Polymer Solid-Phase Grafting at Temperature Higher than the Polymer

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Polymer Solid-Phase Grafting at Temperature Higher than the Polymer Melting Point through Selective Heating Songhe Wang,†,‡ Xiaohong Zhang,‡ Chao Jiang,‡ Haibin Jiang,‡ Yujing Tang,‡ Juan Li,‡ Minqiao Ren,‡ and Jinliang Qiao*,†,‡ †

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China

‡

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S Supporting Information *

ABSTRACT: A new polymer grafting method, which is a solid-phase reaction but its grafting temperature is higher than the polymer melt point, is reported in this paper. This solid grafting reaction at high temperature can be achieved by using microwave which selectively heats the substance that can absorb microwave. Maleic acid-grafted polypropylene (PP-g-MA) was prepared by this new method. It is well known that polypropylene (PP) is transparent to microwave while maleic anhydride (MAH) can be heated by microwave. Under microwave irradiation, MAH dispersed in the micropores of PP spherical powder can absorb microwave to generate high temperature and cause PP nearby and MAH itself to generate free radicals. MAHgrafted PP (PP-g-MAH) can be prepared without using an initiator. The new method has many advantages that traditional methods do not have. For example, the prepared graft polymer has no initiator and MAH residue and thus is odorless; the prepared graft polymer also has excellent mechanical properties because the graft polymer has not undergone high-temperature degradation. It has also been found that trace amounts of partially reduced graphene oxide can increase the grafting ratio and properties of grafted PP. Furthermore, the monomers which do not absorb microwave can be also grafted onto PP with the help of NaCl which can be heated by a microwave and removed completely with water after grafting. Therefore, this new method is believed to be a general method and suitable to almost all kinds of polymers and monomers.

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INTRODUCTION Graft polymers1,2 have been used widely in polymer recycling,3,4 polymer modification,5,6 polymer composites,7−10 and so forth; therefore, the polymer grafting reaction has been studied extensively in both academia and industry.11−14 However, the existing methods are all not good enough to make graft polymers with high performance and low cost. For example, the most widely used “melt grafting method”15−19 must use peroxide as the initiator, which brings grafted polymers poor performance and high cost because peroxide is usually expensive and can cause polymer degradation or crosslinking. Furthermore, the peroxide residual, together with the ungrafted monomer, always make the grafted polymer have color and odor, which is not only harmful to our health but also limits its application.20 The solid-phase grafting method can overcome the drawbacks of the “melt grafting method”. For example, polymer degradation or cross-link can be reduced in solid-phase grafting because the grafting reaction only © XXXX American Chemical Society

happens on a polymer surface; the initiator residual and ungrafted monomer can be washed away after the grafting reaction. Therefore, solid-phase grafting has been studied extensively in the past decades. Currently, maleic acid-grafted polypropylene (PP-g-MA) has been commercialized by solidphase grafting based on extensive study.21−23 However, the existing solid-phase grafting has obvious disadvantages. For example, current solid-phase grafting must be carried out at low temperature, at least below the melting point of the polymer. Solid-phase grafting is usually considered as a lowefficiency process; therefore, ultraviolet light,24,25 γ-ray,13,26 or other special conditions27−30 are usually necessary to use for solid-phase grafting. High pressure means high cost; ultraviolet light grafting can only be applied to the formed polymer Received: December 26, 2018 Revised: April 8, 2019

A

DOI: 10.1021/acs.macromol.8b02737 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules product, not raw material; and γ-ray irradiation grafting will cause polymer degradation. Finally, all existing solid-phase grafting methods also require the addition of initiators which will result in a decrease in polymer properties. Therefore, there is an important research topic in the polymer grafting reaction, that is, “can we develop a solid-phase grafting process which can be conducted at temperature higher than melting point of polymer?” In recent years, microwave radiation has attracted considerable attention for its rapid process with high efficiency.31 In addition, microwave heating is also a characteristic heating mode that cannot be imitated by other heating methods, especially it enables selective heating. Microwave is transparent to most of polymers and can heat the materials with high polarity effectively. Therefore, the polar substance will be selectively heated when microwave is applied to heat a mixture, such as the mixture of PP and maleic anhydride (MAH). Numerous studies have used this selective heating characteristics of microwave to prepare catalysts,32 new materials,33,34 and separation materials.35,36 By using the microwave heating characteristics, we develop a solid-phase grafting process that can be conducted at temperature higher than the melting point of the polymer, which is reported in this paper.

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1010 and 0.05 wt % calcium stearate. PP-g-NaMA pellets were obtained after being extruded and pelletized using a HAAKE twinscrew extruder at 190, 195, 200, 200, 200, and 190 °C. Preparation of PP-Grafted Unsaturated Silane. Vinyl trimethoxysilane was dissolved in ethanol and NaCl was dissolved in deionized water. The silane ethanol solution was then added to a three-necked flask filled with PP powder under vacuum and mechanical stirring. After drying, NaCl aqueous solution was added under vacuum and mechanical stirring. Then microwave irradiation grafting was carried out in the same way of PP-g-MA. Measurements. Graft Ratio. A standard curve was established for the graft ratio of PP-g-NaMA first. The infrared absorption peak area of sodium acrylate characteristic group (−COONa group) at 1562 cm−1 and infrared absorption area of PP internal standard peak at 484−435 cm−1 (peak position is about 460 cm−1) were determined by using mixed samples of sodium acrylate and PP resin as standard samples. The standard curve for the PP-g-NaMA graft ratio was obtained by plotting the ratio of two infrared absorption peak areas versus the content of sodium acrylate. The samples with a thickness of about 100 μm for the graft ratio measured were prepared by hot pressing at 200 °C. The graft ratio was calculated by using a standard curve. The graft ratio of PP-g-MA was obtained through the graft ratio of PP-g-NaMA prepared by the PP-g-MA sample with NaOH. The surface grafting ratio of PP-g-vinyl trimethoxysilane was obtained by using energy spectrum [energy-dispersive system (EDS)]. The mass fraction of Si in a certain area was measured and the graft ratio of vinyl trimethoxysilane was further calculated. MFR was measured according to ASTM D1238-2004 with MI-4 melt index instrument (GOETTFERT, Germany) at 230 °C under a load of 2.16 kg. Infrared spectroscopy of sample was measured by using Fourier transform infrared of Thermo Scientific Nicolet IS 50. Samples (100 μm thick) molded at 1 MPa and 180 °C for 30 s were used. The rheology test was carried out at 200 °C under a nitrogen atmosphere by using a strain-controlled rotary rheometer (ARES-2 KFERTN1-FCO-HR, American Scientific Rheometer). Parallel plates with a diameter of 25 mm and plate spacing of 1 mm were used. The frequency range used was 0.017−100 rad/s with strain 5%. The sample was 1.5 mm thick prepared by hot pressing. The samples for mechanical property testing were prepared by injection molded. Izod notched impact strength was tested in accordance with ASTM D256-2010, the bending properties were tested according to ASTM D790-2010, and the heat distortion temperature was tested in accordance with ASTM D648. The differential scanning calorimetry (DSC) measurements were performed with a Diamond DSC differential scanning calorimeter at scan rates of 10 °C/min in a flowing N2 atmosphere, and the sample weight was ca. 8 mg. Indium was used as the standard sample. As for the isothermal crystallization process, the samples were heated to 200 °C at 100.0 °C/min and then the temperature was kept constant at 200 °C for 5 min to eliminate the residual crystals. The samples were then cooled at 100 °C/min to the predetermined crystallization temperature (118, 122, 126, and 130 °C) till the crystallization completed. Optical microscopy observation was conducted by an OLYMPUS BX51 optical microscope, which was equipped with a cross-polarizer, camera system, and programmable heating stage. The surface morphology was observed using a Hitachi S-4800 scanning electron microscope. The WAXD experiment was carried out on the Bruker D8 DISCOVER 2D X-ray diffractometer. The X-ray was generated using an IμS micro Focus X-ray source incorporating a 50 W sealed-tube Xray generator with a Cu target. The wavelength is 0.1542 nm. The power of the generator used for measurement was 45 kV and 0.9 mA. The X-ray intensities were recorded on a VÅNTEC-500 2D detector system with a pixel size of 100 × 100 μm2. The distance from the sample to the detector was 191 mm. The spot size of the beam was 0.5 mm. The exposure time was 2 min. The relative amount of different crystal form was measured from the WAXD profiles. In PP WAXD profiles, (110) at 2θ = 14.1°, (040) at 16.9°, (130) at 18.5°

EXPERIMENTAL SECTION

Materials. All materials used in this study are commercially available. PP spherical powder was supplied by Sinopec Zhenhai petrochemicals with brand name of M60 and melt flow rate (MFR) of 60 g/10 min. MAH, xylene, acetone, and ethyl alcohol were purchased from Xilong Science and Technology Co., Ltd. Sodium hydroxide (NaOH) and sodium chloride (NaCl) were purchased from China National Pharmaceutical Co., Ltd. Graphene oxide (GO) aqueous suspension was purchased from Nanjing JCNANO Technology Co., Ltd. Vinyl trimethoxysilane was kindly supplied by Tokyo Chemical Industry Co., Ltd. PP-g-MAH (CA 100) and PP-gMAH (CMG 9801) were supplied by French Arkema and Jiayirong Compatibilizer Jiangsu Co., Ltd., respectively. α nucleating agent NA11 was obtained from Japan ADEKA Co., Ltd. 2,2,6,6Tetramethylpiperidine oxide (TEMPO) was provided by J&K Technology Co., Ltd. Antioxidant 1010 and calcium stearate were provided by BASF and Tianjin Institute of Fine Chemical Engineering, respectively. Preparation of PP-g-MA. MAH was dissolved in acetone and added to PP spherical powders which were filled in a three-necked flask under vacuum and mechanical stirring. After acetone was evaporated, the mixture of MAH and PP spherical powder was transferred to a pressure-resistant glass bottle filled with nitrogen under atmospheric pressure or 0.1 MPa (Figure S1A in Supporting Information). The sample was heated by microwave for a certain period of time in a 700 W household microwave oven and then was washed three times with deionized water using a suction filtration device to ensure complete removal of unreacted MAH monomer. After drying, PP-g-MA pellets were obtained by mixing the sample with 0.1 wt % 1010 and 0.05 wt % calcium stearate, followed by being extruded and pelletized using a HAAKE twin-screw extruder at 190, 195, 200, 200, 200, and 190 °C. To increase the grafting reaction temperature further, 0.05% of GO aqueous solution and 0.05% of ascorbic acid by mass of PP were added to MAH and dispersed on PP powder together. The concentration of GO aqueous solution is 0.1 wt %. After drying in a blast drying oven at 80 °C for 2 h, the GO was prereduced. Then, microwave irradiation grafting was carried out in the same way above. Preparation of PP-g-NaMA. NaOH solution was added to a three-necked flask filled with PP-g-MA powder under vacuum and mechanical stirring to react for 6 min, followed by washing three times with deionized water. The dried sample was mixed with 0.1 wt % of B

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Figure 1. Temperature vs time curve of MAH (A) and NaCl (B) under microwave irradiation. are the principal reflections of the α-crystal form of PP while (300) at about 15.9° is the principal reflection of the β-crystal form. The crystallinity and relative content of different crystal form of PP can be calculated from the following equations.37,38

Kβ =

indicated by Figure 1A. Furthermore, our experimental results showed that PP could remain in solid state as long as the reaction time was shorter than 10 min even the graft temperature was higher than the melt point of PP. That is to say, the heat generated in 10 min was not enough to melt PP because the amount of MAH used in the “microreactor” is comparatively small; therefore, the grafting reaction could carry out at above 206 °C for 10 min, which is impossible for the conventional grafting reaction. There are two possible mechanisms for this grafting reaction. One is that MAH > 206 °C can cause PP-generating free radicals, and these PP free radicals can graft MAH nearby onto PP molecule chain because the tertiary carbon in PP without antioxidant is unstable and easy to generate free radicals whenever its temperature was over 200 °C, as reported by the literature.39 The other possible mechanism is that both PP and MAH can generate free radicals at 206 °C and MAH free radicals can cause PP to generate free radicals further for grafting. Our experimental results proved that MAH could also generate free radicals under our experimental condition. It could be found from Figure 2 that the peak intensity of MAH at 1710 cm−1

Aβ(300) Aα(110) + Aα(040) + Aα(130) + Aβ(300)

Xall = 1 −

(1)

A amorphous ∑ (A amorphous + Acrystalline)

(2)

where Kβ expresses the relative amount of β-crystal form with respect to α-crystal form; Aα(300) represents the area of the (300) reflection peak; Aα(110), Aα(040), and Aα(130) represent the areas of the (110), (040), and (130) reflection peaks, respectively; and Aamorphous is the area of the amorphous peak. In this work, a curve fitting software was used to calculate the peak intensities of WAXD profiles. SAXS measurements were carried out on Nanostar SAXS system with a sample-to-detector distance of 1047 mm. The X-ray generator was operated at 100 mA and 40 kV using Cu Kα radiation (0.1542 nm). A 100 μm × 100 μm two-dimensional position-sensitive detector was used with each virtual cell element 3 mm apart. The scattering intensity was stored in a 1024 × 1024 data array. The gel content was measured by extracting samples with xylene at 120 °C for 6 h. Approximately 0.3 g of granules was cut into small pieces and placed in a preweighed stainless steel fine wire mesh. After extraction, the sample was dried in a vacuum oven at 120 °C for 6 h to reach constant weight. The gel content was calculated as the percentage ratio of the final weight of the polymer to its initial weight and was determined by the average value of two samples. Molecular weight distribution and long-chain branching (LCB) content were determined at 25 °C by using Polymer Char’s size exclusion chromatography (SEC). The eluent was 1,2,4-trichlorobenzene and a calibration curve was constructed using narrow molecular weight distribution polystyrene standards ranging from a molecular weight of 1820 to 12 200 000 g/mol. Samples were injected in a volume of 200 μL at a concentration of 1.0 mg/mL for molecular weight measurement by a IR detector, viscometry detector, and light scattering detector. Temperature of NaCl and MAH during microwave irradiation was measured by using a thermocouple placed in the middle of sample bottle (Figure S1B). Pure MAH and 1:1 volume ratio mixture of NaCl with microwave transparent PP powder were used for the measurement. The relationship between the heating time and temperature of the samples was recorded as shown in Figure 1.

Figure 2. FTIR spectra of PP, PP + MAH, and PP + MAH + TEMPO before and after microwave heating.

gradually decreased with the extending microwave heating time, indicating that MAH could indeed generate free radicals, which could be caught by the free-radical scavenger TEMPO. The literature and experimental results indicate that microwave can heat MAH to 206 °C and these hot MAH could induce itself and PP nearby to produce free radicals; therefore, MAH can be grafted onto a PP molecular chain without using any initiator. Because PP does not melt during the grafting reaction, the ungrafted MAH residual can be completely removed by water washing while MAH grafted on PP molecule

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RESULTS AND DISCUSSION Preparation of PP-g-MA and Reaction Mechanism of Selective Heating. The PP powder used in this study had many micron-scale pores and a large specific surface area (Figure S2). During microwave irradiation, MAH dispersed in the pores of PP powder could be rapidly heated to >206 °C as C

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radicals further caused PP to branch or cross-link. Furthermore, the gel content test results showed that there was no gel in PP-g-MA; therefore, it should be LCB that caused MFR to reduce. The rheological test results further prove the existence of LCB in PP. The testing results of dynamic viscosity are shown in Figure 4. As can be seen from Figure 4A, the complex viscosity of PPg-MA prepared with 5% MAH and 3 min microwave heating increased only at low frequencies compared with PP, and the complex viscosity of PP-g-MA prepared with 5% MAH and 5 min microwave heating is higher than that of PP over the entire frequency range. It can be also found from Figure 4B that tan δ values of both PP-g-MA samples are lower than that of PP at low frequencies. It is clear that PP-g-MA samples prepared in this work contain LCB; therefore, they exhibited higher viscosity and higher elasticity than PP, especially at low frequency. The results of SEC test (Figure S3) confirm the existence of LCB in the samples with microwave irradiation. Their mean-square radius of gyration (Rg) values clearly deviated from linear PP, indicating that they indeed contained LCB. The calculated gpcBR value showed that the average LCB level could be increased when longer microwave irradiation time was used.40,41 Therefore, the LCB content can be increased by prolonging the microwave irradiation time. To sum up, different from conventional melt grafting methods, the selective heating grafting method not only avoids the serious degradation of PP but also causes PP branching. Therefore, the prepared PP-g-MA materials are all polar PPs with high melt strength. The above results also showed that increasing the MAH dosage and prolonging the microwave heating time cannot effectively improve the graft ratio of the grafting reaction. Therefore, we must find new approach to effectively improve the graft ratio for this new grafting method. It is obvious that high grafting reaction pressure can reduce the high-temperature gasification of MAH; therefore, using high pressure should be an effective method for the increasing graft ratio. The experimental results show that when the pressure of the reaction vessel was increased by 0.1 MPa. It has been found that the graft ratio of the sample prepared with 5% MAH and 5 min microwave heating could increase from 0.5 to 0.7%, that is, an obvious increase of 40%. Apparently, increasing the grafting reaction pressure is proved to be an effective way to increase the graft ratio. Furthermore, it has been found that the graft ratio can be enhanced more effectively by increasing pressure at higher grafting reaction temperature. According to the literature42 and our previous work,43 graphene temperature can reach over 1000 °C under microwave irradiation. We have found that the prereduced GO loaded on the surface of the polymer sponge could be reduced perfectively, while the structure of the polymer material remained intact under microwave irradiation for 5 s. Obviously, if a small amount of prereduced GO was added to PP powder together with MAH for microwave irradiation, higher temperature can improve the graft ratio of MAH while PP could remain solid. The experimental result proved that the graft ratio can increase 20% from 0.5 to 0.6% and PP remained solid when 0.05 wt % of prereduction GO was used for the sample with 5% MAH under microwave irradiation for 5 min; the obtained sample was named PP-g-MA-rGO. Furthermore, the graft ratio could be increased by 100−1.0% by 0.1 MPa pressure increase. It is indicated that the pressurization is more favorable for the grafting reaction at high temperature when the selective

can be hydrolyzed to MA simultaneously. The prepared PP-gMA had no ungrafted MAH and initiator residues and thus was odorless. Grafting Ratio and Molecular Structure of PP-g-MA. The effect of MAH dosage and microwave heating time on grafting ratio is shown in Figure 3. It can be found that grafting

Figure 3. Effect of MAH dosage and microwave heating time on the graft ratio.

ratio of MAH on PP increased when the MAH concentration and microwave heating time increased as the microwave heating time was less than 10 min. However, the grafting rate of MAH on PP no longer increased or even slightly decreased when the microwave heating time exceeded 10 min. It is interesting to know why this happens. Before the microwave heating time reached 10 min, the grafting reaction could happen between PP free radicals and MAH. However, when the grafting reaction time exceeded 10 min, MAH grafted on PP molecules could be still heated by microwave and caused nearby PP molecules to produce free radicals, which could not cause MAH grafting onto PP because MAH had either been grafted onto PP or had been vaporized. Therefore, the grafting rate of MAH on PP no longer increased. For PP-g-MAH with a higher grafting ratio, the grafting ratio is even slightly decreased when the microwave heating time exceeded 10 min because the high temperature caused by MAH can eliminate the small amount of MAH grafted on PP. Furthermore, the PP free radicals could make PP branching or cross-linking when there is no graftable MAH near those free radicals. Further experimental results have proved that branching or a cross-linking reaction did happen during microwave heating. More research works must be carried out to provide direct pieces of evidence for above explanation. Table 1 shows the MFR of the PP-g-MA samples as a function of microwave heating time. It can be found that MFR of PP-g-MA gradually decreased as the extending microwave heating time, giving proofs that PP did undergo branching or a cross-linking reaction. Moreover, the higher the MAH dosage, the bigger the reduction degree of MFR is, indicating that MAH did make PP to generate free radicals and these free Table 1. Effect of the Microwave Irradiation Time on MFR of PP (g/10 min) microwave irradiation time (min) MAH concentration 3% MAH concentration 5% MAH concentration 7%

0 60 60 60

3 60 60 58

5 58 55 52

7 55 51 50

10 53 43 41

12 47 42 40 D

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Figure 4. Effect of microwave irradiation and frequency on (A) complex viscosity η* and (B) tan δ of PP-g-MAs and PP.

heating grafting method is used because high temperature will rapidly gasify MAH and pressurization can effectively control the occurrence of gasification. It is obvious that PP-g-MA with a high graft ratio can be prepared by using the selective heating method without an initiator. Besides, mechanical properties of PP-g-MA prepared by the new process should be superior to those prepared by the traditional melt grafting method because PP did not undergo melting degradation. The following experimental results can prove such inferences. Mechanical Properties and Crystallization Behavior of PP-g-MA. Table 2 shows mechanical properties of PP and Table 2. Mechanical Properties and Relative Amount of βCrystallite (Kβ) in PP and Different PP-g-MA Samples sample PP PP-g-MA PP-g-MA (+0.1 MPa) PP-g-MA-rGO PP-g-MA-rGO (+0.1 MPa)

flexural modulus (GPa)

Izod notched impact strength (J/m)

heat distortion temperature (°C)

Kβ (%)

1.48 1.53 1.57

17.1 21.4 20.6

87.6 91.6 93.0

0 43.4 25.5

1.61 1.63

17.8 18.6

95.2 97.6

8.4 17.4

Figure 5. WAXD profiles of PP, PP-g-Mas, and commercial PP-gMAHs.

or even increase. Trace amounts of graphene could reduce the β crystal content of PP-g-MA samples because graphene can promote PP to form alpha crystals,44,45 which can compete with β crystal growth. However, the β crystal content of PP-gMA samples can be increased when the graft ratio is increased if the same amount of graphene is used. When the MA graft ratio was 0.6%, the β crystal content of PP-g-MA was only 8.4%; the β crystal content could be increased to 17.7% when the graft ratio was increased to 1.0% and toughness of PP-gMA samples increased with their β crystal content. Temperature also has an important influence on the crystallization behavior of PP-g-MA as shown in Figure 6, which are isothermal crystallization curves of PP-g-MA samples at different temperatures. Figure 7 shows the β crystal content Kβ and crystallinity (Xall) of PP-g-MA samples crystallized

different PP-g-MA samples. It can be found that all PP-g-MA samples have higher stiffness, toughness, and heat resistance than PP, which is completely different from the grafted PP prepared by the conventional melt grafting method. What is particularly surprising is that all PP-g-MA samples showed no reduction in toughness and the impact strength of PP-g-MA is even 25% higher than that of PP. Why can such an abnormal phenomenon appear? Figure 5 is WAXD profiles of PP, PP-g-MA, PP-g-MA-rGO, and two commercial PP-g-MAH samples. It can be found that there are four obvious diffraction peaks for PP and two commercial PP-g-MAH samples, corresponding to 2θ values of 14.1°, 17.0°, 21.3°, and 22.0°. Obviously, only an α monoclinic crystal exists in PP and two commercial PP-g-MAH samples, CA 100 and CMG 9801, exist. However, the four grafted samples prepared by selective heating method all have obvious diffraction peaks at 2θ = 15.9° corresponding to β hexagonal system of PP, indicating that they all contain β crystal forms with its relative amount (Kβ) of 43.4, 25.5, 8.4, and 17.7%, respectively. Obviously, different from two commercial PP-gMAH samples, all PP-g-MA samples prepared by selective heating method contain considerable amount of β-crystallite; therefore, the toughness of PP-g-MA samples did not decrease

Figure 6. WAXD profiles of PP-g-MA crystallized isothermally at different temperatures. E

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Figure 7. Relative content of β-form crystal and crystallinity (Xall) of PP-g-MA crystallized isothermally at different temperatures.

Figure 8. FTIR spectra of PP, PP-g-MA, and PP-g-NaMA samples.

isothermally at different temperatures calculated from the data of Figure 6. It is obvious that β crystals can be obtained by isothermal crystallization at 90−140 °C and maximum Kβ can be obtained at 110 °C. It is well known that the stiffness of β-PP should be lower than that of alpha PP.46−48 Why does PP-g-MA containing a large amount of β crystals still have higher flexural modulus than PP? It can be found from Table 3 that all PP-g-MA

be observed, proving that all MA grafted on the PP molecular chain had become sodium maleate (NaMA). It is well known that inorganic sodium salts, such as sodium benzoate and sodium aryl phosphate (NA11) are all crystalline nucleating agents for PP,49,50 which can improve crystallization behavior and mechanical properties of PP. It is interesting to find out if NaMA grafted onto PP molecular chain can also affect the mechanical properties and crystallization kinetics of PP. The mechanical properties of PP, PP-g-MA, and PP-gNaMA samples are shown in Table 4. It can be found that the

Table 3. Crystallization Parameters of PP, PP-g-MA, and Commercial PP-g-MAH sample

Tc (°C)

crystallinity (%)

Tm (°C)

PP PP-g-MA PP-g-MA (+0.1 MPa) PP-g-MA-rGO PP-g-MA-rGO (+0.1 MPa) commercial PP-g-MAH (CA100) commercial PP-g-MAH (CMG9801)

116.5 118.3 119.1 118.7 119.5 118.1 118.3

41.8 43.2 43.7 43.4 44.1 42.1 42.4

162.7 162.7 162.8 163.1 163.3 162.9 162.8

Table 4. Mechanical Properties of PP, PP-g-MA, and PP-gNaMA

sample PP PP-g-MA PP-g-MA (+0.1 MPa) PP-g-NaMA PP-g-NaMA (+0.1 MPa) PP + 0.1% NA11 PP + 0.3% NA11

samples prepared by the new selective heating method have higher crystallization temperature, crystallinity, and melting point than PP. It is clearly that PP-g-MA prepared by the new method not only contains β crystal but also has thicker lamellae and higher crystallinity; therefore, the prepared PP-gMA not only has high toughness but also has high stiffness and high heat resistance. It is worthy to find out in the future why PP-g-MA can induce β crystals in PP whereas traditional grafted PP cannot. Preparation and Crystallization Kinetics of PP-gNaMA. The prepared PP-g-MA in this study is similar to the traditional solid-phase grafted product. MA was grafted on the surface and pores of PP powders. Therefore, PP-g-NaMA can be prepared easily by adding NaOH aqueous solution to PP-gMA powder. Two PP-g-NaMA samples were prepared by adding NaOH aqueous solution to PP-g-MA powders prepared by using 5% MAH and microwave heating for 3 and 5 min. Figure 8 is the Fourier transform infrared (FTIR) spectrum of PP-g-MA and PP-g-NaMA. As shown, the absorption peaks of these samples at wavelengths of 1500−2000 cm−1 are significantly different. PP-g-MA samples showed symmetry and antisymmetric absorption peaks of CO bond at 1780 and 1830 cm−1, and absorption peaks of carboxyl groups at 1710 and 1555 cm−1, indicating that the prepared samples were indeed PP-g-MA. In the infrared spectrum of PP-g-NaMA samples, only the carboxyl absorption peak at 1555 cm−1 could

flexural modulus (GPa)

flexural strength (MPa)

Izod notched impact strength (J/m)

heat distortion temperature (°C)

1.48 1.53 1.57

35.1 35.8 35.9

17.1 21.4 20.6

87.6 91.6 93.0

1.67 1.70

38.1 38.2

18.3 17.6

98.8 99.5

1.63 1.71

36.1 36.5

17.0 16.7

95.1 99.6

stiffness and heat resistance of PP-g-NaMA samples have been improved significantly compared with PP. In fact, PP-g-NaMA with good toughness has stiffness and heat resistance similar to those of PP modified by excellent α nucleating agent, NA11. Obviously, polar PP-g-NaMA has mechanical properties similar to those of the nonpolar PP modified by commercial α nucleating agent NA11. Therefore, the crystallization behavior of PP-g-NaMA should be special and it is necessary to conduct an in-depth study. PP-g-MA and PP-g-NaMA samples prepared by 5 min microwave heating and MAH content of 5% were studied by DSC and polarized light microscopy. It can be found from Table 5 and Figure S4 that the crystallization temperature, crystallinity, and melting point of PP-g-MA and PP-g-NaMA samples are all higher than those of PP, and PP-g-NaMA samples have much higher crystallization temperature, crystallinity, and melting point than PP-g-MA. PP-g-NaMA with a high melting point should be thick lamellar, which has been confirmed by the one-dimensional correlation function of the SAXS test (Figure S5). Furthermore, polarized microscopy F

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properties similar to those of the modified PP by a nucleating agent. Preparation of PP Grafted Unsaturated Silane. To find out if the selective microwave heating method can be used for grafting reaction of nonpolar monomers, unsaturated silane was selected as grafting monomer to prepare PP grafted unsaturated silane. It is obvious that heat generated by unsaturated silane is insufficient to cause PP to generate radicals under microwave irradiation because the polarity of unsaturated silane is much weaker than that of the MAH. Our experiment has proved that NaCl could be heated over 210 °C in 4 min under microwave irradiation (Figure 1B). Furthermore, NaCl can be washed off with water after completing grafting reaction. Therefore, NaCl was selected to use together with unsaturated silane to prepare PP-gunsaturated silane (PP-g-Sil). SEM photograph in Figure S10 shows that unsaturated silane has grafted onto PP and graft ratio is 0.85 wt %. This result shows that selective heating method is a new polymer grafting method with wide adaptability.

Table 5. Crystallization Parameter of PP, PP-g-MA, and PPg-NaMA sample

Tc (°C)

crystallinity (%)

Tm (°C)

PP PP-g-MA PP-g-NaMA

117.2 122.7 125.4

41.4 44.1 47.8

162.7 163.2 164.3

photographs as shown in Figure S6 indicate that PP-g-NaMA has spherulites size similar to those of NA11-modified PP, which is much smaller than that of PP. Therefore, it is reasonable that PP-NaMA has high stiffness and high heat resistance, which is similar to NA11-modified PP. To understand why PP-g-NaMA has such excellent performance, its crystallization kinetics was also studied. Figure S7 is isothermal crystallization curves of PP, PP-g-NaMA and commercial nucleating agent NA11-modified PP samples at different crystallization temperatures. All samples have only single α-crystallization peak during isothermal crystallization. By converting the isothermal curves in Figure S7 into the curve of relative crystallinity versus crystallization time (Figure S8), it can be found that the crystallization rate of PP-g-NaMA is faster than that of PP, but slower than that of NA11-modified PP. According to the Avrami equation, the semicrystallization time t1/2, the total crystallization rate constant K of each sample, and the Avrami index n value can be obtained. Figure S9 shows the Avrami curves of PP, PP-g-NaMA samples, and NA11-modified PP under different isothermal crystallization conditions. The slope and intercept of the linear part of the curves were used to calculate corresponding Avrami index n and rate constant K as shown in Table S1. Meanwhile, the semicrystallization time (t1/2) of typical samples at different isothermal crystallization temperatures was present in Figure 9.

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CONCLUSIONS A new polymer solid-phase grafting method with wide adaptability, selective heating method conducted at temperature higher than the polymer melting point, has been successfully developed. Several new grafting polymers with excellent performance have been prepared by using this new method. • Different from traditional polar PP, PP-g-MA with high melt strength prepared by this new method does not contain any initiator and MAH residue. • The new PP-g-MA also has high toughness because of a large number of β crystals in it. PP-g-MA is a highperformance polar PP with high stiffness, high toughness, high heat resistance, and no odor. • Polar PP-g-NaMA with high melt strength prepared by reacting PP-g-MA with NaOH aqueous solution is colorless and has mechanical properties similar to those of NA11-modified PP. This universal grafting method can be used to prepare many kinds of grafted polymers, which can be applied in polymer recycle, polymer modification, polymer composites, and so forth. Hopefully, this work can inspire some further explorations for both academia and industry in different research areas.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. Dependence of the t1/2 on crystallization temperature for PP, PP-g-MA, PP-g-NaMA, and NA11-modified PP.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02737. Reaction flasks for microwave grafting and temperature measurement; SEM images of the surface and internal pores of PP spherical powder; curves of molecular weight distribution and square radius of gyration (Rg) of samples determined by SEC, WAXD profiles of PP, PPg-MA, and PP-g-NaMA; sketch and enlarged images of one-dimensional correlation function K(z) for PP, PP-gNaMA, and PP-g-NaMA; polarized light microphotographs of PP, PP-g-Mas, and PP-g-Mas; isothermal crystallization curve; curve of relative crystallinity versus time of isothermal crystallization at different temper-

It can be found that the crystallization temperature and crystallinity of PP-g-NaMA were higher than PP while its Avrami index n is as same as that of the PP. It shows that the nucleation mode of PP-g-NaMA is different from that of nucleating agent-modified PP but is the same as PP. Moreover, PP-g-NaMA has a faster crystallization rate than PP at higher temperatures (122, 126 and 130 °C) but has a slower crystallization rate at lower crystallization temperature (118 °C). This may be the main reason that PP-g-NaMA has high toughness. However, more research works must be carried out to clarify why PP-g-NaMA prepared by our new process has G

DOI: 10.1021/acs.macromol.8b02737 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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atures and plots of isothermal crystallization at 118, 122, 126, and 130 °C for PP, PP-g-NaMAs, and PP with NA11; EDS data of PP-g-vinyl trimethoxysilane prepared by NaCl-assisted microwave heating; and isothermal crystallization kinetic data of PP, PP-g-NaMA, and NA11-modified PP samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jinliang Qiao: 0000-0002-2608-6223 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ph.D. Programs Foundation of SINOPEC Beijing Research Institute of Chemical Industry.

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DOI: 10.1021/acs.macromol.8b02737 Macromolecules XXXX, XXX, XXX−XXX