Colloid Polym Sci https://doi.org/10.1007/s00396-017-4220-8
ORIGINAL CONTRIBUTION
Effect of annealing on microstructure and tensile properties of polypropylene cast film Yueming Ren 1 & Hao Zou 1 & Shaojie Wang 1 & Jianye Liu 1 & Dali Gao 1 & Changjiang Wu 1 & Shijun Zhang 1
Received: 15 August 2017 / Revised: 18 October 2017 / Accepted: 22 October 2017 # Springer-Verlag GmbH Germany 2017
Abstract In this work, the effect of annealing on microstructure and tensile properties of polypropylene (PP) cast film was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), SEM, etc. The SAXS data showed that the elevating annealing temperature led to the increase of thickness of lamellae phase and amorphous phase. The orientation of lamellae along MD direction after annealing was proved with the SAXS data. Moreover, these annealed samples had an extra low-temperature melting peak in the DSC curves and it moved to a higher temperature obviously with increasing the annealing temperature. More importantly, the relationship between the yield strength and the lamellar thickness was built through testing the tensile properties and calculating the Thomson-Gibbs equation, respectively. It revealed that the yield strength first increased rapidly and grew slowly when the lamellar thickness increased to a certain extent. A detailed physical model was also proposed to depict the change of microstructure during annealing.
Keywords Annealing . Polypropylene . Tensile properties . Yield strength . Lamellar thickness
* Changjiang Wu
[email protected] * Shijun Zhang
[email protected] 1
SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China
Introduction In the past decades [1–6], much work had been done to investigate the microstructure of semi-crystalline polymers during annealing. E.W. Fischer et al. [7] proposed the model for the effect of annealing on the structure of drawn polymers. It indicated that the initial disordered structure rearranged into the ordered structure, explaining the long period of drawn polymers increased during annealing. H. Bai et al. [8] investigated the effect of annealing on the microstructure of injectionmolding bar of β-iPP through DSC, SAXS, and so on. It was very interesting that the thickness of both amorphous phase and rigid amorphous phase increased, while that of the crystalline phase decreased slightly in their experiments. For semi-crystalline polymers, it is almost impossible to reach the thermodynamic equilibrium state when they are rapidly cooled down from melt state to room temperature, forming an imperfect packing density and conformational structure. In comparison, annealing is thought to be an effective method to change the structure and properties of semi-crystalline polymer via healing of defects and diminishing the stress and strain. As one of the most important annealing factors, the annealing temperature between glass transition temperature and melting temperature plays a vital role in changing the structure and properties of polymer. In addition, the annealing time is also regarded as an important annealing factor. Z. Ding et al. [4] studied the effect of annealing time on tensile properties and thermodynamics of PP films. T. Wu et al. [5] designed a rapid annealing method, involving a short-time thermal stimulus treatment at high temperature and then cooling in the air, which improved the stress-strain behaviors of PP films. Y. Shang et al. [9] investigated the effect of annealing time on the microstructure of PP, and the results revealed that there should exist a characteristic annealing time, which should be affected by annealing temperature. In fact, the crystalline structure changed slightly only
Colloid Polym Sci
beyond 10 min, which was enough for annealing the PP films. However, changing the annealing temperature could get over the time constraints. At the same time, the effects of flow orientation of polymers as well as the chemical structure of polymer chains on the structure of polymer should be considered [10]. The above mentioned Bdefect^ means the paracrystalline structure with the small monoclinic crystal whose melting point is related to the thickness of lamellae. As has already been stated by other workers [2, 4, 11], the change of small crystal during annealing could be observed by the way of formation of low-temperature melting peak in the curve of DSC. According to the kinetic crystallization theory of polymer, the lamellar thickness in the small crystal could be calculated by the Thomson-Gibbs equation [12] (as follows) on basis of the melting point of lamellae (Tm). 2σe 0 ð1Þ T m ¼ T m 1− lΔh Where T 0m represents the melting temperature of the infinitely large crystal (the equilibrium melting temperature), σe means the top and bottom specific surface free energy, l is the lamellar thickness, and Δh represents the bulk heat of fusion per cubic centimeter. In addition, the thickness of lamellae could also be obtained from the SAXS data [13]. However, the crystallization of polymer was occurring at the higher degrees of supercooling; the distribution of lamellae is so broad that the average lamellar thickness obtained from SAXS could not trace the change of thinner lamellae during annealing. Besides the change of lamellar thickness during annealing, the tensile properties also vary in this process as we can see from many papers [5, 14, 15]. Prior to yielding, deformation of semi-crystalline polymer is accomplished primarily by the deformation of the amorphous phase, which is rubberlike [16]. At the yield point, the polymer chains in the amorphous region find it increasingly difficult to slide past each other, and continued deformation of the polymer results mainly from the crystalline phase. Polymer crystal can deform by slip, twinning, or strain-induced martensitic phase transformations [17]. Coarse slip occurs at larger deformation on a few parallel planes, resulting in the fragmentation of lamellae into blocks which thus slide past each other. As the deformation continued, the fragmentation of lamellar blocks occurred, resulting in the formation of oriented microfiber structure. When the annealing temperature is over 80 °C, the double yield is found in the stress-strain curves [11], where the second yield infers that the coarse slip occurred at larger deformation. As the Table 1 The main index of polypropylene
annealing temperature increased, the second yield point shifts to the lower strain due to the increase of crystallinity and decrease of amorphous phase during annealing. As we all know, the influence of annealing-induced microstructural change on the mechanical behaviors of PP has been a focus of attention in the past few years [1, 18–22]; however, the microstructural change during annealing is not clear. 1. Despite substantial interest to the kinetics of growth of the low-temperature shoulder on DSC traces of polypropylene film, this phenomenon remains the subject of debate [18]. In this work, some new methods were used to reveal the mechanism of microstructural change during annealing. First, the XRD with hot stage was employed to simulate the annealing process of polypropylene film. The obtained results can tell us how the microstructure changes during annealing, which is actually a meltrecrystallization process. Second, the Thomson-Gibbs equation was used to trace the process of subsidiary lamellae based on the data obtained by fitting DSC. These data indicate that the melt-recrystallization process is related to the annealing temperature. Therefore, the detailed model about the microstructural change of polypropylene during annealing is proposed. 2. It is generally accepted that the improved tensile properties are associated with the perfection and growth of crystal structure, but few study had been done on the relationship between microstructure and tensile properties. Okane et al. [23] studied the relationship between stress and lamellar thickness that obtained by SAXS. However, in this work, this relationship was built in a different perspective, which is that the lamellar thickness is calculated by ThomsonGibbs equation based on the data of DSC. Therefore, the reason, which leads the phenomenon of extra yield that is not mentioned in literature, was found. That is the fragmentation of thin lamellae or subsidiary lamellae.
Experimental section Materials The commercial isotactic polypropylene used in this study was provided by SINOPEC. Its main index is listed in Table 1. The melting point Tm and the crystallization temperature Tc were measured using differential scanning
Trade name
Supplier
MFR (g/10 min) (230 °C-2.16 kg)
Density (g/m3)
Tm (°C)
Tc (°C)
Mw
PDI
F300C
SINOPEC
3.4
0.91
162
120
400,000
5.03
Colloid Polym Sci
calorimetry (DSC) at a rate of 10 °C/min. The polydispersity index (PDI) and the molecular weight were obtained using GPC. Sample Preparation The cast films were prepared using a lab multilayer cast film unit from LabTech in Sweden, which equipped with 42.5 cm width slit die. The extrusion was carried out at 230 °C, and the extruder screw speed is 50 rpm. An air knife was mounted close to the die to provide air to the film surface. The chill roll temperature is 70 °C, and the thickness of the cast film is about 100 μm. The cast films were placed in the oven at 90, 110, 120, 130, 140, and 150 °C for 12 h in the vacuum environment. Then, they were cut into the rectangular splines with 15 mm width along the MD direction and put into a sealed bag. The sample is named as follows: the cast film before annealing is called PPL0 and the cast film after annealing at T°C for 12 h is called PPL0AT, such as PPL0A90. Characterization Differential scanning calorimetry A PerkinElmer Pyris I DSC with a mechanical refrigerator was used to investigate the thermal properties of PP cast films. The temperature and heat flow were calibrated using standard materials such as benzoic acid and indium. The dried samples were encapsulated in a sealed aluminum pan with the sample weight of ~ 5 mg. All samples were first melted at 200 °C for 3 min to erase the thermal history, then cooled to 50 °C, and the subsequent DSC heating traces were recorded with a rate of 10 °C/min. Wide-angle X-ray diffraction WAXD experiments were conducted with a Rigaku X-ray diffractometer D/max-2500 (CuKa, λ = 0.154 nm, 40 kV, 200 mA, reflection mode). The experiments were performed with a 2θ range of 10–30°, at a scanning rate of 6 °C /min and a scanning step of 0.02°. The XRD instrument with hot stage was employed to simulate the annealing process, as we can see from Fig. 1. The sample was firstly heated to 150 °C from room temperature (RT) with the rate of 10 °C/min, then the sample was kept at 150 °C for 10 minutes. Finally, the sample was cooled to room temperature by natural cooling. The XRD data were collected in the following point: RT 141M, 150M, 150A1, 150A10, and CD, the scanning time was 30 s. Note: RT represents room temperature; 141M means 141 °C in the heating process; 150M means 150 °C in the heating process; 150A1 means keeping 150 °C for 1 min;
Fig. 1 The schematic of PP in the annealing process
150A10 means keeping 150 °C for 10 min; CD represents cooling down to room temperature. The degree of crystallinity ((X)%) for different samples was calculated by X¼
Ic 100% Ic þ Ia
ð2Þ
Where Ic is the intensity of diffraction peak of crystalline phase and Ia represents the intensity of diffuse peak of the amorphous phase. Small-angle X-ray scattering The small-angle X-ray scattering (SAXS) patterns were collected via a Bruker AXS Nanostar system. The instrument is equipped with a microfocus copper anode at 45 kV/0.65 mA. The MONTAL OPTICS and a VANTEC 2000 2D detector were located at 104.7 mm distance from the samples, which were originated from cast films. The orientation factor f of crystallites is calculated according to Herman’s orientation function [24]: f ¼
1 2 3 cos ∅ −1 2
cos ∅ ¼ 2
ð3Þ
π
∫02 I ð∅Þsin∅cos2 ∅d∅ π
∫02 I ð∅Þsin∅d∅
ð4Þ
Where 〈cos2∅〉 is called the orientation parameter and I(∅) represents the intensity of lamellae as the ∅angle changes. Scanning electron microscopy To clearly observe the lamellar structure, the un-annealed cast film and the PP cast film annealed at 130, 140, and 150 °C were used in the SEM examination. The PP cast films were immersed for 2 h or more in an acid etchant solution [25] (1 wt% KMnO4, 33.5 wt% H3PO4, and 65.8 wt% H2SO4 to
Colloid Polym Sci
Fig. 2 2D-SAXS profile of cast film which was non-annealed (a) annealed at 130 °C (b) 140 °C (c), and 150 °C (d).
preferentially etch the amorphous part of the polymer in the spherulites and expose the remaining crystalline lamellar structure. The films after etching were washed in the mixed solution of sulfuric acid, water, and hydrogen peroxide in the volume ratio of 2:7:1. Then, they were washed with water and ethanol for 2 min in order to remove any residual etchant. After drying, the films were coated with a very thin layer of gold to eliminate any undesirable charging effects during SEM observation. The instrument used in this study was a Hitachi S4800 microscope. The voltage of the electron beam used for SEM observation is around 2.0 kV. The SEM images were obtained using a secondary electron detector and magnifications ranging from 20 to 50 k.
annealed sample and annealed samples at 130, 140, and 150 °C. There was only a diffuse ring in Fig. 2a, indicating the spherulite or aggregates of the non-oriented lamellae. After annealing, the diffuse ring became intense along the MD direction, implying the orientation of lamellae increases in the MD direction. The related explanation about this phenomenon will be discussed later. The Iq2-q profile was depicted on the basis of SAXS data (Fig. 3). The long period, which is determined from the q in the profile of Iq2-q in Fig. 3a according to Eq. (5), reflects the sum of the thickness of lamellae and amorphous phase in the ideal two-phase model only consisting of crystal and amorphous phase.
Tensile properties
Lb ¼ 2π=qmax
Uniaxial tensile tests were performed in accordance with ASTM: D882-12 using a SANS CMT6104 at room temperature. The specimens having a rectangular geometry of 15 mm width and 20 mm long were stretched at a speed of 100 mm/ min. The load of the sensor is 500 N.
Results and discussion Microstructural evolution of lamellar structure during annealing SAXS was applied to explore the microstructural change during annealing. Figure 2 showed the 2D-SAXS profile of nonFig. 3 a The SAXS profile of cast films annealed at 130, 140, and 150 °C and b un-annealed cast film
ð5Þ
Where qmax means the values of q in x-axis, which is corresponding to the peak value of Iq2 and Lb represents the long period corrected by Lorentzian according to the Bragg’s equation. There were three peaks in un-annealed samples as we can see from Fig. 3b, where n = 1, 2, and 3 is related to the first, second, and third order scattering peak, respectively. The q1, q2, and q3 where q1:q2:q3 = 1:2:3 involves that a lamellar structure is corresponding to the value of x-axis of the three scattering peaks. As Fig. 3a shows, as the annealing temperature increased, the first order scattering peak gradually moved to the low-angle region; it indicates that the long period including crystal phase and amorphous phase increased gently. The value of half peak width, which reflects the regularity of
Colloid Polym Sci Table 2 data
The long period and orientation calculated based on SAXS
Parameter
PPL0
PPL0A130
PPL0A140
PPL0A150
Lb (nm) Orientation ( f )
10.7
17.6
18.4
21.6
0.063
0.076
0.085
0.116
lamellar spacing, was getting smaller, and the value of intensity becomes bigger as the annealing temperature increased, which prove that the regularity of size and interlayer spacing of lamellae becomes better. That is why the samples at higher annealing temperature have multiple scattering peaks. The value of Lb (long period), which is reflected in the first order scattering peak, was calculated as shown in Table 2. The result in Table 2 showed that the Lb increased from 10.7 to 21.6 nm with increasing the annealing temperature. The increase of long period indicates the increase of interlayer lamellar spacing. The orientation ( f ) of the samples annealed at 150 °C was much larger than that of the non-annealed samples (f was changed from 0.063 to 0.116). It inferred the increases of the number of lamellae in MD direction, which was consistent with the result that was observed in the profile of 2D-SAXS. In addition, in order to clearly illustrate the variation of crystal and amorphous phase dividedly due to the annealing temperature increase, the 1D electron density correlation function (Fig. 4a, b) was calculated from [26, 27] K ðzÞ ¼ q2 I ðqÞcosqz dq
ð6Þ
Where q is scattering vector, I(q) is the scattering intensity, and z represents the distance in real space, normal to lamellae. As shown in the Fig. 4a, there are one peak and two troughs, which all move to longer distance as annealing temperature increases, in un-annealed samples and annealed samples at different annealing temperatures from 130 to 150 °C. The straight line, which existed from initial position to the first trough, whose slope in annealed samples were all bigger than that in un-annealed samples.
Fig. 4 SAXS correlation function of a cast films at different annealing temperature and b annealed samples at 150 °C in zaxis
More information could be obtained through the method described in Fig. 4b, where Lc represented the mean distance between the centers of adjacent lamella, Lm/2 represented the mean distance between the centers of lamellae and adjacent amorphous phases, dc represents the mean thickness of lamella (dc is the value of z of the point of intersection of the inclined straight line and the tangent of the first trough), and t represents the mean thickness of boundary layer. The calculated data is shown in Table 3, where da represented the mean thickness of the amorphous phase (da = Lc−dc). The value of Lc, Lm, dc, and da all increased with the increase of annealing temperature, which indicated that the thickness of lamellae and the amorphous phase increase. In addition, compared to the thickness of lamellae, the thickness of boundary layer (t) was too small to neglect. The SEM micrograph was used to determine the type of the morphology in the studied samples. Figure 5 was the SEM pictures of the etched cast films treating under different annealing conditions. Figure 5a is the SEM image of the nonannealed iPP sample. The lamellar structure was hard to be observed because the fine lamella had been mostly etched. However, the annealed iPP samples at 130, 14, and 150 °C for 12 h revealed a cross-hatched morphology (Fig. 5b–d) consisting of radial and transverse lamellae. The thickness of the lamellae increased with increasing the annealing temperature. The in situ XRD profile is shown in Fig. 6a; the variation of crystallinity of PP in the annealing process is shown in Fig. 6b. When the temperature rose to 150 °C from the room temperature, the crystallinity of PP decreased, which indicated that the crystal had been melted in the heating process. When the temperature was held at 150 °C for 10 min, the crystallinity of PP increased, indicating that the recrystallization of molecular chains occurred. Then, the PP cast film was dropped to room temperature by natural cooling; the crystallinity rose rapidly, which suggested recrystallization still existed. The annealing of PP was actually a melt-recrystallization process based on the above results.
Colloid Polym Sci Table 3 The related data obtained by the correlation function of different samples Samples
PPL0
PPL0A130
PPL0A140
PPL0A150
Lm/nm Lc/nm dc/nm da/nm
10.6 11.1
16.2 17.6
17.3 18.3
19.8 21.3
4.2 6.9
5.0 12.6
5.1 13.2
6.5 14.8
Thermal dynamic of lamellar structure in cast film The thermodynamic property of all samples was tested by DSC as Fig. 7a shows. An extra low-temperature melting peak, which moved to a higher temperature and was obvious as the annealing temperature increased from 90 to 140 °C, was observed in the annealed samples and disappeared in sample annealed at 150 °C. This phenomenon was also observed in other papers [2, 6, 11, 28], E.W. Fischer et al. [6] found that an extra low-temperature peak existed in the curves of DSC of annealed PE crystal and the peak decreased with the extending of annealing time; it indicated that the thinner crystal was melted to recrystallize for thicker lamellae during annealing. A similar phenomenon was observed in our experiments. In addition, the WAXD patterns of some samples were shown in Fig. 7b, and it turned out the crystal form of polypropylene did not change during annealing. Therefore, the above phenomenon might be due to the formation of imperfect lamellae or the thinner lamellae in the samples annealed at a lower temperature, which changed into perfect or thicker lamellae at higher annealing temperature. Fig. 5 SEM micrograph of cast films after etching for a nonannealed, b annealed at 130 °C, c at 140 °C, and d 150 °C
In order to investigate the change of thinner lamellae of cast films during annealing, the analysis software (Peak Fit 4) of DSC had been used to deconvolute DSC peaks of samples, and the Pearson IV function was used to fit the data. The original curve and fitting curve of DSC of the sample annealed at 140 °C are shown in Fig. 8a, where the curve of fitting1 and fitting2 represented the melting peak of thinner lamellae and originally lamellae, respectively. As Fig. 8b showed, the half peak width of melting fit peak of thinner lamellae in samples was narrower, and the peak value was higher with the increase of annealing temperature. It inferred that the thinner lamellae were melted to recrystallize for the thicker lamellae, and the distribution of recrystallization-lamellae was more uniform. ΔH1, which is calculated by peak area, means the melting enthalpy of thinner lamellae. As shown in Fig. 9a, the value of ΔH1 increased with the increase of annealing temperature. It indicated that the thickness of thinner lamellae increased, and the distribution of thinner lamellae was more uniform. The relationship between the melting point of polymer and the thickness of lamellae of a single crystal of polymer could be described using the following Thomson-Gibbs equation. For polypropylene [29] σe=10−6J/cm2,Δh = 1.54 × 102J/cm3. With reference to the thermodynamic equilibrium melting point (T 0m ) of PP in literature [30], 443.2 K is used as the T 0m of PP in our paper. The above equation could also be reformed in the way as follows: l¼
2σe T 0m Δh T 0m −T m
ð7Þ
Colloid Polym Sci
Fig. 6 The in situ XRD profile of PP cast film at different annealing process
Fig. 7 a DSC heating curves of the cast films at different annealing temperature for 12 h and b WAXD patterns of cast films at annealing temperature from 130 to 150 °C (t = 12 h)
Tm could be obtained through fitted data of DSC, then the thickness of thinner lamellae (l) could be calculated by the above equation. As Fig. 9b showed, the thickness of thinner lamellae increased with the annealing temperature increased. The above result suggested that the thinner lamellae had been melted to recrystallize for thicker lamellae.
Schematic model of lamellar structure during annealing The changes of microstructure during annealing was schematically drawn in Fig. 10. In the un-annealed sample, the thinner lamellae were located between the original lamella. In the heating process of annealing, the ordered molecular chains Fig. 8 a The fusion peaks of different lamellae separated by an analysis software of DSC and b the melting fit peak of thinner lamellae in samples annealed at 130, 140, and 150 °C, the value of half peak width are given in the profile as an inset
exhibited a disordered state. After annealing, the random polymer chains were rearranged to form the thicker lamellae and grew on the lateral of original lamellae. As is known, the thicker the lamellae, the higher its melting point. Therefore, the position of extra melting peak moved to a higher temperature until it was overlapped with the melting peak of original lamellae as the annealing temperature increased. Moreover, because of the lateral growth of original lamellae, the main melting peak in DSC curve was slightly offset to the right with the increase of annealing temperature. As Fig. 5a shows, the distance between two adjacent lamellae in an un-annealed sample was bigger than that in the annealed samples. However, the value of long period in an un-annealed sample was smaller than that in the annealed
Colloid Polym Sci Fig. 9 a The variation of ΔH1 with the annealing temperature increases and b the variation of l with the annealing temperature increases
Fig. 10 Schematic representation of microstructural change of polypropylene during annealing
sample. So it indicated that there were some aggregates including the original lamellae and the fine lamellae, whose thickness was smaller than thinner lamellae, in an unannealed sample. A little lamella was observed in the picture (Fig. 5a) of SEM, because the most of the fine lamella, which was also observed in the stress-strain curves of an un-annealed sample, was etched by the etchant. However, after annealing,
Fig. 11 The stress-strain curves along MD of cast films at different annealing temperature (t = 12 h); strain rate = 100 mm/min
the fine lamellae were melted to recrystallize for thinner lamellae in the lower annealing temperature, which was observed in Fig. 7a; then, the thinner lamellae were melted to recrystallize for thicker lamellae in the higher annealing temperature. As a result, the mean thickness of the lamellae and the amorphous phase were increased during annealing, which was the reason for the increase of long period. It was consistent with the calculated result by SAXS data. Compared to un-annealed sample, the degree of orientation of annealed sample was greatly enhanced, which was related to melt-recrystallization of crystal during annealing. The orientation factor in the original sample was relatively small; on the one hand, the crystallinity of original sample was lower, and on the other, the noise of sample was more serious during the SAXS testing due to the existence of fine lamellae. However, after annealing, the thinner lamellae was melted to recrystallize the thicker lamellae, which lead to the increase of crystallinity, and the crystal of recrystallization was oriented along the direction of alignment of residual crystalline [31]. Besides, the noise of sample becomes less slightly in the process of testing of SAXS. Therefore, the orientation factor (in Table 2) of the sample increased from
Colloid Polym Sci Fig. 12 a The relationship between the first yield stress and the annealing temperature and b the relationship between the yield strength and l (the thickness of the thinner lamellae)
0.063 to 0.116. In fact, the orientation of samples was caused by the shear flow [32], rather than the annealing. Effect of annealing on tensile properties of precursor films of polypropylene Figure 11 shows the engineering stress-strain curves of samples in MD direction, and it was clear that there were three stages in the deformation of all samples. First of all, the stress increased linearly with the increase of strain, which was called elastic deformation. In this procedure, the deformation of cast film was uniform and can be recovered after removing the external force. Then, the stress reached the maximum value, which was named yield strength, and decreased rapidly, next, remained constant. We called this process, where the plastic deformation was occurred in precursor films of polypropylene with neck, stress softening. However, there was an extra peak in this process in the samples from PPL0 to PPL0A130. Finally, the stress increased gradually as the strain increased called stress hardening, while the growth rate in PPL0A140 and PPL0A150 was faster than that in other samples due to the existing thicker lamellae under higher annealing temperature. The relationship between the first yield strength and annealing temperature is shown in Fig. 12a. It can be seen that the first yield strength increased continuously. The yield strength that corresponding to the slip or rotation of lamellae was decided by tie molecules between lamella of cast films of polypropylene. Therefore, the increase of yield strength may be the reason that the increase in the number of tie molecules Fig. 13 The microstructural change of cast films annealed at a lower temperature and b higher temperature during tensile process
caused by the increase of crystallinity during annealing. In addition, as Fig. 12b shown, initially, the yield strength increased rapidly and then its trend became gentle. It inferred that the yield strength should become constant when the thickness of lamellae was large enough. For annealed sample at 140 and 150 °C, because the thickness of thicker lamellae was similar with the original lamellae, so the yield strength means the second yield strength. The semi-crystalline polymer showed different deformation under different strain [16]. Figure 13 presents the plastic deformation process from the macroscopic and microscopic perspectives in the sample annealed at 110 and 150 °C. The entanglement of molecular chains in the amorphous region, whose interaction force was weaker, was not as dense as that in crystal, so the deformation of the amorphous region occurred first under a certain stress. The slip among lamella occurs in the lower stress due to the molecular chains of the amorphous region was elongated or slips. Meanwhile, the sample reached the first yield with the fine slip among lamella. For the sample annealed at lower temperature, the extra yield was observed in the process of stress softening, it seemed that the broken of the thinner lamellae could account for this. As the strain increased further, the stress hardening occurred due to the broken of the original lamellae. However, for the sample annealed at higher temperature, because the thinner lamellae were melted to recrystallize for the thicker lamellae, there was no obvious yield phenomenon in the process of stress softening in Fig. 11. Indeed, the stress hardening was in relation with the broken lamellae after annealing.
Colloid Polym Sci
Conclusions In summary, the changes of microstructure and characterization of tensile properties of polypropylene during annealing was investigated clearly. Firstly, it was shown that the mean thickness of lamellae and the amorphous phase were increased in annealed samples. There were some aggregates including the original lamellae and the thinner lamellae in un-annealed sample, which was observed in the curve of DSC. It seemed that when the sample was annealed at a certain temperature, the thinner lamellae, which was observed in the DSC curves, was fused for recrystallizing to form more perfect lamellae. Moreover, the lateral growth on original lamellae was the reason that the offset of main melting peak in DSC. As a result, the mean thickness of lamellae and the amorphous phase increased, which lead to the increase of long period. The partial lamellae were oriented in MD direction in the annealed sample, which might be due to preferred orientation during the process of melt-recrystallization or the later growth. It was also observed in the picture of SEM. In addition, the thickness of the thinner lamellae was calculated by the Thomson-Gibbs equation according to the melting point of the thinner lamellae, and the relationship between macroscopic tensile properties and microscopic lamellar size was built to explain the phenomenon that the extra yield occurred. For the sample annealed at lower temperature, the formation of the extra yield was due to the broken of the thinner lamellae. When the annealing temperature was high enough, the thinner lamellae was melted to recrystallize for more perfect lamellae, the extra yield was not obvious. Therefore, there was an extra peak in stress-strain curves of un-annealed sample or the sample annealed at lower temperature in the process of stress softening. Moreover, the first yield strength of samples also increased after annealing, which might be due to the increase of the number of tie molecules caused by the increase of crystallinity during annealing. In addition, the yield strength first increased rapidly and grew slowly when the lamellar thickness increased to a certain extent. It inferred that the yield strength should become constant when the thickness of lamellae was large enough. Acknowledgements The authors would like to thank the support from the National key R&D Program of China (No.2016YFB0302001).
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17. Compliance with Ethical Standards 18. Conflict of interest The authors declare that they have no conflict of interest.
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