adhesion science at virginia tech: joining forces for the 21st century

1. Adhesion Science at Virginia Tech:Joining Forces for the 21st Century David A. Dillard Director, Center for Adhesive and Sealant Science Professor, Engineering Science & Mechanics Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 CIT Virginia Materials Showcase Herndon, Virginia 6 June 2002

3. CASS: An Interdisciplinary Center

4. CASS Thrust Areas CASS Thrust Areas Hot Melts PSAs Sealants Thermosets Waterbornes Bioadhesion Microelectronics Transportation & Infrastructure CASS Thrust Leaders Chip Frazier Tim Long John Dillard Tom Ward Rick Davis Brian Love Ravi Saraf Jack Lesko

5. Technical Scope of CASS

6. Hot Melts: Synthesis Faculty Members: Tim Long (Chemistry) 540-231-2480 Students Huiying Kang Sponsors Eastman, CASS Objectives: To introduce thermo-reversible bonds into high performance macromolecules resulting in exceptionally low melt viscosities, but final mechanical and thermal properties of infinitely high molecular weight polymers. Approach Use of telechelic non-covalent polymers Telechelic means oligomers with bifunctionality She is using ionic groups to give interaction Telechelic means oligomers with bifunctionality She is using ionic groups to give interaction

7. Multiple Hydrogen Bonding in Multiphase Systems

8. RNA and DNA: Exquisite Examples of Tailored Intermolecular Interactions

9. Pyrimidine and Purine Derivatives: DNA Heterocyclic Bases

10. Effect of End Groups on the Melt Viscosity as a Function of Temperature This is the linear plot of the rheology. The melt viscosity of MHB-PI was much higher than PI with the other end-group. The melt viscosity rapidly degreased and at 70 oC, the melt viscosity was neally close to non-functionalized PI. This data may suppose to the dissociation temperature is 70deg.This is the linear plot of the rheology. The melt viscosity of MHB-PI was much higher than PI with the other end-group. The melt viscosity rapidly degreased and at 70 oC, the melt viscosity was neally close to non-functionalized PI. This data may suppose to the dissociation temperature is 70deg.

12. Thermosets: Polyurethane Formation Faculty Members: Chip Frazier (WSFP) 540-231-8318 Students Sudipto Das Sponsors USDA Objectives: To characterize the cure chemistry of the pMDI in contact with wood Approach Use labeled phosgene and labeled polyamine to produce double labeled polymeric isocyanate Use solids NMR to detect urethane

13. Chemistry and Morphology of the Wood/pMDI Bondline

14. The Wood/pMDI Bondline

15. The Wood/pMDI Bondline The double label gives complimentary nitrogen and carbon spectra that allow us to detect the urethane formation with wood. The carbon spectrum above reveals the urethane.The double label gives complimentary nitrogen and carbon spectra that allow us to detect the urethane formation with wood. The carbon spectrum above reveals the urethane.

16. Waterborne: Stabilization Faculty Members: Rick Davis (Chem E) 540-231-4578 K. Van Cott (Chem E) W. Ducker (Chemistry) Students J.L. Streeter, D.B. Henderson Sponsors ASCEF, NSF Objective: Develop new block copolymer steric stabilizers that will provide better control of colloid stabilization for waterborne adhesives than existing technology. Approach Using biodegradable polyamino acids from E. coli sources as block co-polymer steric stabilizers Current methods for colloid stabilization of particles in waterborne adhesive formulations have several disadvantages Insufficient stabilization leads to poor viscosity control Poor ability to disperse inorganic particles in waterborne adhesive formulations Current methods for colloid stabilization of particles in waterborne adhesive formulations have several disadvantages Insufficient stabilization leads to poor viscosity control Poor ability to disperse inorganic particles in waterborne adhesive formulations

17. Structure-Rheology Relationships of Waterborne Adhesives Inorganic particle aggregation in water is more pronounced than aggregation of polymer colloids due to stronger attractive dispersion forces. Aggregation leads to the formation of a 3-D network that increases suspension viscosity, shear thinning, and can result in thixotropy.

18. Adhesive Application Processes - Shear Rate and Viscosity �Windows� Coating Process Shear Rate, sec-1 Viscosity, ?, cp Spray 103-104 400-1000 Brush 103-104 1000-40,000 Caulk 100-103 ? 105 (pipe flow) Volumetric Flow Rate, i.e. the Production coating rate ? ?P/?

19. Steric Stabilization Gives the Lowest Possible Viscosity at the Highest Particle Volume Fraction

20. Brush Forming Copolymers Make the Best Steric Stabilizers Anchoring requires strong binding of anchor block to substrate Tail block extension due to osmotic pressure requires solubility More efficient than homopolymer - lower concentration required Insensitive to plant or process related fluctuations in electrolyte concentration and pH It has been known for a number of years that brush-forming block copolymers in which the tail blocks are water-soluble and uncharged make the most efficient and effective particle stabilizers. The adsorbed layer thickness ? needs to be thick enough to overcome attractive van der waals forces between any two such coated surfaces, thereby keeping the particles dispersed, thus preventing them from aggregating. Our estimates of the thickness required to overcome the attractive van der waals forces comes from the well known DLVO theory. The brush layer thickness needed lies between 5-20 nm for particles most commonly used in waterborne adhesives.It has been known for a number of years that brush-forming block copolymers in which the tail blocks are water-soluble and uncharged make the most efficient and effective particle stabilizers. The adsorbed layer thickness ? needs to be thick enough to overcome attractive van der waals forces between any two such coated surfaces, thereby keeping the particles dispersed, thus preventing them from aggregating. Our estimates of the thickness required to overcome the attractive van der waals forces comes from the well known DLVO theory. The brush layer thickness needed lies between 5-20 nm for particles most commonly used in waterborne adhesives.

21. Adsorption Studies Show that Proline-Glutamic Acid Copolymers Should Be Efficient Steric Stabilizers We have shown that the nonionic but water-soluble PRO, when mixed with GLU in water, does not adsorb onto alumina particles whereas the negatively charged GLU adsorbs strongly with positively charged alumina particles at pH < 9. These results show that the PRO-GLU diblock copolymer should form the self-assembled brush layers (as shown in the figure) that will function as efficient steric stabilizers for alumina particles and other particles with related surface chemistries such as zinc oxide.We have shown that the nonionic but water-soluble PRO, when mixed with GLU in water, does not adsorb onto alumina particles whereas the negatively charged GLU adsorbs strongly with positively charged alumina particles at pH < 9. These results show that the PRO-GLU diblock copolymer should form the self-assembled brush layers (as shown in the figure) that will function as efficient steric stabilizers for alumina particles and other particles with related surface chemistries such as zinc oxide.

22. Biosynthesis of PRO-GLU Diblock and Advantages of Polyamino Acids We are working with Prof. Kevin Van Cott in the Chemical Engineering Department at VA Tech on making the PRO-GLU diblock copolymers using genetically engineered E.Coli bacteria. These polymers are made using well-known fermentation technology. After fermentation, the PRO-GLU diblock copolymer is recovered and purified using well-established techniques. With these engineered bacteria, we can control the structure of the copolymers with high accuracy and have the capability of making a variety of different copolymers that can be tailored to adsorb onto any particle surface.We are working with Prof. Kevin Van Cott in the Chemical Engineering Department at VA Tech on making the PRO-GLU diblock copolymers using genetically engineered E.Coli bacteria. These polymers are made using well-known fermentation technology. After fermentation, the PRO-GLU diblock copolymer is recovered and purified using well-established techniques. With these engineered bacteria, we can control the structure of the copolymers with high accuracy and have the capability of making a variety of different copolymers that can be tailored to adsorb onto any particle surface.

23. Expectations of First Generation PRO-GLU Diblock Act similar to surfactants on latex Short tail block No foaming GLU block is soluble We expect that the PRO-GLU diblock currently under development: will be very water-soluble will not foam when solutions are shaken (meaning that the copolymers will not form micelles) should form adsorbed layers on alumina particles that are about 5 nm or more in thickness. This should be thick enough to at least partially stabilize alumina particles in water. One of our collaborators, Prof. Ducker in the Chemistry Department at VA Tech, has developed an atomic force microscope technique to measure the repulsive steric forces created the adsorption of the PRO-GLU diblock onto alumina.We expect that the PRO-GLU diblock currently under development: will be very water-soluble will not foam when solutions are shaken (meaning that the copolymers will not form micelles) should form adsorbed layers on alumina particles that are about 5 nm or more in thickness. This should be thick enough to at least partially stabilize alumina particles in water. One of our collaborators, Prof. Ducker in the Chemistry Department at VA Tech, has developed an atomic force microscope technique to measure the repulsive steric forces created the adsorption of the PRO-GLU diblock onto alumina.

24. Biomedical /PSA: Transdermal Delivery Faculty Members: Tim Long (Chem) 540-231-2480 Students Allison Sagle Sponsors ASCEF Objective: Develop tools to synthesize and characterize acrylic PSAs for transdermal drug delivery Approach Transdermal nutrient delivery protocols were developed for the delivery of folic acid from acrylic pressure sensitive adhesives and hydrogels.

25. Folic Acid: Impetus for Transdermal Nutrient Delivery Systems

26. In Vitro Permeation: Synthetic Skin as a Model Testskin II is an artificially living skin system that simulates the epidermis and dermis very closely.

27. Construction of Transdermal Diffusion Cells

28. Reversible Adhesion Faculty Members: Tom Ward (Chem) 540-231-5867 Students Jianli Wang Sponsors CASS Objectives: Develop reversible adhesion through the use of switchable surfaces Approach Develop novel, stable heterogeneous polymer brushes that can be reversibly altered to affect adhesion

29. What�s the Difference?

30. CFABC Polymer Brushes

31. CFABC Polymer Brushes

32. Chain Conformation Adjustment

33. Reversible Contact Angle

34. Analysis and Design of Adhesives Faculty Members: Eric Johnson (AOE) 540-231-6699 Students Vinay Goyal Sponsors NASA-LaRC Objectives: Develop appropriate analysis tools for predicting failure of adhesive bonds Approach Develop decohesion elements capable of addressing the strength and fracture properties of adhesive bonds. These finite elements start out bonded, but may debond when failure criteria is reached. Capable of addressing both the initiation and propagation of debonding.

35. Modeling the Fracture Process Using Decohesion Finite Elements

36. Decohesion Finite Element

37. Decohesion Finite Elements

38. Single Lap Joint Failure Analysis

39. Finite Element Results

40. Microelectronic: Conductive Adhesives Faculty Members: David Dillard (ESM) 540-231-4714 Students Shuangyan Xu Sponsors Emerson & Cuming, Motorola, CASS Objectives: Develop means to characterize impact (e.g. drop) performance of conductive adhesives at a material level Approach Develop an improved method to conduct drop tests of microelectronic components Develop falling wedge technique to measure impact fracture performance at a material level and correlate with TTSP studies

41. Component Bonded to PCB with ECA

42. Drop Tower Setup A Dynatup, model 730-I drop tower Two 30� polycarbonate wedges are used to split samples apart Samples are mounted vertically and secured at the base of the drop tower with setscrews No load cell is required for this method

43. A Photographic Frame of a Falling Wedge Test

44. Impact DCB Results Summary

45. Tan d vs. Gc

46. Research Collaboration Models

47. Benefits from Partnership Adhesive supplier New perspectives on adhesion assessment New test procedures for quantifying adhesion and impact resistance Closer working relationship with adhesive user through mutual interactions toward a common goal Contacts for future employment

48. Benefits from Partnership Adhesive user Assessment of engineering properties of adhesive New test procedures for quantifying adhesion and impact resistance Closer working relationship with adhesive producer through confidence gained through interactions Contacts for future employment

49. Benefits from Partnership University Funded research activity Publications Educating student Expand research in new adhesive application Extend test procedures for quantifying adhesion and impact resistance Closer working relationship with adhesive supplier and user Contacts for future employment and research interactions

50. Facilitating Partnership Interaction Clear research goal and plan Each partner visits each site Monthly phone or video conferences Coordination of material flow E&C: Model adhesives to Motorola and VT Motorola: Fabricated specimens to VT VT: Results, analysis, and failed specimens to sponsors

51. The Importance of Research

52. Education to Meet the Industry Needs