Polymer Degradation and Stability 85 (2004) 555e558

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Ultrasonic degradation of polybutadieneand isotactic polypropylene

Jayanta Chakraborty, Jayashri Sarkar, Ravi Kumar, Giridhar Madras)

Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

Received 10 July 2003; received in revised form 8 September 2003; accepted 15 September 2003

Abstract

The ultrasonic degradation of polybutadiene and isotactic polypropylene in solution was studied at different temperatures and indifferent solvents. The time evolution of molecular weight distribution was determined experimentally through gel permeationchromatography. Degradation rate coefficients were determined from amodel based on continuous distribution kinetics and assuming

mid-point chain scission. The variation of rate coefficients with vapour pressure and kinematic viscosity was also investigated.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Ultrasound; Degradation; Polybutadiene; Isotactic polypropylene; Solvent effect; Continuous distribution kinetics

1. Introduction

Polymers can be degraded thermally by pyrolysis [1]or in solution [2] but the processes are energy intensive.The need for alternative techniques that would reduceenergy requirements for the degradation process isimportant. The use of ultrasound, photo and chemicalmethods is less energy-intensive polymer degradation.Further, the mechanism by which they interact with thepolymeric systems can help in gaining insight into thedegradation pathways or mechanisms.

Polymers undergo degradation when they are sub-jected to ultrasound irradiation of high intensity. Severalworkers have investigated the ultrasound degradation ofpolymers, which has been summarized by Price [3]. Thebreaking of the chemical bonds is due to the cavitation inthe medium. Cavitation is the formation and violentcollapse of small bubbles or voids in the liquid as a resultof pressure changes in the medium. This leads to shearingforces of sufficient magnitude that they can cause therupture of bonds. The sound waves do not directlyinteract with the polymer but they act on the solventcausing the growth and rapid collapse of micro-bubbles

) Corresponding author. Tel.: C91-80-309-2321; fax: C91-80-360-

0683.

E-mail address: giridhar@chemeng.iisc.ernet.in (G. Madras).

0141-3910/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2003.09.021

resulting in high shear gradient. It has been shown thatthis shear force is roughly similar to the force required tobreak chemical bonds in polymers [4,5]. Another uniquefeature of ultrasonic degradation is the fact that, in con-trast to all chemical and thermal decomposition reac-tions, the ultrasound depolymerization is a non-randomprocess which produces fragmentation at the mid-pointof the chain [6]. The existence of limiting molecularweight, below which degradation by ultrasound does nottake place, has the additional effect in that the molecularweight distribution initially broadens and before nar-rowing during degradation.

The effect of various parameters like temperature,ultrasound intensity, dissolved gases [7] and polymerconcentration [8] on the ultrasonic degradation of poly-styrene has been investigated. A primary factor in theultrasonic degradation of polymers is the effect of sol-vent in the degradation rate. While some workers [9,10]report no effect of solvent on the degradation of cel-lulose nitrate and polyisobutylene, several investigatorshave found an effect of solvent on the ultrasonicdegradation of dextran [11], poly(ethylene glycol) [12],hydroxypropyl cellulose [13], poly(alkyl methacrylate)s[14], poly(vinyl acetate) [15] and polystyrene [8]. Whilethe ultrasonic degradation is influenced by polymeresolvent interaction parameters like FloryeHuggins andHuggins constant [8], the major factors that influence

556 J. Chakraborty et al. / Polymer Degradation and Stability 85 (2004) 555e558

the degradation are the kinematic viscosity and vapourpressure of the solvent [9].

Though polybutadiene and polypropylene are impor-tant commercial polymers, there are no studies on theultrasonic degradation of these polymers. We are un-aware of any study on the ultrasonic degradation ofisotactic polypropylene and polybutadiene over an ex-tensive temperature range or the effect of solvents ondegradation of these polymers. The objective of thisstudy is to present new experimental data for the ultra-sonic degradation of these polymers and determine thedegradation rate coefficients using a model developedearlier [16]. The variation of the rate coefficients withtemperature is attributed to the change in the kinematicviscosity and vapour pressure.

2. Experiments

2.1. Degradation reaction

The characteristics and properties of the polybutadi-ene and polypropylene used in the reaction are providedelsewhere [17,18]. The reactionwas carried out in a 100-mlbeaker. Approximately 80 ml of polymer solution(2 g/l) was taken each time and the beaker was held witha clamp-stand assembly in a constant temperature waterbath (G 1 (C). Ultrasound was coupled directly to thereaction system by a horn type sonicator (Vibronics,India) with voltage and frequency of 180 V and 25 kHz,respectively. The beaker was wrapped with aluminiumfoil to the sonic horn to minimize solvent evaporation.The experiments were conducted by covering the beakerto ensure that no air entered the system during degra-dation. No crosslinking was observed because the GelPermeation Chromatography (GPC) chromatograph didnot show any molecular weight above the initialmolecular weight. Calculated amounts of solvents wereadded to maintain the concentration of polymer solu-tion constant inside the reaction system. Samples weredrawn at regular intervals through a syringe andanalysed by GPC.

The ultrasonic degradation of polybutadiene was in-vestigated at 32, 50, 60 and 70 (C with o-dichloroben-zene as solvent and in chlorobenzene, toluene, p-xylene,chloroform, tetrahydrofuran and benzene at 32 (C.The ultrasonic degradation of isotactic polypropylenewas investigated at 80, 90, 113, 133 and 155 (C witho-dichlorobenzene as solvent. Because the initial molec-ular weight of polypropylene was only 125,000, it wascompletely dissolved even at 70 (C. This was confirmedby dissolving the polymer at 70 (C and 155 (C. Boththe solutions showed the exact same GPC chromato-gram confirming that polypropylene remained soluble inall experimental conditions.

The model to determine the degradation rate co-efficient requires the limiting molecular weight for thedegradation of the polymer. Several experiments wereconducted for 10 h and no detectable change in themolecular weight was noticed after 6 h. Further, theexperimental data indicated that the ratio of the limitingmolecular weight to the molecular weight after 120 minof degradation was approximately 0.8 in all cases. Thisratio determines the limiting molecular weight of thepolymer for the experiments conducted for only 3 h. Thevariation of the limiting molecular weights for the poly-mers with reaction temperature was measured and theplot was linear (Fig. 1). This observation was consistentwith the observations reported in the literature [7] forthe variation of the limiting molecular weight with tem-perature for the ultrasonic degradation of polystyrene intoluene.

2.2. Polymer analysis

The molecular weight distributions of the polymersamples were measured by Gel Permeation Chromato-graphy (GPC). The HPLCeGPC system consists ofan isocratic pump (Waters, USA), mixed B columns(Polymer Labs, UK) and evaporative light scatteringdetector (Polymer Labs, UK). The columns were main-tained at 150 (C using a column heater (Eldex, USA)and the system was calibrated with polystyrene stand-ards. The flow rate of the eluent, o-dichlorobenzene, wasmaintained at 0.7 ml/min and the flow rate of nitrogento the light scattering detector was maintained at 0.8 ml/min for enhanced sensitivity. The molecular weights aredetermined by using appropriate K and a values forpolypropylene or polybutadiene and polystyrene.

20 40 60 80 100 120 140 1602.5

3.0

3.5

4.0

4.5

5.0

Lim

iting

MW

, xl

Temperature (oC)

Fig. 1. Variation of the limiting molecular weight with temperature in

o-dichlorobenzene for the ultrasonic degradation of isotactic poly-

propylene (,) and polybutadiene (�).

557J. Chakraborty et al. / Polymer Degradation and Stability 85 (2004) 555e558

3. Theoretical model

Because the polymer breaks at the mid-point

PðxÞ�!kðxÞ 2Pðx=2Þ ðAÞ

PðxÞ represents the polymer species and pðx; tÞ is themolecular weight distribution of the species. No repoly-merization of the degraded species was observed in theexperiments. This was confirmed by the GPC chromato-gram that showed no molecular weight products higherthan the initial distribution. The population balanceequation for the above reaction (A) is [16]

vpðx; tÞvt

¼ �kðxÞpðx; tÞC2

ZNx

kðxÞpðx#; tÞd x� x#

2

� �dx#

ð1Þ

The degradation is assumed to be first order with thepolymer concentration p(x,t) and the degradation rate,k(x), is assumed to be of the form kðxÞ ¼ kðx� xlÞ,where the xl represents the limiting molecular weight[16]. This ensures that the rate coefficient, k, is inde-pendent of x and becomes zero when x ¼ xl and nofurther degradation takes place. Applying the moment

operationRN0 xðnÞ½ � dx, to the above equation yields

dpðnÞ

dt¼ kpðnC1Þð21�n � 1Þ � kpðnÞxlð21�n � 1Þ ð2Þ

p(0) and p(1) represent the molar and mass concentra-tions of the polymer, respectively and can be obtainedby putting n ¼ 0 and 1 in Eq. (2). For n ¼ 0

dpð0Þ

dt¼ kpð1Þ � kpð0Þxl ð3Þ

Solving Eq. (3) with the initial condition, pð0Þðt ¼ 0Þ ¼pð0Þ0 , gives

ð pð1Þ � pð0Þ0 xlÞ

ð pð1Þ � pð0ÞxlÞ

" #¼ ekxlt ð4Þ

The number average MW, Mn is defined as p(1)/p(0), andthe above equation reduces to

ln H ¼ kxlt; H ¼ ðxl �M�1n0 Þ

ðxl �M�1n Þ

� �ð5Þ

4. Results and discussion

Fig. 2a and b shows the variation of H with time forthe ultrasonic degradation of isotactic polypropyleneand polybutadiene, respectively. The semi-log plots arenearly linear, confirming the validity of Eq. (5). The

degradation rate coefficients were determined from theregressed slopes. The rate coefficients, k (!108 molg�1 s�1), for isotactic propylene decreased from 0.94 to0.28 as the temperature increased from 80 to 155 (C. Therate coefficients, k (!108 mol g�1 s�1), for polybutadi-ene decreased from 0.76 to 0.25 as the temperatureincreased from 32 to 70 (C. As the rate coefficientsdecrease with an increase in temperature, an Arrheniusplot would yield negative activation energies and wouldnot have any physical meaning. The decrease of de-gradation rate with an increase of temperature is similarto that observed for mechanical breakage of polymers.As the temperature of the solution increases, a largequantity of the solvent vapour enters the cavitationbubbles during their expansion and exerts a cushioning

0 20 40 60 80 100 1200.0

0.5

1.0

1.5

2.0

Time(min)

a

0 20 40 60 80 100 120 140 160 180 2000.0

0.5

1.0

1.5

2.0

2.5

3.0

Ln(H

)

Time(min)

b

ln(H)

Fig. 2. Semi-log plot of the variation of H with sonication time in

o-dichlorobenzene for the ultrasonic degradation of: a. isotactic

polypropylene (legend: - 80 (C; � 90 (C; : 113 (C; ;; 133 (C;¤ 155 (C) b. polybutadiene (legend: - 32 (C; � 50 (C; : 60 (C;; 70 (C).

558 J. Chakraborty et al. / Polymer Degradation and Stability 85 (2004) 555e558

effect [3] during the collapse leading to diminishing of theintensity of the shock wave, reducing the jet velocity [19]leading to reduced degradation at higher temperatures.The same phenomena can be used to explain the decreaseof the degradation rate coefficients with an increase in thevapour pressure of the solvent (Fig. 3a). While kinematicviscosity of the solvent was assumed to play a role in theultrasonic degradation [20,21], a more detailed study [16]indicated that this could be a crucial parameter. Bettertransmission of the shock waves in solution of higherkinematic viscosity [16] is a probable reason that explainsthe increase of degradation rate coefficients with in-creasing kinematic viscosity (Fig. 3b).

The limiting molecular weight obtained for thedegradation of polybutadiene and polypropylene is

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.2

0.4

0.6

0.8

1.0

k x

108

(mol

g-1

s-1 )

Vapor pressure(bar)

k x1

08 (m

ol g

-1s-

1 )

0.000 0.002 0.004 0.006 0.008 0.0100.0

0.2

0.4

0.6

0.8

1.0

Kinematic viscosity (St)

a

b

Fig. 3. Variation of the ultrasonic degradation rate coefficients with: a.

vapour pressure of the solution; b. kinematic viscosity of the polymer

solution. Legend: , polypropylene in o-dichlorobenzene at different

temperatures, � polybutadiene in o-dichlorobenzene at different

temperatures, : polybutadiene in chlorobenzene at 32 (C, ;

PB(Benzene) at 32 (C, ¤ polybutadiene in THF at 45 (C, +polybutadiene in toluene at 45 (C, < polybutadiene in chloroform

at 45 (C, = polybutadiene in xylene at 45 (C.

similar to the limiting molecular weight obtained forthe ultrasonic degradation of polystyrene [7,8], poly(vinyl acetate) [15] and poly(methyl methacrylate) [22].However, the degradation rate coefficients for poly-butadiene and polypropylene are lower than thedegradation rate coefficients of polystyrene [8], poly(vinyl acetate) [15] and poly(methyl methacrylate) [22].The variation of the rate coefficients with increase intemperature, vapour pressure and kinematic viscosity isconsistent with the observations for other polymers inthe literature [3,7,15,22].

5. Conclusions

The ultrasonic degradation of two commerciallyimportant polymers, polybutadiene and isotactic poly-propylene, has been investigated. The evolution of themolecular weight was determined by gel permeationchromatography and a model based on continuous dis-tribution kinetics was used to determine degradationrate coefficients. The degradation rate decreased withincreasing temperatures, increasing vapour pressure anddecreasing kinematic viscosity of the solvents.

Acknowledgements

The authors thank the Department of Science andTechnology, India for financial assistance.

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