Temperature Effect on the Conformation Transition of Ultra-high Molecular Weight Polyethylene/Polypropylene Blends Undergoing Continuous Volume Extensional Flow:A Mesoscopic Simulation

WANG Junxia ,YAN Shilin ,YU Dingshan＊

(1.Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education and Key Laboratory of High Performance Polymer-based Composites of Guangdong Province,School of Chemistry,Sun Yat-Sen University,Guangzhou510275,China;2.Hubei Key Laboratory of Theory and Application of Advanced Materials Mechanics,Wuhan University of Technology,Wuhan 430070,China)

Abstract: Due to the multiformity and complexity of chain conformation under external flow and the challenge of systematically investigating the transient conformation and dynamic evolution process of polymer chains at the molecular level by means of present experimental techniques,a universal description of both chain conformation and dynamics with respect to continuous volume extensional flow (CVEF) is still absent.Taking into account the temperature effect,we performed dissipative particle dynamics (DPD) simulations with the particles corresponding to the repeat units of polymers over a wide temperature range and analyzed the correlation with the conformational properties of ultra-high molecular weight polyethylene/polypropylene(UHMWPE/PP) blend in response to the CVEF.With time evolution,the polymer chains become highly oriented parallel to the flow direction instead of the initial random coiling and self-aggregation.It is found that a high temperature is necessary for more substantial compactness to take place than low temperature.The low-k plateau and low-k peak in structure factor S(k) curves suggest a low degree of conformational diversity and a high degree of chain stretching.It is also concluded that the intra-molecular C-C bond interaction is the main driving force for the dynamics process of the chain conformations undergoing CVEF,where the motion of the alkyl chains is seriously restricted owing to the increase in bond interaction potential,resulting in a reduction of the difference in diffusion rates among alkyl chains..

Key words: temperature effect;dissipative particle dynamics;ultra-high molecular weight polyethylene;polypropylene;volume extensional flow;chain conformation;blends

1 Introduction

Recently,a novel eccentric rotor extruder (ERE)capable of generating a continuous volume extensional(or elongational) flow (CVEF)in the whole conveying,mixing and plasticizing process has emerged.The ERE consists of a number of eccentric rotor plasticizing and conveying units instead of traditional screws.Accurately,it undergoes extensional deformation in addition to shear deformation,but it is dominated by extensional flow.It is widely believed that this technique has various advantages over shear flow,including better dispersive and distributive mixing.In particular,the ERE could generate the volume extensional deformation and rolling effects,which could engender a continuous change of polymer volume and rotate by themselves,guaranteeing the material in the“core”part of the block close to the heat source,improving the mass and heat transfer,promoting melting process,and eventually,yielding a defect-free material after extrusion.Considerable efforts have been directed towards the fabrication of polymer-based blends and composites with molecular weights numbering in the millions and extremely long polymeric chains,

e g

,ultra-high molecular weight polyethylene(UHMWPE).For example,the morphologies and properties of UHMWPE/organmodified montmorillonite(OMMT) nanocomposites were characterized to investigate the effects of CVEF on the dispersion of OMMT layers and the properties of UHMWPE/OMMT nanocomposites.It was found that the ideal dispersion of OMMT in the UHMWPE matrix obviously improved the crystallinity and the mechanical properties at a certain concentration of OMMT loading,indicating that a lower OMMT addition could lead to an effective strengthening and toughening on the mechanical properties of UHMWPE.Extensive investigation into the effects of the intercalated and exfoliated OMMTs on the microstructure,crystallization process and dynamic rheological behavior of UHMWPE/polypropylene(PP) nanocomposites were carried out by Lin

et al

and the results showed that OMMT caused heterogeneous nucleation in the blends,leading to a high crystallization temperature.Yin

et al

showed that effective and homogeneous dispersion of carbon nanotubes (CNTs) into intractable UHMWPE was realized by using ERE without adding any other additives or solvents.The influence of CNTs loading,mixing time and rotation rate on the microstructure,crystallization properties,tensile strength and rheological properties were comprehensively studied.Based on the research described above,the melt mixing process under the synergy of ultrasonic wave and CVEF has been developed and the effects of ultrasonic power and sonication time on UHMWPE/OMMT and UHMWPE/CNTs composites have been experimentally discussed.

When processed by the ERE we have shown that nascent UHMWPE powder can be successfully extruded at relatively low temperatures around 230 ℃,forming semitransparent and uniform samples.With increasing temperature the UHMWPE samples become more transparent and flat,resulting in better plasticizing behavior,as revealed by scanning electron microscope of the morphology.A universal description of both chain conformation and dynamics with respect to CVEF is still absent.The absence is due in part to the multiformity and complexity of chain conformation under CVEF and in part from the challenge of systematically investigating the transient conformation and dynamic evolution process of polymer chain at the molecular level by means of present experimental techniques.Dissipative particle dynamics (DPD)methods have become extremely helpful in providing generic properties of macromolecular systems,such as UHMWPE.Consequently,in the present study here,special care was taken on the DPD simulations over a wide temperature range and the analysis of the correlation with the conformational properties of UHMWPE in UHMWPE/PP blends in response to CVEF,taking the temperature effect into account.

2 Computational details

The simulations are carried out using Accelrys Materials Studio software (USA) over a wide range of temperature,from 350 to 550 K.The temperatures were monitored by a Nose thermostat with a

Q

ratio of 1.0.In DPD method,a repeat unit,[-CH-CH-]for UHMWPE,[-CH-CH(CH)-]for PP,was represented by beads and a polymer chain can be described by the number (

n

) of beads,

n

=

M

/(

M

×

C

),where

M

is the molar mass of the polymer,

M

is the molar mass of the repeat unit and

C

is the characteristic ratio of polymer.The UHMWPE/PP blend(90/10)model contained 15164 beads,where every UHMWPE chain was composed of 13 914 beads with a total molecular weight of 3.0×10and every PE chain consisted of 250 beads with a total molecular weight of 7.3×10.The interaction parameter (

a

)between UHMWPE and PP was 25.61.The geometry optimization was performed by smart algorith,followed by initial isothermal-isometric(NVT) with Berendsen thermostat and subsequent isothermalisobaric(NPT) dynamic simulation with Souza-Martins method.The electrostatic interaction and van der Waals interaction were calculated using Ewald summation and bead-based method,respectively.The CVEF was simultaneously driven by pressures in

X

,

Y

and

Z

directions(

P

=

P

=1.0 MPa,

P

=-2.0 MPa) and the

X-Y

plane (

P

=

P

=0.005 MPa)in a cubic box of 300×300×300 Å.In a homogeneous and isotropic DPD system,the pressure employs the radial distribution,

g

(

r

),as follows:

Fig.1 Simulation snapshots of UHMWPE/PP blends under CVEF obtained every 60 ps:(a) 0 ps;(b) 60 ps;(c) 120 ps;(d) 180 ps;(e) 240 ps;(f)300 ps.UHMWPE chains are shown in green and PP chains in blue.(g) schematic representation of CVEF and the velocity gradient

Fig.2 Time-dependent evolutions of (a) cell length and (b) cell angle in relation to CVEF at 503 K

where

k

is the Boltzmann constant,

T

is the temperature,

ρ

is the bead density of the simulated system,

r

is the unit length and

f

(

r

) is the conservative force as a function of distance

r

.The first term is ascribed to kinetic contribution to the pressure and the second term corresponds to the potential contribution.The

g

(

r

) can be presented by the following equation:

where

a

is the parameter of conservative force.

3 Results and discussion

Firstly,the dynamic process of chain conformation under CVEF at a high temperature of 503 K(230℃),which is the same as the temperature applied in the ERE process,was analyzed as displayed in Fig.1.The snapshots were visually captured every 60 ps (6×10time steps).Fig.1(a) shows the initial relaxed configuration in

X-Z

plane and it provides an evidence of the existence of self-aggregation of the PP chains.With time evolution,the UHMWPE and PP chains were elongated and aligned parallel to the flow direction (

Z

direction).After 300 ps,the polymer chains were highly oriented and the self-aggregates disappeared.There is a marked preference for the polymer chains to have the same direction,implying that not only the macromolecular chain of UHWMPE tended to be arranged parallel to each other,but also the small molecular chain of PP,to which they are attached,preferred to pack closely together,leaving an ordering feature and somewhat uniform layer.Subsequently,the time-dependent evolution of the cell length and angle at a high temperature of 503 K are analyzed and displayed in Fig.2.It can be seen that with time evolution,lengths of

a

and

b

were barely changed in comparison with the dramatic increase of the length of

c

due to the extension along

z

direction.The cell angle fluctuated around 90° with the increase of simulation time.For better elucidation of the dependence of the chain conformation on temperature,we extended our simulations involving ERE conditions(503 K) to a wide range of temperature (set as 453,483,503,523 and 553 K).Table 1 lists the cell parameters of the UHMWPE/PP blends after 300 ps at the various temperatures.With increasing temperature from 453 to 553 K,the length of

c

decreased and lengths of

a

and

b

increased correspondingly.This implies somewhat degree of compactness.

Fig.3 Time evolutions of Rg over the simulation temperature range

To check the underlying cause of the change in chain conformation caused by the temperature variation,a series of evolving orientation order were characterized by the radius of gyration (

R

) evolution of UHMWPE and PP at the various temperatures,as illustrated in Fig.3.

R

,which describes the extension of a polymer chain in space,is an important parameter to characterize the chain conformation.where

r

and

r

.are the position vectors of each segment in a polymer chain and the center of mass for the whole chain.When temperature was 503 K,there was an overall increase in the value of

R

with time,indicating that the UHMWPE and PP chains became straighter and aligned with simulation time evolution,which is in accordance with the dynamic process of chain conformation change illustrated in Fig.1.In the cases of 453,483,523 and 553 K simulations,the changes in

R

occurred in a similar manner to that observed for the 503 K simulation.It is noteworthy that the 553 K condition produced a marked reduction of

R

in the final state of 300 ps,which suggests that a high temperature results in more substantial compactness,as evident from Table 1.To gain more insight into the thermally-induced changes that occurred with respect to temperature,the temperature-dependence of the radial distribution function

g

(

r

) was plotted in Fig.4(a).The larger the peak in a

g

(

r

) curve,the more ordered arrangement between molecules and the stronger interaction between the beads.The

g

(

r

) exhibited similar peaks at the same locations (

r

=11 Å) for all temperatures.The large value of

r

indicated that the polymer chains were loosely packed.The intensity of the peak in the

g

(

r

)curves was relatively larger at 453 K than the four higher temperature studied.The decrease of intensity in

g

(

r

) can be ascribed to the interaction between the UHMWPE and PP chains,which will be discussed in detail in the subsequent section.Additionally,the structure factor,

S

(

k

),is a good indicator to investigate the structural order of a solid or liquid and is depicted in Fig.4(b).The definition of the

S

(

k

) is given by the following expression in Eq.(1):

Fig.4 (a) the RDFs and (b) structure factor,S(k),with respect to CVEF at various temperatures

where

ρ

is the bulk density,

N

is the total number of atoms in the system,

dr

is the distance interval,

k

is the scattering wave-vector.For all temperatures,the structural shift decreased from sharp peaks occuring at

k

=0.06 Åto a plateau when

k

≥0.6 Ådue to chain ordering because the first peak in

S

(

k

) curves encodes the information of long-range ordering.The peaks in the

S

(

k

) curves generally shift to higher

S

(

k

)values with increased temperature,resulting from a small amount of chain overlap or entanglement in long-chain systems.Consequently,under CVEF,the low-

k

plateau and low-

k

peak suggest a low degree of conformational diversity and a high degree of chain stretching,highlighting the stability and reliability of the independent simulations for the given temperature range.

Fig.5 (a) the MSD plots as a function of temperature;(b) and (c)time evolution of potential energy at varying temperature

The diffusion coefficient can be calculated using the Einstein relation,as shown in Eq.(5):

where the quantity in brackets is the average mean square displacement (MSD) of the particles or beads in time

t

,and

r

is the vector coordinate of the center of mass of particle

i

.MSD is the distance of the beads moving from their original position to the second moment of their distribution in a defined time span.Fig.5(a) describes the MSD plots over the temperature range studied.The slopes of the MSD plots showed a decrease of diffusivity with increasing temperature.As one would expect,the difference in the diffusivity became less and less pronounced with increasing temperature.From an interaction perspective,two factors should be taken into consideration:one is the change in the non-bond interactions between alkyl chains,and the other to be considered is the flexibility of the alkyl chains.The non-bond energy measured by the van der Waals (VDW) interaction energy,in Fig.5(b),revealed some degree of fluctuation for every temperature.It clearly indicates that the overall VDW energy of the system at the varying temperatures exhibited a rate dependent feature,showing a slight increase from 453 to 553 K.The VDW energy of the process at 453 K drops down so fast before 100 ps and it gained 115 kcal/mol lower than that at 503 K.Fig.5(c) depicts the time evolution of bond energy(attributed to valence energy) at various temperatures.Increasing the temperature caused a decrease of bond energy,from 2.20×10at 453 K to 1.56×10kcal/mol at 553 K,in the final state of 300 ps.It is noted that the VDW interaction was relatively weaker than the bond energies.These phenomena tell us that the motion of the alkyl chains was seriously restricted owing to the stronger bond interaction potential than VDW interaction potential,even though the change in the VDW interactions would be responsible for the reduction of the difference in diffusion among the chains.Furthermore,it may be concluded that the bond interaction was the main driving force for the dynamic process of chain conformation when undergoing CVEF,which dominated the decrease of intensity in

g

(

r

).

4 Conclusions

In summary,the conformation transition of UHMWPE and PP chains in a 90/10 blend undergoing CVEF was evaluated in detail with DPD simulation,in which their dependence on temperature was included in the consideration,to our knowledge,for the first time.Under ERE conditions,the polymer chains evolved from self-aggregated configurations to highly oriented structure.Most notably,a higher temperature causes a somewhat greater compactness.The low-

k

plateau and low-

k

peak in structural factor,suggested a low degree of conformational diversity and a high degree of chain stretching.By analyzing the time evolution of the VDW energy and bond energy at varying temperatures,we concluded that the bond interaction was the main driving force for the collapse process when undergoing CVEF,in which the motion of alkyl chains was seriously restricted owing to the change in bond interaction potential,causing a reduction of the difference in diffusion rates among alkyl chains and a decrease in intensity of

g

(

r

).