Reactive Block Copolymers as Versatile Compatibilizers - American

Chapter 24

Reactive Block Copolymers as Versatile Compatibilizers 1

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Leticia Flores-Santos , Alfonso González-Montiel , and Enrique Saldívar-Guerra 2

Downloaded by COLUMBIA UNIV on March 9, 2013 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch024

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CID, Centro de Investigación y Desarrollo Tecnológico, S. A. de C.V., Av. de los Sauces No 87 Mz 6, Parque Industrial Lerma, Lerma, Edo. Mexico, CP 52000, México Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, Saltillo, Coahuila 25100, México

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A series of block copolymers synthesized via CRP containing epoxy functional groups are used as compatibilizers for a variety of polymer blends. The CRP technique allows fine tuning of the architecture, molecular weight of reactive and non reactive block, the amount of functional groups and the composition of the non reactive block in order to obtain an optimal compatibility between two non miscible polymers. A selection based on the performance of these copolymers as compatibilizers in polycarbonate and polyethylene terephtalate blends will be addressed and a correlation of the average particle size observed in optical or transmission electron microscopy with the intrinsic variables of the compatibilizer (Mn, composition, etc.) is also discussed.

342

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.


343 The blending of polymers provides a powerful route for obtaining materials with improved property/cost performance. Since most blended polymers are immiscible, compatibilization is required to obtain maximum synergy. This strategy is usually more cost effective and less time-consuming than the development of new monomers and/or new polymerization routes, as the basis for entirely new polymeric materials. An additional advantage of polymer blends is that a wide range of material properties is within reach by merely changing the blend composition (7). Compatibilization of polymer blends can be achieved using specialized additives which are typically macromolecular species exhibiting interfacial activity in heterogeneous polymer blends (2, 3). Usually the compatibilizers are copolymers with a block or graft structure with one constitutive block/graft miscible with one blend component and a second block/backbone miscible with the other blend component. Other option for compatibilization is the addition of a reactive polymer, miscible with one blend component and reactive towards functional groups attached to the second blend component, which results in the in-situ formation of block or grafted copolymers. This technique has certain advantages over the addition of pre-made block or grafted copolymers. Usually reactive polymers can be generated by free radical copolymerization or by melt grafting of reactive groups onto a polymer backbone. Furthermore, reactive polymers only generate block or grafted copolymers at the site where they are needed, i.e. at the interface of an immiscible polymer blend. By the early 1970's compatibilizers based on maleated polypropylene were available for the manufacture of polyolefm based composite materials. The maleic anhydride domain of these compatibilizers can react with nucleophilic groups in polyamides, polyesters and polycarbonates, and with amino silanes (4). The maleic anhydride graft occurs naturally on the ends of the polypropylene chain, obtaining a non-random structure. Attempts to apply the analogous solution to styrenic systems of blends have been without success, mainly because the architecture of the compatibilizers is random, there are no separate domains, and therefore, there is no domain that is compatible with the polystyrenic phase of the blends. As a result, the required improvement in the physical properties of the blends is not achieved, and, indeed, sometimes there is even a degradation of physical properties compared to the same alloy without the proposed compatibilizer (5, 6, 7).

Block Copolymers as Compatibilizers In order to obtain well defined block copolymers to be used as compatibilizers, several approaches can be taken; one of them is the use of living polymerization processes (S). In living polymerization processes, termination reactions are suppressed or significantly reduced, allowing the formation of



344 block copolymers. It is possible to produce block copolymers by anionic polymerization, However due to the sensitivity of the process to impurities such as water and its intolerance of many functional groups, the process presents limitations. A more promising technique for producing block copolymers with a large variety of monomers is that based on living or quasi-living free radical polymerization (P). This can be achieved by adding to an otherwise standard free radical polymerization recipe, a chemical agent that significantly reduces the extent of irreversible termination or chain transfer reactions, providing a living or quasi-living character to the polymerization, which is also called "controlled polymerization" or "controlled free radical polymerization" (CRP) (70). There are several chemical routes for CRP polymerizations but most of them are limited in an industrial practice because they require chemical agents that are not readily commercially available in the market. Among these techniques, one that is particularly useful, and for which the required chemical agents are available in the market, is a quasi-living free radical polymerization controlled by nitroxides (nitroxide mediated radical polymerization, NMP), and derivatives thereof (like alkoxyamines (77)), which act as stable free radicals capping polymeric growing radicals and uncapping them in a fast and reversible way, allowing for short periods of propagation through monomer-addition steps (12-29).

Compatibilizers Design Compatibilizers presented in this work were designed as block copolymers with functional groups for in-situ compatibilization. In this way a variety of monomers can be incorporated, obtaining a controlled microstructure and composition. These diblocks (shown in figure 1) have one block miscible with one component of the polymer blend, and another block that contains functional molecules like anhydride, epoxy or acid, which can react with a variety of polymers. CRP allows control over the molecular weight of both blocks and over the degree of functionality in the second block. The advantages of this strategy are that functionality is obtained where it is needed and that molecules can be designed to have a variable degree of functionality to match different applications. These advantages are supposed to render compatibilizers superior to other molecules containing functional groups that are random copolymers or are produced via reactive extrusion. The functional copolymers obtained via CRP can be used as compatibilizers for several polymer systems, examples are: polystyrene, high impact polystyrene and polystyrene compatible polymers (polyphenylene oxide) with glass fiber and polar fillers. They can also compatibilize blends of styrene, high impact polystyrene or styrenic copolymers (ABS, SAN, St-co-MMA, etc.) with polycarbonates and polyamides to name a few.


345 Block

A


Miscible Portion

Control over length + versatile composition: PS, P(S-w-BuA), Ρ (SAN)

Block Β Reactive portion

Control over length and functionality Varying types and degrees of functionality: ΜΑΗ H> epoxy acid Alternate (MAFI), random and end location of functionality

Figure 1. Schematic representation of diblock copolymers which can be used as compatibilizers.

Experimental Polymers used in the blend studies were commercial grades obtained with different manufacturers: Amorphous PET was acquired from Eastman (EASTAR copolyester 6763, Mn=26,000); Polycarbonate was acquiredfromGE (Lexan 121, Mn=19520, PDI=2, MFI @300°C, 2160g=27.5 g/10min); St-coM M A (CET 115, Mn=81326, PDI=2.6 ) copolymer and polystyrene (HF777, Mn=96059, PDI=2.1) were acquired from RESIRENE. The NMP regulators are 4-hydroxy-2,2,6,6-tetramethylpiperidin-l-oxyl (4-hydroxy-TEMPO) from CIBA, and alkoxyamine CGXPR330 provided by CIBA (30). These regulators were used as received. Styrene from Aldrich was washed with a NaOH solution in order to remove the inhibitor and dried with anhydrous sodium sulfate.

Instrumentation. Molecular weight was determined using a Waters 410, RI detector, THF eluent, 1.0 mL/min, at 40°C; Styragel linear columns HR 5, HR 4, HR 3, using polystyrene as a standard. Molecular weights of the first block of the copolymers


346 and total are experimentally determined whereas Mn of the second block is calculated as the difference between Mn of the diblock minus Mn of the first block.


Compatibilizers Synthesis. The synthesis of the compatibilizers proceeded in two steps. First a reactive block copolymer was synthesized using either NMP regulators in a bulk-batch process at temperatures between 120-150°C. The monomers ratio and the controlling agent concentration were determined by the amount of epoxy groups desired in each diblock, and the desired molecular weight. In the case of 4hydroxy TEMPO, BPO was used as an initiator at a constant ratio ([4-hydroxy TEMPO]/[BPO]= 1.3:1. The resulting polymer of the first block was dissolved in the monomers used for the second block. The second block was synthesized in the same bulk-batch process at the same temperature range to reach a final conversion according to the desired molecular weight of the diblock. The general structure of the reactive copolymer is depicted in Figure 1. The copolymer is composed of a reactive block with epoxy functionality and a block that is typically a homopolymer.

Blending conditions. PET and PC were dried overnight in a vacuum oven at 65 °C and 100°C respectively, prior to processing them. PET/PSt-co-MMA blends were prepared by direct melt blending of the compatibilizer and the polymers in a H A A K E mixer at 150°C and 60rpm. PC/compatibilizers and PC/PSt blends were also prepared by direct melt blending in a HAAKE mixer at 270°C and 60 rpm.

Microscopy. Specimens for microscopy were prepared by trimming samples with a razor blade to form blocks of approximately 5 x 5 x 2 mm. These blocks were further trimmed to the shape of a truncated pyramid. An RMC MT-X ultramicrotome was used to obtain thin sections (50 nm thick) from these blocks. Sections were analyzed in an OLYMPUS M50 optical microscope in the case of large size particles or with a JEOL TEM microscope operating at 120 kV. Phase contrast between the different components of the blend was achieved by staining with Ruthenium oxide to distinguish the styrenic phase.


347

Results and Discussion


In this work, two polymer blends will be discussed: PET/PSt-co-MMA and PC/PSt. The compatibilizers are diblock copolymers with one functional block, capable of reacting with PET or PC and a second block miscible with St-MMA or PSt. Table 1 depicts the variety of compatibilizers with different molecular weights at the first and the second block and also with different amounts of epoxy groups used as compatibilizers. For both blends, the resulting average particle size of the dispersed phase is discussed in terms of the compatibilizers structure.

Table I. Compatibilizers of the general structure presented in figure 1. Functional groups refers to the number of epoxy molecules in block one.

First Compatibilizer Block ID Mn 1 2 3 4 5 6 7 9

7826 15269 15965 20212 23156 24144 27526 27682

First Block Epoxy Diblock Diblock PDI Groups Mn PDI 1.13 1.10 1.13 1.12 1.17 1.14 1.19 1.10

11 2 10 21 24 28 34 9

16782 50648 25329 92618 91460 67697 49768 93273

1.20 1.50 1.21 1.54 1.41 1.55 1.50 1.54

PET/PSt-co-MMA blends. Blends were prepared (according to the experimental procedure) in a 80/20/3 weight ratio of amorphous PET, St-MMA and compatibilizer. Figure 2 shows photomicrographs of the resulting blends. It is possible to observe that the non compatibilized blend has particles whose mean diameter is of 1630nm, while using different compatibilizers, particle sizes can be reduced to lOlOnm and down to 347 nm depending on the compatibilizers structure. Since the structure and composition of the compatibilizers can be controlled, relations between the compatibilizers properties and their performance, seen through particle size in this case, can be found. In the group of graphs shown in Figure 3, the effect of the molecular weight of the functional block and the nonfunctional block, and the number of functional molecules on the particle size can be analyzed. In Figure 3A, it can be observed that there is no apparent relationship between the molecular weight of the functional block


348


Β

Figure 2. Three examples of PET/St-co-MMA blends. A. Optical microscopy photograph of the non compatibilized blend showing particles with a mean diameter of1630nm; B. Optical microscopy photograph of a compatibilized blend showing particles with a mean diameter of 101 Onm; C. TEM photograph of a compatibilized blend showing particles with a mean diameter=347nm.

and the particle size of PSt-co-MMA. Figure 3B shows that two of the three best blends have a low molecular weight of the nonfunctional blockwhile one has a relatively high molecular weight, so a low molecular weight in this particular block might help to obtain smaller particle sizes. Finally, the number of functional groups in the compatibilizer is the key to obtaining small particles. Figure 3C shows that the three best compatibilizers have an average number of epoxy groups per chain of about ten. This number of epoxy groups seems to be the ideal number for the specified blend.



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Figure 3. Graphs showing the influence of the number average molecular weight of the functional block (A), the number average molecular weight of th nonfunctional block (B) and the number offunctional groups (C) on the mea diameter ofPSt-co-MMA particles observed in optical microscopy or TEMof PET/PSt-co-MMA blends. The best three blends (the ones with smaller particl size) are grouped in an oval.



350 It is generally recognized that in ternary blends of two homopolymers (A and B) with a copolymer A-B the molecular weight of the copolymer blocks should be higher than the molecular weights of the corresponding homopolymers, so as to maximize segmental repulsion between the individual blocks compared with the parent homopolymers (7). It has also been reported that the emulsifying effect of a block copolymer in such blends increases as the molecular weight of the copolymer increases up to a certain point where the tendency seems to reverse. This is understood by considering the possibility of micelle formation as the copolymer molecular weight increases leading to a three state equilibrium among copolymer chains adsorbed at the interface, chains homogeneously mixed in the bulk phases and copolymers at micelles within the bulk phases (57). However, the results in Figure 3B point out that two of the best dispersions are achieved when the molecular weight of the nonfunctional block is in the low range. These observations can be explained if we consider the case where the blocks of the copolymer are chemically different from but miscible with the corresponding homopolymers. In this particular case exothermic interactions may develop and provide additional driving force for the block to be anchored into the parent miscible homopolymer. Then strong interface adhesion can be promoted independently of the relative molecular weight of the block/homopolymer pairs (7,52). Therefore, it seems that for the PSt-coMMA/PET system a low molecular weight in the polystyrene block (nonfunctional block) of the compatibilizer helps to localize the copolymer at the interface. Similar observations have been reported in a ternary A/B/C-D system where the homopolymer pair of PPO and Phenoxy have been compatibilized with a PS-b-PMMA copolymer (55). In that particular case the molecular weight of the PMMA block needed to be twice as short as the Phenoxy homopolymer to localize the copolymer at the interface. In a second set of blends, the influence of the amount of compatibilizer in the blend is studied, using one of the compatibilizers detected as optimum in the 80/20/3 (PET/PSt-co-MMA/compatibilizer) blends. In this second set, a fixed ratio of PET/St-co-MMA of 80/20 was used and different amounts of the compatibilizer,from0.5 to 5 were evaluated. Figure 4 shows the results of varying the amount of the compatibilizer. It is apparent from this figure that by increasing the amount of compatibilizer a smaller average diameter of PSt-co-MMA in the blends is obtained. For this blend, the use o f , 5 wt% of compatibilizer leads to a broad distribution of particle sizes, having very small particles together with large ones, therefore, these results point out that it is better to use 3 wt% of the compatibilizer to obtain a more homogeneous distribution of particle sizes (see Figure 2C).


351 PET/PSt-co-MMA blends with different amoounts of compatibilizer 500 450 ~400 350


οι 300 οι Ε 250 η Ό 200 c (0 150 01

Σ 100 50 2 3 Compatibilizer (%ppr)

Figure 4. Influence of the amount of compatibilizer in the particle size of PSt-co-MMA in PET/PSt-co-MMA blends.

Polycarbonate/Polystyrene blends. In contrast with the approach taken with PET/pSt-coMMA, in this case the dispersion of the compatibilizer in polycarbonate was studied first. Polycarbonate and compatibilizers were mixed in a 70/30 weight ratio and the samples were analyzed using optical or transmission electron microscopy. Figure 5 shows optical microscopy photographs and TEM photographs of three blends. These images show how particle sizes vary from 19 micrometers down to 136 nm, depending on the structure and composition of the compatibilizer. A l l of the compatibilizers are diblock copolymers containing epoxy as the functional group in the reactive block. But the presence of blocks and adequate reactive groups (epoxy) does not guarantee a good interaction between the compatibilizers and polycarbonate. Since composition and molecular weight of the compatibilizers can be controlled and modified, the relationship between the compatibilizer and its dispersion in polycarbonate can be studied.



352

Β

Figure 5. Three examples ofPC/compatibilizers blends. A. Optical microscop photograph showing particles with a mean diameter of Ifym; Β. TEM photograph showing particles with a mean diameter of528nm; C. TEM photograph showing particles with a mean diameter-136nm.



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If we analyze Figures 6A and 6B, which show the effect of the molecular weight of the reactive and non reactive block, we observe that the compatibilizer with the worst performance is the one that combines a low molecular weight in both blocks. Analyzing the effect of the number of functional groups of the compatibilizer on its dispersion (Figure 6D), we observe that an increase in the amount of epoxy groups favors the dispersion of the compatibilizer in polycarbonate. Two of the compatibilizers showing the best dispersions in PC were evaluated in PC-PSt blends at different PC/PSt ratios and maintaining constant the amount of compatibilizer: 60/40/1.5,75/25/1.5 and 90/10/1.5. In the case of the uncompatibilized 60/40 blend, we observe a phase morphology in the co-continuity region, but when compatibilizer Β is added, we get a good dispersion, obtaining an average particle size of 2.5 micrometers (see figure 7). Decreasing the amount of polystyrene in the blends, diminishes the polystyrene particle size, so, in the case of the 90/10 blend, the uncompatibilized blend shows an average particle size of 0.43, while the compatibilized blend allows us to obtain an average particle size of 330 nanometers.Overall the presence of both compatibilizers seems to reduce the interfacial tension of the system and help the dispersion of PSt domains.

Conclusions Reactive block copolymers can be effectively synthesized by CRP (controlled radical polymerization) technologies. The reactive block copolymers act as compatibilizers for PET/PSt-co-MMA and PC/PSt polymer blends. Since CRP allows a control of the molecular weight of both blocks in the copolymer and of the amount of functional groups in the compatibilizers, the influence of these variables on the dispersion of the components of the blend could be analyzed and optimum compatibilizers were found. The technology is very cost effective and will lead to commercial compatibilizers based on the novel block copolymers

References 1. 2.

Koning, C.; Van Duin, M.; Pangoulle, C.; Jérôme, R. Prog. Polym. Sci. 1998, 23, 707-757. Utracki, L.A. Polymer Blends Handbook; Kluwer Academic Publishers, 2002, 295-414.


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Reactive Block Copolymers as Versatile Compatibilizers - American

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