Article pubs.acs.org/Macromolecules
Orientation and Birefringence Compensation of Trunk and Graft Chains in Drawn Films of Cellulose Acetate-graf t-PMMA Synthesized by ATRP Hirofumi Yamanaka, Yoshikuni Teramoto, and Yoshiyuki Nishio* Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ABSTRACT: We examined the molecular orientation and optical anisotropy that are induced by the stretching of cellulose acetate (CA)-graf t-poly(methyl methacrylate) (PMMA) films, in connection with the side-chain contents. The copolymers were prepared by atom transfer radical polymerization. The molecular characterization was successfully performed by NMR and GPC analyses, before and after deliberate cleavage of the PMMA grafts. Even if the molecular weight of the grafts increased, the dispersity index remained below 1.2. The molecular orientation behavior in uniaxially drawn films of the copolymers was estimated by a fluorescence probe technique. The measurement revealed a declining tendency of the orientation development with an increase in degree of the molar substitution (MS) of MMA unit of the copolymers. In comparison of birefringence (Δn) at a given stage of elongation of the films, the value of Δn decreased rapidly with increasing MS, which eventually led to the conversion from a positive Δn for pristine CA (acetyl DS = 2.15) to a negative one for the composition containing >65 wt % PMMA that corresponded to the highest MS in the graft series used. This type of graft copolymer may deserve to be a high-functional material whose optical anisotropy is controllable variously in terms of birefringence compensation between the oriented trunk and graft chains.
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INTRODUCTION Graft copolymerization of cellulosics is a traditional way to improve their original properties. A current advanced use of the grafting technique has been in the design of environmentally conformable and/or biocompatible materials such as cellulosegraf t-poly(hydroxyalkanoate)s;1−4 for instance, the degradation and thermal properties of the materials were widely controllable by altering the copolymer composition. Meanwhile, in the perspective of many-faceted applications of cellulosic graft copolymers, the practical properties and functions as solids can change sensitively according to the state of polymer chain orientation, which is furnished in the manufacturing process into some dimensional products (e.g., films and filaments). Therefore, even for such multicomponent materials, it is important to clarify the relationship between the molecular orientation behavior and the particularly mechanical and optical properties. In the preceding paper,5 we examined the molecular orientation and optical anisotropy that were induced by stretching cellulose acetate-graf t-poly(L-lactide) (CA-g-PLLA) films in connection with the length of the grafted side chains, by means of techniques of fluorescence polarization and birefringence (Δn) measurements. In that series, the orientation development was declined monotonically with increasing content of the PLLA side-chain component. Overall, the molecular orientation distribution obeyed a common type of prolate ellipsoid with a variable axial ratio, irrespective of the copolymer composition. As did each film sample of pristine CA © 2013 American Chemical Society
(degree of acetyl substitution (DSAc), 2.15) and PLLA homopolymer, the CA-g-PLLA films exhibited consistently positive optical anisotropy (Δn > 0) upon drawing. Regarding the Δn vs % elongation plots, however, we noticed a certain nonmonotonic change in location level of the data, with varying parameter of the side-chain length. The observation was explicable by assuming the locally different orientation manners of the attached PLLA chain-segments; viz., the lactyl units placed in close proximity to the graft joint would be arranged perpendicular to the trunk chain that is mainly aligned in the draw direction, whereas the lactyl units apart from the joint would orient preferably in the draw direction. The former situation makes a negative contribution and the latter does a positive one to the total birefringence. In view of those results, it may be possible to control not only the magnitude but also the polarity of the birefringence of deformed graft copolymers, with an adequate combination of two ingredients that have polarizability units of mutually opposite signs. In a special case, the polymer materials would maintain the optical isotropy of naught birefringence regardless of the chain orientation, as has been demonstrated for random copolymers6−8 and compatible polymer blends.9−14 In the present paper, we provide insight into the molecular orientation and optical anisotropy for CA-graf t-poly(methyl Received: January 23, 2013 Revised: March 30, 2013 Published: April 8, 2013 3074
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stirring. Subsequently, 6.3 mL (50.6 mmol) of BriBBr in 10 mL of THF was slowly dropped into the solution over 1 h at 0 °C in an ice− water bath. The mixture was further stirred for 24 h in a water bath (30 °C). After the esterification, the reaction mixture was added dropwise into a vigorously stirred, large excess of methanol. The crude product of macroinitiator (CAmBBr) obtained as the reprecipitate was dissolved in acetone, and the solution was added dropwise into methanol. This purification process was repeated twice. The precipitate was dried for 24 h at 40 °C in vacuo and stored in a nitrogen-substituted desiccator. Graft Copolymerization of MMA and Subsequent Hydrogenolysis. CAmBBr (1 g, 1.75 mmol as to the initiating site) was added to a 500-mL three-necked round-bottom flask (flask-1) equipped with a magnetic stirrer bar. After it was sealed with a rubber septum, the flask was vacuum-degassed for 30 min and backfilled with nitrogen; this was repeated three times. Then, 340 mL of NMP was added to the system to dissolve the macroinitiator with stirring under vacuum degassing. After the macroinitiator was completely dissolved, MMA (18.5 mL, 175 mmol) was added. On the other hand, in a 50mL two-necked egg-plant flask (flask-2), 125.5 mg (0.88 mmol) of CuBr, 370 μL (1.75 mmol) of PMDETA, and 15 mL of NMP were added and the system was vacuum-degassed for 1 h and backfilled with nitrogen. Subsequently, the mixture of CuBr, PMDETA, and NMP was withdrawn from the flask-2 with a degassed syringe and added to the flask-1. These procedures were performed at 20 °C so far. The system was stirred for 1 min at 20 °C and the flask-1 was immersed in a water bath thermostated at 70 °C with successive stirring. In parallel with the ATRP graft reaction, in another 50-mL two-necked egg-plant flask (flask-3), 940 μL (3.5 mmol) of TBT and 9.4 mL of NMP were added and the mixture was degassed by three freeze−pump−thaw cycles after 10 min of stirring, and then the flask was backfilled with nitrogen. After the ATRP graft reaction continued over a prescribed timeperiod, the contents in the flask-3 were withdrawn with degassed syringes and added to the flask-1. The reaction mixture was further stirred for 1 h on exposure to air for terminating ATRP and also making hydrogenolysis. Similar hydrogenolysis was performed for a solution of CAmBBr in THF, for the purpose of obtaining a dehalogenated product (CAmBH) of the macroinitiator. The crude reaction mixture (∼350 mL) containing the objective graft product (CAmB-g-PMMA) or CAmBH was diluted with 150 mL of acetone and then passed though an activated alumina column to remove copper complex. After that, the mixture was added dropwise into a vigorously stirred, large excess of distilled water. The precipitate thus formed was recovered by centrifugation, dissolved in acetone, and dropped into a stirred, large excess of methanol. Again the precipitate was dissolved in acetone and filtrated with a glass fiber filter GFP (Kiriyama Glass Works Co.; pore size, 0.8 μm). The filtrate was dropwise added into methanol and the precipitate as a thoroughly purified product was dried for 24 h at 40 °C in vacuo. Extraction of PMMA Side-Chains via Alkaline Hydrolysis of Ester Linkage. In a 300 mL egg-plant flask, 0.3 g of the graft copolymer CAmB-g-PMMA obtained above was suspended and stirred in 45 mL of 2 M NaOH aqueous solution at 70 °C. After 48 h, the suspension was cooled to 20 °C and mixed with 90 mL of aqueous HCl (1 M), followed by stirring for another 2 h. Subsequently, a solid mixture of the liberated PMMA and cellulose (restored by deesterification) was filtered off. After that, the PMMA fraction was extracted from the polymer mixture with 100 mL of acetone and filtrated. The PMMA/acetone solution obtained as filtrate was condensed and dried at 40 °C in vacuo and provided for GPC and NMR analyses. Film Preparation and Stretching. A 5.0 wt % polymer solution in N,N-dimethylfornamide (DMF) was prepared at 20 °C for each of the graft copolymer products. A small amount of BBS was dispersed into the respective solutions so as to give a concentration of ∼1.0 × 10−4 mol/L in the dried solid matrix. The solutions were cast into a film form of adequate size onto a glass plate through evaporation of DMF at 50 °C under reduced pressure ( 0) upon stretching of their films. The macroinitiator CAmBBr0.60 and its dehalogenated one CAmBH0.60 also showed a positive sign in
(8b)
As exemplified in Figure 6, the films of unmodified CA2.15, and those of CAmBBr0.60 and its dehalogenated product CAmBH0.60 as well, showed a high level of molecular orientation upon stretching, so as to make a certain fit of the ⟨cos2ω⟩ vs ε plot to the theoretical curve of the affine deformation. The capability of these polymers to undertake such a high orientational action may be attributed principally to their semirigid carbohydrate backbone. In contrast, the stretched films of comparatively flexible PMMA presented a lower level of molecular orientation; they were liable to undergo a plastic flow in the drawing process at Tg. Concerning the graft copolymer series, the ⟨cos2ω⟩ vs ε plots were located below the corresponding plots obtained for the trunk polymer per se, CA2.15 or CAmBH0.60. The degree of orientation showed a substantially monotonic decrease with an increase in MS, when compared at a given stage of elongation. In the drawing of the copolymer samples, deservedly, the tensile stress should have been applied not only onto the cellulosic trunk chains but also onto the attached PMMA sidechains. It is then plausible to assume that the stress component exerted on the flexible side-chains less contributed to the molecular orientation in the draw direction. In addition, the higher the MS, the lower the density of intermolecular interactions such as hydrogen bonding and/or entanglement between the trunk polymers should be. Accordingly, the molecular mobility as a whole of the graft copolymers of higher MS becomes enhanced so that the orientation relaxation in the drawing process would be more noticeable. Using the fluorescence polarization technique, it is possible to obtain information about not only the degree but also the type of molecular orientation in the noncrystalline regions of polymer solids.24,37 A convenient method for estimating the type of molecular orientation is to construct a plot of the fourth moment against the second moment of orientation, and then to search for conformity of the plot to the theoretical relationship between the moments calculated in terms of a potential model of the orientation distribution. Examples of the construction of the ⟨cos4 ω⟩ vs ⟨cos2 ω⟩ plot are shown in Figure 7, with the data for films being deformed to ε of 20 ± 3%. A continuous line in the figure represents a relationship derived by assuming that the molecular orientation distribution obeys a type of 3080
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(1) As far as the four products of CAmB0.60-g-PMMA were concerned, Δn decreased monotonically with increasing MS, when compared at a given stage of elongation of the films. (2) Regarding the composition CAmB0.60-g0.15-PMMA6.84 of the highest MS in this series, the drawn films showed a negative sign in the birefringence over the range of elongation explored. (3) The compositions of CAmB0.60-g0.11-PMMA5.26 and CAmB0.60-g0.15-PMMA6.84, assuming diametrically opposite characters in the sign of birefringence, contain graft fractions (wPMMA listed in Table 1) of 0.635 and 0.694, respectively, both well over 60 wt % PMMA of Δn° < 0. According to the ⟨cos2 ω⟩ estimation (Figure 6), the copolymer samples of higher MS were inferior in the growth level of molecular orientation on drawing. This may be largely responsible for the suppression in rising rate of the Δn vs ε curve with increasing MS (observation 1). However, observation 2 of the negative birefringence for CAmB0.60-g0.15PMMA6.84 demonstrates that, in the present copolymer system, there definitely occurs a compensation effect resting on the positive and negative contributions of the oriented CAmBH and PMMA, respectively, to the overall birefringence. Comparing the two data for plain PMMA and CAmBH0.60 in Figure 8, we can assume |Δn°| of PMMA to be roughly comparable to or somewhat larger than that of CAmBH0.60. In interpretation of observation 3, therefore, it may be relevant to consider different manners of local alignment for the PMMA side-chains of the drawn copolymers concerned. Namely, MMA units fairly apart from the graft joint would be arranged parallel to the draw direction and hence serve as a negative anisotropic unit of polarizability. Contrarily, the other MMA units adjacent to the anchoring site (isobutyryl) on the CA trunk would be collocated perpendicular to the carbohydrate polymer; these vicinal MMA units take a rather positive part in the birefringence compensation. Then it follows that the critical wPMMA leading to a negative birefringent material exceeds 0.5 to an appreciable extent. In addition, it might be worth noting an effect of molecular weight of PMMA on the segmental orientation. Namely, the grafted PMMA chains are rather lower in molecular weight (Mn ≈ 3000−4000), while the reference PMMA homopolymer in Figures 5−8 has Mw around 90000 as mentioned in the Experimental Section. Formerly, we observed a somewhat lower Tg (∼95 °C) for a low-molecular-weight PMMA (Mn = 13000),38 in comparison with the conventional data 100−110 °C. For this grade of PMMA having lower Mn, it is natural to assume that the polymer bulk shows a poor ability of molecular orientation on stretching, because of the serious orientation relaxation with less chain-entanglement. Nevertheless, the grafted PMMA chains were surely oriented to a decent degree; undoubtedly, this was accomplished with the aid of the cellulosic trunk as a substantial sustainer. On the basis of the above observations, we can readily deduce for the graft series CAmB0.60-g-PMMA of DSgraft = 0.1− 0.15 that a composition of MS ≈ 6.0 (corresponding to wPMMA ≈ 0.65) should realize a perfect zero-birefringence phenomenon for the deformed film, in which the positive and negative contributions of CAmBH (+vicinal MMA units) and PMMA, respectively, to the overall birefringence will cancel each other completely, no matter how much the molecular orientation is induced.
Figure 8. Plots of birefringence vs % elongation for film samples of CAmB-g-PMMA copolymers and their component polymers and related ones. Numerals denote MS values for the CAmB-g-PMMA series. Inset shows the data for the graft copolymers on an enlarged scale.
the birefringence, upon similar stretching. However, as seen in the figure, Δn data obtained for the CAmBH0.60 films were situated in a seriously lower level, in contrast to the higher values observed for the CAmBBr0.60 films. As has been illustrated in Figure 6, both of the CA derivatives imparted a high level of molecular orientation development and the status was rather superior in the CAmBH0.60 medium; hence, the difference in the birefringence behavior between the two must be attributed to the structural distinction as to their side-chains. Namely, a high polar C−Br moiety in the side-chain segment of CAmBBr0.60 would make a large component of polarizability parallel to the principal axis of the drawn sample and offer a considerable positive contribution to the total Δn, while the isobutyryl moiety without Br basically acts as a negative element of the orientation birefringence. In this context, a constant decrease of orientation birefringence with increasing DS has been found for cellulose acetate13 and mixed alkyl esters (acetate propionate and acetate butyrate).14 Such a DSdependence of birefringence observed for high-substituted cellulose alkyl esters is presumably owing to a configurational habit characteristic of the ester carbonyls; the axis of their polarizability would be arranged almost perpendicular to the carbohydrate backbone. Consequently, it can be said that the covalently attached ester moieties diminish, more intensely, the intrinsic birefringence Δn°, rather than affect the orientation function (3⟨cos2 ωS⟩ − 1)/2 in eq 9. On the other hand, drawn PMMA films showed a definitely negative optical anisotropy in agreement with an intrinsic birefringence data15 of Δn° = −50.7 × 10−4. Thus, the graft copolymers are undoubtedly composed of two polymers (CAmBH and PMMA) providing intrinsic birefringences of mutually opposite signs. In Figure 8, the following observations on the Δn vs ε data are worth noting, particularly with respect to the dependence on the copolymer composition. 3081
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CONCLUSIONS
Graft copolymers composed of CA and PMMA were successfully prepared over a range of compositions by ATRP. Detailed molecular characterization for the graft copolymer (CAmB-g-PMMA) products was also accomplished with a success of determining the major structure parameters MS, DPs, DSgraft, and molecular weights, through NMR and GPC analyses before and after adequate cleavage of the PMMA grafts. While the molecular weight Mn of the PMMA component increased above 3000, the dispersity index (Mw/ Mn) remained below 1.16. This result indicates that the graft chains were produced via a well-controlled ATRP mechanism. The copolymers were totally noncrystalline and their solutioncast films were regarded as intimately incorporated composites showing no phase separation of the trunk and side-chain components. The molecular orientation and optical anisotropy induced by uniaxial stretching of film specimens were investigated for the graft copolymers by techniques of fluorescence polarization and birefringence measurements. Upon stretching, any film imparted a positive orientation function, i.e., f = (3⟨cos2ω⟩ − 1)/2 > 0, which increased with the extent of elongation. The orientation development was declined monotonically with increasing MS; however, the varying MS parameter never affected the orientation distribution pattern with constant obedience to an orthodox ellipsoidal model, which was deduced from the statistical ⟨cos4ω⟩ vs ⟨cos2ω⟩ relationship. The two polymers used as the trunk and graft, CA (or CAmBH) and PMMA, showed mutually opposite signs in the intrinsic birefringence, and the drawing of films of the respective graft copolymers provided the orientation-induced birefringence changeable in a range of magnitude and even in polarity, depending on the MS or wPMMA. It was thus confirmed that there appeared an effect of compensation resting on the positive and negative contributions of the oriented trunk and graft chains, respectively, to the overall birefringence. However, the inversion in polarity of the birefringence did not reflect a simple additive property in quantity of the constituent polymers. The observation was explicable by assuming the locally different orientation manners of the attached PMMA chain-segments; viz., some MMA units placed in vicinity to the graft joint would be arranged perpendicular to the trunk chain that is mainly aligned in the draw direction, whereas other MMA units apart from the joint would orient preferably in the draw direction. The former situation makes a positive contribution and the latter does a negative contribution to the total birefringence. However, further investigation using more wide-ranging copolymer compositions in DSgraft and DPs will be preferable for detailed elucidation of the compensation effect. The optical anisotropy of the present graft copolymers composed of CA and PMMA is delicately controllable by changing the copolymer composition, which is of great significance for designing optical retardant films including a specialty that shows a zero-birefringence nature irrespective of the elongation and ensuing molecular orientation growth. Such materials exhibiting various anisotropies by simple processing may be also useful in many-faceted prospective applications of polymers including cellulosics.
Article
AUTHOR INFORMATION
Corresponding Author
*(Y.N.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially financed by a Grant-in-Aid for Scientific Research (No. 23580228 to Y.T.) from the Japan Society for the Promotion of Science (JSPS).
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REFERENCES
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