Influence of Polyhedral Oligomeric Silsesquioxane Structure on the

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Influence of Polyhedral Oligomeric Silsesquioxane Structure to the Disentangled State of Ultra High Molecular Weight Polyethylene Nanocomposites during Ethylene in situ Polymerization Wei Li, Tao Chen, Chao Guan, Dirong Gong, Jingshan Mu, Zhongren Chen, and Qi Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 21 Jan 2015 Downloaded from http://pubs.acs.org on January 24, 2015

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Industrial & Engineering Chemistry Research

Influence of Polyhedral Oligomeric Silsesquioxane Structure to the Disentangled State of Ultra High Molecular Weight Polyethylene Nanocomposites during Ethylene in situ Polymerization Wei Li1,*, Tao Chen1, Chao Guan1, Dirong Gong1, Jingshan Mu1, Zhong-ren Chen1, Qi Zhou 2 1

Department of Polymer Science and Engineering, School of Material Science and

Chemical Engineering, Ningbo University, Ningbo, 315211, Zhejiang. P. R. China 2

Department of Chemical Engineering, Ningbo University of Technology, Ningbo

315016, PR China.

Corresponding authors: Wei Li, E-mail address: [email protected], FAX: 0086057487609983

ACS Paragon Plus Environment


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Abstract Three

kinds

of

polyhedral

oligomeric

silsesquioxane

(Methyl-POSS,

cyclohexyl-POSS and phenyl-POSS) were chosen to adsorb [3-t-Bu-2-O-C6H3CH= N(C6F5)]2TiCl2 catalyst (FI catalyst) in order to synthesize ultra-high molecular weight polyethylene (UHMWPE)/POSS nanocomposites according ethylene in situ polymerization. It was shown that the characteristic of “living’’ polymerization of the FI catalyst would be maintained when alkyl-POSS was incorporated. However, the presence of alkyl-POSS could decay the catalyst activity. The incorporation of alkyl-POSS led to an increase of the crystallinity and the lamellae thickness of nascent UHMWPE, as observed by differential scanning calorimetry. It indicated that the alkyl-POSS would be used as a nucleating agent during chain growth procedure. Interestingly, all the nanocomposites exhibited low starting storage modulus value which was a reflection of disentangled structure in the nascent UHMWPE nanocomposites. However, polymer chains were prior to entangle with the incorporation of alkyl-POSS. Rheology and crystallization studies were used to discuss the formation mechanism of entangled structure in the synthesized UHMWPE/POSS nanocomposites. Keywords UHMWPE nanocomposites; disentangled; in-situ polymerization


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1.

Introduction

Ultra high molecular weight polyethylene (UHMWPE), with molar weight exceeding 1 million, is well-known for its ultra toughness, low moisture uptake and excellent chemical stability

[1]

. For many applications it is highly desirable to improve the

UHMWPE crystallization rate, toughness/stiffness balance, functional ability as well as processability

[2]

. Incorporation nanoparticles into UHMWPE matrix can

significantly improve its performance if an effective dispersion of nanoparticles can be employed [3]. However, commercial available UHMWPE contains large number of entanglements. Usually, 1 mg polymer with molar weight of million contains up to 1014 entangled chains [4]. Because of the large numbers of entanglements, mobility of the UHMWPE chains is limited seriously, resulting in an extremely high melt viscosity of the polymer

[5-8]

. Hence, the nanoparticles are not easy to achieve

effective dispersion in the UHMWPE matrix by physical blending methods [9, 10]. For optimizing the material properties it is of great interest to implement nanoparticles dispersion directly into ethylene polymerization to benefit from the low viscosity of the liquid polymerization media. Catalytic polymerization in the presence of fillers (in situ polymerization) is used extensively to produce conventional polyolefin compounds. Catalyst for ethylene polymerization is introduced into a gallery of filler after which polyethylene forms in situ and the aggregation morphology delaminates

[11]

. Kaminsky

[12]

reviewed the progress of polyolefin

nanocomposites synthesized by in situ polymerization. So far, lots of nanoparticles have been successfully incorporated into the conventional polyolefin with uniformed



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dispersion state, affording polyolefin with many special properties. Yet, considerably less is known with respect to UHMWPE nanocomposites synthesizing by in situ polymerization [12-15]. Stürzel M [16] prepared UHMWPE/graphene nanocomposites by polymerization filling using single-site chromium catalyst immobilized on functionalized graphene nanosheet. The nanocomposites exhibited an unusual simultaneous improvement in stiffness, elongation at break and effective nucleation of polyethylene crystzllization at only 1wt% graphene content. Rastogi [17] used several nanoparticles (TiO2, ZrO2 and hydroxyapatite) to support FI catalyst for preparing UHMWPE nanocomposites. The high-surface area of the nanoparticles, coupled with controlled reaction conditions, favored the growth of polyethylene chains with a reduced number of entanglements, resulting in an improvement of the processability. Recently, we had synthesized UHMWPE/polyhedral oligomeric silsesquioxane (POSS) nanocomposites

[18]

. FI catalyst was immobilized on the disilanolisobutyl

POSS by reaction with hydroxyl, resulting in the closest distance between the POSS-OH molecules and the nascent UHMWPE chains. It was found that the characteristic of “living’’ polymerization of the FI catalyst was retarded. There is a strong interaction between the POSS-OH particles and the nascent UHMWPE chains, resulting in an increased entanglement density of the synthesized UHMWPE. In this work, three kinds of alkyl-POSS (methyl-POSS, cyclohexyl-POSS and phenyl-POSS)

were

chosen

to

adsorb

the

FI

catalyst

for

synthesizing

UHMWPE/POSS nanocomposites. These alkyl-POSS have no reacted functional group with FI catalyst. Hence, the FI catalyst can only physically adsorb on the


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alkyl-POSS. It may make the environment of chain propagation different with that in the POSS-OH/FI catalyst system

[18]

. The influence of alkyl-POSS to the FI catalyst

polymerization behaviors was compared. We aim to know the influence of alkyl-POSS to the formation of disentangled state of the nascent UHMWPE in the physical bonded catalyst system. In particular, POSS dispersion state, crystalline behaviors and rheology of the synthesized UHMWPE/POSS nanocomposites were discussed for understanding the formation mechanism of disentangled state. 2. Experiments 2.1 Materials [3-t-Bu-2-O-C6H3CH=N(C6F5)]2TiCl2 (FI catalyst) used in the investigation was synthesized according to literatures

[19,20]

. Methyl-POSS, cyclohexyl-POSS and

phenyl-POSS were purchased from Hybrid Company (USA) and dried for 24 h before used. Figure 1 depicted the structure of the FI catalyst and the alkyl-POSS. Polymerization-grade ethylene and nitrogen were purchased from Fangxin Ningbo Corp. (Ningbo, China) and purified by filtering through Mn molecular sieves and subsequent molecular sieves of 5 Å. Methylaluminoxane (MAO, 10 wt% solution in toluene) was purchased from Albermarle Chemical Inc. (USA). Toluene (Ningbo Chemical Reagents Co., China) were purified over sodium/benzophenone ketyl and distilled prior to use. All manipulations were made under the nitrogen atmosphere using Schlenk techniques. 2.2 Preparation of alkyl POSS combined FI catalyst (FI/alkyl-POSS catalyst) 500 mg of alkyl-POSS and 83.6 mg of FI catalyst were dissolved in 50 ml of toluene



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for 1 h at room temperature, respectively. After that, the FI catalyst solution was added into the alkyl POSS solution drop by drop. The mixture was stirred for 24 h at room temperature in the glove box. Then, the dissolution was termed as FI/methyl-POSS, FI/cyclohexyl- POSS and FI/phenyl-POSS catalyst, respectively. 2.3 Ethylene solution polymerization Solution ethylene polymerization was carried out in a 100 mL glass reactor, equipped with a mechanical stirrer. The reactor temperature was set to 30 °C. 50 mL of toluene were added into the reactor. 1 µmol of the catalyst was introduced into the reactor under nitrogen purging after the injection of appropriate MAO as cocatalyst. The polymerization then took place under a continuous ethylene flow to meet 1 bar at a stirring rate of 100 rpm. The obtained polymer was precipitated and washed with acidified (2 wt % hydrochloric acid) ethanol, filtered, and dried at 50 °C under vacuum for 12 h. 2.4 Characterization and Analysis 2.4.1 Gel permeation chromatography (GPC) measurements Weight average molar mass (Mw) and molecular weight distribution (MWD) of synthesized samples were determined at 150 °C by a PL-GPC 220 type high temperature chromatograph equipped with three Plgel 10 µm Mixed-B LS type columns. 1,2,4-Trichlorobenzene (TCB) was employed as the solvent at a flow rate of 1.0 mL/min. The calibration was made by polystyrene standard EasiCal PS-1 (PL, where the molecular weight can reach to 6200000). This polystyrene standard can measured molecular weight of PE up to 2000000. (Typical measuring data is shown in


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the supporting information) 2.4.2 Differential scanning calorimetry (DSC) measurements DSC was performed with a DSC-7 instrument (PerkinElmer Corp., USA) to measure the melting point and the crystallinity of polyethylene. Samples (ca. 8 mg) were first heated to 160 °C at a rate of 10 °C/min and then cooled to 50 °C at the same rate. The second heating cycle was conducted at the same heating rate in order to observe the difference of crystalline behaviors between the nascent polymer and melting crystallized polymer. Melting temperature was taken at the peak of the endotherm. Crystallinity was calculated by comparison with the heat of fusion of a perfectly crystalline polyethylene, i.e. 289 J/g

[21]

. Tm1 denoted as the melting point in the first

heating scans; Tm2 meant the melting point in the second heating scans; Tc indicated the crystallization point in the cooling scans; Xm1 suggested the crystallinity in the first heating scans; Xm2 meant the crystallinity in the second heating scans. 2.4.3 Isothermal crystallization Isothermal crystallization study of polymers was carried out under nitrogen atmosphere in the Hyper DSC 8500 instrument (PerkinElmer Corp., USA). Samples of approximately 8 mg were used for each run. The samples were first heated from 30 °C to 160 °C at a rate of 50 °C /min, and then held for 5 min to erase any previous thermal history. After that, the samples were rapidly cooled to the designated crystallization temperature (150 °C /min) and kept at that temperature until the crystallization completed. To insure the integrity of the isothermal crystallization process and the reliability of the calculated kinetic data, proper temperatures for



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isothermal crystallization were selected. For this reason, the isothermal crystallization measurements of each sample were carried out at a temperature range of 118 °C-120°C. Generally, the Avarami equation was used to analyze the isothermal crystallization kinetics of polymers:

Where Xc was the relative crystallinity at time t. na was the Avrami exponent, which revealed the nucleation mechanism. Ka was the Avrami crystallization rate constant. Since Avarami analysis described the early stages of polymer crystallization, Tobin theory which involved phase-transformation kinetics with growth site impingement, was therefore proposed to improve the fitting results at the later stages of crystallization:

Where Kt was the Tobin crystallization rate constant and nt was the Tobin exponent governed by different types of nucleation and growth mechanisms. 2.4.4 X-ray diffraction measurements (XRD) XRD measurements were carried out on a Bruker GADDS diffract meter with an area detector operating under 40 kV and 40 mA, using Cu Kα radiation (λ =0.154nm). 2.4.5 Transmission election microscopy (TEM) characterization Morphology of the synthesized nanocomposites was investigated by transmission election microscopy (TEM, Tecnai F20, USA). All the characterizations were made at


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the room temperature. 2.4.6 Thermal gravimetric analyses (TGA) TGA were recorded on a SDT 2960 Simultaneous TG-DTA instrument under N2 flow (100 mL/min) with a heating rate of 10°C /min. 2.4.7 Rheological measurements Rheological studies were performed on a strain-controlled rheometer, HAAKE III instruments. A disk of 8 mm diameter and 1 mm thickness was compressed under 20 MPa at 120 °C for 30 mins and was used in all rheological studies

[5, 17]

. The disk

between the parallel plates of the rheometer was heated to 170 °C under a nitrogen environment. The rheology experiments started after waiting for the thermal stabilization at 170 °C (∼5 min). Dynamic time sweep test was performed to follow the entanglement formation at a fixed frequency of 1 rad/s and strain in the linear viscoelastic regime of the polymer. Dynamic frequency sweep test was performed at a fixed strain of 1% when the dynamic time sweep test was completed. 3. Results and discussion 3.1 Ethylene polymerization with FI/methyl-POSS, FI/cyclohexyl-POSS and FI/phenyl-POSS catalyst and FI catalyst Table 1 summarizes the results of ethylene polymerization with the catalysts. A decrease of activity in the FI/alkyl-POSS catalyst system is shown compared to the homogeneous FI catalyst. Furthermore, the activity of homogeneous FI catalyst gradually increase with the elongation of polymerization time, while the activity of FI/alkyl-POSS catalyst system presents opposite trends with that of homogeneous FI



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catalyst. It indicates that the alkyl-POSS can make the active centers of FI catalyst unstable during polymerization. The catalyst activity decreases in the order of FI catalyst, FI/methyl-POSS, FI/cyclohexyl-POSS and FI/phenyl-POSS. It can be due to the increased electron density of oxygen atoms of the POSS molecules, caused by the gradually increased electronic donating effect of methyl, cyclohexyl and phenyl [22]. The synthesized UHMWPE and UHMWPE/nanocomposites have narrow molecular weight distribution (MWD) (Table 1). It is quite different with that in the disilanolisobutyl POSS/FI catalyst systems where a broad MWD (can reach to 5.0) is shown

[18]

. It indicates that physical adsorption between alkyl-POSS and FI catalyst

cannot obviously influence the chemical environment of titanium active centers during

ethylene

polymerization.

Furthermore,

the

of

Mw

synthesized

UHMWPE/POSS-CH3 and UHMWPE/POSS-C6H12 nanocomposites is less than that of pure UHMWPE. However, the Mw of UHMWPE/POSS-C6H6 is higher than that of pure

UHMWPE. The methyl-POSS and FI/cyclohexyl-POSS can achieve

homogeneous state in the toluene. The homogeneous state can still be maintained when the POSS reacts with the FI catalyst. Hence, steric hindrance to catalyst active centers does not present in the FI/methyl-POSS catalyst system

[24]

. As a result, the

decreased of Mw is probably caused by the decayed catalyst activity

[25]

. Yet, the

solubility of FI/phenyl-POSS in the toluene is poor. More aggregation of phenyl-POSS can be found during catalyst preparation. The FI catalyst will adsorb into the POSS aggregators thereof. Thus, the diffusion barriers of monomers will hinder the β-H chain transfer process, resulting in an increase of the Mw and a slightly


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increase of the polydispersity index (MWD) [15]. The synthesized polymer presents a high melting temperature (>140 oC) during the first heating scans. Such a high melting temperature is normally found for “chain-extended” polyethylene crystals [5]. Moreover, the high melting temperature of 141 oC is lost on the second heating where a melting temperature of 134 oC is measured (Typical DSC traces are shown in Figure 2. All of the DSC data are list in the supporting information). It suggests the synthesized polyethylene may have disentangled structure [26, 27]. Interestingly, the crystallinity of UHMWPE/POSS-C6H12 and UHMWPE/POSS-CH3 nanocomposites obviously increases compared with that of pure samples. It indicates that the POSS particles can be used as a nucleating agent during chain growth procedure

[28]

. Moreover, the crystallinity and the melting point

of UHMWPE/POSS-C6H6 are low, probably due to the worse dispersion state of phenyl-POSS in the UHMWPE matrix. 3.2 Dispersion state of alkyl-POSS in the UHMWPE matrix It is shown in the table 1 that the Mw of synthesized polymer are up to 1 million in all of the catalyst system after 40 min of polymerization (The GPC traces are shown in the supporting information). It indicates that the UHMWPE can be obtained. For the polymer synthesized at 40 min of polymerization, the POSS loading in the polymer matrix is around 1.0 wt%. This will diminish the influence caused by the concentration variance of nanofiller. Thus, the polymer synthesized at 40 min of polymerization was used in the following study. XRD can be used to determine the aggregation of nanoﬁllers [28, 29]. Figure 3 shows



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the XRD patterns of pure nascent UHMWPE, POSS and the nanocomposites for 2θ from 4˚ to 60˚. For UHMWPE and its nanocomposites, two main reﬂections can be observed at 2θ=21˚ and 2θ=24˚, which are attributed to the 110 and 200 crystal planes, respectively [30]. A shoulder peak can be observed at 2θ=19.5˚ which is the reflection of amorphous phase of UHMWPE. Figure 3 shows that polymer matrix does not have any peak from 7˚ to 20˚ where the alkyl-POSS have typical reflections in this region. The reflections of nanocomposites are similar with that of pure UHMWPE. It indicates that the aggregation of the alkyl-POSS clusters is completely disrupted through ethylene in situ polymerization

[30, 31]

. However, the data from XRD are not

sufficient to reflect the dispersion state of alkyl-POSS in the UHMWPE matrix due to the low incorporation concentration of alkyl-POSS (around 1.0 wt%). Therefore, TEM images are further used to observe the dispersion state of alkyl-POSS. Figure 4 shows the TEM images of UHMWPE/POSS nanocomposites. It is shown that methyl-POSS and cyclohexyl-POSS have the exfoliated dispersion state in the UHMWPE matrix with only several tens nanometers of nanofiller. This can be due to the good solubility of methyl-POSS and cyclohexyl-POSS in the toluene. However, the aggregators of phenyl-POSS are large. The size of aggregators can reach to several hundred nanometers in the UHMWPE/POSS-C6H6 nanocomposites. It can be attributed to the worse solubility of phenyl-POSS in the toluene. This worse solubility of phenyl-POSS cannot achieve uniformed distribution of FI catalyst in the phenyl-POSS aggregators. As a result, phenyl-POSS cannot reach to the exfoliated dispersion state when the ethylene polymerization occurred.


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3.3 Disentangled state of UHMWPE/POSS nanocomposites Dynamic time sweep test is used to characterize the disentangled state of the synthesized polymers [5, 6]. Figure 5a plots modulus buildup data for the synthesized UHMWPE/POSS nanocomposites. Similar to UHMWPE, all the nanocomposites exhibit low starting modulus values which is a reflection of the less number of entanglement per chain. With the formation of entanglements during time, where the driving force is towards the thermodynamically stable melt state, the elastic modulus increases with time. The increase in the modulus buildup with time in figure 5a suggests an initial disentangled state of the synthesized polymers [5-8]. The time required for a disentangled polymer to reach 98% of its maximum plateau modulus is termed the total buildup time (tm). The tm of nanocomposites is less than that of pure UHMWPE which indicates that the nanocomposites require less time to reach the thermodynamic melt state from its nascent disentangled state [5, 17]. Figure 5b plots the normalized storage modulus buildup data of the nanocomposites synthesized at 40 min of polymerization. Generally, chains acquire maximum number of entanglement at G’max in a thermodynamically stable melt. Therefore, GNt indicates the present number of entanglement. It is a fraction of the total number of entanglement in the thermodynamically stable melt. In this way, the higher value of GNt indicates the higher entanglement density and lower molecular weight between entanglements. The nascent UHMWPE nanocomposites present increased tm and GNt=0 value in the order of UHMWPE/POSS-CH3, UHMWPE/POSS-C6H6 and UHMWPE/POSS-C6H12. It suggests that polymer chains are prior to disentangle



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following this order. In other words, the UHMWPE/POSS-C6H12 nanocomposites can achieve the most disentangled state in the nanocomposites. Generally, high molecular weight with narrow molecular weight distribution have more physical entangle points which will require more time to reach the thermodynamic melt state from its disentangled

state.

In

our

results,

the

synthesized

UHMWPE/POSS-C6H6

nanocomposites have the highest molecular weight among all the nanocomposites. However, the nascent UHMWPE/POSS-C6H6 nanocomposite does not spend the longest time to reach its thermodynamic melt state. Similar results can be found in UHMWPE/POSS-C6H12 and UHMWPE/POSS-CH3 nanocomposites where similar molecular weight of polymer requires obviously different tm to reach to the thermodynamic melt state. This can be due to the polymerization conditions which synthesize nascent polymer with different initial disentangled state. As a result, we will further discuss the formation mechanism of disentangled UHMWPE chains during in situ polymerization. 3.4 The formation mechanism of disentangled structure of UHMWPE chains during in situ polymerization. It is worth to mention that favored crystallization rate over chain growth rate and sufficient distance between growing chains are the key points for the preparation of disentangled UHMWPE in the homogeneous catalyst systems [5-8]. However, in the nanocomposites systems, the influence parameters to the disentangled degree will be more complex due to the interaction between nanofillers and polymer matrix.


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3.4.1 The crystallization ability of UHMWPE/POSS nanocomposites The crystallization rate is difficult to measure during the polymerization. Polymerization temperature and polymerization environment are very important for the nascent polymer chains to be crystallized. In this work, the polymerization conditions are the same in all nanocomposites systems. Thus, the influence of alkyl-POSS to the crystallization ability of UHMWPE may influence the crystallization rate. Isothermal crystallization behaviors of the polymers are conducted in the thermodynamic equilibrium, thus are investigated for quantitative study of the crystallization ability of the polymer chains based on the Avrami and Tobin methods [32-34]

.

Figure 6a shows the typical isothermal crystallization curves of polyethylene at different crystallization temperatures (Tc). The crystallization exothermic peaks become flatter and move to the higher value upon increasing Tc. Meanwhile, the time to complete crystallization is also increased. Xt can be obtained from the area under the exotherm up to time t, divided by the total exothermic peak area:

Where dHc is the enthalpy of crystallization released during an infinitesimal time interval dt. Thus the development of Xt with crystallization time for polyethylene can be established (Figure 6b). Table 2 summaries the fitting results of isothermal crystallization. Tobin method has the best consistency between the data and the model due to the smallest derivation indicating that the chain impinging effect cannot be ignored during the crystallization [35].



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Generally, t1/2 of a polymer is taken as a measure of the overall rate of crystallization. Table 2 shows that t1/2 of all polymer increase with increased Tc. This verifies that the crystallization takes place by a nucleation-controlled mechanism

[35]

.

The values of nt, generally approximately to na +1, also remain close to 2, indicating the simultaneous occurrence of two-dimensional crystal growths with heterogeneous nucleation

[36]

. The crystallization rate constant kα and kt is increased as the

incorporation of alkyl-POSS. It is possible that alkyl-POSS act as nuclei for the initial nucleation and subsequent growth of crystallites. Furthermore, the crystallization rate constant kα and kt increases in the order of UHMWPE, UHMWPE/POSS-CH3, UHMWPE/POSS-C6H12 and UHMWPE/POSSC6H6, indicating that the crystallization ability of the polymer is increased following the same order. To recall, the polymerization rate of FI/alkyl-POSS catalyst system decreases in the order of FI catalyst, FI/methyl-POSS, FI/cyclohexyl-POSS and FI/phenyl-POSS catalyst. It indicates that polymer propagation rate follows the same trends. Thus, the UHMWPE chains should become more disentangled in the order of UHMWPE/POSS-CH3,

UHMWPE/POSS-C6H12

and

UHMWPE/POSS-C6H6.

However, in the present work, UHMWPE/POSS-C6H12 achieves the most disentangled state. 3.4.2 Interaction between alkyl POSS and UHMWPE matrix Dynamic frequency sweep tests are conducted after the samples reach to the thermodynamically stable melt state. Figure 7 shows that incorporation of alkyl-POSS particles obviously influences the rheological properties of the synthesized UHMWPE


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nanocomposites where the storage modulus becomes less dependent of frequency [37]. It indicates that physical adsorption of polymer chains on alkyl-POSS particles is noticeable

[37, 38]

. The physical adsorption between the UHMWPE chains and the

alkyl-POSS particles can hinder the mobility of growing chains, leading to an enhancement of chain overlapping

[38]

. Moreover, the frequency dependence of

storage modulus is similar in the UHMWPE/POSS-CH3 and UHMWPE/POSS-C6H6 nanocomposites. However, the storage modulus of UHMWPE/POSS-C6H12 nanocomposites turns to more dependent on the sweep frequency, especially in the low frequency zone (reflecting the relaxation of long chains). It indicates that the physical adsorption between POSS and UHMWPE matrix is weak in the UHMWPE/POSS-C6H12 nanocomposites compared with other nanocomoposites. This weak adsorption effect may be due to the large steric hindrance of cyclohexyl on the POSS molecules which will push the polymer chains far from the Si-O-Si cage. Therefore, the decreased physical adsorption between POSS and UHMWPE chains hinder the overlapping of polymer chains, resulting in a disentangled structure. Figure 8 compares the tan delta behavior of the resins in the melt. The tan delta (G’’/G’) quantifies the balance of the loss and elastic properties. A high value of tan delta indicates that the polymer material has poor recovery characteristics since the chains relax slowly after being stressed. From a processing point of view, this value can indicate how well a material will recover after being deformed. A material that withholds stress histories may lead to melt flow instabilities such as melt-fracture. [39] Figure 8 shows that the tan delta of UHMWPE nanocomposites is obviously less



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than that of pure UHMWPE. It suggests that better recovery characteristics of UHMWPE nanocomposites can be achieved due to the large number of polymers chains

adsorbed

on

the

POSS

particles.

Moreover,

the

tan

delta

of

UHMWPE/POSS-C6H12 nanocomposites presents more dependence on the frequency which has the same orderliness with storage modulus, indicating a worse adsorption effect between UHMWPE and POSS-C6H12 nanofillers [39]. 3.4.3 The formation of disentangled structure in the nascent UHMWPE matrix Rastogi et al

[5-8]

had studied a method to reduce the number of entanglements in

nascent UHMWPE by using a homogeneous single-site catalyst. The formation of disentangled structure was explained based on the completion between chain crystallization and propagation. Ethylene polymerization was conducted at dilute catalyst concentration and low temperature. The dilute catalyst concentration would ensure spatial distance between the polymer chains growing on active sites. While, the low synthesized temperature was favorable to chain crystalline compared with chain growth. Thus, the chain overlaps process can be hindered, resulting in a decrement of chain entanglements in the synthesized UHMWPE [5]. Figure 9 presents a view for the proposed formation procedure of entangled structure in the nascent UHMWPE matrix when nanofillers are introduced into the polymerization media: FI catalyst centers are surrounded by many alkyl-POSS particles before polymerization occurs. Thus, many nascent UHMWPE chains will physically adsorb on the alkyl-POSS through the van der waals interaction. The strong adsorption between the UHMWPE chains and the alkyl-POSS particles can


Page 19 of 38

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pack the motion-hindered chains more compact, leading to an enhancement of chain overlapping

[38]

. This makes the nascent UHMWPE chains turn to entangle with the

incorporation of alkyl-POSS [5, 18]. Thus, the physical adsorption between nanofiller and UHMWPE matrix takes more influence on the formation of entangled structure of nascent UHMWPE matrix in the in situ polymerization systems. 4. Conclusions Methyl-POSS, cyclohexyl-POSS and phenyl-POSS are chosen to adsorb the FI catalyst using to synthesize UHMWPE/POSS nanocomposites. Combined with our previous work [18], we conclusively show that: firstly, the catalyst activity decreases in the order of FI catalyst, FI/methyl-POSS, FI/cyclohexyl-POSS and FI/phenyl-POSS, due to the increased electronic donating effects of the POSS molecules. Secondly, the solubility of nanofiller in the polymerization media is very important in the physical absorption catalyst system for achieving exfoliated nanocomposites. The aggregation morphology of POSS could only be delaminated when the catalyst was diffused into the gallery of POSS. The increased solubility of cyclohexyl-POSS, phenyl-POSS and methyl-POSS makes the aggregator of alkyl-POSS in the synthesized UHMWPE matrix decrease in the same order. The methyl-POSS can achieve the best dispersion state with only several tens nanometers of aggregators. Finally, more entangled structures are synthesized when the adsorption between alkyl-POSS particles and UHMWPE chains is enhanced. In this work, nascent UHMWPE nanocomposites become entangled in the order of UHMWPE/POSS-C6H12 and UHMWPE/POSSC6H6 and UHMWPE/POSS-CH3.



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Page 20 of 38

Acknowledgements. Funding from the Project of Natural Science Foundation of China (No. 21206078, No. 51203081, No. 21304050), the Natural Science Foundation of Zhejiang Province (LQ12B06003), the Natural Science Foundation of Ningbo (2012A610087), Zhejiang Province Department of Education Fund (Y201223797), the Key Innovation Team of Zhejiang Province (2011R50001-5,-11), Sponsored by K.C.Wong Magna Fund in Ningbo University, Ningbo Key Laboratory of Specialty Polymers 2014A22001 are gratefully acknowledged. Supporting

Information

Available

Typical

original

GPC

data

of

UHMWPE/POSS-CH3 synthesized at 40 min of polymerization are shown in Figure S1. GPC traces of UHMWPE and UHMWPE nanocomposites synthesized at 40 min of polymerization are shown in Figure S2. The DSC data of all the synthesized polymers are list in Table S1. This information is available free of charge via the Internet at http://pubs.acs.org/. References 1. Kurtz, S.M. The UHMWPE Handbook, 2nd ed.; Academic Press: New York, 2009. 2. Stein, H.L. Engineering Materials Handbook; Willey: New York, 1998. 3. Ren, P.G.; Di, Y.Y.; Zhang, Q.; Li, L.; Pang, H.; Li, Z.M. Composites of Ultrahigh-Molecular-Weight

Polyethylene

with

Graphene

Sheets

and/or

MWCNTs with Segregated Network Structure: Preparation and Properties. Macromol. Mater. Eng. 2012, 297, 437. 4. Sharma KG. Easily processable ultra high molecular weight polyethylene with narrow molecular weight distribution. Ph.D. Thesis, Eindhoven University of


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catalysis

as

an

efficient

tool

for

the

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preparation

of

polyethylene/carbon nanotube nanocomposites: effect of the catalytic system on the coating morphology. J. Mater. Chem. 2007, 17, 2359-2366. 14. Pavlidou, S.; Papaspyrides, C.D. A review on polymer-layered silicate nanocomposites. Prog Polym Sci. 2008, 33, 1119-1198. 15. Paul, D.R.; Robeson, L.M. Polymer nanotechnology: Nanocomposites. Polymer 2008, 49, 3187-3204. 16. Stürzel, M.; Kempe, F.; Thomann, Y.; Mark, S.; Enders, M.; Mülhaupt, R. Novel graphene UHMWPE nanocomposites prepared by polymerization filling using single-site catalysts supported on functionalized graphene nanosheet dispersions. Macromolecules 2012, 45, 6878-6887. 17. Ronca, S.; Forte, G.; Tjaden, H.; Yao, Y.F.; Rastogi, S. Tailoring molecular structure via nanoparticles for solvent-free processing of ultra-high molecular weight polyethylene composites. Polymer. 2012, 53, 2897-2907. 18. Li, W.; Guan, C.; Xu, J.; Mu, J.S.; Gong, D.R.; Chen, Z.R.; Zhou, Q. Disentangled UHMWPE/POSS nanocomposites prepared by ethylene in situ polymerization. Polymer 2014, 55,1792-1798. 19. Mitani, M.; Mohri, J.; Yoshida, Y.; Saito, J.J.; Ishi, S.; Tsuru, K.; Matsui, S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S.; Matsugi, T.; Kashiwa, N.; Fujita T. Living polymerization of ethylene catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate ligands. J Am Chem Soc. 2002, 124, 3327-3336.


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20. Makio, H.; Kashiwa, N.; Fujita, T. FI catalysts: A new family of high performance catalysts for olefin polymerization. Adv Synth Catal. 2002, 344, 477-493. 21. Abbasi, S.H.; Hussein, I.A.; Parvez, M.A. Nonisothermal crystallization kinetics study of LDPE/MWCNT nanocomposites: effect of aspect ratio and surface modification. J Appl Polym Sci. 2011, 119, 290-296. 22. Zhang, Y.W.; Ye, Z.B. Homogeneous polyhedral oligomeric silsesquioxane (POSS)-supported

Pd-diimine

complex

and

synthesis

of

polyethylenes

end-tethered with a POSS nanoparticle via ethylene "living" polymerization. Chem Commun. 2008, 10, 1178-1180. 23. Li, W.; Adams, A.; Wang, J.D.; Blümich, B.; Yang, Y.R. Polyethylene/ palygorskite nanocomposites: Preparation by in situ polymerization and their characterization. Polymer 2010; 51, 4686-4697. 24. Mckenna, T.F.L.; Martino, A.D.; Weickert, G.; Soares, J.B.P. Particle growth during the polymerisation of olefins on supported catalysts, 1-nascent polymer structures. Macromol React Eng 2010, 4, 40-64. 25. Choi, Y.Y.; Soares, J.B.P. Supported hybrid early and late transition metal catalysts for the synthesis of polyethylene with tailored molecular weight and chemical composition distributions. Polymer. 2010, 51, 4713-4725. 26. Corbeij-Kurelec, L. On the borderline between Solid and Melt Thesis, Eindhoven Univ of Technology, 2001. 27. Rastogi, S.; Kurelec, L. Polymorphism in polymers; its implications for polymer crystallisation. J Mater Sci. 2000, 35, 5121-5138.



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28. Perrin, F.X.; Panaitescu, D.M.; Frone, A.N.; Radovici, C.; Nicolae, C. The influence of alkyl substituents of POSS in polyethylene nanocomposites. Polymer 2013, 54, 2347-2354. 29. Zhou, Q.; Wang, Z.; Shi, Y.; Fang, J.; Gao, H.; Loo, L.S. The migration of POSS molecules in PA6 matrix during phase inversion process. Appl Surf Sci 2013, 284, 118-125. 30. Gopakumar, T.G.; Lee, J.A.; Kontopoulou, M.; Parent, J.S. Influence of clay exfoliation

on

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montmorillonite/polyethylene

composites. Polymer 2003, 43, 5483-5491. 31. Wei, Z.; Lu, Y.L.; Meng, Y.; Zhang, L.Q. Improved understanding of in-situ polymerization of zinc dimethacrylate: The solid bulk polymerization. Polymer 2012, 53, 1409-1417. 32. Li, W.; Guan, C.; Xu, J.; Chen, Z.R.; Jiang, B.B.; Wang, J.D.; Yang, Y.R. Bimodal/broad polyethylene prepared in a disentangled state. Ind Eng &Chem Res 2014, 53, 1088-1096. 33. Avrami, M. Kinetics of phase change I-General theory. J. Chem. Phys. 1939, 7, 1103-1112. 34. Tobin, M.C. Theory of phase-transition kinetics with growth site impingment.1. Homogeneous nucleation. J. Polym. Sci. Poly. Phys. 1974, 12, 399-406. 35. Shi, X.M.; Wang, J.D.; Jiang, B.B.; Yang, Y.R. Influence of nanofiller dimensionality on the crystallization behavior of HDPE/Carbon nanocomposites. J. Appl. Polym. Sci. 2013, 128, 3609-3618.


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36. Guan, C.; Yang, H.Q.; Li, W.; Zhou, D.Y.; Xu, J.; Chen, Z.R. Crystallization behavior of ultrahigh-molecular-weight polyethylene/polyhedral oligomeric silsesquioxane nanocomposites prepared by ethylene in situ polymerization. J. Appl. Polym. Sci. 2014, 131, 19, 40847. 37. Zhang, Q.H.; Lippits, D.R.; Rastogi, S. Dispersion and rheological aspects of SWNT in ultrahigh molecular weight polyethylene. Macromolecules. 2006, 39, 658-666. 38. Li, W.; Jiang, B.B.; Buda, A.; Wang, J.D.; Blumich, B.; Yang, Y.R.; Zheng, J. An NMR investigation on the phase structure and molecular mobility of the novel exfoliated polyethylene/palygorskite Nanocomposites. J Polym Sci Part B: Polym Phys 2011, 48, 1363-1371. 39. Soares, J.B.P.; Abbott, R.F.; Kim, J.D. Environmental stress cracking resistance of polyethylene: The use of CRYSTAF and SEC to establish structure-property relationships. J Polym Sci Part B: Polym Phys 2000, 38, 1267-1275.



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Page 26 of 38

Figure legends Figure 1 Molecular structure of catalyst, alkyl-POSS used in the present work. Figure 2 The DSC traces of UHMWPE and its nanocomposites. (a) is the first heating scan, (b) is the second heating scan. Figure 3 XRD patterns of UHMWPE and UHMWPE/POSS nanocomposites synthesized

at

40

min

of

polymerization,

(a)

UHMWPE/POSS-CH3;

(b)UHMWPE/POSS-C6H12;(c)UHMWPE/POSS-C6H6. Figure 4 TEM images of UHMWPE/POSS nanocomposites synthesized at 40 min of polymerization,

(a)

UHMWPE/POSS-CH3;

(b)

UHMWPE/POSS-C6H12;

(c)

UHMWPE/POSS-C6H6. The magnification is 40000. Figure 5 Dynamic time sweep test at 170 oC for UHMEPE and UHMWPE/POSS nanocomposites at a constant frequency 10 rad/s and strain 1%. GN t is normalized storage modulus, normalized by G’max (maximum plateau modulus in the modulus buildup). ‘b’ plots the normalized data of ‘a’ . Figure 6 (a) Typical DSC curves of isothermal crystallization of polymers at different crystallization temperatures. (b) Typical development of relative crystallinity (Xt) with crystallization time. Here, the polymer was UHMWPE. Figure 7 Dynamic frequency sweep data for UHMWPE and UHMWPE/POSS nanocomposites at 170 oC and constant strain 1%. Figure 8 Flow recovery comparisons of UHMWPE and UHMWPE nanocomposites. Figure 9 Proposed procedure for the formation of disentangled structure in the nascent UHMWPE matrix.


Page 27 of 38

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Table 1 Polymerization results of the synthesized UHMWPE nanocomposites.a Time Samples

Activity×106b

Entry min

POSS Mw in PEc *10

4

Xm1 Xc Xm2

Mn MWD *10

4

% %

%

1

10

0.80

0.00 104.1 38.0

2.7 45.2 43.2 37.5

2

20

0.84

0.00 129.2 49.2

2.6 45.1 43.1 37.3

3

30

0.99

0.00 155.1 62.0

2.5 45.3 43.0 37.0

4

40

1.53

0.00 164.3 60.7

2.7 45.2 43.0 37.0

5

60

1.15

0.00 189.5 73.1

2.6 45.3 43.0 37.0

6

10

0.99

2.92 105.8 31.8

3.3 48.6 46.3 40.3

7

20

0.91

1.58 133.4 46.9

3.6 48.5 46.1 39.9

8

30

0.77

1.24 148.8 50.0

2.9 48.4 46.0 39.8

9

40

0.76

0.94 159.0 61.2

2.6 48.1 45.8 39.7

10

60

0.68

0.71 180.0 82.0

2.1 48.4 45.9 39.6

11

10

0.98

2.94 106.9 40.3

2.7 48.4 46.2 40.3

12

20

0.85

1.69 131.5 50.7

2.6 48.4 46.1 40.2

POSS-C6H12 13

30

0.72

1.33 140.1 61.0

2.3 48.2 45.9 39.7

14

40

0.72

1.00 156.0 70.1

2.2 48.4 46.0 39.7

15

60

0.57

0.84 178.0 83.6

2.1 48.1 45.8 39.8

16

10

0.71

4.09 112.5 32.0

3.5 45.3 43.4 37.1

17

20

0.67

2.16 142.1 44.6

3.2 45.3 43.2 37.2

18

30

0.58

1.66 151.0 51.7

2.9 45.2 43.0 37.2

19

40

0.59

1.08 180.2 64.3

2.8 45.3 43.0 37.2

20

60

0.49

0.98 220.5 79.6

2.8 44.8 42.8 37.2

UHMWPE

UHMWPE/ POSS-CH3

UHMWPE/

UHMWPE/ POSS-C6H6

a

Polymerization conditions: Temperature 30°C, [Al]/[Ti] molar ratio=16000.

b

Catalyst activities = ×106 g PE/h·bar·mol [Ti];

c

The load of POSS was determined by TGA analysis with the heating temperature

ranging from 50 °C to 600 °C and a heating rate of 10°C/min.



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Page 28 of 38

Table 2 Kinetic Parameters of Avrami and Tobin Models for the polymers. Avrami UHMWPE

Ka(min-1)

na

Tobin R2

Kt(min-1)

nt

R2

t1/2

118

2.8386 1.1928 0.9909 4.0056 2.0094 0.9990 0.2470

119

2.0815 1.5173 0.9982 3.0850 2.5581 0.9992 0.3110

120

1.3386 1.2088 0.9960 2.1120 2.0406 0.9992 0.4360

UHMWPE/POSS-CH3 118

2.8368 1.1354 0.9927 4.1571 1.9389 0.9996 0.2440

119

2.1794 1.1225 0.9911 3.2344 1.9300 0.9993 0.3080

120

1.5577 1.1174 0.9969 2.3559 1.8938 0.9995 0.4150

UHMWPE/POSS-C6H12 117

3.1005 1.2319 0.9913 4.3324 2.0524 0.9989 0.2290

118

2.4579 1.2258 0.9964 3.4397 2.0680 0.9992 0.2890

119

1.8437 1.2069 0.9929 2.5967 2.0498 0.9993 0.3850

UHMWPE-C6H6 117

3.6535 1.2161 0.9911 5.1275 2.0132 0.9994 0.1960

118

3.0738 1.1951 0.9944 4.3335 2.0081 0.9995 0.2310

119

2.4688 1.1648 0.9988 3.5078 1.9862 0.9996 0.2840


Page 29 of 38

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Figure 1 211x99mm (300 x 300 DPI)



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Figure 7 141x115mm (300 x 300 DPI)



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Figure 8 127x107mm (300 x 300 DPI)


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Influence of Polyhedral Oligomeric Silsesquioxane Structure on the

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