Ultradrawing and Ultimate Tenacity Properties of Ultrahigh Molecular Weight Polyethylene Composite Fibers Filled with Nanosilica Particles with Varying Specific Surface Areas

1Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education, Key Laboratory for the Green Preparation and Application of Functional Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China 2Graduate School of Material Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan 3Department of Materials Engineering, Kun Shan University, Tainan 71070, Taiwan


Introduction
As a kind of extremely significant and strategic material, ultrahigh molecular weight polyethylene (UHMWPE) fibers have attracted much attention for the last three decades, since they exhibit significantly higher tenacity but lower density values than those of other high performance fibers, such as carbon and aramid fibers [1][2][3][4][5].Polyethylene fibers [1, are typical high performance fibers produced using the gel spinning processing method from flexible polymer chains.Remarkable progress has been made in the improvement of these high performance fibers since then; however, the highest tensile strengths and moduli achieved for UHMWPE fibers are still well below the broad range of theoretical tensile strengths and moduli reported for the UHMWPE perfect crystals [1].The highest tenacity of commercially available UHMWPE fibers reaches as high as 45 g/den [32]; however, this obtained strength is still far below the theoretical achievable strength, 372 g/den reported for the perfect polyethylene crystal [16].The key element in obtaining high-strength UHMWPE fibers is to find a way to draw the as-prepared gel specimens to an ultrahigh draw ratio after the gel spinning process.The drawability of the as-prepared gel specimens was found to depend significantly on the compositions of solutions from which gels were made [6,7,33].Several authors [12][13][14][15]33] reported that the drawing temperature and rate 2 Journal of Nanomaterials could markedly affect the maximal achievable draw ratio and tensile properties of solution-grown UHMWPE samples.In addition to the gel solution compositions and drawing conditions, it is generally recognized that the conditions used in the formation process after spinning and/or solution casting of gel solutions can also have a significant influence on the morphology, microstructure, and drawing properties of the specimens formed during the above-mentioned processes [7,9,14,[17][18][19][20][21][22][23].
Our recent investigations [24][25][26][27] found that the achievable draw ratios (achievable ) of UHMWPE/nanofillers as-prepared fibers prepared near the optimal UHMWPE concentration improve to a maximal value as their nanofillers contents reach an optimal value, respectively, in which, the nanofillers (e.g., carbon nanotube (CNT) [24], attapulgite [25], nanosilica and/or their functionalized nanofillers [26], and functionalized bacterial cellulose [27]) with extremely high specific surface areas can serve as efficient nucleation sites and facilitate the crystallization of UHMWPE molecules into crystals but with lower melting temperatures (  ) and/or evaluated smaller crystal thickness (  ) values during their crystallization processes.Presumably, the crystals with lower   and/or evaluated smaller   values obtained at proper plain and/or modified nanofiller contents can be melted and pulled out of folded lamellar crystals relatively easily during ultradrawing processes and hence this results in higher drawability and orientation of the UHMWPE/nanofillers or UHMWPE/modified nanofillers fibers.The maximal achievable draw ratios of UHMWPE/ nanofillers or UHMWPE/modified nanofillers as-prepared fiber specimens and the tensile strengths of the drawn UHMWPE/nanofillers or UHMWPE/modified nanofillers fiber specimens are significantly higher than those of the plain UHMWPE as-prepared and drawn fiber specimens prepared at the same draw ratios of UHMWPE concentrations but without addition of the nanofillers and/or modified nanofillers, respectively.The ultimate tensile strength values of UHMWPE/purified attapulgite, UHMWPE/functionalized CNT, UHMWPE/functionalized nanosilica, and UHMWPE/functionalized bacterial cellulose drawn fibers prepared using one-stage drawing process at 95 ∘ C can reach 4.7, 5.8, 7.0, and 7.1 GN m −2 , respectively, which is about 1.74, 2.15, 2.59, and 2.63 times of that of the corresponding plain UHMWPE drawn fibers prepared at the same optimal UHMWPE concentration, formation, and drawing condition but without incorporation of modified nanofillers.
The above results clearly suggested that nanofillers with high specific surface areas can serve as efficient nucleation sites for crystallization of UHMWPE molecules and improve the ultradrawing and ultimate tensile properties of UHMWPE/nanofiller fibers.Among these nanofillers, nanosilica particles are cheap and commercially available for a wide range of specific surface areas.In this study, the ultradrawing and ultimate tensile properties of the UHMWPE/nanosilica and UHMWPE/functionalized nanosilica fibers with a wide range of specific surface areas were systematically investigated.The maximal achievable  and ultimate tensile strength values obtained for the best prepared UHMWPE/functionalized nanosilica as-prepared fibers are even higher than those of the best prepared UHMWPE/modified attapulgite, UHMWPE/functionalized CNT, and UHMPE/functionalized bacterial cellulose as-prepared fibers prepared at the optimal modified attapulgite, functionalized CNT, and functionalized bacterial cellulose contents, respectively [24][25][26][27].Specific surface area, morphological and Fourier transform infrared analyses of the original and functionalized nanosilica specimens, and/or investigations of thermal, orientation factor, and ultimate tensile properties of the as-prepared and drawn UHMWPE/ functionalized nanosilica fiber specimens were performed to understand the above improved ultradrawing and ultimate tensile properties of the UHMWPE/functionalized nanosilica as-prepared and/or drawn fibers.

Materials and Sample Preparation.
The UHMWPE GUR-4120 resin used in this study is associated with a weight average molecular weight (Mw) of 5.0 × 10 6 , which was kindly supplied by Celanese (Nanjing) Diversified Chemical Corporation, Nanjing, China.Three types of nanosilica particles (Merck SSA-100, SSA-300, and SSA-600) used in this study were purchased from Lu Ming Nanomaterials Corporation, Dalian, China.The specific surface areas of SSA-100, SSA-300, and SSA-600 nanosilica (NSI) particles were quoted as 90-105 m 2 /g, 285-305 m 2 /g, and 580-610 m 2 /g, respectively, by Lu Ming Nanomaterials Corporation.Functionalized nanosilica (FNSI) particles were prepared by grafting maleic anhydride grafted polyethylene (PE g-MAH ) molecules onto NSI particles in ultrasonicated mixtures of decalin, NSI, and PE g-MAH at 170 ∘ C for 1 hour, in which, PE g-MAH resin was purchased from Langfang Plastic Corporation, Langfang, China.The nanosilica and functionalized nanosilica particles prepared above are referred to as NSI  and FNSI  m , respectively, in the following discussion, in which, the superscript  denotes the quoted specific surface areas of virgin NSI nanosilica particles and the subscript  denotes the weight ratio of PE g-MAH to NSI  used in the preparation processes of FNSI  m functionalized nanosilica particles.Table 1 summarized designations and compositions of typical nanosilica and functionalized nanosilica particles prepared in this study.
Varying contents of NSI  and FNSI  m particles together with UHMWPE resin were dispersed and dissolved in decalin at 135 ∘ C for 1.5 hours, in which 0.1% di-t-butyl-p-cresol was added as an antioxidant.The UHMWPE, UHMWPE/NSI  , and UHMWPE/FNSI  m gel solutions prepared above were then fed into a temperature-controlled hopper and kept as hot homogenized gel solutions before spinning.The hot homogenized gel solutions were then gel-spun using a conical die with an exit diameter of 1 mm at an extrusion rate of 1000 mm/min and an extrusion temperature of 170 ∘ C. A water bath and a winder with 70 mm in diameter were placed at a distance of 520 mm and 810 mm from the spinneret exit, respectively.The extruded gel fibers were cooled in a temperature-conditioned atmosphere and then quenched into a water bath for about 1 minute, where the temperature of the air atmosphere and water bath was controlled at 5 ∘ C. The quenched fibers were then extracted in n-hexane bath for 5 minutes to remove the residual decalin solvent.The extracted fiber specimens were then dried in air for 30 minutes to remove the remaining n-hexane solvent before any drawing run.The UHMWPE, UHMWPE/NSI  , and UHMWPE/FNSI  m as-prepared fiber specimens prepared above are referred to as F 100 , F 100 NSI   , and F 100 FNSI  m- as-prepared fiber specimens, respectively, in the following discussion, in which, the superscript  denotes the quoted specific surface areas of varying NSI  particles used to prepare NSI  and FNSI  m particles in F 100 NSI   and F 100 FNSI  m- as-prepared fiber specimens, respectively; the subscript 100 denotes one hundred parts of UHMWPE resins used in the as-prepared fibers;  denotes the weight ratio of PE g-MAH to NSI  used in the preparation processes of FNSI  m fillers, while the subscript  denotes parts of NSI  or FNSI  m fillers used in per hundred parts of UHMWPE resins in the as-prepared fibers.Table 2 summarized designations of typical UHMWPE, UHMWPE/nanosilica, and UHMWPE/functionalized nanosilica as-prepared fiber specimens and the corresponding compositions of gel solutions used in the gel spinning processes.

Fourier Transform Infrared Spectroscopy.
Fourier transform infrared (FTIR) spectroscopic measurements of NSI  or FNSI  m specimens with varying specific surface areas were recorded on a Nicolet Avatar 360 FTIR spectrophotometer at 25 ∘ C, wherein 32 scans with a spectral resolution 1 cm −1 were collected during each spectroscopic measurement.Infrared spectra of NSI  or FNSI  m film specimens were determined using the conventional KBr disk method.Alcohol and decalin solutions containing NSI  or FNSI  m particles, respectively, were cast onto KBr disk and dried at 60 ∘ C for 30 minutes.The cast films used in this study were prepared sufficiently thin enough to obey the Beer-Lambert law.

Morphological Analyses.
In order to understand the morphology on the surfaces of NSI  or FNSI  m particles with varying specific surface areas prepared in Materials and Sample Preparation, NSI  particles were dispersed in alcohol, while FNSI  m particles were dispersed in decalin to have a better dispersed morphology before examination.Before morphological analyses, ten micrograms of NSI  or FNSI  m particles was added and ultrasonicated in 10 mL alcohol and decalin at 25 ∘ C for 5 minutes, respectively.The dispersed particles were then dried onto a carbon-coated copper grid under ambient conditions prior to morphological analyses.The cast NSI  or FNSI  m particles were then examined using a Philip transmission electron microscope (TEM) model Tecnai G20 operated at 200 kV.

Specific Surface Area Analyses. A Laser Particle Size
Analyzer model BT-9300H (Dandong Bettersize Instruments Corporation, Dandong, China) was used to study the specific surface areas of NSI  or FNSI  m particles with varying specific surface areas.Before analyses, ten micrograms of NSI  or FNSI  m particles was added and ultrasonicated in 10 mL alcohol and decalin at 25 ∘ C for 5 minutes, respectively.The specific surface areas of NSI  or FNSI  m particles were then measured by placing the ultrasonicated solutions prepared above in the curette of the Laser Particle Size Analyzer at 25 ∘ C.

Thermal and Orientation Factor
Analyses.Thermal properties of all as-prepared fiber specimens were performed on a Du Pont differential scanning calorimeter (DSC) model 2000.All scans were carried out at a heating rate of 20 ∘ C/min under flowing nitrogen at a flow rate of 25 mL/min.Samples weighing 0.5 mg and 15 mg were placed in the standard aluminum sample pans for determination of their melting temperature (  ) and percentage crystallinity (  ) values, respectively.The percentage crystallinity values of the as-prepared fiber specimens were estimated using baselines drawn from 40 to 200 ∘ C and a perfect heat of fusion of polyethylene of 293 J/g [28].
In order to understand the ultradrawing properties of UHMWPE, UHMWPE/NSI  , and UHMWPE/FNSI  m asprepared fiber specimens, the lamellar thickness (  ) values of the above as-prepared fibers were evaluated from their   values using Hoffman and Weeks' equation [28,29] given in (1) as follows, in which, an equilibrium melting temperature (   ) of 145.5 ∘ C, a perfect heat of fusion (Δ  0 ) of 293 J/g, and a folded surface free energy (  ) of 9 × 10 −6 J/cm 2 of polyethylene crystals [28] were used for evaluation of   values of UHMWPE, UHMWPE/NSI, and UHMWPE/FNSI asprepared fiber specimens: The orientation factor ( 0 ) values of UHMWPE, UHMWPE/NSI  , and UHMWPE/FNSI  m as-prepared and drawn fiber specimens were measured using a sonic velocity orientation instrument model SCY-III, which was purchased from Donghuakaili Chemicals and Fiber Technology Corporation, Shanghai, China.Before testing, the fiber specimen with 60 cm in length was wound and clamped on a testing device with a span of 40 cm. 0 values of the as-spun and drawn fiber specimens were then measured at 25 ∘ C. A minimum of five samples of each specimen were tested and averaged during  0 measurements. 0 values were evaluated using (2) as suggested by Xiao and coauthors [30]: where  is the sonic velocity of the as-prepared or drawn UHMWPE fiber specimen and   is the sonic velocity of the fully unoriented sample, taken as 1.65 km/s [30].

Drawing and Tensile Properties of Fiber Specimens.
The UHMWPE, UHMWPE/NSI  , and UHMWPE/FNSI  m fiber specimens used in the drawing experiments were cut from the dried as-prepared fibers and then stretched on a Gotech tension testing machine model GT-TFS-2000 equipped with a temperature-controlled oven.The fibers are 150 mm in length, which were wound and clamped in a stretching device and then stretched at a crosshead speed of 20 mm/min and a constant temperature of 95 ∘ C. The draw ratio of each fiber specimen was determined as the ratio of the marked displacement after and before drawing.The marked displacement before drawing was 27 mm.The tensile properties of the as-prepared and drawn fibers were determined using a Hung Ta tension testing machine model HT-9112 at a crosshead speed of 20 mm/min.A minimum of five samples of each specimen were tested and averaged during the tensile experiments.

Results and Discussion
3.1.Fourier Transform Infrared Spectroscopy.Figure 1 illustrates typical Fourier transform infrared (FTIR) spectra of nanosilica (NSI  ), functionalized nanosilica (FNSI  m ), and maleic anhydride grafted polyethylene (PE g-MAH ) specimens.PE g-MAH specimen exhibited two distinctive absorption bands centered at 1711 and 1791 cm −1 , which were generally attributed to the motion of O-C=O and C=O stretching vibrations of maleic anhydride [31] (see Figure 1(a)).As shown in Figures 1(b), 1(f), and 1(j), there are three distinguished absorption bands centered at 1097, 1635, and 3442 cm −1 corresponding to the motions of Si-O-Si stretching, H-O-H bending, and Si-OH stretching vibrations [31], respectively, which were found in the spectra of NSI 100 , NSI 300 , and NSI 600 specimens.It is interesting to note that the peak magnitudes of Si-O-Si stretching, H-O-H bending, and Si-OH stretching bands of NSI  specimens increased significantly as their quoted specific surface areas increased from 100 to 300 and 600 m 2 /g (see Figures 1(b), 1(f), and 1(j)).The significant increase in the magnitude of Si-O-Si stretching, H-O-H bending, and Si-OH stretching bands of NSI  specimens is attributed to the increased amounts of Si-O-Si, H-O-H, and Si-OH groups exposed on NSI  particles with higher specific surface areas.
After grafting PE g-MAH to NSI 100 , NSI 300 , and NSI 600 particles, the peak magnitudes corresponding to H-O-H bending and Si-OH stretching bands of FNSI  m specimens reduced significantly as the weight ratios of PE g-MAH to NSI  increased (see Figures 1(b) to 1(e), 1(f) to 1(i), and 1(j) to 1(m)).In fact, as shown in Figures 1(d) to 1(e), 1(h) to 1(i), and 1(l) to 1(m), H-O-H bending and Si-OH stretching bands originally present in NSI  specimens disappeared almost completely as the weight ratios of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 of FNSI 100 m , FNSI 300 m , and FNSI 600 m specimens were equal to or more than 3, 6, and 9, respectively.In the meantime, a new absorption band centered at around 1228 cm −1 corresponding to the motion of ester C-O stretching vibration [31] was found in the spectra of FNSI 100 m , FNSI 300 m , and FNSI 600 m specimens (see Figures 1(c) to 1(e), 1(g) to 1(i), and 1(k) to 1(m)).In contrast, the absorption bands centered at 1711 and 1791 cm −1 corresponding to the motion of C=O and O-C=O stretching vibrations of maleic anhydride gradually reappeared as the weight ratios of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 of FNSI  2(a), 2(d), and 2(g)).After modification by PE g-MAH , some translucent resins were found attaching on the surfaces of NSI 100 , NSI 300 , and NSI 600 particles, wherein the amounts of attached translucent resins increased gradually as the weight ratios of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 increased, respectively (see Figures 2(b) to 2(c), 2(e) to 2(g), and 2(h) to 2(i)).As evidenced by FTIR analyses in the previous section, the attached translucent resins were most likely the grafted PE g-MAH molecules, which were firmly bonded to NSI 100 , NSI 300 , and NSI 600 particles by the reaction of the maleic anhydride groups of PE g-MAH resins with the hydroxyl groups of NSI 100 , NSI 300 , and NSI 600 particles, respectively.In fact, the translucent resins were found fully surrounding and overwrapping on NSI 100 , NSI 300 , and NSI 600 particles, as the weight ratios of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 were greater than 3, 6, and 12, respectively (see Figures 2(c), 2(f), and 2(i)).

Specific Surface Area Analyses of 𝑁𝑆𝐼 𝑥 and FNSI 𝑥
m Particles.The values of specific surface areas of NSI  and FNSI  m particles are summarized in Figure 3 and Table 1.The specific surface areas of NSI 100 , NSI 300 , and NSI 600 particles were evaluated at around 100, 300, and 600 m 2 /g (i.e., 102.3, 303.9, and 601.7 m 2 /g), respectively.After modification by PE g-MAH , the specific surface areas of FNSI 100 m , FNSI 300 m , and FNSI 600 m particles reached a maximal value at 129.8, 335.8, and 630.7 m 2 /g, respectively, as the weight ratios of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 approached an optimal value at 3, 6, and 9, respectively.Presumably, the beneficial effect of PE g-MAH contents on specific surface areas of FNSI  m particles is attributed to the increase in grafted amounts and specific surface areas of PE g-MAH on NSI  particles during their functionalized processes.However, PE g-MAH molecules may agglomerate, bundle, entangle together, and overwrap NSI  particles, as PE g-MAH molecules are superfluous and can no longer graft onto NSI  particles.As evidenced by morphology analyses in the previous section, some translucent resins were found fully surrounding and overwrapping on NSI  particles (see Figures 2(c), 2(g), and 2(i)), as the weight ratios of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 were more than 3, 6, and 9, respectively.Based on this premise, it is reasonable to infer that the overwrapped FNSI  m particles exhibit relatively lower specific surface areas than those FNSI 100 m3 , FNSI 300 m6 , and FNSI 600 m9 particles grafted with proper amounts of PE g-MAH resins.and UHMWPE/FNSI (F 100 FNSI  m- ) as-prepared fiber series specimens are summarized in Figure 4  m- ) as-prepared fibers reduced to a minimal value, as their NSI  and/or FNSI  m contents reached an optimal value, respectively, in which   and   values of F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375as-prepared fibers prepared at the optimal FNSI  m contents at 0.075, 0.05, and 0.0375 phr, respectively, were significantly lower than those of the corresponding F 100 NSI 100 0.1 , F 100 NSI 300 0.0625 , and F 100 NSI 600 0.05 as-prepared fibers with an optimal NSI  content at 0.1, 0.0625, and 0.05 phr, respectively.However,   values of F 100 NSI   and/or F 100 FNSI  m- as-prepared fibers increased to a maximal value, as NSI  and/or FNSI  m contents reached their corresponding optimal values, respectively, wherein   values of F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375as-prepared fibers prepared at their optimal FNSI  m contents, respectively, were significantly higher than those of the corresponding F 100 NSI 100 0.1 , F 100 NSI 300 0.0625 , and F 100 NSI 600 0.05 as-prepared fibers prepared at their optimal NSI  contents, respectively.Moreover, it is worth noting that F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375as-prepared fibers prepared at the optimal FNSI  m contents exhibited another minimal   (or evaluated   ) but other maximal   values as their FNSI 100 m , FNSI 300 m , and FNSI 600 m were modified using an optimal weight ratio of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 at 3, 6, and 9, respectively (see Figures 5(j As evidenced by specific surface area and TEM analyses in the previous sections, NSI  and/or FNSI  m particles are with a wide range of relatively large surface areas per volume, which make them in close proximity to a large fraction of the UHMWPE matrix.Apparently, even very small contents of dispersed NSI  and/or FNSI  m particles can serve as efficient nucleation sites for UHMWPE molecules during their gel spinning processes.These efficient nucleation sites of NSI  and/or FNSI  m particles then facilitate the crystallization of UHMWPE molecules into crystals with thinner lamellar thickness and/or lower   values during their crystallization processes.After grafting PE g-MAH to NSI 100 , NSI 300 , and NSI 600 particles, the properly modified FNSI  m particles with even higher specific surface areas are likely to disperse better in UHMWPE and serve as more effective sites for nucleation of UHMWPE molecules during their gel spinning processes than NSI  particles.As a consequence, F 100 FNSI 100 m- , F 100 FNSI 300 m- , and F 100 FNSI 600 m- as-prepared fiber specimens exhibit significantly higher   but lower   (or evaluated   ) values than the corresponding F 100 NSI 100  , F 100 NSI 300  , and F 100 NSI 600  as-prepared fiber specimens prepared with the same NSI  contents but without modification by PE g-MAH , respectively.Moreover, the minimal   (or evaluated   ) values obtained for F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared fibers prepared at the optimal FNSI  m contents and weight ratio of PE g-MAH to NSI  , respectively, reduced significantly as the specific surface areas of FNSI  m particles increased, while the highest   values obtained for F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared fibers increased consistently as the specific surface areas of their FNSI  m particles increased.

Achievable Draw Ratios of the As-Prepared Fibers.
Figure 5 summarized the achievable draw ratio (achievable ) values of F 100 , F 100 NSI   , and F 100 FNSI  m- as-prepared fiber specimens prepared at varying NSI  and/or FNSI  m contents, respectively.For comparison purposes, achievable  values of the best prepared UHMWPE/functionalized carbon nanotube (FCNT) as-prepared fibers (i.e., F 100 C f2-0.1 specimens) obtained in our previous investigations [24] were also summarized in Figure 5, in which, functionalized carbon nanotubes are with relatively high (i.e., 272.7 m 2 /g) but significantly lower specific surface areas than those of FNSI 300 m and FNSI 600 m particles prepared in this study.After addition with NSI  and/or FNSI  m particles in UHMWPE, the achievable  values of F 100 NSI   and/or F 100 FNSI  m- as-prepared fibers increased initially and reached a maximal value as their NSI  and/or FNSI  m contents approached an optimal value, respectively, in which the achievable  values of F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375as-prepared fibers prepared at the optimal FNSI  m contents at 0.075, 0.05, and 0.0375 phr, respectively, were significantly higher than those of the corresponding F 100 NSI 100 0.1 , F 100 NSI 300 0.0625 , and F 100 NSI 600 0.05 as-prepared fibers prepared at the optimal NSI  contents at 0.1, 0.0625, and 0.05 phr, respectively.Moreover, it is worth noting that F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375as-prepared fibers prepared at the optimal FNSI  m contents exhibited other maximal achievable  values at 176, 289, and 361, respectively, as their FNSI 100 m , FNSI 300 m , and FNSI 600 m particles were modified using an optimal weight ratio of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 at 3, 6, and 9, respectively.It is further interesting to note that the highest achievable  values obtained for the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared fibers prepared at the optimal FNSI  m contents and weight ratios of PE g-MAH to NSI  improved significantly as the specific surface areas of their FNSI  m particles increased.In fact, the maximal achievable  value (i.e., 361) obtained for F 100 FNSI 600   as-prepared fiber is about 2.05 and 1.25 times of those of F 100 FNSI 100 m3-0.075 and the best prepared UHMWPE/FCNT as-prepared fibers and is 2.85 times of that of F 100 as-prepared fiber without addition of original and/or modified nanosilica particles.

Orientation Factor Analyses of the As-Prepared and
Drawn Fiber Specimens.Typical orientation factor ( 0 ) values of F 100 , F 100 NSI   , and F 100 FNSI  m- as-prepared and drawn fibers are summarized in Figure 6.No significant difference in  0 values was found for F 100 , F 100 NSI   , and F 100 FNSI  m- as-prepared fibers.As expected,  0 values of F 100 , F 100 NSI   , and F 100 FNSI  m- fibers increased consistently as their draw ratios increased.After addition of NSI   and/or FNSI  m- particles,  0 values of drawn F 100 NSI   and/or F 100 FNSI  m- fibers were significantly higher than those of drawn F 100 fibers with the same draw ratios. 0 values of drawn F 100 NSI   fibers with a fixed draw ratio reached a maximal value as their NSI 100 , NSI 300 , and NSI 600 contents approached the optimal values at 0.1, 0.0625, and 0.05 phr, respectively.Similarly,  0 values of each drawn F 100 FNSI  m- fiber series specimen reached a maximal value as their FNSI 100 m , FNSI 300 m , and FNSI 600 m contents approached an optimal value at 0.075, 0.05, and 0.0375 phr, respectively, in which,  0 values of drawn F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375fibers prepared at the optimal FNSI  m contents were significantly higher than those of the corresponding drawn F 100 NSI 100 0.1 , F 100 NSI 300 0.0625 , and F 100 NSI 600 0.05 fibers prepared with the same draw ratios and at an optimal NSI  content, respectively.Moreover, it is worth noting that F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375drawn fibers prepared at the optimal FNSI  m contents exhibited other maximal  0 values as their FNSI 100 m , FNSI 300 m , and FNSI 600 m particles were modified using an optimal weight ratio of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 at 3, 6, and 9, respectively.It is further interesting to note that the maximal  0 values obtained for the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared fibers prepared at the optimal FNSI  m contents and weight ratios of PE g-MAH to NSI  improved significantly as the specific surface areas of their FNSI  m particles increased.As evidenced by thermal and lamellar thickness analyses,   (or evaluated   ) values of F 100 NSI   and/or   F 100 FNSI  m- as-prepared fibers reduced to a minimal value, as their NSI  and/or FNSI  m contents reached an optimal value, respectively.Moreover, the lowest   (or evaluated   ) values obtained for the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared fibers prepared at the optimal FNSI  m contents and weight ratio of PE g-MAH to NSI  reduced significantly as the specific surface areas of their FNSI  m particles increased.Presumably, these crystals with lower   and/or evaluated   values can be melted and pulled out of folded lamellar crystals relatively easily during the ultradrawing processes, and hence this results in higher drawability and orientation of the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375fibers, in which, the drawability and orientation of the best prepared fibers improved significantly as the specific surface areas of FNSI 100 m3 , FNSI 300 m6 , and FNSI 600 m9 present in F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared and/or drawn fibers increased.However, the amounts of coagulated NSI  and/or FNSI  m particles are likely to increase significantly when their NSI  and/or FNSI  m contents are higher than certain values, respectively.These coagulated NSI  and/or FNSI  m particles can slide against each other and serve as the defects for stress concentration during the drawing processes of F 100 NSI   and F 100 FNSI  m- as-prepared fibers and hence lead to an early breakage and/or significant reduction in achievable  and  0 values of the resulting drawn fibers.
Based on these premises, it is reasonable to understand that the achievable  values of F 100 NSI   and F 100 FNSI  m- as-prepared fibers and  0 values of the drawn F 100 NSI   and F 100 FNSI  m- fibers with a fixed draw ratio reduce significantly when their NSI  and/or FNSI  m contents are higher than the specific optimal value, respectively.

Morphological Analyses of the As-Prepared and Drawn
Fibers. Figure 7 exhibits typical SEM micrographs of the as-prepared and drawn F 100 , F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375fibers with various draw ratios.Many demarcated drawn "microfibrils" were found paralleling the drawing direction of the drawn F 100 , F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375fibers as their draw ratios increased, wherein the thicknesses of these drawn micro-fibrils reduced significantly as the draw ratios increased.Moreover, more and thinner "micro-fibrils" were found on the surfaces of F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared and/or drawn fibers with the same draw ratio as the specific surface areas of their FNSI 100 m3 , FNSI 300 m6 , and FNSI 600 m9 particles increased.It is not completely clear what accounts for the interesting demarcated "micro-fibril" morphology found on the surfaces of F 100 , F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared and drawn fibers.Presumably, during the ultradrawing processes, many of the UHMWPE kebab crystals with relatively thinner lamellar thickness values can be unfolded and pulled out of the crystal lamellae in an easier way than those kebab crystals with thicker lamellar thickness values.The unfolded UHMWPE molecules pulled out from the kebab lamellae can then gradually transform into the oriented "micro-fibrils" during their ultradrawing processes.As evidenced by DSC analyses in the previous section,   (or evaluated   ) values of F 100 NSI   and/or F 100 FNSI  m- as-prepared fibers were significantly lower than that of F 100 as-prepared fiber and reached a minimal value, as their NSI  and/or FNSI  m contents approached an optimal value, respectively, in which, the lowest   (or evaluated   ) values obtained for the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared fibers prepared at the optimal FNSI  m contents and weight ratio of PE g-MAH to NSI  reduced significantly as the specific surface areas of their FNSI  m particles increased.Based on these premises, it is reasonable to infer that the "micro-fibrils" found on the surfaces of as-prepared and/or drawn F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375fibers are more and thinner than those of the corresponding asprepared and/or drawn F 100 fibers with the same draw ratios but without addition of any "nuclear" nanofillers.By the same analogy, more and thinner "micro-fibrils" are expected to be found on the surfaces of F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375as-prepared and/or drawn fibers as the specific surface areas of their FNSI  m particles increased.
3.8.Tensile Properties.Tensile strength (  ) and modulus () values of F 100 , F 100 NSI   , and F 100 FNSI  m- as-prepared fibers prepared at varying draw ratios are illustrated in Table 3.For comparison purposes,   and  values of the best prepared UHMWPE/FCNT (i.e., F 100 C f2-0.1 ) as-prepared fiber obtained in our previous investigation [24] were also summarized in Table 3 m9-0.0375drawn fiber is about 1.7 and 1.5 times of those of the F 100 FNSI 100 m3-0.075 and the best prepared UHMWPE/FCNT drawn fiber specimens, respectively, and is about 2.3 times of that of the best prepared UHMWPE drawn fibers prepared at the same optimal UHMWPE concentration and drawing condition but without addition of any nanofiller.
The mechanical properties of the drawn specimens are generally believed to depend mainly on the degree of orientation of the drawn specimens, as their molecular weights are constant [16,34].As evidenced by orientation analyses in the previous section, at a fixed draw ratio,  0 values of drawn F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375fibers prepared at the optimal FNSI  m contents were significantly higher than those of the corresponding F 100 NSI 100 0.1 , F 100 NSI 300 0.0625 , and F 100 NSI 600 0.05 fibers prepared at an optimal NSI  content, respectively.Moreover,  0 values of F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375drawn fibers were always higher than those of other F 100 FNSI 100 m-0.075, F 100 FNSI 300 m-0.05, and F 100 FNSI 600 m-0.0375fibers prepared with the same draw ratios and FNSI contents but modified using an optimal weight ratio of PE g-MAH to NSI 100 , NSI 300 , and NSI 600 other than 3, 6, and 9, respectively.In fact, the maximal  0 values obtained for the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375drawn fibers improved significantly as the specific surface areas of their FNSI  m particles increased.These results clearly suggest that a good orientation of UHMWPE molecules along the drawing direction positively affects the tensile properties of F 100 , F 100 NSI   , and F 100 FNSI  m- fibers.Excellent orientation and ultimate tensile properties of UHMWPE/nanofiller fibers can be prepared by the ultradrawing of F 100 FNSI  m- asprepared fibers with optimal contents of the best prepared FNSI 100 m3 , FNSI 300 m6 , and FNSI 600 m9 particles well dispersing in their as-prepared fibers.Moreover, the specific surface areas of well-dispersed functionalized nanofillers in UHMWPE/functionalized nanofiller fibers can positively affect their ultradrawing, orientation, ultimate tensile properties, and "micro-fibrils" morphologies.

Conclusions
As evidenced by FTIR and TEM analyses, PE g-MAH molecules were successfully grafted onto nanosilica particles with varying specific surface areas through the reaction of the hydroxyl groups of nanosilica particles with the maleic anhydride groups of PE g-MAH molecules during their functionalized processes.The specific surface areas of FNSI 100 m , FNSI 300 m , and FNSI 600 m functionalized nanosilica particles reached a maximal value at 129.8, 335.8,

Figure 6 :
Figure 6: The orientation factor ( 0 ) values as-prepared and drawn fibers with varying draw ratios.

Table 1 :
Designations, compositions, and specific surface areas of nanosilica particles (NSI  ) and functionalized nanosilica particles (FNSI  m ) prepared in this study.

Table 2 :
Designations, melting temperatures (  ), percentage crystallinity (  ), and evaluated lamellar thickness (  ) values of UHMWPE, typical UHMWPE/nanosilica, and UHMWPE/functionalized nanosilica as-prepared fiber specimens and corresponding compositions of gel solutions used in the gel spinning processes.
Figure 2exhibits typical TEM micrographs of NSI  and FNSI  m particles.Typical irregular particle feature with dimensions of 250-350, 150-200, and 50-80 nm in diameter was observed for NSI 100 , NSI 300 , and NSI 600 particles (see Figures m specimens are attributed to the reaction of the hydroxyl groups of NSI 100 , NSI 300 , and NSI 600 particles with the maleic anhydride groups of PE g-MAH molecules during their functionalized processes.The reappearance of O-C=O and C=O stretching bands of maleic anhydride groups is most likely due to the overdosage of PE g-MAH during the functionalized processes of FNSI 100 m , FNSI 300 m , and FNSI 600 m particles.3.2.MorphologicalAnalyses of   and FNSI  m Particles.
and Table 2.A main melting endotherm with   and   at 142.7 ∘ C and 65.1%, respectively, was found for F 100 specimen.After incorporation of NSI  and/or FNSI  m particles in UHMWPE,   (or evaluated   ) values of F 100 NSI   (i.e., F 100 NSI 100  , F 100 NSI 300  , and F 100 NSI 600  ) and/or F 100 FNSI  m- (i.e., F 100 FNSI 100 m- , F 100 FNSI 300 m- , and F 100 FNSI 600 . As expected,   and  values of the drawn F 100 , F 100 NSI   , and F 100 FNSI  m- fibers improve consistently as their draw ratios increase.It is worth noting that   and  values of drawn F 100 NSI   and F 100 FNSI  m- fibers are significantly higher than those of the corresponding drawn F 100 fibers with the same draw ratio but without addition of NSI  and/or FNSI  m particles.Similar to those found for their  0 values,   and  values of drawn F 100 NSI   and F 100 FNSI  m- fibers reach a maximal value as their NSI  and/or FNSI  m contents approach the optimal values at 0.075, 0.05, and 0.0375 phr, respectively, in which   and  values of drawn F 100 FNSI 100 MAH to NSI 100 , NSI 300 , and NSI 600 at 3, 6, and 9, respectively.It is interesting to note that the maximal   and  values obtained for the best prepared F 100 FNSI 100 m3-0.075, F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375drawn fibers prepared at the optimal FNSI  m contents and weight ratio of PE g-MAH to NSI  increased significantly as the specific surface areas of FNSI  m particles increased.For instance, the ultimate   values of best prepared F 100 FNSI 100 m3-0.075, F 100 C f2-0.1 , F 100 FNSI 300 m6-0.05 , and F 100 FNSI 600 m9-0.0375fibers reached 4.4, 5.1, 7.1, and 7.6 GPa, respectively, as the specific surface areas of FNSI 100 m3 , FCNT, FNSI 300 m6 , and FNSI 600 m9 particles increased from 129.8 to 272.7, 335.8 and to 630.7 m 2 /g, respectively.The ultimate   value of the best prepared F 100 FNSI 600