synthetic strategies for the generation of polymer monolayers grafting via immobilized monomers

Transcription

1 synthetic strategies for the generation of polymer monolayers I I I I "grafting-to" chemisorption: groups of the polymer are reacted with suitable surface sites grafting via immobilized monomers "grafting-from" surface initiated polymerization: chains are grown from initiator monolayers

2 grafting via immobilized monomers general considerations first step: growing chain reacts with surface groups grafting-to second step: the chain continues to grow away from the surface grafting-from step 2: grafting-from step 1: grafting-to "grafting-in-between

3 grafting via immobilized monomers state of the art: very few publications! mechanism not understood no molecular picture - K. ollen, V. Kaden, K. Hamann, Angew. Makromol. Chem. 6 (1969) 1 anchored monomer: MAA physisorbed on Ti 2 resuls: ~ 1-2 mg/m 2 or 1-2 nm film thickness, no molecular weights given R. Laible, K. Hamann, Adv. Colloid Interface Sci. 13 (1980) 65 Si anchored monomer: MPS on silica results: ~ 1-2 mg/m 2 or 1-2 nm film thickness, no molecular weights given MPS M. Chaimberg, Y. Cohen, AIChE Journal 40 (1994) 294 anchored monomer: vinyl silane (VS) on silica results: ~ 1-3 mg/m 2 or 1-3 nm film thickness, low molecular weights good description of the macroscopic kinetics Si VS

4 Grafting with immobilized monomers: system description monomers + corresponding acrylates Si thacryloyl-propyl trimethoxysilane (MPS) Si -thacryloyl--methylpropyl trimethoxysilane (MPS) substrates silicon wafers porous silica µ-glass beads Si + T=393K, toluene reflux Et 3 Si, toluene AIB, * Si m + free polymer n * H H H silica gel surface silica gel surface silica gel surface

5 grafting-in-between influence of the polymerization time: layer thickness conditions: 60 C, bulk styrene, [AIB] = 3 mmol/l system 25 Si 2 -MPS Si styrene C C AIB thickness [nm] film thickness first increases then levels off at ~15-20 nm polymerization time [h]

6 grafting-in-between influence of the polymerization time: molecular weights conditions: 60 C, bulk styrene, [AIB] = 3 mmol/l M n,m w [10-5 g/mol] M w M n M w /M n ~ 1.8 molecular weights remain constant during polymerization polymerization time [h]

7 grafting-in-between influence of the polymerization time: graft densities conditions: 60 C, bulk styrene, [AIB] = 3 mmol/l, silica gel 0,025 graft density (µmol/m²) 0,020 0,015 0,010 0,005 0, reaction time (h) graft densities: ~ 20 nmol/m 2 or ~ 3% of immobilized monomers for comparison: grafting-from yields ~ 1 µmol/m 2 ~ 40% of initiating sites

8 grafting-in-between influence of the silica concentration on grafted amount polymerization conditions: 60 C, bulk styrene, [AIB] = 3 mmol/l before after surface modification grafted amount of polymer [g PS / g Si x ] 0,7 0,6 porous silica gel 0,5 0,4 0,3 0,2 0,1 0, silica gel concentration [mg/l] grafted amount polymer [g PS / g Si x ] 0,20 µ glass beads 0,15 0,10 0,05 0, glass bead concentration [mg/l]

9 grafting-in-between discussion I: is surface-attached monomer consumed through multiple attachment points?

10 grafting-in-between variation of the monomer graft density: dilution of surface-attached monomer by an inert silane Si polymerization conditions: silica gel, styrene/toluene 1/1 v/v, 60 C graft density (µmol/m²) 0,025 0,020 0,015 0,010 0,005 0,000 0,0 0,3 0,6 0,9 1,2 graft density MPS (µmol/m²) dramatic reduction of polymerizable groups does not influence graft density multiple attachment is not the limiting factor

11 How many of the surface groups are active when diluted? 40 efficiency (%) ,0 0,2 0,4 0,6 0,8 1,0 'active' silane

12 grafting-in-between discussion II: what limits film formation? "grafting-to" or "grafting-from"?

13 grafting-in-between influence of the molecular weight of the attached chains Γ [µmol/m2] conditions: silica gel, styrene/toluene 1/1 v/v, 60 C 0,12 0,10 0,08 0,06 0,04 0,02 0, c(aib) [mmol/l] M n [10 3 g/mol] gr. amount [g PS/gSi 2 ] 3,0 2,5 2,0 1,5 1,0 0,5 0, c(aib) [mmol/l] decreasing initiator concentration higher molecular weight decreasing graft density no influence layer thickness

14 Filling up of layers with low molecular weight polymers 1st step: grafted polymer (g PS / g Si x ) 0,6 0,5 0,4 0,3 0,2 0,1 0,0 graft density after one reaction graft density after two reactions 2ST-PS-1 2ST-PS-2 2ST-PS-3 2ST-PS-4 2ST-PS-5 growth of long polymer chains; variation of the polymerization time 2nd step: filling up with short chains sample

15 mechanism "grafting-to" step is limiting the overall process film thickness / grafted amount depends on the product of molecular weight and graft density of the surfaceattached polymer

16 functional monolayers through grafting-in-between

17 Active Ester Monolayers: 3, PSae Attachment of Fluorescence Dyes 2,5 2,0 H abs (a.u.) 1,5 1,0 0, thacryloyl-β-alanine succinimideester (MA-C2-AE) 0, wavelength (nm) H 2 1-Aminomethyl-pyren (AMP)

18 Polyamino-Surfaces: Immobilization of Catalysts phtalimid-precursor glass particle + H 2 H ts R Ru CH 2 ts olymer monolayer GRUBBs catalyst (2 nd generation) Cy 3 P Cl Cl Ru s H Cl Ru s Cl s F. Michalek, Prof. Dr. W. Bannwarth s

19 Photolinking of polymer molecules to solid surfaces hν

20 Concept promotion of the growth of endothelial cells on heart valve surfaces through surface-attached polymer layers toxic GDA groups compatibilizing polymer layer GDA heart valve (collagen) GDA = H C (CH 2 ) 3 C H

21 Strategy: benzophenone monolayers C H C H polymer chain hν and other products Si Si. Prucker, C. aumann, J. Rühe, C. Frank and W. Knoll; JACS, 121; (1999) triplet formation through n,π*-excitation (~350 nm) and hydrogen transfer radical-radical coupling: reaction with almost any C,H-bond of a polymer molecule in the vicinity leads to surface attachment of the polymer

22 Photochemistry of benzophenone triplet formation upon n,π* excitation biradical reacts with C,H bonds H C 350 nm C C C τ = 100 µ s H C hydrogen abstraction C C H recombination

23 Experimental procedure surface with photoreactive groups film deposition Film generation: spin-coating dip-coating doctor-blading spray-coating casting painting... illumination surface-attached polymer monolayer extraction

24 Film thickness example: polymer: polystyrene film thickness of spin coated film: 120 nm irradiation time: 3 h after 15 h extraction with toluene only chains can be attached that have contact with the surface 15 d max [nm] M w [10 6 g mol -1 ] R g. Prucker, C. aumann, J. Rühe, C. Frank and W. Knoll; JACS, 121; (1999)

25 15 d max [nm] R g (bulk) [nm] R (bulk) = M g film thickness of surfaceattached layer increases linearly with the coil size (radius of gyration) of the deposited polymers w

26 Cell adhesion tests I cells: human umbilical vein endothelial cells (HUVEC) CH 2 CH 2 C PEtx: Et homogenous dense coverage of the surface with vital cells n 10x CH 2 CH C PAAm: H 2 only single cells or small agglomerates visible; abnormal cell morphology n 20x

27 Cell adhesion tests II cells: endothelial cells from a sheep CH 2 CH 2 PEtx: homogenous coverage of the surface with vital cells C n Et CH 2 H H H 2 Chitosan in some areas weak cell adhesion; partial delamination n delaminated area

28 From monolayers to networks higher film thicknesses: nm µm mm [ ] improved mechanical strength additional synthetic strategies

29 Improved adhesion non-attached networks: - float off the surface if contact with a solvent - no polymeric residues solvent adhesive failure substrate network surface-attached networks: - only destructive removal possible - after failure polymeric residues remain on the surface solvent substrate network cohesive failure

30 Strategy cast R I I I I I I I I I I I I I I I I I I substrate R I R I R R R I networks via simultaneous initiation from the surface and in solution I C C Si C C polymerization is started simultaneously in the bulk and from the surface network is formed within the cast that serves as reaction vessel cast determines the shape and thickness of the surface attached polymer network (µm - mm range)

31 etwork towers: Drying/Swelling surface-attached attached network reference cast glass substrate swelling drying small towers (~3 x 5 mm) can be dried and swollen without damage to interface reference sample (nonattached) fails immediately drying

32 etwork towers: adhesion cast substrate cohesive failure

33 Copolymers with photocrosslinkable groups CH 2 anthracenes benzophenones H H H Et statistical copolymers: hydrophilic/hydrophobic/pel photogroup content: 1-20%

34 Polymer networks attached to polymeric substrates photocrosslinkable overcoat simultaneous crosslinking and surface attachment via pendant benzophenone units polymeric substrate (e.g. polyurethane) swelling in water (2h) ~ 1 mm ca. 20 µm

35 Polymer networks on porcine heart valve surfaces PEtx-BP Reference: PEtx PEtx-BP swollen in water Reference: Polymer is washed off UV 2.5 mm extraction in water Surface-attached polymer networks can be formed on a biologic surface etwork shows good adherence and swelling behavior Jürgen Rühe, Lehrstuhl Chemie & Physik von Grenzflächen 35