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Thermal Response in Crystalline Iβ Cellulose: A Molecular Dynamics Study
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. (Biokompositer)
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. (Biokompositer)ORCID iD: 0000-0001-5818-2378
2007 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 111, no 30, 9138-9145 p.Article 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. Vol. 111, no 30, 9138-9145 p.
Keyword [en]
cellulose, molecular dynamics, thermal expansion
National Category
Physical Chemistry Polymer Chemistry
URN: urn:nbn:se:kth:diva-9474DOI: 10.1021/jp072258iISI: 000248315700062ScopusID: 2-s2.0-34548219478OAI: diva2:114133
QC 20100618Available from: 2008-11-17 Created: 2008-11-06 Last updated: 2010-07-05Bibliographically approved
In thesis
1. Crystalline cellulose in bulk and at interfaces as studied by atomistic computer simulations
Open this publication in new window or tab >>Crystalline cellulose in bulk and at interfaces as studied by atomistic computer simulations
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. vi, 67 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2008:69
Cellulose, molecular dynamics simulation, interfaces
National Category
Chemical Sciences
urn:nbn:se:kth:diva-9570 (URN)978-91-7415-166-4 (ISBN)
Public defence
2008-12-10, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
QC 20100621Available from: 2008-11-17 Created: 2008-11-17 Last updated: 2012-02-23Bibliographically approved

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