AFM Observations on Dimensional Changes of Individual Cellulose Aggregate Fibrils under Variable Humidity Environments

Transcription

1 2006 International Conference on Nanotechnology, April 26-28, 2006, Atlanta, GA AFM Observations on Dimensional Changes of Individual Cellulose Aggregate Fibrils under Variable Humidity Environments Presented by: Jung Myoung Lee, John A. Heitmann and Joel J. Pawlak Ph.D. student Department of Wood & Paper science NC State University Your logo here

2 Outline Introduction Motivation and Objectives Experimental Results and Discussion Conclusions Acknowledgments

3 Introduction

4 Bio-based materials The demand to increase the sustainability of material is driving a new concepts in the design and fabrication of materials. - Cellulose, All-cellulose, and Green nano-composites However, physical and mechanical properties of the materials often change significantly with environmental factors. - Bio-based materials by moisture content - Synthetic materials by temperature changes Characterization techniques and analytical modeling methods are needed to engineer advanced cellulose composites to ensure long-term structural integrity over the design life-time.

5 Dimensional instability of paper Instability of paper to surrounding environmental factors affects the paper quality. - In-plane deformations (cockling, misregister) - Out-of-plane deformations (curl, roughening, fiber-rising) Papermaking fiber is a complicated biocomposite and its hygroexpansivity is correlated with paper.

6 Macro- to Nano-scopic views Cellulose fibril aggregate (15nm in diameter) Paper Cellulose aggregate fibril (CAF) Fiber Alava (2006), Fahlén (2005) and Lu (1995) Cellulose microfibril

7 Tapping-mode AFM Feedback loop Detector electronics Split photodiode detector Controller electronics Laser Scanner X, Y Z Cantilever & Tip Sample AFM is a versatile tool for imaging materials at the nanoscale in virtually native environments. Contrasts in the AFM image give valuable information on dimensions of the material. It gives a real-time or dynamic analysis. Scanning parameters should be considered. Topographic information

8 Tapping-mode AFM Feedback loop Detector electronics Split photodiode detector Controller electronics Laser Scanner X, Y Z Cantilever & Tip Sample AFM is a versatile tool for imaging materials at the nanoscale in virtually native environments. Contrasts in the AFM image give valuable information on dimensions of the material. It gives a real-time or dynamic analysis. Scanning parameters should be considered. Topographic information

9 Scanning parameters Uncertainties - Instrument setting & noise - Image processing procedures Weak damping soft tapping Tip/sample interaction - Setpoint ratio -Scan size -Scan rate setpoint 0 Strong damping hard tapping setpoint 0

10 Motivation The present work was motivated by the need to understand the response of nano-scale cellulosic fibers to changes in environmental conditions (relative humidity). The fiber s behavior is important for engineering bio-based materials (paper, natural composites, and nano-composites).

11 Objectives To develop a well-characterized method for the metrological imaging (length, width, height, and cross sectional area) of cellulose aggregate fibrils under varying relative humidity conditions. To take account of AFM instrument parameter, the effect of probe/surface convolutions, and actual surface topographical characteristics.

12 Experimental

13 Materials Standard height gratings - 1D arrays of rectangular SiO 2 5µm A B (A) 20 nm grating (B) 102 nm grating (C) 500 nm grating C 10 µm 10 µm scan size

14 Cellulose aggregate fibril (CAF) Optical (A to C) & AFM image (D) A B 100µm C 100µm D 100µm 5µm (A) Valley beater (x500); (B) PFI milling filtercake (x500); (C) PFI milling filtrate(x500); (D) PFI milling filtrate (20 μm)

15 Specifications of CAF Optical image AFM image (10 µm) 500X μm Height (nm) μm 7.5 μm 0 5µm X (μm) Length (µm) : 50 to 500 Thickness (nm) : 50 to 500 Width (nm) : 100 to 5000 Aspect ratio (l/d) : 2,000 to 10,000

16 Preparation of slide glass sample of CAF Cleaning of a slide glass - EtOH and 1 N HCl (1:1, v/v) - Ultrasonic (three times) Dropping 10 µl of diluted CAF solution into 20 spots in the cleaned slide glass Rotated for AFM Imaging Conditioned for one week - 23 C and 50 % RH

17 Experimental setup Humidity generator system Installed In a paper testing room (23 C and 50%RH) Profiles on humidity with saturated solutions(23 o C) 88% RH 50% RH to 23% RH(cycle 1) 23% RH to 98% RH(cycle 2) 98% RH to 23% RH(cycle 3) 23% RH to 98% RH(cycle 4) RH % % RH III II IV I 27% RH Time(min.)

18 Humidity cycles Forced change (FC) & Free decay (FD) Conditioned at 50 % RH Forced change (FC) Only Conditioned at 50 % RH Pump to 85 % RH for 2 hr. Pump to 85 % RH for 2 hr. Free decay to 50 % RH for 24 hrs. Pump to 25 % RH for 2 hr. Pump to 25 % RH for 2 hr. Pump to 85 % RH for 2 hr. Free decay to 50 % RH for 24 hrs.

19 Image reconstruction procedure Algorithm 5 µm 5 µm 5 µm 5 µm

20 Rapid feature analysis (RFA) technique Width Z (a) Z (b) Z` = f(x) Length Fitted baseline Z = 0 X (a) X (b) Height = Area / width

21 Results & Discussion

22 Comparison of analysis techniques 2µm 102 nm Grating Mean area (nm 2 x 1000), Mean height (nm) Manual-100 RFA-100 RFA-400 Area Height Width Mean width (μm) Manual-100 & RFA-100 (4 µm 2.5 µm) RFA- 400 (4 µm 10 µm) Dimensions of 1D-grating standard were well characterized based on the RFA technique

23 Effect of image pre-processing 20 nm 20 nm 102 nm 500 nm A B C D nm Scan area (10 µm 10 µm) was most effective to apply the baseline correction

24 Effect of scan area Mean area (nm 2 x 1000), Mean height (nm) Area Height Width 20 nm 102nm 500 nm Mean width (μm) 0 TGZ01 TGZ02 TGZ02* TGZ03* 4 μm X 4 μm 10 μm X 10 μm The scan area with 10 µm 10 µm was selected to evaluate other scanning parameter 0.0

25 Effect of scan rate nm grating, 10 µm 10 µm (scan area) Height (nm) Area (μm 2 x 1000) Area Width Height Width (μm) Scan rate (Hz) Scan rate has a significantly impact on the dimensions of the step feature.

26 Effect of setpoint ratio Fixed drive amplitude (0.187 V), Three setpoints (0.15, 0.375, 0.5 V) Height (nm) Area (nm 2 x 1000) Area Width Height Width (μm) Setpoint ratio (A setpoint /A 0 ) A decrease in the setpoint ratio (hard tapping) resulted in a substantial decrease in the measured dimensions compared to higher setpoint ratio (soft tapping).

27 Effect of setpoint ratio Three drive amplitude (0.14, 0.187, 0.3 V), fixed setpoint (50% damping) Height (nm) Area (nm 2 x 1000) Area Width Height Width (μm) Setpoint ratio (A setpoint /A 0 ) Drive amplitude (0.25 value) and setpoint (50% damping) were closest to the nominal step height.

28 Dimensions of CAF with optimized scanning parameters Mean area (nm 2 x 1000) Setpoint ratio Setpoint ratio Area Width Height Mean width (μm) Mean height (nm) Each fresh engagement - 10 µm 10 µm - 1 Hz, 400 resolutions , 50% damping - 50 % RH, 23 C A B C D E Trials Variations over five engagements were 12 nm 2 in area (2 %), 10 nm in height (3 %), and 5 nm in width (0.2%).

29 Standard grating as a function of RH FC & FD FC only Start FC (1) - high RH FD (2) - to ambient FC (3) - low RH FD (4) - to ambient Changes in area (%) Changes in height (%) Changes in width (%) RH (%) RH (%) Changes in area (%) Changes in height (%) Changes in width (%) RH (%) RH (%) Start FC (1) - high RH FC (2) - low RH RH (%) RH (%)

30 Dimensional changes of CAF Mean area (nm 2 x 1000) RH (%) Start FC (1) FD (2) FC (3) FD (4) FC (5) FD (6) FC (7) FD (8) R 2 = Mean height (nm) RH (%) Start FC (1) FD (2) FC (3) FD (4) FC (5) FD (6) FC (7) FD (8) R 2 =0.65 Area and height are found to increase with increase in relative humidity, Such changes are expected due to the hygro-scopic nature of cellulose.

31 Dimensional changes of CAF Mean width (μm) Start FC (1) FD (2) FC (3) FD (4) FC (5) FD (6) FC (7) FD (8) R 2 =0.16 Mean length (μm) Start FC (1) FD (2) FC (3) FD (4) FC (5) FD (6) FC (7) FD (8) R 2 = RH (%) RH (%) Unexpectedly, the length of the CAF was found to shrink as the relative humidity increased.

32 AFM images of CAF 50% RH 72% RH 44% RH Length: µm Length: µm Length: µm

33 Conclusions

34 Scan rate and dampening were found to significantly affect measured dimensions. Variations attributed to experimental error in the height, width, and area were less than 3 %. Changes in the width of the CAF were not correlated with changes in the RH, while changes in the height and area of the CAF were positively correlated with the RH. The CAF showed reduced swelling in the width, which may be attributed to the attachment of the CAF to the glass slide.

35 The length of the CAF exhibited negative longitudinal hygro-expansion coefficient (reduce in length as RH increased). This could be attributed to the simple conservation of volume of the CAF as it undergoes a strain, or to the specific arrangement of cellulose chains within the CAF. CAF exhibit unexpected hygro-expansion properties, which must be carefully considered when engineering composite materials.

36 Acknowledgements McIntire-Stennis Program for financial support of this research Dr. Dimitris S. Argyropoulos, Dr. Martin A. Hubbe Forest material research group members

37 Thank You! PRESENTED BY Jung Myoung Lee, John A. Heitmann, Joel J. Pawlak* Department of Wood & Paper Science, North Carolina State University, Raleigh, NC, USA Tel: , fax: Corresponding author;