Manipulation of multiphase morphology in the reactive blending system OBC/PLA/EGMA

Abstract

In recent years, biodegradable polymers derived from bioresources have received more and more attention. Making a new type of blend by compounding biodegradable polymer with a petroleum-based polymer is now an important method of preparation for such polymers. A key issue to dominate the quality of a polymer blend is manipulation of multiphase morphology. In this study, a bio-degradable polymer, poly(lactic acid) (PLA), was blended with a new thermoplastic elastomer, olefin block copolymer (OBC), through melt mixing using ethylene-glycidyl methacrylate (EGMA) as a compatibilizer. In this blending system, only physical interaction exists between OBC and EGMA, but chemical reaction occurs between PLA and EGMA. This significant asymmetry of interaction and compatibility offers the opportunity for yielding special dispersed-phase structures during melt blending. By altering the blending sequence, EGMA amount, blend time, and OBC hard-segment content, some interesting substructures of dispersed-phase were achieved such as core–shell, subinclusions, co-continuous, salami and micelle. This study offers good insight into designing the multiphase morphology via competition between compatibilization and intra-particle reaction.

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Blending of existing polymers provides a cost-effective method to modify the properties of polymer materials and meet the increasing requirements of applications.¹ There has been considerable scientific and industrial interest in multiphase morphology of polymer blends, because it plays a crucial role in determination of the properties. Theoretically, the morphology of polymer blends is predominantly affected by thermodynamic factors, i.e., interfacial tension between polymer pairs.^2,3 For ternary mixtures that contain two minor phases and a matrix phase, there are mainly three types of thermodynamically equilibrium morphologies: core–shell morphology, stack or partially encapsulated morphology, and isolated morphology.² The structure that is formed can be perfectly predicted by some thermodynamic parameters such as the spreading coefficient, which is calculated by interfacial tension.² However, because of the complicated situation during melt blending, the influence of processing conditions should be taken into consideration. Some thermodynamically non-equilibrium structures that are controlled by kinetic factors, such as composition, viscosity⁴ and elasticity,⁵ can be frozen in blend materials. For example, subinclusion or a composite droplet microstructure has been observed in poly(methyl methacrylate) (PMMA)/polypropylene (PP)/polystyrene (PS) blend,⁶ PS/styrene-butadiene rubber (SBR)/polyethylene (PE) blend⁷ and high density polyethylene (HDPE)/PS/PMMA blend,⁸ in which a few PMMA objects were encapsulated in a dispersed PS phase. However, through quiescent annealing, dual coalescence between not only composite droplets themselves but subinclusions within droplets occurred.⁸ Although some inclusion substructures are formed in polymer blends prepared by common blending (without in situ reaction), they are not uniform throughout the matrix phase and are thermodynamically stable, and are thus possibly destroyed during the second stage of processing, e.g., the molding process, and less likely to present the property advantages of specific phase morphology.

The introduction of chemical reaction during melt blending has been widely proven to be an effective way to control the morphology of multicomponent polymer blends. The as-generated morphology features a small domain size, good stability and complicated phase structure. A number of research groups have proposed the effect of in situ generated block/graft copolymers for improving interfacial adhesion, decreasing interfacial tension and reducing dispersed particle size via hindering inter-particle coalescence, which is known as in situ compatibilization.^9–12 However, besides the compatibilizing effect, the copolymers also have a significant effect on the formation and evolution of versatile phase patterns in blends. Horiuchi et al.³ changed the phase morphology of polyamide-6 (PA6)/polycarbonate (PC)/poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) blend, in which PA6 is the matrix phase, whereas PC and SEBS are minor phases, from a stack-type to a partially encapsulated-type and then to a capsule-type by gradually incorporating more amount of maleinated SEBS (SEBS-g-MA). This change is caused by a significant decrease in interfacial tension between PA6 and SEBS, as predicted by the spreading coefficient model. Copolymers were also reported to stabilize metastable microstructures.^12,13 Leibler et al.¹³ prepared a blend of PA6 and random copolymer of ethylene and maleic anhydride (PE–MAH). A thermodynamically stable co-continuous morphology was obtained with two continuous phases, one PA6/PE-g-PA6 and another unreacted PE. The composition polydispersity and randomness of in situ grafted copolymers were suggested to contribute to the formation of such a unique multiphase morphology. Another interesting role that copolymers play is to introduce substructure within dispersed-phase domains due to their self-assembly nature. Jerome et al.^14–16 utilized anhydride end-capped polystyrene-b-polyisoprene (PS-b-PIP-anh) to compatibilize PA12 and PS. Upon changing PS/PIP ratio in the diblock copolymer, various phase patterns ranging from core–shell, vesicular structure (PA12 core is covered by a self-assembled multilayer of in situ generated PA12-b-PIP-b-PS) to irregular cellular structure, which were formed by the aggregation of core–shell particles, emerged. In other studies,^17,18 salami-like substructure has been found in a ternary mixture of poly(lactic acid) (PLA), ethylene/n-butyl acrylate/glycidyl methacrylate (EBA–GMA) terpolymer and a zinc ionomer of ethylene/methacrylic acid (EMAA–Zn) copolymer, where EMAA–Zn droplets are trapped in the EBA–GMA domain due to its chemical interaction with the terpolymer and incompatibility with PLA. Generally, in reactive blending, copolymers are generated at the interface region and thus can cause further changes in interface morphology, composition and interfacial tension to alter the compatibility and interfacial interaction between components.¹⁹ Moreover, with the increasing amount of generated copolymers, their dispersion and distribution behaviors would also change.^20–22 These might, on the one hand, complicate the quantitative analysis and theoretical prediction involved in reactive blending, while on the other hand, can allow for a versatile and sophisticated control over the multiphase morphology.

Among the various dispersed-phase structures, salami-like microstructure in multiphase blends are particularly attractive, where dispersed spherical domains in the matrix phase are heavily filled with subdomains of another component. High-impact polystyrene (HIPS) owes its superior impact toughness to this special microstructure, which is formed in situ when styrene is polymerized in polybutadiene (PB)/styrene solution.²³ The multiphase morphology consisting of salami-type dispersed droplets was also found in a few other blends, e.g., PMMA/poly(ethylene-co-propylene),²⁴ poly(2-chlorostyrene)/PS²⁵ and poly(ethyl acrylate)/poly(ethyl methacrylate)/PMMA,²⁶ which were all prepared by the in situ polymerization way. However, heavily filled salami-type microstructures obtained via a molten blending technique has rarely been reported in the literature. In our present study, typical salami-like microstructures are achieved in a ternary, reactive blending system consisting of olefin block copolymer (OBC) as the matrix phase, poly(lactic acid) (PLA) as the minor phase (dispersed phase), and a random copolymer of ethylene and glycidyl methacrylate (EGMA) as the compatibilizer. OBC is a novel polyethylene thermoplastic elastomer (TPE) with promising prospects. For sustainable development concerns, combining PLA with the more widely used petrochemical-based polymers such as OBC may not only compensate for their drawbacks but be of great value for environmental conservation by reducing the demand for oil resources. In the design of blend systems containing PLA and petrol-based polymer, PLA was often chosen as the matrix phase, due to its great advantage of biodegradability. PLA acting as the minor phase to construct blend systems is beneficial for extending its application areas, for example reinforcement of soft TPE.

Two types of commercially available OBC (INFUSE 9507 and 9530, Dow Chemical) were used as the matrix resin. One contains 15.3 mol% octane co-monomer and 12 wt% hard blocks and is named OBC12. The other with these two parameters as 10.4 mol% and 35 wt%, respectively, is named OBC35. The weight-average molecular weight (M_w) and polydispersity index (PDI, the ratio of M_w to number-average molecular weight, M_n) are 90.8 kg mol⁻¹ and 2.48 for OBC12, and 78.6 kg mol⁻¹ and 2.60 for OBC35, respectively. The melt index of OBC12 is 5.1 g/10 min (190 °C, 2.16 kg) and that of OBC35 is 5.0 g/10 min. EGMA has 24 wt% methyl acrylate, 8 wt% glycidyl methacrylate and 68 wt% ethylene. It has a M_w value of 141 kg mol⁻¹ and PDI value of 2.32. PLA (4032D, Nature Works LLC) contains 1.2–1.6% D-isomer lactide with M_w of 142 kg mol⁻¹ and PDI of 1.35. A series of ternary blends were prepared by melt mixing in a Haake Rheometer (HAAKE Polylab OS, USA) with a rotation speed of 60 rpm at 190 °C. Premixing was carried out for 4 min and the second-stage mixing for 5 min. The time for one step mixing was 5 min as well. The phase morphology was observed with an FEI Inspect F field-emission scanning electron microscope (FE-SEM, USA) on cryo-fractured samples, and a transmission electron microscope (TEM, Tecnai G2 F20, USA) was used for samples cryo-microtomed at −100 °C and strained by RuO₄ for 10 min.

According to Fig. 1, OBC is a block copolymer of alternative hard and soft polyethylene blocks synthesized by “chain shuttling polymerization”. The “hard” and “soft” blocks are termed according to the “crystal” and “amorphous” aggregation structure they would form, respectively. The hard blocks contain a lower level of α-octane co-monomer and less short branches, while soft ones are of much higher α-octane co-monomer content and more branches.²⁷ Due to the great difference of chain polarity between OBC and PLA, these two components are mutually incompatible, consistent with the morphology result shown in the SEM image of the OBC/PLA 80/20 blend. Large spherical PLA particles with micron size are dispersed in OBC and their interfaces appear to be clear and sharp. The ethylene segment (–E) in the compatibilizer, EGMA, is expected to entangle with the ethylene blocks on the backbone of OBC. They show good compatibility, as proven by SEM observation. Although there are some large EGMA domains observed, most of the particles are quite small and the interfaces are too indistinct to be seen clearly. Rheological test¹⁹ also indicates strong interaction or chain entanglement between these two polymers, as seen in Fig. S1.† Another segment in EGMA, the –GMA, is widely reported to form chemical bonds with PLA, i.e., epoxide groups of –GMA react with the terminal carboxyl groups of PLA.^{17,18,28–30} Thus, blending of these two polymers generates the in situ grafted copolymer, where EGMA acts as the backbone with PLA as grafting chains (EGMA-g-PLA). No appreciable phase segregation can be seen in the micrograph for the PLA/EGMA blend mixed for 20 min. Indeed, numerous nanometer-scaled EGMA-g-PLA micelles generated in molten PLA/EGMA blend,²⁹ indicating excellent compatibility between these two polymers. Furthermore, the interfacial tensions measured by contact angle tests are distinctly different, with values of 13.1, 5.2 and 1.2 mN m⁻¹ for OBC/PLA, OBC/EGMA-g-PLA and PLA/EGMA-g-PLA pairs, respectively.³¹ There is significant asymmetry of interfacial interaction and compatibility between the three polymer pairs among OBC, EGMA and PLA, which may induce preferential location of EGMA and EGMA-g-PLA in PLA phase domains. As a result, substructured dispersed-phase microstructures in such ternary blends are highly desired. Subsequently, we attempt to manipulate the dispersed-phase microstructure in ternary OBC/PLA/EGMA blends by altering various conditions like melt mixing sequence, EGMA amount, reactive grafting level and OBC hard block content.

Fig. 1 Schematic showing the interaction types and phase morphologies in three polymer pairs among OBC, EGMA and PLA (the scale bars in SEM micrographs represent 5 μm).

Fig. 2 demonstrates the morphology of OBC12/PLA/EGMA 80/14/6 ternary blends prepared with three mixing sequences: for the methods of (a) and (c), EGMA was premixed with PLA and OBC, respectively, then a third component was added, while PLA, OBC and EGMA were simultaneously compounded together in method (b). As OBC and EGMA can be stained by RuO₄, which has been reported to be able to stain saturated polymers containing HDPE,³² bright areas in the TEM images should be PLA-rich regions. When PLA and EGMA were mixed first and then added to OBC, a typical salami morphology, quite similar to that in HIPS,²³ was observed, as shown in Fig. 2(a), with dispersed domains strongly filled with smaller-sized, light subdomains of PLA (diameter of 0.1–0.4 μm). The EGMA-g-PLA (dark) forms thin lamellar with a width of 30–100 nm between subdomains and on the outside around the dispersed particles. The darker areas in the background are crystalline lamellas of OBC hard blocks. In a one-step-mixed sample, however, most dispersed droplets appear to not be typical salami but a quasi-salami microstructure, where several large irregular-shaped PLA subdomains are segregated by an EGMA-g-PLA phase and some interfaces between these two phases are indistinct, as shown in Fig. 2(b). However, in some other smaller droplets, there is no distinct substructure observed. Moreover, the average diameter of dispersed particles decreases from 1.1 μm (Fig. 2(a)) to 0.7 μm (Fig. 2(b)). When the two-step method with EGMA premixed with OBC was utilized, the average dispersed domain size further decreased to 0.5 μm. In Fig. 2(c), although a few quasi-salami structured domains can still be seen, the majority of the dispersed particles are discrete simple droplets with the size much smaller than that of the substructured ones.

Fig. 2 TEM micrographs of ternary OBC12/PLA/EGMA 80/14/6 blends prepared upon different mixing sequences: (a) PLA and EGMA were mixed first and then OBC was added, (b) three components were blended together simultaneously, and (c) OBC and EGMA were premixed then blended with PLA (the scale bar is 1 μm).

By controlling blending sequences, the diffusion and migration of EGMA during melt mixing can be kinetically controlled. The main difference of the three strategies is that the difficulty of EGMA reaching and reacting with PLA increases from the first method to the third one. For the first method, EGMA reacts with and is trapped in PLA during premixing. The extent of reaction is very low due to the short premixing time,²⁹ which is also illustrated by the TEM images presented in Fig. S2.† Further reaction occurs within PLA rich domains during the second-stage blending. Some of the generated EGMA-g-PLA migrates to OBC/PLA surfaces to reduce interfacial tension, but most of them are arrested in PLA together with the unreacted EGMA, making a separated layer zone in salami-like dispersed domain. However, the situation in one-step blending is clearly different. EGMA has to diffuse through the OBC bulk to the interface first, react with PLA at the interface and migrate into PLA rich domain for further reaction. As a consequence, more EGMA-g-PLA than that in OBC12/(PLA/EGMA) resides at the interface and less within the dispersed domain, inducing dispersed particle size decrease and PLA subdomain size increase and the generation of simple core–shell droplets. Moreover, the difficulty for the reaction of EGMA with PLA increases and the content of in situ formed copolymers may decrease. Similarly, EGMA might diffuse through OBC to PLA when the third method is used, and the process should be more difficult due to the pre-entanglement of EGMA with OBC. This results in the production of more particles without substructure and less quasi-salami domains, suggesting the highest ratio of interface EGMA-g-PLA to subincluded reacted and unreacted EGMA. From discussions above, it can be inferred that the formation of ultimate multiphase morphology should be dominated by the competition of two processes, i.e., compatibilization and intra-particle reaction. The former drives compatibilizing phase (EGMA) to locate at the interface, reducing interfacial energy, elevating affinity between different component phases, and decreasing size of dispersed-phase domains; the latter, referring to in situ reaction that occurred in the dispersed-phase droplets during melt blending process, making EGMA to migrate into dispersed particles and react with PLA. From the viewpoint of salami pattern formation in our ternary blends, the former process should be restrained, while the latter needs to be promoted.

Besides altering the mixing order, varying initial PLA/EGMA ratio can also control EGMA migration into PLA domain. Ternary blends with different EGMA content are prepared by the first method and their morphologies are shown in Fig. 3. When the EGMA content is slightly reduced, from 6 wt% (Fig. 2(a)) to 4 wt% (Fig. 3(a)), the salami microstructure still remains. Further decreasing the EGMA content to 2 wt% results into a typical subinclusion microstructure, with nanometer-sized (50–100 nm) EGMA-g-PLA micelles dispersing in PLA rich particles, as shown in Fig. 3(b). Phase inversion occurs when the previous discontinuous PLA phase takes over the matrix within the dispersed domain. The microstructure develops into the classical core–shell one, i.e., the PLA core packaged by an EGMA-g-PLA shell, when the EGMA content is as low as 0.4 wt%, suggesting a compatibilization-dominated morphology formation process. Clearly, it is of top priority for EGMA-g-PLA trapped in PLA to migrate to the interface of OBC and PLA and meet the requirement of reducing interfacial tension. With the EGMA content gradually decreasing, the extent of reaction between PLA and EGMA after first step mixing may increase, resulting in a simultaneous decrease of intra-particle reaction and EGMA content during the second-stage processing.

Fig. 3 TEM micrographs of ternary OBC12/(PLA/EGMA) blends prepared upon premixing of PLA and EGMA then blending with OBC for various compositions: (a) 80/(16/4), (b) 80/(18/2), and (c) 80/(19.6/0.4) (the scale bar is 1 μm).

To better understand the importance of intra-particle reaction in the formation of salami morphology, we prolonged the time for premixing PLA and EGMA to 20 min, for ensuring a close complete reaction between them, and then blended the mixture with OBC. From TEM images demonstrated in Fig. 4, it is found that the as-formed OBC12/(PLA/EGMA) 80/(14/6) blend exhibits a completely different phase morphology, as compared to the 4 min-premixed sample. Considerably smaller PLA domains without substructure are dispersed in the OBC matrix. Because the reaction extent is quite high during premixing, no intra-particle reaction in the second stage of melt processing can occur as reactive blending but it behaves as common blending of three components. Moreover, the size of the dispersed domain is considerably small (50–300 nm), indicating that reacted PLA makes up a high percentage of total PLA and a considerable amount of graft copolymers directly form micelles. When the intra-particle reaction is totally suppressed, normal in situ compatibilizing behavior of copolymers is observed in the blending system. In addition, most of the dispersed particles are not purely spherical, which is probably caused by the fast processing induced deviation from the equilibrium state. It is therefore clear that in situ reaction between EGMA and PLA is essential for the formation of salami-like cellular domains in OBC12/PLA/EGMA blends. Comparing the morphology of the complete salamis (Fig. 2(a)) with that of the incomplete ones (Fig. 2(b) and (c)), it can be reasonably inferred that salami-type subdomains are generated through EGMA simultaneously reacting with PLA component and cutting PLA domain. The driving force of this process is to generate more EGMA/PLA interfaces because the chemical reaction can lead to a significant free energy drop. Moreover, the in situ grafted copolymers are located at the surface of subdomains and stabilize the microstructure.

Fig. 4 TEM micrographs of ternary OBC12/(PLA/EGMA) 80/(14/6) blend by premixing of PLA/EGMA for 20 min: (a) low magnification and (b) high magnification (the scale bar is 1 μm for (a) and 500 nm for (b)).

Fig. 5 indicates the effect of OBC hard block content on the multiphase morphology of OBC/(PLA/EGMA) blends that were prepared by the two-step procedure with PLA and EGMA premixed first and then blended with OBC. The two types of OBC are specifically chosen such that they have similar molecular weights, PDI and especially the same melt index, to ensure that the comparison is only between block fractions. The clear interfaces between the dispersed phase and matrix phase can be found for the case of OBC12, as shown in Fig. 5(a). However, the interfaces are difficult to be recognized when OBC35 was used as the matrix phase (see Fig. 5(b)). This phenomenon is attributed to better compatibility between OBC35 and EGMA, which is also shown in Fig S1.† TEM technique was utilized to reveal the internal microstructure of the dispersed particles. A co-continuous substructure is observed in dispersed domains, because neither gray zone (EGMA-g-PLA and unreacted EGMA) nor white zone (PLA) can be encapsulated completely by the other one. Moreover, the average size of the dispersed droplets was 0.9 μm, smaller than that of OBC12 blends. Because the viscosity of OBC12 is not less than that of OBC35 in a wide range of sweep frequency (0.01–100 Hz), as tested by a melt rheometer (see Fig. S3†), the reason for particle size decrease should be more compatible at the interface because of a stronger interaction between EGMA and OBC35. In addition, because OBC35 contains harder ethylene blocks, it can be proposed that the hard blocks entangle more tightly with the –E segments in EGMA. The strong interaction between the OBC35/EGMA pair, on the one hand, drives the EGMA phase (reacted and unreacted) to the interface, resulting in a domain size decrease, on the other hand, delays the in situ reaction between PLA and EGMA and depresses the formation of cellular subdomains.

Fig. 5 SEM images of (a) OBC12/(PLA/EGMA) 80/(14/6) and (b) OBC35/(PLA/EGMA) 80/(14/6) blends. (c) TEM micrograph of OBC35/(PLA/EGMA) 80/(14/6) blend (the scale bar is 3 μm for SEM images and 1 μm for TEM image).

A summary of substructure patterns in OBC/PLA/EGMA ternary blends is schematically shown in Fig. 6. Fine salami domains are generated under the following conditions: (a) the content ratio of EGMA to PLA is relatively high, i.e. 3/7 or 2/8, (b) the EGMA-g-PLA makes up a high percentage of total components containing EGMA, and (c) the graft copolymers are generated by in situ reaction in the dispersed-phase domain. When the content of EGMA is too low to form a continuous phase, the generated graft copolymers would self-assemble into micelles in the PLA phase, resulting in a typical subinclusion microstructure. The substructure disappears and a classical core–shell microstructure is obtained, further reducing the EGMA content. On the other hand, by changing the mixing sequence and increasing the content of OBC hard blocks, the formation of in situ grafted copolymer becomes difficult. In the one-step method with OBC/EGMA premixed samples, the delay of EGMA migrating into PLA and reacting with it leads to a lower content of graft copolymers in the dispersed domain. Similarly, stronger interaction between OBC matrix and EGMA also drives the EGMA components to locate at the interface, delaying both the reaction and substructure formation. Another important factor for microcellular salami domains is in situ reaction during the melt blending process. A blend prepared by premixing PLA and EGMA for a sufficiently long time exhibits the phase morphology fashion of micelles, and nanometer-scaled core–shell droplets directly disperse in the OBC matrix.

Fig. 6 Scheme of substructures of dispersed-phase in ternary OBC/PLA/EGMA blends prepared under different conditions.

The results elucidated above indicate that one can manipulate a series of substructures of dispersed-phase domains in reactive blending multiphase mixture in which significant asymmetry of interaction exists between different component pairs, i.e., chemical interaction of dispersed phase/compatibilizer and physical entanglement of the matrix phase/compatibilizer. The competition between compatibilization and intra-particle reaction leads to preferential distribution and reaction of the compatibilizing phase composed of unreacted EGMA and graft copolymers at the interface or inside the dispersed-phase domains. When the former process is restrained by kinetically arresting EGMA in PLA first, intra-particle reaction occurs accompanied with salami substructure generation. Moreover, a reversed entrapment of EGMA in OBC hinders the latter process and accelerates the compatibilizer to locate at the interface, resulting in a compatibilizing-dominated morphology. Similarly, high graft copolymer content before second-stage mixing is favorable for compatibilization.

In conclusion, it is suggested that the reaction situation of EGMA with PLA plays a dominant role in the formation of dispersed-phase substructures due to the competition between two issues, compatibilization and intra-particle reaction, which drives the compatibilizing components to locate at an interface region or make them migrate and/or retain in dispersed-phase droplets. Using a two-step mixing procedure with PLA and EGMA premixed, a series of microstructure including core–shell, subinclusion and salami is obtained sequentially as the amount of EGMA was increased. The salami microstructure would develop in a micellar fashion only when the extent of PLA/EGMA after the first mixing stage is prominently increased, even if the EGMA content is sufficiently high. On the other hand, altering the mixing sequence and increasing the OBC hard block content mildly depressed the formation of salami-like microstructure, resulting in quasi-salami or co-continuous substructure of the dispersed phase. Thus, the favorable conditions for fine salami morphology were demonstrated unambiguously.

Acknowledgements

Financial support from the NSFC (51373108 and 21574088) and the Science & Technology Department of Sichuan Province (2013TD0013) are gratefully appreciated.

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Footnote

† Electronic supplementary information (ESI) available: Plot of η* at 190 °C vs. time for OBC12/EGMA, OBC35/EGMA, and OBC35/PLA; TEM micrographs of PLA/EGMA 7/3 binary blend; a plot of η* at 190 °C vs. frequency for OBC12, OBC35, and PLA/EGMA 7/3. See DOI: 10.1039/c5ra12271f

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