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Crystalline cellulose in bulk and at interfaces as studied by atomistic computer simulations
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. (Biokompositer)
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Cellulose is a linear polysaccharide, serving as reinforcement in plant cell walls.Understanding its structure and properties is of importance in the developmentof nanostructured cellulose materials. The aim of this thesis is to address thisquestion by applying the computer simulation technique Molecular Dynamics(MD) onto an atomistic model of a native crystal form of cellulose.A molecular model of crystalline cellulose Iβ was developed and simulatedwith the GROMACS simulation software package.Temperature dependence of the crystal bulk model was investigated. A gradualtransition was observed between 350 K and 500 K in concordance with experimentalresults. The high temperature structure differed from the originalstructure in terms of crystal cell parameters, hydrogen bonding network andelastic modulus.Spin-lattice relaxation times, T1, from solid-state Nuclear Magnetic Resonancespectroscopy were compared with values calculated from the dynamics ofthe C4-H4 vector in MD simulations. Calculated T1 compared well with experimentallyobtained, suggesting well reproduced dynamics. Moreover, a differencein T1 of about a factor 2 was found for C4 atoms at surfaces parallel to differentcrystallographic planes. This supports a proposed explanation regarding anobserved doublet for C4 atoms in the NMR spectrum.Interaction energies between crystalline cellulose and water and 6− hydroxyhexanal(CL) were determined from simulations. Water was found to interactstronger with cellulose than CL. Moreover, the effect of grafting CL onto surfacecellulose chains was examined. For both water and CL interfaces, grafting ledto increased interaction. Electrostatic interactions were dominating in all cases,however grafting increased the importance of van der Waals interactions.The experimental approach to investigate polymer desorption by pulling itfrom a surface by the use of Atomic Force Microscopy (AFM) was enlightenedwith a modelling study. A single cellulose octamer was pulled from a cellulosecrystal into water and cyclohexane. Resulting pull-off energies proved a clearsolvent effect, 300 − 400 [kJ/mole] in cyclohexane and 100 − 200 [kJ/mole] inwater.In general, MD was shown to be useful when applied in combination withfeasible experimental techniques such as NMR and AFM to increase the fundamentalunderstanding of cellulose structure and properties.

Abstract [sv]

Cellulosa förstärker cellväggen i växter i form av nanostrukturerade och mycketstarka fibriller. För utvecklingen av nya cellulosamaterial från dessa fibriller ären förståelse för cellulosans struktur och egenskaper viktig. Syftet med dennaavhandling är att med hjälp av en atomistisk modell och molekyldynamiskadatorsimuleringar (MD) öka kunskapen om cellulosa på atomär nivå.En atomistisk modell av kristallin cellulosa Iβ utvecklades och simuleradesmed simuleringsprogrampaketet GROMACS.Temperaturberoendet hos kristallin cellulosa i bulk undersöktes. Mellan350 K och 500 K skedde en gradvis kristallin strukturomvandling. Vid högre temperaturhade cellulosan annorlunda kristall-enhetscellsparametrar, vätebindingsmönsteroch elastisk modul jämfört med orginalstrukturen.Systemet cellulosa-vatten har stor praktisk betydelse. Spinn-gitter-relaxationstiderT1 beräknades därför från dynamiken hos C4-H4-vektorn i MD-simuleringaroch jämfördes med värden uppmätta med fastfas-NMR. De beräknadevärdena stämde väl överens med de experimentella och dynamiken vid ytan kanantas vara välreproducerad i modellen. Dessutom kunde en skillnad i T1 meden faktor 2 för C4-atomer på ytkedjor vid olika kristallografiska plan påvisas.Simuleringsresultaten stödjer därmed en tidigare föreslagen förklaring till endubblett för C4-atomer i cellulosans NMR-spektrum.Växelverkansenergier mellan cellulosa och polymeren PCL är intressant förnanokompositmaterial. Därför bestämdes växelverkansenergier mellan kristallincellulosa och vatten och cellulosa och 6-hydroxyhexanal (CL). Växelverkan mellancellulosa med vatten visade sig vara större än mellan cellulosa och CL.Ympning av CL-molekyler på cellulosaytan ledde till ökad växelverkan för såvälgränsytor mot vatten som mot CL. Elektrostatisk växelverkan dominerade vidsamtliga gränsytor, även om CL-ympning orsakade ökad andel av van der Waalskrafter.Polymerdesorption kan undersökas med hjälp av atomkraftmikroskopi (AFM).Ett simulerat experiment med MD utfördes därför genom att en cellulosaoktamerdrogs från en cellulosayta in i vatten eller cyklohexan. Det krävdes avsevärtmindre energi att dra loss oktameren i cyklohexan (300−400 kJ/mol) jämförtmed vatten (100 − 200 kJ/mol). Resultaten analyserades i termer av specifikväxelverkan mellan cellulosaoktameren och identifierbara kemiska grupper påcellulosaytan.MD har stor potential att öka förståelsen för cellulosa på molekylär nivå.MD-simuleringar kan inspirera experimentella mätningar genom upptäckter avnya fenomen. MD kan dessutom tillföra nya aspekter vid analys av experimentellaresultat. Det har i avhandlingen demonstrerats för metoder som NMR,AFM, mekanisk provning och mätning av termisk utvidgning

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , p. vi, 67
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:69
Keyword [en]
Cellulose, molecular dynamics simulation, interfaces
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-9570ISBN: 978-91-7415-166-4 (print)OAI: oai:DiVA.org:kth-9570DiVA, id: diva2:126412
Public defence
2008-12-10, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100621Available from: 2008-11-17 Created: 2008-11-17 Last updated: 2012-02-23Bibliographically approved
List of papers
1. Thermal Response in Crystalline Iβ Cellulose: A Molecular Dynamics Study
Open this publication in new window or tab >>Thermal Response in Crystalline Iβ Cellulose: A Molecular Dynamics Study
2007 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 111, no 30, p. 9138-9145Article in journal (Refereed) Published
Abstract [en]

The influence of temperature on structure and properties of the cellulose Iβ crystal was studied by molecular dynamics simulations with the GROMOS 45a4 force-field. At 300 K, the modeled crystal agreed reasonably with several sets of experimental data, including crystal density, corresponding packing and crystal unit cell dimensions, chain conformation parameters, hydrogen bonds, Young's modulus, and thermal expansion coefficient at room temperature. At high-temperature (500 K), the cellulose chains remained in sheets, despite differences in the fine details compared to the room-temperature structure. The density decreased while the a and b cell parameters expanded by 7.4% and 6%, respectively, and the c parameter (chain axis) slightly contracted by 0.5%. Cell angles α and β divided into two populations. The hydroxymethyl groups mainly adopted the gt orientation, and the hydrogen-bonding pattern thereby changed. One intrachain hydrogen bond, O2'H2'···O6, disappeared and consequently the Young's modulus decreased by 25%. A transition pathway between the low- and high-temperature structures has been proposed, with an initial step being an increased intersheet separation, which allowed every second cellulose chain to rotate around its helix axis by about 30°. Second, all hydroxymethyl groups changed their orientations, from tg to gg (rotated chains) and from tg to gt (non-rotated chains). When temperature was further increased, the rotated chains returned to their original orientation and their hydroxymethyl groups again changed their conformation, from gg to gt. A transition temperature of about 450 K was suggested; however, the transition seems to be more gradual than sudden. The simulated data on temperature-induced changes in crystal unit cell dimensions and the hydrogen-bonding pattern also compared well with experimental results.

Place, publisher, year, edition, pages
Washington: ACS Publications, 2007
Keyword
cellulose, molecular dynamics, thermal expansion
National Category
Physical Chemistry Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-9474 (URN)10.1021/jp072258i (DOI)000248315700062 ()2-s2.0-34548219478 (Scopus ID)
Note
QC 20100618Available from: 2008-11-17 Created: 2008-11-06 Last updated: 2017-12-14Bibliographically approved
2. Dynamics of Cellulose-Water Interfaces: NMR Spin-Lattice Relaxation Times Calculated from Atomistic Computer Simulations
Open this publication in new window or tab >>Dynamics of Cellulose-Water Interfaces: NMR Spin-Lattice Relaxation Times Calculated from Atomistic Computer Simulations
Show others...
2008 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 112, no 9, p. 2590-2595Article in journal (Refereed) Published
Abstract [en]

Solid-state nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy has often been used to study cellulose structure, but some features of the cellulose NMR spectrum are not yet fully understood. One such feature is a doublet around 84 ppm, a signal that has been proposed to originate from C4 atoms at cellulose fibril surfaces. The two peaks yield different T1, differing by approximately a factor of 2 at 75 MHz. In this study, we calculate T1 from C4-H4 vector dynamics obtained from molecular dynamics computer simulations of cellulose Iβ-water interfacial systems. Calculated and experimentally obtained T1 values for C4 atoms in surface chains fell within the same order of magnitude, 3-20 s. This means that the applied force field reproduces relevant surface dynamics for the cellulose-water interface sufficiently well. Furthermore, a difference in T1 of about a factor of 2 in the range of Larmor frequencies 25-150 MHz was found for C4 atoms in chains located on top of two different crystallographic planes, namely, (110) and (10). A previously proposed explanation that the C4 peak doublet could derive from surfaces parallel to different crystallographic planes is herewith strengthened by computationally obtained evidence. Another suggested basis for this difference is that the doublet originates from C4 atoms located in surface anhydro-glucose units with hydroxymethyl groups pointing either inward or outward. This was also tested within this study but was found to yield no difference in calculated T1.

Place, publisher, year, edition, pages
Washington: ACS Publications, 2008
Keyword
cellulose-water interface, spin-lattice relaxation times, molecular dynamics
National Category
Physical Chemistry Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-9475 (URN)10.1021/jp074641t (DOI)000253511900006 ()2-s2.0-41549090192 (Scopus ID)
Note

QC 20100618

Available from: 2008-11-17 Created: 2008-11-06 Last updated: 2017-12-14Bibliographically approved
3. Molecular modeling of interfaces between cellulose crystals and surrounding molecules: Effects of caprolactone surface grafting
Open this publication in new window or tab >>Molecular modeling of interfaces between cellulose crystals and surrounding molecules: Effects of caprolactone surface grafting
2008 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 44, no 11, p. 3662-3669Article in journal (Refereed) Published
Abstract [en]

A technical problem in cellulosic nanocomposite materials is the weak interaction between hydrophilic cellulose and hydrophobic polymer matrices. One approach to solve this difficulty is to chemically graft monomers of the matrix polymer onto the cellulose surface. An important question is to understand the effect such surface modification has on the interfacial properties. Semi-empirical approaches to estimate work of adhesion based on surface energies do not provide information on specific molecular interactions. Details about these interactions were obtained using molecular dynamics (MD) simulation. Cellulose interfaces with water and caprolactone medium were modeled with different amounts of grafted caprolactone. The modification lead to an increased work of adhesion between the surface and its surrounding medium. Furthermore, the MD simulations showed that the interaction between cellulose, both modified and non-modified, and surrounding medium is dominated by Coulomb interactions, predominantly as hydrogen bonds.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2008
Keyword
cellulose, molecular dynamics simulation, surface modification
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-9476 (URN)10.1016/j.eurpolymj.2008.08.029 (DOI)000261722600033 ()2-s2.0-55149089482 (Scopus ID)
Note
QC 20100618Available from: 2008-11-17 Created: 2008-11-06 Last updated: 2017-12-14Bibliographically approved
4. Force Pulling of Single Cellulose Chains at the Crystalline Cellulose-Liquid Interface: A Molecular Dynamics Study
Open this publication in new window or tab >>Force Pulling of Single Cellulose Chains at the Crystalline Cellulose-Liquid Interface: A Molecular Dynamics Study
2009 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 25, no 8, p. 4635-4642Article in journal (Refereed) Published
Abstract [en]

Pulling single cellulose molecules from a crystalline cellulose surface has been modeled by molecular dynamics (MD) simulations of the experimental procedure used in atomic force microscopy (AFM). Specifically, the aim of the study was to investigate cellulose interactions at desorption. Simulations were performed in both water and the organic solvent cyclohexane. Moreover, the effects of initial octamer conformation and orientation with respect to the surface chains were studied. A strong effect from the solvent was observed. In cyclohexane, normal forces of 200-500 pN and energies of 43.5 +/- 6.0 kJ/mol glucose unit were required to pull off the octamer. The normal forces in water were substantially lower, around 58 pN, and the energies were 18.2 +/- 3.6 kJ/mol glucose unit. In addition, the lateral components of the pull-off force were shown to provide information on initial conformation and orientation. Hydrogen bonds between the octamer and surface were analyzed and found to be an important factor in the pull-off behavior. Altogether, it was shown that MD provides detailed information on the desorption processes that may be useful for the interpretation of AFM experiments.

Keyword
Single molecule pulling, molecular dynamics, cellulose
National Category
Polymer Chemistry Dentistry
Identifiers
urn:nbn:se:kth:diva-9478 (URN)10.1021/la803915c (DOI)000265281700064 ()2-s2.0-65249157941 (Scopus ID)
Note
QC 20100618. Updated from manuscript to article in journal.Available from: 2008-11-18 Created: 2008-11-06 Last updated: 2017-12-14Bibliographically approved

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