Evaluation of the Puncture Behavior, Tear Behavior, and Essential Work of Fracture of iPP Film with Continuous and Ordered β‑Transcrystallinity

Abstract

This study investigates the puncture and tear fracture behavior of isotactic polypropylene (iPP) films with an ultrahigh fraction of continuous and ordered β-transcrystallinity (β-TC), in comparison with α-spherocrystal, α-transcrystallinity, and β-spherocrystal films. In puncture tests, where the loading is applied perpendicular to the film surface, the β-TC film withstood a displacement exceeding 10 mm without rupture, absorbing substantially more energy than the other films. In tear tests, where the loading is applied parallel to the film surface, the β-TC film displayed pronounced ductile tearing characteristics, including significant yielding and strain-hardening, indicative of superior in-plane tear resistance. Quantitative evaluation using the Essential Work of Fracture (EWF) method further revealed that the specific essential work of fracture (w_e)a measure of intrinsic fracture toughnessreached 38.229 kJ/m² for the β-TC film, representing an 81.39% enhancement over the β-spherocrystal film. These findings systematically establish a clear correlation between the ordered β-transcrystalline morphology and the multidimensional fracture resistance of iPP films, highlighting a promising strategy for the design of high-performance polymer films.

1. Introduction

Resistance to puncture and tear are crucial mechanical properties for film materials across various fields of application, including protective equipment, packaging, and separators within lithium-ion batteries, among others. − After the global spread of Covid-19, masks and protective clothing made from polymers were proven to be the effective shield to safeguard lives against potential viruses. − Packaging films for goods and materials are constantly subjected to potential puncture and tear from both external and internal forces during transit. − Polyolefin separators between the positive and negative poles are perpetually at risk of being penetrated by the dendritic deposition of lithium metal, which can lead to a short circuit and potentially cause an explosion within the battery. − However, most current research on the characterization of mechanical performance focus on conventional methods such as tensile tests, shear tests, and bending tests, among others. − The mechanical properties that closely related to practical applications, such as puncture resistance and tear resistance, are seldom evaluated and reported for film materials exhibiting plastic behavior. The fundamental mechanisms linking the material’s microstructure to puncture and tear behavior require further exploration and elucidation.

Regarding puncture behavior, it has been reported that Leslie et al. tested the needle puncture resistance of some protective gloves and glove liners made from elastomer materials, aiming to compare the resistance performance to needle penetration of the glove products. Dolez and colleagues investigated the puncture resistance of protective clothing against medical needles and proposed a step-by-step analysis of the puncture mechanism of the film material from a qualitative perspective. , Additionally, Wilson-Fahmy and his team conducted a series of studies on puncture protection geomembrane materials, emphasizing that the thickness and structure of the material are crucial for protection. , Nguyen et al. investigated the resistance to puncture of elastomer materials used in protective clothing and found that the puncture behavior with conical probes was controlled by local deformation and the failure strain. In the limited number of studies on puncture resistance, most focused on elastomer materials with elastic deformation, lacking a specialized study on the puncture properties of ductile film materials with plastic deformation.

Regarding tear behavior, various research groups have conducted a series of experiments to ascertain the relationship between the tear resistance of polyolefin films and the material’s crystalline structure. − A.C. Chang et al. investigated the deformation process associated with ductile tear behavior in blown films made of polyethylene blended with polypropylene, demonstrating that the lamellar morphology near the propagating crack significantly influences the tear resistance of the blown film. R.-Y. Wu and his team investigated the tear resistance of multilayered plastic films and discovered that it was closely related to the large plastic deformation zone near the crack area. Furthermore, George et al. characterized the tear behavior of isotactic polypropylene and nitrile rubber blends and observed that the increase in tear strength was largely dependent on the morphology of the blends. Chang and his colleagues conducted research on the tear strength of ductile polyolefin films, convincingly demonstrating that the tear performance is significantly influenced by process conditions, which affect crystalline morphology and lamellae orientation. In fact, while the factors influencing tear behavior had been largely addressed by previous researchers, little effort had been made to create new materials by applying these scientific principles.

Previously in our laboratory, a novel kind of isotactic polypropylene (iPP) film which almost purely constructed of β-transcrystallinity (β-TC), namely β-TC film, was successfully manufactured. , The most significant characteristic of this material was its continuous and highly ordered crystalline morphology, in which the β-lamellae exhibited an obvious oriented alignment. Our previous works have primarily focused on the crystal formation mechanisms and pore-forming capabilities of the film, and have confirmed the orderly arrangement and regular morphology of β-lamellae. Moving beyond our previous work, this study reveals its superior puncture and tear resistance and, through quantitative EWF analysis, elucidates the mechanistic role of the ordered lamellar architecture in governing energy dissipation and fracture toughness. According to the literature researches listed above, the puncture and tear behavior were both closely related to the inner structure and crystalline morphology of film materials. This leads one to wonder: how does this newly developed film material perform in terms of puncture and tear resistance, and could its peculiar crystalline morphology bring about any breakthroughs in these application areas? To further explore the potential performance and application scenarios of β-TC film, it is necessary to conduct systematic research focused on the puncture and tear behavior of the film.

In this study, our custom-built puncture test apparatus and right-angle tear experiments were conducted on the newly developed β-TC film, and the results were compared with iPP films constructed of differing crystalline structures. Additionally, the method of essential work of fracture (EWF) was employed to quantitatively evaluate the fracture toughness of the β-TC film. The theoretical correlation between crystalline morphology and resistance to puncture and tear was elucidated, and the structure–property relationships of the specific film were discussed in detail.

2. Experimental Section

2.1. Materials

The raw material was consumer-grade isotactic polypropylene, specifically the F401 model, produced by Yangzi Petroleum and Chemical Co., Ltd. (Nanjing, China). It had an isotactic index of 98% and a melt flow index (MFR) of 2.9 g/10 min (at 230 °C, under a load of 2.16 kg), with a weight-average molecular weight (M _w ) of approximately 300,000. A common β-nucleating agent, TMB-5 (an aryl amide compound), was employed in this study and was kindly provided by Shanxi Province Institute of Chemical Co., Ltd. (Taiyuan, China). Additionally, an α-crystal nucleating agent, grade NΑ-11, manufactured by ADEKA Corporation (Japan), was utilized to induce the crystalline structure of the comparison samples. Furthermore, a chemical reagent known as E51 (glycidyl ether of bisphenol A) was procured from Wuxi Resin Plant Completed (Jiangsu, China), with an epoxide equivalent of 185–192 g/eq Another chemical reagent, MOCA (4,4′-methylene-bis­(2-chloroaniline)), used as a curing agent, was purchased from Changshan Beier Chemical Co., Ltd. (Zhejiang, China). These two chemical reagents were introduced to synthesize the base material of epoxy resin, for uniform dispersion of nucleating agents.

2.2. Sample Preparation

The manufacturing method for β-TC films is briefly described as follows: First, the β-nucleating agent was uniformly dispersed into the interior of solid substrates with a perfectly flat surface. Then, the prepared pure iPP film was placed between two solid substrates containing the nucleating agent to form a sandwich structure. Next, the structure was heated to melt the central iPP film, followed by cooling to 130 °C where isothermal crystallization was maintained for 30 min. During this process, the nucleating agents located at the interfaces adjacent to the iPP film induced β-crystal growth inward from both sides, forming the unique morphology of the β-transcrystallinity, as shown in Figure . Finally, the assembly was cooled to room temperature, and the solid substrates on both sides were removed to obtain the β-TC film.

1.

Schematic diagram of the manufacturing process and internal crystal structure of β-TC films. The left part shows the sandwich structure with an iPP film between two substrates contained with β-nucleating agent, and the right part denotes the magnification of a single β-transcrystallinity.

To conduct a comparative analysis of the crystalline structures of β-TC films, three distinct comparative samples were prepared based on different combinations of crystal types and crystalline morphologies. Specifically, these included: β-spherocrystal films (β-SC films), which shared the β phase but exhibited spherocrystalline morphology; α-transcrystalline films (α-TC films), composed of the α phase with transverse crystalline morphology; and α-spherocrystal films (α-SC films), composed of the α phase with spherocrystalline morphology. The nomenclature and distinctions of the four film samples are summarized and presented in Table . To eliminate any extraneous experimental variables, all four sample films were prepared using the same experimental parameters and thermal treatment process, with the exception of the nucleating agent, ensuring that the only differences among the four films were in their crystal structures and morphologies. For a more detailed description of the fabrication procedure for these four films, please refer to the Supporting Information.

1. Standardized Nomenclature, Crystalline Form and Morphology, and Nucleating Agent Content of the Four iPP Films Discussed in This Study.

nomenclature	crystal form	crystal morphology	nucleating agent content
α-SC film	α-phase	spherulite	0.1 wt % α-nucleating agent
α-TC film	α-phase	transcrystallinity	0%
β-SC film	β-phase	spherulite	0.1 wt % β-nucleating agent
β-TC film	β-phase	transcrystallinity	0%

2.3. Characterization and Test

2.3.1. Puncture Test

The puncture behavior of film samples was characterized using our custom-built puncture device, detailed instruction can be seen in Figure S1 of the Supporting Information. Before initiating the puncture test, all the films were meticulously cut into round specimens with a diameter slightly less than 30 mm using a cutter. Subsequently, the specimens were placed between two clamps of the device. The clamps were attached to an electronic universal tensile machine (SANS CMT4104, MTS Systems Co. Ltd., China) via a connecting base, and the sensor was set to a capacity of 500 N. At the commencement of the puncture test, the steel probe affixed to the connector was positioned as close to the film surface as possible without making direct contact. The compression mode was then selected to regulate the probe’s downward movement at a speed of 50 mm/min. The puncture tests were conducted on each kind of film utilizing three different steel needles with diameters of 1 mm, 2 mm, and 4 mm; all other experimental procedures and conditions remained consistent. In practice, each kind of film underwent testing with five samples under identical conditions.

2.3.2. Tear Test

Tear behavior of the film samples was characterized using the right-angle tear test method, which is based on the standard of QB/T 1130–91. For specific experimental setup and sample dimensional details, please refer to Figure S2 in the Supporting Information. Prior to the initiation of the experiment, the samples to be tested were first cut to the same size using a right-angle tear cutter and then fixed with a clamp on an electronic universal tensile tester (SANS CMT4104, MTS Systems Co. Ltd., China). The experiment was conducted in tensile mode with a 500 N sensor and a controlled clamp movement speed of 20 mm/min. During the experiment, the load of the sensor and the displacement of the clamp were recorded in real-time by the supporting computer software. According to the test criterion, the right-angle tear strength (noted as T_s) was calculated as follows.

$${\mathrm{T}}_{\mathrm{s}}=\frac{\mathrm{F}}{\mathrm{d}}$$ 1

The parameter F in the formula denotes the maximum load in Newtons (N) recorded by the sensor during the right-angle tear test, whereas the parameter d represents the thickness of the sample film to be tested in millimeters (mm). For each sample, five parallel tests were conducted, and the average value was computed using the aforementioned formula.

2.3.3. The Essential Work of Fracture (EWF) Test

The EWF test is a method that enables quantitative analysis of material toughness and serves as a critical indicator for characterizing the material’s resistance to crack damage. − To assess the resistance of iPP films using this method, the film was initially shaped into a deeply double-notched specimen, known as the DDENT specimen, whose characteristics can be found in Figure S3. of the Supporting Information. In practice, a series of DDENT specimens with different ligament lengths (l) were prepared for each of the four types of films, and then were subjected to tensile stress.

Since the EWF method requires that the ligament lengths of the specimens satisfy the condition 3t < l < W/3, after calculation, the values of l for the DDENT specimens of each film were set to 1, 2, 3, 4, 5, and 6 mm. The subsequent test to determine the essential work of fracture was conducted using an electronic universal tensile tester (Instron 5567, Instron Corporation, USA) equipped with a 500 N sensor. The test was carried out in tensile mode with a controlled clamp movement speed of 5 mm/min and an experimental temperature of 25 °C. During the test, the sensor load and displacement distance were recorded in real-time by computer software. Five parallel DDENT specimens were prepared for each ligament length, and testing was conducted under identical conditions to minimize experimental error.

2.3.4. Polarized Optical Microscopy (POM)

The film samples were placed under a polarizing microscope (BX51, Olympus Corporation, Japan) to observe their crystal structures at 100–200× magnification. The microscope was equipped with a digital imaging camera and a crossed polarizer. To more visually observe the difference in crystal morphology of different film samples, the optimal observation is conducted from the cross-sectional direction of the film. Consequently, the sample to be observed must be sliced and sectioned from the side of the film and fixed on a slide with a thickness of 20 μm using a microtome (YD-2508, Jinhua Yidi Medical Equipment Co., Ltd., China).

2.3.5. Two-Dimensional Small-Angle X-ray Scattering Measurement (2D-SAXS)

To examine the microscopic structural changes of the film samples, the destruction areas were characterized using a two-dimensional small-angle X-ray scattering device (Xeuss 2.0 system, Xenocs Scientific Instruments Technologies, France). The X-rays in the instrument were generated from Cu Kα rays produced by a high-energy electron stream bombarding a Cu target, using a GeniX3D CuULD (Xenocs SA, France), with a wavelength of 0.154 nm. The system was equipped with a scatter-free slit technique, which allows the size of the X-ray spot on the sample surface to be controlled within 0.5 × 0.5 mm² to accurately characterize small deformation areas. The scattered X-ray signal was collected by a dedicated detector (Pilatus 300 K, Dectris, Switzerland) with an area of 680 × 680 pixels and an average pixel size of 0.172 nm. The source-detector (SD) distance between the sample and the detector was 2500 mm, with an exposure time of 300 s per sample.

2.3.6. Two-Dimensional Wide-Angle X-ray Diffraction Measurement (2D-WAXD)

To examine the changes in the crystalline structure of the film samples before and after the tests, and to explain the differences in their properties in terms of aggregation states, the samples were tested using a two-dimensional wide-angle X-ray diffractometer (D8 DISCOVER, Bruker, Germany), which was also equipped with a two-dimensional surface detector (VANTEC 500). The specific instrument settings for the test procedure were in transmission mode with a scanning voltage of 40 kV, a scanning current of 40 mA, a scanning angle range of 5°–45°, a scanning rate of 0.03°/s, and a step size of 0.02°.

3. Results and Discussion

3.1. Crystal Morphology of β-TC Film

To emphasize the crystal morphology of β-TC iPP film and to discuss the crystalline similarities and differences among other films more intuitively, polarized micrographs of four types of films were presented side by side in the cross-sectional direction, as shown in Figure . Despite being made from the same material of isotactic polypropylene, the different processing methods resulted in distinct crystalline structures.

2.

Cross-sectional polarized optical microscopy (POM) images of α-SC film (a), α-TC film (b), β-SC film (c), and β-TC film (d), revealing their distinct crystalline morphologies.

The lateral crystalline morphology of the α-SC film containing an α-phase nucleating agent is shown in Figure (a). Numerous tiny α-spherulites, induced by the high-density nucleating agent particles via heterogeneous nucleation, are randomly dispersed throughout the crystal region. The lamellae growing from adjacent nuclei impinge on one another before full development, resulting in clusters of fine, dust-like spherulites. In contrast, the α-TC film presented in Figure (b) exhibits a transverse crystalline morphology, with the lamellae growing perpendicular to the film surface. Although its overall morphology resembles that of the β-TC film, it consists of α-phase transcrystallinity rather than β-phase.

For the β-phase films, the β-SC film (Figure c) displays spherulites comparable in number and size to those in the α-SC film, with the primary distinction being the crystal formthe β-spherulites exhibit a characteristic yellow color under polarized light due to their negative birefringence. Distinct from the other three films, the β-TC film, the focal point of this study (Figure d), features a bright yellow crystalline morphology with a highly regular pattern across its lateral surface. The intense yellow color arises from the strong negative birefringence of β-transcrystallinity and the high β-crystal content. Notably, the lamellae within the β-TC film are arranged in a regularly ordered and symmetric manner, with numerous transcrystalline layers connecting to form a coherent domain and nearly all oriented perpendicular to the film surface, reflecting a remarkable degree of structural uniformity.

By examining the distinct crystalline morphologies and lamellar arrangements of the films, it can be inferred that different internal structures will inherently affect the mechanical properties and fracture behavior. In fact, the fundamental mechanical properties of the four types of films were systematically tested, in which the β-TC film exhibited superior toughness. The detailed performance specifications and a brief analysis are provided in Table S1 of the Supporting Information. This paper focuses on the effects of different crystal morphologies on puncture and tear resistance, along with a quantitative analysis of fracture toughness.

Additionally, the overall crystallinity and α/β-crystal content of the film samples were determined using differential scanning calorimetry (DSC) data, with the results presented in Table . As shown in the table, the crystallinity of the four films is nearly identical, and the β-crystal content of both β-phase films exceeds 90%. Consequently, the experimental variables for the four film samples are limited to differences in crystal form and crystal morphology.

2. Crystallinity and α/β-Crystal Fraction of the α-SC, α-TC, β-SC, and β-TC Films .

specimens	X c (%)	F α(%)	F β (%)
α-SC film	57.6 ± 1.6	100	0
α-TC film	55.3 ± 2.1	100	0
β-SC film	56.5 ± 2.3	9.3 ± 0.6	90.7 ± 0.6
β-TC film	58.3 ± 1.5	7.2 ± 0.8	92.8 ± 0.8

^aAll films underwent the same isothermal crystallization process

3.2. Puncture Behavior and Mechanism of β-TC Film

Film materials frequently experience transverse shear stress and longitudinal compressive stress during practical service. Among these, the longitudinal compressive stressesespecially under puncture loadingposes the greatest challenge. Therefore, maintaining material performance and preventing failure under such conditions throughout the material’s lifecycle requires high puncture resistance.

With four types of film samples subjected to puncture tests using probes of different diameters, the key parameters of puncture strength and puncture energy have been determined from the load–displacement curves. Specifically, the puncture strength is calculated as the peak load divided by the sample thickness, and the puncture energy as the area under the curve divided by the sample thickness.

The load–displacement curves obtained from puncture tests on the four film samples using probes of different diameters are shown in Figure . With a 1 mm probe, the film samples with different crystalline morphologies exhibited no significant differences in their puncture response, and all fractured within 1–3 s after probe contact, as shown in Figure (a). However, the curves show that the two α-phase films fractured shortly after yielding, whereas the two β-phase films underwent a prolonged plastic deformation stage after yielding before penetration. Due to the small probe diameter, no strain hardening was observed in any of the film samples after yielding. When the probe diameter was increased to 2 mm, distinct puncture failure modes were observed among films with different crystalline morphologies, as shown in Figure (b). Specifically, the two α-phase films did not exhibit significant yielding behavior; instead, the load continued to increase until both films fractured at their peak loads. In contrast, the two β-phase films exhibited significant yielding, with the β-TC film undergoing a prolonged plastic deformation process after yielding.

3.

Representative load–displacement curves obtained during puncture tests of α-SC, α-TC, β-SC, and β-TC films tested with steel probes of 1 mm (a), 2 mm (b), and 4 mm (c) in diameter, respectively. The shaded area under each load–displacement curve represents the total energy absorbed during puncture.

When the probe diameter reached 4 mm, the load–displacement curves of the film samples displayed significant differences, as shown in Figure (c). The two α-phase films still exhibited brittle fracture similar to that observed with the 2 mm probe, with virtually no yielding. Meanwhile, the β-SC film exhibited a typical plastic deformation process after yielding. Notably, the β-TC film exhibited markedly higher toughness. This sample displayed two yielding events. The first yielding was similar to that of the β-SC film, which could be attributed to their shared β-crystal form and similar crystallinity. Subsequently, a strain-hardening stage followed the first yielding, during which the puncture strength increased until the second yielding. Based on the POM images presented above, it is plausible that the distinct puncture behavior of the β-TC film is closely associated with its unique transcrystalline morphology and the ordered arrangement of β-lamellae.

Integration of the load–displacement curves up to a given displacement gives the area under the curve, which corresponds to the puncture energy absorbed at that displacement. The resulting energy–displacement curves are presented in Figure . With a 1 mm probe, as shown in Figure (a), the two α-phase films absorbed relatively limited puncture energy, whereas the two β-phase films absorbed more energy, particularly the β-TC film. At a probe diameter of 2 mm, as shown in Figure (b), the energy absorption trend was similar to that at 1 mm; however, the β-TC film again exhibited the highest puncture energy, suggesting that its transcrystalline structure may begin to contribute to enhanced toughness and puncture resistance as the probe diameter increases. When the probe diameter reached 4 mm, as shown in Figure (c), the advantage of the β-TC film’s unique crystalline structure became evident: it absorbed more than twice the puncture energy of any other film, demonstrating substantially higher puncture resistance. The comprehensive experimental data, including puncture strength, puncture displacement, and puncture energy, are summarized in Table .

4.

Representative energy-displacement curves obtained during puncture tests of α-SC, α-TC, β-SC, and β-TC films tested with steel probes of 1 mm (a), 2 mm (b), and 4 mm (c) in diameter, respectively.

3. Specific Values of Puncture Strength, Puncture Energy, and Puncture Displacement for α-SC, α-TC, β-SC, and β-TC Films Tested with Steel Probes of 1, 2, and 4 mm in Diameter.

puncture test	puncture strength (kN/m)			puncture energy (mJ)			puncture displacement (mm)
diameter	1 mm	2 mm	4 mm	1 mm	2 mm	4 mm	1 mm	2 mm	4 mm
α-SC film	37.7 ± 0.2	111.8 ± 0.8	264.7 ± 1.4	7.2 ± 0.1	35.2 ± 0.7	116.2 ± 2.3	1.55 ± 0.04	2.59 ± 0.05	4.04 ± 0.07
α-TC film	34.8 ± 0.4	105.1 ± 0.7	251.8 ± 1.5	6.7 ± 0.1	37.2 ± 0.8	167.9 ± 3.1	1.62 ± 0.05	2.79 ± 0.06	5.47 ± 0.10
β-SC film	23.7 ± 0.3	59.5 ± 0.6	147.7 ± 1.1	9.0 ± 0.2	31.1 ± 0.8	193.3 ± 4.5	2.13 ± 0.05	2.82 ± 0.06	6.83 ± 0.12
β-TC film	23.3 ± 0.2	58.4 ± 0.6	156.6 ± 1.2	10.9 ± 0.3	45.4 ± 1.1	491.7 ± 9.8	2.66 ± 0.06	3.83 ± 0.08	13.05 ± 0.17

The postpuncture morphology of the film samples could be observed by the extent of plastic deformation in the cross-section. With a probe diameter of 4 mm, the piercing pressure was sufficiently moderate to clearly reveal the influence of crystal form and morphology on the puncture properties. As shown in Figure , the plastic deformation after the puncture test differed significantly among the samples, and the characteristics correlated well with the behavior in the energy-displacement curves presented above.

5.

Representative photographs showing the lateral fracture morphology of α-SC film (a), α-TC film (b), β-SC film (c), and β-TC film (d) after puncture testing with a 4 mm diameter probe. The α-SC (a) and α-TC (b) films exhibit typical brittle fracture, whereas the β-SC (c) and β-TC (d) films display extensive ductile fracture.

The puncture fracture behavior revealed that both the α-SC and α-TC films exhibited typical brittle fracture characteristics: the probe-induced bulges remained unpenetrated at the apex but fractured circumferentially at the base. The extent of plastic deformation throughout this process was notably limited. In contrast, the β-SC and β-TC films underwent fracture by the piercing of bulges at their apex after reaching the limit of plastic deformation. Notably, as shown in Figure (d), the β-TC film sustained a puncture displacement exceeding 10 mm under the same loading condition, highlighting its exceptional puncture resistance, which can be attributed to the continuous and highly ordered arrangement of β-lamellae.

To understand the outstanding puncture resistance of the β-TC film at the microscopic level, 2D-SAXS was employed to examine the crystalline orientation in the puncture region, as shown in Figure . As the probe diameter increased, the scattering pattern of the β-TC film gradually transformed from a broad diamond shape into a markedly narrower one, indicating that the crystalline orientation in the punctured region became increasingly pronounced. This can be attributed to the fact that a larger probe diameter engages a greater volume of β-transcrystallinity, which likely enables more lamellae to dissipate puncture energy through structural rearrangement. This deformation-induced alignment is indicative of the toughness enhancement associated with the ordered and continuous β-transcrystallinity. Consequently, the enhanced crystalline orientation likely contributed to the increased plastic deformation, which in turn helps to explain the substantial enhancement in puncture energy of the β-TC film relative to the other films.

6.

Photographs of the failure morphology of β-TC film after puncture testing with probes of 1 mm, 2 mm, and 4 mm in diameter, and the corresponding 2D-SAXS patterns acquired from the plastically deformed region around the puncture hole.

Taken together, these results demonstrate the superior puncture resistance of the β-TC film. Although no significant increase in puncture strength was observed compared with the other films, the puncture energya more direct indicator of toughnessexhibited a more than 2-fold enhancement, particularly against less sharp penetrators, which are more representative of common puncture threats in packaging and protective applications. This behavior implies that packaging or protective materials based on β-TC film can more effectively resist puncture by dissipating energy through extensive plastic deformation without rupture, thereby better preserving the integrity of the contents and preventing contamination from external agents.

3.3. Tear Behavior and Mechanism of β-TC Film

Polymer films often experience transverse shear stresses and longitudinal compressive stresses in practical applications, where tearing is the most common failure mode induced by transverse shear stresses. Consequently, it is necessary to examine the tearing behavior of iPP films and assess the impact of different crystalline morphologies on tear resistance in the transverse direction. Subsequently, the tear behavior of the film samples was studied using the standard right-angle tear tests.

In the right-angle tear test, the primary indicators were tear strength and tear energy, which characterized the material’s tear resistance in terms of force and work, respectively. Tear strength was calculated using eq , in which the effect of specimen thickness was eliminated, based on the load–displacement curve shown in Figure . Tear energy was defined as the work consumed by the specimen during the tearing process, serving as a measure of the material’s fracture toughness and its capacity for plastic deformation.

7.

Representative load–displacement curves obtained during tear tests of α-SC, α-TC, β-SC, and β-TC films, respectively. The shaded area under each load–displacement curve represents the total energy absorbed during the tear process.

From the load–displacement curves, it is evident that both α-phase films displayed a rapid and abrupt fracture behavior. This is characterized by a rapid increase in load to a peak, followed by a sharp decline and fracture, without any significant yielding. Given the tightly stacked lattice structure and cross-hatched lamellae morphology of α-crystals, it can be inferred that the films made of α-phase crystals are relatively less ductile due to the limited movement of the lamellae and molecular chains when resisting tear failure. In contrast, the β-phase films, despite showing a decrease in the maximum load during tearing, exhibited improved deformation behavior beyond the elastic stage. The β-phase films all exhibited a more prolonged plastic deformation and a notable yielding behavior in the curve. Furthermore, the β-TC film exhibited strain-hardening behavior after yielding, as evidenced by a gradual increase in load with continued displacement. This phenomenon can be attributed to the progressive adjustment of the regularly aligned lamellae within the β-TC film, which likely transform into fibrous structure as the deformation proceeds.

As shown in Figure , the tear energy absorbed by the films during the right-angle tear test and how the energy absorption evolved as tearing progressed. The slope of the curves indicated that the two α-phase films exhibited greater tear strength at fracture. However, due to their limited failure displacements, the area under the load–displacement curve was smaller, resulting in less energy absorbed during the tearing process. This is consistent with their abrupt fracture behavior. In contrast, the damage zone near the tip of the β-SC film transformed into a continuous yield region as the crack laterally expanded. This plastic deformation could lead to an increase in the failure displacement of the film during tearing, thereby allowing it to absorb more tear energy.

8.

Full-range energy–displacement curves from the tear tests of α-SC, α-TC, β-SC, and β-TC films, with the region enclosed by the dashed rectangle enlarged in the inset for clarity.

For the β-TC film, the area under the load–displacement curve was significantly larger than that of the other films, indicating a pronounced ductile tearing behavior and absorbing several times more energy during the measurement. Moreover, the tear energy of the β-TC film exhibited a linear increase with increasing displacement, suggesting that progressive microstructural changes within the film were involved in the strain hardening process. This is likely because the internal lamellae of the β-TC were arranged with regularity prior to the onset of tearing. The abundance of continuous and ordered lamellae enabled the tearing energy to be gradually absorbed through steady structural evolution during the tearing process, thereby contributing to its exceptional tearing resistance. Further experimental details of the tear test are summarized in Table .

4. Specific Values of Tear Strength, Tear Energy, and Tear Elongation at Break for α-SC, α-TC, β-SC, and β-TC Films Obtained from the Right-Angle Tear Test.

specimens	tear strength (kN/m)	tear energy (mJ)	elongation at break (%)
α-SC film	159.2 ± 0.9	101.7 ± 1.1	8.18 ± 0.07
α-TC film	123.8 ± 1.1	172.2 ± 1.7	14.5 ± 0.12
β-SC film	75.3 ± 0.8	246.5 ± 2.5	25.99 ± 0.29
β-TC film	126.9 ± 1.3	1903.6 ± 15.9	171.78 ± 1.85

From the perspective of failure morphology analysis, it was observed that the distinct crystalline composition and morphology of the films resulted in significant variations in failure patterns. The experimental results shown in Figure indicated that the α-phase films exhibited brittle fracture, primarily at the notch tips of the specimens, with negligible prior deformation. These fractures typically manifested as sharp fold lines. In contrast, the β-SC film exhibited ductile fracture, characterized by straightening of the specimen, followed by stress whitening at the notch tip and crack initiation. The cracks then propagated in a direction perpendicular to the tensile force, and ultimately, the specimens were torn, revealing a stress-whitened zone where plastic deformation had taken place at the fracture site.

9.

Representative photographs showing the front-view fracture morphology after the right-angle tear test of α-SC film (a), α-TC film (b), β-SC film (c), and β-TC film (d). The red dashed line indicates the crack path and the stress-whitened region.

Although the β-TC film also exhibited ductile fracture, its fracture mechanism differed from that of the β-SC film. The primary distinction was that after the specimen was straightened and stress whitening appeared in the middle region, cracks did not emerge at the notch tip area. Instead, the entire film continued to undergo significant plastic deformation, resulting in necking, with the central portion thinning and elongating, and ultimately forming a long strip before being pulled apart. Direct observation revealed that the tearing behavior of iPP films with different crystalline morphologies differed significantly. This not only corroborated the enhanced toughness of the β-phase but also indicated that the well-ordered lamellae in the β-TC film can further enhance this toughness.

By characterizing the middle region of the film specimens using 2D-WAXD before and after the right-angle tear test, it was clearly observed how the crystalline structure of the material varied during the tearing process, as shown in Figure . Prior to the test, it was evident that the α-SC and α-TC films exhibited characteristic diffraction rings at 2θ angles of 14.1°, 16.9°, and 18.6°, confirming that the crystal structure within both specimens was of the α-form; in contrast, the β-SC and β-TC films displayed characteristic diffraction rings at a 2θ angle of 16.1°, indicating that the crystal structure within these two specimens was of the β-form. − It was also notable that the diffraction rings of all film specimens were isotropic.

10.

2D-WAXD patterns acquired from the central region of the fracture zone before and after the right-angle tear test of α-SC film (a), α-TC film (b), β-SC film (c), and β-TC film (d). The bright diffraction rings correspond to different crystal form, and the azimuthal intensity distribution indicates the degree of crystal orientation.

After the right-angle tear test, the diffraction patterns of the α-SC and α-TC films showed little change: the positions of the diffraction rings remained unchanged, although they became slightly diffuse. This indicated that the crystalline form within the film did not transform during the tearing process, and only a small fraction of crystals was fragmented under the applied force. In contrast, the diffraction patterns of β-SC and β-TC films exhibited significant changes after the tear test. Notably, both the position and number of diffraction rings shifted from the characteristic β-form rings to those of the α-form, indicating that a crystal phase transition occurred within the plastically deformed region in the center of the specimen. Moreover, the diffraction rings changed from isotropic rings to diffraction arcs, indicating the reorganization and orientation of the lamellae within the two β-phase films. This orientational effect was more pronounced in the 2D-WAXD patterns of the β-TC film, as evidenced by its greater diffraction intensity. Taken together, the experimental results indicated that a larger number of lamellae were involved in the phase transformation within the β-TC film, and the degree of orientation after the tearing process was also higher. This was the fundamental reason for the higher tear strength and greater tear energy absorption of the β-TC iPP film.

These results demonstrate the superior tear resistance of the β-TC film, particularly in terms of tear energya key indicator of toughness. In practical applications, this means that when subjected to transverse crack propagation, the material is more likely to maintain its structural integrity rather than undergo sudden failure, thereby enhancing product safety and reliability. This characteristic is particularly valuable for flexible packaging subjected to lateral stresses during handling and transportation, where the ability to resist crack propagation helps prevent localized damage from escalating into complete package failure.

3.4. The EWF Research of β-TC Film

The EWF method originated from Broberg’s fracture theory, was proposed by Cotterell et al., and subsequently refined by Karger and Wong, and has become a standard method for quantitatively evaluating the toughness and resistance to crack extension of polymeric materials. − This method is performed using specific DDENT specimens with different ligament lengths (see Figure S3 in Supporting Information). As the test begins with the eligible specimens, cracks will form and expand in the ligament region toward the center due to the presence of notches on both sides. This crack growth continuously consumes fracture work. , In this process, according to the different causes and locations of fracture work consumption, the total fracture work can be separated into two parts: the essential work of fracture (W_e), and the nonessential work of fracture (W_p). The physical meanings of the two symbols are as follows: W_e signifies the energy required to create a new surface as the crack extends, which is involved in the inner fracture region during the crack propagation. In contrast, W_p represents the energy needed for the plastic deformation, which is dissipated in the outer region around the crack. In the theory of the EWF method, the relationship between W_e and W_p can be expressed by the following equation. ,

$${\mathrm{W}}_{\mathrm{f}}={\mathrm{W}}_{\mathrm{e}}+{\mathrm{W}}_{\mathrm{p}}={\mathrm{w}}_{\mathrm{e}}\mathrm{tl}+{\mathrm{\beta w}}_{\mathrm{p}}\mathrm{t}{\mathrm{l}}^2$$ 2

In the formula, the parameter W_f represents the total fracture work, while W_e and W_p represent the essential work of fracture and nonessential work of fracture, respectively. Since the DDENT specimens may vary in thickness during the preparation process, this variation would affect the measurement of the fracture work values. To more accurately characterize the difference in the intrinsic properties of the material, the thickness factor must be removed from the experimental results. Consequently, the specific fracture work is defined as the fracture work divided by the cross-sectional area of the specimen, which is calculated using the following equation.

$${\mathrm{w}}_{\mathrm{f}}=\raisebox{1ex}{${\mathrm{W}}_{\mathrm{f}}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{tl}$}\right.={\mathrm{w}}_{\mathrm{e}}+{\mathrm{\beta w}}_{\mathrm{p}}\mathrm{l}$$ 3

In the formula, w_f denotes the total specific fracture work, while w_e and w_p represent the specific essential work of fracture and the specific nonessential work of fracture per unit area in the ligament region, respectively. Among the other parameters, β signifies the shape factor of the plastic deformation region, t denotes the thickness of the specimen, and l denotes the ligament length.

Following puncture and tearing tests that demonstrated the exceptional fracture toughness of β-TC iPP film, a detailed analysis of the failure process was conducted to determine its essential work of fracture, thereby quantifying the influence of the internal crystalline morphology on its toughness.

Since the DDENT specimens prepared from the two α-phase films fractured in a brittle manner during testing and valid load–displacement curves meeting the standard requirements could not be obtained, the following analysis in this section focused on the essential work of fracture of the β-TC film, using the β-SC film as a comparative sample.

As shown in Figure , the load–displacement curves recorded during the test revealed that the β-TC film and its comparison specimen, the β-SC film, exhibited broadly similar trends. Overall, the curves could be divided into two stages. In the first stage, the load increased rapidly to a peak value, accompanied by significant yielding of the specimen. In the second stage, the precracks on both sides propagated stably toward the center of the specimen, and the load descended at a near steady rate with increasing displacement until the specimen fractured. A comparison of the test curves for β-TC film and β-SC film specimens at different ligament lengths revealed a high degree of similarity among the curves in each series. Specifically, the tensile modulus before and after yielding was almost the same in magnitude, whereas the yield stress, elongation at break, and the area under the curve all increased with increasing ligament length. Moreover, the β-TC film exhibited a smoother load variation during crack extension.

11.

Representative load–displacement curves from the essential work of fracture (EWF) tests on DDENT specimens of β-SC film (a) and β-TC film (b) at various ligament lengths. All curves exhibit excellent linearity, validating the applicability of the EWF method.

If the area under each curve is calculated by integration, the total work of fracture for specimens with different ligament lengths can be determined. According to eqs , the total fracture work is equal to the sum of the essential work of fracture and the nonessential work of fracture. Dividing both sides of eq by the specimen thickness and ligament length yields the specific total fracture work, w_f, which is equal to the sum of the specific essential work of fracture (w_e) and βw_p l, as shown in eq . When considered collectively as a single variable, βw_p represents the specific plastic deformation work, i.e., the energy dissipated through plastic deformation per unit volume of material in the plastic zone surrounding the crack. This parameter thus characterizes the material’s capacity for plastic deformation during crack extension. It should be noted, however, that this parameter is not an intrinsic material property; it can be influenced by the size and shape of the plastic deformation zone in the specimen, as well as by the testing conditions. Consequently, greater attention should be directed to w_e, the specific essential work of fracture, which depends only on the composition and internal structure of the material. Physically, it represents the work dissipated at the crack tip during fracture, reflecting the resistance to crack extension. This parameter can thus be used to characterize the most intrinsic fracture toughness of the material.

A series of experiments on DDENT specimens provided a set of ligament length (l) and corresponding specific total work of fracture (w_f) values. By plotting these data points and performing linear regression analysis, a straight line was obtained in accordance with eq . From the intercept and slope of this line, the specific essential work of fracture (w_e) and the specific plastic deformation work (βw_p) of the tested specimens were determined, respectively. As shown in Figure , the linear fitting results for β-SC and β-TC film specimens, revealing a clear linear relationship for each data set, with correlation coefficient (R ²) exceeding 98%. This indicates excellent agreement between the experimental data and the theoretical analysis. According to the theoretical formula, the slope of the fitted line corresponds to the specific plastic deformation work (βw_p). Extrapolating of the line to l = 0 yields the specific essential work of fracture, w_e, from the intercept on the vertical axis.

12.

Scatter plot of specific work of fracture versus ligament length and the corresponding linear fit for DDENT specimens of β-SC film (a) and β-TC film (b), derived from the curves in Figure . Both linear fits exhibit a high correlation coefficient (R ² > 0.98).

The final calculation gave the specific plastic deformation work, βw_p, of the β-TC film as 15.289, and its specific essential work of fracture, w_e, as 38.229. For the comparative β-SC film, the corresponding values of βw_p and w_e were 14.619 and 21.076, respectively. More detailed data are summarized in Table . It is evident that the specific essential work of fracture value of the β-TC film is approximately 81.39% higher than that of the β-SC film. On one hand, this suggests that the intrinsic properties of the material can be significantly enhanced through the modulation of crystalline morphology, resulting in superior fracture toughness. On the other hand, it demonstrates that the ordered lamellar structure within the β-TC film promotes more efficient energy dissipation, thereby imparting significantly enhanced resistance to crack extension.

5. Specific Total Work of Fracture (w_f), Specific Plastic Deformation Work (βw_p) and Specific Essential Work of Fracture (w_e) for β-SC and β-TC Films Obtained from the EWF Test.

specimens		β-SC film	β-TC film
specific total work of fracture w f (mJ)	l = 1 mm	32.75 ± 0.22	55.11 ± 0.36
	l = 2 mm	49.63 ± 0.35	69.06 ± 0.59
	l = 3 mm	68.13 ± 0.72	82.46 ± 0.73
	l = 4 mm	82.95 ± 0.93	96.28 ± 0.91
	l = 5 mm	95.95 ± 0.98	116.84 ± 1.05
	l = 6 mm	104.41 ± 1.01	130.69 ± 1.38
specific plastic deformation work βw p (kJ/m2)		14.619	15.289
specific essential work of fracture w e (mJ)		21.076	38.229

4. Conclusion

In this paper, we examined the performance of polypropylene β-TC film in comparison with α-SC, α-TC, and β-SC films, focusing on fracture behavior and toughness, to explore how the distinctive aggregated structure of high-content, continuous, and ordered β-transcrystallinity enhances the mechanical properties of the material. We investigated the performance of iPP films with various crystalline morphologies from multiple perspectives, including puncture resistance in the through-thickness direction and tear resistance in the in-plane direction, in line with the practical application scenarios of the films. Furthermore, we employed the essential work of fracture (EWF) method, which partitions the total work of fracture into essential and nonessential components, and conducted a quantitative analysis to assess the intrinsic toughness enhancement attributable solely to variations in crystalline morphology. The specific findings of the study are as follows:

• The puncture test revealed that the β-TC films exhibited enhanced puncture resistance with increasing probe diameter. Specifically, they exhibited an exceptional prolonged plastic deformation process after yielding, and the puncture energy absorbed was more than doubled compared with that of the comparative films. Small-angle X-ray scattering (SAXS) characterization revealed that the microstructure of the β-TC film developed a pronounced crystalline orientation in the puncture region, indicating that a substantial amount of puncture energy was dissipated through the collective movement of lamellae.

• In the right-angle tear test, the β-TC films displayed a distinctive fracture morphology, markedly different from the brittle fracture of α-phase films and even from the β-SC film with the same crystalline form. Specifically, the β-TC films exhibited an exceptionally prolonged plastic deformation and pronounced necking, and the tear energy absorbed was increased several-fold compared with that of the comparative specimens. WAXD characterization revealed that the β-to-α phase transition and the higher degree of lamellar orientation in the β-TC film during tearing dissipated a substantial amount of energy, thereby contributing to its excellent tear resistance.

• By decomposing the total work of fracture into the essential work of fracture, which is determined by the internal structure of the material, and the nonessential work of fracture, which is associated with extrinsic plastic deformation, it was calculated that the essential work of fracture of the β-TC film was enhanced by 81.39% compared to the β-SC film. Given that the two films share the same crystalline form, this significant enhancement in intrinsic fracture toughness can be primarily attributed to their different crystalline morphologies, particularly the ordered lamellar arrangement in the β-TC film.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acspolymersau.6c00075.

• Detailed preparation process for the film samples, instructions for the custom-built puncture device, illustration of the tear test apparatus, geometry of the DDENT specimen, fundamental mechanical properties of the films (PDF)

The authors declare no competing financial interest.