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Crystallization of low-density polyethylene embedded inside zinc oxide nanoparticle percolating network.

INTRODUCTION

Crystallization of semicrystalline polymers in confined environments attracts increasing attentions recently due to scientific interests and potential applications in engineering sectors (1-3). As semicrystalline polymer chains are restrained in nanoscopic space, they exhibit completely different crystallization behavior and resulting crystal morphologies, which in turn govern their physical properties (4). For instance, confined polymer crystals generally exhibit highly anisotropic properties, resulting in broad potential applications in electronic devices and barrier materials (5-8).

A few methods have been employed to examine the crystallization behavior of semicrystalline polymers under confined environments. Semicrystalline block copolymers including crystalline amorphous and crystalline crystalline copolymers are one of most powerful strategies. Crystallizable blocks crystallize within the block copolymer domains established by self-assembly in melt below their order disorder transition temperatures. Size of the domains with kinds of morphologies such as spherical, gyroidal, cylindrical, and lamellar varies in the range of several nanometers to tens of nanometers depending on molar mass and composition of the copolymers. Moreover, since microdomains are spontaneously produced in block copolymers, no additional techniques are required for the formation of confined environments, which facilitates their applications. So far, crystallization of block copolymers has been widely investigated (9-22). Loo and coworkers have reported that crystallization of polyethylene (PE) from discrete spheres of 25 nm follows first-order kinetics with homogeneous nucleation, in contrast to the sigmoidal kinetics in PE bulk (9), (10). However, the mechanical strength of the polymer microdomains is poor, which are readily broken by crystallization in some cases, leading to the failure of constraint. Loo and coworkers reported that crystallization destructed microdomains of block copolymers as segregation strength was below the critical value (approximately three times of segregation strength at order disorder transition) (23).

There has been another facile strategy to fabricate polymer ultrathin films for investigating crystallization of semicrystalline polymers under quasi-two-dimensional confinements (24-28). Polymer films with thickness from several nanometers to hundreds of nanometers can be readily fabricated by existing approaches such as spincoating, drop-coating. Wang and coworkers have reported that as crystallizing from a 20 nm layer fabricated by layer-multiplying coextrusion process, poly(ethylene oxide) (PEO) aligns as single and high-aspect-ratio lamellae. Formation of such PEO crystals led to two orders of magnitude reduction in the gas permeability (6), (29).

Inorganic material templates have also been employed to confine crystallization of polymers. When lineal polyethylene was filled into alumina nanoscopic cylindrical pores, crystallization in larger pores (diameter = 62 nm) was dominated by homogeneous nucleation but heteroge-neous nucleation prevailed in smaller pores (diameter = 48 nm) (30), (31). As PE intercalated into galleries of layered silicates (gallery distance is 1.5 nm), crystallization of PE was drastically retarded, The melting temperature of PE was decreased due to smaller lateral size of PE crystals (32-34). In a previous work, we found that when poly(ethylene oxide) was confined in Prussian Blue nanoshells of 65 nm, crystallization of PEO was completely suppressed due to curvature structures (35). Inorganic material templates are ideal candidate for polymer constraints due to superior mechanical strength. However, filling of polymers into templates is of big difficulty because of poor mobility of polymers and weak interaction between polymer and templates, which strictly limits their applications.

Nanoparticles are considered as building blocks of various hierarchical architectures with numerous applications in photonic materials (36-39). As distributed in polymer matrices, nanoparticles form various structures depending on their concentration. Nanoparticles are distributed as clusters in polymer matrices at low concentration. As increasing nanoparticle concentration, they begin to contact to each other to form continuous pathway. Such geometric transition from isolated clusters to infinite network is known as percolation transition. The critical nanoparticle concentration is termed as percolation threshold (40). The percolation transition is accompanied with abrupt changes in physical properties of the polymer/nanoparticle hybrids i.e., electrical conductivity increasing (or resistivity decreasing) by several orders of magnitude. In a previous work, we investigated electrical properties of low-density polyethylene reinforced with zinc oxide (ZnO) fabricated via melt compounding. It was found that the electrical resistivity of LDPE/ZnO composites decreased stepwise and torque applied during melting compounding rose abruptly as ZnO content reached 60 vol%, derived from the formation of ZnO percolating network (41). Such percolating network of ZnO provides a novel environment for investigating crystallization of polymers under constraint. In this work, we investigated the crystallization behavior of LDPE embedded inside ZnO percolating network. The results showed that LDPE inside ZnO percolating network exhibited quite different crystallization behavior as compared to pristine LDPE.

EXPERIMENTAL

Preparation

LDPE/ZnO composites were prepared by melt compounding commercial low-density polyethylene pellets (Cosmothene LDPE, polyolefin, Singapore, bulk density of LDPE is 0.917 g/cm3) with ZnO particles in a Brabender mixer. ZnO nanoparticles of ti 200 nm were supplied by Nanostructrued and Amorphous Materials (Los Alamos, NM, bulk density of ZnO is 5.6 g/c[m.sub.3]). LDPE pellets were dried in an oven at 80 [degrees] C for 4 h before use. To avoid thermal degradation of the polymer matrix leading to coloring of products, the mixing time was set to 15 min at 200 [degrees] C at rotation rate of 40 rpm. Pristine LDPE and LDPE/1.15 vol% ZnO were also prepared under similar conditions for the purpose of comparison.

X-Ray Diffraction

X-ray diffraction (XRD) measurements were performed with a Philip X'pert diffractometer equipped with Ni-filtered Cu KY radiation, having a wavelength of 0.154 nm. The fabricated LDPE/ZnO blends were hot pressed at 200 C under 10 MPa into plates with thickness of 1 mm. The squares of 10 x 10 x 1 m[m.sub.3] were cut out of the plates for XRD measurement. The specimens were amounted on sample holders using a pieces of plasticene. The diffractometer was scanned in 20 range of 5-30 [degrees] and the scanning rate used was 1.2 [degrees]/min. Deconvolution profile fitting and integration of diffraction peaks were carried out using Origin 8.0 software.

Differential Scanning Calorimetry

Crystallization behavior of LDPE and its composites was investigated with a Perkin-Elmer DSC-7 Instrument under nitrogen atmosphere. Temperature was calibrated with indium prior to the test. The samples (about 5 mg) were sealed in aluminum pans. The samples were cooled rapidly to the designed temperatures from molten (180t) after eliminating previous thermal history, and keep at this temperature until crystallization completed. The change in heat flow versus time was recorded. Three specimens from different location of each sample were measured for checking repeatability of DSC measurement.

RESULTS AND DISCUSSION

Structure of LDPE/ZnO Composites

In a previous work, we investigated the electrical properties of LDPE/ZnO composites prepared using melt compounding [411. The results revealed that the electrical resistivity of LDPE/ZnO composites decreased stepwise and torque applied during melting compounding rose abruptly as ZnO content reached 60 vol%, accounting for the formation of ZnO percolating network in LDPE/ZnO composites. Microstructure of LDPE inside ZnO percolating network was investigated using X-ray diffratometry (XRD). Figure 1 shows XRD curves of pristine LDPE, LDPE/ 1.15 vol /0 ZnO and LDPE/60 vol% ZnO composites. There are no diffraction peaks for ZnO particles in the range investigated, and XRD curve for ZnO particles is not shown here. The XRD patterns of LDPE and its composites show two distinct (110) and (200) reflection peaks associated with the orthorhombic structure of polyethylene, illustrating that the addition of ZnO particles does not change the crystal type of LDPE. A broad shoulder peak at ~ 19.3 [degrees] derived from LDPE amorphous region is also observed. The crystalline peaks of polyethylene can be discerned from the amorphous region by deconvolution profile fitting. The inset shows the representative profile fitting curve for the XRD pattern of LDPE/60 vol% ZnO composite. The degree of crystallinity (X) for LDPE and its composites is determined from the integral of crystalline and amorphous peaks of polyethylene using the following equation (42):

[X.sub.t]=([I.sub.110]+1.46[I.sub.200])/([I.sub.110]+1.46[I.sub.200]+0.75[I.sub.a])x100% (1)

where [I.sub.110], [I.sub.200] and [I.sub.a] are the integral area of the reflection peaks from (110) and (200) crystal planes as well as the amorphous region of polyethylene, respectively. The X value of pristine LDPE is determined to be 33.1%, as shown in Table 1. As compared to pristine LDPE, LDPE/ 1.15vol% ZnO exhibits slightly larger X (34.2%), while much smaller X was observed for LDPE/60 vol% (27.0%), demonstrating that LDPE/60 vol% ZnO has less perfect crystalline structure than LDPE/1.15 vol% ZnO. It is well known that polyethylene macromolecules fold up and assemble into three-dimensional spherullite of tens of micrometer in a free space upon crystallization. In LDPE/ 60 vol% ZnO, LDPE resides inside the percolating network of ZnO nanoparticles. The growth of polyethylene crystals is hindered by such spatial confinement to some degree, resulting in imperfect crystalline structure and lower degree of crystallinity. Indeed we failed to acquire a polarized optical microscopy image for LDPE/60 vol% ZnO. This result is in good agreement with one of our work reported previously on poly(ethylene oxide) confined inside Prussian Blue nanoshells, where crystallization of PEO is completely suppressed in 65 nm Prussian Blue nanoshells (35).

TABLE 1. The degree of crystallinity for neat LDPE and its composites.

Sample           Neat LDPE  LDPE/1.15   LDPE/60
                             vol% ZuO  vol% ZuO

The degree of         33.1       34.2        27
crystallinity/%


Isothermal Crystallization Kinetics

From XRD results, it can be seen that only orthorhombic structure of polyethylene exists in pristine LDPE and its composites. Therefore conventional DSC technique can be used to investigate crystallization behavior of LDPE and its composites.

Figure 2 shows the heat flow traces of pristine LDPE and its composites during isothermal crystallization at various crystallization temperatures. It is noticed that crystallization of LDPE/60 vol% ZnO occurs in a similar temperature range as pristine LDPE. It has been commonly accepted that as semicrystalline polymers are confined in isolated nanoscopic domains established by block copolymers, only at much lower temperatures (high supercoolings) can crystallization initiate due to homogenous nucleation inside each isolated microdomain (9), (10). Therefore, the relatively high crystallization temperature of LDPE/60 vol% ZnO is likely due to the strong nucleation effect of ZnO particles on LDPE crystallization. Similar results have also been reported for intercalated PE/ montmorillonite composites (32-34).

The relative degree of crystallinity can be evaluated according to the equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [Q.sub.t] and [Q.sub.8] are the heat generated at time t and infinite time, respectively, and dH/dt is the heat flow rate. The plots of the relative degree of crystallinity against crystallization time are shown in Fig. 3. For the purpose of comparison, the plots at [T.sub.c] = 98 [degrees] C in Fig. 3 are compiled in Fig. 4. It is obvious that crystallization of LDPE/ 60 vol% ZnO is much quicker than those of pristine LDPE and LDPE/1.15 vol% ZnO. Figure 5 shows half-crystallization time of LDPE and its composites as a function of crystallization temperature. It is found that at elevated crystallization temperatures LDPE/60 vol% ZnO has much shorter half-crystallization time than pristine LDPE, indicating quicker crystallization rate, while slightly longer half-crystallization time is observed at lower crystallization temperature, showing slower crystallization rate. Possible reason is that at elevated crystallization temperature crystallization is dominated by nucleation process. In LDPE/60 vol% ZnO, LDPE segments sit inside percolating network of ZnO particles that are inclined to absorb LDPE segments and initiate crystallization process due to large surface area, resulting in quicker crystallization rate. While crystallization is controlled by crystal growth process at lower crystallization temperatures. Diffusion of LDPE segments towards crystallization sites is retarded due to geometric confinements in LDPE/ 60 vol% ZnO, leading to slower crystallization rate. Moreover, LDPE/1.15 vol% ZnO shows shorter half-crystallization time than pristine LDPE at each crystallization temperature, demonstrating quick crystallization rate. This is in accordance with results of semicrystalline polymer/filler composites at low filler loading (43-53).

The isothermal crystallization kinetics can be analyzed in terms of Avrami theory (54):

1 - X(t) = exp(-[K.sub.t]) (3)

In[-In(1-X(t))] = In K + n In t (4)

where K is the crystallization rate constant and n is Avrami exponent whose value is dependent on the mechanism of nucleation and the form of crystal growth. The crystallization rate constant K and Avrami exponent n can be obtained from the intercept and slope from the lineal region of the plots of In[In(1X(t)] against In t, respectively. Figure 6 shows the plots of In[-In(1 - X(t))] versus In t for LDPE and its composites. The values of n and K are summarized in Table 2 and Fig. 7. It is found that the values of n for LDPE and LDPE/1.15 vol% ZnO vary ranging from 2.8-3.2, indicating three-dimensional growth of polyethylene crystals with heterogeneous nucleation. While the values of n for LDPE/60 vol% ZnO are in range of 1.8 to 2.0, illustrating quasi-two-dimensional crystallization with heterogeneous nucleation. Although confined inside ZnO percolating network, LDPE phase is still spatially continuous, which is morphologically similar to gyroidal structures generated by block copolymers to some degree, where Avrami exponent with value of 1.7 is obtained (55). The values of Avrami exponents in the range of 1.6-1.9 have also been reported for the polyethylene confined in alumina cylinders of 15-110 nm (31). When polyethylene was confined inside the galleries of montmorillonite (<1.5 nm), the Avrami exponent was about 2 at high crystallization temperatures (32). Therefore, Avrami exponent of about 2 of LDPE embedded inside the ZnO network implies that crystallization of embedded LDPE is restrained to some extent.

TABLE 2. Isothermal crystallization parameters for pristine LDPE
and its composites.

Sample     Crystallization    n     K  [T.sub.1/2]  [DELTA] E
               temperature                   (min)   (kj/mol)
             ([degrees] C)

LDPE                    90  3.0  9.13        0.423     -352.2

                        92  3.0  7.74        0.447

                        94  2.9  3.49        0.583

                        96  2.9  1.33        0.804

                        98  2.9  0.25        1.501

                       100  2.8  0.04        2.601

                       102  2.8  0.01        5.020

LDPE/1.15               90  3.1  22.5        0.325     -264,5
vol% ZnO
                        92  3.1  12.5        0.393

                        94  3.2  7.04        0.573

                        96  3.1  2.38        0.787

                        98  3.2  0.61        1.186

                       100  3.1  0.13        2.290

                       102  3.1  0.03        4.270

LDPE/60                 90  1.9  2.56        0.520      -96.4
vol% ZnO
                        92  1.9  2.42        0.535

                        94  1.9  2.35        0.543

                        96  1.9  1.49        0.682

                        98  2.0  1.21        0.757

                       100  1.9  0.92        0.870

                       102  1.8  0.57        1.107


It is obvious from Fig. 7 and Table 2 that the crystallization rate constant, K, of LDPE/60 vol% ZnO is much larger than that of LDPE bulk at elevated temperature, indicating quicker crystallization rate. Whereas, at lower crystallization temperature, LDPE/60 vol% ZnO has much smaller K than LDPE bulk, demonstrating slower crystallization rate. Moreover, LDPE/1.15 vol% ZnO shows larger crystallization rate constant than pristine LDPE at each crystallization temperature, illustrating quicker crystallization rate. These are consistent with the results of half-crystallization time.

The crystallization rate constant K can be empirically expressed in terms of Arrhenius equation (56):

[K.sub.I/n = [K.sub.0]exp(-[DELTA]E/R[T.sub.C]) (5)

where Ko is pre-exponential factor, R is the universal gas constant, and AE is the activation energy. The activation energy can be determined from the lineal region of the plots of (1/n)lnK versus 1/TT, as shown in Fig. 8. It is apparent from Fig. 8 that only at high crystallization temperature range plots of (1/n)lnK versus 1/Tc are lineal. The value of crystallization activation energy of LDPE/60 vol% ZnO is 96.4 kJ/mol, much larger than that of neat LDPE (352.2 kJ/mol). As mentioned above, growth of PE crystal is confined inside ZnO network in LDPE/60 vol% ZnO, and diffusion of PE segments towards crystal sites is hindered by ZnO network, leading to high crystallization activation energy.

CONCLUSIONS

We have investigated the crystallization behavior of low-density polyethylene embedded inside the percolating network of ZnO nanoparticles (LDPE/60 vol% ZnO), and compared with those of LDPE bulks (pristine LDPE and LDPE/1.15 vol% ZnO). The results reveal that LDPE embedded inside ZnO percolating network exhibits noticeably different crystallization behavior in contrast to LDPE bulk. The degree of crystallinity of LDPE/60 vol% ZnO is much lower than that of LDPE/1.15 vol% ZnO, showing that the former possesses less perfect crystalline structure. LDPE/60 vol% ZnO has much shorter half-crystallization time at high crystallization temperature than LDPE bulk, while at lower crystallization temperature slightly longer half time is observed. Isothermal crystallization kinetics study shows that the Avrami exponent of LDPE/60 vol% ZnO is in range of 1.8-2.0, indicating quasi-two-dimensional crystallization with heterogeneous nucleation, contrasting to about 3 for LDPE bulk. Finally, crystallization activation energy of LDPE/60 vol% ZnO is by far larger than that of LDPE bulk due to geometric confinement effect.

Correspondence iv: Guo-Dong Liang; e-mail: Igdong@mail.sysusedu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 21074151.

DOI 10.1002/pen.23065

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

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Guo-Dong Liang, (1), (2) Ting-Ting Liu, (1) Wang-Ping Qin, (1) Fang-Ming Zhu, (1), (2) Qing Wu (1), (2)

(1.) DSAPM Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People's Republic of China

(2.) PCFM Lab, OFCM Institute, Sun Yat-Sen University, Guangzhou, 510275, People's Republic of China
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Author:Liang, Guo-Dong; Liu,Ting-Ting; Qin, Wang-Ping; Zhu, Fang-Ming; Wu, Qing
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jun 1, 2012
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