ARTICLE pubs.acs.org/Biomac
Cellulose Microfibrils Grafted with PBA via Surface-Initiated Atom Transfer Radical Polymerization for Biocomposite Reinforcement Shuzhao Li,†,‡ Miaomiao Xiao,† Anna Zheng,‡ and Huining Xiao*,† † ‡
Department of Chemical Engineering, University of New Brunswick, Fredericton, NB Canada School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, China
bS Supporting Information ABSTRACT: Immobilizing poly(butyl acrylate) (PBA) on cellulose microfibrils (CMFs) by atom transfer radical polymerization (ATRP) of butyl acrylate (BA) on the surface of 2-bromoisobutyryl-functionalized CMF generated highly hydrophobic microfibrils (CMF-PBA) with a hard core and a softshell structure. TGA and static water contact angle results suggested that the surfaces of the modified CMF samples were not completely covered by PBA chains until the molecular weight of grafts became sufficiently long. The GPC results indicated that the grafts with low molecular weight showed controlled/ “living” characteristics of the surface-initiated ATRP; however, there existed more side reactions with the increase in molecular weights. Biocomposites consisting of polypropylene (PP) and CMF-PBA samples exhibited significantly improved compatibility, interface adhesion, and mechanical properties with the increase in PBA graft length. The findings confirmed that the longer grafts facilitated the better entanglement of PBA grafts with PP macromolecules and thus further improved the mechanical properties.
’ INTRODUCTION Recently, natural fibers have been extensively employed for biobased products because of their biodegradability, low density, wide availability, cost effectiveness, and superior mechanical properties.14 Cellulose microfibrils (CMFs) derived from natural cellulose fibers possess all of the properties of cellulose but have a higher specific surface than cellulose. Therefore, much attention has been paid to the CMF-based biocomposites because of the potential reinforcing capability of CMF offered by the inherent high strength of CMF and because of CMF’s low weight, biodegradability, and renewability.59 However, the key drawbacks of CMF as a reinforcement are the incompatibility with most hydrophobic polymeric matrices, the limitation of processing temperature, and the water absorption.6,10 The presence of absorbed water in CMF decreases the interfacial adhesion to most polymeric matrices.11 Water absorption also induces a plasticizing effect, leading to dimensional instability and poor mechanical properties.12 Grafting of a hydrophobic polymer onto CMF could significantly enhance the hydrophobicity of the CMF, and thus the adhesion of the modified CMF to the polymeric matrix could be improved.6,1315 CMF can be submitted to a specific surface modification performed physically, physicochemically, or chemically. Coupling agents or compatibilizers can also be applied.16 Siqueira et al.6 grafted N-octadecyl isocyanate (C18H37NCO) onto nanowhiskers and microfibrillated cellulose, and they also prepared polycaprolactone (PCL) nanocomposite film by casting. It proved that the grafting method could improve the mechanical properties of the nanocomposites. The grafting of vinyl monomers onto cellulose and other cellulosic fibers has r 2011 American Chemical Society
been extensively studied.1725 Takacs et al.23 modified two kinds of cellulose (cotton and linter) by grafting vinyl monomers with long paraffin chains through a direct irradiation grafting technique but obtained only a limited improvement of mechanical properties. To tailor precisely molecular weight (MW) and molecular weight distribution (MWD) for various applications, controlled polymerization techniques are required. Recently, much attention has been devoted to single-electron transfer living radical polymerization (SET-LRP)2629 and atom transfer radical polymerization (ATRP) because both approaches can provide greater monomer diversity and less stringent reaction conditions in comparison with living anionic/cationic polymerization. SETLRP has several advantages such as ultrafast polymerization and ultrahigh MW, predictable MW evolution and distribution, perfect retention of chain-end functionality, and notably colorless reaction mixtures and colorless polymers.30,31 Over the past decade, ATRP has been extensively studied for grafting polymerization of vinyl monomers onto cellulose and cellulose derivatives in a “living”/controlled manner. In previous studies, for different purposes, the synthesis of cellulose-based grafting polymerization by ATRP has been carried out in either a heterogeneous or a homogeneous medium.3238 In this work, CMF was modified by grafting with poly(butyl acrylate) (PBA) chains using the surface-initiated ATRP (SI-ATRP) in an attempt to improve the compatibility and dispersion of the Received: June 12, 2011 Revised: July 27, 2011 Published: July 28, 2011 3305
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Biomacromolecules CMF in a PP matrix. Butyl acrylate (BA) was chosen as the monomer for the ATRP on the surface of the 2-bromoisobutyrylfunctionalized CMF (CMF-Br). The grafted polymer, PBA, is hydrophobic and expected to enhance the interface adhesion between CMF and PP. Furthermore, because PBA has a low glass-transition temperature (Tg) and a soft chain, a core/shell structure can be formed after grafting PBA onto CMF. One of the reasons for using ATRP is to control the length of grafted chains and to investigate the influence of chain length on the hydrophobicity of the modified CMF and the mechanical properties of PP composites. In our previous studies,39 the SI-ATRP of BA on CMF was optimized with the CuIBr/PMDETA system. In the current study, we prepared CMF-PBA samples with various tailored MWs and investigated the changes of controllability and side reactions of SI-ATRP with the increase in MW. In addition, the compatibility and interface adhesion of PP composites consisting of PP and modified CMF with different lengths of PBA grafts were studied and compared.
’ EXPERIMENTAL SECTION Materials. CMF, supplied by Aldrich and derived from cotton linters using mechanical treatment, with a mean particle size of 60 μm and a mean bulk density of 0.3 g/cm3 was dried in a vacuum oven at 60 °C for 24 h. BA, also from Aldrich, was first washed with a dilute NaOH solution to extract the inhibitor, then washed with distilled water and dried using a vacuum oven before use. CuIBr (from Aldrich) was purified through three steps: stirring it in acetic acid, washing it with methanol, and then drying the CuIBr in an oven. 1-Methyl-2-pyrrolidinone (NMP, from Aldrich) was purified by distillation over CaH2. Triethylamine (TEA), 4-(dimethylamino)pyridine (DMAP), 2-bromoisobutyrylbromide (BIBB), N,N,N0 ,N0 ,N00 -pentamethyldiethylenetriamine (PMDETA), and toluene were used as received from Aldrich. Membranes used for filtration were from Spectrum, and the approximate molecular weight cut off (MWCO) of the membranes is 1000 Da. The commercial polypropylene (PP), EPS30R (MFR = 3.0 g/10 min, Qilu Petrochemical), was dried in an oven at 80 °C for 12 h prior to use. Synthesis of CMF-Br. Dried CMF powder (10 g), 250 mL of NMP solvent, 0.1 mol of TEA, and 0.05 mol of DMAP were added to a threenecked round-bottomed flask, respectively. NMP solution (50 mL) containing 0.092 mol of BIBB was then added dropwise over 1.5 h at 0 °C after the flask contents were degassed and backfilled with nitrogen for three cycles. The reaction proceeded under nitrogen at 0 °C for 24 h and then at room temperature for 48 h. The crude product was then filtered and washed with ethanol to remove the impurities. The macroinitiator, CMF-Br, was then dried at 60 °C in a vacuum oven for 24 h prior to use for ATRP. General Procedure for the SI-ATRP of BA on CMF-Br. In a typical example, the polymerization of BA was carried out under nitrogen in a dried three-necked round-bottomed flask (50 mL) equipped with a magnetic bar. CMF-Br (1 g, 0.8 mmol of Br), PMDETA (0.167 mL, 0.8 mmol), CuIBr (0.115 g, 0.8 mmol), and 20 mL of toluene were added to the flask, which was then sealed with a rubber septum and glass stoppers. The flask contents were then cycled between vacuum and nitrogen three times to remove oxygen. Degassed BA (4.45 mL, 0.032 mol) was added using a syringe. The flask was then immersed in an oil bath heated to 90 °C. After heating was stopped, the reaction mixture was cooled to room temperature and purified by filtration and washing with methanol several times to remove Cu complexes. Cleavage of PBA Grafts from CMF-PBA Surfaces. To investigate the MW and polydispersity of PBA grafts on the CMF surfaces, we extracted CMF-PBA samples overnight in methanolic HCl to cleave covalently bound PBA from the CMF-PBA. The fibers were then filtered
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off and rinsed with distilled water. The solution of the extracted material was analyzed by gel permeation chromatography (GPC). It is noteworthy that after the CMF-PBA is hydrolyzed, the linear polymer is actually poly(acrylic acid) (PAA) because of the hydrolysis of the ester groups, which have been hydrolyzed to be carboxyl groups. Therefore, the GPC results represented the MWs of PAA, which are proportional to those of PBA. Preparation of PP-Based Biocomposites. The preparation of PP-based biocomposites took place in a twin-screw extruder (KESUN, diameter: 35 mm, L/D ratio: 32) at 180 °C and 80 rpm. Before compounding, PP and CMF-PBA samples with different chain lengths were dried at 70 °C for 10 h in an air-circulated oven and then mixed with 0.2% of an antioxidant, Irganox 1010 (Ciba), in a high-speed mixer. The premix was added to the extruder to prepare the biocomposite sample. The extrudate from the die was quenched in a water tank at 2030 °C and then palletized. Characterization. The infrared characterization was performed in the transmission mode using a Spectrum 100 FT-IR spectrometer (Perkin-Elmer) with 200 scans between 4000 and 400 cm1 in potassium bromide (KBr). The onset decomposition temperature of all CMF samples was determined by thermogravimetrical analysis (TGA) using a SDT Q600 from TA Instruments. The decomposition measurements were obtained between 50 and 600 °C at a scanning rate of 20 °C/min under a helium atmosphere. A 300 MHz Varian Unity 400 spectrometer was used for 1H NMR analysis to reveal the chemical structure of hydrolyzed polymer. The number-average molecular weight (Mn) and MWD of PAA, obtained by hydrolysis of the CMF-PBA samples, were measured on a GPCMax VE201 GPC (Viscotek) equipped with three columns (PAA202-204-205) and three detectors: viscometer, light scattering, and IR detector. We used 0.05 N sodium nitrate aqueous solution as an eluent; the flow rate was 1 mL/min. Calibration was based on narrow MW poly(ethylene oxide) standards. Water static contact angle measurements were conducted using a JC200A instrument (PowerEach, China) with a digital photo analyzer. With a syringe, a water drop was dropped carefully onto the pellet of the modified and unmodified CMF samples. The compatibility of the modified CMF samples with the PP matrix was analyzed with a JEOL JSM6400 digital scanning electron microscope (SEM). The samples were brittle fractured in liquid nitrogen, and the fractured surfaces were observed using the SEM. The CMF-PBA samples were also observed using a Leica digital light microscope (LM) (Leica DM-R upright microscope with transmitted light from a halogen lamp and epi-fluorescence from a mercury lamp). The sample was cut into sections of 0.5 to 1 μm and embedded in acrylic resin, and then observed under UV in the range of 340380 nm. The tensile and flexure measurement of all blend samples and neat PP were carried out using a Universal tensile test machine (ZWICK ZO 20/TN2S) according to ASTM D638 and ASTM D256, respectively. The tensile and flexure tests were performed at a crosshead speed of 50 and 2 mm/min, respectively. Izod impact test was carried out using an impact tester (JJ-20) with a 0.818 kg hammer according to ASTM D256. Five specimens were tested for each sample.
’ RESULTS AND DISCUSSION As described in our previous work,39 prior to the grafting reactions, the CMF-Br initiators were prepared by immobilizing BIBB on the CMF surface. Also, the molar amount of Br immobilized on the surface of CMF-Br was tailored for designing the MW of PBA grafts by ATRP. In the grafting polymerization experiments, four samples, including CMF-PBA-10, CMF-PBA20, CMF-PBA-40, and CMF-PBA-100 with targeted degree of 3306
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Scheme 1. Schematic of Forming PBA on the Surface of Brominated CMF via ATRP
Figure 2. TGA curves of the CMF sample and CMF-PBA samples.
Figure 1. FT-IR spectra of the CMF sample and CMF-PBA samples.
polymerization (DP) of 10, 20, 40, and 100, respectively, were prepared. All grafting reactions of BA with CuIBr/PMDETA as a catalyst system were conducted in toluene at 95 °C for 24 h, which are the optimized reaction conditions for surface-initiated ATRP of BA on CMF in accordance with our previous findings.39 The grafting polymerization is shown in Scheme 1. Figure 1 shows the FT-IR spectra of the original CMF, CMFPBA-10, CMF-PBA-20, CMF-PBA-40, and CMF-PBA-100 samples. As a result of in situ polymerization of BA by ATRP, an additional strong absorption band shows at 1738 cm1 on all curves of the four CMF-PBA samples, which is attributed to the stretching vibration of carbonyl groups in PBA. Furthermore, the peak intensity of the CMF-PBA samples around 2900 cm1 increases because of the addition of CH2 and CH3 groups to the CMF substrates. It is also found that the strong absorption at 1640 cm1, which is ascribed to intermolecular HOH stretching,40 decreases as the DP of PBA increases. This result indicates a decrease in HOH interactions when the PBA macromolecules were introduced onto the surface of the CMF. Therefore, it can be confirmed that PBA was immobilized on the CMF’s surface. The influence of the grafted PBA chains on the thermostability of the CMF-PBA samples was investigated with TGA. In particular,
the grafting yield of PBA was quantified, based on TGA results, to estimate the conversion of BA monomers. Thermogravimetric curves of the unmodified CMF sample and all of the CMF-PBA samples are shown in Figure 2. A Table summarizing the thermostability and contact-angle results of the CMF sample and CMFPBA samples are reported in Part 1 of the Supporting Information. From the results of Figure 2 and the table in Part 1 of the Supporting Information, it can be seen that the content of grafted PBA increases with the increase in the tailored DP of PBA. The results indicate that the higher the tailored DP of PBA, the longer the PBA grafts can be obtained because the same macroinitiators were employed for all CMF-PBA samples. The onset decomposition temperature (Td) of the unmodified CMF sample is 315 °C. In comparison, the Td values of the CMF component of CMF-PBA-20, CMF-PBA-40, and CMF-PBA-100 samples (the temperatures of initial weight lost of the curves in Figure 2) are all >335 °C, which means that the thermostability of the modified CMF samples was increased because of the introduction of long PBA macromolecules chains. Moreover, it can be found that the longer the grafts attached to the CMF substrates, the better the stability of the modified CMF, which suggests that long PBA grafts benefit the thermostability of the modified CMF. However, the curve of CMF-PBA-10 shows a contrary tendency. The curve deviated from the baseline at 230 °C corresponding to the Td of BIBB, which was undetectable on the curves of the other CMF-PBA samples. Furthermore, the first breaking point of CMF-PBA-10, related to the degradation of the CMF component, took place at 300 °C, which is lower than that of the unmodified CMF sample. The result implies that the surface of CMF-PBA-10 might not be completely covered by PBA grafts because of the short PBA chains. Consequently, the unstable BIBB, immobilized by esterification with hydroxyl groups, was the first component of CMF-PBA-10 to decompose, causing the deviation from the baseline at 230 °C. To study the possibility of controlling the length of the grafts on the surface of CMF, we hydrolyzed all of the CMF-PBA samples. The hydrolyzed polymers are water-soluble because the PBA grafts were hydrolyzed to PAA. Figure 3 shows the 1H NMR spectrum of the hydrolyzed polymer from CMF-PBA. The chemical shifts at δ 1.61.8 and δ 2.02.2 are mainly attributed to the CH2 (b) and CH (a) groups, respectively, which indicates the structure of PAA. The GPC analysis of the polymers, obtained from the hydrolysis of four CMF-PBA samples, was performed. The results related to the MW and MW distribution of the CMF-PBA 3307
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Figure 3. 1H NMR of PAA hydrolyzed from the CMF-PBA-40 sample.
Figure 5. GPC trace of the polymers cleaved-off from the CMF-PBA100 sample.
Figure 4. GPC traces of the polymers cleaved-off from the CMF-PBA samples prepared using the same macroinitiator.
samples are listed in the table in Part 1 of the Supporting Information. Figure 4 presents the main GPC peaks of all cleaved-off polymers from the modified CMF samples. Two narrow and monomodal peaks are observed on the curves of CMF-PBA-10 (DP10) and CMF-PBA-20 (DP20) in Figure 4; however, the curve of CMF-PBA-40 (DP40) shows a broad peak with a tail of high MW corresponding to smaller retention volume, implying the existence of recombination termination reactions as the chain lengths increased. In contrast, two small shoulders appear on the curve of CMF-PBA-100 (DP100) corresponding to the shorter and larger retention volume, respectively, which demonstrates the existence of three different polymerization situations. As expected, the lower MW could be attributed to either the steric inaccessibility of bromine atoms or the loss of bromine atoms from the PBA chains. The first small shoulder, corresponding to the smaller retention volume, demonstrates the existence of a small portion of PBA with a relatively high MW. This result implies that recombination reactions between propagating macroradicals take place particularly at high-tailored MWs even under controlled/“living” free radical polymerization conditions. In our heterogeneous reaction systems, when the DP was low (such as at 10 and 20), the propagating molecular chains on the same CMF substrate were not long enough to combine with each other. Therefore, few recombination termination reactions occurred. The resulting MWDs of DP10 and DP20 are narrow, as seen in Figure 4.
However, with the increasing molecular chain length, more propagating radicals could approach and combine with each other, and thus a tail appears on the curve of DP40 at the highMW portion. When DP was further increased to 100, the resulting molecular chains were so long that the end parts of the grafted chains became more flexible in the solvent, and thus the probability of the free radical collision was enhanced. Consequently, if the DP is too high (i.e., 100), then the couple termination reactions of PBA chains could be generated by the recombination of living chains from the same CMF substrate or from the different CMF substrates. However, the latter could form cross-linked agglomerates, which tend to limit the application in reinforcing the mechanical properties of biocomposites. Figure 5 presents the GPC trace of cleaved-off polymers from CMF-PBA-100, which further reveals the existence of side reactions. From the envelope of the GPC trace, three MW fractions (peaks ac in Figure 5) can be observed. The main MW fraction (peak b in Figure 5) with an Mn value of 9600 is ∼74% of the total, showing that most of the PBA chains are “living.” Of the side reactions, the high-MW fraction (peak a, ∼17% of the total) with an Mn value of 17 300 is much higher than the low-MW fraction (peak c, ∼9% of the total) with an Mn value of 2700. Consequently, recombination reactions dominate in the side reactions in the current heterogeneous ATRP systems. SEM was also employed to characterize the CMF sample and modified CMF samples. Figure 6 shows the SEM images of the unmodified CMF sample and CMF-PBA samples. After the CMF substrate was grafted with PBA using controlled ATRP, the surface and pores of the modified CMF were covered with a thin layer of PBA. From the images of CMF-PBA-10, CMF-PBA20, and CMF-PBA-40 (Figure 6bd), it can be seen that the surfaces of three CMF-PBA samples are smoother in comparison with the rough surface of the CMF sample (Figure 6a). However, the CMF-PBA-100 sample (Figure 6e) appears to be cross-linked because of the recombination termination reactions occurring among the different CMF substrates, which is in accordance with the broad distribution and the appearance of the high-MW fraction in the GPC results. In fact, the cross-link reactions among CMF powders were confirmed by the existence of some 3308
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Figure 6. SEM images of the surfaces of the CMF sample and modified CMF samples.
agglomerates dispersed in acetone suspension as well. Consequently, the coupling termination reactions are non-negligible, even in the controlled polymerization, especially in the heterogeneous ATRP system. The hydrophobicity of a modified CMF sample is vital for improving the compatibility of the CMF in a PP matrix. The hydrophobicity of the CMF-PBA samples was investigated by measuring the static water contact angles; the results are summarized in the table in Part 1 of the Supporting Information. The figures of contact angles of the CMF sample and CMF-PBA samples are reported in Part 2 of the Supporting Information. It is obvious that the contact angle of the CMF-PBA-10 sample is increased to a great extent in comparison with that of the original CMF on which the water droplet absorbed or flattened immediately once it contacted the surface (due to the strong hydrophilicity of the unmodified CMF). When the amount of PBA was further increased, the contact angles of the CMF-PBA-20 sample
Figure 7. Light microscope (LM) images, under UV light, of the sections of the CMF-PBA-40 sample in the PP matrix. 3309
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Figure 8. SEM images of the fractured surface of the unmodified CMF and modified CMF samples in PP matrix (a) PP/CMF, (b) PP/CMF-PBA-10, (c) PP/CMF-PBA-20, and (d) PP/CMF-PBA-40.
and CMF-PBA-40 sample unceasingly increase to 117.7 and 137.2°, respectively, demonstrating a further increase in hydrophobicity. However, by comparing the CMF-PBA-40 and CMFPBA-100 samples, it can be found that the contact angles are almost the same. The results suggest that the surfaces of CMFPBA-10 and CMF-PBA-20 samples were not completely covered with PBA grafts until the chain length of the PBA grafts was increased to that of the PBA grafts on the surface of the CMFPBA-40 sample. As expected, once the surfaces of the CMF substrates are completely covered by PBA grafts, the contact angles tend to remain the same. Figure 7 shows the dispersion of the CMF-PBA-40 in the PP matrix revealed by LM under the UV light. CMF-PBA-40 is fluorescent under the UV (at 488 nm), leading to the evident differentiation of the CMF-PBA-40 from the PP matrix. As shown in Figure 7, the CMF-PBA-40 is uniformly dispersed in the PP matrix with a tropism resulting from the shear in twinscrew extruder; the individual CMF is also observed, implying that the modified CMF dispersed well in the PP matrix. Because of the limited resolution of the LM, the interfaces between the CMF-PBA-40 and the PP matrix could not be clearly observed. Therefore, the samples were further observed under an SEM. Figure 8 shows the SEM images of PP/CMF (Figure 8a), PP/CMF-PBA-10 (Figure 8b), PP/CMF-PBA-20 (Figure 8c), and PP/CMF-PBA-40 (Figure 8d) composites, respectively. As can be seen from Figure 8a, the unmodified CMF separates from the PP matrix, and the interface adhesion between the surface of the CMF and the PP matrix appears to be very poor. In contrast, even with very short PBA grafts on the surface of CMF-PBA-10, the interface adhesion between the surface of the modified CMF and PP matrix was obviously improved, which is confirmed by the reduction of void in the interface of PP/CMF-PBA-10
(Figure 8b). From Figure 8b, it can be seen that the CMF was pulled out of the PP matrix, and part of the CMF stands out. This is because the CMF and PP are not chemically bonded and the mechanical strength of CMF is higher than that of PP. As a result, the CMF could not be fractured simultaneously with PP. With longer PBA grafts introduced on the surface of CMF-PBA-20 (Figure 8c), the interfacial bonding between the CMF and PP matrix became stronger, although the CMF could still be pulled out. As can be seen from Figure 8d, CMF-PBA-40 is integrated with the PP matrix and broken together with PP after the composite was brittle fractured in liquid nitrogen; in addition, the interface between the CMF-PBA-40 surface and the PP matrix became much dimmer. The results suggest that the hydrophobicity modification of the CMF by grafting short hydrophobic polymer chains on the surface indeed increases the CMF’s compatibility with hydrophobic polymeric matrix; however, to increase the interface adhesion between CMF and PP matrix, we should introduce long grafts onto the CMF surface to provide the entanglement of grafts with matrix chains. Mechanical properties of neat PP and biocomposites (PP/ CMF, PP/CMF-PBA-10, PP/CMF-PBA-20, and PP/CMFPBA-40) were studied and compared (except for PP/CMFPBA-100 because of its agglomeration leading to its poor dispersion in PP matrix). Figure 9 presents the performance of the above five samples in terms of flexure, tensile, and impact strengths. Compared with neat PP, the impact strength of PP/ CMF increases slightly, whereas both the flexure and tensile strength decrease to a great extent. This could be due to the poor compatibility between PP and CMF as well as potential moisture adsorption induced by CMF. Apparently, by compounding PP with CMF-PBA-10, which was grafted with short PBA chains on the surface, the mechanical properties of the resulting PP-based 3310
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’ ASSOCIATED CONTENT
bS
Supporting Information. Thermostability and contact angle results of the CMF sample and CMF-PBA samples and contact angles of the CMF sample and CMF-PBA samples. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 1-506-4533532. E-mail:
[email protected].
Figure 9. Mechanical properties of neat PP and biocomposites.
biocomposite are improved though the overall improvement is limited. Moreover, with the increase in the MW of PBA, the mechanical properties of biocomposite further increase, which is contributed to the fact that longer PBA chains facilitate the better entanglement with PP, and thus interface adhesion can be further improved. In contrast, the mechanical properties of biocomposite, formed by compounding CMF-PBA-40 and PP, increase significantly (the impact resistance in particular). This behavior is not only attributed to the hydrophobicity of CMF-PBA-40 but also is attributed to the uniform distribution of CMF-PBA-40 in the PP matrix; the distributed CMF-PBA-40 has a coreshell structure in which the thin elastic PBA shell can absorb impact energy.
’ CONCLUSIONS Surface-initiated ATRP facilitated the preparation of PBA grafts with targeted MW, when CMF-Br macroinitiators, obtained by the esterification of hydroxyl groups on CMF with BIBB, are employed. The polymerization of BA on the surface of the CMF substrate was confirmed by the characterization of FTIR. Moreover, TGA results showed an increase in the thermostability of the CMF modified with long-chain PBA grafts. The hydrophobic modification of the CMF was further confirmed by water contact angle measurements, which indicated that the contact angle increased with the extending of the chain length of PBA grafts until the MW was high enough to cover the surface of the CMF. The GPC characterization suggested that the surfaceinitiated ATRP was controllable even in the heterogeneous system if the grafts on the surface of the CMF were short; however, it was unavoidable to generate the recombination termination reactions or even cross-linking when the length of PBA grafts on the surface of the CMF was long. The enhanced hydrophobicity of the modified CMF improved the dispersion and compatibility of the CMF-PBA in the PP matrix significantly. The compatibilities of the PP composites were also characterized using SEM. The results showed that long grafts on the surface of the CMF facilitated the entanglement of the grafts with the PP matrix, and thus the interface adhesion was enhanced. The mechanical property results indicated that the flexure, tensile, and impact resistance of the CMF-PBA samples were all improved, especially for CMF-PBA-40, in which, outside the CMF, the long PBA grafts formed a shell, which acted as an elastic layer to adsorb impact energy.
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