Supramolecular structure of polypropylene fibers formed with addition of functionalized graphene oxide

Abstract

Pimelic acid and calcium hydroxide were used to attach calcium pimelate to the surface of graphene oxide. The additive was mixed with isotactic polypropylene granulate. Neat polypropylene and polypropylene with functionalized graphene oxide was extruded into fibers under laboratory conditions. The gravity spun fibers containing different concentration of the additive and the fibers taken at various velocities were obtained. Morphology and elemental composition of functionalized graphene oxide were studied by means of scanning electron microscopy and energy dispersive X-ray spectroscopy. The structure of fibers was examined by means of differential scanning calorimetry and wide-angle X-ray scattering. The ability of calcium pimelate supported on the surface of the graphene oxide to nucleate the β-form of polypropylene was revealed. A considerable amount of the β-form crystals was obtained in the gravity spun fibers. In the fibers taken at moderate and higher velocities the β-form disappeared. The structure of the fibers extruded with the additive was similar to the structure of the fibers extruded from neat polypropylene. At moderate velocities, the content of mesophase in the structure was high. At higher velocities, the crystalline structure built only from α-form crystals was obtained. The paper presents a discussion of the changes observed in the fiber structure in connection with polypropylene nucleation.

Keywords

polypropylene fibers structure β-form nucleating ability graphene oxide functionalization

Due to several advantageous properties, easy processing and relatively low price, polypropylene fibers and textiles have gained great interest. Various products including mono- and multifilaments, staple fibers, tapes, fibrillated fibers as well as spun-bonded and melt-blown nonwovens manufactured worldwide by many producers have been widely used in interior and technical textiles.¹

Various techniques were developed to manufacture polypropylene textiles. The most convenient and commonly used method for the formation of mono- and multifilaments is the classical melt spinning during which polypropylene powder or pellet is heated above its melting point and then extruded into the air through fine orifices of the spinneret. In the air, the thin streams are intensively cooled and subject to an intense stretching. Finally, the solidified filaments are collected at a take-up device.²

Numerous investigations into melt spinning of polypropylene fibers revealed that formation conditions have great influence on the fiber structure.^3–6 Depending on the formation parameters of the fibers, a semicrystalline structure rich in smectic mesophase and/or well-ordered crystalline phase was observed. The high content of mesophase was recorded in fibers extruded at high extrusion temperature,⁷ in fibers manufactured from polypropylene with low molecular weight,⁸ in fibers spun at low take-up velocity^9,10 and in fibers intensively cooled in water with the addition of ice or in the mixture of dry ice and acetone.^11–14 In contrast, a high content of crystalline phase was obtained in fibers extruded at low extrusion temperature and taken at high take-up velocities.^10,15 It was discovered that the crystalline phase is built from the most thermodynamically stable monoclinic α-form crystals. Other polymorphic forms of polypropylene, the trigonal β-form with the greatest impact strength, the largest elongation at break and the highest heat deformation temperature as well as the orthorhombic γ-form, were not detected in commercial fibers.^16–18

To enhance the polymer processability and improve the properties of the products, modified polypropylene fibers are usually applied. The most common technique involves application of additives which are incorporated into the ﬁbers during their formation. For many years, the commonly used additives were organic low molecular weight substances such as wax, parafﬁn or silicone oils, fatty acids, nonionic surfactants, fillers, pigments, flame retardants, photostabilizers and several other compounds.^19–24

The additives blended with polypropylene melt interact with the polymer chains during fiber solidification and/or affect the conditions of polypropylene crystallization.^25–29 Certain additives, including pigments and inorganic fillers, possess the nucleating ability and considerably increase the nucleation density.^30,31 Some additives belong to selective nucleating agents, which promote the growth of the β-form crystals.³²

The activity of β-nucleating agents during formation of fibers is strongly influenced by formation parameters. In examinations the influence of various spinning and drawing parameters on the β-form content was observed.³³ It was revealed that the content of the β-form is strongly dependent on cooling medium and the cooling rate and is less dependent on the melting temperature.³⁴ In studies on fibers colored with γ quinacridone pigment the high content of the β-form was obtained only in spun fibers taken at the low take-up velocities at low extrusion temperature.³⁵ With the increase of the take-up velocity, the β-form content systematically decreased, so in the fibers taken at higher velocities, the crystalline structure containing only the α-form was detected.

In recent years fine dispersed additives in the form of nanoparticles have been applied for modification of polypropylene fibers.^36–38 The addition of nanoparticles, such as inorganic fillers, montmorillonite, carbon black, carbon nanotubes or graphene, allowed to obtain the speciﬁc properties of fibers by low additive concentration.^39–43

Due to the unique combination of outstanding thermal, mechanical and electrical properties, graphene and its derivatives have attracted tremendous attention worldwide.^44,45 Several attempts to form nanocomposite fibers containing graphene derivatives have yielded promising results.^46–48

In examinations the graphene oxide obtained by oxidation of graphite powder was often applied.^49,50 The graphene oxide built from the graphene flakes of various thicknesses is formed by splitting graphite without destroying the unique honeycomb structure. As a result of oxidation between the carbon layers and on their edges several oxygen-containing functional groups are introduced. The groups prevent the restoration of van der Waals interaction between the graphene layers, enhance interactions with the polar polymer matrices, limiting at the same time dispersion in the non-polar polymers.

To enhance the compatibility of graphene oxide with non-polar polymers and improve its dispersability in polypropylene matrix the graphene oxide is usually functionalized. For this purpose several compounds with long aliphatic chains were attached to oxygen-containing groups. As a result of functionalization the interfacial adhesion between graphene oxide and the polypropylene was increased and the nucleation ability of the graphene oxide in polypropylene crystallization was improved.^51,52 By funtionalization with calcium pimelate, one of the best known and effective nucleating agents promoting growth of the beta-form of polypropylene, the selective nucleating ability of the beta-form was achieved.⁵³

In our investigations the calcium pimelate was connected with carboxylic groups of graphene oxide as a result of reaction with pimelic acid in the presence of calcium hydroxide. In previous examinations this functionalized graphene oxide was characterized and its selective nucleating ability in polypropylene crystallization in quiescent state was confirmed.⁵⁴ During the investigations, which are presented in the present paper, the functionalized graphene oxide was added as an additive by the formation of the polypropylene fibers. The fibers were extruded in laboratory conditions and taken at different winding speeds. The influence of formation parameters on fibers’ supramolecular structure is analyzed.

Experimental

Materials

The investigations were performed for nanocomposite polypropylene ﬁbers formed by means of a laboratory twin-screw extruder EHP-2x16S supplied by Zamak Mercator (Poland). The extruder was coupled with a 32-hole spinneret with the diameter φ of 0.2 mm. The ﬁbers were extruded from the melt at a temperature of 230℃ and cooled in air at room temperature of 20℃. The gravity spun fibers extruded at velocity 4.4. m/min were collected without drawing below the spinneret in a container at a distance of 0.5 m. Additionally the series of fibers were collected at the final winding device located at a distance of 1.5 m from the spinneret. The fibers wound at different speeds—25, 50, 100, 200 and 400 m/min—were obtained.

Commercial isotactic polypropylene Moplen HP462R (MFI = 25 g/10 min) supplied by LyondellBasell was applied. The additive was graphene oxide functionalized with pimelic acid.

The graphene oxide was prepared in laboratory conditions from the graphite powder through chemical oxidation by means of the modified Hummers’ method.⁵⁵ As a raw material, graphite powder flakes with a particle size <20 µm, purchased from Sigma-Aldrich, were applied. The suspension of 20 g of graphite powder in 460 cm³ of sulfuric acid was stirred in an ice bath for 30 min. Subsequently, 60 g of potassium permanganate was slowly added to the solution. The solution was then warmed to 35℃ and stirred for 2 hours. Then, 920 cm³ of distilled water was added. To remove the remnants of permanganate, 800 cm³ of warm distilled water (60℃) and 500 cm³ of the 3% aqueous solution of hydrogen peroxide were added. Finally, graphene oxide was centrifuged and rinsed with distilled water and acetone.

In the next stage, graphene oxide was functionalized with pimelic acid. Functionalization was carried out by mixing graphene oxide with pimelic acid in the presence of calcium hydroxide and heating the mixture in an aluminum vessel at 120℃ for 1.5 h.

The functionalized graphene oxide was compounded with the polypropylene resin to obtain a masterbatch containing 5%_wt of a fine dispersed additive. The blending was performed in the molten state at the temperature 230℃ in the barrel of the laboratory extruder. During fibers formation the masterbatch was compounded with the appropriate amount of the polypropylene granulate to obtain a mixture with the concentration of 0.1, 0.5 and 1%_wt.

Methods

Scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS)

To evaluate morphology and elemental composition of graphene oxide and the product of its functionalization, scanning electron microscopy (SEM) was applied. The investigations were performed using the Phenom ProX scanning electron microscope coupled with the PhenomWorld EDS (energy dispersive X-ray spectroscopy) detector. Micrographs were recorded in the image mode. The EDS measurements were performed in the map mode. On the basis of SEM microphotographs the fiber diameter was determined. The diameter was calculated as the average of 10 measurements.

Wide-angle X-ray scattering (WAXS)

The crystalline structure of the nanocomposite fibers was studied by means of wide-angle X-ray scattering (WAXS). The measurements were performed using the URD 6 Seifert diffractometer equipped in a copper target X-ray tube (λ = 0.154 nm). CuK_α radiation was monochromatized with Ni filter and pulse-height analyzer. WAXS curves were recorded in the symmetrical mode in the angular range 3–40°, with a step of 0.1 and the registration time of 20 s per step. The investigated samples were powdered and pressed into a sample holder. The WAXS curves, corrected for background scattering and normalized to the same integrated intensity in the whole registration range, were decomposed into parts arising from crystalline, meso and amorphous phases. The analysis was conducted using the WAXSFIT software.^56,57

Differential scanning calorimetry (DSC)

Thermal properties of the nanocomposite fibers were studied by mean of differential scanning calorimetry (DSC). The investigations were carried out using an analytical system (TA Instruments, USA) with a calorimeter (MDSC 2920) with the refrigerated cooling system. The analysis of the DSC curves was performed by means of the Universal V4.5A software supplied by TA Instruments. The samples were heated from −40℃ to 210℃ at a rate of 10°/min in nitrogen purge (flow 40 ml/min). Based on the curves, the melting temperature and the enthalpy of transitions were determined. The content of ordered phases was calculated as the ratio of measured enthalpy to the melting enthalpy of the fully crystalline polymer. According to the literature ΔH_m = 207 J/g for the α-form was assumed.⁵⁸

Results

Functionalized graphene oxide

The representative SEM microphotograph of graphene oxide before functionalization is presented in Figure 1. The graphene oxide forms aggregates with the external dimensions in the order of several dozen micrometres. The aggregates have rippled structure and are constituted from irregular flake-like crystals formed from the layers of oxidized graphite sheets.

Figure 1.

SEM micrograph of the aggregates of graphene oxide before functionalization.

During oxidation, the oxygen-containing functional groups are introduced into the regular carbon structure of the graphite. In the investigated samples, the content of oxygen atoms reaches almost 40% after the oxidation. At the same time, the average atomic ratio C/O reaches the high value of 1.5 (Figure 2).

Figure 2.

EDS spectra for graphene oxide aggregates.

The SEM micrographs of graphene oxide functionalized with pimelic acid in the presence of calcium hydroxide are presented in Figure 3. The micrographs also show clearly visible particles of other materials besides graphene oxide aggregates. The particles are formed from tiny granules which in certain places create big, spatially extended aggregates. The dimensions of the aggregates vary from a few to several dozen micrometres. Smaller aggregates are randomly distributed and attached to the surface of graphene oxide. Bigger aggregates located in the free space are loosely connected to the surface of graphene oxide.

Figure 3.

SEM micrograph of the aggregates of functionalized graphene oxide; (a) small aggregates of calcium pimelate attached to the surface of graphene oxide; (b) aggregate of calcium pimelate.

The EDS analysis clearly shows that the aggregates contain calcium atoms, the main component of calcium pimelate produced during the graphene oxide functionalization. The tiny aggregates attached locally to the surface of graphene oxide contain a small percentage of calcium (Figure 4(a)). In this case, the analysis takes into the account the carbon and oxygen atoms forming both graphene oxide and calcium pimelate. For the big aggregates constituted only from pure calcium pimelate, the atomic percentage of calcium atoms is much higher and exceeds 25%.

Figure 4.

EDS spectra of samples after functionalization for: (a) tiny aggregates of calcium pimelate attached to the surface of graphene oxide; (b) aggregates of calcium pimelate.

Gravity spun fibers

WAXS

WAXS diffraction patterns of gravity spun fibers extruded from the polypropylene without additives and from the polypropylene containing various amounts of functionalized graphene oxide are presented in Figure 5.

Figure 5.

X-ray diffraction patterns of gravity spun fibers.

Several diffraction peaks (110) at 14.1°, (040) at 16.9°, (130) at 18.4°, (111) at 21.2° and (041) at 21.8° attributed to monoclinic α-form occur in the WAXS pattern of the fibers extruded from neat polypropylene. In the WAXS patterns of the fibers manufactured with the addition of functionalized graphene oxide, beside diffraction peaks corresponding to crystals of α-modification, we can also observe a main diffraction peak (300) of the β-form of polypropylene located at 2θ = 16.1°. For the fibers with the low content 0.1% and 0.5% of functionalized graphene oxide, the intensity of this peak is very high. For the fibers containing the high concentration of the additive (1%) the intensity of the (300) peak decreases and is comparable to the intensity of crystal peaks attributed to the α-modification.

An example of curve decomposition for the gravity spun fibers extruded with addition of 0.1% of additive is presented in Figure 6.

Figure 6.

WAXS pattern decomposition for gravity spun fibers (additive concentration 0.1%).

The presence of diffraction peaks of both crystalline forms and their different intensity distribution proves that the gravity spun fibers with the additive contain, with different contribution, crystallites of both α- and β-modifications of polypropylene. The contribution of β-modification to the crystalline phase of the polymer matrix of the fibers is characterized by K_β parameter, determined according to equation (1).⁵⁹

K_{β} = \frac{I_{β (300)}}{I_{β (300)} + I_{α (110)} + I_{(040)} + I_{(130)}}

(1)

where: I_β₍₃₀₀₎ is the intensity of the main diffraction peak characteristic for (300) plane of β-form, I_α₍₁₁₀₎, I_α₍₀₄₀₎ and I_α₍₁₃₀₎ are intensities of the strongest peaks characteristic for (110), (040) and (130) planes of α-form.

Table 1 presents the diameter and structural parameters of the gravity spun fibers determined by the WAXS measurements: K_β value and crystallinity index. The crystallinity index was calculated as the ratio of the integral intensity contained in all crystalline peaks to the total integral intensity scattered by the sample in the whole recorded range.

Table 1.

Structural parameters of gravity spun fibers

Concentration of functionalized graphene oxide %	Fiber diameter φ, µm	Crystallinity index %	K_β value –
0	212 ± 5.5	47.5	–
0.1	215 ± 6.0	49.6	0.72
0.5	210 ± 4.5	50.7	0.75
1	216 ± 4.8.	52.2	0.32

The diameter of the gravity spun fibers formed with and without additive regardless of the additive concentration is practically the same. The crystallinity index for the fibers extruded without additive is relatively high and equals 47.5%. As a result of adding the additive, the crystallinity index grows slightly with the increment of additive concentration. The K_β value reaches its highest level at the two additive concentrations: 0.1 and 0.5%. For the highest additive concentration the K_β value is more than two times lower than the value determined at the concentration of 0.5%.

DSC

The DSC curves recorded during heating of the gravity spun fibers from neat polypropylene as well as polypropylene with functionalized graphene oxide are presented in Figure 7.

Figure 7.

DSC curves of gravity spun polypropylene fibers without additives and polypropylene fibers modified with graphene oxide at 0.1%; 0.5% and 1.0%, respectively.

On all the curves strong endothermic peaks, corresponding to the melting of polypropylene α crystals, are observed. For the fibers extruded without the additive, the melting temperature is 165.6℃ (Table 2). For the ﬁbers formed with the addition of functionalized graphene oxide the melting temperature slightly increases and achieves the highest value of 167.2℃ for the fibers containing 0.5% additive.

Table 2.

Values of characteristic temperatures and degrees of crystallinity determined for gravity spun fibers

Sample	Temp. of melting β-iPP		Temp. of melting α-iPP	Degree of crystall.
Sample	T_m1 _β, ℃	T_m2 _β, ℃	T_m _α, ℃	κ _DSC, %
PP 100%	–	–	165.6	49.2
PP/GO (0.1%)	141.4	149.6	166.2	49.6
PP/GO (0.5%)	142.5	150.7	167.2	50.2
PP/GO (1.0%)	142.5	150.9	166.3	52.2

PP: polypropylene; GO: graphene oxide.

For the fibers extruded with functionalized graphene oxide, except the melting peak attributed to the α-form, two less intensive endothermic peaks with a minimum at temperatures of approximately 141–142℃ and approximately 150–151℃, are visible. The peaks correspond to the melting of the β-form of isotactic polypropylene. The total amount of ordered phases in the fibers calculated on the basis of DSC measurements increases monotonically from 49.2% for pure polypropylene gravity-spun fibers to 52.2% for the fibers containing 1% additive (Table 2). The crystallinity measured on the basis of DSC curves corresponds to values determined from WAXS patterns.

Fibers taken at different velocities

WAXS

The WAXS patterns registered for the series of fibers extruded from neat polypropylene and taken at different velocities are presented in Figure 8. Similar patterns registered for the fibers spun with addition of functionalized graphene oxide are shown in Figure 9.

Figure 8.

WAXS patterns of fibers extruded from pure polypropylene without additives.

Figure 9.

WAXS patterns of fibers extruded from polypropylene with addition of functionalized graphene oxide (additive concentration 0.5 %).

The WAXS patterns of both series of fibers taken at velocities 25, 50 and 100 m/min are dominated by two broad peaks. The peaks correspond to the mesomorphic form of polypropylene and are located at 2θ = 15° and 21°, respectively. In the WAXS patterns of the fibers taken at 200 m/min the crystalline peaks (110), (040), (130), (111) and (041) attributed to the α-form appear. The crystalline peaks overlap the broader mesophase peaks. For the fibers taken at the highest velocity (400 m/min), the mesophase peaks become invisible and only sharp crystalline peaks characteristic for the α-form occur.

Based on decomposition of the WAXS curves into parts arising from the crystalline, meso and amorphous phases, the content of the ordered phases in the fibers was calculated. An example of curve decomposition for the fibers extruded from neat polypropylene taken at 200 m/min is presented in Figure 10. The parameters determined for the fibers extruded with and without the additives are presented in Table 3.

Figure 10.

WAXS pattern decomposition for fibers extruded from neat polypropylene taken at 200 m/min.

Table 3.

Structural parameters of fibers taken at different velocities obtained from WAXS measurements

Fibers	Take-up velocity	Fiber diameter	Content of ordered phase
Fibers	V, m/min	φ, µm	x_o, %
PP 100%	25	29.8 ± 2.1	41.8
	50	24.2 ± 1.4	42.3
	100	21.0 ± 1.0	42.9
	200	17.7 ± 0.9	47.2
	400	13.0 ± 0.7	52.5
PP/GO (0.5%)	25	30.5 ± 2.2	42.0
	50	24.3 ± 1.4	43.5
	100	21.5 ± 1.1	43.1
	200	17.8 ± 0.9	47.9
	400	13.2 ± 0.8	51.4

PP: polypropylene; GO: graphene oxide.

The diameter of the fibers formed with and without additive is the same and decreases with the increase of the winding velocity. The content of the ordered phase for both types of fibers changes with the increase of the take-up velocity. In the fibers formed without the additive the ordered phase content increases from 41.8% for the lowest velocity to 52.5% for the highest one. Similarly, for the fibers extruded with the additive, the ordered phase content increases from 42% to 51.4%.

DSC

DSC curves registered during heating of the fibers extruded from pure polypropylene and taken at different velocities are presented in Figure 11.

Figure 11.

DSC curves of polypropylene fibers without additive as a function of take-up velocity.

Independently of the take-up velocity, one strong endothermic peak is visible on the curves. The peak is associated with melting of the α-form of polypropylene. For the fibers taken at the lowest velocity (25 m/min) the melting temperature is 164.1℃ and it fluctuates slightly for the fibers formed at higher velocities (Table 4).

Table 4.

Values of characteristic temperatures and enthalpies of transitions determined for neat polypropylene fibers taken at different take-up velocities

Take-up velocity of PP 100% fibers	Temperature of recrystall.	Enthalpy of recrystall.	Temperature of melting	Enthalpy of melting	Content of ordered phase
Take-up velocity of PP 100% fibers	T_r , ℃	ΔH_r , J/g	T_m, ℃	ΔH_m, J/g	κ _α, %
25 m/min	102.4	14.39	164.1	98.28	40.5
50 m/min	102.9	12.25	163.8	98.27	41.6
100 m/min	112.6	13.65	164.7	94.43	39.0
200 m/min	124.2	6.30	162.5	102.41	46.4
400 m/min	107.2	0.81	164.4	105.09	50.4

PP: polypropylene.

On the DSC curves of the fibers, besides the endothermic peak attributed to the melting of the crystalline structure, a broad exothermic peak with relatively low intensity between 75℃ and 150℃ is visible. This peak results from the transition, including partial melting and recrystallization, of the mesophase into the crystalline phase. The enthalpy of the discussed effect decreases significantly with the increasing take-up velocity.

The DSC curves of the series of fibers spun with the addition of functionalized graphene oxide are shown in Figure 12.

Figure 12.

DSC curves of polypropylene fibers modified with functionalized graphene oxide taken at different take-up velocities.

The shape of the DSC curves in Figure 12 is the same as for the polypropylene fibers formed without the additive. Similarly, a strong endothermic peak associated with the melting of the α-form is visible in all the samples, regardless of the take-up velocity. Additionally, between 75℃ and 150℃ there is a broad exothermic peak with relatively low intensity associated with mesophase transition. As the take-up velocity increases, the intensity of the exothermic peak decreases gradually so at the highest velocity of 400 m/min the peak is no longer visible.

The characteristic temperatures and enthalpies of the transitions determined on the basis of the DSC curves are presented in Table 5. Similar to the fibers formed from neat polypropylene, the increase of the take-up velocity leads to the decrease of the enthalpy of mesophase transition and the content of the ordered phases increases. Compared with the fibers taken from neat polypropylene, the values related to the content of the ordered phase determined for the fibers containing the additive are a little higher.

Table 5.

Values of characteristic temperatures and enthalpies of transitions determined for polypropylene fibers modified with functionalized graphene oxide taken at different take-up velocities

Take-up velocity of PP/GO(0.5%) fibers	Temperature of recrystall.	Enthalpy of recrystall.	Temperature of melting	Enthalpy of melting	Content of ordered phase
Take-up velocity of PP/GO(0.5%) fibers	T_r , ℃	ΔH_r , J/g	T_m, ℃	ΔH_m, J/g	κ, %
25 m/min	103.7	14.22	164.4	99.69	41.5
50 m/min	105.2	9.15	164.1	97.41	42.9
100 m/min	106.9	8.59	164.1	99.41	44.1
200 m/min	124.7	6.65	164.3	103.01	46.8
400 m/min	–	–	164.6	104.86	50.9

Discussion

The fiber structure is formed as a result of polypropylene crystallization which proceeds below the spinneret during cooling and solidification of an extruded stream. At melt spinning, crystallization occurs under nonisothermal conditions and high molecular orientation. The cooling rate and orientation strongly affect the mechanism and kinetics of crystallization. For the gravity spun fibers, the cooling rate and the orientation are very low. In these conditions the crystallization proceeds at a relatively high temperature, which favors the formation of the highly crystalline structure.

In the fibers extruded without the additive, crystallization is initiated on homogeneous nuclei and the crystalline structure containing only the most stable α-form is formed. In the case of the fibers containing functionalized graphene oxide, the additive particles act as nucleating agents and participate in the formation of the heterogeneous nuclei. Calcium pimelate supported on aggregates of graphene oxide stimulates the formation of the β-nuclei, which afterwards grow and form a structure containing crystals of two, α and β, polymorphic modifications.

The presence of the β-form is visualized both on the WAXS diffraction patterns and DSC thermograms. On the WAXS patterns, the intensity of the (300) peak characteristic for the β-form of polypropylene is comparable with the intensities determined for the polypropylene crystallized in the presence of other known β-nucleating agents. On the DSC curves, the presence of the β-form was evidenced by two exothermic peaks occurring at the temperatures below the melting temperature of the monoclinic α-form. Similar peaks were observed in the earlier investigations, for the samples containing the instable β-form. The peaks are attributed to two kinds of recrystallization taking place during the heating of the samples rich in the β-form, ββ-recrystallization within the β-phase leading to the perfection of the β-form crystal structure and βα-recrystallization which involved the transition of the β-form into the more stable α-form.⁶⁰

For the gravity spun fibers, the content of the β-form crystals depends on the additive concentration. The maximum contribution of β crystals was observed for the medium functionalized graphene oxide content. For both the lower and higher additive concentrations, the contribution of β-crystals was lower. A similar tendency was observed earlier in the investigations into β-nucleation of isotactic polypropylene in the presence of γ‐quinacridone pigment.⁶¹

The structure of fibers taken at various velocities changes. At very low velocities, 25, 50 and 100 m/min, in the fibers extruded both with and without the additive, a less ordered mesophase is formed instead of the crystalline phase. At 200 m/min, the fiber structure containing both mesophase and crystalline phase structure is obtained. At the highest velocity of 400 m/min, the mesophase completely disappears and only the crystalline phase built from α-crystals is formed.

The formation of mesophase in the fibers taken at low to moderate velocities was repeatedly observed. In these particular cases, the formation window of take-up velocities resulting in mesophase formation was determined.⁶²

At higher velocities, the influence of the orientation increases and affects greatly the nucleation mechanism and overall crystallization kinetics.⁶³ Under the influence of molecular orientation the polypropylene chains became partially oriented along the flow direction. The isotropic nucleation followed by the growth of spherulites, occurring commonly during crystallization of quiescent melt, changes into anisotropic nucleation. Inside the solidified fibers, the nuclei built from a bundle of partially extended chains are formed. Above a certain velocity, the density of the active nuclei increases rapidly and is many orders of magnitude higher compared with crystallization in the quiescent state. In these conditions the quick growth of the polypropylene α-crystals is induced.

Conclusions

Oxygen-containing groups formed during graphite oxidation enable the functionalization of the graphene oxide with compounds with the selective nucleating ability. As a result of reaction of the carboxylic groups of graphene oxide with pimelic acid in the presence of calcium hydroxide, calcium pimelate is formed. The aggregates of calcium pimelate attached to the surface of graphene oxide show the ability to nucleate the β-form of polypropylene. The nucleating ability of the additive is clearly visible in the case of the gravity spun fibers in which—in its presence—the structure with a considerable amount of the β-form crystals is obtained. As for the fibers taken at moderate and higher take-up velocities, the influence of the additive on polypropylene crystallization is lost. At the high molecular orientation, the numerous nuclei built from a bundle of partially extended chains are formed and the heterogeneous nucleation on additive crystals loses its importance. As a result, the fiber structure with the additive is formed similarly to the fibers extruded from neat polypropylene independently of the additive addition. In the fibers taken at moderate velocities, a structure rich in mesophase is formed. At higher velocities, the crystalline structure built only from α-form crystals is obtained.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Jan Broda

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