pure PCL exhibits two peaks: a sharp band centered at 1726 cm−1, which corresponds to PCL in its crystalline conformation, and a shoulder band at 1736 cm−1 attributed to amorphous PCL. With the increase in the ER content, another band contribution was observed at 1709 cm−1. The presence of this band confirms the vibration of hydrogen-bonded carbonyl groups. However, this band only appears in the blends when the ER concentration is above 40 wt %. As the epoxy content increases, it was noted that the intensity of crystalline bands decreased, whereas those of the amorphous increased indicates that PCL blocks are in the amorphous state in the thermoset.28 In addition, the band at 1736 cm−1 was found slightly shifted to the lower wavenumbers with increasing the content of ER thermoset. This indicates that the interaction between ER and PCL was only observable when

Figure 4. Infrared carbonyl stretching region of ER/PCL-b-P2VP thermosets at 100 °C. D

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Macromolecules 1709 cm−1. The intensity of this band increases with increasing concentration of ER. However, this peak appears only above 40 wt % of ER blends. Hence, it can be concluded that PCL blocks in ER/PCL-b-P2VP blends exhibit a weaker ability to form hydrogen bonds at low ER concentrations. This is due to the formation of relatively strong intermolecular hydrogen bonds between ER and P2VP. Moreover, the crystalline peak of PCL centered at 1726 cm−1 was no longer observable because of the melting of the crystalline components of the block at PCL at 100 °C. These results imply that the hydroxyl groups of ER form hydrogen bonds with P2VP preferentially at all concentrations, whereas PCL can take part in intermolecular interactions only at higher ER content. Otherwise, it can be stated that competitive hydrogen-bonding interactions exist in ER/PCL-b-P2VP blends. Phase Behavior. Thermal properties of ER/PCL-b-P2VP blends were examined by measuring the glass transition temperature (Tg) of all blend compositions. Figure 5 shows

increasing block copolymer content. The reduction in Tg in blends corresponding to 60 wt % or above ER is due to the dissolution of PCL blocks at this composition. This can be attributed to the miscibility between ER/P2VP, and ER/PCL components due to the formation of strong intermolecular hydrogen-bonding interactions. Melting point (Tm) depression is the characteristic of a miscible polymer blend involving hydrogen-bonding interactions. The Tm of the block copolymer components in the blend decreases with an increasing content of the hydrogen-bonding donor blocks. The pure PCL-b-P2VP diblock copolymer exhibits melting points, Tm (PCL) at 56 °C, attributable to the crystalline PCL block. The Tm of PCL shifts substantially to a lower temperature with an increasing content of ER. It is clearly displayed that the Tm of PCL remains almost unchanged in the blends with very low ER concentration. This shows that there is no or very limited miscibility between PCL and ER at lower ER contents. The melting peak corresponding to the crystalline phases reduces in its intensity and eventually disappears at 50−60 wt % ER blends, and no melting peaks are observed thereafter, which is due to the miscibility of both PCL and P2VP with ER at higher ER contents. The cooling scan of the blends is shown in Figure 6. The pure PCL-b-P2VP block copolymer exhibits a crystallization

Figure 5. DSC thermograms of the heating curves of neat epoxy, PCLb-P2VP diblock copolymer and ER/PCL-b-P2VP thermosets at 20 °C/min.

the DSC thermograms of the ER/PCL-b-P2VP blends. Block copolymer PCL-b-P2VP contain two immiscible blocks, which should exhibit two Tgs corresponding to PCL blocks and P2VP blocks, respectively. It is noted that a single Tg is the most widely and conventionally used criterion to examine the miscibility blend. The binary blends of ER/P2VP29 and ER/ PCL30 are miscible over the entire composition range due to interassociation hydrogen bonding between the hydroxyl group of phenolic and the pyridine group of P2VP and the carbonyl group of PCL, respectively. Here, the Tgs of the ER/PCL-bP2VP blends show variations. The Tg of PCL blocks was not detectable from the DSC curves under the current experimental conditions. The MDA-cured ER shows a glass transition (Tg) at 179 °C and corresponding to P2VP blocks is at 102 °C in the pure block copolymer, as displayed in Figure 5. It can be seen that the Tg of the blends shifts to lower temperatures with

Figure 6. Crystallization curves of PCL-b-P2VP diblock copolymer, neat epoxy, and ER/PCL-b-P2VP thermosets during cooling.

peak at 24 °C, whereas the MDA-cured ER/PCL-b-P2VP blends show a crystallization peak higher than that of the block copolymer. Blends with 40 wt % above ER did not display a crystallization peak for the PCL blocks, which shows that the crystallization is hindered. The increase in crystallization peak temperature upon the addition of ER may be due to the relaxation of PCL blocks caused by the formation of strong hydrogen bonds between P2VP and ER in lower ER content blends. Based on the DSC results, it can be concluded that microphase separation exists only due to PCL, which has a weaker hydrogen bond interaction than P2VP. Also, at lower ER contents, the hydroxyl group is insufficient to form two E

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Macromolecules hydrogen-bonding integrations namely, ER/P2VP, and ER/ PCL. Therefore, PCL has a higher chance to phase separate at these concentrations. This was also evidenced by TEM, which will be discussed later. It is also noticed that ER/PCL binary blends exhibit strong hydrogen-bonding interactions and good miscibility over the entire composition range (Supporting Information). The formation of new carbonyl band at 1709 cm−1 (Figure S1) and the shift in ER Tg to low temperature values as well as depression in PCL melting pint (Figure S2) even at very low ER concentrations indicate that ER/PCL binary blends show good interaction/miscibility. On the other hand in ER/PCL-bP2VP blends, these changes occurs only at very high ER concentrations (60 wt % and above). This is due to the competitive hydrogen-bonding interactions present in the blends and are composition-dependent. Moreover, it has been reported that P2VP reacts with ER and participate in the cross-linking reactions.23 However, our experimental evidence (DSC and FTIR) show that there is a strong hydrogen-bonding interaction (and hence miscibility) exists between P2VP pyridine groups and hydroxyl groups in ER. Moreover these results are also consistent with the P2VP miscibility with other strong proton donor polymers such as poly vinyl alcohol.15,25 It is assumed that only small amounts of P2VP engages in the cross-linking reaction (as stoichiometric amounts of MDA was added as the major curing agent), and the majority of P2VP involves in strong hydrogen-bonding interactions with ER. Self-Assembly of Nanostructured Thermosets. All the blends were transparent before curing, hence indicating the macroscopic homogeneity of the mixtures in the molten state. TEM in Figure 7 shows the morphologies of block copolymer modified thermosets obtained in ER/PCL-b-P2VP blends. The light continuous regions are the cross-linked ER matrix, which is linked with the P2VP blocks of the block copolymer, whereas the white regions are PCL domains. TEM images of ER/PCLb-P2VP blends show the order-to-disorder (hexagonal cylinders to lamellar to distorted lamellar) morphological change with increase in the ER content. Pure block copolymer exhibits a well-ordered hexagonal cylindrical morphology (Figure 7a). After the addition of ER into the block copolymers, the blends show a variety of morphological changes. As shown in Figure 7b, 10 wt % ER content in the blends, hexagonal packed cylinders with a “core-shell” structure31 can be observed. At 20 wt % of ER, lamellar morphology was observed. At these ER contents, self-ordered structures are caused by microphase separation of the PCL blocks from ER/P2VP combined phase. The TEM micrograph (Figure 7c−g of 30/70 up to 60/40 ER blends also show lamellar morphology with three different phases. From Figure 7c−e three phases can be clearly visualized and these consist of (1) the ER-rich phase which forms the matrix despite the high block copolymer concentration; (2) the dark nodules, correspond to the strongly hydrogen-bonded ER with P2VP, and the bright domains represents the weak interacting and microphase-separated PCL blocks. For blends with more than 60 wt % of ER, the long-range order of lamellae was deteriorated and form a distorted worm-like structure. Homogenous morphology is observed above 80 wt % of ER content. The transition from initial microphase separated morphology to the homogeneous morphology above 80 wt % of ER is composition dependent and based on the strength of competitive hydrogen-bonding interaction.

Figure 7. TEM images of ER/PCL-b-P2VP thermosets (a) Pure PCLb-P2VP block copolymer and (b) 10/90, (c) 20/80, (d) 30/70, (e) 40/60, (f) 50/50, (g) 60/40, (h) 70/30, (i) 80/20, and (j) 90/10 mixtures. The light continuous regions correspond to the mixed phase of cross-linked ER matrix, and P2VP blocks of the block copolymer; the white regions correspond to PCL.

The SAXS profiles of the blends at room temperature are given in Figure 8. The SAXS patterns exhibit scattering peaks at the q-positions of 1:√3:√7:√9:...√25 relative to that of the first-order peak (i.e., q/q* = 1:√3:√7:√9). From these spacings, it is evident that the PCL-b-P2VP forms a wellordered hexagonally packed structure in bulk with a long period of 30 nm corresponding to the spacing between neighboring P2VP and PCL microdomains. At 10 wt % ER blend also shows ordered hexagonally structure, which is consistent with the TEM images. The 20−60 wt % blends give multiple peaks at 1:2:3:4:5 q/q*, which corresponds to the lamellar morphologies. One can recognize the characteristic profile of a lamellar structure with up to 5 orders of scattering maxima located at integer multiples of the first-order peak position. Like TEM images, the SAXS profiles are consistent with a well-separated, regularly spaced lamellar morphology for blends with 20−60 wt % ER. Lamellae are very regular and form large grains F

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Figure 8. SAXS profiles of ER/PCL-b-P2VP blends.

extending over several micrometers. The q space of the peaks shifts to lower q as the concentration of ER is increased. This result shows that there is a systematic increase in the size of the phase separated domain, which implies the progressive incorporation of ER. Above 60 wt % ER, the blends show only weak and broad peaks, and the broadening of the peak indicates the deterioration of long-range ordered structures indicating a near-homogeneous morphology as observed in 80−90 wt % ER/PCL-b-P2VP blends in Figure 7, parts i and j). The formation mechanism of different self-assembled nanostructures in ER/PCL-b-P2VP blends at different compositions is schematically shown in Figure 9. Pure triblock copolymer exhibited a hexagonal cylindrical morphology as shown in Figure 9a. Since the blocks in the block copolymers have the general tendency to phase separate, they exhibit an amphiphilic characteristic, which is caused by the restriction due to the presence of a covalent bond between the chemically different blocks, resulting in microphase separated structures. When a homopolymer is complexed with a diblock copolymer, involving competitive hydrogen-bonding interactions, the weakly hydrogen-bonded block is excluded from the homogeneous region due to the high entropic penalty for conformational distortion. Here, by addition of more homopolymer, microphase separation takes place due to the self- assembly of the elementary block copolymer; i.e., it selectively swells the blocks due to the competitive hydrogen bonding, which results in phase separation. The blend system contains a block copolymer PCL-b-P2VP and MDA-cured ER. It was observed that there is a morphological transition from hexagonally packed cylinders to hexagonal cylinders with a core−shell morphology upon curing, and further transformation to lamellae and finally to homogeneous blend with the incorporation of ER resin and subsequent curing. This is due to the miscibility of the block copolymers and also CHIPS, which appears to be the driving force to produce the uniform microphase separated structures. According to Hamley,32 during microphase separation, the minority block is segregated from the majority block forming ordered and uniformly spaced nanodomains. The shape and order of the segregated domains in a diblock is determined by the volume fraction of the minority block (e.g. MA ) and block incompatibility. The regular periodic phases for A-B diblock copolymers with increasing MA include the body-centered cubic

Figure 9. Schematic representation of epoxies modified with various weight percentage of PCL-b-P2VP diblock copolymer. Key: (a) hexagonal cylindrical morphology of pure block copolymer, (b) hexagonal cylinders with core−shell morphology at 10 wt %, and (c) lamellae at 20 wt % of epoxies in ER/PCL-b-P2VP blends.

of A spheres in a B matrix, hexagonally packed A cylinders in a B matrix, bicontinuous gyroid and lamellae of A and B blocks. Here, the PCL-b-P2VP block copolymer has hexagonally packed P2VP cylinders (minority component as per molecular weight) in the PCL matrix. For the pure block copolymer, which is originally in the cylindrical phase, the hydrogenbonding interaction with ER is thus expected to induce structural transformations, which is analogous to block copolymers in in analogy with block copolymer selective solvent systems. The addition of ER into the system increases the volume fraction of P2VP ( MP2VP), which controls the transition of the hexagonally packed cylindrical morphology to hexagonal cylinders with a core−shell morphology that is schematically shown in Figure 9b. At 10 wt %, the added ER and P2VP interacts very strongly whereas PCL blocks, which are repelled by P2VP, have a relatively weak hydrogen bonding with ER. Therefore, the added ER form ER/P2VP single phase (as a core within the P2VP cylinders), whereas the weakly interacting PCL phase separates as the matrix. The addition of further ER resin, which preferentially reacts with P2VP blocks, results in a higher MP2VP. This in turn, results in the formation of the lamellar morphology in the 20/80 blends and higher compositions. Figure 9c shows a representation of ER/PCL-bP2VP blends with 20−70 wt % of the ER. G

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Macromolecules

ER/PCL increases at higher concentrations, which results in twisted lamellae and homogeneous blends. The formation of various composition-dependent microphase separated morphologies in the ER/PCL-b-P2VP blends can be explained based on the relative strength of hydrogen bonding between the different pairs in the system.

Between 20 and 60 wt % (Figure 7c−g), mixed ER and P2VP become the continuous phase and the non reacted PCL domains become the dispersed phase due to the very low molecular weight of the block copolymer and the interactions of P2VP block with ER. The lamellae consisting of two continuous phases representing the hydrogen-bonded ER/ P2VP and the PCL and the neat cross-linked ER, respectively, are demonstrated. Here, ER and the P2VP block of the block copolymer interact very strongly to form a single phase, whereas the PCLblock gets microphase separated due to the weak interaction with ER compared to P2VP blocks. As the concentration of ER increases again, the microphase morphology varies, displaying matrix-dispersed wormlike morphology obtained in 60 wt % of ER (Figure 7d). As the concentration reaches 60−80 wt % ER, the interface between the ER/P2VP and ER/PCL microphases become less distinct. The interaction of ER between P2VP and PCL together leads to the formation of disordered and homogeneous matrix at very high compositions of ER. This is due to the hydrogen-bonding interactions of ER with PCL along with P2VP, due to the availability of free hydroxyl groups. In other words, ER behaves as a common solvent for both P2VP and PCL blocks. The nanostructures observed in these complexes are completely different from the cylindrical microphase of the pure PCL-bP2VP block copolymer, implying that competitive hydrogenbonding inetarctions along with the curing process has caused a morphological transition from the cylindrical structure of the neat block copolymer to a highly ordered nanostructure. The presence of hydrogen-bonding interactions in the blends enhances the miscibility of the blocks and facilitates the phase separation, which in turn affects the properties of the blends. There are unequal hydrogen-bonding interactions of ER with both P2VP and PCL, and at lower ER content the unreacted PCL phase separated, which altogether leads to the formation of various nanostructures in ER/PCL-b-P2VP blends. The coexistence of three kinds of hydrogen bonds, i.e., interassociated ER/P2VP, ER/PCL pairs and selfassociated ER/ER, will bring about the competition of the ER distribution among P2VP and PCL microdomains. Moreover, the self-association of ER hydroxyl interaction tunes the possibility to adjust the self-organized structures in nanoscale through ordered-disordered transitions. The geometry of the structures formed in the blends is determined, to a large extent, by the competition between P2VP and PCL blocks in regards to hydrogen bonding with ER. The blends were prepared through a complete curing process at very high temperatures, the thermosets are all cross-linked and therefore morphologies obtained here and are assumed to be kinetically stable and reproducible. As such these types of highly ordered nanostructures can be potentially used for the development of highly functional lithographic substrates or making nanoarrays.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00687. Experimental section with IR spectra and DSC thermograms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(N.V.S.) Telephone: +61 3 5227 2670, Email: nisavs@deakin. edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The SAXS measurements were carried out on the SAXS beamline at the Australian Synchrotron, Victoria and we would like to thank Dr Nigel Kirby for his assistance on the SAXS. We also thank Australian Institute of Nuclear Science and Engineering Ltd for the reseach support. The present work was carried out with the support of the Deakin Advanced Charecterisation Facility.

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CONCLUSIONS The fabrication of nanostructured thermosets via CHIPS in ER/PCL-b-P2VP blends was studied. In ER/PCL-b-P2VP blends, the hydroxyl groups of ER can selectively interact with both the P2VP and PCL blocks, leading to the formation of composition-dependent microphase separated morphologies in these blends. The disparity of strongly interacting ER/P2VP pairs and weakly associated ER/PCL pairs results in compositionally dependent microphase separation and the formation of cylindrical, hexagonal cylinders with core−shell structures, and lamellae at lower ER content. The miscibility of ER/P2VP and H

DOI: 10.1021/acs.macromol.5b00687 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b00687 Macromolecules XXXX, XXX, XXX−XXX