Effects of Sodium Salt and Sorbitol-Derivative ... - ACS Publications

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Effects of Sodium Salt and Sorbitol-Derivative Nucleating Agents on Physical Properties Related to Crystal Structure and Orientation of Polypropylene Wonchalerm Rungswang,*,† Kraipop Thongsak,‡ Attaphon Prasansuklarb,§ Korakot Plailahan,∥ Phutsadee Saendee,† Supagorn Rugmai,⊥ and Watcharee Cheevasrirungruang† †

SCG Chemical Co., Ltd., Siam Cement Group (SCG), 10 I-1 Road, Map Ta Phut Industrial Estate, Muang District, Rayong Province 21150, Thailand ‡ SCG Performance Co., Ltd., Siam Cement Group (SCG), 1 Siam Cement Rd., Bangsue, Bangkok 10800, Thailand § SCG Plastics Co., Ltd., Siam Cement Group (SCG), 1 Siam Cement Rd., Bangsue, Bangkok 10800, Thailand ∥ Thai Polypropylene Co., Ltd., Siam Cement Group (SCG), 10 I-1 Road, Map Ta Phut Industrial Estate, Muang District, Rayong Province 21150, Thailand ⊥ Synchrotron Light Research Institute (Public Organization), P.O. Box 93, Nakhon Ratchasima 30000, Thailand ABSTRACT: Although various types of nucleating agents (NA) have been used in polypropylene (PP) for commercial products, the effects of sodium salt and sorbitol based NAs on polymer structures and the relation to the physical properties have been rarely reported. The present work reveals structure and orientation of PP crystal with direct comparison of two types of NAs, i.e., sodium salt and sorbitol-derivative, via X-ray scattering techniques. The results show how those crystal structures are related to the properties of injected-mold PP, with the emphasis on the compressive strength and shrinkage.

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Tc about 10 °C higher than neat PP. They also showed that the SPES possesses higher nucleation ability than the sorbitol-based NA.15 However, the linkage between the crystal structures and the properties of the end-product related to the function of NA has not been clarified. The present work reveals the effects of two types of NA, i.e., SPES and 1:3,2:4-di(methylbenzylidene) sorbitol (DMBS), on the physical properties, focusing on the compression strength and the shrinkage, of the injected-mold PP. The relationships of those properties and the crystal structures were investigated by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering diffraction (WAXD). Several structural parameters were evaluated to investigate the role of the NAs on the polymer crystal in detail.

INTRODUCTION Polypropylene (PP) is one of the commodity polymers which has been extensively used in a wide range of applications due to high stiffness, good thermal properties, and low cost.1,2 However, some properties, such as clarity, shrinkage, and strength, are needed to be modified with polymer additives, in order to serve some specific applications. The nucleating agent (NA) is one of the common additives which has been used to tailor the properties of the polymeric products by means of manipulating the polymer crystallization, morphology, and dimension of the polymer crystal.3−6 Among various types of commercial NAs, sodium salt and sorbitol derivative are conventionally used for PP. Each NA gives unique properties to the end product.7 Sodium phosphate-ester salt (SPES) is one of the sodium salt NAs which has been generally used due to low cost and high efficiency in accelerating polymer crystallization. However, due to the rod-like morphology, the anisotropic properties of the product are inevitable.8,9 Yoshimoto et al. have revealed that the SPES promotes the growth of the PP lamellar crystals in the epitaxial orientation with regard to the SPES surface. This is resulted from the crystallographic matching of the helix pitch of the PP crystal and the SPES lattice. Consequently, the PP lamellae are oriented perpendicular to the SPES surface which is normally aligned along the flow direction.10 In case of the sorbitol-derivative NA, it is soluble in the polymer melt at high temperature during processing, such as in an injection-molded process. Then, it recrystallizes, forming the network above the crystallization temperature (Tc) of the polymer during cooling.11−14 As a result, the polymer crystallization is accelerated while minimizing the anisotropicity. Quan et al. have shown that the SPES and the sorbitol NAs exhibit shifting © 2014 American Chemical Society

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EXPERIMENTAL SECTION Materials. Mw and Mn of the PP sample were determined by GPC to be 178 000 g/mol and 26 000 g/mol, respectively. SPES and 1,3:2,4-bis-O-(4-methylbenzylidene) sorbitol (DMBS) were used as received. Characterization. Mw and Mn were measured by gel permeable chromatography (GPC), Polymer Laboratories (Agilent), model PL-GPC 220. 1,2,4-Trichlorobenzene (TCB) containing 0.025 wt % of Santonox as an antioxidant was used as the mobile phase with 1.0 cm3/min of flow rate at 140 °C. SAXS and WAXD measurements were carried out at Received: Revised: Accepted: Published: 2331

November 18, 2013 January 14, 2014 January 17, 2014 January 17, 2014 dx.doi.org/10.1021/ie4039136 | Ind. Eng. Chem. Res. 2014, 53, 2331−2339

Industrial & Engineering Chemistry Research

Article

Figure 1. SEM micrographs of nucleating agents: (a) SPES and (c) DMBS including their representative schematic drawings: (b) for (a) and (d) for (c).

the plate specimen was cut in injection direction (ID) and transverse-injection direction (TD) with 1 mm of the width (Figure 9). For determining shrinkage of the specimen, the square specimen with the size of 133 mm × 133 mm × 2 mm was prepared by injection molding. The specimen was conditioned at 25 °C and 50% of relative humidity for 24 h. Then, the specimen was measured for the dimension in both the ID and the TD, in which the percentages of the specimen shrinkage were reported as compared to the mold dimension.

BL2.2 of the Siam Photon Laboratory, Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, Thailand. The 2D-SAXS patterns were recorded by the CCD Mar SX165, and the q value, defined as q = (4π/λ) sin(θ/2) (θ: the scattering angle), was calibrated by silver behenate (AgBh) having a spacing of 5.84 nm. The 2D-WAXD patterns were recorded by the Image Plate Mar345, and the calibration was done by titanium dioxide (TiO2) having a spacing of 0.35 nm. The X-ray wavelength, λ, was tuned at λ = 0.155 nm. Thermal property profiles were measured by differential scanning calorimeter (DSC) (DSC 823 METTLER TOLEDO) under N2 atmosphere. For an isothermal treatment of Avrami’s plot, a thermal history was first erased at 200 °C for 5 min followed by rapidly decreasing the temperature to isothermal temperatures (Tiso) at 100 °C/min cooling rate. Then, the sample was isothermally held at that Tiso for 15 min. A plot of storage modulus (G′) and temperature in Figure 2 was obtained from rheometer, Discovery Hybrid DHR3, TA Instrument, with 5 °C/min cooling rate from 200 °C and 0.1 rad/s of angular frequency. Specimen Preparation. NA was mixed with PP powder in the twin-screw extruder at 500 ppm and 2000 ppm for SPES and DMBS, respectively. Those NA contents are the optimum based on our study. SPES-PP, DMBS-PP, and N-PP are abbreviated for SPES-added, DMBS-added PP and neat PP, respectively. The 1-mm-thick plate specimen was prepared by injection molding with the injected and the molded temperatures at 180 and 50 °C, respectively. For the compression test,

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RESULTS AND DISCUSSION It has been known that the orientation and the morphology of the polymer crystals are induced by the morphology of NA. Thus, at first, the morphology of NAs needs to be clarified. Figure 1 shows the SEM micrographs of the SPES and the DMBS. The results show the morphology of the SPES as rodlike structure, having the diameter of almost a micrometer (Figure 1a,b). For the DMBS, the size is much smaller than the SPES with similar morphology as shown in Figure 1c,d. These indicate that a spherulite or a cluster size of the lamellar crystals should be smaller in the case of the DMBS-PP. Consequently, the haze of the DMBS-PP (17.5%) is the smallest value, followed by SPES-PP (37.9%) and N-PP (64.0%), respectively. This is a unique property of the soluble NA which leads to the efficient NA-dispersion. For the sorbitol-based NA, it has been known that the NA is formed in a three-dimensional fibril network when it is cooled down to a certain temperature, socalled gelation temperature (Tgel), from the melt. Shepard et al. 2332

dx.doi.org/10.1021/ie4039136 | Ind. Eng. Chem. Res. 2014, 53, 2331−2339


Article

−ln(1 − X(t )) = Kt n

reported that the Tgel of methylbenzylidene sorbitol (MDBS) in PP depends on the added concentration, ranging from 130 °C to almost 200 °C.16 In our case, the Tgel of the DMBS is approximately 160 °C as observed in the sharp increase of the storage modulus, in which the Tc of the PP crystals is approximately 140 °C (Figure 2). It should be noted that the

(1)

where X(t ) =

t dH . dt dt ∞ dH . dt 0 dt

∫0 ∫

(2)

X(t) is the fractional crystallinity at time t which is calculated from the enthalpy of crystallization (Figure 3a,c) starting from the beginning of the crystallization (t = 0) to time t, where it is normalized with the total crystallinity (eq 2). K and n are the crystallization rate constant and the Avrami’s exponent, respectively. Those parameters can be determined by an intersection and a slope of ln(−ln(1 − X(t))) vs ln(t) plot as shown in Figure 3b,d. Half-time crystallization (t1/2) can be defined as the time for the crystallization progressing to half of the total crystallinity. By substituting X(t) with 0.5, t1/2 can be calculated by the following equation:

K=

ln 2 n t1/2

(3)

The crystallization-kinetic parameters of the isothermal crystallization at 130 and 132 °C are shown in Table 1. It is clear that the NAs dramatically increase the crystallization rate of the N-PP, approximately 104−105 times. Moreover, the Tc values of the SPES-PP and the DBS-PP are shifted about 15 °C higher and the crystallinity values are increased about 4−5% in comparison with the N-PP. It should be noted that the SPESPP shows faster crystallization than the DMBS-PP, at the same levels of Tc and crystallinity. This is important since the fast crystallization normally leads to low haze value of the PP specimen due to large amount of crystal nucleation being formed. Consequently, this leads to smaller size of the spherulytic crystal obtained.21,22 However, in this case, even

Figure 2. Storage modulus (G′) profiles as a function of temperature for N-PP (●), SPES-PP (▲), and DMBS-PP (■).

temperature used for the injected-mold specimen is 180 °C which is higher than the Tgel of the DMBS. Thus, it is confirmed that the network formation of the DMBS occurs in the DMBS-PP specimen. Crystallization kinetics is one of the important factors which manipulates the structure of the PP crystals. In order to study this in detail, several crystallization kinetic parameters are needed to be evaluated via Avrami’s equation,17−20

Figure 3. Isothermal DSC profiles of N-PP (solid black line), SPES-PP (dashed red line), and DMBS-PP (dashed blue line) at (a) 130 °C and (c) 132 °C, and their Avrami’s plots; (b) for (a) and (d) for (c). 2333



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Table 1. Avrami’s Parameters Including Transition Temperatures and Crystallinity of N-PP, SPES-PP, and DMBS-PP Tiso = 130 °C samples N-PP SPES-PP DMBS-PP

Tma (°C) 156 159 158

Tca (°C) 116 130 128

crystallinityb (%) 45.0 49.2 49.6

K130

t1/2 (s) −8

1.58 × 10 4.94 × 10−3 3.57 × 10−4

541 20 32

Tiso = 132 °C n 2.80 1.65 2.19

K132 −8

1.55 × 10 4.51 × 10−4 5.00 × 10−5

t1/2 (s)

n

940 36 51

2.57 2.07 2.42

Measured from DSC profiles of second heat at 10 °C/min of heating rate. bObtained from crystalline−amorphous curve fitting of 1D-WAXD profile. a

Figure 4. (A) Schematic drawing describing specimens prepared for X-ray scattering experiments and (B, a) directions in a 2D-SAXS pattern and (B, b) its schematic model representing crystal lamella.

morphology from the skin to core is assumed. Thus, the crystal structure of the specimen is measured as average. For these cases, the 2D-SAXS results appear to have the intensity preferentially peaked in the meridional direction (Figure 4B(a)). The patterns reflect the stack of the lamellar crystal being oriented perpendicular to the meridional direction (or its normal vector (nL) being aligned parallel to the meridional direction) as shown in Figure 4B(b). Figure 5 shows the 2D-SAXS patterns of the N-PP (Figure 5a−c), the SPES-PP (Figure 5d−f), and the DMBS-PP (Figure 5g−i) in ND, TD, and ID, respectively. It should be noted that the scattering streak in the equatorial direction in the ID and the TD patterns might originate from the scattering of the specimen edge. The results reveal that the preferential orientation of the lamellar crystals in the perpendicular

though the DMBS-PP performs at a slower crystallization rate than the SPES-PP, the haze value of the DMBS-PP is still smaller. This indicates that the small size and the good dispersion of the DMBS are dominant which renders the smaller spherulite size. Furthermore, the n value which implies the crystal dimension suggests that the SPES-PP has less dimensional structure of the crystal spherulite than the DMBSPP. To analyze the polymer crystal in depth, 2D-SAXS and -WAXD techniques were used. Each specimen was measured in three directions based on the injected flow direction (IFD) or polymer flow direction, i.e., injection direction (ID), normal to injection direction (ND) and transverse-injection direction (TD) as shown in Figure 4A. It should be noted that, due to the thin specimen, the minimum gradient of the crystal 2334



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Figure 5. 2D-SAXS patterns of N-PP (a, b, and c), SPES-PP (d, e, and f), and DMBS-PP (g, h, and i) in ND, TD, and ID, respectively.

direction to the IFD, or z direction, observed in the ND and the TD patterns already exists in the N-PP (Figure 5a,b). This might be resulted from the orientation of the PP chain under shear during the injection. As the NAs were added, the accumulated SAXS intensity for the ND and the TD patterns appear to have more preferentiality in the meridional direction. This is quite interesting for the ID patterns, in which the NA seems to play a role. In case of the N-PP, the preferential accumulation of the scattering intensity in the meridional direction was clearly observed, while the accumulations in the meridional and equatorial directions were seen in the SPES-PP. For the DMBS-PP, the circular pattern was clearly seen, indicating the existence of the randomly orientated lamellae in the y−z plane. This result suggests that the soluble DMBS added during the injection process leads to the random orientation of the lamellar crystals in planes perpendicular to the IFD. To quantitatively analyze the crystal structures in details, several structural parameters were calculated from the patterns in Figure 5. The degree of orientation of the lamellar crystals was estimated from the full-width at the half-maximum (fwhm) of intensity distribution along the radial scan of the 2D patterns

(Figure 5). The lamellar repeating period (D), as schematically shown in Figure 4B(b), was calculated from Bragg’s law (D = 2π/q). The mean core thickness, T0, the mean lamellar thickness, ⟨T⟩, and the long period, L, were estimated from the 1D electron density correlation function, K(z), which is defined by the following equation:23 K (z) = ⟨[η(z′) − ⟨η⟩][η(z + z′) − ⟨η⟩]⟩ =2

∫0

∞

(π )−1q2I(q) cos(qz) dq

(4)

where ⟨⟩ indicates the ensemble average. η(z) and ⟨η⟩ are the electron density along the lamellar normal and the averaged electron density, respectively. The values of T0, ⟨T⟩, and L were determined from the K(z) profiles, as shown in Figure 6a. The results in Table 2 clearly show that NAs induce higher lamellar orientation perpendicular to IFD, as indicated by the smaller fwhm of the scattering peaks. The SPES-PP has the lowest fwhm, indicating the highest degree of the orientation. This might be resulted from the NA morphology. The rod-like shape with the high aspect ratio of the SPES can induce preferential orientation in the parallel direction to the IFD. As it 2335



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Furthermore, the dimension of the lamellar crystals is modified by the NA. Figure 6a reveals that the K(z) profile of the N-PP is significantly shifted as compared to those of the SPES-PP and the DMBS-PP. This results in the smaller values of T0, ⟨T⟩, and L for N-PP (Table 2), suggesting that the NAs increase the thickness of the PP lamellar crystals. As shown earlier, the degree of orientation of the lamellar crystals can be estimated by the fwhm of the scattering peaks. An alternative approach can also be used to quantitatively analyze the lamellar orientation, i.e., the Hermans orientation function ( f) calculated from the 2D-WAXD pattern. In general, the f function is defined as follows:24,25 3⟨cos2 φ⟩ − 1 2

f=

2

⟨cos φ⟩ =

∫0

π /2

∫0

(5)

I(φ)cos2 φ sin φ dφ π /2

I(φ) sin φ dφ

(6)

where φ is the angle between the interested axis (or c-axis which is parallel to the chain axis as shown in Figure 4B(b)) and the reference direction (IFD in this case). The f value is zero in the case of the random orientation. The f value is the unity and −0.5 in the cases of the interested axis oriented parallel and perpendicular to the reference direction, respectively. The f values can be separately calculated for the crystal axes, in which the ω, μ, and σ are defined as the angles with respect to a*, b, and c axes of the IFD, respectively. Thus, the f functions are defined as26 fa * = Figure 6. Profiles of electron density correlation function K(z) (a), showing structural parameters of crystal lamella, i.e., T0, ⟨T⟩, and L, to be extracted, and (b) K(z) of N-PP (solid black line), SPES-PP (dashed red line) and DMBS-PP (dashed blue line).

Table 2. Structural Parameters Obtained from 2D-SAXS Patterns of N-PP, SPES-PP, and DMBS-PP in ND, ID, and TD samples N-PP: ND N-PP: ID N-PP: TD SPES-PP: ND (φ = 270°) SPES-PP: ND (φ = 180°) SPES-PP: ID SPES-PP: TD DMBS-PP: ND DMBS-PP: ID DMBS-PP: TD

T0 (nm)

⟨T⟩ (nm)

L (nm)

D (nm)

fwhm (°)

3.23 3.54 2.99 3.78

4.16 4.28 3.73 4.34

10.02 10.00 9.03 11.00

6.27 6.26 6.01 6.60

72.3 78.2 65.5 40.7

3.79

4.34

11.00

6.99

35.0

3.23 3.79 3.97 3.29 3.17

4.10 4.30 4.41 4.10 4.22

10.00 11.00 11.00 10.00 10.34

6.62 6.62 6.77 7.27 6.55

40.2 45.8 48.0 NA 47.5

3⟨cos2 ω⟩ − 1 2

(7)

fb =

3⟨cos2 μ⟩ − 1 2

(8)

fc =

3⟨cos2 σ ⟩ − 1 2

(9)

and fa * + fb + fc = 0

(10)

f b can be directly calculated from the scattering of the (040) plane in eqs 8 and 11 and fc is calculated from the scattering peaks of (040) and (110) planes as indicated in eqs 9 and 12. Then, the fa* is obtained from eq 10. ⟨cos2 μ⟩ = ⟨cos2 φ(040)⟩

(11)

⟨cos2 σ ⟩ = 1 − 1.099⟨cos2 φ(110)⟩ − 0.901⟨cos2 φ(040)⟩ (12)

Figure 7 shows the calculated Hermans orientation functions. From the results, fc is positive while f b and fa* are negative for all the samples, indicating the preferentially oriented lamellae perpendicular to the IFD. fc of the SPES-PP has the highest value followed by the DMBS-PP and the N-PP, respectively. This suggests that the degree of orientation of the hPP chains in the lamellar crystals with respect to the IFD is highest for the SPES-PP and lowest for the N-PP. In other words, the result confirms that the SPES induces the lamellar preferential orientation in the perpendicular direction to the IFD. It is important to note that even DMBS is soluble in the polymer

is well-known for the PP crystallization with the SPES, the lamellar crystal of PP epitaxially grows on the SPES surface. In other words, the lamellar crystals are oriented perpendicular to the SPES surface (or nL in parallel direction to the SPES surface). Yoshimoto et al. have revealed that the helical pitch of PP molecules and the repeating period of the SPES crystals are comparable. Thus, this promotes the crystal growth of PP in the epitaxial manner on the SPES crystal surface. 10 2336



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and the TD as shown in Figure 8a, in which the cut specimens were then compressed longitudinally (Figure 8b). The results in Figure 9 show that, for all samples, the compression

Figure 7. Hermans orientation function ( f) of N-PP, SPES-PP, and DMBS-PP in crystallographic axes: a* axis (fa*) (■), b axis ( f b) (▲), and c axis ( fc) (●) in the ND. Figure 9. Compression strength of N-PP, SPES-PP, and DMBS-P; in the ID (●) and the TD (▲) and shrinkage; in the ID (○) and the TD (△).

melt during the specimen injection, and the crystal orientation is also promoted as compared to N-PP. This might be due to the orientation of the DMBS network at high temperature as reported by Sreenivas et al.27 Furthermore, another similar case was shown by Yamaguchi et al. N,N′-dicyclohexyl-2,6naphthalenedicarboxamide, NU-100, which can be melted above 280 °C, was added in the hPP for the extrusion process under high shear. After cooling from the melt, NU-100 recrystallized to a needle-like crystal which is oriented along the extrusion flow direction (EFD). The result showed that the lamellar orientation was promoted to preferentially orient perpendicular to the EFD.28 To investigate effects of the lamellar orientation on the physical properties, the compression strength and the shrinkage of the specimens were measured in both the ID and the TD. For the compression test, the injected sheet was cut in the ID

strengths in the TD are significantly higher than those in the ID. This indicates that the lamellae oriented parallel to the compression direction yield a higher compression resistance than the others (Figure 8b). It has been suggested that the crystal lamellae with the PP chains aligned in the parallel direction to the external force, i.e., compressive force in this case, tend to be deteriorated due to the crystallographic slip.29 This might be the reason why the lamellae oriented perpendicular to the compression direction (or PP chains oriented parallel to the compression direction) in the ID is weaker in this case. For the compression strength in the TD, the SPES-PP shows a slightly higher value than that of the

Figure 8. Schematic drawings describing (a) specimen prepared and (b) compressive force direction for compression test. 2337



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DMBS-PP, while that of the N-PP is the lowest. This may be explained from the lamellar orientation. fwhm and fc of the SPES-PP indicate the highest orientation of the crystal lamellae in the direction perpendicular to the IFD. In other words, for the TD, the SPES-PP has the lamellae with the highest degree of orientation parallel to the compression direction, followed by the DMBS-PP and the N-PP, respectively. Furthermore, it should be noted that the lamellar thickness and length of the NPP are small in comparison with the SPES-PP and the DMBSPP, and this might contribute to the low compression strength as well. In case of the ID, the compression strength of the DMBS-PP is the highest one, while the N-PP is still the lowest (Figure 9). The dimension of the lamellar crystals might play a role in this case, as observed from the ⟨T⟩ and D values in the ND that the DMBS-PP have the largest values (Table 2). These should be related to the high compressive resistance in the ID. It is important to note that the difference of the compressive strength between the ID and the TD is small for the DMBS-PP, as compared to those of the N-PP and the SPES-PP. This might result from the random orientation of the lamellae in the ID in case of the DMBS-PP. For the shrinkage, it has been found that the shrinkage in the TD is normally higher than that in the ID for the injected-mold polymers.30 Furthermore, the rod-shape fillers, especially the reinforced fiber, promotes the shrinkage in the TD.31 It is known that the shrinkage is a result of the gradual crystallization after sudden cooling in the mold. Thus, as shown in the schematic illustration of the oriented lamellae in Figure 4B(b), the volumetric change of the injected-mold specimen in the direction perpendicular to the nL should be higher due to the packing of the folded polymer chains, which causes the volumetric shrinkage in the TD. As seen in Figure 9, the shrinkage values of all samples in the TD are higher than those in the ID. The shrinkage of the SPES-PP in the TD was found to have the highest value, which is consistent with the highest degree of orientation of the lamellae perpendicular to the ID. However, similar to the case of the compression strength, it should be noted here that the difference of the shrinkage values between the ID and the TD for the DMBS-PP is the smallest, which might result from the random orientation of the lamellar crystals in the ID.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +66-3891-2828. Fax: +66-3868-4676. E-mail: wonchalr@ scg.co.th. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from Thai Polyethylene Co., Ltd., (TPE). The authors would like to acknowledge all researchers and staff at SLRI who kindly facilitated the X-ray experiments. The author (W.R.) would like to thank Dr. Taiyo Yoshioka and Pimsai Tanphibal for their support on the X-ray scattering analysis.

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REFERENCES

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CONCLUSIONS

The structure and orientation of the PP crystals were induced by the NAs. It was found that the SPES promotes the lamellae oriented perpendicular to the IFD, resulting in the high compressive strength and shrinkage value in the TD. For the DMBS, random orientation of the PP crystals was found in the ID, which might come from the soluble characteristic in the polymer melt of the DMBS. This random orientation results in more isotropic properties as indicated by low directional dependence of shrinkage and compressive strength. Furthermore, it was found that the size of the lamellar crystals might be one of the factors contributing to the compressive resistance in the ID, where the lamellar orientation has less effect. The lamellar thickness and the repeating period of the DMBS-PP are found to be largest, resulting in its high compressive strength in the ID. 2338



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