Influence of thermally induced self‐assembly shish‐kebab crystal on charge transport behaviour in polypropylene/elastomer blends

Funding information National Nature Science Foundation of China, Grant/Award Numbers: 51677127, 51677128, 51707132, 51707133 Abstract This paper reports on the influence of thermally induced self-assembly shish-kebab crystal on charge transport behaviour in polypropylene (PP) and PP/propylene-based elastomer (PBE) blends. The film samples with the shish-kebab crystal were prepared by adding a β nucleating agent TMB-5 under thermally induced self-assembly. Polarised optical microscope (POM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were performed to understand the crystallisation characteristics of samples. Carrier trap distribution was analysed by isothermal surface potential decay (ISPD) method, and DC breakdown strength measured as well. The decrease of shish-kebab crystal size, the reduction of trap level, the increase of shallow trap density and the decrease of deep trap density were obtained with the TMB-5 content. The hopping distance of charges decreased with the shallow trap density increasing. The DC breakdown strength for PP/PBE/TMB-5 was higher than that for PP/TMB-5. It is suggested that deep traps are formed on the clear shish-kebab crystal boundaries, while shallow traps are induced by the elastomer and the un-crystallisation nucleating agents. Both the carrier trap formation on the crystal boundary and the physical channel orientation of the shish-kebab crystals affect the charge transport behaviour.


INTRODUCTION
Polypropylene (PP) is widely considered as a promising candidate for high voltage direct current (HVDC) cable insulation due to its excellent electrical properties, thermal properties and recyclability [1,2]. Propylene-based elastomer (PBE), which has good compatibility with PP, is selected to modify the stiffness of pure PP [3,4]. However, the electrical performance of PP/PBE blend degrades with the addition of elastomer.  [5][6][7][8]. However, the large concentration of nanoparticles could lead to the agglomeration, which introduces the physical defects into the insulation. The application of these nanocomposites has been seriously limited found that a small amount of β nucleating agent added into PP/elastomer blend could be well-dispersed and improve the electrical properties [9][10][11]. Accordingly, the influence of the crystallisation characteristics of materials on their electrical properties is widely concerned. It has been reported in our previous work that the shish-kebab crystal structure could be formed in PP/PBE blends expect for the ordinary spherulitic crystal structure [12]. The samples with shish-kebab crystal performed better insulation properties. Hence exploring the shish-kebab crystal formation process and its influence on the electrical properties is a matter of cardinal significance. Charge transport behaviour has attracted much attention in the research field of electrical insulation materials [5][6][7][8][9][10][11][12]. That is a microcosmic expression to evaluate electrical properties of materials, which is closely related to the microstructure and the crystal structure of materials [13]. The physical defects (molecular weight distribution, free volume etc.) and the chemical defects (broken chains, branched chains, additives and by-products etc.) in insulation polymer act as carrier traps and their distribution has great impact on the charge transport [14]. In addition, it is reported by M.D. Min et al. that the deep traps are significantly responsible for the DC breakdown strength [15]. Therefore, investigating the influence of shish-kebab crystal on charge transport behaviour for polypropylene/elastomer blends is vital important and that may be helpful for the novel method of PP modification by introducing the shish-kebab crystal.
In this work, the aryl amide derivative β nucleating agent TMB-5 was selected to induce shish-kebab crystals, the TMB-5 could be first self-assembled into fibrous structure and then as a template directed the epitaxial crystallisation of PP into hybrid β-form shish-kebab crystals at high temperature [16]. Samples with different elastomer content and nucleating agent content were prepared by hot press method. Polarised optical microscopy (POM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were employed to understand the crystal morphology. Isothermal surface potential decay (ISPD) method was carried out to obtain the trap distribution. DC breakdown strength was measured to estimate the electrical property as well. The possible mechanism of the shish-kebab crystal tailoring charge transport in PP/PBE blends has been demonstrated on the basis of its crystallisation characteristics. Obtained results revealed that the shish-kebab crystals were induced by the thermally induced self-assembly method, and its influence on charge transport behaviour for polypropylene/elastomer blends was decided by both the carrier trap formation and the physical channel orientation of the shish-kebab crystals.

Sample preparation
The base material is commercially available isotactic polypropylene (PPH-T03) purchased from Sinopec Maoming Company, China. PBE (Vistamaxx 6202, Exxon Mobil Corporation, USA) was added into PP to modify its flexibility. Aryl amide derivative β nucleating agent TMB-5 (Shanxi Chemical Research Institute, Co., Ltd., Taiyuan, China) was added in order to form shishkebab crystal. The content of elastomer was 0, 10, 20, 30 wt% and that of β nucleating agent was selected at 0, 0.05, 0.1 and 0.3 wt%, respectively. The PP and the PBE pellets were firstly cleaned by ethyl alcohol, then were dried together with TMB-5 powder for 24 h at 60 • C in a vacuum chamber. An internal mixer was used to uniformly blend the mixture at 175 • C, 30 r/min for 5 min. The obtained mixtures were heat treated at 230 • C for 5 min in order to control the formation of the fibrous β nucleating agents by self-assembly, then were pressed under 16 MPa. The dimension of the blend samples was 90 mm × 90 mm, and the thickness of the sample was 100 ± 10 µm and 20 ± 5 µm to satisfy different experiments. In order to easily index the blends with different contents of elastomer and nucleating agent, the description of the material is shown in Table 1, where x is the weight ratio of PBE and y is the weight ratio of TMB-5. As an example, PB10-0.1 stands for PP blended with 10 wt% PBE and 0.1 wt% TMB-5.

Experimental methods
POM was employed to observe the crystal morphologies of samples. The small test sample pieces with the thickness of 20±5 µm were firstly etched in a strong oxidation acid solution, then the etched samples were washed by water and dried in a vacuum chamber. The crystal morphologies were obtained via a POM (59XF, Shanghai Optical Instrument Factory No.1). The detailed experimental procedure could be found in our early publication [12]. XRD analysis was performed in order to investigate the crystal forms of samples. A Rigaku diffractometer (D/MAX-2500, Japan) with Cu target K α radiation (K α = 1.54 Å) was used, and the measurements were carried out at room temperature. The recorded angle range was 10 • ∼35 • with a scan rate of 2 • /min in XRD patterns. Tuner-Jones formula was used to calculate the relative content of β-crystal, K β [17], where A β(300) is the diffraction intensity of β(300) plane at diffraction angle 2θ = 16.1 • . A α(110) , A α(040) and A α(130) are the diffraction intensities of α(110), α(040) and α(130) planes at diffraction angles 2θ = 14.1 • , 16.9 • and 18.5 • in the XRD pattern, respectively. DSC measurement was conducted to estimate the melting and the crystallisation behaviour of samples. A Perkin Elmer DSC7 was used. The sample was firstly heated up from 25 to 200 • C, then was cooled down to −30 • C, and finally was reheated to 200 • C. When the termination temperature was reached, it should be kept for 5 min to make the sample be heated uniformly. The first heating process was to remove the thermal history, and the cooling and the second heating process were to obtain the crystallisation and the melting curves. The heating/cooling rate was 10 • C/min and the measurements were taken under nitrogen atmosphere. The crystallinity of the sample was calculated as well by, where ΔH m is the melting enthalpy of sample, ΔH m0 is the standard fusion heat of α-PP (178 J g -1 ) or β-PP (170 J g -1 ) [9]. ISPD method was employed to analyse carrier trap distribution and charge transport behaviour of test samples. The schematic diagram of the experimental platform including the electrode arrangement and the test circuit is shown in Figure 1. The detailed description of the ISPD platform could be found in our early publication [3]. The potential of the needle electrode was ±5.7 kV, and that of the grid electrode was ±2.8 kV. Every sample was charged for 15 min at 40 • C, and the relative humidity was 20%±2%. A Kelvin type electrostatic voltmeter (Trek347, Trek Co ltd., USA) was used to record the surface potential of the test sample, five specimens were measured for every type of sample.
Carrier trap distribution could be obtained from the ISPD measurement. The surface potential decay process is associated with the charge de-trapping behaviour closely [3]. The time dependent trap level ΔE could be expressed as [18], where E c is the band edge, E m is the demarcation energy defined as the border between emptied and occupied traps [19], k = 1.38 × 10 −23 J/K is the Boltzmann's constant, T is the Kelvin temperature in K, ν is the attempt to escape frequency, and t is the decay time in s. The trap density N is related to all the surface potential decay process, and is expressed as [19], where N(ΔE) is the trap density occupied by carriers at trap level ΔE, ε 0 = 8.85 × 10 −12 F m −1 is the permittivity of vacuum, ε r is the relative permittivity of the material, q = 1.60 × 10 −19 C is the elementary charge, L is the thickness of the sample in m, and U s is the surface potential in V. Accordingly, the relationship between N and ΔE is employed to characterise the carrier trap distribution of the corona charged sample [20]. DC breakdown strength of samples was measured by a breakdown test system. The test sample was sandwiched by a pair of semicircle electrodes, and was immersed into transformer oil to prevent surface flashover. The radius of the electrode was 12.5 mm, and one of the electrodes was connected to the DC power source (HTC 10 kVA/100 kV, Wuhan Sanxin Huatai Electrical Testing Equipment Co., Ltd., China), the other was grounded. The rate of the rise in the applied voltage was 0.5 kV/s, and the DC power source would be removed when the insulation breakdown occurred in the sample. The test was performed at room temperature. The average breakdown strength of at least 15 specimens for every type of sample was recorded.

Crystal structure characterisation
The crystal morphology observed by POM is shown in Figures 2-5. In pure PP as shown in Figure 2(a), the randomly distributed spherulite structures are formed. It is also clearly shown that the average size of spherulites is ∼80 µm and the spherical crystal boundary is clear. With the addition of TMB-5, the fibrous structure has been formed by self-assembly because of the high temperature treatment. The folded chains of PP grow around the fibrous structure and the shish-kebab crystal structure is formed, as the reference [21] reported. The length of the shish-kebab crystal for PP-0.05 is ∼70 µm, while the width is ∼25 µm, as shown in Figure 2(b). With the increase of TMB-5 content, the size of the shish-kebab crystal decreases, as compared Figure 2(c,d) with Figure 2(b). The length and the width of the shish-kebab crystal for PP-0.1 are ∼25 µm and ∼10 µm, respectively. The length and the width of the shish-kebab crystal for PP-0.3 are ∼15 µm and ∼5 µm, and the crystal boundaries are un-clear. That is because the space around the crystal centres is not enough for the crystal formation with the nucleating agent concentration increasing. A small amount of shish-kebab crystals are observed in PB10, as shown in Figure 3(a). The molecular chains of PBE prepared by metallocene catalysis method is short, hence the molecular chains are easy to stretch and act as fibrous nucleation centres to induce the formation of shish-kebab crystal [12]. The length of the shish-kebab crystal induced by PBE is ∼130 µm and the width is ∼30 µm, which is much larger than that induced by TMB-5. When the TMB-5 is added into PB10, the shish-kebab crystals induced by PBE are disappeared, and the small-size shish-kebab crystals are observed in Figure 3(b,d). The length and the width of the shish-kebab crystal for PB10-0.05 are ∼35 µm and ∼15 µm. The size of the shish-kebab crystal for PB10-0.1 increases to ∼70 µm in length and ∼30 µm in width. The POM image for PB10-0.3 is similar to that for PP-0.3, where the shish-kebab structure is un-clear and almost only the fibrous nucleation centres are observed. It indicates that heterogenous nucleation is easier to happen than homogeneous nucleation. The crystal of PB20 is a little shorter than that of PB10, as depicted in Figure 4(a). With the TMB-5 content increasing, the change trend of the shish-kebab crystal size for PB20/TMB-5 is in agreement with that for PB10/TMB-5, as shown in Figure 4(b-d). It is observed that the boundary of the shish-kebab crystal is clear when the nucleating agent content is 0.05 and 0.1 wt%. The crystal sizes of PB20-0.05 and PB20-0.1 are larger than that of PB10-0.05 and PB10-0.1. The shish-kebab size of PB20-0.3 is also short and narrow, which is similar to that of  Figure 5(a). The average diameter of the spherulite for PB30 is similar to that for pure PP, but the crystal boundary becomes un-clear. It illustrates that the formation process for the spherulite is restricted by the amorphous PBE when the elastomer content reaches to the upper limit of compatibility [12]. With the increase of TMB-5 content added into PB30, the change trend of the shish-kebab crystal size for PB30/TMB-5 is also in agreement with that for PB10/TMB-5 and PB20/TMB-5, as shown in Figure 5 [9][10][11]. Figure 6 shows the XRD patterns of PP and its blends. Only the α-crystal is observed in pure PP, as shown in Figure 6(a). The relative content K β of β-crystal for the sample with only 0.05 wt% TMB-5 content increases to 58.55% by the introduction of β nucleating agents. It reveals that the TMB-5 has strong ability of het- With the content of the TMB-5 increasing, the K β increases, as the diffraction intensity peak of β (300) for PP/TMB-5 becomes stronger. However, the growth rate is slowed down, which is due to the limitation of the crystallisation ability for PP/TMB-5. The addition of a great amount of nucleating agent into PP may cause agglomeration and the space for crystallisation is limited. Hence the additive amount of nucleating agent should be controlled within a certain concentration. A weak peak of β (300) is observed in PB10 and PB20 while no β-crystal peak is obtained in PB30, as shown in Figure 6(b,d).
That means low content of PBE added into PP can induce a small amount of β-crystals, which is related to that the shishkebab crystals are formed in PB10 and PB20 but not in PB30. For PP/PBE/TMB-5, the K β increases with the increase of the TMB-5 content and decreases with the increase of the PBE. It indicates that the nucleating agent can promote the β-form crystallisation while the elastomer can restrict the formation process of crystallisation. Figure 7 depicts the melting processes for pure PP and its blends, and the melting temperature T m is shown in the figure. It is observed that pure PP and PP/PBE blends have one melting peak corresponding to the α-crystal. The β-crystal for PP/PBE blends measured in XRD is not observed as a single endothermic peak because the K β is just ∼5%. With the elastomer content increasing, the T m raises and the melting range becomes wide. The melting process starts at 155.1 • C and ends at 168.7 • C in pure PP, while the melting range of PB30 is from 149.9 to 169.9 • C. It reveals that the crystal size for pure PP is more uniform than that for PP/PBE blends [22,23], which is due to the shish-kebab crystal formation and the restriction from the elastomer in the crystallisation process in PP/PBE blends. With the addition of TMB-5, a new endothermic peak appeared at ∼150 • C is the characteristic melting temperature of β-crystal. The T m of β-crystal increases with the TMB-5 content, which means the β-crystal content increases as the XRD results show. In addition, the melting peak of β-crystal gradually becomes sharp with the TMB-5 content, indicating that the grain size distribution becomes more uniform. In particular, the melting peak of β-crystal is divided into two peaks when the content of TMB-5 is 0.05 wt% in PP/PBE/TMB-5. That means two types of β-crystals appear, which are respectively used to form the shish-kebab crystal and the ordinary spherulite. The β-form shish-kebab crystals and spherulites have different stabilities, therefore, the divided two β-crystal melting peaks are caused by the melting of these two types of crystal forms. Moreover, the peak separation is obtained in PB20-0.1 as well, and the melting range of PB30-0.1 is wide, which is similar to the peak separation. That may be the result of the restriction by the large amount of PBE in the formation process of the crystal. The crystallisation temperature T c and the crystallinity of the β-crystals, the α-crystals and the both crystals (X β , X α and X c ) calculated from formula (2) are plotted in Figure 8. It can be seen that the T c increases with the TMB-5 content increasing and decreases with the increase of the PBE content. The introduction of nucleating agents can form a large number of nucleating centres and promote the crystallisation process [24]. The addition of PBE restricts the crystallisation. Due to the nucleating agent is a type of β nucleating agent, the X β increases and the X α decreases with the TMB-5 content increasing. The X c increases slightly due to the addition of nucleating agents, while the X c decreases with the PBE content. In a word, the introduction of β nucleating agent can improve the crystallinity of pure PP and PP/PBE blends, but the addition of elastomer can reduce the crystallinity.

Charge transport behaviour
In ISPD method, it is assumed that the dissipation of trapped charge is induced by the de-trapping from both shallow and deep traps, hence the density of trapped charge can be expressed by [3], where N(t) is the density of total trapped charge, N s and N d are the density of charge captured by shallow and deep traps, k s and k d are de-trapping rate for shallow and deep traps. The surface potential decay process of the sample is recorded, which is closely related to the release of charge from both shallow and deep traps [25]. The variation in surface potential with time is proposed as a bi-exponential function [3,26], where A, B are fitting parameters related to the potentials excited respectively with charges captured by shallow and deep traps, α, β are the parameters with respect to the de-trapping rates of charges captured by shallow and deep traps. Therefore, two peaks are formed respectively in the trap distribution curve, which are referred to as shallow and deep trap centres. The typical distribution of carrier traps by taking hole trap of PB10/TMB-5 as an example is shown in Figure 9. In order to better demonstrate the influences of the elastomer and the nucleating agent on carrier trap distribution, the blue and the red dotted lines are separated as shown in Figure 9, which represent the distributions for shallow and deep traps. The energy levels and densities for shallow trap centre and deep trap centre (the blue and the red peak shown in Figure 9) for every sample are recorded, the average results and the data scatter of the multiple measurements for both positive and negative charging conditions are shown in Figure 10. The hole trap levels for samples are shown in Figure 10(a). Pure PP has the highest deep trap level of hole at 1.009 eV, and the error range is small, which indicates that the charges are difficult to be de-trapped in PP. With the increase of TMB-5 content, the deep trap level of hole decreases, but that for PP-0.05 is 1.007 eV and close to that for pure PP. The high deep trap level is maintained when a small amount of TMB-5 is added into PP. It has been demonstrated that deep traps are mainly formed on the boundaries of crystal and amorphous region in semi-crystalline polymers [27,28]. According to the POM results, the shish-kebab crystal boundaries are clear in the PP-0.05, hence deep traps in a high level are formed. For PP/PBE blends, the deep trap level of hole reduces with the PBE content. The addition of PBE makes the boundary of crystal region fuzzy, leading to the decrease of the deep trap level. For PP/PBE/TMB-5 with low content of PBE (10 and 20 wt%), the variation scope of deep hole trap level is small and the trend is to increase first and then decrease. The highest deep trap level of hole appears in PB10-0.05 and PB20-0.05. That is similar to the deep trap level change in PP/TMB-5. As the POM results show, the clear crystal region boundaries are formed in the blend with a few TMB-5 added, hence the deep traps appear and the deep trap level is remained at a high level. The error range for PB20/TMB-5 is larger than that for PB10/TMB-5, which is decrease. For PB30/TMB-5, the deep trap level of hole decreases sharply. The reason is that the boundaries of crystal and amorphous region are indiscernible with the large amount of PBE added. The deep trap level for PB30/TMB-5 decreases with the increase of the nucleating agent content, and that for PB30-0.3 reduces to 0.907 eV. It indicates that when the content of elastomer increases to a certain extent, the introduction of the shish-kebab crystal has little effect on maintaining the high deep trap level of the material. In addition, the deep trap level of all the samples with 0.3 wt% TMB-5 content is the lowest, which is in good agreement with the indistinct crystal boundaries observed in POM. Large amount of both elastomer and nucleating agent are not conducive to the formation of deep traps. The shallow trap level of hole for pure PP is also the highest. With the addition of PBE and TMB-5, the shallow trap level of hole for samples decreases. Shallow traps are mainly formed in the amorphous region [28,29]. The PBE is a type of amorphous polymer, hence it can introduce physical defects into the sample and make the shallow traps become shallower. A part of the TMB-5 added into the sample cannot induce the crystal formation as nucleation centres and remain in the amorphous region acting as shallow traps, and the agglomeration may occur. Therefore, the shallow trap level is reduced by the large amount of TMB-5 addition. In particular, a slight increase peak is observed in PB10-0.05, PB20-0.05 and PB30-0.05, and even the shallow hole trap level for PB10-0.05 at 0.8985 eV is a little higher than that for pure PP at 0.8983 eV. Considering the melting curves, all of these samples have separated β-crystal melting peaks as shown in Figure 7. It is inferred that the deeper shallow traps are easy to be produced when the β-crystals of the shishkebab structure and the spherulite structure appear simultaneously.
The density distribution of hole trap is shown in Figure 10(b). With the increase of the PBE content, the deep trap density of hole for PP/PBE/TMB-5 decreases and the shallow trap density increases. According to the decrease of X c with the addition of PBE shown in Figure 8, a large amount of shallow hole traps are introduced into PB20/TMB-5 and PB30/TMB-5 by PBE because the interfaces presented between the elastomer and the PP matrix can introduce small voids or cracks, which can act as shallow traps in the samples [29]. As a result, the shallow hole trap density increases sharply in PP/PBE/TMB-5 with high elastomer content. With the addition of TMB-5, the deep hole trap density increases first and then decreases, and the shallow trap density increases. That means the nucleating agent has dual influences on trap density. On the one hand, the TMB-5 acts as the nucleation centre and promote the crystallisation. Both the shish-kebab crystals and the spherulites in β-form (shown in Figures 6 and 8 that the K β and the X β increase) can introduce deep traps on the boundaries of the crystal and the amorphous regions. On the other hand, the TMB-5 also acts as defects in the amorphous region when it does not initiate crystallisation. Therefore, the addition of a small amount of nucleating agent can induce to form the appropriate size shish-kebab crystals and spherulites, and to improve the deep trap density. The excessive nucleating agent addition will lead to the formation of shallow traps.
The changes of the electron trap level and density for PP and its blends with that of the PBE and TMB-5 content is shown in Figure 10(c,d). The change trend for electron trap is similar to that for hole trap. With the addition of PBE and TMB-5, the trap level tends to reduce, the deep trap density decreases and the shallow trap density increases. However, the trap level and the deep trap density can maintain at a high level when the elastomer and the nucleating agent are added into PP with a low content (as the PBE content is 10-20 wt% and the TMB-5 content is 0.05-0.1 wt%).
Charge transport behaviour is decided by trap distribution. Hopping conduction model is a possible mechanism for the charge transport of PP. In this model, it is assumed that the measured surface potential decay is mainly determined by charge transport through the sample bulk, hence the bulk conductivity σ of the sample could be estimated by [30], The relationship between the σ and the electric field E f for a hopping conduction can be expressed by [30], where δ is the average hopping distance, ϕ is the average hopping barrier height. The measurement results are fitted by numerical fitting method according to the formula (7) and (8).
The average hopping distances for all the samples are obtained and listed in Table 2, which is in the reasonable range of 5-20 nm, hence it is proposed that the hopping conduction model is suitable to describe the charge transport behaviour for PP and its blends in this work. The δ for pure PP is 20.84 nm, which is in agreement with the previous study [31]. It has been reported that the charges trapped by shallow traps can de-trap easily, thus they are prone to hop between shallow traps. The overall charge hopping process in the bulk is considered to be mainly determined by the shallow traps [32]. With the content of PBE and TMB-5 increasing, the δ decreases because the trap level reduces and the shallow trap density increases, thus the charge transport is encouraged. In particular, some of the samples have larger δ as compared with other blends with the same PBE content, such as PP-0.05, PB10-0.1. PB10-0.05, PB20-0.1, PB30-0.1. Another reason for the extraordinary growth of the δ is considered.
In pure PP, the crystalline with spherulites and the amorphous region are simultaneously existed. When the β nucleating agents are added into PP or PP/PBE blends, the lamellas grow along the punctiform and fibrous nucleation centres, hence the spherulites and the shish-kebab crystals in β-form are formed. The crystalline region is extended with the TMB-5 content. Figure 11 shows the schematic diagram of microscopic morphologies for PP/PBE/TMB-5 blend. In the bulk of the samples, the spherulites and shish-kebab crystals are randomly distributed, as shown by a schematic diagram in Figure 11(a). In a small part of the sample, the shish-kebab crystals are distributed parallel to (Figure 11(b)) or perpendicular to (Figure 11(c)) the direction of the electric field E f , whose direction is set up as shown in Figure 11(b) to influence the charge transport. The spherulites are isotropic while the shish-kebab crystals are oriented. Hence the orientation of the shish-kebab crystal can influence the charge transport. The electron transport is taken as an example to analyse the effect of the shish-kebab crystal orientation on charge transport in PP/PBE/TMB-5 blend. Firstly, when the shishkebab crystals are parallel to the direction of the electric field E f generated with the implanted charges, as depicted in Figure 11(b), physical charge transport channels are formed in the amorphous region. The electrons captured by the surface traps migrate to the grounded electrode easily with time. The surface potential decays rapidly, hence the calculated trap level is low and the δ is underestimated. Secondly, when the shish-kebab crystals are perpendicular to the E f , as shown in Figure 11(c), the migration of electrons to the grounded electrode is hindered by the shish-kebab barriers. The electrons have to move around the obstacles and then migrate towards the grounded electrode. The surface potential decays slowly, thus the trap level is highly calculated and the δ is overestimated. In the above δoverestimated samples (PP-0.05, PB10-0.1. PB10-0.05, PB20-0.1, PB30-0.1), a large number of physical channels perpendicular to the E f appear and restrain the charge transport. In a word, the influence of the shish-kebab crystal on charge transport behaviour for PP and its blends is depended upon its orientation on the basis of the trap formation on the crystal boundary and the physical channel orientation.  Figure 12 shows the dependence of the DC breakdown strength upon the content of TMB-5 varying as a function of PBE content. The average value and error bar of breakdown strength are obtained from at least 15 experimental results for every type of sample, which is a reliable method confirmed by previous studies [33,34]. Moreover, although the deviation in Figure 12 is significant, the trend of the breakdown strength for samples with the addition of elastomer and nucleating agent is remarkable. The breakdown strength of virgin PP is 377 kV mm. With the TMB-5 content increasing, the DC breakdown strength for PP/PBE/TMB-5 tends to decrease monotonically. When PBE is added into PP, the breakdown strength of PP/PBE blend decreases. It has been reported that the increase in the density and level of deep traps contributes to the improved dielectric breakdown performance [35], hence the decline of the DC breakdown strength is due to the reduce of the deep trap level and density for both hole and electron with the TMB-5 and PBE, as shown in Figure 10. Particularly, when the TMB-5 content is larger than 0.05 wt%, the breakdown strength of PP/PBE/TMB-5 is higher than that of PP/TMB-5. That is also closely related to the orientation of the shish-kebab crystals.

DC breakdown strength
The breakdown paths are generally formed in the amorphous regions due to the loose structure. The disordered shish-kebab crystals may form a channel to encourage or form a barrier to hinder the breakdown, which has similar influence as that for the electron transport shown in Figure 11(b,c). The addition of PBE may prevent the orderly arrangement of the shish-kebab crystals as the direction is parallel or perpendicular to the E f . More randomly arranged shish-kebab crystals are likely to be formed in the PP/PBE/TMB-5 blends than that in PP/TMB-5 blends. Hence the breakdown pathways are partly hindered and the breakdown strength of PP/PBE/TMB-5 blends is a little higher.

CONCLUSIONS
The influence of thermally induced self-assembly shishkebab crystal on charge transport behaviour in polypropylene/elastomer blends has been investigated. The main conclusions can be summarised as follows, 1. Clear shish-kebab crystals are formed by adding a small amount of nucleating agent. Both the spherulites and the shish-kebab crystals are formed in β-form, hence K β and X β increases while X α decreases with the TMB-5 content. 2. With the addition of the TMB-5, the trap level for PP/PBE/TMB-5 decreases and the trap density of shallow traps increases monotonically. The average hopping distance appears to decrease with the addition of PBE and TMB-5. 3. Deep traps are formed on the clear shish-kebab crystal boundaries, while shallow traps are induced by the elastomer and the un-crystallisation nucleating agents. The charge transport is also influenced by the orientation of shish-kebab crystals. 4. The DC breakdown strength decreases with the increase of TMB-5 content, and that for PP/PBE/TMB-5 is higher than that of PP/TMB-5.
In summary, it is found that the introduction of the thermally induced self-assembly shish-kebab crystal into PP/elastomer blend would provide a novel route for tailoring the insulation performance in the future.