Impact of two different Cellulose Nanoreinforcements on the Melting and Crystallization Behavior of Polycaprolactone

Transcription

1 Impact of two different Cellulose Nanoreinforcements on the Melting and Crystallization Behavior of Polycaprolactone Gilberto Siqueira, Carole Fraschini, Julien Bras, Alain Dufresne, Robert Prud homme and Marie-Pierre Laborie Geneva, October 14 th

2 Background Research Objective Outline Materials and Methods Results and Discussion Cellulose/PCL morphology Crystallization kinetics (Avrami and Secondary Nucleation Theory) Conclusions

3 Background & Objective

4 Nanofillers & PCL Melting Crystallization PCL/ nanoclay crystallization 1,2 For low clay/carbon nanotubes content, fillers act as a nucleating agent, accelerating the crystallization of the polymer. At high clay content, fillers delay crystallization, possibly due to reduced mobility. PCL/ carbon nanotubes crystallization 3,4 Decrease of avrami exponent n or dimensionality of the crystal growth. 4 Carbon nanotubes expedited the crystallization process of PCL. 1. Jimenez G. Et al. 1997, J. of Appl. Pol. Sci. 64: Di Maio E et al. 2004, Polymer 45: Wu & Chen 2006 Poly. Eng. & Sci. 46(9): Chen & Wu 2007 Poly. Degr. & Stability 92(6):1009.

5 Nanocellulose & PCL Melting Crystallization In PEO/ CNW nanocomposites 1 : Xc constant up to 10 % loading; decreased with further loading Smaller and more numerous spherulites were observed The whiskers acted as nucleating agents while limiting crystal growth In PP/ CNW models 4 Nucleating effect Transcrystalline layer 5 In MFC reinforced PVA 2 and PLA 3 slight increase in Xc with small amounts of MFC (1 to 5 wt%) nucleating effect 1. Azizi Samir et al. 2004, Polymer 45(12): Lu J. et al. 2008, Comp. Part A 39(5): Suryanegara L. et al. 2009, Comp. Sci. & Techno. 69: Grey D., 2008, Cellulose, 15, 297.

6 Objective It is important to understand the impact of nanocellulose melting/crystallization behavior and kinetics of polymers - Fundamental standpoint: understanding interactions, morphology, and miscibility in the melt etc. - Practical standpoint: processing of nanocomposites. Objective: To evaluate the impact of different cellulose nanofillers on the melting/crystallization behavior of Polycaprolactone. How the shape and nature of the cellulosic filler might influence the morphology and crystallization kinetics of the matrix? 6

7 Cellulose Nanocrystals vs MFC Acid Hydrolysis Mechanical disintegration Individual crystals Web-like structure Straight Long and flexible 7 Cellulose nanocrystals from Sugar beet pulp Azizi Samir et al., 2004 Dimensions: Length: nm; Diameter: 4 15 nm. Rånby and Ribi (1950) Terminologies: whiskers, cellulose nanowhiskers (NCW), cellulose nanocrystals (CNX or CNC), Nanocrystals of cellulose (NCC). MFC from Opuntia ficus-indica Malainine et al., 2005 Dimensions: Length: micrometer scale; Diameter: nm. Herrick et al. and Tubark et al. (1983) Terminologies: microfibrillated cellulose (MFC), microfibrils, nanofibrillated cellulose (NFC), cellulose nanofibers (CNF).

8 Materials & Methods Polycaprolactone Microfibrillated Cellulose (MFC) & Cellulose Whiskers (CNWs) Chemical Modification : Isocyanate Grafting on Cellulose Morphological Characterization and Isothermal Crystallization Studies

9 Raw Materials Sisal-based Cellulose fibers (Agave sisalana), originating from northeast Brazil, were purchased in Mariana (Minas Gerais, Brazil) poly(ε-caprolactone) PCL O O n M n = 42,500 g.mol -1 M w = 65,000 g.mol -1 9

10 Raw Materials Acid Hydrolysis with sulfuric acid Microfluidizer TEM of sisal whiskers Dimensions of Whiskers Length: 215 nm 67 nm Diameter: 5 nm 1 nm L/D 43 TEM of sisal MFC Dimensions of MFC Length: µm scale Diameter: 52 nm 15 nm 10

11 Chemical Modification Method I Solvent Exchange Temp. -10 C Nanoparticles in dichloromethane Nanoparticles in toluene Nanoparticles in water Solvent Exchange Nanoparticles in acetone Isocyanate Temp. 110 C 30 min Temp. 50 C 30min Nanoparticles in acetone Temp. 90 C 75 min Add. Toluene Method II Nanoparticles in toluene 11 XRD showed that the cellulose crystalline structure was not affected by grafting Grafting was demonstrated by XPS (O/C and N/C ratios) Siqueira et al. (Langmuir 2010) HO O HO C O N H O OH n-octadecyl isocyanate OH O O OH HO OH n

12 Chemical Modification Fiber MFC Whiskers 2 to 10 m².g -1 ~ 2% OH accessible (Trejo-O reilly et al. 1997) x10 51 m².g cellulose chains 3.1% OH accessible x m².g cellulose chains 21% OH accessible microfibril Fiber 12 Siqueira et al. (Langmuir 2010)

13 Methods: DSC Analysis Pure PCL, 12% wt PCL/MFC, 12% wt PCL/CNW Isothermal crystallization studies with Perkin-Elmer DSC-7 Heat to 90 C at a heating rate of 100 C/min Stay at 90 C for 20 minutes Quench (100 C/min) to T c and maintain until crystallization is complete T c ranged from C with a spacing of 2 C. Heat at 10 C/min to 90 C Heat flow as a function of time during isothermal crystallization Melting temperature (T m ) and endotherm upon second heat The degree of crystallinity (X c ) 13

14 Nanocomposite Morphology Melting and crystallization behavior Crystalline structure analysis Equilibrium melting point

15 DSC Melting Behavior endo endo b T m1 =62.4 o C Tc = 42 C Heat Flow (W/g) a b c T m1 =62.4 o C T m2 =62.0 o C T m2 =63.5 o C T m2 =64.5 o C PCL PCL-CNW PCL-MFC Temperature ( C) Broadening and splitting of the melting upon addition of nanocellulose Effect is more marked with MFC than CNW More heterogeneous crystals, other crystalline structure and/or a melting-recrystallization phenomena in nanocomposites? 15

16 Melting/Crystallization Behavior 16 Sample T c ( C) X c T m1 ( C) T m2 ( C) PCL PCL-CNW PCL-MFC * * * * X c increases with addition of MFC and CNWs Effect more marked with CNWs than MFC Intensity (a.u.) MFC CNW PCL-MFC PCL-CNW PCL θ (degree) No new crystalline structure Nanofillers enhance PCL crystallization - nucleating effect? - related to surface area?

17 Equilibrium Melting Point Hoffman-Weeks Plot R 2 = R 2 = T o m T m 1 o l = ( Tm Tc ) γ = γ l * T m ( o C) R 2 = γ : thickening coefficient l: final lamellar thickness l*: critical nucleus size PCL - T m 0 = 79 o C PCL-CNW - T m 0 = 70 o C PCL-MFC - T m 0 = 69 o C T c ( o C) T c Addition of MFC and CNWs depress the T m0 of PCL Suggests melt miscibility of modified nanocellulose and PCL Nucleating effect Sample T o m ( C) γ PCL PCL-CNW PCL-MFC

18 Isothermal Crystallization kinetics Avrami Kinetics: Bulk Crystallization Theory Lauritzen-Hoffman Kinetics: Secondary Nucleation Theory

19 Isothermal Crystallization: Raw Data 19 MFC and CNWs expedite PCL crystallization Slightly faster with MFC than with CNW Nanofiller enhance PCL crystallization - nucleating effect - not just related to surface area - related to surface chemistry? Confinement?

20 Avrami Modelization 1 X = exp k t [ ( t) n ] X t is the relative crystallinity, t is the crystallization time k is the crystallization rate constant (min -1 ) n is the Avrami exponent Increasing T c results in slower crystallization: nucleation dominated domain; Good fit to Avrami equation up to 70% relative crystallinity. 20

21 [ ln( 1 )] = nln t ln k X t ln + Avrami Modelization n PCL PCL-CNW PCL-MFC T c ( C) Avrami parameter varies between 2 and 3: heterogeneous growth with truncated spherulite; No clear effect of nanocellulose on n. 21

22 Avrami Parameters 22 k is increased ca. 100 fold and t 1/2 decreased 10 fold by adding nanocellulose k is larger and t 1/2 is smaller with MFC than with CNW

23 Activation Energy for Crystallization 1.0 1/n ln(k) R 2 = R 2 = R 2 = n ( ln k) = ln k0 E RT a c -3.0 Sample E a (kj/mol) PCL PCL-CNW PCL-MFC PCL 317 PCL-CNW 264 PCL-MFC 242 1/T c (x10-3 ) Energetics of crystallization are facilitated with addition of CNW and MFC Effect more marked with MFC than with CNWs 23

24 Secondary Nucleation Theory: Lauritzen-Hoffman Transport Nucleation G = G 0 * U exp R( T T K g c ) Tc ( T ) f ln(1/t 1/2 ) + U * /[R(T c -T )] R 2 = R 2 = R 2 = PCL PCL-CNW PCL-MFC 1.0x x x x10-4 1/(T c.( T).f ) G : linear lamellar growth rate G 0 : growth rate constant U * : diffusional activation energy (1500 cal.mol -1 ) T : temperature at which motion ceases (30k below Tg) K g : nucleation parameter or free energy necessary to form a nucleus of critical size T: supercooling f: correction factor

25 Secondary Nucleation Theory: Lauritzen-Hoffman PCL PCL-CNW PCL-MFC G 0 (min -1 ) 5.307x x x10 4 K g (K 2 ) 1.434x x x10 4 σσ e (erg 2 /cm 4 ) K g 4bσσ et = β k H 0 m 0 f K g decreases when adding CNW and MFC (effect of MFC>CNW) Decrease of energy barrier of secondary nucleation (MFC>CNW) G 0 decreases when adding CNW and MFC (effect of MFC> CNW) Molecular chain mobility significantly reduced by nanocellulose (effect of MFC> CNW) 25

26 Conclusions Surface area effect: heterogeneous nucleation effect Overall degree of crystallinity is greater in nanocomposites (CNW than MFC) Energetics of bulk crystallization lowered by nanocellulose Crystallization expedited Surface chemistry effect? Compatibility (T 0 m depression) Energetics of nucleation slightly favored with MFC compared to CNW (K g and lamellar surface energies) Confinement / transport effect? Growth rate constant significantly lowered by nanocellulose Effect more marked with MFC (entanglements) than CNW 26

27 Thank you for your attention! Acknowledgements ALBAN Program for the financial support (PhD fellowship) of Dr. Gilberto Siqueira Grenoble Polytechnic Institute for financial support (sabbatical) of Dr. Marie-Pierre Laborie