Article pubs.acs.org/Macromolecules
Exploring the Role of Supramolecular Associations in Mechanical Toughening of Interpenetrating Polymer Networks Seyedali Monemian and LaShanda T. J. Korley* Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States S Supporting Information *
ABSTRACT: Model “supramolecular IPNs” were developed via the formation of a hydrogen-bonded, supramolecular network of 2-ureido-4-[1H]-pyrimidinone (UPy) telechelic poly(ethylene-co-1-butene) (SPEB) in the presence of photopolymerizable, hydroxyl-terminated polybutadiene (HTPB). The role of a supramolecular elastomeric phase in mechanical toughening of IPNs was explored through (1) dynamic dissociation and reassociation of the noncovalent, UPy supramolecular associations, and (2) interphase formation. While an ∼4× increase in tensile toughness of the HTPB matrix was observed through incorporation of 10 wt % ethylene−propylene rubber (EPR)as a conventional elastomeric toughening agentinto HTPB, it was shown that adding the same amount of supramolecular elastomer SPEB to HTPB led to ∼600× enhancement in tensile toughness. Strain rate-dependent mechanical response and fractography studies revealed that this dramatic toughness enhancement was due to dissociation/reassociation of the dynamic UPy linkages in the elastomeric phase that facilitated dilatational yielding of the IPN. This toughness enhancement was only observed in combination with the existence of strong interfacial coupling between the matrix and supramolecular phase as revealed by transmission electron microscopy and dynamic mechanical analysis. By exploiting noncovalent dynamics and interfacial control in interpenetrating networks, pathways are envisioned toward a new class of tough materials.
1. INTRODUCTION Interpenetrating polymer networks (IPNs) are multicomponent materials in which one component (in the case of semiIPNs) or all the components (in the case of full-IPNs) are cross-linked according to simultaneous or sequential polymerization processes.1 This unique class of polymer blends exhibits a superior combination of the characteristics of its components, attributed to the tight entanglement of the polymeric networks and the phase interpenetration.2 Although to a lesser extent than in conventional polymer blends, the IPNs phase-separate to varying degrees induced by both entropic and enthalpic penalties.3 The phase-separation of IPNs is affected by different parameters, namely the interaction between components, the amount of each component, rate of polymerization process, and the cross-link density.4 While immiscible polymer blends exhibit a weak interphase, the formation of entangled network architecture in IPNs results in extended interphase boundaries.5,6 The applications of IPNs range are quite diverse, including ion exchange resins, adhesives, and noise- and vibration-damping materials.1 Significant research efforts have focused on using the concept of IPN formation to fabricate rubber-toughened materials.2 The main feature required for IPNs to exhibit considerable toughness is that the size of the elastomeric phase should be in the range of 50−500 nm, which is in contrast with the usual case for polymer blends where the domain range is often above © XXXX American Chemical Society
500 nm for optimum toughness. This small feature size of elastomeric domains in IPNs leads to dual phase continuity, which reduces the probability of interfacial debonding under the strain and contributes to the toughening process.1 In IPNs, as constrained multicomponent materials in intimate contact, each component can display disparate mechanical characteristics, and thus, IPNs are considered a viable platform to examine the interaction between the components with respect to mechanical toughness. The elastomeric component utilized as a toughening agent in multicomponent polymeric systems, such as IPNs, dissipates the energy through relaxation of stress transferred from the rigid matrix and, in this way, enhances the mechanical toughness.7 Supramolecular polymers, which are composed of individual molecular units assembled through noncovalent reversible interactions, can relax imposed stresses via the mechanism of dissociation and reassociation of the supramolecular motifs8−11 further than the conventional reptation mechanism.12 It is anticipated that, by utilizing supramolecular elastomers as toughening agents, this dynamic dissociation and reassociation mechanism may enhance mechanics. These supramolecular characteristics hold untapped potential for use in interReceived: August 6, 2015 Revised: September 18, 2015
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DOI: 10.1021/acs.macromol.5b01752 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
oxide (IRGA CURE 819, Ciba) (absorbance band in the range of 300−440 nm) as the photoinitiator in anhydrous chloroform was prepared and added to the solution mixture. The photoirradiation process was carried out in air due to simplicity and to simulate potential application conditions; a high concentration of photoinitiator (10 wt %) was used to overcome the oxygen inhibition effect.16 Films were cast from a mixture solution (3 w/v % in chloroform) containing photoinitiator onto a glass substrate and dried overnight under vacuum at room temperature. UV-irradiation of each sample was achieved using an OmniCure Series 1000 (Lumen Dynamics, U.S.) at an irradiation intensity of 33.4 ± 2.0 mW/cm2 and maximum emitting wavelengths in the range of 320−390 nm for 0, 5, 20, and 40 min. The nomenclature of the studied samples was chosen in the format of HTPB wt %-SPEB wt %-UV irradiation time; for example, HTPB90SPEB10-20 represents an IPN sample with a 90/10 HTPB/SPEB w/w % composition, which was UV-irradiated for 20 min. The thickness of each sample was 50 μm with irradiation penetration of 94% as measured by UV intensity detector. Control IPNs HTPB/PEB, HTPB/EPR, and ATPB/SPEB with weight ratios of 90/10, 70/30, 50/ 50, 30/70, and 10/90 w/w % were also prepared via UV-irradiation at the same time points to reveal the influence of dynamic supramolecular associations and interfacial hydrogen bond formation on the mechanical toughness of supramolecular IPNs. 2.3. Mechanical and Structural Characterization. The tensile properties of the supramolecular IPNs were measured using a Zwick Roell Testing System with a 100 N load cell. Samples were cut into rectangular shapes of 3 mm width × 50 μm thickness with a gauge length of 10 mm. The reported values were the average of five measurements for each sample. Each sample was uniaxially stretched at room temperature at a continuous strain rate of 5%/min. The strain rate-dependent tensile studies were performed at strain rates of 5, 50, and 500%/min. For hysteresis analysis, the loading and unloading cycles were held at a constant 3% strain, while the loading rate was fixed at 500%/min to maintain a constant loading history. However, unloading was carried out at two different strain rates of 5%/min and 500%/min for each sample. Scanning electron microscopy (SEM) was utilized to investigate the phase-separated morphology of the supramolecular IPNs samples and their damage zone after uniaxial tensile testing to failure. The samples were cryo-microtomed at −120 °C, stained by osmium tetroxide vapor for 40 min, and then sputter-coated with a 10 nm thick gold layer. SEM was conducted on a JEOL JSM-6510LV operating at 15 kV with image analysis using ImageJ software. Dynamic mechanical analysis (DMA) was used to study the interactions between phases in the supramolecular IPNs. DMA was performed using a TA Instruments Q800 at fixed frequency of 1 Hz with a heating rate of 3 °C min−1 at temperature range of −80 to 120 °C. The experiments were carried out under tension at a constant displacement of 10 μm on samples of approximately 15 mm × 3 mm × 50 μm. In preparation for transmission electron microscopy (TEM), the IPN samples were cryo-microtomed (Leica Ultramicrotome UC6) using a Diatome cryodiamond knife at −120 °C to obtain thin sections (50−70 nm thickness). These thin sections were vapor-stained with 1 wt % osmium tetroxide aqueous solution for 40 min to selectively stain the polybutadiene phase. TEM was utilized to probe the interphase of the supramolecular IPNs, and images were obtained using a FEI Tecnai G2 Twin Spirit operating at an accelerating voltage of 100 kV. TEM image analysis was done utilizing ImageJ software.
penetrating polymer networks (IPNs), and it is envisioned that incorporation of supramolecular motifs into IPNs will allow for a new level of control over mechanical, physical, and morphological properties in these versatile materials. Although the incorporation of supramolecular motifs has been explored extensively for potential biomedical applications where specific, directional, and dynamic noncovalent interactions led to the development of stimuli-responsive and selfhealing hydrogels,13 potential pathways toward mechanically tough, interpenetrating networks via incorporation of a supramolecular phase have received limited attention. In this study, the ureidopyrimidinone (UPy) supramolecular motif9 was telechelically attached to an oligomeric saturated poly(ethylene-co-1-butene) core to form the dynamic IPN phase. We explored the impact of the UPy supramolecular associations on the IPN interphase and morphology, and the associated mechanical response. With this approach, insight into the interplay between a dynamic noncovalent supramolecular elastomeric network and a covalently linked network was obtained in the development of mechanically robust IPNs.
2. EXPERIMENTAL SECTION 2.1. Materials. Hydroxyl-terminated polybutadiene (HTPB) [Mn = 3000 g/mol, Đ = 2.1, 20% vinyl, 40% cis, and 40% trans configuration] as unsaturated backbone oligomers was purchased from Scientific Polymer Products Incorporated (U.S.), and hydroxylterminated poly(ethylene-co-1-butene) (PEB) [Mn = 3000 g/mol, Đ = 2.2] as a hydrogenated polybutadiene oligomer was supplied by Kraton L2203 grade. An alkyl-terminated polybutadiene (ATPB) (Figure S1) [Mn = 5000 g/mol, Đ = 2.2, 20% vinyl, 40% cis, and 40% trans configuration] as a control for HTPB was purchased from SigmaAldrich. Ethylene-propylene rubber (EPR) [Mooney viscosity = 36 (at 100 °C), 60% ethylene, 40% propylene] as a conventional elastomer was purchased from Scientific Polymer Products Incorporated (U.S.), and used as a control for supramolecular phase. HTPB, PEB, ATPB, and EPR were used as received. All the other chemicals were obtained from Sigma-Aldrich and used without further purification. Poly(ethylene-co-1-butene) functionalized with UPy groups (SPEB) formed the supramolecular elastomeric phase of the IPNs, and was synthesized by following an established protocol with slight modifications.9 The detailed synthetic pathway was reported before in our recent publication14 (Scheme S1). In brief, a 5-fold excess of 1,6-hexane diisocyanate was added to 2-amino-4-hydroxy-6-methylpyrimidine to yield the supramolecular linkage, 2(6-isocyanato hexylaminocarbonyl-amino)-6-methyl-4-[1H]-pyrimidinone (UPy), with a 98% yield. Then, the hydroxyl-terminated elastomeric core (PEB) was reacted with excess UPy in dimethylformamide catalyzed by a few drops of dibutyltin dilaurate. The reaction was completed in ∼2 h as determined by disappearance of the isocyanate peak at 2265 cm−1 via Fourier transform infrared (FTIR) spectroscopy. The product was then purified via treatment with silica to remove unreacted UPy in the presence of dibutyltin dilaurate at 60 °C in chloroform for 1 h. Silica-bound UPy was subsequently filtered, yielding the pure SPEB supramolecular elastomer. 2.2. Preparation of Supramolecular IPNs and UV Irradiation Procedure. The supramolecular IPNs were prepared using a sequential approach.1 Although according to the classical definition of IPNs1 the studied system may be called semi-interpenetrating polymer networks (semi-IPNs) because only one phase forms the chemically cross-linked network, the supramolecular polymer also forms a physical network through dimerization and stacking of UPy linkages15 so we simply designated these systems as “interpenetrating polymer networks (IPNs)”. First, SPEB and HTPB were dissolved separately in anhydrous chloroform as a good solvent to form 3 w/v % solutions. Then, the two solutions were mixed at different weight ratios of 90/10, 70/30, 50/50, 30/70, and 10/90 HTPB/SPEB w/w %. A 10 wt % solution of phenylbis(2,4,6-trimethyl benzoyl)phosphide
3. RESULTS AND DISCUSSION In this investigation, we targeted the examination of the mechanical toughness of supramolecular IPNs that combine dynamic and covalent interactions. UPy-functionalized telechelic poly(ethylene-co-1-butene) was chosen as the model supramolecular elastomer phase due to its ability to form strong and directional interchain hydrogen bonds. The covalently assembled network phase was comprised of hydroxylterminated polybutadiene containing pendant vinyl groups B
DOI: 10.1021/acs.macromol.5b01752 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (A) Mechanical toughness of supramolecular IPNs and the controls at 5, 20, and 40 min UV-irradiation time and (B) engineering stress− strain curves of 20 min UV-irradiated supramolecular IPNs.
sequential assembly of HTPB and SPEB revealed three regimes of mechanical response as a function of composition. In Regime I, yielding behavior and the highest modulus were observed for supramolecular IPNs with SPEB as the minor component (HTPB90-SPEB10-20; HTPB70-SPEB-30-20). HTPB90SPEB10-20 showed obvious yielding with an extensibility of ∼80% and a modulus of ∼209 MPa. Yielding is the point at which a material ceases to deform elastically in a recoverable manner and undergoes permanent irreversible plastic deformation.19 The yielding of the matrix is the primary dissipation mechanism before failure and in this way contributes to the improvement of mechanical toughness.20 The morphology of the damage zone of supramolecular IPNs samples stretched to failure will be probed to find correlation between the observed yielding and toughening mechanism of the supramolecular IPNs (Fractography section). Increase of the SPEB component to 30 wt % (HTPB70-SPEB30-20) led to less dramatic yielding, and lower extensibility (∼40%) and Young’s modulus (∼91 MPa) compared to HTPB90-SPEB10-20. In contrast, supramolecular IPNs at 50/50 w/w% composition (HTPB50SPEB50-20) exhibited a lower Young’s modulus (∼9 MPa) with no obvious yielding in Regime II. Shifting to SPEB as the major component (Regime III), HTPB30-SPEB70-20 and HTPB10-SPEB90-20 displayed the lowest extensibility and modulus among the studied IPNs with extensibility values ∼35% and Young’s modulus lower than 9 MPa. Thus, it is observed that mechanical toughness increased abruptly in Regime I by adding 10−30 wt % dynamic supramolecular component (SPEB), and then, it decreased substantially in Regimes II and III with the inversion to SPEB as the major phase. Prior art has reported a similar trend in elastomertoughened IPNs where toughness enhancement showed a linear increase up to a certain level of elastomeric content and then a plateau or decrease beyond this level.19,21,22 To probe the dynamic effect of the minor supramolecular elastomeric phase, we compared the mechanical toughness of HTPB90-SPEB10-20 with that of HTPB90-EPR10-20, which is an analog IPN containing ethylene-propylene rubber (EPR) as the minor phase. A substantial enhancement was observed in the tensile toughness of the matrix (∼600× for HTPB90SPEB10-20 (Figure S2, Table S1)) compared to ∼4× enhancement for the control IPN that contained EPR as a
suitable for UV-initiated cross-linking. Implications on IPN assembly and mechanical response were probed via this structural interplay. 3.1. Mechanical Response. 3.1.1. Effect of UV-Irradiation Time and Composition. Initial studies explored the tensile response (Figure S2, Table S1) of a series of HTPB/SPEB supramolecular IPNs with compositions ranging from 90/10 to 10/90 w/w % with a focus on the influence of UV cross-linking time on the overall toughness (area under the stress−strain curve). Figure 1A shows the toughness as a function of irradiation time (5, 20, 40 min). It has been shown previously that, beyond 40 min, the PB network is too tightly cross-linked for mechanical enhancement via supramolecular associations.14 At an irradiation time of 5 min, the supramolecular IPNs exhibited low (
