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Exploring Biopolymer-Clay Nanocomposite Materials by Molecular Modelling
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0001-8986-526X
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, bio-nanocomposites made from two alternative biopolymers and montmorillonite (Mnt) clay have been investigated by molecular modelling. These biopolymers are xyloglucan (XG) and chitosan (CHS), both of which are abundant, renewable, and cost-effective. After being reinforced by Mnt clay nanoparticles, the polymer nanocomposites gains in multifunctionality and in the possibility to register unique combinations of properties, like mechanical, biodegradable, electrical, thermal and gas barrier properties. I apply molecular dynamics (MD) simulations to study the interfacial mechanisms of the adhesion of these biopolymers to the Mnt nanoplatelets at an atomic level.

For the XG-Mnt system, a strong binding affinity of XG to a fully hydrated Mnt interface was demonstrated. It was concluded that the dominant driving force for the interfacing is the enthalpy, i.e. the potential energy of the XG-Mnt interacting system. The adsorbed XG favors a flat conformation with a galactose residue in its side chain that facilitates the adsorption of the polymer to the nanoclay.

 The XG adsorption was found do depend strongly on the hydration ability of counterions. The binding affinity of XG to Mnt was found to be strongest in the K-Mnt/XG system, followed by, in decreasing order, Na-Mnt/XG, Li-Mnt/XG, and Ca-Mnt/XG. The competing mechanism between ions, water and the XG in the interlayer region was shown to play an important role.

The dimensional stability upon moisture exposure, i.e. the ability of a material to resist swelling, is an important parameter for biopolymer-clay nanocomposites. While pure clay swells significantly even at low hydration levels, it is here shown that for the XG-Mnt system, at a hydration level below 50%, the inter-lamellar spacing is well preserved, suggesting a stable material performance. However, at higher hydration levels, the XG-Mnt composite was found to exhibit swelling at the same rate as the pure hydrated Mnt clay.

For the CHS-Mnt system, the significant electrostatic interactions from the direct charge-charge attraction between the polymer and the Mnt clay play a key role in the composite formation. Varying the degree of acetylation (DA) and the degree of protonation (DPr) resulted in different effects on the polymer-clay interaction. For the heavily acetylated CHS (DA > 50%, also known as chitin), the strong adhesion of the neutral chitin to the Mnt clay was attributed to strong correlation between the acetyl functional groups and the counterions which act as an electrostatic “glue”. Similarly, the poor adhesion of the fully deprotonated (DPr = 0%) neutral CHS to the clay is attributed to a weak correlation between the amino functional group and the counterions.

The stress-strain behavior of the CHS-Mnt composite shows that the mechanical properties are highly affected by the volume fraction of the Mnt clay and the degree of exfoliation of the composite. The material structure has a close relationship to the material properties.

Biopolymer-clay nanocomposites hold a bright future to replace petroleum-derived polymer plastics and will become widely used in common life. The theme of the thesis is that further critical improvements of these materials can be accomplished by development of the experimental methods in conjunction with increased understanding of the interactions between polymer, clay, water, ions, solutions in the polymer-clay mixtures provided by molecular modelling. 

Abstract [sv]

I denna avhandling har molekylär modellering och molekyldynamisk (MD) simulering använts för att studera modellsystem för bio-nanokompositer bestående av montmorillonit-lera samt två olika sorters biopolymerer – xyloglukan (XG) och kitosan (CHS). Båda dessa polymerer är naturligt förekommande och mycket vanliga. De är dessutom förnyelsebara och kostnadseffektiva. Då polymererna förstärkts med nanopartiklar av montmorillonit får det resulterande kompositmaterialet en unik kombination av egenskaper såsom mekaniska, elektriska, termiska och barriär egenskaper etc. Genom att använda molekyldynamiska (MD) simuleringar, studeras här växelverkan mellan dessa biopolymerer och lernanopartiklar (Mnt) på grundläggande atomistisk detaljnivå.

Mellan XG och Mnt i ett fullt hydrerat system kunde stark bindningsaffinitet påvisas. Den dominerande drivkraften för affiniteten var entalpi, d.v.s. potentiell växelverkansenergi. Den adsorberade XG-kedjan antar en platt konformation på ytan. Ett förslag utifrån simuleringsresultaten var att galaktosresidyn i xyloglukanets sidokedja underlättar adsorptionen till lerytan.

Simuleringarna kunde också visa att adsorption av XG till Mnt beror starkt på motjonernas hydreringsförmåga. Bindningsaffiniteten mellan XG och Mnt var som starkast i K-Mnt/XG- systemet. Därefter följde, i minskande ordning, Na-Mnt/XG, Li-Mnt/XG och Ca-Mnt/XG. Det kunde visas att strukturen vid gränsytan styrs av konkurrerande mekanismer mellan joner, vatten och XG.

Dimensionsstabilitet vid fuktexponering, d.v.s. förmågan hos ett material att motverka svällning, är en viktig egenskap för biopolymer-lernanokompositer. Ren lera sväller signifikant även vid låga fukthalter. Dock kunde MD simuleringar visa att ett modellsystem av XG-Mnt behåller sitt ursprungliga interlamellära avstånd vid hydreringsnivåer under 50%, vilket indikerar ett stabilare material. Vid högre hydrering uppmättes dock svällningen vara densamma som för ren lera.

I CHS-Mnt-systemet visade det sig att direkt elektrostatisk växelverkan med signifikant styrka mellan laddningar på polymer och Mnt-yta spelar störst roll för kompositformeringen. Olika effekt på polymer-lerväxelverkan uppnåddes genom att variera acetyleringsgraden (DA) respektive protoneringsgraden (DPr). För den tungt acetylerade CHS-polymeren (DA > 50%, även kallad kitin) visade sig den starka vidhäftningen bero på korrelation mellan acetylgrupperna och motjonerna som i sin tur verkade som ett elektrostatiskt “lim”. På liknande sätt kunde den svaga vidhäftningen mellan fullt deprotonerad (DPr = 0%) neutral CHS och lera förklaras med en betydligt svagare korrelation mellan aminogrupperna och motjonerna.

Spänning-töjningsbeteendet hos CHS-Mnt modellen visar att dess mekaniska egenskaper beror kraftigt på volymsandelen Mnt och graden av exfoliering i kompositen. Materialets struktur är nära relaterat till materialegenskaperna.

Framtiden för nanokompositer av biopolymerer och lera är ljus då de kan komma att ersätta oljebaserade plaster och användas frekvent i våra dagliga liv. Materialen kommer successivt förbättras genom utveckling av experimentella metoder i kombination med molekylmodellering för ökad förståelse för växelverkan mellan polymer, lera, vatten, joner och lösningsmedel. 

Abstract [zh]

本论文利用分子动力学模拟技术研究了两种备选生物大分子与蒙脱土(Montmorillonite, Mnt)(一种粘土)组成的生物纳米复合材料,分别是木葡聚糖(Xyloglucan, XG)/蒙脱土和壳聚糖(Chitosan, CHS)/蒙脱土。木葡聚糖与壳聚糖都是自然界广泛存在的生物大分子,资源丰富且取材面宽,提取及加工成本低廉,加之可以生物降解并可再生,是优秀的生物复合材料备选原料。经过蒙脱土纳米颗粒加固后,这些基于生物大分子的复合材料将获得多功能且有多种独特特性相结合的优点,比如,更好的力学性能,生物可降解,良好的导电性能,传热性能和屏蔽气体与液体侵扰的能力等等。论文中,我们采用分子动力学模拟的方法着重对生物大分子与蒙脱土在界面上的粘附相互作用机理进行了深入探讨。 

首先,对于木葡聚糖/蒙脱土纳米复合材料,我们发现糖分子与土分子间有着很强的天然亲和力。研究证明它们之间的这种相互作用,热焓是主要的推动力,也就是糖和土分子间的相互作用势能。含有半乳糖残基的木葡聚糖分子(本文中亦称天然木葡聚糖分子)吸附到粘土表面后,分子构型呈现扁平状,半乳糖残基似有辅助木葡聚糖大分子吸附到粘土颗粒上的作用。 

进一步研究发现,木葡聚糖分子在粘土表面上的吸附与溶液中抗衡离子的水和作用密切相关。在钾离子平衡的糖/粘土系统中,糖分子与土分子的相互作用最强,钠离子平衡的糖/粘土系统次之,紧接着是锂离子平衡的糖/粘土系统,最弱的是钙离子平衡的糖/粘土系统。研究发现,离子,水分子,以及糖分子在粘土层间的竞争机制在糖分子的粘附过程中起着重要的作用。 

材料暴露于潮湿环境中的尺寸稳定性,也就是材料抗肿胀的能力是生物大分子/蒙脱土所构成的复合材料的重要参数。蒙脱土自身即使在很低的潮湿环境下就会有明显地膨胀现象,然而,对木葡聚糖/蒙脱土复合材料来说,尺寸稳定性可以在水和值低于50%以下有效保存。其夹层尺寸的稳定保持暗示了材料在这个程度的潮湿环境下的稳定性。然而,当水和值高于50%时,木葡聚糖/蒙脱土复合材料将出现明显的肿胀现象,表现在夹层尺寸的明显增大,且其膨胀速率与粘土自身的膨胀速率逐渐趋于相当水平。 

其次,对于壳聚糖/蒙脱土复合材料,我们发现由电荷-电荷间直接产生地强烈的静电吸引作用是壳聚糖分子与蒙脱土分子相互粘附并构成复合材料的关键因素。通过改变壳聚糖分子的乙酰化程度(Degree of acetylation, DA)和质子化程度(Degree of protonation, DPr),糖分子与土分子的相互作用有着显著地不同。对于乙酰化程度(DA)高于50%的壳聚糖分子(亦成为甲壳素分子chitin, CHT),电中性的甲壳素分子与土分子间的强吸附作用源于乙酰基功能团与抗衡离子的强相关性。抗衡离子此时扮演着类似于“电子胶”的作用,可以有效地将电中性的甲壳素分子与土分子粘结在一起。类似地,当质子化程度最低时,亦即壳聚糖分子完全非质子化,即呈现电中性时,较差的糖/土吸附作用源于氨基功能团与抗衡离子的较弱的相关性。 

进一步对壳聚糖/蒙脱土复合材料的分子系统进行应力应变计算发现,复合材料的力学性能直接受蒙脱土体积分数和其剥离程度的影响,通常,粘土的体积分数越大体系的力学性能越高,且剥离程度对材料的整体性能也有直接影响。因此,材料的结构与其性能的表征有着密切联系。

 我们相信生物大分子与蒙脱土构成的生物复合材料有着光明的前景,可以取代石油提取物制成的塑料材料,并将能够广泛应用在日常生活中。通过实验技术的改善和应用分子模拟技术对复合材料体系中生物大分子,蒙脱土分子,水分子,离子,溶液环境等混合物质相互作用的理解增加,这种可再生的新材料将会得到重要改进,这也是整本论文的主旋律。

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , x, 62 p.
Series
TRITA-BIO-Report, ISSN 1654-2312 ; 2015:11
Keyword [en]
bio-nanocomposite, interfacial mechanism, binding affinity, dimensional stability, counterions, hydration, stress-strain behavior, xyloglucan, chitosan, chitin, molecular dynamics, montmorillonite.
Keyword [zh]
生物复合材料,界面机理,分子间亲和力,材料尺寸稳定性,抗衡离子,水合作用,应力应变表现, 木葡聚糖,壳聚糖,甲壳素,分子动力学模拟,蒙脱土
Keyword [sv]
bio-nanokompositer, gränsytor, affinitet, dimensionsstabilitet, jonhydrering, spännings-töjningsbeteende, xyloglukan, kitosan, kitin, molekyldynamisk, montmorillonit.
National Category
Materials Chemistry Theoretical Chemistry Physical Chemistry
Research subject
Chemistry; Fibre and Polymer Science; Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-166299ISBN: 978-91-7595-550-6 (print)OAI: oai:DiVA.org:kth-166299DiVA: diva2:811733
Public defence
2015-06-04, FB 52,AlbaNova Universitetetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Bio-nanocomposites
Note

QC 20150520

Available from: 2015-05-20 Created: 2015-05-07 Last updated: 2015-06-04Bibliographically approved
List of papers
1. Molecular dynamics simulation of strong interaction mechanisms at wet interfaces in clay-polysaccharide nanocomposites
Open this publication in new window or tab >>Molecular dynamics simulation of strong interaction mechanisms at wet interfaces in clay-polysaccharide nanocomposites
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2014 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 25, 9541-9547 p.Article in journal (Refereed) Published
Abstract [en]

Bio-composites comprised of the polysaccharide xyloglucan (XG) and montmorillonite (MTM) clay has potential as a 'green' replacement of conventional petroleum-derived polymers in the packaging industry. These materials have been shown to possess excellent material properties, even in high relative humidity. Although interfacial interaction between XG and MTM, and the molecular structure of XG can be identified as key parameters for the complex formation process and the resulting tensile properties, these properties are usually difficult to address using experimental methods. Here we use molecular dynamics (MD) simulations to study the adsorption of fully atomistic models of both native and chemically modified XG to MTM clay surfaces in explicit water. We show that the driving force for adsorption is a favorable change in enthalpy, and furthermore that native XG adsorbs stronger than modified XG. This highlights the importance of molecular structure details to molecular adhesion. The present study provides insights into the molecular scale adsorption mechanisms and can therefore help in designing routes for further improvements of bio-composite materials.

Keyword
Force-Field, Side-Chain, Xyloglucan, Cellulose, Nacre, Water
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-148351 (URN)10.1039/c4ta01459f (DOI)000337774100015 ()2-s2.0-84902665538 (Scopus ID)
Funder
Swedish Research Council, VR 2013-4058
Note

QC 20140805

Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2015-05-20Bibliographically approved
2. Molecular Adhesion at Clay Nanocomposite Interfaces Depends on Counterion Hydration-Molecular Dynamics Simulation of Montmorillonite/Xyloglucan
Open this publication in new window or tab >>Molecular Adhesion at Clay Nanocomposite Interfaces Depends on Counterion Hydration-Molecular Dynamics Simulation of Montmorillonite/Xyloglucan
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2015 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 16, no 1, 257-265 p.Article in journal (Refereed) Published
Abstract [en]

Nacre-mimetic clay/polymer nanocomposites with clay platelet orientation parallel to the film surface show interesting gas barrier and mechanical properties. In moist conditions, interfacial adhesion is lowered and mechanical properties are reduced. Molecular dynamic simulations (MD) have been performed to investigate the effects of counterions on molecular adhesion at montmorillonite clay (Mnt)-xyloglucan (XG) interfaces. We focus on the role of monovalent cations K+, Na+, and Li+ and the divalent cation Ca2+ for mediating and stabilizing the Mnt/XG complex formation. The conformation of adsorbed XG is strongly influenced by the choice of counterion and so is the simulated work of adhesion. Free energy profiles that are used to estimate molecular adhesion show stronger interaction between XG and clay in the monovalent cation system than in divalent cation system, following a decreasing order of K-Mnt, Na-Mnt, Li-Mnt, and Ca-Mnt. The Mnt clay hydrates differently in the presence of different counterions, leading to a chemical potential of water that is highest in the case of K-Mnt, followed by Na-Mnt and Li-Mnt, and lowest in the case of Ca-Mnt. This means that water is most easily displaced from the interface in the case of K-Mnt, which contributes to the relatively high work of adhesion. In all systems, the penalty of replacing polymer with water at the interface gives a positive contribution to the work of adhesion of between 19 and 35%. Our work confirms the important role of counterions in mediating the adsorption of biopolymer XG to Mnt clays and predicts potassium or sodium as the best choice of counterions for a Mnt-based biocomposite design.

National Category
Biochemistry and Molecular Biology Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-159975 (URN)10.1021/bm5014525 (DOI)000347864200025 ()2-s2.0-84921266287 (Scopus ID)
Note

QC 20150304

Available from: 2015-03-04 Created: 2015-02-12 Last updated: 2017-12-04Bibliographically approved
3. Hydration and dimensional stability of the intercalated galleries in xyloglucan/montmorillonite nanocomposites studied by molecular dynamics simulations
Open this publication in new window or tab >>Hydration and dimensional stability of the intercalated galleries in xyloglucan/montmorillonite nanocomposites studied by molecular dynamics simulations
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The outstanding properties of biological composite nacre materials have for a long time inspired research and development of man-made bionanocomposites. One of the most recent nacre-mimetic bionanocomposites comprising xygloglucan (XG) and montmorillonite (Mnt) clay has been investigated by related model systems through Molecular dynamics (MD) simulations. The expansion of the inter-gallery of the XG-Mnt composites when exposed to water, has been found to be a key issue for the material property. In order to shed light on the mechanism for this swelling behavior we have investigated the relation between the hydration and the dimensional stability of the inter-gallery in XG-Mnt composites, exploring also the role of the dynamic state of the polymer XG for the dimensional change. We find that at a hydration level below 50%, XG-Mnt possesses good dimensional stability, suggesting a constant performance of the material, while at a hydration level of 75%, the expansion ratio of the composite is found to be slightly smaller than the swelling of Mnt clay. At the four-layer hydrate formation with a hydration level of 100%, the swelling ratios of clay and the2composite reach the same value, suggesting a critical point of losing dimensional stability. We conclude that the strong adhesion between the polymer XG and the Mnt clay is the main driving force for the preservation of the stability at lower hydration conditions, while the dynamics of the XG polymer is related to the losing of dimensional stability for the composite at higher hydration levels. The ramification of these results in terms of moisture sensitivity of the material is briefly discussed.

Keyword
organic polymers, interfacial water, inter-gallery distance, counterions, bionanocomposites, moisture, relative humidity, interlay spacing, basal spacing
National Category
Materials Chemistry Theoretical Chemistry Physical Chemistry
Research subject
Materials Science and Engineering; Chemistry; SRA - E-Science (SeRC); Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-166284 (URN)
Projects
Bio-nanocomposite Xyloglucan/Montmorillonite clay
Note

QS 2015

Available from: 2015-05-07 Created: 2015-05-07 Last updated: 2015-05-20Bibliographically approved
4. Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay
Open this publication in new window or tab >>Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Molecular dynamics simulations have been performed to investigate molecular adhesion of chitin and chitosan oligomers to montmorillonite (Mnt) clay at different degrees of acetylation (DA, 0%, 20%, 40%, 60%, 80% and 100%) and different degree of protonation (DPr, 0%, 50%, 100% mimicking pH > 6.5, pH = 6.5, pH < 4, respectively) under fully hydrated conditions. Although the Mnt surface is negatively charged and a variation in DA also implies going from a positively charged oligomer at DA = 0% to a neutral oligomer at DA = 100%, the simulations show unexpectedly variation of the total molecular adhesion as a function of DA. From our analysis we propose that this quantitatively similar adhesion arise from two different mechanisms. At low DA, the oligomer is rich in positively charged amino groups interacting strongly with the negatively charged surface by direct electrostatic interaction. On the other hand, at high DA, electrically neutral acetyl groups are strongly correlated with the Na+ counter ions, which are in all cases stuck at the surface and the counter ions seem to act as ‘glue’ between the acetyl groups and the Mnt. However, when protonation was decreased, adhesion was significantly lowered. The reason is conclued by differences in charge distributions of the respective functional groups. A further investigation on the intramolecular hydrogen bonds formed in CHT or CHS shows that the adsorbed conformation of the polymer is also highly affected by DA. This work provides fundamental insights into adhesion mechanisms and is, of potential importance for the development of polymer-clay based composite materials.

Keyword
molecular modelling, bio-nanocomposite, intramolecular hydrogen bonds, interfacial adhesion
National Category
Theoretical Chemistry Materials Chemistry Physical Chemistry
Research subject
Fibre and Polymer Science; Chemistry; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-166292 (URN)
Projects
Bio-nanocomposite Chitosan/Montmorillonite
Note

QS 2015

Available from: 2015-05-07 Created: 2015-05-07 Last updated: 2015-05-20Bibliographically approved
5. Stress-strain behavior in chitosan-montmorillonite nanocomposites studied by molecular dynamics simulations
Open this publication in new window or tab >>Stress-strain behavior in chitosan-montmorillonite nanocomposites studied by molecular dynamics simulations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We have performed molecular dynamics (MD) simulations to study the mechanical properties of bionanocomposites composed of chitosan (CHS) and montmorillonite (Mnt) clay. The stress-strain behavior and the Young’s modulus are calculated to estimate the mechanical properties of the material. Our results show that the mechanical properties of the CHS-Mnt composites are determined by many factors. The volume fraction and the degree of exfoliation of the clay platelets play the key roles. Meanwhile, the molecular adhesion between the polymer CHS and the Mnt at the wet interface is also a main factor. The stress-strain curve of the partially exfoliated CHS-Mnt composite shows significantly larger stiffness than the fully exfoliated one due to the volume fraction of clay is higher in the former case. The stiffness is slightly improved by adding more polymer in the fully exfoliated complex. We conclude that a higher volume faction of the Mnt is an essential premise to fabricate a high-performance composite material. The composite material structure has been found highly relevant to the mechanical properties. In addition, a strong molecular adhesion between the polymer and the clay would be of great importance for the mechanical properties in the composite material. The work provides insight into how to predict the mechanical properties in polymer-clay nanocomposites and may therefore be helpful for the design of bionanocomposite materials.

Keyword
Young’s modulus, interface thickness, volume fraction, interlayer distance, degree of exfoliation
National Category
Materials Chemistry Physical Chemistry Theoretical Chemistry
Research subject
Chemistry; Fibre and Polymer Science; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-166293 (URN)
Projects
Bio-nanocomposite Chitosan/Montmorillonite
Note

QS 2015

Available from: 2015-05-07 Created: 2015-05-07 Last updated: 2015-05-20Bibliographically approved

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