In-situ polymerization of different molecular weight polyethylene glycol and maleic acid onto carbon fiber for reinforcing polyamide 66

ABSTRACT The interfacial adhesion character highly depends on the molecular interactions on the interface, including bond combination and molecular entanglement, and mobility for sharing load. In situ polymerization of different molecular weight polyethylene glycol (PEG) and maleic acid (MA) is used to form a hyperbranched polyester (HBPE) layer on an electrochemically oxidized carbon fiber (OCF) surface. It is found that more terminal hydroxyl groups are formed under the reaction at 280°C than that at other temperature. The branch extent and molecule chain length on the HBPE sizing reach balance by choice of 2000 molecular weight PEG monomer. HBPE as a film has uniform coverage on the OCF surface. The interlaminar shear strength of polyamide 66 composite reinforced with the sized OCF by in situ polymerization of 2000 molecular weight PEG and MA at 280°C is the optimal, which is enhanced to 90.5 MPa by 169% compared to unoxidized fiber and by 70% compared to OCF. The significant interfacial property improvement is attributed to the abundant terminal hydroxyl groups and appropriate chain length of the formed HBPE sizing, which enhanced optimally the interaction of fiber/sizing and sizing/matrix by balancing the contribution of molecular entanglement, deformation and covalent interconnection to interface. GRAPHICAL ABSTRACT


Introduction
Carbon fiber (CF) reinforced thermoplastic composite has efficient moulding character and unique welding manner, so that it has great advantages in scale, such as low cost andefficient repair and recovery [1][2][3]. However, for low reactivity, high viscosity and high process temperature of the resins, new methods need to be found for CF surface treatment and sizing according to the resin's specific properties.
Some research works have been done for developing sizing agents of CF for reinforcing thermoplastic matrices. One thought is to select high-temperature-resistance thermoplastic resins such as polyimide, polyether ether ketone and so on [4][5][6]. However, the molecular structure of this kind of sizing agents is usually low activity and rigid, making the molecular chain segment movement difficult to achieve good wettability with CF or good compatibility with thermoplastic matrix. Additionally, these rigid structure sizing agents usually need specific organic solvents to dissolve, which worsens the processing environment and makes it difficult to achieve large-scale industrial application. The other thought is to select flexible and polar thermoplastic resins such as polyurethane, polyamide and so on [7][8][9]. Anyway, the interfacial properties were still low, which indicates their combination with CF still needs to be improved.
Recently, hyperbranched polymers terminated with multiple functional groups, have received more and more attention, and been employed to modify CF to enhance the interface bonding with epoxy matrix [10,11]. The hyperbranched structure like octopuses affords more adhesive forces. Through changing molecular weight, the branchy structure and functional group activity could be adjusted to improve the interfacial properties. Tang et al. implemented modification of CF by different molecular weight branched polyethylenimine (PEI), and the results showed that the interfacial shear strength (IFSS) of epoxy composites with CFs modified by low molecular weight PEI was better than that modified by high molecular weight PEI [12]. Similarly, Han et al. study also showed high molecular weight of PEI as grafting agent of CF decreased the IFSS of the reinforced epoxy composite. The reason they ascribed the above phenomenon to was that the higher molecular weight of PEI, the more serious steric hindrance, which resulted in the less chemical linking between PEI and CF or PEI and epoxy matrix [13]. Since the adhesive properties could be adjusted by branch structure and functional groups, the hyperbranched polymers could be used as CF sizing agent for reinforcing thermoplastic resin because the thermostability and chemical reactivity can be adjusted through selecting monomers and processing condition. Besides, since the thermoplastics are much easier to deform, compared to chemical interconnection, molecular entanglement and deformation of the sizing agent must become more important to the interface strength of thermoplastic resin composites than that of thermosetting resin composites [14].
Compared to coating, in situ polymerization that directly grows polymers on CF surface provides a possibility to combine more tightly with CF surface. Li et al. used hexachlorocyclotriphosphazene and melamine to in situ polymerize on CF surface in DMF in a batch reactor, and the results showed that the interlaminar shear strength (ILSS) of reinforced thermoplastic copoly(phthalazinone ether sulfone)s matrix with the modified CF was increased 23.2% [15]. However, the procedure is time-consuming and not environmentally friendly, making it difficult for the method to match CF industrial production line. For adapting continuous processing of CF, a more convenient and efficient technique needs to be developed by choosing water-soluble monomers and suitable process conditions. Meanwhile, more importantly the grown layer should be thermostable and chemically reactive for satisfying the sized CF to enhance thermoplastic matrix.
Our previous work reported that sizing CF by in situ polymerization of maleic acid (MA) and glycerol could remarkably enhance the interface of the reinforced polyamide 66 [16]. Considering thermoplastic resin like polyamide 66 (PA66) is usually flexible, the interface should have adaptive deformation under stress. Therefore, in this work, different molecular weight polyethylene glycols (PEG) are chosen instead of glycerolto in situ polymerize to with MA on CF surface. Choice of low functionality PEG with different molecular weights to react with MA is intended to control moderate chemical interconnection and mobility at the interface, while bringing more molecular entanglement and deformation. The motivation of this step is to gain an insight into whether the contribution balance of molecular entanglement, deformation and covalent interconnection to the interphase is a key to achieving the maximum of the interfacial adhesion for thermoplastics.

Materials
polyacrylonitrile-based CF (12k, not commercially-available) was from the production line in Hengshen Co., Ltd., China, and it was not carried out any surface treatment and labelled BCF in this paper. PA66 resin (Zytel, 101 L-NC010) was purchased from Dupont Co., Ltd., USA, and its melting temperature is 262°C. NH 4 HCO 3 , PEG300 (600, 2000 and 6000), glycerol, MA and 2,2,2-trifluoroethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received.

Electrochemical oxidation of BCF
Electrochemical oxidation of BCF was carried out referencing to our previous work [16] as shown in Figure 1. The current density was set as 6 A m −2 , and the oxidated BCF was accordingly named as OCF.

Sizing CFs by in situ polymerization
OCF was sized by in situ polymerization of PEG and MA referencing to our previous work [16] as shown in Figure 2. The concentration of monomer water solution was 2.5 wt.% and the mole ratio of MA and different molecular weight PEG was 1:2. Before polymerization, the impregnated in monomer solution and dried OCF was labelled as MSOCF. Then the MSOCF was sized by in situ polymerization of PEG and MA at 235 °C, and the products were correspondingly named OCF300-235, OCF600-235, OCF2000-235 and OCF6000-235, according to the molecular weight of PEG. In addition, the polymerization was also carried out at 280°C, and the products were correspondingly named OCF300-280, OCF600-280, OCF2000-280 and OCF6000-280, according to the molecular weight of PEG. Besides, PEG2000 was chosen to in situ polymerize with MA at the temperature of 200, 260 and 300°C, and the samples were correspondingly named OCF2000-200, OCF2000-260 and OCF2000-300, respectively.
In comparison with PEG, high functionality glycerol was chosen to in situ react with MA onto OCF at 280°C referencing to our previous work [16], and other conditions were same to the above process, and the product was correspondingly named OCFglyc-280. Solo PEG2000 was also used to coat OCF without polymerizing for further clarifying the effect of polymerizate hyperbranched structure as sizing on the ILSS of composite with the sized OCFs.

Preparation of composites
Unidirectional composite laminates were prepared referencing to our previous work [16] as shown in Figure 3. According to ASTM D3529-76, the fiber weight percent in the composite laminate was measured as 60 ± 1%.

Reactions of the monomers
Differential scanning calorimeter (DSC) (Q20, TA Instruments Co., USA) was used to characterize the reaction heat between PEG2000 as representative and MA in nitrogen at the range of 30-290 °C. The heating rate was 5°C min −1 .

Sizing content of CFs
Weight method was used to evaluate the sizing contents on OCF. Before sizing, OCF tow with 10 m length was weighed and labeled as m 0 . Then the sized OCF by in situ polymerization of PEG and MA was weighed and labeled as m 1 . The sizing content (S) is calculated by the following equation:

Surface chemical structure and morphology of CFs
The surface chemical structure of CFs was characterized by X-ray photoelectron spectrometer (XPS) (Escalab 250Xi, Thermo Fisher Scientific Co., USA) with a 1486.6 eV Al Kα X-ray source. Approximately 1 × 10 −9 Torr of base pressure was used in the sample chamber and 20 eV pass energy was employed. In addition, CFs were ground into powder and blended with KBr, and then were characterized by Fourier transform infrared spectrometer (FTIR) (Nicolet 8700, Thermo-Electron Co., USA) with 4 cm −1 resolution. Scanning electron microscope (SEM) (JSM-7500F, JEOL Ltd., Japan) was employed for investigating the surface morphology of CFs.

Contact angle measurement
The matrix of PA66 particles were firstly dissolved in 2,2,2-trifluoroethanol solvent at 60°C, and then the solution was dropped on a single CF filament and the size of drop was controlled in 50-80 μm. After the solvent being removed, the single filament was heated at 270°C for 5 min. During this process, the resin globule fully melted and then the contact angle between the fiber and PA66 resin was observed and measured with an optical microscope (CX41, Olympus Co., Japan).

ILSS test
Before test, composite specimens were firstly fixed and then cut as well as cooled with running water with a precision cutting saw (SYJ-200, Shenyang kejing auto-instrument Co., Ltd., China). Then the samples were obtained as a size of width-to-thickness ratio of 5 and length-to-thickness ratio of 10. According to ISO 14,130:1997(E) standard, the composite ILSS measurement was performed with a WDW 3020 instrument, at a crosshead speed of 1 ± 0.005 mm min −1 and a span-to-thickness ratio of 5. Then ILSS values were obtained by testing at least five specimens per batch.

Cross-sectional morphology
Firstly, composites were fractured after ILSS and exhibited their cross sections, and then were sputter-coated with gold before measurement. Their cross-sectional morphologies were observed by SEM (S-4800/SU8010, HITACHI Ltd., and JSM-7500F, JEOL Ltd., Japan).

Thermal reactions between monomers
DSC was employed to study the reactions between PEG2000 as representative and MAin nitrogen, as shown in Figure 4. Firstly, an endothermic peak occurs at 135°C, which represents the forming of maleic anhydride by the the intramolecular dehydration reaction of MA as shown in Reaction 1.

Reaction 1
Then the profile tends to be slightly exothermic at 202°C, and it ends at 215°C. A following giant endothermic peak untimely terminated the above exothermic process. As reported in some literatures [17][18][19], hydroxy and anhydride easily reacted below 100 °C to generate monoester, but were difficult to form diester and multi-ester at such low temperature. Therefore, the tiny exothermic trend at 202 °C corresponds to theproducing of monoester betweenPEG2000 and maleic anhydride as shown in Reaction 2.

Reaction 2
Further analyses were performed on the prominent sharp endothermic peak at 215°C at the DSC curve. From some literatures, unsaturated double bond on maleic anhydride can react with active hydrogen atom especially above the temperature of 190°C, due to dramatical improvement of the unsaturated double bond activity at high temperature [20,21]. Thus, the giant endothermic peak at 215°C is attributed to the addition reaction between the double bond of monoester and the active hydroxyl of excess PEG2000.
Moreover, there is possibly further esterification between carboxyl of monoester and hydroxyl of excess PEG at 215°C. Therefore, HBPE polymerizate with terminal hydroxyl groups isgenerated as shown in Reaction 3.

In situ polymerization of different molecular weight PEG and MA on OCF surface
Effects of in situ polymerization of different molecular weight PEG and MA at 235 and 280°C on the sizing structure on OCF surface were studied by XPS. As shown in Figure 5 and Table 1, BCF has the main peaks at 284.6 and 285.1 eV, which are ascribed to sp 2 C and sp 3 C peaks , respectively. Besides, there occur peaks at 286.1-286.3, 286.9 and 288.3-289.7 eV, which are attributed to hydroxyl or ether, anhydride and carboxyl or ester groups , respectively [22][23][24]. BCF owns relative low contents of functionalized carbon atoms. Compared to BCF, the hydroxy content on OCF becomes lower, but the contents of carboxyl and anhydride tend to be higher. This is because of the conversion of hydroxyl to carboxyl or anhydride during electrochemical oxidation treatment. Besides, the totals of functionalized carbon atom on OCF exceed that on BCF. After being sized by in situ polymerization of different molecular weight PEG and MA at 235 and 280°C, the C1 peaks show dramatic change and their concentrations vary significantly with different PEG molecular weights. With the molecular weight of PEG increasing, the hydroxyl content on the sized OCFs by in situ polymerization of PEG and MA at 235°C firstly increases gradually and then decrease. The hydroxyl content on OCF2000-235 reaches to the maximum by 360% compared to that on OCF. Whereas the contents of carboxyl and anhydride initially keep constant, and then increase significantly when molecular weight of PEG exceeds 2000. Besides, with the molecular weight of PEG increasing, the activated carbon content on OCF6000-235 is increased most significantly by 51% compared to that on unsized OCF. This indicates HBPE sizing with multi-terminal hydroxy has been generated on OCF surface at 235°C. The variation trend of groups on OCF surface is abscised to the difference of the chemical structure and molecular weight of PEG monomer. When the molecular weight of PEG is less than 2000, the activity of PEG is basically unaffected during polymerization. This is verified by the independence of the contents of carboxyl and anhydride on OCF300-235, OCF600-235 and OCF2000-235 on the molecular weight increasing of PEG. This means the reaction extent of PEG and MA is basically not influenced by the lower molecular weight of PEG, which makes the formed HBPE sizing on OCF2000-235 still possess abundant branch points and terminal hydroxy groups. Besides, with molecular weight of PEG increasing, the ether content and chain length of PEG increases, which makes the ether/hydroxy contents and chain length on the resulted HBPE sizing achieve the maximum at 2000 molecular weight PEG. However, when the molecular weight of PEG exceeds 2000, the large volume and complexity of PEG-6000 molecules hindered the activity of terminal hydroxy groups on PEG during the in situ polymerization [25]. This inhibited the reaction of hydroxy with anhydride or carboxyl among monomer or oligomers, consequently leading to decrease of the branch points and terminal hydroxyl groups of the formed HBPE sizing on OCF6000-235, but increase of its molecule chain length. Schematic descriptions of sizing structure on OCF surface from in situ polymerization of different molecular PEG with MA are presented in Figure 6.
When the temperature is increased to 280°C, the contents of hydroxyl on the sized OCFs by in situ polymerization of different molecular weight PEG and MA are all higher than that at 235°C. Meanwhile, the ratio of C=C to C-C and the content of anhydride both become lower than that that at 235°C. This indicates that high-temperature polymerization of PEG and MA prompted the reaction between hydroxy and anhydride or double bond and improved the reaction extent of addition and esterification among the monomers or oligomers. Therefore, HBPE sizing with higher branch extents and terminal hydroxyl groups is produced on OCF surface, along with lower remaining anhydride groups. Besides, the change trend of functional groups on the sized OCFs at 280°C with different molecular weight of PEG increasing is same as that at 235°C. This further verifies over-high molecular weight of PEG is less active, consequently resulting in the increase of anhydride and the decrease of the branch points and terminal hydroxyl groups on the formed HBPE sizing. Besides, the anhydride groups on the sized OCFs by the in situ polymerization of different molecular weight PEG and MA despite 235 or 280°C are higher than that in situ polymerization of high functionality glycerol and MA. This is due to the lower functionality and longer molecule chain of PEG, leading to relatively lower reactivity of terminal hydroxy of PEG during polymerization. As a result, the branch points and terminal hydroxy content of the HBPE sizing by in situ polymerization of PEG and MA are lower than that in situ polymerization of glycerol and MA. From the above analysis, the branch extent and molecule chain length on the HBPE sizing can achieve balance by choice of 2000 molecular weight PEG and MA to in situ polymerize.
The surface characteristics of different CFs are shown in Figure 7. Some furrows appear on BCF surface (Figure 7a). After electrochemical oxidation, amounts of finer and  Figure 7f) both become smoother, which is possibly related to thicker sizing film because of higher viscosity of higher molecular weight. This indicates that in situ polymerization of different molecular weight PEG and MA on OCF surface can achieve to size evenly.
In order to evaluate the effects of in situ polymerization of different molecular weight PEG and MA on OCF surface wettability, the contact angles between various CFs and PA66 resins are analyzed as shown in Table 2. Compared to BCF, the contact angle  between OCF and the PA66 decreases, which means that the wettability between fiber and PA66 resin is improved after the oxidation of BCF. When the OCF was sized by in situ polymerization of different molecular PEG and MA, the wettability between sized OCF and PA66 resin is better, especially OCF2000-280 owning a minimum contact angle with PA66 resin. This indicates that PEG2000 and MA are appropriate monomers for in situ sizing to enhance the wettability between OCF and PA66 resin.

Effects of temperature on in situ polymerization of PEG and MA on OCF surface
In order to fully investigate the effects of temperature on the in situ polymerization, PEG2000 and MA were chosen to in situ polymerize on OCF surface at different temperature in the range of 200-300°C, and these sized OCFs were analysed with XPS. As shown in Figure 8 and Table 3, MSOCF without polymerization has higher ratio of C=C to C-C and hydroxy, but lower anhydride groups. After in situ polymerization of the monomers at 200°C and getting rid of unreactive monomers, OCF2000-200 owns the lower ratio of C=C to C-C and fewer hydroxy groups, but more anhydride contents than that of MSOCF. This indicates there occurs the addition reaction between the double bond on maleic anhydride with the active hydroxyl on PEG2000, which leads to the decrease of C=C and hydroxy groups. The increase of anhydride group is ascribed to the lower esterification rate between maleic anhydride and PEG2000 than their addition reaction rate. As a result, the more anhydride groups remained on the formed HBPE sizing of OCF2000-200. With the increasing of in situ polymerization temperature of PEG2000 and MA, the ratio of C=C to C-C gradually decreases. It is worth noting that the hydroxy/ether content firstly gradually increases at the temperature lower than 280°C and then decreases when the temperature rises to 300°C. This is consistent with the FTIR as shown in Figure 9. Whereas the anhydride group content exhibits an opposite trend with hydroxy content and becomes higher at the temperature more than 280°C. This indicates 280°C is the optimal for the synergistic effect of addition and esterification between PEG and MA. Too high temperature is averse to the esterification, which results in the decrease of branch points and terminal hydroxyl groups on the HBPE sizing of OCF2000-300, along with more remaining anhydride. Figure 10 shows the ILSS of composites reinforced with the sized OCFs by in situ polymerization of different molecular PEG with MA at 235 and 280°C. Compared to the unsized OCF, the ILSS of the composites with the sized OCFs by in situ polymerization of PEG300 and MA at 235 decreases. Then with the molecular weight of monomer PEG continual increasing, the ILSS of the composites with the sized OCFs by in situ polymerization of PEG and MA at 235°C firstly sharply increases, then keeps a plateau region, and finally decreases fast. The optimal ILSS of these composites is obtained at 2000 molecular weight PEG, and reaches 57.8 MPa by 72% compared to BCF and by 9% compared to OCF. This slight increase means the interaction at the interface of OCF/sizing/resin was still needed to be improved. With in situ polymerization temperature of PEG and MA increasing to 280°C, the ILSS of composites reinforced with the sized OCFs in situ polymerization of different molecular  PEG with MA is all higher than that at 235°C. This is consistent with the monotonous increase of ILSS profile of composites with in situ polymerization temperature gradually increasing from 200 to 280°C as shown in Figure 11. However, when the temperature rises more than 280°C, the ILSS value of corresponding composite decreases. Back to Figure 10, interestingly, the ILSS profile initially has a continual increasing without plateau region when the molecular weight of monomer PEG is less than 2000, and then decreases dramatically with PEG molecular weight exceeding 2000. The maximum of ILSS of these composites is same at 2000 molecular weight PEG, reaching 90.5 MPa by 169% compared to BCF and by 70% compared to OCF, which even exceeds the ILSS of composite with the sized OCF by in situ polymerization of glycerol with MA at 280°C. This indicates that high-temperature polymerization of PEG and MA makes more significant contribution to improving of the interfacial adhesion between CF and PA66 matrix. From above, PEG2000 is moderate molecular weight monomer to in situ polymerize with MA for sizing OCF and enhancing its interfacial adhesion with PA66 matrix. To confirm the dramatical contribution of HBPE polymerizate to the ILSS rather than a solo PEG monomer, a solo PEG monomer was used to size OCF and the ILSS of the sized CF reinforced PA66 matrix is measured as only 24.6 MPa, much lower than that with the sized OCFs by in situ polymerization of PEG and MA. This verifies that in situ  polymerization of PEG and MA is an effective method to improve the interfacial property of CF/PA66 composites. Contrast to other different molecular weight polymer sized CFs as shown in Table 4, HBPE sized OCF by in situ polymerization of 2000 molecular weight PEG and MA in our study dramatically improved the interfacial properties. The sizing content of HBPE sized OCFs by in situ polymerization of different molecular weight PEG of 300, 600, 2000 and 6000 with MA is 1.6, 1.8, 2.0 and 2.0%. Besides, the sizing agent of OCFglyc-280 is 2.0%. Combined to the data in our previous work, the influence of sizing agent on the ILSS of the composites is negligible. Figure 12 shows the cross sections of the composites with HBPE-sized OCFs by in situ polymerization of different molecular weight PEG and MA. As shown in Figure 12a, much fiber pulling out from the matrix occurs on the cross section with unsized OCF. When the OCF was sized by in situ polymerization of 300 molecular weight PEG and MA at 235°C (Figure 12b), the fiber pull-out on the cross section still exists a lot, which is consist with its low interfacial properties. With PEG molecular weight increasing, the fiber pull-out on the cross section of OCF600-235/PA66 composite (Figure 12c) obviously decreases. Interestingly, when the molecular weight increases to 2000 (Figure 12d), composite with OCF2000-235 presents flat fracture cross section and less fiber pull-out, which suggests the strong interaction between the sized fiber and PA66 resin. However, with the molecular weight increasing more than 2000, amounts of fibers were pulled out again on the cross section of OCF6000-235/PA66 composite (Figure 12e). With in situ polymerization temperature of different molecular PEG with MA increasing to 280°C (Figure 12 f-12), the cross section with the sized OCFs exhibits sharply reduced fiber pull-out compared to that at 235°C. This is consistent with the increased ILSS of the composites with sized OCFs by high-temperature polymerization of PEG and MA as shown in Figure 10. Thus, the more beneficial influence of hightemperature polymerization of monomers on the interfacial adhesion of the composites is confirmed. Especially, after being sized by in situ polymerization of PEG2000 and MA at 280°C, composite with OCF2000-280 ( Figure 12h) hardly presents fiber pull-out, which means the good stress transfer from the PA66 resin to the OCF2000-280. However, when the molecular weight of PEG was over-high (Figure 12i), there appear some fiber pull-out and voids again on the cross section of OCF6000-280/PA66 composite. This verifies the interface adhesion between the OCF6000-280 with PA66 resin decreases.

Interfacial reinforcing mechanism
Effects of in situ polymerization of different molecular weight PEG with MA onto OCF at different temperature on the interfacial properties of the composites with sized OCFs mainly depend on the formed HBPE chemical structure and chain length, which affects the interaction between CF and the sizing and the sizing with PA66 matrix and the interface deformation with load increasing. When the molecular weight of PEG is less than 2000, there is abundant hydroxyl groups and branch points formed on the HBPE sizing structure, which brings about more opportunities for the sizing to form covalent bonding with the carboxyl groups from OCF [30,31]. In addition, with molecular weight of PEG increasing from 300 to 2000, the molecule chain length of formed HBPE sizing structure also increases. The longer molecule chain length of sizing as on OCF2000-235 not only benefits the molecular entanglement between the sizing with PA66 matrix compared to that as on OCF300-235 as shown in the Figure 13a,, but also affords larger  deformation to achieve more effective stress transfer between resin and fiber. This can explain the significant increase of ILSS of the composite with sized CF by in situ polymerization of PEG2000 and MA. The enhancement effect is more significant at the higher temperature polymerization of 280°C. As aforementioned, there are more hydroxy and anhydride groups on sized OCFs at 280°C than that at 235°C, which leads to the stronger chemical linking between the sizing with OCF or PA66 resin. This is reasonable explanation why the ILSS of the composites with sized OCFs at 280°C is higher than that at 235°C. However, when the molecular weight of PEG exceeds 2000, the large volume and complexity of PEG6000 molecules hindered the activity of terminal hydroxy groups on PEG during the in situ polymerization. This led to less hydroxyl groups and branch points formed on the HBPE sizing structure as shown in our XPS, and less chemical linking between the sizing and the carboxyl groups from OCF, consequently decreasing the interfacial adhesion of OCF6000-235/280 reinforced PA66 composites as shown in the Figure 13c. Meanwhile, it is worth noting that the overhigh hydroxy content and branch points on the sizing is also not beneficial to further improving the interfacial adhesion. This is verified by the lower ILSS of composite with sized OCF by in situ polymerization of high functionality glycerol with MA that that by in situ polymerization of PEG2000 and MA. From above analysis, choice of 2000 molecular weight PEG to in situ polymerize with MA onto OCF can achieve a stronger synergistic effect of chemical interconnection and molecular entanglement and deformation at the interface of CF/sizing/resin. This further clarifies that the contribution balance of molecular entanglement, deformation and covalent interconnection to the interphase is a key to achieving the maximum of the interfacial adhesion.

Conclusions
In the present work, CF surface is firstly electrochemically oxidized and then sized by in situ polymerization of different molecular weight PEG and MA. X-ray photoelectron spectrometer shows the formed HBPE layer owns more terminal hydroxyl groups at the polymerization of 280°C than that at other temperature. Moreover, the branch extent and molecule chain length on the HBPE sizing reach a balance by choice of 2000 molecular weight PEG and MA to in situ polymerize. SEM shows the good overage homogeneity of HBPE sizing on CF surface. The ILSS of polyamide 66 composite reinforced with the sized CF by in situ 2000 molecular weight PEG and MA at 280°C is the optimal, which is enhanced to 90.5 MPa by 169% compared to unoxidized CF and by 70% compared to oxidized CF, which even exceeds that of the composite reinforced by the sized CF by in situ polymerization of high-functionality glycerol with MA. The significant interfacial property improvement is attributed to the abundant terminal hydroxyl groups and appropriate chain length of the formed HBPE sizing, which enhances optimally the interaction of CF/sizing and sizing/matrix by balancing the contribution of molecular entanglement, deformation and covalent interconnection to interface. This work indicates that in situ of polymerization of PEG and MA on CF surface can maximally enhance the mechanical strength of CF/PA66 resins by proper molecular weight choice of PEG.