Polymers and nano-composites

Transcription

1 Polymers and nano-composites Michael Wübbenhorst Laboratorium voor Akoestiek en Thermische Fysica, Katholieke Universiteit Leuven Celestijnenlaan 200D, B-3001 Leuven, Tutorial: Novel Applications of Broadband Dielectric Spectroscopy Rome, 30 August 2009

2 Introduction Why DRS to study polymer systems? 1. Polymers show various relaxation phenomena Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

3 Introduction - Why DRS to study polymer systems? 2. They are (more or less) insulators Losses due to molecular relaxations >> electrical conduction 3. DRS easily to use and powerful technique (extremely broad dynamic range Broadband Dielectric Spectroscopy) 4. Many other reasons Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

4 Relaxations in polymer materials More introduction: DRS basic theory (see also previous lectures) [link] Various representation of complex dielectric spectra [link] Classification of relaxation processes (classical denotation) Dipoles in polymers (part II of this lecture) Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

5 Classification of relaxation processes Classical denotation loss δ γ β α Primary relaxation = dynamic glass transition T secondary relaxation, usually sub-t g processes Widely accepted schema for amorphous polymers September 16, 2009

6 Classification of relaxation processes Deviations from this standard notation Case 1: some semi-crystalline polymers (PE, PP) loss δ γ β α Primary relaxation = dynamic glass transition T Here, α-relaxation relates to molecular relaxation in crystalline phase (rotator-phase mode) September 16, 2009

7 Classification of relaxation processes Deviations from this standard notation Case 2: liquid-crystalline polymers loss γ β α δ Primary relaxation = dynamic glass transition T Very confusing denotation, see later September 16, 2009

8 Outline Introduction Part 1: Polymer dynamics as seen by Dielectric Spectroscopy Length- and time scales of dynamic processes in polymers Relaxation phenomena in various polymer architectures Part 2: DRS using dielectric probes Part 3: Dielectric relaxations in nano-composites Separate lecture on Friday Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

9 Polymer dynamics: Length- and time scales Large 10-2 m Slow 10 6 s material properties interfaces boundaries Macroscopic interactions organisation Mesoscopic molecular structure CH 3 O CH 3 O CH 3 O H 3 C O O CH 3 O CH 3 Molecular Small m Fast s September 16,

10 Polymer dynamics: molecular vibrations Fast dynamics molecular vibrations (t < s) trajectory t of a carbon atom in a dense liquid (T>T g ) vibrational displacements around discrete (lattice) positions x jumps between discrete (lattice) positions slower than vibrational motions

11 Polymer dynamics: molecular vibrations Fast dynamics molecular vibrations (t < s) FTIR absorption spectrum of two LC polymers 6 (a) en 9-3 (b). anhydride moiety acid/ester moiety September 16,

12 Polymer dynamics: relaxation processes Relation between polymer dynamics and dielectric relaxations local motions, e.g. simple bond rotations segmental motions (dynamic glass transition) chain relaxation (Rouse, Reptation) < 1 nm 2 < ξ < 10 nm 10 < ξ < 200 nm increasing relaxation time, characteristic length scale September 16, 2009

13 1. Local Relaxation Processes Local processes (τ > s) Energy U ΔU rotation angle discrete rotations of bonds resulting in local conformational changes possible in liquid, amorphous solid and crystalline state September 16, 2009

14 1. Local Relaxation Processes Temperature dependence of relaxation time: Arrheniuslaw thermal activation of τ log f α (Hz) max α β τ β-process E = kt ( ) a T τ exp /T (1/K) T-dependence τ(t) ofα α and β- process in a poly(ether ester) [Mertens et al. 1999, pol. 1a] September 16, 2009

15 2. Segmental dynamics dynamic glass transition Medium slow fast dynamics : glass transition (10-4 s < τ < s) t=t 0 t=t 0 +τ cooperative motions of a few monomer units (polymer segments) at T>T g September 16,

16 2. Segmental dynamics dynamic glass transition typical f-t-dependence of an amorphous polymer α-process = main-relaxation ε ohmic conduction β α segmental relaxations (dynamic glass transition) γ September 16,

17 2. Segmental dynamics dynamic glass transition Curved τ(t) vs. 1/T dependence: Vogel-Fulcher-Tammann law (VFT) E V τ ( T ) = τ exp k( T TV ) Rationalization of VFT law: Temperature dependent length scale ξ=ξ(t) of cooperatively rearranging regions (CRR) (Adam and Gibbs, 1965) typical for dynamics of supercooled liquids (glassforming materials) : α-relaxation τ [s] ,2-propanediol ξ glass transition temperature T g usually T V + 50K WLF VFT equation /T ξ September 16,

18 3. Whole chain dynamics Slow dynamics involving reconfiguration of whole chains viscosity For very long chains (M w > 20000g/mol) high morelaxation of an entanglement network PMMA film, 10x stretched just above T g heating again to T > T g : retraction of the film to original size 10 < ξ < 200 nm September 16,

19 3. Whole chain dynamics Normal relaxation Normal relaxations, e.g. cis-polyisoprene End-to-end vector fluctuations cause dielectric normal relaxation chain relaxation self-diffusion of whole polymer chain τ nm depends on molecular mass M τ M ν ν = 3.7 for M > 10 4 ν =2 for M < 10 4

20 Relaxation phenomena in various polymer architectures Numerous polymer architectures known, e.g.: Linear chain polymers (Polyolefines, PS, PC, PMMA ) Side-group polymers (hyper)branched polymers, polymer networks Copolymers, e.g. random and block-cp s Classification based on functionality: Liquid-crystalline polymers Polyelectrolytes, Polymer electrolytes Ferro-, piezo-, pyroelectric and NLO-active polymers Polyamphiphilics. Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

21 Relaxation Processes in specific polymers Linear polymers, flexible: poly(ethylene) - PE poly(dimethylsiloxane) PDMS poly(ethylene oxide) Bisphenol-A PC Linear polymers, (semi) stiff: aromatic polyesters (PET, PEN) Vectra These architecture might result in Amorphous polymers Semi-crystalline polymers Liquid crystalline polymers Side-group polymers polystyrene PS poly(methyl methacrylate) PMMA polysiloxane, functionalized aliphatic polycarbonates September 16, 2009

22 Linear, flexible polymers Low density polyethylene (LDPE) ε" LDPE + x% DBANS β α 0% 0.1% 0.5% 1.0% T g : ~ -35 C no intrinsic dipole moment only weak dielectric i relaxations due to partial oxidation (C=O) dielectric probes γ T [ C] Linear, flexible polymers September 16, 2009

23 Amorphous, linear flexible polymers Bisphenol A Polycarbonate -8 log(τ [s]) -6 α β BPA-PC -4 E a : 41 kj/mol log(τ [s]): T g = 155 C /T β-relaxation involves several backbone bonds September 16, 2009

24 Amorphous, linear flexible polymers PDMS No explicite dielectric β- process found T g = C log(τ [s]) -6-4 Me Me Me -2 Si O Si O Si O Me Me Me 0 n /T [1/K] September 16,

25 Amorphous, linear flexible polymers Poly(ether esters) 10a 0.1 γ β tanë 0.01 Li + /1a temperature [K] γ-process: twist motion of ether groups September 16,

26 Linear, semi-stiff polymers Aromatic polyesters, PET and PEN - semicrystalline 1 loss dlnε ε/dlnf 0.1 1H 3H 7A 7B 9A PET β 2 PEN α PEN: additional process between α and β-relaxation 0.01 β β Temperature [ C] September 16,

27 Linear, semi-stiff polymers Aromatic polyesters, PET and PEN H 3H 7A 7B 9A β 2 α O C O O C O C H 2 CH 2 dlnε/dlnf 0.01 β β Temperature [ C] September 16,

28 Linear, semi-stiff polymers Aromatic polyesters (3) Vectra B950 E a [kj/mol] log(τ 0 ) δ γ β A Arrhenius processes, but only 2 of them are local relaxations September 16, 2009

29 Linear, semi-stiff polymers Analysis of bond rotation contributions Vectra B950 various crankshaft motions September 16, 2009

30 Amorphous, linear side-group polymers Atactic Polystyrene % 0.1% 0.5% 1.0% α log(τ [s]) PS + 1% pna, α PS + 1% pna, β* PS + 0.5% pna, α PS + 0.5% pna, β PS % pna, α PS % pna, β PS + 0.1% pna, α PS + 0.1% pna, β PS + 0.1% DBANS, β PS + 0.5% DBANS, β PS + 1% DBANS, β tanδ 10-2 β PS 10-3 PS + x% DBANS β α PS β (DBANA) E a : kj/mol /T β-process: E a ~ 25 kj/mol crankshaft mechanism T[ C] September 16,

31 Amorphous, linear side-group polymers Side-group polysiloxane log g(τ [s]) /T [1/K] β-process: E a ~ 20 kj/mol September 16,

32 Amorphous, linear side-group polymers Polysilane September 16,

33 Amorphous, linear side-group polymers Poly(methyl methacrylate), PMMA log(τ [s]) E a : 109 kj/mol log(τ [s]): a-pmma 80 kj/mol log(τ [s]): α β dielectric β-process 2 E a [kj/mol] log(τ 0 ) /T β September 16,

34 Amorphous, linear side-group polymers Poly(methyl methacrylate), PMMA tanδ (25 Hz) 0.1 β β α (isotactic) α log(τ [s]) α α α (a-pmma) α (s-pmma) α (i-pmma) β (i-pmma) β (s-pmma) β (a-pmma) i-pmma, 58 nm a-pmma, 158 nm s-pmma, 79 nm Temperature [ C] /T [1/K] large influence of stereoregularity: i-pmma: E a ~ 70kJ/mol September 16,

35 Amorphous, linear side-group polymers Molecular mechanism of dielectric β-process -5 [s]) log(τ E a = 70.0 kj/mol s-pmma, M n = E a = 82.3 kj/mol nm 9.0 nm 8.8 nm β 2 : cooperative process involving the backbone β 1 : noncooperative process 0 2 main components contribute to dielectric β-process in PMMA 1000/T [1/K] September 16,

36 Liquid crystalline polymers Liquid crystalline polycarbonates O O O O O n (CH 2 ) m O NO 2 September 16,

37 Liquid crystalline polymers Liquid crystalline polycarbonates (2) β m 0 O -2 SC I S A O O A α S γ O (CH 2 ) m O O n ç [s] -4 λ 1 Î 2 λ 2 M -6 Î 1 γ /T 3 VFT-type relaxations (related to segmental dynamics)! Symbols λ 1 and λ 2 introduced for clarity NO 2 September 16,

38 Liquid crystalline polymers - Polysiloxanes Me 3 SiO Me Me Me Si O Si O Si O SiMe 2 t-bu Me Me n 1 Me 3 SiO SiMe 2 O SiMe 2 Me Me Me Si O Si O Si O SiMe 2 t-bub Me Me n SiMe 2 O SiMe 2 (CH 2 ) m O (CH 2 ) p CH 3 4 CB10, m = 10 5 CB CB p = 4 3 p = 9 CN 6 CB CB CB5, m = 5 September 16,

39 Liquid crystalline polymers - Polysiloxanes Again, 3 VFT-type relaxations observed λ 1 log(τ [s]) -6-4 α β γ Our present interpretation schema -2 λ /T [1/K] September 16,

40 Dielectric probes: a powerful tool for the study of glass transition dynamics in apolar polymers in bulk and ultra-thin film geometry Michael Wübbenhorst Co-authors: S. Napolitano, K. Kessairi, S. Capaccioli Lab. voor Akoestiek en Thermische Fysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven PolyLab-CNR and Dipartimento di Fisica, Università di Pisa, Largo. B. Pontecorvo 3, Pisa, 56127, Italy Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

41 Outline Dielectric probes why do we need them? First applications Effect of probe length on dielectric response Polystyrene molecular l mass effects Dielectric probes in ultrathin polymer films Summary & conclusions Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

42 Motivation Dielectric spectroscopy Versatile tool for study of molecular dynamics in condensed matter Exceptionally large frequency range: Hz are possible Increasing popularity due to availability of turn-key instrumentation for more than 15 years One essential prerequisite α o molecular dipoles αe α a Orientational polarisation Relaxation phenomena IR VIS/UV Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

43 Dipole moments Some dipole moments of simple molecules Compound Dipole moment Cm H 2 O 6.1 HF 6.4 HCl 3.6 PVC - + HBr 2.6 H C - Cl H CH 4 0 C μ CCl 4 0 C CO 2 0 H Cl NH CH 3 Cl C 2 H 5 OH 5.7 PVDF H + (C 2 H 5 ) 2 O 3.8 F C - C 6 H 5 Cl 5.7 C C F C 6 H 5 NO Dipole moments in polymers Unfortunately, there are several interesting systems that lack strong polar groups, e.g. PE, PP and PS introduction of dipoles necessary H F - F

44 Outline Dielectric probes why do we need them? First applications of dielectric probes Effect of probe length on dielectric response Polystyrene molecular l mass effects Dielectric probes in ultrathin polymer films Summary & conclusions Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

45 DBANS in apolar polymers Synthesis of DBANS (O. van den Berg) Both fluorescent and strong dipole moment Compatible with aromatic and aliphatic environment O. van den Berg, W. G. F. Sengers, W. F. Jager, S. J. Picken, and M. Wübbenhorst, Macromolecules, vol. 37, pp , Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

46 DBANS in apolar polymers Various polymer/probe combinations: ε" 10-1 PP + x% DBANS α 0% 0.1% 0.5% 1.0% Isotactic PP Temperature [ C] PS + DBANS: - weak intrinsic polarity of pure PS LDPE: LDPE: almost no detectable α-process in pure PE! Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

47 DBANS in apolar polymers Proof of linearity between Δε and c probe Excellent reproducibility of structural relation time for all systems Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

48 Characterization of immiscible polymer blends Dielectric probes in more complex systems: Binary blends of PE and PP W. G. F. Sengers, O. van den Berg, M. Wübbenhorst, and A. D. Gotsis, "Mapping of relaxation processes in immiscible apolar polymer blends using dielectric spectroscopy," Polymer, vol. 46, pp , Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

49 First summary Addition of dielectric probes amplifies the dielectric relaxation associated with the dynamic glass transition (α-process) No effect of DBANS on local (secondary) relaxation processes found Detailed study of cooperative dynamics in complex systems enabled Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

50 Outline Dielectric probes why do we need them? First applications of dielectric probes Effect of probe length on dielectric response Polystyrene molecular l mass effects Dielectric probes in ultrathin polymer films Summary & conclusions Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

51 Effect of probe length DBANS appeared to work well for PP, PE and PS Question: Can we learn anything about the length scale of the α-relaxation by varying the size of the probe? O 2 N O 2 N 6.4 Å 12.8 Å N DBANA (pna) Synthesis of probe molecules with different length of the rigid part by varying the number of aromatic rings (1 3). N DBANS O 2 N DHANS 19.6 Å N Mixing of probe with PS in solution; Preparation of samples for DRS by film casting. O. van den Berg, M. Wübbenhorst, S. J. Picken, and W. F. Jager, J. Non-Cryst. Solids, vol. 351, pp , Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

52 Effect of probe length Dielectric loss ε (T) at f =13 khz ε" % 01% 0.1% 0.5% 1.0% α ε" % 01% 0.1% α 0.25% 0.5% 1.0% β α β? β PS + x% DBANS PS + x% DBANA T [ C] T [ C] DBANS and DHANS: selective amplification of α-proces, no significant effect on β-process DBANA (short probe): amplification of both α-proces and additional probe relaxation (β*) below T g Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

53 Effect of probe length Short probe, further analysis Δε 0.2 PS + DBANA Δα Δβ 0.1 Δε α (PS) probe concentration [% m/m] Contribution of probe response Δε μ 2 to α and β*-process scales linearly with c probe Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

54 Effect of probe length Short probe, further analysis Arrhenius-type process (β*) with higher activation energy than native β-process of PS Kink in E a (T) close to volumetric T g Temperature dependence in τ β* contains contribution from thermal expansion Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

55 Effect of probe length Angular fluctuations of the probe molecules: two possible scenario s large probe: short probe: T > T g long probe large angular fluctuations in "viscous" matrix amplification of α-process short probe large angular fluctuations contribution to α-process T < T g small angular fluctuations ti large angular fluctuations, ti restricted t no dielectric response β*-process free-volume probe Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

56 Rotational diffusion Do DBANS & DHANS behave as true rotational probes? Rotational diffusion time for Brownian rigid rods: τ l 3 (l = probe length) single-exponential process ( Debye relaxation) log(τ τ [s]) α PS relaxation time at high frequencies proportional to probe length -2 PS pure 0 PS + 0.5% pna PS + 0.5% DBANS PS + 0.5% DHANS /T [1/K] Relation τ l 3 holds well Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

57 What about the spectral shape of the α-process? Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

58 Effect of probe length on spectral shape From experiment: Glass transition dynamics shows usually non-exponential kinetics Rationalisation by spatially heterogenous dynamics ξ Averaging over different environ- ments yields KWW behaviour. φ () t = exp ( t τ) logε β Δ ε * ε = ε + 1 a [ + ( iωτ ) ] b slow fast ε ω m HN nhn log(ω) ε ω Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

59 Effect of probe length on spectral shape Probe rotational diffusion in a heterogeneous scenario Case 1: probe size ~ ξ - low temperatures - small probe molecules Individual probe response reflects heterogeneity Case 2: probe size >> ξ - high temperatures - large probe molecules Probe averages over spatial heterogeneities Debye relaxation expected Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

60 Dynamics of a-pp Recent work on atactic (amorphous) PP K. Kessairi, S. Napolitano, S. Capaccioli, P. A. Rolla, and M. Wübbenhorst, Macromolecules, vol. 40, pp , ε'' 260 K 273 K 303 K 10-3 Tremendous enhancement of dielectric α-process Exclusive coupling of DBANS with glass transition dynamics 0, frequency [Hz] frequency [Hz] 143 K ε'' DBANS 1% α 10-2 log g[τ/s] pure PP 0.2% DBANS 0.5% DBANS 1% DBANS -2 τ α β T [K] pure 10-3 Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst /T [1/K] τ β

61 Dynamics of a-pp Probe concentration and T-dependence of shape parameter a HN 1,50 logε * Δ ε 0,0 % ε = ε + a HN 0,2 % 1+ 1,25 0,5 % 1.0 % 100 1,00 DEBYE limit a [ ( iωτ ) ] b 0, ,50 ε ω m HN nhn log(ω) ε ω 0, T[K] Pure a-pp: broad α-process (a HN ~ 0.5) a-pp+1%dbans: almost single exponential (Debye) process at high temperatures Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

62 Summary and conclusions Rigid-rod type polar molecules with variable length (0.6 nm, 1.3 nm, 2.0 nm) of their stiff core were used as "dielectric probes" in polystyrene, polypropylene and polyethylene Dependent on probe length, the dielectric probe response was found to sense specific molecular motions:» shortest probe (0.6nm): strong mobility in glassy state of PS large angular diffusion within free volume holes» medium and large probes: predominant contribution to glass transition dynamics amplification of α-relaxation Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst

63 Summary and conclusions (2) Potential application of the dielectric probe approach for the study of complex materials was demonstrated (M w -dependence of PS, polymer blends) Further potential of dielectric probes lies in the investigation of dynamics in ultrathin polymer films Laboratorium voor Akoestiek en Thermische Fysica Michael Wübbenhorst