Rheology, Non-Isothermal Crystallization Behavior, Mechanical and Thermal Properties of PMMA-Modified Carbon Fiber-Reinforced Poly(Ethylene Terephthalate) Composites

Poly(ethylene terephthalate) (PET) composites containing carbon fiber (CF) or polymethyl methacrylate (PMMA)-grafted carbon fiber (PMMA-g-CF) were prepared by melt compounding. The rheology, non-isothermal crystallization behavior, and mechanical and thermal properties of pure PET, PET/CF and PET/PMMA-g-CF composites were investigated. The results show that the addition of CF or PMMA-g-CF significantly increases the storage modulus (G′), loss modulus (G″), and complex viscosity (η*) of the composites at low frequency. The Cole-Cole plots confirm that the surface modification of CF leads to a better interaction between the CF and PET, and then decreases the heterogeneity of the polymeric systems, which is confirmed by the SEM observation on the tensile fracture surface of the composites. Non-isothermal crystallization analysis shows that the CF or PMMA-g-CF could serve as nucleation agent to accelerate the crystallization rate of the composites, and the effect of PMMA-g-CF is stronger than that of CF. The result is further confirmed by the analysis of the crystallization activation energy for all composites calculated by the Flynn-Wall-Ozawa method. Moreover, the tensile and impact strength and the thermal stability of the composites are improved by CF, while the incorporation of PMMA-g-CF further enhances the tensile and impact strength and thermal stability.

The SEM images of untreated CF and PMMA-g-CF are shown in Figure 2. As shown in Figure  2A, it can be observed that the untreated CF with a few narrow grooves is relatively neat and smooth. In contrast, the surface of the PMMA-g-CF in Figure 2B becomes rough and is wrapped by a thick layer of polymer. This indicates that PMMA is chemically grafted on the carbon fiber, generating a layer of PMMA particles on the carbon fiber's surface. The rougher surface on the grafted carbon fiber is expected to be of benefit to improve the adhesion of grafted CF and the matrix, which then improves the mechanical properties of the composites.  Figure 3 shows the wide-scan XPS spectra of different elements on the carbon fiber's surface before and after treatment. As shown in Figure 3, the content of carbon decreases from 92.54% to 70.97% and the content of oxygen increases from 7.46% to 29.03% after the modification of MMA. In addition, the surface atomic O/C ratios increase from 0.08 to 0.41, suggesting the change of the polarity of the CF surface. To further investigate the content of functional groups on the CF surface, the C1s spectra is peak fitted into four peaks and the results are shown in Table 1 and Figure 4. It is clearly seen that the percentages of the C-OH, C=O, and O=C-OR functional groups on the CF surface after modification increases from 11.6%, 1.3%, and 5.2% to 19.3%, 3.5%, and 18%, respectively, suggesting the successful grafting of PMMA.
In short, through the analysis of FTIR, SEM, and XPS results, it can be concluded that the surfaces of CFs have been grafted with PMMA. The SEM images of untreated CF and PMMA-g-CF are shown in Figure 2. As shown in Figure 2A, it can be observed that the untreated CF with a few narrow grooves is relatively neat and smooth. In contrast, the surface of the PMMA-g-CF in Figure 2B becomes rough and is wrapped by a thick layer of polymer. This indicates that PMMA is chemically grafted on the carbon fiber, generating a layer of PMMA particles on the carbon fiber's surface. The rougher surface on the grafted carbon fiber is expected to be of benefit to improve the adhesion of grafted CF and the matrix, which then improves the mechanical properties of the composites. The SEM images of untreated CF and PMMA-g-CF are shown in Figure 2. As shown in Figure  2A, it can be observed that the untreated CF with a few narrow grooves is relatively neat and smooth. In contrast, the surface of the PMMA-g-CF in Figure 2B becomes rough and is wrapped by a thick layer of polymer. This indicates that PMMA is chemically grafted on the carbon fiber, generating a layer of PMMA particles on the carbon fiber's surface. The rougher surface on the grafted carbon fiber is expected to be of benefit to improve the adhesion of grafted CF and the matrix, which then improves the mechanical properties of the composites.  Figure 3 shows the wide-scan XPS spectra of different elements on the carbon fiber's surface before and after treatment. As shown in Figure 3, the content of carbon decreases from 92.54% to 70.97% and the content of oxygen increases from 7.46% to 29.03% after the modification of MMA. In addition, the surface atomic O/C ratios increase from 0.08 to 0.41, suggesting the change of the polarity of the CF surface. To further investigate the content of functional groups on the CF surface, the C1s spectra is peak fitted into four peaks and the results are shown in Table 1 and Figure 4. It is clearly seen that the percentages of the C-OH, C=O, and O=C-OR functional groups on the CF surface after modification increases from 11.6%, 1.3%, and 5.2% to 19.3%, 3.5%, and 18%, respectively, suggesting the successful grafting of PMMA.
In short, through the analysis of FTIR, SEM, and XPS results, it can be concluded that the surfaces of CFs have been grafted with PMMA.  Figure 3 shows the wide-scan XPS spectra of different elements on the carbon fiber's surface before and after treatment. As shown in Figure 3, the content of carbon decreases from 92.54% to 70.97% and the content of oxygen increases from 7.46% to 29.03% after the modification of MMA. In addition, the surface atomic O/C ratios increase from 0.08 to 0.41, suggesting the change of the polarity of the CF surface. To further investigate the content of functional groups on the CF surface, the C1s spectra is peak fitted into four peaks and the results are shown in Table 1 and Figure 4. It is clearly seen that the percentages of the C-OH, C=O, and O=C-OR functional groups on the CF surface after modification increases from 11.6%, 1.3%, and 5.2% to 19.3%, 3.5%, and 18%, respectively, suggesting the successful grafting of PMMA.
In short, through the analysis of FTIR, SEM, and XPS results, it can be concluded that the surfaces of CFs have been grafted with PMMA.      Figure 5 shows the complex viscosity (η*) of pure PET, PET/CF, and PET/PMMA-g-CF composites vs. the angular frequency (ω). It can be seen that the complex viscosity values of all the samples decrease gradually with increasing ω, exhibiting the pseudoplastic behavior. Meanwhile, the values of η* increase with the addition of CF or PMMA-g-CF within our research scope. This phenomenon indicates that the addition of CF or PMMA-g-CF disturbs the mobility of polymer chains in the melt and then increases the complex viscosity. In comparison with pure PET and the composites, it is clearly seen that the composites with PMMA-g-CF have larger viscosity due to the       Figure 5 shows the complex viscosity (η*) of pure PET, PET/CF, and PET/PMMA-g-CF composites vs. the angular frequency (ω). It can be seen that the complex viscosity values of all the samples decrease gradually with increasing ω, exhibiting the pseudoplastic behavior. Meanwhile, the values of η* increase with the addition of CF or PMMA-g-CF within our research scope. This phenomenon indicates that the addition of CF or PMMA-g-CF disturbs the mobility of polymer chains in the melt and then increases the complex viscosity. In comparison with pure PET and the composites, it is clearly seen that the composites with PMMA-g-CF have larger viscosity due to the  Figure 5 shows the complex viscosity (η*) of pure PET, PET/CF, and PET/PMMA-g-CF composites vs. the angular frequency (ω). It can be seen that the complex viscosity values of all the samples decrease gradually with increasing ω, exhibiting the pseudoplastic behavior. Meanwhile, the values of η* increase with the addition of CF or PMMA-g-CF within our research scope. This phenomenon indicates that the addition of CF or PMMA-g-CF disturbs the mobility of polymer chains in the melt and then increases the complex viscosity. In comparison with pure PET and the composites, it is clearly seen that the composites with PMMA-g-CF have larger viscosity due to the stronger interaction between the PMMA-g-CF and PET matrix, which is confirmed by the SEM analysis of the tensile fracture surfaces of the PET/PMMA-g-CF composites below. stronger interaction between the PMMA-g-CF and PET matrix, which is confirmed by the SEM analysis of the tensile fracture surfaces of the PET/PMMA-g-CF composites below. The storage modulus (G′) and the loss modulus (G′′) versus frequency (ω) for all the test samples are shown in Figure 6. It is observed from Figure 6A that the G′ values of the composites increase with the addition of CF or PMMA-g-CF in the whole frequency region, which indicates that the melt strength increases. In addition, the G′ value of the PET/PMMA-g-CF composite is larger than that of the pure PET and PET/CF composite, especially in the low-frequency region. It is well known that the storage modulus represents the elastic response of a material. As shown in Figure 6A, PMMA-g-CF has a significant reinforcing effect on the G′ of the PET, which means that the PET/PMMA-g-CF composite can store large amounts of energy during deformation, resulting in the improvement of the Charpy impact strength. The result is consistent with the mechanical measurements described below. Figure 6B shows the similar trend of G′′ to that for G′. It is generally believed that the homogeneity of polymer melts or solutions can be characterized by the plot of G′ versus G′′, which is named the modified Cole-Cole plot by Harrell and Nakajima [23], and the slope value of 2 obtained from the G′ versus G′′ plot indicates a homogeneous and isotropic polymer solution or melt [24][25][26]. The plots of G′ versus G′′ for pure PET, PET/CF, and The storage modulus (G ) and the loss modulus (G ) versus frequency (ω) for all the test samples are shown in Figure 6. It is observed from Figure 6A that the G values of the composites increase with the addition of CF or PMMA-g-CF in the whole frequency region, which indicates that the melt strength increases. In addition, the G value of the PET/PMMA-g-CF composite is larger than that of the pure PET and PET/CF composite, especially in the low-frequency region. It is well known that the storage modulus represents the elastic response of a material. As shown in Figure 6A, PMMA-g-CF has a significant reinforcing effect on the G of the PET, which means that the PET/PMMA-g-CF composite can store large amounts of energy during deformation, resulting in the improvement of the Charpy impact strength. The result is consistent with the mechanical measurements described below. Figure 6B shows the similar trend of G to that for G .

Rheological Properties
Polymers 2018, 10, x 6 of 16 stronger interaction between the PMMA-g-CF and PET matrix, which is confirmed by the SEM analysis of the tensile fracture surfaces of the PET/PMMA-g-CF composites below. The storage modulus (G′) and the loss modulus (G′′) versus frequency (ω) for all the test samples are shown in Figure 6. It is observed from Figure 6A that the G′ values of the composites increase with the addition of CF or PMMA-g-CF in the whole frequency region, which indicates that the melt strength increases. In addition, the G′ value of the PET/PMMA-g-CF composite is larger than that of the pure PET and PET/CF composite, especially in the low-frequency region. It is well known that the storage modulus represents the elastic response of a material. As shown in Figure 6A, PMMA-g-CF has a significant reinforcing effect on the G′ of the PET, which means that the PET/PMMA-g-CF composite can store large amounts of energy during deformation, resulting in the improvement of the Charpy impact strength. The result is consistent with the mechanical measurements described below. Figure 6B shows the similar trend of G′′ to that for G′. It is generally believed that the homogeneity of polymer melts or solutions can be characterized by the plot of G′ versus G′′, which is named the modified Cole-Cole plot by Harrell and Nakajima [23], and the slope value of 2 obtained from the G′ versus G′′ plot indicates a homogeneous and isotropic polymer solution or melt [24][25][26]. The plots of G′ versus G′′ for pure PET, PET/CF, and It is generally believed that the homogeneity of polymer melts or solutions can be characterized by the plot of G versus G , which is named the modified Cole-Cole plot by Harrell and Nakajima [23], and the slope value of 2 obtained from the G versus G plot indicates a homogeneous and isotropic polymer solution or melt [24][25][26]. The plots of G versus G for pure PET, PET/CF, and PET/PMMA-g-CF composites are shown in Figure 7. One can see that the PET/CF and PET/PMMA-g-CF composites exhibit a deviated curve. The introduction of CF decreases the slope producing the deviation from the master curve corresponding to the PET matrix, indicative of the increased heterogeneity in the system. However, the deviated extent of the PET/PMMA-g-CF composite decreases. This phenomenon suggests that surface modification of CF improves the compatibility between the CF and PET, and then decreases the heterogeneity of the polymeric systems. PET/PMMA-g-CF composites are shown in Figure 7. One can see that the PET/CF and PET/PMMAg-CF composites exhibit a deviated curve. The introduction of CF decreases the slope producing the deviation from the master curve corresponding to the PET matrix, indicative of the increased heterogeneity in the system. However, the deviated extent of the PET/PMMA-g-CF composite decreases. This phenomenon suggests that surface modification of CF improves the compatibility between the CF and PET, and then decreases the heterogeneity of the polymeric systems.

The Non-isothermal Crystallization Behavior and Melting Behavior
The crystallization curves of pure PET, PET/CF, and PET/PMMA-g-CF composites at various cooling rates (5, 10, 20 and 40 °C /min) are shown in Figure 8. As shown in Figure 8, it is not difficult to find that the curves become wider and the peak temperature (Tp) shifts to the lower temperature as the cooling rate increases gradually. The parameters from the non-isothermal crystallization exotherms for all samples are summarized in Table 2. According to Table 2, it can be seen that the onset temperature of crystallization (To) and the peak temperature (Tp) for all samples shift to lower values with the increasing cooling rate. This may be because the crystals have had less time to nucleate and grow as the cooling rate increased. For a given cooling rate, the To and Tp values of PET are lower than that of PET/CF and PET/PMMA-g-CF composites, which is an indication that the presence of CF or PMMA-g-CF can act as nucleating agent to increase the crystallization rate of PET. This can be attributed to the heterogeneous nucleation effect of the CF or PMMA-g-CF, which facilitates the crystallization of PET chains when the composite is cooled down from the melting temperature.

The Non-isothermal Crystallization Behavior and Melting Behavior
The crystallization curves of pure PET, PET/CF, and PET/PMMA-g-CF composites at various cooling rates (5, 10, 20 and 40 • C/min) are shown in Figure 8. As shown in Figure 8, it is not difficult to find that the curves become wider and the peak temperature (T p ) shifts to the lower temperature as the cooling rate increases gradually. The parameters from the non-isothermal crystallization exotherms for all samples are summarized in Table 2. According to Table 2, it can be seen that the onset temperature of crystallization (T o ) and the peak temperature (T p ) for all samples shift to lower values with the increasing cooling rate. This may be because the crystals have had less time to nucleate and grow as the cooling rate increased. For a given cooling rate, the T o and T p values of PET are lower than that of PET/CF and PET/PMMA-g-CF composites, which is an indication that the presence of CF or PMMA-g-CF can act as nucleating agent to increase the crystallization rate of PET. This can be attributed to the heterogeneous nucleation effect of the CF or PMMA-g-CF, which facilitates the crystallization of PET chains when the composite is cooled down from the melting temperature.    To better understand the effect of CF and PMMA-g-CF on the crystallization and melting behavior of pure PET, the crystallization and melting behavior of PET/CF and PET/PMMA-g-CF composites are characterized by using DSC cooling and heating thermograms, as shown in Figure 9. The detailed crystallization parameters, such as crystallization temperature (T c ), melting temperature (T m ), and degree of crystallinity (X c ), are shown in Table 3. As shown in Table 3, we can see that the composites show higher T c and X c values than that of the pure PET, and the values of T c and X c for PET/PMMA-g-CF composites are higher than that of the PET/CF composites, which can be due to the nucleation effects of the CF and PMMA-g-CF, and then improve the crystallization of the PET matrix. In addition, the PET/PMMA-g-CF composites show higher T m values than that of pure PET or PET/CF composites. It can be concluded that higher crystal perfection caused by the PMMA-g-CF results in a higher T m of the PET matrix.

Non-Isothermal Crystallization Kinetics
Generally, the Avrami equation [27] can be used to describe non-isothermal crystallization, and is defined as:

Non-Isothermal Crystallization Kinetics
Generally, the Avrami equation [27] can be used to describe non-isothermal crystallization, and is defined as: where X t is the relativity crystallinity at crystallization time t, n is the Avrami exponent, and Z is the crystallization rate constant. Considering the temperature is constantly changing during a non-isothermal process, Jeziorny [28] suggested that the value of the crystallization rate parameter Z should be corrected by the cooling rate β as follows: where Z c is the corrected Jeziorny crystallization constant. According to Equation (2), the plots of log[−ln(1 − X t )] against logt for PET, PET/CF, and PET/PMMA-g-CF composites are shown in Figure 10. The values of n and Z can be obtained by the slopes and the intercepts of lines, respectively, and the values of Z c can be calculated by Equation (3). The values of n and Z c are listed in Table 4. As shown in Figure 10, the plots of PET, PET/CF, and PET/PMMA-g-CF composites are fairly linear, although they deviate at the beginning and the end, which is usually attributed to the induced period during initial crystallization and secondary crystallization at the end of crystallization.     It can be seen from Table 4 that the n values for pure PET range from 2.76 to 3.32, which means a three-dimensional spherical growth and homogeneous nucleation. After adding the CF or PMMA-g-CF, the n values are found to be in the range of 2.72-3.50. This result suggests that the addition of CF or PMMA-g-CF does not significantly change the nucleation mechanism and crystal growth of the PET matrix. In addition, as shown in Table 4, higher Z c values are obtained for the PET/CF and PET/PMMA-g-CF composites than those of pure PET under the same cooling rate. Usually, a higher Z c value means faster crystallization rate of the matrix at the same cooling rate [29]. Therefore, this behavior shows that the incorporation of the CF or PMMA-g-CF can act as a nucleating agent and increase the crystallization rate of the composites.

Crystallization Activation Energy
Usually, the reliable values of the effective activation energy of the non-isothermal crystallization of polymer can be evaluated by the differential iso-conversional method of Flynn-Wall-Ozawa [30]. The Flynn-Wall-Ozawa equation is expressed as follows: where R is the universal gas constant, β i is the cooling rate, E a is the activation energy for a certain conversion, and T a,i refers to the temperature at a certain conversions and cooling rate. Figure 11 shows the analyses of PET, PET/CF, and PET/PMMA-g-CF composites by the Flynn-Wall-Ozawa method. By plotting of lnβ i against 1/T a,i , where i = 2%, 4%, 6%, 8%, 10%, 15%, . . . , 90%, 92%, 94%, 96%, 98%, and the E a values can be calculated from the slope of the straight lines in Figure 11. Figure 12 shows the dependence of the effective activation energy E a on the relative degree of crystallinity for PET, PET/CF, and PET/PMMA-g-CF composites. It is found that the E a values of PET, PET/CF, and PET/PMMA-g-CF composites increase with increasing the extent of the relative crystallization, indicating easier crystallization occurs at lower relative crystallinity. In addition, for a given conversion, the E a values of PET/CF and PET/PMMA-g-CF composites are much lower than that of the pure PET, and the E a value of PET/PMMA-g-CF composite is lower than that of the PET/CF composite. It is known that the lower the E a value, the higher the crystallization ability of the polymer becomes [31]. The results indicate that the addition of CF or PMMA-g-CF has a strong nucleating effect on the PET matrix during the crystallization process, and the effect of PMMA-g-CF is stronger than that of CF due to the enhancement of interfacial interaction between the PET matrix and CF through the surface treatment of PMMA.
where R is the universal gas constant, βi is the cooling rate, Ea is the activation energy for a certain conversion, and Ta,i refers to the temperature at a certain conversions and cooling rate. Figure 11 shows the analyses of PET, PET/CF, and PET/PMMA-g-CF composites by the Flynn-Wall-Ozawa method. By plotting of lnβi against 1/Ta,i, where i = 2%, 4%, 6%, 8%, 10%, 15%, …, 90%, 92%, 94%, 96%, 98%, and the Ea values can be calculated from the slope of the straight lines in Figure  11.  Figure 12 shows the dependence of the effective activation energy Ea on the relative degree of crystallinity for PET, PET/CF, and PET/PMMA-g-CF composites. It is found that the Ea values of PET, PET/CF, and PET/PMMA-g-CF composites increase with increasing the extent of the relative crystallization, indicating easier crystallization occurs at lower relative crystallinity. In addition, for a given conversion, the Ea values of PET/CF and PET/PMMA-g-CF composites are much lower than that of the pure PET, and the Ea value of PET/PMMA-g-CF composite is lower than that of the PET/CF composite. It is known that the lower the Ea value, the higher the crystallization ability of the polymer becomes [31]. The results indicate that the addition of CF or PMMA-g-CF has a strong nucleating effect on the PET matrix during the crystallization process, and the effect of PMMA-g-CF is stronger than that of CF due to the enhancement of interfacial interaction between the PET matrix and CF through the surface treatment of PMMA.  Figure 13 shows the SEM images of the tensile fracture surfaces of the PET/CF and PET/ PMMAg-CF composites. As can be seen in Figure 13A, the PET/CF composites show an obvious interface region between the CF and the PET matrix, and the surface of the carbon fiber is smooth. These features suggest insufficient adhesion between the untreated CF and the PET matrix. However, it is easy to find that there is a close connection between the PMMA-g-CF and the PET matrix in Figure  13B, which can result in a better interfacial adhesion between the PMMA-g-CF and the PET matrix. The better interfacial adhesion between the treated CF and PET matrix is believed to be of benefit to improve the mechanical properties of the composites.  Figure 13 shows the SEM images of the tensile fracture surfaces of the PET/CF and PET/ PMMA-g-CF composites. As can be seen in Figure 13A, the PET/CF composites show an obvious interface region between the CF and the PET matrix, and the surface of the carbon fiber is smooth. These features suggest insufficient adhesion between the untreated CF and the PET matrix. However, it is easy to find that there is a close connection between the PMMA-g-CF and the PET matrix in Figure 13B, which can result in a better interfacial adhesion between the PMMA-g-CF and the PET matrix. The better interfacial adhesion between the treated CF and PET matrix is believed to be of benefit to improve the mechanical properties of the composites. Figure 13 shows the SEM images of the tensile fracture surfaces of the PET/CF and PET/ PMMAg-CF composites. As can be seen in Figure 13A, the PET/CF composites show an obvious interface region between the CF and the PET matrix, and the surface of the carbon fiber is smooth. These features suggest insufficient adhesion between the untreated CF and the PET matrix. However, it is easy to find that there is a close connection between the PMMA-g-CF and the PET matrix in Figure  13B, which can result in a better interfacial adhesion between the PMMA-g-CF and the PET matrix. The better interfacial adhesion between the treated CF and PET matrix is believed to be of benefit to improve the mechanical properties of the composites.

Mechanical Properties
The Charpy impact strength and tensile strength of pure PET, PET/CF, and PET/PMMA-g-CF composites are shown in the Figure 14. As shown in Figure 14, for the CF-reinforced composites, not only does the tensile strength increase with the addition of CF, but the Charpy impact strength also increases. The results imply that the incorporation of CF improves both the rigidity and the toughness as well, which maybe because of the reinforcement effect of the CF. In addition, the addition of PMMA-g-CF further improves the tensile strength and the Charpy impact strength of PET. This may contribute to the addition of PMMA-g-CF increasing the interfacial adhesion between the two phases, which allows for a more efficient stress transfer under stress conditions. The result is consistent with the SEM results above.

Mechanical Properties
The Charpy impact strength and tensile strength of pure PET, PET/CF, and PET/PMMA-g-CF composites are shown in the Figure 14. As shown in Figure 14, for the CF-reinforced composites, not only does the tensile strength increase with the addition of CF, but the Charpy impact strength also increases. The results imply that the incorporation of CF improves both the rigidity and the toughness as well, which maybe because of the reinforcement effect of the CF. In addition, the addition of PMMA-g-CF further improves the tensile strength and the Charpy impact strength of PET. This may contribute to the addition of PMMA-g-CF increasing the interfacial adhesion between the two phases, which allows for a more efficient stress transfer under stress conditions. The result is consistent with the SEM results above.

Thermogravimetric Analysis (TGA)
Thermal stability of the pure PET, PET/CF, and PET/PMMA-g-CF composites is investigated by TGA. The TG and DTG curves of the decomposition temperature for pure PET, PET/CF and PET/PMMA-g-CF composites under N2 are shown in Figure 15. As shown in Figure 15, the curves of PET and its composites show one step of mass loss. The decomposition temperature for 10% mass loss (T10%) and the temperature at the maximum mass loss rate (Tmax%) are listed in Table 5. It can be observed in table 5 that the T10% and Tmax% of the PET are increased with the addition of CF or PMMAg-CF. The results demonstrate that both the CF and PMMA-g-CF could increase the initial thermal degradation temperature and the thermal stability of PET. In addition, the Tmax% of PET/PMMA-g-CF composite is higher than that of PET/CF composite, showing higher thermal stability. This behavior could be attributed to the excellent heat stability and thermal conductivity of the surface treatment of CF. The incorporation of PMMA-g-CF into the PET matrix can effectively act as a strong barrier to

Thermogravimetric Analysis (TGA)
Thermal stability of the pure PET, PET/CF, and PET/PMMA-g-CF composites is investigated by TGA. The TG and DTG curves of the decomposition temperature for pure PET, PET/CF and PET/PMMA-g-CF composites under N 2 are shown in Figure 15. As shown in Figure 15, the curves of PET and its composites show one step of mass loss. The decomposition temperature for 10% mass loss (T 10% ) and the temperature at the maximum mass loss rate (T max% ) are listed in Table 5. It can be observed in table 5 that the T 10% and T max% of the PET are increased with the addition of CF or PMMA-g-CF. The results demonstrate that both the CF and PMMA-g-CF could increase the initial thermal degradation temperature and the thermal stability of PET. In addition, the T max% of PET/PMMA-g-CF composite is higher than that of PET/CF composite, showing higher thermal stability. This behavior could be attributed to the excellent heat stability and thermal conductivity of the surface treatment of CF. The incorporation of PMMA-g-CF into the PET matrix can effectively act as a strong barrier to prevent the diffusion of volatile decomposed products out of the PET/PMMA-g-CF composites during the thermal degradation process.
Thermal stability of the pure PET, PET/CF, and PET/PMMA-g-CF composites is investigated by TGA. The TG and DTG curves of the decomposition temperature for pure PET, PET/CF and PET/PMMA-g-CF composites under N2 are shown in Figure 15. As shown in Figure 15, the curves of PET and its composites show one step of mass loss. The decomposition temperature for 10% mass loss (T10%) and the temperature at the maximum mass loss rate (Tmax%) are listed in Table 5. It can be observed in table 5 that the T10% and Tmax% of the PET are increased with the addition of CF or PMMAg-CF. The results demonstrate that both the CF and PMMA-g-CF could increase the initial thermal degradation temperature and the thermal stability of PET. In addition, the Tmax% of PET/PMMA-g-CF composite is higher than that of PET/CF composite, showing higher thermal stability. This behavior could be attributed to the excellent heat stability and thermal conductivity of the surface treatment of CF. The incorporation of PMMA-g-CF into the PET matrix can effectively act as a strong barrier to prevent the diffusion of volatile decomposed products out of the PET/PMMA-g-CF composites during the thermal degradation process.

Conclusions
In this work, poly(ethylene terephthalate)(PET) composites containing carbon fiber (CF) or polymethyl methacrylate (PMMA)-grafted carbon fiber (PMMA-g-CF) were prepared by melt compounding. The rheology, non-isothermal crystallization behavior, and mechanical and thermal properties of pure PET, PET/CF, and PET/PMMA-g-CF composites were investigated by a parallel-plate rheometer, differential scanning calorimetry (DSC), tensile and impact tests, and thermogravimetric analysis (TGA), respectively. The results show that the addition of CF or PMMA-g-CF significantly increases the storage modulus (G ), loss modulus (G ), and complex viscosity (η*) of the composites at low frequency. The Cole-Cole plots confirm that the surface modification of CF leads to a better interaction between the CF and PET, and then decreases the heterogeneity of the polymeric systems, which is confirmed by the SEM observation on the tensile fracture surface of the composites. Non-isothermal crystallization analysis shows that the CF or PMMA-g-CF could serve as a nucleation agent to accelerate the crystallization rate of the composites, and the effect of PMMA-g-CF is stronger than that of CF. Moreover, based on the analysis of the crystallization activation energy calculated by the Flynn-Wall-Ozawa method, for a given conversion, the values of E a for pure PET, PET/CF, and PET/PMMA-g-CF composites decrease in the order of: pure PET > PET/CF > PET/PMMA-g-CF, suggesting that the crystallization rate of PET has been significantly accelerated after the addition of CF or PMMA-g-CF. In addition, it is found that the addition of CF or PMMA-g-CF increases the Charpy impact strength and tensile strength of the PET matrix. TG and DTG curves of pure PET, PET/CF, and PET/PMMA-g-CF composites are shifted to higher temperature. The result indicates the composites have better thermal stability. Author Contributions: The paper was designed and conceived by all of the authors. The first author (G.L.) who is a PhD student, conducted the entire experimental work under the supervision of Y.Z. (project leader). The co-authors, D.L., M.L., X.Z., contributed to the analysis and discussion of the experimental results. The manuscript was written by G.L.