Elastomers and Composites

The Rubber Society of Korea

Elastomers Compos. 2021; 56(2):92-99

pISSN: 2092-9676, eISSN: 2288-7725

DOI: https://doi.org/10.7473/EC.2021.56.2.92

Article

Change of Crystalline Properties of Poly(ethylene-co-vinyl acetate) according to the Microstructures

Sung-Seen Choi¹^,^†, Yu Yeon Chung¹

¹Department of Chemistry, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, Republic of Korea

^†Corresponding author E-mail: sschoi@sejong.ac.kr

© Copyright 2021 The Rubber Society of Korea. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received: Jun 16, 2021; Revised: Jun 28, 2021; Accepted: Jun 30, 2021

Published Online: Jun 30, 2021

Abstract

Microstructure-dependent changes in the crystalline properties of poly(ethylene-co-vinyl acetate) (EVA) was investigated using various EVAs at different VA contents via X-ray diffraction (XRD). The parameters analyzed herein were percentage crystallinity (X_c), interplanar crystal spacing (d_hkl), crystal stack size (D_hkl), and the number of crystal plane piles (N_hkl). The X_cs of [110] and [200] crystals were 21.0-4.1 and 6.7-1.4%, respectively, and they decreased by approximately 2.3 and 0.7% for every mol% of the VA content, respectively. The X_c ratios of the [110] and [200] crystals were approximately 3. The d₁₁₀s and d₂₀₀s values were 0.41-0.42 and 0.37-0.38 nm, respectively. The D₁₁₀s and D₂₀₀s values were 9.56 -21.92 and 7.00-16.42 nm, respectively. The d_hkls increased with an increase in the VA content, whereas the D_hkls decreased. The N₁₁₀s and N₂₀₀s were 22.7-51.3 and 18.3-43.2, respectively, and they decreased by increasing the VA content. EVA with the same VA content showed different crystalline properties as per the suppliers, and some EVAs deviated from the average trends. This could be explained by the difference in their microstructures such as the sizes and distribution uniformity of the ethylene sequences in EVA chains.

Keywords: poly(ethylene-co-vinyl acetate); crystalline properties; microstructures; X-ray diffraction

Introduction

Poly(ethylene-co-vinyl acetate) (EVA) is a copolymer of ethylene and vinyl acetate (VA), and the VA units are randomly distributed throughout the ethylene polymer backbone. EVA is a transparent thermoplastic copolymer, and it becomes more flexible and transparent by increasing the VA content. EVA has crystalline structures due to the ethylene sequences, and its properties and application fields are mainly depending on the VA content.^1-6 The crystalline structures are influenced by the VA units (Scheme 1). Crystallinity and melting points of EVA decrease with increase in the VA content.⁷ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) are common analysis techniques to investigate crystalline properties of polymeric materials.^8-22 It is well known that degree of crystallinity influences on properties of polymers, and XRD is widely used for analysis of the crystallinity of polymeric materials.

Scheme 1. Crystalline structure formed by the ethylene block of EVA.

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Length of the ethylene sequence of EVA is restricted by positions of the VA units as shown in Scheme 2. Paraffin wax has stacked lamella crystals due to relatively short methylene sequence, while polyethylene (PE) has chain-folding crystalline structures due to long methylene sequence.^23-27 EVA should have stacked lamella crystals like paraffin wax because of relatively small ethylene block size. In general, EVAs have two crystalline structures of [110] and [200]. Crystalline structure and ethylene block size of an EVA should be influenced not only by the VA content but also by positions of the VA units in an EVA chain. Thus, crystalline structures and ethylene block sizes of EVAs with the same VA content may be different each other if the distribution patterns (or uniformities) of the VA units are different as described in Scheme 2.

Scheme 2. Microstructures of EVA depending on the positions of vinyl acetate units.

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In this study, crystalline properties of various EVAs with the VA contents of 3.3-17.8 mol% (9.5-40 wt%) were analyzed using XRD and influence of microstructures of the EVAs on the crystalline properties was investigated. Percentage crystallinity (X_c), interplanar crystal spacing (d_hkl), and crystal stack size (D_hkl) were compared as the crystalline properties. Three sets of EVAs having the same VA contents were employed to examine the influence of microstructures on the crystalline properties in detail. Relative superiority for the percentage crystallinity and the number of crystal plane piles (N_hkl) of [110] and [200] crystals was also investigated.

Experimental

Twelve EVAs with different VA contents of three suppliers were used: four EVAs of Aldrich Co. (USA) (marked to EVA 12, EVA 18, EVA 25, and EVA 40 with the VA contents of 12, 18, 25, and 40 wt%, respectively), four EVAs of DuPont Co. (USA) (EVA 770, EVA 450, EVA 360, and EVA 265 with the VA contents of 9.5, 18, 25, and 28 wt%, respectively), and four EVAs of Hanwha Co. (Republic of Korea) (EVA 1315, EVA 1316, EVA 1317, and EVA 1328 with the VA contents of 15, 18, 22, and 28 wt%, respectively). Weight percentages (wt%) of the VA contents were converted to the mole percentages (mol%) (Table 1). Three EVAs of EVA 18, EVA 450, and EVA 1316 have the same VA content of 6.7 mol%. Two sets of EVAs (EVA 25/EVA 360 and EVA 265/EVA 13128) have the same VA contents of 9.8 and 11.2 mol%, respectively.

Table 1. EVAs Used in This Study

EVA	Vinyl acetate content (mol%)
EVA 770	3.3
EVA 12	4.3
EVA 1315	5.4
EVA 18	6.7
EVA 450	6.7
EVA 1316	6.7
EVA 1317	8.4
EVA 25	9.8
EVA 360	9.8
EVA 265	11.2
EVA 1328	11.2
EVA 40	17.8

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Crystalline properties of EVA samples were measured using XRD. XRD analysis was performed using a D-max 2500/PC diffractometer (Rigaku Co., Japan) with Cu-K_α radiation. The accelerating voltage and electric current were 40 kV and 100 mA, respectively. The samples were scanned from 10° to 35° at a scan speed of 2°/min. The degree of crystallinity determined by XRD was calculated as the percentage of the scattered intensity of the crystalline phase over the sum of the scattered intensities of the crystalline and amorphous phases. The percentage crystallinity was acquired from the peak at 12° to 28° using XRD analysis software (Mdi Jade 6.0).

The percentage crystallinity (X_c, %) was calculated using equation (1).

X c (%) = 100 × I c / (I c + I a)

(1)

where I_c and I_a are the sums of the intensities of the crystalline and amorphous regions, respectively. The interplanar crystal spacing of (hkl) planes, d_hkl was calculated according to Bragg’s law of equation (2).

d h k l = λ / 2 sin θ

(2)

where λ is the wavelength of the X-ray (Cu-K_α = 0.154 nm) and θ is the Bragg angle. The stack (or particle) size of crystal was calculated according to Scherrer formula of equation (3).

D h k l = K λ / (B 1 / 2 cos θ)

(3)

where D_hkl is the interfacial distance (perpendicular to the plane of the crystal particle), K is the Scherrer constant (0.89 is selected), λ is the wavelength of the X-ray, B_1/2 is the FWHM (radian), and θ is the Bragg angle.

Results and Discussion

Raw XRD pattern was smoothed for following deconvolution process to separate the crystalline and amorphous regions. The smoothed curve was deconvoluted to one amorphous and two crystalline peaks as shown in Figure 1. Two crystalline peaks at 2θ = 21.4 and 23.7° corresponding to [110] and [200] crystals, respectively, were observed. Full width at half maximum (FWHM) of the crystalline peak was also measured. The Bragg angles (2θ) and FWHMs of EVAs were summarized in Table 2. Ranges of the Bragg angles of [110] and [200] peaks were 21.04-21.68 and 23.26-23.92°, respectively. Ranges of the FWHMs of [110] and [200] peaks were 0.76-1.67 and 0.99-2.29°, respectively. The FWHMs of [110] peaks were narrower than those of [200] ones. The FWHM of [110] peak tended to increase as the VA content increased.

Figure 1. XRD pattern and deconvolted peaks of EVA 18. The XRD pattern was deconvoluted into the amorphous and crystalline peaks. FWHM denotes the full width at half maximum of the peak.

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Table 2. Bragg Angles (2θ) and FWHMs of EVAs Obtained from the XRD Analysis Results (°)

EVA	2θ		FWHM
EVA	[110]	[200]	[110]	[200]
EVA 12	21.45	23.68	0.765	1.191
EVA 18	21.42	23.40	0.919	0.978
EVA 25	21.62	23.92	1.028	1.112
EVA 40	21.04	23.26	1.672	2.294
EVA 770	21.68	23.88	0.761	1.032
EVA 450	21.44	23.56	0.924	1.049
EVA 360	21.66	23.78	0.924	1.249
EVA 265	21.24	23.48	0.936	1.195
EVA 1315	21.44	23.54	0.844	1.220
EVA 1316	21.54	23.86	0.878	1.195
EVA 1317	21.26	23.61	1.141	1.195
EVA 1328	21.10	23.31	1.178	1.605

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The X_c variation with the VA content was plotted as shown in Figure 2. The total X_c of [110] and [200] crystals decreased by about 3% every 1 mol% of the VA content. The individual X_cs of [110] and [200] crystals decreased by about 2.3 and 0.7%/mol%, respectively. After 10 mol% of the VA content, the decrement was very slight. There were some cases largely deviated from the fitted curve. For EVAs with 9.8 mol% of the VA content (EVA 25 and EVA 360), the X_cs of EVA 25 were lower than the fitted line but those of EVA 360 were much higher than the fitted line. If the VA units are evenly distributed, there will be few large ethylene blocks and crystalline structures may not be well developed. If the VA units of an EVA with 10.0 mol% of the VA content are absolutely evenly distributed, the ethylene block size is ‘9’ of ~VA-(CH₂CH₂)₉-VA~. Difference in the melting points of paraffin wax and its ester is about 15°C, and the total number of ethylene units not to participate in crystal formation by the prevention of VA units in both sides is about ‘8’.^28,29 Thus, the number of ethylene sequence to participate in the crystal formation is just ‘1’ for an EVA with the VA content of 10.0 mol% when the VA units are completely evenly distributed. Hence, it can be expected that distribution uniformity of the VA units of EVA 25 will be greater than that of EVA 360.

Figure 2. Variation of the total, [110], and [200] crystallinities of EVA with the vinyl acetate content. The squares, circles, and triangles stand for the EVAs of Aldrich Co., DuPont Co., and Hanwha Co., respectively. The solid, open, and crossed symbols indicate the total, [110], and [200] crystallinities, respectively. The three dash lines are the curve fitted lines less than 10 mol% of the vinyl acetate content for the total, [110], and [200] of y = -2.98x + 37.68 (r = 0.975), y = -2.27x + 28.61 (r = 0.977) and y = -0.72x + 9.06 (r = 0.945), respectively.

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There are two crystalline structures of ethylene blocks of ~VA-(CH₂CH₂)_n-VA~ (n > 20 and 15 < n < 20) in EVA.²⁹ The large and small ethylene block sizes may belong to the [110] and [200] crystals, respectively. The X_c ratio of [110] and [200] crystals (X_c[110]/X_c[200]) was plotted as a function of the VA content as shown in Figure 3. The X_c ratio on the whole slightly decreased with increase in the VA content. It means that crystal formation of the big ethylene block was more decreased than that of the small one by increasing the VA content. Except for three EVAs (EVA 18, EVA 265, and EVA 1328), the X_c ratios were about 3. The X_c ratio of EVA 18 (6.7 mol% of the VA content) was larger than those of the other EVAs. This is because EVA 18 has higher [110] crystallinity as well as lower [200] one compared to the other EVAs with the same VA content as shown in Figure 2. This implies that there are longer ethylene sequences in EVA 18 due to more inhomogeneous distribution of the VA units. The X_c ratios of EVA 265 and EVA 1328 (their VA contents are the same of 11.2 mol%) were much lower than those of the other EVAs since they had relatively higher X_cs of the [200] crystal. Thus, EVA 265 and EVA 1328 should have more homogeneous distribution of VA units than the other EVAs.

Figure 3. Variations of the crystallinity ratio of [110]/[200] of EVA with the vinyl acetate content.

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Variations of the d_hkl and D_hkl were plotted as a function of the VA content as shown in Figures 4 and 5, respectively. The d₁₁₀s and d₂₀₀s were 0.410-0.422 and 0.372-0.382 nm, respectively (Figure 4). The d₁₁₀s were greater than the d₂₀₀s and the difference was about 0.04 nm. The d₁₁₀ and d₂₀₀ on the whole increased as the VA content increased, and the increment of d₁₁₀ was slightly larger than that of d₂₀₀. This means that distance between EVA chains becomes longer by prevention of the VA units. The D₁₁₀s and D₂₀₀s were 9.564-21.918 and 6.997-16.417 nm, respectively (Figure 5). The D₁₁₀ was greater than the D₂₀₀, and the difference became smaller by increasing the VA content. The D₁₁₀ and D₂₀₀ on the whole decreased with increase in the VA content, and the decrement of D₁₁₀ was greater than that of D₂₀₀. The decreased D_hkls can be explained by reduction of the number of ethylene sequences participating in the crystal formation. Some EVAs of the same supplier had similar D_hkls though their VA contents are very different. For instance, EVA 450, EVA 360, and EVA 265 (6.7-11.2 mol% of the VA content) had similar D₁₁₀ of 17.32-17.09, and EVA 1315, EVA 1316, and EVA 1317 (5.4-8.4 mol% of the VA content) had similar D₂₀₀ of 13.16-13.45. The D_hkls of EVAs with the same VA content showed big difference according to the suppliers. For EVA 18, EVA 450, and EVA 1316 with the same VA content of 6.7 mol%, the order of D₁₁₀ was EVA 1316 > EVA 18 ~ EVA 450, whereas that of D₂₀₀ was EVA 18 > EVA 450 > EVA 1316. For EVA 25 and EVA 360 with the VA content of 9.8 mol%, the D₁₁₀ of EVA 360 was greater than that of EVA 25, whereas the D₂₀₀ of EVA 360 was smaller than that of EVA 25. Thus, it can lead to a conclusion that the D_hkls are largely influenced by the VA content and the distribution uniformity of VA units.

Figure 4. Variations of the interplanar crystal spacings (d_hkls) with the vinyl acetate content. The squares, circles, and triangles stand for the EVAs of Aldrich Co., DuPont Co., and Hanwha Co., respectively. The solid and open symbols indicate the d₁₁₀ and d₂₀₀, respectively. The dash lines are the curve fitted lines for the d₁₁₀ and d₂₀₀ of y = 7.010 × 10^-4x + 0.409 (r = 0.637) and y = 4.954 × 10^-4x + 0.373 (r = 0.485), respectively.

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Figure 5. Variations of the interfacial distances (D_hkls) with the vinyl acetate content. The squares, circles, and triangles stand for the EVAs of Aldrich Co., DuPont Co., and Hanwha Co., respectively. The solid and open symbols indicate the D₁₁₀ and D₂₀₀, respectively. The dash lines are the curve fitted lines for the D₁₁₀ and D₂₀₀ of y = -0.751x + 23.092 (r = 0.901) and y = -0.522x + 17.625 (r = 0.782), respectively.

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The number of crystal plane piles (N_hkl) was calculated using equation (4).³⁰

N h k l = D h k l / d h k l

(4)

The N₁₁₀s and N₂₀₀s were 22.7-51.3 and 18.3-43.2, respectively. Figure 6 shows variations of the N₁₁₀ and N₂₀₀ with the VA content. The N₁₁₀ decreased with increase in the VA content by about twice every 1 mol% of the VA content, and it will be lower than 10 when the VA content is higher than 24.8 mol%. This implies that not only the ethylene block size but also the number of EVA chains participating in the [110] crystal formation decreased by increasing the VA content. The N₂₀₀ did not show notable change until about 10 mol% of the VA content and then showed a decreased trend. The N₁₁₀s were generally larger than the N₂₀₀s, but EVA 18, EVA 25, and EVA 1317 showed the reverse trends. The larger N₁₁₀s than N₂₀₀s implies that the number of EVA chains with long ethylene sequences is greater than that of EVA chains with short ones. On the contrary to this, for EVA 18, EVA 25, and EVA 1317, the number of EVA chains with long ethylene sequences might be smaller than that of EVA chains with short ones.

Figure 6. Variations of the numbers of crystal plane piles (N₁₁₀ and N₂₀₀) of the [110] and [200] crystalline peaks of EVA with the vinyl acetate content. The squares, circles, and triangles stand for the EVAs of Aldrich Co., DuPont Co., and Hanwha Co., respectively. The solid and open symbols indicate the crystalline structures of [110] and [200], respectively. The dash lines are the curve fitted lines for [110] and [200] crystals of y = -1.86x + 56.15 (r = 0.900) and y = -1.42x + 47.07 (r = 0.787), respectively.

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The N₁₁₀s and N₂₀₀s of EVAs with the same VA content also showed different trends according to the suppliers. For EVA 18, EVA 450, and EVA 1316 with the same VA content of 6.7 mol%, the N₁₁₀ of EVA 1316 was greater than those of the others whereas the N₂₀₀ of EVA 1316 was smaller than those of the others. And total N_hkls of the N₁₁₀ and N₂₀₀ were also different each other. The total N_hkls of EVA 18, EVA 450, and EVA 1316 were 85.2, 82.3, and 80.3, respectively. Flexibility and transparency of EVA with smaller N_hkls might be better than those of EVA with larger ones. For EVA 25 and EVA 360 with the same VA content of 9.8 mol%, the N₁₁₀ of EVA 360 was greater than that of EVA 25 but the N₂₀₀ of EVA 360 was smaller than that of EVA 25 though the total N_hkls were the same of 76.7 and 76.6 for EVA 25 and EVA 360, respectively. For EVA 265 and EVA 1328 with the same VA content of 11.2 mol%, both the N₁₁₀ and N₂₀₀ of EVA 265 were greater than those of EVA 1328. The total N_hkl of EVA 265 (76.3) was greater than that of EVA 1328 (58.5) by 1.3 times. This denotes that EVA 1328 would be more flexible and transparent than EVA 265.

Figure 7 shows variation of the N₁₁₀/N₂₀₀ ratio with the VA content. Except for some EVAs, the N₁₁₀/N₂₀₀ ratios were about 1.2. This implies that the number of EVA chains participating in [110] crystal formation is greater than that of EVA chains participating in [200] one by about 20%. EVA 12 had the greatest N₁₁₀/N₂₀₀ ratio of 1.41. The N₁₁₀/N₂₀₀ ratios of EVA 450, EVA 18, and EVA 25 were nearly 1.00 (1.03, 0.97, and 0.98, respectively). The N₁₁₀/N₂₀₀ ratio of EVA 1317 (0.94) was lower than the others. EVAs with the average low N₁₁₀/N₂₀₀ ratio might have the long ethylene sequences less than the short ones. Especially, EVA 1317, EVA 25, and EVA 18 would have the long ethylene sequences less than the short ones as discussed previously.

Figure 7. Variation of the N₁₁₀/N₂₀₀ ratio with the vinyl acetate content. N₁₁₀ and N₂₀₀ are the numbers of crystal plane piles of [110] and [200], respectively. The squares, circles, and triangles stand for the EVAs of Aldrich Co., DuPont Co., and Hanwha Co., respectively.

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Influence of the VA content and distribution uniformity of EVA on the crystalline properties was summarized in Table 3. The total X_c notably decreased by increasing the VA content, while it slightly decreased by increasing the distribution uniformity. The X_c[110]/X_c[200] ratio was more affected by the distribution uniformity than the VA content. The d₁₁₀ and d₂₀₀ slightly increased by increasing the VA content, but the distribution uniformity did not affect the d₁₁₀ and d₂₀₀. The D₁₁₀ and D₂₀₀ decreased by increasing the VA content, whereas the D₂₀₀ slightly increased by increasing the distribution uniformity. Variations of the N₁₁₀ and N₂₀₀ according to the VA content and distribution uniformity showed similar trends to those of the D₁₁₀ and D₂₀₀. The N₁₁₀/N₂₀₀ ratio slightly decreased by increasing the distribution uniformity, but the VA content did not affect the N₁₁₀/N₂₀₀ ratio.

Table 3. Influence of the Microstructures on the Crystalline Properties of EVA^*

Crystalline property	VA content	Distribution uniformity
Total crystallinity	↓↓↓	↓
X_c[110]/X_c[200]	↓	↓↓
d₁₁₀	↑	---
d₂₀₀	↑	---
D₁₁₀	↓↓	↓
D₂₀₀	↓	↑
N₁₁₀	↓	↓
N₂₀₀	↓	↑
N₁₁₀/N₂₀₀	---	↓

↑: slightly increased, ---: not influenced, ↓: slightly decreased, ↓↓: decreased, ↓↓↓: largely decreased

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Conclusions

Two crystalline peaks at 2θ = 21.0-21.7° and 23.2-24.0° of the [110] and [200] crystals of EVA were corresponded to the large and small ethylene block sizes, respectively. The X_c ratios of [110] and [200] were about 3 except for some EVAs, and it on the whole slightly decreased with increase in the VA content of EVA. The total X_c decreased by about 3%/mol%, while the individual X_cs of [110] and [200] decreased by about 2.3 and 0.7%/mol%, respectively. Formation of the crystalline structures of EVA became more difficult by increasing the distribution uniformity. The d₁₁₀s and d₂₀₀s were 0.410-0.422 and 0.372-0.382 nm, respectively, and they on the whole increased as the VA content increased. The D₁₁₀s and D₂₀₀s were 9.564-21.918 and 6.997-16.417 nm, respectively, and they on the whole decreased with increase in the VA content. The N₁₁₀s and N₂₀₀s were 22.7-51.3 and 18.3-43.2, respectively, and the N₁₁₀ decreased with increase in the VA content by about 2/mol%. The N₁₁₀/N₂₀₀ ratios were about 1.2 except for some EVAs. For some EVAs, the crystalline properties were largely deviated from the average trends. EVAs with the same VA content of different suppliers showed some different crystalline properties. These were explained by difference in their microstructures, especially degree of the distribution uniformity. Difference in the crystalline properties due to the microstructures could influence on properties of EVAs such as flexibility and transparency.

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