CHEMICALRESEARCH, AD-A23 5 434DEVELOPMENT El - !ENGINEERINGCENTER CRDEC-TR-254

PREDICTING POLYMER PROPERTIESBY COMPUTATIONAL METHODS

I. POLYVINYL CHLORIDE ANDITS HOMOLOGS

P.N. KrishnanR.E. Morris

COPPIN STATE COLLEGEBaltimore, MD 21216

G.R. FaminiA. Birenzvige

RESEARCH DIRECTORATE

., ,, January 1991

Sr l U.S. MYARMAE NfTVL ,,. ........ --- MUNITIONS

CHEMICAL COMMAND

Aberdeen Proving Ground, Maryland 21010-5423

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I. AGECt USE ONLY (VLem 2bl . REPORT DATE 3. REPORT TYPE AND DATES CERED1991 January Final, 90 May - 90 Jun

4 mu- D SU TM L FUNDING NUMIERS

Predicting Polymer Properties by Computational MethodsI. Polyvinyl Chloride and Its Homologs

PR-1C162622A553L6. AUTHOR(S)

Krishnan, P.N., and Morris, R.E. (Coppin State College);Famini, G.R., and Birenzvige, A. (CRDEC)

7. PERFORMING ORGANIZATION NAME(S) AND AOORESS(ES) 6. PERFORMING ORGANIZATION

Coppin State College, Baltimore, MD 21216 REPORTNUMBER

CDR, CRDEC, ATTN: SMCCR-RSC, APG, MD 21010-5423 CRDEC-TR-254

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13. ABSTRACT (Maximum 200 wordt)

A semiempirical quantum mechanical approach [semiempirical minimum neglectdifferential overlap (MNDO) Hamiltonian] has been applied to predict the heat offormation, dipole moment, polarizability, and solubility of polyvinyl chloride(PVC) and several of its homologs. Some of the physical properties of longchain polymers can be estimated from the calculations, using the dimer as a model.To the best of our knowledge, this study is the first attempt to predict polymerproperties from relatively small molecule properties.

1. SUIUICT TERMS S. NUMBER OF PAGES

MNDO Polyvinyl chloride Computational chemistry 31Heat of formation Dipole moment Polarizability 16. PRCE cooESolubility

17. SECURITY CLASSIFICATION 16. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. UMITATION OF ABSTRACTOf REPORT OIF THIS PAGE Of ABSTRACT

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PREFACE

The work described in this report was authorized under ProjectNo. 1C162622A553L, CB Defense Assessment Technology. This work was startedin May 1990 and completed in June 1990.

The use of trade names or manufacturers' names in this report doesnot constitute an official endorsement of any commercial products. This reportmay not be cited for purposes of advertisement.

Reproduction of this document in whole or in part is prohibitedexcept with permission of the Commander, U.S. Army Chemical Research,Development and Engineering Center, ATTN: SMCCR-SPS-T, Aberdeen ProvingGround, MD 21010-5423. However, the Defense Technical Information Centerand the National Technical Information Service are authorized to reproduce thedocument for U.S. Government purposes.

This report has been approved for release to the public.

Ac knowl edgments

The authors are grateful to Edward Sommerfeldt and Emmanuel Owusu-Sekyere for providing continuous assistance relating to the use of thecomputer at Coppin State College, Baltimore, MD. It is indeed a pleasure toacknowledge the great debt owed to Gilbert 0. Ogonji for his continuous support,understanding, and encouragement throughout this project. Special thanks aregiven to Dennis J. Reutter for providing advice and offering valuablesuggestions.

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CONTEiTS

PAGE

1. INTRODUCTION ........................................... . . ...... 7

2. METHODS OF COMPUTATION .......................................... 7

3. RESULTS AND DISCUSSIONS ............. ...................... 8

3.1 Molecules Investigated ... 8...................... ..... 83.2 Geometry Optimization ............... ................. ...... 103.3 Heat of Formation ............................. o................ 10

4. RCOD IONS ............................... o .......... 15

LITERATURE CITED .................................................... 17

APPENDIXES

A. OPTIMIZED GEOMETRIES OF VARIOUS MOLECULES ................... 19

B. MOLECULAR ELECTROSTATIC POTENTIAL ANDMOLECULAR LIPOPHILIC POTENTIAL MAPS ....................... 27

5

LIST OF FIGURES AND TABLES

Figure

Change in Hf/Monomer Units ......................... 12

Tables

1 Molecules Investigated .. ..... *....... ............ *........ 9

2 Molecular Heat of Formation .............................. 11

3 Dipole Moment ....* ........................... ..*** 13

4 Polarization Volume, Molecular Volume, and PolarizabilityIndex ................................................... ***** 14

5 Solubility .......................................... ..... 16

6

PREDICTING POLYMER PROPERTIESBY COMPUTATIONAL METHODS

I. POLYVINYL CHLORIDE AND ITS HOMOLOGS

1. INTRODUCTION

Microencapsulation is a process of encasing (encapsulating) a chemicalby a high molecular weight polymeric material. The encapsulated chemical iscalled "fill material," whereas the encapsulating polymer is called "shell. "1Microencapsulation is used in many diverse industries [e.g., paper (carbonlesscarbon paper), pharmaceutical (time released medication and targeted drugapplication), agricultural (time released herbicides and pesticides), andperfume (scratch and sniff)]. The properties and applications of themicrocapsule greatly depend on the characteristics of its shell. Thepossibilities of using microencapsulation on the chemical/biological battlefieldhas been raised.

Presently, development of new applications for microencapsulationdepends either on personal experience or a "hit and miss" approach to determinethe nature of the shell material. This report describes the initial results ofa study attempting to develop a systematic methodology using computationalchemistry for screening polymeric shell materials for their desired properties.

Recent development of affordable high speed computer hardware and"user friendly" software makes it possible to apply computational methods topredict physical and chemical properties that are timely and expensive toobtain experimentally in the laboratory. Also, the increased environmentalawareness and the proliferation of regulations for environmental protectionmake the computational approach much more attractive. The application of onecomputational method [Modified Neglect Differential Overlap (MNDO)] to predictcertain physical properties of polyvinyl chloride (PVC) and certain vinylchloride-ethylene copolymers is described in this report.

The computational methods used in this work are described in Section 2.The results and the discussion of the results are given in Section 3. Thereport closes with a summary of the "experimental" work and recommendations forfuture work.

2. METHODS OF COMPUTATION

Because of the limited computer resources existing, it is impracticalor even impossible to calculate the properties of the polymer. Thus, theproperties of shorter chains (monomer, dimer .... pentamer) were calculated inan attempt to determine how the properties of the chain change with anincreasing number of monomer units and how many monomer units are required toestimate the polymer properties.

The molecules examined are large, making abinitio calculationsprohibitive. Therefore, semiempirical techniques employing MNDO Hamiltonian 2

have been used to calculate the physical properties. The molecular geometry ofeach molecule was optimized (see Appendix A) to give a minimum energyconformation. Next, the MOPAC program3 was used to calculate the heat of

7

formation, dipole moment, and polarization volume. The molecular volumes werecalculated from the optimized geometries, using the Hopfinger algorithm4 thatis incorporated in the Molecular Modeling Analysis Display System (MMADS).5,

6,*

The molecular electrostatic potential (MEP) and the molecular lipophilicpotential (MLP) for each molecule were calculated on a VdW Surface of themolecule.2 The MEP was computed using the appropriate columbic potential(V(r)) in equation 1

v3.

where qi is the formal charge of atom i and ri is the distance from atom i tothe point being examined. The MLP was determined using an analogous procedure.However, instead of qi, the Hansch-Leo octanol/water partition coefficientvalues (li) for specific fragments were used

L P 0 (2)1.

where Lp(r) is the lipophilic potential, and li is the octanol/watercoefficient for atom i (from Hansch-Leo fragment table), and ri is the same asin equation 1.7 Both MEP and MLP were examined on a Tektronic 4105 Color-graphic terminal. The calculations were performed on either a micro Vax II atU.S. Army Chemical Research, Development and Engineering Center (CRDEC) or onthe Coppin State College (Baltimore, MD) Vax 780, both running under the VMSoperating system.

3. RESULTS AND DISCUSSIONS

3.1 Molecules Investigated.

As stated before, the purpose of the current investigation is todetermine if semiempirical methods could be used to estimate the differentchemical and physical properties of polymeric materials used as shell materialin microencapsulation. Because polymers are large molecules containing hundredsof atoms, it is desirable to predict these quantities from calculationsperformed on smaller molecules. Hence, calculations were performed on differentoligomers (e.g., monomer, dimer, trimer, etc.) of vinyl chloride. In addition,compounds representing different degrees of chlorine substitution in thepolymeric chain were chosen to examine how these substitutions affect thedifferent chemical and physical properties. Table 1 lists the moleculesinvestigated in the present study.

* Molecular volume is that taken up by a molecule in space by adding the van de

Waal's contribution for each atom and subtracting out the overlap volume.

8

Table 1. Molecules Investigated

No. Molecule Name Molecular Structure

1. Vinyl Chloride (Monomer) CH2==CHC1

2. Ethyl Chloride CH3CH2Cl

3. 1.3-Chloro Butane (Dimer) CH2ClCH2CHClCH3

4. 1,3,5-Chloro Hexane (Trimer) CH2C1CH2CHClCH2CHC1CH3

5. 1,3,5,7-Chloro Octane (Tetramer) CH2ClCH 2CHCICH 2CHClCH2CHCICH3

6. 1,3,5,7,9-Chloro Decane (Pentamer) CH2ClCH2CHClCH2CHCICH2CHClCH2CHClCH3

7. 1-Chloro Butane CH2ClCH2CH2CH3

8. 2-Chloro Butane CH3CH2CHClCH3

9. 1-Chloro Hexane CH2ClCH2CH2CH2CH2CH3

10. 2-Chloro Hexane CH3CHCICH2CH2CH2CH3

11. 3-Chloro Hexane CH3CH2CHClCH2CH2CH3

12. 1,3-Chloro Hexane CH2ClCH2CHClCH 2CH2CH3

13. 1,5-Chloro Hexane CH2ClCH2CH2CH2CHClCH3

14. 1,7-Chloro Octane CH-2CCH2CH2CH2CH2CH2CHCICH3

15. 1,5,7 Chloro Octane CH2CICH2CH2CH2CHClCH2CHCICH3

16. 1,7-Chloro Decane CH2CICH2CH2CH2CH2CH2CHCICH2CH2CH3

17. 1 ,9-Chloro Decane CH2CICH2CH2CH2CH2CH2CH2CH2CHCICH3

18. 1,5,9-Chloro Decane CH2ClCH2CH2CH2CHCICH2CH2CH2CHClCH3

9

3.2 Geometry Optimization.

The properties of a compound depend strongly on its geometry. Thus,the first task was to determine the geometric structure of the molecule, whichwas determined by the method of energy minimization utilizing approximatemethods (CRUDE). The calculated geometry was inspected visually to insurelinearity of the hydrocarbon chain. If the chain was not linear, the dihedralangles between the four carbon atoms were adjusted to 1800. Then, MOPAC wasused to obtain the final optimized geometry. These corrected geometries wereused to calculate the various properties described below. The optimizedgeometries are presented in Appendix A.

3.3 Heat of Formation.

The calculated and experimental values of heat of formation for themolecules studied are compared in Table 2. As can be seen, the agreementbetween calculated and experimental values is quite good. For oligomers ofvinyl chloride, Sinke and Stull predicted that the heat of formation willdecrease by 22.6 Kcal/mol/monomer unit added even though they did not presentany data to support this claim. However, our calculations show that theadditional heat of formation for each additional monomer unit is about halfthat value (when ethyl chloride is taken as a monomer). Furthermore, itappears that this incremental increase is leveling off to around 13.5 Kcal/molefor each monomer addition after two units (see the figure).

3.4 Dipole Moment.

The distribution of charge in a molecular system has greatsignificance for its reactivity and is a function of its structure. Thedipole moment is the principal experimental quantity related to the chargedistribution in the molecule. The calculated and experimental values of dipolemoment for the molecules studied are compared in Table 3. Only a fewexperimental values were found in the literature. As can be seen, these valuesagree to within 0.5 Debye with the calculated values. Because the calculateddipole moment is a function of the postulated geometry of the molecule, thisagreement supports the validity of the optimized geometries.

3.5 Polarizability.

Polarizability is an expression of the ability of an externalelectric field to induce a dipole moment (i.e. a charge separation in themolecule). As with dipole moment, very few experimental values are reportedin the literature. For those molecules for which experimental values could befound, the agreement with the calculated values is reasonably good, as canbe seen in Table 4.

Because it is an expression of the induced dipole moment,polarizability can be expected to depend on the size of the molecule. However,for oligomer composed of increasing numbers of the same or similiar buildingblocks, we can expect that the polarizability per unit volume will be similiar.The last column in Table 4, the polarizability index, is an expression of"unit polarizability" and is calculated by dividing the calculated polarization

10

Table 2. Molecular Heat of Formation

Calculated ExperimentalAHf AHf

No. Molecule Name (Kcal/mol) (Kc I/mol)

1. Vinyl Chloride (Monomer) 4.94 8.07 + 0.3310-12

2. Ethyl Chloride -28.8 -26.1 + 0.5110-12

3. 1,3-Chioro Butane (Dimer) -44.02

4. 1,3,5-Chloro Hexane (Trimer) -58.15

5. 1,3,5,7-Chloro Octane (Tetranier) -71.92

6. 1,3,5,7,9-Chloro Decane (Pentamer) -85.60

7. 1-Chloro Butane -38.49 -36.94 + 0.28110-12

8. 2-Chloro Butane -38.90 -38.53 + 2.010-12

9. 1-Chioro Hexane -47.44 -48.59 + 0.3610-12

10. 2-Chioro I-exane -45.17 -48.80 + 1.710-12

11. 3-Chioro Hexane -44.18

12. 1,3-Chioro Hexane -52.42

13. 1,5-Chloro Hexane -53.83

14. 1,7-Chior-o Octane -63.35

15. 1,5,7-Chloro Octane -68.04

16. 1,7-Chioro Decane -71.55

17. 1,9-Chloro Decane -72.78

18. 1,5,9-Chioro Decane -77.56

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Table 3. Dipole Moment

Calculated ExperimentalDipole Moment Dipole Moment

No. Molecule Name (Debye) (Debye)

1. Vinyl Chloride (Monomer) 1.70 1.459

2. Ethyl Chloride 1.55 2.0510

3. 1,3-Chloro Butane (Dimer) 2.11

4. 1,3,5-Chloro H-exane (Trimer) 2.04

5. 1,3,5,7-Chioro Octane (Tetramer) 2.86

6. 1,3,5,7,9-Chloro Decane (Pentamer) 3.39

7. 1-Chloro Butane 2.16 2.0510

8. 2-Chloro Butane 2.12 2.0410

9. 1-Chloro Hexane 2.05

10. 2-Chloro Hexane 2.09

11. 3-Chioro Hexane 2.05

12. 1,3-Chloro Hexane 2.23

13. 1,5-Chloro Hexane 2.18

14. 1,7-Chloro Octane 2.28

15. 1,5,7-Chloro Octane 2.06

16. 1,7-Chloro Decane 2.49

17. 1,9-Chloro Decane 2.33

18. 1,5,9-Chloro Decane 2.08

13

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volume (Column 3) by the calculated molecular volume (Column 5). As can beseen, the polarizability indices of all the molecules studied, except ethylchloride, fall within 3% of the average value of 0.016.

3.6 Solubility.

The distribution of electrons around the molecule determine thereactivity of the molecule. In general, a homogenous distribution indicateslow reactivity, but a concentration of charge at certain regions will indicatehigh reactivity. Similarly, a homogenous charge distribution will indicate lowsolubility in a polar solvent such as water.

By plotting the electrostatic and lipophilic potential around themo.,.cule, we can qualitativ yssess the chemical reactivity and polarsolubility of the compound.V -18 The MEP and MLP maps for the moleculesinvestigated indicate that they all have very low solubility in water, whichis confirmed in Table 5. As expected, the solubility in water diminisheswith an increasing number of carbon atoms in the chain. Similarly, the MEPmaps indicate that the molecules investigated have very low reactivity, whichis also expected.

4. CONCLUSIONS

The results of this semiempirical study indicate that oligomers assmall as the dimer can be used as a model to reasonably predict at leastsome of the physical properties of PVC. It appears that vinyl chloride cannot be used as a model for PVC due to the large deviation of the heat offormation. Ethyl chloride is eliminated as a potential model because of thediscrepancy in the dipole moment and polarizability index. The MEP and MLPmaps confirm the similarity between the dimer and the larger chain PVCmolecules. These maps also confirm the dissimilarities between the vinylchloride, ethyl chloride, and longer chain PVC molecules. This is the firstknown attempt to predict the behavior of polymers based on calculations ofthe properties of shorter chain molecules. The vinyl chloride-ethylenecopolymers investigated here show similar properties to the PVC.

5. RECOMMENDATIONS

This study shows that computational chemistry can be used toestimate physical and chemical properties of polymeric compounds. The semi-empirical computer model used in this effort is the MNDO. However, othercomputer models, which are available, should be investigated. Determiningwhich model is best suited to predict each property is the subject of an ongoinginvestigation and will be reported upon at a later date. We recommend that amethodology to estimate the solubility of polymeric materials in differentmedia be developed with similar calculations performed on other polymericmaterials. To investigate the predictability of spectral excitation frequenciesfor polymeric materials, these calculations should be extended to the abinitiolevel.

15

Table 5. Solubility

No. Molecule Name Experimental Solubil1ty WIW)

1. Vinyl Chloride (Monomer) 0.2711

2. Ethyl Chloride 0.44711

3. 1,3-Chloro Butane (Dimer)

4. 1,3,5-Chloro Hexane (Trimer)

5. 1,3,5,7-Chloro Octane (Tetramer)

6. 1,3,5,7,9-Chloro Decane (Pentamer)

7. 1-Chloro Butane 0.1111

8. 2-Chloro Butane 0.1011

9. 1-Chloro Hexane 0.00813

10. 2-Chloro Hexane

11. 3-Chloro Hexane

12. 1,3-Chloro Hexane

13. 1,5-Chloro Hexane

14. 1,7-Chloro Octane

15. 1,5,7-Chloro Octane

16. 1,7-Chloro Decane

17. 1,9-Chloro Decane

18. 1,5,9-Chloro Decane

16

LITERATURE CITED

1. Gutcho, M.H., Micro Capsules and Other Capsules-Advances Since1975, Noyes Data Corporation, Park Ridge, NJ, 1979.

2. Dewar, M.J.S., and Thiel, W., "Ground States of Molecules, TheMNDO Method," J. Am. Chem. Soc. Vol. 99, p 4899 (1977).

3. Stewart, J.J.P., MOPAC: A General Molecular Orbital Package,FJSRL-TR-88-0007, Frank J. Seiler Research Laboratory, U.S. Air Force Academy,Colorado Springs, CO, December 1988.

4. Hopfinger, A.J., "A Quantitative Structure Activity RelationshipInvestigation of Dehydrolfate Reductase Using Molecular Shape Analysis,"J. Am. Chem. Soc. Vol. 102, p 7126 (1980).

5. Leonard, J.M., A User's Guide to the Molecular Modeling Analysisand Display System, CRDEC-TR-030, U.S. Army Chemical Research, Developmentand Engineering Center, Aberdeen Proving Ground, MD, January 1989, UNCLASSIFIEDReport.

6. Famini, G.R., Using Theoretical Descriptors in StructuralActivity Relationships, I. Molecular Volume, CRDEC-TR-88031, U.S. ArmyChemical Research, Development and Engineering Center, Aberdeen Proving Ground,MD, January 1988, UNCLASSIFIED Report.

7. Hornsch, C., and Leo, A., Substituent Constants for CorrelationAnalysis in Chemistry and Biology, Wiley-Interscience, New York, NY, 1979.

8. Sinke, G.C., and Stull, D.R., "Heats of Combustion of SomeOrganic Compounds Containing Chlorine," J.P. Chem. Vol. 62, p 397 (1958).

9. Hirgill, J.A., Coop, I.E., and Sutton, L.E., "Electron Diffractionand Electric Dipole Moment Investigations of Some Derivatives of Ethylene,"Trans. Farraday Soc. Vol. 34, p 1518 (1938).

10. CRC Handb ok of Chemistry and Physics, 68th ed., CRC Press, Inc.,Boca Raton, FL, 1987-88.

11. Riddick, J.A., Bunger, W.B., and Sakamo, T.K., Orqanic Solvents:Physical Properties and Methods of Purification, II. Techniques of hemistry,4th ed., Wiley Publications, New York, NY, 1986.

12. Pedley, J.B., Naylor, R.D., and Kirby, S.P., Thermo ChemicalData of Organic Compounds, Chapman and Hall, Ltd., New York, NY, 1986.

13. Wright and Shaffer, "Anthelmintic Tests of Hydrocarbons,"Arm. J. Hvg. Vol. 16, p 330 (1932).

14. Bonaccorsi, R., Pullman, A., Scrocco, E., and Tomasi, J.,Chem. Phys. Lett. Vol. 12, p 662 (1972).

17

15. Kollman, P., McKelvey, J., Johansson, A., and Rothenberg, S.,"Theoretical Studies of Hydrogen Bonded Dimers," J. Am. Chem. Soc. Vol. 99,p 955 (1975).

16. Kollman P., and Rothenberg, S., "Theoretical Studies of Basicity,Proton Affinities, Li Affinities and H-Bond Affinities of Some Simple Bases,"J. Am. Chem. Soc. Vol. 99, p 1333 (1977).

17. Scrocco, E., and Tomasi, J., "Electronic Molecular Structure,Reactivity and Intermolecular Forces," Adv. Quantum Chem. Vol. 11, p 115 (1978).

18. Tomasi, J., "Use of Electrostatic Potential as a Guide toUnderstanding Molecular Properties," Chemical Applications of Atomic andMolecular Electrostatic Potentials, Polltzer, P. and Truhlar, D.G., Eds.,Plenum, New York, NY, 1981.

19. Luque, F.J., Illas, F., and Orozco, M., "Comparative Study ofthe Molecular Electrostatic Potential Obtained from Different Wavefunctions;Reliability of Semi-Empirical MNDO Wave Function," J. Comp. Chem. Vol. 11,p 416 (1990).

18

APPENDIX A

OPTIMIZED GEOMETRIES OF VARIOUS MOLECULES

CL I

VINYL CHLCIRUDE (MOEtcFR)

19

CLS

1, 3 - CHLcwM DUANE (Dowe1r)

1, 3P 5 - OILM HEXANS (Trimr)

Appendix A 20

CL 12 CLIO

1, 3, 5o, 7 - CHUMR OCTANE (Tetramer)

1, 3# So 7P 9 - CHLMDECANE (Pentamer)

1- CHOILUANE

Appendix A 21

CL5

1 - aICAOIEXANE

Appendix A 22

2 - CHLaC"LEXANE

CL?

3 - CHOILHEXANE

CLO

1, 3 - cHLconmxANEAppendix A 23

CL

1, 5 - cHvoHEXANE

1is 7 -e C4 wC2xA

1So5 7 - CHUM OCTANEAppendix A 24

1, 7 -CHLoIM DWE~CN

1. 9 - CHLCM DICAN

1, 5, 9 - afLCM DWCANAppendix A 25

Bl ank

26

APPENDIX B

MOLECULAR ELECTROSTATIC POTENTIAL ANDMOLECULAR LIPOPHILIC POTENTIAL MAPS

The potential at a point on the molecular surface is represented bycolor coding; red is used to represent the most negative potential and blue,the most positive. The other colors of the spectrum represent values betweenthe two extremes. Therefore, through visual inspection of the molecularelectrostatic potential maps, one can equate significant charge separationswith large polarization (reactivity).

Similarly, the molecular lipophilic potential maps represent insolubleportions of the molecule with violet and soluble portions with red. Again,visual inspection is used to determine the solubility of the molecule.

27

REP: 1, 3-Chloro Butane (Dimer)

Appendix B 28

1EP 13,5,79-Chioro Decane (Pentazer)

Appendix B 29

MLP: 1, 3-Chloro Butane (Diner)

Appendix B 30

MWP: 1, 3,,.7,9-Chloro Decane (Pentamer)

Appendix B .31