The UHMWPE resin, with a viscosity-average molecular weight ($$\overline{M}$$ _v) of 1.9×10⁶ g/mol, was provided by the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences. The paraffin oil A360B was purchased from Total Energies with a flash point of 250 °C. A mixture of UHMWPE resins and paraffin oil in a 1:9 (W:W) proportion was dissolved using a twin-screw extruder at temperatures of 133 and 200 °C before being extruded through a flat mouth die of dimensions 30 mm × 0.5 mm. The extruded material was then cooled via mirrored rolls, to obtain the initial UHMWPE gel films. Following this, a 4-h extraction with n-hexane solvent was performed, the resulting UHMWPE low-entangled films were vacuum-dried for 6 h at 70 °C. During these processes, the films were carefully maintained in a taut state to prevent shrinkage.

In situ SAXS and WAXD measurements were performed at the BL16B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), with X-ray wavelength set at 0.124 nm, and an exposure time of 4 s was employed. Prior to testing, blank samples were tested to obtain the background diffraction patterns for both SAXS and WAXD. The patterns for 2D-SAXS and 2D-WAXD were captured using Pilatus 900K and Pilatus 2M detectors with a resolution of 172 μm × 172 μm. The sample-to-detector distances for SAXS and WAXD modes were calibrated to 242 and 1676 mm, respectively, using silver behenate (AgBe) and cerium dioxide (CeO₂) for standard samples. We used a Linkam, MFS350 stretching device to perform uniaxial stretching at temperatures of 100, 110, 120 and 130 °C with a stretching speed of 5mm·min⁻¹. During stretching, we simultaneously recorded stress-strain curves and X-ray data. The length of the film between the stretching fixtures was 15 mm. All X-ray signals were normalized to the beam fluctuations. The WAXD and SAXS data were analyzed using FIT2D software.^[

In situ ultra-small-angle X-ray scattering (USAXS) measurement was conducted at the BL10U1 beamline station of the SSRF. The 2D-USAXS patterns were recorded using Dectris Eiger 4M detector with a pixel resolution of 75 μm × 75 μm and an exposure time of 4 s. The distance between the sample and the detector for USAXS was calibrated using a bovine tendon standard sample, resulting in a distance of 27600 mm. The stretching parameters were consistent with those of the in situ SAXS/WAXD measurements. The in situ USAXS data were synchronously collected during the stretching process of the samples.

The morphology characterization of UHMWPE films under different stretching temperatures and strains was performed using SEM (SU70) at 5 kV. To prepare the SEM samples, UHMWPE film samples were heated to 105−115 °C in an n-octane solvent to etch the surfaces and remove the amorphous component for a clearer visibility of the film's crystal structure on the surface. The samples were then rinsed three times with anhydrous ethanol and dried. Finally, after the samples were prepared, a gold sputter coating was applied to the surface for imaging.

Samples weighing 5−8 mg were held in a standard aluminum crucible by a dry nitrogen gas stream, and the thermal properties of UHMWPE films were recorded using NETZSCH Polyma DSC21400A. Prior to the actual measurements, a blank sample test was performed to eliminate any signal interference except that of the samples. The samples were heated and scanned from 25 °C to 200 °C at a ramp rate of 10 °C/min. The enthalpy of melting for 100% crystalline polyethylene is 293 J/g.^[

The viscosity characterization of UHMWPE low-entangled films was performed using the rheometer (Haake Mars60). Rheological experiments were conducted on UHMWPE films (dimensions of 20 mm in diameter and 0.3 mm in thickness) using fixed strain and frequency time scans. The rheological testing process proceeds as follows: employing a parallel plate test system, the modulus test is selected with a constant strain of 0.025%, a temperature of 180 °C, a frequency of 1 Hz, and a test duration of 30 min.^[

The UHMWPE low-entangled films with reserved shish crystals and those without were first stretched to the strain of 500%, 350%, 300% and 300% with tensile equipment (Linkam TST350) at a speed of 5 mm·min⁻¹ at 100, 110, 120 and 130 °C, and then cooled to room temperature naturally. The same strains were chosen for both films with and without reserved shish crystals at the same stretching temperatures. After cooling the samples, we used the Linkam TST350 to test the breaking strength and Young's modulus of the necking section at a speed of 10 mm·min⁻¹.

Fig. 1(a) displays the SAXS pattern of the UHMWPE film and the stretching direction, where q₁ was vertical to the stretching direction and q₂ was parallel to the stretching direction. Due to the principle of reciprocal space, the scattering signal along the q₁ direction represents the scattering signal of the crystal along the stretching direction, while the scattering signal along q₂ represents the scattering signal of the crystal vertical to the stretching direction.^[

Fig 1  (a) The SAXS patterns of UHMWPE films; (b) The scattering intensity of crystals in the stretching direction; (c) The scattering intensity of crystals in the vertical stretching direction.

Download:  Full-size image | High-res image | Low-res image

$$I_{q1}={\int }_{0.03}^{0.3}{\int }_{-160\text{°}}^{160\text{°}}I\left(q,\varphi \right)\mathrm{d}q\mathrm{d}\varphi$$

1

$$I_{q2}={\int }_{0.05}^{0.5}{\int }_{160\text{°}}^{20\text{°}}I\left(q,\varphi \right)\mathrm{d}q\mathrm{d}\varphi$$

2

The average shish length (L_shish) and the distribution of shish crystals ($$B_{\varphi }$$ ) was calculated using the Ruland’s streak method by analyzing the streak along the equator line direction in the USAXS patterns.^[

$$B_{\mathrm{obs}}=\frac{1}{⟨L_{\mathrm{shish}}⟩s}+B_{\varphi }$$

3

When the scattering intensity distribution fit the Gaussian functions; then, the equation becomes:

$${B_{\mathrm{obs}}}^2={\left(\frac{1}{⟨L_{\mathrm{shish}}⟩s}\right)}^2+{B_{\varphi }}^2$$

4

where s is the scattering vector, s = 2sinθ/λ.

The intensity distribution curve concerning 2θ is obtained by one-dimensional integration of the 2D-WAXD pattern. Peaking analysis is subsequently applied to the curve to calculate the proportions of the amorphous and crystalline regions, then calculates the crystallinity. The formula of calculation is as follows, where A_c and A_a represent the areas under the crystalline and amorphous peaks of the I(2θ)~2θ curve.

$$X_{\mathrm{c}}=\frac{A_{\mathrm{c}}}{A_{\mathrm{c}}+A_{\mathrm{a}}}$$

5

The lateral size of the crystals was determined using the Scherrer equation. Where K is the shape factor, generally set to 0.89 in polymer science, λ is the wavelength, θ is the Bragg diffraction angle, and β is the half-peak width at maximum intensity.^[

$$L_{hkl}=\frac{K\lambda }{\beta \cos \theta }$$

6

The crystal orientation of UHMWPE films was determined using the Herman's method. In this study, the stretching direction of the UHMWPE film was taken as the reference direction. Assuming the crystal plane as hkl, the orientation parameter can be expressed as follows:

$${<{\mathrm{c}\mathrm{o}\mathrm{s}}^2\varphi >}_{hkl}=\frac{{\int }_0^{\text{π}/2}I\left(\varphi \right){\mathrm{c}\mathrm{o}\mathrm{s}}^2\varphi \mathrm{s}\mathrm{i}\mathrm{n}\varphi \mathrm{d}\varphi }{{\int }_0^{\text{π}/2}I\left(\varphi \right)\mathrm{s}\mathrm{i}\mathrm{n}\varphi \mathrm{d}\varphi }$$

7

where, ϕ is the azimuth angle, and I(ϕ) is the scattering intensity along the azimuth angle. The orientation $$f$$ can be defined as follows:

$$f=\frac{3{<{\mathrm{c}\mathrm{o}\mathrm{s}}^2\varphi >}_{hkl}-1}{2}$$

8

When f=−0.5 and 1, it represents that the normal of the reflection plane is parallel or vertical to the reference direction, respectively; f=0 when the orientation is random.

The sample information for the UHMWPE films stretched at a rate of 5 mm·min⁻¹ at temperatures of 100, 110, 120 and 130 °C is shown in Table 1. UPE-dt133 represents the UHMWPE low-entangled film with reserved shish crystals, UPE-dt200 represents the UHMWPE film without reserved shish crystals.

Table 1  Dissolution and stretching temperature of UHMWPE gel films.

Dissolution temperature (°C)	Stretching temperature (°C)	Abbreviation
133	−	UPE-dt133
	100	UPE-dt133st100
	110	UPE-dt133st110
	120	UPE-dt133st120
	130	UPE-dt133st130
200	−	UPE-dt200
	100	UPE-dt200st100
	110	UPE-dt200st110
	120	UPE-dt200st120
	130	UPE-dt200st130

Download:  CSV

To characterize the degree of entangled molecular chains of the UHMWPE gel films, viscosity measurements were performed using shear rheology measurements. The viscosity values of UPE-dt200 was 2.2×10⁴ Pa·s, while UPE-dt133 had a viscosity of 1.2×10⁴ Pa·s, both significantly lower than the viscosity of UHMWPE compression molding samples (which had a viscosity of 1.0×10⁷ Pa·s).^[

Fig. 2 presents the SEM morphology of UPE-dt200 and UPE-dt133. It can be observed that UPE-dt200 is composed of randomly distributed lamellae, UPE-dt133 is composed of not only randomly distributed lamellae but also multiple shish crystals, indicating that the shish crystal in the resin have been successfully reserved. Comparing the morphology of UHMWPE films with reserved shish crystals prepared by compression molding^[

Fig 2  SEM morphology of (a) UPE-dt200 and (b) UPE-dt133.

Download:  Full-size image | High-res image | Low-res image

DSC results can also confirm the successful reservation of shish crystals in UPE-dt133. As shown in Fig. 3, the melting point of UPE-dt200 reaches 138.9 °C due to the randomly distributed lamellae observed in the SEM. UPE-dt133 has two high temperature melting points at 140.1 and 142.7 °C. The high-temperature melting point of 142.7 °C corresponds to the reserved shish crystals during the gel molding, while the melting point of 140.1 °C corresponds to the lamellae that grew around the reserved shish crystals.^[

Fig 3  DSC curves of UPE-dt200 and UPE-dt133.

Download:  Full-size image | High-res image | Low-res image

The above SEM, DSC and Viscosity results all confirmed the successful reservation of shish crystals in UHMWPE films prepared by gel molding. Our subsequent investigation focuses on the mechanisms of structural evolution of UPE-dt200 and UPE-dt133 stretched at different temperatures. Fig. 4 shows the stress-strain curves of UPE-dt200 and UPE-dt133 stretched at different temperatures. UHMWPE films all show the typical three stages of elastic deformation, yield, and strain hardening when stretched at different temperatures. UPE-dt133 shows higher stress values than UPE-dt200 at different stretching temperatures. This suggests that the reserved shish crystals could significantly increase the breaking strength and Young's modulus of UHMWPE films. Additionally, it was found that UPE-dt133 has a faster increase of the stress during the hot stretching, which makes it enter the strain hardening stage earlier (200%−300%). This result indicates that the reserved shish crystals promote the transmission of stress between crystals, particularly during the strain hardening stage. As a result, UPE-dt133 has a more significant increase of stress before and after the strain hardening stage compared to UPE-dt200. It is notable that for both UPE-dt200 and UPE-dt133, when stretched at higher temperatures (120 and 130 °C), the stress is lower than those at 100 and 110 °C, this result is due to the increased mobility of molecular chains at higher temperatures.

Fig 4  Stress-strain curves of UPE-dt200 and UPE-dt133 hot stretched at different temperatures.

Download:  Full-size image | High-res image | Low-res image

To further investigate the effect of reserved shish crystals on the structural evolution and the properties during the hot stretching of UHMWPE low-entangled films. Fig. 5 shows the in situ 2D SAXS patterns of UPE-dt200 and UPE-dt133 stretched at different temperatures. As the stretching proceeds, both UPE-dt133 and UPE-dt200 evolve from initial scattering rings to ellipses and then to shuttle shapes along the equatorial direction. This indicates that the randomly oriented lamellae in the films gradually align along the stretching direction. Notably, no distinct kebab long-period signals were observed in the 2D SAXS patterns. This lack of long-period signals could be due to the presence of residual porosity in the low-entangled films caused by the extraction and drying process after gel molding, and consequently, during the hot stretching process, stretching-induced filling of these pores occurs with the fragmentation and recrystallization of lamellae, indicating that the shish-kebab crystals formed in the process of hot stretching of gel films are not as periodic as the shish-kebab crystals formed by traditional stretching of gel fibers.^[

Fig 5  In-situ 2D SAXS patterns of UPEdt200 and UPEdt133 stretched at different temperatures: (a) UPE-dt200st100, (b) UPE-dt200st110, (c) UPE-dt200st120, (d) UPE-dt200st130, (e) UPE-dt133st100, (f) UPE-dt133st110, (g) UPE-dt133st120, (h) UPE-dt133st130.

Download:  Full-size image | High-res image | Low-res image

Fig. 6 shows the scattering intensity ratio of crystals along the stretching direction (q₁) to those along the vertical direction (q₂) for UPE-dt200 and UPE-dt133 stretched at different temperatures. The initial Iq₁/Iq₂ for UPE-dt200 and UPE-dt133 show no significant increase, suggesting that the reserved shish crystals have not yet aligned along the stretching direction. As the stretching proceeds, UPE-dt133 shows faster increase in Iq₁/Iq₂ compared to UPE-dt200, this suggests that the reserved shish crystals in UPE-dt133 accelerate the crystal alignment along the stretching direction. When stretched at higher temperatures (12 and 130 °C), there is a more rapid increase of Iq₁/Iq₂, this result also confirmed the increased mobility of the molecular chains when stretched at 120 and 130 °C, which allows more chains to participate in the crystal growth and evolution. A similar trend in the scattering intensity is observed for UHMWPE high-entangled films with reserved shish crystals prepared by the compression molding.^[

Fig 6  Ratio of scattering intensity of crystals in the stretching direction (q₁) to crystals in the vertical stretching direction (q₂) for UPE-dt200 and UPE-dt133 stretched at different temperatures.

Download:  Full-size image | High-res image | Low-res image

To understand more clearly the changes of the growth and evolution of shish-kebab crystals, we observe the growth and evolution of shish-kebab crystals on a larger scale by in situ USAXS. Fig. 7 shows the in situ 2D USAXS patterns for UPE-dt200 and UPE-dt133 stretched at different temperatures. As the stretching proceeds, both UPE-dt200 and UPE-dt133 show shuttle shapes along the stretching direction. And UPE-dt133 shows more distinct patterns during the later stages of stretching. The shuttle shape observed in UPE-dt200 patterns is attributed to the gradual evolution of shish crystals formed along the stretching direction, while the shuttle shape observed in UPE-dt133 is attributed to the alignment of reserved shish crystals and newly formed shish crystals along the stretching direction.

Fig 7  In-situ 2D USAXS patterns of UPE-dt200 and UPE-dt133 stretched at different temperatures: (a) UPE-dt200st100, (b) UPE-dt200st110, (c) UPE-dt200st120, (d) UPE-dt200st130, (e) UPE-dt133st100, (f) UPE-dt133st110, (g) UPE-dt133st120, (h) UPE-dt133st130.

Download:  Full-size image | High-res image | Low-res image

Fig. 8 shows the average lengths (L_shish) and distributions (B_Ф) of shish crystals along the stretching direction for UPE-dt200 and UPE-dt133 stretched at different temperatures. UPE-dt200 shows a gradual decrease in L_shish and B_Ф. This is due to that the molecular chains are easily disentangled from the amorphous region and involved in the evolution from lamellae to shish-kebab crystals during the initial stage of hot stretching, during the later stage of hot stretching, only new, shorter shish crystals can be formed from the remaining molecular chains which are difficult to disentangle,^[

$$L_{\mathrm{shish}}$$

$$L_{\mathrm{shish}}$$

Fig 8  Average shish length and distribution angle of UPE-dt200 and UPE-dt133 stretched at different temperatures.

Download:  Full-size image | High-res image | Low-res image

Fig. 9 shows the in situ 2D WAXD patterns for UPE-dt200 and UPE-dt133 stretched at different temperatures, with the stretching direction being the meridian direction. The (110) and (200) crystalline planes of the PE orthogonal crystal system can be found from the inside out. As the stretching proceeds, the diffraction signals along the meridional direction gradually weaken, while the diffraction rings along the equatorial direction increase gradually, forming diffraction arcs and finally transforming into diffraction points.

Fig 9  In situ 2D WAXD patterns of UPE-dt200 and UPE-dt133 stretched at different temperatures: (a) UPE-dt200st100, (b) UPE-dt200st110, (c) UPE-dt200st120, (d) UPE-dt200st130, (e) UPE-dt133st100, (f) UPE-dt133st110, (g) UPE-dt133st120, (h) UPE-dt133st130.

Download:  Full-size image | High-res image | Low-res image

Fig. 10 shows the crystallinity, the lateral size and orientation changes of the (110) crystal plane of UPE-dt200 and UPE-dt133 stretched at different temperatures. Before hot stretching, the crystallinity of UPE-dt133 was consistently higher than that of UPE-dt200, indicating that the reserved shish crystals acted as nucleation points for crystal growth during the isothermal process. Additionally, before the hot stretching, higher crystallinity was observed at 100 and 110 °C, which were more conducive to crystal growth during the isothermal process.^[

Fig 10  (a, b) Crystallinity, (c, d) (110) crystalline plane lateral size and (e, f) orientation changes for UPE-dt200 and UPE-dt133 stretched at different temperatures.

Download:  Full-size image | High-res image | Low-res image

In the preceding SAXS results, we investigated that the faster increase of Iq₁/Iq₂ for UPE-dt133 than UPE-dt200 stretched at different temperatures was attributed to the accelerated transformation of crystals along the stretching direction because of reserved shish crystals. This finding is further supported by the WAXD results. Figs. 10(e) and 10(f) show the orientation changes of (110) crystal plane along the stretching direction for UPE-dt133 and UPE-dt200 stretched at different temperatures. During the initial stages of stretching, the orientation increased continuously, while in the later stages of stretching, it approached a plateau. Notably, the orientation reached values greater than 0.9, which indicates a highly oriented crystal structure. Before reaching a strain of 200%, the UPE-dt133 showed a faster orientation change than UPE-dt200 at all stretching temperatures except 100 °C. This indicates that the reserved shish crystals can induce crystal orientation along the stretching direction during the early stages of stretching, and the lower orientation observed for UPE-dt133 stretched at 100 °C is attributed to the lower temperature, which has a decreased mobility of molecular chains compared to 110−130 °C, resulting in a slower induction of crystal orientation by reserved shish crystals. While reaching a strain of 200%, UPE-dt133st100 exhibited a faster increase in orientation compared to UPE-dt200st100, which is due to the crystals growing around the reserved shish crystals gradually aligned along the stretching direction. Furthermore, UPE-dt133 reached a highly oriented crystal structure faster than UPE-dt200 stretched at different temperatures. And UPE-dt133 can reached a highly oriented crystal structure faster when stretched at 120 and 130 °C because of the increased mobility of molecular chains. The WAXD results indicate that stretching at 120 and 130 °C resulted in faster evolution for shish-kebab structures compared to 100 and 110 °C, and the crystals aligned more easily along the stretching direction. This further confirms that the reserved shish crystals can induce the formation and evolution of shish-kebab crystals, particularly when stretched at 120 and 130 °C. In contrast, the UHMWPE high-entangled film with reserved shish crystals prepared by compression molding exhibit limited orientation. In some cases, they even show lower orientation than films without reserved shish crystals.^[

To provide a clearer description of the structural evolution in UHMWPE low-entangled films with reserved shish crystals stretched at different temperatures, Fig. 11 shows the SEM results of UPE-dt200 and UPE-dt133 before stretching, stretch to 100%, and 300%. Before hot stretching, the UPE-dt200-0% shows a morphology with disordered lamellae, while the UPE-dt133-0% shows a morphology with shish-kebab crystals. UPE-dt133-100% showed shish-kebab crystals and gradually aligned along the stretching direction when stretched at different temperatures. Conversely, UPE-dt200-100% mainly showed lamellae arrangements, which indicates that the reserved shish crystals induced the formation of shish-kebab crystals during the initial stages of stretching. UPE-dt133-300% shows highly oriented shish-kebab structures when stretched at different temperatures, some of these well-oriented shish-kebab structures are gradually transformed into shish crystals. In contrast, UPE-dt200-300% shows that the shish-kebab crystals along the stretching direction are not as uniform as those in UPE-dt133-300%. The SEM results show that stretching at 120 and 130 °C resulted in more orderly alignment of crystals along the stretching direction compared to stretching at 100 and 110 °C, which is consistent with the SAXS and WAXD results. The average long period and lamellae thickness of crystals measured from lots of SEM images and the results are shown in Table S1 in the electronic supplementary information (in ESI), which further indicates that the reserved shish crystals can promote the formation of well-organized shish-kebab crystals, and the shish-kebab crystals have a tendency to transform into shish crystals when stretched at 120 and 130 °C.

Fig 11  SEM morphologies of UPE-dt200 and UPE-dt133 stretching to 100% and 300% at different temperatures. (a) UPE-dt200-0%, (b) UPE-dt133-0%, (c₁) UPE-dt200st100-100%, (d₁) UPE-dt200st110-100%, (e₁) UPE-dt200st120-100%, (f₁) UPE-dt200st130-100%, (g₁) UPE-dt133st100-100%, (h₁) UPE-dt133st110-100%, (i₁) UPE-dt133st120-100%, (j₁) UPE-dt133st130-100%, (c₂) UPE-dt200st100-300%, (d₂) UPE-dt200st110-300%, (e₂) UPE-dt200st120-300%, (f₂) UPE-dt200st130-300%, (g₂) UPE-dt133st100-300%, (h₂) UPE-dt133st110-300%, (i₂) UPE-dt133st120-300%, (j₂) UPE-dt133st130-300%. Horizontal direction is the stretching direction.

Download:  Full-size image | High-res image | Low-res image

The above results indicate that the reserved shish crystals can accelerate the structural evolution of UHMWPE films. To further confirm the effect of reserved shish crystals on UHMWPE low-entangled films, Figs. 12(a) and 12(b) show the DSC curves for initial films and films stretched to different strains at 100, 110, 120 and 130 °C, respectively. The DSC curves of films with reserved shish crystals (dark curves) show a significant shift towards a higher temperature peak compared to those without shish crystals (light curves) at different temperatures from 100% to 300%. This indicates that the reserved shish crystals can accelerate the evolution from lamellae to shish-kebab crystals during the hot stretching. Additionally, the DSC results also show that the melting peaks are broader when stretched at 120 and 130 °C compared to 100 and 110 °C, which is also caused by the increased mobility of molecular chains when stretched at 120 and 130 °C, which allows more molecular chains to participate in the crystal growth and evolution, resulting in more shish-kebab crystals transform into highly-oriented shish crystals that show higher temperature peak. Compared to the UHMWPE film with reserved shish crystals prepared by compression molding,^[

Fig 12  DSC curves of UPE-dt200 and UPE-dt133 stretched to different strains at different temperatures and original UHMWPE films. (a) 100 and 110 °C; (b) 120 and 130 °C.

Download:  Full-size image | High-res image | Low-res image

Figs. 13(a) and 13(b) show the breaking strength and Young's modulus of UPE-dt133 and UPE-dt200 after stretching at different temperatures. It can be seen that UPE-dt133 stretched at different temperatures shows higher breaking strength and Young's modulus compared to UPE-dt200. This indicates that the reserved shish crystals give a significant improvement in the mechanical properties of the films, which is consistent with the above conclusion that the reserved shish crystals can induce the structural evolution of UHMWPE gel films. Additionally, the breaking strength and Young's modulus show a higher value when stretched at 120 and 130 °C compared to 100 and 110 °C because of the shish-kebab crystals and the oriented lamellae show a greater alignment along the stretching direction due to the increased mobility of molecular chains, which result in a more organized crystal structure after the hot stretching. Compared to the UHMWPE high-entangled films with reserved shish crystals prepared by compression molding,^[

Fig 13  (a) Breaking strength and (b) Young’s modulus after stretching at different temperatures for UPE-dt200 and UPE-dt133.

Download:  Full-size image | High-res image | Low-res image

The results of SAXS/USAXS/WAXD, SEM, DSC, breaking strength and Young's modulus stretched at different temperatures indicate that the reserved shish crystals can accelerate the formation and evolution of shish-kebab crystals, and the increased mobility of molecular chains can further promote the structural evolution of shish-kebab crystals. Fig. 14 shows the mechanism of structural evolution of UHMWPE low-entangled films with reserved shish crystals during hot stretching. Before crystallization, there are molecular chains and reserved shish crystals in UPE-dt133. During the crystallization, these reserved shish crystals act as nucleation to induce the growth of shish-kebab crystals, as a result, UPE-dt133 shows shish-kebab crystals and lamellae before stretching, while UPE-dt200 only shows disordered lamellae. In the middle stages of stretching, UPE-dt200 shows a gradual alignment of lamellae along the stretching direction when stretched at 100 and 110 °C, and the lamellae tend to align more easily along the stretching direction when stretched at 120 and 130 °C due to the increased mobility of molecular chains. As the stretching proceeds, UPE-dt200 shows evolution from lamellae to shish-kebab crystals with size reduced, and the disentangled molecular chains promote the formation of new short shish-kebab crystals, while stretching at 120 and 130 °C, in addition to the evolution from oriented lamellae to shish-kebab crystals, some highly oriented shish-kebab crystals gradually transform into highly oriented shish crystals. In contrast, in the middle stages of stretching, UPE-dt133 shows that the shish-kebab crystals growing from the reserved shish crystals gradually align along the stretching direction with lamellae oriented when stretched at 100 and 110 °C, while stretched at 120 and 130 °C, the shish-kebab crystals and the oriented lamellae show greater alignment along the stretching direction due to the increased mobility of molecular chains, resulting in small-sized shish-kebab crystals, and the oriented lamellae gradually transformed to shish-kebab crystals by the effect of the alignment of shish-kebab crystals formed by reserved shish crystals along the stretching direction. As the stretching proceeds, UPE-dt133 shows further orientation of shish-kebab crystals that growing from the reserved shish crystals, and further transforming into highly oriented shish crystals. Additionally, some oriented lamellae surrounding the reserved shish crystals gradually transformed into new shish-kebab crystals. While stretched at 120 and 130 °C, these shish-kebab crystals growing from reserved shish crystals and newly formed shish-kebab crystals complete their transformation from shish-kebab crystals to highly oriented shish crystals, and the highly oriented lamellae during the middle stretching not only transform into shish-kebab crystals, but also a portion of the regular shish-kebab crystals derived from lamellae undergo further transformation into shish crystals, the transformation of shish crystals derived from retained shish crystals and lamellae leads to highly oriented and well-developed crystal structures in films after stretching at 120 and 130 °C. UHMWPE films with reserved shish crystals, prepared via gel molding, exhibit enhanced structural evolution and properties in comparison to those prepared via compression molding. The addition of paraffin oil and reserved shish crystals significantly reduces molecular chain entanglement during the processing of these films. Consequently, shish crystals align and orient more rapidly during hot stretching, resulting in a more improved shish-kebab structure as evidenced by WAXD and SEM results. Conversely, compression-molded films with reserved shish crystals show a lower content of induced shish crystals, with a gradual increase in L_shish. In contrast, the L_shish in gel-molded films initially increases and then decreases, suggesting the induction of numerous new shish-kebab crystals during hot stretching. This abundance of highly oriented shish-kebab crystals in gel-molded films contributes to improved thermal properties, breaking strength, and Young’s modulus. Specifically, the melting point of gel-molded films after stretching is higher (142 °C to 143 °C) compared to compression-molded films (138 °C to 139 °C), with a 150% increase in breaking strength and a 600% increase in Young's modulus.^[

Fig 14  Structural evolution mechanism of UHMWPE low-entangled films with and without reserved shish crystals stretched at different temperatures: (a) UPE-dt200-before crystallization, (b) UPE-dt200-100°C, 110 °C pre-stretching, (c) UPE-dt200-100°C, 110 °C mid-stretching, (d) UPE-dt200-100°C, 110 °C post-stretching, (e) UPE-dt200-120°C, 130 °C pre-stretching, (f) UPE-dt200-120°C, 130 °C mid-stretching, (g) UPE-dt200-120°C, 130 °C post-stretching, (h) UPE-dt133-before crystallization, (i) UPE-dt133-100°C, 110 °C pre-stretching, (j) UPE-dt133-100°C, 110 °C mid-stretching, (k) UPE-dt133-100°C, 110 °C post-stretching, (l) UPE-dt133-120°C, 130 °C pre-stretching, (m) UPE-dt133-120°C, 130 °C mid-stretching, (n) UPE-dt133-120°C, 130 °C post-stretching.

Download:  Full-size image | High-res image | Low-res image