polymers

Article Different Dependence of Tear Strength on Film Orientation of LLDPE Made with Different Co-Monomer

Yi Ren 1,2 , Ying Shi 2, Xuerong Yao 3, Yujing Tang 3 and Li-Zhi Liu 2,* 1 School of Science, Shenyang University of Technology, Shenyang 110027, China; [email protected] 2 Advanced Manufacturing Institute of Polymer Industry, Shenyang University of Chemical Technology, Shenyang 110027, China; [email protected] 3 SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100176, China; [email protected] (X.Y.); [email protected] (Y.T.) * Correspondence: [email protected]; Tel.: +86-186-0027-6275

 Received: 19 January 2019; Accepted: 26 February 2019; Published: 6 March 2019 

Abstract: Crystal orientations, tear strength and shrinkage of Linear Low-Density (LLDPE) films made with different processes (compressed, cast and blown) were investigated. The films were made with three different LLDPE resins, respectively, which have similar density and molecular weight but are made with different comonomers (1-, 1- and 1-octene), in order to investigate if tear strength in Machine Direction (MD) of the LLDPE films made with different comonomer has similar dependence on crystal orientation. Our study indicates that the films made of 1-hexene and 1-octene based LLDPE resins have significantly higher intrinsic tear strength and less decrease in MD tear strength for a given film orientation. That is, for a given orientation in MD, the MD tear drops dramatically for films made with butene-based resin but much less decrease for the films made with hexene and octene-based resins. The shrinkage property at high temperature shows a good correlation with crystal orientation and the fraction of the crystals melted at this temperature.

Keywords: LLDPE; tear strength; orientation; film; shrinkage

1. Introduction Linear Low-Density PolyEthylene (LLDPE) is a random copolymer of and α-olefin made with Ziegler-Natta or metallocene catalysts [1]. LLDPE has a huge market worldwide and is used in many different industries. In fact, a very significant portion of LLDPE resins are used in polymer film industry, mainly for packaging applications. For such applications, mechanical properties, like tear and puncture strength, are very important. It is well known that the films made with LLDPE resins can have better tear and dart strength than the films made with other type of polyethylene, due to higher tie chain concentration introduced by Short Chain Branching (SCB) [2]. For similar branching degree and molecular weight, the tear strength of the film made of LLDPE with longer SCB (hexene and octene comonomers) is significantly better than the films made with butene comonomer [3]. Tie chain concentration is very important to the mechanical property of LLDPE films [4]. Generally speaking, for LLDPE with similar density (or branching degree), the tie chain concentration increases with molecular weight and SCB length determined by comonomer type (1-butene, 1-hexene or 1-octene) [5]. That is, the intrinsic tear strength of compressed films made of LLDPE with different comonomers can be very different [6]. From the manufacturing point of view, LLDPE films can be made with different processes, such as, blown and cast film processes, Machine Direction Orientation (MDO), double-bubble and biaxial

Polymers 2019, 11, 434; doi:10.3390/polym11030434 www.mdpi.com/journal/polymers Polymers 2019, 11, 434 2 of 13 stretching processes [7]. Studies have shown that the mechanical properties of LLDPE films in different directions can be very different, depending on film orientation [8]. It is well known that the tear strength in Machine Direction (MD) is usually significantly lower than that in Transverse Direction (TD) [9] for LLDPE films. With the increase in crystal orientation, the difference between the tear strength in MD and TD directions becomes more pronounced. In fact, the mechanical failure of the films is closely related to film orientation, such as, cracks are easier to be initiated in MD direction where crystals are preferably aligned [10]. Therefore, for most applications, minimized orientation is desired in order to achieve more balanced tear strength in MD and TD. Reducing crystal orientation is also very helpful to maximize dart and puncture strength. However, it remains an open question if the dependence of tear strength on crystal orientation for the films made of LLDPE resins with different α-olefin comonomer is same. In fact, many experiments need to be carried out in order to draw the conclusion about the dependence of tear strength on film orientation. In our previous work published in Chinese [11], two of the three film samples studied in the present work were also studied to mainly quantify the differences of tear strength obtained with Elmendorf method at very high deformation speed and with type C (right angle) method at a very slow deformation speed, as the type C method is widely used in China although it is invalid for many film applications. In fact, an in-depth tensile study of polyethylene films at very slow and very fast speeds was also carried out by our group [12]. The different crazing behavior and structural transitions at lamellar level were observed during slow and very fast stretching processes. In the present work, three LLDPE resins with similar density (or branching degree) and molecular weight but different comonomer type (1-butene, 1-hexene and 1-octene, respectively) were used to make films with different processes (compressed, cast and blown). The very different processes can ensure the orientation difference for the films made with the same resin is large enough. The films were investigated for tear strength and crystal orientation. Different dependence of MD tear strength on crystal orientation in MD is observed for the films made with LLDPE resins synthesized with different comonomers.

2. Materials and Methods

2.1. Materials LLDPE resins selected in this study are octene-based 2045 G from Dow Chemical (DOW, Midland, TX, United States), hexene-based 9030 (SINOPEC, Tianjin, China) and butene-based 9085 from Sinopec (SINOPEC, Tianjin, China). All materials were made with Ziegler-Natta catalyst. The resins have very close density; the molecular weight and molecular weight distribution of the resins are also very close, as shown in Table1. The mole faction of the comonomer for each resin is also listed in Table1.

Table 1. The characteristics of the resins used.

Comonomer Density Comonomer Sample MWD MW Type (g/cm3) Content (mol%) 2045G octene 0.920 3.85 1.27 × 105 2.4 9030 hexene 0.919 4.74 1.10 × 105 3.8 9085 butene 0.920 4.21 1.33 × 105 3.9

2.2. Film Preparation Blown films were made with COLLIN-BL180-400 (COLLIN, Maitenbeth, Germany) blown film machine with a processing temperature at 195 ◦C. The screw speed was 30 revolutions per minute (RPM) and the diameter for the die was 80 mm. The blow-up ratio (BUR) was 1.27, which is defined as the ratio between the diameter of the final bubble and the die diameter. The take up ratio (TUR) is 22.5, which is defined as the ratio of the speed at the take-up roll to the speed of the feed at the die exit. The frost line height (FLH) was 85, 63 and 95 mm for 2045G, 9030 and 9085, respectively, with a Polymers 2019, 11, 434 3 of 13 cooling temperature of 20 ◦C for dual-lip air ring. The resultant film thicknesses were 30, 28, 28 µm for 2045G, 9030 and 9085, respectively. Cast films were prepared by LabTec-LCR400 (LabTec, Praksa, Thailand) multilayer co-extrusion cast film machine. The barrel and die temperature was 230 ◦C and 235 ◦C, respectively. The temperature of cooling roller was 30 ◦C. The thicknesses of films were 56, 66 and 60 µm for 2045G, 9030 and 9085, respectively. Compressed films were prepared by LabTec-LPS50 (LabTec, Praksa, Thailand). The pre-weighed pellets were evenly placed in a mold and melted at 180 ◦C for 5 min and then compressed under 5 MPa for 10 min. The film was then cooled to room temperature at a rate of 15 ◦C/min. The film thickness obtained with this process is around 150 µm.

2.3. Differential Scanning Calorimetry (DSC) Thermal behaviors of all the resins and the films were studied by Q100 system (TA, New Castle, DE, USA). The tested sample (5 mg) was sealed in an aluminum holder and was heated up from 25 ◦C to 200 ◦C at a rate of 10 ◦C/min with the protection of nitrogen gas. The melting behavior of the films can be studied with the first heating of the films. The resin samples were kept at 200 ◦C for 5 min and were then cooled down to 25 ◦C at 10 ◦C/min to investigate the non-isothermal crystallization of these resins. The resin samples were subsequently heated back to 200 ◦C at the same rate to investigate their melting behaviors.

2.4. Small Angle X-ray Scattering (SAXS) The data were collected with NANOSTAR U SAXS instrument (Bruker, Karlsruche, Germany) operated at 40 kV and 650 µA. The data collection time for each sample was 180 min. The wavelength (λ) used was 0.154 nm (Cu Kα) and pinhole collimation system was employed to tune the X-ray beam. A Hi-Star area detector (Bruker, Madison, WI, United States) with 1024 × 1024 pixels and each pixel being 100 µm was used for data collection. The distance between the sample and the detector was 1070 mm, with a covered q-rang from 0.09 to 2.0 nm−1, where q represents the scattering vector (q = 4πsinθ/λ and 2θ represents the scattering angle). The collected data were corrected for air background before any analysis. Evaluation of Herman’s orientation of the films was also carried out e based on the SAXS data with MD as the reference direction.

2.5. Elmendorf Tear Strength The Elmendorf tear strengths of the films were measured by PROTEAR (Thwing-Albert, West Berlin, NJ, USA) according to the standard of ASTM-D-1922. Film specimens were cut to form a rectangle 76 mm in width by 63 mm in length (tearing direction). Two sets of specimens were cut from each film sample so that their sides are parallel to MD and TD directions, respectively, for the corresponding MD and TD tear strength test. Ten specimens were prepared and tested for each measurement in order to get a good average on tear strength. The film specimen is clamped between two clamps and pre-notched by a razor blade at the bottom between the clamps. The uncut portion of the film is 43 mm in notching direction. The tear measurement is done by releasing the pendulum aligned horizontally to tear the un- notched portion of the film. The loss energy due to tearing the specimen is used to calculate the Elmendorf tear resistance.

3. Results and Discussion Although the LLDPE resins studied in this work have similar density and molecular weight, the crystallization behavior can still have appreciable differences, as LLDPE is basically a mixture of the molecules with different polyethylene sequence distribution which can be different for these resins, besides the difference in co-monomer type. These differences can lead to different crystallization behavior during a same cooling process and different subsequent melting behavior. The differences in crystallization dynamics and the corresponding melting behavior of the resins can be studied with Polymers 2018, 10, x FOR PEER REVIEW 4 of 14

Although the LLDPE resins studied in this work have similar density and molecular weight, the crystallization behavior can still have appreciable differences, as LLDPE is basically a mixture of the Polymersmolecules2019, 11, 434with different polyethylene sequence distribution which can be different for these resins,4 of 13 besides the difference in co-monomer type. These differences can lead to different crystallization behavior during a same cooling process and different subsequent melting behavior. The differences DSC andin crystallization the learning dynamics would be and very the helpful corresponding for understanding melting behavior different of the film-making resins can processesbe studied used in thiswith work. DSC and the learning would be very helpful for understanding different film-making processes used in this work. 3.1. Non-Isothermal Crystallization of the LLDPE Resins and Thermal Property of the LLDPE Films Made of the Resins3.1. Non-Isothermal With Different Crystallization Processes of the LLDPE Resins and Thermal Property of the LLDPE Films Made of the Resins With Different Processes The non-isothermal crystallization during a cooling at 10 ◦C/min from melt and a subsequent The non-isothermal crystallization during a cooling at 10 °C/min from melt and a subsequent heating at 10 ◦C/min were studied with DSC for the three LLDPE resins made with different heating at 10 °C/min were studied with DSC for the three LLDPE resins made with different comonomers. The cooling and subsequent heating traces are shown in Figure1a,b. As shown comonomers. The cooling and subsequent heating traces are shown in Figure 1a, b. As shown in in thisthis figure, figure, the the crystallization crystallization temperature during during the the cooling cooling process process for for2045G, 2045G, 9085 9085 and and9030 9030 ◦ samplesample is 105.4, is 105.4, 107.0 107.0 and and 110.5 110.5C, respectively,°C, respectively, indicating indicating that that 9030 9030 with with a hexenea hexene co-monomer co-monomer has a strongerhas crystallinea stronger abilitycrystalline than ability 9085 andthan 2045G 9085 samples.and 2045G The samples. 2045 resin The with2045 octeneresin with comonomer octene has the weakestcomonomer crystalline has the ability. weakest The crystalline resins were ability. studied The resins as received were studied and without as received further and treatment; without the differencefurther seen treatment; from the theDSC difference study seen can from be attributed the DSC study to the can difference be attributed in PE to sequence the difference distribution in PE or possiblesequence additive distribution effect or or both possible of the additive effects. effect In fact, or both the melting of the effects. behavior In fact, typically the melting cannot behavior be affected that muchtypically by cannot possible be additivesaffected that and much can by provide possible useful additives information and can provide to differentiate useful information the three to resins. differentiate the three resins. Figure 1(b) does show that 9030 has a significant higher melting point Figure1b does show that 9030 has a significant higher melting point than the other two materials, than the other two materials, suggesting this resin is very likely to have more high-density fraction. suggesting this resin is very likely to have more high-density fraction. On the other hand, the heating On the other hand, the heating thermogram of 9030 resin also shows less melting ◦in the thermogramtemperature of 9030 range resin from also 90 showsto 110 less°C than melting the inother the two temperature materials, rangesugge fromsting 90that to fewer 110 Csmall than the othercrystals two materials, were formed suggesting during thatthe cooling fewer smallprocess. crystals It is also were noticed formed that duringdouble themelting cooling peaks process. (120.4 It is ◦ also noticedand 123.7 that °C) doubleare observed melting for peaks2045G (120.4resin as and seen 123.7 in FigureC) are1(b). observed The melting for 2045Gbehavior resin of 9085 as seenis in Figuresimilar1b. The to melting2045 G but behavior with a lower of 9085 melting is similar point to (121.6 2045 °C). G but The with results a lower suggest melting that 2045G point is (121.6likely ◦C). The resultsto have suggesta bi-model that distribution 2045G is likelyin terms to of have the ashort bi-model chain branching distribution of the in termsmolecules, of the while short the chain branchingother oftwo the materials molecules, are whilenot. The the crystallinity other two materials of 2045G, are 9085 not. and The 9030 crystallinity is 42.4, 39.7 of 2045G,and 37.8%, 9085 and 9030 isrespectively. 42.4, 39.7 and 37.8%, respectively.

Figure 1. DSC cooling (−10 ◦C/min) thermograms (a) of the resins studied and the subsequent melting Figure 1. DSC cooling (−10 °C/min) thermograms (a) of the resins studied and the subsequent thermogramsmelting thermograms (b). (b).

LLDPELLDPE films films made made by by blown blown and and cast cast filmfilm processesprocesses were were also also studied studied with with DSC DSC technique. technique. As shownAs shown in Figure in Figure2, the melting2, the melting behavior behavior of any ofof the any films of the is quite films different is quite fromdifferent its corresponding from its meltingcorresponding thermogram melting obtained thermogram from the same obtained resin butfrom underwent the same the resin non-isothermal but underwent crystallization the during the cooling at 10 ◦C/min (Figure1b). Double melting peaks are observed for all three blown films (one peak and a shoulder for 9030). For the cast film process, only one melting peak is observed for the film made with 9030, but one melting peak and one shoulder are present for the films made of 2045G and 9085 resins. For a cast film process, the extruded film immediately reaches a cooling roll, so crystallization finishes very fast. While for our blown film process, the frost line high was from 60 to 100 mm, suggesting that the crystallization finished at a slow process. This may explain why double melting peaks above 110 ◦C (Figure2) are observed for the blown films. The slow crystallization Polymers 2018, 10, x FOR PEER REVIEW 5 of 14

164 non-isothermal crystallization during the cooling at 10 °C/min (Figure 1b). Double melting peaks 165 are observed for all three blown films (one peak and a shoulder for 9030). For the cast film process, 166 only one melting peak is observed for the film made with 9030, but one melting peak and one 167 shoulder are present for the films made of 2045G and 9085 resins. For a cast film process, the 168 extruded film immediately reaches a cooling roll, so crystallization finishes very fast. While for our 169 blown film process, the frost line high was from 60 to 100 mm, suggesting that the crystallization Polymers 2019, 11, 434 5 of 13 170 finished at a slow process. This may explain why double melting peaks above 110 °C (Figure 2) are 171 observed for the blown films. The slow crystallization process allowed the component with a strong 172process crystalline allowed ability the component to crystallize with first a strongand the crystalline component ability with toa weak crystallize crystalline first andability the to component crystallize 173with later a weak to form crystalline smaller ability crystals to repre crystallizesented laterby the to lower form melting smaller peak crystals of the represented observed double by the peaks. lower 174 As can be seen from the dash line at 110 °C, the average melting point for the blown films is higher melting peak of the observed double peaks. As can be seen from the dash line at 110 ◦C, the average 175 (further away from the line) than the average melting point for the cast films (close to the line). The melting point for the blown films is higher (further away from the line) than the average melting point 176 direct comparison of the melting thermograms for the cast and blown films made of the same resin for the cast films (close to the line). The direct comparison of the melting thermograms for the cast 177 are shown in Figure 3. It can be seen clearly from this figure that more crystals with even lower and blown films made of the same resin are shown in Figure3. It can be seen clearly from this figure 178 melting point are formed during blown film process than cast film process, as can be seen in the 179that moretemperature crystals range with evenright lowerabove melting90 °C (the point dash are line formed). The during crystals blown with filmlower process melting than point cast could film ◦ 180process, affect as the can shrinkage be seen in property the temperature of the films range at elevated right above temperature, 90 C (the which dash will line). be Theaddressed crystals in withlater 181lower section. melting point could affect the shrinkage property of the films at elevated temperature, which will be addressed in later section.

182 Polymers 2018, 10, x FOR PEER REVIEW 6 of 14 Figure 2. DSC heating thermograms of LLDPE films made with blown film process (a) and cast film 183 processFigure (b). 2. DSC heating thermograms of LLDPE films made with blown film process (a) and cast film 184 process (b).

185 Figure 3. DSC heating thermograms of the cast and blown films made with different LLDPE resins 186 Figure 3. DSC heating thermograms of the cast and blown films made with different LLDPE resins (a) 2045G, (b) 9030 and (c) 9085. 187 (a) 2045G, (b) 9030 and (c) 9085.

188 3.2. Structure and Orientation of LLDPE Films Made with Different Resins and Different Processes 189 The SAXS patterns of the LLDPE films processed by the three different processes for each resin 190 are shown in Figure 4. As expected, no crystal orientation is seen for the compressed films, while 191 weak crystal orientation is observed for the cast film, suggested in Figure 4 by more scattering in 192 MD than in TD direction. During the cast and blown film processes, some oriented lamellar stacks 193 with their normal direction being approximately along MD direction are formed, which leads to 194 more SAXS in MD direction than in TD direction. In fact, most anisotropic SAXS scattering patterns 195 (two-dots scattering patterns) are observed in case of the blown films (Figure 4), indicating that 196 stronger crystal orientation is formed for the blown films. 197 The information on lamellar packing can also be obtained from the scattering images. The 198 average scattering profiles over 360° are shown in Figure 5. The scattering profiles show that a 199 well-defined scattering peak is obtained for the films made with 2045G, indicating that a 200 well-organized lamellar packing structure is formed in these films. Less organized lamellar packing 201 structure is formed in the films made of 9085 resin by showing less well–defined scattering peaks 202 and least organized lamellar packing is formed in the films made with 9030 resin, as no 203 well-defined scattering peak is present. Another important structural parameter on the lamellar 204 stacks is the average long period, meaning the average total thickness of crystalline lamellae and 205 amorphous lamellae or the average interdistance of neighboring crystalline lamellae. The actual 206 long period can be obtained with a few different approaches [13,14]. In the present work, 207 Lorentz-correction was applied to the scattering curves (not shown) to evaluate the long period, as 208 scattering maxima is absent for some of the scattering profiles. The long periods for the 9 films made 209 with different resins and by different processes are shown in Figure 6. It can be seen from this figure 210 that the long period of the film made with the same resin can vary a lot with different film process. 211 The long period for the compressed film is the largest, while it is the smallest for the cast film. This 212 trend is understandable considering the speed of the cooling for the different processes. Generally

Polymers 2018, 10, x FOR PEER REVIEW 7 of 14 speaking, thicker crystalline lamellae (higher melting point) and larger long period is formed for a slower crystallization process and vice versus. For the compressed films, the films were cooled down gradually inside an oven, thus it was supposed to be much slower than the other two Polymersprocesses.2019 ,In11 ,the 434 previous thermal section, it was addressed that the cooling for the blown6 film of 13 process was supposed to be slower than the cast film process. In fact, this is confirmed by the SAXS study based on the larger long period of the blown film than the cast film made of the same resin. 3.2. Structure and Orientation of LLDPE Films Made with Different Resins and Different Processes It is also seen from Figure 6 that for the same film process, the long period of the film made with The2045G SAXS is the patterns smallest, of the while LLDPE the filmslong period processed is the by largest the three for different the film processes made with for 9030 each resin. resin areAlthough shown the in Figure long period4. As expected, of film made no crystal with orientationbutene-based is seen 9085 for is thevery compressed close to the films, film whilemade weak with crystaloctene-based orientation 2045G is observedfor the same for the process cast film, (Figure suggested 6), there in Figure is a huge4 by moredifference scattering in intrinsic in MD than tear instrength TD direction. for these During two films the cast (will and be blown discuss film in processes,later section), some implying oriented that lamellar the films, stacks made with theirwith normalvery similar direction resins being in approximatelyterms of density along (cryst MDallinity) direction and are molecular formed, whichweight leads and to with more similar SAXS instructural MD direction parameter than inlike TD long direction. period, In can fact, have most very anisotropic different SAXS tear scattering strength. patterns In fact, (two-dotstie-chain scatteringconcentration patterns) (not reflected are observed by typical in case ofstructures the blown) or films the (Figureconnected4), indicating degree among that stronger the crystals crystal at orientationchain level is a formed very important for the blown governing films. factor for intrinsic tear strength of polyethylene film.

FigureFigure 4. SAXSSAXS images images of of LLDPE LLDPE films films made by the th threeree different processes with different resins.

The information on lamellar packing can also be obtained from the scattering images. The average scattering profiles over 360◦ are shown in Figure5. The scattering profiles show that a well-defined scattering peak is obtained for the films made with 2045G, indicating that a well-organized lamellar packing structure is formed in these films. Less organized lamellar packing structure is formed in the films made of 9085 resin by showing less well–defined scattering peaks and least organized lamellar packing is formed in the films made with 9030 resin, as no well-defined scattering peak is present. Another important structural parameter on the lamellar stacks is the average long period, meaning the average total thickness of crystalline lamellae and amorphous lamellae or the average interdistance of neighboring crystalline lamellae. The actual long period can be obtained with a few different approaches [13,14]. In the present work, Lorentz-correction was applied to the scattering curves (not shown) to evaluate the long period, as scattering maxima is absent for some of the scattering profiles. The long periods for the 9 films made with different resins and by different processes are shown in Figure6. It can be seen from this figure that the long period of the film made with the same resin can vary a lot with different film process. The long period for the compressed film is the largest, while it is the smallest for the cast film. This trend is understandable considering the speed of the cooling for the different processes. Generally speaking, thicker crystalline lamellae (higher melting point) and larger long period is formed for a slower crystallization process and vice versus. For the compressed Figure 5. The average SAXS profiles (over 360o) obtained from the SAXS images shown in Fig 4 for films, the films were cooled down gradually inside an oven, thus it was supposed to be much slower the films made with (a) 2045G, (b) 9030 and (c) 9085. As an example, the azimuthal SAXS scattering than the other two processes. In the previous thermal section, it was addressed that the cooling for the profiles crossing the scattering maxima for LLDPE 9085 film are shown in figure (d), which is used blown film process was supposed to be slower than the cast film process. In fact, this is confirmed for evaluation of Herman crystal orientation function with MD direction as a reference direction. by the SAXS study based on the larger long period of the blown film than the cast film made of the same resin. It is also seen from Figure6 that for the same film process, the long period of the film made with 2045G is the smallest, while the long period is the largest for the film made with 9030 resin. Although Polymers 2018, 10, x FOR PEER REVIEW 7 of 14 speaking, thicker crystalline lamellae (higher melting point) and larger long period is formed for a slower crystallization process and vice versus. For the compressed films, the films were cooled down gradually inside an oven, thus it was supposed to be much slower than the other two processes. In the previous thermal section, it was addressed that the cooling for the blown film process was supposed to be slower than the cast film process. In fact, this is confirmed by the SAXS study based on the larger long period of the blown film than the cast film made of the same resin. It is also seen from Figure 6 that for the same film process, the long period of the film made with 2045G is the smallest, while the long period is the largest for the film made with 9030 resin. Although the long period of film made with butene-based 9085 is very close to the film made with octene-based 2045G for the same process (Figure 6), there is a huge difference in intrinsic tear strength for these two films (will be discuss in later section), implying that the films, made with very similar resins in terms of density (crystallinity) and molecular weight and with similar structural parameter like long period, can have very different tear strength. In fact, tie-chain concentration (not reflected by typical structures) or the connected degree among the crystals at chain level is a very important governing factor for intrinsic tear strength of polyethylene film.

Polymers 2019, 11, 434 7 of 13

the long period of film made with butene-based 9085 is very close to the film made with octene-based 2045G for the same process (Figure6), there is a huge difference in intrinsic tear strength for these two films (will be discuss in later section), implying that the films, made with very similar resins in terms of density (crystallinity) and molecular weight and with similar structural parameter like long period, can have very different tear strength. In fact, tie-chain concentration (not reflected by typical structures) or the connected degree among the crystals at chain level is a very important governing factorFigure for intrinsic 4. SAXS tear images strength of LLDPE of polyethylene films made by film. the three different processes with different resins.

FigureFigure 5. 5. TheThe average SAXS profilesprofiles (over(over 360360◦o)) obtainedobtained from from the the SAXS SAXS images images shown shown in in Figure Fig 4 for Polymersthethe films films2018, 10made made, x FOR with with PEER (a) (a) REVIEW2045G, 2045G, (b) (b ) 9030 9030 and and (c) (c) 9085. As As an an example, example, the the azimuthal SAXS SAXS scattering scattering 8 of 14 profilesprofiles crossing the scatteringscattering maximamaxima forfor LLDPE LLDPE 9085 9085 film film are are shown shown in in figure figure (d ),(d which), which is usedis used for forevaluation evaluation of Hermanof Herman crystal crystal orientation orientation function function with with MD MD direction direction as aas reference a reference direction. direction.

Figure 6. The long period of cast, blown and compressed films made with 2045G, 9030 and 9085 resins. Figure 6. The long period of cast, blown and compressed films made with 2045G, 9030 and 9085 The mechanical properties of polyethylene filmsresins. closely relate to crystal orientation which can be evaluated with the 2D SAXS. The crystal orientation can be quantitatively evaluated with azimuthal intensityThe profilemechanical crossing properties the scattering of polyethylene maxima withinfilms closely a selected relate q-range to crystal as shown orientation in Figure which4 for can thebe 2Devaluated scattering with pattern the 2D obtained SAXS. fromThe compressedcrystal orientation 2045G film.can be The quantitatively azimuthal SAXS evaluated scattering with profilesazimuthal crossing intensity the scatteringprofile crossing maxima the are scattering shown inmaxima Figure within5d for LLDPEa selected 9085 q-range film for as example.shown in TheseFigure azimuthal 4 for the scattering2D scattering profiles pattern are usedobtained for evaluation from compressed of Herman 2045G orientation film. The function azimuthal with SAXS MD directionscattering as profiles a reference crossing direction. the scattering maxima are shown in Figure 5(d) for LLDPE 9085 film for example. These azimuthal scattering profiles are used for evaluation of Herman orientation function with MD direction as a reference direction. Herman’s orientation function [15], f, is defined as follows:

1 2 f =−()3cosφ 1 (1) 2 π 2 I ()φφsin cos2 φφ d 2 φ 0 cos = π (2) 2 I ()φφφsin d 0 where φ is azimuthal angle shown in Figure 4. For perfect orientation with a reference direction in orientation direction, f equals to 1, while f equals to −1/2 for a perfect orientation with a reference direction 90 degree off the orientation direction. For a random system without preferred orientation, f equals to zero. For our data processing, MD was used as a reference direction; the selected q range for generating azimuthal profiles for orientation evaluation was 30 pixels. The calculated Herman’s orientation values for the cast and blown films made with each of the resins are listed in Table 2. It can be seen from this table that the film made with blown film process has significantly larger orientation than the corresponding cast film.

Table 2. Herman’s Orientations function for all the films study

Sample designation Compressed Cast Blown 2045G 0 0.048 0.178 9030 0 0.024 0.146 9085 0 0.047 0.1328

3.3. Intrinsic Tear Strength and MD Tear Strength for Oriented Films As discussed above, films made by industry processes (cast or blown) are oriented. The mechanical properties of the films are very much dependent on film orientation. The intrinsic tear

Polymers 2019, 11, 434 8 of 13

Herman’s orientation function [15], f, is defined as follows:

1  D E  f = 3 cos2 φ − 1 (1) 2

π R 2 2 D 2 E 0 I(φ) sin φ cos φdφ cos φ = π (2) R 2 0 I(φ) sin φdφ where φ is azimuthal angle shown in Figure4. For perfect orientation with a reference direction in orientation direction, f equals to 1, while f equals to −1/2 for a perfect orientation with a reference direction 90 degree off the orientation direction. For a random system without preferred orientation, f equals to zero. For our data processing, MD was used as a reference direction; the selected q range for generating azimuthal profiles for orientation evaluation was 30 pixels. The calculated Herman’s orientation values for the cast and blown films made with each of the resins are listed in Table2. It can be seen from this table that the film made with blown film process has significantly larger orientation than the corresponding cast film.

Table 2. Herman’s Orientations function for all the films study.

Sample Designation Compressed Cast Blown 2045G 0 0.048 0.178 9030 0 0.024 0.146 9085 0 0.047 0.1328

3.3. Intrinsic Tear Strength and MD Tear Strength for Oriented Films As discussed above, films made by industry processes (cast or blown) are oriented. The mechanical properties of the films are very much dependent on film orientation. The intrinsic tear strength for films without any orientation is a better way to compare the tear strength for the films made with different LLDPE resins. In the present work, compressed films were made for Elmendorf tear test; the normalized intrinsic tear strength values for the compressed films made with the three LLDPE resins are listed in Table3, together with the MD and TD tear strength values of the cast and blown films made with the three LLDPE resins.

Table 3. The intrinsic Elmendorf tear strength and MD and TD Elmendorf tear strength of the films made with the three LLDPE resins.

Tear Strength (mN/µm) Sample Intrinsic Tear Blown Cast Designation (mN/µm) MD TD MD TD 2045G 127 411 182 198 237 9030 93 287 154 191 176 9085 41 219 44 105 93

For most film applications, balanced MD and TD tear strengths are desired, as the mechanical failure of a film is usually determined by the weak strength direction, such as, puncture and tear. However, practically, it is not easy to achieve the balanced mechanical properties, as crystals in the film made with typical film making processes have a preferred orientation direction. It is well known that once a film is oriented, the tear strength in the oriented direction drops significantly. A film gets damaged easier in the direction where crystals are oriented, leading to lower tear strength in the orientation direction. This trend can also be seen from Table3 for the films studied in this work. The tear strength in MD direction can be significantly smaller than its intrinsic tear strength, due to the preferred orientation in MD, while TD tear strength can be much larger than its intrinsic tear Polymers 2019, 11, 434 9 of 13 strength. As MD tear is more important for a film performance, the MD tear strength is plotted against intrinsic tear strength in Figure7. It can be seen from Table3 and Figure7 that the intrinsic tear strength increases significantly with increasing the short chain length determined by the co-monomer type. The intrinsic tear strengths of the films made with LLDPE with octene and hexene co-monomers are remarkably better than the film made with LLDPE with butene co-monomer. More importantly, the MD tear strength of cast and blown films made with butene co-monomer drops dramatically relativePolymers to2018 the, 10, corresponding x FOR PEER REVIEW intrinsic tear strength but the tear strengths in MD, relative to10 theof 14 corresponding intrinsic tear strength, holds better for the cast and blown films made with hexene and octene co-monomers.

Figure 7. Intrinsic tear strength vs. tear strength in MD for the films made with the three LLDPE resins.

3.4. MDFigure Tear Strength7. Intrinsic Dependence tear strength on Crystalvs. tear Orientationstrength in MD and Co-monomerfor the films Typemade of with LLDPE the three LLDPE resins. Usually, the more oriented a film is, the more loss its tear strength in MD direction bears. As cast and3.4. blown MD Tear films Strength made Dependence with different on Crysta resinsl Orientation usually have and different Co-monomer orientation Type of LLDPE degree even under the same processing condition, it is difficult to evaluate the difference in the dependence of MD tear Usually, the more oriented a film is, the more loss its tear strength in MD direction bears. As strength on crystal orientation for the films made by different LLDPE resins. In the present work, cast and blown films made with different resins usually have different orientation degree even three films with different orientation degree were made for each of the three resins with butene, under the same processing condition, it is difficult to evaluate the difference in the dependence of hexene and octene co-monomer, respectively, which makes such an evaluation possible. As shown MD tear strength on crystal orientation for the films made by different LLDPE resins. In the present in Figure8, the MD tear strength decreases quickly with increase in crystal orientation for the films work, three films with different orientation degree were made for each of the three resins with made with any of the resins. It seems that the MD tear strength decreases faster with increase in butene, hexene and octene co-monomer, respectively, which makes such an evaluation possible. As crystal orientation for the films made with LLDPE with butene comonomer. The different response shown in Figure 8, the MD tear strength decreases quickly with increase in crystal orientation for the to orientation can be better viewed in Figure9 from the plot of normalized MD tear strength (by films made with any of the resins. It seems that the MD tear strength decreases faster with increase its the intrinsic tear strength) versus crystal orientation. It can be seen from Figure9 that at a given in crystal orientation for the films made with LLDPE with butene comonomer. The different orientation degree, the film made with a longer short chain branching always has a larger MD tear response to orientation can be better viewed in Figure 9 from the plot of normalized MD tear strength. The length of short chain branching plays a key role in the dependence of MD tear strength strength (by its the intrinsic tear strength) versus crystal orientation. It can be seen from Figure 9 on crystal orientation. Figure9 clearly shows that as increase in crystal orientation the MD tear strength that at a given orientation degree, the film made with a longer short chain branching always has a of the films made with butene based LLDPE drops much faster than films made with the hexene and larger MD tear strength. The length of short chain branching plays a key role in the dependence of octene based LLDPE. As increase in crystal orientation, the MD tear strength for the films made with MD tear strength on crystal orientation. Figure 9 clearly shows that as increase in crystal orientation hexene based LLDPE decreases slightly faster than the film made with the octene based LLDPE. As the MD tear strength of the films made with butene based LLDPE drops much faster than films shown in Figure9, for Herman orientation value of 0.05, the MD tear strength still remains 77% of its made with the hexene and octene based LLDPE. As increase in crystal orientation, the MD tear intrinsic tear strength for the film made with 2045G with octene co-monomer, while it drops to 47% of strength for the films made with hexene based LLDPE decreases slightly faster than the film made its intrinsic tear strength for the film made with 9085 with butene co-monomer. In another word, the with the octene based LLDPE. As shown in Figure 9, for Herman orientation value of 0.05, the MD MD tear strength for the films made with 2045G and 9030 with hexene and octene co-monomers are tear strength still remains 77% of its intrinsic tear strength for the film made with 2045G with octene co-monomer, while it drops to 47% of its intrinsic tear strength for the film made with 9085 with butene co-monomer. In another word, the MD tear strength for the films made with 2045G and 9030 with hexene and octene co-monomers are much less sensitive to orientation than the film made with 9085 with butene co-monomer. It should also be pointed out that the compressed films with significantly larger thickness than films made with other processes were used for evaluation of intrinsic tear strength. As the cast films have very weak orientation, especially for the two films made with 2045G and 9030, the averaged tear strength in MD and TD direction can be used to approximately compare with its corresponding intrinsic tear strength obtained from the compressed films. It can be seen from Table 3 that the average from 9030 cast film is very similar to

Polymers 2019, 11, 434 10 of 13 much less sensitive to orientation than the film made with 9085 with butene co-monomer. It should alsoPolymers be pointed 2018, 10 out, x FOR that PEER the REVIEW compressed films with significantly larger thickness than films made11 of with14 other processes were used for evaluation of intrinsic tear strength. As the cast films have very weak the obtained intrinsic tear strength and the average from 2045G is about 20% smaller than the orientation, especially for the two films made with 2045G and 9030, the averaged tear strength in MD obtainedPolymers intrinsic 2018, 10, x FORtear PEER strength. REVIEW The larger difference for 2045G does not change the11 aboveof 14 and TD direction can be used to approximately compare with its corresponding intrinsic tear strength conclusion on tear dependence on film orientation but make it more solid. In fact, smaller 316obtained the from obtained the compressed intrinsic tear films. strength It can and be the seen average fromTable from3 2045Gthat the is averageabout 20% from smaller 9030 than cast filmthe is intrinsic tear strength for 2045G leads to the conclusion that the tear strength in MD direction for 317very similarobtained to theintrinsic obtained tear intrinsicstrength. tear The strength larger difference and the averagefor 2045G from does 2045G not ischange about the 20% above smaller the film made with octene based co-monomer (2045G) is even less sensitive to crystal orientation 318than theconclusion obtained on intrinsic tear dependence tear strength. on Thefilm larger orientation difference but make for 2045G it more does solid not. changeIn fact, smaller the above 319than described above. conclusionintrinsic on tear tear dependence strength for on2045G film leads orientation to the conclu but makesion that it more the tear solid. strength In fact, in smaller MD direction intrinsic for tear 320 the film made with octene based co-monomer (2045G) is even less sensitive to crystal orientation strength for 2045G leads to the conclusion that the tear strength in MD direction for the film made with 321 than described above. octene based co-monomer (2045G) is even less sensitive to crystal orientation than described above.

322 Figure 8. MD tear strength vs. crystal orientation for the films made with the three resins. Figure 8. MD tear strength vs. crystal orientation for the films made with the three resins. 323 Figure 8. MD tear strength vs. crystal orientation for the films made with the three resins.

324

325 FigureFigure 9.Figure Normalized9. Normalized 9. Normalized MD MD tearMD tear tear strength strength strength (by(by ( itsby its its correspondingcorresponding corresponding intrinsic intrinsic tear tear strengthstrength strength)) asas as aa functionafunction function of of of 326 crystalcrystal orientation.crystal orientation. orientation. 3.5. Shrinkage Property at High Temperature vs. Crystal Orientation and Melting Behavior 3273.5. Shrinkage3.5. Shrinkage Property Property at High at High Temperature Temperature vs. vs. Crystal Crystal Orientation Orientation and and Melting Melting Behavior 328 TheThe film Thefilm shrinkage film shrinkage shrinkage values values values at at different differentat different temperatures temperatures temperatures areare are shown shown in in FigureFigure 10. 10 .ItIt It cancan can bebe be seen seen seen from from from 329thisthis figure figurethis thatfigure that the that the shrinkage theshrinkage shrinkage measured measured measured from from from thethe the blownbl blownown filmsfilms films is is obviouslyobviously larger larger than than thethe the cast cast cast films films films ◦ 330forfor all all threefor three all seriesthree series series of of samples. samples. of samples. The The blown blown blown films films films start startstart to to toshrinkage shrinkage shrinkage at at 90 at90 °C,°C, 90 whwhileC,ile while the castcast the films films cast do filmsdo not not do 331 . notshrink shrinkshrink at atthis at this thistemperature. temperature. temperature. This This Thiscan can be can beinterprete interpreted be interpretedd based based on based on the the thermal onthermal the thermaldatadata shown data inin earlyshownearly section section in early. 332 As shown in Figure 3 that more crystals with lower melting point are formed during blown film As shown in Figure 3 that more crystals with lower melting point are formed during blown film 333 process than cast film process, as indicated by the dash line at 90 °C in that figure. This is why the process than cast film process, as indicated by the dash line at 90 °C in that figure. This is why the

Polymers 2018, 10, x FOR PEER REVIEW 12 of 14

blown films shrink at 90 °C, while the cast films hold pretty well. In fact, the average melting point forPolymers the blown2019, 11 ,films 434 is higher than the average melting point for the cast films, as indicated by the11 fact of 13 that the overall melting for the blown film is within a higher temperature range than the cast films (Fig 3).The blown film made with octene based 2045G also has more crystals with significantly section.Polymers As 2018 shown, 10, x FOR in FigurePEER REVIEW3 that more crystals with lower melting point are formed during12 blown of 14 lower melting point (below 110 °C, see Figure 3) than the other two blown films, leading to largest film process than cast film process, as indicated by the dash line at 90 ◦C in that figure. This is why the 334 shrinkageblown films at 110 shrink °C for at 2045G 90 °C, blownwhile thefilm. cast films hold pretty well. In fact, the average melting point blown films shrink at 90 ◦C, while the cast films hold pretty well. In fact, the average melting point 335 for. Onthe blownone hand, films crystal is higher orientation than the playsaverage a big melting role forpoint the for shrinkage the cast films,property. as indicated On the otherby the hand, fact for the blown films is higher than the average melting point for the cast films, as indicated by the fact 336 thethat shrinkage the overall property melting at fora given the blown temperature film is within can be a also higher affected temperature by the crystalrange than fraction the castwith films low that the overall melting for the blown film is within a higher temperature range than the cast films 337 melting(Fig 3).The point. blown The shrinkage film made at with110 °Coctene for all based cast 2045Gand blown also hasfilms more made crystals with the with three significantly different (Figure3). The blown film made with octene based 2045G also has more crystals with significantly 338 resinslower is meltingplotted pointin Figure (below 11 110as a°C, function see Figure of crystal3) than orientation.the other two It blown is seen films, from leading this figure to largest that lower melting point (below 110 ◦C, see Figure3) than the other two blown films, leading to largest 339 althoughshrinkage the at data 110 is°C from for 2045G both blowncast and film. blown films made with three different LLDPE resins, the shrinkage at 110 ◦C for 2045G blown film. 340 shrinkage. On correlates one hand, linearly crystal orientationwith crystal plays orientation. a big role for the shrinkage property. On the other hand, 341 the shrinkage property at a given temperature can be also affected by the crystal fraction with low 342 melting point. The shrinkage at 110 °C for all cast and blown films made with the three different 343 resins is plotted in Figure 11 as a function of crystal orientation. It is seen from this figure that 344 although the data is from both cast and blown films made with three different LLDPE resins, the 345 shrinkage correlates linearly with crystal orientation.

Figure 10. The shrinkage at different temperatures for the cast and blown films made with the three FigureLLDPE 10. resins. The shrinkage at different temperatures for the cast and blown films made with the three LLDPE resins. On one hand, crystal orientation plays a big role for the shrinkage property. On the other hand, 346 the shrinkage property at a given temperature can be also affected by the crystal fraction with low ◦ 347 meltingFigure point. 10. The The shrinkage shrinkage atat 110differentC for temperatures all cast and for blown the cast films andmade blown with filmsthe made three with different the three resins 348 is plottedLLDPE in Figure resins. 11 as a function of crystal orientation. It is seen from this figure that although the data is from both cast and blown films made with three different LLDPE resins, the shrinkage correlates linearly with crystal orientation.

Figure 11. The shrinkage at 110 °C vs. crystal orientation for all cast and blown films made with three different resins. 349 Figure 11. The shrinkage at 110 ◦C vs. crystal orientation for all cast and blown films made with three different resins. 350 Figure 11. The shrinkage at 110 °C vs. crystal orientation for all cast and blown films made with three 351 different resins.

Polymers 2019, 11, 434 12 of 13

4. Conclusions In the present work, the films made with very similar resins in terms of density (crystallinity) and molecular weight but different co-monomer type (butene, hexene and octene, respectively) were studied with different techniques. The long period of film made with butene-based 9085 is very close to the film made with octene-based 2045G but there is a huge difference in intrinsic tear strength for these two films due to different co-monomer type. That is, very similar lamellar packing structure (crystallinity/density and long period) does not necessarily make similar tear strength of the LLDPE films., The tie-chain concentration (not reflected by typical structures) or the connected degree of the crystals at chain level is an important governing factor for intrinsic tear strength of polyethylene films. MD tear strength for the films made with LLDPE with butene co-monomer decreases the fastest as increase in crystal orientation, such as, for a given Herman orientation value at 0.05, the MD tear strength for the film made with octene co-monomer can still remain 77% of its intrinsic tear strength, while it drops to 47% of its intrinsic tear strength for the film made LLDPE with butene co-monomer (Table4). As increase in crystal orientation, the MD tear strength for the films made with hexene based LLDPE decreases slightly faster than the film made with the octene based LLDPE at a given orientation value.

Table 4. Summary of the key results obtained from the films made with the three resins.

Sample 2045G 9030 9085 Comonomer type Octene Hexene Butene Density (g/cm3) 0.920 0.919 0.920 MW(×105) 1.27 1.10 1.33 Long period (nm) Cast 15.1 17.7 15.6 Blown 13.6 14.9 13.9 Intrinsic tear (mN/µm) 237 176 93 MD tear /intrinsic tear at the orientation of 0.05 77% 75% 47%

For shrinkage property at elevated temperatures, on one hand, crystal orientation degree plays a big role. The shrinkage increases linearly with increase in crystal orientation, confirmed by the results from the cast and blown films made with LLDPE resins with different co-monomers (butene, hexene and octene, respectively). On the other hand, at a certain elevated temperature, the shrinkage property can also be affected by the crystal fraction with low melting point. In fact, our study shows that more crystals with lower melting points (say around 90◦C) are formed during blown film process than cast film process, especially for films made with bi-model LLDPE resin (2045G).

Author Contributions: Conceptualization and experimental design: L.L.; Mechanical/thermal experiment and data analysis: Y.R. and Y.S.; Film preparation: X.Y.; Y.S. and Y.R.; X-ray experiment and data analysis: Y.T.; Draft Preparation: Y.R.; Y.S.; X.Y. and Y.T. Final revision: L.L. Funding: This research received no external funding. Acknowledgments: The authors would like to thank Chunxia Luo at BRICI SINOPEC for her help with GPC measurements and Dali Gao at BRICI SINOPEC for his help with film preparations. Conflicts of Interest: The authors declare no conflict of interest.

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